Renewable Energies
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Renewable Energies
Edited by Jean-Claude Sabonnadière
This Page Intentionally Left Blank
Renewable Energies
This Page Intentionally Left Blank
Renewable Energies
Edited by Jean-Claude Sabonnadière
First published in France in 2007 by Hermes Science/Lavoisier entitled: Nouvelles technologies de l'énergie volumes 1 et 3 © LAVOISIER, 2007 First published in Great Britain and the United States in 2009 by ISTE Ltd and John Wiley & Sons, Inc. Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address: ISTE Ltd 27-37 St George’s Road London SW19 4EU UK
John Wiley & Sons, Inc. 111 River Street Hoboken, NJ 07030 USA
www.iste.co.uk
www.wiley.com
© ISTE Ltd, 2009 The rights of Jean-Claude Sabonnadière to be identified as the author of this work have been asserted by him in accordance with the Copyright, Designs and Patents Act 1988. Library of Congress Cataloging-in-Publication Data Energies renouvelables. English Renewable energies / edited by Jean-Claude Sabonnadière. p. cm. Originally published: Paris : Hermes science, 2006: under title: Nouvelles technologies de l'énergie, Pt. 1. Energies renouvelables. Includes bibliographical references and index. ISBN 978-1-84821-135-3 1. Renewable energy sources. I. Sabonnadière, Jean-Claude II. Title. TJ808.N6913 2009 621.042--dc22 2009017410 British Library Cataloguing-in-Publication Data A CIP record for this book is available from the British Library ISBN: 978-1-84821-135-3 Printed and bound in Great Britain by CPI Antony Rowe, Chippenham and Eastbourne.
Table of Contents
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xvii
Chapter 1. Photovoltaic Electricity Production. . . . . . . . . . . . . . . . . .
1
Jean-Claude MULLER
1.1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2. Photovoltaic conversion . . . . . . . . . . . . . . . . . . . . 1.2.1. I-V characteristics of a cell and conversion output . . 1.3. Cells with a crystalline silicon base. . . . . . . . . . . . . . 1.3.1. Raw silicon . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.2. Monocrystalline silicon . . . . . . . . . . . . . . . . . . 1.3.2.1. Techniques for growing monocrystals . . . . . . . 1.3.2.2. Record for cells on monocrystals . . . . . . . . . . 1.3.3. Multicrystalline silicon . . . . . . . . . . . . . . . . . . 1.3.3.1. Techniques for growing multicrystals . . . . . . . . 1.3.3.2. Improvement in performance of cells created from multicrystals . . . . . . . . . . . . . . . . . . . . . . . . 1.3.4. Silicon in self-supported ribbon . . . . . . . . . . . . . 1.3.4.1. Growing techniques . . . . . . . . . . . . . . . . . . 1.3.4.2. Prospects . . . . . . . . . . . . . . . . . . . . . . . . . 1.4. Cells in thin films . . . . . . . . . . . . . . . . . . . . . . . . 1.4.1. Polycrystalline silicon . . . . . . . . . . . . . . . . . . . 1.4.2. Nanocrystalline and amorphous silicon . . . . . . . . . 1.4.2.1. State of the art and new prospects . . . . . . . . . . 1.4.2.2. Industrial applications . . . . . . . . . . . . . . . . . 1.4.3. Marriage of crystalline and amorphous technologies .
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1.4.4. Other emerging thin-film materials . . . . . . . . . . . . . . . 1.4.4.1. Materials with a cadmium–tellurium base . . . . . . . . . 1.4.4.2. Materials with a base of indium–copper–selenium (CIS) (copper selenate) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.5. Prospects for thin films . . . . . . . . . . . . . . . . . . . . . . 1.5. Photovoltaic market . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.1. Stimulation of production by political intervention . . . . . . 1.5.2. First beneficial effects on production and power of the installations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.3. Adaptation of the product to the market: cost of watt and kilowatt hour PV . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6. Prospects for photovoltaic electricity development . . . . . . . . 1.7. Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Chapter 2. Photovoltaic Systems Connected to the Grid . . . . . . . . . . . .
25
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Seddik BACHA and Daniel CHATROUX
2.1. Problems of photovoltaic power generation connected to the grid 2.2. General remarks on connection . . . . . . . . . . . . . . . . . . . . . 2.2.1. Interfacing with the grid. . . . . . . . . . . . . . . . . . . . . . . 2.2.2. General remarks on control . . . . . . . . . . . . . . . . . . . . . 2.3. Physical architectures . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1. Central inverter . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2. Individual inverter . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3. Row inverters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.4. Multiple row inverters . . . . . . . . . . . . . . . . . . . . . . . . 2.3.5. Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Constraints related to supplying energy to the utility grid . . . . . 2.4.1. Quality of the energy supplied . . . . . . . . . . . . . . . . . . . 2.4.2. Security . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2.1. Security regarding the grid . . . . . . . . . . . . . . . . . . . 2.4.2.2. Security with respect to installation . . . . . . . . . . . . . . 2.5. Algorithmic architectures . . . . . . . . . . . . . . . . . . . . . . . . 2.5.1. The search for MPPT . . . . . . . . . . . . . . . . . . . . . . . . 2.5.2. Control of the inverter grid and the global chain . . . . . . . . 2.6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7. Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Chapter 3. Solar Heating. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vii
45
Christophe MARVILLET
3.1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1. Some history. . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. Some basic calculations . . . . . . . . . . . . . . . . . . 3.1.3. The performance of solar heating devices . . . . . . . 3.2. Available energy from the sun. . . . . . . . . . . . . . . . . 3.2.1. The apparent motion of the sun. . . . . . . . . . . . . . 3.2.2. Evaluation of sunlight received by a collector . . . . . 3.3. Flat solar panels . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1. Different technologies of thermal solar collectors. . . 3.3.2. Evaluation of the performance of solar collectors . . . 3.3.3. Selective coatings for collectors and glazing. . . . . . 3.4. Solar heating systems . . . . . . . . . . . . . . . . . . . . . . 3.4.1. Individual and collective solar water heaters . . . . . . 3.4.2. Combined solar systems for the heating of buildings . 3.5. Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Chapter 4. Solar Thermodynamic Power Stations . . . . . . . . . . . . . . . .
63
Alain FERRIÈRE
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Concentrating solar power technologies . . . . . . . . . . 4.1.1. Why concentrate solar radiation? . . . . . . . . . . . 4.1.2. Concentrating systems . . . . . . . . . . . . . . . . . . 4.1.2.1. The parabolic concentrator (or dish). . . . . . . . 4.1.2.2. The tower concentrator . . . . . . . . . . . . . . . 4.1.2.3. The cylindrical-parabolic concentrator (or trough concentrator) . . . . . . . . . . . . . . . . . . . . 4.1.3. Components for production of heat and conversion into electricity. . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.3.1. The solar receiver. . . . . . . . . . . . . . . . . . . 4.1.3.2. Heat transfer fluid . . . . . . . . . . . . . . . . . . 4.1.4. Storage and hybridization . . . . . . . . . . . . . . . . 4.2. The state of the art . . . . . . . . . . . . . . . . . . . . . . . 4.2.1. First generation solar stations and exploratory work 4.2.2. Second generation solar power stations: precommercial prototypes. . . . . . . . . . . . . . . . . . . . 4.3. Prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1. Strategy for penetrating the market . . . . . . . . . . 4.3.1.1. Power stations of the future and research efforts 4.3.1.2. Conclusions . . . . . . . . . . . . . . . . . . . . . . 4.4. Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . .
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Chapter 5. Wind Systems Technology . . . . . . . . . . . . . . . . . . . . . . .
103
Régine BELHOMME, Daniel ROYE and Nicolas LAVERDURE
5.1. Introduction: wind power today . . . . . . . . . . . . . . . . . . . . . . . 5.2. Description of a wind generator . . . . . . . . . . . . . . . . . . . . . . . 5.2.1. Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2. Constitution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Operation of a wind turbine . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1. Controls of energy conversion . . . . . . . . . . . . . . . . . . . . . 5.3.2. Control at the turbine level . . . . . . . . . . . . . . . . . . . . . . . 5.3.2.1. Action of the wind on the turbine blades . . . . . . . . . . . . . 5.3.2.2. Control methods at the turbine level. . . . . . . . . . . . . . . . 5.3.3. Mechanical system – transmission of the power. . . . . . . . . . . 5.3.4. Controls at generator and transmission network levels – different types of wind power generator systems . . . . . . . . . . . . . . 5.3.4.1. Fixed speed systems – squirrel cage asynchronous machines . 5.3.4.2. Variable speed systems . . . . . . . . . . . . . . . . . . . . . . . 5.4. Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Chapter 6. Integration of Wind Turbine Generators into the Grid . . . . .
143
Régine BELHOMME, Daniel ROYE and Nicolas LAVERDURE
6.1. Connection to the grid . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.1. Voltage at the point of connection . . . . . . . . . . . . . . . . . . . 6.1.2. Currents in steady state . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.3. Short circuit currents . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.4. Voltage profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.5. Voltage quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.5.1. Slow variations in voltage . . . . . . . . . . . . . . . . . . . . . . 6.1.5.2. Sudden changes in voltage . . . . . . . . . . . . . . . . . . . . . 6.1.5.3. Flicker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.5.4. Harmonics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.5.5. Disturbances of signals transmitted on the grid . . . . . . . . . 6.1.6. Stability and protection design . . . . . . . . . . . . . . . . . . . . . 6.1.6.1. Management in normal and abnormal regimes . . . . . . . . . 6.1.6.2. Managing voltage sags (FRT “fault-ride-through” or LVRT). 6.1.6.3. Interaction with the protection design . . . . . . . . . . . . . . . 6.1.7. Auxiliary system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.7.1. Regulation of voltage and reactive compensation . . . . . . . . 6.1.7.2. Regulation of frequency . . . . . . . . . . . . . . . . . . . . . . . 6.1.7.3. Operating on a separate grid and reconstitution of grids . . . . 6.1.8. Variability and unpredictability of production . . . . . . . . . . . .
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Table of Contents
6.1.9. Other solutions for connection problems . . . . . . . . . . . . 6.1.9.1. Reinforcement of the grid . . . . . . . . . . . . . . . . . . . 6.1.9.2. Power shedding . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.9.3. Coordination with other production methods . . . . . . . 6.1.9.4. Load control . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.9.5. Systems of reactive compensation and of voltage control 6.1.9.6. Systems for managing voltage sags . . . . . . . . . . . . . 6.1.9.7. Systems for energy storage . . . . . . . . . . . . . . . . . . 6.1.9.8. Short circuit current limiting devices . . . . . . . . . . . . 6.1.9.9. Other equipment . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Comparison of technologies and conclusion . . . . . . . . . . . . 6.3. Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4. Appendix: symbol table . . . . . . . . . . . . . . . . . . . . . . . . 6.4.1. Parameters and physical variables . . . . . . . . . . . . . . . . 6.4.1.1. Time variable . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.1.2. Turbine, blades . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.1.3. Mechanical system . . . . . . . . . . . . . . . . . . . . . . . 6.4.1.4. Induction and synchronous generators . . . . . . . . . . . 6.4.1.5. DC bus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Chapter 7. Marine Energy Resources Conversion Systems . . . . . . . . . .
181
Bernard MULTON, Alain CLÉMENT, Marie RUELLAN, Julien SEIGNEURBIEUX and Hamid BEN AHMED
7.1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2. Electricity productivity from marine resources . . . . . . . . . . . 7.2.1. Energy sources from the sea . . . . . . . . . . . . . . . . . . . 7.2.1.1. Solar heat. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.1.2. Wind energy . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.1.3. Ocean wave energy. . . . . . . . . . . . . . . . . . . . . . . 7.2.1.4. Tidal currents . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.1.5. Continuous ocean currents . . . . . . . . . . . . . . . . . . 7.2.1.6. Osmotic energy . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.1.7. Ocean biomass . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.1.8. Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.2. General technical-economic aspects . . . . . . . . . . . . . . . 7.3. Ocean wave generator systems (WEC: wave energy converters) 7.3.1. Wave energy characteristics . . . . . . . . . . . . . . . . . . . 7.3.2. Diversity in conversion systems . . . . . . . . . . . . . . . . . 7.3.3. Systems with breakwater ramps . . . . . . . . . . . . . . . . . 7.3.4. Oscillating water column (OWC) systems . . . . . . . . . . . 7.3.5. Systems with wave activated bodies. . . . . . . . . . . . . . .
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7.4. Tidal energy converters (TEC) . . . . . . . . . . . . . . . . 7.4.1. Characteristics of tides and other marine currents. . . 7.4.2. Tidal power production systems with dams . . . . . . 7.4.3. Systems for recovering energy from marine currents . 7.5. Other conversion systems . . . . . . . . . . . . . . . . . . . 7.5.1. Offshore wind power generators . . . . . . . . . . . . . 7.5.2. Ocean thermal energy converter (OTEC). . . . . . . . 7.6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7. Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Chapter 8. Small Hydropower . . . . . . . . . . . . . . . . . . . . . . . . . . . .
227
Raymond CHENAL, Aline CHOULOT, Vincent DENIS and Norbert TISSOT
8.1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2. What is small hydropower? . . . . . . . . . . . . . . . . . . . . . . . 8.3. Hydraulic energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4. The exploitation of hydraulic force . . . . . . . . . . . . . . . . . . . 8.4.1. Description of a typical scheme . . . . . . . . . . . . . . . . . . 8.4.2. Different types of schemes encountered . . . . . . . . . . . . . 8.4.3. Different kinds of turbines . . . . . . . . . . . . . . . . . . . . . 8.4.3.1. The Pelton turbine . . . . . . . . . . . . . . . . . . . . . . . . 8.4.3.2. The Francis turbine . . . . . . . . . . . . . . . . . . . . . . . . 8.4.3.3. The diagonal turbine . . . . . . . . . . . . . . . . . . . . . . . 8.3.3.4. The Kaplan turbine . . . . . . . . . . . . . . . . . . . . . . . . 8.4.3.5. The waterwheel for water from above. . . . . . . . . . . . . 8.4.3.6. The Banki or crossflow turbine. . . . . . . . . . . . . . . . . 8.4.3.7. The inverted Archimedes screw . . . . . . . . . . . . . . . . 8.4.4. Particular applications of small hydro . . . . . . . . . . . . . . . 8.4.4.1. Turbines in drinking water networks . . . . . . . . . . . . . 8.4.4.2. Turbines in wastewater networks . . . . . . . . . . . . . . . 8.4.4.3. Recovery of energy in desalination plants . . . . . . . . . . 8.5. Potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.1. Worldwide small hydropower . . . . . . . . . . . . . . . . . . . 8.5.2. European-wide small hydropower . . . . . . . . . . . . . . . . . 8.5.3. Possibilities for development of small hydropower in Europe 8.6. Research & Development in small hydropower . . . . . . . . . . . 8.6.1. Development of equipment adapted to each site. . . . . . . . . 8.6.2. Development of variable speed. . . . . . . . . . . . . . . . . . . 8.6.3. Development in generators . . . . . . . . . . . . . . . . . . . . . 8.6.4. Development in control-command. . . . . . . . . . . . . . . . . 8.6.5. Inflatable weirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.6.6. Water intake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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8.7. Environmental aspects of small hydropower . . . . . . . . . . 8.7.1. Initial state of the milieu . . . . . . . . . . . . . . . . . . . . 8.7.2. Setting phase. . . . . . . . . . . . . . . . . . . . . . . . . . . 8.7.2.1. Setting of a small power plant for its integration into the ecosystem . . . . . . . . . . . . . . . . . . . . . . . . . . 8.7.2.2. Flow of materials and equipment . . . . . . . . . . . . 8.7.3. Principal inputs and outputs during the operating phase . 8.7.3.1. Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.7.3.2. Materials carried by the watercourse . . . . . . . . . . 8.7.3.3. Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.7.3.4. Electricity production . . . . . . . . . . . . . . . . . . . 8.8. Policies favoring small hydropower . . . . . . . . . . . . . . . 8.8.1. R & D program . . . . . . . . . . . . . . . . . . . . . . . . . 8.8.2. Rate measures . . . . . . . . . . . . . . . . . . . . . . . . . . 8.9. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.10. Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Chapter 9. Geothermal Energy Production . . . . . . . . . . . . . . . . . . . .
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Florence JAUDIN and Laurent LE BEL
9.1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2. Geothermal energy: why, for whom and how? . . . . . . . 9.2.1. The types of resources used . . . . . . . . . . . . . . . . 9.2.1.1. Fissured and/or porous volcanic formations . . . . 9.2.1.2. Aquifers of sedimentary basins. . . . . . . . . . . . 9.2.1.3. Superficial formations . . . . . . . . . . . . . . . . . 9.2.1.4. Deep formations (with low permeability). . . . . . 9.2.2. End-use . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.2.1. Heat network system . . . . . . . . . . . . . . . . . . 9.2.2.2. Heat pump (HP) system . . . . . . . . . . . . . . . . 9.2.2.3. Electricity production . . . . . . . . . . . . . . . . . 9.2.3. Other uses . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3. Geothermal heat pump systems . . . . . . . . . . . . . . . . 9.3.1. Current situation and tendencies . . . . . . . . . . . . . 9.3.2. The principle of the heat pump . . . . . . . . . . . . . . 9.3.2.1. Classification of heat pumps . . . . . . . . . . . . . 9.3.2.2. Coefficient of performance (COP) . . . . . . . . . . 9.3.3. Extracting heat from the ground . . . . . . . . . . . . . 9.3.3.1. Drawing calories from groundwater . . . . . . . . . 9.3.3.2. Horizontal and vertical in-ground heat exchangers 9.3.4. Development prospects and potential . . . . . . . . . .
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9.4. Direct production of heat . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.1. Current situation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.2. Geothermal heating networks. . . . . . . . . . . . . . . . . . . . . 9.4.2.1. The theoretical doublet and the associated heating network . 9.4.2.2. Geothermy experience in the Paris basin . . . . . . . . . . . . 9.4.2.3. Technological developments . . . . . . . . . . . . . . . . . . . 9.4.3. Prospects and potential for development . . . . . . . . . . . . . . 9.4.3.1. The objectives of the revival in Ile-de-France . . . . . . . . . 9.5. Electricity production . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.5.1. Current contribution of geothermal energy to the production of electricity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.5.2. Exploitation of geothermal resources . . . . . . . . . . . . . . . . 9.5.2.1. Naturally producing reservoirs . . . . . . . . . . . . . . . . . . 9.5.2.2. Enhanced geothermal systems . . . . . . . . . . . . . . . . . . 9.5.3. Development potential . . . . . . . . . . . . . . . . . . . . . . . . . 9.6. Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.7. Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Chapter 10. Biofuels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
329
Frédéric MONOT, Jean-Luc DUPLAN, Nathalie ALAZARD-TOUX and Stéphane HIS
10.1. The place of biofuels in the energy environment . . . . . . . . . . . . 10.1.1. A favorable environment. . . . . . . . . . . . . . . . . . . . . . . . 10.1.2. Principal characteristics of systems today. . . . . . . . . . . . . . 10.1.3. Main advantages and disadvantages associated with biofuel use 10.1.4. The situation of biofuels in the world . . . . . . . . . . . . . . . . 10.1.4.1. The influence of the Common Agricultural Policy (CAP) . . 10.1.5. Prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2. Current systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.1. Biodiesel systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.1.1. Raw materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.1.2. Production processes . . . . . . . . . . . . . . . . . . . . . . . . 10.2.2. The bioethanol system . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.2.1. Raw materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.2.2. Production procedures . . . . . . . . . . . . . . . . . . . . . . . 10.3. Future systems: use of lignocellulose . . . . . . . . . . . . . . . . . . . 10.3.1. Characteristics of components in vegetable lignocellulose. . . . 10.3.2. The BtL system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3.2.1. Main constraints of the process . . . . . . . . . . . . . . . . . . 10.3.2.2. Conditioning of biomass . . . . . . . . . . . . . . . . . . . . . . 10.3.2.3. Gasification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3.2.4. Treatment of syngas . . . . . . . . . . . . . . . . . . . . . . . .
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10.3.2.5. Fuel synthesis: Fischer–Tropsch and hydrocracking 10.3.2.6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 10.3.3. The bioethanol system . . . . . . . . . . . . . . . . . . . . 10.3.3.1. Main constraints of the process . . . . . . . . . . . . . 10.3.3.2. Pretreatment . . . . . . . . . . . . . . . . . . . . . . . . 10.3.3.3. Enzyme hydrolysis . . . . . . . . . . . . . . . . . . . . 10.3.3.4. Ethanol fermentation . . . . . . . . . . . . . . . . . . . 10.3.3.5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 10.4. Economic and environmental balance of biofuel production systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4.1. Economic aspects . . . . . . . . . . . . . . . . . . . . . . . 10.4.1.1. The competitiveness of biofuels . . . . . . . . . . . . 10.4.1.2. The ethanol system . . . . . . . . . . . . . . . . . . . . 10.4.1.3. Cost of ETBE production . . . . . . . . . . . . . . . . 10.4.1.4. Biodiesel . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4.1.5. New fuel systems . . . . . . . . . . . . . . . . . . . . . 10.4.2. Results of analyses of life cycle of biofuels. . . . . . . . 10.5. Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Chapter 11. Biogas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
397
Pierre LABEYRIE
11.1. Introduction: biogas, “the renewable natural gas” . . . . . . . 11.2. Naturally occurring biogas . . . . . . . . . . . . . . . . . . . . . 11.3. Production organized by humans . . . . . . . . . . . . . . . . . 11.4. History of anaerobic digestion . . . . . . . . . . . . . . . . . . . 11.5. Anaerobic digestion . . . . . . . . . . . . . . . . . . . . . . . . . 11.5.1. Management of the anaerobic digestion process . . . . . . 11.5.1.1. The effect of temperature . . . . . . . . . . . . . . . . . 11.5.1.2. Effect of pH . . . . . . . . . . . . . . . . . . . . . . . . . 11.5.1.3. Dynamics of the bacteria populations . . . . . . . . . . 11.5.1.4. Mixtures of substrates or codigestion . . . . . . . . . . 11.6. Anaerobic digestion installations or biogas units . . . . . . . . 11.6.1. Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.6.1.1. Digesters functioning with a continuous introduction of substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.6.1.2. Discontinuously functioning digesters (batch) . . . . . 11.6.2. Examples of recent agricultural anaerobic digestion installations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.6.2.1. Mr Claudepiere’s installation: liquid system . . . . . . 11.6.2.2. The GAEC Oudet installation: liquid system . . . . . .
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11.6.2.3. GAEC of the Chateau installation (under completion): mixed system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.6.2.4. Pierre Lebbe installation: solid system . . . . . . . . . . 11.7. Uses of biogas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.7.1. Thermal engine cogeneration . . . . . . . . . . . . . . . . . . 11.7.2. Exclusively thermal use . . . . . . . . . . . . . . . . . . . . . 11.7.2.1. Use of a boiler . . . . . . . . . . . . . . . . . . . . . . . . . 11.7.2.2. Use of a production system for cooling . . . . . . . . . . 11.7.3. Fuel production . . . . . . . . . . . . . . . . . . . . . . . . . . 11.7.3.1. PSA (pressure swing adsorption) purification . . . . . . 11.8. Conclusion: renewable natural gas and its challenges . . . . . . 11.9. Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Chapter 12. Energy Production from Wood. . . . . . . . . . . . . . . . . . . .
427
Frédéric DOUARD
12.1. Introduction: what is wood energy?. . . . . . . . . 12.2. Overview of wood fuels. . . . . . . . . . . . . . . . 12.2.1. Logs . . . . . . . . . . . . . . . . . . . . . . . . . 12.2.2. Densified wood logs or compacted logs . . . . 12.2.3. Briquettes. . . . . . . . . . . . . . . . . . . . . . 12.2.4. Wood pellets . . . . . . . . . . . . . . . . . . . . 12.2.5. Wood chips. . . . . . . . . . . . . . . . . . . . . 12.2.6. Industrial chips . . . . . . . . . . . . . . . . . . 12.2.7. Grindings from recycled wood . . . . . . . . . 12.2.8. Ground bark . . . . . . . . . . . . . . . . . . . . 12.2.9. Sawdust and wood chips . . . . . . . . . . . . . 12.2.10. Wood powder . . . . . . . . . . . . . . . . . . 12.2.11. Roasted wood . . . . . . . . . . . . . . . . . . 12.2.12. Wood charcoal . . . . . . . . . . . . . . . . . . 12.2.13. Spent pulping liquors and paper mill sludge 12.3. Principles of conversion of wood into energy . . . 12.3.1. Combustion . . . . . . . . . . . . . . . . . . . . 12.3.2. Pyrolysis . . . . . . . . . . . . . . . . . . . . . . 12.3.3. Gasification . . . . . . . . . . . . . . . . . . . . 12.4. Generators of thermal energy from wood . . . . . 12.4.1. Domestic technologies . . . . . . . . . . . . . . 12.4.1.1. Hearths, ovens and other open fireplaces . 12.4.1.2. Closed hearth fireplaces . . . . . . . . . . . 12.4.1.3. Heating and cooking stoves . . . . . . . . . 12.4.1.4. Wood-fired boilers . . . . . . . . . . . . . . 12.4.1.5. Pellet stoves . . . . . . . . . . . . . . . . . .
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12.4.1.6. Pellet boilers . . . . . . . . . . . . . . . . . . . . . . 12.4.1.7. Domestic wood chip boilers . . . . . . . . . . . . . 12.4.2. Housing complexes or industrial technologies . . . . 12.4.2.1. Hot air boilers and generators with fixed grilles . 12.4.2.2. Boilers with moving or mobile grilles . . . . . . . 12.4.2.3. Boilers with rotating conical grilles . . . . . . . . 12.4.2.4. Boilers with vibrating grilles . . . . . . . . . . . . 12.4.2.5. Boilers with rolling grilles . . . . . . . . . . . . . . 12.4.2.6. Bottom draft or air injection boilers . . . . . . . . 12.4.2.7. Boilers with boiling fluidized beds . . . . . . . . . 12.4.2.8. Boilers with circulating fluidized beds . . . . . . 12.5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.6. Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . .
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List of Authors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Preface
1.1. Electric energy Energy, which was the basis of the industrial revolution, has had an exponential growth since the birth of industrial electricity at the end of the 19th century. The discovery of the rotating field by Nikola Tesla and the invention of the transformer allowed the expansion of three phases: alternating currents for generation, transmission, and delivery and uses of electric energy in the cheapest way. At the beginning of the 20th century the development of large electric power grids enabled many countries throughout the world to bring the benefits of electric energy to their citizens, while intensively developing the industrial and tertiary applications of electricity. This growth led to generalization of the use of electric energy in domestic applications and all sectors of industry. Generation, transportation, transmission and distribution of electric energy were considered to be such strategic operations by most countries that they decided to build them as monopolistic state companies in order to control their development. These decisions and the heavily capitalistic nature of generating and transmission systems led to a vertical integration of electric energy utilities for economic reasons. This is the classic paradigm that for more than a century has allowed the creation of an industry that reached the heights of its power in a slow but constant improvement of the reliability of equipment, the main objective being to ensure the supply of electricity to domestic and industrial customers connected to the grid. The tremendous growth of electric energy consumption during the middle of the last century led to the construction of very large and complex electric power systems (for instance, in the French grid, more than 66,000 MW are flowing at any time in about 1,300,000 km of lines and cables). Figure 1 shows a schematic representation of these systems.
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Figure 1. The historic paradigm
All electric power systems have been built and operated according to this diagram from the beginning, and for almost all, of the 20th century. Their organization and their operation from generation to the consumer were integrated inside only one, generally monopolistic, private or public company. The deregulation that started at the beginning of the 1980s introduced a tremendous change by imposing the unbundling of the functions of generation, transmission and distribution. This change introduced a new mode of organization according to a broken up model we shall describe in the next paragraph as the new paradigm. 1.2. The new paradigm The goal aimed at by the instigators of the deregulation of electric systems has always been to promote a new organization of the electric systems in order to create the conditions of commercial competition among all the stakeholders: the aim is to lower the price of electric energy supply for consumers. The setting up of new regulations took place in a context where geographic constraints were prominent due to the territorial settlement of power grids that were in fact naturally monopolistic. However, the new system instituted in the UK at the beginning of the 1990s, and then in the USA, is now installed in almost all developed countries, although with some difficulties of adaptation. As a matter of fact, the systems designed and built on to be operated as an integrated company in a well-defined territory must today be operated on a continental scale without prior modification of the infrastructures of transmission and interconnection.
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1.2.1. The energy system in a deregulated context The major functions which must be carried out in order to satisfy consumer demand are the same as previously described but with a different mode of interaction, and they are operated by different actors according to the five essential links described in Figure 2.
Figure 2. The new organization of electric systems
This new organization creates a total hierarchical independence between the producers and the other members of the system. The producers of course are in charge of the generation. They sell to consumers according to several kinds of contracts for short or medium term delivery. The traders are the link between generation and consumption, taking into account the power transmission capacity through transmission and distribution grids. The power grids must ensure the transmission of energy to the consumers according to the trading exchanges they had with the producers, insuring equitable treatment for all producers. Independent system operators for transmission and distribution grids are in charge of this duty. This new set of concepts generates not only drastic changes in the economic conditions of the electric system’s operation, but also significant technical changes that favor the new technologies with dispersed generation, especially renewable energy technologies that will continue to be the foundation of consumer power. 1.3. Dispersed generation The economic issues arising from the new regulations incite the big consumers to buy internal generation units in order to smooth the price fluctuations coming from deregulation. They are of course small generation units of limited power,
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generally connected to distribution grids. These units will be able to control the power imported by the consumer, who will be able to sell his energy on the market by injecting it into the grid if price conditions are favorable. The development of a large number of small generation units from solar, wind, small hydraulic or thermal units, with combined heat and power, alongside classical large generation machines, will superimpose a new phenomenon on the normal operation of the distribution grids: bidirectional energy flow, generally intermittent and random according to wind and sun production. This phenomenon will create new difficult problems such as the management of these energies while keeping the grid security at the required level. The new paradigm, which integrates dispersed generation with the economic environment, will lead to a new operation scheme for electric power systems, one that will increasingly be a substitute for the present scheme as described in Figure 3.
y
y
Figure 3. The new electric power system
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1.4. The new energy generation and management Independently of these new schemes of grid operation, in the next chapters we shall describe the main characteristics of the emerging generation technologies that will be installed on the grid of the next decades. We shall review the new energy technologies in terms of the types of renewable energy. The second volume, Low Emission Technologies and Energy Management, will deal with reduced emission technologies and energy saving. This volume will include various forms of solar energy such as photovoltaic, thermal and thermodynamic energy conversion. Wind technologies are in full development today. The chapters dedicated to them describe the state of the art, taking into consideration the question of insertion into the grids of a large quantity of this intermittent energy. Energy from the sea will complete this general view, with a chapter discussing very small hydraulic plants that become of interest when fossil energies become more and more expensive. We shall then provide an analysis of geothermal energy, followed by energy from biomass. This will entail a full description of biofuels and biogas, and especially energy from wood as a substitute to fossil energy heating. Volume 2 will be dedicated to energy storage, low emission technologies and energy management. It will start with the new generation of nuclear energy, which is presently at a crossroads of its history with these future new generations. It will then analyze combined heat and power generation, by which it is possible to produce heat and electricity jointly as a complement to heat generation in order to improve the efficiency of heat plants. A very careful analysis of the economic conditions of the operation of combined heat and power will lead to a description of the economic conditions for which this technology is advantageous. Energy storage will describe the various means and methods of storage in association with intermittent energies like photovoltaic and wind energy plants. Their efficiency and cost are strongly dependent upon their operational facilities and investment costs. A projection of the use of hydrogen as an energy vector similar to electricity will give readers a comprehensive view of the way to create, store and transport this gas,
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which is generally improperly considered as very dangerous. Its future using fuel cells as a conversion facility allows us to foresee significant development of this energy vector with a large set of applications. Finally, we take up the very important subject of energy management, control of energy demand, and energy saving. We will describe positive energy houses, low consumption public and domestic lighting, and power moderation by control of the load from the grid. Jean-Claude SABONNADIÈRE
Chapter 1
Photovoltaic Electricity Production
The production of electricity by conversion from light with the aid of photovoltaic cells has soared dramatically in the beginning of this new millennium, with a rate of increase of more than 35% a year for six years and a record of 66% in 2004. New technological approaches are always necessary to make low cost and high energy conversion efficiency cells so that photovoltaic energy from now until 2020 can cross the competitiveness threshold in comparison with other sources of electricity production. Producing cells based on crystalline silicon is still the most advanced technology on the technological and the industrial levels. In fact, silicon in the form of silica is one of the most abundant elements on earth, being perfectly stable and non-toxic. The current direction of Research & Development in the photovoltaic field is based on developing new processes for growing multicrystalline silicon, the reduction of thickness and the improvement of the devices. It is also based on the emergence of cells in thin layers of silicon or other semiconductor compounds. The integration of photovoltaic modules into electricity production installations in remote sites, in the form of large power plants or photovoltaic roofs connected to the grid, will increasingly require intelligent systems for the control and management of energy, and therefore for high performance power electronic components.
Chapter written by Jean-Claude MULLER.
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1.1. Introduction In 20 years, the price of the photovoltaic watt has dropped considerably. From more than €100 in l975, it has fallen today to about €2 for PV modules and to about €6 for a system connected to a grid. Industrial production is in almost exponential growth (1.6 GW in 2005 while it was about 50 MW in 1990) and there are predictions of a 200-fold increase in production by 2030. However, in spite of the immense distance it has already come, solar electricity has not yet reached the point where it can be competitive compared to other sources of electricity production. ,
Figure 1.1. World market position of photovoltaic energy at the end of 2004 (source: AIE)
This will require new technological approaches in order to reduce production costs while increasing output efficiency. Furthermore, there are still two technological barriers to break for cell production to be on the one hand less energyintensive and on the other hand to be free of environmental repercussions. Cell production with a solid multicrystalline silicon base has been evolving for the following reasons. One is the considerable increase in the cost of the raw material: electronic quality silicon has gone from $25/kg at the end of 2004 to more than $100/kg 18 months later. Other reasons are to reduce the thermal requirements
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by using increasingly faster growing techniques (continuous casting) and the fact that the silicon is increasingly less purified (the supply of microelectronic cast-offs has begun to be limited).1 This lowering of quality results in an alteration of the volume transport properties of photo-generated charges, so that in the future the devices must be increasingly thin, up to a thickness of some 100 microns, in order to compensate for the reduction of lifespan in the base. In other respects, this development will make it possible to reduce the quantity of material used.
Figure 1.2. Distribution of world production in terms of models2
1.2. Photovoltaic conversion After obtaining the professional applications market for remote sites, being in a good position for electrifying rural populations in developing countries, solar electricity has asserted itself in industrialized countries as a complementary source to classic electric energy sources. Before the middle of the century, it will contribute in a considerable way, even in a decisive way, to controlling the greenhouse effect. The photovoltaic effect was discovered by Antoine Becquerel in 1839, 57 years before his grandson Henri discovered radioactivity. The photovoltaic effect is obtained by the absorption of photons in a material possessing at least a possible transition between two levels of energy (semiconductor). In order to be able to transfer an electron from the valence band to the conduction band, by creating a hole in the former, an energy, for example of 1.1 eV 1 This production represents close to 62% of sales in 2004, with 72% more in monocrystalline silicon, more than 3.4% in amorphous silicon and a rise in power of heterostructures in thin layers on crystalline with 4% of the market in 2004. 2 Photon International, April 2005 (available at www.photon-magazine.com); The Solar Power Letter, Cythelia, April 2005 (available by subscription at www.cythelia.fr).
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for crystalline silicon, or 1.7 eV for amorphous silicon, is necessary. The photons absorbed from an energy higher than this gap can create an electron–hole pair, with the electron in the conduction band and the hole in the valence band. To obtain a current, we separate the electron and the hole by creating an electric field in a semiconductor, a p-n diode. Zone n has an electron excess, zone p has an excess of holes, giving rise to an electric field separating the charges created by the photovoltaic effect. A difference in potential is established at the terminals of the photovoltaic cell. For silicon, we obtain a p zone by doping it with boron and an n zone by doping it with phosphorus. 1.2.1. I-V characteristics of a cell and conversion output
Figure 1.3. Principle of photovoltaic conversion and definition of conversion output
We arrive at the notion of energy conversion output of the cell (see Figure 1.3), which is the ratio between the electric power delivered and the incident power: Rdt = Voc (V) Isc (mA/cm2 ). FF/P incid (W/cm2 )
[1.1]
The short circuit current Isc is equal to the photo-generated current for V = 0, Voc is the open circuit voltage and FF is a form factor linked to the quality of the cell.
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By convention, the conversion output of a cell is always measured for an incident power of 100 mW/cm2 and a solar spectrum corresponding to a sun under AM1.5 (that is, for the sun making an angle with the horizon such that the thickness of the atmosphere effectively crossed is equal to one and a half times the thickness normalized to 1 (Air Mass One, AM1) for the sun at its zenith). 1.3. Cells with a crystalline silicon base 1.3.1. Raw silicon Silicon, the basis of the entire modern electronic industry, is obtained by reduction of silica in an electric oven, furnishing the “metallurgic” material, whose purity is about 98%. This is in turn purified in the form of trichlorosilane or silane gas. After pyrolysis of the latter, the material obtained serves as the starting point for growth. 1.3.2. Monocrystalline silicon 1.3.2.1. Techniques for growing monocrystals It is possible to produce monocrystals by the Czochralski (Cz) method for 30 cm diameter and a length longer than 1 m. It should be noted that by beginning with 1 kg of silica, we do not obtain more than 100 g of monocrystalline silicon for a considerable expenditure of energy, in the order of MWh. Furthermore, half of the crystal will be lost in the course of cutting it into slices of 300 μm thickness. It is p type, that is doped with boron, at a concentration of between 1016 and 1017 atoms cm-3, in order to obtain a resistivity on the order of 0.1 to 1 ȍ.cm. This choice results from a compromise between a lowest possible resistivity and a moderated doping in order to avoid degradation of the diffusion length of the photo-generated carriers. Subsequently, it is still necessary to create a potential barrier necessary for the collection of charges, that is the emitter of the photovoltaic cell, which will not be described in this chapter. 1.3.2.2. Record for cells on monocrystals With such Cz microelectronic materials, it is possible to obtain energy efficiencies in the laboratory of more than 22% for Cz (FhG-ISE, Germany) and the record of 24.7% for FZ (Float-Zone growing method) held by the University of New South Wales, Australia.
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Figure 1.4. Structure of the best performing laboratory cell (source: Photovoltaics Special Record Center UNSW [GRE 92])
1.3.3. Multicrystalline silicon 1.3.3.1. Techniques for growing multicrystals In a polycrystal, the monocrystals are separated from each other by perturbed zones and grain boundaries, and each of the smaller crystals does not have the same orientation as its neighbor. The growing techniques that assure the formation of a colony structure with large crystals (from this comes the term multicrystalline silicon, mc-Si) is favored, in order to limit the harmful effects of the grain boundaries. This directional solidification technique, where blocks of more than 250 kg can be obtained by controlled cooling of the silicon in fusion in a mold of appropriate material (often quartz), is faster and less costly in terms of energy than a growth of one Cz or FZ ingot (24 to 48 hours as opposed to several weeks for the latter).
Figure 1.5. Principle of growth by Polix directional solidification developed by Photowatt (source: Photowatt S. A. FR)
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However, this constrained and dislocated material is also contaminated by the residual impurities in the silicon made from the cast-offs of the microelectronic industry. The leading mc-Si materials have come from Silso de Wacher in Germany since 1975, followed in France by Polix of Photowatt. In the USA, the best known materials are the Semix of Solarex (which became BP-Solar) and the HEM of Crystal Systems. During the 1990s, the Japanese offered a new material produced by continuous casting in an electromagnetic crucible (Osaka Titanium Co. [OTC], now Sitix of Sumitomo). In France, Madylam of Grenoble has also explored this possibility of production in a cold crucible (Emix) like Photowatt (PolixEM).
Figure 1.6. Principle of silicon growth by continuous casting in an electromagnetic crucible (source: Madylam FR)
1.3.3.2. Improvement in performance of cells created from multicrystals The industrial conversion outputs, which were of the order of 8 to 10% before 1980, are now from 16 to 17% for large blocks of 10 x 10 or 15 x 15 cm2. This progress is explained by the constant improvement of the quality of the materials and by the growth in understanding of the faults and residual impurities. Currently, we know perfectly well how to counteract the harmful effects of most of the faults of the crystallographic and certain metallic elements on the electric
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activity of the cell by introducing hydrogen at the time, for example, of the deposition of a nitride layer serving as a non-reflecting surface. However, the efficiency of this neutralization will depend on the interaction of these faults and impurities with carbon and oxygen according to their respective content in these materials.
Figure 1.7. Progression of conversion outputs in the laboratory and in industry for cells with a multicrystalline silicon base (source: NREL USA [SOP 99])
We can also purify the silicon bars at the time of a high temperature thermal treatment using an effect where impurities migrate toward sites located outside of the active zones where they may be trapped (a phenomenon better known by the term getter). Controlling these phenomena has allowed the InESS Laboratory of the CNRS to obtain a record efficiency of 16.7% on Polix bars of Photowatt by rapid thermal annealing in a lamp oven, that is, by way of a reduction of the thermal load and an environmentally friendly cell development process. Nevertheless, sawing these large blocks remains an onerous operation and leads to a fairly considerable loss of material, hence the trend toward ultra-thin bars (currently in the neighborhood of 200 μm in production at Photowatt, with a mechanical limitation of bar manipulation around 100–120 μm). The alternative would be to resort to silicon that can be used without requiring cutting operations.
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1.3.4. Silicon in self-supported ribbon 1.3.4.1. Growing techniques The techniques for producing silicon in self-supported ribbons were very appealing at the technological level. They could be obtained by capillarity between two carbon jaws or by growing them on a film or a carbon mesh. During the 1990s, these bands were the subject of much development at the research level and in some cases have reached the point of a design for precommercial production. However, all these “ribbon” techniques were widely penalized by a linear growth speed (usually from several cm/min to several tens of cm/min), imposed by the criteria of solidification that determine the size of the grains and the purity of the material by segregation of the impurities.
Figure 1.8. Process of drawing of silicon bands by capillarity across a mold allowing the production of octagonal tubes of silicon, currently in industrial production RWE Schott Solar at Alzenau (DE)
1.3.4.2. Prospects To this day these reasons have practically eliminated ribbons (2% of the world production), except in the USA, where EFG-type bands produced by the Mobil Solar Corporation (now RWE Schott Solar) continue in the form of nonagons or tubes of silicon, in order to increase production capacity.
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Other industrial companies, such as Evergreen Solar and Ebara in the USA, rely on a reduction of the thickness of the band (< 150 μm). The best conversion efficiencies for this technique are about 15%. On the other hand, in Germany the band is perceived more as a substrate for thin films which are laid down or epitaxied. In this case the crystallinity and the purity are less important factors, so they can be produced at very great speed and lower cost. 1.4. Cells in thin films 1.4.1. Polycrystalline silicon The technology of thin films of crystalline silicon deposited on a substrate whose development will largely be determined by the cost of the silicon consists of depositing a thin layer of polycrystalline silicon 10–40 μm thick on various substrata of slightly purified metallurgical silicon, quartz, ceramic or metal. These thin layers should make it possible to obtain conversion efficiencies of the same order as a cell on solid material. This is done by means of an optical confinement and a repelling opposing field for the minority carriers. Also, because the distance the carriers have to travel is lower, we can accept a lesser degree of purity for the basic material. For several years, thin films of crystalline silicon have raised major interest in Europe and in the USA, where Astropower (in 1998, 10 years after announcing it) was to have reached a pilot production of cells on thin layers of an undisclosed compositional ceramic with a depositing procedure that uses an alloy of silicon in liquid phase. This success has led many laboratories to work on the fabrication of films of Si in the gaseous phase on foundations of silicon, ceramic, or even glass. Two techniques are currently in competition: growing by epitaxy in liquid phase (LPE), whose main drawback is the use of a crystalline substrate (19 to 21% efficiency in the laboratory), and depositing in the solid phase. We will cite the pyrolytic decomposition of silane and hydrogen on a hot filament of tungsten (LPICM from CNRS), allowing the deposition of microcrystalline silicon (μc-Si) on a glass substrate at 500ºC at relatively high speeds (> μm/h). We also mention the deposition in vapor phase (CVD) at temperatures > 800ºC in the presence of a gas containing silicon (silane or chlorosilane) with the possibility (CNRS-PHASE) of a more rapid growth with deposit speeds of the order of 10 μm/mn with CVD assisted by heating lamps (RT-
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CVD) when the substrate is heated only during the depositing at temperatures much higher than those of the gas.
Figure 1.9. Growth of silicon thin film on ceramic by RT-CV (source: CNRS-PHASE (now CNRS-InESS) [SIF 04])
However, the efficiency remains less than 10% and reaches values of 13 to 16% only with a recrystallization to increase the size of the grains. On a ceramic foundation an output of 9.5% was reached (see Figure 1.10 in which the yields of all types of cells in thin layers of silicon were given according to the size of the grains).
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Figure 1.10. Progression of conversion outputs in the laboratory with cells of polycrystalline silicon base according to size of the grains (source: 12th PVSEC International Conference, Korea)
1.4.2. Nanocrystalline and amorphous silicon 1.4.2.1. State of the art and new prospects Since 1975, extensive research has been carried out on the use of noncrystallized silicon, in an amorphous state, whose hanging bonds are saturated by hydrogenation. This material offers three major advantages. One is a strong absorption coefficient that allows a very small thickness, of the order of microns, thus lowering the risk of a silicon shortfall. Another is low energy consumption during the production cycle with an energy break-even time of less than a year. The third is the capability of being deposited on large surface units of the order of 1 m2 for a single piece. On the other hand, its two weak points are the conversion efficiency and degradation when exposed to light (the Staebler–Wronski instability), which are increasingly better overcome by technological artifices, such as, in particular, a superimposition of two p-i-n structures in tandem or of three very thin active layers. Thus, light degradation has been reduced from 30% to 10%. The simplest structure of a cell of amorphous silicon (see Figure 1.10) is composed of a boron doped zone, an intrinsic zone, and a phosphorous doped zone (p-i-n).
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Each element may be connected directly in series without the need for external connections. Although outputs in the laboratory have reached 20% in double or triple p-i-n, industrial outputs have stagnated for years, remaining below 10%, so that very promising studies have again been taken up in the field of nano- or microcrystalline materials, and even of nearly amorphous (micromorphous) materials, with conversion outputs already higher than 10% (see Figure 1.10).
Figure 1.11. Cell with silicon base: a) amorphous silicon with p-i-n structure; b) cross-sectional view of a module (source: Solems Corporation)
In fact, these studies have shown that layers of μ-crystallines or μ-morphs of hydrogenated silicon deposited using radio frequency discharge make it possible to create cells that do not show any degradation when exposed to light. The idea is to stack up a classic p-i-n a-Si:H cell with an extremely thin cell several micrometers thick of the type p-i-n μc-Si:H (at the Neuchâtel Microtechnic Institute) or p-i-n μ-morph Si:H (at the Palaiseau Polytechnic School). 1.4.2.2. Industrial applications The very first commercialization involved the solar calculator market by Sanyo at the beginning of the 1980s, then came other applications for the general public
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such as hurricane or garden lamps, then photovoltaic kits and security systems. The successive improvements in the process contributed to an improvement in efficiency, over a period of 20 years, from 4% for non-encapsulated p-i-n cells to 8% for encapsulated cells. More recently the development of triple junctions, of which the latter is doped with germanium, has made it possible to reach stabilized efficiencies of 12% in the laboratory and 10% in industrial pilot programs. If small systems for mass markets were the main trend for the amorphous form for many years, flexible products (with triple junctions such as those offered by Canon) and building trade applications have been targeted since the end of the l990s. In this way tiles, clapboard, and other materials for roofs and facades incorporating amorphous cells have been developed in cooperation with building trade professionals. 1.4.3. Marriage of crystalline and amorphous technologies The a-Si/crystalline silicon heterostructures (HIT of Sanyo [TAG 00]) are probably the future solution for amorphous materials; in fact this union with crystalline silicon (since charge transport occurs in a good material) makes it possible to obtain open circuit voltages of more than 700 mV and a very high conversion efficiency: more than 21% in the laboratory (with a structure that does not degrade over time and for which the manufacture of the heterojunction is at low temperature). These structures, having a strong growth in production, comprise in the order of 3% of the current market for PV.
Figure 1.12. Cell with heterojunctions a-Si/c-Si 4.4; other emerging thin layer materials
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1.4.4. Other emerging thin-film materials The intensive research over many years for materials other than those with a silicon base has finally, towards the end of this decade, reached the stage of industrialization. In 2002, these other materials occupied a market share that was still negligible (0.9%), equal to that for silicon ribbons (see Figure 1.2). 1.4.4.1. Materials with a cadmium–tellurium base Until recently, it was admitted that the CdS–CdTe process represented one of the most promising approaches for earthbound photovoltaics. The value of 1.45 eV for the forbidden energy band of CdTe is ideally adapted to the solar spectrum and its very large absorption coefficient means that almost the complete spectrum is absorbed to a depth of 2 μm, thus allowing the use of relatively impure materials whose diffusion length for the minority carriers does not exceed several micrometers. The technological development of techniques for deposition by screening and electrolytic methods led BP Solar to the installation of a line of 10 MW commercial products. The majority of cells with CdTe use one layer of n type CdS as an entry window for light and as an electrical potential barrier (heterojunction CdS–CdTe). Although the recent results (10.5% obtained by Matsushita on a module of 1.375 cm2) are very encouraging, the environmental problems associated with the use of cadmium harmed the attempted development of this technology and it is at risk due to the same restrictions as apply to the use of cadmium batteries. 1.4.4.2. Materials with a base of indium–copper–selenium (CIS) (copper selenate) Alloys with an indium copper selenate base CuInSe2 were studied most by Boeing, then by NEREL in the USA, Matsushita in Japan and in Europe by Siemens Solar (ex-ARCO) and ZSW in Stuttgart. Diselenium of copper and indium (CIS) is a type I-III-VI compound material, with a very promising chalcopyrite structure, since the theoretical efficiency of the heterojunction (n) CDS–(p) CuInSe2 is about 25%. Cells with a base of quaternary chalcopyrite compounds of the type Cu(Ga,In)(Se,S)2 have recently attained record conversion efficiencies of 18.8% in the laboratory (NEREL) and despite the known difficulties of managing this technology on a large scale, outputs of 12.8% have been obtained by ZSW on 730 cm2. Industrial production has been ready at Siemens Solar since July 1998 with the release of a module of 10 W. The main improvements came from widening the forbidden band of CuInSe2 (EG = 1.02 eV) by using alloys of the type CuGaSe2 and CuInS2. The absorption coefficient is 100 to 1,000 times greater than that of crystalline silicon in the range of 1.1 to 2.6 eV.
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Figure 1.13. Cell with a Cu–In–Se base
Among the many deposition methods that have been tried, we will note: the coevaporation of three elements, the selenization of In and Cu films, the electrochemistry developed in France by IRDEP3, and the spray pulverization technique. The first method is the one that allowed Siemens-Solar to reach the highest efficiencies. The last one cited is certainly the most simple to apply but its output is fairly small. Because of the environmental problems raised by the presence of Cd in the window layer, the scientific community is currently looking to replace it by safer materials, such as transparent semiconductor oxides like ZnO. 1.4.5. Prospects for thin films In conclusion, thin films of microcrystalline Si of CdTe and CuInSe2 are the serious outsiders. They are capable, in theory, of outstripping the combined output performances and cost of solid crystalline Si. Their development, known for new μ-morph cells, will take a relatively long time to move from research to being considered for commercialization, probably beyond 2010. We will mention that very high output cells (silicon, compounds III-V), as well as concentration systems, remain attractive subjects, but they do not have much potential in the European market. As for “exotic” cells, they have an appeal from the point of view of basic research. However, the most attractive thing is that of electrochemical batteries with a base of TiO2 and of coloring; they have little chance 3 Common Laboratory CNRS-EDF.
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of a large scale market since they are made up of a liquid electrolyte and present certain instabilities even though their laboratory output exceeds 10%. Similarly, cells with a base of organic polymer materials that increase in output (currently from 3 to 5%) suffer from a very limited lifespan (from about 500 to 1,000 hours to a residual output of 1%) that restricts them to the sector of ephemeral photovoltaic compounds that, keeping in mind that they have a very low predicted cost, will open other market niches.
Figure 1.14. Cell with a polymers base [BRA 01]
1.5. Photovoltaic market Having gained the market for professional applications in remote sites and services to rural populations in developing countries, solar electricity now has hopes of being confirmed as complementary to classic electric energy sources throughout the world before the middle of the century, and of contributing in a fairly sizable way to managing the greenhouse effect (the Kyoto Accords). Photovoltaic technology has developed only very slowly by progressively conquering market niches in developing countries, across several professional applications (remote emergency signal terminals, remote detection, alarm, emergency pathway lighting, and booming in telecommunication relays, etc.), with the profits realized on the first applications allowing further development in the more far-reaching applications. With this growth (see Figure 1.1 for the period up to
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1998), it should have taken 25 years for the annual production to reach 5,000 MW/year or the equivalent of 0.005% of global raw energy consumption predicted for that period. In other words, photovoltaic energy conversion of solar energy would have remained a novelty or, at best, the energy for remote sites. Widespread political action was therefore necessary and expected. This came from Germany, then from Japan, and more recently from the European Union. In fact, the global dynamism of the research and of the photovoltaic industry (see the Preface) is strongly linked to voluntary programs in these developed countries and led to numerous experiments for systems integrated into architecture at the beginning of the 1990s in Germany and to a wide diffusion of private installations connected to the grid, and, especially, since 1995 in Japan, with NEDO programs. 1.5.1. Stimulation of production by political intervention According to the recommendations of the European Parliament, European Commission presented in its “White Book” an action plan for a large increase in renewable forms of energy from 2006 until 2010. In fact a doubling of renewable forms of energy in Europe from 6% in 2006 to 12% was planned for 2010. This very ambitious goal, implying a 22% share for electricity from renewable sources, is matched by the combination of photovoltaic installations in member countries of 3,000 MW, implying an annual growth rate of almost 30% in the next 10 years (rate confirmed since 1999 by an average rate of 35% a year). Although this planned photovoltaic contribution (3 GW) represents only a relatively modest share of the global energy mix, the goal of the white book in fact represents a formidable challenge for the entire European photovoltaic industry (3,000 MW, that is 3 million cells of 100 cm2, assuming for simplicity 10% efficiency). Until 2004 the cumulative world production was actually only around 2.5 GW (for an annual electricity production of 2.5 TWh for 1,000 hours of full sunshine a year). The distribution of this power actually installed, according to the most recent estimate of the AIE, gives something like a little less than half to Japan, or 1,150 MW; 800 MW for Germany, which dominates in Europe; and more than 15% for the USA (350 MW). To reach these goals for combined European production of 3 GW from 2005 until 2010, the annual production capacity of the plants installed in Europe should go from 50 MW currently to more than 600 MW in the year 2010.
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, ,
Figure 1.15. The progression of combined installed power [AIE 03, AIE 04, AIE 05]. First beneficial effects on production and power of the installations for Japan, Germany and the USA
1.5.2. First beneficial effects on production and power of the installations We note, first of all, that of the combined 2.5 GW installed worldwide, a large share was installed in Japan and Germany as mentioned above, while the other part, installed in developing countries, benefited most often from aid programs (World Bank, non-governmental organizations, etc.) Moreover, for too long France, engaged only in the area of application in remote sites, was very slow in connecting to the grid. On a global level, the rate of growth for remote systems is, on average, 15% a year. This includes systems for village applications in developing countries, isolated residences in developed countries, and commercial complexes. If the first has a huge potential that depends on international financing, the second, even though cost effective, will rapidly be saturated (this is already the case in Switzerland). The third, with considerable growth potential today represents more than 20% of global installations compared with 15% for non-commercial remote systems. Unless there is an unlikely jolt in the decentralized finance systems for developing countries, a sustained growth rate of nearly 30% a year will not come from the isolated systems. To reach the goals of the white book, there is therefore no choice but to push the use of photovoltaic power in our own countries through voluntary policies of integration on building surfaces. In this way Germany and Japan are the two uncontested leaders of this segment that represents 60% of the market. Germany, where more than 80% of the PV installations are in private dwellings connected to the grid, is able to jumpstart a program of 100,000 solar roofs subsidized to 70%, a program that has been emulated in Italy and in the
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Netherlands. In Japan, the new NEDO program, with the first program of 70,000 roofs already half-completed, is still more ambitious, with a combined installation of 5 GW on roofing by around 2010; that is, the equivalent of a large nuclear power plant. In fact, the Japanese authorities insist that only such a widespread program will be able to precipitate the lowering of costs that would really allow a change of scale. This program has had an immediate favorable effect on national cell producers (Kyocera, Sharp, Sanyo), who have doubled and then tripled their production capacity (see Figure 1.16). Germany, which was supposed to provide its own supply up till now on the world market, supports Q-Cells in its effort to increase capacity in order to follow the Japanese. Q-Cells in Germany, which started up its production in 2002 with 8 MW, has announced an increase from 150 MW to 360 MW in 2005 with the launching of its fourth production line.
Figure 1.16. Classification of the 10 premier industrial producers of photovoltaics at the end of 2003 [WAL 04]
In France, Photowatt, which has a capacity of the order of 30 MW, sells almost all its production for export (more than 50% to Germany) because the national market is extremely small (less than 5%). Photowatt has tried to follow the movement and has announced it would go to 100 MW three years from now and to 200 MW in five years, preferably at the Bourgoin-Jallieu site. In the realm of large photovoltaic power stations that are effectively demonstration operations with 5% of the market, the USA has launched, since around 1985, photovoltaic power stations of 1 to 6 MW (in California).
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This has been followed by Italy with ENEL, and by Spain, along with the European Community, by connecting a dozen power stations of 100 kW to 3 MW to the grid in the course of the last 10 years. Unfortunately, these demonstration programs for the moment principally demonstrate that use of photovoltaic currents is still too expensive (by a factor of four) compared with the watts per hour produced by fossil energies. 1.5.3. Adaptation of the product to the market: cost of watt and kilowatt hour PV If photovoltaic energy is one day to represent a significant share of electricity production in Europe, it should follow the example of the microcomputer revolution. It should begin with a multitude of private producers–consumers that are spread out but connected to existing grids, rather than with large-size solar power plants, because photovoltaic energy is, first and foremost, a decentralized energy source and available on everyone’s door step. The annual world industrial production of photovoltaic modules is well under way, with a progression of world module production that should continue to rise by 30% a year until 2015 (see Figure 1.17), then move at a slightly less consistent pace of 25% a year. However, solar electricity has not yet crossed the threshold that allows it to be competitive compared with other sources of electricity production. It still needs an effort of a factor of three to reach the cost of an installed watt on the same size as that of an installed nuclear watt on the order of 2€. ,
w ,
Figure 1.17. Predictable development of the PV market until 2030 according to an EPIA study
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With respect to the cost of the production of a kilowatt hour, estimated (for 2000) to be in the range of around €0.30 for southern Europe and €0.60 for northern Europe,4 we are not very far from the intersection with the leading edge cost ceiling of the kilowatt hour for southern Europe and from now till 2020 for northern Europe. Photovoltaïc energy will be competitive with the cost of the nuclear kilowatt hour, at 3 cents, around 2030–2040.
Figure 1.18. Predictable evolution of the price of the kWh PV until 2040 according to an EPIA study for PV Track (Source: RWE Energy AG and RSS gmbH)
1.6. Prospects for photovoltaic electricity development Therefore, materials with a solid silicon base should inevitably evolve towards substrata cut more thinly or obtained by continuous casting in order for this technology to remain competitive in the next 10 years with materials on thin films with a silicon base or semiconductor compounds CIS, not forgetting nano- or microcrystalline materials. In fact, it is by 2010–2015 that most experts foresee a growth in power of industrial production with a thin films base, before the emergence of new designs for cells with bases of polymer or organic materials. The world market is soaring and the combination of installations (currently probably more than 2.5 GW) in remote sites on the one hand (isolated dwellings, water pumping, marine marking lights, telephone relays, etc.) or, on the other hand, 4 Eurec Position Paper, Eurec Agency, BE-1040 Brussels, Ed. 2000; reprinted in 2002.
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connected to the grid (power stations, and, especially, photovoltaic roofs) continue to grow so that by 2020 and 2040 photovoltaic energy should be able to represent 1% and 14% respectively of world energy consumption. 1.7. Bibliography [AIE 03] AGENCE INTERNATIONALE DE L’ENERGIE, World Energy Outlook, IEA Annual Report, O.C.D.E., Paris, 2003. [AIE 04] AGENCE INTERNATIONALE DE L’ENERGIE, World Energy Outlook, IEA Annual Report, O.C.D.E., Paris, 2004. [AIE 05] AGENCE INTERNATIONALE DE L’ENERGIE, World Energy Outlook, IEA Annual Report, O.C.D.E., Paris, 2005. [BRA 01] BRABEC C. J., FROMHEZ T., HUMMELEN J. C., PADINGER F., SARICIFTCI N. S., SHAHEEN S. E., “Efficient organic plastic solar cells”, Applied Physics Letters, vol. 8, New York, p. 503–513, 2001. [GRE 92] GREEN M. A., Solar Cells, Operating Principles. Technology and System Applications, University of New South Wales, Kensington (Australia), 1992. [SIF 04] SIFFERT P., SLAOUI A., “Polycrystalline silicon films for electronic devices” in KRIMMEL E., SIFFERT P. (eds.), Silicon: Evolution and future of a technology, Springer-Verlag, Berlin, p. 49–72, 2004. [SOP 99] SOPORI B. (ed.), Proceedings of the 9th Workshop on Crystalline Solar cell Materials and processes, NREL, Golden, 1999. [TAG 00] TAGUSHI M. et al., “HIT Cells – High Efficiency crystalline Si cells with Novel Structure”, Progress in Photovoltaics Research and Applications, vol. 8, series 5, John Wiley & Sons, Chichester, p. 503–513, 2000. [WAL 01] JÄGER-WALDAU A. et al., Status Report 2004 – Energy End-Use Efficiency and Electricity from Biomass, Wind and Photovoltaics in the European Union, EC Joint Research Centre, Ispra, 2004.
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Chapter 2
Photovoltaic Systems Connected to the Grid
2.1. Problems of photovoltaic power generation connected to the grid In the field of power generation systems based on renewable energy, photovoltaic (PV) generation is the most expensive in terms of installed peak power (in the order of €3,000/kWc). This has a direct impact on the cost of the produced energy (between €250 and €350 per MWh), Photovoltaic power also generates the most CO2 emissions per kilowatt hour produced (60–250 g CO2/KWh) [ADE 05]. These two characteristics essentially come from the process of photovoltaic cell manufacturing. There are two tools available to mitigate this problem: reduce the cost of production or increase the energy output. In any case, cost reduction techniques that act on the technology and the production process are in direct opposition to obtaining higher conversion outputs. Therefore, it is necessary to find a useful optimum. Market effects and technological progress have led to predictions of cost reduction by a factor of 3 to 4 by 2020. If we insist on using the criterion of installation cost purely at the level of market price, this reduction will still not be enough to make this type of energy production competitive compared to other types of production, conventional or otherwise.
Chapter written by Seddik BACHA and Daniel CHATROUX.
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Renewable Energy Technologies
Finally, like wind power, solar power is characterized by its intermittence, making it necessary either to provide an often cumbersome storage system (not connected to the grid) or a grid connection. The replacement of the storage battery every three/four years in an autonomous photovoltaic system costs almost as much as the PV generator, effectively doubling the cost of the energy. In an objective analysis, it is necessary to take into consideration several factors at play: – the reduction of the joule losses for transmitting the energy due to proximity to the consumers, – the significant reduction of greenhouse emissions compared to conventional means of production, – the relatively well distributed availability of sunlight, – the possibility of easy integration into urban architecture, – technological progress in manufacturing the cells, – the deferment or reduction of investments connected with the increase of production and distribution capacity, – the possibility of eventual participation in auxiliary services of the grid, essentially by means of the reactive power, – the negligible effect on the functioning of the distribution grid, – in a more general context, the rise in the price of fossil fuels, – the high social acceptance, – if, in the past, a PV system barely produced break-even energy compared to what it took to build, this is no longer the case today, where we have reached a factor of 5 on a lifespan of 20 years. These points explain in part the current trend of the grid connected PV, even if it is strongly encouraged by direct or indirect subsidies. In this way, global photovoltaic energy production has seen great progress because of installations connected to the grid. In fact, these represented 77% of systems installed in the entire world at the end of 2003 and, during 2003 and 2004, they represented respectively 90% and 93% of systems utilized (see Table 2.1).
Photovoltaic Systems Connected to the Grid
,
,
, ,
,
, ,
, , ,
, ,
,
27
, , , , ,
, , , , ,
, ,
, , ,
, , ,
, , ,
, , , , ,
, , ,
,
, ,
,
,
,
, ,
, ,
, , , ,
, , ,
, ,
,
,
, , , , ,
,
,
, , , ,
,
,
,
,
,
,
,
,
,
, , , , ,
,
,
,
,
,
,
Table 2.1. State of the locations of PV generation in 2004 [IEA]
,
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Renewable Energy Technologies
In view of the cost of an installed kW, the fundamental principle for all production systems with a photovoltaic base connected to the grid, and for a given group of panels, is to produce a maximum amount of kWh. In general, this means an energy management approach based on: – maximum power extraction (MPPT: Maximum Power Point Tracking), – transfer of this energy with a maximum efficiency to the grid and/or the loads, – continuity of service, faced with degradation of either the cells or the grid, – other services may also be assured without significant addition of equipment such as the generation/absorption of reactive energy exchanged with the grid. To assure this management of the energy flow, power electronic converters are used as interfaces. It is also appropriate to mention the control of these devices. This chapter is devoted to these aspects and focuses on the physical architectures and algorithmic architectures, that is, to the control aspects. 2.2. General remarks on connection There are a large number of topologies for connecting to the grid. They depend on the type of system (single phase or three phases), the contract for supplying energy (total or partial repurchase), the installation (old or new), etc. [EDF 03]. Figure 2.1 gives the two main schemes. The first type of interface injects only the production excess (Figure 2.1a), while the second (Figure 2.1b) injects the entire photovoltaic production. This second diagram is more advantageous for the producer because of the repurchase costs by the EDF; for example, in 2005: 0.1525 €/kWh in a metropolitan area and €0.305 in the Caribbean and French Reunion islands, Saint Pierre and Miquelon and, finally, Corsica. In any case, if the subsidy policy for the kWh produced were to disappear, the first option would be more advantageous. 2.2.1. Interfacing with the grid If we make an abstraction of the classic elements of switching, of protection (overloads, grounding, etc.), of metering and of filtering, the electric system is composed of two elements: the PV panels and the related power electronics (PE). The PE takes care of the interfacing of power, that is the safe connection of the two systems, the PV panel and the grid. Figure 2.2 describes the main functions of this power interface.
Photovoltaic Systems Connected to the Grid
29
Figure 2.1. Connection to the grid: a) with resale of the excess; b) with resale of the total
This interface will have the following functions: – for the PV cells: to extract the maximum power (the most common case) or to control this power to the limit of what is available (more rare) and finally to meet the static and dynamic constraints (currents, voltages and admissible variations), – for the grid: to inject the available active power, possibly to control the reactive power exchanged with the grid, to assure the power quality of the injected energy (harmonics rate of the injected currents), possibly to aid in the regulation of voltage or frequency (islanded system), to assure the functioning of anti-islanding protection and, finally to control the connection/disconnection operations.
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Renewable Energy Technologies
Figure 2.2. Specifications for interfacing of power electronics
2.2.2. General remarks on control The control system connected to the PV generator has the role of directing the various physical elements constituting this generator, mainly the power electronic converters. Figure 2.3 shows the various pieces. The various stages are spelled out as follows: – a MPPT module that determines the references of output currents (Ip*) or voltages (Vp*) of the PV panel, these references corresponding to the point of maximum power, – a module for generation of the references for the currents to be injected into the grid (Id* and Iq*, respectively active and reactive) with respect to the voltage of the grid, the voltage of the possible DC bus (Vbus) or the output power of the panels, and finally of the other variables), – a platform for the set up and the generation of rules of control of the power electronic switches, – a module for “other functions” that may impose one mode of functioning with respect to another, in order to manage the disconnection protection, the start-up and shutdown, for example. 2.3. Physical architectures Since the generating element produces a characteristic low voltage DC, the connection to the grid will require a particular conversion, the most popular consisting of two stages, a DC/DC converter and an inverter for the connection to the grid. These two structures are coupled using a DC bus.
Photovoltaic Systems Connected to the Grid
31
Figure 2.3. Diagram of the control
Depending on the output, the desired cost and reliability, and the possibility of managing the increase or decrease of installed power, several schemes for connection and combination of the modules may be considered. A photovoltaic system connected to the grid is generally composed of: – a combination of modules, which supplies a power that depends on sunshine, – a security system, assuring the protection of people, equipment and connection. The connection is made onto the grid at the user site; that is, in the case of residential installations, usually a low voltage grid 230/400 V, – an inverter that converts the DC current supplied by the solar panels into AC current. Various configurations are possible: - a single DC/AC converter, - two converters (DC/DC + DC/AC or DC/AC + low frequency transformer), - several converters (DC/AC + high frequency transformer + AC/DC + DC/AC, for example). The choice of configuration will depend on the structure of the PV system being operated. The configurations of the PV systems connected to the grid may be of four types (Figure 2.4).
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Figure 2.4. The various configurations
2.3.1. Central inverter Several lines of modules are connected directly to a centralized inverter. This structure requires the placing of a diode in series that prevents all reverse current in an elementary grouping of modules connected in series, forming one branch of parallel connection. 2.3.2. Individual inverter This module is connected to a small inverter (certain inverters allow connection of up to five modules in series). The inverters are then all connected in parallel onto the grid. 2.3.3. Row inverters This line of modules is connected to an inverter (since certain inverters allow direct connection in two or three parallel lines). The inverters are connected in parallel to the grid. 2.3.4. Multiple row inverters A DC/DC converter is joined to a series combination of modules. The DC/DC converters, in parallel, are connected to a central DC/AC converter.
Photovoltaic Systems Connected to the Grid
33
2.3.5. Conclusion The PV architectures vary depending on the installation power, and also according to the country. In fact, the design for row inverters has tended to become the standard in Germany and France in medium power domestic systems. In the Netherlands, the individual inverter is undergoing intense research. The future tendency is for multirow inverter design. This design seems to allow a reduction of price and increase in output of the systems, especially for high power systems [CRA 04, MEI 01]. The advantages and the disadvantages of the different architectures are summarized in Table 2.2 below [MER 05]. Architecture
Advantages
Disadvantages
Central inverter
– better DC/AC conversion output (linked to voltage conversion stage) – simplified maintenance (a single inverter) – the lowest inverter cost
– availability (no redundancy) – absent modularity, no possibility of expansion – single module (power losses by series and parallel coupling, complex MPPT) – DC cabling (HV protection equipment, many connectors and junction boxes)
– AC cabling (low current, standard) – maximum modularity, easy expansion possible – independence of modules (fewer power losses from coupling) – high continuity of service (strong redundancy and parallelism) – safe maintenance
– the lowest DC/AC conversion output – coexistence of the inverters and their safety functions – complex maintenance (locating repair work and increased risk of failure) – increased cost of the inverter
Individual inverter
– modularity by blocks, good expansion potential – continuity of service Row inverter – good conversion output – good inverter cost – the most used solution today Multirow inverter
– limitation in inverter power – power losses from series coupling
– modulation by blocks, expansion – availability (no redundancy in the possible DC/AC converter) – high DC/AC conversion output – power losses by series coupling – good inverter cost
Table 2.2. Comparison of different PV architecture systems connected to the grid
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Renewable Energy Technologies
2.4. Constraints related to supplying energy to the utility grid Supplying electric energy to the grid is allowed only with formal authorization. This authorization is granted only if the supplier can prove that its installation meets conditions that deal, on the one hand, with the quality of energy supplied and, on the other hand, with security. The interested reader can find more detailed information concerning connection requirements in the S. Mercier’s Master Research thesis [MER 05], on which the following sections are based. 2.4.1. Quality of the energy supplied A PV installation receives authorization to send energy to the utility grid if the quality of energy furnished is guaranteed. The certification of the system also takes this aspect into account. The principal problem that an inverter supplying power to the grid can produce is the presence of currents of frequencies other than the frequency operating on the grid (usually 50 Hz in Europe). All inverters produce such components and therefore filtering is necessary. However, high frequencies are much easier to filter than low frequencies. To ensure that an inverter does not generate low frequencies, its internal switching frequency must definitely be higher than the frequency of the grid (PWM inverters). The standards in effect limit the total harmonic distortion (THD) of the signals supplied by the inverter. The standardization is based on the European directive 89/336/CCE that deals with electromagnetic compatibility (EMC). If the equipment conforms to the EMC norms, it will have the label CE (European Community). Another problem that an inverter can present is injection of DC components into the grid. In order to be certified, the inverter may not supply a direct current, even in the case of a breakdown of one of its semiconductors. An isolation transformer at the output of the inverter makes it possible to prevent injection of direct current into the grid. This injection current must be lower than 5 mA (standard). 2.4.2. Security There are manuals giving the specific requirements for the connection of PV modules into the LV grid. The use of certified equipment is required. The equipment used must conform to the standards of electric safety. The standardization is based on the European Directive 73/23/EEG that deals with electric safety. A PV
Photovoltaic Systems Connected to the Grid
35
installation connected to the grid may present dangers both at the level of the grid and of the installation itself. 2.4.2.1. Security regarding the grid As far as the grid is concerned, the main safety problem occasioned by PV systems is the risk of functioning as an island. An island is a part of the distribution grid that is isolated from the rest of the grid. This situation is encountered when the distributor wants to disconnect this part from the grid, for example in order to carry out a repair or because a fault has been detected in this part of the grid. If the island continues to be fed by small energy suppliers, this can be as dangerous for people as it is for the equipment. Therefore, each inverter should recognize when it is operating as an island and disconnect from the grid as quickly as possible when this situation arises. Furthermore, the inverters should not interact among themselves so that another inverter is not recognized as the grid. The detection of islanding is possible only if there is an imbalance between the power consumption, active or reactive, of the grid users and the power production. To reduce the risk of nondetection, it is necessary to monitor both the voltage level of the grid and its frequency. In order to be certified, the equipment should meet a series of standardized tests. Only the German standard DIN VDE 0126 is recognized at present, but other standards are now being proposed. The methods for detection of islanding are, for the simplest ones, called “passive,” based on monitoring of the voltage and frequency of the grid. The inverter is connected to the grid by a PLL (phase locked loop) so as to create a reference of output current. The verification of frequency and peak value of the grid voltage is made at this permanent location, and the operation of the inverter can be blocked if the grid voltage disappears or is modified. There are other methods for detecting the isolated functioning of the inverters and they are integrated in commercial equipment. The most complicated ones use redundant methods and the principles of detection known as “active” are based on the injection of impulses into the grid and the measurement of the response (impedance measurement, for example). When many inverters are used (individual inverter architecture), problems of proximity interaction between the inverters can occur and the detection of isolated operations can suffer. 2.4.2.2. Security with respect to installation 2.4.2.2.1. Safety of the equipment Safety of the equipment is assured first of all by adequate sizing of the cabling. The dimensions of the cables depend on the distance between the cables, the
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Renewable Energy Technologies
location, the ambient temperature, and the nature of the cable insulation. It also depends on the type of current being carried, AC or DC. Proper sizing of the cables makes it possible to avoid the risk of fire. The use of cables giving a maximum voltage drop of 3% for both alternating and direct current circuits is recommended. Furthermore, cables are generally sized to carry a fault current (Isc_fault) equal to twice the short circuit current of the source (Isc), not including a safety factor. Moreover, each circuit joined to a system of modules, whether distribution or terminal, should have its own safety equipment, assuring electric protection (against overloads, insulation faults, and high voltages), sectioning (making it possible to separate and isolate a circuit or device from the rest of the installation so as to guarantee the safety of people), and control (serving to establish or interrupt the flow of current in the terminal circuit in normal conditions and in overload situations). 2.4.2.2.2. Safety of personnel The metallic conductors of installations, accessible to people can produce dangerous voltages in the case of a fault (indirect contact). They should be grounded, but this measure is not sufficient. The other measures to be taken depend on the grounding system. In some cases this system is imposed by requirements but in others is left to the user’s choice. There are three distinct neutral systems, designated by two letters: GG (grounded on system and customer side), GN (grounded on system side, neutral on customer side), IG (insulated or impedance grounded). The first defines the position of the neutral of the secondary of the distribution transformer, and the second indicates the position of the metallic conductors of the enclosures. The safety equipment is different for the three systems. All buildings fed by public LV distribution grids (domestic, small public service sector, small workshop) are required by the inter-ministerial decree of 2/13//70 to use the GG system. However, the neutral system can be different upstream and downstream from the inverter, whether it is in the presence of galvanic insulation or not. If galvanic insulation is not present between upstream and downstream of the inverter, the neutral systems must be identical. If insulation is present, the grounding systems may be different. The galvanic isolation may be obtained by a transformer with separate windings. General outline A diagram of the general principle of PV generation with direct connection, without batteries and with protection, could be as follows.
Photovoltaic Systems Connected to the Grid
37
Figure 2.5. Diagram of protection for a generator; certain protections may be internal in the inverter [BOU 04]
Lightning protection Lightning protection calls for the installation of the following layouts: – interconnection of the components by a 25 mm2 copper conductor, – single grounding of the components (principle of EMC), – interconnection with devices for current flow at the time of direct hits (if present: lightning rod, hanging wires, etc.), – floating PV cabling (not grounded: attention to the neutral system), – PV/inverter connection with reinforced connection, – limitation of the area of the cabling loops (EMC), – bipolar lightning arrestors on DC circuits (zinc oxide varistor with integrated thermal disconnection, between the terminal and the ground): very close to the modules (if the distance between the PV field and inverter > 10 m) and at the input of the inverter, – lightning arrestors on AC circuits (modular types for GG systems with high power flow into the distribution grid) between phases and ground: in AC output from the inverter and to the distribution panel for interior installation (optional). Protection for material and persons The protection of equipment and personnel employs: – a sectional power interrupter that is bipolar, at the output of the PV installation. It allows intervention on the installation in complete safety, and protection for the installation in the case of short circuit if it has fuses,
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Renewable Energy Technologies
– a permanent insulation controller usually integrated into the inverter and used because of the presence of the neutral system IG downstream from the inverter. It supplies a visual and/or audio alarm after the first insulation fault, – a differential circuit breaker (magneto-thermal), used because of the presence of the neutral system GG upstream from the inverter. The detection of short circuits and protection against overloads are assured. The circuit breaker also serves as a device for control and sectioning. The required triggering at the first fault is assured by a differential device with residual current (DDR). The protections used imply that the inverter has complete galvanic isolation and they are appropriate to the grounding systems employed. Additional protections are internal to the inverter. The presence of a transformer within the inverter influences not only the safety measures, the grounding to be adopted within the system and the control of the DC injection into the grid, but also the size, the weight, the simplicity of installation and the cost of the material. The transformer may be low frequency (LF) or high frequency (HF). Use of a high frequency transformer makes it possible to reduce the transformer size. However, the HF transformer, placed upstream from the inverter, does not prevent injection of DC into the grid. A device for limiting the injection of DC will be necessary. Inverters with a LF transformer have efficiencies in the order of 90%, while those with a HF transformer reach about 92% and, without isolation, 94% [ALO 04]. 2.5. Algorithmic architectures 2.5.1. The search for MPPT A PV cell produces output characteristics that are non-linear and vary greatly according to the output current, radiation and temperature. This results in a maximum power point that is derived using two parameters (voltage and current). MPPT tracking makes it possible to reach this point.
Photovoltaic Systems Connected to the Grid
(a)
(b) Figure 2.6. Curves of a half-module MSX-56 as a function of the radiation (simulation): a) I–V curves; b) P–V curves
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(a)
(b) Figure 2.7. Curves of a half-module MSX-56 as a function of the temperature (simulation): a) I–V curves; b) P–V curves
The different techniques for reaching the maximum power point are more or less robust, requiring different numbers of measurements, and they generally encounter the presence of several bends of local maxima in the case of degraded modes or of parametric derivatives of a group of interconnected panels.
Photovoltaic Systems Connected to the Grid
41
The general principle is like the search for an extremum of a characteristic containing bends of zero derivative. If the power measured is P, a change ǻP in the curve for a variation ǻI in the power–current plane (P,I) should assure at each instant the relation: Sign (ǻP), Sign (ǻI) < 0. It should be noted that we can also make the same argument in the voltage-power plane (P,V). 2.5.2. Control of the inverter grid and the global chain Figure 2.8 shows the control overview for the connection stage to the grid in a three phase case. Since the inverter is supposed to assure the transfer of available energy, this is usually done by imposition of UDC of the DC bus voltage to a reference value UDC*. The inverter itself should also control the power factor.
Figure 2.8. Diagram of inverter control for connecting to the grid
There are several possibilities for controlling the energy flow in a PV system. Here we will illustrate an example where the MPPT is controlled by a DC/DC stage producing an output on the DC bus. The inverter itself assures the regulation of the DC bus voltage, transferring the power entering the DC bus to the grid, with local losses [VAL 01]. Figure 2.9, below, shows the behavior of the system under different scenarios: at t = 0.5 s variable sunshine, the MPPT module serves its function, the voltage of the
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Renewable Energy Technologies
DC bus increases and by an injection of supplementary active power to the grid the inverter brings this voltage to its nominal value. At t = 1.5 s and t = 2 s, perturbations appear on this voltage. They are immediately compensated by an adjustment of the active power injected. At t = 2.5 s the inverter ceases to work solely on the active power and injects reactive power as well.
Figure 2.9. Reaction of the PV chain to different inputs
2.6. Conclusion Because of the high cost of the installed kW, the main purpose of photovoltaic generators connected to the grid is to produce the maximum amount of energy. This choice requires an energy management system using interfaces with power electronics joined to algorithmic architectures in order to: – extract the maximum power from the photovoltaic field, – transfer this energy with maximum efficiency into the grid and/or the loads, – assure continuity of service in the face of degradations on the part of the cells or the grid. The four classic configurations of PV systems connected to the grid are a central inverter, row inverters, individual inverters and multirow inverters.
Photovoltaic Systems Connected to the Grid
43
In addition to the constraints of connection to the grid, quality of energy supplied and security for equipment and people, photovoltaic systems include specific safety features with respect to the risk of islanding and the risks associated with lightning. The specific algorithmic architecture is the technique of maximum power point tracking of the photovoltaic field that should be especially robust with respect to the tracking of local maxima of the photovoltaic field. 2.7. Bibliography [ADE 05] ADEME, Bilan carbone®, Calcul des facteurs d’émission, ADEME, Angers, 2005 (available at http://www2.ademe.fr/). [ALO 04] ALONSO ABELLA M., CHENLO F., “Choosing the right inverter”, Renewable Energy World, James and James, vol. 7, n° 2, p. 132–146, March/April 2004. [BOU 04] BOULANGER P., Guide de rédaction du cahier des charges techniques des générateurs photovoltaïques connectés au réseau, ADEME, 2004 (available in pdf format at http://www.mctparis.com/fr/images_db/Guide_Ademe_PVC.pdf). [CRA 04] Cramer G. et al., “PV system technologies: State of the Art and Trends in Decentralised Electrification”, Refocus, vol. 5, 1, p. 38–42, 2004. [EDF 03] EDF/GDF Services, Accès au réseau basse tension pour les installations photovoltaïques, Paris, 2003 (available at http://www.edfdistribution.fr). [IEA] http://www.iea-pvps.org. [MEI 01] MEINHARDT M., CRAMER G., BURGER B., ZACHARIAS P., “Multi-StringConverter with reduced specific costs and enhanced functionality”, Solar Energy, vol. 69 sup. 6, p. 217–227, July/December 2001. [MER 05] MERCIER S., Architecture de systèmes PV de forte puissance, Mémoire de Master de Recherche, Laboratoire d’Electrotechnique de Grenoble – CEA, September 2005. [VAL 01] VALERO J., Sur les convertisseurs de l’EP dédiés à l’interfaçage des systèmes de production décentralisée – Etude du cas de la génération photovoltaïque, mémoire de DEA de l’Institut National Polytechnique de Grenoble, September 2001.
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Chapter 3
Solar Heating
3.1. Introduction 3.1.1. Some history Historically, the use of solar energy to heat a fluid was surely one of the first means of taking advantage of solar radiation. The first notable achievements date from the 18th century, using complex concentrating equipment requiring the tracking of the sun’s trajectory. One of the first solar ovens, designed by Antoine Lavoisier, reached the remarkable temperature of 1,750°C and enabled the smelting of liquid metals. The oven included a lens of 1.32 m in diameter. During the 19th century there were many attempts to produce a solar boiler to generate steam for steam engines used in the production of mechanical power. The first prototypes were built in France and were operational at Tours between 1864 and 1878. The French engineers Mouchot and Pifre made significant progress thanks to their design of large truncated conical and parabolic collectors. In 1910, in California, AG Eneas installed a collector 10 m in diameter for the production of mechanical energy needed for a pump. For similar applications in 1912, Shuman and CV Boys, in Meadé, Egypt, built a set of collectors of cylindrical-parabolic shape, 62 m long with a total surface area of 1,200 m2. The power of this solar engine reached 37 to 45 kW.
Chapter written by Christophe MARVILLET.
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After the First World War and during the subsequent 50 years, much work was done to improve the design of the collectors and concentrators resulting in two separate families of technology. The first is a centralized system using a field of heliostats (mirrors to follow the trajectory of the sun), focusing the radiation on a tower. The second is a decentralized system using parabolic collectors, a system of Fresnel lenses or cylindrical-parabolic collectors. Another area of interest has been the production of domestic hot water, for which initial installations were completed in the 1930s. The beginning of industrial solar water heaters came in the early 1960s and spread rapidly throughout the world. The most common installation consists of flat collectors used as a thermo-siphon with an absorber of 3 to 4 m2, a chamber for hot water storage from 150 to 180 liters, and a chamber for cold water storage, all of which are integrated onto a single platform in order to obtain a compact design to install on a roof or on land close to the building. Today more than 30 million square meters of solar collectors are installed across the world for this application. Another type of solar water heater is a forced convection system in which the collectors are on the roof and the storage chamber in a utility area of the building. More recently, the use of hot water produced by solar collectors for space heating has grown in Europe over the last 10 years. This promising application is based on the combination of two heat sources (solar and an auxiliary boiler or a heat pump) and the use of a low temperature thermal distribution system (floor heating for example) operating in the building. More specialized in terms of use are solar ovens made of concentrating collectors in a very wide range of shapes, enabling the heating of containers for cooking food. These technologies, developed over the last 30 years, have been widely distributed in China (300,000 installations), India and more recently in African countries. These devices should allow a significant reduction in the consumption of wood as fuel for these uses. We recall that for many sub-Saharan countries, wood is still the main source of energy (up to 60% of the primary energy consumption) and this has led to massive deforestation in the regions near urban areas. The heating of air by solar radiation remains a very small application, but it is worth recalling several applications using heated air. In the field of agriculture, solar dryers can make use of this technique. For housing, the preheating of air can lead to a reduction in consumption of traditional heating sources (such as furnaces, fireplaces).
Solar Heating
47
Figure 3.1. Solar collectors of parabolic and cylindrical-parabolic shape
Figure 3.2. Examples of solar crucibles
3.1.2. Some basic calculations The sun can be considered to be a sphere of very high temperature gas with a diameter of 1.39 x 109 m. At a distance from the Earth of 1.5 x 1011 m, it takes solar energy 8 minutes and 20 seconds to reach the Earth’s surface. The sun, from the point of view of thermal radiation, can be viewed as a black body at a temperature of 5,762 K. The temperature in the central region of the sun is much higher and has been estimated to be between 8 x 106 and 40 x 106 K. The sun behaves like a fusion reactor, transforming hydrogen into helium. The total energy emitted by the sun represents 3.8 x 1020 MW, corresponding to an average surface power density of 63 MW/m2. Only a tiny portion of this solar
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radiation is intercepted by the Earth. This is estimated at 1.7 x 1014 kW. This power received at the Earth’s surface corresponds to an average power per unit of surface area (not including the effect of the atmosphere) of 1.367 kW/m2. This value is known as the solar constant. This constant is not in itself enough to describe the solar energy resource available on the ground. Indeed many effects and parameters tend to drastically reduce the power received by a collector positioned at ground level. Among the main factors are as follows: – The location of the field of collectors (particularly the latitude) determines the apparent motion of the sun and therefore its orientation. This also determines the length of the solar day. The solar constant implies that the solar radiation is oriented perpendicularly to the collectors. Obviously, another orientation to the sun would result in an instantaneous solar energy flux which could be significantly lower. – The atmosphere – which is characterized by a thick layer of air, which absorbs some visible radiation, aerosols and dust which absorb and refract radiation, clouds, which also absorb radiation – is a region of complicated energy interactions. Thus, while the incident energy flow outside the atmosphere is characterized by a directional radiation, the solar flux collected at the ground can be broken down into two terms: a direct radiation, and a diffuse radiation whose sum, the global radiation, varies with the season and with the geography of the location. To represent the solar energy potential at one point in the world, over a year, we define the average daily solar radiation (kWh/m2/day) or the solar radiation (kWh/m2). This second term represents the energy received annually on a horizontal plane. The first term is equal to the second term divided by the number of days in the year. The estimated value of the average daily solar radiation on the globe is shown in Figure 3.3. 3.1.3. The performance of solar heating devices Solar heating is one of the most efficient methods of recovering solar energy. It is estimated, in fact, that an average power of 30 to 60 W/m2 can be obtained using a solar collector. For comparison, we can obtain an estimated average power of 3 to 15 W/m2 with a photovoltaic sensor and 0.12 W/m2 from biomass (another form of energy recovery from the sun).
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Figure 3.3. Average daily solar radiation on a horizontal plane in kWh/m2/day
The design and sizing of solar heating equipment requires three steps: – an assessment of the energy potential available at the installation of the collectors, – a characterization of collector yields depending on their design, orientation and the specific characteristics of the materials of which they are composed, – an assessment of the performance of the complete system, which typically includes extra collectors, storage manifolds, piping connections, circulators and auxiliaries. These main steps are detailed in the following section. 3.2. Available energy from the sun 3.2.1. The apparent motion of the sun All solar installations require, for their design and sizing, knowledge of the apparent motion of the sun with respect to a fixed point on the Earth, where the collectors are located. The Earth rotates on its axis in the same direction of rotation as it rotates around the sun. An important parameter is the declination of the sun (į), which is the angle formed by a line from the Earth to the sun with the plane of the equator. The
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declination changes over the course of the year and is zero at the equinoxes (March 21 and October 21). The maximum values are at the solstices: 23° 27’ for the summer solstice and í23° 27’ for the winter solstice. This declination is calculated by the following formula in which the parameter J is the day of the year counted from March 21: sin G = 0.4sin 2 J S / 365
[3.1]
For any given day, the declination of the sun is known. If the Earth is assumed to be fixed, the sun describes a circle of constant angular velocity around the axis OZ of the Earth (Figure 3.4). The sun travels from east to west, and we call true noon the time at which the trajectory of the sun crosses the meridian of the desired location. This point corresponds to the origin of the angle hour (AH). True times of rising and setting can easily be found from the expression cos AH
where
G
tanM /tanG
[3.2]
is the latitude of the location in question.
Figure 3.4. Definition of important parameters describing the apparent motion of the sun
The position of the sun at any given moment is characterized, as represented in Figure 3.5, by the height, HS (the angle between the horizon and the sun–Earth axis), and the azimuth, AS (angle with respect to due south). These two quantities are found from the time angle, AH, the solar declination, and the latitude of the given location from the following expressions: sin HS
sinM .sinG + cosM .cosG .cos AH
[3.3]
sin AS
cosG .sin AH / cos HS
[3.4]
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Figure 3.5. Definition of the azimuth and the height of the sun
The apparent motion can be calculated at any point in the world and can be represented, as in Figure 3.6, by rectangular coordinates – height/azimuth – at different times of the year for different parts of the world (for example, Europe at a latitude of 45°, a tropical zone at a latitude of 20°). These figures illustrate the basic data needed to calculate the available solar energy. These include the length of a solar day, and the height of the sun, for which we can easily note the very marked change during the year at 45° latitude, and a much smaller variation in the tropics.
Figure 3.6. Apparent movement of the sun in two specific locations (latitude 45q north; tropical 23.4q north)
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This mode of representation also has the advantage of allowing us to assess the effects of shade; that is, the impact of homes or trees or any nearby barriers that reduce the collector radiation. For this evaluation, it is sufficient to represent the different obstacles in the height–azimuth plane as shown in Figure 3.7.
Figure 3.7. Evaluation of periods of shade for a collector with different obstacles located nearby
3.2.2. Evaluation of sunlight received by a collector
,
,
Table 3.1. Variations of the direct sunlight received by collectors for different degrees of cloudiness
The sunlight received by a collector, struck perpendicularly by the sun’s rays, excluding the effect of the atmosphere, is equal to 1,367 W/m2. Before reaching the ground, solar radiation in the visible range is absorbed by the atmosphere (about 17%), is reflected by the clouds (23% on average) and is diffracted by aerosols out of the atmosphere (6% on average) and towards the ground (16% on average). The average direct sunlight on the globe is no more than 38% of the incident flow. Of course, these average values around the world do not reflect the very large disparities associated with atmospheric variations in location and time. We see in Table 3.1 the wide variation in the direct and diffuse radiation received by a collector for various cloud conditions.
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The value of direct solar flux impinging on a horizontal plane can be deduced from the solar constant, adjusted by an index of clarity dependent on the average climatic conditions at the location in question. These indices range from 0.8 for very clear weather to 0.3 for overcast conditions. In other respects, incident solar flux is not usually perpendicular to the horizontal plane or to the plane of the collector. The instantaneous solar flux received must be projected on a horizontal plane as a function of the angle HS, designating the height of the sun. The solar flux received by a horizontal plane is therefore a sinusoidal function of time, corresponding to the time variation of the height of the sun during the day, assuming that the index of clarity remains constant. For any plane, the solar flux received is calculated as a function of the angle between the normal to the collector and the line in the Sun–Earth direction. The orientation from the horizontal and orientation relative to due south result in flux curves that are much more asymmetric, as we can see for collectors of different orientations in Figure 3.8. ,
,
Figure 3.8. Solar flux (W/m2) received at Geneva on March 15 with a clear sky as a function of solar time for different orientations. HOR (horizontal collector), S (vertical facing south), E (vertical facing east), W (vertical facing west), S45 (inclined 45° from south)
3.3. Flat solar panels
Most applications involving domestic solar heating of water or air use flat solar or vacuum collectors. The following discussions will focus on these technologies and we refer the reader to section 3.4 for discussion of parabolic or other types of concentrators.
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3.3.1. Different technologies of thermal solar collectors
Flat collectors using liquid with no glazing (Figure 3.9a) are usually made of a black absorbent plastic non-selective polymer coating without a thermally insulated backing. These low-cost collectors lose significant heat to the environment, which limits their use to low temperature applications such as swimming pools, water temperature boosters in fish farming applications, industrial heating, and so on. In cold climates, they are exclusively used in the summer because of the high heat losses. Flat panels with liquid and with glazing (Figure 3.9b) include an absorbing black plate (frequently covered with a selective coating) fixed within a framework between single or double glazing and a back insulating slab. Copper tubes soldered to the back of the collector allow the circulating liquid to be heated. These devices, which limit thermal losses, allow applications at moderate temperatures for the heating of domestic water, the heating of buildings and for industrial processes.
Figure 3.9. Different liquid solar collector technologies: a) collector without glass; b) liquid collector with glass; c) collector with tubes under vacuum
Vacuum collectors (Figure 3.9c) are composed of an absorber covered by a selective coating and sealed in a vacuum in a glass tube. The thermal losses are extremely small and these collectors are well adapted to applications requiring medium and high temperatures, such as the production of domestic hot water and the heating of buildings, particularly in cold climates. The collectors on the market use a
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heat pipe to extract the heat collected at the absorber, where the head of the heat pipe is cooled by a circulating liquid. Finally we consider collectors using air, produced for space heating as shown in Figure 3.10. The collector consists of an absorbent wall located near the outer wall of the building so that airflow, circulating by natural convection, can flow in the open space between the wall and the absorber. The solar flux absorbed is thus transferred by convection into the air before it is distributed into the heating ducts.
Figure 3.10. An example of implementation of an air solar collector for preheating in a building
3.3.2. Evaluation of the performance of solar collectors
The energy balance in a non-glazed collector (Figure 3.11a) is expressed by the equality between the fraction of a solar flux, GS, absorbed by the collector where: – G is the incident solar flux W/m2 (dependent, as we noted previously, on the solar constant corrected for the effect of orientation of the collector with respect to the sun–Earth axis and the index of clarity at the time and place being considered), – the coefficient D corresponds to the absorptivity of the coating of the collector,
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– S represents the useful surface area of the collector; and the two following terms: – a term Q, representing the “useful” thermal power, which represents the heating of water circulating in the tubes on the back of the absorber, – a term corresponding to the heat loss on the back due to conduction through insulation and heat loss through the front of the absorber by radiation and by convection with the surrounding air. This term is characterized by an exchange coefficient, h, for both sides and by the difference ǻT between the average water temperature and the temperature of the surrounding environment. So, for a non-glazed collector, from energy conservation we can write:
D GS
Q h'TS
[3.6]
In the case of a flat glazed collector, the glass absorbs a fraction of the incident solar flux and the energy balance is written taking into account the characteristics of the glass which has a transmittivity W .
D GSW
Q h'TS
[3.7]
The efficiency of the glazed solar collector is defined as the ratio of the useful output power, Q, and the solar power received by the collector, GS. The efficiency is then equal to:
K DW h'T / G
[3.8]
This function is illustrated by the curve in Figure 3.11b, with the abscissa representing the efficiency and the ordinate being the ratio ǻT/G. We see from the caption of this figure that the efficiency of the collector decreases as its temperature increases from room temperature. Three important points should be noted: – Under conditions where ambient temperatures and the collector temperature are the same (ǻT = 0), the efficiency is equal to the product DW , two strictly optical parameters, representing the collector and the glazing. – The slope of the curve (and hence the degradation of the gain) is determined by the heat transfer coefficient between the absorber and the atmosphere (radiation, conduction and convection). – The intersection of the efficiency curve and the abscissa corresponds to a zero output condition of the collector and a maximum temperature of the absorber called the stagnation temperature.
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Figure 3.11. Energy balance of a collector and efficiency curves a) flat panel; b) efficiency curves of solar collectors; c) comparison of efficiencies for different technologies
Comparison of the different collector technologies (Figure 3.11c) clearly highlights the fact that collectors without glass have reduced optical losses but higher thermal losses, which leads to a rapid decline in gain. The use of this type of collector is limited to low temperature applications. The vacuum collectors and those with glazing, although having higher optical losses, maintain high performance due to reduced thermal losses.
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3.3.3. Selective coatings for collectors and glazing
Solar radiation can be likened to that emitted by a black body with a temperature near 6,000 K so that solar collectors, and especially absorbers, receive radiation mostly in the visible range. On the other hand, the radiation from the absorber, according to Planck’s law (Figure 3.12a) í for temperatures between 300 and 400 K í is located in the infrared range, below 3 μm. Depositing a selective coating (oxides of chromium or nickel, called black nickel) produces an absorptivity close to 1 in the visible spectrum (Figure 3.12b) resulting in maximum collection of solar radiation and an absorptivity D close to 0 (i.e. a reflectivity U close to 1). Under these conditions in the IR range, the flux emitted by the absorber is quite low. The selective coatings reduce optical and thermal losses of the collector and improve its efficiency. Thus we define the performance of selective coatings by the ratio of their absorptivity in the visible range to their emissivity in the IR range. Some values of this index are given in the table in Figure 3.12b. Some glasses (particularly those that are rich in iron oxide) have the peculiarity of having a transmittivity close to 1 in the visible range. This results in low optical loss of solar radiation during transmission through the glass. In the IR, however, the transmittivity decreases rapidly, resulting in low thermal losses through what is commonly known as the greenhouse effect. 3.4. Solar heating systems
There are basically two types of solar heating systems: individual solar water heaters (CESI) upon which a solar collector can be installed and combined solar systems (CSS) for space heating. 3.4.1. Individual and collective solar water heaters
The water heater, operating as a thermo-siphon, consists of a solar collector and a storage tank located above it. The water in the collector, heated by solar radiation, rises in the circulation loop and gradually fills the tank with hot water. The denser cold water from a cold water tank flows into the loop. This system, without pump or circulator, is especially reliable, but it has limitations because it is cumbersome.
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Figure 3.12. Table of absorber and selective glazing: a) monochromatic emission of a black body as a function of wavelength: case of a black body at 6,000 K representing the sun, case of a black surface at 300í400 K representing an absorber; b) absorbtivity ( D ) and emissivity ( H ) of a selective surface as a function of the wavelength ( O c 3P m ); c) transmittivity of selective glass as a function of wavelength
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Solar water heaters with forced circulation consist of a collector and reservoirs that are separated for better architectural integration (a collector and reservoirs that are separate from each other so as fit better into the architecture). Only the collector is located outside and the storage tank is placed under the ridge of the roof. Several systems are used: – direct systems with an exhaust heat exchanger, – direct systems that can be used in areas without the risk of freezing. They include collectors, a storage tank, a hydraulic system (circulating pump, one-way and other valves) and control equipment. These devices will be drained during winter if there is a risk of freezing, – heat-exchanger systems (Figure 3.13), the system most commonly used in Europe, protect the system from freezing through the use of an appropriate coolant fluid, generally of the water–glycol type. They are distinguished from the previous types by the presence of a heat exchanger core in the storage reservoir. This exchanger is fed by the reactor coolant, which is heated in the collector. An expansion chamber in the cooling loop prevents excess pressure associated with changes in fluid volume, – drainable systems are similar in design to the direct systems. However, to avoid any risk of freezing, drainage equipment is integrated into the systems so that the parts located outside can be drained.
Figure 3.13. Individual solar hot water system with heat exchanger
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3.4.2. Combined solar systems for the heating of buildings
Interest in space heating by solar collectors has grown strongly in Europe over the past decade. These devices simultaneously provide the production of domestic hot water and space heating. We represent, in Figure 3.14, the changing heating requirements for the building and the available solar resources over a year. There are clearly three distinct periods: – during the winter, the solar energy captured is inadequate to ensure that requirements are met and supplementary energy is needed, – a period between winter and summer, where solar energy can provide almost all needs, – a summer period, when the solar energy captured exceeds the requirements. Thus, in the graph, zone 2 represents the usable solar energy during the year, zone 1 the auxiliary energy, which can come from a boiler or a heat pump, and zone 3 the non-recoverable surplus solar energy. Some basic rules must be followed in the design of these facilities: – the heat distribution equipment in the building must operate at low temperature: radiators with large surface areas or heated floors are preferred, – devices protecting the loop from overheating the coolant are necessary; stagnation temperatures can reach values greater than 150°C.
Figure 3.14. Variation over a year of building heating requirements and available solar radiation
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3.5. Bibliography [CAB 00] CABIROL T., Chauffage de l’habitat et énergie solaire, Edisud, La Calade, 2000. [EIC 03] EICKER U., Solar Technologies for Buildings, John Wiley & Sons, Chichester, 2003. [MAR 05a] MARVILLET C., “La maîtrise de l’énergie dans l’habitat” Clefs, revue du CEA, N° 50/51, p. 149, Winter 2004–2005. [MAR 05b] MARVILLET C., “Les gisements d’énergie restant importants” Clefs, revue du CEA, N° 50/51, p. 141, Winter 2004--2005. [WEI 03] WEIZZ W., Solar Heating Systems for Houses, AIE, James and James, London, 2003.
Chapter 4
Solar Thermodynamic Power Stations
Introduction The term “thermodynamic power plants” refers to the whole group of techniques for transforming the energy radiated by the sun into high temperature heat, then converting this heat into mechanical and electrical energy by means of a thermodynamic cycle machine coupled with an electric generator. The first generation of solar power stations arose in the 1970s. Most of the prototypes produced power of less than 10 MW, were financed by public funds and were used experimentally by research organizations to evaluate the technical choices of the principal subsystems. In Europe especially, the struggle against climate change has recently launched research efforts aimed at producing electricity or other energy solutions while minimizing CO2 emissions. Furthermore, the rise in the price of gasoline, which continues today, caused uncertainties regarding the world supply of energy. The scenarios for primary energy production, particularly studied by the international energy agency, forecast greater reliance on renewable sources other than hydraulic by 2050. The use of concentrating solar technologies to enlarge the “energy mix”, is under serious consideration because of several specific advantageous characteristics which we are going to detail in this chapter. Concentrating solar technologies convert direct solar radiation and furnish primary high temperature heat. Solar heat can be stored temporarily, so as to be independent of passing clouds, or for periods of several hours, to provide utilization beyond daylight illumination of the panels. Hybridization, with a fossil or biomass Chapter written by Alain FERRIÈRE.
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heat source, allows greater availability of installations and guaranteed production of heat. This heat is converted to electricity using high efficiency thermodynamic cycles. These cycles, well understood by the electricity production industry, are not specific to solar technologies, and this is an additional advantage. Today, environmental impact is an important criterion for the choice among competing technologies. With an emission rate of less than 20 kgCO2/MWh, solar thermodynamic energy, from this point of view, is of the same order as hydro electricity (4 kgCO2/MWh) or nuclear (6 kgCO2/MWh), and incomparably better than photovoltaic electricity (100 kgCO2/MWh) or electricity from carbon combustion (900 kgCO2/MWh). The figures cited here refer to the emissions linked to the construction of stations and the loss from the extraction of combustibles. Compared to conventional technologies for producing heat (other than nuclear), each square meter of the collector installed under a radiation of 2,000 kWh/m2/year reduces the emission from 250 to 400 kg of CO2 per year. The payback period for energy return1 from concentrating solar installations is only 6 months2. Their lifespan is estimated to be from 25 to 30 years, and part of their composition at the end of their life is reusable (steel, glass). Finally, this type of new technology, which requires the deployment of large collection surfaces in order to be put into operation, represents an important potential for job creation. The European association ESTELA3 estimates, for example, that 65,000 jobs will be created directly from now until 2020 due to the installation of a capacity of 30,850 MW [ARI 03]. With the help of incentives adopted by several European and world nations, second generation solar power stations came into being in 2006. They are precommercial power stations of modest size, constructed mainly with private funds and utilized by industrial consortiums. These power stations employ proven technologies, well known in industry. In Europe, the projects are concentrated in Spain, where solar thermal electricity is sold at 0.216 €/kWh. Thus, a solar power station with a saturated steam Rankine cycle tower rated at 11 MW was inaugurated in the spring of 2006 (PS10, Abengoa Solar, Seville). A plant of 50 MW with cylindrical-parabolic collectors and seven hours storage was commissioned in December 2008 near Guadix (Andasol I, SolarMillennium/ACS Cobra). Several units of 50 MW each are in construction near Seville and Ciudad Real. In all, more than 500 MW are planned in Spain.
1 The duration of utilization of an installation necessary to produce the energy required for its construction. 2 Source: Concentrating Solar Power Now, the German Environment Ministry, 2002. 3 ESTELA: European Solar Thermal Electricity Association.
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In this chapter we will list the properties of the places where power stations using solar technologies have been put into operation. We will detail their essential components, discussing the criteria for their selection and their sizing. Finally, we will examine the paths currently being followed by researchers and industrialists to advance their performance, to lower production costs, and to promote these technologies. We have adopted a resolutely optimistic style, but without concealing technical drawbacks or masking scientific difficulties. Our first objective is to inform the reader. If the opinion he/she forms by reading this chapter is that solar thermal stations merit a place alongside other solutions for the production of energy in the future, we will have exceeded our hopes. 4.1. Concentrating solar power technologies 4.1.1. Why concentrate solar radiation? The solar flux intercepted by the Earth’s surface, considered as a black body, is about 1,350 W/m2 outside the atmosphere (the solar constant). Solar radiation is attenuated when passing through the atmosphere by absorption and diffusion. The solar flux occurring on the surface of the earth in desert regions is about 1,000 W/m2. When solar radiation lights the surface of a solid or liquid material, a fraction is reflected, a fraction is transmitted, and the rest is absorbed in the surface. This absorbed fraction is then converted into heat and the temperature of the body increases. The maximum temperature attained by an opaque body subjected to solar radiation depends essentially on three parameters: the intensity of the solar radiation, the factor of absorption of the illuminated surface and the thermal losses caused by the elevation of the body’s temperature. The thermal losses are established by convective exchange and by the thermal radiation of the receiving surface toward the surrounding environment. These losses are proportional to the surface that is illuminated. An equilibrium is established between the solar power absorbed and the thermal power lost. To serve as a simple illustration, let us consider the case of a black body4 which receives solar power Psol and which is placed in an environment at absolute zero. It reaches an equilibrium temperature Teq such that: Psol
4 HV STeq
[4.1]
Thus, a black body with a surface S equal to 1 m2 which absorbs a solar power of 1,000 W reaches a maximum temperature Teq = 91.4ºC. The non-concentrating solar thermal technologies are thus reserved for the production and utilization of low
4 Perfectly absorbing body (Į = 1) and perfectly emitting (İ = 1).
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temperature heat, typically between 30ºC and 80ºC. Residential heating or air conditioning are the most recent common applications of these technologies. In order to put a thermodynamic cycle into operation efficiently and to produce electricity economically, it is necessary to operate at the highest possible temperature. Concentrating solar radiation is an efficient solution to this need. It draws on more sophisticated, though generally more expensive, technologies, which require great care in their application on an industrial scale. Concentrating radiation has the effect of increasing incident solar power without increasing the receiving surface. Receiving more solar power, the same surface S attains a higher temperature with increased losses and thus reach thermal equilibrium. The graph in Figure 4.1 (solid line) shows the equilibrium temperature reached by a black body as the concentration5 increases. , , , , ,
Figure 4.1. Temperature as a function of concentration
A model that is not much more complicated but more realistic enables the temperature level of usable heat after conversion of solar radiation to be estimated. We introduce the output of a mirror or a receiver and express the thermal loss from radiation toward a surface with a temperature close to the ambient temperature. The curve in Figure 4.1 (dotted line) shows that we can convert 90% of the solar power into usable heat at a temperature in the neighborhood of 400ºC if the concentration is about 100 times that of the solar radiation. According to this model, a concentration factor of 500 or 1,000 enables a temperature higher than 600ºC, or 5 Concentration is expressed by a non-dimensional number. The product of this factor multiplied by the direct instantaneous radiation gives the density of the radiated flux delivered by the installation in W/m2. In the estimated calculations, direct radiation is taken by convention as 1,000 W/m2.
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800ºC, respectively, to be reached. This example illustrates the value of solar concentration whenever the production of high temperature heat is desired, and it also shows the order of magnitude of the necessary concentrating factor according to the level of temperature desired. The concentrating systems operating in solar thermodynamic power stations have concentrating factors averaging 50 to 1,000. 4.1.2. Concentrating systems Among the numerous optical devices permitting the deflection of the sun’s rays in order to concentrate them, we will consider here only those that lend themselves to industrial operation for average or high power and that lead to the production of heat at a temperature higher than 250ºC. For example, optical lenses of polished glass will be omitted as too heavy, too expensive and of limited size. We will also eliminate acrylic Fresnel lenses as having a low unit power and being too fragile for prolonged use in a harsh exterior environment (wear and tear by particles, clogging). The optical devices considered here operate as reflecting surfaces consisting of mirrors, which are industrial products of excellent quality for which a market already exists. The geometric nature of the active surfaces and the complexity of the structures supporting the mirrors define the concentrating systems. Three families of solar concentrating systems can be distinguished: stations with cylindrical-parabolic collectors (named troughs), stations with a tower and central receiver, and parabolic-motor systems (named dish-engine systems). The related concentrating devices are shown schematically in Figure 4.2. The devices are distinguished by their elementary dimensions (and therefore their power), their optical and thermal performances, and their cost (see Table 4.1).
Figure 4.2. Principles of the main solar concentrating systems: a) cylindrical-parabolic concentrator; b) tower concentrator; c) parabolic concentrator
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High temperature solar heat (25–1,800ºC) is produced with excellent thermal efficiency, higher than 70%. On the downside, the conversion into electricity draws on a conventional thermodynamic cycle. Therefore, the efficiency from solar electricity conversion is between 20% and 30%, depending on the operating technology. According to the estimates of the GEF6, the investment cost of an installation is valued between 2,800 €/kWh (20–80 We station with cylindricalparabolic collectors and Rankine cycle) and 4,000 €/kWh (40 to 200 MW tower station with a combined cycle), and it reaches 14,000 €/kWh for a decentralized unit of the dish-Stirling type of 10 to 25 kW. According to the same sources, the cost of electricity produced under favorable conditions – that is with a radiation higher than 2,000 kWh/m2 /year – is found in the range of 0.16 to 1.24 €/kWh for a large station and in the order of 0.30 €/kWh for a dish-Stirling. Technology 7
Nominal thermal efficiency Power of the installation Working temperature 8
Cost of the solar field
Total cost of the investment
Cylindrical-parabolic
Tower
Parabola
70%
73%
75%
80–300 MWth
10–100 MWth
1–100 kWth
270–450°C
450–1,000°C
600–1,200°C
29
210–250 €/m
2.8–3.5 €/We
2
140–220 €/m
~150 €/m2
3–4 €/We
10–14 €/We
Table 4.1. Current characteristics of concentrating devices
4.1.2.1. The parabolic concentrator (or dish) It is well known that all concave reflecting surfaces behave as concentrators when they are illuminated by solar radiation. A snowy valley, a slightly hollowed limestone cliff, a room with white walls are all concentrators, of rather paltry performance but one plainly perceived by our eyes. In order to concentrate the radiation in measurable degrees, it is necessary to adopt regular geometric forms for the concentrator. The first of these is the sphere, or part of a sphere. There is a large number of systems using spherical solar concentrators; some of them make it possible to reach high temperatures. Even though there is no reason to exclude it a priori from the field of study, no spherical concentrator has been used in the designs of a modular solar electric converter. Furthermore, the difference from the parabolic
6 Source: GEF (Global Environment Facility) Scientific and Technical Advisory Panel, 2001. 7 This efficiency is the fraction of the available incident solar energy leaving the receiver in the form of thermal energy during operation. 8 Source: Solar Thermal Power Plants, EUREC-Agency, 2000. 9 This price includes the cost of the tubular receptor.
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concentrator is minimal, and most of the discussion concerning the latter is applicable to the spherical configuration. The parabolic concentrator, that is, a parabola of revolution, is the ideal reflecting surface for best concentrating luminous rays. The requirement is to orient the axis of the parabola permanently in the direction of the sun. The solar rays reflected by the parabola thus converge toward a zone of maximum concentration, the focal point. The need to move the parabola along the two axes of rotation in order to assure tracking of the sun’s daily course limits the dimension of this type of installation. The largest completed prototype reaches 400 m2, which amounts to a real technological feat10. Most of the mobile parabolas are in the range of 50–100 m2. Besides the displacement system, which must be both precise and sturdy, a major technological difficulty is fabrication of a parabolic mirror. Single parabolic mirrors in polished and silvered glass are used for diameters of less than 2 m. For larger diameters, other less costly and, in particular, lighter solutions are adopted. A metallic film stretched over a drum in which a partial vacuum is maintained takes a concave form close to that of the parabola. This solution was tested but abandoned because it was too fragile and its had a short life span. Today the preferred solution is a shell of plastic or of a material composed of a glass and polymer resin, on which elementary mirrors of thin flexible glass less than a millimeter thick are glued. The parabolic shell, light and sturdy, is fabricated by molding, using well-established technology, for example as used in pleasure boat construction. The shell may consist of various identical segments assembled like a corolla (the petals of a flower). The average concentrating factor obtained at the focal point of a parabola exceeds about 1,000, which makes it possible to produce heat at a very high temperature, typically 700ºC or higher11. In practice, the limitation on the temperature comes from the materials operating in the solar absorber placed at the focal point. The conversion into electricity is achieved by a thermodynamic cycle of very high efficiency owing to the elevated temperature of the hot source. For this range of powers, it is the Stirling cycle that is favored by the designers of parabolicengine systems. The gas of the cycle’s work is also the heat transfer fluid that collects heat in the solar absorber. Currently helium or hydrogen are used. The latter performs better thermally, but operation is more delicate because of its tendency to escape and because of the level of risk associated with its use. The engine, as well as the electric generator, is placed at the focal point of the parabola. A water circuit cools the cycle and releases low temperature heat into the surrounding air by means 10 “Big Dish”, Australian National University, Canberra, Australia. 11 The solar concentration is not uniform on the focal patch: it is higher in the center. It is demonstrated that the maximum solar concentration is 46,000, which exceeds all needs. In practice, the optical imperfections of the mirrors and the limitations of the angular tolerance of the receivers mean that the maximum concentration barely exceeds 20,000.
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of a radiator-convector. The parabolic-engine module thus constituted is a solar electricity converter that operates with the course of the sun. The minimum threshold of usable radiation is about 300 W/m2 for the 10 kW Eurodish module produced by the German manufacturer SBP-SOLO12. This is held to be an attractive and very efficient solution. The instantaneous efficiency of solar electricity conversion is greater than 22% (29% for the 25 kW module produced by the American manufacturer SES13), which is excellent. The specific cost of investment, which is still high (14 €/W for the Eurodish module), will be reduced by at least half when a market exists for this type of installation and the manufacture of a product line can be started. These machines sustain numerous cycles of breakdowns/outages and the components of the receiver are subjected to violent thermal shocks. The cost of utilization and maintenance is high. The parabolic-engine systems are intended in the first place for the decentralized production of electricity. The search for hybrid solutions, in which a non-intermittent heat source (fossil or biomass, for example) takes over for the solar source when radiation disappears, is an essential ingredient for capturing market share. Decentralized cogeneration of electricity and heating or cooling, entirely conceivable with this kind of machine, also works from the perspective of penetrating targeted markets. In any case, centralized production of electricity is not totally excluded from the field of application. The modularity of these systems makes it possible to envisage a progressive rise in installed power capacity on the same site, spreading the investments out over a long period of time, which facilitates financing. This point of view seems to be shared by the American manufacturer SES, which in 2006 announced very daring plans for installing thousands of 25 kW modules in California to create solar power stations of 300 MW and higher. To conclude this section dedicated to parabolic concentrators, it is appropriate to add several words about solar furnaces. In the same way as previously described, the concentrator of a solar furnace is generally a parabolic mirror, or by default a spherical mirror. Solar furnaces are distinguished from parabola engine systems in that the concentrator mirror is fixed and it is lit by a flat primary illuminated mirror to reflect solar rays along the optical axis. This mobile mirror is called a heliostat. The high cost of solar ovens, a result of having two reflecting surfaces, restricts their use to investigative work rather than industrial applications. These are remarkable research instruments intended for experimental work on the utilization of high temperature solar heat. The immobility of the focal point allows it to accommodate complex experimental devices, equipped with numerous measurement instruments. Moreover, the easy access to a fixed focal point allows the experimenters to make the close observations and frequent interventions that characterize research work. 12 The Schlaich Bergermann and Partner (SBP) Society is associated with the motors manufacturer Stirling SOLO to produce the Eurodish module. 13 Stirling Energy Systems.
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The research work carried out with solar furnaces greatly exceeds electric production. The research concerns the synthesis of new materials, the characterization of their properties at high temperature, and studies of solar processes for producing new energy solutions (hydrogen). 4.1.2.2. The tower concentrator To avoid the limitations of size, and therefore of power, encountered with the parabola, without giving up too much performance in terms of concentration, the reflecting collection surface was developed, placing elementary mobile mirrors on the ground. As with heliostats for solar furnaces, these heliostats are operated using movement along two axes. They follow the course of the sun and direct the solar radiation toward a single point which is the focal point of the installation, placed at the top of a tower. The precision of the orientation of the heliostats, the absorption of the radiation between a heliostat and the focal point, and the height of the tower are the three factors determining the dimensions of the installation. A total surface for the field of heliostats of 200,000 m2 constitutes a reasonable limit, and enables installed thermal powers of around 200 MW to be achieved. The distribution of the heliostats and the height of the tower are optimized so that the entire surface of the mirrors remains visible from above.
a)
b)
Figure 4.3. Examples of configurations of the heliostat field for tower concentrators; the tower is placed at the beginning of the marks – position 0.0: a) lateral configuration (north); b) circular configuration
In median latitudes, the tower is placed on the border of the field of heliostats (to the north or the south, according to the earth’s hemisphere). In low latitudes (< 35º)
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it occupies a more central position. The schematics in Figure 4.3 illustrate these two configurations. The geometry of the solar receiver’s entrance should be adapted to the configuration of the heliostats field, lateral or circular. The shade the tower casts over the field of mirrors directly affects the efficiency of the concentrator. Moreover, the tower is a construction whose impact must be taken seriously. Therefore an unimposing structure with slim lines is preferred. The practical versions are constructions of concrete or metallic beams. In comparison with the ideal parabola, optical performances are clearly lower. The collecting surface takes on the form of the terrain, generally flat. This is a modest surface, consisting of lightly focused heliostats. Its effectiveness in concentrating solar radiation varies according to the position of the sun. The average concentration factor of tower concentrators is typically higher than 500, which makes it possible to reach a high temperature (> 600ºC) for the heat source of the thermodynamic cycle of solar electricity conversion. The expensive components are the heliostats and the central receiver placed at the focal point. A lowering of the cost of the heliostats is sought by increasing the unit size. Today there are commercial heliostats of 120 m2. Studies are being made of prototypes of 200 m2 and more. An interesting approach is that of mechanically coupled heliostats, which leads to economies in the mechanization. We can also cite studies, mainly in Australia, of designs for concentrators made of mini towers (10 to 20 m in height) distributed in a field of heliostats of small dimensions and low cost. The solar receiver has been developed from a complex technology that must show itself capable of overcoming a double constraint imposed by the input of power in the form of highly concentrated radiation and by the intermittent nature of the source. It is a matter of transferring a very high density of flux and resisting extreme variations of power and temperature during cloudy periods. Studies of the component materials of receivers are essential for making these components reliable. Lowering the cost of the heliostats, increasing the temperature, and a gain in performance and reliability of the central receivers are the principal goals in the research work carried out in the field of tower concentrators. The impressive performance of tower power stations in the high power range makes them destined for centralized electricity production. 4.1.2.3. The cylindrical-parabolic concentrator (or trough concentrator) We present here a solution that is a little more economical than the tower concentrator, yet gives a creditable performance. Its simplicity, its relatively moderate cost and the reliability it has already achieved place this system in the best position for a model to be put to immediate use. The concentrator mirror is a parabolic section cylinder. It has only one direction of curvature; its manufacture and its set up are thus considerably simplified. Installed along the north–south axis, the mirror is moved by a single rotation which
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makes it swing from east to west so as to be lit by solar radiation according to the optical axis of the parabola. Concentration is obtained on a line occupied by the tubular receiver in which the heat transfer fluid circulates. The diagram in Figure 4.4 illustrates this design. The collector thus created is a modular system, simple to install, which offers great usage flexibility (see Figure 4.5). The receiver and the mirror are formed in one piece. Flexible couplings allow the modules to be connected in series. Very great power can be installed by attaching numerous modules to each other. The limitation comes from thermal losses and pressure drops, which increase with the length of the tubing necessary to carry the heat transfer fluid in the solar field. The number of modules placed in series and in parallel is optimized in order to minimize the pumping costs and thermal losses.
Figure 4.4. Tracking the sun by a cylindrical-parabolic collector
Figure 4.5. Mounting a piece of the tubular receiver on the cylindrical-parabolic collector
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Progress in research is needed to lower the cost of the structural support of the mirrors and gain several percent of conversion efficiency at the level of the receiver by improving the optical properties of the tubes and, at the level of the thermodynamic cycle, by raising the output temperature of the solar field. Fresnel linear mirror collectors offer an alternative solution to cylindricalparabolic concentrators. Illustrated in Figure 4.6, this system uses flat mirrors arranged in parallel plates which incline by rotation so as to light a fixed tubular receiver. Simpler, and above all cheaper, but with a less satisfactory performance in terms of concentration, they are attractive, for example, for preheating the transfer fluid. Current work aims at directly producing steam in the receiver tube of a linear Fresnel system. The former Belgian company SolarMundo contributed to the development of this design. This work is continued by the German company Solar Power Group in association with Man Ferrostaal. A test loop of about 500 kW is under qualification at the PSA in Spain. This concept has also been strongly supported by the Australian government and a team from the University of Sydney is promoting it in association with the company Solar Heat and Power. The application consists of installing a field of CLFR14 collectors to partially preheat the water in the Rankine cycle of the Lydell Power Station (NSW), a coal fired combined cycle station. About 500 kW of collectors manufactured by the SHP15 company have already been installed on the site, which will accommodate a total solar capacity of 36 MW (see Figure 4.6). Several companies were recently created with the objective of building and operating large plants using this technology in the range 20–200 MW (Ausra, 2006, Novatec Biosol, 2005). Today these companies operate small demonstration plants of about 1 or 2 MW. To conclude this section devoted to concentrating systems, let us point out that it may prove worthwhile to combine several concentrating technologies in the same solar station. In this way the greatest benefit of each concentrating system can be drawn upon in the optimal manner. For example, it might be wise from the point of view of economy to use a CLFR or cylindrical-parabolic concentrator in the first stage of heating, where high power of moderate temperature is needed, and then to transfer the fluid, preheated in this way, to the central receiver of a tower concentrator to raise its temperature there with complementary power delivered under high heat flux. The complexity which results from the multiplication of providers and the simultaneous behavior of several systems in use must be carefully thought out and balanced against the gain in productivity expected from such a mixed device.
14 Compact Linear Fresnel Reflector. 15 Solar Heat and Power Pty Ltd (Australia).
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Solar incident radiation Secondary reflector Fresnel primary mirror Absorber tube
a)
b) Figure 4.6. Fresnel linear mirror concentrator: a) diagram of the principle; b) example of the installation in Australia (source: SHP)
4.1.3. Components for production of heat and conversion into electricity The heart of a thermodynamic solar power plant consists of the solar receiver, which is associated with the heat transfer fluid. It is here, in effect, that the transformation of solar radiation into heat takes place, and the temperature attained by the transfer fluid is essential for conversion downstream in the thermodynamic cycle. The nature of the transfer fluid used (liquid, gas or two-phase) and the utilization temperature under consideration largely determine the general design of the station and the size of its essential components (receiver, storage, eventual exchanger with the fluid of the thermodynamic work cycle). There are several designs for solar receivers and different heat transfer fluids and thermodynamic
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Renewable Energy Technologies
cycles have been suggested and tested for converting concentrated solar energy into electricity. The result is a great variety of conversion models that would be too long and tedious to detail here. To try to clarify this situation and to give the reader a more comprehensive view of the problem, we will confine ourselves to presenting here the best candidates for these components, stressing their principal qualities and their eventual major faults. 4.1.3.1. The solar receiver The majority of receivers work by indirect heating. In these systems, the receiver captures the solar radiation and it is cooled by internal circulation of fluid. This heat transfer fluid functions to cool the solar receiver efficiently so that it reaches its output temperature without causing too high a temperature at the walls, which is where thermal loss takes place. This is accomplished by ensuring the best exchange coefficient between the fluid and the material of the receiver, and good heat transfer in the walls. The simplest and the most frequently encountered example is a tube receiver, whose design is inspired by numerous conventional boilers. It can be found associated with all types of concentrators (parabolic, tower and cylindricalparabolic). The diagram of a tubular receiver with a cylindrical-parabolic collector is shown in Figure 4.7. A glass exterior tube results in a drop in the radiative losses of the internal absorber tube. The space between the two tubes is maintained as a vacuum, which cuts losses by convection. The glass–metal connection is a delicate point of this system. It should allow the expansion of the two materials without breaking the seal. Industries that have experienced this problem have come up with efficient solutions (Solel, Schott).
Figure 4.7. Design of tubular receiver for cylindrical-parabolic collector (source: Luz)
Figure 4.8a illustrates absorption into a tubular receiver. Another example, less frequent and reserved for the case where the fluid is a gas, is that of volumetric receivers (Figure 4.8b). A porous or microgrooved wall is heated by solar radiation. The gas is introduced – or sucked in, in the case of ambient air –at the irradiated face
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and it is progressively heated by circulating in the pores. The surface that undergoes the radiating thermal losses maintains a temperature that is lower than the output temperature of the gas. These receivers therefore have a relatively good performance at very high temperature, but their design is delicate and their usage is proprietary because of the complexity and the cost of the materials employed.
Figure 4.8. Modes of absorption and temperatures of fluid and absorber: a) tubular receiver; b) volumetric receiver
In direct heating systems, it is the fluid that absorbs most of the solar radiation. In the case of a liquid the flow takes the form of a film along a wall. In the case of a gas, a gas–solid flow is created by introducing particles that increase the absorption and reduce the transmission of the radiation. These receiver devices have been tested, but they are not used in current designs for solar stations because their operation is difficult to stabilize as a result of the difficulty in controlling the flow. 4.1.3.2. Heat transfer fluid As we have already indicated, the choice of heat transfer fluid is paramount. It determines the maximum admissible temperature, it directs the choice of the technology and the materials of the receiver, and it largely determines the possibility and convenience of storage. It can also be advantageous if the transfer fluid is the working fluid of the thermodynamic cycle, in this case economizing on the exchanger.
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Oils are single-phase fluids that provide a suitable coefficient of exchange. Their temperature range is limited to about 400ºC. They lend themselves to storage in multilayered chambers. Mineral oils, which are very flammable, have been abandoned in favor of synthetic oils. Frequently used in industry, they are produced in great quantity. Their use necessitates precautions in order to avoid environmental damage in case of leakage. It is currently the most commonly used fluid in power stations with cylindrical-parabolic collectors and Rankine cycles, for which an example of an installation is illustrated in Figure 4.9. The oils are chemically neutral with respect to steel tubes. Experience shows, however, that outgassing of hydrogen is produced as a result of slow decomposition of the oil; the hydrogen migrates across the steel and partially breaks the vacuum of the receiver tubes. The thermal performance of the installation degrades after some 15 years of intensive use. Liquid metals – especially liquid sodium – have been abandoned for safety reasons and because of their very negative impact on the environment. Other fluids are now used when we wish to increase the temperature of the primary heat produced by the solar concentrator beyond 400ºC. Three candidates remain: a mixture of molten salts, water vapor or air.
Figure 4.9. The principle of a solar power station with cylindrical-parabolic collectors with oil (400ºC) with storage in melted salt and Rankine cycle (source: Flabeg)
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The salts in question here have a sodium and potassium nitrate base. They have a eutectic point of about 120°C to 220°C depending on their composition; they are liquid at higher temperatures. They offer a good exchange coefficient and have a high density, which also makes them very good fluids for storage, better than oils. The experiments that have been carried out (Themis, Solar Two) have not shown any corrosion of the steel, and the stability of the salts tested is excellent. The major drawback is related to solidification at ambient temperature, which introduces serious and expensive constraints. It is imperative to treat the salt circuits during shutdowns at the station, by preheating or maintaining their temperature above 250°C. Molten salts are used in tubular receivers; their exit temperature reaches 650ºC (verified at Solar Two). Their use with a tower concentrator and a Rankine cycle constitutes a model that is already proven, represented by the diagram in Figure 4.10. A commercial demonstration project based on this design, named Solar Three, is proposed in Spain by the Ghersa group in association with the SENER company (see section 4.2). For its part, the Italian research center ENEA proposes using a molten salt as the heat transfer and storage fluid in a cylindrical-parabolic collector power station. ENEA’s objective is to build a 20 MW solar unit to provide back-up heat in the steam cycle of a conventional combined cycle station in Sicily (Archimedes project). This technology is under qualification by ENEA in a test loop.
Figure 4.10. Diagram of the principal of a tower station with molten salt (Solar Two, Barstow, USA)
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Renewable Energy Technologies
Now let us examine the case of direct steam generation using a solar concentrator. Liquid water really is an ideal heat transfer fluid. It has an excellent exchange coefficient and possesses a strong heat transfer capacity. Moreover, it is perfectly neutral with respect to the environment, abundant and practically free. However, it is necessary to apply high pressure to maintain liquidity when the temperature rises, which poses problems regarding the thickness of the metal and the air tightness of the circuits. Vaporizing the water in the receiver offers the advantage of absorbing a large quantity of energy corresponding to the latent heat. Furthermore, an evaporator functions at a constant temperature. Another advantage is that the two-phase system water/steam is also the working fluid of the Rankine type cycle. Everything seems to favor this fluid, at least as long as it is confined to saturated steam, because, unlike liquid water, dry steam has a low heat transfer coefficient and lends itself poorly to thermal storage. Superheating the steam complicates the design of the receiver by imposing a separator stage and recirculation upstream. This is not justified except when electricity production is achieved using solar irradiation in a Rankine steam cycle. In practice, stability in the functioning of such a receiver is difficult to control, which may result in undesirable variations of operation of the cycle downstream. In a two-phase operation, it is necessary to pay attention to correct molding of the walls of the tubes, which demands precise control of the flow. We could aim for a high temperature and thus attempt a high output cycle with low storage capacity in a chamber of pressurized water. The price to be paid is withstanding the pressure, which can reach 100 bars and result in burst tubes, mainly in the central receiver. It is especially important to optimize the central control of power stations. The problem is better controlled in the case of a cylindrical-parabolic concentrator. The tests carried out on a pilot loop at the PSA encouraged the INDITEP Spanish promoters of the project to back it. They propose installing a 4.7 MW station with cylindrical-parabolic collectors and superheated steam; a diagram of the principle is shown in Figure 4.11. Moreover, the tower station PS10, brought into service in 2006, uses, in a more conservative but nonetheless very convincing fashion, a central receiver and a 250ºC, 40 bar saturated steam cycle (see section 4.2).
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81
,
,
,
,
,
Figure 4.11. Diagram of the principle of a power station with cylindrical-parabolic collectors and direct generation of steam (Inditep Project, 4.7 MWe, CIEMAT, Spain)
Finally, air is a candidate to consider seriously. This is not because of its thermophysical properties, which make it a very poor heat transfer fluid at atmospheric pressure, but by virtue of its supply and management, both of which are easy, at least in appearance, and its capability, shared with all the simple gasses, of reaching very high temperatures while maintaining perfect stability. Used at atmospheric pressure, it can be advantageously taken in close to the receiver and it transfers its thermal energy downstream to the secondary circuit or to a bed of ceramic or mineral balls. Volumetric receivers alone offer the possibility of efficiently heating air at atmospheric pressure. Discarding warm air and taking in external air at a very low temperature is less efficient. It is preferable to feed the receiver with recycled air. The design for such a system is complex and its implementation proves to be costly, especially because of the very large output of air to be circulated. The diagram in Figure 4.12 illustrates this principle applied in the case of a tower station. No installation has gone beyond the small scale experimental stage. The reason air is attractive resides in the fact that it can feed the expansion turbine of a Brayton cycle, provided that it is first compressed between 7 and 15 bars, depending on the situation, and that it reaches a sufficient temperature, generally higher than 1,000ºC.
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Figure 4.12. Diagram, of the principle of a tower station with atmospheric air heat transfer
This design is very promising because it uses a cycle and machines of very high output in a power range adapted to tower concentrators. As we will see in the following section, it is also intrinsically hybrid; the combustion chamber of the gas turbine adds auxiliary power to the solar source in order to guarantee rated conditions. Illustrated by a diagram of the principle in Figure 4.13, this innovative tower station design may prefigure a solar power station of the future. In this case a closed receiver is used. Experiments at modest power have already shown that a volumetric receiver equipped with a quartz window makes it possible to reach a temperature of 1,000ºC. Raising the power and making the pressurized air receivers reliable constitute the major goal in the work currently being done on this configuration. The use of metals with a nickel base and ceramic materials with a base of silicon is recommended. We come up against the limits of the ability of materials to withstand the temperature and sometimes we face the problems of oxidation by air at these temperature levels, especially in the presence of traces of water vapor. 4.1.4. Storage and hybridization The possibility of storing heat from the sun is a significant advantage compared to the direct production of electricity by photovoltaic cells or wind power. Storage technologies in the form of tangible heat in fluids or in solid materials are currently well understood. The efficiency is higher than 95% and the cost is moderate, typically 10–30 €/kWh_th or 25–75 €/kWh_el.16 The installation availability is accrued, typically from 1,800–2,000 hours/year without storage to 4,000 hours/year with storage. Seasonal storage, studied in the laboratory, has been considered in chemical form. 16 By way of comparison, electricity storage by pumping costs 70 €/kWh_el with an efficiency of less than 80%.
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The hybridization of conversion systems with a fossil or biomass heat source makes it possible to further increase the capacity factor17, mainly by increasing the operation time in winter without oversizing the solar field. This technique consists, for example, of placing a boiler in parallel with the solar field to furnish thermal energy to the cycle during periods of low sunshine. Another design consists of putting the boiler in series on the primary circuit so as to raise the temperature of the heat transfer fluid by a complementary contribution of very high temperature heat coming from combustion. To illustrate this concept, a diagram of the principle of a solar/fossil hybrid station with a high temperature gas cycle and combined cycle is given in Figure 4.13. The production of base electric power can be achieved: a capacity factor of 100% can be obtained, but with a low annual solar fraction18: 10–20% with a radiation of 2,015 kWh/m2/year (Seville, Spain); 10–28% with radiation of 2,790 kWh/m2/year (Daggett, California) [BUC 04]. Hybridization guarantees continuous production while avoiding the construction of a power station using a fossil source in parallel with a 100% solar station. In the ISCC19 design, solar heat delivered at 350–400ºC by a cylindrical-parabolic field is injected into the downstream cycle (Rankine) of a combined cycle, in parallel with a conventional boiler. We save on storage and the solar energy is converted with a high efficiency. In this design the solar fraction remains small (< 10%).
Figure 4.13. Hybrid configuration: tower station with solar receiver with pressurized air (800ºC), with fossil auxiliary and combined cycle 17 The capacity factor is the ratio annual electric energy produced/(nominal power x length of the year). 18 The annual solar fraction is the ratio of thermal energy issued from the solar receiver/total thermal energy furnished to the cycle in a year. 19 ISCC: Integrated Solar Combined Cycle.
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Renewable Energy Technologies
4.2. The state of the art 4.2.1. First generation solar stations and exploratory work These are prototype power stations of low power, financed by public funds and used on an experimental basis by public research organizations. In these projects, now terminated, the objective is to test and prove the principal technical choices, to evaluate the performance of the models and their components. Prototypes of tower stations with powers of less than 11 MW were used in this way on an experimental basis from 1980 to 2000. For example, the 2.5 MW Themis station, installed in Targasonne in France and used from 1983 to 1986, demonstrated the validity of the concept of a molten salt heat transfer fluid and salt storage. This concept was taken up and followed at Barstow in California with the Solar Two experiment (see Figure 4.14). Previously, in the USA, the direct production of steam in the central receiver of a tower had been validated by the Solar One experiment in California, 1982–1985.
Figure 4.14. Solar Two Power Station, Barstow, USA, 1997–2000. Solar field 75,000 m2, molten salt heat transfer fluid and storage, Rankine cycle 12.4 MW
Certain current installations use only components for the conversion of solar energy into heat and exclude the connection between the primary solar loop and the utilization loop. The utilization cycle downstream is rightly considered to be a conventional component whose future improvements do not come under the work
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carried out by solar specialists. Atmospheric air receivers, for example, have been tested on the scale of several hundreds of thermal kilowatts. Some experimental work on storage also has the objective of constructing specific prototypes: storage in a bed of solid particles, storage in concrete with flooded tubes. Two principal sites host these experiments: Almeira’s solar platform in Spain, made by the CIEMAT research center, and in the USA the Sandia Laboratories’ test center in Albuquerque (New Mexico). The parabolic-Stirling system, intended to decentralize production of electricity by using units of low power (10 to 25 kW), have been explored since 1985, notably by the DLR in Europe and by Sandia in the USA. Small industries soon became interested in this concept and have contributed to the progress of this system by making the components that depend on leading edge power technology reliable. This is the case, for example, of Schlaich Bergermann and Partners in Germany and of Stirling Energy Systems in the USA. Today, several preindustrial prototypes of the parabolic-Stirling type are used experimentally and for demonstration. There are seven examples of the SES 25 kW prototype: six in Albuquerque, USA, used by Sandia Labs, and one at Johannesburg, South Africa, used by Eskom. In Europe, six Eurodish 10 kW units produced by SBP-SOLO (see Figure 4.15) have been installed in four countries: two in the DLR-PSA in Germany, one at the University of Seville in Spain, one at the CESI, Italy, one in Wurzburg in Germany, and the last at the CNRS-Odeillo in France. New actors like Infinia Corp in the USA and Sunmachine in Germany propose small dish-Stirling systems of 2–3 kW for domestic applications.
Figure 4.15. Preindustrial prototype Eurodish, 10 kW, produced by SBP-Solo (Germany)
1980
5 MWth
Molten salt
8,261
222
61
AIE
1981
2.7 MWth
Liquid sodium
4,616
111
43 Sodium
CEE
1981
1 MWe
Sat. steam
4,193
182
55
Molten salt
Storage
DOE
Net capacity
Tower height (m)
(Sicily)
No. hours elec.
Adrano
Surface area (m²)
(Spain)
Heat transfer fluid
EURELIOS
Tabernas
Year
SSPS
Albuquerque (NM, USA)
Promoter
CRTF
Place
Renewable Energy Technologies
Installation
86
Molten salt
Sunshine
Nio (Japan)
–
1981
1 MWe
Sat. steam
12,912
807
69
Molten salt
Solar One
Barstow (CA, USA)
DOE
1982
10 MWe
Steam surr
72,527
1818
80
Oil
1983
1 MWth
Sat. steam
1,584
30
20
–
7 CIEMA 1983 T MWth
Sat. steam
11,880
300
80
Molten salt
EDF/AF 2.5 ME/CN 1983 MWe RS
HITEC salt
10,796
201
Molten salt
75,527
STEOR
CESA-1
Thémis
Solar Two
Kern County ARCO (CA, USA) Power Tabernas (Spain) Targasonne (France)
Barstow (CA, USA)
DOE
1995
12.4 MWe
101. Molten 5 salt
1,818 54
Table 4.2. Principal prototype installations with a central receiver
Molten salt
Heat transfer fluid
Qutput temp. (ºC)
Solar field area (m2)
1980
50 kWe
(Gilotherm) oil
250
1,176
UE + USA
1981 500 kWe
(Santotherm) oil
300
2,672
Daggett
Luz
1984 14 MWe
Oil
–
–
SEGS II
Daggett (CA, USA)
Luz
1986 30 MWe
(ESSO 500) oil
316
190,338
SEGS III/IV
Kramer Junction (CA, USA)
Luz
1987 30 MWe
Oil
349
230,300
SEGS V
Kramer Junction (CA, USA)
Luz
1988 30 MWe
Oil
349
250,500
SEGS VI
Kramer Junction (CA, USA)
Luz
1989 30 MWe
(Dowtherm A) oil
390
188,000
SEGS VII
Kramer Junction (CA, USA)
Luz
1989 30 MWe
Oil
390
194,280
SEGS VIII
Harper Lake (Cal. USA)
Luz
1990 80 MWe
Oil
390
464,340
SEGS IX
Harper Lake (CA, USA)
Luz
1991 80 MWe
Oil
390
483,960
LS-3
Tabernas (Spain)
CIEMAT-UE
1997
–
(Syltherm 800) oil
420
685
DISS
Tabernas (Spain)
Iberinco/ CIEMAT/UE
1998
–
Steamsuper
400
3,838
SKAL-ET
Kramer Junction (CA, USA)
Solar Millennium, Flagsol, SBP
2004
–
Oil
400
4,350
El Nasr
Egypt
NREA
2004
–
Sat. Steam
175
1,900
Place
Promoter
Installation
Net capacity (MW)
87
Year
Solar Thermodynamic Power Stations
COSS
Vignola (Corsica, France )
CEA/ CNRS/ AFME
SSPS-DCSACUREX
Tabernas (Spain)
SEGS I
Table 4.3. Principal prototype installations with cylindrical-parabolic collectors
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Renewable Energy Technologies
The SEGS20 power stations located in the Mojave Desert in California are the only solar power stations to have been used industrially for more than 20 years. Designed and constructed by the Luz company, thanks to Israeli and American capital, and helped by advantageous fiscal measures, they have a total capacity of 354 MW distributed in nine units, each of 14 to 80 MWh. Incentives having been suspended, no station has been constructed since 1991. The Luz company’s bankruptcy put the brakes on the industrial development of this technology. The SEGS stations use the technology of cylindrical-parabolic collectors with oil as the heat transfer fluid, which is the most mature technology (see Figure 4.16). Over the years, the experience acquired by those implementing the technology has allowed a great deal of progress and the costs of implementation and maintenance have fallen considerably. Today two electricity production and distribution companies share the use of these installations and the proceeds from the sale of the electricity: Southern California Edison and FPL Energy. Production is about 800 GWh/year. Several builders have followed up on the expertise of Luz and have pursued industrial development of the technology: Solel in Israel, Schlaich Bergermann and Partner in association with Flabeg and Schott in Europe, and Duke Solar (who became Solargenix and is now part of the Acciona group) in the USA.
Figure 4.16. SEGS power station, USA, 354 MW. Cylindrical-parabolic collectors with oil, Rankine cycle
The totality of the work carried out for nearly 30 years by public or semipublic research organizations in the USA, Israel, Germany and Spain, with the goal of improving the components of solar power stations, has made it possible to bring progressive technological improvements and has contributed to the regular lowering of installation costs and increasing of efficiency. Two large testing platforms have 20 SEGS: Solar Electricity Generation System.
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Heat transfer fluid
Oil
Superheated system
Molten salt
Saturated steam
Reference
Andasol
Inditep
Solar Three
PS-10
PS-1021
Place
Spain
PSA22
Spain
Spain
Project size
50 MWe
Current state
In progress
Maturity Commercial
4.7 MWe
17 MWe
In progress Projected Pilot
Pilot
Atmospheric Air under air pressure
Parabolic–Stirling
CRS hybrid
Central receiver tower (CRS)
Technology
Cylindrical-parabolic
hosted most of the pilot installations: La Plataforma Solar de Almeria (PA) of CIEMAT in Spain, and the Sandia solar testing center in Albuquerque in New Mexico (USA). European Union research programs contributed a great deal of experimental work carried out at the PSA. Table 4.4 indicates the estimated or measured performances on these pilot projects or on industrial installations.
Hydrogen
Solgate
Eurodish
Spain
PSA
PSA
11 MWe
10 MWe
14.7 MWe CC23
10 kWe
In progress
Suspended
Pilot
Pilot
Pilot
Precommercial
2 MWth
Phoebus 2.5 MWth
Solgate 230 kWe
Eurodish 10 kWe
None
N
Completed Completed
Max. installed capacity
SEGS 80 MWe
DISS 0.8 MWth
Solar Two 11 MWe
Storage
Molten salt
None
Molten salt
Cycle
Rankine
Rankine
Rankine
Rankine
Rankine
Brayton
Stirling
LEC (€/kWh)
0.172
0.162
0.183
0.241
0.234
0.14724
0.281
Water/steam Ceramic bed reservoir
Table 4.4. Current performance of the systems mentioned
21 PS-10 is mentioned twice because this pilot project station, initially using air, was modified in 2004 by the designers who have changed to a water/saturated steam station. 22 Plataforma Solar de Almeria, Spain. 23 Combined cycle: Brayton (gas turbine) + Rankine (steam turbine). 24 Cost of uniquely solar heat. The total cost solar + fossil is 0.10 €/kWh. Assumption of fossil fuel at 15 €/MWh.
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4.2.2. Second generation solar power stations: precommercial prototypes First generation power stations, designed from 1975 to 1990, have been succeeded today by the second generation of solar installations based on already validated designs, sometimes with several technical improvements. Political incentives adopted chiefly in Spain and in several states in the southwest of the USA (California, Arizona and Nevada) are opening new industrial development prospects for power stations with the hope of penetrating the electricity market. A solar concentrator technologies industry is being developed in Europe and in the USA. Plans for second generation stations have wide appeal for private investment and have been launched by industrial investor groups (Solar Millennium, ACS, Acciona), manufacturers and construction companies (Abengoa, Ghersa, Inabensa, Flabeg, Schott, Solargenix), engineering firms (Flagsol, SBP, SENER) and users (APS). In the USA, at the end of 2005 Solargenix put a 1 MW power station in service in Arizona with cylindrical-parabolic collectors and incorporating an organic Rankine cycle. In 2006 the same company began the construction in Nevada of a 65 MW station that will include a field of cylindrical-parabolic collectors of 300,000 m2 and will occupy a ground surface of 1.4 km2. The emblematic projects in Europe are PS10, Andasol and Solar Three, all three located in Spain, where the repurchase rate of thermodynamic solar electricity recently reached 0.269 €/kWh for installations limited to 50 MW power. The first two examples are in operation, while the construction of Solar Three has not yet started. Designed and used by Abengoa Solar, PS10 is a tower station with direct generation of saturated steam, pressurized water storage and saturated steam cycle (see Figure 4.17). It is installed at Sanlucar, near Seville, where the direct annual solar source reaches 2,015 kWh/m2/year. The solar field has 624 heliostats of 121 m2. The receiver is a cavity formed by four panels of 5 m x 12 m tubes, the thermal storage has a capacity of 15 MWh, the steam is furnished to the cycle under 40 bars at 250ºC. The rated power is 11 MW and the annual production is estimated to be 23 GWh/year. The station occupies 600,000 m2 of land. Construction was completed at the end of June 2006 (see Figure 4.18). Abengoa is constructing a second station of similar technology but of slightly higher power, named PS20. Andasol is an industrial project for a station with cylindrical-parabolic collectors and oil heat transfer fluid, illustrated by the diagram in Figure 4.19. The distinctive feature of this station is very large storage (7 h) and an oversized solar field (500,000 m2) relative to the rating (50 MW). The goal is to produce electricity as close as possible to the load curve of the network, any time from 9 a.m. to 11:00 p.m. This station is installed near Guadix. It has been in operation since December 2008.
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Figure 4.17. Diagram of the principle of the power station PS10 (Spain)
Figure 4.18. The power station PS10 under construction near Seville (Spain) May 2006: a heliostat (121 m2) lights the solar receiver at the top of the tower (100 m)
Solar Three is a tower station with a molten salt heat transfer fluid and molten salt storage, coming directly from the Thémis and Solar Two experiments. The only differences are that steam generation will be located in the tower and salt pumps will be flooded in storage tanks (see Figure 4.19). A field of oversize mirrors 264,825 m2 (2750 heliostats of 96.3 m2) and a storage of 15 hours will supply the Rankine cycle with a rated power of 17 MW. According to the designer SENER, the estimated annual production of Solar Three under radiation of 2,060 kWh/m2/year will be 105,566 MWh. The capacity factor thus attained will be very high: 71%.
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Figure 4.19. Solar Two and Solar Three: two similar designs
Figure 4.20 shows the sites in Spain and projects currently studied or proposed. Some 20 projects for 100% solar or hybrid stations are under study in the world, which represents a solar production capacity of 2,000 MW. Most of them use today’s most mature technology: cylindrical-parabolic collectors with oil heat transfer fluid. Projects for solar power stations in developing countries (India, Algeria, Morocco) benefit from development aid funds granted by the World Bank. The proposed technology is hybrid ISCC (see section 4.1.3). Table 4.5 shows the main projects. The most advanced is the Algerian station of Hassi R’mel, located 400 km south of Algiers and proposed by the New Energy Algeria (NEAL) consortium. It consists of installing a field of cylindrical-parabolic solar collectors with oil coupled to a 150 MW gas combined cycled station. The solar power installed will be 25 MW.
Solar Thermodynamic Power Stations
Location USA
Solar capacity 1,000 MWe 515 MWe
Spain
15 MWe 50 MWe
Technology cylindrical-parabolic + oil cylindrical-parabolic + oil tower + molten salt tower + 1 atmosphere air
Israel
100 MWe
cylindrical-parabolic + oil
South Africa
100 MWe
tower + molten salts
Egypt
30 MWe
cylindrical-parabolic + oil and ISCC
Algeria
25 MWe
cylindrical-parabolic + oil and ISCC
Morocco
20 MWe
cylindrical-parabolic + oil and ISCC
Italy
20 MWe
cylindrical-parabolic + molten salts
Germany
1 MWe
tower + 1 atmosphere air
Table 4.5. Solar power station projects worldwide (solar capacity)
Figure 4.20. Plans for solar power stations in Spain by 2015
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4.3. Prospects 4.3.1. Strategy for penetrating the market Solar concentrating systems collect only direct solar radiation, while their nonconcentrating homologs and photovoltaic installations capture diffuse radiation as well. Direct solar radiation represents between 50% and 90% of the total solar radiation. More abundant in a geographical zone commonly called the sun belt, indicated in Figure 4.21, the direct solar radiation resource is considerable on the planetary scale. Included in the solar belt are 70 cities of more than a million inhabitants and numerous countries expected to have a high rate of development in the near future.
Figure 4.21. The sun belt, regions of the globe for which the average annual direct radiation25 exceeds 2,000 kWh/m2/year (source: Pharabod & Philibert [PHA 91])
The solar resource is not the only criterion for the selection of potential sites for installing high power solar power stations. Since their purpose is to furnish electricity to the grid, ease of connection to the latter is a major criterion. Because of the large surface areas and civil engineering work needed to deploy the mirrors, it is necessary to use land without much of a slope (average incline less than 4%) and without a conflict of interest with other uses. Protected natural spaces, industrial 25 The numbers generally given in atlases refer to global radiation (direct + diffused) incident on a horizontal plane.
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95
zones or those near habitation, for example, are to be avoided. Detailed studies based on satellite imagery are conducted in order to better evaluate the potential of concentrating solar technologies for electricity production. The example given in Figure 4.22 illustrates the case of the Mediterranean regions. This study carried out by the DLR and ISET reveals that the potential for electricity production in the desert regions of the Sahara and the Arabian peninsula reaches 30 MW/km2, a value three times higher than in the most favorable regions of Mediterranean Europe (maximum of 10 MW/km2).
Figure 4.22. Potential for electricity production by thermodynamic solar means in Mediterranean regions (source: ISET, 2003 [KAB 03])
The issue of water resources must be examined with care: washing the reflecting surfaces and water cooling the thermodynamic cycles contribute to conversion efficiency but increase the water requirements. Air cooling for Rankine cycles is possible, but at the price of a drop in production of about 6%. Scenarios worked out by the International Energy Agency concerning primary energy production indicate a significant increase on the part of renewable sources other than hydraulic in the energy mix by 2030 (see Figure 4.23).
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Renewable Energy Technologies , , , , , , , ,
Figure 4.23. Evolution of primary energy production (source: C. Philibert, AIE, 2006)
The scenarios for market penetration and the growth of thermodynamic solar power stations, developed by researchers (the European ECOSTAR program) and by the sector industries (the European association ESTELA), lead to a 5,000 MW installed capacity by 2015 (see Figure 4.24). , , , , , , , , ,
Figure 4.24. Scenario for growth in the market for thermodynamic solar energy
The desert regions of the globe (the Sahara, the southwest of the USA, Australia, South Africa), receiving up to 2,900 kWh/m2/year of direct radiation, provide a large part of the necessary surface area for a massive production of electricity using concentrating technologies. It is therefore pertinent to assume that the contribution of these technologies in the medium or long term offers a genuine alternative to the consumption of fossil resources.
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Even if it does not settle the issue of energy independence for countries with high consumption, concentrating solar energy does make it possible to deal with the hard-to-control fluctuations in the fossil resources market26. Energy independence is then reduced to the problem of transmission of the energy, similar to the current problem of moving fossil fuels toward the places of consumption or transformation. Figure 4.25 illustrates a scenario studied by the DLR and ISET in which the electricity produced in high radiation locations would be moved toward southern Europe by HVDC lines. It shows that the losses on line, estimated at 15% of the power transmitted, are largely compensated for by the increase in production linked to the strong radiation in these production regions compared with that of the areas of consumption. The Algerian ISCC type electricity station proposed for Hassi R’mel is represented in this diagram (see Figure 4.26). For populations not connected to the electricity distribution grid, the implementation of decentralized devices for producing low power energy by concentrating solar technologies represents an essential opportunity. In this application framework, with an annual efficiency of around 18%, parabolas of 50 to 80 m2 equipped with Stirling motors placed at the focal point and directly fed by solar heat supply an appropriate, flexible response, cleaner with respect to CO2 emissions and more efficient than photovoltaic devices.
Figure 4.25. Trans-Mediterranean grid for electricity transmission. The dots containing an “S” indicate the locations of solar electricity production (DLR ISET, 2003 [KAB 03] scenario) 26 The price of crude oscillated between $8 and $140/barrel between 1985 and 2008.
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Beyond European borders, the increasing electricity needs of countries with strong sunlight, needs that for some are linked to their soaring economy or for others to the need for air conditioning, open up prospects for applications that encourage broad research and development efforts with the aim of improving still further the already remarkable performance of solar electric stations.
Figure 4.26. Plan for ISCC power station at Hassi R’mel (150 MW) and the distribution grids in the Maghreb and in Europe (source: NEAL, Algeria)
4.3.1.1. Power stations of the future and research efforts Models of the thirde generation of power stations present even higher technological risks. Before they can be exploited on an industrial scale (50– 200 MW) intermediary stages of development on a pilot scale (1–10 MW) are necessary. Plans for research on pilot installations are financed by national and international public programs27 and appeal mainly to national research centers. These have formed alliances of laboratories, such as SunLab in the USA28 or SolLab in Europe29, to share their respective expertise. According to one study carried out in 2004 by the European ECOSTAR program [DER 05], the research work to be done 27 US Department of Energy (DOE), General Administration of Research of the European Commission (EC-DGXII), German Ministry of the Environment (BMU). 28 SunLab is an alliance between Sandia Labs and the National Renewable Energy Laboratory (NREL). 29 SolLab is an alliance created in 2004 between PROMES-CNRS (France), ETHZ (Switzerland), CIEMAT (Spain) and DLR (Germany).
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toward lowering the cost of solar electricity primarily concerns concentrators, storage devices and receivers. Table 4.6 indicates the principal innovations researched in these three areas, gains in performance and corresponding terms. Component
Concentrators
Storage
Innovations
Gains in performance/ reduction in cost
Term
Size: 120 m2 to 200 m2
Investment reduced
< 5 years
Thin glass/front face mirror
Reflectivity 93.5%
5–10 years
Dust repellent mirrors
Reduced maintenance
> 10 years
Autonomous heliostats
Investment reduced
< 5 years
Molten salts with thermocline (one tank)
Investment reduced
5–10 years
Ionic liquids at ambient temperature
Parasitic consumption for conditioning
> 10 years
Concrete with modular structures
Rapid charge/discharge
5–10 years
Phase-changing material (DSG)
Less space
> 10 years
Fixed or fluidized bed (gas/solid)
High temp. storage (stations with air, hybrid, or gas cycles)
10 years
Steam reservoirs
Receivers/ absorbers
< 5 years
Absorbant/selective deposits
Higher temperature
5–10 years
Control instabilities of flow
Efficiency of transfer
–
Pressure drops
Parasitic consumption
–
Windows, secondary optics
Output
–
Table 4.6. Principal innovations coming from work on R&D priorities
The design and development tools developed by research centers over the course of 20 years have attained a level of reliability that allows us to make projections for performance and cost using the data accumulated by the experiments as a system of reference. The results of the European ECOSTAR program are appropriately instructive. For all models the irradiation of the site is a determining factor. The cost of electricity produced by a station functioning under conditions of 2,900 kWh/m2/year instead of 2,000 kWh/m2/year is reduced by 31%. By locating projects of the same power (50 MWe) on the same site (Seville, Spain) and with an identical electrical production constraint (full power from 9.00 am to 11.00 pm or the hours of sunlight), performances of different technologies of distinct maturity can be analyzed.
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Technology Heat transfer fluid
Cylindrical-parabolic
Tower with central receiver (CRS)
CRS hybrid
Oil
Superheated steam
Molten salts
Saturated steam
Air at 1 atmosphere
Pressurized air
Area of the solar field (m2)
442,035
448,191
458,160
465,032
522,900
152,000
Electrical power (MW)
50 (1 x 50 MWe)
47 (10 x 4.7 MWe)
51 (3 x 17 MWe)
55 (5 x 11 MWe)
50 (5 x 10 MWe)
58.7 (4 x 14.7 MWe)
34.4
34
35.9
33.8
41.8
29.4
Water/steam reservoir 0.4 h
Ceramic fixed bed 3h
–
3,019
3,989
1,622
Implantation (m2/kW)
Storage
Investment in €/kW, of which: solar field, tower, receiver, storage, group indirect costs Capacity factor Solar fraction
Two tanks of molten salt 3h 3,530
2,840
Two tanks of molten salt 3h 3,470
51% – – 8% 22% 19%
64% – – – 17% 19%
36% 3% 15% 3% 24% 19%
38% 5% 14% 4% 20% 19%
35% 5% 13% 13% 15% 19%
22% 8% 11% – 39% 20%
28.5%
21.7%
33%
26.4%
33%
55.1%
–
100%
100%
100%
100%
100%
19%
Rankine
Rankine
Rankine
Rankine
Rankine
Brayton
Efficiency of cycle
37.5%
26%
38%
30.3%
34%
44.7%
Annual output of solar electricity
14%
9.9%
16%
13.6%
13.5%
19.1%
LEC (€/kWh)
0.172
0.187
0.155
0.168
0.179
0.13930
13–29%
23–38%
11–25%
21–33%
25–37%
17–28%
0.12–0.15
0.11–0.14
0.12–0.14
0.11–0.13
0.11–0.14
0.093–0.115
Cycle
Impact of innovations LEC 2.01531 (€/kWh)
Table 4.7. Estimated performance for the systems referred to with an installed power of 50 MW and exploited under irradiation of Seville (2,014 kWh/m 2/year)
30 Uniquely solar electricity. Total solar + fossil cost: 0.082 €/kWh. 31 LEC estimated after the impact of innovations. The effect of mass production, not considered here, leads to a 30% supplementary reduction of the LEC.
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Table 4.7 collects the results essential to this study for two cylindrical-parabolic models (oil or water/steam and four tower models (molten salts, saturated steam, air at 1 atm and pressurized hybrid air). The annual solar electricity efficiency comes to between 13.5% and 19.1% except for the model with direct generation of superheated steam (9.9%), which is penalized by the output of an inefficient cycle (26%) due to the small dimensions of the reference system (4.7 MW). The least advanced devices present the greatest potential for cost reduction, which places all the technologies in the same bracket of production cost of €0.11 to 0.15/kWh by 2015. The deployment of 5,000 MW in 2015 will bring a supplementary reduction of 31% in the cost of solar electricity which will then be between 0.06 €/kWh and 0.073 €/kWh for 100% high power solar stations in climates like that of Seville. In the event of a slight rise in the cost of fossil or nuclear electricity, which is probable, the competitive range, estimated at 0.05–0.07 €/kWhe in 2015, will be attained. Hybrid stations with pressurized air and gas turbines constitute an attractive special case with a total production cost (solar + fossil) of 0.082€/kWhe and a margin of reduction from 17 to 18% as a result of progress in gas receivers. 4.3.1.2. Conclusions Thermodynamic solar power stations are embryonic. They have a formidable potential for production in areas where normal direct radiation exceeds 2,000 kWh/m2/year. They also offer several advantages in comparison with other technologies for producing renewable electricity: – high energy output and the prospect of an increase that will place them at the forefront of technologies using renewable sources by 2015, – the possibility of storing solar energy, intermittent by nature, in the form of tangible or latent heat for short term local use, or in the form of chemical storage (hydrogen for example) for uses that vary by season or are distant from the place of production, – the possibility of hybridization with the use of fossil sources enables a transition strategy to be built, therefore envisaging from the start progressive use of increasing amounts of renewable energy in the energy mix, – the life cycle of concentrating solar installations is very favorable in comparison with conventional technologies or the use of other energy resources. Current levels of investment costs (2,500 to 3,500 €/kW) and exploitation and maintenance costs bring the cost of producing electricity into the range of 0.15 to 0.20 €/kWh depending on the technology, under irradiation of 2,000 kWh/m2/year (Mediterranean climate). To make these technologies competitive with conventional fossil, nuclear or hydraulic technologies, a 50–60% reduction in production costs
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must be achieved by 2015. The players in and promoters of solar concentrating power development recommend, through the Global Market Initiative (GMI) launched in Bonn in 2004, the installation of a capacity of 5,000 MW by 2015. The passage from the scale of several megawatts (prototypes) to several dozen megawatts (commercial stations) will make it possible to reduce the costs of use and maintenance. Finally, the pursuit and intensification of targeted research work on concentrators, receivers and storage devices will lead to a significant gain in the performance of the installations. The combination of deployment, increase in size of the units and progress in performance will lead to a reduction in the cost of thermodynamic solar electricity by a factor of 3 to 5 by 2015. Solar stations installed first in the regions of the globe with strong annual direct irradiation will then be competitive with conventional technologies that use fossil resources. Solar–fossil hybridization, technically possible and being used or projected in several developing countries, offers a particularly attractive transition strategy: a smooth transition toward an energy future that is, as it were, more respectful of the environment? 4.4. Bibliography [ARI 03] ARINGHOFF R., AUBREY C., BRAKMANN G., TESTKE S., Solar Thermal Power 2020, European Solar Thermal Power Industry Association (ESTIA), Birmingham, 2003. [BUC 04] BUCK R. et al., “Solar Gas Turbine Systems: Design, Cost and Perspectives”, 12th SolarPACES International Symposium, Oaxaca, Mexico, 6–8 October 2004. [DER 05] PITZ-PAAL R., DERSCH J., MILOW B., ECOSTAR Roadmap Document, DLR, 2005 (available at: http://www.pre.ethz.ch/documents/). [KAB 03] KABARITI M., MÖLLER U., KNIES G., “Trans-Mediterranean Renewable Energy Cooperation TREC for development, climate stabilisation and good neighbourhood”, DLR and ISET, Amman, 2003. [PHA 91] PHARABOD F., PHILIBERT C., “Luz solar power plants: success in California and worldwide prospects”, Deutsche forshungsanstalt für Luft und Raumfahrt e.V., IEA, Cologne, 1991. [SAR 03] SARGENT AND LUNDY LLC CONSULTING GROUP, Assessment of Parabolic Trough and Power Tower Solar Technology Cost and Performance Forecasts, SL-5641, Chicago, 2003.
Chapter 5
Wind Systems Technology
5.1. Introduction: wind power today Wind energy is booming throughout the world. Table 5.1 shows the recent trend in installed power in some typical countries [BEL 01, WWE 06]. Worldwide, by the end of 2005, there were 58,982 MW installed. Europe is still the market leader (with growth of 18% in 2005), but North America (36% growth in 2005) and especially Asia (48% growth in 2005 thanks to India and China) are now growing faster. 2000 (MW)
2004 (MW)
2005 (MW)
Germany
Country
5,432
16,629
18,428
Spain
2,235
8,263
10,027
USA
2,568
6,725
9,149
India
1,150
3,000
4,430
Denmark
2,181
3,124
3,128
Great Britain
391
888
1,353
Netherlands
444
1,078
1,219
China
302
764
1,260
France
69
386
757
Table 5.1. Total installation of wind energy in a number of countries
Chapter written by Régine BELHOMME, Daniel ROYE and Nicolas LAVERDURE.
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Wind energy currently accounts for 2.8% of European electrical energy consumption [EWE 06]. Installed power is projected to reach 75,000 MW in 2010 and 180,000 MW in 2020. Several scientific and technological factors are contributing to this growth: – wind energy is abundant; the wind is inexhaustible. It is therefore a truly renewable resource, – wind energy is clean; wind turbines do not produce pollution. They do not release harmful substances into the environment and do not produce waste. They therefore contribute to a substantial reduction in greenhouse gasses, – the wind power industry has great potential in terms of manufacturing and installation jobs. However, a key factor is still the political will displayed by governments to encourage investors through incentives (Production Tax Credit in the USA, renewable energy law in China, purchase obligations at attractive prices in several European countries including France). Wind energy is a function of the wind speed, which makes this type of energy intermittent and difficult to schedule. Its insertion into the electricity grid causes problems and specific constraints that must be taken into account by the network operators. In fact, as long as this type of production remained marginal, the constraints were limited. For a long time, the only “constraints” on wind farms were to produce power when possible and not to degrade the quality of voltage on the system. Today, wind farms, due to significant development and continuing growth in terms of installed power, are subject to increasingly stringent technical requirements set out in the rules for connection to the networks defined on the initiative of network managers. The feasibility and quality of responses to these new requirements strongly depends on the structure and technology of wind power generation systems. 5.2. Description of a wind generator 5.2.1. Principle Wind turbines are used to convert wind energy into electrical energy [BUD 00]. This conversion is done in two stages:
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105
– at the level of the turbine, which removes a portion of the kinetic energy of the wind available for conversion into mechanical energy, – at the level of the generator, which receives mechanical energy and converts it into electrical energy which is then transmitted to the utility grid. There must therefore be balanced conversion and transmission of energy given that there is only the possibility of inertial storage at the price of acceleration of the turbine. The general operation is shown in Figure 5.1.
Figure 5.1. Principle of energy conversion
5.2.2. Constitution A typical wind turbine is composed of several elements that are presented in Figure 5.2. One mast, or tower, supports the nacelle (1) and turbine (16). It is important that the tower be high due to the increase in wind speed with height. It is tubular and houses a ladder or even an elevator. The nacelle (1), partially soundproofed (6), (9), with a metal skeleton (5), houses the generator (3) and cooling system (2), the gearbox (8) and various pieces of electronic control equipment (4) that allow control of the various orientation mechanisms and the overall operation of the turbine. The gearbox has a slow shaft (12) supporting the turbine (16), and a high speed shaft (1,000 to 2,000 rpm). It is equipped with a mechanical disk brake (7), to which the generator is coupled (3). The gearbox is equipped with a cooling system (13) for the oil.
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Figure 5.2. Example of wind energy system (GE)
The turbine (16) generally has three blades (15) used to capture wind energy and transfer it to the slow shaft. An electromechanical system (14) generally allows orientation of the blades to control the torque of the turbine and regulate the rotation speed. The blades also work as an aerodynamic brake by unfurling or only by the rotation of their extremities. A mechanism using electrical servomotors (10), (11) helps to orient the nacelle to the wind. An anemometer and wind vane on the roof of the nacelle supply the data necessary to the control system to guide the turbine and to trigger or stop it depending on the wind speed. 5.3. Operation of a wind turbine 5.3.1. Controls of energy conversion The operating conditions of a wind turbine depend mostly on wind conditions over which no action is possible. We can only take action by limiting, sometimes as efficiently as possible in certain operating conditions, and always precisely controlled under other conditions, the energy actually converted by the turbine and then by the electric generator before transfer to the network. It is therefore necessary to control certain quantities. Control of the power provided by the system can be carried out at each of the two energy conversion
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levels in different ways depending on the operating conditions (see next section): – at the turbine level, primarily to limit the power in the case of strong winds, several methods are used, – at the generator level, especially for variable speed structures that will be seen later. This allows, for example, the capture of energy in the case of light and moderate winds to be optimized. Several options are available. They require simultaneous control of quantities directly affecting the functioning of the generator (currents, speed), and the requirements of the operating system (bus voltage, DC currents in the system interfacing with the network). The control system of a wind generator can be broken down into two essential functional levels (Figure 5.3): – control systems for the physical quantities and associated monitoring and protection. This ensures control and regulation of quantities to values of limits issued by the higher level of control, – the system management operating modes, making rules for instruction and management of protection.
Figure 5.3. General control structure of a wind turbine
For more general cases we should add a third level corresponding to the management of the entire wind farm depending on the constraints required by network operations.
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5.3.2. Control at the turbine level 5.3.2.1. Action of the wind on the turbine blades The action of moving air will be translated by forces applied to each point of the surface. The blades have an airfoil shown in the diagram in Figure 5.4. We note in particular the following: – extrados: top of the blade, – intrados: underside of the blade, – cord: length A of the profile from edge to edge, – angle of pitch ß (inclination of the axis of reference in relation to the plane of rotation).
Figure 5.4. Characteristic elements of a blade
The profiles are generally plane-convex (the underside is flat while the top surface is convex) or biconvex (underside and the top surface are convex). They are standardized and the parameters are well defined. Because of the rotation of the blade, the section located at a distance r from the hub is subjected to both the incident wind, of velocity V and the relative wind speed U coming in the opposite direction of rotation speed ȍT:
U = r :T
[5.1]
The resulting speed W of the apparent wind is expressed as: JJG JG JG W V U
[5.2]
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The velocity makes an angle of attack \ with the plane of rotation. This angle is expressed as
\
arctan
v u
[5.3]
We then introduce the so-called angle of incidence D between the reference axis of the blade and the direction of the apparent wind:
D \ E
[5.4] V
W
U U r
T
Figure 5.5. Wind direction on a section of the blade
The action of the relative wind on an airfoil gives rise on a section of blade of width dr and length of cord l at a distance r from the axis of rotation, to a resultant force dF.
Figure 5.6. The forces applied on an element of blade
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We can decompose the resultant force dF in the following manner: – lift dL, normal to the apparent wind direction, – drag force dD parallel to the wind direction. It can also be broken down in another way: – axial thrust dFa perpendicular to the plane of rotation, – tangential thrust dFt in the direction of rotation. It is easy to deduce expressions for the tangential and axial thrust depending on the lift and drag from the previous diagram:
dFt
dLsin\ dDcos\
[5.5]
dFa
dLcos\ dDsin\
[5.6]
It is the torque resulting from all tangential forces that causes the turbine to rotate. The magnitudes of these two forces dL and dD are expressed as a function of two coefficients, the lift coefficient CL and the drag coefficient CD: dL
1 UW 2 dACL 2
[5.7]
dD
1 UW 2 dACD 2
[5.8]
with:
DA = l(r)dr
area of the blade section,
L (r)
length of the cord times the distance r from the axis of rotation,
CL
lift coefficient (dimensionless),
CD
drag coefficient (dimensionless).
These coefficients CL and CD are highly dependent on the angle of incidence
D (Figure 5.7). For low D angles, the airflow along the blade is laminar and is
faster on the top surface than on the bottom. The resulting low pressure on the top surface creates lift. It is this force that raises an airplane and allows it to fly. Here it propels the blade forward. If angle D increases, the lift increases up to a certain
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point and then the flow becomes turbulent. The resulting lift resulting from the low pressure on the top surface disappears. This phenomenon is called aerodynamic stall.
Figure 5.7. Variation of the coefficients of lift and thrust with the angle of incidence
However, it is not only lift and stall that blade designers are concerned about. They also pay a lot of attention to air resistance, also known in aerodynamics as drag. In general, the drag increases if the area exposed to the direction of airflow increases. Here, this phenomenon will appear for large angles D . 5.3.2.2. Control methods at the turbine level As shown by the expressions for forces given previously, these forces increase rapidly with the apparent wind, and the corresponding power can quickly become greater than the rated power of the machine. Therefore, we must limit the torque. Regulation of the torque, and therefore the power captured by the turbine, is essentially accomplished by acting on the lift, which mainly depends on the angle of incidence D . As such, power control will be achieved by action on D . There are essentially two methods of control. They serve primarily to limit the power captured for strong winds, but one of them can also intervene to facilitate the rotation of the turbine.
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Figure 5.8. Methods of controlling the power captured by the turbine
5.3.2.2.1. Control by passive stall The angle of pitch, E , is fixed. The angle D naturally increases with the speed, V, of the incident wind if the rotational speed is nearly constant. This increase causes an increased drag (coefficient CD) and progressive stall of the blade (Figure 5.9). The torque is maintained roughly constant ( D1 D D 2 ) to total stall ( D 3 D ) (sharp drop in CL and rapid growth in CD), where it falls quickly. The power is thus successfully limited.
Advantages This simple and normally robust concept does not have an intervening mechanical or auxiliary electrical system.
Disadvantages The power captured by the turbine is just a function of the wind speed and the rotational speed which must remain nearly constant. There is therefore no ability to adapt. In the event of a fault in the network, if the energy captured cannot be transmitted, it is necessary to have sufficiently large brakes to absorb the kinetic energy of the turbine and energy captured during braking even if there is a transmission problem, which assumes a braking system on the shaft of the turbine itself (very high braking torque).
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Figure 5.9. Passive stall
Typically, manufacturers expect to be able to use the extremities of the blades as air brakes by having them pivot by 90° in an emergency. In this case, the mechanical braking system can be mounted behind the gearbox where the torque is lower, and it is used only as a “parking” brake. An emergency brake may also be provided by the generator provided that a resistive electrical energy recovery circuit is connected in an emergency (resistive brake). Furthermore, in normal operation the generator must be able to slow down the turbine and to impose stall when the wind speed increases. This may necessitate dimensions that are larger then those corresponding to the nominal conditions. The curve of variation of the wind power available on the total surface area swept out by the blades of the turbine is shown in Figure 5.10. This power is expressed by: Pv
1 U SbV 3 2
[5.9]
where:
U is the volumetric density of the air (1,225 kg/m3 at 15°C and 1,013 mbar), Sb is the surface swept by the blades (m2), V is the filtered wind speed at the hub of the turbine or average speed of the wind through the surface Sb (m/s).
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Figure 5.10. Typical power curve under passive stall (Pn = 660 kW)
The power captured by the turbine can be expressed as a function of this available power by introducing a factor dependent on the aerodynamic conditions of the blades (that is the wind speed V and the rotational speed ȍT), which could be calculated by conducting a full study on the effect of the wind on the blades: PT
O
1 U SbV 3CP O 2
:T RT V
[5.10]
[5.11]
with
Cp
power coefficient (dimensionless),
O
speed ratio (dimensionless),
ȍT
rotational speed of the turbine (rad/s),
RT
radius of the turbine (m).
5.3.2.2.2. Pitch control The angle D may be strongly reduced (or increased) by rotation of the blades by hydraulic or electrical actuators, therefore by increasing (or decreasing) the pitch angle E by a few tens of degrees (generally 20 to 30°). The aerodynamic forces acting on the blades are therefore reduced (lift and drag). The torque can be kept almost constant and can be canceled by unfurling the blades like a flag ( E = 90°). In this way, the power may be limited to its nominal value.
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Advantages The decrease in the angle of incidence, D , to zero or negative values reduces all the aerodynamic forces on the blades, which greatly reduces the force due to high wind speeds. As the strength of axial thrust is also diminished, forces on the tower are reduced. This advantage is further amplified by the fact that we can operate at variable rotation speeds, and by the fact that the excess energy during a gust, in which the change was too sudden for the mechanism control to offset, can be stored in the inertia of the rotor by varying its speed (if the generator allows), while the power transmitted remains practically constant. The mechanical brake is therefore a “parking” brake.
Figure 5.11. Stall by controlling the pitch angle
Disadvantages The energy required for actuators must be transmitted to the hub of the turbine. If the actuators are electric, carbon brushes and slip rings are required, which are subject to wear and require maintenance. As before, power captured by the turbine can be expressed as a function of the available power by introducing a factor depending on the aerodynamic conditions of the blades, therefore of the wind speed V, the rotational speed ȍT and the pitch angle E , where CP is a dimensionless power coefficient: PT
1 U SbV 3CP O , E 2
[5.12]
The coefficient Cp passes through a maximum for E = 0° and a particular value of the speed ratio O opt. For values of variable wind velocity such that PT remains below Pn, the rated power (zone 1 in Figure 5.12), it is possible to impose a variation of the rotational speed ȍT such that O remains equal to O opt and the captured power is at its maximum value.
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, ,
,
,
Figure 5.12. Typical power curves with pitch angle control
Figure 5.13. Power curves with pitch angle control
5.3.3. Mechanical system – transmission of the power
The power captured by the turbine is transmitted to the generator. If the generator is a conventional electric machine, a gearbox is inserted into the transmission to change the speed. The gear, in addition to adjusting the speed, introduces a certain elasticity between what happens at the input (fluctuating primary source) and the output (generator and network). Mechanical oscillations may result, as we will see later. The gearbox introduces problems of weight (about 19 metric tons for NORDEX’s 500 kW N80-2 turbine), and most of the maintenance issues can be attributed to this. Some manufacturers have therefore sought to eliminate the gearbox in the so-called direct drive systems, or even just make it smaller [BOH 03, LIL 04]. This then requires a specific electrical generator capable of turning at the same speed as the turbine and thus having a large number of pole pairs.
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The complete mechanical system therefore includes the following: – the blades (usually three in number), – the hub on which the blades are mounted, with the possibility of rotation where the angle of pitch is adjustable, – a slow shaft on which the hub is fixed, – the gearbox (except for direct drive systems) of ratio N, – a rapid shaft, where there is a gearbox, which is coupled to the rotor of the generator. The model will be different depending on whether the structure has a gearbox (MAS and MASDA structures) or not (MS structure). It has been highlighted, particularly in constant speed wind turbine systems, that there is an interaction between the mechanical system (turbine, transmission, rotor assembly) and the electricity network [SAL 03]. This interaction plays an important role in the stability of the system during severe transient events such as the loss and reestablishment of the voltage due to a fault on the network and manifests itself in the form of a torsional oscillation of the transmission system, especially in the presence of a gearbox. The great apparent flexibility the transmission is largely the consequence of the difference in speed between the slow shaft that supports the hub of the turbine and the fast shaft to which the generator rotor is coupled, due to the high ratio (50 to over 100, depending on the rated power of the system). The other causes of flexibility are the consequences of the architecture of the transmission itself: its mounting on a gearbox with flexible “suspension” using shock absorbers [HAU 00] to reduce the transmission of vibration and noise, therefore, softly mating on the rotor fast shaft to mitigate the transmission of noise and tolerate a very slight misalignment of the gears. The structural and dynamics properties of the transmission are also used to ensure mechanical damping of forces exerted on the turbine if there is a fault in the electrical system. The characteristics of the “flexible” transmission generally include everything between the blade system, which represents the bulk of the inertia of the rotating system, and the generator rotor. It is therefore necessary to model everything. Some authors [LED 01, ROD 00] have proposed models separating each of the blades, the hub with the slow main shaft and the gearbox, and the fast shaft. A simplified model recombines the blades of the turbine by making them into an equivalent single mass. To further simplify matters, it can be assumed that the attachment between the blades and slow shaft is rigid and can be represented by a single mass. This produces a model with two masses, the most widely used in the literature [LED 01, PET 01]. It has been demonstrated that this model was sufficient to represent the behavior of a wind turbine in a transient state [LED 01, SAL 03].
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N Figure 5.14. Two mass mechanical model
The equations describing the model are as follows: CT
JT
d ZT DT :T Ctors N dt
Ctors
Kt NTT TG De N :T :G
Ctors
JG
Ttors
N TT T G
d ZG DGZG CG dt
[5.13] [5.14] [5.15] [5.16]
When operating in a specific state, the transmission has an angular twist and stores potential energy: Wtors
1 K tTtors 2 2
[5.17]
The two inertias store kinetic energy: Wkin
1 1 J T :T2 J G :G2 2 2
[5.18]
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If there is sharp decline in the generator torque as a result of a fault in the electrical grid, for example, torsion reduces and some of the potential energy is converted into kinetic energy, which corresponds to an acceleration of the system, in particular the generator, which has the lower moment of inertia. Data for the mechanical transmission of a 660 kW turbine are presented in the appendix at the end of Chapter 6. The system has a resonant frequency almost the same as the oscillation frequency of the generator rotor: fOG
1 2S
Kt JG
2.39 Hz
[5.19]
The antiresonant frequency coincides almost exactly with the oscillation frequency of the turbine: fOT
1 2S
N 2 Kt JT
0.95 Hz
[5.20]
The resonant frequency is a little farther away at fr = 2.57 Hz. These characteristics can cause dangerous situations, particularly for variable speed systems, where the resonant frequency can be excited during particular operating conditions. 5.3.4. Controls at generator and transmission network levels – different types of wind power generator systems
5.3.4.1. Fixed speed systems – squirrel cage asynchronous machines We have previously seen that the simplest method of power control for the turbine, the passive stall, requires a constant rotation speed to operate properly. Wind systems characterized by a practically constant rotational speed, regardless of the wind speed, have therefore been developed. They are composed of asynchronous generators rotating above the synchronous speed : sync 1 s , where s is the slip of the machine (s < 0 in this case). This deviation is very small since the nominal slip of an asynchronous generator of 1.5 MW is about 1%. For reasons of size and cost, a normal machine is most often designed for a rated speed of 1500 rpm, which requires a gearbox with a high ratio of about 30 to 100 depending on the rating.
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Figure 5.15. Induction machine equivalent circuit
The squirrel cage induction machine can be represented by the classical equivalent circuit of Figure 5.15. The equations of the model referred to the stator and expressed in a stationary reference frame are written: dMs °°vs Rs is dt ® ° 0 R i dM R jZM R R R °¯ dt
[5.21]
with expressions for the flux linkage: °M s ® °¯M R
Ls is Lm iR
[5.22]
Lm is LR iR
In these expressions, vs , is , iR , M s , M R are rotating vectors (phasors) representing the magnitude of the system of three-phase quantities and are defined by the transformation:
X
2S 4S 2 ª j3 j3 ,e < «1, e 3 « ¬
ªx º º « a» » < « xb » »¼ « » ¬ xc ¼
[5.23]
The expression for the electromagnetic torque is obtained from a balance of active power: * elm
p p I m Im ª is < iR º ¬ ¼
where Im is the imaginary part of the phrase in brackets.
[5.24]
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Using the results from the model, the expression for the torque is obtained as: 2
* elm # p
ZRW R 1 § Lm · ¨ ¸ LR © Ls ¹ 1 ZRVW R 2
§ vs · ¨ ¸ © Zres ¹
2
[5.25]
with
vs
Zres
# M s , ZR
Zres p<:G
g
LR RR
[5.26]
The mechanical power input to the machine is expressed as: Pm
:G <* elm # :G
[5.27]
The power transmitted to the stator is written: Pts
ZRes p
*elm # Ps
[5.28]
For small values of slip corresponding to super-synchronous operation: 2
§ L · 1 § vgrid p¨ m ¸ ¨ © Ls ¹ RR ¨© Z grid
* elm
2
· ¸ sZ grid with s 0, s max # 1% ¸ ¹
[5.29]
The power dissipated in the rotor is: PR
s
ZRes p
* elm
[5.30]
This structure is commonly called the “Danish design”. It is presented in Figure 5.16. It is particularly simple and robust. Note the presence of the gearbox between the turbine and generator and the possibility of a control system for adjusting the angle of the blades. Here the network connection is classified as direct. This is, in effect, normal operation. To reduce the high magnetization current drawn when connecting to the system, a static AC power converter is used in order to gradually bring up the
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terminal voltage of the machine. It is then short-circuited. There is also a capacitor bank for reactive power compensation to maintain the magnetization of the machine. At present, this configuration is only offered by the German manufacturer Siemens (Bonus) [EIS] and the French manufacturer Vergnet [MO], and as “entry level products” by a few other companies (Nordex, Made, Mitsubishi, etc.).
Figure 5.16. Asynchronous machine configuration
Advantages The advantages are: – a standard robust machine, – low cost, – no power electronics for the connection.
Disadvantages The disadvantages are: – captured power not optimizable, – noise and maintenance of the gearbox, – no reactive power control, – no control of the magnetization of the machine in the event of default on the network.
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5.3.4.2. Variable speed systems We have previously seen that the method of power control by adjusting the angle of the blades allows operation at variable rotational speeds. Variable speed wind turbine systems will help to optimize the conditions for operation of the turbine. Various structures are proposed. 5.3.4.2.1. Variation of 10% to 16% above synchronous speed – asynchronous machine control using an external rotor resistance The rotor contains a three-phase insulated winding. A control resistance with a power electronic interface is built into the rotor of the machine. The control of this interface provides a variable rotor resistance. This has the effect of allowing a variation in the velocity of the order of 10% above synchronous speed, significantly increasing the energy captured and reducing the effects of power oscillations caused by fluctuations of wind speed. The machine model is identical to that of the squirrel cage machine, the resistance of the rotor being increased by RC, the value of the control resistance carried by the rotor. For small values of slip above synchronism, the torque is expressed as: 2
2
§L · § vRes · 1 p¨ m ¸ ¨ ¸ sZRes , s max # 10% to 16% © Ls ¹ RR RR © ZRes ¹
* elm
[5.31]
The power dissipated in the rotor is increased: PR
s
ZRes p
* elm
[5.32]
This structure retains the same disadvantages as the asynchronous configuration. Nevertheless, we can consider it as a forerunner of modern variable speed technologies. This structure has been proposed by the Danish manufacturer Vestas (Optislip system [VES]) for its entry level product and is still offered by the Indian manufacturer Suzlon [SUZ].
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Figure 5.17. Asynchronous machine with rotor resistance configuration
The network connection is the same as that of the fixed speed system with a squirrel cage generator.
Advantages The advantages are: – variable speed operation (10% to 16% or more above synchronism), – robustness, – low power electronic converter.
Disadvantages The disadvantages are: – captured power is not optimizable, – noise and maintenance of the gearbox, – no reactive energy control, – magnetization of the machine not controllable in the event of a network fault.
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5.3.4.2.2. Variation of 30% above synchronous speed – doubly fed asynchronous machine In this case, the stator of the generator is directly coupled to the network and the rotor with its three-phase winding connected to the same network through slip rings with a power electronic interface and generally a transformer. The doubly fed wound rotor induction machine can be represented by the equivalent circuit of Figure 5.18.
v1
LV 1
R1
i1
m dT j
dt
M1
dM1 dt
LV 2 L'm
R2
i2
dM 2 dt
v2
Figure 5.18. Equivalent circuit of doubly fed wound rotor induction motor
The model referred to the rotor in the rotor reference frame is written: °°v1 ® °v °¯ 2
dM1 dT j M1 dt dt dM R2 i2 2 dt
R1 i1
[5.33]
The flux linkage expressions are: °M1 ® °¯M2
L1 i1 Lmc i2
[5.34]
Lmc i1 L2 i2
The stator quantities come from the corresponding rotor values as:
M1
1 M s e jT , v1 m
1 vs e jT , i1 m
mis e jT
where m is the turns ratio between rotor and stator windings.
[5.35]
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The expression for electromagnetic torque is obtained from writing the balance of real power: * elm
pLmc I mag ª i1 < i2 º ¬ ¼
[5.36]
We make a change of reference by a simple rotation. This redefines the model in a rotating reference frame called (d, q) as in the case of the synchronous machine.
Figure 5.19. Location of axes for dq study
We define the position of the rotating reference frame with respect to the rotor:
T RT
D R DT
[5.37]
We then have:
M1 dq M1 rotor e jT
RT
and M1 rotor
M1 dq e jT
[5.38]
RT
The model in the rotating reference frame is now written as: °°v1 ® °v °¯ 2
dM1 jZresM1 dt with Z dM 2 jZRM 2 R2 i2 dt
R1 i1
p:G and Z ZR
Zres
[5.39]
The rotor oscillations are defined as follows:
ZR
dT RT dt
[5.40]
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We choose the reference in the direction of the stator flux
127
M1 . We retain only the
two variables i2 and M1 expressed as: i1
1 M1 Lmc i2 and M2 L1
V
1
Lmc M1 V L2 i2 L1
[5.41]
with Lmc2 L1 L2
In order to be able to use this model, we project these vector equations onto the d and q axes. The general expression is written as: x
X d jX q
[5.42]
The reference frame is linked to a rotating vector. It rotates at a synchronous speed and at steady state all vectors are fixed. Their components are constants. In transient operation the locations of the variables change relative to each other and their components Xd and Xq are known values. Orienting the stator flux vector along the d axis allows us to cancel the component of flux along the q axis:
I1d
M1
I1q
0
[5.43]
The equations are then written: °V2 d ° ® °V ° 2q ¯
°V1d ® °V1q ¯
R2 I 2 d V L2 R2 I 2 q V L2
R1 L1d
dI1d dt
R1 L1q ZresI1d
dI 2 d Lc dI ZR V L2 I 2 q m 1d dt L1 dt dI 2 q dt
ZR V L2 I 2 d
Lc ZR m I1d L1
[5.44]
[5.45]
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I1d I1q
Lmc I 2 d L1 I1d Lc m I 2q L1
[5.46]
The expression for electromagnetic torque becomes: * elm
p
Lmc I 2q L1
[5.47]
We can note that at steady state we have: V1d # 0 V1q # ZresI1d # v1
[5.48]
The equations of the rotor show in the two cases the terms introduced by the coupling and the back emf. These perturbation terms are continuous and are proportional to ZR .
Principles of vector control The previous equations show that the principal stator quantities can be controlled via the rotor currents for the case in which we can make the approximations:
P1 # V1q I1q #
L'm V1q I 2 q L1
2 1 V1q L'm Q1 # V1q I1d # V1q I 2 d L1 Zres L1
[5.49]
with
I1d # V1q / Zres ZR
Zres p:G
[5.50]
It can be seen that the torque and the stator active power are controlled by the current I2q, while the stator reactive power can be controlled by the current I2d.
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The mechanical power absorbed by the machine is expressed as: Pm
:G * elm # :G CG
[5.51]
The possibility of supplying the rotor enables it to operate with a much higher slip than in the previous configurations where the rotor power rotor could only be dissipated and the machine can operate as a generator both above and below synchronism if the excitation to the rotor is reversible (Figure 5.20): P1 #
ZRes p
* elm , P2 # s
ZRes p
* elm with s # r30%
[5.52]
For the vast majority of wind turbines proposed on the market, this interface is now composed of two voltage source pulse width modulated inverters with switches that can be controlled to open or close (usually these are IGBTs) and which can also work as a rectifier [BHO 99, ROD 02].
Figure 5.20. Regions of operation of a doubly fed induction generator in a wind power system
The general configuration and control scheme for the doubly fed induction generator is presented in Figure 5.21. This structure is currently and aggressively under development by several wind turbine manufacturers. Among the principal companies on the market are Vestas [VES], GE Wind Energy [GE], Gamesa [GAM], Repower [RE] and Ecotecnia
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[ECO]. Some also offer high-end high-power models, such as the German manufacturer Nordex [NLD]. The lack of a squirrel cage reduces the electromagnetic and electromechanical damping phenomena and may increase the magnitude of the electric and magnetic oscillations in the rotor. Control of the rotor side inverter makes it possible to control the converted rotor power. The aim is to control the torque of the generator and thus obtain the desired rotational speed. The dynamics of electrical and mechanical quantities are very different; it is advantageous to control the machine by a multilevel structure with nested loops [PETE 03].
Figure 5.21. Control system for doubly fed induction generator
The torque and the flux are controlled through very fast internal current loops. This control is executed in a rotating reference frame [EHR 01, HOF 00, PEN 97]
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(flux oriented vector control [HOF 00, FOR 02, LED 01, RAM 02] or voltage vector control [PER 99]). Consequently, the torque control is usually achieved using a slower external loop. The target speed can be calculated from the wind speed to optimize operation at low and medium wind speeds and to obtain operation at constant speed (in general) for high wind speeds when the captured power is limited. Control on the inverter side of the network allows regulation of power transfer. This is to ensure the transfer of rotor power, controlling the DC bus voltage to a value ensuring the proper operation of the inverter. This control is achieved by control of the DC current in the inverter and the three-phase current. Voltage control provides a reference value of current from which we infer the correct three-phase (or transformed) current provided by current controllers [EHR 01, HOF 00, RAM 02]. It is therefore necessary to insert a three-phase inductor at the terminals of the inverter. A modification in the calculation of the reference currents allows for the integration of other functions: control of reactive power, active filtering [GUF 00]. This structure can thus provide reactive power either from the stator of the machine or at the interface of the inverter with the network.
Advantages The advantages are: – a standard machine, – variable speed operation (± 30% compared to synchronous speed), – optimization of captured power at low wind speed (zone 1 in Figure 5.12), – power electronics sized at 30% of nominal power, – connection of the machine which is easier to manage, – management of energy conversion and reactive power, – magnetization of the machine which is controllable in the event of a fault in the network.
Disadvantages The disadvantages are: – noise and maintenance of the gearbox, – contact slip rings/brushes to power the rotor winding, – the price of power electronics, – complex control.
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5.2.4.2.3. Change from 0 to 100% of the rated rotational speed – synchronous machine with wound or permanent magnet rotor (SM) Different types of synchronous machines can be applied in this configuration. Currently it is essentially different types of multipolar synchronous machines which are used to eliminate or reduce the size of the gearbox [BOH 03, LIL 04, TOU 00]. Manufacturers are now leaning toward synchronous machines with permanent magnets rather than the wound rotor type. As the frequency and magnitude of the voltage produced by this generator depend on its speed, the power electronics interface is inserted before connecting to the network.
Figure 5.22. Simplified equivalent circuit of the synchronous machine
A simplified vector model of a round rotor synchronous machine (non-salient pole) with a wound field or with magnets in a rotating reference frame is written: Vs
* elm
Rs is Ls
dM fs dis dT dT j Ls is j M fs dt dt dt dt
pIm ª is( s ) <M fs( s ) º ¬ ¼
[5.53]
[5.54]
Using a coordinate system whose real axis (d) is oriented in the direction of the vector M fs (axis of the rotor pole), when we project the vector equations on the d and q axes, we obtain the general relation: x
X d jX q
[5.55]
This allows us to obtain a model of the machine (the Park model), which takes into account the irregularities of the air gap (rotor salient poles in the case of a
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wound field machine, different magnet configurations) by the introduction of two stator inductors, Ld and Lq. In cases where M fs is constant, we can write: °°Vsd ® °V °¯ sq
dI sd dT Lq I sq dt dt dI sq dT dT Rs I sq Lq Ld I sd I fsd dt dt dt Rs I sd Ld
[5.56]
In this case the electromagnetic torque is obtained by a balance of real power as: * elm
p ª Ld Lq I sq I sd I fsd I sq º ¬ ¼
[5.57]
If the machine has a constant air gap, Ld synchronous inductance.
Lq
Ls where LS is the
The Park transformation allows us to easily switch between three-phase variables and transformed variables:
ªXd º « » «¬ X q »¼
ª xa º « » « xb » «¬ xc »¼
ª 2S · 2S · º § § cos ¨ T « cos T bcos ¨ T » xa ½ ¸ 3 ¹ 3 ¸¹ » ° ° 2« © © <® xb ¾ 3« 2S · 2S · » ° ° § § sin ¨ T « sin T sin ¨ T » x 3 ¸¹ 3 ¸¹ ¼ ¯ c ¿ © © ¬ ª « « cos T 2« § 2S « cos T 3 « ¨© 3 « § 2S « cos ¨ T 3 ¬« ©
sin T 2S · § ¸ sin ¨ T 3 ¹ © 2S · § ¸ sin ¨ T 3 ¹ ©
º » » ·» ª X d º ¸ »<« X » ¹ » ¬« q ¼» ·» ¸» ¹ ¼»
[5.58]
[5.59]
This is the position of the rotor relative to the stator (electrical angle). For the wind turbines offered on the market with permanent magnet generators, the interface now consists of two voltage source inverters with controlled turn-on and turn-off capabilities (usually these are IGBTs) and which may operate as a rectifier. For those offered with a wound field (and therefore the possibility of adjusting the magnitude of the back emf) and a cage damper winding, we can find on the machine
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side of the converter a structure involving a diode bridge rectifier and voltage chopper. In any case, there is no direct link between the generator and the electricity network which can ensure a decoupling between the behavior of the turbine– generator system on the one hand and the behavior of the network on the other hand. For wound field synchronous machines the existence of the excitation circuit requires an additional converter, and therefore a cost and additional losses [GRA 96]. Most of the time rotating exciters are an “inside-out” synchronous machine (stationary field winding and rotating three-phase winding and rectifier), which avoids the introduction of slip rings and brushes. The presence of a cage damper winding on the rotor allows for the protection of the field winding (electromagnetic screen), and improves the stability of the system but complicates modeling of the machine and analysis of its transient behavior. A synchronous machine with permanent magnets has no external excitation circuit. Its size can be reduced by using a smaller pole pitch. The lack of a damper winding causes some stability problems during overloads or transients. The modeling is very simple. This structure is currently less popular than the doubly fed induction machine. For several years the German manufacturer Enercon [ENE] has been providing a full range of wind turbines with synchronous generators with multipolar rings and direct drive (large diameters), and with wound field and diode rectifiers. The French manufacturer Jeumont Industrie [JEU] proposed an original solution with a disk shaped permanent magnet generator with direct drive and a pulse width a modulated rectifier. The Japanese manufacturer Mitsubishi [MIT] provides a line of wind turbines with permanent magnet generators and direct drive. The Finnish manufacturer Winwind [WIN] offers permanent magnet generators with a gearbox of low ratio (~ 5 or 6) with a reduced footprint. The Spanish manufacturer Made [MAD] offers a line of wind turbines with brushless synchronous generators and the American manufacturer GE Wind Energy [GE] offers a range of wind turbines with permanent magnet synchronous generators, in both cases with low numbers of pole pairs and a gearbox. The general control structure for a permanent magnet synchronous machine is presented in Figure 5.23. The control of the rotor side converter (here an inverter) allows control of the converted power. The dynamic torque control of the synchronous machine by control of the PWM converter need not necessarily require a control algorithm based on a rotating reference frame. It is often the real three-phase currents that are
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controlled. In the case of configurations with diode rectifiers and choppers, it is the rectified current which is controlled. The control of the grid-side inverter allows control of the transfer of power. It is clear that this control (transfer of the active power and control of the DC bus voltage) is generally identical in principle and configuration to that of the doubly fed asynchronous machine. Control of the reactive power supplied to the network will always be present a priori in this structure at this level.
Figure 5.23. Control of permanent magnet synchronous machine
Advantages The advantages are: – variable speed operation over the entire speed range, – management of energy conversion and reactive power, – optimized power capture at low wind speed (zone 1 in Figure 5.12), – easily controlled connection of the system, – the possibility of decoupling between the behavior of the turbine and the network, – the possibility of the absence or reduction of the gearbox.
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Disadvantages The disadvantages are: – usually a special machine, – a large diameter machine in some cases, – power electronics sized to 100% of power rating, – the price of power electronics and of the machine. There are many points in common between the two families of variable speed machines with regard to control and management. They concern different regulation loops (current control, DC bus voltage control, speed control of the generator, control of the reactive power exchanged with the network) and the system of supervision (management of start-up and shut-down, management of output depending on the wind). 5.4. Bibliography
Bibliographic references [BEL 01] BELHOMME R, NASLIN C., G. BESLIN, ALBRIEUX N., “Wind Farms and Networks – Main Technical Issues”, International Conference on Power Generation and Sustainable Development, Liege, October 2001. [BEL 04] BELHOMME R., CORENWINDER C., “Wind Power Integration in the French Distribution Grid Network Regulations and Requirements” NWPC’04, Goteborg, March 2004. [BEL 05a] BELHOMME R., BOUSSEAU P., NAVARRO E., BADELIN A., DEGNER T., ARNOLD G., BERENGUER A., CHOCARRO I., MATERAZZI-WAGNER C., WHITE S., HATZIARGYRIOU N., TSELEPIS S., NERIS A., LEFEBVRE D., “Case Studies on the Integration of Renewable Energy Sources into Power Systems,” 10th Kasseler Energy Systems Symposium Technology 2005, Kassel, Germany, 10/11 November 2005. [BEL 05b] BELHOMME R., WHITE S., HATZIARGYRIOU N., TSIKALAKIS A., LEFEBVRE D., RANCHIN T., FESQUET F., ARNOLD G., TSELEPIS S., NERIS A., DEGNER T., TAYLOR P., “Deliverable D7.3-Distributed generation on European islands and weak grids,” Project European DISPOWER, available D7.3, Document No. Del_2005_0079, www.dispower.org, December 2005. [BOH 03] BOHMEKE G., “Development and operational experience of the wind energy WWD-1 converter”, EWEC 2003 Conference, Madrid, in June 2003. [BOL 03] BOLIK, SM, R&D DEPARTMENT VESTAS WIND SYSTEM, “Grid requirements: challenges for wind turbines”, Fourth International Workshop on Largescale Integration of Wind Power and Transmission Networks for Offshore Wind, Billund, Denmark, October 2003.
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[BOU 03] BOUSSEAU P., GAUTIER E., GARZULINO I., JUSTON P. H., BELHOMME R., “Grid impact of different technologies of wind turbine generator systems”, 2003 Conference, Madrid, in June 2003. [BOU 04] BOUSSEAU P., FESQUET F., BELHOMME R., NGUEFEU S., THAI T.-C., “Solutions for the grid integration of wind farms – a survey”, Proceedings of EWEC 2004 (European Wind Energy Conference), London, 22/25 November 2004. [BOU 06] BOUSSEAU P., BELHOMME R., MONNOT E., LAVERDURE N., BOËDA D., ROYE D., BACHA S., “Contribution of Wind Farms to Ancillary Services”, CIGRE Session 2006, Paper C6-103, Paris, 27 August/1 September, 2006. [BUD 00] BUDINGER M., LERAY D., DEBLEZER Y., “Eoliennes et vitesse variable”, La Revue 3EI N°20, March 2000. [CAI 04] CAIRE R., Gestion de la production décentralisée dans les réseaux de distribution, PhD Thesis, Institut National Polytechnique de Grenoble, April 2004. [CAN 00] CANARD J. F., Impact de la génération d’énergie dispersée dans les réseaux de distribution, PhD Thesis, Institut National Polytechnique de Grenoble, December 2000. [CEI 01] COMITÉ ELECTROTECHNIQUE INTERNATIONAL, “Aérogénérateurs – Partie 21: Mesurage et évaluation des caractéristiques de qualité de puissance des éoliennes connectées au réseau”, Norme Internationale CEI61400-21, 1st edition, December 2001. [CER 04] CER COMMISSION FOR ENERGY REGULATION, “Wind Farm Transmission Grid Code Provisions”, CER/ 04/237, Ireland, July 2004. [CON 05] CONSORTIUM DEWI/E.ON NETZ/EWI/RWE TRANSPORTNETZ STROM/VE TRANSMISSION, “Energiewirtschaftliche Planung für die Netzintegration von Windenergie in Deutschland an Land und Offshore bis zum Jahr 2020”, Deutsche Energie-Agentur GmbH (DENA), www.dena.de, March 2005. [CRU 01] CRUZ I., AVIA F., ARIAS F., ARRIBAS L. M., FIFFE R.P., CIEMAT-DER, “Assessment of different energy storage systems for wind energy integration”, EWEC 2001, Copenhagen, July 2001. [DIT 03] DITTRICH A., “Grid voltage fault proof doubly fed induction generator system”, EPE 03, Toulouse, 2003. [EDF 04] EDF, “GESER – génération d’eau chaude synchronisée avec les energies renouvelables”, Journal “L’INDEPENDENT”, 16 May 2004. [EHR 01] EHRENBERG J., ANDRESEN B., REBSDORF A., “Digitally controlled wind turbines in megawatt size with doubly-fed induction generator without position sensor” Magazine Elektronik, 2001. [ELK 04] ELKRAFT SYSTEM, ELTRA, “Wind turbines connected to grids with voltages above 100 kV – technical regulation for the properties and the regulation of wind turbines”, Regulation TF 3.2.5 registered with the Danish Energy Authority, December 2004. [EON 03] EON NETZ, “Grid code for high and extra high voltage”, EON Netz GmbH, www.eon-netz.com, August 2003.
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[ERI 03] ERINMEZ I. A., on behalf of CIGRE WG B4.19, “Static Synchronous Compensator (STATCOM) for Arc Furnace and Flicker Compensation”, Electra, N° 211, p 58–65, 2003. [EWE 06] EWEA – THE EUROPEAN WIND ENERGY ASSOCIATION, “Europe’s energy crisis – the no fuel solution”, EWEA Briefing 2006, www.ewea.org, February 2006. [FAG 05] FAGAN E., MCARDLE J., SMITH P., STRONGE M., “Development of grid code provisions for wind generators in Ireland”, CIGRE Symposium on Power Systems with Dispersed Generation, Paper No. 003, Athens, April 2005. [FEL 02] FELD G., LAVABRE M., “Conversion d’énergie à partir d’une éolienne en vitesse variable”, La Revue 3EI N° 31, December 2002. [FOR 02] FORCHETTI D., GARCIA G., VALLA M. I., “Vector control strategy for a doubly-fed stand-alone induction generator”, IECON 2002 Conference Proceedings, 2002. [GUF 00] GUFFON S., Modélisation et Commandes à Structure Variable de Filtres Actifs de Puissance, PhD Thesis, INP Grenoble, 2000. [HAR 03] HARTGE S., DIEDRICHS V., “Ride-through capability of Enercon wind turbines”, Fourth International Workshop on Large-scale Integration of Wind Power and Transmission Networks for Offshore Wind, Billund, 20/21 October 2003. [HAU 00] HAU E., Wind Turbines: Fundamentals, Technologies, Application, Economics, Springer, Berlin, 2000. [HEI 98] HEIER S., Grid Integration of Wind Energy Conversion Systems, John Wiley & Sons, New York, 1998. [HOF 00] HOFMANN W., RABELO B., “Optimised power flow on wind power plants with the doubly-fed induction generator”, PEMC 2000 Conference Proceedings, 2000. [HOP 01] HOPFENSPERGER B., ATKINSON D. J., “Doubly-fed AC machines: classification and comparison”, EPE 2001 Conference Proceedings, 2001. [JAU 04] JAUCH C., SØRENSEN P., BAK-JENSEN B., “International Review of Grid Connection Requirements for Wind Turbines”, Proceedings of NWPC’04, Gothenburg, ¼ March 2004. [KAL 06a] KALSI S. S., MADURA D., ROSS M., INGRAM M., BELHOMME R., BOUSSEAU P., ROGER J.-Y., “Operating experience of a superconductor dynamic synchronous condenser”, Session CIGRE 2006, Paris, 27 August/1 September, 2006. [KAL 06b] KALSI S., ROSS M., FESQUET F., BOUSSEAU P., ROGER J.-Y., BELHOMME R., “Enhancement of a wind-farm electrical system with a Superconducting dynamic synchronous condenser”, European Wind Energy Conference (EWEC), Athens, 27 February/2 March 2006. [KEY 04] KEY T., “Wind Power Integration Technology Assessment and Case Studies”, EPRI Technical Report, 2004.
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[LAV 02] LAVERDURE N., BACHA S., ROYE D., RAISON B., DUMAS F., “Elements of modelling of wind power systems with energy management – two structures in comparison”, IECON 02, Seville, 2002. [LAV 04] LAVERDURE N., ROYE D., BACHA S., BELHOMME R., “Mitigation of voltage dips effects on wind generators”, EWEC 2004 Conference, London, 22/25 November 2004. [LAV 05] LAVERDURE N., Sur l’intégration des générateurs éoliens dans les réseaux faibles ou insulaires, PhD Thesis, Institut National Polytechnique de Grenoble, 9 December 2005. [LED 01] LEDESMA P., Analisis dinamico de sistemas electricos con generacion eolica, PhD Thesis, University of Leganes, Spain, 2001. [LIL 04] LILJA M., RISSANEN I., “Multibrid low speed synchronous generator with frequency converter”, NWPC 2004, Gothenburg, March 2004. [LUN 01] LUNDSAGER P., BINDNER H., CLAUSEN N.-E., FRANDSEN S., HANSEN L. H,. HANSEN J. C., “Isolated systems with wind power”, Main Report, Risø National Laboratory, Roskilde, 2001. [MAR 04] MARVIK I., BLORGUM T., INAESS B., UNDELAND T., GJENGEDAL T., “Control of a wind turbine with a doubly-fed induction generator after transient failures”, Nordic Workshop on Power and Industrial Electronics (NORPIE 04), Trondheim, 14/16 June 2004. [MAT 04] MATEVOSYAN J., ACKERMANN T., BOLIK S., SÖDER L., “Comparison of international regulations for connection of wind turbines to the network”, Proceedings of NWPC’04, Gothenburg, 1/4 March 2004. [MIL 03] MILLER N. W., “Power system dynamic performance improvements from advanced control of wind turbine-generator”, Fourth International Workshop on Largescale Integration of Wind Power and Transmission Networks for Offshore Wind, Billund, 20/21October, 2003. [MIN 03a] MINISTERE DE L'ECONOMIE, DES FINANCES ET DE L'INDUSTRIE, “Décret n° 2003-229 du 13 mars 2003 relatif aux prescriptions techniques générales de conception et de fonctionnement auxquelles doivent satisfaire les installations en vue de leur raccordement aux réseaux publics de distribution”, Journal Officiel de la République Française, No. 64, p. 4589–4592, http://www.legifrance.gouv.fr, March 2003. [MIN 03b] MINISTERE DE L'ECONOMIE, DES FINANCES ET DE L'INDUSTRIE, “Arrêté du 17 mars 2003 relatif aux prescriptions techniques de conception et de fonctionnement pour le raccordement à un réseau public de distribution d’une installation de production d’énergie électrique”, Journal Officiel de la République Française, No. 93, p. 7005–7008, http://www.legifrance.gouv.fr, April 2003. [MIN 03c] MINISTRE DELEGUE A L’INDUSTRIE, “Arrêté du 22 avril 2003 modifiant l’arrêté du 17 mars 2003 relatif aux prescriptions techniques de conception et de fonctionnement pour le raccordement à un réseau public de distribution d’une installation de production d’énergie électrique”, Journal Officiel de la République Française, p. 7904, http://www.legifrance.gouv.fr, May 2003.
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[MIN 03d] MINISTERE DE L'ECONOMIE, DES FINANCES ET DE L'INDUSTRIE, “Décret n° 2003-588 du 27 juin 2003 relatif aux prescriptions techniques générales de conception et de fonctionnement auxquelles doivent satisfaire les installations en vue de leur raccordement au réseau public de transport de l'électricité”, Journal Officiel de la République Française, No. 151, p. 11110–11113, www.legifrance.gouv.fr, July 2003. [MIN 03e] MINISTERE DE L'ECONOMIE, DES FINANCES ET DE L'INDUSTRIE, “Arrêté du 4 juillet 2003 relatif aux prescriptions techniques de conception et de fonctionnement pour le raccordement au réseau public de transport d'une installation de production d'énergie électrique”, Journal Officiel de la République Française, No. 201, p. 14896–14902, www.legifrance.gouv.fr, August 2003. [MUL 98] MULTON B., “L’énergie sur la terre: ressources et consommation. Place de l’énergie électrique”, La Revue 3EI N° 14, p 29–38, September 1998. [NAV 05] NAVARRO E., BADELIN A., SCHLÖGL F., ROHRIG K., MACKENSEN R., ARNOLD G., CHOWDHURY M. H., TAMZARTI A., BELHOMME R., BOUSSEAU P., GAUTIER E., BERENGUER A., CHOCARRO I., MATERAZZI-WAGNER C., HASLINGER M., TRAJANOSKA B., POSPISCHIL W., “Deliverable 7.2. DG in European interconnected grids”, Projet européen DISPOWER, Livrable D7.2, Document No. del_2005_0071, www.dispower.org, December 2005. [NII 04] NIIRANEN J., “Voltage Dip Ride Through of a Doubly-Fed Generator Equipped with an Active Crowbar”, Nordic Wind Power Conference, Chalmers University of Technology, Gothenburg, 1/2 March 2004. [OHR 03] ÖHRSTRÖM M., SÖDER L., “The use of fault current limiters as an alternative to substation upgrade when there is a need to increase the available short-circuit power”, Fourth International Workshop on Large-scale Integration of Wind Power and Transmission Networks for Offshore Wind, Billund, October 2003. [PED 06] PEDROSA E., “Gamesa eolica technology – latest developments”, EWEC 2006, Athènes, 27 February/2 March, 2006. [PEN 97] PENA R., CLARE J. C., ASHER G. M., “Implementation of vector control strategies for a variable speed doubly-fed induction machine for wind generation system”, EPE 1997 Conference Proceedings, 1997. [PER 99] PERESADA S., TILLI A., TORRIELLI A., “Robust output feedback control of a doubly-fed induction machine”, IECON 1999 Conference Proceedings, 1999. [PER 04] PERDANA A., CARLSON O., PERSSON J., “Dynamic response of grid-connected wind turbine with doubly-fed induction generator during disturbances”, Nordic Workshop on Power and Industrial Electronics (NORPIE 04), Trondheim 14/16 June 2004. [PETE 03] PETERSSON A., Analysis, Modeling and Control of Doubly-Fed Induction Generators for Wind Turbines, Department of Electric Power Engineering, Chalmers University of Technology, Gothenburg, 2003 [PETR 03] PETRU T., Modeling of Wind Turbines for Power System Studies, Department of Electric Power Engineering Chalmers, University of Technology, Gothenburg, 2003.
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[PUL 00] DE BATTISTA H., PULESTON P. F., MANTZ R. J., CHRISTIANSEN C. F., “Sliding mode control of wind energy systems with DOIG power efficiency and torsional dynamics optimization”, IEEE Transactions on Power Systems, vol. 15, no. 2, May 2000. [RAM 02] RAMOS C. J., MARTINS A. P., ARAUJO A. S., CARVALHO A. S., “Current control in the grid connection of double-output induction generator linked to a variable speed wind turbine” IECON 2002 Conference Proceedings, 2002. [RED 04] REDDY N., “D-VAR and SuperVAR technology presentation”, American Superconductor, Step Group Meeting, San Diego, March 2004. [ROD 00] RODRIGUEZ AMENEDO, J. L., Analisis Dinàmico y Diseño del Sistema de Control de Aeroturbinas de Velocidad Variable con Generador Asincrono de Doble Alimentacion, PhD Thesis, Madrid, 2000. [ROD 02] RODRIGUEZ-AMENEDO J. L., ARNALTE S., BURGOS J. C., “Automatic generation control of a wind farm with variable speed wind turbines”, IEEE Transactions on Energy Conversion, vol. 17, no. 2, June 2002. [RTE 05] RTE, “Référentiel technique de RTE”, Indice 1, www.rte-france.com, June 2005. [SAL 03] SALMAN S. K., TEO A. L., “Windmill modeling consideration and factors influencing the stability of a grid-connected wind power-based embedded generator”, IEE Transactions on Power Systems, vol. 18, N° 2, May 2003. [SCH 06] SCHELLINGS V., “GE Energy 2.X, field experience and improvements”, EWEC 2006, Athens, 27 February/2 March 2006. [SEI 05] SEI – SYSTEMES ENERGETIQUES INSULAIRES, Insertion d’éoliennes dans un réseau isolé, EDF, www.edfdistribution.fr, June 2005. [TOU 00] TOUNZI A., “Utilisation de l’énergie éolienne dans la production de l’électricité”, La Revue 3EI N° 20, March 2000. [VIT 04] VITHAYATHIL J., on behalf of CIGRE WG B4.35, “Thyristor controlled voltage regulators: non-conventional static reactive power regulation”, Electra, N° 212, p 74–83, 2004. [WWE 06] WWEA – WORLD WIND ENERGY ASSOCIATION, Worldwide wind energy boom in 2005: 58.982 MW capacity installed, Press Release, www.wwindea.org, March 2006.
Documentation and constructor websites [ENE] Documentation ENERCON: www.enercon.de [GAM] Documentation GAMESA: www.gamesa.es [GE] Documentation ENRON: www.wind.enron.com [IDS] Documentation IDS: “Converter systems for variable speed windpower plants from 600 kW to 2500 kW”: www.idsag.ch [JEU] Documentation JEUMONT INDUSTRIE: www.jeumont-framatome.com [MIT] Documentation MITSUBISHI: www.mhi.co.jp
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[MTO] Documentation MTORRES: www.mtorres.es [NORDEX] Documentation NORDEX: www.nordex-online.com [VES] Documentation VESTAS: www.vestas.com [WIN] Documentation WINWIND: www.winwind.fi [ZEP] Documentation ZEPHYROS: www.zephyros.com
Websites for the operators of French networks [ERD] EDF Distribution, EDF réseau de distribution: www.edfdistribution.fr. [RTE] RTE EDF Transport, Réseau de transport: www.rte-france.com.
Chapter 6
Integration of Wind Turbine Generators into the Grid
6.1. Connection to the grid The insertion of wind power, and of decentralized power in general, into transmission and distribution grids creates a certain number of problems and constraints because these grids were not initially designed to accommodate this kind of production. One example is seen in distribution networks that are essentially “passive” (having to do only with consumers) and where the power flow is unidirectional (from the high voltage grid, through the transformer to the consumer installations, passing along medium or low voltage arteries or terminals). With the arrival of decentralized production, the situation has changed, with fairly significant consequences that have left it to the people involved (the grid administrators and, depending on the country, governments, regulators, etc.) to define the needs or technical rules for connecting to the grids [BEL 01, BEL 04]. In France, for example, the broad outline of these connection rules is given in governmental decrees and ministerial orders [MIN 03a, MIN 03b, MIN 03c, MIN 03d, MIN 03e]. They are spelled out in technical reference books for transmission and distribution grid managers, as well as in contracts for access to the grid and the conventions of connection and use [ERD, RTE].
Chapter written by Régine BELHOMME, Daniel ROYE and Nicolas LAVERDURE.
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The most recent development in wind power has produced some constraints on grids specific to this type of production, due both to the uncertain and fluctuating nature of production (function of wind speed) and to the technologies used (power electronic converters, doubly fed asynchronous machines, etc.) These constraints have led to adapting of the connection rules or defining specific rules for this kind of production. Initially, as a result of its “capricious” character, wind power was placed under rules that were definitely less severe than those for other more classic means of production, and it even enjoyed a certain number of exemptions. However, with the growth in the rate of wind turbine energy’s penetration, the connection requirements have “toughened” and specific rules in this direction have appeared in different counties. In the following pages we will review different problems and constraints generated by the interfacing of wind power, while limiting ourselves to those that directly touch on the wind producer or wind power project developer. In fact, most of these problems are also encountered in decentralized production in general. In all cases, we will mention aspects specific to wind power and we will briefly compare the behavior of the different wind power technologies previously described. It goes without saying that the complete treatment of interfacing wind power into electric systems involves other considerations such as taking wind power into account in planning and installation of the grid, or in the development and use of other means of production (including centralized production). However, these issues are beyond the scope of this work and will not be covered here. 6.1.1. Voltage at the point of connection In general, there are requirements for the voltage at the point of connection depending on the power of the production installation. Table 6.1 illustrates the requirements in France. It gives the power limits of production installations for integration into low voltage and medium voltage (LV and MV) distribution networks and into high voltage (HV) transmission networks [MIN 03a, MIN 03b, MIN 03d, MIN 03e]. These requirements apply as much to wind power as to other means of production.
Integration of Wind Turbine Generators into the Grid Network BT
HTA HTB
Voltage Limits
Main Levels
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Power Limit
U d 1 kV (single-phase connection)
230 V
P d 18 kVA
U d 1 kV (three-phase connection)
400 V
P d 250 kVA
1 kV < U d 50 kV
15 kV, 20 kV, 33 kV
P d 12 MW
50 kV < U d 130 kV
63 kV, 90 kV
P d 50 MW
130 kV < U d 350 kV
150 kV, 225 kV
P d 250 MW
350 kV < U d 500 kV
400 kV
P > 250 MW
Table 6.1. Voltage levels at the point of connection in France
6.1.2. Currents in steady state A wind farm (whatever technology is used for the generators) injects active power into the grid at the point of connection. Therefore, the power flows are modified, as well as the currents in the equipment (lines, cables, etc.). The first requirement to be respected concerns the values of the currents in the steady state, which must not exceed the maximum admissible values for the different pieces of equipment in the grid, whatever the configuration, and the operating point of the wind farm and the electric system into which it is inserted. For high power wind farms or high rates of penetration (on a medium voltage grid, for example), instances of congestion can also appear at high voltage levels (the transmission grid, for example). The affected zone will be larger as the injected power is higher. 6.1.3. Short circuit currents In the case of a fault in the grid, depending on the technology being used, the wind farm can generate a short circuit current that will increase the short circuit current circulating in the grid and feeding the fault. Here the requirement is not to exceed either the maximum admissible values for each piece of equipment in the grid and the conductors, or the interruption capability of the protection equipment. Figure 6.1 [BOU 03] illustrates the contribution to short circuit current of different wind technologies. However, it does not represent new technologies like the doubly fed asynchronous machines and synchronous machines equipped with capabilities for regulating voltage sags discussed in section 6.1.6.
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Figure 6.1. Contribution of wind technologies to short circuit current
Squirrel cage doubly fed asynchronous machines (not equipped with the capability of regulating voltage sags) contribute to the short circuit current on the grid. In the example in Figure 6.1, for a fall in voltage of about 80%, they generate a short circuit current equal to more than three times their nominal current. However, this current decreases very rapidly (about 300 ms) to below nominal current. When they do have voltage sag regulating capabilities (LVRT – “low voltage ride through”), the contribution of doubly fed asynchronous machines to the short circuit current depends on the strategy of the LVRT used (see section 6.1.6). Thus they can contribute to it but limit the short circuit current they generate, or not contribute to it at all (disconnection of the stator).
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Depending on the control–command strategies of their power electronic interface, synchronous machines either do not contribute to the short circuit current or they contribute to the extent of the limitation of current implemented in the converters. The use of this type of machine therefore enables us to solve problems of short circuit current that are too high. In Figure 6.1, the current limitation is fixed at 1.2 times the nominal current. A study of the strategy employed and its impact on short circuit current is thus necessary for each machine by different manufacturers. 6.1.4. Voltage profile As already mentioned, connecting a wind farm modifies the power flow in the grid and thus the design or the voltage profile as well. In general, voltage increases at the point of interface as illustrated in Figure 6.2 for a wind farm connected at 20 kV on a distribution line 5 km from the HV/MV transformer [BOU 03].
Figure 6.2. Voltage profile on a distribution line
Thus, the requirement is to maintain voltage at every point within the limits of maximum and minimum admissible voltage, whatever the configuration and the operation point of the wind farm and the grid. This requirement must be guaranteed not only for the voltage level where the wind installation connects, but also at lower voltage levels. Besides reinforcing the grid, managing the reaction produced or consumed by the wind farm at the point of connection can provide a solution to voltage design problems. This will be discussed in section 6.1.5. Another solution consists of acting on the active power injected by the farm: reduction, or temporary or permanent limitation of active power, to limit the voltage rise [BEL 05b].
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6.1.5. Voltage quality By the nature of their primary energy (wind) and the technologies used, wind turbines are particularly likely to affect voltage quality in the grids that receive them. The impact generally depends on the “strength” of the grid, that is to say on its short circuit capability: the weaker it is at the point of connection, the more the voltage quality can be degraded. Voltage quality can thus become a critical point in certain grids. In order to allow grid managers to respect their commitments with respect to all the grid’s users, voltage quality must be preserved and the problems that affect it must be limited. 6.1.5.1. Slow variations in voltage The power produced by a wind turbine is a function of the wind speed and therefore varies along with it, causing “slow” variations of the voltage at the point of interface. Depending on the regulating capability of the different wind turbine technologies, these variations can be attenuated, indeed filtered (see section 6.1.7). 6.1.5.2. Sudden changes in voltage Sudden changes or rapid variations in voltage can occur at the time of connecting or disconnecting the wind turbines (and their transformers) to the grid, at the time of changing the means of connection (star-delta) of certain generators, etc. [BEL 01]. More exactly: – asynchronous machines, at the time of interfacing with the grid, absorb a considerable quantity of reactive power to magnetize the alternator. Therefore they generate a significant current called the starting current that causes a fall in voltage. To make up for this drawback, the builders equip these machines with current limiting systems (based on resistance or variable impedance) that intervene at the time of connecting and then are short circuited. For high power machines, the effectiveness of these systems can be limited and problems can arise for weak systems, but they can be resolved by choosing machines of lower power, – for doubly fed asynchronous machines and synchronous machines, the presence of power electronic converters makes a smooth connection possible and therefore does not lead to voltage surges, – whatever the kind of wind turbine technology, energizing the transformers generates a strong starting current at the time of connection for magnetization, and this causes a surge in voltage. On a strong grid (sufficient short circuit capability), the drop in voltage should not exceed regulated limits, but this is not always the case on a weak grid for high power wind turbines (and therefore equally high power
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transformers). Again the simplest solution is to choose wind turbines with weaker power, but this is not necessarily the best solution, especially at the economic and environmental levels. 6.1.5.3. Flicker Variations in voltage linked to variations in wind speed, successive starts and stops1, the shadow of the mast2 and the gradient of the wind3 can lead to the generation of flicker. Indications of the severity of flicker are PST (“probability short term”), a short term flicker amounting to 10 minutes, and PLT (“probability long term”), a long term flicker amounting to 120 minutes [CEI 01]. Flicker must be limited and the maximum amounts for the indicators are generally specified in the technical rules for connection. Among the different wind turbine technologies, squirrel cage asynchronous machines are the ones that pose the most problems in terms of flicker or fluctuations in voltage. Nevertheless, an action on the angle of attack, ß, can make it possible to reduce the flicker produced and thus improve the performances of this type of machines [BEL 05b]. For doubly fed asynchronous machines and synchronous machines, the presence of their power electronic converters makes it possible to a certain extent to “filter” the variations in power that are the source of the flicker or to achieve an adjustment of the voltage that reduces them [BEL 05b, LAV 05]. 6.1.5.4. Harmonics Electronic power switches are potential sources of harmonics. Depending on the technologies used, wind turbines may therefore generate harmonics. Thus, the risks of the emission of harmonics: – are very small for squirrel cage asynchronous machines directly connected to the grid, because they do not contain power electronics except at the time of interface with the grid when constant frequency voltage converters, eventually Triacs, can be used [LAV 05], – exist for doubly fed asynchronous machines and synchronous machines connected by a power electronic interface.
1 Notably if the wind speed falls below the start-up speed or if it exceeds the disconnecting speed. 2 Resulting from an oscillation in power linked to a repetitive passage of the blades in front of the mast (minimal power generated). 3 The wind speed generally increases with altitude, bringing an oscillation in power with the rotation of the blades (for a triple blade turbine, maximum power is generated when one of the blades is in the “high” position).
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In fact, the levels of harmonics depend on the technology of the switches used. The increasingly frequent use of forced switch converters, like the IGBT (insulated gate bipolar transistor), and modulation by the pulse width modulation (PWM) method cause practically no generation of low order harmonics (Figure 6.3). The frequencies generated are in the order of several kilohertz (switching frequency of the converter). However, this is not the case with the older technologies based on thyristors that generate low order harmonics (Figure 6.4).
Figure 6.3. Harmonic emissions: forced commuted converters [BEL 01]
Figure 6.4. Harmonic emissions: load commutated converter [BEL 01]
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Harmonic emission is therefore a point to consider, particularly on weak grids, and it must be limited. Maximum amounts are generally specified in the technical rules of connection. 6.1.5.5. Disturbances of signals transmitted on the grid Signals can be transmitted on grids for the purpose of communication and/or remote control. For example, in France, TCFM (centralized remote control with audible frequency) is used on the MV and LV grids to transmit the costs to multirate meters. The most used frequency for TCFM is 175 Hz. Decentralized production interface and therefore wind turbine interface can also modify the “impedances” of the grid and therefore are capable of disturbing the transmission of these signals. In order to guarantee the proper functioning of the applications that depend on it, it is necessary to verify the impact of the connection on the level of the transmitted signal and, should the case arise, a passive or active filter should be installed. For asynchronous machines, as much for squirrel cage machines as for doubly fed machines, the item to take into consideration is the asynchronous generator (along with its capacitors) directly connected to the grid. This type of machine is capable of disturbing signals transmitted on the grid and a case by case study is necessary because the risk also depends on the grid. For synchronous machines, the risk depends on the technology of the converters used. For forced commutated converters (IGBTs), their impedance can be considered as infinite and they should not disturb the rate signals. For load commutated converters assisted by the grid (thyristors), the impedance is not infinite. The risk of disturbance exists and also depends on the grid. A case by case study thus will be necessary. 6.1.6. Stability and protection design In electricity grids there are constant variations in voltage and frequency resulting from variations of loads, of generation, and of anticipated or unanticipated events. Most of the time these variations are minimal, but in certain cases they can definitely be more significant. Several “regimes” of functioning can thus be distinguished [MIN 03a, MIN 03b, MIN 03d, MIN 03e]: – the normal regime in the course of which the fundamental characteristics of the electric system remain within so-called normal limits; in particular the frequency and the voltage are maintained within their normal range, statutory or normative, at every point of the system,
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– the abnormal regime, in the course of which certain characteristics (for example voltage or frequency) for a limited time, go outside of the limits or fixed states of the normal regime. There can also be situations that are highly disturbed (at times only locally) following a major disturbance: faults on the grid, leading to voltage sags; significant loss of production, leading to drops in frequency, loss of transmission, capacity, etc. 6.1.6.1. Management in normal and abnormal regimes Limits to both voltage and frequency in normal and abnormal regimes are often specified in the technical reference books, grid codes or distribution codes for grid managers. Wind farms should be able to function in these normal and abnormal voltage and frequency areas while remaining connected to the grid. Requirements for active and reactive power produced (or consumed) under these conditions are generally specified in the technical connection rules. 6.1.6.2. Managing voltage sags (FRT “fault-ride-through” or LVRT) With classical technologies, in the case of voltage sags, wind turbines are generally protected by disconnecting themselves from the grid faster than other means of production. These disconnections cause production losses that can aggravate the situation in a grid already made fragile by the incident at the origin of the voltage sag, and therefore they can have very harmful consequences. Studies on this subject were made in several European countries (Germany [CON 05], Ireland [FAG 05] and France [NAV 05]), for different scenarios of massive wind power development and for faults in the transmission grid. The studies show that following the fault, the voltage sag can affect a very large region and this can lead to a significant loss in production for high wind turbine penetration rates if the wind generators do not have enough capability to regulate the voltage sag. They also show that the region impacted can be reduced when wind turbines contribute to regulating the voltage. These studies have led grid managers to define the requirements for regulating the voltage sag of wind turbines [MAT 04]. These requirements can differ fairly widely from one country to another. By way of example, Figure 6.5 shows the magnitude of the voltage sags that were defined by EON Netz [EON 03] in Germany and RTE [MIN 03b] in France and which the wind generators must be able to withstand without disconnecting.
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Figure 6.5. Examples of voltage sag magnitudes
Assuming different protection schemes (notably of under-voltage and overspeed), the simulations carried out in [BOU 03] on the various classical wind turbine technologies show that (Figure 6.6) depending on the depth and duration of the voltage sag, squirrel cage asynchronous machines can lose stability. On the other hand, after eliminating the fault, the remagnetization of the generator requires a significant quantity of reactive power that causes a large inrush current (noted in Figure 6.1) and considerably slows down the reestablishment of voltage. Furthermore, this characteristic of squirrel cage asynchronous machines can lead to limiting of their penetration rate (notably in islanded grids [SEI 05]).
Figure 6.6. Voltage and rotor speed for a short circuit cleared in two stages
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For machines equipped with power electronics (doubly fed asynchronous machines and synchronous machines) but not equipped to regulate voltage sags, the risks of loss of stability are not entirely non-existent, but they are reduced by the action of power electronic converters. After eliminating the fault, voltage reestablishes itself rapidly because, on the one hand, synchronous machines do not need reactive power and, on the other hand, the reactive power needs for remagnetization of doubly fed asynchronous machines are compensated for by the reactive power produced by the converters within the limits of their size. In order to meet the voltage sag management requirements formulated by grid managers, some solutions have been studied and/or developed. The essential points are [BOU 04, LAV 04]: – to limit the increase in the speed of the turbine rotation so as not to trigger the over-speed protection, – to maintain the stability of the machine, – for machines equipped with power electronic converters, to maintain the DC bus voltage at an acceptable level. We now review the solutions for the different wind turbine technologies. Squirrel cage asynchronous machines experience complete voltage sags at their terminals and the only means of action consists of acting on the angle of pitch of the blades to limit the increase in the speed of rotation without, however, being able to prevent it. The rotation speed and the stator currents can trigger protection and therefore disconnect the wind turbine. The possibilities for improving the management of voltage sags are therefore very limited and do not enable this functionality to be guaranteed. The solution then consists of combining them with additional equipment such as those described in section 6.1.9. For doubly fed asynchronous machines, combined with the action on the blade pitch, two strategies have been studied and compared in [LAV 04, LAV 05]: – the addition of a “crowbar” circuit to the rotor (a circuit that “closes” the rotor on a load resistance at the time of the fault) and the disconnection–reconnection sequence of the stator of the machine (this strategy is currently the one most used by manufacturers), – the addition of a load circuit on the DC bus and modification of the algorithm controlling the current, the flux and the torque of the machine during the voltage sag.
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Results show that the two strategies allow us to manage voltage sags to a large extent. However, with the first strategy, large values of certain quantities can still be observed (for example, rotor speed, voltage on the DC bus) and, according to the thresholds adopted, there can be a risk of triggering certain protection devices. With the second strategy, the increase in these quantities remains limited, but large variations in current and power are nevertheless still observed. For synchronous machines connected by electronic power converters, the strategy studied in [LAV 04, LAV 05] consists of adding a load resistance on the DC bus of the converters (to dissipate the power produced) and keeping the voltage of the DC bus constant. Results show that this “fairly limited” adaptation of the structure allows complete disconnection between the machine and the grid, which makes this kind of machine capable of undergoing voltage sags without any particular difficulty. In practice, even if developments in this field continue, several manufacturers (for example, Enercon, Ge, Nordex, Repower, Vestas, Gamesa) have already offered wind turbines equipped with regulating capacity for voltage sags [BOL 03, HAR 03, MIL 03, PED 06]. 6.1.6.3. Interaction with the protection design As has been discussed already, the connection of a generating installation, including wind turbines, modifies power flow, short circuit currents, impedances of the grid, etc. This can therefore interfere with the functioning of the protection design of the grid, and, in this case, the safety of people and property cannot be assured. Various aspects must be considered. Besides the risk of exceeding the interrupting capability of the protection devices that was previously mentioned, the connection of a wind farm can: – affect the sensitivity of the protection design (non-detection of certain faults), – affect its selectivity (disconnection of a larger part of the grid than necessary to eliminate a fault, – cause untimely disconnections on the healthy parts of the grid, – disturb the devices for locating the fault installed on the distribution grids, – interact with the automatic reconnecting sequence, etc. Therefore, it is important to verify proper functioning of the protection design of the grid following connection and to take the necessary measures if this happens, which can make it necessary to review regulation of the grid protection, to change the protection (for example in order to raise the interrupting capability), to install
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new types of protection (for example directional protection on the medium voltage lines), etc. Another aspect concerns coordination of the internal protection of the wind farms with the protection system of the grid. To this end, grid managers provide the requisite information to the producer, even a set of specifications, in order to allow him to define and regulate his protection appropriately. In certain cases, for example on distribution grids, the generating installations should be equipped with a decoupling protection intended to disconnect the installation under certain conditions such as: – functioning as a separate grid without the fault, – phase-to-ground fault, phase-to-phase fault, or phase-to-neutral fault, – risk of faulty synchronization or synchronizing out of phase (for example in the case of automatic reclosing), – certain faults on the transmission grid. Uncoupling protection is generally based on criteria for disengaging from over/under voltage and over/under frequency. Nevertheless more elaborate criteria can sometimes be implemented (jump vector, frequency variation rates, etc.). Certain uncoupling protections even use automatic remote switching of the beginning of the line or at the high/medium voltage transformer. 6.1.7. Auxiliary system The auxiliary system is necessary to guarantee safe and reliable operation of the electricity systems. It is controlled by grid managers on the basis of contributions furnished mainly by the production installations connected to the grids. Among the auxiliary systems we can cite: – voltage regulation, – frequency regulation, – black start, – reconnection of the grid. Because of the unpredictable and fluctuating nature of their primary energy (wind) and the technologies used, wind turbines in general have a limited capability for contributing to the auxiliary system. This results in greater demands on other production groups to assure this contribution. This problem is further exacerbated by the great variability in wind turbine production. The problem has an even greater
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impact on frequency regulation (to maintain the production–consumption balance) and, for high penetration levels, requires a greater amount of active power reserves (secondary and tertiary reserves) [BOU 06, CON 05]. 6.1.7.1. Regulation of voltage and reactive compensation Voltage regulation is carried out by means of production or consumption of reactive power. Reactive power “transmits” very poorly on a grid, meaning that the corresponding auxiliary system must be supplied locally by the means available. This is why requirements in terms of voltage regulation or of production/absorption of the reactive power are specified in the rules for connection. In transmission grids, wind farms generally must participate in regulating the voltage by supplying and absorbing reactive power. The modalities for this regulation are specified in the grid codes or technical reference books. However, for wind turbines, adaptations are sometimes permitted according to what is possible for them compared with classic production methods. These adaptations can imply recourse to localized technical solutions, allowing them to reach the same performance, such as installing capacitor banks or more sophisticated power electronic equipment [MIN 03e]. In distribution grids, the requirements vis-à-vis decentralized production and therefore wind farms as well, are generally more ”frustrating”. Most of the time they only consist of requiring a constant reactive power at the point of connection. Nevertheless, with the growth in penetration rate, the requirements are becoming more and more strict. Thus, in France, the ministerial decrees [MIN 03b, MIN 03c] governing connection with the distribution grid foresee the possibility of grid managers requiring a contribution to regulation of the reactive power and even to voltage regulation, according to the size of the production installations. These requirements are summarized in Table 6.2, which gives, depending on the power and the type of production installation, the minimum constructive capabilities required for furnishing or absorbing reactive power and the possible types of regulation. The type of regulation actually implemented is determined by the grid manager.
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Constructive capacities minimum regulation of Q
Type of regulation possible determined by the grid manager
LV
Q 0
Q constant
P 1 MW
0 Q 0.4 Sn
Q constant
1 MW < P 10 MW
– 0.1 Sn Q 0.5 Sn
Regulation of Q; adjustment of V on demand by the grid manager
10 MW < P
– 0.2 Sn Q 0.6 Sn
Regulation of V or Q; installation can be equipped with a voltage regulator
Asynchronous generators
0 Q 0.4 Sn
Capacitor banks
Table 6.2. Constructive capacities and regulation of voltage/reactive power4
Among the wind technologies, only squirrel cage asynchronous machines consume reactive power and the value of this depends on the operating point (in particular on the voltage at the terminals of the machine and the active power produced). Therefore, they do not have the capability to regulate voltage. Compensation for the reactive power they consume and provision of additional reactive power to satisfy the demands of the grid manager are generally assured by capacitor banks. As such, the reactive power varies by steps whose size depends on the size of the capacitor bank units. This solution mentioned in the technical rules of connection (see Table 6.2) can be sufficient to satisfy the requirements in terms of furnishing or absorbing the reactive power. Nevertheless, in certain cases, better management of the reactive power may be required, necessitating the implementation of more sophisticated solutions than those presented in section 6.1.9. On the other hand, this solution does not necessarily allow us to solve the problem of too high voltage after connection and it does not permit us to assure a precise voltage regulation if that is required.
4 With P: active installed power; Q: reactive power; V: voltage; Sn: nominal apparent power.
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Doubly fed asynchronous machines and synchronous machines, thanks to their interface with power electronics, have the possibility of regulating the reactive power produced or consumed at their terminals and also of carrying out accurate regulation of the voltage. The limits and the performance of the regulation depend on the size of the converters and the strategies of control–command implemented [BOU 06]. The response time for regulation of the voltage or the reactive power is very short (several ms). Several manufacturers have already offered machines capable of contributing to regulation of voltage or reactive power, sometimes even in the absence of wind and thus in the absence of active power production (“zero power voltage regulation”) [SCH 06]. If voltage regulation at the point of the farm’s connection is required, then a regulation strategy coordinated with the voltage at the terminals of the different wind turbines of the farm should be installed. 6.1.7.2. Regulation of frequency Wind farms have a limited capability for participating in regulation of frequency and active power reserves. Until recently, their contribution was rarely required. With the growth in the penetration rate of wind turbine energy, stronger and stronger requirements have appeared in different countries. Certain grid managers require a contribution only in the case of over-frequency in the form of a more or less rapid reduction of active power produced [EON 03, JAU 04, MAT 04]. Others are beginning to require a “complete” frequency regulation (increase or reduction of the active power produced), sometimes comparable to that of classic decentralized production means [CER 04, ELK 04]. This type of regulation requires the wind turbines to keep an active margin of power, for example by acting on the angle of pitch of the blades. In other words, they may not produce all the power that the wind would allow, resulting in a loss of production. This practice goes against what has until now been accepted operation, in which the output was maximized and all the available power was injected into the grid. In France, wind turbines have not yet been required to participate in frequency regulation and in primary and secondary reserves [MIN 03b, MIN 03e]. Nonetheless, on islanded grids, this non-contribution occurs at the expense of power shedding and limits the penetration of wind turbines [MIN 03b, SEI 05].
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To evaluate the capacities of the different wind technologies for participating in primary regulation of frequency, a study was carried out in [BOU 06, LAV 05]. The results show that: – for squirrel cage asynchronous machines, the only means of action is control of the power of the turbine by the intermediary of the pitch angle of the blades. The implementation of an appropriate strategy allows asynchronous machines to keep a margin of active power and to deliver this reserve at the time of a frequency variation on the grid. The wind turbine’s action is very rapid (in comparison with the reaction of other production groups). It can therefore take part in primary regulation. However, the slip of the machine cannot be controlled, and at the slightest variation in frequency in the grid, the machine finds itself at a new operating point. Finally, the interest in using this type of wind turbine is limited to a sufficiently high load (or generated power) so that the scheduled power, taking into account the reserve, is not too small, – for doubly fed asynchronous machines (DFAM), two strategies have been studied: on the one hand, by acting on the pitch angle for the operating points of sufficiently high load, and on the other hand by using the turbine rotor as a means of inertial energy storage with a partial load (control of the speed of the generator to extract the maximum power of the turbine at a given angle of pitch). Simulations show the feasibility of the two strategies: the DFAM structures can participate in regulation of frequency for a large load as much as for a partial load, but with a severe restriction on the minimum attainable speed of rotation. However, we can state that the machines are subjected to significant mechanical constraints, – for synchronous machines connected by power electronics, the same two strategies have been tested: action on the angle of pitch with high load and use as a flywheel at partial load. The simulations show that synchronous machines can participate in frequency regulation, whether there is a partial load (without the restrictions of the DFAM on minimum rotation speed) or a full load. The release of power is made with a small transient and in a relatively short time (less than a second), thanks to the performance of the controls. This type of machine therefore offers the best performance. The principal difficulty resides in detecting the frequency, which does not take place naturally through a variation in the speed of the machine or of the slip. Figure 6.7 [BOU 06] illustrates the contribution to frequency regulation by acting on the angle of pitch for doubly fed asynchronous machines and synchronous machines.
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Figure 6.7. Contribution to frequency regulation by action on angle of pitch
6.1.7.3. Operating on a separate grid and reconstitution of grids By their nature, wind turbines by themselves do not have the capability of supplying loads in a separate grid5. Solutions are being studied but are not yet at the prototype stage (see for example the work of Econnect [BEL 05a, BEL 05b] section 6.1.9.4). Therefore, wind turbine farms are generally exempt from contributing to operation as a separate grid. They are also exempt from participating in the reconstitution of the grid, that is, to the restoration of normal grid function following a total breakdown caused by a widespread incident [RTE 05]. The reconstitution of the grid in its first phases is similar to the construction and maintenance of separate grids. The generation 5 That is, a portion of the grid which, in exceptional circumstances, functions while disconnected from the principal supply grid [RTE 05].
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elements that participate in this must follow very strict requirements [RTE 05], some of which cannot be satisfied by wind turbines at this time. 6.1.8. Variability and unpredictability of production Wind power technologies have evolved a great deal and we can now solve a large number of problems linked to interfacing with the grid. However, they do not solve the problem of variability and unpredictability of wind speed and therefore of production, nor of the necessary increase of active power reserves (secondary and especially tertiary) to address this. Apart from power shedding, solutions are needed either by improving the methods of predicting what can be generated and/or by providing wind farms with additional equipment (like energy storage) or other equipment already present on the grid (other means of generation, load, etc.) 6.1.9. Other solutions for connection problems In this section we will briefly review some other solutions to the problems of interfacing wind turbines with the grid [BOU 04]. 6.1.9.1. Reinforcement of the grid Grid reinforcement is the first solution for addressing problems generated by interfacing generation, and, in particular, those associated with wind power. This can imply replacing pieces of grid equipment by others, sized appropriately (lines, cables, transformers, protections, etc.). It can also imply installing new equipment (transformers, protection, etc.), even the construction of new grid sections (new HV/MV towers, new transmission lines, etc.). On the one hand, depending on the size of the reinforcements required, the solution can be very costly and sometimes take a long time to implement (for example the construction of new transmission lines). On the other hand, reinforcement of the grid does not solve all the problems, such as, for example, problems linked to regulating frequency, and to the variability and unpredictability of the wind turbine. We note that, depending on the country, the costs of reinforcing the grid may or may not be at the producer’s expense. We can in fact distinguish two principal approaches: – the so-called “shallow costs” method, where the producer pays only the costs of interfacing between his installation and the point of attachment to the grid;
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– the so-called “deep costs” method, where the producer pays not only the costs of connection but also all the required costs of grid reinforcement. Intermediary approaches can also be applied, as in France, in which the producer pays the costs of interfacing and the required costs of reinforcement up to the high voltage level (not included). For example, for a farm interfacing in HVA, he will pay the required reinforcement costs on the HVA grid and into the HVB/HVA transformer, but not into the HVB grid. 6.1.9.2. Power shedding Another solution relates to shedding of wind power production or the limitation of penetration rates. Shedding production allows us to solve problems linked to the variability and unpredictability of wind power, to the stability of the grid, and to the regulation of frequency and voltage. It also allows us to solve problems of maximum current in steady state and in short circuit, problems of congestion on the transmission lines, and problems of voltage profile and also voltage quality, since they largely depend on the power produced. Shedding production can happen in different ways: – permanent limitation of the injected power (which eventually can be modified in the course of the life of the farm), – periodic planned limitations, – non-periodic limitations planned and communicated in advance (one or several days, several hours), – limitation in real time which needs an appropriate communication system. Implementation at the farm level can be carried out, for example, by disconnecting the wind turbines (the simplest way) or by acting on the angle of pitch of the blades (if the technology permits). 6.1.9.3. Coordination with other production methods The coordination of wind farms with other production methods could allow us to solve a certain number of problems: – compensation to a certain extent for the variations in wind power produced more or less short term and thus for the variability and unpredictability of wind power, – contribution to regulating the frequency and to the primary and even secondary reserve,
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– contribution to regulating voltage/reactive power when the interfacing points of wind power and other production methods are close by, – congestion on the transmission grids (depending on the location of the farm and the generation equipment). Applications of this type already exist; for example: – wind–diesel systems on small isolated or insular grids [LUN 01], – combination of wind with hydraulic power in the Scandinavian countries [KEY 04]. More generally, work has already been carried out on the development of management systems for several decentralized production methods (including wind power) so as to optimize operation while meeting the requirements of the grid operators and facilitating use of the grids. An example is given by the AGCU (Advanced Grid Control Unit) system studied within the scope of the European project Dispower [NAV 05]. This system was tested in the Deiderode region in Germany on four wind turbines and one cogeneration plant. The results are promising. 6.1.9.4. Load control The control of loads on the grid is a new method being investigated for facilitating the integration of decentralized production, and in particular of wind power, into the grid. Load control can be used to respond to a certain number of constraints linked to the injection of wind power: – compensation for the variations (slow) in the wind power produced, – regulation of frequency and contribution to the reserve in active power, – congestion in the grid and temporary problems of capacity to receive (maximum currents, voltage profile), – operation as a separate grid, etc. Various applications have already been considered and tested [LUN 01, KEY 04] with heating systems or water heating for domestic hot water [EDF 04], pumping water, or different types of loads that present a certain “inertia” or a low priority for consumers. In particular, in the context of the European project Dispower, Ecconect carried out studies on the application of load control in order to, on the one hand, increase the penetration rate of wind power on a weak rural grid in the north east of England and, on the other hand, to allow operation as a separate grid of a wind farm [BEL 05a, BEL 05b]. One experiment in particular was carried out and a load control
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strategy was associated with a synchronous compensator in order to test the feasibility of operation of a wind turbine farm in Northumberland (England) as a separate grid. 6.1.9.5. Systems of reactive compensation and of voltage control As has already been discussed, machines equipped with power electronic converters can contribute to the supply and absorption of reactive power to the limits of capacity linked to their size. Therefore, they can participate in voltage regulation without requiring additional equipment. This is not the case for squirrel cage asynchronous machines or for asynchronous machines with external rotor resistance control, which can only consume reactive power and in a manner that depends on the operating point. These types of machines therefore require the use of additional equipment for the control of reactive power and of voltage. The first system is obviously capacitor banks, the solution used most widely, and already discussed above. However, there are other more sophisticated systems [BOU 04] that allow more subtle variations or other functionalities (for example, for voltage quality) and which can be better adapted or required in some situations. We will cite only some of these and refer the reader to the specialized literature for a more detailed description [BOU 04, ERI 03, RED 04, VIT 04]: – the TSC (thyristor-switched capacitor) comprises a capacitor bank in series with thyristors mounted in opposition and a filter inductor (that also serves as a disconnect circuit in order to limit the risk of destruction of the capacitors by harmonics). The TSC is often used as a means of regulating the voltage dynamically in the case of voltage fluctuations that are not too rapid. The response time for a TSC is in the order of 20 ms, – the SVC (static VAR compensator) has been used for a long time on grids to reduce flicker and imbalance in the presence of fluctuating loads such as arc furnaces, rolling mills, foundries, etc. This technology is mature and its use for reactive compensation and voltage regulation can be foreseen. The SVC is generally composed: - of three branches of reactances controlled by thyristor (thyristor controlled reactor (TCR)) Y connected to form a variable three-phase inductance capable of absorbing, in a continuous fashion, reactive power between íQL and 0, - of one three-phase capacitor bank, Y connected, that produces a fixed quantity, QC, of reactive power. Depending on the reactive power requirements at the interface point, the TCR automatically adjusts its consumption of reactive power, allowing the SVC to make an adjustment either of reactive power or of voltage within the limits of its capacity, that is of –QL to +QC. Several versions of the SVC exist:
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– The STATCOM (static compensator), like its alternative the D-STATCOM, is a “voltage source” whose amplitude, phase and frequency are completely controllable. It consists of a capacitor connected to the grid, through a voltage converter (voltage sourced converter or VSC), that is a power electronic interface of IGBTs with PWM control (pulse width modulation) placed in series with an inductor (Figure 6.8). In theory, the STATCOM also allows us to reduce the flicker linked to variations in reactive power consumed by asynchronous machines. With the constant progress in semiconductors, it is now possible to construct high power STATCOMs (around several dozen MVA) with a very good dynamic response, that is, with time constants in the order of a millisecond.
Figure 6.8. Diagram of the principle of a STATCOM
– A hybrid version of the STATCOM (American Superconductor’s D-VAR) consists of a small STATCOM combined with banks of switched capacitors that it controls. This coordinated operation allows a wider and more continuous area of operation of supply and absorption of the reactive power. For example, with a STACOM of 4 MVA and capacitor bank of 4 MVAR, it is possible to obtain a continuous reactive power variation between –4 MVAR and +8 MVAR. This configuration can also be used to regulate the voltage. – The synchronous condenser is a synchronous alternator dedicated to the production and absorption of reactive power. It allows continuous regulation of the voltage, like all synchronous alternators. Synchronous condensers have been partially supplanted by the static compensators previously presented. However, development of new types of compensators could make them advantageous for applications of regulating voltage/reactive power in association with wind turbine farms. In particular, American Superconductor has developed a synchronous
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condenser with superconductivity whose applications for wind power and potential performance is under examination [KAL 06a, KAL 06b]. 6.1.9.6. Systems for managing voltage sags As has previously been discussed, for doubly fed asynchronous machines and synchronous machines connected by power electronics, manufacturers have already offered machines on the market with the capability of managing voltage sags (LVRT), meeting the requirements of grid managers. On the other hand, this is not the case for squirrel cage asynchronous machines and asynchronous machines with control by external rotor resistance, and specific systems of LVRT have been developed for wind turbine farms using these technologies. The principle of LVRT devices is to maintain voltage at the wind farm level at times of severe voltage sags originating from the grid in order to avoid having the wind farms disconnect because of the under-voltage criterion and thus lose stability. Such a system for a wind farm generally consists of two parts: a series reactance called “decoupling” and a shunt device for producing or absorbing reactive power (Figure 6.9):
Figure 6.9. Diagram of the principle of an LVRT device
The decoupling reactance artificially inserts an “electric length” between the fault that strikes the grid and the interface point of the wind farms. It limits the extent of the voltage sags at this point and delays the lowest voltage level being reached. It can be inserted permanently or temporarily (for example totally short circuited, or partially, in the case of normal operation). The reactive injection device is placed near the wind farms downstream of the resistance. It maintains voltage during the fault by producing reactive power so that the voltage at the terminals of each wind turbine remains sufficiently high. Generally, a device for regulating the voltage is also inserted near the grid. The performance of these solutions depends in particular on the size of the reactance and the response time of the reactive injection. As for the reactance, the
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larger it is, the better it will perform in case of a fault, but the greater the risk of disturbing the function during normal operation. In every case, the entire installation (wind farm + LVRT system) must be examined in great detail so as to ensure that the sizes and various control systems lead to satisfactory operation in all the different configurations, especially under normal operation and for different types of faults. At the present time, LVRT systems are mainly prototypes. The studies that have been carried out are based on simulations and the test results are not yet sufficient to evaluate their performance. 6.1.9.7. Systems for energy storage There are a large number of technologies and storage systems [CRU 01]. Depending on their characteristics and size, energy storage systems can make it possible to resolve some of the constraints linked to the insertion of wind power and in particular they must make it possible to: – mitigate to a certain degree the variability and unpredictability of the power generated by the wind farms by guaranteeing power produced on different time scales depending on the storage quantity (minutes, hours, sometimes days), – smooth out rapid fluctuations in power and therefore voltage, and in this way improve the voltage quality (flicker, voltage fluctuations), – participate in regulating frequency and voltage and contribute to primary and even secondary reserves, – contribute to the LVRT capacity of the wind farms or guarantee the power produced on the grid after the elimination of the fault in the case when the wind farms disconnect, – solve temporary problems of congestion or of too-high current in the steady state by limiting the power injected into the grid and by storing surplus energy to restore it when the problem has disappeared. With respect to the case of combining energy storage with wind power production installations, the best potential candidates among “classic” batteries are lead/acid batteries and eventually nickel/cadmium, such as Redox-Flow and Sodium-Sulfur devices, the latter having the net advantage of being compact. Nevertheless, in the current context, solutions based on energy storage are still very expensive and therefore very little used. 6.1.9.8. Short circuit current limiting devices Various solutions are possible for solving the problems of short circuit currents that are too high. On the one hand, at the level of the wind farms themselves, we
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have seen that certain wind turbine technologies equipped with a power electronic interface allow us to limit the short circuit current they generate (see section 6.1.3). Moreover, the internal grid of the wind farm can be sized to reduce the contribution to the short circuit current, for example for a farm interfaced with the transmission grid, and raising the reactance of the MV/HV transformer up to 20%. However, the efficiency of this approach is limited and strongly depends on the constraints of the wind farm. On the other hand, the short circuit current limiting device can be used either in the wind farm internal grid or in the upstream grid [OHR 03] in order to limit: – the peak current in the case of a response time in the order of 3 ms (systems with IGBTs or GTOs with an appropriate control–command, fuse, superconductor device), – the current turnoff (systems with IGBT, GTO or thyristors with an appropriate control–command, fuse, superconductor devices, rapid disconnectors, etc.). Certain current limiters insert a series reactance in the grid during the short circuit in order to limit the current. 6.1.9.9. Other equipment There are also other pieces of equipment for resolving specific constraints linked to the interfacing of decentralized or wind production; for example: directional protection that allows a reduction in the number of false disconnects, antiharmonic filters, passive or active filters intended to prevent disturbances of transmission signals on the grid (for example the TCFM in France), etc. 6.2. Comparison of technologies and conclusion Wind turbines allow conversion of energy from wind into electrical energy. Several designs exist; four are currently on the market. The simplest of them (squirrel cage asynchronous machines) allowed wind energy to soar in the l980s. The advent of power electronics then allowed the development of new configurations, called variable speed, for an additional cost estimated at 50%. Since the wind power field is extremely competitive and in constant evolution, there are few documents that allow us to establish a precise technical evaluation and comparison of the various structures of wind generators offered on the market [LAV 02, LAV 05]. The manufacturers give little information on their respective technologies, particularly if the solution adopted is a machine especially adapted to the application. As for generators, there is information about their output. The output
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for the different wind systems varies very little with the type of machine used [HAU 00], the difference being essentially the use or otherwise of power electronics. The system with doubly fed asynchronous machines uses a relatively standard machine and the converters connected to the rotor are generally sized for the range of the machine’s power, which, in practice, divides their price by three with respect to converters sized for nominal power. The reduced number of pairs of poles (two to three generally) forces the presence of a high ratio gear. The principal disadvantage of this solution is the cost and regular maintenance of the gears as well as the relative fragility and necessary maintenance of the supply of the rotor winding with rings and brushes. Moreover, the direct connection of the stator of the machine to the grid introduces a strong coupling between the turbine/generator system and the grid, which makes the entire setup delicate in case of disturbances. For direct drive systems with synchronous machines, the current situation illustrates the supremacy of a wound rotor synchronous machine, allowing good voltage regulation and the possibility of use with a simple diode rectifier (ENERCON); the consequence of this choice is a machine of large diameter (in the order of 4 to 6 m for 1.5 MW, 10 m for 4.5 MW) due to the large number of poles. This situation is undergoing change with the existence and development of several solutions: synchronous machines with permanent magnets are already in use (Jeumont Industrie, Mitsubishi, Winwind) or at an advanced stage of development (Zephyros consortium with ABB, Winwind). The principal advantages of the solution consisting of a machine with magnets is a higher flux density under the poles, resulting in a more compact machine, with fewer losses, smaller cooling system and the possibility of making a machine that is completely closed [DEV 01]. The basic structure of the synchronous machine, thanks to the decoupling of the machine and the grid, guarantees better performance vis-à-vis disturbances affecting either of them since it is possible to prevent these being transmitted using appropriate control at the DC bus level. An interconnection between wind farms of this type is also foreseeable at the DC bus level. Regarding the problematic situation of interfacing with the grid, the development of decentralized production in grids led in the 1990s to the definition of technical interface conditions. Initially, because of the unpredictable character of wind energy and the technologies used (power fluctuations and difficult to predict), wind farms often had as the sole “constraint” to produce when it was possible to produce and not to degrade the voltage quality on the grid. They were often subject to modified interface requirements, “weakened” requirements, and certain requirements were even dispensed with.
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Today, because of considerable and still increasing development in terms of installed power, the technical interface conditions have evolved and wind farms are subject to increasingly strict technical interface requirements. This is why a more and more pressing need for solutions to facilitate their integration into the grid has arisen. For power electronic technologies (doubly fed asynchronous machines and synchronous machines connected to the grid through power electronic switches), the tendency observed today is to give them new constructive capabilities and new control–command strategies to allow them to respond by themselves (without the aid of additional external equipment) to requirements experienced in technical interface conditions and to follow the development of the latter. We can cite by way of example the regulating capabilities for voltage sags, which new machines now have, and the possibilities of regulating voltage and reactive power that are mentioned in the product data sheets. This development is not possible for squirrel cage asynchronous machines, given their intrinsically limited capabilities. However, even if doubly fed asynchronous machines and synchronous machines currently dominate the market and are the technologies used most often in wind park designs, squirrel cage asynchronous machines still represent a large proportion (in installed power) of wind power currently in operation. This statement is all the more true for insular grids where squirrel cage asynchronous machines are clearly in the majority and where wind farm designs use this technology very widely. The tendency then is to add additional equipment (such as those described in section 6.1.9) that gives them the capabilities and functions that they lack. 6.3. Bibliography Scientific references [BEL 01] BELHOMME R, NASLIN C., G. BESLIN, ALBRIEUX N., “Wind Farms and Networks – Main Technical Issues”, International Conference on Power Generation and Sustainable Development, Liege, October 2001. [BEL 04] BELHOMME R., CORENWINDER C., “Wind Power Integration in the French Distribution Grid Network Regulations and Requirements” NWPC’04, Goteborg, March 2004. [BEL 05a] BELHOMME R., BOUSSEAU P., NAVARRO E., BADELIN A., DEGNER T., ARNOLD G., BERENGUER A., CHOCARRO I., MATERAZZI-WAGNER C., WHITE S., HATZIARGYRIOU N., TSELEPIS S., NERIS A., LEFEBVRE D., “Case Studies on the Integration of Renewable Energy Sources into Power Systems,” 10th Kasseler Energy Systems Symposium Technology 2005, Kassel, Germany, 10/11 November 2005.
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[BEL 05b] BELHOMME R., WHITE S., HATZIARGYRIOU N., TSIKALAKIS A., LEFEBVRE D., RANCHIN T., FESQUET F., ARNOLD G., TSELEPIS S., NERIS A., DEGNER T., TAYLOR P., “Deliverable D7.3-Distributed generation on European islands and weak grids,” Project European DISPOWER, available D7.3, Document No. Del_2005_0079, www.dispower.org, December 2005. [BOH 03] BOHMEKE G., “Development and operational experience of the wind energy WWD-1 converter”, EWEC 2003 Conference, Madrid, in June 2003. [BOL 03] BOLIK, SM, R&D DEPARTMENT VESTAS WIND SYSTEM, “Grid requirements: challenges for wind turbines”, Fourth International Workshop on Largescale Integration of Wind Power and Transmission Networks for Offshore Wind, Billund, Denmark, October 2003. [BOU 03] BOUSSEAU P., GAUTIER E., GARZULINO I., JUSTON P. H., BELHOMME R., “Grid impact of different technologies of wind turbine generator systems”, 2003 Conference, Madrid, in June 2003. [BOU 04] BOUSSEAU P., FESQUET F., BELHOMME R., NGUEFEU S., THAI T.-C., “Solutions for the grid integration of wind farms – a survey”, Proceedings of EWEC 2004 (European Wind Energy Conference), London, 22/25 November 2004. [BOU 06] BOUSSEAU P., BELHOMME R., MONNOT E., LAVERDURE N., BOËDA D., ROYE D., BACHA S., “Contribution of Wind Farms to Ancillary Services”, CIGRE Session 2006, Paper C6-103, Paris, 27 August/1 September, 2006. [BUD 00] BUDINGER M., LERAY D., DEBLEZER Y., “Eoliennes et vitesse variable”, La Revue 3EI N°20, March 2000. [CAI 04] CAIRE R., Gestion de la production décentralisée dans les réseaux de distribution, PhD Thesis, Institut National Polytechnique de Grenoble, April 2004. [CAN 00] CANARD J. F., Impact de la génération d’énergie dispersée dans les réseaux de distribution, PhD Thesis, Institut National Polytechnique de Grenoble, December 2000. [CEI 01] COMITÉ ELECTROTECHNIQUE INTERNATIONAL, “Aérogénérateurs – Partie 21: Mesurage et évaluation des caractéristiques de qualité de puissance des éoliennes connectées au réseau”, Norme Internationale CEI61400-21, 1st edition, December 2001. [CER 04] CER COMMISSION FOR ENERGY REGULATION, “Wind Farm Transmission Grid Code Provisions”, CER/ 04/237, Ireland, July 2004. [CON 05] CONSORTIUM DEWI/E.ON NETZ/EWI/RWE TRANSPORTNETZ STROM/VE TRANSMISSION, “Energiewirtschaftliche Planung für die Netzintegration von Windenergie in Deutschland an Land und Offshore bis zum Jahr 2020”, Deutsche Energie-Agentur GmbH (DENA), www.dena.de, March 2005. [CRU 01] CRUZ I., AVIA F., ARIAS F., ARRIBAS L. M., FIFFE R.P., CIEMAT-DER, “Assessment of different energy storage systems for wind energy integration”, EWEC 2001, Copenhagen, July 2001. [DIT 03] DITTRICH A., “Grid voltage fault proof doubly fed induction generator system”, EPE 03, Toulouse, 2003.
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[EDF 04] EDF, “GESER – génération d’eau chaude synchronisée avec les energies renouvelables”, Journal “L’INDEPENDENT”, 16 May 2004. [EHR 01] EHRENBERG J., ANDRESEN B., REBSDORF A., “Digitally controlled wind turbines in megawatt size with doubly-fed induction generator without position sensor” Magazine Elektronik, 2001. [ELK 04] ELKRAFT SYSTEM, ELTRA, “Wind turbines connected to grids with voltages above 100 kV – technical regulation for the properties and the regulation of wind turbines”, Regulation TF 3.2.5 registered with the Danish Energy Authority, December 2004. [EON 03] EON NETZ, “Grid code for high and extra high voltage”, EON Netz GmbH, www.eon-netz.com, August 2003. [ERI 03] ERINMEZ I. A., on behalf of CIGRE WG B4.19, “Static Synchronous Compensator (STATCOM) for Arc Furnace and Flicker Compensation”, Electra, N° 211, p 58–65, 2003. [EWE 06] EWEA – THE EUROPEAN WIND ENERGY ASSOCIATION, “Europe’s energy crisis – the no fuel solution”, EWEA Briefing 2006, www.ewea.org, February 2006. [FAG 05] FAGAN E., MCARDLE J., SMITH P., STRONGE M., “Development of grid code provisions for wind generators in Ireland”, CIGRE Symposium on Power Systems with Dispersed Generation, Paper No. 003, Athens, April 2005. [FEL 02] FELD G., LAVABRE M., “Conversion d’énergie à partir d’une éolienne en vitesse variable”, La Revue 3EI N° 31, December 2002. [FOR 02] FORCHETTI D., GARCIA G., VALLA M. I., “Vector control strategy for a doubly-fed stand-alone induction generator”, IECON 2002 Conference Proceedings, 2002. [GUF 00] GUFFON S., Modélisation et Commandes à Structure Variable de Filtres Actifs de Puissance, PhD Thesis, INP Grenoble, 2000. [HAR 03] HARTGE S., DIEDRICHS V., “Ride-through capability of Enercon wind turbines”, Fourth International Workshop on Large-scale Integration of Wind Power and Transmission Networks for Offshore Wind, Billund, 20/21 October 2003. [HAU 00] HAU E., Wind Turbines: Fundamentals, Technologies, Application, Economics, Springer, Berlin, 2000. [HEI 98] HEIER S., Grid Integration of Wind Energy Conversion Systems, John Wiley & Sons, New York, 1998. [HOF 00] HOFMANN W., RABELO B., “Optimised power flow on wind power plants with the doubly-fed induction generator”, PEMC 2000 Conference Proceedings, 2000. [HOP 01] HOPFENSPERGER B., ATKINSON D. J., “Doubly-fed AC machines: classification and comparison”, EPE 2001 Conference Proceedings, 2001. [JAU 04] JAUCH C., SØRENSEN P., BAK-JENSEN B., “International Review of Grid Connection Requirements for Wind Turbines”, Proceedings of NWPC’04, Gothenburg, ¼ March 2004.
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[KAL 06a] KALSI S. S., MADURA D., ROSS M., INGRAM M., BELHOMME R., BOUSSEAU P., ROGER J.-Y., “Operating experience of a superconductor dynamic synchronous condenser”, Session CIGRE 2006, Paris, 27 August/1 September, 2006. [KAL 06b] KALSI S., ROSS M., FESQUET F., BOUSSEAU P., ROGER J.-Y., BELHOMME R., “Enhancement of a wind-farm electrical system with a Superconducting dynamic synchronous condenser”, European Wind Energy Conference (EWEC), Athens, 27 February/2 March 2006. [KEY 04] KEY T., “Wind Power Integration Technology Assessment and Case Studies”, EPRI Technical Report, 2004. [LAV 02] LAVERDURE N., BACHA S., ROYE D., RAISON B., DUMAS F., “Elements of modelling of wind power systems with energy management – two structures in comparison”, IECON 02, Seville, 2002. [LAV 04] LAVERDURE N., ROYE D., BACHA S., BELHOMME R., “Mitigation of voltage dips effects on wind generators”, EWEC 2004 Conference, London, 22/25 November 2004. [LAV 05] LAVERDURE N., Sur l’intégration des générateurs éoliens dans les réseaux faibles ou insulaires, PhD Thesis, Institut National Polytechnique de Grenoble, 9 December 2005. [LED 01] LEDESMA P., Analisis dinamico de sistemas electricos con generacion eolica, PhD Thesis, University of Leganes, Spain, 2001. [LIL 04] LILJA M., RISSANEN I., “Multibrid low speed synchronous generator with frequency converter”, NWPC 2004, Gothenburg, March 2004. [LUN 01] LUNDSAGER P., BINDNER H., CLAUSEN N.-E., FRANDSEN S., HANSEN L. H,. HANSEN J. C., “Isolated systems with wind power”, Main Report, Risø National Laboratory, Roskilde, 2001. [MAR 04] MARVIK I., BLORGUM T., INAESS B., UNDELAND T., GJENGEDAL T., “Control of a wind turbine with a doubly-fed induction generator after transient failures”, Nordic Workshop on Power and Industrial Electronics (NORPIE 04), Trondheim, 14/16 June 2004. [MAT 04] MATEVOSYAN J., ACKERMANN T., BOLIK S., SÖDER L., “Comparison of international regulations for connection of wind turbines to the network”, Proceedings of NWPC’04, Gothenburg, 1/4 March 2004. [MIL 03] MILLER N. W., “Power system dynamic performance improvements from advanced control of wind turbine-generator”, Fourth International Workshop on Largescale Integration of Wind Power and Transmission Networks for Offshore Wind, Billund, 20/21October, 2003. [MIN 03a] MINISTERE DE L'ECONOMIE, DES FINANCES ET DE L'INDUSTRIE, “Décret n° 2003-229 du 13 mars 2003 relatif aux prescriptions techniques générales de conception et de fonctionnement auxquelles doivent satisfaire les installations en vue de leur raccordement aux réseaux publics de distribution”, Journal Officiel de la République Française, No. 64, p. 4589–4592, http://www.legifrance.gouv.fr, March 2003.
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[MIN 03b] MINISTERE DE L'ECONOMIE, DES FINANCES ET DE L'INDUSTRIE, “Arrêté du 17 mars 2003 relatif aux prescriptions techniques de conception et de fonctionnement pour le raccordement à un réseau public de distribution d’une installation de production d’énergie électrique”, Journal Officiel de la République Française, No. 93, p. 7005–7008, http://www.legifrance.gouv.fr, April 2003. [MIN 03c] MINISTRE DELEGUE A L’INDUSTRIE, “Arrêté du 22 avril 2003 modifiant l’arrêté du 17 mars 2003 relatif aux prescriptions techniques de conception et de fonctionnement pour le raccordement à un réseau public de distribution d’une installation de production d’énergie électrique”, Journal Officiel de la République Française, p. 7904, http://www.legifrance.gouv.fr, May 2003. [MIN 03d] MINISTERE DE L'ECONOMIE, DES FINANCES ET DE L'INDUSTRIE, “Décret n° 2003-588 du 27 juin 2003 relatif aux prescriptions techniques générales de conception et de fonctionnement auxquelles doivent satisfaire les installations en vue de leur raccordement au réseau public de transport de l'électricité”, Journal Officiel de la République Française, No. 151, p. 11110–11113, www.legifrance.gouv.fr, July 2003. [MIN 03e] MINISTERE DE L'ECONOMIE, DES FINANCES ET DE L'INDUSTRIE, “Arrêté du 4 juillet 2003 relatif aux prescriptions techniques de conception et de fonctionnement pour le raccordement au réseau public de transport d'une installation de production d'énergie électrique”, Journal Officiel de la République Française, No. 201, p. 14896–14902, www.legifrance.gouv.fr, August 2003. [MUL 98] MULTON B., “L’énergie sur la terre: ressources et consommation. Place de l’énergie électrique”, La Revue 3EI N° 14, p 29–38, September 1998. [NAV 05] NAVARRO E., BADELIN A., SCHLÖGL F., ROHRIG K., MACKENSEN R., ARNOLD G., CHOWDHURY M. H., TAMZARTI A., BELHOMME R., BOUSSEAU P., GAUTIER E., BERENGUER A., CHOCARRO I., MATERAZZI-WAGNER C., HASLINGER M., TRAJANOSKA B., POSPISCHIL W., “Deliverable 7.2. DG in European interconnected grids”, Projet européen DISPOWER, Livrable D7.2, Document No. del_2005_0071, www.dispower.org, December 2005. [NII 04] NIIRANEN J., “Voltage Dip Ride Through of a Doubly-Fed Generator Equipped with an Active Crowbar”, Nordic Wind Power Conference, Chalmers University of Technology, Gothenburg, 1/2 March 2004. [OHR 03] ÖHRSTRÖM M., SÖDER L., “The use of fault current limiters as an alternative to substation upgrade when there is a need to increase the available short-circuit power”, Fourth International Workshop on Large-scale Integration of Wind Power and Transmission Networks for Offshore Wind, Billund, October 2003. [PED 06] PEDROSA E., “Gamesa eolica technology – latest developments”, EWEC 2006, Athènes, 27 February/2 March, 2006. [PEN 97] PENA R., CLARE J. C., ASHER G. M., “Implementation of vector control strategies for a variable speed doubly-fed induction machine for wind generation system”, EPE 1997 Conference Proceedings, 1997. [PER 99] PERESADA S., TILLI A., TORRIELLI A., “Robust output feedback control of a doubly-fed induction machine”, IECON 1999 Conference Proceedings, 1999.
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[PER 04] PERDANA A., CARLSON O., PERSSON J., “Dynamic response of grid-connected wind turbine with doubly-fed induction generator during disturbances”, Nordic Workshop on Power and Industrial Electronics (NORPIE 04), Trondheim 14/16 June 2004. [PETE 03] PETERSSON A., Analysis, Modeling and Control of Doubly-Fed Induction Generators for Wind Turbines, Department of Electric Power Engineering, Chalmers University of Technology, Gothenburg, 2003 [PETR 03] PETRU T., Modeling of Wind Turbines for Power System Studies, Department of Electric Power Engineering Chalmers, University of Technology, Gothenburg, 2003. [PUL 00] DE BATTISTA H., PULESTON P. F., MANTZ R. J., CHRISTIANSEN C. F., “Sliding mode control of wind energy systems with DOIG power efficiency and torsional dynamics optimization”, IEEE Transactions on Power Systems, vol. 15, no. 2, May 2000. [RAM 02] RAMOS C. J., MARTINS A. P., ARAUJO A. S., CARVALHO A. S., “Current control in the grid connection of double-output induction generator linked to a variable speed wind turbine” IECON 2002 Conference Proceedings, 2002. [RED 04] REDDY N., “D-VAR and SuperVAR technology presentation”, American Superconductor, Step Group Meeting, San Diego, March 2004. [ROD 00] RODRIGUEZ AMENEDO, J. L., Analisis Dinàmico y Diseño del Sistema de Control de Aeroturbinas de Velocidad Variable con Generador Asincrono de Doble Alimentacion, PhD Thesis, Madrid, 2000. [ROD 02] RODRIGUEZ-AMENEDO J. L., ARNALTE S., BURGOS J. C., “Automatic generation control of a wind farm with variable speed wind turbines”, IEEE Transactions on Energy Conversion, vol. 17, no. 2, June 2002. [RTE 05] RTE, “Référentiel technique de RTE”, Indice 1, www.rte-france.com, June 2005. [SAL 03] SALMAN S. K., TEO A. L., “Windmill modeling consideration and factors influencing the stability of a grid-connected wind power-based embedded generator”, IEE Transactions on Power Systems, vol. 18, N° 2, May 2003. [SCH 06] SCHELLINGS V., “GE Energy 2.X, field experience and improvements”, EWEC 2006, Athens, 27 February/2 March 2006. [SEI 05] SEI – SYSTEMES ENERGETIQUES INSULAIRES, Insertion d’éoliennes dans un réseau isolé, EDF, www.edfdistribution.fr, June 2005. [TOU 00] TOUNZI A., “Utilisation de l’énergie éolienne dans la production de l’électricité”, La Revue 3EI N° 20, March 2000. [VIT 04] VITHAYATHIL J., on behalf of CIGRE WG B4.35, “Thyristor controlled voltage regulators: non-conventional static reactive power regulation”, Electra, N° 212, p 74–83, 2004. [WWE 06] WWEA – WORLD WIND ENERGY ASSOCIATION, Worldwide wind energy boom in 2005: 58.982 MW capacity installed, Press Release, www.wwindea.org, March 2006.
Manufacturers and documentation [ENE] Documentation ENERCON: www.enercon.de
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[GAM] Documentation GAMESA: www.gamesa.es [GE] Documentation ENRON: www.wind.enron.com [IDS] Documentation IDS: “Converter Systems for Variable Speed Windpower Plants from 600 kW to 2500 k”: www.idsag.ch [JEU] Documentation JEUMONT INDUSTRIE: www.jeumont-framatome.com [MIT] Documentation MITSUBISHI: www.mhi.co.jp [MTO] Documentation MTORRES: www.mtorres.es [NORDEX] Documentation NORDEX: www.nordex-online.com [VES] Documentation VESTAS: www.vestas.com [WIN] Documentation WINWIND: www.winwind.fi [ZEP] Documentation ZEPHYROS: www.zephyros.com
French system operators [ERD] ERDF, Electricité Réseau Distribution France: www.erdfdistribution.fr. [RTE] RTE, Réseau de transport d’Electricité: www.rte-france.com.
6.4. Appendix: symbol table 6.4.1. Parameters and physical variables 6.4.1.1. Time variable
t : Time 6.4.1.2. Turbine, blades Parameters RT :
Turbine radius
Sb :
Turbine apparent area
CP :
Power coefficient
Variables
U:
Air density
V :
Wind speed at turbine hub level
E:
Wedge angle of turbine blades
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O:
Turbine speed ratio
ȍT :
Turbine rotation speed
PT :
Turbine pickup power
6.4.1.3. Mechanical system Parameters N:
Gear ratio
De :
Gear friction coefficient at high speed level
Kt :
Stiffness coefficient at high speed level
JT :
Turbine moment of inertia
DT :
Slow speed shaft friction coefficient
JG :
Generator moment of inertia
DG :
High speed shaft friction coefficient
Variables CT :
Turbine torque
Ctors : Torsion torque CG :
Generator torque
TT :
Slow shaft angle position
TG :
Rapid shaft angle position
6.4.1.4. Induction and synchronous generators Parameters Pn :
Generator nominal power
I Sn :
Generator nominal currents
m:
Transformation ratio
Lm , L'm :
Mutual cyclic inductances
LV 2 , LVR :
Rotor phase leakage inductance, real and referred to the rotor
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L2 , LR :
Rotor cyclic inductances, real and referred to the stator
LVS , LV 1 :
Stator phase leakage inductance, real and referred to the rotor
LS , L1 :
Stator cyclic inductances
Ld , Lq :
Stator direct and transverse cyclic inductances for a synchronous machine
p:
Generator pole pair number
RS , R1 :
Stator resistances, real and referred to the rotor
R2 , RR :
Rotor resistance, real and referred to the stator
V
L2 1 m
LS L R :
Leakage coefficient
Variables i2 , i R :
Rotor currents, vectors real and referred to the stator
iS , i1 :
Stator currents
v S , v1 :
Stator voltage
v2 , v R :
Rotor voltage, real and referred to the stator
M S , M1 :
Stator flux, real and referred to the rotor
M2 ,M R :
Rotor flux, real and referred to the stator
M fS :
Stator flux generated by excitation
ī elm :
Electromagnetic torque
ȍG :
Rotation speed
PtS :
Air gap generator power
PS
P1 :
Stator active power
QS
Q1 :
Stator reactive power
P2
PR :
Rotor active power
Q2
QR :
Rotor reactive power
Pm :
Mechanical power
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T:
Rotor–stator electrical shift angle
Z:
Mechanic angular velocity
ZR :
Rotor angular velocity
fR :
Rotor frequency
g:
Slip
6.4.1.5. DC bus Parameters C:
Decoupling capacitor
Rd :
Load resistance
Variables UC :
DC frequency inverter voltage
Chapter 7
Marine Energy Resources Conversion Systems
7.1. Introduction The annual solar energy impacting the Earth’s surface represents more than 6,000 times the entire human consumption of primary energy. Since oceans occupy more or less two thirds of the Earth’s surface, they capture, transfer and accumulate enormous quantities of energy, but they also play a major role in the climatic equilibrium of the planet. Life, which has evolved in the waters of the Earth, uses some of these renewable resources and at the same time constitutes an important reserve of biomass and a system for the biological transformation of energy. The massive use of seaweed for the industrial production of methane or even hydrogen from solar radiation is under investigation. Various estimates of the possible energy to be extracted have been given, giving special consideration to the various kinds of environmental impact [PEL 02]. They show evidence of a considerable potential that can contribute to satisfying a large part of human needs, the principal difficulty being linked to the fact that the areas of marine production are usually distant from the areas of consumption. Besides the non-renewable sources (fossil fuels, including methane hydrates) which are found underneath the oceans and which we will not discuss in this chapter, maritime expanses capture a large quantity of solar energy and provide access to various forces of nature (wind, waves, marine currents, etc.). Their waters also store an immense quantity of heat that can be partially used in the favored areas Chapter written by Bernard MULTON, Alain Julien SEIGNEURBIEUX and Hamid BEN AHMED.
CLÉMENT,
Marie
RUELLAN,
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to address problems associated with intermittence. The winds and thermal marine currents (continuous) have their origin directly in solar radiation. As for ocean waves, they are a byproduct of the action of the wind on the surface of the waters. Tidal currents (alternating) are created by the gravitational interaction of Earth– moon–sun and are not associated with solar radiation. Finally, marine geothermics (heat coming from the Earth’s core), even though it no doubt represents a considerable resource, is not discussed in this chapter. It could well be utilized using the same methods as on land, but the difficult conditions of the marine environment as well as the depth of the sea and the distance to the coast mean that it is not currently considered in estimating global resources. The use of marine biomass is also not discussed in this chapter. Except for the very particular and marginal case of tidal plants (dams for recovering the potential energy of tides), the production technologies for renewable marine electricity are still quite recent. The use of wind power in the offshore zone really began in 1991 at Vindeby (Denmark) and the experience accumulated has begun to be significant [IEA 05]. Systems for recovering thermal energy from the sea are still in the demonstration stage. The situation for marine currents is the same as that for ocean waves. Even if there are enough of these demonstrations to have been tested, they are still in the experimental phase, sometimes preindustrial, and they probably require several more decades to develop competitive technologies, as much in terms of cost as reliability. In this chapter we will address available resources and then the principal technological solutions for their conversion into electricity. Offshore wind energy production will be mentioned very briefly, since wind generation technologies are covered in Chapters 5 and 6. As a point of reference, let us first recall that the current annual human consumption of primary energy (including estimated non-commercial sources) is approximately 140 x 1012 kWh (140,000 TWh) or 12 Gtoe (ton of oil equivalent). The worldwide production of electricity in 2004 was 17,000 TWh. Moreover, this figure for primary consumption does not really correspond to human needs, since the losses, in all the conversion chains leading to final use, are very high and contribute widely to the exhaustion of non-renewable primary (energy) materials. On the other hand, a large part of humanity still has only very limited access to energy. To establish an order of magnitude corresponding to the needs that would be realistic, let us start with the principle that the 10 billion inhabitants who may one day live on the Earth consume as much final energy as a French citizen now does (2.9 toe a year). That means we will need about 300,000 TWh. This figure, no doubt fantastic, can nevertheless serve as a reference to compare with the available renewable sources in the marine environment.
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7.2. Electricity productivity from marine resources 7.2.1. Energy sources from the sea 7.2.1.1. Solar heat The oceans and seas constitute an immense collector of solar radiation that stores and transfers a gigantic quantity of heat, thus contributing significantly to the thermal equilibrium of the planet. We may consider using part (the smallest possible to minimize the impact) of this energy resource in the areas where temperature variations are sufficiently large to consider employing thermodynamic machines at a favorable cost (see section 7.5.2). Sufficient temperature gradients are found in equatorial and tropical regions. The surface temperatures there can reach values between 25 and 30ºC in a layer 100 to 200 m thick and their seasonal variations are not markedly different. At a depth of between 200 and 400 m, the temperature suddenly diminishes (thermal barrier) and then progressively declines to 4ºC at 1,000 m. There are charts [LEN 05] that show divergences in temperature between the water at the surface and at a depth of 1,000 m. The raw solar energy resource captured annually by the oceans is enormous, in the order of 400 x 1015 kWh, of which only a tiny part is accessible and, as we have already said, there is no question of drawing massive amounts from this natural cycle due to its crucial role in the stability of the global climate. Considering the global output of all the deep cold currents (30 Mm3/s) and the fact that conceivable thermodynamic machines require a very high discharge of cold water (about 2 m3/s per MW) and weak conversion efficiency in the order of 3% (principally due to the small difference in temperature between the hot and cold sources), we reach a maximum annual potential of 80,000 TWh, which is renewable [AVE 02, AVE 94]. 7.2.1.2. Wind energy The wind at the surface of the sea gives off an energy estimated at 500 x 1012 kWh. Again, there is a limit on its use that must not be exceeded so as not to influence atmospheric circulation and climate. The global offshore potential [BES 02] is estimated at 37,000 TWh (including sites between +/-72º latitude and at least 30 m in depth) to be nearly three times the world production of electricity. In Europe, the net potential (electric energy converted by wind generators of about 100 m in diameter at a rate of one per km2) has been estimated [MAT 95] at about 3,000 TWh, considering areas where the depths are less than 40 m and at most 40 km from the shore. This hypothesis has been revised down, by revising the constraints [EWE 99], to about 310 TWh annually, which remains considerable in comparison to European production of about 2,600 TWh. As a comparison, the European land potential was estimated at 630 TWh in this same study. It is in the offshore region that the most significant growth in production is expected in the long term. Production at sea in fact offers two considerable advantages: better
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productivity (winds stronger, more regular and having more energy, from 3,000 to more than 4,000 equivalent hours of full power) and a reduction in the visual impact. On the other hand, like other marine sources, it requires transmitting electricity to the shore, toward the places of consumption, and that means a higher cost, and is the reason for a limit of 40 km distance from shore. In 2005, the two largest offshore wind power parks in Denmark are Horns Rev and Nysted, which total 160 MW and 158 MW respectively, out of a total of sea installations of 785 MW. The Danish energy plan Energi 21, developed in 1996, set as its objective the installation of 4,000 MW offshore by the year 2030. They would therefore supply the equivalent of about 40% of Danish electricity consumption. 7.2.1.3. Ocean wave energy Resulting from the effect of the wind on the ocean surface (the power of the waves is roughly proportional to the power of 5 of the wind speed), the energy from waves represents, according to the World Energy Council (WEC) [THO 04], a net available quantity of 140 to 700 TWh/year, or 1 to 5% of the annual world demand in electricity. The recoverable energy could even reach 2,000 TWh/year with more efficient conversion systems. There are [THO 04] maps of the world that specify the orders of magnitude of average annual power from the raw ocean waves source, quantified in kilowatts per meter of wave front with orders of magnitude between 10 and 100 kW/m (Cape Horn). However, it is necessary to consider the fairly low conversion efficiency (in the order of 10% average over the year) of current recovery systems. The European Atlantic coasts are especially well exposed with values of 30 to 60 kW/m. 7.2.1.4. Tidal currents The order of magnitude of the energy dissipated annually by the tides is estimated at 22,000 TWh. The recoverable part in dam systems is estimated at 600 TWh/year, of which 48 are in Europe and 20 TWh/year in France [BOU 04, FER 06]. A dozen estuary sites alone allow an annual production of close to 200 TWh, of which more than 100 TWh are in the sea of Okhotsk on the east coast of Siberia. Today, around the world, three tidal plants produce 0.57 TWh annually. The recovery of kinetic energy from marine currents by turbines further increases the recovery potential, since it is not necessary to construct dams, whose environmental impact is controversial; these sites therefore are much more numerous. The global potential has not yet been completely estimated, but it is without doubt higher than that of dam systems. In Europe alone, it is estimated at about 50 TWh, considering only the regions near the coasts, where the currents are rapid enough for an acceptable technical-economic scale of turbines.
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7.2.1.5. Continuous ocean currents These currents, as distinct from tidal currents, result from a complex agitation of the ocean waters principally due to temperature and salinity gradients. Bearing in mind the flow rate and the global volume, we can calculate that it takes about 1,000 years for a complete mixing of the ocean waters [BON 98]. Only some currents are fast enough to be usable; this is the case with the Gulf Stream, which gives an average flow rate of 30 Mm3/s with speeds between 1.2 and 2.7 m/s, while the equatorial currents have speeds more in the range of 0.2 to 0.3 m/s. The global resource is hard to estimate. The Gulf Stream alone offers a kinetic power of more than 30 GW and about 300 TWh annually. However, there is no question of massive extraction of its energy without serious climatic consequences. 7.2.1.6. Osmotic energy The recombination of fresh water with salinated seawater produces a diffusion phenomenon (osmosis) that can liberate energy. Use of a specific membrane enables us to obtain an osmotic pressure due to the diffusion of the fresh water toward the seawater, a usable pressure for driving a turbine. The conversion methods [JON 03], conceived at the beginning of the 1970s, are still in the planning stage, not very far advanced (Norwegian company StatKraft Hydropower Company) and we have chosen not to present them. This source of energy is renewable since it is solar energy, at the moment of the evaporation of the seawater, that assures the natural cycle of evaporation, condensation, return of fresh water to the saline ocean. In terms of environmental impact, we know that the arrival into the ocean of fresh water and the associated nutrients play an important role for marine animals, so we must evaluate the acceptable threshold of removal. The world energy resource is difficult to estimate, since it depends on the exploitable rivers and the annual variability of their outflow. The annual quantity of water that flows from land toward the oceans represents about 40 x 10 12 m3 [BON 98] or an average outflow of 1.3 Mm3/s. The annual gross world osmotic resource would be, on this basis, 30 x 1012 kWh. For example, the Mississippi, whose average outflow is from 18,000 m3/s, carries a gross source of 420 TWh, of which 160 TWh would be recoverable with technological solutions now foreseen [THO 00]. By allowing a capping of the discharges during periods of flood and partial usage of this source, taking into account the fact that the waters are filled with a large quantity of solid and organic materials, the Mississippi River alone has the capability to generate a huge annual amount of energy. We find the same order of magnitude in other rivers such as the Amazon, the Ganges, the Danube, etc.
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7.2.1.7. Ocean biomass The phytomass or vegetable ocean biomass represents a primary raw production of about 450 x 1012 kWh (600 in land regions) [BON 98]. On land, the primary part, realistically exploitable, is estimated at about 7,000 TWh or 1.5% of the raw production, and it is about 1,000 TWh that are actually used. Marine biomass has not yet been exploited and it is difficult to determine what is truly recoverable in an ecological and economically acceptable fashion. 7.2.1.8. Evaluation
Primary exploitable part Recoverable electric energy
Continuous ocean currents
Marine biomass
Osmotic
Global estimate
Ocean solar
Waves
400,000
?
450
>> 500
80
30
4,000
1?
4
100
2
0.3?
80
0.3?
1
(K 2%)
(K 30%)
(K 25%)
Tides Currents
Hydraulic cycles
Estuaries
Annual energy 1012 kWh (103 TWh)
Wind power
Table 7.1 sets out an overall evaluation of oceanic resources. The figures should certainly not be taken literally; it is about the order of magnitude. The recoverable parts are often debatable; the preceding text enables us to get an idea of the margin of uncertainty. For comparison, the annual world production of electricity in 2005 was 17,000 TWh.
22
0.6
1.8
37 0.75 0.1? 0.6 (K 37%) (K 35%) (K 30%)
0.9
Table 7.1. Order of magnitude of renewable energy sources and their exploitable part as primary energy, then electrical with the means of production whose efficiency Ș has been established
7.2.2. General technical-economic aspects With all the sources mentioned, there is no raw material consumed at the time of conversion to energy. The cost of electricity production therefore will be due only to the amortization of the investment, to the costs of maintenance and upkeep, to the life expectancy of the system and to the cost of its dismantling, and also to the duration of its full power equivalent operation. In fact, a good part of the cost of
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equipment, especially the electromechanical parts, is directly linked to peak power. In the case of strongly changing intermittent sources (ocean waves, wind), it is not economically profitable to size the system for extreme energy situations (storms) as, even if the power from the source would be very high, the cumulative duration of production at these levels of intensity is not generally sufficient to make up for the higher cost of investment. This is why the recovery systems should clip the top of the power received but also stop producing during critical situations. Taking into account the intermittence, clipping of the top, outages, and, of course, the productivity conditions of the site, we get an annual full power production duration equivalent generally much less than 8,760 hours. In offshore wind power, the value of this equivalent production duration typically lies between 3,000 and 4,000 hours. The tidal phenomena, besides the fact that they are very predictable, have well defined effects that allow 2,000 to 4,000 hours of this equivalent. Finally, the exploitation of thermal energy from the seas would allow continuous function at full power, except during maintenance operations. In the case of offshore technologies, the interaction of the centennial1 waves and other caprices of the severe marine environment, such as rogue waves, requires special sizing and experience that at present is still very limited. Moreover, this experience comes largely from offshore petroleum and gas extraction technologies. The difficulty of maintenance operations at sea also requires high levels of reliability to be achieved, and consequently, once again, sufficient experience is needed. In other words, the marine energy sector needs real help to take off, because the installation of specific technologies cannot be achieved without numerous setbacks, which are needed in order to learn how to overcome them. The maturation of wind power technologies has in the end taken about a century. It could undoubtedly have been achieved faster if there had been more will, but probably we could count in decades the time necessary to trigger a falling economic learning curve because of market attractiveness. Figure 7.1 shows an example of such learning curves published in the EPRI (Electric Power Research Institute) report in the USA [BED 05a]. The report [WAV 03] brings to light a similar variation in production costs for ocean wave generator systems with respect to weather (a range of 40 to 55 c€/kWh in 1980 and 5 to 11 c€/kWh in 2001). Most of the technologies mentioned are not mature and sometimes they are extremely disparate, as is still the case for ocean wave generator devices, so the investment costs are still often imprecise. Nevertheless, we will give several investment cost values in specific chapters on the different technologies, knowing full well that they will change considerably if they develop.
1 Having a probability of occurring once every 100 years.
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Figure 7.1. Example of economic learning curves [BED 05a]
7.3. Ocean wave generator systems (WEC: wave energy converters) 7.3.1. Wave energy characteristics Waves are formed and maintained locally by the wind and take the same direction as it does; their period rarely exceeds 8 seconds. The wave spreads beyond the region where the wind has given it birth, with slower oscillations, typically 10 seconds, with a long wavelength (150 m) and a speed of propagation (or phase velocity) of around 14 m/s. The period can reach 25 seconds and the length of the wave 900 m in a heavy sea (36 m/s). Ideally (pure sinusoidal wave approximation and infinite depth), the length of the wave is linked to the phase velocity and to the period by the classic relation:
O
[7.1]
c.T
In deep water and for long wavelengths, the propagation velocity c and wave period T are functions of the wavelength c
g.O and T 2S
2S .O g
[7.2]
The depth of the sea also plays an important role in the sense that shallow depths favor the dissipation of energy by bottom friction and wave breaking. Thus, on
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arriving on shore, waves have generally lost a large part of their mechanical energy. Unfortunately for the simplicity of characterization, a field of waves is composed of multiple billows that do not all spread out in the same direction. A simple way of characterizing a random sea state [GOD 00] is to limit ourselves to only two parameters: a crest to trough wave height Hs (significative height calculated as the average of the third of the highest waves, sometimes denoted by H1/3) together with a period Tp denoting the period of the spectral peak. These two parameters permit us to represent a random sea state by referring to a standard model of spectral distribution of the energy. We generally estimate the period of the stability of this sea state in nature to be from one to three hours. Measurement buoys moored for a very long period on a given site allow us to draw up statistics and establish monthly or annual probability distribution diagrams (or “scatter diagrams”) of Tp and Hs, such as the one in Figure 7.2, or in the form of a table defining the cumulative number of hours per year as a function of Tp and Hs. Such databases already exist for many regions of the world. The characteristics of producible power for wave energy converters are then defined as a function of Tp and Hs by a production matrix that allows us to estimate easily the annual productivity on a given site by the “intersection” of the two matrices.
Figure 7.2. Examples of distribution diagram of Tp and Hs of the wave generation resource at a particular site (Gorringe in Spain) [BUR 05]
The power of a pure traveling wave (perfectly sinusoidal and unidirectional) can be calculated quite easily if the depth of the milieu in which it propagates is supposed to be infinitely deep (in practice larger than half a wavelength). Under these conditions, we can show [CLE 02] that the power (average) transmitted per unit width of the wave front in the direction of propagation is expressed as: Pw
ȡ.g 2 2 .H .T | 980.H 2 .T (W/m) 32ʌ
[7.3]
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where ȡ is the density of the water, g the acceleration due to gravity, H the crest to trough height of the waves and T its period. With seawater (density of 1,024 kg/m3), the coefficient 32 ʌ equals about 980 in SI units. For an irregular sea whose spectrum is specified by the significant wave height Hs and period Tp, we obtain:
Pw | 420.Hs2 .Tp (W/m)
[7.4]
These expressions can seem a priori to be surprising, since we would expect a power proportional to the inverse of the period. The result of expressions [7.3] and [7.4] is due to the behavior of gravity waves (see expressions [7.1] and [7.2]). Thus, we can show that the power per unit of size of wave front is indeed proportional to T or c. The resource is sometimes characterized by its average height Hm rather than Hs, and its average period of passage to zero Tz, rather than Tp. In this case, the respective equivalences with the significative height Hs and period Tp are given by: Hs # 1.6.H m and Tp # 1.4.TZ
Besides the fact that waves undulate at a low frequency and cause instantaneous fluctuations of power, the average power of the source fluctuates considerably as a function of the sea state, from calm to stormy weather. If we apply the expression [7.4] to a random wave, we obtain the following orders of magnitude of linear power for a “fairly calm sea” (5 s, 0.6 m) 0.7 kW/m and for a “very heavy sea” (15 s, 18 m) 2 MW/m. It is the extent of this scale of incident wave power that shows the challenge facing the engineers who must design systems that can work for sea states of low to medium power (the most frequent) while surviving extreme sea states whose colossal power is capable of ruining huge offshore platforms or ships of very heavy tonnage. The recoverable power then depends on the size of the captured wave and the efficiency of the conversion device. The wave energy converters often have a behavior relatively suited to a given type of wave and, even if advanced control allows optimizing of the extraction of power under varied conditions, their characteristics of output power are not really proportional to Hs2 Tp. When an elongated floating object (such as a ship) is impelled by the waves, it undergoes movements according to its six degrees of freedom: three in translation (surge, heave, sway) and three in rotation (pitch, roll, yaw). The surge, sway and heave are respectively the movements of translation with respect to the longitudinal axis, the transverse axis and the vertical axis. The roll, pitch and yaw are the movements of rotation with respect to the longitudinal, transverse and vertical axes. To partially measure the complexity of the waves’s characteristics, Figure 7.3 gives an example of the height fluctuation over a period of one year.
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Figure 7.3. Example of fluctuations of significative height Hs in one year (1999)
, , , , , , ,
Figure 7.4. Example of fluctuations of the instantaneous power of the source over 350 seconds (on the y axis, an arbitrary scale; on the x axis, time in seconds)
Figure 7.4 gives an example of the resource’s power fluctuations over several minutes (instantaneous fluctuations). In the last figure, we can observe variations in the scale of the period Tp but also wave clusters at a lower frequency, called wave groups, a phenomenon well known to swimmers and surfers.
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7.3.2. Diversity in conversion systems
Technologies often have a very long history. This is so in the case of recovering energy from ocean waves. We will cite two patents (Girard 1799 and Barrufet 1885), in which the inventors tried to design machines to use the heaving movement of the waves (vertical component). Barrufet suggested an ingenious machine made up of independent floats transmitting their alternating movements to a common transmission shaft in continuous rotation, thereby evening out the local fluctuations. In addition to the very strong constraints of the marine milieu, one of the difficulties consists precisely in trying to resolve problems of fluctuations, at least on the scale of the period Tp. Since the 1970s, numerous devices have been conceived, patented and/or tested [WDK 01], but it is not possible to make an inventory of them here. However, except for the unclassifiable ones, we can suggest a simplified classification [CLE 02]: – Systems with a breakwater: the seawater breaks on a ramp, crosses its threshold and fills a reservoir placed behind. It is then passed through a low head turbine to return to the sea. These systems offer the advantage of evening out the power and obtaining a relatively regular production, or at least one that does not fluctuate with the waves. Not long ago, they were located on the coast, using the natural configuration of the land to create the basin at the least cost (a lagoon, for example), but now there are floating systems. – Systems with an oscillating water column: placed on the shore or on floating machines, a cavity open to the action of the waves by a submerged mouth has its free internal surface oscillate like a liquid piston. The air of the cavity is then alternately expelled, then admitted, by an exit channel to the atmosphere. A turbine, whose direction of rotation does not change with respect to the direction of the air circulation, is placed in the channel. The turbines drive an alternator that in this way produces continues energy. – Systems with wave activated bodies: many systems with floats on the surface or submerged floats have been conceived. Some of them, presenting several bodies animated by the waves, use the relative movement of these bodies – we therefore talk of an internal reference (or on-board reference). Others use the movement of a single body with respect to a fixed reference (generally the sea bed by the intermediary of fixed structures or taut moorings) – we then speak of an external reference. A second criterion is that of location, according to whether the devices are on the shoreline, in the sea but near shore with a sea depth of less than 50 m to allow anchorage, or offshore. The shoreline production plants must satisfy numerous
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constraints, notably adaptation to the topology of the site, but also its acceptability, their main advantage residing in the ease of connecting to the electricity grid. The near shore or offshore systems, enable us to consider ocean wave generators that are more standardized and benefit from a greater energy resource. They usually consist of identical modules installed as a farm, electrically connected together in order to coordinate electricity production by requiring only one common cable to the coast. They are also called wave power farms. Generally, ocean wave generators, by taking part of the resource, absorb the waves by reducing the peak to trough height. Inasmuch as wave characteristics and recovery technology play an important role, we can estimate that 10 to 30% of the resource can be recovered. The characterization in watts per meter of the wave front makes it difficult to estimate the recoverable power per unit of marine surface. Several estimates have been made, especially with the Pelamis system, which we will describe later. For example, in a milieu whose source is about 75 kW/m, we can extract 30 MW per km2 of occupied maritime space and an annual energy of 110 GWh/km2, or about six times more than offshore wind power, but the space is occupied more densely than for wind power. The investment costs of ocean waves generators near their maturity is estimated today (2006) in the range of 1 to 3 €/W and the production costs, depending of course on the technology and the local conditions, could go in several decades from now from some 10 c€/kWh to several c€/kWh. Ocean wave generators could take advantage of the offshore wind power development, especially the electricity transmission systems to land. 7.3.3. Systems with breakwater ramps
We will cite two systems: the Tapchan (tapered channel) constructed in 1985 on the coast on the Toftestallen site in Norway, with a power of 350 kW and shut down in 1991 following a storm, and the Wave Dragon, a floating system whose prototype of 1:4.5 scale was placed in service in 2003 in Denmark. The Tapchan principle is described in Figure 7.5. The effectiveness of filling the reservoir depends on the height of the waves relative to that of the breakwater ramp, but it depends just as much on the effects of the tides. The most recent solutions allow a better adaptation to the height of the waves by using a breakwater ramp including a multistage capture system along the ramp (Seawave Energy – SSG – Seawave Slot-cone Generator).
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Figure 7.5. Principle of the Tapchan breakwater system
The ocean wave generator Wave Dragon (www.wavedragon.net) is floating and moored, its height of flotation adjustable with respect to the characteristics of the waves. The dimensions of the scaled version 1 are 300 m (distance between the extremes of the arms), 170 m (length) and 17 m in height, of which 3 to 6 m are above the level of the sea. The total mass is 33,000 tons with a reservoir capacity of 8,000 m3. Its maximum power is 7 MW with an annual productivity of 20 GWh for an average resource of 36 kW/m. Thus the number of hours equivalent to full power reaches 2,800. The water is passed through a turbine low head (a priori Kaplan). It is advantageous to use several turbines of low power (here 16 to 20), rather than only one, which improves the output with respect to the available discharge. The resulting efficiency curve is thus greatly improved. Permanent magnet generators of variable speed make it possible to further increase the global efficiency. In the reduced scale system, with the same maximum installed power, going from a system with two Kaplan turbines of fixed speed to a system with 16 Kaplan turbines of variable speed increases the annual energy from 1.79 to 2.04 GWh and reduces the Pmaxium/Paverage ratio, an indicator of the production fluctuations from 12.4 to 10.1 [SCH 05, SOE 04]. On the British Atlantic coast bordering Wales, the installable production capacity per km2 of ocean surface is about 28 MW. This is even higher on the Irish and Scottish coasts. The first machine of the scale 1:1 has been installed in 2007 in Wales (Milford Haven). Figure 7.6 shows two schematic views of the Wave Dragon device and Figure 7.7 shows a photograph of the device with reduced scale (20 kW, 237 tons, 58 m x 33 m, reservoir of 55 m3).
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Figure 7.6. Views from above and in profile of the floating Wave Dragon system, courtesy of Wave Dragon (www.wavedragon.net) [SOE 05]
Figure 7.7. Photograph of the Wave Dragon with reduced scale (58 x 33 m), courtesy of Wave Dragon
7.3.4. Oscillating water column (OWC) systems
This may be one of the most used principles, and, of course, it is borrowed from nature, in which “blow holes” found in rocky coasts clearly reveal the inflow and outflow of air trapped in a cavity subjected to the fluctuations of the waves.
Figure 7.8. Oscillating column device LIMPET [MAC 04], courtesy of Wavegen
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As examples of coastal systems, we cite in particular the Kvaerner column (Norway) that operated between 1985 and l988 (total height 25 m, internal free surface 56 m2 , turbine 500 kW at 1,500 rpm); the European pilot project Pico, from the name of the island in the Azores, in Portugal (chamber 12 x 12 m of 8 m height, 400 kW, Wells variable pitch turbine); the Energetech converter of 1 MW in Australia (modular system with parabolic collector: 35 m opening); and the LIMPET (Land Installed Marine Powered Energy Transformer) of the Wavegen company, installed on the coast of the Isle of Islay, Scotland, having a power of 500 kW. Finally, we cite the installations in the ports of Haramashi and Sakata, Japan, as well as that in Kerala, India.
Figure 7.9. Wavegen turbo generator with Wells turbines being installed (group of 500 kW), courtesy of Wavegen [MAC 04]
Here we will give several details of the Wavegen company’s LIMPET system, installed at Islay [FOL 05]. The first prototype (1991) had a power of 75 kW. A second version of 500 kW was put in service in 2001 and is connected to the electricity grid of the UK. The present system captures the variations in pressure of three water columns offering a total capture surface of 169 m2. The turbo generator consists of two Wells turbines with fixed pitch of 2.6 m in diameter in counter rotation, each one directly driving a doubly fed asynchronous generator of 250 kW running at variable speed (700 to 1,400 rpm). The Wells turbine has the advantage
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of a torque motor that does not change sign with the direction of the air circulation. The Isle of Pico uses, on the other hand, a Wells turbine with variable pitch. Acoustical sound, known to disturb the turbines, is attenuated here by a specific acoustic chamber. The Wells turbines make use of the alternating air flow produced by the oscillations of the columns’ free water surface by which means they can optimize absorption for maximum power recovery. In the LIMPET system, the turbines have a fixed pitch, their efficiency curve, related to the “tip speed ratio” Ȝ Ȝ = v/(R.:) where v is the average velocity of the air flow, and r: is the peripheral velocity at the blade tips), is in the form of a pointed bell. As in the case of wind power, and for the same reasons, the variable speed allows us to better maximize the output of the turbine in terms of average air flow. The peak output is higher than 70% at steady state, but it falls at least 30% in a cycle. The high moment of inertia of the turning parts (1,200 kg/m2 in this example) produces a smoothing out of the fluctuations in the power of the resource. The Wavegen company offers various applications using its design, especially for creating a breakwater, integrating several generators of the LIMPET type. Let us note that the Australian Energetech1 MW converter uses a variable pitch turbine (Denniss-Auld type. Like the Wells turbine, its torque keeps the same sign at the times of inversions of flow) connected to a squirrel cage asynchronous generator (twelve poles) of variable speed (two three-phase converters with pulse width modulation: one on the machine side and one on the grid side). The system is anchored and placed on feet adapted to the depth of the location. Figure 7.10 shows the array of the Energetech sea wave generator’s power characteristic.
,
,
Figure 7.10. Power curves (in kW) of the Energetech ocean waves generator
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Finally, among the offshore floating and anchored oscillating column systems, we can list: – the Sperboy (UK) made of a float 4 m in diameter under which are tubes (oscillating water columns) descending to 12 m under the surface of which are the conversion system containing the compression chambers and an ensemble of four horizontal turbine generators delivering a total maximum power of 140 kW, – the Mighty Whale (Jamstec: Japan Marine Science and Technology Center) is a prototype ship of 50 m by 30 m and 12 m in height, put into service in 1998. The maximum power is 110 kW for Hs = 8 m and Tp = 10 to 15 s. The conversion system comprises three chambers of oscillating columns joined to three groups of asynchronous Wells turbine generators (1 x 50 kW and 2 x 30 W) of variable speed (300 to 1,800 rpm). 7.3.5. Systems with wave activated bodies
One of the best known representatives of this vast family is the Salter Duck, named after its inventor, designed in the 1970s. Its principle is to use off-center asymmetric watertight caissons, rolling around an axis, activating hydraulic pumps. This principle has encountered numerous technical difficulties, especially associated with keeping its long axis face to the waves. Even though this system has received numerous improvements, it now seems to have been surpassed by other devices; only the future can determine whether they survive by “natural selection”. Let us list some of these devices before going into more detail on the characteristics of the Pelamis: – Power Buoy (Ocean Power Technologies, Inc., USA) [TAY 03] consists of an immersed buoy, part fixed taut anchorage, part oscillating to the rhythm of the waves; the relative movement is absorbed in order to be converted into electricity. Its presence under water is indicated only by a marker buoy for navigation. This system, whose natural frequency makes its performance very sensitive to the period of the waves, requires a specific control to maximize the extraction of energy, including, in particular, a predictive behavior. The conversion device includes a pump, an accumulator and a hydraulic motor driving an electric generator. Some versions of a direct linear electromagnetic generator are also being studied. A model of 40 kW was tested between 1997 and 2002; it is 9 m high and has a diameter of 1.5 m at the level of the float, for a mass of 2,140 kg. Models of 150 and 250 kW are being considered. On 20,000 m2, 40 buoys of 250 kW enable installation of a production capacity of 10 MW.
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– Archimedes Wave Swing (AWS, Netherlands): the pilot version, tested in 2004 off the coast of Portugal, has a nominal power of 1 MW (average over a cycle) and peak of 2 MW. Each production unit is immersed at 8 m beneath the surface, a cylindrical float of height 21 m and 9.5 m in diameter is placed in oscillation by the waves and compresses the air trapped between itself and a cylinder anchored to the bottom. It drives the mobile part of a direct permanent magnet linear synchronous generator (1 MN, 2 m/s). Thanks to a power electronic converter and an adaptive control, the system allows optimal use of wave periods between 9 and 20 seconds. Thanks to the effect of mechanical resonance, a 1 m waves amplitude can give movements of 7 m (maximum run stroke of the generator). Contrary to the final solution being considered, which will be anchored via cables, the experimental device is placed on a pedestal (a sunken barge at the site) that allows us to retrieve it easily. The commercial version is expected to have a diameter of l2 m, a maximum stroke of 12 m and an average power of 4.75 MW. – SEAREV (Autonomous electric system for recovery of wave energy, project carried out by the Ecole Centrale of Nantes). The Searev design consists of a completely enclosed float, of which one pendulum mass (400 tons) is placed in oscillation by the indirect oscillations of the waves [BAB 05]. In the prototype version, it is the hydraulic jacks that absorb the pendulum and fill the accumulators. A mechanical control in real time, piloted by an on board computer, enables the pendulum system to be maintained in a state of parametric resonance in spite of the irregular character of excitation from the waves. Hydraulic motors drive asynchronous generators for a maximum power of 500 kW. A completely electric direct drive solution is also under study [RUE 05]. – Pelamis (Ocean Power Delivery, Scotland, http://www.oceanpd.com/). The Pelamis system [YEM 00], now commercialized (model P750), consists of a group of four floating metallic cylinders linked together by three articulations of two degrees of freedom, and resembling a snake 4.6 m in diameter and a with total length of 123 m (700 tons, of which 380 tons are steel). The behavior of the system makes it follow, more or less, the distortion of the free surface, which in fact makes it a “profiler” rather than a resonator like the previous systems. This general shape allows the system to adapt to extremely varied sea states and to make good use of their energy. In each articulation (see Figure 7.11) there are four hydraulic jacks, two of which use and absorb the heave (vertical) and the two others the sway (transverse). These pumps accumulate energy in the form of oil under pressure in a reservoir (100 to 350 bars). Two hydraulic motors that turn regularly each drive an asynchronous generator of 125 kW at 1,500 rpm (fixed speed). Thus, the device offers the advantage of smoothing out a naturally fluctuating energy.
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Its life expectancy is 15 years. The maximum electric power is 750 kW (three articulations each comprise two generators of 125 kW).
Figure 7.11. Ocean waves generator Pelamis P 750 (750 kW), detail and cross-section of the active articulation [HEN 05]. Courtesy of Ocean Power Delivery Ltd
Figure 7.12 shows the chart of power with respect to two characteristic parameters, Hs and Tp, of the waves. On the plot of Figure 7.12, at constant source period, we note an increase and then a cutting off of the peak, necessary for sizing the system. The architecture of Pelamis leads to a relatively manageable behavior, meaning that the recovered power does not follow the increase in power of the wave generator with the period.
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h
In 2008, three P750 units were installed 5 km off the coast of Portugal to form an experimental ocean wave power farm of 2.2 MW for a total of 8 M€ . In principle, a surface of 1 km2 allows installation of a production capacity of 8 to 30 MW. As in the case of offshore wind farms, the units, spaced at about 150 m, are organized in clusters.
h
Figure 7.12. Pelamis electric power: a) chart of electric power of the Pelamis P750 (750 kW); b) curve function of Hs for a period of 8 s (courtesy of Ocean Power Delivery Ltd)
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7.4. Tidal energy converters (TEC) 7.4.1. Characteristics of tides and other marine currents
The use of tidal energy is ancient, as numerous tide mills show. Recovery can be made in its various potential forms, these being variation in sea level and exploitation via a dam, and kinetic forms, these being direct exploitation of currents by turbines placed directly in the flow like submarine wind turbines, also called hydraulic turbines. These two principles also give rise to two families of tide motor systems. One of the advantages of tides is their great predictability, which facilitates planning and allows better insertion of such production systems into the grids, even if there is not a correlation between availability and demand. With dam systems, the effect of storage upstream and downstream can be put to use for creating storage, eventually by pumping in the advantageous parts of the cycle. There are different kinds of tides, depending on the location. The most energetic ones have about two cycles per day with a period of about 12.5 hours and amplitude that varies appreciably in a sinusoidal manner over a cycle. The tidal range or amplitude of the tides (between high and low tides) can vary considerably with respect to the place and the coefficient of the tide (20 to 120). The coefficient of the tide is a factor applied to determine the amplitude of the tide with respect to a reference value specific to each site. For example, the reference tidal range for the bay of Mont Saint Michel is 11 m; the greatest coefficient is reached at the equinoxes, and reaches a maximum tidal range of 13 m. This French region figures among the premier sites in the world, the greatest known tidal range reached being in the Bay of Fundy, Canada, with a value of 16 m. Figure 7.13 issued by [DAV 04] shows variations in the tide coefficient in the course of one year at a site as well as examples of variations in coastal tide currents in a cycle. Tidal currents are more intense at shallow depths. Their intensity is maximum at the surface, reaching a value of zero at the bottom from the effect of the “boundary layer”. The change of the speed v with respect to the distance z, measured from the surface, is of the type: ªzº v = v0 . « » ¬H¼
D
[7.5]
where v0 is the speed on the surface and H is the depth (typical value of D = 1/7).
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Tidal phenomena are not the only sources of marine currents. The rotation of the Earth, temperature variations, density gradients (a function of salinity) create continuous currents, such as the Gulf Stream, whose average annual speeds (there are actually seasonal variations), in certain places and on the surface, are high enough to allow us to consider commercial exploitation. This is more particularly the case off the Florida coasts where we see currents reaching speeds of 2 m/s. We can thus call tidal currents alternating currents and the others direct currents. We stress that the direct currents play a determining role in the climate and they already seem to be affected by global warming, therefore we cannot consider their large scale use without taking the ecological consequences into account.
Figure 7.13. Examples of variation of tide coefficient: a) in one year (2001, Concarneau); b) variation in speed of current in a tide cycle [DAV 04] (courtesy of HydroHelix and Saipe)
7.4.2. Tidal power production systems with dams
Dam construction modifies the configuration of the shoreline site involved as well as the ecosystem, and requires high investments. For these reasons, there have been very few such constructions in the world and the largest, by far, is the tidal power station of La Rance [BAN 97]. Placed in service in 1966, its annual production is about 0.540 TWh, or more than 90% of the tidal electric power
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produced worldwide. Its 24 groups of 10 MW are integrated into a dam that also serves as a road linking St Malo to Dinard (see Figure 7.14). There have been only a few such high power constructions (higher than 1 MW), in Canada and China, of 20 MW and 5 MW, respectively. Other projects have been expected for several decades, among them one in the Bay of Fundy, Canada, where a plant of 5,300 MW is considered, and one on the Severn, England, with 216 turbines with a total power of 8,640 MW. However, these projects, already very old, may never be completed. In all, 87 GW have undergone feasibility studies in the world for an annual production of 190 TWh. However, there is one exception: the Sihwa dam (12.7 km long), which was constructed at the beginning of the 1999s in South Korea for other reasons entirely than energy production, and which uses a reservoir with a 173 km2 surface. It should be equipped with a tidal power station of 250 MW, making it the most powerful in the world (10 MW more than the one in La Rance). KOWACO (Korea Water Resources Corporation), the existing user of the dam, has systematically studied the construction of a plant whose construction costs represent only about 25% of the total cost of an installation of this type, US$250 million ($1/W instead of $4/W).
Figure 7.14. Diagram of the “La Rance” tidal power station (240 MW) [ABO 05b]
The working of a tidal power station is similar to that of hydropower stations, since the height of the waterfall is not great, but, unlike them, two choices are available: simple or double acting. In the simple acting cycle, the sluices of the dike are open during the rising tide and allow the basin to be filled. When the level of the sea has gone down enough for the height of the fall to be sufficient, the sluices are opened so the water returning to the sea drives a turbine. This is the principle applied in ancient tidal mills. We recall that hydraulic power is expressed by: PH = ȡghQ (W)
[7.6]
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when ȡ is the density of the water, h is the height of the fall and Q the outflow in m3/s. Knowing that the height of the fall varies as the basin is emptied and that the sea level height changes (approximately sinusoidally), it seems clear that it is important to apply a strategy for optimizing the sluice opening times to get the most out of the source. The double acting cycle makes it possible to use the available power equally well during rising and falling tides. Therefore reversible turbines are needed (that work in both directions of current). Such groups are used by the La Rance power station [ABO 05b]. Optimal usage is quite complicated. For example, it could be advantageous to pump water when the tide is going out to increase the level on shore and to release it later at an advantageous height of the waterfall. For tides of small and average range, only a simple action cycle is used: – during the rising tide, all the sluices are opened and the estuary is filled with a very small time-lag, due to the transit time for the water, depending on the sea level, – once high tide is reached, the sluices are closed and the sets function as pumps to raise the level of the water in the estuary (for example, by 2 m), which, in this particular cycle, exceeds the maximum level of the sea, – during the falling tide, when the level of the sea reaches close to its average level, the equipment is triggered to act as turbines and production takes place with good smoothing out of the level until the sea rises to approximately its average level. The energy dispensed at the time of pumping is thus restored by a factor of almost two, thanks to the increase in the height of the waterfall. With strong tides (coefficients higher than 105, about 20% of the tides at Saint Malo), the double action cycle is the most effective: – during the rising tide, and even a little past high tide, the water passes through a turbine and produces electricity, – the equipment is then stopped, the sluices are opened to speed up the process of filling the basin until the moment when the sea reaches its average level, – finally, we return to the functioning of a simple action cycle when the water accumulated in the basin passes through the turbine until the sea returns to its average level. In the double action cycle, the level of water in the basin varies in an almost sinusoidal way with a phase displacement of a quarter of a period with respect to that of the sea and its maximum value remains lower than that of the sea. The experience acquired now allows us to automate the functioning and to get the most out of the installation. The price of kWh return, all charges included, is among the lowest, at about 3 c€ [BAN 97].
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The hydraulic turbine generator set of the La Rance plant are bulb shaped with a horizontal axis; they comprise Kaplan turbines (5.35 m diameter) with four blades and variable pitch, each coupled directly to a synchronous machine, itself electrically joined to the grid without an electronic converter (fixed speed of 93.75 rpm, 32 pairs of poles). In the Sihwa power station (under construction), the technology will be noticeably the same with 10 turbine generator sets of 25.4 MW, an annual production of 0.55 TWh, or the equivalent of 2,100 hours of full power (2,250 hours for the La Rance plant). Artificial lagoons
One way of dealing with topographical constraints may be to create an artificial lagoon and therefore an offshore tidal power station with a dam. This is what the Tidal Electric Ltd company proposes. Feasibility studies were carried out, like that in the Bay of Swansea in the UK, where there is an area of 5 km2 in which the depth to the bottom at low tide is between 1 and 5 m, making it possible to construct a circular jetty. The average accessible power is proportional to the square of the amplitude of the tides and the surface of the basin. Thus, in the Bay of Swansea, with a lagoon of 5 km2 and tide amplitudes reaching 7 m, we can hope for a power of 60 MW and an annual energy of about 0.187 TWh with bidirectional sets using the double action cycle previously described for the La Rance plant. The total investment is estimated at between 70 and 110 M€. However, this technology remains to be proven, particularly with respect to durability of the jetty. 7.4.3. Systems for recovering energy from marine currents
The recovery technique using free currents is very close to that of wind generators, but whereas for the former the direction of currents is constant, in the latter the sense is alternating (effect of the tide) or direct, and the turbines are located in salt water (this technology has already been proven in fresh water, especially in Amazonian rivers). We thus have two large families of turbines according to whether the axis of rotation is vertical or horizontal, but we also find other more original technologies such as one using flat oscillating wings that function like the tail of marine mammals, or even floating systems shaped like paddle wheels. Apart from the projects for extracting energy from the Gulf Stream (Floridahydro or Ocean Energy Inc.) with colossal power, 400 turbines of 15 MW or even 4,000 turbines of 2.5 MW [HOO 05], most of the current projects deal with tidal currents. Therefore, we will focus our attention on technologies adapted to alternating tidal currents, knowing that they are for the most part adaptable to direct currents.
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The mechanical power, which closely equals the electrical power (because the efficiency of generators is close to 100%), of a turbine in free flow is expressed by:
P=
1 .Cp .ȡ.v3 .S (W) 2
[7.7]
where Cp is the coefficient of hydrodynamic power, limited by Betz law to 59%, ȡ the density of water, v the speed of the current (m/s) and S the surface swept by the turbine [MUL 04]. When the speed of the currents varies sinusoidaly, the power of the source varies with the cube of this speed. Even if the fluctuations in speed cannot be compared to those of wind, it would be advantageous to top off the power above the nominal. This speed is an important parameter of technical-economic optimization, determined with respect to the characteristics of each site. Off the coast of Brittany, currents have a maximum speed of 0.5 to 3 m/s, at Raz Blanchard (point of the Hague) they reach 5 m/s and, in certain fjords of Norway, peak speeds are measured at 7.8 m/s. With a minimum hydrodynamic output (Cp) of 30% (this is pessimistic: values in the water are more to be between 0.35 and 0.5), for 3 and 7 m/s the power per unit surface area of the turbine equals 4 and 50 kW/m2, respectively. The turbines’ characteristics have exactly the same appeal as those of wind power or those operating in ocean wave generators with oscillating columns. Figure 7.15 highlights the interest in having the rotation speed vary to maximize power recovery when the current speed changes according to the cycle of the tide. This same figure shows the characteristics of electrical power obtained by a turbine ensemble with variable pitch and an electrical generator with variable speed. The variable speed makes it possible to fully exploit the turbine’s potential below the rated speed (here 2.4 m/s) and the variable pitch makes it possible to cut off the power above that value.
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Figure 7.15. Array of power characteristics of a hydropower generator: a) with respect to rotation speed; b) with respect to current speed. Graph of typical power of a hydropower generator including a topping off of power [DAV 04] (courtesy of HydroHelix and Saipem)
In communication [DAV 04], the authors stress that there is an optimum in rated speed of the current (from which the power is leveled) in order to minimize the cost of sizing a hydropower generator. Although the kinetic power appreciably increases with the cube of the speed of the current, it is worthwhile to top off the power of the full power turbine (beyond the so-called nominal speed), making it possible to optimize the size of the complete system in view of the fact that high speed currents rarely occur. Figure 7.16 shows the effect of “rated” speed of the current, above which the power is leveled on the annual energy productivity. The study highlights an economic optimum speed of around 2.2 m/s, the value for which the energy produced would have the least cost of return.
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, , , ,
Figure 7.16. Influence of nominal speed (of the current) of the sizing on the annual energy productivity and on the investment cost reduced to production (“ratio of the energy cost”): a) source from the turbine kWh/m2/year; b) ratio of the energy cost [DAV 04] (courtesy of HydroHelix and Saipem)
Four categories of marine hydrogenerators can be distinguished: – with a turbine with horizontal axis, – with a turbine with vertical axis, – with beating or oscillating wings, – with a flat paddle wheel.
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Figure 7.17. Types of marine hydrogenerators: a) turbine with horizontal axis (HydroHelix); b) turbine with vertical axis (Gorlov); c) with oscillating wings (Stingray)
The systems envisaged [BED 05b] are nearly all totally immersed, except for that of HydroGen (http://www.hydro-gen.fr/), which uses floating paddle wheels on the surface. The low power immersed generators can be floating, placed under a barge, making it possible to regulate the problem of variations in height due to the tides themselves more easily, and to keep the close to the surface at all times. They can also be floating, carried by a buoy and moored, as proposed by the SMD Hydrovision company (http://www.smdhydrovision.com/) or Ponte de Archimede with its KOBOLD turbine with vertical axis (www.pontediarchimede.com). This principle has already been used in tributaries. The hydrogenerators can also be placed on the sea bed on a gravity base (Lunar Energy: www.lunarenergy.co.uk and HydroHelix) or carried by a single metal pier like those of MCT Ltd (Seaflow, www.marineturbines.com). In fact the choice of bearing structures is mainly dictated by the depth and the nature of the sea bed. Figure 7.18 (MCT) [FRA 04] shows, in a simplified fashion, the various support technologies. The single pier is reserved for depths between 20 and 40 m. It is dug into the bottom (sand) by pile driving. Coming from the petroleum or offshore gas exploitation sector, the jacket structures, made of soldered tubes assuring a broad hold on the bottom, allow access to greater depths and/or a rocky bed. These are also the technologies used or considered in offshore wind energy.
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Figure 7.18. Various hydrogenerator technological support solutions [FRA 04]; courtesy of Marine Current Turbines TM Ltd
The various prototypes or systems at the preindustrial stage have powers from several tens to several hundreds of kilowatts. We plan to analyze more precisely the hydropower hydroturbines of MCT (Marine Current Turbines Ltd), which have been the subject of very advanced research (Seaflow project); a highly detailed report of the studies [SEA 05] has been published. The Seaflow project consisted of installing in the Bristol Channel (at a depth of 20 to 30 m) an 11 m diameter bi-blade (composite blades) turbine with variable pitch to 180º on a steel pier 42.5 m long and 2.1 m in diameter (mass 80 metric tons). The nacelle can move the length of the pile, facilitating maintenance. The maximum power, 300 kW, is reached with the tides at the equinox. The turbine drives, through a mechanical gear box (ratio 1:70), a squirrel cage asynchronous generator of 450 kVA (690 V and three pairs of poles) with variable speed (two three-phase converters with pulse width modulation back to back via a DC bus (760 to 1,100 V). As in the case of wind turbines, bi-blade rotors are less efficient than tri-blades and they induce more force pulsations, but they allow mechanical simplification and lower cost, at least in this phase of development. The trials carried out since summer 2003 have highlighted the large power fluctuations (30 to 50 kW) due to the blades passing in front of the piers (about every 2 s). MCT predicts using mono piers supporting two bi-blade hydrogenerators of 500 kW, each one placed on each side of the pier, whose flow will no longer be perturbed by the passage of the blades in front of the pier. Figure 7.19 shows two photographs of the MCT Seaflow system with the emerged nacelle (for maintenance) and immersed nacelle (when working), as well as an electrical diagram of the entire system including the emergency backup (batteries
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and electrical generator), the directional control of the blades, and, of course, the power conversion chain.
a)
b) Figure 7.19. MCT’s Seaflow Hydrogenerator: a) photographs (in maintenance and in service) [FRA 04]; b) functional electrical diagram [SEA 05]; courtesy of Marine Current Turbines TM Ltd
An estimate of the production potential was carried out [BAH 04] in the region between the Channel Island Alderney and the point of the Hague (strong currents). The diameter of the turbines was defined such that the lowest point would be at a distance from the sea bed equal to 25% of the depth and the highest point at 7 m
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beneath the surface (a value taking into account the wave trough of 4 m and 3 m of uncertainty). The complete hypotheses are defined in [BAH 04] with double rotor machines (MCT type) of powers 12, 24 and 38 MW (respective diameters of 14, 20, and 25m), a cumulative installed power of 3,243 MW annual producible energy reaching 7.4 TWh, which gives an annual full power equivalent of 2,280 h. The diversity of vertical axis machines is described well in [ABO 05a]. These machines do not require orientation with respect to the current and they can be combined easily in matrix fashion. They have a low tip speed ratio (Ȝ = R/v), resulting in low optimum rotation speeds. Three categories of turbines are presented here: Darrieus (among them the Kobold), Gorlov (see Figure 7.17) and those coming from the LEGI (INPG research laboratory) whose blade tips include a break, making it possible to reduce the turbulence and improve the hydrodynamic output. We will then present the Italian company Ponte di Archimede’s Enermar project, under test since 2002 in the Strait of Messina (between Sicily and Italy), where currents reach 1.5 to 2 m/s with a depth of 20 m. The Kobold turbine is suspended from a floating platform 10 m in diameter anchored at 150 m from the shore. The turbine is 3 carbon fiber blades 5 m in length (height) and 40 cm of cord and the diameter is 6 m. It drives, via a gear (ratio 90:1), a triphased synchronous generator with four poles (1,500 rpm) The maximum power (130 kW) is predicted with a current of 3 m/s and for a rotation speed of 18 rpm. However, the tests have been carried out only with a current of 1.6 m/s and the power is only 16 kW. Oscillating systems with hydrofoil wings are the final family of marine hydrogenerators, of which the Stingray device (http://www.engb.com) is the most successful representative [EB 02, EB 03, EB 05]. The attack edges of one or more of the parallel hydrofoil wings are placed facing the current. The attack angle of the wings is controlled so that their lift is maximum and remains in the same direction as the movement. Hydraulic jacks absorb the movement by compressing oil in a high pressure reservoir. The oil is passed through a turbine in a hydraulic motor that drives a variable speed electrical generator. The attack angle of the blades requires that they be optimized continuously (hydraulic control) during the vertical oscillations to maximize recovery. The ensemble is designed to be set and anchored on the sea bed. A prototype of 150 kW with a single wing (actually two aligned halfwings) has been built. It occupies a ground area of 280 m2 and weighs 35 metric tons (185 tons including the ballast). The half-wings are 7 m in length and they have a 3 m cord, offering a total support surface of 42 m2. The support arm, 11 m in length, allows oscillations of +/-50º or a vertical flutter of about 17 m and a capture section facing the current of 235 m. Movement control requires a precise optimization of the change of angle, especially of the period. In a current of 2 m/s, a cycle period of 24 s leads to a power of 117 kW. The system has been tested and a 500 kW version with three superimposed wings is planned.
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j
w
a)
b)
c)
Figure 7.20. Stingray system (Engineering Business Ltd) with hydrofoil wing: a) design; b) photograph of the prototype; c) artist’s view of a sub-marine farm [EB 03]
7.5. Other conversion systems 7.5.1. Offshore wind power generators
Since wind power generators are the topic of a specific chapter in this work, we will mention only some specific qualities associated with marine applications, especially regarding electromechanical structures and the transmission of electricity to land. The experience accumulated since the installation of the first farm in 1991 (Vindeby, Denmark) and the growth of power of individual machines (up to 5 MW)
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make it possible to reach investment costs of 1.3 to 2.5 €/W. Although they are higher than on land (about 1 €/W), they remain compatible with economical production, since the winds at sea are stronger and more regular than on land. Thus, at present [in 2005], we can obtain annual production durations of 3,000 to 4,000 hours (about 2,000 on land). At the end of 2005, of the 57,800 MW wind power installations in the world, 785 MW were at sea, with several very large installations, like Horns REV (Denmark) with a power of 160 MW (80 machines of 2 MW). The depth of the sea and the distance from shore are the factors that increase costs. At the current state of the technology, the turbine of an offshore wind plant has a diameter of between 80 and 120 m for rated powers from 2 to 5 MW. The generators are still, for the most part, connected to a gear box. Variable speed is almost a system given and the conversion devices are fundamental to doubly fed machines. Since the maintenance costs offshore are very high and likely to lead to considerable losses of productivity, especially if the meteorological conditions do not allow rapid intervention, it is important to minimize the frequency of the operations and maximize the reliability. A breakdown of the mechanical gear can require employing replacement means that are clumsy and costly [BUS 01]. Generators with direct drive (synchronous with a large number of poles and thus high torque), especially with permanent magnets, should increasingly occupy the offshore wind power market. Currently, accessible depths are from 10 to 20 m. Beyond that, the technologies are still prohibitively expensive. The cost of bearing structures and installation at sea represents a high part of the total cost (0.6 to more than 1 €/W). The cost of installing electric conversion and transmission is typically 0.3 €/W for distances between several and 20 km. These costs, added to those of the machines (about 1 €/W) limit the economic feasibility of offshore farms. These are composed of a large number of wind generators that are then regrouped in clusters: as a star cluster or a string cluster. The major difference between the two types of regrouping is associated with the availability of energy from the generators, especially in the case of breakdown of the cables, less reliable for a string cluster in which the connection cable is common to all the units [MUL 04]. On the other hand, each star cluster requires a platform. The sizing of the cables and transformers differs according to the architecture and the power of the generators. For example, for a star architecture, we can avoid the use of transformers if the voltages of the generators are sufficiently high (Windformer ABB design) [DAH 00]. For the transmission of the energy produced by high power wind power farms (several hundred MW), relatively distant from the points of high voltage (several tens of km), connection can be carried out by direct or alternating current. Depending on the distance and various criteria of accessibility and reliability, there are a number of solutions to choose from.
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Transmission systems using high power DC (HVDC) offer the advantage of desynchronizing the sources. The levels of DC voltage reach 145 kV or +/- 145 kV (symmetric DC voltages) with cables of 600 to 700 mm2. The most mature technologies operate, at each extremity of the line, a thyristor rectifier-inverter with phase control, but they generate harmonics, consume the reactive power, and must therefore be connected to passive or active compensation devices (Figure 7.21). At present, alternating current solutions predominate, since the distances between the farm and land connection remain sufficiently small (less than 15 km). DC transmissions should, however, end up prevailing with increase in distance and in power, and lowering of the costs of electronic power converters.
o
o
o
o
Figure 7.21. HVDC transmission systems in DC with thyristors: a) with passive filtering; b) with active filtering [COUR 02] (Areva T&D)
The possibilities offered by IGCT and IGBT for high power allow consideration of DC transmission with converters with pulse width modulation (active rectifiersinverters) that require less filtering and enable control of the reactive power, but these technologies are very expensive. In the case of alternating current transmission, the active compensation devices of the parallel and series type (UPFC: Unified Power Flow Controller) allow reduction of the effects of the fluctuations of wind power injected on a grid that is too small with respect to the peak power [COU 02], most especially if a device for energy storage is involved. If the machines are of
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variable speed, the number of AC/DC and DC/AC converters may vary according to whether a generator converter or a cluster converter is connected, even a single converter for the entire park. In the first case, the speed of each turbine may be individually controlled with respect to the wind. In the second and third cases, we act indirectly on the cluster or the farm as a whole, meaning that the wind conditions must be homogenous, which is frequently the case at sea. As far as the offshore aspects of civil engineering are concerned, we may consult the article [RUE 04] in which various solutions are considered with respect to the depth and the nature of the bottoms (sand on clay, sand on rocky substratum, sediments, etc.). Again we encounter the solutions used in the case of submarine turbines (Figure 7.19) with the possibility of floating structures. The local characteristics of the waves, of the currents, of the tidal range and, of course, of extreme winds, all play an important role in the sizing of the bearing structures [MOR 04]. When the depth increases, the necessary mass of steel and concrete become prohibitive from the point of view of economics and of energy (energy needed for the construction). Study [RUE 04] shows that the mass of steel per MW begins at 80 metric tons per MW for a depth of almost zero and then grows by 3.6 to 4.6 t/MW per meter of depth. For example, for the support of a wind turbine in the offshore zone in Languedoc Roussillon (sedimentary bottom at 50 m), the proposal has been for a jacket structure, anchored by four anchors 7.5 m in diameter and 15 m in length. 1,400 metric tons of steel are necessary to support a wind generator of 5 MW (axis at 126 m above the bottom), or an energy expenditure of 10 GWh (2,000 h of full power production). For this reason, above a certain depth, floating structures are considered. Various designs are presented in [RUE 04]. One of them seems promising; it involves a tensioned leg platform (TLP); the turbine is mounted on a one-piece mast with an immersed float with three prongs maintained by tubular anchored cables [HEN 02]. The main advantage lies in the small quantities of materials required. Figure 7.22 shows two floating offshore wind turbine designs [HEN 02].
Figure 7.22. Examples of designs for offshore floats [HEN 02]: a) the TLP principle; b) semi-submersible vessel, autostable and anchored
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The machines’ density is limited by the fact that the distance between them must be sufficient to limit the effects of wakes. Spaces of six to 10 times the diameter of the turbines lead to power densities of 5 to 8 MW/km2, or annual productivities of between 16 GWh and 32 GWh/km2, depending on the intensity of the winds. The Horns Rev Example
The Danish Horns Rev farm, with a power of 160 MW, brought into service at the end of 2002 in the North Sea, lets us provide a concrete example. Located at a distance between 14 and 20 km from shore and on depths of 6.5 to 13.5 m, its investment cost was from 268 M€ (1.68 €/W), of which 40 M€ was for connection. Its annual production is more than 600 GWh (or 3,750 hours annually, the average wind speed equals 9.7 m/s) for an occupied marine surface area of 20 km2 (5 x 4). The farm includes 80 Vestas V80 2-MW tri-blade wind turbines with a doubly fed generator, spaced at 560 m. The wind turbines, including their own 690 V–33kV transformers, are arranged as 10 string clusters, joined two by two and connected to the conversion station by 150 kV three-phase 50 Hz. The energy is then transmitted to land via a 21 km cable. The highly capacitive behavior of the cable is compensated for by the reactances placed on land. The voltage at the level of the conversion station is usually 165 to 169 kV and may go down to 122 kV. The 160 MVA offshore transformer is equipped with a load regulator allowing operation of between 122 and 170 kV. The conversion station at sea is located on a 1,200 metric ton steel platform 14 m above sea level. This platform is a parallelepiped superstructure with a surface of 20 x 28 m and 7 m high that includes, besides the transformer, various pieces of security equipment: disconnectors, diesel auxiliary generator, raised helicopter platform, emergency boat, etc. The wind generators are mounted on mono piers 4 m in diameter, sunk from 22 to 24 m into the bottom and emerging at 9 m or of lengths of between 37 and 47 m in relation to the depth. The dimensioning of the bearing structures was made to withstand maximum waves of 8 m and an average signifacitive height value of 1 m. 7.5.2. Ocean thermal energy converters (OTEC)
The idea of employing a small temperature difference, especially in the oceans, to make thermodynamic machines work, is attributed to D’Arsonval (1881). Georges Claude, in the years 1920 to 1930, tried several experiments at sea, of which one produced energy for several days, but numerous technical difficulties prevailed over his courageous attempts [MAR 80]. The availability of the resource (perfectly continuous), which would make it possible to provide a basic, and even
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adjustable, production, was one of his major advantages. Various other projects have been considered, but few have come through up until now. Among those that have, we will cite the OTEC power station in Hawaii, brought into service in 1979, producing 18 kW of electricity [VEG 99] for notably 8,000 hours a year, depending on maintenance stoppages. To minimize environmental disruption, we estimate that the density of power removal should not exceed 0.2 MW/km2 [AVE 02]. The principle of OTEC devices is based on a turbine machine using the expansion of a fluid evaporated due to the effect of a hot source (surface water temperature: 26 to 30ºC) and then condensed thanks to a cold source (temperature of deep water pumped and brought to the level of the machinery). Therefore, it is necessary to pump from depths in the order of 800 to 1,000 m to make use of a temperature variation that is as high as possible, yet without expending an excessive quantity of energy for the pumping process. Carnot’s theoretical efficiency is expressed by:
Șc
1
Tmini Tmaxi
[7.8]
where the maximum and minimum temperatures are expressed in Kelvin. Therefore, we obtain, with Tmini # 277 K (4D C and Tmaxi # 301 K (28D C) . Moreover, there must be considerable outflow of hot water and cold water, of the order of 2 to 3 m3/s per MW. The related pumping losses (of hot as well as of cold water) are all the smaller as load losses due to flow in the tubes are reduced. Therefore it is essential, especially for a long pipeline of cold water, to have a large conduit diameter. Economic optimization (including energy expense of pumping, cost of piping) leads to sections in the order of 1.8 m2 per MW or diameters of 1.5 m for 1 MW and 15 m for 100 MW. The energy dissipated for pumping the water then represents between 10% and 30% of the energy produced by the turbine. The result of all this is a net electricity production efficiency of about 2.5 to 3%. Since the source is renewable, this low efficiency does not cause an over consumption of non-renewable raw materials, but “only” a costly over sizing of the thermodynamic machine. Until now, one of the major brakes on the development of OTEC technologies, apart from their lack of maturity and technical difficulties such as the resistance of the very long pipe, is the investment cost, which remains high (4–12 €/W). OTEC power stations may be built on shore, which simplifies the problems of electricity transmission, but increases the pumping cost, since the length of the
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conduits has to be increased, especially for cold water. This turns out to be technically unacceptable if the depth does not decrease fast enough near the shore. In other cases, the conversion plant could be built on a barge. Figure 7.23 shows the main constituents of such a plant [GAU 04]. Conduits of great length and diameter can be built in high density polyethylene for diameters up to 1.5 m, or even in reinforced plastic or lightened concrete for higher diameters up to 30 m. With such small temperature differences, the thermodynamic converters may be of two types: open or closed cycles. The open cycle machines use seawater directly and make it possible to avoid exchangers that are very cumbersome and risk being clogged from the effect of biofouling (growth of microorganisms). The principle consists of having two turbines work in a belt of very high pressure (2 to 3 kPa or 20 to 30 mbar) created with vacuum pumps. The water from the hot source, at such pressures, vaporizes and drives the turbine, then condenses on contact with the cold source. The fresh water obtained by condensation can then be used as a value added byproduct. The major disadvantage of the open cycle resides in the large dimensions of the low pressure turbine. To produce 1 MW, a turbine diameter of 8 m is required [RAV 02], and increases to the prohibitive size of 80 m for powers of about 100 MW. Here the simplicity of the open cycle finds its limit. It is now no longer considered except for low power (less than a MW). The closed cycle, for its part, at the price of using a fluid of lower boilingpoint, such as ammonia, and exchangers with a large surface, allows the use of a turbine of smaller dimension (diameter 1 m for 1 MW) [RAV 02]. The exchangers should have surfaces of the order of 10,000 m2 per MW and resist corrosion and biofouling. Materials used are titanium and aluminum. The battle against biofouling requires, if we want to avoid using chemical products (chlorine, for example), almost continuous mechanical washing. Whatever the cycle, the turbine drives a generator (synchronous, even asynchronous) through a gear box, in the case of open cycles, or the turbine is very slow. Variable speed may be considered to improve the output in the case of power variation, but there are other current difficulties (pumping conduits, resistance of interfacing to the plant in the presence of waves, corrosion and biofouling of the exchangers, etc.).
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System for anchoring and recovery of the cold water
Figure 7.23. Diagram of a floating plant for ocean thermal energy recovery (courtesy of ECRIN) [AU 04]
If these methods can be put to long term use, OTEC power stations could first be used in islands in tropical and equatorial regions, where basic electricity production from fossil sources is increasingly expensive. These first steps could lead to technical developments and improvements to bring about a significant lowering of costs and possible applications connected to large electricity grids. 7.6. Conclusion
The potential for marine energy resources is enormous and their variety is such that we can envisage that in most regions of the globe they could make a very significant contribution to the production of electricity and also of hydrogen and
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fresh water, as well as being used for cooling, etc. The technological solutions are still in full development. Most of these technologies have not yet overcome the constraints of nature and of the market, and there generally remains considerable research and development to be done. All things considered, the economic learning curve has hardly begun, and the potential for reducing investment costs of most marine technologies is very high, all of which let us predict that very attractive production costswill eventually be achieved. Thus, it is most important to develop a sufficient public will to conduct experiments as fast as possible under the real and difficult conditions of the marine environment, while obviously taking risks. In fact, as in all other areas, there will have to be failures (constructive) in order to make progress and reach real industrial maturity. The electricity produced by the conversion systems that we have just reviewed often fluctuates and is more or less predictable. This is not a problem as long as the proportion of energy injected into the grid remains modest, on condition of appropriate management of the grid. A greater recourse to forms of energy that are fluctuating and hard to predict in the long term (wind and wave generators), on the other hand, gives rise to external economic considerations. It is then necessary to install other means of production or storage capable of responding to demand and of working with a better adjustment of consumption to production. We note, however, that weather prediction has made great progress and is widely used in the field of wind power. It can be the same for ocean wave power. As for marine currents, they are perfectly predictable and the conversion of thermal energy from the ocean is destined to function as “base loading”. We have seen that it is in the offshore area that the resources are the most abundant. The transmission of electricity to land and associated conversions then weigh significantly on the total investment cost. The mutualization of these infrastructures for different but complementary conversion systems (wind and ocean waves, even marine current) can make it possible to reduce, in the final analysis, the production costs. We know that all large scale energy transformations disturb the environment to some extent. The aggression is especially intense when it is usage of non-renewable resources (which do not register in the natural cycles on our timescale), but also in certain cases when renewable resources are used. Even so, in order to realize sizable economies of scale, this is often what we try to do. Nevertheless, production that is small scale and on the site of consumption represents a solution of least disturbance, but at sea there are few consumers and this is not possible. It is therefore indispensable to conduct serious environmental impact studies and, since the ecosystem is of a complexity whose secrets we have not begun to penetrate, it would then seem reasonable to avoid all massive energy transformations. The rates of extraction should remain low. This is why reductions in consumption constitute one
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of the priorities of lasting development. Finally, the last of the environmental aspects: the impact of climate change on energy production beginning with renewable resources. The effects of climate change are very noticeable at sea and we have already begun to worry about their consequences, especially in terms of the size of wind and ocean wave production systems [HAR 05]. It is especially important to consider that the violence of extreme storms is increasing, and therefore not size the generation systems too close to the present characteristics of the resources in order to maintain a certain robustness considering the future changes of these characteristics. 7.7. Bibliography [ABO 04] ABONNEL C. et al., “Energie des Mers”, Conférence pour l’OPECST (Office parlementaire d’évaluation des choix scientifiques et technologiques), ECRIN, Assemblée Nationale, Paris , 22 p., 20 October 2004. [ABO 05a] ABONNEL C. et al., “Some Aspects of EDF Modelling and Testing Activities, within its Marine Current Energy Research and Development Project Hydroliennes en mer”, Work Shop, Aalborg, April 2005. [ABO 05b] ABONNEL C., “Some Aspects of the RanceBulb Turbines after almost 40 years’service – Feedback of experience”, Second CA-OE, Work Shop, Uppsala, November 2005. [AVE 94] AVERY W., WU C., Renewable Energy from the Ocean. A Guide to OTEC, Oxford University Press, Oxford, 1994. [AVE 02] AVERY W. H., “Ocean Thermal Energy Conversion (OTEC)”, Encyclopedia of Physical Science and Technology, vol. 11, p. 123–160, 2000. [BAB 05] BABARIT A., Optimisation hydrodynamique et contrôle optimal d’un récupérateur d’énergie des vagues, PhD Thesis, Ecole Centrale de Nantes, 2005. [BAH 04] BAHAJ A. S., MYERS L., “Analytical estimates of the energy yield potential from the Alderney Race (Channel Islands) using marine current energy converters”, Renewable Energy, 29, Elsevier, p.1931–1945, 2004. [BAN 97] BANAL M., L’énergie marémotrice, REE, n °8, p. 6–7, September 1997. [BED 05a] BEDARD R., HAGERMAN G., PREVISIC M., SIDDIQUI O., THRESHER R., RAM B., Offshore Wave Power Feasability Demonstration Project, E2I Report, EPRI Global WP 009 – US Rev 1 (Electric Power Research Institute), 14 January 2005. [BED 05b] BEDARD R. et al., Survey and characterization tidal in stream energy conversion (TISEC) devices, EPRI Report TP-004 NA, November 2005. [BES 02] BESLIN G., “Eolien Offshore: attentes, espoirs et réalités”, Colloque GEVIQ’2002, Marseille, p.43–46, 12–13 June 2002.
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[BON 98] BONNET J. F., Les apports solaires implicites dans les activités humaines. PhD Thesis, ENSAM, Centre de Bordeaux, 1998. [BOU 04] LE BOULLUEC M., “L’exploitation de la ressource hydrocinétique: état de l’art”, Seatech Week Conf. Brest, October 2004. [BUR 05] BURGER M., GARDNER F., “A wave energy converter performance standard WPS”, Second CA-OE (Coordination Action on Ocean Energy) Work Shop, Aalborg, 5–6 April 2005. [BUS 01] VAN BUSSEL G. J. W., HENDERSON A.R., “State of the Art and Technology Trends for Offshore Wind Energy: Operation and Maintenance Issues”, Proc. Offshore Wind Energy Special Topic Conf., Brussels, p. 1–4, December 2001. [CLE 02a] CLÉMENT A. et al., “Wave energy in Europe: current status and perspectives”, Renewable and Sustainable Energy Reviews, Pergamon, 6, p.405–431, 2002. [CLE 02b] CLÉMENT A., Propagation des ondes de gravité, course at Ecole Centrale de Nantes, 2002. [COU 02] COURAULT J., “Energy Collection on Offshore Wind Farm”, GIRCEP, March 2002. [DAH 00] DAHLGREN M., FRANK H., LEIJON M., OWMAN F., WALFRIDSSON L., “Windformer. Production à grande échelle d’électricité éolienne”, Revue ABB, n °3, p.31– 37, 2000. [DAV 04] DAVIAU J. F., MAJASTRE H., GUENA F.,. RUER J, “Divers aspects de l’exploitation de l’énergie des courants marins”, Seatech Week Conf. Brest, October 2004. [EB 02] ENGINEERING BUSINESS LTD, Research and development of a 150 kW tidal stream generator, contract report n °1, ETSU T/06/00211/00/REP, 2002. [EB 03] ENGINEERING BUSINESS LTD, Stingray tidal stream energy device – Phase 2, contract report n °2, ETSU T/06/00218/00/REP, 2003. [EB 05] ENGINEERING BUSINESS LTD, Stingray tidal stream energy device – Phase 3, contract report n °2, ETSU T/06/00230/00/REP, 2005. [EWE 99] EWEA (European Wind Energy Association), Wind Force 10. A blue print to achieve 10% of the world electricity from wind power by 2020, report 1999. [FER 06] DAL FERRO B., “W&T: scale of the opportunity”, Séminaire franco-britannique sur les Énergies Marines, CDROM proc, Le Havre, 19–20 January 2006. [FOL 05] FOLLEY M., CURRAN R., BOAKE C., WHITTAKER T., “Performance investigations of the LIMPET counter-rotating Wells turbine”, Second CA-OE, Work Shop, Uppsala, November 2005. [FRA 04] FRAENKEL P., “Marine Current Turbines: feedback on experience so far”, Seatech Week Conf. Brest, 20–21 October 2004. [GAU 04] GAUTHIER M., “L’énergie thermique des mers”, Revue trimestrielle du réseau ECRIN, n °57, p. 16–19, September 2004 (available at http://www.ecrin.asso.fr).
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[GOD 00] GODA Y., “Random Seas and Design of Maritime Structures”, Advanced Series on Ocean Engineering, vol. 15, 2nd edition, July 2000. [GUI 01] GUYON E., HULIN J. P., PETIT L., Hydrodynamique physique, EDP sciences, CNRS Editions, Paris, 2001. [HAR 05] HARRISON G. P., WALLACE A. R., “Climate sensitivity of marine energy”, Renewable Energy 30, Elsevier, Oxford, p.1801–1817, 2005. [HEN 02] HENDERSON A. R., LEUTZ R., FUJII T., “Potential for Floating Offshore Wind Energy in Japanese Waters”, Proceedings of The Twelfth (2002) International Offshore and Polar Engineering (ISOPE) Conf., Kitakyushu, May 2002. [HEN 05] HENDERSON R., “State of the Art Hydraulics for the Pelamis Wave Energy Converter”, Second CA-OE, Work Shop, Uppsala, November 2005. [HOO 05] HOOVER M., “The Gulf Stream Energy Project: Thinking Outside the Gearbox”, Workshop on Alternative Energy Tech. Savannah, 12/13 May 2005. [JON 03] JONES A. T., FINLEY W., “Recent development in salinity gradient power”, IEEE OCEANS 2003 Proceedings, vol. 4, p. 2284–2287, 22–26 September 2003. [LEN 05] LENNARD D., “Ocean Thermal Energy Conversion”, Survey of Energy Resources, available at www.worldenergy.org. [MAC 04] MACKIE G., “Wavepower an operator experience”, Seatech Week Conf. Brest, October 2004. [MAR 80] MARCHAND P., “L’énergie thermique des mers”, in BIZEC R. F. (ed..), La recherche sur les énergies nouvelles, Le Seuil , Paris, 1980. [MAR 97] MARTIN J., “Energies éoliennes”, Techniques de l’Ingénieur, Traités Energétique, 1997. [MAT 95] MATTHIES H. G. et al., “Study of Offshore Wind Energy in the EC (Joule I project)”, Verlag Natürliche Energie, Brekendorf, 1995. [MOR 04] MORGAN C., “Offshore Wind – State of the Art”, Seatech Week Conf. Brest, October 2004. [MUL 04] MULTON B., ROBOAM X., DAKYO B., NICHITA C., GERGAUD O., BEN AHMED H., “Aérogénérateurs électriques”, Techniques de l’Ingénieur, Traités de Génie Electrique, D3960, November 2004. [PEL 02] PELC R., FUJITA R., “Renewable energy from the ocean”, Marine Policy Revue, Elsevier, n °26, p. 471–479, 2002. [PIZ 00] PIZER D. J., RETZLER C. H., YEMM R. W., “The OPD Pelamis: Experimental and numerical results from the hydrodynamic work program”, Wave Energy Conferences, Alborg 2000. [RAV 02] RAVINDRAN M., RAJU A., “The Indian 1 MW demonstration OTEC plant and the development activities”, IEEE Oceans Apos, vol. 3, p. 1622–1628, October 2002.
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[RUE 04] RUER J., PIMENTA DE MIRANDA W., “Influence des conditions locales de site sur la conception des éoliennes offshore. Etude ADEME pour des éoliennes ancrées au fond et perspectives pour des éolienne flottantes”, Seatech Week Conf. Brest, October 2004. [RUE 05] RUELLAN M., ROZEL B., BEN AHMED H., MULTON B., BABARIT A., CLÉMENT A., “Pre-design of direct electrical PTO for the SEAREV wave energy device”, Second CAOE (Coordination Action on Ocean Energy) Workshop, Uppsala, November 2005. [SOE 04] SOERENSEN H. C., “World’s first offshore wave energy converter Wave Dragon connected to the grid”, Proc. of 19th World Energy Congress, Sydney, p. 12, September 2004. [SOE 05] SOERENSEN H. C., MADSEN E. F., CHRISTENSEN L., KOFOED J. P., FRIGAARD P., KNAPP W., “The results of two years testing in real sea of Wave Dragon”, Proc. of 6th European Wave and Tidal Energy Conference, Glasgow, August/September 2005. [SCH 05] SCHILLING R., “Water Turbines for Overtopping Wave Energy Converters”, Second CA-OE Work Shop, Uppsala, November 2005. [SEA 05] “Seaflow. Pilot project for the exploitation of marine currents”, European Commission Report, n° EUR21606, 2005. [STE 05] STENZEL T. et al., “Offshore wind experiences”, International Energy Agency Report, June 2005. [TAY 03] TAYLOR G. W., “Wave Energy Commercialisation”, London Energy Group, 3rd Annual Alternative Energy Seminar, 10 December 2003. [THO 00] THORSEN T., “Salinity Energy”, United Nations Atlas of the Oceans (available at http://www.oceansatlas.org/). [THO 04] THORPE T., “Wave Energy”, in WORLD ENERGY COUNCIL (ed.), 2004 Survey of Energy Resources, 20th edition, Elsevier, Oxford, p.401–417, 2004. [VEG 99] VEGA L. A., “Ocean Thermal Energy Conversion (OTEC)”, OTEC, December 1999. [WAV 03] WAVENET REPORT, “Results from the work of the European Thematic Network on Wave Energy”, ERK5-CT-1999-2001, 2000–2003, March 2003 (available at www.wave-energy.net/index3.htm). [WDK 01] RAPPORT DANOIS, “Bølgekraftforeningens Konceptkatalog”, Revue des concepts de générateurs fonctionnant à partir de la houle, April 2001 (available at www.waveenergy.dk). [YEM 00] YEMM R. W., HENDERSON R. M., TAYLOR C. A., “The OPD Pelamis WEC: Current Status and Onward Programme”, Wave Energy Conferences, Alborg, 2000.
Chapter 8
Small Hydropower
8.1. Introduction Hydropower has played an essential role in the development of humanity. The first hydraulic machines appeared more than 2,000 years ago both in the Mediterranean basin and in China. Their great diversity attests to the ingenuity and skill of the artisans who built them. These machines, in relieving man of painful and repetitive tasks, played a vital role in industrial evolution. The simplest form of hydraulic machine is the waterwheel used in ancient Greek and Roman civilizations. A first advancement appeared in the second century BC, when the wheel was no longer fed from beneath, but rather by its upper part, giving a production output of a few tens to several hundreds of watts. Up to the middle of the 19th century, these small hydraulic schemes had evolved only a little and supplied mechanical energy to several tens of thousands of mills, saw mills, tanneries, etc. The first real turbines appeared during the 19th century, about 1820, with the Fourneyron turbine that would later evolve to become the Francis turbine. The Pelton turbines appeared around 1880 and the Kaplan made its appearance at the beginning of the 20th century. This development essentially took place on the European continent to satisfy the growing need for mechanical energy for industries in full development. The countries of the continent had to develop their own solutions to respond to the growing mechanization of British industries that were benefiting from Watt’s steam engine. Thereafter, the development of large power stations for energy production Chapter written by Raymond CHENAL, Aline CHOULOT, Vincent DENIS and Norbert TISSOT.
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and of electricity networks allowed enterprises to obtain an abundant energy source that was reliable and affordable. This advancement led the users of these small stations to relinquish their own means of energy production in favor of this “convenient” form of energy. The Swiss, for example, at the beginning of the 20th century had more than 7,000 hydraulic schemes (waterwheel or small turbines). In 1985, the date of a new census report, this number had gone down to about 1,000. For several years there has been renewed interest in hydropower as a result of, on the one hand various oil crises, the continued growth in the demand for and the limit of the availability of fossil fuels that has led to great price instability, and, on the other hand, the engagement of a certain number of countries in the Kyoto Protocol for the reduction of greenhouse gases. These two elements have pushed governments to encourage the development of means of electricity production from renewable energies. Hydraulic energy has, in this context, an especially important role to play. Currently it represents globally an installed electricity output of more than 740,000 MW for an annual production of nearly 2,800 TWh, representing about one third of the world potential that is economically realizable. The share of hydraulic energy in world electricity production (16,660 TWh/year) is about 16%. In 2001, the annual production of energy from hydraulics in the 25 countries of Europe was about 385 TWh, representing about 13% of total European energy production. However, the potential of the large power stations, whether in Europe or in other industrialized countries, is either already being used or difficult to develop because of environmental constraints. An increase in hydraulic production cannot take place except by modernizing equipment that is reaching the end of its life and, especially, by the development of small hydropower schemes, marginalized during these last decades in favor of the creation of large hydropower schemes. The rest of this chapter will treat only specific aspects of small hydraulic energy production. First we will define what is hidden behind the term “the small hydropower plant” before taking up hydraulic energy and its conversion into electrical energy. The different kinds of equipment allowing this conversion will then be treated, with the various applications specific to small hydro plants on a world level, as well as the European level, being examined. The three last parts will be devoted to environmental aspects, to the directions of research and development having priority in the field of small hydro, and to various policy aspects necessary for the development of this clean and renewable energy source.
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8.2. What is small hydropower? At the present time there is no satisfying definition that allows us to determine if a hydropower plant is small or large. This differentiation depends on a multitude of criteria, such as the output of the scheme and its size or technical or economic characteristics. The criterion currently used for defining small hydropower plants is that of output, but many variants are in use. UNIPEDE (International Union of Producers and Suppliers of Electricity), the European Commission, ESHA (European Small Hydropower Association) as well as several other member countries have defined a scheme of less than 10 MW as being small. However, Italy fixes this limit at 3 MW, the UK at 5 MW and France at 8 MW. If this differentiation, linked to the criterion of output, has the advantage of being the simplest and may, from a technical point of view, be allowed, it is already more difficult to accept if we consider a scheme from an economic or environmental point of view. This is even truer for low-head schemes using important discharges. The two examples below demonstrate the limitation of a definition based only on output. The Karlstor power plant (see Figure 8.1), located on the Neckar near Heidelberg in Germany, is equipped with two turbines, each using 70 m3/s and developing a total output of 3,100 kW. The diameter of the runners is 3.4 m and the construction of the plant required nearly four years of work.
Figure 8.1. Karlstor power plant, 2 x 1,550 kW (ESHA)
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The La Douve II power plant (see Figure 8.2) uses treated water from a wastewater treatment plant in the Swiss Alps. The plant, with an output of 75 kW, as well as the turbine and the complement of electrical equipment, was fully assembled in a work shop in several months. The whole thing was brought in by helicopter in little less than half an hour, since the site was inaccessible by road.
Figure 8.2. Douve II power plant (Switzerland), 75 kW, installed by helicopter delivery (MHyLab, Switzerland)
According to the criterion of output alone, these two power plants are both small plants. However, it seems clear that the techniques and timeframes are completely different, and that each of the plants imposes its own appropriate technology. Therefore, the construction of small hydraulic schemes requires implementation of equipment specifically developed for this area that satisfies three fundamental criteria: high level of performance, maximum reliability, and simplicity, allowing easy maintenance by non-specialists. As a result, the equipment composing a small plant cannot simply be a geometric reduction of large units. A small plant is thus a plant that, for technical and economic reasons, may not be the geometric reduction of a large plant.
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8.3. Hydraulic energy The conversion of hydraulic energy into mechanical energy is done by the intermediary of a turbine. This, like the hydraulic wheel, is an engine and it produces only mechanical energy. Moreover, this is the form that was used for a long time as a direct drive for machines before the arrival of the electric generator. The use of this energy by a turbine requires not only a certain amount of water, but also a difference in altitude. It is based on the transformation of the potential and kinetic energy in water into mechanical energy, then into electrical energy. Consequently, it is possible only in places that offer a sufficient discharge and a difference in levels. This difference in levels may be natural, when the water is collected at a height and then led to the turbine, located below it, or artificial, when a head is created or augmented by installing a weir or dam on a river in an appropriate spot. Various applications are currently under development for using other forms of hydraulic energy. We could cite, for example, hydropower stations that exploit the kinetic energy of marine currents. The available hydropower, Ph, for a scheme is given by the following formula: Ph = ȡ Q gH (W) where: ȡ Q gH
the density of water (~ 1,000 kg/m3 at 10°C; kg/m3), is the discharge of the scheme (m3/s), is the specific hydraulic energy.
This is expressed as: gH
1
U
p1 p2
1 2 2 c1 c2 Z1 Z 2 2
(J/kg)
In the preceding equation, the term p represents the pressure, c the speed and Z the altitude with respect to a reference level. The indices 1 and 2 represent the entry point and the exit point of the turbine. They are defined according to precise international standards (see Figure 8.3).
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Allowing that the levels Z0 and Z3 are submitted to the same atmospheric pressure and that the flow speeds into these basins are zero (see Figure 8.3), the formula below may be simplified: gH
g 'Z gHr
c22 2
(J/kg)
where: 'Z
represents the difference in altitude between point 0 and point 3 (m),
gHr represents the energy loss in the pipe between point 0 and point 1 due to friction and the geometry of the pipe (narrowing, elbows, etc.). It is determined by empirical formulas or measurements at the site and proportional to the square of the flow speed in this pipe, c22 represents the residual kinetic energy at the outlet of the turbine and 2 which cannot be recovered.
Figure 8.3. Principle of a hydraulic scheme
The electric output supplied to the grid is expressed by: Pel
Ph Kt Km K g Kt
(W)
where:
Ph
represents the hydropower as previously defined,
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Kt represents the efficiency of the turbine. This efficiency varies as a function of the discharge and the head, Km represents the efficiency of various mechanical parts such as bearings, speed increaser, etc. This efficiency is practically constant, K g represents the efficiency of the generator. This varies according to the mechanical output of the turbine, Kt represents the efficiency of a possible transformer. 8.4. The exploitation of hydraulic force Hydraulic energy is transformed into electrical energy in hydroelectric schemes. These are characterized both by the difference in levels used and by their capacity to store a volume of water. A particularly complete example of a scheme is shown in Figure 8.4, and its main features are described below. However, in reality, it is very rare to find all of these features together.
Figure 8.4. Diagram of a complete scheme (Federal Office of Conjectural Questions, Switzerland)
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Along with power plants using natural sites, small hydropower also has been growing in certain “alternative” areas, where it is possible to take advantage of hydraulic energy that would otherwise be dissipated. This may be the case in drinking water networks, raw or treated wastewater networks, rain water networks and irrigation networks, or in certain industrial processes, such as desalination of seawater. 8.4.1. Description of a typical scheme A typical scheme includes: – a weir, which maintains a minimum elevation on the upstream level. In accumulation schemes, the weir makes it possible to create a storage space, – a fish-pass, which allows the migration of water fauna, – a water intake, which withdraws water from the upstream level. The intake is protected from floating trash by a rack furnished with a cleaning device, possibly working automatically, – a sand separator. Certain rivers, especially in the mountains, are full of particles in suspension such as sand, for example. These particles, associated with great flow speeds can badly erode certain parts of the turbine. The sand separator allows the particles in suspension to settle. An emptying valve is open during periods of strong discharge in order to clear out the deposits accumulated in the basin, – a water chamber that makes it possible to guarantee that the penstock will be in water at all times, – a penstock that takes the water from the water chamber to the power plant. It is made of synthetic material when the head is low or of steel for a high head, – a power plant, or a small sized building that accommodates the collection of electromechanical equipment such as the control, the connection to the electricity grid, the alternator, the turbine, and the inlet valve. 8.4.2. Different types of schemes encountered A hydraulic scheme is characterized by two features. The first is the possibility or otherwise of storing a volume of water to be used later, at times of high electricity demand or when the supply of water is insufficient. These are called “accumulation power plants”, or “run of river power plants”. Small hydropower generally uses run of river schemes, generating “ribbon” energy. The weir serves only to maintain a minimum level for the water intake, but does not allow storage. Consequently, these schemes are very dependent on the hydrologic
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conditions of the river. The term “run of river” is often incorrectly ascribed to lowhead schemes, which is an oversimplification. In fact, the term “run of river” designates all schemes that cannot provide storage. The second criterion is the heads, which can be classified into three categories. The limits in heads may vary according to whether we are talking about small or large hydro. We will show below the limits commonly applied to small hydropower: – high-head schemes for heads higher than 60 m, – medium-head schemes, for heads between 25 m and 250 m, – low-head schemes, for heads of less than 30 m. ,
Figure 8.5. Operation range of the turbines
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8.4.3. Different kinds of turbines Depending on the available head, turbines operate in two different ways: – action turbines, which use the hydraulic energy made available only in the form of kinetic energy. These turbines have out-of-water runners that turn in an atmospheric pressure housing or in a slight vacuum, – reaction turbines, which use the hydraulic energy made available in the form of pressure and kinetic energy. The runners of these turbines are submerged in the flow. The type of turbine to set in the flow depends not only on the head, but also on the available discharge. The different types are described later. The stated efficiencies are valid only for machines developed in the laboratory. 8.4.3.1. The Pelton turbine This turbine is an action machine that is especially appropriate for high-head applications (from 60 m to about 1,000 m) with variable discharge, its efficiency not being particularly sensitive to the variation in discharge. The runner, composed of buckets, is driven by one or more jets coming from nozzles. The number of nozzles varies between 1 and 6 according to a relationship linked to the head and the discharge. The nozzles regulate the discharge arriving on the runner with a needle. The water is then returned into the river. Small Pelton turbines can reach a mechanical efficiency equal to or even higher than 90%. Figure 8.6a shows the inside of a Pelton turbine. The runner composed of buckets can be seen in the center and the three nozzles at the sides. Figure 8.6b gives a view of the same scheme seen from the outside.
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a)
b) Figure 8.6. Savièse power plant, equipped with a Pelton turbine with three nozzles: a) inside view of the Pelton turbine; b) inside view of the plant (Commune of Savièse, Switzerland)
8.4.3.2. The Francis turbine This turbine is a reaction machine whose ambit in the heads extends from 25 to about 250 m. It is preferred to Pelton turbines for heads that are higher than 60 m when the discharge is especially important and not very variable. The water arrives in a spiral casing and then goes into mobile guide vanes before arriving at the runner. At the runner outlet the water goes into the draft tube before being returned to the river. The guide vanes are used to regulate the discharges. This type of turbine is also frequently seen in very old low-head schemes with a configuration as a water chamber that is without a spiral casing. Since their rotation speed is very low and their adaptability to variations in discharge is relatively poor, they have been replaced by Kaplan turbines, which came on the market in the 1940s. Although this type of turbine is often seen in small schemes, it is, because of its geometric complexity, ill adapted to small hydropower. Small Francis turbines may reach a mechanical efficiency of 92%.
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Figure 8.7 gives a sectional view of a Francis turbine. The runner is seen in the center, the guide vanes at the sides, and the spiral casing that distributes the water arriving from the penstock.
Figure 8.7. Enlarged view of a Francis turbine (MHyLab, Switzerland)
8.4.3.3. The diagonal turbine This turbine is also a reaction machine with an application ambit from 25 to 100 m. It can also replace Pelton turbines for heads between 60 and 100 m when the discharge is especially large and variable. The water arrives in a spiral casing then goes into a guide vane system before arriving at the runner. At the exit from the runner, the water goes into a draft tube before being returned to the river. Unlike the Francis turbine, it is possible to set two regulating devices. As for the Francis turbine, the first is the guide vanes, and the second is the blades of the runner, whose orientation can be modified. This double regulation allows greater adaptability as much for variations in the head as for the discharge. There also are diagonal turbines with a single or with two regulating devices. This type of turbine is relatively new and is not frequently seen in small hydropower schemes. 8.3.3.4. The Kaplan turbine This turbine is also a reaction machine whose ambit in the heads goes from 2 to about 30 m. There are two large categories of Kaplan turbines: turbines with a spiral casing and axial turbines which in particular include bulb turbines. This turbine can also be differentiated by the manner in which it makes regulations. If both the guide vanes and the runner can be adjusted at the same time, we are speaking of the Kaplan turbine or of an axial turbine with double regulation. If only the guide vanes
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are adjustable, we mean the propeller turbine. If the runner is adjustable and the guide vanes fixed, then we mean the Kaplan turbine or the axial turbine with single regulation. As for the diagonal turbine, the double regulation allows better adaptation to variations of the head and the discharge. In the case of the Kaplan turbine with a spiral casing, the course of the water is similar to that of the diagonal or Francis turbine. In the case of an axial turbine, the water flows into the axis of the supply pipe and goes through the guide vanes before arriving at the runner. When it leaves the runner, the water goes into the draft tube before being returned to the river. Figure 8.8 shows an axial turbine under development, set up on a test stand.
Figure 8.8. Axial turbine under development set up on a test stand (MHyLab, Switzerland)
8.4.3.5. The waterwheel for water from above The waterwheel, which has existed for more than 2,000 years, is not strictly speaking a hydraulic turbine, given its method of operation. In the case of a turbine, kinetic energy and/or continuous pressure in water is used, but the waterwheel uses only the mass of water coming to fill each bucket.
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A hydraulic wheel built according to modern techniques has an efficiency of around 70%. This turbine is adapted to relatively low heads and discharges, with outputs of several tens of kilowatts. The largest wheels built have diameters of about 10 m. This type of machine often makes it possible to revive, at low cost, abandoned schemes and to preserve the legacy of these sites. 8.4.3.6. The Banki or crossflow turbine The Banki or crossflow is an action turbine, implying that the energy available to the runner is only kinetic. This means of energy conversion would place it in the ambit of high heads; however, it is predominantly encountered in low-head schemes. As already cited, action turbines require a height for immersing the runner such that the downstream level is always beneath the low point of the runner. Although this height is not problematic when we speak of high heads, this is not the case in the area of low heads, where it can very easily represent more than 10% of the usable difference in levels. The efficiency at a crossflow shaft is about 80%. If the height of the immersion is taken into account, its total efficiency rises, at best, to 72%, while the efficiency of a Kaplan turbine developed in the laboratory is about 90%. In the case of rural electrification projects in developing countries, this turbine is attractive for its simplicity of construction and maintenance, facilitating its upkeep by local resources. In industrialized countries, its use is justified only if it represents the only solution for making the site profitable. 8.4.3.7. The inverted Archimedes screw The Archimedes screw has a certain similarity to the waterwheel. Like the wheel, it uses only the mass of water to drive the screw. Its efficiency is about 70%. These turbines are adapted to relatively low heads and average discharges. They have some tens or hundreds of kilowatts in output. Considering their mediocre performance, they, like the crossflow, are not to be used except as a last resort, when no other scheme can make the site profitable. 8.4.4. Particular applications of small hydro The turbines previously discussed may, in particular, be used in the applications described in the following section.
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8.4.4.1. Turbines in drinking water networks In mountainous areas there is often a strong downward slope between the catchment area and the consumers. This results in a pressure that is too great for the water supply network and that must be dissipated by a pressure breaker before going into the reservoir at the head of the supply system. Yet it is often technically and economically possible to make use of this pressure in small turbines. The water is thus optimally used because it produces energy before being consumed. Figure 8.9 shows schematically the different possibilities for turbines with drinking water.
h
Figure 8.9. Description of the various possibilities for turbine use in a drinking water network (Les Electriciens Romands, Switzerland)
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The environmental impact caused by the power plant is positive in other respects: for one thing, the catchment equipment, basins and pipes have in any case to be installed to satisfy the population’s water requirements, and, for another, the energy produced is renewable. Figure 8.10 shows the Troistorrents power station in Switzerland. It is installed on top of the commune’s supply reservoir. Since it is located in a tourist area, a special effort was made to integrate it into the landscape, and the plant is indistinguishable from the neighboring chalets.
Figure 8.10. Troistorrents power plant located on the commune drinking water reservoir; example of integration into the local landscape (MHyLab, Switzerland)
8.4.4.2. Turbines in wastewater networks There are two possibilities for using turbines in wastewater (see Figure 8.11).
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Figure 8.11. Possibilities for turbine use with wastewater before and after the wastewater treatment plant (MHyLab, Switzerland)
The first is located above the wastewater treatment plant. In this case, the sanitation system for a built-up area, located at a height, ends up in a sieving and surge tank. The wastewater is then taken by a penstock to the wastewater treatment plant, located in the valley, where it is passed through a turbine before being treated. The second is located after the wastewater treatment plant. In this case, the wastewater treatment plant is located at a height and it is the purified water that is taken down to the valley by a forced pipe to end up in a turbine scheme before being discharged back into a lake or river. We resort to this method mainly when the river into which the release must take place (in altitude) has too low an output and the dilution is not sufficient, or when there is no river close to the wastewater treatment plant. In both cases, the environmental impact from the turbine is positive, especially in the case of a turbine set after the wastewater treatment plant, since improving the oxygenation of the water on leaving the power plant is possible. Moreover, no supplementary authorization is necessary.
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8.4.4.3. Recovery of energy in desalination plants Seawater desalination plants operate according to the principle of inverse osmosis, which consists of compressing the water with a semi-permeable membrane above the osmotic pressure (about 80 bars). The pure water is collected behind the membrane while the residue of liquid water containing salt, always at high pressure, can be passed through a turbine in order to recover part of the energy used for the initial compression. 8.5. Potential 8.5.1. Worldwide small hydropower The worldwide potential of small hydropower is difficult to evaluate because the definition (based on the maximum output of the scheme) of a small power plant varies from country to country. The World Energy Council estimates that in 2000 the installed capacity in small hydro globally was 37,000 MW (representing an estimated annual production of 148,000 GWh) and that this capacity could easily reach 55,000 MW (representing an estimated annual production of 220,000 GWh) in 2010. There is considerable potential on the African, Asian, and South American continents. In the majority of these regions, the main problem currently is the lack of financial means, while small hydro would be a response to the electrification needs of rural and peripheral areas. 8.5.2. European-wide small hydropower The economically feasible potential for small hydropower1 in Europe is estimated at more than 110,000 GWh/year (63,000 GWh/year in the Europe of the Fifteen,2 7,900 GWh/year in the 10 countries that joined the Union in 2003,3 26,700 GWh/year in the three other candidate countries4 and 13,900 GWh/year in Switzerland and Norway). More than 18,000 power plants are in operation in all of these countries taken together. 1 Source: ESHA, European Small Hydropower Association, www.esha.be. 2 Germany, Austria, Belgium, Denmark, Spain, Finland, France, Greece, Ireland, Italy, Luxembourg, the Netherlands, Portugal, the UK and Sweden. 3 Cyprus, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, the Czech Republic, Slovakia and Slovenia. 4 Bulgaria, Romania and Turkey.
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For most of the countries mentioned above, the total production is 50,900 GWh/year for a total installed output of almost 14,000 MW, which represents about 1.4% of the total European electricity production. The distribution of this potential varies greatly according to the country. More than 60% of the potential of Europe of the Fifteen, Norway and Switzerland is already being exploited. However, the figure is only 30% in the 10 new member countries and 5% in the candidate countries. 8.5.3. Possibilities for development of small hydropower in Europe The directive on renewable energies RES-e, adopted in 2001, specifies that the European Union must supply 22% of its total electrical energy production from renewable sources by 2010. The 10 countries that joined the Union in 2003 must produce 11% of their electrical energy production from renewable sources by 2010. In 2020, the contribution of renewable energy to electricity production must be more than 33%. For the Europe of the Fifteen, these objectives imply the creation of about 5,000 MW in small hydropower, causing the annual production of 40,000 GWh to go to 55,000 GWh by 2010. 8.6. Research & Development in small hydropower The market in small hydropower is occupied by two types of enterprises. The first is the industrial groups that furnish equipment for the large hydraulic schemes (> 10 MW). In this context they carry out a lot of development in the laboratory and the entities charged with the development of small turbines directly profit from these results. The second type is composed of small and medium size enterprises. Generally they do not have the financial means to conduct such Research & Development programs and they produce equipment without being able to test its performances. As already suggested, small hydropower requires specific development with two goals in view: – improvement in productivity, – improvement in the integration of the schemes. Improvement in productivity can be obtained only by optimization of equipment and construction techniques to obtain schemes and equipment that are simple and
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reliable, implying low maintenance costs. This equipment must, moreover, also be highly efficient so as to increase the number of kWh produced per financial unit injected into the project. The main R & D directions in small hydropower, presented below, give only a few possibilities that are important, but they are not exhaustive. They are connected with electromechanical aspects and civil and environmental engineering. Several of these directions are interdisciplinary. Thus, for example, many developments in civil engineering or in electromechanical equipment also have a large environmental component. 8.6.1. Development of equipment adapted to each site Most often, small and medium companies develop a range of standard machines or they simplify the hydraulic characteristics of large machines empirically without checking the consequences. The result is that the machine supplied is only rarely the best from the point of view of hydraulics. The consequences can be many: loss of production, cavitation, reduced life time of the equipment, etc. However, since it is not financially possible to develop a laboratory model for each new small turbine according to the characteristics of the site it is intended for, it is necessary to develop systematically in the laboratory, models of turbines adapted to small hydro. This method is the only one that subsequently allows all turbines, in the required domain, to be exactly dimensioned according to the specific data of the scheme. It also enables us to guarantee a high level of efficiency, with optimum hydrodynamic behavior that assures not only the reliability and longevity of the turbine, but also future production. The developments coming out of R & D programs should also facilitate the rehabilitation of existing sites or the setting of new sites, taking account of accrued environmental constraints. In this context, we can cite in particular immersed turbines that do not require a building and imply only very little maintenance. 8.6.2. Development of variable speed Low-head sites often present significant discharge variations, especially during floods. Therefore the operating conditions of the machine are very variable, implying an increased risk of cavitation and sometimes a considerable degradation of performance. The solution in this case is to adapt the rotation speed of the machine in order to regain the same optimal operating point as during nominal head and discharge. However, and contrary to strongly held beliefs, variable speed allows
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only a very slight adaptability of the scheme to discharge variations for a constant blade opening. The economic gains of the variable speed can be significant and should be considered in the technical and economic analyses of the projects. In other respects low-head turbines often operate at rotation speeds that are relatively low. To reduce the number of generator poles, it may be economically necessary to drive it by a system of mechanical speed increasers (belt or teeth gear systems). Another solution consists of coupling the generator directly to the turbine and multiplying the frequency with an electronic converter. The efficiency of the converter is equal, even superior, to that of the mechanical speed increaser. Important synergies may be developed in this field with wind energy, which has made use of this technique for a long time. 8.6.3. Development in generators As previously explained, the low-head turbines have rotation speeds that are relatively low. In order to limit the losses in the conversion systems (speed increaser or frequency converter), we can couple the turbine directly to a generator with a large number of poles. However, these machines have the disadvantage of being very large. Nevertheless, new generators having a large number of pole pairs and yet compact are in the process of coming into use. Improvement in the efficiency level of generators also remains a major subject of research and development, especially for generators with permanent magnets. 8.6.4. Development in control-command Control-command systems go from a technology based on programmable microcontrollers to systems that use advances in computing and new methods of communication that allow the development of total management systems, highly modular, adaptable to present equipment and to the specific needs of each scheme or type of turbine. Various technologies are used, such as the CAN-Bus technology, which enables the system to have a decentralized “intelligence”, able to function even in the case of a fault in the power plant unit by authorizing, in particular, direct communication between the instruments and a management of events independent from the power plant unit.
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Significant development has also come about in teleprocessing systems that make it possible to acquire information about the functioning of the power plant, especially the alarms, from a distance by a communication system (fiber optics, mobile phone or radio link). In certain cases, these systems make it possible to resolve the problem at a distance from the fault and to restart the machine without having to go to the site. 8.6.5. Inflatable weirs Inflatable weirs are an especially economical way to be able to revamp existing weirs. The creation of a new weir is also possible by this method, but it implies building a concrete base on the river bed to ensure that the floating weir is anchored to the bottom The height of these weirs is between 1 and 4 m at most. Furthermore, beyond a certain height, the weir may not be inflated with air, but with water, the Archimedes force becoming too great. This solution presents an interesting advantage: the possibility of deflating the weir during flooding, thereby making it possible to limit the risk of flooding upstream. 8.6.6. Water intake Although it is important for the proper operation of the scheme, the water intake often remains neglected at the time of construction. The water intake has two main roles: to guarantee that water enters the pipe in the most efficient possible way, and to prevent floating debris (leaves, branches, rubbish, etc.) from reaching the turbine. The classic solution consists in furnishing the intake with a trash rack to stop the debris before it enters the intake pipe. However, this solution has the inconvenience of making it necessary to clean the rack at regular intervals and to eliminate trash that has been collected. Several solutions are under development. Among these are submerged water intakes, such as the Tyrolean intake, which has the particular feature of not being very much affected by floating debris and creates almost no visual impact since it is beneath the surface of the river. These water intakes can also be cleaned easily by a special operation of the turbine.
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8.7. Environmental aspects of small hydropower Since it comes from a renewable resource, small hydropower is considered to be positive from some environmental points of view, and to be negative from others. This divergence can be explained by the fact that all human intervention has an impact on the environment; today solutions with different levels of complexity and cost are available to reduce this impact. 8.7.1. Initial state of the milieu The impact study of a small hydropower plant on the environment comes back to the following question: “How would the milieu (river, local environment, etc.) function if it were not infringed upon?” Two main types of site with potential for small hydropower are considered here: – natural rivers, modified or canalized, – water networks, whether drinking water, wastewater (treated or not) or irrigation. Rivers are characterized by a dynamic function: – hydraulic regime (flood, low water, frozen, etc.), – carrying of material, – ecosystems more or less diversified (plants, fish, invertebrates), – human intervention (channeling, introduction of new species, pollution, etc.). Water networks are characterized by an infrastructure composed of pipe systems, reservoirs, etc., and by a primary function other than producing electricity: supply of drinking water, collecting and clearing, discharging wastewater, etc. Different laws apply to these sites (laws on energy, environment, water, fish and also historic monuments, etc.), specific to each country. 8.7.2. Setting phase 8.7.2.1. Setting of a small power plant for its integration into the ecosystem The small power plant must respect, even improve, ecologically, technically and/or economically, the initial state of the site in which it is integrated. Thus, it is imperative that it respects: – the primary function of the infrastructure in the case of water networks,
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– the dynamics of the river: the growth of vegetation, of invertebrates and fish (migration), hydrology conditions, etc. In order to make possible and optimize electricity production, and thus the use of resources, the milieu undergoes transformations such as: – the construction of a dam or weir, of a water intake, of channels, – the creation of a reservoir or pond for the new weir, – the placing of new pipes, – the short circuit of a section of the river bed, between the water intake and its return. If no measures are taken, these schemes can affect the ecosystem of the milieu more or less significantly with, among others: – the interruption of the continuum of the watercourse, isolating populations and preventing migration, even reproduction, – the monotonization of the milieu, with the regularization of discharges, of the watercourse bed (its depth, banks, bottom, etc.), – the reduction in flow speed in the watercourse, with more sedimentation, the accumulation of pollutants, the growth of algae, the modification of the oxygen content, water temperature, etc. To limit the negative impact on flora and fauna, the setting of the site should take its inspiration from the natural watercourse and basins, with the goal of avoiding the disappearance of biotopes and even favoring the creation of new ones. One of the rules is to avoid monotony, whether in terms of depth, width, or slope of the banks, in order to create all sorts of niches, small islands, backwaters and other shaded areas. The vegetation planted on the site should be diversified and adapted to biotopes. It should be noted that certain schemes have succeeded to the point of being classed as protected zones (see Figure 8.12).
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Figure 8.12. Holding basin created by a weir (Switzerland) (MHyLab, Switzerland)
In the short-circuited section, there is now a discharge, called a residual or reserved discharge, in order, for example: – to maintain a certain continuum of the watercourse, – to ensure the life of the flora and fauna, and their diversity, – to supply underground water, – to create alluvial spaces, – to ensure the cleanliness of the watercourse. This residual discharge is defined by specific laws for each country, based, for example, on hydrology data or on the depth of the watercourse and its speed. Unfortunately, the multiplicity of these definitions demonstrates their weaknesses. In fact, each site being unique, it is difficult to give a formula for calculating the residual output that is systematically applicable. Moreover, although a small discharge is favorable to one particular wild animal and one particular plant, a large discharge can be harmful to them, and the reverse. Thus, variability of this residual discharge seems like the best compromise for assuring the life of the ecosystem. To re-establish the continuity of the watercourse, a device for crossing the weir or dam specific to the site is set (depending on the obstacle type, the hydrology of the water stream, migrating species, the available space, etc.). Its efficiency depends on a number of parameters: water flow speed, depth, integration into the milieu, etc. Two types of devices may be cited: – the by-pass, or lateral channels of a natural type (see Figure 8.13), – fish ladders, which divide the heads into several steps that the fish can cross, alternating with small basins (see Figure 8.14).
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Figure 8.13. Example of a fish pass of natural type (BOKU: University of Natural Resources and Applied Life Sciences, Vienna, Austria)
Figure 8.14. Example of a fish ladder (BOKU: University of Natural Resources and Applied Life Sciences, Vienna, Austria)
Other devices, such as racks, bubble screens, etc., are installed to prevent fish from getting into the turbines or conduits. This is done in a manner that avoids injuries, fatal or not, due to sharp variations in pressure. Another final step in site setting may consist of creating recreational areas that contribute to its local acceptance (see Figure 8.10). It is understood that each site has its specific impact on the flora and fauna, and therefore specific measures for addressing them.
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8.7.2.2. Flow of materials and equipment As in the case of all construction (housing, industry, infrastructures, etc.), the setting phase of a small power plant implies certain kinds of environmental impact due to the flow of materials. With a regard for sustainable development, we will therefore try to limit the consumed energy while being attentive to the origin of the materials and equipment, to their quality and to their life cycle. In this regard, we can stress that small hydropower favors the intervention of local enterprises made possible thanks to the technical and economic optimization of its components. The life time of the equipment on the whole being long (more than 50 years), if they have been chosen correctly to begin with, in other respects, the number of components that need to be changed in the operating phase is limited. Furthermore, we can note that the construction of small hydropower plants generally generates less construction waste and does not require installation of roads. 8.7.3. Principal inputs and outputs during the operating phase
Figure 8.15. Principal flows during the operating phase for a small hydropower plant and milieu
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8.7.3.1. Water The issue of water is crucial in a small hydropower plant project. In terms of quality, the main risk of pollution is connected with the use of oil in the blades and other mechanical systems, hence the use of biodegradable oil and even the elimination of all oil devices (the current practice in drinking water networks). In terms of quantity, we stress that turbined water is returned in full to the watercourse or water network. As a result, given the limited availability of water, all the equipment should guarantee optimum performance, including the turbine, the fish pass, etc., up to the electrical connection, and to whatever kind of hydraulic system is used, in order to produce maximum electricity beginning with outputs that are usable technically, economically and ecologically. 8.7.3.2. Materials carried by the watercourse The watercourse carries all sorts of materials: leaves, branches, bags, plastic bottles and other non-natural detritus, which are stopped at the racks of the power plant. Laws particular to each country define the future of these materials: elimination of all of them, restitution to the watercourse, selection of natural materials, etc. In this way, cleanup due to power stations turns out to be ecologically beneficial for many watercourses. 8.7.3.3. Noise Rotating machines and other equipment can be relatively noisy. However, it is easy to take preventive measures by soundproofing the plant. It should be recalled that the better the equipment performs, the less noisy it is. 8.7.3.4. Electricity production Finally it can be stressed that the production of electricity does not involve atmospheric emissions. Thus, integrated by substitution into the European interconnected electricity network, each hydroelectric GWh produced corresponds to a reduction of 480 metric tons of carbon dioxide (CO2) emissions. 8.8. Policies favoring small hydropower Europe and many other countries have established often ambitious goals for themselves for supplying part of their electricity by production means coming from renewable energy. These goals can be reached only if there is a favorable, voluntary, and long-term political framework in place.
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Two directions to consider for small hydropower are as follows: – financing of R & D programs and demonstration projects, – rate pay-back model allowing development of small hydropower. 8.8.1. R & D program We have already stressed previously the importance of research and development in the field of small hydro, not only to obtain quality equipment that is also simple, reliable and high performing, but also to give the promoter a performance description that conforms to reality. These guarantees are indispensable not only from the technical point of view, since they assure the longevity of the equipment, but also from the environmental point of view, since the water resources are thus used in the optimal way. The implementation of pilot schemes completes this program by allowing the verification on real sites of solutions developed in the laboratory. These R & D aspects are not limited only to technical aspects, but should also integrate the environmental component. As has already been mentioned, the costs of such development unfortunately remain high and are therefore not easily accessible to active SME (small and medium enterprises) in the field. Consequently, it is indispensable that research programs be made accessible to all. This objective can be realized in two ways. The first is to finance individual projects of various suppliers in the fields of electromechanical engineering (turbines, generators, automatic control systems, etc.) or in civil engineering (fish pass, water intake, etc.) This method can lead not only to a redundancy of certain developments since each will want to develop his/her range of solutions, which will finally be very similar to each other, but also to the development of solutions specific to sites that are not usable in other cases. Furthermore, this approach will not allow wide diffusion of the solutions that are found, which is contrary to the spirit of programs of support and encouragement. The second solution is to set up several centers of research and competence in small hydro charged with development work and with making the results available to all who require it. This approach was developed for hydraulic machines in Switzerland within the framework of the Energy 2000 program, since a laboratory for small hydraulics had been created and still supplies a technology that is proven
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and guaranteed to all who request it. A similar approach can be considered for other fields connected to small hydraulics. 8.8.2. Rate measures The cost price of the kWh produced varies greatly according to the head and the output of the scheme. In Figure 8.16 we demonstrate that the lower the head and the output, the higher the cost. Since these schemes represent the largest potential remaining in Europe, it is therefore indispensable to put in place tools that allow remunerating the energy produced at a price that makes the development of these plants favorable. We can point out that in the countries applying market prices for the repurchase of electricity produced from renewable resources, the development of new schemes has been strongly reined in. Furthermore, the simple comparison of the price of electricity produced from renewable resources and from conventional systems is not accurate since in general neither their external costs are taken into consideration, nor the development of long-term energy policies.
Figure 8.16. Variation of the cost price (in c€), according to the power of the plant and its ambit in the heads
Consequently, the development of new schemes must come from establishing rate measures that enable small schemes to be made profitable. Furthermore, the rate should be guaranteed for the long term to give investors confidence and allow them to obtain the necessary bank financing. In fact, small hydraulics, in the manner of other renewable energies, is characterized by major investments that are amortized over periods of 20-40 years. It should be pointed out that the higher the purchase price, the more the realizable economic potential becomes important. A high rate
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also allows excellent measures for environmental integration to be guaranteed. To be efficient, the guaranteed price measures should be accompanied by a performance bonus motivating the owners to optimize their sites. A second tool, already in place in several European countries, requires certain participants (producers, suppliers, consumers) to produce or acquire electricity from renewable energy sources. These individuals are then free to choose their preferred method for reaching these objectives at the best cost. If this tool enables reflection of the market price, it does not, however, allow optimal development of the renewable energies and does not financially encourage investors to build new schemes. In certain respects it amounts to a minimal program. 8.9. Conclusions Hydropower is a renewable energy, local and also modular, being the only one that can serve as an energy accumulator with an excellent output and able instantly to adapt to consumer needs. Its potential can still be widely developed and could cover a significant share of world energy consumption. In this context, small hydropower whose potential consists of new schemes and sites to be rehabilitated, has an important role to play, as much in developing countries as in industrialized ones. Although this resource has been exploited for several hundreds of years and the technology is perfectly mature for large schemes, small hydropower still requires considerable development work aimed at placing specific equipment on the market. These should be simple, reliable, and have high performance levels and guarantees, attainable only by equipment coming from R & D. Small hydropower is a local resource. By supplying consumers located in proximity to the power plant, it removes congestion from the grid and consequently enables transmission losses to be limited. Building small hydropower plants also generates local activity, whether in the construction or the operating phase, aspects that are especially important in outlying areas. Small hydropower is environmentally friendly because it: – returns all the water taken from the river without changing its quality, – offers a favorable balance in consumed energy per kWh produced, – participates in the reduction of greenhouse gas emissions, the electricity production being without atmospheric emissions.
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Emphasis cannot be placed on the great potential of small hydropower unless voluntary policies are put in place for encouraging the development of schemes that use natural resources in an optimal fashion, from the technical point of view as much as from the economic and ecological points of view. 8.10. Bibliography Bibliographic references [AIE 04] AGENCE INTERNATIONALE DE L’ENERGIE, Key World Energy Statistics 2004, AIE, Paris, 2004. [BAN 06] BANSARD J. F. (THEE), DENIS V. (MHYLAB), CHOULOT A. (MHYLAB), SASSO A. (SASSO S.N.C.), Développement systématique des petites turbines axiales: le projet SEARCH LHT, MHyLab, 2006. [BUR 97] BURGER P. K., GROSS H., Petites centrales hydrauliques sur l’eau potable, Diane 10, Petites centrales hydrauliques, Programme Energie 2000, Office fédéral de l’Energie, Berne, Switzerland, 1997. [CHE 95] CHENAL R., VUILLERAT C.-A, RODUIT J., L’Eau usée génératrice d’Electricité, Dossier technique et étude du potentiel, Diane 10, Petites centrales hydrauliques, Programme Energie 2000, Office fédéral de l’Energie, Berne, Switzerland, 1995. [COM 99] COMMISSION EUROPEENNE, Energie pour l’avenir, les sources d’énergies renouvelables, Livre blanc établissant une stratégie et un plan d’action communautaire, European Commission, Luxembourg, 1999. [DEN 98] DENIS V., “The role of the hydraulic laboratory in small hydro development”, Hydropower & Dams, 4, p. 50–52, 1998. [DIR 01] “Directive 2001/77/EC of the european parliamant and of the council”, Official Journal of the European Communities, europa.eu/eur-lex/pri/en/oj/dat/2001/, 2001. [ESH 00] ESHA, Rapport Blue Age, www.esha.be, 2000. [ESH 03] ESHA, Report on Small Hydropower Statistics, General Overview of the Last Decade (1990-2001), www.esha.be, 2003. [ESH 04] ESHA, Small Hydropower Situation in the New EU Member States and Candidate Countries, www.esha.be, 2004. [ESH 05b] ESHA, Environmental Integration of Small Hydropower Plants, www.esha.be, 2005. [ESH 05c] ESHA, State of the Art of Small Hydropower in the EU-25, www.esha.be, 2005. [EUR 05] EUROSTAT, Statistique européenne de l’énergie, epp.eurostat.cec.eu.int, Luxembourg, 2005.
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[HYD 05] Hydropower and Dams World Atlas, International Journal on Hydropower & Dams, Sutton, 2005. [MOR 97] MOREL N., ROULET C. A., L’énergie au futur, Editions d’en-bas, Lausanne, 1997. [PAQ 00] PAQUIER S., Histoire de l’électricité en Suisse, la dynamique d’un petit pays européen, Editions Présent-Passé, Geneva, 2000. [RES 03] RESEAU THEMATIQUE EUROPEEN DE LA PETITE HYDRAULIQUE, ESHA, Reserved Flow – Short Critical Review of the Methods of Calculation, www.esha.be, 2003. [RES 05a] RESEAU THEMATIQUE EUROPEEN DE LA PETITE HYDRAULIQUE, ESHA, MHyLab, Proposals for a European Strategy of Research, Development and Demonstration for Renewable Energy from hydropower, www.esha.be, 2005. [RES 05b] RESEAU THEMATIQUE EUROPEEN DE LA PETITE HYDRAULIQUE, ESHA, FAQ “Frequently Asked Questions” ou Questions souvent posées sur la petite hydraulique, www.esha.be, 2005. [RES 05c] RESEAU THEMATIQUE EUROPEEN DE LA PETITE HYDRAULIQUE, ESHA, Guide pour la Réalisation de projets de Petite Hydroélectricité, available in English, French, German and Swedish, www.esha.be, 2005 [RES 05d] RESEAU THEMATIQUE EUROPEEN DE LA PETITE HYDRAULIQUE, ESHA, Brochure on the Environmental Aspects of the Small Hydroelectric Plants, www.esha.be, 2005. [RES 05e] RESEAU THEMATIQUE EUROPEEN DE LA PETITE HYDRAULIQUE, ESHA, Reserved Flow – Effects of Additional Parameters on Depleted Stretch, www.esha.be, 2003. [RES 05f] RESEAU THEMATIQUE EUROPEEN DE LA PETITE HYDRAULIQUE, ESHA, Checklist, Liste des points importants à analyser avant d’installer une petite usine hydro-électrique, available in English, French, German and Italian, www.esha.be, 2005. [RES 06] RESEAU THEMATIQUE EUROPEEN DE LA PETITE HYDRAULIQUE, ESHA, Innovations is our Business, Brochure sur les innovations en matière d’ingénierie, www.esha.be, 2006. [TAN 96] TANNO G., “Pico-centrales, les toutes petites centrales à installer soi-même”, Diane 10, Petites centrales hydrauliques, Programme Energie 2000, Office fédéral de l’Energie, Berne, Switzerland, 1996. [VON 97] VON MOOS L., LEUTWILER H., “Aperçu général sur les petites centrales hydrauliques, Aspects économiques et écologiques”, Diane 10, Petites centrales hydrauliques, Programme Energie 2000, Office fédéral de l’Energie, Berne, Switzerland, 1997. [ZAU 96] ZAUGG C., LEUTWILER H., “Petites Centrales Hydrauliques et Ecologie des Eaux, Analyse de la Situation”, Diane 10, Petites centrales hydrauliques, Programme Energie 2000, Office fédéral de l’Energie, Berne, Switzerland, 1996.
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[ZAU 97] ZAUGG C., PEDROLI, J. C., “Poissons et petites centrales hydrauliques, Solutions avantageuses de franchissement pour les poissons et la microfaune aquatique”, Diane 10, Petites centrales hydrauliques, Programme Energie 2000, Office fédéral de l’Energie, Berne, Switzerland, 1997.
Specific sites [SMA] www.smallhydro.ch [MHY] www.mhylab.com [ADE] www.ademe.fr [HYD] www.hydropower.org [RET] www.retscreen.org [MIC] www.microhydropower.net
Chapter 9
Geothermal Energy Production
9.1. Introduction Geothermal energy is the thermal energy contained in the Earth, and geothermal energy production covers all the processes which enable the extraction and industrial processing of this internal heat. It is partly due to an inherited process (the gravitational energy from the formation of the planet) and partly due to the continual decay of radioactive materials (U, Th and K) over a long period of time, which are contained within the continental crust, at an average thickness of 30 km. It is transferred – in stable parts of the globe – by conduction towards the surface of a relatively low flow of heat (65 mW/m2) creating an average thermal gradient of 30°C/km. The internal structure of the Earth is such that 99% of its mass is subjected to temperatures exceeding 1,000ºC and only 0.1% has a temperature lower than 100ºC, the extremes being more than 6,000ºC for the internal part of the core and an average of 15ºC for the surface of the planet, where humans evolved. The internal production of heat being estimated at 20,000 GW, the flow of terrestrial heat – which lets out a thermal power in the range of 42,000 GW – is leading to a very slow cooling of the Earth. However, the internal parts of the Earth are moving and, whatever model describes this dynamic, the transfer of convective heat accompanies the movements of matter that affect the Earth’s mantle, which behaves, on the geological timescale, Chapter written by Florence JAUDIN and Laurent LE BEL.
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as a strongly viscous fluid. Moreover, in certain specific regions of the globe, the fluids linked to the Earth’s crust are placed in circulation by convective systems. They extract and transfer to the surface the heat of the rock structures they encounter. Surface manifestations (thermal anomalies, hot springs, geysers, etc.) signal the presence of geothermal reservoirs made of permeable geological volumes containing aqueous fluids ready to give off thermal energy, once brought to the surface. Geothermal energy makes it possible to produce either electricity or heat; each method of taking advantage of heat from the Earth uses distinct technologies for different applications. Industrial geothermal technology consists of getting access to this natural resource, generally with the help of bored wells of greater or lesser depth, and taking advantage of it either directly, by direct heat exchange when there is a corresponding demand (heating, industrial process, etc.) or by conversion, in the case of electrical energy production or cooling. Thermodynamic heat pump (HP) systems allow use of ground heat even when it is at an insufficient temperature for direct use. This geothermal HP system has been developing rapidly because it is very capable of being modular in terms of scale of usage, and the methods currently used for obtaining heat from the ground make it possible to create operations everywhere, whatever the geologic environment might be. 9.2. Geothermal energy: why, for whom and how? Heat from the Earth, present everywhere, shows up in different geological environments; the modern techniques that are used make it possible to exploit this energy over a very wide range of temperatures, for diversified uses that can meet the needs of users at all levels. 9.2.1. The types of resources used A geothermal resource is made up of one heat source and one rock formation. The heat source can either be the local terrestrial thermal flow or a magmatic intrusion at very high temperature relatively close to the surface (5 to 10 km). The rock formation is to varying degrees compact, porous, fissured or fractured (Figure 9.1).
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Figure 9.1. Pores and fractures in rocks (from Bodelle and Margat) (a rock is said to be permeable when the pores and fissures are interconnected, allowing the circulation of a fluid)
In certain regions of the globe, fluids placed in circulation, thanks to convection systems, extract and transfer heat from the rock formation to the surface, either from a shallow depth when the geothermal gradient is abnormally high (compression tectonic regime or extension regime) or greater depths when the geothermal gradient is close to the average gradient. In other regions, the highly permeable layers allow the exchange of heat with the contained fluids (stationary or moving), which are thus brought to a temperature conforming to the local gradient. Finally, in general, the thermal equilibrium attained in the most superficial layers reproduces the average of annual atmospheric temperatures. These phenomena lead us to propose a geological typology of the current resources and the potential of geothermic energy: – fissured and/or porous volcanic formations (convection systems of active zones with high geothermic gradient), – base that is more or less fractured and more or less “dry” in the stable continental platforms (conduction or conduction/convection combination), – sedimentary formations with variable porosity and permeability, which may be productive or dry (conduction), – the (near substrate) shallow ground (thermal equilibrium). It is the enthalpy (kJ/kg) of these systems (a thermodynamic function that comes from the internal energy of the fluid properties and one of whose indicators is the temperature) that is theoretically going to determine the usage of the source in the two large energy systems: electricity production and heat production. High enthalpy (t > 160ºC) and average enthalpy (90ºC < t <160ºC) allow the generation of electricity by conversion. Low enthalpy (30ºC < t < 90ºC) is limited to direct usage by heat transfer. Finally, very low enthalpy (t < 30ºC) involves intermediary
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thermodynamic systems to allow the production of heat and/or cooling at the temperature levels necessary for the final use. The flow of heat from the Earth and the energy stored in the crust mean that geothermal sources are almost limitless and renewable when they are properly used. It is only technical and economic difficulties that, as of now, limit access to certain types of resources. 9.2.1.1. Fissured and/or porous volcanic formations What is referred to as steam fields are naturally produced reservoirs containing geothermal fluids at high or average enthalpy, whose temperature is generally between 125 and 250ºC, and whose physical state is often a coexistence between phases (liquid and steam). The behavior of the natural thermodynamic system is controlled either by steam (two large geothermal fields known in the world: The Geysers in the USA and Larderello in Italy) or by liquid (the more usual case). When steam pressure controls the system, the reservoir produces dry steam or superheated steam. In the other case, the reservoir usually produces either saturated steam or a liquid–steam mixture. At the lowest temperatures, the reservoir produces geothermal fluid directly in liquid form.
Figure 9.2. Schematic drawing of a geothermal field producing steam (the impermeable layer that covers the reservoir maintains the pressure and avoids the escape of steam. There can be natural escapes by fractures or faults, with production of volcanic gas emissions and heat sources on the surface)
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These types of reservoirs are found in regions that are volcanic or that have had recent magma production, also called “active zones”, usually near the borders of lithospheric plates. A magma intrusion at high temperature constitutes a heat source that heats up the surrounding territory. When this territory contains permeable layers, it constitutes a geothermal reservoir of high temperature and usually of limited depth. In Bouillante in Guadeloupe1 the first French power plant for producing geothermal electricity was put in service over this type of reservoir. 9.2.1.2. Aquifers of sedimentary basins Sedimentary basins usually harbor layers that are porous and permeable (limestone, sandstone, sand, etc.). They constitute aquifers (the water can circulate) of greater or lesser depth (Figure 9.3). The temperature of these layers is in direct relation to their depth; it can reach 50 to 100ºC between 1,000 and 2,000 m deep. In France, the Dogger aquifer in the Parisian region was developed in the 1980s.
Figure 9.3. Schematic drawing of the principle of an aquifer with sedimentary basin (of the Paris basin type)
1 French West Indies.
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9.2.1.3. Superficial formations Superficial formations are geological formations of the near ground usually having a depth between 0 and 100 m. These formations may be porous or fissured and they may or may not contain aquifers. In general, the thermal equilibrium reached, in the most superficial layers, reproduces the average of the annual atmospheric temperatures (10 to 15ºC in the range of the first 100 m). These formations are the basic target of heating/air conditioning devices by geothermal heat pumps2 (GHP) that are currently being developed. From the Earth’s surface to several meters of depth, the energy essentially comes from solar radiation and the migration of rain water into the Earth. Above some 10 m, in our European latitudes, the energy comes essentially from natural thermal flux. The relatively low temperatures at these depths also enable production of summertime cooling for buildings. 9.2.1.4. Deep formations (with low permeability) The thermal flux coming from a deep origin and the average mass thermal capacity of the Earth’s crust are such that the quantity of heat stored in the first 5 km of the crust is considerable. Extraction of a fraction of this heat is possible. At these depths of several kilometers, we encounter either rocks with a base that is more or less fractured, or sedimentary rocks from thick basins with variable characteristics of porosity and permeability. The extraction of part of the heat contained in these formations is possible by artificially stimulating them near wells in order to make the circulation of fluid and thermal exchanges between the fluid and the surrounding rock possible. The ensemble of systems thus enhanced is called enhanced geothermal systems (EGS). A research program has been underway for more than 15 years, in France at Soultz-sous-Forêts, in Alsace, on a granite formation. 9.2.2. End-use In terms of end-use, the classification by large systems is the following: – direct heat exploitation system: exploitation of aquifers (generally by doublet) for direct use of the heat by industry and for heating residential complexes, – heat pump (HP) system: for supplying heat and, eventually, cooling, to ensure the heating and air conditioning needs of the large service sector, housing complexes or individual housing,
2 Also called ground source heat pumps (GSHP).
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– electricity system: geothermal power plants for producing electricity by thermoelectric conversion. 9.2.2.1. Heat network system The production of heat is the most direct form of exploitation of geothermal fluids. It usually addresses low enthalpy fluids that do not allow electricity production. These fluids are either extracted directly from geological reservoirs, or they come from geothermal electric power plants (thermal effluents). In this latter case, the installation is a device for geothermal cogeneration. Heat does not hold up economically under long distance transportation; this energy production lends itself to uses than can be located near the source (several kilometers). Urban heating by heat network is, on a worldwide scale, the second greatest user of geothermal heat (after swimming and mineral baths establishments). In this case we are talking about low temperature or low enthalpy geothermal energy. The term direct exploitation of geothermal heat is also employed. Water extracted from the ground may be reinjected into the aquifer where it originated by means of a second well; we call this a geothermal doublet. The principle of the geothermal doublet was developed in the 1980s in the Parisian region for urban heating operations exploiting the Dogger aquifer. We should mention that geothermal water does not circulate in the heat distribution network. In fact, an exchanger is placed between the circuit of geothermal water and the heat distribution network. The transfer of heat is made at the level of this exchanger, then the geothermal water is sent back to its natural reservoir. 9.2.2.2. Heat pump (HP) system The HP system is a device that moves heat from a low temperature heat source to a higher temperature heat sink. This transfer requires a certain amount of work that corresponds to an energy consumption (electric or other). The attraction of the heat pump is that the final usable quantity of energy is higher than the energy consumed by the transfer. The energy source from which the process draws the required heat or cold may be: – shallow aquifers, – the ground itself, via devices such as vertical geothermal probes (collectors), layers of horizontal tubes (flat ground collectors), or thermoactive foundations.
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9.2.2.3. Electricity production Electricity is produced by turbine action (direct or not) on steam extracted from the subsurface. In this case we call it a geothermal system of high energy or high enthalpy or high temperature. If the natural fluid is not in the form of steam, but at a sufficient temperature, a binary cycle may allow the production of electricity by vaporization of an intermediary fluid that, in vaporizing, will enable powering of a generator. This type of installation produces a considerable quantity of residual heat that can be used in various applications. 9.2.3. Other uses Besides the three main systems cited above, there are multiple uses of geothermal power (Figure 9.4). Therapeutic baths use both water and its heat, but, traditionally in France, unlike other countries in the world, this type of use is not classified in terms of production statistics as geothermal usage.
Figure 9.4. Principal uses of geothermal energy with respect to temperatures (source: http://www.geothermie-perspectives.fr)
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9.3. Geothermal heat pump systems The operation of a thermodynamic conversion device makes it possible to make use of the energy of the ground in order to bring it to the temperature levels that are usable for covering heating and/or cooling needs. The heat pump system exploits the energy from low temperature ground to ensure heating and/or air conditioning needs under satisfactory energy and environmental conditions. This is an efficient energy option for ensuring comfort needs in the service sector, the small collective and the residential. This system uses shallow ground, that is, usually a layer between 0 and 100 m deep and whose temperature is usually between 10 and 15ºC depending on the geographical location. 9.3.1. Current situation and tendencies In France, the consumption of heating and/or cooling represents close to 30% of the energy consumption of the service sector and more than 75% of the residential sector (source: DGEMP). The geothermal heat pump involves individual installations, the service sector, and collective housing. The energy performance of these devices contributes to the reduction in the use of fossil fuels and greenhouse gas emissions. Geothermal heat pumps instead use the natural heat of the substrate that is available throughout the locality. In its report in 2007 the EuroObservER3 estimates the total number of geothermal heat pumps (GHP) to be near 600,000 units in the European Union, representing an installed capacity in the order of 7,326 MWth.4 Sweden has the largest number with more than 270,111 units, or a cumulative power of 2,431 MWth. That eclipses the French total (83,856 units, or 922 MWth), the German total (90,517 units, or 995 MWth), the Austrian total (40,151 units, or 664 MWth) and the Finish total (33,612 units, or 721 MWth). For its part, Switzerland had 33,000 units in 2004. These numbers reflect very different penetration rates from one country to another; in fact, this represents about 95% of new residences in Sweden, 40% for Switzerland, and around 5% in France.
3 7th report EuroObservER (http:/www.energies-renouvelables.org). 4 MWthermal.
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9.3.2. The principle of the heat pump A heat pump makes it possible to extract energy (calories) from a low temperature heat source (called a cold source) and to reinsert it at a higher temperature in a heat sink (called a hot source). The heat pump is made of a closed circuit in which is circulated a refrigerant that changes state according to the components it passes through: 1) the heat removed from the exterior is transferred to the refrigerant fluid, which evaporates, 2) the compressor takes in the vaporized refrigerant fluid, 3) the compression raises the temperature of the refrigerant fluid, 4) the refrigerant fluid gives off its heat to the heating circuit, 5) the refrigerant fluid condenses and returns to a liquid state, 6) an expansion valve lowers the pressure of the refrigerant fluid, which goes on to begin the cycle again. The attraction of the system resides in the fact that the energy necessary for the transfer is recovered and added to what has been taken out of the cold source.
Figure 9.5. Diagram of the functioning of a compression heat pump (vapor compression heat pump). Heat production case (source: BRGM im@ge)
9.3.2.1. Classification of heat pumps There are various heat pump systems based on the basic operating principle, with different performance and application possibilities.
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The heat pump may recover energy from different environmental conditions: – in air: exterior air or extracted air, – in water: groundwater, standing water, – in the soil and the ground through various devices. We are speaking of: – geothermal heat pumps, when the extraction of heat is carried out in the soil, the ground or groundwater, – aerothermodynamic heat pumps, when the heat is drawn directly from the ambient air, the exterior air, or the interior of the house. When we speak of a heat pump model, the first term means the origin of the heat extraction and the second means the manner of heat distribution. The heat pump can also reinsert energy in different ways: – in the water of a heating and/or cooling circuit (floor heating/cooling, radiators, water and ventilation-convectors, etc.), – in the air of an air diffusion circuit. Some procedures for energy collection have been developed by the manufacturers. The main difference is associated with the nature of the fluid circulating in the collectors and emitters of heat, and therefore the technology of the heat pumps used. We may list: – heat pump with fluid intermediary, constituting three circuits: - the closed circuit of the refrigerant of the heat pump, - the collector circuit or subterranean water (for certain kinds of collectors, we use water with an antifreeze product added), - the circuit that supplies the emitters with hot water, – heat pump with direct contact, made of a single circuit: the refrigerant circulates in the heat pump, the collectors and the emitters of heat, – mixed heat pump, made of two circuits, - the circuit of the refrigerant of the collectors and the heat pump, - the circuit of hot water of the emitters. Depending on the elements previously described, different commercial names are currently proposed (Table 9.1).
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Type of catchment
Horizontal buried collectors
Vertical buried collectors
Ground water
Liquid collector
Type of HP
Emitter fluid
Designation
Fluid refrigerant
HP with direct contact
Fluid refrigerant
Soil/soil
Fluid refrigerant
Mixed HP
Water
Soil/water
Glycol water
HP with intermediary fluid
Water
Glycol water/ water
Glycol water
Glycol water/ water
Water (in the case of deep collectors)
HP with intermediary fluid
Water/water
Water
HP with intermediary fluid
Water/water
Table 9.1. Some geothermal heat pump configurations
The energy compensation required to ensure passage of thermal energy from a cold source (low temperature) to a hot source (higher temperature) may be supplied: – in mechanical form: these are compression heat pumps; the energy compensation corresponds to the mechanical energy supplied to the compressor (electric or gas motor, etc.), – in thermal form: these are absorption heat pumps; they are most often used to produce cold. The energy compensation is put into play by using, in one part of the system, the phenomenon of absorption of a gas in a fluid and, in the other part, the inverse phenomenon of desorption (the pairs most used now are ammonia–water or lithium bromide–water). Electric compression heat pumps are currently used almost exclusively for low to medium power geothermal heat pump devices. The choice of a heat pump depends on what is to be done with it (heating, heating and cooling, etc.), the type of building (new, renovated) and its use (housing, service sector, industry or agriculture, for greenhouse heating or fish culture, for example).
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9.3.2.2. Coefficient of performance (COP) The COP translates the energy performance of a heat pump. It is expressed as the ratio between the quantity of heat energy it produces and the energy consumed to carry out the transfer of energy: – for an ideal heat pump, the theoretical COP or Carnot output is expressed by the formula: COPCarnot = T cold source / (T hot source – T cold source) when T are the temperatures expressed in Kelvin, – for a heat pump functioning in heating mode: COP = Pth /Pconsumed where Pth is the thermal power supplied to the hot source and Pconsumed is the power consumed to carry out the transfer, – the performance in cold mode is expressed by the coefficient of energy efficiency (CEE): CEE = Pcold /Pconsumed Several types of COP may be defined: – the “instantaneous” COP, which in general is the value measured by the manufacturer at a given trial temperature. Recently the use of certifications and labels to guarantee HP performance has been increasing rapidly in France, – the total heat pump COP, which takes into account the electric consumption, of the auxiliaries serving the heat pump (extraction and/or circulating pumps), – the total COP of the installation, which takes into account the energy losses (notably for the distribution grid) that do not contribute to local heating. The type of heat emitter to the building interior and its sizing also influence the total performance of the installation. Mainly it is low temperature floor heating (temperature less than 28ºC), low temperature radiators (temperatures generally between 45 and 50ºC; previously installed radiators may be suitable), or ventilationconvectors connected to the circuit for heating water. The COP of the installation will depend: – on the temperature of the “cold” source and the distribution system of the heat (hot source), – on the energy consumption of the auxiliaries (pumps, etc.),
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– on the rated output of the machine, – on the sizing of the HP with respect to the needs and operating conditions, – on the control system of the installation group. Manufacturers are constantly working on improving the performance and reliability of heat pumps. They offer a range of products adapted to new construction (low temperature emitters) as well as for renovation (higher temperature emitters).
Figure 9.6. COP measured on geothermal heat pumps (measured at the Toess Heat Pump Test Centre; source: http://www.wpz.ch)
The appeal of the substratum as the “cold” source resides in the fact that its temperature remains constant over the seasons in a cold period (the case of a great demand for heating), and it will have a temperature higher than the air outside. By inversion of the cycle, certain heat pumps alternatively can produce heat or cold; this is called a reversible machine. The name CER (coefficient of efficiency of refrigeration) refers to the efficiency of the HP when it produces cold: – environmental performance: geothermal heat pump devices with an electric compressor do not emit waste into the atmosphere. Therefore, these devices contribute to limiting CO2 emissions (and other polluting gases as well), which is particularly attractive in an urban area when these devices replace fuel oil or gas boilers, – economic advantages: a geothermal heat pump is usually characterized by its low usage cost compared with a system using fossil fuels. The investment cost is still higher than a traditional solution; however, in a new building this surcharge is fairly small compared with a traditional gas or fuel oil installation. This surcharge is still less if we include a cooling option that brings additional comfort for the same investment cost. Geothermal heat pumps seem especially well adapted to individual
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new homes using floor heating/cooling emitters. For bigger systems, several criteria may contribute to making the geothermal heat pump system less expensive than a traditional solution, such as, for example, gas heating connected to a cooling system: - use of a single machine for producing heat and cold, - reduced energy costs, particularly if the energy transfer can be used, - economy of utility space, since only one location is used instead of two locations, or one for heating and another exterior utility space for the cooling system. This criterion is especially important in the case of urban projects (office buildings, service sector buildings) for which economy of space is very important, - reduced maintenance costs since there is only one machine to maintain. Besides the particular case for which architectural integration prevails (no cooling system in the roof), the analysis of the total cost and consideration of environmental criteria make it possible to justify the implementation of a heating pump solution. In fact, heating and cooling systems using geothermal heat pumps answers in a positive way the principal goals of public administration in the matter of controlling energy demand, energy efficiency, and environmental impact (reduction of greenhouse gas emissions). A significant effort in terms of performance and installation quality, as well as on investment costs, will allow this technology to occupy a predominant place in the market in the coming years, – production of heat and cold: - in winter operation, the geothermal heat pump extracts stable heat from the ground and injects the heat at a usable temperature into the heating circuit of the building (air, water loop), - in summer operation, a reversible heat pump allows extraction of excess energy from the building interior, which is thus cooled; the excess heat (the heat extracted as well as the heat coming from the power of the compressor) is transferred to the ground, - another option is direct cooling (also called geo-cooling), which consists of directly using water at the natural temperature of the ground (with the interposition of an exchanger), to assure cooling, - finally, operation as a “thermal refrigerating pump” allows the simultaneous production of heat and cold (intended for appliances that control the environment of different parts of the building). The aquifer layer constitutes the source of energy from which the system draws the required heat or cold. 9.3.3. Extracting heat from the ground The term geothermal heat pump encompasses various devices allowing the exploitation of the energy of the ground to supply a heat pump, with different conditions of operation. The main devices currently used are presented in the
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following sections. We should stress that the size of the operations is very variable since the majority of the devices can be implemented for an individual installation as well as in a small complex. 9.3.3.1. Drawing calories from groundwater The first geothermal heat pumps developed in the 1980s traditionally used groundwater extracted from a water table using a drill hole or a well.
Figure 9.7. Capturing groundwater
The performance is very good and steady since the calories are extracted from groundwater whose temperature is usually between 10 and 13ºC in our European latitudes. The heat pump can therefore cover all heating needs, and even contribute to the production of domestic hot water. The groundwater taken from the water table (aquifer) must be disposed of once it has been cooled. If there is no natural location nearby (a river), this necessitates a second drilling for reinjection. The drill holes for extraction and reinjection should be designed and positioned so as to avoid risks of thermal interference. This positioning is a function of the characteristics of the water table (determined by a hydrogeological study). The drill holes should be made according to the specifications of the professionals working at the state of the art in this area. Different drilling techniques may be used, depending on the terrain and the required depth. A first approximation of the output of water necessary for a system can be made using the formula: output (m3/h) = Pth x 0.7/(1.16 x dT) where Pth is the thermal power required for the heating (kW), and dT is the difference between the temperature of the extracted groundwater and the temperature of the water after passing through the heat pump.
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277
It is therefore advisable to know the minimum temperature in winter of the extracted water to define an acceptable temperature difference so as not to risk freezing in the evaporator. These devices are applicable for fulfilling the heating needs in a private (single) habitation or a small housing complex, in the service sector, and also for agricultural applications such as greenhouses or fish farming. They also enable provision of cooling in summer or air conditioning (reversible HP). The aquifers are accessible at depths that go from several tens to some hundred meters; but it is also possible, for larger operations, to make deeper drill holes to reach aquifers at higher temperatures. Unfortunately, the distribution of groundwater resources is not uniform throughout a region. Investigating the potential for aquifers for geothermal uses will provide the necessary technical information (http://www.geothermie-perspectives.fr, espace régional). Geothermmal energy from tunnels and mines is a variation on groundwater exploitation. Tunnels drain large quantities of water that are most often emptied to the exterior of the galleries by canals that drain into watercourses. The temperature of this water can reach 20 to 40ºC, depending on the nature and the thickness of the rocks. Considerable outflows can be extracted, as is the case in Switzerland. In fact, in Switzerland, which is rich in tunnels, a study of some 15 of these works has shown outputs that go from 360 to 18,000 liters a minute with temperatures between 12 and 24ºC. Such a resource, connected to heat pumps, can be used for the heating of public and private buildings. 9.3.3.2. Horizontal and vertical in-ground heat exchangers Currently, in France, horizontal ground heat exchangers are the most widespread. These systems are the least expensive, but they require a sufficient surface area. Therefore, they are mainly reserved for heating individual houses. The vertical configurations have been well developed in other countries and some versions have been developed in France. These systems are more expensive and perform slightly better. Their footprint is very small, therefore they are appropriate for heating individual houses, but especially for housing developments and office buildings that are constrained by the surrounding available surface area (source: ADEME).
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Renewable Energy Technologies
9.3.3.2.1. Horizontal collectors In this case, it is a matter of placing horizontal ground heat exchangers at a depth that is usually between 0.8 and 1.5 m; this type of device is also called “tube layers”.
Figure 9.8. Horizontal in-ground heat exchangers
Since horizontal ground heat exchangers are normally laid at a maximum depth of only 1.5 to 3 m, climate plays an important role in this type of heat exploitation. The Earth serves, so to speak, as a solar energy accumulator. In this specific case, geothermal energy strictly speaking plays only a secondary role. In this case, the ground is used as a natural seasonal accumulator; in fact, the energy essentially comes from sunshine and the migration of rainwater into the ground. The power that can be extracted from the ground depends on its nature and on the altitude; average values of 15 watts “collectible” per linear meter of buried tube, or 20 to 30 W/m2, are typically found. This type of collector (ground heat exchangers) consists of tubes of polyethylene or copper sheathed with polyethylene, in which a liquid circulates (often glycol water). The total length of the tubes exceeds several hundred meters; they are formed into loops usually 40 to 50 cm apart (depending on the exposure of the ground; if the collector is in the shade, this separation should be greater) to avoid the risk of the ground’s freezing. This type of collector, connected to heat emitters of low temperature floor heating, also allows summer cooling, by simple circulation of liquid in the collectors. We estimate the surface area required for installing such an exchanger to be from 1.5 to 2 times the living space to be heated. This ground surface should be open (no trees or weatherproof surfacing).
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Another variant for exploiting the heat from the superficial part of the ground is the system called the “Canadian well”. The principle of this type of installation is to use the thermal inertia of the ground to carry out a preliminary treatment of the air that ventilates the buildings. The Canadian well consists of having a part of the new, renewing air go through tubes laid in the Earth at a depth of some 1 to 2 m before entering the house. In winter, the ground, at this depth, is warmer than the exterior temperature: the cold air is therefore preheated during its passage through the tubes. In summer, the ground is colder than the exterior temperature: this “well” therefore uses the relative coolness of the ground to cool the air entering the residence. The calculation for a Canadian well depends on several parameters: the volume of the house, the output of air required in winter and in summer, the choice of ventilation for the house (natural aeration, etc.), the architecture (bioclimatic, materials, insulation, greenhouse, etc.), the nature of the soil (sand, clay, water table, etc.), the space available for burying the pipe, the geographical location, etc.
a)
b) Figure 9.9. Canadian well: a) principle; b) installation of a very large Canadian well for the zenith of the great Dijon that uses 700 m of buried pipes
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Renewable Energy Technologies
9.3.3.2.2. Vertical ground heat exchangers or geothermal probes The second option consists of buried vertical ground heat exchangers, also called geothermal probes.
Figure 9.10. Geothermal probe
The geothermal probes (vertical collectors) are made of two polyethylene tubes forming a U, installed and sealed up in a drill hole. Water with antifreeze added is circulated in a closed circuit. The constant temperature of the ground makes it possible to obtain a good annual performance coefficient and a long lifetime in the heat pump, if it has been correctly sized. The power derived from such collectors varies according to the nature of the terrain (Table 9.2). The natural humidity of the ground improves the thermal conductivity and guarantees a good contact between the probe and the subsoil. When a geothermal probe penetrates a water table whose speed of flow exceeds several centimeters a day, the usable quantity of heat noticeably increases.
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281
For installations of small and medium size, no thermal recharge is normally necessary. The auto-regeneration is continuous. Large installations can be thermally recharged as needed from simple backup systems, for example solar collectors. Length of geothermal probe by kW of power of heat (m)
Thermal conductivity
Extraction power
(W/m K)
(W/m)
COP = 3
COP = 3.5
Hardened rocks with high thermal conductivity
> 3.0
70
19.5
10
Hardened or loose rocks saturated with water
1.5–3.0
50
13
14
Poor quality substratum (dry, loose rocks)
< 1.5
20
33
36
Gravel, sand, aquifer
1.8–2.4
55–65
10–12
11–13
Clay, silt, wet
1.7
30– 0
17– 2
18–24
Gravel, sand, dry
0.4
< 20
> 33
> 36
Limestone mass
2.8
45–60
11–15
12–16
Sandstone
2.3
55–65
10–12
11–13
Gneiss
2.9
60–70
9.5–11
10–16
Granite
3.4
55–70
9.5–12
10–13
Basalt
1.7
35–55
12–19
13–20
Substratum
Table 9.2. Geothermal probes: summary sizing (extracted from Information Technology documents/geothermal probes, tube layers, http://www.geothermal-energy.ch)
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Geothermal probe fields Geothermal probe fields are an adaptation of the principle of the geothermal probe for large buildings. Building
Bogn Engiadina
Meister + Co. AG
Centre d’études Gerzensee
Chestonag Automation AG
Location Canton
Scuol Grisons
Wollenrau Schwyz
Gerzensee Berne
Seengen Argovie
Thermal plant – 38,900
Industrial 3,050 30,000
Var. premises 4,900 –
Offices 1,600 –
40 150 29 Shale
32 135 32 Molasses
32 150 30 Molasses
4 250 41–50 Molasses
5 x 60 kW 2 x 67 kW
PAC + machine refrig.: 1 x 52 kW
2 x 114 kW
5 x 14 kW
4,162
470
667
150
0
75
Unknown
44
Gas/heating oil consumption (MWh/year)
1,049
0
40
0
Placed in service
1993
1995
2002
2000
Type of location Reference surface (m2) Building volume (m3) Probes Number Depth (m) Specific power (W/m) Type of rock Heat pumps (HP) Geothermal probes Heat recovery Thermal demand (MWh/year) Refrigeration demand (MWh/year)
Table 9.3. Technical characteristics of four fields of probes in Switzerland (Info-Géothermie, nº 8, September 20045)
5 http://www.geothermal-energy.ch.
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283
Deep geothermal probes Deep geothermal probes are also an adaptation to large buildings based on the principle of the geothermal probe. These devices can reach depths of 500 to 2,000 m, where the temperatures can rise as high as 70ºC. In the case of closed probes, water circulates in a system of tubes in a closed circuit. The semi-closed systems allow integration of subterranean water into the circulation system. In both cases, the heated heat transfer fluid may be used at the surface directly or by a heat pump for the production of energy for space heating and clean water. Geothermal foundations These are structures built into the ground or in contact with the ground, intended to support a construction. These elements are usually concrete or reinforced concrete. Their special property is the double purpose of the concrete elements in contact with the ground, serving both as a foundation and for the production of energy in the form of heat and cold. A network of polyethylene tubes can be installed in the interior of these foundations. The circulation of a heat transfer fluid then allows the exchange of heat between the ground and the foundations. In this case, these structures are called geothermal or thermoactive foundations.
Figure 9.11. Geothermal foundations
There are several tens of examples of installations operating on this principle in Europe. In Switzerland, these structures are called energy geostructures or “energy post systems”. In these types of foundation, the piping is connected to the heating or cooling system of the building by a hydraulic circuit that has a heat pump. In this way, the land under the building is used in winter as a natural heat producer. In summer, such
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systems can be used for industrial cooling and air conditioning. In this case, the heat removed by the foundation is transferred into the ground and stored for use in winter. The energy capacity of posts in the ground, 40 cm in diameter, can represent – depending on the distance between the posts and the parameters of the water table – 30 to 50 W of thermal power or cooling the energy extracted, possibly from 40 to 90 kWh per meter of length of the energy post per year. With respect to the cost of the energy, the combination of uses for cooling and heating is especially economical. For the price of one precise set of plans and with careful execution, such a system requires practically no maintenance.
Characteristics of the energy post system of the Fully Centre Scolaire Type of building Reference surface Net volume heated Number of classes
Minergie 2,635 m2 7,018 m3 20
Total number of driven posts Number of posts equipped Average depth Exchanger in the posts Output of circulation by post Specific energy drawn off from the posts Specific energy drawn off annually
118 41 23.2 m Tube in double U 3101/h 50 W/m 75 kWh/m
Power of the heat pump with condenser (four modules) Annual performance coefficient Utilization of the energy Energy demand - for heating - for cooling
56 kW 3.8 Heating and cooling 92,225 kWh/year 50,000 kWh/year
Table 9.4. Characteristics of energy post systems of the Centre Scolaire de Fully, Valais Switzerland (source: Info-Géothermie, nº 7, December 20036)
6 http://www.geothermal-energy.ch.
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285
9.3.4. Development prospects and potential After a first fruitless attempt at the end of the l970s with the Perche program, the HP market in France has seen a revival since l997. The market has an annual growth in double digits, which has further accelerated since 2005, with the establishment of increased tax credits for HPs. Annual sales for geothermal heat pumps have gone from 7,700 units in 2002 to more than 13,000 units in 2005 (AFPAC data). Since l997, the revival of HPs in France has emphasized a strategy of promoting quality, intended to avoid repeating the errors of the past. This strategy is now on the verge of succeeding and new devices have been in place since 2007, such as the quality label NF PAC for machines, the quality charter QualiPAC for fitters, and the drilling quality charter for vertical heat exchangers QualiForage for drillers. In the area of performance, GHPs are a good answer to future energy issues, as well as for fulfilling the objectives of the Kyoto protocol. In this way we can show that a high performing HP, accurately sized, installed, and regularly maintained, makes it possible to achieve economies in primary energy even in comparison with the highest performing fossil fuel heat solutions. Further, on-site measurements have shown that this objective has held. Regarding the CO2 emissions resulting from supplying the heating needs of a dwelling, heat pumps are also in a very good position since they make it possible to reduce emissions from 15 to 75% in comparison with classic heating methods. In order to further the development of HPs in France, numerous financial measures have been developed. They can be implemented by the State at the national level – this is the case with tax credits or reduced rates of the Value Added Tax, or at the regional level, when territorial collectives or ADEME7 offer local subsidies to encourage its development. Finally, certain organizations like EDF or ANAH8 also offer, under certain conditions, financial incentives for acquiring a HP. There are also technical measures like the Aquapac guarantee for groundwater drillings to supply GHP, the drilling quality charter for vertical heat exchangers QualiForage for drillers installing vertical geothermal probes, decision-making tools that allow fast and easy access to information on the groundwater resources that can be used with GHP. The GHP network therefore seems to enjoy exceptional conditions for long-term establishment in France. However, success will come by providing essential quality control to ensure user confidence in this technology. Everywhere in Europe initiatives have been taken to encourage the development of GHP, because they appear to be one of the inevitable solutions for the reduction 7 ADEME: Agency for Environment and Energy Management. 8 ANAH: the national agency for accommodation.
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Renewable Energy Technologies
of consumer energy consumption and CO2 emissions associated with heating. In Europe, the leader technology is the vertical geothermal probe, far ahead of doublets in water tables and medium temperature geothermal use. This recent evolution, initiated in Sweden, Austria and Switzerland, is explained by the possibility of using this technology everywhere, but also by the arrival on the market of less expensive heat pumps that offer better performance coefficients. The leader countries remain those of central and northern Europe, but the possibility of cooling offered by these systems, making it possible to reduce the time of investment return, should allow the countries of the south and new members of the European Union to make even more rapid progress on installed power. The market for heating and cooling is very large since it represents more than 40% of the primary energy consumption in Europe. For three years the European Renewable Energy Council (EREC) has initiated an intense lobbying campaign for geothermal use by the European Geothermal Energy Council (EGEC) in order to support the development of heat production using a heating/cooling directive equivalent to that already existing for electricity and for biofuels. These efforts will bear fruit very soon and have encouraged the financing of a series of actions financed within the framework of the IEE9 program. Today we can attest that in Europe there is a profusion of initiatives that have at their core a system of geothermal heat pumps, destined to accelerate the progress that has already been made. 9.4. Direct production of heat The direct use of heat is another form of industrial use of geothermal liquids. It usually involves liquids (fluids) of low enthalpy that do not allow the production of electricity (tºC < 90ºC). These liquids are extracted either directly from geological reservoirs or, more rarely, from thermal effluents coming from geothermal power plants producing electricity. In this latter case, the installation constitutes a geothermal cogeneration device. Since the heat does not economically lend itself to transportation beyond several kilometers, this type of geothermal usage is limited to locations near the source. District heating by a heat network is one of the main applications, on an industrial scale, for this renewable thermal energy source. 9.4.1. Current situation Globally, the totality of direct geothermal installations in 2004 represented an installed power of 27 GW using (producing) 72 TWh of geothermal heat – 9 IEE: the Intelligent Energy Europe program is the EU’s tool for funding action to improve these conditions and move towards a more energy intelligent Europe.
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287
including, for certain countries, the operation of large collections of heat pumps – for an average load rate of 30%. About 20.2% (14.7 TWh) of this amount of energy is annually devoted to household heating, and the largest part (11.3 TWh) is distributed via heat networks with a total power of about 3.4 GWh (average load rate of 38%). Eight countries, mainly European, account for these data (data for Russia is at this time incomplete): – four countries are clearly in the lead of geothermal use in district heating networks, each year using more than 1 TWh (1,000 GWh) of geothermal heat: Iceland, China, Turkey and France (Table 9.5), Number of geothermal heating networks
Installed power (MWth)
Quantity of geothermal energy used (GWh/year)
Iceland
> 30
1,350
4,780
China
>6
550
1,775
Turkey
14
645
1,670
France
34
243
1,120
Countries
Table 9.5. Countries annually using more than 1 TWh of geothermal heat in urban heating networks
– four other countries (Hungary, the USA, Germany and Italy) use geothermal energy in a significant way for their heating networks (Table 9.6). Number of geothermal heating networks
Installed power (MWth)
Quantity of geothermal energy used (GWh/year)
Hungary
9
103
282
USA
20
84
219
Italy
8
74
167
Germany
7
90
163
Countries
Table 9.6. Countries using geothermal energy in a significant way in urban heating networks
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9.4.2. Geothermal heating networks Putting a geothermal reservoir to use requires accessing the hydrothermal resource (a geothermal liquid is water possibly containing mineral salts and gas taken to TºC) with the aid of a production well; extracting and transferring the thermal energy of the geothermal liquid to the primary circuit of a heating network via a thermal (heat) exchanger; and, generally, with environmental constraints and concern for management of the reservoir, returning the geothermal liquid to the reservoir by an injection well at a temperature of (T – ' T)ºC. 9.4.2.1. The theoretical doublet and the associated heating network The geothermal loop thus constituted is found in “doublet” wells as a drainage pump submerged in the production well (except in the case of artesian wells) and an injection pump as part of the surface installations next to the heat exchanger, the electric equipment, the filters and metrology instruments. The production and injection wells have classic completions, derived from petroleum works. The casings used are generally made of carbon steel (API K55), but completions using fiberglass are now being used. The two wells are usually drilled from the same platform and are separated so that the impact in the production zones of the aquifer are far enough away for thermal transfer not to occur during use. The theoretical power, P, of the geothermal loop expressed in MWth is evaluated by the equation: PMWth
Q0 (average standard output in kg/s) u 'T u 0.004184
and the quantity of energy supplied to the exchanger, E, is expressed in TJ/year by: E TJ/year
Q (average annual output in kg/s) u 'T u 0.1319
or otherwise in MWh/year as: E MWh/year
Q (average annual output in kg/s) u 'T u 36.64
A geothermal doublet can constitute the main source of energy of a heating network, but it is usually associated with a “conventional” energy installation that
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289
ensures auxiliary thermal needs and help or, in the case of district heating networks with superheated water, that can bring the temperature of the primary network to a temperature much higher than that of the head of the production well and the output of the exchanger (Figure 9.12). Geothermal heating networks are therefore multisources or hybrids. The degree of hybridization can be relatively high, as in the case of the geothermal doublet association – fuel oil boiler – cogeneration with natural gas and incineration of household waste in Créteil, France.
Figure 9.12. Geothermal doublet connected to its auxiliary system and to the heating network serving the users’ substations
9.4.2.2. Geothermy experience in the Paris basin In France, the installed capacity is 330 MWth, and the annual production about 1,500 GWh (129,000 toe, 10th world ranking in 2003), 63% (950 GWh) coming from the 31 Ile-de-France sites currently in use, the balance being produced mainly in the Aquitaine basin. France was one of the pioneer countries in the development of geothermal energy use on aquifers of sedimentary basins to supply heating networks for a housing complex. Constructed between 1969 and 1987, the 34 Francilian geothermal system units serve 29 urban heating networks (153,800 house equivalents) and, with the exception of a triplet at Melun l’Almont, function with doublets (system comprising a production well and an injection well) connected to the Dogger aquifer in the Paris basin (Figure 9.13).
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Renewable Energy Technologies
The Dogger is a predominantly limestone aquifer, contained within the marly borders of the Lias and the Malm. It is characterized by average (10–30 Dm) to high (> 30 Dm) transmittivity. It is accessed at a depth of between 1,500 and 2,000 m, usually by submerged pumps (however, there are several artesian wells) producing an average of 64 l/s (33–97) per well of mineral water at 20 g/l (10–39), at a temperature of 73ºC (56–85). These parameters allow a doublet to have, on average, a power of 10 MWth (calculated on the basis of a return temperature of 30ºC). The reactive character of the liquids with regard to the wells was evident very early with the appearance of large sulfur deposits formed at the expense of the metal of the tubing. This created technical difficulties for geothermal plants in the Paris area, but a correction was quickly provided by the injection of organic corrosion inhibitors at the bottom of the wells. Financial difficulties and the reduction of benchmark energy costs, begun in 1985, led to progressive closure, beginning in 1989, of 16 of the 47 sites in operation at the peak in 1987. Thus, the burden of loans made during a high inflationary period, even though alleviated by later renegotiations, was too burdensome for certain housing groups (up to 50% of the operating expenses).
Figure 9.13. Principle of a geothermal heat network drawing from the Dogger aquifer in the Paris basin
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9.4.2.3. Technological developments 9.4.2.3.1. Hybridization of energy sources The enthalpy levels of geothermal liquids are not always sufficient to satisfy at the peak, or even at the trough, the requirements of heating networks. The addition of auxiliary systems, complementary or supplementary, at times gives a multisource character to geothermal installations, offering both the flexibility and economic profitability of hybrid systems and the virtues of the use – when possible – of additional renewable energy sources. In the case of heat networks in the Paris area, the considerable hybridization of the energy sources (major contribution from natural gas, the principal fuel of auxiliary boiler rooms and cogeneration units) is made at the expense of the renewable energy source, by proceeding in a manner that is not proportional to the increase in needs associated with extending the networks. A sensible hybridization should allow enlarging the field of application of low enthalpy geothermal use to the extent that it can be mixed with high power systems. For these, the energy efficiency resulting from this hybridization is greater than the sum of the efficiencies of the systems functioning independently. The installation of cogeneration systems with gas engines or turbines (41 production units) in 17 of the 29 geothermal networks for a total electric power of 125 MW has, on the one hand, led users to modify the operating conditions of certain geothermal sites (artesian production substituted for pumping production) and, on the other hand, directly caused the average rate of geothermal coverage in the networks to go from 74% to 57%, the average rate for networks with cogeneration power units (> 1.8 MW) being about 47% (Table 9.7). In this way, the average annual contribution of a geothermal doublet in Ile-deFrance satisfying the needs of a heat network is in the order of 2,800 MWh. Produced by natural gas, this energy would cause an annual emission of 6,600 metric tons of CO2 (224,000 metric tons for the Paris area’s plants).
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Renewable Energy Technologies
Operations
Dept.
Alfortville
94
Date put Temp. Output in head of in m3/h service well in ºC 1986
73
275
Housing equivalents
MWh Geo
Rates of Geo coverage %
4,415
43,155
78
Blanc Mesnil North
93
1983
66
175
2,754
25,471
74
Bonneuil sur Marne
94
1986
79.3
280
3,078
25,519
66
Cachan
94
1984
70
360
4,605
49,028
85
Champigny
94
1985
78
280
6,644
58,552
71
Chelles
77
1987
69
280
3,601
16.917
38
Chevilly La Rue – L’Hay les Roses
94
1985
72.6
560
9,783
72,580
58
Clichy-sous-Bois
93
1982
71
180
3,794
15,572
33
Coulommiers
77
1981
85
230
2,106
24,752
94
Créteil
94
1985
78.9
300
12,303
56,466
37
Epinays/Sénart
91
1984
72
250
5,105
49,874
78
Fresnes
94
1986
73
250
5,351
32,335
48
La Courneuve North
93
1983
58
200
2,393
21,666
73
La Courneuve South
93
1982
56
180
2,822
12,472
35
Le Mée sur Seine
77
1978
72
134
4,856
21,155
35
Maisons Alfort 1
94
1985
73
300
4,505
36,673
65
Maisons Alfort 2
94
1986
74
260
4,329
20,755
39
Meaux Beauval et Collinet
77
1983
75
400
13,529
58,384
35
Meaux Hôpital
77
1983
76
130
3,761
20,674
44
Melun l’Almont
77
1971
72
260
5,238
44,593
68
Montgeron
91
1982
72.5
220
1,749
16,881
77
Orly 1 and 2
94
1984
75
355
6,651
62,046
75
Ris-Orangis
91
1983
72
190
225
16,239
58
Sucy-en-Brie
94
1984
78
200
2,152
25,167
94
Thiais
94
1986
76
250
4,352
43,539
87
Tremblay-en-France
93
1984
73
275
4,212
45,562
87
Vigneux
91
1985
73.2
240
3,430
33,579
66
Villeneuve-SaintGeorges
94
1987
76
350
4,303
34,411
65
Villiers-le-Bel
95
1985
67
230
2,959
21,699
60
Table 9.7. Principal operations in service in Ile-de-France (source: ADEME-Valor)
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9.4.2.3.2. Enhancing low energy aquifers The arrival of absorption heat pumps at the level of industrial maturity represents significant technological progress in the area of the hybridization of energy sources involving geothermal science, since it allows both the enhancement of low energy aquifers (30–50ºC) and the use, alongside geothermal sources, of other renewable thermal energy sources, like household waste, or solid, liquid or gas biomass fuels. Heat pumps with aqueous solutions of lithium bromide (LiBr, an absorbing fluid), by taking advantage of the affinity of water vapor (fluid refrigerant) for these systems, make it possible to extract thermal energy from a geothermal fluid at a given temperature and to deliver it to a heat network at a higher temperature (from 20 to 60ºC depending on the number of stages). The diagram of the coupling of a geothermal doublet and a heat network via an absorption HP built at Thisted in Denmark (Figure 9.14) shows: – the evaporator where the energy taken from the geothermal fluid at 44ºC is extracted by the vaporization of water, – an absorber where the energy produced by absorption of the water vapor by a concentrated solution (62%) of LiBr is supplied to the heat network at 61ºC, – a generator (or concentrator) activated by an exterior thermal energy source at 150ºC which reconcentrates the solution of diluted LiBr (58%) by vaporization of water, – a condenser that supplies energy produced by condensation of water vapor to the heat network. The geothermal fluid is reinjected into the reservoir at 19ºC. The thermal energy transmitted to the heat network by the absorber and condenser, each responsible for about 50% of the delivery, is the sum of the energy extracted from the geothermal loop and the energy supplied to the generator by the exterior source (superheated water or steam). Apart from the pump connecting the absorber to the generator, an absorption HP does not have any moving mechanical parts. This kind of coupling, involving an absorption heat pump that uses the stable heat of cogeneration from the incineration of household waste, was tested for the first time at Thisted (Jutland, Denmark), with financing from the European Union.
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Figure 9.14. Coupling between a geothermal doublet and a heat network via an absorption heat pump (Thisted, Denmark)
On this site, over an underground completion designed by Dong S/A and surface installations designed by Houe and Olsen, an installation with a total power of 20 MWth was erected between 1982 and 1988. It involved a geothermal doublet exploiting a very high quality aquifer 1,250 m deep. The geothermal loop, activated by pumping, puts out 42 kg/s and functions between 44ºC and 19ºC. In this way it delivers 4 MWth to the evaporator of the absorption HP, which receives 6 MWth from waste incinerator cogeneration from elsewhere (or from its own gas boilers) via a loop of superheated water (150– 125ºC). The power of 10 MWth is applied on return from the urban heat network (140 kg/s), whose temperature goes from 44ºC to 61ºC. To the €11 million initially invested during this period, it is appropriate to add €1.5 million that served, in 2001, to finance a development bringing the installation power to 15 MWth by increasing the output of the geothermal loop to 55 kg/s and adding a second absorption HP. The reinjection temperature of the geothermal fluid then took place at 9.5ºC. The quantity of geothermal energy used by the Thisted network was 22 GWh in 2004.
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This demonstration plant soon encouraged industrial systems, the main ones being the geothermal power plants of Neubrandenbourg (Germany, 1989), Erding (Germany, 1990), Pyrzyce (Poland, 1996), Klaipeda (Lithuania, 2000) and Mszczonow (Poland, 2000), all using absorption heat pumps with LiBr. Projects are currently being evaluated, notably one in Latvia on an aquifer at 48ºC. For Erding (Germany) and for Mszczonow (Poland), the geothermal fluid is water of sufficient quality that after having given off thermal energy to the heat pump evaporator and undergone a slight chemical and biological treatment, it can feed the municipal drinking water distribution networks directly. More precisely, at Erding (Figure 9.15), the geothermal fluid is produced at 65ºC with an output of 24 kg/s. First it gives off energy on return from the municipal heat network via three thermal exchangers (1.7 MW), then enters the evaporator of an absorption heat pump at 45 ºC, which subtracts 2.8 MW from it (in other respects, it receives 4.2 MW from a circuit of water superheated by a gas boiler) and goes back out of the HP at 20ºC to be sent, after treatment (chemically to eliminate iron and manganese, by ozone to treat bacteria), to the drinking water distribution circuits.
Figure 9.15. Geothermal installations at Erding that furnish energy to a heat network as well as to thermal baths, and the direct supply to the drinking water distribution network from geothermal fluid
This installation delivers more than 10% of the heat consumed by the heat network and furnishes about 30% of the drinking water.
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Heat storage The direct role of geothermal energy in a hybrid system of thermal energy generation connected to a heat network can be increased considerably when the aquifers involved are incorporated into the designs for seasonal storage/retrieval of heat. In France, although two projects were planned in the 1980s, only one was implemented. The Sarcelles pilot project was planned in 1984 and was intended to store, during the summer season, part of the heat (25,000 MWh) from a household waste incinerator plant feeding a heat network with superheated water (from 180ºC), distributing 250,000 MWh annually. The construction was supposed to comprise a heat exchanger of 10 MW attached to the primary network of the household waste incinerator plant, two vertical wells at Néocomien (890–940 m deep), 340 m apart, equipped with six 5/8” filters and gravel packs that would make it possible to establish a storage and retrieval loop functioning between 90ºC and 50ºC (the temperature of the return from the secondary network of urban heat at 45ºC) under a maximum pressure of 50 bars. For a number of reasons, the Sarcelles project has not been built. On the other hand, the Plaisir/Thiverval-Grignon demonstration project, conceived in 1975, had an operational phase conducted by the SNEA and the CEA between 1985 and 1990. The artist’s drawing in Figure 9.16 shows the design of the underground installation consisting of four wells. A central well serves to transport the water at high temperature (180ºC) to an aquifer chosen to be the reservoir during the phases of storage and retrieval, and the three peripheral wells (constructed in the aquifer at about 90 m from that of the central well) make it possible to extract “warm” water during the storing operation, and the injection of water at 75ºC during the pumping for retrieval.
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Figure 9.16. Diagram of the zone of the wells for the Thiverval-Grignon heat-storage project
The connection between the wells dug into the Néocomien aquifer (485 m deep and initial temperature of 25ºC), the household waste incinerator of Plaisir and the associated heat network is made by two exchangers with plates, allowing them, respectively, to carry, for storage, injection water to the central well at 180°C by means of the transfer circuit of the household waste incinerator and to reheat the return water from the heat network to 130ºC at the time of retrieval. A specification manual established the operating conditions for the demonstration pilot plant: injection power of 10.4 MW (injection output: 25 kg/s at 180ºC and extraction output: 23 kg/s at 70ºC) and retrieval power of 10 MW, corresponding to an output of 33 kg/s. The expected storage capacity was 23,000 MWh and the efficiency hoped for was 65%. The trials gave contrasting results. Those made under cold conditions, then under warm conditions (40–70ºC), demonstrated that the hydraulic behavior of the aquifer conformed to the predictions, but also indicated the necessity of improving the geochemical treatment of the fluids. The first storage of 3,600 MWh allowed a recovery of 429 MWh (12%).
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During the storage trial at 180ºC, 8,035 MWh were injected, but the sand that entered the system led to deterioration of the turbo-pump of the central well, preventing the main heat recovery test. Finally, some fairly satisfactory trials from a doublet made of two peripheral wells between 120ºC and 80ºC showed that the objective of storage at 180ºC was too ambitious (technological limits in the pumping systems of the time) and that an objective of 140ºC would probably be better. Currently in Germany there are two hybrid systems (cogeneration on geothermal installations) that have proceeded the development and testing of technologies for seasonal recharging of aquifers. At Neubrandenbourg, the connection of a geothermal power plant of 15 MWth and a combined gas cycle installation by thermal exchangers was created in 2004 to optimize the consumption of fuel outside the heating period, when the thermal energy, used only for domestic hot water, could be a secondary and reliable coproduct from the electrical energy generated. The initial 1987 installation comprised two doublets exploiting the sandstone aquifer of Rhétien, located at a depth between 1,200 and 1,300 m. The geothermal fluid (54ºC) was produced at 40 kg/s to feed the heat network of Rostocker Strasse via an absorption HP. Recent developments have included a complete renovation of one of the well doublets of the geothermal power plant (a slight deepening, completion of the fiberglass casing, new pumps and new well heads) so that it could become a reversible doublet allowing thermal loading and unloading operations, as well as the interconnection of the Rostocker Strasse heat network and that of the cogeneration plant. It is anticipated that between April and September, the injection of geothermal fluid into the aquifer at an output of 28 kg/s at 80ºC by the cogeneration network (power of 4 MW) will furnish a thermal recharge of 12,000 MWh, into the direct environment of the injection well of which 73% (8,800 MWh) will be restored to the Rostocker Strasse network under a varying load of 4.0 to 2.9 MW in the winter. On a more modest scale, the tri-generation system (electricity, heat and cold) of the Reichstag buildings in Berlin, whose construction was completed at the end of 2002, used the coupling of a cogeneration engine with methyl ester of vegetable oils (EMVO) of 3.2 MW and two distinct low energy aquifers via absorption heat pumps with a total power of 2 MWth (Figure 9.17).
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This hybrid system from renewable energy sources was designed to cover 90% of the “heat” demand, 60% of the cold demand and 82% of the electricity demand, while assuring 37% of peak electricity load.
Figure 9.17. Diagram of aquifers used as hot and cold tiers, one below the other, directly beneath the Reichstag in Berlin
The lower aquifer, at a depth of 300 m, is equipped with a doublet that can support a reversible loop with an output capable of 84 kg/s, and play the role of the “hot” tier. The (upper) aquifer, at a depth of 60 m, receives a more complex completion (2 x 5 wells), that can support a reversible output of 51 kg/s and fulfill the role of the cold tier. In summer, cogeneration produces electricity, heats the lower aquifer, which goes from 20ºC to 65ºC, and feeds the cooling components (two networks: 6–12ºC and 16–19ºC) which, by their absorbers, progressively take on the temperature of the higher aquifer of 5ºC to 30ºC. In winter, cogeneration produces electricity, delivers heat directly to the high temperature heat circuit (90–60ºC) and to the absorption heat pump generators which, in removing energy from the lower aquifer, progressively cools it from 65ºC to 20ºC. Simultaneously, the higher aquifer is cooled to 5ºC by direct thermal exchange with the ambient air. Simulations, confirmed by the first site results, indicate that the cold tier should store 4,250 MWh during the summer period in order to restore 93% (3,940 MWh) of this energy during summer, while the hot tier would store 2,650 MWh in summer in order to restore 77% (2,050 MWh) in winter. In Denmark, the owner of the geothermal loop of the Thisted installation (see above) conducted simulations to evaluate the output of seasonal storage of heat, both
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with the current completion, modified by the addition of a pump located in the injection well, in order to make the doublet reversible, and with a completion of three wells, two of which would be equipped with submersible pumps. In the former configuration with reversible doublet, the aquifer supply to the vicinity of the production wells at 72ºC could restore, in about 1,000 hours, 36% (6,400 MWh) of the 17,500 MWh of energy to be stored from the thermal recharge supplied by cogeneration during 4.5 months of the summer season. With a three wells completion with the same injection and the same geothermal fluid at 77°C, the restitution rate will be of 70.4% (13,900 MWh) on a period of 2,750 hours. 9.4.3. Prospects and potential for development Compiling the quantities of geothermal heating and the total quantities of energy used by the heat networks of the six main European users of geothermal energy of the “Europe of the 32” makes it possible to affirm that, apart from Iceland, the rate of usage in geothermal technology of the European heat networks is at best a few percent; the average for the 32 countries considered is estimated at 1.5% (Table 9.8). Countries
QGTH (GWh)
QȈ (GWh estimated)
QGTH/QȈ (%)
4,780
4,840
98.7
167
4,650
3.6
1,120
36,400
3.1
Hungary
282
15,110
1.9
Germany
163
92,000
0.2
Turkey
1,670
n.c.
–
“Europe of the 32”
8,500
555,600
1.5
Iceland Italy France
Table 9.8. Estimate of rates of geothermal equipment of the European heat networks
Even if not all European countries have the exceptional geothermal resources that Iceland does, nevertheless, just as they pay attention to household waste and biomass, which have already gained a certain place in heat networks, they could reconsider geothermal energy as an alternative energy source that allows a total increase in the efficiency of using renewable energies, thus working for the protection of the environment and for economic growth.
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9.4.3.1. The objectives of the revival in Ile-de-France Analysis of the current economic, energy and environmental situation has encouraged French public officials to study the conditions for a revival of geothermal use in Ile-de-France (Paris and the surrounding area) including, on sites pre-selected according to geologic and market criteria, either the creation from scratch of geothermal doublets and associated heat networks or – when it seems technically possible – substituting geothermal doublets for a certain number of heaters(boilers) with fossil fuels in the existing heat network. On the 89 sites judged favorable among the 223 sites initially considered, some 30 – a little more than half of which already have heat networks – have been studied in greater detail. The results, which are quite encouraging, support the view that the annual delivery of heat from a geothermal source by urban heating networks could reach, under certain conditions, 3,500 GWh for Ile-de-France alone by 2015. 9.5. Electricity production 9.5.1. Current contribution of geothermal energy to the production of electricity The installed geothermal power in the world is about 8.9 GW (1.2% of the installed electric power in the world), distributed over 24 countries in the following way: 6 countries in Europe for 1,024 MW, 6 in the Americas for 3,920 MW, 7 in Asia for 3,390 MW, 3 in Oceania for 441 MW and 2 in Africa for 134 MW. The annual production of about 56.8 TWh (satisfying 0.4% of worldwide needs) is produced by a total number of about 470 installations and involves some fifty million inhabitants. According to the SER10, this puts geothermal power in fourth place in the systems for electricity production from renewable energy, after hydraulic (2,750 TWh), biomass (250 TWh), and wind (160 TWh). However, the excellent average annual load rate (in the order of 73%) gives geothermal systems the attractive advantage of production as a base unit, while wind systems, depending on an intermittent resource, offer only an average worldwide rate estimated at 20%. It is only at the beginning of the 21st century that the European countries without high enthalpy resources have joined the club of countries producing electricity by geothermal means. Thus, at the beginning of 2001, Austria put the first small binary cycle unit (1 MW) into service, using a karstic aquifer at 106ºC. Germany followed
10 SER: Syndicat des Energies Renouvelables (Union for Renewable Energy).
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this example in November 2003 by inaugurating a binary converter (230 kW) grafted onto a conventional geothermal doublet for urban heating using water at 98ºC. In the USA, 189 units equipping 66 production centers (California, Nevada, Utah and Hawaii) represent a capacity of 2,540 MW and produce 17.8 TWh/year covering 0.5% of the national needs. The majority of the production (84%) is produced by direct cycle geothermal power plants (condensing turbines with dry steam – The Geysers – and with steam separated after simple, double or triple vaporization (flash), while the plants with binary cycle or combined cycles contribute to the net production. After the USA, if we limit ourselves to an installed capacity higher than 100 MW, come the Philippines (1,931), Mexico (953), Indonesia (797), Italy (790), Japan (535), New Zealand (435), Iceland (202), Costa Rica (163) and El Salvador (151). For its part, with an installed capacity of 15 MW in Guadeloupe, France ranks n° 18 among the producer countries. 9.5.2. Exploitation of geothermal resources 9.5.2.1. Naturally producing reservoirs Today geothermal electricity is produced from geological reservoirs containing geothermal fluids of high or medium enthalpy, whose temperature usually is between 130ºC and 250ºC and whose physical state is often the coexistence between phases (liquid and steam). The behavior of a natural thermodynamic system is controlled either by steam (two large geothermal fields known in the world: The Geysers in the USA and Larderello in Italy) or by liquid (the most usual case). When steam pressure controls the system, the reservoir produces dry steam or superheated steam. In the other case, the reservoir produces either saturated steam or a liquid-steam mixture. At its lowest temperatures, the reservoir (whose aquifers are hot) produces the geothermal fluid directly in liquid form. In the first two cases, electricity is produced by geothermal power plants using direct cycle thermoelectric conversion units (Figures 9.18 and 9.19: turbines with atmospheric escape or condensation) from the geothermal steam obtained either directly at the head of production wells or after the separation of the liquid phase, or binary cycle units (Figures 9.20 and 9.21: Rankine cycle and Kalina cycle) where a
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thermal exchanger allows the transfer of energy from the geothermal fluid to a working fluid (organic fluid, ammonia water solution, etc.) whose steam is then worked by a turbine.
Figure 9.18. Device with atmosphere escape
Figure 9.19. Device with double vaporization and condenser
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Figure 9.20. Binary device with working fluid (Rankine cycle)
Figure 9.21. Device with Kalina cycle (KCS 34 g)
Recently (1998-2003), combined cycle geothermal power plants (Figure 9.22: combined cycle) linking atmospheric escape turbines and binary cycle energy recuperators were developed, notably in New Zealand. By making it possible to enhance the final energy usage of the different effluents, they achieved the highest worldwide efficiency.
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Figure 9.22. “Combination” device (using geothermal steam and work fluid)
Finally, hot aquifers (90–130ºC) allow energy production by using binary cycle production units. 9.5.2.1.1. High enthalpy (t > 160ºC) An examination of the diagram of the principle of Unit no. 1 of the geothermal power plant of Bouillante in Guadeloupe (4.7 MW) and its comparison with the entropic diagram (Figure 9.23) makes it possible to understand the mechanical and thermodynamic functioning of a condensation turbine fed by steam separated from the hydrothermal fluid in two vaporization chambers (double flash). The geothermal reservoir, whose producing zones are located between 300 and 1,000 m deep, contains a fluid with a salinity of 18 g/l with low concentration of CO2 (0.1% by mass) brought to a temperature of about 250ºC, and whose enthalpy is 1,150 kJ/kg. This is typical for a high enthalpy source in a recent volcanic environment. The production well makes it possible to bring the fluid to the surface where it undergoes variations of state described by the sequence 1-9. From the well heads (1) where the output of the fluid is controlled, a water/steam mixture coming from the vaporization taking place in the well is conducted to the separator (2) that regulates a
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first production of steam at 160ºC, and under 6.3 bars, directed toward the high pressure entrance of the turbine (3). The separated water (6) feeds a second vaporization chamber that generates a new steam at 107ºC and under 3 bars. This is introduced into the low pressure section of the turbine (8). The relaxation of the vapors in the turbine is regulated by a mixing barometric condenser fed by sea water that maintains a vacuum of 75 to 100 mb (4). The water separated from the second vaporization chamber (9) rejoins the mixture of condensed steam/sea water in a tank, and the ensemble is taken to the sea at a maximum temperature of 45ºC. The turbine drives an alternator via a gear box.
a)
b) Figure 9.23. a) Diagram of the principle of unit 1 of the geothermal power plant of Bouillante (Guadeloupe); b) corresponding entropic diagram
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In 2003, the annual production of electric energy of this unit rose to 23 GWh. With the second production unit with a capacity of 11 MW, placed in service in 2005, the user Géothermie Bouillante SA satisfied 8.5% of the electric energy needs of Guadeloupe annually. 9.5.2.1.2. High enthalpy: special case of supercritical fluids An examination of the enthalpic diagram of the water (Figure 9.24), that follows the evolution of the enthalpy, the temperature, the density, and the composition of the liquid-steam mixture of a geothermal fluid, suggested that access to reservoirs containing fluids in the supercritical area (P > 220 bars and T > 374ºC) would be useful in the region in the upper right quarter of the diagram, where the enthalpy of the fluid is higher than 2,800 kJ/g, and the temperature, higher than 450ºC, for the reservoir pressures between 240 and 300 bars. In fact, under these conditions, the adiabatic decompression – without excessive conduction cooling – that would be subjected to the supercritical fluid in order to be taken to the surface in the form of superheated steam (path F-G) would allow the production of a large quantity of steam with high enthalpy.
,
,
,
,
,
Figure 9.24. Enthalpic diagram of H2O
,
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The theoretical per unit gain of power (per well), compared with an electricity production by conventional geothermal means, would be on the order of a factor of 10 (49 MW versus 5 MW), as illustrated in Table 9.9, comparing the data relative to an installation using a “conventional” geothermal reservoir to those of an installation exploiting a production well with a comparable completion accessing a “supercritical” reservoir. In the first case, the geothermal steam is worked in the turbine directly and, in the second, the superheated steam yields its energy to the working fluid of a binary cycle installation. In the two cases, the output of the turbine is virtually identical. Conventional reservoir
Supercritical reservoir
Parameters “reservoir” (0)
P0 (bara) T0 (°C) h0 (kJ/kg) q0 (m3/s)
30 235 2,807 0.67
260 550 3,328 0.67
Parameters “well head” (tp)
Ptp (bara) Ttp (°C) htp (kJ/kg) mtp (kg/s)
25 224 2,802 10
195 503 3,260 55
Parameters “turbine” (ad)
Pad (bara) Tad (°C) mad (kg/s) P (MWe)
7.7 183 10 5
170 495 53 49
Table 9.9. Specific characteristics of installations with single well serving either a conventional geothermal reservoir or a supercritical fluid reservoir
In Iceland in 2001, based on the existence of high temperature geothermal sources (380ºC at 2,200 m at Nesjavellir, in the south west of the island), an Icelandic industrial consortium launched a feasibility study of an Iceland Deep Drilling Project (IDDP), with the aim of demonstrating the possibility of exploiting deep geothermal reservoirs (of about 5 km) containing fluids at temperatures between 400ºC and 600ºC. The main results of the feasibility study published in May 2003 indicate that it is possible, in the Iceland geological context, to drill wells 5,000 m deep and recover fluids at temperatures of 400ºC to 600ºC by utilizing an internal 4” liner. Since the composition of the fluid to be recovered is difficult to predict, because of the current imperfection of the thermodynamic models relative to systems more complex than
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that of pure H20, the experts have indicated that among the 12 potential candidate sites for a deep drilling (identified in the geothermal fields of Reykjanes, Nesjavellir and Krafla), the preference would be given to the site that, a priori, could produce fluids with a high meteoric component and low mineral content. A site on the Reykjanes field was chosen in 2005 to carry out the first drilling. Although the drilling reached 3,100 m at the beginning of 2006, technical difficulties led the operators to transfer the project to the region of Krafla. 9.5.2.1.3. Medium enthalpy (90ºC < t < 160ºC) Volcanic environments These do not deliver only to developers of high enthalpy reservoirs. The peripheral zones of such reservoirs are also favorable for generating electricity via the implementation of binary cycle production units. The world’s first geothermal power plant calling on the Kalina cycle was put into production in Husavik, Iceland in July 2000. Intended to supply 75% of the needs of the city (2,500 inhabitants), it draws its geothermal source from artesian wells located 20 km to the south that deliver 90 l/s of a fluid at 121ºC. After 15 months in operation, it appeared that the mastery of the complex process of vaporization and reconcentration of an 82% ammonia water solution was not particularly a problem and that there was no sign of escape to cause concern, no sign of risk for people and/or the environment. Performance tests carried out at the end of this period showed that the net electric power obtained, 1.7 MW, corresponded well to the 2.0 MW of the scheduled power, keeping in mind the correction to make for a shortfall of 3ºC in the temperature of the heat source (121ºC instead of 124ºC). This success, allowed the designer of the process (Exergy Inc.), the owner of the power plant (Orkuveita Husavikur) and two engineering firms involved (Power Engineers Inc., Exorka) to fuel the debate over the comparison of the energy output of the Kalina cycle and the organic fluid Rankine cycle. However, this was marred by corrosion problems affecting the turbine. The turbine, originally an axial flux KKK, was replaced by a GE radial flux turbine, apparently satisfactory. Deep aquifers Widely distributed in the continental platforms and generally well recognized by petroleum operators, deep aquifers can constitute geothermal sources for producing electricity from the moment that the parameters of temperature, porosity and permeability allow the production, at the surface, of a fluid with sufficient output
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and enthalpy. Temperatures above 90ºC and outputs of some tens of l/s are the parameters desired for binary cycle production. If, for the moment, projects are still in the technical-economic demonstration stage, still there are a few countries of the European Union, notably Germany, that include incentives for developing such projects in their energy policy. Historically, it is in Australia that the first small geothermal installations were placed over aquifers to supply isolated farms with electricity. Thus, at Mulka (south Australia), a 1,300 meter well drilled in the Great Artesian Basin in 1904 was reconditioned in 1985 and equipped with an organic fluid binary cycle device. An artesian output of 10 l/s of water at 86ºC allows producing 10 to 15 kW. Further to the north, in Queensland, a part of the electric demand (250 kW) of the city of Birdsville is supplied by an isopentane binary cycle installation of 100 to 150 kW, supplied by an artesian well 1,220 m deep, producing 30 l/s at 97.5ºC. However, it is Austria that in 2001 became the first country of the European Union to produce electricity from a low energy reservoir. On the one hand, it is the karstic aquifer of the upper Jurassic of upper Austria that supplies – via a production well reaching 2,472 m deep – the binary cycle power plant of Altheim. An output of 8 l/s obtained by pumping lightly mineralized geothermal water (1.3 g/l) at a temperature of 106ºC allows the generation of a raw power of 1 MW furnished by a turbine driven by Solkatherm steam (azeotropic mixture of HFC Solkane). A deviated reinjection well allows the return of water to the aquifer at 1,700 m from its point of extraction. On the other hand, it was also in 2001 that the thermal park of Blumau in Styrie was equipped with a 250 kW ORMAT binary cycle converter capturing the energy of a geothermal fluid (50 l/s at 110ºC) coming from an aquifer located between 2,350 and 2,500 m deep in the paleozoic karstic dolomites. In Germany, the establishment of a relatively favorable guaranteed purchase price for electricity from geothermal sources in 2000, its adjustment to the increase by the Renewable Energy Act of 2004 giving 15 cents €/kWh for an installed power less than 5 MW, and the setting up by the BMU (ministry charged with the economy) of “ZIP” programs for stimulating investment in geothermal use resulted in a flowering of projects using deep aquifers and binary cycle production units (Gross Schönebeck, Landau, Offenbach/Pfalz, Speyer, Bruschsal, Bremerhaven, Unterhaching, etc.). Among these, the project now under development at Unterhaching in Bavaria is an example of what is now going on in Germany. Following the demonstration in 2000, by the regional geological service of Bavaria, of the strongest geothermal gradient ever found in Germany (12.5ºC/100 m at Altdorf, near Landshut), a systematic compilation of data of more than 1,000
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exploration drillings for gas and petroleum was undertaken. It gave way to the projection of an atlas of Bavarian geothermal sources that especially stressed the potential of the karstic aquifer of Malm (upper Jurassic) at the south of the Molassic basin of Bavaria. Encouraged by the initiatives taken in Rhine-Palatinat , and with the support of the BMU through its investment program ZIP, in 2002 the municipality of Unterhaching, launched a geothermal cogeneration project by creating the Geothermie Unterhaching GmbH & Co. KG. This is a project for covering – via an urban heat network – the heat needs of 22,000 inhabitants of the commune and for generating electricity during the periods when the call for thermal heating is reduced or even zero. The project, of a total cost estimated at 50 million Euros, is aimed at exploiting the potential of the Malm reservoir by operating a geothermal power plant than can deliver a power of 38 MWth and/or by converting this thermal energy into electricity by a Kalina cycle production unit (Siemens/Cryostar consortium) that has a maximum capacity of 3.4 MW.
,
,
,
Figure 9.25. Simplified diagram of the geothermal cogeneration installation of Unterhaching (Germany)
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9.5.2.2. Enhanced geothermal systems Geothermal convection resources, linked to an abnormal geothermal gradient, notably induced by active volcanic activity or deeper magma activity, are not uniformly distributed close to the Earth’s surface. Therefore, today they limit a generalized use of geothermal energy for electricity production to certain specific regions. The thermal flux coming from a deep source and the average mass capacity of the Earth’s crust are such that the quantity of heat stored in its five first kilometers is considerable. The extraction of a fraction of this heat is possible whenever the characteristics of permeability – natural or stimulated – allow an exchange with the water (heat transfer fluid) circulating between the wells. Thus, from 1 km3 of granite initially at 200ºC, it is theoretically possible to produce an electric energy on the order of 100 GWh/year for 20 years by cooling it by only 20ºC. Hydraulic stimulation of a well has the goal of pressurizing it in order to cause either a fracturing of the rock or an opening of existing fractures. In the latter case, if the tectonic state is right, a shearing of the fractures can be provoked, leading to an irreversible increase of the permeability (auto-shoring of the fractures). Depending on the intensity of the stimulation, it can affect the milieu in a more or less distant field. The ruptures caused in the solid mass and the reactivation of existing fractures are the beginning of microsismic events that are measured at the time of the stimulations. The hydraulic stimulation of a well is most often accompanied by a thermal effect in the closest well that can help improve the connection of the well to the surrounding solid mass. Chemical stimulation is another approach for increasing the permeability around the well by chemical action on the fillings of the fractures in order to cause these to break up. Future geothermal sites are therefore potentially rocky masses with temperatures between 130ºC and 200ºC, and therefore located, depending on the value of the geothermal gradient, between 3,000 and 5,000 m deep. At these depths we encounter rock bases, more or less fractured, whose distribution around the world is ubiquitous (the stable continental platforms), that have been and still are the object of important research for demonstrating the industrial feasibility of the concept of generalized deep geothermal use. These hot base rocks have been targeted since the first research work in 1970 in the USA, then, in virtually the same way, in the 1980s in Japan, the UK, Germany, Sweden and France, and, more recently in Australia (2000) and Switzerland (2001). The first experiments took place in the USA. Between 1973 and 1995, teams at the Los Alamos National Laboratory conducted trials at Fenton Hill, New Mexico,
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between wells spaced from 150 to 300 m drilled in a granodiorite with a strong geothermal gradient (65ºC/km). The heat exchanger, created by hydraulic stimulation at 3,500 m depth and at 250ºC, turned out to have a strong hydraulic impedance and the output of the loop never exceeded 10 1/s of fluid at 182ºC, values that do not allow considering industrial applications. The reservoir of Fenton Hill is considered to be the prototype of the closed HDR system, whose parameters limit interest, in the current state of technologies for stimulation. In Japan since 1984, the NEDO developed its own site for a parallel experiment like that of Fenton Hill. At Hijiori, a series of wells dug into a distorted granodiorite allowed the creation of two semi-deep exchangers (1,800 and 2,200 m) in an environment that was very hot (250–270ºC) and open. The circulation tests made from 1991 to 2000 highlighted the low impedance of the system, but the high rate of losses (~25%) did not allow a true loop to be established. In the UK, British teams assembled around the Camborne School of Mines between 1980 and 1993 created a heat exchanger of 80ºC and 2,200 m deep in the granite of Rosemanowes Quarry in Cornwall. A circulation between wells, maintained for several months, showed a high impedance and a high rate of loss (about 25%) for the outputs tested (10-15 l/s). The projected was not developed at 6,000 m as had been planned. In Germany, data concerning the temperature, the system of fractures, the distribution of constraints and the hydraulic behavior of the base directly below Bad Urach (Baden Württenberg) were acquired by the municipality of that power station during tests carried out between 1977 and 1992 over a single well drilled in metamorphic rocks, successively at 3,334, 3,488 and, finally 4,444 m, a depth where the temperature is 170ºC. The fact that the site was submitted to a tectonic regime of horizontal shearing confirmed the municipality in its project of creating a heat exchanger at 4,400 m that would produce a fluid to feed a binary cycle power plant of 4.5 MW, using an injection well and two production wells. Construction on a new deep well (4,285 m), however, was suddenly stopped at 800 m in May 2004, following unexpected technical and financial difficulties. It is in France, at Soultz-sous-Forêts, that the European Union and the French and German energy agencies have decided to turn their attention. This is on a project conceived by BRGM and BGR. Since l987, the granite under a sedimentary cover (1,400 m thick) of the northern part of graben du Rhine (Figure 9.26) has been the object of an investigation conducted in successive stages by a group of European participants (mainly French, German, Italian, Swiss, with some British and Swedish participation):
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– a first phase (1987–1997) was the drilling of almost 2,000 m that made it possible to identify the characteristics of the natural fracturing of the granite at Soultz-sous-Forêts, as well as a take a first measure of the temperature (140ºC). Hydraulic stimulation of this well in 1991 then demonstrated the capacity of the solid mass to respond with an increase of permeability, which was reproduced in 1993 on the same well deepened to 3,600 m (160ºC) and in 1995 on a second well drilled, at a distance of 450 m from the first, to a depth of nearly 3,900 m (170ºC). For four months a loop allowed the production at the surface, with an output of 25 kg/s, of a fluid at 142ºC, that could deliver, via a thermal exchanger, a power of 10 MWth before being reinjected into the deep reservoir,
Figure 9.26. Schematic representation of the graben in the Soultz-sous-Forêts region
– the second phase (1999–2007), consists of establishing that the same characteristics are found at a depth of 5,000 and allowing the installation of an electricity generation unit fed by two producer wells each with an output of 50 l/s of a fluid at 185ºC that will be reinjected between 50ºC and 100ºC via an injector well, – the third phase (2007–2008) consists of: 1) achieving the connection among the three wells by chemical and/or hydraulic stimulation, 2) conducting a long term circulation test, 3) tracking the hydraulic-thermal-chemical behavior of the loops,
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4) setting up a first binary cycle electricity generator (Turboden/Cryostar consortium, radial flux turbine), with a net capacity of 1.5 MW, to demonstrate the feasibility of electricity production (Figure 9.27). The capacity of the production unit will be brought to 6 MW by adding 4.5 supplementary MW.
,
Figure 9.27. Geothermal exploitation of granite at 5 km deep at Soultz-sous-Forêts
In Switzerland, the Geopower Basel AG industrial consortium formed by eleven partners, directed by Geothermal Explorers Ltd., designed a €52 million project that will undertake the completion defined by Soultz-sous-Forêts. The goal is to install a triplet of wells (1 injector and 2 producers at 5,000 m) at the southern edge of the Rhine valley near the city of Basel, to supply a pilot demonstration of a geothermal cogeneration of 6 MWe and 17 MWth serving a part of the city. The coupling with a gas turbine could bring the electricity capacity of the hybrid system to 14 MW. The first well at a little more than 5,000 m deep was completely finished in November 2006, allowing the almost immediate output of hydraulic stimulation (December). In the course of putting it into service (up to 50 l/s under 300 bars), induced seismic events led the operators to temporarily interrupt their injection operations.
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,
,
,
,
,
,
Figure 9.28. Simplified diagram of the thermal cogeneration project at Basle (Switzerland)
Similarly, in Australia an evaluation of the continent’s potential was made in 1994. On the basis of a study of the data of 3,475 deep wells, the Australian geological service (AGSO), in collaboration with the Australian National University (ANU), the University of New South Wales (UNSW) and the Australian association of electricity producers (ESAA), estimated the extractible energy potential to be 22,500 EJ (7,500 years of Australian consumption), of which 80% would be in the subgroups of the Eromanga Basin, including the Cooper basin, located within southern Australia and Queensland, the balance being other small basins, including Sydney’s. It is the government of South Australia that gave the green light to an important project by launching in October 2000 a call for bids for permits for geothermal exploration on three tracts covering the greatest geothermal anomalies of the Cooper Basin, where granodiorites forming the infrastructure of the large artesian basin are famous for having temperatures on the order of 250ºC at less than 4 km deep. It was Geodynamics Ltd. (GDY) that won and was awarded the permit.
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In August 2002, GDY presented its technical plan in three phases: 1) verification of the concept aimed at demonstrating the possibility to create, by hydraulic stimulation, a deep heat exchanger (about 5,000 m) and to extract energy by a hydraulic loop maintained between two bore holes for several months, 2) construction of a commercial demonstration pilot including a third well and a binary cycle electric production unit of 13 MW, 3) construction of a commercial installation of 275 MW calling for 31 wells making up a system of 16 satellites (16 injectors and 21 producers) distributed over 6.25 km2. After an introduction at the Sydney stock exchange (ASX) in September 2002, GDY between February 2003 and January 2004 successfully carried out the drilling and the hydraulic stimulations of a first deep well (4,421 m): a deep heat exchanger was thus created at an environment of 250ºC. After new stock market maneuvers and the creation of a company for the commercial implementation of the license for a Kalina cycle, GDY in July 2004 undertook the drilling of a second well located 500 m from the first. It reached a depth of 4,350 m after re-cutting the stimulated areas from the first well, but the loss in the well of part of the shaft and the failure of repeated maneuvers to recover it and/or bypass it minimized the success. During 2005, in parallel with the attempts to save the well, GDY tried to perfect the hydraulic connections of the deep heat exchanger, but in June 2006 had to decide to suspend operations and declare the loss of the well. At the end of 2006, GDY made it known that it was willing to continue the project by drilling a third well of larger diameter that will serve as a production well in the industrial phase, called from then on the Hotrock 40 Program (40 MW). 9.5.2.2.1. New generation geothermal systems All the projects for deep geothermal exploitation in hot base rocks call for a stimulation of a well that is more or less massive in order to create deep thermal heat exchangers. This modification of the permeability can theoretically be made on all geothermal systems (volcanoes of active zones, bases of stable platforms or sedimentary basins). It led the US Department of Energy to call systems thus modified Enhanced Geothermal Systems (EGS). Projects launched in 2002 by Shell El Salvador in association with Geotermica Salvadorena SA involve the stimulation of weakly producing wells and aim to hydraulically stimulate parts of the geothermal field of Coso (California) in order to
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allow it to increase the currently installed capacity (236 MW) by 15 MW. These projects are based on the industrial use of this new concept. The European Union has recognized the merit of this new approach in geothermy by launching a significant cooperation initiative, ENGINE (ENhanced Geothermal Innovative NEtwork for Europe), the coordination of which was assigned to BRGM11. 9.5.3. Development potential The development potential for a renewable energy system depends, these days, not only on investment costs and production costs of the system itself, and therefore on its competitiveness compared with other conventional and/or renewable systems, but also more and more on the environmental benefits that it is capable of producing. The investment and production costs of geothermal energy depend on many parameters, notably on the geology of the prospective region and on the quality of the resource exploited (enthalpy, presence of incondensible gas, type of dissolved substances, etc.), on the means of production (cycle, direct, binary or combined), on the installed capacity (effect of size) and on the country in which it is generated (indirect costs). On the environmental level, geothermal electricity production contributes, in a significant way, to the reduction of greenhouse gas emissions in spite of the sometimes elevated content of incondensable gases (CO2, H2S, NH3 and CH4) that can be contained in fluids of certain geothermal fields at high enthalpy (up to 5% in weight with, on average, 90% CO2). The protocols for capture and reinjection into reservoirs of these gases are not well understood and their use allows the production of electricity with a minimum of gaseous discharge. In the USA in 1991, a time when the installed electric capacity had reached its peak (2.8 GW), the quantities of CO2 emitted into the atmosphere by geothermal power plants were on the order of 55 tons of CO2 per GWh (55 g/kWh) produced. A more recent estimate on a total number of 6.6 GW (82% of the worldwide installed pool) noted a bracket of between 4 and 740 g/kWh for a weighted average of 122 g/kWh, making the production of electricity by geothermal use on average to be 2.5 to 7.5 times less polluting compared, respectively to the combined gas cycle,
11 http://engine.brgm.fr.
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petroleum, and coal. By cutting back on the production to 73% of the total installations, the average CO2 emissions comes to 55 g/kWh. The generalized use of reinjection techniques and of binary or combined cycle power plants would allow bringing back the emission levels of CO2 and SO2 (coming from H2S) to virtually zero (the optimum being that of a binary cycle power plant that emits up to 2,000 time less carbon than a natural gas plant) and would limit, in parallel fashion, the risks linked to the decline of the resource and to associated subsidence phenomena. Geothermal resources allowing the industrial production of electricity have been the subject of various evaluations. As in the mining sector, notions of proven, probable and possible reserves have are used. The methods employed are varied and give results that are sometimes very divergent. Sometimes the authors go back to their work and make significant revisions. Thus, the Icelanders have – at first – estimated at 1,420 GW, the worldwide potential for geothermal sources, both identified and still to be discovered, allowing the implementation of conventional production techniques of electric energy. They take it to 2,840 GW by considering a massive development of innovative technologies (EGS), notably using new thermoelectric conversion cycles (ORC and Kalina), making it possible to take advantage of low energy sources. The corresponding anticipated annual production was then estimated at respectively 11,200 TWh and 22,400 TWh, or 4 to 8 times higher than worldwide hydro production in 2003. Much more modest estimates were advanced at the same time by the Americans, who considered that the accessible worldwide potential with conventional techniques would be about 80 GW, notably in Central America (22 GW), Indonesia (16 GW), east Africa (10 GW) and the Philippines (8 GW), and that it could be developed in thirty years. The development of technologies of drilling and stimulation of the permeability of the reservoirs (EGS) could bring this potential to 138 GW capable of producing 1,100 TWh/year, representing 7.2% of the world electric production of 2002 (about 15,300 TWh) or, more optimistically, 4.8% of the electric energy that would be consumed in 2025 (about 23,000 TWh). Very recently Valgardur Stefansson (2005) revised his estimates of 1998 in presenting the results of two very different approaches, one based on volumes (energy stored in a volume of rock affected by an extraction factor), giving a high estimated limit, the other based on models constrained by parameters directly derived from the geothermal fields being used, giving a low limit.
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From this study – which is, in fact, a synthesis of the Icelandic and American views – it comes out that the lower limit of the potential of geothermal resources allowing the generation of electricity by using “conventional” methods (t > 130ºC) would be 50 GW, that the potential of resources identified up to the present day would be on the order of 200 GW, and that the higher limit of the absolute potential would be in the range of 1,000–2,000 GW. The development of geothermal sources for producing electricity from high enthalpy fluids today depends neither on technology nor on environmental constraints, but rather on access to a resource that is unequally distributed throughout the world (but still partially contributing: about 10%) and on the economic competition compared with fossil or nuclear energy sources dominating the electricity markets. Managing environments that are deep and not very permeable would allow more generalized use of the geothermal resource and would likely lead to its soaring on the worldwide level. 9.6. Glossary A Aquifer: geological formation containing temporarily or permanently mobilizable water, made of permeable rocks and capable of restoring the water naturally and/or by exploitation. We may distinguish: – aquifer with free layer; aquifer resting on bed that is not very permeable, surmounted by a zone not saturated in water (the first groundwater layer in the subsoil is called the water table), – confined aquifer (or captive layer): in a captive layer, the groundwater is confined between two formations that are not very permeable. When a drilling reaches a captive layer, the water rises in the drill hole. Artesian basin: geological structure, often of very large dimension, in which the water is under pressure. When a well reaches a captive layer, the water goes up into the drill hole; this water rising can go as far as water gushing above the surface of the ground; the well is then called an artesian well.
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C Calorie: the calorie is an ancient unit of heat. The unit is defined as the quantity of heat necessary to raise the temperature of 1 g of water 1ºC, between 14.5ºC and 15.5ºC. The international unit of energy is the Joule (J); 1 calorie = 4.18 Joules. Cementing: the cementing of a tube in a drill hole consists of cementing the ring shaped space between the tubing and the natural wall of the well. The purpose of this cementing is to protect the quality of the underground water (in order to avoid mixing the waters of different levels and the infiltration of surface water). Not to be confused with the cementing of a geothermal probe: in this case, the cementing consists of sealing the ground heat exchanger in the well by the filling with a cement especially prepared for this purpose. Cogeneration: conjoined production of heat and power (for electricity production). One speaks of the cogeneration station or the heat-power station. Compressor: component of the heat pump in which the refrigerant is compressed, which has the effect of raising its temperature. The compressor works with an electric motor or a thermal motor (most often, gas, for large installations). Condensation: change of state of a fluid, that goes from gas to liquid, releasing energy to the medium. Condenser: component of the heat pump in which the refrigerant releases its heat to the fluid of the heating circuit. Conduction: action of transmitting gradually heat, electricity Convection: movement of a fluid, with transport of heat, under the influence of temperature differences. D Diffusion: capacity of a material (milieu) to transmit a heat flow. Dogger: main geothermal aquifer exploited in the Paris region. It is located between 1,500 and 2,000 m deep and contains a water of varying temperature, depending on the depth, of 65ºC to 85ºC. The Dogger corresponds to ancient deposits (175 to 154 million years old) with a predominance of limestone of the middle Jurassic period. The water content of this aquifer is heavily mineralized (6.5 to 35 g/l). Geothermal aquifer systematically exploited by “doublet” drill holes.
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Doublet (geothermal): set of two connected drill holes, one intended for the production of geothermal fluid, the other for the reinjection of the fluid into the aquifer where it originated. This configuration presents several advantages: – absence of discharge into the environment (closed loop circuit), – continuity of the hydraulic output, – stability of the working pressures. E EGS: Enhanced Geothermal System or Engineered Geothermal System. In French it is most often translated by “système géothermal stimulé”. Enthalpy: quantity used to calculate the energy exchanged at the time of a change of state. Evaporator: component of the heat pump in which the energy taken from the ground in the subsurface transferred to the refrigerant that vaporizes. Evaporation: change of state of the fluid that goes from liquid to gas, removing energy in a milieu that it cools. F, G Floor heating, low temperature: a heat emitter made of tubes in which a liquid circulates and gives off heat to the rooms to be heated. Built into a layer of concrete, it is sized so that the surface temperature remains moderated (about 23ºC). Floor heating can also provide house cooling; it is then called a heating-cooling floor. Geostructure: subterranean ground foundations or foundations on posts, equipped with conduits serving as thermal exchangers for the production of energy, called energy posts systems. Their special quality is the double application of components covered with concrete in contact with the sub-soil, both serving both as the foundation and for energy production in the form of heat and/or cold. Geothermal gradient: name given to the value of the temperate increase as a function of the depth. The geothermal gradient is on the order of 3ºC/100 m in stable zones; it can reach 10ºC/100 m in active zones. The so-called stable zones are essentially sedimentary basins and deep magmatic rocks. The active zones are linked to plate tectonics. Geothermal posts: when the construction of a building requires a foundation on concrete posts (for bearing purposes), it is possible to equip these posts with
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collectors (polyethylene tubes buried in the concrete). The collectors are then connected to a heat pump. They are also called geostructures. Geothermal power station: the ensemble of surface equipment allowing the recuperation of heat (energy) contained in the geothermal fluid and the outflow toward the distribution circuit of the energy (heat or electricity). Also includes all the equipment necessary for this transfer (systems of regulation, exchangers, pumps, etc.). All this equipment may be grouped in the same building. Glycol water: water mixed with an antifreeze with glycol base, currently utilized in buried sensors connected to a heat pump. H, K Heat exchanger: equipment allowing a hot fluid to release its heat to a colder fluid. In a heat pump, there are two types of heat exchangers: condenser and evaporator. Hydrogeology: one of the branches of Earth science, the object of which is a knowledge of the geological and hydrological conditions and the physical laws that govern the origin, the presence of movements, and the properties of groundwaters. Application of this knowledge to the human actions on these groundwaters, notably to their potential, their catchment, and their protection. kW (kilowatt): expresses a power: 1 kW = 1,000 W. kWh (kilowatthour): unit of energy equal to the energy consumed by an appliance of 1,000 W (or 1 kW) in 1 hour. Not to be written as kW/h (frequent error). (1 m3 of water whose temperature is lowered by 5ºC delivers 5.106 calories, or 5.8 kWh.) O ORC (Organic Rankine Cycle): ORC machines are made of a closed secondary circuit with heat transfer fluid with low boiling point, used for the generation of electricity by turbine. The heat transfer of the primary circuit, that is, the geothermal water circuit, is achieved by a heat exchanger. P Permeability: ability of a milieu to allow a fluid to pass through. Piezometric level: free level of the water observed in a well or drill hole compared to a reference leveling.
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Porosity: intrinsic characteristic of soils: is also related to the volume of the holes over the total volume of the rock, and is expressed as a percentage: The total porosity of a rock is very variable: from l to 50% (porosity is not be confused with permeability; if the holes in the earth are not interconnected, the water cannot circulate). R Reducing valve: heat pump component in which the pressure of the refrigerant is reduced. Refrigerant: fluid confined in the HP that assures, at the time of changes in state (gas, liquid), the transfer of heat. The working of thermodynamic machines (refrigerator, heat pump) is based on the capability of refrigerants to vaporize and condense at ambient temperature. Refrigerating circuit: closed circuit containing a refrigerant. The four main components of the refrigerating circuit are the compressor, the condenser, the pressure regulator and the evaporator. S Soft water: water containing a small quantity of dissolved matter, tasteless. T Tep or TEP (ton of oil equivalent: the equivalent of a ton of oil is a unit of measure of energy currently used by energy economists to make comparisons among different kinds of energy. It is the energy produced by the combustion of a ton of average oil, which represents about 11,600 kWh. The English also use the boe (barrel of oil equivalent), which is about 0.135 of tep, according to the equivalence: 1 tep = about 7.3 barrels (the barrel is a measure of capacity equaling 159 liters). Mtep: Mega tep = 106 tep = 1,000,000 tep. We utilize the Joule or the ton of oil equivalent (tep) to compare the different forms of energy. In France, the conversion coefficients are fixed by the Energy Observatory, using the method common to the international organizations (AIE, European Commission, UN, World Energy Council). Tertiary: buildings of the tertiary, the service and administrative sectors. The distribution of heating and/or air conditioning needs of these buildings can be different according to the zone of activity: offices, hospitals, etc. Therm (th): 1 th = 106 cal. Thermal conductivity: capability of a material to conduct (transport) thermal energy (heat).
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Thermodynamics: branch of physics dealing with relations among the thermal and mechanical phenomena. Transmissibility: parameter that regulates the output of water that flows, by unit of size of the aquifer, under the effect of the hydraulic gradient unit, including the thickness of the aquifer. Transmissivity makes it possible to estimate the output a drill hold can catch. V, W Ventilator-convector: emits heat or cooling by air connected to a circuit of air heated or cooled by the HP. It filters and diffuses the air of the room by a fan. W (watt): the watt is the legal power unit. It corresponds to the quantity of energy consumed or produced by unit of time, or one Joule per second. Its symbol is W. We often use multiples of this symbol: kW (kilowatt), with 1 kW equal to 1,000 W. Wh (watthour): this is the unit of work equivalent to 3,600 Joules. It is the work accomplished by one power of 1 watt during 1 hour. We use it most often with multiples expressed in kWh (kilowatthour), in MWh (megawatthour) or TWh (terawatthour) with 1 MWh = 1,000 kWh and 1 TWh = 1 million kWh. Example: one light bulb of 20 watts functioning for 50 hours consumes 1,000 Wh, or 1 kWh (kilowatthour). Z Zone: – non-saturated: zone of the subsoil between the surface of the ground and the surface of a free layer – saturated: zone of the subsoil in which water completely occupies the interstices of the rocks, forming, in an aquifer, a layer of subterranean water. 9.7. Bibliography [ADEME] ADEME, Les pompes à chaleur géothermique, Guide pratique habitat individuel, http://www.ademe.fr/particuliers/Fiches/pacg/rub5.htm. [ADEMEB] ADEME BRGM, http://www.geothermie-perspectives.fr. [AFPAC] ASSOCIATION http://www.afpac.org.
FRANÇAISE
POUR
LES
POMPES
À
CHALEUR,
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[ASS 00] ASSAD P., “Environmental assessment of geothermal power production, an overview”, 76th Annual Meeting of the Pennsylvania Academy of Science, in K. Shyamal (ed.), Journal of the Pennsylvania Academy of Science, vol. 73, p. 172, 2000. [BAR 02] BARIA R., BAUMGAERTNER J., GÉRARD A., JUNG R., GARNIS J., “European HDR research programme at Soultz-sous-Forets (France), 1987-1998”, 4th International Hot Dry Rock Forum, draft papers, in Geophysik, Geologisches Jahrbuch, Sonderhefte, vol. 1, p. 61-68, 2002. [BERA 06] BÉRANGER B., Les pompes à chaleur, Eyrolles, Paris, 2006. [BERN 04] BERNIER J., La pompe à chaleur: déterminer, installer, entretenir, Pyc livres, Paris, 2004. [BRG 04] COLLECTIF BRGM-ADEME, La géothermie, collection Les Enjeux des Géosciences, BRGM-ADEME, Orléans/Angers, 2004. [CAT 99] CATALDI R., HODGSON S.F, LUND J.W., Stories from a heated earth, our geothermal heritage, Geothermal Resources Council, Special Report, vol. 19, Davis, USA, 1999. [COU 89] COUDERT J.-M., JAUDIN F., La géothermie: du geyser au radiateur, BRGM, Orleans, 1989. [DUC 02] DUCHANE D., “The history of HDR research and development”, in R. Baira, J. Baumgaertner, A. Gérard., R. Jung (eds.), 4th International Hot Dry Rock Forum, draft papers (modified), in Geophysik, Geologisches Jahrbuch, Sonderhefte, vol. 1, p. 61-68, 2002. [EGEC] EUROPEAN GEOTHERMAL ENERGY COUNCIL, http://www.egec.org. [EHPA] EUROPEAN HEAT PUMP ASSOCIATION, http://www.ehpa.org. [ENGINE] Projet Européen ENGINE, http://engine.brgm.fr/ (ENhanced Geothermal Innovative Network for Europe). [FER 98] FERRANDES R., La chaleur de la terre: De l’origine de la chaleur à l’exploitation des gisements géothermiques, ADEME, Paris, 1998. [FLY 97] FLYNN T., “Geothermal sustainability, heat utilization, and the advanced binary technology solution”, Transactions, Geothermal Resources Council, Davis, USA, vol. 21, p. 489-496, 1997. [GAB 00] GABAUER A., “Geowarme für Erding–Das Projekt und seine Geschite”, Geothermische Energie, n° 30-31, 2000. [GEN 00] GENTIER S., “La Géothermie”, Manuel de mécanique des roches, Tome 2, Les applications, Presses de l’école des mines, Paris, 2000. [GEO] LA GEOTHERMIE EN ILE-DE-FRANCE, http://www.ile-de-france.drire.gouv.fr/ (rubriques sous-sol). [GEO] LA GEOTHERMIE A CHEVILLY-LARUE/L’HAŸ-LES-ROSES, www.semhach.fr. [GEO] SOCIETE SUISSE POUR LA GEOTHERMIE, http://www.geothermal-energy.ch.
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[GEO] GEODYNAMICS LIMITED, http://www.geodynamics.com.au (company exploring Hot Dry Rock Geothermal Energy in Australia). [GER 06] GÉRARD A., GENTER A., KOHL T., LUTZ P., ROSE P. (DIR.), “The deep EGS (Enhanced Geothermal System) project at Soultz-sous-Forêts”, Geothermics, vol. 35, Issues 5-6, p. 473-714, 2006. [GHPC] GEOTHERMAL HEAT PUMP CONSORTIUM (US), http://www.geoexchange.org/ about/ compare.htm. [GROUNDHIT] PROJET EUROPÉEN GROUNDHIT, http://www.geothermie.de/groundhit (Ground Coupled Heat Pumps of High Technology). [GROUNDREACH] PROJET EUROPÉEN GROUND-REACH, http://groundreach.fizkarlsruhe.de/en (Reaching the Kyoto Targets by Ground Source Heat Pumps). [IGA] INTERNATIONAL GEOTHERMAL ASSOCIATION, http://iga.igg.cnr.it. [IGL 00] IGLESIAS E., BLACKWELL D., HUNT T., LUND J., TAMANYU S., KIMBARA K., “Géothermal”, International Geothermal Association (IGA), Pisa, 2000, Proceedings of the World Geothermal Congress, Kyushu, Japan, 28 May-10 June 2000. [IGSHPA] INTERNATIONAL GROUND SOURCE HEAT PUMP ASSOCIATION, http://www.igshpa. okstate.edu. [KAR 03] KARMAN F.H., LORBERER A., PATZAY G., RYBACH L., “Géothermics”, Hungarian Geothermal Association (HGA) and International Geothermal Association (IGA), vol. 32, n° 4-6, p. 371-377, 2003, Proceedings of the European Geothermal Congress 2003, Szeged, Hungry, 25-30 May 2003. [LAP 01] LAPLAIGE P., LEMALE J., Energie géothermique, Traité Génie énergétique BE 8 590, Techniques de l’Ingénieur, Paris, vol. BE, 2001. [LEM 98] LEMALE J., JAUDIN F., La géothermie: une énergie d’avenir, réalité en Ile-deFrance, F. Brenière (ed.), with the collaboration of Y. Benderitter, P. Laplaige and R. Ferrandes, ARENE, Paris, 1998. [LUN 04] LUND J.W., “100 Years of Geothermal Power Production”, Geo-Heat Center Bulletin, vol. 25, n° 3, September 2004. [MIN 06] MINISTERE DE L’INDUSTRIE, PPI “chaleur”, DGEMP, DIDEME, Paris, 2006. [OBS 06] OBSERVATOIRE DE L’ENERGIE, L’énergie en France, Repères, Paris, 2006. [PRE 06] PREVOT H., Les réseaux de chaleur, Rapport du Conseil général des Mines au Ministère de l’Industrie; www.industrie.gouv.fr/energie/publi/pdf/rapport-prevot.pdf, 2006. [SOULTZ] PROGRAMME EUROPEEN DE GEOTHERMIE PROFONDE A SOULTZSOUS-FORETS, http://www.soultz.net/fr/. [VUA 99] VUATAZ F.-D., Proceedings of the European Geothermal Conference Basle’99, Basel, Peter Lang, Neuchâtel, 28-30 September 1999.
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[WER 06] WERNER W., Euroheat and Power Conference, Brussels, 22 June 2006. [WOR 05] World Geothermal Congress, Proceedings of the World Geothermal Congress, Antalya, Turkey, available at: http://geothermal.stanford.edu/standard/, 24-29 April 2005.
Chapter 10
Biofuels
10.1. The place of biofuels in the energy environment The energy situation in recent years has been particularly favorable to the development of systems offering alternatives to oil. The continuous increase in the price of the barrel between 2003 and the first part of 2008, the increased dependence of consumer countries on oil compared with the limited number of producer countries, a wide realization of the finite character of this energy resource, and the strong pressure of public opinion to develop energy systems that would be more environmentally friendly were equally motivating forces in coming up with alternatives, notably in the transportation sector. In the range of solutions considered, biofuels are certainly the ones with the most accelerated development in recent years and today, after more than 20 years of industrial development, they hold promise for the future. Recent developments in this field are harbingers of widespread use of biofuels – up until now limited to certain countries (Brazil, the USA, etc.) – on a worldwide scale and with a potential positioning on the world market. These new prospects could nevertheless have limitations, especially because of constraints of resources and costs. The future of biofuels will probably therefore consist of setting up new systems taking advantage of lignocellulose material (wood, straw).
Chapter written by Frédéric MONOT, Jean-Luc DUPLAN, Nathalie ALAZARD-TOUX and Stéphane HIS.
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10.1.1. A favorable environment Driven by the three main players in the field, Brazil, the USA and, to a lesser extent, Europe, the worldwide prospects for the development of biofuels are promising. Since the first projects promoting biofuels were launched following the two oil crises of the 1970s, the situation has developed significantly: 1) Biofuels have acquired real status as a complement to petroleum-based fuels. In fact, energy policies at the beginning of the l980s were marked by the will to find substitutes for petroleum, and in this respect biofuels were the focus of the greatest hopes. The ethanol development program (Proalcool program) in Brazil best demonstrated this vision. Today the developments in biofuel systems have shown that they will certainly never entirely substitute for petroleum. Biofuels are a leading edge alternative to gasoline or diesel which, in certain cases, can be very significant. Moreover, their method of use as a mixture is a strong point, because, unlike other alternative fuels (NGV, LPG), they can take advantage of current fuel distribution networks and be used without special adaptation of vehicles (notably for incorporation rates of less than 5%, as is the case in Europe). 2) Initially used for their ability to reduce the emission of pollutants from vehicle exhaust pipes, biofuels are now developed especially for their positive contribution to reducing greenhouse gases, notably in the area of transportation, for which the alternatives appear to be limited. The widespread use of catalytic converters, as well as the related improvement in petroleum fuels and the management of combustion in motors, has in other respects allowed considerable gains over the level of emission of pollutants from automobiles without having recourse to biofuels. 3) Biofuels profit from renewed support from the public authorities who have introduced very ambitious objectives: 5.75% (percentage in energy) in 2010 and 10% in 2020 of fuel consumption in European transportation, and from 4% in 2010 to 20% in 2030 of fuel consumption in US transportation. Nevertheless, the realization of these objectives, which represents a real breakthrough, will not take place without difficulties. Based on systems in place today, this will require, in particular, good management of the competition with the food supply sector for use of land. Furthermore, the volumes of byproducts inevitably generated will amount to considerable quantities that have to find a use if the specific objectives for 2010 and 2020 are to be achieved. 4) Biofuels are identified as attractive new markets for agriculture. In an environment where support for agriculture in the northern countries is regularly denounced by the southern countries in the European market, new markets for agricultural products are being sought. From this point of view, biofuels offer an opportunity. They represent an outlet that is relatively protected from international competition. In fact, biofuel markets are for the moment very limited, and real
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economic competition with petroleum products remains limited because of support by the public authorities for these systems. However, it is appropriate to stress that this situation may change since the European and American biofuel markets have become very attractive. Thus, some countries like Brazil for ethanol and, to a very small extent, Malaysia or Indonesia for palm oil are going to try, for a time, to export their production at a low price to these new potentially lucrative markets. 10.1.2. Principal characteristics of systems today Today there are two types of biofuels: ethanol, which is used in gasoline engines, and fatty acid methyl esters (FAME) of vegetable oils, intended for use in diesel engines. Ethanol is the most widely used biofuel, its production rising to about 37 Mt in 2005, produced essentially in Brazil and the USA, and of which 80% was used as fuel. The production of FAME was in the order of 4 Mt in 2005, produced mainly in Europe. Today ethanol is produced from two main plant sources: sugar plants (sugar cane, sugar beets) and starchy plants (wheat, corn). These different systems all go through a fermentation stage that transforms the sugars into ethanol and a distillation stage that separates the alcohol from water. Some of the systems generate significant quantities of byproducts: the sugar cane system is, from this point of view, the most beneficial, since the principal byproduct, bagasse, is used as an energy source, notably for the distillation; the sugar beet system generates 0.75 tons of byproducts per ton of ethanol (essentially pulp), which currently is used in the animal feed market; finally, the wheat or corn systems produce 1 ton of stillage (or DDGS – Distiller’s Dry Grain with Solubles) per ton of ethanol, which is also intended for the animal feed market. Ethanol can be used in pure form, as a mixture or as ether (ETBE – ethyl tertbutyl ether), produced by reaction with isobutylene from refineries. The use of pure ethanol or a very strong concentration (for example, 85% or E85) requires a specific adaptation of the vehicle (injection systems, engine regulation, compatibility with plastics and joints, specific strategies for cold engine starts for pure ethanol). However, most automobile manufacturers have put vehicles on the market that can operate alternatively with E85 or gas, for example in Brazil or the USA. On the other hand, for weaker concentrations, no adaptation is necessary. It can be used in concentrations varying from 5% to 10%, as in the USA. Ethanol is usually used in the form of a mixture of a relatively weak concentration (from 5% to 10%). This usage can cause technical difficulties; in the presence of small traces of water in ethanol, the phases of gasoline and alcohol can separate (the de-mixing phenomenon); the addition of pure ethanol to gasoline
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increases its propensity to evaporation (increase in the vapor pressure). The use of ETBE overcomes these disadvantages. The FAMEs are vegetable oil products made, for example, from canola, sunflower, soy or even palm. In the case of oil from grinding grains (canola, soy, sunflower), a solid residue (the oil cake) is produced (1 to 1.5 tons of oilcake per ton of oil). This is usually reserved for animal feed. Not adaptable for supplying modern diesel engines directly, vegetable oils must be transformed in a transesterification process with alcohol (methanol at present), which results in methyl esters of vegetable oils and glycerine (0.1 ton of glycerine per ton of FAME). The role of this byproduct in the final value of this system is therefore far from negligible. Once more, in the case of a high volume production of FAME, particular attention should be paid to the development of the glycerin market, which is relatively limited (world production in 2005 was about 0.8 Mt a year, of which 370,000 tons already came from the production of FAME). As with ethanol, FAME can be used in pure form or as a mixture. Use in pure form requires adaptations of the vehicle, which limits its widespread use. FAME is currently used mainly as a mixture varying in strength from a few percent up to 30%. In the historical dynamics of biofuels, new products are now appearing that could, in the near future, play a role that is not inconsiderable. They are: – methyl esters of animal oils. The idea here is to substitute, for example, animal fat coming from slaughter houses, for vegetable oils. The European or world potential for this kind of conversion is obviously limited. Nevertheless, it could make sense in certain special local situations. By way of example, in France, the national potential for this type of product was estimated at about 200,000 t/year, – fatty acid ethyl esters of vegetable oil (FAEE). This variant of FAME uses ethanol in the place of methanol for the synthesis of esters. Ethanol is a product coming from natural gas or from carbohydrates (often named bioethanol). The final product has properties that are entirely equivalent to those of FAME. This product is for the moment foreseen in two regions of the world: Brazil and Europe. In Brazil, this system, which is in the development phase, takes advantage of the large availability of inexpensive bioethanol. In Europe the aim is mainly to introduce bioethanol in diesel fuel. In fact, the European market of biofuels is at present, and will be even more in the future, dominated by a strong demand for diesel fuel. The development of biofuels in Europe therefore mainly means the development of the FAME system. However, the logic of optimizing space designated for the production of biofuels is leading to the development of ethanol (a better yield per hectare). The temptation to introduce ethanol into diesel oil is thus very strong. However, ethanol is not soluble in diesel fuel and does not have good physical
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properties for direct use in diesel combustion. Using EEVO is therefore a way of allowing introduction of ethanol into diesel fuel. It should nevertheless be noted that the quantities of ethanol used are relatively small (15% by mass) and, above all, that the final product is likely to be more expensive in Europe than FAME because of the difference in price between ethanol and methanol (ratio of about a factor of 2 at the beginning of 2006), – diesel fuel from a synthesis based on the hydrogenation of vegetable oil. This product, currently being developed by the Finnish company Neste Oil, called NextBtL®, should be further developed in the coming years. At least two plants of about 170,000 tons are operational in projects in Europe (Finland, Austria). The particular feature of this product is that it is a very high quality diesel oil and, especially, the process allows a certain flexibility in the oil to be used. In fact, unlike FAME, for which the European standard requires canola oil as the major raw material, a large variety of oils can be used for the production of biodiesel NextBtl®: palm, soy or even animal fat oils – a very desirable flexibility. It should also be stressed that investors in these biofuel production units are petroleum companies. This is a new phenomenon. Up until now, the main investors in this sector came mainly from the agro-feed sector. 10.1.3. Main advantages and disadvantages associated with biofuel use The advantages of biofuels are well known: an alternative to petroleum in the transportation sector and an improved environmental footprint. On this last point, it is especially for their ability to reduce greenhouse gas emissions that their widescale use is foreseen today: for pure use, the gain in terms of greenhouse gas emissions can go up to 90% for the most efficient systems (sugar cane). Furthermore, this gain is of the same order of magnitude in terms of fossil energy consumption as compared with petroleum fuels (a comparison that takes into account all of the steps in the life cycle of the biofuel). Nevertheless it should be stressed that even if the greenhouse gas effect advantage seems certain, it is difficult to quantify it in a precise way, as much because of agricultural practices that can vary significantly among different regions, countries or continents, as for methodological reasons or difficulties in estimating the N2O emissions on cultivated fields; this is a greenhouse gas three times more noxious than CO2 for climate warming. The main disadvantages are a result of certain agronomic properties of biofuels and their cost. The yields per hectare of the main processes are relatively low: 1 tep/ha for FAME from canola or sunflower, 1 to 2 tep/ha for ethanol from wheat or corn, and,
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finally, 3 to 4 tep/ha for ethanol from sugar beets and sugar cane. Furthermore, a certain number of agronomic constraints mean that not all the plants can be cultivated on all land under the same conditions. Consequently, the massive development of biofuels will entail competition with the food system for land use that will have to be arbitrated. Moreover, introducing a significant quantity of byproducts has the risk of saturating the market, which will have, as a minimum, the consequence of making the cost of biofuel production more expensive. Thus, 10% substitution for gasoline and diesel fuel consumption in Europe and the USA will require more than 40% of the arable land in these countries [FUL 04]. These figures show very well the limits of the biofuel systems developed today and the necessity of considering new systems if the ambitious objectives (above 10%) for substitution of petroleum fuels by biofuels are to be attained. The most promising method makes use of lignocellulose material: according to a study published in the USA in April 2005 [PER 05], nearly 1.3 million tons of biomass can be produced a year. Transformed into biofuels, they would represent the equivalent of nearly 30% of the current petroleum consumption of the USA. Finally, the cost of production of biofuels is higher than the price of fossil fuels (before tax), even if, in recent years, due to the high price of the barrel of oil, the difference has been reduced (Table 10.1). The especially low cost of ethanol in Brazil should be noted. EtOH1 Europe
EtOH Brazil
EtOH USA
MEVO2 Europe
Petroleum fuels 25 $/bl
Petroleum fuels 50 $/bl
0.4-0–6 €/l
0.23 €/l
0.3 $/l
0.35–0.65 €/l
0.2 $/l
0.4 $/l
19–29 €/GJ
11 €/GJ
14 $/GJ
10.5–20 €/GJ
6 $/GJ
12 $/GJ
Table 10.1. Comparison of prices of different biofuels (according to [FUL 04])
The development of biofuels therefore still requires subsidies provided by governments. During the last few years, when the price of oil has been high, the demand increased and the prices of ethanol in the USA and Brazil and of canola oil in Europe were especially high: in the order of $3/gallon (or almost $1,000/t in May 2006) for ethanol in the USA and $800/t for canola oil in Europe.
1 EtOH: ethanol. 2 EMHV: methyl esters of vegetable oils (or FAME).
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10.1.4. The situation of biofuels in the world The volumes of biofuel production worldwide are given in Figure 10.1.
Figure 10.1. Volumes of biofuel production worldwide
In Brazil, ethanol consumption can be divided into three periods: a period of growth between 1975 and 1990, a period of relative stagnation between 1990 and the beginning of the 2000s, and a new period of growth between 2000 and 2006 (Figure 10.2).
Figure 10.2. Progress of ethanol consumption in Brazil in thousands of liters
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The government program Proalcool, which was established following the oil crisis of the 1970s, was essential to the development of the system of ethanol produced from sugar cane in this country. Between 1973 and 1990 it included three components: a guaranteed volume of ethanol bought by the national petroleum company Petrobras, a guaranteed price of ethanol, and incentives for investing in new plants by use of preferential interest rates and subsidies for purchase of vehicles running on pure ethanol. As a result of this program, in three decades, sugar cane production in Brazil increased by a factor of almost 4 (80 Mt/year in 1970 and nearly 300 Mt/year in 2008) and ethanol fuel consumption between 1975 and today went from around 300,000 tons/year to about 12 Mt in 2008. The oil counter-crisis of 1986 and the national company Petrobras’s success in discovering oil fields have somewhat weakened the main argument for the development of the system, namely, independence from oil imports. In particular, the fall in the price of oil in 1986 made the price support for ethanol purchase untenable for public finances because of the very large price difference between the price of gasoline and that of ethanol. Market changes in sugar, which were more attractive for the sugar cane producers, also played a not insignificant role. During the 1990s, the program therefore underwent major reform following difficulties in ethanol supply; the sugar/ethanol producers found an economic advantage on the international sugar markets. The main modifications were the following: – orientation toward a mixture by the retraction of specific subsidies for the purchase of vehicles running on pure ethanol, – since 1997 and 1999, “opening” the ethanol market and the end of price guarantees, – volumes consumed in part guaranteed with obligatory ethanol content in gasoline from 22 to 24% fixed by the government, – almost complete elimination of taxes from the sale of ethanol. The Brazilian automobile fleet today still includes nearly 3 million specialized cars and some 16 million vehicles running on a mixture with gasoline. Furthermore, the government has relaunched, via a tax deduction, subsidies on the purchase of vehicles of the Fuel Flexible Vehicle (FFV) type that can run either with pure ethanol or with a mixture. It is important to stress here the determining role played by the introduction of the FFV at the beginning of the 2000s on the progress of ethanol fuel consumption (Figure 10.3).
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Figure 10.3. Development in sales of new vehicles made for ethanol in Brazil (source: Anfavea) (by the end of 2005, the FFV represented almost 70% of the total market for vehicles with controlled ignition)
The total consumption of ethanol as a fuel rose by almost 12 Mt in 2005 (or close to 40% of national gasoline consumption). The ethanol production sector in Brazil continued to grow during the following years (15 Mt produced in 2007). The dynamism of Brazil on the ethanol market attracts even foreign investors. The declared goal is to export ethanol on a new world market: that of biofuels. To this end, infrastructures such as terminal ports and pipelines have been built or planned. The first market anticipated is Japan, whose government is now studying the possibility of imposing an ethanol content in gasoline (from 3% to 10%) and which has very limited production capability. Eventually, the USA and Europe are also anticipated to participate in this market. However, let us recall that currently, the import of ethanol into these countries is subject to duties of the order of 0.15–0.2 $/l, which limits economic attractiveness. Brazil is not only interested in ethanol, and in 2003 it launched a national program for the use of FAME. The main source of vegetable oils for this production is naturally soy, of which Brazil is the second largest producer in the world, but other sources are also considered, such as castor oil. In the case of Brazil, the aim is at the present time is a mixture of 5% by 2013. Finally, another particularity: it is anticipated that methanol will be replaced by ethanol in the production of FAME to produce ethyl esters of vegetable oils (EEVO). The USA is now the biggest consumer of ethanol fuel: production, essentially from corn, rose from about 8.3 Mt in 2003 up to 19.4 Mt in 2007. This trend should continue, especially following the incentives given by the US government in the
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version of the Energy Policy Act voted for in the summer of 2005. Nevertheless, it is important to stress that unlike Brazil, this consumption is not very big on the overall scale for the USA: this volume corresponds to about 2.6% of fuel consumption in 2007. The use of ethanol is regulated by two main documents: the Clean Air Act of 1970, modified in 1990, and the Energy Policy Act of 1978, regularly amended or subsequently expired. The Clean Air Act, modified in 1990, made it obligatory for commercial gasoline to contain oxygenated components in areas where the air quality did not conform to federal norms, with a minimum oxygen content (2% and 2.7% overall). Until recently, refiners were using MTBE (methyl tert-butyl ether) to reach this objective. The use of this product, for reasons of public health, was forbidden in a certain number of states, including California in 2003. MTBE was then replaced by ethanol. This explains, in large part, the importance of the increase in demand these last few years. In other respects, we should stress that a modification of the standards on volatility was introduced to allow gasohol (a mixture of 10% ethanol/90% gasoline in splash blending) to be used in this framework. In a parallel fashion, the Energy Policy Act includes fiscal advantages accorded to ethanol. This tax exemption, accorded since 1978 and regularly revised, was extended to 2007 for mixtures of 10%, 7.7% and 5.7%, averaging a reduction in three stages of the fiscal exemption at 53, 52, and 51 cents/gallon of ethanol (14.0, 13.7 and 13.5 cents/l) in 2001, 2003 and 2005, respectively. For comparison, the gasoline tax was, at the end of 2007, in the order of 40 cents/gallon (10.6 cents/l). In addition to these fiscal deductions, a number of states offer their own tax exemptions that can be in the order of 20 cents/gallon (5.3 cents/l.) Besides these fiscal advantages, public support for biofuels has been reinforced in recent years: the US agriculture policy, established in 2002, was marked by the financial aid accorded to biofuels, which could take different forms and for which the total bill could go above $150 million per year over the period 2003/2006 (title IX of the Farm Bill 2002); the President Bush’s energy policy (Energy Bill), already cited, includes, notably, a significant plan for promotion of biofuels with the ambitious objective of increasing production from nearly 12 Mt in 2005 to 22.5 Mt in 2012, mainly by measures of obligatory incorporation of biofuel components. The USA is also beginning to take an interest in FAME. Ignored for a long time by the rules regulating the usage of biofuels, FAME shows up very clearly today and is part of the alternatives to diesel fuel engines mainly used by public fleets; their use gives them access to financial aid, as do a certain number of other alternatives to petroleum. FAME is used most often today in a 20% mixture (B20). The US administration has agreed, beginning January 1, 2005, to a tax exemption for FAME
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of up to 1 cent per percent of mixture; for B20, which is the mixture most often used, the tax exemption rises to 20 cents. The development of biodiesel production in the United States is in very strong growth: from less than 100,000 t in 2004, production has reached about 300,000 t in 2005. This strong growth should continue: in March 2006 the production capacity of biodiesel in the USA was estimated at 1.2 Mt, with an increase in capacity predicted in the coming 18 months of the same order of magnitude. A standard specifying the quality of FAMEs has also been established. This does not include a constraint on the iodine index (an index that measures the degree of saturation of the ester), unlike the European standard (the iodine index should remain lower than 120). This is because the USA is trying to take advantage of their soy production, of which they are the major producer in the world, which produces esters with a high iodine index (about 135). It should also be noted that this European specification will in the future block the use (and therefore the importation) of esters from soy oil (unless, of course, if it is modified). Europe exhibits a certain slowness in comparison with the sometimes very broad programs that have been carried out in Brazil and the USA. Only France has, over the last 20 years, had a relatively steady policy in this area. However, in the current decade, the leading place has been taken by Germany for FAME and by Spain for ethanol (Table 10.2). Unlike the situation in Brazil or the USA, the European market for fuels is marked by a growing domination of diesel fuel consumption (60% of the fuel consumption in Europe). This tendency explains in part the rapid development of FAME rather than ethanol. The production of FAME in Europe has thus grown very significantly in recent years, reaching 3 Mt in 2005 (the annual rate of growth was 35% from 2000 to 2005). Three countries have been mainly responsible for this growth: France, Germany and Italy. Germany in 2003 became the largest European producer and consumer of FAME, with a production of 715,000 tons. In 2005, its production was about 1.7 Mt This rapid progress is essentially the result of a very advantageous fiscal policy: total exemption of excises for FAME without a quota, unlike France, which has always had a partial tax exemption on a given volume subject to call for tenders. This fiscal scheme is undergoing development and the establishment of a biodiesel tax coupled with an incorporation obligation has been put in place. This measure makes it possible to limit financial losses for the State.
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1,669
France
492
Italy
396
Czech Republic
133
Poland
100
Austria
85
Slovakia
78
Spain
73
Denmark
71
UK
51
Slovenia
8
Estonia
7
Lithuania
7
Latvia
5
Greece
3
Malta
2
Belgium
1
Cyprus
1
Portugal
1
Sweden
1
TOTAL
3,184
Table 10.2. Production of FAME in Europe 2005 (kt)
France, the leader in this area until 2001, produced about 500,000 tons of FAME in 2005. In autumn 2005, the French government reiterated its intention to see France become the top European producer of biofuels. The incorporation objective for biofuels of 5.75% (energy content) for 2010 in the 2003 European directive on the promotion of biofuels was brought forward to 2008; for 2010 the objective is to reach 7% of biofuels, and then 10% in 2015. In line with these commitments, many calls for tenders entitling them to a partial tax exemption were launched and the construction of several new production units was planned, for a total investment estimated at more than €3.5 billion. In other respects, as in Germany, development of a fiscal plan was initiated: a “quasi-obligation” for the incorporation of biofuels
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was established on January, 1 20053 in parallel with a reduction of the tax exemption (by way of example, for FAME it went from 33 €/hl in 2005 to 25 €/hl in 2006). The objective here is to succeed in supporting the development of biofuels while limiting the losses of fiscal returns for the State budget. The last in the trio of leading countries, Italy, produced nearly 320,000 tons of FAME in 2004 (or a growth of 30% compared with 2002), of which about 25% was used for heating. Finally, it is appropriate to mention that the countries that have more recently joined the European Union already have capabilities in place and bring a real will to become significant players in this field, especially the Czech Republic and Poland. In 2005, the European production of ethanol fuel represented a total quantity of about 750,000 t, compared with 950,000 t consumed: 200,000 t of ethanol were therefore imported. The production was mainly in Spain, Sweden, Germany and France. In these countries, with the exception of Sweden, ethanol was not used directly but was transformed into ETBE, which is itself mixed with gasoline. In Europe there is an obligation to conform to a standard for the properties of fuels and especially for volatility, and ETBE avoids the phenomenon of de-mixing in the presence of traces of water in the storage tank. A longtime leader of the ethanol system in Europe, France, which produces as much ethanol as it consumes, today has been surpassed by Spain, Sweden and Germany, as Figure 10.4 shows. The quotas accorded by the French state in 2005 entitling companies to a partial tax exemption for ethanol rose to 134,587 t of ethanol in the form of ETBE with gas, and 72,416 t of pure ethanol to be used in a direct mixture with so-called low volatility gasoline. In 2005 consumption was essentially in the form of ETBE, for which the quotas therefore were almost attained (114,000 t). On the other hand, sufficient quantities of ethanol, taking advantage of a partial tax exemption for incorporation of low volatility gasolines, were not put on the market. The public authorities therefore tried to promote the use of ethanol through various actions, and notably in 2006: 1) an experiment carried out in Rouen on the use of pure ethanol in a mixture with low volatility gasolines, 3 General tax on polluting activities: tax before being paid by the fuel distributers, of which the amount is proportional to the quantity of fuels sold and the difference between a percentage of incorporated biofuels to be attained and that which has in fact been attained. This tax is zero if the objective percentage for biofuel incorporation has been attained.
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2) promotion of the use of E85 with the establishment of an experimental fleet of vehicles used for 12 months by the General Council of Marne, with scientific aid entrusted to Ademe and to IFP.
Figure 10.4. Production and consumption of ethanol in Europe in 2005 (kt) (source: eBIO for ethanol and wine surplus, French Ministry of Agriculture for France, [BAL 06] for the conversion factors)
Among other countries, Spain has shown itself to the most dynamic in Europe. The UK consumes almost as much ethanol as France, only from imports. Sweden today consumes more ethanol than it produces because of the quasi-generalization of E5 since the beginning of 2003, and because of the development of E85 for some 50 service stations. Finally, it should be noted that between 2004 and 2007, the production of ethanol fuel in Europe went from 500,000 to 1,700,000 tons. The use and the taxation of biofuels in Europe today are based on several European community regulations: – European Directive 98/70/CE on the quality of fuels authorized the incorporation into gasoline of up to 5% ethanol and of up to 15% ETBE (see Directive 85/538/CE), and of up to 5% of FAME in diesel fuel, for small sales, sold at the pump. Higher rates are completely compatible with current engines, but this information must be advertised at the pump, – the directive on the promotion of biofuels (2003/30/CE) sets increasing objectives for biofuel consumption in the transportation area. This consumption should represent a least 2% in 2005 and 5.75% (percentages measured in energy) in 2010 of the total consumption of gasoline and diesel fuel used in transportation. These objectives are suggested and not required; nevertheless, the Member States must inform the Commission of the measures taken to reach them,
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– Directive 2003/96/CE on taxation has the goal of giving Member States the possibility of partially or completely exempting excise taxes on biofuels. The fiscal policies of biofuels remain the responsibility of each country. Today, many Member States have obtained an exemption, allowing the elimination of taxation on biofuels; this can go from 30% to 100% of excise taxes enacted on petroleum fuels. 10.1.4.1. The influence of the Common Agricultural Policy (CAP) Besides the European Directives already cited and the particular fiscal policy of each of the Member States, the CAP has had and continues to have a significant impact on the economics of biofuel systems. In fact, in 1992 a reform of the CAP was decided upon. The objective, to get production under control, was put into effect by using a double mechanism: a reduction in the guaranteed intervention price and the initiation of direct grant on site (on the basis of average yields). For the large cultivation sector (grains, mainly oleaginous), access to these direct aids was permitted on condition of a land freeze (from the notion of fallow land bonus). The sugar beet–ethanol system does not enter into the picture (system of price guarantees per quota about 42 €/t). The introduction of the notion of industrial freeze opened the possibility of collecting this grant in the case of putting these lands into cultivation for non-food purposes. From the campaign of 2000-2001 (Berlin accord), this freeze was fixed at 10% of cultivated land, and the sum of the compensatory aide at 63 €/ton (or about 350 €/ha for an average location), for grains and oleaginous plants. The goal of these grants is to guarantee compensation for the user’s loss of revenues. By way of illustration, the freeze on these lands represents about 6 million hectares in Europe. In France, this is nearly 1.5 million hectares which have been laid fallow. Thus, part of the lands has been virtually “reserved” for the production of biofuels. The future of these systems is thus directly linked to the CAP, since a modification of the amount of fallow land or simply of the sum total of the compensation grant will have a direct impact on the attractiveness for a user to develop this non-food cultivation. A few years ago, a new orientation of the CAP was decided for application from the beginning of 2005 to 2013. The stated goal for this reform is to separate the grants from the level of production by orienting the products toward less quantity and more quality. Each grower will receive a unique grant for his production. Its dispersal is subject to meeting the regulations for environmental standards and public health. The cost of the aid is calculated on a historical basis. In other respects, a grant of 45 €/ha was accorded for energy cultivation. It is justified by the beneficial effect in terms of greenhouse gas these cultivations bring. This grant is capped each year for the entire Europe of the 25.
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It should be noted that at the end of 2005 the European Commission issued a biomass action plan, a large part of which concerns biofuels. In 2006, the following actions were undertaken: – the European Directive of 2003 for the promotion of biofuels will be revised with the objective of studying more precisely the possible establishment of a requirement to incorporate biofuels into petroleum fuels, the revision of the objectives to be reached per country, the necessary establishment of certification procedures for a durable production of biofuels on the entire system, – the Member States will be encouraged to promote the use of second generation biofuels from lignocellulose (wood, straw), – a proposal for a legal arrangement for the promotion of the purchase of vehicles capable of operating with a large biofuel content is to be accomplished, – a study of the potential for reducing the CO2 emissions linked to the use of biofuels in dedicated fleets is to be carried out, – within the framework of the negotiations on international commerce, the conciliation of endogenous production and the possibility of biofuel imports should be pursued, – a modification of the current specifications on biodiesel will be proposed in order to allow use of a wider spectrum of resources than currently used (essentially canola oil) and to allow use of esters of ethanol in place of methanol in the process, – a study of the current limits of incorporation of biofuels in petroleum fuels (limit on the vapor pressure for ethanol and limit on the content of biodiesel for diesel fuel) is to be carried out, – the petroleum industry must justify, from the technical point of view, current practices for the use of biofuels in the mixture that can be interpreted as barriers to their diffusion. The practices of these industries will be monitored to assure the absence of discriminatory action for biofuels, – access to the European market for biofuels for developing countries should be encouraged within the framework of international trade agreements underway. A new factor in the world energy environment is the return to high barrel prices; for some years, many other countries have foreseen the launching of ambitious national programs favoring biofuels. A number of Latin American countries can be cited that are following the Brazilian example for ethanol. China and India are also in position to implement a policy promoting the use of biofuels, especially ethanol, in the gasoline mixture, with contents of between 5 and 10%. The main palm oil producing countries, Malaysia and Indonesia, should not be overlooked, and especially support the use of biofuels by fixing incorporation objectives of from 2 to 5% of the consumption of diesel fuel until 2008–2010.
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10.1.5. Prospects To find solutions to the problems of cost, limits in production volumes (competition with land use for crops) and management of volumes of coproducts of biofuels, Research & Development work is underway to devise new systems. They use lignocellulose materials (wood, straw), a more abundant resource and cheaper than food crops. Two options are considered: one that produces ethanol, developed in recent years, especially in North America, and one that allows the production of biofuel from synthesis by the Fischer–Tropsch (FT) process, mainly considered in Europe. These two systems could furthermore be combined on a single industrial site, easily assimilated by a biorefinery, the waste products from the ethyl model serving to supply the part that is gasification-synthesis FT. This industrial combination would thus generate a gasoline fuel and a diesel fuel at the same time, as do today’s refineries. This option also allows production of “biokerosene” to feed the air transportation sector, which offers the opportunity of reducing greenhouse gas emissions in this area where the alternatives to petroleum remain limited. 10.2. Current systems 10.2.1. Biodiesel systems Biofuels adapted to diesel engines are currently produced, especially in Europe, from vegetable oil from rapeseed or sunflower. Formerly, this oil, after purification, could be used directly as a fuel, notably in the first diesel engines at the beginning of the last century. Since then, the progress made in these technologies and the performance of the engines have demanded stronger requirements in fuel quality. This is why, apart from limited specific applications, vegetable oil must be modified in order to be incorporated into fuel for automobiles. 10.2.1.1. Raw materials Vegetable oils that are often still used exclusively for food are obtained from grains of various oleaginous agricultural raw materials. The world production is higher than 100 Mt/year. The most abundant of these oils are soy and palm oils (> 60%), followed by rapeseed and sunflower (> 20%). Grains are first triturated, then oil obtained by extraction is refined. For fuel applications, this refining is somewhat simplified, notably by eliminating the decolorizing and deodorizing stages.
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The main subproduct is oil-cake, composed of proteins, usable for animal feed (Table 10.3). Soy
Palm
Rapeseed
Sunflower
Agricultural yield (grains) (t/ha)
3.4
30
3.1
1.7
Oil (t/ha)
0.7
6.3
1.2–1.3
0.7
Oil-cakes (t/ha)
2.7
–
1.8–1.9
0.6–0.7
Table 10.3. Grain, oil and oil-cake yields from various agricultural resources (according to [BAL 06])
Vegetable oils are fatty or liquid substances composed in a very large part of triglycerides (> 95%). These glycerides consist of one molecule of glycerol esterified with three (tri) molecules of fatty acids. Depending on the agricultural source, the fatty acids are differentiated by the length of the carbon chains and their degree of saturation (Table 10.4). It is this chain length that makes the product compatible with potential use as a diesel biofuel. Fatty acid
*C16:0
C18:0
C18:1
C18:2
C18:3
Palm (%)
Stearic (%)
Oleic (%)
Linoleic (%)
Linolenic (%)
Rapeseed
5
2
59
21
9
Soy
10
4
23
53
8
C22:0
Sunflower
6
5
18
69
< 0.5
C22:1
Palm
44
6
38
10
< 0.5
C14:0
Others C20:1 C22:1
* CX: Y – X is the number of carbon atoms and Y is the number of double bonds in the fatty chain.
Table 10.4. Composition of vegetable oil fatty acids
10.2.1.2. Production processes To manufacture a biodiesel fuel, the vegetable oil must be made to react with methanol to transform it into an “ester”, a methyl ester of vegetable oils (FAME or biodiesel), better known in France by the name of diester®. The main byproduct is glycerol (Figure 10.5).
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Figure 10.5. Production of biofuel from rapeseed grain (source: IFP)
10.2.1.2.1. Methanol production Methanol is a chemical product obtained from the gas from the synthesis CO + H2. This gas itself essentially comes from natural gas: methane. Methanol is therefore not a renewable resource. It is most often used today for the manufacture of formaldehyde, acetic acid and methyl terephthalate, but also for MTBE, which is a fuel. 10.2.1.2.2. The transesterification reaction Biodiesel fuel, or methyl esters of vegetable oils (FAME), is obtained by the transesterification of triglycerides with methanol, in the presence of a catalyst, according to the balanced reaction given in Figure 10.6.
Figure 10.6. Transesterification of a vegetable oil
10.2.1.2.3. Homogenous catalysis processes These are the processes that are most frequently used and up until 2006 were the only industrial processes.
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Several types of catalysts are possible: alkaline, acidic and others. The acid catalysts are not very active and can pose problems of corrosion in installations. The industrial catalysts most frequently used are alkaline: soda and sodium methylate. The processes may be discontinuous or continuous. The oldest processes, discontinuous, which are limited in capacity, are generally more expensive in terms of investment and especially in operation. This is why the new processes are generally continuous and reach more than 100,000 t/year or even 200,000 t/year of production capacity in biofuel. In both cases, the reaction principle is the same (Figure 10.7).
Figure 10.7. Main reaction of biofuel production by homogenous catalysis
Vegetable oil, the excess methanol and the catalytic solution are mixed in one or more reactors in series under low pressure (<3 bars) and at moderate temperature (45 to 90ºC). The residence time is optimized to reach thermodynamic equilibrium. It is necessary to separate the methyl esters from the glycerol by decantation and eliminate the salts from the catalyst, most often by a water bath. The catalyst and the glycerol are neutralized with a mineral acid and the water is distilled. The elimination of water allows a glycerol purity between 80 and 90% to be reached. The excess methanol is recovered and recycled (Figure 10.7). The efficiency in yield of esters (FAME or biodiesel) depends on the catalyst and the process, but is often higher than 98.5%. The losses are explained by the formation of alkaline soaps resulting from reactions between the fatty acids and the catalyst (Figure 10.8).
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Figure 10.8. Diagram of the principle of the process for production of biodiesel fuel with homogenous catalysts
10.2.1.2.4. Heterogenous catalysis This new process, which functions continuously, has been operational since 2006. The first industrial installation is located at Sète, in the south of France. The catalyst is an aluminate developed by IFP which is produced in the form of an extruded material (Figure 10.9). This catalyst is stable and, unlike the homogenous catalyst, it is not eluted with the other products and does not need to be extracted from the reactive milieu.
X A12O4 Figure 10.9. Heterogenous catalyst for biodiesel fuel production (source: IFP)
Using such a heterogenous catalyst offers the advantage of eliminating expensive treatments for neutralizing the acid and for purification. The principle of the process
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is simpler (Figure 10.10) and the glycerol recovered by simple decantation is of higher purity than that coming from the homogenous process (> 98%).
Figure 10.10. Diagram of the principle of the process for production of biodiesel fuel by heterogenous catalysis (source: IFP)
The reaction temperatures are higher than those used with homogenous catalysts, from 180 to 220ºC, for pressures between 40 and 60 bars. Two transesterification reactors are installed in series. The excess methanol is eliminated by partial evaporation between the two reactors, allowing a change in the chemical balance, and the yield approaches 100% (Figure 10.11). The final excess of methanol is eliminated by distillation before recycling. The glycerol obtained is sufficiently pure to be used directly in many applications.
Figure 10.11. Industrial heterogenous catalysis process for production of biodiesel fuel (source: IPF/Axens)
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10.2.1.2.5. Use of biodiesel FAME in fuels [BAL 06, KNOT 05, MIT 04] The main physical properties of methyl esters of vegetable oils (FAME) make them compatible with use in diesel engines. This is not the case for pure vegetable oils (Table 10.5). In fact, biodiesel has a very good cetane index and a density close to that of oil-based diesel fuels. The viscosity, which has an impact on wear and tear and engine performance is also suitable. The presence of oxygen in the molecule favors good combustion, reducing particle emissions and unburned hydrocarbons. Finally, the lubricating power of biodiesel mixed with low sulfur diesel has been demonstrated. Biodiesel also has disadvantages, such as a lower energy content than that of diesel, a stability in time (thermal degradation and oxidation) that is highly dependent on the starting oil and its degree of unsaturation. Its characteristics when cold are close to those of classic diesel fuels but they are incompatible with extreme cold (below í20ºC). Sometimes there are higher emissions of nitrogen oxides [KNOT 05]. The advantages and disadvantages of biodiesel vary according to the rate of incorporation in diesel fuel. The most studied and best known are respectively 5% (B5) and 30% (B30), for which vehicle operation was shown to be very good. Methyl esters of vegetable oil are subject to a specific European standard: CEN 142 14. Diesel fuel
Methyl esters of rapeseed
Rapeseed oil
820–860
880–885
920
Viscosity at 40 °C (mm2/s)
2–4.5
4.5
30.2
Cetane index
> 49
50
35
Flash point (°C)
> 100
170–180
decomposition above 320°C
Specific density (kg/m3)
Table 10.5. Main physical properties of diesel fuel, rapeseed oil and VOME (according to [BAL 06])
10.2.1.2.6. Alternative paths, other processes, other raw materials By using ethanol in place of methanol in the manufacture of esters, researchers have produced an ethyl ester of vegetable oil (FAEE), an attractive solution for producing a biofuel 100% from biomass, allowing the introduction of agricultural
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ethanol into diesel fuel. Nevertheless, this approach requires significant modifications in the processes because of the differences in chemical and physical properties of the two alcohols. With respect to engines, FAEE has already been tested and proved to be satisfactory. Animal fats, lard, tallow and other byproducts of slaughter have chemical compositions close to vegetable oils and are most often less expensive fats. Depending on their origin, these products have impurities, but their use for producing methyl (or ethyl) esters is entirely possible. However, it should be noted that the resource remains limited: in France it is estimated to be a few hundred thousand tons. Vegetable oils and animal fats can also be used to produce a diesel fuel by a route other than transesterification. This involves using hydrogen to eliminate the carboxylic groups from the triglycerides at a temperature of about 300ºC. We then obtain paraffin carbonaceous chains, propane, water and some byproducts. Catalytic hydrotreatment is a process already used with petroleum hydrocarbons. It is thus already planned to use a refinery, which disposes of hydrogen, either with a process dedicated to vegetable oils or animal fats, or on an existing process such as a mixture with petroleum components. The necessity of using hydrogen and the low added-value of the byproduct formed, propane, are the main disadvantages to this approach. 10.2.2. The bioethanol system The possibility of producing ethanol by fermentation has been known since antiquity, and its use as a fuel was already foreseen by Henry Ford when he created the celebrated Model T. However, in the past few years, first in Brazil and now in the USA, what is currently called bioethanol, as opposed to petrochemical ethanol, has seen a remarkable resurgence of interest, since it is the main biofuel produced and consumed in the world. It is intended essentially for engines with controlled ignition, either after direct incorporation (for example, in Brazil, the USA and Sweden), or after transformation into ethyl tert-butyl ether or ETBE (for example, in Spain and France). 10.2.2.1. Raw materials Two types of agricultural resources are currently transformed into ethanol: – sugar plants such as sugar cane or sugar beets, which contain saccharose, – starchy plants, such as corn or wheat, the grain of which mostly consists of starch.
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Saccharose is a dimer of glucose and fructose, while starch is a homopolymer of glucose molecules connected by osidic linkages in Į-1.4 with branches in Į-1.6. The production regions of these vegetables are dependent upon climatic, agronomic and economic conditions. The final yield in ethanol is essentially a function of the source considered and its content in sugars convertible to ethanol, also called fermentables (Table 10.6). Sugar cane
Sugar beet
Corn (grain)
Wheat (grain)
Agricultural yield (t/ha)
80–90
65–75
7.2–8.5
7.2–8.5
Content in sugar or starch (% mass)
14–15
15
72
62–65
Yield in ethanol (t/ha)
5.6–6.4
5.2–5.6
2.3–2.7
2.1–2.5
Table 10.6. Ethanol yield from various agricultural resources (according to [BAL 06])
10.2.2.2. Production procedures 10.2.2.2.1. Different stages Ethanol production processes consist of three stages: transformation of the raw material into fermentable sugars, their fermentation into ethanol, and the extraction of the ethanol from the fermentation broth, mainly by distillation. Sugar plants and starchy plants lead to processes that differ in the first stage upstream from the ethanol fermentation [POU 06]. In the first case, it is a matter of extracting the saccharose, itself fermentable, while, in the second case, it is necessary to hydrolyze the starch into glucose. Another characteristic of these processes is that they cause byproducts from the non-fermentable fraction of the raw material, byproducts of a variable nature depending on the source used and the transformation procedure. In all cases, the key stage is the ethanol fermentation in the course of which the sugars are converted into ethanol and CO2 according to the following equation: C6H12O6 ĺ C2H5OH + 2CO2 The theoretical weight yield of the reaction, still called the Gay-Lussac yield, is 0.51. However, this theoretical yield is never reached since one part of the sugar is used for the growth of microorganisms and another part is converted into byproducts such as glycerol, succinic acid and some higher alcohols (fusel oils). For this reason, the maximum weight yield, or Pasteur yield, is 0.48, or 94.7% of the theoretical yield. The microorganisms of choice for this fermentation are yeasts of the
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Saccharomyces type, more particularly of the species Saccharomyces cerevisiae, which have as their main advantages: – their tolerance for strong ethanol content (increase up to 10 to 12% v/v and viability up to 15%), – their resistance to high osmotic pressures and relatively acid pH, – their acceptance as GRAS (Generally Recognized as Safe) microorganisms, – their capability of being easily separated from the fermentation broth, – their stability (constant physiological characteristics) and their robustness, which make them microorganisms easily capable of being used on an industrial scale. Saccharomyces use most of the hexoses (C6 sugars such as glucose), but also several disaccharides such as saccharose. Besides the use of selected strains, the bioethanol-producing manufacturers have improved processes that are now often continuous, sometimes multistage, processes, in order to reduce the size of the reactors and to increase productivity. An important goal is to obtain the most concentrated possible fermentation broth to lower the distillation cost. The last stage consists of extracting ethanol from the fermentation broth, then, in most cases to dehydrate it, notably when it is supposed to be mixed with gasoline. The first stage of classic distillation leads to the azeotrope (96% ethanol, 4% water), and is followed either by azeotropic distillation using a solvent carrier such as cyclohexane, or dehydration on a molecular sieve. The distillation of ethanol has been the object of multiple improvements aimed at reducing its energy cost, for example distillation in multieffect vacuum or mechanical recompression of vapors. 10.2.2.2.2. Ethanol production from sugar plants Figure 10.12 presents successive unitary operations leading from sugar cane or sugar beets to ethanol. The extraction of the saccharose present in these vegetable cells is identical to that used in the sugar industry and is done either by pressing or using a hot water bath on the grain. A stage to concentrate the juices obtained is often included to improve their preservation before fermentation. The residues of distillation, or vinasse (stillages), are rich in minerals and can be a good soil additive. The bagasses represent the lignocellulose part obtained after extraction of the saccharose from the cane. They are mainly used to cover the energy needs of the production units for ethanol from sugar cane and thus contribute greatly to a
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favorable CO2 footprint for this system. They can even contribute to supplying energy for external use.
Figure 10.12. Diagram of the process for ethanol production from sugar plants
The production of ethanol from sugar beets follows the same diagram, obtaining pulp in place of bagasse. However, sugar beets produce much lower quantities of lignocellulose residues. In other respects, let us note that the cultivation of sugar beets is limited geographically to several regions of Europe. 10.2.2.2.3. Ethanol production from starchy plants The starch content in grains of wheat or corn is made of D-glucopyranose (glucose) units essentially connected by glycosidic linkages that must be hydrolyzed in order to produce a substrate usable by yeast. This stage is accomplished with the aid of enzymes, enzymatic hydrolysis completely replacing the chemical hydrolysis using strong acids previously used. This hydrolysis is carried out in two stages: a) liquefaction by the action for about 1 hour at high temperature (100–105ºC) of Į-amylases that cut the starch chains in a random fashion, b) hydrolysis by the action for 4 to 5 hours of glucoamylases that free glucose from the oligomers produced by the liquefaction. From a more global point of view, there are two ways of producing ethanol from corn-grain: dry-milling and wet milling (Figure 10.13). In dry-milling processes, the grains are ground before mashing and hydrolysis. In wet-milling processes, which
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come from starch production, the lipid fraction (oil coming from the germ) and protein fraction (gluten) are extracted in stages before hydrolysis. At present, the new processes for the production of ethanol fuel are mainly dry-milling processes, which are most easily implemented. The result is a byproduct, Distiller’s Dried Grain with Solubles (DDGS), which comes from the stillage and is used as animal feed.
Figure 10.13. Diagram of ethanol production from corn grain
10.2.2.2.4. The use of ethanol in fuels The main physical properties of ethanol (Table 10.7) make it compatible with use in engines with controlled ignition. In fact, it has a very high octane index and a density close to that of gasoline. The presence of oxygen in the molecule favors good combustion. Ethanol also has disadvantages, such as a lower energy content than gasoline, the formation of azeotropes with light hydrocarbons that increase the vapor pressure of the mixture, a high miscibility with water that can cause problems of de-mixing in mixtures with hydrocarbons. In other respects, its high latent heat of evaporation has a beneficial effect on the efficiency of filling the combustion chamber, but it can cause difficulties in starting in the cold. Finally, its use imposes supplementary constraints on the materials.
Biofuels
Ethanol Molecular mass (g/mol)
ETBE
357
Gasoline (standard)
46.07
102
102.5
Volume mass (kg/m )
794
750
735–760
Latent heat of vaporization (kJ/kg)
854
321
289
Boiling point (°C)
78.4
72.8
30–190
26,805
35,880
42,690
RON4
111
117
95
MON5
92
101
85
3
Lower heating value (kJ/kg)
Table 10.7. Principal physical properties of ethanol and ETBE (after [BAL 06])
These potential disadvantages depend on the ethanol content in the gasoline. The European automobile fleet is compatible with use of fuels containing 5% ethanol (E5). In the USA, the ethanol content in gasoline can go up to 10% (E10), and in Brazil mixtures containing about 24% ethanol (E24) are distributed. The use of fuels with a high ethanol content, for example, mixtures of 85% ethanol (E85), make it possible to solve some of the problems already mentioned. These fuels can be used for dedicated vehicles, as in Brazil or Sweden, in dedicated fleets that can run on pure ethanol. Furthermore, some automobile manufacturers produce Flexible Fuel Vehicles (FFV) that can run equally well with all kinds of mixtures going from E85 to pure gasoline. This emerging market has had great success in Brazil and Sweden, and is beginning to be proposed by US automobile makers. The solution adopted by some European countries, notably France and Spain, is to convert ethanol into ethyl tert-butyl ether or ETBE. ETBE is obtained by adding ethanol to isobutylene. It contains 45% ethanol (by mass) and the incorporation of 5% ethanol in the gasoline corresponds to 11% (v/v) of ETBE. By replacing ethanol with ETBE, the problems of de-mixing with water as well as those associated with the high latent heat of vaporization are resolved and, in addition, ETBE has very favorable characteristics for fuel applications. Furthermore, the incorporation of 4 RON: research octane number. 5 MON: motor octane number.
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ETBE has a beneficial effect on the emission of exhaust pollutants and hydrocarbons by evaporation. However, without contesting the virtues of ETBE, it must also be admitted that the problems associated with the direct incorporation of ethanol can find solutions other than conversion to ETBE (see the USA, etc.). In other respects, conversion to ETBE requires a product coming from fossil sources whose availability probably is not equal to the ethanol production foreseen and which increases the global CO2 balance. Finally, ETBE could itself be subject to environmental constraints similar to those imposed on MTBE (methyl tert-butyl ether), as the source of aquifer pollution that led to its progressive abandonment in the US and to its replacement by ethanol. 10.3. Future systems: use of lignocellulose 10.3.1. Characteristics of components in vegetable lignocellulose Lignocellulose biomass represents the most abundant renewable source of carbon on our planet. It is composed mainly of the three polymers of the vegetable cell wall: cellulose, hemicellulose and lignin. These are present in variable proportions according to the plant under consideration (Table 10.8). They are tightly connected to each other in the two different layers of the wall, thus forming a rigid matrix difficult to take apart [ODO 06]. Cellulose (%)
Hemicellulose (%)
Lignin (%)
Softwood
35–40
25–30
27–30
Hardwood
45–50
20–25
20–25
Wheat straw
33–43
20–25
15–20
Table 10.8. Polymer composition of lignocellulose plants
Cellulose is a linear homopolymer of D-glucopyranose units connected by ß-1.4 osidic linkages (Figure 10.14). The degree of polymerization (DP) depends on the origin of the cellulose, and it is generally between 1,500 and 10,000, even 15,000 in cotton fibers. There are hydrogen linkages between the adjacent glucose molecules, so that each of them is positioned at 180º with respect to its neighbor, resulting in a ribbon structure that is very stable. The cellulose chains are connected among themselves by other hydrogen linkages to form microfibrils of cellulose. The microfibrils are joined as fibrils to
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form cellulose fibers. In the cellulose chains, amorphous zones can alternate with crystalline zones. The higher the degree of crystallinity, the more resistant the cellulose is to chemical reactions [MAN 99].
Figure 10.14. Structure of cellulose
The hemicelluloses are heteropolysaccarides of lower DP than cellulose, linear or branched and easier to hydrolyze than cellulose. Depending on the sugar components (hexoses, pentoses, uronic acids), these are mainly galacto-glucomannans, arabino-glucuro-xylans or glucurono-xylans. The hemicelluloses are often adsorbed on the cellulose microfibrils and form covalent linkages among themselves, notably by the intermediary of ferulic acids (ester linkage) in the case of grasses, and with lignin. Lignin is a heteropolymer that is strongly cross-linked, made of three aromatic alcohols: coniferyl, coumaryl and sinapyl alcohol (Figure 10.15). They form amorphous heterogenous structures that are joined to hemicelluloses via ferulic acid and thus form a cement at the heart of the lignocellulose material. The composition of the lignocellulose materials is very variable depending on the plant under consideration (Tables 10.9 and 10.10). For this reason, chemical and/or thermal treatments free different compounds, of which certain are easier to add to or to transform than others. As a result, the processes of fractionation or transformation of the plant should be based on the raw material to be treated.
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Figure 10.15. Chemical structure of lignin. Shown in the frame are the precursors: coumaryl alcohol (R1 = R2 = H), coniferyl alcohol (R1 = OCH3; R2 = H), sinapyl alcohol (R1 = R2 = OCH3) (adapted from [KIR 87])
Spruce Poplar (P. deltoid) Birch Wheat straw Corn stalks/leaves Cane bagasse
Glucan
Xylan
Mannan
Arabinan
Lignin
Ashes
42
6
14
1
27
1
34
12
2
-
22
<1
38 37 36
19 18 19
1 0 <1
2 3
23 18 12
1 6 5
43
21
-
1
19
1
Table 10.9. Composition of sugars in lignocellulose plants, as a percentage with respect to the dry material [PUL 93]
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10.3.2. The BtL system Biomass to Liquid (BtL) is a so-called thermochemical method which consists of a succession of operations intended to produce, in the future, synthetic fuel from biomass. It has four main steps: conditioning of the biomass, gasification, treatment of the synthesized gas and synthesis of the fuel (Figure 10.16). Some of these operations have already been demonstrated for producing an automobile fuel from coal (CtL) or natural gas (GtL). Gasification was in practice during the Second World War, especially in Germany, to counter the difficulties of supply and the high price of oil. Gasification from coal in South Africa allowed them to confront the oil embargo during apartheid. The current energy environment, the eventual exhaustion of the oil supply and the desired limit of CO2 emissions from fossil sources is leading toward the strengthening of research to produce a synthetic automobile fuel of renewable biological origin. The great appeal of this approach is that its theoretical capability accept all the forms of biomass which, despite their wide dispersion, are abundant on the planet.
Figure 10.16. Diagram of the principle of gasification from biomass associated with the process of fuel synthesis
10.3.2.1. Main constraints of the process Unlike coal and natural gas, biomass contains a high amount of oxygen (> 40%), which reduces its energy density by as much. Furthermore, lignocellulose materials, the agricultural or forest vegetable products, wood and wastes, are found in very diverse physical forms (Figure 10.17) which should be pretreated or conditioned in order to transform them in an industrial gasification process.
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Figure 10.17. Diversity of the usable lignocellulose resources
The gasification technologies that are already widely in use for producing heat or electricity from coal and petroleum products were not designed to accept lignocellulose products. They are not necessarily adapted for producing gas of a composition compatible with its transformation into fuel. A fairly complex treatment of the synthesized gas (CO + H2) is a necessary step to adjust the ratio H2/CO acceptable for its transformation into fuel and to eliminate the impurities and inert gases (CO2). The process of fuel synthesis, called Fischer–Tropsch, has been used on the industrial scale for application to a synthesized gas from methane or coal, but has not yet been commercialized for gas from biomass. 10.3.2.2. Conditioning of biomass This step has the objective of transforming the vegetable resource into a material that is homogenous and injectable into a gasifier. Although tests have been conducted for injecting raw biomass that was simply dried and ground, it is most often necessary to use thermal and mechanical transformations that lead either to a liquid/solid mixture or slurry, or to a finely divided solid. There are two main methods: pyrolysis and roasting. 10.3.2.2.1. Pyrolysis Under the action of heat, the cellulose, hemicellulose and lignin undergo primary cracking, rupture of carbon–carbon and carbon–oxygen bonds, followed by secondary recombination reactions. Pyrolysis thus leads to a partial decomposition
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of the carbon products of the biomass in three phases: solid (coal), liquid (bio-oil), and gas (mainly CO2, CO, H2 and CH4). The distribution among these three phases depends mainly on the temperature, the speed of heating (Table 10.10) and the residence time. Speed of heating
Temperature
Majority product (material output)
Energy output
Slow pyrolysis
< 50°C/min
350°C
carbon (< 35%)
60%
Rapid pyrolysis
> 100°C/s
500°C
liquid (50–80%)
75%
Pyro-gasification
> 100°C/s
> 800°C
gas (> 70%)
80%
Table 10.10. Characteristics of different types of pyrolysis (energy output: energy content of the products/energy content of the biomass)
The best known application of pyrolysis is the manufacture of charcoal, carried out with low heating rates (< 50ºC per minute), a temperature of about 350ºC and residence time of several hours. This technology, also called carbonization, is attractive since it produces a solid that could, after grinding, be finely decomposed in order to be injected into a gasifier. Unfortunately, it is a slow pyrolysis whose output of materials and energy is fairly low. In a more recent development, rapid pyrolysis produces mostly liquid or bio-oil (Figure 10.18), with greater or smaller quantities of coal (Table 10.11). Such a liquid or liquid/solid mixture is, a priori, easily transportable and injectable into a gasifier. The conditions required to maximize their production consist of imposing high densities of heat flow on the biomass and limiting the secondary reactions of cracking of the vapors in the gaseous phase by minimizing their temperature and their residence time.
Figure 10.18. Principle of rapid pyrolysis at 500ºC for one second
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To begin with, the biomass should be ground up to some degree. The temperature is always around 500ºC, and the residence time around one second. The effluents are rapidly cooled to the ambient temperature. Several technologies studied at the pilot level have been proposed: fluidized bed (Dynamotive), circulating fluidized bed (Ensyn, VTT), cutting cone (Twente University, BTG), cyclone (TNO) and twin vice ablative reactor (Pytech). In terms of output, the different processes have similar performances (Table 10.11). To be adapted to final fuel production, these technologies should be able to be scaled-up to several tons or tens of tons per hour. They should also be autonomous in energy, for example capable of using the gas produced to generate the necessary heat for the pyrolysis. Output in bio-oil (%)
Output in gas (%)
Output in coal (%)
Dynamotive wood fluidized bed
70
13
17
Dynamotive straw fluidized bed
58
24
18
Process
Load
Technology
FZK
straw
double screw
54
22
25
TNO
wood
cyclonic
68
15
17
BTG
wood cutting cone
70
15
15
Table 10.11. Technologies and estimated outputs of different processes of rapid pyrolysis
The fluidized bed and the circulating fluidized bed are the most widespread technologies on the pilot scale. They are probably the easiest to scale-up. Dynamotive already makes use of one installation of about 400 kg/h (Figure 10.19). An alternative solution developed by the University of Twente and BTG is based on the effect of the centrifugal force generated by the rotation of a cone. BTG uses an installation of 200 kg/h that seems difficult to scale-up. However, an industrial plant with a capacity of 50 tons/h should be built in Malaysia.
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Figure 10.19. Diagram of the principle of the Dynamotive process
Because of the high presence of water (from 20 to 50%) and oxygen, the bio-oils have a heating power close to biomass, about 18 kJ/kg. Their mass volume, about 1,200 kg/m3, is, on the other hand, higher than that of wood, 600 kg/m3, and much higher than that of straw. They contain several hundreds of different chemical products in very variable proportions, notably phenols, sugars, alcohol, organic acids, and aromatic compounds. They have the property of not being completely miscible with either water or with petroleum hydrocarbons. It should be noted that research is being carried out to improve these oils by hydrogenation in order to make fuel. This direct method, even if it is attractive, would appear to be difficult because of the quantities of hydrogen necessary, and especially because of the chemical nature of the compounds, which is very far from that of classic automobile fuels. 10.3.2.2.2. Roasting (“torrefaction”) Wood that has been thermally transformed (roasted) has been the object of several studies for specific applications since the 1980s. First considered as a substitute for charcoal, these studies then showed that the resulting alteration of wood treated in this manner, especially for the least desirable species, had very good resistance to attack by fungi and certain insects. In fact, this treatment makes the material more durable and therefore makes it suitable for house construction but it also makes it less resistant to grinding. This property would be very attractive for obtaining a finely cut solid adapted to certain gasification technologies.
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Roasting, also called “cross linking”, can be similar to a final drying. It is characterized by a succession of thermal reactions obtained in the course of the progressive increase in temperature to a final level between 240ºC and 300ºC. The residence time is from several minutes up to one hour, depending on the temperature. At the end of the operation, the wood should be kept under an inert atmosphere until it has cooled down to the ambient temperature. Carried out at much lower temperatures, roasting is therefore a process requiring much less energy than pyrolysis. It is the Dutch research organization ECN, the Energy Research Center of the Netherlands, that has most recently promoted in Europe interest in heat-treated wood for energy applications. The research has the objective of cocombustion of thermally transformed wood in an electric power plant. Roasting is a method of preparation that is simple and attractive in terms of energy (Table 10.12), since the combustion of the small amount of gas produced supplies the necessary heat for the process (Figure 10.20). Currently work is being conducted to optimize the different stages of roasting, grinding, and injection into a gasifier, notably those functioning under pressure.
Figure 10.20. Diagram of the principle of the roasting process (source: ECN)
Firing speed Thermo-transformation < 50°C/min
Temperature
Majority product (material output)
Energy output
240–300°C
solid (> 85%)
> 90%
Table 10.12. Characteristics of wood roasting (energy output: energy content of the products/energy content of the biomass)
10.3.2.3. Gasification [KNOE 05] Unlike pyrolysis, which takes place in the absence of a reacting gas, gasification takes place in the presence of a reactive gas (water vapor, oxygen) and produces a
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synthesis gas (“syngas”) containing mainly H2, CO, CO2, H2O, CH4, but also, unfortunately, carbon or inorganic impurities. The mixture thus obtained is at present used industrially, from coal or petroleum, for combustion in cogeneration power plants for electricity and heat. Such a mixture has been used in the past for automobile applications; it was known as “gazogen”. In the framework of a “BtL” model, which has the goal of producing a liquid fuel for the Fischer–Tropsch catalytic reaction, the constraints on the synthesized gas are stricter than for cogeneration. The maximum output of CO and H2 is desired, of course, but with a ratio H2/CO adapted to the synthesis of the fuel. It is necessary to minimize both CO2 and CH4 production that are inert for fuel production and, overall, to eliminate the impurities that poison the Fischer–Tropsch catalyst. 10.3.2.3.1. Principle of biomass gasification The thermal and chemical degradation of the conditioned biomass, solid or liquid, takes place in several stages, which are a function of temperature. Even in the absence of an oxidizing agent, above 600ºC the biomass decomposes thermally, mainly into gas but also into charcoal and mineral salts. In the presence of water vapor or oxygen, up to 900 or 1,000ºC a virtually complete oxidation of carbon into CO and CO2 is accomplished. Even at these temperatures, certain aromatic carbon products, called tars, are difficult to oxidize. In the absence of a catalyst, it is necessary to bring these products to about 1,200 or 1,300ºC to eliminate them (Figure 10.21). This temperature is also favorable for simultaneous elimination of methane.
Figure 10.21. Principle of gas decomposition and gasification of biomass (source: CEA/IFP)
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By a more attentive examination of the chemistry occurring, we can state that several types of reactions take place simultaneously. They are distinguished by their homogenous character (gas phase) or heterogenous character (gas, solid phases), but also by their exothermal character (heat production) and endothermal character (heat consumption). By way of a first approximation, we can consider the heterogenous reactions as being those based on carbon (Table 10.13). In the heterogenous phase, carbon is transformed into a mixture of CO, H2, CO2, H2O and CH4, since these react among themselves in the gaseous phase. Globally, the production of CO and H2 is endothermic and requires addition of heat obtained by combustion of the biomass as CO2 and H2O. These compounds, like CH4, represent losses of carbon and hydrogen for the production of fuel by the Fischer–Tropsch synthesis. Heterogenous reactions: C + O2 o CO2
Exothermal
Loss of carbon
C + ½O2 o CO
Exothermal
Gain in carbon
C + H2O CO + H2
Endothermal
Gain in carbon and hydrogen
C + CO2 2 CO
Endothermal
Gain in carbon
C + 2H2 CH4
Exothermal
Loss of hydrogen
CO + ½O2 oCO2
Exothermal
Loss of carbon
H2 + ½O2 o H2O CO + H2O CO2 + H2
Exothermal
Loss of hydrogen
Exothermal
Loss of carbon, gain in hydrogen
CH4 + H20 CO + 3H2
Endothermal
Gain in carbon, gain in hydrogen
Homogenous reactions:
Table 10.13. Reactions (simplified) occurring at the time of gasification
In other respects, a loss of hydrogen is produced to the detriment of CO by the reaction called the water gas shift: CO + H2O CO2 + H2 The energy needed for gasification (combustion) and the water gas shift reaction produce a large amount of CO2, which is a significant loss of carbon (Table 10.13). This is the main explanation for the low output in possible synthetic fuel (< 20%). This output could be increased only by addition of external energy, for example electrical, a solution proposed in France by the CEA [CLA 06], and/or an addition of hydrogen.
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10.3.2.3.2. Possible technologies for biomass gasification No specific biomass gasification technology has come on the industrial stage. Commercial processes produce syngas from methane, coal or oil. In certain cases, addition of vegetable or even animal wastes have been gasified (for example in Puertollano, Spain) to produce energy. The processes proposed for biomass have come from those developed for fossil sources. Besides the generic aspects seen in the previous section, the main problems of biomass gasification for synthetic fuel are: biomass supply in solid or liquid form, the destruction of tars and the management of inorganic compounds. In the presence of a catalyst, it is theoretically possible to eliminate tars at temperatures lower than 1,200ºC. In fluidized bed and circulating fluidized bed technologies, heat transfer, and often a catalyst (olivine, dolomite, etc.), is used. The heat is brought into the heart of the gasification itself or in a separate reactor where the combustion takes place (Figure 10.22). The gasification temperatures are between 700 and 900ºC. Unfortunately, current performances of the catalysts are not sufficient to eliminate tars to an acceptable level for the fuel system. Moreover, above 900ºC, phenomena of agglomeration of the solid because of the fusion of ashes pose significant problems for the functioning of these technologies.
Figure 10.22. Diagram of the principle of a circulating fluidized bed reactor with separate combustion (source: CEA/IFP)
In the absence of a catalyst, the destruction of tars occurs only for temperatures greater than 1,200-1,300ºC. At these temperatures, obtained in entrained-flow technologies (Figure 10.23), the ashes (metal) are fused; they should be recovered at the bottom of the gasifier. Part of these ashes may also be vaporized and found in the synthesized gas itself. These technologies have the advantage of eliminating methane.
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The supply systems and the burners should be adapted to the characteristics of the pyrolysis oils or the finely ground solids of the roasting process. Today, entrained flow systems seem to be the best adapted for producing syngas for fuels. We recall in particular the ones that function with oxygen (absence of nitrogen) and under pressure (as is the case for the Fischer–Tropsch process). Such facilities should be designed with appropriate materials.
Figure 10.23. Diagram of the principle of an entrained flow reactor
10.3.2.4. Treatment of syngas Syngas from a gasifier can be adapted to the Fischer–Tropsch reaction. This involves purifying the gas, but also increasing the ratio H2/CO and reducing the CO2. We recall the following main steps: filtration, the water gas shift, stripping and, finally, the removal of particles (final filtering). On leaving the gasifier, the nature and the quantity of the possible impurities varies depending on the composition of the biomass (wood or straw) and the gasification technology. These are solid particles, organic products, nitrogen compounds, sulfur, halides, notably alkaline metals. Even in very small quantities, these impurities poison the water gas shift or the Fischer–Tropsch catalyst. They can also be corrosive to installations. Filtration eliminates solid particles, carbonates, refractory oxides, alkaline particles or aerosols. The technologies used are well known for certain compounds, but more experiments are needed to ensure accurate elimination of some, notably sodium and potassium.
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The water gas shift reaction: CO + H2O CO2 + H2 converts one part of the CO content in the syngas into H2 in order to obtain a correct H2/CO ratio. This is a catalytic reaction which, depending on the catalyst used, can also enable hydrolysis of part of the nitrogen and sulfur compounds. Stripping can be carried out using water, amines, methanol or other solvents. It involves eliminating acids such as HCl, HF, HCN, H2S, as well as NH3 and COS. The main product eliminated by these baths, using a chemical solvent (amine) or physical solvent (methanol) is CO2. The CO2 is inert on the Fischer–Tropsch catalyst, but its presence considerably increases the size of the installations and their cost. Finally, trapping solids used in a sufficient quantity will eliminate the last traces of gaseous pollutants in order to obtain a syngas with the specifications of the Fischer–Tropsch catalyst. 10.3.2.5. Fuel synthesis: Fischer–Tropsch and hydrocracking The Fischer–Tropsch reaction, named after two German chemists, was discovered in the 1920s. It enables the production of carbon chains of variable lengths that are then suitable for cracking to obtain diesel fuel and kerosene. This reaction is used industrially in South Africa after gasification of coal and, more recently, in Malaysia, from natural gas. The reaction is exothermal and can be written diagrammatically as follows: CO + 2H2 –CH2 + H2O Two types of catalysts are possible, based on iron or cobalt base [PAR 98]: – in the presence of an iron-based catalyst, the Fischer–Tropsch reaction is accompanied by the water gas shift: CO + H2O CO2 + H2. The synthesis is particularly oriented toward the production of alkenes; – in the presence of a cobalt-based catalyst, the synthesis produces many alkanes from ethane, C2H6 (two carbon atoms) up to C80H162 (80 carbon atoms). At ambient temperature, paraffins with a number of carbon atoms higher than 16 are waxes. To obtain a diesel fuel, it is necessary to adjust the length of the chain to between 10 and 20 carbon atoms. This catalytic operation, hydrocracking, also converts the linear chains into isomeric branched chains whose solidification temperatures are lower.
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The diesel fuel obtained in this way does not contain sulfur, nitrogen or aromatics. It has a cetane index of the order of 70, which is much higher than that of petroleum diesels, which are of the order of 50. Therefore, it is a fuel of very high quality for vehicles. Furthermore, its use allows significant reduction in polluting emissions compared with classic diesel. There are several commercial Fischer–Tropsch technologies, some of which have reached the industrial or pilot stage (Shell, Statoil, Sasol, Exxon, BP, Conoco, Rentech, IFP/ENI – Figure 10.24, Syntroleum).
Figure 10.24. General view of a Fischer–Tropsch pilot (source: IFP/ENI)
10.3.2.6. Conclusion The BtL system for producing automobile fuel from lignocellulose biomass by thermochemical means is supported by proven processes for coal, oil and natural gas. It is a question of adapting industrial gasification, used up until now for the production of heat and electricity, to biomass technologies. The processes of conditioning the biomass by pyrolysis or roasting are the subject of several research and development projects. This involves making the biomass homogenous, transportable, and easily injectable into a gasifier. These processes are not yet commercial, but are already the subject of pilot demonstrations in Germany (FZK), the Netherlands (BTG), Canada (Dynamotive) and Finland (VTT). Energy efficiency that is accompanied by a reduction of carbon losses will be an essential element of their growth.
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Even if significant research efforts are made to adapt gasification fluidized bed technologies to biomass, significant progress remains to be accomplished. The entrained flow technologies that work at high temperature, under oxygen, and, most often, under pressure, seem to be the best adapted to the BtL system. Some of them, those working under fossil load (Texaco, Lurgi, Shell, Conoco-Philips, Future Energy), should in the future be able to gasify biomass. Others, specifically developed for lignocellulose (Choren), should see a strong development in the future. The Fischer–Tropsch synthesis has been known for a long time. Commercial units operate in South Africa and Malaysia using coal and natural gas. It is now a matter of purifying the syngas of specific impurities that come from biomass, tars and inorganic materials to be in a position to produce a large quantity of high quality biofuel, environmentally clean and adapted to the vehicles of today andthe future. To achieve effective production of such a fuel by 2015–2020, all of these operations should be optimized in order to minimize energy losses, maximize the material footprint and, in the final analysis, reduce production costs. 10.3.3. The bioethanol system The different stages leading to the production of ethanol from lignocellulose materials are identical to those used on starchy plants, that is, a preparation including hydrolysis of polysaccharides into fermentable sugars, followed by fermentation and extraction of the ethanol obtained. However, the particular and more complex composition of the raw material as well as its structure make the first part of the process more arduous. For this reason, production processes have not yet reached the commercial stage and consequently research and development programs have been set up in recent years. 10.3.3.1. Main constraints of the process Unlike starch, polysaccharide is easily extracted from grains of corn or wheat, by hydrolysis by means of enzymes. It does not contain fermentable glucose; polymers with vegetable walls contain different constituents, of which some are not fermentable. Furthermore, the polymer constituents are tightly interlocked, forming a rigid structure, difficult to decompose. As a result, the processes take into consideration the following characteristics: – cellulose, like starch, is made of glucose, but the osidic ß-1.4 linkages are more difficult to hydrolyze than the Į-1.4 bonds. Moreover, the structure of cellulose is
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often crystalline and interactions among the cellulose chains considerably complicate the enzyme hydrolysis process, requiring the conjoined action of several enzyme activities. At present, the cost of this hydrolysis is one of the principal obstacles to the development of the production of ethanol from cellulose, – the hemicelluloses often contain high proportions of pentoses that are not used by the Saccharomyces cerevisiae strains used for ethanol fermentation. The result is a relatively low maximum output, since the fermentable sugars come only from the cellulose fraction, – lignin is not convertible into ethanol, – cellulose, hemicelluloses and lignin are organized such that the accessibility of the cellulolytic enzymes to the cellulose is limited. Consequently, it is necessary to apply a pretreatment to de-structure the matrix made of these three polymers, – at the time of pretreatment, it is necessary to be careful to avoid creating compounds from the degradation of the sugars in order, for one thing, not to affect the total outcome in ethanol, and, for another, not to form molecules capable of exercising toxic effects on the yeasts (hydroxymethylfurfural, furfural and acetic acid) or inhibiting the enzymes, – these polymers are insoluble in water whereas the aqueous phase is the favorite milieu of microorganisms and enzymes. As a result, concentrations in substrates (and thus in products) often remain weak (in general, less than 15% of dry materials). 10.3.3.2. Pretreatment As mentioned above, the pretreatment operation aims to act on the lignocelllulose material in order to make it hydrolyzable, while minimizing the formation of products susceptible to exercising an inhibiting effect on fermentation. It follows the conditioning of the raw material, which is a technically simple operation, such as grinding for straw or cutting into shavings for wood. It is noteworthy that, while this aspect is often passed over, this conditioning can have an impact on the efficiency of the subsequent operations. Several types of pretreatments have been proposed and tested, at least at the laboratory stage, often based on a total or partial solubilization of the hemicelluloses and/or the lignin [MOS 05]. The hemicelluloses are hydrolyzed into monomers or oligomers of low DP under heat and in the presence of diluted acid, while the lignin can be extracted under heat, notably in basic conditions or in the presence of certain organic solvents. At the present time, two methods of pretreatment have been developed to a pilot stage: cooking in the presence of diluted acid (acid prehydrolysis) and steam explosion.
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In acid pre-hydrolysis, the material is placed in the presence of a mineral acid (0.3 to 2% sulfuric or hydrochloric acid), at a moderate temperature (in the neighborhood of 150ºC), for about 15 to 20 minutes. A small part of the lignin can also be accessed and products coming from its decomposition released. Processes comprising a second stage at higher temperature (240ºC for several minutes) have also been studied. In fact it seems that the conditions of this pre-hydrolysis are moving toward higher temperatures and shorter reaction times. The NREL (National Renewable Energy Laboratory, located in the USA) recommends operating at 240ºC for 2 to 3 minutes in reactors assuring optimum contact between solid and liquid phases. Steam-explosion consists of bringing the material to high pressure (15 to 23 bars) and temperature (180 to 240ºC) in the presence of steam for a short time, then effecting a sudden relaxation back to atmospheric pressure in order to deconstruct the lignocellulose matrix. During the high temperature period, part of the hemicellulose is hydrolyzed and the lignin goes into fusion. This is recondensed in a different form at the time of rapid decompression. The short duration of the operation limits the formation of degradation compounds. This technology has been developed up to the commercial stage (SunOpta and Iogen processes). A variation consists of operating in the presence of diluted acid or after placing the material in contact with acid, in order to obtain a solubilization of hemicelluloses and better accessibility of cellulose. The two technologies represent a significant cost in the process, especially in terms of investment costs, since they require equipment that resists pressure and corrosive acid conditions, and in terms of energy consumption (heat and steam). In the two methods of pretreatment described, hemicelluloses are soluble and, if necessary, can be recovered by hot washing and solid/liquid separation. Finally, let us note that other processes, such as ammonia fiber explosion (AFEX), Organosolv or phophoric acid treatments are being studied in depth. 10.3.3.3. Enzyme hydrolysis Most of the processes envisaged favor the enzyme method rather than the chemical one for hydrolysis of the cellulose [OGI 99] because: – economic estimates show it to be less expensive, – they produce lower amounts of effluents, – the conditions are mild and not sources of corrosion, – they offer the prospect of considerable improvement. Cellulose biodegradation occurs in the environment. It is a fundamental stage in the carbon cycle. It is effected by many microorganisms, bacteria and fungi
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[LYN 02], and can take place as aerobiosis (for example, on the surface of the ground) and as anaerobiosis (for example, in animal rumen). There are three classes of complementary cellulase enzyme activities that come into play for total hydrolysis of cellulose into glucose (Figure 10.25): – exo-glucanases, essentially cellobiohydrolases, which attack the ends of the cellulose chains to free cellobiose residues (dimer of ß-1.4 linked glucoses) in the case of cellobiohydrolases, – endo-glucanases, which cut the cellulose chains in a random fashion (especially at the level of the amorphous zones), – E -glucosidases, which hydrolyze cellobiose, as well as oligomers with low DP, into glucose. It is necessary to have these three classes of enzyme activity present for total hydrolysis of the cellulose. These different enzymes act in synergy, endo-glucanases creating supplementary sites for cellobiohydrolases, ß-glucosidases acting on the cellobiose, capable of inhibiting the exo- and endo-glucanase. Currently all these enzymes are called cellulases. With a number of the cellulases, there is, besides the catalytic domain, an anchoring to the cellulose which facilitates the action of the enzyme. In addition to the intermolecular synergies, there also are intramolecular synergies between these domains, the domain of entanglement capable of having a complementary action in breaking down the structure of the cellulose fibrils.
Figure 10.25. Diagram illustrating the action of the cellulase enzymes on the cellulose microfibrils
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The production of cellulases can be either extracellular, as for fungi of the genus Trichoderma, known for their strong rate of cellulase secretion, or linked to the bacterial wall, as for bacteria of the genus Clostridium. The enzyme complexes synthesized by the latter are called cellulosomes. They are very efficient on cellulose, but they have never been commercially produced, mainly for reasons related to the producing microorganisms (slow growth and low enzyme production). Mixtures of cellulases secreted by Trichorderma contain the three classes of cellulase enzymes. Trichoderma reesei is the species of choice, selected for industrial production of these enzymes. The improvement of cellulase production, as well as their activity, has been and still is the object of many studies, because the cost of enzyme hydrolysis contributes greatly to the final cost of cellulosic ethanol. Progress largely depends on genetic improvements of the producing strains and of cellulolytic enzymes. The use of these enzymes is simple. The Trichoderma reesei cellulases work best at temperatures in the neighborhood of 50ºC and pH between 4 and 5. The action time is usually relatively long (48 to 72 hours). The insolubility of the substrate limits the usable concentration; the reaction mixture should stay homogenous. Besides the standard cellulases, other enzymes can participate in the process of hydrolysis of raw material. This is the case for hemicellulases or other enzymes such as ferulate esterases that act on the ester linkages involving ferulic acid. In fact, even in the case of pretreatments which solubilize hemicelluloses, there remains a residual fraction which tends to counteract the action of the cellulases. Here it is notable that the microorganisms which decompose the lignocellulose biomass, such as Trichoderma, secrete enzyme mixtures equally rich in hemicellulases. 10.3.3.4. Ethanol fermentation The final product of cellulose hydrolysis is glucose, which can be converted into ethanol in the same way as in processes using starchy plants. It is good practice to eliminate the solid part that contains lignin right at the beginning, unless hydrolysis and fermentation take place simultaneously (see below). The main differences come from the initial concentration in glucose, which is often lower because it is limited by the content of dry materials before hydrolysis, and the possible presence of inhibitor compounds (furfural, hydroxymethylfurfural and acetic acid, for example) coming from certain pretreatment processes. 10.3.3.4.1. Pentose fermentation Only the cellulose part, 50% of the raw material, is transformed into ethanol, because yeasts of the genus Saccharomyces do not use pentoses derived from hemicelluloses. Many studies are also aimed at converting these pentoses into ethanol in order to increase the total output of the process [JEF 06].
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The direction of research that seems most obvious is to produce the desired attribute in the-alcohol producing yeasts. This is done by introducing certain genes that are present in other yeasts, such as Pichia stipitis, capable of assimilating xylose. However, the metabolic and energy flow of the microorganism host are then greatly modified, and it is necessary to control the level of expression (that is, the stages that go from the gene to the protein) of these genes to balance these different flows. Novel pentose-utilizing Saccharomyces strains have been obtained, and their performances are high enough to be positively considered for possible industrial use. In other respects, the productivity is often markedly decreased by the sequential use of glucose and then xylose. Other studies have tried to introduce genes, this time of bacterial origin, into another bacterium, Zymomonas mobilis, which has strong alcohol-producing capabilities. In this case too, the appropriate genes could be transferred. However, the strains obtained have not yet seen industrial development, mainly because of the difficulty of using Zymomonas mobilis on a large scale (frequent contamination by foreign germs, sensitivity to the composition of the fermentation medium, etc.) Another method is to use microorganisms which use a broad spectrum of sugars, bacteria that are easy to transform genetically if possible, and to give them alcoholproducing capability by introduction of the necessary genes. These can be genes that code for the enzymes of Zymomonas mobilis (also a bacterium) responsible for the stages of transformation of the sugars into ethanol (pyruvate decarboxylase and alcohol dehydrogenase). In this way it was possible to completely orient the flow of carbon toward the synthesis of ethanol in the presence of microorganisms such as Escherichia coli. These research projects have not yet seen commercial development. 10.3.3.4.2. Simultaneous hydrolysis and fermentation – integration of the processes A simplification of the process consists of simultaneously carrying out the enzyme hydrolysis and the ethanol fermentation (SSF procedure – simultaneous saccharification and fermentation, or SSCF – simultaneous saccharification and cofermentation, in the case of strains using pentoses and hexoses, as opposed to the SHF process for separate hydrolysis and fermentation). This configuration has many advantages, the main ones being: – a smaller investment since a single reactor is used, – better hydrolysis since the cellobiose and the glucose are consumed as they appear in the milieu, thus not being able to accumulate higher than inhibiting concentrations for the cellulases, – a lowering of the risk of contamination of the hydrolysate – rich in glucose – by foreign germs since the hydrolysate is not transported from the hydrolyzer to the fermenter, nor is it stored.
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On the other hand, the main disadvantages of the SSF are the different optimum temperatures for the cellulases (45 to 50ºC) and the yeasts (30 to 35ºC) and the necessity of operating with a charge of relatively low concentration since it includes a large solid fraction (in the SHF, lignin can be separated before fermentation), which limits the final concentration of ethanol in the fermentation milieu and can increase the distillation cost. Nevertheless, it appears that the processes currently proposed favor the SSF processes, notably in the case of alcohol-producing microorganisms capable of fermenting the hemicelluloses since then there is a significant integration of the main material flow, that is to say the flow of carbon material. Whatever process is considered, current studies are increasingly trying to integrate these flows in order to minimize the aqueous waste in the case of biological processes, and to optimize the energy consumption [OLO 08]. In the case of processes for production of ethanol from lignocellulose biomass, the main coproducts are lignin and, eventually, hemicelluloses. We have seen that many research projects were devoted to the construction of strains capable of transforming pentoses into ethanol. The integration that is being most pursued consists of using microorganisms capable of hydrolysis of the cellulose and converting the sugars obtained into ethanol. This is CBP (Consolidated BioProcessing), an attractive concept being studied and aimed at constructing yeast strains that express cellulases. In other respects, lignin is often seen as being further used in the form of an energy source, essentially for ethanol production units, which could then be self-sufficient, even with a surplus, from the energy point of view. However, other methods for added use of these two coproducts, with a view to obtaining bioproducts, are also being studied. 10.3.3.5. Conclusion The procedures for production of ethanol from lignocellulose biomass are the object of a great deal of research and development work [LIN 06]. At this time these concern mainly the optimization of enzyme hydrolysis, fermentation of pentoses, and integration of the process. The research and development is highly multidisciplinary and it mobilizes many methods and important industrial products from the world of agrobusiness and enzymes. These processes are not yet commercial, but there are many pilot plants such as those of the NREL (capacity of 1 t/day of raw material – a non-integrated process) in the USA, of IOGEN in Canada (capacity of 40 t/day of raw material), and of Etek in Sweden (capacity of 2 t/day of raw material). Since 2006, plenty of new pilot and demonstration plants have been scheduled and will be ready before 2010. Abengoa Bioenergy has installed a pilot plant capable of treating more than 60 t/day of wheat straw beside a new ethanol production unit that will treat the grain. A new concept is appearing here, that of the biorefinery capable of treating the entire plant and of producing various products
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besides fuel, as petroleum refineries do from crude petroleum. Furthermore, this same term biorefinery is also adopted by the petroleum refineries for a different approach, since it is based on the use of biomass in a petroleum refinery. Here it is about having relatively flexible processes that accept loads of a varying nature. Whatever processes finally see the light of day, lignocellulose biomass is evoking a growing interest and the placing of bioethanol from lignocellulose on the market seems to be a question of only a few years, 2012 being the most optimistic, that is to say, tomorrow. In parallel, intensive research projects are in progress on other “advanced biofuels” such as oils from algae or biobutanol. 10.4. Economic and environmental balance of biofuel production systems One of the limits to the development of biofuels is their cost compared with their equivalents from fossil. Nevertheless, the pressure of public opinion on the questions of depletion of finite resources of raw material or the degradation of the environment linked to human activity is getting stronger and stronger. In this context, the environmental performance of products or processes becomes the key element of the decision making process, having the same importance as their cost. This section addresses these two aspects, economic and environmental, in detailing the costs of production of the different biofuels according to the regional situation, as well as their environmental footprints, relying on the results of several studies analyzing the life cycle. 10.4.1. Economic aspects 10.4.1.1. The competitiveness of biofuels Several elements are capable of having a more or less direct influence on the economic competitiveness of biofuels, such as the evolution of the quoted market price of the agricultural materials and byproducts, as well as the currency exchange rate. The quoted market price of agricultural raw materials has a much greater impact on the economy of biofuels than do the different dynamics of the energy market. To demonstrate this, two examples may be cited: – variations in the world sugar market had and will continue to have a large impact on the ethanol fuel development program in Brazil. At the beginning of the 1990s, the conjunction of the low price of oil and the high price of sugar directed the use of sugar cane toward the food market and created a situation of low supply for ethanol,
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– in 2004–2006, the rise in the price of canola oil in Europe, linked in part to the strong and sudden demand seen in Germany for the manufacture of biodiesel, altered the profitability of this system whose competitiveness could have been assured with a crude petroleum price slightly higher than $50/b. As was mentioned already, the manufacture of fuels from vegetable sources is accompanied by that of byproducts and their value and changes in their price influence the biofuel economy. By way of example, the drop in the market rate for glycerine by a factor of 2 between April 2003 and April 2005 was felt in a reduction in the economic profitability of the biodiesel system. Regarding the exchange rate, it is worthwhile to recall that the price of oil is posted in dollars, and that of biofuels produced in Europe in euros. At a constant price of a barrel of crude, the more the euro appreciates with respect to the dollar, the more the price of the barrel expressed in euros is reduced and the more biofuels are penalized compared with petroleum fuels. Finally, in order to correctly estimate an equilibrium price, it is absolutely necessary to take into consideration the energy content of each of these fuels, just as it will certainly be recommended to take into account in the near future their environmental impact, in particular on the emission of greenhouse gases. To appreciate in a more concrete manner the competitiveness of biofuels vis-àvis petroleum products, the example of the FAME is a good one. Figure 10.26 retraces, between August 1990 and August 2004, the comparative fluctuation of the European prices of canola oil (the cost of the oil represents almost 90% of the final price of FAME) and diesel.
Figure 10.26. Comparative fluctuation of prices of canola oil and diesel in Europe ($/t) (source: Oilworld, Platt’s)
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It shows that there was relative competitiveness of FAME compared to diesel only during autumn 2000. This period was marked by an oil price of the order of $35/b, a low price for vegetable oil ($330/t) and a weak euro compared with the dollar ($0.85/€). Subsequently, the price of vegetable oil and diesel followed opposite trends: a rise for vegetable oil, especially following the strong demand for FAME on the German market, and the fall of diesel. The situation in the summer of 2005 was very different. In summer 2005, the prices quoted in Germany gave an average price of 735 €/t for FAME, compared with a diesel price of the order of 500 to 550 $/t, or 417 to 460 €/t. The price of biodiesel is about 1.5 times higher than for diesel despite a barrel costing $50 or $60. This situation is mainly the consequence of a high price of vegetable oil maintained by a very dynamic European market for FAME fuels (a growth of more than 30% over the three previous years) and a strong euro with respect to the dollar. A high barrel price would not be the only guarantee of the competitiveness of biofuels compared with petroleum products, even if it remains a determining factor. 10.4.1.2. The ethanol system Ethanol is by far the top biofuel product in the world. The main ethanol fuel producer countries are the USA, Brazil and, to a smaller degree, Europe. This hierarchy means that today Brazil and the USA profit from the best economy of scale effects and thus offer the lowest costs for ethanol production in environments that are very different. 10.4.1.2.1. Production costs of ethanol in Brazil The lowest production cost of ethanol is obtained in Brazil, to a slight extent in India, and potentially in all tropical regions that produce large quantities of sugar cane. Thus, in Brazil, the price of hydrous ethanol (92% ethanol, 8% water), usable only in dedicated vehicles, per unit volume, went down to below the price of gas in 2002 and 2003 (about $0.20/l). In the same way, in India, the price of ethanol, again on a volumetric base, approached the price of gas in 2002. The prices stated in 20042006 were strongly dependent on variations in the price of sugar. This situation has certainly changed more recently with the opening of ethanol purchase or sale contracts at the New York Board of Trade. The details of average production costs of ethanol in Brazil are given in Table 10.14.
Biofuels Building
0.25
Machinery
1.38
Total investment
1.63
Labor
0.62
Insurance/maintenance
0.58
Raw material
11.76
Other
2.78
Gross production cost
17.37
Stillage credit
383
1
Net production cost
16.37
Export cost (fob Sao Paolo)
18.37
Import cost (cif Rotterdam)
23.37
Import cost (cif Rotterdam) (€/hl)
19.48
Import tax on non-enatured ethanol (€/hl)
19.20
Transportation in France (€/hl)
1
Delivery cost to refinery (€/hl)
39.68
€1 = $1.20; $1 = 3 Reals.
Table 10.14. Production cost of ethanol in Brazil ($/hl) and delivery cost in Europe (€/hl) in 2004 (source: FO Litchs)
Since the 1990s, great progress has been made in the production of ethanol. Bagasse, for example, is used as a raw material in cogeneration units for electricity and heat. In 2006, the production costs for ethanol at 92% are about $0.16/l. Anhydrous ethanol, usable in a mixture with gasoline, has a production cost that is slightly higher. On an energy content basis, the price of production of ethanol in Brazil is increasing to about $8/GJ, while the cost of gasoline corresponding to a barrel of crude petroleum, at $25–30, is between $6 and $8/GJ. Brazilian ethanol is already almost competitive with gasoline. It is all the more so with a barrel price between $60 and $65, which corresponds to an energy cost for gasoline of $14 to $16/GJ. The cost of Brazilian ethanol in Europe was about 40 €/hl in 2006 when we take into account the transportation costs and the import taxes on denatured ethanol. On an energy content comparison, the cost of Brazilian ethanol imported into Europe ($18–$19/GJ) approached that of gasoline calculated at $60–$65 a barrel ($14– $16/GJ).
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We should remain cautious about these values. In recent years, we have seen a rise in the market price because of the strong demand for ethanol in the transportation sector and a very fluctuating price of crude oil. 10.4.1.2.2. Cost of ethanol production in the USA Ethanol is produced mainly from corn in the USA. Two types of processes can be implemented depending on the nature of the targeted markets for the byproducts: a “wet” grinding process and a “dry” grinding process. The byproducts, and therefore the nature of the markets for these byproducts, have a capital importance for the economy of ethanol production: the volume generated from byproducts represents about 55% total of the raw material introduced into the process and its added value can represent up to 50% of the price of the raw material. Each of these methods has advantages and disadvantages. The dry method has the advantage of offering a smaller initial investment and easier implementation; on the other hand, the stillage byproduced has a low added value (it is essentially intended for the animal feed market). On the other hand, the so-called wet method, more complex to carry out, requires a larger investment; it is also used for producing a syrup rich in fructose, mainly intended for the food market, especially for sweetened drinks or cooking products. With the rise in the production of ethanol fuel observed in the USA since the beginning of the 2000s, a tendency that is not expected to weaken in the coming years, the so-called dry methods are becoming preponderant on the US market. In fact, these processes are establishing themselves because they are dependent only on the added value of ethanol and the DDGS in an expanding market, unlike the multitude of byproducts generated by the so-called wet methods, for which the fluctuating markets are not able to absorb the excess produced by the new large units now being set up. In recent years all the new installations have production capacities of more than 100,000 t/year, in effect bringing a scale that results in a lowering of production costs. The distribution of the costs, represented in Figure 10.27, is noticeably the same for the two types of processes. It makes the importance of the position of the raw material obvious, despite being attenuated by revenues from the sale of byproducts. In 2006, the production costs from the dry milling processes rose to $1.20/gallon, of which $0.80/gallon is attributable to the raw material. Investment costs are in the order of $1.10/gallon. In absolute terms, the cost of the raw material is higher with the wet process; on the other hand, the revenues from the sale of the byproducts are greater.
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Figure 10.27. Distribution of production costs of ethanol from corn (source: H. Shapouri)
Taking these revenues into account, the net production costs of ethanol from corn are obviously equivalent for the two types of processes. It is in the order of $0.24/l, to which is added the estimated costs of the financial amortization of the capital at $0.05/l, to arrive at a cost price of $0.29/l (Table 10.15). Net cost of raw material
0.13
Cost of raw material
0.23
Credit of the byproducts
(0.11)
Operating cost
0.11
Labor/administration/maintenance
0.05
Chemicals
0.03
Energy
0.04
Return on investment
0.05
Total production cost
0.29
Table 10.15. Estimate of production costs of ethanol in the USA ($/l) (September–October 2005) (source: AIE)
This cost is representative of the large plants in the USA, where we should recall that the price of ethanol over the 12 years before 2006 was established at an average of $0.30/l. The cost expressed in $/l in Table 10.15 is to be compared to a price in
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Renewable Energy Technologies
the order of $0.18 to $0.25/l for gasoline with the price of the barrel of oil varying between $25 and $30, depending on the local conditions with regard to refining. Based on energy content, the difference in cost between gasoline and ethanol is increased by the lower energy content of ethanol. For instance, for a barrel price of $25 to $30, the energy costs are established at $6–$8/GJ for gasoline and in the order of $14/GJ for ethanol. Thus, the cost of ethanol per unit of energy in the USA is still approximately twice as high as the price of gasoline, with the price of a barrel of oil at $25 to $30, but it can become competitive when the barrel price of oil goes above $60. It remains the case that with the strong demand in ethanol currently observed, the price is changing and establishing itself at a cost of $2.80/gallon (May 5, 2006), or close to $0.75/l or $933/t. 10.4.1.2.3. Cost of ethanol production in Europe Because of the much smaller units of a sector that is much less structured than in the USA or Brazil and because of the much higher price of raw materials, the cost of producing ethanol is much higher in Europe. By way of example, in France, one of the leading countries for ethanol fuel production in Europe, the average plant size in 2005 was in the order of 20,000 t/year. It is also important to note that the raw materials used – wheat or sugar beets – are different from those used in Brazil or the USA. For wheat, the procedure is similar to that for corn, already described in the case of the USA. For beets, the configuration is close to that of Brazil. An estimate of average costs of ethanol production is given in Table 10.16. Raw material
0.3–0.4
Production cost
0.2–0.3
Credit from byproducts Total cost
í0.07 0.43–0.63
Table 10.16. Estimate of average production cost of ethanol (€/l) from wheat/beets (source: IFP from AIE, FO Litch)
These costs are given only as an indication and do not reflect the diversity of the situations found in Europe. Related back to the energy content, the cost of ethanol in Europe is thus higher than the cost of gasoline ($17 to $22.50/GJ for ethanol from wheat, $20–$24/GJ for ethanol from beets compared to $6–$8/GJ for gasoline with the barrel at $25–$30 and $14–$16/GJ with the barrel at $60–$65).
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10.4.1.3. Cost of ETBE6 production The cost of production of one ton of ETBE, calculated with a price of raw materials (ethanol and isobutene) of 295 €/t, rises to €397. Also entering into this evaluation are fixed charges at a level of 46 €/t, while other variable expenses (utilities, catalyst, labor, etc.) are from 56 €/t. By way of example, the investments for a unit producing 80,000 t/year of ETBE from FCC containing 16% isobutene are in the order of M$ (to the limit of the capacity of the production units). The energy expenses attributable to the manufacture of one ton of ETBE are for electricity at 14 kWh and steam from 0.1–1 t, while around 60 m3 of water for cooling is used. The “raw material” represents 75% of the cost price of ETBE. For this reason, the slightest fluctuations in this item explain the variations observed in cost price of ETBE. 10.4.1.4. Biodiesel Unlike ethanol, whose use as a fuel is widespread throughout the world, biodiesel is a biofuel produced and consumed mainly in Europe, where this system is in rapid expansion because of objectives set by the European Union with regard to biofuels and because of the growing importance of diesel in the demand for fuel. 10.4.1.4.1. Cost of biodiesel production in Europe The change in the price of vegetable oils and methanol over the course of recent years has already been discussed. As far as the synthesis of methanol from natural gas is concerned, the investments in the capacities of the plants are, by way of example, between M€90 and M€124, corresponding to an all inclusive investment of 189 to 227 M€, for a day to day production capacity of 2,500 t. The production costs of biodiesel by homogenous catalysis (Table 10.17) lead to several comments. The first concerns the significant impact of the price of raw materials, especially oils, on the production costs of biodiesel. Depending on the size of the industrial unit, it represents between 76 and 84% of the total production cost. This latter is moderately sensitive to the effect of scale since it is lowered only by about 10% when the unit capacity is multiplied by a factor of 10. The sale of glycerine generally compensates for the purchase of chemicals, including methanol. Based on the price of this raw material, an estimated average over the years 2003–2004, the cost of methanol came in at a level of €20 per ton of biodiesel.
6 Source: Axens.
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Capacity (t/year)
10,000
35,000
70,000
100,000
460.0
460.0
460.0
460.0
Chemicals
43.9
42.1
42.1
42.1
Energy
10.1
6.9
6.9
6.9
Investments (amortization over 20 years)
27.8
32.9
20.7
15.9
Labor
41.6
15.6
7.8
5.3
Maintenance/administration
17.00
18.6
15.0
13.6
Oil
7
Total cost
600.4
576.1
552.5
543.8
Sale of glycerine
í46.0
í46.0
í46.0
í46.0
Cost of biodiesel
554.4
530.1
506.5
497.8
€1= $1.30
Table 10.17. Production costs of biodiesel (€/t) (source: Lurgi)
In the case where the sites of pressure and extraction of the oleaginous grains (trituration) and synthesis of biodiesel are mixed, the costs of items such as labor and maintenance are appreciably lowered. Glycerine is a major byproduct in the manufacture of biodiesel, representing 10% by mass of FAME production. This shows the importance of the market for this byproduct on the economic profitability of this system. Glycerine currently adds value to the price of between 400 and 800 €/t according to the purity of the product and the state of the market. In other words, the sale of glycerine allows a reduction of the cost price of EMVO of some 40 to 80 €/t. This market is therefore important for the economic profitability of the system. This is why developments in the process of esterification have the aim, among others, of producing the purest possible glycerine in order to draw maximum revenue. The costs of purification of glycerine from industrial units producing biodiesel by homogenous catalysis are indicated in Table 10.18. The purification consists of distillation, hence the importance of the energy balance line comparable or higher than the investment budget line. The effect of scale is significant since the price for purifying one ton of glycerine varies from 281 €/t to 173 €/t when the installation capacity is multiplied by 3, going from 3,200 t/year to 9,500 t/year. These production figures correspond respectively to an annual manufacture of biodiesel from 32,000 t to 95,000 t. 7 Average price of canola seed oil in the period 2003–2004. In 2005, it was in the region of 600 €/t.
Biofuels Capacity of glycerine production (t/year) Chemicals Energy Investment (amortization over 10 years) Labor Maintenance/administration Total cost
3,200
389
9,500
0.3
0.3
77.1 109.2 54.4 40.5 281.5
77.1 53.8 18.0 23.8 173.0
€1 = $1.30
Table 10.18. Cost of purification of glycerine (€/t) (source: Lurgi)
The cost prices of FAME manufactured by heterogenous catalysis are slightly lower than those obtained by homogenous catalyst, with identical production capacity, greater than or equal to 100,000 t/year, and for the same costs of raw material.8 The gains come from the resale of glycerine of better quality and from the raw material costs because of the improvement in total output. On the other hand, the operating conditions are stricter in temperature and pressure and lead to fixed charges and utility expenses that are a little higher. It remains the case that, in the end, if the volumes of FAME production grow too much, the glycerine market would be saturated quite quickly, and the value of the glycerine greatly reduced. In summary, the cost of biodiesel production in Europe, based on energy content, varies between $15 and $22/GJ when the price of vegetable oil is between 400 (average over 2003–2004) and 600 €/t (€1 = $1.3). At 400 €/t oil, biodiesel becomes competitive with diesel produced from crude at $60–$65/b. It should be noted that the strong demand for biodiesel in Europe has caused the price of canola oil to climb greatly. In May 2006 these were at their highest historic prices: $780/t or 614 €/t (€1 = $1.3). 10.4.1.5. New fuel systems New systems for production of biofuels have the objective of lowering the price as much as possible, but also of increasing the supply. They are mainly systems aiming at producing fuels from lignocellulose biomass. The production costs of the two methods, namely enzyme hydrolysis for obtaining ethanol and gasification for obtaining diesel fuels, are for the moment relatively uncertain, since no installation on an industrial scale has, as yet, been built. Nevertheless, a certain number of estimates have been made. 8 Source: Axens.
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Studies on ethanol production by enzyme hydrolysis have been made mainly in the USA. They show production costs attainable at term by this method to be in the order of $0.19/l ethanol (cost of raw material: 30%, other operating costs: 30%, investment cost: 40%, for a unit producing 260,000 tons of ethanol/yr) compared with $0.29/l previously cited. This big gain, in the order of 35%, should nevertheless be considered as tentative, since it is obtained based on the hypotheses of gains that are still uncertain. The actual costs would be in the order of $0.36 to $0.60/l of ethanol for a unit producing 160,000 tons of ethanol/year. The production of synthetic diesel by the Fischer–Tropsch process has been studied mainly in Europe, where the demand for diesel fuel is strong. The announced production costs are in the range of $0.50 to $0.75/l of diesel (35% for the investment, 35% for the raw material and 30% for the other operating costs for a unit of the order of 100,000 t/year). These costs are relatively large in comparison with the production costs for petroleum fuels or FAME which are $0.21 and $0.35 to $0.65 /l, respectively. The investment expenses in fact weigh heavily. As an example, a unit of 100,000 t/year would cost about M$300, or $3,000/t, against M$90 for a unit of 160,000 t/year of ethanol, or about $600/t. Table 10.19 will serve as a reminder of the orders of magnitude of the investments and the cost prices of the products from the gas to liquid (GTL) and coal to liquid (CTL) models. Investment ($/t)
Product prices ($/b)
GTL
400–700
14 to 23 for gas at $0.5/Mbtu
CTL
900–1,200
–40
Table 10.19. Economic elements relative to the GTL and CTL systems
At this stage, it is important to note that one of the obstacles to the development of this type of process is the high total investment cost. By way of comparison, the order of magnitude for the investment in a refinery is between $10,000 and $15,000 x b-1 x J-1, or 150 to 300 $/t. 10.4.2. Results of analyses of life cycle of biofuels Numerous studies to measure the environmental impact of the production and use of biofuels have been carried out. These studies, called “from well to wheel”, compare greenhouse gas emissions and energy consumption on the totality of the life cycle of the fuel for biofuel solutions and traditional solutions using petroleum
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fuels. The other kinds of impact mentioned above are studied far more rarely because the uncertainties are larger and these impacts are very dependent on local conditions where the pollutants are emitted. This is why, in this early period, only greenhouse gas “from well to wheel” emissions and the consumption of fossil energy will be covered here. The results of two studies, in particular, will be discussed in the following pages: – the study carried out in France in 2002 by the Price Water House Coopers company on behalf of Ademe (Agence de l’environnement et de la maîtrise de l’énergie – Agency on the Environment and Management of Energy) and Direm (Direction des ressources énergétiques et minières – Management of Energy and Mining Resources), – the European study carried out in collaboration by the Common Research Center of the European Commission (JRC), Concawe, which is the European association of petroleum companies treating questions linked to the environment, and Eucar, which coordinates the research and development of the European association of automobile manufacturers. The results of these two studies regarding greenhouse gas emissions are presented in Table 10.20. JRC/Eucar/Concawe9
60%
Ethanol from beets
Gain with respect to the reference
85.9
Reference (g/km)
Gain with respect to the reference
34.4
Uncertainty
Reference (g/km)
Ethanol from wheat
g/km traveled
g/MJ traveled [alt. Covered]
Ademe/Direm
167
40-55
196
15%
33.6
85.9
61%
116
7-7
196
41%
Ethanol from lignocellulose
nd
nd
nd
49
17-6
196
75%
Ethanol from sugar cane
nd
nd
nd
nd
nd
nd
92%
FAME canola
23.7
79.3
70%
93
37-37
164
43%
EMVO sunflower
20.1
79.3
75%
68
24-23
164
59%
nd
nd
nd
22
12-6
164
87%
BTL
nd = non-determined; * Reference: petroleum fuels
Table 10.20. Greenhouse gas emissions (expressed in grams of CO2 equivalent) pertaining to various biofuels “from well to wheel” 9 This table presents only one part of the results, allowing their comparison with those of the Ademe/Direm study.
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Renewable Energy Technologies
Several comments can be made from an analysis of these results: – In general, the use of biofuels allows a significant reduction of greenhouse gas emissions compared with conventional solutions. Nevertheless, this conclusion needs to be tempered by the fact that biofuels are most often used as a mixture with relatively low content (5 to 10% with a maximum of 24% in Brazil10). The total benefit to be expected, in terms of the greenhouse effect, will therefore be less than 5% if only traditional biofuels are used. However, even if these figures seem low at first glance, it is important to note that they can be obtained in a sector whose growth is very difficult to manage and that the options offering the same benefit in a relatively short time are few. – If, from a qualitative point of view, the use of biofuels allows a certain gain in terms of greenhouse gas emissions, this gain remains difficult to quantify with precision; it also depends on the N2O emissions, whose influence is especially important since this gas is three times more noxious in terms of the greenhouse effect than CO2. Now, these N2O emissions themselves depend on the quantities of fertilizers used, climate conditions, soil quality, etc., and therefore are difficult to evaluate in a precise manner. It is important to underline that between the two studies there are important differences of methodology regarding the attribution of the impacts when coproducts are produced. The France study on behalf of Ademe/Direm uses mass pro rata. On the other hand, the JRC/Eucar/Concawe study adopts the so-called substitution method which consists of attributing the totality of the impacts to a single biofuel and then deducting a “credit” once the ensemble of impacts has been evaluated. This credit is estimated by making the hypothesis that the byproduct allows avoidance of greenhouse gas emissions linked to the manufacture of this product alone. As an example, for ethanol from wheat, the value added byproducts for animal feed is the DDGS. Byproduction allows avoidance of the greenhouse gas emissions that would have been generated if it had been necessary to produce an equivalent amount of animal feed from wheat. This last calculating method is clearly less favorable than that used in the Ademe/Direm study, since, for example, for ethanol from wheat, taking into account the uncertainties encountered (Table 10.6), the gain in terms of greenhouse effect can be zero, even less unfavorable compared with classic petroleum systems. – The most important gain in terms of the greenhouse effect is obtained by transforming lignocellulose materials into biofuels. In fact, in this value added to the wood or straw, part of the initial load is often used to produce the utilities necessary 10 This is for three main reasons: because they represent a certain surcharge with respect to petroleum, because, by this manner of operating as a mixture, they avoid the development of a dedicated distribution system and thus take advantage of the petroleum fuels system already in existence, and, finally, because the volumes of biofuels produced can, with difficulty, cover the totality of transportation demand. In Brazil, hydrous ethanol is used without adding gasoline.
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for the procedure, especially the heat, thus allowing substantial gains in terms of emissions. – The studies agree that the best result is obtained when ethanol is produced from sugar cane. The use of the bagasse to generate the heat needed for distillation greatly contributes to this result. – From the results in Table 10.21, presenting the energy balance, several elements stand out. First of all, the production of biofuel is accompanied by a consumption of significant quantities of fossil energy. In consequence the final expected gain compared with the “petroleum fuels” solution will never be 100%. According to the hypotheses made and the systems studied, this gain can go from 10% to 90%. The extent of this range indicates that we should be prudent when the subject of gains in terms of energy consumption linked to biofuels usage is addressed. Furthermore, just as in the case of greenhouse gas emissions, the expected gain is going to depend on the methodology applied. We will therefore remark, again, that the methodology of the Eucar/JRC/Concawe study is unfavorable. We will nevertheless note that these balances are not fixed and we can foresee improvement in the fossil “energy” balances of production units for biofuels, either by energy economy programs or even by using renewable energy resources. It is interesting to note that, contrary to what may generally be guessed, greenhouse gas emissions are not necessarily proportional to the fossil energy consumption of a system. In particular, for the ethanol from beets system of the JCR/Eucar/Concawe study, it will be observed that the gain in greenhouse gas emissions is of the order of 40%, while in terms of fossil energy consumption, it is only 19%. This difference is mainly attributable to the use of fertilizers. The two studies cited converge on the interest of using FAME and diverge with respect to ethanol. In fact, the two studies show that the FAME system is more favorable in terms of fossil energy consumption than the ethanol from wheat or from beets systems. On the other hand, while the Ademe/Direm study advances a relatively important advantage in terms of fossil energy consumption linked to the use of ethanol (about 60%), the JRC/EUcar/Concawe study arrives at a result that is clearly less favorable (an advantage of only about 15%). As for greenhouse gas emissions, the most important gains in terms of consumption of fossil energy are obtained for the systems based on lignocellulose materials and sugar canes (ethanol from sugar cane).
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Renewable Energy Technologies
Reference (g/km)
Gain with respect to the reference
MJ from fossil/km
Reference* (g/km)
Gain with respect to the reference
JRC/Eucar/Concawe
MJ from fossil/MJ
Ademe/Direm
Ethanol from wheat
0.489
1.15
57%
224
255
12%
Ethanol from beets
0.488
1.15
58%
206
255
19%
Ethanol from lignocellulose
nd
nd
nd
58
255
77%
Ethanol from sugar cane
nd
nd
nd
nd
nd
91%
FAME canola
0.33
1.09
70%
72
212
66%
FAME sunflower
0.32
1.09
71%
53
212
75%
nd
nd
nd
11
212
95%
BTL *Reference: Petroleum fuels
Table 10.21. Fossil energy consumption for various fuels, “from well to wheel”
10.5. Bibliography [BAL 06] BALLERINI D., Les Biocarburants – Etat des lieux, perspectives et enjeux du développement, Technip, Paris, 2006. [CLA 06] CLAUDET G., DUPLAN J.-L., SEILER J.-M., “Produire un carburant par transformation thermochimique de la biomasse”, Revue de la défense nationale, April 2006. [FUL 04] FULTON L., HOWES T., HARDY J., Biofuels for Transport – An International Perspective, International Energy Agency publication, 2004. [JEF 06] JEFFRIES T. W., “Engineering yeasts for xylose metabolism”, Curr. Op. Biotechnol., 17, 320–326, 2006. [JRC 05] JRC/EUCAR/CONCAWE, Well to wheel analysis of future automotive fuels and powertrains in the European context, Report, http://ies.jrc.cec.eu.int/wtw.html, 2005. [KIR 87] KIRK T. K., FARELL R. L., “Enzymatic combustion: the microbial degradation of lignin”, Ann. Rev. Microbiol, 41, 465–505, 1987. [KNOE 05] KNOEF H. et al., Handbook Biomass Gasification, BTG, Enschede, 2005. [KNOT 05] KNOTHE G., GERPEN J. V., KRAHL J., The Biodiesel Handbook, AOCS Press, Champaign, 2005.
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[LIN 06] LIN Y., TANAKA S., “Ethanol fermentation from biomass resources: current state and prospects”, Appl. Microbiol. Biotechnol, 69, 627–642, 2006. [LYN 02] LYND L. R., WEIMER P. J., VAN ZYL W. H., PRETORIUS I. S., “Microbial cellulose utilization: fundamentals and biotechnology”, Microbiol. Mol. Biol. Rev., 66, 506–577, 2002. [MAN 99] MANSFIELD S. D., MOONEY C., SADDLER J. N., “Substrate and enzyme characteristics that limit cellulose hydrolysis”, Biotechnol. Progr., 15, 804–816, 1999. [MIT 04] MITTELBACH M., REMSCHMIDT C., Biodiesel, the Comprehensive Handbook, M. Mittelbach Publisher, Graz, 2004. [MOS 05] MOSIER N., WYMAN C., DALE B., ELANDER R., LEE Y. Y., HOLTZAPPLE M., LADISCH M., “Features of promising technologies for pretreatment of lignocellulosic biomass”, Bioresource Technol., 96, 673–686, 2005. [ODO 06] O’DONOHUE M. J., DEBEIRE P., “Fractionnement de la biomasse lignocellulosique en synthons”, in P. Colonna (ed.), La chimie verte, Lavoisier, Paris, p. 21–39, 2006. [OGI 99] OGIER J. C., BALLERINI D., LEYGUE J. P., RIGAL L., POURQUIÉ J., “Production d’éthanol à partir de biomasse lignocellulosique”, Oil and Gas Sci. Technol, review of the IFP, 54, 67–94, 1999. [OLO 08] OLOFSSON K., BERTILSSON M., LIDEN G, “A short review on SSF – an interesting process option for ethanol production from lignocellulsic feedstocks” Biotechnol. for Fuels, 1, 7, 2008. [PAR 98] PARMALIANA A. et al., Natural Gas Conversion, Studies in Surface Science and Catalysis, vol. 119, 1998. [PER 05] PERLACK R. D. et al., Biomass as Feedstock for a Bioenergy and a Biomass Industry: the Technical Feasability of a Billion Ton Supply, US Department of Energy, Washington, April 2005. [POU 06] POURQUIÉ J., “Bioéthanol: comparaison des sources amidon, saccharose et lignocellulose”, in P. Colonna (ed.), La chimie verte, Lavoisier, Paris, p. 395–417, 2006. [PRI 02] PRICE WATERHOUSE COOPERS, Bilans énergétiques et gaz à effet de serre des filières de production des biocarburants en France, ADEME/DIREM report, http://www.industrie. gouv.fr/energie/renou/biomasse/ecobilan-synthese.pdf, 2002. [PUL 93] PULS J., “Substrate analysis of forest and agricultural residues”, in J. N. Saddler (ed.), Bioconversion of forest and agricultural residues, C.A.B. International, Wallingford, p. 13–32, 1993.
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Chapter 11
Biogas
11.1. Introduction: biogas, “the renewable natural gas” Biogas from the degradation of organic matter is a gas fuel composed of a single hydrocarbon, methane. As such, it belongs to the “natural gas” family, whose energy is supplied essentially by methane. Different kinds of biogas may be distinguished, depending on the source of the organic material which they come from, and the contaminations that they have been exposed to. 11.2. Naturally occurring biogas In nature many sources of biogas are found which arise without voluntary human action: – gas from swamps and lakes, – enteric gas from the animals, – gas from buried organic matter (discharges, etc.). These have evoked the curiosity of humans, who have tried to understand the phenomenon and ways to harness it. Studies conducted on biogas reserves beneath deep water in Lake Kivu, Rwanda estimate there to be more than
Chapter written by Pierre LABEYRIE.
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50,000 million Nm3 of methane1. The annual production from organic waste removed by erosion is estimated at 250 million Nm3 of methane. An inventory of natural biogas deposits is far from being made. Dumps for household and other wastes produced by human activities generate very large quantities of biogas from the buried biomass. This production is known but at best only partially captured and little taken advantage of. By way of an example, the “Garaff” dump in Barcelona (Catalonia) produces about 54 million Nm3 of methane a year.
Figure 11.1. Collection at the Montech, Tarn and Garonnne CET
11.3. Production organized by humans The importance of these resources has led to the placing of catchment systems for existing deposits, and also of anaerobic digesters capable of reproducing the observed phenomena by adapting them to the organic substrates identified. These installations often came out of the necessity of treating the liquid or solid wastes in order to reduce pollution while producing usable energy. The depletion of fossil energy resources today is leading to the development of renewable energy sources and to the installation of anaerobic digestion units principally for energy.
1 Nm3: unit of measurement used for gases. It is the used volume in normal conditions, defined as a temperature of 0°C and pressure of 760 mmHg or 14.503 PSI.a.
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The desire to reduce greenhouse gas emissions is also leading to the study of methods for biogas collection and for producing energy without depletion of fossil fuels using anaerobic digestion installations.
Figure 11.2. Household waste digester at Amiens (source: Valorga)
11.4. History of anaerobic digestion The first records of the exploitation of biogas go back to the 10th century BC, when the uses of biogas for heating baths is mentioned. The use of biogas is found in Persia in the 16th century. Then in the 17th century, Van Helmont and, in 1776, Alessandro Volta identified biogas as a gas fuel composed of methane from the fermentation of waste. One of the first installations for anaerobic digestion was constructed in a leper colony at Bombay, India, in 1859. Humans have tried to capture the biogas produced in marshes or lakes. It seems that China has more than a thousand years experience in the collection and transportation of biogas in bamboo pipes. In Europe, since the work of the Italian Volta in 1776 and the identification of methane by Sir Humphrey Davy in 1808, the construction of installations for anaerobic digestion has taken place, starting with farm animal wastes (manure), then domestic wastewater, industrial effluents and household garbage. Some of the constructions are very rustic while others are imposing industrial plants.
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Figure 11.3. Illustrations of methane collection (1887) (Pl.XXX Ford Madox Brown: Dalton collects methane; fresco, Manchester City Hall)
11.5. Anaerobic digestion [LAG 79] Whether it involves a naturally occurring situation or a dedicated installation, anaerobic digestion corresponds to a series of biochemical reactions involving different microorganisms with metabolic variations. The main metabolic stages are the following: 1) hydrolysis: the carbon macromolecules that constitute the living matter are first hydrolyzed in order to produce small soluble molecules; 2) acidogenesis: the products of hydrolysis are decomposed into alcohols, low molecular weight organic acids, volatile fatty acids (acetogenic acid), H2 and CO2; 3) acetogenesis: conversion of the products of acidogenesis into acetogenic acid, H2 and CO2; 4) methanogenesis: 70% of the methane is produced from acetogenic acid. The acetoclastic bacteria are strictly anaerobic. H2O + CH3COO ĺ CH4 + 2HCO3 Another metabolic method produces the remaining 30% from hydrogen and carbon gas: 4H2 + CO2 ĺ CH4 + 2H2O
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Figure 11.4. Methods of anaerobic digestion
The metabolic methods of the different phases of anaerobic digestion are complex and the microorganisms involved are hard to identify. New genome sequencing technologies and polymerase chain reaction techniques are the only ways to move towards identification, if only partial, of the populations contained in the substrata of anaerobic digestion, as well as the metabolic pathways present in these bacterial populations, since only part of the bacteria or archaebacteria present can be cultivated. The genotyping (polymorphism of the RNA ribosomes, access to the metagenome) obtained from a standard sampling could serve as a reference in order to identify the genes and therefore the metabolic functions affecting dysfunction of the anaerobic digestion process. 11.5.1. Management of the anaerobic digestion process [FAR 95] The stability of the process used and expectation of the quantities and qualities of the gas product, the effluent product, are the important elements that combine to bring about technical and economic success. Since the methanogenic bacteria are strictly anaerobic, anaerobiosis should be strictly applied.
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11.5.1.1. The effect of temperature Like all biological processes, anaerobic digestion is very sensitive to temperature. Three temperature zones can be distinguished for which bacterial populations are effective: – the psychrophilic zone for temperatures lower than 20ºC, – the mesophilic zone for temperatures between 25 and 35ºC, – the thermophilic zone for temperatures above 45ºC. Above 70ºC, the usual bacterial populations are inactive. A drift in the temperatures brings a modification of the bacterial populations, the variations best adapted to the conditions of the milieu taking the lead. ,
,
,
s Figure 11.5. Effect of temperature on the speed of anaerobic digestion
Increasing the operating temperature of an anaerobic digestion installation is one method of adapting it to increase the substrate to be treated. This link between anaerobic digestion and temperature leads to different approaches, depending on the relevant climate conditions. Isolation of digesters is the rule in Europe, but this is not the case in the intertropical zone. Concentration of the substrate is a necessity if it has to be heated. Non-concentrated wastewater may be treated if the climate is hot.
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Figure 11.6. Production of gas as a function of temperature
11.5.1.2. Effect of pH For the process to work well, the pH should remain close to 7.5. For liquid or solid manure, the system regulates itself with good buffer ability. For other substrates (whey, silage), it is necessary to monitor this parameter and, eventually, to intervene (by adding lime). In fact, at the time of the acidogenesis and acetogenesis phases, if the hydrogen formed is not used to produce methane with CO2, the accumulation of hydrogen can block fermentation. 11.5.1.3. Dynamics of the bacteria populations The temperature and pH play a major role. This especially affects the speed of multiplication of the bacteria populations. They develop, of course, in relation to the substrates present, but they are also eliminated with the effluent at the end of fermentation. In order to avoid slowing down of the biological process when new substrates are introduced, we try to fix the microorganism populations in the digesters or to recirculate the effluents to inseminate the entering substrates. The most advanced digester techniques pay close attention to this effect. 11.5.1.4. Mixtures of substrates or codigestion Many studies have looked at the methanogenic potential of substrates. We can take as a base the products given in Table 11.1. Substrates
Biogas liters/kg
CH4 content
CO2 content
kJ/m3
Sugars
790
50
50
17,800
Proteins
700
71
30
24,900
1,250
68
33
23,700
Fatty matter
Table 11.1. Methods observed in 10 installations in Germany
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The substrate mixtures give results that cannot be reduced to a simple rule of proportions The organic wastes usually treated have methanogenic potential between 170 and 740 liters of CH4/kg of organic matter.
Figure 11.7. Production of methane from various types of waste
The mixture of substrates enables an increasing in the energy production by treating the substrates which, alone, are more difficult to degrade. This is especially the case for fatty waste from agro-feed. Addition of 5% of organic agro-industrial waste to liquid pork manure can bring an increase in biogas production of 200 to 300% depending on the products. Research into efficient cosubstrates constitutes a market in countries that have developed anaerobic digestion. The substrates most studied are bleaching clay from oil mills and fatty wastes. Some substrates, such as clays, also play a supporting role in the fixing of bacterial populations. Research on cosubstrates is one of the principal areas in the study of new establishments of anaerobic digestion. Codigestion also offers the advantage of linking the treatment of organic industrial material to agricultural uses that recycle their effluents in agriculture. The multiplication of these installations (more than 3,500 in Germany in 2006) enables the transfer of material to be limited.
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11.6. Anaerobic digestion installations or biogas units Anaerobic digestion units, from the end of the 19th century, were initially designed to eliminate dangerous organic pollution, in particular from the sewage of big cities (the Acheres station treats the urban sludge of Paris). Research in energy production is more recent. For a long time, the biogas produced was considered a nuisance and not a resource. The first regulations in the last 30 years in France instituted the obligation to destroy biogas by combustion. It is only recently that the rules have been modified in favor of taking advantage of the biogas. 11.6.1. Techniques [SCH 01, WEL 02] The techniques implemented for anaerobic digestion of organic waste first sought to solve the mechanical problems of the fluids, such as: – introducing a flow of organic matter, dissolved to some extent, with particles in suspension into an airtight enclosure, – guaranteeing a specified residence time in the enclosure, – extracting the effluent and draining it under acceptable conditions. The following techniques are classified according to different criteria: – techniques that treat the matter continuously and discontinuously, – techniques that involve one or several stages, – dry or wet systems (less than 20% of dry material). 11.6.1.1. Digesters functioning with a continuous introduction of substrates 11.6.1.1.1. Infinitely mixed digester This is the type of digester that is most used. Variations come in the method of agitation. For agricultural installations, the agitator is fixed to the vertical wall or inclined. For digesters of industrial or urban sludge, the agitation is often in the center, eventually with an eductor tube. The agitation can also be due to a gas lift. The purpose of the agitation is to homogenize the digestion and avoid the formation of a surface crust from the particles in suspension.
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u
Figure 11.8. Principle of the completely mixed digester
11.6.1.1.2. Digester for recirculation of sludge (contact) In this case, the objective is to recycle the bacteria that are drained with the effluent into a classic infinitely mixed bath. Thus, the density of microorganisms is increased at the time of the introduction of new substrates.
u
Figure 11.9. Principle of the digester for recirculation of sludge
11.6.1.1.3. Digester with fixed cells This type of digester enables acceleration of the decomposition of organic material into biogas. The bacteria populations colonize the supports and are introduced slowly with the effluent. The main problem with fixed cell digesters is possible clogging. This technique is used for very liquid substrates.
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u
Figure 11.10. Principle of digester with fixed cells
11.6.1.1.4. High rate sludge (UASB) digester This is used a great deal for industrial and urban liquid substrates with a high soluble organic content.
u
Figure 11.11. Principle of the UASB digester
11.6.1.1.5. Piston digester The piston digester, unlike the infinitely mixed ones, preserves the introduction sequence. It allows for better management of the residence time. The substrate is immersed the whole time, so there is no risk of the substrates floating. On the other hand, the risk of clogging is not negligible. 10% of German digesters are of this type.
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Figure 11.12. Principle of the piston digester
11.6.1.2. Discontinuously functioning digesters (batch) The recovery of organic farm waste is not always carried out in a continuous state. In culture on manure, it is regularly recovered, but the time interval varies from several weeks to several months, depending on the mode of operation. Discontinuous digesters enable a response to these variations. Long considered primitive, discontinuous anaerobic digestion has been developing, especially with respect to the demands of organic agriculture. 11.6.2. Examples of recent agricultural anaerobic digestion installations We describe below the four agricultural biogas installations built in France in 2000. 11.6.2.1. Mr Claudepiere’s installation: liquid system The EARL2 Les Brimbelles operates in the commune of Mignéville, in Lorraine. It is a dairy farm, a conventional organic farm of 60 milk cows, or 107 UGB (measurement unit) counting the calves.
Figure 11.13. Anaerobic digestion unit of Mr Claudepierre (EARL Les Brimbelles) 2 Entreprise Agricole à Responsabilité Limitée (Agricultural Company Limited).
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In bringing its farm buildings into compliance with standards, EARL set up an installation for anaerobic digestion structured around the following objectives: – protection of the environment and water quality by the treatment of farming effluents, – creation of a service activity (delivery of renewable energy to the EDF3 grid), – production of milk of superior quality demanded by the Blamont cooperative for producing its Munster AOC “Val de Weiss”. Currently the installation annually treats all of the effluents from production as well as from the whey. The treated effluents are then spread over the farmlands (110 ha). The biogas has added value in cogeneration. The electricity produced is all sold to EDF. The heat is used for heating the digester, the residences, and the dairy, as well as for drying hay. The total investment for this installation was up to €160,000, of which €80,000 is subsidized at 60% for bringing the buildings into compliance and €80,000 at 50% for the biogas. The operators received aid from the PMPOA (for building compliance), from ADEME4 and the Regional Council. They estimated at 2,500 hours the time spent setting up the project and doing some of the work themselves. The installation produces an annual revenue of about €14,000 (€7,800 from the sale of electricity and €6,200 from savings in heat and fertilizer). The annual operating expenses, mainly for maintenance of the cogenerator, go up to €2,000. The payback time on the investment is six years.
Figure 11.14. Principle of the installation 3 Electricité de France. 4 Agence de l’Environnement et de la Maîtrise de l’Energie.
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Year of construction Waste for treatment
Tank volumes Anaerobic digestion
Cogeneration
Bovine liquid manure Black waste, green and white waters Whey Digester Storage of the digested material Residence time in the digester Temperature Gas production Electric power Thermal power Electricity production Heat production
2002–2003 1,200 m3/year 80 m3/year 540 t/year 235 m3 338 m3 80 days 30qC 350 m3/day 21 kW 42 kW 100,000 kWh/year 200,000 kWh/year
Table 11.2. Technical data from EARL
Investment details (anaerobic digestion and cogeneration) Cogeneration module Propane line Mixer pump Geo-membrane Stainless steel door Control box Exhaust fan Propane tank Total equipment purchases Study and construction permits Rental of earth moving equipment and trench Insulation Concrete and materials Total civil engineering expenses Heating and ECS connection External biogas connection Internal biogas connection Electric connection Propane connection Sundry small items Total internal connections EDF connection Conformity and certification Placing into service Total
Cost (€) 20,092 1,853 8,417 3,649 2,045 945 570 700 38,271 2,552 625 3,540 11,795 23,935 11,541 3,625 6,555 12,017 545 3,436 37,721 16,742 250 3,500 12,5119
Table 11.3. Details of investments for Mr Claudepierre’s anaerobic digestion unit
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11.6.2.2. The GAEC5 Oudet installation: liquid system The GAEC Oudet (three associates) operates in the Clavy Ewarby commune in the Ardennes. It is a dairy farm of 65 milk cows, or 107 UGB including calves. The livestock is housed in free stalls with covered feeding troughs with selflocking feeding gates. The rest of the herd is in deep litter. All the herd spends 7 months inside, and only 3 hours a day inside for the remaining 5 months, and thus produce about 1,100 m3 of liquid manure and 1,245 tons of manure a year. The buildings already conformed to standards before the anaerobic digestion project.
Figure 11.15. GAEC Oudet anaerobic digestion unit
GAEC designed and constructed its installation in association with Fachverband Biogas. The inauguration took place in May 2005. Currently, the installation treats 6 to 7 tons of cosubstrates daily: 90% liquid manure, 9% grain refuse and 1% grass. The installation's effluents are spread over the farmland (188 ha). Biogas is a value added product from cogeneration. The electricity produced is all sold and the heat is used for heating the digester and the operators’ two houses.
Figure 11.16. Diagram of operation of the GAEC Oudet anaerobic digestion unit 5 Groupement Agricole d’Exploitation en Commun (Agricultural Association).
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The total investment for this installation was €201,400 in 2004. The users received funds from the Regional Council, ADEME, the General Council, and the Credit Agricole Bank, up to 59% of the total investment. They estimated the time it took to build the project, doing some of the work themselves, to be 3,000 hours. The installation brings in an annual revenue of about €20,100 (95% in electricity and 5% in fertilizer savings). The annual operating expenses, mainly for maintenance of the cogenerator, approaches €2,260. The payback time on the investment is six years. Technical Data – GAEC OUDET Construction year Waste to be treated
2004 1,100 m3/year 300 t/year 25 t/year 600 m3 300 m3 1,000 m3 60 days 40°C 550 m3/day 30 kW 60 kW 250,000 kWh/year 500,000 kWh/year
Bovine liquid manure Grain waste Grass Digester Storage of gas Storage of digested material Residence time in the digester Temperature Gas production Electric power Thermal power Electricity production Heat production
Volumes
Anaerobic digestion
Cogeneration
Table 11.4. Technical data for GAEC Oudet
Investment details (2005)
Cost (€)
%
Earthwork
4,830
2
Digester (concrete, heat, covering, siding, other)
80,640
42
Technical location/housing
8,100
4
Pumps, mixers
27,700
14
Piping, hydraulic circuit
9,130
4
Desulfurization
300
–
Cogenerator
35,000
17
Electricity
13,700
7
Control panels
4,770
2
Connection to the grid
2,030
1
Assembling the project
6,400
3
Heating of the houses Total
8,840
4
201,440
100%
Table 11.5. Detail of investments for GAEC Oudet
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11.6.2.3. GAEC of the Chateau installation (under completion): mixed system
Figure 11.17. Anaerobic digestion unit of the GAEC of the Chateau under construction
Within the framework of bringing its farm buildings into code, the GAEC put in place an anaerobic digestion project designed to meet the following objectives: – environmental protection and water quality, – creation of a service activity: delivery of electricity to the grid and heat to the neighborhood, – receiving the public. The installation should treat 6 to 7 m3 of cosubstrates daily, in the form of liquid manure, wastes from grains, and energy cultures. The installation's effluents are spread over the farmlands. The biogas will have added value in cogeneration. The energy produced will all be sold to EDF. The heat will be used for heating the digester and three houses, as well as several public buildings (under consideration). Work began in July 2005. The total investment for the anaerobic digestion unit is €360,000. The owners should receive aid from the PMPOA (applied to the building standards), ADEME and the Regional Council. The installation should provide, counting only the resale of electricity, an annual revenue of about €55,000 (at 0.11 cents € per kW). The annual expenses will rise to €33,000. The return time on the investment should therefore be eight years, counting 50% in grants, and without counting the revenue from the sale of heat and savings in fertilizer.
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Figure 11.18. Principle of the Etrepigny GAEC anaerobic digestion unit
Technical Data – GAEC of the chateau Years of construction
2005–2006
Waste to be treated
Bovine liquid manure Grain wastes Grass clippings Corn silage, used Cooking oils
1,200 m3/year 150 t/year 1,000 t/year 560 t/year 50 t/year
Volumes
Digester piston Digester tank Storage tank
100 m3 736 m3 736 m3
Anaerobic digestion
Residence time in the digester piston Residence time in the digester tank Temperature of the digesters Gas production
15 days 90 days 0°C 945 m3/day
Cogeneration
Electric power Thermal power Electricity production Heat productions
77 kW 154 kW 500,000 kWh/year 100,000,000 kWh/year
Table 11.6. Technical data for the Etrepigny GAEC
11.6.2.4. Pierre Lebbe installation: solid system Mr and Mrs Lebbe are producers of organic goat’s cheese and beer. Their farm is located in the High Pyrenees, at Villefranque, and has 120 head of caprins (a goat-
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like animal). Their farm produces between 340 and 370 m3 of manure a year. The manure output is taken four times a year, 90 m3 each time, corresponding to a useful amount of manure output.
Loading a digester pit-boat
Figure 11.19. Mr Lebbe’s installation
EARL Lebbe manages a flock of 140 goats for cheese production at the farm. The farm also produces 6,000 bottles of artisanal beer a year which, like the cheese, has the “organic” label. The agricultural area in use is 24 hectares. All of the feed for the goats as well as the barley for beer production is produced on this land. EARL has four full-time employees. With respect to caring for the flock, the animals are in the goat shed all year, on straw. The manure is removed from the building four to five times a year. All of the animal waste is therefore in the form of manure, representing about 400 to 450 m3 per year. In addition, the enterprise produces 300 liters of waste from the treatment room each day (green and white waters) during the lactation period (11 months a year). Formerly, the manure was stacked next to the goat shed before being spread. When it rained, the surroundings were unmanageable, the liquid flowed away from the manure, diminishing its nutritive value. It was necessary to modify the management of the manure and to make a concrete slab. Without any aid to improve the farm buildings, Mr and Mrs Lebbe tried to make their investment profitable and turned to anaerobic digestion.
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They looked everywhere for information, especially from abroad, with the association of German farmers producing biogas (Fachverband Biogas) through the Eden (Energies Development Environment) company. Mr Lebbe became an administrator of the Eden company, which brings together members with different competences in terms of renewable energy who are open to developing energy practices. In this way EARL Lebbe began an anaerobic digestion project in 2000: – the first objective was to make a place for storing manure, and thus make the farm surroundings clean, – the second imperative was not to increase the work load required for filling the tanks. These two goals were achieved, although there were many unanticipated problems which, although not too severe, did take up time.
Figure 11.20. Principle of the EARL Lebbe installation
The anaerobic digestion unit is of the “discontinuous” type, inspired by the research of Ducellier-Isman, which consists of storing the manure in silage-type containers equipped with a plastic air tight cover. The operation’s three boat-type manure containers were set up in such a way that they can be used as discontinuous
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digesters. These were covered by a PVC tarp with installation of a system to immerse the manure, a system for draining the fluid, a heating slab with a biogas heating system, insulated with sprayed polyurethane. The three digesters are filled in turn. Each time the operator removes manure, he loads it into the digester. He closes the digester, covers the manure with water (notably green and white) in which the skirts of the PVC tarp have been doused in order to assure air tightness. Wooden bars are placed across the trenches every 60 cm to keep the manure from floating and from interfering with the water tightness of the hydraulic joint. Then the manure is left to ferment for about 6 months. At the end of the fermentation period, the water is drained from the digester and the digested manure can be spread. The heating slab enables the manure to be kept at an appropriate temperature. An option for recirculating the water was planned because of possible heating. This recirculation system also allows the bacteria populations to be stirred and spread over all of the working trenches. Using three manure digesters makes it possible to take the digested manure out only during the times for spreading it. This avoids setting up a temporary manure storage area and limits the nitrogen losses. Furthermore, in this nitrogen rich fermented manure, the nitrogen is in the form of ammonia, a form easily assimilated by plants. After composting, the manure is ripe (the color is fairly black) and spreadable in the same way as traditional manure. Pierre Lebbe estimates two hours of time dedicated to closing the trenches, besides the obvious time already allotted to spreading the manure: – The first benefit obtained is maintaining the nitrogen in the manure and its passage into the wastewater that immerses the manure. These waters, which will have turned black after composting, are used for the nitrogen fertilization of grains and meadows. – The second benefit comes in its use of biogas: - with the savings of 6,000 liters of fuel oil for heating the homes and dairy (under construction), - with self-sufficiency in gas for kilning the malt and beer making (under construction), - with a substantial reduction in the electricity bill (production of electricity with a cogenerator from the Eden company), - future use of natural gas for the vehicles.
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Financially, the installation cost €40,000 (without labor) and benefitted from a grant of €15,000 within the framework of a French territorial use contract. Currently, the annual revenue of the installation can be estimated as €5,000 (€4,000 in fuel savings and €1,000 savings in fertilizers), bringing a payoff on the investment in 10 years. Counting all of the available energy, 36,500 m3 of biogas/year (minimum estimate) at 55% methane producing 20,000 m3 of CH4, and if a liter of fuel oil is 50 cents euro, the annual revenue is €10,000 and, in four years, the anaerobic digestion installation has been amortized. Description
Cost
Manure housing
Walls Joints Power shovel Materials
8,667 3330 868 6,261
Anaerobic digestion
Tarps Insulation of soils Heating slab Insulation of walls Reinforced walls Plastic accessories Metal accessories Gas accessories Tank Insulation of tank Connection of the tank Various Cord Cogenerator renovation
3,300 3,300 2,213 780 1,628 3,855 1,268 700 433 1,460 818 818 420 1,700
Total
41,463 Table 11.7. Financial aspects of EARL Lebbe
Besides these financial savings, the farm is clean. It considerably reduces greenhouse gas emissions without depleting fossil carbon and all of the fertilizer value of the effluents and waste is recovered.
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11.7. Uses of biogas Organic material which ferments in the absence of air produces biogas: a mixture of methane (60%) and carbon gas (40%).
Figure 11.21. Composition of natural gas
This composition is very close to raw fossil natural gases. The composition of biogas is much simpler: a single hydrocarbon, methane and much less H2S than in natural Lacq gas. We can observe the presence of H2 linked to anaerobic digestion, but in very low proportions this often signals a malfunction and, eventually, blockage in the anaerobic digestion process. We observe several trace elements linked to contamination. Heavy metals are generally absent since they are precipitated in the presence of H2S. The uses of biogas can also be the same as for fossil natural gas. Those most used are the ones that require only a small amount of or no purification. It should be used in a specific locations, close to the production site. Currently, its major added value comes from cogeneration.
Figure 11.22. Composition of biogas
11.7.1. Thermal engine cogeneration Conditions of the sale of electricity: the July 2006 order regarding the new rate fixed for electricity from agricultural biogas enables an improvement in the profitability of anaerobic digestion units whose added value is cogeneration. Table
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11.8 summarizes the conditions of purchase of electricity produced from biogas for power smaller than 150 kWh. The abbreviation “E” corresponds to the total energy available. In the case of cogeneration, 30% of the energy is used as electricity, and 60% of thermal energy remains for use, assuming that there is a loss of 10%. Rate Rate in metropolitan France for power
<150 kW
Bonus/premium/subsidy for anaerobic digestion Bonus/premium/subsidy for energy efficiency
9 c€/kWh 2 c€/kWh
If E <40% If E >75%
0 c€/kWh 3 c€/kWh
Table 11.8. Details of the rate for purchase by EDF of electricity produced from biogas
Between 40 and 75%, the price is proportional to the amount of energy added. In other words, the rate varies between 11 and 14 c€/kWh. The operation of an electricity generating plant produces electricity and heat. Cogeneration is the term for electricity generating systems equipped with heat exchangers for recovering heat. Most engines can be used in cogeneration, so the total efficiency of the installation can then be more than 90%, but it is necessary to be able to use the hot fluid obtained. Cogenerators may be equipped with two types of thermal motors: – Dual-fuel engines: these are diesel engines with auto-ignition of the fuel by compression. Methane alone is not flammable under these conditions, unlike diesel fuel. Therefore, these groups run with a mixture of the following composition: 90– 95% biogas and 5–10% diesel fuel, which allows auto-ignition of the mixture. Given the annual operation time of this equipment, the effective consumption of diesel fuel remains high. This type of equipment is mainly recommended for modifying a set that is already on the site. – Engines with ignition control: these sets, derived from gasoline engines with carburetors, run exclusively on biogas and are the most widely seen in European installations. There are two types of electricity generators: – Synchronous: this type of generator works in parallel with the electricity grid, and also as a backup in case of power outages on the electricity grid. – Asynchronous: the cost is lower and the lifetime longer than a synchronous system, but this type of cogenerator does not enable functioning as a backup in case of a power outage on the electricity grid.
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One part of the heat produced (about 30%) is reserved for heating the digester, which must be maintained at a temperature close to 39ºC in mesophilic operation. The excess heat can be used in a heating network for domestic or commercial purposes.
Figure 11.23. Asynchronous biogas engine (30 kW, GAEC Oudet)
Figure 11.24. Example of heating network (GAEC Oudet commercial space)
11.7.2. Exclusively thermal use 11.7.2.1. Use of a boiler This method of adding value is the easiest to put into place. It is sufficient to use burners adapted to this type of fuel. Sensors of biogas quality (% CH4) are now necessary in order to adapt the conditions of combustion and to meet pollution emission standards.
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11.7.2.2. Use of a production system for cooling Thermal value added use in the form of heat is attractive during the winter period in Europe. During the summer and in hot countries, it can be attractive to produce cooling. There are different systems which enable production of cooling from gas, notably absorption systems (refrigerators using gas or petroleum). The production of electricity, heating, and cooling can be implemented in a single ensemble, called tri-generation. Such an ensemble was designed for the airport in Bordeaux, but biogas was replaced by fossil gas at the last minute for economic reasons, at a time when long-term development did not enter into the decision. 11.7.3. Fuel production
Figure 11.25. Purification process
The use of biogas as a fuel, that is, as a substitute for natural fossil gas (NVG: natural vehicle gas), requires purifying the biogas to NVG standards. Therefore, it is necessary to obtain a dry gas with 97% CH4, without H2S or any impurities, and it must be odorized. Various techniques are used worldwide and in the thousands of vehicles running in Europe. New Zealand was the pioneer country in the years from 1970 to 1990. Water washing: the CO2 and H2S are dissolved in water in a scrubbing tower, and the remaining methane is dried and compressed to 200 bars.
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This is the type of purification most used in Europe. All the installations constructed in France were built on this model: Amiens in 1990, Lille, Tours, Chambery.
Figure 11.26. Biogas purification unit of the Amiens plant for anaerobic digestion of household waste
11.7.3.1. PSA (pressure swing adsorption) purification The use of materials with adapted adsorbing properties (zeolites) under pressure gives excellent results. This technique is generally use as a final purification step and for safety after washing with water, as well as for drying the gas. These installations can also do well in supplying energy for transportation, and in injecting biogas into the natural gas grid, as long as the quality of the gas in the network is respected. This is already a practice in several European Union countries.
Figure 11.27. Helsingborg Clarifier in Sweden (output 350 m3/h)
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11.8. Conclusion: renewable natural gas and its challenges Among all the fossil fuels used, only methane has a renewable version, biogas. A feature of this renewable version is that it can be manufactured from all kinds of non-ligneous organic wastes. This type of substrate is present everywhere there is human activity. Therefore, the first important characteristic is the fact that renewable natural gas is accessible everywhere in the world, unlike fossil natural gas, which is present in only a few sites. The consequences are many: – produced on-site, there is no need to transport it long distances from production sites to consumer sites. Interconnection then only adds a measure of security and solidarity among the users, – produced everywhere, there can be no issue of greed and dramatic conflicts, as is the case today for gas and oil fields. This is a major element for peace in the world, – produced from renewable organic substrates, its production is renewable. There is no depletion of stocks as for fossil resources. The consumption of renewable natural gas has no inflationary effects, unlike the exhaustion of fossil reserves. Renewable natural gas can also be produced from non-ligneous cultures. In this case, its appeal depends on the production techniques of these cultures. The more energy consuming they are, the less attractive. The use of polluting components will obviate the environmental appeal of this type of production. The production techniques for renewable natural gas are simple, and the techniques for purifying the gas to the natural gas standards of the distribution network are simple as well. There is no prohibitive scale effect. Small producers and large ones can coexist. Anaerobic digestion is accessible to all and cannot be used to create energy monopolies. Methanization does not create dependence. It is a development tool, as much for the most developed countries as for the poor ones. Education and technical training are the main tools for its development. Renewable natural gas is produced from carbon from the atmosphere fixed by the photosynthesis of plants. There is no circulation of fossil carbon that has been contained in the earth for millions of years. With renewable natural gas, such activities as transportation, by land and air, can be pursued because they do not produce major pollution of the atmosphere. Propulsion by natural gas is already operational. Airlines (Tupolev), helicopters, trains (Linkoping-Stockholm) and many commercial or private vehicles already run on fossil and/or renewable natural gas. Motorization by methane is operational.
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Besides its energy use, renewable natural gas production and its purification open the way to carbon chemistry and the production of CO2 with very low environmental impact. 11.9. Bibliography [ATEE] www.biogaz.atee.fr. [BIOGAS] www.biogas.ch. [EDEN] www.eden-enr.org. [FACHVERBAND] www.fachverband-biogas.de. [FAR 95] DE LA FARGE B., Le biogaz, Masson, Paris, 1995. [LAG 79] LAGRANGE B., Biomethane, Edisud, Aix-en-Provence, 1979. [SCH 01] SCHULTZ H., EDER B., Biogas praxis, Ökobuch Verlag, Fribourg, 2001. [WEL 02] WELLINGER A. et al., Biogas handbuch, Verlag Wirz AG, Aarau, 2002.
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Chapter 12
Energy Production from Wood
12.1. Introduction: what is wood energy? Wood energy is a term that designates both wood fuel and energy systems using woody plants. Wood is the result of photosynthesis, that is, of the production of carbohydrates from solar energy. It is the third highest ranking energy source used in the world after oil and coal. It is now fully accepted that its reasonable exploitation contributes to maintaining the biochemical balance of the planet (carbon-neutral renewable vis-à-vis the greenhouse effect, very low sulfur contact, etc.) Not only is its combustion neutral, but the substitution of wood for fossil fuels avoids depletion of carbon, which is not always neutral but positive. As a result of successive oil crises, the interest in wood energy has given rise to new techniques, combining the management of combustion and automation of the loading of fuel. These new technologies use a wide range of fuels (log, briquette, granule, chip, bark) and also industrial products such as wood powder or waste from paper mills. Wood energy represents 50% of European and French renewable energy and thus it amounts to the largest share of the renewable energy market. This chapter will briefly present three overviews: – wood fuels, – basic principles of converting wood into energy, – heat generators on which the overwhelming majority of applications will be based. Chapter written by Frédéric DOUARD.
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Figure 12.1. Carbon cycle (source: Itebe)
Figure 12.2. Traditional wood energy in forest of Chaux, France (source: Itebe)
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12.2. Overview of wood fuels 12.2.1. Logs The log is the wood fuel used most in developed countries for individual heating applications. They are used in fireplaces, heating stoves, cooking stoves and manualfed boilers. In France this represents more than 80% of wood energy consumption, or the equivalent of nearly 8 million tons of oil. Logs are packaged in split billets (or quarters) 25, 33 and 50 cm in length. The most used unit of measure is the apparent volume, the stère: this is a group of logs a meter long, piled in order to make a cube 1 m high (35 cubic feet). The energy content of the logs is between 1,000 and 1,800 kWh per stère, depending on the density of the wood and its humidity. Logs are most often produced by manual techniques, but since the late l990s there have been more or less automatic machines that can saw and split logs productively and working conditions are much safer. The wood has to be thoroughly dried in manually fed equipment to guarantee proper burning. It is best to dry it under shelter for 6 to 12 months after splitting and sawing to reach a humidity of less than 25%. Wood with a high humidity rating results in poor combustion which pollutes and dirties the home, the exchanger, and the chimney flue, and can prematurely damage the equipment.
Figure 12.3. Agricultural saw-splitter, Pilke, Finland
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Figure 12.4. Platform for a saw-splitter, Pinosa, Italy
12.2.2. Densified wood logs or compacted logs Densified wood logs are made by hydraulic or mechanical presses, from dry or dried chips or sawdust. They take the form of cylinders or blocks from 7 to 10 cm in diameter and 20 to 30 cm in length. Their unit weight varies from 0.5 to 2 kg. Their energy content averages 4,600 kWh per ton for a humidity of 10%. They are packaged for retail sale as films or in cartons. It is also possible to have them delivered on pallets. These logs are used by manual feeding for domestic heating, but also in pizzerias, bakeries or restaurants.
Figure 12.5. Compacted logs, Holzbrik, Hertz, Austria
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Figure 12.6. Continuous press, Cimaj, France (source: Itebe)
This is a fuel conditioned and adapted to urban life since it is clean and takes up little space. Dry and ready for immediate use, it can be purchased on the street as needed and can be kept for a long time without any problem. 12.2.3. Briquettes Briquettes are the little sisters of compacted logs. They are manufactured according to the same principles, but at a reduced size: 5–6 cm in diameter and in length. They are produced in woodworking mills to reduce the volume of scraps from planning machines and are used in automatic furnaces.
Figure 12.7. Briquette press, Gross, Germany [KAL 01]
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Figure 12.8. Furnace for a complex fed with chips, Trondheim, Norway (source: Itebe)
12.2.4. Wood pellets Wood pellets are made from sawdust or finely ground wood. They are produced in industrial units from extruders from dried wood. This very dense fuel has a very high calorific value (4,700 kWh/ton). It comes in the form of cylinders from 6 to 9 mm in diameter and 3 cm in average length. Being very fluid, it is a very flexible fuel for wood stoves and automatically fed furnaces. It can be transported by trucks equipped with a pneumatic system to silos where other solid fuels cannot go. It has the lowest ash rate in the biomass market, from 0.5 to 1%, making maintenance very easy. It can be delivered in bulk – a form adapted for a furnace with a silo – or purchased in 15 kg bags – suitable for a wood stove. The production of this fuel, which has been growing since the beginning of the 1980s, has seen dazzling growth since 2004 because it corresponds perfectly to current consumer needs. The pellet is well on the way to becoming a vehicle of global trade in wood energy.
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The rapid development is currently being driven by the domestic sector and by green electricity power stations in certain European countries. However, the sustained increase in the price of petroleum lowers the threshold of profitability of the pellet in the collective and tertiary sectors as well.
Figure 12.9. Granule press, CPM, Savoiepan, France
Figure 12.10. Granulation factory, Hansa Graanul, Estonia
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12.2.5. Wood chips Logging and pruning produce a large number of branches and tree tops that can be shredded to supply automatic furnaces. Some trees that do not have other material value can be shredded in their entirety. Finally, the cultivation of trees specifically for energy can also serve as a primary chip material. The wood is chopped into lengths by a waste wood chipper in the forest or on a platform. The usual unit of measure is the apparent cubic meter of volume of the chips. Wood chipped in this way is voluminous and hard to transport because of its natural irregularity. It requires special storage facilities, with buried silos and removal equipment, which restrict it to high consumption users because of the relatively high investment costs. The chip is particularly sought after in complexes of furnaces or industrial furnaces where it figures as a very competitive fuel. Its energy characteristics, while good (2,500 to 3,500 kWh/t), are variable, since they depend on its moisture content. This can vary from 25 to 45%. The forestry chip is a product that can dry in 2 to 4 months under shelter by auto-warming. Its particle size allows it to be used in all types of fireboxes with automatic feed, from 15 kW to several hundred megawatts.
Figure 12.11. Forestry chips, place d’armes de Bière, Switzerland (source: Itebe)
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Figure 12.12. Mobile chipper Logset, Finland
12.2.6. Industrial chips Like branches, the chips from sawing lumber can be ground in the form of chips to feed automatic furnaces. The energy content of these chips is between 2,200 and 3,500 kWh per ton for a moisture variation of 25 to 50%. They are generally coarser than forestry chips and are not recommended for screw-fed installations.
Figure 12.13. Chips from a sawmill, Dold Sawmill, Germany (source: Itebe)
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Figure 12.14. Residue grinder at Randan, Puy-de Dôme, Weima, Germany (source: Itebe)
These residues are also a resource of choice for paper mills and particle board factories, and only a small part of these products are for energy. There are other residues, in enterprises producing a second transformation of wood (woodworking shops), but they are virtually all used for the needs of these enterprises themselves 12.2.7. Grindings from recycled wood Scrap wood is, by definition, wood at the end of its life. It has several origins: – factories for the second transformation of wood with residues of treated wood, – recycling centers with pieces of furniture, door frames and other wooden objects, – building enterprises with wood from workshops and demolition, – collection of packaging with palettes, crates, and other materials. Two major categories of recycled wood may be distinguished: – wood that is clean or with a small amount of mortar and concrete: wood that has undergone only mechanical transformation or chemical treatment without heavy metals or halogenation with fire extinguishing products. It can be used after extraction of iron in modern automatic furnaces with controlled combustion, without
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requiring supplementary smoke treatment: this includes pallets, crates, pressed wood panels, planks, etc., – wood treated with halogenated products or containing heavy metals must follow specific elimination systems such as incineration: this includes railroad ties, cable poles, plastic-coated furniture, outdoor furniture and certain construction woods, such as those covered with lead paint.
Figure 12.15. Recycled wood grinder, Denmark (source: Itebe)
Figure 12.16. Pallet grinder, Denmark (source: Itebe)
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The energy content of recycled wood grindings is good, averaging between 3,300 and 3,900 kWh/ton for moisture from 20 to 40%. The ash rate can be high, because of the impossibility of removing 100% of the metals. Recycled wood is used in furnaces with grates or with a fluidized bed because of its size heterogenity and its possible high ash rate. 12.2.8. Ground bark Removing bark from wood is practiced to improve the conditions for sawing as well as to ensure the discharge of chips in paper making. Softwoods are almost all debarked, which is not the case for all hardwoods. Barks are coarse products, damp, usually dirty, and they have a high mineral content. The resulting ash rates from the combustion of bark can therefore reach 7% in the case of oak, for example. It is strongly recommended to grind the barks of softwood trees or poplars, and to remove stones, before using them in a furnace. The barks of hardwood trees can be used raw because they are less fibrous and the type of debarked wood used produces small pieces. Barks have the same characteristics of density and calorific power as wood itself, from 2,800 to 1,800 kWh per ton for a humidity variant of 40 to 60% during heating. The volume of bark produced can represent up to 15% of the volume of the log for some softwood trees. Energy is not the only product from bark, which is also commercialized as mulch, improving vineyards and used as a substrate for humus or treatment of biosolids.
Figure 12.17. Softwood bark (source: Itebe)
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Bark can be used only in furnaces with heated grates or fluidized beds because of its high moisture and high ash rate. For the most part, bark is transported in large volume conveyor trucks to minimize transportation costs. Bark in fact is a low energy fuel so the transportation cost per unit of energy is high. This fact must be taken into account in boiler room design with respect to access, unloading, and rate of filling the silo. Although bark was the main fuel for wood furnaces in European communities only five years ago, because of its ease of collection, this tendency is now being reversed in favor of wood pellets, which are more abundant. 12.2.9. Sawdust and wood chips We have seen earlier that sawdust and shavings are used in the manufacture of compressed logs, briquettes and pellets: in effect this is how they are used commercially. Chips, for example, are less economical products to transport given their low density (0.12 to 0.15). Damp sawdust from sawmills is hard to burn in fireboxes with fixed grates, excluding them from the largest part of the small housing complex furnaces market in rural areas.
Figure 12.18. Dry shavings from planing (source: Itebe)
These products are therefore burned on site by the companies themselves when they are not transformed. Fixed grate or bottom draft fireboxes are then used for the dry products. Damp products are used with moving, even vibrating, grates since it is hard for the combustive fuel of fixed fireboxes to strike through it.
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12.2.10. Wood powder Pulverized wood exists virtually only in Sweden. It comes from sawdust or other scraps of raw wood finely ground to less than 1 mm, then dried to less than 10%. It is used as a fuel by injection into reclassified conventional fuel oil furnaces, and the wood needs only a small amount of special treatment. Its calorific power is high: 4,700 kWh/t.
Figure 12.19. Wood powder burner, Talloil, Sweden
12.2.11. Roasted wood Roasted wood comes in the form of partially charred chips. This easily flammable fuel with high calorific power (4,500 kWh/t) has, for the moment, found few applications and is used as a substitute for wood charcoal for barbecues or as a carbon cofuel for electricity production. 12.2.12. Wood charcoal Charcoal comes from the carbonization process (a slow pyrolysis) to eliminate most of the gas from wood. This process is almost always carried out on the artisan,
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even ancestral, scale, the gas products being liberated into the atmosphere without any benefit, generating pollution and significant energy waste. The energy conversion efficiency does not exceed 15% in traditional kilns. There are, however, industrial processes, such as pyrolysis towers with circulation and recycling of the gas in the wood to dry it, but they are seldom used. The largest quantities are produced in hot and humid areas on site in the forest in traditional kilns or in bell furnaces near logging roads. In arid countries, this production is sedentary. European production is very low. Known in rich countries only for leisure barbecuing use, or in its active form for manufacturing filters, charcoal is for hundreds of millions of people a basic energy resource for cooking food or for the operation of metallurgic factories, notably in Brazil.
Figure 12.20. Charcoal burner, Senegal (source: Itebe)
12.2.13. Spent pulping liquors and paper mill sludge Spent pulping liquors and paper mill sludge are formed during extraction of the cellulose to produce paper fibers in turning them into paper paste. They contain residues of lignin, many organic products, water, and chemical products used in their extraction. These products can be used as fuels within the paper production process itself, to produce steam and electricity.
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This electricity production can be carried out on the grate by cocombustion with other lignin products such as bark or with charcoal, since these products have little dry material. Gasification is also under study. 12.3. Principles of conversion of wood into energy 12.3.1. Combustion Direct combustion in heating installations has until now dominated the processes for the conversion of energy from wood. The combustion installations serve to produce heat, which is also used for secondary energy (in the form of steam, for example), final energy (in heating networks) or else useful energy (radiant heat for cooking, for example). In all other procedures for conversion into energy, biomass generates secondary fuels such as synthetic gas, liquid fuels from pyrolysis, or roasted wood and charcoal. These secondary fuels are usually transformed either by a process of subsequent combustion (internal combustion engine, combustion chamber of a gas turbine or in some kind of heating installation). Thus, the processes for taking advantage of wood energy currently used always include at least one combustion stage which takes place either by direct combustion of the fuel or by combustion of a secondary fuel from wood. Combustion corresponds to oxidation of the fuels by oxygen, an exothermic reaction. This reaction is the result of a complex chain of chemical transformations of the ligneous matter subjected to conditions of temperature and oxygenation that result in reducing this matter into three agents: heat, combustion gases and minerals.
Figure 12.21. Combustion system (source: Verenum)
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The combustion of wood can thus be divided into three stages: drying, gasification and, finally, oxidation of the combustible gases and of the solid carbon. There is also an ephemeral intermediary phase of pyrolysis just before the gasification, but it is hardly observable.
Figure 12.22. Staging of the temperatures in combustion of wood (source: Itebe)
Drying is a specific stage for biomass fuels containing water: this phase is carried out by ventilation and drying of the fuel. It is an endothermic phase. Pyrolysis and gasification of the complex molecules into simple molecules that can be oxidized is also an endothermic phase that takes place in a large temperature range, depending on the complexity of the molecules to be broken up. Finally, oxidation, an exothermic reaction, reduces the gases (CO, CxHy and H2) and the charcoal into simple elements, carbon dioxide and water.
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Figure 12.23. Chemical process of wood combustion (source: Verenum)
This thermal process can be carried out completely depending on a certain number of conditions and only under these conditions will the combustion of the wood be complete, the efficiency high, the production of pollutants minimal and the smoke invisible except in cold weather.
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Figure 12.24. Materialization of the stages of wood combustion in a furnace (source: Veremum)
The conditions of this efficient combustion are: – the presence of three basic elements: fuel (containing hydrocarbons), oxidizer and heat, – a separation of the phases: drying, pyrolysis/gasification, combustion, – a sufficient temperature: 650 to 950ºC, – the right fuel mixture (wood gas)/oxidizer (secondary air), – a sufficient reaction time (residence time), – an excess of air approaching the optimal conditions of the boiler and the stoichiometric value (control of air seals and regulation). Modern wood furnaces reach combustion efficiencies higher than 90% for dry wood. 12.3.2. Pyrolysis Pyrolysis is a thermal operation decomposing the organic molecules of wood into simpler elements. This endothermic destruction begins at 150 to 200ºC, depending on the nature of the wood, and continues to 600ºC or more. In “normal”
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wood combustion, a brief pyrolysis operates under the effect of the flame coming from the burning of the first gases to escape from the wood and under the action of incandescent radiation of the charcoal. However, the objective of controlled pyrolysis is not to burn the wood, but to obtain intermediate fuels. The operation takes place in the absence of air in order to avoid the formation of a flame. We can distinguish long pyrolysis at moderate temperature (400ºC, “roasting” and curing) and rapid pyrolysis at higher temperatures (400 to 600ºC). Rapid pyrolysis, now under experimentation, enables the synthesis of renewable hydrocarbons, still called second generation biofuels, to be used as high energy liquid biofuels capable of use in vehicles. Many types of reactors have been and are being tested for rapid or flash pyrolysis, and fluidized bed reactors are among the most highly developed and are the only ones to be industrialized.
Figure 12.25. Pyrolysis oil. Dynamotive, Vancouver, Canada
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Figure 12.26. Diagram of the principle of the Dynamotive procedure
12.3.3. Gasification The gasification of wood enables the transformation of solid wood into gas fuel. This gas is composed essentially of carbon monoxide, hydrogen, methane, ashes and tars coming from pyrolysis. The gasification of wood is obtained in the presence of a mixture of steam and oxygen. It is in fact the injection of cold and humid air over the bed of charcoal that creates a reduction of the carbon atoms into carbon monoxide and hydrogen, leading in tandem to the formation of methane according to several reactions of oxidation and reduction. There are several types of gasifiers with fixed beds, with co- or counter-currents and with fluidized beds. The traditional processes of gasification with air at low temperature and low pressure uses a fixed bed with co-current, that is, the fuel comes down from the reactor and the gases exit in the same direction downward after having crossed the bed of charcoal embers. They produce a poor gas composed of carbon monoxide, hydrogen and almost 50% nitrogen and carbon dioxide gases, which are non-
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combustible. We say that the gas is poor due to its low calorific power of between 4.5 and 58 MJ/mN3, but its composition is adapted to the conventional combustion engines for the production of electricity and heat. The energy efficiency from conversion of gasifiers with co-current is in the order of 70 to 80%. These gasifiers must work using dried wood of uniform size. They are adapted to low electric power installations of less than 500 kW.
Figure 12.27. Xylowatt Platform, Belgium
Figure 12.28. Fixed bed gasification with co-current (source: UCL-Xylowatt)
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Fixed bed gasification on a large scale uses the counter-current principle, that is, the gas does not pass over embers but rises across cool fuel. This product produces a gas of higher energy quality and can use damper wood, taking advantage of the heat brought by the rising gases. Electric power plants of several tens of electric megawatts can thus be supplied with gas by these large size installations.
Figure 12.29. Counter-current gasification reactor, Harbore, Volund , Denmark
Fixed bed gasifiers are finally undergoing experimentation and, like all boilers with the same bed, they will be used for installations of much greater power.
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12.4. Generators of thermal energy from wood 12.4.1. Domestic technologies Domestic technologies are the most common, and these comprise the majority of the market, both European and worldwide. The applications are domestic heating and food preparation. 12.4.1.1. Hearths, ovens and other open fireplaces Ovens and hearths are usually made by artisans and the great majority are of a very ancient design. Very similar ovens were already producing ceramics and pottery in Asia 15,000 years ago. This kind of equipment worked exclusively by radiation from the oven for the purpose of heating a room or cooking on the hearth. Most of the thermal production of the fuel, coming from the flames and representing 70% of the energy produced, escaped into the atmosphere because it was not transferred by a surface or an exchanger. Thus, the simplicity of such devices resulted in a heat recovery efficiency that was simply disastrous.
Figure 12.30. Diagram of the principle of an open hearth fireplace, France
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Figure 12.31. Pizzeria oven, Marana Forni, Italy
Furthermore, the total absence of a means of controlling the conditions of combustion (fuel and temperature) produces very low efficiency and local pollution from unburned hydrocarbons, carbon monoxide and unburned dust particles. In developed countries, these devices are classified as leisure or traditional equipment rather than as appropriate equipment for heating or cooking. In poor countries, these open hearth installations represent the majority of usage and, except collective, commercial, artisanal or industrial installations, domestic installations are of a very rudimentary design. 12.4.1.2. Closed hearth fireplaces These devices, originating in France, usually of cast iron, appeared in the 1970s at the same time as electric heating. They brought the beginning of a solution with respect to the limited ability to regulate air from open hearths and they improve combustion temperature. Thus, we can begin to speak of efficiency of combustion and heating. On the other hand, the differentiation of the stages of combustion (drying, gasification and combustion) is not managed in these low volumes. Only a few manufacturers have successfully integrated a small post-combustion chamber of refractory material, allowing some secondary air to oxidize the combustion gases at the best temperatures. Regarding heat recovery, radiation from the glass added to convection from the source is eventually recovered by a gain in hot air directed toward other rooms.
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Figure 12.32. Closed hearth fireplace (source: Ademe-Eliope)
Figure 12.33. Closed hearth with post-combustion, Buderus, Germany [KAL 01]
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The comfort of using these devices is low, partly because of the strong temperature gradient in front of the device, and partly because of the combustion quality, which generates a lot of unburned material and requires daily maintenance (insufficient temperatures and poor distribution of primary air). 12.4.1.3. Heating and cooking stoves There are two types: convection and radiation. 12.4.1.3.1. Convectors Convectors are closed stoves made of metal, sometimes lightly covered by an accumulating material, which radiates around them but without much thermal capacity. Their main advantages are convection and its speed of action, the investment cost, and their transportability, allowing them to be moved if necessary. There are many varieties and aesthetic aspects play an increasingly large role in their popularity. With respect to combustion, the performance is slightly better than that of closed hearth fireplaces because the hearths have greater confinement (reduced dimensions) and better airtightness (the small size of the doors). With respect to heat recovery, the performance is also higher. The fact that they are entirely in the room produces a high rate of radiation and a less direct escape of gas tends to reduce the speed of escape of the gas, giving them more time for transferring the heat. On the other hand, the fact that these devices are more airtight allows their operation to be slowed down, resulting in reduced polluting conditions and low gasification temperatures with the production of tar.
Figure 12.34. Wood burning stoves. Spectrum, Pacific Energy, Canada
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Figure 12.35. Cooking stove. Binder, Austria
12.4.1.3.2. Radiant heating stoves Radiant heating stoves, or accumulation stoves, are by design and use diametrically opposed. Whereas the convector is used in an impromptu manner, the radiating stove can be used only by planning in advance. Its large thermal capacity leads to prolonged heating, but requires a long time for preheating. These devices are equipped with a classical closed firebox and they differ from convectors in two ways: live combustion of short duration and maximum heat recovery. The accumulation takes place around the smoke flue, which makes a long circuit in the interior of the mass of the stove before escaping. This circuit, calculated with precision according to its losses of load, allows heat to transfer to the ceramic flue, then to the radiating surface of the stove, covered with tiles, bricks or other accumulating material. The calculations are made by expert artisans so that the gases’ escape temperature is just a little higher than 100ºC, to avoid eventual condensation, but in particular to obtain maximum recovery of the energy produced. This recovered energy can then be restored to the living space in 12 hours. For this reason, accumulation stoves are very heavy and some of them weigh several tons: these are devices to be placed in the center of the house, and they constitute excellent central heating for houses with good heat circulation and low thermal capacity. These are immovable stoves and it is advisable to build the house around them rather than try to put them in place once the house has been built.
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Regarding combustion, being able to accumulate heat and restore it in 12 hours means it is not necessary to build a permanent fire: a live fire with good, hot combustion twice a day makes it possible to maintain great and very homogenous comfort in the house, thanks to infrared radiation. These stoves have been a traditional design in central Europe for centuries, and are related to the ancient Roman furnaces, known as hypocausts, which is why they are called hypocaust stoves by some Roman–Swiss colleagues. Finally, and so that we can speak a little about cooking, an oven can be connected to these stoves near the flame for those who love wood-fire cooking, as long as the fire is made before the meal! 12.4.1.4. Wood-fired boilers Wood-fired boilers have changed a lot in 30 years and the only resemblance to their coal-burning ancestors is in outward appearance. Derived from cast iron coalburning boilers during the middle of the last century, they have progressively integrated the differences required by the combustion of wood. Wood combustion is more like gas combustion than that of a solid. For sufficiently heated wood, 85% of its anhydrous mass is transformed into gas, reducing by 15% direct similarities with the combustion of coal. The volume of the firebox, for example, must be capable of accommodating this volume of gas, plus the volume of steam from drying. The differentiation of the phases of combustion in manually fed devices takes place first in the boilers. Thus we have come from a single chamber, where all the steps of combustion were unfortunately mixed together, to a succession of chambers: a storage chamber for the fuel, the firebox, a post-combustion chamber, and finally the exchanger. This differentiation came about in the l980s with the socalled turbo boilers, with downward flame and forced draft. This technology has succeeded boilers with upward combustion and natural draft, which do not allow differentiation of the stages or the presence of an efficient exchanger because of the energy loss it brings about. Nevertheless, boilers with inverse draw and natural draw have been able to maintain themselves by good management of the phenomenon and by precision of installation with respect to the draw in the flues, whose importance is critical throughout in the case of natural draw. Inverted combustion boilers, such as the upward combustion boiler, are always available on the market. However, the problems of corrosion in the fuel chamber due to too little drainage of the gases during the pauses, problems that have now been solved, have led many manufacturers to the principle of horizontal combustion.
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Figure 12.36. Diagram of wood-fired boiler installation with accumulation (source: Itebe)
Figure 12.37. Diagram of high efficiency wood boiler (Schmid, Switzerland) [BUH 89]
Horizontal combustion boilers have confronted the problems of container corrosion by kinetics that are more natural. Well-researched post-combustion chambers have been added, allowing some manufacturers to claim 90% efficiency! Such efficiency really depends on the equipment design and also on the way it is
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used. Therefore, a good number of manufacturers guarantee performance and maintenance of their equipment only for hydraulic installations with accumulation. On the same principle as accumulation heating stoves, firing it up well once a day proves to work well as much for combustion as for comfort. The accumulation in fact makes it possible to guarantee, beyond combustion performance, heating durations unsurpassed with continuous feed. 12.4.1.5. Pellet stoves The pellet stove is an example of the advantages of electronics. Thanks to a regular fuel that is dry and fluid, and to the electrification of the stove controls, electronics can regulate, according to options, all of the operations: thermostatic control of the power by adjusting the intakes of air and wood from a programmed algorithm, automatic or remote ignition, shut-off following a long slow-down period. This regulation, combined with the simplicity of natural combustion of the wood pellet and an air or water heat exchanger, gives the pellet heating stove the performance level of a good automatic boiler while at the same time it is used domestically and has the appearance of a wood fire. This realistic aesthetic feature has a lot to do with the success of pellet heating stoves, as well, of course, as their ability to operate on their own between two feeds for 48 to more than 100 hours, depending on the model and the installation. These stoves are convection stoves par excellence, but they can also work as a boiler with a water circuit (different ranges). We should note the noise sometimes caused by the ventilation of the combustion, particularly with certain manufacturers. From a technical point of view, the pellet is taken out of the reservoir by a worm that serves as a firewall when it goes up and when it is in front of a discharge chute for the fuel. Other models insert the pellet into a crucible with a push screw, after passing into a firewall lock. This crucible has the ability to automatically discharge the ashes. For other burners with discharge chutes, daily cleaning out of the ashes is necessary because they are sensitive to ash accumulation, which eventually interferes with combustion and blackens the window. A low ash rate, less than 0.5%, is required for a number of materials, a level that corresponds to the German Pellet Din Plus standard. Manufacturers have recently developed burners with discharge chutes that have a mobile grille in order to space out the maintenance operations, even to allow pellet feed with a higher rate of ashes. All these screws are sensitive to the presence of sawdust in the fuel, and standardization recommends a maximum level of 0.7% of the mass, otherwise the screws are blocked by the accumulation of sawdust.
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Finally, it should be emphasized that pellet stoves operate with a draft that is close to zero (0.1 mm CE) and do not require a chimney that is piped or very high. Some duct manufacturers have even suggested systems of vent ducts to be placed in the walls as for gas or oil furnaces.
Figure 12.38. Envirofire, pellet stove, Canada
Figure 12.39. Thermorossi pellet stove, Italy
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12.4.1.6. Pellet boilers Pellet boilers are the boiler version of stoves, the difference being that supply is done from a silo fed in bulk by a pneumatic mobile tanker. The installations can thus easily have a year without service and therefore in practice approaches the convenience of fuel oil. Manufacturers have competed in ingenuity in recent years by inventing all kinds of prefabricated silos, rigid or flexible, that the user can locate inside or outside their house, in the basement, upstairs, even buried under the flowers in the garden. The transfer from the product to the boiler works either by rigid or flexible screws or by suction, thus enabling the fuel to overcome obstacles between the silo and the boiler. There are many burner technologies, and the performance of pellet boilers routinely exceeds 90%, reaching 100% PCI for those that use condensation. The maintenance on these boilers is minimal, at the very most requiring a few interventions a year plus an annual cleaning.
Figure 12.40. Cutaway view of a pellet boiler, Hargassner, Austria
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Figure 12.41. Condensation pellet boiler, Okofen, Austria
12.4.1.7. Domestic wood chip boilers Although feeding and combustion in pellet boilers is very simple, there are aspects that are very delicate in the case of wood chips. The wood chip is a rough fuel that packs down, caves in and does not flow. For this reason there are very few solutions for getting them into the silo except for traditional dumping. In order to retrieve them automatically, manufacturers have installed specialized scrapers that scrape out the chips in the direction of the transfer screw. A firewall well and several other security devices (thermal vanes, sequencing of the batching screw) form a barrier to the potential rising fire.
Figure 12.42. Small domestic wood chip boiler, Hertz, Austria
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Figure 12.43. KWB wood chip boiler, Austria
Combustion is easy as long as the fuel does not exceed 35% moisture content, which remains a requirement in all fixed grille domestic boilers. A limited number of manufacturers offer movable grille boilers that enable consumption of wood with up to 45% moisture, but at a higher investment cost. The dropping and scraping out of these small boilers is done automatically for most of the new models, guaranteeing optimal performance of the heat exchangers. With regard to the regulation of combustion, more and more manufacturers, especially the Austrians, offer the option of regulation with an exhaust gas oxygen sensor, enabling control of the rate of oxygen in the flue gas and regulating the combustion automatically, especially when the fuel varies. 12.4.2. Housing complexes or industrial technologies The technologies for industrial wood combustion have been used for centuries in businesses in all sectors. They have been greatly modernized over the last 30 years, made automatic, and now they are used mainly in wood companies and local housing complexes. 12.4.2.1. Hot air boilers and generators with fixed grilles Fixed grille hot air boilers or generators operate on the same principle as domestic wood chip boilers. They can be easily supplied from sources other than
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wood chips, for example from shavings or dry sawdust in woodworking shops by means of a pneumatic conveyor to the batching screw.
Figure 12.44. Diagram of a Köb wood chip boiler, Austria
Figure 12.45. Wood chip boiler for Schid complex, Switzerland [NUS 97]
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Some fixed grilles are flat and do not tolerate a relative humidity of 35%, but the large majority of fixed grilles from this point on are made of a bowl-shaped firebox, with built in bars of a cast iron and chrome alloy that the primary air passes through, drying the wood and cooling the grille. This technique was derived from fixed coal hearths of the Volcan type. Ash removal from bowl-shaped boxes is by air. In other words, it is the primary air that blows the ashes from the sides of the bowl and the user must clean out the surplus once a week or once a month, depending on the situation, and depending especially on the amount of wood ash. This is because these grilles are sensitive to clogging of the bars. Once they are blocked, they no longer allow the primary air to pass through and lend all its force to the fire, whose combustion is now disturbed and, finally, no longer cools the grille or the bed of ashes. The ashes begin to melt and form scoria (slag), then glass, if cleaning is delayed. It is for this reason that mobile or moving grille fireboxes are preferred for fuel with an ash rate higher than 3%. The pneumatic cleaning out of flues offered as an option by most manufacturers is recommended starting from several hundred kW in order to guarantee a good permanent heat exchange. Fixed grille boilers are available from 15 to 5,000 kW. 12.4.2.2. Boilers with moving or mobile grilles Constructed on the same general principle as fixed grille boilers, these horizontal boilers are equipped to handle high humidity, 60% for some of them, and ash rates higher than 5% and considerably more. They are made of large quantities of refractory concrete to give them high thermal capacity and the ability to dry the wood as it is inserted. The drying takes place from the vault that overhangs the grille, heated by the radiation from the embers on the grille that in turn radiates over the fuel as it enters. The grille acts as a wave that causes the fuel to be slowly rotated over itself in order to agitate it and make it move forward. The surface of the grille has an area proportional to the power of the boiler for a given PCI, its length and its conveying speed are calculated so that once the fuel is completely spent it is entirely reduced into minerals before falling into the ash pan. The moving grilles today are practically horizontal and consist of hundreds of bars of chrome cast iron some 30 cm long and about 12 cm wide, placed on the axes and moved by hydraulic cylinders. Ash elimination from these burners is always mechanized, and automatic cleaning is also advised. Ash removal is accomplished by dry or wet means, the dry means being more demanding in terms of the rate of unburned matter, but guaranteeing an easier means of agronomic added value because of the pulverized nature of the product obtained, rather than mud. Mobile grille boilers can also be chosen in order to use dry fuels with a high rate of ash, such as culled wood or even straw from grains. In this case, specific adjustments are necessary to give greater
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cooling of the grille that has been subjected to high temperatures, and to avoid melting of the ashes. For the same reason, the refractory vault is greatly reduced in length in order to limit the temperature of the firebox. Mobile grille boilers are available from 25 kW to 25 MW.
Figure 12.46. Diagram of a moving grille boiler, Jämforsen, Sweden
Figure 12.47. Boiler of the Pinnggau heating grid, Kohlbach, Austria
12.4.2.3. Boilers with rotating conical grilles A unique design by the Finnish manufacturer Sermet, then taken over by Wärtsilä, combines the firebox for pushing fixed grilles in its center and a mobile grille on the considerable surface of a cone, radiated by a vault of the same shape. The cone is a rotating one, some of the circulating grilles turning in the opposite direction to the others in order to make the product inserted in the middle pour down, like an erupting volcano. Such a drying surface allows this technology to
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achieve efficient consumption of fuel that is two-thirds water, where all other grille technologies would have been extinguished.
Figure 12.48. Diagram of the Wärtsilä biogrille, Finland
Figure 12.49. Wärtsilä boiler, Finland [PAA 00]
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Available in a range from 2 to 25 MW, this technique enables the burning of bark, peat or wood coming straight out of the forest or sawmills in the middle of winter. 12.4.2.4. Boilers with vibrating grilles The conical boiler is a transition from horizontal boilers and vertical boilers which, starting from 25 MW, has completely replaced the former because of technical limitations. The vertical grille boiler which is the simplest and also the most recent in design has a vibrating grille. Made of two or four overlapping plates, slightly inclined and moving backwards and forwards, the vibrating grille is adapted to fine fuels with low ash. It is simple to maintain and cools by primary air or by the water in the boiler. The absence of large quantities of ash in the fuel and mechanical joints under the grille allows preheating of the primary air up to 300ºC and thus acceptance of very damp fuel.
Figure 12.50. Diagram of the principle of the Velund vibrating grille, Denmark
12.4.2.5. Boilers with rolling grilles This technology, developed by the Babcock firm for burning coal, has been reemployed for co-combustion with dry biomass. A derating of the power of the boiler
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comes combined with the lowering of the PCI introduced, but the reuse of such technologies for biomass is relatively easy without too large an investment. It enables high power generators (65 MW), often unused following the reduction in the market share for coal, to be made available
Figure 12.51. Diagram of the Babcock Ignifluid rolling grille boiler
12.4.2.6. Bottom draft or air injection boilers These boilers are vertical or horizontal boilers, but without a grille. Boilers for fuel oil that are converted to the horizontal, or for wood that have a base plate for the vertical, these boilers burn fine wood particles that are sprinkled or showered. Injected at the top or the middle of the firebox, the particles fall lightly because of their low weight, and they are consumed once they reach the bottom of the furnace. These boilers are very simple, very inexpensive, without thermal capacity, without mechanics and often their ashes are cleaned manually in the establishments where they are used. Bottom draft boilers are the simplest: an airflow carries in the sawdust or shavings by a simple pipe, the gases from the flue gas are put in cyclonic motion at the exit in order to clean out the flying cinders. Injection boilers use powder burners and, in the case of wood, consume the powder from ground wood or fine
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particles from sanding, especially in factories building pressed wood panels. These boilers are used in a range of powers that are relatively low, from 1 to 15 MW.
Figure 12.52. Diagram of the Seeger bottom draft boiler
12.4.2.7. Boilers with boiling fluidized beds When the grilles undergo too many constraints because of the power demanded or the temperatures required, especially for producing high pressure steam and electricity, boilers with boiling fluidized beds take over.
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In a boiler with a hot fluidized bed, air is introduced into the bottom of the furnace to fluidize the bed with the combustible. The thickness of this bed usually goes up to 1 m. The dimension of the particles may go up to 10 cm. For reasons of overheating the firebox around the bed, this technology is limited to 20 MW.
s
Figure 12.53. Diagram of the principle of the Sandwell furnace with boiling fluidized bed, USA
12.4.2.8. Boilers with circulating fluidized beds At higher powers, from 25 to 600 MW, the circulation of the bed helps to distribute heat from the furnace, making it possible to reach levels of pressure for the steam produced that are more favorable for the production of electricity. The Alholmens Kraft power plant at Pietarsaari in Finland (500 MWth) reaches temperatures of 545ºC after superheating and pressures of l65 bars, generating an electrical efficiency greater than 40% for an effective power of 240 MW. Boilers with fluidized beds are also marvelous devices with universality and flexibility because they can use virtually all possible fuels, separately or together, making security of supply of the installations and the economics of the projects much more certain. For example, at Grenoble, the only French installation with a fluidized bed to use wood, the heating company uses it in cocombustion, depending on market conditions, with household waste, animal meal, fuel oil, natural gas, charcoal and wood for its supply.
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Figure 12.54. Diagram of a Kvaerner boiler with circulating fluidized bed, Finland [EUBIO 03]
Figure 12.55. Boiler room of the postern at Grenoble, heating company, France
12.5. Conclusion At the dawn of the 21st century, the bio-energy system is still, for most consumers and even professionals, a universe that is complex, diverse, operated by empirical methods and often having a backward-looking image.
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If it is true for wood that the fuels and the technologies are many, it is also true for petroleum fuels, except that in this case, all the systems come from the same source, the industrial refinery. Therefore, it was easier for this sector to focus its research and training efforts and the processes were developed in the hands of a very limited number of players. For biomass, a resource widely available for hundreds of millions of people, the coordination of research or training is never imposed until the scientific knowledge relating to the environment reveal practical inefficiencies or, on the contrary, good for the climate! The difficulty of understanding this sector also comes from the fact that it is shared between two worlds: that of forestry and farming and that of energy specialists, two worlds that are culturally very different and have never really mixed; the professional structures recently created are only beginning to work together. At the crossroads of the economy and the environment, now profiting from some 30 years of technological developments carried out for the most part in the small and medium enterprises (SMEs) sector, wood energy is in the process of entering the world energy market: this is proof of its new maturity and its unanimous attraction for the world market. 12.6. Bibliography [BUH 89] BÜHLER R., Chauffages centraux au bois, Office fédéral des questions conjoncturelles, Berne, 1989. [EUBIO 03] EUBIO, Eubio VTT Processes Biomass Co-firing: An Efficient Way to Reduce Greenhouse Gas Emissions, Jyväskylä, Finland, 2003. [HAR 03] HARTMANN H., THUNEKE K., HÖLDRICH A., ROSSMANN P., Handbuch Bioenergie – Kleinanlagen, Fachagentur Nachwachsende Rohstoffe e. V., Gülzow, 2003. [KAL 01] KALTSCHMITT M., HARTMANN H., “Energie aus Biomasse”, Grundlagen, Techniken und Verfahren, Springer Verlag, Berlin, 2001. [MAR 99] MARUTZKY R., SEEGER K., Energie aus Holz und anderer Biomasse Grundlagen, Technik, Entsorgung, Recht, DRW-Verlag, Stuttgart, 1999. [NUS 97] NUSSBAUMER T., Energie aus Holz: Vergleich der Verfahren zur Produktion von Wärme, Stromund Treibstoff aus Holz, Bundesamt für Energiewirtschaft (BEW) ENET, Berne, 1997. [PAA 00] PAANANEN M., “Wood fuel basic information pack”, Jyväskylä Science Park, Jyväskylä, 2000.
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List of Authors
Nathalie ALAZARD-TOUX IFP Rueil-Malmaison France
Raymond CHENAL MHyLab Montcherand Switzerland
Seddik BACHA LEG Grenoble France
Aline CHOULOT MHyLab Montcherand Switzerland
Régine BELHOMME EDF DRD Clamart France
Alain CLÉMENT LMF Ecole centrale de Nantes France
Hamid BEN AHMED SATIE Antenne de Bretagne de l’ENS de Cachan Bruz France
Vincent DENIS MHyLab Montcherand Switzerland
Daniel CHATROUX CEA Grenoble France
Frédéric DOUARD ITEBE Lons le Saunier France Jean-Luc DUPLAN IFP Lyon Vernaison France
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Alain FERRIÈRE PROMES Font-Romeu France Stéphane HIS IFP Rueil-Malmaison France Florence JAUDIN BRGM Orleans France Pierre LABEYRIE EDEN Colomiers France
Jean-Claude MULLER InESS – CNRS Louis Pasteur University Strasbourg France Bernard MULTON SATIE Antenne de Bretagne de l’ENS de Cachan Bruz France Daniel ROYE LEG Grenoble France
Nicolas LAVERDURE Lycée Pierre Termier Grenoble France
Marie RUELLAN SATIE Antenne de Bretagne de l’ENS de Cachan Bruz France
Laurent LE BEL BRGM Orleans France
Jean-Claude SABONNADIÈRE LEG Grenoble France
Christophe MARVILLET INES Le Bourget-du-Lac France
Julien SEIGNEURBIEUX SATIE Antenne de Bretagne de l’ENS de Cachan Bruz France
Frédéric MONOT IFP Lyon Vernaison France
Norbert TISSOT MHyLab Montcherand Switzerland
Index
A Acetogenesis, 400, 403 Acidogenesis, 400 Apparent motion of the sun, 48, 49, 50 Aquifer, 265-311, 320-325, 358 Architectures, 28-42 B Bagasse, 331, 355, 360, 393 BGR, 313 Biodiesel, 333, 339, 344-351, 381-394 Bioethanol, 332, 352, 354, 373, 380 Biofuel, 329-393 Biogas, 397-425 Biomass to Liquid (BtL), 361 Boiler, 83, 45, 46, 61, 289, 291, 295, 421, 439, 445, 455-470 BRGM, 270, 313, 318, 325, 326 Briquette, 431
Concentration, 5, 16, 66, 69-74, 305, 331, 377, 379 Concentrator, 67-80, 90, 293 Conduction, 4, 56, 261, 263, 307 Connection, 26, 28-43, 76, 84, 94, 104, 121-124, 131, 135, 143-163, 170, 214217, 234, 253, 297, 298, 312, 314, 410 Cooper basin, 316 Crystalline silicon, 1, 4, 5, 10, 14, 15 D-E Diffusion, 5, 15, 18, 65, 185, 255, 271, 344 Dogger, 265, 267, 289, 290, 321 EGS, 266, 317 EMVO, 298, 388, 391 Enthalpy, 263-268, 286, 291, 301-310, 318, 320 Enzyme, 374-390, 395 Ethanol, 330-345, 351-357, 373-395 Exergy Inc., 309
C F Canadian well, 279 Cellulases, 376-379 Cellulose, 358-362, 373-379, 395, 441 Circulating fluidized beds, 469 Collector, 45, 48-60, 64, 73-79, 183, 196, 271, 272, 278 Combined cycle, 68-83, 92, 302, 304, 319
FAME, 331-351, 381-394 Fischer-Tropsch, 345, 362, 367, 368, 370373 Flicker, 149, 165-168 Frequency, 13, 29, 31-38, 119, 132, 139, 149-174, 180, 190, 191, 198, 214, 247
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G Gasification, 361, 366, 394, 442, 447 GDY, 317 Generators, 42, 119, 133-139, 145-158, 169, 174, 179, 183, 193-199, 206-221, 247, 255, 299, 420, 427, 461, 467 Geothermal energy, 261-268, 278, 287-312, 318 power plant, 266, 286, 295-311, 318 probe, 267, 280-286, 321 Geysers, 262 Glycerin, 332 Grid, 25 Grille, 457-467 Ground bark, 438 H, I Harmonics, 29, 149, 150, 165, 215 HDR, 313, 326 Heat exchanger, 323 Heat pump, 267, 282, 293 Heat transfer fluid, 69-92, 283, 312, 323 Heating of buildings, 54, 61 Hemicellulose, 358, 362, 375 Husavik, 309 Hydropower scheme, 228 Hydropower, 204, 207-210, 228-234, 257-259 Inverter, 30-43, 130-135, 180, 215
Monocrystalline silicon, 5 MPPT, 28, 30, 33, 38, 41 Multicrystalline silicon, 3-10 O-Q Ocean biomass, 186 Ocean thermal energy, 220 Pellet boiler, 459, 460 stove, 457, 458 Pelton turbine, 227, 236-238 Permeability, 310, 312, 314, 319, 323 Photovoltaic conversion, 3 electricity, 1-22 systems, 25 Phytomass, 186 Porosity, 263, 266, 309, 324 Protections, 37, 38, 156, 162 Pyrolysis, 5, 362-372, 440-447 R Rankine, 64-100, 303-309, 323 Reactive power, 26, 29, 42, 122-136, 148, 152-171, 180, 215 Real power, 126, 133 Roasted wood, 440 Rosemanowes, 313 S
J-L Kaplan turbine, 194, 205, 237-240 Lignocellulose, 329, 334, 344, 345, 354362, 372-379, 389-395 Log, 427-430, 438 M-N Maximum power extraction, 28 Methane, 181, 347, 362-369, 397-404, 418-424, 447 Methanization, 424 Methanol, 332, 337, 344-351, 371, 387
Sawdust, 430-440, 457, 462, 467 SEGS, 88 Silicon, 1-23, 82 Small hydropower, 228-238, 244-258 Solar flux, 48-56, 65 Solar heating devices, 48 Solar power stations, 63, 64, 70, 88, 90101 Stability, 79-81, 117, 134, 153, 154, 163, 167, 183, 189, 322, 351, 354, 401 Synchronous machine, 126, 132-135, 145-179, 205 System operator, 177
Index T
U-Z
Thermal solar collectors, 54 Tidal currents, 185, 202, 206 Torrefaction, 365 Trichoderma, 377 Turbine in drinking water networks, 241 in wastewater networks, 242
Wood charcoal, 440 chips, 434 pellets, 432
477
