Small and Micro Combined Heat and Power (CHP) Systems: Advanced Design, Performance, Materials and Applications (Woodhead Publishing Series in Energy)

Small and micro combined heat and power (CHP) systems

© Woodhead Publishing Limited, 2011

CHP-Beith-Pre.indd 1

5/6/11 11:05:11 AM

Related titles: Stand-alone and hybrid wind energy systems (ISBN 978-1-84569-527-9) Wind power generation is fast becoming one of the leading renewable energy sources worldwide, with increasing penetration of stand-alone and hybrid wind energy systems, particularly in distributed, isolated, and community power networks. Advanced energy storage and grid integration systems are required to provide secure, reliable power supply to the end user. This book provides a comprehensive overview of the development of stand-alone and hybrid wind energy systems, as well as energy storage and buildinggrid-integration systems. Chapters cover the design, construction, monitoring, control and optimisation of stand-alone and hybrid wind energy technologies, and the continuing development of these systems. Solid oxide fuel cell technology (ISBN 978-1-84569-628-3) High temperature solid oxide fuel cell (SOFC) technology is a promising power generation option which features high electrical efficiency and low emissions of environmentally polluting gases such as CO2, Nox and SOx. It is ideal for distributed stationary power generation applications where both high-efficiency electricity and high-quality heat are in strong demand. This book presents a systematic and in-depth narrative of the technology from the perspective of fundamentals, providing comprehensive theoretical analysis and innovative characterisation techniques for SOFC technology. The book covers the development of SOFC technology, from cell materials and fabrication, to performance analysis and stability and durability issues. Waste to energy (WTE) conversion technology (ISBN 978-0-85709-011-9) Increasing global consumerism and population has led to an increase in the levels of waste produced. Energy recovery represents a useful, additional option in support of an improved and sustainable waste management hierarchy, in local energy supply and wider energy security. Advanced conversion technologies coupled with advanced pollution control systems can be employed to convert these calorific waste materials into clean energy, which can be integrated into buildings, distributed energy networks and national electricity grids. This book provides an integrated, comprehensive reference on waste to energy (WTE) conversion technology. Details of these books and a complete list of Woodhead’s titles can be obtained by:  visiting our web site at www.woodheadpublishing.com  contacting Customer Services (e-mail: [email protected]; fax: +44 (0) 1223 832819; tel.: +44 (0) 1223 499140 ext. 130; address: Woodhead Publishing Limited, 80 High Street, Sawston, Cambridge CB22 3HJ, UK)

If you would like to receive information on forthcoming titles, please send your address details to: Francis Dodds (address, tel. and fax as above; e-mail: francis.dodds@ woodheadpublishing.com). Please confirm which subject areas you are interested in.

© Woodhead Publishing Limited, 2011

CHP-Beith-Pre.indd 2

5/6/11 11:05:11 AM

Woodhead Publishing Series in Energy: Number 18

Small and micro combined heat and power (CHP) systems Advanced design, performance, materials and applications Edited by Robert Beith

Oxford

Cambridge

Philadelphia

New Delhi

© Woodhead Publishing Limited, 2011

CHP-Beith-Pre.indd 3

5/6/11 11:05:11 AM

Published by Woodhead Publishing Limited, 80 High Street, Sawston, Cambridge CB22 3HJ, UK www.woodheadpublishing.com Woodhead Publishing, 1518 Walnut Street, Suite 1100, Philadelphia, PA 19102-3406, USA Woodhead Publishing India Private Limited, G-2, Vardaan House, 7/28 Ansari Road, Daryaganj, New Delhi – 110002, India www.woodheadpublishingindia.com First published 2011, Woodhead Publishing Limited © Woodhead Publishing Limited, 2011 The authors have asserted their moral rights. This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. Reasonable efforts have been made to publish reliable data and information, but the authors and the publisher cannot assume responsibility for the validity of all materials. Neither the authors nor the publisher, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming and recording, or by any information storage or retrieval system, without permission in writing from Woodhead Publishing Limited. The consent of Woodhead Publishing Limited does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from Woodhead Publishing Limited for such copying. Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. ISBN 978-1-84569-795-2 (print) ISBN 978-0-85709-275-5 (online) ISSN 2044-9364 Woodhead Publishing Series in Energy (print) ISSN 2044-9372 Woodhead Publishing Series in Energy (online) The publisher’s policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp which is processed using acidfree and elemental chlorine-free practices. Furthermore, the publisher ensures that the text paper and cover board used have met acceptable environmental accreditation standards. Typeset by Replika Press Pvt Ltd, India Printed by TJI Digital, Padstow, Cornwall, UK

© Woodhead Publishing Limited, 2011

CHP-Beith-Pre.indd 4

5/6/11 11:05:11 AM

Contents

Contributor contact details

xiii

Woodhead Publishing Series in Energy

xvii

Preface

xxi

Part I Introduction to small and micro combined heat and power (CHP) systems 1

Overview of small and micro combined heat and power (CHP) systems J. Knowles, Barbreck Services, UK

1.1 1.2 1.3 1.4 1.5 1.6 1.7

Introduction to cogeneration – a short history Types of technology and potential applications Energy efficiency improvement Cost benefits and emissions reduction Grid connection Barriers to combined heat and power (CHP) Future trends

3 5 11 12 13 15 16

2

17



Techno-economic assessment of small and micro combined heat and power (CHP) systems A. D. Hawkes, Imperial College London, UK

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8

Introduction The economics of combined heat and power (CHP) Techno-economics for onsite generation A specific modelling methodology Case study: micro combined heat and power (CHP) Future trends Sources of further information and advice References

17 18 21 23 28 39 40 41

3

© Woodhead Publishing Limited, 2011

CHP-Beith-Pre.indd 5

5/6/11 11:05:11 AM

vi

Contents

3

Thermodynamics, performance analysis and computational modelling of small and micro combined heat and power (CHP) systems T. T. Al-Shemmeri, Staffordshire University, UK



3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 4

4.1 4.2 4.3 4.4 4.5 4.6 4.7 5 5.1 5.2 5.3 5.4

Introduction Types of combined heat and power (CHP) systems Thermodynamics of cogeneration Performance analysis of cogeneration cycles Theory of heat exchangers Worked example Computational modelling of a combined heat and power (CHP) cycle Analysis of the computational model of the combined heat and power (CHP) system Case study: system performance of a biogas-driven small combined heat and power (CHP) system in a sewage works Sources of further information and advice References and further reading Integration of small and micro combined heat and power (CHP) systems into distributed energy systems J. Deuse, GDF-SUEZ – Tractebel Engineering, Belgium Distributed energy resources (DER) The value of distributed generation Conditions for profitable decentralized generation Evaluating the ‘full value’ of being network connected Recommendations to distribution system operators (DSO) and regulators Acknowledgement References Biomass fuels for small and micro combined heat and power (CHP) systems: resources, conversion and applications H. Liu, University of Nottingham, UK Introduction Characterisation of solid biomass fuels Biomass conversion technologies Current development of small and micro scale biomass combined heat and power (CHP) technologies

42 42 43 44 48 48 51 54 55 60 68 68

70 70 73 75 78 81 87 87

88 88 91 94 107

© Woodhead Publishing Limited, 2011

CHP-Beith-Pre.indd 6

5/6/11 11:05:11 AM

Contents

5.5 5.6 5.7

Conclusions Acknowledgements References

vii

116 116 117

Part II Development of small and micro combined heat and power (CHP) systems and technology 6

Internal combustion and reciprocating engine systems for small and micro combined heat and power (CHP) applications R. Mikalsen, Newcastle University, UK

6.1 6.2 6.3 6.4 6.5 6.6 6.7

Introduction Types, properties and design of engine Engine operating characteristics and performance Installation and practical aspects Commercially available units Conclusions References

7

Microturbine systems for small combined heat and power (CHP) applications J. L. H. Backman and J. Kaikko, Lappeenranta University of Technology, Finland



125 125 126 133 138 140 145 145 147

7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8

Introduction Cycle performance Types and properties of microturbine components Operation Manufacturers and applications Future trends Sources of further information and advice References

147 152 160 166 172 174 176 176

8

Stirling engine systems for small and micro combined heat and power (CHP) applications J. Harrison, E.ON Engineering, UK

179



8.1 8.2 8.3 8.4 8.5

Introduction Definition of a Stirling engine Why Stirling engines are suited to micro combined heat and power (CHP) The Stirling cycle Types of Stirling engine

179 180 181 183 188

© Woodhead Publishing Limited, 2011

CHP-Beith-Pre.indd 7

5/6/11 11:05:11 AM

viii

Contents

8.6

Development of Stirling engines for micro combined heat and power (CHP) applications Micro combined heat and power (CHP) design and system integration Applications and future trends Sources of further information and advice References

8.7 8.8 8.9 8.10 9

9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9 10

10.1 10.2 10.3 10.4 10.5 10.6 10.7

Organic Rankine cycle (ORC) based waste heat/waste fuel recovery systems for small combined heat and power (CHP) applications J. Larjola, Lappeenranta University of Technology, Finland Introduction Principle of the organic Rankine cycle (ORC) process Typical process heat sources and operating ranges for organic Rankine cycle (ORC) systems Benefits and disadvantages of organic Rankine cycle (ORC) process as compared to water-based systems Selection of working fluid for organic Rankine cycle (ORC) systems Process system alternatives Background and summary of commercial development and exploitation Efficiency and typical costs for current organic Rankine cycle (ORC) plants References Fuel cell systems for small and micro combined heat and power (CHP) applications D. J. L. Brett, University College London, UK, N. P. Brandon and A. D. Hawkes, Imperial College London, UK and I. Staffell, University of Birmingham, UK Introduction Fundamentals of operation, types and properties of fuel cells Fuel cell systems Operating conditions and performance Commercial development and future trends Sources of further information and advice References

189 199 203 205 205

206

206 206 207 213 219 221 223 230 231 233

233 234 239 246 253 257 257

© Woodhead Publishing Limited, 2011

CHP-Beith-Pre.indd 8

5/6/11 11:05:11 AM

Contents

11

11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8 11.9 11.10 11.11 11.12 11.13 11.14 11.15 12

Heat-activated cooling technologies for small and micro combined heat and power (CHP) applications K. Gluesenkamp and R. Radermacher, University of Maryland, USA Introduction Introduction to small-scale trigeneration Types of cooling systems and their applications Open sorption cycles: desiccant dehumidification Closed sorption cycles: absorption and adsorption heat pumps Steam ejector cycle Component-specific efficiency and effectiveness metrics System-wide performance and efficiency metrics Advantages and limitations of heat-activated cooling Future trends Sources of further information and advice References Bibliography Appendix 1: Nomenclature and abbreviations Appendix 2: Notes on terminology Energy storage for small and micro combined heat and power (CHP) systems A. Price, Swanbarton Ltd, UK

12.1 12.2 12.3 12.4 12.5

Introduction Types of energy storage (ES) systems Applications of electrical energy storage Applications for combined heat and power (CHP) systems Grid services applications and relationship to combined heat and power (CHP) 12.6 Electrical vehicles 12.7 Large-scale and small-scale storage – conceptual planning 12.8 The development and application of thermal storage 12.9 Future trends 12.10 Sources of further information and advice

ix

262

262 263 267 269 279 284 285 290 296 297 299 301 303 305 305 307 307 308 309 315 317 318 318 318 321 322

Part III Application of small and micro combined heat and power (CHP) systems 13

325



Micro combined heat and power (CHP) systems for residential and small commercial buildings J. Harrison, E.ON Engineering, UK

13.1

Introduction

325 © Woodhead Publishing Limited, 2011

CHP-Beith-Pre.indd 9

5/6/11 11:05:11 AM

x

Contents

13.2 13.3

Basic issues and energy requirements Types of system for residential and small commercial buildings Domestic applications for micro combined heat and power (CHP) Small commercial buildings and other potential applications Advantages and limitations Future trends Sources of further information and advice References

13.4 13.5 13.6 13.7 13.8 13.9 14

326 329 331 336 341 344 345 345



District and community heating aspects of combined heat and power (CHP) systems J. Clement, Aars District Heating, Denmark, N. Martin, Shetland Heat Energy and Power, UK and B. Magnus, COWI, Denmark

14.1 14.2 14.3 14.4 14.5 14.6 14.7 14.8 14.9

Introduction How to get started Heat sources Pipework installation issues and design considerations Control system and consumer installations Case study: Lerwick, Shetland Case study: Aars, Denmark Future trends Sources for further information and advice

347 348 349 351 353 359 360 363 364

15

Small combined heat and power (CHP) systems for commercial buildings and institutions R. Boukhanouf, University of Nottingham, UK

365



15.1 15.2 15.3

Introduction Basic issues and energy requirements Small combined heat and power (CHP) use in commercial buildings and institutions 15.4 Small-scale combined heat and power (CHP) technology 15.5 Application of small-scale combined heat and power (CHP) technology in buildings 15.6 Performance analysis and optimisation 15.7 Merits and limitations of small-scale combined heat and power (CHP) 15.8 Future trends 15.9 Sources of further information and advice 15.10 References

347

365 366 368 369 377 386 389 390 392 393

© Woodhead Publishing Limited, 2011

CHP-Beith-Pre.indd 10

5/6/11 11:05:11 AM

Contents

16

Small and micro combined heat and power (CHP) systems for the food and beverage processing industries P. S. Varbanov and J. J. Klemeš, University of Pannonia, Hungary

16.1 16.2

Introduction Food processing and energy requirements – examples for specific food and drink industries 16.3 Heat and power integration of food total sites 16.4 Types of small and micro combined heat and power (chp) suitable for the food industry 16.5 Established combined heat and power (chp) technologies for the food industry 16.6 High-efficiency technologies in theoretical and demonstration stages 16.7 Integration of renewables and waste with food industry energy demands 16.8 Potential applications 16.9 Future trends 16.10 Sources of further information and advice 16.11 References 17



17.1 17.2 17.3 17.4 17.5 17.6 17.7 18

Biomass-based small and micro combined heat and power (CHP) systems: application and status in the United Kingdom A. V. Bridgwater, A. Alcala and M. E. Gyftopoulou, Aston University, UK UK energy policy and targets Renewables and combined heat and power (CHP) in the UK Technical challenges for small-scale biomass combined heat and power (CHP) systems Capital costs for small-scale biomass combined heat and power (CHP) systems Conclusions Acknowledgement References



Thermal-engine-based small and micro combined heat and power (CHP) systems for domestic applications: modelling micro-CHP deployment K. Mahkamov, Northumbria University, UK

18.1

Introduction

xi

395

395 396 397 400 404 406 411 414 419 421 423

427

427 429 450 452 455 456 456

459 459

© Woodhead Publishing Limited, 2011

CHP-Beith-Pre.indd 11

5/6/11 11:05:11 AM

xii

Contents

18.2

Prime movers deployed in micro and small combined heat and power (CHP) systems Product development in the micro and small combined heat and power (CHP) market Overview of the method for estimation of economical and environmental benefits from deployment of micro combined heat and power (MCHP) technology in buildings Heat demand modelling Electrical demand Performance mapping Economic and environmental analysis References

18.3 18.4

18.5 18.6 18.7 18.8 18.9

Epilogue



R. Beith, Beith & Associates, UK



Index

460 470

480 483 490 492 500 508 510

514

© Woodhead Publishing Limited, 2011

CHP-Beith-Pre.indd 12

5/6/11 11:05:11 AM

Contributor contact details

(* = main contact)

Editor and Epilogue

Chapter 3

Robert Beith Beith & Associates Ltd 11 Links Avenue Felixstowe Suffolk IP11 9HD UK

T. T. Al-Shemmeri Faculty of Computing, Engineering and Technology Staffordshire University Stafford ST18 0AD UK

E-mail: [email protected]

E-mail: [email protected]

Chapter 1

Chapter 4

J. Knowles Barbreck Services Station Road Muthill Perthshire PE5 2AR UK

J. Deuse Rue de Corbais 51 B1435 Hévillers Belgium E-mail: [email protected]

E-mail: joeknowles@barbreckservices. co.uk

Chapter 2 A. D. Hawkes Grantham Institute for Climate Change Imperial College London South Kensington London SW7 2AZ UK

formerly of Power System Consulting, Department of Power and Gas GDF-SUEZ Tractebel Engineering Avenue Ariane 7 B1200, Brussels Belgium

E-mail: [email protected]

© Woodhead Publishing Limited, 2011

CHP-Beith-Pre.indd 13

5/6/11 11:05:11 AM

xiv

Contributor contact details

Chapter 5

Chapters 8 and 13

H. Liu Department of Architecture and Built Environment Faculty of Engineering University of Nottingham University Park Nottingham NG7 2RD UK

J. Harrison Technology Consultant E.ON Engineering Newstead Court Sherwood Park Annesley NG15 0DR UK

E-mail: [email protected]

E-mail: jeremy.harrison@microchap. info

Chapter 6

Chapter 9

R. Mikalsen Sir Joseph Swan Institute for Energy Research Newcastle University Newcastle upon Tyne NE1 7RU UK

J. Larjola LUT Energy Lappeenranta University of Technology P.O. Box 20 FI-53851 Lappeenranta Finland

E-mail: [email protected] E-mail: [email protected]

Chapter 7 J. L. H. Backman* and J. Kaikko LUT Energy Lappeenranta University of Technology P.O. Box 20 FI-53851 Lappeenranta Finland E-mail: [email protected] [email protected]

Chapter 10 D. J. L. Brett* The Centre for CO2 Technology University College London London UK E-mail: [email protected]

N. P. Brandon Department of Earth Science and Engineering Imperial College London London UK

© Woodhead Publishing Limited, 2011

CHP-Beith-Pre.indd 14

5/6/11 11:05:11 AM

Contributor contact details

A. D. Hawkes Grantham Institute for Climate Change Imperial College London South Kensington London SW7 2AZ UK

Chapter 14

E-mail: [email protected]

E-mail: [email protected]

I. Staffell Department of Chemical Engineering University of Birmingham Birmingham UK

Chapter 15

Chapter 11 K. Gluesenkamp* and R. Radermacher 4164 Martin Hall University of Maryland College Park, MD 20742 USA E-mail: [email protected] [email protected]

Chapter 12 A. Price Swanbarton Limited The Old Cake House Dairy Farm Pinkney Malmesbury Wiltshire SN16 0NX UK

xv

J. Clement Aars Fjernvarmeforsyning a.m.b.a. Dybvad Møllevej 1 DK-9600 Aars Denmark

R. Boukhanouf Department of Architecture and Built Environment Faculty of Engineering University of Nottingham Nottingham NG7 2RD UK E-mail: rabah.boukhanouf@nottingham. ac.uk

Chapter 16 Petar Sabev Varbanov* and Jirˇ í Jaromír Klemeš Centre for Process Integration and Intensification – CPI2 Research Institute of Chemical Technology and Process Engineering Faculty of Information Technology University of Pannonia Egyetem u. 10 8200 Veszprém Hungary E-mail: [email protected] [email protected]

E-mail: [email protected]

© Woodhead Publishing Limited, 2011

CHP-Beith-Pre.indd 15

5/6/11 11:05:11 AM

xvi

Contributor contact details

Chapter 17

Chapter 18

A. V. Bridgwater*, A. Alcala and M. E. Gyftopoulou Bioenergy Research Group Aston University Birmingham B4 7ET UK

K. Mahkamov School of Computing Engineering and Information Sciences Ellison Building Northumbria University Newcastle upan Tyne NE1 8ST UK

E-mail: [email protected] [email protected]

E-mail: khamid.mahkamov@ northumbria.ac.uk

© Woodhead Publishing Limited, 2011

CHP-Beith-Pre.indd 16

5/6/11 11:05:11 AM

Woodhead Publishing Series in Energy

1 Generating power at high efficiency: Combined cycle technology for sustainable energy production Eric Jeffs 2 Advanced separation techniques for nuclear fuel reprocessing and radioactive waste treatment Edited by Kenneth L. Nash and Gregg J. Lumetta 3 Bioalcohol production: Biochemical conversion of lignocellulosic biomass Edited by K.W. Waldron 4 Understanding and mitigating ageing in nuclear power plants: Materials and operational aspects of plant life management (PLiM) Edited by Philip G. Tipping 5 Advanced power plant materials, design and technology Edited by Dermot Roddy 6 Stand-alone and hybrid wind energy systems: Technology, energy storage and applications Edited by J.K. Kaldellis 7 Biodiesel science and technology: From soil to oil Jan C.J. Bart, Natale Palmeri and Stefano Cavallaro 8 Developments and innovation in carbon dioxide (CO2) capture and storage technology Volume 1: Carbon dioxide (CO2) capture, transport and industrial applications Edited by M. Mercedes Maroto-Valer 9 Geological repository systems for safe disposal of spent nuclear fuels and radioactive waste Edited by Joonhong Ahn and Michael J. Apted

© Woodhead Publishing Limited, 2011

CHP-Beith-Pre.indd 17

5/6/11 11:05:11 AM

xviii

Woodhead Publishing Series in Energy

10 Wind energy systems: Optimising design and construction for safe and reliable operation Edited by John D. Sørensen and Jens N. Sørensen 11 Solid oxide fuel cell technology: Principles, performance and operations Kevin Huang and John Bannister Goodenough 12 13

Handbook of advanced radioactive waste conditioning technologies Edited by Michael I. Ojovan Nuclear reactor safety systems Edited by Dan Gabriel Cacuci

14 Materials for energy efficiency and thermal comfort in buildings Edited by Matthew R. Hall 15 Handbook of biofuels production: Processes and technologies Edited by Rafael Luque, Juan Campelo and James Clark 16 Developments and innovation in carbon dioxide (CO2) capture and storage technology Volume 2: Carbon dioxide (CO2) storage and utilisation Edited by M. Mercedes Maroto-Valer 17 Oxy-fuel combustion for power generation and carbon dioxide (CO2) capture Edited by Ligang Zheng 18 Small and micro combined heat and power (CHP) systems: Advanced design, performance, materials and applications Edited by Robert Beith 19 Hydrocarbon fuel conversion technology: Advanced processes for clean fuel production Edited by M. Rashid Khan 20 Modern gas turbine systems: High efficiency, low emission, fuel flexible power generation Edited by Peter Jansohn 21 Concentrating solar power (CSP) technology: Developments and applications Edited by Keith Lovegrove and Wes Stein 22 Nuclear corrosion science and engineering Edited by Damien Féron 23 Power plant life management and performance improvement Edited by John Oakey

© Woodhead Publishing Limited, 2011

CHP-Beith-Pre.indd 18

5/6/11 11:05:11 AM

Woodhead Publishing Series in Energy

xix

24 Direct-drive wind and marine energy systems Edited by Markus Mueller 25 Advanced membrane science and technology for sustainable energy and environmental applications Edited by Angelo Basile and Suzana Nunes 26 Irradiation embrittlement of reactor pressure vessels (RPVs) in nuclear power plants Edited by Naoki Soneda 27 High temperature superconductors (HTS) for energy applications Edited by Ziad Melhem 28 Infrastructure and methodologies for the justification of nuclear power programmes Edited by Agustín Alonso Santos 29 Waste to energy (WtE) conversion technology Edited by Marco Castaldi 30 Polymer electrolyte membrane and direct methanol fuel cell technology Volume 1: Fundamentals and performance Edited by Christoph Hartnig and Christina Roth 31 Polymer electrolyte membrane and direct methanol fuel cell technology Volume 2: In situ characterisation techniques Edited by Christoph Hartnig and Christina Roth 32 Combined cycle systems for near-zero emission power generation Edited by Ashok Rao 33 Modern earth buildings: Materials, engineering, construction and applications Edited by Matthew R. Hall, Rick Lindsay and Meror Krayenhoff 34 Handbook of metropolitan sustainability: Understanding and improving the urban environment Edited by Frank Zeman 35 Functional materials for energy applications Edited by John Kilner, Stephen Skinner, Stuart Irvine and Peter Edwards 36 Nuclear decommissioning: Planning, execution and experience Edited by Michele Laraia 37 Nuclear fuel cycle science and engineering Edited by Ian Crossland 38 Electricity transmission, distribution and storage systems Edited by Ziad Melhem

© Woodhead Publishing Limited, 2011

CHP-Beith-Pre.indd 19

5/6/11 11:05:11 AM

xx

Woodhead Publishing Series in Energy

39 Advances in biodiesel preparation: Second generation processes and technologies Edited by Rafael Luque and Juan Antonio Melero 40 Biomass combustion science, technology and engineering Edited by Lasse Rosendahl 41 Ultra-supercritical coal power plant: Materials, technologies and optimisation Edited by Dongke Zhang 42 Radionuclide behaviour in the natural environment: Science, impacts and lessons for the nuclear industry Edited by Horst Geckeis and Christophe Poinssot

© Woodhead Publishing Limited, 2011

CHP-Beith-Pre.indd 20

5/6/11 11:05:12 AM

Preface

A preface has the function of introducing scope, intention and method of a literary or other work. The broad scope in this book is to review in depth the prospects and opportunity for applying small- and micro-chp systems as energy providers for domestic residences and smaller institutional, commercial and industrial buildings. But you might ask ‘surely we are already well provided for in this area of services?’ We have modern gas fired hot water boilers which can be up to 90% efficient and we have a readily available mains electricity supply which we can conveniently ‘tap’ as needed, even though conversion efficiency is low. However, we are in a time of change where ‘energy resources’ have suddenly become an important and challenging issue and the scenario needs a re-think. In the twentieth century we were provided with a stable national electricity system by the government-owned CEGB (Central Electricity Generating Board) for England and Wales and equivalents in Scotland and Northern Ireland. These systems had slowly grown from many small local generators mid century, to regional and then a national grid network. Supply was based mainly on large, typically 500 MW size per unit, coal fired electricity generators backed by ample cheap coal resources. In parallel we had moved from local ‘coal gas’ producers (with gasometers for gas storage) to a national supply pipeline fed from the extensive gas wells in the North Sea. Then both gas and electricity were privatised by the Thatcher Government in the cause of ‘competitiveness’ towards the end of the century and the ‘dash for gas’ for new cheaper generation changed the supply balance. Nearly every home is now linked to the electricity network and over 80% to the gas grid. The twenty-first century has brought concerns about long-term sustainability of fossil fuels as our initially vast North Sea reserves are depleting and by 2020 this source may provide as little as 10% of our needs, which means reliance on long cross border pipelines and shipped LNG (Liquefied natural gas). These routes also have cost rise implications. Secondly, the major issue of ‘climate change’ has resulted in national energy policies which seek a broad and ongoing reduction in fossil fuel usage. The IPCC (International Panel of

© Woodhead Publishing Limited, 2011

CHP-Beith-Pre.indd 21

5/6/11 11:05:12 AM

xxii

Preface

Climate Change) who operate under UN auspices, have promoted reduction of carbon discharges to atmosphere for all nations who are partners, of which there are over 190 countries. In the UK the government has responded by planning to generate a total of 15% of our energy needs through renewable sources by 2020. This is a huge challenge! To build up adequate capacity of such renewable energy plant will take time, and so will the building of possible new nuclear plant. But fossil energy reduction and carbon discharge reductions can also come from measures such as ‘energy saving’ and ‘using energy more efficiently’. Of course, the obvious energy savers, such as insulating buildings, low energy electrical appliances and light bulbs, are already being implemented. But using energy more efficiently nearly always means more complicated systems and results in increased equipment cost which is not always readily accommodated. However, applying CHP (combined heat and power) plant is one key example of how to increase efficiency. What happens is that electricity is produced from a conventional turbine/engine but then the waste heat is also collected for use in a process or for space heating. Overall efficiency typically rises from, say, 33% to 60–90% at peak. The problem is to find a heat demand local to the plant and cost goes up with this added equipment. But what was more expensive and more complicated and unacceptable in the days of cheap and readily available fuel may now be justified. Extra plant cost and complexity can be offset by better fuel economics and availability. As a matter of interest 2008 statistics show that the UK had installed 5569 MWe of CHP capacity of all sizes (which apart from heat duty) actually provided 7% of UK electricity supply. While units below 1 MWe only provided 218 MWe of this total, this represented only 1192 plants. There is, therefore, a huge scope for growth in this size range. While many of the existing CHP plants in this country are of multi MW size and are mainly large process/industrial plant related, the opportunity for CHP is now also being further considered and applied in the small and micro size range which is the subject of this book. There is no strict size definition, but typically ‘micro’ size could be 2–10 kW and be aimed mainly at domestic type applications, whereas ‘small’ CHP plants cover a wider range of applications from say 50 kW up to a few MW and are directed at larger establishments such as multi-dwelling blocks, hotels, hospitals, educational and community centres, commercial buildings, etc., and may even be suitable for small industrial sites. It is an application area where previously it would have been easier to pipe in gas and electricity, but where now it is definitely of interest to consider small- or micro-CHP, which would increase overall efficiency of fuel usage, avoid the 4–7% grid line losses and provide a measure of self sufficiency. The technology can also be used with biomass type fuels saving conventional gas and heating oil.

© Woodhead Publishing Limited, 2011

CHP-Beith-Pre.indd 22

5/6/11 11:05:12 AM

Preface

xxiii

There are government plans towards eventually building up locally controlled supply systems, often called DES (distributed energy systems), which really means local industry and communities building up their own supply network, which can still be supplemented by the grid when needed. Small- and micro-CHP would fit well in this scenario with other local energy sources. It should be pointed out that small- and micro-chp is on the agenda in many parts of the world. At the ‘micro scale’ Japan and now Germany and the USA, in particular, have already marketed conventional engine technology, using neatly contained and ‘outside located’ packages for domestic use. There are many tens of thousands of installations. UK has focussed more on developing Stirling or heat pump technology for domestic ‘in house’ application, with imminent commercial breakthrough. The Netherlands, Austria and Denmark are also contenders. The latter country is particularly relevant in applications for district heating for communities. All these and many other countries are also using ‘small scale’ CHP for all sorts of industrial, commercial, community and residential applications with mostly conventional engine technology, but with growth of micro turbines in North America. Involvement on such an international scale will ensure steady growth of this technology area. So the case for considering small- and micro-CHP in new build and even sometimes for replacement situations has been established. But there is the need for careful review and assessment of each potential application in terms of the mix and, pattern and optimisation of electricity and heat demand with time of day and year. Existing large scale CHP often has a big heat load as the main process demand and small electricity output for sale; the ratio can be as high as 6 to 1. New fuel cell designs can, however, offer a 1 to 1 ratio. Each installation can be different and has to be assessed so that the right type of equipment is installed for the duty. The aim of this book is to provide, from a wide range of experts, the widest coverage and description that we can of the different CHP technologies available and their typical applications. In addition, the aim is to produce enough background information for a serious assessment of performance capability, so that equipment installers can identify the most likely route for their particular duty. We have also tried to clarify the ‘state of art’ of each technology area. Some processes are still in the final stages of development even though major energy suppliers have imminent plans for marketing. Others are already in the market and being applied. The method of presentation of the book is to group the chapters into three parts. Part I includes an initial overview, followed by chapters on technicoeconomic appraisal, performance analysis and computational modelling, performance integration and possible integration into DES, and use of biomass fuels in CHP systems.

© Woodhead Publishing Limited, 2011

CHP-Beith-Pre.indd 23

5/6/11 11:05:12 AM

xxiv

Preface

Part II goes into more detail on each of the various technologies and fuels. There are seven chapters covering such areas as internal combustion reciprocating engines, microturbine systems, Stirling cycle engine systems, Organic Rankine Cycle systems, fuel cell systems, heat-activated cooling technologies, and energy storage methods for small and micro scale systems. Then in Part III we turn attention to areas of application with chapters on micro-chp for residential and small commercial buildings, small CHP for community-based district heating applications, small CHP systems for commercial buildings and institutions, small systems for the food industry, biomass-based CHP systems, and thermal engine-based CHP systems for domestic applications. It is not necessarily a book to read from cover to cover, but rather the purpose is to provide a range of information, some or more of which will be directly relevant to each reader’s application, and which will then provide a positive starting point for deciding whether and how to go ahead with small- or micro-CHP. It also provides a broad background to the technology for those wishing to update their knowledge on this subject which is one of growing importance. Robert Beith Beith & Associates Ltd Felixstowe

© Woodhead Publishing Limited, 2011

CHP-Beith-Pre.indd 24

5/6/11 11:05:12 AM

Epilogue R. B e i t h, Beith & Associates, UK

Some closing comments are necessary to try and summarise, from the book’s contents, how small- and micro-CHP systems are currently positioned in the energy scene and to anticipate how they could develop in the future. I think the first point to make is that it is clearly demonstrated from the 18 chapters that there are a wide range of applications and equally a wide choice of technologies to fulfil the services needed. Each technology has specific characteristics which best suit certain duty requirements. It may be helpful if I present an outline of the market and technology as I see it.

Review of micro combined heat and power (CHP) Looking first at the micro-chp scene and the Stirling cycle, this has been developed over many years aimed at the domestic market and particularly in the UK. Initial units have been trialled in houses and it is considered that most of the problems have been resolved, such as low noise and cleanliness for in-house installation, acceptable maintenance cycles, reliability, high overall efficiency (even up to 95%). Cost is now apparently coming down to acceptable levels, but perhaps the main challenge has been how to match the typical duty of say 1 kW electricity and 5–7 kW heat output against a widely varying house demand. It seems that this electrical duty matches house ‘base load’ quite well and peak load can be ‘mains topped’. But the heat duty on earlier units was marginal and it is now accepted sometimes to add a simple supplementary heater. Two main electricity suppliers are said to be planning marketing of Stirling cycle equipment in the near future. Some larger units may be applied in bigger establishments but the market here is said by one author to be mature but small, compared with the potentially very large domestic scene. Domestic scale reciprocating gas/oil-fired micro-CHP units are already commercialised and being sold, mainly in Japan (say 20 000 per annum) and in limited quantities in the United States and Germany. These are sized for 2–5 kW, but the units have to be located outside because of noise, exhaust, etc. Another technology being developed for domestic and other services is the fuel cell. One UK developer is moving towards commercialisation in terms of technical design but it is believed that the cost is still high and demonstration of product life (target say 40 000 hours) is still not fully 510 © Woodhead Publishing Limited, 2011

CHP-Beith-Epilogue.indd 510

4/5/11 9:44:18 AM

Epilogue

511

established. The technology is attractive with the typical duty of 1:1 ratio of electricity to heat and with much higher electricity output than a typical domestic Stirling cycle unit. The package is more a ‘system’ than a ‘unit’. It will, of course, have many applications other than just domestic services. So, for the domestic scene the technological groundwork is being well laid, but there has not yet been big market impact. There could be significant advances and perhaps a breakthrough in the near future, but two issues are important for this: (i) all domestic equipment has to be in standardised and ‘easy to install’ form for builders to understand, and (ii) householders will need to realise that these new systems operate in a different fashion and to adjust to this. For example, existing gas boilers are oversized so that they can bring temperature up quickly and then be turned off, whereas new microchp systems produce the heat more steadily but at a lower rate.

Review of small combined heat and power (CHP) Looking next at the ‘small’-CHP range, we find a number of process and equipment routes are being exploited. Perhaps the most widely used equipment is the gas/oil reciprocating engine, typically in size ranges of the order 50–2000 kWe. They are applied in many establishments and organisations (e.g. in many universities, offices, hospitals, community centres, etc.) and reported also in many food preparation processes, such as in the dairy, meat and sugar industries. It is likely there will be other industrial applications not reported in this book. Application tends to be where energy requirement is ‘round the clock’ and the plant duty is sized as near normal full load as possible, particularly for electricity supply as this is the expensive ‘buy in’ commodity. Usually in residential installations only low temperature heat is required for radiator systems, but this is not the case for many industries. There are applications in supermarkets, also, where the excess heat is used to drive lithium bromide, absorption refrigeration systems for cold stores. This makes for good economics where the heat output is large in relation to electricity and is not all required for heating. Supermarket sizes can typically be 1200 kWe. There are also a number of steam cycle plants typically in the 100–1000 kWe range, which mainly use biomass fuelling. Some of the smaller ones may be biogas-fuelled reciprocating engine plants. There are a range of such fuels, but typically wood (pellets or other), agricultural wastes, energy crops, or such as straw wastes are used. These plants tend to be in rural locations near biofuel sources to save transport costs. For instance, smaller plants may be fired from gas produced at a farm site anaerobic digestor (AD) or from manure processing. The pattern here is use of local available cheap fuels for local energy supply and carbon reduction is obviously one of the factors. Another equipment combination, also mainly using biofuels, is the use of

© Woodhead Publishing Limited, 2011

CHP-Beith-Epilogue.indd 511

4/5/11 9:44:18 AM

512

Small and micro combined heat and power (CHP) systems

the ORC (organic Rankine cycle) to produce electricity, using low boiling point fluids to drive a turbine. Low level waste heat from another source can drive the process. It is not thought economic below, say, 10 kWe but it is reported that 130 plants have been installed over 16 countries typically in the 200 kWe range. Another contender is the gas turbine/generator. One characteristic of these is the low electricity-to-heat ratio (can be 1:5 with only about 20% electricity efficiency) and also the physical size. They can be a good choice and competitive for larger-scale industrial/process plant situations where a big and steady process heat requirement exists and where spare electricity can be sold off; and maintenance costs are low. But at much smaller scale this route is possibly more expensive than the duty can support. However, TGs are available, mainly from US suppliers, in sizes from 30 kWe to 1000 kWh. We should not forget district heating (DH) applications and, looking at Denmark’s experience, where they have maximised this energy route, it can be noted that 60% of their population are served by DH driven by CHP. Systems often start smaller and are extended, but typically units will be 1 MW upward. They are sized to suit the housing needs in an area. They are usually gas-fired engines or at bigger scale steam turbines, sometimes using wood chip furnaces. Water heat stores are used for balancing on the heat duty side but there is no problem with electricity load not matching local needs, as this goes to the grid.

Conclusion and outlook So it is clear there are a wide range of technologies, systems and applications for small-CHP and steady growth in a number of areas. There is still a need to custom design each application – few are identical – but the equipment components are generally readily available. On the fuel side, gas is still the main resource, but there is a growing move towards biomass fuels and a need to improve standardisation/specification of these fuels on a more national basis. The cost of these fuels is also a variable to be taken into account, but of course this is also the case for fossil fuels. Finally, it seems clear that small-scale CHP will grow steadily and probably faster as experience develops from existing plants and as application is encouraged by energy and carbon saving benefits. But it is still essential to assess and design each application carefully from heat and electricity duty requirements to ensure a viable system, and this may limit expansion rate. Also, as was indicated for the micro-chp market, there is a need for owners/ users to realise they may need to operate in a different way than they would with the old formula of plenty of cheap fuel! Certainly this energy technology has good Government support with targets

© Woodhead Publishing Limited, 2011

CHP-Beith-Epilogue.indd 512

4/5/11 9:44:18 AM

Epilogue

513

for CHP plant availability (all sizes) to almost double by 2020; and with incentives in terms of feed-in tariff (FIT) payments at the domestic scale, together with benefits in potential carbon discharge reductions. I noted, for instance, in one chapter that typically there could be a 10–20% carbon saving with the Stirling ‘system’ compared with conventional domestic gas-fired equipment. So in my view the future scenario for small- and micro-CHP is definitely encouraging.

© Woodhead Publishing Limited, 2011

CHP-Beith-Epilogue.indd 513

4/5/11 9:44:18 AM

1

Overview of small and micro combined heat and power (CHP) systems

J. k n o w l e s, Barbreck Services, UK

Abstract: Though considered a modern energy efficient technology, combined heat and power (CHP) has its genesis in the factories of the industrial revolution during the late nineteenth century. It lost its place in industry to central electricity generation and national grids, but through the latter part of the last century, with the development of reliable prime movers and the advent of microprocessor-based controls, resurgence has taken place. Now, at the start of the twenty-first century, a combination of ever increasing energy use and the growing realisation of the damage done to our environment is demanding ever more innovative and efficient technologies. CHP is adopting these new technologies and applying microturbines, Stirling engines and fuel cells alongside the latest developments in more conventional systems. Key words: cogeneration, CHP, combined heat and power, energy efficiency, history, power, electricity, generation.

1.1

Introduction to cogeneration – a short history

It is not difficult to identify the genesis of CHP, except that the term combined heat and power would never have been uttered as the nineteenth century drew to a close. The industrial revolution brought about the mechanisation of manufacture across the whole spectrum of industry. It was driven by energy derived principally from the combustion of coal to generate steam which was converted into mechanical power in a steam engine. In a late nineteenth-century factory, this power would be transmitted via a system of shafts and pulleys to a variety of machines. A relatively small inventive step by the Victorian engineers recognised that the waste heat from power generation could be used within the factory to provide heating in winter, and in some cases to facilitate manufacturing processes. At the start of the twentieth century, mechanical power was progressively replaced by electricity, still generated within the factory and utilising waste heat. This was the early form of CHP as we know it today. It wasn’t long before excess power from factory-based CHP was ‘exported’ to local dwellings and other businesses close to the factory.

3 © Woodhead Publishing Limited, 2011

CHP-Beith-01.indd 3

4/5/11 9:15:53 AM

4

Small and micro combined heat and power (CHP) systems

Tom Casten,1 a pioneer of modern cogeneration technologies in the USA, points out that in 1902 the average efficiency of power generation in the industrial USA was 66%. One hundred years later, the best engineers using the latest technologies have striven to reduce the efficiency of delivered electricity to 33%. Of course, the early industrial power generation quotes a CHP efficiency of 66%; power generation devices of the time could only achieve 10–12%, so the major benefit was in the utilisation of the heat. The improvements from this low level of generating efficiency were in the centralisation of generation in large coal and later nuclear plants with efficiency of power generation exceeding 40%, and it is the remoteness of these plants which introduces grid losses that reduce the overall efficiency of power at the point of use. The formation of national grids made all of this possible, but to the detriment of the overall energy efficiency achieved by factory-based power generation systems. Strangely, it was the advent of a national grid in England which gave rise to another innovative CHP solution. In 1926 the young Oscar Faber,2 a trained structural engineer, won the first contract for his newly formed engineering consultancy. These were times of great unrest with high unemployment and frequent riots in the streets of London. In the reconstruction of the Bank of England in Threadneedle Street, Oscar Faber was asked to install power generation deep inside the building as the Bank feared that the new national grid could be a target in industrial disputes and wished to remove its dependence on external and centralised generation. A state of the art diesel engine power plant was installed, but Oscar had to deal with the problem of removal of the waste heat of power generation. Instead of simply ventilating the power hall, he installed a heating system for the building to utilise this heat, and even incorporated a system of pipes within the dome of the Bank to dissipate some of the excess heat to atmosphere – one of the first documented uses of CHP in a commercial building. In industry, CHP retained a place throughout the twentieth Century where onsite generation made sense for process, economic or security reasons. It is interesting to note that in the Act of Parliament which established the United Kingdom’s CEGB (Central Electricity Generating Board), reference is made to the need for industrial use to be found for the waste heat from power generation, but this was largely forgotten in the drive for ever larger and more efficient power stations which were sited on the coalfields to reduce the transportation costs of the fuel. Some industries have a very long history, not just in CHP, but in the use of renewable fuel in CHP. From the 1950s major modernisation of the cane sugar industry across the world saw the 1

Tom Casten of Trigen Inc. in a speech to the WADE conference, Amsterdam 2002. From the biography of Oscar Faber by his son, John Faber. Faber, J. (1989) Oscar Faber. Quiller Press, Shrewsbury. 2

© Woodhead Publishing Limited, 2011

CHP-Beith-01.indd 4

4/5/11 9:15:53 AM

Overview of small & micro combined heat & power (CHP) systems

5

efficient use of the cane waste (called bagasse) to generate steam, some of which was used in the refining process, but the main use was to drive steam turbines which in turn powered the mill. These were relatively low efficiency single stage turbines dedicated to each of the mechanical processes of sugar extraction and refining. Today, a second generation of modernisation using high efficiency multi-stage turbines allows the same amount of bagasse to power the mill electrically and export up to 70% of the electricity produced to the local grid. Other industrial sectors that have naturally utilised waste product or heat in CHP include oil refineries and steel works. In the 1970s and 1980s, the widespread availability of natural gas allowed many energy intense industries to return to CHP as a means of reducing their energy costs. Based mainly on modern industrial gas turbines, a new level of sophistication began to be introduced through integrated control systems incorporating the earliest programmable electronic devices. As the cost of these systems dropped over time, they became accessible and useable on smaller generators, specifically reciprocating gas engine driven units, and CHP then became available in an ever increasing range of sizes and applications. As with many modern technologies, small-scale CHP was made possible by the advent of microprocessors. Then in the 1990s other innovations began to emerge which further broadened the variety and scope of CHP. The micro gas turbine showed great promise through the use of modern materials, air bearings and operation of an effective recuperative cycle where heat energy is exchanged between the hot exhaust and the air leaving the compressor before combustion to increase significantly the cycle efficiency. As the twenty-first century approached, the scale of CHP reached down to the true micro level, becoming available to help heat and power our homes. Among the technologies employed in micro-CHP is the Stirling engine, invented by the Reverend Robert Stirling, a minister of the Church of Scotland, as long ago as 1816. Practical applications of the Stirling engine were slow to emerge, though Philips undertook the development of the engine in the 1940s, one variation of which was the reverse Stirling cycle, used to produce low temperature liquid gases in cryogenics. As a heat engine, the Stirling engine is an external combustion machine which makes it suitable for the combustion of any fuel type, as the working internal parts of the engine never see the products of combustion. It also works very effectively at the small scale size required for domestic micro-CHP where it has found its first mass-market application.

1.2

Types of technology and potential applications

We have already seen that every form of power generation technology, from steam engines and steam turbines, through the range of reciprocating and rotating internal combustion engines of all sizes, then down to the micro

© Woodhead Publishing Limited, 2011

CHP-Beith-01.indd 5

4/5/11 9:15:53 AM

6

Small and micro combined heat and power (CHP) systems

sized gas turbines and external combustion engines, can be moulded into some form of combined heat and power. It might be thought that the only logical reason for the combustion of a fuel, fossil or renewable, should be the generation of power, and that the ‘waste’ heat element of this power generation should always be utilised. But to achieve such energy utopia, the technology has to be matched to applications. Table 1.1 lists the overall categories of CHP; all types below 2 MW fall in the range of ‘small or micro-chp’. The simple definition of CHP is a system where the waste heat of power generation provides beneficial use. It therefore follows that a primary need for the siting of any CHP system is where there is a demand for the heat output. Heat from a CHP system can be provided in any number of useful forms from high pressure steam to low pressure hot water; as hot gas or heated air; and through the use of absorption or adsorption refrigeration processes as chilled water or sub-zero brine for low temperature refrigeration applications. A further factor in application matching is the heat-to-power ratio of the site or process to be served by the CHP. In many cases it is advantageous to match as closely as possible these needs in the CHP system output; however, where a very large heat demand exists, it is common to export power from the system to the grid to ensure a thermal match. Less common is the export of heat due to transmission difficulties, though all district heating or cooling schemes do exactly that. The heat-to-power ratio can also determine which CHP technology is most appropriate. For example, an application requiring large amounts of steam will almost certainly be best served by a gas turbine where almost all of the heat energy is found in the high temperature exhaust which is ideal for steam generation. In a building which needs to be air conditioned, a trigeneration system based on a gas engine and lithium bromide absorption chiller is most likely to provide the best system match. Table 1.1 CHP base technologies CHP power Power range generation (applied to CHP) technology

Power efficiency range (%)

CHP efficiency (peak) (%)

CCGT* Gas turbine Steam turbine Reciprocating engine Micro-turbine** Fuel cell Stirling engine

30–55 20–45 15–40 25–40 25–30 30–40 10–25

85 80 75 95 75 75 80

20 MW to 600 MW 2 MW to 500 MW 500 kW to 100 MW 5 kW to 10 MW 30 kW to 250 kW 5 kW to 1 MW 1 kW to 50 kW

* Combined cycle gas and steam turbines ** Micro-turbines are small, radial flow gas turbines

© Woodhead Publishing Limited, 2011

CHP-Beith-01.indd 6

4/5/11 9:15:53 AM

Overview of small & micro combined heat & power (CHP) systems

7

The key factors that determine the best CHP solution are therefore the type of thermal energy to be produced, and the heat-to-power ratio of the application. One other factor will influence the decision on technology, and that is the scale of the system which we will consider in the following paragraphs.

1.2.1 Large-scale CHP It is useful to outline the status of large-scale CHP so that the status of small- and micro-CHP can be put into perspective. Large-scale CHP covers the widest range of thermal and power output, and currently accounts for up to 90% of the installed capacity worldwide. Most large CHP installations are on industrial sites and include some very large installations in the range 300 MW to over 1 GW at large oil refineries. As the definition of large-scale CHP embraces anything over 2 MW, both the number and variety of applications are significant. The simplest form of large-scale CHP is found where the exhaust gas from an industrial gas turbine can be applied directly to heat a process, for example in the drying of bulk chemicals. The process must be tolerant of the products of combustion, which, from a modern gas turbine operating on natural gas, are exceptionally low in harmful emissions. More common in large industrial processes is the need for steam as a flexible and easily transported heat medium. The natural provider of large quantities of steam from waste heat is again the industrial gas turbine which in some applications can be operated in combined cycle with a steam turbine, the latter often receiving steam from the process or passing out to the process. A diminishing number of large-scale CHP systems have a steam turbine as the source of power. Exceptions to this are systems where heat can be recovered into steam from a high temperature process, or more commonly now when renewable fuels are burnt in the steam raising plant. Reciprocating engines also have a place in large-scale CHP, in particular where the heat-to-power match is relatively low. District heating systems will normally employ engines for that reason, to benefit not only from the low heat-to-power ratio, but also from the much higher efficiency of the engine which can extend operating periods against a seasonal heat demand. When making comparisons between gas turbine and gas engine CHP solutions, there are additional factors that may benefit the latter technology. The gas turbine performance is affected directly by the ambient air temperature at the compressor inlet, and significant derating of the turbine takes place as temperatures rise. Both power and efficiency are affected by any temperature change, whereas for most gas engines, a constant power and efficiency can be obtained at temperatures up to 30–35 °C. By actively cooling the intake air to the gas turbine, performance can be restored, generally at the cost of

© Woodhead Publishing Limited, 2011

CHP-Beith-01.indd 7

4/5/11 9:15:53 AM

8

Small and micro combined heat and power (CHP) systems

some thermal or electrical energy. The other drawback of the gas turbine is its relatively poor efficiency against part-load, whereas the gas engine will lose only a few percentage points between full load and 50%. So the best application for the gas turbine is a constant load with high heat-to-power ratio. A seldom deployed but highly effective solution for large-scale systems is to use a combination of a gas turbine with a number of gas engines. The turbine can be selected to provide the bulk of the heat demand whilst running at full load to provide a base level of power generation. The gas engines provide flexibility to cater for both electrical and heat demand fluctuations, and serve to increase the electrical generating efficiency of the combined system. Many large-scale systems are installed where there is a very significant heat demand, greatly in excess of the electrical demand of the site or process. The efficient production of electrical power in these CHP installations makes it viable to export excess power to the grid thereby raising the overall efficiency of the network.

1.2.2 Small-scale CHP Possibly the widest range of technologies and applications for CHP fall into the category of small-scale. The term small-scale can in itself, however, be confusing, but for the purpose of this section we will assume an individual CHP unit size of typically greater than 100 kW and less than 2 MW power output, and systems that are generally smaller than 10 MW where multiple power generators are installed. The prime mover technologies introduced for large-scale systems all find their place at the smaller scale, though there is a predominance of reciprocating gas engines in this area where their relatively low cost and superior generating efficiency come to the fore. Indeed, the wide range of outputs available from modern engines employing essentially the same technology extends all the way down into the micro-CHP sector. At the higher end of the small-scale range, generating efficiencies in excess of 40% have been achieved, and even at smaller sizes the power efficiency closely matches that of delivered centrally generated power. So in energy efficiency terms the heat recovered is a genuine bonus, and here again the reciprocating engine has excelled in CHP applications, achieving total energy efficiencies of up to 95% when used to heat air. A further reason for the relative dominance of the reciprocating engine is the close thermal match it can achieve for applications in the commercial and public building sector, both as CHP and in trigeneration systems. Turbines should not, however, be ignored, and there are now gas turbines available with good generating efficiency at around 2 MW power output. At the other end of the spectrum are a number of micro-turbine developments

© Woodhead Publishing Limited, 2011

CHP-Beith-01.indd 8

4/5/11 9:15:53 AM

Overview of small & micro combined heat & power (CHP) systems

9

of up to 250 kW which achieve respectable efficiency levels through the use of the recuperated cycle where waste heat from the exhaust is recycled to the compressed air before combustion. Steam turbines also have their place, again mainly in heat recovery applications, but at low power, cycle efficiency remains low. There are even reciprocating and radial type steam engines appearing in the market which, though exhibiting typically low electrical generating efficiencies, can find niche applications in small thermal plants burning renewable or waste fuels. Developed decades ago for the US space program, fuel cells are now beginning to find land-based power generation application. There are many types of fuel cell either in the market or under development, and it is a technology that will still take many years to reach maturity and commercial viability. The principle of the fuel cell is simple, and indeed the antithesis of the process of electrolysis which uses electrical potential to split water into oxygen and hydrogen. In the fuel cell, these elements recombine across a membrane to form water, and in doing so release electrons which provide the power output of the cell. As neither pure oxygen nor hydrogen is cheaply available, current fuel cells operate on air and natural gas or similar hydrocarbon fuel from which the hydrogen is extracted in thermochemical processes. These processes account for the inefficiency of the fuel cell which is largely recoverable as heat which makes the fuel cell a useful CHP technology.

1.2.3 Trigeneration Many CHP systems serve seasonal demands such as the heating of buildings, and therefore only provide beneficial use during the colder months of the year. CHP in buildings has grown mainly in the densely populated areas of the northern hemisphere. However, the growing trend to air condition buildings allows other technologies to be added to CHP which utilise its thermal output to provide the cooling required in air conditioning. The principles of absorption refrigeration were first discovered back in 1777, and the first commercial machine was built in 1850 using the Aqueous Ammonia cycle. It was not until the 1920s that the lithium bromide cycle used in current water chillers became practical, and by fits and starts, the technology has developed into a flexible range of products to maximise the potential for CHP-based solutions. In the process, water is the refrigerant, and lithium bromide in solution is the absorbent. The driving principle of absorption refrigeration is that at a high vacuum, water will evaporate at a very low temperature. The application of heat to a weak solution of lithium bromide in water will drive off water vapour and produce a strong solution. Allowing the water vapour to pass into a vessel at even lower pressure, it is there condensed as it cools the chilled water, and recombines with the

© Woodhead Publishing Limited, 2011

CHP-Beith-01.indd 9

4/5/11 9:15:53 AM

10

Small and micro combined heat and power (CHP) systems

strong solution of lithium bromide, creating once again a weak solution. A simple low energy pump boosts the lithium bromide solution back to the higher pressure in the evaporator part of the system. Chilled water is used extensively in air conditioning systems in buildings, and in some industrial applications for process cooling. Absorption chillers therefore find increasing use in CHP systems, which are commonly referred to as trigeneration – where power, heat and cooling are provided from the same system. The measure of performance of a chiller system is the Coefficient of Performance (CoP) and is the ratio of the cooling produced by the process divided by the energy (heat in the case of an absorption system) input to the process. The simplest form of chiller used in CHP converts hot water into chilled water at a CoP of approximately 0.7. This is a single effect chiller, referring to the heat input being used only once to evaporate water from the lithium bromide solution. Where a higher grade of heat is available, as is the case from a steam generating CHP system, a double effect absorption chiller can be employed. In this device, the heat can be cascaded to provide secondary evaporation, boosting the CoP to around 1.3. Of course, the efficiency of the steam generation process limits the total amount of heat available, but on balance the system can provide more cooling from a more compact absorption chiller. A relatively recent innovation has seen a new form of lithium bromide absorption chiller in trigeneration systems. This has been termed the multi-energy chiller, and when matched to a gas engine CHP system it uses hot water from the jacket system in a single effect process and the exhaust gas separately in double effect to achieve a CoP of up to 1.05. The design of this chiller will also permit supplementary heat input to increase further the cooling output. A limitation of the lithium bromide process is the temperature to which the refrigerant (H­2O) can cool the chilled water, which is generally limited to a minimum of 5 °C. Where a lower temperature is required by an industrial process, then the original Aqueous Ammonia process needs to be employed. In this process, ammonia is the refrigerant and water is the absorbent. Early, non-CHP applications of this process included the freezing of food and manufacture of ice cream. To achieve very low temperatures, only high grade heat from the CHP system can be used, and the lower the temperature to be achieved, the lower the CoP of the chilling system will be. For example, to achieve frozen food storage temperatures, the system would operate at a CoP of 0.2 or lower.

1.2.4 Micro-CHP At the smallest end of the CHP spectrum, there are technologies that can be applied to small buildings and individual homes. These are micro-CHP

© Woodhead Publishing Limited, 2011

CHP-Beith-01.indd 10

4/5/11 9:15:53 AM

Overview of small & micro combined heat & power (CHP) systems

11

systems, not to be confused with the micro-turbines discussed earlier, though very small gas turbines could be employed in micro-CHP. More common technologies entering this part of the market are small reciprocating engines, Stirling engines and fuel cells. Reciprocating engines of similar output to a small family car can be used to provide CHP to small buildings in the private and public sectors, and have been developed to operate not only on natural gas, but both gas and liquid biofuels. Smaller engines with outputs below 5 kW might be applied to large homes, though very few houses will have a constant demand for this level of power generation. Small systems can also be applied to apartment blocks providing a miniature form of district heating, though where more than 15 homes are connected the technology is more appropriately small and not micro scale. In the more common domestic environment of the average family home, the Stirling engine has found its niche. Being an external combustion engine, there is a greatly reduced level of wear of the moving parts, so potential for very high reliability. It also operates extremely quietly as the combustion system resembles that of the familiar domestic boiler. Several of the pioneering manufacturers in this field have adopted the Sterling engine to produce a direct replacement device for the domestic boiler in both floor and wall mounted variants. Small fuel cells have also been developed for use in this sector of the market, and though the technology is still expensive to install, in time viability and performance will improve making this clean, efficient and virtually silent technology more widely applicable. All micro-scale technologies suffer to an extent from lower power generation efficiencies making it crucial to ensure a good match to the thermal demand of the application when considering micro-CHP. To encourage uptake and further development of micro-CHP, it is common to find financial incentives from governments or utility companies and, in turn, wider uptake will allow manufacturers to invest to reduce the cost of their product.

1.3

Energy efficiency improvement

The environmental benefit that CHP in all its forms brings is based solely on the energy efficiency improvement that is produced from generating power at a location where the heat energy which would otherwise be wasted can be used beneficially. There is a valid argument to be made that in every instance where a fuel is consumed, be it of fossil or renewable origin, we should seek to firstly produce power (electricity) from the high temperature combustion of the fuel, and secondly recover as much of the lower grade heat for the benefit of the location in which the fuel is consumed. This would require us to move all thermal power generation into our towns and cities

© Woodhead Publishing Limited, 2011

CHP-Beith-01.indd 11

4/5/11 9:15:53 AM

12

Small and micro combined heat and power (CHP) systems

where not only is the power consumed, but a benefit can be obtained from the waste heat in processes, comfort heating and cooling. Most nations have concentrated their power generation in large and remote locations so the implications of reversing this legacy policy are immense. However, while we need to burn fuels for power and heat, CHP will remain the most logical, economical and environmental way to do so. In thermal power generation systems, the driver for larger power plant was the need to strive for higher power generating efficiency. And it was the correct strategy when there were no concerns about fuel supply and little understanding of the damaging effect of CO2 on our environment. The highest efficiency achieved by a super-critical steam generating power plant is around 40%, and by the largest current gas turbine something less than 50%. When transmission losses and part-load operation are taken into account, more than 60% of the thermal energy of the fuels used to generate power is wasted. Similar generating efficiencies are available from the best CHP prime mover technologies, so from a purely energy use point of view, CHP will increasingly become the first choice when new thermal power plant is required. The range of heat uses from CHP is unlimited as has been described earlier, and only thermal processes which require intense heat such as metal refining and certain chemical processes cannot benefit directly. As has also been seen, the overall efficiency of a CHP system depends on the grade of heat required by the process to be served. High pressure steam systems will have the lowest overall efficiency whereas systems required to heat ambient air can recover heat from all waste sources including radiant heat from hot surfaces to achieve extremely high efficiency. When we consider the value of high efficiency achieved by CHP, it is the cost of the energy displaced by the CHP system which must be considered. The offset cost of imported electricity is generally the most valuable component, followed by the value of any electricity exported. The recovered heat on the other hand has a relatively low value equivalent only to the cost of producing the same amount of heat in a modern boiler or direct fired heater system. It is generally considered that the output of the absorption chiller in a trigeneration system offsets imported electricity used to power vapour compression chillers.

1.4

Cost benefits and emissions reduction

As we have seen, the electricity produced by the CHP system has the highest value, and in most cases will confer the highest element of cost benefit. Heat value, however, often offers the benefit which drives the investment decision which emphasises the importance of maximising the use of heat output from CHP. Not to be forgotten in the calculation of the cost benefits

© Woodhead Publishing Limited, 2011

CHP-Beith-01.indd 12

4/5/11 9:15:53 AM

Overview of small & micro combined heat & power (CHP) systems

13

of CHP, however, is the relatively high cost to maintain the operation and performance of the CHP equipment. When compared to the cost of providing the same energy from alternative sources, these elements provide a simple evaluation of the viability of CHP. However, there are other considerations in the decision-making process. In the case of a new installation, the high initial cost of the CHP system may be partially offset by cost savings elsewhere, for example in heating and cooling equipment, or the cost of a grid connection. Where the CHP system performs a secondary role, such as providing standby power, the cost of a standby generator may be saved. In certain installations, the CO2 output from the CHP system can have a value providing it can be captured and utilised. A good example is in horticulture where cleaned exhaust gas is used to introduce additional CO 2 into greenhouses, aiding plant development. A less direct benefit from CO2 is becoming available through trading mechanisms designed to stimulate reductions in emissions. Initially only large energy users and larger CHP installations could economically benefit from these schemes, but ultimately, all CO2 saved by CHP may find a value. For smaller systems, this may come through taxation benefits, feed-in tariffs, or by valuing the recovered heat energy. Where they are applied, such measures can significantly impact the evaluation of the benefits of a CHP installation. Though generally considered a clean technology, the potential for CHP to produce significant levels of other pollutant emissions is not being ignored. Driven by ever tightening standards set in the USA and Europe, prime mover manufacturers have progressively reduced levels of NOx, CO and, where relevant, SOx and particulates either by improved combustion technologies or the addition of post combustion treatment of the emitted gases. The penalty is, however, a more complex and costly CHP installation with potentially higher operating costs.

1.5

Grid connection

One of the most important aspects of CHP installation design is to provide an economic but safe connection to the electricity grid. Almost all CHP systems operate connected to the grid and rules have been developed by almost all grid operators which ensure the safety of both the CHP installation and its connection. There are a number of key considerations in designing the connection.

1.5.1 Protection The connection rules are written to ensure that under a fault condition, the grid is fully protected whether the fault is with the CHP installation or on the grid itself. Various parameters are constantly measured at the

© Woodhead Publishing Limited, 2011

CHP-Beith-01.indd 13

4/5/11 9:15:54 AM

14

Small and micro combined heat and power (CHP) systems

point of connection of the CHP installation to determine whether a fault is developing. Frequency and voltage in any electrical network can vary over time; however, a significant change may indicate a developing fault, and if not reversed, then action will be taken to disconnect the CHP system from the grid if the limits of variation set by the connection code are exceeded. More certain indications of failure of an electrical system can be detected. A significant vector shift may be measured and used to determine an imminent fault requiring disconnection, and for larger systems the rate of change of frequency (RoCoF) is frequently employed. Though CHP systems can fail and require to be disconnected to protect the grid, more common on smaller scale systems are events on the grid which alter the measured parameters sufficiently to instigate operation of the protection devices. When deciding where to install the protection equipment, the possibility of operating the CHP system in island mode should be considered, so that the CHP can provide power to the site or part of the site during a prolonged grid outage. By carefully designing the connection together with a means of managing the CHP load, it is possible to achieve a seamless transfer from grid to island operation and back.

1.5.2 Synchronisation CHP systems can employ either synchronous or asynchronous generators. The latter are generally applied to smaller systems, and as they need to be connected to the grid to operate, they require no synchronisation equipment. All other systems need to be synchronised in order to connect to the grid. For rotating synchronous generators, the requirement is to closely match the key parameters of the generator to those of the grid before making the connection. Voltage needs to be closely set, but any small variation at the instant of connection is eliminated as the grid voltage dictates the connected level. Frequency needs to be very closely matched and, most importantly, phase angle needs to be within a few degrees before connection is made. The synchronisation system operates the governor on the prime mover to bring these parameters within connection tolerance, and modern electronic synchronisers operate fully automatically to achieve this.

1.5.3 System Fault Level Analysis Any changes to the system connected to the grid will have an effect on the fault level at the point of connection to the grid and beyond. In most countries the grid has been developed principally to operate in one direction, and has progressively become loaded to a point where any change can have major impacts. Distributed or embedded generation can be beneficial in reinforcing a weak grid, but such reinforcement can introduce local problems which

© Woodhead Publishing Limited, 2011

CHP-Beith-01.indd 14

4/5/11 9:15:54 AM

Overview of small & micro combined heat & power (CHP) systems

15

require action to ensure the continued safe operation of the system. Though it is generally the larger CHP installations that will have the biggest impact, even modest installations on particular sections of a grid can cause problems, an example being the addition of a CHP system to a city centre grid which already has significant CHP or renewable generation capacity connected. It is important, therefore, to know the impact that the installation of a CHP system will have at the point of connection, and to commission a study of the effect on the grid of the changes. Most network operators require such studies to be carried out before licensing onsite generation, and they often provide a service to conduct the necessary analysis. Once the design of the connection is approved and the system installed, it is again normal for the network operator to require a test of the system and its protection to be witnessed by their engineers before generation in parallel can be permitted.

1.5.4 Inverters Connection of small CHP systems to the grid can also be made through an inverter, and most fuel cells and micro-turbines already connect by this method. The fuel cell generates d.c. power, and requires the inverter to convert this to a.c. In the case of the micro-turbine, its high rotational speed required the use of high frequency generators, and the inverted is utilised to convert this to grid frequency. More recently, inverter connection of specifically designed generators has been developed as a potentially simpler method of connecting smaller conventional CHP systems. Similar protection and connection issues need to be addressed for inverter systems.

1.5.5 Harmonics There is a limit to the harmonic distortion that can be tolerated at the point of connection, and again the network operator has set standards which must be complied with. Most CHP systems are unlikely to cause harmonic problems, but in rare cases, the connected system may be at or near the limit, and due account of the effect of adding a CHP system will need to be taken. The advent of inverter connections will potentially increase the incidence where harmonic studies are required.

1.6

Barriers to combined heat and power (CHP)

In almost all markets where CHP has been introduced, there have been legislative barriers to overcome, though it must be admitted that with good connection codes in place and more enlightened authorities, these have largely been removed. A particular difficulty in the past was to be allowed

© Woodhead Publishing Limited, 2011

CHP-Beith-01.indd 15

4/5/11 9:15:54 AM

16

Small and micro combined heat and power (CHP) systems

to connect CHP to the grid, and though much of the resistance of network operators has been removed, there is often a very cautious approach applied which might deter some developments. Where such caution is suspected, it is important to properly interrogate the network operator’s findings and to jointly seek an economic solution to any problem encountered. Most CHP developments are on a relatively small scale, and therefore have minimal local environmental impact, particularly where natural gas is the fuel. However, as more renewable fuels are consumed, there may be issues of storage, emissions and waste handling to be addressed to satisfy planning requirements. There is a trend towards the use of such fuels in larger district heating CHP systems which may also find resistance at community level. Perhaps the greatest barrier to CHP remains that of potential users and developers who may have concerns over the technology and the complications it may bring, or who have financial expectations that CHP cannot fulfil. We continue to see volatility in energy markets, and though the recent trend has been towards sustainably higher energy costs, there can still be sufficient price movement over a short period to deter investment. Any CHP development has to be considered on the basis of its lifecycle cost, but some commercial and industrial users have much shorter horizons and require short payback cycles.

1.7

Future trends

Predicting the future is always dangerous, but in the growing knowledge and acceptance of the negative effects of our carbon society and its likely demise, CHP will surely feature in any energy future where thermal as well as electrical demands exist. The fuels of the future will be hard won, therefore their efficient use will demand that CHP has a continuing role. It has often been said that CHP is a bridge technology to a renewable future, but it should be recognised more as a foundation on which the future can be built. For the immediate future, there are a number of exciting options including CHP networks where a number of CHP installations can be interlinked to satisfy the total thermal demand of a large site or collection of applications. In this concept, each application generates power to satisfy the immediately local thermal demand, and the excess electricity is consumed by the applications which have a power demand which does not match their thermal needs. There are also opportunities to integrate renewable technologies with CHP. For example, solar thermal has been used to supplement heat input to packaged trigeneration systems, boosting cooling output when it is most needed. The largest future opportunities are the tens of thousands of small to medium existing buildings and businesses that could benefit from CHP, and the millions of households where fully developed and deployed microCHP can help change the future.

© Woodhead Publishing Limited, 2011

CHP-Beith-01.indd 16

4/5/11 9:15:54 AM

2

Techno-economic assessment of small and micro combined heat and power (CHP) systems A. D. H a w k e s, Imperial College London, UK

Abstract: This chapter introduces the economics of small-scale CHP systems. Beginning with a review of how decentralised energy resources can achieve economic value, it then discusses the sometimes ill-defined concept of techno-economics, and the variety of modelling techniques that underpin it. An optimisation method that pinpoints the key characteristics of commercially successful CHP is presented and applied to the case of microCHP in the UK, demonstrating that the heat-to-power ratio prime mover is the key driver of economic and environmental performance. Finally, the emerging tension between CHP and heat pumps is discussed in relation to future stringent emissions reduction targets. Key words: CHP, cogeneration, economics, techno-economics, optimisation, policy, CO2.

2.1

Introduction

Small-scale combined heat and power (CHP) is a class of technologies that have the potential to simultaneously tackle a number of energy policy aims surrounding the economics, environmental impacts and security of energy supply. Perhaps foremost among its credentials is the perceived ability to reduce greenhouse gas emissions arising from energy consumption at low private and social cost. As such, and given broader national and international efforts to mitigate climate change, CHP has benefitted from recent industry and political attention. In this process it has become apparent that tools to further assist development and introduction of systems could aid decision making and push this technology more into the mainstream. This chapter presents a techno-economic modelling framework designed to assist investors, CHP technology developers and policy makers in achieving conception to commissioning of ‘successful’ CHP products and installations. The modelling and supporting analysis is used to weigh up the ability of the technologies to meet commercial and policy aims, and to gauge the suitability and effectiveness of policy instruments in encouraging uptake where that uptake will aid in meeting policy objectives. CHP is a distinctive technology. Along with energy efficiency measures, 17 © Woodhead Publishing Limited, 2011

CHP-Beith-02.indd 17

4/5/11 9:16:15 AM

18

Small and micro combined heat and power (CHP) systems

it is one of the very few ‘interventions’ to address all three primary aims of energy policy convincingly. Both energy efficiency measures and CHP can be economically rational investments in that they can pay back within their lifetimes, and can reduce greenhouse gas and pollutant emissions, and improve energy security by reducing demand and diversifying and decentralising supply. But despite these benefits, the extent to which CHP can be deemed as ‘successful’ depends greatly on site-specific characteristics and the nature of the technology applied, not to mention an assortment of ‘external’ uncontrollable factors such as relative electricity and fuel prices, the speed and nature of change in the broader energy system, and the business models by which CHP systems are introduced. This chapter does not intend to tackle all of these issues in detail, but rather focuses on a specific facet of the economics of CHP. In order to achieve this, it is split into four main parts: a discussion of the general economic opportunities and challenges faced by decentralised energy resources, discussion of the concept of ‘techno-economic’ analyses, description of a technique to scope and assess CHP technologies, and a case study of an important subset of small-scale systems – micro-CHP for residential applications. It is hoped that this will give readers insight into the fundamentals of decentralisation, with specific focus on a method for development and assessment of CHP systems.

2.2

The economics of combined heat and power (CHP)

In any discussion of the economics of CHP, the topic of the relative price of electricity and fuel cannot be overlooked. This quantity is expressed via a metric known as the ‘spark spread’, which is a measure of the gross income a generator can expect to receive when they sell one unit of electricity after purchasing the fuel necessary to produce that electricity. For the case of CHP, the spark spread concept needs to be altered slightly to cater for the fact that CHP also produces heat, which has economic value. If we assume that the value of heat from the CHP is identical to the cost of producing heat in a boiler, it is possible to derive the following relationship between CHP spark spread, price and efficiencies: Êh ˆÊ ˆ spark ark spread ar pr CHP = EP – GP + Á o – 1˜ Á GP˜ he Ë he ¯ Ë hb ¯

2.1

where EP is the electricity price, GP is the gas price, and he, ho and hb are the CHP electrical efficiency, CHP overall (i.e. heat plus power) efficiency, and competing boiler efficiency, respectively. In the case where ho ≈ hb, this reduces to equation 2.2:

© Woodhead Publishing Limited, 2011

CHP-Beith-02.indd 18

4/5/11 9:16:15 AM

Techno-economic assessment of small and micro CHP systems

Ê ˆ spark ark spread ar pr CHP = EP – Á GP˜ Ë ho ¯

19

2.2

and in the case of an extremely well-engineered system, where ho ≈ 1, the relationship further reduces to: spark Gap = EP – GP

2.3

Both Equations 2.2 and 2.3 are independent of the electrical efficiency of the generator, which is substantially different from the case of electricity-only generators for which the standard spark spread metric (where no value is afforded to heat) is usually calculated. Furthermore, the quantity in equation 2.3 is sometimes referred to as the ‘spark gap’, and is a relatively common measure of the competitiveness of CHP at any given point in time. Of course, CHP spark spread or spark gap does not include all the necessary information about the economics of CHP, and other sources and limitations of value can come from, for example, emissions trading permits, levies or levy exemptions for CHP, operational expenses, connection, distribution, the ability to utilise thermal energy, etc. Nonetheless, spark gap is a useful metric for a first approximation, and as stated above no discussion of CHP economics would be complete without it. Before moving on, it is worth briefing touching on the risk faced by investors in CHP as a result of fluctuation in the CHP spark spread. As one would expect, it varies from year to year, as displayed in Fig. 2.1. This figure

Electricity price (7/kWh) gas price (7/kWh)

Price/value (7/kWh)

CHP Spark spread (7/kWeh) 0.1

0.05

0 1998

2000

2002

2004 Year

2006

2008

2.1 Historical electricity and gas price of the UK from European Commission (2010), plotted against calculated CHP spark spread.

© Woodhead Publishing Limited, 2011

CHP-Beith-02.indd 19

4/5/11 9:16:15 AM

20

Small and micro combined heat and power (CHP) systems

shows fairly wide variation in the CHP spark spread, primarily as a result of electricity price drops after full energy market liberalisation in 2001. The situation has since improved, but the historical volatility certainly does not lend itself to this increased investment. In essence, exposure to the CHP spark spread does represent a risk, which is a noteworthy barrier for CHP uptake. In addition to the customary spark spread economics, there are a range of less tangible factors that bear upon the economics of CHP, both at each individual site and further upstream. These stem from the fact that CHP is a class of technologies that are a subset of the broader ‘decentralised’ or ‘distributed’ generation (DG) concept. Such systems are often attributed a variety of advantageous characteristics relating to their economics. Whilst 207 separate benefits can be identified according to Lovins et al. (2002), they typically fall into one of the following categories: 1. The ability to utilise waste heat. Specific to the case of CHP and integral in the preceding discussion of CHP spark spread, co-location of supply and demand allows thermal energy that would otherwise be wasted to be utilised onsite or nearby for some productive purpose. 2. Modularity and speed of installation. Unlike large centralised generators, distributed generators are modular and have short lead times for installation. Therefore, all other aspects being equal, there is less financial risk involved in investing in a large number of decentralised generators than one of their centralised counterparts. 3. Reduced requirement for upstream infrastructure. Installing generation assets near the point of demand implies that less centralised generation and transmissions and distribution (T&D) infrastructure will be required upstream, thus reducing total final cost. 4. Reduction in electricity T&D losses. Similarly to (3), siting electricity generation at or near the point of demand means that some T&D losses (which currently account for approximately 7% of all electricity generated in the UK) will be avoided because some electricity does not need to be transmitted. 5. Backup power or uninterruptable power supply (UPS). The ability to install generation onsite offers the possibility to provide backup power that improves the reliability of the total power supply (i.e. supplementing standard power system reliability). DG also offers flexibility in meeting reliability needs, where specific customers can be afforded appropriate levels of reliability depending on the value of the load being served. 6. Diversification of primary energy sources. A wide range of primary fuels are available for DG systems (e.g. wind, biomass, solar, wasteto-energy). Such diversification has evident energy security benefits, along with further ‘portfolio’ benefits of hedging risk via the ability to

© Woodhead Publishing Limited, 2011

CHP-Beith-02.indd 20

4/5/11 9:16:15 AM

Techno-economic assessment of small and micro CHP systems

21

switch from one primary energy source to another when operating an aggregation of DG resources. 7. Access to ‘stranded’ or ‘distributed’ primary energy sources. The small scale and relative portability of generators means they can be located where the fuel is. For example, wind power needs to be located in windy places, and some bio or waste-to-energy sources may benefit from avoiding transportation of the fuel. The precise value of all of these benefits is challenging to estimate, as it depends on many factors relating to both the technology and site. Some studies have attempted to quantify them, but wide error bars are usually conceded, and results can rarely be generalised. In spite of the potential benefits, market rules and regulations can hinder the ability of smaller generators to access the value they create, via preventing market access on equitable terms or complicating permitting, installation and commissioning processes. In some senses such barriers may be reasonable as some DG technologies will not create value in one or more of the listed categories, and it is arguable that each installation needs to be judged on its own merits. This leads to one of the most important challenges with regard to DG: the uncertainty of performance or suitability of a particular technology in a specific setting creates the wish to assess each installation. This leads to imposition of measures such as permitting and licensing, impeding uptake and sometimes creating regulatory risk for the installer. As such, even installations that are likely to be acceptable and perform well are subject to procedures that discourage managers from investing time in even the first stages of scoping DG projects. Clearly the array of opportunities and challenges facing CHP are substantial, and not all can be addressed here. Instead we focus on one specific aspect: the techno-economics of CHP, beginning with an exploration of definitions and the scope of applicable techniques, followed by presentation of a modelling methodology and a case study of residential micro-CHP.

2.3

Techno-economics for onsite generation

Conventional power system models are rarely appropriate for analysing onsite generation. This is because they do not adequately capture the dynamics of site energy demand, and rarely consider the economics of ‘behind-the-meter’ generation or the range of site-specific constraints a typical installation faces. To address their shortcomings, models designed to analyse the economic and environmental performance of onsite generation technologies such as CHP are becoming more common. Whilst the aims, assumptions and structures of these models vary greatly, most can be classified as ‘techno-economic’, based on either simulation or optimisation approaches.

© Woodhead Publishing Limited, 2011

CHP-Beith-02.indd 21

4/5/11 9:16:15 AM

22

Small and micro combined heat and power (CHP) systems

Before delving into the details of a specific modelling approach, it is useful to explore the term ‘techno-economics’, which is only loosely defined and yet widely applied. In its most general form, it refers to studies that cross the disciplinary boundary between the physical sciences and economics. Many techno-economic studies are typified by a technical characterisation of an engineering system and the environment in which it operates, accompanied by modelling of its technical performance given particular state/s of input parameters (e.g., factors relevant to the performance and lifetime of the system). The results of such analyses are designed to be quantities that can be attributed economic value (e.g., annual electricity and fuel consumptions, maintenance requirements, etc). The economic part of the analysis takes these values and calculates standard metrics such as net present value, equivalent annual cost, payback periods, or risk-related metrics for the investment. Whilst this all-purpose description is very straightforward, the methods by which techno-economic modelling is performed vary greatly from study to study, and each approach presents specific strengths and weaknesses. A primary distinction between modelling methodologies lies in comparison of simulation and optimisation. Simulation models performance over time using a set of (potentially temporally interdependent) physical relationships. Conversely, optimisation models performance (potentially over time) where some variables are not fixed and can be adapted in order to maximise or minimise some defined objective, subject to constraints. Either modelling approach can be very simplistic or extremely sophisticated. Optimisation approaches can be further disaggregated into those regarding system design, and those regarding system operation. Equally, simulation approaches exist in a variety of forms and consider many different boundaries for analysis. The primary advantage of simulation is that it can be more easily used to explore transients in the technology itself and in the environment in which it operates, because highly non-linear interactions and even first-principles relationships can be readily incorporated into models. Whilst techno-economic optimisation rarely attempts to capture such detail explicitly, it has the advantage that technical and/or operational parameters do not need to be fixed. This means that it is possible to explore the characteristics of the technology, its control and/or its operational environment that lead to the ‘best’ outcome, as defined by the objective function of the optimisation. For this chapter, a unit commitment optimisation model based on steadystate efficiency characterisation has been chosen to assess the performance of CHP. For the interested reader, a definition and review of various unit commitment methodologies can be found in Padhy (2004). Dynamic/transient performance of the CHP is emulated via a set of operational constraints, adding elements of system and building response into the formulation. This optimisation approach is intended to provide useful information regarding the

© Woodhead Publishing Limited, 2011

CHP-Beith-02.indd 22

4/5/11 9:16:15 AM

Techno-economic assessment of small and micro CHP systems

23

best system design and control, and can be used to explore the performance potential of emerging technologies. The superior thermal demand/response modelling of credible simulation models is drawn upon to create fixed thermal demand profiles for use in the case study presented later.

2.4

A specific modelling methodology

As discussed above, in order to investigate CHP economics, optimisation is applied. The inputs and outputs of this process are described in Fig. 2.2. The ‘processing’ element in Fig. 2.2 forms the core computational effort of the modelling, and is a mixed integer linear programming method implemented using Visual C++.NET and CPLEX 10. The model has been named CODEGen, which stands for ‘Cost Optimisation of Decentralised Energy Generation’. The mathematical formulation of the optimisation problem is not presented here, and the reader is referred to Hawkes et al. (2009a) for a complete description. Instead, a narrative of the basis of choice of the optimisation objective function and a conceptual outline of the mathematics are provided in the sub-sections below.

2.4.1 Choice of the central performance metric This section explores the choice of the objective (i.e., the primary performance metric to minimise or maximise) of the optimisation through consideration Inputs Technical inputs

Processing

Country-specific inputs

Techno-economic characterisation Capital cost Installation cost Maintenance cost Start/stop cost Minimum set point Min up/down time

Outputs

Performance assessment Economic conditions electricity/gas cost discount rates Energy demand (Heat and power) 5 minute – 1 hour precision

Maximum installed cost Codegen model Mixed integer linear programming

Optimum operating strategy Annual energy cost Energy consumed Greenhouse gas emissions

Ramp limits Capacity

Sensitivity analyses

2.2 The CODEGen model: inputs, outputs and flow diagram for optimisation modelling framework.

© Woodhead Publishing Limited, 2011

CHP-Beith-02.indd 23

4/5/11 9:16:16 AM

24

Small and micro combined heat and power (CHP) systems

of the modelling aims. The modelling framework must of course be tuned to these aims, which are: ∑

to understand the potential of CHP technology, and use it to assess the key market drivers, and assess the ability to meet policy aims; ∑ to investigate key technical parameters to understand their influence on economic and environmental credentials, with the aim of improving the knowledge of system developers; ∑ to use the modelling results to critique current, proposed and potential new policy and regulation surrounding the introduction of CHP. In one sense the aims all speak to the primary considerations of energy policy: economics, environment, and energy security and the ability of CHP to contribute to beneficial outcomes in these three areas. However, in order for the technologies to be commercially successful, consumers must adopt them in large numbers, so deployment pathways become relevant. Deployment models for microgeneration are considered to gain an understanding of what metrics may drive adoption/diffusion and demarcate successful CHP products. This leads to choice of a primary performance metric for the optimisation modelling (i.e. an objective function), and further assessment metrics. CHP commercial exploitation pathways The driving force in a CHP investment decision depends greatly on which stakeholder is making that investment. It order to gain understanding of appropriate metrics for modelling of the systems, it is useful to consider which actors are associated with each potential route to market. Watson (2004) developed a set of deployment models for microgeneration in general, and applied it in Sauter and Watson (2007) to investigate social acceptance of microgeneration. The deployment pathways they developed are: ∑

Plug-and-play. Where the decision to invest in microgeneration is taken autonomously by the building owner or occupier who independently finances it. ∑ Company control. Where more passive consumers provide a site for the system which is owned and/or operated by an energy service company (ESCO) or energy supplier. ∑ Community microgrid. Where a group of individuals and/or businesses group together to provide some of their collective energy needs, and own and may operate the units. As discussed in Hawkes et al. (2009a), the company control pathway is possibly the most effective for mass market introduction of efficiency and other measures in the built environment because it largely takes the investment decision out of the hands of the dwelling occupier, and therefore allows that

© Woodhead Publishing Limited, 2011

CHP-Beith-02.indd 24

4/5/11 9:16:16 AM

Techno-economic assessment of small and micro CHP systems

25

decision to be more explicitly economically driven than in the plug-and-play pathway.1 This is also true in comparison with the community microgrids pathway, but in this case decisions may also be driven by the community’s particular needs and the potential for bolstering of the local economy in ways such as increased employment. Community microgrids are also less likely to constitute a mass market, although they may be a stepping-stone that proves concepts and raises the profile of successful approaches, leading to further uptake. Between the three pathways, only ESCO (or energy suppliers acting as ESCOs; the company control pathway) actors can almost always be assumed to be economically rational, making investment decisions based on classic parameters such as net present value. Therefore the success of the company control deployment pathway in allowing CHP to reach a mass market can be closely linked to the economic advantage the systems offer. Likewise with the plug-and-play pathway and community microgrids, other factors relating to diffusion of innovations as discussed in Rogers (2003) will also play a role, but economics is likely to remain central to decision making. Central performance metric It is clear that economics forms the central concern in terms of achieving a mass market for CHP under all three deployment pathways. It is not the sole issue, but where other factors such as social prestige of ‘greenness’ are addressed, economic profitability will be critical to achieving a large market share. For CHP, attitudes and expectations regarding performance are inextricably linked to those of incumbent heating and electricity systems. Therefore, the investment decision for CHP should be considered in comparison with that of the competing reference grid/boiler systems, and it is the capital cost difference between the two options (that provide an essentially comparable service) that the potential adopter faces. Therefore, the primary metric chosen to address these issues is the net present value of the CHP system with respect to the competing reference system. This metric is calculated as the discounted value of the annual profits the ECSO could obtain from installing CHP in the customer’s dwelling and operating it over its lifetime. The ESCO makes profit by charging the customer the same amount that they would have been charged if they had used the incumbent grid/boiler reference system. Therefore this metric is cost-neutral for the dwelling 1

It should be noted that provision of quality information is of great importance in the plug-and-play model. This is because household owners/occupiers frequently do not have the time or knowledge to assess the economics of a particular CHP installation. Provision of such advice by independent experts could go a long way to achieving appropriate uptake of micro-CHP under a plug-and-play model.

© Woodhead Publishing Limited, 2011

CHP-Beith-02.indd 25

4/5/11 9:16:16 AM

26

Small and micro combined heat and power (CHP) systems

occupier in that it assumes gains afforded through reduced operational costs are offset by the leasing cost (i.e., the annualised capital cost) of the CHP equipment. The net present value of the ESCO’s profits can act as a guide for maximum installed capital costs the ESCO would pay for the CHP system. The reader should also note that the metric could equally be interpreted as the maximum capital cost a dwelling occupier or community would pay for outright purchase of the CHP system in the plug-and-play or community microgrid models, if they were to accept the chosen cost of capital. The chosen central performance metric caters directly to the company control pathway, and also has relevance to the other deployment pathways. Importantly, it is formulated to avoid the issue of incorporating uncertain capital costs into economic calculations, indicating the maximum allowable capital cost rather than guessing at a specific capital cost. Carbon dioxide (CO2) performance metrics In addition to the economic metric, further gauges are required to understand the capability of CHP to aid in achieving the goals of energy policy. At present, foremost among these goals is reduction of greenhouse gas emissions. For greenhouse gas emissions reduction, the adopted metric is straightforward: the annual CO2 savings provided by the CHP system when compared to that of the competing reference system. This measure considers only the operational greenhouse gas emissions. Lifecycle emissions due to manufacturing, fuel chain, disposal/recycling are not included and for these aspects readers are referred to Pehnt (2008). The CO2 metric is calculated based on the results of the optimisation. Therefore it relates to a situation where the economic performance of the system has been optimised, leading to the CO2 related result. This implies that the primary driver for CHP adoption is assumed to be economic, and the influence of CHP on CO 2 is consequential.

2.4.2 Description of the optimisation problem The complete mathematical formulation for the optimisation problem will not be presented here, and the reader is referred to published descriptions of the method in Hawkes et al. (2009a). Instead, a brief description of the objective function, decision variables, and constraints of the optimisation are provided to clarify the conceptual framework. The objective function of the optimisation is the annual cost of meeting a given energy demand profile, which is minimised. The energy demand profiles consist of heat and electricity demand, and are represented by a set of ‘sample days’ that are deemed to adequately characterise the complete annual demand. Depending on the CHP application, the demands are represented as

© Woodhead Publishing Limited, 2011

CHP-Beith-02.indd 26

4/5/11 9:16:16 AM

Techno-economic assessment of small and micro CHP systems

27

5-minute, half hourly, or hourly demand over each sample day, with more peaky demand profiles (such as residential demand) requiring finer temporal resolutions to properly capture demand dynamics following Hawkes and Leach (2005). The decision variables are: ∑

the output of the CHP prime mover in each time period (kWhe, kWhth), split piecewise by level of prime mover output to allow characterisation of non-linear efficiency profiles, ∑ the output of the supplementary thermal system in each time period (kWth), ∑ import of grid electricity in each time period (kWhe), ∑ charge to and discharge from electricity storage in each time period (kWhe), ∑ charge to and discharge from thermal energy storage in each time period (kWhe), ∑ yes/no variable to determine whether to switch on/off in each period. Optimisation constraints are: ∑ ∑ ∑ ∑ ∑ ∑

electricity demand must be met in each time period, thermal energy demand must be met in each time period, or exceeded by a small margin, the capacity of each system must not be exceeded, and charge/discharge rates for storage must not be exceeded, the minimum operating point of each system must be respected, maximum ramp rates of each system must be respected, constraints to impose start/stop costs.

The primary economic metric can be calculated from the value of the optimised objective function. This is achieved by calculating the cost of meeting the same energy demand with a defined reference system (e.g. a condensing boiler and grid electricity) and subtracting the value of the optimised objective function. The result is the annual saving provided by the system and, where a certain lifetime is assumed, the net present value of that annual saving can be calculated. This net present value is the primary economic metric as discussed above; it is the amount a rational investor would pay for the micro-CHP system over-and-above what they would pay for the competing reference system. Once this mathematical formulation is implemented, it may be used to explore a variety of technical, economic and policy-related aspects of CHP. In the following sections the case of micro-CHP for residential applications in the UK is considered, from the point of view of an investor/policy maker, and then a technology developer, each of which have differing interests. The CO2-related performance is also considered, culminating in a synthesis of

© Woodhead Publishing Limited, 2011

CHP-Beith-02.indd 27

4/5/11 9:16:16 AM

28

Small and micro combined heat and power (CHP) systems

the key characteristics of micro-CHP installations that are more likely to be commercially successful.

2.5

Case study: micro combined heat and power (CHP)

This case study applies the technique discussed above to examine an interesting emerging class of CHP technologies: that of micro-CHP for residential applications. These are essentially ‘home heating solutions’ designed to replace existing systems such as boilers or furnaces, and can provide both space heating and hot water, in addition to some electricity. The potential market for such systems is large, with ten-of-millions of boiler replacements occurring each year in Europe alone according to Micro-Map (2002). There are several examples of their development, demonstration, and commercialisation in, for example, Tokyo Gas Co. (2005) and Anon. (2007, 2008). The main prime mover technologies for micro-CHP are polymer electrolyte fuel cells (PEMFC), solid oxide fuel cells (SOFC), internal combustion engines (ICE), and Stirling engines. Each of these core technologies have individual limitations and performance characteristics, and they are at very different stages of development. This case study focuses on two key near-to-medium term technologies for Europe: ICEs and SOFCs. Specific results are based on the energy demand characteristics, energy prices and CO2 emissions rates of the UK in 2009, but the technique could be applied to any country and any time period. A summary of the input parameters for the case study is as follows. At the time of writing the average annual dwelling electricity demand in the UK was 4.3 MWh, and average annual total thermal demand was approximately 18 MWh, although wide variation around these values is observable. Energy prices were approximately 10p/kWh (U$0.15/kWh) for electricity and 2.5p/ kWh (U$0.0375/kWh) for gas. Grid-embodied CO2 rates were approximately 0.52 kgCO2/kWh, and gas was assumed to embody 0.19 kgCO2/kWh (net calorific value). Finally, details of the technical limitations and performance characteristics of the micro-CHP systems investigated can be found in Hawkes et al. (2009b). The primary distinction between prime movers in terms of techno-economics is their heat-to-power ratio (HPR), as discussed below. SOFCs have the lowest HPR of about 1 kWthh to 1 kWeh (1:1), whilst PEMFCs, ICEs and Stirling engines exhibit HPRs of approximately 2:1, 3:1, and 8:1, respectively. Finally, the lifetime of each system is assumed to be 10 years (equivalent to the average service life of the existing boiler stock), and the service interval is assumed to be the same as for a boiler (once per year, at an additional cost of £25 relative to a boiler service). The following analysis takes these inputs and applies them in the modelling methodology in two distinct ways: firstly, to inform investors regarding the

© Woodhead Publishing Limited, 2011

CHP-Beith-02.indd 28

4/5/11 9:16:16 AM

Techno-economic assessment of small and micro CHP systems

29

key drivers and potential of each prime mover technology, and secondly, to inform system developers regarding which technical aspects upon which to focus research and development.

2.5.1 Techno-economic assessment for investors and policy makers It has been widely reported that there is a relationship between the thermal demand met by CHP units and their economic viability, with high and consistent thermal demand often associated with positive results. Whilst it is not expected that micro-CHP is an exception to this rule, it is informative to investigate the extent to which annual demand influences value, given the distinctive nature of residential tariffs and demand. It is also informative to investigate the influence of annual electricity demand. Figure 2.3 displays the variation in economic results for two key microCHP technologies with respect to the dwelling’s annual thermal demand (for this figure annual electricity demand has been held constant at the UK ICE Flat Bungalow Terrace Semi-detached Detached Linear fit (all)

1200 Maximum cost difference between micro-CHP system and boiler system (£)

SOFC

1000

1200

1000

800

800

600

600

400

400

200

200 y = 0.025x – 63, R = 0.94

0

5

10

15

y = 0.008x + 686, R = 0.73

0 20 25 30 5 10 15 Annual thermal demand (MWh/year)

20

25

30

2.3 Sensitivity of economic performance of two micro-CHP systems (ICE – internal combustion engine, and SOFC – solid oxide fuel cell) to annual thermal demand in the target dwelling. Each subplot contains data for five typical UK construction types, and a linear fit to all data points.

© Woodhead Publishing Limited, 2011

CHP-Beith-02.indd 29

4/5/11 9:16:16 AM

30

Small and micro combined heat and power (CHP) systems

mean value of 4.3 MWh/year). Therefore the only source of variation in each plot is the annual thermal demand profile applied. Thermal demand scenarios correspond to each of the five dwelling construction types reported on in the UK census, with each construction type denoted by three markers corresponding to ‘existing’, ‘refurbished’ and ‘new’ insulation standards (i.e., decreasing thermal demand). Inspection of the figure reveals that the dependence of the economic result on annual thermal demand is evident in all cases, but is more obvious in the case of the micro-CHP prime mover with higher heat-to-power ratio (i.e., the internal combustion engine). The ICE is more exposed to lack of thermal demand, demonstrated by the relatively steep slope of the linear fit in the left subplot. The fuel cell-based system shows a more consistently positive economic result which does not change significantly as annual thermal demand decreases (with corresponding shallower slope on the linear fit). The key inference here is that SOFC-based systems are more economically resilient to changes in annual thermal demand than ICE-based systems. Figure 2.4 displays the sensitivity of the economic result to the dwelling’s annual electricity demand (plotted across three cases of annual thermal demand; SOFC

ICE Low thermal demand Average thermal demand High thermal demand Linear fit (all)

Maximum cost difference between micro-CHP system and boiler system (£)

1200

1200

1000

1000

800

800

600

600

400

400

200

200 y = 0.06x + 112, R = 0.73

0

0

2500

5000

y = 0.13x + 225, R = 0.97

0 7500 10000 0 2500 5000 Annual electricity demand (kWh/year)

7500

10000

2.4 Sensitivity of economic performance of two micro-CHP systems (ICE – internal combustion engine, and SOFC – solid oxide fuel cell) to annual electricity demand in the target dwelling. Each subplot contains data for five typical UK construction types, and a linear fit to all data points.

© Woodhead Publishing Limited, 2011

CHP-Beith-02.indd 30

4/5/11 9:16:17 AM

Techno-economic assessment of small and micro CHP systems

31

an existing flat – ≈13 MWh/year (low); average existing terrace – ≈18 MWh/ year (average); and an existing detached house – ≈28.5 MWh/year (high)). These thermal demands include both space heating and domestic hot water demand. It is apparent from this figure that the fuel cell-based system shows strong dependence of economic result on annual electricity demand; much stronger than its sensitivity with respect to annual thermal demand observed in Fig. 2.3. Conversely, the engine-based system shows approximately the same sensitivity to annual thermal demand as it does to annual electricity demand. It can be deduced that electricity demand and thermal demand are both important for a positive economic result for the engine-based system, but onsite electricity demand is the primary driving factor for the fuel cellbased system. Overall the results in Figs 2.3 and 2.4 can be synthesised to arrive at a key conclusion for micro-CHP: those prime movers that produce heat are more likely to be more influenced by the level of annual thermal demand. Lack of thermal demand, or significant heat production for a given electricity output (i.e., a high heat-to-power ratio of the micro-CHP prime mover), corresponds to a constraint on system operation. Essentially these ‘thermal constraints’ limit the ability of a system to benefit economically from displacing onsite electricity demand. Conversely, the economic performance of systems that produce less heat (i.e., low heat-to-power ratio) is driven more by annual electricity demand. This relates to the fact that thermal constraints do not interfere so much with their operation, and they can therefore access the value associated with generating electricity to displace onsite demand which would otherwise have been met by more expensive grid electricity. Lack of onsite electricity demand obviously limits the ability of the system to displace it and gain access to this value. In summary, primary value for micro-CHP lies in generating to displace onsite electricity demand; access to this value is enabled by the presence of such demand, and the presence of thermal demand and/or application of a prime mover with a low heat-to-power ratio to avoid thermal constraints on operation.

2.5.2 Techno-economic assessment for technology developers Choice of prime mover nameplate capacity is an important concern for micro-CHP systems developers in terms of where their product fits into the market, and those concerned with assessing the economic credentials of micro-CHP. Therefore the sensitivity of economic performance to capacity choice is investigated here. This is important because examination of a single capacity system may miss valuable opportunities for micro-CHP developers to scale up or scale down their products. Figure 2.5 displays the difference in value between the micro-CHP system and the reference grid-boiler system

© Woodhead Publishing Limited, 2011

CHP-Beith-02.indd 31

4/5/11 9:16:17 AM

32

Small and micro combined heat and power (CHP) systems

(i.e., the central performance metric) for ICE and SOFC-based micro-CHP, with sensitivity of results to annual energy demands of three constructiontype variants. Figure 2.5 shows the notable variation in value between the two microCHP technologies with respect to competing conventional boiler systems. For a 1 kWe SOFC-based system operating in a mean demand situation (i.e., the terraced house in the right subplot) an investor with 12% cost of capital would pay approximately £800 (US$12002) more than what they would pay for the competing boiler. Conversely, a rational investor would only pay approximately an extra £400 (US$600) for the modelled ICE system.3 Moreover, Fig. 2.5 demonstrates that the current economic situation presents a challenge for any micro-CHP developer. This is because the manufacturing ICE

Maximum installed cost difference between boiler-only and micro-CHP (£)

2500

Flat Terrace Detached

2500

2000

2000

1500

1500

1000

1000

500

500

0

0

1

2

0 3 4 0 1 Prime mover capacity (kWe)

SOFC

2

3

4

2.5 Sensitivity of micro-CHP system value to prime mover capacity. Plotted for two prime mover types (ICE – internal combustion engine, and SOFC – solid oxide fuel cell) and for three dwelling construction types in the UK. 2

The exchange rate at the time of writing was applied here, where £1GBP ≈ US$1.50. Whilst one cannot draw conclusions regarding comparison between the economic attractiveness of the two technologies based on this result (because manufacturing and installation costs for systems incorporating each prime mover type are not yet readily observable), it is clear that fuel cell system developers have more breathing space. 3

© Woodhead Publishing Limited, 2011

CHP-Beith-02.indd 32

4/5/11 9:16:17 AM

Techno-economic assessment of small and micro CHP systems

33

cost of including the micro-CHP prime mover and balance of plant (in addition to the boiler) in the system is likely to be considerable, and a challenge for system developers to attain with an allowable margin of less than £1000 (US$1500) including any potential profit. As such, early markets are likely to focus on houses with larger demands which have access to higher value, up to £1300 (US$1950) per installation, and on installations driven by noneconomic aspects such as environmental benefits. Also with regard to Fig. 2.5, it is instructive to note how the value per kWe installed changes with increasing prime mover capacity. Almost all the plotted lines have the largest positive slope between 0 kWe and 1 kWe prime mover capacity. As prime mover capacity increases thereafter, value per additional kWe installed decreases. Therefore there is probably no justification in providing a product to the UK residential market with prime mover capacity greater than approximately 1 kWe. This result does vary between technologies; the value per kWe installed for fuel cell-based systems is clearly not as sensitive to increasing capacity as the engine-based systems. At the far end of the scale, a comparatively large SOFC-based system has a value of roughly £2400 (US$3600) more than the competing boiler, and this could be a reasonable manufacturing proposition if economies of scale entail cheaper production (per kWe) of larger systems. Regardless of such possibilities, micro-CHP systems with 1 kWe capacity present the best per kWe installed value, and are probably the best proposition for a mass market which is dominated by single-residence dwellings in a mild climate. Aside from choice of capacity, many other high-level technical characteristics are of interest to micro-CHP system developers. For example, Fig. 2.6 displays the sensitivity of the economic result to a range of maximum allowable ramp rates. Ramp rates are important particularly for fuel cell-based systems, which may face durability issues under regular thermal cycling, and the developer faces a choice regarding whether or not to invest time in improving ramping performance. This question can be answered in that the figure clearly shows that variation in the maximum allowable ramp rate of the micro-CHP system does not have a significant influence on economic credentials. Only at very low maximum ramp rates below 20 Watts per minute is any influence discernable, and even then it is minor, corresponding to less than 5% of the value of the fuel cell-based micro-CHP systems. From a technology developer’s point of view, this means that they should not invest in creating systems that are able to ramp up and down quickly in response to changing conditions. Rather, they should focus on development of somewhat predictive control systems that can forecast when operation will be required and/or profitable. In a single residential dwelling this is relatively easily achieved, where the user programs a desired temperature profile, thus giving the system much advance warning of expected modes of operation.

© Woodhead Publishing Limited, 2011

CHP-Beith-02.indd 33

4/5/11 9:16:17 AM

34

Small and micro combined heat and power (CHP) systems

Maximum installed cost difference between boiler-only and micro-CHP (£)

900 800 700 ICE 600

PEMFC SOFC

500

Stirling

400 300 200 100

0

0.05 0.1 0.15 Maximum ramp rate (kWe/minute)

0.2

2.6 Sensitivity of economic result to the maximum ramp rate for four micro-CHP technologies (ICE – internal combustion engine, PEMFC – polymer electrolyte membrane fuel cell, SOFC – solid oxide fuel cell, and Stirling engine). Results plotted for 1 kWe system operating in an average terraced house.

For the final example of the use of techno-economics to provide information to developers, Fig. 2.7 presents the sensitivity of the case for investment to minimum operating point (i.e., a proxy for turndown ratio) for four micro-CHP systems. Once again a clear differentiation is apparent between the prime mover technologies. The low heat-to-power ratio fuel cell-based systems are much more resilient to limited turndown characteristics than the higher heat-to-power ratio engine-based systems. For example, the Stirling engine-based system is unable to provide a positive case for investment if it cannot turn down below 0.4 kWe, whilst the fuel cell-based systems retain value even when they cannot turn down at all. The result in Fig. 2.7 presents a challenge for micro-CHP prime mover developers, because achieving efficient turndown in small systems is problematic, where the conventional wisdom is that balance of plant (BoP) energy consumptions become dominant and system efficiency drops off rapidly. The majority of 1 kWe engine-based units in field trials and commercially available (in Japan) are not able to turn down at all. They offer on/off operation only. Whilst this engineering decision could have been taken for a variety of technical reasons, the results of the analysis presented suggest turndown should be considered as a valuable system characteristic, even if that turndown involved operation at a few selected set-points. Should BoP

© Woodhead Publishing Limited, 2011

CHP-Beith-02.indd 34

4/5/11 9:16:17 AM

Techno-economic assessment of small and micro CHP systems

Maximum installed cost difference between boiler-only and micro-CHP (£)

1200

35

ICE PEMFC

1000

SOFC Stirling

800

600

400

200

0

0

0.2

0.4 0.6 0.8 Minimum operating point (kWe)

1

2.7 Sensitivity of economic result to the minimum operating point for four micro-CHP technologies (ICE – internal combustion engine, PEMFC – polymer electrolyte membrane fuel cell, SOFC – solid oxide fuel cell, and Stirling engine). Results plotted for 1 kWe system operating in an average terraced house.

components be improved to the point where turndown to near-zero output whilst maintaining efficiency is possible, particularly for fuel cell-based systems, substantial additional value would become available to the owner/ operator, arguably increasing the value of the unit to the developer. This and the previous sub-sections have presented examples of how technoeconomic optimisation modelling might be used to assess the economic performance CHP. The following two sub-sections build on this analysis from a different perspective; that of the stakeholder who considers environmental performance to be important. A policy maker may be one example of such a stakeholder.

2.5.3 CO2 emissions reduction performance Figure 2.8 shows the sensitivity of annual CO2 emissions reductions to system nameplate capacity for the two key micro-CHP systems. There is clear differentiation between these prime mover technologies, with the fuel cell-based system offering a definitive performance advantage. Similarly to the results relating to the relative economics of micro-CHP systems presented in the previous sections, these CO2-related results can be attributed to the different heat-to-power ratios between the technologies. Those technologies

© Woodhead Publishing Limited, 2011

CHP-Beith-02.indd 35

4/5/11 9:16:18 AM

36

Small and micro combined heat and power (CHP) systems

with a low heat-to-power ratio prime mover are less constrained by lack of thermal demand and are subsequently able to generate more electricity and thereby gain CO2 credit for offsetting more grid electricity. A further point of interest from Fig. 2.8 is that the SOFC-based microCHP is able to virtually eliminate the entire operational CO2 footprint of the dwelling when a 4 kWe system is installed. Whilst such large nameplate capacity systems may be prohibitively expensive in the near term, the result still presents an interesting possibility for delivering housing in the line with the UK’s current low carbon policy. For example, the 4 kWe system could single-handedly achieve the UK government’s 2050 80% emissions reduction target for that dwelling. However, extension of this concept to a large number of dwellings is problematic because the result is underpinned by a high CO2 rate for grid electricity, which would not be the case if fuel cell micro-CHP produced a large portion of this grid electricity. It is also instructive to consider the sensitivity of the CO2 result to annual thermal demand (holding electricity demand constant), and annual ICE

5000

5000 Flat Terrace

4500

SOFC

4500

Annual CO2 reduction (kgCO2)

Detached 4000

4000

3500

3500

3000

3000

2500

2500

2000

2000

1500

1500

1000

1000

500

500

0

0

1

2

0 3 4 0 1 Prime mover capacity (kWe)

2

3

4

2.8 Sensitivity of CO2 reductions to prime mover capacity. CO2 reduction is with respect to the reference grid/boiler system, and plotted points are for two prime mover types (ICE – internal combustion engine, and SOFC – solid oxide fuel cell) and three dwelling construction types in the UK.

© Woodhead Publishing Limited, 2011

CHP-Beith-02.indd 36

4/5/11 9:16:18 AM

Techno-economic assessment of small and micro CHP systems

37

electricity demand (holding thermal demand constant), once again mirroring the economic analysis. Figures 2.9 and 2.10 present these sensitivities for 1 kWe systems, operating in each of the five dwelling construction types and across ‘existing’, ‘refurbished’, and ‘new’ building thermal performance standards. Figure 2.9 shows that annual thermal demand can be important in microCHP achieving CO2 reductions. This is particularly apparent for the higher heat-to-power ratio (HPR) ICE-based systems. In contrast, the SOFC-based system provides reductions largely independent of annual thermal demand. These two cases relate once again to HPR; the ICE engine (higher HPR) is unable to provide reductions for low annual thermal demands because its operation is frequently curtailed by thermal constraints, whilst the SOFC-based prime mover (low HPR) can operate almost all the time because it rarely encounters these constraints regardless of the thermal demand scenario. Figure 2.10 shows the CO2 reduction achievable by 1 kWe micro-CHP systems across a range of annual electricity demand scenarios. This figure ICE

1800

Flat Bungalow Terrace Semi-detached Detached Linear fit (all)

1600 1400 Annual CO2 reduction (kgCO2)

1800

SOFC

1600 1400

1200

1200

1000

1000

800

800

600

600

400

400

200

200 y = 0.041x –307, R = 0.94

0

5

10

15

y = 0.011x + 1136, R = 0.80 0 20 25 30 5 10 15 20 25 30 Annual thermal demand (MWh/year)

2.9 Sensitivity of CO2 performance of two micro-CHP systems (ICE – internal combustion engine, and SOFC – solid oxide fuel cell) to annual thermal demand in the target dwelling. Each subplot contains data for five typical UK construction types, each with three insulation levels, and a linear fit to all data points.

© Woodhead Publishing Limited, 2011

CHP-Beith-02.indd 37

4/5/11 9:16:18 AM

38

Small and micro combined heat and power (CHP) systems ICE

1800

Low thermal demand Average thermal demand High thermal demand Linear fit (all)

Annual CO2 reduction (kgCO2)

1600

1600

1400

1400

1200

1200

1000

1000

800

800

600

600

400

400 200

200 y = 0.01x + 402, R = 0.12 0

SOFC

1800

0

2500

y = 0.02x + 1270, R = 0.49

0 5000 7500 10000 0 2500 5000 Annual electricity demand (kWh/year)

7500

10000

2.10 Sensitivity of CO2 performance of two micro-CHP systems (ICE – internal combustion engine, and SOFC – solid oxide fuel cell) to annual thermal demand in the target dwelling. Each subplot contains data for five typical UK construction types, each with three insulation levels, and a linear fit to all data points.

demonstrates that there is little reliance of CO2 reduction on annual electricity demand for any specific construction type. This can be contrasted with the equivalent economic result presented in Fig. 2.4, which showed great dependence of economic result on annual electricity demand, particularly for low HPR prime movers. The reason for this contrasting CO2 result is that displaced grid electricity CO2 rates are the same regardless of whether generation is consumed onsite or exported to the grid (conversely, in the economic case, export attracts a lower value than onsite generation under central estimate values).

2.5.4 Key characteristics of commercially successful micro CHP The conclusion to be drawn from the case study of residential micro-CHP can be summarised as follows. The economic value of these systems (with respect to competing boiler systems) is driven by the combination of the ability to produce electricity and presence of onsite electricity demand to

© Woodhead Publishing Limited, 2011

CHP-Beith-02.indd 38

4/5/11 9:16:18 AM

Techno-economic assessment of small and micro CHP systems

39

be met. It is limited by excessive thermal production and/or lack of thermal demand in the target dwelling. For appropriately sized prime movers, heatto-power ratio is the key technical metric that speaks to these issues because it captures the propensity of a prime mover to deliver electricity despite low thermal demands. At times of low thermal demand a micro-CHP prime mover with a low heat-to-power ratio will be able to continue operating (and displacing expensive electricity import) where a higher heat-to-power ratio technology will be required to modulate or switch off. The driving forces of CO2 reduction for micro-CHP are identical to those of economic value, with the exception that the annual electricity demand of the target dwelling is not important. This is because the CO2 credit obtained for displacing imported electricity is identical to that for exported electricity. Conversely, for the economic case, the value of displacing onsite electricity demand is substantially higher than the value of electricity exported to the grid. On the whole, technologies with low heat-to-power ratios, low thermal capacity, contributing to serving dwellings with larger annual electricity and thermal demands are best placed to access economic value. For CO2 reductions, technologies with low heat-to-power ratio, low thermal capacity, serving dwellings with larger annual thermal demand are best placed to provide savings. Therefore appropriately sized fuel cell-based micro-CHP technologies, which have the lowest heat-to-power ratios of the investigated systems, benefit from all of these attributes and provide the best performance. The SOFC-based systems as characterised here provide the largest allowable installed cost difference (with respect to competing boiler systems) and the greatest potential CO2 emissions reduction. Higher heat-to-power ratio ICEbased systems still exhibit competitive performance credentials, but are more challenged to provide savings when thermal demands are low. The primary caveat to these statements is that the final mass-manufactured installed costs of each technology are not yet observable. For example, the relative advantages of SOFCs may be less relevant if the final commercial products are much more expensive than internal combustion engines, which is arguably likely to be the case. Investigation of potential final installed cost of systems is beyond the scope of this chapter, but the analysis above has been designed to be applicable regardless of this.

2.6

Future trends

On the whole, small-scale CHP is a relatively established technology, with a variety of systems already commercially available, and with a growing market share. But the range of technologies captured by the term CHP is vast, spanning from the well-established internal combustion engine through to the most sophisticated bio-energy or fuel cell-based systems. As such, there are

© Woodhead Publishing Limited, 2011

CHP-Beith-02.indd 39

4/5/11 9:16:18 AM

40

Small and micro combined heat and power (CHP) systems

always emerging technologies under the CHP banner, and much potential for scale-up or scale-down of established systems. Over the past decade perhaps the most interesting development has been that of micro-CHP, which has not yet entered the market in force except in Japan, where internal combustion engines dominate. New technologies, particularly fuel cells as discussed in this chapter, could form an important part of this market in future. Indeed, several developers are planning market launch of relevant products within the next few years. Of course, history tells us that many of these systems may not turn out to be viable or may require more development. Looking further into the future, the international low carbon aspirations may force attention to be directed more towards technologies that can meet long-term greenhouse gas emissions reduction targets. CHP fuelled by fossil fuel-based gases will always be challenged in this regard, because even perfectly efficient utilisation of relatively clean natural gas results in production of approximately 0.19 kgCO2/kWh, so there is little room for achieving reductions of the magnitude required. Therefore alternative fuels and technologies must be considered in the future. At the time of writing, much attention is focused on two potential routes to deeper carbon reductions. These are the mass market introduction of heat pumps, and the decarbonisation of piped natural gas for use in CHP or boilers/furnaces. Given inherent limitations in decarbonising piped gas by injection of waste-to-energy and biomass sources, it seems likely that the key competitor to CHP in the coming decades will be heat pumps (for space and water heating applications). Nevertheless, heat pumps face their own set of unique challenges, primarily surrounding installation and technology cost and upstream infrastructure impacts, and as yet there is no considered solution to these issues. Research is required into classes and combinations of demand-side technologies that can meet low carbon targets, and it is fitting that CHP remains a consideration in this regard due to its potential for fuel flexibility, relatively low cost, and arguably complementary upstream infrastructure impacts. Ultimately CHP is likely to form part of a diversified solution to meeting energy needs, where its informed use in combination with alternative technologies such as heat pumps could serve to meet stringent carbon aspirations whilst simultaneously minimising infrastructure investment requirements in coming decades.

2.7

Sources of further information and advice

Interested readers may find the following bodies of work of interest: ∑

IEA ECBCS Annex 42 on Residential Cogeneration, investigating the performance of several micro-CHP systems using experimental data and building simulation approaches: available at http://cogen-sim.net/.

© Woodhead Publishing Limited, 2011

CHP-Beith-02.indd 40

4/5/11 9:16:19 AM

Techno-economic assessment of small and micro CHP systems

∑ ∑

41

IEA ECBCS Annex 54 on the Assessment of Microgeneration Technologies, a new Annex investigating integration of microgeneration technologies. The CHPA-funded report questioning ‘all-electric’ heating approaches versus combined heat and power in Speirs et al. (2010).

2.8

References

Anon. (2007). ‘Ceres Power funding to develop manufacturing’. Fuel Cells Bulletin 2007(7): 8. Anon. (2008). ‘CFCL invests in German facility, wins Nuon order’. Fuel Cells Bulletin 2008(4): 9–10. European Commission. (2010). ‘Eurostat Database: Energy Prices’. Retrieved 24 March 2010, from http://epp.eurostat.ec.europa.eu/portal/page/portal/energy/introduction. Hawkes, A. D. and M. A. Leach (2005). ‘Impacts of temporal precision in optimisation modelling of micro-combined heat and power’. Energy 30(10): 1759–1779. Hawkes, A. D., D. J. L. Brett and N. P. Brandon (2009a). ‘Fuel cell micro-CHP technoeconomics: Part 1 – model concept and formulation’. International Journal of Hydrogen Energy 34(23): 9545–9557. Hawkes, A. D., I. Staffell, D J. L. Brett and N. P. Brandon (2009b). ‘Fuel cells for micro-combined heat and power generation’. Energy & Environmental Science (Royal Society of Chemistry) 2(7): 729–744. Lovins, A., E. K. Datta, T. Feiler, K. R. Rábago, J. N. Swisher, A. Lehmann and K. Wicker (2002). Small Is Profitable: The Hidden Economic Benefits of Making Electrical Resources the Right Size. Snowmass, CO: Rocky Mountain Institute. Micro-Map (2002). Mini and Micro CHP – Market Assessment and Development Plan: Summary Report. London: FaberMaunsell Ltd. Padhy, N. P. (2004). ‘Unit commitment – a bibliographical survey’. IEEE Transactions on Power Systems 19(2): 1196–1205. Pehnt, M. (2008). ‘Environmental impacts of distributed energy systems – the case of micro cogeneration’. Environmental Science & Policy 11(1): 25–37. Rogers, E. M. (2003). Diffusion of Innovations. New York: Free Press. Sauter, R. and J. Watson (2007). ‘Strategies for the deployment of micro-generation: implications for social acceptance’. Energy Policy 35(5): 2770–2779. Speirs, J., R. Gross, S. Deshmukh, P. Heptonstall, L. Munuera, M. Leach and J. Torriti (2010). Building a roadmap for heat: 2050 scenarios and heat delivery in the UK, A Report by Imperial College London and University of Surrey for the Combined Heat and Power Association, London. Tokyo Gas Co. (2005). Sales of the residential gas engine cogeneration system ‘ECOWILL’ and establishment of the optional agreement ‘Residential cogeneration system contract’. Tokyo, Japan. Watson, J. (2004). ‘Co-provision in sustainable energy systems: the case of microgeneration’. Energy Policy 32(17): 1981–1990.

© Woodhead Publishing Limited, 2011

CHP-Beith-02.indd 41

4/5/11 9:16:19 AM

3

Thermodynamics, performance analysis and computational modelling of small and micro combined heat and power (CHP) systems

T. T. A l - S h e m m e r i, Staffordshire University, UK

Abstract: Combined heat and power (CHP) production compared with single generation of power has two advantages: it helps to improve the utilisation of energy production and to reduce pollution. Cost-efficient operation of a CHP system can be planned using an optimisation model based on the thermodynamic principles involved in the behaviour of the system’s working fluid and operating conditions of the system’s components. However, the calculations are complex, lengthy and prone to errors. Hence a computerised model was written with the aim of helping the designer to examine the influence of the various parameters involved on the overall efficiency/utilisation of the plant under different conditions. The model in this chapter examines the effect of the varying demand on heat and power and calculates the performance parameters and the overall utilisation factor of the plant at any power/heat ratio. Finally, a case study is presented to demonstrate the system performance of a CHP system driven by biogas internal combustion engine. Key words: CHP, cogeneration, power generation, thermodynamic modelling.

3.1

Introduction

Fuels (such as coal, oil, natural gas, biomass and biogas) are burnt to release energy which is then harnessed to serve some useful purpose. The most basic form of the released energy is heat (as in a domestic boiler) and this can then be distributed via a heat-exchanger and a circulating fluid to be used for water and space heating. Good domestic boilers have good thermal efficiency in the range of 60–80% or even higher, so some heat is lost depending on the design, maintenance of the burner and on the type of fuel. In power plants, heat is used to drive steam turbines which are in turn coupled to an alternator thus producing electricity. In this mode, typically the system is only 30–40% efficient, hence two-thirds of the energy in the fuel is wasted. Most power stations are designed and built for the sole purpose of producing electricity, and all heat is dumped to the atmosphere. If these two requirements are well planned and the choice of system or location is carefully selected, it is possible to make use of the reject heat 42 © Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 42

4/5/11 9:16:49 AM

Thermodynamics, performance analysis & computational modelling

43

from the power utility, and hence improve the returns from the fuel, hence increase the efficiency of the system. This method of producing/using two outputs is known as cogeneration or combined heat and power (CHP). In CHP systems, the waste heat is used to provide process heat or space heating/ cooling for industrial facilities, district energy systems, and commercial buildings. By recycling this waste heat, cogeneration systems achieve typical effective electric efficiencies of 50–80% – a dramatic improvement over the average 33% efficiency of conventional fossil-fuelled power plants. The higher efficiencies of cogeneration improve productivity by reducing fuel costs and reduce greenhouse emissions.

3.2

Types of combined heat and power (CHP) systems

A cogeneration unit consists of the following three basic components: ∑

a primary mover in which fuel is converted into mechanical and/or thermal energy ∑ a generator to transform the mechanical energy into electricity ∑ a heat recovery system to collect the produced heat. Gas engines are normally used in situations where the heat is used for spatial heating. When higher temperature heat is needed, e.g. for process heating, gas turbines tend to be more appropriate. Traditionally, gas engines have been used for small-scale applications (200 kWe–5 MWe), while gas turbines and steam turbines have been used for large-scale applications (> 5 MWe). In recent years, however, the micro turbine (30 kWe–0.5 MWe) has come onto the market and entered use for many small-scale applications. Prime movers for CHP systems can be any of the following options: ∑ Steam turbines ∑ Gas turbines ∑ Internal combustion engines ∑ Solar systems ∑ Biomass external combustion systems ∑ Stirling engines. In conventional thermal power plants, only about one-third of the energy in the coal or oil appears as electrical power – two-thirds of the energy is thrown away in the form of lukewarm water, in cooling towers, to rivers or to the sea. Alteration of the design and operation of an electrical power plant to cogenerate useful heat and work improves energy utilisation. The heat must be provided at the correct temperature for space and hot water requirements of domestic, commercial and public buildings, or alternatively steam may be provided to meet industry’s needs for processes.

© Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 43

4/5/11 9:16:49 AM

44

Small and micro combined heat and power (CHP) systems

The design of CHP systems will depend on the individual application. In order for CHP systems to be economically viable, it is essential to exploit every possibility of minimising cost by a fully logical approach both in the area of generation and in the transport and distribution of heat. Among other considerations are the following parameters: ∑ period of use of the existing generating plant ∑ electrical and thermal load capacities of the appropriate plant ∑ load variations, daily, diurnal, diversity of household, industrial, commercial loads ∑ flow temperatures in the supply network related to pipe diameter required ∑ methods used for metering, operation of tariffs, collection of charges. There are three general areas of cogeneration: ∑ combined heat and power for district heating ∑ combined heat and power for commercial buildings ∑ combined heat and power for industry. The reject heat from a conventional power plant is at too low a temperature for district heating purposes, but with some design changes, it can be raised with some loss in the production of electricity. From the point of view of electricity production the result is a loss of efficiency but, when combined electricity and useful heat production is considered, there is a considerable increase in the overall effectiveness over the same quantities of electricity and heat produced from the separate sources. As a general guide the overall efficiency of a domestic cogeneration (CHP/DH) system can be of the order of one and a half times greater than the efficiency from separate sources. Such an increase results in large projected fuel savings. For any CHP/DH scheme to be viable, the savings in fuel costs must be sufficient to provide for the extra capital costs of the scheme together with any inducements necessary to persuade potential customers to switch from their existing methods of heating. Industrial cogeneration (CHP) has been widely applied in industry for the production of electricity and process heat, especially in areas that have a high demand for process heat. In certain cases it may be necessary to utilise a boost fired burner to ensure the correct thermal quality of process heat as required by the consumer. If a large company has suitable electrical and process heat requirements, then it may be more economical to operate and maintain their own CHP system.

3.3

Thermodynamics of cogeneration

When designing CHP installations there are a number of design issues, design variables and operating factors to consider to achieve optimum plant

© Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 44

4/5/11 9:16:49 AM

Thermodynamics, performance analysis & computational modelling

45

performance. In order to analyse CHP systems, it is necessary to review the thermodynamics of the basic stand-alone power processes and the heat producing system and then to consider the CHP alternative. In this chapter typical analysis is based on the steam plant cycle as it shares the working fluid with the process heat. The following subsections present three scenarios in which such plants can operate.

3.3.1 Scenario one: power only Consider the basic power plant. The sole purpose is to convert a proportion of the input energy transferred to the working fluid into useful work. The remaining portion of the heat is rejected to rivers, lakes, oceans, or the atmosphere as waste heat. Wasting a large amount of heat is a price we have to pay to produce work, because electrical or mechanical work is the only form of energy on which many engineering devices can operate. Power only cycles have typical efficiencies in the region of 35%. Figure 3.1 shows a schematic diagram of the simple power-only plant operating on the basic Rankine cycle with superheated steam. The figure also displays the temperature–entropy property diagram of the cycle. It can be seen that this simple cycle is made up of four components/processes. The focus in this cycle is on the turbine unit, process 3–4, which represents the power output developed. Hence, optimising this process is vital to getting the best out of the system. The net power output from this system is given in terms of steam mass flow rate and enthalpies as:

Wnet = ms ¥ [(h3 – h4) – (h2 – h1)]

3.1

Where Wnet is the net work output from the system, ms is the mass flow

Boiler

3

Turbine 3

T 4

2

Condenser

2 1

Pump

4

1 S

3.1 A simple ‘power-only’ plant and its T-s cycle.

© Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 45

4/5/11 9:16:49 AM

46

Small and micro combined heat and power (CHP) systems

rate system, hx is the enthalpy of steam at stake point xT (x represents any number corresponding to the cycle Fig. 3.3).

3.3.2 Scenario two: heat only Many systems or devices, however, require energy input in the form of heat, called process heat. Some industries that heavily rely on process heat are chemical, pulp and paper, oil production and refining, steel making, food processing, and textile industries. Process heat in these industries is usually supplied by steam at 5–7 bar and 150–200 °C. Energy is usually transferred to the steam by burning coal, oil or natural gas in a furnace. The process heat output from this system is given in terms of steam mass flow rate and enthalpies as (see Fig. 3.2): Qprocess = ms ¥ [(h3 – h4)]



3.2

The generation of high quality steam for process heating only is generally efficient, however, as in any practical situation, it suffers from losses due to: ∑ inefficient heat transfer surfaces ∑ losses in distribution ∑ irrecoverable thermal energy of the steam at exit from process heater. In spite of the above losses, a typical heat-only scheme has efficiency in excess of 80%.

3.3.3 Scenario three: combined heat and power In practice, industries that use large amounts of process heat also consume a large amount of electric power. Therefore, it makes economical as well as engineering sense to use the already existing work potential to produce PRV

3

T

Boiler

3

4 2

2 Pump

1

Process heater

4

1 S

3.2 A simple ‘process heating only’ plant and its T-s cycle.

© Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 46

4/5/11 9:16:49 AM

Thermodynamics, performance analysis & computational modelling

47

power while utilising thermal energy for process heat, instead of letting it go to waste. Such a plant is called a cogeneration plant. In general, cogeneration is the production of more than one useful form of energy (such as process heat and electric power) from the same energy source. A schematic of a typical steam-turbine cogeneration plant is shown in Fig. 3.3. Under normal operation, some steam is extracted from the turbine at some predetermined intermediate pressure (point 5). The rest of the steam expands to the condenser pressure (point 6) and is then cooled at constant pressure. The heat rejected from the condenser represents the waste heat for the cycle. At times of high demand for process heat, all the steam is routed to the process heating units and none to the condenser. The waste heat is zero in this mode. If this is not sufficient, some steam leaving the boiler is throttled by an expansion or pressure-reducing valve (PRV) to the extraction pressure (point 5) and is directed to the process heating unit. Maximum process heating is realised when all the steam leaving the boiler passes through the PRV. No power is produced in this mode. When there is no demand for process heat, all the steam passes through the turbine and the condenser, and the cogeneration plant operates as an ordinary steam power plant.

1

3

Boiler

2

Turbine

PRV 11

4

5

6

Process heater

10 Mixing Pump 2

7

Condenser

9

Pump 1

8

3.3 Combined heat and power plant.

© Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 47

4/5/11 9:16:50 AM

48

Small and micro combined heat and power (CHP) systems

3.4

Performance analysis of cogeneration cycles

The following parameters are considered in evaluating the performance of a CHP. Referring to the CHP cycle in Fig. 3.3, the net power output from this system is given in terms of steam mass flow rate and enthalpies as: Wnet = ms ¥ [(h3 – h6) – (h9 – h8) – (h7 – h10)]

3.3

The process heat output from this system is given in terms of steam mass flow rate and enthalpies as: Qprocess = ms ¥ [(h4 – h7)]

3.4

The work ratio gives an indication of the proportion of useful power to that produced in the turbine. WR = [(h3 – h6) – (h9 – h8) – (h7 – h10)]/[(h3 – h6)]

3.5

The utilisation factor in cogeneration is used in place of the thermal efficiency, to describe the ratio of useful energy output divided by the energy input: Utilisation tilisation ffactor actor =

Network orkk output + P or Pro rocess heat eat heat supplied

3.6

The specific steam consumption (SSC) indicates the relative size of plant, i.e. mass flow per unit power output. This determines the compactness of the system so it is desirable to have this as low as possible, as less steam used to generate power implies less energy is wasted in pumping, heating, etc. SSC =

3.5

3600 kg/kWh Network or ork

3.7

Theory of heat exchangers

A heat exchanger is a device used for transferring heat from a hot fluid to a cold fluid. There are three different types of heat exchangers, depending on the geometry and the way in which the two fluids interact: ∑ ∑ ∑

double-pipe heat exchangers shell-and-tube heat exchangers cross-flow heat exchangers (see Fig. 3.4).

Heat exchange between a hot fluid and a cold fluid across the metal boundary of a heat exchanger is represented by equations: Cold fluid: Qa = mair ¥ Cpair ¥ DTair

3.8

Hot fluid: Qw = mwater ¥ Cpwater ¥ DTwater

3.9

heat exchanger Qex = AUm DTm

3.10

© Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 48

4/5/11 9:16:50 AM

Thermodynamics, performance analysis & computational modelling

49

Thi

DT1 Cross flow Tho

dq Tco

DT2

Tube flow

Tci

3.4 Cross-flow heat exchanger and a typical temperature profile.

where Q is the total heat transfer rate, A is the total internal contact area, and Um is the mean overall coefficient of heat transfer. There are three methods for evaluation/sizing heat exchangers.

3.5.1

The logarithmic mean temperature difference (LMTD)

The LMTD is defined by the four temperatures involved as follows: DT1 – DT2 ln((DT1 /DT2 )

3.11

where: DT1 = Th,i–Tc,o and DT2 = Th,o–T c,i

3.12

DTln =

3.5.2

The modified LMTD

This is applicable to other types of heat exchangers, with factors taken from correction charts to allow for deviation from the double pipe LMTD values. In most practical designs the two fluids will not flow in pure co-current, counter-flow or cross-flow fashion but will be some combination of all three. In the common ‘shell and tube’ heat exchanger, the temperature profile is further complicated by the fact that the shell side flow is not in one direction due to the presence of baffles. Baffles are installed to increase shell side fluid velocities and mixing and hence improve the shell side heat transfer coefficient. Clearly the simple logarithmic mean temperature difference equation cannot be directly applied in these cases and a correction factor (F) has to be applied to DTm (LMTD) for a simple double pipe heat exchanger. The analysis of multipass and cross-flow geometries is usually presented graphically utilising two system characteristic temperature ratios. Z=

Ti – To t –t ,P= i o to – ti Ti – t i

3.13

© Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 49

4/5/11 9:16:51 AM

50

Small and micro combined heat and power (CHP) systems

where T and t denote the shell-side and tube-side temperatures, respectively, and i and o denote inlet and outlet respectively. The rate of heat transfer (Q) is given by Q = U ¥ A ¥ DTm ¥ F

3.14

Correction factor charts for common geometric are available in standard textbooks on heat transfer or heat exchangers.

3.5.3

The number of transfer units (NTU)-effectiveness method

This method for evaluating the performance of heat exchangers has the advantage that it does not require the calculation of the logarithmic mean temperature difference. its main use is to calculate achievable outlet temperatures by an existing heat exchanger of known area and construction upon a change in operating requirement or duty. The method depends on the evaluation of three dimensionless parameters: the effectiveness, the NTU and the capacity ratio, these are defined in the following paragraphs. Effectiveness (e) This is defined as the ratio of actual heat exchange to the maximum possible heat transfer Q Actual al heat ttransfer = maximum possible heat tra ransfer Qmax . where Qmax = (m Cp)min (Thi – Tci) . Q = m h Cph (Thi – Tho) . Q = m c Cpc (Tco – Tci)

e=

3.15 3.16 3.17 3.18

hence

e=

m h C Cpph (Thi – Tho ) m Cp (T – Tci ) or c c co  p )min  p )min (mC (mC min (Th hii – Tci ) min (Th hii – Tcci )

3.19

depending on which data are available. Number of transfer units (NTU) This is defined as: . NTU = UA/(m Cp)min

3.20

© Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 50

4/5/11 9:16:51 AM

Thermodynamics, performance analysis & computational modelling

51

The Capacity ratio This is defined as: . . R = (m Cp)min/(m Cp)max

3.21

It can be shown that for any heat exchanger, the effectiveness is:

e = Function of (NTU, capacity ratio, system geometry)

The analytical solutions to the above for various geometries are represented graphically by [R = Cpmin/Cpmax]. It is possible to use this method in three different ways: with any two unknowns, the third can be predicted: ∑ knowing the effectiveness and the NTU to predict the capacity ratio ∑ knowing the effectiveness and the capacity ratio to predict the NTU ∑ knowing the NTU and the capacity ratio to predict effectiveness. The NTU, capacity ratio (R), and the effectiveness are inter-related. They are often presented in the form of a chart for a specfic heat exchanger; so knowing any two quantities will help find the third quantity from the chart. Then, by calculating the Cmin/Cmax and the NTU, the effectiveness can be read from these charts. Once the effectiveness has been found, the heat load is calculated by:

Q = Effectiveness ¥ Cmin



3.6

¥ (Hot temperature in – Cold temperature in)

3.22

Worked example

In a CHP plant (see Fig. 3.3), steam enters the turbine at 7 MPa and 500 °C. Some steam is extracted from the turbine at 500 kPa for process heating. The remaining steam continues to expand to 10 kPa. Steam is then condensed at constant pressure and pumped to the boiler pressure of 7 MPa. At times of high demand for process heat, some steam leaving the boiler is throttled to 500 kPa and is routed to the process heater. The extraction fractions are adjusted so that steam leaves the process heater as a saturated liquid at 500 kPa. It is subsequently pumped to 7 MPa. The mass flow rate of steam through the boiler is 10 kg/s. Disregarding any pressure drops and heat losses in the piping, and assuming the turbine and the pump to be 100% isentropic, determine: (a) the rate at which process heat can be supplied when the turbine is bypassed, (b) the power produced when the process heater is bypassed, and (c) the rate of process heat supply and power output, when the PRV is

© Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 51

4/5/11 9:16:51 AM

52

Small and micro combined heat and power (CHP) systems

closed, 50 percent of the steam is extracted from the turbine at 500 kPa for process heating and the remaining 50% expands through to the condenser.

Solution In this worked example, there are three scenarios of power and/or heat to be investigated. In order to evaluate each, it is important to determine the behaviour of the working fluid (water/steam) during the processes involved. The behaviour of water/steam undergoing various processes is best examined by following a property chart, and the best chart to use in such situations is the temperature-entropy chart for steam, as shown in Fig. 3.5. The CHP cycle is marked by locating the known points and condition at key points in the plant under a given working condition. Conditions such as operating pressure and temperature will help locate the various joints. The T-s chart, once completed, will allow the user to find the enthalpy (heat content) at each point, according to the analysis carried out in Section 3.4. Steam tables can also be used to find more accurate values, but using steam tables is time consuming compared with locating points on the T-s charts, and using charts is standard practice for designers and engineers. However, there is an even better method, using look-up tables or fitting equations to the behaviour of water/steam. Using computer-aided macros, it is nowadays possible to carry out such complex calculations, but for the

1,2,3

500

400 T (°C)

4

300

200

10 11

100

7

9

5

8

0 0

6 2

4 6 s (kJ kg–1 K–1)

8

10

3.5 Temperature-entropy chart for combined heat and power plant.

© Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 52

4/5/11 9:16:52 AM

Thermodynamics, performance analysis & computational modelling

53

sake of explaining how the performance of a ChP is evaluated, this worked example is completed by manual calculations base on properties found from the steam chart and tables. First of all, determine the enthalpy values at key points in the cycle: h1 = h2 = h3 = h4 = 3410.3 kJ/kg h5 = 2738.2 kJ/kg (¥5 = 0.995) h6 = 2153 kJ/kg (¥6 = 0.819) h7 = 640.23 kJ/kg h8 = 191.83 kJ/kg h9 = h8 + vf8 (P9 – P8) = 91.83 + 0.00101(7000 – 10) = 191.83 + 7.061 = 198.9 kJ/kg h10 = h7 + vf7 (P10 – P7) = 640.23 + 0.00109(7000 – 500) = 640.23 + 7.1 = 647.33 kJ/kg h11 =

m10.h10 + m9.h9 5 ¥ 647.33 + 5 ¥ 198.9 = = 423.11 kJ/kg m10 + m9 5+5

(a) Heat only The maximum rate of process heat is achieved when all the steam leaving the boiler is throttled and sent to the process heater and none is sent to the turbine: m· 4 = m· 7 = m· 1 = 10 kg/s; m· 3 = m· 5 = m· 6 = 0 Thus, Q· r.max = m· 1(h4 – h7) = 10[(3410.3 – 640.23)] = 27.700 MW (b) Power only When no process heat is supplied, all the steam leaving the boiler will pass through the turbine and will expand to the condenser pressure of 10 kPa, that is: m· 3 = m· 6 = m· 1 = 10 kg/s and m· 2 = m· 5 = 0 maximum turbine work is: W· t = m· 1(h3 – h6) = (10)[(3410.3 – 2153)] = 12.573 MW

© Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 53

4/5/11 9:16:52 AM

54

Small and micro combined heat and power (CHP) systems

W· p = m· (h9 – h8) = (10)(198.9 – 191.83) = 0.070 MW W· Net = W· t – W· p = 12.503 MW (c) For a cogeneration operation (i) Process heat: Q· P = m· 5(h5 – h7) = 5.0(2738.2 – 640.23) = 10.489 MW (ii) Power: Wp = m5(h10 – h7) + m6(h9 – h8) = 5(647.23 – 640.23) + 5(198.9 –191.83) = 0.071 MW Wt = m· 3(h3 – h5) + m· 6(h5 – h6) = 10 (3410.3 – 2738.2) + 5.0 (2738.2 – 2153) = 9.647 MW Wnet = 9.647 – 0.071 = 9.576 MW (iii) The input heat supplied and utilisation factor: Q· in = m· 1(h1 – h11) = (10)[(3410.3 – 423.11)] = 29.872 MW

e=

3.7

Q p + Wnet 10.489 + 9.576 = = 0.67 Qin 29.872

Computational modelling of a combined heat and power (CHP) cycle

it is clear that analysing a combined heat and power cycle is a lengthy process. especially when optimum performance is to be achieved, the search for optimum operating conditions is determined by very many parameters. in order to assess how variable conditions affect a typical cycle, computational software is essential. To allow amendments to be made to cycle conditions, and to assess results in detail, an excel program was created. The software approach has the added advantage of eliminating errors and improving the accuracy of calculations. in this chapter it was decided to investigate the influence of varying some key conditions in the cycle and examine their effect on the performance of the ChP plant. The variable conditions included the boiler pressure superheated temperature, heat extraction pressure, turbine and pump isentropic efficiency, and percentage of heat extracted. To enable this to be evaluated in detail, it is important that only one variable is changed at a time, with all other conditions remaining constant. The program was created in Microsoft Excel 2007. Firstly, the steam table data were copied into Excel (Keenan et al., 1969).

© Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 54

4/5/11 9:16:52 AM

Thermodynamics, performance analysis & computational modelling

55

Using a combination of ‘if’, ‘Vlookup’ and ‘And’ functions, the steam table properties were extracted from the tables for the key points within the cycle. The isentropic efficiency of a pump or turbine is defined as the ratio of the actual work output vs the work output if the process between inlet and outlet is isentropic. Pump efficiency can be adjusted, amending h2 and h8 data as appropriate. The following calculations were used: h2 = h1 +

h2¢ – h1 Ep

h8 = h7 +

h8¢ – h7 Ep

Turbine efficiency can be adjusted, changing h5 and h6 data. The following calculations were used: h5 = h4 – Et(h4 – h¢5) h6 = h4 – Et(h4 – h¢6) The enthalpy value at h¢5 can be accurately calculated, even if the process heat inlet falls inside or outside of the saturation line, i.e., whether mixed phase or superheated. This process uses an ‘if’ function. if the dryness fraction of the extract process heat is mixed phase (i.e. x5 < 1), then h5 is calculated using the dryness fraction calculation below: h¢5 = hf + x5hfg If the dryness fraction is above 1 (i.e. superheated), then a lookup function matches the entropy value at s4, with the data in the extract process heat steam table at the superheated condition. Some interpolation goes ahead, and the exact value for h5 can be calculated.

3.8

Analysis of the computational model of the combined heat and power (CHP) system

The software developed for the combined heat and power plant was verified against manual calculations, and the results are compared and shown in Table 3.1. The maximum error between manual and Excel program methods is only a fraction of a percent. Such small errors can be explained by inconsistent rounding of numbers in the manual calculations. The simulation package developed here proved a very useful tool to investigate the various parameters involved in an accurate and fast way. One area for improvement is that the excel program could use the industrial formulation for calculation of properties of water and steam (known as ‘IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam (IAPWS-IF97)’). This would increase cycle analysis availabilities, as there are instances where the steam tables have limited data. © Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 55

4/5/11 9:16:53 AM

56

Small and micro combined heat and power (CHP) systems

Table 3.1 Verification of simulation results against manual calculations Designation

Manual method

Program method

Units

h 1 h2’ h 2 h 3 h 4 x 5 h5’ h 5 x 6 h6’ h 6 h 7 h8’ h 8

191.83 192.83 192.83 482.212 3410.3 1.05 2877.94 2877.94 0.8197 2153.27 2153.27 762.83 771.594 771.594

191.83 kJ/kg 192.739 kJ/kg 192.739 kJ/kg 482.166 kJ/kg 3410.3 kJ/kg 1.047 2877.974 kJ/kg 2877.974 kJ/kg 0.820 2153.174 kJ/kg 2153.174 kJ/kg 762.810 kJ/kg 771.594 kJ/kg 771.594 kJ/kg

Error (%) 0 0.047 0.047 0.0095 0 0.29 0.0012 0.0012 0.037 0.0045 0.0045 0.0026 0 0

Notes

Ideal Real

Ideal Real Ideal Real Ideal Real

3.8.1 Variable 1: superheat temperature It can be noticed that by increasing the superheated boiler exit temperature from 300 °C to 600 °C, the process heat available (at constant process heat pressure) increases by 32%, from 8.59 MW up to 11.35 MW. Pump work remains constant, regardless of the superheated maximum temperature. As the superheated boiler exit temperature increases from 300 °C to 600 °C, the net work increased by 53% and the utilisation factor increased by 5% (see Fig. 3.6). It is clear that, to maximise the process heat available, the boiler exit temperature should be superheated. The hotter the boiler exit gas, the more process heat will be available.

3.8.2 Variable 2: pump efficiency Keeping all other points constant, adjusting the pump efficiency can be analysed in isolation. It is clear that the pump work increases dramatically as the pump isentropic efficiency drops. A decrease in pump efficiency of 90% to 70% implies higher pumping power and less net work output, but has little effect on the utilisation factor of the plant. However, a drop in pump efficiency below 50% will have a more serious effect (see Fig. 3.7). Service and maintenance is therefore key in ensuring the plant performance is maintained at the highest level.

3.8.3 Variable 3: turbine efficiency Keeping all other points constant, adjusting the turbine isentropic efficiency results in some concerning results (Fig. 3.8). A drop in turbine efficiency has

© Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 56

4/5/11 9:16:53 AM

Thermodynamics, performance analysis & computational modelling

57 1.000

35.000

0.900 30.000 0.800 25.000

0.700

Heat/work (MW)

0.600

20.000

Process heat Net work Input heat supplied Utilisation factor

15.000

0.500 e 0.400

0.300

10.000

0.200 5.000 0.100 0.000 0.000 300 320 340 360 380 400 420 440 460 480 500 520 540 560 580 600 Superheat temperature (°C) Max. pressure

Intermediate Min. pressure pressure

Pump efficiency

Turbine efficiency

Max. Extraction temperature ratio

70 bar

10 bar

100%

100%

300–600 °C

0.1 bar

50%

3.6 The superheat temperature influence on CHP performance.

a large effect on the turbine work available. A drop in efficiency from 90% down to 70% results in a drop in turbine work of 28.5%. Not only that, a drop in turbine efficiency has a direct effect on process heat available and net work, resulting in a reduction of the utilisation factor. During this analysis, pump work and input heat supplied remain constant.

3.8.4 Variable 4: extraction ratio Keeping all other points constant, adjusting the extraction ratio provides an interesting analysis (Fig. 3.9). As expected, dropping the extraction ratio reduces the utilisation factor. At an extraction ratio of 100% (i.e. maximum process heat), the utilisation factor is 1. Also, as the extraction ratio increases,

© Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 57

4/5/11 9:16:53 AM

58

Small and micro combined heat and power (CHP) systems

9.000

0.700

8.900

0.600

8.800

0.500

Net work

8.700

Utilisation factor

0.400

MW

Pump work (kW)

e

8.600

0.300

8.500

0.200

8.400

0.100

8.300

0.000 10

20

30

40 50 60 70 Pump efficiency (%)

80

90

100

Max. pressure

Intermediate Min. pressure pressure

Pump efficiency

Turbine efficiency

Max. Extraction temperature ratio

70 bar

10 bar

10–100%

100%

500 °C

0.1 bar

50%

3.7 Pump efficiency influence on CHP performance.

the process heating available also increases. No great surprises here. However, the extraction ratio also has a direct impact on the turbine work. As the extraction ratio increases, more process heat becomes available. However, turbine work decreases.

3.8.5 Variable 5: maximum pressure – boiler Keeping all other points constant, adjusting the maximum boiler pressure reveals some important information (Fig. 3.10). Increasing the boiler pressure and keeping the superheated temperature constant reduces the process heating available. Conversely, turbine work increases. An increase from 10 to 50 bar for maximum pressure, results in an increase from 5.088 to 8.439 MW of turbine work. This is an increase of 40%. However, increasing

© Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 58

4/5/11 9:16:53 AM

Thermodynamics, performance analysis & computational modelling

59

35.000

0.700

30.000

0.600

25.000

0.500 Process heat Net work Input heat supplied Utilisation factor

MW

20.000

0.400 e

15.000

0.300

10.000

0.200

5.000

0.100

0.000

0.000 10

20

30

40 50 60 70 Turbine efficiency (%)

80

90

100

Max. pressure

Intermediate Min. pressure pressure

Pump efficiency

Turbine efficiency

Max. Extraction temperature ratio

70 bar

10 bar

100%

10–100%

500 °C

0.1 bar

50%

3.8 Turbine efficiency influence on CHP performance.

the maximum pressure from 50 to 100 bar results in a turbine work increase from 8.439 to 9.378 MW. A rather disappointing 11% increase, based on a doubling of maximum pressure. As the maximum pressure increases, input heat supplied reduces slightly. Conversely, pump work and utilisation factor increase slightly.

3.8.6 Variable 6: heat extraction pressure – process heat Keeping all other points constant, adjusting the process heat extraction pressure reveals some somewhat surprising information (Fig. 3.11). Increasing the process heat extraction pressure from 10 to 80 bar results in a tiny increase in process heat available of just 1.1%, from 10.576 to 10.697 MW. However, as the process heat extraction pressure increases, turbine work decreases quite significantly, whereas the utilisation factor decreases marginally.

© Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 59

4/5/11 9:16:54 AM

60

Small and micro combined heat and power (CHP) systems

35.000

1.200

30.000

1.000 Process heat Net work

25.000

Input heat supplied

0.800

Utilisation factor

e

20.000 MW

0.600

15.000 0.400 10.000

0.200

5.000

0.000

0.000 10

20

30

40 50 60 70 Extraction ratio (%)

80

90

100

Max. pressure

Intermediate Min. pressure pressure

Pump efficiency

Turbine efficiency

Max. Extraction temperature ratio

70 bar

10 bar

100%

100%

500 °C

0.1 bar

10–100%

3.9 Extraction ratio influence on CHP performance.

3.9

Case study: system performance of a biogasdriven small combined heat and power (CHP) system in a sewage works

This case study was reported as part of a Masters degree thesis submitted by Brian Rose, who worked on this project. The project was concerned with the design, supply and commissioning of a CHP plant for the treatment of sewage waste. A significant amount of digester gas is generated as a by-product and, following a feasibility study, it was decided to install a CHP plant to enable the site to become totally self-sufficient in heat and power. Anaerobic digestion is a process in which organic matter is broken down naturally by bacterial action. As the name suggests, the process takes place in the absence of air, and in conditions of warmth and darkness. Anaerobic

© Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 60

4/5/11 9:16:54 AM

Thermodynamics, performance analysis & computational modelling 35.000

61 0.680

0.670

30.000

0.660 25.000 0.650

Process heat Net work

20.000

Input heat supplied

0.640

MW

Utilisation factor

e 0.630

15.000

0.620 10.000 0.610 5.000

0.000 Max. pressure

0.600

0.590 10

20

30

40 50 60 70 Boiler pressure (bar)

Intermediate Min. Pump pressure pressure efficiency

10–100 bar 10 bar

0.1 bar

100%

80

90

100

Turbine efficiency

Max. Extraction temperature ratio

100%

500 °C

50%

3.10 Maximum boiler pressure influence on CHP performance.

digestion is commonly used to convert residues from farming, food manufacture and human waste. Sewage sludge is supplied to a heated vessel (the digester) where fermentation takes place in an oxygen-free environment. Microbial bacteria, which reside in the digester, feed on the organic matter and gases are evolved. The resulting digester gas is approximately 65% methane, 35% carbon dioxide, and has a gross calorific value in the range 20–23 MJ/m3. This level of energy content is sufficiently high to power an internal combustion engine.

3.9.1 Choice of prime mover Following an extensive feasibility study, a lean-burn spark ignition engine was selected as the prime mover for the CHP plant. The main advantages of the lean burn engine deemed relevant to this application were:

© Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 61

4/5/11 9:16:54 AM

62

Small and micro combined heat and power (CHP) systems

35.000

0.700

30.000

0.600

25.000

0.500 Process heat Net work

20.000 MW

Input heat supplied

0.400

Utilisation factor

e

15.000

0.300

10.000

0.200

5.000

0.100

0.000



10

20

30 40 50 60 Extraction pressure (bar)

70

80

0.000

Max. pressure

Intermediate Min. pressure pressure

Pump efficiency

Turbine efficiency

Max. Extraction temperature ratio

80 bar

10–80 bar

100%

100%

500 °C

0.1 bar

50%

3.11 Process heat extraction pressure influence on CHP performance.

∑ low emissions levels attainable with lean-burn ∑ heat-to-power ratio closely matched the requirements of the process ∑ high-grade heat suitable for the digestion process ∑ high levels of machine reliability ∑ relatively low capital cost compared to a gas turbine. Each engine is an 8-cylinder turbo-charged lean-burn engine, operating at a constant speed of 1000 rev/min with a compression ratio of 9:1 and a displacement of 142.5 litres. The engine is coupled to a 6-pole synchronous alternator, which generates 1.49 MWe rated electrical power at 11 kV and 50 Hz. A total of five engines are installed, giving the installation a maximum installed capacity of approximately 7.5 MWe. In practice, however, only three or four engines are operated at any given time to allow for routine

© Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 62

4/5/11 9:16:54 AM

Thermodynamics, performance analysis & computational modelling

63

engine maintenance and servicing. Each engine is fitted with a specially enlarged carburettor orifice to handle the increased gas flows through the engine. The engine is tuned for lean-burn operation, i.e. 100% excess air (equivalence ratio f = 2.0). A compact plate-type heat exchanger is employed to recover rejected heat from the engine-cooling jacket (Fig. 3.12). The heat exchanger is arranged so that it receives return water from the digester process at 59 °C and raises its temperature to about 70 ºC prior to further heating in the exhaust heat economiser. High-grade heat is recovered from the engine exhaust gas by means of an extended surface type heat exchanger. Exhaust gases at 360 °C are used to produce hot water at a temperature of 77 °C as required for the digester process.

3.9.2 Engine performance An initial appraisal of the engine thermal efficiency at full load capacity indicates a heat-to-power ratio of 1.56:1, representing a thermal efficiency Exhaust

Tout = 88°C

M

Economiser Qr = 1093 kW Gas engine BP = 1575 kW Alternator output = 1500 kW

Tin = 78.6°C Tout = 77 °C Digester m = 22 kg/s

Tin = 59 °C

Jacket circuit

M

Heat exchanger Qr = 782 kW Oil cooler

Intercooler circuit

key to symbols M

Control valve Pump

2-Stage radiator Note: mass and energy flowsat 100% design capacity

3.12 CHP system flow diagram.

© Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 63

4/5/11 9:16:55 AM

64

Small and micro combined heat and power (CHP) systems

of 39.1%, when calculated on the saturated lower calorific value of the fuel = 32.9 MJ/m3 (@ 39.1 MJ/m3 dry gross saturated lower calorific value). When the engine is operating at part load, the gross thermal efficiency falls to approximately 28.9% at 22% load rating. The inter-relationships between efficiency heat rejection and thermal efficiency of the engine are illustrated in Fig. 3.13. Approximately 1.87 MW of high-grade heat is available when the engine operates at its maximum capacity of 1.57 MW brake power. The overall energy balance for the engine operating at 100% capacity can be illustrated in the form of a Sankey diagram (Fig. 3.14).

3.9.3 Heat recovery system The requirement for process heat is 1.7 MW at an outgoing temperature of 77 °C (±3 °C). The temperature of the process water is raised in two Jacket heat Intercooler heat

Exhaust heat Thermal efficiency

1200

Lube oil heat

45 40

1000

30

Heat rejection (kWth)

800

25 600 20

Thermal efficiency (%)

35

15

400

10 200 5

0 0

500

1000 1500 Shaft power (kWb)

0 2000

3.13 Engine efficiency and heat rejection.

© Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 64

4/5/11 9:16:55 AM

Thermodynamics, performance analysis & computational modelling

65

Exhaust losses 1092 kW (27%)

High-grade heat Fuel input 4037 kW(100%)

Useful work Low-grade heat

Radiation 50 kW(1.2%)

High-grade heat

Shaft power 1577 kW (39.1%)

Jacket Lube oil Intercooler heat cooling heat 144 kW(3.6%) 391 kW(9.7%) 783 kW(19.4%)

3.14 Sankey diagram for a single generation scheme (engine at 100% rated capacity without heat recovery).

stages by passing it first through the jacket water heat exchanger then finally through the exhaust heat economiser. The heat exchanger employed is a compact type plate heat exchanger, which has a multiple pass arrangement. This type of heat exchanger has a very small hydraulic radius in the fluid flow channels, which reduces the effect of the laminar fluid film at the interface between the fluid and the heat exchanger. The overall heat transfer coefficient is enhanced by the use of turbulence-inducing patterns within the fluid flow channels of the heat exchanger. As a result, very high energy transfer rates per unit area can be achieved when compared to a traditional shell and tube type heat exchanger. The compact heat exchanger (CHE) employed in this particular application operates under the conditions given in Table 3.2. The overall heat transfer coefficient was very high (9798 W/m2K), as would be expected for this type of heat exchanger. The overall effectiveness was quite low, but this could be attributed largely to the higher thermal capacity ratio of the fluid streams. It can be shown that heat exchanger effectiveness decreases with increasing thermal capacity ratio (where thermal capacity ratio R = Cpmin/Cpmax). The approach temperature was also higher than would be expected of a CHE unit, but this was unavoidable in this particular application. Cooling of the engine coolant would result in high thermal stresses in the engine.

© Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 65

4/5/11 9:16:55 AM

66

Small and micro combined heat and power (CHP) systems

Table 3.2 Compact heat exchanger performance (at full load condition) Parameter

Units

Fluid Flow rate kg/s Inlet temperature °C Exit temperature °C Specific heat capacity KJ/kgK Thermal capacity ratio – Surface area m2 Heat exchanged kW Overall heat transfer coefficient, U W/m2K Overall effectiveness e Approach temperature °C

Hot stream

Cold stream

50% glycol 20.6 88 73.9 3.435 1.184 13.2 996.4 9798 0.52 17.4

water 21.1 59 70.6 4.066

Table 3.3 Economiser performance (at full load condition) Parameter

Units

Fluid Flow rate kg/s Inlet temperature °C Exit temperature ∞C Specific heat capacity KJ/kgK Thermal capacity ratio – Surface area m2 Heat exchanged kW Overall heat transfer coefficient, U W/m2K Overall effectiveness e Approach temperature °C

Hot stream

Cold stream

Flue gases 3.02 338 118 1.11 0.273 137.4 736.8 43 0.883 38.9

water 21.1 70.6 79.2 4.066

3.9.4 Exhaust Heat Recovery The exhaust heat exchanger is an extended surface heat exchanger, which consists of a double bank of finned tubes inserted in the flue gas stream of the engine. Exhaust gases flow over the external surfaces of the finned tubes, which contain the heat transfer media. The overall heat transfer coefficient was very low (U = 43 W/m2K), mainly due to large hydraulic radius of the fluid passages in the heat exchanger. The economiser was specially designed for low flow resistance on the flue gas side of the economiser, due to a limitation on the exhaust system backpressure of 508 mm H2O. The overall effectiveness though was very high (e = 0.883), this being due to the low thermal capacity ratio between the flue gas and the water (see Table 3.3).

3.9.5 Overall system performance With the incorporation of heat recovery equipment, the performance of the combined heat and power system becomes 82% at maximum design capacity (Fig. 3.15). Using biogas fuel to operate the system displaces emissions from combustion of fossil fuels from centralised power generation plants.

© Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 66

4/5/11 9:16:55 AM

Thermodynamics, performance analysis & computational modelling Exhaust losses 285 kW(7.1%)

67

Overall thermal efficiency = 82%

Fuel input 5982 kW (100%)

High-grade heat

Economiser heat recovery 737 kW(18.3%)

Power

Electrical power 1577 kW(39.1%)

Jacket heat High-grade heat recovery 998 kW(24.7%)

Radiation 50 kW(1.2%)

Intercooler heat 391 kW(9.7%)

Radiator heat rejection 297 kW(4.9%)

3.15 Sankey diagram for a cogeneration scheme (engine at 100% rated capacity with heat recovery). Table 3.4 Emissions reduction per generator Pollutant

Biogas CHP emissions (g/kWh)

Centralised power generation (g/kWh)a

Emissions reduction due to power + heat (tonnes/year)

CO2 NOx SO2

696 0.15 0.04

720 1.9 5.2

15,488 30 74

a

The data given for centralised power generation assumes a mix of 50% coal, 47% gas, and 3% oil, inclusive of 5% transmission losses. A boiler efficiency of 80% was assumed in the calculation of emissions reductions due to heat recovery.

The degree of displacement is further improved by the incorporation of heat recovery to offset fuel consumption, which would otherwise be consumed in process heating. The overall displacement effect is summarised in Table 3.4, and assumes that the plant would be operational for 8760 hours per annum.

3.9.6 Conclusions ∑

The use of biogas fuel in this installation demonstrates the viability of a renewable energy source. This system not only reduces dependency on

© Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 67

4/5/11 9:16:55 AM

68

∑ ∑

∑ ∑ ∑

Small and micro combined heat and power (CHP) systems

fossil fuel, but also exhibits a high level of thermal efficiency through heat recovery consumption local to the point of generation. The installation enables the sewage treatment processing plant to be self-sufficient in its energy needs, i.e. no energy is imported from the national grid. The installation uses a free source of energy which would otherwise vent to atmosphere, which, if imported as premium natural gas, would cost £389,000 per generator (based on 3536 MWh imported at 1.1 p/ kWh tariff). This type of installation offers further diversity in sources of energy, and helps reduce national dependence on finite reserves of traditional fossil fuel. Due to its high level of efficiency, the installation is exempt from the climate change levy taxation of 0.15 p/kWh, producing an annual saving of £2,900 per generator. This particular installation uses 100% renewable energy and is therefore exempt, producing an annual saving of approximately £43,700 per generator based on 10% of the potential 1.32 GWhe energy produced per annum.

In economic terms, the plant is able to exploit a free energy source, which would otherwise be vented to atmosphere from the digestion process. As a result, the sewage treatment plant is self-sufficient in energy, and is in fact able to export electrical energy to the national grid at times of low in-house demand.

3.10

Sources of further information and advice

http://www.theiet.org/publishing/books/renewable/cogeneration.cfm http://www.chpa.co.uk/ http://www.cibse.org/index.cfm?go=page.view&item=385 http://www.publications.parliament.uk/pa/cm199899/cmselect/ cmenvaud/159/9022306.htm http://www.carbontrust.co.uk/emerging-technologies/current-focus-areas/ pages/micro-combined-heat-power.aspx http://www.iea.org/files/CHPbrochure09.pdf

3.11

References and further reading

Boyce, M.P. (2010) Handbook for Cogeneration and Combined Cycle Power Plants, 2nd edn. ASME Publications, New York. Keenan, J. H., Keyes, F.G., Hill, P.G. and Moore, J. G. (1969) Steam tables, John Wiley & Sons, New York.

© Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 68

4/5/11 9:16:56 AM

Thermodynamics, performance analysis & computational modelling

69

Kehlhofer, R., Hannemann, F., Stirnimann, F. and Rukes, B. (2009) Combined-Cycle Gas and Steam Turbine Power Plants, 3rd edn. PennWell Corp., Tulsa, OK. Horlock, J. H. (2001) Combined Power Plants: Including Combined Cycle Gas Turbine (Ccgt) Plants. Krieger Publishing Company, Malabar, FL.

© Woodhead Publishing Limited, 2011

CHP-Beith-03.indd 69

4/5/11 9:16:56 AM

4

Integration of small and micro combined heat and power (CHP) systems into distributed energy systems

J. D e u s e, GDF-SUEZ – Tractebel Engineering, Belgium

Abstract: During the last decades electrical systems have undergone a transformation, which is still underway and is supposed to lead from the ‘vertical integration’ model to a fully open electricity market. This process presently overlaps with the development of distributed energy resources (DER) that brings under a single concept electricity generation, storage of energy and demand response, all at small scale. DER development raises a number of issues for the different stakeholders. However, it is obviously an opportunity for new, emerging market players, but it brings also new perspectives for incumbents. Participation of DER in the different energy markets requires the setting up of upgraded structures for aggregating small energy sources. This chapter summarizes why dispersed generation is innovating in the electrical power system context. It shows how distributed generation can represent value for the system and it evaluates the significant economic advantage of being interconnected. It concludes with recommendations to wire companies and to regulatory bodies. Key words: distributed energy resources (DER), distributed generation (DG), distribution system operators (DSO), smart grids, regulation.

4.1

Distributed energy resources (DER)

4.1.1 Initial developments in power systems Before addressing the issue of integration of DER in the power system, it is useful to recall briefly the development of the production of mechanical energy during the nineteenth century as well as the transition to electricity as a multi-purpose energy resource. From the beginning of the industrial revolution, with the introduction of the steam engine, ‘power generation’ meant ‘centralization’. This was the result of the physical and technical principles that lie behind these complex processes and the ‘hierarchy’ of the different forms of energy. These physical principles are still present today and have induced a strong opposition between production, on the one hand, and consumption, on the other. Historically the steam machine was built in front of each workshop where the mechanical power generated was distributed to different machines 70 © Woodhead Publishing Limited, 2011

CHP-Beith-04.indd 70

4/5/11 9:18:58 AM

Integration of small & micro combined heat & power systems

71

using a system consisting of transmission shafts, pulleys and belts. The size of the production machine was already significantly larger than the average power consumer process in the workshop. Therefore, ‘centralization’ of power generation existed prior to the birth of the electricity sector. When electricity development started, things did not change immediately. The structure of the distribution of the mechanical power in workshops remained more or less as it was, partly because electrical machines were extremely costly. Thus the steam engine was replaced by a single electric motor. However, a certain concentration took place upstream, because it was no longer necessary to place generation and utilization of energy side by side. The complexity of the energy transformation process leading to electricity production provoked the development of larger factories. The flexibility of the transmission and distribution of electricity, particularly using alternating current, allowed the increasing separation of power generation and the various industrial processes. Within the factory, the electricity generating units were installed in the same building, called the ‘power plant’, in French ‘la centrale électrique’, an expression where the notion of centralization is obvious. The security of supply for the different workshops was strengthened as they were fed by all these units operating in parallel rather than being dependent on a single local steam engine. This shows that electricity is inherently an activity organized about the network. The next step consisted in the development of the connection between neighbouring factories to ensure the sharing of reserves. This permitted the same level of reliability to be reached while limiting the cost of development of the system. The principles that justify network interconnection were already at work. This is due to the relatively lower cost of high voltage (HV) networks compared to the cost of power plants. It is important to note that distributed generation (DG), which is a component of DER, is not a return to a solution or situation from the past. It is really a new era for electrical systems. Indeed, for the first time since the beginning of the electricity generating industry, one can consider that for certain applications and certain primary energy sources, the production of electricity by very small units could become profitable in the short term. The DG label is less a question of size than a matter of ratio. Indeed a generation unit of 10 MW installed in a large interconnected system can be considered as a decentralized generation, whereas the same unit installed in a system of 100 MW of peak power, would probably be considered as ‘centralized’. Soon it will be possible to consider combined production of heat and electricity (cogeneration) facilities designed for being installed in single family houses and leading to profitable operation without any incentives beyond ‘green certificates’. Within 10 to 20 years it may also be the case for photovoltaic conversion. Never in the past has it been possible to use energy conversion processes of such a small size. Today, it is becoming possible

© Woodhead Publishing Limited, 2011

CHP-Beith-04.indd 71

4/5/11 9:18:58 AM

72

Small and micro combined heat and power (CHP) systems

to generate electricity with generators having a size which is comparable to the size of load components.

4.1.2 Reliability of supply The economic value of local generation is highest when it is interconnected with the network. This is a direct consequence of the fundamental properties of any electrical system requesting, among other things, the real-time adjustment of the balance between production and consumption both for active and reactive powers. This requires specific technical solutions that the liberalization of the energy market has made more complex. Reliability of supply does not play the central role it played under the ‘vertical integration’ paradigm for the planning of generation. Most often in the present situation of the electricity market there is no longer any clear standard for setting the requested over-capacity in terms of generation which should be available to ensure the reliability of supply objectives. At best it is developed at generation company level. The need to build a new plant is only determined by the market.1 Such a position, however, is not absolute. In the United States, PJM, for example, checks on a yearly basis the evolution of future performance in terms of reliability.2 In the description that follows, to simplify the approach and as a first approximation, the system will be considered using the ‘vertical integration’ paradigm. The reliability of supply consists of two complementary aspects: the adequacy on the one hand and security on the other. The first, adequacy, means that the system is able to generate and transmit power from generating plants to the load for a set of standard situations including ‘normal’ and ‘abnormal’ situations which must also be carefully defined. ‘Abnormal’ means that the system is weakened due to the unavailability of some of its elements, whether these be the result of maintenance activities or the occurrence of sudden unexpected ‘events’, hence the concept of ‘secured events’.3 The second, security, means that the system must be robust for guaranteeing a stable operation when faced with these secured events. This robustness is based on the concept of preventive safety margins that must be respected during system operation. In the case of more serious incidents, the preservation of the integrity of the power system involves specific operational procedures and also the deployment of automatic countermeasures known as defence plans. This decomposition of the reliability concept into adequacy and security aspects is valid independently of the size of the network. Ensuring the reliability of any system involves, therefore, specific means allowing for the adjustment in the short, medium and long terms of the active and the reactive power balance. Indeed, voltage and frequency stability depends highly on the balance between production and consumption. The network physically

© Woodhead Publishing Limited, 2011

CHP-Beith-04.indd 72

4/5/11 9:18:58 AM

Integration of small & micro combined heat & power systems

73

aggregates loads and production. The safe operation of the electrical power system assumes a ‘sufficient control’ of generation. This is implemented through the ‘aggregation’ of the various generating plants by means of communication channels (this is an extension of the earlier practice, but now considering smaller units) or it is based on ‘statistical’ principles (micro cogeneration status can be stochastically assessed according to weather forecasts for example at lower cost). Furthermore, demand response can be included as an innovative manner of stabilizing system operation.

4.2

The value of distributed generation

Successful integration of distributed generation can result from consistent and progressive actions that consider the technical aspects first, then the market architecture and of course the associated regulatory framework. The methodology that was developed as part of the EU-DEEP research project4 started from a simple assumption: most of the physical properties of electrical systems cannot be circumvented; as a result, the in-depth analysis of system behaviour is the route to the identification of solutions that can be effectively deployed. This then allows the selection of the most effective market mechanisms. In particular it is fundamental to propose solutions that are able to reveal the actual value that DER can represent for the network, and for the system beyond the potential gains that it allows in terms of externalities. However, the ‘sustainable’ economy of distributed generation, without incentives, has not yet been demonstrated. According to new actors, distributed generation has many advantages which should be remunerated. For traditional players, especially distribution system operators (DSO), the connection of distributed generation raises many questions, such as voltage setting, operation of protection, risks related to island operation (‘anti-islanding’ protection) and the increase in short-circuit power. A lack of clarity results also from the appellation ‘distribution network’. In some cases it means medium and low voltage networks, but for others it means a network from as high a voltage as 132 kV down to the low voltage network. But this can also be a consequence of the attitude of some of the market players who are not willing to play this new game.

4.2.1 Technical aspects In fact, most of the issues relating to distributed generation can be solved without systematically causing additional costs for the network, especially for production that does not violate the design criteria of the distribution network. The assessment that the ‘value’ DER represents for the network is based

© Woodhead Publishing Limited, 2011

CHP-Beith-04.indd 73

4/5/11 9:18:58 AM

74

Small and micro combined heat and power (CHP) systems

on the comparison of islanded and interconnected situations. Fair comparison of solutions supposes similar performance in terms of actual reliability of supply and comparable technical and economic contexts. For example, an energy balance between generation and consumption set up on an annual basis has no meaning, when considering the operation of the electrical power system. Determining the advantages or disadvantages of DER requires the application of specific methodologies for the assessment of costs and benefits of various options under consideration. These cost–benefit studies should integrate all positive and negative consequences, in particular those related to the distribution networks. Making the approach more relevant requires taking into account the increasing penetration rates of DER. This helps distinguish local issues typically related to the distribution network which appear first, then systemic issues that concern the system as a whole. This leads to investigations about power system control in normal as well as in abnormal situations. This includes the impact of the extremely high penetration proportion of DER on the behaviour of the system under emergency conditions that require significant technical upgrades.

4.2.2 The value for the system Three main issues must be addressed: how to get DER into the wholesale energy market, how to build upgraded use of system charge schemes and rules for transmission and distribution companies, and finally how to determine support and incentive programmes, if they are deemed useful for society or absolutely necessary to initiate a promising technology. The participation of DER in the wholesale market means taking part in the primary markets, spot market as well as ancillary services market, but also to secondary markets, forward markets as well as to various hedging instruments. Due to the small size of DER units, the accession to these markets requires new solutions, including aggregation, to allow them to reach the required scale. The additional value of DER is essentially related to its position in the system.5 Time interval metering of load and generation, associated with appropriate treatment (‘profiling’ individual contribution or via ex post detailed analyses) would allow the determination of the footprint of the local generation or local consumption on the network. Similar charging solutions already exist for transmission costs, the compensation of losses and for congestion management, such as the ‘competitive locational price’. Another example is the ‘TRIAD’ concept as applied by NGET in the UK.

© Woodhead Publishing Limited, 2011

CHP-Beith-04.indd 74

4/5/11 9:18:58 AM

Integration of small & micro combined heat & power systems

75

4.2.3 Market regulation The profitable integration of DER remains questionable for the smallest ones. Indeed economies of scale, here expressed in terms of mass production, do not seem sufficient for the time being to lead to competitive generation costs compared to traditional generating units. Presently, the price of electricity for low voltage customers includes two main components: the energy price and ‘use of network’ charges. These latter are obviously maximum at low voltage, as all layers of the network are playing a role. If DER is operating at the right moment, that is to say during peak consumption in its neighbourhood, it represents a significant value for the network. New pricing methodologies mentioned above are able to remunerate local generation in a network that is dominated by the load and vice versa.5,6 The sustainable economy of DER is closely linked to the regulatory framework. Innovative regulatory environments may limit in the long term the cost of distribution networks. Distribution of electrical energy is expensive. A minimum density of customer is required to justify the development of a network. However, as soon as a client is connected to a network it has, as a client, additional benefits as a producer as well as a consumer. The price of the network, paid through ‘use of system charges’ mechanisms is certainly high, but still generally highly competitive when compared to islanded operation. The next section is an attempt to evaluate this additional value.

4.3

Conditions for profitable decentralized generation

Sustainable development of DG, combined heat and power units in the short term and renewable energy units in the longer term means the adequate optimization of the different components of the installation, but also of the interaction with the external system. Indeed the present chapter shows that islanding operation cannot be profitable except in regions of very low density of customers. Cogeneration of heat and power is most often the best approach for increasing the overall efficiency of a plant and for reducing the consumption of primary energy. But this is seldom true as far as the profitability of the installation is concerred. This is due in large part to the capital intensive character of electricity generation. Furthermore, fairly long amortization periods are usually considered necessary in the electrical supply industry. For example, combined cycle plants based on combustion turbines, heat recovery steam generators and condensing steam turbines are amortized in 15 years, classical coal units are amortized in 20 years and this could be even longer for new nuclear power plants as these are now built for a 60-year operational

© Woodhead Publishing Limited, 2011

CHP-Beith-04.indd 75

4/5/11 9:18:58 AM

76

Small and micro combined heat and power (CHP) systems

lifetime. This has to be compared with other branches of the industry where profitability is expected in a fairly shorter period of about 3–5 years. Reducing generating costs means taking the right action on various parameters that must be optimized as follows: ∑ the initial investment must be kept as low as possible; ∑ the overall efficiency of the installation must be high enough; ∑ the efficiency of electricity generation is less important than the overall efficiency and the reduction of initial investment has to be prioritized; ∑ a good design based on heat demand is the main objective and the minimum demand for heat is particularly important; ∑ the installation must be viewed in connection with the electricity network and the electricity power system. For small or micro-units, the scale effect that led to multi MW power plants up to 1600 MW turbo-generators, becomes the mass production in fully automatic factories. The design of the unit must be such that overall efficiency is kept as high as possible at an acceptable cost. As an example, Stirling conversion units that until recently were characterized by fairly low efficiencies can now be designed with an overall efficiency of about 40%7. However, such high figures can only be obtained at very high costs that are only compatible with the space industry. Lower efficiency machines using equivalent technical solutions, like free-piston and oscillating permanent magnet generator with efficiencies about 20% can be used for making cogeneration boilers. The additional cost required to turn a boiler into a micro-CHP must be kept as low as possible and the marginal cost of each additional efficiency point for electricity generation must be carefully checked. Present condensing boilers are characterized by a nominal efficiency of about 108% (base: low heating value of the fuel) and by a seasonal efficiency of about 90%. The part of the investment within the cost of heat is extremely low (for large industrial boilers it is of the order of 1%). It is of utmost importance to keep the overall efficiency as high as possible. In fact due to the increased complexity of the conversion process, including the increased weight of the boiler, it is generally not possible to maintain the efficiency figures of conventional boilers. It is nevertheless possible today to reach a nominal efficiency slightly above 100% with Stirling boilers. It is here important to note the advantage of the Stirling conversion: its ability to use various primary fuels such as natural gas, fuel-oil and even wood pellets. The degradation of the overall efficiency with micro-turbines can be significant, mainly for smaller machines. This is due to the considerable mechanical difficulty of reducing the clearance between fixed and rotating parts of the machines. This is definitely a disadvantage for the smaller machines. The aforementioned characteristics are to be tuned at the design

© Woodhead Publishing Limited, 2011

CHP-Beith-04.indd 76

4/5/11 9:18:58 AM

Integration of small & micro combined heat & power systems

77

stage of the generating unit with the best possible balance between reliability, efficiency and cost. The next step concerns the optimization of the installation. The case of industrial sites, such as the chemical industry, will be considered as an example. Such installations are generally supposed to feed heat at different pressures. These installations are most often designed as follows. The cogeneration unit is designed based on the minimum heat requirement, corresponding to summer conditions. The additional supply of heat is generated by classical boilers. When natural gas is used as primary energy, the initial conversion takes place in a combustion unit coupled to a synchronous machine which generates electricity. The heat content of exhaust gases is converted into steam in a heat recovery steam generator which is often equipped with additional burners that are sometimes able to be switched to autonomous operation in case of a trip of the combustion turbine (this requires a fresh air supply with auxiliary fans). When heat must be supplied to different processes under different pressure conditions, steam turbines are often used, backpressure units or condensing units with extraction of steam located at different stages of the steam expansion. These steam turbines are also coupled to a synchronous, sometimes asynchronous generator, supplying electricity to the industrial steam network. To keep the reliability of the installation at an acceptable level, but at as low investment cost as possible, pressure reducers are installed in parallel and are operated in case of steam turbine trips. Relief valves are also used at different pressure levels in order to keep the required minimum flow in the plant in case of process trip for making the operation of pressure reducers stable. Usually cogeneration plants are designed to operate at full load for about 8760 hours per year, excluding maintenance period. For micro cogeneration, which can be viewed as the last cogeneration segment of customers that are not yet equipped, things are a little bit more complicated because the equivalent full load duration is most often not high enough (about 3500 hours of full load equivalent period for micro-CHP in the UK). Figure 4.1 gives the evolution of the mean cost of electricity as a function of the full load duration, from 1000 to 8760 hours per year. The installation is supposed to operate for 15 years with maintenance costs corresponding to 5% per year (this is practically equivalent to three majors overhauls, each of them corresponding to one-third of the initial cost, which is usually the case for internal combustion machines). Natural gas price is 750/MWh, electrical efficiency 35%, total efficiency 85%, competing boiler yearly efficiency 90%, cogeneration investment 72000/kW, and the selected discount rate is 8%. For domestic customers, an optimized design, based on the minimum demand of heat, would lead to a fairly limited installed power for heating the daily consumption of hot water. For example, for a family of two adults

© Woodhead Publishing Limited, 2011

CHP-Beith-04.indd 77

4/5/11 9:18:58 AM

78

Small and micro combined heat and power (CHP) systems 7400 CHP generation cost (EUR/MWh)

Investment 7350

Fuel

7300 7250 7200 7150 7100 750 70 1000

2000

3000 4000 5000 6000 7000 Full load working hours per year (hours)

8000

8760

4.1 Generation cost versus full load working hours.

and two children, 1.5 kWh output for heat is sufficient. This is significantly lower than the 6 kWh of heat output of future Stirling boilers. This figure is most often the result of another approach which is based on the electrical side of the installation. Indeed the objective is to feed the home in electricity for compensating approximately the yearly consumption (mean value estimated at about 3500 kWh per year in England), hence the power of the generator of about 1 kWe. This power is lower than the equivalent peak consumption of a domestic customer; consequently, at first sight, it cannot be harmful for the network. This means that the design of the network is not seriously compromised, it would seem.

4.4

Evaluating the ‘full value’ of being network connected

What is the ‘full value’ of being interconnected to the network? A methodological approach has been set up to answer this question in EUDEEP Deliverable D11.8 This value is in relation to the large flexibility for balancing load and generation when a site is connected to the network. Indeed the short-term power peak can reach the maximum subscribed power of the site (e.g., up to about 15–20 kW for domestic clients) whereas the peak consumption for the network, considering diversity, is only about 1.5  kW (at system level) or 3 kW (at distribution transformer level) for mean European domestic customers (assuming that electricity is not used for house heating). The basic assumption is to evaluate the performance of the installation presenting an equivalent reliability of supply. The reference is given by the network performance (valid for the region where the considered site is located). Then a simplified model of the site is set up, with the site operating as an island. The size of the machine is selected for being able to feed the

© Woodhead Publishing Limited, 2011

CHP-Beith-04.indd 78

4/5/11 9:18:58 AM

Integration of small & micro combined heat & power systems

79

load. The demand is supposed to be given by interval metering. The ‘time window’ could be a quarter of an hour, half an hour, etc. The number of machines operating in parallel is selected to achieve the required performance in terms of reliability of supply. The full operational details are not necessarily taken into account as orders of magnitude only are considered. The present worth evaluation for the whole expected operational life of the installation is calculating taking into account different assumptions (investment, operational and maintenance costs, discount rate, utilization of the plant, etc.). This permits the determination of the expected mean costs of electricity.9 The comparison of this cost with the ‘real’ price of electricity, including network costs, gives an evaluation of the ‘true value’ of being network connected. Given the experience of different experiments that have been implemented within the EU-DEEP project, it is not necessarily straightforward to develop this type of ‘equivalent’ installation. Some adjustments must be made to make this comparison possible. This is particularly the case for domestic installations. This is essentially due to the lack of diversity when a single installation is considered, the difference between load and generation functions (load and generation curves) and finally due to the the large instantaneous variations that usually characterize load behaviour. Technical performance during operation is evaluated by determining the dynamic behaviour of the site disconnected from the network, based on measurements made in the field using sufficiently high sample rates (e.g., up to 50 samples per second). The methodology has been used for determining an order of magnitude of the ‘full value’ of being connected to the distribution system. It corresponds to the difference between the total cost of electricity supplied by the system and the mean generating cost of electricity produced locally, in islanding. The determination has been made assuming that the islanded installation presents the same performance as the distribution network in terms of overall reliability of supply. In the presented examples the mean system reliability has been set at about 99.99 to 99.995, which corresponds to the performance of a network presenting ‘good reliability’. This methodology has been used to develop five cases based on the tests implemented within the EU-DEEP project. Two are based on ‘market segments’ experiments implemented in Grenoble (internal combustion cogeneration plant with islanded capabilities – two variants, one with three or four generators and another one with a combination of UPS, CHP and one or two backup generators) and in Athens (a trigeneration plant based on a micro-turbine with storage and two variants with respectively 100% and 50% loading factor). The three additional cases have been selected to complete the picture: one ‘big’ site characterized by a high diversity of demand (Kapodistrian University of Athens) and two cases inspired from the domestic customer installations in the Berlin test where 10 micro-CHP systems have

© Woodhead Publishing Limited, 2011

CHP-Beith-04.indd 79

4/5/11 9:18:58 AM

80

Small and micro combined heat and power (CHP) systems

been installed and remotely operated for one year. However, the specific issues of islanded operation for such sites require characteristics that are not found in the Berlin site tests. Therefore two fictitious cases have been built. The first one considers a CHP installation, based on an efficient fuel cell able to work marginally in open cycle. The second one considers a combination of photovoltaic (PV) and cogeneration with different PV investments. This gives a large diversity of situations allowing for extrapolating to futuristic plants. The results are summarized in Figs 4.2 and 4.3. Figure 4.2 considers the first sites corresponding to generation of some hundreds of kW up to MW. Costs figures are given assuming two values corresponding to two different levels of reliability of supply. For each case the upper value (square) corresponds to the installation fulfilling the reliability performance; the lower value (diamond) corresponds to the second best solution in terms of reliability. This permits a range of costs to be set up for which the performances are ‘acceptable’. In general the second site corresponds to the same installation where one generator has been removed. This is the case for the Grenoble SM (synchronous machine), Grenoble UPS with one or two backup synchronous generators and for the NTUA inspired tests. For Kapodistrian University (NKUA) two different installations have been considered; the first one is characterized by 21 units (defined by the strict application of the reliability

900 800 700

7/MWh

600 500 400 300 200 100 0

G

n re

ob

le

SM

10

kW

Gr

o en

bl

e

U

PS

10

kW Nk

ua

4

M

W

El

ec

Nk

ua

4

M

W

CH

N

P

a tu

10

0

kW

(5

0%

)

Nt

ua

10

0

kW

(1

00

%

)

4.2 Generation costs evaluated for islanded sites.

© Woodhead Publishing Limited, 2011

CHP-Beith-04.indd 80

4/5/11 9:18:58 AM

Integration of small & micro combined heat & power systems sofc chp sofc Elec.

81

res-chp

800

7/MWh

600 400 200 0

2009

2015

Long run

4.3 Generation costs evaluated for small customer islanded sites.

calculation) and the second one by 12 units (this is more in line with practical approaches). The generation costs are in that case determined for CHP as well as for electricity generation. Figure 4.3 summarizes the results for the domestic customer installations. The solid oxide fuel cell (SOFC) case includes the cost reductions that are expected for the near future, whereas the renewable energy source and combined heat and power solution (RES-CHP) presents results assuming the extension of an already existing installation. Both these installations are marginally able to supply the electrical energy with the reliability objectives set up initially. These figures must be compared with the cost of electricity delivered by the power system: for example, in the low voltage network, 7160–200/ MWh (taxes included), where ‘use of system’ charges represent about 45 to 50%. This indicates an advantage when compared to islanded operation of the order of about 7200/MWh minimum, except for the case of NTUA characterized by 100% utilization, which is not fully realistic when assuming islanded operation.

4.5

Recommendations to distribution system operators (DSO) and regulators

Autonomous operation of small size sites is generally not competitive when compared to the network connection. Hence, in general, DER will be connected to the distribution network. The rational integration in the system supposes fully open collaboration between DSO and regulatory bodies. Technical analyses have shown that efficient and sustainable solutions exist. They assume first that new designs have been defined, and second that adequate regulatory frameworks have been deployed. Both these aspects are summarized below as recommendations for DSO and regulators.

© Woodhead Publishing Limited, 2011

CHP-Beith-04.indd 81

4/5/11 9:18:58 AM

82

Small and micro combined heat and power (CHP) systems

4.5.1 New designs for distribution networks Context Increasing the proportion of DER in the distribution network can have significant impacts on the network infrastructure; the type and the size of the considered DG are of utmost importance. DER has the potential to deliver services to DSOs and transmission system operators (TSOs) via entities aggregating multiple small resources dispersed in the distribution network. It is expected that these opportunities will increase in the future when medium voltage (MV) and low voltage (LV) distribution networks lose their unconditional adequacy (the ‘fit and forget’ principle being no longer fulfilled). For the sake of clarity it is important to distinguish ‘thick’ from ‘thin’ distribution networks (respectively, operating HV-MV-LV or MV-LV grids only). In ‘thick’ distribution systems the range of services that can be delivered today or in the short term is broader. In ‘thin’ distribution systems, the range of services that can be delivered is limited to balancing services for TSOs. Future services that could be delivered by DER in MV and LV distribution networks are related to limited contribution to voltage control or reactive power compensation, and not to power flow management. For the more critical constraints, such as voltage control in ‘N-1’ situations, the corrective action corresponds rather to generation curtailment than to active management of sources. Active management is an appealing solution but it supposes the existence of control margins, otherwise it means limitation to DER output. Design criteria for distribution networks might be upgraded if limitations to DER output cannot be accepted. Increasing the ‘DER hosting capacity’ of the network, whether or not active management is used, requires the setting up of new design criteria for developing or exploiting distribution networks. This is necessary because margins are needed if reduction in power injection is to be avoided as much as possible. This means that ‘exogenous’ objectives are necessary for fixing limits: limitation of peak generation for each customer in connection with peak load (diversity included), objective in terms of penetration for DER, limits set to generation control, etc. The impact of an increased number of DER on the cost of the system depends on the types of DER considered and on the network where they are connected. The additional investment costs due to DER integration depend on energy policy choices, including the associated operational rules that are imposed, including the possibility to control power injection in case of contingencies. ‘Exogenous’ objectives, in close connection with the more general objectives of energy policies, are necessary for fixing these targets: limitation of DER generation power per connection, objective in terms of penetration for DER, limits set for generation control in normal and abnormal conditions such as fault

© Woodhead Publishing Limited, 2011

CHP-Beith-04.indd 82

4/5/11 9:18:59 AM

Integration of small & micro combined heat & power systems

83

handling and DER’s ‘fault-ride-through’ capability, etc. This would lead to extensions to grid codes to comprise equipment with lower power ratings than today. New design New design criteria for distribution networks can easily be developed as soon as clear objectives are defined. These objectives should be defined outside of the electrical supply industry, but with its participation. The exact sharing between design upgrades and active management is an integral part of the process. The key aspects here are the existing voltage control margins, homogeneity of the feeders resulting in similar voltage behaviour, DER characteristics, its size, its design and the relationship between load and local generation (i.e. coincidence between the peak generation and peak demand or vice versa). As a result, when determining the ‘hosting capacity’ (the proportion of DER that can be operated in the system without inconvenience), it is important to use the network design criteria as the reference, but also to consider the specificities of the considered DER in connection with the local conditions prevailing in the network. Distribution networks developed using the traditional ‘fit-and-forget’ principle often exhibit operational margins that allow for accepting a significant proportion of DER, particularly in urban and semi-urban networks. The extension of the ‘fit-and-forget’ principle should be based on the ‘reinterpretation’ of network design criteria. As voltage drops along feeders, the voltage set point is traditionally adjusted near to the upper limit at the feeding HV-MV substation, allowing at the same time system losses to be reduced. In the presence of local generation and in the case of non-homogeneous location of load and generation along feeders, voltage profiles can increase and decrease along the various feeders depending on the coincidence between load and generation on them. The reference voltage must therefore be adjusted downwards which allows the ‘fit and forget’ approach to be preserved. This supposes, however, sufficient regularity in terms of behaviour of the load and generation customers. Existing margins can be expanded using active voltage management when a certain degree of homogeneity exists between feeders, or even more when considering active management of individual DER sources. When the voltage margins following the ‘fit-and-forget’ principle are insufficient, one needs to dynamically change the voltage control settings in the HV-MV substation. However, this supposes that the load shapes of the different feeders exhibit similar voltage characteristics as significant voltage lack of homogeneity can push the voltage outside of the acceptable range. Due to this lack of homogeneity, or if the DER installations lay outside of the network design rules, i.e. the size of the DER installations is not at all in line with the mean

© Woodhead Publishing Limited, 2011

CHP-Beith-04.indd 83

4/5/11 9:18:59 AM

84

Small and micro combined heat and power (CHP) systems

demand of the clients in the neighbourhood, there is a risk of technical issues such as over-voltages or overload of network elements. This implies that either penetration ratios should be reduced in these types of networks, or active management of sources is needed, including occasional generation curtailment. In fact, an optimal design for distribution could be characterized by active voltage management during normal conditions, and active DER management for ‘N-1’ contingency situations only. Technical objections to DER in distribution are often true in principle, but do not often materialize when real contexts and realistic parameters are considered. Protection schemes as used in the distribution systems are not adequate in the presence of DER. This is one of the critical questions with the protection of micro grids. However, as long as distribution networks are involved, due to the physical properties of the power system, the operation of protection schemes is dominated by the short-circuit power supplied by the HV network. Furthermore, small DER units installed in low voltage generally do not supply short-circuit power, hence they cannot disturb the operation of protection. For radial networks (MV and LV part of distribution), the EU-DEEP project proposed new design rules that take care of one of the most critical issues: voltage control in rural networks. Indeed, distribution networks experience voltage drops/rises on the different circuits as a function of their respective loading. Control or compensating equipment is therefore provided to offset the resultant variation in voltage. The basic requirements leading to a ‘flexible’ system assume full ‘non-homogeneity’ for local generation and consumption. The main consequences of such distribution network design rules are: ∑

HV–MV distribution substations become able to operate at nominal power to or from the distribution network. ∑ Full ‘non-homogeneity’ between feeders can be accepted without issues in terms of voltage control. ∑ HV–MV substations do operate at nominal medium voltage under all circumstances, which is slightly lower than in the present situation. ∑ Distribution transformers are set at their nominal transformer ratio, at least for MV feeders, allowing changes of their operating point from consumption to generation, and vice versa. ∑ In low load density regions, where distribution networks can be near to voltage drop limits, reinforcement of the system may be needed using, for example, larger cross-section lines or cables. For massive DER deployment in distribution networks or for the connection of quite large lumped DER in the network, several technical issues must be faced:

© Woodhead Publishing Limited, 2011

CHP-Beith-04.indd 84

4/5/11 9:18:59 AM

Integration of small & micro combined heat & power systems

85

∑

Rural networks might be limited by voltage control issues in the presence of DER. ∑ Urban networks might be limited by thermal capacity as well as fault level issues, whenever rotating machines are connected. ∑ In general, large amounts of DER connected to the network will lead to more complex power flows in the distribution network.

4.5.2 New regulatory frameworks Net tariffs can be used provisionally at small scale by default of adequate metering systems. Some regulatory arrangements implement net metering, hence the ability to reduce the ‘use of system’ charges when installing a DER. This can represent a significant part of the revenues needed to cover the costs of the installation. This becomes an issue for DSO when faced with a significant penetration of such DER. Present ‘use of system’ tariffs are generally built on two terms: fixed charges related to the subscribed peak demand and variable energy-based charges proportional to the consumption. Owing to the lack of metering infrastructure, the above terms are estimated on a specified time window (typically one year), the sites where generation and consumption remain balanced throughout this period do not pay for the ‘use of system’. This can deeply affect the financial status of DSO. Given the fact that hosting of DER units can sometimes lead to increased costs for the DSO, they should not be ‘punished’ for hosting more DERs. Distribution network charges schemes must be urgently revisited: since net pricing is not cost reflective, DSO’s business could be at risk under large deployment of DER. Therefore, a sustainable framework has been proposed for allocating distribution network investment costs to customers and distributed generation. These charges must be transparent and nondiscriminatory, meaning that ideally each market participant (load/generation) must be charged on the basis of a good estimation of the real costs that they impose on the distribution network. A new efficient ‘use of system’ charges method, based on a ‘marginal’ approach has been developed allowing for unveiling the footprint of load or generation on the distribution network infrastructure. The impact of load and generation must be determined separately as they play symmetrical but complementary roles. Ideally they should be determined for all upstream elements in the network. But such an implementation requires large-scale deployment of smart metering with automatic meter reading and heavy ex post data treatment. New EU targets are pushing towards more renewable energy and energy efficient distributed CHP units. With demand flexibility, these trends will change the way electricity is generated, transported and used. The integration of DER poses a valid challenge to both industry and regulators. Estimation

© Woodhead Publishing Limited, 2011

CHP-Beith-04.indd 85

4/5/11 9:18:59 AM

86

Small and micro combined heat and power (CHP) systems

of their technical and cost impacts remains an issue. So far, most regulators have remained fairly passive and short-sighted, whereas it has been shown that engineering models can adequately estimate both the technical and some of the economic aspects of this integration. A crucial and still open question is, then, to what extent and for what purpose can normative engineering models provide relevant information for economic network regulation? System operators have the responsibility to develop and to maintain the transmission and distributions networks following exogenously defined targets. New conditions mean new targets in relation to updated energy policy objectives. This is a process that must be catalyzed by regulatory bodies. The most suitable regulatory environment must be implemented with longterm objectives for a smooth, sustainable development of the distribution networks. Long-term planning of the distribution system is an essential part of the activities of a distribution network operator. Normative models are an attempt to model the planning problem without duplicating the industry planning process. Attractive from a regulatory point of view, they help the regulator overcome the difficulties resulting from the lack of information. These norm models are just special cases of engineering cost functions leading to a high-level representation of the considered network. However, they can include all distribution network cost drivers in order to enable finding the right balance between these cost drivers and the investment required in the network. By doing so, the connection and reinforcement costs, as well as the benefits obtained with DER, could be quantified from a system point of view. Efficient ‘use of system’ tariffs for distribution should consider: ∑ ∑ ∑ ∑ ∑ ∑

Bills of customer based on kW and kWh components of the supply. A clear separation of ‘use of system’ tariffs from incentives is mandatory. For sites equipped with DER, load and generation must be treated independently. This allows for the deployment of ‘use of system’ tariffs that are able to reveal the value of DER (or loads in a part of the network dominated by generation) as ‘network replacement’. But for being applicable down to low voltage networks, the method requires large-scale metering systems as well as ex post data management. This asks for simplification. Different stages of simplification are possible. However, caution must be exercised to keep the ‘essence’ of the tariff during this simplification process. Furthermore, equality of treatment principles must also be integrated to avoid penalizing customers due to their location in the network, like the remote end of feeder; and ‘use of system’ tariffs must be made stable from one year to the next.

© Woodhead Publishing Limited, 2011

CHP-Beith-04.indd 86

4/5/11 9:18:59 AM

Integration of small & micro combined heat & power systems

4.6

87

Acknowledgement

This work has been partly funded by the European Commission as part of EU-DEEP, a European Project supported by the Sixth Framework Programme for Research and Technological Development.

4.7

References

[1] Strbac, G. and Jenkins, N. (2002) ‘Network Security of the Future UK Electricity System’, Report to Performance and Innovation Unit (PIU). [2] Lambert, J.D. (2001) Creating Competitive Power Markets: The PJM Model, PennWell Corporation, Tulsa, OK. [3] Ofgem/DTI (2004) Planning and Operating standards under BETTA, An Ofgem/ DTI consultation document, Vols 1 and 2, July. [4] EU-DEEP Project: www.eu-deep.com [5] Deuse, J., Grenard, S., Benintendi, D., Agrell, P.J. and Bogetoft, P. (2007) ‘Use of system charges methodology and norm models for distribution system including DER’, CIRED 19th Int. Conf. on Electricity Distribution, Vienna. [6] Deuse, J. and Purchala, K. (2009) ‘DER profitability, distribution network development and regulation’, CIRED 20th Int. Conf. on Electricity Distribution, Prague. [7] Sunpower (2010) High performance free-piston Stirling engines. Available at: http:// www.sunpower.com/lib/sitefiles/pdf/productlit/Engine%20Brochure.pdf [8] EU-DEEP Deliverable D11, ‘Determination of the value of being network connected application to 5 cases’, downloadable from: www.eu-deep.com/ [9] Willis, H.L. and Scott W.G. (2000) Distributed Power Generation: planning and evaluation, Marcel Dekker, New York.

© Woodhead Publishing Limited, 2011

CHP-Beith-04.indd 87

4/5/11 9:18:59 AM

5

Biomass fuels for small and micro combined heat and power (CHP) systems: resources, conversion and applications

H. L i u, University of Nottingham, UK

Abstract: Section 5.1 outlines the significance of biomass as a fuel in the world energy supply scene and defines biomass and bioenergy categories. This section also notes the scope and extent of biomass as a renewable energy resource, how it fits in the CO2 cycle and the different types of biomass available. In Section 5.2 the characterisation methods of solid biomass fuels are explained. Section 5.3 goes into detail on the various biomass energy conversion technologies available, of which there are many. Finally, in Section 5.4, having laid the ground concerning the biomass options available for use as an energy resource, current developments in the small- and micro-scale CHP systems are explored and some commercial applications cited. Key words: biomass combined heat and power, biomass CHP, biomass conversion technologies, biomass characterisation, biomass fuels.

5.1

Introduction

Because it is expected that conventional fossil fuels such as oil and natural gas will eventually run out, alternatives that provide the usefulness, flexibility and economy of these fossil fuels have been sought for many years. In addition, many scientists, environmentalists, governments and other non-government organisations believe that the accelerated utilisation of fossil fuels over the past decades is the main cause of ‘global warming’ and this forces us to look for cheap and environmentally friendly alternatives to fossil fuels more urgently than ever before (IEA WEO 2009). One of these alternatives is as close as the kitchen waste or the plants outside – ‘biomass’: a source of energy that is both as old as humankind and as new as the morning paper. Biomass resources are increasingly used as alternative fuels for transportation, space heating and power generation because of the persistent high energy prices and pressures on carbon dioxide (CO2) mitigation. Biomass is a very broad term which is used to describe material of recent biological origin that can be used either as a source of energy or for its chemical components. As such, biomass includes trees, crops and other plants, as well as agricultural and forest residues. Biomass also includes many materials that 88 © Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 88

4/5/11 9:19:53 AM

Biomass fuels for small & micro combined heat & power systems

89

are considered as waste by society such as food and drink manufacturing effluents, sludges, manures, industrial (organic) by-products and the organic fraction of household waste. In many ways biomass can be considered as a form of stored solar energy. The energy of the sun is ‘captured’ through the process of photosynthesis in growing plants (Klass 2004). There are a number of common terms related to ‘biomass’, some of which have been misused or misunderstood on occasions and are worth further clarifications here (Klass 2004): ∑ ‘Bioenergy’: the general term for energy derived from biomass; ∑ ‘Biofuel’: a solid, gaseous, or liquid fuel produced from biomass; ∑ ‘Biogas’: a medium-energy-content gaseous fuel, generally containing 40–80 vol% methane, produced from biomass by anaerobic digestion; ∑ ‘Landfill gas’: a medium-energy-content fuel gas, rich in methane and carbon dioxide produced by landfills that contain municipal solid wastes and other waste biomass; ∑ ‘Energy crops’: plants grown specifically for energy use.

5.1.1 Biomass – a renewable energy resource Both burning biomass and burning fossil fuels release carbon dioxide (CO2) to the atmosphere. However, there is a vital difference between the two cases: burning fossil fuels releases CO2 that has been locked up for millions of years in the ground, affecting the natural CO2 cycle and resulting in an increase in the CO2 concentration in the atmosphere. By contrast, burning biomass simply returns to the atmosphere the CO2 that was absorbed as the plants grew over a relatively short period of time (a few years to ca. a decade). The same amount of CO2 which was absorbed from the air via the photosynthesis process while the biomass plant was growing is released back into the air when biomass is burned and there is no net release of CO2 to the atmosphere, i.e. it is CO2-neutral, if the cycle of growth and harvest is sustained. Therefore, biomass can be considered as a renewable energy resource (Fig. 5.1). Some net release of CO2 would take place if the production (planting, harvesting, processing) or transportation of the biomass fuel involved the use of fossil fuels. This part of CO2 can be significant for some biofuels which have low energy ratios (Hoefnagels et al. 2010, Cherubini 2010). There are many types of biomass and they can be grouped by different methods in different countries. The IEA Bioenergy Education Website on Biomass and Bioenergy (IEA Bioenergy 2010a) groups biomass into categories of woody biomass, non-woody biomass and organic wastes. Woody biomass mainly includes: ∑

forest residues, e.g. thinning, pruning or any other leftover plant material after cutting;

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 89

4/5/11 9:19:53 AM

90

Small and micro combined heat and power (CHP) systems

Atmospheric CO2, water and sunlight

Cycle time: Ca. 1–10 years Carbon dioxides released back to the atmosphere

Converted into new plant material via photosynthesis

Harvested and used as a fuel

5.1 Biomass is a renewable energy resource.

∑ ∑

fuel wood, e.g. logs or any other form to be used in small stoves; wood waste from wood-processing industry, e.g. bark, sawdust, shavings and offcuts; ∑ short rotation forestry, e.g. willow, poplar and eucalyptus; ∑ woodlands/woody urban biomass, e.g. tree trimmings, the green and woody portion of municipal solid waste. Non-woody biomass mainly includes: ∑

agricultural crops, e.g. various annual and perennial non-woody energy crops such as Miscanthus, Switchgrass, traditional agricultural crops such as maize/corn, rapeseed, and sunflowers which can be used as animal/ human food and liquid biofuels production feedstock; ∑ crop residues, e.g. rice or coconuts husks, maize cobs and cereal straw; ∑ processing residues, e.g. bagasse from sugar cane processing and olive marc from olive oil extraction. Organic waste biomass mainly includes: ∑ ∑

animal wastes, e.g. manure from pigs, chickens and cattle; sewage sludge, domestic and municipal sewage from mainly human waste;

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 90

4/5/11 9:19:53 AM

Biomass fuels for small & micro combined heat & power systems

∑

91

organic wastes produced by households and institutional buildings such as paper, food, leather and vegetable wastes.

5.1.2 Biomass potential to the world energy supply At the present time, the world is heavily reliant on fossil fuels for energy supply – over 80% of the current annual world energy consumption (~500 Exajoules) comes from coal, petroleum oil and natural gas (EIA IEO 2010). However, the reserves of fossil fuels are finite and subject to depletion as they are consumed – millions of years are required to form fossil fuels in the earth. Currently, biomass is the fourth largest energy resources after coal, oil and natural gas – the current rate of the world biomass energy consumption is estimated to be in the region of 50 EJ/y which is about 10% of the world total primary energy consumption (IEA Bioenergy 2009). Biomass is the only natural, renewable carbon resource known that is large enough to be used as a substitute for fossil fuels. Some projections have shown that the world’s bioenergy potential seems to be large enough to meet global energy demand in 2050 (Ladanai and Vinterbäck 2009). The annual global primary production of biomass is equivalent to the 4,500 EJ of solar energy captured each year (Ladanai and Vinterbäck 2009) and the rate of energy storage by land biomass is in the order of 3000 EJ/y (Larkin et al. 2004). However, only a small portion of the global energy stored by biomass can be considered as sustainable biomass resources, which is generally believed to be in the region of 250 EJ/y, being equivalent to about 50% of the current global energy consumption (Ladanai and Vinterbäck 2009, European Biomass Industry Association 2010). The future potential of the global biomass energy depends on many factors, such as land availability and productivity, and the advancement of biomass conversion technologies (Fischer and Schrattenholzer 2001, Hoogwijk et al. 2003, Demirbas et al. 2009, Ladanai and Vinterbäck 2009). But it is reasonable to assume that biomass could sustainably contribute between a quarter and a third of the future global energy mix (IEA Bioenergy 2009).

5.2

Characterisation of solid biomass fuels

Proximate analysis, ultimate analysis and calorific value are commonly used to characterise solid biomass fuels. The proximate analysis serves as a simple means for determining the behaviour of a solid biomass fuel when it is heated. It determines the contents of moisture, volatile matter, ash and fixed carbon of the fuel. On the other hand, the main purpose of an ultimate analysis is to determine the elemental composition of the solid fuel

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 91

4/5/11 9:19:53 AM

92

Small and micro combined heat and power (CHP) systems

substance. The calorific value of a fuel is a direct measure of the chemical energy stored in the fuel. Due to the inhomogeneous nature of solid biomass fuels, it is notoriously difficult to prepare small quantities (in the order of grams) of representative samples for biomass characterisation tests. Therefore, strict sampling and preparation procedures specified by the British/European standard BS EN 14778-1:2005 have to be followed by any proximate, ultimate and calorific value tests of solid biomass fuels described below.

5.2.1 Proximate analysis Since an appreciable amount of water vapour is released when a solid biomass fuel is heated to above the boiling temperature of water, the first parameter of a proximate analysis is the moisture content of the fuel. The moisture content is determined by drying solid biomass samples at 105 °C in air atmosphere until constant mass is achieved and percentage moisture calculated from the loss in mass of the sample. Standard procedures for the determination of moisture content of solid biomass fuels are specified by three British/European standards BS EN 14774-1:2009, BS EN 14774-2:2009 and BS EN 14774-3:2009. Another major loss occurs when a solid biomass fuel is heated in a covered crucible or in other apparatus which prevents the oxidation of the carbon residue. This loss is referred to as the volatile matter and constitutes the second parameter of the proximate analysis. Volatile matter is determined with the sample being heated out of contact with ambient air at 900 °C for 7 minutes. Standard procedures for the determination of volatile matter of solid biomass fuels are specified by the British/European standard BS EN 15148:2009. If the remaining residue is further combusted, the residue left after the combustion is called ash, and the weight loss on combustion is referred to as ‘fixed carbon’. Fixed carbon and ash contents constitute the third and fourth parameters of the proximate analysis. This part of proximate analysis, i.e. the combustion of the residue, is carried out in a furnace at 550 °C and the standard procedures are specified by the British/European standard BS EN 14775:2009. It is not always practical to conduct the above various determinations stepwise. Therefore, one set of samples could be used for the moisture content determination, and another set of samples for the combined moisture and volatile matter loss, and still another set of samples for ash determination.

5.2.2 Ultimate analysis The main purpose of ultimate analysis is to determine the elemental composition of a solid biomass fuel. The main elements of solid biomass fuels include

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 92

4/5/11 9:19:53 AM

Biomass fuels for small & micro combined heat & power systems

93

carbon (C), hydrogen (H), nitrogen (N), sulphur (S) and oxygen (O) but for some solid biomass fuels chlorine (Cl) and other elements may also be of interest. Nowadays, the ultimate analyses of sold biomass fuels are usually carried out with fully automated instruments. The instrumental method for the determination of total carbon, hydrogen and nitrogen contents in solid biomass fuels is described by the British/European standard CEN/TS 15104:2005, whereas the methods for the determination of the total sulphur and total chlorine content in solid biomass fuels are specified by the British/European standard CEN/TS 15289:2006. Sometimes, the determinations of the major elements (Al, Ca, Fe, Mg, P, K, Si, Na and Ti) and the minor elements (As, Cd, Co, Cr, Cu etc.) of solid biomass fuels are also necessary and required. The British/European standards CEN/TS 15290:2006 and CEN/TS15297:2006 describe and specify the corresponding methods and procedures.

5.2.3 Calorific value The calorific value of a fuel is the number of heat units evolved when unit mass (or unit volume in the case of a gas) of a fuel is completely burned and the combustion products are cooled to 298 K. This definition of calorific value includes the provision that the products of combustion are cooled to 298 K which means the sensible heat and the latent heat of condensation of the water produced during combustion are included in the heat liberated. Therefore, the calorific value of the fuel is designated as ‘gross calorific value (GCV)’ or ‘high heating values (HHV)’. However, with many industrial applications, the latent heat of condensation is not given up and the total heat liberated per unit mass (or volume) of the fuel is less. The calorific value in the case where the water remains as vapour is designated as ‘net calorific value (NCV)’ or ‘low heating value (LHV)’. The gross calorific value of a solid biomass fuel is usually determined experimentally by a bomb calorimeter, whereas the net calorific value of the fuel is usually calculated from the gross calorific value and the ultimate analysis of the fuel. Specific experimental procedures and calculation formulae are detailed by the British/European standard BS EN 14918:2009. Many solid biomass fuels contain high moisture content which greatly affects the net calorific value as illustrated by Fig. 5.2 and Table 5.1 (Larkin et al. 2004).

5.2.4 Calculation of analyses to different bases The analytical data of solid biomass fuels may be reported on different bases including as analysed (air-dried, ad), as-received (ar), dry basis (db) and dry and ash-free (daf). Most analytical values on a particular basis may be converted to any other basis by multiplying it by the appropriate formula

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 93

4/5/11 9:19:53 AM

94

Small and micro combined heat and power (CHP) systems 20 NCV (MJ/kg, as received)

18 16 14 12 10 8 6 4 2 0 0

10

20 30 40 50 60 Moisture content (as received), %

70

80

5.2 Effect of moisture content on the net calorific value of a biomass fuel. Table 5.1 Average net calorific value of solid biomass fuels (Larkin et al. 2004) Fuel

Net calorific value



(GJ/tonne)

(GJ/m3)

Wood (green, 60% moisture) Wood (air-dried, 20% moisture) Wood (oven-dried, 0% moisture) Grass (fresh-cut) Straw (as harvested, baled) Domestic refuse (as collected) Coal (UK average)

6 15 18 4 15 9 28

7 9 9 3 1.5 1.5 50

given in Table 5.2, after insertion of the numerical values for the symbols. However, for some parameters (H, O and net calorific value) there is a direct involvement of the moisture content. Further details on the calculation of analyses to different bases are described by the British/International standard CEN/TS 15296:2006. Table 5.3 shows the proximate, ultimate analyses and gross calorific values of selected solid biomass fuels (Gaur and Reed 1995). The proximate and ultimate analyses of more solid biomass fuels can be found in many references such as Gaur and Reed (1995), Annalmalai and Puri (2007), and biomass databases such as Phyllis (2010).

5.3

Biomass conversion technologies

Most biomass materials are initially solid, bulky and expensive to transport over appreciable distances. They often contain high moisture content and decompose rather quickly, so few of them are good long-term energy stores. If

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 94

4/5/11 9:19:54 AM

Biomass fuels for small & micro combined heat & power systems

95

Table 5.2 Formulae for calculation of results to different bases* Given

Wanted As analysed (air dried, ad)

As received (ar)

Dry (d)

Dry, ash, free (daf)

As analysed (air dried, ad)

1

100 – Mar 100 – Mad

100 100 – Mad

100 100 – (Mad + Aad )

As received (ar)

100 – Mad 100 – Mar

1

100 100 – Mar

100 – (Mad + Aad ) 100

Dry (d)

100 – Mad 100

100 – Mar 100

1

100 100 – Ad

Dry, ash, free (daf)

100 – (Mad + Aad ) 100

100 – (Mar + Aar ) 100

100 – Aad 100

1

*M – moisture content (%), A – ash content (%) Table 5.3 Proximate, ultimate analyses and gross calorific values of selected solid biomass fuels (dry basis) (Gaur and Reed 1995) Name

Fixed Volatiles Ash C carbon (%) (%) (%) (%)

H (%)

O (%)

N S GCV (%) (%) (MJ/kg)

Wood Ponderosa pine 17.17

82.54

0.29 49.25

5.99 44.38 0.06 0.03 20.02

Energy crop poplar

16.35

82.32

1.33 48.45

5.85 43.89 0.47 0.01 19.38

Processed biomass Plywood 15.77

82.14

2.09 48.13

5.87 42.46 1.45 0.00 18.96

agricultural wheat straw

19.80

71.30

8.90 43.20

5.00 42.18 0.61 0.11 17.51

coal Pittsburgh seam 55.80

33.90

10.30 75.50

5.00

4.90 1.20 3.10 31.75

biomass fuels are to compete with our present fossil fuels, they must be able to meet the demand for appropriate forms of energy at competitive prices. The most important criteria for new forms of energy are their availability and transportability. The premium fossil fuels – oil and natural gas – are valued because their energy can be stored with little loss and made available where and when we need it. Biomass energy conversion technologies can be classified in terms of either the conversion process they use or their end product. The following biomass conversion processes will be briefly discussed: ∑

Thermochemical processes Combustion



to produce

heat

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 95

4/5/11 9:19:55 AM

96

∑ ∑

Small and micro combined heat and power (CHP) systems

Gasification Pyrolysis Biological processes  Fermentation  Anaerobic digestion Chemical/Mechanical processes  

to produce to produce

fuel gas bio-oil or charcoal

to produce to produce to produce

bio-ethanol biogas bio-diesel

5.3.1 Combustion of solid biomass fuels Direct combustion is the most common way of converting biomass to energy – both heat and electricity – and worldwide it already provides over 90% of the energy generated from biomass (Van Loo and Koppejan 2007). Direct combustion of solid biomass fuels is well understood, relatively straightforward, commercially available, and can be regarded as a proven technology. Biomass combustion systems can be easily integrated with existing infrastructure. In theory, it is possible to burn any type of biomass but in practice combustion is feasible only for biomass with moisture content of lower than ca. 50% (Goyal et al. 2008). Combustion of a solid biomass fuel can be characterised by a three-stage process (Fig. 5.3). The first stage is drying, i.e. the evaporation of any water in the fuel. This stage does not produce energy but consumes energy. Then in the combustion process itself, there are two further stages: the volatile release (devolatisation) and combustion stage – the volatile matter is released as a mixture of vapours (CO, CO2, H2, H2O, CxHy, etc.) as the temperature of the fuel rises, the combustion of volatile matter produces the flame seen around the burning solid fuel; and the char combustion stage – the solid which remains consists of char together with any inert matter and the char (mainly carbon) burns to produce CO2, whilst the inert matter becomes clinker, slag or bottom ash. A feature of solid biomass fuels is that three-quarters or more of their energy is in the volatile matter (unlike coal, where the fraction is usually

H 2O

Volatiles burn in gas phase to form CO2, CO, H2O, etc.

CO Æ CO2 CO

O2 C Æ CO ASH

Drying

Devolatisation (Pyrolysis): volatiles evolved, char remains

Char combustion

5.3 Stages of solid biomass fuel combustion.

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 96

4/5/11 9:19:56 AM

Biomass fuels for small & micro combined heat & power systems

97

less than half). The design of any biomass combustion facility should ensure that the volatile matter of the fuel is burnt to completion. The heterogeneous chemical reactions of biomass char with oxygen from air are also important – the char combustion stage is the slowest of the three stages described above. To achieve overall high combustion efficiency, a sufficient combustion time has to be provided for the char combustion stage. Most solid biomass materials can be burnt to produce heat for use in situ or at not too great a distance. However, simple physical processing, involving sorting, chipping, compressing, air-drying, etc., is usually necessary for most solid biomass materials to be used in efficient combustion processes. Some raw solid biomass materials require further pre-treatment before they can be used in conventional combustion plants. For example, household waste, as collected, is not an ideal fuel for combustion because of its variable contents, high moisture content and low calorific value. But it can be burned in specially designed MSW (municipal solid waste) combustion (incineration) plants or converted to RDF (refuse-derived fuel) or d-RDF (densified refuse-derived fuel) to be burned alone or co-fired with coal in conventional combustion plants (Larkin et al. 2004). Modern systems for burning solid biomass fuels are as varied as the solid biomass fuels themselves, ranging in size from small stoves through domestic

5.4 A 50 kW wood pellet boiler installed at the Department of Architecture and Built Environment, University of Nottingham, UK.

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 97

4/5/11 9:19:56 AM

98

Small and micro combined heat and power (CHP) systems

5.5 An Arimax Bio Energy boiler burning woodchips for small district heating in Finland.

space and water heating systems to large boilers producing megawatts of heat and/or electricity (Figs 5.4 and 5.5). Van Loo and Koppejan (2007) provide a comprehensive review of biomass combustion basic principles and industrial applications and therefore it is an indispensible reference for biomass combustion researchers and practitioners. Biomass combustion produces a range of pollutants including carbon monoxide, nitrogen oxides, particulates, smoke, etc., and can have potential operational problems whether burning in dedicated combustion plants or co-firing with coal (Demirbas 2005, Van Loo and Koppejan 2007, Khan et al. 2009, H. Liu et al. 2010a). However, biomass combustion can be CO2neutral if the cycle of biomass growth and harvest is sustained.

5.3.2 Biomass gasification Thermochemical gasification is the conversion by partial oxidation at elevated temperature of a carbonaceous feedstock such as biomass into a gaseous energy carrier (Liu and Neubauer 2010). This gas contains carbon monoxide, carbon dioxide, hydrogen, methane, higher hydrocarbons such as ethane and ethene, water, nitrogen (if air is used as the oxidising agent) and various contaminants such as small char particles, ash, tars and oils. The partial oxidation can be carried out using air, oxygen, steam or a mixture of them. Air is a cheap and widely used gasification agent but air gasification of biomass produces a low calorific value gas (ca. 3–6 MJ/Nm3) which is suitable

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 98

4/5/11 9:19:56 AM

Biomass fuels for small & micro combined heat & power systems

99

for boiler, engine and turbine operation but not for pipeline transportation due to its low energy density (Wang et al. 2008). Oxygen gasification of biomass produces a medium calorific value gas (ca. 10–15 MJ/m3) suitable for limited pipeline distribution and as synthesis gas for conversion, for example, to methanol and gasoline. Such a medium calorific value gas can also be produced by steam gasification. Gasification is not a new process and has been around for over a century (BEF 2010, Liu and Neubauer 2010). ‘Coal gas’, the product of coal gasification, was widely used in the UK and elsewhere for many decades. ‘Wood gas’, the product of wood gasification, was used for heating, lighting and even as vehicle fuel. Both ‘coal gas’ and ‘wood gas’ were gradually superseded by natural gas from around 1930. But during the Second World War, over a million biomass gasifiers were built for the civilian sector while the military used up all the gasoline. The oil crisis of the 1970s also sparked a renewed interest in biomass gasification systems all over the world (BEF 2010). There are many different designs of gasifiers (Basu 2010), but with the same set of main gasification reactions: those of hot steam and oxygen interacting with the solid fuel (Liu and Neubauer 2010). The gasification reactions can only proceed at elevated temperatures (a few hundred to over a thousand degrees Celsius) and pressures (from a little above atmospheric pressure to 30 times this). Similar to the combustion stages shown in Fig. 5.3, the gasification process begins with the drying of the solid fuel to evaporate moisture, followed by the release of the volatiles from the heated solid, leaving the char. Volatiles and char in turn undergo partial oxidation reactions with steam and oxygen, resulting in the combustible gas. Figure 5.6 compares the three thermochemical conversion processes of biomass: combustion, gasification and pyrolysis. Although biomass gasification has been practised for over 100 years, so far it has had a very limited commercial impact on the energy market due to competition from other fuel sources and other energy forms. The past decade has seen a major resurgence of interest in biomass gasification processes mostly due to environmental and political pressures required of CO2 mitigation Solid biomass feedstock Greater than stoichiometric air/O2

Limited air/O2

No air/O2

Combustion

Gasification

Pyrolysis

Heat + flue gas + ash

Gas + ash + tar

Gas + oil + char

5.6 Comparison of thermochemical conversion processes of biomass.

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 99

4/5/11 9:19:57 AM

100

Small and micro combined heat and power (CHP) systems

measures. Very few biomass gasification processes have proved economically viable, although the technology has progressed steadily. However, there is sufficient expertise and knowledge now available to have a high level of confidence in modern gasification processes. Small gasification plants (<300 kW) are now available commercially, often combined with gas engines driving small generators, and demonstration plants in the range 10–30 MW have been in operation since the mid-1990s (Reed and Gaur 2001, Knoef 2005, Wu et al. 2008, Liu and Neubauer 2010, Alauddin et al. 2010). The overall conversion efficiency from the energy of the solid biomass fuel to that of the resulting gas varies widely, from as little as 40% in relatively simple systems, to 70% or more in the most sophisticated plants (Liu and Neubauer 2010).

5.3.3 Biomass pyrolysis Pyrolysis is the thermal decomposition occurring in the absence of oxygen, and it is also the initial step in combustion and gasification processes where it is followed by total or partial oxidation of the primary products. For many centuries, charcoal has been produced by pyrolysis of wood. The process of charcoal making involves slowly heating wood in the near absence of air, typically at 300–500 °C, until the volatile matter has been driven off. The residue is the charcoal – a fuel which has about twice the energy density of the original and burns at a much higher temperature. Depending on the moisture content and the efficiency of the process, 4–10 tonnes of wood are required to produce one tonne of charcoal, and if no attempt is made to collect the volatile matter, up to three-quarters of the original energy content can be lost (Larkin et al. 2004). Today this conventional pyrolysis process to produce charcoal is termed as ‘slow pyrolysis’. Slow pyrolysis of biomass leads to less liquid and gaseous product and greater char production (Goyal et al. 2008). The term pyrolysis is now normally applied to the processes where the aim is to collect the volatile components and condense them to produce a liquid fuel or bio-oil. Bio-oil is composed of a very complex mixture of oxygenated hydrocarbons, and like crude fossil oil can be used for heat production, power generation and synthesis gas production as well as the production of a range of chemicals (Mohan et al. 2006, Goyal et al. 2008). Bio-oil can also be upgraded to transportation fuels (Balat et al. 2009). The main benefit of the pyrolysis process, when compared to combustion and gasification, is that a liquid fuel is easier to transport than either solid or gaseous fuels (Goyal et al. 2008). This means that the pyrolysis plant does not have to be located near the end-use point of the bio-oil, but can instead be located near the biomass resource supply, which results in considerably lower fuel transportation costs. High transportation costs are one of the limiting

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 100

4/5/11 9:19:57 AM

Biomass fuels for small & micro combined heat & power systems

101

factors for the construction of large-scale biomass power plants, which have higher efficiencies and lower emissions than small-scale plants. The pyrolysis reaction is relatively complex and results in non-equilibrium products, making their properties hard to predict. The products and their properties are dependent on the process temperature, the period of heating, ambient conditions, the presence of oxygen, water and other gases, and the nature of the feedstock. In general, lower process temperature and longer heating periods result in the production of charcoal, high temperature and longer heating periods increase the biomass conversion to gas, and moderate temperature and short heating periods are optimum for producing liquids (Bridgwater et al. 1999, IEA Bioenergy 2010a, 2010b). Figure 5.7 shows a widely accepted simplified model of the principal pyrolysis pathways (Bridgwater et al. 1999). Current trends in the research and development of pyrolysis are focused on the so-called ‘fast pyrolysis’. Fast pyrolysis is a thermal decomposition process that occurs in the absence of oxygen, at moderate temperatures with a high heat transfer rate to the biomass particles and a short hot vapour residence time (in the order of a few seconds) in the reaction zone. In fast pyrolysis biomass decomposes to generate mostly vapours and aerosols and some charcoal. After cooling and condensation, a dark brown mobile liquid is formed which has a heating value about half that of conventional fuel oil. The essential features of a fast pyrolysis process for producing liquids are (Bridgwater et al. 1999): ∑

very high heating and heat transfer rates at the reaction interface, which usually requires a finely ground biomass feed; ∑ carefully controlled pyrolysis reaction temperature of around 500 °C and vapour phase temperature of 400–450 °C; ∑ short vapour residence times of typically less than 2 seconds; rapid cooling of the pyrolysis vapours leads to the bio-oil product; ∑ the main product, bio-oil, is obtained in yields of up to 75 wt% on dry feed basis, together with by-products, char and gas, which are used

Biomass

Medium heating value gas

High heating rate High temperature

Aerosols + vapours

High heating rate Low temperature

Char + low heating Low heating rate value gas

5.7 Biomass pyrolysis pathways (Bridgwater et al. 1999).

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 101

4/5/11 9:19:57 AM

102

Small and micro combined heat and power (CHP) systems

within the process to provide the process heat requirements so there are no waste streams other than the flue gas and ash. In comparison with combustion and gasification, biomass pyrolysis is in the early state of development. Although biomass fast pyrolysis technologies for liquid fuel production have been successfully demonstrated at small scale, and several large pilot plants or demonstration projects (up to 200 ton/day biomass feed design capacity) are in operation or at an advanced stage of construction, they are still relatively expensive compared to fossil-fuel-based energy technologies and thus face economic and other non-technical barriers when trying to penetrate the energy markets (IEA Bioenergy 2010b).

5.3.4 Biomass fermentation to produce ethanol Fermentation is an anaerobic biological process in which sugars (e.g. C6H12O6) are converted to alcohol by the action of micro-organisms, usually yeast. The required product, ethanol (C2H5OH) is separated from other components by distillation. Figure 5.8 shows the block diagram of the basic steps involved with biomass fermentation (DOE 2010). Fermentation requires sugar, so the obvious source is sugar-cane. Brazil’s PRO ALCOOL programme – producing ethanol from sugar residues – was the world’s largest commercial biomass programme in the twentieth century (Larkin et al. 2004). Plants whose main carbohydrate is starch (for example, potatoes, corn and wheat) require initial processing to convert the starch to sugar. This route is followed in the US where the main feedstock is corn. The US ethanol industry relies almost exclusively on corn, consuming 20% of the available corn supply in 2006 (EIA ISA-Biomass 2010). Other types of plant matter, called cellulosic biomass and made up of very complex sugar polymers, are under research and development as a feedstock for bioethanol production. Specific feedstocks of cellulosic biomass currently under Biomass handling

Enzyme production

Biomass pre-treatment

Cellulose hydrolysis

Ethanol

Glucose fermentation

Ethanol recovery

Pentose fermentation

Lignin utilisation

5.8 Basic steps of biomass fermentation (DOE 2010).

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 102

4/5/11 9:19:57 AM

Biomass fuels for small & micro combined heat & power systems

103

research and development for bio-ethanol production include: agricultural residues, forestry wastes, municipal solid waste, food processing and other industrial wastes, and energy crops. The main components of these types of biomass are 40–60 wt% of cellulose, 20–40 wt% of semi-cellulose and 10–24% of lignin, depending on the biomass sources. The lignin part remains as residual material after the sugars in the biomass have been converted to ethanol. It contains a lot of energy and can be burned to produce steam and/ or electricity for the biomass-to-ethanol process. Despite various efforts to develop cellulosic ethanol in many parts of the world, further significant progress in terms of both technologies and energy policies are needed to commercialise and make cellulosic ethanol a viable economic option in the future (EIA ISA-Biomass 2010, Gnansounou and Dauriat 2010, Talebnia et al. 2010, Binod et al. 2010, Fang et al. 2010, Gnansounou 2010, Alvira et al. 2010). The liquid resulting from fermentation contains about 10% ethanol, which is distilled off. The heating value of bio-ethanol is about 27 MJ/kg, which is about 70% of the heating value of normal petrol (~40 MJ/kg). Unlike methanol, ethanol cannot simply substitute for petrol, but it can be used as a gasoline extender in gasohol which is a mixture of petrol (gasoline) and ethanol. A few years ago, cars without engine modification could use the gasohol containing ethanol up to 26%. But now many cars have been equipped to enable them to run on E85, a mixture of 85% ethanol and 15% gasoline (WhatGreenCar 2010). With suitable engine modifications (retuning, etc.), ethanol can also be used directly. A complete process of fermentation requires a considerable heat input but this can usually be supplied by biomass residues. The conversion efficiency of fermentation is low, but the technology is comparatively simple and the plant cost is low. When the inputs are residues or surpluses and the output is a desirable product, low efficiency of fermentation may be a price worth paying (Larkin et al. 2004). Ethanol production from biomass fermentation has been commercially available for over a decade and widely practised all over the world (Nigam and Singh 2011). World production of ethanol increased from ca. 37 billion litres in 2005 to 73.9 billion litres in 2009 (Bioethanol 2010). The world’s top five bioethanol producers in 2009 were: ∑ USA – 40 130 million litres (ca. 54% of the world total) ∑ Brazil – 24 900 million litres (ca. 34% of the world total) ∑ China – 2050 million litres (ca. 3% of the world total) ∑ Canada – 1348 million litres ∑ France – 1250 million litres. However, present world-wide bio-ethanol production relies heavily on food crops such as corn, grain and sugarcane, but the limited supply of these crops can lead to competition between their use in bio-ethanol production and food

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 103

4/5/11 9:19:57 AM

104

Small and micro combined heat and power (CHP) systems

provision. Cellulosic biomass is the most promising feedstock considering its great availability and low cost, but the large-scale production of bio-ethanol from cellulosic feedstock is yet to be demonstrated and commercialised (Balat 2011, Nigam and Singh 2011).

5.3.5 Anaerobic digestion Anaerobic digestion, like pyrolysis, occurs in the absence of air; but in this case the decomposition is caused by bacterial action rather than high temperatures. It is a biochemical process which takes place in almost any biological material, but is favoured by warm, wet and airless conditions. It occurs naturally in decaying vegetation on the bottom of ponds, producing the marsh gas which bubbles to the surface and can catch fire. It also occurs in landfill sites of municipal solid wastes and purpose-built anaerobic digesters of wet wastes. Landfill gas is produced in a landfill site due to anaerobic digestion taking place over a period of years. The landfill site itself acts as the digester and the operator has very limited control of the digestion process (Larkin et al. 2004). The gas generated in purpose-built anaerobic digesters of wastes is specifically termed as ‘biogas’ which is a result of anaerobic digestion of wastes taking place over a period of days to weeks in the digesters. With the purpose-built digesters, the anaerobic digestion process can be carefully controlled by the operation temperature of the digester (Larkin et al. 2004). Both landfill gas and biogas contain a mixture of mainly methane and carbon dioxide and can be used to produce heat or electric power, or in many cases both. Biogas can also be upgraded as transport fuel as demonstrated in Switzerland and Sweden (Börjesson and Mattiasson 2007, Poeschl et al. 2010). The basic steps of anaerobic digestion are illustrated in Fig. 5.9 (Appels et al. 2008). Despite the successive steps, the hydrolysis step, which degrades both insoluble organic material and high molecular weight compounds such as lipids, polysaccharides, proteins and nucleic acids, into soluble organic substances (e.g. amino acids and fatty acids), is generally considered as the rate limiting step. The overall rate of digestion, the composition of the final gas and residuals are affected by various parameters including pH, alkalinity, temperature, retention times and the design of the digester as well as the nature of the feedstock (Appels et al. 2008, Ward et al. 2008). Wet biomass wastes, such as dung and sewage, are especially well suited for the anaerobic digestion process. However, some organic materials, such as lignin, cannot be effectively digested by the anaerobic digestion process. Therefore, woody biomass wastes which contain a significant amount of lignin should not be used in an anaerobic digester – gasification is a better option to convert woody biomass to a gaseous fuel. The advantages of anaerobic digestion when compared to thermochemical processes are that as

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 104

4/5/11 9:19:57 AM

Biomass fuels for small & micro combined heat & power systems

105

Suspended organic matter Hydrolysis Soluble organics Acidogenesis

Volatile fatty acids

Acetic acid

Acetogenesis

Methanogenesis

CH4 + CO2

H2, CO2

Methanogenesis

5.9 Basic steps in the anaerobic digestion process (Appels et al. 2008).

a by-product it also produces a concentrated nitrogen fertiliser, and serves as a means of waste neutralisation, which would otherwise be dumped in the environment. Because of this, small-scale digesters are well suited for integration in small rural farms, and have been used extensively in several developing countries like China (Jiang et al. 2007) and India (Rao et al. 2010). The technology of using anaerobic digestion to produce biogas is well developed and the last decade has brought about huge steps forward, in terms of maturation of biogas technologies and economic sustainability for both small- and large-scale biogas plants (Holm-Nielsen et al. 2009). Now there are a range of digesters commercially available. However, the capital cost of a digester may be out of reach for a typical small farmer in some developing countries (Larkin et al. 2004). Nevertheless, there is considerable potential for biogas production from anaerobic digestion in Europe, as well as in many other parts of the world (Holm-Nielsen et al. 2009, Jiang et al. 2007, Rao et al. 2010).

5.3.6 Biodiesel production Biodiesel production from oilseed crops such as soya, rapeseed, sunflower, corn, etc., involves chemical and/or physical processes. The first step of biodiesel production from oilseed crops is to produce vegetable oils from the feedstock and the second step is to produce the biodiesel from the vegetable

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 105

4/5/11 9:19:58 AM

106

Small and micro combined heat and power (CHP) systems

oils. The direct use of vegetable oils as fuel in compression ignition engines is problematic due to their high viscosity (about 11–17 times greater than petroleum diesel fuel) and low volatility (Atadashi et al. 2010). Vegetable oils do not burn completely and form carbon deposits in the fuel injectors of diesel engines and therefore conversion of the vegetable oils to biodiesel is preferred. There are two main processes for extracting oil from biomass seed feedstock: mechanical press extraction and solvent extraction. In mechanical press extraction, the oil seed feedstock is first heated to about 40–50 °C. The oil seed is then crushed manually or automatically (e.g. in a screw press). After most of the oil is removed, the remaining seed meal can be used as an animal feed. The solvent process extracts more of the oil contained in the oil seed feedstock but requires more costly equipment. The process uses a solvent (most commonly hexane) to dissolve the oil. After extraction, a distillation process separates the oil from the solvent. The solvent condenses and can be recycled and reused in the process. Solvent extraction produces vegetable oil with a higher degree of purity than the mechanical press process. Several methods are available for producing biodiesel from vegetable oils but currently the method of choice is a chemical process, called ‘transesterification’ (Ma and Hanna 1999, Singh and Singh 2010). The main purpose of the transesterification process is to lower the viscosity of the oil. In the transesterification process, vegetable oil reacts with an alcohol (methanol or ethanol), usually in the presence of a catalyst, and reduces its viscosity, resulting in a higher quality fuel, biodiesel. Biodiesel can also be produced from recycled oil or fat (Enweremadu and Mbarawa 2009). It is estimated that about half of the biodiesel industry can use recycled oil or fat. Biodiesel production from waste oils from chip shops is already a reality in the UK. Argent Energy (UK) Limited operates the UK’s first large-scale biodiesel plant which started the production of biodiesel in March 2005 using recycling tallow and used cooking oil (Argent Energy Limited 2010). The energy content of biodiesel is 10–12% lower than petroleum diesel (~38 GJ/tonne vs. ~42 GJ/tonne). Biodiesel has many advantages, including (EBB 2010): ∑ It is non-toxic and biodegradable. ∑ It reduces the emission of harmful pollutants from diesel engines. ∑ It has a high cetane number (above 100, compared to only 40 for diesel fuel). Cetane number is a measure of a fuel’s ignition quality. The high cetane numbers of biodiesel contribute to easy cold starting and low idle noise. ∑ Blends of 20% biodiesel with 80% petroleum diesel can be used in unmodified diesel engines. Biodiesel can also be used in its pure form

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 106

4/5/11 9:19:58 AM

Biomass fuels for small & micro combined heat & power systems

107

but may require certain engine modifications to avoid maintenance and performance problems. ∑ The use of biodiesel can extend the life of diesel engines because it is more lubricating and, furthermore, power output is relatively unaffected by biodiesel. ∑ Biodiesel replaces the exhaust odour of petroleum diesel with a more pleasant smell of popcorn or French fries. Biodiesel production has increased dramatically over the past few years. Biodiesel is now widely available in many countries. The top five biodiesel producers in 2009 were (Biodiesel 2010): ∑ Germany – 2859 million litres (16% of the world total) ∑ France – 2206 million litres (12% of the world total) ∑ United States – 2060 million litres (11% of the world total) ∑ Brazil – 1535 million litres (9% of the world total) ∑ Argentina – 1340 million litres (7% of the world total).

5.4

Current development of small and micro scale biomass combined heat and power (CHP) technologies

Biomass is best suited for decentralized, small-scale and micro-scale combined heat and power (CHP) systems* due to its intrinsic properties, in particular its low calorific values in comparison with fossil fuels (Table 5.1). On one hand, small-scale and micro-scale biomass CHP systems can reduce transportation costs of biomass and provide heat and power where they are needed. On the other hand, it is more difficult to find an end-user for a great amount of heat produced in larger CHP systems (Eriksson et al. 2007). Over the past decade or so, the development of small-scale and micro-scale biomassfuelled CHP systems has been carried out by many researchers all over the world. Dong et al. (2009) have provided a review of small- and micro-scale biomass CHP technologies; in particular, those based on organic Rankine cycle (ORC) power generation. Despite various research efforts, small- and micro-scale biomass CHP systems still suffer from undesirable economics and technical uncertainties which require considerable technical advances in the future. With the expected continued rise in gas and electricity prices and the advances in the development of biomass conversion technologies and biomass fuel supply infrastructure, biomass-fuelled CHP systems can only become more economically competitive. *EU Cogeneration Directive 2004/8/EC defines ‘small-scale’ CHP units as those with electrical power output capacities less than 1000 kWe, whereas ‘Micro-scale’ CHP units are those with electrical power output capacities less than 50 kWe.

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 107

4/5/11 9:19:58 AM

108

Small and micro combined heat and power (CHP) systems

5.4.1 Introduction of biomass CHP technologies Biomass CHP technologies can be grouped in different ways such as by the biomass conversion technology or by the prime mover. In this chapter, they are grouped by the biomass conversion technology. Various combinations of biomass conversion technologies and prime movers have been used with biomass-fuelled CHP systems as shown in Table 5.4. The biomass conversion technologies, namely combustion, gasification, pyrolysis, fermentation, anaerobic digestion and biodiesel production were discussed in the Section 5.3, whereas the prime movers, namely steam turbines, steam engines, Stirling engines, organic Rankine cycle turbines, gas turbines and internal combustion engines, are characterised in other chapters of this book, and therefore, the following discussion on the biomass CHP technologies will focus only on the specific features of the combinations of the biomass conversion technologies and the prime movers.

5.4.2 Combustion-based biomass CHP technologies Combustion-based biomass CHP technologies are the dominant ones that are used from large-scale power plants to micro-scale CHP systems. The heat released from biomass combustion is used to produce steam, hot air or organic working fluid vapour which can drive a steam turbine, a steam engine, a hot air gas turbine or an ORC turbine to generate electricity. Biomass conversion technology: combustion and prime mover – steam turbine/steam engine Of all the technologies listed in Table 5.4, ‘combustion’ and ‘steam turbine’ are the most widely used combination, particularly for large-scale and Table 5.4 Biomass CHP technologies (modified from Dong et al. 2009) Biomass conversion technology

Prime mover

Combustion

Steam engine; Steam turbine; Stirling engine; Organic Rankine cycle (ORC) turbine; Hot air gas turbine

Gasification

Steam turbine; Internal combustion engine; Gas turbine/micro-turbine; Fuel cell

Pyrolysis

Internal combustion engine

Biochemical/biological processes (fermentation and anaerobic digestion)

Internal combustion engine

Chemical/mechanical processes (biodiesel production)

Internal combustion engine

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 108

4/5/11 9:19:58 AM

Biomass fuels for small & micro combined heat & power systems

109

medium-scale biomass-fuelled CHP systems (Masters 2004). Indeed, the majority of current world biomass power plants are based on ‘combustion’ and ‘conventional steam turbine Rankine cycle’ (Masters 2004). Steven’s Croft Biomass Power Station in Scotland (rated 44 MWe) of E.ON, which started its commercial operation in 2007, is a good example of a dedicated biomass power plant based on the technologies of ‘combustion’ and ‘steam turbine’ (E.ON 2010). The Markinch Biomass CHP plant in Scotland (rated 50 MWe) of RWE is another example of the application of the technologies, which is under construction and scheduled to start operation in 2012 (RWE 2010). The prohibitive cost of transporting dispersed and bulky biomass feedstock over long distance often limits the capacity of this type of biomass power/CHP plants to the order of 100 MWe, which results in the power generation efficiency to be in the region of 20%. If the same technologies were applied to small- and micro-scale biomass CHP systems, it would lead to both unacceptable low power generation efficiency and high capital costs (Pritchard 2002). The minimum capacity of biomass-fired steam turbine CHP is likely to be around 2 MWe, whereas for biomass-fired steam engine CHP it is likely to be around 200 kWe (BIOS 2010a, 2010b). Biomass conversion technology: combustion and prime mover – ORC turbine The combination of ‘combustion’ and ‘ORC turbine’ is receiving more and more attention in the development of small- and micro-scale biomass CHP systems. Instead of water, ORC uses organic chemicals with favourable thermodynamic properties as working fluids. Further details of ORC are discussed in Chapter 9. Despite the fact that ORC is linked with low electrical efficiency which is likely to be in the region of 6–17% for small- and micro-scale power/CHP systems, new applications of ORC are consistently explored due to its possibility to utilise the low-level waste heat from other processes (Schuster et al. 2009). The organic working fluid evaporates at lower temperatures than water and therefore ORC CHP systems can work at lower temperatures and pressures than the conventional steam Rankine cycle (Dong et al. 2009). For this reason, ORC is particularly suitable for small-scale and micro-scale biomass-fired CHP systems. Biomass-fired ORC-based CHP plants with capacity higher than 200 kWe have been demonstrated and in operation across Europe for almost a decade (Dong et al. 2009). Two particular demonstration plants are worth mentioning: the 400 kWe Admont CHP plant and the 1000 kWe Lienz biomass CHP plant (Admont 2001, Obernberger et al. 2002, BIOS 2010c). For both ORCbased biomass CHP demonstration plants, the biomass combustion furnace/ thermal oil boiler and the ORC process were coupled by a thermal oil cycle, whereas silicon oil was selected as the organic working fluid (Fig. 5.10).

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 109

4/5/11 9:19:58 AM

110

Small and micro combined heat and power (CHP) systems Turbine Thermal oil cycle

Generator G (directly coupled)

Thermal oil boiler

ORC process

Biomass

Evaporator Furnace

Air pre-heater

Silicon oil pump

Regenerator Condenser

District/process heating

Flue gas Economiser Combustion air

5.10 Schematic of the ORC-based biomass CHP plant of Lienz (Obernberger et al. 2002, BIOS 2010c).

The condensation of the silicon oil takes place at a temperature level that allows the heat recovered to be utilised with the hot water feed temperature of about 80–90 °C. The net electric efficiency of the Admont CHP plant was reported to be 7.4% for year 2000 but was expected to increase to 10–11% soon afterwards (Admont 2001). The net electric efficiency of the 1000  kWe Lienz biomass CHP plant amounted to 18% at nominal load and about 16.5% at 50% partial load at feed water temperatures of 85 °C (Obernberger et al. 2002). This underlines the excellent partial load behaviour of the technologies, which is especially relevant for heat-controlled operation of a CHP plant. Some manufacturers of ORC plants now offer commercial products of biomass-fired ORC-based CHP plants with capacity in the range of ca. 200  kWe and 2000 kWe. For example, Turboden (2010) has had 139 biomass-fired ORC-based CHP plants built or under construction in 15 countries by 20 September 2010 – these plants are within the size range of 200–2200 kWe. Turboden actually manufactured and supplied the ORC of the Lienz biomass CHP plant (Obernberger et al. 2002). The development of micro-scale biomass ORC-based CHP systems has received more and more attention over the last few years, partly due to the increasing demand for the building applications of micro-scale CHP systems. H. Liu et al. (2010b) have developed a micro-scale biomass ORC-based CHP system at the University of Nottingham in the UK. The main components of the CHP system, schematically shown in Fig. 5.11, include a pellet biomass boiler, a hot water cycle and an organic Rankine cycle. Due to safety

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 110

4/5/11 9:19:58 AM

© Woodhead Publishing Limited, 2011 T

Pump

P

T Heat exchanger Generator

P

T

T

T

Sight glass

Flow meter

P

Vacuum pump

T

T

P

P

P

T

Tap water cooling system

T

T

Turbine and generator

5.11 Schematic of biomass-fired micro-scale CHP system with ORC developed at the University of Nottingham, UK (H. Liu et al. 2010b).

Valve

Filling loop

Flow meter

Pressure expansion

Pressure gauge Drain cock

P Pressure transducer

T Thermocouple

Biomass boiler

Hot water pump

AAV

ORC fluid circulation

Recuperator

Pressure gauge

Receiver vessel

Hot water circulation

Heat exchanger Condenser Liquid level

CHP-Beith-05.indd 111

4/5/11 9:19:58 AM

112

Small and micro combined heat and power (CHP) systems

concerns with the potential users of micro-scale CHP systems, the biomass boiler and the organic Rankine cycle are coupled with a hot water cycle, instead of a thermal oil cycle (Fig. 5.10) which is normally used for larger biomass ORC-based CHP systems such as those developed by Turboden (2010). Two hydrofluoroethers (HFEs), HFE 7000 and HFE 7100, which are environmentally friendly with non-ozone depletion and low global warming potentials, have been tested as the ORC working fluid. The latest experimental results have proved the feasibility of the CHP system, produced 0.8 kW of electricity, and achieved ca. 2% of electric power generation efficiency and 88% of overall CHP efficiency. Further improvement of the CHP system and its main components, in particular the ORC turbine, is under way and better electric power generation efficiency is expected in the future. The selection and optimisation of the ORC turbines is of considerable importance to the development of micro-scale biomass-fired ORC-based CHP systems as ORC turbines greatly affect the organic Rankine cycle efficiency and electric power generation efficiency of the CHP systems. However, at the present time, micro-scale turbines for organic fluids with capacity smaller than 10 kWe are not commercially available and many researchers have to use replacement turbines with their micro-scale ORC-based CHP systems (Qiu et al. 2010). Scroll expanders are one of the popular choices for the replacement micro-scale ORC turbines (Peterson et al. 2008, Wang et al. 2009, Lemort et al. 2009, Quoilin et al. 2010, G. Liu et al. 2010), whereas modified air motors were used by H. Liu et al. (2010b) and Qiu et al. (2010). Further technical and economic advances on ORC turbines are needed before micro-scale ORC-based CHP systems can be commercialised. Biomass conversion technology: combustion and prime mover – Stirling engine Stirling engine-based small- and micro-scale biomass CHP systems have been researched for more than a decade. Podesser (1999) reported the laboratory testing results of a biomass Stirling engine with air (nitrogen) as working gas and a shaft power of 3 kW. Biomass Stirling engine CHP systems rated 35 kWe and 70 kWe had been developed and demonstrated by researchers in Austria and Denmark (Obernberger et al. 2003, Biedermann et al. 2004, BIOS 2010d). A 35 kWe plant was put into operation in 2002 and was successfully tested for more than 10 000 hours, and a 70 kWe plant has been in operation since autumn 2003 (BIOS 2010d). The overall electrical efficiency and the overall CHP efficiency of the 35 kWe and 70 kWe plants were reported to be 11.7% and 88.3%, respectively (BIOS 2010c). A field test with seven 35 kWel plants has been initiated and the first demonstration plants were put into operation in summer 2005 (BIOS 2010d). Although the demonstrated biomass Stirling engine CHP system is ‘not available on the market at the

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 112

4/5/11 9:19:58 AM

Biomass fuels for small & micro combined heat & power systems

113

moment’, it is claimed that the CHP system ‘will be commercially available within the next few years’ (BIOS 2010d). Figure 5.12 shows the schematic of the small-scale biomass Stirling engine CHP system (BIOS 2010d). A US company, Sunpower (Sunpower 2010) started their research and development into micro-scale Stirling engine-based biomass CHP systems in the 1990s and tested units as small as 1 kWe. Sunpower (2010) specialises in the development and delivery of a variety of free-piston Stirling engines at power levels ranging from 35 We to 7.5 kWe which have now been demonstrated by using a variety of heat inputs including biomass, liquid and gaseous fossil fuel burners, solar concentrators and heat pipes. A German company (Sunmachine 2010) is commercialising its Stirling engine-based biomass CHP systems – ‘SUNMACHINE® Pellet’ which is rated 1.5–3.0 kWe. Recently, a SUNMACHINE® Pellet CHP system was tested in laboratories by Thiers et al. (2010). Although the performance of the CHP unit was not as good as the manufacturer had claimed, its electric power generation efficiency of 14.3% is far better than many of its competitors for such small biomass CHP systems. Biomass conversion technology: combustion and prime mover – hot air gas turbine The combination of ‘combustion’ and ‘hot air gas turbine’ has been adopted by some research groups and developers of biomass CHP systems. The recently launched commercial product of Talbotts Generators Ltd, BG25 TCS (25 kWe and 80 kWt), is based on this combination of technologies: hot clean air heated by biomass combustion is used to drive a turbocharger and power turbine combination to generate electricity (Talbotts 2010). Gaderer et al. (2010) reported the developments at the Institute for Energy Air pre-heater Hot heat exchanger

Air input

Regenerator Economiser Flue gas

Generator Air Biomass

Furnace

G Stirlingengine Cooler

Heat consumers

5.12 Schematic of the small-scale biomass Stirling engine CHP of BIOS (2010d).

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 113

4/5/11 9:19:59 AM

114

Small and micro combined heat and power (CHP) systems

Systems, Technical University of Munich, Germany, of biomass-fired hot air gas turbine and a fluidised bed wood combustor with integrated high temperature heat exchanger out of structured steel tubes for indirect firing of micro-turbines at 100 kWe. Figure 5.13 shows the process flow diagram of the CHP system. Externally fired small- and micro-scale gas turbine CHP systems using biomass fuels have also been investigated by Cocco et al. (2006), Kautz and Hansen (2007) and Al-attab and Zainal (2010a, 2010b). Two key components of a biomass CHP with an externally fired air turbine are the high temperature heat exchanger which may need to be made of special materials, either metals or ceramics, and the turbine which usually is of a standard gas turbine (Kautz and Hansen 2007). There seems to have been no investigation into the development of biomass CHP systems with hot air turbines with capacity smaller than 10 kWe, which would be ideal for building applications. This may be partly due to the lack of micro-scale turbines smaller than 10 kWe on the commercial market.

5.4.3 Gasification-based biomass CHP technologies Gasification-based biomass CHP technologies are probably the second most popular category of technologies for large- to micro-scale biomass CHP systems. For large-scale CHP systems, such as power plants, biomass gasification can be combined with gas turbines and/or steam turbines Combustion air Flue gas 200 °C

Counter-current air preheater

FB combustor

High Temp HX

Biomass fuel

Process air 240 °C Recuperator 900 °C G

C

T

Air

5.13 Process flow diagram of the biomass-fired hot air gas turbine with fluidised bed combustion (Gaderer et al. 2010).

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 114

4/5/11 9:19:59 AM

Biomass fuels for small & micro combined heat & power systems

115

(integrated gasification combined cycle – IGCC) as adopted by some global manufacturers of power generation units/components such as General Electric (GE Power 2010). For small- and micro-scale biomass CHP systems, when gasification is adopted as the biomass conversion technology, there are three main choices for the prime mover: gas turbines, internal combustion engines and fuel cells. The success of the small- and micro-scale gasification-based CHP systems will depend largely on the quality of the combustible gases produced by biomass gasification – there are specific requirements for each prime mover in terms of gas quality and cleanliness (Liu and Neubauer 2010). Gas cleaning and conditioning of the combustible gases produced by biomass gasification can be very costly and energy intensive for small- and micro-scale gasification-based CHP units (Liu and Neubauer 2010). Community Power Corporation (CPC), the Modular Bioenergy Company of the United States, has developed and commercialised biomass CHP systems which are based on the technologies of gasification and internal combustion engine with capacity ranging from 25 kWe to 75kWe (CPC 2010). Each of CPC’s CHP systems consists of a gas production module which contains a downdraft gasifier and gas cleaning equipment, etc., and a power generation engine which is either a spark ignition engine or a compression ignition engine. Schmitt Enertec GmbH of Germany (Schmitt Enertec 2010) commercially supplies biomass gasification and internal combustion engine-based CHP systems with capacity between 250 kWe and 1000 kWe. The specification of these CHP systems indicates that the electric power generation efficiency is ca. 26%, whereas the overall CHP efficiency is ca. 80% (Schmitt Enertec 2010). The combination of biomass gasification with a solid oxide fuel cell (SOFC) and/or micro gas turbine for small-scale CHP was assessed using AspenplusTM process simulation software by Fryda et al. (2008). Karellas et al. (2008) investigated an innovative allothermal biomass gasification process – the Biomass Heatpipe Reformer (BioHPR), and its coupling with micro turbine and SOFC systems. M. Liu et al. (2010) carried out the feasibility study of applying biomass gasification to a SOFC system for distributed power/CHP generation in rural areas of China. Their preliminary system calculations indicated that an electrical efficiency of 32% and an overall CHP system efficiency of 59% were achievable with a 100 kWe biomass gasification-SOFC CHP system. The development of biomass gasification-based micro-scale CHP systems is less advanced in comparison to the development of biomass combustionbased micro-scale CHP systems. This is partly due to developers’ and the general public’s perceptions and concerns about the safety issues of biomass gasification. The technical and economic challenges of biomass gasification may also have contributed to this phenomenon, considering that biomass combustion is a mature and well-developed biomass conversion technology.

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 115

4/5/11 9:19:59 AM

116

Small and micro combined heat and power (CHP) systems

5.4.4 Other biomass CHP technologies There are various other technologies available for biomass combined heat and power generation, such as using bio-ethanol/bio-oil/biodiesel and biogas to fuel internal combustion engines and gas turbines/micro-turbines. As pointed out in Section 5.3, many internal combustion engines can burn a blend of gasoline and bio-ethanol, a blend of petroleum diesel and biodiesel, and biogas, and hence internal combustion engine-based CHP systems can be fuelled by liquid biofuels and biogas. A UK-based company, Ener-G Combined Power, commercially supplies biogas-fuelled CHP units within the size range of 30 kWe to 1950 kWe and biodiesel-fuelled CHP units with the size range of 130 kWe to 385 kWe (Ener-G 2010). Two 385 kWe biodiesel CHP systems are to be installed in the Museum of Liverpool and expected to be in operation in summer 2011 (CHPA 2010). Over the past decade, many turbine/micro-turbine manufacturers have developed gas turbines and microgas turbines which can successfully burn biogas such as the micro-turbines made by the Capstone Turbine Corporation of the United States (Capstone 2010). Capstone now commercially supplies biogas-fuelled micro-turbine power generating units within the size range of 30 kWe to 1000 kWe.

5.5

Conclusions

As a renewable energy resource, biomass has great potential to meet the world’s primary energy demand in the future. There are a range of technologies available which can convert bulky biomass feedstock into convenient forms of energy/fuels. Great advances have been made over the past decade with the development of small- and micro-scale biomass CHP systems with some of them having entered the commercial service stage. The demand for these CHP systems is set to continue to increase due to the combination of the mounting CO2 mitigation pressure, the expected price rises and the exhaustion of premium fossil fuels. The development and implementation of micro-scale biomass-fuelled CHP systems, particularly for those with capacity in the order of a few kWe, which are ideal for building applications, are still hindered by technical and economic barriers. Significant research efforts are urgently needed and should be accelerated in order that the distributed micro-scale biomass-fuelled CHP systems can be demonstrated, commercialised and widely deployed in the near future.

5.6

Acknowledgements

The author wishes to acknowledge the financial support from the UK Technology Strategy Board (TPQ3082A), UK EPSRC (EP/E020062/1, EP/

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 116

4/5/11 9:19:59 AM

Biomass fuels for small & micro combined heat & power systems

117

F038070/1 via ERA-NET Bioenergy) and the Carbon Trust (Project No. 089-051) for his groups’ research on biomass CHP, biomass gasification and biomass combustion at the University of Nottingham. The contributions of past and present research fellows (Dr L. Dong, Dr G. Qiu and Dr W. Zhang) and PhD students (Mr F. Daminabo, Ms Y. Shao, Mr J. Li and Mr I. Ul Hai) to the funded research projects are also acknowledged.

5.7

References

Admont (2001), Biomass fired CHP plant based on an ORC cycle – Project: ORC-STIAAdmont, http://ec.europa.eu/energy/res/sectors/doc/bioenergy/chp/bm_120_98.pdf, accessed 20 September 2010. Al-attab K. A. and Zainal Z.A. (2010a), Performance of high-temperature heat exchangers in biomass fuel powered externally fired gas turbine systems, Renewable Energy 35, 913–920. Al-attab K. A. and Zainal Z.A. (2010b), Turbine startup methods for externally fired micro gas turbine (EFMGT) system using biomass fuels, Applied Energy 87, 1336–1341. Alauddin Z. A. B. Z., Lahijani P., Mohammadi M. and Mohamed A. R. (2010), Gasification of lignocellulosic biomass in fluidized beds for renewable energy development: a review, Renewable and Sustainable Energy Reviews 14, 2852–2862. Alvira P., Tomás-Pejó E., Ballesteros M. and Negro M. J. (2010), Pretreatment technologies for an efficient bioethanol production process based on enzymatic hydrolysis: a review, Bioresource Technology 101, 4851–4861. Annalmalai K. and Puri I. K. (2007), Combustion Science and Engineering, CRC Press, Boca Raton, FL. Appels L., Baeyens J., Degrève J. and Dewil R. (2008), Principles and potential of the anaerobic digestion of waste-activated sludge, Progress in Energy and Combustion Science 34, 755–781. Argent Energy Limited (2010), http://www.argentenergy.com/, accessed 18 September 2010. Atadashi I. M., Aroua M. K. and Aziz A. A. (2010), Biodiesel separation and purification: a review, Renewable Energy 36, 437–443. Balat M. (2011), Production of bioethanol from lignocellulosic materials via the biochemical pathway: a review, Energy Conversion and Management 52, 858–875. Balat M., Balat M., Kırtay E. and Balat H. (2009), Main routes for the thermo-conversion of biomass into fuels and chemicals – Part 1: Pyrolysis systems, Energy Conversion and Management 50, 3147–3157. Basu P. (2010), Biomass Gasification Design Handbook, Academic Press, New York. BEF (2010), Biomass Energy Foundation website: http://www.woodgas.com/history. htm, accessed 4 October 2010. Biedermann F., Carlsen H., Obenberger I. and Schöch M. (2004), Small-scale CHP plant based on a 75 kWel Hermetic eight cylinder Stirling engine for biomass fuels – development, technology and operational experiences, 2nd World Conference and Exhibition on Biomass for Energy, Industry and Climate Protection, May, Rome, Italy. Binod P., Sindhu R., Singhania R. R., Vikram S., Devi L., Nagalakshmi S., Kurien N., Sukumaran R. K. and Pandey A. (2010), Bioethanol production from rice straw: an overview, Bioresource Technology 101, 4767–4774.

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 117

4/5/11 9:19:59 AM

118

Small and micro combined heat and power (CHP) systems

Biodiesel (2010), http://www.biofuels-platform.ch/en/infos/production.php?id=biodiesel, accessed 18 September 2010. Bioethanol (2010), http://www.biofuels-platform.ch/en/infos/production.php?id=bioethanol, accessed 16 September 2010. BIOS (2010a), BIOS Bioenergiesysteme GmbH: http://www.bios-bioenergy.at/en/ electricity-from-biomass/steam-turbine.html, accessed 6 October 2010. BIOS (2010b), BIOS Bioenergiesysteme GmbH: http://www.bios-bioenergy.at/en/ electricity-from-biomass/screw-type-engine.html, accessed 6 October 2010. BIOS (2010c), BIOS Bioenergiesysteme GmbH: http://www.bios-bioenergy.at/en/ electricity-from-biomass/orc-process.html, accessed 12 October 2010. BIOS (2010d), BIOS Bioenergiesysteme GmbH: http://www.bios-bioenergy.at/en/ electricity-from-biomass/stirling-engine.html, accessed 8 October 2010. Börjesson P. and Mattiasson B. (2007), Biogas as a resource-efficient vehicle fuel, Trends in Biotechnology 26, 7–13. Bridgwater A. V., Meier D. and Radlein D. (1999), An overview of fast pyrolysis of biomass, Organic Geochemistry 30, 1479–1493. BS EN 14778-1:2005, Solid biofuels – Sampling – Part 1: Methods for sampling, British Standards Institution, London. BS EN 14774-1:2009, Solid biofuels – Determination of moisture content – Oven dry method Part 1: Total moisture – Reference method, British Standards Institution, London. BS EN 14774-2:2009, Solid biofuels – Determination of moisture content – Oven dry method Part 2: Total moisture – Simplified method, British Standards Institution, London. BS EN 14774-3:2009, Solid biofuels – Determination of moisture content – Oven dry method Part 3: Moisture in general analysis sample, British Standards Institution, London. BS EN 14775:2009, Solid biofuels – Determination of ash content, British Standards Institution, London. BS EN 14918:2009, Solid biofuels – Determination of calorific value, British Standards Institution, London. BS EN 15148:2009, Solid biofuels – Determination of the content of volatile matter, British Standards Institution, London. Capstone (2010), Capstone Turbine Corporation: http://www.capstoneturbine.com/prodsol/ solutions/rrbiogas.asp, accessed 23 September 2010. CEN/TS 15104:2005, Solid biofuels – Determination of total content of carbon, hydrogen and nitrogen – Instrumental methods, British Standards Institution, London. CEN/TS 15289:2006, Solid biofuels – Determination of total content of sulphur and chlorine, British Standards Institution London. CEN/TS 15290:2006, Solid biofuels – Determination of major elements, British Standards Institution, London. CEN/TS 15297:2006, Solid biofuels – Determination of minor elements, British Standards Institution, London. CEN/TS 15296:2006, Solid biofuels – Calculation of analyses to different bases, British Standards Institution, London. Cherubini F. (2010), GHG balances of bioenergy systems – overview of key steps in the production chain and methodological concerns, Renewable Energy 35, 1565–1573. CHPA (2010), Combined Heat and Power Association (CHPA) press release: http://www. chpa.co.uk/media/760ae75d/21%2009%202010%20-%20Huhne%20highlights%20

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 118

4/5/11 9:19:59 AM

Biomass fuels for small & micro combined heat & power systems

119

key%20role%20of%20CHP%20-%20FINAL.pdf, accessed 14 October 2010. Cocco D., Deiana P. and Cau G. (2006), Performance evaluation of small size externally fired gas turbine (EFGT) power plants integrated with direct biomass dryers, Energy 31, 1459–1471. CPC (2010), CPC website, http://www.gocpc.com/, accessed 21 September 2010. Demirbas M. F. (2005), Potential applications of renewable energy sources, biomass combustion problems in boiler power systems and combustion related environmental issues, Progress in Energy and Combustion Science 31, 171–192. Demirbas M. F., Balat M. and Balat H. (2009), Potential contribution of biomass to the sustainable energy development, Energy Conversion and Management 50, 1746–1760. DOE (2010), US Department of Energy, Energy Efficiency and Renewable Energy, http:// www1.eere.energy.gov/biomass/abcs_biofuels.html#prod, accessed 17 September 2010. Dong L., Liu H. and Riffat R. B. (2009), Development of small-scale and micro-scale biomass-fuelled CHP systems – a literature review, Applied Thermal Engineering 29, 2119–2126. EBB (2010), European Biodiesel Board: http://www.ebb-eu.org/biodiesel.php, accessed 6 October 2010. EIA IEO (2010), International Energy Outlook 2010, http://www.eia.doe.gov/oiaf/ieo/ pdf/0484(2010).pdf, Energy Information Administration, Official Energy Statistics from the US Government, accessed 16 September 2010. EIA ISA-Biomass (2010), US Energy Information Administration – Independent Statistics and Analysis, http://www.eia.doe.gov/oiaf/analysispaper/biomass.html, accessed 15 September 2010. Ener-G (2010), Ener-G Combined Power: http://www.energ.co.uk/index313.aspx#P3, accessed 14 October 2010. Enweremadu C. C. and Mbarawa M. M. (2009), Technical aspects of production and analysis of biodiesel from used cooking oil – a review, Renewable and Sustainable Energy Reviews 13, 2205–2224. E.ON (2010), Steven’s Croft Biomass Power Station: http://www.eon-uk.com/generation/ stevenscroft.aspx, accessed 8 October 2010. Eriksson O., Finnveden G., Ekvall T. and Bjorklund A. (2007), Life cycle assessment of fuels for district heating: a comparison of waste incineration, biomass- and natural gas combustion, Energy Policy 35, 1346–1362. European Biomass Industry Association (2010), http://www.eubia.org/215.0.html, accessed 2 August 2010. Fang X., Shen Y., Zhao J., Bao X. and Qu Y. (2010), Status and prospect of lignocellulosic bioethanol production in China, Bioresource Technology 101, 4814–4819. Fischer G. and Schrattenholzer L. (2001), Global bioenergy potentials through 2050, Biomass and Bioenergy 20, 151–159. Fryda F., Panopoulos K. D. and Kakaras E. (2008), Integrated CHP with autothermal biomass gasification and SOFC–MGT, Energy Conversion and Management 49, 281–290. Gaderer M., Gallmetzer G. and Spliethoff H. (2010), Biomass fired hot air gas turbine with fluidized bed combustion, Applied Thermal Engineering 30, 1594–1600. Gaur S. and Reed T. B. (1995), An atlas of thermal data for biomass and other fuels, NREL, http://www.scribd.com/document_downloads/3120309?extension=pdf, accessed 2 August 2010.

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 119

4/5/11 9:19:59 AM

120

Small and micro combined heat and power (CHP) systems

GE Power (2010), http://www.gepower.com/prod_serv/products/gasification/en/overview. htm, accessed 22 September 2010. Gnansounou E. (2010), Production and use of lignocellulosic bioethanol in Europe: current situation and perspectives, Bioresource Technology 101, 4842–4850. Gnansounou E. and Dauriat A. (2010), Techno-economic analysis of lignocellulosic ethanol: a review, Bioresource Technology 101, 4980–4991. Goyal H. B., Seal D. and Saxena R. C. (2008), Bio-fuels from thermochemical conversion of renewable resources: a review, Renewable and Sustainable Energy Reviews 12, 504–517. Hoefnagels R., Smeets E. and Faaij A. (2010), Greenhouse gas footprints of different biofuel production systems, Renewable and Sustainable Energy Reviews 14, 1661–1694. Holm-Nielsen J. B., Al Seadi T. and Oleskowicz-Popiel P. (2009), The future of anaerobic digestion and biogas utilization, Bioresource Technology 100, 5478–5484. Hoogwijk M, Faaij A, van den Broek R, Berndes G, Gielen D and Turkenburg W. (2003), Exploration of the ranges of the global potential of biomass for energy, Biomass and Bioenergy 25, 119–133. IEA Bioenergy (2009), IEA Bioenergy Annual Report, http://www.ieabioenergy.com/ DownLoad.aspx?DocId=6507, accessed 3 August 2010. IEA Bioenergy (2010a), Education website on biomass and bioenergy, http://www. aboutbioenergy.info/index.html, accessed on 3 August 2010. IEA Bioenergy (2010b), Task 34: Pyrolysis, http://www.pyne.co.uk/index.php?_id=76, accessed 15 September 2010. IEA WEO (2009), IEA World Energy Outlook 2009, http://www.worldenergyoutlook. org/, accessed 1 August 2010. Jiang J., Sui J., Wu S., Yang Y. and Wang L. (2007), Prospects of anaerobic digestion technology in China, Tsinghua Science and Technology 12, 435–440. Karellas S., Karl J. and Kakaras E. (2008), An innovative biomass gasification process and its coupling with microturbine and fuel cell systems, Energy 33, 284–291. Kautz M. and Hansen U. (2007), The externally fired gas turbine (EFGT-cycle) for decentralized use of biomass, Applied Energy 84, 795–805. Khan A. A., De Jong W., Jansens P. J. and Spliethoff H. (2009), Biomass combustion in fluidized bed boilers: potential problems and remedies, Fuel Processing Technology 90, 21–50. Klass D. L. (2004), Biomass for renewable energy and fuels, in Encyclopaedia of Energy, Volume 1, Elsevier Amsterdam, pp. 193–212. Knoef H. A. M. (2005), Handbook Biomass Gasification, BTG Biomass Technology group BV, The Netherlands. Ladanai S. and Vinterbäck J. (2009), Global Potential of Sustainable Biomass for Energy, Department of Energy and Technology, Swedish University of Agricultural Sciences Report 013. Larkin S., Ramage J. and Scurlock J. (2004), Bioenergy. In Renewable Energy – Power for a Sustainable Future (Boyle G., ed.), Oxford University Press, Oxford, pp 106–146. Lemort V., Quoilin S., Cuevas C. and Lebrun J. (2009), Testing and modeling a scroll expander integrated into an organic Rankine cycle, Applied Thermal Engineering 29, 3094–3102. Liu G., Zhao Y., Li L. and Shu P. (2010), Simulation and experiment research on wide ranging working process of scroll expander driven by compressed air, Applied Thermal Engineering 30, 2073–2079. Liu H. and Neubauer Y. (2010), Gasification. In High Temperature Processes in Chemical

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 120

4/5/11 9:20:00 AM

Biomass fuels for small & micro combined heat & power systems

121

Engineering (Lackner, M., ed.), Verlag Process Eng Engineering GmbH, Vienna, pp. 361–408. Liu H., Qiu G., Shao Y. and Riffat S. B. (2010a), Experimental investigation on the flue gas emissions of a domestic biomass boiler under normal and idle combustion conditions, International Journal of Low Carbon Technologies 5, 88–95. Liu H., Qiu G., Shao Y., Daminabo F. and Riffat S. B. (2010b), Preliminary experimental investigations of a biomass-fired micro-scale CHP with organic Rankine cycle, International Journal of Low Carbon Technologies 5, 81–87. Liu M., Aravind P. V., Woudstra N., Cobas V. R. M. and Verkooijen A. H. M. (2010), Solid oxide fuel cell integrated with biomass gasification for power generation for rural areas in China, http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=05348036, accessed 13 October 2010. Ma F. and Hanna M. A. (1999), Biodiesel production – a review, Bioresource Technology 70, 1–15. Masters G. M. (2004), Renewable and Efficient Electric Power Systems, Wiley, New York. Mohan D., Pittman C. U. Jr. and Steele P. H. (2006), Pyrolysis of wood/biomass for bio-oil: a critical review, Energy & Fuels 20, 848–889. Nigam P. S. and Singh A. (2011), Production of liquid biofuels from renewable resources, Progress in Energy and Combustion Science 37, 52–68. Obernberger I., Thonhofer P. and Reisenhofer E. (2002), Description and evaluation of the new 1000 kWel organic Rankine cycle process integrated in the biomass CHP plant in Lienz, Austria, Euroheat & Power 10/2002. Obernberger I., Carlsen H. and Biedermann F. (2003), State-of-art and future development regarding small-scale CHP systems with a special focus on ORC and Stirling engine technologies, International Nordic Bioenergy 2003 Conference, Jyväskylä, Finland. Peterson R. B., Wang H. and Herron T. (2008), Performance of a small-scale regenerative Rankine power cycle employing a scroll expander, Proc. IMechE Part A: J. Power and Energy 222, 271–282. Phyllis (2010), ECN Phyllis – the composition of biomass and waste, http://www.ecn. nl/phyllis/, accessed 4 October 2010. Podesser E. (1999), Electricity production in rural villages with a biomass stirling engine, Renewable Energy 16, 1049–1052. Poeschl M., Ward S. and Owende P. (2010), Prospects for expanded utilization of biogas in Germany, Renewable and Sustainable Energy Reviews 14, 1782–1797. Pritchard D. (2002), Biomass combustion gas turbine CHP, ETSU B/U1/00679/00/REP. DTI Pub URN No. 02/1346. Talbott’s Heating Ltd. Qiu G., Liu H. and Riffat S. B. (2010), Selection of expanders for micro-scale ORC-based CHP systems, Paper SE-066, SET2010, – 9th International Conference on Sustainable Energy Technologies, Shanghai, China, 24–27 August. Quoilin S., Lemort V. and Lebrun J. (2010), Experimental study and modeling of an Organic Rankine Cycle using scroll expander, Applied Energy 87, 1260–1268. Rao P. V., Baral S. S., Dey R. and Mutnuri S. (2010), Biogas generation potential by anaerobic digestion for sustainable energy development in India, Renewable and Sustainable Energy Reviews 14, 2086–2094. Reed T. and Gaur S. (2001), A Survey of Biomass Gasification 2001, 2nd edn, The Biomass Energy Foundation, Franktown, CO. RWE (2010), Markinch Biomass CHP: http://www.rwe.com/web/cms/en/429432/rwenpower-renewables/sites/projects-in-construction/biomass/markinch-biomass-chp/

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 121

4/5/11 9:20:00 AM

122

Small and micro combined heat and power (CHP) systems

project-details/, accessed 8 October 2010. Schmitt Enertec (2010), http://www.schmitt-enertec.com/wood_gasification/woodgas_ power.htm, accessed 21 September 2010. Schuster A., Karellas S., Kakaras E. and Spliethoff H. (2009), Energetic and economic investigation of Organic Rankine Cycle applications, Applied Thermal Engineering 29, 1809–1817. Singh S.P. and Singh D. (2010), Biodiesel production through the use of different sources and characterization of oils and their esters as the substitute of diesel: a review, Renewable and Sustainable Energy Reviews 14, 200–216. Sunmachine (2010), http://www.sunmachine.de/produkte.php, accessed 21 September 2010. Sunpower (2010), http://www.sunpower.com/index.php?pg=25, accessed 21 September 2010. Talbotts (2010), Talbotts Generators Limited, http://www.biomassgenerators.com/docs/ BG25%20Brochure.pdf, accessed 12 October 2010. Talebnia F., Karakashev D. and Angelidaki I. (2010), Production of bioethanol from wheat straw: an overview on pretreatment, hydrolysis and fermentation, Bioresource Technology 101, 4744–4753. Thiers S., Aoun B. and Peuportier B. (2010), Experimental characterization, modeling and simulation of a wood pellet micro-combined heat and power unit used as a heat source for a residential building, Energy and Buildings 42, 896–903. Turboden (2010), http://www.turboden.eu/en/products/products-chp.php, accessed 20 September 2010. Van Loo S. and Koppejan J. (2007), The Handbook of Biomass Combustion and Cofiring, Earthscan, London. Wang H., Peterson R. B. and Herron T. (2009), Experimental performance of a compliant scroll expander for an organic Rankine cycle, Proc. IMechE Part A: J. Power and Energy 223, 863–872. Wang L., Weller C. L., Jones D. D. and Hann M. A. (2008), Contemporary issues in thermal gasification of biomass and its application to electricity and fuel production, Biomass and Bioenergy 32, 573–581. Ward A. J., Hobbs P. J., Holliman P. J. and Jones D. L. (2008), Optimisation of the anaerobic digestion of agricultural resources, Bioresource Technology 99, 7928–7940. WhatGreenCar (2010), http://www.whatgreencar.com/bioethanol.php, accessed 6 October 2010. Wu C., Yin X., Ma X., Zhou Z. and Chen H. (2008), Design and operation of a 5.5 MWe biomass integrated gasification combined cycle demonstration plant, Energy & Fuels 22, 4259–4264.

© Woodhead Publishing Limited, 2011

CHP-Beith-05.indd 122

4/5/11 9:20:00 AM

6

Internal combustion and reciprocating engine systems for small and micro combined heat and power (CHP) applications

R. M i k a l s e n, Newcastle University, UK

Abstract: This chapter reviews the internal combustion engine technology and its application in small and micro combined heat and power (CHP) systems. Prior to considering CHP systems, basic design aspects are examined in some depth in relation to small-scale stationary engines. Specific aspects of two main engine types, spark ignition and compression ignition engines, are next reviewed and compared. The use of such engines in small- and micro-CHP systems is then carefully considered, including aspects of system energy balance, heat recovery and operational control. Finally, some commercially available internal combustion engine based micro-CHP units are reviewed, and their performance and design solutions are discussed. Key words: internal combustion engine, micro-CHP, design, operation, performance.

6.1

Introduction

After more than a century of continuous development, internal combustion engine technology is mature and well established. Internal combustion engines provide excellent fuel conversion efficiencies and high power-to-weight ratios, leading to their widespread use in a range of applications, including transport, stationary power generation, and combined heat and power (CHP) systems. Such engines further provide excellent dynamic properties during varying load demands, and are, unlike many competing technologies, suitable for scaling down to small sizes. The latter is underlined by their current wide use: from micro-scale units for radio-controlled model cars to ship propulsion with engines several storeys high, all working according to the same basic principle. For micro-CHP applications, the excellent efficiency at small scale is one of the main advantages of internal combustion engines (compared to, for example, Stirling engines or organic Rankine cycle systems). In the size range of 1–5 kW electric power output, suitable for a single family house, the fuel efficiency of internal combustion engines is currently leading in the marketplace. 125 © Woodhead Publishing Limited, 2011

CHP-Beith-06.indd 125

4/5/11 9:20:24 AM

126

Small and micro combined heat and power (CHP) systems

Such engines are well suited for use with natural gas and can therefore take advantage of the well-developed gas supply infrastructure in many countries. Other advantages of internal combustion engines for micro-CHP applications include low capital cost, easy maintenance and a widely developed service infrastructure, whereas noise, vibration and exhaust gas emissions are the main challenges in relation to use in domestic applications. This chapter will discuss internal combustion engine technology and its suitability for use in micro-CHP systems. Important design aspects will be reviewed and the operation and performance of internal combustion engine based micro-CHP systems presented. Information and data from commercially available units will be presented as examples. For more comprehensive background reading on internal combustion engines, their design and operating principle, Stone (1999) provides an accessible introduction to the subject while Heywood (1988), one of the standard textbooks, offers a considerable level of detail.

6.2

Types, properties and design of engine

Internal combustion engines are commonly divided into ‘Otto engines’ and ‘Diesel engines’, after the two German inventors. The Diesel engine is a direct injection compression ignition engine, wherein air is compressed in the cylinder and fuel is injected at high pressure to self-ignite and burn. Otto engines utilise a spark plug to ignite a pre-mixed charge after compression in the cylinder. Figure 6.1 illustrates typical spark ignition and compression ignition designs. Figure 6.1(a) shows a ‘pent-roof’ combustion chamber, commonly used in spark ignition engines, which gives a compact combustion chamber and allows large valve areas, minimising gas flow losses. Figure 6.1(b) shows a Diesel engine ‘bowl-in-piston’ combustion chamber, where the fuel will be injected into the bowl, mix with the compressed air, and burn. Although the Otto and Diesel colloquial terms are useful to denote the operating principle of the engine (spark ignition or compression ignition), there is some potential for confusion with the Otto or Diesel ‘textbook’ thermodynamic cycles, commonly taught in introductory thermodynamics courses. In practice, most modern stationary engines, both Otto and Diesel engines, operate with close to constant volume combustion. The air-standard Diesel thermodynamic cycle, characterised by constant pressure combustion, would provide a very poor representation of a modern compression ignition engine. Both spark ignition and compression ignition engines can use either a two stroke or a four stroke operating cycle, of which the four stroke cycle is by far the most common. Two stroke engines are found in a limited number of applications, interestingly at the outer ends of the power spectrum. For small engines where power density is critical, such as chainsaws or other hand-held

© Woodhead Publishing Limited, 2011

CHP-Beith-06.indd 126

4/5/11 9:20:24 AM

Internal combustion and reciprocating engine systems

127

(a)

(b)

6.1 Common internal combustion engine cylinder designs: (a) spark ignition engine combustion chamber, (b) compression ignition engine combustion chamber.

tools, two stroke engines are used since they provide higher power output for a given engine size. The two stroke cycle is also used in the largest marine engines, which can be several storeys high and where each cylinder has a power output in the megawatt range. However, since the four stroke cycle provides superior performance in most applications between these extremes, only four stroke engines will be discussed here.

© Woodhead Publishing Limited, 2011

CHP-Beith-06.indd 127

4/5/11 9:20:25 AM

128

Small and micro combined heat and power (CHP) systems

Often, engines for CHP systems are based on existing engine models from, for example, portable power generators or automotive engines. Using existing, mass-produced designs and converting them to CHP operation can clearly reduce cost significantly but is, as will be argued below, not necessarily the best solution to obtain optimum performance. Particularly for the micro-CHP size range (1–5 kW), most existing engines are aimed for transport or similar applications, where low weight and small size are essential and fuel efficiency is less critical.

6.2.1 Basic design considerations An engine designer faces a number of performance requirements, the most important being good fuel efficiency, high power density, and low exhaust gas emissions levels. In a stationary system, efficiency and emissions will be of highest importance, with the requirements for high power density being somewhat relaxed compared to, for example, a car engine. Some of the fundamental design variables at hand are: design speed, number of cylinders, cylinder bore, stroke length and compression ratio. These parameters provide the basic design outline of the engine, and their influence will be discussed briefly here. Figure 6.2 shows the significant influence of the compression ratio on the cycle efficiency. A high compression ratio is therefore desirable. However, in practice it is limited by the fuel knock limit (in spark ignition engines), metallurgical limits (since high compression leads to high in-cylinder gas pressures), and heat losses and the formation of temperature-dependent emissions (since high compression leads to high gas temperatures). Also shown are typical fuel efficiency ranges for small (~5 kW) spark ignition (petrol) and compression ignition (Diesel) engines. (Note that the actual efficiency obtained from a real engine depends on numerous other factors in addition to compression ratio, and this is for illustration only.) Worth noticing is the significant potential for efficiency improvement in current engines compared to the theoretical limit (the ideal cycle). The main factors limiting efficiency include mechanical friction, heat losses to the combustion chamber surface, and deviations from the ideal cycle (including volume change during combustion).1 1

One concept with large potential and currently receiving significant research attention is the lean-burn, homogeneous charge compression ignition (HCCI) concept. This is a pre-mixed engine in which the charge is compressed until self-ignition, giving very fast and close to constant volume combustion. The use of a lean charge, i.e. with an excess of air compared to that necessary for combustion, reduces peak temperatures, and thereby heat losses and emissions formation. However, problems in controlling the ignition timing have prevented this concept from seeing commercial application.

© Woodhead Publishing Limited, 2011

CHP-Beith-06.indd 128

4/5/11 9:20:25 AM

Internal combustion and reciprocating engine systems

129

0.9 0.8 0.7 Ideal otto cycle

Efficiency

0.6 0.5 Diesel engine

0.4 0.3 0.2

Spark ignition engine

0.1 0 0

10

20

30 40 Compression ratio

50

60

70

6.2 Ideal (air standard) Otto cycle efficiency and typical fuel efficiencies for small-scale internal combustion engines for microCHP systems.

In addition to the compression ratio, the design of the cylinder and combustion chamber are key variables. Typically, approximately 15–20% of the fuel energy is lost to in-cylinder heat transfer, and the combustion chamber should be designed to minimise these losses. Engines which require high power densities typically have low stroke-to-bore ratios, i.e. a short stroke in relation to the piston diameter and the swept volume. For example, a Honda CB1000R motorcycle has a stroke-to-bore ratio of 0.73; a VW Golf 1.6 engine has a stroke-to-bore ratio of 0.96. A short stroke allows high speed operation (see the next paragraph), and thereby high power output, but gives a less compact combustion chamber (at piston top dead centre, the combustion chamber has the shape of a flat disc). This penalises efficiency due to the surface area of the combustion chamber being large in relation to its volume, in turn giving high heat transfer losses. For this reason, engines optimised for high efficiency typically use a higher stroke-to-bore ratio, which gives a more compact combustion chamber with a more favourable volume to surface area ratio. For example, a Volvo TAD1240 300 kW diesel generator set has a stroke-to-bore ratio of 1.15; large marine engines can have stroke-to-bore ratios of up to 4. The design speed of the engine relates directly to the power output, and thereby the power density, of the engine since it dictates the number of power strokes carried out per unit of time. In the extreme case, material loads (on crank system bearings, piston rings, valves, etc.) limit the maximum speed of the engine. However, frictional losses and gas flow losses scale overproportionally with the speed, and the trade-off between speed and energy

© Woodhead Publishing Limited, 2011

CHP-Beith-06.indd 129

4/5/11 9:20:25 AM

130

Small and micro combined heat and power (CHP) systems

losses is what effectively determines the design speed. In practice, the mean piston speed2 of an engine is the more important variable. Automotive-type engines have maximum mean piston speeds of around 15 m/s, whereas high-performance motorcycle or racing engines can reach above 20 m/s (the Honda CB1000R motorcycle engine has a mean piston speed of 20 m/s at full speed). Engines used for power generation or marine propulsion typically have mean piston speeds around 7–9 m/s. Hence, in order to minimise the friction and the flow losses during the gas exchange stroke (engine breathing losses) in applications where power density is not critical, one would choose a low mean piston speed. As can be seen, a number of interrelated design variables influence engine performance. However, one important trade-off in the initial design stage is that between engine power density and fuel efficiency. If a penalty in power-to-weight ratio can be accepted, which is often the case in stationary systems, an engine can be designed for optimal fuel efficiency by adopting a longer stroke and lower mean piston speed. For this reason, one would expect better performance from an engine purpose-designed for a CHP system compared to one adapted from, for example, an automotive engine. It should be noted that this is a simplified representation of a complex, multivariable optimisation problem; for a more detailed discussion on these aspects, the reader can refer to Heywood (1988) or Stone (1999).

6.2.2 Spark ignition engines: specific aspects In spark ignition engines, the combustion process is governed by the flame propagation in the compressed charge. As the piston approaches top dead centre, a spark plug ignites the charge near the centre of the combustion chamber, and the flame spreads towards the outer regions of the combustion space. As the engine compression ratio dictates the end-of-compression charge temperature, the maximum compression ratio that can be used in spark ignition engines is fundamentally limited by the resistance to self-ignition (knock resistance) of the fuel-air mixture. Engine knock is the phenomenon where the fuel-air mixture that is not yet reached by the flame front ignites spontaneously due to the high pressure from the compression and initial stages of combustion. Such self-ignition gives a violent pressure rise which can produce very high peak pressures, and thereby high mechanical loads on the engine, in particular on the piston rings. To avoid knock when using 2 Mean piston speed, vp, is a function of engine speed, N, and stroke length, S, such that vp = 2SN. For example, an engine with 80 mm stroke running at 3000 rpm (50 Hz) has a mean piston speed of 8 m/s.

© Woodhead Publishing Limited, 2011

CHP-Beith-06.indd 130

4/5/11 9:20:25 AM

Internal combustion and reciprocating engine systems

131

standard fuels, most commercial spark ignition engines use compression ratios of around 10. Spark ignition engines used in micro-CHP systems most often run on natural gas, due to its low cost, high availability and well-developed supply infrastructure. Such engines can also run on a range of different fuels, including standard petrol fuel, hydrogen (H2), landfill or anaerobic digester gas, syngas from biomass gasification, etc. This is, however, less common in small- and micro-CHP plants due to cost and/or fuel quality issues. Alcohol fuels, such as methanol and ethanol, are also attractive options for use in spark ignition engines due to their favourable combustion properties (high knock resistance) and the possibility of production from bio-crops, but these have not yet seen widespread use in micro-CHP systems. The performance of a spark ignition engine depends heavily on the fuel properties, in particular the flame speed. A high flame speed is desirable, in order to provide a rapid, close to constant volume combustion and for operational stability. The flame speed depends on the fuel type, but also on the fuel-air ratio (the richness of the mixture) and on the in-cylinder conditions. For hydrocarbon fuels, the highest flame speed is around stoichiometric conditions; operation with very lean mixtures may lead to penalties in fuel efficiency due to reduced flame speed and poor ignition qualities. The same may be the case for low-quality fuels, such as syngas or landfill gas, which have high fractions of inert species such as N2 or CO2. One option to enhance the combustion and improve performance is to increase the gas velocities and turbulence in the combustion chamber. The flame speed depends heavily on the turbulence levels in the charge, and the gas flow fields in the cylinder are therefore usually optimised through careful design of the ports, valves and combustion chamber. Also, the use of a compact combustion chamber reduces the required flame travel distance, therefore giving a more rapid combustion and improving performance. The main emissions from spark ignition engines are nitrogen oxides (NOx), carbon monoxide (CO), and unburnt hydrocarbons (HC). One challenge in pre-mixed engines is that the (homogeneous) fuel-air mixture will distribute evenly throughout the combustion chamber. This includes crevices, in which the mixture may be cooled down such that it cannot burn efficiently, or the flame cannot reach (the flame is quenched). This leads to incomplete combustion and increased emissions of unburned hydrocarbons and carbon monoxide, as well as efficiency reductions since part of the fuel energy is not utilised. The three-way catalyst has become the industry standard for reducing spark ignition engine exhaust gas emissions, and is widely used, particularly in automotive engines. It can provide reduction of all three major emissions species, NOx, CO, and HC, within a single unit. However, in order to operate efficiently, it requires stoichiometric engine operation, i.e. no excess

© Woodhead Publishing Limited, 2011

CHP-Beith-06.indd 131

4/5/11 9:20:25 AM

132

Small and micro combined heat and power (CHP) systems

oxygen in the exhaust gas, and it can therefore not be used with lean-burn engines. To ensure efficient catalyst operation, the engine fuel-air ratio must therefore be controlled accurately within a narrow range, and closed loop control of the fuel injection rate is required. This is achieved by using an oxygen sensor at the engine exhaust outlet to adjust the fuel injection rate to obtain stoichiometric conditions. For engines operating primarily in steady state, such as those found in CHP systems, very efficient catalyst operation can be achieved, with above 90% reduction for all three species. Transient engine operation (i.e. load and speed variations) reduces the efficiency of the three-way catalyst somewhat due to difficulty in maintaining a steady fuel-air ratio.

6.2.3 Compression ignition engines: specific aspects Compression ignition (Diesel) engine operation is fundamentally different from that of spark ignited engines in that the fuel is injected after compression and self-ignites due to the high gas temperature in the combustion chamber. This eliminates the problem of knock, since only pure air is present in the cylinder during the compression stroke, and therefore allows significantly higher compression ratios to be used. The compression ratio in Diesel engines is limited by the pressure and temperature loads on the cylinder structure, and by the formation of temperature-dependent emissions at high combustion temperatures. Compression ignition engines can use a range of fuels, including standard diesel fuel, heavy fuel oils, biodiesel and vegetable oils. However, the engine performance depends heavily on the quality of the fuel. Requirements for Diesel engine fuels include good ignition and combustion properties, in order to avoid excessive ignition delays and poor combustion efficiency, but also good handling properties, in particular a viscosity suitable for efficient supply through the engine injection system. The latter has been a challenge with many fuels aimed to replace standard (fossil) diesel, such as bio-oils. For these fuels, pre-processing and the use of additives are required to ensure stable properties under varying temperatures and good injection properties. In the marine industry, this has long been the case with low-quality heavy fuel oils requiring thermal viscosity control before being fed to the engine. As auto-ignition of the injected fuel is required, the fuel must, unlike fuels for spark ignition engines, have good self-ignition properties. Unlike the flame propagation process in spark ignition engines, Diesel combustion rate is governed by diffusion combustion, and the availability of oxygen in the vicinity of the fuel sprays is critical. The properties of the fuel injector determine the spray characteristics and therefore play a key role in the performance of the engine. Moreover, the fuel must have good spray and atomisation properties, to ensure efficient combustion and to avoid excessive

© Woodhead Publishing Limited, 2011

CHP-Beith-06.indd 132

4/5/11 9:20:25 AM

Internal combustion and reciprocating engine systems

133

emissions formation. High levels of in-cylinder gas motion increases fuel-air mixing and improves performance, and Diesel engines therefore often use bowl-in-piston combustion chamber designs, which enhance gas velocities significantly by forcing the air charge rapidly into the bowl at the end of the compression stroke (this is known as squish). Since the fuel and air is mixed in the cylinder and over a very short time, utilisation of all the oxygen in the intake air is not possible. Diesel engines therefore always operate lean of stoichiometric, i.e. with an excess of air. However, due to the higher compression ratios used, and the resulting higher temperature levels, the formation of temperature-dependent emissions, most notably nitrogen oxides, in Diesel engines is comparable to that of spark ignition engines. (The exact NOx levels can vary significantly in both engine types and depend heavily on operational parameters such as injection/ignition timing.) Emissions of CO and HC are negligible in Diesel engines; however, particulates emissions represent a major challenge. Particulate matter, or soot, poses significant health risks, and is a particular problem with fuels having poor ignition and combustion characteristics, such as bio-oils. Particulate emissions comprise complex hydrocarbons that have not been fully oxidised, and can be reduced by increasing the combustion temperatures, for example through advancing the start of injection. However, while this increases the oxidation of soot particles, the higher temperatures lead to increased NOx formation, and also influences fuel efficiency, hence there is a trade-off. (This Catch-22-like situation, that measures to reduce particulates emissions tends to increase NOx, and vice versa, is known as ‘the Diesel dilemma’.) Both NOx and particulate emissions are formed in the fuel spray in Diesel engines, and the injection properties therefore have very high influence on the emissions. Much research effort is going into the improvement of fuel injection systems, and there is a clear trend towards higher injection pressures to give a finer fuel spray and better fuel break-up and atomisation. Further strategies, which have been made possible with modern, high-speed and electronically controlled injection systems, include multiple injections and rate shaping of the injector flow. External means for reducing Diesel engine emissions include reduction catalysts for NOx, which are not normally used in small-scale systems due to high costs, and particulate filters, which are increasingly being used in automotive and other small-scale systems. The Diesel version of the SenerTec Dachs micro-CHP system, described in more detail below, is fitted with a particulate filter.

6.3

Engine operating characteristics and performance

Electrical efficiency is of key importance in all heat engines. However, for use in micro-CHP systems, effective recovery of waste heat is also

© Woodhead Publishing Limited, 2011

CHP-Beith-06.indd 133

4/5/11 9:20:25 AM

134

Small and micro combined heat and power (CHP) systems

critical in order to provide a good total utilisation of the fuel energy. Further, the operational flexibility of the plant is of high importance, in particular whether efficient part load operation is possible to meet varying load demands.

6.3.1 Energy balance, efficiency and heat recovery As a rule of thumb, the energy supplied to an internal combustion engine through the fuel can be divided into three parts: approximately one-third is converted into mechanical work; approximately one-third is lost as heat in the exhaust gases; and approximately one-third is lost as friction and heat losses within the engine. Most of the latter is carried away as heat by the cooling water circuit and lubrication oil cooler (if applicable), but a part of this energy is lost as radiation, convection and other non-recoverable energy losses. The ratio of mechanical output to the fuel input energy is known as engine efficiency. For many applications this is the only performance parameter of interest, as the mechanical work is often the only energy that is utilised (the exhaust gas and cooling heat is simply dumped). While in CHP systems heat losses are also utilised, engine efficiency is still of paramount importance, since the electrical output that can be fed back to the grid essentially determines the economic viability of an investment in a CHP system over a conventional boiler. The efficiency that can be achieved in internal combustion engines varies significantly with engine size, design, fuel type and operational conditions. The 5.5 kW Dachs micro-CHP unit, discussed in more detail below, has an electrical efficiency of 27–30%, while the 4.7 kW ecopower unit achieves 24% efficiency. Notably, internal combustion engines are suitable for scaling down to small sizes without excessive efficiency penalties; the Honda Freewatt unit achieves an efficiency of 22% at 1.2 kW electrical output. For comparison, Diesel engine or spark ignited natural gas-based generator sets with 300–500 kW output achieve efficiencies around 40%; the largest low-speed marine propulsion engines are the most efficient internal combustion engines and can achieve 50% efficiency. Figure 6.3 shows a typical configuration of an internal combustion engine-based CHP system. Recovery of heat from the engine cooling system can be achieved by adapting the heat exchanger in the cooling circuit (through which the cooling heat is otherwise dumped). The cooling water has a temperature of 85–95 °C, and this is maintained nearly constant over the load range by control of the cooling water flow rate. If the engine has a separate lubrication oil cooler, recovery of this heat can be carried out in series with the cooling water; the lubrication oil typically has a temperature slightly lower than that of the cooling water circuit. In larger

© Woodhead Publishing Limited, 2011

CHP-Beith-06.indd 134

4/5/11 9:20:25 AM

Internal combustion and reciprocating engine systems Exhaust gas H/E

Hot water supply

To exhaust pipe

Exhaust outlet

Engine cooling circuit

Cooling water H/E Cold water inlet

135

Electric output +   –

Combustion engine

6.3 Common internal combustion engine based CHP system configuration.

systems, turbocharger cooling can also provide a small amount of heat recovery. Recovery of exhaust gas heat is carried out using an exhaust gas heat exchanger, and heat can be recovered as hot water or low-pressure steam at 100–120 °C. The exhaust gas temperature can vary significantly over the engine load range, from above 600 °C at full load down to around 300 °C at idle. The temperature even at low loads is sufficiently high for effective heat utilisation; however, these temperature variations must be taken into account when designing the energy recovery system. Condensing heat exchangers can be utilised to recover the latent heat in the steam in the exhaust stream. However, the use of condensing boilers is normally limited to natural gasfired systems due to problems of corrosion when using other fuels. Figure 6.4 shows a typical breakdown of the energy balance in an internal combustion engine over the load range. It can be seen that the engine efficiency, i.e. the fraction of mechanical output to the fuel input energy, drops rapidly below approximately 50% load; this is a typical characteristic of internal combustion engines. Over the load range, there is a relatively stable fraction of non-recoverable losses of between 5 and 10% of the fuel input energy. At idling (0% load), there is no mechanical output and fuel is supplied merely to overcome internal friction and keep the engine running. Most of this friction work is dissipated as heat through the cooling system. As the load increases, the engine efficiency improves and the fraction of energy lost to the exhaust and cooling system is reduced. At full load, the energy distribution is approximately 37% mechanical work, 33% exhaust heat, and 21% cooling system heat.

© Woodhead Publishing Limited, 2011

CHP-Beith-06.indd 135

4/5/11 9:20:26 AM

136

Small and micro combined heat and power (CHP) systems

Fraction of input energy (%)

60 Cooling system

50

Mechanical output 40 30

Exhaust heat

20 Non-recoverable losses 10 0 0

20

40 60 Engine load (%)

80

100

6.4 Typical energy balance in an internal combustion engine based CHP system (Wilbur 1985).

6.3.2 Operational optimisation and load control On commissioning, the operational strategy of the engine must be established. While the major engine design variables are fixed, some flexibility exists in the possibility to adjust operating parameters, most importantly fuel injection rate and ignition or start-of-injection timing. Modern engines use an electronic control unit (ECU) to control operational variables. One advantage of this is that the settings can be adjusted according to the operating conditions, such that the performance of the engine is optimised for any given speed or load level. Internal combustion engine-based generator systems commonly run at constant speed, allowing them to supply alternating current at the required frequency directly to the grid. To allow varying speed operation, a frequency converter and power electronics are usually required in order to condition the electric power output. This adds cost and introduces losses in the electric circuit, but gives the flexibility of allowing the engine power output to be adjusted over the full load range. Figure 6.5 shows a typical performance map for a spark ignition engine. (Engine torque is shown as brake mean effective pressure (bmep), which is a common measure for ‘specific torque’ and proportional to the shaft torque.) The power output is the product of speed and torque, hence a load change can be effectuated through a torque change at constant speed, a speed change at constant torque, or a change in both variables. Superimposed on the efficiency map are two lines of constant load, where Load 1 at 2500 rpm illustrates the design operating point, and Load 2 represents an approximate 40% load reduction from Load 1.

© Woodhead Publishing Limited, 2011

CHP-Beith-06.indd 136

4/5/11 9:20:26 AM

Internal combustion and reciprocating engine systems 1000

Load 1 Load 2

800 Torque (kPa bmep)

137

22%

24% 28% 30%

600

24% 27%

400

22% 16% 13%

200 0 0

1000

2000 3000 Engine speed (rpm)

4000

5000

6.5 Typical performance map for a spark ignition engine (Heywood 1988).

It can be seen that changes in torque have a large influence on engine efficiency, and it is clear that, from the combustion engine perspective, having a constant speed engine is the least desirable option if operation over a range of loads is required. (Consider changing from Load 1 to Load 2 at constant speed.) An imaginary constant-torque engine with variable speed would be preferred, since the engine efficiency is less sensitive to speed changes. Clearly, the ability to vary both load and speed both increases the load range available and provides more flexibility and operational optimisation possibilities. It should, however, also be noted that one may not have complete freedom to choose any combination of torque and speed for a given load, since this also depends on the electric machine being able to utilise the output efficiently. While the load and/or speed of the engine is controlled through the fuelling rate, the spark timing, or start-of-injection timing in Diesel engines, is used exclusively for operation optimisation purposes. At any operating point, i.e. for any given speed and load, there is an optimum ignition/injection timing which will give highest fuel efficiency (e.g. 15 crank angle degrees before top dead centre). In practice, the ignition/injection timing is retarded (delayed) slightly compared to this optimum value, in order to reduce NOx formation. Even small variations in the ignition timing and the start of combustion have large influence on the peak combustion temperatures, and therefore on NOx formation. Retarding the ignition/injection (e.g. by 5 crank angle degrees) produces major reductions in NOx formation while giving only a modest penalty in fuel efficiency. (Due to the large influence of engine tuning on NOx formation, a single engine model can be supplied in standard mode and in low-NOx mode, the only difference being the software settings.)

© Woodhead Publishing Limited, 2011

CHP-Beith-06.indd 137

4/5/11 9:20:26 AM

138

6.4

Small and micro combined heat and power (CHP) systems

Installation and practical aspects

Unlike large-scale systems, small- and micro-CHP systems are usually supplied in a ready-to-install package, requiring only the connection of fuel and exhaust lines, electrical grid connection, and heating circuit connection. In many cases, a micro-CHP system can directly replace an existing boiler in a system; however, some limitations exist related to the installation. Due to the noise and vibration from an engine system, as well as the relatively large size of the units, internal combustion engine-based micro-CHP systems are generally not intended for installation inside the living area of domestic residences. Further, if the CHP unit is replacing a conventional boiler, the increased fuel and exhaust flows for a constant heat output (since the CHP system produces electricity in addition to heat) must be taken into account. In the integration of the CHP unit in the building heating system, it is critical that the return temperature from the heating circuit be below that prescribed by the manufacturer. Too high a return temperature may lead to thermal overload in the combustion engine, which may be critical for the engine. The maximum allowable return temperature for internal combustion engine-based micro-CHP systems is typically 60–70 °C. Also, in order to maximise fuel utilisation, in particular when using a condensing exhaust heat exchanger, a sufficiently low return temperature is required.

6.4.1 Electrical connection A three-phase (400 V) electrical connection is commonly required, but smaller systems (around 1 kWel) can also use single phase (230 V) connections. The micro-CHP system is usually connected in parallel with other consumers, so that the net difference in usage and production at any time can be supplied back to the grid. Although the net electrical usage can be registered by a standard meter installed by the utility, a separate meter for supplied electrical energy is commonly installed to take into account differences in electricity prices for usage and supply as well as subsidies for micro-CHP generation. The electrical connection always includes an (automatic) emergency switch allowing the CHP system to be taken off the grid in case of, for example, maintenance work by the utility, in order to avoid individual CHP units continuing to supply power to the grid in such a case. Additional power electronics can be installed to allow operation of the CHP system in stand-alone mode as an emergency power supply. If there is a power outage, both the CHP system and consumers will be disconnected from the grid and the CHP unit then re-started to supply off-grid electric power. Both the Dachs and ecopower micro-CHP units (described below) can be supplied with such a system.

© Woodhead Publishing Limited, 2011

CHP-Beith-06.indd 138

4/5/11 9:20:26 AM

Internal combustion and reciprocating engine systems

139

6.4.2 Operational control Since the output of a CHP system will only rarely be able to match exactly the electrical and heating demand in a building, an operational scheme must be decided upon. There are three options: heat-led operation, electricity-led operation, or a combination of the two. In the most commonly used heat-led mode, the CHP system will be controlled to meet the heating demand in the installation, much like a conventional boiler. The electricity produced will be fed to local consumers or supplied to the grid according to the production rate and independent of the local electricity usage. Heat-led operation is favoured in most cases due to the flexibility of the electrical supply; any excess electric energy from the CHP system can be fed back to the grid at any time, hence no energy is ‘lost’. In heat-led operation, the CHP system will be designed to meet a given minimum heating load, and, depending on the plant, a supplementary boiler or thermal energy storage can be used to meet demand peaks and to allow stable and continuous operation of the CHP system. In particular for small applications, a well-matched hot water storage will allow better utilisation of the CHP unit. By allowing the CHP system to charge the heat storage at times of low demand (such as night time), the system can run in steady state mode for most of its operational time and does not need to follow the dynamically changing building heat demand. In electricity-led operation, the CHP system is controlled to meet the building electrical power demand at any time. This type of operation may be advantageous in applications with periods of high electrical loads, and where peak-load electric energy is more expensive. Under electricity-led operation, efficient utilisation of the produced heat becomes critical in order to maintain good total system efficiency. The use of heat storage is common if the base heat load is lower than the production rate, in order to avoid dumping heat energy. A combination of heat-led and electricity-led operation is also possible, and, if matched to a building’s energy demands, may give a better total efficiency than one of the above alone. Optimal operation of a CHP system is a highly complex problem, since it depends on the efficiency map of the heat engine, the fuel price, the electricity purchase and feedback tariffs (both of which often vary over the course of a day), and the electrical and heat demand profiles of the building. Internal combustion engine-based microCHP systems allowing variable speed operation, such as the ecopower unit described below, can provide efficient generation with variable electrical and heat output, and can therefore provide valuable flexibility for operational control and optimisation. Constant-power units, such as the Dachs, may require energy storage in order to provide good total efficiency in plants with highly varying load demands.

© Woodhead Publishing Limited, 2011

CHP-Beith-06.indd 139

4/5/11 9:20:26 AM

140

Small and micro combined heat and power (CHP) systems

6.4.3 Maintenance, reliability and availability Being a mature and well-tested technology, internal combustion engine systems are generally reliable, but require regular maintenance and therefore regular scheduled outages. Despite this, availability levels of above 95% are commonly reported for stationary engine systems. One advantage of internal combustion engines used in stationary power generation is the highly favourable operating regime. Engines used in CHP systems run under stable conditions, often at constant load and speed, and over long periods of time. This is a major advantage in terms of maintenance, wear rates and reliability compared to, for example, automotive engines, which operate under constantly varying load demands, speed variations and with frequent cold starts. For this reason, the lifetime of a stationary engine is many times that of one used in a vehicle. Regular engine maintenance includes changing of lubrication oil, filters, engine coolant, spark plugs, etc. Recommended maintenance intervals are typically: every 3500–4000 h for natural gas fuelled engines; every 2700 h for operation on heating oil; every 1400 h for biodiesel operation; and every 750–1000 h for engines running on vegetable oils (Thomas, 2007). Some manufacturers have service plans which include major overhauls after (typically) between 15 000 and 40 000 running hours, and this can include changing piston and piston rings, fuel injectors, bearings, seals, etc.

6.5

Commercially available units

This final section presents an overview of some commercially available internal combustion engine-based micro-CHP systems. Some reported design and performance data will be presented; however, it should be noted that detailed information is protected industrial intellectual property and is therefore not publicly available.

6.5.1 SenerTec Dachs German company SenerTec is the market leader for micro-scale CHP systems in Europe, having installed more than 20 000 units of their Dachs micro-CHP system, shown in Fig. 6.6. While the system is suitable for larger domestic residences, it is believed that the main market is larger buildings with high heating demands, such as hotels, schools, etc. To meet any required energy demand, the manufacturer can supply systems comprising several units operating in parallel. The Dachs can be supplied for several different fuels, including natural gas, propane, heating oil, biodiesel, and rapeseed oil, of which the natural gas version is the most sold. In addition to models for different fuels, SenerTec

© Woodhead Publishing Limited, 2011

CHP-Beith-06.indd 140

4/5/11 9:20:26 AM

Internal combustion and reciprocating engine systems

141

6.6 SenerTec Dachs micro-CHP unit. (Illustrations courtesy of SenerTec Kraft-Wärme-Energiesysteme GmbH (SenerTec, 2010). Reprinted with permission.)

© Woodhead Publishing Limited, 2011

CHP-Beith-06.indd 141

4/5/11 9:20:27 AM

142

Small and micro combined heat and power (CHP) systems

also supplies a range of accessories, including versions that can supply emergency power in case of a power outage, as well as systems aimed for stand-alone power supply in remote applications. Table 6.1 shows technical data for some of the Dachs models. The different versions are based around the same basic structure, with engine modifications implemented to adapt the system to the different fuels. Heat recovery is carried out using a combined cooling system and exhaust gas heat exchanger. The exhaust gas temperature at the system outlet is 150  °C in the standard versions; however, a condensing heat exchanger can be supplied with the unit. The condensing heat exchanger gives an additional 1.4–2.3 kW thermal output by cooling the exhaust to 55 °C, thereby significantly increasing the thermal and total efficiency of the unit. Fuel utilisation is generally good for all versions, with electrical efficiency above 26%. Independent tests have confirmed the efficiency values: Thomas (2008) reported that 28% electrical efficiency and 89% total efficiency were achieved for the G5.5 model in laboratory tests for a German environmental certification (i.e. slightly exceeding the performance claimed by the manufacturer).

Table 6.1 SenerTec Dachs micro-CHP technical data (SenerTec 2010; Thomas, 2007) Model

G5.5

G5.5 G5.0 condensing low-NOx

Engine

Single-cylinder, four-stroke

Fuel

Natural gas

G5.0 HR 5.3 low-NOx, condensing

Heating oil

Displacement

578 cm3

Bore

90 mm

Stroke Compression ratio

HR 5.3 condensing

91 mm 19.5

N/A

Operating speed

2450 rpm

Electric output 5.5 kW

5.0 kW

Heat output

12.5 kW 14.8 kW

12.3 kW

Maintenance interval

3500 h

Electrical efficiency

27

Thermal efficiency

61

72

63

74

59

66

Total efficiency 88

99

89

100

89

96

Dimensions

5.3 kW 14.6 kW

10.5 kW 11.9 kW 2700 h

26

30

Width: 72 cm, length: 107 cm, height 100 cm, weight 530 kg.

© Woodhead Publishing Limited, 2011

CHP-Beith-06.indd 142

4/5/11 9:20:27 AM

Internal combustion and reciprocating engine systems

143

Maintenance requirements for the system vary depending on the fuel used, with recommended service intervals every 3500 h with natural gas down to every 1400 h for the biodiesel and rapeseed oil models. With regular maintenance, the design service life of the unit is 80 000 h. The Diesel engine versions (HR models) of the Dachs are fitted with a soot filter to reduce particulates emissions. The natural gas fuelled models are lean burn and therefore, unlike e.g. the ecopower unit described below, do not use a three-way catalyst. An emissions comparison between these two systems can be found below. There are only minor differences in price for the natural gas and heating oil models: at the time of writing, the recommended retail price from SenerTec is around 721,000, with the condensing heat exchanger costing an additional 72000. For other models, such as that for vegetable oil fuel or for off-grid power generation, the prices are somewhat higher.

6.5.2 PowerPlus Technologies ecopower The ecopower micro-CHP system is developed by PowerPlus Technologies, a subsidiary of Vaillant, one of Europe’s largest boiler manufacturers. With 4.7 kW electrical output the unit is similar to the Dachs, however, with one significant difference in that the ecopower provides variable electric output, from 4.7 kW down to 1.3 kW. This is achieved through engine speed control, and a frequency converter is implemented to condition the power output for supply to the grid. Table 6.2 shows the basic design data for the ecopower micro-CHP unit. The price of the unit is similar to the Dachs, at approximately 720 000. The ecopower includes a condensing heat exchanger in the standard version, and achieves electrical and total efficiencies of 24.7% and 88.9%, respectively,

Table 6.2 Ecopower micro-CHP unit design and operational data (Thomas, 2007; PowerPlus Technologies, 2010) Engine

Single cylinder, four stroke, spark ignition

Fuel Displacement Bore Stroke Compression ratio Operating speed Electric output Heat output Design service life Maintenance interval Dimensions

Natural gas or propane 272 cm3 73 mm 65 mm 12.8 1200–3600 rpm 1.3–4.7 kW 4.0–12.5 kW 40 000 h (80 000 h with major overhaul) 4000 h (or at least yearly) Height 108 cm, width 76 cm, depth 137 cm, weight 395 kg.

© Woodhead Publishing Limited, 2011

CHP-Beith-06.indd 143

4/5/11 9:20:27 AM

144

Small and micro combined heat and power (CHP) systems

Output (kW)

at full load (Thomas, 2008). The gas engine is designed for stoichiometric operation and fitted with a three-way catalyst, which gives very low exhaust gas emissions levels at steady state operation. Thomas (2008) notes that the emissions are somewhat higher during transients. Thomas (2007) presented test results for the ecopower; these are summarised in Fig. 6.7. It can be seen that excellent part load efficiencies are achieved, with electrical efficiency of above 20% over the full load range. The same is the case for the thermal efficiency, and the wide load range of the unit makes ecopower a very flexible option in plants where varying output is desired or required. In his comprehensive benchmark testing of different micro-CHP systems, Thomas (2007) further measured exhaust gas emissions in the ecopower and also compared these to measurements taken from the SenerTec Dachs. The ecopower produced CO emissions levels of 0.1 mg/Nm3 and NOx emissions levels of 8.4 mg/Nm3 NOx (at 5% O2); these are very low and show the effectiveness of the three-way catalyst. For the Dachs, which operate in lean burn mode and without a catalyst, the measurements were 0 mg/Nm3 CO and 500–600 mg/Nm3 NOx. It should, however, be noted that these tests were done with the standard, natural gas fuelled Dachs model (G5.5). According to SenerTec, the low-NOx model (G5.0) achieves a 50% reduction in NOx compared to this, but at the cost of slightly lower electrical efficiency, as shown above.

14 12 10 8 6 4 2 0 1000

Thermal

Electrical 1500

2000

2500 Speed (rpm)

Efficiency (%)

80

3000

3500

4000

Thermal

60 40

Electrical

20 0 1000

1500

2000

2500 Speed (rpm)

3000

3500

4000

6.7 PowerPlus Technologies ecopower CHP unit performance data (Thomas, 2007).

© Woodhead Publishing Limited, 2011

CHP-Beith-06.indd 144

4/5/11 9:20:27 AM

Internal combustion and reciprocating engine systems

145

6.5.3 Honda Ecowill/Freewatt The Honda Ecowill micro-CHP unit has been available for a number of years in Japan, and sales have reached more than 50 000 units. It is also available in USA, where it is marketed under the name Freewatt. The US version is based around the same engine but has minor design differences. The system is currently being introduced to Europe in a collaboration between Honda and Vaillant. The electric power output is 1 kW (1.2 kW in the USA), and it is therefore suitable for smaller dwellings than the Dachs or the ecowill. The system is based around a single-cylinder, water-cooled four stroke engine with 163 cm3 displacement, running on natural gas. The thermal output is 3.25 kW, and system efficiencies are 22.5% electrical and 85.5% total. The engine is configured for stoichiometric operation and the system uses a three-way catalyst to control emissions. Very low emissions levels can therefore be expected; however, detailed information from the manufacturer or independent tests are not available. Service is required only every 6000 hours (Slowe, 2006). The unit is 640 mm wide, 380 mm deep and 940 mm high, and weighs 83 kg. Installation cost is approximately £5,600 (Harrison, 2010).

6.6

Conclusions

Engines for micro-CHP systems will without question continue to develop into its own specialist niche within the massive landscape of internal combustion engine systems. From the first academic studies of internal combustion engine-based micro-CHP which typically used converted engines from other applications, we are now seeing purpose-designed engines being developed for such systems. As this technology becomes more mature and more widely used, the performance of these engines will likely continue to improve. Internal combustion engine-based micro-CHP systems can provide efficient and reliable power generation with low installation costs, and if the problems of vibration and noise can be overcome, such systems have the potential to take a large share of the micro-CHP market.

6.7

References

Harrison J. (2010) Micro combined heat and power web site. http://www.microchap. info Heywood, J.B. (1988) Internal combustion engine fundamentals. McGraw-Hill, Maidenhead. PowerPlus Technologies (2010) Company web site. http://www.ecopower.de SenerTec (2010) Company web site. http://www.senertec.de. Slowe, J (2006) Micro-CHP: Global industry status and commercial prospects. In: 23rd World Gas Conference, Amsterdam. © Woodhead Publishing Limited, 2011

CHP-Beith-06.indd 145

4/5/11 9:20:27 AM

146

Small and micro combined heat and power (CHP) systems

Stone, R. (1999) Introduction to internal combustion engines. MacMillian, New York. Thomas, B. (2007) Mini-Blockheizkraftwerke. Vogel Buchverlag, Würzburg. Thomas, B. (2008) Benchmark testing of Micro-CHP units. Applied Thermal Engineering, 28, 2049–2054. Wilbur L.C. (1985) Handbook of Energy Systems Engineering. Wiley, Chichester.

© Woodhead Publishing Limited, 2011

CHP-Beith-06.indd 146

4/5/11 9:20:27 AM

7

Microturbine systems for small combined heat and power (CHP) applications

J. L. H. B a c k m a n and J. K a i k k o, Lappeenranta University of Technology, Finland

Abstract: Microturbines are small-size gas turbines with high potential for distributed energy systems. The chapter discusses the characteristic features of microturbines in combined heat and power (CHP) generation under 100 kWe. It begins by introducing the challenges for the design in the microturbine scale and analyzing the factors that affect the performance in CHP operation. The chapter then describes the types and properties of the main microturbine components and considers operational viewpoints. Finally the chapter presents the current manufacturers and future trends for the microturbine industry. Key words: microturbines, combined heat and power (CHP), performance, radial compressor, radial inflow turbine.

7.1

Introduction

Microturbines are small-size gas turbines. They have been developed based on the principles of larger industrial systems, but using development results in the automotive turbocharger and gas turbine research as well as solutions in auxiliary power units. After the development with automotive gas turbines stalled, the idea of using microturbines for power and CHP applications was introduced in the 1990s and stronger commercial products have been available since 2000. A microturbine cycle in the simplest construction comprises a compressor, combustion chamber, turbine and generator. Air is compressed in the compressor to a higher pressure (3–5 bars), fuel is burnt in the combustion chamber using part of the compressed air and the combustion gases enter the turbine (typically at 900–950 °C). The gases are expanded in the turbine, which produces more power than the compressor consumes. This surplus power is converted to shaft power to drive the generator (Fig. 7.1(a)). The gases exhausting from the turbine can be at high temperature. It is advantageous from the fuel-to-power conversion efficiency point of view to use a recuperator where heat from the combustion gases is transferred to the air before combustion (Fig. 7.1(b)). Before discharging the gases to the atmosphere, their thermal energy is recovered for external heating purposes in 147 © Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 147

4/5/11 9:20:54 AM

148

Small and micro combined heat and power (CHP) systems

HRHE

HRHE

REC

C

T

G

C

(a)

T

G

(b)

7.1 Flow diagram of (a) simple gas turbine cycle and (b) recuperative cycle. C is compressor, T turbine, G generator, REC recuperator and HRHE heat recovery heat exchanger.

CHP operation. The current microturbines apply recuperated configuration and generate power at approximately 30% efficiency, but without the recuperator the efficiency would be only half of this. In CHP the total efficiency of heat and power generation is typically in the range of 75–85% depending on the application. The simplest arrangement is to have the compressor, turbine and generator on a single shaft. In the two-shaft arrangement the first turbine drives the compressor on one shaft, and the second turbine powers the generator on another. The single-shaft type dominates the market and is also the focus of this chapter. We have chosen to use 100 kW of produced electricity as the upper limit of the microturbine power range. Some define it as below 1000  kW (Pritchard, 2002), some below 100 kW and even below 15 kW (Dong et al., 2009; Malmquist, 2006). The target applications in the industry are microturbines that produce the electricity and heat required for the processes or water heating. In hospitals, office and factory buildings the microturbine gives electricity as well as summer absorption chilling and winter heating. In some electricity critical manufacturing processes, microturbines may provide the primary power with UPS battery backup (Soares, 2007). Microturbines are well suited for distributed generation applications due to their flexibility in connection methods. In distributed generation reliable operation is important since these locations may be remote from the grid and in case of failures the loss of power may have large financial consequences (EEA, 2008). The total equipment costs for microturbine-based CHP applications (producing hot water) are around $1600–1700/kWe (EEA, 2008). The fuel cost can be as high as 75% of all costs incurred during the

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 148

4/5/11 9:20:54 AM

Microturbine systems for small CHP applications

149

operational lifetime, while the other costs comprise mainly service and capital costs (Boyce, 2006). The overall production of microturbines (below 100 kWe) during 2010–2015 is predicted to be in the range of 1000–1500 units (Soares, 2008).

7.1.1 Gas turbine development Gas turbines were developed at the end of the nineteenth century, but due to the low unit efficiencies and elementary combustion technology they did not generate power until the first decades of the twentieth century. The first commercial gas turbines were made in the 1940s and 1950s, but the technological breakthrough came in the 1960s. The gain in efficiency is clearly seen in Fig. 7.2 and it seems that there is no major improvement to be foreseen in the efficiency of the industrial gas turbine. The efficiency increase is due to improvements in the internal flow characteristics in turbines and compressors as well as increases in the turbine inlet temperatures. The polytropic efficiencies have been approaching unity and the turbine inlet temperatures have become closer to that of the combustion process itself. The specific price of the gas turbine unit decreases with increasing size. This enables investment in more complex systems and using more advanced materials. As a result, the efficiencies of the gas turbines also increase with increasing size (Fig. 7.3).

7.1.2 General challenges with microturbine scale The availability of commercial microturbines is scarce as the manufacturing capacity has not reached the economical requirements. It is therefore necessary to contemplate the reasons for this. 40

Electric efficiency (%)

38 36 34 32 30 28 26 24 22 20 1960

1970

1980

Year

1990

2000

2010

7.2 Development of gas turbine efficiencies.

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 149

4/5/11 9:20:54 AM

Small and micro combined heat and power (CHP) systems Efficiency

Electric efficiency (%)

45

Speed

100000

40

10000

35

1000

30

100

25

10

20 0.01

0.1

1 10 Net power (MW)

100

Speed (rpm)

150

1 1000

7.3 Power and efficiency of current gas turbines over 1 MW (Siemens, 2010) and below (Capstone, 2010a).

Volume Industrial gas turbines are serving municipalities or factories and are very large in size (power output currently up to 340 MW), which also contributes to high efficiency as the fluid paths are relatively large. Also, the pressure of the combustion is quite high, usually between 20 and 40 bars leading to pressure and expansion ratios of the same magnitude in an open gas turbine cycle. The through-flow velocity is kept constant in all the stages in the axial compressor. This works in favour of the designer as the stages can be designed on similar principles, but it also means that the channel area must decrease with the increasing density of the fluid. In the industrial gas turbines one can find the narrowest flow paths in the last stage of the axial compressor. The blade heights vary from hundreds to some ten millimeters as the gas turbine power decreases from hundreds to a few megawatts. If the power is less than 100 kilowatts this analogy makes the last stage blade to be only a few millimeters. The manufacture of such small parts is possible, but the ratio between the flow channel area and the circumference will become very small and thus the efficiency will be low. The gaps between the rotating wheels and the stationary casing are usually sealed up with contactless seals, such as labyrinth seals, to reduce mechanical losses and ensure a long life. Therefore, there always exist small paths where the air can leak out. The smaller the turbomachinery, the relatively larger the gaps become. Speed Turbomachine design is governed by defining the pressure ratio, volume flow and angular speed. The single machine element, a centrifugal or an axial stage is defined by its specific speed Ns © Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 150

4/5/11 9:20:55 AM

Microturbine systems for small CHP applications

Ns =

w qv 2p N qv = D hs0.75 D hs0.75

151

7.1

where w is angular speed (rad/s), N is rotational speed (1/s), qv is inlet volume flow (m3/s) and Dhs is stage isentropic enthalpy change (J/kg). Results of an extensive study on specific speeds for various turbines, compressors and pumps are shown in Fig. 7.4 (Balje, 1981), where the compressor types and contours of constant isentropic efficiency are plotted for single-stage compressors against specific speed and specific diameters Ds. The diagram shows that in order to maintain high efficiency, the specific speed Ns cannot be varied extensively. When the turbomachine is made smaller in size, the work done in the fluid is still the same and the denominator in Equation 7.1 does not change. Instead, the volume flow is smaller with the decreased power and consequently the speed has to be increased. it therefore can be stated for machines with the same pressure ratio that the speed of a turbomachine increases inversely proportionally to the square root of its power. This effect can also be seen in Fig. 7.3. Efficiency in principle, there should be no thermodynamic reasons not to have as efficient turbomachinery in the microturbine as at the industrial scale. But

Specific diameter Ds(–)

100.0 85% 80% 75% 70% 60% 10.0

Radial

Mixed flow Axial

1.0

0.1 0.1

1.0 Specific speed Ns(–)

10.0

7.4 Efficiency values of single stage compressors. Adapted from Balje (1981).

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 151

4/5/11 9:20:55 AM

152

Small and micro combined heat and power (CHP) systems

we need to use the same fluids and part of the machinery is rotational. From the efficiency point of view, the fluid Reynolds number should be discussed and it is in general defined as Re =

cd v

7.2

where c is velocity (m/s), d is characteristic length (m) and n is kinematic viscosity (m2/s). There is a critical Reynolds number magnitude of about 2 · 105 above which the viscosity-related losses are constant (Baskharone, 2006). For example, if the Reynolds number related to the impeller blade height and circumferential velocity drops from 2 · 105 to 2 · 104, the efficiency of the centrifugal compressor decreases ten percentage points (casey, 1985). The aforementioned challenges in the microturbine scale are summarized in Fig. 7.5, which depicts the dependency of compressor and turbine efficiencies on the microturbine size.

7.2

Cycle performance

all actual gas and microturbines are based on an ideal Brayton (Joule) cycle that contains compression, heat addition, expansion and heat rejection. in the ideal cycle, all these processes take place without loss of energy so they are reversible. The compression and expansion are also adiabatic so that no heat transfer occurs, and hence they are isentropic (entropy remains unchanged). Heat is added to and removed from the cycle by heat transfer so the amount and composition of the working gas remain constant throughout the cycle. 88

Isentropic efficiency (%)

Turbine 84 Compressor 80

76

72 0

25

50 75 100 Microturbine electric output (kW)

125

150

7.5 Effect of microturbine power output on compressor and turbine efficiency (Galanti and Massardo, 2010).

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 152

4/5/11 9:20:56 AM

Microturbine systems for small CHP applications

153

An extensive analysis for the ideal Brayton cycle and its variants is given in the classical textbooks of thermodynamics and turbomachinery such as Moran and Shapiro (2010) and Saravanamuttoo et al. (2008). In the actual microturbines, losses and dissipation of energy occur in each component. The compressor and turbine can be considered adiabatic with high accuracy, but as a result of losses entropy is increased in these processes. Fluid friction causes pressure losses in various components such as combustion chamber, heat exchangers as well as inlet and outlet ducting. The heat is added to the cycle in the combustion chamber by burning fuel in the air flow. As a result, the working gas composition and, to a small degree, its amount change. Typically, microturbines apply the so-called open cycle where the compressor draws ambient air and combustion gases are discharged back to the atmosphere. The interpretation is that the atmosphere completes the series of processes into a cycle. The T-s diagram for the ideal Brayton cycle and actual microturbine cycle with the same maximum temperature is presented in Fig. 7.6(a). The added heat in the combustion chamber is fully available in the form of hot combustion gases. Instead, the heat exchangers where heat is transferred through a separating wall feature a temperature difference between the hotter and colder fluids. Recuperators, heat recovery heat exchangers (for water heating, for instance) and intercoolers are of this type. Mechanical losses are incurred in the rotating shaft (or shafts in a multiple shaft unit) and conversion losses in the generator and power conditioning. The electric output is further decreased by the auxiliary power needs of the control system and possible fuel compressor, for instance. In CHP applications, the heat recovery is implemented after the power cycle. As a result, it affects the electricity generation by increasing the pressure losses in the hot-gas path. The microturbine configurations that will be considered here are the simple cycle, recuperated cycle, and the cycle with intercooling, recuperation and reheat. All these cycles are of the open type and they generate power and heat. The number of shafts has no relevance in this type of design analysis. The simple cycle is the most straightforward configuration containing only the compressor, combustion chamber, turbine, generator and heat recovery heat exchanger as the main components (Fig. 7.6(a)). In the simple cycle combustion gases are discharged from the turbine, and consequently from the power cycle, at a high temperature. This contributes to the recoverable heat but penalizes the efficiency of electricity generation. To increase electric efficiency, the standard solution for current microturbines is to use recuperation (Fig. 7.6(b)). In the recuperated cycle the heat of the turbine discharge flow is utilized to pre-heat the compressed air before combustion. This reduces the fuel amount necessary to achieve the desired combustion temperature, but also penalizes power output slightly due to increased

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 153

4/5/11 9:20:56 AM

154

Small and micro combined heat and power (CHP) systems T

Ideal Actual

3

4

1

3

2

1

2

G

5

5

4 s

(a) T 4

Ideal Actual 5

3

7 2 1

6

6

2

3

4 G

7

1

5 s

(b) T

6

5

Ideal Actual

8

7

9

11 4 3

2 1

10

10

11

4 1 2

5

6 7

8

3

(c)

G

9 s

7.6 T-s diagram and flow diagram for (a) ideal Brayton cycle and the actual simple microturbine cycle, (b) actual recuperated cycle and (c) actual intercooled, recuperated and reheated cycle.

pressure losses. The recuperator performance is determined using the term effectiveness (thermal ratio) which is the ratio of the actual heat transfer rate and theoretically maximum rate.

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 154

4/5/11 9:20:56 AM

Microturbine systems for small CHP applications

155

While the recuperated microturbine represents current practice in the microturbine segment, more complex configurations are being developed to boost the performance to new levels and/or create new applications (aTT, 2010). To give an indication of the performance potential, a case that combines intercooling, recuperation and reheating is considered (Fig. 7.6(c)). an intercooler between the compressor stages counteracts the temperature rise, hence reducing the power need for compression. Similarly, an additional combustion chamber between the turbine stages helps to maintain the temperature level, increasing the power output from expansion. Both these methods increase the net output from the unit and when used together with recuperation, electric efficiency too. in this presentation, four performance parameters have been selected for characterizing the performance of the microturbines in cHP operation: electric efficiency he, specific power Psp, total efficiency htot, and power-toheat ratio s. Electric efficiency is an established parameter also in CHP operation focusing on the most valuable species of energy, electricity. it is the ratio of the net electric output Pe to fuel input Ff.

he =

Pe Ff

7.3

Total efficiency (overall energy conversion efficiency) takes into account the useful heat output too, and the efficiency is the ratio of the electric and heat outputs to fuel input.

htot =

Pe + F h Ff

7.4

Fuel input is determined here using the lower heating value (LHV) for the fuel, which does not include the latent heat of condensation of the water vapour in the combustion gases. By using the lower heating value it is thereby assumed that no condensation of water occurs. Ff = qmf (LHV + hf)

7.5

The sensible enthalpy hf of the incoming fuel is also taken into account for completeness, although its value is insignificant compared to LHV, especially for liquid and gaseous fuels. Power-to-heat ratio proportions the products of operation, net electric output and useful heat output. although it can be determined on the basis of electric and total efficiencies, and therefore brings no additional value, it is a convenient parameter for assessing the applicability of the microturbine for a case with given heat and electricity loads.

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 155

4/5/11 9:20:56 AM

156

Small and micro combined heat and power (CHP) systems

s =

Pe = 1 F h httot ot –1 he

7.6

Specific power is determined by dividing the net electric output by compressor mass flow. Psp =

Pe qmc

7.7

it characterizes the size of the unit for given power, and is therefore indicative of the price, too. This is especially valid for the recuperated microturbines with expensive gas-to-gas recuperators: a decrease in specific power increases the mass flow through the recuperator, hence increasing its heat transfer rate and price. microturbine performance depends mainly on the turbomachinery (compressor and turbine) efficiencies, turbine inlet temperature and compressor delivery pressure, indicated commonly using compressor pressure ratio. While increasing the efficiencies and inlet temperature contribute to the performance unambiguously, the selection of the pressure ratio can be seen more as an optimization task. The optimum depends on the other parameters, but also on the desired criterion. an analysis will be given on the effect of compressor pressure ratio and turbine inlet temperature on the performance of the selected configurations (simple cycle Sc, recuperated cycle Rc, and the cycle with intercooling, recuperation and reheat icRRc). While values for turbine inlet temperatures are typically in the range of 900–950 °c for current microturbines, slightly lower and higher values are also used to indicate their effect on performance. The analysis is based on iSo operating conditions (15 °c, 101.3 kPa, 60%) and using natural gas. The fuel is in practice sulfur-free and therefore the amount of recoverable heat is not limited by the condensation of sulphuric acids. consequently, the exhaust gas temperature after the heat recovery boiler is selected as low as 70 °c. in the intercooler, the minimum temperature for outlet air is 30 °c. However, the risk of condensation onto the cold surfaces of the air channels is taken into account by using a temperature margin of 10 °c between the air outlet temperature and saturation temperature. The simulation has been performed using commercial heat balance modelling software iPSEpro. Table 7.1 presents the main input values for the simulation, which can be considered typical for the current microturbines. The simulation results are collected in Fig. 7.7. as Fig. 7.7 shows, an optimum pressure ratio exists to maximize electric efficiency for all configurations. Maximum specific power is attained at a certain pressure ratio too, but it differs from the value for best efficiency. The difference between the values is so high that selecting one as the

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 156

4/5/11 9:20:57 AM

Microturbine systems for small CHP applications

157

Table 7.1 Main input values in the simulation, all values in % Recuperator effectiveness Compressor polytropic efficiency Turbine polytropic efficiency Mechanical efficiency (shaft) Electromechanical efficiency (generator and inverter) Intercooler airside pressure loss Recuperator airside pressure loss Combustion chamber pressure loss Recuperator exhaust gas side pressure loss Heat recovery heat exchanger pressure loss (gas side) 300

30 20 10 SC 0

0

RC

ICRRC

Specific power (kJ/kg)

Electric efficiency (%)

40

250 200 150 100

2 4 6 8 10 12 14 Compressor pressure ratio (a)

SC 0

RC

ICRRC

2 4 6 8 10 12 14 Compressor pressure ratio (b)

1.4

90 80 70 60 SC 0

RC

ICRRC

2 4 6 8 10 12 14 Compressor pressure ratio (c)

Power-to-heat ratio (–)

Total efficiency (%)

50 0

100

50

90 83 84 98 90 1 1 3 3 1

1.2 1 0.8 0.6 0.4 0.2 0

SC 0

RC

ICRRC

2 4 6 8 10 12 14 Compressor pressure ratio (d)

7.7 Effect of compressor pressure ratio on (a) electric efficiency, (b) specific power, (c) total efficiency and (d) power-to-heat ratio for simple cycle (SC), recuperated cycle (RC), and intercooled, recuperated and reheated cycle (ICRRC). The curve parameter is turbine inlet temperature with values of 800, 900 and 1000 °C. Increase in temperature improves all performance parameters.

design basis incorporates compromises with respect to the other. Typically, the performance of the microturbines is optimized for (or near) maximum efficiency. Increase in turbine inlet temperature increases the electric efficiency

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 157

4/5/11 9:20:57 AM

158

Small and micro combined heat and power (CHP) systems

and specific power as well as their optimum pressure ratios. However, the increase in pressure ratio is moderate except for the electric efficiency of the simple cycle. Also, the curves for efficiency and specific power are flat, and as a consequence, the impact of pressure ratio on performance remains low around the optimum. An exception here is the recuperated cycle with strong decrease in electric efficiency especially at pressure ratios lower than the optimum. Compared to the simple cycle, recuperation increases the electric efficiency and decreases the optimum pressure ratio considerably. Electric power output decreases mildly due to the pressure losses in the recuperator, thus reducing the specific power output. The intercooled, recuperated and reheated cycle offers the highest electric efficiency among the studied configurations. This is a result of several contributing factors. Intercooling reduces the compression power need while additional heat supply increases turbine power yield. As a result, the net output of the unit increases. At the same time recuperation recovers most of the additional fuel input. This type of configuration enables the efficiency levels of large industrial gas turbines (around 40%) to be reached without using high pressure ratios or advanced turbine inlet temperatures with the required cooling and material technology. The boosting nature of the power output is indicated by the superior specific power compared to the other cycles. The pressure ratio has only a mild effect on the total efficiency of all studied configurations. This is characteristic for thermal power engines in CHP operation: using fuel energy as the basis, what cannot be converted to mechanical work can be largely recovered as useful heat. All heat cannot be utilized, and the main factors here for lowering total efficiency are the stack losses that take into account the heat of the exhaust gases (relative to fuel energy) when exiting the heat recovery. In the studied range of pressure ratios, the simple cycle possesses the lowest stack losses while the intercooled, recuperated and reheated cycle has the highest. Both cycles show decreasing losses with decreasing pressure ratio. In contrast, the stack losses for the recuperated cycle increase significantly as the pressure ratio decreases, which is the result of decreasing fuel power. This is indicated by the highest variation in total efficiency among the configurations. Other factors that lower the total efficiency include those losses in the process that cannot be recovered in the working fluid as heat. Typical examples of these are losses that occur in electricity conversion. The greater share of the fuel power is obtained as electricity, the higher these losses are. However, for the studied configurations the amount of conversion losses is low compared to the stack losses and the latter governs the behaviour of the total efficiency. In general, microturbines and gas turbines consume large amounts of air with respect to their power output, and therefore the stack losses are typically higher than with the reciprocating engines or steam power plants.

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 158

4/5/11 9:20:57 AM

Microturbine systems for small CHP applications

159

The power-to-heat ratio depends on the electric efficiency and total efficiency as indicated by Equation 7.6. However, in practice its behaviour is very similar to the electric efficiency even for the recuperated cycle with the highest variation in total efficiency. The pressure ratios for maximum electric efficiency produce very accurately highest power-to-heat ratio, too. Based on Fig. 7.7, a pressure ratio of 3–4 is beneficial for high electric efficiency of the recuperated microturbine. As a consequence, commercial microturbines usually apply a single-stage radial compressor made of aluminium, which is capable of a pressure ratio up to 5 with air. Higher pressure ratios would require multi-stage compression, and this could be the case for the intercooled, recuperated and reheated cycle. For the simple, non-recuperated cycle the optimum pressure ratios are far too high for single-stage radial turbomachinery. Axial multi-stage compressors would be capable of producing high pressure ratios but they are not used in microturbines due to their low efficiency in small size, as described in Section 7.1.2. When considering the prerequisites for operation, turbine outlet temperature must also be taken into account. An increase in turbine inlet temperature and decrease in pressure ratio both increase the temperature at turbine outlet, which sets demands especially for the recuperator materials. Current cost-effective metallic recuperators allow exhaust gas temperatures up to 650 °C. For the recuperated cycle with turbine inlet temperature of 800, 900 and 1000 °C, the limiting pressure ratio below which the exhaust gas temperature exceeds 650 °C is 2.3, 3.5 and 5.3, respectively. The narrow margin for the feasible pressure ratios must be taken into account in the design and operation of the microturbines with metallic recuperators. For instance, the design pressure ratio may be selected slightly higher than the optimum without a significant loss in electric efficiency, as can be seen in Fig. 7.7. At part-load operation the pressure ratio decreases, and therefore turbine inlet temperature may be reduced to maintain the temperature above the limiting value at turbine outlet. For the intercooled, recuperated and reheated cycle, the temperature exceeds the limit through the whole range of studied pressure ratios, except for the case with the lowest turbine inlet temperature (800 °C) where the limiting pressure ratio is 6.3. Finally, it must be noted that turbomachinery efficiencies, recuperator effectiveness and various pressure losses have a strong impact on microturbine performance, too. This is due to the fact that the net output of the microturbine is essentially a difference of two large factors, compressor and turbine power. The above analysis is based on compressor polytropic efficiency of 83%, turbine polytropic efficiency of 84% and recuperator effectiveness of 90%. For instance, an individual increase of five percentage points in each parameter value at turbine inlet temperature of 900 °C would result in increasing the maximum electric efficiency of the recuperated cycle by 2.5, 2.4 and 3.2 percentage points, respectively.

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 159

4/5/11 9:20:57 AM

160

7.3

Small and micro combined heat and power (CHP) systems

Types and properties of microturbine components

7.3.1 Compressors In the compressor, the rotation work is converted to pressure with rotary and stationary blades. First, the rotor wheel accelerates the flow and the absolute velocity over the blade. During this work also the static pressure usually rises when the relative velocity over the rotor blade decreases. The stator slows down the velocity and changes a greater part of the dynamic pressure to static pressure. In a typical compressor the absolute velocities are several hundred metres per second. In an axial compressor the flow enters an annular channel axially. First there is the rotor cascade and then the stator cascade. After several rotor and stator cascades the flow enters the diffuser where the remaining kinetic energy is partially transformed into static pressure rise. The diffuser is normally an evenly expanding channel. High efficiencies are obtained by using advanced computational fluid dynamics (CFD) which is extensively used by compressor manufacturers around the world. The first computer programs for axial compressors were written in the early 1960s. Today there are quite good possibilities to achieve isentropic efficiencies close to 90% in commercial series production. In the axial compressor, the area of the channel decreases with the rise of pressure and density to keep the axial velocity of the flow near constant. Because the flow out of the compressor is axial, the whole machine is quite simple to fit to the surrounding system. In a centrifugal compressor (Fig. 7.8), the flow enters axially, but is then turned radially out from the impeller. The flow turning is the reason why it is also called the radial compressor. The static pressure rises due to the decrease in the relative velocity, but also due to the centrifugal forces. After the impeller the flow enters a radially annular diffuser in which there can be blades. Because the radius increases, the diffuser can also be made without blades (parallel diffuser), where the flow is slowed down by the free vortex flow. The high complexity of the flow, especially in the rotating impeller, makes CFD modelling challenging. The radial compressor needs a gathering body (volute) to assemble the flow to the next stage or to the outlet piping. Usually the volute is a logarithmically increasing passage that ends in a circular outlet which is easy to connect to the piping. The velocities in the volute should be kept low to keep the pressure losses small. As noted in previous chapters, the size of the microturbine is small and the best application is a recuperated cycle, where the pressure ratio is between 3 and 4. This pressure requirement is best suitable for a centrifugal compressor which can be made with a single stage unit. The axial compressor would need several stages even with lower pressure ratios and there have not been approaches to manufacture so small axial compressors.

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 160

4/5/11 9:20:57 AM

Microturbine systems for small CHP applications

161

7.8 Main parts of the centrifugal compressor. 1 impeller, 2 vaneless diffuser, 3 volute and 4 inlet guide vanes. Photo LUT Energy.

If the electric power of a gas turbine is 100 kW, the power of the compressor is close to the same magnitude. The isentropic efficiency of a centrifugal compressor can be 85%, but the smaller the compressor the lower the efficiency. The efficiency of a 100 kW compressor is around 80% and with a 10 kW compressor it is closer to 70%. As the compressor consumes as much as the electricity power is, the compressor efficiency does have a strong influence on the overall performance. The design principles of the compressors (and turbines) are based on fluid dynamics of the work done. The basic thermodynamic equations together with experimental data can quickly give rough results about the main geometry and the speed of the compressor. These methods have been used and enhanced by measuring the performance of the compressor. The complex flow phenomena with the impeller of the compressor make the accurate design more laborious. Since the early 1990s the use of CFD and development of more accurate measurement methods have contributed to improving the efficiency of the centrifugal compressor. There are currently several commercial software packages available which allow the design of a well-performing compressor.

7.3.2 Turbines Turbine design is critical in the microturbine cycle, because the power generated in the turbine determines the amount of electrical power produced

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 161

4/5/11 9:20:58 AM

162

Small and micro combined heat and power (CHP) systems

by the machine. A turbine operating at high efficiency can be the factor that makes the cycle economically viable. We consider here two main types of turbines: the axial turbine and radial inflow turbine. The choice of the turbine type depends on many factors, including manufacturing costs, flow paths of the cycle, rigidity of the shaft, leakage questions and foreign particles in the flow. The axial turbine is commonly used in the commercial applications of medium or large size turbomachines, but in small machines radial inflow turbines are more often used. Radial turbines (Fig. 7.9), are widely used with small size turbochargers as they can be mounted back to back with the impeller of the centrifugal compressor. The turbine operates in a similar way as the compressor, but the flow direction is reversed. The combustion gases enter through the volute, the flow is accelerated in the nozzle vanes and the high velocity flow rotates the rotor wheel. In this impeller, the flow coming from the radial direction is turned and the flow exits in the axial direction. The axial turbine stage has two blade cascades. The nozzle cascade is stationary and is used to increase the velocity of the flow (Fig. 7.10). The rotor cascade in turn reverts the flow velocity to work. The flow in the axial turbine is less complex than in the radial turbine, and therefore one can achieve higher efficiencies. Also foreign particles travel through the axial turbine without great problems. In the radial inflow turbine this can be a source of serious erosion, because the flow direction is inwards. There can be a situation where the flow forces thrust the foreign particles to swirl into the rotor impeller passage. The problem arises from the fact that the centrifugal force pushes the particles back out from the impeller. If the particles are stuck to circulate back and forth between the impeller and stator, there will very rapidly be serious erosion, especially in the rotating impeller blades (Wilson, 1991).

7.9 Radial turbine. 1 rotor, 2 stator nozzles and 4 volute. Photo LUT Energy.

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 162

4/5/11 9:20:58 AM

Microturbine systems for small CHP applications

163

7.10 One stage of an axial turbine. The flow enters the stationary nozzle cascade (above) and sets the rotor cascade (below) in motion. Photo LUT Energy.

7.3.3 Heat exchangers for internal heat recovery Heat exchangers are an essential part of microturbines irrespective of the cycle design. Commonly, heat exchangers are used to transfer heat from the turbine discharge flow into the power generation cycle, and in CHP operation to recover heat after the power cycle for external heating purposes. Some emerging configurations use intercoolers for cooling air between the compressor stages. We focus on the heat exchangers for internal heat recovery, recuperators and regenerators. A recuperator is a gas-to-gas heat exchanger where a significant amount of heat of the hot exhaust gas (typically around 650 °C) is transferred to the compressed air (150–200 °C) before it enters the combustor (Soares, 2007). The most important performance parameter of the recuperator is the effectiveness that proportions the actual amount of transferred heat to the theoretically maximum amount. While the use of a recuperator is a prerequisite for achieving competitive electric efficiencies, it is also a subject for techno-economic optimization: an increase in effectiveness improves the efficiency strongly, but also increases the required heat transfer surface (investment costs) and pressure losses for a given design. Because of the gas-to-gas heat transfer, the recuperator is inherently characterized by low overall heat transfer coefficient, and hence large heat transfer surface. As a consequence, it is usually the most expensive single component of the microturbine.

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 163

4/5/11 9:20:58 AM

164

Small and micro combined heat and power (CHP) systems

The design and manufacture of the recuperators is a challenging task as they operate under high pressure and temperature differentials. Also, the connections (interfaces) of the recuperator must be carefully designed to avoid unnecessary pressure losses. The design criteria for maximum heat transfer surface and minimum pressure loss in the channels direct the heat exchanger design in favour of plate heat exchangers, as the traditional tubular heat exchangers would be too massive and expensive. Current compact plate heat exchangers apply primary surface design where the flow channels are composed of a stack of formed plates. Due to their form, the plates constitute the fin effect without the additional layer of sheet metal fins between the plates as is the case with plate-fin recuperators, hence the name primary surface. Instead of brazing, the plates are pressed together to reduce thermal stresses in the structure. The primary surface design is featured by a large heat transfer surface area with respect to the heat exchanger volume, which results in compact size. The effectiveness values are up to 90% and the hydraulic diameters of the flow channels are in the order of a few millimeters. Despite the very narrow passages, fouling is typically not a problem because the combustion gases of gaseous and liquid fuels are practically particulate-free. Also, most microturbines apply oil-free bearings, which eliminate the risk of oil contamination on the hot side of the recuperator. The recuperator design has two distinct shapes, an annular one surrounding the turbomachinery/combustor and a cubic one adjacent to the unit. The working temperature of the stainless steel recuperators is limited to 650 °C and superalloys increase the range to 800 °C. Ceramic materials allow temperatures in excess of 900 °C (Acuñas et al., 2010), but their commercial use is still at an elementary stage. Instead of a recuperator, a regenerator can be used where heat transfer takes place using a disk, usually of ceramic material, that rotates slowly (compared to the microturbine shaft) and is alternately exposed to hotter and colder flows. The exhaust heat warms first the section confronting the flow and as this part turns to the air channel, the heat is transferred to the pressurized air. The two flow channels are sealed from each other to minimize leakage flow from the high-pressure (air) to low-pressure (combustion gas) side. The leakage can be further reduced by using an intermittent operation where the heat transfer matrix is stationary and airtight most of the time, but experiences a periodical movement to maintain heat transfer. Compared to the recuperator, the regenerator can provide higher effectiveness, up to 98%, smaller pressure losses and with ceramic material higher operating temperature, but it requires separate rotation machinery to spin the disc (Wilson Solarpower, 2010). Although recuperators and regenerators are essential for high electric efficiency, they reduce the useful heat output from the microturbine.

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 164

4/5/11 9:20:58 AM

Microturbine systems for small CHP applications

165

7.3.4 Combustors The role of the combustor is to provide a homogeneous combustion gas flow to the turbine with even temperature distribution (to avoid hot spots) and with small pressure loss. In a microturbine only part of the air flow is needed for the combustion process while the rest is mixed with the combustion gases to maintain the temperature of the gas mixture below limits set by the materials. Before mixing, the excess air is used to cool the combustor walls. Both single can type and annular combustors are used. While the former are typically used with separate recuperators, the latter allow for size reduction and adapt well in the integrated design with annular recuperators. The microturbines that run on gaseous fuels apply lean premixed (dry low NOx) combustion where part of the combustion air is mixed with the fuel before the combustion zone and the rest is introduced into the combustion process in stages. Compared to the conventional diffusion type combustion with fuel and air separately injected into the flame zone, this arrangement reduces local temperature peaks in the flame, thereby lowering the formation of thermal nitrous oxides (NOx), the dominating NOx formation mechanism with microturbines. However, an advanced combustor design is necessary to ensure flame stability. The combustion process in the microturbine is continuous unlike with reciprocating engines, for instance. This is favourable for the control of combustion and also for the completeness of combustion reactions. The combustion temperatures are also moderate compared to the reciprocating engines or larger gas turbines. Together with lean premixed combustion technology, these factors contribute to the very low emissions of NOx, carbon monoxide (CO) and unburnt hydrocarbons (UHC) in the current microturbines.

7.3.5 Bearings There are stringent requirements for bearings to operate reliably for long periods with high rotational speeds. Oil-lubricated bearings are widely used in turbochargers and are highly reliable, but they need a separate system for oil pumping, filtering and cooling. It is also difficult to prevent oil from penetrating into the exhaust gas, which causes fouling in the recuperator and limits the applicability of the exhaust gas in CHP operation. In microturbines this system will increase the investment and maintenance costs. Gas bearings allow the shaft to rotate on a thin layer of ambient air where friction is low and no external delivery system is needed (Larjola, 1988). Gas bearings offer oil-free operation with practically no need for service. The main disadvantage is the metal contact of the shaft and bearing during

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 165

4/5/11 9:20:58 AM

166

Small and micro combined heat and power (CHP) systems

start-ups and shutdowns, which may with an intermittent operation cycle require overhaul sooner than expected. During the last ten years especially, gas foil bearings have gained significant records with considerable amount of operating hours (Capstone, 2010a). Also active magnetic bearings (AMB) have been developed with the advances in power electronics to be a serious alternative with high speed machinery. Originally developed in the 1980s (SMM, 1984), the bearings offer contact free operation for the shaft. The bearings have two actuators for each of the two radial bearings and one actuator for the axial bearing. The magnetic forces are induced with DC current and the system uses electrical sensors to measure the position of the shaft.

7.3.6 Variable frequency drive The microturbine electric generator produces a high frequency AC current of the respective rotational speed. If the speed is 90 000 rpm (1500 rps), the frequency is 1500 Hz which is 25–30 times higher than the normal electric net frequency. This high frequency current is rectified to DC current and then converted to 50 or 60 Hz AC current. The use of power electronics is necessary, but it also brings benefits to the microturbines. The units can be operated at the best possible speed if the load changes. Moreover, as the frequency conversion is performed electronically, the turbomachinery design is not affected by the grid frequency. The industrial gas turbines operate at constant speed, and therefore they differ considerably from each other depending on the frequency of the grid.

7.4

Operation

This section focuses on the recuperated microturbine of single-shaft configuration due to its prevailing role in the market.

7.4.1 Determination of the operating point The operating point of the single-shaft microturbine is determined by the interaction between the compressor and turbine. The role of the compressor is to provide a pressure increase which subsequently causes the flow in the turbine. In practice, the pressure ratio and flow depend on the ambient conditions, rotational speed and turbine inlet temperature of the unit. The properties of the compressor and turbine can be described using four parameters: pressure ratio p, mass flow qm, rotational speed N, and efficiency h (isentropic or polytropic). The interdependencies between the parameters are usually presented using characteristics (performance maps) that are specific to each turbomachinery type and design. The presentation

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 166

4/5/11 9:20:58 AM

Microturbine systems for small CHP applications

167

of the characteristics varies, and Fig. 7.11 depicts one commonly used form. In the figure the subscripts 1 and 2 refer to the compressor inlet and outlet, respectively. Similarly, 3 and 4 refer to the turbine inlet and outlet. The parameters apply total state values where the speed of the working media is also taken into account and this is indicated by the subscript 0. The parameters N / T01 hc

p02 p01

Su

e rg

lin

e

N

/

T 01

qm T01 (a)

hT

N/

p01

T 03

qm T03 p03

N/

T 03

(b)

p03 p04

7.11 Example of (a) compressor and (b) turbine characteristics. Adapted from BorgWarner (2010).

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 167

4/5/11 9:21:00 AM

168

Small and micro combined heat and power (CHP) systems

for mass flow and speed have correction terms for the inlet pressure and temperature so the maps are applicable also for other inlet conditions than the reference case (Saravanamuttoo et al., 2008). Due to its role of increasing pressure, the compressor is susceptible to flow surge if the pressure increase becomes too high for the given speed. The limiting line is presented in Fig. 7.11 as the surge line. Surging is an unstable process with cyclic flow separation, delivery pressure collapse, flow reversal and reattachment. In addition to collapsing the mass flow and pressure delivery, surging strains or even damages the structure, and must therefore be avoided. To ensure stable operation, a safety margin is applied against surging conditions. At high enough mass flow rate the constant speed parameter lines start to descend steeply. This indicates approaching the choking of the flow. At choked conditions the flow reaches the sonic velocity in the compressor and thereby the maximum flow rate for a given speed. Choking is not harmful for the compressor, but the pressure delivery and efficiency of operation decrease strongly. For the turbine, a wider range for pressure ratio and mass flow is available than plotted in the figure for a given speed. However, the limited presentation is convenient for microturbine use where the compressor and turbine are interconnected. In this case the pressure ratio and mass flow rate typically both increase with increasing speed. For a single-shaft configuration, the turbine mass flow data can be presented on the compressor map, and the operating point is determined by the intersection of the compressor and turbine curves. This enables the effect of ambient conditions and control methods on the transition of the operating point and, for instance, the surge risk to be studied graphically. Figure 7.12 is a schematic presentation of the resulting constant parameter curves, showing the principal directions for the operating point as a result of reducing single parameters of T01, T03 and N. The figure shows that a decrease in ambient temperature moves the operating point closer to the surge area, while decreasing turbine inlet temperature has an adverse effect. Changes in rotational speed move the operating point somewhat parallel to the surge line. The efficiency of the compressor is typically maintained best in the direction of the speed change.

7.4.2 Effect of ambient conditions An open microturbine takes in surrounding air and therefore any changes in ambient conditions (temperature, pressure, humidity) affect the performance of the unit. Temperature, in particular, has a pronounced effect. An increase in ambient temperature decreases the mass flow through the microturbine, which has a negative effect on the net output. At the same time compressor

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 168

4/5/11 9:21:00 AM

Microturbine systems for small CHP applications

Surge line

T0

169

3 /T 01

Reduction of p02/p01

T01 T03

N

/√

T

01

N

qm√T01/p01

7.12 Effect of single parameter change on the operating point.

Relative value, 100% = design

110%

100%

90%

80% Power Efficiency 70% –30

–20

–10

0 10 20 Temperature (°C)

30

40

50

7.13 Effect of ambient temperature on power output and electric efficiency at nominal load (relative values). Adapted from Capstone (2010a).

discharge temperature is increased, thus impairing the prerequisites for the recuperator. As a result, generating efficiency is also decreased. A decrease in ambient temperature results in increasing output and efficiency. Depending on the capacity of the components (power electronics, for instance), power output may be limited below a certain temperature. Figure 7.13 presents the typical effect that ambient temperature has on the microturbine performance in this case. The power output and efficiency are given relative to the corresponding design values.

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 169

4/5/11 9:21:00 AM

170

Small and micro combined heat and power (CHP) systems

Ambient pressure, or altitude, also has an effect on the performance, but compared to temperature, it is of minor importance for most applications. As an example, an increase of 1000 m in altitude (decrease of 115 mbar in pressure) decreases the power output by 11% (EEA, 2008).

7.4.3 Control methods When the microturbine is connected parallel to the electricity grid, the control system typically aims at following the heat load. The reason for this is that the heat demand must be satisfied locally while electricity deficit/surplus can be balanced out using the network. In grid-independent operation electric load must be followed to prevent any imbalance. Surplus heat can easily be discharged to the atmosphere in the form of exhaust gases, but possible deficit must be covered by a supplementary system, such as a parallel boiler. Thermal storage is frequently used to compensate the diversity of supply and demand for both operational modes. The output level (electric or heat) of the microturbine is reduced by lowering the rotational speed of the shaft. At the same time turbine inlet temperature is reduced to prevent temperature increase of the combustion gases at the recuperator inlet. As can be seen in Fig. 7.12, this affects the operating point of the microturbine, and consequently the mass flow rates, pressure levels and component efficiencies. The main contributing factor for output reduction is the decreased mass flow rather than heat supply temperature, and therefore this control mode maintains power generation efficiency at part load better than the conventional method with turbine inlet temperature as the only control parameter. However, efficiency reduction cannot be avoided using this control mode either. Figure 7.14 presents the typical behaviour of electric efficiency at part load, with the performance parameters given relative to the corresponding design values. The strong decrease in electric efficiency contributes to the heat output, but in practice heat transfer rate decreases. This is due mainly to the decreased exhaust mass flow rate. As a result, the system experiences a slight reduction in total efficiency, too.

7.4.4 Emissions The main pollutants from the microturbines are NOx, CO and UHC. While NOx is the result of thermal NOx formation, CO and UHC are indicative of incomplete combustion. Microturbines that operate on gaseous fuels apply lean premixed combustion which is one of the key factors for low NOx emissions. At full load and using natural gas as fuel, the guaranteed emission levels (at 15% O2) are below 15 ppmv and in some cases even below 9 ppmv, with the lowest figure of 4 ppmv. The UHC emissions are of the same magnitude but CO emissions

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 170

4/5/11 9:21:00 AM

Relative electric efficiency, 100% = design

Microturbine systems for small CHP applications

171

100%

80%

60%

40%

20%

0%

0

20

40

60 Load (%)

80

100

7.14 Effect of electric load on electric efficiency (relative values). Adapted from EEA (2008).

are somewhat higher, up to 40 ppmv (EEA, 2008; Capstone, 2010a; Turbec, 2010). This is attributable to the use of lean premixed technology that has a tendency to increase CO emissions. At part load all emissions typically increase. The noise in microturbines is generated primarily internally by flow vortices, blading, magnetic fields and combustion (Kolanowski, 2004). The current microturbine technology with a recuperator and contactless bearings effectively attenuate the transmission of noise through exhaust gases and microturbine casing. As a result, the noise levels are reasonably low, at full load the acoustic emission at 10 m distance is typically 65–70 dBA. An additional sound insulation in the inlet channel can be used to further decrease noise (Capstone, 2010a; Turbec, 2010).

7.4.5 Other operational viewpoints Microturbines operate best with premium fuels such as natural gas, liquefied petroleum gas (LPG), diesel and kerosene. In case of low-grade fuel gases such as biogas or industrial waste gases, the fuel often contains acid gas components, oils or particulates that may require fuel pre-treatment (EEA, 2008). The high-speed generator of the microturbine can be used as a motor, which makes the start-up of a single shaft microturbine very straightforward: the motor starts to rotate the shaft and increases the speed until compressor delivery reaches the conditions for self-sustaining combustion. Fuel is introduced in the airflow and the mixture is ignited. After this fuel power

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 171

4/5/11 9:21:00 AM

172

Small and micro combined heat and power (CHP) systems

is increased together with the speed of the shaft (now controlled by the generator) until nominal output is achieved. In grid-independent operation, a battery is used as power storage for start-up. Microturbines have relatively short start-up and shutdown times, with full power or complete stop in a couple of minutes after initiating the sequence. At start-up, power delivery begins in around 30 s (Kolanowski, 2004). One significant feature of the variable frequency drive is that during short periods of no-load, the speed can be lowered to the idle level where combustion still occurs but no net electricity is generated. From this operation mode power can be increased very quickly. Avoiding bringing the shaft to a halt also increases the life of the bearings, as the stopping and starting pose the most wearing conditions for both gas and magnetic bearings. Microturbines can be operated in a variety of ways. They can run in parallel with the electric grid or as a stand-alone system, following the electric or heat load. Single units can be interconnected to increase capacity or reliability. The engines are equipped with control and protection systems that allow remote operation and interconnection as well as monitoring/diagnostics of the units. The single-shaft microturbines feature essentially only one rotating part. Together with gas or magnetic bearing technology, this provides the possibility for low maintenance needs together with high reliability and life of the units. A typical preventive maintenance schedule includes periodic inspection and minor replacements with the major overhaul due every 30  000 to 40  000 turbine run hours. In the overhaul the shaft with the attached components, and possibly the combustor are replaced (Capstone, 2010a; Turbec, 2010). The life estimates for the main parts vary between 40 000 and 80 000 hours, but due to the late emergence of the microturbines, these estimates are currently being tested in real operating conditions.

7.5

Manufacturers and applications

The market for microturbines is still in the developmental stage. The largest manufacturer, Capstone Inc., has sold 5000 units (Capstone, 2010b). Although the figures also include units above 100 kW, the total power is less than 500 MW, which is equivalent to one to two large industrial gas turbines. There have also been some changes among the manufacturers. Calnetix Power Solutions (a company based in California, USA) acquired in 2007 Elliott Energy Systems (a subsidiary company of Ebara Corporation of Tokyo, Japan) that was manufacturing 100 kW microturbines and in 2010 Capstone (California, USA) acquired the TA 100 microturbine production line from Calnetix.

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 172

4/5/11 9:21:00 AM

Microturbine systems for small CHP applications

173

7.5.1 Capstone Capstone Turbine Corporation is one of the world’s leading manufacturers of microturbines, and was first on the market with commercially viable air bearing turbine technology. The company has shipped thousands of microturbines to customers worldwide and the systems have logged millions of documented runtime operating hours. Capstone makes various sizes of microturbines: 30, 65 and 200 kW. Products based on the three turbine models are also available in configurations between 100 and 1000 kW. Capstone is predicted to sell annually around 700 units below 100 kW in the coming years (Soares, 2008). The Capstone microturbine comprises a single-stage centrifugal compressor, a radial inflow turbine and a recuperator (Fig. 7.15). It can be operated on natural gas, biogas, flare gas, diesel, propane and kerosene. The microturbines can be used for producing electric power only or as a CHP unit or also providing cooling (Capstone, 2010a).

7.5.2 Turbec Turbec has its roots in the development of small gas turbines for automotive use in the early 1970s, where a permanent magnet high-speed generator was designed (Malmquist, 1988). Turbec was founded in 1998 and has its head office in Italy. The first commercial T100 unit was delivered in 2000. The microturbine comprises a single-stage centrifugal compressor, a radial inflow turbine and a recuperator. The compressor pressure ratio is 4.5 (Turbec, 2010). The microturbine uses a combustor that can run on various fuels such as natural gas, diesel, ethanol and biogas. The electrical efficiency using natural gas is 33% with a 1 percentage point uncertainty. The shipped units

7.15 Capstone microturbine. Air enters the compressor around the generator and exits into the recuperator on the outer periphery. Warmed air is led to the combustor, combustion gases rotate the turbine and exhaust gases exit through the secondary side of the recuperator (NASA, 2003).

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 173

4/5/11 9:21:00 AM

174

Small and micro combined heat and power (CHP) systems

of Turbec microturbines have more than 3 million operating hours (Turbec, 2010). Annual sales are predicted to be around 100 units (Soares, 2008).

7.6

Future trends

7.6.1 Externally-fired microturbines Biofuels are especially suitable for distributed energy systems, as their transportation is often economically feasible only over short distances. Microturbines can easily utilize biogas but interest is currently focusing on direct biomass combustion too, due to simple and widespread combustion technology. Taking the combustion gases directly to the turbine poses a challenge for operation and maintenance because of the fouling nature of the combustion products. Therefore, there have been many suggestions to use externally-fired microturbines (EFMT) where the heat is transferred to the power cycle by using a heat exchanger (Fig. 7.16). The heat exchanger inevitably increases the investment costs and sets limitations on the heat supply temperature, but the benefit is that the outlet flow from the microturbine is hot air. When using gas or magnetic bearings and thus having no risk of oil-contamination, this air can be used directly for space heating, for instance. One of the first examples is Talbott’s Heating (Pritchard, 2002, 2005) who has developed and field-tested a 100 kW EFMT unit with 17% electric efficiency. Another demonstration is by Compower (Malmquist, 2006)

HRHE

REC

HE C

T

G

7.16 Flow diagram of an externally-fired microturbine cycle. C compressor, T turbine, G generator, REC recuperator, HE heat exchanger and HRHE heat recovery heat exchanger.

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 174

4/5/11 9:21:01 AM

Microturbine systems for small CHP applications

175

who has focused on smaller, under 15 kW scale. Currently most major microturbine manufacturers have expressed their interest in and target to introduce externally-fired microturbine versions into the market.

7.6.2 Fuel cells and microturbines Studies with fuel cell technology combined with a gas turbine cycle are promising very high electric efficiencies compared to conventional gas turbine cycles (Backman et al., 2004). The development of the fuel cell technology has been slower than expected and the integration of gas turbines is still at an elementary stage. With the solid oxide fuel cell (SOFC), the exhaust gas temperature is as high as 800 °C, which is suitable for a recuperated gas turbine cycle. The recent approaches include a dynamic performance analysis to use the Turbec T100 recuperator with a fuel cell set-up (Ferrari et al., 2010). The fuel cell may either be operated at ambient pressure or pressurized, which determines the method of integration. In many designs the fuel cell is pressurized with a pressure of around 3 bars, which suits well the recuperated gas turbine cycle (Figs 7.1(b) and Fig. 7.17). If the fuel cell operates at ambient pressure, the gas turbine can be integrated using the external heat source as shown in Fig. 7.16, where the hot exhaust SOFC/Gas turbine Natural gas Recuperator Exhaust

Fuel cell Anode Cathode Inverter DC AC DC C

AC

High speed generator 60000 rpm Gas turbine

T

Direct connection

Air

7.17 Gas turbine integrated to a fuel cell (Backman et al., 2004).

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 175

4/5/11 9:21:01 AM

176

Small and micro combined heat and power (CHP) systems

gases from the fuel cell are heating the compressed air entering the turbine. This additional heat exchanger lowers the electric efficiency compared to the pressurized operation, but the air entering the turbine is clean and there are no significant erosion problems.

7.6.3 Pushing the envelope There are projects within various research institutes to design and construct small microturbines. Provided they could be operated with a reasonable efficiency and sold at an affordable price, there could be a breakthrough for household microturbines. Micro Turbine Technology MTT b.v. in the Netherlands is developing a recuperated microturbine CHP concept that will provide 3 kW electrical and 15 kW thermal power for consumer as well as small and medium enterprise markets (MTT, 2010). It has been demonstrated to give 2.7 kW electricity at 12.2% electrical efficiency (Visser et al., 2010). Further development is currently focused on at least 16% efficiency and the researchers believe that the technology could reach efficiencies beyond 20%. Researchers at the Research Center of Ceramic Engines in Saint Petersburg have developed a new turbo machinery, the tunnel turbine made of ceramics. Their design at the smallest size is 200 W (Soudarev et al., 2008). MIT has conducted research on micro-electrical-mechanical systems (MEMS) based microturbines (Epstein, 2003). Due to the small size, the efficiency is around 6%.

7.7

Sources of further information and advice

Compressor and turbine design software is now widely available as commercial products, but their use does require good knowledge of fluid mechanics and turbomachinery. Below are listed examples of well-known software providers. Advanced Design Technology: http://www.adtechnology.co.uk/products/ ANSYS CFX: http://www.ansys.com/products/fluid-dynamics/cfx/ CFTurbo: http://www.cfturbo.de/en/ Concepts NREC: http://www.conceptsnrec.com/ ANSYS FLUENT: http://www.fluent.com/ PCA Engineers Limited: http://www.pcaeng.co.uk/ SoftInWay: http://www.softinway.com/

7.8

References

Acuñas A, Huete J and Amallobieta I (2010), ‘Recent Advances in High Temperature Ceramic Regenerators for Externally Fired Gas Turbines. Theoretical and Experimental

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 176

4/5/11 9:21:01 AM

Microturbine systems for small CHP applications

177

Results’, ASME Paper GT2010-23806, ASME Turbo Expo 2010, Glasgow, United Kingdom. ATT, Agile Turbine Technology (2010), Publications. Available from: http://www.agileturbine.com/publications/index.html. Backman J, Reunanen A, Esa H, Punnonen P, Honkatukia J and Larjola J (2004), ‘Concept Design of a Solid Oxide Fuel Cell Gas Turbine’, 3rd International Conference on Heat Powered Cycles, 11–13 Oct 2004, Larnaca, Cyprus. Balje O E (1981), Turbomachines, A Guide to Design, Selection and Theory, John Wiley & Sons, New York. Baskharone E (2006), Principles of Turbomachinery in Air-Breathing Engines, Cambridge University Press, Cambridge. BorgWarner (2010), Turbo-Facts. Available from: http://www.turbodriven.com/en/turbofacts/designAndFunction.aspx. Boyce M P (2006), Gas Turbine Engineering Handbook, 3rd edition, Gulf Professional Publishing, Oxford. Capstone (2010a), Products. Available from: http://www.capstoneturbine.com/prodsol/products/. Capstone (2010b), News. Available from: http://www.capstoneturbine.com/ news/story.asp?id=573. Casey M (1985), ‘The effect of Reynolds number on the efficiency of centrifugal compressor stages’, Journal of Engineering for Gas Turbines and Power, 107, 541–548. Dong L, Liu H and Riffat S (2009), ‘Development of small-scale and micro-scale biomass-fuelled CHP systems – a literature review’, Applied Thermal Engineering, 29, 2119–2126. EEA, Energy and Environmental Analysis (2008), Technology Characterization: Microturbines. Available from: http://www.epa.gov/chp/documents/catalog_chptech_ microturbines.pdf. Epstein A (2003), Millimeter-Scale, MEMS Gas Turbine Engines, ASME Paper GT200338866, ASME Turbo Expo 2003, Atlanta, USA. Ferrari M, Sorce A, Pascenti M and Massardo A (2010), Experimental Investigation of the Dynamic Performance of a Micro Gas Turbine Recuperator including Innovative Cycle Configurations, ASME Paper GT2010-22299, ASME Turbo Expo 2010, Glasgow, United Kingdom. Galanti L and Massardo A (2010), Thermoeconomic Analysis of Micro Gas Turbine Design in the Range 25–500 kWe, ASME Paper GT2010-22351, ASME Turbo Expo 2010, Glasgow, United Kingdom. Kolanowski B F (2004), Guide to microturbines, Fairmont Press, Lilburn, GA. Larjola J (1988), Basic properties of gas lubricated, tilting-pad journal bearing, Conference on High Speed Technology, 21–28 August, Lappeenranta, Finland. Malmquist A (1988), Conference on High Speed Technology, 21–28 August, Lappeenranta, Finland. Malmquist A (2006), Swedish Micro-CHP Solution – ‘Externally Fired Microturbine System’, 2nd World Pellets Conference, Jönköping, Sweden, 30 May–1 June. Moran M J and Shapiro H N (2010), Fundamentals of Engineering Thermodynamics, 6th edition, Wiley, Hoboken, NJ. MTT, Micro Turbine Technology (2010), Available from: http://www.mtt-eu.com/. NASA (2003), Research & Technology reports. Available from: http://www.grc.nasa.gov/WWW/RT/RT2002/5000/5960weaver.html. Pritchard D (2002), Biomass Combustion Gas Turbine CHP, ETSU B/U1/00679/00/

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 177

4/5/11 9:21:01 AM

178

Small and micro combined heat and power (CHP) systems

REP. Available from: http://webarchive.nationalarchives.gov.uk/+/http://www.berr. gov.uk/files/file14922.pdf. Pritchard D (2005), Biomass Fuelled Indirect Fired Micro Turbine, B/T1/00790/00/00/ REP, DTI/Pub URN 05/698. Available from: http://webarchive.nationalarchives.gov.uk/tna/+/http://www.dti.gov.uk/renewables/ publications/pdfs/bt100790.pdf/. Saravanamuttoo H I H, Rogers G F C, Cohen H and Straznicky P V (2008), Gas Turbine Theory, 6th edition, Pearson Prentice Hall, Upper Saddle River, NJ. Siemens (2010), Siemens Gas Turbines. Available from: http://www.energy.siemens.com/us/en/power-generation/gas-turbines/. SMM, Société de Mécanique Magnétique (1984), Application of Active Magnetic Bearings in the Machine Tool Industry, Vernon Cedex, France. Soares C (2007), Microturbines: Applications for Distributed Energy Systems, Elsevier Butterworth-Heinemann. Soares C (2008), Gas Turbines: A Handbook of Air, Land, and Sea Applications, Elsevier Butterworth-Heinemann, Maryland Heights, MO. Soudarev A, Konakov V, Morozov N, Ovidko I and Semenov B (2008), Novel ShrinkageFree Structural Ceramic Materials for Gas Turbine Applications, ASME Paper GT2008-50549, ASME Turbo Expo 2008, Berlin, Germany. Turbec (2010), Products. Available from: http://www.turbec.com/products/products. htm. Visser W, Shakariyants S and Oostween M (2010), Development of a 3 kW Micro Turbine for CHP Applications, ASME Paper GT2010-22007, ASME Turbo Expo 2010, Glasgow, United Kingdom. Wilson D G (1991), The Design of Gas Turbine Engines, Thermodynamics and Aerodynamics, IGTI/ASME. Atlanta, GA. Wilson Solarpower (2010), Wilson Heat Exchanger. Available from: http://www.wilsonturbopower.com/index.php/nonsolarapps/heatexchanger.html.

© Woodhead Publishing Limited, 2011

CHP-Beith-07.indd 178

4/5/11 9:21:01 AM

8

Stirling engine systems for small and micro combined heat and power (CHP) applications J. H a r r i s o n, E. ON Engineering, UK

Abstract: Stirling engines have for many years been considered as the most promising technology for micro-CHP applications. As external combustion engines permitting close control of the combustion process, their characteristics of low emissions, high efficiency, reliability, extended service intervals, low noise and vibration levels are all well suited to the demands of micro-CHP in individual homes. This chapter describes the fundamentals of the Stirling cycle and explains how the technology meets the needs of, primarily, existing homes with reference to leading Stirling engine products either commercially available or under development and provides a view on future developments. Key words: Stirling engine, Carnot efficiency, micro-CHP, domestic energy systems, microgeneration.

8.1

Introduction

As has been explained previously, micro-CHP is not just a scaled down version of conventional CHP technology. The technical, environmental and economic challenges are significantly more onerous. Micro-CHP is seen as a ‘drop-in’ replacement for gas boilers in central heating systems; it must therefore perform at least as well as the technology it attempts to displace. This implies that micro-CHP must achieve long life, long service intervals, low noise and vibration, low emissions and, to recover the additional investment, high efficiency. Stirling engines are, at least in theory, well suited to fulfil these requirements and the majority of true micro-CHP systems are currently based on external combustion technology as their characteristics are best suited to this stationary, constant running application. This chapter will describe the fundamentals of the Stirling engine and discuss how the characteristics of the technology and specific Stirling-based products aim to provide a cost effective solution to the challenges of domestic CHP. It will also outline the history of Stirling engine development and provide a view on likely development trends.

179 © Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 179

4/5/11 9:22:34 AM

180

8.2

Small and micro combined heat and power (CHP) systems

Definition of a Stirling engine

The Stirling engine is an external combustion engine, a type of engine which allows continuous, controlled combustion potentially resulting in very low pollutant emissions and high combustion efficiency. It can operate without valves or an ignition system, thus permitting long service intervals and low running costs. External combustion engines separate the combustion process (which is the energy input to the engine) from the working gas, which undergoes pressure fluctuations and hence does useful work. As the combustion process is used to provide a continuous heat input to the working gas, it is more controllable and potentially more efficient, cleaner and quieter than internal combustion engines. External combustion engines also have the potential for long life and service intervals similar to the annual maintenance of a gas boiler. In its simplest form the Stirling engine comprises cylinder, regenerator, piston and displacer as shown in the Fig. 8.1. Fuel is burned continuously outside the engine to maintain one end of the cylinder at high temperature while the opposite end is cooled by circulating water around it. Power is derived from the pressure fluctuations acting on the working piston, as a fixed volume of working gas (sealed within the engine) is alternately heated and cooled, forcing it back and forth between the two temperature zones via

Heat Regenerator Displacer Seal Cooling water in

Cooling water out Working piston Piston connecting rod Displacer connecting rod

8.1 Schematic of a Stirling engine.

© Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 180

4/5/11 9:22:34 AM

Stirling engine systems for small and micro CHP applications

181

the regenerator. The working gas is moved by the displacer, which is 90º in advance of the working piston. The sinusoidal waveform of the power output results in low vibration and noise levels. Thermal efficiency is enhanced by the regenerator, a heavy matrix of fine wires that acts as a repository for heat extracted from the working gas during the cooling pass to be returned on the heating pass. One significant difference between ICE (internal combustion engines) and Stirling engines is that, in an ICE, it is possible to adjust power virtually instantaneously by controlling the fuel supply. This makes ICE ideal for automotive applications where rapid variations in power are required. However, there is a significant time delay between fuel input and power output in a Stirling engine, as there is usually a substantial amount of heat stored in the hot end, which continues to transfer energy to the working gas long after the fuel supply to the burner is cut. Although this is not a concern in stationary applications, which do not require instantaneous power variation, it is a consideration for control that there is a delay of the order of minutes between a thermostat calling for heat, the availability of heat and finally the output of power. In addition, the stored energy must be passed to the heat distribution system before the engine shuts down at the end of a heating cycle, both to avoid wasting useful thermal energy and to avoid potential damage to the engine itself. All Stirling engines have two pistons (conceptually), one of which (known as a ‘displacer’), shuttles the working gas between the hot and cold zones whilst the other (known as the ‘working’ piston) is subject to the resulting pressure changes and does work to drive the engine. Heat is applied to the hot end of the engine by, for example, a gas burner and is removed from the cold end by means of a flow of cooling water, typically the water in a central heating system. The hot end can be as high as 800 °C and the cold end around 80 °C. The displacer shuttles the working gas between the hot end and cold end of the piston without doing any work. The resulting alternate heating and cooling of the working gas causes pressure changes which in turn exert force on the working piston which drives a generator through the crank mechanism.

8.3

Why Stirling engines are suited to micro combined heat and power (CHP)

In most texts it is stated that the key characteristics of Stirling engines which align with the requirements for micro-CHP applications are long life, long service intervals, high efficiency, low noise and vibration and low emissions. Whilst these are indeed key characteristics, the reality in execution is rather more complex.

© Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 181

4/5/11 9:22:34 AM

182

Small and micro combined heat and power (CHP) systems

8.3.1 Long life and extended service intervals The requirement for long life relates to the very long operating periods required of stationary heating systems (typically around 20 000–30 000 hours for the typical 10-year life expectancy of a central heating boiler) around ten times that of a typical automotive IC engine. As far as service intervals are concerned, it is not just the cost of the service, but the frequency and intrusiveness of a service that is important; it would, for example, not be acceptable to have to undertake an oil change every 200 hours (less than a fortnight) equivalent to the 10 000 mile service intervals common to car engines. As an external combustion engine, with a continuous combustion process there is no need for complex valving, timing gear, spark ignition and the many other service limiting components characteristic of automotive IC engines. However, the need to avoid oil migration into the upper cylinder and the consequent dry lubrication approach mean that seals and bearings are subjected to severe operating conditions. So far the WhisperGen is the only Stirling micro-CHP system to have demonstrated significant numbers of systems in the field demonstrating lifetimes in excess of 20 000 hours together with the requisite annual service intervals. It remains to be seen whether the other Stirling products approaching market which are higher efficiency and higher stressed, will achieve the same service characteristics.

8.3.2 Noise and vibration Although the noise and vibration of Stirling engines is very much less than that of IC engines, it is extremely challenging to produce any reciprocating engine with a noise level acceptable in the occupied space of a home operating more or less continuously during the heating season. As the name suggests, the WhisperGen unit was considered relatively silent in its original application as a marine APU where the alternative would be a conventional diesel IC engine. In domestic micro-CHP applications the early WhisperGen products performed more than adequately for installation in utility rooms and garages, but were only installed in the occupied space within acoustic enclosures which performed satisfactorily when properly constructed. However, when installed in inappropriate locations or without adequate acoustic isolation, a number of users found the background noise disturbing. Unfortunately noise is a very subjective phenomenon and current guidance tends to err on the side of caution in recommending installation in utility rooms even though the sound level has been significantly reduced and is not significantly different from a conventional boiler. The wall-mounted MEC LFPSE derivatives with their characteristic 50 Hz hum are believed to suffer from noise transmission through the supporting structure and pipework

© Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 182

4/5/11 9:22:34 AM

Stirling engine systems for small and micro CHP applications

183

with unpredictable noise breakout within the home, although customers seem to find the overall noise level generally acceptable. It can therefore be concluded that, whilst there is still some way to go in producing Stirling engines with completely unobtrusive noise levels, current products are within the boundaries of customer expectations and acceptability.

8.3.3 Low emissions One of the primary benefits of micro-CHP is its ability to reduce overall GHG (greenhouse gas) emissions. The effective CO2 emissions associated with the production of electricity are equivalent to the lost opportunity cost of heat. In other words, electricity is produced at an effective rate of 0.22 kgCO2/kWh; this is a significant saving compared with the displaced grid electricity supply (0.568 kgCO2/kWh) with regard to GHG emissions. However, the very high temperatures in Stirling burners tend to produce higher levels of NOx than gas boilers which can operate adequately with lower flame temperatures. In the event that legislation is introduced in this area, NOx emissions could be mitigated, for example, by means of exhaust gas recirculation (EGR) without the need for catalytic converters which would be required for ICE products.

8.3.4 High efficiency Although, as will be discussed later, electrical efficiency is not necessarily the determining factor in assessing the commercial viability of a micro-CHP product, the higher the electrical efficiency (provided it is not achieved at the expense of total efficiency) is of significant benefit as it is the value of electricity produced which justifies the investment in micro-CHP. The high efficiency theoretically achievable with Stirling engines,1 although demonstrated in laboratory prototypes, has yet to materialise in mass market products. It appears that delivery of very high electrical efficiency imposes economic and production challenges which has meant that all domestic scale units so far at or near market have electrical efficiencies of less than 15%. However, it can be seen from the discussion below that significantly higher efficiencies are readily achievable and it is likely that Stirling engines for domestic CHP applications will continue to develop for some time, just as ICE engines have done in automotive applications.

8.4

The Stirling cycle

In a Stirling engine there is a continuous input of heat to the hot end of the cylinder, together with a continuous cooling of the cold end of the cylinder.

© Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 183

4/5/11 9:22:35 AM

184

Small and micro combined heat and power (CHP) systems

In micro-CHP applications, the hot end is heated by a very high temperature fuel-fired burner (typically natural gas), whilst cooling is provided by the primary circuit in a conventional, hydronic (water-based) central heating system. In other words, the heat from the gas burner is transferred to the central heating flow to the radiators through the Stirling cycle in the engine, rather than directly through a conventional heat exchanger as would be found in a gas boiler. As the Stirling engine is an external combustion engine, the gas within the pressurised cylinder is completely sealed from the atmosphere and the combustion process. It can be seen from the sectional diagram that heat can only be transferred from the burner to the working gas by conduction through the (normally finned) walls of the cylinder, one of the most challenging constraints of the Stirling engine as it is very difficult to transfer heat to the working gas or indeed to extract it given the relatively small surface areas available for heat exchange. The function of the regenerator is to optimise the efficiency of the cycle and will be described in more detail later. The PV (pressure/volume) diagram in Fig. 8.2 shows the theoretical thermal cycle of a Stirling engine, beginning at point 1. At this point the working gas is at its maximum volume and minimum temperature and is contained within the cold end of the cylinder. A displacer shuttles the gas into the hot end of the cylinder without doing any work and without any temperature increase. In reality, of course, work has to be done to compress the cold gas into the hot end space and inevitably the temperature does rise before reaching point 2. The practical implications of this are discussed later. At point 2, heat is added to the gas and both temperature and pressure rise towards point 3. From point 3 to point 4, the hot gas expands, exerting

P (pressure)

Vmin

Vmax

3 4

Tmax

2 1

Tmin

V(volume)

8.2 Theoretical Stirling cycle.

© Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 184

4/5/11 9:22:35 AM

Stirling engine systems for small and micro CHP applications

185

a force on the working piston. It is this stage that produces useful power from the engine and is known as the power stroke. At point 4 the gas is at its maximum volume and is then shuttled back to the cold end where it is cooled, ready to commence the next cycle. In theory the work done by the engine is illustrated by the shaded area between the two isotherms and vertical lines of constant volume.

8.4.1 Efficiency and other performance constraints The diagram and cycle described above illustrate an idealised and greatly simplified process. In reality there are inevitable compromises in terms of practical design and construction. Even in this idealised diagram it can be seen that the regenerator has a certain volume which, by definition, cannot be part of the swept volume; its volume is referred to as ‘dead volume’. The WhisperGen engine, for example, is a double acting design in which each of the four cylinders is linked to its neighbour with a pipe running from the hot end of one to the cold end of the adjacent cylinder; the gas contained within these pipes also constitutes dead volume. Thus, in practice, the actual thermal cycle in a typical Stirling engine is more accurately represented by the central dark elliptical shape in the middle of the graph shown in Fig. 8.3. As dead volume is reduced, the true cycle tends towards the larger (pale) ellipse which extends to the isothermal boundaries of the ideal cycle. This can be achieved for flat plate exchangers, but these tend to have very low specific power and are of little practical use for micro-CHP applications. Also, as was mentioned earlier, the gas does not actually undergo distinct

Vmax

P (pressure)

Vmin

3 4

Tmax

1

Tmin

2

V (volume)

8.3 Stirling cycle constraints.

© Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 185

4/5/11 9:22:35 AM

186

Small and micro combined heat and power (CHP) systems

steps, but more a gradual change from one stage to another. Only in the event of discontinuous motion of the pistons can the ideal cycle be approached.2 In this case the oval is stretched as shown by the arrows; in practice, this can be imitated by complex mechanical linkages or, in the case of the linear engine, quite possibly by means of electronic actuation of the displacer at least. The theoretical efficiency of any heat engine is limited, according to Carnot’s principle, by the relative temperatures of the hot and cold ends of the engine. To achieve maximum efficiency, it is desirable to achieve as high as possible a hot end temperature and as low as possible a cold end temperature. In practice, the maximum temperature is constrained by the availability of suitable materials to working temperatures of around 800 s°C. It should be borne in mind that the temperature of the working gas will always be lower than the material required to conduct heat to it, and that the flame temperature will be even higher than that of the conducting material. Generally high temperature alloys such as Inconel are used which are reasonably good conductors, able to withstand repeated thermal cycling and very high pressures and which can be worked albeit with some difficulty and at high cost. The cold end temperature is constrained simply by the temperature at which the ‘waste’ heat (actually the useful heat as far as the domestic heating system is concerned) is required. Some modern heating systems have oversized radiators or use underfloor coils and operate at relatively low temperatures (30–40 °C); as with heat pumps the efficiency of microCHP systems is enhanced in this application. However, one of the main attractions of micro-CHP is that it can be used as a ‘drop-in’ replacement for existing homes, the majority of which (in the UK at least) are fitted with radiators designed for higher water temperatures (60–80 °C). In this case, the efficiency is marginally reduced as the DT is reduced from, say 760 °C (=800–40 °C) to 720 °C (=800–80 °C). In some instances, efficiencies quoted for Stirling engines under development in laboratory conditions are based on a cold end temperature which, although technically feasible, often using cold mains water, is of little relevance to their likely performance in the field. The choice of working gas significantly influences efficiency; ideally the gas should have a low specific heat such that a given amount of heat input produces the maximum possible pressure increase. Both hydrogen and helium are well suited to this application although both are very ‘leaky’, i.e. difficult to contain; hydrogen actually leaks through the metal of the engine housing. For this reason the highest efficiency Stirling engines tend to use helium as a working gas, although nitrogen is an effective compromise used in the WhisperGen engine, for example. The regenerator, generally some kind of matrix of fine wires, serves to

© Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 186

4/5/11 9:22:35 AM

Stirling engine systems for small and micro CHP applications

187

store heat between the heating and cooling phases of the cycle; the more efficiently the regenerator can capture the heat, store it and transfer it back to the gas in each cycle, the higher, potentially, the efficiency of the engine. The Carnot efficiency limit referred to above can only be even theoretically achieved when the regenerator has an efficiency of 100%. It might therefore appear that a large, high capacity regenerator should be used in all cases to maximise efficiency. However, the volume of the regenerator comprises dead volume, and the matrix presents a parasitic pressure drop to the cycle, so there are practical compromises which need to be accepted. In some higher efficiency engines, multi-stage regenerators are used with varying matrix spacing between the hot and cold ends optimised to the flow of expanding or contracting gas. In addition to the unavoidable compromises to the theoretically achievable efficiency of the engine design, there are further losses caused by pumping losses (in the movement of the working gas through pipes, etc.), frictional losses in the bearings and drive mechanism as well as the frictional loss in the gas seals. This latter is a key challenge to all Stirling engines, but is particularly onerous for engines using helium or other leaky gases. It is one of the reasons that Stirling engines require such high precision manufacturing tolerances to ensure that seals function effectively, but without exerting excessive frictional forces on the cylinder wall. It can be seen from Fig. 8.2 that, where the work done in each cycle is indicated by the shaded area, then that area can be enlarged by increasing the swept volume (the difference between Vmin and Vmax), by increasing the pressure range, by increasing the temperature range (Tmax – Tmin), or any combination of these factors. Increasing the working temperature and pressure will result in a higher power density, whereas a larger volume simply represents an engine with a higher cubic capacity, just like in an IC engine. Whilst not all of these parameters explicitly increase the efficiency, in practice the frictional losses do not increase proportionally in line with power, so that a Stirling engine with a higher power density or a higher power will tend to be more efficient. As explained earlier, a high upper temperature is in any case desirable to achieve a high Carnot efficiency. In micro-CHP applications, the final power output from the Stirling engine is required as electricity, rather than mechanical power, so that some form of generator is an essential component. These tend to be either induction generators (actually induction motors as these are mass produced and relatively inexpensive) or synchronous generators. Although the latter are more electrically efficient, they are more expensive and require power electronics to synchronise to the grid, resulting in additional losses, cost and bulk. One drawback of induction generators is that they derive their reactive power from the network to which they are connected; whilst this simplifies

© Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 187

4/5/11 9:22:35 AM

188

Small and micro combined heat and power (CHP) systems

synchronisation (as they automatically adjust to the frequency of the reactive power supply), it also means that the engine is tied to the grid throughout each cycle. Although the nominal rotational speed of 1500 rpm (or 3000 rpm) results in a synchronised frequency of 50 Hz, this does not necessarily mean that the power cycle of the Stirling engine delivers power evenly at a constant speed, indeed it is clear from Figs 8.2 and 8.3 that this is not the case. As a result, single cylinder Stirling engines may accelerate and decelerate to speeds alternately above and below the synchronous speed of the generator within each cycle, consequently producing in excess of their mean power at one point, whilst drawing power (motoring) at another. Not only does this impose undesirable loads on the engine and generator, it also produces wasteful heat in the generator, further reducing its efficiency. As has been discussed earlier, in order to realise maximum efficiency it is desirable to achieve a high maximum temperature. Although Stirling engines in micro-CHP applications can derive their heat input from virtually any high temperature heat source, in practice micro-CHP products are currently focused on mass market applications, notably those with a relatively low cost (natural gas) fuel supply. Whereas in a gas boiler, the gas is simply used to heat the central heating water at around 80 °C, the Stirling engine requires combustion temperatures well in excess of 800 °C in order to transfer heat effectively to the working gas through the limited heat exchange area available. It is a fundamental law of physics that high temperature energy is more useful than low temperature; it is therefore clear that it is rather wasteful to heat up the ambient air/fuel mixture from, say 10 °C to 1000 °C entirely by burning additional fuel, when the exhaust from the engine can be well over 500 °C and is simply recovered in a low temperature heat exchanger to add additional heat to the central heating circuit at a mere 80 °C. High efficiency engines thus use a recuperator (also known as an air pre-heater) to recover heat from the high temperature exhaust gas and transfer it to the incoming combustion air supply. However, this component is often omitted due to cost, bulk or because it constrains the ability to make use of EGR. It can thus be seen that a number of factors, practical and theoretical, constrain the efficiency which can be achieved in Stirling engines. However, even current micro-CHP products which achieve efficiencies of up to 15% show clear promise of a potential evolution as the technology matures in this application.

8.5

Types of Stirling engine

All Stirling engines fall into one of the following two basic categories: ∑

Kinematic Stirling engines have a crank arrangement to convert the reciprocal piston motion to a rotational output, say to drive a generator. The displacer is actuated through some form of mechanical linkage. © Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 188

4/5/11 9:22:35 AM

Stirling engine systems for small and micro CHP applications

∑

189

Free-piston Stirling engines (FPSE) have no rotating parts. In the majority of cases, output power is taken from a linear (usually permanent magnet) alternator attached to the piston, while the displacer is actuated by the pressure variation in the space beneath the piston.

In theory the LFPSE (linear free piston Stirling engine) is much simpler as it contains fewer moving parts.  In practice, the challenges of differential expansion and linear generator design have so far proved a major obstacle to commercialisation. Stirling engines can be further characterised by the three typical configurations of the displacer and working pistons, known as alpha, beta and gamma. In the alpha type, the working gas shuttles between two pistons. One piston carries out compression in the cold space and the other, expansion in the hot space. A sub-division of the alpha type is the double-acting type, where useful work is done by symmetrical pistons. In the beta type, both compression and expansion are carried out by the working piston, the working gas being shuttled between hot and cold spaces in the same cylinder by means of a (non-working) displacer. The third version is the gamma type in which the working piston is placed in a separate cylinder. It has been shown that the beta type is inherently more efficient than the others,3 but as will be explained later, high efficiency alone is not necessarily a desirable goal. Indeed, measures which improve efficiency may have undesirable consequences both in technical and economic terms. Clearly there is little point in achieving a high efficiency if the production costs are so high that it could never be recovered from energy savings. For example, it is possible to improve the Carnot efficiency of a Stirling engine by using discontinuous motion of the piston. Practical implementation of this feature is possible using electromagnetic actuation of the displacer, and is to some extent simulated in the conventional crank arrangement of some engines. However, fluctuations in rotation of the working piston give rise to other complications, particularly variations in electrical output and high electrical losses as well as obvious increases in noise, vibration and mechanical stress. Thus, the quest for high efficiency has economic and performance implications which may be undesirable. Indeed, the appearance of the WhisperTech engine with an inherently low electrical efficiency, but with reliability and production cost parameters in line with market requirements, can be seen as a major landmark in the commercialisation of micro-CHP.

8.6

Development of Stirling engines for micro combined heat and power (CHP) applications

In the 1970s the quest for improved efficiency promised by Stirling technology led to attempts to develop automotive power units. However, the © Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 189

4/5/11 9:22:35 AM

190

Small and micro combined heat and power (CHP) systems

lack of controllability required complex gas valving arrangements with high parasitic losses, somewhat defeating the point of the exercise. Even after the abandonment of Stirling as direct drive automotive unit, problems with lubrication and working gas pressure loss remained significant obstacles. Due to the very high working gas pressures, particularly with ‘leaky’ gases such as helium (selected for their small molecular size and heat transfer efficiency), leakage to atmosphere could only be contained by high loss seals. Shaft seals were initially subject to side forces, as were the piston seals, so efforts were made to minimise lateral forces with patented mechanisms (e.g. Carlqvist), reducing wear as well as frictional losses. At the same time as gas was leaking out, lubricating oil had a tendency to migrate into the hot end of the cylinders where it carbonised on the heat exchange surface, reducing heat transfer, power and efficiency. The solution was to incorporate the generator within the hermetic envelope of the engine so that the crankcase was at mean operating pressure. The only penetrations of the crankcase were electrical leads so that no high pressure (high friction loss) moving seals are required. This feature is now found on almost all micro-CHP Stirling engines, although the Solo engine (10 kWe) simply provides constant top-up of leaking helium by provision of a cylinder of compressed gas, replenished annually. The problem of oil contamination was resolved by use of dry seals located within the cooler area of the cylinder using Teflon type materials. This section describes a number of leading Stirling engine technologies and their individual approaches to overcoming some of the key technical challenges.

8.6.1 WhisperGen kinematic Stirling engine The WhisperGen micro-CHP unit is marketed in the UK by the energy company E.ON, in the Netherlands by Magic Boiler Company and in Germany by Sanevo. It is a four cylinder unit which overcomes the issues discussed earlier relating to the single cylinder asynchronous unit and leads to smooth, vibration free operation, with noise levels similar to a domestic freezer. The MkV unit, incorporating a supplementary burner, was introduced in 2005 to provide additional flexibility, making the unit suitable for larger homes. This variant also incorporated an integral acoustic enclosure which made kitchen installation possible. In January 2008, WhisperGen announced the establishment of a joint venture (EHE) with Spanish white goods manufacturer Mondragon CC to mass produce units for the European market; in late 2009, the first units (designated EU1) were dispatched from their production facility at Tolosa in Northern Spain. The WhisperGen micro-CHP unit, from its earliest example as a dieselfired DC battery charger for marine applications, arguably represents the first

© Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 190

4/5/11 9:22:35 AM

Stirling engine systems for small and micro CHP applications

191

modern commercially available Stirling engine and is certainly the first viable micro-CHP product. Its success lies less in any claimed efficiency, more in its construction using conventional materials and production techniques which allow realistic manufacturing costs to be achieved. The engine uses relatively low pressure nitrogen, low head temperature and a low efficiency regenerator, all of which result in an electrical efficiency of no more than 11%. However, the relatively low investment cost is readily recovered with these performance parameters and the key benefit of reliability and extended service intervals leads to a viable product. It is essentially a four-cylinder double-acting, alpha-type engine, similar to the configuration of the larger (10 kWe) Solo engine. However, its novelty lies in the mechanism for translating the reciprocating motion of the pistons into rotation suitable for connection to a conventional generator. Whereas the Solo engine uses conventional cross heads with oil lubrication, the WhisperTech unit uses an ingenious ‘wobble-yoke’ mechanism, the centre of which is linked through a nutating bearing to the generator rotor. In 1999, following extensive economic and market studies,4 the UK consultancy EA Technology demonstrated the commercial viability of the unit. Laboratory tests validated the key performance parameters and demonstrated the endurance through accelerated life testing.5 It was decided to continue the commercialisation of the unit and limited field trials in the heating season of 2000–2001 further validated the anticipated performance.6 However, a number of recommendations from the trials were implemented in a modified product which was installed in 20 homes in the north west of England and ran through the 2001–2002 heating season, with a further trial of 30 homes in the north west and eastern England. These trials effectively demonstrated the performance of the WhisperGen unit in a range of typical UK homes, and helped to confirm the identified target markets.7 However, in 2006, E.ON took the decision to postpone their previously announced mass market launch pending the availability of mass manufactured products from the factory in Europe. It is expected that, following proving trials during the 2009–2010 heating season, products may again become available in late 2010. The EU1 will have an electrical output of 1 kWe, together with a thermal output of 7 kWt from the engine, plus a further 7 kWt from the supplementary burner. An outside temperature controller is used to ensure the majority of heat is delivered by the engine, thus maximising run hours, unless the ambient temperature is extremely low, in which case the supplementary burner is brought into operation. It is recommended for homes with a design heat loss between 5 and 12 kWt; the additional 2 kWt capacity is intended to allow for simultaneous provision of space and domestic hot water. To avoid excessive cycling, homes with a design heat loss of less than 7 kWt should be provided with a primary thermal store. The engine no longer includes

© Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 191

4/5/11 9:22:36 AM

192

Small and micro combined heat and power (CHP) systems

an air pre-heater in order to reduce cost and size of the production units; the consequent loss of efficiency has been compensated for by a number of minor modifications including an increase in working pressure and a higher efficiency regenerator. The unit (Fig. 8.4) is floor-mounted, about the size of a dishwasher and weighs around 140 kg, making it suitable only for installation at ground level on a solid floor.

8.6.2 MEC (Microgen) linear free piston Stirling engine The Microgen unit, developed by BG Group from a US (Sunpower) design, is a LFPSE which is intended for wall-mounting; it contains a supplementary burner which enables it to meet the full heating requirements for even larger homes. Following disposal by BG Group in 2007, development of the Microgen unit was taken over by MEC, a consortium of gas boiler companies (Viessmann, Baxi, Vaillant, Remeha) and Sunpower. Each of the boiler companies is expected to market their own variant of the microCHP unit incorporating the MEC engine. Trial versions of the unit being demonstrated in the field make use of engines produced in Japan, although mass manufacture of engines has recently been transferred to China. Earlier versions produced 1 kWe from the linear generator, together with 3 kWt heat output, implying an efficiency of around 20%. However, current models have the same electrical output, but with a higher thermal output (6 kWe), implying an electrical efficiency of around 13%. Figure 8.5 shows a cross section through the upper cylinder, with the characteristic Inconel fins designed to conduct heat effectively through the

8.4 WhisperGen EU1 1 kWe micro-CHP (display shows status and power output).

© Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 192

4/5/11 9:22:36 AM

Stirling engine systems for small and micro CHP applications

193

8.5 MEC LFPSE cutaway.

relatively small surface area of the hot end of the cylinder. The lower heat exchanger transfers heat to the cooling circuit. The multiple diaphragms are designed to strengthen the working piston which is subjected to significant pressure fluctuations. The unit is equipped with a very large supplementary burner of around 18 kWt depending on the manufacturer. In the case of the Remeha unit, the supplementary burner is used to provide all the domestic hot water as in a combination boiler. Although this overcomes some of the inefficiencies of hot water production in summer common to all boilers and particularly poor for high thermal inertia Stirling engines, it also means that the total annual run hours will be significantly reduced, limiting the economic value of the electricity generated as a by-product of heat production. The Sunpower engine on which it is based is a beta-type, linear free piston Stirling engine (LFPSE) with an integral linear alternator. The freepiston arrangement eliminates the need for a crank mechanism, and avoids a potential source of failure. LFPSE units are thus fundamentally elegant in engineering terms, but in reality a number of technical challenges remain, which detract somewhat from their simplicity. Firstly, linear alternators are not commonly available, and developers are thus faced with the prospect of parallel development of engine and generator. The gas bearings, which avoided another potential point of failure, have, for practical reasons, been dropped in favour of diaphragm springs as used on the Infinia engine. There are also fundamental challenges as the displacer is operated by resonance and is thus

© Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 193

4/5/11 9:22:36 AM

194

Small and micro combined heat and power (CHP) systems

tuned to the operating frequency of the engine. This can cause difficulties during start-up and stopping and several LFPSE engines are known to suffer from the piston ‘slamming’ against the cylinder ends during these periods. Similar problems may arise where grid frequency is subject to significant fluctuations. In addition, the need for very small clearances between the components of the alternator (to minimise losses) causes problems due to differential expansion as the engine heats up. The two boiler manufacturers planning to launch the product in 2010, Baxi in the UK and Remeha in the Netherlands are both producing what is claimed to be a wall-mounted boiler replacement as this represents the largest market share in both countries. However, both units are very much larger than wall-mounted boilers and require special lifting gear to manoeuvre the unit into place and support it during fixing. Figure 8.6 shows the Baxi unit complete with the industry standard Gianoni stainless steel heat exchanger (and supplementary burner) located above the primary (engine) burner. The engine assembly is suspended from the central plate by an arrangement of springs and a 9 kg counterweight assembly is similarly suspended to counteract the resonant vibration which is inherent in any LFPSE engine design.

8.6.3 Infinia (STC) linear free piston Stirling engine The Infinia (formerly known as STC) LFPSE is now being used in a collaboration between Ariston (formerly MTS), Bosch and Enatec in Europe as well as Rinnai in Japan. Rinnai will produce the LFPSE module for integration into micro-CHP packages by the other partners for the European market, with a trial of 1000 units planned for 2008–2010. Rinnai will also produce a packaged unit for the Japanese market. Bosch initially produced a wall-mounted version similar to the MEC configuration, but has subsequently adopted a common design approach with Ariston in producing an integrated LFPSE, supplementary burner and hot water storage cylinder in a floor-mounted package the size of a fridgefreezer (Fig. 8.7). The current Bosch product is being trialled in the UK in a project managed by Worcester Bosch. No firm dates have been announced for commercial launch so far. The LFPSE component is rather similar to that of the MEC variants, with similar electrical output, although a somewhat lower claimed electrical efficiency, estimated to lie between that of the WhisperGen and MEC units.

8.6.4 Disenco kinematic Stirling engine The Disenco unit is a kinematic design with an electrical output of around 3 kWe, significantly higher than other domestic products. The high electrical

© Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 194

4/5/11 9:22:36 AM

Stirling engine systems for small and micro CHP applications

195

8.6 Baxi Ecogen 1 kWe micro-CHP unit.

output enhances payback of the unit (which is anticipated to be significantly higher than the other 1 kWe products), although this makes the unit susceptible to the recoverable value of exported power from the unit unless it can achieve high utilisation such as in a small hotel*. In January 2008, Disenco announced a manufacturing partnership with Autocraft to produce the core * The new FiT (Feed in Tariff) introduced in April 2010 in the UK would make a very significant beneficial impact on the economic viability of this product and others with similarly high electrical power outputs; it allows for a subsidy of 10p/kWh of all power generated regardless of whether it is consumed on site or exported in addition to an export payment of 3p/kWh.

© Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 195

4/5/11 9:22:37 AM

196

Small and micro combined heat and power (CHP) systems

8.7 Ariston 1 kWe micro-CHP unit.

engine, with packaging into a micro-CHP product by Malvern boilers and recently announced marketing deals with Endesa and Centrica. The design concept originated in the Swedish TEM design and subsequent Sigma Elektroteknisk developments in Norway, based on the ingenious Carlqvist crank mechanism. This mechanism, in conjunction with two balancing generators, converted reciprocating action into rotary motion with negligible lateral forces on the piston seals. The beta-type TEM 1–75 incorporated finned tube heater head and air pre-heater, both components contributing to the high mechanical (and consequently electrical) efficiency. This engine drove two permanent magnet generators to produce a total of 1 kWe DC power. In 1994, it was decided to develop a unit with higher electrical output, partly funded by the EU THERMIE programme. The PCP1-

© Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 196

4/5/11 9:22:37 AM

Stirling engine systems for small and micro CHP applications

197

130 was a considerably larger machine and the DC generators were replaced by two 3-phase 400 volt induction units, delivering a total of 3 kWe (Fig. 8.8). This design retained high pressure helium as a working gas, two-stage regenerators, the very expensive finned tube heater head and an Inconel air pre-heater. It was able to demonstrate electrical efficiencies in excess of 20%, even with grid-tied induction generators which were subsequently found to inhibit performance due to the fluctuating power output of the single cylinder design. A number of approaches were proposed which could easily have raised the electrical efficiency into the upper twenties, but Sigma were unable to continue development due to the collapse of their main shareholder, Ocean Power in the USA. However, since the project was taken over by Disenco in the UK, the performance of the original design has been severely compromised, not least by the abandonment of the Carlqvist mechanism; Disenco have opted instead for a more conventional rhombic drive crank mechanism. It is unclear whether this is an oil-lubricated crank or whether it uses sealed roller bearings, but if the former it would be quite an achievement to have overcome the problems associated with this approach, namely oil migration into the upper cylinder and carbon deposition on the cylinder walls leading to breakdown of the heat transfer process. They have yet to demonstrate significant run hours in the laboratory or field although one unit is claimed to have operated for an undisclosed period in a single installation in the UK. The unit is able to modulate output from 0.5–3 kWe electrical and 12– 17.4 kWt, with an implied electrical efficiency of little more than 15%, again

(Burner and pre-heater omitted for clarity)

Finned tube heater head

Flow and return connections Power outlets

Twin induction generators

Twin induction generators Crank cover facilitates removal of crank mechanism

8.8 Sigma 3 kWe micro-CHP unit.

© Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 197

4/5/11 9:22:38 AM

198

Small and micro combined heat and power (CHP) systems

illustrating how electrical efficiency tends to be compromised as products approach commercial launch. Field trials had been announced for late 2009, but following prolonged financial difficulties, in March 2010 Disenco was placed into administration.

8.6.5 Sunmachine One of the key characteristics of Stirling engines is their ability to make use of virtually any heat source, although the majority of Stirling engine developers have understandably focused their activities on capturing the substantial gas boiler replacement market. The Sunmachine, however, is an exception, being intended as a micro-CHP system capable of burning biomass as a fuel. It is a 3 kWe product with a claimed electrical efficiency of 20% and thermal efficiency of 70%, manufactured in Germany. Wood pellets are burned directly above the finned tube heater head, similar to the TEM design. However, previous attempts at this configuration have not been successful due to problems with sooting of the heat exchanger which quickly becomes ineffective. It is unclear if or how Sunmachine have actually overcome this issue as little public information is available. Sunmachine plan to launch a biogas version of their product in 2010, although this is a far less technically challenging technology, differing little from natural gas variants; the biggest challenge is finding a supplier of biogas! The biomass product has been repeatedly announced at a market price of around 723,000 in 2005, but which had risen to 735,000 by 20088.

8.6.6 Cleanergy AB (formerly Solo) Although within the definition of micro-CHP, the Solo micro-CHP unit with an electrical output of 9 kWe and a thermal output of 26 kWt, is clearly targeted at commercial rather than residential installations. The Solo V160 engine is a 2-cylinder alpha-type Stirling engine designed to run on biogas and can modulate electrical power between 2 and 9 kWe with corresponding thermal output between 8 and 26 kWt. The working gas is helium and the engine achieves an electrical efficiency of over 22% (LCV). Naturally it is a substantial piece of kit (1280 ¥ 700 ¥ 980) weighing 460 kg. In the V160, air is pre-heated and mixed with gas to improve efficiency; EGR is used to reduce emissions by recirculating a proportion of the exhaust gases into the combustion process which slows down and spreads the reaction in a process known as flameless oxidation (FLOX™). Although oil changes are claimed to be limited to 40 000 hours, the piston rings need replacement every 4000–6000 hours. It is also believed that the helium charge needs replenishment on a regular basis with a consumable cylinder being provided with every unit.

© Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 198

4/5/11 9:22:38 AM

Stirling engine systems for small and micro CHP applications

8.7

199

Micro combined heat and power (CHP) design and system integration

In order to extract the maximum value of micro-CHP it is necessary to optimise the overall performance of the micro-CHP package within the energy system of the application. This means that not only must the performance of the Stirling engine be optimised in its own right, but that the ancillary components in the micro-CHP package as well as its interaction with the domestic energy system be carefully considered. It is, in many cases, necessary to accept compromises in efficiency in order to achieve a robust and practical micro-CHP system. This point has been discussed previously in the context of achieving high electrical efficiency of Stirling engines by operating at high temperature and pressures, but it is equally relevant at the system level, where the micro-CHP interfaces with what is in most cases a sub-optimal heat distribution system.

8.7.1 Design for thermal efficiency Although the key value of micro-CHP is determined by its ability to generate high value electricity instead of less valuable heat, it is also necessary to ensure that the total efficiency, that is electrical and thermal efficiency combined, is not unnecessarily compromised. As discussed earlier, the specific carbon dioxide emission of micro-CHP generation is adversely impacted if electricity is generated at the expense of a high thermal loss. So considerations of micro-CHP economics and environmental benefits require careful consideration of the balance of heat and electricity outputs. In the earlier discussion of Stirling engine efficiency, it was explained that the efficiency of the Stirling engine is fundamentally determined by the difference in temperature between the hot and cold ends of the working cylinder; the Carnot efficiency is constrained by this temperature difference. The efficiency of the engine in turn determines the electrical efficiency of the micro-CHP package. However, for optimum overall efficiency, the thermal efficiency must not be neglected. Based on the high efficiency Sigma Stirling engine which includes an air pre-heater, it can be seen in Fig. 8.9 that the residual heat in the flue gases is passed to a heat exchanger in the return water from the primary heating circuit as it is at too low a temperature to add significantly to the temperature of the flow from the engine. In early versions of the WhisperGen engine, flue gases passed first through a relatively low capacity air pre-heater and then to the heat exchanger in the primary circuit. In the first case it is apparent that the return water temperature is increased by this process, adding to the thermal energy recovery, but resulting in a smaller temperature difference in the engine. However, in both cases it was concluded that the small loss

© Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 199

4/5/11 9:22:38 AM

© Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 200

4/5/11 9:22:38 AM

Two-way meter

EMS

Consumer unit

U/O frequency U/O voltage protection

Natural gas supply

Exhaust fan condenser

Condensate drain Electrical supply to house

Cooling fan

Flow boiler

Outdoor air supply

8.9 Diagram of Sigma Stirling engine micro-CHP package showing arrangement of heat exchangers.

BTC heating control

Exhaust to flue

Stirling engine systems for small and micro CHP applications

201

in electrical efficiency was more than compensated for by the significant increase in thermal efficiency due to the potential for condensation of the flue gas in the low temperature return heat exchanger. Indeed, the potential efficiency increase between non-condensing and condensing operation undergoes a step change of up to 10%, just as with gas boilers. However, it is essential to closely control the amount of heat input to the heat exchanger from the supplementary burner which, in the WhisperGen unit, utilises this ‘secondary’ heat exchanger as its only means of delivering heat to the primary central heating flow; it is otherwise quite possible that the rise in temperature in the primary flow can result in raising the temperature of the cold end rather than lowering it, causing inefficiency and potentially damage to the engine. The precise balance of energy recovery is rather complex, depending on the temperatures at the various stages of the Stirling process and can vary greatly depending on the operating temperature, degree of air pre-heat and capacity of the flue gas heat exchanger. Again, as with gas boilers, there is some concern that thermal efficiencies achieved under idealised test conditions are not replicated in the field. Condensing boilers which can readily achieve efficiencies in excess of 90% in the laboratory do not benefit from low return temperatures (the prerequisite for condensing, high efficiency operation) under full load conditions except with well-designed radiators or during the initial warm-up period at the start of each programmed heating cycle. For this reason, in some instances an additional heat exchanger is located after the flue gas heat exchanger as a secondary air pre-heater. This has the effect of virtually guaranteeing condensing operation as the incoming air will, almost inevitably, be below 20 °C whenever space heating is required, providing a cold enough heat exchanger surface to ensure condensation occurs.

8.7.2 System design Although the micro-CHP unit is essentially a replacement for a gas boiler, it has peculiar characteristics which mean that it cannot be treated in exactly the same way either from a design, installation or operational point of view. It may be a ‘drop-in’ replacement as regards the home, but the designer, the installer and the householder need to understand a number of key issues if the system is to perform at its best and to avoid disappointment for the user. One of the key requirements of a micro-CHP system as discussed earlier is to maximise the number of run hours each year in order to generate as much valuable electricity as possible. Not only does this imply a more accurately designed system, it also means that the householder may need to become accustomed to a somewhat different control approach. For example, optimum start controls which are designed to achieve

© Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 201

4/5/11 9:22:38 AM

202

Small and micro combined heat and power (CHP) systems

comfort conditions regardless of outdoor air temperature, are regarded with some suspicion by householders who cannot understand why the heating system comes on at different times every day (as a function of outdoor air temperature) even though they have set what they believe to be a start time for the central heating. In other words they fail to distinguish between setting the programmer to turn on the heating system at, say, 07:00 and setting the programmer to make sure the house is at the desired temperature by 07:00. In the optimised start programmer the control algorithm calculates the required start time based on the thermal inertia of the building, the necessary temperature lift (i.e. difference between current and desired temperature) and the heat loss deduced from the outdoor air temperature. Of course, it is perfectly possible to design a micro-CHP system to perform in largely the same way as a gas boiler; the majority of microCHP packages now incorporate some form of supplementary burner with a significantly higher thermal output than the prime mover, so it is quite simple to use this capacity to achieve the rapid heat up which householders are accustomed to. However, this results in sub-optimal operation in which the potential value of electricity generation is squandered simply to avoid control complexities or to avoid the need to accommodate a HWC in the system. In the case of the Remeha product, domestic hot water is provided entirely by the supplementary burner, replicating the performance of a combi-boiler, but typically losing the opportunity to generate 1000 kWh of electricity annually, worth around £100. The WhisperGen unit, although configured as an integrated system boiler linked to a conventional hot water cylinder, offers users the facility to control the supplementary burner (which is triggered by the rate of increase in flow temperature, a proxy for the ability of the system to meet the instantaneous thermal demand) by means of a timed delay. The longer the delay, the less heat is provided by the supplementary burner and the more is provided by the engine, resulting in longer run hours and enhanced electricity production. However, this enhanced electricity production is achieved at the expense of longer recovery times of the hot water in the HWC for example; this can be effectively overcome by incorporating a larger capacity HWC, but this has implications for cost, space and standing losses. As always, it is a matter of compromise between theoretical efficiency and pragmatism. As with condensing boilers, the thermal efficiency of Stirling engine micro-CHP is constrained by the return temperature which should ideally be kept at a temperature low enough to induce condensation and thus recover latent heat from the condensate in the flue gases. However, unlike heat pumps the performance of these units is not significantly compromised by the need to deliver DHW at a relatively high temperature as they are

© Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 202

4/5/11 9:22:38 AM

Stirling engine systems for small and micro CHP applications

203

easily capable of delivering hot water at similar temperatures to a gas boiler.*

8.7.3 Interface design Very often, existing central heating systems with radiators, particularly open-vented systems, contain substantial amounts of sediment and other undesirable contaminants in the primary heating circuit. Although this impacts on the performance of conventional boiler heating systems with a reduction in heat transfer capability and increased pump power, the problem is more acute for micro-CHP systems. Stirling engine heat exchangers are particularly susceptible to the accumulation of contaminants in their ‘cooling circuit’ (the primary heating circuit as far as the house is concerned) as they need to transfer substantial amounts of heat through a relatively small area (at the cold end of the piston) and consequently tend to have very fine heat exchanger elements which are relatively prone to clogging.

8.8

Applications and future trends

Stirling engines have historically been identified as the leading micro-CHP technology for individual homes, a role for which they are showing ever increasing promise. However, given the increasing availability of low cost, reliable and efficient ICE-based products suitable for small commercial applications, it is difficult to see how the larger output Stirling products, such as the Solo unit, can continue to compete. After all, the very characteristics which make Stirling engines attractive to householders are of less relevance to those installations where a plant room is available and service can be carried out on a normal commercial basis. Chapter 13 describes the potential for the various micro-CHP technologies in more detail, but as a rule, the key characteristics of Stirling micro-CHP including their relatively high heat-to-power ratio, make them eminently suitable for the majority of the existing UK housing stock with average annual thermal demands of around 18,000 kWh and where thermal and electrical demands are in similar proportions to the outputs of the Stirling technology, namely around 6:1. *Early Stirling engine-based micro-CHP products tended to limit the flow temperature to around 70 °C in order to maintain a cold end temperature that would avoid any possible damage to the piston seals which were considered a vulnerable component. The availability of improved materials and better temperature control have largely overcome this issue and current units operate at similar temperatures to gas boilers, although performance is enhanced by maintaining the highest possible temperature difference between hot and cold end.

© Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 203

4/5/11 9:22:38 AM

204

Small and micro combined heat and power (CHP) systems

Stirling engine micro-CHP is a particularly effective solution to the provision of low carbon heat and power in those homes for which it is difficult to identify alternative energy efficiency solutions, for example, homes which have solid walls and are thus difficult to insulate further, or larger homes which have already had all cost-effective measures applied, but which still have significant thermal needs. These latter also tend to have adequate space to accommodate what are today, relatively bulky floor-mounted Stirling-based micro-CHP products, often with separate utility rooms. However, there is a practical need for different configurations to meet demands of different types of homes. An increasing proportion of gas boilers installed in the UK are wall-mounted and combi-boilers are becoming increasingly popular; although the MEC-based products claim to be wallmounted, their weight in excess of 100 kg makes them very difficult to handle and install. It remains to be seen whether lighter units will be developed or whether Stirling engines will be limited to those applications where either a floor-mounted unit is acceptable or a substantial structural wall is available. And, whilst the Remeha unit already addresses the challenge of a combi configuration, the reduced potential for electrical generation may limit the economic viability to properties with larger thermal demands than the average UK home. It is quite possible that the different drivers in other markets may result in the development of products with characteristics readily transferable to those of the UK. So, whilst German homes often incorporate spacious basement areas which can readily accommodate existing Stirling products along with the performance enhancing thermal storage vessel without difficulty, Dutch homes often include the boiler within the roof space due both to space constraints and to facilitate flueing; they may not incorporate any thermal storage at all. It is no surprise therefore that the Remeha product is configured as a wall-mounted, combi package. Along with these ongoing developments to match the demands of different configurations, there is likely to be significant development in the performance of Stirling engine technology; there is certainly substantial theoretical room for increased electrical efficiency as has been demonstrated by numerous laboratory prototypes. We should also expect to see the production cost fall in response both to increased volumes and to compete with alternative products such as high performance fuel cells as they reach the market. If Stirling engines are to remain competitive they must do so on the basis of low initial cost; they are incapable of achieving the electrical efficiencies of SOFC technologies. In the longer term, beyond say 2030, there is some concern regarding the availability of natural gas or other fossil fuels for Stirling engines. Against this, external combustion engines generally are able to make use of a wide range of fuels, at least in theory; already a limited number of

© Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 204

4/5/11 9:22:38 AM

Stirling engine systems for small and micro CHP applications

205

biomass products are emerging and it is relatively simple to convert burners from natural gas to biogas (simple combustion is less of a challenge than combustion in an ICE). However, it should not be assumed that the rather limited biofuel resources likely to become available on a global scale will necessarily be supplied for combustion in the domestic heating sector. It may be, however, that limited quantities of natural or biogas will be used in hybrid domestic systems, perhaps incorporating heat pumps making use of intermittent renewable electricity, backed up by gas-fired micro-CHP during periods of peak demand or capacity shortfall.

8.9

Sources of further information and advice

∑ Information on Stirling engine micro-CHP including links to manufacturers’ websites: http://www.microchap.info/stirling_engine.htm ∑ General information on micro-CHP and related technologies with links to additional resources: http://www.microchap.info ∑ General information on microgeneration technologies with links to manufacturers and additional resources: http://www.microgeneration-oracle.com/index.htm ∑ Links page to government and institutional websites providing information on energy issues as well as organisations active in the field of distributed energy in general and CHP in particular: http://www.microchap.info/LINKS.HTM

8.10 1 2 3 4 5 6 7 8

References

Egnell, R. & Gabrielsson, R. (1991), Alternativa motorer, NUTEK, Stockholm. Kolin, I. (1991), Stirling motor, Zagreb University. Finkelstein, T. (1998), Stirling Conference, Osnabruck. Winstanley, R. et al. (1999), WhisperTech WG800 Stirling engine micro CHP economic viability report, EA Technology client confidential report. Winstanley, R. (1999), WhisperTech WG800 laboratory tests, EA Technology client confidential report. Winstanley, R. (2001), Micro CHP sheltered field trial report, EA Technology client confidential report. Green, R. (2002), WhisperTech alpha trials report, EA Technology client confidential report. Donau Zeitung, 10 September 2008.

© Woodhead Publishing Limited, 2011

CHP-Beith-08.indd 205

4/5/11 9:22:38 AM

9

Organic Rankine cycle (ORC) based waste heat/waste fuel recovery systems for small combined heat and power (CHP) applications J. L a r j o l a, Lappeenranta University of Technology, Finland

Abstract: This chapter describes the principle of the organic Rankine cycle (ORC), suitable heat sources, the benefits of ORC when compared to the water vapour process, the different ways to build an ORC, a historical review, some present day manufacturers and guidelines to estimate efficiency and specific price. ORC is a power plant process, where the working fluid is an organic fluid instead of water. It can be used to produce electricity and heat from various fuels and heat sources, such as biogas, waste heat, biomass, etc. The typical electric power range of ORC is from 140 kW to 2500 kW. Key words: organic Rankine cycle, ORC, waste heat, biogas, biomass, combined cycle, CHP.

9.1

Introduction

The organic Rankine cycle (ORC) is a Rankine process, where the working fluid is an organic fluid instead of water, which is used in conventional steam power plants. ORC power plants are often operated in cogenerating mode, i.e., the thermal power discharged by the condenser is used in an industrial process or to heat environments (these plants are called combined heat and power (CHP) plants). Alternatively, by condensing the working fluid at a lower temperature, only electricity can be obtained (condenser heat is rejected to the atmosphere or to lake/sea water).

9.2

Principle of the organic Rankine cycle (ORC) process

The Rankine cycle is named after William John Macquorn Rankine (1820– 1872). The conventional steam power plant cycle is called the Rankine cycle, where the working fluid is water. If the heat source is at high temperature, and the design power sufficiently high, water is an excellent working fluid. It has exceptional thermal stability, excellent heat transfer properties, it is 206 © Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 206

4/5/11 9:23:10 AM

ORC based waste heat/waste fuel recovery systems

207

non-toxic, non-combustible, inexpensive, has zero ODP (ozone depletion potential) and zero GWP (global warming potential). However, if the heat source is at low or medium temperature, and/or if the power capacity of the plant is low, selecting an organic fluid instead of water as the working fluid gives several benefits. The principle of the ORC has been well known for decades. Most of the present ORC plants are based on conventional turbine technology, which includes shaft seals, a reduction gear, an air cooled generator and a lubricating oil system. Thermal energy is transferred to the process either directly, or by means of a thermal-oil circuit. More recently a new turbo-generator concept has been developed and brought to commercial realization: the pump, the electrical generator and the turbine share a single shaft, while the working fluid also serves as the lubricant and coolant for the electrical generator. This system configuration allows for completely hermetic design, with no need for shaft seals or a separate lubrication system. The main idea in most ORC plants is to produce electric power from a relatively low temperature heat source and/or relatively small amounts of waste heat or heat produced by burning limited amounts of fuel difficult to use in other processes. For low temperature waste heat applications, ORC technology provides many benefits compared to the conventional steam Rankine process. Typical low temperature heat sources for ORC applications are geothermal heat and the waste heat from combustion engines, gas turbines and many industrial processes. Typical high temperature sources with limited capacity are burning of biomass, landfill gas, or biogas and heat generated by concentrating solar collectors. The greatest number of ORC plants are probably sold in the power range 300–2000 kW, but they are also made in the range 100 to 22 000 kW, and there are also low efficiency micro-ORC in the power range 0.5–5 kW. One of the most notable advantages of a Rankine cycle employing an organic substance as the working fluid is that the fluid remains in the superheated vapour phase throughout the entire expansion in the turbine, thus avoiding the well-known problem of condensation in the low-pressure stages typical of all steam turbines. A recuperator is therefore normally used to de-superheat the vapour entering the condenser, and to preheat the fluid entering the boiler. Figures 9.1 and 9.2 illustrate the technology of typical ORC power plants dating back to the 1980.

9.3

Typical process heat sources and operating ranges for organic Rankine cycle (ORC) systems

9.3.1 Industrial waste heat In many industrial processes significant amounts of hot gases are produced, and the heat cannot be utilized. If this heat could be converted into electricity,

© Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 207

4/5/11 9:23:10 AM

© Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 208

4/5/11 9:23:10 AM

Feed pump Turbine (single stage) Gearbox

Generator or compressor

Vaporizer

Preheater Separator

Waste heat stream

Diverter valve

To stack

9.1 Working principle of a conventional ORC process without thermo-oil circuit (Sundstrand 1983). The diagram shows how the liquid process fluid is pumped from the condenser hotwell to the regenerator to be preheated, and from the regenerator to the vaporizer where it is converted to vapour by means of hot heat source gases. This high temperature and high pressure organic vapour expands in the turbine and produces electricity by means of a generator. After exiting the turbine the vapour goes through the regenerator (where it is de-superheated) to the condenser, where it is converted back to a liquid state.

Boost pump

Hotwell

Heat rejection to cooling source

Condenser regenerator

Waste gas out

ORC based waste heat/waste fuel recovery systems

209

9.2 Process shown in Fig. 9.1 in reality. Upper detail shows generator and gearbox, lower detail turbine, recuperator and condenser. Power 500 kW (Sundstrand 1983).

significant savings can be attained (see Table 9.1). Figure 9.3 shows an application, where hot gases from several ceramic kilns are collected and introduced to an ORC plant.

9.3.2 Gas turbines and combustion engine exhaust gases Exhaust gases from gas turbines and internal combustion engines can be recovered effectively by means of an ORC power plant, thus considerably increasing the overall energy conversion of the combined system, much like in modern large combined-cycle power plants. In such large gas turbine plants and also in very big motor power plants, high efficiency is obtained by using a steam process with several pressure levels. However, if the gas turbine power is below 5–7 MW and motor power below 8–12 MW, a water vapour process is difficult to achieve with good efficiency, and an ORC process may be more favourable. An example of an ORC power plant run with exhaust gas from a 2 MW gas engine is given in Fig. 9.4. The minimum gas turbine power for present day economically feasible ORC is probably about 0.5 MW and minimum motor power probably about 1 MW (future

© Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 209

4/5/11 9:23:10 AM

210

Small and micro combined heat and power (CHP) systems

Table 9.1 Some suitable present-day industrial waste heat sources, and economical feasibility according to one ORC-manufacture (Vescovo 2009) Industry/application Cement Float glass

Steel: flat products (rolling mill)

Unit

Heat source Kiln and clinker Oven exhaust Preheating oven cooler gas gas exhaust gas Plant capacity 2,500 500 6,000 Tons per day Electricity costa 0.09 Wasted thermal power 12   in exhaust gasb Thermal power to ORC 11 Thermal power to 1   thermal users Net ORC electric 1.6   production Net electricity 12,800   productiond

0.095 5

0.06 13

7/kWh MW

4.7 0.3

13 0

MW MW

1

2.4

MW

8,000

19,200

MWh/y

1.3 1.1 2.6

2.4 1.5 4.3

Million 7 Million 7 Million 7

–40,000

–40,000

–40,000

7/y

1,152,000 240,000 1,352,000

760,000 72,000 792,000

1,152,000 0 1,112,000

7/y 7/y 7/y

Capital expenditure indications ORC cost 1.8 Balance of plantc 2.6 Total cost 4.8   (+10 % project   management) Annual cash flowsd Operational   expenditure Cash flow – electricity Cash flow – heate Net cash flow

Resultsf Profit before tax 4 3.7 4.4 Internal rate of return 25% 27% 23%   (10 years) Net present value 75,333,129 73,310,109 74,091,971   (10 years) Avoided CO2 9,664 5,520 12,096   emissionsg

Tons per year

Notes: a These values include incentives if any; differences are due to total power installed, nation, etc. b Assuming to cool down the gas to 150/160 °C. c Including heat recovery exchangers and civil works – estimated by reputable suppliers. d Assuming 8,000 operating hours/year. e Assuming a heat valorization of circa 0.03 7/kWh. f Assuming discount rate of 5%. g Assuming 0.63 kg of CO2/kWh electric and 0.2 kg of CO2/kWh thermal (from CH4 combustion).

© Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 210

4/5/11 9:23:10 AM

ORC based waste heat/waste fuel recovery systems

211

9.3 ORC power plant (500 kWe), heat source of which are hot gases collected with pipes from several ceramic kilns. Early 1980 design (Sundstrand 1983).

9.4 ORC power plant (145 kW) run with exhaust gas from a 2 MW gas engine installed in 2008. Left, ORC process module; right, the exhaust gas boiler (Tri-O-Gen 2010).

© Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 211

4/5/11 9:23:11 AM

212

Small and micro combined heat and power (CHP) systems

development may decrease these limits). Many of these smaller motors are run with biogas, which makes this application particularly favourable for the environment.

9.3.3 Biogas from industrial and municipal waste streams The gas produced in landfills is similar to biogas, but may contain components that are harmful for gas turbines and combustion engines. Larger plants may be provided with a gas cleaning system, and this gas can be used without problems in gas engines or in gas turbines. However, if the amount of gas in a landfill is small, it can be used without cleaning directly as a heat source for an ORC plant. An example of an ORC plant run with unclean landfill gas is given in Fig. 9.5. Of course, efficiency is lower compared to gas motor (in the order of 20–22% instead of 30–40% for gas motors).

9.3.4 Biomass in agriculture and in forestry In agriculture and forestry huge amounts of combustible waste are produced, such as bark, wood chips, sticks, straw, etc. However, usually it is not economically feasible to transport this waste over long distances. Thus a power plant that can utilize the biomass near the production site is required. In many cases there is not enough biomass for a conventional steam power

9.5 ORC power plant (150 kW) run with unclean landfill gas (Tri-O-Gen 2006).

© Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 212

4/5/11 9:23:11 AM

ORC based waste heat/waste fuel recovery systems

213

plant (minimum size ca. 3 MW), and thus an ORC plant is a good solution. The economical feasibility of the biomass plant depends largely on the collecting and storage costs of fuel. The optimal range for this cost limits the capacity of the power plant to quite low values in many cases. Figure 9.6 shows how electricity and district heat can be generated from biomass.

9.3.5 Using geothermal heat High temperature geothermal heat may be used in the water vapour process (there are a number of different processes). However, if the temperature level and/or amount of heat power are limited, an ORC process may be a better alternative (see Fig. 9.7). Also, if geothermal steam is ‘dirty’ and/or sour, an indirect ORC process is more favourable. Thus a very large number of ORC plants are in use in geothermal applications, two examples of which are shown in Fig. 9.7.

9.3.6 Micro-ORC in distributed energy system By selecting a suitable working fluid and by using high speed technology with a hermetic turbogenerator, ORC power plants may be realized in very small power sizes (below 5 kW) with reasonable or even good efficiency. Ormat has already for decades made small very low efficiency ORC units for special applications (Bronicki 1988) (see Fig. 9.8). In these cases reliability is the only requirement and a mtbf (mean time between failures) of over 200 000 h has been obtained. However, by manufacturing the micro-ORC unit in large series, it should be possible to obtain a price level that would make it suitable as a household CHP power plant, that is, it would be like a reversed heat pump. Local fuels, like biogas or biomass could be used as a heat source to produce electricity and heat for a private house or a farmhouse (Colonna and Pasquale 2008). This type of project is in the starting phase.

9.4

Benefits and disadvantages of organic Rankine cycle (ORC) process as compared to waterbased systems

9.4.1 Benefits of low heat of vaporization The typical ORC process is presented in the temperature–entropy diagram in Fig. 9.9. The process is idealized by neglecting pressure and heat losses. The steps are: ∑ ∑

expansion of vapour in turbine (from 1 to 2) removal of superheating in recuperator (from 2 to 3) © Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 213

4/5/11 9:23:11 AM

© Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 214

4/5/11 9:23:11 AM

Thermal oil (300–250 °C) to ORC evaporator 2220 kW thermal

ORC turbogenerator HER400

Hot water (60–90 °C) to ambient heating ~ 1780 kW thermal

9.6 Diagram showing production of electricity and district heat from biomass (Bini and Manciana 1996). Top right: biomass ORC plant in Kopfling, Austria 200 kW; bottom right: biomass ORC plant in Althofen, Austria 1500 kW (www. turboden.eu).

Biomass powered boiler

Power output 400 kW electric

Air-cooler

© Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 215

4/5/11 9:23:11 AM

Cooled geothermal fluid

Injection well

Injection pump

Motive fluid pump

9.7 Principle of ORC using geothermal heat as heat source, and two example plants in Iceland (top right) and in Costa Rica (bottom right) (www.ormat.com, Ormat 2008).

Hot geothermal fluid

Production well

Preheater

Vaporizer

Generator

Turbine

Condenser

Air-cooled binary geothermal power plant

© Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 216

4/5/11 9:23:11 AM

Fuel control panel

Fuel inlet

Control cables for fuel panel Thermostat

Organic fluid

Condensate feed pipe

Alternator Canister Feed pump

Turbine wheel

Condensate outlet

Condenser

Chimney

9.8 Principle of micro-ORC plant for special applications, and two example plants: pipeline gate valve station in Alaska (top right) and Telecom link in Chile (bottom right) (www.ormat.com).

Burner

Vapour generator

Electrical output filtered DC

Electrical cabinet including rectifier

Vapour

Vapour inlet to turbine Input AC cables to rectifier

Turbine nozzle

Vapour inlet to condenser

Vacuum valve

ormat© Energy Converter (OEC)

ORC based waste heat/waste fuel recovery systems

217

T Critical point 1 6

7 Superheated vapour

Wet vapour

2

Liquid

5 4

3

S

9.9 Simplified ORC process in temperature–entropy diagram. It should be noted that typical organic vapour remains superheated in expansion, in contrary to water vapour, which is moist after expansion.

∑ condensing (from 3 to 4) ∑ pressure increase of the liquid (from 4 to 5) ∑ preheating of the liquid in recuperator and boiler (from 5 to 6) ∑ vaporizing in boiler (from 6 to 7) ∑ superheating in boiler (from 7 to 1). When comparing ORC and water vapour processes, there is a clear difference in relative magnitude of vaporization heat. If the temperature of the heat source gas is limited (e.g. exhaust gas), this high vaporization heat limits the obtainable pressure and superheating in the case of water vapour process. When the organic fluid is selected properly (vaporization happens near the critical point), the relative magnitude of vaporization heat is small, and we can obtain significantly higher average temperature and thus higher process efficiency. This is presented in Fig. 9.10.

9.4.2 Benefits of low enthalpy drop in turbine The specific enthalpy drop of organic vapours in turbine is small when compared to water vapour. This makes turbine design easy. In most cases a single stage turbine with reasonable tip speed is sufficient. The specific

© Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 217

4/5/11 9:23:11 AM

Small and micro combined heat and power (CHP) systems

E

Temperature

218

a xh

us

tg

as

O

rg

a

c ni

flu

id

Water

Total enthalpy

9.10 Comparison of ORC and water vapour process in temperature– total enthalpy diagram. Relative magnitude of vaporization heat (horizontal line) of water vapour is high, which results in reduced average temperature when compared to ORC.

enthalpy drop in the high efficiency turbine stage is about the square of tip speed. In the case of water vapour, three turbine stages are required in most cases in order to obtain high efficiency. In addition to this, if the power is small, the optimum speed of a water vapour turbine may be unrealistically high. Turbine power is mass flow multiplied by specific enthalpy drop. Thus low enthalpy drop means high mass flow and high volumetric flow, which makes the turbine design stage easier (larger relative blade height). We may introduce specific speed Ns of the turbine stage: Ns =

w qv2 D hs0.75

9.1

where w is the angular velocity of shaft, qv2 is the volumetric flow at turbine outlet and Dhs is the isentropic drop of specific enthalpy. In order to obtain good turbine stage efficiency, Ns must be higher than 0.3–0.5 (depending on turbine design). This condition is easily obtained with organic vapour with reasonable shaft speed, but in the case of water vapour (high enthalpy drop, low volumetric flow) we must either use a very high rotational speed, or accept low Ns or use a multistage turbine. Also, a multistage turbine may be necessary in the case of water vapour in order to avoid too high a tip speed of the turbine rotor.

© Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 218

4/5/11 9:23:12 AM

ORC based waste heat/waste fuel recovery systems

219

The disadvantage in ORC turbine design is the low sound velocity in organic fluids. Thus, despite the reasonable low gas velocities, supersonic velocities cannot usually be avoided in single stage ORC turbines (Hoffren et al. 2002, van Buijtenen et al. 2003, Turunen-Saaresti et al. 2006, Colonna et al. 2008). This is becausse, for example, 250 m/s is clearly subsonic for air, but supersonic for most organic fluids. Velocities in this range must be used in turbine in order to obtain good efficiency. Despite the supersonic velocity problems, it is easy to make a small ORC (e.g. 25 kW) with a high efficiency single stage turbine, whereas water vapour process must be made in most cases with a three or four stage turbine, thus resulting in a practical minimum size of 2000–3000 kW.

9.4.3 Benefits of good dielectric properties Most organic working fluids have good dielectric properties. This means that generator windings may be exposed to the organic vapour, in contrast to water, which is a poor insulator, and may cause a short circuit in windings.

9.4.4 Disadvantages of ORC with large size plants In large power plants, the specific price is lower for the water vapour process than for the ORC. Thus, if the power plant size is big enough, say 4000–5000 kW or bigger, the water vapour process is in most cases cheaper and more efficient than the ORC. If power is high enough, we can afford a multistage turbine, and relatively large vaporization heat problem can be avoided by using several pressure levels (e.g. large combined power plants use up to three pressure levels in order to maximize the average temperature level in the water vapour process using exhaust gas heat from a gas turbine). Limiting power depends on the heat source temperature. If the heat source temperature is very high, the water vapour process may be favourable down to 2000 kW; if the heat source temperature is low, the ORC may be favourable up to 5000–10 000 kW.

9.5

Selection of working fluid for organic Rankine cycle (ORC) systems

9.5.1 Thermal stability The theoretical thermal efficiency of ORC is not very much affected by the working fluid. It is almost entirely defined by the temperature difference between condenser and heat source. However, many practical properties of the working fluid affect the obtainable efficiency. One is the thermal stability. The process fluid should not decompose in the temperatures used, or this

© Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 219

4/5/11 9:23:12 AM

220

Small and micro combined heat and power (CHP) systems

decomposition should be extremely small. Otherwise the main products of decomposition, non-condensing gases, fill the condenser and decrease efficiency. Other harmful products of decomposition (depending on fluid) are carbonous particles, or glue-like polymers (siloxanes), or in the case of fluoro-containing fluids some toxic compounds. For most organic fluids, temperatures higher than 400 °C cause problems. Decomposition is affected by many factors, e.g. certain metals may act as catalyst. Thus the best way to find the limits of thermal stability of a fluid is to run it a long time in a test loop, where conditions and duration of high/low temperature correspond approximately to the real process (Calderazzi and Colonna 1997).

9.5.2 Volumetric flow In most ORC plants it is favourable to use a single stage turbine. In order to obtain good efficiency, the specific speed of the turbine should be within certain limits. Because the specific enthalpy drop does not vary very much with typical organic fluids, the easiest way to adjust the specific speed is to select suitable volumetric flow at the turbine outlet, see Equation 9.1. This does vary very much with different fluids, depending on the vaporization pressure. For small ORC plants it is favourable to select a fluid with high volumetric flow, because this will ease the turbine design (not too high speed).

9.5.3 Pressure in condenser If the pressure in the condenser is higher than the atmospheric pressure, leakage of air into the process is impossible. However, volumetric flow in the turbine will be low. If the pressure in the condenser is very low, air will easily leak to the process, and vacuum suction must be used in order to avoid pressure increase in the condenser, resulting in a decrease in efficiency. Also, one problem is that air (oxygen) is a catalyst for thermal decomposition; even ‘safe’ temperatures in the evaporator might become problematic if oxygen is present there.

9.5.4 Environmental aspects In general, working fluid should have zero ODP (ozone depletion potential) and low GWP (global warming potential). Preferably it should be non-toxic and non-combustible. Also, of course, it should be inexpensive. Usually all the requirements cannot be met simultaneously. Thus many working fluids used are, for example, combustible and also toxic to some extent. Table 9.2 presents the properties of some working fluids used in present day plants.

© Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 220

4/5/11 9:23:12 AM

ORC based waste heat/waste fuel recovery systems

221

Table 9.2 Properties of some working fluids used in present day plants (Heinimö and Jäppinen 2005) Working fluid

n-Pentane

Octamethyltri-siloxane Toluene (Silicone oil)

Chemical formula

C5H12

C8H24O2Si3

C 7H 8

Molecular weight (g/mol)

72.2

236.5

92.1

Density (kg/m3)

630 (20 °C)

820 (25 °C)

870 (20 °C)

Smelting point (°C)

–129

–86

–95

Boiling point (°C)

36

152–153

111

Auto ignition point (°C) 309

418

480

Ignition limits in air (vol.%)

1.5–7.8

0.9–13.8

1.1–1.7

Viscosity (mPas)

0.224 (25 °C)

0.820 (25 °C)

0.560 (25 °C)

Heat of vaporization at room temperature 25 °C (kJ/kg)

366

Safety aspects

Flammable, toxic

413

Flammable, toxic to some extent

Flammable, toxic to some extent

9.5.5 Dry or wet expansion Most organic fluids experience dry expansion in the turbine, and their degree of superheating will increase. This makes the control of the vaporizer easy: in most cases it is sufficient to ensure that the vapour entering the turbine is dry; complex control of the degree of superheating is not needed.

9.6

Process system alternatives

There are several alternatives in building an ORC plant. ∑

The vaporizer may be run directly by the heat source fluid, or by a thermo-oil circuit. ∑ The turbine may be connected directly to the generator, or a reduction gear used. ∑ The feed pump may be connected directly to the turbine, or a separate, multistaged pump run with an electric motor may be used. ∑ The generator may be air cooled or exposed to the working fluid vapour and cooled with it. ∑ The generator may be connected directly to the network and run at synchronous speed, or it may be connected through an inverter, which permits variable speed of the generator.

© Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 221

4/5/11 9:23:12 AM

222

Small and micro combined heat and power (CHP) systems

∑

The process may be hermetic, or provided with one or several shaft seals, having potentially some leakage. ∑ The turbine bearings may be lubricated with process fluid, magnetic bearings may be used, or the shaft may be provided with external, oil lubricated bearings (usually the working fluid cannot be mixed with oil; significant reduction of thermal stability may result). ∑ The condenser heat may be used for heating purposes (CHP), or condenser heat may be rejected to the air (or water). The thermo-oil circuit has been widely used, for example by Turboden, GMK and also Ormat in certain applications. The benefit is that in most cases the customer is able to build the thermo-oil circuit, and the ORC manufacturer can connect the ORC energy converter directly to this circuit. Several thermooils can be used, e.g. Therminol 66. However, if the heat source is at limited temperature, this extra circuit decreases the ORC vapour temperature and thus decreases the efficiency. Also, the maximum permitted temperature of thermo-oil, if it is lower than that of the ORC fluid, may limit the maximum temperature of the circuit. The thermo-oil circuit also increases investment expenses to some extent. If the working fluid is exposed directly to the boiler, the boiler must be particularly designed for the organic fluid. In this design process one important target is to avoid hot spots at the sides of the working fluid (if heat source gas is very hot). This means that the ORC manufacturer must either also supply the boiler or at least participate in the boiler design. An obvious benefit is a more simple system and higher efficiency if the heat source temperature is limited. The conventional system in turbine design is to use a high speed turbine, shaft seal, reduction gear and air-cooled, synchronous generator. However, the turbine shaft seal may be a difficult component, and an oil system is needed for the reduction gear. In so-called high speed technology solution, turbine, generator, and feed pump are connected directly to each other, and provided with process fluid lubricated bearings. In this case the generator windings are exposed to the organic fluid vapour, and the generator is connected to the network through an inverter. Because of the lack of outgoing shafts, the turbogenerator can be made fully hermetic. Probably the first of this type of plants made were the micro-ORC plants supplied by Ormat at the beginning of the 1960s (Bronicki 1988). Larger high speed technology plants with high pressure ratio circuits were developed by Lappeenranta University of Technology (Larjola 1984, 1988), resulting later as a commercial plant in co-operation with Tri-O-Gen b.v. (van Buijtenen et al. 2003, van Buijtenen 2009).

© Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 222

4/5/11 9:23:12 AM

ORC based waste heat/waste fuel recovery systems

9.7

223

Background and summary of commercial development and exploitation

9.7.1 Historical review of early development stages 1960–1984 The basic idea behind the ORC is quite old. The first plants were built at the beginning of the 1960s. The majority of the plants built between 1960 and 1984 are shown in Table 9.3. It can be seen that two of the companies, Ormat and Turboden, are still major ORC manufactures today. Excluding chloro-fluoro-carbons, the most popular working fluids at that time were chlorobenzenes, fluorinol 85 and toluene.

9.7.2 Examples of current technology provided by present day manufacturers Several hundred ORC plants have been built during the last decade. Because nowadays it is very important to decrease CH4 emissions and to use all waste heat in electricity production, it is expected that the number of ORC plants will further increase quickly. CH4-rich gases (biogas, landfill gas) produced by the decay of organic material should be burned, or biomass should be burned before it produces CH4, and in this conversion ORC is very suitable, as stated earlier. Some present day ORC manufacturers are described below in alphabetical order: GMK – Gesellschaft für Motoren und Kraftanlagen mbH, Germany GMK was founded in 1994, and it makes ORC plants for industrial waste heat, geothermal and biomass applications. Heat transfer from heat source to ORC power conversion module is made with a thermo-oil circuit (see Fig. 9.11). Typical power range is from 500 kW to 5 MW. GMK has supplied about 10 ORC plants (see Fig. 9.12) (situation May 2010) (http://www. gmk.info/). Ormat Technologies Inc Ormat was founded in Israel, and was making micro-ORC plants as long ago as 1960, introducing production of industrial size ORC plants in 1980 (Bronicki 1988). Ormat concentrates mainly on geothermal applications, and its main factories are nowadays in the US (its head office is located in Reno, Nevada). The total number of ORC plants supplied is over 2000. Ormat also makes quite large plants, and ORC plants using biomass heat or industrial

© Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 223

4/5/11 9:23:12 AM

© Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 224

4/5/11 9:23:12 AM

1980–1983 1979

22 1 1 1 1 1 1 1 1 1

R114 92 Toluene 320 Toluene 323 R114 85 R11 160 13 R11 24 280 11.6 C8F16 R114 105 15.3

1979 1982 1980 1982

1969 1972 1974 1975 1977 1977 1978–1982 1979 1979

1966 1967 1968

1961 1960–1984

Year of introduction

1 1 1 1 1 1 5 1 1

1 1 1

1 n. 3500

Number of units

R11 116 11.6 Fluorinol 85 288 48.3 R11 123 13.3 Toluene 274 22 R113, A 90 12.4 Fluorinol 50 321 55.2 Toluene 251–274 16–22 Toluene Tetrachloro- 110 0.7 ethylene R114 R11 88 6.2

Japan IHI, Tokyo 475 USA Thermo Elektron Co., Waltham 108 Japan IHI, Tokyo 500 USA Sundstrand, Rock ford, Illinois 600 Japan IHI, Tokyo 500 USA Thermo Elektron Co., Waltham 34 USA Sundstrand, Rockford, Illinois 600 USA Sundstrand, Rockford, Illinois 200 Italy Germmindustria S.n.c, Milan 40 Israel Ormat turbines, Yavne 50–600 USA Mechanical Technology 500 Incorporated, Latham, NY Germany 21 100 170 500 Germany Kali-Chemie, Hanover, 30 Spillingwerk, Hamburg 130 Italy Gemmindustria, S.n.c, Milan 45 Italy Franco Tosi S.p.A, Legnano 500

Parameters P max, bar

R11 Trichloro- 200 benzene R11 120 12.5 R12 R11 121 12.8

Process T max °C

USA Trukline gas Co., Houston 347 Several Ormat turbines, Yavne 0.2–3 countries Japan IHI, Tokyo 190 USSR 750 Japan IHI, Tokyo 3800

Country Manufacturer Output, kW Fluid

Table 9.3 Historical review of the early stages of ORC development 1960–1984

© Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 225

4/5/11 9:23:12 AM

USA Baber-Nichols Co., Denver, Co Japan Mitsui Engineering & 14000 Shipbuilding Co. USA Mechanical Technology 1000–1500 Incorporated, Latham USA Ormat turbines, Yavne, Israel 5000 France Betin & Cie, Creusot-Loire 1000 Japan Mitsubishi Heavy Industries Ltd 3280 Italy Turboden S.n.1, Milan 100 100 Finland Lappeenranta University of 100 Technology 8.1 50 6.7 0.9 0.8 22

75 330 90 173 130 170

R114 Fluorinol 85 R11 Dichloro- benzene C8H10 R114

7.9

130

R113

34

260

R12, R113 R114 isobutane Fluorinol 85

1 1

1 1 1 1

3

1

1984 1984

1982 1983 1983 1984

1981–1984

1981

226

Small and micro combined heat and power (CHP) systems

9.11 One typical ORC power conversion module from GMK (http:// www.gmk.info/).

2 3

1

5

4

9.12 Example of GMK ORC plant connected to diesel engine. 1. Diesel exhaust gases heat up the thermo-oil in heat exchanger 2, and thermo oil, circulated by pump 4 vaporizes the ORC working fluid in vaporizer 5. ORC power conversion module (turbine, generator, condenser, feed pump) is situated between vaporizer and diesel engine (http://www.gmk.info/).

waste heat are mainly connected to their heat source by thermo-oil circuit (see Fig. 9.13). Power of energy conversion modules delivered range from 200 kW to 22 MW (http://www.ormat.com/).

© Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 226

4/5/11 9:23:13 AM

© Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 227

4/5/11 9:23:13 AM

Heat recovery unit

Storage/expansion tank

Preheater

Vaporizer

Turbine

Condenser

Recuperator

Motive fluid pump

Generator

Recovered energy generation (REG) using OEC

9.13 Flow diagram of Ormat ORC plant producing electricity from industrial waste heat or from biomass. Thermooil is preheating and vaporizing the working fluid in separate heat exchangers (http://www. ormat.com/).

228

Small and micro combined heat and power (CHP) systems

Tri-O-Gen B.V., Goor, The Netherlands Tri-O-Gen made its first plant in 2003. However, its roots are deeper, as mentioned earlier: the high speed design is based on development work at Lappeenranta University of Technology, where the first prototype of this type was made in 1984. Since 2004 Tri-O-Gen has supplied about 10 plants mainly to biogas and landfill gas applications (situation May 2010). Plants are based on high speed technology and on hermetic circulation process. No thermo-oil circuit is used; working fluid vaporizer is run directly with the heat source gas (see Fig. 9.14) (http://www.triogen.nl/). The principle of the ORC based on hermetic high speed technology is shown in Fig. 9.15 (van Buijtenen 2009). Heat source gas vaporizes the working fluid in the vaporizer, hot vapour expands in the turbine, and superheating is removed in the recuperator, which preheats the fluid going to the vaporizer. Turbine, generator and feed pump are directly connected to each other, and this common shaft is provided with process fluid lubricated bearings. Because of high speed, the feed pump needs only one stage, and the generator, cooled with the process fluid, is small compared to the power. The process is hermetic, because there are no shaft outlets. The size of the standard power conversion module is 160 kW, but it may be

9.14 ORC power conversion module from Tri-O-Gen (left). Heat source gas (in this case exhaust gas of a biogas engine) is introduced directly to the process fluid vaporizer (grey in the middle) (Trio-O-Gen 2006).

© Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 228

4/5/11 9:23:13 AM

ORC based waste heat/waste fuel recovery systems

Bearings Pump

229

Frequency inverter

Generator Turbine

Process fluid as gas

Electricity into grid

Cooling For ex. district heating grid Heating energy

Process fluid as liquid

9.15 Process diagram of the Tri-O-Gen ORC plant based on high speed technology (Tri-O-Gen 2006).

9.16 Power conversion module of typical Turboden ORC plant (Bini and Costa 2010).

provided with several parallel turbogenerators, thus giving a power range up to 800 kW. Turboden S.r.l., Brescia, Italy Turboden made its first plant in 1984. The design is based on the thermo-oil circuit. Heat is introduced with thermo-oil to the power conversion module (Figs 9.16 and 9.17). The turbine is reported to have very high efficiency. In

© Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 229

4/5/11 9:23:13 AM

230

Small and micro combined heat and power (CHP) systems

9.17 Schematic of Turboden power conversion module (Bini and Costa 2010). Typical flow diagram of Turboden plant in biomass CHP application is presented in Fig. 9.6.

many references the heat source is biomass. Typical plant size is from 400 to 2000 kW. Turboden has supplied in total 139 ORC plants for biomass applications, 13 for waste heat applications and 3 for geothermal applications (situation March 2010). Nowadays Turboden is a part of Pratt & Witney Power Systems (http://www.turboden.eu/en/home/).

9.8

Efficiency and typical costs for current organic Rankine cycle (ORC) plants

It is difficult to give accurate figures for ORC plant efficiency, because it is very dependent on cycle upper (T2) and cycle lower (T1) temperature. Upper temperature may vary significantly due to heat source temperature, and lower temperature may vary due to condensing connections: is the condensing heat used for district heating (CHP), or is it rejected to ambient air or to sea or lake water? Thus a good method is to calculate Carnot efficiency on the basis of known upper and lower temperature, and then compare this to the given ORC plant efficiency horC (= electric power/input heat power). This ratio may be called the ORC process efficiency hpros and is a measure of plant quality. Suppose that the upper temperature of the working fluid is 320 °C Æ T2 = 593 K, and the lower or condenser temperature is 50 °C Æ T1 = 323 K. Supposing that the given ORC plant efficiency is 21%, it can be obtained from Equation 9.2 that hpros = 46%. T horC = hpros ÈÍ1 – 1 ˘˙ T 2˚ Î

9.2

© Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 230

4/5/11 9:23:14 AM

ORC based waste heat/waste fuel recovery systems

231

In general the ORC plant efficiency is much lower than the theoretical maximum Carnot efficiency. In most cases ORC plant efficiency is in the order of 18–22%. The above mentioned hpros is in the range of 44–50% for many plants. Thus hpros can be set at 50% in order to estimate hORC for a particular case. It should be noted that if a thermo-oil circuit is used, the upper temperature of thermo-oil must be at least 5–10 °C higher than T2. And T1 must be at least 5–7 °C higher than the upper temperature of the condenser cooling water. Some manufacturers give specific prices in public papers (Duvia et al. 2009, van Buijtenen 2009, Vescovo 2009) for ORC. The specific price is very dependent on what is included and what the heat source is. If the heat source is biomass, and a burning system is included, expenses are of course higher than if the heat source is industrial waste heat. Speaking in very rough figures, in favourable conditions, specific prices of complete plants fall in the range 71600–45007/kWe (year 2009). However, before a real feasibility study is made, one should get a quotation for that particular case.

9.9

References

Bini, R. and Costa, A.: Turboden marketing material, 2010. Bini, R. and Manciana, E.: Organic Rankine cycle turbogenerators for combined heat and power production from biomass. Energy conversion from biomass fuels; current trends and future systems. Munich, Germany, 22–23 October 1996. Bronicki, L.: Experience with high speed organic Rankine cycle turbomachinery. Proceedings of Conference on High Speed Technology, 21–24 August 1988, Lappeenranta, Finland. Lappeenranta University of Technology, Department of Energy Technology, Publ. No ENTE D-15, pp. 47–61, 1988. Calderazzi, L. and Colonna, P.: Thermal stability of R-134a, R-141b, R-13I1, R-7146, R-125 associated with stainless steel as a containing material. International Journal of Refrigeration, Volume 20, pp. 381–389, 1997. Colonna, P. and Pasquale, D.: Micro-ORC turbogenerator: a viable option for domestic cogeneration. Delft University of Technology, Process and Energy Department, Delft, The Netherlands, Technical Report ET-2272, April 2008. Colonna, P., Harinck, J., Rebay, S. and Guardone, A.: Real-gas effects in organic Rankine cycle turbine nozzles. Journal of Propulsion Power, Volume 24, pp. 282–294, March–April 2008. Duvia, A., Guercio, A., Rossi di Schio, C.: Technical and economic aspects of biomass fuelled CHP plants based on ORC turbogenerators feeding existing district heating networks. Biomass Conference 2009, Hamburg 2009. Heinimö, J. and Jäppinen, E.: ORC-technology in distributed electricity production (in Finnish). Lappeenranta University of Technology. Research report EN B-160. 2005. Heinimö, J., van Buijtenen, J.P., Backman, J., Ojaniemi, A., Malinen, H.: High Speed ORC technology for distributed electricity production. 2nd World conference on biomass for energy, industry and climate protection, Rome, Italy, 10–14 May 2004. Hoffren, J., Talonpoika, T., Larjola, J., Siikonen, T.: Numerical simulation of real-gas flow

© Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 231

4/5/11 9:23:14 AM

232

Small and micro combined heat and power (CHP) systems

in a supersonic turbine nozzle ring. ASME Journal of Engineering for Gas Turbines and Power, Volure 124, pp. 395–403, April 2002. http://www.gmk.info/ http://www.ormat.com/ http://www.triogen.nl/ http://www.turboden.eu/en/home/index.php Larjola, J.: ORC-plant with high-speed gas lubricated turbogenerator. VDI-Berichte 539: ORC-HP-Technology (proceedings of VDI-Seminar: ORC-HP-Technology, 10–12 September 1984, ETH Zurich), pp. 697–705. Dusseldorf 1984. Larjola, J.: ORC power plant based on High Speed Technology. Proceedings of Conference on High Speed Technology, 21–24 August, 1988, Lappeenranta, Finland. Lappeenranta University of Technology, Department of Energy Technology, publ. No ENTE D-15, pp. 63–77, 1988. Ormat Technologies, Inc. 2008 Sustainability Report. Sundstrand: ORC Leaflet, 1983. Tri-O-Gen: Marketing material 2006. Tri-O-Gen: Marketing material 2010. Turboden company presentation, Codice doc09Z00158_e, 2010. Turunen-Saaresti, T., Tang, J., van Buijtenen, J., Larjola, J.: Experimental and numerical study of a real-gas flow in a supersonic ORC turbine nozzle. Paper No. GT2006-91118, ASME Turbo Expo, Barcelona, 8–11 May 2006. van Buijtenen, J.P., Larjola J., et al: Design and validation of a new high expansion ratio radial turbine for ORC application. 5th European conference on turbomachinery, Prague, March 2003. van Buijtenen, J.P.: The Tri-O-Gen organic Rankine cycle: development and perspectives. Journal of the IDGTE Power Engineer, Volume 13, Issue 1, March 2009. Vescovo, R.: ORC recovering industrial heat; power generation from waste energy streams. Cogeneration and on site power production, March–April 2009.

© Woodhead Publishing Limited, 2011

CHP-Beith-09.indd 232

4/5/11 9:23:14 AM

10

Fuel cell systems for small and micro combined heat and power (CHP) applications

D. J. L. B r e t t, University College London, UK, N. P. B r a nd o n and A. D. H a w k e s, Imperial College London, UK and I. S t a f f e ll, University of Birmingham, UK

Abstract: Fuel cells are electrochemical energy conversion devices that turn chemical fuel directly into electrical power as well as generating heat. They operate at high efficiency and can be applied across a wide range of applications. Micro-combined heat and power (CHP) is one area in which fuel cells are expected to have a particularly significant impact with the potential for lowering energy cost and CO2 emissions in the residential housing sector. This chapter looks at the technological aspects of fuel cells applied to micro- and small-scale CHP applications as well as examining the state of commercial development and future trends. Key words: fuel cell, stack, heat and power, system.

10.1

Introduction

Fuel cells are electrochemical devices that convert the chemical energy of a fuel directly into electricity and heat without involving the process of combustion. A simplistic view of a fuel cell is a cross between a battery (chemical to electrical generator) and a heat engine (chemical to heat to generator) (Hawkes et al., 2009a). There are a number of fuel cell technologies with very different designs, each suited to different applications; however, they all share the characteristics of high efficiency, no moving parts, quiet operation, and low or zero emissions at the point of use. In addition, modular stack design means that there are no technical limitations on minimum capacity, which is a problem for mechanical heat engines. The range of fuel cell applications and size of the potential markets are enormous, including: battery replacement in small portable electronic devices, prime movers and/ or auxiliary power units in vehicles, large scale (megawatt) electrical power generation and high-reliability backup power. Fuel cells are also well suited to providing combined heat and power (CHP) at both the ‘micro’ scale, relevant to domestic and small commercial loads (ca. 1–5 kWe) and ‘small’ scale, suitable for larger commercial and municipal applications (tens of 233 © Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 233

4/5/11 9:23:36 AM

234

Small and micro combined heat and power (CHP) systems

kWe). It is the small- and micro-scale CHP application for fuel cells that is the subject of this chapter.

10.2

Fundamentals of operation, types and properties of fuel cells

10.2.1 Fundamentals of operation Individual fuel cells consist of an anode, electrolyte and cathode, and are electrically connected in series to form a ‘stack’. Electrically conductive interconnectors (or bipolar plates) are used to distribute fuel and oxidant to the individual cells, and to electrically connect them together. Coolant fluid can also be distributed through channels in the interconnects, or through additional plates inserted between cells. Taking the low temperature polymer electrolyte fuel cell (PEFC) operating on hydrogen as an example, Fig. 10.1 shows the basic operation. Electrons are stripped from the incoming hydrogen at the cell anode, forming ions which pass through a conductive electrolyte to combine with oxygen at the cathode. The stripped electrons produce an electric current through the external circuit and do useful work. The exact reactions that occur depend on the type of fuel cell (as several technologies exist), but for hydrogen fuelled cells, the overall reaction is the same as that of fuel combustion: H 2 + 1/2O2 Æ H2O.

e–

e–

Interconnect

Electrolyte

H2

H2 –

e–

H+

e–

H+

+ O2

O2 H 2O

Cathode

H 2O

Anode

10.1 Illustration of fuel cell operation (two cells in a stack) taking the hydrogen fuelled polymer electrolyte fuel cell (PEFC) as an example.

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 234

4/5/11 9:23:37 AM

Fuel cell systems for small and micro CHP applications

235

Figure 10.2 (a) illustrates the voltage and power dependence on current output for a generic fuel cell. An increase in current density (current per unit area of each cell) results in a decrease in operating voltage due to internal losses in the system. Power output initially increases with current and reaches a maximum at point D, above which the decreasing voltage and increasing losses in the system result in loss of electrical power output, although the heat generated continues to increase. The nominal operating point is around point C, typically ~ 2/3 to ¾ of the open circuit voltage (OCV) of the cell (which is around 1 V for a PEFC operating on hydrogen). The point of operation is a trade-off between electrical efficiency and capital cost (Ang et al., 2010), and for a CHP system the requirement to service the heat load is also a factor in determining the operating point. Considering the whole fuel cell system, Fig. 10.2(b) shows how the electrical and thermal efficiency varies with electrical load. In contrast to heat engines, which have a maximum efficiency at their nominal operating point, fuel cells are known to have excellent ‘turn-down’ performance, i.e. reducing the electrical load results in higher electrical efficiency, to a point. However, since there are components that require electrical supply (e.g. (a)

Stack power

Cell voltage

D

C

B A

The Elect

rical

rma

l

Thermal and electrical system efficiency

Stack electrical efficiency

(b)

Current density

10.2 Illustration of the operating range of a fuel cell, showing (a) stack voltage and power and (b) electrical and thermal efficiency. Labelled operation points are described in the text.

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 235

4/5/11 9:23:37 AM

236

Small and micro combined heat and power (CHP) systems

sensors, actuators, control system), and their load is constant regardless of the power delivered by the fuel cell, this parasitic load degrades the system efficiency at low electrical load. There is a point B where the parasitic load equals the power delivered by the fuel cell and the system therefore has ‘zero efficiency’. In a similar sense, high temperature fuel cell stacks such as solid oxide fuel cells (SOFCs) need to generate heat to maintain their operating temperature, and so have a lower bound on their operational window below which the stack is no longer thermally self-sustaining and begins to cool. There is therefore a practical lower limit below which the system cannot operate. The exact point will depend on the size, shape and materials used to construct the SOFC, but is typically of the order of 20% of the nominal operating point. It can be seen from Fig. 10.2(b) that as the electrical load on the fuel cell increases, the thermal efficiency increases and the electrical efficiency decreases. The way in which the heat-to-power ratio (HPR) of the fuel cell varies with electrical load will depend very much on the system design, but will generally tend to increase when subjected to heavy electrical loading. However, it should be remembered that the heat-to-power ratio of the system can also be controlled at any fuel cell operating point by varying the fuel utilisation and the amount of heat generated in the afterburner. Hydrogen is the ideal fuel for most fuel cell types based on performance and durability; however, it is not practical for direct use in homes since no hydrogen generation or distribution infrastructure currently exists. Instead, hydrocarbons (particularly natural gas) are seen as the ideal fuel for microCHP systems, as these can be reformed into hydrogen at the point of use (Hawkes et al., 2009a). Natural gas is low cost, abundant (for the time being), and has extensive infrastructure throughout Western Europe. All commercial fuel cell micro-CHP systems are fuelled by natural gas, LPG or kerosene. However, academic studies have also described systems running off gasified coal (Marquez et al., 2007), diesel (Lindström et al., 2009), biogas from waste (Kiros et al., 1999; Spiegel et al., 1999), biomass (Xuan et al., 2009), and biologically produced hydrogen from sugary waste (Penfold, 2004; Redwood, 2008). One of the criticisms of fossil fuelled micro-CHP is that it may be limited to a 20–30-year window of opportunity, after which the scarcity of natural gas and decarbonisation of centralised electricity would make it unattractive (Harrison, 2008). The potential to operate fuel cells on various types of biogas would, however, offer a solution to this problem, whilst giving profound reductions in CO2 emissions, assuming a sustainable and carbon-neutral production route could be found (Redwood, 2008; Das and Veziroglu, 2001; Wakayama and Miyake, 2001).

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 236

4/5/11 9:23:37 AM

Fuel cell systems for small and micro CHP applications

237

10.2.2 Types of fuel cell stack There are more than a dozen distinct fuel cell technologies under academic and commercial development; however, only few are suitable for domestic small- and micro-CHP. For this application the fuel cell stack must have (at least the potential for) low cost manufacture and long operating lifetime in suboptimal conditions, particularly with regard to impurities in the hydrogen fuel. There are also considerations about safety, practicality and cost-effectiveness to consider, which favour those technologies that are well established and commercially demonstrated, and offer high operating efficiency. Four fuel cell technologies have been applied to CHP applications: ∑ ∑ ∑ ∑

PEFC: polymer electrolyte fuel cells;1 SOFC: solid oxide fuel cells; PAFC: phosphoric acid fuel cells; AFC: alkaline fuel cells.

While these technologies share the same operating principles outlined in the previous section, there are some fundamental differences in the way they achieve their electrochemical reactions. Three of the characteristic differences are the diverse materials they are made from, their range of operating temperatures and the fuels they can tolerate. Table 10.1 summarises the typical materials of construction of each fuel cell type, along with their operating conditions and tolerances to fuel impurities. Domestic CHP systems based on PEFC and SOFC stacks have received intense research and commercial development over the last decade. There are at least a dozen major companies actively pursuing this market, and products have been deployed in large-scale field trials throughout Japan, South Korea and Germany. The majority of this chapter will therefore focus on these two technologies. Despite being developed 10–20 years earlier, PAFCs and AFCs failed to retain substantial commercial interest due to difficulties in overcoming high manufacturing cost and low lifetime, respectively. No significant products have been developed for the domestic CHP market; however, they possess many of the desired characteristics and have been demonstrated at the 1–10 kWe scale operating on natural gas as CHP units (Ghouse et al., 2000; Independant Power Technologies Ltd, 2006).

10.2.3 Materials for PEFCs and SOFCs PEFC technologies use precious metal electrocatalysts (e.g. platinum) to ensure adequate electrode reaction kinetics for a high power output. Specialised 1

PEFC is also referred to in the literature as PEMFC (proton exchange membrane) and SPFC (solid polymer).

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 237

4/5/11 9:23:37 AM

238

Small and micro combined heat and power (CHP) systems

Table 10.1 General operating characteristics of each fuel cell technology, taken from (Staffell, 2010) SOFC

PAFC

AFC

Electrodes

Pt, Ru, C, PTFE

Ni, LSM

Pt, C, PTFE

Pt or Ni, C, PTFE

Electrolyte

Solid polymer (PFSA)

Ceramics: YSZ, LSM

Liquid H2SO4 Liquid KOH

Interconnect

Graphite, steels Chromium Graphite alloys, steels

Graphite, metal or plastic

Operating temperature

30–100 °C

500–1000 °C

200–250 °C Must remain >70 °C

50–200 °C

Fuels

H2

H2, CO

H2

H2

Sulphur (as S, H2S)

< 0.1 ppm

< 1 ppm

< 50 ppm

?

CO

< 10–100 ppma

Fuel

< 0.5–1%

< 0.2%

CO2

Diluent

Diluent

Diluent

< 100–400 ppm or < 0.5–5%b

CH4

Diluent

Fuel/Diluentc Diluent

Diluent

NH3

Poison

< 0.5%

?

Fuel tolerance

PEFC

< 4%

Abbreviations: PTFE (polytetrafluoroethylene – better known as Teflon™), PFSA (perfluorosulfonic acid – for example Nafion™), YSZ (yttria-stabilised zirconia), LSM (lanthanum-strontium-managanate). a Standard Pt anode catalysts can only withstand CO concentrations up to 10 ppm, and PtRu alloys up to 30 ppm (Song, 2002). These limits can be extended by bleeding air into the anode and using alternative bi-layer catalysts (Uribe et al., 2004; Ball and Thompsett, 2002). b CO2 tolerance is highly dependent on the cell design. Strongly bonded nickel and silver electrodes with a circulating electrolyte can be tolerant, while platinum and carbon with an immobilised electrolyte are highly sensitive. c Internal reforming is possible with SOFC anodes, making desulphurised natural gas a viable fuel. The long lifetimes required for domestic CHP operation have not yet been demonstrated by these systems though.

graphite powders and resins are used, along with the fluorinated polymer electrolytes found in modern chlorine electrolysis cells. High purity hydrogen fuel is required to avoid performance degradation, as the electrocatalysts are easily poisoned by carbon monoxide and other impurities. Management of fuel impurities and hydration of the polymer electrolyte requires relatively complex and expensive engineering solutions, and current work is aimed at relaxing these strict requirements by increasing the operating temperature to over 100 °C (Zhang et al., 2006).

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 238

4/5/11 9:23:37 AM

Fuel cell systems for small and micro CHP applications

239

SOFCs instead use ceramics and specialist chromium alloys or steels that can withstand high temperatures. For micro-CHP applications there is a trend to move from the high temperature region (850–1000 °C) used in early large-scale systems, into the so-called intermediate temperature (IT) range of 500–750 °C (Brett et al., 2008). This allows a wider range of materials to be used, giving cheaper fabrication and improved resilience to cycling ceramic components between ambient and operating temperatures. Lower temperature operation also affords more rapid start-up and shut-down, reduced corrosion rate of metallic components, more robust construction through the use of compressive seals and metallic interconnects as well as the advantage of greatly simplified system requirements (Brett et al., 2008).

10.3

Fuel cell systems

10.3.1 Fuel cell stack Figure 10.3 shows a generic fuel cell system for CHP applications running on reformed hydrogen from natural gas. The major balance-of-plant (BoP) items are shown (i.e., the auxiliary components that enable the fuel cell to operate); however, there are many additional components required for

Fuel in Fuel processor, CO removal and H2S removal

Purge

Inverter DC/AC

DC power

Ejector

Air humidifier

HEX Tank

Compressor/ blower

230 VAC

Air in

Water reservoir

Exhaust

Fuel cell

Make-up water Pump

Heat load

Condenser

HEX

Exhaust

Reservoir

Pump

Pump

Pump Low temp radiator, space heat or HEX to tank

Water cycle

Fuel cycle

Thermal cycle

Air cycle

Electrical power

10.3 Schematic diagram of a stationary fuel cell CHP system.

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 239

4/5/11 9:23:37 AM

240

Small and micro combined heat and power (CHP) systems

operation such as pressure valves, mass flow controllers, sensors, control systems, etc. that are not included. The key issues for each of the main components of the system are now described. The combination of individual fuel cells into an interconnected ‘stack’ represents the main and most expensive component of the entire system. The stack is composed of individual cells (anode, electrolyte and cathode) electrically interconnected using bipolar plates that also distribute reactant to the electrodes. PEFC stacks are usually interspersed with cooling plates that circulate demineralised water, while SOFC stacks rely on air for cooling.

10.3.2 Fuel processor Converting natural gas into an acceptably pure supply of hydrogen requires several processing stages, as outlined in Fig. 10.4. The required stages for each type of fuel cell stack are integrated into a single, compact fuel processing unit, along with the thermal management systems and a steam generator to supply water vapour to the reformer and shifter (Echigo et al., 2004; Yasuda et al., 2001). An SOFC fuel processor is typically composed of a desulphuriser and pre-reformer; whereas PEFCs require a larger reformer, and additional shift reactor and gas clean-up stage, all of which add to the cost and complexity, and erode the total system efficiency. The ability of SOFCs to operate at high efficiency on hydrocarbon fuels is a major advantage over low temperature fuel cells, particularly for micro-CHP applications. Natural gas (or other hydrocarbon fuels) can be converted to hydrogen via a range of processes, including: steam reforming, partial oxidation and autothermal reforming (Kolb, 2008). Steam reforming is generally the preferred method as it produces higher concentrations of hydrogen, and thus requires up to 30% less hydrocarbon fuel (Hubert et al., 2006). The hydrogen rich steam leaving the reformer will contain a proportion of CO and sulphurous compounds (usually converted to H2S in the reforming stage) from the fuel source and added odorants. Both of these molecules are poisonous to PEFCs, while SOFCs are able to use CO as a fuel but remain highly sensitive to many sulphur-containing molecules (Lohsoontorn et al., 2008a; 2008b). These must therefore be removed, usually by reacting with ZnO or adsorption using activated carbon. These units will need to be changed periodically, adding to the maintenance cost of the system. Other desulphurisation techniques exist, but most are not suitable for such smallscale applications (Kolb, 2008). For PEFCs, the level of CO entering the fuel cell needs to be reduced to the order of <100 ppm in order not to poison the Pt-based anode electrocatalyst. The output from the reforming stage contains an appreciable quantity

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 240

4/5/11 9:23:37 AM

Fuel cell systems for small and micro CHP applications Function

Methods

Natural gas

Output gas composition

Remove the sulphur based odorants added Hydroto natural gas for safety desulphurisation, Desulphuriser selective reasons: adsorption. Al ZnO + H2S æÆ 25æ °C ZnS + H2O Catalytically process Steam reforming, methane into hydrogen partial oxidation, with steam and an autothermal absence of oxygen: reforming. Ni-Al /Pt-Pd CH4 + H2O æ ææ CO + 3H2 650–850 650– 850 Æ °C C Improve the hydrogen yield and reduce concentration of the waste carbon monoxide:

High-and lowtemperature shift.

Reformer

Shift reactor

241

C

95

%

2H 6,

CH 1% 4 , 4% CO 2

10% CO, 10% CO2, 0.5–1% CH4

0.5–1% CO, 15% CO2

SOFC

PAFC

Cu-Zn/Fe-Cr Cu Zn/Fe-Cr Zn/FeCr CO + H2O æ æææ 350– 350–450 450 °C (HT) (HÆ T) CO2 + H2 175–300 °C ((L LT)

Preferential Reduce CO concentration oxidation, to ppm levels: pressure swing adsorption, Pt-Ru/Rh-Al P Pt-R t-Ru/ u/Rh Rh-A -All CO + 1 2 O2 æ ææ 150– 150–200 200 Æ °C CO2 methanisation. Reduce CO2 concentration to ppm levels: CO2 + Ca( Ca(OH) C a(OH OH))2 æÆ 25æ °C

CO removal

Soda-lime adsorption, CO2 scrubber regenerative amines, electroCaCO2 swing adsorption.

10 ppm CO, 15% CO2

10 ppm CO, 100 ppm CO2

PEFC

AFC

+ H2O

10.4 An overview of fuel processing for fuel cell systems. Each stage is highlighted in bold down the centre of the diagram and given with the most common processing methods. A description of each stage is given at the far left, along with the ideal reactions for the primary method. Indicative ranges of gas composition after each stage are given to the right. Following the stages down from natural gas to each type of fuel cell on the right indicates which processing stages are required. Reproduced from Staffell (2010).

of CO (typically ~10%) (Brett et al., 2007). CO produced in the steam reforming process can be converted subsequently to CO 2 in a shift reactor, reducing the CO concentration to the order of <1%. Techniques such as selective oxidation of CO and oxygen bleed can be used to reduce the CO concentration to safe levels for the PEFC, at the expense of cost and efficiency, respectively. the fuel processor for a PefC is therefore a relatively complicated system, each stage requiring tight temperature control and thermal management. A high degree of thermal integration is required, as the optimal temperatures for each reaction range from ambient to several hundred degrees, and attaining high

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 241

4/5/11 9:23:39 AM

242

Small and micro combined heat and power (CHP) systems

thermal efficiency is paramount to the overall efficiency of the fuel cell CHP system (Woods and Cuzens, 2001). While this is relatively straightforward for large systems operating at steady state, dynamic operation and the size of small- and micro-CHP fuel processors are much more of a challenge. In particular, the turn-down ratio (the minimum sustainable power output relative to the maximum) is compromised since it is difficult to maintain thermal homogeneity due to the relatively large surface-to-volume ratio of smaller units. SOFC systems have a clear advantage over PEFCs here, since they are capable of fully internally reforming hydrocarbon fuels. Studies have, however, shown that a small pre-reformer can improve SOFC performance by damping the effects of the strongly endothermic reforming reaction that can lead to excessive thermal stresses in the SOFC stack and shorten the operating lifetime (Schmidt, 2006). The most striking difference between fuel processors from major manufacturers is the choice of reforming method; the majority choose steam reforming, although Plug Power (USA) and Hexis (Switzerland) use the other methods listed in Fig. 10.4 (Hubert et al., 2006). The main benefit of steam reforming is the high concentration of hydrogen in the output reformate – 70–80%, compared with 50–60% for autothermal and even less for partial oxidation; which consequently gives the highest operating efficiency (Woods and Cuzens, 2001). The drawbacks are that the highly endothermic reaction (–250 kJ mol–1 CH4) and high operating temperature (up to 800 °C) prevent the rapid start-up and transient performance that can be achieved with other methods (Woods and Cuzens, 2001; Yasuda et al., 2001).

10.3.3 Reactant delivery systems Since small- and micro-CHP systems are not pressurised, a blower rather than a compressor is used to supply air to the system. The air blower is the main parasitic electrical load on the system, the power requirement scaling with the mass flow rate of air delivered. The mains gas pressure that enters a home, and certainly the pressure of tanked storage, is usually sufficient to operate a small- or micro-CHP system. The high temperature operation of SOFCs means that the fuel and air entering the stack need to be pre-heated to a level that avoids thermal shock to the ceramic components of the stack. Therefore, appropriately sized heat exchangers are required to heat the reactant streams and raise steam if a reformer is used. Silent operation is often cited as a benefit of fuel cells. While it is true that the fuel cell stack operates silently, the blowers, pumps, water evaporation generators, burner, etc., required to operate it will generate a noise level similar to that of a conventional condensing boiler. However, compared to

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 242

4/5/11 9:23:39 AM

Fuel cell systems for small and micro CHP applications

243

mechanical CHP engines, fuel cell systems should be noticeably quieter; ca. 40–55 dB for fuel cell systems compared with around 95–99 dB for CHP engine-based technologies (Garche and Jörissen, 2003).

10.3.4 Water management High purity water is required for steam reforming and to maintain adequate hydration of the electrolyte membrane in PEFCs to ensure proton conduction. Heat exchanger condensers remove water from the exit of the stack, harvesting heat and supplying process water to these components. An ion-exchange pack is often used to ensure the purity of the water before feeding it back in the cell, since cation contamination can increase the rate of degradation of membrane electrolytes and certain anions affect the electrocatalysts.

10.3.5 Heat management Heat recovery from fuel cell stacks is markedly different depending on the stack technology. High temperature SOFC stacks are cooled by excess air flow over the cathode, which is then combusted with unconsumed fuel in an afterburner. This heat is used to pre-warm the gas inlets to the stack and maintain reformer temperature, and the excess is passed through a condensing heat exchanger to provide hot water for the home (Hawkes et al., 2009a). Low temperature stacks are cooled by circulating a liquid through cooling plates interspersed through the stack, which is then passed through a liquidliquid heat exchanger. The low operating temperature of PEFC systems means that heat output is limited to 60–65 °C, and a boiler is needed to produce hot water at higher temperatures, if required (Osaka et al., 2005).

10.3.6 Heat storage The hot water output is stored within a large, well insulated tank, which is gradually filled by the low capacity fuel cell throughout the day. These heat stores improve on conventional hot water cylinders by promoting thermal stratification with mixing valves and buffer zones. The majority of the water is stored as a warm buffer (~45 °C) that is used as an intermediate heat exchanger between the generator and the central heating system. A smaller tank sits in the centre of the store, holding around a quarter of the water at a higher temperature for direct consumption (Staffell et al., 2010). The heat stores supplied with fuel cell micro-CHP systems range from 75 to 750 L, the upper end of which would traditionally be recommended for houses with five or more bathrooms (Staffell, 2010). The space required

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 243

4/5/11 9:23:39 AM

244

Small and micro combined heat and power (CHP) systems

by such a tank poses a problem for installation in smaller houses, so they are currently installed in basements or outside. A 600 L tank would hold around 28 kWhth – just over half a day’s requirement from a typical British house.

10.3.7 Auxiliary burner In cool climates such as the UK, houses are notoriously poor at retaining their heat and there is generally lower demand for electricity as air conditioning is not widespread (Colella, 2002). The annual average HPR of UK houses is around 5.5:1, which is more than triple that of most fuel cell CHP systems (Utley and Shorrock, 2008; Hawkes et al., 2009a). It is unlikely that a significant portion of the UK housing stock will be retrofitted with the exceptional insulation needed for a tenfold reduction in space heating demand,2 so an additional heat source is required to prevent the fuel cell owner from experiencing a loss in comfort in winter. It would take the fuel cell alone several hours to replenish the typical sized hot water tank, so an auxiliary boiler is also required when the household demands a lot of hot water (Osaka et al., 2005). A condensing boiler is therefore integrated into commercial micro-CHP systems. Japanese PEFC and SOFC systems are typically backed up by a 42 kWth gas burner, while the leading European PEFC systems contain a 15 kWth condensing boiler (Staffell, 2010). Combining the two devices, rather than installing them separately, offers lower installation costs (as one device rather than two must be connected to the property’s gas supply), and offers the potential for integrated control of both devices. The overall system controller could theoretically operate the two devices in a complementary fashion to vary the system HPR, and increase thermal efficiency by burning anode off-gas more efficiently or supplying heat to the fuel processor. This level of integration is not seen at present, and in some cases the auxiliary boiler can in fact hinder the performance of the fuel cell, with both devices fighting to produce hot water at times of high demand.

10.3.8 Inverter and power electronics The fuel cell stack produces a low voltage, high current direct current (DC) output, typically around 20–40 V for a 1 kWe system. An inverter and power conditioning unit is therefore required to convert this into a stabilised 2

By bringing UK houses up to the German ‘Passive House’ standard, space heating requirements could be reduced from around 11 MWh to 1 MWh per year. This would give an annual heat demand of 4–6 MWh per year including hot water, which could be provided by a 1 kW fuel cell alone (Utley and Shorrock, 2008; Dorer et al., 2005).

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 244

4/5/11 9:23:39 AM

Fuel cell systems for small and micro CHP applications

245

alternating current suitable for electrical appliances and for export to the grid. The typical efficiency for an inverter at the sub-10 kWe level is 85–95% (Staffell, 2010). Most small- and micro-CHP systems also integrate with the national electricity grid so that excess power can be exported at a profit to the household. As with heat storage, this allows a substantial improvement to the utilisation and economic benefit of operating a fuel cell. Storing the excess power output in batteries is an alternative that has been used in the past; however, the increase in capital cost makes it uneconomical if export is available (Staffell et al., 2008).

10.3.9 Control system Although not shown in Fig. 10.3 the control system for a fuel cell system plays a vital role in ensuring safe, efficient and reliable long-term operation. The level of sophistication and complexity will be greater if the system is designed to respond to the dynamics of the electrical and thermal load, rather than operating at a constant set point and relying on thermal storage and load balancing from the grid. The control electronics, sensors and actuators all represent a constant parasitic load (e.g. ~50 We for a 1 kWe PEFC stack), which has a marked impact on system efficiency at low part load and should therefore be minimised for both capital and operating efficiency reasons.

10.3.10 The installed system The largest market for fuel cell micro-CHP is in residential housing where the typical commercial model is for the fuel cell system to generate all of the heating and hot water and the majority of the electricity needed by a typical home. Stack sizes for such systems are of the order of a few kWe, with approximately the same amount of heat generated; an integrated condensing boiler is used to supply the remaining heat demand not serviced by the fuel cell. The vast majority of condensing boilers in Europe are wall mounted; since residential fuel cell micro-CHP is intended to be a boiler replacement (plus some electricity generation), wall-mounted units with a similar form and volume are the design objective of most developers (e.g., Ceres Power, UK); although larger units, typically sited in garages and integrated with large hot water storage units, are also on trial (e.g., Hexis, Switzerland). There are constraints on start-up and shut-down of fuel cell micro-CHP systems, approximately an hour of pre-heating is required for PEFC systems (incorporating a fuel processor), and as much as 12 hours for SOFCs. Sophisticated control logic is therefore used to predict when best to operate,

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 245

4/5/11 9:23:39 AM

246

Small and micro combined heat and power (CHP) systems

and many SOFC systems are run continuously, modulating to minimum power output when there is little or no demand.

10.4

Operating conditions and performance

10.4.1 Conversion efficiency Fuel cells offer significantly higher electrical efficiency than CHP engines, and can rival modern combined cycle gas turbine (CCGT) systems. However, their total efficiency (heat and power) is currently lower than engines, largely because of their relative immaturity and difficulties in capturing low-grade waste heat (Hawkes et al., 2009a). Figure 10.5 plots the electrical and thermal efficiency of 48 fuel cell CHP systems, compared to the traditional alternatives available in the UK (Staffell, 2010). It is seen that the efficiency of most fuel cell systems is 5–30% above the best available heat and electricity generation methods; and 20–50% above the average systems currently in place.

Average alternative 60%

Best alternative PEMFC SOFC

Electrical efficiency (HHV)

50%

40%

CC

GT

Av e

po

ra

we

30%

ge

rs

gr

tat

id

ion

s

m

ix

20%

Co

Av e

10%

ra

0% 0%

20%

ge

Be

nd

st

en

he

sin

ati

ng

gb

40% 60% Thermal efficiency (HHV)

0% 0%

+2

+1

+3

+5

0% 0%

+4

0%

oil

er

80%

100%

10.5 Thermal and electrical efficiency of fuel cell CHP systems, plotted against lines that connect the electrical and thermal efficiency of the traditional alternatives available in the UK. Reproduced from Staffell (2010).

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 246

4/5/11 9:23:39 AM

Fuel cell systems for small and micro CHP applications

247

An aspect of generating efficiency that is often overlooked is the energy that is consumed in the process of producing power. For example, power stations in the UK consume 2% (gas), 5% (coal) and 9% (nuclear) of the electricity they generate (MacLeay et al., 2008), and electricity consumption by condensing boilers and renewable microgeneration can prove to be significant (Staffell et al., 2010; Carbon Trust, 2007). This is also the case with fuel cell micro-CHP systems, as energy is required by the pumps, fans, inverter, system controller and fuel processor; all of which is neglected if only the stack is considered in isolation. These additional losses reduce the electrical efficiency of a whole system by one-fifth to one-third, as illustrated by the example in Fig. 10.6. Leading SOFC systems have demonstrated electrical efficiencies as high as 40–50% HHV when operating on natural gas (New Energy Foundation, 2010; Todo et al., 2008; Ceramic Fuel Cells Limited, 2009). Their efficiency in real-world operation is somewhat lower though, with average efficiencies of 35% electrical and 71–74% total seen during residential demonstrations in Japan (New Energy Foundation, 2010). Fuel processing incurs greater losses in low temperature PEFC systems, and so electrical efficiencies are lower than for SOFCs. Large-scale demonstrations in both Japanese and German homes have shown that 27–32% electrical, and 70–75% total efficiency can be consistently achieved (New Energy Foundation, 2009; Franke, 2010).

10.4.2 Effects of dynamic operation The average efficiencies experienced during field trials are typically 2–6% lower than the values quoted by manufacturers, which are measured at steady state when operating at full power. The dynamic nature of energy demand from individual homes means that fuel cells cannot always operate at their peak efficiency, and that energy is wasted in starting and stopping the unit and by generation occurring when there is no demand from the house. Electrical efficiencies in particular are profoundly influenced by the operating pattern of the CHP device over the course of time, which makes real-world performance diverge from that of controlled laboratory tests (Kuhn et al., 2008). Part load performance Most technologies have a lower efficiency when run at partial load, either due to incomplete fuel combustion or higher ancillary power drains (Ida, 2008; Thomas, 2008; Beausoleil-Morrison, 2007), and this is also the case for fuel cells as explained in Section 10.2. Systems are usually operated well below their peak power to improve their durability and efficiency, so in

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 247

4/5/11 9:23:39 AM

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 248

4/5/11 9:23:40 AM

24

15

Burner

Fuel processor

components which require 5% of gross output.

(35/80). This can be broken down into a fuel utilisation of 73% ((80-22)/80),

hth = 45%

22

30

Fuel cell stack

35

Aux. loads

33

hel = 30%

Inverter

of 30%.

80

giving a net electrical efficiency

units of heat are also extracted.

AC with an efficiency of 90%,

Total stack efficiency is 81%, as 30

(35/(80-22)).

The remainder is converted to

used to power the auxiliary

converting hydrogen into electricity

and an electrical efficiency of 60%

The DC output of the stack is

The stack itself is 44% efficient at

10.6 A typical breakdown of the overall efficiency of a micro-CHP system. Reproduced from Staffell (2010).

NG in: 100

76

overall system efficiency.

the high temperature unit, improving

15 units of heat are recovered from

inefficiency is attributed to the stack.

recycled from the anode gas, but this

22 units of hydrogen are also

converting natural gas into hydrogen.

The fuel processor is 80% efficient at

Fuel cell systems for small and micro CHP applications

249

practice when power output decreases, the electrical efficiency also declines. Figure 10.7 plots the electrical and thermal efficiency of 12 different fuel cell systems (as opposed to stacks) at different levels of power output. It is

Normalised electrical efficiency (whole system)

110% 1 8

100%

10

90%

7

6 80%

11

5

1 Plug Power (6 kW PEFC) 2 H-Power (5 kW PEFC) 3 Tokyo Gas LIFUEL (1 kW PEFC) 4 Mitsubishi Kepco (12 kW SOFC) 5 Toshiba (1 kW PEFC) 6 ENEFARM (1 kW PEFC) 7 ENEFARM (1 kW PEFC) 8 ENEFARM (1 kW PEFC) 9 Kyocera (700 W SOFC) 10 Confidential (PEFC) 11 Sulzer Hexis (1 kW SOFC)

3 70% 4 60% 2 50% 9

40% 0%

20%

40% 60% 80% Normalised power output

100%

160% 1

Normalised thermal efficiency

140% 8

120% 100%

6

80%

5 2

60%

7

40% 20% 0% 0%

20%

3

4 1 H-Power (5 kW PEFC) 2 Tokyo Gas (1 kW PEFC) 3 enefarm (1 kW PEFC) 4 enefarm (1 kW PEFC) 5 enefarm (1 kW PEFC) 6 Kyocera (700 W SOFC) 7 Confidential (PEFC) 8 Plug Power (6 kW PEFC)

40% 60% 80% Normalised power output

100%

10.7 Whole system electrical and thermal efficiency (including fuel processing, inverter and parasitic loads) of different fuel cell CHP systems, measured against power output. The efficiency of each system is presented relative to its full-power efficiency (Staffell, 2010).

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 249

4/5/11 9:23:40 AM

250

Small and micro combined heat and power (CHP) systems

clear that across nearly all PEFC and SOFC products, electrical efficiency falls as power output decreases, and the thermal efficiency of domestic-scale systems is either constant or also falls. On/off cycling and start-up Low temperature fuel cells are expected to operate intermittently in people’s homes, starting up and shutting down on most days in response to levels of demand (Hamada et al., 2006; Osaka et al., 2005). Many SOFCs are not able to cope with this due to the materials degradation caused by thermal cycling, and so current systems have to operate continuously. Electricity is required during start-up and shut-down procedures to power the pumps, blowers and electronic systems which provide adequate stack conditions. A long period of pre-heating is also required to raise the generator’s mass up to operating temperature. Although PEFC stacks may be able to operate from ambient temperature, the fuel processor must be heated to several hundred degrees before hydrogen can be produced. The amount of energy required to start a fuel cell micro-CHP system over the course of a year has not been widely studied; however, it is thought to be significantly greater than for other micro-CHP technologies. A 12-month field trial of a Baxi Beta (Staffell, 2010) and laboratory tests on a Vaillant system (Arndt, 2007) (both PEFC) suggest that 1–2 kWh of natural gas are required per kWe of capacity each time the fuel cell is started from cold. In comparison, the Carbon Trust estimated that 0.5 kWh of heat and 75 Wh of electricity is required by a Stirling engine, and condensing boilers incur a penalty of 0.17 kWh gas consumption for every start-up (Carbon Trust, 2007). The gas consumed in pre-heating the fuel cell system is effectively wasted, as the heat embodied in the generator is not transferred to the house in useful ways (Kuhn et al., 2008). Systems are typically located away from the main living areas of the house due to constraints on space or noise. Most systems are located in basements, garages or outside, where any heat lost through radiation or conduction will be useless. If the fuel cell is located in an occupied area of the house, any heat dissipated to the surroundings will be of use during the heating season, but will be unwanted during the summer months when only hot water is required. The annual seasonal efficiency of the fuel cell (as reported in field trials) will therefore be lower than when measured at steady state, as the additional gas and electricity consumed during start-up and shut-down will be accounted for. In the Baxi field trial, pre-heating the system accounted for 3% of the total consumption, and so is not a negligible effect. Two additional consequences are that system efficiencies will be slightly higher during winter months when longer periods of heating are required, and that fuel cells will be better

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 250

4/5/11 9:23:40 AM

Fuel cell systems for small and micro CHP applications

251

suited to houses with higher demands for space heating which can guarantee fewer on-off cycles (Carbon Trust, 2007). Utilisation of generated energy The production of electricity and heat in a fuel cell are inseparably linked, and the heat-to-power ratio of most fuel cell systems lies between 0.8 and 1.6 (excluding the auxiliary burner). The relatively inflexible operation of micro-CHP systems therefore means that much of the energy they generate can go to waste if there is insufficient storage capacity. The always-on SOFC systems trialled in Japan have suffered particularly due to the combination of limited thermal demand in Japanese homes, and the use of small 70 L storage tanks. Even though the units operated with 36–39% thermal efficiency, nearly half of the heat they generated was dumped to the atmosphere, meaning the thermal utilisation efficiency averaged only 23%. These issues and a similar loss of performance relative to manufacturers’ specifications were reported in the only published field trial of a PEFC system in a UK house (Staffell, 2010), again raising the issue of correctly sizing and siting these technologies.

10.4.3 Reliability, availability and lifetime The functional lifetime is a crucial and contentious issue for the commercialisation and economic viability of fuel cell micro-CHP systems, and is one of the characteristics which varies most between designs. The high capital cost of the fuel cell stack means that frequent replacement is not economically justifiable, and the impact of gradual degradation on the efficiency can have serious impacts on the output and thus value of the system (Hawkes et al., 2009b). The industry-wide target of 40 000 hours continuous operation has remained elusive for nearly a decade (Larminie and Dicks, 2003; Horwitz, 2008), only being attained in the field by industrial-scale PAFC systems from UTC and Fuji. Figure 10.8 shows that the demonstrable lifetime of PEFC systems is gradually moving towards this target, but SOFC and AFC appear to have stagnated with openly published reports of stack tests not lasting for more than 10 000–20 000 hours for SOFC, or 5000–10 000 hours for AFC. Japanese manufacturers of ENE-FARM systems expect that their latest generation of PEFC stacks is now able to meet the 40 000 hour target (Panasonic Corporation, 2008); however, as none of these units have been operating for more than two years in the field it is impossible to verify their claims yet. The longest reported lifetimes so far from the Japanese field trials have been around 20 000 hours (FuelCell Japan, 2008; Tabata et al., 2009; Homma, 2008; Bessho, 2008).

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 251

4/5/11 9:23:40 AM

252

Small and micro combined heat and power (CHP) systems

80 AFC

PAFC

SOFC

PEFC

70 PAFC: 58 000 hours, + 3% per year

Lifetime (thousand hours)

60

50

40

30

20

10

PEFC: 20 000 hours, + 11% per year

SOFC: 10 500 hours, + 1% per year AFC: 7000 hours, + 1% per year

0 1995

1997

1999

2001

2003

2005

2007

2009

10.8 The improvement in demonstrated stack and system lifetimes of different fuel cell technologies over the past 15 years. Data points indicate individual results reported in the literature, and weighted exponential fits are shown for each technology, with a label giving the rate of improvement, and estimated average lifetime as of 2009.

The longest SOFC lifetimes were demonstrated by Siemens-Westinghouse in the late 1990s (George, 1997; Hoogers, 2002). Evaluations of fuel cells which are based on this achievement make the leap of faith that if two cells can operate at steady state in a laboratory for 69 000 hours, then a complete system in the dynamic environment of a house should be able to as well (Pehnt and Fischer, 2006). The preceding decade of research has not managed to achieve this stiff technical challenge, and system lifetimes have remained under 15 000 hours since. The latest Japanese roadmap for SOFC technology predicts that 10 000–20 000 hour lifetimes should be attainable by 2015, and that 40 000 hours is not expected until after 2020 for domestic systems (New Energy Foundation, 2010; Kogaki, 2007). Although manufacturers outside of Japan are more optimistic about these time scales, none have yet demonstrated a product that is close to achieving these targets (Dow, 2009; Hexis, 2007). Currently, both PEFC and SOFC stacks lose power at a rate between 0 and 5% per thousand hours, depending on the design and materials used by

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 252

4/5/11 9:23:40 AM

Fuel cell systems for small and micro CHP applications

253

each manufacturer (Staffell, 2009; de Bruijn et al., 2008). Reduced catalytic activity in the cells and reformer, combined with increasing cell resistance causes a gradual drop in output voltage, and thus power output. This can shorten stack lifetime, but mechanical deterioration of the cells is usually the limiting factor. Large-scale field trials have revealed that the reliability of micro-CHP systems falls short of expectations, and that the fuel cell stack no longer requires the greatest attention. These complex systems are currently filled with novel and relatively untried components, which has resulted in relatively simple problems causing numerous forced shutdowns. Analysis of nearly 1000 PEFC and SOFC systems has shown that the mean time between failure (MTBF) is currently in the region of 5–10 000 hours, meaning that most systems experienced at least one failure per year (New Energy Foundation, 2009; 2010). Tentative data suggest that MTBF has increased substantially (towards 30 000 hours) now that these systems have been commercially launched, and so are produced on a larger scale and have become more standardised; however, this will need to be confirmed once more recent data are released.

10.5

Commercial development and future trends

Fuel cells have been under development for the past 30–50 years, but are still relatively immature in commercial markets. Fuel cells are often seen as lagging behind other micro-CHP technologies, ‘forever 5 years away from commercialisation’ (Kho, 2005; Rechtin, 2006; O’Sullivan, 2006). However, Japanese manufacturers began to roll the first units off automated production lines in 2009, marking the long-awaited transition towards mass production. With over 10 000 domestic micro-CHP units already operating in Japan and annual sales expected to double this in 2011, the commercialisation of fuel cells has already begun. A survey of small stationary fuel cell developers showed that of the 11 000 CHP and backup-power systems installed up until 2009, 85% were based on PEFCs, and the remainder were SOFCs (Adamson, 2009).

10.5.1 Major manufacturers, products and demonstrations Three countries are leading the demonstration and commercialisation of fuel cells: Japan, Germany and South Korea. The USA is a key player in the fields of industrial-CHP and fuel cell vehicles, but has seen little development of domestic scale CHP due to an unattractive financial and regulatory landscape. A series of large-scale demonstration programmes have been carried out in Japan since 2002, and have so far resulted in the installation of 3352 PEMFC

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 253

4/5/11 9:23:41 AM

254

Small and micro combined heat and power (CHP) systems

and 210 SOFC units into public homes (New Energy Foundation, 2010). The ENE-FARM brand of PEFC systems were launched after these trials, emerging from extensive collaboration between Panasonic, ENEOS (Sanyo) and Toshiba. They are based on PEFC stacks ranging from 0.7 to 1.0 kWe electrical output (0.9–1.4 kWth) and are packaged with a fuel processor for either natural gas, LPG or kerosene, and a hot water tank with integrated boiler. ENE-FARM is the culmination of over a decade of collaborative research and demonstration by Japanese fuel cell manufacturers and energy distribution companies, who respectively produced the stacks and fuel reformers. These companies agreed to collaborate on the development and commercialisation of the ENE-FARM in the realisation that the problems that had to be overcome were too great for one company to achieve alone. System manufacturers decided on, and then published specifications for standardised balance of plant (BoP), and individual companies then openly competed to develop these components (Ueda, 2007). This collaborative strategy attained an almost four-fold decrease in BoP costs, whilst improving durability and readying the whole supply-chain for mass production (Tanaka, 2008). Japan is also at the forefront of SOFC development with companies such as Kyocera, Nippon Oil and TOTO engaged in residential demonstrations of their 1 kWe-class micro-CHP systems. Despite the vastly different stack operation, these devices are externally similar to the ENE-FARM, with a small unit containing the stack, fuel processor and electronics, and a hot water tank and backup burner packaged separately. The government roadmap aims for fundamental materials research and residential demonstrations to continue until at least 2012 before commercialisation in 2015–2020 (New Energy Foundation, 2010). In Germany, the Callux residential field trials began in 2008 with three manufacturers: Baxi Innotech (PEMFC), Hexis and Vaillant (both SOFC). Approximately 100 units have been installed so far, and a total of 800 will be installed by 2012, and operated for up to three years in German homes (Franke, 2010). Elsewhere, Ceramic Fuel Cells (Australia), Acumentrics (USA), Hyosung (S. Korea), GS Fuel Cell (S. Korea), Plug Power (USA) and Ceres Power (UK) all have active programmes for fuel cell CHP systems and most plan to commercialise in the early 2010s.

10.5.2 Current and future costs Fuel cells are the most expensive form of micro-CHP at present. There is still limited information on the price of most systems, due to commercial secrecy and low production volumes. Between 2005 and 2009, the average price of 0.75–1 kWe ENE-FARM systems decreased from £36 000 for early

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 254

4/5/11 9:23:41 AM

Fuel cell systems for small and micro CHP applications

255

demonstrations to £15 500 at the time of commercial launch (Staffell and Green, 2009). In comparison, leading SOFC demonstration systems from Hexis and Kyocera cost a minimum of £30 000 for 0.7 and 1.0 kWe systems (Yamamoto, 2008; Singhal, 2007). It must be remembered that these are pre-commercial designs produced by skilled engineers rather than production lines, so mass production will bring with it substantial cost reductions. However, it is expected that ten years of deployment and cost reduction is required before unsubsidised prices reach £5 000 per kWe (Staffell and Green, 2009). Table 10.2 presents the actual sale prices of seven models of fuel cell system which were collated in Staffell (2010). No clear trend can be seen between technologies, as the differences in price are currently dominated by production volumes and system capacity. Excluding the larger PAFC and AFC

AFC

PAFC

SOFC

PEMFC

Table 10.2 Known sale prices for fuel cell micro-CHP systems. All prices have been converted to 2009 Euros with the following exchange rates: ¥145, $0.80, 1325 won to 71, and 2.5% annual inflation System

Year

Price

Description

Panasonic (0.75 kW)

2011

719 000

Current sale price in Japan for the latest ENE-FARM model.

Eneos, Toshiba 2009 (0.7 kW)

722 500

Initial sale prices for ENE-FARM models in Japan.

Panasonic (1.0 kW)

2009

723 900

Systems included a backup boiler and hot water tank, plus other ancillaries.

GS Fuel Cell, 2008 Fuel Cell Power, Hyosung (all 1 kW 2007 systems)

780 000

Given as the current system price in 2008 (only available in limited trials in South Korea).

770 000

Given as the individual price for the 70 demonstration units delivered in 2007.

Plug Power (5 kW)

2001–03 755 000– 85 000

The average purchase and installation costs during the US Department of Defense field trials.

Kyocera (0.7 kW)

2009

Sulzer Hexis (1 kW)

2000–05 ~755 000 Mentioned as the cost of demonstration systems. The later Galileo model was described as ‘less costly’, but no price was given.

UTC and Fuji (100+ kW)

2001–08 72800– 5400 per kW

The average sale price of industrial CHP systems.

(5–10 kW)

2006

Quoted price from an anonymous manufacturer for a hydrogen fuelled CHP system.

~770 000 Mentioned in the METI technology per kW roadmap and by Kyocera during the demonstration project.

710 000 per kW

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 255

4/5/11 9:23:41 AM

256

Small and micro combined heat and power (CHP) systems

systems, it is clear that ENE-FARM are offered at the lowest price, which is understandable as they are the most commercially developed micro-CHP system. In addition to publishing current prices, the manufacturers and agencies involved in the leading fuel cell demonstrations have laid out their expectations and targets for each technology, which are summarised in Table 10.3 (Staffell, 2010). It could be argued that these manufacturers are best placed to make predictions as they currently have the most experience with commercialising micro-CHP systems. The projections in Table 10.3 are substantially higher than those given by other sources; they are both closer to current sale prices, and have far less aggressive timetables for cost reduction. A striking feature is that neither the Japanese government, nor the manufacturers of PEFC or SOFC systems expect prices to fall below ¥400 000 (£2000) even in 10–20 years’ time. Table 10.3 Expectations and targets given by the manufacturers and government bodies involved with world-leading fuel cell demonstrations Systems South Korea

Cost/Price per system

Production volume

Description

2008

756 000

100

Expected price during the third and final year of the current demonstration project.

2010

712 000

2012

78000

10 000 cumulative

Target price set by the Ministry of Knowledge Economy.

2004

714 500

10 000 p.a.

Estimated manufacturing cost for ENEFARM systems made by the manufacturers.

2012

75000–8000 50 000 p.a.

2015

73500–5000 500 000 p.a.

The METI technology roadmap for production cost of residential cogeneration systems.

2015

73500

Target cost stated in the Korean national action plan.

PEFC

Japan

Year

200 000 p.a.

2020– 72750 2030

SOFC

Japan

Panasonic’s target price for systems set in 2008. The METI technology roadmap for production cost of residential cogeneration systems.

2008

~73800

Mass production

Kyocera’s expected retail price for systems (including hot water tank).

2015

77000/kW

Several thousand p.a.

The METI technology roadmap for residential cogeneration systems.

2020– 72750/kW 2030

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 256

4/5/11 9:23:41 AM

Fuel cell systems for small and micro CHP applications

10.6

257

Sources of further information and advice

Nice and Strickland (2009) and Merewether (2005) give an overview of fuel cell theory, while Rayment and Sherwin (2003) and Larminie and Dicks (2003) provide a rich and detailed discussion. Further information on the different components of fuel cell systems is available in the literature (Garche and Jörissen, 2003; Blomen and Mugerwa, 1994).

10.7

References

Adamson K-A (2009) Fuel Cell Today Small Stationary Survey. Available at: http:// www.fuelcelltoday.com/online/surveys, accessed May 2009. Ang S M C, Brett D J L & Fraga E S (2010) A multi-objective optimisation model for a general polymer electrolyte membrane fuel cell system. Journal of Power Sources, 195 No. 9, 2754–2763. Arndt U (2007) ‘Investigation of a Vaillant Fuel Cell Euro 2 at the Technical University of Munich’, in Beausoleil-Morrison I (ed.) Experimental Investigation of Residential Cogeneration Devices and Calibration of Annex 42 Models. International Energy Agency. Ball S & Thompsett D (2002) Ultra CO Tolerant PtMo/PtRu anodes for PEMFCs. Materials for Fuel Cells and Fuel Processors. Solid State Ionics. Beausoleil-Morrison I (2007) Experimental Investigation of Residential Cogeneration Devices and Calibration of Annex 42 Models. A Report of Subtask B of FC+COGENSIM. International Energy Agency. Bessho T (2008) New Models of Residential PEMFC Cogeneration Systems. Fuel Cell Seminar & Exposition. Phoenix, Arizona. Available at: http://tinyurl.com/mm3nng Blomen L J M J & Mugerwa M N (1994) Fuel Cell Systems, Springer. Brett D J L, Aguiar P, Brandon N P & Kucernak A R (2007) Measurement and modelling of carbon monoxide poisoning distribution within a polymer electrolyte fuel cell. International Journal of Hydrogen Energy, 32 No. 7, 863–871. Brett D J L, Atkinson A, Brandon N P & Skinner S J (2008) Intermediate temperature solid oxide fuel cells. Chemical Society Reviews, 37 No. 8, 1568–1578. Carbon Trust (2007) Micro-CHP Accelerator: Interim Report. Available at: http://tinyurl. com/lrlmd9 Ceramic Fuel Cells Limited (2009) BlueGEN: Modular Generator – Power + Heat. Available at: http://tinyurl.com/paxjh4, accessed August 2009. Colella W G (2002) Combined Heat and Power Fuel Cell Systems. PhD thesis, Department of Engineering Science, University of Oxford. Das D & Veziroglu T N (2001) Hydrogen production by biological processes: a survey of literature. International Journal of Hydrogen Energy, 26 No. 1, 13–28. de Bruijn F A, Dam V A T & Janssen G J M (2008) Review: durability and degradation issues of PEM fuel cell components. Fuel Cells, 8 No. 1, 3–22. Dorer V, Weber R & Weber A (2005) Performance assessment of fuel cell microcogeneration systems for residential buildings. Energy and Buildings, 37 No. 11, 1132–1146. Dow B (2009) Company Update. Ceramic Fuel Cells Limited. Available at: http://tinyurl. com/mc3b4y Echigo M, Shinke N, Takami S & Tabata T (2004) Performance of a natural gas fuel

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 257

4/5/11 9:23:41 AM

258

Small and micro combined heat and power (CHP) systems

processor for residential PEFC system using a novel CO preferential oxidation catalyst. Journal of Power Sources, 132 No. 1–2, 29–35. Franke A (2010) Baxi Innotech: PEM fuel cells for the Callux programme. Hyrdogen & Fuel Cells for Clean Cities. Birmingham, UK. FuelCell Japan (2008) 20,000 Hours = Continuous Run Time of JOMO’s Residential FC Cogeneration System. Available at: http://www.fcpat-japan.com/News2008-3. html#73, accessed September 2008. Garche J & Jörissen L (2003) ‘PEMFC fuel cell systems’, in Vielstich W, Lamm A & Gasteiger H A (eds) Handbook of Fuel Cells – Fundamentals, Technology and Applications. John Wiley & Sons, vol. 4. George R A (1997) SOFC Combined Cycle Systems for Distributed Generation. American Power Conference. Chicago. Available at: http://tinyurl.com/mhxsg2 Ghouse M, Abaoud H & Al-Boeiz A (2000) Operational experience of a 1 kW PAFC stack. Applied Energy, 65, 303–314. Hamada Y, Goto R, Nakamura M, Kubota H & Ochifuji K (2006) Operating results and simulations on a fuel cell for residential energy systems. Energy Conversion and Management, 47 No. 20, 3562–3571. Harrison J (2008) What is Microgeneration? Claverton Energy Group Conference. Bath, UK. Available at: http://www.claverton-energy.com/what-is-microgeneration.html Hawkes A D, Staffell I, Brett D & Brandon N (2009a) Fuel cells for micro-combined heat and power generation. Energy & Environmental Science, 2, 729–744. Hawkes A D, Brett D J L & Brandon N P (2009b) Fuel cell micro-CHP techno-economics: Part 2 – Model application to consider the economic and environmental impact of stack degradation. International Journal of Hydrogen Energy, 34 No. 23, 9558–9569. Hexis (2007) Swiss company Hexis tests fuel cell system in the field. Available at: http:// tinyurl.com/mx7nfz, accessed July 2009. Homma T (2008) The latest Fuel Cell news in Japan, June 2008. Available at: http:// www.fcdic.com/eng/news/200807.html#3, accessed October 2008. Hoogers G (2002) Fuel Cell Technology Handbook, CRC Press. Horwitz J (2008) Fuel Cell mCHP on the Threshold of Success. Fuel Cell Seminar & Exposition. Phoenix, Arizona. Available at: http://tinyurl.com/mqp9bx Hubert C-E, Achard P & Metkemeijer R (2006) Study of a small heat and power PEM fuel cell system generator. Journal of Power Sources, 156 No. 1, 64–70. Ida H (2008) Optimization of Japanese heat pump systems in the moderate climate region. 9th IEA Heat Pump Conference. Zürich, Switzerland. Available at: http:// tinyurl.com/ykl2dre Independant Power Technologies Ltd (2006) PULSAR-6 Product Brochure. Moscow. Kho J (2005) Fuel Cell Follies. Red Herring. San Mateo, California. Available at: http:// www.redherring.com/Home/14402 Kiros Y, Myrén C, Schwartz S, Sampathrajan A & Ramanathan M (1999) Electrode R&D, stack design and performance of biomass-based alkaline fuel cell module. International Journal of Hydrogen Energy, 24 No. 6, 549–564. Kogaki T (2007) Japan’s Approach to the Commercialization of Fuel Cell and Hydrogen Technology. Fuel Cell Seminar & Exposition. San Antonio, USA. Available at: http:// tinyurl.com/lnue32 Kolb G (2008) Fuel Processing: for Fuel Cells, Wiley. Kuhn V, Klemes J & Bulatov I (2008) MicroCHP: overview of selected technologies, products and field test results. Applied Thermal Engineering, 28 No. 16, 2039– 2048.

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 258

4/5/11 9:23:41 AM

Fuel cell systems for small and micro CHP applications

259

Larminie J & Dicks A (2003) Fuel Cell Systems Explained, John Wiley & Sons, Ltd. Lindström B, Karlsson J A J, Ekdunge P, De Verdier L, Häggendal B, Dawody J, Nilsson M & Pettersson L J (2009) Diesel fuel reformer for automotive fuel cell applications. International Journal of Hydrogen Energy, 34 No. 8, 3367–3381. Lohsoontorn P, Brett D J L & Brandon N P (2008a) The effect of fuel composition and temperature on the interaction of H2S with nickel-ceria anodes for solid oxide fuel cells. Journal of Power Sources, 183 No. 1, 232–239. Lohsoontorn P, Brett D J L & Brandon N P (2008b) Thermodynamic predictions of the impact of fuel composition on the propensity of sulphur to interact with Ni and ceria-based anodes for solid oxide fuel cells. Journal of Power Sources, 175 No. 1, 60–67. MacLeay I, Harris K & Michaels C (2008) ‘Chapter 5: Electricity’. Digest of UK Energy Statistics. National Statistics. Marquez A I, Ohrn T R, Trembly J P, Ingram D C & Bayless D J (2007) Effects of coal syngas and H2S on the performance of solid oxide fuel cells: Part 2. Stack tests. Journal of Power Sources, 164 No. 2, 659–667. Merewether E A (2005) Alternative Sources of Energy – An Introduction to Fuel Cells. US Geological Survey Bulletin 2179. Available at: http://pubs.usgs.gov/bul/b2179/ New Energy Foundation (2009) Report data from the Large Scale Residential Fuel Cell Demonstration Project in 2008. Available at: http://happyfc.nef.or.jp/pdf/20fc.pdf (in Japanese). New Energy Foundation (2010) Solid Oxide Fuel Cell Empirical Research. Available at: http://sofc.nef.or.jp/topics/pdf/2010_sofc_houkoku.pdf (in Japanese). Nice K & Strickland J (2009) How Fuel Cells Work. Available at: http://tinyurl.com/ ltsu5y, accessed June 2009. O’Sullivan J B (2006) Fuel cells: The last 45+ years. Fuel Cell Seminar & Exposition. Honolulu, Hawaii. Available at: http://tinyurl.com/nbsxr4 Osaka N, Nishizaki K, Kawamura M, Ito K, Fujiwara N, Nishizaka Y & Kitazawa H (2005) Development of Residential PEFC Co-generation Systems. Fuel Cell Seminar & Exposition. Palm Springs, California. Available at: http://tinyurl.com/nbko3c Panasonic Corporation (2008) Panasonic Develops New Fuel Cell Cogeneration System for Home Use. Available at: http://tinyurl.com/5fk325, accessed June 2009. Pehnt M & Fischer C (2006) ‘Environmental Impacts of Micro Cogeneration’, in Pehnt M, Cames M, Fischer C, Praetorius B, Schneider L, Schumacher K & Voß J-P (eds) Micro Cogeneration: Towards Decentralized Energy Systems. Springer. Penfold D (2004) A Dual Anaerobic System for Bioremediation of Metal and Organic Waste. PhD Thesis, School of Biosciences, University of Birmingham. Rayment C & Sherwin S (2003) Introduction to Fuel Cell Technology. University of Notre Dame, USA. Available at: http://tinyurl.com/ndghpk Rechtin M (2006) Fuel cell vehicles? Don’t hold your break; Somehow, success always seems to be just 20 years away. Tire Business. Arkon, Ohio. Redwood M D (2008) Bio-hydrogen and biomasss-supported palladium catalyst for energy production and waste-minimisation. PhD Thesis, School of Biosciences, University of Birmingham. Schmidt D S (2006) Status of the Acumentrics SOFC Program. Fuel Cell Seminar & Exposition. Honolulu, Hawaii. Available at: http://tinyurl.com/lhy3or Singhal S C (2007) Innovative Solid Oxide Fuel Cell Systems for Small Scale Power Generation. Tenth Grove Fuel Cell Symposium. London, UK. Song C (2002) Fuel processing for low-temperature and high-temperature fuel cells:

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 259

4/5/11 9:23:41 AM

260

Small and micro combined heat and power (CHP) systems

challenges, and opportunities for sustainable development in the 21st century. Catalysis Today, 77 No. 1–2, 17–49. Spiegel R J, Preston J L & Trocciola J C (1999) Fuel cell operation on landfill gas at Penrose Power Station. Energy, 24 No. 8, 723–742. Staffell I (2009) A review of small stationary fuel cell performance. Available at: http:// wogone.com/iq/review_of_fuel_cell_performance.pdf Staffell I (2010) Fuel cells for domestic heat and power: Are they worth it? PhD Thesis, Chemical Engineering, University of Birmingham. Available at: http://etheses.bham. ac.uk/641/ Staffell I & Green R J (2009) Estimating future prices for stationary fuel cells with empirically derived learning curves. International Journal of Hydrogen Energy, 34 No. 14, 5617–5628. Staffell I, Green R & Kendall K (2008) Cost targets for domestic fuel cell CHP. Journal of Power Sources, 181 No. 2, 339–349. Staffell I, Barton J, Blanchard R, Hill F, Jardine C, Brett D, Baker P, Brandon N, Hawkes A, Infield D, Kelly N, Leach M, Matian M, Peakcock A, Sudtharalingam S & Woodman B (2010) UK Microgeneration. Part II: Technology Overviews. Proceedings of the Institute of Civil Engineers: Energy, 163 No. 4, 143–165. Tabata T, Yamazaki O, Shintaku H & Oomori Y (2009) ‘Degradation Factors of Polymer Electrolyte Fuel Cells in Residential Cogeneration Systems’, in Büchi F N, Inaba M & Schmidt T J (eds) Polymer Electrolyte Fuel Cell Durability. Springer. Tanaka R (2008) Update on Residential Fuel Cells Demonstration and Related Activities in Japan. Fuel Cells KTN: Fuel cells for distributed generation. Available at: http:// tinyurl.com/qqpp2s Thomas B (2008) Benchmark testing of Micro-CHP units. Applied Thermal Engineering, 28 No. 16, 2049–2054. Todo Y, Ishikawa H, Uematsu H, Gonda I, Sumi H, Okuyama Y, Usui Y, Komatsu D & Furusaki K (2008) Development of Solid Oxide Fuel Cells at NGK Spark Plug Co., Ltd. Fuel Cell Seminar & Exposition. Phoenix, Arizona. Available at: http:// tinyurl.com/n3c36e Ueda T (2007) Japan’s Approach to the Commercialization of Fuel Cell/Hydrogen Technology. 7th Steering Commitee Meeting. Sao Paulo, Brazil, International Partnership for the Hydrogen Economy. Available at: http://tinyurl.com/m9dtr5 Uribe F A, Valerio J A, Garzon F H & Zawodzinski T A (2004) PEMFC reconfigured anodes for enhancing co tolerance with air bleed. Electrochemical and Solid-State Letters, 7 No. 10, A376–A379. Utley J I & Shorrock L D (2008) Domestic Energy Fact File. London, BRE. Available at: http://tinyurl.com/y95fxpu Wakayama T & Miyake J (2001) ‘Hydrogen from Biomass’, in Miyake J, Matsunaga T & Pietro A G S (eds) Biohydrogen II: An Approach to Environmentally Acceptable Technology. Elsevier. Woods R R & Cuzens J E (2001) Autothermal Reforming of Natural Gas: A Key Technology for Fuel Cells. International Gas Research Conference (IGRC 2001). Amsterdam. Available at: http://tinyurl.com/kvwacw Xuan J, Leung M K H, Leung D Y C & Ni M (2009) A review of biomass-derived fuel processors for fuel cell systems. Renewable and Sustainable Energy Reviews, 13 No. 6–7, 1301–1313. Yamamoto A (2008) Activities toward Commercialization of Fuel Cell/Hydrogen Technology in Japan. Fuel Cell Seminar & Exposition. Phoenix, Arizona. Available at: http://tinyurl.com/mz3rov © Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 260

4/5/11 9:23:41 AM

Fuel cell systems for small and micro CHP applications

261

Yasuda I, Miura T, Fujiki H, Komiya J, Shirasaki Y & Seki T (2001) Development of Highly-Efficient and Compact Fuel Processors for PEFC Applications. International Gas Research Conference (IGRC 2001). Amsterdam. Available at: http://tinyurl. com/knpevo Zhang J, Xie Z, Zhang J, Tang Y, Song C, Navessin T, Shi Z, Song D, Wang H, Wilkinson D P, Liu Z-S & Holdcroft S (2006) High temperature PEM fuel cells. Journal of Power Sources, 160 No. 2, 872–891.

© Woodhead Publishing Limited, 2011

CHP-Beith-10.indd 261

4/5/11 9:23:41 AM

11

Heat-activated cooling technologies for small and micro combined heat and power (CHP) applications

K. G l u e s e n k a m p and R. R a d e r m a c h e r, University of Maryland, USA

Abstract: This chapter covers the technologies available for small-scale combined heat and power (CHP) systems to utilize recovered heat for air conditioning or refrigeration purposes. Five classes of technology are described and discussed within the context of small-scale CHP: (1) solid desiccant dehumidification, (2) liquid desiccant dehumidification, (3) adsorption (solid adsorbent-based) heat pumps, (4) absorption (liquid absorbent-based) heat pumps, and (5) steam ejector cycles. In this context of small-scale CHP, performance metrics for each class of system are discussed, and performance comparisons are made to the ‘baseline’ systems most commonly used to provide air conditioning and refrigeration today. Key words: cooling, desiccant, heat-activated, thermally-driven, dehumidification, absorption, adsorption, sorption, CCHP, trigeneration, polygeneration, integrated energy system.

11.1

Introduction

Although heat-activated cooling has been used for well over a century, and even predates vapor compression systems in its development, today the majority of space cooling and refrigeration is provided by vapor compression systems. Decades of intense development and deployment of vapor compression systems has led to efficient and affordable products. However, at sites where CHP is put in place, improved energy efficiency, reduced emissions and cost advantages can be realized by utilizing heat-activated cooling. This chapter focuses on equipment which can utilize the thermal byproducts of small-scale power generation in order to generate a chilled working fluid for space cooling and/or refrigeration, all with a minimal usage of electrical power.1 Further, since electricity generation consumes high grade heat and makes available low grade (or ‘waste’) heat, cooling equipment that makes use of low grade heat is emphasized. 1

Here, ‘minimal’ is used to mean: comparable to the amount of power that would be required to distribute a similar amount of heat (e.g. by pumps and fans); and substantially less power than would be required to run a vapor compression system.

262 © Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 262

4/5/11 9:24:11 AM

Heat-activated cooling technologies for small and micro CHP

263

Beginning with an introductory overview of small-scale trigeneration (11.2), this chapter then describes the available types of systems and some of their applications (11.3–11.6). In order to evaluate the energy consumption of these systems, appropriate performance metrics must be defined. Along with a discussion of such metrics, illustrative comparisons are made between heat-activated cooling technologies (HACT) and their more common ‘baseline system’ counterparts (11.7–11.8). Finally, the advantages and limitations are summarized (11.9), future trends in HACT are discussed (11.10), and further sources of information are provided (11.11).

11.2

Introduction to small-scale trigeneration

Trigeneration is the use of a single fuel source to simultaneously provide heating, cooling and electrical or mechanical power. This chapter deals with technologies that provide cooling with a primary input of heat. More specifically, the heat source is heat recovered as a co-product of electricity generation, and the cooling can be space cooling or refrigeration (or a combination). This means that the cooling device’s driving heat source is at a relatively low temperature: typically 60–120 °C (140–250 °F) for smallscale CHP units. Generally, industrial installations recover heat as saturated or superheated steam, while residential units recover heat as liquid water well below the boiling point at atmospheric pressure. Although some large engines are designed to be water cooled with low-pressure steam (ebullient systems), cooling jacket temperatures for small internal combustion engines are generally in the range of 65–85 °C (150–185 °F). Due to the ‘single-input, multiple-output’ nature of trigeneration, and due to the variety of outputs, accounting for the fuel efficiency of such systems is more complicated than for ‘single-input, single-output’ devices. The simplest system-wide metric is the overall fuel utilization rate (sometimes called the ‘combined efficiency’ or ‘overall efficiency’). This metric is based on the First Law of Thermodynamics, and is defined as all useful energy products divided by the fuel input, giving a value that is always between 0 and 1.2 For the heat-activated component of a trigeneration system, the simplest metric is the thermal coefficient of performance, or thermal COP. This is also a First Law calculation, dividing the useful cooling energy by the heat input energy (sometimes also accounting for electrical parasitics). However, since the thermal COP is bounded by the Second Law of Thermodynamics, it does not necessarily have to be less than 1. These metrics, as well as more sophisticated ones, are discussed in Sections 11.7 and 11.8. While there is no universally accepted definition of ‘small’ in this context, 2

See Section 11.8.1 for a discussion of a possible exception to this when the LHV of fuel is used.

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 263

4/5/11 9:24:11 AM

264

Small and micro combined heat and power (CHP) systems

the term ‘small-scale’ can be taken to apply to installations producing less than 100 kW of electric power. However, 100 kW installations have a lot in common with larger, typically industrial installations, and heatactivated cooling products and information for these applications (such as large absorption chillers) are widely available. On the other hand, microscale devices (under ~5 kW in electrical output), have unique properties, challenges, and requirements, with comparatively few products and little information are available on them. Additionally, micro-scale CHP systems (those providing only heat and electric power) have seen a recent surge in installations, particularly in Japan, Germany and the UK where governments have recognized a role for residential CHP in increasing energy efficiency. Thus a new market for micro-heat-activated technologies is being created. For all these reasons, this chapter makes a special effort to address technologies appropriate for micro-trigeneration.

11.2.1 Prime movers There is a wide variety of prime movers being used and developed for small CHP applications. The most commonly used technologies for micro-CHP are the spark ignition reciprocating internal combustion engine (SI-ICE) and, for systems over a few tens of kWelec, microturbines (MT) and compression ignition engines (CI-ICE). Intense development and rapid improvements are being seen in Stirling engines (SE) and both proton exchange membrane fuel cells (PEMFC) and solid oxide fuel cells (SOFC). Additionally, organic Rankine cycle (ORC) systems are under development. For the external combustion systems (Stirling and ORC), fuel flexible operation (including biofuels) and hybrid solar/fuel-fired systems are being explored. Data important to CCHP are shown for each of these prime movers in Table 11.1, including the typical sizes, efficiencies and heat recovery temperatures. The primary heat recovery options for most prime mover technologies include two sources of comparable magnitude and widely different temperatures (such as the cooling jackets and exhaust for an ICE, or the cold side cooling fluid and burner flue gas for a Stirling engine). Additional minor contributions to heat recovery can be made by water-cooling the lubricating oil, watercooling the alternator/generator, and carefully insulating the cabinet to capture heat radiated and convected from the engine. These minor contributions can add up to an additional 10–15% of the fuel energy being captured. A discussion of the available temperatures of heat recovery is very important when considering heat-activated heat pumps, because each of these devices has some minimum and some optimal driving heat temperature, as discussed in Section 11.3. There are several options for capturing the available heat, and all involve tradeoffs between temperature and efficiency: in general, the higher the heat recovery temperature, the less heat will be recovered,

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 264

4/5/11 9:24:11 AM

Heat-activated cooling technologies for small and micro CHP

265

Table 11.1 Characteristics of small-scale CHP prime movers important to heatactivated equipment SI-ICE

CI-ICE

SE

PEMFC

SOFC

ORC

MT

Capacity (kWelec)

>0.5

>5

>0.5

>1

>1

>1

>25

Electrical efficiency (LHV)

15–30%

20–40% 10–30%

30–35%

30–50%

8–15%

15–35%

Overall fuel utilization factor (LHV)

80–90%

80–90% 70–90%

75–85%

75–85%

70–80% 75–85%

Heat:electricity 1.7–4.5 ratio range

1.0–3.5

1.4–8

1.2–1.8

0.5–1.8

6–7

Lower temperature (°C)/(°F) and type

65–75 150–170 Cooling jackets

65–85 150–185 Cooling jackets

40–75 60–80 105–170 140–175 Gas Exhaust cooler

–

10–50 – 50–120 Condenser

Higher temperature (°C)/(°F) and type

480–730 250–500 200–300 >700 900–1350 480–930 390–570 >1300 Exhaust Exhaust Flue gas Reformer (if equipped)

700–1000 1300– 1800 Exhaust

~200 ~390 Flue gas

1.2–4.5

~260 ~500 Exhaust

but the more value the heat will have for driving a heat-activated device. In designing a CHP heat recovery system, the heat recovery loop can be a series circuit or two parallel (split-stream) circuits; if a series circuit is chosen, then it can either flow to the high-temperature (e.g. exhaust) component first or last. Since small CHP units on the market today are primarily designed for low temperature, heating season uses, most utilize a series circuitry which cools the high-temperature component first. The tradeoffs are summarized in Table 11.2.

11.2.2 Overview of cooling technologies and applications Low-grade heat is typically an inevitable byproduct of electricity production. Its capture and distribution are therefore relatively straightforward, requiring only heat exchangers and piping to capture and distribute this heat energy that would otherwise be dissipated to the environment. This straightforward configuration is known as cogeneration or CHP, and a schematic is shown in Fig. 11.1 with numerical values typical of a system based on a small internal combustion engine (ICE). On the other hand, using waste heat to cool a working fluid below ambient temperature requires additional equipment. Since three useful products are produced (electricity, usable heat, and space cooling or refrigeration), this

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 265

4/5/11 9:24:11 AM

266

Small and micro combined heat and power (CHP) systems

Table 11.2 Heat recovery circuitry options and tradeoffs for small-scale ICE, PEMFC, SE and ORC CHP prime movers Series circuit

Parallel circuit

Order of recovery

High-temperature component first

Low-temperature component first

(Separate loops)

Effect on electrical efficiency

None (ICE/PEMFC) Lower h (SE/ORC)

None (ICE/PEMFC) None (ICE/PEMFC) Higher h (SE/ORC) Highest h (SE/ORC)

Effect on heat recovery efficiency

Higher hHR

Lower hHR

Lower hHR

Effect on heat recovery temperature

Lower T

Higher T

One very high stream One moderate stream

Comments

Currently most common circuitry

Return T must be Increases complexity sufficiently cool and parasitic loads

Electricity (15–30%)

Input

Fuel (100%)

Prime mover

Recovered heat (50–65%) Exhaust losses (5–15%) Convection and radiation (3–7%)

Desired outputs

Heat losses

11.1 Energy flow schematic of a generic CHP (cogeneration) system. Numerical efficiency ranges are on an HHV basis; they are representative of a small ICE prime mover with heat recovered from both the exhaust and the cooling jackets, and with cabinet insulation to lessen convection and radiation losses from the prime mover surfaces.

is called trigeneration or combined cooling, heat and power (CCHP). The flow of energy involved in CCHP is shown in Fig. 11.2. Note that the heat rejected from the heat-activated cooling device (HACD) is likely still hot enough to be used for preheating domestic hot water (DHW), swimming pool heating, industrial process pre heating, or other uses. However, the amount of heat rejected by the HACD may exceed the demand for such low-grade heat (especially during the cooling season), and thus most is likely to be rejected to the environment. The output region enclosed by dotted lines in Fig. 11.2 indicates this variable utility of the heat rejected from the HACD. The ability to replace electrical consumption with the use of low-grade heat allows a trigeneration system to provide cooling in addition to electricity,

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 266

4/5/11 9:24:11 AM

Heat-activated cooling technologies for small and micro CHP Electricity

Input

Fuel (100%)

Prime mover

Recovered heat

Cooling

267

Desired outputs

HACT Heat rejection

Exhaust losses Convection and radiation

Heat losses

11.2 Energy flow schematic of a generic CCHP (trigeneration) system. Heat rejected from the HACT may still be at a high enough temperature to be useful, e.g. as DHW preheating or swimming pool water heating, or it may be considered a heat loss if rejected directly to the ambient.

without increasing fuel consumption. It also has the advantage of increasing the system utilization rate during the cooling season. Small-scale installations in the 5–100 kW range generally serve industrial facilities, commercial facilities or larger residential complexes. Micro systems (under 5 kW electric) are generally used for residential or small commercial applications. In general, any of the three energy products produced by a trigeneration system can either be utilized directly at the site of generation or be exported off-site. In the case of small-scale systems, electricity is typically exported easily (by selling electricity back to the grid). Although thermal energy is not usually viable for export in small systems, it is relatively easy to store (e.g. in a hot water tank). On the other hand, electricity storage is generally expensive. For installations independent of the grid, electrical storage is still possible (e.g. in batteries), but as long as a grid is available to absorb the excess electricity, then ‘net metering’ is the most cost-effective and energy efficient solution for dealing with times of excess electricity production. In organizing the discussion of heat-activated cooling devices, a choice must be made between a classification scheme based on applications, and one based on cycle types and working fluids. In the interest of clarity, the sections on system types are organized around cycle types and working fluids, with comments regarding applications incorporated where appropriate. Additionally, Table 11.3 has been provided for easy cross reference between applications and the appropriate corresponding cycle types.

11.3

Types of cooling systems and their applications

Heat-activated cooling technologies can most broadly be classified as either open or closed cycles. In open cycles, atmospheric air is the working fluid,

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 267

4/5/11 9:24:12 AM

© Woodhead Publishing Limited, 2011

Sorption cycles

>50

H2O/CaCl2 75–135

60–100

H2O/LiCl

70–120

Methanol/activated carbon

55–150

55–220

Water/Zeolite (various)

Zeolite (various)

60–120

Water/Silica gel (various)

>90

120–150

Single effect NH3/H2O

Silica gel

75–105

Tregeneration (°C)

–20 to 20

–

–

–

–

–25 to 20

>0 to 20

>0 to 20

–25 to 20

>0 to 20

Tcooling (°C)

unknown

–

–

–

–

>100 W

>100 W

>3 kW

>100 W

>10 kW

Cooling capacity (total)

Cooling

Characteristics for applications

Single effect H2O/LiBr

Thermo-mechanical Ejector (various)

Open

Closed

Cycle types

Desiccant dehumidification

Adsorption heat pump

Absorption

Table 11.3 Matrix connecting cooling cycles with applications

Solid

Liquid

CHP-Beith-11.indd 268

4/5/11 9:24:12 AM

<1 g/kg

to 1 g/kg

to 5 g/kg

–

–

–

–

–

Remaining moisture content in air

–

–

Has been considered

>35 kW

Under devel.

>3 kW

–

–

–

–

–

Latent cooling capacity

Dehumidification

Heat-activated cooling technologies for small and micro CHP

269

and it is drawn into the system and exhausted out of it with only partial or no recirculation. In closed cycles, the working fluid is circulated repeatedly through the cycle without any loss of mass, allowing various working fluids to be chosen based on their thermodynamic properties, such as water, ammonia, alcohols, hydrocarbons, salt solutions, and other fluids. Open cycles discussed in this chapter are liquid and solid desiccant processes. Closed cycles discussed are absorption, adsorption and steam ejector heat pump cycles.

11.3.1 Background on sorption processes Nearly all open and closed thermally-driven cycles for cooling and dehumidification rely on sorption processes (with the exception of the thermomechanical ejector cycle). Sorption processes can be divided into absorption and adsorption. In absorption, absorbate molecules are dissolved into (i.e. incorporated into the bulk of) an absorbent material, changing the chemical makeup of the absorbent. On the other hand, the adsorption process involves adhesion of molecules to a surface at a gas–solid interface due to van der Waals (secondary) bonding (e.g. hydrogen bonding), leaving the chemical makeup of the adsorbent unchanged. This means that the absorption capacity of absorbent media scales with absorbent volume; whereas adsorption capacity of adsorbent media scales with interfacial surface area. The sorption rates of both adsorption and absorption processes scale with interfacial surface area, since mass transfer can only occur at the gas–solid or gas–liquid interface.

11.4

Open sorption cycles: desiccant dehumidification

A desiccant material can be an absorbent or an adsorbent. Absorbent desiccant materials are generally aqueous salt solutions, although triethylene glycol has been proposed (water is also used as an absorbent in closed absorption cycles, described in Section 11.5). In contrast, adsorbent desiccant media are created by either (1) using packed beds of solid adsorbent grains or pellets or (2) applying a solid desiccant such as silica gel or activated carbon as a thin coating to high surface area materials such as a honeycomb of aluminum or a finned heat exchanger. The coated finned heat exchanger approach allows heat to be added to (and removed from) the desiccant material by an internal heat transfer fluid such as water, while the packed bed and honeycomb approaches require that an airstream be used to regenerate the desiccant. Although a wide variety of materials have at least some affinity for water, additional properties are important to make a material practical as a desiccant. Particularly important are (1) a large water capacity per unit weight (and per unit volume) of desiccant material, (2) chemical and physical stability over many cycles of sorption and regeneration, and (3) resistance or immunity

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 269

4/5/11 9:24:12 AM

270

Small and micro combined heat and power (CHP) systems

to contamination with dust and other contaminants present in the process and regeneration air streams. Additionally, the specific characteristics of the material’s sorption isotherms must match the desired application (e.g. regeneration temperature and desired process air humidity ratio). There are both liquid and solid hygroscopic materials that make good desiccants. The hygroscopic nature of the sorbent surface lowers the vapor saturation pressure of water at the surface. When the desiccant surface vapor pressure is lower than the partial pressure of water in the air over the surface, sorption of water will take place from the air to the desiccant. Sorption continues until the desiccant becomes saturated (i.e. until the vapor pressure differential between the surface and the surrounding air equalizes, achieving equilibrium). The desiccant can then be dried, or regenerated, by heating it to raise the vapor pressure above the surrounding air’s water vapor partial pressure. By cyclically saturating a desiccant material with moisture from the process air, regenerating (drying) the desiccant material in a heated regeneration air stream (which is exhausted outdoors), and cooling the desiccant with process air for sorption again, moisture can be removed from a building’s process air and exhausted to the surroundings. Three realizations of this basic principle are shown in Fig. 11.3. The primary input of energy to a desiccant system is heat to drive the desorption (regeneration) process, and 0.02

RH = 0.8 RH = 0.6 RH = 0.4

Humidity ratio (kgH2O/kgdry

air)

Saturation line Ambient

0.015

0.01

A

RH = 0.2

B

C

Setpoint

Pressure = 101.3 kPa 0.005 10

20

30 T (°C)

40

50

11.3 Airside psychrometric state points for three possible dehumidification processes, all of which cool and dehumidify ambient air to the same setpoint. Path ambient A-B-setpoint: conventional process, which removes moisture by cooling the air along the saturation line and reheating; path ambient-C-setpoint: rotary solid desiccant wheel with sensible-only cooling coil; path ambient-setpoint: liquid desiccant with sorption occurring directly on chilled wet desiccant cooling coils.

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 270

4/5/11 9:24:12 AM

Heat-activated cooling technologies for small and micro CHP

271

fortunately the temperature required to regenerate many desiccants (i.e. the temperature of the desiccant isotherm at which the desiccant vapor pressure exceeds the ambient or regeneration water partial pressure) is within reach of small-scale CHP systems.

11.4.1 Solid (adsorptive) desiccant cycles

1

Regeneration inlet

4

2

Process outlet

3

Indoor loads

5

Cooling coil

Process inlet

6

Desiccant wheel

Regeneration outlet

Regenerator

The solid desiccant cycle is typically arranged as a desiccant-coated rotary heat exchanger (commonly referred to as a desiccant wheel) which rotates very slowly through process and regeneration air streams. A basic cycle is shown in Figs 11.4 and 11.5. Since the process air must first cool the desiccant before the desiccant can begin to dehumidify it, the first few degrees of

11.4 Basic desiccant wheel schematic. 0.02

0.8 0.6 RH = 0.4

RH = 0.2 30 °C

Humidity ratio (kgH2O/kgdry

air)

25 °C

6

20 °C

0.015

1

15 °C

4

0.01

Pre

2 3 0.005 10

20

30

40

T (°C)

50

5 ssu

60

re

=1 01. 3

kPa

70

80

11.5 Air side psychrometric state points for a basic desiccant wheel, for typical indoor and outdoor summer conditions in the Southeastern US. Regeneration inlet is assumed to be outgoing ventilation air.

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 271

4/5/11 9:24:13 AM

272

Small and micro combined heat and power (CHP) systems

rotation into the dehumidification section are actually devoted to sensible cooling of the desiccant (and the medium to which it adheres or within which it is packed). However, due to the slow rotation speed of the wheel, this has only a small effect on the process air temperature. The outlet air conditions are not uniform around the wheel, but they become homogeneously mixed downstream, and these mixed averages are depicted in Fig. 11.5. Solid desiccant materials and substrates There are many substances that are hygroscopic, but relatively few of these have the large surface area required to serve as a practical desiccant. The surface is where a desiccant actually interacts and exchanges moisture with the surrounding airstream. Thus it makes sense to maximize the surface area of a given volume (or mass) of desiccant material. This is achieved through selecting materials with high porosity. Several materials have been developed to serve as practical desiccants. An important distinction exists between the zeolites and all other desiccants (such as silica gel, activated carbon, and activated alumina). Zeolites have Silica gel Zeolite Activated carbon

T

m ed

T

lo

Desiccant moisture content (kgH2O/kgadsorbent)

w

0.4

0.2

h

T hig

0 Increasing water vapor pressure at desiccant surface

11.6 Qualitative equilibrium isotherms (for illustrative purposes only) for adsorption of water by silica gel, zeolite, and activated carbon. Each isotherm shows, for a given desiccant temperature, the equilibrium adsorbed moisture content as a function of the vapor pressure at the adsorbent surface (this corresponds to the partial pressure of water when the desiccant is in thermal equilibrium with air). Isotherms are shown for three adsorbent temperatures for silica gel, and for clarity only one is shown for zeolite and activated carbon.

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 272

4/5/11 9:24:13 AM

Heat-activated cooling technologies for small and micro CHP

273

uniform pore size due to their crystalline pore structure (Ruthven, 1984, p. 9), while the other desiccants have a distribution of pore sizes centered on some mean value. For all desiccant types, typical pore diameters range from a few Angstroms to a few tens of Angstroms (Ruthven, 1984, p. 4). Every desiccant material has a set of equilibrium sorption isotherms with a characteristic shape in a graph of moisture content vs. surface vapor pressure. A qualitative comparison of the equilibrium isotherm shapes for the most common solid desiccants is shown in Fig. 11.6. From this figure, the relative characteristics of each desiccant are apparent. Compared with other water-adsorbing materials, silica gel has the widest operating range (though large swings in surface vapor pressure are required for large moisture removal) and a large capacity. The uniform pore size of zeolite is evident in the relatively sharp transition in moisture content that occurs over a narrow region of vapor pressure. A wide variety of zeolites are available, and depending on the type of zeolite, this transition can occur at higher or lower vapor pressure, and may correspond to a larger or smaller jump in moisture content. Thus zeolites have a fairly narrow operating range, but a high level of moisture transport can be accomplished with fairly small swings in vapor pressure. The one shown in Fig. 11.4 can be seen to have a high affinity for water at very low vapor pressures, making it effective at deep drying, although its utility in removing latent load for occupied spaces would be limited. The other important aspect to consider when looking at isotherm data for a substance is the temperature dependence of the isotherms. For an example of such a chart, see Fig. 11.13 in Section 11.5.2. The hygroscopic nature of a desiccant means that, when the desiccant is relatively dry, water vapor will spontaneously adsorb onto its surface in an exothermic reaction. In fact, this will occur even at temperatures above the psychrometric dewpoint of the surrounding air (another way of saying this is that the vapor pressure at the desiccant surface is much lower than that over a surface of liquid water at the same temperature). The reversible nature of this reaction is equally important: heating the desiccant can desorb the water off the desiccant surface (a phase change requiring a heat input equal to the heat of sorption). Importantly, the temperature required to regenerate (i.e. dry) many desiccants is well below the boiling point of water, and within reach of the low-temperature waste heat recovery utilized in small- and micro-CHP systems. Thus, by exposing a desiccant material alternately to a process air stream (i.e. one to be dehumidified) and to a hot exhaust airstream (which is exhausted to the environment), the desiccant can be alternately saturated with moisture and regenerated. This is the basis of the basic solid desiccant process depicted in Figs 11.4 and 11.5.

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 273

4/5/11 9:24:13 AM

274

Small and micro combined heat and power (CHP) systems

Desiccant wheel configuration options For solid desiccant systems, a large number of configurations are possible when designing the internal components of a desiccant wheel unit. Every desiccant system has the essential components of a desiccant and a regenerator, and a system could be designed with single or multiple stages of these. In addition, single or multiple heat recovery exchangers (which transfer heat but not moisture), evaporative coolers, and sensible cooling coils or evaporators can be chosen. The number of possible combinations and sequences is enormous, and only a relatively small number have been studied in detail. Starting with the most basic cycle, some configurations that have already been explored and implemented are described below. Basic cycle The most basic configuration of a desiccant wheel consists of simply an adsorbent-coated wheel rotating between two air streams, with a sensible cooling coil (or evaporator) to remove the sensible load, as shown in Fig. 11.4. The rotation speed of the wheel is generally low enough (~1 rotation per minute) that the sensible energy transferred from the regeneration airstream to the process airstream is small. However, the water vapor adsorbed onto the desiccant surface in the process stream undergoes a phase change from vapor to an adsorbed quasi-liquid (an adsorbed substance generally has internal energy somewhere between its solid and liquid forms at the given temperature). The release of that latent heat raises the process air temperature, and in practice this sensible heat gain is roughly equal to the latent energy removed from the process stream. In this sense the desiccant wheel is a constant enthalpy device that exchanges latent energy for sensible energy, simultaneously dehumidifying and heating the process airstream. Even in this most basic configuration, a desiccant wheel can be very helpful. Since the process stream is dehumidified already, the evaporator/ cooling coil need only provide sensible cooling. Thus it only has to cool the process air to the desired dry bulb temperature, rather than all the way to the (much cooler) desired dew point temperature, improving the cooling system COP and capacity. Cycle enhancements Although heating the process air is undesirable, it does provide valuable opportunities. In fact, often the temperature of the dehumidified air leaving the wheel is so high as to be significantly above ambient temperature, allowing sensible heat exchange with the ambient air through a run-around loop, heat pipe, thermosiphon, or sensible wheel. Additionally, a large efficiency gain

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 274

4/5/11 9:24:13 AM

Heat-activated cooling technologies for small and micro CHP

275

2

Evaporative cooling

6

3

Regeneration inlet

5

4

Indoor loads

1

7

Cooling coil

8

Sensible wheel

Process inlet

9

Desiccant wheel

Regeneration outlet

Regenerator

can be achieved by utilizing outgoing ventilation air as the heat sink for this sensible exchange, then subsequently heating it for use as the regeneration air stream. This can simultaneously reduce the sensible load remaining for the sensible cooling coil and reduce the regeneration heat required to regenerate the desiccant. In terms of packaging, a particularly convenient and common configuration is to include a sensible energy exchange wheel in series with the desiccant wheel, as shown schematically in Fig. 11.7 and on a psychrometric chart in Fig. 11.8. This wheel exchanges sensible energy without transferring moisture, preheating the regeneration air before it reaches the heater, and cooling the dehumidified process air before it reaches the conditioned space, cooling coil, evaporator, or air handling unit. This simultaneously reduces

Process outlet

11.7 Schematic of enhanced desiccant wheel with sensible exchange wheel and evaporative cooling. 0.02

25 °C

0.8 0.6 RH = 0.4 30 °C

RH = 0.2

1

air)

Humidity ratio (kgH2O/kgdry

9

20 °C

0.015

15 °C

6 5

0.01 4

0.005 10

8

7

20

2

3

30

Pressure = 101.3 kPa

40

T (°C)

50

60

70

80

11.8 Representative average air side state points for enhanced desiccant wheel (with sensible exchange wheel and regenerationside evaporative cooling) in psychrometric chart.

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 275

4/5/11 9:24:14 AM

276

Small and micro combined heat and power (CHP) systems

the sensible load required of the cooling coil and reduces the amount of heat required for regeneration. Once a sensible wheel has been added to the system, another possible enhancement is to add evaporative cooling to the regeneration side, before the sensible wheel (as also shown in Fig. 11.7). Although this may adversely affect the regeneration of the desiccant wheel, it also introduces the possibility of cooling the dehumidified process stream down to the wet bulb temperature of the regeneration inlet air. In one configuration, the outgoing ventilation (i.e. building exhaust) air is available for sequential use, first as a heat sink for the heat of adsorption, and next as the regeneration air stream. With this configuration, under the right climatic conditions and with a sufficiently effective sensible wheel, the dehumidified air can even be cooled below the setpoint temperature of the space, completely abdicating the need for any cooling equipment beyond the desiccant wheel itself. In practice, however, the climatic conditions amenable to this scenario are not very common, and even sophisticated desiccant wheel configurations for space cooling of occupied buildings are generally installed with supplementary sensible cooling equipment (such as vapor compression cooling). An absorption or adsorption heat pump could also be used for the supplemental sensible cooling. Evaporative cooling of the regeneration air, given sequential use of the regeneration air as heat sink and for regeneration, does have an adverse effect on the regeneration process. This adverse effect can be simply expressed in two scenarios: (1) for a fixed amount of heat input to the regenerative heat exchanger, it increases the RH of the regeneration air (and thus diminishes the moisture removal capacity of the wheel), or (2) for a fixed amount of moisture removal, it increases the required heat to the regenerator. Thus, in cases where a desiccant wheel installation has excess moisture removal capacity, evaporative cooling and sequential use of the regeneration air is an attractive option for reducing supplemental HVAC capacity requirements. In cases where the desiccant wheel does not have excess moisture removal capacity, this adverse effect can be circumvented by using a parallel approach to evaporative cooling/heat sinking and regeneration. In a parallel arrangement, outgoing ventilation air is evaporatively cooled, passes over the sensible wheel, and is exhausted. Meanwhile a separate air stream (perhaps outdoor air) is used for regeneration. This opens additional possibilities for providing total (sensible and latent) cooling with a solid desiccant humidifier without supplementary equipment. Additional configurations have been explored, including two-stage (parallel) regeneration for improved thermal COP (Kodama et al., 2003); dual desiccant wheels and dual cooling coils, and multiple-stage (serial) regeneration stages for more complete dehumidification (Henning et al., 2007); three- and fourwheel arrangements for high humidity climates (Kodama et al., 2003); and many more.

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 276

4/5/11 9:24:14 AM

Heat-activated cooling technologies for small and micro CHP

277

11.4.2 Liquid (absorptive) desiccant processes Compared with solid desiccants, liquid desiccants have the advantage of very high specific moisture capacity: in fact, LiCl will absorb 10 times its own mass of water if allowed to reach equilibrium in a 90% RH environment (ASHRAE, 2005). However, the degree to which the absorption process approaches the equilibrium state depends on the amount of surface area exposed to air and the contact time with air. A quiescent liquid/air interface is not sufficient to achieve practical absorption rates. Thus, in order to increase surface area, most liquid desiccant systems either spray the desiccant into an air stream, or pass the air stream through an extended surface contact medium through which a falling film of desiccant falls. In the case of spraying, small droplet sizes clearly have an excellent ratio of surface area to volume, and smaller droplets also fall more slowly, achieving a longer residence time in the air stream. However, any spraying process creates a distribution of droplet sizes, and when the average vertical velocity of the falling droplets is comparable to or slower than the airflow velocity, many smaller droplets will be carried upward with the air stream, requiring additional filtering equipment to prevent them entering the process air stream. This filtering becomes a necessity even for falling film systems, as high local velocities through the media create very small liquid droplets that can be carried with the air stream. Two more advantages of liquid desiccants are that they can be regenerated with a lower temperature than most solid desiccants (although recently developed zeolites can also achieve low regeneration temperatures), and the liquid desiccant solution can be heated and cooled independently of the process and regeneration air streams (e.g. in liquid-to-liquid heat exchangers). This prevents the heat of regeneration from being dumped into the process air stream, and allows unheated air (ambient or building exhaust air) to be used for regeneration. This is shown in Fig. 11.9. Additionally, separating the desiccant heating and cooling processes presents the opportunity for recuperative heating of the diluted absorbent with the strong absorbent where the solution temperatures overlap. Liquid desiccant solutions The most common solution used in liquid desiccant dehumidification is aqueous LiCl, LiBr, CaCl2 and triethylene glycol are possibilities that have been experimented with. A liquid desiccant’s vapor pressure is basically proportional to its temperature and absorbent concentration. An equilibrium isotherm of LiCl is shown in Fig. 11.10.

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 277

4/5/11 9:24:14 AM

278

Small and micro combined heat and power (CHP) systems

Air-air HX Regeneration air outlet

Process air outlet

Air-air HX

Qcooling to ambient P2 Qheating

Process air inlet P1

Regeneration air inlet

from prime mover

Desiccantdesiccant HX

P3

11.9 Schematic of one possible liquid desiccant dehumidification system. Pump P3 flows just enough to maintain fixed solution concentrations in the sumps of the conditioner and regenerator. It is also possible to have cooling coils immersed in the process air stream, with the desiccant spray forming films over them.

12 LiCl Desiccant moisture content (kgH2O/kgabsorbent)

10 8 6 4 2 0 10

30 50 70 Relative humidity (%)

90

11.10 Equilibrium isotherm for aqueous LiCl. Isotherm will shift up with a decrease in solution temperature (or down with an increase in temperature).

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 278

4/5/11 9:24:14 AM

Heat-activated cooling technologies for small and micro CHP

279

11.4.3 System integration options Several choices remain in designing a desiccant wheel or liquid desiccant installation. The desiccant dehumidifier is generally configured to be in series with the unit that provides sensible cooling to the space (a cooling coil, evaporator, or air handling unit). In some cases, however, the desiccant unit can be installed in parallel with and independent of the existing HVAC system. This would make sense for a retrofit where, for example, a sensible heat recovery wheel is incorporated into the desiccant wheel unit. For a given desiccant dehumidification unit, where the stages of desiccation, regeneration, heat recovery, and evaporative cooling are in place, some decisions remain about how the unit is integrated with the building’s HVAC system: ∑ Choice of regeneration air stream source: outdoor air, outgoing ventilation (exhaust) air stream, or some combination. ∑ Choice of process air stream source: outdoor makeup air, recirculated conditioned air, or some combination. Schinner and Radermacher (1999) have performed detailed modeling of a combined desiccant/absorption system, demonstrating promising potential for such a system. The most efficient arrangement was predicted to be the use of building ventilation exhaust air for desiccant regeneration, and makeup (outdoor) air for the desiccant process air.

11.5

Closed sorption cycles: absorption and adsorption heat pumps

The two kinds of closed sorption cycles are absorption and adsorption heat pump cycles.3 Absorption refers to the sorption of a solvent into the bulk of a fluid or material, while adsorption refers to the sorption of a solvent onto the surface of a material. Of these, the absorption cycle is very well established (indeed the first form of mechanical ice production was based on an absorption cycle, invented in 1846 by Ferdinand Carré) and today there are many commercialized absorption heat pump products. Adsorption heat pumps also have a very long history, and adsorptive processes have been used extensively in open desiccant dehumidification systems (not to mention separation processes in the chemical and pharmaceutical industries). However, only a few commercial products are available today, and the adsorption heat pump is a technology currently under intense development. It is instructive to compare heat-driven heat pumps to the basic reverse 3

Throughout this chapter, the term heat pump is used in the generic sense to refer to devices that transfer heat from a lower temperature source to a higher temperature sink, whether the intended application is cooling or heating.

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 279

4/5/11 9:24:14 AM

280

Small and micro combined heat and power (CHP) systems

Rankine (mechanical vapor compression) heat pump cycle, which is overwhelmingly the most widespread heat pump technology today. In the reverse Rankine cycle, a mechanical compressor compresses refrigerant vapor to a high temperature and pressure state, a condenser cools (and therefore condenses) the refrigerant at constant pressure, an expansion valve expands the refrigerant adiabatically, and an evaporator heats (thereby evaporating) the refrigerant at constant pressure back to vapor to complete the cycle. Absorption and adsorption heat pumps differ from the reverse Rankine cycle by replacing the mechanically powered compressor with thermally powered sorption equipment, taking advantage of the temperature dependence of sorption isotherms to achieve a high-pressure refrigerant vapor state. The various types of absorption and adsorption heat pumps are defined and distinguished by the sorption materials and methods they use to transform low-pressure refrigerant vapor into high-pressure vapor.

11.5.1 Absorption heat pumps The vast majority of absorption heat pumps use either water/LiBr or ammonia/ water as their refrigerant/absorbent working pair. The LiBr-based system has a better COP, but to prevent damage to the system by freezing of the water refrigerant, its evaporator temperature cannot go below 0 °C. Moreover, the solubility characteristics of LiBr in water are such that absorber cooling must be performed at relatively low temperature and/or low concentration to avoid crystallization. In contrast, the ammonia/water system is able to achieve the much lower temperatures required for refrigeration and freezing applications, but because of the toxicity of ammonia vapor, these systems must be either large enough to justify the overhead of safety measures (as in the case of large commercial refrigeration), or small enough to circumvent regulations related to the mass of ammonia in the system (as in the case of domestic refrigerators). Manufacturers associated with absorption systems with cooling capacity under 20 kW (~6 RT) include Broad (China), Yazaki (Japan), ClimateWell (Sweden), Rotartica (Spain), Robur (Italy), and Pink GmbH (Germany). Fundamentally, absorption heat pumps accomplish the feat of compressing refrigerant vapor with only a very small amount of mechanical input by pumping the refrigerant to high pressure as a liquid rather than compressing it as a vapor.4 The crucial element of the design, then, is how to convert low 4 The efficiency of a pump or compressor can be defined as the required work input per unit of fluid mass flow rate. The drastically higher efficiency of pumping liquid compared with compressing gas can be easily explained. Compared with the vapor phase, a refrigerant’s liquid phase has a much higher density, higher viscosity, and lower compressibility. Higher density means a smaller device with potentially lower fluid velocities are possible, higher viscosity means less leakage will occur, and most importantly, lower compressibility means less flow work (pressure-volume work) must be done.

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 280

4/5/11 9:24:14 AM

Heat-activated cooling technologies for small and micro CHP

281

pressure vapor into a liquid, and high pressure liquid into a vapor. These two tasks are accomplished with heat-powered absorption and desorption. An absorption heat pump is best shown schematically as if superimposed on a Dühring diagram, as shown in Fig. 11.11, where increasing height corresponds to increasing pressure, horizontal distance to temperature, and each diagonal line is a particular absorbent equilibrium concentration. In an absorption heat pump, vapor leaves the evaporator as it is absorbed into strong solution in the absorber. This releases the refrigerant’s latent heat of absorption, so that the absorber must be actively cooled.5 Eventually, as the absorbent becomes nearly saturated with refrigerant, it is pumped to a high pressure desorber (also called a generator), first being pre-heated in the solution heat exchanger. In the desorber, heat is added to the dilute solution, evaporating the refrigerant, making the solution stronger and driving high pressure refrigerant vapor to the condenser. The remaining strong liquid solution need only be cooled and depressurized to absorb refrigerant vapor again. The solution is pre-cooled in the solution heat exchanger (HX), adiabatically cooled through a throttling valve, and arrives back at the absorber to complete the absorption/desorption cycle. The balance of the system (evaporator, condenser, and expansion valve) operates as in a reverse Rankine (i.e. vapor compression) cycle. An important efficiency improvement to the absorption cycle is achieved by the generator-absorber heat exchange cycle, commonly known as the Qcondensation Condenser

Qdesorption Refrigerant (vapor)

Desorber

Weak solution

Refrigerant (liquid)

Solution HX W

Refrigerant (two-phase) Evaporator

Qevaporation

Refrigerant (vapor)

Strong solution High pressure side Low pressure side

Absorber

Qabsorption

11.11 Schematic of an absorption heat pump cycle. 5

The absorber will be cooled towards the ambient dry bulb temperature in an air-cooled system, and towards the ambient wet bulb temperature in a water-cooled system with cooling tower.

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 281

4/5/11 9:24:15 AM

282

Small and micro combined heat and power (CHP) systems

GAX cycle. The GAX cycle can be thought of as the logical extension of extending the solution heat exchanger into the desorber (generator) and absorber. The latent heat of absorption which is gained by the absorber is used to provide heat to the desorber, and the latent heat of desorption released by the desorber cools the absorber. In this way, less heat needs to be added to the desorber, and similarly less heat needs to be rejected to the ambient from the absorber. This is only possible if temperatures overlap between the desorber and absorber, generally precluding GAX from water/ LiBr systems. The most important consideration when choosing a cycle type for a cogeneration system is the available waste heat temperature. Since every heat-activated cooling system has a minimum regeneration temperature, and since many small cogeneration systems have a relatively low waste heat temperature, many cooling technologies are impractical for use with small CHP. For example, a micro-CHP device may produce hot water at 80 °C, just hot enough to drive a single-effect absorption chiller but not nearly hot enough for a double- or triple-effect chiller. For applications with low heat recovery temperature, it may be necessary to utilize a half-effect absorption cycle. The half-effect cycle requires an additional absorber, desorber, solution heat exchanger, and solution pump, and also has a lower COP than the single-effect cycle. However, the required driving temperature is significantly lower.

11.5.2 Adsorption heat pumps In common with the absorption cycle, the adsorption heat pump replaces the mechanical compressor of the reverse Rankine cycle with sorption equipment. However, adsorption is used instead of absorption, and no solution pump is needed. Compared with absorption heat pumps, adsorption heat pumps are less well developed and generally have a lower thermal COP, but have a wider range of possible working pairs (see Table 11.3), a wider range of regeneration temperatures, and can have lower parasitic power requirements. One major challenge of adsorption heat pumps is packaging the adsorbent to achieve good heat and mass transfer within a reasonable volume and mass. Manufacturers associated with small adsorption systems include Jiangsu Shuangliang (China) Mayekawa (Japan), and SorTech AG (Germany). In an adsorption heat pump, shown in Figs 11.12 and 11.13, two adsorbentcoated heat exchangers (or adsorbent beds) are placed in separate sealed chambers. Each coated heat exchanger has piping to allow heating or cooling water to flow within. Thus, by heating one of the heat exchangers (the desorber), refrigerant will be desorbed from its surface, pressurizing its chamber until the upper valve opens, allowing refrigerant vapor to pass to the condenser (which is cooled by ambient-temperature water). The condensed refrigerant

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 282

4/5/11 9:24:15 AM

Heat-activated cooling technologies for small and micro CHP

Condenser

Active or passive check valves Desorber

Expansion orifice

Adsorber

283

Heat transfer fluid temperatures: Low (e.g. 10 °C) Moderate (e.g. 30 °C) High (e.g. 70 °C) Refrigerant pressures:

Evaporator

Low (e.g. 0.01 atm) High (e.g. 0.04 atm)

11.12 Adsorption heat pump schematic.

6

fr ig e tio ran n t lin e

X

=

0

2 .1

6 X

=

0

7 .0

4 X

=

0

ra

Re

=

3

4 .0

X

tu

8

X

2 0.

sa

Vapor pressure (kPa)

12

X

4

55

4

= =

0

2 .0

0.

0

2

14

49 Refrigerant Tsaturation (°C)

16

43 37 31

3

25

2

19

1.5

13

1 –0.0033

–0.0031

–0.0029 –1/Tadsorbent (–K–1)

–0.0027

7 –0.0025

11.13 Dühring diagram for water on silica gel 3A, plotted from state equation given in Ng et al. (2001). X denotes the value of kgH2O/ kgadsorbent for each isostere (line of constant adsorbent loading). Dotted lines: simple (non-recuperative) adsorption cycle operating with the following conditions: Tdesorption = 80 °C (176 °F, –0.00283 in diagram); Trejection = 32 °C (90 °F, –0.00328 in diagram), corresponding to a pure water vapor pressure of 4.76 kPa; Tevaporation = 10 °C (50 °F), corresponding to a pure water vapor pressure of 1.23 kPa.

drips by gravity to an expansion orifice or valve and adiabatically cools upon expansion. The expansion process generates two-phase refrigerant, with the liquid refrigerant evaporating from the surfaces of the evaporator to provide cooling capacity. The pressure differential across the expansion orifice is maintained by cooling the second coated heat exchanger (the adsorber) with

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 283

4/5/11 9:24:15 AM

284

Small and micro combined heat and power (CHP) systems

ambient-temperature water, thus causing refrigerant vapor to adsorb onto its coated surface. This arrangement can be maintained until the desorber approaches a ‘dry’ state and the adsorber approaches a saturated state. The adsorber/desorber roles of the two coated heat exchangers are then reversed, allowing the process to continue. Thus the adsorption heat pump is a periodic cooling device – although when operating properly, an excess of liquid refrigerant will build up on the evaporator during the peak sorption period, which buffers the capacity delivered by the evaporator during adsorption/ desorption switching periods. The necessity of periodically heating and cooling the adsorber heat exchangers and associated piping accounts for the generally lower thermal COP of adsorption systems compared with absorption systems, since heating this ‘dead mass’ consumes driving heat (which is then rejected to the ambient in the next adsorption phase) without contributing additional cooling capacity. However, since no solution pump is required for an adsorption heat pump, the potential exists for improving the ECOP compared with absorption systems. Also, the wide variety of adsorbents available and in development, particularly a diversity of zeolites, hold much promise for enabling adsorption heat pumps to efficiently operate under a wide range of conditions, including utilizing much lower heat source temperatures than required by absorption systems. Refrigerant/adsorbent working pairs that have been used for adsorption heat pumps include water/silica gel, water/zeolite, ammonia/activated carbon, butane/silica gel, methanol/silica gel and others. Since adsorption involves van der Waals bonding, and since hydrogen bonding is the strongest form of van der Waals bonding, the use of small polar molecules such as water, ammonia and methanol as refrigerants tends to result in the best performance.

11.6

Steam ejector cycle

Whereas sorption heat pumps replace the compressor of a VCC with a set of thermally-driven sorption equipment, the ejector cycle replaces the compressor with a boiler, feed pump, and ejector: a mechanically simple device with no moving parts (Fig. 11.14). However, although the ejector cycle (Fig. 11.15) is attractive for its simplicity, it does not achieve as high a performance as sorption heat pumps. Also, currently there are no commercially available cooling products based on the ejector cycle, although there is much ongoing research. The ejector cycle most commonly uses water as a working fluid, although various compounds have been tried (see Sun, 1999). The steam ejector refrigeration cycle was utilized widely during the 1930s, and until recently, very little further development has been pursued. Current areas of research include improving the COP and developing systems that can operate with lower boiler temperatures (to enable waste heat and solar firing).

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 284

4/5/11 9:24:15 AM

Heat-activated cooling technologies for small and micro CHP Throat

Mixing Converging/diverging nozzle

285

Subsonic

chamber

diffuser

Primary or ‘motive’ fluid

Mixed Outlet

Shock

Fluid velocity

Secondary or ‘entrained’ fluid

Sonic velocity

Ejector length

11.14 Schematic of an ejector with fluid velocity profiles. Solid double line: fully mixed fluid; solid single line: primary fluid; dotted line: secondary fluid (figure adapted from Sun, 1999). Qdriving Boiler Pr ehe at

W

Pressure

er

Qrejected

Condenser

e Ej

ct

or

Pre -co ole r

Evaporator Qevap Temperature

11.15 Ejector cycle schematic, shown with two optional pre-heating and pre-cooling heat exchangers for improved capacity and COP.

11.7

Component-specific efficiency and effectiveness metrics

The figures of merit most commonly used for describing the performance of cooling devices are the COP, thermal COP, and the specific cooling power. These metrics are useful for sorption heat pumps and dehumidification systems

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 285

4/5/11 9:24:16 AM

286

Small and micro combined heat and power (CHP) systems

that provide sensible or total (sensible and latent) cooling. However, some desiccant dehumidification configurations provide latent cooling without sensible cooling, and a distinct approach must be taken to evaluate their effectiveness.

11.7.1 Thermal coefficient of performance (COP) and Carnot COP The thermal COP is analogous to the more familiar COP used with vapor compression cycle (VCC) heat pumps. However, whereas the VCC COP is the ratio of delivered heat or cooling power to the electrical or mechanical power input, the denominator in the thermal COP is the thermal power input. The advantages of HACTs are obscured by the fact that this important distinction is often overlooked. It is important to remember the more thermodynamically valuable nature of electricity compared with fuel or heat when comparing VCC COPs to thermal COPs. In this regard, if it is assumed that three units of fuel (i.e. high-temperature heat) are required to produce one unit of electricity, then an electrically-driven heat pump with a COP of 3 will have equivalent source fuel utilization rate to a fuel-fired, heat-activated heat pump with a COP of only 1. In the case of waste-heat-activated cooling technologies, the accounting is even more favorable towards the HACT. For example, imagine that facility ‘A’ utilizes a CCHP system which incorporates a waste-heat-driven HACD with a COP of 0.5, while facility ‘B’ utilizes grid electricity and a VCC heat pump with a COP of 4. Depending on factors such as equipment utilization rate, the assumed conversion efficiency of the electric grid, climate, and many others, facility ‘A’ might turn out to use less source fuel energy than facility ‘B’ to meet the same loads throughout the year (it will likely also eliminate expensive demand charges on its electric bill which are usually incurred during the hottest days of the year). This highlights the need to take a system-wide approach to system performance, rather than simply focusing on the COP values for different cooling technologies. Another perspective on thermal COP is provided by comparing it with the Carnot limit for a given regeneration temperature. This is shown for typical system COPs in Fig. 11.16. System-wide approaches to measuring efficiency are addressed in Section 11.8. Figure 11.16 shows the typical ranges of firing temperature and COP for absorption, adsorption and ejector cycles. Of the plotted technologies, four are commercially available: (1,2) single-effect H2O/LiBr or NH4/H2O machines, (3) double-effect H2O/LiBr machines, and (4) adsorption machines with various working pairs. The various working pairs for adsorption machines are discussed in Section 11.5.2. Although the potential exists for large COP improvements with multi-stage adsorption machines (see Douss and Meunier,

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 286

4/5/11 9:24:16 AM

Heat-activated cooling technologies for small and micro CHP

287

ol

in

g

n

je re

Adsorption Single effect 1 cycles H2O/LiBr

Double effect NH3/H2O Single effect NH3/H2O

0.5 0

Triple effect H2O/LiBr

Double effect H2O/LiBr

T

co

ct

1.5

T

Thermal COPcooling

2

io

Th e CO rm P al c

co ar ol n in g fo ot = r 35 = 10 °C °C and

2.5

Half effect H2O/LiBr 35

75

Ejector cycles

115 155 195 Regeneration temperature (°C)

235

11.16 Thermal COP vs. regeneration temperature for various HACT.

1989), this possibility is left out of the figure since experimental results for such a machine are not well established. An interesting note to Fig. 11.16 is that the COP range for a given absorption technology is flat – the COP for single-effect H2O/LiBr absorption, for example, does not increase with increasing firing temperature. This is a consequence of the working fluid properties. If a higher heat source temperature is available, utilizing a double-effect system does increase COP, but the COP for double-effect is also flat. This dependence of performance on working fluid pair (and not heat source temperature) is in fact a general characteristic of sorption heat pumps. The region in Fig. 11.16 for adsorption cycles displays a diagonal tilt because it represents a family of working pairs. Although not sorption-based, the ejector cycle region also represents a family of working fluids and geometries. The Carnot COP of a VCC heat pump is the best possible performance achievable between two source/sink temperatures, given a mechanical or electrical input. It is derived in every thermodynamics fundamentals textbook. The Carnot thermal COP of a HACD is derived in Herold et al. (1996) by coupling a Carnot power production cycle to a Carnot heat pump cooling cycle. It is assumed that (1) the work produced by the power cycle equals the work consumed by the cooling cycle, and (2) the heat rejection temperature of the power cycle equals the heat rejection temperature of the cooling cycle. There are thus three temperatures involved in calculating the Carnot thermal COP for cooling: the low temperature (i.e. evaporator) heat source (T0), the heat rejection temperature (T1), and the high temperature heat source (T2). The resulting expression is given in Equation 11.1, where the first quotient is the Carnot efficiency for a power cycle and the second is the Carnot COP for a cooling cycle:

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 287

4/5/11 9:24:16 AM

288

Small and micro combined heat and power (CHP) systems

Ê T – T1ˆ Ê T0 ˆ COpcarnot,thermal, cooling = Á 2 Ë T2 ˜¯ ÁË T1 – T0 ˜¯

11.1 This efficiency limit can equivalently be applied to an engine-driven VCC device or to a directly heat-activated cooling device. Thus, considering the various energy conversion and transmission losses incurred by centrally generating power and distributing it to utility customers’ VCC systems, it is not surprising that a directly-fired HACD should be able to demonstrate source-energy efficiency comparable to or better than conventional VCC systems. Of course, this is especially the case when the HACD is driven by low-grade heat that is rejected from an on-site power cycle or collected by a solar collector. Although not discussed in detail in this chapter, it is important to note that most cooling devices are also capable of running in a heating mode during the heating season. The Carnot thermal COP for heating mode is given by Equation 11.2. Ê T – T0 ˆ Ê T1 ˆ COpcarnot,thermal, heating = Á 2 Ë T2 ˜¯ ÁË T1 – T0 ˜¯



= COPCarnot,thermal,cooling + 1

11.2

The fact that the heating COP (even for a system far from the Carnot ideal) is always greater than 1 means that the installation of a heat-activated heat pump can provide benefits both during the heating and cooling seasons. For heating use, the evaporator would be heated by the ambient, and the heat rejected by the condenser and adsorber would be used for space heating. For example, with a heating thermal COP of 1.5, an adsorption machine could provide 1.5 kW of space heating at 35 °C (95 °F) for every kW of heat produced at higher temperature by the prime mover. The thermal COP of heat-activated machines can generally be expressed as a simple ratio of useful thermal output to the thermal input, as in Equation 11.3, but can also include the parasitic loads (e.g. electricity to run pump and fan motors) required by the device. Further, there are at least two ways of accounting for parasitic loads: as subtracted from the cooling energy (Equation 11.4) or added to the regeneration energy (Equation 11.5).



COpcooling =

Qcooling Qregeneration

COpcooling =

Qcooling – Wparasitics Qregeneration

COpcooling =

Qcooling Qregeneration + Wparasitics

11.3



11.4



11.5

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 288

4/5/11 9:24:17 AM

Heat-activated cooling technologies for small and micro CHP

289

In any equation accounting for parasitic loads, the parasitic could also be weighted by the electric conversion efficiency, giving the amount of fuel that must be burned to supply the parasitic load. Taking this approach, Equation 11.6 could be substituted into Equation 11.4 or 11.5 above to replace the parasitic term to achieve an energy efficiency accounting using only thermal terms: Wparasitics helectrical

11.6 The performance of a heat-activated cooling device can also be expressed as the electric COP, or ECOP, which neglects thermal inputs and is the cooling output divided by the electrical parasitics, as shown in Equation 11.7. This definition makes sense if the heat driving the system is assumed to be truly free, and it demonstrates the reduction in electricity consumption of a HACD compared with a VCC. That the ECOP of a HACD exceeds the COP of a VCC is a necessary (but not necessarily sufficient) condition for demonstrating improved energy efficiency of the HACD. In practice, the electrical parasitics involved in running an absorption chiller are not insignificant, but the ECOP of an absorption system still easily surpasses the COP of a VCC. The ECOP of an adsorption chiller is generally even more favorable since pressurization of the working fluid is accomplished thermally rather than mechanically. There is no theoretical limit to the ECOP; indeed HACD have been built that require no electricity at all. Qparasitics,equiv =

ECOPcooling =

Qcooling Wparasitics

11.7 Thus, there are at least six possibilities for how to calculate the efficiency of a heat-activated sensible or total cooling device on a First Law basis. Further, when the choice of lower heating value vs. higher heating value is considered for the efficiency term in Equation 11.6, there are at least eight calculation possibilities (and the continuous range of numerical values that can be used for that efficiency term means the possibilities are infinite). Additionally, any thermal COP value can be divided by the Carnot thermal COP value to obtain a Second Law efficiency (which is always between 0 and 1). No definition is inherently better than another. Clearly, however, one must be careful in assessing efficiency claims, and should clearly state assumptions when reporting efficiencies, as a manipulative or careless choice of efficiency definition can have misleading results.

11.7.2 Desiccant dehumidification effectiveness Defining the effectiveness of a desiccant dehumidification process is complicated by the fact that a desiccant wheel can be useful without © Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 289

4/5/11 9:24:17 AM

290

Small and micro combined heat and power (CHP) systems

actually providing any reduction in the energy content (enthalpy) of the process air. For example, as noted in Section 11.4, the process air exiting a desiccant-coated wheel experiences essentially no change in enthalpy, as it is sensibly heated to the same degree it is latently cooled. Thus, for the most basic desiccant wheel configuration (Figs 11.7 and 11.8), if any one of Equations 11.3–11.5 were applied, the cooling provided would be zero! Thus these definitions are clearly not helpful for the basic configuration. Since the primary objective of dehumidification is moisture removal, Equation 11.8 makes intuitive sense, and is commonly used. Parasitic energy consumption could be subtracted from the latent heat removal term, or added to the regeneration term, following the pattern of Equations 11.4–11.6. However, as the primary ‘parasitic’ effect of a desiccant wheel may be an increase in pressure drop that must be overcome by the ventilation fan, or perhaps an increase in air flow rate, defining the parasitic load may not be straightforward. Another possibility is to define the dehumidification effectiveness relative to the ideal of removing all moisture from the process air (Equation 11.9). However, drier is not always better if an ideal humidity level has been reached, and this definition does not provide fair comparisons among different ambient conditions.



e dW,1 =

Qlatent Qregeneration

e dW,2 =

Qlatent w – w out = in Qlatent,max w in



11.8

11.9 The effectiveness definitions for dehumidification in Equations 11.8 and 11.9 can be useful for comparing one dehumidifying device to another under similar conditions, where dehumidification is the only objective. However, cycle enhancements to remove sensible load, such as adding a sensible wheel to a desiccant wheel system, will be under-appreciated by these definitions. For a desiccant dehumidification configuration that provides total cooling, Equations 11.3–11.5, or a system-wide performance metric, would be more appropriate.

11.8

System-wide performance and efficiency metrics

Attempting to define and interpret the component-specific efficiencies for each component in a CCHP system can be difficult, and sometimes the fuel consumption to meet a given set of loads can be improved by sacrificing one component’s individual performance to benefit another’s (e.g., in a combined

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 290

4/5/11 9:24:18 AM

Heat-activated cooling technologies for small and micro CHP

291

cycle power plant, the Brayton topping cycle’s exhaust gas temperature and pressure are increased to benefit the Rankine bottoming cycle and improve the plant’s overall thermal efficiency). Thus, in many cases it makes more sense to calculate the system-wide performance, capturing the interactions and synergies among components into a single metric which can be compared with alternative integrated systems.

11.8.1 Energy content of fuel There are basically two alternatives to account for the fuel input to a system: the lower heating value (LHV) basis or the higher heating value (HHV) basis. Each alternative is commonly used, and the choice is fairly arbitrary since results in either basis are readily interconverted. However, arbitrariness does not mean unimportance; indeed the choice of heat value should always be stated when reporting any performance metric that involves the fuel energy, such as system efficiency. Unfortunately, this choice is often not specified when efficiency values are reported, leading to much confusion and difficulty in comparing systems. Both heating value definitions include all the sensible energy removed from an exhaust stream by cooling it to 25 °C (77 °F). However, the LHV assumes that all water vapor formed by combustion remains vapor, while the HHV assumes that all water vapor condenses. HHV is typically 7% to 11% higher than LHV for hydrocarbon fuels. Consider the energy accounting consequences for the CHP system shown in Fig. 11.1 if the fuel basis is changed from HHV to LHV. The electrical and hot water energy flows (i.e. kW or Btu/hr) out of the system will clearly not be altered by this change in fuel accounting, so their efficiencies will be increased by an amount inversely proportional to the decrease in fuel energy (e.g. from 18% HHV electrical efficiency to 20% LHV electrical efficiency, corresponding to a 10% lower LHV energy content than HHV energy content). On the other hand, in order to arrive at a proper First Law energy balance on the system, the exhaust flow needs to be evaluated with respect to a reference state, and this reference state must correspond to the reference state used in the fuel heating value definition. For a prime mover with a sufficiently effective exhaust gas heat exchanger and a relatively low heat recovery temperature, it is possible to cool the exhaust gas below its dew point (about 60 °C/140 °F for stoichiometric combustion of natural gas in air), thereby transferring latent heat from the exhaust to the heat recovery fluid. Thus the use of the LHV reference state can allow the exhaust mass flow out of the system to be considered a heat flow into the system, making possible an overall fuel utilization rate greater than 1.

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 291

4/5/11 9:24:18 AM

292

Small and micro combined heat and power (CHP) systems

11.8.2 System-wide metrics The most commonly used system-wide performance metric is the fuel utilization factor shown in Equation 11.10.6 This is a simple First Law accounting of the useful energy flows (regardless of temperature or type) obtained from the system, divided by the fuel input. While straightforward to calculate and understand, it does not account for the thermodynamic or economic value of any outputs. FuF =

Welectric + Qcooling + Qheating Qfuel,hhV or lhV

11.10 An accounting of the energy flows based on the Second Law of Thermodynamics is shown in Equation 11.11, where the heating and cooling flows are weighted by the theoretical efficiency of a Carnot heat pump operating between the relevant source and sink temperatures (i.e. the second quotient of Equation 11.1), and electrical power and fuel are equivalent to their First Law values. Although this definition is derived directly from thermodynamics, is does not always correspond with the realities of economics, or with the actual efficiencies of alternative heating and cooling methods. Welectric +

Qcooling Qheating + hcarnot, heating hcarnot, cooling Qfuel,hhV or lhV

h2nd law = 11.11 In the US, the 1978 PURPA regulations define a combined efficiency for qualifying cogeneration facilities, given in Equation 11.12. The factor of 2 in this definition has no direct thermodynamic justification, and the resulting efficiency value will generally fall between the values calculated by the FUF and the Second Law efficiency.



hpuRpa =

Welectric +

Qheating 2

Qfuel,hhV or lhV



11.12

6

The fuel utilization factor adds together types of energy of very different value (electricity and low-grade heat) without any weighting or correction factor to normalize them. It is therefore misleading (although commonplace) to refer to it as the ‘overall efficiency’ or ‘combined efficiency.’ This terminology distinction for CCHP systems is analogous to VCC heat pump performance being characterized by a ‘coefficient of performance’ rather than an efficiency, since the VCC COP similarly does not correct for the difference in value between its numerator and denominator. Incidentally, since the thermal COP of a HACD is a ratio of comparable energies, it could legitimately be called an efficiency, unlike the COP of a VCC. This, again, underscores the incomparability of VCC COP and thermal COP values.

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 292

4/5/11 9:24:18 AM

Heat-activated cooling technologies for small and micro CHP

293

The fuel utilization efficiency shown in Equation 11.13 is a practical definition for comparing different systems.7 It weights each energy flow by how it would be produced by an alternative conventional device. Importantly, this means that all four terms in Equation 11.13 (i.e. work, fuel, heating, and cooling) are converted to units of energy of comparable value, making it more thermodynamically justifiable than Equation 11.10 from an exergetic standpoint. It is, however, up to the discretion of the user to choose baseline system efficiencies, making it less thermodynamically rigorous than Equation 11.11. Also note that Equation 11.13 is not directly bounded by the First Law, and can exceed 1. The calculation is straightforward when assuming baseline devices that are fuel-fired (such as a furnace, where hheater = 0.8 (LHV) would be a reasonable choice). However, if a VCC device is chosen as a baseline system, then the overall fuel utilization of that device (rather than its COP) must be considered, which also requires an assumption be made for electric grid efficiency. For example, if the baseline cooling device is chosen to be a VCC air conditioner, an appropriate choice for hAC, assuming an air conditioner with COP of 3 and a grid efficiency of 1/3 (LHV), would be hheater = (3) (1/3) = 1 (LHV). Of course a similar procedure applies to calculating hheater for an electrically-driven space-heating heat pump. Welectric +

Qcooling Qheating + hheater, fuel–basis hac,fuel–basis Qfuel,hhV or lhV

hfuel utilization = 11.13 The fuel chargeable to power (FCP) is based on the premise that the CHP system heat output displaces the use of a boiler, with electricity being produced as a byproduct. Under this premise, it makes sense to calculate an electrical efficiency where the fuel that would have fired a boiler is subtracted from the fuel the CHP system consumed. This can be generalized to include cooling outputs, as in Equation 11.14. Fcp = Qfuel,hhV or lhV –

Qheating Qcooling – hboiler hVcs

11.14 In common usage, the FCP ignores cooling and is often expressed as a heat rate (i.e. a ratio of inputs to outputs). In Equation 11.15 it is generalized to include cooling and expressed as the FCP efficiency, a ratio of inputs to outputs (the inverse of the FCP heat rate).

7

Here, fuel utilization efficiency is defined distinctly from the fuel utilization factor. In common usage, these terms are used interchangeably, and additional explanation is required to distinguish them.

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 293

4/5/11 9:24:19 AM

294

Small and micro combined heat and power (CHP) systems

hFcp = Qfuel –

Welectric Qheating

–

Qcooling

11.15

hheater,fuel–basis hac,fuel–basis The FCP efficiency metric (Equation 11.15) is very similar to the fuel utilization efficiency (Equation 11.13), except that the fuel chargeable to heating and cooling is subtracted from the inputs rather than credited to the outputs. A comparison of these two metrics is shown in Fig. 11.17. There are three pairs of lines in Fig. 11.17. Each pair compares the fuel utilization efficiency with the FCP efficiency, and each pair uses a different independent variable while holding others constant. The pair with crosses varies the electrical efficiency while holding fixed the heating and cooling efficiencies; the pair with squares varies the heating efficiency while fixing the electrical and cooling efficiencies; and the pair with pluses varies the cooling efficiency while fixing the electrical and heating efficiencies. A similar exercise can be carried out for the several other efficiency definitions given in this section, and each definition will exhibit distinct values and trends. This reinforces the importance of a critical approach to evaluating and reporting efficiency values.

Efficiency metric value

1 0.8

hFU = f (Welectric)

0.6

hFCP = f (Welectric) hFU = f (Qheating)

0.4

hFCP = f (Qheating) hFU = f (Qcooling) hFCP = f (Qcooling)

0.2 0 0 0.2 0.4 0.6 Independent variable as fraction of fuel input

11.17 Comparison of fuel utilization rate (Equation 11.13) and fuelchargeable to power efficiency (Equation 11.15), assuming hheater, fuel– basis = 0.8 and hAC,fuel–basis = 1.05. Solid lines: fuel utilization efficiency. Dotted lines: FCP efficiency. Crosses: as functions of electrical output of system, holding fixed, as fractions of fuel input, Qheating = 0.6 and Qcooling = 0; squares: as function of Qheating holding fixed Welectric = 0.25 and Qcooling = 0; pluses: as function of Qcooling holding fixed Welectric = 0.25 and Qheating = 0.

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 294

4/5/11 9:24:19 AM

Heat-activated cooling technologies for small and micro CHP

295

11.8.3 Comparative performance relative to baseline systems Comparing to baseline systems is a more empirical approach, of a less thermodynamically fundamental nature than metrics such as the COP. It can often be more useful when a direct comparison to conventional systems is desired. However, any comparative performance estimate can easily have a wide span of outcomes, depending on the choice of baseline system, the assumptions made about the baseline system, and the way that various energy flows are accounted for. One choice for comparing a proposed CCHP system to a baseline one is to use a metric such as the fuel utilization efficiency (Equation 11.13), and the other is to build models of the proposed system and the baseline system to evaluate the relative performance by detailed system simulation. A simple example of this approach is shown in Figs 11.18–11.20, in which the fuel required to meet one unit of loads in a residence is calculated for three systems. For all cases it is assumed that there is a 4:1 ratio of cooling 35% 0.76 Btu fuel

3.0 COPAC

Elec.

hel

0.8 Btu Cool air

losses 80% hwater heater

0.25 Fuel

0.2 DHW

11.18 Zero-order model of baseline non-CHP system with VCC cooling device and fuel-fired water heater with efficiency of 80%. The conversion efficiency of fuel to electricity is chosen to be typical for a Rankine-cycle-dominated electric grid. Net fuel consumption: 1.01 units per unit loads. rted Expo id r g to

0.07 fuel offset 4.5

25% 0.76 Btu Fuel

COPAC

Elec. hel

Waste heat

hHR

Hot water

90%

losses

eDW

0.8 Btu Cool air

Dry air 0.2 DHW

11.19 Zero-order model of CCHP system with desiccant wheel and VCC. The desiccant wheel is assumed to boost the heat pump COP from 3 to 4.5 by separating sensible and latent cooling, and to provide 20% of the driving heat as sensible cooling via an advanced configuration such as that shown in Fig. 11.7. Excess electricity produced offsets production by the grid, assumed to have an efficiency of 35%. Net fuel consumption: 0.69 units per unit loads.

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 295

4/5/11 9:24:19 AM

296

Small and micro combined heat and power (CHP) systems

25% 1.99 Btu Fuel

hel

Elec.

Waste heat

Exporte

90% hHR

d d to gri

Offsets 1.42 Btu fuel

0.7 Hot water

COPabs

0.8 Cool air

0.2 DHW losses

11.20 Zero-order model of CCHP system with absorption chiller. Electricity produced offsets production by the grid, assumed to have an efficiency of 35%. Net fuel consumption: 0.57 units per unit loads.

to DHW loads. The height of each box is proportional to the energy it represents. Figure 11.18 shows a zero-order model (i.e. components are described by fixed values without any independent variables) of a baseline system consisting of a grid-electricity-powered VCC and a fuel-fired hot water heater. The baseline system requires 1.01 units of fuel (0.76 off-site and 0.25 on-site) for every unit of loads. A simple zero-order model of a CCHP system with desiccant cooling is shown in Fig. 11.19. This system requires 0.76 units of fuel on-site for every unit of load, but also is a net producer of electricity. When credit is issued for this production (assuming it offsets grid electricity that would have been produced with an efficiency of 35%), it requires only 0.69 net units of fuel. A zero-order model of a CCHP system with absorption cooling is shown schematically in Fig. 11.20. This system requires more on-site fuel consumption, but also produces a significant excess of electricity. When credit is issued for this electricity, it only consumes 0.57 net units of fuel for each unit of load. This simple example demonstrates the potential for fuel savings with CCHP systems, and demonstrates the basic methodology of using system modeling to compare system alternatives. Costs can easily be assigned to components and to imported and exported energy to make economic as well as energetic performance comparisons.

11.9

Advantages and limitations of heat-activated cooling

Heat-activated cooling and small-scale CHP systems have a synergistic combination. Without available waste heat, heat-activated cooling technologies © Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 296

4/5/11 9:24:20 AM

Heat-activated cooling technologies for small and micro CHP

297

must be direct-fired, and in that case they struggle to compete with conventional cooling methods, both on a cost and an energy basis. Small-scale CHP, without any heat-activated cooling device, often struggles to demonstrate sufficient operating hours throughout the year to justify the capital expenditure on CHP equipment, and may demonstrate only marginally improved energy efficiency compared with state-of-the-art standard energy solutions. However, combining small-scale CHP with HACT can increase both system operating hours and system fuel efficiency. Thus, integration with CHP can justify the energetic and cost performance of HACT, and vice versa. The integrated system performs better than the sum of its parts. On the other hand, heat-activated cooling for small-scale CHP does suffer some limitations. Most importantly, adding more components to a CHP system increases the initial capital cost, in a technology where high initial cost is a fundamental challenge. Secondly, system complexity is also increased, and many heat-activated cooling technologies do not yet have commercial products that are well proven in terms of reliability and longterm performance.

11.10 Future trends With the proliferation of small-scale CHP installations and an increased emphasis globally on fuel efficiency, small-scale heat-activated cooling technologies have a promising long-term outlook. Although current CHP manufacturers for domestic and commercial CHP applications will probably first succeed in the climate zones most amenable to CHP (i.e. climates with long, heating-dominated seasons), manufacturers will be looking for ways to expand to other regions. As costs for both CHP systems and HACT come down, and as the efficiency benefits are more fully appreciated, HACT can help justify the economics of installation to a much wider set of markets, including residences in cooling-dominated climate regions, commercial establishments with large latent HVAC loads, and industrial applications with large cooling loads. In general, the capacity of a HACD increases with higher regeneration temperature; while a CHP prime mover has a higher overall efficiency with a lower heat recovery temperature, which extracts the most possible energy from the exhaust and minimizes convective and radiative losses. This fundamental compromise requires technological advances that enable high thermal COPs with lower regeneration temperatures (e.g. advanced desiccant polymers for desiccant wheels and advanced regenerative adsorption heat pumps). As energy efficiency requirements for HVAC equipment continue to become more stringent, manufacturers are exploiting various technologies proven to improve efficiency, and most of these interact positively with HACT. For example, a VCC system set up for separate sensible and latent

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 297

4/5/11 9:24:20 AM

298

Small and micro combined heat and power (CHP) systems

cooling can be supplemented with a desiccant dehumidifier. Utilization of low temperature difference (DT) heat exchangers enhances the incentive to keep air conditioning evaporator temperatures high, and thus it becomes even more important to perform dehumidification separately from sensible cooling. Higher standards of thermal comfort will incentivize systems that operate efficiently at part load, since they may have to be sized larger to meet higher design-day requirements, and in this area absorption chillers excel. A combination of higher thermal comfort standards and tighter building envelope will increase ventilation requirements, which in turn enhances the benefits of dedicated dehumidification equipment. Further, a CCHP system such as a liquid dessicant/absorption system with on-site power generation can abdicate the need for vapor compression systems altogether, greatly reducing peak electricity demand and reducing fuel consumption. Finally, fuel-flexible external combustion engines and waste-heat or direct-fired HACT can provide a hedge against uncertainty of any one fuel source. Many HACT have proven to be more efficient solutions to cooling. The four primary barriers to widespread adoption are first cost, system complexity, inherent efficiency limits, and parasitic power consumption. 1. The generally higher first cost of HACT is the most important barrier to more widespread adoption. The fundamental solutions to this are a major shift in energy prices, which would more dramatically reward energy efficiency over first cost, and economies of scale as more units are made, lowering first cost. 2. Complexity of HACT is often cited as a development barrier, but this issue amounts merely to an engineering challenge which can be overcome given sufficient incentive to commercialize a product. 3. A more fundamental issue with HACT is the limits set by Carnot efficiency – but as this chapter has demonstrated, this is primarily an apparent shortfall, resulting from the erroneous direct comparison of thermal COP and mechanical/electrical COP. Furthermore, when fuel is used sequentially, first in a prime mover or process requiring high temperature, and next in a HACD (or when low-temperature solar heat is used), then the use of HACT can boost the overall fuel efficiency well above what is achievable by conventional systems. 4. A final issue with HACT is parasitic power consumption, which tends to increase as pumps and heat exchangers are added to a HACD to improve the thermal COP. If parasitic power consumption is not smartly managed in the design phase, the electrical consumption of a HACD can end up being comparable to the conventional system it replaces – but this potential pitfall can be overcome by good design. Thus, many of the HACT discussed in this chapter are ready for deployment today, and many more could quickly become widespread given appropriate changes in energy priorities. © Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 298

4/5/11 9:24:20 AM

Heat-activated cooling technologies for small and micro CHP

299

11.11 Sources of further information and advice 11.11.1 General – trigeneration Wu and Wang (2006) provide an overview of CCHP technologies for microup to large-scale systems. The European Union-funded PolySMART project (Polygeneration with advanced Small and Medium scale thermally driven Air-conditioning and Refrigeration Technology) has many publications available at www.polysmart.org. Neil Petchers’ Combined Heating, Cooling & Power Handbook (2003) contains two chapters (within Section VII) on absorption cooling and desiccant dehumidification (including solid desiccant beds) and how they are relevant to CHP. Review articles on solar cooling are also relevant due to the interest in low-temperature regeneration shared by solar and small-scale CCHP applications – see Hwang et al. (2008), Kim and Infante Ferreira (2008), and Anyanwu (2003, 2004). Onovwiona and Ugursal (2006) provide an overview of residential CHP prime movers.

11.11.2 General – heat-activated cooling devices The IEA Heat Pump Program Annex 34, ‘Thermally Driven Heat Pumps for Heating and Cooling,’ is scheduled to be completed in 2011 (www. annex34.org). A US Department of Energy-funded report covers thermally activated technologies: see TIAX (2004). Also, the IEA Solar Heating and Cooling Program Task 25, ‘Solar-Assisted Air-Conditioning of Buildings’ dealt largely with low regeneration temperature heat-activated devices, and was completed in 2004 – see Henning (2007) and www.iea-shc.org/task25. Pons et al. (1999) provide a comparative compilation of the performances of various sorption systems for heating, cooling, and refrigeration applications. The International Sorption Heat Pump Conference, originally dedicated to absorption machines, has been expanded to encompass all sorption cooling and dehumidification processes, as well as cogeneration and fundamentals of heat and mass transfer. It is held in various countries approximately every three years since 1982, including Seoul, Korea in 2008 and Padua, Italy in 2011. Many compilations of these conference proceedings are available, including Radermacher et al. (1994), Nikanpour and Hosatte (1996), and Schweigler et al. (1999).

11.11.3 Desiccant dehumidification Munters Corporation produces desiccant-based dehumidification products for industrial, commercial, and residential applications. Brundrett’s Handbook of Dehumidification Technology (1987) provides an overview of traditional desiccant and other dehumidification processes, although specific information

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 299

4/5/11 9:24:20 AM

300

Small and micro combined heat and power (CHP) systems

on low regeneration temperature processes is hard to find. For information on enhanced desiccant processes, including those that handle sensible load, recent academic literature is the best source (see bibliography for starting points). Also the ASHRAE Handbook: Systems (2008, Chapter 23) contains a helpful overview of desiccant dehumidification.

11.11.4 Absorption heat pumps Absorption Chillers and Heat Pumps by Herold et al. (1996) covers the theory and operation of absorption devices. The ASHRAE Handbook: Refrigeration (2006, Chapter 41) contains an overview of absorption chillers, and the ASHRAE Handbook: Fundamentals (2005, Chapter 22) deals with the basics of sorption fundamentals. The IEA Heat Pump Program Annex 24, ‘Absorption Machines for Heating and Cooling in Future Energy Systems’ was completed in 1999 – see www.heatpumpcentre.org – and among their publications is a list of absorption equipment manufacturers. Proceedings from the International Sorption Heat Pump Conference (held every three years) along with articles in the academic literature – particularly in the International Journal of Refrigeration (published by the International Institute of Refrigeration) – provide additional resources.

11.11.5 Adsorption heat pumps There is not a comprehensive reference for adsorption heat pumps, and perhaps the most helpful way to discover more about them is to take a combined approach: for adsorption fundamentals (which are well established owing to their importance in chemical processing), several helpful books are available; and for heat pump application-specific information, more recent review articles in the academic literature provide information. Coverage of adsorption fundamentals can be found in Ruthven’s Principles of Adsorption and Adsorption Processes (1984) and R.T. Yang’s Adsorbents: Fundamentals and Applications (2003). Naturally these volumes do not contain information on the most recent advances in adsorbents (nor do they directly address HACT), but nevertheless have excellent and clear discussions of the classifications of adsorbents, their thermodynamics, equilibrium models, and kinetics. Basmadjian’s Little Adsorption Book: A Practical Guide for Engineers and Scientists (1997) provides a more succinct overview of the fundamentals. With respect to recent advances, Sumathy et al. (2003), Lambert and Jones (2005) and Demir et al. (2008) provide excellent reviews of adsorption heat pump working pairs and cycle enhancements, and also provide useful comparisons to other heat-activated cooling technologies. Power Partners, Inc. is currently manufacturing water/silica gel units in North America of 30

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 300

4/5/11 9:24:20 AM

Heat-activated cooling technologies for small and micro CHP

301

tons and larger (Power Partners, Inc., 2010). Finally, Yong and Wang (2007) review over 100 patents related to adsorption heat pumps, most issued since 2000.

11.11.6 Ejector refrigeration cycle Well-established ejector performance studies for boiler temperatures of 120–140 °C are readily available; see Chunnanond and Aphornratana (2004a). For low firing temperature ejector cycles, see Sun (1999), Godefroy et al. (2007), Chunnanond and Aphornratana (2004b), Yapıcı and Ersoy (2005) Yapıcı and Yetişen (2007), and Meyer et al. (2009).

11.11.7 Performance metrics Petrov et al. (2004) provide an excellent discussion of efficiency metrics in the context of CCHP systems with case study examples. Carnot COPs and efficiencies of various heat-activated devices are derived in Herold et al. (1996). Tozer and James (1997) derive performance limits for multiple effect machines, and Meunier et al. (1997) discuss the theoretical performance limits for adsorption heat pumps. For desiccant wheels, Mandegari and Pahlavanzadeh (2009) discuss conventional metrics and propose new ones.

11.12 References American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE) (2005) ASHRAE Handbook: Fundamentals. American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE) (2006) ASHRAE Handbook: Refrigeration. American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE) (2008) ASHRAE Handbook: Systems. Anyanwu, E. E. (2003) ‘Review of solid adsorption solar refrigerator I: an overview of the refrigeration cycle’, Energy Conversion and Management, 44(2), 301–312. Anyanwu, E. E. (2004) ‘Review of solid adsorption solar refrigeration II: an overview of the principles and theory’, Energy Conversion and Management, 45(7–8), 1279–1295. Basmadjian, D. (1997) The little adsorption book: a practical guide for engineers and scientists, Boca Raton, FL: CRC Press. Brundrett, G. W. (1987) Handbook of dehumidification technology, London: Butterworths. Chunnanond, K. and Aphornratana, S. (2004a) ‘An experimental investigation of a steam ejector refrigerator: the analysis of the pressure profile along the ejector’, Applied Thermal Engineering, 24(2–3), 311–322. Chunnanond, K. and Aphornratana, S. (2004b) ‘Ejectors: applications in refrigeration technology’, Renewable and Sustainable Energy Reviews, 8(2), 129–155. Demir, H., Mobedi, M. and Ülkü, S. (2008) ‘A review on adsorption heat pump: problems and solutions’, Renewable and Sustainable Energy Reviews, 12(9), 2381–2403.

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 301

4/5/11 9:24:20 AM

302

Small and micro combined heat and power (CHP) systems

Douss, N. and Meunier, F. (1989) ‘Experimental study of cascading adsorption cycles’, Chemical Engineering Science, 44(2), 225–235. Godefroy, J., Boukhanouf, R. and Riffat, S. (2007) ‘Design, testing and mathematical modelling of a small-scale CHP and cooling system (small CHP-ejector trigeneration)’, Applied Thermal Engineering, 27(1), 68–77. Henning, H.-M. (ed.) (2007) Solar-assisted air-conditioning in buildings: a handbook for planners, 2nd rev. edn, London: Springer. Henning, H.-M., Pagano, T., Mola, S. and Wiemken, E. (2007) ‘Micro tri-generation system for indoor air conditioning in the Mediterranean climate’, Applied Thermal Engineering, 27, 2188–2194. Herold, K. E., Radermacher, R. and Klein, S. A. (1996) Absorption Chillers and Heat Pumps, Boca Raton, FL: CRC Press. Hwang, Y., Radermacher, R., Al Alili, A. and Kubo, I. (2008) ‘Review of solar cooling technologies’, HVAC & R Research, 14, 507–528. Kim, D. S. and Infante Ferreira, C. A. (2008) ‘Solar refrigeration options – a state-ofthe-art review’, International Journal of Refrigeration, 31(1), 3–15. Kodama, A., Hirose, T. and Okano, H. (2003) Low-temperature heat driven adsorptive desiccant cooling improved for the use in humid weather, translated by Saha, B., Akisawa, A. and Koyama, S., Fukuoka, Japan. Lambert, M. A. and Jones, B. J. (2005) ‘Review of regenerative adsorption heat pumps’, Journal of Thermophysics and Heat Transfer, 19(4), 471–485. Mandegari, M. A. and Pahlavanzadeh, H. (2009) ‘Introduction of a new definition for effectiveness of desiccant wheels’, Energy, 34, 797–803. Meunier, F., Poyelle, F. and LeVan, M. D. (1997) ‘Second-law analysis of adsorptive refrigeration cycles: the role of thermal coupling entropy production’, Applied Thermal Engineering, 17(1), 43–55. Meyer, A. J., Harms, T. M. and Dobson, R. T. (2009) ‘Steam jet ejector cooling powered by waste or solar heat’, Renewable Energy, 34(1), 297–306. Ng, K. C., Chua, H. T., Chung, C. Y., Loke, C. H., Kashiwagi, T., Akisawa, A. and Saha, B. B. (2001) ‘Experimental investigation of the silica gel-water adsorption isotherm characteristics’, Applied Thermal Engineering, 21(16), 1631–1642. Nikanpour, D. and Hosatte, S. (1996) Conference Proceedings: International Absorption Heat Pump Conference, Montreal, Canada, September 17-20, 1996, Québec: CANMETEDRL, Natural Resources Canada. Onovwiona, H. I. and Ugursal, V. I. (2006) ‘Residential cogeneration systems: review of the current technology’, Renewable and Sustainable Energy Reviews, 10(5), 389–431. Petchers, N. (2003) Combined heating, cooling & power handbook: technologies & applications: an integrated approach to energy resource optimization, Lilburn, GA: Fairmont Press. Petrov, A. Y., Zaltash, A., Labinov, S. D., Rizy, D. T., Liao, X. and Radermacher, R. (2004) Evaluation of different efficiency concepts of an integrated energy system (IES), Anaheim, CA: American Society of Mechanical Engineers, 347–356. Pons, M., Meunier, F., Cacciola, G., Critoph, R. E., Groll, M., Puigjaner, L., Spinner, B. and Ziegler, F. (1999) ‘Thermodynamic based comparison of sorption systems for cooling and heat pumping’, International Journal of Refrigeration, 22(1), 5–17. Power Partners, Inc. (2010) ‘Eco-Max Adsorption Chillers’, [online], available at: http:// www.eco-maxchillers.com/ (accessed June 2010). Radermacher, R., Herold, K., Miller, W., Perez-Blanco, H., Ryan, W. and Vleit, G. (1994) Proceedings of the International Absorption Heat Pump Conference, translated by New Orleans, Louisiana: American Society of Mechanical Engineers, viii, 534 p. © Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 302

4/5/11 9:24:20 AM

Heat-activated cooling technologies for small and micro CHP

303

Ruthven, D. M. (1984) Principles of adsorption and adsorption processes, New York: Wiley. Schinner Jr, E. N. and Radermacher, R. (1999) ‘Performance analysis of a combined desiccant/absorption air-conditioning system’, HVAC and R Research, 5, 77–84. Schweigler, C., Summerer, S., Hellman, H.-M. and Ziegler, F., eds. (1999) Proceedings of the International Sorption Heat Pump Conference, March 24-26, 1999, Munich, Germany, Lang Offsetdruck GmbH. Sumathy, K., Yeung, K. H. and Yong, L. (2003) ‘Technology development in the solar adsorption refrigeration systems’, Progress in Energy and Combustion Science, 29(4), 301–327. Sun, D.-W. (1999) ‘Comparative study of the performance of an ejector refrigeration cycle operating with various refrigerants’, Energy Conversion and Management, 40(8), 873–884. TIAX (2004) Review of Thermally Activated Technologies.A Distributed Energy Program Report for the US DOE Office of Energy Efficiency and Renewable Energy, TIAX LLC. Tozer, R. M. and James, R. W. (1997) ‘Fundamental thermodynamics of ideal absorption cycles’, International Journal of Refrigeration, 20(2), 120–135. Wu, D. W. and Wang, R. Z. (2006) ‘Combined cooling, heating and power: a review’, Progress in Energy and Combustion Science, 32(5–6), 459–495. Yang, R. T. (2003) Adsorbents: fundamentals and applications, Hoboken, NJ: WileyInterscience. Yapıcı, R. and Ersoy, H. K. (2005) ‘Performance characteristics of the ejector refrigeration system based on the constant area ejector flow model’, Energy Conversion and Management, 46(18–19), 3117–3135. Yapıcı, R. and Yetişen, C. C. (2007) ‘Experimental study on ejector refrigeration system powered by low grade heat’, Energy Conversion and Management, 48(5), 1560–1568. Yong, L. and Wang, R. Z. (2007) ‘Adsorption refrigeration: a survey of novel technologies’, Recent Patents on Engineering, 1(1), 1–21.

11.13 Bibliography Alefeld, G. and Radermacher, R. (1994) Heat conversion systems, Boca Raton, FL: CRC Press. Angrisani, G., Minichiello, F., Roselli, C. and Sasso, M. (2010) ‘Desiccant Hvac system driven by a micro-CHP: experimental analysis’, Energy and Buildings, 42(11), 2028–2035. Bejan, A. (1982) Entropy generation through heat and fluid flow, New York: Wiley. Castro, J., Oliva, A., Carlos David, P.-S. and Oliet, C. (2008) Recent developments in the design of a new air-cooled, hot-water-driven H2O-LiBr absorption chiller, New York: American Society of Heating, Refrigeration and Air Conditoning Engineers, 288–299. Castro, J., Oliva, A., Perez-Segarra, C. D. and Oliet, C. (2008) ‘Modelling of the heat exchangers of a small capacity, hot water driven, air-cooled H2O-LiBr absorption cooling machine’, International Journal of Refrigeration, 31, 75–86. Dinçer, I. (2003) Refrigeration Systems and Applications, New York: John Wiley and Sons.

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 303

4/5/11 9:24:20 AM

304

Small and micro combined heat and power (CHP) systems

Goodheart, K. A. (2000) Low Firing Temperature Absorption Chiller System, unpublished thesis, University of Wisconsin-Madison. Gulen, S. C. (2010) ‘A proposed definition of CHP efficiency’, POWER Magazine, June. Huangfu, Y., Wu, J. Y., Wang, R. Z. and Xia, Z. Z. (2007) ‘Experimental investigation of adsorption chiller for micro-scale BCHP system application’, Energy and Buildings, 39(2), 120–127. Jin, W., Cun Nan, L., Jian Hua, L., Si Chang, L. and Jun, C. (2009) A New Air-Conditioning System of Liquid Desiccant and Evaporation Cooling, Power and Energy Engineering Conference, 2009. APPEEC. Kim, D. S. and Infante Ferreira, C. A. (2003) Solar Absorption Cooling, 1st progress report for NOVEM, October, Delft University of Technology. Kong, X. Q., Wang, R. Z. and Huang, X. H. (2004) ‘Energy efficiency and economic feasibility of CCHP driven by Stirling engine’, Energy Conversion and Management, 45(9–10), 1433–1442. Liao, X. (2004) The development of an air-cooled absorption chiller concept and its integration in CHP systems, unpublished thesis, University of Maryland. Mei, V. C., Chen, F. C., Lavan, Z., Collier, R. K. and Meckler, G. (1992) An assessment of desiccant cooling and dehumidification technology, Oak Ridge National Laboratory, US Department of Energy. Mitsubishi Plastics (2008) ‘Zeolitic water vapor adsorbent AQSOA’, available at: http:// www.aaasaveenergy.com/products/001/pdf/AQSOA_1210E.pdf Nayak, S. M. (2005) Experimental and theoretical investigation of integrated engine generator–liquid desiccant system, unpublished thesis, University of Maryland. Nayak, S. M., Hwang, Y. and Radermacher, R. (2009) ‘Performance characterization of gas engine generator integrated with a liquid desiccant dehumidification system’, Applied Thermal Engineering, 29(2–3), 479–490. Nia, F. E., van Paassen, D. and Saidi, M. H. (2006) ‘Modeling and simulation of desiccant wheel for air conditioning’, Energy and Buildings, 38(10), 1230–1239. Niu, J. L. and Zhang, L. Z. (2002) ‘Effects of wall thickness on the heat and moisture transfers in desiccant wheels for air dehumidification and enthalpy recovery’, International Communications in Heat and Mass Transfer, 29(2), 255–268. Peltier, R. V. (2001) ‘How efficient is ‘efficiency’?’, POWER Magazine, 145(2), 105. Peltier, R. (2010) ‘Plant efficiency: begin with the right definitions’, POWER Magazine, February. Saha, B. B., Akisawa, A. and Koyama, S. (2003) Proceedings of the International Seminar on Thermally Powered Sorption Technology, December 4–5, 2003, Chikushi Campus, Kyushu University, Fukuoka, Japan. Srikhirin, P., Aphornratana, S. and Chungpaibulpatana, S. (2001) ‘A review of absorption refrigeration technologies’, Renewable and Sustainable Energy Reviews, 5(4), 343–372. Sumathy, K., Huang, Z. C. and Li, Z. F. (2002) ‘Solar absorption cooling with low grade heat source – a strategy of development in South China’, Solar Energy, 72(2), 155–165. Wang, R. Z. (2001) ‘Adsorption refrigeration research in Shanghai Jiao Tong University’, Renewable and Sustainable Energy Reviews, 5(1), 1–37. Zhang, X. J., Dai, Y. J. and Wang, R. Z. (2003) ‘A simulation study of heat and mass transfer in a honeycombed rotary desiccant dehumidifier’, Applied Thermal Engineering, 23(8), 989–1003.

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 304

4/5/11 9:24:20 AM

Heat-activated cooling technologies for small and micro CHP

305

11.14 Appendix 1: Nomenclature and abbreviations AC – air conditioning CCHP – combined cooling, heating and power CHP – combined heating and power CI – compression ignition COP – coefficient of performance DHW – domestic hot water DW – desiccant wheel FC – fuel cell FCP – fuel chargeable to power GAX – generator-absorber exchange HACT – heat-activated cooling technology HACD – heat-activated cooling device HHV – higher heating value of a fuel HR – heat recovery HVAC – heating, ventilation and air conditioning HX – heat exchanger ICE – internal combustion engine IEA – International Energy Agency, founded 1974 by the OECD LHV – lower heating value of a fuel ORC – organic Rankine cycle MT – microturbine PEMFC – proton exchange membrane fuel cell PM – prime mover, the primary fuel conversion device in a CHP system PURPA – Public Utilities Regulatory Policies Act of 1978 RH – relative humidity RT – refrigeration ton, equivalent to 3517 W or 12 000 Btu/hr of cooling capacity SEER – seasonal energy efficiency ratio SI – spark ignition SOFC – solid oxide fuel cell T – temperature (°C) or (°F) or (K) or (R) VCC – vapor compression cycle W – work, usually electrical energy or power (kW) e – effectiveness (dimensionless) h – efficiency (dimensionless) w – humidity ratio (gH2O/kgdry air)

11.15 Appendix 2: Notes on terminology ‘Trigeneration’ and ‘combined cooling, heating and power (CCHP)’ are used interchangeably in this chapter. Additional equivalent terms in use include

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 305

4/5/11 9:24:20 AM

306

Small and micro combined heat and power (CHP) systems

‘combined heating, cooling and power (CHCP)’ and ‘cooling, heating and power (CHP)’ (in this chapter CHP is used to mean combined heat and power, exclusive of cooling). More comprehensive terms in use include polygeneration and integrated energy systems (IES). ‘Heat-activated cooling technology/technologies (HACT)’ and ‘heatactivated cooling device(s) (HACD)’ are used interchangeably, as appropriate, in this chapter. Additional equivalent terms in use include ‘heat-driven’, ‘thermally-activated’, and ‘thermally-driven’, combined with either ‘cooling’, ‘refrigeration’, ‘chillers’, and/or ‘technology/device’, giving rise to an abundance of equivalent potential abbreviations such as TAT, TAC, TDC, etc.

© Woodhead Publishing Limited, 2011

CHP-Beith-11.indd 306

4/5/11 9:24:20 AM

12

Energy storage for small and micro combined heat and power (CHP) systems

A. p r i c e, Swanbarton Ltd, UK

Abstract: Several types of energy storage technologies are described and assessed for their suitability for applications alongside CHP systems. The technical and commercial requirements are described. Distributed energy storage is also a means of providing grid or network services which can provide an additional economic benefit from the storage device. Electrical energy storage is shown to be a complementary technology to CHP systems and may also be considered in conjunction with, or as an alternative to, thermal energy storage. Key words: energy storage, electricity storage, thermal storage.

12.1

Introduction

The use of energy storage on power systems has been limited due to technical and commercial considerations. Early power systems relied on extensive use of batteries, mostly of the lead acid accumulator type, in order to provide stability and simplify operation. The change from direct current local systems to interconnected alternating current systems caused the demise of night storage accumulators as AC to DC conversion was limited to rotary convertors. Today large-scale energy storage is mostly represented by pumped hydroelectricity storage – with a worldwide capacity of more than 100 GW – and some network connected battery storage and other types of energy storage systems. Non-grid connected systems tend to have more examples of battery storage – as shown by remote farms or ranches using batteries to run lighting circuits when the generator is not running. The predominant use of smaller scale energy storage is for uninterruptible power supplies (UPS) – systems based on batteries, flywheels, and fuel cells are all in use. There is considerable interest in energy storage – the perceived applications include energy management, islanding capability and power trading.

12.1.1 Energy management applications Energy management applications include storing ‘off peak’ generated electricity for use at a later time, where ‘off peak’ may refer to time of day 307 © Woodhead Publishing Limited, 2011

CHP-Beith-12.indd 307

4/5/11 9:24:51 AM

308

Small and micro combined heat and power (CHP) systems

(such as overnight), times when a power market price for energy is low or where a producer may not have a use or demand for the energy. Popular attention is drawn to energy management because of the perceived need to store surplus electricity which may be generated from mini farms or other remote resources.

12.1.2 Islanding capability applications Islanding capability applications would enable a small power system comprised of a generator, store and load to operate either as part of a network, or independently in the event of a network failure. A co-generation project, where a CHP installation operates in a grid connected mode will often be required to disconnect itself from the network in the event of a network failure. This disconnection is required to protect the network (and those working on it) from power re-energising a section of line which might be expected to be inactive. In order to provide islanding capability, a local cogeneration project would need to have isolated systems for disconnection and a safe, and approved means of reconnecting to the network when the external power supply is restored.

12.1.3 Power trading Power trading is the action of buying energy from the market at a low cost and selling it at a higher cost at a later time. Sometimes known (incorrectly) as energy arbitrage, as it is an inter-temporal process, the financial parameters involved require a significant knowledge of, and investment in, energy trading.

12.2

Types of energy storage (ES) systems

12.2.1 Definition and scope Although energy can be stored in many forms, for the purposes of this chapter, the scope will be limited to those devices that will be charged by electrical means and when discharged, convert the stored energy back to electrical energy. This scope excludes the use of hydrogen and fuel cell systems which are described in another chapter. Energy storage is defined as the conversion of electrical energy from a power network into a form in which it can be stored until converted back to electrical energy. For completeness, a section has been included on thermal energy storage and the overlap between this and electrical energy storage.

© Woodhead Publishing Limited, 2011

CHP-Beith-12.indd 308

4/5/11 9:24:51 AM

Energy storage for small and micro CHP systems

309

12.2.2 Overview of types Energy storage systems can be classified in many ways, by type of storage medium, by size, applications or commercial status (Table 12.1). Many energy storage technologies are modular, and assembled from a number of components arranged in series and parallel. By its nomenclature, a battery is a collection of cells – the largest batteries assembled are in the order of tens of MW and hundreds of MWh, although most would be in the size range up to about 10 MW. Capacitors for energy storage tend to be grouped in smaller installations by power rating, and their storage capacity is limited. Electrolysers and fuel cells have been installed in MW size installations and, where there is large-scale fuel storage, the energy capacity can be measured in hours or days. Flywheel systems are also becoming larger. While industrial machines are rated in tens or hundreds of kW, installations of 10 or 20 MW are now under construction.

12.3

Applications of electrical energy storage

An energy storage device can be used to: ∑ ∑

provide power over short durations – seconds or minutes provide power over longer durations – minutes to hours or days

In order to keep a power system stable, generation must equal demand plus losses at all times. If generation falls, voltage and frequency drop, machines slow down, lights dim and eventually the system fails. At the other extreme, if generation exceeds demand and losses, the effects can be catastrophic. A short duration interruption, even of an AC cycle (0.02 seconds at 50 Hz) can cause damage to computers, computer controlled equipment and precision machinery. Customers protect themselves against this occurrence by installing uninterruptible power supplies (UPS).

12.3.1 Types of uninterruptible power supply (UPS) Most commercial UPS systems consist of three sub-systems: ∑

an energy storage device, usually a flywheel or battery with an AC/DC power conversion system ∑ switchgear and isolators ∑ a separate generator which can be started to provide resilience of supply beyond the energy storage capacity of the storage device. These can be installed to protect a single load, such as a computer, or to protect larger installations such as offices, shops or factories. UPS systems to protect domestic computers are readily available from

© Woodhead Publishing Limited, 2011

CHP-Beith-12.indd 309

4/5/11 9:24:51 AM

© Woodhead Publishing Limited, 2011

CHP-Beith-12.indd 310

4/5/11 9:24:51 AM

Not yet available

e.g. MeOH, chemical hydrides

Chemical

Other storage media

Commercially Power quality available Standby power Commercially available Power quality Commercially available Multi functional Commercially Multi functional available Commercially available Multi functional Early stage of commercialisation Multi functional Early stage of Remote area commercialisation applications Early stage of commercialisation

Electro- Low temperature Lead acid chemical batteries NiCd Lithium cells High temperature Sodium batteries sulphur Sodium nickel chloride Flow batteries Zinc bromine Vanadium Hydrogen Electrolyser/fuel cycle cell combination

Mechanical Potential energy Pumped hydro Mature in storage medium Compressed air energy Mature technology, but storage (CAES) limited commercial take-up Kinetic energy in Low speed Commercially storage medium flywheels available Advanced Commercially flywheels available

No

Yes

Yes Yes Yes

Yes Yes Yes

Yes

Yes

Yes

Potential

No

No

Power quality, reliability Energy management, reserve Energy management, reserve Uninterruptible power supplies Power quality/ Energy management

Yes

Power quality

Early stage commercial Some commercial examples

Electrical Capacitors Super conductors

Capacitors and ultracapacitors Superconducting Magnetic Energy Storage (SMES)

Relevance to small-scale CHP and distributed generation

Type Sub-group Examples Development Typical status applications

Table 12.1 The development status of energy storage devices

© Woodhead Publishing Limited, 2011

CHP-Beith-12.indd 311

4/5/11 9:24:51 AM

Thermal

Hot water Commercially available Ceramics Commercially available Molten Early stage of commercialisation salt/ steam Ice Commercially available Thermal storage Under development and heat engines

Peak shaving Peak shaving Integration of renewables Peak shaving Energy management Yes Potential

Yes Yes Yes

312

Small and micro combined heat and power (CHP) systems

computer stores. They are installed between the domestic wall socket and the computer using standard plugs. Care needs to be taken to ensure that the UPS has the power capacity (kW) required to sustain the computer, or other device, as well as the energy rating to provide resilience against the duration of the power interruption. Typically this is of the order of 15 minutes, which is sufficient to close down a computer safely. Increasing the discharge duration increases the cost of these systems significantly. Modern computers are now often rated at lower powers than earlier machines, thereby either increasing the resilience time, or decreasing the cost of the energy storage system. These devices are usually based on valve regulated lead acid batteries, which provide a cost effective solution with low maintenance. In many instances, a commercial or industrial scale UPS is supplemented by a standby generator, which is started after 1 or 2 minutes. This continues to supply the load for as long as there is fuel. For some of those larger installations, flywheels are used as an alternative to batteries. The flywheel might be integrated with the standby generator, which improves reliability by directly linking the mechanical systems. While UPS solve the technical problems caused by interruptions to the power supply, some equipment requires a very stable, continuous power supply with no fluctuation to the expected smooth voltage profile. This higher performance is achieved by specifying increased standards for the power conversion system. For many small systems, this will be hidden from the user, but will be of significance for medium and larger-scale projects.

12.3.2 Types of energy storage technologies for integration with CHP systems Smaller-scale systems in use for small- and micro-CHP projects are unlikely to make use of larger-scale technologies such as pumped hydro or underground compressed air energy storage directly, but if the CHP systems are connected to the grid network, they will, of course, be making indirect use of such systems. This point is often overlooked – that introducing electricity storage at any point in the network has an impact through the system. The conceptual difficulty is that finance does not always follow the flow of electrons in an equitable manner. The location and ownership of the storage device are prime considerations in the financial analysis of the storage device. The installer or operator may not gain an economic return on the whole value of the investment.

12.3.3 Mechanical systems Flywheels have been available as UPS for many years, but some developers are now offering these as energy storage devices in their own right. Flywheels

© Woodhead Publishing Limited, 2011

CHP-Beith-12.indd 312

4/5/11 9:24:51 AM

Energy storage for small and micro CHP systems

313

are typically characterised as either high speed or low speed. As the energy storage capacity is proportional to the square of the rotational speed, increasing the speed is an effective method of increasing the capacity of the system. Flywheel systems would have a high cycle lifetime, low maintenance costs and would have simple requirements in terms of siting. The interface to the power network is usually based on a variable frequency power conversion system, which can accommodate changes in the flywheel speed, but producing a constant frequency power output. Although the maximum energy storage capacity of the flywheel is fixed by the parameters of the rotary mass, the power rating and hence the discharge time can be varied, so that it is feasible to consider flywheels with a low power rating but a discharge time measured in terms of minutes or even hours. Figure 12.1 shows a matrix of large scale flywheels operating in combined mode connected to the power network to provide 1 MW of power for frequency regulation. Figure 12.2 illustrates the size of a single 50 kW flywheel system. There are currently two large-scale compressed air energy storage (CAES) plants operating. Figure 12.3 shows the 110 MW CAES plant built in McIntosh, Alabama, USA that has been operating since 1991. Although large scale (100+ MW) compressed air systems are considered out of scope for small-scale CHP applications, MW scale compressed air installations are now being considered. EPRI expects to run a demonstration MW scale CAES plant using air storage in above ground pipes (derived from the oil and gas industry). Figure 12.4. illustrates a typical system diagram.

12.1 A 1 MW flywheel energy store system under construction in New York State (source: Beacon Power Corporation).

© Woodhead Publishing Limited, 2011

CHP-Beith-12.indd 313

4/5/11 9:24:52 AM

314

Small and micro combined heat and power (CHP) systems

12.2 A 50 kW demonstration flywheel.

12.3 CAES Plant, Alabama (source: EPRI).

© Woodhead Publishing Limited, 2011

CHP-Beith-12.indd 314

4/5/11 9:24:52 AM

Energy storage for small and micro CHP systems

315

Exhaust

Heat exchanger Inlet Clutch

Clutch Motor/generator

Compressor Combustor fuel

Turbine/ expander

Underground air storage

12.4 Diagram of a typical CAES system.

12.4

Applications for combined heat and power (CHP) systems

12.4.1 Establishment scale CHP systems are usually installed on premises where there is a constant heat load, and, ideally, reasonably constant electrical load. Many CHP systems installed in the past 20 years have considered the electricity generated as a useful byproduct and not the main reason for installing the plant. The purpose of integrating energy storage is to optimise the generation and onsite consumption of electricity with the use of the storage device and energy exchanges with the network. The analysis is dependent on a number of factors: ∑ ownership and operation of the CHP plant ∑ ownership and operation of the energy storage plant ∑ whether the installation is a net product or consumer of electricity ∑ the commercial arrangements for import and export of electricity. The costs may be considerably lower to install a new CHP/storage system than retrofitting storage to an existing installation. More recently, with the

© Woodhead Publishing Limited, 2011

CHP-Beith-12.indd 315

4/5/11 9:24:52 AM

316

Small and micro combined heat and power (CHP) systems

drive towards sustainability, more businesses and organisations are striving to become energy neutral or self-sufficient in energy. Such considerations can influence decisions, making it possible to consider the use of storage even when the investment is not rewarded by payback.

12.4.2 Domestic scale For small installations, such as houses or apartment blocks, installing storage may have severe impacts on the economics of the heating system. As CHP installations may receive feed in tariffs for on-site production under the arrangements for micro power, there is little incentive to use on-site storage unless the import purchase price exceeds the value of the feed in tariff when multiplied by the inverse of the efficiency of the storage device. The Japanese research group CRIEPI (Central Research Institute of the Electric Power Industry) is undertaking active development work to introduce battery energy storage into all-electric houses which use heat pumps as the heating source. The control system will also be adapted to permit the introduction of electric and plug-in hybrid vehicles.

12.4.3 The economic case If such CHP systems are not eligible for feed-in tariff support, then the financial calculation is simplified, and the benefit can be expressed in terms of the difference between the value of exported power against the price of imported power.

For example: Exported power



25% round trip efficiency loss



1 kWh = £0.06

1 kWh to store = 0.75 kWh output

£0.06 = £0.08 0.75 On peak purchase price of imported power = £0.12/kWh

Effective cost of stored power =

Such economics ignore the following: ∑ capital cost of the energy storage equipment ∑ capital cost of the installation ∑ annual operating costs (maintenance, insurance, rates, rent of additional land) ∑ trading costs ∑ other grant aid. Integrating CHP with electrical energy storage is therefore only likely to happen if there are very simple trading arrangements (for example, based on

© Woodhead Publishing Limited, 2011

CHP-Beith-12.indd 316

4/5/11 9:24:52 AM

Energy storage for small and micro CHP systems

317

net metering), or if the installation is large enough to justify either trading costs directly or when traded through an aggregator.

12.4.4 Costs of storage The cost of an energy storage installation is dependent on a number of factors. For a relatively simple installation, such as a domestic UPS system where a device may be purchased from a catalogue and installation costs are low, it is easy to estimate the overall project cost. As soon as the project enters a bespoke phase, costs have a tendency to increase as specialist skills and equipment are required. Costs will be lower when established technologies such as lead acid batteries or ice storage are used. Advanced batteries, such as lithium chemistries, high temperature batteries or flow batteries will have much higher costs. The costs of the power conversion system and switchgear may be between 50 and 100% of the cost of the battery.

12.5

Grid services applications and relationship to combined heat and power (CHP)

Grid services fall into two broad groups ∑ ∑

localised services, such as voltage control system services, such as frequency regulation and reserves.

Distributed generation can have positive and negative effects on the operation of a distribution network. Traditionally, distribution networks have been installed on the basis of a radial system, with power generated centrally and distributed to users through the distribution network. Voltages were set so that the consumers at the end of each distribution line were served with an electrical supply that was within the regulated limits (230 V ± 10%). Adding distributed generation to the extremities of the network can significantly alter the voltage at points on the network. The power electronics associated with a storage device can be used to mitigate these effects, maintaining the voltage within the defined limits. However, such services are the responsibility of the distribution company and a private owner of storage would not normally be expected to provide these services. The transmission system operator has a duty to maintain the system frequency within defined limits. In order to do this, the system must be kept in balance, that is, generation must match demand, plus losses. The system is balanced for energy across trading periods (30 minutes in Great Britain) and constantly for power. In Great Britain, the National Grid Company instructs generating companies and demand customers to increase and decrease their supply and demand to maintain the balance. These participants use a variety

© Woodhead Publishing Limited, 2011

CHP-Beith-12.indd 317

4/5/11 9:24:52 AM

318

Small and micro combined heat and power (CHP) systems

of different contracts depending on the type of service being provided. Examples of these services include: ∑ ∑

frequency response short-term operating reserve.

An owner of a CHP plant could contract for these services, but the addition of a storage plant makes it easier to provide those services without necessarily imposing on the operation of the CHP plant. In many instances it may be possible to provide these services for a few hours each day.

12.6

Electrical vehicles

There is considerable interest in the use of electric vehicles as a means of decarbonising the power network. The assumption is that the energy used to charge the electric vehicles will be generated from the sustainable resources. Operators of CHP plant, especially where there is a variable demand for electricity, may be able to balance the heat and electrical load, particularly at off-peak periods, by charging electric vehicles. Many sustainable resources, such as solar photovoltaic and wind are variable and not continuous and it is appropriate to consider the use of storage as a means of not spilling electricity. However, this should be balanced against the use of the grid as a sink for such electricity and as a source for generation. An example of a battery system is shown in Fig. 12.5. The vanadium battery is charged from an associated PV panel. The stored energy is used exclusively to charge a fleet of electrically powered motorbikes.

12.7

Large-scale and small-scale storage – conceptual planning

Many different models can be postulated for the installation of energy storage and these also relate to its integration with CHP. Whilst the most impact is obtained by placing the storage as close to the end consumer as possible (as this aggregates the benefit of storage from the consumer back to the source of generation), economies of scale may mean that the specific costs of installing storage at the consumer’s point of connection is high. Savings could be made by introducing storage at higher levels in the distribution network, not only in specific equipment and capital costs, but also in trading and other commercial costs.

12.8

The development and application of thermal storage

This chapter has considered the case of using a storage medium as the intermediate stage between electricity charge and discharge. It is quite © Woodhead Publishing Limited, 2011

CHP-Beith-12.indd 318

4/5/11 9:24:52 AM

Energy storage for small and micro CHP systems

319

12.5 Vanadium battery (source: Cellstrom).

possible to use a storage device which uses thermal storage as the intermediate medium, and such thermodynamic cycles are under development. One modification is to use heat as the charging method, and this is adapted in the molten salt storage system typically installed in conjunction with solar concentrating thermal plants (see Fig. 12.6). The concentrated solar radiation is used to raise the temperature of a molten salt, which is contained in wellinsulated containers. During periods of darkness, the molten salt is used to raise steam which drives a turbine to generate power. Another modification is to use the heat directly, without recourse to generating electricity. This reduces the efficiency loss of the overall system. This is similar to the principle of the night storage heater, which was popular in Britain in the 1960s. Bricks were heated by electricity at night, and an arrangement of vents was used to allow the heat to warm the house over the next 17 hours. The premise was economics – the electricity generated at night from base load generating plant (coal and nuclear) was at lower cost than supplying power from peaking plant during the afternoon and early evening peak. Customers received an incentive in that night time electricity was at lower cost than the normal day time rate. Domestic thermal storage is still available, and the aesthetics and performance of the devices have improved considerably. Where a self-producer of electricity has a heat requirement,

© Woodhead Publishing Limited, 2011

CHP-Beith-12.indd 319

4/5/11 9:24:53 AM

320

Small and micro combined heat and power (CHP) systems

(a)

(b)

12.6 (a) Parabolic trough collectors, Almeira Spain (source: SPIE). (b) Experimental solid media storage unit with a capacity of 400 KWh (source: SPIE).

it would be quite feasible to use thermal energy storage as a means of time shifting energy production to suit the consumer’s demand profile. A further variation is to use off-peak – or surplus – electricity to produce ice. During peak periods, air can be blown over the ice or through cooling loops to cool buildings, saving on the use of the full air conditioning load.

© Woodhead Publishing Limited, 2011

CHP-Beith-12.indd 320

4/5/11 9:24:53 AM

Energy storage for small and micro CHP systems

321

Some commercial buildings, such as supermarkets and cold stores could also use the large thermal version of deep freezers and refrigerators as a proxy store. Thermal storage appears to the grid as a demand customer and can therefore contribute to the supply of resources by shutting off demand, and also to respond to high frequency events by switching on, when necessary. A recently announced development in the United States links a 5 MW concentrated solar power plant with a thermal storage plant at the ‘Solar Zone’ which is part of the University of Arizona Science and Technology Park. This large project has an estimated cost of more than $32 million.

12.9

Future trends

The smart grid is seen as a way of integrating all forms of generation and demand to operate the network in the most efficient manner. Some include a vision of control of industrial loads, even at and before domestic level and the incorporation of a smart meter which has time of day pricing. At the distribution and transmission level the transformers, switchgear and lines and cables are controlled to ensure that lines are run under optimum conditions and variable generation is used effectively. This complex vision can be simplified by the use of storage at key points in the network. American Electric Power (AEP) propose the introduction of Community Energy Storage (see Fig. 12.7) where storage is located at the local distribution transformer. The store acts as a local buffer, aggregating distributed generation and demand on the circuits downstream of the share.

12.7 Community Energy Storage: 5 kW, 20 kWh Community Energy Storage by Redflow Technologies Ltd.

© Woodhead Publishing Limited, 2011

CHP-Beith-12.indd 321

4/5/11 9:24:53 AM

322

Small and micro combined heat and power (CHP) systems

The Community Energy Storage is controlled by the network, charged and discharged to provide a stable power flow from the utility into, or out of, the battery. AEP see that this buffer will protect this network from the large and variable power flows that may arise from the introduction of electric vehicles and domestic charging.

12.10 Sources of further information and advice Electricity Storage Association: www.electricitystorage.org Department of Energy and Climate Change: www.decc.gov.uk Feed in tariffs: see DECC announcement http://www.decc.gov.uk/en/content/ cms/news/pn10_010/pn10_010.aspx (accessed 1 February 2010). Energy Saving Trust: www.energysavingtrust.org.uk Numerous renewable energy equipment suppliers.

© Woodhead Publishing Limited, 2011

CHP-Beith-12.indd 322

4/5/11 9:24:53 AM

13

Micro combined heat and power (CHP) systems for residential and small commercial buildings

J. H a r r i s o n, E.ON Engineering, UK

Abstract: The principal market for micro-CHP is as a replacement for gas boilers in the 18 million or so existing homes in the UK currently provided with gas-fired central heating systems. In addition there are a significant number of potential applications of micro-CHP in small commercial and residential buildings. In order to gain the optimum benefit from micro-CHP, it is essential to ensure that an appropriate technology is selected to integrate with the energy systems of the building. This chapter describes the key characteristics of the leading micro-CHP technologies, external and internal combustion engines and fuel cells, and how these align with the relevant applications. Key words: Stirling engine, internal combustion engine, fuel cells, microCHP.

13.1

Introduction

It is widely held that the principal market for micro-CHP is as a direct replacement of around 12 million of the 18 million gas central heating boilers, representing potential annual sales of up to 1.5 million units. This is based on the premise of simple economic payback of the investment cost of the product recoverable within an acceptable period for a 1 kWe Stirling engine micro-CHP package.1 Whilst a useful metric, it is rather simplistic, does not hold true for other micro-CHP technologies, and takes no account of potential applications in other building types. Micro-CHP derives its principal environmental and economic benefit from generating electricity as a byproduct of an existing thermal load. It replaces the gas boiler in a hydronic central heating system, producing space and water heating just as the boiler might do. It requires a primary fuel input and does not claim to be renewable, but is a low carbon and extremely energy efficient technology. As might be expected, the micro-CHP product has a higher initial cost than the boiler it replaces and must recover this cost from the value of the electricity it produces. Clearly, the more electricity that can be produced from a given thermal load, the higher the electrical output and the consequent operational income. However, it is almost invariably true 325 © Woodhead Publishing Limited, 2011

CHP-Beith-13.indd 325

4/5/11 9:28:33 AM

326

Small and micro combined heat and power (CHP) systems

that the higher the electrical output, the higher the capital cost so that here is both a compromise in the quest for electrical efficiency in producing a product at an appropriate cost and a natural match for different technologies to different thermal loads. The principal technologies considered relevant to mass market micro-CHP are internal combustion engines (ICE), external combustion engines (such as Stirling engines) and fuel cells. Of the latter, the high electrical efficiency and potential for low capital cost make SOFC (solid oxide fuel cells) the most promising fuel cell technology, although these are still at a relatively early stage of development and there are no commercially available products in the UK or Europe. Each of these technologies has characteristics making it more or less applicable to differing building types and thermal load profiles. However, micro-CHP is still a relatively immature technology and products continue to be developed to fulfil the requirements of various market sectors, able to compete more effectively with alternatives such as gas boilers, heat pump and larger-scale CHP technologies.

13.2

Basic issues and energy requirements

Although there are certain characteristics which make applications more or less attractive and viable, there are also some fundamental criteria which must be fulfilled to make an application viable. These include fuel availability, adequate thermal demands and an accommodating regulatory environment.

13.2.1 Economic rationale for micro-CHP The most fundamental economic factor required for viability of micro-CHP is that it must be able to recover the initial investment from the value of the electricity produced in lieu of heat from the primary fuel. It therefore holds true that the value of electricity must exceed that of heat by a significant amount. The ratio of electricity and gas prices is commonly known as ‘spark spread’ and, whilst it is generally the case that this ratio is around 3:1, reflecting in part the dominance of gas as a primary fuel input to central generating plant, it is not the case in countries such as Norway and Sweden where there is no widely available natural gas network, but an abundance of low cost hydroelectric power. In countries such as the UK, however, where retail electricity prices are around 11p/kWh and gas 3.5p/kWh, the cost of heat produced in a gas boiler with 90% efficiency is just under 4p/kWh. Thus, assuming the total efficiency of the micro-CHP unit to be equivalent to that of the gas boiler, each unit of electricity produced costs the consumer the lost opportunity cost of one unit of heat (4p), but is worth the value of displaced electricity

© Woodhead Publishing Limited, 2011

CHP-Beith-13.indd 326

4/5/11 9:28:33 AM

Micro CHP systems for residential and small commercial buildings

327

(11p) assuming further that the electricity is consumed within the home and not exported. Each unit of electricity thus generates a net income of 7p; an annual production of 3000 kWh is worth up to £210. For domestic investments, this could justify a marginal capital cost of around £600 for the mass market of consumers who require a three-year payback and up to £1000 for a smaller proportion of the market who are satisfied with a payback of five years. Considering that the average UK householder moves home every seven to eight years on average, even an investment of £1500 would seem rational and early adopters of other microgeneration technologies such as PV have been prepared to invest with paybacks in excess of a century as an environmental gesture! Figure 13.1 illustrates the simplified concept of electricity generation at the expense of heat for the major micro-CHP technologies. The importance of this is that the value of each unit of electricity produced is the same (in both economic and environmental terms) regardless of the technology or its electrical efficiency; a higher electrical efficiency product will generate more units of electricity for a given heat load (and will consequently consume proportionately more gas) so that its overall income is higher, but the value per kWh of electricity is exactly the same. This belies the widely held, but erroneous view that low efficiency micro-CHP technologies are somehow environmentally inferior to higher electrical efficiency products. Indeed, the reality for currently available, high electrical efficiency ICE products is that they do not achieve the same total efficiency as gas boilers so that the saving per unit electricity generated is reduced by the cost of additional gas burned. It is of course necessary that for economic viability, the micro-CHP unit must run for enough hours each year to generate sufficient electricity to

Solid oxide fuel cell

IC engine

Heat Electricity Loss

Stirling engine

Gas boiler 0%

20%

40%

60%

80%

100%

13.1 Economic rationale for micro-CHP.

© Woodhead Publishing Limited, 2011

CHP-Beith-13.indd 327

4/5/11 9:28:34 AM

328

Small and micro combined heat and power (CHP) systems

recover the initial investment. That requires an extended heating period; not necessarily a high peak demand, but preferably a long moderate heat demand in order to make full use of the heat demand to generate electricity for at least 2500 hours annually. However, technologies currently under development such as fuel cells with very low heat-to-power ratios may be able to achieve economic viability for homes with much smaller thermal demands so that some SOFC technologies are able to operate continuous baseload to meet domestic hot water (DHW) demand with an electrical output of 1 kWe every hour of the year.

13.2.2 Fuel availability The majority of micro-CHP products are designed to operate using natural gas as a fuel input and this represents the substantial market in Europe. Although it is possible to operate micro-CHP using alternative fuels such as liquid petroleum gas (LPG), fuel oil and even biofuels,2 these are relatively expensive and do not constitute a significant potential market. It is therefore natural that access to a natural gas network is an essential requirement for mass market micro-CHP applications. In the UK the majority (18 million of the 24 million or so homes) are indeed connected to the natural gas network and are equipped with gas-fired central heating. Germany and the Netherlands also have a majority of heating systems based on hydronic natural gas central heating.

13.2.3 Regulatory environment Although not essential, it is certainly preferable that the energy market is liberalised, allowing connection of micro-CHP units to the LV electricity network and being able to provide rewards for the export of electricity to the grid. In Japan, where it is not possible to recover value for any excess generation exported to the grid, products are designed to deliver sub-optimal performance to avoid export. Although such products are made viable by the application of grants and other subsidies, the liberalised markets of many European states offer a much better prospect of sustainable economic viability.

13.2.4 Technical requirements for micro-CHP viability In addition to these criteria for economic viability, there are also physical constraints on the technologies themselves as they must be suitable for installation either in the home or in other appropriate buildings. Micro-CHP is intrinsically different from conventional CHP in that it serves the highly volatile thermal and electrical loads in individual homes

© Woodhead Publishing Limited, 2011

CHP-Beith-13.indd 328

4/5/11 9:28:34 AM

Micro CHP systems for residential and small commercial buildings

329

in contrast to the more stable demands arising from the diversified loads characteristic of larger systems serving multiple homes or other building types. Therefore it is implicit that power cannot be wholly provided from CHP and that such systems will, in the majority of circumstances, be grid connected and exchange electrical energy with the network. Residential electrical loads fluctuate from below 100 We and can peak at over 20 kWe. Another complication is that residential heating systems typically operate for only around 2500 hours per year, much less than the normal CHP criteria (typically 6000 hours) for commercial viability. In addition, most manufacturers recognise it is not acceptable either practically or economically to service units more often than once annually, such as is required for gas boilers. Taken together these constraints impose severe technical demands on micro-CHP which are only just being resolved. For commercial buildings, nursing homes and similar buildings it is normal to have space allocated to a plant room which may contain bulky, relatively noisy equipment with regular access for service; in such applications ICE technology may be most appropriate particularly as this technology is able to deliver relatively high electrical efficiencies from a wide range of manufacturers. For individual homes, however, it is necessary to incorporate the micro-CHP product within the occupied space so that noise, vibration and physical bulk must be limited, as must intrusive service access requirements. It is therefore not possible to consider all technologies as equally suitable for all applications, although as a rule, technologies suitable for domestic installation are likely to be suitable also for commercial plant room applications subject to the necessary cost criteria.

13.3

Types of system for residential and small commercial buildings

Although all micro-CHP products have the common characteristic of producing heat and power from a primary fuel, fossil or otherwise, there are many different technologies acting as the ‘prime mover’, each with its own particular characteristics which make it more or less suitable for any given application. Alternatives, discussed in greater detail elsewhere in this book, include various types of engines, fuel cells, turbines and novel devices such as thermo-electric convertors.

13.3.1 External combustion engines External combustion engines separate the combustion process (which is the energy input to the engine) from the working gas, which undergoes pressure fluctuations and hence does useful work. The continuous, controlled external combustion process offers significant advantages in terms of low

© Woodhead Publishing Limited, 2011

CHP-Beith-13.indd 329

4/5/11 9:28:34 AM

330

Small and micro combined heat and power (CHP) systems

emissions, high efficiency, low noise and vibration and potentially long life and extended service intervals, although it must be conceded that these characteristics have yet to be demonstrated in the relatively immature products currently approaching commercialisation. Stirling engines have long been considered the leading micro-CHP technology, but have so far failed to reach the market in significant numbers, although it is expected that both WhisperGen and Baxi will launch their mass-produced 1 kWe products on the UK market during 2010 through the energy companies E.ON and British Gas, respectively. Both products have an electrical output around 1 kWe and electrical efficiencies approaching 15% and with overall efficiencies similar to those of gas boilers. For a typical UK home they would be expected to generate around 3000 kWh of low carbon electricity with a value of up to £200 if all the generated power were consumed on site and an additional £300 from the proposed FIT (feed-in tariff) subsidy. However, it should be noted that promoters of these technologies recommend their installation in larger homes with higher than average thermal demands in order to maximise the income from electricity generation and minimise payback; such homes, with an annual thermal demand of greater than 18 000 kWh constitute around half of the gas centrally heated homes in the UK.

13.3.2 Internal combustion engines Internal combustion engines inject fuel and air into the cylinders where combustion occurs.  The resulting temperature and pressure changes of the fuel/air mixture (which is also the working gas) act on the piston to produce useful work. This is mature technology, able to draw on extensive experience in both stationary and automotive applications, although the characteristic high emissions, noise and vibration as well as high service requirements inherent in this technology, raise significant challenges for micro-CHP applications. However, current products available from as little as 1 kWe and with a range of outputs up to 50 kWe (the upper limit of microgeneration*) seem to have substantially overcome each of these challenges and there are around 100 000 Honda Ecowill (1 kWe) units installed in Japanese homes and over 10 000 Baxi Dachs (5 kWe) units installed in large homes and similar buildings in Europe. With an electrical efficiency in excess of 25%, the electrical output of these products in many homes can be quite substantial, possibly as much as 4000 kWh per year and generating an income of around £240† (excluding *As defined by the UK Government in the Climate Change and Sustainable Energy Act 2007. † As the total efficiency of the leading current ICE product is significantly less than that of a gas boiler, the higher production cost of each unit of electricity results in a net income of only 6p.

© Woodhead Publishing Limited, 2011

CHP-Beith-13.indd 330

4/5/11 9:28:34 AM

Micro CHP systems for residential and small commercial buildings

331

subsidies which could add another £400 in the UK). The relatively low heat to power ratio of 3:1 requires the inclusion of a supplementary burner in the micro-CHP package, but makes installation in lower thermal demand homes economically viable.

13.3.3 Fuel cells In a fuel cell, the chemical energy within the fuel is converted directly into electricity (with byproducts of heat and water) without any mechanical drive or generator. Generally speaking they have a much higher electrical conversion efficiency than other technologies, particularly in the case of SOFC technologies, but are relatively inflexible in performance, requiring more or less continuous operation to avoid thermal cycling and the consequent induced mechanical stresses. The leading SOFC technology with an electrical efficiency of 60% and a heat to power ratio of 1:2 operates to provide DHW (domestic hot water) throughout the year, any requirement for space heating being met by the supplementary burner included within the package. In theory, this product would be technically suitable for every gas heated home in the country, although it is a relatively bulky product, requiring a substantial thermal store to capture the ‘waste’ thermal energy in the form of hot water and is thus probably best suited to homes with the necessary available space.

13.3.4 Other novel technologies There are numerous experimental technologies which may at some future date result in useable products. These include thermo-ionic and thermoelectric technologies which utilise temperature difference acting on metals or semi-conductors to produce electricity together with thermo-photovoltaic units which convert the radiant energy emitted by the burner to produce electricity using infra-red sensitive PV cells.

13.4

Domestic applications for micro combined heat and power (CHP)

There is currently a limited number of prime mover technologies on which micro-CHP systems suitable for individual homes can be based. Given the relative technical immaturity of micro-CHP, it is not always currently possible to identify a micro-CHP product ideally matched to the specific application. The mass market for micro-CHP is for stationary, on-grid applications, primarily in domestic buildings ranging from individual detached houses to multi-storey, apartment blocks. It should be acknowledged that, for some of these applications, particularly very high density apartments, some form

© Woodhead Publishing Limited, 2011

CHP-Beith-13.indd 331

4/5/11 9:28:34 AM

332

Small and micro combined heat and power (CHP) systems

of communal CHP system may be more appropriate from an economic and environmental perspective, although even in such instances, the desire for independence and personal control may make micro-CHP the preferred option from the user’s point of view. As noted earlier, by far the largest potential market for micro-CHP is for installation in individual homes which currently have gas-fuelled hydronic central heating systems. The technology is particularly relevant to provide heating in larger, less energy efficient homes where all practical, cost effective measures have already been taken to reduce energy cost, but where a significant thermal demand remains, so-called ‘hard to treat’ homes.

13.4.1 Complementary applications of micro-CHP in housing Currently available micro-CHP products simply provide space and water heating together with electricity. This limits their operating hours to periods when space heating is required in winter plus an additional limited duration outside the heating season, perhaps an hour or so daily, when there is still a demand for hot water. Some commentators have suggested measures to enhance the performance of their micro-CHP concepts by extending the potential operating hours to meet other heat-based loads. In theory, opportunities do exist for more complex, hybrid packages including cooling to make use of the heat output and thus generate more electricity outside the heating season. However, in practice, absorption cooling has yet to be demonstrated successfully at this scale. Still others propose the application of the generated electricity to power a vapour compression heat pump to maximise the thermal output of the unit, either on a continuous basis or for peaking purposes; this same principle could be applied as a fuel arbitrage measure in the longer term when the dominant domestic heating technology may be air source heat pumps. During periods of limited electricity availability from intermittent renewable sources, effectively the micro-CHP unit becomes a component of a bivalent heat pump heating package.

13.4.2 The existing homes market It has already been explained that the micro-CHP technologies currently under consideration are assumed to replace the boiler in a hydronic system. The majority of UK homes (18 million out of 25 million) are equipped with such central heating systems, as are those of the Netherlands and Germany, generally assumed to be the other two key European markets. At the same time the majority of the 1.5 million gas boilers sold each year are for replacement of boilers in existing systems when they reach the end of their useful life, typically after 10–15 years. It therefore seems logical to focus

© Woodhead Publishing Limited, 2011

CHP-Beith-13.indd 332

4/5/11 9:28:34 AM

Micro CHP systems for residential and small commercial buildings

333

marketing activities on this substantial market provided that the micro-CHP technology matches the thermal characteristics of the home. Not only do existing homes tend to have relatively higher thermal demands than newer, better insulated homes, but the heat-to-power ratio also aligns well with that of Stirling-based systems currently being introduced to the market. Around 85% of energy use within the home is for space and water heating, the remaining 15% is electricity3, as shown in Fig. 13.2. Thus the output of the micro-CHP unit, matching as it does this ratio, has the potential to meet the majority of the home’s energy requirements in a most cost-effective manner. One other key issue in the existing home sector is that of ‘hard to treat’ homes; many existing homes have already had all practical insulation measures applied, so that the only viable way of further improving the efficiency of the home is by installing an energy saving energy system such as micro-CHP or heat pumps. Yet other homes are constructed in such a way that precludes the application of cost-effective insulation measures, such as homes with solid walls, particularly listed buildings or those with attractive external features. Of course, one further compelling reason to focus on this market is that there is an established route to market with logistics, installers and marketing resources already delivering 1.5 million boilers annually. This should allow micro-CHP to make significant impact without requiring the long lead times characteristic of central plant alternatives such as nuclear power or carbon capture and sequestration (CCS); micro-CHP delivers power from day one of installation and an installation rate of 1.5 million a year represents around 1.5 GWe additional generating capacity annually, equivalent to one modern nuclear power station which would require 10 years to build and would not generate a single kilowatt hour until fully completed. Cooking 5% Lights and appliances 10%

Water heating 23%

Space heating 62%

13.2 Domestic energy consumption by end use.

© Woodhead Publishing Limited, 2011

CHP-Beith-13.indd 333

4/5/11 9:28:34 AM

334

Small and micro combined heat and power (CHP) systems

Scope of micro-CHP in existing homes The criteria for economic viability have already been discussed, outlining the requirement for payback within a reasonable period. This payback is determined largely by the thermal demand of the property in question, a minimum of 18 000 kWh being considered the threshold for lower efficiency Stirling engines. As can be seen from Fig. 13.3 which shows the distribution of gas consumption in all UK homes, this represents a target market of around 9 million homes; more electrically efficient micro-CHP technologies may become viable in smaller homes. At the extreme, products such as the CFCL SOFC unit with a heat-to-power ratio of 1:2 may be able to operate continuously to cover domestic hot water demand alone, so making it suitable for all homes with a thermal demand in excess of 2600 kWh, virtually the entire housing stock. Such products would be provided with an integral supplementary boiler to provide space heating, so it would also be able to meet the full thermal demand of even the largest homes. Objections to micro-CHP There remain those who object to the installation of current micro-CHP technologies on the basis that we should wait until more (electrically) efficient products become available. This belief is flawed; such inaction would waste the opportunity to make immediate, if modest, savings whilst we await the improved products and, as explained earlier, micro-CHP products, as gas boilers, undergo a regular replacement cycle, allowing the early products to be replaced with the enhanced performance models in due course. Still others believe we should not install any fossil-fuelled products at all, but instead await the arrival of low carbon renewable energy to fuel heat pumps,4 failing to appreciate the timescales involved and the urgent need to make the best use of our current finite gas resources until the ultimate low carbon future is attained. Another instance of the better being the enemy of the good! It is also worth considering the likely roadmap from our currently installed domestic heating systems dominated by gas-fired hydronic central heating to one in which heat pumps incrementally displace those gas boilers. There is likely to be a very significant, some would say catastrophic, shortfall in electrical generating capacity within the next decade*; this capacity shortfall would be compounded by a rapid shift to an electrified heat sector. The parallel introduction of heat pumps requiring additional electrical generating capacity and micro-CHP, which might contribute to that capacity, seems to be one effective means of achieving both decarbonised heat and avoiding *Both E.ON and EdF in the UK have indicated an anticipated shortfall in generating capacity of around 45 GWe by 2016 if they are forced to close existing coal-fired power plants to comply with the EU LCPD Directive.

© Woodhead Publishing Limited, 2011

CHP-Beith-13.indd 334

4/5/11 9:28:34 AM

CHP-Beith-13.indd 335

4/5/11 9:28:35 AM

© Woodhead Publishing Limited, 2011 0

500 000

1 000 000

1 500 000

2 000 000

2 500 000

Gas consumption band (kWh per annum)

13.3 Number of UK domestic gas consumers by consumption.

Number of customers in consumption

300 0 600 0 900 0 12 000 15 000 18 000 21 000 24 000 27 000 30 000 33 000 36 000 39 000 42 000 45 000 48 000 51 000 54 000 57 000 60 000 63 000 66 000 69 000 72 000 75 000 150 000 300 0 0 450 0 00 600 0 000

336

Small and micro combined heat and power (CHP) systems

overload of the electricity generation and distribution networks in the short to medium term.

13.4.3 The new-build housing market When designing and building new homes, there is the opportunity to make decisions based on the optimum combination of high performance construction and an integrated energy system. Too often, unfortunately, the innate conservativism of the construction industry and their assumptions about public aspirations, result in the major housing developers choosing to construct very poorly performing homes. They are then forced to resort to ‘addon’, sometimes tokenist (usually disproportionately costly) microgeneration solutions to comply with the increasingly stringent building regulations. Under such a scenario it is possible that micro-CHP systems will continue to be installed in traditionally constructed homes for some years although it is clear that by 2016, when zero carbon homes become mandatory, it will no longer be possible to justify micro-CHP using fossil fuels.

13.5

Small commercial buildings and other potential applications

This section considers the potential for micro-CHP units (possibly in multiple modular configurations) in small commercial buildings including residential, office, educational and other relevant applications. It can be seen from Table 13.1 that there is a much broader range of technologies which may be suitable for non-domestic applications, although the potential number of viable installations is significantly less. In addition to these stationary applications, there is also considerable global potential for micro-CHP in mobile and remote stand-alone configurations, but these are considered outside the scope of this publication. Such applications might include cabin heaters for trucks, range extenders for electric vehicles, off-grid residential and other stand-alone applications such as auxiliary power units (APU) in marine and military systems. Indeed the WhisperGen micro-CHP unit began life as a diesel-fired APU/heating system for marine applications. However, it should be recognised that the relatively low electrical efficiency of Stirling engines makes them less than ideal for applications where the primary concern is electrical generation; fuel cells are better suited to such applications.

13.5.1 UK commercial market The UK small commercial market is more fragmented than the domestic market and has a much smaller overall potential. However, there are a

© Woodhead Publishing Limited, 2011

CHP-Beith-13.indd 336

4/5/11 9:28:35 AM

© Woodhead Publishing Limited, 2011

CHP-Beith-13.indd 337

4/5/11 9:28:35 AM

Existing homes

� � � �

Offices

Emergency

Laundrettes

�

�

�

�  � �  �

�  �  �

�  �

�  �  �

�  �

�  �

�  �

�  �  �

�  �  �

5 kWe ic engine

�  �

�  �

�  �

�

�  �  �

�  �  �

15 kWe ic engine

�  �

�  �

�  �  �

�  �  �

50 kWe ic engine

Note: The stars indicate the suitability of each technology to the respective application – the more stars the better the suitability.

�

�

�

�  �

�  �  �

Schools

�

�

Restaurants

�

�  �

�

�

�  �  �

�  �  �

2 kWe sofc

Hotels

�

�  �

1 kWe sofc

�  �

�  �  �

1 kWe ic engine

Sheltered

New homes

1 kWe Stirling

Table 13.1 Preferred technologies for micro-CHP applications

338

Small and micro combined heat and power (CHP) systems

number of niche markets where micro-CHP may find applications that are less sensitive to initial investment costs than the domestic market. Such areas are where there is a large demand for hot water and lighting throughout the year, prime examples being small hotels and hairdressing establishments. However, it is almost inevitable that commercial establishments which do not have a residential component are destined to have relatively low running hours simply because they are only occupied for around one-third of the day during the heating season. Schools are even less attractive due to the long holiday periods which further reduce the potential for extended operating hours, although in most cases it is possible to make use of thermal storage to extend operating hours of the CHP system during term time well beyond the occupied period by storing heat for later (or peaking) use. However, it is important to consider the likely use of the electricity generated as avoided import is clearly more valuable than the price attributed to export; it is therefore necessary to match generation to consumption as closely as possible. The following types of commercial applications, although not exhaustive, should provide an impression of the potential for micro-CHP technologies in respective building types. The summary is intended to illustrate relevant technologies, but due to the rather varied size of offices, hotels, etc., each case needs to be considered on its own merits. Applications worth consideration include the following. ∑

Hotels, where thermal demand and thus economic viability may be assessed on the basis of number of bedrooms. Each room will have a space heating demand plus a demand for sanitary hot water; on an aggregate basis it is likely that the diversified demand will be similar to a house for multiple room establishments. The prime market is probably the 13 0005 or so small hotels with 4–15 bedrooms for smaller 1–3 kWe electrical output units; the larger ICE-based units are well suited to hotels with more rooms and, although this text is focused on micro-CHP, there is clearly an overlap with larger-scale CHP plants depending on the size and thermal demands of the establishment in question. ∑ Residential and nursing homes as well as sheltered flats provide an excellent potential for micro-CHP due to the relatively high continuous thermal demands throughout the year for both space and water heating. In the majority of cases it is likely that micro-CHP units in the range of 5–15 kWe corresponding to a thermal output of 12–30 kWt would be able to achieve adequate run hours to attain a good economic return. There are in the region of 16 000 such establishments in the UK. ∑ Restaurants and pubs may have high space heating requirements, but over relatively short periods and hot water requirements tend to be light. A micro-CHP unit is unlikely to be utilised for more than 2000 hours per year, resulting in rather poor paybacks. However, if there are live-in

© Woodhead Publishing Limited, 2011

CHP-Beith-13.indd 338

4/5/11 9:28:35 AM

Micro CHP systems for residential and small commercial buildings

339

tenants or rooms to let, such buildings would be prospective installations with the same characteristics as a large house. ∑ Offices, where economic viability is assessed on the basis of floor area and implied thermal demand. Although in terms of numbers, offices appear to offer a substantial market (there are over 100 000 offices with floor areas less than 100 m2) ,6 heating needs are relatively low due to the high level of internal gains from computers, lighting and other equipment. There is little requirement for water heating, giving no summer load for the CHP unit. Even in winter there is potentially a mismatch between electrical use and heating demand, since, although the electrical load in offices is fairly constant throughout the occupied period, the heating demand will be at its highest just before the start of occupancy.7 There is a further challenge to viability as offices are rarely owner occupied and there is little incentive for landlords to include micro-CHP or indeed to provide any energy efficiency measures. ∑ Emergency service buildings (police, fire and ambulance stations) which are continuously occupied and with a constant electrical demand as well as similar thermal requirements to large homes may provide a significant opportunity for micro-CHP, particularly if the unit is capable of stand-alone operation, clearly a benefit for such establishments; there are around 12 000 in the UK. ∑ Laundrettes, hairdressers and similar premises with a significant hot water and electrical demand; there are around 11 000 of these in the UK. ∑ There are also around 10 000 small schools which may benefit from micro-CHP, although it is difficult to justify the investment from the relatively short-run hours unless the building is also used for community purposes during the evenings and school holidays, or in the case of boarding schools which may effectively be considered as student halls of residence and be suitable for small-scale CHP or become part of larger CHP schemes across a campus, for example. In summary, the potential for small commercial applications suggests a market in excess of 100 000 premises. This aligns reasonably well with the gas consumption profile data which indicate a market of around 107 000 consumers with relevant gas consumptions. Depending on the size of microCHP unit under consideration, it is possible that more than one unit may be installed in some kind of cascade arrangement as shown in (Fig. 13.4). Although the larger micro-CHP units including ICE-based products can achieve high electrical efficiencies, extended service intervals and low capital cost due to their level of industrial maturity, it is possible that as production volumes of smaller units increase, their cost may become competitive with these larger units and provide a greater degree of operational flexibility. This

© Woodhead Publishing Limited, 2011

CHP-Beith-13.indd 339

4/5/11 9:28:35 AM

340

Small and micro combined heat and power (CHP) systems

13.4 Multiple EC Power 15 kWe ICE units in sheltered housing scheme.

may be particularly true for SOFC where the basic fuel cell stack may be effectively scaled by modularisation with relatively little impact on capital cost other than the balance of plant components. Assuming a replacement rate of 5% (as for domestic heating systems), and an average of two micro-CHP units per application, this represents a potential market of 5000 units per year in the small commercial sector, very much smaller than the domestic market, but conceivably less technically challenging and addressable on the basis of ‘rational’ economic decisions.

© Woodhead Publishing Limited, 2011

CHP-Beith-13.indd 340

4/5/11 9:28:36 AM

Micro CHP systems for residential and small commercial buildings

13.6

341

Advantages and limitations

13.6.1 Competing technology solutions Micro-CHP clearly has a significant role to play in a range of applications, but it is important to ensure that the appropriate technology is selected for each application. This may be Stirling engines for larger thermal demand properties, fuel cells for lower energy consuming homes and we should not be afraid to acknowledge that, in some cases, some other form of energy system altogether may be more appropriate. Micro-CHP is most cost effective where there is an adequate heat load to justify the investment and where a natural gas supply is available, but in other cases alternative technology solutions may be required. For example, practical constraints of physical size and issues with provision of mains gas supply combined with the low thermal demand of individual apartments within high rise blocks might favour community heating in preference to micro-CHP, whereas alternative technologies such as heat pumps or biomass heating are potential options in rural areas where no gas supply is available. Community heating District heating (DH), also known as community heating (CH) is relatively uncommon in the UK, with only 1% market share. This is to some extent a reflection of the high level of owner-occupation and the desire to have independent control, allied to a traditional focus in the UK on low first cost. It is also difficult to see how the conflicting demands of DH (which logically requires all homes in an area to be connected for economic viability) and a competitive market (which demands that all customers may choose their energy supplier and their energy system) can be resolved. Various studies8 have identified up to 5 million homes within areas with sufficiently high heat densities to justify district heating.* The areas identified tend to be central urban sites with high rise apartments, unsuitable for micro-CHP both from a heat loss perspective and due to the physical size and construction of the properties. Micro-CHP and DH can therefore be considered complementary rather than competing technologies, although, where practical, micro-CHP is preferable on economic and environmental grounds.9 Figure 13.5 illustrates the relative merits of various micro-CHP technologies compared with conventional gas boiler central heating plus either solar *

Note that the viability is extremely sensitive to discount rate due to the high initial cost. The 5 million figure (which includes input from the 2002 PB Power study) assumes 6%, whereas, for a rate of 9% about 400 000 and for 12%, less than 200 000 homes would be viable.

© Woodhead Publishing Limited, 2011

CHP-Beith-13.indd 341

4/5/11 9:28:36 AM

342

Small and micro combined heat and power (CHP) systems Boiler (70%) PV Micro CHP (10%) Micro CHP (20%) Micro CHP (50%)

Boiler (86%) Wind Micro CHP (15%) 500 kW 0%

9000 Boiler (70%)

Annual CO2 emissions (kG)

8500 8000

Boiler (86%)

7500

We 500 k

Boiler + PV

4000

h

7000 6500

M CHP (SE)

6000 5500 M CHP (FC)

5000 4500 4000



0%

5%

10%

15%

20%

13.5 Comparative annual CO2 emissions for micro-CHP and CH/CHP.

PV or micro wind for a typical home with an annual thermal demand of 18 000 kWh and 6000 kWh electrical demand. For each of these technology combinations, the annual CO2 emissions can be seen on the vertical axis. It clearly demonstrates the environmental benefits of micro-CHP compared with these more expensive and less effective technologies, even for the lowest efficiency, Stirling engine-based micro-CHP products.10 Figure 13.5 also shows the emissions for an alternative, small (500 kWe) CHP plant connected to a small community heat network. The major disadvantage of such a system in efficiency terms is the inevitable heat distribution losses; the horizontal axis shows the assumed heat distribution losses from zero to 20%, a figure of 10% being fairly typical for a welldesigned modern system. However, the same point made above in reference to the long lead times of central electricity generating plant could also be levelled against CH as it too requires substantial, expensive and timeconsuming infrastructure investment before any useful energy is delivered at all. Whereas micro-CHP offers incremental investment risk, large-scale community heating, like nuclear, CCS, etc., requires a level of market certainty over a long period which has not so far been forthcoming. However, one major advantage of CH over micro- and small-scale CHP schemes is that, whilst the latter are largely confined to gas–fired applications, larger-scale

© Woodhead Publishing Limited, 2011

CHP-Beith-13.indd 342

4/5/11 9:28:36 AM

Micro CHP systems for residential and small commercial buildings

343

systems are able to make use of both energy from waste and a wide range of alternative fuels and provide opportunities for fuel switching in response to availability. Other microgeneration technologies Depending on the space and water heating requirements of the home, the availability of a natural gas supply or alternative fuels and a host of other factors, there may be alternative microgeneration technologies which may offer alternative or complementary benefits to micro-CHP. Such technologies include biomass boilers, air and ground source heat pumps, solar thermal, micro-wind and micro-hydro. However, one of the key benefits of microCHP compared with technologies such as solar thermal, PV and microwind, is that it is a non-discretionary purchase. In other words, micro-CHP replaces an essential component of any home, namely the central heating boiler. So, as with heat pumps and biomass boilers, the investment cost of micro-CHP needs to be considered as an incremental cost compared with the alternative gas boiler, whereas the discretionary technologies need to justify their entire cost. After all you don’t need a PV system, but you do need space heating. Conventional central plant Figure 13.6 illustrates the comparative electrical and thermal efficiencies of the main micro-CHP technologies. For example, SOFC micro-CHP has an electrical efficiency of 50% and a thermal efficiency of 40% in this diagram. These are overlaid on the alternative, conventional central generating plant 90

Thermal efficiency (%)

80

Stirling engine

70

IC engine

60

PEM fuel cell

50 40

Solid oxide fuel cell

30 20 10 0 0

10

20 30 40 Electrical efficiency (%)

50

60

13.6 Comparative efficiencies of micro-CHP and conventional options.

© Woodhead Publishing Limited, 2011

CHP-Beith-13.indd 343

4/5/11 9:28:36 AM

344

Small and micro combined heat and power (CHP) systems

option providing electricity at an efficiency of 45% and 35% (representing CCGT and UK average delivered efficiencies) and heat provided by gas boilers, with state-of-the-art condensing gas boilers and typical UK gas boilers delivering efficiencies of 90% and 70%, respectively. The solid line represents the best available conventional technology, whilst the broken line is more representative of typical UK practice. Despite the variation in relative electrical and thermal efficiencies, there is no case in which microCHP performs worse than centrally generated electricity and gas boiler central heating.

13.7

Future trends

As micro-CHP enters the commercial phase, the products themselves are reaching a level of maturity which primarily requires developments to be of a ‘design for manufacture’ nature rather than addressing fundamental issues of performance and reliability. Considerations of product acceptability are also being addressed with a focus on issues such as control, interfacing with the consumer and education to ensure householders gain the most value from the operation of their micro-CHP products. The products also need to be adapted to the peculiar requirements of each market, and technology development of higher efficiency, smaller, quieter, cheaper products continues. One particularly encouraging development is the rapid progress currently being made by fuel cells which are already demonstrating exceptionally high levels of electrical efficiency, leading to the potential for application in the majority of homes. At the same time, it is becoming apparent that the implementation of micro-CHP can be accelerated by the availability of ‘enabling technologies’ such as advanced controls and metering which improve the performance within the home and which allow the true value of micro-CHP generation to be realized. Whilst there is no one technology which can overcome the challenges of UK energy policy, there is no doubt that micro-CHP can deliver a substantial and possibly the greatest individual contribution to the joint goals of eliminating fuel poverty, carbon mitigation, competitiveness and security of supply within the housing sector. There is a need to develop a range of technology solutions to meet the range of needs of different dwelling types, households and logistic constraints. It is therefore likely that new products will continue to be introduced to the market to provide householders with a range of complementary options including heat pumps, micro-wind, biomass and solar thermal technologies. Perhaps most importantly, aside from the specific developments of enhanced performance products, additional products from a variety of manufacturers and other inevitable developments of a maturing technology, there is a growing recognition that micro-CHP and other technologies must not be viewed as

© Woodhead Publishing Limited, 2011

CHP-Beith-13.indd 344

4/5/11 9:28:36 AM

Micro CHP systems for residential and small commercial buildings

345

isolated components of the energy system; we must consider their synergies. The simplistic advocacy of individual technologies whether larger-scale CHP, conventional central plant solutions or alternative microgeneration technologies is giving way to an understanding that there is no one technology which is able to meet the energy demands of all consumers and applications and that we need a diversified portfolio encompassing all available low carbon, energy efficient technologies. But more than this, the technologies may not just complement one another; they may depend on one another. As we enter an era which is likely to see the emergence of very large-scale intermittent renewable generation, we need to consider the ability to arbitrage fuels, swap loads between fuel types and ideally to design systems in which, for example, the electrical output from gas-fired micro-CHP can be used to support the intermittent output of wind generation either directly through the VPP (virtual power plant) concept, or indirectly by supplying electricity to drive heat pumps when wind power is constrained. In summary, micro-CHP should be seen as a medium-term transitional technology supporting the implementation of an electrified heat sector as we move towards a zero carbon future, but may also retain a long-term, even permanent role in support of such a system.

13.8

Sources of further information and advice

∑

General information on micro-CHP and related technologies with links to manufacturers and additional resources: http://www.microchap.info ∑ General information on microgeneration technologies with links to manufacturers and additional resources: http://www.microgenerationoracle.com/index.htm ∑ Links page to government and institutional websites providing information on energy issues as well as organisations active in the field of distributed energy in general and CHP in particular: http://www.microchap.info/ LINKS.HTM

13.9

References

1 Harrison J and Redford S (2001), Potential benefits of micro CHP, Energy Saving Trust. 2 Harrison J. (2003), Micro CHP in rural areas, Renewable Energy World. 3 Department of Trade and Industry (1999), UK energy sector indicators. 4 McKay D (2009), Sustainable Energy without the hot air, UIT, Cambridge. 5 EA Technology (2000), Micro CHP – Review of emerging technologies, products, applications & markets. EA Technology report. 6 Herring H, Hardcastle R and Phillipson R (1998), Energy use and energy efficiency in UK commercial and public buildings up to the year 2000, Stationery Office Books. © Woodhead Publishing Limited, 2011

CHP-Beith-13.indd 345

4/5/11 9:28:36 AM

346

Small and micro combined heat and power (CHP) systems

7 Moss K (1994), Energy consumption in public and commercial buildings, BRE Information Paper, IP 16/94. 8 BRE (2003), The Potential for Community Heating in the UK, Carbon Trust. 9 Harrison J (2002), ‘Options for upgrading residential CHP’, COGEN Europe Conference Paper. 10 Harrison J (2007), ‘Micro CHP and microgeneration’, Claverton Energy Group Conference.

© Woodhead Publishing Limited, 2011

CHP-Beith-13.indd 346

4/5/11 9:28:36 AM

14

District and community heating aspects of combined heat and power (CHP) systems

J. C l e m e n t, Aars District Heating, Denmark, N. Ma r t i n, Shetland Heat Energy and Power, UK and B. Ma g n us, COWI, Denmark

Abstract: This chapter focuses mainly on the district and community heating aspects of small CHP systems, the heat sources and control systems and contains issues to be considered before a new district heating system is to be established. Further, this chapter contains some of the preconditions for getting started and overall design considerations. These experiences are based on projects established primarily in Denmark and the UK and with case studies from Aars in Denmark and Lerwick in Shetland. Key words: district heating, improved efficiency, fuel flexibility, CO2 reduction, CO2 neutral heating, heat and energy storage.

14.1

Introduction

District heating (DH) is a very old principle and was already used by the Romans some 2000 years ago. Of course, it has developed since then, but in principle it remains a central heating system expanded to contain more than just one house, and the heating is today transported from boiler to house in pre-insulated pipes. It is a relatively simple system and the principles are easy to understand, but still district heating is often called one of the best kept secrets because it is buried under the ground and very few people know about it. Even in Denmark, where 60% of all houses are connected to district heating systems, very few people knows about district heating in detail and many do not even know from where the heating comes. One of the reasons for this situation is that very few problems arise with the system, the limited space demand and almost no maintenance in the home. When designed according to the recognized recommendations, the system is extremely robust and the number of hours when there is no heating is very limited. In areas with district heating there are only a limited number of individual stacks and therefore the air quality is high and nobody thinks about the heating of the house. It is just something you have as long as you pay the bill and can be compared to buying your water or your electricity and who knows where these services come from? One of the main reasons for establishing district heating is that heating 347 © Woodhead Publishing Limited, 2011

CHP-Beith-14.indd 347

4/5/11 9:29:43 AM

348

Small and micro combined heat and power (CHP) systems

of houses and preparation of hot tap water do not need high-level energy carriers like fossil fuels or electricity but can use low-level sources as surplus energy from, for example, power production and industrial processes. When used as a byproduct from power production, the fuel efficiency is more than doubled, from typically 35–45% in power plants to 85–95% in combined heat and power (CHP) plants. Further, the district heating system gives a better fuel flexibility and enables a number of local resources such as straw, wood, different waste products, biogas, sun, wind, geothermal energy, etc., to be used for heating. This is good for the local economy and the overall environment as the energy efficiency is improved and the emissions from the heat generation can be controlled and limited due to the central generators and monitoring possibilities that exist in a centralized installation of a certain size range. District heating water has a further advantage: it is easy to store and, with a storage tank, daily variations in heat consumption can easily be handled. It is therefore not necessary to have generation capacity available for the peak demand and the heat production can be stable or, if there is surplus energy from a process at any time, it can be stored in the storage tank.

14.2

How to get started

In Denmark, district heating covers more than 60% of the housing stock which is among the highest rates in the world. The last 40% of Danish houses are in natural gas areas, are in rural locations or in small towns with fewer than 100 houses. They are supplied by individual boilers with natural gas and oil but also wood pellets and straw boilers are used. The high connection rate to district heating was helped along by the two energy crises in the 1970s when oil prices more than doubled, and a political wish to become less dependent on oil and coal from abroad. Energy planning was introduced in all major towns and cities in Denmark, where areas were dedicated to district heating and some cities also introduced compulsory connection to DH systems. Many of the big DH schemes were owned and operated by municipalities but often the establishment of a new district heating system was started by individuals who had the energy, charisma and enthusiasm for the idea of having a local and flexible heat supply. DH is comparable in cost to other heat sources and often cheaper, but some of the small gas-fired CHP plants have experienced a change in the preconditions with higher gas prices and lower electricity prices and they can only put the deficit on the heating cost. Preconditions for getting a DH system up and running include: ∑ getting customers committed to take the heating ∑ a heat source that can provide competitive energy prices ∑ political support as the municipality has to give permits for the © Woodhead Publishing Limited, 2011

CHP-Beith-14.indd 348

4/5/11 9:29:43 AM

District and community heating aspects of CHP systems

349

establishment of the system and they do also have control over a number of buildings that could become good customers ∑ financial resources to handle the investments which are long term and with a long payback period ∑ an organization with knowledge of handling a project like this, both setting up the preconditions for the project, the design and operation, and developing a plan. When a DH project starts, it must be recognized that it is as long-term, ongoing project. You can finish your first, second and third stages, but you will never finish developing your DH system. When the system in the town is fully developed, you can start thinking about connecting to neighbours and connection to new heat sources or developing new heat sources. Lerwick and Aars are two good examples (explained in more detail in Sections 14.6 and 14.7) but almost every district heating company can improve or expand, and this will continue with the development of technology and knowledge.

14.3

Heat sources

The heat sources available for district heating have developed rapidly in recent years. Economy, environment and sustainability are important drivers when it comes to choosing the right heat source. In the 1960s and 1970s fossil fuels were cheap and easily available and there was limited focus on the environment. However, first the energy crises in the 1970s and then the slow depletion of natural gas (Denmark will not be self-sufficient in gas from around 2016) and the global warming commitments have changed the focus. Now sustainability and security of supply are the most important factors when setting up a new system, but also in developing an existing system. With the development of wind farms the electricity market will experience frequent imbalances, therefore even electricity can be a sustainable heat source when used at the right time. Large-scale electric boilers or heat pumps can help balance electricity consumption with generation, and DH systems with storage tanks can easily adapt to these variations.

14.3.1 Size considerations It is evident there is a considerable difference between summer load and winter load, the actual difference depending on the temperature and wind factors, but in the UK and Denmark the factor of heat demand is probably 4–5 times larger during winter compared to summer. During the summer the need for heating is very limited and heat produced is primarily used for heating tap water and heat loss from the pipe system. When designing the plant, a general principle is the heat source with the lowest production price has the highest investment cost. Therefore the © Woodhead Publishing Limited, 2011

CHP-Beith-14.indd 349

4/5/11 9:29:43 AM

350

Small and micro combined heat and power (CHP) systems

plant should consist of a base plant able to supply, e.g., 60% of the peak load and a peak and reserve plant covering the last 40%. A rule of thumb, 60% of the peak load will cover 90% of the yearly energy production and therefore it is of little importance what the production cost is with the peak load installations.

14.3.2 Examples of heat sources The following is a short introduction to a number of possible heat sources. Biomass Biomass is often available locally and can consist of straw, wood chips, wood pellets, shredded roots, and other residues from forestry and farming. Most commonly these plants are heat only but there are a few CHP plants. Biogas In Denmark there are only a few plants using biogas, but in Germany and other parts of Europe the number of such plants is increasing. They provide gas from landfill sites, composting facilities and anaerobic digestors accepting a range of green and wet wastes. Biogas, which is mainly methane based, can be used in, e.g., gas engines producing both power and heating resulting in a very high efficiency. Waste to energy Energy from waste is very common in Denmark and provides a considerable amount of the district heating. Most plants are built as CHP installations, but because of the very corrosive flue gases the steam generators can only operate to steam temperatures around 400–440 °C. The efficiency is therefore relatively low if these plants are only producing electricity and even an optimized plant will not achieve more than ~30% efficiency. However, the combination of heat and power production can give efficiencies above 95% and with flue gas condensation even above 100%. This is possible because the efficiency is calculated with the lower heating value and these fuels contain some moisture. Fossil fuel Typically fossil fuel plants use light fuel oil or natural gas. These plants are used as peak and reserve load installations as the investment cost is low and © Woodhead Publishing Limited, 2011

CHP-Beith-14.indd 350

4/5/11 9:29:43 AM

District and community heating aspects of CHP systems

351

the operation relatively simple. Natural gas is also used in CHP plant with gas engines but these are often smaller plants and have relatively high heating prices as the gas prices have risen and electricity prices gone down. Other heat sources The other heat source in Danish district heating systems are geothermal, with only a few plants running but more are currently under development. Waste heat from industrial processes and solar heating are growing fast at the moment and these systems can count as an energy sawing initiative.

14.4

Pipework installation issues and design considerations

District heating pipes have the prime purpose of distributing the energy to customers with the lowest possible heat and pressure. The physical layout of a district heating network has to be based on the location of the consumers as well as geographical conditions of the area to cover ground levels of the landscape, etc. At the same time the pipes have to be protected against corrosion and damage from other utilities, etc. The standard pipes are today insulated with polyurethane foam and have containment pipes of steel or polyethylene. Branch pipes are often flexible and the dimensions depend on the differential pressure available over the length of the pipe and the overall condition of the DH system. If the DH system is old, it can contain some dirt and therefore very small pipes (< 16 or 20 mm) should be avoided but new branch pipes are made down to 12 mm inside diameter in the media pipe. These very small diameters gives very little heat loss and a cold pipe takes limited time to heat up, but if the main pipe contains some dirt the clogging of the branch is a risk factor. Older pipe systems are of different quality and, depending on the water quality and general operating conditions, they can be either in good condition or very bad condition. To clean the water particle filters are installed as part flow filters, but if a pipe is clogged there is no alternative but to flush the pipes. New pipes are of a high quality and there will be no problems with them if they are installed according to the specifications and not damaged by other utility companies. Some problems have been caused by cable diggers who are just ploughing their cables into the ground with little or no consideration for other pipes. Regarding the installation of pipework in the ground, there can be problems in installing pipes in streets and built-up areas as there are usually many existing services already laid in the ground. On some occasions re-routing of existing systems is necessary or the heating pipework may have to be installed deeper. It is also possible to drill a ‘tunnel’ and drag the pipe through. These

© Woodhead Publishing Limited, 2011

CHP-Beith-14.indd 351

4/5/11 9:29:43 AM

352

Small and micro combined heat and power (CHP) systems

problems become more difficult with bigger pipe diameters, but the pipes used are flexible and tough and can be manipulated satisfactorily. It is true to say that there are usually some problems, but potential consumers have always been connected regardless of such problems.

14.4.1 Pipe size considerations The optimal sizing of the main pipes is difficult as the pipes have a lifetime of 40 years and therefore need to be designed for a future load compared to when they are installed. The initial pipe design has to include a development plan for the main pipes going from the plant to the areas being supplied, and new areas to be connected must include a plan for how and when customers are to be connected. When an area is developed it is therefore a big advantage if the connection rate is high from the beginning as the heat loss is then optimized. The installation cost is also a lot cheaper if consumers are connected when the mains pipes are installed. To get people connected, customer information and promotion is therefore essential and often combined with a discount rate for connections carried out during the initial installation of the pipes. If the connection rate is too low, the system will operate with a high heat loss but, if the connection rate on the other hand is too high compared to the connection plan, the pipes might need to be upgraded before the end of the lifespan. To get some heat load on the system from the beginning, it is a good idea to concentrate on the larger consumers such as hospitals, swimming pools, sports centres, schools and public buildings and areas developed with new housing, and initially connect them as they will ensure a good flow in the pipes. If the areas with the larger consumers are developed first, it also ensures that the system will work and a reasonable base load is applied to the system. If there is little knowledge about district heating, it is also an advantage to have proven the system with an initial installation before large-scale expansion, as mouth-to-mouth promotion is often one of the most efficient ways of getting new customers.

14.4.2 Consumer education As the capacity of the pipes is linked closely to the temperature difference between flow and return, it is important to get the consumers to cool the water and extract as much heat as possible. The heat loss in the return pipes also depends on the temperature, so the consumers are therefore an integral part of the DH system and it is important to get them to follow some simple rules to make the overall system operate with the lowest losses and cost. These rules include the following.

© Woodhead Publishing Limited, 2011

CHP-Beith-14.indd 352

4/5/11 9:29:43 AM

District and community heating aspects of CHP systems

353

∑

Install radiators designed for District Heating, e.g. 60–70 °C flow temperature and return at 30–40 °C. ∑ Use all the radiators in the house to ensure the best cooling of the DH water. ∑ Keep doors closed to rooms where you want lower temperatures. ∑ Avoid temperatures lower than 16 °C when operating with lower temperatures during night-time. This is to avoid big changes in heat demand during the day, as the maximum heat demand is what pipes and heat sources are designed for. Heat loss from the house is very limited at lower temperatures, therefore nobody gains much by going to lower house temperatures. Basically it is more economical to operate at the system design duty than at low load. ∑ Give a bonus to those who can achieve good cooling. Further, a good customer relationship can be an advantage as an aware customer keeps an eye open and if there are, e.g., leaks in the system, they can help find them. If an aware customer sees an area with no snow during the winter period or steam coming from underground, they would know this could come from a DH pipe and be an important informant for the operators.

14.5

Control system and consumer installations

The commitment of the system is to ensure all costomers have sufficient heating. The temperature, differential pressure and static pressure need to be sufficient to heat the house and make hot water within the safe temperature parameters at all times. In the design of a district heating network, the following basic items must be observed and considered. Figure 14.1 outlines the general overall principles of a DH supply unit with a CHP unit as the base unit and oil-fired boiler as backup, the pressurising system and supply of treated water. Figure 14.2 outlines the principles of a DH supply unit also including a heat storage tank for optimized operation of the producing units and able to average the daily variations. Figure 14.3 outlines a standard consumer installation with direct space heating by means of radiators and hot tap water supply through a plate heat exchanger. The supply to customers can also include a heat exchanger to ensure the water in the radiators and the distribution pipes are separated. For example, in Aars it is a direct system as shown in Fig. 14.3, but in Lerwick, Shetland, all customers are supplied through a heat exchanger.

14.5.1 Pressurizing and static pressure The pressure in the district heating network must be calculated and designed on the basis of the ground levels of the area covered. The pressure in the

© Woodhead Publishing Limited, 2011

CHP-Beith-14.indd 353

4/5/11 9:29:43 AM

354

Small and micro combined heat and power (CHP) systems

Engine

Oil boiler

Consumers

B

Pressurising system Water treatment Raw water

14.1 Boiler station with a CHP unit as the base unit.

Engine

Oil or biomass boiler

Heat storage

Consumers

B

Pressurising system Water treatment Raw water

14.2 Boiler station with a heat storage tank.

pipes must ensure there is water coverage at the highest locations under all operating conditions, preventing air and raw water intake at defect connections or consumer installations. Normally the pressure in a distribution network is also kept below 10 barg at all locations. If the network needs

© Woodhead Publishing Limited, 2011

CHP-Beith-14.indd 354

4/5/11 9:29:44 AM

© Woodhead Publishing Limited, 2011

CHP-Beith-14.indd 355

4/5/11 9:29:44 AM

Main valves

F

T

T Differential pressure control valve

Temperature control valve

Plate heat exchanger

Space heating

T

T

Cold tap water

Hot tap water

T

Radiators with thermostatic valves T

Radiators with thermostatic valves

14.3 Standard consumer installation with direct space heating by means of radiators and hot tap water supply through a plate heat exchanger.

District heating network

Strainer

Energy meter

Radiators with thermostatic valves

356

Small and micro combined heat and power (CHP) systems

to cover areas with a large variation in ground level it has to be separated into pressure zones by large heat exchanger units or by booster pumps and pressure reduction valves. The ‘static pressure’ is the pressure that remains in the district heating network when the system is at rest, e.g. with all pumps stopped. The static pressure should be kept at a level that gives approx. 1 barg at the highest ground level in the network. The pressurizing system is in most cases brought as a pre-assembled unit with a pressure-less reservoir, with pumps to add water to the network at low pressure and mechanical valves to drain water at high pressure. The design capacity of the pressurizing system is based on the water volume in the network, enabling the system to drain water at a sufficient rate when the whole network is heated up from standstill, and again add sufficient water if the system cools down as a result of a break down, lack of heat power, etc. It is also essential that the reservoir of the pressurizing system is designed to prevent oxygen uptake in the water. Differential pressure The differential pressure between flow and return pipe in the network creates the flow rate through the customer installations. The lowest differential pressure at the individual customer depends on the pressure loss in their internal installations, but normally 0.3 bar is sufficient. The differential pressure in the network is established by the main pumps installed at the DH plant. The main pumps are designed for highest possible energy efficiency on the loads that occur in the highest number of running hours during the year. In the design process loads for summer and winter are considered as well as the redundancy or backup philosophy, and the number of pumps is determined, i.e. two 100% capacity pumps, or maybe three 50% capacity pumps. The location of the main pumps also has to be considered in the system layout. Normally the best solution will be to locate the pumps in the return pipe pumping through boilers, heat exchangers, etc., as this will keep a pressure on the producing units securing against steam generation in the system.

14.5.2 Heat storage tank In DH networks based on CHP units, biomass boiler units or maybe surplus energy from local wind turbines, it is of great benefit to incorporate a heat storage tank to equalize the difference between production and demand in the network. CHP units can be operated at full load during periods with high electricity prices, and relatively slow reacting biomass or waste-to-energy boilers often run optimal at constant load, regardless of the peak loads in the

© Woodhead Publishing Limited, 2011

CHP-Beith-14.indd 356

4/5/11 9:29:44 AM

District and community heating aspects of CHP systems

357

network. The heat storage tank is in its simplest form an insulated pressurized tank with diffusers at the top and bottom to make the flow in and out of the tank at a low velocity, thus building up a layer division between hot and cold water (Fig. 14.4).

14.5.3 Water quality in the network In order to reduce risk of corrosion, leaks, deposits and bacterial growth in the district heating network, it is essential to establish and maintain the right quality of the circulated water. In smaller DH networks the makeup water is normally prepared in a pre-assembled softening unit where the calcium and magnesium salts in the water are exchanged with sodium salt which do not cause the disadvantages of hard water. Adding special chemicals raises the pH value in the water and prevents growth of bacteria. The pH value is monitored and additional chemicals dosed on a continuous basis, keeping the pH value at approx. 9.5.

14.4 Heat storage in Aars to account for daily variations in heat consumption, size 800 m3.

© Woodhead Publishing Limited, 2011

CHP-Beith-14.indd 357

4/5/11 9:29:45 AM

358

Small and micro combined heat and power (CHP) systems

14.5.4 Flow temperature The flow temperature in the DH network should be kept as low as possible to reduce the losses throughout the year. The temperature should, on the other hand, be high enough to ensure sufficient heating and hot tap water for customers located at the furthest point in the network. Space heating Space heating depends on the dimensions of radiators, and district heating often requires larger radiators than for a traditional boiler system as the DH system is more efficient when the return temperature is as low as possible. These radiators are, though, only marginally bigger and can be combined with under floorheating as they can operate at flow temperatures as low as 30 °C. This will ensure a very low return temperature. A traditional consumer boiler installation often operates with a high return temperature as this is needed to protect the boiler itself from corrosion, and the ‘losses’ from both flow and return pipes is within the premises thus all adding to the heating. Losses from district heating are mainly located in the exterior network and this is reduced by low return temperatures. So when including existing consumer space heating installations, there might be a need to increase radiator sizes and there must be focus on removing all bypasses in the installations. Hot tap water The hot tap water must be kept above 55 °C to limit the risk of bacteria growth. This is often the dimensioning criteria for the district heating temperatures during summer operation and consumer installations should also be designed with consideration to where in the network they are located. If close to the plant the temperatures and pressure are higher and direct heat exchangers can easily be used, but at some distance the temperatures will be lower and the differential pressure also lower therefore, so a hot water tank might be the best solution as the operation of the DH system needs to take account of the designed consumer installations to avoid customer complaints.

14.5.5 Customer billing system Different billing systems have been used in DH systems through the ages, previously based on items such as consumed cubic meters, area of heating surfaces, floor area, etc. Due to temperature drop in the widespread network, different flow temperatures were available to consumers thus giving a different energy price. The availability of cheap electronic energy meters have nowadays resulted in energy-based billing giving the same price per

© Woodhead Publishing Limited, 2011

CHP-Beith-14.indd 358

4/5/11 9:29:45 AM

District and community heating aspects of CHP systems

359

energy unit to all customers, normally combined with a standing charge based on size of installation and maximum heating demand. Also the customer’s ability to return the water at a low temperature has in recent years been integrated in the tariffs. It is important to get the right balance between the different elements of the billing system to support incentives such as cooling, and still to keep the district heating energy price attractive compared to other sources.

14.6

Case study: Lerwick, Shetland

Designed in 1998, installation started in 1999 with main pipes to some parts of the town where the main customers and some of the larger ones were located, such as hospitals, swimming pools, sports centres, schools and public buildings and areas developed with new housing. This system is one of the most successful district heating systems in the UK and during its 10-year life span has now covered most of the town of Lerwick with more than 1000 customers. The system started as a result of heat from a 6.5 MW Energy from Waste plant built during the same period as the initial design and establishment of the DH system. The main drivers were the Shetland Islands Council and the Shetland Charitable Trust ensuring capital. The Trust had funds accumulated from oil activities from the 1970s and was allowed to invest funds to benefit the Shetland economy. District heating schemes are very capital intensive requiring a large investment at the start. This tends to be the main obstacle to the development of such schemes when payback is over a long term. The most difficult task to retrofit a district heating scheme in an existing town in the UK is to obtain low return temperatures. Buildings were originally designed on a 82 °C supply and a 71 °C return. As the quality of customers’ systems was questionable it was decided that there would be no direct feeds into a building heating system so that heat exchangers would be required, meaning the supply temperature would have to be above 87 °C. In addition the connection rate was expected to be low to start with until the scheme proved itself, so 95 °C was decided upon to allow for heat losses. Whilst a return temperature of around 70 °C was anticipated in general, it was found that many systems were overdesigned and that they could run at lower temperatures. An additional return line was run to the swimming pool which only needed to heat water to 30 °C and could lower the overall water from the town at a beneficial tariff. Initially the return was 65 °C but with new build and refurbishments over 10 years the return is down to 55 °C and it is hoped that this will be reduced to 50 °C over the next 10 years. Lowering the supply temperature to 90 °C is also a possibility. The scheme had problems at the start with many objectors against the scheme primarily because it was expensive and an unfamiliar technology.

© Woodhead Publishing Limited, 2011

CHP-Beith-14.indd 359

4/5/11 9:29:45 AM

360

Small and micro combined heat and power (CHP) systems

Falling oil prices did not make the marketing easy. By the following year oil prices had risen steeply and word of mouth on how good the system was starting selling the scheme, outstripping the local ability to connect, which produced a long waiting list. By 2010 the scheme had over 50 km of pipe and about 50% of the households within reach of the pipes connected. Most of the largest heat users are now connected. Non-domestic customers are 10% of the total but account for 60% of the heat consumed. At two of the largest consumers, arrangements were made to take over the former boilers and keep them ‘in mothballs’ ready for use to feed into the scheme should flow from the Energy from Waste (EFW) plant or boilers have to be interrupted for maintenance purposes. As the installations had backup and were designed for worst case scenarios, they had sufficient spare capacity. It is anticipated that the lifespan of the boilers being kept in standby will be prolonged for many years. By 2006 the demand had grown so large that the backup oil boilers were consuming enough oil to meet morning peakloads to justify a 12 MWh thermal storage tank to store surplus heat during the early hours which was being dumped by coolers. Right from the start electronic ultrasonic heat meters were installed. Whilst initially read from an external plug, the meters had radio modules installed that could relay most of the readings back to the office thus enabling accurate billing and monitoring of cooling by customers. The scheme reached the output of the EfW plant in 2010 and numerous new sources of heat are being examined including front runner of wind turbines feeding into a large thermal storage tank. The potential increase of output from the EfW plant is also possible by the installation of an internal water jacket provided more waste becomes available. Whilst the scheme has brought environmental benefits in reducing emissions and landfill, it is the economic benefits that initially justified the scheme and these have been exceeded. With the reduction in imported fuels where money would have left the islands, reduced maintenance and capital costs of large customers and jobs created, economic returns (after deducting all costs and adding resultant benefits) of between 15 and 25% have been achieved.

14.7

Case study: Aars, Denmark

The DH system was established in 1955 with initially 26 customers signing up to the system. The initiative was taken by some local entrepreneurs who could see the local benefits from investment in this plant. Low heating costs and a safe supply of heat was essential to develop the town further. The first system was based on fossil fuels and the pipes were steel pipes in concrete ducts insulated with Rockwool and Leka stones, but from the 1960s the first pre-insulated pipes were installed. Oil was the main heat source, but the oil crisis influenced the design towards the development of alternative fuel

© Woodhead Publishing Limited, 2011

CHP-Beith-14.indd 360

4/5/11 9:29:45 AM

District and community heating aspects of CHP systems

361

sources and the Municipality and the District Heating Company developed plans for a Waste to Energy (WtE) plant (Fig. 14.5). The first WtE plant (6.5 MW) was established in 1985 and commissioned in 1986 together with a coal-fired (5 MW) peak and reserve boiler. The site was in the outskirts of the town and a transmission pipe was installed to supply the former oilfired boiler stations. As the town expanded, the new areas were connected to district heating via a heating plan and therefore the connection rates in these areas were 100%. This was very efficient and gave a good utilization of the pipes and the base load from the WtE plant was used during most of the year. The waste amount also increased and in 1995 the next incinerator was installed increasing the capacity by more than 100%. This was a combined heat and power line and included a 3 MW turbine together with a 10 MW heat capacity. This capacity was too big for the summer load and coolers had to be installed as there was a waste treatment obligation. The summer load only allows one incinerator to be in operation and the challenge was then to further expand the district heating area. Compulsory connection was approved by the municipality and during a 10-year period all houses within the district heating area had to connect. Now all houses in Aars are connected and in 2009 a neighbouring town was connected via a 7.5 km transmission pipe (Fig. 14.6). The transmission pipe has a special design to both take account of heat loss and pumping loss. It is a twin pipe with a DN125 in the flow giving high flow velocity and a DN150 in the return giving low pressure loss. Both

14.5 The Energy from Waste plant in Aars, seen from the east at the entrance to the town.

© Woodhead Publishing Limited, 2011

CHP-Beith-14.indd 361

4/5/11 9:29:45 AM

362

Small and micro combined heat and power (CHP) systems

14.6 A twin pipe with two different dimensions. This is the 7.5 km transmission pipe between Aars and Hornum in Denmark.

pipes are built into the same insulation dimension as a DN150/DN150 pipe to ensure optimum insulation, resulting in a temperature loss in the flow pipe of only 1 °C during winter operation in the 7.5 km long transmission pipe. The neighbouring town has a gas engine CHP plant and, in combination with the Energy from Waste plant in Aars, there is a possibility only to operate the engines when the electricity market is out of balance and giving high prices for regulating power. Further, a 1 MW electric boiler is installed and the purpose of this is to use some of the surplus electricity from the wind turbines. When operated it is at negative electricity prices, at down to – £150/ MWh. In total 4300 customers are now connected to the DH system and a few of the other neighbouring towns with gas engine CHP plants are also interested in connecting to Aars as the heating cost is low, there is still a little surplus energy during the summer period and the two incinerator lines could easily operate for longer periods. The project economies of these projects are to be investigated in 2010.

© Woodhead Publishing Limited, 2011

CHP-Beith-14.indd 362

4/5/11 9:29:45 AM

District and community heating aspects of CHP systems

14.8

363

Future trends

14.8.1 Temperature optimization To limit heat losses from the pipes, many DH systems are now introducing temperature optimization. The principle is to supply only the necessary temperature during summer and winter load most customers can operate their heating system with a flow temperature of 60 °C, but the temperature in the pipes depends on the flow rate and the insulation of the pipes. Therefore most systems have been operated on the safe side with too high temperatures resulting in unnecessary heat loss from the pipes. Computer programs can simulate the temperatures from the load situation and with control measurement the program can be balanced according to the actual conditions and afterwards the flow temperature and pressure from the plant optimized to the load situation.

14.8.2 Balancing of the electric grid As a result of the introduction of wind farms and the unpredictable electricity generation, the heating systems can be used as load dumps when there is overflow in the electricity market. Electric boilers are easy to operate and can react quickly. Although they only generate, e.g., 1 MW of heat when using 1 MW of electricity, heat pumps could improve the power factor considerably but the regulating capability is not yet as good as the boiler, but a combination of the two systems could be a possibility.

14.8.3 Pipe dimensioning Pipe dimensioning has mostly been relatively conservative, based on flow rates and velocity but branch pipes close to the boiler station could be made with smaller dimension because they have a high differential pressure available. This could also be used in the pipes instead of in a regulating valve.

14.8.4 Conversion from gas to district heating Conversion of natural gas areas to district heating is one of the new trends in Denmark as the natural gas resources are running out and the government is now supporting these conversions. Heat pump solutions are probably the new heat source for rural consumers and those who are not close to a district heating system as they can be made to primarily use electricity at off-peak periods or even at electricity overflow from wind turbines.

© Woodhead Publishing Limited, 2011

CHP-Beith-14.indd 363

4/5/11 9:29:45 AM

364

14.9

Small and micro combined heat and power (CHP) systems

Sources for further information and advice

∑ ∑ ∑

Danish Board of District heating, http://www.dbdh.dk/ Danish District Heating Association, http://www.danskfjernvarme.dk/ SHEAP Shetland Heat Energy and Power, District Heating Manager Neville Martin, http://www.sheap-ltd.co.uk/ ∑ COWI, http://www.cowi.com ∑ Aars District Heating, Director Jan Clement, homepage (only in Danish) www.aarsfjv.dk, or email [email protected], phone +45 9998 8070.

© Woodhead Publishing Limited, 2011

CHP-Beith-14.indd 364

4/5/11 9:29:45 AM

15

Small combined heat and power (CHP) systems for commercial buildings and institutions

R. B o u k h a n o u f, University of Nottingham, UK

Abstract: Small-scale CHP has a huge potential to deliver energy savings and be an effective carbon mitigation strategy in commercial buildings and institutions. This chapter starts with a brief discussion about energy requirements, trends, and the regulatory frameworks driving energy efficiency in these types of buildings. Then details on technical and operational characteristics of small-scale CHP technology are given with emphasis on implementation in different types of buildings. Finally, future prospects and ways to support the technology are discussed. Key words: small-scale CHP, energy consumption, commercial buildings, institution buildings, energy efficiency, engines.

15.1

Introduction

The UK’s long-term manifesto on reducing carbon emissions is supported by the Climate Change Act 2008 that sets out a target of cutting greenhouse gas emission by at least 34% and 80% by the years 2020 and 2050, respectively, against the 1990 baseline (UK Parliament, 2008). This means significant emission cuts will have to be realised in each of the three main sectors of the economy responsible for the bulk of greenhouse gas emissions: the power generation sector, the transport sector, and the building sector. The latter consumes about 45% of the total primary energy and contributes to a corresponding amount of pollutants emission. The UK has a stock of about 1.8 million non-domestic buildings, which account for up to 18% of total CO2 emissions. These buildings consume approximately 300 TWh of energy a year, predominantly for the provision of space heating and cooling, hot water, and power for lighting and office equipment, as shown in Fig. 15.1 (Elsadig, 2005). Energy consumption of the existing building stock can vary widely as it is influenced by the design of the building envelope and the efficiency of the heating, ventilation and air conditioning (HVAC) equipment (Baird et al., 1984). The UK Building Regulation Part L2, conservation of fuel and power in non-domestic buildings, and the EU’s Energy Performance of Buildings Directive (EPBD), are the main tools for change towards sustainability in 365 © Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 365

4/5/11 9:30:00 AM

366

Small and micro combined heat and power (CHP) systems Other 7%

O ffi tio

7%

%

3%

t6

n

en

ng

ra

pm

oli

ge

ui

fri

eq

Co

ce

Re

Space heating 32%

Ventilation 3%

Cooking

4%

Water heating 15%

Lighting 23%

15.1 Energy use in non-domestic buildings.

the building sector. The EPBD requires display energy certificates (DECs) in public and commercial buildings and energy performance certificates (EPCs) to be made available at point of sale or rent. The EPCs rating of a building carries, among other benefits, help to identify poorly operated buildings and a list of remedies that can be taken to improve the overall energy performance of the building (EU Parliament, 2002). The UK building regulations are continually evolving for tighter building construction and operation standards. For instance, Part L of the proposed 2011 building regulations will include 20% improvements in building energy performance. Eventually, it is intended to have building regulations enforcing a target of zero carbon emissions for domestic and non-domestic buildings in 2016 and 2019, respectively.

15.2

Basic issues and energy requirements

In the UK, information regarding energy consumption in non-domestic buildings is generally not easily available and not explicitly compiled compared to the industrial and domestic sectors. This is due in part to the diverse nature of buildings with different physical forms and sizes that fall in this category and which are used in a wide range of activities including industrial, commercial, public service, transport and agricultural sector. Figure 15.2 shows a sample of non-domestic buildings with different economic

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 366

4/5/11 9:30:00 AM

Small CHP systems for commercial buildings and institutions

Others 23%

367

Retail 22%

Leisure 6% Offices 17%

Hospitals 6% Schools 10%

Hotels and restaurants 16%

15.2 Energy use in the commercial sector by building type.

activities and the proportional energy consumption (Pérez-Lombard et al., 2008). Furthermore, these types of buildings would often be serviced by plant and equipment with varied power capacities, presenting a major difficulty for collecting accurate and reliable statistical data and producing standardised energy consumption patterns. The implementation of the DECs scheme may offer an opportunity to improve the national database and establish an accurate benchmark of energy consumption in this sector of the economy. Recent energy consumption statistics in the UK non-domestic building stock normalised by floor area (i.e., kWh/m2) show an increase of about 5% between 1990 and 2008. This upward trend is mainly a reflection in increased electrical energy consumption that follows from the overall increase in the commercial sector floor space. The improvement in efficiency of building services equipment slowed down the increase in energy use for air conditioning, refrigeration, lighting, IT, and long opening hours in large retail stores. It is increasingly recognised that investment in the energy efficiency of buildings has, in addition to cutting the cost of energy used and supporting efforts for climate change mitigation, the benefit of improving occupants’ productivity and health (Scrase, 2000). The deployment of commercially available renewable energy and low carbon technologies to provide space heating, air conditioning and lighting could contribute to substantial carbon savings in non-domestic buildings. However, the high cost of deployment of renewable energy technologies (PV, heat pumps, etc.) makes returns on investment unattractive to potential investors. Combined heat and power

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 367

4/5/11 9:30:00 AM

368

Small and micro combined heat and power (CHP) systems

(CHP) is, however, a proven and reliable technology that can reduce carbon emission and be cost effective.

15.3

Small combined heat and power (CHP) use in commercial buildings and institutions

The adoption of CHP systems as an energy supply option in industry and district heating schemes is well established in the UK and the developed countries. CHP systems can be found in all sectors of the UK economy, from individual dwellings to heavy industries and processes and large district heating schemes. In 2008, the UK total CHP electricity generation capacity stood at around 5.5 GWe with a heat-to-power ratio of 1.87 and an overall thermal efficiency of 67.2% (DUKES, 2008). Most of the power is generated from large-scale CHP plants installed in the industrial sector (oil, gas, chemicals and paper industries). Electrical power generated from CHP plants installed in the building sector, however, accounts for only around 344 MWe, representing just over 7 per cent of the UK’s total CHP capacity (DUKES, 2008). Because of the small-scale nature of the CHP plants installed in buildings, these constitute over 90% of the total number of CHP installations. To reduce carbon emissions and help deliver the Climate Change Programme, the UK has a target of achieving at least 10 GW of Good Quality CHP electrical power capacity by the end of 2010. However, this seems unlikely to be met on current trends as growth in installed CHP capacity has stagnated in the last few years. The use of small-scale CHP plants to provide heat and power in the building sector was first introduced in buildings where there is year round demand for both heat and power such as sport centres, hospitals and hotels. Current applications for small CHP in buildings has been extended to encompass universities, schools, tower buildings, sheltered housing schemes, military bases and even farms. Most small-scale CHP plant are built as complete units on a common frame with enclosure with electrical power rating ranging from as low as 30 kW to up to 2 MW. Table 15.1 gives a summary of the total number of small-scale CHP schemes and power generation capacity in different types of commercial buildings and institutions (DUKES, 2008). It can be seen that the largest proportion of the CHP capacity is installed mainly in hospitals, whereas leisure centres and hotels have the largest number of installations. Generally, the basic common requirement for a small-scale CHP is to satisfy the electricity and heat demand in a building with operation in excess of 4500 hours per year or about 14–16 hours/day to be economic (CIBSE, 2007). The traditional intermittency operation of a boiler, to provide heat in a building only when required, is not compatible with the mode of operation of a CHP system. A small-scale CHP should operate continuously and be

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 368

4/5/11 9:30:00 AM

Small CHP systems for commercial buildings and institutions

369

Table 15.1 Number and capacity of CHP installations in the building sector (DUKES, 2008) Building type

Number of schemes

Electricity Heat capacity capacity (MWe) (MWth)

Leisure Hotels Health Residential group heating Universities Offices Education Government estate Retail Other (agriculture, airports, domestic buildings)

394 254 187 40 41 17 17 17 17 3

50 36 124.2 27.7 50 15 10 15.9 4.6 10.5

54.8 45.4 190.4 61 83.8 12 17.7 18.6 3.4 18.7

Total

987

344

505.7

sized to have a heat-to-power ratio comparable to that of the building it intends to service. A departure from full-load operating conditions impacts negatively on the thermal efficiency of a CHP system and hence results in longer payback period. Correspondingly, the outlet temperature level should be suitable for the heating application. For instance, the minimum required temperatures in building applications vary from 40 °C, where underfloor heating is used, to 80 °C for conventional radiators. Thus, a temperature of approximately 100 °C can be regarded as the sufficient output temperature of a CHP system. Therefore, while the overriding aim of installing and sizing a CHP system will be to supply all normal electricity and heat load, in practice this is not always realistic and supplementary grid power and heat from a standby boiler will be considered as part of an overall design strategy.

15.4

Small-scale combined heat and power (CHP) technology

CHP technologies are traditional power generation equipment where the attendant heat from fuel energy conversion into mechanical or electrical energy is recovered for heating or cooling purposes. In this way, smallscale CHP systems can convert up to 80% of the energy in the fuel (GCV) into electrical power and useful heat. This compares favourably with the conventional method of supplying heat and power to buildings from a heatonly boiler and grid. Most small-scale CHP units used in buildings come as packaged plant built around an engine with all components assembled and ready for connection to

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 369

4/5/11 9:30:01 AM

370

Small and micro combined heat and power (CHP) systems

a building’s heating and hot water circuit and electrical distribution panels. Approximately 30% of engine fuel energy input is converted to mechanical power to drive the electric generator, where the conversion into electricity occurs with little loss. Of the remaining energy released in the engine, over 50% can be recovered as useful heat using a heat recovery system. The heat recovered for small-scale CHP is in the form of low temperature hot water (LTHW) at between 70 and 90 °C. A further 10% of available heat can also be recovered at low temperatures of 30 to 50 °C. Figure 15.3 shows a Sankey diagram of energy balance for a small-scale packaged CHP system. Other associated equipment with a small-scale CHP system include a control and monitoring panel that operate the engine start-up and shut-down sequences to ensure safe functioning, an exhaust system with compatible materials, and an acoustic enclosure to attenuate noise emanating from the plant.

15.4.1 Heat engine types The heat engine, also known as the prime mover and the term ‘engine’ will be used throughout this chapter, is the main component of a CHP system. The heat engine, which is referred to here as simply engine, is used to drive an electric generator to convert fuel energy into electricity. Commonly, there are three types of engines used in small-scale CHP for buildings. Reciprocating internal combustion There are two well-known types of reciprocating engine used in small-scale stationary power generation and CHP applications: the spark ignition (Otto cycle) engine and the compression ignition (Diesel cycle) engine. The main difference between the two types is the method of igniting the fuel. Spark Power 30% Useful heat 50%

er

ed

heat

Fuel energy 100%

Heat losses 20%

Reco

v

15.3 Energy balance of a small-scale CHP.

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 370

4/5/11 9:30:01 AM

Small CHP systems for commercial buildings and institutions

371

ignition engines use a spark plug to ignite a pre-mixed air/fuel mixture introduced into the cylinder. In Diesel engines, the fuel is injected into the cylinder at high pressure after the introduced air has been compressed to a high pressure, raising its temperature to the auto-ignition temperature of the fuel. The thermal efficiency of internal combustion engines depends primarily on the compression ratio of the fuel–air mixture in the cylinder with a typical range of 12–24 for diesel engines and 9–12 for spark ignition engines. This makes diesel engines achieve thermal efficiencies of up to 45% whereas that of its spark ignition counterpart is limited to about 35%. The spark ignition engines are based on automotive or marine (industrial) engine derivatives converted to run on gaseous fuel such as natural gas. These engines are available in sizes from down to 20 kW and up to 5 MW electrical power output. The automotive derived engines are usually rated below 200  kW electrical power output rating whereas the industrial counterparts have a higher power output rating. In addition to CHP applications, diesel engines are widely used to provide standby power in sites such as hospitals and data servers that require uninterruptible power supply to protect critical services. Converted diesel engine standby generators for small-scale CHP are often limited to sites where the electrical power demand is in excess of 500 kW and run on diesel fuel or diesel/gas dual fuel. The reciprocating engines have proven reliability, low maintenance requirements and good service life. Depending on the engine size, a full engine overhaul is only required after achieving between 20 000 and 50 000 running hours. The economic benefit of a small-scale CHP is enhanced by effective recovery and use of the thermal energy rejected from the engine. Approximately 60–70% of the inlet fuel energy appears as heat energy, a proportion of which can be recovered from the engine exhaust gas, jacket cooling water, lubrication oil and turbocharger cooling water, and electric generator coolant. The exhaust gas leaves the engine at a typical temperature range of 450–650 °C, contains about 10–30% of the fuel energy while heat in the engine jacket coolant accounts for about 30% of fuel energy input. The heat recovered from the engine is generally in the form of low temperature hot water (90 °C) or low steam at a pressure lower than 2 barg. Figure 15.4 shows the main components and heat flow paths in a gas-fired internal combustion engine. Reciprocating engines for small-scale CHP are a well developed and proven technology. They offer low initial cost, easy to start up, good reliability and availability with a proper maintenance schedule. Better control of the combustion process and use of exhaust catalysts have led to a significant improvement in greenhouse gas (GHG) emissions. However, because a large proportion of the heat energy rejected is of low grade, the ability of the engine

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 371

4/5/11 9:30:01 AM

372

Small and micro combined heat and power (CHP) systems

Fuel input

3-phase Control power panel supply

Exhaust gases

Low temperature hot water supply Exhaust heat exchanger

Internal combustion engine

Hot water return

Lubrication oil cooler

Water jacket cooler

15.4 A simple layout diagram of an internal combustion gas engine for small-scale CHP.

to produce steam is limited. However, this is less critical in commercial and institutional buildings. Gas turbines Gas turbines are an established technology that is widely used by utility and industrial power generators. These are often modified aero-engines where part of fuel (e.g., natural gas or gas-oil) energy released in the combustion chamber is converted to electrical power at an efficiency ranging from 20 to 35%. Combustion gases exiting the turbine at a temperature over 450 °C are a source of high grade thermal energy. The rejected heat from the exhaust gases can be recovered as steam or hot water in an unfired or fired heat recovery boiler. Depending on size and operating properties of the gas turbine, the recoverable heat-to-power generation ratio can vary between 1.5 and 3. The heat-to-power ratio can be increased to over 6 by further fuel combustion in the waste heat recovery boiler to take advantage of excess oxygen content of the exhaust gases from the gas turbine. Hence this is very useful for sites with variable heat loads. The available high quality waste heat, high reliability, and low maintenance costs per unit of power generation make gas turbines the favourite candidates for industrial and commercial CHP applications. Gas turbines can be used in small-scale CHP systems with power output ranging from just under 1 MW to a few MWs. However, the number of sites suitable for gas turbine-powered CHP systems in commercial buildings and institutions is usually limited to those with high pressure steam demand and electricity loads above 1 MW. In practice, gas turbines form a very small percentage of all CHP systems

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 372

4/5/11 9:30:01 AM

Small CHP systems for commercial buildings and institutions

373

installed in buildings and are mainly confined to large hospitals where a small proportion of heat demand is in the form of steam used for medical equipment sterilisation. Micro-gas turbines Micro-gas turbines are small combustion gas turbines with power output ranging from 30 kW to over 200 kW. The scaling down of gas turbine technology impacts negatively the heat and combustion processes which reduces the turbine power output and efficiency. Hence micro-gas turbines often operate at very high shaft speed (up to 100 000 rpm) and use electronic power inverters for power conditioning to generate a.c. voltage and at grid frequency. Equally, a heat recuperator is usually fitted in the exhaust hot gases to pre-heat compressed combustion air to reduce fuel consumption and achieve efficiencies of up 30%. Micro-gas turbines also offer the advantages of compact size, low weight per unit power, multi-fuel capability and ease of emissions control. Figure 15.5 shows a schematic diagram of a microgas turbine. The introduction recently of packaged micro-gas turbines has offered comparable capital costs to reciprocating internal combustion engines with the benefit of lower maintenance costs and high availability, offsetting the lower electrical efficiency. The main application for micro-gas turbines is in packaged small-scale CHP which can be installed as single or multiple units, achieving an overall efficiency higher than 80%. Micro-gas turbines can also be used for emergency or standby power generation as well as a mechanical drive for pumps and compressors. Typical performance characteristics Combustion air

Turbine

Compressor

Generator

Exhaust gas

Heat exchanger

Fuel

CC Electricity supply

Recuperator

Hot water/ steam supply

15.5 A simplified diagram of a gas-fired micro-gas turbine CHP system.

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 373

4/5/11 9:30:01 AM

374

Small and micro combined heat and power (CHP) systems

of the main engines used in small-scale CHP plants are summarised in Table 15.2.

15.4.2 Integration of small-scale CHP into building services Heating and hot water services Heat supply in buildings is mostly required for space heating and hot water, and heat is recovered from small-scale CHP systems at about 80 °C when reciprocating internal combustion engines are used. Where steam is required, this can be generated directly from the exhaust gas heat recovery heat exchange of the engine or a micro-gas turbine. A working fluid (e.g., water) is circulated in a closed loop pipe work that includes backup boilers to transfer heat to the site heat loads. A plate heat exchanger, located on the return feed of the heating system of the building, offers a practical method of interface between the CHP header pipe and the heating circuit, as shown in Fig. 15.6. This series connection arrangement in particular offers an easy method of incorporating packaged CHP units into existing boiler-only heating systems in buildings. In new CHP installations where the CHP system is sized to provide a substantial amount of heat demand, then the CHP unit can be installed in parallel with the backup boilers. Regardless of the adopted installation arrangement, the CHP unit should operate as lead boiler to maximise the number of running hours and the boilers provide a spare heating capacity to be used at times of peak load. Packaged CHP units using internal combustion engines are usually designed to operate within a range of water supply and return temperatures (e.g., 80/70  °C ± 3 °C) and any deviation from this range will cause the engine controls to shut down the unit. Hence the building heating system must be correctly sized in terms of heat dissipation, water flow rates, water supply and return temperature to eliminate frequent start and stop cycles. Table 15.2 Type and properties of heat engines used in packaged CHP for buildings Engine

Electrical Electrical Overall Heat grade power range efficiency efficiency (kW) (%) (%)

Gas turbine

>900

30–40

65–90

High temperature (steam)

Internal combustion 20–15 000 engine

30–45

65–90

Low and medium temperature

Micro-gas turbine

20–25

75–85

Low, medium temperature and capable of raising steam

30–200

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 374

4/5/11 9:30:01 AM

Small CHP systems for commercial buildings and institutions Main circulating pump

Isolating Header valve pump

375

Hot water return

Plate heat exchanger Header pipe

CHP unit 1

Hot water supply

CHP unit 2

Back up boiler Fuel supply

Circuit breaker

Building electrical distribution panel To building electricity loads

Connection to grid

15.6 Small-scale CHP heating circuit interface with building services.

Electric services The selection of a small-scale CHP unit for buildings is usually based on supplying the building with as much heat and electrical power as economically viable to satisfy onsite energy loads. In most cases small-scale CHP systems in buildings will be operated in parallel with the public electricity supply network to export or import power as the electricity load profile of the site dictates. Connecting the CHP unit to a distribution network introduces a new source of energy which may increase the ‘fault level’ in the distribution network (i.e., the fault current that may flow when a fault occurs) and render its detection and isolation difficult. The power generated by a small-scale packaged CHP would commonly be three-phase voltage at 415 V or 11 kV and frequency of 50 Hz. Alternatively, a step-up transformer could be used to increase a generator voltage from 415 V and 11 kV to supply electricity to site. Figure 15.7 shows a simplified single line diagram of a CHP electrical connection. Prior to connecting a CHP plant to a low voltage grid, it is the building operator’s responsibility to inform the distribution network operator (DNO) and approval must be obtained before the connection can take place. The power generation at low voltage in a small-scale CHP in commercial and institution buildings means that connection to the grid will be mostly covered

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 375

4/5/11 9:30:02 AM

376

Small and micro combined heat and power (CHP) systems

CHP unit

Building distribution panel (415 V)

Circuit breaker

Building loads

Voltage transformer Connection to grid (11 kV) Circuit breaker Grid

15.7 A simplified single line diagram of a small-scale CHP electrical connection.

under Electricity Association Engineering Recommendation G59/1. The engineering recommendations set out the conditions for the generator to maintain fault duration, voltage and frequency variations within statutory limits. When run in parallel mode, the CHP must be equipped with adequate protection equipment so that the plant is automatically disconnected in the event of an electrical failure or fault. In building sites where backup power is required to maintain critical service operation during an outage of local area power distribution systems, smallscale packaged CHP can operate in island mode by shedding non-critical loads. However, for an islanding mode of operation, the electric generator must be of synchronous type and the CHP plant must also be fitted with grid synchronisation equipment which would increase the cost of the system. Management of exhaust gases Small-scale CHP plant rooms are often located in the basement or rooftop of buildings with strong mounting platform. In common with boilers, adequate volumetric flow rates of ambient air is drawn in from an outdoor intake to the plant room for combustion and ventilation of the plant room. The exhaust system is then used to discharge the combustion products away and at an outlet point from which it cannot be re-circulated into the building ventilation system. Hence often exhaust duct outlets are extended to building

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 376

4/5/11 9:30:02 AM

Small CHP systems for commercial buildings and institutions

377

roof level with due consideration for compatible duct material, insulation and vibration isolation.

15.5

Application of small-scale combined heat and power (CHP) technology in buildings

Commercial and institution buildings are complex structures with challenging operational requirements. Energy performance of a building depends on adequate use of passive energy strategies such as improved building envelope, maximising use of day time lighting, passive cooling and ventilation as well as its interface with the heating and power services. The UK and EU have made strong commitments to reducing GHG emissions from buildings through tougher building construction standards and integration of renewables and energy efficient systems (UK Parliament, 2008). This carrot-and-stick approach rewards building operators by earning higher display energy certificate (DEC) rating, avoiding climate change levy on consumed fuel and generated power, and attracting renewables obligation certificates (ROCs) when biomass fuel is used. Hence, small-scale CHP is currently the technology of choice for the provision of heat and power in new and refurbished commercial buildings and institutions, as it is a more cost effective technology than renewables.

15.5.1 CHP systems in large office buildings The increase in energy consumption in UK commercial buildings is partly due to rapid growth in floor space with offices, for instance, occupying almost twice as much floor space as in 1970 (Scrase, 2000). Despite improvement in energy efficiency of engineering services, this has led to a rapid growth in energy consumption particularly for space air conditioning, IT equipment and light. The good practice energy consumption in offices in the UK ranges from 112 kWh/m2 per year for naturally ventilated buildings, to 348 kWh/m2 per year for prestigious air conditioned buildings (BRE, 1998). The energy consumed in prestigious office buildings is heavily skewed towards the use of electricity to run traditional vapour compression chillers for space air condition. Hence the proportion of energy used as heat compared to electricity varies widely in commercial office buildings. On average the heat-to-electricity demand ratio ranges from 0.54 for prestigious office buildings to 2.4 for naturally ventilated buildings. Hence CHP systems become attractive in office buildings with high heatto-power ratio as demand for space heating is important. Existing packaged CHP using internal combustion gas engines have a heat-to-power ratio in the range of 1 to 2 which can service this type of building adequately. Equally, in prestigious office buildings where substantial demand for electricity

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 377

4/5/11 9:30:02 AM

378

Small and micro combined heat and power (CHP) systems

for space air conditioning, thermally activated absorption chillers using waste heat from the CHP system can practically replace traditional vapour compression chillers to reduce overall energy consumption. It is estimated that deployment of small-scale CHP in commercial office buildings could reduce overall CO2 emissions in this sector by 5–22% (BRE, 1996).

15.5.2 CHP systems in higher education institutions The higher education sector has been at the forefront of innovation in building sustainability as capital allocations by the higher education funding authorities are increasingly tied to the achievement of sustainability targets. The UK university estate is large and diverse which has a built floor area approaching 25.4 million m2 of floor space (Building, 2009). In 2006, the total energy consumption of the higher education institutions (HEI) was about 8.2 GWh which represents roughly 3.5% of the total energy consumption for the UK service sector. The average energy consumption intensity across all the institutions stands approximately at 287 kWh/m2. This average value is, however, above the 162 kWh/m2 recommended in the best practice benchmark of Higher Education Environmental Performance Improvement (HEEPI) (Ward et al., 2008). Energy rating of all new and refurbished buildings in this sector is expected to achieve a BRE Environmental Assessment Method (BREEAM) rating of ‘good’ or ‘excellent’, whereas the long-term target is to work towards zero net carbon emissions which is planned to be introduced in 2019. Electricity and gas are the main forms of energy used in HEI buildings, accounting for 37.6% and 53.5%, respectively. Assuming that gas is used mainly for space heating and hot water, then the ratio of heat to power demand is about 1.4. Therefore, HEI buildings present a favourable electrical-to-total thermal ratio for integration of small CHP systems. Small CHP use in the education sector has had a successful track record in many institutions. Table 15.3 shows a sample of current small-scale CHP schemes in university campuses in the UK (DUKES, 2008). It is shown that gas-fired reciprocating engines are overwhelmingly the technology of choice for this type of building. The following describes two typical gas-fired small-scale CHP schemes used in higher education institutions. Small-scale CHP at the University of Edinburgh Since 2003, the University of Edinburgh has installed three small-scale CHP systems on its campuses. One of the CHP systems was installed in 2005 on George Square campus to provide heat, power and cooling. The £7m CHP system investment comprises one 1.6 MW power output gas-fired

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 378

4/5/11 9:30:02 AM

Small CHP systems for commercial buildings and institutions

379

Table 15.3 Current CHP schemes in university campuses (DUKES, 2008) University site

Engine

Generating Town/city capacity (kWe)

Coventry University, Charles Ward Building (A Block)

IC engine

306

Coventry

Coventry University, Graham Sutherland Building (M Block)

IC engine

211

Coventry

Coventry University, William Morris Building

IC engine

211

Coventry

Coventry University, Armstrong Siddeley Building

IC engine

206

Coventry

Coventry University, Ellen Terry Building

IC engine

306

Coventry

Lancaster University

Gas turbine 1400

Lancaster

University of East Anglia (Plain Campus)

IC engine

3054

Norwich

University of Bath, Scheme 2, Stv

IC engine

189

Bath

University of Bristol, CHP 2

IC engine

1160

Bristol

University of Bristol, CHP 1

IC engine

501

Bristol

University of Dundee, Main CHP Boilerhouse

IC engine

3000

Dundee

University of Edinburgh, King’s Buildings

IC engine

2700

Edinburgh

University of Edinburgh, Pollock Halls of Residence

IC engine

526

Edinburgh

University of Edinburgh, George Square campus

IC engine

1644

Edinburgh

University of Southampton

IC engine

2826

Southampton

University of Surrey

IC engine

1040

Guildford

University of Ulster, Jordanstown Campus

IC engine

1000

Newtownabbey

University of Ulster, Gibbet Hill Energy Centre

IC engine

400

Coventry

University of Warwick, CHP Boilerhouse (CHP 2)

IC engine

4197

Coventry

University of York

IC engine

979

York

University college London, Gower Street Heat & Power Ltd

IC engine

2944

London

GE Jenbacher 612 internal combustion engine; two 6 MW and one 3 MW low-temperature hot water backup boilers; a 600 kW absorption chiller exploiting byproduct heat to cool specialist laboratories in summer; and a 75 m3 thermal store (Somervell, 2006). The CHP engine thermal output is

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 379

4/5/11 9:30:02 AM

380

Small and micro combined heat and power (CHP) systems

1.7 MW, a fuel to electrical energy conversion efficiency of 42% and an overall efficiency of 86%. Although the CHP runs in parallel with the grid, the building energy controls have been set up so that the CHP operates on no-exporting mode by supplying all site power loads while retaining a small inflow from the grid. The current feed-in tariff makes exporting surplus electricity to grid unattractive. In its first full year, the combined CHP and cooling system saved £220 000 in energy costs and 1250 tonnes of CO2 emissions. It is projected that by further optimisation of electricity generation, annual savings of nearer £500 000 could be realised. A simplified schematic layout of the CHP system is shown in Figure 15.8. Small-scale CHP at the University of Dundee Steady growth in electricity consumption at Dundee University has led to identifying that small-scale CHP represented the most realistic mechanism for controlling electrical energy costs and an initial feasibility study was undertaken in 1988. However, it was not until 1995 when an improvement in heat-to-power ratio demand profile, coupled with lower gas prices and rising electricity costs, allowed the CHP project to be launched. The small-scale CHP plant consists of three Jenbacher J320G5-BO5, 4 stroke 20 cylinder spark ignition gas-fired engines rated at 1002 kW power output each. Each engine drives a 3.3 kV alternator at 1500 rpm (DUUSCo, 2009). Heat recovery from exhaust gas, first stage turbo intercooler and jacket water is passed by plate heat exchangers into the returning of existing district heating system water before it enters the boiler plant. When water conditions permit, all the available heat is utilised in the pre-heating function, otherwise some of the jacket heat is used to heat nearby buildings, and any surplus is dissipated to atmosphere by heat damping coolers. Electrical power from the CHP plant is then fed into the university’s 11 kV distribution network via a 4 MVA voltage step up transformer. The CHP plant is fully computerised and is set to operate in load-following mode with output level set to minimise the electricity importation rate. Each engine generator unit is fitted with a comprehensive monitoring system which is connected via modem link with the system suppliers who are contracted for a long-term maintenance agreement. As the boiler plant forms part of an academic teaching building, noise and vibration generated by the engines become an important issue to be resolved. Hence an acoustic enclosure and vibration isolation equipment were fitted to prevent transmission through the building structure. The noise level emanating from air fans of the outside heat damping units also needed to be kept below 37 dB(A) as these were in proximity to adjacent residential property. Hence the cooling units were fitted with multiple slow running

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 380

4/5/11 9:30:02 AM

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 381

4/5/11 9:30:03 AM

CHP engine

Glycol circuit

85 °C

Building loads

415 V

Heat dump heat exchanger

75 °C

95 °C

Plate Heat exchanger

60 °C

Thermal store 75 m3

Circulating pumps

Backup boiler 3 3 MW

Backup boiler 2 6 MW

Backup boiler 1 6 MW

60 °C

Hot water 80 °C supply

15.8 A simplified schematic layout of the CHP and cooling (trigeneration) system at Edinburgh University.

Grid connection

11 kV local grid

Gas supply

Absorption chiller

80 °C

Chilled water supply

382

Small and micro combined heat and power (CHP) systems

fans each having two silencers, and electronic speed control ensures that the fans start and speed up progressively to avoid any sudden changes in the perceived sound level.

15.5.3 CHP systems in health care buildings Health care buildings are the most energy intensive buildings in the commercial and institution sector as they are usually occupied 24 hours per day, all year round and they require a careful control of the internal climate. In the UK, existing health care buildings are often old and operating outside the energy performance indicator of 55–65 GJ/1003 of heated volume. In 2008, energy consumption in National Health Service (NHS) England buildings amounted to about 9465 GWh, of which 45% was electricity and causing around four million tonnes of CO2 to be discharged to the atmosphere (NHS, 2009a,b). The health care buildings’ typical energy demand for thermal energy for space heating, hot water and electrical power is often required simultaneously most of the time with an average load factor above 80%. Therefore, hospitals buildings represent an ideal case for the application of CHP technologies. Table 15.4 shows a sample of existing small-scale CHP scheme installations of less than 2 MW power output in hospitals across the UK (DUKES, 2008). All the listed CHP installations are gas-fired internal combustion engines. An example of a typical gas-fired small-scale CHP scheme has been installed in Royal Shrewsbury hospital. In 2004, an energy audit of the hospital commissioned by Shrewsbury and Telford NHS Trust showed the primary energy consumption stood at around 100 GJ /100m3, nearly double the NHS target indicator. The study identified small-scale CHP as the best way of improving the hospital’s energy efficiency (EnerG, 2007). In 2007, a small-scale CHP system using a gas-fired Caterpillar internal combustion engine with a rated power output of 1150 kW was installed. The CHP system provides nearly 1.7 MW of thermal energy in the form of both medium temperature hot water and steam at 120 °C. This is supplemented by a dual-fired backup boiler. The unit is also connected to a 700 kW absorption chiller, installed as part of the CHP scheme. The scheme was delivered through a fully funded performance contract with annual savings of £780k guaranteed for 15 years (EnerG, 2007). Similarly, a small-scale CHP has been installed in a purpose-built energy centre at Birmingham Heartlands, a major general hospital managed by Heart of England NHS Foundation Trust, to provide heat, power and cooling. The CHP project cost was £5 million and financed by EnerG Combined Power Ltd through Public Private Partnership agreement with a £403  000 grant from the Carbon Trust under the Community Energy Programme. The project is estimated to save £688 000 a year and cut emissions of CO2 by

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 382

4/5/11 9:30:03 AM

Small CHP systems for commercial buildings and institutions

383

Table 15.4 Small-scale CHP installations in hospitals (DUKES, 2008) CHP Site name

Engine

Generating Town/City capacity (kWe)

Cannock Chase Hospital

IC engine 303

Cannock

Royal Devon and Exeter Hospital

IC engine 1016

Exeter

Stoke Mandeville Hospital

IC engine 979

Aylesbury

Leighton Hospital

IC engine 225

Crewe

Leighton Hospital

IC engine 600

Crewe

Dewsbury District Hospital

IC engine 480

Dewsbury

Pontefract General Infirmary

IC engine 409

Pontefract

Milton Keynes General Hospital (Phase 2)

IC engine 152

Milton Keynes

Milton Keynes General Hospital (Phase 1)

IC engine 185

Milton Keynes

Biggart Hospital

IC engine 54

Prestwick

Guildford Nuffield Hospital

IC engine 165

Guildford

Sussex Nuffield Hospital

IC engine 165

Brighton

Southmead Hospital

IC engine 979

Bristol

University Hospital of North Tees

IC engine 1550

Stockton on Tees

Gwynedd Hospital

IC engine 330

Bangor

Northampton General Hospital (CHP 2)

IC engine 211

Northampton

Northampton General Hospital (CHP 1)

IC engine 409

Northampton

Northampton General Hospital (CHP 3)

Gas turbine

Northampton

105

Causeway Hospital

IC engine 900

Coleraine

Rochdale Infirmary

IC engine 300

Rochdale

Poole Hospital (CHP 2)

IC engine 210

Poole

Poole Hospital (CHP 1)

IC engine 400

Poole

Royal Manchester Children’s Hospital

IC engine 145

Manchester

Salisbury District Hospital

IC engine 400

Salisbury

Western General Hospital, Lothian Universities NHS Trust

IC engine 1003

Edinburgh

The Princess Royal University Hospital

IC engine 995

Orpington

St Anthonys Hospital

IC engine 168

North Cheam

Norfolk and Norwich University Hospital

IC engine 979

Norwich

United Bristol Healthcare Trust

IC engine 979

Bristol

Glenfield Hospital

IC engine 570

Leicester

Dorchester County Hospital

IC engine 370

Dorchester

Lincoln County Hospital

IC engine 1413

Lincoln

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 383

4/5/11 9:30:03 AM

384

Small and micro combined heat and power (CHP) systems

Table 15.4 Continued CHP Site name

Engine

Generating Town/City capacity (kWe)

Bedford Hospital

IC engine 300

Bedford

Montagu Hospital Mexborough

IC engine 110

Mexborough

Lagan Valley Hospital,

IC engine 306

Lisburn

Forth Park Maternity Hospital

IC engine 95

Kirkcaldy

Royal Infirmary of Edinburgh

IC engine 1029

Edinburgh

1627 tonnes per year (EnerG, 2007). The CHP systems has a 1165 kW power output capacity MTU gas-fired engine with a heat output of 1272 kW and is also equipped with steam raising capability, backup boilers and a 300 kW absorption cooling system which operates off the gas engine waste heat and serving to meet the air conditioning demand during warmer months of the year.

15.5.4 CHP systems in leisure and recreation buildings Leisure, recreation and accommodation buildings have a steady demand for heat for space heating, catering and showering for up to 24 hours a day, making CHP very cost effective. Where swimming pool facilities are installed, the heat and electricity demand increase substantially specifically for heating water and ventilating the pool hall, and the case for CHP becomes even more compelling (Carbon Trust, 2008). Actual energy consumption in leisure and recreation buildings with pool facilities could be as high as 237 kWh/m2 and 1336 kWh/m2 per year for heat and electricity, respectively, compared to good practice of 152 kWh/m2 and 573 kWh/m2, respectively (ETSU, 2001). The leisure and recreation industry has been an excellent example for application of CHP to lower energy costs. Current small-scale CHP systems in the UK’s leisure and recreation buildings contribute more than 20 MW to the total of UK electricity generation capacity. With a few exceptions, most of the plants use gas-fired internal combustion engines with power output ranging from as low as 40 kW to over 500 kW (DUKEs, 2008). For instance, David Lloyd Leisure has more than 46 small-scale CHP plants with another dozen units planned for the next few years in new buildings or as retrofit to existing building services. It is projected that electrical and heat output of 73 GWh and 115 GWh a year, respectively, will be supplied with energy bill savings of £0.9 million and reduction in carbon emissions of 16 000 tonnes a year (EnerG, 2008).

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 384

4/5/11 9:30:03 AM

Small CHP systems for commercial buildings and institutions

385

15.5.5 CHP systems in supermarkets and hypermarkets A typical supermarket in the UK uses about 1200 kWh of electricity and 260 kWh of gas per square meter of floor sales area per year compared to the industry benchmark of 915 kWh/m2 for electricity and 200 kWh/m2 for fossil fuels (CIBSE, 2004). Figure 15.9 presents a breakdown of a typical UK supermarket energy consumption. It is shown that approximately 75% of the energy is used for powering vapour compression refrigerators/freezers and HVAC equipment, whereas energy consumption for hot water is very modest. Hence, implementation of CHP in the classical way would result in underutilisation and limited primary energy savings. To increase the utilisation time, excess heat generated by the CHP plant would be used to drive an absorption chiller to displace grid-electricity operated food cooling equipments. The application of an integrated CHP/absorption scheme (trigeneration) in the supermarket can increase the attractiveness of small-scale CHP by utilising available waste heat to shift cooling from an electricity load to a thermal load in a very cost effective way (Zogg and Brodrick, 2005). The use of integrated small-scale CHP with heat driven absorption chiller is also an environmentally benign way that would provide heat, power and cooling/refrigeration at a substantial reduction in CO2 emissions. This could be realised in addition to reducing overall energy consumption, by using alternative refrigerants to hydrofluorcarbons (HFCs) with zero or low global warming potential, such as water and ammonia, with further benefit Others

tw

at

er

3%

Ho

Bakery 11% 3%

Lighting 11%

Refrigeration 49%

hvac 23%

15.9 Breakdown of energy consumption in UK supermarkets.

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 385

4/5/11 9:30:03 AM

386

Small and micro combined heat and power (CHP) systems

of gaining additional BREEAM points. Table 15.5 shows an example of current small-scale CHP installation in supermarkets in the UK (DUKES, 2008).

15.6

Performance analysis and optimisation

Energy consumption patterns in commercial and institution buildings are complex and depend on type, size and activity carried out in the building. Hence a successful implementation of a small-scale CHP scheme requires conducting a detailed feasibility study of the building’s energy consumption patterns. This usually identifies accurately the building’s demand for all heat energy grades, electrical power and cooling. Then a daily and seasonal load profile for each form of energy demand must be established to inform selection of type and energy capacity of the CHP engine. One of the fundamental requirements in considering CHP for buildings is to have a steady and concurrent demand for electrical power and heat/ cooling all year round. This usually consists in running the CHP plant as ‘lead boiler’ to maximise the number of running hours. As a rough guide, a viable small-scale CHP scheme using internal combustion engines requires to run for about 14–16 hours a day and 4500 hours a year (CIBSE, 2007). This is usually achieved by selecting a CHP plant engine with a heat-to-power ratio that matches as closely as practical that of the building. While CHP systems have a higher capital and maintenance cost than using heat-only boilers and grid, it is more efficient than the separate supply of heat and power and savings in energy cost would usually suffice to realise payback periods of 3–5 years. The financial attractiveness of a CHP installation is further enhanced when a complete replacement of existing boilers is considered as its cost is partly offset against the displaced equipments. Most small-scale CHP systems used in buildings are gas-fired plants and purchased fuel constitutes the main running cost of the plant. Small-scale Table 15.5 Small-scale CHP installations in supermarkets (DUKES, 2008) CHP Site name

Engine

Generating capacity (kWe)

Town/City

Sainsbury’s – Cromwell Road CHP 1 Sainsbury’s – Richmond Sainsbury’s – Greenwich Sainsbury’s – Brighton Sainsbury’s – Romford Sainsbury’s – Burnley Tesco – Carmarthen Tesco – Gloucester Tesco – Swansea

IC IC IC IC IC IC IC IC IC

422 211 211 321 211 211 238 238 237

London Richmond Greenwich Brighton Romford Burnley Carmarthen Brockworth Swansea

engine engine engine engine engine engine engine engine engine

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 386

4/5/11 9:30:03 AM

Small CHP systems for commercial buildings and institutions

387

CHP in buildings could use between 700 MWh and 25 GWh a year of gas for a 30 kW micro-gas turbine of 20% efficiency and a 2 MW internal combustion gas engine of 35% efficiency, respectively. The unit price of gas fuel and maybe of electricity import/export will be determined by individually negotiated contracts with an energy supply utility and will be greatly influenced by the pattern and volume of gas and power consumed (ETSU, 1983). The economic benefit of CHP is hence very sensitive to variation in displaced electricity and purchased fuel prices, the difference of which is referred to as ‘spark spread’. A high ‘spark spread’ encourages market uptake and installation of new plants as better investment yields can be achieved. Small-scale packaged CHP systems for buildings are designed to operate continuously and supply as high a proportion of the building’s energy demand as possible. In general, three main strategies could be considered.

15.6.1 Base heat load supply operation mode In this mode of operation, the CHP is sized to supply base heat demand and a backup boiler to meet heat demand peaks. This allows the engine to run continuously and at full-rated power output without the need for heat damping or thermal storage facilities. However, unless a site has a flat thermal load profile (i.e., a small peak-trough difference), adopting such a strategy will not yield the full economic and environmental benefits of using CHP. It is also most likely that only a small fraction of the site’s electricity demand will be met by the CHP and the grid will be used to supply the deficit in power demand. This strategy is, however, susceptible to increases or decreases in base load demand, making CHP uneconomical.

15.6.2 Base electricity load supply operation mode Generally, it is desirable that a CHP plant is sized to supply the building’s base load electricity demand. In this way, all electricity generated is used onsite and peaks in power demand will be provided from the grid. This, however, requires a careful assessment as the CHP plant thermal output will be higher than that of the building’s base heat load demand. Hence to be economically feasible, the CHP engine should be selected with as high a thermal efficiency as possible and means heat damping to atmosphere becomes necessary.

15.6.3 Cost-led operation strategy In this strategy various types of CHP plants will be considered. The economic evaluation centres on maximum accrued savings on hourly operation basis

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 387

4/5/11 9:30:03 AM

388

Small and micro combined heat and power (CHP) systems

and taking into account the amortisation of the plant investment capital and running cost. one of the most probable ChP selection scenarios in this case is to size the ChP plant above the building’s heat base load demand and operating at full load for a maximum number of hours per day. This, however, will require provision of short-time thermal storage, backup boilers, heat damping, engine modulation and import and export of power to the grid to smooth out as much as practically possible the heat and power demand profiles. It is important the extra costs resulting from this additional equipment are accounted for at the evaluation stage. other operating possibilities in this cost driven strategy could also include running the ChP plant for coverage of peak load power demands. This is because the cost of imported electricity varies through the day and can be substantially higher during winter months’ peak hours. on the other hand, the cost of electricity generated by the ChP plant would remain unchanged as gas tariffs remain constant through the day (CIBSE, 1999). hence the marginal cost saving of ChP generated electricity during peak times may justify such a strategy.

15.6.4 CHP system performance indicators The high energy performance of a ChP system over traditional heat-only boiler and power from the grid is derived from the ability of the ChP plant to recover a large part of the attendant heat from the conversion of fuel to electrical energy that is wasted in centralised power station. hence the energy performance of a CHP plant is characterised by a thermal efficiency, he, which indicates the amount of fuel converted to electricity, and the overall efficiency, hChP, which is a measure of the primary energy savings that can be obtained compared to separate generation of heat and power. These can be expressed as follows:

he =

Pe Qf

hChP =

15.1 Pe + Qu Qf

15.2

where, Pe is the electrical output, Qu is the useable heat energy recovered and Qf is the total fuel energy input based on gross calorific value of the fuel. A further performance indicator that is specific to CHP systems is the heat-to-power ratio which is a useful property when selecting a ChP plant to match a specific building heat and power load. This is given as: hPR =

Qu Pe

15.3

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 388

4/5/11 9:30:04 AM

Small CHP systems for commercial buildings and institutions

389

Therefore the amount of primary energy that can be saved using a ChP plant compared to supplying heat and power separately from a boiler and grid, can be evaluated as (Bruno 2005): Ê ˆ Es = Pe Á 1 + hPR – 1 ˜ hB he ¯ ËhG

15.4

where hG and hB are the average thermal efficiency of a centralised power station and boiler efficiency, respectively. finally, a simple estimate of the energy cost savings per unit of electrical power supplied by ChP plant and assuming that all heat recovered is used onsite, may be given as follows: Ê ˆ Cs = Cp + Cm + Á hPR – 1 ˜ Cg he ¯ Ë hB

15.5

where Cp, Cm and Cg are the unit cost of purchased electricity from grid, maintenance cost per unit of power generated from the ChP and unit cost of fuel, respectively. It is assumed, however, that the same type of fuel is used in the boiler and ChP.

15.7

Merits and limitations of small-scale combined heat and power (CHP)

Small-scale ChP has the potential to impact positively on the main pillars of the uk’s energy policy: security of supplies, diversity of energy sources, sustainability and affordability. It is a proven technology that can cut primary energy consumption and Co2 emissions from the building sector by as much as 30% compared to providing heat and power from a heat-only plant and grid electricity, respectively (Carbon Trust, 2004). Since the introduction of DEC, uk organisations are increasingly looking to improve their carbon footprint and on-site small-scale ChP is being promoted as the most cost effective available technology that can help achieve better EPC ratings. Small-scale ChP in buildings is a form of distributed generation that is often configured to operate in parallel with the electricity grid to allow power flow to and from the building. In the event of grid failure, it can thus operate as an emergency backup generator to maintain efficient operation of the building’s critical services. Equally important, small-scale ChP can improve stability of the local electricity network and benefit in the process large electricity utilities to reduce standby electric power generation capacity necessary to meet peak load demands. Good quality CHP receives further government financial support through Climate Change Levy (CCL) exemption on fuels and generated electricity, business rates exemption for ChP equipment, and earning renewable energy

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 389

4/5/11 9:30:04 AM

390

Small and micro combined heat and power (CHP) systems

certificates (ROCs) when used in conjunction with renewable fuels. It can also impact positively on an organisation’s public relations and marketing profile for being perceived as technologically innovative and environmentally friendly. Despite all the technical and environmental credentials of small-scale CHP, market penetration is still limited. One of the main factors is high initial capital outlay compared to heat-only plants, making it difficult for commercial or public organisations to allocate funds for non-core business. Until recently, commercial and public organisations have made little commitment to consider long-term coordinated energy management strategies which impacted negatively on the small-scale CHP market. Furthermore, because suitability of small-scale CHP is site-specific, it makes the economic viability of a scheme directly linked to the site having a consistent base load demand for power and heat/cooling. This often presents a problem in the summer months when the need for heating is reduced, whilst the electricity demand remains fairly constant. Hence a detailed energy audit for any site will have to be carried to size the CHP for optimum performance, a process requiring a long lead time. Adding to this, the risk generated by unpredictable variation in prices of fuel, electricity, and high maintenance costs mean that initial estimates of a payback period cannot be guaranteed. This is exacerbated further by the competitive nature of the UK electricity market as small-scale CHP generators cannot compete on electricity prices. This has had the effect that small-scale systems being undersized so as to limit exporting electricity to the grid which may undermine the scheme’s viability. Small-scale CHP are usually installed in building basements or roof tops and have to comply with existing noise, vibration and air pollution regulations which entails installing acoustic enclosures, silencers and vibration absorbers to avoid transmission of vibrations through the building structure. Those plants located close or inside urban areas can impact negatively on local air quality. Internal combustion engines are the most common for CHP in buildings and can have high emissions of nitrogen oxides and unburned hydrocarbons which may be perceived as an additional risk about the technology and its benefits.

15.8

Future trends

Favourable energy prices (large spark spread) during the 1990s allowed growth in electricity generating capacity of CHP to increase by over 90% by 2001 (DUKES, 2008). Since then energy market conditions have changed markedly and with the reforms of the UK energy sector, through the introduction of the New Electricity Trading Arrangements (NETA), and the increase in gas prices led to a deterioration in ‘spark spread’. Thus, there has been a decline

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 390

4/5/11 9:30:04 AM

Small CHP systems for commercial buildings and institutions

391

in investment in new CHP plants in general. As investment opportunities, small-scale CHP projects in buildings are also considered small business ventures with associated high initial costs and high risk, rendering them not very attractive for long-term investors. Furthermore, the competitive UK electricity market meant small-scale CHP operators (under 1 MW) have only been offered very low prices to export surplus electricity. This has had the effect of undersizing CHP plant designs to supply the building electricity demand instead of meeting all the building heat load and exporting excess power, undermining the full economic and environmental potential of smallscale CHP (Hinnells, 2008). The upward trend in energy consumption in commercial buildings and institutions is set to continue in the future because of expansion of built area and associated energy needs. The energy policies of the UK and other EU countries have put in place mechanisms for promoting energy efficiency in buildings, developing new technologies for energy generation, managing energy demand, and raising social awareness for a sustainable long-term economic growth (Pérez-Lombard et al., 2008). Small-scale CHP forms part of this energy policy with a huge potential to deliver energy savings and thus cost and emission reductions through to 2050 targets. This is reflected in a number of studies with a wide range of projection estimates. For instance, it has been estimated that with favourable energy market conditions, power output capacity of small-scale CHP in the commercial buildings and institutions sector could increase at a rate of 700 MW a year by 2020, even with no further government support. An additional financial incentive of £10/MWh could, however, stimulate this growth to 1.76 GW a year (BEER, 2005, Defra, 2004). It is intended that introduction of further framework policies such as the European EPBD, UK Climate Change Act, Building regulations of 2011, Carbon Reduction Commitment (CRC) should impact positively over time on market uptake of small-scale CHP and other renewable technologies. The CRC will require, for example, organisations that consume more than 6000 MWh of electricity a year to participate in the scheme from April 2010. It is estimated that around 5000 public and private sector organisations ranging from retail, leisure and manufacturing companies through to local authorities, universities and NHS Trusts will have to buy and surrender carbon allowances to cover their annual emissions from 2011. The UK has one of the most competitive markets for electricity generation and supply but at the same time no effective mechanisms have yet been put in place to develop a heat supply market (Toke and Fragaki, 2008). Recently, there has been some positive signs that suggest that the situation may change. This mainly stems from the acknowledgement that implementation of the Climate Change Act would require a new vision towards provision of heat for space heating and hot water in buildings, which accounts for approximately

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 391

4/5/11 9:30:05 AM

392

Small and micro combined heat and power (CHP) systems

50% of all energy used in buildings. This is led by the publication of the UK government of a consultation document on the introduction of a Renewable Heat Incentive (RHI) scheme with the aim to be implemented in 2011. Under the proposed scheme, financial support will be offered to a range of technologies including CHP of all scales that is fuelled by renewable fuels such as biomass and biogas (DECC, 2010). Such a scheme should give necessary financial impetus to advance market penetration of existing gasfired internal combustion engine CHP and develop new technologies such as organic Rankine cycles that run on biomass fuels. The steady growth in energy consumption and the pressing need to cut the amount of CO2 discharges to atmosphere from commercial buildings and institutions have underpinned the energy efficiency strategy for this sector, with energy efficient technologies forming one of the pillars of this strategy. Among the proposed solutions, good quality small-scale CHP is the tried and tested technology which is readily available as packaged units of a wide range of heat and power ratings. Finally, installation of small-scale CHP in buildings can enhance energy certification, CRC, achieve building regulations standards and with the introduction of further support such as RHI, small-scale CHP will achieve the shortest payback period, making it the first consideration for investors and owners of new and refurbished buildings in the future.

15.9

Sources of further information and advice

15.9.1 Institutions ∑ ∑ ∑ ∑ ∑ ∑ ∑

The Carbon Trust: www.carbontrust.co.uk CIBSE CHP group: www.cibse.org Energy Saving Trust: www.est.co.uk Combined Heat and Power Association: www.chpa.co.uk EU Energy Performance for Buildings Directive: http://europa.eu Cogen Europe: http://www.cogeneurope.eu Department of Energy and Climate Change (DECC), http://www.decc. gov.uk/ ∑ World Alliance for Decentralised Energy (WADE) ∑ US Clean Heat & Power Association (USCHPA): http://www.uschpa. org/ ∑ US Department of Energy: http://www.energy.gov

15.9.2 Small-scale CHP installer/manufacturers ∑

GE Jenbacher: www.jenbacher.com – Internal combustion engines manufacture

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 392

4/5/11 9:30:05 AM

Small CHP systems for commercial buildings and institutions

393

∑ ∑ ∑ ∑ ∑ ∑ ∑

Capstone: www.capstoneturbine.com – Micro-gas turbine manufacture Cogenco: www.cogenco.co.uk – CHP specialist Ener.G Combined Power Ltd: www.enerG.co.uk – CHP specialist ec Power: www.ecpower.co.uk – CHP specialist Caterpillar: www.cat.com – Internal combustion engines manufacture Dalkia: www.dalkia.co.uk – Internal combustion engines manufacture Wärtsilä Corporation: www.wartsila.com – Internal combustion engines manufacture ∑ Baxi-SenerTec UK: www.baxi-senertec.co.uk – Mini and micro-CHP provider

15.10 References Baird G, Donn M R, Pool F, Brander W, Seong Aun C (1984), Energy Performance of Buildings. Boca Raton FL: CRC Press, pp. 25–51. BERR (2005), Future Energy Solution – Renewable Heat and Heat from CHP plants – study and analysis report, ED02137 Published version. Available from: www.berr. gov.uk/files/file21141.pdf (accessed March 2010). Bruno J C (2005), Technical Module on Combined Heat and Power, The European Green Building Programme. Available from: www.eu-greenbuilding.org (accessed, March 2009). Building (2009), Cost model: Universities. Available from: www.building.co.uk, June 2009 edition (accessed March 2010). Building Research Establishment (BRE) (1996), Potential carbon emission savings from Combined Heat and Power in buildings. IP4/96, London. Building Research Establishment (BRE) (1998), Non-Domestic Building Energy Fact File, Global Atmosphere Division (GAD) of the Department of the Environment, Transport and the Regions, London. Carbon Trust, Action Energy Programme (2004), Good Practice Guide 388: Combined Heat and Power for Buildings, London. Carbon Trust (2008), Swimming pools: a deeper look at energy efficiency: In depth technology, Guide CTG009. Available from: www.carbontrust.co.uk (accessed February 2010). Chartered Institution of Building Services Engineers (CIBSE) (1999), Small scale CHP for buildings, CIBSE Application Manual: AM12, London. Chartered Institution of Building Services Engineers (CIBSE) (2004), Guide F: Energy efficiency in buildings, London. Chartered Institution of Building Services Engineers (CIBSE) (2007), Carbon saving with CHP, CPD supplement magazine, June. DECC (2010), Renewable Heat Incentive (RHI) – Consultation on the proposed RHI financial support scheme. Available from: www.decc.gov.uk (accessed March 2010). Defra (2004), Cambridge Econometrics Modelling Good Quality Combined Heat and Power Capacity to 2010: Revised Projections, A final report submitted to Department of Environment, Food and Rural Affairs (DEFRA), London. Digest of United Kingdom Energy Statistics (DUKES) (2008), Department for Business, Enterprise and Regulatory Reform (BERR), London. Dundee University Utility Supply Co (DUUSCo) (2009), University of Dundee CHP,

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 393

4/5/11 9:30:05 AM

394

Small and micro combined heat and power (CHP) systems

available from: http://www.dundee.ac.uk/duusco/duusco~1.htm (accessed March 2010). Elsadig A K (2005), Energy Efficiency in Commercial Buildings, Master of Science dissertation, Department of Mechanical Engineering, University of Strathclyde, UK. EnerG Combined Power Ltd (2007), Case study: Hospitals. Available from: www.enerG. co.uk (accessed November 2009). EnerG Combined Power Ltd (2008), Next Generation looks to CHP for cost and carbon savings. Available from: www.enerG.co.uk (accessed, January 2010). Energy Technology Support Unit (ETSU) – Energy Efficiency Office (1983), Good Practice Guide 1: Guidance notes fro the implementation of small scale packaged CHP, Department of the Environment, London. Energy Technology Support Unit (ETSU) – Energy Efficiency Office (2001), Good Practice Guide 78: Energy use in sports and recreation buildings, Energy Consumption. Department of the Environment, London. European Parliament and the Council of the EU Union (2002), Directive 2002/91/EC, Energy Performance of Buildings. Hinnells M (2008), Combined heat and power in industry and buildings, Energy Policy Journal, 36, 4522–4526. NHS Sustainable Development Unit (2009a), NHS Carbon Reduction Strategy for England. Available from: Www.Sdu.Nhs.Uk (accessed January 2010). NHS Sustainable Development Unit (2009b), NHS England Carbon Emissions: Carbon Footprint Modelling to 2020, Available from: www.Sdu.Nhs.Uk (accessed January 2010). Pérez-Lombard L, Ortiz J, Pout C (2008), A review on buildings energy consumption information, Energy and Buildings, 40, 394–398. Scrase I (2000), White Collar CO2: Energy consumption in the service sector, The Association for the Conservation of Energy, London. Somervell D (2006), University of Edinburgh’s sustainable future: CHP key to low-carbon strategy. Available from: www.ed.ac.uk (accessed 14 July 2009). Toke D, Fragaki A (2008), Do liberalised electricity markets help or hinder CHP and district heating? The case of the UK, Energy Policy Journal, 36, 1448–1456. UK Parliament (2008), Climate Change Bill. Available from: www.parliament.uk (accessed 20 September 2009). Ward I, Ogbonna A, Altan H (2008), Sector review of UK higher education energy consumption, Energy Policy Journal, 36, 2939–2949. Zogg R, Brodrick J (2005), Using CHP systems in commercial buildings, American Society of Heating, Refrigerating and Air-Conditioning Engineers Journal, 33–34.

© Woodhead Publishing Limited, 2011

CHP-Beith-15.indd 394

4/5/11 9:30:05 AM

16

Small and micro combined heat and power (CHP) systems for the food and beverage processing industries

P. S. V a r b a n o v and J. J. K l e m e š, University of Pannonia, Hungary

Abstract: This chapter provides an overview of CHP energy technologies for the food and beverage processing industries on the small and micro scale. The chapter starts by providing the right context in terms of key energy demand properties of food processing sites as well as with techniques of energy integration. Next it proceeds to describe the key small- and microCHP technologies, starting from the established ones and continuing further with experimental and developing technologies represented by fuel cells. The chapter concludes with a summary of the future trends and information for further reading. Key words: small- and micro-CHP systems, food and beverage processing industry, energy efficiency and minimisation.

16.1

Introduction

The world population is growing fast and this imposes a number of global challenges, mainly the simultaneous increase in the demands for food, water, and waste management. Resource demands are not only rising, but also feature strong interactions competing for energy. Food processing refers to the activities converting raw food materials to final consumable products. Foods are processed for the purpose of enhancing quality, taste, nutritional value, as well as shelf life. Processing methods include cooking, preserving, packaging, storage and distribution. One of the main features of the food industry is that, besides the target products, certain amounts of waste are released, most of them of organic composition. Therefore, the food industry has the unique property to simultaneously impose energy demands and potentially offer a number of streams from which energy can be generated. Therefore efficient supply and use of energy in the food industry, including efficient combinations of waste management and energy generation, are of paramount importance to give the food industry a competitive edge. A very important aspect of energy supply for the food industry is the need for local generation, especially for smaller-scale plants, thus avoiding potentially costly energy 395 © Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 395

4/5/11 9:32:43 AM

396

Small and micro combined heat and power (CHP) systems

carrier transport operations, and resulting in a ‘distributed generation’ architecture. The objectives of this chapter are to first address the basic concepts and issues concerning CHP systems in the food industry, followed by a more in-depth coverage of the issues of energy supply and integration of smallerscale CHP technologies.

16.2

Food processing and energy requirements – examples for specific food and drink industries

Many food companies are significant energy users due to the need for heating and cooling of their products during manufacture and storage. The energy for heating is normally consumed as thermal energy from the combustion of fossil fuels to generate steam and hot water. Cooling and refrigeration generally consume electricity, which also powers other equipment, lighting, ventilation, etc. Within the dairy industry, the split of thermal energy consumption is typically around 80% for heating and 20% for cooling (Elkin and Stevens, 2008). There are various processes needed for food and beverage production. The following sections present a brief overview of some of them from the point of view of energy demand and supply. The food sector, including slaughterhouses, contributes to the energy consumption by industry. For example, in Flanders, the food sector consumes about 2% of the total industrial energy (Genné and Derden, 2008). In the UK the food industry uses about 126 TWh/y which is equivalent to 14% of the energy consumed by UK businesses (Elkin and Stevens, 2008; DEFRA, 2006).

16.2.1 Sugar production An extended discussion of energy and water management in the sugar industry was provided by Urbaniec and Klemeš (2008). The authors point out that in recent decades, the world output of sugar has tended to exceed the demand, causing a decrease in investments in new sugar factories. For economic and environmental reasons, however, there is a constant need for reconstruction of sugar factories – mostly to increase the production rate and reduce energy waste and emissions. In order to obtain the main product, that is, crystalline sugar with a negligible water content, water must be separated thermally with minimum energy expenditure determined by its specific heat of evaporation of 2258 kJ or 627 kWh per 1 kg water at atmospheric pressure. A factory example has been provided with processing capacity of 4800 t beet per day. The specific fuel consumption is about 3.40  kg oil equivalent (at 41 MJ/kg) per 100  kg beet, resulting in a total energy consumption of 2278 MW for the total site.

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 396

4/5/11 9:32:43 AM

Small & micro CHP systems for food and beverage processing

397

16.2.2 Meat processing At most slaughterhouses, the largest part of electricity demand (50–65%) comes from refrigeration operations. The consumption of energy in slaughterhouses is also closely connected to the use of hot water, whose temperature is usually activity dependent. Various process steps require the availability of water at lower or elevated temperatures (4–7 °C, 40 °C, 55 °C and 90 °C) (Genné and Derden, 2008). Refrigerated areas include chills, freezers and cold stores, which entail electricity consumption even during non-production periods. The slaughtering industry can be subcategorised on the basis of the type of animals that are slaughtered: large animals (e.g., pigs, cattle, horses and sheep) and small livestock (e.g., poultry). Most slaughterhouses are smalland medium-sized enterprises (SMEs) and have a labour-intensive character. The electricity consumption varies widely and is very plant specific, e.g. 7–60 kWh/pig, 20–310 kWh/cattle, 0.4–1.2 kWh/piece of poultry. The varying use of external energy (fuel) to heat water (20–90%) and pig scalding and singeing (50–75%) are other key issues.

16.2.3 Processing of agricultural crops Small-to medium-scale plants for production of refined sunflower oil (Klemeš et al., 1998) feature two types of heating utilities, steam/dowtherm and steam/water totalling about 1 MW of use, and two cooling utilities, cooling water and ice water totalling about 0.8 MW. The same source reports similar utility demands also for sites extracting raw sunflower oil, where the added complication is the operation using a volatile and flammable extraction agent. An example from crystalline glucose production (Klemeš et al., 1998) indicates utility demands after heat integration of about 2.5–3 MW for steam and 0.7 MW for cooling water. Operating plants in this process widely use vapours bleeding from a multiple-stage evaporation system for concentrating the water–glucose solution. A further example is given by a case study of a Whisky Distillery (Kemp, 2007; CADDET, 1994). This reports a set of energy demands split among power (12 MW), heating (40–50 MW) and cooling (15–20 MW).

16.3

Heat and power integration of food total sites

Energy recovery is not only an energy supply or CHP issue, it is also important for two reasons. First, applying energy recovery improves the performance of the concerned processes, reducing the utility requirements and thus making the task of energy resource supply easier. Second, advanced CHP

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 397

4/5/11 9:32:43 AM

398

Small and micro combined heat and power (CHP) systems

technologies for supplying utilities make extensive use of energy recovery and heat integration (Klemeš et al., 2010). As a whole, food processing is characterised by relatively low temperatures of process streams (rarely above 120–140 °C), by small numbers of hot streams (some with varying final temperatures, for example secondary condensate of multiple-stage evaporation systems), by low boiling point elevation of food solutions, by intensive deposition of scale in evaporator and recovery systems, and by seasonal performance. The application of heat integration is hindered by some specific technological and design requirements, for example direct steam heating, difficulties in cleaning heat exchanger surfaces and high utility temperatures.

16.3.1 Basic heat integration In the late 1970s the push for better industrial energy efficiency led to the development by Linnhoff et al. (1982) of the basis of pinch technology – now considered the cornerstone of heat integration. The discovery of the heat recovery pinch was a critical step in the development of heat exchanger network (HEN) synthesis. The methodology has been further developed and the most recent state-of-the-art has been presented elsewhere (Klemeš et al., 2010). The main idea behind the formulated HEN design procedure was to obtain, prior to the core design steps, guidelines and targets for HEN performance. This procedure is based on established thermodynamic theory. The hot and cold streams for the process under consideration are combined to yield a hot composite curve representing, collectively, the process heat sources (the hot streams); and a cold composite curve representing the process heat sinks (the cold streams). For a specified minimum allowed temperature difference DTmin, the two curves are combined in one plot (Fig. 16.1) providing a clear thermodynamic view of the heat recovery problem. The overlap between the two composite curves represents the heat recovery target. The overlap projection on the heat exchange axis represents the maximum amount of process heat being internally recovered. The vertical projection of the overlap indicates the temperature range where the maximum heat recovery should take place. The targets for external (utility) heating and cooling are represented by the non-overlapping segments of the cold and hot composite curves. The heat recovery targets are further supplemented by targets for heat transfer area, capital cost and total cost. As a further step, the pinch design method (Linnhoff and Hindmarsh, 1983) for synthesising HENs has been developed featuring algorithmic simplicity and efficient management of problem complexity. The method has evolved into a complete suite of tools for heat recovery and design techniques for energy efficiency, including guidelines for modifying and integrating a number of energy-intensive processes. © Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 398

4/5/11 9:32:43 AM

Small & micro CHP systems for food and beverage processing

399

200

T (°C)

150

Pinch

100

50

DTMIN = 10 °C

0 QC,MIN = 328

QREC = 5912

QH,MIN = 1168

DH (kW)

16.1 Composite curves for heat recovery targeting.

16.3.2 Total site energy integration Besides the heat integration of individual processes, it is also possible to obtain heat recovery and power cogeneration targets for entire sites consisting of more than one production process. The procedure is based on thermal profiles of heat sources and heat sinks for the entire site that are called total site profiles (TSPs) (Dhole and Linnhoff, 1993; Klemeš et al., 1997). An example of TSPs together with the total site composite curves is shown in Fig. 16.2. As sugar plants are mostly not isolated, but interconnected with power plants and in many cases ethanol distilleries, as well as may provide heating for surrounding civic settlements, the total site methodology has been successfully applied. It typically considers a total site comprising sugar beet or cane delivery, storage, pre-processing and processing, packaging and serving the nearby villages or towns. An example of this setup is shown in Fig. 16.3. Additionally, locally installed boilers, consuming traditional fossil-based fuels, biomass or waste, can also help to meet the process energy requirements, when demand is high or other sources are unavailable. Heating/cooling and power not required by one unit can be fed to a local grid system, and then passed to another unit that is unable to meet its demands locally. The grid system can distribute power (electricity) as well as heating agents such as hot water or steam. In geographic locations where air conditioning is required, a cooling distribution main could also be provided.

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 399

4/5/11 9:32:43 AM

400

Small and micro combined heat and power (CHP) systems From boilers 4

250

3

200 Temperature (°C)

VHP

HP

1 2

MP

2 150 3

1

LP

100

50 –25

CW 4 –20

–15

–10

–5 0 5 Enthalpy (MW)

10

15

20

25

16.2 Total site profiles and total site composite curves.

16.4

Types of small and micro combined heat and power (chp) suitable for the food industry

Decentralised power generation combined with heat supply (CHP) is an important technology for improving energy efficiency, security of energy supply and reduction of CO2 emissions. This section provides an overview of advanced energy technologies which can contribute to the more efficient energy supply of food industries via in-house generation. Many governments encourage micro-CHP system deployment in order to enable meeting international and domestic targets on carbon emissions. Recognising the importance of the issue, the UK government has adopted a policy of incentives to support the development of the technology. As recently as February 2010 (CHPA, 2010) a feed-in tariff of 10 pence per kWh has been adopted in the United Kingdom. This scheme concerns all micro-CHP units with capacity below 2 kW, regardless of whether or not renewables are used. Micro-CHPs are already being developed for applications in SFH (small family houses) and MFH (medium family houses) and SMEs due to their technical and performance features, including high overall efficiency above 90%, low maintenance requirements as well as very low noise levels and low emissions of NOx, COx, SOx and particulates. An assessment has been done of the performance enhancement when applying micro-CHP in residential applications, which compares the new technologies with a base case employing a condensing boiler of 90% efficiency and network electricity (from the grid), which costs approximately 680 £/y (Peacock and Newborough, 2005). The study showed sizable theoretical savings in both cost

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 400

4/5/11 9:32:43 AM

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 401

4/5/11 9:32:43 AM

Bio-fuels

Bio-fuels

UNIT 2

UNIT 3

GT

Fossil fuels

UNIT 4

Nuclear

16.3 Locally integrated energy sector (LIES) CHP (after Perry et al., 2008).

Fossil fuels

UNIT 1

Fossil fuels

Bio-fuels

Fossil fuels

Renewables

Electricity

UNIT 5

UNIT 6

Electricity Steam Hot water Cooling utility

402

Small and micro combined heat and power (CHP) systems

and carbon foot print (CFP). The CO2 emissions saving was evaluated as up to 9% for Stirling engines and 16% for fuel cells (FCs), while the energy cost savings reached up to 14.6% for Stirling engines and up to 40% for operating an FC at excess electricity generation and selling it to the grid. It should be pointed out that FCs still need further development to achieve longer service availability and lower investment costs, at sufficient levels to be applied in the food industry. However, the possible savings in fuel and emissions from a separate source (Slowe, 2006) look very promising (Fig. 16.4). A market analysis and field tests in Germany and the UK (Berger et al., 2006; Forster, 2006; Carbon Trust, 2005) were evaluated focusing on energy producers and conversion techniques with a high development status. These products are either close to market introduction, undergoing EC certification or have already been matured. The following technology types are discussed in relation to possible application in the food industry: ∑ reciprocating engines ∑ microturbines (electric power below 250 kW) ∑ Stirling engines ∑ fuel cells.

16.4.1 Internal combustion – reciprocating engines Reciprocating engines are prime movers very extensively used in the food industries. These engines are suitable for smaller food processing sites Gas engine, 5 kWe

Gas engine, 1 kWe PEMFC, 1 kWe Low efficiency stirling-rankine engine, 1 kWe High efficiency Stirling engine, 1 kWe SOFC 1 kWe 0

0.2

0.4 0.6 CO2 savings (t/y)

0.8

1

16.4 Annual microCHP CO2 savings compared to grid electricity and boiler alternatives (after Slowe, 2006).

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 402

4/5/11 9:32:43 AM

Small & micro CHP systems for food and beverage processing

403

with relatively lower power-to-heat ratios (PHR). A US market evaluation estimates the share of the food industry as almost 10% of all applications of reciprocating engines on the market for the year 2000 (Energy Solutions Center, 2004). A typical reciprocating engine (diesel, gas, multiple fuel) and a generator linked to the engine can efficiently produce electricity, over wide power ranges. Gas engines are most suitable for back-up applications. Diesel engines are recommended for continuous base load use. One of their main drawbacks is their relatively noisy operation, which for industrial applications can be compensated using proper noise insulation. The moving parts require regular maintenance, which results in certain costs – up to 0.01–0.015 (US$/y 2000) US$/kWh (Onsite Sycom Energy Corp., 2000) and CFP (Perry et al., 2008). CHP based on reciprocating engines are more applicable to stable energy demands with fewer peaks in the electricity and heat consumption profiles (Alanne and Saari, 2004). The most efficient performance can be achieved by the proper selection of the size of the internal combustion engine, the capacity of thermal and electrical storage systems and the operation scenario on the energy performance of the entire micro-CHP system (Onovwiona et al., 2007).

16.4.2 CHP using microturbines Gas turbines with electrical power generation from 25 to 250 kW, usually referred to as microturbines, can be also used for cogeneration. Such a facility generally consists of a generator, a compressor, a combustion chamber, and a turbine connected by a shaft, with a heat recovery module linked to the turbine exhaust. The high temperature of the turbine exhaust gas (450–550 °C) enables considerable heat cogeneration. Among other advantages, they feature low noise levels, small size and lower emission levels (especially NOx) compared with reciprocating engines (Soares, 2007). Gas and liquid fuels are suitable for microturbines. However, microturbines have low electrical efficiency, especially on part load, while capital and maintenance costs are rather high. Therefore CHPs with microturbines are most applicable for high steam production, with fixed output volume and similar applications.

16.4.3 External combustion – Stirling engines The Stirling engine is a reciprocating engine with its cylinder closed and combustion taking place outside the cylinder. Stirling engines are characterised by rather low emissions (especially NOx) and lower noise levels. External combustion also requires less maintenance which favourably influences the carbon footprint of the technology. The Stirling engine is usually quiet because the combustion is not explosive and it can use almost any combustible fuel

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 403

4/5/11 9:32:43 AM

404

Small and micro combined heat and power (CHP) systems

and any source of heat, including biomass. This type of CHP has rather low electrical efficiency, about 25–30% when natural gas is used as a fuel. When solid fuels (e.g., biomass) are used, the efficiency can even be as low as 15%, making them suitable for serving energy demands with low power-to-heat ratios. Their low efficiency supports their use as back-up power supplies rather than for continuous use (Peacock and Newborough, 2005).

16.4.4 Fuel cells (FC) A FC produces electricity electrochemically, by combining fuel and atmospheric oxygen. Lower-temperature FCs need pure hydrogen (e.g., proton exchange membrane fuel cell (PEMFC), phosphoric acid fuel cell (PAFC) while higher temperature fuel cells (solid oxide fuel cells (SOFC) and molten-carbonate fuel cells (MCFC)) can also process gases containing carbon monoxide. Hydrogen can be obtained from various fuels by means of a reforming process. The electrical efficiency of these systems can be as high as 45–55% (Alanne and Saari, 2004; Varbanov et al., 2006). If pure hydrogen is used, the only direct emission is water. If reforming is used, CO2 and a minimal amount of oxides of sulphur and nitrogen are formed, depending on the fuel. Other benefits are noiselessness, reliability, modularity, and rapid adaptability to load changes. Modularity brings substantial scalability and may substantially reduce the economy of scale effect (Yamamoto, 2000; Varbanov and Klemeš, 2008).

16.5

Established combined heat and power (chp) technologies for the food industry

This section discusses specific established CHP technologies already having application in the food industry or having the potential to be applied.

16.5.1 Rankine cycle technologies Siddhartha Bhatt and Rajkumar (2001) and several other authors presented combined heat and power studies relating to cane sugar factories. Part of the fuel energy supplied to the boiler is transferred to steam turbines through high-pressure steam that in turn powers the turbine and generator. This separation of the combustion from the working fluid enables steam turbines to operate with a variety of fuels including natural gas, solid waste, coal, wood, wood waste and agricultural byproducts. The capacity of commercially available steam turbines typically ranges from 50 kW to over 250 MW. The lower end of this range is suitable for smaller-scale CHP. Although steam turbines are competitively priced compared to other prime movers, the costs of a complete boiler/steam turbine CHP

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 404

4/5/11 9:32:43 AM

Small & micro CHP systems for food and beverage processing

405

system are relatively high on a per kW basis. This is because steam turbines are typically sized with low power-to-heat ratios, and have high capital costs associated with the fuel and steam handling systems and the custom built nature of most installations.

16.5.2 Gas turbine-based CHP Gas turbines (GT) for industrial use are generally available in a wide interval of capacities ranging from 500 kW to 250 MW and can operate on a variety of fuels such as natural gas, synthesis gas, landfill gas and fuel oils (US EPA, 2010). In recent years US and European companies have started also offering micro gas turbines (see Section 16.4.2). These include Turbec SpA (TURBEC, 2010), as well as Capstone Turbine Corporation (CAPSTONE, 2010) and Calnetix Power Solutions (CALNETIX, 2010) in the United States. Most gas turbines typically operate on gaseous fuel with liquid fuel as a backup. Gas turbines can be used in a variety of configurations including simple cycle generating only power, CHP – where its exhaust heat is used for generating process steam or direct process heating, and combined cycle operation in which the GT exhaust is used to power steam turbines for generating additional power. The steam cycle placed below the gas turbine in the combined cycle arrangement is often referred to as the ‘bottoming cycle’, while the GT is the ‘topping cycle’. Another variation is to extract process steam at intermediate levels from the combined cycle arrangement. The capabilities of this technology are very broad and flexible, allowing serving food processing sites from different sectors and of different scales. Most GT exhausts allow generating high pressure (HP) steam at 40 bar and higher. By allowing the bottoming Rankine cycle to extract steam at intermediate pressures and to eventually employ condensing steam turbines, a very wide range of demands with different power-to-heat ratios can be served. Much of the gas turbine-based CHP capacity currently existing in the United States and the European Union consists of large combined-cycle CHP systems that maximise power production for sale to the grid. Simple-cycle CHP applications are common in smaller installations, typically less than 40 MW. The suitability of employing GT-based CHP at food processing sites depends on the demand parameters and the nature of the raw materials used. For instance, for cane-based sugar plants one of the primary objectives is to maximise the utilisation of the bagasse. In such case the preferred method in Brazil is by simple Rankine cycle-based CHP. However, it is possible to utilise the bagasse in a GT-based system, first deriving synthesis gas via gasification. While examples of established use of bagasse gasification in sugar industries are difficult to find, theoretical works and small-scale experiments are already under development (Pellegrini and de Oliveira Jr, 2007; Rodrigues et al., 2003).

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 405

4/5/11 9:32:43 AM

406

Small and micro combined heat and power (CHP) systems

16.5.3 Reciprocating engines Reciprocating engines, discussed previously, are also used for energy applications in the food industry. An EU-FP6 project report (POLYSMART, 2009) describes the use of a diesel engine in a trigeneration application, using the lower-temperature recovered heat in an absorption chiller and the higher-grade heat is used for generating process steam. Spark ignition (SI) engines use spark plugs with a high-intensity spark of timed duration to ignite a compressed fuel–air mixture within the cylinder. They are available in sizes up to 5 MW, running mostly on natural gas, but propane, gasoline and landfill gas can also be used. Diesel engines (compression ignition, or CI engines), are very efficient power generation options. They operate on diesel fuel, heavy oil, or recently biodiesel has become an option. Dual fuel engines, using predominantly natural gas with a small amount of diesel pilot fuel, are also installed. Higher-speed diesel engines (1200 rpm) are available in capacities up to 4 MW, while lower-speed diesel engines (60–275 rpm) can be larger – up to 65 MW. Reciprocating engines start quickly, follow load well, have good part-load efficiencies and are generally highly reliable. The overall energy generation availability can be enhanced by deploying multiple reciprocating engines. Reciprocating engines are well suited for applications that require hot water or low-pressure steam.

16.6

High-efficiency technologies in theoretical and demonstration stages

As discussed in Section 16.2, certain food industry sectors, e.g. meat processing, have energy demands with high PHR. In order to use the fuels efficiently, there are two principal ways of satisfying the demands of such sites: to use CHP technologies with high overall efficiency but lower PHR, utilising somehow the extra heat generated, or to employ technologies with high PHR and high electrical efficiency. This section addresses situations in the food industry where higher electrical efficiencies are required for processes. Fuel cell (FC) systems are one of the most promising emerging technologies for power generation, which have the potential to replace conventional methods by offering higher energy conversion efficiencies and significantly lower greenhouse gas emissions (Varbanov and Friedler, 2008). They have not yet been widely commercialised due to high capital costs and the need to improve their reliability and lifespan. The achievable electrical efficiency for FCs ranges from 40% up to 60% and above. There are several classes of FCs with small variations for each type. The most researched types include proton-exchange FC (PEFC), phosphoric acid FC (PAFC), molten-carbonate FC (MCFC), and solid-oxide FC (SOFC).

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 406

4/5/11 9:32:43 AM

Small & micro CHP systems for food and beverage processing

407

There are many factors influencing the FC efficiency, of which the operating temperature is the most important. Based on an analysis by Yamamoto (2000), it can be seen that the correlation between the efficiency and the operating temperature is very pronounced, differing by more than 20% between the PEFC and SOFC (Fig. 16.5). Another important feature is that the exhausts of hotter FCs are available at temperatures above 500 °C, making them suitable for integration with bottoming cycles and cogeneration. Higher temperatures favour higher potential for further power generation or cogeneration from the FC exhausts. Any drop in the temperature drastically decreases this potential. The principle and main parameters of a MCFC are discussed next as an example FC technology.

16.6.1 Molten-carbonate fuel cells (MCFC) The MCFC is one of the fuel cell types operating at high temperatures (550– 700 °C). It is considered suitable mainly for stationary power generation. In an MCFC, natural gas or a gaseous low molecular weight hydrocarbon-containing fuel is fed to the anode. To the cathode, a mixture of oxygen (from the air) and carbon dioxide (from the anode) is introduced. This is a continuous process. The electrolyte is a mixture of molten alkali metal carbonates, usually a binary mixture of potassium and lithium, or lithium and sodium carbonates. The molten carbonates are kept in a ceramic matrix of LiAlO2 (Larminie and Dicks, 2000). At operating temperatures of 550–700 °C, the carbonates are highly conductive to carbonate ions (CO32–) providing ionic conduction. The moving ions inside the electrolyte, along with the electrons in the outer circuit, complete the electric circuit for power production. The high operating temperature of the MCFC gives an opportunity for greater fuel flexibility, use of less expensive electro-catalysts and high overall system operating efficiencies. 60

hMax (%)

50 40 30 20 10 0

80 °C (PEFC)

200 °C (PAFC)

700 °C (MCFC)

1000 °C (SOFC)

16.5 Variation of FC efficiency with operating temperature (after Yamamoto, 2000).

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 407

4/5/11 9:32:44 AM

408

Small and micro combined heat and power (CHP) systems

The basic configuration of a MCFC system consists of a fuel cell stack, an internal reforming chamber, a catalytic combustor and inlet/outlet pipes for the various process streams. The process flow diagram of the system is shown in Fig. 16.6, Natural gas and water from the supply system enter the fuel cell reformer where they get reformed internally (indirect reforming) by the heat content of the exothermic fuel cell reaction. The endothermic reforming reaction taking place in the fuel reformer produces carbon monoxide (CO) and hydrogen (H2). The anode exhaust is sent to a catalytic combustor where the unused fuel is burned in the presence of air taken from the ambient for going through the cathode. The amount of air provided must be more than the stoichiometric air requirement for the fuel combustion. The flue gases are then introduced in the cathode after cooling them to a suitable temperature. Otherwise, at high temperatures, electrolyte evaporation and material corrosion would occur. Carbon dioxide, oxygen and electrons, returning from the external electric circuit, react together in the cathode to produce carbonate ions (CO32–). These migrate from the cathode to the anode via the electrolyte matrix, where they react with the hydrogen from the anode stream to produce water, carbon dioxide and electrons. The electrons pass from the fuel electrode (anode) to the oxidant electrode (cathode) via the electric circuit, external to the electrolyte matrix. The reactions occurring at the anode and the cathode oxidise the hydrogen and reduce CO2 to carbonate ions. In addition, a water gas shift reaction also occurs at the anode resulting in additional production of hydrogen. An example MCFC of the described type has been analysed (Varbanov et al., 2006), generating 2.32 MW of electricity. It needed 0.1 kg/s of

408 °C Reformer

Methane

551 °C

25 °C 25 °C

Water

650 °C Anode Air 25 °C

Catalytic combustor 873 °C

630 °C

Electrolyte matrix

670 °C Cathode exhaust 1

Cathode

670 °C Cathode exhaust 2

670 °C Cathode exhaust 3

16.6 Simple MCFC process flow diagram (after Varbanov et al., 2006).

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 408

4/5/11 9:32:44 AM

Small & micro CHP systems for food and beverage processing

409

methane, equivalent to 5.002 MW energy input, resulting in 46.38% electrical efficiency.

16.6.2 Fuel cell combined cycles (FCCC) using MCFC and solid oxide fuel cells (SOFC) MCFCs feature an important thermodynamic limitation. Operation at temperatures higher than 700 °C leads to falling gains in the fuel cell performance. This happens because of increased electrolyte loss from evaporation and corrosion of materials at high temperatures. Similarly, SOFCs also have certain limitations on their operating temperature and efficiency, mainly in terms of suitable materials and their durability. Therefore, any further increases in fuel utilisation efficiency should be sought in a different direction. One especially interesting option is to consider the formation of a fuel cell combined cycle (FCCC) for power generation, which can be eventually employed for stand-alone power generation, as well as for cogeneration. The main reason for considering this option is the significant amounts of high-grade waste heat released by MCFC and SOFC systems. The literature sources on combining fuel cells with gas and steam turbines clearly illustrate the potential to achieve high power and cogeneration efficiencies as well as economic viability (Massardo and Bosio, 2002; Karvountzi et al., 2004; Kurz, 2005; Varbanov et al., 2006). The potential to use the high-temperature exhaust for process heat cogeneration from SOFC or MCFC can be evaluated in two steps following the hierarchy common for process design: 1. Appropriate heat integration within the FC. This would maximise its fuel efficiency without altering the design and give an estimate of the further heat flows available for steam generation. 2. Steam generation and utilisation in a steam network for process heating or more power generation. Analysing the MCFC system shown in Fig. 16.6 reveals significant potential for cogeneration. Using pinch analysis (Linnhoff et al., 1982; Klemeš et al., 2010), it is shown that the MCFC heating and cooling demands define a threshold problem with net utility demands only for cooling (Fig. 16.7). A similar and more thorough analysis of a SOFC (Varbanov and Klemeš, 2008) has also been performed. The SOFC flowsheet is shown in Fig. 16.8. The pinch analysis of the SOFC system produced the composite curves shown in Fig. 16.9. A more complete analysis of the heat-integrated SOFC, combined with generation of hot water or steam, showed remarkable CHP efficiency of over 70% (Fig. 16.10). This value could be eventually increased further if

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 409

4/5/11 9:32:44 AM

410

Small and micro combined heat and power (CHP) systems 1027

No heating required

827 DT = 50 °C = DTmin

T (°C)

627

Hot composite curve

427

227

27

Cold composite curve

0

1000

2000

DH (kW)

3000

Qcooling = 1718.09 kW

16.7 Composite curves for the MCFC from Fig. 16.4 (DTmin = 50 °C).

3055.0 kW Power

1664.6 kW

Air 25.0 °C 2.28 kg/s

AP

700.0 °C

0.12 kg/s CH4 25 °C

SOFC

Heat loss 1639.6 kW 985.0 °C 2.77 kg/s

Reformate 900.0 °C

Fuel heat 6001.7 kW

Afterburner 850.0 °C 0.49 kg/s Reformer FP 100.0 °C 1000.4 kW 1227.9 kW reaction 403.4 kW heating

Water/steam P = 1 atm

173.6 kW Loss 2677.0 kW Cooling

932.2 °C

V EC 3119.1 kW

303.6 kW

130.0 °C

0.36 kg/s 25 °C

SG

Cogenerated heat (water or steam) – varied in the analysis Water (demineralised) Key: AP: Air preheat SG: Steam generation

FP: Fuel preheat EC: Exhaust cooling

Exhaust

V: Vaporiser (water)

16.8 SOFC flowsheet (after Varbanov and Klemeš, 2008).

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 410

4/5/11 9:32:44 AM

Small & micro CHP systems for food and beverage processing

411

1000

T (°C)

800 600 400 Hot CC

200

Cold CC 0 0

2000

Q (kW)

4000

6000

16.9 Composite curves for the SOFC integration (after Varbanov and Klemeš, 2008).

Efficiency (%)

70

CHP efficiency hot water

65 60

SOFC 51% stand-alone efficiency

Power generation CHP

55 50 0

5

10 Steam pressure (bar)

15

20

16.10 Overall efficiencies of the combined SOFC and steam system arrangement (after Varbanov and Klemeš, 2008).

the SOFC design is optimised and the high temperature exhausts could be cooled to lower temperatures than 130 °C.

16.7

Integration of renewables and waste with food industry energy demands

The comparatively low temperatures make food processing very attractive for utilising various renewables, including ground heat via heat pumps as well as solar thermal energy. There are a number of waste and renewable energy sources which are available to provide local heating and cooling, and in doing so can reduce the costs and the greenhouse gas emissions of industrial sites. The focus of this section is on those sources suitable for application to the food industry. The technologies and sources considered include heat pumps, biomass and solar energy.

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 411

4/5/11 9:32:44 AM

412

Small and micro combined heat and power (CHP) systems

16.7.1 Heat pumps Heat pumps are used to upgrade low temperature heat from sources such as ambient air, exhaust air, ground soil, ground rock, groundwater and surface water, to higher temperature heat outputs which can be used for space or process heating. In the food and beverages industry heat pumps are widely used for cooling and refrigeration, mainly utilising their heat sink side. The two principal technologies used in heat pumps are compression and absorption (Laue, 2006). A compression heat pump consists of an evaporator, compressor, condenser and an expansion valve. They are driven mainly by electricity, but can also be powered by direct drive prime movers – gas or diesel engines, gas or steam turbines. Each heat pump is characterised by its coefficient of performance (COP), which depends on the input temperature of the heat source, and the output temperature required. Under the most usual conditions the COP varies between 4 and 5 for temperature lift about 20–30 °C, but may slip below 4 for higher temperature lift values. In the food and drink industry, besides for refrigeration as mentioned, heat pumps can also be applied for process heating. An ideal match would be between a low-temperature heat source or cooling demand below the pinch and a moderate-temperature heating demand above the pinch. However, low-grade geothermal energy can also be used.

16.7.2 Biomass Biomass is another renewable energy source that can provide heat and can reduce the production of greenhouse gases. In many cases it can be readily stored and used when needed. In other cases biomass is a side product or waste from the main production processes, e.g. bagasse and molasses in sugar processing. Depending on the form in which biomass is available, different methods can be applied for utilising it. First, and by far the most common, is direct firing of biomass when it is available in a sufficiently dry condition. However, if higher moisture and/or potentially dangerous components are present, it may be preferable to apply anaerobic digestion of the biomass material, obtaining biogas, which can be further utilised in gas-fired boilers, gas turbine variants (including microturbines) as well as in fuel cells. The biomass renewable energy source can also be used in gasification processes for the production of heat and power. A gasification system thermally converts biomass to synthesis gas, which can then be used to produce fuels, products, power and hydrogen (US DOE, 2008). Currently most large-scale gasification systems research is pursued, but smaller modular gasification systems are also planned. These systems would operate in the range of 5 kW to 5 MW (CIWM, 2008), and would provide heat and power from localised

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 412

4/5/11 9:32:44 AM

Small & micro CHP systems for food and beverage processing

413

sources of biomass. The gasification product, composed primarily of carbon monoxide and hydrogen, is cleaned and then used in gas turbines or internal combustion engines. In addition to the power produced, waste heat can be directed to district heating based systems.

16.7.3 Using solar energy Solar energy is another renewable source that can be used for both heating and cooling. Solar heating is a mature technology that has proven to be reliable and cost-competitive since solar water heaters were introduced over 30 years ago. Although these systems have been centred mainly on producing heat and power for individual or large residential/commercial buildings, there has also been widespread interest in using concentrated solar heat systems for industrial applications (Rantil, 2006). In active solar heating systems, water, or another heat transfer fluid, is circulated through a duct and heated by transfer from direct solar radiation on the collector panel. Various designs of collectors are utilised in order to concentrate the solar radiation on the fluid duct and to maximise solar gains. The amount of heat energy captured per square metre of collector surface area varies with design and location but typically can range from 300 to 800 kWh/(m2y). Some designs use a heat transfer fluid that, when warmed, flows to a storage tank or a heat pump where the heat is further upgraded and used by consumers. An interesting study illustrating the integration of solar captured heat is presented by Atkins et al. (2010). The described example presents a New Zealand dairy milk powder plant located in the central North Island. The case study has been conducted to explore the possible retrofit with the integration of an industrial solar thermal system to the milk powder plant. A 1000 m2 evacuated tube solar collector field is assumed. Evacuated tubes have been considered the most appropriate collector design in this case to fit the required temperature levels and flow rates. However, the overall findings can be generalised to other collector types as well. The plant operates from the beginning of August through to the end of April, when there is a three month shut down due to low milk production in the winter months. This period of no production also coincides with the lowest ambient temperatures and solar radiation levels. The plant data have been analysed using pinch analysis using minimum temperature difference DTmin = 5 °C. The hot and cold utility targets obtained are 1674 and 2927 kW, respectively, with the pinch located at 45 °C for the hot streams and 40 °C for the cold streams. They consider several options for placing the captured solar heat, where the main challenges are how to ensure appropriate placement of the solar heat utility above the pinch and at the same time to maximise the solar collector efficiency. The most beneficial

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 413

4/5/11 9:32:44 AM

414

Small and micro combined heat and power (CHP) systems

scenarios found were: (i) solar heat integration at constant capture flow and varying temperature resulting in reduction of the hot utility by 14% and of the cold utility by 4%; and (ii) solar integration alone – above the pinch at constant capture flow resulting in 7% hot utility reduction.

16.8 Potential applications There is an enormous potential to apply small-scale CHP in the food industry. In this section two case studies are presented. First, an application of a high-temperature fuel cell to powering a Japanese brewery is presented to outline the practical potential of this new technology. Second, a novel idea for integrating smaller-scale industrial plants and other energy consumers/ providers is covered to underscore the importance of taking advantage of local context and potential energy partnerships for successfully implementing distributed generation.

16.8.1 MCFC applied to a Japanese brewery A Brewery in Japan owned by Kirin Beer operates a 250 kWe molten carbonate fuel cell developed by US-based FuelCell Energy together with Marubeni Corporation in Tokyo. It is reported as the world’s first fuel cell to run on digester gas from a waste treatment plant (Kirin Brewery, 2003). The MCFC has been supplying roughly 4% of the electric power and about 1% of the steam for the brewery Toride Plant in Tochigi Prefecture since early April 2003. Marubeni has been responsible for the entire package of investment and for installation, maintenance and operational management of the fuel cell, with the support of the New Energy & Industrial Technology Development Organization (NEDO). The fuel used consists of digester gas (mainly methane) generated by anaerobic treatment at the brewery’s waste treatment plant. All the power generated by the fuel cell and the steam generated from waste heat recovery is supplied for process heating to the brewing plant. Using the fuel cell in cogeneration mode gives a dramatic improvement in overall efficiency to 72% or better (i.e. power generating efficiency of >47% and waste heat recovery efficiency of over 25%).

16.8.2 Locally integrated energy sectors Total site targeting has provided a method for analysing the heat sources and sinks from more than one process, as discussed in Section 16.3.2 (Klemeš et al., 1997). This methodology can be adopted for the analysis of heating and cooling requirements in an enlarged geographical area, which is referred to as a locally integrated energy sector (LIES) (Perry et al., 2008). The LIES

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 414

4/5/11 9:32:44 AM

Small & micro CHP systems for food and beverage processing

415

concept can be very useful for application to the food industry, especially taking into account the relatively smaller scale of many food production plants in Europe. The following example illustrates the concept. Consider two small-scale industrial processing plants, coupled with a hospital complex and a group of residential dwellings and office complexes. The process streams of the industrial processes are shown in Tables 16.1 and 16.2. The grand composite curves, built for DTmin = 20 °C from the data for plants A and B, are shown in Figs 16.11 and 16.12. Table 16.1 Process plant A stream data Stream

Name

Tsupply (°C)

Ttarget (°C)

DH (MW)

CP (kW/°C)

1 2 3 4 5

A2 A1 A5 A6 A7

170 150 25 70 30

80 55 100 100 65

5.000 6.477 1.500 0.750 5.250

55.5556 68.1818 20.0000 35.0000 150.0000

Hot Hot Cold Cold Cold

Table 16.2 Process plant B stream data Stream

Name

Tsupply (°C)

Ttarget (°C)

DH (MW)

CP (kW/°C)

1 2 3 4 5 6

B1 B2 B3 B4 B5 B6

200 20 100 150 60 75

80 100 120 40 110 150

10.000 4.000 10.000 8.000 1.000 7.000

83.3333 50.0000 500.0000 72.7273 20.0000 93.3333

Hot Cold Cold Hot Cold Cold

165 145 125 T (°C)

105 85 65 45 25

0

1

2

3 4 5 Enthalpy (MW)

6

7

8

16.11 Process grand composite curve–process plant A (DTmin = 20 °C).

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 415

4/5/11 9:32:44 AM

416

Small and micro combined heat and power (CHP) systems 225 205 185

T (°C)

165 145 125 105 85 65 45 25

0

1

2

3

4 5 6 Enthalpy (MW)

7

8

9

10

16.12 Process grand composite curve–process plant B (DTmin = 20 °C). Table 16.3 Process Stream data of hospital complex (Plant C) Stream

Name

Tsupply (°C)

Ttarget (°C)

DH (kW) CP (kW/°C)

1 2 3 4 5 6 7 8 9 10 11

Soapy water Condensed steam Laundry sanitary water Laundry Boiler feed water Sanitary water Sterilisation Swimming pool water Cooking Heating Bedpan washers

85 80 25 55 33 25 30 25 30 18 21

40 40 55 85 60 60 121 28 100 25 121

23.85 96.4 17.7 77.4 7.2 77 12.74 151.68 59.5 100.8 5

Hot Hot Cold Cold Cold Cold Cold Cold Cold Cold Cold

0.53 2.41 0.59 2.58 0.24 2.2 0.14 50.56 0.85 14.4 0.05

Plant A has no net heating requirements, indicated by the lack of abovethe-pinch area, but has a large net cooling requirement below the pinch – excess heat that has to be removed. Consequently there is approximately 4 MW of heat which is available at temperatures of around 120 °C. This heat is available at a temperature sufficient for the production of steam. Plant B requires an external heating source above the pinch, again at a temperature of around 120 °C, and also has a small amount of excess heat which has to be removed by cooling below the pinch. This excess heat, of approximately 1.5 MW, could be used to produce hot water at temperatures of around 70 °C – sufficient to use in the residential and office areas of the LIES, provided the transportation distances are not too long. The LIES under consideration also includes a hospital complex – Plant C (Herrera et al., 2003). The process stream data for this unit is shown in Table 16.3 and the process grand composite curve with a DTmin = 20 °C, derived

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 416

4/5/11 9:32:44 AM

Small & micro CHP systems for food and beverage processing

417

from this original data, is shown in Fig. 16.13. In this particular unit, there is a heat sink of around 400 kW above the pinch which requires supply of external heating. The final unit in the energy sector is composed of a group of residential dwellings and office complexes (Plant D). The heating requirements in this mixed unit are aggregated hot water requirements and space heating. The process stream data are given in Table 16.4 and the process grand composite curve, again at DTmin = 20 °C, in Fig. 16.14. The total site profiles diagram of the LIES is given in Fig. 16.15. The heat sources and heat sinks of the involved processes have been combined to produce the overall total site sink profile and total site source profile. Without integration, the LIES would need to dispose of 6.2 MW of heat, and 17.5 MW of heat would have to be supplied from external heating sources (e.g. fossil fuels). A possible scenario (Scenario 1) for integration in the LIES is shown in Fig. 16.16. In this scenario, a hot water main at a temperature of 60 °C to 40 °C is provided for extracting heat from the total site source profile and supplying heat to the total site sink profile. The amount of generated heat for the hot water supply is 5.5 MW, and the amount of heat that has to be supplied by the hot water is also 5.5 MW. The remaining heat required by the LIES is 12.0 MW, giving a heat reduction of 5.5 MW due to heat 145 125

T (°C)

105 85 65 45 25

0

0.05

0.10

0.15 0.20 0.25 Enthalpy (MW)

0.30

0.35

0.40

16.13 Process grand composite curve of hospital complex (Plant C, DTmin = 20 °C). Table 16.4 Process Stream data of residential and office complex (Plant D) Stream

Name

Tsupply (°C) Ttarget (°C) DH (MW)

CP (kW/°C)

1 Hot 2 Hot

District heating Hot water

15 15

133.333 76.9232

60 80

6.000 5.000

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 417

4/5/11 9:32:44 AM

418

Small and micro combined heat and power (CHP) systems

85 75 T (°C)

65 55 45 35 25 0

2

4

6 Enthalpy (MW)

8

10

12

16.14 Grand composite curve of residential and office complex (Plant D, DTmin = 20 °C). 160 140

Sinks

Sources

T (°C)

120 100 80 60 40 20 10

5

0

5 10 Enthalpy (MW)

15

20

16.15 Site profiles for the LIES.

integration in the LIES. The external heat required could be provided by a boiler using a carbon neutral fuel such as biomass or waste combustion unit. In this particular case the heat for the hot water system is supplied by the small-scale food processing plants, and the recipients of the heat are the hospital and residential and office complexes. If this heat (the 5.5 MW) was no longer available, then it would need to be supplied from another source. A second scenario, Scenario 2, is shown in Fig. 16.17. In this case a steam main at 125 °C has been added to the system. The heat source from the LIES has now been split between steam (3.1 MW) and hot water (2.4 MW). On the sink side, 8.4 MW of steam is supplied to the hospital and residential and office complexes, and 5.5 MW of hot water. The amount of heat to be supplied by the boiler or waste combustion system remains at 12.0

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 418

4/5/11 9:32:44 AM

Small & micro CHP systems for food and beverage processing

419

200 180 160

Sources

140 T (°C)

120 100 80

Sinks

60 40 20 0 10

5

0

5 10 Enthalpy (MW)

15

20

16.16 Scenario 1 – Total site profiles. 200 180

Sources

160

T (°C)

140 120 100 80 Sinks

60 40 20 0 10

5

0

5 10 Enthalpy (MW)

15

20

16.17 Scenario 2 – Total site profiles.

MW. Economics is a significant factor in the final design potential. Installing both a hot water main and a steam main over the geographical area of the LIES may be an economical option. This needs to be further evaluated in relation to changing energy/capital cost ratios. The recent developments in coping with fluctuating sources (typical for most renewables) and demands have been presented by Varbanov and Klemeš (2010).

16.9

Future trends

A number of possible future configurations for smaller-scale CHP technologies and their users are possible and have good probability for development in the

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 419

4/5/11 9:32:44 AM

420

Small and micro combined heat and power (CHP) systems

future. Here several possible future directions of development are outlined from the viewpoint of their potential for improving the security of energy supply as well as the overall performance of the industrial processes and the local energy sectors.

16.9.1 Technology development trends Regarding technology development, the current state of the art for microCHP is the Stirling engine, which is very useful and provides security and availability due to its maturity. However, due to its lower power-to-heat ratio (about 0.5) it is not very suitable for industrial applications, which necessitates further developments and a paradigm shift. Particularly fuel cell technology should be more aggressively pursued. It may have significant positive implications on the CHP market as long as sufficient durability, availability and maintainability of the fuel cells can be ensured. Switching to different technologies will also lead to a different power-to-heat ratio, which is quite high (about 1 to 2 for fuel cells). An overview has been presented by Kuhn et al. (2008).

16.9.2 Strategic issues Recently, attention has shifted towards security of supply (van Soest et al., 2006). It has become clear that having energy resources brings influence, and many countries are not hesitating to use that influence when they have it. The energy exporting countries on the one hand realise that because of the ever growing energy demand world-wide, the energy markets are sellers markets these days. They are in a position to sell to the highest bidder and to those users who are willing to accept long-term contracts. The energy importing countries realise that energy is key for economic development, and that short-term scarcities may hamper economic growth for longer periods. Decentralisation This is becoming more and more popular as an idea, especially with the development of new energy conversion technologies making available high efficiency at lower and lower costs. An example of such a technology with a great potential is fuel cells. The generation of the heat and power at the premises of the consumer or close nearby has two fundamental advantages: minimisation of energy transportation losses and minimisation of infrastructure failures or disturbances.

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 420

4/5/11 9:32:44 AM

Small & micro CHP systems for food and beverage processing

421

The further integration of renewables and maximum waste utilisation This is another matter of strategic importance. It is closely related to the issues of energy security, environmental protection and sustainable development. In particular, maximising waste utilisation according to an appropriately formulated waste management hierarchy has the potential to provide a win-win development by simultaneously reducing waste disposal and the consumption of external energy resources. A good example of waste utilisation has been provided in Section 16.8.1. Locally integrated energy sectors The locally integrated energy sectors discussed in this chapter provide another opportunity – to extend process energy efficiency, waste management and renewables integration to the scope of local communities and towns, thus enabling more efficient utilisation of company resources and symbiosis with other companies and residential areas.

16.10 Sources of further information and advice The material provided in this chapter highlights the major CHP technologies with an emphasis on smaller-scale ones. However, it is by no means exhaustive. Therefore, this section supplies additional references to provide guidelines for more detailed and comprehensive studies.

16.10.1 Institutional resources A large number of institutional resources are available over the world. The ones found most useful by the authors are listed next. The US EPA has compiled a ‘Catalogue of CHP Technologies’ available over the Internet (US EPA, 2010). It contains a number of sections starting with an introductory overview of CHP technologies and accompanied with more detailed analyses of each technology in terms of main characteristics and the potential for their application, also including various CHP efficiency definitions used in industry, research and various standards and regulations. Another useful US information resource is the ‘California Distributed Energy Resources Guide’ (CDERG, 2010). This is a public benefit website providing information regarding technologies for distributed energy generation, to which the small- and micro-CHP technologies inherently belong. The website contains various sections – e.g. ones dedicated to background and overview, equipment analysis (including prominent suppliers), information on ongoing research, an extensive library of real-life case studies, costing, policy incentives, market analysis, and others.

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 421

4/5/11 9:32:44 AM

422

Small and micro combined heat and power (CHP) systems

From the European resources, the Irish Combined Heat and Power Association has compiled a set of resources on CHP technologies including overviews and examples of installed running plants (ICHPA, 2010). The UK Carbon Trust (Carbon Trust, 2010a) also offers an extensive website dedicated to CHP technologies with a special section on micro-CHP (Carbon Trust, 2010b). The section offers an extensive report and guide in PDF format for companies and organisations on the issues of understanding and choosing the right combination of micro-CHP technologies. The guide puts a special emphasis on CO2 emissions minimisation. The Carbon Trust website does require free-of-charge user registration before allowing downloads.

16.10.2 Books An interesting textbook on microturbines is provided by Soares (2007). It describes the various microturbine types, their applications, and their requirements for installation, maintenance and repair. It also discusses the combination of microturbines with fuel cells for forming combined cycles as well as other integration options. Knowles et al. (2004) present another useful resource. The book provides an in-context overview of small-scale energy technologies in terms of renewable energy utilisation, technology options such as wind turbines, hydro, microCHP (Stirling engines, microturbines, fuel cells and their hybrids), biomass gasification, and electricity distribution issues. Pehnt et al. (2005) describe CHP and micro cogeneration from a different perspective, emphasising societal and social issues, emissions reduction, economic viability and market analysis.

16.10.3 Journals There are a variety of peer-reviewed journals available. Those regularly publishing on CHP and micro-CHP issues include: ∑

Applied Thermal Engineering <www.elsevier.com/wps/find/journaldescription.cws_home/630/ description#description> ∑ Energy – the International Journal <www.elsevier.com/wps/find/journaldescription.cws_home/483/ description#description> ∑ Chemical Engineering Transactions <www.aidic.it/CET> ∑ Renewable and Sustainable Energy Reviews <www.elsevier.com/wps/find/journaldescription.cws_home/600126/ description#description> ∑ Clean Technologies and Environmental Policy <www.springer.com/environment/sustainable+development/ journal/10098> © Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 422

4/5/11 9:32:44 AM

Small & micro CHP systems for food and beverage processing

423

∑

Journal of Cleaner Production <www.elsevier.com/wps/find/journaldescription.cws_home/30440/ description#description> ∑ Journal of Clean Technologies and Environmental Policy <www.springer.com/environment/sustainable+development/ journal/10098> ∑ Resources, Conservation and Recycling <www.elsevier.com/wps/find/journaldescription.cws_home/503358/ description#description>

16.10.4 Conferences There are also a number of conference options to choose from that cover CHP in the food industry and also smaller-scale cogeneration. However, these issues are not always completely coherently covered by a specific conference. A small selection of high-profile conferences is provided next. The Total Food series of biennial, international conferences was initiated in 2004 by the Royal Society of Chemistry Food Group and the Institute of Food Research, Norwich. The aim of Total Food is to debate global research and development relevant to exploiting the whole food crop rather than the limited proportion that is consumed at present. The conference considers the topics of energy supply and efficiency in food processing. The series of conferences Process Integration, Modelling and Optimisation for Energy Saving and Pollution Reduction (PRES), organised annually since 1999, <www.conferencepres.com>, provides a forum for exchanging information and views on energy technologies and emissions minimisation from a wide spectrum of industries, including increasing participation of food processing engineers and researchers. The other prominent forums include: ∑ European Symposium on Computer Aided Process Engineering (ESCAPE), organised annually since 1992, <www.cape-wp.eu> ∑ AIChE International Congress on Sustainability Science and Engineering (ICOSSE) organised since 2009, <www.aiche.org/IFS/Conferences/index. aspx> ∑ Dubrovnik Conference on Sustainable Development of Energy, Water and Environment Systems (SDEWES), organised biennially since 2002, <www.sdewes.fsb.hr>

16.11 References Alanne, K., Saari, A., 2004. Sustainable small-scale CHP technologies for buildings: the basis for multi-perspective decision-making. Renewable and Sustainable Energy Reviews, 8(5), 401–431.

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 423

4/5/11 9:32:45 AM

424

Small and micro combined heat and power (CHP) systems

Atkins, M., Walmsley, M. R. W., Morrison, A. S., 2010. Integration of solar thermal for improved energy efficiency in low-temperature-pinch industrial processes. Energy, 35(5), 1867–1873. Berger, S., Raabe, E., Zernahle, O., 2006. Market analysis for microCHP – application of microCHP in the power range between 1 and 5 kWel, Report, Berliner Energieagentur GmbH, Germany. CADDET, 1994. Center for the Analysis and Dissemination of Demonstrated Energy Technologies, Integrated Heat Recovery in a Malt Whisky Distillery. Project No. UK-94-509. CALNETIX, 2010. Calnetix Power Solutions. [Online] <www.tapower.com>, accessed 08/06/2010. CAPSTONE, 2010. Capstone Turbine Corporation [Online] <www.capstoneturbine. com>, accessed 08/06/2010. Carbon Trust, 2005. The Carbon Trust’s Small-Scale CHP field trial update, CTC513. Carbon Trust, 2010a. Combined Heat and Power. [Online] <www.carbontrust.co.uk/ emerging-technologies/technology-directory/pages/combined-heat-power.aspx>, accessed 10/06/2010. Carbon Trust, 2010b. Micro Combined Heat and Power Accelerator. [Online] <www. carbontrust.co.uk/emerging-technologies/current-focus-areas/pages/micro-combinedheat-power.aspx>, accessed 10/06/2010. CDERG, 2010. California Distributed Energy Resources Guide. [Online] <www.energy. ca.gov/distgen/index.html>, accessed 09/06/2010. CHPA, 2010. Government cash reward for microCHP. A Combined Heat and Power Association press release, 5 February 2010. [Online] <www.chpa.co.uk/news/press_ releases/2010/01.02.10%20Government%20cash%20reward%20for%20microCHP. pdf>, accessed 07/06/2010. CIWM, 2008. Energy from Waste: A Good Practice Guide, The Chartered Institution of Waste Management, Northampton. DEFRA, 2006. Food Industry Sustainability Strategy. [Online] <www.defra.gov.uk/ foodfarm/policy/foodindustry/documents/fiss2006.pdf>, accessed 28/07/2010. Dhole, V. R., Linnhoff, B., 1993. Total site targets for fuel, co-generation, emissions, and cooling. Computers & Chemical Engineering, 17, S101–S109. Elkin, D., Stevens, C., 2008. Environmental and consumer issues regarding water and energy management in food processing. In: Handbook of Water and Energy Management in Food Processing. Edited by J Klemeš, R Smith and J.-K. Kim. Woodhead Publishing Limited, Cambridge, pp. 29–44. Energy Solutions Center, 2004. Reciprocating Engines [Online], <www.energysolutionscenter. org/distgen/AppGuide/Chapters/Chap4/4-1_Recip_Engines.htm>, accessed 29/07/2010. Forster, H., 2006. Ideal solutions with warm-up. Energiespektrum, 11, 28–30. Genné, I., Derden, A., 2008. Water and energy management in the slaughterhouse. In: Handbook of Water and Energy Management in Food Processing. Edited by J Klemeš, R Smith and J.-K. Kim. Woodhead Publishing Limited, Cambridge, pp. 805–815. Herrera, A., Islas, J., Arriola, A., 2003. Pinch technology application in a hospital. Applied Thermal Engineering, 23, 127–139. ICHPA, 2010. Welcome to the Irish Combined Heat and Power Association. [Online] <www.ichpa.com/index.php>, accessed 09/06/2010. Karvountzi, G. C., Price, C. M., Duby, P. F., 2004. Comparison of molten carbonate and solid oxide fuel cells for integration in a hybrid system for cogeneration or tri-generation. ASME, Advanced Energy Systems Division (Publication) AES, 44, 139–150. © Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 424

4/5/11 9:32:45 AM

Small & micro CHP systems for food and beverage processing

425

Kemp, I.C., 2007. Pinch Analysis and Process Integration (a second edition of Linnhoff, B., Townsend, D. W., Boland, D., Hewitt, G. F., Thomas, B. E. A., Guy, A. R., Marsland, R. H. User Guide on Process Integration for the Efficient Use of Energy, IChemE, Rugby (1982, last edition 1994), Butterworth-Heinemann/IChemE Series. Kirin Brewery, 2003. Japanese brewery MCFC powered by digester gas. Fuel Cells Bulletin, 2003(7), 4. Klemeš, J., Dhole, V. R., Raissi, K., Perry, S. J., Puigjaner, L., 1997. Targeting and design methodology for reduction of fuel, power and CO2 on total sites, Applied Thermal Engineering, 17(8/10), 993–1003. Klemeš, J., Kimenov, G., Nenov, N., 1998. Application of pinch-technology in food industry. CHISA’98/1st Conference PRES’98, Prague, Lecture F6.6 [136]. Klemeš, J., Friedler, F., Bulatov, I., Varbanov P., 2010. Sustainability in the Process Industry: Integration and Optimization. McGraw-Hill Professional, New York. Knowles, M., Burdon, I., Beith, B., 2004. Micro Energy Systems: Review of Technology, Issues of Scale and Integration. John Wiley and Sons, New York. Kuhn, V., Klemeš, J., Bulatov, I., 2008. MicroCHP: overview of selected technologies, products and field test results. Applied Thermal Engineering, 28(16), 2039–2048. Kurz, R., 2005. Parameter optimization on combined gas turbine–fuel cell power plants. ASME Journal of Fuel Cell Science and Technology, 2, 268–273. Larminie, J., Dicks, A., 2000. Fuel Cell Systems Explained. Wiley, Chichester. Laue, H. J., 2006. Heat pumps. In: Renewable Energy, Vol. 3C. Edited by C. Clauser, T. Strobl, and F. Zunic. Springer, Berlin. Linnhoff, B., Hindmarsh, E., 1983. The pinch design method for heat exchanger networks. Chemical Engineering Science, 38(5), 745–763. Linnhoff, B., Townsend, D. W., Boland, D., Hewitt, G. F., Thomas, B. E. A., Guy, A. R., Marsland, R. H., 1982. (last edition 1994). User Guide on Process Integration for the Efficient Use of Energy, IChemE, Rugby. Massardo, A. L., Bosio, B., 2002. Assessment of molten carbonate fuel cell models and integration with gas and steam cycles. Journal of Engineering for Gas Turbines and Power, 124, 103–109. Onovwiona, H. I., Ugursal, V. I., Fung, A. S., 2007. Modeling of internal combustion engine based cogeneration systems for residential applications. Applied Thermal Engineering, 27, 848–861. Onsite Sycom Energy Corp., 2000. The market and potential for combined heat and power in the commercial/institutional sector. US Department of Energy, Energy Information Administration. Peacock, A., Newborough, M., 2005. Impact of microCHP systems on domestic sector CO2 emissions. Applied Thermal Engineering, 25, 2653–2676. Pehnt, M., Cames, M., Fischer, C., Praetorius, B., Schneider, L., Schumacher, K., Voß, J-P., 2005. Micro Cogeneration: Towards Decentralized Energy Systems, Springer, Berlin. Pellegrini, L. F., de Oliveira Jr, S., 2007. Exergy analysis of sugarcane bagasse gasification. Energy, 32(4), 314–327. Perry, S., Klemeš, J., Bulatov, I., 2008. Integrating waste and renewable energy to reduce the carbon footprint of locally integrated energy sectors. Energy, 33(10), 1489–1497. POLYSMART, 2009. Meat processing factory in Baza POLYSMART Milestone 6.8, [Online] <www.polygeneration.org/cms/upload/concepts/trigeneration/CS_1.4.3.pdf> accessed 13/04/2010. Rantil, M., 2006. Concentrating solar heat – kilowatts or Megawatts? Seminar ‘Renewable

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 425

4/5/11 9:32:45 AM

426

Small and micro combined heat and power (CHP) systems

heating and cooling – from RD&D to deployment’, International Energy Agency, April 2006, [Online] <www.iea.org/Textbase/work/workshopdetail.asp?WS_ID=243>, accessed 17/05/2010. Rodrigues, M., Walter, A., Faaij, A., 2003. Co-firing of natural gas and biomass gas in biomass integrated gasification/combined cycle systems. Energy, 28(11), 1115– 1131. Siddhartha Bhatt, M., Rajkumar, N., 2001. Mapping of combined heat and power systems in cane sugar industry. Applied Thermal Engineering, 21, 1707–1719. Slowe J., 2006. MicroCHP to increase energy efficiency: emerging technologies, products and markets, Delta Energy & Environment Affiliation, [Online] <mail.mtprog.com/CD_ Layout/Day_2_22.06.06/1400-1545/ID33_Slowe_final.pdf>, accessed 17/05/2010. Soares, C., 2007. Microturbines Applications for Distributed Energy Systems, Elsevier Inc., Maryland Heights, MO. TURBEC, 2010. Turbec SpA Corporate web site. [Online] <www.turbec.com/company/ distributors.asp>, accessed 08/06/2010. Urbaniec, K., Klemeš, J., 2008. Water and Energy Management in the Sugar Industry. In: Handbook of Water and Energy Management in Food Processing. Edited by J Klemeš, R Smith and J.-K. Kim. Woodhead Publishing Limited, Cambridge, pp. 863–884. US DOE, 2008. US Department of Energy, 2008, Energy Efficiency and Renewable Energy, [Online] <www1.eere.energy.gov/biomass/fy04/fundamental_biomass_gas. pdf>, accessed 17/07/2008. US EPA, 2010. Catalog of CHP Technologies. [Online] <www.epa.gov/chp/basic/catalog. html>, accessed 08/06/2010. van Soest, J. P., Bartholomeus, P., Overdiep, H., Klimbie, B., 2006. From microopportunities to macro-changes. Micro CHP in Perspective. [Online] <www.jpvs.nl/ downloads/micro chp in perspective.pdf>, accessed 10/06/2010. Varbanov, P., Friedler, F., 2008. P-graph methodology for cost-effective reduction of carbon emissions involving fuel cell combined cycles. Applied Thermal Engineering, 28(16), 2020–2029. Varbanov, P., Klemeš, J., 2008. Analysis and integration of fuel cell combined cycles for development of low-carbon energy technologies. Energy, 33(10), 1508–1517. Varbanov, P. and Klemeš, J., 2010. Total sites integrating renewables with extended heat transfer and recovery. Heat Transfer Engineering, 31(9), 733–741. Varbanov, P., Klemeš, J., Shah, R. K., Shihn, H., 2006. Power cycle integration and efficiency increase of molten carbonate fuel cell systems. Journal of Fuel Cell Science and Technology, 3(4), 375–383. Yamamoto, O., 2000. Solid oxide fuel cells: fundamental aspects and prospects. Electrochimica Acta, 45, 2423–2435.

© Woodhead Publishing Limited, 2011

CHP-Beith-16.indd 426

4/5/11 9:32:45 AM

17

Biomass-based small and micro combined heat and power (CHP) systems: application and status in the United Kingdom

A. V. B r i d g w a t e r, A. A l c a l a and M. E. G y f t o p o u l o u, Aston University, UK

Abstract: This chapter discusses the current state of biomass-based combined heat and power (CHP) production in the UK. It presents an overview of the UK’s energy policy and targets which are relevant to the deployment of biomass-based CHP and summarises the current state for renewable, biomass and CHP. A number of small-scale biomass-based CHP projects are described while providing some indicative capital costs for combustion, pyrolysis and gasification technologies. For comparison purposes, it presents an overview of the respective situation in Europe and particularly in Sweden, Finland and Denmark. There is also a brief comment about novel CHP technologies in Austria. Finally it draws some conclusions on the potential of small-scale biomass CHP in the UK. Key words: biomass, small-scale combined heat and power (CHP), renewable energy, bioenergy.

17.1

UK energy policy and targets

In February 2003 the Government published the Energy White Paper (1) setting a target of 20% of electricity produced from renewables by 2020, and biomass is categorised in Government energy policy as part of renewables. Onshore and offshore wind and biomass could be the largest contributors to the renewables generation mix in 2020. The Government accepted the recommendation of the Royal Commission on Environmental Pollution to reduce carbon emissions by 60% by 2050 and stated their commitment to a target of 10 GWe of ‘good quality’ combined heat and power (CHP) capacity being installed by 2010. CHP is defined as simultaneous generation of usable heat and power in a single process, with the advantage of offering much higher overall energy efficiency (up to 85%) compared to electricity production only (rarely exceeds 35%). CHP that is certified as ‘good quality’ is exempt from the Climate Change Levy imposed on electricity and gas sales for non-residential energy users. Biomass is defined as ‘the biodegradable fraction of products, waste and 427 © Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 427

4/5/11 9:42:36 AM

428

Small and micro combined heat and power (CHP) systems

residues from agriculture (including vegetal and animal substances), forestry and related industries, as well as the biodegradable fraction of industrial and municipal waste’ (2).The Biomass Task Force Report by Sir Ben Gill published in October 2005 stressed that ‘Biomass is unique as the only widespread source of high-grade renewable heat’ (3).The report challenged the Government to recognise that biomass heat can save carbon and help deliver the climate change agenda at a cost favourable to other options. The Government’s stated their agreement with the Biomass Task Force Report, specifically with the conclusion that renewable heat provides important opportunities and is a particularly efficient way of cutting carbon emissions, provided that there is a secure market for the heat generated (4). At the same time the Government acknowledged the contribution that biomass can make to renewable electricity targets through co-firing or dedicated electricity generation. A working group, the Biomass Implementation Advisory Group, has been formed by the UK Department for Environment, Food and Rural Affairs (Defra) to oversee progress in implementing the recommendations. In the framework of these recommendations a five-year new Bio-energy Capital Grant Scheme targeted at biomass heat and CHP was launched on 29 December 2006. This supported the installation of biomass-fuelled heat and CHP projects in the industrial, commercial and community sectors, worth £10–15m in England over the first two financial years to March 2008. The first two rounds of the Bio-energy Capital Grant Scheme allocated grants to project developers and organisations investing in heat and/or electricity generating projects fuelled by energy crops and other biomass feedstocks creating new bio-energy markets. Round six was announced in December 2009 and was open for applications until 31 March 2010. Microgeneration is defined as the small-scale production of heat and/or electricity from a low carbon source (5). Solar, micro-wind, micro-hydro, heat pumps, biomass, micro combined heat and power (micro-CHP) and small-scale fuel cells are all considered microgeneration technologies. MicroCHP is defined as CHP with an electrical capacity of less than 50 kW (6). A number of programmes and actions have been adopted by the Government to support microgeneration: ∑ The Low Carbon Buildings Programme launched by the DTI in April 2006 superseding the previous Clear Skies Initiative and Solar PV programmes (7). The programme provides grants for a number of microgeneration technologies for householders, community organisations, schools, the public sector and businesses. An accredited installer and product are prerequisites for grant eligibility. A list is available from Clear-Skies (http://www.clear-skies.org/). ∑ A field trial of micro-CHP units run by the Carbon Trust (funded by the Government), monitoring the energy and financial savings. The Carbon Trust provides finance for carbon-reduction projects. © Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 428

4/5/11 9:42:36 AM

Biomass-based small and micro CHP systems

429

∑

A Value Added Tax (VAT) rate reduction on micro-CHP units from 17.5% to 5% since April 2005. ∑ The introduction of new Building Regulations in April 2006, with changes to the regulations on energy conservation encouraging the use of low or zero carbon (LZC) systems to combat climate change. ∑ The UK-wide Community Energy Programme (now closed) provided funding to promote public sector community heating through capital grants to install new schemes and refurbish outdated infrastructure and equipment, primarily using CHP technology.

In July 2006 the Government published its energy review (8), in which it was concluded that on present policies, the UK is on course to exceed its target under the first commitment period of the Kyoto Protocol, this is, to cut overall greenhouse gas emissions by 12.5% on 1990 levels throughout the period 2008–12. However, the growing energy consumption combined with higher levels of electricity generation from coal has led to higher carbon emissions. As a result further action has to be taken in order to achieve the goal of cutting carbon emissions by 60% by 2050 following the recommendation of the Royal Commission on Environmental Pollution. Renewable energy is an integral part of the Government’s strategy for tackling climate change. The key support mechanism for the expansion of renewable electricity is the Renewables Obligation (RO) on all electricity suppliers in the UK to supply a specific proportion of electricity from eligible renewables. This has succeeded in bringing forward major and minor developments of the most economic forms of renewable energy, in particular onshore wind, landfill gas, and co-firing of biomass in coal-fired power stations. The level of the RO is due to rise from 10% by 2010 to about 15% in 2015–16. Although not given special focus in the energy review, biomass is seen as a key component of the general mix of renewables and will make an important contribution, particularly as a source of distributed energy. At the same time the introduction of the EU Emissions Trading Scheme (ETS) creates a strong economic incentive for biomass technologies by putting a price on carbon. Ignoring climate change will eventually damage economic growth and risk irreversible disruption to economic and social activity. Strong, early action is needed to tackle climate change with initiatives such as investment in higher energy efficiency and low or zero carbon emission technologies of which biomass and biomass-fuelled CHP are key contributors.

17.2

Renewables and combined heat and power (CHP) in the UK

In these sections we look first at the broad picture of electricity produced from renewables, then in Section 17.2.2 we review heat from renewable © Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 429

4/5/11 9:42:36 AM

430

Small and micro combined heat and power (CHP) systems

biomass-based heat production. In Section 17.2.3 we comment on CHP in the UK, then in Section 17.2.4 we focus on the renewable-based CHP.

17.2.1 Renewables and biomass electricity generation in the UK

Electricity Generation from biomass (GWh)

According to the UK Energy Digest (9), the largest contribution to renewables in input terms for 2008 (over 81%) is from biomass, followed by large-scale hydroelectricity production. Only 12.2% of renewable energy comes from renewable sources other than biomass and large-scale hydro (solar, wind, small-scale hydro and geothermal aquifers). Total electricity generation from renewables, excluding non-biodegradable waste in 2008 equalled 21 597 GWh showing a 10% increase compared to electricity generation in 2007 and accounting for 5.5% of the total electricity generated. With biomass derived fuels such as landfill gas, sewage sludge and municipal solid waste, a substantial proportion of the energy content is lost in the process of conversion to electricity. Therefore, although in input terms the contribution of biomass to renewable electricity generation in 2008 was over 81%, in output terms it was 43.1%; this accounts for 9315 GWh and 2.4% of the total electricity generation in the UK that year. In 2007, some 9270 GWh of electricity were generated from biomass, comprising 47.2% of the renewable electricity generation. Recent years have reached a plateau in the total electricity generated from biomass, as shown in Fig. 17.1, with 10 000 9000 8000 7000 6000 5000 4000 3000 2000 1000 2001

2002

2003

2004

Landfill gas Municipal solid waste combustion Animal biomass

2005

2006

2007

2008

Sewage sludge digestion Co-firing with fossil fuels Plant biomass

17.1 Electricity generation from biomass in the UK (GWh).

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 430

4/5/11 9:42:36 AM

Biomass-based small and micro CHP systems

431

a slight increase from 2007 to 2008 due to the contribution of landfill gas, animal and plant biomass, with a small decrease in co-firing biomass with fossil fuel. A list of currently operational and under development biomass electricity projects is presented in Table 17.1. Sewage gas and landfill gas plants are not included in the list although they make a significant contribution. This Table 17.1 Biomass electricity projects in the UK (9, 10, 11) Generating station name

Capacity Fuel (kW)

Technology

Operator company

Date commissioned

Balcas Timber 2450

Sawdust and woodchips

Bioflame

MSW

Incineration

Bioflame Ltd.

01/11/2007

Chestnut Bio 980 Power Ltd (formally Ecoenergy ltd)

Biomass

 

Chestnut Bio Power Ltd

01/09/2006

Eccleshall Biomass

Miscanthus Combustion – Eccleshall Microturbines Biomass Ltd

01/08/2007

Elean 40 000 Business Park

Straw

Combustion – Vibrating grate steam cycle

EPR Ely Ltd

01/09/2000

Eye Power Station (Fibropower)

Poultry litter

Combustion – Moving grate steam cycle

EPR Eye Limited (previously Fibropower Ltd)

01/07/1992

Fawley Waste 8600 to Energy Plant

MSW

Combustion – Pyros 01/06/2001 FB Environmental Limited

Glanford 16 700 Power Station (Fibrogen)

Poultry litter

Combustion – Moving grate steam cycle

EPR Glanford 01/11/1993 Limited (previously Fibrogen Ltd)

Goosey Lodge 16 000 Power Plant

Biomass – Animal waste

Combustion – FB

Wykes 01/10/2000 Engineering Co.(Rushden) Ltd.

J E Hartley Ltd

594

Biomass

 

J E Hartley Ltd

01/06/2007

Knypersley Renewable Generator

7200

Biomass

 

Yorkshire Generation Company Limited

01/08/2007

600

2645

14 316

Balcas Timber 01/04/2005 Ltd

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 431

4/5/11 9:42:36 AM

432

Small and micro combined heat and power (CHP) systems

Table 17.1 Continued Generating station name

Capacity Fuel (kW)

Technology

Longma Thorn

400

Recycled vegetable oil

Reciprocating Longma engine Biofuels Ltd

LPL Hockwold

400

Recycled   cooking oil

Reg Bio 01/01/2007 Power UK Ltd

Mossborough 300 Hall Farm

Clean wood

Gasification

Biomass Engineering Limited

01/01/2005

Old Manor House

100

Biomass

 

J B Adams (Farms) Ltd

01/10/2006

PDM Group Widnes

9500

MBM

Combustion – FB

Granox Ltd

01/09/2000

Peabody 239 Trust, BEDZED

Waste timber slurry and food

Reciprocating Peabody Trust 01/05/2002 engine

UPM Shotton 19 655 Paper Boiler 7

Biomass BFB (Sludge from paper mill)

Slough Electricity Contracts Ltd

Packging and wood waste

Combustion – Slough Heat FB and Power

Thetford 41 500 Power Station

Poultry litter

Combustion – Moving grate steam cycle

EPR Thetford 01/10/1998 Limited (Fibrothetford Ltd)

Wilton 10 Biomass Gen station

35 220

Energy crops, sawmills waste, recycled wood

Combustion – Boilers

Sembcorp Utilities (UK) Ltd

01/01/2007

Dowhill Farm

120

Biodiesel?

 

James Crawford

01/07/2005

Stevens Croft

42 260

Energy crops and wood chips

Combustion – E.ON UK plc. Microturbines

Poultry litter

Combustion – The Westfield 01/10/2000 FB Biomass Plant/EPR Scotland Ltd

35 000

The Westfield 12 500 Biomass Plant

Operator company

UPM Kymmene (UK) Ltd

Date commissioned 01/07/2006

01/10/2006

01/04/1993

01/06/2007

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 432

4/5/11 9:42:36 AM

Biomass-based small and micro CHP systems

433

table does not claim to be exhaustive; however, it does represent the best of our knowledge at this time. This is a dynamic area and new projects are likely to be forthcoming over the next few years.

17.2.2 Biomass heat generation in the UK

Thousand tonnes of oil equivalent heat generation

According to a study conducted by Future Energy Solutions (12), in 2005 the production of heat accounted for approximately 30% of total energy consumed in the UK excluding transport. The main provider of heat is natural gas, only 1% is generated from renewable energy sources. On the other hand, 94% of renewable heat comes from biomass. Figure 17.2 shows that the level of heat generated from biomass has been steadily increasing since 2005 after a short decline due to tighter emissions controls discouraging the burning of waste wood. Domestic use of wood is still the main contributor to renewable heat generation whereas the input of industrial wood waste has decreased from 225.2 thousand tonnes of oil equivalent (ktoe) in 2001 to just 93.1 ktoe in 2005 rising again in 2008 to 107.6 ktoe. Residential wood fuel systems are increasing in popularity under the DTI’s Low Carbon Buildings Programme. They vary from stand-alone stoves of 6–12 kWth fuelled by logs or pellets providing space heating for a room, to boilers larger than 15 kWth connected to central heating and hot water systems fuelled by pellets, logs or chips. 800.0 700.0 600.0 500.0 400.0 300.0 200.0 100.0 –

2001

2002

2003

2004

2005

2006

2007

2008

Landfill gas

Sewage sludge

Wood combus-domestic

Wood combus-industrial

Animal biomass

Plant biomass

Municipal solid waste combustion

17.2 Biomass derived fuels used for heat generation in the UK.

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 433

4/5/11 9:42:37 AM

434

Small and micro combined heat and power (CHP) systems

Capital costs depend on the type and size of the specific system and installation and commissioning costs. For a stand-alone room pellet-fuelled heater the price range is around £1500–£3000 which is just the cost of buying the unit. A typical 20 kWth pellet boiler would cost around £5000 to install including the cost of the flue and commissioning, whereas a 20 kWth manual log feed system would be slightly cheaper (13). Fuel supply is the principal operating cost for a biomass heat system. It includes sourcing the fuel, processing the fuel to required specifications (e.g. chipping, sieving and drying for wood chips) and transporting the fuel. The bulky nature of biomass in combination with its lower energy density compared to that of fossil fuels results in higher transport costs. The Biomass Energy Centre (14) published indicative domestic scale delivered biomass fuel costs (pence per kWh, 2010): ∑ wood chips: 2.3 p/kWh ∑ wood pellets: 3.9 p/kWh ∑ natural gas: 4.1 p/kWh ∑ electricity: 13.3 p/kWh. Small-scale (up to 350 kWth) biomass heat systems are more viable in areas not connected to a natural gas line, where their higher capital cost is offset by the lower cost of biomass fuels compared to the cost of oil and liquid petroleum gas (LPG). However, due to increased oil and gas prices, significant potential has arisen in all areas for larger-scale heating projects of more than 350 kW, e.g. community heating, particularly where there is a high or predictable heat demand.

17.2.3 CHP in the UK Combined heat and power offers higher carbon savings per tonne of fuel than the current centralised scheme for power generation. In CHP the heat generated is recovered and utilised for industrial processes, community heating or space heating. It is noteworthy that heat demand is the driver in CHP, and electricity is a useful by-product. Therefore heat projects have the potential to become CHP projects. The economics of the energy markets may alter this balance in the future. According to the 2008 UK Energy Digest (15) some 1439 CHP schemes were operating in the UK with an installed capacity of 5469 MWe generating 27 911 GWh of electricity (7.24% of the total electricity generated in the UK) and 52 197 GWh of heat. This represents a 72 MWe increase in installed capacity equivalent to a 1% increase in electricity generation between 2004 and 2008. In terms of electrical capacity, schemes larger than 10 MWe represent 83% of the total installed capacity. However, in terms of number of schemes, the largest share is in schemes less than 1 MWe (81%).

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 434

4/5/11 9:42:37 AM

Biomass-based small and micro CHP systems

435

With regard to buildings, the largest proportion (from the capacity point of view) is in the health sector, mainly hospitals, whereas leisure and hotels account for more than half the total number of schemes, as shown in Table 17.2. In fact, these three sectors account for 85% in terms of number of units installed, 61% of all electricity capacity and 57% of heat capacity. The major fuel used in CHP schemes in 2008 was natural gas. It comprised 71.1% of the overall fuel used, whereas renewable fuels comprised only 3.7% including sewage gas, other biogases, municipal waste and refusederived fuels. The breakdown of the fuel input for CHP schemes in the UK is depicted in Fig. 17.3. Table 17.2 Number and capacity of all CHP schemes installed in buildings by sector in 2008 (16) Number of schemes

Electrical Heat capacity capacity (MWe) (MWth)

Leisure 394 Hotels 254 Health 187 Residential group heating 40 Universities 41 Offices 17 Education 17 Government estate 17 Retail 17 Airports 3

50.0 36.0 124.2 27.7 50.0 15.0 10.0 15.9 4.6 10.5

54.8 45.4 190.4 61.0 83.8 12.0 17.7 18.6 3.4 18.7

Total

344.0

505.7

987

3.32% 20.06%

1.75%

3.71%

71.16% Coal

Fuel oil

Natural gas

Renewable fuels

Other fuels

17.3 Fuel input for CHP in the UK in 2008.

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 435

4/5/11 9:42:37 AM

436

Small and micro combined heat and power (CHP) systems

Micro-CHP is defined as CHP units with an electrical capacity of less than 50 kW. Micro-CHP units are typically operated as conventional boilers, providing space heating and warm water in residential or commercial buildings. However, unlike boilers, micro-CHP units generate electricity together with the heat at very high efficiencies. Most units operate in grid-parallel mode, so that the building can either receive electricity from or export to the network. Of the 24 million households in the UK, as many as 14 to 18 million are thought to be suitable for micro-CHP units comprising approximately 20 GW of electrical capacity in total (17).

17.2.4 Renewable CHP generation in the UK As was mentioned in the previous section, only 3.7% of the fuels used for CHP generation in 2008 were renewable, i.e. sewage gas, other biogases, municipal waste and refuse-derived fuels, resulting in the generation of 838 GWh of electricity and 1566 GWh of heat. Figure 17.4 shows that the increase in CHP generation from renewables for the period up to 2007 is slow, with an average of a 2% increase for heat and 7.43% increase for electricity. A list is presented in Table 17.3 showing the current biomass CHP plants in the UK. Sewage gas, landfill gas plants and biomass co-firing in coal-fired power stations are not included in the table. Table 17.3 is not claimed to be exhaustive. It is certain that there are projects in development with undisclosed details and even some of the projects in development included in the list could be facing problems with planning and environmental permissions and

CHP generation from renewables (GWh)

3000.0

2500.0

2000.0

1500.0

1000.0

500.0

–

2001

2002

2003

2004

Electricity

2005

2006

2007

2008

Heat

17.4 CHP generation from renewables in the UK (GWh).

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 436

4/5/11 9:42:38 AM

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 437

4/5/11 9:42:38 AM

Deeside – North East Wales

Innogy Cogen Ltd Alstom/Foster (Aylesford) CHP – Wheeler C, D ME20 7DL

Holsworthy Devon Andigestion Ltd (part of the Summerleaze Group)

UPM Shotton Paper Boiler 7

Npower Cogen Ltd (Aylesford) CHP – C, D

Holsworthy (Summerleaze) Biogas Plant

Biomass CHP (Exus Energy or B9 Energy Biomass Ltd)

Kilwaughter Chemical Co., Larne, N. Ireland

CHP Enclosure, 21 Sandmartin Way, BEDZED, Wallington, Surrey

Kilwaughter Chemical Co

Peabody Trust, BEDZED

Biomass CHP (Exus Energy or B9 Energy Biomass Ltd)

Biomass CHP (Exus Energy or B9 Energy Biomass Ltd)

Blackwater Valley Blackwater Valley Museum Museum, N. Ireland

Aker Solutions

Aker Solutions

Meadowhead Road Irvine KA11 5AT

UPM Caledonia Paper mill

Manufacturer/ Developer

Location

Generating station name

Table 17.3 Biomass based CHP plants in the UK (9, 11) Feedstock

Downdraft fixed air gasification

Downdraft fixed bed air gasification

Downdraft fixed bed air gasification

Anaerobic Digestion

BFB

Waste timber slurry and food

Wood chips

Wood chips

Manure, litter and food waste

Sludge from de-inking process

Bubbling Waste paper Fluidised Bed fibre and sludge

Bubbling Waste paper Fluidised Bed fibre and sludge

Technology

1500

Gas engines 239

Gas engines 300

Gas engines 200

Gas engines 2700

Steam turbines

20 000

 

400

 

23 000  

Operational

In development (News of grant in 2009)

Installed in 1998 Recommissioning

Installed in 2001 Recomissioned in 2005

Operational

Prime mover Output Output Status (kWe) (kWth)

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 438

4/5/11 9:42:38 AM

Jepson Bros, Biomass Culcheth, Cheshire Engineering Ltd.

Little Woolden Hall Farm

Biomass Engineering Ltd.

Biomass Engineering Ltd

Preston

ECOS Millenium Centre, N. Ireland

Biomass Engineering Ltd

Cumbria

Biomass Engineering Ltd

Biomass Engineering Ltd

Ballymena ECOS Centre

 

Stoke

Old Manor House Banbury

Mossborough Hall Biomass Farm Rainford St Engineering Ltd Helens

Mossborough Hall Farm

Manufacturer/ Developer

Location

Generating station name

Table 17.3 Continued

Downdraft fixed bed air gasification

Downdraft fixed bed air gasification

Downdraft fixed bed air gasification (8250 kWe gasifiers)

Downdraft fixed bed air gasification (4250 kWe gasifiers)

Downdraft fixed bed air gasification (12 250 kWe gasifiers)

Downdraft fixed air gasification

Downdraft fixed air gasification

Technology

2 ¥ Gas engines

300

Wood chips

Wood chips

Clean wood

Wood chips

Reclaimed wood

IC engine

85

Gas engines 65

Gas engines 2000 X4

Gas engines 1000 ¥2

Gas engines 3000 ¥6

170

 

 

 

250

Installed in 2005 Operational

Installed in 2000 Not operational

In development 2007/08

In development

In development Q3 2007

Operational

Operational

Prime mover Output Output Status (kWe) (kWth)

Biomass/Wood Gas engines 100 chips

Clean wood

Feedstock

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 439

4/5/11 9:42:38 AM

Longlands Lane, Margam, Port Talbot

Courtauld Road, Burnt Mills Industrial Estate, Basildon

University of East Integrated Energy Gasification Anglia, Norwich, Utilities and NR4 7TJ, UK Refgas

Aberdeen

Courtauld Road

UEA Norwich

Seaton Energy Centre

Integrated Energy Utilities

Integra Developments

Eco2

Sawmills residues, forestry residues

IC engine?

 

Steam turbine

Steam turbine

Gas turbine

Gas

Gas Turbine

2500

 

1060

1400

4400

2000

13 800  

6000

280

55–75

Microturbine 250

Virgin forestry IC engine woodchips

Mechanical MSW Biological Treatment (MBT) and AD

Combustion

Pyrolysis and RDF from gasification mixed commercial, domestic and civic amenity waste

Western Wood Energy Plant

Compact Power/ HLC Henley Burrowes (both went bust)

London Borough of Livingstone

Wood chips

Waste leather

Pyrolysis and Clinical, gasification municipal waste

Downdraft fixed bed air gasification

London Borough of Livingstone Resource Recovery Facility

Compact Power (site acquired by Ethos Recycling/ Cyclamax)

Biomass Engineering Ltd.

Newton-leWillows

Downdraft fixed bed air gasification

Avonmouth Refuse Transfer Station, Kings Weston Lane BS11 0YS Bristol

Biomass Engineering Ltd.

British Leather Corporation

Compact Power Avonmouth

 

Construction by 2008

Comissioning Nov 2009

Permission granted July 2008

Opened in Sept 2009

Failed

Installed in 2001

Installed in 2002 Test operation

Test in 2004. Plant planned for 2005

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 440

4/5/11 9:42:38 AM

Great Thickthorn Purepower Farm, Bedforshire energy

Huntingdon, Cambridgeshire

Kimbolton Road, Ravensdaen, Bedforshire

Great Thickthorn Farm

Huntingdon

Kimbolton Road

Boughton Pumping Station, Nottinghamshire

Boughton Pumping Station

Combustion

Anaerobic Digestion

Pyrolysis

Pyrolysis

Pyrolysis

Rural Generation Downdraft Ltd. fixed bed air gasification

Birds Green REG Bio-Power Rattlesden Bury St UK Ltd Edmunds, Suffolk IP30 0RT

REG Bio-Power/ Agri-gen UK Ltd

Purepower energy

Purepower energy

LPL – Hockwold

Bentwaters Building 89 Power Generation Woodbridge IP12 Facility 2TW

Enterprise House, Loucetios Energy Combustion Unit G, Forge Way, Brown Lees Industrial Estate, Knypersley, Staffordshire ST8 7DN

Pyrolysis

Knypersley Renewable Generator

Intervate

Yorkshire

Technology

Esholt Sewage

Manufacturer/ Developer

Location

Generating station name

Table 17.3 Continued

IC engine

IC engine

IC engine

IC engines

 

Forest residues, wood chips

IC engine

Waste cooking IC engines oil

95

400

2000

1000

5000

4500

7200

 

200

 

 

 

 

Operational

 

 

 

Commissioned late 2009

Prime mover Output Output Status (kWe) (kWth)

Food and farm IC engines waste

Mixed waste wood

Mixed waste wood

Waste wood

Jatropha oil

Sewage

Feedstock

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 441

4/5/11 9:42:38 AM

Laragh, Enniskillen, Co Fermanagh

Balcas Timber

Bedfordia Biogas Milton Park Milton Ltd Ernest Bedford MK44 1YU

Granox Ltd, Desoto Road, West Bank Dock Estate, Widnes, Cheshire

PDM Group Widnes

 

Wykes Engineering

Goosey Lodge Wykes Power Plant NN10 Engineering 9LU

Tallbott’s

Tallbott’s

Goosey Lodge Power Plant – A,C,D

Harpers Adams University College

Eccleshall Raleigh Hall Biomass Farming Industrial Estate, Eccleshall, Stafford Forest residues, SRC

Integrated Anaerobic Digestion

 

95

95

 

Steam turbines

Steam turbines

200

200

200

786

2700

5500

Operational

 

Installed in 2006 Operational

Installed in 1992 Test operation

Installed in 1996 Not operational

10 000 Operational

16 000  

Microturbine 100

Microturbine 2000

IC engine

IC engine

Food and farm IC engines waste

Sawdust and woodchips

Bubbling MBM, food Fluidised Bed waste

Bubbling MBM, food Fluidised Bed waste

Combustion

Combustion

SRC

Willow chips

Rural Generation Downdraft Ltd. fixed bed air gasification

Enniskillen College

Enniskillen College, Londonderry, N. Ireland

Willow chips

Brook Hall Estate Brook Hall Estate, Rural Generation Downdraft Londonderry, Ltd. fixed bed air N. Ireland gasification

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 442

4/5/11 9:42:38 AM

Location

Manufacturer/ Developer

Bracknell Biomass   CHP Berkshire

Somerset

Charlton Energy Ltd. CHP Somerset

Bracknell Town Centre Biomass CHP

Buckland down

Charlton Energy Ltd. CHP Somerset

Ridgeway Grain Membury Hungerford Berkshire RG17 7TJ

Ridgeway Grain

Gasification

Combustion

Unit 5b Thorn Business Park Rotherwas HR2 6JT

Longma Thorn

 

Combustion

Rotating kiln gasifier

Biomass combustion boiler

Anaerobic Digestion

Integrated Anaerobic Digestion

Technology

Dowhill Farm – D Dowhill Farm Dowhill Girvan Ayrshire KA26 9JP

 

Turrif, Aberdinshire AB53 8BP

Biogask

Bedfordia Biogas Westwood,   Ltd Higham Park, Rushden, Northamptonshire

Generating station name

Table 17.3 Continued

Recycled industrial and agricultural wood

Recycled vegetable oil

7000

1100

340

Gas turbine

IC engines

3000

400

120

Reciprocating 7000 engines

Steam turbine

Waste cooking IC engines and vegetable oil

SRC and woody biomass

Wood chips

Food and farm waste?

1500

 

7000

7000

4500

 

Awaiting planning permission

Ongoing research

Post planning permission 2007

Failed (Nontechnical reasons)

 

Prime mover Output Output Status (kWe) (kWth)

Food and farm IC engines waste?

Feedstock

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 443

4/5/11 9:42:38 AM

Energy crops

Steam turbine

Food and farm IC engines waste?

Integrated Anaerobic Digestion

Combustion

South Shropshire The Business Park   Biowaste Digester Coder Road –D Ludlow Shropshire SY8 1XE

 

Fluidised bed/ Packging, Steam Vibrating woodchips and turbines grate boilers wood waste

Roves Farm, Wiltshire, Roves Energy

Slough Electricity 342 Edinburgh Contracts Ltd Avenue Slough

Roves Farm, Wiltshire, Roves Energy

 

35 000

2500

 

5000

 

Operational

Operational

444

Small and micro combined heat and power (CHP) systems

therefore may not come to fruition. On the other hand this is a dynamic area and new projects are likely to be forthcoming over the coming years. Table 17.3 can be rearranged in the form of charts for easier interpretation as in Figs 17.5–17.8. Figure 17.5 demonstrates that large projects (over 1 MWe) dominate the UK’s biomass CHP scene followed by schemes smaller than 400 kWe. This is of particular importance for market research of future opportunities in the CHP market because it dictates the current trend of biomass CHP appeal, thus the most likely market in which continued growth could be achieved as well as areas in which help is needed in order to promote future growth. Another 4% 32% 58%

6%

<400 kWe

400 kWe<X<1000 kWe

>1000 kWe

No data

17.5 UK biomass CHP arranged by size (%).

No data

Micro-turbines

IC engines

Steam turbine

Gas turbine 0

5

10

15 Number of units

20

25

30

17.6 UK biomass CHP arranged by prime mover.

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 444

4/5/11 9:42:39 AM

Biomass-based small and micro CHP systems

445

16 14

Number of units

12 10 8 6 4 2 0

Food and farm waste

Wood waste

SRC and clean wood

Vegetable oil Other waste

17.7 UK biomass CHP arranged by feedstock. 6%

12%

19%

13%

13%

37% Fluidised bed

Anaerobic digestion

Gasification

Pyrolysis

Combustion (no details)

No data

17.8 UK biomass CHP arranged by technology (%).

factor to consider when analysing units by output or size is the modularity of the small-scale schemes and the possibility of growing and thus achieving higher outputs by simply adding modules. Examples of small-scale modular biomass CHP systems are detailed in Sections 17.2.5–17.2.7.

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 445

4/5/11 9:42:39 AM

446

Small and micro combined heat and power (CHP) systems

Figure 17.6 shows a trend towards the use of internal combustion engines as prime movers when compared to any turbine technology. As expected for a mature technology, it dominates the market as a result of its reliability and expected performance. A similar situation is reflected in the figures specifically for steam turbines. It is important to mention that the micro-turbines referred to in this study are from Talbott’s, whose technology hold is on air-driven micro-turbines. The figure also exposes the need for continued research efforts into turbine technology, in order to fill the technical gap between turbines and IC engines. This would undoubtedly bring down costs and promote its use. In Fig. 17.7 the advantage is for short rotation coppice (SRC) and clean wood to be used as feedstock for biomass CHP in the UK. All other feedstocks are a waste variety, that is, food and farm waste, wood waste, recovered vegetable oils and other waste. This leads to further re-arranging into three distinct categories: ∑ ∑

clean fuel (SRC and clean wood) represents 32% fuels of waste origin (food, farm, wood and vegetable oil) stands for 51% ∑ other waste corresponds to the remaining 17%. If, on the other hand, all wood-derived fuel sources are grouped together, then the figures change as follows: ∑ wood fuels (SRC, clean and waste wood) will now represent 53% ∑ waste fuels (food and farm waste and vegetable oils) is now 30% ∑ other waste remains at 17%. Once more, this fact is important in terms of future opportunities as the market of wood-derived fuels and for the disposing (or making use) of waste seems to be one of the main drivers towards biomass CHP. Biomass gasification is an established technology that has been in the commercial market for some time now. In Fig. 17.8 the dominance of gasification over the other technologies is acknowledged. Combustion technologies, anaerobic digestion and pyrolysis are lagging behind gasification in terms of number of units in service and thus market penetration. It is significant to note that, despite its suitability for the disposal of organic matter, anaerobic digestion has not taken a significant proportion of the UK’s biomass CHP market, in a marked difference with other European countries. Summarising the findings of Table 17.3, the UK biomass CHP market can be characterised by a typical facility being larger than 1000 kWe, using waste as fuel in a gasifier feeding an internal combustion engine.

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 446

4/5/11 9:42:39 AM

Biomass-based small and micro CHP systems

447

17.2.5 Examples of past and present small-scale biomass gasification companies Biomass engineering Biomass Engineering (18) is a company that has succeeded in the fields of design, manufacturing and installation of biomass gasification systems. Using energy crops, waste wood, wood chippings and forestry wood as fuel, they have managed to secure the following projects. ECOS Millennium Centre, Ballymena, N. Ireland (19) A 65 kWe gasification system comprising a downdraft gasifier, char and ash removal, gas conditioning and cooling, with wood preparation (cutting, drying, conveying) and power generation with modified diesel engine, was provided to Ballymena Borough Council, Northern Ireland in 1999. The main objective was to supply heat and electricity for the ECOS Millennium Centre as part of a series of renewable initiatives built into the project. Over 3500 hours of operation on a wide variety of wood types was obtained, with continuous test runs of up to 10 hours (specified by the client). The gasifier has now been dismantled. Mossborough Hall, Merseyside (20) This is a 300 kWe CHP demonstration plant partially funded by the DTI. The plant is on a mixed use farm with a green waste licence and it is used to gasify chipped logs and clean waste wood. It operates unmanned with remote monitoring and comprises a wood-based downdraft gasification system with dry gas cleaning and two gas engines. The electricity produced is exported to the grid. Radiated heat and diluted exhaust gases are used for fuel drying. Little Woolden Hall Farm, Culcheth, Cheshire Biomass Engineering has installed and commissioned an 85 kWe CHP gasification system at a farm site in Culcheth, Cheshire for Jepson Bros Ltd, which comprises woodchip feeding, gasifier and power generation. Heat is used for wood drying and the electricity is sold to the grid. Biomass CHP Biomass CHP is another company heavily involved in biomass CHP projects in the UK based on gasification technologies. Among their projects are the following. © Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 447

4/5/11 9:42:39 AM

448

Small and micro combined heat and power (CHP) systems

Blackwater Valley Museum, Benburb (21) A 200 kWe/400 kWth biomass gasification plant designed by Exus Energy (formerly known as B9 Energy and now part of Biomass CHP) is located at the Blackwater Valley Museum, Benburb, Northern Ireland. It uses sawmill wood chips to provide heating for the museum and electricity for about 400 homes. The wood chips are dried from about 50% water content to 10–15% using waste heat from the engine cooling system before being fed into a downdraft gasifier. The producer gas is cleaned, cooled, mixed with air and fed into the engine with 10% diesel for ignition purposes. The engine heat exhaust is recovered via heat exchangers. The resulting hot water is then pumped to the radiators in the museum for space heating via an underground heating network. Overall energy efficiency is 70% and the capital cost has been estimated at about 7280 000. Beddington ZED (22) The Beddington Zero Energy Development Project (BedZED) was a novel mixed development (housing and offices), completed in 2002, owned by the Peabody Trust, London’s largest housing association. Biomass CHP designed, installed and commissioned a 130 kWe CHP unit which provided the site with electricity and heat in the form of hot water, distributed around the site via a district heating system of super-insulated pipes. The plant used off-cuts from tree surgery waste that would otherwise go to landfill. The CHP plant is no longer in operation and the system has been dismantled. Rural Generation Ltd Gasification is the main technology employed by Rural Generation Ltd in the following projects. Brook Hall Estate, Londonderry, Northern Ireland (23) Brook Hall Estate has been operating a wood-fuelled gasifier on the farm since 1996. Heat and electricity were initially produced from forestry residues, while willow coppice was being established. Wood chips or willow chips are fed to a downdraft gasifier linked to a modified Iveco diesel engine. The system can produce 95 kWe and 200 kWth. It ran in batch mode and typically operated for 12–14 hours per day producing electricity, which was exported to the UK National Grid while the thermal energy was used to dry cereals from the farm. The heat was also used in the winter months to dry willow chip that would subsequently be used in the gasifier. The project is currently not operational (24).

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 448

4/5/11 9:42:39 AM

Biomass-based small and micro CHP systems

449

Boughton Pumping Station The station houses a biomass CHP plant that runs on wood chip forest residue. The plant was installed in May 1999 but began operating regularly in autumn 2000 due to problems with the fuel source. Waste wood chips were converted into gas, which was then mixed with 20% diesel to fuel the engine. (Diesel is required for ignition and lubrication purposes.) The engine ran for 8 hours a day, providing 95 kW electricity and a thermal output of around 200 kW. Around 60% of the electricity generated was exported to the UK National Grid. This plant is not operational at the moment (24).

17.2.6 Examples of past and present small-scale biomass pyrolysis + gasification examples Compact power Compact Power, Avonmouth, Bristol (23) This project has been operating for 5 years. It processes 800–1000 kg of clinical and/or municipal waste per hour. The waste is first pyrolysed in two tubes and the char residue is steam gasified in a close coupled gasifier with the product gas mixed directly with the pyrolysis gas. The combined gas is then burned in a cyclone burner with the exhaust providing heat for the pyrolysis chamber and a steam boiler. A maximum of 280 kW of power and 2.5 MW of heat can be generated from the process. The unit is a demonstration for larger capacity units that will be built on a modular system with multiple pyrolysis tubes. The emissions performance is exceptionally good because of the good control of the gas combustion process and the high temperature. Dioxin levels have been measured at less than 0.003 ng/nm3. The unit is fully licensed for commercial use by the UK Environment Agency.

17.2.7 Examples of past and present small-scale biomass combustion examples Talbott’s Talbott’s Biomass Energy System (25) specialises in the development of combustion based energy systems. An example is presented below: Harper Adams University College (26) A biomass indirectly fired air turbine system developed by Talbott’s Heating is being demonstrated at a leading agricultural college in Shropshire, Harper Adams University College. The CHP demonstration plant provides 100 kW of electricity and 200 kW of heat employing a Brayton Cycle that uses

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 449

4/5/11 9:42:39 AM

450

Small and micro combined heat and power (CHP) systems

clean hot air for turbine operation. The electrical energy is sold back to the National Grid providing an extra income for the college. The thermal energy provides heating and hot water for the university buildings. The plant was officially launched in spring 2006.

17.3

Technical challenges for small-scale biomass combined heat and power (CHP) systems

Biomass CHP systems are subject to a number of technical problems relating to fuel and operation and are therefore often criticised. Both the biomass source and the technical challenges facing the prime movers are outlined in the following sections.

17.3.1 Difficulties associated with biomass fuels The more frequent difficulties associated with biomass fuels are described below. Energy efficiency, energy balance and carbon neutrality Although the burning of biomass releases the same amount of CO2 previously captured within the organic matter, the production and transportation stages of biomass do add to the greenhouse effect. The extent to which a particular biofuel has an impact on the global energy balance is still subject to debate, some claiming minimal or no CO2 reduction while others propose at least moderate reductions. Biodiversity and land use There is mounting concern that the increased use of biofuels may displace the native ecosystem into cropland for fuels. The loss of biodiversity is not only a moral pressure, but a technical one as well, as the crops will have a reduced ability to cope with blights. Food shortages The above concern also underlines the fear of food shortages if energy crops replace food crops. However, there is optimism that increases in food crop productivity and yields, together with better land use management and higher yields from second-generation biofuels (non-food-based crops) will overcome this issue.

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 450

4/5/11 9:42:39 AM

Biomass-based small and micro CHP systems

451

Supply and security of supply issues Biomass as a fuel is still, to a great extent, a developing market. In particular solid biomass has to deal with concerns associated with its bulky nature and large space requirements, widely dispersed nature, high handling and transport costs, low heating value or energy density, issues related to biomass conditioning or fuel preparation prior to conversion (such as drying and specific size requirements) and finally harmful emissions during the conversion process. Liquid biofuels face similar but substantially lesser difficulties, summarised in the steps normally taken towards avoiding/minimising contamination, upgrading, differences when compared to conventional liquid fuels and the discharge of harmful emissions such as smoke, particulates, and NOx. In terms of issues related to security of supply, a recent report by IEA in 2007 states that ‘the implications of the use of biofuels for global security as well as for economic, environmental, and public health need to be further evaluated’ (27).

17.3.2 Difficulties associated with prime movers IC engines These are the preferred prime movers for small-scale biomass CHP systems, mainly because of their higher efficiency when compared to other prime movers, fuel flexibility and robustness. However, a number of issues have been identified for use with biomass derived fuels, mostly related to fuel properties such as lower heating value and higher water content, which will in turn de-rate the engine output. Fuel quality is a major issue including high viscosity, particulates, stability, low temperature properties, acidity and consistency. General issues related to IC engines are the amount of vibration that needs to be damped, as well as issues with low frequency noise and maintenance, which is more frequent when compared to a turbine. Turbines The selection of turbines as prime movers is hindered by the very limited number of units claiming to be able to run on biomass fuels, as this technology is at an early development stage. The turbine efficiency will be negatively affected when fed by a lower heating value fuel and may not be possible at all in extreme cases. In addition, turbines tend to be more expensive than IC engines and maintenance is more technically demanding. Fuel conditioning is also important when operating a turbine with biomass derived fuel, especially in regard to removing solid particulates due to their corrosion and erosion effect. There is also a possibility that the nitrogen components in the biomass derived fuel will contribute to an increase in NOx emissions levels, although

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 451

4/5/11 9:42:39 AM

452

Small and micro combined heat and power (CHP) systems

generally speaking the emissions rates are lower when compared to an IC engine. Other general issues associated with turbines as prime movers in general are the higher noise levels, large amount of hot flue gases to handle, lower efficiency when compared to IC engines, low efficiency when operating at part load and low efficiency at high ambient temperatures. There are also concerns over durability and reliability that can only be overcome with very long-term trials and experience.

17.4

Capital costs for small-scale biomass combined heat and power (CHP) systems

17.4.1 Small-scale biomass gasification CHP There are some published data for costs for small scale biomass gasification systems: Biomass Engineering’s cost data are in the range of £2105/kWe for a 1000 kWe including installation but excluding biomass storage, dryer and shredder (28). Biomass CHP has two installations of 100 kWe and 200 kWe installations at Blackwater Valley and a 130 kWe installation at Beddington. These provide cost estimates of £1400/kWe to £2400/kWe installed. There is no size indication or systems included (29). Previous references showed that Kara Energy Systems in the Netherlands had gasification equipment available across a wide range of capacities; however, recent research shows that the company no longer offers biomass gasification equipment.

17.4.2 Small-scale biomass pyrolysis CHP The ‘Techno-economic assessment of power production from the Wellman and BTG fast pyrolysis processes’ paper by Peacocke et al. (30) has cost data, although for a significantly larger and more complex system. The cost of approximately £700–£950/kWe is given for a large scale (2600 kWe) biomass fuelled pyrolysis plant from wood reception to liquid storage, although excluding power generation. This compares well with data from Bridgwater et al. from 2002 (31).

17.4.3 Small-scale biomass combustion CHP Similar to small-scale biomass gasification and pyrolysis, there is very little published data for small-scale biomass CHP combustion systems. Talbott’s novel concept of a direct hot air micro-turbine estimated total capital costs of £2500/kWe for a 30 kWe installation, whereas a 500 kWe would be in the range of £1000/kWe (32, 33). There is no indication of what is included in this estimate.

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 452

4/5/11 9:42:39 AM

Biomass-based small and micro CHP systems

453

17.4.4 Small-scale biomass CHP in Europe An overview of typical technological characteristics of all the available small-scale biomass CHP technologies in Europe is given in Table 17.4 (34). The document also points out that although diesel and gas engines are a very well-established technology, few employ biomass derived fuels, which is claimed to account for only 1% of all biomass CHP technologies in operation. Biomass fed micro-turbines are under development. The UK has two mayor players in this field: Talbotts and Bowman Power Systems. Outside the UK, Ingersoll-Rand, Capstone (US based), Turboden and Turbec are well-known names of micro-turbine developers. A Capstone unit is being developed for biomass derived liquids at the University of Florence, Italy (see http://www.bioliquids-chp.eu/). Stirling engines are at the initial stage of market introduction in the UK. Intensive research is being carried out at the research institutions in Denmark, Austria and Germany, with commercial applications on sight in the short term (35). The organic Rankine cycle (ORC) is still relatively new in electricity generation but has already achieved a breakthrough in the market and is reported in detail in Chapter 9. Steam engines in the scale range between 0.3 and 10 MWe have been installed in Europe, but in the capacity range below 300 kWe the technology is relatively unproven (34).

17.4.5 European policy There are two relevant European policies affecting biomass CHP systems. The first is the Directive on the Promotion of Cogeneration based on Useful Heat Demand in the internal energy market (Directive 2004/8/EC), and the second is the Directive on the Promotion of Electricity produced from Renewable Table 17.4 Typical technological characteristics from production of small scale CHP  

Capacity range (kWe)

Diesel/gas engine

15–10 000 30–38

45–50

75–85

85–100

25 000–60 000

Microturbine

25–250

15–35

50–60

75–85

85–100, steam

50 000–75 000

Stirling engine

10–150

15–35

60–80

80–90

60–80

50 000–60 000

ORC turbine 200–1500 10–20

70–85

85–95

80–100

?

Steam engine

40–70

75–85

85–120

>50 000

20–1000

Electrical Thermal Overall Heat Lifetime (h) efficiency efficiency efficiency production (%) (%) (%) (°C)

10–20

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 453

4/5/11 9:42:40 AM

454

Small and micro combined heat and power (CHP) systems

Energy Sources in the internal electricity market (Directive 2001/77/EC). Both directives encourage the reduction in CO2 emissions through novel and efficient technologies. A number of common barriers for further biomass CHP/DH implementation were identified in surveys: ∑

low electricity prices combined with the fact that customers are not willing to pay extra for green electricity; ∑ lack of standardisation on the definitions and the ways of action; ∑ competitive priced fuels; ∑ immature biomass markets; ∑ informational barriers, i.e. lack of public awareness and information/ dissemination.

17.4.6 Novel biomass CHP technology in Austria Austria is one of the pioneers in developing and implementing novel biomass CHP technologies, including ORC, Stirling engines and both steam and gas engines. The OPET document ‘Micro and small-scale CHP from biomass (<300kWe)’ (34) details the pioneering activities of Austria in developing biomass CHP plants. It mentions an ORC cycle (400 kWe) successfully installed in 1999 in a wood manufacturing company in Admont and how the technology has been improved to a degree that a successful demonstration biomass district heating plant in Lienz, with a 1000 kWe capacity was installed in 2002. In the document there is a reference to the first Austrian small-scale biomass CHP plant based on a Stirling engine, with a nominal electric capacity of 30 kWe. Finally, it also mentions an innovative approach towards a steam engine based small biomass CHP, with a screw-type engine of 700 kW that started up in late 2003.

17.4.7 Small-scale biomass CHP in Sweden, Denmark and Finland The experience of the Scandinavian countries in CHP is varied, denoting developments influenced by regional policies (36). CHP in Denmark represents around 70% of annual power production and provides 80% of district heating needs; however, small scale is defined as anything below 99 MWe capacity. Nevertheless, most Danish decentralised plants are below 10 MWe. Around 18% of the total electricity production and 27% of the heat destined to district heating comes from small-scale CHP. The Danish government has had a sustained goal of incentives (subsidy) for small-scale CHP and encourages the use of indigenous fuels such as biogas, straw and wood. A similar situation arises with heat production in Finland, where CHP

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 454

4/5/11 9:42:40 AM

Biomass-based small and micro CHP systems

455

accounts for a 76% share for district heating. There are about 30 small scale (less than 20 MWe) CHP plants in Finland running on peat, biomass or natural gas. Most are conventional Rankine cycle, which is production of superheated steam to be used in a steam turbine coupled to a generator. Peat is by far the main fuel, although it is normally co-fired with wood. New plants under development will continue the same trend in relation to size and main fuel. In Sweden the contribution of CHP to the heat demand only reached about 10% in 2000, despite a well developed district heating network. In 2001, CHP in Sweden accounted for 7% of total power production. Most CHP in Sweden is large scale, limiting small scale to few applications, notably emergency back-up, landfill gas sites and small industrial applications (like sawmills) used to reduce external consumption. Thus small-scale biomass CHP does not represent a significant share of the heat and power market in Sweden.

17.5

Conclusions

The UK energy policy and associated targets are the main drivers behind both CHP and biomass-based CHP. As an integral part of the UK effort in reducing CO2 emissions, biomass is regarded as a key component in the energy mixed required to comply with the emissions reduction targets. A number of programmes have been developed by the UK government in order to promote renewable, including biomass use, and in particular for the CHP market including grants, pilot programmes and VAT reductions. Other schemes, however, have not seen continuity, in particular the Community Energy Programme in which district heating was being promoted. Although representing about 81% of all renewable contribution in the UK, biomass accounts for only 2.4% of all electricity generated and about 1% of overall heat produced. Since 2005 the electricity generation attributed to biomass has apparently reached a plateau in the UK market (Fig. 17.1). At the same time, the amount of heat produced using biomass has increased 32% since 2005. Recent studies show that the cost of biomass is becoming competitive (37). The 2008 UK Energy Digest suggests that there are around 1500 CHP schemes from all fuels including fossil and renewables in the UK, largely employing natural gas as fuel. CHP schemes in the UK represent 7.2% of all power generated, yet renewable fuels including sewage, biogas, municipal solid waste and refuse-derived fuels account for only 3.7% of all CHP schemes. In terms of number of schemes, the leisure sector leads followed by hotels and health care related buildings. Together these add up to 85% of the UK’s current CHP market. A number of micro-CHP units operating on natural gas are available

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 455

4/5/11 9:42:40 AM

456

Small and micro combined heat and power (CHP) systems

commercially in the UK, as reported in other chapters. This interest is driven by the high market potential, since a large proportion of UK households are considered suitable for micro-CHP units. The Government supports microCHP via a series of initiatives such as VAT rate reduction from 17.5% to 5%, reaping the environmental and social benefits, such as fuel saving and a reduction of greenhouse gas emissions. However, biomass-fired micro-CHP units are still under development and far from mass market penetration. The number of small-scale biomass CHP plants in the UK is gradually increasing under the Governments’ support for renewable energy sources. Energy from biomass (excluding waste) still makes a small contribution to the UK’s energy balance producing about 1.5% of electricity and 1% of heat. The slow progress could be explained by the combination of high investment costs, requirement for long-term agreements (both electricity and biomass supply), reliability problems and lack of general familiarity with biomass technologies. Increasing oil and gas prices and the attractive electricity sales prices for renewable sources might help to overcome many of the problems that have previously hindered the commercialisation of biomass-fired CHP in the size range up to 5 MW. At present a typical biomass CHP unit in the UK is larger than 1000 kWe, using some form of waste as feedstock for a gasifier using a conventional internal combustion engine for power generation. Both gasification and pyrolysis are equally applicable but there are insufficient reference cases to persuade investors and councils to invest in what is perceived as higher risk technologies. The new technology of indirectly fired micro-turbines is showing considerable promise for sizes up to 100 kW although there are few operational systems. The viability of CHP is strongly dependent on the availability of a suitable market for the heat which accounts for the seasonal variations in demand. In conclusion, biomass and particularly the high efficiency offered by CHP can offer good savings in carbon emissions while increasing the power and heat market security by the diversification of sources and technologies. Supported by fiscal measures, technological advances and taking advantage of other countries’ experience, the number of biomass-based CHP systems in the UK is expected to rise in all sectors in the short to medium term.

17.6

Acknowledgement

This work was partially supported by the European Commission through the Bioliquids CHP project (http://www.bioliquids-chp.eu/).

17.7

References

1 Department of Trade and Industry (2007), Meeting the Energy Challenge, A White Paper on Energy, Norwich, The Stationery Office. © Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 456

4/5/11 9:42:40 AM

Biomass-based small and micro CHP systems

457

2 EU Parliament (2001), Directive 2001/77/EC of 27 September 2001 on the promotion of electricity from renewable energy sources in the internal electricity market, Official Journal of the European Communities, p. L283/33. Available from: http:// eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2001:283:0033:0040:EN:P DF (Accessed 16 November 2009). 3 Biomass Task Force (2005), Report to Government, The Biomass Task Force/ Defra. 4 Department of Trade and Industry (2006), The Government’s Response to the Biomass Task Force Report, DTI/Defra. 5 Office of Public Sector Information, (2004), Energy Act 2004, Part 2, Chapter 1, Section 82. Available from: http://www.opsi.gov.uk/acts/acts2004/ukpga_20040020_ en_9 (Accessed 16 November 2009). 6 EU Parliament (2004), Directive 2004/8/EC of 11 February 2004 on the promotion of cogeneration based on a useful heat demand in the internal energy market and amending Directive 92/42/EEC, Official Journal of the European Communities, p. L52/50. Available from: http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=O J:L:2004:052:0050:0050:EN:PDF (Accessed 16 November 2009). 7 Department of Energy and Climate Change (2009), Low Carbon Building ProgrammePhase 2. Available from: http://www.lowcarbonbuildingsphase2.org.uk/index.jsp (Accessed 16 November 2009). 8 Department of Trade and Industry (2006), The Energy Challenge – Energy Review Report 2006, Norwich, The Stationery Office. 9 Department of Energy and Climate Change (2009), Digest of United Kingdom Energy Statistics (DUKES). Available from: http://www.decc.gov.uk/en/content/ cms/statistics/publications/dukes/dukes.aspx (Accessed 16 November 2009). 10 UK Energy Research Centre (2009), BIOENERGY (Online). Available from: http:// www.ukerc.ac.uk/support/tiki-index.php?page=ESBioenergy&structure=Environme ntal+Sustainability (Accessed 17 November 2009). 11 Ofgem (2009), A List of Stations Over 50kW Accredited for the RO and CCL (Online) Available from: http://www.ofgem.gov.uk/Sustainability/Environment/RenewablStat/ Documents1/Accreditation_OVER50kw.xls (Accessed 17 November 2009). 12 Future Energy Solutions (2005), Renewable Heat and Heat from Combined Heat and Power Plants – Study and Analysis, Published Version 1, Oxfordshire, AEA Technology. 13 Department of Energy and Climate Change (2009), Low carbon building programme, Biomass. Available from: http://www.lowcarbonbuildings.org.uk/micro/biomass/ (Accessed 17 November 2009). 14 Biomass Energy Centre (2010), Fuel cost per kWh. Available from: http://www. biomassenergycentre.org.uk/portal/page?_pageid=75,59188&_dad=portal&_ schema=PORTAL (Accessed 9 March 2009). 15 Department of Energy and Climate Change (2009), Energy Statistics: Combined Heat and Power. Available from: http://www.decc.gov.uk/en/content/cms/statistics/ source/chp/chp.aspx (Accessed 16 November 2009). 16 Department of Energy and Climate Change (2009), Digest of United Kingdom Energy Statistics 2009, Chapter 6: Combined Heat and Power, London, The Stationary Office. 17 COGEN Europe (2005), Micro-CHP Fact Sheet United Kingdom. Available from: http://www.internet-public-library.org/carbon-reduction/combined-heat-and-powerfact-sheet.pdf (Accessed 17 November 2009).

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 457

4/5/11 9:42:40 AM

458

Small and micro combined heat and power (CHP) systems

18 Biomass Engineering Ltd (2009), Biomass Engineering UK – Home. Available from: http://www.biomass.uk.com/ (Accessed 16 November 2009). 19 Biomass Engineering Ltd (2009), Biomass and Waste Gasification. Available from http://www.supergen-bioenergy.net/Resources/user/docs/7%20Peacocke%20 R-026%20SuperGen%20June%202005%20v1.pdf (Accessed 16 November 2009). 20 Green Energy UK (2009), Mossborough Hall. Available from: http://www.greenenergy. uk.com/Item.aspx?ITEM_ID=57 (Accessed 16 November 2009). 21 Energie Cites (2001), Biomass Wood Gasification-Cogeneration – Armagh, United Kingdom (Online). Available from: http://www.energie-cites.org/db/armagh_139_ en.pdf (Accessed 16 November 2009). 22 Peabody (2009), BedZED (Online). Available from: http://www.peabody.org.uk/ media-centre/case-studies/bedzed.aspx (Accessed 16 November 2009). 23 Kwant K W and Knoef H (2004), Status of Biomass Gasification in the Countries Participating in the IEA and GasNet, IEA Bioenergy Gasification/EU Gasification network. 24 Doran M (2007), Rural Generation Ltd, personal communication, 26 February. 25 Talbott’s Biomass Energy Systems (2009), Talbott’s Biomass Energy Systems. Available from: http://www.talbotts.co.uk/ (Accessed 16 November 2009). 26 Centre for Rural Innovation at Harper Adams University College (2006), Harper Adams develops a state-of-the-art generator. Available from: http://www.harper-adams. ac.uk/sustainability/doc/Kirby_S_Biomass_Generator.pdf (Accessed 16 November 2009). 27 International Energy Agency (2007), Contribution of Renewables to Energy Security, IEA Head of Publication Services, Paris. 28 Renewables East (2007), Commercial assessment. Advanced conversion technology (Gasification) for biomass projects. Juniper Consultancy Services, England. 29 Lensu T and Alakangas E (2004), Small-scale electricity generation from renewable energy sources. Organisations for the Promotion of Energy Technologies (OPET), Finland. 30 Peacocke G V C, Bridgwater A V and Brammer J G (2006), ‘Techno-economic assessment of power production from the Wellman and BTG fast pyrolysis processes’, in Science in Thermal and Chemical Biomass Conversion (Volume 2), Edited by A V Bridgwater and D G B Boocock, CPL Press, Newbury, pp. 1785–1802. 31 Bridgwater A V, Toft A J and Brammer J G (2002), ‘A techno-economic comparison of power production by biomass fast pyrolysis with gasification and combustion’, Renewable and Sustainable Energy Reviews, 6, 181–248. 32 Pritchard D (2003), Biomass combustion gas turbine CHP, Talbott’s Heating Ltd. 33 Lymberopoulos N (2004), Microturbines and their application in bio-energy, Centre for Renewable Energy Sources, Greece. 34 Organisations for the Promotion of Energy Technologies (2004), ‘Micro and smallscale CHP from biomass (<300kWe)’, Organisations for the Promotion of Energy Technologies (OPET), Finland. 35 Department of Trade Industry (2003), ‘Energy from biomass - a mission to Austria and Denmark’, Department of Trade Industry, London. 36 Organisations for the Promotion of Energy Technologies (2004), ‘Small scale biomass CHP technologies. Situation in Finland, Denmark and Sweden’, Organisations for the Promotion of Energy Technologies (OPET), Finland. 37 Department of Energy and Climate Change (2010), ‘Biomass prices in the heat and electricity sector in the UK’. E4tech, England.

© Woodhead Publishing Limited, 2011

CHP-Beith-17.indd 458

4/5/11 9:42:40 AM

18

Thermal-engine-based small and micro combined heat and power (CHP) systems for domestic applications: modelling micro-CHP deployment

K. M a h k a m o v, Northumbria University, UK

Abstract: In order to reduce demands on central energy supplies and to reduce carbon emissions, a number of technologies are being developed for application in domestic and commercial buildings. Micro (or domestic) and small CHP is one of such technologies. First, Section 18.2 describes types of thermal engines used as prime movers in CHP systems. Section 18.3 briefly reviews recent CHP appliance developments with thermal engines. Section 18.4 gives an overview of general principles used in modelling MCHP deployment in a household and estimation of benefits. Sections 18.5 and 18.6 demonstrate principles of obtaining typical heat and power demand profiles in an average household. Section 18.7 presents information on experimental performance mapping of a MCHP system on the test rig simulating a hydronic heating system in a semi-detached house. Finally, Section 18.8 provides an example of how the operation of an MCHP was simulated within a household, taking into account information generated on heat and electricity demands and data on the experimental performance of the MCHP system. Additionally, a summary of estimation of economical and environmental benefits from deployment of a MCHP system is presented in this section. Key words: micro and small CHP, thermal engines, deriving heat and electricity demand profiles, experimental performance mapping, analysis of benefits.

18.1

Introduction

There is a need nationally to use less energy and in the short term, energy efficiency across all sectors will be one of the most effective tools against the predicted environmental effects. Depending on the area, improved efficiency can come from behavioural changes (e.g. minimising energy waste) and implementing novel and advanced technology. The latter include more efficient and ‘cleaner’ industrial process equipment, transport with lower emissions/mile, buildings with improved passive thermal properties, improvement in the efficiency of electrical appliances, reduction of standby power in electronic devices, distributed generation, etc. 459 © Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 459

4/5/11 9:42:59 AM

460

Small and micro combined heat and power (CHP) systems

One of the key targets for increased efficiency for many years has been buildings. Vast quantities of energy are used for controlling the climate within both residential homes and commercial properties. Over 50% of greenhouse gas emissions in the UK come directly or indirectly from buildings. A number of technologies were earmarked for promotion within this sector and these included conventional products such as condensing boilers, energy saving lights and cavity wall insulation, but also micro-generation technology such as photovoltaics and, the subject of this paper, micro- and small-CHP (combined heat and power) units. Small gas and vapour/steam turbines, internal combustion engines and Stirling engines with power up to 50 kW and different types of fuel cells are all considered as contenders for application as heat-to-electricity converters in small- and micro-CHP units. Each of the above energy converters has advantages and limitations of design and operational features. There is a continuous ongoing debate between specialists as to which technology is most suitable and statements are made suggesting that there might be a niche market for each, or most of, these technologies. It is not a goal in this chapter to compare technical and economical performance of the various above converters, but to concentrate on providing a deeper insight into micro- and small-CHP technology, based on thermal engines. The experience accumulated and the technology evolved make it possible to make significant and rapid progress in the development of commercial and domestic CHP systems. A number of industrial companies across the world have announced readiness for commercial production of their micro- and small-CHP systems with the power ranging from 0.5 to 50 kWel.

18.2

Prime movers deployed in micro and small combined heat and power (CHP) systems

18.2.1 Rankine and organic Rankine cycle machines In the Rankine cycle, water is converted into the relatively high pressure and temperature water vapour (steam) in a special high temperature heat exchanger (boiler/steam generator). This vapour is then used to expand and drive a prime mover. After the expansion process, vapour may be partially converted into the liquid and at the end of expansion this relatively low pressure and temperature liquid–vapour mixture is circulated through a low temperature heat exchanger (condenser) in which all vapour is converted into the liquid state due to the cooling process. This liquid then is returned to the boiler/steam generator using a small feed pump. In organic Rankine cycles (ORC) special organic fluids with lower boiling temperatures are used instead of water. The thermo-physical properties of such fluids allow

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 460

4/5/11 9:42:59 AM

Thermal-engine-based small and micro CHP systems

461

a greater power output to be achieved for the same identical dimensions of the prime mover and of heat exchangers and the working fluid mass flow. Prime movers are usually reciprocating piston (Rankine cycle), scroll or screw (organic Rankine cycle) mechanism machines. Figure 18.1 shows schematic of a double-acting piston arrangement in which steam or vapour is supplied in turn into cylinders on the left and right. When steam is directed into the left cylinder, this coincides with a discharge of vapour from the right cylinder. The pressure of steam drives pistons which are connected to a linear rotor of a linear alternator, used to convert kinetic energy of linear piston motions into electricity. A scroll engine employs two identical involute spiral shape scrolls which are mated together inside the casing forming concentric spiral shapes (see (Fig. 18.2). During expansion, vapour formed in heat exchangers enters a casing through the inlet port in the centre of the casing and expands in crescent-shaped gas pockets forcing the orbiting scroll to orbit around the fixed scroll in the casing. The orbiting scroll is connected to the rotating shaft with a rotor of an alternator. During a single orbit, expansion of vapour takes place simultaneously in several gas pockets formed by two scrolls, 2

3

4

1

10

9

8

7

6

5

18.1 A schematic of the steam/vapour engine with reciprocating pistons and a linear alternator (the schematic is courtesy of OTAG [1]). 1 – the linear converter; 2 – the high pressure steam pipe; 3 – the right cylinder; 4 – the steam generating heat exchanger; 5 – the burner; 6 – connection to grid/load; 7 – the reciprocating core of the linear generator; 8 – valve; 9 – stator of the linear generator; 10 – the left cylinder.

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 461

4/5/11 9:42:59 AM

462

Small and micro combined heat and power (CHP) systems

18.2 A scroll engine.

18.3 Schematic of a rotary engine.

providing smooth, continuous operation of the engine. After expansion the fluid/vapour mixture leaves the casing through the exhaust port. The rotary screw engine layout is illustrated in Fig. 18.3. The engine consists of two intermeshing screws or rotors (also called male and female

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 462

4/5/11 9:42:59 AM

Thermal-engine-based small and micro CHP systems

463

rotors, respectively) which trap gas between the rotors and the compressor case. Both rotors are encased in a housing provided with gas inlet and outlet ports. High pressure vapour enters the casing and expands forcing two meshed rotors into rotational motion. The male rotor is connected to a rotor of an alternator. In both cases, engine lubricant is mixed with vapour and also used for cooling and sealing purposes.

18.2.2 Internal combustion engines Internal combustion engines have been in commercial production since the second half of the nineteenth century and presently this is a mature and very well-established technology. The engines used for CHP are usually four-stroke machines with one or several cylinders depending on the power output required. The fuel is natural gas from mains, biofuels, biogas (formed in bio-digestion) or syngas (produced as a result of biomass gasification). In these engines a combustion chamber in each cylinder is formed by the top of the piston, the head of the cylinder with built-in inlet and exhaust valves, and a cylinder liner (see Fig. 18.4). In micro-CHP systems natural gas is usually used as fuel and engines deployed are a single cylinder spark ignition engine. The spark plug is also built in the head of the cylinder. In order to avoid overheating of components during the engine’s operation, there are water jackets in the cylinder and in its head. Engines used for CHP have, as a rule, a conventional crank-shaft drive mechanism, placed in the crank-case below the cylinder/s. In such a mechanism the piston is connected to the shaft using a rod and the reciprocating motion of the piston is converted into rotational motion of the shaft. Natural gas and air are mixed in the inlet manifold before entering the cylinder. The cycle starts when the piston is in its top position in the cylinder and can be split into four stages: induction stroke – air-gas in (see Fig. 18.4(a)); compression stroke – all



(a)

(b)

(c)

(d)

18.4 Operational principles of a four-stroke spark ignition engine.

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 463

4/5/11 9:42:59 AM

464

Small and micro combined heat and power (CHP) systems

valves are closed (see Fig. 18.4(b)); combustion and expansion stroke (see Fig. 18.4(c)); exhaust stroke – combustion products are forced out of the cylinder (see Fig. 18.4(d)).

18.2.3 Stirling engines The Stirling engine is the common name for an external combustion heat engine which works on a closed regenerative cycle. Various design configurations have been developed for a wide range of applications which include solar dish/engine installations, biomass systems, air independent submarine propulsion systems, on-board power systems for deep-space missions, etc. This sub-section is focused only on Stirling engines developed for smalland micro-CHP purposes. The operational principle of this engine remains the same as for any heat engine: the useful power output work generated during expansion of the working fluid should be greater than the work used to compress the working fluid during the working cycle. The major feature of the Stirling engine is that the working fluid is sealed inside the engine’s volume and heat is transferred to and from the working fluid and from and to the internal gas circuit, respectively, through specially designed heat exchangers, namely a heater and a cooler, which are parts of the engine’s design and maintained at constant high and low temperatures, respectively. Figure 18.5 presents an example of a design arrangement of a Stirling engine. In this arrangement the engine has two variable working spaces, namely expansion and compression spaces, located in separate cylinders, namely ‘hot’ and ‘cold’ cylinders with their own pistons. These working spaces are connected to each other via channels of three special heat exchangers integrated into the design of the engine, namely the heater, the regenerator and the cooler. The walls of the ‘hot’ cylinder and the heater are kept at elevated temperature, using some heat source (combustion of fuels, nuclear heat, concentrated solar radiation, etc.). Similarly, the walls of the ‘cold’ cylinder and the cooler are kept at a lower temperature (e.g., using a cooling water jacket). The movements of the pistons are synchronised and arranged in such a way that the ‘hot’ piston leads by approximately 90–120 degrees of the crank-shaft angle. Such motions of the pistons result in the gas being located mainly in the ‘hot’ cylinder during the expansion stroke (heat is introduced into the cycle) and in the ‘cold’ cylinder during the compression stroke (heat is rejected from the cycle). Heat exchangers for the introduction of heat (a heater) and its rejection (a cooler) should have a surface-to-volume ratio as large as possible. The regenerator, which is placed between the heater and the cooler, represents a porous medium enclosed in the metallic casing. This porous medium is made from a material with a high heat capacity and should ideally have an

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 464

4/5/11 9:42:59 AM

Thermal-engine-based small and micro CHP systems

465

P (MPa)

V(m3)

P (MPa)

Crankshaft angle (deg)

V(m3)

Crankshaft angle (deg)

18.5 An example of a Stirling engine design arrangement and of a variation of the gas pressures and volumes in the ‘hot’ and ‘cold’ cylinders of the engine and pressure–volume diagrams.

infinite radial and zero axial conductivities. It acts as a heat sponge: heat is transferred to the material of the regenerator and stored when the working fluid flows from the ‘hot’ to ‘cold’ zone. The stored heat then is returned to the working fluid when it flows in the opposite direction. Thermo-insulation is usually used to separate the porous medium from the walls of its casing in order to reduce heat losses. The volumes of heat exchangers and of pipe joints are so-called ‘dead’ volumes since these decrease the compression ratio, which is the ratio of maximum and minimum volumes of the engine. In order to obtain high power output and efficiency, it is necessary to keep the compression ratio of the engine as high as possible. In Stirling engines the typical value of the compression ratio is about 2. Figure 18.5 also shows examples of the variation of the volumes in the ‘hot’ and ‘cold’ cylinders, the gas pressure of such engines and their pressure–volume diagrams over a single cycle. The difference in areas of the pressure–volume diagrams for the ‘hot’ and ‘cold’ cylinders represents work produced in the cycle (the indicated cyclic work). The product of the indicated cyclic work and the number of cycles completed per second is called the indicated power of the machine.

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 465

4/5/11 9:43:00 AM

466

Small and micro combined heat and power (CHP) systems

The thermodynamic efficiency of the ideal Stirling cycle with an ideal regenerator is equal to the efficiency of the Carnot cycle. In practice, the effective efficiency of existing Stirling engines is somewhat close to, or even less than, that of spark ignition engines, although there is great potential for improvement. Stirling engines can use any source of heat: fossil fuels (liquid, gaseous, coal), radioisotopes, burning of metals and renewable energy (hydrogen; biomass fuels in solid or gaseous state from wood, straw, etc., sewage; landfill gases; direct or accumulated solar radiation heat). When using fossil fuels, a continuous combustion process takes place in the burners and using different techniques it is possible to achieve a very low level of pollutants in flue gases. There is no explosive combustion of fuel in the cylinders and burning occurs at relatively low pressures. Furthermore, as there is no valve-train mechanism, the noise in an operating Stirling engine is significantly lower in comparison with internal combustion engines. When necessary, special drive mechanisms can be used to provide an almost ideal dynamical balancing of the engine, which virtually eliminates vibrations during its operation. The variation of the engine’s torque is significantly smoother than that in internal combustion engines. The fraction of the heat rejected from the cycle into the cooling system of the engine is approximately double that in internal combustion engines. This heat then can be used for CHP purposes. Gases with a high heat capacity and low viscosity, such as hydrogen and helium, are used in machines with a high performance. The pressure inside the engine may be up to 220 bar or greater, hence special attention should be paid to the sealing to prevent the leakage of these gases. To restrict costs, engines may be designed to run using air, nitrogen and other easily available gases. However, this results in an increase in the dimensions of the engine for the given level of the maximum pressure in the cycle and power output. Lubrication is another challenge when designing Stirling engines. It is undesirable to have an oil lubricant or its vapour inside cylinders and heat exchangers, since the lubricant may burn off and form a film deposit on the internal surfaces of the internal gas circuit of the engine. This will then dramatically decrease the heat transfer between the heat exchangers and the working fluid. An additional negative effect will be an increase in the hydraulic resistance of the regenerator, since it will be blocked by particulate matter in the products of the oil burning. Hence, in engines with oil lubrication of the drive mechanism, it is necessary to use special sealing techniques to prevent lubricating oil from passing to the cylinders. As far as the pistons are concerned, they have guiding and sealing rings made from fluoroplastic materials, similar to those used in compressors. When using piston rings made of this type of material, there is no need to specially lubricate them using conventional oils.

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 466

4/5/11 9:43:00 AM

Thermal-engine-based small and micro CHP systems

467

The ‘hot’ cylinder, the heater and the casing of the regenerator are made of a stainless steel type material in order to withstand high temperatures and pressures in the cycle (up to 1000 °C and 220 bar, respectively), which makes Stirling engines more expensive in their production than internal combustion engines. Another challenge is in the complexity of the control system of Stirling engines. If necessary, the control of the machine is achieved by the combination of a reduction in the heat output from the combustion chamber, pressure in the cycle or alternatively by increasing the dead volume of the engine. There are three main layouts of modern Stirling machines – so-called alpha, beta and gamma configurations. Fig. 18.6 shows a ‘V’ type alpha single-acting engine, which has two separate cylinders with pistons. Both the pistons are used to produce work from the engine. When using a conventional crank-drive mechanism the phase-angle in the motion of pistons is equal to the angle between the axes of cylinders, hence the cylinders are arranged at an angle of 90–120 degrees. An alpha engine can be designed to be a double-acting machine, in which the compression space of each cylinder is connected through the channels of the heat exchangers to the expansion space of the adjacent cylinder (see Fig. 18.7). Such configuration allows nearly double the power output for the same swept-volume of cylinders. Figure 18.8 presents a general scheme of the beta-type Stirling engine in which two pistons are installed in a single cylinder. The top piston is not

18.6 A V-type, single-acting alpha-configuration Stirling engine.

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 467

4/5/11 9:43:00 AM

468

Small and micro combined heat and power (CHP) systems

18.7 Arrangement of a double-acting 4-cylinder Stirling engine.

18.8 A beta-type Stirling engine.

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 468

4/5/11 9:43:00 AM

Thermal-engine-based small and micro CHP systems

469

used to produce useful work and its only function is to control the flow of the working fluid from the compression to the expansion space and viceversa and this piston is called a displacer. The useful work in the cycle is produced by the second piston which is called a power piston. Figure 18.8 shows one of the drive mechanisms used in such engine configurations, which is called a rhombic drive. This mechanism allows an ideal balance of dynamic forces to be achieved and takes the forces from the sides of the pistons. This results in reducing the wear of the sealing and the guiding piston rings. An ideal dynamic balancing also eliminates vibration during the operation of the engine. Finally, Fig. 18.9 shows the gamma configuration engine in which the compression space is split into two parts. The first part is located in the ‘hot’ cylinder under the displacer and the second part is located above the power piston in the ‘cold’ cylinder. Both the parts are connected to each other by channels made in the casing of the cylinders or by pipes. The design configurations of Stirling engines described above have kinematical drive mechanisms. Figure 18.10 demonstrates the general concept of a free-piston Stirling engine. In this concept the engine does not have a kinematical drive mechanism. The engine is usually of a beta or gamma type with a displacer and piston in a single cylinder and operates as a thermal auto-oscillating system. Pistons perform reciprocating motions, which are in

18.9 A gamma-type Stirling engine.

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 469

4/5/11 9:43:00 AM

470

Small and micro combined heat and power (CHP) systems Linear alternator

Stirling engine Regenerator

Rear flexure stack

Mover

Front flexure stack

Piston

Displacer

18.10 A general scheme of a free-piston Stirling engine (schematic courtesy of ENATEC Micro Cogen B.V. [11]).

fact forced oscillations caused by the varying pressure of the working fluid in the working spaces of the engine and the forces from mechanical or gas springs. The power piston is a ‘rotor’ of a linear alternator. Its reciprocating motion in the stator generates electrical power.

18.3

Product development in the micro and small combined heat and power (CHP) market

Micro- and small-CHP is essentially the scaling of the CHP principal for use in single/small groups of domestic dwellings or small commercial properties. These smaller units are led by heat demand for space heating or hot water, with electricity generated as a by-product, the opposite of most large-scale CHP plants. Hence, they are designed as replacements for conventional central heating units. The electricity produced is used within the household with any excess potentially exported to the grid. Currently, the technology of such units is still in its relative infancy, along with their deployment. The main benefit of micro-CHP is the primary energy savings from the elimination of electrical transmission losses and high operating efficiency of the system. This could bring considerable savings in fuel usage, emissions and energy bills. A number of technologies are under development for this application, including small external (e.g. Stirling and Rankine Cycle) and internal combustion engines (usually these are four-stroke spark ignition gas engines). For the associated final product to be successful, it must prove appropriate for the market. This includes such factors as cost, capacity, heat/power ratio, efficiency, reliability, size, noise, etc. The following is a brief summary of the analysis of the technology status within the sector.

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 470

4/5/11 9:43:00 AM

Thermal-engine-based small and micro CHP systems

471

18.3.1 Rankine cycle micro combined heat and power (MCHP) The Rankine cycle is based on external combustion and characterised by the working fluid undergoing a phase change (from liquid to gas) which can be utilised to achieve high power densities. The most familiar Rankine engine is the steam engine where water is boiled by an external heat source to cause an expansion and the generation of pressure which can be used to produce useful work. MCHP production units implementing the Rankine cycle are entering the market and Otag released the Lion Powerblock unit in Germany [1]. This is a double-acting free-piston steam engine with a linear alternator with outputs of 0.3–2 kWel and 3–16 kWth with overall efficiency of about 94%. The operating frequency is 40–75 Hz and noise level is between 48 and 54 dB. The dimensions of the MCHP is 126 mm (H) ¥ 620 mm (D) ¥ 830 mm (W) (see Fig. 18.11) and its weight is 195 kg. Other products being developed include a 1 kWel Kingston being developed by Genlec Energetix Ltd (Capenhurst, UK). This MCHP unit is based on organic Rankine cycle with a scroll engine (see Fig. 18.12) [2] and its overall and electric efficiencies are 90 and 10%, respectively. Cogen Microsystems (Australia) has developed a Rankine cycle Cogen Micro unit based on a reciprocating steam engine (see Fig. 18.13) [3]. There

18.11 Rankine cycle Lion MCHP (photo courtesy of Otag [1]).

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 471

4/5/11 9:43:01 AM

472

Small and micro combined heat and power (CHP) systems

18.12 Kingston 1 kWel MCHP unit (photo courtesy of Genlec Ltd [2]).

18.13 2.5 kWel Cogen Micro MCHP unit (photo courtesy of Cogen Microsystems [3]).

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 472

4/5/11 9:43:01 AM

Thermal-engine-based small and micro CHP systems

473

are domestic (2.5 kWel, 11/22 kWth) and small commercial (10 kWel and 44 kWth) versions of Cogen CHP systems, both with an overall efficiency of 90%. Dimensions and weight of the domestic version are 870 mm (H) ¥ 600 mm (W) ¥ 400 mm (D) and 60 kg. The size of the commercial version is 960 mm (H) ¥ 800 mm (W) ¥ 600 mm (D) and its weight is 175 kg.

18.3.2 Internal combustion engine (ICE) MCHP Currently, only the ECOWILL micro-CHP unit [4] is commercially available which is manufactured by Honda on the basis of the GE160V natural gas fuelled internal combustion engine. The engine is four stroke, water cooled single cylinder with overhead valves, with a cylinder displacement of 163  cm3. The system has 1 kWel and 2.8 kWth outputs with the option of an external auxiliary boiler. Dimensions of the system are 580 mm (W) ¥ 380 mm (D) ¥ 880 mm (H) and its weight is 83 kg. The overall efficiency of the system is 85.5% and power generation efficiency is 22.5%. It uses a catalytic converter along with an elaborate acoustic attenuation system to conform to the market requirements, although it might still not be suitable for indoor installation in its current configuration. In Japan a federation of three major organisations in the gas energy industry intends to sell a total of 235,000 units of ECOWILL by 2011. A variation of Ecowill, Freewatt, is sold in the USA. Freewatt has slightly greater power (1.2 kWel) and heat outputs and is sold complete with a heating system: a boiler for hot water or a furnace for hot air [5]. There are several other ICE-based CHP units, such as Baxi SenerTec Dachs (Germany) [6] and XRGI 15 of EC Power (Denmark) [7] with an electrical power output greater than 4 kWel, and these can be classified as mini-CHP units. The Baxi Dachs Mini CHP system is based on a four stroke, water cooled single cylinder internal combustion engine, which was specially developed by Fichtel and Sachs AG (Germany). Electrical and thermal outputs are 5.5 kWel and 12.5 kWth, respectively, with the electrical and total efficiencies being 30% and 79–92%. Dimensions are 720 mm (W) ¥ 1070 mm (D) ¥ 1000 mm (H) and the weight is 530 kg. The service interval is 3500 hours. XRGI 15 (see Fig. 18.14) is based on the four stroke, four cylinder water cooled Toyota gas engine. The swept volume of cylinders is 2237 cm3 and the engine is equipped with the oxidation catalyst. The electrical and thermal outputs are 6–15.2 kWel and 17–30 kWth, respectively. The total efficiency of the system is 92% with the electrical efficiency being 27–30%. Dimensions of the power system (only) are 1250 mm (H) ¥ 750 mm (W) ¥ 1110 mm (D) and the weight is 700 kg.

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 473

4/5/11 9:43:01 AM

474

Small and micro combined heat and power (CHP) systems

Control panel Heat distributor Power unit

18.14 XRGI 15 Mini CHP system based on Toyota gas engine (photo courtesy of EC Power A/S [7]).

3

4

5 2

1 6

18.15 1 kWel Whispergen Mk 5 MCHP unit (scheme courtesy of Whisper Tech Ltd, New Zealand [8]). 1 – alternator; 2 – engine; 3 – burner assembly; 4 – auxiliary burner; 5 – heat recovery heat exchanger; 6 – air fans.

18.3.3 Stirling cycle MCHP A number of Stirling engine-based units are being developed by Whisper Tech (New Zealand) [8], Disenco Energy (UK) [9], Microgen Engine Corporation (a consortium of the gas boiler companies Viessmann, Baxi, Vaillant and Remeha with Sunpower) [10], Enatec Micro-Cogen (Netherlands) [11] and Sunmachine [12]. The nominal electrical and thermal outputs vary between 1–3 kWel and 5–36 kWth respectively. Whispergen Mk 4/5 is shown in Fig. 18.15. This is built around a four

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 474

4/5/11 9:43:01 AM

Thermal-engine-based small and micro CHP systems

475

cylinder double-acting Stirling engine with a wobble-yoke drive mechanism (see Fig. 18.16). The system has two burners: main and auxiliary. The main burner is to transfer the heat into the working fluid (nitrogen) inside the engine through heat exchangers integrated in the cylinder heads. The auxiliary burner is used to boost the heat output during large heat demand periods. Both burners are of a premix surface type. The generator is a four pole single phase induction generator. The electrical and thermal outputs are 1 kWel and 5.5–7 kWth (up to 13–14 kWth), respectively. The dimensions of the MCHP unit are 491 mm (W) ¥ 563 mm (D) ¥ 838 mm (H) and its dry weight is 148 kg. E-ON in the UK supplied a relatively small number of Whispergen Mk 4 and Mk 5 MCHP units to households in the south of England and currently work is going on with manufacturing partners towards a full market roll-out of mass-produced units [13]. Efficient Home Energy Sl (or EHE, based in the Basque region of Spain), which is a joint venture between Whisper Tech

18.16 A ‘wobble-yoke’ drive mechanism of a Stirling engine in Whispergen (scheme courtesy of Whisper Tech Ltd, New Zealand [8]).

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 475

4/5/11 9:43:01 AM

476

Small and micro combined heat and power (CHP) systems

Limited (a subsidiary of the New Zealand company Meridian Energy Ltd.) and the Mondragón Corporation, has commenced a large-scale production of Whispergen MCHP units for the EU market (exclusive UK) [14]. A 3 kWel MCHP unit from DISENCO is still in the development stage and little information on the technical specification is available on this product. Figure 18.17 shows a schematic of this system which has approximately the same dimensions as a Whispergen (the size of a domestic dish washer) and is built around a single cylinder beta-type Stirling engine which uses helium as a working fluid and runs at a speed of 3000 rpm. The engine used in this MCHP was derived from the engine developed by Sigma Elektroteknisk (Norway). In order to deliver 3 kWel output, the engine should be pressurised to about 70–80 bar. A proposed thermal output is 15–18 kW th (up to 30 kWth in conjunction with a thermal storage) which makes the overall efficiency of the system 90%. Figure 18.18 schematically shows components of this MCHP unit. The Microgen MCHP unit is built on the basis of the extensively modified free-piston Stirling engine, which was originally designed by SUNPOWER Inc (USA), and it has a 1 kWel output [3] (see Fig. 18.19). Field trial results were encouraging, with the demonstration of net electrical efficiencies of over 15% in conjunction with an overall efficiency of 90%. Figure 18.20 demonstrates the wall-mounted Baxi Ecogen MCHP system [15] built around Microgen’s free-piston Stirling engine. The thermal output is 6–6.5 kW th (up to 24 kWth with an auxiliary burner), its dimensions are 920 mm (H) ¥ 426

18.17 A schematic of 3 kWel MCHP appliance (scheme courtesy of DISENCO [9]).

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 476

4/5/11 9:43:01 AM

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 477

4/5/11 9:43:01 AM

18.18 Components of 3 kWel MCHP (scheme courtesy of DISENCO [9]).

Return from heat store (cool) Flow to heat store (hot) Gas in

Fly wheel end cover

Crank case

Helium cooler (heat exchanger)

Pre-heat and burn Gas valve

Water overpressure valve

Water pump

Hot water from engine

Dampers

Engine mounting bars

Generator

Gas controller

Premix gas/air fan

Heat exchanger condenser

478

Small and micro combined heat and power (CHP) systems

18.19 A 1 kWel free-piston Stirling engine from Microgen (UK): the scheme on the right shows the engine with installed counter-weight dynamic balancing system (schemes courtesy of Microgen Engine Corporation [10]).

18.20 1 kWel Baxi Cogen MCHP unit.

mm (W) ¥ 425 mm (D) and the weight is 115 kg. The overall efficiency of the system is about 98% or greater (in condensing mode). Enatec, in collaboration with Rinnai (Japan), Bosch Thermotechnik (Germany), Merloni TermoSanitari (Italy), plan to carry out Europe-wide field

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 478

4/5/11 9:43:01 AM

Thermal-engine-based small and micro CHP systems

479

tests of their MCHP appliance. The system is built around a modification of a 1 kWel free-piston Stirling engine originally developed by Infinia Corporation (USA) (see Fig. 18.21). Enatec is developing two versions of the MCHP system also called HRe boilers (see Fig. 18.22), one of which has a built-in

18.21 1 kWel free-piston Stirling engine used in Enatec MCHP unit (photo courtesy of ENATEC Micro Cogen B.V. [11]).

18.22 Enatec 1 kWel MCHP unit; the unit has built-in heat storage tank (photo courtesy of ENATEC Micro Cogen B.V. [11]).

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 479

4/5/11 9:43:01 AM

480

Small and micro combined heat and power (CHP) systems

heat storage tank. The thermal output is 4–35 kWth (when auxiliary burner is on), the overall efficiency of the system is >98 in condensing mode. Finally, a 3 kWel Stirling engine MCHP unit is being built and sold by Sunmachine GmbH (Germany). It is built around a gamma (or alpha)-type Stirling engine with a wood pellet burner. The system is equipped with a 50 l-wood pellet storage tank. The electrical and thermal outputs are 1.7–3 kW el and 6.5–10.5 kWth. Electrical and overall efficiencies are 20% and >85%, respectively. Dimensions of this wood pellet MCHP system are 1160 mm (D) ¥ 760 mm (W) ¥ 1590 mm (H) and its weight is 410 kg. There is also the SOLO V161 Stirling CHP system (see Fig. 18.23) which was commercially available in Europe a few years ago [16]. This unit was produced by Stirling Systems GmbH and had 2–9.5 kWel and 8–26 kWth electrical and thermal power outputs, respectively. The SOLO V161 Stirling CHP was used for small commercial applications. The system was built around an alpha (v-type) Stirling engine which uses helium as a working fluid. The crankcase of the engine was unpressurised and separated from the internal gas circuit using special rod seals.

18.4

Overview of the method for estimation of economical and environmental benefits from deployment of micro combined heat and power (MCHP) technology in buildings

The method used for estimation of benefits from deployment of MCHP is described in detail in [17]. The micro- or small-CHP systems are located within or close to the property which utilises the electricity generated, with the recovered heat used to provide space heating and domestic hot water. A grid connection offers the possibility of frequency synchronisation with electricity import/export, should power demand not match the generation of the unit. The MCHP is usually developed as a drop-in replacement for the conventional hydronic central heating boiler. The potential direct benefits of the system are economy for the consumer, savings in primary energy and reduced emissions. The micro-CHP system generates electricity from the fuel, source (mainly natural gas, but also biofuels and diesel or heating oils can be used) with a high overall efficiency due to the heat recovery system. From the differential in price between electricity and fuel, noticeable savings can be made. Also, with the appropriate metering and trading arrangements in place, excess generation can be exported back to the grid for which the consumer would be compensated. Due to the high operating efficiency of around 90%, compared with the 40% of the national electricity supply, the system results in considerable savings in primary energy and resources. From the savings in primary energy, which currently are fossil

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 480

4/5/11 9:43:01 AM

Thermal-engine-based small and micro CHP systems

481

18.23 A 9.5 kWel CHP unit on the basis of an alpha V-type SOLO V-161 Stirling engine (photo courtesy of SOLO Kleinmotoren GmbH, Germany).

fuels, there are reduced greenhouse gas emissions. The magnitude of these benefits depends greatly on the deployment scenario, with the greatest gain achieved when the CHP system provides the maximum feasible amount of the thermal and electrical demands. Therefore, the system must suit the

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 481

4/5/11 9:43:02 AM

482

Small and micro combined heat and power (CHP) systems

situation in which it is employed. The numerous factors involved include the following: ∑

electrical and thermal demand profiles which depend on climatic conditions; occupancy size and pattern; dwelling type and house fabric details; equipment used and attitude towards energy use ∑ the design of the MCHP system which can be specified by its capacity, heat/power ratio, efficiency and performance characteristics, thermal storage used (type, size, efficiency), electrical stores (type, size, efficiency); heat delivery system and control system and regime ∑ differential in fuel and grid electricity prices, emission factors of fuel and grid electricity; electricity export tariff ∑ installation and maintenance costs.

Due to the vast number of variables involved, it is very complex to make generalised predictions about the local economic and environmental impact of installing a micro-CHP unit. A number of previous papers have already attempted to model the effects of different micro-CHP units. Thus Newborough assessed the benefits of using a generalised MCHP system without referring to a specific technology [18]. Peacock and Newborough analysed the effect of 1 kWel Stirling engine and 1- and 3 kWel fuel cell MCHP on energy flows in the UK electricity supply industry and their impact on domestic CO2 emissions based on the expected average efficiencies of the systems [19, 20]. Cockroft and Kelly compared the performance of four micro-CHP technologies based on an air-source heat pump, fuel cell, Stirling engine and internal combustion engine using general performance characteristics of the above, such as average efficiencies and heat-to-power ratio [21]. Hawkes and Leach highlighted the importance of temporal precision in describing heat and power demand date when assessing CO2 emission reductions with implementation of MCHP systems [22]. According to their investigations, economic and environmental benefits can be considerably overestimated through use of coarse data. This is due to the averaging effects which ignore the significance of peaks and troughs in demand and their effects on energy import/export. However, the calculations in the above publications were performed using only averaged values of the efficiency of MCHP units. Peackock and Newborough attempted to take into account simplified dynamic characteristics of a Stirling engine MCHP during the start-up and shut-down processes [23]. In this chapter it is proposed to predict economic and ecological benefits using the experimental performance maps of the MCHP in various scenarios in the modelling process. This approach is demonstrated using the Whispergen Mk 3 MCHP as an example. First, in order to assess the various competing MCHP technologies, suitable heat and electricity demand profiles in dwellings must be quantified. As the unit is designed for use within a single dwelling,

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 482

4/5/11 9:43:02 AM

Thermal-engine-based small and micro CHP systems

483

it will be subject to large fluctuations in both electrical and thermal loads, as well as their concurrency, due to the transient nature of a household’s energy requirements. Then, to evaluate the performance of the unit, a spreadsheet model of the hydronic heating system was constructed with the use of the developed daily heat and electrical loadings for weekdays and weekends for different heat demand bands.

18.5

Heat demand modelling

In the domestic sector, the average annual demand amounts to approximately 17 MWht and 4.6 MWhe of thermal and electrical energy, respectively [18]. This gives an annual heat-to-power ratio of 3.7. However, these are subject to significant variations dependent on a number of factors with little direct correlation to dwelling size. This makes it difficult to define a ‘typical home’ for the investigation from which savings predictions can be applied to the UK’s housing stock as a whole. For the purposes of these investigations, specific scenarios have been defined, based on common UK ‘components’. Due to the seasonal variations in household demand, the performance over a year has been calculated by producing daily profiles typical of each month of the year. The results from each of these profiles have then been weighted accordingly and combined to produce an overall picture. To ensure sufficient precision, the modelling procedure for this study have been event based, building up the demand profile on a 1 min time base from predicted events and operation of equipment. The proposed scenarios are based on the type of house that might be most likely to install a micro-CHP unit. This was perceived to be a reasonably modern, well insulated property where the occupants have an interest in the environment and energy saving measures. From statistics produced by the Office of the Deputy Prime Minister (ODPM) in 2004, the most common type of housing was semi-detached (33% of the total housing stock) and this forms the basis of the scenario investigated for two storey semi-detached dwelling with a footprint of 64 m2. The size of the dwelling is based on national averages in 2003 for housing of that type [24]. Trends in house size show significant downsizing has occurred over the last few decades and this is predicted to continue. It was assumed in modelling that the house is occupied by a medium-sized family, consisting of two middle age adults and two children, with a typical full-time work and schooling occupancy pattern as defined in Table 18.1. The household thermal demand consists of three components [18]: ∑

Space heating – supplied by the central heating system with extreme seasonal variation, dependent on climatic conditions, dwelling design, heat delivery mechanism, occupancy patterns and perception of comfort.

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 483

4/5/11 9:43:02 AM

484

Small and micro combined heat and power (CHP) systems Table 18.1 Occupancy pattern for modelling procedure Day

Active occupancy periods

Weekdays Monday–Friday

07:00–08:00

Weekend Saturday–Sunday

18:00–23:00 09:00–23:00

Total occupied time/week

∑

∑

58 hours

Domestic hot water – mainly supplied by the central heating system, but sometimes by electric auxiliary heaters. Little variation with season, but dependent on household appliances, occupants’ attitude to energy use, occupancy size and occupancy pattern. Cooking – supplied by number of appliances which may be fuelled by different fuels. Slight variation with season but dependent on cooking appliances, occupants’ attitude to energy use, occupancy size and occupancy pattern.

In this analysis, in order to characterise the thermal requirements of the proposed scenario, the following factors are included in the modelling process: ∑

The house is sited in the south east of England, where the Whispergen units are being initially installed. The house’s front faces south with a set of double glazed windows installed on the main south and northfacing walls. There are four vinyl windows in each of two main walls with 1 m ¥ 0.91 m and 2 m ¥ 0.91 m dimensions and a 13 mm air gap. The interior wall has a finish made of 100 mm bricks and the house has a gypsum board ceiling and wooden floor. Thermal performance is in line with Part L of the 2000 building regulations [25]. ∑ Internal design temperature is 21 °C. ∑ The house has a conventional hydronic central heating system consisting of a gas-fired central non-condensing boiler (h = 0.8) supplying a network of appropriately sized radiators and 150 litre hot water storage cylinder. ∑ Domestic hot water is supplied entirely from the hot water cylinder with appliances including hot fill washing machine, cold fill automatic dishwasher, standard shower, bath and washbasins. ∑ Cooking is with the use of gas-fired hobs, oven and grill supplemented by an electric microwave, kettle and toaster.

18.5.1 Space heating In order to calculate the space heating requirements of the dwellings throughout the year, a model was constructed in ‘Hot 2XP’, a program developed by the

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 484

4/5/11 9:43:02 AM

Thermal-engine-based small and micro CHP systems

485

Daily thermal space heating demand (kWh)

CANMET Energy Technology Centre [26] to analyse energy flows within buildings. Taking into account all the appropriate thermal gains (solar, occupants, equipment, cooking), building details and historical weather data, the variations in space heating requirements have been predicted as illustrated in Fig. 18.24. As can be seen, there is a significant variation between the months with no supplementary heating required during the summer months June to September and annual heat demand is 13,172 kWh. The program also calculates the required size of space heating system, based on the maximum rate of heat loss. The result for the semi-detached house model was 5 kWth. This corresponds to the combined thermal output of the radiators when operating, fully open, at their specified supply and return temperatures, i.e. the magnitude of the maximum instantaneous thermal demand from the space heating system. The Whispergen Mk 3 unit has a thermal output of approximately 6 kWth which meets the heat demand in the semi-detached house. The conventional control system of a central heating system consists of a ‘time led’ operation, with defined periods coinciding with the occupancy, between which the system follows the heat demand within the property. The space heating demand is monitored by a simple bi-metallic thermostat positioned within the property. This leads to a profile with a heat-up period and cycling once the design temperature has been reached. The magnitude of the demand is varied depending on the positions of the individual radiator valves. A common energy saving addition to the system is thermostatic radiator valves (TRVs) which automatically modulate the output of individual radiators to match the requirement of the room. The majority use the principal 45 40 35 30

Annual total = 13 172 kWh

25 20 15 10 5 0 Jan

Feb

Mar

Apr May

Jun Jul Month

Aug

Sep

Oct

Nov

Dec

18.24 Daily space heating requirements (averaged on a monthly basis).

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 485

4/5/11 9:43:02 AM

486

Small and micro combined heat and power (CHP) systems

of thermal expansion with an appropriate mechanism to steadily close the valve as the room approaches the desired temperature. They save energy by reducing the overheating of individual rooms and, if the boiler is capable of modulating its output, reducing the cycling of the boiler. In terms of the demand profile, TRVs lead to a shape illustrated in Fig. 18.25. The exact proportions and shape depend on a number of factors, such as the initial room temperature, the building’s thermal inertia, performance of the TRVs and current climatic conditions. In particular, the steady state will be subject to fluctuating thermal gains, e.g. solar or occupancy, and this will vary with the control system’s response to these. However, to simplify the modelling, the profile may be split into two stages as shown in Fig. 18.26: the initial heating period where the design temperature is reached followed by the steady state where the temperature is maintained. As the occupancy pattern is based around full-time work, the dwelling will be empty for a significant period in the middle of the day during the week; therefore there will be two timed periods of operation. At the weekend, there is a less prescribed pattern with the likelihood that the property is occupied all day, resulting in a single long period of operation. The occupied periods correspond to the steady state part of the profile (area 2) in Fig. 18.26, when the design temperature is maintained. Therefore the system will come into operation earlier to actually raise the temperature to that required. Taking into account the marked difference between weekdays and the weekend,



1

2

3

Thermal power

Room temperature

On

Off

Time

18.25 Idealised space heating demand profile with TRVs. 1 – The heat up period where the valves are fully open and the system operates at maximum power; 2 – The valves start to close, reducing thermal power, as the temperature approaches the set point; 3 – Steady state reached where thermal power balances heat loss from building.

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 486

4/5/11 9:43:02 AM

Thermal-engine-based small and micro CHP systems

1

487

2

Thermal power

Room temperature

On

Off

Time

18.26 Space heating demand profile for modelling purposes. 1 – Heat up period where valves are fully open and system operates at maximum power; 2 – Steady state reached where thermal power balances heat loss from building. Table 18.2 Daily space heating demand (kWh) Month

Semi-detached Actual

Banded

January

40

40

February

27

29

March

29

April

15

May

2

June

0

July

0

August

0

September

0

October

1

19 1.5

0

1.5

November

23

19

December

31

29

a representative profile for each type of day is required for each month. However, due to the symmetry of variation throughout the year, months with similar thermal demands can be grouped together so reducing the number of profiles required (see Table 18.2). Appropriate banding is shown in Fig. 18.27, which breaks the year into five different heating categories.

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 487

4/5/11 9:43:03 AM

488

Small and micro combined heat and power (CHP) systems

Daily thermal space heating demand (kWh)

45 Actual 40

Banded

35 30 25 20 15 10 5 0 Jan

Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month

18.27 Daily space heating bands used for modelling.

This grouping requires ten separate daily profiles for the dwelling. These are generated using the occupancy pattern in conjunction with the banded daily requirements and output ratings of the systems from the CANMET program. The results demonstrate there is a large variation in the steady state power required from the central heating boiler.

18.5.2 Domestic hot water An average person in the UK uses 49 litres of hot water per day, with little seasonal variation [27]. The actual consumption is linked to the composition of the household, e.g. single or multiple occupiers, over or under 60 years of age, young or dependent children. It also depends on the equipment used and their specification, e.g. automatic washing machine, high flow rate power shower, etc. For the prescribed scenario of a couple with two dependent children utilising an automatic dishwasher, the average daily consumption rises slightly to 50 litres per person, resulting in a household consumption of 200 litres per day. This requires an annual thermal input of 4245 kWhth. As stated, this is supplied from the hot water storage cylinder within the gas-fired central heating system. This thermal storage method uses a heat exchanger within the cylinder to transfer heat from the closed loop boiler circuit to the surrounding water in the cylinder when the central heating system is active. This is regulated by

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 488

4/5/11 9:43:03 AM

Thermal-engine-based small and micro CHP systems

489

a thermostat on the tank which restricts the maximum temperature to 60 °C. Domestic hot water is drawn off the top of the cylinder, as required, which is then refilled from the cold header tank. This system effectively creates a buffer between demand and generation, providing hot water outside the operation of the central space heating system. The size of the buffer, i.e. storage cylinder, is governed by the overall system design and the demands placed upon it. The capacity specified in the scenario is 150 litres which is around average with a capacity of 8.73 kWhth, assuming an increase of the temperature of water from 10 °C in mains to 60 °C in the cylinder. This part of the thermal demand forms the base load for the system as it demonstrates little seasonal variation, therefore, only two daily profiles are required; weekday and weekend. The same profiles will also be applied to both scenarios. To form the profiles using an event-based method, the uses and their requirements must be identified along with an appropriate schedule. The majority of hot water is required for one of five main activities. These are summarised in Table 18.3 along with their proposed characteristics for the purpose of modelling. There are obviously further miscellaneous uses, e.g. car washing, which are not daily and make them difficult to integrate into the model. However, they are relatively small and can be eliminated without affecting the accuracy of the model. The schedule and frequency of the above events can be calculated by considering the activities of a household throughout the day and the estimated daily usage of 200 litres. The two proposed profiles are shown in Fig. 18.28. As expected, the weekend profile is more distributed throughout the day whilst the weekday profile shows concentrations around the limited occupancy periods.

18.5.3 Cooking Cooking requires high temperature inputs that are not supplied by a central heating system. However, the heat produced does contribute to the thermal gains of the property, slightly reducing the auxiliary space heating required. If Table 18.3 Use of domestic hot water DHW Use

Flow rate Average (l/min) duration/ event (min)

Total volume/ event (l)

Thermal power required (kW)

Energy used (kWh)

Automatic washing machine Showering Bathing Hand washing dishes Hand and face washing

6 5 6 6 4

12 35 54 12 4

20.9 17.4 20.9 20.9 13.9

0.696 2.030 3.135 0.696 0.232

2 7 9 2 1

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 489

4/5/11 9:43:03 AM

490

Small and micro combined heat and power (CHP) systems

25 Weekday Weekend Thermal power (kW)

20

15

10

5

0 00.00

03.00

06.00

09.00

12.00 Time

15.00

18.00

21.00

18.28 Domestic hot water thermal power requirements.

the thermal loads due to cooking are met by electricity, the electrical demand profile will demonstrate large fluctuations due to the high power requirements of the thermal devices. As the main cooking appliances in the scenarios are gas-fired, the effects on the electricity profile will be less dramatic.

18.6

Electrical demand

It was assumed in the modelling process that the household electrical demand consists of three types of load: ∑

Base load – a constant load of about 100 We due to the standby power consumption of most electrical goods, plus the cyclic load from refrigeration devices. This is present all day, throughout the year, with little variance. ∑ Biased load – a load that occurs on most days at similar or predictable times, e.g. lighting, television, cooking appliances. These arise through external influences or habitual behaviour. ∑ Elective load – a load operated primarily at the occupier’s discretion, e.g. washing machine, PC. These are harder to predict due to their more irregular nature. The daily electrical profile is made up of a combination of these loads, with the peaks occurring during periods of coincidence. The profile as a whole demonstrates little seasonal variation, except with changes in lighting periods and seasonal equipment. Taking the average annual electricity consumption at 4.6 MWhe [18], the daily usage is approximately 12.6 kWhel.

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 490

4/5/11 9:43:03 AM

Thermal-engine-based small and micro CHP systems

491

As with the domestic hot water, to form the profiles using an event-based method, the electrical loads and their characteristics must be identified along with an appropriate schedule. Recent years have seen significant improvements in the efficiency of household appliances. With increased public awareness through advertising and literature on the subject, ‘A’ rated appliances are becoming the standard for new and replacement purchases. From previous research on the subject regarding ownership and energy consumption trends [28, 29], the details of the appliances to be included in the model, along with their usage patterns and consumption, were compiled and these are shown in Table 18.4. This table contains only a certain number of the electrical loads present within a household. However, these were thought to be the most influential for the investigation due to either their peak power or usage pattern. In order to create the daily profiles, a suitable schedule for operation must also be created. Due to the slight seasonal variation, it was decided to use the approach adopted for the space heating profiles and generate a pair of profiles, weekday and weekend, for each of the heating categories. This still captures the variation to a suitable degree as it follows a similar symmetric variation through the year. A number of electrical appliances within the household are not used daily, as becomes evident from Table 18.4. This creates problems when generating Table 18.4 Electrical appliances included in profile model Category

Appliance

Category Energy consumption

Usage pattern

Peak power

–

Base load

Base

876 kWh/year

Permanent

100 W

Cold

Fridge-freezer

Base

584 kWh/year

Permanent

270 W

Wet

Washing machine

Elective

1.21 kWh/cycle 274 cycle/year

2.2 kW

Electric tumble dryer

Elective

1.76 kWh/cycle 148 cycle/year

2.5 kW

Dishwasher

Biased

1.46 kWh/cycle 251 cycle/year

2.2 kW

Televisions

Biased

190 kWh/year

6.5 hrs/cycle

80 W

PC

Elective

500 kWh/year

3 hrs/cycle

500 W

Vacuum cleaner

Elective

50 kWh/year

1 hr/week

1.5 kW

Lawnmover

Elective

30 kWh/year

1 hr/week (seasonal)

1.5 kW

Iron

Elective

100 kWh/year

0.5 hr/cycle

2 kW

Lighting

Lighting

Biased

440 kWh/year

3–10 hrs/day

1 kW

Cooking

Electric kettle

Elective

250 kWh/year

4–5 cycles/day

2.5 kW

Electric toaster

Elective

70 kWh/year

2–3 cycles/day

1.5 kW

Microwave

Elective

75 kWh/year

300 cycles/year 1.3 kW

Brown

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 491

4/5/11 9:43:03 AM

492

Small and micro combined heat and power (CHP) systems

single daily profiles to be applied to extended periods. To overcome this, the operation of these non-daily loads will be evenly spread over 52 weeks to create weekday/weekend profiles to achieve an effect similar to the usage pattern prescribed. As before, the actual daily schedule was created by considering the activities of a household throughout the day and translating this using the acquired appliance information into an electrical demand profile. Examples of those generated are shown in Fig. 18.29. The weekend profile shows a more distributed load with 14.3 kWh of total and 5.35 kW of peak loads against 13.8 kWh and 4.58 kW of total and peak loads, respectively, in weekdays. However, both profiles have similar peaks in the evening due to the high concurrent use of appliances in this period.

18.7

Performance mapping

To produce a comprehensive performance map of the Whispergen Mk 3 MCHP unit, the following key areas were focused on: ∑ response to heat demand (start-up and run-down characteristics); ∑ maximum outputs (electrical and thermal); ∑ modulation capabilities; ∑ full and part load efficiencies; ∑ emission analysis. The Whispergen Mk 3 unit has been installed within a test rig which consists of a small-scale mock-up of a conventional hydronic space heating

6.0 Weekday 5.0 Electrical power (kW)

Weekend 4.0 3.0

2.0

1.0

0.0 00.00

03.00

06.00

09.00

12.00 Time

15.00

18.00

21.00

18.29 Electrical profiles for pairing with 40 kWh heating band.

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 492

4/5/11 9:43:04 AM

Thermal-engine-based small and micro CHP systems

493

system (theoretical output of 6 kWth) and the test rig contains a number of meters: Ultrasonic heat meter (Landis Gyr+ 2WR5) – measures the flow rate through the radiators and the heat supplied. ∑ Data logging via RS232 – PC connection to CHP unit itself to log main parameters including electrical output. ∑ Gas meter – monitors gas consumption to calculate overall efficiency. ∑ Gas analyser (RBR Ecom KD) – analyses exhaust composition using heated sample system with probe placed directly in the exhaust flue of the unit.

∑

The heat demand for the system, usually indicated by a combination of a time switch and thermostat, was simplified to a time switch which allows simulation of timed periods of demand. The variables within the rig during experiments were periods of heat demand (duration and pattern), set point temperature of hot water in Whispergen Mk 3 (60–75 °C) and position of radiator valves. A number of experimental configurations were executed whilst monitoring the system. These have been analysed with the relevant information, figures and characteristics extracted for inclusion in the relevant following section. One of the important characteristics that affects the unit’s provision of heat and power is how fast it switches into operation after the heat demand is signalled. This is influenced by a number of factors, such as the control strategy, thermal mass of the cylinder walls and burner output; as with all external combustion engines, it must warm up before it will begin to operate. To evaluate this aspect, the thermal and electrical outputs at the start and finish from a number of cycles were examined and compared.

18.7.1 MCHP performance as a response to heat demand: start-up Start-up characterirics of the MCHP unit were recorded from a number of separate cycles for a 30-minute period following a heat demand signal. In all these cycles the central heating system was starting from room temperature. The results demonstrated significant variation in the time taken to reach a semi-steady state; between 10 and 20 minutes. All of the profiles have very similar shapes and gradients, hence the variation arises from the initial delay in the system before it appears to start. The source of this delay is believed to be internal diagnostics by the control system with dependence on external variables such as the air and current central heating water temperatures. From examining a single pair of electrical and thermal profiles, distinct periods in the start-up procedure can be identified, as shown in Fig. 18.30.

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 493

4/5/11 9:43:04 AM

Small and micro combined heat and power (CHP) systems 6

1500

5

Thermal output

4

Electrical power (W)

1000 Electrical output 500

0

Preheating engine Initial diagnostics

3 2 1

Engine comes to operation

0 Alternator is used as a motor to start the engine

–500

–2 –3

Diagnostic run of the engine –1000

–1

Thermal power (kW)

494

0

5

10 15 20 Time from startup (min)

25

30

–4

18.30 Breakdown of Whispergen Mk 3 start-up characteristic.

18.7.2 MCHP performance as a response to heat demand: run down A similar comparison was made between the profiles recorded after the end of the heat demand. Governed by the control system, the unit operates on a low power level for approximately 30 minutes after the heat demand ceases. Compared with start-up, the profiles are equally consistent in terms of their shape and gradient, but much more so in regards to timing, with two shutdown procedures apparent depending on the power level prior to shutdown. Notable thermal energy is supplied after the shutdown of the engine as the heat stored within the cylinder walls is transferred to the exhaust heat exchanger by the continuing operation of the burner fan. An examination of the run-down characteristic is given in Fig. 18.31.

18.7.3 MCHP performance as a response to heat demand: maximum outputs The maximum outputs were measured by operating the system at 75 °C with the radiators fully open for a three hour heat demand period. This produced the profiles shown in Fig. 18.32. The thermal power reached 80% of its maximum after 20 minutes, plateauing at approximately 6.25 kW after 2 hours while the electrical power reached its steady peak of 850 W after

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 494

4/5/11 9:43:04 AM

Thermal-engine-based small and micro CHP systems

495

1500

6 5

Thermal output

4 Engine continues to operate at low power level

Dissipation of internal heat

Burner fan stops

3 Thermal power (kW)

Electrical power (W)

1000

2

500 Electrical output

1 0

0 Engine stopped with gradual withdrawal of heat

–1 –2

–500

–3 –1000

0

10

20 30 40 Time from end of heat demand (min)

50

60

–4

18.31 Breakdown of Whispergen Mk 3 run-down characteristic.

8 6

1000

0

4

Electrical output Thermal output

500

0

50

100

2 150

200

–500 –1000

0 250 –2 –4

Time (min)

Thermal power (kW)

Electrical power (W)

1500

–6

18.32 Maximum outputs of WhisperGen Mk 3 unit.

20 minutes. From examination of the gas consumption, this consisted of a single burner firing rate with the variation due to the system ‘warming up’.

18.7.4 MCHP performance as a response to heat demand: modulation capabilities As the thermal demand within the household will vary and often be less than the unit’s maximum output, it is important to analyse modulation capabilities

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 495

4/5/11 9:43:04 AM

496

Small and micro combined heat and power (CHP) systems

of the system. To investigate this, two of the radiators were three-quarters closed, the set point temperature reduced to 65 °C and a two hour heat demand period programmed. This produced the profiles shown in Fig. 18.33. From these results, it becomes apparent that the unit can only operate continuously on two separate power levels. In this run, the unit operated at full power to bring the flow temperature close to that of the set point, after which it switched to a lower level. However, this still provided too much thermal energy causing the flow temperature to continue rising. This resulted in the engine switching off entirely for a 12-minute period while the temperature dropped. The pattern was then repeated when the engine restarted. The unit is capable of little modulation, with only steady state thermal outputs of 5.3 kWth and 6.25 kWth. This will produce a high cycling rate during long periods of low thermal demand which will be detrimental to the performance of the unit.

18.7.5 Efficiency per cycle and power level efficiencies The economic performance of the unit will be governed by both its overall efficiency and heat-to-power ratio (HPR). This is due to the significant difference in gas and electricity prices. The efficiency and HPR can be examined on two levels: ∑

Per cycle basis using the total output energies and gas consumption for the period. This can be used to compare modulated and non-modulated cycles. Power level basis using the recorded trends within a cycle. This can be used to analyse the full and part load efficiencies.

∑

Thermal power (kW)

7

70

Set point temperature

6

60

5

50 40

4 3

Thermal output

2

Flow temp

30

1 0

–1

0

20

40

60

20

Unit switches off for a period 80 100 Time (min)

120

140

10 160

0

Water flow temperature (°C)

For comparison, the profiles from Figs 18.32 and 18.33 were analysed along with their gas consumption data. The results are presented in Table 18.5. The figures show a significant difference in both the efficiencies and ratios for the compared cycles. As anticipated, the modulated cycle with its engine

–10

18.33 Modulated thermal output of Whispergen Mk 3 unit.

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 496

4/5/11 9:43:04 AM

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 497

4/5/11 9:43:04 AM

2 hours

Modulated

24.74 kWh 14.99 kWh

2.312 m3 1.401 m3 1.120 kWh

2.564 kWh

Input energy* Total electricity generated

Gas consumed

*Based on a calorific value of 10.7 kWh/m for natural gas

65 °C 3

3 hours

Full power

75 °C

Heat demand Set point duration temperature

Cycle

Table 18.5 Example cycle efficiencies

11.207 kWh

19.984 kWh

Total heat generated

10.00

7.79

82.23%

91.4%

Heat-to-power Overall ratio efficiency

498

Small and micro combined heat and power (CHP) systems

shutdown period is nearly 10% less efficient. Also, the proportions of heat generated are greater, further decreasing the comparative economic value. The instantaneous efficiency of the unit was analysed for the two steady state power levels, namely full and intermediate power levels. This was achieved using two 3 hour runs at 65 °C and 75 °C and their gas consumption. The data used to calculate the efficiency were taken once the outputs had stabilised. The results are shown in Table 18.6. Both power levels have high overall efficiencies of over 90% but the major difference lies in the heatto-power ratio. This is almost doubled when the unit switches from full to intermediate power. Hence, the possible economic returns will be severely diminished if the unit operates at the intermediate power level for extended periods due to the reduced proportion of electricity produced.

18.7.6 Emissions analysis As one of the major drivers behind micro-CHP development is the reduction of harmful gas emissions, the composition of the Whispergen’s exhaust is very important. This was investigated using the gas analyser included in the test rig. The combustion process inputs are natural gas and air. Therefore, the components of flue gas are primarily made up of compounds of oxygen, nitrogen, hydrogen and carbon. The efficiency of the combustion process can also be determined from the combustion products. This was achieved using the oxygen and/or CO2 measurement in conjunction with pre- and postcombustion gas temperatures. The calculation was performed automatically by the gas analyser. The exhaust composition and combustion efficiency was logged at two-minute intervals for a number of experimental runs. An example from a three hour run is shown in Fig. 18.34. The gas analyser relies on an electrochemical reaction for its readings and must be purged at regular intervals during continuous measurement, hence the short gaps in readings. The actual amount of carbon dioxide produced depends on the type and amount of fuel burnt, hence this was a function of gas consumption rather than the burner design. For natural gas, this was 0.19 kg/kWh. Carbon monoxide is the product of incomplete fuel combustion from the lack of oxygen. As the Stirling engine design is based on continuous external combustion, which is easily optimised, the combustion process is highly efficient as indicated. This results in little formation of carbon monoxide once the process has stabilised. The principal source of NOx emissions in natural gas appliances is the oxidation of excess atmospheric nitrogen at temperatures above 1100 °C. Modern applications require the minimisation of NOx through advanced burner design. Emissions can be greatly reduced by rapid or complete mixing of the gas/air fuel supplies and encouraging the internal recirculation of combustion

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 498

4/5/11 9:43:04 AM

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 499

4/5/11 9:43:04 AM

3 hours

3 hours

Full

Intermediate

850 W (11.1%)

7.640 kW 6.227 kW

0.714 m3/hour 0.582 m3/hour 400 W (6.4%)

Electricity power

Gas Input power* consumption rate

*Based on a calorific value of 10.7 kWh/m3 for natural gas

65 °C

75 °C

Heat demand Set point duration temperature

Power level

Table 18.6 Power level efficiencies

5.3 kW (85.1%)

6.25 kW (81.8%)

Thermal power

13.25

7.35

91.54%

92.88%

Heat-to-power Overall ratio efficiency

500

Small and micro combined heat and power (CHP) systems

140

NOx (ppm) CO (ppm) CO2 (%) Combustion efficiency (%)

*Peak 1960 ppm

120 100 80 60 40 20 0

0

50

100 Time (min)

150

200

18.34 Exhaust composition analysis of Whispergen Mk 3 unit.

products. This minimises the peak temperatures in the combustion zone. The Whispergen Mk 3 unit has peak NOx emissions of over 120 ppm. During steady state operation this level is about 80 ppm. Typical NOx emission level in conventional burners is between 110 and 150 ppm. In burners with the fully pre-mixed combustion process, the NOx emission level is reduced to 45–75 ppm whilst advanced burners, such as catalytic stabilised and catalytic, have NOx emissons less than 5–10 ppm [30].

18.8

Economic and environmental analysis

To evaluate the performance of the unit, a spreadsheet model of the hydronic heating system was constructed with the use of the daily developed heat and electrical loadings for weekdays and weekends for different heat demand bands. The mapped characteristics of the unit were applied, and the system performance calculated for each of the pairs of electrical and thermal demand profiles. The time step in the modelling process has been to one minute. This is necessary in order to make it possible to reflect the gradual increase or decrease in the thermal and electrical outputs when the engine is switched on or off. The results were weighted according to the banding strategy to form a picture of the unit’s performance over a typical year in each scenario. These were then analysed for efficiency and the anticipated economic and environmental savings of installing a micro-CHP. In the modelling process the following physical features of the heating system were taken into account: ∑

Thermal inertia – As well as the thermal mass of the house and hot water

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 500

4/5/11 9:43:05 AM

Thermal-engine-based small and micro CHP systems

501

cylinder, the water and associated pipe work within the heating circuits have their own mass. This affects the lag present within the system. The heat capacity of the test rig was determined by examining the rate at which the radiators heated during the experimental runs in conjunction with the thermal input and dissipation calculated. This capacity was then scaled to each scenario according to the maximum space heating output of each system. Additional thermal mass was also added to model the 5 kW heating circuit through the hot water cylinder which is not present in the test rig. ∑ Individual space and tank thermostats – Most heating systems contain a distribution valve which directs the hot water from the boiler to where it is needed. If no further space heating is required, all the water from the boiler is directed to the hot water cylinder and vice versa. This was included in the model along with the effects when the valve position changes, mixing standing water from one circuit with the active circuit. ∑ Variable heat loss – As the inside of the house heats up, the heat loss through the fabric will increase in proportion. The heat losses from the house in various seasons, at design temperature, were obtained from the CANMET program results. These were then used to model variable heat loss for each season based on the current internal and appropriate ambient temperatures. ∑ Standby losses from hot water cylinder – While the hot water is stored in the cylinder, an amount of heat is lost through the walls depending on the temperature of the water. This can be reduced through insulation, although not totally removed. An appropriate loss was included in the model in line with current British Standards for hot water cylinders. The effect of thermal stratification within the cylinder was also taken into account for modelling the performance of the heat exchanger. To assess the economic benefits of the Whispergen Mk3 unit, the associated costs must be compared to the base situation of a conventional boiler with all the household electricity imported from the grid. In conjunction with the performance of the Whispergen Mk 3 unit, the most important factor is the differential between gas and electricity prices. For analysis, the standard ‘British Gas’ gas and electricity tariffs for 2005 in the London area were used. In this example, benefits from the new feed-in tariff scheme were not included in the calculations. These, along with most other tariffs, consist of two rates: a higher rate for the first x units each quarter followed by a much lower rate. To ease the costing, these were averaged out on the estimated household usage. The result was a kWh unit cost of 3p and 10.6p for gas and electricity, respectively. This leads to a significant difference which will maximise the value of the micro-CHP generation.

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 501

4/5/11 9:43:05 AM

502

Small and micro combined heat and power (CHP) systems

A further value is generated by any rebate for the export of excess electricity through the appropriate network and metering interface. The rebate per exported unit is generally significantly less than the import rate due to the embedded infrastructure costs, etc. A rate of 3p per unit was used for the analysis as an average value. The main environmental benefit comes from reduced CO2 emissions through high efficiency of localised generation. From analysis of the electrical grid and its generation facilities, the amount of CO2 released per unit reaching the consumer can be estimated and the figure used in the calculations was 0.43 kg/kWh. A similar figure was produced for the combustion of natural gas, which was 0.19 kg/kWh. The reduction in CO2 emissions was calculated using these figures and the assumption that the electricity exported from the household was ‘CO2 free’. The performance of the unit is strongly related to how well it can match its thermal output to that required. The band 1 winter days had the highest rate of heat loss from the building and therefore required the most space heating input. In conjunction with the hot water cylinder, this provided a large enough thermal sink to prevent the system flow temperature from rising above that at which the Whispergen Mk 3 unit shuts down. This lead to long run times with little cycling, resulting in a high efficiency with a constant electrical output. However, simulation results demonstrate that the temperature of the water in the hot tank drops below an acceptable level. The various system temperatures for the weekday in this band are shown in Fig. 18.35. The corresponding electricity import/export profile shows mainly concurrency between generation and demand resulting in little export, maximising the value of the electricity generated. The high thermal demand also allowed the unit to operate at full power for significant periods which minimised the heat-to-power ratio. The graph illustrating the electrical distribution is shown in Fig. 18.36. The economic and environmental savings are inherently dependent on duration of operation resulting in days of highest thermal demand having the largest attributable savings. In contrast, as the space heating demand decreases towards the middle of the year due to reduced heat loss through the fabric of the building, the system does not have enough thermal dissipation for the Whispergen Mk 3 to run continuously at even its intermediate power setting. This resulted in the constant cycling shown in Fig. 18.37 by the system temperatures for a mid-season weekend profile. It can be seen that the MCHP does not maintain the hot water temperature in the storage tank. The operating efficiency for this profile showed a marked 5% decrease compared to its band 1 equivalent. The intermediate power operation also increases the heat-to-power ratio which further diminishes the savings. The corresponding electricity profile (Fig. 18.38), showed the expected import/export fluctuations, partly due to

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 502

4/5/11 9:43:05 AM

Thermal-engine-based small and micro CHP systems 90 80

Cylinder temp Radiator flow temp System flow temp House temp

House design temp

Temperature (°C)

70

503

Active occupancy period

60 50 40 30 20 10 0 00:00

03:00

06:00

09:00

12:00 Time

15:00

18:00

21:00

18.35 System temperatures for semi-detached heating band 1 weekday. Cylinder Temp – the average temperature of the water in the hot water tank; Radiator flow temp – the temperature of the water coming out of the radiators; System flow temp – the temperature of the water coming out the MCHP. 6.0

Active occupancy period

Demand 5.0

Import/export

Electrical power (kW)

4.0 3.0 2.0 1.0 0.0 00:00 –1.0

03:00

06:00

09:00

12:00

15:00

18:00

21:00

Time

18.36 Electricity profile for semi-detached heating band 1 weekday.

cycling but also to the reduced electricity demand during the middle of the day. The Whispergen Mk 3 operates at flow temperatures considerably lower than a conventional boiler to maximise the efficiency of and protect the

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 503

4/5/11 9:43:05 AM

504

Small and micro combined heat and power (CHP) systems

90

Cylinder temp Radiator flow temp System flow temp House temp House design temp

80

Temperature (°C)

70 60

Active occupancy period

50 40 30 20 10 0 00:00

03:00

06:00

09:00

12:00 Time

15:00

18:00

21:00

18.37 System temperatures for semi-detached heating band 3 weekend.

5.0

Active occupancy period Demand

4.0 Electrical power (kW)

Import/export 3.0

2.0

1.0

0.0 00:00 –1.0

03:00

06:00

09:00

12:00

15:00

18:00

21:00

Time

18.38 Electricity profile for semi-detached heating band 3 weekend.

Stirling engine from overheating and therefore it would be desirable to modify the existing heating system by switching to low-temperature radiators. Conventional radiators can still be used though they are designed to operate with a higher temperature difference of ~50–60 °C while the Whsipergen Mk 3 provides the thermal output restricted to a temperature difference of

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 504

4/5/11 9:43:06 AM

Thermal-engine-based small and micro CHP systems

505

~40  °C as found in the simulation. Due to this lower temperature of the water coming out of the MCHP system, there is also restricted heat transfer in the hot water cylinder. During the summer months when the need for space heating is practically removed, the unit operates periodically to heat the hot water cylinder. However, without the load sink of the space heating system and the limited transfer to the cylinder itself for the reasons above, the flow temperature quickly rises causing the unit to shut down. Therefore, the summer profiles consist of an excess number of short, intermediate power cycles resulting in relatively low overall efficiency, high heat-to-power ratio and little production of electricity. As in previously considered cases, the MCHP is not capable of maintaining the high temperature level in the storage tank. This kind of operation is shown in Fig. 18.39. As anticipated, the annual variation in daily savings basically mirrors the thermal demand of the household, as is apparent in Fig. 18.40. Due to their origin, the CO2 and financial reductions are directly related. During the four summer months, the savings drop to a lowly 4% meaning little difference is seen between the CHP and conventional system. A similar variation profile is apparent in the unit’s efficiency, although with reduced magnitude. The lack of reasonable thermal demand-following capabilities results in an estimated total of 2078 cycles per year, averaging over five a day. This is acutely detrimental on the service interval of the unit as well as its expected life. 90 80

Temperature (°C)

70 60

Active occupancy period

Cylinder temp Radiator flow temp System flow temp House temp House design temp

50 40 30 20 10 0 00:00

03:00

06:00

09:00

12:00 Time

15:00

18:00

21:00

18.39 System temperatures for semi-detached heating band 4 weekend.

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 505

4/5/11 9:43:06 AM

Small and micro combined heat and power (CHP) systems 100

18

90

16

80

Daily saving and ratio

20

14

70

12

60

10

50

8

40

6

Heat-to-power ratio

30

4

CO2 saving (%) Economic saving (%)

20

2

Efficiency (%)

506

10

Efficiency

0

0 Jan

Feb Mar Apr May Jun Jul Month

Aug Sep

Oct

Nov Dec

18.40 Annual variation of Whispergen Mk 3 performance for semidetached scenario.

Over the year, the unit satisfied 16.9% of the household’s electrical requirements in response to the thermal demand. This relatively low figure illustrates the mismatch between the annual heat-to-power ratios of 8.85 for the unit and 2.04 for the demand. The ratio for the modelled demand is lower than the average but it is typical for the modelled modern house due to the trends in house design and electrical appliance ownership. Of the electricity generated, 26.5% was exported to the grid. This indicates a reasonable level of concurrency between thermal and suitable electrical demands to absorb the electricity generated. The annual savings amount to 9% and 9.3% in monetary and CO2 terms respectively. The household was modelled using the ten representative heat/ power profiles analysed above, providing simulation results on a per day basis. These are summarised in Table 18.7 along with a weighted annual summation. One of the major problems with this particular MCHP system (the Whispergen Mk 3) was its small range of thermal modulation. Modern high efficiency boilers commonly have continuous modulation capabilities with a ratio of 1:3. This is compared to a ratio of 1:1.2 with only two discrete values available for the Whispergen Mk 3 unit. Excessive cycling occurs as a result. The operation of the Whispergen Mk 3 unit could be improved by adapting the existing heat delivery systems within the household to greater suit the characteristics of the unit. This could be achieved in a number of ways, such as the use of larger conventional radiators, taking into account the lower temperature differential when calculating the output, or forced convection

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 506

4/5/11 9:43:06 AM

© Woodhead Publishing Limited, 2011

1 2 3 4 5 1 2 3 4 5

Annual summary

Weekend

Weekday

6.168 4.660 3.607 1.289 1.149 8.020 7.403 5.212 1.498 0.976 1159

Electricity generated (kWh)

47.860 36.862 32.969 16.605 13.460 75.627 55.120 43.292 15.964 11.207 10254

60.859 47.602 42.464 20.860 17.477 94.411 73.515 57.851 20.962 14.778 13279

Thermal energy Gas generated consumption (kWh) (kW)

Table 18.7 Summary of semi-detached simulation results

Heating band Heating band

CHP-Beith-18.indd 507

4/5/11 9:43:06 AM

2 6 7 5 4 4 12 12 5 4 2078

On/off cycles 9.606 11.045 11.582 12.574 12.426 8.004 8.990 10.288 11.309 12.914 4187

1.410 1.348 0.885 0.179 0.247 2.197 2.548 1.650 0.397 0.05 307

Electricity Electricity imported exported (kWh) (kWh) 7.759 7.910 9.140 12.879 11.716 9.430 7.445 8.307 10.657 11.488 8.85

Heat-topower ratio 88.776 87.229 86.136 85.785 83.591 88.599 85.049 83.845 83.306 82.433 85.950

15.5 11.9 10.1 5.8 4.4 16.0 12.8 10.2 5.1 4.1 9.3

2.46 1.71 1.31 0.53 0.37 3.47 2.31 1.53 1.45 0.27 411

14.00 11.48 9.39 5.44 4.15 14.52 12.11 9.43 4.96 3.17 9

Efficiency Economic CO2 savings (%) saving (kg) (%) (%)

508

Small and micro combined heat and power (CHP) systems

to increase the radiator heat dissipation via fans, although this may produce an undesirable environment in terms of noise and excessive air motion. A thermal storage system for space heating could be used as well for the domestic hot water, so allowing long, highly efficient periods of operation [31]. Most importantly, it should be highlighted that the above relatively conservative results were obtained without taking into about the new feed-in tariff scheme which is currently in place (£0.1 for each kWhel for cogenerated power and an additional £0.03 for each kWhel exported to the grid). If this new scheme is taken into account, then savings produced by a MCHP appliance will increase significantly. Finally, the system’s operation on ‘intermediate power’ level may have reduced the value of the electrical efficiency in these tests on the Mk 3 unit. The intermediate power mode is disabled in the currently available Mk 5 version of the unit and the engine operates at full power and only switches on and off. Further, the Mk 5 version of the MCHP is equipped with an auxiliary burner to meet higher levels of heat demand and utilises a low emission, pre-mix, steel mesh burner with NOx levels in the 20–40 ppm range.

18.9 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

18.

References

http://www.otag.de, OTAG Vertriebs GmbH & Co.KG, 17 September 2010. http://www.energetixgroup.com, Energetix Group plc, 12 September 2010. http://www.cogenmicro.com, Cogen Microsystems, 15 September 2010. http://www.hondanews.com, HONDA, 12 December 2008. http://www.freewatt.com, 16 August 2010. www.baxitech.co.uk, Baxi-SenerTec UK, 14 August 2010. www.ecpower.co.uk, EC Power A/S, 15 September 2010. http://www.whispergen.com/, Whisper Tech Ltd, 29 April 2010. http://www.disenco.com, DISENCO ENERGY plc, 12 July 2010. http://www.microgen-engine.com, Microgen Engine Corporation, 12 May 2010. http://www.enatec.com, ENATEC Micro Cogen b.v., 12 September 2010. http://www.sunmachine.com, 16 August 2009. http://www.eonenergy.com/At-Home/Products/Technology-And-Initiatives/ WhisperGen.htm, E.ON, 12 December 2008. http://www.ehe.eu, 16August 2010. http://www.baxi.co.uk/ecogen, 16 August 2010. http://www.stirling-engine.de/engl, Stirling Engine Systems GmbH, 12 December 2008. Veitch, D.C.G. and Mahkamov, K. 2009. Assessment of economical and ecological benefits of using a Stirling domestic CHP unit based on its experimental performance. Proceedings of the Institution of Mechanical Engineers, Part A, Journal of Power and Energy, 223/7: 783–798. Newborough, M. 2004. Assessing the benefits of implementing micro-CHP systems in the UK. Proceedings of Institute of Mechanical Engineers, Part A: Journal of Power and Energy, 218(4): 203–218. © Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 508

4/5/11 9:43:06 AM

Thermal-engine-based small and micro CHP systems

509

19. Peacock, A.D. and Newborough, M. 2006. Impact of micro-combined heat-and-power systems on energy flows in the UK electricity supply industry. Energy, 31(12): 1804–1818. 20. Newborough, M. and Peacock, A.D. 2005. Impact of micro-CHP systems on domestic CO2 emissions. Applied Thermal Engineering, 25(17–18): 2653–2676. 21. Cockroft, J. and Kelly, N. 2006. A comparative assessment of future heat and power sources for the UK domestic sector. Energy Conversion and Management, 47(15–16): 2349–2360. 22. Hawkes, A. and Leach, M. 2005. Impacts of temporal precision in optimisation modelling of micro-combined heat and power. Energy, 30(10): 1759–1779. 23. Peacock, A.D. and Newborough, M. 2007. Controlling micro-CHP systems to modulate electrical load profiles. Energy, 32(7): 1093–1103. 24. http://www.planningportal.gov.uk, 29 April 2008. 25. http://www.rics.org, 29 April 2008. 26. http://www.sbc.nrcan.gc.ca/software_and_tools/hot2xp_e.asp, 29 April 2008. 27. Estimates of hot water consumption from the 1998 EFUS. Implications for the modelling of fuel poverty in England. A summary report presenting data from the 1998 EFUS produced by the BRE Housing Centre on behalf of DTI and DEFRA. June 2005. 28. Newborough, M. and Augood, P. 1999. Demand-side management opportunities for the UK domestic sector. Generation, Transmission and Distribution, IEE Proceedings, 146(3): 283–293. 29. Fawcett, T., Lane, K. and Boardman, B. et al. 2000. Lower carbon futures for European households, Environmental Change Institute, Oxford University. http:// www.eci.ox.ac.uk/research/energy/lcfpublications.php, 29 April 2008. 30. Joynt, B. and Wu, S. 2000. Nitrogen oxide emissions standards for domestic gas appliances, Chapter 5. February 2000 Environment Australia. http://www.environment. gov.au/atmosphere/airquality/publications/residential/, 9 September 2008. 31. Haeseldonckx, D., Peeters, L., Helsen, L. and D’Haeseleer, W. 2007. The impact of thermal storage on the operational behavior of residential CHP facilities and the overall CO2 emissions. Renewable and Sustainable Energy Reviews, 11(6): 1227–1243.

© Woodhead Publishing Limited, 2011

CHP-Beith-18.indd 509

4/5/11 9:43:06 AM

Index

absorption, 279 absorption heat pumps, 280–2 absorption refrigeration, 9–10 active magnetic bearings, 166 adsorption heat pumps, 282–4 air pre-heater, 188 AMB see active magnetic bearings anaerobic digestion, 104–5 anaerobic digestor, 511 APU see auxiliary power units Aqueous Ammonia cycle, 9 Argent Energy (UK) Limited, 106 ash, 92 auxiliary burner, 244 auxiliary power units, 336 baffles, 49 bagasse, 5 Baxi SenerTec Dachs, 473 bearings, 165–6 active magnetic bearings, 166 gas, 165–6 oil-lubricated, 165 Beddington Zero Energy Development, 448 BedZED see Beddington Zero Energy Development beverage processing see food and beverage processing Bio-energy Capital Grant Scheme, 428 bio-oils, 100, 101, 132, 133 biodiesel production, 105–7 bioenergy, 89 biofuel, 89 biogas, 89, 104, 212, 350 biogas-driven small CHP system in sewage works, 60–8 choice of prime mover, 61–3 CHP system flow diagram, 63 engine performance, 63–4 efficiency and heat rejection, 64 Sankey diagram for a single generation scheme, 65 exhaust heat recovery, 66

economiser performance, 66 heat recovery system, 64–6 compact heat exchanger performance, 66 overall system performance, 66–7 emissions reduction per generator, 67 Sankey diagram for cogeneration scheme, 67 biomass, 212–13, 350, 412, 427–8 categories, 89–91 non-woody biomass, 90 organic waste biomass, 90–1 woody biomass, 89–90 combined heat and power technologies, 108–16 combustion-based, 108–14 gasification-based, 114–15 illustration, 108 other, 116 combustion and prime mover, 109–14 biomass-fired hot air gas turbine process, 114 biomass-fired micro-scale CHP system with ORC, 111 hot air gas turbine, 113–14 ORC-based biomass CHP plant of Lienz, 110 organic Rankine cycle turbine, 109–12 small-scale biomass Stirling engine CHP of BIOS, 113 steam turbine/steam engine, 108–9 Stirling engine, 112–13 conversion technologies, 94–107 anaerobic digestion, 104–5 basic steps in anaerobic digestion process, 105 basic steps of biomass fermentation, 102 biodiesel production, 105–7 biomass thermochemical conversion processes, 99 conversion processes, 95–6 fermentation to produce ethanol, 102–4 gasification, 98–100 pathways of biomass pyrolysis, 101

514 © Woodhead Publishing Limited, 2011

CHP-Beith-Index.indd 514

4/5/11 9:44:36 AM

Index pyrolysis, 100–2 current development of small and micro CHP, 107–16 fuels for small and micro combined heat and power systems, 88–116 renewable energy resource, 89–91 illustration, 90 solid biomass fuels, 91–4, 95, 96–8 50 kW wood pellet boiler, 97 Arimax Bio Energy boiler, 98 average net calorific value, 94 calculation of analyses to different bases, 93–4, 95 calorific value, 93, 94 characterisation, 91–4, 95 combustion, 96–8 combustion stages, 96 formulae for calculation of results to different bases, 95 moisture content effect on biomass fuel net calorific value, 94 proximate, ultimate analyses and gross calorific values, 95 proximate analysis, 92 ultimate analysis, 92–3 world energy supply, 91 biomass-based small and micro CHP systems application and status in UK, 427–56 technical changes, 450–2 UK energy policy and targets, 427–9 capital costs, 452–5 European policy, 453–4 novel biomass CHP technology in Austria, 454 small-scale biomass CHP in Europe, 453 small-scale biomass CHP in Sweden, Denmark and Finland, 454–5 small-scale biomass combustion CHP, 452 small-scale biomass gasification CHP, 452 small-scale biomass pyrolysis CHP, 452 technological characteristics from small scale CHP production, 453 difficulties associated with biomass fuels, 450–1 biodiversity and land use, 450 energy efficiency, energy balance and carbon neutrality, 450 food shortages, 450 supply and security of supply issues, 451 difficulties associated with prime movers, 451–2 IC engines, 451 turbines, 451–2 past and present small-scale biomass gasification companies, 447–9

515

Beddington Zero Energy Development (BedZED), 448 Biomass CHP, 447–8 Biomass Engineering, 447 Blackwater Valley Museum, Benburb, 448 Boughton Pumping Station, 449 Brook Hall Estate, Londonderry, N. Ireland, 448 ECOS Millennium Centre, Ballymena, N. Ireland, 447 Little Woolden Hall Farm, Culcheth, Cheshire, 447 Mossborouh Hall, Merseyside, 447 Rural Generation Ltd, 448–9 past and present small-scale biomass pyrolysis + gasification examples, 449–50 Compact Power, 449 Harper Adams University College, 449–50 Talbott’s Biomass Energy System, 449–50 renewables and combined heat and power in the UK, 429–50 biomass-based CHP plants in the UK, 437–43 biomass derived fuels used for heat generation, 433 biomass electricity projects, 431–2 biomass heat generation, 433–4 CHP in UK, 434–6 CHP schemes installed in buildings by sector (2008), 435 electricity generation from biomass, 430 fuel input for CHP (2008), 435 increase in CHP generation from renewables, 436 renewable CHP generation, 436–46 renewables and biomass electricity generation, 430–3 UK biomass CHP arranged by feedstock, 445 arranged by prime mover, 444 arranged by size, 444 arranged by technology, 445 Biomass Energy Centre, 434 biomass fermentation, 102–4 biomass gasification, 98–100, 446 thermochemical conversion processes, 99 Biomass Implementation Advisory Group, 428 biomass pyrolysis, 100–2 fast pyrolysis, 101–2 pathways, 101 slow pyrolysis, 100 Biomass Task Force Report, 428 boiler station, 354 bottoming cycle, 405

© Woodhead Publishing Limited, 2011

CHP-Beith-Index.indd 515

4/5/11 9:44:36 AM

516

Index

bowl-in-piston combustion chamber designs, 133 Brayton cycle, 152, 153, 449–50 Brazil’s PRO ALCOOL programme, 102 BS EN 14775:2009, 92 BS EN 14918:2009, 93 BS EN 15148:2009, 92 BS EN 14774-1:2009, 92 BS EN 14774-2:2009, 92 BS EN 14774-3:2009, 92 BS EN 14778-1:2005, 92 Building Regulations (2006), 428 CAES see compressed air energy storage Calnetix Power Solutions, 172, 405 capacitors, 309 capacity ratio, 51 Capstone Inc., 172 Capstone Turbine Corporation, 173, 405 carbon dioxide central performance metric, 26 emissions reduction performance, 35–8 CO2 performance of two micro-CHP systems to annual thermal demand in target dwelling, 37, 38 CO2 reductions to prime mover capacity, 36 Carbon Reduction Commitment (CRC), 391 Carbon Trust, 428 Carnot COP, 286–9 Carnot cycle, 465 Carnot efficiency, 187, 189, 199, 230 Carnot’s principle, 186 Casten, T., 4 CCHP see combined cooling, heat and power cellulosic biomass, 102–3, 104 CEN/TS 15104:2005, 93 CEN/TS 15289:2006, 93 CEN/TS 15290:2006, 93 CEN/TS 15296:2006, 94 CEN/TS 15297:2006, 93 Central Electricity Generating Board, 4 CFD see computational fluid dynamics choking, 168 CHP see combined heat and power CHP systems see combined heat and power systems clean wood, 446 Cleanergy AB, 198 CODEGen see Cost Optimisation of Decentralised Energy Generation Coefficient of Performance, 10, 412 Cogen Microsystems, 471, 473 cogeneration see combined heat and power cogeneration plant, 46–7 combined cooling, heat and power, 266 combined heat and power, 427 grid services applications, 317–18

systems applications, 315–17 costs of storage, 317 domestic scale, 316 economic scale, 316–17 establishment scale, 315–16 combined heat and power systems, 3–16, 43, 510–13 applications in fuel cell systems, 233–56 commercial development and future trends, 253–6 fuel cell systems, 239–46 fundamentals of operation, types and properties of fuel cells, 234–8 operating conditions and performance, 246–53 barriers, 15–16 biomass fuels, 88–116 biomass conversion technologies, 94–107 current development, 107–16 solid biomass fuels characterisation, 91–4 cogeneration, 3–5 commercial exploitation pathways, 24–5 community microgrid, 24 company control, 24 plug-and-play, 24 conclusion and outlook, 512–13 cost benefit and emissions reduction, 12–13 district and community heating aspects, 347–63 Aars, Denmark case study, 360–2 control system and consumer installations, 353–9 future trends, 363 heat sources, 349–51 Lerwick, Shetland case study, 359–60 pipework installation issues and design considerations, 351–3 preconditions, 348–9 energy efficiency improvement, 11–12 energy storage, 307–22 applications, 315–17 electrical energy storage application, 309–15 electrical vehicles, 318 energy management applications, 307–8 future trends, 321–2 grid services applications and relationship, 317–18 islanding capability applications, 308 large-scale and small-scale storage – conceptual planning, 318 power trading, 308 thermal storage development and application, 318–21 types of systems, 308–9

© Woodhead Publishing Limited, 2011

CHP-Beith-Index.indd 516

4/5/11 9:44:36 AM

Index food and beverage processing industries, 395–421 established CHP technologies for food industry, 404–6 food processing and energy requirements, 396–7 future trends, 419–21 heat and power integration of food total sites, 397–400, 401 high-efficiency technologies, 406–11 integration of renewables and waste, 411–14 potential applications, 414–19 suitable types of small and micro CHP for food industry, 400, 402–4 future trends, 16 grid connection, 13–15 harmonics, 15 inverters, 15 protection, 13–14 synchronisation, 14 system fault level analysis, 14–15 heat-activated cooling technologies, 262–306 advantages and limitations of heatactivated cooling, 296–7 closed sorption cycles, 279–84 component-specific efficiency and effectiveness metrics, 285–90 cooling systems and their applications, 267–9 future trends, 297–9 open sorption cycles, 269–79 small-scale trigeneration, 263–7 steam ejector cycle, 284–5 system-wide performance and efficiency metrics, 290–6 integration into distributed energy systems, 70–86 conditions for profitable decentralised generation, 75–8 distributed energy resources, 70–3 distributed generation value, 73–5 evaluating the ‘full value’ of being network connected, 78–81 recommendations to distribution system operators and regulators, 81–6 internal combustion engine technology and application, 125–45 commercially available units, 140–5 installation and practical aspects, 138–40 operating characteristics and performance, 133–7 types, properties and design, 126–33 review of micro combined heat and power system, 510–11 review of small combined heat and power system, 511–12 Stirling engine applications, 179–205

517

applications and future trends, 203–5 definition, 180–1 development for micro CHP applications, 189–98 micro CHP design and system integration, 199–203 Stirling cycle, 183–8 suited to micro CHP, 181–3 types, 188–9 techno-economic assessment, 17–41 case study: micro CHP, 28–39 economics, 18–21 future trends, 39–41 modelling methodology, 23–8 onsite generation, 21–3 thermal-engine-based, 459–508 deployed prime movers, 460–70 economic and environmental analysis, 500–8 economical and environmental benefits estimation, 480–3 electrical demand, 490–2 heat demand modelling, 483–90 performance mapping, 492–500 product development, 470–80, 481 thermodynamics, performance analysis and computational modelling, 42–68 analysis of computational modelling, 55–60 case study: biogas-driven small CHP system in sewage works, 60–8 cogeneration thermodynamics, 44–7 computational modelling, 54–5 heat exchangers theory, 48–51 performance analysis of cogeneration cycles, 48 temperature-entropy chart, 52 types of systems, 43–4 worked sample, 51–4 types of technology and potential applications, 5–11 base technologies, 6 large-scale CHP, 7–8 micro-CHP, 10–11 small-scale CHP, 8–9 trigeneration, 9–10 see also specific type of CHP system Community Energy Programme, 455 Community Energy Storage, 321–2 community heating, 341–3 Community Power Corporation, 115 competitive locational price, 74 compressed air energy storage plants, 313, 314 system diagram, 315 compression heat pump, 412 compression ignition engines, 132–3 compression ratio, 465

© Woodhead Publishing Limited, 2011

CHP-Beith-Index.indd 517

4/5/11 9:44:36 AM

518

Index

compression stroke, 133 computational fluid dynamics, 160 computational modelling CHP, 54–5 analysis, 55–60 extraction ratio efficiency influence on CHP performance, 60 maximum boiler pressure influence on CHP performance, 61 process heat extraction pressure influence on CHP performance, 62 pump efficiency influence on CHP performance, 58 simulation results vs manual calculations, 56 superheat temperature influence on CHP performance, 57 system flow diagram, 63 turbine efficiency influence on CHP performance, 59 variable 1: superheat temperature, 56 variable 2: pump efficiency, 56 variable 3: turbine efficiency, 56–7 variable 4: extraction ratio, 57–8 variable 5: maximum pressure – boiler, 58–9 variable 6: heat extraction pressure – process heat, 59 condensing connections, 230 conventional central plant, 343–4 COP see Coefficient of Performance correction factor, 49 Cost Optimisation of Decentralised Energy Generation, 23–8 inputs, outputs and flow diagram for optimisation, 23 CPC see Community Power Corporation customer billing system, 358–9 Dachs, 139, 140, 143 dead volume, 185 DEC see display energy certificates decentralised/distributed generation concept, 20 characteristics, 20–1 defence plans, 72 DER see distributed energy resources desiccant dehumidification, 269–79 effectiveness, 289–90 desiccant wheel, 271, 275 configuration options, 274 DG see distributed generation Diesel dilemma, 133 diesel engines, 126, 132–3, 403, 406 direct combustion, 96 direct heating, 341–3 direct hot air microturbine, 452 Directive on the Promotion of Cogeneration base on Useful Heat Demand, 453

Disenco kinematic Stirling engine, 194–8 Sigma 3 kWe micro-CHP unit, 197 displacer, 181, 184, 189, 469 display energy certificates, 366 distributed energy resources, 70–3 initial developments in power systems, 70–2 supply reliability, 72–3 distributed energy systems conditions for profitable decentralised generation, 75–8 distributed energy resources, 70–3 initial developments in power systems, 70–2 supply reliability, 72–3 distributed generation value, 73–5 market regulation, 75 technical aspects, 73–4 value for the system, 74 evaluating the ‘full value’ of being network connected, 78–81 generation costs evaluated for islanded sites, 80 evaluated for small customer islanded sites, 81 vs full load working hours, 78 recommendations to distribution system operators and regulators, 81–6 new designs for distribution networks, 82–5 new regulatory frameworks, 85–6 small and micro combined heat and power systems integration, 70–86 distributed generation, 71 value, 73–5 distribution system operators, 73 district heating, 512 Aars, Denmark case study, 360–2 Energy from Waste plant in Aars, 361 transmission pipe between Aars and Hornum, 362 combined heat and power systems, 347–63 Lerwick, Shetland case study, 359–60 preconditions, 348–9 control system and consumer installations, 353–9 boiler station with a CHP unit as the base unit, 354 boiler station with a heat storage tank, 354 customer billing system, 358–9 heat storage in Aars, 357 heat storage tank, 356–7 pressurising and static pressure, 353–6 standard consumer installation, 355 water quality in the network, 357 flow temperature, 358 hot tap water, 358 space heating, 358

© Woodhead Publishing Limited, 2011

CHP-Beith-Index.indd 518

4/5/11 9:44:36 AM

Index future trends, 363 balancing of the electric grid, 363 conversion from gas to district heating, 363 pipe dimensioning, 363 temperature optimisation, 363 heat sources, 349–51 biogas, 350 biomass, 350 fossil fuel, 350–1 size considerations, 349–50 waste to energy, 350 pipework installation issues and design considerations, 351–3 consumer education, 352–3 pipe size considerations, 352 DSO see distribution system operators E-ON, 475 economics techno-economic assessment, 18–21 onsite generation, 21–3 UK electricity and gas price plotted vs CHP spark spread, 19 ECOWILL micro-CHP unit, 473 ECU see electronic control unit Efficient Home EnergyS1, 475–6 EFMT see externally-fired microturbines EFW see Energy from Waste plant electric efficiency, 155 electrical vehicles, 318 electricity generation, 327 electricity grid connection, 13–15 electricity-led operation, 139 electrolysers, 309 electronic control unit, 136 Emissions Trading Schemes, 429 ENE-FARM, 251, 254 energy crops, 89 energy efficiency improvement, 11–12 Energy from Waste plant, 360 energy management, 307–8 energy performance certificates, 366 Energy Performance of Buildings Directive, 365–6 energy service company, 25 central performance metric, 25–6 energy storage combined heat and power systems applications, 315–17 costs of storage, 317 domestic scale, 316 economic scale, 316–17 establishment scale, 315–16 electrical energy storage application, 309–15 50 kW demonstration flywheel, 314 Beacon’s 20 MW flywheel under construction, 313

519

CAES Plant, 314 CAES system diagram, 315 mechanical systems, 312–15 technologies for integration with CHP systems, 312 uninterruptible power supply, 309–12 electrical vehicles, 318 vanadium battery, 319 future trends, 321–2 community energy storage, 321 small and micro CHP systems, 307–22 energy management applications, 307–8 energy storage, 307–21 grid services applications and relationship, 317–18 islanding capability applications, 308 large-scale and small-scale storage – conceptual planning, 318 power trading, 308 thermal storage development and application, 318–21 parabolic trough collectors and experimental solid media storage unit, 320 types of systems, 308–9, 310–11 definition and scope, 308 energy storage devices development status, 310–11 Energy White Paper, 427 engine efficiency, 134 engine knock, 130 EPBD see Energy Performance of Buildings Directive EPC, 366 energy performance certificates ESCO see energy service company ETS see Emissions Trading Schemes EU -DEEP project, 79, 84 external combustion engines, 180, 326, 329–30 externally-fired microturbines, 174–5 flow diagram, 174 extraction ratio, 57–8 influence on CHP performance, 60 Faber, O., 4 FCP see fuel chargeable to power fit-and-forget principle, 83 fixed carbon, 92 flameless oxidation, 198 FLOX see flameless oxidation flywheels, 309, 312–13, 314 food and beverage processing, 395 energy requirements, 396–7 agricultural crops processing, 397 meat processing, 397 sugar production, 396 established CHP technologies, 404–6 gas turbine–based CHP, 405 Rankine cycle technologies, 404–5

© Woodhead Publishing Limited, 2011

CHP-Beith-Index.indd 519

4/5/11 9:44:37 AM

520

Index

reciprocating engines, 406 food total sites heat and power integration, 397–400, 401 basic heat integration, 398–9 heat recovery targeting, 399 locally integrated energy sector CHP, 401 total site energy integration, 399–400, 401 total site profiles and composite curves, 400 future trends, 419–21 decentralisation, 420 further integration of renewables and maximum waste utilisation, 421 locally integrated energy sectors, 421 strategic issues, 420–1 technology development trends, 420 high-efficiency technologies in theoretical and demonstration stages, 406–11 fuel cell combined cycles, 409–11 fuel cell efficiency variation with operating temperature, 407 molten-carbonate fuel cells, 407–9 overall efficiencies of combined SOFC and steam system arrangement, 411 simple MCFC process flow diagram, 408 potential applications, 414–19 locally integrated energy sectors, 414–19 MCFC in a Japanese brewery, 414 renewables and waste integration with food industry energy demands, 411–14 biomass, 412–13 heat pumps, 412 using solar energy, 413–14 small and micro CHP systems applications, 395–421 suitable types of small and micro CHP systems, 400, 402–4 annual microCHP CO2 savings, 402 CHP using microturbines, 403 external combustion–Stirling engines, 403–4 fuel cells, 404 internal combustion–reciprocating engines, 402–3 fossil fuels, 91, 350–1 free-piston Stirling engine, 470 Freewatt, 473 fuel cell combined cycles, 409–11 fuel cell systems, 239–46 auxiliary burner, 244 commercial development and future trends, 253–6 current and future costs, 254–6 expectations and targets given by manufacturers and government bodies, 256 known sale prices, 255

major manufacturers, products and demonstrations, 253–4 control system, 245 dynamic operation effects, 247–51 on/off cycling and start-up, 250–1 part load performance, 247, 249–50 utilisation of generated energy, 251 whole system electrical and thermal efficiency, 249 fuel cell stack, 239–40 stationary fuel cell CHP system, 239 fuel cells, 234–8 fundamentals of operation, 234–6 materials of PEFCs and SOFCs, 237–8 operating characteristics, 238 operating range, 235 operation, 234 types of stack, 237 fuel processor, 240–2 fuel processing, 241 heat management, 243 heat storage, 243–4 installed system, 245–6 inverter and power electronics, 244–5 operating conditions and performance, 246–53 conversion efficiency, 246–7 demonstrated stack and system lifetimes, 252 overall efficiency, 248 reliability, availability and lifetime, 251–3 thermal and electrical efficiency, 246 reactant delivery systems, 242–3 small and micro combined heat and power applications, 233–56 water management, 243 fuel cells, 9, 175–6, 233, 234–8, 309, 326, 331, 404, 406–7, 510–11 fuel chargeable to power, 293–4 fuel processor, 240–2 fuel utilisation efficiency, 293 fuel utilisation factor, 292 gas boilers, 201, 204 gas engines, 403 gas turbine/generator, 512 gas turbines, 7–8, 149, 372–3, 405 gasification product, 413 gasification system, 412 GCV see gross calorific value Genlec Energetix Ltd, 471 geothermal heat, 213, 215 global warming potential, 207, 220 Grenoble synchronous machine, 80 Grenoble UPS, 80 gross calorific value, 93 GWP see global warming potential

© Woodhead Publishing Limited, 2011

CHP-Beith-Index.indd 520

4/5/11 9:44:37 AM

Index HACD see heat-activated cooling device HACT see heat-activated cooling technologies harmonic distortion, 15 health care buildings, 382–4 heat, 42 heat-activated cooling device, 266 heat-activated cooling technologies closed sorption cycles, 279–84 absorption heat pump cycle, 281 absorption heat pumps, 280–2 adsorption heat pump schematic, 283 adsorption heat pumps, 282–4 Dühring diagram for water on silica gel 3A, 283 component-specific efficiency and effectiveness metrics, 285–90 desiccant dehumidification effectiveness, 289–90 thermal coefficient of performance and Carnot COP, 286–9 thermal COP vs regeneration temperature for various HACT, 287 open sorption cycles, 269–79 airside psychrometric state points for basic desiccant wheel, 271 airside psychrometric state points for dehumidification processes, 270 desiccant wheel schematic, 271 enhanced desiccant wheel, 275 equilibrium isotherm for aqueous LiCl, 278 liquid desiccant dehumidification system, 278 liquid desiccant processes, 277–8 qualitative equilibrium isotherms, 272 representative average air side state points for enhanced desiccant wheel, 275 solid desiccant cycles, 271–6 system integration options, 279 small and micro combined heat and power systems, 262–306 advantages and limitations, 296–7 cooling systems and their applications, 267–9 future trends, 297–9 sorption processes, 269 small-scale trigeneration, 263–7 cooling technologies and applications, 265–7 energy flow schematic of a generic CCHP (trigeneration) system, 267 generic CHP (cogeneration) system, 266 heat recovery circuitry options and trade-offs, 266 matrix connecting cooling cycles with applications, 268 prime movers, 264–5

521

small-scale CHP prime movers, 265 steam ejector cycle, 284–5 ejector cycle schematic, 285 ejector with fluid velocity profiles, 285 system-wide performance and efficiency metrics, 290–6 comparative performance relative to baseline systems, 295–6 energy content of fuel, 291 fuel utilisation rate and fuel-chargeable to power efficiency, 294 system-wide metrics, 292–4 zero-order model baseline non-CHP system with VCC cooling device and fuel-fired water heater, 295 CCHP system with absorption chiller, 296 CCHP system with desiccant wheel and VCC, 295 heat engine types, 370–4 gas turbines, 372–3 micro-gas turbines, 373–4 reciprocating internal combustion, 370–2 heat exchanger network, 398 heat exchangers, 163–4 heat exchangers theory, 48–51 cross-flow heat exchanger and typical temperature profile, 49 logarithmic mean temperature difference, 49 modified logarithmic mean temperature difference, 49–50 number of transfer units-effectiveness method, 50–1 capacity ratio, 51 effectiveness, 50 NTU, 50 heat extraction pressure – process heat, 59 influence on CHP performance, 62 heat-led mode, 139 heat-led operation, 139 heat management, 243 heat pumps, 412 heat recovery pinch, 398 heat sources, 350–1 heat storage, 243–4 heat storage tank, 356–7 heat-to-power ratio, 28, 236, 496 HEN see heat exchanger network HFE see hydrofluoroethers high heating values, 93 higher education institutions, 378–82 homes market, 332–6 micro combined heat and power systems, 332–6 objections, 334, 336 scope, 334

© Woodhead Publishing Limited, 2011

CHP-Beith-Index.indd 521

4/5/11 9:44:37 AM

522

Index

Honda CB1000R, 129 Honda Ecowill/Freewatt, 145 hospitals, 383–4 hot tap water, 358 Hot 2XP, 484–5 HPR see heat-to-power ratio HRe boilers, 479–80 hydrofluoroethers, 112 hypermarkets, 385–6 IAPWS-IF97 see IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam, 55 ICE see internal combustion engines IGCC see integrated gasification combined cycle indicated power of machine, 465 industrial cogeneration, 44 Infinia linear free piston Stirling engine, 194, 196 Ariston 1 kWe micro-CHP unit, 196 integrated gasification combined cycle, 115 internal combustion engines, 28, 39, 181, 326, 330–1, 463–4 commercially available units, 140–5 Honda Ecowill/Freewatt, 145 installation and practical aspects, 138–40 electrical connection, 138 maintenance, reliability and availability, 140 operational control, 139 micro combined heat and power, 473–4 operating characteristics and performance, 133–7 common internal combustion engine based CHP system configuration, 135 energy balance, efficiency and heat recovery, 134–6 energy balance in internal combustion engine based CHP system, 136 operational optimisation and load control, 136–7 performance map for a spark ignition engine, 137 PowerPlus Technologies ecopower, 143–4 CHP unit performance data, 144 ecopower micro-CHP unit design and operational data, 143 SenerTec Dachs, 140–3 micro-CHP technical data, 142 micro-CHP unit, 141 specific aspects, 130–3 compression ignition engines, 132–3 spark ignition engines, 130–2

technology and application in small and micro CHP systems, 125–45 types, properties and design, 126–33 basic design considerations, 128–30 common designs, 127 ideal Otto cycle efficiency and typical fuel efficiencies, 129 inverters, 15 isentropic efficiency, 55 islanding capability, 308 landfill gas, 89, 104, 212 large combined heat and power system, 7–8 large office buildings, 377–8 leisure and recreation buildings, 384 LFPSE see linear free piston Stirling engine LHV see low heating value LIES see locally integrated energy sector linear free piston Stirling engine, 189, 193 Lion Powerblock unit, 471 liquid desiccant processes, 277–8 liquid desiccant dehumidification system, 278 liquid desiccant solutions, 277–8 LMTD see logarithmic mean temperature difference locally integrated energy sector, 401, 414–19, 421 hospital complex process grand composite, 417 process stream data, 416 plant A process grand composite, 415 process stream data, 415 plant B process grand composite, 416 process stream data, 415 residential and office complex grand composite curve, 418 process stream data, 417 site profiles, 418 scenario 1, 419 scenario 2, 419 logarithmic mean temperature difference, 49 modified, 49–50 Low Carbon Buildings Programme, 428, 433 low heating value, 93, 155, 291 lubrication, 466 Magic Boiler Company, 190 maximum pressure – boiler, 58–9 influence on CHP performance, 61 MCFC see molten carbonate fuel cell MEC LFPSE see Microgen linear free piston Stirling engine micro combined heat and power system review, 510–11 micro combined heat and power systems, 436

© Woodhead Publishing Limited, 2011

CHP-Beith-Index.indd 522

4/5/11 9:44:37 AM

Index basic issues and energy requirements, 326–9 economic rationale, 326–8 electricity generation, 327 fuel availability, 328 regulatory environment, 328 technical requirements for viability, 328–9 case study, 28–39 characteristics of commercially successful micro CHP, 38–9 CO2 emissions reduction performance, 35–8 CO2 performance of two micro-CHP systems to annual thermal demand in target dwelling, 37, 38 CO2 reductions to prime mover capacity, 36 economic results variation of two key micro CHP technologies to annual electricity demand in target dwelling, 30 economic results variation of two key micro CHP technologies to annual thermal demand in target dwelling, 29 sensitivity of economic result to maximum ramp rate for 4 microCHP technologies, 34 sensitivity of economic result to minimum operating point for 4 micro-CHP technologies, 35 sensitivity of micro-CHP system value to prime mover capacity, 32 techno-economic assessment for investors and policy makers, 29–31 techno-economic assessment for technology developers, 31–5 competing technology solutions, 341–4 annual CO2 emissions for micro-CHP and CH/CHP, 342 community heating, 341–3 conventional central plant, 343–4 efficiencies of micro-CHP and conventional options, 343 other microgeneration technologies, 343 deployment modelling, 459–508 domestic applications, 331–6 complementary applications in housing, 332 domestic energy consumption by end use, 333 existing homes market, 332–6 new-build housing market, 336 UK gas consumers by consumption, 335 residential and small commercial systems, 325–45 advantages and limitations, 341–4 future trends, 344–5

523

small commercial buildings and other potential applications, 336–40 emergency service buildings, 339 hotel, 338 laundrettes, hairdressers, and similar premises, 339 multiple EC power 15 kWe ICE units, 340 offices, 339 preferred technologies, 337 residential and nursing homes, 338 restaurants and pubs, 338–9 small schools, 339 UK commercial market, 336, 338–40 types of system for residential and small commercial buildings, 329–31 external combustion engines, 329–30 fuel cells, 331 internal combustion engines, 330–1 other novel technologies, 331 micro-gas turbines, 373–4 Micro Turbine Technology, 176 Microgen linear free piston Stirling engine, 192–4, 195 Baxi Ecogen 1 kWe micro-CHP unit, 195 cross section through the upper cylinder, 193 Microgen MCHP unit, 476 microgeneration, 428 microturbine systems components types and properties, 160–6 axial turbine, 163 bearings, 165–6 centrifugal compressor, 161 combustors, 165 compressors, 160–1 heat exchangers, 163–4 radial turbine, 162 turbines, 161–3 variable frequency drive, 166 cycle performance, 152–9 compressor pressure ratio effect, 157 main input values in the simulation, 157 T-s and flow diagram of Brayton cycle and actual recuperated cycle, 154 future trends, 174–6 externally-fired microturbines, 174–5 flow diagram of externally-fired microturbines, 174 fuel cells and microturbines, 175–6 gas turbine integrated to a fuel cell, 175 gas turbine and recuperative cycle flow diagram, 148 gas turbine development, 149, 150 efficiencies, 149 power and efficiency, 150 general challenges with microturbine scale, 149–52 efficiency, 151–2

© Woodhead Publishing Limited, 2011

CHP-Beith-Index.indd 523

4/5/11 9:44:37 AM

524

Index

microturbine power output effect on compressor and turbine efficiency, 152 single stage compressors efficiency values, 151 speed, 150–1 volume, 150 manufacturers and applications, 172–4 Capstone, 173 Capstone microturbine, 173 Turbec, 173–4 operation, 166–72 ambient conditions effect, 168–70 ambient temperature effect on power output and electric efficiency, 169 compressor and turbine characteristics, 167 control methods, 170 electric load effect on electric efficiency and heat output, 171 emissions, 170–1 operating point determination, 166–8 other operational viewpoints, 171–2 single parameter change effect on operating point, 169 small combined heat and power applications, 147–76 microturbines, 403 modelling methodology, 23–8 central performance metric, 23–6 CHP commercial exploitation pathways, 24–5 CO2 performance, 26 inputs, outputs and flow diagram for optimisation, 23 optimisation problem, 26–8 Modular Bioenergy Company, 115 molten carbonate fuel cell, 404, 407–8, 409–11 applied to a Japanese brewery, 414 composite curves for MCFC, 410 simple MCFC process flow diagram, 408 multi-energy chiller, 10 net calorific value, 93 NETA see New Electricity Trading Arrangements new-build housing market, 336 New Electricity Trading Arrangements, 390 non-woody biomass, 90 NTU see number of transfer units number of transfer units, 50–1 ODP see ozone depletion potential open cycle, 153 optimisation, 26–8 constraints, 27 decision variables, 27 optimisation models, 22

ORC see organic Rankine cycle ORC-based biomass CHP plant, 109–12 biomass-fired micro-scale CHP system with ORC, 111 illustration, 110 ORC process efficiency, 230 organic Rankine cycle, 107, 453, 460, 512 commercial development and exploitation, 223–30 current technology provided by present day manufacturers, 223, 226–30 Gesellschaft für Motoren und Kraftanlagen mbH, Germany, 223, 226 GMK ORC plant connected to diesel engine, 226 GMK ORC power conversion module, 226 historical review of early development stages (1960–1984), 223 ORC power conversion module from Tri-O-Gen, 228 Ormat ORC plant producing electricity, 227 Ormat Technologies Inc, 223, 226–7 plants built between 1960 and 1984, 224–5 power conversion module of typical Turboden ORC plant, 229 process diagram, 229 schematic of Turboden power conversion module, 230 Tri-O-Gen B.V., Goor, The Netherlands, 228–9 Turboden S.r.l., Brescia, Italy, 229–30 conventional process generator, gearbox, recuperator and condenser, 209 working principle, 208 efficiency and typical costs for current ORC plants, 230–1 large size plants disadvantages, 219 ORC process vs water-based systems benefits, 213, 217–19 good dielectric properties, 219 low enthalpy drop in turbine, 217–19 low heat of vaporisation, 213, 217, 218 process comparison in temperature– total enthalpy diagram, 218 simplified ORC process in temperature– entropy diagram, 217 power plant (145 kW) run with exhaust gas, 211 (150 kW) run with unclean landfill gas, 212 (500 kWe) heat source are hot gasses collected with pipes, 211 process principle, 206–7, 208, 209

© Woodhead Publishing Limited, 2011

CHP-Beith-Index.indd 524

4/5/11 9:44:37 AM

Index process system alternatives, 221–2 small CHP applications, 206–31 typical process heat sources and operating ranges, 207–13 biogas from industrial and municipal waste streams, 212 biomass in agriculture and in forestry, 212–13, 214 electricity production and district heat from biomass, 214 gas turbines and combustion engine exhaust gases, 209, 211–12 geothermal heat, 213, 215 industrial waste heat, 207, 209, 210–11 micro-ORC in distributed energy system, 213, 216 micro-ORC plant principle for special applications, 216 ORC principle using geothermal heat, 215 present-day industrial waste heat sources, and economical feasibility, 210 working fluid selection, 219–21 dry or wet expansion, 221 environmental aspects, 220–1 pressure in condenser, 220 thermal stability, 219–20 volumetric flow, 220 working fluids properties used in present day plants, 221 organic Rankine cycle machines, 460–3 organic waste biomass, 90–1 Otto engines, 126 ozone depletion potential, 206–7, 220 PAFC see phosphoric acid fuel cell PEFC see polymer electrolyte fuel cells PEMFC see proton exchange membrane fuel cell phosphoric acid fuel cell, 404 photovoltaic, 80 PHR see power-to-heat-ratios pipework installation, 351–3 polymer electrolyte fuel cells, 28, 234 power generation, 70–1 power piston, 469, 470 power plant, 71 power stroke, 185 power-to-heat-ratios, 403 power trading, 308 PowerPlus Technologies ecopower, 143–4 CHP unit performance data, 144 ecopower micro-CHP unit design and operational data, 143 pressure-reducing valve, 47 prime movers, 264–5, 461 process heat, 46

525

proton exchange membrane fuel cell, 404 pump efficiency, 56 influence on CHP performance, 58 radial compressor, 160 radial turbines, 162 Rankine cycle, 45, 206, 455, 460 Rankine cycle machines, 460–3 Rankine cycle micro combined heat and power, 471–3 Rankine cycle technologies, 404–5 reciprocating engines, 402–3, 406 reciprocating internal combustion, 370–2 recuperator, 163–4 Remeha, 194, 202 renewable energy, 429 renewable energy certificates, 389–90 renewable energy source and combined heat and power solution, 81 Renewable Heat Incentive scheme, 392 Renewable Obligation, 429 RES -CHP see renewable energy source and combined heat and power solution Reynolds number, 152 RHI see Renewable Heat Incentive scheme rhombic drive, 469 ROC see renewable energy certificates rotary screw engine, 462–3 Rural Generation Ltd, 448–9 Sanevo, 190 Sankey diagram, 64 cogeneration scheme, 67 single generation scheme, 65 scroll engine, 461, 462 Second Law of Thermodynamics, 292 SenerTec Dachs, 140–3 micro-CHP technical data, 142 micro-CHP unit, 141 simulation models, 22 small combined heat and power system review, 511–12 small combined heat and power systems, 8–9 application in buildings, 377–86 breakdown of energy consumption in UK supermarkets, 385 cooling trigeneration system at Edinburgh University, 381 health care buildings, 382–4 higher education institutions, 378–82 hospitals, 383–4 large office buildings, 377–8 leisure and recreation buildings, 384 supermarkets, 386 supermarkets and hypermarkets, 385–6 university campuses, 379 commercial buildings and institutions, 365–92

© Woodhead Publishing Limited, 2011

CHP-Beith-Index.indd 525

4/5/11 9:44:37 AM

526

Index

basic issues and energy requirements, 366–8 energy use in commercial sector by building type, 367 energy use in non-domestic buildings, 366 future trends, 390–2 merits and limitations, 389–90 number and capacity of CHP installations in the building sector, 369 microturbine systems, 147–76 components types and properties, 160–6 cycle performance, 152–9 future trends, 174–6 manufacturers and applications, 172–4 operation, 166–72 organic Rankine cycle systems applications, 206–31 benefits and disadvantages vs waterbased systems, 213–19 commercial development and exploitation, 223–30 efficiency and typical costs for current ORC plants, 230–1 process principle, 206–7 process system alternatives, 221–2 typical process heat sources and operating ranges, 207–13 working fluid selection, 219–21 performance analysis and optimisation, 386–9 base electricity load supply operation mode, 387 base heat load supply operation mode, 387 cost-led operation strategy, 387–8 system performance indicators, 388–9 small-scale technology, 369–77 electrical connection, 376 energy balance, 370 gas-fired micro-gas turbine CHP system, 373 heat engine types, 370–4 heat engines types and properties, 374 heating circuit interface with building services, 375 integration into building services, 374–7 internal combustion gas engine, 372 small-scale biomass CHP, 453 small-scale biomass combustion CHP, 452 small-scale biomass gasification CHP, 452 small-scale biomass pyrolysis CHP, 452 SOFC see solid oxide fuel cells solar energy, 413 solid biomass fuels, 91–4, 95, 96–8 characterisation, 91–4, 95 average net calorific value, 94

calculation of analyses to different bases, 93–4, 95 calorific value, 93, 94 formulae for calculation of results to different bases, 95 moisture content effect on biomass fuel net calorific value, 94 proximate, ultimate analyses and gross calorific values, 95 proximate analysis, 92 ultimate analysis, 92–3 combustion, 96–8 50 kW wood pellet boiler, 97 Arimax Bio Energy boiler, 98 stages, 96 solid desiccant cycles, 271–6 basic cycle, 274 cycle enhancements, 274–6 desiccant wheel configuration options, 274 solid desiccant materials and substrates, 272–3 solid oxide fuel cells, 28, 39, 81, 175, 236, 326, 404, 409–11 flowsheet, 410 integration composite curves, 411 overall efficiencies of combined SOFC and steam system arrangement, 411 Solo engine, 190, 191 see also Cleanergy AB SOLO V161 Stirling CHP system, 480 solvent refined coal, 446 space heating, 358 spark gap, 19 spark ignition engine, 130–2, 406, 463–4 spark spread, 18–19, 326, 387 specific steam consumption, 48 squish, 133 SRC see solvent refined coal steam ejector cycle, 284–5 steam engine, 453 steam tables, 52–3 steam-turbine cogeneration plant, 47 Stirling conversion, 76 Stirling cycle, 510 Stirling cycle micro combined heat and power, 474–80, 481 Baxi Cogen MCHP unit, 478 Enatec MCHP unit, 479 free-piston Stirling engine from Microgen, 478 free-piston Stirling engine in Enatec MCHP unit, 479 V-type SOLO V-161 Stirling engine, 481 Whispergen Mk 5 MCHP unit, 474 ‘wobble-yoke’ drive mechanism in Whispergen, 475 Stirling engines, 5, 28, 403–4, 453, 464–70 alpha engine, 467

© Woodhead Publishing Limited, 2011

CHP-Beith-Index.indd 526

4/5/11 9:44:37 AM

Index applications and future trends, 203–5 beta-type engine, 467–9 definition, 180–1 design arrangement and pressure-volume diagrams, 465 development for micro CHP applications, 189–98 Ariston 1 kWe micro-CHP unit, 196 Baxi Ecogen 1 kWe micro-CHP unit, 195 Cleanergy AB, 198 Disenco kinematic Stirling engine, 194–8 Infinia linear free piston Stirling engine, 194, 196 Microgen linear free piston Stirling engine, 192–4, 195 Sigma 3 kWe micro-CHP unit, 197 Sunmachine, 198 WhisperGen kinematic Stirling engine, 190–2 double-acting 4-cylinder engine, 468 gamma configuration engine, 469 general scheme beta-type engine, 468 free-piston engine, 470 layouts of modern Stirling engines, 467–70 micro CHP design and system integration, 199–203 design for thermal efficiency, 199–201 interface design, 203 Sigma Stirling engine micro-CHP package, 200 system design, 201–3 schematic illustration, 180 small and micro CHP applications, 179–205 Stirling cycle, 183–8 constraints, 185 efficiency and other performance constraints, 185–8 theoretical Stirling cycle, 184 suited to micro CHP, 181–3 high efficiency, 183 long life and extended service intervals, 182 low emissions, 183 noise and vibration, 182–3 types, 188–9 free-piston Stirling engines, 189 kinematic Stirling engines, 188 V-type, single acting alpha-configuration, 467 Sunmachine, 198 Sunpower engine, 193 superheat temperature, 56 influence on CHP performance, 57 supermarkets, 385–6 surging, 168 system fault level analysis, 14–15

527

T-s chart, 52 Talbott’s Heating, 174–5 techno-economics, 22 assessment for investors and policymakers economic results variation of two key micro CHP technologies to annual electricity demand in target dwelling, 30 economic results variation of two key micro CHP technologies to annual thermal demand in target dwelling, 29 assessment for technology developers sensitivity of economic result to maximum ramp rate for 4 microCHP technologies, 34 sensitivity of economic result to minimum operating point for 4 micro-CHP technologies, 35 sensitivity of micro-CHP system value to prime mover capacity, 32 assessment of combined heat and power systems, 17–41 case study: micro CHP, 28–39 economics, 18–21 future trends, 39–41 modelling methodology, 23–8 onsite generation, 21–3 thermal coefficient of performance, 286–9 thermal-engine-based small and micro CHP systems deployed prime movers, 460–70 four-stroke spark ignition engine operational principles, 463 internal combustion engines, 463–4 Rankine and organic Rankine cycle machines, 460–3 rotary engine schematic, 462 scroll engine, 462 steam/vapour engine schematic, 461 Stirling engines, 464–70 domestic applications, 459–508 economic and environmental analysis, 500–8 economical and environmental benefits estimation, 480–3 electrical demand, 490–2 base load, 490 biased load, 490 elective load, 490 electrical appliances in profile model, 491 electrical profiles for pairing with 40 kWh heating band, 492 heat demand modelling, 483–90 cooking, 484, 489–90 daily space heating bands used for modelling, 488

© Woodhead Publishing Limited, 2011

CHP-Beith-Index.indd 527

4/5/11 9:44:37 AM

528

Index

daily space heating demand, 487 daily space heating requirements, 485 domestic hot water, 484, 488–9 domestic hot water thermal power requirements, 490 idealised space heating profile with TRVs, 486 occupancy pattern for modelling procedure, 484 space heating, 483, 484–8 space heating demand profile, 487 use of domestic hot water, 489 performance as response to heat demand efficiency per cycle and power level efficiencies, 496–8, 499 emissions analysis, 498, 500 maximum outputs, 494–5 modulation capabilities, 495–6 run down, 494 start-up, 493–4 performance mapping, 492–500 breakdown of Whispergen Mk 3 rundown characteristic, 495 breakdown of Whispergen Mk 3 start-up characteristic, 494 cycle efficiencies, 497 exhaust composition analysis of Whispergen Mk 3 unit, 500 maximum outputs of Whispergen Mk 3 unit, 495 modulated thermal output of Whispergen Mk 3 unit, 496 power level efficiencies, 499 product development, 470–80, 481 3 kWel MCHP appliance, 476 Cogen Micro MCHP unit, 472 components of 3 kWel MCHP appliance, 477 internal combustion engine MCHP, 473–4 Kingston 1 kWel MCHP unit, 472 Rankine cycle Lion MCHP, 471 Rankine cycle micro combined heat and power, 471–3 Stirling cycle MCHP, 474–80, 481 XRGI 15 Mini CHP system based on Toyota gas engine, 474 semi-detached heating band annual variation of Whispergen Mk 3 performance, 506 electricity profile 1 weekday, 503 electricity profile 3 weekend, 504 summary of simulation results, 507 system temperatures 1 weekday, 503 system temperatures 3 weekend, 504 system temperatures 4 weekend, 505 thermo-insulation, 465 thermochemical gasification, 98

thermodynamics cogeneration, 44–7 combined heat and power, 46–7 heat-only plant, 46 power-only plant, 45–6 ‘process heating only’ plant and its T-s cycle, 46 schematic of typical stream-turbine cogeneration plant, 47 simple ‘power-only’ plant and its T-s cycle, 45 thermostatic radiator valves, 485 topping cycle, 405 total site profiles, 399 total site targeting, 414 transesterification, 106 transmission system operators, 82 TRIAD, 74 trigeneration, 9–10, 263–7 TRV see thermostatic radiator valves TSO see transmission system operators TSP see total site profiles Turbec, 173–4 Turbec SpA, 405 turbine efficiency, 56–7 influence on CHP performance, 59 uninterruptible power supplies, 307 types, 309, 312 United Kingdom Building Regulation Part L2, 365 United Kingdom-wide Community Energy Programme, 429 university campuses, 379 UPS see uninterruptible power supplies utilisation factor, 48 vanadium battery, 318, 319 vapour compression cycle, 286 vertical integration, 72 volatile matter, 92 Volvo TAD1240, 129 VW Golf 1.6 engine, 129 waste, 350 Waste to Energy plant, 361 water management, 243 Whispergen, 474–5, 476, 492–3 WhisperGen kinematic Stirling engine, 182, 185, 186, 190–2, 194, 199, 201, 202 EU1 1 kWe micro-CHP unit, 192 WhisperGen micro-CHP unit, 336 WhisperTech engine, 189 woody biomass, 89–90 working piston, 189 XRGI 15, 473

© Woodhead Publishing Limited, 2011

CHP-Beith-Index.indd 528

4/5/11 9:44:37 AM