Energy Resources and Systems, Volume 2: Renewable Resources

Energy Resources and Systems Tushar K. Ghosh Mark A. Prelas Energy Resources and Systems Volume 2: Renewable Res...

Author: Tushar K. Ghosh

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Energy Resources and Systems

Tushar K. Ghosh

Mark A. Prelas

Energy Resources and Systems Volume 2: Renewable Resources

123

Tushar K. Ghosh Nuclear Science & Engineering Institute University of Missouri, Columbia Lafferre Hall E 2434 65211 Columbia Missouri USA [email protected]

Mark A. Prelas Nuclear Science & Engineering Institute University of Missouri, Columbia Lafferre Hall E 2434 65211 Columbia Missouri USA [email protected]

ISBN 978-94-007-1401-4 e-ISBN 978-94-007-1402-1 DOI 10.1007/978-94-007-1402-1 Springer Dordrecht Heidelberg London New York Library of Congress Control Number: 2009928307 c Springer Science+Business Media B.V. 2011 No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission from the Publisher, with the exception of any material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Cover design: eStudio Calamar S.L. Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com)

Preface

Energy is the lifeblood of civilization. Access to relatively inexpensive and plentiful energy has been and will continue to be its driving force. In the past few years, the reality of the fragile nature of an oil dominated energy infrastructure has become apparent. Civilization is engaged in a life and death struggle to redefine its primary energy resources and to find transitional solutions without invoking chaos. Time is relatively short and it will be in the hands of the current generation of students to solve. The authors began working on Energy Resources and Systems in 1996. The goal of the authors was to provide a comprehensive series of texts on the interlinking of the nature of energy resources, the systems that utilize them, the environmental effects, the socioeconomic impact, the political aspects and governing policies. Volume 1 on Fundamentals and Non Renewable Resources was published in 2009. It blends fundamental concepts with an understanding of the non-renewable resources that dominate today’s society. The second volume of Energy Resources and Systems is focused on renewable energy resources. Renewable energy mainly comes from wind, solar, hydropower, geothermal, ocean, bioenergy, ethanol and hydrogen. Each of these energy resources is important and growing. For example, high-head hydroelectric energy is a well established energy resource and already contributes about 20% of the world’s electricity. Some countries have significant high-head resources and produce the bulk of their electrical power by this method (e.g., Norway-over 98%, Paraguay100% and Brazil-85%). However, the bulk of the world’s high-head hydroelectric resources have not been exploited, particularly by the underdeveloped countries. Low-head hydroelectric is unexploited and has the potential to be a growth area. Wind energy is the fastest growing of the renewable energy resources for the electricity generation. Solar energy is a popular renewable energy resource. About 891015 watt (W) of solar energy is absorbed annually by the earth’s land mass and oceans. However, this only translates to around 1,000 W m2 spread over the earth’s surface area. The diffuse nature of solar energy has limited its growth because it is difficult to base systems on resources with a low energy density. v

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Geothermal energy is viable near volcanic areas. Iceland for example has taken advantage of its geothermal resources in that it produces 24% of its electricity and heats 87% of its buildings with it. Bioenergy and ethanol have grown in recent years primarily due to changes in public policy meant to encourage its usage. Energy policies stimulated the growth of ethanol, for example, with the unintended side effect of rise in food prices. Hydrogen has been pushed as a transportation fuel. The chapters on advanced energy resources were included in Volume 2 in the initial outline. However, the size of Volume 2 became too large, so it was decided to split renewable energy resources and advanced energy resources into separate volumes. The new Volume 3, on nuclear advanced energy resources and nuclear batteries, consists of fusion, space power systems, nuclear energy conversion, nuclear batteries and advanced power, fuel cells and energy storage. Volume 4 will cover environmental effects, remediation and policy. Solutions to providing long term, stable and economical energy is a complex problem, which links social, economical, technical and environmental issues. It is the goal of the four volume Energy Resources and Systems series to tell the whole story and provide the background required by students of energy to understand the complex nature of the problem and the importance of linking social, economical, technical and environmental issues. One thing is for certain, the business as usual model is losing favor. The historic positions of political parties and environmental groups of supporting only a selective group of energy resources have significantly changed. Even public attitude has changed with a widespread acceptance of renewable resources and nuclear energy. Part of this change is due to a more analytical approach to assessing the broad based risks of each type of energy resource and having confidence in risk management approaches. There appears to be a universal recognition that the future mix of energy resources will have to be diverse thus mitigating the risks associated with each individual energy resource. We would like to acknowledge the assistance of our students Jason B. Rothenberger, Daniel E. Montenegro, and Eric D. Lukosi, who read, edited, and commented upon various parts of the manuscript. Columbia, Missouri, USA May 12, 2010

Tushar K. Ghosh Mark A. Prelas

Contents

1

Wind Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.2 Harvesting Energy from Wind . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.3 Wind Resource Map .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.4 Land Area Requirement . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.5 Energy and Power from Wind. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.5.1 Betz Limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.6 Capacity Factor for a Wind Turbine . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.7 Energy Production .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.8 Turbine Types .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.8.1 Horizontal Axis Wind Turbines (HAWTs) . . . . . . . . . . . . . . . . . 1.8.2 Vertical Axis Wind Turbines (VAWTs) .. . . . . . . . . . . . . . . . . . . . 1.9 Comparison Between Turbines . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.10 Cost of Electricity from Wind Energy . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.10.1 Payback Time for Wind Energy Systems . . . . . . . . . . . . . . . . . . . 1.10.2 Cost Reduction Efforts.. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.11 Effect of Capacity Factor .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.12 Industrial Wind Turbines . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.13 Basic Principles of Wind Resource Evaluation . .. . . . . . . . . . . . . . . . . . . . 1.14 Wind Farm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.14.1 Offshore Wind Farm . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.15 Small Wind Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.16 Low Frequency Noise form Wind Turbines . . . . . .. . . . . . . . . . . . . . . . . . . . 1.16.1 Sound Intensity.. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.16.2 Sound Pressure .. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.16.3 The Sound Pressure Level . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.17 Wind Energy and Intermittency .. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1.18 Summary .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .

1 1 6 8 16 20 21 23 24 25 26 32 35 38 43 45 47 48 48 51 53 58 61 63 64 64 66 67 69 vii

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2 Solar Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.1 Energy from the Sun .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.2 Energy Transfer to the Earth . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.2.1 Seasonal Variation . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.2.2 Height of the Sun in the Sky.. . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.3 Energy and the Sun . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.4 Use of Solar Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.4.1 Solar Thermal Energy . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.5 Concentrating Solar Power (CSP) . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.5.1 Trough Systems . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.5.2 Power Tower Systems . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.5.3 Dish/Engine Systems . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.6 Solar Thermal Molten Salt Technology . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.7 Photovoltaics .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.7.1 PV Theory .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.7.2 The Efficiency of Photovoltaic Cells . . . .. . . . . . . . . . . . . . . . . . . . 2.7.3 The “Sun” Value . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.7.4 Effect of Thickness of the Cell . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.7.5 The Effect of Temperature .. . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.7.6 Effect of Dopant Concentration . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.8 From Cells to Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.9 Solar Cell Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.9.1 Semiconducting Materials for Solar Cell . . . . . . . . . . . . . . . . . . . 2.10 Multijunction Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.11 Hybrid Power Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.12 Solar Lighting .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 2.13 Summary .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .

79 79 80 81 83 83 85 87 102 102 104 104 107 110 112 120 129 131 131 134 136 136 137 140 141 141 142 144

3 Hydropower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 3.2 Hydropower Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 3.3 Hydropower System Construction Methods.. . . . .. . . . . . . . . . . . . . . . . . . . 3.3.1 Impoundment.. . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 3.4 Hydroturbine .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 3.4.1 Impulse Turbine .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 3.4.2 Reaction Turbine .. . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 3.5 Selection of Turbines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 3.6 Run-of-the-River Hydropower Systems . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 3.7 Small Hydroelectric Power System . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 3.7.1 Components of a Small Hydro Power System . . . . . . . . . . . . . . 3.8 Micro-Head Hydropower Systems . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 3.8.1 Selection of Turbine for Small or Micro Head Systems . . . 3.9 Pumped Storage Hydropower System . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .

157 157 161 164 164 168 169 174 185 185 186 188 190 191 192

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3.10 Calculation of Power from Water Flow . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 3.10.1 Local Head Losses . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 3.10.2 Head Losses in Open Channels . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 3.11 Hydropower System Efficiency . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 3.12 Fish Ladder and Fish Passage in Hydropower Systems . . . . . . . . . . . . . 3.13 Summary .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .

194 200 202 205 205 207 210

4 Geothermal Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 4.2 Resource Identification . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 4.3 Geothermal Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 4.4 Applications.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 4.4.1 Electricity Generation.. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 4.4.2 Direct Use of Geothermal Energy .. . . . . .. . . . . . . . . . . . . . . . . . . . 4.4.3 Ambient Ground Heat/Geothermal Heat Pump . . . . . . . . . . . . 4.5 Summary .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .

217 217 219 223 224 227 238 243 256 258

5 Ocean Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 5.2 Wave Power .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 5.3 Theory .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 5.3.1 Linear Wave Theory . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 5.3.2 Energy Transport and Power . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 5.3.3 Applicability of Linear Wave Theory . . .. . . . . . . . . . . . . . . . . . . . 5.3.4 Significant Wave Height . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 5.4 Wave Power .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 5.4.1 Tapered Channel (TAPCHAN) . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 5.4.2 Terminator Device/Oscillating Water Column.. . . . . . . . . . . . . 5.4.3 Point Absorber . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 5.4.4 Attenuator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 5.4.5 Overtopping Devices. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 5.5 Tidal Current Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 5.5.1 Tidal Barrage Method.. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 5.5.2 Principles of Operation . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 5.5.3 Tidal Lagoons . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 5.5.4 Tidal Fence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 5.5.5 Tidal Turbine Method .. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 5.5.6 Linear Lift Mechanism or Oscillating Hydroplane Systems . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 5.5.7 Venture Based Systems . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 5.6 Tidal Farm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .

267 267 268 271 273 276 280 280 282 282 283 284 285 289 292 296 296 301 302 302 307 307 309

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5.7

Ocean Thermal Energy Conversion (OTEC) . . . . .. . . . . . . . . . . . . . . . . . . . 5.7.1 Closed-Cycle OTEC System . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 5.7.2 Open-Cycle OTEC System . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 5.7.3 Hybrid OTEC System. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 5.7.4 Components of an OTEC System . . . . . . .. . . . . . . . . . . . . . . . . . . . 5.7.5 Byproducts of OTEC System . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 5.8 Summary .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .

309 312 315 317 317 318 318 320

6 Bioenergy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 6.2 Energy Source of Biomass . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 6.3 Composition of Biomass . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 6.3.1 Lignocellulosic Biomass. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 6.3.2 Hemicellulose . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 6.3.3 Lignin .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 6.4 Types of Biomass .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 6.5 Biomass Resources, Land Requirement, and Production .. . . . . . . . . . . 6.5.1 Energy Crops Production Area . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 6.5.2 Lignocellulosic Based Biomass . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 6.5.3 Land for Biomass . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 6.6 Wood Fuel.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 6.6.1 Unit of Wood .. . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 6.6.2 Wood Burning .. . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 6.7 Use of Biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 6.7.1 Process Heat and Steam Generation . . . .. . . . . . . . . . . . . . . . . . . . 6.7.2 Electric Power Generation .. . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 6.8 Biomethane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 6.9 BioFuels .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 6.9.1 Biodiesel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 6.9.2 Biofuel Production Method.. . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 6.10 Biofeedstock for Industrial Chemicals. . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 6.11 Summary .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .

327 327 333 335 336 338 338 338 341 344 345 347 356 360 362 362 363 369 378 381 385 386 389 393 396

7 Ethanol .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 7.2 Ethanol Production from Corn . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 7.2.1 Structure, Types, and Composition of Corn . . . . . . . . . . . . . . . . 7.2.2 Processing of Corn .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 7.2.3 Fermentation Process . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 7.2.4 Byproducts from Corn Processing . . . . . .. . . . . . . . . . . . . . . . . . . . 7.2.5 Comparison Between Dry Mill and Wet Mill Processes .. . 7.3 Sugar Crop Fermentation.. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 7.4 Corn Versus Sugarcane .. . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .

419 419 424 425 430 435 436 436 442 443

Contents

7.5

xi

Production of Ethanol from Cellulosic Biomass .. . . . . . . . . . . . . . . . . . . . 7.5.1 Pretreatment .. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 7.5.2 Hydrolysis .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 7.5.3 Fermentation and Process Integration .. .. . . . . . . . . . . . . . . . . . . . 7.6 Energy Balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 7.7 DDGS Market.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 7.8 Water Requirements for Corn Growing . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 7.9 Fuel Ethanol Quality Comparison . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 7.10 E-Diesel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 7.11 Summary .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .

443 445 449 454 458 464 464 465 468 468 471

8 Hydrogen Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.2 Hydrogen Economy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.3 Hydrogen Demand.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.4 Hydrogen Internal Combustion Engine .. . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.5 Hydrogen Production Methods . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.5.1 Reforming of Natural Gas . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.5.2 Biomass Gasification . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.5.3 Reforming of Biofuel . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.5.4 Hydrogen from Coal . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.5.5 High-Temperature Water Splitting . . . . . .. . . . . . . . . . . . . . . . . . . . 8.6 Nuclear Energy for Hydrogen Production .. . . . . . .. . . . . . . . . . . . . . . . . . . . 8.6.1 Water Electrolysis .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.6.2 High Temperature Electrolysis (HTE) of Steam.. . . . . . . . . . . 8.6.3 Thermochemical Water Splitting . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.7 Solar Energy for Hydrogen Production .. . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.7.1 High-Temperature Water Splitting-Solar Concentrators . . . 8.7.2 Solar Reforming of Natural Gas. . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.7.3 Thermochemical Solar Cycle. . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.8 Electrolytic Process .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.8.1 Alkaline Electrolysis .. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.9 Thermochemical Hybrid Cycles . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.10 Hydrogen from Wind Energy . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.11 Hydrogen from Biomass . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.12 Photolytic Processes .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.12.1 Photobiological Water Splitting . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.12.2 Photocatalytical Processes . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.13 Cost of Hydrogen Production . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.14 Hydrogen Storage.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.14.1 High Pressure Cylinder . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.14.2 Liquid Hydrogen Storage System .. . . . . .. . . . . . . . . . . . . . . . . . . . 8.14.3 Carbon and Other High Surface Area Materials .. . . . . . . . . . .

495 495 497 497 500 501 503 508 508 509 509 510 510 512 515 543 544 545 548 555 555 555 557 560 560 560 563 566 566 568 573 575

xii

Contents

8.14.4 Clathrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.14.5 Hydrides .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.15 Comparison of Hydrogen Storage Capacity . . . . . .. . . . . . . . . . . . . . . . . . . . 8.16 Hydrogen Delivery Methods . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 8.17 Summary .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .

576 576 586 586 587 590

Appendices .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . Appendix 1: Wind Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . Appendix 2: Solar Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . Appendix 3: Hydropower .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . Appendix 4: Geothermal Energy . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . Appendix 5: Ocean Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . Appendix 6: Bioenergy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . Appendix 7: Ethanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . Appendix 8: Hydrogen .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .

631 631 646 659 661 667 670 711 715

Index . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 719

Chapter 1

Wind Energy

Abstract The kinetic energy of wind is harvested using wind turbines to generate electricity. Among various renewable energy sources, wind energy is the second most technologically advanced renewable energy source; hydropower is the first. Although there is a significant potential for converting wind energy to electricity, a number of issues must be addressed before it can be used to its full potential. Wind blows in every corner of the earth; however, it does not blow constantly. In addition, it must maintain a certain speed to be effective for running a wind turbine and generating electricity. In this chapter, various aspects of wind energy including types of wind turbines, onshore and offshore wind farms, the cost of wind energy, and steps necessary for installing a wind turbine are discussed.

1.1 Introduction The utilization of wind energy can be dated back to as early as 5000 B.C., when wind energy propelled boats were sailing along the Nile River. By 200 B.C., the use of windmills in China for pumping water was documented. Vertical-axis windmills with woven reed sails were used for grinding grain in Persia and the Middle East. During that time period, the primary applications were for grain grinding and water pumping. Between 1850 and 1970, over six million, mostly small (one horsepower or less) wind mills were installed in the U.S. alone for conversion of the wind energy to the mechanical energy. The primary use was water-pumping for stock watering and meeting the water needs of farms and homes. Very large windmills, with rotors up to 18 m in diameter, were used to pump water for the steam railroad trains that provided the primary source of commercial transportation in areas where there were no navigable rivers. The historical perspective of wind energy development has been discussed by a number of researchers [1–12]. Wind energy got a big boost following the OPEC (Organization of Petroleum Exporting Countries) Oil Embargo of 1973, when several countries started investing T.K. Ghosh and M.A. Prelas, Energy Resources and Systems: Volume 2: Renewable Resources, DOI 10.1007/978-94-007-1402-1 1, © Springer Science+Business Media B.V. 2011

1

2

1 Wind Energy

Fig. 1.1 History of installed wind power capacity in the USA

in wind power related technologies. Federal and state tax incentives and aggressive government research programs triggered the development and use of many new wind turbine designs. A wide variety of small-scale models became available for home, farm, and remote areas. A new market for wind energy generated electricity, wind farms, began in the early 1980s. In the USA, the Public Utility Regulatory Policies Act of 1978 promoted wind farms. This legislation required utilities to buy electricity from private, non-utility individuals and developers that are using renewable energy resources for electricity generation. California, USA, led the use of wind energy, which was nick named as The Great California Wind Rush, when thousands of wind mills were delivered to the wind program in California in the early eighties. By 1997, nearly 2% of California’s electricity was generated by the wind. As the cost of the technology continued to decline, other areas in the USA, namely the Great Plains, Pacific Northwest and Northeast, began development of wind farms. As can be seen from Fig. 1.1, the installed capacity of wind energy in the USA has increased dramatically over the last several years. Modern wind turbines using advanced technologies are able to produce electricity at a affordable cost for homes, businesses, and even utilities. In the late nineteenth century, the multi-blade windmill design was introduced to generate electricity. A discussion on the current status of the wind energy can be found in various articles [13–24]. Wind power continues to prosper as new turbine designs are helping to reduce costs of wind power and make wind turbines economically viable in more places in the world. Worldwide capacity of wind energy for electricity production has increased from 58,982 MW in 2005 to about 93,864 MW in 2007; an increase

1.1

Introduction

3

Table 1.1 Installed wind power capacity of various countries (end of year data) Rank 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

Nation Germany Spain United States India Denmark (incl. Faroe) China Italy United Kingdom Portugal France Netherlands Canada Japan Austria Australia Greece Ireland Sweden Norway Brazil Egypt Belgium Taiwan South Korea New Zealand Poland Morocco Mexico Finland Ukraine Costa Rica Hungary Lithuania Turkey Czech Republic Iran Rest of Europe Rest of America Rest of Asia Rest of Africa and Middle East Rest of Oceania World total

Source: Global Wind Energy Council [25]

2005 (MW) 18;000 10;028 9;149 4;430 3;132 1;260 1;718 1;332 1;022 757 1;219 683 1;061 819 708 573 496 510 267 29 145 167 104 98 169 83 64 3 82 77 71 18 6 20 28 23 129 109 38 31 12 59,091 MW

2006 (MW) 20;621 11;615 11;603 6;270 3;136 2;604 2;123 1;963 1;716 1;567 1;560 1;459 1;394 965 817 746 745 572 314 237 230 193 188 173 171 153 124 88 86 86 74 61 55 51 50 48 163 109 38 31 12 74,223 MW

132000 2009

2008

94122 2007

2006

2005

47686 2004

2003

31164 2002

24320 2001

18039 2000

1999

9663

0

1998

20000

7475

40000

13696

60000

39290

80000

59004

100000

74133

120000

109000

140000

1997

Cumulative Installed Capacity (MW)

Prediction

Actual

160000

160000

1 Wind Energy

2010

4

Year Fig. 1.2 Projected increase in world wind power installed capacity (Adapted from Global Wind Energy Council [25])

of about 35% in 3 years. This amount currently contributes less than 1% of worldwide electricity use. However, the wind generated electricity accounts for 23% of electricity use in Denmark, 6% in Germany and approximately 8% in Spain. The current wind-powered electricity generation capacity of various countries is shown in Table 1.1. Europe is leading the world in utilization of wind energy. The European Wind Energy Association (EWEA) is projecting Europe’s wind-generating capacity to be about 75,000 MW in 2010 and 180,000 MW in 2020. The increase in the installed capacity in the world is shown in Fig. 1.2. Also, several countries are evaluating their wind energy potential to further increase the generation capacity. The activities related to wind energy in various countries are discussed by a number of researchers [26–56]. The USA is also making a major push towards the use of more wind energy. The installed capacity in the USA has almost doubled to 40,180 MW in 2010 from that in 2008. This is becoming possible as legislators in several states are trying to increase the share of electricity generation by renewable energy sources. The current wind power capacity in various states of the USA is shown in Fig. 1.3. However, it is clear from Fig. 1.4 that wind energy is still contributing only a small fraction of total electricity generation in any state in the USA.

1.1

Introduction

5

Fig. 1.3 Installed wind power capacity in the USA as of 2010 (Courtesy of the National Renewable Energy Laboratory [57])

Washington 2.0% Oregon 3.5%

Montana 1.7% Idaho 1.6%

Wyoming 1.6%

North Dakota 1.8% South Dakota 2.6 Nebraska 0.7%

California 2.6%

Colorado 1.3%

New Mexico 3.9%

VT 0.2%

Minnesota 4.6% Wiscons 0.2%

New York 0.6% Pennsylvania 0.2%

lowa 5.5%

Kansas 2.3% Oklahoma 2.6%

ME 0.6%

Illinois 0.3

Tennessee 0.1% > 1,000 MW 100 MW-1,000 MW < 100 MW

Texas 2.0% Alaska 0.1% State generation as reported by the Energy Information Agency

Hawaii 1.3%

Wind generation estimated by AWEA, using capacity totals as of end 2007

Fig. 1.4 Percentage of electricity produced from wind energy by various states in the USA (Courtesy of American Wind Energy Association [58])

6

1 Wind Energy

1.2 Harvesting Energy from Wind The term wind energy or wind power is referred to the process by which the wind is captured to generate electricity. About 1–2% of 174,423,000,000,000kW h of energy that the sun radiates to the earth per hour are converted into wind energy. That is about 50–100 times more than the energy converted into biomass by all plants on the earth. Wind blows due to the uneven heating of the atmosphere by the sun, the irregularities of the earth’s surface, and rotation of the earth. Wind flow patterns depend on the earth’s terrain, oceans, and vegetative coverage. Locally, buildings, plants and mountains control the wind pattern and also the speed. The wind flow or the kinetic energy in the wind is harvested by wind turbines to generate either mechanical power or electricity. Wind turbines first convert the kinetic energy in the wind into the mechanical power, which then rotates a shaft to generate electricity. The mechanical power can also be used for other tasks, such as for grinding grain or pumping water. The wind rises from the equator and moves north and south to the higher layers of the atmosphere. In the equator, there will be a low pressure area close to ground level attracting winds from the north and south. At the poles, there will be high pressure due to the cooling of the air. Once air has been set in motion by these pressure gradients and the rotation of the earth, it undergoes an apparent deflection called the Coriolis force. Around 30ı latitude, in both hemispheres, the Coriolis force prevents the air from moving much farther. At this latitude there is a high pressure area, as the air begins sinking down again. As shown in Fig. 1.5, air is deflected to the right by the Coriolis force in the northern hemisphere as it moves

Fig. 1.5 A demonstration of the Coriolis force

1.2

Harvesting Energy from Wind

7

from high to low pressure. In the southern hemisphere, air moves from high to low pressure and is deflected to the left by the Coriolis force. The amount of deflection that air makes is directly related to both the speed at which the air is moving and its latitude; both are important factors in designing wind mills. In 1835, Gustave-Gaspard Coriolis, a French scientist, first described mathematically this phenomenon, giving his name to the Coriolis force. The Coriolis force is a fictitious force exerted on a body when it moves in a rotating reference frame. It is called a fictitious force because it does not appear when the motion is expressed in an inertial frame of reference. The acceleration corresponding to the Coriolis force is given by: aCoriolis D 2. v/ D 2.v /

(1.1)

where is the angular velocity of the rotating reference frame and v is the radial velocity of a particle relative to the center of the rotating reference frame. The Coriolis force can be expressed as: FCoriolis D 2m.v /

(1.2)

where m is the mass of the object causing the Coriolis force. Since the centripetal force is balanced by the Coriolis force on a body at a radial distance r from the center of a frame of reference, rotating with angular velocity , the relationship between and v is given by: v2 D 2v (1.3) r Substituting, v D !r ! D 2 (1.4) The Coriolis parameter .f / is defined as twice the vertical component of the earth’s angular velocity about the local vertical axis, and at latitude , and is given by: f D 2 sin

(1.5)

Due to the bending force of the Coriolis force, the following general direction for the prevailing wind results (Table 1.2). The prevailing wind sets the trend in speed and direction of wind over a particular point on the earth’s surface. The prevailing wind directions are important when determining sites for wind turbines; these should be placed in areas with least obstacles from the prevailing wind directions. Also, local geography should be taken into account since it will influence the performance of turbines.

Table 1.2 Prevailing wind directions Latitude Direction

90–60ı N NE

60–30ı N SW

30–0ı N NE

0–30ı S SE

30 60ı S NW

60 90ı S SE

8

1 Wind Energy

Table 1.3 Classes of wind power density at 10 m and 50 ma 10 m (33 ft) Wind power class 1 2

Wind power density (W=m2 ) <100 100–150

3

150–200

4

200–250

5

250–300

6

300–400

7

>400

50 m (164 ft) Speedb m/s (mph) <4.4 (9.8) 4.4 (9.8)–5.1 (11.5) 5.1 (11.5)–5.6 (12.5) 5.6 (12.5)–6.0 (13.4) 6.0 (13.4)–6.4 (14.3) 6.4 (14.3)–7.0 (15.7) >7.0 (15.7)

Wind power density (W=m2 ) <200 200–300 300–400 400–500 500–600 600–800 >800

Speedb m/s (mph) <5.6 (12.5) 5.6 (12.5)–6.4 (14.3) 6.4 (14.3)–7.0 (15.7) 7.0 (15.7)–7.5 (16.8) 7.5 (16.8)–8.0 (17.9) 8.0 (17.9)–8.8 (19.7) >8.8 (19.7)

a

Vertical extrapolation of wind speed based on the 1/7 power law. b Mean wind speed is based on the Rayleigh speed distribution of equivalent wind power density. Wind speed is for standard sea-level conditions. To maintain the same power density, speed increases 3%/1,000 m (5%/5,000 ft) of elevation [59].

Based on the wind speed, wind resources are categorized into seven classes. A wind-class refers to a range of wind power density and speed that describes the energy contained in the wind. The wind power classes are given in Table 1.3.

1.3 Wind Resource Map Areas designated as Class 4 or greater are suitable for the power generation using wind turbines that are currently available commercially. Areas with wind Class 3 areas may be suitable for future generation technology. Class 2 areas are marginal and Class 1 areas are unsuitable for wind energy development. The wind map for a country, a state, or a local area is generally available from the local government or designated authority. The wind resource map for the state of Missouri, USA is shown in Fig. 1.6. In the map, areas designate as 1, 2, or 3 are referred to as the wind class for that particular area. Class 3 wind that is essential for economical use of wind turbine is not available in most of the regions in Missouri, USA as shown in the wind map. This suggests that not necessarily all the areas within a country are suitable for wind power generation. The wind maps of the Europe and the USA are shown in Figs. 1.7 and 1.8, respectively. Similar maps for Denmark and England are shown in Figs. 1.9 and 1.10, respectively. Denmark is one of the leaders in utilization of wind power in Europe, and England is making rapid progress in utilization of wind power. It is considered that England may have the best wind resources among Europeans countries. Although several countries are considering wind energy for electricity generation,

1.3

Wind Resource Map

9

Fig. 1.6 Annual average wind power for Missouri, USA (Adapted from Wind Energy Resource Atlas of the United States [59])

according to Elliott [60], a major barrier to the deployment of the wind energy in many regions of the world is the lack of reliable and detailed wind resource data. Availability of this data is essential for the government and industry to identify wind power generation potential and to act on that knowledge. Wind resources assessment from the 1980s to early 1990s were used to produce wind atlases for the U.S. and the Europe [61, 62], and a worldwide wind resource map [63]. These maps have identified general areas of good wind resources and provided the basis for preliminary estimates of the wind-energy potential around the world. The US Department of Energy had also developed a worldwide wind resource map in 1985 using wind data compiled in 1980. It should be noted that much of the data underestimated the wind resources because of exposure problems and inadequate maintenance of the wind speed sensors (anemometers) that were use at that time. Many areas shown as Class 1 during the time of these studies actually have a better wind class than indicated. The availability of land in Class 4 and above is also critical, since this will determine the actual harvesting of wind energy of any country or region. As shown in Fig. 1.11, only a small percentage of land in the USA has above Class 3 wind on an annual basis. The land area for Class 3 wind is shown in Fig. 1.12.

10

1 Wind Energy

Fig. 1.7 Wind resource map of Europe (Printed with permission from Hau [3])

Even if a land mass has Class 3 or above wind, not necessarily all the land mass will be available for installation of wind turbines. Figure 1.13 shows the availability of land for installation of wind turbines if various restrictions are taken into account. In the US, when considering potential land area, 100% of federal and state environmentally sensitive lands were excluded from such consideration. These lands include parks, monuments, US Forest Service lands, wilderness areas and wildlife refuges. The available windy land area was further reduced when the primary lands

Wind Resource Map

Fig. 1.8 Wind resource map of the USA (Courtesy of National Renewable Energy Laboratory [57])

1.3 11

12

1 Wind Energy

Fig. 1.9 Wind resource map of Denmark (Printed with permission from Danish Energy Agency, Energy & Environmental Data [65])

used by a region was excluded. About 100 Quads of energy would be generated by this estimate with modest restriction. The percentage of type of land areas that would be excluded is given below: • • • • •

Forest – 50% excluded Agriculture – 30% excluded Range – 10% excluded Mixed Agriculture and Range – 20% excluded Barren – 10% excluded

1.3

Wind Resource Map

13

Annual mean wind speed at 25m above ground level [m/s] > 10.0 8.5 − 10.0 7.0 − 8.5 5.5 − 7.0 < 5.5

km 25 0

125 km

Fig. 1.10 Wind energy resource map for England (Adapted from The UK Wind resource Wind energy fact sheet 8) [64]

• Wetland – 100% excluded • Urban – 100% excluded • Water – 100% excluded Because of the change in exclusion methodologies, areas available for wind energy development are always changing.

Fig. 1.11 Percentage of land area with Class 3 or higher wind resources (Adapted from Elliot and Schwartz [66])

14 1 Wind Energy

Wind Resource Map

Fig. 1.12 Area with Class 3 wind in the USA (Courtesy of National Renewable Energy Laboratory) [57]

1.3 15

16

1 Wind Energy

Fig. 1.13 Wind energy potential in the contiguous US (Source: Schwartz and Elliot [67])

1.4 Land Area Requirement Once a site is found to be suitable for wind energy development, the availability of that particular land should be explored. The primary objective of a wind project design is to locate the wind turbines in the best wind sites to maximize energy production. A number of software packages are available to determine the placement of wind turbines at eligible sites (Wind Energy Finance, National Renewable Energy Laboratory, USA; RETScreen International Wind Energy, Canada; Job and Economic Development Model, EERL, US Department of Energy). Wind turbines are typically arranged in single or multiple rows, depending on the size and shape of the land. A single row is most often used on ridgelines and hilltops where the flat land is very limited. The distance between rows in complex terrain is typically dictated by the terrain characteristics. Multiple rows can be used in a broader and flatter land. In both cases, rows are laid out to be as perpendicular as possible to the prevailing wind direction(s). The main consideration in placing the wind turbines is the interference of one wind turbine from another turbine. The interference of a turbine by a downwind from another turbine is called the “wake effect” or “array effect”. If turbines are closely spaced, they will experience higher wake-effect-induced energy losses. Although wide spacing between wind turbines would maximize energy production, this would increase land requirement and other infrastructure requirements (i.e., cabling, roads). Therefore, the turbine spacing must be optimized to minimize the cost. The distance between wind turbines (between turbine rows and between turbines within a row) is commonly described in terms of rotor diameters. For example, if a

1.4

Land Area Requirement

17

Fig. 1.14 Spacing of wind turbines in a wind farm

project design is described as having 3 by 10 spacing, it means that the turbines are generally spaced 3 rotor diameters apart within rows, and the rows are spaced 10 rotor diameters apart (see Fig. 1.14). For a project using wind turbines with a 70 m (230 ft) rotor diameter, this would mean spacing the turbines 210 m (690 ft) apart within a row, and 700 m (2,300 ft) apart between rows. However, 3 by 10 spacing is not a fixed parameter. As can be seen from Table 1.4, the spacing can vary depending on the type of land and the location of the wind farm. For a wind farm, several wind turbines must be put together to generate the required power or electricity, increasing the total land requirements. Due to the low energy density of the wind energy, large land areas are required compared to conventional sources such as coal and nuclear. The land requirements for a 1;000 MWe system for various energy resources are given in Table 1.5. The land area was determined by local requirements and climate conditions (wind availability factors ranging from 20% to 40%). The energy density of fossil and nuclear fuel allows relatively small areas, 1–4 km2 . A number of assessments on land requirements for a wind farm are available in the literature from various agencies and are discussed below. According to the American Wind Energy Association (AWEA), in open flat terrain, the land area required is approximately 50 acres per MWe [58]. The land area also depends on the

18

1 Wind Energy

Table 1.4 Examples of turbine spacing in several wind farms in the USA Project location Madison, NY

Land use and type Farmed hilltop

Wethersfield, NY

Open farmland, north south ridgeline

Fenner, NY

Meyersdale, PA

Mixed farmland and woodlots on broad hill Ridgetop, farmland

Searsburg, VT

Forested ridgeline

Turbine layout Circular row of 7 turbines along hill rim Single north-south row of 10 turbines perpendicular to prevailing wind direction Variable layout of 20 turbines Single northeast-southwest row of 20 turbines Single northeast-southwest row of 11 turbines on ridge

Turbine spacing (rotor diameters) 4 Diameters between turbines 3 Diameters between turbines

Generally 5–7 diameters between turbines 3 Diameters between turbines Variable (1.5–3.5 diameters) between turbines

Source: Turbine verification program [68]

Table 1.5 A comparison of land requirement for a 1,000 MW(e) power plant Energy resource Land area Fossil and nuclear sites: 1–4 km2 Solar thermal or photovoltaic (PV) parks: 20–50 km2 (a small city) Wind fields: 50–150 km2 Biomass plantations: 4;000–6;000 km2 (a province) Source: Nuclear power advantages, Limited Environmental Impacts [69]

size of the wind turbine and the capacity factor. For example, a 1,000 MWe baseload plant would require over 3,400 MWe of widely dispersed wind generation capacity due to its low capacity factor of about 30%. Therefore, the land requirements should be calculated for installation of a 3,400 MWe system [70]. The USA National Renewable Energy Laboratory estimated that the “footprint” for a wind turbine would be typically between 0.25 and 0.50 acres (1,012 and 2;023 m2) per turbine. This estimate does not include the 5–10 turbine-diameter of spacing requirement between wind turbines. Also, “footprint” of land represents the land that has to be taken out of production to provide space for turbine towers, roads, and support structures. The area within the perimeter of the wind farm will be larger due to spacing of the turbines, but is still useable by the farm. According to the British Wind Energy Association (BWEA), a typical wind farm of 20 turbines would require an area of about 1 km2 , but only 1% of the land area would be used to house the turbines, electrical infrastructure and access roads; the remainder can be used for other purposes, such as farming or as a natural habitat.

Land Area Requirement

Fig. 1.15 A comparison of land area requirements for power plants based on the capacity and energy resources (Printed with permission from Gagnon et al. [72])

1.4 19

20

1 Wind Energy

To generate 10% of Britain’s electricity from the wind would require constructing around 12,000 MWe of wind energy capacity. The land requirement would be between 80,000 and 120,000 ha (0.3–0.5% of the UK land area). Less than 1% of this (800–1,200 ha) would be used for foundations and access roads, the other 99% could still be used for productive farming [71]. It should be noted that 1 ha is 10;000 m2. Gagnon et al. [72] assessed the land requirements for various types of electricity generation systems. Their finding is shown in Fig. 1.15.

1.5 Energy and Power from Wind The kinetic energy of wind is converted to mechanical or electrical energy using wind turbines. The amount of energy captured by the rotor depends on the density of the air, the rotor area, and the wind speed. This is schematically shown in Fig. 1.16. Power that is obtained from wind flowing at a certain speed may be calculated by assuming that a parcel of air is moving towards a wind turbine at a velocity of v as shown in Fig. 1.16. The kinetic energy (KE) of the air stream is given by: KE D

1 1 2 mv D a Va v2 2 2

(1.6)

Rotor Diameter (D)

where, m is the mass of the moving air, a is the density of the air, and Va is the volume of the air parcel. Since the air parcel will be swept away by the turbine, the

High velocity High kinetic energy

Fig. 1.16 Working principle of a wind turbine

Low velocity Low kinetic energy

1.5

Energy and Power from Wind

21

cross sectional area of the rotor (AT ) interacting with the air parcel and the velocity of air are critical in determining the power production, which can be expressed as: P D

1 a AT v3 2

(1.7)

This yields the theoretical power in a free flowing stream of wind. The actual power that is obtainable from a wind turbine is given by: 1 P D .CP "g "b / a AT v3 2

(1.8)

In Eq. 1.8: P D power in watts (746 W D 1 hp) (1;000 W D 1 kW) a D air density (about 1:225 kg=m3 at sea level, decreases with altitude) AT D rotor swept area, exposed to the wind .m2 / CP D coefficient of performance, also called power coefficient v D wind speed in meters/s .20 mph D 9 m=s/ "g D generator efficiency "b D gearbox/bearings efficiency The maximum theoretical value of CP possible is 0.593. This is also known as the Betz limit. The practical value of CP is in the range of 0.35–0.40. A value of greater than 0.80 is possible for "g if a permanent magnet generator or grid connected induction generator is used. The efficiency of gearbox and bearings can be greater than 95%.

1.5.1 Betz Limit Betz, a German physicist, in 1919, theoretically determined that a wind turbine can only convert 16/27 (59.3%) of the kinetic energy of the wind into mechanical energy by turning a rotor [73]. This is known as the Betz Limit or the Betz’s Law. Since the velocity at the rotor inlet (v1 ) is different from that at the outlet (v2 ), an average velocity was used to calculate the mass of the air streaming through the rotor per second (see Fig. 1.17) as follows: m D a AT

.v1 C v2 / 2

(1.9)

where Œ.v1 C v2 /=2 is the average wind speed through the rotor area. The power extracted from the wind by the rotor using the average wind speed is given by: P D

1 2 m v1 v22 2

(1.10)

22

1 Wind Energy

Fig. 1.17 Change in the velocity in a wind turbine 0.6 0.5

P/P0

0.4 0.3 0.2 0.1 0.0 0

0.2

0.4

V2 / V1

0.6

0.8

1

Fig. 1.18 Betz limit

Substitution of Eq. 1.9 into Eq. 1.10 provides: P D

a v21 v22 .v1 C v2 /AT 4

(1.11)

The total power in the wind .P0 / streaming through exactly the same area, AT , in the absence of the rotor can be written as: P0 D

a 3 v AT 2 1

(1.12)

The ratio of the two powers is given by: " 2 # P v2 1 v2 1 D 1C Po 2 v1 v1

(1.13)

1.6

Capacity Factor for a Wind Turbine

23

A plot of P =Po as a function of v2 =v1 is shown in Fig. 1.18. It shows that the curve passes through a maximum at 0.593 when v1 =v2 D 1=3. Therefore, according to Eq. 1.13, the maximum value for the power extracted from the wind is 0.593 or 16/27 of the total power in the wind.

1.6 Capacity Factor for a Wind Turbine The capacity factor of a wind turbine is the actual energy output for the year divided by the energy output if the turbine operated at its rated power output for the entire year. The output from a wind turbine depends on the wind speed through the rotor. The relationship between wind speed and rated power, called a power curve, is shown graphically in Fig. 1.19. The turbine starts to produce power only when a certain wind speed is reached (called cut-in wind speed). As the wind speed increases, the power output increases sharply. Similarly, at lower wind speeds, the power output drops off sharply. However, if the wind speed is above a certain value, the wind turbine is forced to remain idle. This is known as cut-out wind speed. The “rated wind speed” is the wind speed at which the “rated power (RF)” is achieved. There are two main options if the wind speed is above the rated wind speed. In one option, the power output above the rated wind speed is mechanically or electrically maintained at a constant level using an advanced control system. However, this is rather costly as the rotation of blades is hard to control. In the other option, the wind turbine is cut off from power production. Using the power curve, it is possible to determine roughly how much power will be produced at the average or mean wind speed prevalent at

Fig. 1.19 Dependence of power output of a turbine on wind speed

24

1 Wind Energy

a site. The power curve shown in Fig. 1.19 indicates that the turbine would produce about 20% of its rated power at an average wind speed of 15 mph (or 20 kW if the turbine was rated at 100 kW).

1.7 Energy Production While wind turbines are most commonly classified by their rated power at a certain wind speed, annual energy output is the most important measure for evaluating a wind turbine. The payout time for the wind turbine will depend on its energy production. The energy production can be calculated from the power production using: Energy D Power Time

(1.14)

In order to calculate expected energy output, the capacity factor of the turbine should be known. A reasonable capacity factor would be between 0.25 and 0.30. A very good capacity factor would be 0.40. Capacity factor is very sensitive to the average wind speed. When using the capacity factor to calculate estimated annual energy output, it is extremely important to know the capacity factor at the average wind speed of the intended site. The power curve can also be used to find the predicted power output at the average wind speed at the wind turbine site. By multiplying the rated power output by the capacity factor and the number of hours in a year, (8,760 h), an estimate of annual energy production can be obtained for a 100 kW turbine producing 20 kW at an average wind speed of 15 mph. The annual energy production would be: 100 kW.RP/ 0:20.RCF/ 8760.h/ D 175; 200 kWh where, RP is the rated power, RCF is the rated capacity factor, and h is hour. For accurate estimate of energy production, the wind distribution of the site should be known. If such data is not available, there are two common wind distributions functions that are used to make energy calculations for wind turbines: the Weibull distribution and a variant of the Weibull distribution, called the Rayleigh distribution that is thought to be more accurate at sites with high average wind speeds. Energy output is also influenced by the wind turbine design features, including cut-in and cut-out speeds. In most commercial operations, the turbine is shut down at the cut-out-speed to protect the rotor and drive train machinery from damage. Therefore, the wind turbine must be designed based on the characteristics of the site. Although the capacity factor of some wind turbines installed around 2006 approached 45% (Fig. 1.20), the average remained around 26%. The installed capacity of wind energy in the USA and its capacity factor is shown in Table 1.6. Only a small improvement in the averaged capacity factor is noticed from 2002 to 2006.

1.8

Turbine Types

25

Fig. 1.20 The improvement in capacity factor of wind turbines over the years (Courtesy of Wiser and Bolinger [74]) Table 1.6 The installed capacity of wind energy in the USA and the capacity factor

Year 2002 2003 2004 2005 2006

Installed capacity in MW 4,685 6,374 6,740 9,146 11,575

Net electricity generation in thousands kilowatt-hour 10,354,279 11,187,467 14,143,741 17,810,549 26,589,137

Capacity factor* 25.22936 20.03621 23.95523 22.23013 26.2228

Calculated based on 365 days, 24 h of continuous operation [74]

The increased capacity factor will lead to higher reliability and availability of the wind power and will reduce the need for stand-by excess capacity or energy storage systems.

1.8 Turbine Types Wind turbines can be divided into two categories based on the axis about which the turbine rotates: • Horizontal Axis Wind Turbines (HAWTs) • Vertical Axis Wind Turbines (VAWTs) The HAWTs generally can be designed for higher power. This is possible due to higher rotor diameters that can be used when designing HAWTs. The turbine capacity depends on the rotor diameter as shown in Fig. 1.21. As shown in Fig. 1.22, the rated power output from a single wind turbine has increased steadily over the past several decades. Currently, a single wind turbine

26

1 Wind Energy

Fig. 1.21 Relationship between power capacity and the rotor diameter (Printed with permission from Danish Wind Industry Association [75])

can theoretically generate 5 MW. This increase became possible due to development of wind turbines with a large rotor diameter. A rotor diameter of 124 m has been designed providing a power output of 5 MW.

1.8.1 Horizontal Axis Wind Turbines (HAWTs) The blades of horizontal-axis wind turbines spin in a vertical plane. During rotation, blades move more rapidly over one side, creating a low pressure area behind the blades and a high pressure area in front of it. The difference between these two pressures creates a force which causes blades to spin. The HAWTs have the main rotor shaft and electrical generator at the top of a tower, and are pointed into the wind. Small turbines are pointed by a simple wind vane, while large turbines generally use a wind sensor coupled with a servo motor. Most have a gearbox, which turns the slow rotation of the blades into a quicker rotation that is more suitable for generating electricity. The basic structure of a HAWT is shown in Fig. 1.23. There are several advantages of horizontal wind turbines. These are discussed below. • The design and location of blades provide a better stability of the structure. • The ability to pitch the rotor blades in a storm minimizes the damage.

1.8

Turbine Types

27

5000 kW f124 m

2000 kW f80 m

500 kW f 40 m 50 kW f15 m

1980

600 kW f50 m

100 kW f20 m

1985

1990

1995

2000

2003

Fig. 1.22 The increase in the wind turbine size over the years (Printed with permission from Leithead [23])

• The use of a tall tower allows access to stronger wind in sites with wind sheer and placement on uneven land. • The manufacturing cost can be less because of higher production volume, larger sizes and, in general, higher capacity factors and efficiencies. The disadvantages are: • tall towers and long blades (up to 180 ft long) are difficult to transport, • higher install costs, and • higher maintenance costs. Horizontal axis wind turbines are most widely used for commercial power generation. Currently three blade rotor systems are preferred; however, in the past both one blade and two blade wind turbines have been designed and tested (see Fig. 1.24). One-blade and two-blade wind turbines generate 15% and 5% less power than threeblade wind turbines, respectively. However, the main issue for using one-blade or two-blade systems is the stability of the turbine. More sophisticated and costly control mechanisms are necessary to make one-blade or two-blade turbines stable during rotation. Wind turbines with more than three blades (multi-blade) have also been explored, but no significant gain in costs or stability of multi-blade systems over three-blade turbines was achieved. Various multi-blade horizontal axis wind turbines and other proposed designs are shown in Fig. 1.25.

28

1 Wind Energy

Rotor Blade

Swept Area of Blades

Rotor Diameter

Nacelle with Gearbox and Generator

Hub Height Tower

Underground Electrical Connections (Front View)

Foundation (Side View)

Fig. 1.23 The basic schematic diagram of a horizontal axis wind turbine (Courtesy of European Security Network [76])

Fig. 1.24 One-blade and two-blade horizontal axis wind turbines (Printed with permission from Hau [3])

1.8

Turbine Types

29

Fig. 1.25 Various types of horizontal axis wind turbine

1.8.1.1 Wind Turbine Components The basic components of a wind turbine are as follows: • • • • • • • • • • • •

Nacelle Rotor blades Hub Low speed shaft Gearbox High speed shaft with its mechanical brake Electrical generator Yaw mechanism Electronic controller Tower Anemometer Wind vane

A diagram of a wind turbine with its components is shown in Fig. 1.26. A brief description of these components is given below.

Nacelle The nacelle contains the key components of a wind turbine, including the gearbox, and the electrical generator.

Rotor Blades The rotor blades capture the wind and transfer its power to the rotor hub. A 1,000 kWe wind turbine has rotor blades that are about 27 m (80 ft) in length. The blades or “rotors” catch the wind and cause the movement of the blades that turns

30

1 Wind Energy

Fig. 1.26 Various components of a wind turbine (Courtesy of National Renewable Energy Laboratory [57])

the shaft. The generator then turns this movement into electricity. Blades come in many sizes; as shown in Fig. 1.22, the longest blades in use today are about 62 m long (rotor diameter is 124 m). Hub The hub of the rotor is attached to the low speed shaft of a wind turbine. Low Speed Shaft The low speed shaft of a wind turbine connects the rotor hub to the gearbox. The rotor rotates at about 19–30 rotation per minute (rpm) in a 1,000 kWe wind turbine. The shaft contains pipes for the hydraulics system to enable the aerodynamic brakes to operate. Gearbox Gears connect the low-speed shaft to the high-speed shaft and increase the rotational speeds from about 19 to 30 rotations per minute (rpm) to about 1,000–1,800 rpm, which is required by most generators to produce electricity. The recent design uses “direct-drive” generators that operate at lower rotational speeds and do not need gear boxes.

1.8

Turbine Types

31

High Speed Shaft with Its Mechanical Brake This drives the generator and employs a disc brake, which can be applied mechanically, electrically, or hydraulically to stop the rotor in emergencies.

Electrical Generator The generator converts the mechanical energy of the rotating shaft into electrical energy.

Yaw Mechanism The turbines must face upwind for power production. The yaw drive is used to keep the rotor facing into the wind as the wind direction changes. Downwind turbines don’t require a yaw drive, since the wind blows the rotor downwind.

Electronic Controller The controller starts the machine at the specified wind speed which is generally between 8 and 16 miles per hour (mph) (3.58 and 7.15 m/s) and shuts off the machine at about 55 mph (24.6 m/s). Turbines do not operate at wind speeds above about 55 mph (24.6 m/s) because they might get damaged above this wind speed.

Tower The tower is a high stationary support structure for the wind turbine, so that consistent wind speed can be sustained for the operation of the turbine.

Anemometer It measures the wind speed and transmits wind speed data to the controller.

Wind Vane It measures wind direction and directs the yaw drive to appropriate orientation so that the turbine is properly aligned with respect to the wind direction.

32

1 Wind Energy

1.8.2 Vertical Axis Wind Turbines (VAWTs) The blades of vertical-axis wind turbines spin in a horizontal plane. VAWTs have the main rotor shaft running vertically. Various components of a VAWT are shown in Fig. 1.27. An advantage of this arrangement is that the generator and/or gearbox can be placed at the bottom, near the ground; therefore, a tower is not needed to support the turbine. Also, the turbine does not need to be pointed into the wind. The disadvantages are usually the pulsating torque that is produced during each revolution and the drag created when the blade rotates into the wind. The vertical axis turbines on towers need lower and more turbulent air flow near the ground. This type of condition is difficult to sustain resulting in a lower energy extraction efficiency. A variety of designs for VAWTs have been proposed and are described below.

THRUST BEARING GUY CABLE

ROTOR

BRAKES THRUST BEARING TORQUE SENSOR FLEXIBLE COUPLING SYNCHRONOUS GENERATOR SPEED INCREASER RIGHT ANGLE DRIVE

CLUTCH TORQUE SENSOR

INDUCTION GENERATOR

TIMING BELT

FLEXIBLE COUPLING

Fig. 1.27 Components of a vertical axis wind turbine (Adapted from Reuter and Worstell [77])

1.8

Turbine Types

33

Fig. 1.28 A Darrieus wind turbine (Printed with permission from Hau [3])

1.8.2.1 Darrieus Wind Turbine The most common type of VAWT is the Darrieus wind turbine. The design of these types of turbines looks like an eggbeater (Fig. 1.28). Generally, an external power source is required to start the rotation. The starting torque is very low. In the newer design, three or more blades are used which results in a higher solidity for the rotor. Solidity is measured by blade area over the rotor area. New Darrieus type turbines are not held up by guy wires, but have an external structure connected to the top bearing.

1.8.2.2 Savonius Wind Turbine The Savonius turbine consists of two half-cylinders mounted on a vertical shaft that has an S-shape appearance when viewed from the top. A schematic diagram of the

34

1 Wind Energy

Fig. 1.29 A savonius rotor for wind turbine (Printed with permission from Hau [3])

Savonius rotor is shown in Fig. 1.29. This drag-type VAWT turns relatively slowly, but yields a high torque. Because of the curvature, the scoops experience less drag when moving against the wind. The differential drag causes the Savonius turbine to spin. Most of the swept area of a Savonius turbine is near the ground, therefore, the overall energy extraction efficiency is lower. However, Savonius turbines are cheap and reliable.

1.8.2.3 Other Lift-Type Vertical Axis Configurations Darrieus also proposed another type of vertical axis wind turbine with straight vertical axis blades, called Giromills (Fig. 1.30). A variant of the Giromill, called the cycloturbine (Fig. 1.30b), uses a wind vane to mechanically orient a blade pitch change mechanism. The advantages of vertical axis wind turbines are: • The turbines are easy to maintain because most of their moving parts are located near the ground. • The rotor blades are vertical, therefore, a yaw device is not needed. • The vertical wind turbines have a higher airfoil pitch angle, giving improved aerodynamics while decreasing drag at low and high pressures. • The turbines are better suitable for Mesas, hilltops, ridgelines and passes as these locations can have higher wind speed near the ground. In these places, VAWTs placed close to the ground can produce more power than HAWTs placed higher up.

1.9

Comparison Between Turbines

35

Fig. 1.30 Lift type vertical wind turbine. (a) Giromill wind turbine (b) Cycloturbine (Courtesy of American Wind Energy Association [58])

• The turbine does not need a free standing tower. • The turbines have very high starting torque, therefore, these are better for water pumping. The disadvantages are: • The height and swept area may be limited. • Generally, a flat surface is necessary for their installation, otherwise the installation cost could be higher. • A strong structure is necessary to keep it straight, increasing the cost. • Most VAWTs produce energy at only 50% of the efficiency of HAWTs.

1.9 Comparison Between Turbines Wind turbines may be compared against each other by comparing their coefficient of performance (CP ) against tip speed ratio ( ). Various forces working on a turbine are shown in Fig. 1.31. The coefficient of performance, also known as the power coefficients, is defined by Eq. 1.15. CP D

P P0

(1.15)

36

1 Wind Energy

Fig. 1.31 The angular velocity of a wind turbine rotating in a horizontal axis

where, P and P0 are defined by Eqs. 1.11 and 1.12. Using the swept area and substituting the value of P0 ; P may be expressed as: P D

1 a r 2 v3 CP 2

(1.16)

The performance of the wind turbine depends on the wind speed and the rate of rotation of the rotor. The Tip Speed Ratio () refers to the ratio between the wind speed and the speed of the tip of the wind turbine blades and is expressed as: Tip Speed Ratio; D

Speed of rotor tip vr !r D D Wind speed v v

where, v D the wind speed (m/s) vr D velocity of rotor tip (m/s) r D rotor radius (m) ! D angular velocity (radian/s) and is given by ! D 2f f D frequency of rotation (Hz), Sec1 .

1.9

Comparison Between Turbines

37

Example 1.1. Consider a wind turbine of rotor diameter 20 m, rotating at a speed of one rotation per second. Calculate the tip speed ratio () for this turbine. Solution. Given, f D 1 rotation/s; rotor diameter D 20 m; therefore, radius r D 10 m; calculate !. ! D 2f D 2 1 radian=sec D 2 radian=sec v D !r D 2 10 D 20 m=sec D

20 !r D D 4:19 v 15

The tip speed ratio is an important factor in designing the wind turbine. The rotor must rotate at an optimum speed to maximize its efficiency. If the turbine rotates slowly, it will not catch any wind. The wind will simply pass unperturbed through the gap between the blades. If the turbine rotates at a very high speed, it will behave like a solid wall to the wind passage. Therefore, the turbine design must be optimized. Both the design of the blades (rotor airfoil profile) and number of blades play a critical role in optimization of a turbine’s performance. In general, a high tip speed ratio is desirable since it results in a high shaft rotational speed, which in turn can increase the efficiency of the electrical generator. The optimum value of tip speed ratio .opt / can be approximated by the following expression. opt

!opt r 2 v n

r AT

(1.17)

where n is the number of blades. The ratio (r=AT ) is generally 2, therefore, opt

4 n

(1.18)

The power coefficients (CP ) for various turbines are plotted against tip speed ratio () in Fig. 1.32. As can be seen from this figure, HAWTs with three-blade has the best power coefficient. Among VAWTs, Darrieus turbine has the highest power coefficient, and Savonius rotor has the lowest value. For most of the commercial applications, threeblade HAWTs are preferred. As mentioned earlier, a high tip speed ratio is desirable, but there are a number of disadvantages for operating a turbine at high . 1. A high rotating speed can cause erosion of the blades from impact with dust or sand particles in the air. 2. The level of noise increases, both in the audible and non-audible ranges. 3. The vibration also increases, and there is a chance of a catastrophic failure.

38

1 Wind Energy

Fig. 1.32 A plot of rotor power coefficient as a function of tip speed ratio for various turbines (Printed with permission from Hau [3])

1.10 Cost of Electricity from Wind Energy The cost of a wind energy system has two components: initial installation costs and operating expenses. The initial installation costs include the purchase price of the complete system, including turbine, tower, wiring, utility interconnection or battery storage equipment, and power conditioning unit. Installation costs also include foundations, normally made of reinforced concrete, road construction (necessary to move turbines and sections of the tower to the building site), a transformer (necessary to convert the low voltage (690 V) current from the turbine to 10–30 kV current for the local electrical grid), telephone connection for remote control and surveillance of the turbine, and cabling costs, i.e. the cable from the turbine to the local 10–30 kV power line. The delivery and installation charges, professional fees and sales tax are also part of these overall cost. The total installation costs of a wind energy system are generally expressed on the basis of electricity generation capacity. A grid-connected residential-scale system (1–10 kW) generally costs between $2,400 and $3,000 per installed kilowatt. A commercial system (10–100 kW) costs between $1,500 and $2,500/kW. However, these numbers are very dynamic and depend on a host of variables such as cost of fuels, energy price, metal price, labor costs, etc. Large-scale systems of greater than

1.10

Cost of Electricity from Wind Energy

39

Fig. 1.33 The total installation cost of a wind turbine based on its rated power (Courtesy of Wind Energy Manual, Iowa Energy Center [78])

Fig. 1.34 The history of installed project cost for wind turbines in the USA (Courtesy of Wiser and Bolinger [74])

l00 kW cost in the range of $1,000–$2,000/kW. The installed cost may be lower if multiple units are installed at one location. Figure 1.33 shows the installed cost range as a function of electrical generation capacity. A wind energy system in remote locations generally needs an operating battery storage, resulting in installed cost in the range of $4,000–$5,000/kW. A further analysis of the installed cost from 227 projects around the USA showed an upward trend in 2006–2007 compared to previous years. This analysis is shown in Fig. 1.34. The increase in the installation costs has been attributed to the increased cost of materials and metals.

40

1 Wind Energy

3.0 2003 2.5

2004 2005

2.0

2006 2007

1.5 1.0

S U

U

C an D ad en a m a Fi rk nl a G n er d m a G ny re e Ire ce la nd Ita ly N Ja et pa he n rla n N ds or w Po ay rtu ga Sp l S ai Sw we n itz de er n la nd

0

K

0.5

Fig. 1.35 Investment cost for wind energy systems in selected countries for 2006–2007 (Courtesy of International Energy Agency [79])

A similar trend was observed in other countries (see Fig. 1.35). A major component of the installation cost is the turbine cost. The costs of turbines have decreased by a factor of four since the early 1980s until 2004, but costs have increased by 20–80% by 2006. This is mainly due to the supply tightness of turbines, gear boxes, blades, bearings, and towers. Higher steel and copper prices contributed to this short supply of wind turbine components and their price increase. Total installed costs including turbine ranged from US$ 1,400/kW in the UK to US$ 2,700/kW in Ireland. The second component, operating costs, includes maintenance and service, insurance and other applicable taxes. Estimates for annual operating expenses are 2–3% of the initial system cost. Other estimates are based on the system’s energy production and are equivalent 1–2 cents/kWh of output. The U.S. wind energy industry, electric utilities, and the federal government are working together to develop low-cost, technologically advanced wind turbines. As can be noted from Fig. 1.36, the cost of electricity production from wind energy has decreased significantly in the last decade. The technology improvements have allowed the wind industry to achieve its goal for the cost of electricity generation at an average of 5 cents/kWh by the mid-1990s at sites with 5.8 m/s (13 mph) average annual wind speeds. This cost target was set by the US Department of Energy. New turbine design and development helped to lower the cost to 3.9 cents/kWh by 2006. The target is to achieve a cost of 3.6 cents/kWh by 2012. However, as shown in Fig. 1.37, the average production cost started to increase again in 2006–2007 due to the rising installation costs. Also, a significant variation in the production cost remained. The Energy Efficiency and Renewable Energy

1.10

Cost of Electricity from Wind Energy

41

Fig. 1.36 Cost of electricity produced from wind energy

Fig. 1.37 Average cumulative wind and wholesale power prices over time (Courtesy of Wiser and Bolinger [74])

Division of US Department of Energy conducted a detailed study of costs associated with the production of electricity from wind energy. The cost analysis was carried out under a number of scenarios, as discussed below. The cumulative capacity weighted average wind power price (plus or minus one standard deviation around that price) in each calendar year from 1999 to 2007 is shown in Fig. 1.38. The increase in weighted average wind price in 2006 and 2007 is attributed to higher installation and operating costs of new projects. The number of wind energy systems in the USA increased from 14 to 21, with the installed capacity increasing from 766 to 2,502 MW. Although the overall installation cost increased, as can be seen from Fig. 1.39, the wind power prices did not increase, rather decreased slightly. In Fig. 1.40 is shown the regional variation of wind power price of the projects constructed between 2006 and 2007. The lowest wind generated electricity price was from a Texas project, whereas it was the highest in the eastern region. This regional variation of the wind price is shown in Fig. 1.41.

42

1 Wind Energy

Fig. 1.38 Cumulative capacity-weighted average wind power price over time (Courtesy of Wiser and Bolinger [74])

Fig. 1.39 Wind power prices by commercial operation date (COD) (Courtesy of Wiser and Bolinger [74])

Fig. 1.40 Wind power prices by region in 2007 for projects installed in 2006–2007 (Courtesy of Wiser and Bolinger [74])

The annual operating cost for large onshore turbines worldwide in 2006, including insurance, regular maintenance, spare parts, repair and administration was in the range of US$ 0.014–0.026/kWh. The operating and maintenance costs are considerably higher for offshore wind turbines. For wind powers to be competitive with conventional electricity generators (i.e., coal and nuclear), sites must have

1.10

Cost of Electricity from Wind Energy

43

Fig. 1.41 Wind and wholesale power prices by region for projects installed over 1998–2007 (Courtesy of Wiser and Bolinger [74])

Fig. 1.42 Wind power production costs as a function of the wind resource and investment cost (Courtesy of International Energy Agency [79])

extremely good wind resource as well as nearby grid access. Typical production costs, levelized over turbine lifetime, with a discount rate of 7.5% are shown in Fig. 1.42 for two installation costs; US$ 1,640/kW and US$ 2,300/kW. The production cost was estimated to be from US$ 0.075/kWh to 0.097/kWh, at high to medium quality wind resource sites.

1.10.1 Payback Time for Wind Energy Systems The financial benefit of a wind system investment may be determined by estimating the payback period, which is calculated from the expression given below. Payback time .years/ D

Total annual cost Annual energy cost savings Annual operating costs

44

1 Wind Energy

Example 1.2. Calculate payback time for a 5 kW residential system and a 50 kW commercial system. Solution. It is assumed that the installation cost for residential system is $3,000/kW installed. It is $2,000/kW for the commercial system. The capacity factor is 30% and the cost of electricity is 6 cent/kWh. The installed cost is given by: Residential 5 kW system D $15;000 Commercial 50 kW system D $100;000 The annual electricity generation will be: Residential 5 kW system D 5 365 24 0:30 D 13; 140 kWh Commercial 50 kWsystem D 50 365 24 0:30 D 131; 400 kWh The annual energy-cost savings from both systems would be: Residential $0:06=kWh 13; 140 kWh D $788:50 Commercial $0:06=kWh 100; 000 kWh D $7885:00 Annual operating costs are assumed to be 1.5 cent/kWh. Therefore, annual operating costs are: Residential $0:015=kWh 13; 140 kWh D $197 Commercial $0:015=kWh 131; 400 kWh D $1; 970 The residential payback period will be: $15; 000=.$788:5 $197/ D 25years Commercial payback period: $100; 000=.$7885 $1; 970/ D 17years The above example reflects a simple calculation procedure for the payback period. A more detailed calculation could be performed which would include the following: • • • • •

interest paid on borrowed money insurance utility buy-back, if any state and federal tax benefits wind turbine salvage value, if any

1.10

Cost of Electricity from Wind Energy

Table 1.7 Comparative generating cost in European Union with 10% discount rate (US cent/kWh) (used 1 euro D 1.44 USD)

Gas CCGT Coal – pulverised Coal – fluidised bed Coal IGCC Nuclear Wind onshore Wind offshore

45

2005 4.9–6.5 4.3–5.8 5.0–6.5 5.8–7.2 5.8–7.9 5.0–15.8 8.6–21.6

Projected 2030 with USD 29 43=t CO2 cost 5.8–7.9 6.5–8.6 7.2–9.4 7.9–10.1 5.8–7.9 4.0–11.5 5.8–17.3

Source: World Nuclear Organization [80]

Table 1.8 A compilation of electricity cost (US cent/kWh) from various studies MIT 2003 France 2003 UK 2004 Chicago 2004 Canada 2004 Nuclear 4.2 3.7 4.6 4.2–4.6 5.0 Coal 4.2 5.2 3.5–4.1 4.5 Gas 5.8 5.8, 10.1 5.9, 9.8 5.5–7.0 7.2 Wind onshore 7.4 Wind offshore 11.0

EU 2007 5.4–7.4 4.7–6.1 4.6–6.1 4.7–14.8 8.2–20.2

First 5 gas row figures corrected for Jan 2007 US gas prices of $6.5/GJ (second figure for France & UK columns is using EU price of $12.15/GJ). Chicago nuclear figures corrected to $2,000/kW capital cost. Canada nuclear shows figures for Advanced CANDU Reactor (ACR). Currency conversion at June 2007 [80]

The calculation procedure for an economic analysis of a project of this type has been discussed in Chap. 2 of Volume 1 of this book series. Although the payback time for a commercial wind energy system is close to that of conventional electricity generating systems, still the production cost could be higher than coal and nuclear energy based power plants. Careful selections of sites for wind power systems may compete with coal or nuclear, but such sites are generally rare in most of the countries. A cost comparison among various electricity generating systems in Europe is given in Table 1.7. The World Nuclear Organization also compiled the data reported by various agencies at different countries on the costs of generating electricity from various energy sources. The electricity cost adjusted for 2007 from various studies is given in Table 1.8.

1.10.2 Cost Reduction Efforts As shown in Fig. 1.43, the major cost of a wind power system is the turbine cost. Researchers are working closely with turbine manufacturers to continually improve turbine performance. In addition to laboratories and field tests, researchers provide

46

1 Wind Energy

Insurance (1%)

Legal Cost (2%)

Interest During Construction (2%)

Bank Fees (1%)

Installation (1%)

Development Cost (1%)

Project Management (1%) Grid Connection (6%) Electrical Infrastructure (8%)

Civil works (13%)

Wind turbines (64%)

Fig. 1.43 Cost distribution of various components of a wind power system (The data are averaged from Ref. [81])

Fig. 1.44 Various types of airfoil designed by NREL for use in wind turbine (Courtesy of Advanced aerofoil for wind turbines [82])

independent technical design reviews, analysis and support to meet specific design challenges, and project management. Participating wind turbine manufacturers are also required to contribute a substantial share of research project costs, report regularly on progress, meet specific milestones, and complete project deliverables. There

1.11

Effect of Capacity Factor

47

are two major thrusts for cost reduction: the development of innovative components and subsystems for incorporation into wind turbines, and the development of nextgeneration, utility-grade turbines that use the latest advanced technology. In the USA, National Renewable Energy Laboratory (NREL) is leading the effort to design wind turbine blades to enhance the turbine performance. Since 1984, NREL researchers have developed nine families of thick and thin airfoils, crosssectional blade shapes, for wind turbine blades (Fig. 1.44). Using NREL-designed airfoils on stall-controlled turbine rotors, an increase in energy capture by 23–35% has been observed during field tests. NREL expects the new airfoil family will further improve annual energy capture by 8–10%.

1.11 Effect of Capacity Factor Capacity factor can be used to measure the productivity of a wind turbine and to compare it with other power production facilities. It compares the plant’s actual production over a given period of time with the amount of power the plant would have produced, if it had run at full capacity for the same amount of time and is expressed as: Capacity Factor D

Annual amount of power produced over time Power that would have been produced if turbine operated at maximum output 100% of the time

The capacity factor for several electricity generating power plants is given in Table 1.9. The capacity factor is a major contributor to the cost of electricity from the wind energy. Although the efforts are underway to increase the capacity factor, various studies try to identify the areas that need improvement for reduction of cost. Several researchers have reviewed the wind energy costs and its competitive edge with

Table 1.9 Capacity factor for various electricity generating systems

Fuel type Average capacity factors (%) Nuclear 91.8 Coal (steam turbine) 71.8 Gas (combined cycle) 43.3 Gas (steam turbine) 16.0 Oil (steam turbine) 19.6 Hydro 27.8 Wind 30.4 Solar 19.8 Source: Global Energy Decisions/Energy Information Administration [83]

48

1 Wind Energy

other energy sources [84–87]. However, the best way to assess the cost of the wind energy is to perform a life-cycle assessment of the complete system [88–93]. These studies noted that the payback time is very competitive to other conventional energy sources.

1.12 Industrial Wind Turbines Vestas Wind Systems, Denmark, and General Electric (GE), USA are currently dominating the market for industrial wind turbines, both in the USA and in the world. There are several other manufacturers in the USA and are listed in Table 1.10. As noted in Fig. 1.43, for a wind energy system, most of the cost is for a wind turbine. Accordingly, researchers are working in various areas to improve the turbine performance so that it becomes more efficient and cost effective. The areas of research include development of large turbines [94–97], new materials for turbine blades [98–109], modeling and analysis of wind turbine performance [110–116], airfoil design [117–120], and improvement of the power curve [121–135].

1.13 Basic Principles of Wind Resource Evaluation Wind resources evaluation is a critical element for turbine performance at a given site. Proper siting of the wind turbine is critical as the wind flow itself is seldom steady and consistent. It varies with the time of day, season, height above ground, and the type of terrain. Therefore, the siting of turbines should be in windy locations, away from large obstructions [136–146]. Both the aesthetic impact and wind turbulence, such as wake effects, must be taken into account when selecting a site for wind turbines. In general, an annual average wind speed of 5 m/s (11 mph) is required for gridconnected applications. An annual average wind speed of 3–4 m/s (7–9 mph) may be adequate for non-grid connected electrical and mechanical applications, such as battery charging and water pumping. Wind power density is a useful way to evaluate the wind resource available at a potential site. The wind power density, measured in watts per square meter, indicates how much energy is available at the site for conversion by a wind turbine. It may be noted that the energy available in a wind stream is proportional to the cube of its speed, which means that doubling the wind speed increases the available energy by a factor of eight. According to Jenkins [148], before selecting a site and layout of wind turbines, following attributes of a site should be considered: • A high mean annual wind speed. • A low turbulence (i.e., smooth undisturbed wind flow). • A remote location from habitation.

1.5 MW

2.3 MW

2.5 MW

2.7 MW

1.65 MW

GE 1.5sl

GE 2.3

GE 2.5

GE 2.7

Vestas V82 Vestas V90 Vestas V100 Vestas V90 Gamesa G87 Bonus (Siemens)

1.3 MW

2.0 MW

3.0 MW

2.75 MW

1.8 MW

1.5 MW

GE 1.5s

35.25 m (116 ft) 38.5 m (126 ft) 47 m (154 ft) 44 m (144 ft) 42 m (138 ft) 41 m (135 ft) 45 m (148 ft) 50 m (164 ft) 45 m (148 ft) 43.5 m (143 ft) 31 m (102 ft)

64.7 m (212 ft) 80 m (262 ft) 100 m (328 ft) 85 m (279 ft) 70 m (230 ft) 70 m (230 ft) 80 m (262 ft) 80 m (262 ft) 80 m (262 ft) 78 m (256 ft) 68 m (223 ft)

99.95 m (328 ft) 118.5 m (389 ft) 147 m (482 ft) 129 m (423 ft) 112 m (336 ft) 111 m (364 ft) 125 m (410 ft) 130 m (427 ft) 125 m (410 ft) 121.5 m (399 ft) 99 m (325 ft)

Table 1.10 Manufacturers of wind turbines and various related information Blade Model Capacity lengtha Hub htb Total ht 3,904 m2 (0.96 acre) 4,657 m2 (1.15 acre) 6,940 m2 (1.71 acres) 6,082 m2 (1.50 acres) 5,542 m2 (1.37 acres) 5,281 m2 (1.30 acres) 6,362 m2 (1.57 acres) 7,854 m2 (1.94 acres) 6,362 m2 (1.57 acres) 5,945 m2 (1.47 acres) 3,019 m2 (0.75 acres)

Area swept by blades

13/19

138 mph

194 mph

200 mph

9–19 9/19

179 mph

157 mph

138 mph

177 mph

170 mph

164 mph

184 mph

183 mph

Max blade tip speedc

7.2–15.3

8.8–14.9

8.0–14.4

6.0–18.0

5.5–16.5

5.0–14.9

10.1–20.4

11.1–22.2

Rpm range

12 m/s (27 mph) 11.8 m/s (26 mph) 14 m/s (31 mph) 14.5 m/s (32.5 mph) 15 m/s (34 mph) 13 m/s (29 mph) 11 m/s (25 mph) 15 m/s (34 mph) 15 m/s (34 mph) c. 13.5 m/s (30 mph) 14 m/s (31 mph) (continued)

Rated wind speedd

1.13 Basic Principles of Wind Resource Evaluation 49

2.0 MW

2.5 MW (4 650 KW)

1.25 MW

0.95 MW

38 m (125 ft) 41.2 m (135 ft) 32 m (105 ft) 32 m (105 ft) 44.5 m (146 ft) 46.5 m (153 ft) 46.25 m (152 ft)

Blade lengtha

100 m (328 ft)

60 m (197 ft) 80 m (262 ft) 65 m (213 ft) 73 m (240 ft) 80 m (262 ft)

Hub htb 98 m (322 ft) 121.2 m (398 ft) 97 m (318 ft) 105 m (344 ft) 124.5 m (409 ft) 126.5 m (415 ft) 146.25 m (480 ft)

Total ht 4,536 m2 (1.12 acres) 5,333 m2 (1.32 acres) 3,217 m2 (0.79 acres) 3,217 m2 (0.79 acres) 6,221 m2 (1.54 acres) 6,793 m2 (1.68 acres) 6,720 m2 (1.66 acres)

Area swept by blades

7.8–15.0

9.7–15.5

13.9/20.8

13.9/20.8

11/17

11/17

Rpm range

163 mph

168 mph

156 mph

156 mph

164 mph

151 mph

Max blade tip speedc

c. 15 m/s (c. 34 mph) c. 15 m/s (c. 34 mph) 11 m/s (25 mph) 12 m/s (27 mph) c. 11.5 m/s (c. 26 mph) c. 12.5 m/s (c. 28 mph) 11.2 m/s (25 mph)

Rated wind speedd

Source: Rosenbloom [147] a This figure is actually half the rotor diameter. The blade itself may be about a meter shorter, because it is attached to a large hub b Where different hub (tower) heights are available, the usually used size is presented c Rotor diameter (m) rpm 26:82 d The rated, or nominal, wind speed is the speed at which the turbine produces power at its full capacity. For example the GE 1.5s does not generate 1.5 MW of power until the wind is blowing steadily at 27 mph or more. As the wind falls below that, power production falls exponentially. Vestas models can be found at www.vestas.com, GE models at www.gepower.com/businesses/ge wind energy/en, Siemens Bonus models at www.powergeneration.siemens.com/en/ windpower/products, Suzlon models at www.suzlon.com./product overview.htm, Clipper models at www.clipperwind.com, REpower models at www.repower. de/index.php?id=12&L=1. Enercon, Fuhrl¨ander, Mitsubishi, Nordex, and Ecot`ecnia are also major manufacturers, but their turbines do not appear to be currently used in the U.S

REpower MM92

2.0 MW

Bonus (Siemens) Bonus (Siemens) Suzlon 950 Suzlon S.64/1250 Clipper Liberty

2.3 MW

Capacity

Model

Table 1.10 (continued)

50 1 Wind Energy

1.14

Wind Farm

51

• An area having good access for heavy transportation off a public road. • A high-voltage connection to the distribution network. • A site for enough turbines to spread the administrative and legal cost of the project. • An enthusiastic land owners and general land support. • A site with no impediments or restriction to construction.

1.14 Wind Farm A wind farm is a collection of wind turbines in the same location and is used for the generation of large amount of electricity. One such farm is shown in Fig. 1.45. Even if all the individual wind turbines are rated same, the power production generally varies from one turbine to another turbine. The operation and power management is rather complex and sophisticated electrical circuitry and load management is necessary before feeding to the grid [149]. Due to the variability of the power production and the quality of the power, the integration of the wind farm to the power grid is complicated. A number of studies have addressed this issue and different techniques and methods for the grid integration have been suggested [150– 160]. Although a number of large wind farms have been developed, by themselves, wind farms are not suitable for base-load electricity supply. This is because wind

Fig. 1.45 The wind farm near Palm Springs, California, USA [162]

52

1 Wind Energy

power output is variable and unpredictable with sufficient accuracy as it depends on the wind resources, which cannot be controlled. Therefore, the base load still has to be supplied by coal-fired or nuclear power plants. The design of wind farms is challenging. Steps involved in building a wind farm are given below [161]. 1. Understand Your Wind Resource A site must have a minimum annual average wind speed in the neighborhood of 11–13 mph to even be considered. Local weather data and wind maps for the area should be studied. In the USA, state wind maps are available at http://rredc.nrel.gov/ wind/pubs/atlas/. 2. Determine Proximity to Existing Transmission Lines The existing transmission lines and its availability should be determined. Installation of new high voltage lines can cost thousands of dollars per mile. Whenever possible, availability and access to existing lines should be considered in selecting a site. 3. Secure Access to Land The area should be accessible through roads. Also, the developer should be allowed to make it restricted area during construction period and thereafter if necessary. 4. Establish Access to Capital Building a wind farm is not cheap. On average, the development of the wind power costs around $1 million per megawatt (MW) of generating capacity installed. Therefore, the developer must secure sufficient cash flow for both installation and operation until the generation of revenue. 5. Identify Reliable Power Purchaser or Market Local power purchasers and distributors should be contacted and also a survey of the local market for power should be conducted. 6. Address Sitting and Project Feasibility Considerations Various other factors need to be addressed before finalizing the location and the technical feasibility. These include impact on endangered or protected species (if any), site’s geological suitability, effect of noise and aesthetics issues to the local community, local air traffic, and other issues related to site development, such as roads. 7. Understand Wind Energy’s Economics The economic feasibility and payback time should be determined. 8. Obtain Zoning and Permitting Expertise The county, city, and the state should be consulted for permitting purpose and any concern should be addressed before starting the construction. 9. Selection of Turbine The selection of turbines should take into account the required generation capacity, site specific design criteria, and costs.

1.14

Wind Farm

53

10. Secure Agreement to Meet Operating and Maintenance Needs An agreement should be in place for regular maintenance of the wind turbines and also for emergency response. In order to assure continuous power supply, a separate power generation facility that can be put on-line and ramped up in approximately the same time that wind power diminishes is necessary. Such power generation systems are generally more expensive per unit of electricity generated than base-load generators. Several solutions have been proposed to address this issue. These are: pumped-storage hydroelectricity, and the use of rechargeable flow batteries as a rapid-response storage medium [163]. Vanadium redox flow batteries are currently installed at Huxley Hill wind farm (Australia), Tomari Wind Hills at Hokkaido (Japan), as well as in other non-wind farm applications. A further 12 MWh flow battery is to be installed at the Hill wind farm (Ireland) to address the intermittent power generation.

1.14.1 Offshore Wind Farm Offshore wind farms are generally located about 10 km or more from the land. Offshore wind turbines are less obtrusive than turbines on land. The wind resources in the water are much more consistent compared to land. Also, the average wind speed is usually considerably higher over open water and capacity factors are considerably higher than for onshore locations. Among various countries, Denmark and England are making significant push towards the development of offshore wind energy systems; in addition, a number of other countries in Europe and the USA are also exploring and investing in its development. The offshore wind resource potential in the USA and in Europe are shown in Figs. 1.46 and 1.47, respectively. In the USA, the wind resource increases significantly if the depth is extended up to 900 m. In Europe, a depth of 200 m is sufficient for significant wind power generation. Just like onshore wind farms, a number of wind turbines are lined up in an array to build the offshore wind farm. Such an arrangement is shown in Fig. 1.48. One significant difference between an offshore and an onshore wind farm is that only HAWTs are used in offshore farms since they are easy to install and also they provide highest capacity. The installation of wind turbines offshore involves several steps. These steps are explained schematically in Fig. 1.49. Following identification of a location for an offshore wind farm, the turbines are anchored in the seabed. Various types of anchoring methods have been suggested and are discussed later. However, the current practice is to drive piles (1) into the seabed. Often, the top of the foundation is painted with a bright color to make it visible to ships and has an access platform to allow maintenance teams to dock. The fully assembled turbines are installed by a support structure and secured on the top of poles. The turbine blades (2) rotate around a horizontal hub, which is connected to a shaft inside the nacelle (3). The power generated by turbines is transmitted by

54

1 Wind Energy

Resource Not Yet Assessed Fig. 1.46 Potential offshore wind power in the USA (Courtesy of Robinson [164])

subsea cables (4) to an offshore transformer (5) which converts the electricity to high voltage (33 kV) before connecting to the grid at a substation on land (6). One of the main concerns of offshore wind turbines is the extreme weather conditions to which they will be exposed. In Fig. 1.50 are illustrated these adverse conditions. Turbines must be designed to withstand and survive under these conditions. Wind gusts up to 55 mph (88.5 km/h) are very common. Under these conditions, the blades of the turbine can fold down and remain idle. One of the main challenges of offshore wind turbines is its installation in the seabed. Among several methods for anchoring a wind turbine in the seabed, a monopole is most widely used. The details of a monopole structure used in Horns Rev wind farm, Denmark, are shown in Fig. 1.51. About 22–25 m of the tower is below the sea level and requires the use of corrosion resistant materials. Often stainless steel is used for the tower construction with various corrosion protection measures, such as painting, to ensure long life of various components of the structure. A tripod fixed bottom or a floating structure as shown in Fig. 1.52 has been proposed for installations at greater depths. Currently, the USA is pursuing the floating structure concept, whereas Denmark is developing trifloater concept as shown in Fig. 1.53. Recently, reduced transmission constraints, steadier and more energetic winds, and recent European success are making offshore wind turbines attractive worldwide. Some of the disadvantages include higher development and investment costs, and limited accessibility, resulting in higher capital and maintenance costs. Also, environmental conditions at sea are more severe: more corrosion from salt water

1.14

Wind Farm

55

500km

Wind resources over open sea (more than 10km offshore) for five standard heights 50m 100m 200m 10m 25m ms−1 W m−2 ms−1 W m−2 ms−1 W m−2 ms−1 W m−2 W m−2 ms−1 >1100 >1500 >8.0 >600 >8.5 >700 >9.0 >800 >10.0 >11.0 7.0−8.0 350−600 7.5−8.5 450−700 8.0−9.0 600−800 8.5−10.0 650−1100 9.5−11.0 900−1500 6.0−7.0 250−300 6.5−7.5 300−450 7.0−8.0 400−600 7.5 − 8.5 450−650 8.0 − 9.5 600−900 4.5−6.0 100−250 5.0−6.5 150−300 5.5−7.0 200−400 6.0 − 7.5 250−450 6.5 − 8.0 300−600 <4.5 <100 <5.0 <150 <5.5 <200 <6.0 <250 <300 <6.5

Fig. 1.47 Offshore wind resources in Europe (Printed with permission from Leithead [23])

and additional loads from waves and ice. Additionally, offshore construction is more complicated. As shown in Fig. 1.50, an offshore wind turbine will be subjected to various adverse meteorological conditions. Any construction method must guard the structure against these potential meteorological impacts [166]. Various aspects of offshore wind farms are discussed in the following literature [168–182]. More than ten offshore projects are currently operating worldwide. These projects are listed in Table 1.11. The newly-completed Horns Rev is the largest offshore project in the world. A number of countries are also expressing serious intent in developing their offshore resources.

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Fig. 1.48 An offshore wind farm

Fig. 1.49 A simplified schematic diagram of an offshore wind farm showing its various components (Source: British Wind Energy Association [165]

For offshore wind farms, costs are largely dependent on water depth and distance from the shore. The increase in costs is associated not only with the turbine, but also with foundations, installation, and grid connection. An offshore turbine cost is typically 20% higher than an onshore turbine, and costs for towers and foundations could be 150% more than onshore wind generation. However, offshore wind plant output can be up to 50% greater, due to higher wind speeds. Installation costs in the United Kingdom and Sweden in 2007–2008 were between US$ 2,500/kW and

1.14

Wind Farm

57

Fig. 1.50 Various weather conditions that an offshore wind turbine will encounter (Adapted from Musial [166])

US$ 3,700/kW and production costs at these locations ranged from US$ 0.085 to 0.105/kWh. The percentage of the total cost for a specific component is shown in Table 1.12. The maintenance costs of offshore wind turbines are generally higher than onshore wind turbines. The adverse meteorological conditions cause failures of various components of an offshore turbine more frequently compared to an onshore turbine. In Table 1.13 is shown the percentage failure of various components of an offshore turbine. The generator and blades fail most of the time compared to other components.

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Elevation above Seawater Level

The Horns Rev Turbine 110 m Red blade tips 40 m

Rotor

Pitchable blades Wind measurements (anemometers) Aviational lights Heli-hoist platform

Hub

70 m

Naoelle

Yaw bearings

Cable

61 m

Tower

Personal lift Accommodation

Ladder

Electrical equipment Tower door

Navigational lights

Platform

9m

9m

22-24 m

Seabed

Boat landing

Transition piece

0m

6.5-13.5 m

Foundation

Corrosion protection Tube for cable Cable protection

Scour protection (2 layers of stones)

Trenched cable with optical-fiber cable (connects the turbine to neighbouring turbines or substation)

Driven steel pile 4m

HA50601

Fig. 1.51 Components of a 2 MW offshore wind turbine in Denmark’s Horns Rev offshore wind farm (Adapted from Offshore Wind Collaborative Group [167])

1.15 Small Wind Systems A small wind system can be used on-grid (for a cottage, home, farm, or business) or off-grid (for a boat, RV, cottage, home, farm, business, remote community, or remote station). As mentioned earlier, a wind turbine alone cannot be used for continuous supply of power. The consumer must be connected to an electrical grid.

1.15

Small Wind Systems

59

Fig. 1.52 Various methods for installation of wind turbine offshore (Adapted from Robinson [164])

Fig. 1.53 Various deep sea/ocean anchoring systems. (a) NREL concept (b) Dutch Trifloater concept (Adapted from Robinson [163])

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Table 1.11 Offshore wind farms around the world Location Country Online Vindeby Denmark 1991 Lely (Ijsselmeer) Holland 1994 Tunø Knob Denmark 1995 Dronten (Ijsselmeer) Holland 1996 Gotland (Bockstigen) Sweden 1997 Blyth Offshore UK 2000 Middelgrunden, Copenhagen Denmark 2001 Uttgrunden, Kalmar Sound Sweden 2001 Yttre Stengrund Sweden 2001 Horns Rev Denmark 2002 Frederikshaven Denmark 2003

MW 4:95 2:0 5:0 11:4 2:5 3:8 40 10:5 10 160 10:6

No 11 4 10 19 5 2 20 7 5 80 4

Samsø North Hoyle Nysted Arklow Bank Scroby Sands Kentish Flats Barrow Egmond aan Zee Burbo Bank Lillgrund Beatrice Prince Amalia Thornton Bank Lynn and Inner Dowsing

23 60 158 25:2 60 90 90 108 90 110 10 120 300 194

10 30 72 7 30 30 30 36 25 48 2 60 60 54

Denmark UK Denmark Ireland UK UK UK Holland UK Sweden UK Holland Belgium UK

Totals

2003 2003 2004 2004 2004 2005 2006 2006 2007 2007 2007 2008 2008 2008

587

Turbine type and rating Bonus 450 kW NedWind 500 kW Vestas 500 kW Nordtank 600 kW Wind World 500 kW Vestas 2 MW Bonus 2 MW GE Wind 1.5 MW NEG Micon NM72 Vestas 2 MW 2 Vestas 3 MW,1 Bonus 2.3 MW and 1 Nordex 2.3 MW Bonus 2.3 MW Vestas 2 MW Bonus 2.3 MW GE 3.6 MW Vestas 2 MW Vestas V90 3 MW Vestas V90 3 MW Vestas V90 3 MW Siemens 3.6 MW Siemens 2.3 MW REPower 5 MW Vestas V80, 2 MW REPower 5 MW Siemens 3.6 MW

316

Source: Renewable UK offshore wind [183] Table 1.12 Percentage of total costs for various components of an offshore wind turbine

Components Blade Generator Gearbox Electrical system Control Shaft and bearings Yaw system Pitch mechanism Inverter Brake Parking brake

Percent of total cost 30 28 19 5 5 4 4 3 1 1 <1

Source: Adapted from Rademakers et al. [184]

1.16

Low Frequency Noise form Wind Turbines

Table 1.13 Downtime breakdown

61

Components Blade Generator Gearbox Electrical system Control Yaw system Pitch mechanism Shaft and bearings Inverter Brake Parking brake

Percent of total time 34 32 21 5 2 2 2 1 1 <1 <1

Source: Adapted from Rademakers et al. [184]

On-grid, small wind turbines can help supplement the electricity drawn from the grid and reduce the load on the local electrical utility. The off-grid system can provide electricity to remote locations for both seasonal and year-round uses. In the case of isolated grids (not connected to the national electrical grid), the use of diesel generators or other form of fuel can be reduced. Depending on the wind resource, a small wind energy system can provide power for a number of applications, and are given in Table 1.14. A comparison of costs for small wind turbine systems under various conditions is given in Table 1.15.

1.16 Low Frequency Noise form Wind Turbines One of the major concerns with wind turbines is the low frequency noise that is generated by rotating turbines. Because of low rotational rates of turbine blades, the peak acoustic energy radiated by large wind turbines is in the infrasonic range with a peak in the 8–12 Hz range. For smaller machines, this peak can extend into the low-frequency “audible” (20–20,000 Hz) range because of high rotational speeds and multiple blades. The levels of infrasound radiated by the largest wind turbines are very low in comparison to other sources of acoustic energy in this frequency range such as sonic booms, shock waves from explosions, etc. The danger of hearing damage from wind turbine low-frequency emissions is remote to nonexistent. However, the annoyance is often connected with the periodic nature of the emitted sounds rather than the frequency of the acoustic energy. The noise from wind turbines in most cases is lower than that from other noise sources, such as roads, airports and construction machinery. In urban areas the noise is masked from other activities, however, in rural environments, where the background noise typically is low, the noise can be annoying. Also, in urban areas, the noise can be annoying in the night time. In rural areas, wind turbines may be the only major source of noise. Various engineering techniques, such as proper

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Table 1.14 Small wind turbine applications Small wind turbine category

Typical power rating Typical grid connectivity Typical applications

Battery charging and light seasonal loads 0–1,000 W (0–1 kW) Mostly off-grid, some on-grid Mobile uses (sailboats, recreational vehicles, etc.) Seasonal applications (small cottages, hunting lodges, etc.) Rural and ‘urban perimeter’ residential homes (small loads) Specialty power sources (radar and telecomm devices, measurement instruments, cathodic protection, remote weather stations, etc.) Commercial parks and camps Electric fencing

Residential and heavy seasonal loads 1–30 kW Mostly on-grid , Some off-grid Off-grid rural houses with large lot sizes (usually >1 acre) On-grid rural houses with large lot sizes (usually >1 acre) where DC appliances are driven by wind turbine/batteries or where some electricity is stored on the grid through Net Metering Larger cottages or hunting lodges with significant share of electricity from wind

Commercial, institutional, farms, and remote communities 30–300 kW On-grid , Isolated-grid, or Off-grid On-grid or isolated-grid large farms Off-grid small farms where small wind complements a diesel generator set and/or solar photovoltaics On- or off-grid commercial or institutional buildings Isolated-grid communities where wind is complemented by diesel generators and/or other sources

Source: Canadian Wind Energy Association [185]; Connected to nearby transmission line

insulation of the nacelle can be used to reduce the noise level. The aerodynamic noise, which is dependent on the tip speed, can be controlled by lowering the tip speed to a maximum of about 60 m/s. However, in recent years, the size and also the tip speed of wind turbine are increasing. The current generation of wind turbines is operating at tip speeds up to 80 m/s, indicating that noise again may be a problem with respect to public acceptance. The noise level is generally expressed in decibel unit, which is defined below.

1.16

Low Frequency Noise form Wind Turbines

63

Table 1.15 Cost comparison of various types of small wind turbine systems Small wind turbine category Battery charging Residential and Commercial, and light heavy seasonal institutional, farm, Comparison seasonal loads loads remote communitya Typical power 300–1,000 W Above 1–30 kW Above 30–300 kW rating (0.3–1 kW) $2,800/kW $3,000/kW $2,200/kW Average capital costb of turbine only per unit power Average total $5,000– $6,000/kW $3,300/kW installed $6,400/kW costb per unit power Average annual $40–130/year $1,150/year $3,300/year operations and maintenance costb Typical User Professional Professional installation by 10–15 years 20 years 25 years with major Typical lifetimec component replacement typically after 15 years Requires crane to Comments Larger machines in this raise/lower, making range may require a more difficult and crane to raise/lower, considerably costlier making more to maintain on rough difficult and costlier terrain or in remote to maintain than areas smaller turbines Source: Canadian Wind Energy Association [185] a Costs in northern communities are generally higher than the costs listed here, because of site accessibility; availability and price of cranes; and availability and labour rates of qualified installation and maintenance personnel b All costs in 2004 Canadian dollars c Turbine lifetimes vary significantly depending on operating conditions (e.g. high turbulence winds; extreme dust, cold, or corrosion)

1.16.1 Sound Intensity Sound intensity is defined as the sound power per unit area and is expressed as watts=m2 or watts=cm2 . The measurement of sound is made relative to a standard threshold of hearing intensity I0 , whose value is given below. I0 D 1012 watts=m2 D 1016 watts=cm2

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The most common approach to specify the sound intensity is to express it in terms of decibel, which is defined as below: I I.dB/ D 10 log10 (1.19) I0 The threshold of hearing intensity takes the value of zero decibels (0 dB). The decibel scale can be related to the sound pressure level, which is the most common practice to compare the sound intensity level or the noise.

1.16.2 Sound Pressure The Sound Pressure is the force (N ) of sound on a surface area .m2 / perpendicular to the direction of the sound. Therefore, it is expressed as N=m2 or Pascal (Pa).

1.16.3 The Sound Pressure Level The sound pressure level is expressed in terms of decibel as follows. A reference sound pressure is necessary to define sound pressure level in the decibel scale. The lowest sound pressure possible to hear is approximately 2 105 Pa and the sound pressure level is given by: " The Sound Pressure Level .LW / D 10 log10 D 20 log10

P2 2 Pref P Pref

#

D 10 log

P Pref

2

(1.20)

where, LW D sound pressure level (dB) P D sound pressure (Pa) Pref D 2 105 , reference sound pressure (Pa) If the pressure is doubled, the sound pressure level is increased by 6 dB (20 log 10 (2)). Also, it should be noted that auditory nerves can be permanently damaged from prolonged exposure at 90 dB. A sound pressure level of 120 dB can cause pain and ringing in the ear. A sharp pain and extensive destruction of the auditory nerves occurs at 140 dB. The sound pressure and sound pressure level for various noise sources are shown in Fig. 1.54, and the noise level generated by various components of the turbine is shown in Fig. 1.55.

1.16

Low Frequency Noise form Wind Turbines

65

Sound pressure Jet engine (25 m distance)

Sound pressure level µPa

Threshold of pain

140 dB

100000000 130 120

Jet take-off (100 m away)

10000000 110

Pneumatic hammer

100 Rock group

1000000 90 Heavy truck

80 100000 Conversational speech

10000

Average street traffic

70 60

Business office

50 40

1000 Library

Living

30 room 20

100 10 Bedroom

20

0

Threshold of hearing

Fig. 1.54 Noise levels and sound pressure from various activities (Courtesy of Rogers [196])

The low frequency noise has drawn significant attention of both researchers and public. Little is known about the impact of noise from wind turbines on people living in their vicinity. Although the annoyance from wind turbine noise depends on perceptions, the effect is real and must be addressed. The unexpected high proportion of annoyance could be due to visual interference, influencing noise annoyance, as well as the presence of intrusive sound characteristics. Respondents’ attitude to the visual impact of wind turbines on the landscape scenery was also found to influence noise annoyance [186–196].

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Fig. 1.55 Noise level generated by various parts of a wind turbine (Courtesy of Rogers [196])

1.17 Wind Energy and Intermittency The wind does not always blow at a speed necessary for operating a turbine at its rated power. As a result, the wind power is by nature intermittent. The intermittency of the power could be seen from Fig. 1.56. The issues related to the intermittency of wind power are as follows [198]: • The unpredictability of power output. • The purchase of back-up power from other generators to fill unscheduled gaps when the wind is not blowing. • The production of electricity by many projects during off-peak hours, which can add to over-generation problems, especially at night, when energy demand is lowest. • Wind power’s nature as a non-dispatchable “must take” resource that, therefore, can add to transmission line overloads. • Shifts in power generation among fleets of wind turbines, which can cause voltage collapse within a wind project, thereby, reducing available energy sales.

1.18

Summary

67

Fig. 1.56 Hourly variation of power production of a Nordic Wind Power System (Printed with permission from Holttinen [197])

Several studies noted that when the concentration of wind power in a region is low, the impact on the grid is negligible. If the power transmitted from wind turbines into the grid increases in a region, intermittence and the difficulty of forecasting wind power production would have a real cost associated with this scenario. The intermittency issue could be addressed by having back-up generation, and smart grid design that will allow areas with large wind resources to “spread” their power capability over a larger geographic area. The additional cost would be in the range of 0.2–0.5 cents/kWh for lower percentage of contribution, but could be higher for 20% contribution of power. The UK Energy research center reviewed over 200 international studies to understand the effect of intermittency on renewable power sources [199]. The conclusion was that intermittency need not present a significant obstacle to the development of renewable sources.

1.18 Summary Wind is a renewable energy source, since it will continue to blow as long as there is the sun. Wind energy is mainly used to generate electricity. In 2009, wind energy provided approximately 1.8% of total U.S. electricity generation, totaling about 71 billion kWh. Several countries in Europe including Germany, Spain, UK and Denmark and in Asia such as China and India are utilizing wind power to supply a significant amount of their electricity need. For example, Denmark ranks ninth in the world in wind power capacity, but generates about 20% of its electricity from wind. Significant growth in wind power in recent year has been experienced due to new technologies that have decreased the cost of producing electricity from wind, and tax breaks for renewable energy and green pricing programs. The site for the placement of wind turbines must be carefully selected to optimize their operation. Good sites for wind plants are the tops of smooth, rounded hills, open plains or shorelines, and mountain gaps that produce wind funneling. Wind turbines may be placed both onshore and offshore. Several turbines are generally grouped together in a single location to develop a wind farm. There are

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two types of wind turbines used today, based on the direction of the rotating axis: horizontal axis and vertical-axis. However, horizontal axis wind turbines are mainly used for commercial applications. The size of wind turbines can vary from 100 kW for powering a single home or business to large commercial-sized turbines of capacity of 5 MW. Although the wind energy is one of the most advanced renewable energy sources, issues such as intermittency of the power generation, low level noise, and unsuitability for base-load power are hindering its full scale deployment.

Problems 1. What is wind energy? How is it classified? Why is the wind speed important for wind energy systems? 2. What factors determine the amount of electricity one can generate from wind turbines? 3. What is the difference between availability factor and capacity factor? 4. Is there any limitation on the size of wind turbines? 5. How much electricity can be generated in a 1;000 m2 land with Class 3 wind? 6. Consider that a wind farm is designed for Class 3 wind. What will happen if the wind class drops to 2 or increases above 3? 7. What will happen to power production and to the wind turbine if there is a sudden gust of wind, even for few seconds? 8. What is the production tax credit for wind energy in the USA? 9. Estimate the cost of production of electricity from wind energy. 10. The wind doesn’t blow all the time. What are the consequences of this effect on a utility’s generating capacity? 11. How much energy can wind realistically supply to the U.S.? 12. How much energy can wind supply worldwide? 13. Can wind energy reach its full potential in the U.S.? 14. Wind is a variable energy source. What will be the effect on a single user customer or a utility? 15. What are the economic impacts of using wind energy to a commercial farm or home owners? 16. Calculate the payback time for a wind turbine. Use your electrical bill for the information. 17. What are the environmental impacts of using wind energy?

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18. In cold climate, will ice accumulate on wind turbine blades? Is yes, what are the consequences? 19. What are the factors in determining if an offshore wind farm is feasible? 20. Is there a limitation on how deep a wind turbine could be installed in the sea? 21. Discuss noise generation from wind turbines. 22. It is often estimated that wind energy can supply about 20% of the electricity in the USA and other countries. What is the basis for this 20% estimate? Can it go higher, say 40%? Discuss. 23. How is the intermittency of wind energy addressed for a utility? 24. What is the voltage generated by a wind farm and what are the steps involved in supplying that to the grid? What kind of voltage is fed to the grid? 25. Do wind turbines produce low frequency noise? 26. Do wind turbines frighten livestock? 27. What happens when the wind stops blowing? 28. How strong does the wind have to blow for the wind turbines to work? 29. How fast do the blades turn? 30. Are wind turbines hazardous to birds and bats? 31. Do wind turbines pose a safety hazard? 32. Are there other drawbacks to the use of wind energy? 33. Is wind energy good for the economy? 34. What are the advantages of wind-generated electricity? 35. What are the economic obstacles to greater wind power usage?

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197. Holttinen H (2005) Hourly wind power variations in the Nordic countries. Wind Energy Publication B:B1–B23 198. Asmus P (2003) How California hopes to manage the intermittency of wind power. Electricity J 16(6):48–53 199. UK Energy Research Center (2006) The costs and impacts of intermittency: an assessment of the evidence on the costs and impacts of intermittent generation on the British electricity network. Imperial College London

Chapter 2

Solar Energy

Abstract The sun is the main source of all alternative energies on the earth’s surface. Wind energy, bioenergy, ocean energy, and hydro energy are derived from the sun. However, the term solar energy refers to the energy that is harvested directly from the sun using solar cells, solar concentrators, etc. Although solar energy is abundant on the earth’s surface, harvesting it into a useful energy form is challenging and often costly. Among all of the alternative energy resources, solar energy is most costly for generation of electricity. Solar energy can be used either as a source of thermal energy when using solar concentrators, or for direct electricity generation when using photovoltaics. These systems are discussed in this chapter.

2.1 Energy from the Sun The sun is about 1.4 million kilometers (about 870,000 miles) in diameter and its interior temperature is about 15 million degrees Kelvin (about 27 million degrees F). This high temperature, combined with a pressure that is 70 billion times higher than atmospheric pressure on the earth, creates ideal conditions for fusion reactions. The fusion reaction in the sun involves two hydrogen atoms combining to form a helium atom, releasing the energy in the process. This energy is released in the form of high-energy gamma radiation. As gamma rays radiate from the center to the outside of the solar sphere, they react with the solar media and are transformed into lower-energy radiations, primarily in the visible light and heat portions of the energy spectrum. The sun has been producing energy in this manner for around 5 billion years, and will continue to do so for several more billion years. The basics of solar energy, solar radiation and energy transfer mechanisms have been discussed in several monographs [1–4].

T.K. Ghosh and M.A. Prelas, Energy Resources and Systems: Volume 2: Renewable Resources, DOI 10.1007/978-94-007-1402-1 2, © Springer Science+Business Media B.V. 2011

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2.2 Energy Transfer to the Earth The earth orbits the sun at a distance of 150 million kilometers (93 million miles). Radiation expands outward from the sun at the speed of light, 300,000 km per second (186,000 miles per second), and in the form of electromagnetic radiation. The amount of time it takes for the solar radiation to reach the earth is about 8 minutes. The amount of solar energy reaching a specific location on the surface of the earth at a specific time is called “insolation”, and its value depends on several factors. As shown in Fig. 2.1, if the sun is directly overhead, and the sky is clear, the radiation on a horizontal surface is about 1;000 W=m2 . This is the highest value of insolation possible on the earth’s surface except by concentrating sunlight with devices such as mirrors or lenses. As can be seen from Fig. 2.2, the solar radiation received on the surface is less when the sun is not directly overhead. This is because there is more atmospheric medium between the sun and the surface. Not all energy reaches the earth because some of it is absorbed by the atmosphere present between the sun and the earth.

Fig. 2.1 Energy from Sun

Fig. 2.2 Energy received depends on the earth’s position

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Energy Transfer to the Earth

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Fig. 2.3 Dependence of solar radiation capture on the orientation of the panel

Fig. 2.4 Position of Earth with respect to Sun during a year

The insolation decreases when a surface is not oriented perpendicular to the sun’s rays. This is because the surface presents a smaller cross-sectional area to the sun. This is shown in Fig. 2.3.

2.2.1 Seasonal Variation The earth spins around its axis once per day, and rotates in an elliptical orbit around the sun once per year. The axis around which the earth rotates is tilted to 23:5ı from the solar plane toward the sun at one end of its orbit and away from the sun at the other end (see Fig. 2.4). This gives rise to the seasons: when the earth’s axis is tilted toward the sun, the northern hemisphere receives more direct solar radiation (summer). Six months

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Fig. 2.5 Effect of sunlight on power density on vertical and horizontal surfaces when faced at different angles (Reproduced with permission from Page [5])

Fig. 2.6 Monthly isolation at the equator .0ı /; 30ı N; 60ı N, and 90ı N (Reproduced with permission from Pidwirny [6])

later, when the axis is tilted away from the sun, the southern hemisphere experiences summer. In between are spring and fall, when the tilt of the earth’s axis is neither toward nor away from the sun. This tilt causes the intensity of a beam of sunlight to spread over a relatively larger area. As shown in Fig. 2.5, a tilted surface has a larger area compared to a vertical surface through which a beam of sunlight travels. As a result, the power in the beam on a per unit area basis decreases. Also, the insolation, as shown in Fig. 2.6, varies with Equinox and Solstice. The design of solar based power generating devices must accommodate this variation in insolation.

2.3

Energy and the Sun

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Sun’s path in the Sky June 21

West

North

December 21 PV modules

South

East

Fig. 2.7 Sun’s radiation based on Earth’s rotation (Courtesy of Energy Efficiency and Renewable Energy, US Department of Energy [7])

2.2.2 Height of the Sun in the Sky In the summer, due to the elliptical path of the earth’s rotation around the sun and the rotation around its own axis, it appears that sun is tracing a path high in the sky between rising in the east and setting in the west. In winter, it appears that this path across the sky is much closer to the horizon. These two paths are shown in Fig. 2.7. This has important effects on seasonal insolation and on the design of solar energy based systems.

2.3 Energy and the Sun The sun’s energy sustains the life on the earth. It determines the earth’s surface temperature and supplies virtually all the energy that drives natural global systems and cycles. The sun releases about 46% of its energy as visible light. However, visible light represents only a fraction of the total radiation spectrum; infrared and ultraviolet rays are also significant parts of the solar spectrum. This spectrum is

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Fig. 2.8 The solar spectrum in space and on the earth’s surface (Courtesy of Solar Energy Centre [8])

shown in Fig. 2.8. The solar spectrum in the outer space is significantly different than what is seen on the earth’s surface. It is similar to a black body spectrum. A black body spectrum can be approximated by the Planck’s equation given below. I./ D

2h 3 1 h h i 2 c e . kT / 1

(2.1)

where, I D solar radiance h D Planck constant v D frequency c D speed of light k D Boltzmann constant T D temperature of the black body As the light passes through the atmosphere, some of the light is absorbed or reflected by gases such as water vapor, carbon dioxide, oxygen, and the ozone. Therefore, the solar spectrum on the surface of the earth is different than that in the space. Engineers must consider the spectrum of incident light when designing solar cells. The sun emits virtually all of its radiation energy in a spectrum of wavelengths that range from about 2 107 to 4 106 m. Each wavelength corresponds to a frequency and an energy; the shorter is the wavelength, the higher is the frequency and also the greater is the energy (expressed in eV, or electron volts).

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Use of Solar Energy

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Fig. 2.9 Frequency and photon energy associated with the solar light

Each portion of the solar spectrum is associated with a different level of energy (see Fig. 2.9). Within the visible portion of the spectrum, for example, red light is at the low-energy end and violet light is at the high-energy end. Ultraviolet and infrared belong to the invisible portions of the spectrum. Photons in the ultraviolet region have more energy than those in the visible region. Also, photons in the infrared region, which one feels as heat, have less energy than photons in the visible region.

2.4 Use of Solar Energy Throughout the history, humans have used the heat from sunlight to dry grains, cook food, and heat water and homes [9–14]. The concept and the use of solar thermal energy started in 1767 when the Swiss scientist, Horace de Saussure, invented the world’s first solar collector, or “hot box”. Renowned astronomer, Sir John Herschel, used solar hot boxes to cook food during his expedition to Southern Africa in the 1830s. Today, solar collectors can gather solar thermal energy in almost any climate to provide reliable, low-cost source of energy for many applications including heating water for homes and residential heating systems. Various other industries, such as laundries and food processing companies, also utilize solar energy. In recent years, utilities have begun to use solar thermal energy to generate electricity by using steam turbines. The steam is produced by concentrating the solar energy into a water boiler.

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Fig. 2.10 Energy flow from sun to the earth (Reproduced with permission from Page [5])

An inventory of the solar energy directed towards the earth and the amount received and retained by the earth is shown in Fig. 2.10. A significant amount of the energy is used to sustain the life on the earth; however, a certain amount may be harvested to perform various activities on the earth. A system using solar energy can be designed for both large-scale commercial applications and smaller systems, such as homes. The use of solar energy may be divided into the following categories: • Solar Thermal Energy • Solar Heating – Solar Water Heating – Solar Space Heating – Solar Pool Heating • Electricity Generation Using Solar Concentrators • Photovoltaic Cells • Solar Lighting Businesses, industry, and residential sectors (homeowners) can utilize solar heating technologies for heating and cooling, industrial processes, electricity generation, water heating and daylighting. Homeowners can also produce electricity to operate “off-grid” or to sell the extra electricity to the utilities, depending on local

2.4

Use of Solar Energy

Direct Normal Solar Radiation (Two-Axis Tracking Concentrator)

87

Annual Model estimates of monthly average daily total radiation using inputs derived from satellite and/or surface observations of cloud cover, aerosol optical depth, precipitable water vapor,albedo, atmospheric pressure and ozone resampled to a 40km resolution. See http://WWW.nrel, gov/gis/il_CSP html documentation for more detalis.

kWh/m2/day >9.0 8.5 − 9.0 8.0 − 8.5 7.5 −8.0 7.0 − 7.5 6.5 − 7.0 6.0 − 6.5 5.5 − 6.0 5.0 − 5.5 4.5 − 5.0 4.0 − 4.5 3.5 − 4.0 3.0 − 3.5 2.5 − 3.0 2.0 − 2.5 < 2.0

Produced by the Electric & Hydrogen Technologies & Systems Center - May 2004

Fig. 2.11 Solar radiation on the USA landmass if a two-axis tracking concentration is used (Courtesy of Energy Efficiency and Renewable Energy, US Department of Energy [15])

programs. The use of daylighting in design strategies can help both homes and commercial buildings to reduce consumption of electricity and improve productivity and efficiency of people occupying the space. The surface of the earth does not receive the sunlight or energy uniformly. Therefore, any design of solar energy based systems must take this into account. Data of solar radiation has been complied by various countries and should be used in the design. The average solar radiation received by the USA is shown in Fig. 2.11. However, as shown in Fig. 2.12, the peak hours for maximum exposure to the sun are rather short. The average solar radiation on the earth is shown in Fig. 2.13. A similar irradiation map for the Europe is shown in Fig. 2.14. Significant efforts are underway to further map the solar irradiation around the world using more accurate satellite data.

2.4.1 Solar Thermal Energy Solar thermal technologies involve harvesting energy from the sun for heating water or producing electrical power. Small scale water heating systems use flat plate

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Annual Average Daily Peak Sun Hours 4

4.5

4 4 4

5

4.5 6 5 3

4.7

4.7

5

Fig. 2.12 Annual average daily peak sun hours in various region of the USA (Courtesy of Energy Efficiency and Renewable Energy, US Department of Energy [7])

collectors to capture heat from the sun, while solar thermal electric power plants use various concentrating devices to focus sunlight and achieve high temperatures necessary to produce steam for power generation. Solar heat without concentrating can be used for: • Solar water heating • Solar space heating in buildings • Solar space cooling Worldwide about 126 GW of solar thermal energy are used annually for heating water. The use of solar energy for water heating by the top ten countries is given in Table 2.1. In the USA, the most popular use of the solar energy is for heating swimming pools. About 22,700 units are installed in 2006 that consumed 929 MW (thermal) energy from the sun. Figure 2.15 illustrates the thermal energy harvested from the sun for water and pool heating. The number of solar water and pool heaters installed in the USA are shown in Fig. 2.16 with continued high growth rates projected in 2009–2010. Additionally, the long term extension of the USA federal investment tax credits would provide a significant boost to the market. Several companies in the USA have announced plans to install solar energy based projects for producing electricity. This would further boost the market.

4.0-4.9

6.0-6.9

3.0-3.9

5.0-5.9

Fig. 2.13 Distribution of solar power .W=m2 / around the world (Reproduced with permission from Page [5])

Midpoint of zone value

2.0-2.9

1.0-1.9

2.4 Use of Solar Energy 89

10°W

10°W

5°W

5°W

0°

0°

5°E

5°E

10°E

10°E

15°E

15°E

20°E

20°E

25°E

25°E

60°N

55°N

50°N

55°N 50°N 45°N

Fig. 2.14 Annual solar irradiation in Europe (Courtesy of Meteotest; database Meteonorm [16])

2.April 2008/fd

35°N

Globalstrahlung: Langjähriges Mittel

40°N

40°N

45°N

60°N

35°N

>2420 2381-2420 2341-2380 2301-2340 2261-2300 2221-2260 2181-2220 2141-2180 2101-2140 2061-2100 2021-2060 1981-2020 1941-1980 1901-1940 1861-1900 1821-1860 1781-1820 1741-1780 1701-1740 1661-1700 1621-1660 1581-1620 1541-1580 1501-1540 1461-1500 1421-1460 1381-1420 1341-1380 1301-1340 1261-1300 1221-1260 1181-1220 1141-1180 1101-1140 1061-1100 1021-1060 981-1020 941-980 901-940 861-900 821-860 781-820 741-780 701-740 661-700 621-660 581-620 541-580 501-540 <501

kWh/m2

90 2 Solar Energy

2.4

Use of Solar Energy

91

Table 2.1 Solar hot water installed capacity of top ten countries in the world (giga-watts thermal) Country/European union China European Union Turkey Japan Israel Brazil United States India Australia Jordan (other countries) World Total

Additions in 2007 16:0 1:9 0:7 0:1 0:05 0:3 0:1 0:2 0:1 0 <0:5 20:0

Existing in 2007 84:0 15:5 7:1 4:9 3:5 2:5 1:7 1:5 1:2 0:6 <3:0 126:0

Source: REN21 [17]

Fig. 2.15 Installed capacity of (a) solar water heaters and (b) solar pool heaters in the US (Adapted from Sherwood [17])

2.4.1.1 Solar Water Heating A solar water heater consists of a solar collector that absorbs solar radiation and generates usable thermal energy [19–25]. The thermal energy or heat is then transferred to a fluid medium, such as water, another heat transfer fluid, or air, which flows through the collector [26–32]. The thermal energy can also be stored for night time use or other times when solar radiation is not available [33, 34]. A major application of solar water heating systems is to provide hot water to individual homes. Water can either be heated directly in the collector (direct systems) or

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Fig. 2.16 Number of annual installations of pool and water heating systems in the USA in the last several years (Adapted from Sherwood [18])

indirectly by a heat transfer fluid that is heated in the collector and passes through a heat exchanger to transfer its heat to water. Five types of heating methods can be used to heat domestic and service hot water systems. These water heating systems or methods are: • • • • •

Thermosyphon [35–40], Collector storage systems [41–44], Direct circulation systems, Indirect heating or circulation systems, and Hot air systems.

An auxiliary circulating system, such as a pump, is not necessary for the thermosyphon and collector storage systems. They are called passive systems. The three remaining systems are called active systems and a pump or fan is employed to circulate the fluid. For cold climate applications, freeze protection, recirculation and drain-down are used for direct solar water heating systems and a drain-back is used for indirect water heating systems [45, 46]. Heat pipe systems are also explored for the use in solar water heaters. A heat pipe is a self contained system that transfers heat by evaporation of the working fluid and condensation of vapor. Since there is no moving parts involved the system is more reliable [47–49].

2.4

Use of Solar Energy

93

For all these applications, a solar collector is needed. Three different types of solar collectors are available. • Flat-plate collectors • Evacuated-tube collectors • Integral collector-storage systems Residential and commercial building applications that require temperatures below 94ı C .200ı F/ typically use flat-plate collectors, whereas those requiring temperatures higher than 94ı C .200ıF/ use evacuated-tube collectors.

Flat-Plate Collectors Flat-plate collectors are the most common type of solar collector for waterheating systems and space heating [50–53]. Flat-plate collectors are mainly used for residential water heating and hydronic space-heating installations. Flat plate collectors transfer the heat of the sun to water either directly or through the use of another fluid and a heat exchanger. The amount of solar energy a flat plate collector would receive at various regions of the USA is shown in Fig. 2.17.

Alaska

Average Daily Solar Radiation Per Month JANUARY

Hawaii 5.95

6.42 6.79 5.82

Hawaii, Puerto Rico, and Guam are not Shaded.

San Juan, PR 6.65

Guam , PI 6.41

Two-Axis Tracking Flat Plate Collector Orientation Two-axis tracking flat-plate collector: Data Used to generate this map represent the maximum solar radiation at a site available to a collector. Tracking the sun in both azimuth and elevation, these collectors keep the sun’s rays perpendioular to the collector surface.

This map shows the general trends in the amount of solar radiation received in the United States and its territories. It is a spatial interpolation of solar radiation values derived from the 1961-1990 National Solar Radiation Data Base (NSRDB). The dots on the map represent the 239 sites of the NSRDB. Maps of average values are produced by averaging all 30 years of data for each site. Maps of maximum and minimum values are composites of specific months and years for which each site achieved its maximum or minimum amounts of solar radiation. Though useful for identifying general trends, this map should be used with caution for site-specific resource evaluations because variations in solar radiation not reflected in the maps can exist, introducing uncertainty into resource estimates. Maps are not drawn to scale.

National Renewable Energy Laboratory Resource Assessment Program

kWh/m2/day 10 to 14 8 to 10 7 to 8 6 to 7 5 to 6 4 to 5 3 to 4 2 to 3 0 to 2 none P2xxA01-157

Fig. 2.17 Daily solar radiation averaged over a month in the USA when a two-axis tracking system is used for flat plate collectors (Courtesy of Resource Assessment Program, National Renewable Energy Laboratory [61])

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2 Solar Energy Stainless Steel Fasteners Riveted Corners

Low lron Tempered Glass Low-Binder Fiberglass Insulation Rigid Foam Insulation Secondary Silicone Glazing Seal

Black Chrome or Moderately Selective Black Paint Absorber Coating Copper Absorber Plate

Integral Mounting Channel

Extruded Anodized Type M copper Aluminum Casing Riser Tubes and and Capstrip Manifolds EPDM Vent Primary EPDM Grommets Plugs Glazing Seal 15% Silver Aluminum Backsheet Brazed Joints

Fig. 2.18 Design features of a flat plate collector for heating water (Adapted from SunEarth, Inc. [62])

A typical design of a flat-plate collector has an insulated bottom surface in a metal casing with a dark-colored absorber plate on the top that absorbs most of the solar energy. The insulation prevents the heat from escaping through the back of the collector. A glass or plastic cover called the glazing surface is placed over the absorber plate. The glazing surface traps the heat within the collector. The absorber plate transfers energy from solar radiation to a fluid that circulates within it. Absorber plates are often coated to maximize the solar radiation collection [44–60]. Tubes are placed inside the metal casing to heat liquid or air to temperatures less than 82ı C.180ı F/. A cross-sectional diagram of a flat plate collector is shown in Fig. 2.18.

Liquid Flat-Plate Collectors A schematic diagram of a liquid flat-plate collector is shown in Fig. 2.19. The collector heats the liquid flowing through tubes in or adjacent to the absorber plate. Simplest liquid systems use potable household water, which is heated as it passes directly through the collector and then flows to the house. Solar pool heating also uses the liquid flat-plate collector technology, but the collectors are typically unglazed (Fig. 2.20). Flat plate collectors may be used in three modes: (a) active systems that use a pump to circulate the water or other fluid from the collector to a storage tank,

2.4

Use of Solar Energy

95

Header/Manifold

Metal deck

Flow Solar Panel Pump flow

Fig. 2.19 A tube on sheet solar collector for heating water Fig. 2.20 Unglazed solar collector (Reproduced from Energy Efficiency and Renewable Energy, USDOE [63])

(b) integral collector systems, which replace the absorber plate with large pipes that both absorb solar radiation and store the heated liquid, and (c) thermosiphon systems that use a separate storage tank located above the collector. When liquid is heated it rises naturally to the tank.

Air Flat-Plate Collectors For space heating, air flat plate collectors are mainly used. A general configuration of an air flat plate collector is shown in Fig. 2.21. In flat plate collectors, the cool air is heated directly and the resulting warm air is vented into the space. The absorber plates in air collectors are generally made out of metal sheets; however non-metallic materials have also been used. A fan may be used to blow the air over the collector plates. Air may be heated by natural convection, but the efficiency is lower. Because air conducts heat much less efficiently than a liquid, a larger collecting surface may be necessary [64–68].

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Fig. 2.21 Air flat-plate collectors are used for space heating (Courtesy of Five shades of green energy [63])

Evacuated -Tube Collector

Cross Section

Evacuated Tube Glazing

Outer glass tube Absorbing coating Inner glass tube Fluid tubes Copper sheet Evacuated space

Inflow Reflector Outflow

Fig. 2.22 Evacuated tube collector for heating water or air (Courtesy of Energy Efficiency and Renewable Energy, USDOE [63])

Evacuated-Tube Collectors Evacuated-tube collectors, shown in Fig. 2.22, are designed to reach a temperature in the range of 77ı C.170ı F/–177ı C.350ı F/. This type of temperature is most suitable for heating and a number of other commercial and industrial applications.

2.4

Use of Solar Energy

97

Fig. 2.23 A comparison of solar collectors (Adapted from Seifert [76])

Evacuated-tube collectors are more expensive than flat-plate collectors, with unit area costs about twice that of flat-plate collectors [69–75]. Transparent glass tubes are placed inside the collector below the glazing surface in parallel rows. Each tube contains a glass outer tube and a metal absorber tube attached to a fin. The fins are coated to maximize absorption of solar energy and prevent radiative heat loss as much as possible. A vacuum system is used to remove air from the space between two glass tubes. By this arrangement, conductive and convective heat losses are minimized. A comparison of these three types of solar heat collectors is presented in Fig. 2.23.

Integral Collector-Storage Systems A schematic diagram of the Integral Collector-storage Systems (ICS), also known as “batch” systems is shown in Fig. 2.24. Cold water enters a pipe and can be diverted either to a water heater tank or the batch collector. The batch collector is made of one or more black tanks or tubes in an insulated glazed box. Cold water is preheated in the solar collector and then continues to the conventional backup water heater. The batch collector is a large box holding a tank and covered with a glaze that faces the sun. Water from the solar storage/backup water heater is then fed to the home water pipelines [76–86].

98

2 Solar Energy

Passive, Batch Solar Water Heater

Batch Collector Hot water to house

Spigot drain valve (for cold climates) Bypass valves

Solar storage/ backup water heater

Cold water supply

Fig. 2.24 A schematic diagram of a passive, batch type solar heater (Courtesy of Energy Efficiency and Renewable Energy, USDOE, [63])

ICS systems are simple, reliable solar water heaters. However, care must be taken if they are used in cold climate. The outdoor pipes could freeze in extreme cold weather. The problem with freezing pipes can be overcome by using spigot drain valves. 2.4.1.2 Space Heating of Buildings A solar space-heating system may consists of a passive system, an active system, or a combination of both. Passive systems are typically less costly and less complex than active systems. However, when retrofitting a building, active systems might be the only option for utilization of the solar energy [87–95]. Passive Solar Space Heating Sunrooms or greenhouses are examples of the use of passive solar space heating. Passive solar space heating takes advantage of heat from the sun through various design features [96–99]. Large south-facing windows and materials in the floors or walls that absorb heat during the day and release that at night are used in the design. Passive solar design systems usually have one of the following three designs: Direct gain: In this method, heat is stored into the building materials such as brick, concrete, or floor tiles during the day and is slowly released at night or when ambient temperature drops. Care must be taken to avoid overheating of the space.

2.4

Use of Solar Energy

99

Indirect gain: In this method, various specially designed materials that hold, store, and release heat are used. The materials are located between the sun and living space (typically the wall). Isolated gain: In this approach, the solar energy is remotely collected from the location of the primary living area. For example, a sunroom attached to a house collects warmer air that flows naturally to the rest of the house. Active Solar Space Heating Active solar space-heating systems combine solar collectors with electric fans or pumps to transfer and distribute the collected solar heat [100]. An active system contains an energy-storage system to provide heat when the sun is not shining. Two basic types of active solar space-heating systems are used: • Heating using liquid • Heating using air A general flow diagram of an active solar space heating system is shown in Fig. 2.25. The solar collector is fitted on the roof top. The air or a liquid is heated in the solar collectors and then transported by small electric fans, pumps, or by thermosiphon effects, to a storage unit. Solar heat is stored in water tanks for a liquid heating

SOLAR COLLECTORS (AIR or WATER)

Basic Schematic of Active Solar Space Heating System

MIXING VALVE OR DAMPERS HOUSE HEAT DISTRIBUTION SYSTEM

SOLAR HEAT STORAGE

P-4

P-2 P-3

BOILER or FURNACE

NOTE: P-1, P-2 AND P-3 DENOTE CIRCULATING PUMPS OR FANS

Fig. 2.25 A schematic diagram of an active solar space heating system (Courtesy of Solar Center Information, North Carolina Solar Center [101])

100

2 Solar Energy

system or in rock bins for an air heating system to provide stored heat during sunless periods. The liquid-based systems heat water or an antifreeze solution in a hydronic collector. Air-based systems heat air in an air collector. Air-based solar heating systems usually employ an air-to-water heat exchanger to supply heat to the domestic hot water system, making the system useful in the summer time. Both the systems can transfer the solar heat directly to the interior space, and, if necessary, to a storage system, from which the heat is distributed. An auxiliary or backup system is necessary to provide heat when stored energy is discharged. Liquid systems are more often used when a storage system is included.

Radiant Heating Radiant heating systems involve supplying heat directly to the floor or to panels on the wall or ceiling of a house. The system depends largely on radiant heat transfer. In this method, plastic, rubber or copper pipes embedded in a concrete floor, wall panel, or ceiling tiles are heated by solar energy. They can operate effectively at relatively low temperatures. When solar-heated water circulates through the pipes, the floor, wall, or ceiling heats up and then radiates the heat to the room. A radiant heating system is shown in Fig. 2.26.

Fig. 2.26 Heating of homes using solar radiant heat (Courtesy of Solar Center Information, North Carolina Solar Center [101])

2.4

Use of Solar Energy

101

2.4.1.3 Space Cooling The main application of the solar energy in cooling and refrigeration systems is the regeneration of the working medium [102–106]. These systems can provide yearround utilization of collected solar heat, thereby, significantly increasing the cost effectiveness and energy efficiency of solar installations. These systems are sized to provide 30–60% of building cooling requirements. There are three types of cooling technologies that use the solar thermal energy for regeneration of the working medium: • Absorption Cooling • Desiccant Cooling • Heat Engine/Vapor Compression Cooling (Rankine-Cycle)

Absorption Cooling An absorption chiller is connected to high-temperature collectors such as vacuum tubes or concentrating collectors [107–114]. The absorption chiller operates in a temperature range of 80–120ıC. The chilled working fluid removes the moisture from the air and at the same time cools the air. The diluted working fluid is regenerated using solar collectors. This type of chiller requires some electricity for pumping, has limited efficiency, and requires advanced solar collectors.

Desiccant Cooling In this method, a desiccant, which can be either a solid or a liquid, is used to absorb (in the case of liquid desiccants, such as lithium chloride, lithium bromide, or glycols) or adsorb (in the case of solid desiccants such as silica gel or molecular sieve) the moisture. It is a very effective process for removal of latent heat from the moisture in the air [115–126], thus reduces the refrigeration load. If solid desiccant is used, the dry air (the air from a desiccant unit generally has very low relative humidity) is re-humidified by spraying water into the air to adjust the final relative humidity of the air that is to be vented to the desired space. This way both the relative humidity and the temperature of the space are controlled. Liquid desiccants can control the humidity and temperature within the same unit and further adjustment of humidity is not necessary. The evaporation of water cools down the air. Once the solid or the liquid desiccant becomes saturated with water vapor, it is regenerated by hot air that is heated through solar air collectors or a coil connected to liquid-based collectors.

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2 Solar Energy

Heat Engine/Vapor Compression Cooling (Rankine-Cycle) A heat engine uses a high pressure liquid in a Rankine-cycle to generate electricity. High-temperature solar collectors can be used to heat the fluid and generate the required high pressure. This type of system is used in refrigerated warehouses.

2.5 Concentrating Solar Power (CSP) Concentrating solar power systems use reflective materials, such as mirrors, to concentrate the sun’s heat energy into a boiler to generate steam, which is used to drive a steam turbine and generate electricity [127–160]. Recently, CSP is also used to breakdown metal oxides to its metal counterpart, such as zinc oxide to zinc that is used to split water for producing hydrogen [161–163]. This technology is discussed in details in Chap. 8. There are three kinds of concentrating solar power systems based on how the solar energy is concentrated: • Trough Systems • Power Tower Systems • Dish/Engine Systems

2.5.1 Trough Systems Parabolic, trough-shaped reflectors are used to direct the concentrated solar energy onto a receiver pipe running along the inside of the curved surface (Fig. 2.27). Generally oil is used as the heating fluid. The solar energy heats the oil flowing through the receiver pipe. The solar heat is then used to generate electricity in a conventional steam generator. To generate power on a large scale, a collector field is set up containing many troughs in parallel rows aligned on a north-south axis. This configuration enables the single-axis troughs to track the sun from east to west to ensure that the sun is continuously focused on receiver pipes. A trough system can currently generate about 80 MW of electricity. The aerial view of the Kramer Junction solar plant in California, USA is shown in Fig. 2.28. Trough designs generally incorporate thermal storage, using a heat transfer fluid in its hot phase allowing for electricity generation several hours into the evening. Currently, all parabolic trough plants are “hybrids,” meaning these use fossil fuels to supplement the solar output during periods of low solar radiation. A schematic diagram of a solar power plant at Andulusia, Spain is shown in Fig. 2.29.

2.5

Concentrating Solar Power (CSP)

Fig. 2.27 The structure of a parabolic trough (Courtesy of Energy Efficiency and Renewable Energy, US Department of Energy [164])

103

Concentrator Receiver

Fig. 2.28 The aerial view of the solar power plant using parabolic trough (Courtesy of Energy Efficiency and Renewable Energy, US Department of Energy [164])

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2 Solar Energy

Solar Field

Steam Turbine 2-Tank Salt storage

Solar Superheater

Hot Salt Tank

Condenser

Deaerator Steam Generator Solar Preheater Cold Salt Tank

Low Pressure Preheater

Solar Reheater Expansion Vessel

Fig. 2.29 The schematic of the AndaSol solar power plant (The “AndaSol” solar power plant is located in the Marquesado del Zenete – a wide valley in Andalusia, Spain – converting solar energy into electricity using a parabolic trough collector and a molten-salt thermal storage system. The plant uses 510; 120 m2 SKAL-ET parabolic trough solar field, a 7.5-hour reserve molten-salt thermal storage system and a 49.9 MW-capacity steam cycle) [164]

2.5.2 Power Tower Systems In the power tower system, sun-tracking mirrors (heliostats) are used to focus solar energy on a receiver at the top of a tower. A heat transfer fluid is used to transfer the energy to the receiver, which in turn is used to generate steam. A conventional steam turbine is used to generate the electricity. Molten nitrate salt is generally used as the heat transfer fluid because of its superior heat transfer and energy storage capability. A schematic diagram of a solar power tower system is shown in Fig. 2.30. In the USA, a power tower plant is installed in the Mojave Desert to demonstrate the technology. The plant is designed for 10 MW of electricity. Molten salt at 554ıF.290ı C/ is pumped from a cold storage tank through the receiver where it is heated to about 1;050ı F.565ıC/. The heated salt is stored in the hot storage tank. To generate electricity, the hot salt is pumped to a generator that produces steam. An aerial photograph of a power tower plant is shown in Fig. 2.31.

2.5.3 Dish/Engine Systems A solar Dish-Engine system uses dish shaped parabolic mirrors as reflectors to concentrate and focus the sunlight onto a receiver. The basic components of a dishengine system are shown in Fig. 2.32. The receiver is mounted on individual dish. The sunlight hits the entire dish but is concentrated in a small area so that it can be

2.5

Concentrating Solar Power (CSP)

105

Fig. 2.30 The schematic of a power tower plant system (Courtesy of Energy Efficiency and Renewable Energy, US Department of Energy [164])

more efficiently used. Glass mirrors reflect approximately 92% of the sunlight that hits the dish. The dish structure can track the sun continuously to reflect the beam onto the thermal receiver. The individual dish can be equipped with an engine or generator for direct generation of electricity. A dish/engine system is a standalone unit composed primarily of a collector, a receiver, and an engine. A thermal receiver can be a bank of tubes containing hydrogen or helium as the working fluid for the engine. Heat pipes can also be used to transfer the heat to the engine. The most common type of heat engine used in dish-engine systems is the Stirling engine. Each dish produces 5–50 kW of electricity and can be used independently or linked together to increase generating capacity (See Fig. 2.33).

2.5.3.1 Future Solar Thermal Concentrators Scientists at the University of Chicago, IL, USA, have developed a solar concentrator to deliver solar radiation at a concentration of over 60,000 times the intensity that the earth receives from the sun. The system employs non-imaging optics to focus sunlight on a lens which concentrates the light to an intensity

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Fig. 2.31 The aerial view of the power tower plant at USA (Solar Two-Barstow California, USA Courtesy of Energy Efficiency and Renewable Energy, US Department of Energy [164])

Fig. 2.32 A dish system (Courtesy of Energy Efficiency and Renewable Energy, US Department of Energy [164])

2.6

Solar Thermal Molten Salt Technology

107

Fig. 2.33 Arrangement of a dish engine solar system (Courtesy of Energy Efficiency and Renewable Energy, US Department of Energy [164])

of about 6 kW=cm2 . Potential applications include powering lasers for space communications, destroying toxic chemicals, and manufacturing metals, ceramics, and alloys that are superior to existing materials.

2.6 Solar Thermal Molten Salt Technology The National Renewable Energy Laboratory of the U.S. Department of Energy is investigating new materials for receiving and storing solar heat more efficiently. Various types of chemical compounds, called molten salts, are studied for the heat transfer and heat storage media. The chemical and physical properties of these molten salts are given in Table 2.2. These salts absorb thermal energy during day time, i.e., when the sun is shining. Due to the high heat capacity, these salts can store a significant amount of energy. The phase change materials may be able to store more energy. During heating of these materials, first their sensible heat must reach the melting point and then the materials absorb the latent heat to change from the solid to the liquid phase. During night time, these materials release both the sensible heat and the latent heat. These storage technologies are currently being explored for a number of applications including generation of steam for power plants and for buildings’ heat supply [165–183].

Cold 200 200 200 200 200 200 200

200 250 300 250 265 450 270

Storage medium Sand-rock-mineral oil Reinforced concrete NaCl (solid) Cast iron Cast steel Silica fire bricks Magnesia fire bricks

Liquid Media Mineral oil Synthetic oil Silicone oil Nitrite salts Nitrate salts Carbonate salts Liquid sodium

300 350 400 450 565 850 530

Hot 300 400 500 400 700 700 1,200

Temperature (ı C)

Table 2.2 Storage media for energy storage Solid media

770 900 900 1,825 1,870 2,100 850

Average density .kg=m3 / 1,700 2,200 2,160 7,200 7,800 1,820 3,000 0.12 0.11 0.10 0.57 0.52 2.0 71.0

Average heat conductivity (W/mK) 1.0 1.5 7.0 37.0 40.0 1.5 5.0 2.6 2.3 2.1 1.5 1.6 1.8 1.3

Average heat capacity (kJ/kgK) 1.30 0.85 0.85 0.56 0.60 1.00 1.15 55 57 52 152 250 430 80

Volume specific heat capacity .kWht =m3 / 60 100 150 160 450 150 600 0.30 3.00 5.00 1.00 0.50 2.40 2.00

Media cost per kg (US$/kg) 0.15 0.05 0.15 1.00 5.00 1.00 2.00

4.2 43.0 80.0 12.0 3.7 11.0 21.0 (continued)

Media costs per kWht (US$/kWht ) 4.2 1.0 1.5 32.0 60.0 7.0 6.0

108 2 Solar Energy

Average density .kg=m3 / 2,257 2,110 2,044 2,600

2,160 2,533

Temperature (ı C) 308 333 380 500–850

802 854

Source: Herrmann et al. [184]

Storage medium NaNO3 KNO3 KOH Salt-ceramics .Na2 CO3 BaCO3 =MgO/ NaCl Na2 CO3

Table 2.2 (continued) Phase Change media

5.0 2.0

Average heat conductivity (W/mK) 0.5 0.5 0.51 5.0

520 276

Average heat capacity (kJ/kgK) 200 267 50 420

280 194

Volume specific heat capacity .kWht =m3 / 125 156 85 300

0.15 0.20

Media cost per kg (US$/kg) 0.20 0.30 1.00 2.00

1.2 2.6

Media costs per kWht (US$/kWht ) 3.6 4.1 24.0 17.0

2.6 Solar Thermal Molten Salt Technology 109

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2.7 Photovoltaics Photovoltaic (PV) cells convert the solar energy directly into electricity. PV based systems are commonly known as “solar cells,” and are currently used in a number of devices including calculators, watches, and emergency radios. Large scale units can be used to provide power for pumping water, communications equipment, satellites, and lighting homes. The basic information and discussions of various aspects of photovoltaic are given in several books [185–193]. Various materials have been proposed or are under development for use as photovoltaic, and are discussed in these references [194–203]. Recently, new materials such as organic photovoltaics have been explored for power generation [204–210]. PV cells and modules are found to be very reliable in space applications where they are subjected to much harsher conditions compared to terrestrial applications. The Hubble space telescope and virtually all communications satellites are powered by photovoltaic technology. The PV systems are utilized to power most of the electronic devices in satellites. The design of PV-systems and their applications have been discussed by several researchers [211–223]. PV applications may be divided into following categories: • • • • • •

Simple or “stand alone” PV systems PV with battery storage PV with backup power generator PV connected to the local utility Utility-scale power production Hybrid power systems

The demand for photovoltaics worldwide in 2008 was 5.95 GW, which represented a 110% increase from 2007. The PV market demand in 2007 and 2008 is given in Table 2.3. In the USA, grid-connected PV systems are becoming popular in the residential sector. As shown in Fig. 2.34, the capacity of grid connected PVs dominates the off-grid system.

Table 2.3 Global PV market demand in 2007 and 2008

Countries Germany Spain USA Japan Rest of World Rest of Europe South Korea Italy

Market demand in 2007 (MW) 1,328 650 226 226 226 170 – –

Source: Solarbuzz LLC [224]

Market demand in 2008 (MW) 1,860 2,460 360 230 210 310 280 240

2.7

Photovoltaics

111

Fig. 2.34 Annual capacity of PV systems in the US for last several years (Source: Solarbuzz LLC [224])

20000 Non-residential installation Residential Installation Number of Annual Installations

Fig. 2.35 Number of PV units installed annually in the US both in residential and non-residential sectors (Source: Solarbuzz LLC [224])

15000

10000

5000

0 2004

2005

2006

2007

2008

Year

Almost 19,000 new grid-connected PV systems were installed in 2008. About 90% of these were at residential locations. As can be seen from Fig. 2.35, at the end of 2008, about 69,000 PV installations were operating on the grid in the USA, of which 61,000 were in the residential sector. It may be noted that although the number of units installed in non-residential sectors are much lower than in the residential sector, the total installed capacity is higher for non-residential sectors due to the larger size of PV systems.

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2.7.1 PV Theory Photovoltaic cells use semiconducting materials to capture the energy of sunlight, which is composed of photons. These photons contain an energy corresponding to their wavelengths in the solar spectrum. When sunlight or photons strike a PV cell, three events occur: 1. Photons pass straight through the cell. This depends on the band gap energy of the material (The band gap energy is discussed in the following section). Photons with energy less than the band gap energy pass through the PV cells. 2. Photons reflect off the surface. This depends on the surface characteristics of the material. 3. Photons are absorbed by the PV cell. Only photons with a certain level of energy are able to free electrons from their atomic bonds. By leaving this position, the electron causes a “hole” to form. The electrons from nearby atoms will move into this hole, and the process will continue until it reaches the external electrical circuit. If the energy of the absorbed photons is higher than the band gap energy, sometimes heat is generated, depending on the band structure. The process is shown in Fig. 2.36. Although silicon is most widely used for making PV cells, pure silicon is nearly an insulator. The atomic structure of silicon is shown in Figs. 2.37 and 2.38. Silicon has four electrons in its outer shell. The atoms in crystalline solids are held together by covalent bonds. For silicon, four outer shell electrons of each atom are shared by neighboring atoms. As a result, silicon crystals have no free electrons to make it a conducting metal. However, silicon can be made a semi-conductor by doping it with another element (called dopant), such as boron or phosphorous. Depending on the dopant, silicon can become either p-type or n-type semi-conductor. This is discussed in the following section. Materials of n-type and p-type are best understood by looking at the columns of the periodic table around silicon (Fig. 2.39). Silicon’s atomic structure, since it is in the third row of the periodic table, has two electrons that reside in the 1S shell,

Fig. 2.36 The interaction of photons with a semiconductor material. (a) The atom of the semiconductor material, (b) Interaction of the atom with sunlight or photons, (c) The ijection of electron and creation of the hole, (d) Absorption of energy

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Fig. 2.37 The lattice structure of silicon (Courtesy of Naval Research Laboratory [225])

Fig. 2.38 Silicon structure. (a) A silicon atom (b) Silicon crystal

two electrons that reside in the 2S shell, six electrons that reside in the 2P shell, two electrons that reside in the 3S shell and two electrons that reside in the 3P shell (for a total of 14 electrons shown above). Since the crystalline structure of silicon depends upon covalent bonds (or the sharing of electrons with neighboring atoms), the crystal structure is symmetric due to the bond sharing between silicon atoms. If an impurity atom is substituted in place of a silicon atom, the subtraction or addition of a P shell electron in the crystal matrix changes the nature of the local covalent bond. This leads to the formation of a p-type center when there is an absence of an electron (hole) or an n-type center if there is an addition of an electron (free electron).

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Fig. 2.39 Columns III, IV, V, and VI of the period table

The valence electrons come from the paired electrons in covalent bonds where one electron comes from each atom. The valence electrons contain energies in a range known as the valence band. The energy of electrons in this band has various energy levels, one for each valence electron. The valance band is generally filled and no more valence electrons can be added to the lattice. Some elements have extra electrons that are not held via the covalent bond and can move freely through the lattice. These electrons are called conduction electrons and are responsible for electrical conductivity of metals. Their energies are in another band known as the conduction band that has higher energy level than the valence band. Electrons from the valance band must be excited or energized to move them to the conduction band. The difference between the energy at the bottom of the conduction band and the energy at the top of the valence band is called the band-gap energy of the element. This level of energy is specific to a particular element and crystalline structure. Therefore, the band-gap energy is defined as the amount of energy required to dislodge an electron from its covalent bond. These electrons can be harvested to generate electricity by completing an electrical circuit. No electrons can have energies between the highest energy level in the valence band and the lowest energy level in the conduction band: this band is often called a forbidden band of energies. Conducting metal may not have this forbidden band, whereas semiconducting materials, such as silicon, may have few electrons in the conduction band. The characteristics of insulators, semiconductors, and conductors are shown in Fig. 2.40. Crystalline silicon is mainly used in PV cells. The behavior of the silicon semiconductor when exposed to sunlight is explained in Fig. 2.41. A number of other semiconducting materials have been developed or are under development, however, the basic working principles of all these materials are the same. Pure silicon crystal cannot be used directly to make PV cells. Pure silicon crystal is first changed to either an n-type or p-type semiconductor. An n-type silicon semiconductor is obtained by doping pure silicon with Group V elements

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Fig. 2.40 Energy bands of insulators, semiconductors, and conductors Fig. 2.41 Electron energy of silicon

of the periodic table: phosphorous (P), arsenic (As), or antimony (Sb). Among these elements, phosphorous is most widely used. The purpose of n-type doping is to produce an abundance of mobile or “carrier” electrons in the material. The purpose of p-type doping is to create an abundance of holes. This is done by doping silicon with a group III element of the periodic table, such as boron or aluminum, which is substituted into the crystal lattice. Other important aspects of creating PV cells are the formation of a p-n junction. A p-n junction in silicon is created from a

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b

Silicon doped with phosphorus

Silicon doped with boron

extra electron phosphorus

missing electron or hole boron

Fig. 2.42 Structure of doped silicon (a) n-type: doped with phosphorus (b) p-type: doped with boron (Reprinted with permission from Exell RHB [226])

single crystal with different dopant concentrations diffused across it. Creating a semiconductor from two separate pieces of material introduces a grain boundary between them which would severely inhibit its utility by scattering the electrons and holes. The p-n junction of silicon solar cells is made by diffusing an n-type dopant into one side of a p-type wafer (or vice versa). The crystal structures of silicon following doping with phosphorous and boron are shown in Fig. 2.42. Often, a PV cell needs to be tuned to maximize the capture of photon energy. Special electrical properties of the PV cell—a built-in electric field—provide the voltage needed to drive the current through an external load. There are two main mechanisms for charge carrier separation in a solar cell: • Drift of carriers, which is driven by an electrostatic field established across the device. In p-n junction solar cells, the main mode of charge carrier separation is by drift. • Diffusion of carriers, which is due to the concentration gradient of charges between zones of high and low carrier concentration. The current and voltage generated in a solar cell depend on a host of factors. To calculate the current and voltage from solar cells, an equivalent electrical circuit shown in Fig. 2.43 may be considered. The output current (I ) from a solar cell is given by: I D IP ID ISH where, IP D photon generated current due to photoelectric effect ID D diode current ISH D shunt current

(2.2)

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Fig. 2.43 The equivalent electrical circuit for a solar cell

The diode current, assuming an ideal diode, can be expressed by the Shockley diode equation as: V q D kT 1 (2.3) ID D I0 e where I0 is the saturation current of the diode, q is the elementary charge (1:6 1019 C), k is the Boltzmann constant .1:381023 J=K/, T is the cell temperature in Kelvin, and VD is the measured cell voltage that is either produced (power quadrant) or applied (voltage bias). The shunt current is given by: ISH D

VD RSH

(2.4)

where, RSH is shunt resistance. The output voltage (V ) from the cell is given by V D VD IRS

(2.5)

where, VD is voltage across both the diode and the shunt resistor, and RS is the load shown in Fig. 2.42. Substitution of Eqs. 2.3–2.5 into Eq.2.2 provides i V C IR h q.V CIRS / S I D IP I0 e nkT 1 RSH

(2.6)

A typical I -V curve represented by Eq. 2.6 for a semiconductor when illuminated is shown in Fig. 2.44. Many performances related parameters for the cell can be determined from this I -V curve. These are discussed below.

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Fig. 2.44 A typical I -V curve for a solar cell. (VOC : open circuit voltage, ISC : short circuit current)

2.7.1.1 Short Circuit Current (ISC ) This refers to the current from a solar cell when the top and the bottom (negative and positive leads) leads are connected in a short circuit. In this situation, the impedance is low and the voltage equals zero. 2.7.1.2 Open Circuit Voltage .VOC / The open circuit voltage (VOC ) occurs when there is no current passing through the cell. 2.7.1.3 Maximum Power .Pmax / The power is calculated from P D IV. Therefore, at ISC and VOC , the power will be zero, and the maximum value for power will occur between these two values and is shown in Fig. 2.45. The voltage and current at this maximum power point are denoted as VMP and IMP , respectively. 2.7.1.4 Fill Factor Fill factor is defined as the ratio of the maximum power (Pmax ) to the theoretical power (PT ). The theoretical power is given by the product of open circuit voltage and short circuit current. The fill factor (FF) is considered to be a measure of the quality of a solar cell.

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Fig. 2.45 The maximum power from a solar cell

FF D

Pmax IMP VMP D PT ISC VOC

(2.7)

Both series and shunt resistances, which are discussed below, affect the fill factor. An increase in the shunt resistance .RSH / or a decrease in the series resistance (RS ) has a positive effect on the fill factor, resulting in a greater efficiency. Typical fill factors range from 0.5 to 0.82. 2.7.1.5 Shunt Resistance The efficiency of solar cells is reduced by the dissipation of power across internal resistances. The shunt resistance arises from physical defects (scratches), improper emitter formation, metallization over-firing, or material defects, All of them can provide alternative paths for electrons to flow.

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2.7.1.6 Series Resistance This is caused by metal grids and other similar components of a PV cell. The series resistance may be decreased by using a large cross sectional area for the grid.

2.7.2 The Efficiency of Photovoltaic Cells 2.7.2.1 Theoretical Efficiency For illustrative purposes, the discussion of theoretical efficiency of photovoltaic cells will focus on single crystalline silicon specifically, since it is the material most used for solar energy conversion. When an n-type dopant is added to the semiconductor, the effective energy it takes to raise an electron to the conduction band is decreased (Fig. 2.46). The energy that it takes to raise an electron from the n-type dopant to the conduction band is called the activation energy EAn . There is an activation energy associated with a p-type material EAp. Since a p-type dopant lacks an electron as compared to its neighbor atoms, it will attract electrons into its orbit. The activation energy for a p-type material is the energy needed to raise an electron from the valance band to the p-type dopant. The Fermi level is also an important concept, which is based on the thermodynamics of the semiconductor. If all energy levels were allowed, the Fermi level is the energy at which there is a 50% chance of finding an electron. The presence of a dopant in a semiconductor material will shift the Fermi level. Figure 2.47 illustrates the Fermi level in an n-type and p-type semiconductor. When n-type and p-type semiconductor materials form a junction, this forces the Fermi level of the two layers to coincide and will cause the conduction band and the valance band to bend. The combined effect of the n-type and p-type layers on the conduction and valance band is shown in Fig. 2.48.

Fig. 2.46 The energy structure of a semiconductor with an n-type and p-type dopant. The bandgap Eg , the activation energy for the n-type material and the activation energy of the p-type material are shown

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Fig. 2.47 Band structures and Fermi levels for n-type and p-type doped semiconductors

Fig. 2.48 Band-bending in a p-n junction

The band-bending creates an electric field in the depletion zone. When an ionization event takes place in the depletion zone, an electron is removed from one of the lattice atoms, and the lattice atom becomes an ion, which is still locked in the crystal lattice. Even though the ionized atom cannot move, it can swap electrons with a neighboring atom in the presence of an electric field. The drifting positive charge is called a hole. In the presence of an electric field from the band-bending, the electron/hole pair is separated by the Lorentz force caused by the electric field before they can recombine. This causes charge to flow across the p-n junction and to the external load on the photovoltaic cell (See Fig. 2.49). In the presence of photons with energy greater than the band gap of the material, electron/hole pairs are formed. If the pairs are formed in the depletion zone, the field will separate the electron and hole before they recombine. The electron and hole pair can also diffuse to the depletion zone where they are separated by the electric field before they recombine. The photon energy .E / has to exceed the band-gap energy .Eg / of the material in order for the electron/hole pair to be formed. Any photon

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Fig. 2.49 Operation of photovoltaic device in the presence of photons

energy in excess of the band-gap energy will be lost, so matching the photon source to the band-gap energy of the material is very important. For example, if the photon source is mono energetic, the intrinsic efficiency .i / of the photovoltaic cell can easily be calculated with Eq. 2.8. i D Eg =E

(2.8)

Prelas et al. [227] discussed the theoretical basis for calculating photovoltaic cell efficiency with different photon sources. A specific experiment is described in which a Metal-Insulator-Semiconductor (MIS) solar cell was irradiated with a 60 W=cm2 spectral irradiance source and the efficiency of solar cell was measured (Fig. 2.50). The measured peak efficiency of the MIS solar cell was 38% with photon energy of 1.38 eV. Given that the band-gap of the silicon is 1.1 eV, the intrinsic efficiency of a silicon photovoltaic cell should be, from Eq. 2.8, 79.7%. The difference between the measured efficiency and the intrinsic efficiency are due to a number of factors. First of all, with an MIS silicon solar cell, the depletion zone is only a few micrometers thick so many photons which create electron/hole pairs are wasted because the electron and the holes recombine rather than separate. A second factor could be defects in the crystal which can trap electrons. A third factor could be the contacts between the metal, insulator and silicon which could trap electrons at the interface of the layers. If a low defect crystal can be grown and if high quality contacts between the silicon and metal are made, it is possible for the photovoltaic cell efficiency to closely approach the intrinsic efficiency. A single crystalline silicon solar cell has reached a record 25% efficiency [228]. As will be discussed, the theoretical maximum efficiency of a silicon solar cell is 27%. Thus, losses in semiconductor photovoltaic cells can be minimized. However, the cost of high efficiency photovoltaic cells is high and the cost/efficiency price point is still higher than that of lower cost, low efficiency amorphous silicon photovoltaic cells.

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40

35

Efficiency (%)

30

25

20

15

10

5 1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

Photon Energy (eV) Fig. 2.50 MIS solar cell efficiency at various photon energies

Silicon is the most used material for photovoltaic cells because it’s band-gap is very close to the optimum for extracting energy from the solar spectrum using a single photovoltaic material. The problem with extracting energy from the sun is that the spectrum is a broad band (Fig. 2.8). The sun is very good for color vision but is not optimum for efficient direct energy conversion using photovoltaics. Figure 2.51 shows the fraction of energy that is useable in the solar spectrum for a silicon photovoltaic cell. The effect of the spectrum can be understood by looking at the ratio of the average photon energy to the full width half height of the spectrum .Emean =E/. For the AM2 (Air Mass 2 standard solar spectra, http://rredc.nrel.gov/solar/spectra/, last accessed 7/28/2010), Emean =E is about 1.3 which is indicative of a broad spectral source. For any given spectrum, the efficiency of photovoltaic energy conversion is determined by the variation of the spectral irradiance with photon energy and by the photovoltaic’s band-gap energy. A 100% efficient device is not possible because of the finite width of any know photon spectrum. In other words Emean =E would have to approach infinity to achieve perfect energy conversion. Any photon which has an energy .hv/ less than that of the band-gap of the photovoltaic cell is wasted since electron/hole pairs will not be created by these photons. The power lost because of insufficient photon energy to create an electron/hole pair is: ZEg (2.9) Plost D W .E/dE 0

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IRRADIANCE W/cm2/eV 0.06 0.05 0.04 0.03 0.02 0.01 0 0

0.5

1.0

1.5

2

2.5

3

3.5

4

ENERGY (ev) Fig. 2.51 The spectral irradiance of the solar spectrum (Air Mass 2-AM2). The cross hatched area under the curve is the fraction of the energy which is useable with a 1.1 eV band-gap silicon photovoltaic cell (Courtesy of Prelas et al. [227])

where W .E/ is the irradiance in watts=cm2 =eV. For the solar spectrum, this effect favors low band-gap materials. Competing with the energy lost due to insufficient photon energy is that for photons in which the photon energy exceeds the band-gap energy, the excess energy beyond Eg is lost as heat. Thus, the intrinsic efficiency .i / for photovoltaic conversion, assuming an ideal collection device is, R1 i D

Eg

E

W .E/ Eg dE

R1

(2.10) W .E/dE

0

It is assumed that each photon with energy greater than the band-gap energy will create an electron/hole pair and each electron created will contribute a constant energy, Eg , to the process. The ideal Shockley model for a p-n junction can be used to determine the intrinsic efficiency, which can be written as: Eg i D

R1

Nph .E/dE

Eg

R1 0

Nph .E/dE

D

Eg Eg N.E > Eg / D eg Emean Ntot Emean

(2.11)

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Fig. 2.52 Equivalent circuit model for a photovoltaic cell (Adapted from Prelas et al. [227])

where, Nph D photon flux density in photons=cm2 s eV N.E > Eg / D photon flux with energy greater than Eg Eg D band-gap energy Emean D mean spectral energy eg D fraction of photons with E > Eg To calculate the conversion efficiency, the current on the load from the photon flux, or the current voltage relationship of the converter is modeled using an ideal circuit shown in Fig. 2.52. The two resistors in the model are used to take into account additional loss mechanisms. The resistor Rs is the loss due to finite skin resistance associated with the illuminated surface of the converter. Normally, this can be ignored since the surface is normally metalized with an intricate grid to minimize resistive losses. Rp models recombination losses with the junction. Rl is the load. For a p-n junction, the current voltage characteristics is given by the classical Shockley expression: i h qv I D I0 e nkT 1

(2.12)

where, I D current to the positive terminal V D voltage across the converter I0 D reverse saturation current n D ideality factor (assumed to be 1 for these examples) k D Boltzmann constant and T D absolute temperature in Kelvin With the assumption that each electron from the production of an electron/hole pair is collected, the short circuit current is: Z1 Isc D qA Eg

Nph .E/dE

(2.13)

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where, A is the cross sectional area of the junction. Using Eqs. 2.12 and 2.13 with I0 and Isc given, it has been shown by Loferski [229] that the collection efficiency can be calculated by solving the following three equations: q

q V mp Isc C1 D C1 kT I0 qVmp I0 kT D 1C Isc Vmp qV Isc 1 C mp

e kT Vmp

Pmax

(2.14) (2.15)

kT

Pmax

D A

R1

(2.16)

W .E/dE

0

where, Vmp is the terminal voltage at the maximum power point, and Pmax is the corresponding power. If it is assumed that Rs and Rp are minimal, all of the current will flow through the ideal diode. Equation 2.12 can be solved for the open circuit voltage .Voc / and is given by: Isc kT `n 1 C Voc D q I0

(2.17)

(Note: q is the electrical charge with a value 1:602176487 1019 C. Boltzmann’s constant in electron volts is 8:617343 105 eV/K. At room temperature .300 K/, kT/q is called the thermal voltage and is about 25.85 mV.) From Eq. 2.14, and substituting for the quantity 1 C IIsc0 , Voc D

q qVmp kT `n e kT Vmp C1 q kT

(2.18)

which can be simplified as follows: Voc D Vmp C

kT qVmp `n C1 q kT

(2.19)

Using room temperature for T , Eq. 2.18 becomes: Voc D Vmp C 0:025`n 38:8Vmp C 1

(2.20)

For values of Voc greater than 4, this relationship converges to: Vmp D Voc 0:14

(2.21)

The maximum power delivered occurs when the load impedance matches the dynamic impedance of the junction .Rj /. Using Eq. 2.12 for I.V /; Rj is:

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Rj D

ıV kT qV e kT D ıt qI 0

(2.22)

The maximum power to the load current .Iimp / is: Vmp Vmp qI0 qVmp e kT D Rj kT

Iimp D

(2.23)

If it is assumed that Rs D 0, and RP D 1,

i h qVmp Iimp D Isc I0 e kT 1

(2.24)

Using Ilmp from Eq. 2.23 and plugging it into Eq. 2.24, the resulting equation can be rearranged in the following way: Isc I0 D

qVmp i qVmp qI 0 h Vmp e kT C I0 e kT kT

(2.25)

Dividing Eq. 2.23 by Eq. 2.25, the following expression is obtained: qV

mp Iimp kT D qV Isc I0 1 C kTmp

(2.26)

Rearranging Eq. 2.26: " Iimp D

1

qVmp kT qV C kTmp

#

Isc

I0 1 Isc

(2.27)

It is a good assumption because of the band-gap that provides: I0 Isc Therefore,

" Iimp Š Isc

1

qVmp kT qV C kTmp

(2.28) # (2.29)

The maximum power .Pmax / should be equal to the product of Iimp and Vimp . " q # .Voc 0:14/2 kT (2.30) Pmax D Isc 0:14/ 1 C q.VockT

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The remaining information that is needed is the relationship between the open circuit voltage .Voc / and the band-gap .Eg /. This will fundamentally require the knowledge of the relationship between band-gap and the reverse saturation current .I0 /. From the paper by Loferski [229] the following equation can be used, Jsc D

qE g I0 D c.T / e kT A

(2.31)

where Jsc is the reverse saturation current density. Current (given in amps) can now be transformed into current density J .amps=cm2 / by normalizing with the junction area A. Using the assumption from (2.28) with Eq. 2.17, dividing by A, and using the relationship from (2.31), the following expression is obtained: 3 2 qE g kT 4 Jsc e kT 5 Voc D `n q c.T /

(2.32)

The natural log function can be expanded as follows: kT Jsc Voc D Eg C `n q c.T /

(2.33)

A characteristic value of c.T / for silicon is c.T / D 1:75 106 amps=cm2 . To calculate the maximum efficiency of a photovoltaic cell for converting photons from a source with a known spectrum using a p-n junction, the following four equations are obtained: Z1 Jsc D q

Nph dE

(2.34)

Eg

Voc D Eg C

max

kT `n.5:714 107Jsc / q

Z1 Pin D Nph dE A

Isc A D Pin

"

0

1

q.Voc 0:14/2 kT 0:14/ C q.VockT

(2.35)

(2.36) # (2.37)

In these equations, Pin is the input solar power. In order to find the material (with band-gap Eg / which is best suited for solar energy conversion, a curve can be generated of maximum efficiency as a function of band-gap energy. Using Eqs. 2.34–2.37, the AM2 photon flux density curve is numerically integrated according to Eq. 2.34 from the band-gap energy to infinity to find the short circuit current density .Jsc /.

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Fig. 2.53 Maximum efficiency of a single band-gap solar cell as a function of band-gap energy (Eg /. Silicon and GaAs are shown in the figure

The total input power density is obtained by numerical integration of the total AM2 photon flux density curve. Once Jsc is known, Isc can be found by multiplying Jsc by the area of the photovoltaic cell .A/. I0 is found using Eq. 2.32 and Voc is found using Eq. 2.35. Equation 2.20 is used to solve for Vmp and subsequently Eq. 2.27 can be used to solve for Ilmp . The product of Vmp and Ilmp gives the converted power and the result can then be divided by the total input power to yield a maximum efficiency. The results are shown in Fig. 2.53. The curve for the solar spectrum shows a peak at approximately 30% which is believed to be the upper bound on the ability of a single material junction to convert the solar spectrum. The highest known conversion efficiency for silicon to date has been 25%.

2.7.2.2 Experimental Determination of PV Efficiency Given the variety of solar cells and production methods, the efficiency of a cell is classified by standard methods. A number of instruments are available commercially for testing of PV cells. A major factor when determining the efficiency is the maintenance of a constant intensity of the light to which the cells are exposed. The irradiance intensity is often expressed by the unit, sun value.

2.7.3 The “Sun” Value The solar irradiance measurement unit, One Sun, is defined as equivalent to the irradiance of one solar constant. The solar constant is defined as the irradiance of

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Fig. 2.54 The definition of various global (G) air mass (AM) conditions is depicted here. Most solar reference cells are measured under condition AM 1.5 G (Courtesy of Newport [230])

the sun on the outer atmosphere at a distance of 1 AU (AU refers to Astronomical Unit which is the mean distance between the sun and the earth. 1 AU is 149,597,870.66 km or 92,955,807.25 miles). Generally, a solar cell, known as the “reference solar cell” is calibrated under predetermined conditions. Other cells can be compared against the reference cell to determine the efficiency and other cell characteristics. Often times, a silicon solar cell is calibrated. In the USA, the National Renewable Energy Laboratory (NREL) is one of the laboratories that can certify a reference solar cell. Most terrestrial solar cells are tested under the global AM (air mass) 1.5 G (global) conditions. The definition of air mass is explained in Fig. 2.54. As explained earlier in Eq. 2.37, efficiency () is the ratio of the electrical power output Pout , compared to the solar power input, Pin , into the PV cell. Pin can be taken as the product of the irradiance of the incident light (E), measured in W=m2 or in suns .1;000 W=m2 / under standard test conditions multiplied by the surface area of the solar cell .A; m2 /. D

Pout Pout D Pin EA

(2.38)

Standard test condition specifies a temperature of 25ı C and an irradiance of one sun .1;000 W=m2 / with an air mass 1.5 (AM1.5) spectrum. This condition would correspond to the irradiance and spectrum of sunlight incident on a clear day upon a sun-facing 37ı -tilted surface with the sun at an angle of 41:81ı above the horizon.

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Conventional silicon PV cells have efficiencies between 5% and 15% for conversion of solar energy to usable energy. A number of experimental cells have achieved higher efficiencies, but only under carefully controlled laboratory conditions and with more expensive materials and higher production cost. Efficiency is constantly increasing with various new materials [231–233]. NREL has compiled the progress in the solar cell efficiency over time and is presented in Fig. 2.55.

2.7.4 Effect of Thickness of the Cell Once the sunlight strikes a PV cell, photons must be absorbed by the cell material before the electron excitation takes place. The light of a particular wavelength must penetrate certain depth before it is absorbed. The absorption coefficient determines this depth. The absorption coefficient depends both on the material and the wavelength of light which is being absorbed. Semiconductor materials have a sharp edge in their absorption coefficient, since light which has an energy below the band gap does not have sufficient energy to raise an electron across the band gap. Consequently this light is not absorbed. The absorption coefficient for several semiconductor materials is shown in Fig. 2.56. For photovoltaic applications, a photon energy greater than the band gap is not desirable, as electrons quickly thermalize, back down to the band edges. A conventional solar cell consists of a wafer of silicon that is about 1/50th of an inch thick. Typical cells that are four inches in diameter produce about 1 W of power. These cells are typically grouped into modules of dozens of cells. Modules are further grouped into panels and then arrays, which may produce several kilowatts of power. The construction of a PV cell is shown in Fig. 2.57. As long as light is shining on the cell, the process is repeated: (1) energy from the light is absorbed by electrons and the electrons escape from their orbits, (2) electrons are drawn across the junction in the cell which only permits movement in one direction, and (3) the electrons move through an externally-connected load to recombine with the holes, providing an energy device.

2.7.5 The Effect of Temperature The efficiency of PV cells depends on the temperature at which they are operating. The effect of temperature on an I -V curve is shown in Fig. 2.58. When a PV cell is exposed to higher temperatures, ISC increases slightly, while VOC decreases more significantly, and the overall effect is the reduction in efficiency. As a result, higher temperatures decrease the maximum power output Pmax . As temperature increases, the band gap energy decreases, because the crystal lattice expands and the interatomic bonds are weakened. Therefore, less energy is

Fig. 2.55 Reported timeline of solar cell energy conversion efficiencies (Courtesy of National Renewable Energy Laboratory, USA [234])

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Fig. 2.56 Absorption coefficients of several semi-conductor materials (Courtesy of Wu [235])

Sunlight (photons) Encapsulate seal Top electrical contact P-type material (Boron doped silicon) P/N junction External circuit

Base contact

N-type material (Phosphorous doped silicon)

Fig. 2.57 Basic components of a solar cell (Courtesy of State Energy Conservation Office [236])

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2 Solar Energy

Fig. 2.58 The shift in the I-V curve due to higher temperature Table 2.4 The values of the parameters of Eq. 2.9 for common solar cell materials Parameters Eg .0/; eV a, eV/K b, K

Geranium 0.7437 4:77 104 235

Silicon 1.166 4:73 104 636

GaAs 1.519 5:41 104 204

needed to free an electron and move it to the conduction band. The relationship between temperature and the band gap energy (Eg ) is given by the following equation: Eg .T / D Eg .0/

aT2 .b C T /

(2.39)

Eg .0/ is the limiting value of the band gap at 0 K. The parameters, a and b, are constants and are obtained from the best fit to the experimental data. The values of the parameters, Eg .0/; a, and b for most common semiconductors are given in Table 2.4. A plot showing the temperature effect is given in Fig. 2.59.

2.7.6 Effect of Dopant Concentration The concentration of dopant in a semiconductor used for making n- or p- type semiconductors can change the band gap of the material. High doping densities reduce the band gap, since the distance between two dopants narrows. This is due to the overlapping of two wave functions that form an energy band, rather than forming a

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1.60 GaAs

Band Gap Energy (eV)

1.40 1.20

Silicon 1.00 0.80 Geranium

0.60 0.40

0

100

200

300 Temperature (K)

400

500

600

Fig. 2.59 Change in the band gap energy with temperature for geranium, silicon, and GaAs

discreet energy level. As suggested by Van Zeghbroeck [237], if the impurity level is shallow, this impurity band reduces the energy band of the host material by: 3q 2 Eg .N / D 16 "s

s qN "s k T

(2.40)

where N is the doping density, q is the electronic charge, "s is the dielectric constant of the semiconductor, k is Boltzmann’s constant and T is the temperature in Kelvin. There are several other parameters that can be used to determine the efficiency of a solar cell. These are: thermodynamic efficiency, quantum efficiency, VOC ratio, and the fill factor. These efficiencies are related to various losses that occur in a solar cell, which are known as reflectance, thermodynamic, recombination and resistive electrical losses. As discussed earlier, when a photon is absorbed by a solar cell, it can produce an electron-hole pair. If the electron-hole pair is generated in the depletion region, the built-in electric field drifts the electron and hole apart. As a result, current flows through the device, and is collected. If the electro-hole pair is generated in the n or p regions, the electron and hole drift in random directions and may or may not become part of the photocurrent. However, electrons may again become bound to an atom by giving up its energy. This process is called recombination and results in a loss. Quantum efficiency: The ratio of the number of electron hole pairs that are created and collected to the number of incident photons (expressed as a percentage) is referred to as the quantum efficiency. Overall efficiency: The percent of incident electromagnetic radiation that is converted to electrical power when all the losses are considered.

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Fig. 2.60 Cells, module and array of photovoltaic system (Courtesy of State Energy Conservation Office [236])

2.8 From Cells to Arrays An individual PV cell is the basic unit of a PV system. An individual PV cell typically produces between 1 and 2 W. Therefore, a number of individual cells are connected together to form larger units called modules. Modules are then connected to form larger units known as arrays. Several arrays are next joined together to produce large scale power units. These units are shown in Fig. 2.60. PV systems provide direct current (DC). For off-grid PV systems, these can be used directly, provided appliances can be run in DC. Generally, few appliances are available that run in DC, and these appliances are also costly. Therefore, it is necessary to convert the DC into alternating current (AC). Appliances and lights running on AC are much more common and are generally cheaper. The conversion of DC into AC is generally 80% efficient, resulting in some loss of power. A gridconnected PV system will require a utility interactive DC to AC inverter. A PV system using a DC to AC converter is shown in Fig. 2.61.

2.9 Solar Cell Materials The most important part of a solar cell is the semiconductor layers. There are a number of different materials suitable for making these semiconducting layers, and each has benefits and drawbacks. In addition to the semiconducting materials, solar cells consist of a top metallic grid or other electrical contact to collect electrons from the semiconductor and transfer them to the external load, and a back contact layer to complete the electrical

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Fig. 2.61 An off-grid photovoltaic system (Courtesy of State Energy Conservation Office [236])

circuit. The metallic electrical contact must be oxidation resistant to prolong the life time of the cell. On the top of the complete cell typically a glass cover or other type of transparent encapsulant is placed to seal the cell and keep moisture and debris out. An antireflective coating is needed to keep the cell from reflecting the light back away from the cell.

2.9.1 Semiconducting Materials for Solar Cell Effective PV semiconductors have band-gap energies ranging from 1.0 to 1.6 electron-volts (eV). This level of energy can free electrons without generating extra heat. For example, crystalline silicon’s band-gap energy is 1.1 eV. The entire spectrum of sunlight, from infrared to ultraviolet, covers a range of about 0.5 eV to about 2.9 eV. Red light has energy of about 1.7 eV, and blue light has energy of about 2.7 eV. About 55% energy of the sunlight cannot be used by most current PV cells, because this energy is either below the band gap or carries excess energy that cannot be captured by the cell material. PV materials with different band-gap energies have been developed to capture the various energy levels. Photons with an energy greater than the band gap may be absorbed to create free electrons. Photons with energy less than the band gap pass through the material or generate heat. The band-gap energy of some common PV materials is given in Fig. 2.62.

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Fig. 2.62 Band gap energy of various PV materials

2.9.1.1 Silicon Silicon is the most popular solar-cell material for commercial applications because it is readily available and cheap. However, to be useful in solar cells, it must be refined to 99.9999% purity. Silicon can be used in three different forms in PV cells. • Single Crystal • Amorphous Silicon • Polycrystalline Silicon Thin Film

Single Crystal Single crystal silicon is most efficient because of its uniformity. This uniformity is ideal for efficiently transferring electrons through the material. The amorphous silicon is much cheaper to produce than single crystalline silicon, however, the efficiency is much lower compared to single crystal silicon. Researchers are trying to find ways to minimize the effects of grain boundaries in amorphous and polycrystalline silicon, which reduces the efficiency of silicon as a solar cell.

Amorphous Silicon Amorphous solids, like common glass, are materials in which the atoms are not arranged in any particular order. These do not form crystalline structures, and contain large numbers of structural and bonding defects. Amorphous silicon is commonly used for solar-powered consumer devices that have low power requirements (e.g.,

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wrist watches and calculators). Amorphous silicon also absorbs solar radiation 40 times more efficiently than does single-crystal silicon. A 1 m amorphous silicon film can absorb 90% of the usable solar energy. In addition, amorphous silicon can be produced at a lower temperature and deposited on low-cost substrates reducing the cost. These characteristics make amorphous silicon the leading thin-film PV material.

Polycrystalline Thin Films Polycrystalline thin-film devices require very little semiconductor material and can be manufactured cheaply. Thin films can be deposited on the surface using various techniques. The deposition processes can be scaled up to produce large size wafers. Like amorphous silicon, the layers can be deposited on various low-cost substrates like glass or plastic of virtually any shape, even on flexible plastic sheets. Single crystal cells have to be individually interconnected into a module, but thin-film devices can be made monolithically (as a single unit). Several layers can be deposited sequentially on a glass substrate. These layers are an antireflection coating, a conducting oxide layer, and a back electrical contact.

2.9.1.2 Gallium Arsenide Gallium arsenide (GaAs) is another material proposed as a semiconducting materials for PV cells. It has several advantages over silicon based PV cells. GaAs is especially suitable for use in multijunction and high-efficiency solar cells for several reasons: 1. The GaAs band gap is 1.43 eV, nearly ideal for single-junction solar cells. 2. GaAs has high absorptivity requiring only a few microns thick film to absorb sunlight. (In comparison, crystalline silicon requires a layer 100 m or more in thickness.) 3. GaAs cells can withstand high temperatures and are less sensitive to heat compare to silicon, making it more suitable for solar concentrator type applications. 4. Other types of gallium based semiconductors can be prepared using aluminum, phosphorus, antimony, or indium that have characteristics complementary to those of gallium arsenide, allowing great flexibility in cell design. 5. GaAs is resistant to radiation damage making it very desirable for space applications. The disadvantages are cost and toxicity of raw materials.

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2.10 Multijunction Cells Most common PV devices use a single junction, or interface, to create an electric field. In a single-junction PV cell, only photons whose energy is equal to or greater than the band gap of the cell material can free an electron for an electric circuit. This reduces the efficiency of a single junction cell. One way to address this issue is to use two (or more) different cells, with more than one band gap and more than one junction, to generate a voltage. These are referred to as “multijunction” cells (also called “cascade” or “tandem” cells). Multijunction devices can achieve higher total conversion efficiency, because they can convert more of the energy spectrum of light to electricity [238–244]. A multijunction device is a stack of individual single-junction cells in descending order of band gap .Eg /. The top cell captures the high-energy photons and passes the rest of the photons on to the next layer of lower-band-gap cells. A schematic diagram of a multi-junction cell is shown in Fig. 2.63. The electricity conversion efficiency of commercial solar cells is generally 6–15%. Most of the incoming solar energy, more than 85%, is either reflected or

Fig. 2.63 Working principle of a multi-junction PV cell (a) Basic working principle (b) A commercial cell marketed by Spectrolab California USA

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141

absorbed as heat energy. The absorbed energy increases the working temperature of the solar cells; considerably reducing the cell’s efficiency. A hybrid photovoltaic and thermal (PVT) collector technology has been developed to address this issue. Using water as the coolant, the PV cells are maintained at a lower temperature and the heat is transferred to water to produce hot water for other uses. This increases the overall efficiency of solar energy systems. In recent years, PVT collector systems have been studied extensively by researchers [245–260]. Also, researchers are trying to find new materials that can provide higher efficiency for solar energy conversion. One such material is an organic polymer. The best efficiency reported so far for an organic polymer is about 6%. However, researchers believe that organic polymers have great potential, since these can be modified easily reducing cost of their synthesis. Also, the toxicity of new materials used for their synthesis could be less than their inorganic counter parts. Green et al. [261] provided an excellent summary of the efficiency of the most promising materials for single solar cells and modules. Their findings are given in Appendix XII.

2.11 Hybrid Power Systems The PV arrays can be used more effectively in a hybrid power generation system. A hybrid power system combines a number of electricity production and storage units to meet the energy demand of a given facility or community. In a system such as this, PV arrays, wind turbines, and generators can be added to supply energy in a continuous basis. The development of a hybrid electric system requires the knowledge of the energy demand to be met and the resources available. Energy planners, therefore, must study the solar energy, wind, and other potential resources at a certain location, in addition to the planned energy use. This will allow them to design a hybrid system that best meets the demands of the facility or community. A combined solar and PV system is shown in Fig. 2.64.

2.12 Solar Lighting Most commercial buildings are designed to use electric bulbs to provide interior lighting. Currently, attempts are underway to use sunlight directly for interior lighting via lens collectors, reflective light-pipes, and fiber-optic bundles. Hybrid solar lighting is one such technology. It collects sunlight and routes it through optical fibers into buildings where it is combined with electric light in “hybrid” light fixtures. Sensors keep the room at a steady lighting level by adjusting the electric lights based on sunlight availability. This new generation of solar lighting combines both electric and solar powers at a reduced cost.

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Fig. 2.64 A solar-wind hybrid power generation system (Courtesy of Dixon [286])

2.13 Summary High efficiency, low cost solar cells are necessary for large scale commercial applications of the solar energy for power generation. Several approaches are explored including development of new materials and multi-junction cells. One of the major issues is that most of the new materials are expensive and need toxic hazardous materials for their synthesis. Subsequently, numerous hazardous wastes are generated. Recently nano-materials [262–272] and quantum dots [254–284] are explored as possible materials for solar cells. An excellent review of the literature covering all the aspects of solar energy has been presented by Avi [285].

Problems 1. What is solar energy? Calculate total amount of solar irradiation received by earth on a daily basis. Estimate how much of that can be harvested economically. 2. Discuss which part of the world can potentially harvest most of the solar energy and what is preventing them from doing so.

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3. What is the average monthly solar energy hitting the earth’s surface in your city taking into account average weather patterns for June, September, December and March? What fraction of this energy can be collected with a stationary solar collector during those months? 4. Using Eq. 2.1 duplicate the blackbody curve in Fig. 2.8. 5. You spend the day on a Los Angeles California beach sunbathing on June 21. How much UVB radiation would your skin absorb (290–320 nm) in Joules? 6. What is the difference between a solar collector and a solar panel? 7. Why is the understanding of solar spectrum important for designing of solar cells. 8. What metal(s) are used for reflectors in the solar power tower? 9. What is the efficiency of a typical reflector for the solar spectrum? 10. Explain how solar energy heats hot water. 11. How much hot water at 120ı F can a solar water heater produce in your city per square meter during the months of June, September, December and March? What is the payback time (time it takes for the savings in fuel cost to equal the installation cost) for a solar water heater in your city? 12. Explain how solar energy heats the boiler in a solar power tower. 13. Estimate the cost to install a solar power tower plant in $/KW installed in California. In your city. (State all of the costs and assumptions). 14. Estimate how large a reflector must be in order to concentrate solar energy to 60,000 times. (Assume that solar rays are parallel). 15. Explain the working mechanism of a solar cell. 16. Duplicate the curve in Fig. 2.52. 17. Why does the curve in Fig. 2.52 has a maximum? 18. Explain why the maximum efficiency of a single junction solar cell is slightly below 30% and no higher. 19. Estimate the intrinsic efficiency of a GaAs solar cell (Eg D 1:43 eV). 20. What material do you believe has the highest intrinsic efficiency as a single junction solar cell and why? 21. Estimate the highest possible intrinsic efficiency achievable with a multijunction photovoltaic solar cell. (Provide details on the materials used and the calculations you make.) 22. Estimate the open circuit voltage (Voc ) for a high quality silicon solar cell.

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23. Estimate the cost of a solar photovoltaic cell power plant in $/KW installed in California. What would the cost be in your city? (State all of the costs and assumptions). 24. How does a solar space heating system work? 25. What is radiant solar heating? 26. Estimate the installed cost of a solar space heating system in your city. What is the payback time for a solar space heating system? (State all of the costs and assumptions). 27. Estimate the installed cost of a radiant solar heating system in your city. What is the payback time for a radiant solar heating system? (State all of the costs and assumptions). 28. Estimate the area that a 1 GW electric solar photovoltaic power plant would need in California? How large would it be in your city? 29. Estimate the area that a 1 GW electric solar power tower plant would need in California? How large would it be in your city? 30. Estimate the area that a 1 GW electric integrated solar-wind plant would need in California? How large would it be in your city 31. Describe an integrated solar-wind system and discuss its advantages and disadvantages. 32. What is a solar thermal roof? 33. What is the most economically viable use of solar energy?

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260. Saitoh H, Hamada Y, Kubota H, Nakamura M, Ochifuji K, Yokoyama S (2003) Field experiments and analyses on a hybrid solar collector. Appl Therm Eng 23:2089–2105 261. Green MA, Emery K, Hishikawa Y, Warta W (2010) Solar cell efficiency tables (version 35). Prog Photovoltaics Res Appl 18(2):144–150 262. Manna TK, Mahajan SM (2007) Nanotechnology in the development of photovoltaic cells. In: 2007 ICCEP ’07 International Conference on clean electrical power, pp 379–386 263. Huynh WU, Dittmer JJ, Alivisatos AP (2002) Hybrid nanorod-polymer solar cells. Science 295(5564):2425–2427 264. Gledhill SE, Scott B, Gregg BA (2005) Organic and nano-structured composite photovoltaics: an overview. J Mater Res 20(12):3167–3179 265. Goh RGS, Waclawik ER, Bell JM, Motta N (2007) Organic-nano photovoltaic devices. Mater Aust 40:40–42 266. Fritz KP, Guenes S, Luther J, Kumar S, Sariciftci NS, Scholes GD (2008) IV–VI nanocrystalpolymer solar cells. J Photochem Photobiol, A 195:39–46 267. Conibeer G, Green M, Corkish R et al (2006) Silicon nanostructures for third generation photovoltaic solar cells. Thin Solid Films 511–512:654–662 268. Kalyanasundaram K, Gratzel M (1999) Efficient photovoltaic solar cells based on dye sensitization of nanocrystalline oxide films. In: Max Roundhill D, Fackler JP (eds) Optoelectronic properties of inorganic compounds. Plenum Press, New York, pp 169–194 269. Kang Y, Kim D (2006) Well-aligned CdS nanorod/conjugated polymer solar cells. Sol Energy Mater Sol Cells 90:166–174 270. Kannan B, Castelino K, Majumdar A (2003) Design of nanostructured heterojunction polymer photovoltaic devices. Nano Lett 3:1729–1733 271. Landi BJ, Raffaelle RP, Castro SL, Bailey SG (2005) Single-wall carbon nanotube-polymer solar cells. Prog Photovoltaics 13:165–172 272. Nanu M, Schoonman J, Goossens A (2005) Nanocomposite three-dimensional solar cells obtained by chemical spray deposition. Nano Lett 5:1716–1719 273. Nozik AJ (2002) Quantum dot solar cells. Phys E: Low Dimensional Systems Nanostructures 14(1–2):115–120 274. Robel I, Subramanian V, Kuno M, Kamat PV (2006) Quantum dot solar cells. Harvesting light energy with cdse nanocrystals molecularly linked to mesoscopic tio2 films. J Am Chem Soc 128(7):2385–2393 275. Klimov VI (2008) Carrier multiplication in nanocrystal quantum dots and solar energy conversion. Lasers and Electro-Optics, 2008 conference on quantum electronics and laser science. CLEO/QELS 2008, pp 1–2 276. Lewis NS (2007) Toward cost-effective solar energy use. Science 315(5813):798–801 277. Kennedy M, McCormack SJ, Doran J, Norton B (2009) Improving the optical efficiency and concentration of a single-plate quantum dot solar concentrator using near infra-red emitting quantum dots. Sol Energy 83(7):978–981 278. Hanna MC, Beard MC, Johnson JC, Murphy J, Ellingson RJ, Nozik, AJ (2005) Quantum dot solar cells with multiple exciton generation. 2005 DOE Solar Energy Technologies program review meeting Denver, Colorado, 7–10 Nov 2005. NREL/CP-590-38992 279. Schuler A, Python M, Valle del Olmo M, de Chambrier (2007) Quantum dot containing nanocomposite thin films for photoluminescent solar concentrators. Solar Energy 81(9): 1159–1165 280. Aroutiounian V, Petrosyan S, Khachatryan A, Touryan K (2001) Quantum dot solar cells. J Appl Phys 89:2268–2271 281. Kamat PV (2008) Quantum dot solar cells semiconductor nanocrystals as light harvesters. J Phys Chem C 112:18737–18753 282. Laghumavarapu RB, Moscho A, Khoshakhlagh A, El-Emawy M, Lester LF, Huffaker DL (2007) GaSb/GaAs type II quantum dot solar cells for enhanced infrared spectral response. Appl Phys Lett 90:173125 283. Plass R, Pelet S, Krueger J, Gratzel M, Bach U (2002) Quantum dot sensitization of organic; inorganic hybrid solar cells. J Phys Chem B 106:7578–7580

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284. Sholin V, Olson JD, Carter SA (2007) Semiconducting polymers and quantum dots in luminescent solar concentrators for solar energy harvesting. J Appl Phys 101:123114 285. Avi S (2008) Photovoltaics literature survey (No. 66). Prog Photovoltaics Res Appl 16: 725–730 286. Dixon R (2001) Renewable energy, natural gas, and other hybrid systems: activities within EERE office of power technologies. Morgantown, WV

Chapter 3

Hydropower

Abstract Among all the renewable energy sources, the contribution of hydropower to the worldwide electricity generation is the highest. A hydropower system can be used to generate a few kilowatt of electricity to about 18,000 MW. Although there are a number of advantages and benefits of using hydropower systems, various environmental issues are restricting their development. These issues include depletion of nutrients in the water body, obstruction of the fish ladder for salmon, and fish shearing in turbines. In this chapter, a general description of a hydropower system and its various components are discussed.

3.1 Introduction Hydropower generating plants capture the kinetic energy of falling water, such as from a river and waterfalls, to generate electricity. A turbine and a generator convert the kinetic energy from water, first to mechanical energy and then to electrical energy using a generator. Hydropower is considered a renewable energy source since the water cycle, shown in Fig. 3.1, is a continuous cycle. The description of basic technology has been provided by a number of researchers [1–11]. The Greeks are believed to be the first to use hydropower in about 100 B. C. for grinding wheat into flour. Around 4 A.D., Asia and Europe started utilizing hydropower for milling. Modern hydropower turbines were designed in the mid1700s by a French hydraulic and military engineer, Bernard Forest de B´elidor, who wrote Architecture Hydraulique, in which he described the use of a vertical-axis versus a horizontal-axis turbine. Around 1880, hydropower was used to generate direct-current. The first hydroelectric plant generating alternating current (AC) in the world was located in the United States, Appleton, Wisconsin, in 1882. Hydropower provided about 15% of the U.S. electrical generation in 1907, and its contribution increased to almost 40% in 1940. Currently, about 8–10% of the U.S. electricity comes from hydropower. There are about 80,000 MW of conventional and 18,000 MW of pumped storage electrical power generation capacity in the USA. T.K. Ghosh and M.A. Prelas, Energy Resources and Systems: Volume 2: Renewable Resources, DOI 10.1007/978-94-007-1402-1 3, © Springer Science+Business Media B.V. 2011

157

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Fig. 3.1 The natural water cycle (Courtesy of Energy Information Administration) [12]

Solar energy heats water of the river, ocean, and any other water reservoir open to the atmosphere, causing it to evaporate. As the water vapor rises to the upper atmosphere, and cools down, it condenses into clouds and falls back onto the surface as precipitation (rain water). The water flows through rivers, back into oceans, where it again evaporates and begins the cycle over again. Worldwide generation of hydroelectric power increased by 406 billion kilowatthours between 1994 and 2004, or at an average annual rate of 1.6%. As shown in Fig. 3.2, Canada, China, Brazil, the United States, and Russia, were the five largest producers of hydroelectric power in 2004. Their combined hydroelectric power generation accounted for 51% of the world total. Canada led the world with 334 billion kilowatt-hours or 3.35 quadrillion Btu. China ranked second with 328 billion kilowatt-hours or 3.28 quadrillion Btu, and Brazil was third with 318 billion kilowatt-hours or 3.18 quadrillion Btu. The United States was fourth with 268 billion kilowatt-hours or 2.7 quadrillion Btu, followed by Russia with 165 billion kilowatthours or 1.7 quadrillion Btu.

3.1

Introduction

159

Fig. 3.2 Hydroelectricity generating capacity of top 12 countries in the world (Courtesy of Energy Information Administration [13])

Although hydropower contributes about 3% towards the world’s energy need (Fig. 3.3), the main use of hydropower is to generate electricity. As shown in Fig. 3.4, worldwide approximately 18.6% of the electricity is generated from hydropower. Hydroelectricity also contributes significantly to the total electricity generation in a number of countries. The increase in the contribution of hydroelectricity to the total electricity consumption by the top ten countries in the world from 2005 to 2006 is given in Table 3.1. As can be noted from the table, the increase in generation from hydropower during this time period was rather small. Various environmental issues are restricting its expansion and are discussed in details later in the chapter. As mentioned earlier, hydropower generated almost 40% of the electricity in the USA in 1940, but it was only 6.5% in 2006 (see Fig. 3.5). While thermal-generated electricity is by far the most common, representing over 60% of worldwide electricity generation, in some regions, other energy sources can supply a majority of the electricity. For example, in South America, hydroelectricity accounts for 80% of all electricity produced, which is over four times as much as thermal electricity, and over 50 times as much as nuclear power. Hydropower can contribute significantly to meet the growing demand for electricity worldwide. As can be noted from Fig. 3.6, several regions are utilizing only a small fraction of this resource.

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3 Hydropower

a

Renewables (9%) Nuclear (6%)

Oil (37%) Natural Gas (23%)

Coal (25%)

b

Geothermal (0.2%) Wind (0.3%)

Biofuel (0.2%) Solar PV (0.04%)

Solar heat (0.5%)

Biomass Biomass(4%) (4%) Hydropower Hydropower (3%) (3%)

Fig. 3.3 Contribution of hydropower to the total world energy mix. (a) contribution of the primary sources, (b) breakdown of the renewable portion (Source: Energy Information Administration [13] and World Energy Council [14])

Underutilization of the hydropower resources is mainly in Asia, Africa, and some parts of South America. One of the main reasons for underutilization is the access to these resources. A higher capital investment would be necessary to utilize them. Most developed countries are using a significant portion of their hydropower resources. The economic advantages of hydropower over thermal power systems can be seen in Fig. 3.7. The development and operating costs of hydropower generating

3.2

Hydropower Systems

161

40

Percentage of Global Electricity Supply by Fuel Type

35

39.8

30 25 20 19.6 15

16.1

15.7

10 5

6.7 1.8

0.3 Non-renewables Waste

Renewables

Oil

Nuclear

Hydroelectric

Gas

Coal

0

Fig. 3.4 The contribution of hydropower to the world electricity generation (Source: International Hydropower Association [15])

systems are significantly lower than that of gas turbines and fossil fuel systems. Of the four major electrical power generating systems, the total costs for hydropower systems are the lowest.

3.2 Hydropower Systems Hydropower systems may be divided into two ways: (1) based on its construction methods and (2) based on its size. There are three types of hydropower systems based on construction methods: • Impoundment. • Diversion or run of river. • Pumped storage. Hydropower systems can range in size from 5,000 to 10,000 MW that supply many consumers with electricity to small and micro plants that can meet an individual’s energy needs. These systems are also divided into three categories based on their size.

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Table 3.1 Contribution of hydroelectricity to the total electricity generation of selected countries Total electricity Total hydro electricity generation (billion generation (billion % of the total Country kilowatt hour) kilowatt hour) generated electricity Australia 244.22 16.68 7 Austria 59.31 33.59 57 Belgium 82.94 0.37 0 Brazil 437.26 370.63 85 Canada 612.60 365.30 60 China 3,042.31 429.98 14 France 537.91 57.61 11 Germany 594.66 20.09 3 India 665.30 121.18 18 Italy 292.11 33.13 11 Japan 1,082.24 74.61 7 Mexico 243.29 26.86 11 Nepal 2.70 2.69 100 New Zealand 42.41 23.28 55 Norway 135.02 132.82 98 Pakistan 93.26 32.79 35 Romania 58.25 15.74 27 Russia 964.21 177.01 18 Spain 287.39 27.15 9 Sweden 143.82 65.38 45 Switzerland 64.56 34.85 54 Turkey 181.56 35.44 20 United Kingdom 371.01 5.05 1 United States 4,166.51 248.31 6 Venezuela 110.73 83.03 75 Vietnam 61.02 27.12 44

• Large hydropower • Small hydropower • Micro hydropower Large hydropower Although definitions vary, the US Department of Energy (USDOE) defines large hydropower as facilities that have a capacity of more than 30 MW. Small hydropower The USDOE defines small hydropower as facilities that have a capacity of 100 kW to 30 MW. Micro hydropower A micro hydropower plant has a capacity of up to 100 kW. A small or micro-hydroelectric power system can produce enough electricity for a home, farm, ranch, or village. Each system has its own applications and impact on the eco-system and is described in Table 3.2.

3.2

Hydropower Systems

163 Petroleum (3%)

Others (2.9%)

Hydroelectric (6.5%)

Natural Gas (18.7%)

Coal (49.7%)

Nuclear (19.3%)

Fig. 3.5 Contribution of hydropower to total electricity generation in the USA in 2006 (Source: Energy Information Administration [13])

Fig. 3.6 Hydroelectricity generation potential in different regions of the world (Source: International Hydropower Association [15])

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3 Hydropower

Fig. 3.7 Breakdown of average cost for power production by various systems (Adapted from Wisconsin Valley Improvement Company [16])

4.0 Fuel cost

3.5

Maintenance cost Operation cost

Cents per Killowatt-hour

3.0 2.5

Data 1

2.0 1.5 1.0

Gas Turbine

Hydroelectric

Nuclear

0.0

Fossil Fueled steam

0.5

3.3 Hydropower System Construction Methods The construction method depends mainly on the size of the plant. Other factors that are considered in selecting a particular type of construction method include the geology of the land, terrains, and environmental impacts.

3.3.1 Impoundment Most of the large hydropower systems are impoundment type [17–20]. A dam is used to store river water in a reservoir. Water released from the reservoir flows through a turbine to generator electricity. The water may be released either to meet changing electricity needs or to maintain a constant reservoir level. The basic structure of an impoundment dam is shown in Fig. 3.8. A more detailed construction of a hydroelectric system is given in Fig. 3.9. Some of the largest impoundment hydropower systems constructed around the world are listed in Table 3.3.

3.3

Hydropower System Construction Methods

165

Table 3.2 Service and impact by different types of hydropower systems Type Service Impact • Storage capacity of water • Changes of habitat and Reservoir/Impoundment and energy social impacts through • Annual energy production reservoir inundation • Instant generating capacity • Modification of river flows • Flexibility in providing base • Impacts mainly due to load and peak load services reservoir • Can include irrigation, flood • Need to evaluate cumulative mitigation, water supply, impacts of other water uses environmental management, transportation, ground water recharge, recreation, climate change protection • Base load with limited • Limited flooding and Run-of-river changes to river flows flexibility to follow variation in power demand • Net consumer of electricity • Storage capacity of energy Pumped Storage • Environmental impacts • System security confined to small area • Quality and reliability through ancillary services and peak-load support • Water and power supply • Flow reduction downstream Diversion of diversion • Increase of flow in receiving stream Source: International Hydropower Association [15]

Fig. 3.8 Basic components of an impoundment dam for generating hydroelectricity (Source: Tennessee Valley Authority as reported in Combs [21])

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Fig. 3.9 Longitudinal section of an underground hydropower plant (Courtesy of Munchener Ruck Munich Re Group [22]) Table 3.3 Some large scale impoundment hydropower facilities around the world

Hydropower system Three Gorges dam Itaipu Guri Grand Coulee Sayano-Shushensk Krasnoyarsk Churchill falls La Grande

Location China Brazil/Paraguay Venezuela US Russia Russia Canada Canada

Capacity (MW) 18,200 12,600 10,000 6,494 6,400 6,000 5,428 5,328

Source: International Hydropower Association [15]

The main components that are constructed on site of an impoundment hydropower system are: • Dam • Spillway

3.3.1.1 Dam A dam is built to raise the water level of the river to create a falling water system. This also controls the flow of water. Dams have multipurpose use and are not necessarily built for power generation. Most dams in the USA are built for flood control and irrigation. Only 2,400 of 80,000 existing dams in the USA are used to generate power. Such a low utilization of hydropower resources in the USA is partly due to the strict site requirements for power production. Other uses of the reservoir of a dam are shown in Fig. 3.10.

3.3

Hydropower System Construction Methods

Fig. 3.10 Various uses of reservoir of a dam in the USA (Source: Federal Emergency Management Agency [23])

167

Hydroelectric 2.9% Undetermined 3.8%

Debris Control 0.8% Navigation 0.4%

Tailings & Other 8.0%

Irrigation 11.0 %

Recreation 38.4 %

Fire & Farm Ponds 17.1 % Flood Control 17.7 %

The reservoir of a dam basically stores the energy in the form of potential energy. When water flows down from the reservoir into a turbine, this potential energy is converted to kinetic energy that rotates the turbine blades. The type of dam to be built and its construction method depends on various factors: • The water height of the reservoir • The shape and size of the valley at the proposed construction site, as this will determine the capacity of the hydropower plant • The geology of the valley walls and floor. This will determine the availability of the construction materials and associated costs. The main factor that needs to be considered is the ability of the dam to withstand the pressure of water build up behind it. This will determine the type and capacity, (thereby, the electricity generation capacity) of a dam to be constructed and construction materials. The construction materials should be impermeable to water. Dams can be grouped in two major categories on the basis of the composition of their construction materials: • Embankment dams • Concrete dams

Embankment Dams These types of dams are built with cheap materials such as rock, gravels, earth or clay to hold back the water. The construction materials are used to make a natural settling angle of the material. The central core section is made of concrete, bitumen or clay to prevent water seeping through the dam [24–28]. Various components are shown in Fig. 3.11.

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Fig. 3.11 Cross section through a rockfill dam (Courtesy of Munchener Ruck Munich Re Group [22])

Concrete Dams As the name suggests, concrete is used to construct the dams [29, 30]. Several construction methods of concrete dams are proposed: Concrete Gravity Dam, Roller-compacted Concrete Dam, and Concrete Arch Dam. Basically, the weight of concrete and/or the shape of the dam hold back the water. Concrete is a relatively expensive material and the construction of concrete dams is usually more labor-intensive than the construction of embankment dams. Basic structure of a conventional concrete dam is shown in Fig. 3.12. 3.3.1.2 Spillways When dams are designed, provision must be made to cope with large floods. Spillways are pathways for floodwater to flow over or around the dam so that the dam itself is not breached. Spillways on concrete dams are usually constructed to allow water to flow over the top. Spillways in embankment dams are built at the side of the dam and away from the downstream face (Fig. 3.13). If the water is allowed to flow over the dam, serious damage can occur to rocks or the earth that are used to construct it.

3.4 Hydroturbine Several types of hydroelectric turbines are available for power generation [31–33]. All of them operate on the principle of converting the potential energy stored in water to mechanical energy by rotating a paddle-wheel or a propeller-type runner on

3.4

Hydroturbine

el. 441

169

el. 444 3

1 2 3 4 5 6 7 8 9

1

1 8

,7

:0

Inspection gallery Main inspection gallery Overflow spillway Intake structure Grout curtain Valve chamber Bottom outlet Flip bucket Stilling basin

1 8 4

2

6

9

7

el. 341,5 5 80,6 m

50 m

Fig. 3.12 Cross section of gravity dam (Courtesy of Munchener Ruck Munich Re Group) [22]

the turbine, which then generates electric power. Hydroelectric turbines can rotate either on their vertical or horizontal axis. However, most turbines employed for electricity generation have shafts that are vertically oriented. There are two main types of hydroelectric turbines: • Impulse Turbine • Reaction Turbine The type of hydropower turbine selected for a plant is based on the water head in the reservoir and the flow or volume of water at the site. Other factors that are considered in the selection of the turbine include the depth at which the turbine must be set, its efficiency, and cost.

3.4.1 Impulse Turbine The velocity of the water is used to move the runner of impulse turbines, which generally contain buckets or peddles on a wheel-shaped runner with one or more water jets directed tangentially toward the runner. The water is discharged at atmospheric pressure, after hitting each bucket on the runner. There is no suction on the down side of the turbine. Water flows out from the bottom of the turbine housing.

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3 Hydropower

Fig. 3.13 A hydropower generating dam with a spillway (Source: Energy Information Administration [12])

An impulse turbine is generally suitable for high head, low flow applications. There are mainly three types of the impulse turbine: • Pelton Turbine • Turgo Wheel Turbine • Crossflow or Ossberger Turbine

3.4.1.1 Pelton Turbine In a Pelton turbine, water jets from nozzles strike cups or buckets arranged on the periphery of a runner or wheel, causing the wheel to rotate. The wheel is connected to a shaft, and the rotational motion of the shaft generates electricity via a generator. A Pelton wheel has one or more free jets, discharging water into buckets of the wheel. Pelton turbines are suited for high head, low flow applications. The original design of the Pelton turbine is shown in Fig. 3.14. The turbine has gone through significant modification over the years [34–41]. Pelton turbines are available for

3.4

Hydroturbine

171

Fig. 3.14 The original design of Pelton wheel (Source: Pelton [42])

both large and small hydropower systems. In Fig. 3.15 is shown the design of a current Pelton turbine with its runner. A close view of the cups of a Pelton wheel is given in Fig. 3.16. The arrangements of water jets are shown in Fig. 3.17. The operating range of Pelton turbines is given in Fig. 3.18 as a function of the water head requirements for a specified power output. Pelton turbines can also be used for small hydropower systems. For small systems, a single water jet is typically used (see Fig. 3.19). 3.4.1.2 Turgo Wheel Turbine A Turgo wheel is a modification of the Pelton turbine made by Gilkes [46–48]. The runner is a cast wheel whose shape generally resembles a fan blade that is closed on the outer edges. It also looks like a Pelton wheel cut into half. The water stream

172

Fig. 3.15 Layout of a modern Pelton turbine (Source: Dekarz [43])

Fig. 3.16 Pelton wheel for the Pelton hydro-turbine (Source: Dekarz [43])

3 Hydropower

3.4

Hydroturbine

173

Fig. 3.17 Six jet Pelton turbine (Source: Voith Hydro [44])

2000 1000

Pelton Turbine Standard Pelton Turbine

Head (m)

100

10

0.1

1

10

100

Power Output (MW)

Fig. 3.18 Range for application of Pelton turbines (Source: Voith Hydro [44])

1000

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3 Hydropower

Fig. 3.19 A schematic representation of a small hydropower system using a single jet Pelton turbine (Source: Small hydropower systems [45])

is applied on one side usually at an angle of about 20ı , goes across the blades and exits on the other side. The incoming and outgoing jets do not cross each other, and, therefore, can handle a higher flow rate. The runner and the nozzle of a Turgo wheel is shown in Fig. 3.20. 3.4.1.3 Cross-Flow or Ossberger Turbine A cross-flow turbine, also known as an Ossberger turbine, is shaped like a drum and uses an elongated, rectangular-section nozzle directed against curved vanes on a cylindrically shaped runner [50–52]. The cross-flow turbine allows the water to flow through the blades twice. During the first pass, water flows from the outside of the blades to the inside; the second pass is from the inside back out. The Ossberger turbine designed by Ossberger Co. can be used both in horizontal (Fig. 3.21) and vertical orientations (Fig. 3.22). These turbines can accommodate higher water flow and lower head than the Pelton turbine. A complete Ossberger turbine system is shown in Fig. 3.23. The mean overall efficiency of Ossberger turbines is calculated at 80% for small power outputs over the entire operating range. Efficiencies of up to 86% are measured in the case of medium-sized and bigger units (see Fig. 3.24). The operating range of Ossberger turbines is given in Fig. 3.25.

3.4.2 Reaction Turbine A reaction turbine generates power from the combined action of pressure and moving water. The runner is placed directly in the water stream flowing over

3.4

Hydroturbine

175

Fig. 3.20 A turgo wheel (Source: The encyclopedia of alternative energy and sustainable living [49])

dle

ter

Wa

jet

Runner blades

Fig. 3.21 Inflow in horizontal orientation (Source: Ossberger GmbH Co [53])

nee

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Fig. 3.22 Inflow in vertical orientation (Source: Ossberger GmbH Co [53])

Fig. 3.23 A two cell Ossberger cross flow turbine (Source: Ossberger GmbH Co [53])

3.4

Hydroturbine

177

100

Ossberger turbine

90

ine cis t

urb

70

an

60 50

Fr

Efficiency (η, %)

80

40 30 20 10 0 0

10

20

1/3

30

40

50

60

70

80

90 100

2/3 3/3

Volumetric Flow Rate (Q), % of Total Flow Fig. 3.24 Efficiency of Ossberger and Francis turbines at different admission of flow (Q is percent of total volumetric flow) (Source: OSSBERGER [54])

the blades rather than striking each individually. Reaction turbines are generally preferred over impulse turbines when a lower head but higher flow is available. A variety of reaction turbines are available and these are: • Propeller Turbine – – – –

Bulb turbine Straflo turbine Tube turbine Kaplan turbine

• Francis Turbine • Kinetic Energy Turbine

3.4.2.1 Propeller Turbine Most of the reaction turbines are a propeller type turbine. A propeller turbine generally has a runner with three to six blades in which water impinges continuously

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3 Hydropower

Fig. 3.25 The range of use of an Ossberger turbine (Source: OSSBERGER [55])

at a constant rate. The pitch of the blades may be fixed or adjustable. The major components besides the runner are a scroll case, wicket gates, and a draft tube. Propeller turbines are discussed in the following references [31, 56–61].

Bulb Turbine The Bulb Turbine has the turbine and generator sealed and placed directly in the water stream [62–68]. This type of turbine is suitable for low heads, below 25 m. The near-straight design of the water passage provides both size and cost reductions. Bulb turbines can also operate in reverse flow directions. For very low heads, an extra set of gears is used to increase the rpm of the generator. However, both the generator and set-up gears are placed in an open pit, instead of a bulb. Bulb turbines are available for power output in the range of 10–100 MW. A typical bulb turbine is shown in Fig. 3.26 and its operating range is given in Fig. 3.27.

Straflo Turbine In a Straflo Turbine, the generator is attached directly to the perimeter of the turbine. Straflo is a registered brand name stands for straight-flow (flow straight). A key

3.4

Hydroturbine

179

Fig. 3.26 Cross section of a bulb turbine and generator (Source: Bulb/pit/S-turbines [69])

Head (m)

50

Custom Bulb Turbine

10 Standard pit Turbine

5

Custom pit Turbine 0

0.1

1 Power Output (MW)

10

100

Fig. 3.27 The range of use of bulb turbine (Source: Voith-Siemens hydropower generation [69])

feature of the Straflo turbine is the combination of turbine and generator, requiring less space. A Straflo turbine also consists of a group of axial turbines with a concentrically arranged generator, outside of the flow channel. Various components of a Straflo turbine are shown in Fig. 3.28.

Tube Turbine In a Tube Turbine, the penstock bends just before or after the runner, allowing a straight line connection to the generator. The power output from tube turbines ranges from 20 to 700 kW. These type of turbines have a direct drive configuration where

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Fig. 3.28 A straflo turbine. 1 impeller hub; 2 impeller blade; 3 rotor ring; 4 impeller mantle; 5 diffuser; 6 vanes; 7 Regulierring; 8 suction tube; 9 casing; 10 collector; 11 cover; 12 water collector of the seal ring; 13 cooling water outflow from the seals (Source: Kydd [70])

the turbine and the generator are on the same shaft having common bearings and seals. Figure 3.29 shows the main components of a tube turbine. The key features are given below. • • • • •

Compact structure, turbine and generator with bearings and seals in one unit Installation angle for the unit may vary from vertical to horizontal Stainless steel structure Long service intervals Heads from 5 to 30 m are sufficient for most applications

Kaplan Turbine In a Kaplan turbine, both the blades and the wicket gates are adjustable, allowing for a wider range of operation [72–80]. The basic components of a Kaplan turbine are shown in Fig. 3.30. In this type of turbine, the rotor is attached to the turbine shaft, and rotates at a fixed speed. When the rotor turns, it causes the field poles (the electromagnets) to move past the conductors mounted in the stator. This, in turn, causes electricity to flow and a voltage to develop at the generator output terminals.

3.4

Hydroturbine

Fig. 3.29 Tube turbine [71]

Fig. 3.30 The Kaplan turbine (Courtesy of Hydroelectric Design Center [81])

181

182

3 Hydropower 100

Head (m)

Kaplan Turbines

10 Standard Kaplan Turbines

0.1

1

10

100

1000

Power Output (MW) Fig. 3.31 Application range for Kaplan turbine as suggested by Voith for their turbines (Source: Voith Hydro [82])

The rotational speed of the generator is usually the same as the speed of the turbine, because they are directly connected. Some plant designs incorporate a speed-increasing gearbox between the turbine and the generator. The speed of the turbine is determined by the design and hydraulic conditions. Speeds for Kaplan generators are typically in the range of 50–600 revolutions per minute (rpm). For a generator operating in a 60-Hz system, the rotational speed (in rpm) times the number of field poles on the rotor is always 7,200. The stator is a donut-shaped structure surrounding the rotor. The stator is made up of a steel frame supporting stacked steel laminations. The conductors, called the stator winding, are recessed in slots in this lamination structure. The stator winding is arranged in such a way that when the rotor turns, the field poles pass only a fraction of an inch from it. The movement of the magnet next to the conductor causes electricity to flow in the conductor. Stator windings of larger generators are made up of individual stator coils. Each coil is made of multiple strands of copper, which is insulated with mica held in place with an epoxy or a polyester resin. The individual coils are connected to each other through jumpers and then to a ring bus. The ring bus is connected to the generator leads, which in turn are connected to a power step-up transformer. Finally, the transformer is connected to electrical power grid. Some generator designs use half-coils (bars) in lieu of full coils. The operating range of Kaplan turbines are given in Fig. 3.31.

3.4.2.2 Francis Turbine A Francis turbine has a runner with fixed buckets (vanes), usually nine or more [83–89]. Water is introduced just above the runner and all around it and then falls

3.4

Hydroturbine

183

Fig. 3.32 Cross section of a Francis turbine (Source: Francis turbines [90])

Fig. 3.33 Francis turbine in operation at Niagra Falls (Source: Francis turbines [90])

through, causing it to spin. Besides the runner, the other major components are a scroll case, wicket gates, and a draft tube. The cross-sectional view of a Francis turbine is shown in Fig. 3.32. Niagara Falls, Canada uses Francis turbines for power generation and one such turbine is shown in Fig. 3.33. The operating range of Francis turbines is given in Fig. 3.34.

184

3 Hydropower 1000

Head (m)

100

Custom Francis Turbine Standard Francis Turbine

10 Open Flume Francis Turbine

0.1

10 1 Power output (MW)

100

1000

Fig. 3.34 The operating range of Francis turbines (Source: Francis turbines [90])

Fig. 3.35 A kinetic energy turbine (Courtesy of Free Flow Power Corporation [91])

3.4.2.3 Kinetic Energy Turbine Kinetic energy turbines, also called free-flow turbines, generate electricity from the kinetic energy present in flowing water rather than the potential energy from the head. The systems may operate in rivers, man-made channels, tidal waters, or ocean currents. Kinetic turbines utilize the water stream’s natural pathway. The diversion of water through manmade channels, riverbeds, or pipes is not required. These systems do not require large civil works; rather, existing structures such as bridges, tailraces and channels are sufficient for these turbines. The basic structure of a kinetic turbine is shown in Fig. 3.35.

3.6

Run-of-the-River Hydropower Systems

185

3.5 Selection of Turbines The selection of a turbine depends on several factors. Each turbine operates most efficiently at a certain pressure and flow range. However, often times, the working head determines the turbine types for a specific application. The operating range of various turbines for a given head and flow rate is shown in Fig. 3.36. The same figure can also be used for selection of a turbine type if a certain power output is required.

3.6 Run-of-the-River Hydropower Systems Run-of-the-river plants derive energy from a water flow with minimal disruption of the flow or the surroundings. Also, run-of-the-river hydroelectric plants use the power in the river water as it passes through the plant without causing an appreciable change in the river flow [93–97]. Normally, such systems are built on small dams that impound little water. Several systems have been designed without a reservoir or a dam. A run-of-the-river system may not cause changes in the water quality, such as higher temperature, low oxygen, decreased food production, siltation,

Fig. 3.36 A summary of the range of use of various turbines with certain heads and flow rates (Adapted from St.Onge Environmental Engineering [92])

186 Table 3.4 Large scale run-of-the-river power plants around the world Name of the dam River Location Chief Joseph Dam Columbia River Washington, USA Beauharnois Beauharnois Canal Quebec, Canada Satluj Jal VIdyut Nigam Satluj River Shimla, India Ghazi Barotha Dam Indus River Ghazi, Pakistan La Grande-1 generating station La Grande River Quebec, Canada

3 Hydropower

Capacity 2,620 MW 1,673 MW 1,500 MW 1,450 MW 1,436 MW

increased phosphorus and nitrogen. The presence of decomposition products that result from lubricants, grease, and sealants of a large plant operation is negligible. Run-of-the-river hydroelectric plants also do not normally affect downstream habitat or terrestrial habitat. Other concerns, such as ladder rejection by fishes, are not problems. Although run-of-river plants are small, several plants have been designed that exceeded 1,000 MW. Several 1,000 MW or higher capacity run-of-the-river projects are listed in Table 3.4.

3.7 Small Hydroelectric Power System A majority of run-of-the-river systems are small hydropower (SHP) systems. These systems can generate enough electricity for a single family home, small resorts or a recreational facility. This type of hydropower system does not require a large reservoir [98–109]. Although the USDOE defines SHP system as those having 100 kW to 10 MW generating capacity, there is no international standard for its definition. Most of the countries define their own SHP. For example, India considers SHP up to 15 MW, in China it is up to 25 MW. However, a capacity of up to 10 MW total is generally becoming accepted in Europe and is supported by European Small Hydropower Association (ESHA) and the European Commission. It is estimated that the worldwide installed capacity of SHP is in the order of 61 GW. The percentage distribution of SHP (installed capacity) at various region of the world is given in Fig. 3.37. The actual power produced by various regions is shown in Fig. 3.38. Several countries are pursuing SHP rather aggressively. About 3,000 MW of electricity is generated in the USA from small hydropower systems. There are currently about 2,000 MW of installed small hydro capacity in Canada, contributing about 3% to the total Canadian installed hydroelectric capacity of 67,000 MW. Canada is planning to add about 1,000 MW of SHP in the near future. The Indian Government, through the Ministry of Non-Conventional Energy Sources (MNES), is aggressively pursuing SHP schemes of up to 3 MW from canals, dams, rivers and high-head sites in mountain areas. India has about 114 MW of installed capacity and planning to add another 87 projects with a combined capacity of 94.5 MW. Another 1,344 sites have been identified for development.

3.7

Small Hydroelectric Power System

187

0.4%

Australasia/Oceania Europe

22.3%

North America

6.1%

South America

2.7%

Africa

0.50%

Asia

68% 0

10

20 30 40 50 Regional Contribution (In %)

60

70

Fig. 3.37 World region’s contribution to world’s small hydropower installed capacity (Source: European Small Hydropower Association) [110]

Australasia/Oceania

0.244

Europe

13.6

North America

3.72

South America

1.65

Africa

0.305

Asia

41.5 0

10

20 30 Total Power OutPut (GW)

40

50

Fig. 3.38 Installed generation capacity of small hydropower systems in various regions of the world (Source: European Small Hydropower Association [110])

The installed capacity of small hydropower in China is more than 19 GW with an annual electric output to 64 TWh. According to European Renewable Energy Council (EREC): “In EU-25 about 17,200 Small Hydropower plants are in operation with a total installed capacity of 11 GW. Italy accounts for about 21% of the total Small Hydropower installed capacity in the EU-25, followed by France (17%) and Spain (16%). Poland and the Czech Republic both with 2% of the total EU-25 capacity are the lions of the new EU Member States”.

188

3 Hydropower

Fig. 3.39 Arrangements of a small hydropower system (Courtesy of Energy Efficiency and Renewable Energy [111])

3.7.1 Components of a Small Hydro Power System The basic components of a small hydropower system are shown in Fig. 3.39. Based on a particular site and requirements these components may be different.

3.7.1.1 Feeder Canal Water flows down the feeder canal from the intake to the forebay. The canal is usually made of earth or concrete, and is fitted with a grating to prevent solid objects from being carried away by the stream.

3.7.1.2 Forebay The forebay is a tank that holds water between the feeder canal and the penstock. It must be deep enough to ensure that the penstock inlet is completely submerged, so that air is excluded from the power equipment.

3.7

Small Hydroelectric Power System

189

3.7.1.3 Penstock The penstock is a pipe connecting the forebay to the power house. It pressurizes the water and must be capable of withstanding high pressures. Generally, steel or high-density plastic pipes are used.

3.7.1.4 Power House The power-producing equipment and control devices are housed here. These devices can be operated and monitored either on site or remotely.

3.7.1.5 Tail Race The tail race is the flow of water out of the power house back into the stream.

3.7.1.6 Reserve Power Hydro-electric plants are designed to use only part of the total water flow under normal operating conditions. The reserve flow is the portion of the flow not normally used.

3.7.1.7 Intake The intake is a buffer between the water supply and its hydro-electric plant. It is constructed of earth, masonry, concrete, or riprap. It’s shape is largely determined by the nature of the terrain.

3.7.1.8 Fish Ladder The fish ladder allows salmon to migrate upstream to minimize the biological impact from the power plant.

3.7.1.9 Water Head for Power Production The water head determines the amount of power that can be produced from the given system. The head is the vertical separation between the forebay and the intake of the turbine. This is shown in Fig. 3.40.

190

3 Hydropower

Fig. 3.40 The water head for a small hydropower system (Courtesy of Energy Efficiency and Renewable Energy [111])

There are several advantages of SHP systems that should be considered when comparing them with other alternatives such as wind and solar energy systems. • • • • • • • • • • •

Environmental protection through CO2 emission reduction Proven and reliable technology Reduces the dependency on imported fuels Improves the diversity of energy supply Grid Stability Reduces land requirements Local and regional development Good opportunities for technology export Assists in the maintenance of river basins Technology suitable for rural electrification in developing countries High energy payback ratio

Main disadvantages of a SHP system are its costs and efficiency. Other issues include local environmental impacts which should be fully addressed for these systems before the start of the construction.

3.8 Micro-Head Hydropower Systems The design features of a micro-head hydropower system are basically the same as that of a small hydropower system. Micro-head hydropower systems can utilize a smaller head compared to small hydropower system, and, naturally, produce less

3.8

Micro-Head Hydropower Systems

191

Fig. 3.41 Arrangements for a micro-hydropower system (Source: Idaho National Laboratory [112])

power, below 100 kW, than small hydropower system. A micro-head hydropower system can operate on a flow rate of 2 gal/min (7:5 cm3 =min) and a water head of 2 feet (61 cm) to generate electricity. The voltage generated by this type of system may be sufficient for delivery up to a mile away from the location where it is being used. A description of a micro-head hydropower system is shown in Fig. 3.41. The factors that should be considered before deciding on a micro-head hydropower system are: distance from the power source to the location where energy is required, stream size (including flow rate, output and drop), and other system components, such as inverter, batteries, controller, transmission line and pipelines.

3.8.1 Selection of Turbine for Small or Micro Head Systems The turbine selection for small or micro-head hydropower systems is critical. Most of the turbines used for large hydropower systems are modified to use lower heads. The basic operating principle remains the same. A list of these turbines is given in Table 3.5. A Pelton Turbine might be used at 50 m head for a 10 kW system, but it should be a minimum of 150 m to be considered for a 1 MW system. Generally, the water head is critical in maintaining the required shaft speed of the generator. For most generators, 1,500 rpm is recommended to minimize the speed change between the turbine and the generator. The speed of any given type of turbine tends to decline in proportion to the square-root of the head, so low-head sites need turbines that

192

3 Hydropower Table 3.5 Classification of turbine types for small hydropower application Head classification Turbine type

High (>50 m)

Medium (10–50 m)

Low (<10 m)

Impulse

Pelton Turgo Multi-jet Pelton

Crossflow Turgo Multi-jet Pelton Francis (special type)

Crossflow

Reaction

Francis (open-flume) Propeller Kaplan

Source: Paish [104] 500 Pelton wheel or Turgo wheel 200

0 50 kW

0

10 Francis turbine

kW 50 kW

Net Head (m)

100

20

Crossflow turbine

20 kW

10

10

propeller turbine or Kaplan

kW

3 0.5

1

Discharge

5

10 20

(m3/sec)

Fig. 3.42 Range of use of turbines for small hydro applications (Source: Paish [104])

are inherently faster under a given operating condition. The approximate ranges of head, flow and power applicable to the different turbine types are given in Fig. 3.42 for up to 500 kW power generations.

3.9 Pumped Storage Hydropower System Pumped-storage hydropower systems are operated differently from conventional hydroelectric systems. Although these systems use falling water to generate power in the same manner as a conventional hydroelectric system, water, after falling through the turbine, is collected in a reservoir (called lower reservoir). A reversible turbine that works in both directions is used at the pumped storage facility. When demand for electricity is low, the turbine is operated in the reversible mode to pump

3.9

Pumped Storage Hydropower System

193

Fig. 3.43 Load variation of a pumped storage facility on a typical day of operation

the water back to an upper reservoir. Water from the upper reservoir is released to generate power during periods of peak demand. By pumping water back to the upper reservoir using excess off-peak electricity restores the potential energy of the water [108, 113–126]. The pumped storage systems were originally designed for load leveling, so that these can absorbs/utilize the surplus power from the supply grid during the off-peak period to pump water to an upper reservoir. Then in a peak period, it generates power using the water and thus it levels the load for other generating plants as shown in Fig. 3.43. This type of system is ideal for a large base load power plant such as nuclear power plants. A pumped storage system can be started or stopped quickly depending on the demand. Often time, these operations can be performed remotely, reducing operating costs. It may be noted that conventional hydroelectric power plants are not designed to absorb the excess power. The basic components of a pumped storage system are shown in Fig. 3.44. Pumped storage systems actually are net users of electrical energy. The generation and pumping cycles are usually 90% efficient. When other losses, such as friction loss, turbine efficiency, etc., are considered, the overall efficiency is generally around 75%. However, as noted by the International Hydropower Association, there are several benefits of a pumped storage system: • Improved energy regulation and operation of the supply grid. • Delivers ancillary services to the supply grid, such as standby and reserve duties, black-station start, frequency control, and flexible reactive loading. • Creates environmental benefits such as reduced gaseous emissions and has little environmental impact during its operation. • Allows flexible and rewarding commercial operations across a variety of electrical power supply scenarios. About 127,000 MW of pumped-storage hydro capacity is already operating around the world. Several new projects are under construction that can increase the total pumped-storage capacity to more than 200,000 MW in the next 4 years [128]. In the

194

3 Hydropower Switchyard

Visitors Center

Intake

Reservoir

Elevator

Discharge

Main Access Tunnel Surge Chamber

Powerplant chamber Breakers Transformer Vault

Fig. 3.44 A schematic diagram of a pumped storage plant at the Raccoon mountain operated by the Tennessee Valley Authority (Source: Tennessee Valley Authority [127])

USA, there are 38 plants, 151 generators with a nameplate capacity of 20.35 GW (approximately 2.5% of total installed generating capacity). The European Union (EU) had a net capacity of 38.3 GW pumped storage out of a total of 140 GW of hydropower in 2007. This represented 5% of total net electrical capacity in the EU. The pumped storage capacities of various countries are given in Appendix XIII.

3.10 Calculation of Power from Water Flow The potential energy stored in a body of moving water is used for generating electrical power by allowing the water to fall through a turbine. The potential energy is converted to the kinetic energy, which is then used to turn the turbine and generate power. The potential energy is given by the following expression: E D mg H

(3.1)

where, m is the mass of water, g is the acceleration due to gravity, and H is called head. Head is pressure, created by the difference in elevation between the intake of the pipeline and the water turbine. As shown in Fig. 3.45, head is measured as vertical distance (feet or meters) or as pressure (pounds per square inch, Newtons per square meter). In hydropower calculations, head is generally reported in feet or meters. The power (P ) content of the water may be calculated from the following expression: E m P D D gh (3.2) t t

3.10

Calculation of Power from Water Flow

195

Fig. 3.45 The “fall” and volumetric flow rate of the water body for hydropower generation

where, t is the time and m=t is the mass flow rate of the flowing water. However, it is customary to express power in terms of the volumetric water flow rate. In terms of volumetric flow rate, the power is given by: P D

ghQ 1000

(3.3)

where, P D power, kW Q D volumetric water flow rate (m3=s) h D head, m D density of water, 1;000 kg=m3 g D acceleration due to gravity, 9:81 m=s2 Substitution of density of water into Eq. 3.3 provides the following expression: P D Qhg

(3.4)

Equation 3.3 in British unit may be written as P .kW/ D Where, Q D volumetric water flow rate (ft3=s) h D head, ft

.0:746/Qh 550

(3.5)

196

3 Hydropower

Fig. 3.46 The net head of a small hydropower system (Printed with permission Fritz [129])

D specific gravity of water, 62:4 lb=ft3 g D acceleration due to gravity, 9:81 m=s2 The power calculated from Eq. 3.4 is the theoretical maximum. For calculation of net power, various energy losses must be taken into account. Most important losses are: 1. Head loss 2. Leakage loss around the runner 3. Mechanical loss in the turbine due to mechanical friction between various parts of the turbine. The head loss is calculated by determining the net head, which is defined as the difference between the total head at the entrance to the turbine inlet and the total head at the tailrace. This is shown in Fig. 3.46. The net head can be calculated using the Bernoulli equation. The derivation of Bernoulli equation can be found in any fluid mechanics textbook. The energy head in the water flowing in a closed pipe of circular cross section, under a pressure of P , at a certain point 1, is given by the Bernoulli’s equation: H1 D h1 C

V2 P1 C 1 2g

where, H1 D the total energy head h1 D the elevation above some specified datum plane, P1 D the pressure ” D g D the specific gravity of water

(3.6)

3.10

Calculation of Power from Water Flow

197

D the density of water V1 D the velocity of the water, and g D the gravitational acceleration. In Eq. 3.6, the term h1 represents the potential energy, the term

P1

is the pressure

V12 2g

energy, and the term is the kinetic energy, commonly known as the “Velocity Head”. Equation 3.6 can also be used for an open channel, by replacing the term P1 =g by d1 , the water depth at that point. When water is travelling from Point 1 to Point 2, it loses energy as it flows through a pipe, mainly due to: 1. friction against the pipe wall 2. viscous dissipation as a consequence of the internal friction of flow The losses due to the friction can be determined in terms of “head” (hf ) by applying Eq. 3.6 at both the entrance and the exit of the water flow and is given by: h1 C

P1 P2 V2 V2 C 1 D h2 C C 2 C hf 2g 2g

(3.7)

Darcy and Weisbach [130] suggested the following expression for calculation of head losses due to friction. The equation is valid for incompressible and steady flows through pipes. 2 L V hf D f (3.8) D 2g where, f D friction factor, a dimensionless number L D the length of the pipe, m D D the pipe diameter, m V D the average velocity, m/s, and g D the gravitational acceleration (9:81 m=s2 ). The friction against the pipe wall depends on the wall material roughness and the velocity gradient nearest to the wall. The velocity gradient depends on the type of flow, i.e., laminar or turbulent, which is defined by the Reynolds number and is given by: DV DV NRe D D (3.9) where, D D the pipe diameter, m V D the average water velocity, m/s D the density of water, kg=m3 , D the dynamic viscosity of the fluid, kg /(m s), and D the kinematics viscosity of the fluid, =, m2 =s.

198 Table 3.6 Dynamic and kinematic viscosity of water at different temperatures

3 Hydropower

Temperature (ı C) 0 5 10 20 30 40 50 60 70 80 90 100

Dynamic viscosity .Ns=m2 / 103 1.787 1.519 1.307 1.002 0.798 0.653 0.547 0.467 0.404 0.355 0.315 0.282

Kinematic viscosity .m2 =s/ 106 1.787 1.519 1.307 1.004 0.801 0.658 0.553 0.475 0.413 0.365 0.326 0.294

Both the dynamic and kinematic viscosities are function of temperature. In Table 3.6 is given the values of dynamic and kinematic viscosities for water in the temperature range of 0–100ı C. Generally, the flow is considered laminar when NRe < 2;300, transition flow for 2;300 < NRe < 4;000, and turbulent NRe > 4;000. The value of friction factor, f , is dependent on the Reynolds number. In a laminar flow, f , can be calculated by the equation: 64 64 D VD NRe

(3.10)

32 L V 64 L V 2 D VD D 2g g D2

(3.11)

f D Therefore, head loss, hf , is given by: hf D

For turbulent flow, the surface roughness of the pipe affects the friction factor significantly. The surface roughness is defined as e=D, where e represents the average roughness height of the pipe wall and D is the pipe diameter. The average value of e for different pipe materials is given in Table 3.7. An explicit form for the friction factor is not available. Von Karman [131] suggested the following empirical equation, if the pipe is considered hydraulically smooth but the flow is turbulent. p ! NRe f 1 p D 2 log10 (3.12) 2:51 f

3.10

Calculation of Power from Water Flow

Table 3.7 The value of roughness height, e, for several materials

199

Pipe materials Polyethylene Fiberglass with epoxy Seamless commercial steel (new) Seamless commercial steel (light rust) Seamless commercial steel (galvanized) Welded steel Cast iron (enamel coated) Concrete (steel forms with smooth joints) Riveted steel Concrete Wood stave Cast iron Galvanized iron Asphalted cast iron Commercial steel (wrought iron) Drawn tubing

e .mm/ 0.003 0.003 0.025 0.250 0.150 0.600 0.120 0.180 0.9–4.0 0.3–3.0 0.18–0.9 0.25 0.15 0.12 0.045 0.0015

Source: European Small Hydropower Association (ESHA) [134]

If the pipe is rough and the flow is turbulent, the friction factor may be estimated from the following equation: 1 D p D 2 log10 3:7 e f

(3.13)

Colebrook and White [132] also proposed an expression for calculation of friction factor for the entire range of pipe roughness, which is given below: 1 p D 2 log10 f

2:51 e=D C ıp 3:7 NRe f

! (3.14)

Although the friction factor can be calculated from Eqs. 3.12 and 3.14 by a trial and error method, Moody [133] provided a graphical approach for determining the friction factor. The Moody chart, better known as “Friction Factors For Pipe Flow” is included in Appendix XIII. The use of Moody chart for calculation of friction factor is demonstrated through the following example. Example 3.1. Calculate head loss for a power plant that has a gross head of 100 m, the water flow rate is 10 m3=s, the intake length is 1,000 m, and the pipe diameter is 2 m. Pipe material is welded steel, and assume average water temperature of 10ı C.

200

3 Hydropower

Solution. To calculate the head loss, first the friction factor needs to be calculated. The Moody chart will be used. In order to use the chart, following information is needed: e=D and Reynolds number. From Table 3.7, e D 0:6 mm, therefore, e=D D 0:6=2;000 D 0:0003 To calculate Reynolds number, kinematic viscosity at 10ı C is obtained from Table 3.6 and is 1:307 106 m2 =s. The velocity is given by: V D

Q 4 10 4Q D D 3:18m=s D D 2 =4 D 2 3:14 22

Reynolds number, NRe D DV= D .2 3:18/=1:307 106 D 4:87 106 As can be seen from the Moody Chart (Fig. 3.47), the friction factor, f , is 0.015. The head loss is calculated from Eq. 3.8. 2 1000 3:182 L V D 0:015 D 3:86 m hf D f D 2g 2 2 9:81 Therefore, net head without taking into account other losses is given by: Net head D Gross head Head loss D 100 3:86 D 96:14 m

3.10.1 Local Head Losses Several local head losses also occur when water flowing through a pipe. These losses are due to geometric changes at entrances, bends, elbows, joints, racks, valves and at sudden contractions or enlargements of the pipe section. Losses are expressed as head loss and estimated from rather empirical formulas and are discussed below. These local losses are different for different channels. 3.10.1.1 Head Loss Equations for Closed Channels Entrance Loss hcc e D k1

v2 2g

(3.15)

where, k1 is the entrance loss coefficient for various types of entrances and v is the entrance velocity.

Contraction Loss hcc c D k2

v22 2g

where, k2 is called contraction coefficient, and v2 is the downstream velocity.

(3.16)

103

2

4

103

4

6 8104

2

2

2

4

6 8 105 2

4

2

4

6 8 106

Reynolds Number

6 8105

Smooth pipes

Complete turbulence, rough pipes

4

6 8 104

2

4

6 8 106

4

6 8107

2

2

6 8

4

0.000,01 6 8108

0.000,05

0.0001

0.0002

0.001 0.0008 0.0006 0.0004

0.002

0.004

0.01 0.008 0.006

0.02 0.015

0.03

0.05 0.04

Calculation of Power from Water Flow

Fig. 3.47 Determination of friction factor for Problem 3.1 from the Moody chart

0.010 0.009 0.008

0.015

0.020

0.025

f = 64 Re

flow

0.030

0.040

inar

0.050

0.060

0.070

0.080

0.090

0.100

Lam

Friction Factor

3.10 201

Relative Pipe Roughness (ε/D)

202

3 Hydropower

Expansion Loss A1 2 v21 D 1 hcc ex A2 2g

(3.17)

where, A1 and A2 are the upstream and downstream flow areas and v1 is the upstream velocity.

Bends v2 2g

hcc B D kB

(3.18)

where, kB is bend loss coefficient and v is the velocity at the bend.

Gates and Valves hcc v D kv

v2 2g

(3.19)

where, kv is the gates and valves loss coefficient and v is the velocity at the gates and valves.

Gradual Expansion hcc ge D kge

v21 2g

(3.20)

where, kge is the expansion coefficient and v1 is the upstream velocity. The values of various loss coefficients are given in Appendix XIII.

3.10.2 Head Losses in Open Channels 3.10.2.1 Trash Rack Losses A screen is used at the entrance of both pressure pipes and intakes to capture the floating debris. This results in a head loss, which is generally small. Kirschmer [135] suggested the following formula. hTR D k a

a 4=3 V 2 0 sin b 2g

(3.21)

3.10

Calculation of Power from Water Flow

203

Fig. 3.48 General construction of a trashrack (Courtesy of European Small Hydropower Association (ESHA) [134])

where, hTR D head loss in trash rack k D constant that depends on the geometrical shape of the trash rack grid a D thickness of the grid b D spacing between the grid bar V0 D approach velocity D angle of inclination from horizontal These parameters are shown in Fig. 3.48 along with the value of k for different geometry of the trash rack grid.

Losses at Bends hB D SLB C 2 where, S D longitudinal slope of canal LB D centerline length of bend

bc V 2 rB 2g

(3.22)

204

3 Hydropower

bc D bottom width of canal rB D radius of curvature of bend V D average velocity of flow The longitudinal slope of canal, S , is calculated from the following equation: QD

1 2=3 1=2 AR S n h

(3.23)

where, Q D volumetric flow rate (m3 =s) A D cross sectional area .m2 / Rh D hydraulic radius (m) n D Manning roughness constant. Values for various conditions of a channel are given in Appendix XIII. Values of n various from 0.01 for very smooth channel surface to 0.05 for natural rocky bed surface.

Losses at Entrances

he D 0:05

v22 v2 v21 C 2 2g 2g

(3.24)

where v2 is the velocity in canal and v1 is the velocity upstream from canal entrance.

Contraction

hc D 0:2

v22 2g

(3.25)

where, v2 is the downstream velocity.

Expansion A1 2 v22 hex D 1 A2 2g

(3.26)

where, A1 and A2 are the upstream and downstream flow areas, respectively and v2 is the downstream velocity.

3.12

Fish Ladder and Fish Passage in Hydropower Systems

205

3.11 Hydropower System Efficiency Various energy losses in a hydropower system affect the net power production. These losses are taken into account by multiplying the theoretical power by efficiencies. The hydraulic efficiency (h ) addresses the head loss. The leakage around the runner is taken into account by the volumetric efficiency (v ). The mechanical efficiency (m ) takes into account various losses involving the hydroturbine and is often called the turbine efficiency. The overall efficiency is, therefore, given by: D h v m

(3.27)

The hydraulic efficiency is defined as: h D

Hu H

(3.28)

Where Hu is the head utilized by the runner and H is the net head on the turbine. Efficiencies of around 90–95% can be expected for h . The development of new sealing methods has increased the volumetric efficiency significantly, in the range of 95–98%. Similarly, the efficiency of modern hydroturbines is in the range of 90–95%. The overall efficiency, therefore, is in the range of 75–90%. The net power, therefore, is given by: P D Qhg (3.29)

3.12 Fish Ladder and Fish Passage in Hydropower Systems Although hydropower systems do not discharge pollutants into the environment, it is not free from other adverse environmental effects [136–142]. Efforts to reduce environmental problems associated with hydropower operations, such as providing a safe fish passage and improved water quality, have received considerable attention in the past decade, both at Federal facilities and non-Federal facilities licensed by the Federal Energy Regulatory Commission. Hydropower dams impede the flows of rivers, and, thereby, affect the habitation of various aquatic lives including fishes. Often, the river is the route of migratory fishes. Dams can affect the fish in several ways: they can restrict or delay the fish migration, increase predation, and subject fishes to direct damage and stress. These issues are addressed by designing better fish ladders and passage ways. Upstream fish passages are mainly through the fish ladders. Three main methods have been tried for upstream fish passages: (1) downstream capture followed by manual transportation to upstream. This method is generally costly and labor intensive, however, it can work if the transportation time is short, (2) fish ladders, the most common and rather successful method. These are generally long, since the uphill slope has to be gradual, (3) fish-lift or

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fish-elevator, this method involves collection of fishes in a hopper downstream and holding them there for a certain period of time until the next lift, which can be manual or automatic. The hopper is raised from the tailrace level to the forebay level and fishes are released into the forebay on the upstream side of the dam. Fish lifts can be very effective and are used in many eastern U.S. dams where upstream passage for shads is an issue. The passage of fishes from upstream to downstream also occurs though several other systems: (1) juvenile bypass, (2) spillways, (3) sluiceways and other surface passage, (4) physical transportation, and (5) turbines. Generally, during construction of dams, special provisions are made for juvenile bypass, spillways, and sluiceways for safe passage of fishes. Since 1981, the U.S. Army Corps of Engineers (USACE) has been collecting juvenile salmonids from bypass systems at several dams (McNary Dam on the middle Columbia River and three of the four USACE dams on the lower Snake River) and barging or trucking them downstream. A major concern regarding the use of hydroelectric power is the mortality of turbine-passed fishes, especially among Pacific salmon, steelhead, Atlantic salmon, American shad, and catadromous eels. These fishes must travel from rivers back to the sea after the pawning season. The number of fishes killed, injured, or stressed by turbine passage can vary with species, age, time of year, water temperature, turbine type and operations. Mortality rates over 40% have been reported for juvenile American shad and blueback herring. Mathur et al. [143] reported mortality from zero to about 15% within 1 h of passage among juvenile American shad. A study by Bickford and Skalski [144] that used data from 25 years of Columbia-Snake River System survival studies, found mortality rates between about 7% and 13% for juvenile salmonid. Although the mortality of turbine-passage fish can be mitigated to some extent using the methods mentioned earlier, it can be further lessened by improving passage conditions within the turbine. In order to design a better passage within the turbine, the mechanisms for injuries and mortality among fishes that pass through hydroelectric turbines need to be understood first. The following mechanisms have been suggested: • Rapid and extreme pressure changes (water pressures within the turbine may increase to several times the atmospheric pressure, then drop to subatmospheric pressure, all in a matter of seconds), • Cavitations (extremely low water pressures cause the formation of vapor bubbles which subsequently collapse violently), • Shear stress (forces applied parallel to the fish’s surface resulting from the incidence of two bodies of water of different velocities), • Turbulence (irregular motions of the water, which can cause localized injuries or at larger scales, disorientation), • Strike (collision with structures including runner blades, stay vanes, wicket gates, and draft tube piers), and • Grinding (squeezing through narrow gaps between fixed and moving structures). The locations within the turbine where these events occur are shown in Fig. 3.49.

3.13

Summary

207 ORNL 2000-00571B/abh 3 Gradually 2 Increasing Pressures 1 0

Forebay

Tailrace Water Flow

Draft Tube

Wlcket Gate

Strike and Grinding

Shear Stress

Rapidly Decreasing Pressures and Cavitation

3 2 1 0

Turbulence

Fig. 3.49 Locations within a hydroelectric turbine at which particular injury mechanisms to turbine passed fish tend to be most severe (Cada [145])

The U.S. Department of Energy initiated the Advanced Hydropower Turbine System (AHTS) Program for developing low impact, fish-friendly turbines. The objective of the AHTS program was to design turbines that are environmental friendly, provide safe passage to fishes while maintaining a high efficiency for electricity generation. A turbine designed by the Alden Research Laboratory, Inc./Northern Research and Engineering Corporation (ARL/NREC) under this program is shown in Fig. 3.50. The runner of the new turbine was based on the shape of a pump impeller that minimized the leading edge of the blade and maximized the size of flow passages.

3.13 Summary Among renewable energy sources, hydropower is the leading generator of electricity providing more than 97% of all electricity generated by renewable sources. Other sources including solar, geothermal, wind, and biomass account for less than 3% of

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Fig. 3.50 Schematic of a fish-friendly turbine (Designed by Alden Research Laboratory, and Northern Research and Engineering Corporation. Odeh [146])

renewable electricity production. Hydropower provides about 20% of the world’s electricity and is the main energy source for more than 30 countries. However, the further growth of hydropower systems in a number of developed countries is limited. Hydropower is very efficient because of the high efficiency of hydroturbines, which is 95% and more. Although hydropower systems have significant advantages over conventional coal and nuclear power plants, various environmental issues are restricting their growth. Hydropower can have negative ecological impacts, especially on fisheries and water ecosystems. Large scale hydropower installations can alter river ecosystems, killing fish and affecting the water quality. Various efforts are underway to design new types of hydroturbines that can allow safe passage of fishes during their use.

Problems

209

Problems 1. How does a hydroelectric plant work? 2. What are the advantages and disadvantages of hydroelectric power? 3. What are the obstacles of increasing hydropower systems in the world? 4. What is pumped storage hydropower? What is the difference between traditional pumped storage and closed loop pumped storage hydropower? 5. Is hydroelectric a renewable energy source? 6. What are some environmental benefits of hydroelectric dams? 7. What environmental problems does hydropower pose? 8. Can these environmental problems be solved? 9. What social/political problems are associated with hydropower? 10. Do any laws or regulations prevent the deployment of new hydropower? 11. What is the average cost of building a hydroelectric project? 12. What is the average cost of operating and maintaining a hydroelectric project? 13. What are the key components of a hydroelectric dam for generation of electricity? 14. Compare the efficiency of a hydroelectric power generation system with other conventional power plants. 15. Why is hydropower a renewable source of energy? 16. Explain the factors in determining the types of dam to be constructed in a particular location. 17. Where does hydroelectric power come from? 18. What are other uses of a hydropower system than electricity generation? 19. Is hydroelectric a useful source of energy? 20. What are the advantages and disadvantages of using micro hydro water wheels? 21. Explain key features of various hydroturbines and their selection. 22. Explain the importance of the net head of a hydropower system. 23. Explain the steps involve in designing a micro-head hydropower system. 24. Consider a hydropower system with gross head of 100 ft, pipeline length of 200 ft, head loss of 5 ft, and water flow rate of 200 gal per minute. Calculate the diameter of the pipe.

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25. How the flow rates and head loss are determined? 26. Discuss the economics of a small hydropower system. 27. Why is prevention of fish mortality important? 28. What are different approaches to prevent fish injuries in hydroturbines?

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Chapter 4

Geothermal Energy

Abstract Below the earth’s surface, at its center, there is a body of hot molten rock called Magma. Heat is continually produced at the center from the decay of radioactive materials trapped in Magma during formation of the earth. It is believed that this heat source is going to last for billions of years and the thermal energy can be harvested from this source on a regular basis. As a result, geothermal energy is considered a renewable energy source. The term geothermal comes from the Greek word geo, meaning earth, and thermal, meaning heat. Geothermal energy can be used for a variety of applications including electricity generation, heating buildings, and in heat pumps. In this chapter, we discussed various methods of harvesting and utilization of geothermal energy.

4.1 Introduction The interior of the earth or the earth’s core is extremely hot with a temperature of about 4;200ıC (7;600ıF). Although some of the heat is a relic of the formation of the earth some 4.5 billion years ago, the decay of radioactive materials in the core is the source of the heat. The interior consists of two layers. The inner layer is a solid iron core that is surrounded by the molten rock, called Magma. The heat from this hot inner core flows to the cooler outer crust of the earth. Although it is not possible to capture this vast amount of heat, which is estimated to be 42 1012 W, fortunately several natural geologic processes allow for some of this heat to be concentrated at temperatures and depths favorable for commercial exploitation. The energy from these heat sources is called the geothermal energy source, which is being captured and used in various applications. The temperature distribution at the interior of the earth is shown in Fig. 4.1. The outer core or the Magma is surrounded by a mantle, approximately 2,897 km (1,800 miles) thick. It is made of Magma and rocks. The outermost layer of the earth is called Crust. It is 3–5 miles (4.82–8.05 km) thick under the oceans and 15–35 miles (24.14–56.32 km) thick on the continents. The earth’s crust is made of pieces T.K. Ghosh and M.A. Prelas, Energy Resources and Systems: Volume 2: Renewable Resources, DOI 10.1007/978-94-007-1402-1 4, © Springer Science+Business Media B.V. 2011

217

218

4 Geothermal Energy

Ocean Crust

Mantle

Continental Crust (Note: Thickness of Crust is exaggerated)

930°C 1700°F

Lithosphere Asthenosphere Mesosphere

Outer Core

2760°C (5000°F) 4200°C (7600°F)

Inner Core

6371 km

5140 km 2883 km Mantle Mesosphere 350 km Asthenosphere Lithosphere

100 km

(Note: Figures represent distance from surface of Earth)

Fig. 4.1 Perspective view of earth cross section (Source: Braile [1])

of lands and oceans, called Plates. Magma is extended to the earth’s surface, near the edges of these Plates, where volcanic activities occur. The physical and chemical characteristics of the earth’s inner core are shown in Fig. 4.2. Large bodies of water are trapped in the fissures and pores of the underground rocks and are heated by the earth’s heat. The temperature of the rocks and water get hotter, progressing towards the inner core. The use of heat from this energy source can be used to heat buildings or generate electricity. Geothermal energy is considered a renewable energy source, because water is replenished by rainfall and the heat is continuously produced inside the earth. This is further explained in Sect. 4.3. Various aspects of geothermal energy are discussed in these references [2–16].

4.2

Resource Identification

219 Layering by Chemical Composition

Layering by Physical Properties Hydrosphere Liquid Asthenosphere Solid, but Ductile Lithosphere Solid & Brittle 100 km Thick

Atmosphere gas

Density 1.03 (Ocean) 2.7 (Crust) 3.3 3.6

m 0k 35

Crust 4.3

Mantle

Mesosphere Solid 0 20

Light Colored, Low Density Rock 8 - 70 km Thick

5.7 Dark Colored, High Density Rock 9.7

m 0k

Outer Core 51 5

Liquid

m 0k

14

Core Iron + Nickel

16

Inner Core

6371 km

Solid

Fig. 4.2 Structure of the earth and the origin of magmas (Source: Nelson [17])

4.2 Resource Identification The identification and quantification of geothermal resources require geological, hydrological, geophysical, and geochemical techniques that allow gathering of information regarding the potential use of specific sites. The information is necessary to determine if the site is suitable for development as a geothermal energy source. The preliminary indication of the presence of geothermal resources is given by volcanoes, hot springs, fumaroles, geysers and solfataras. Lumb [18] suggested that any geothermal exploration should address the following items: 1. Identification of geothermal phenomena. 2. Determining if a useful geothermal production field exists. 3. Determining if production wells can be drilled with the highest probability of tapping into the geothermal resource. 4. Estimating the shape, size, and depth of the resource. 5. Classifying of the geothermal field. 6. Locating of productive zones. 7. Determination of the heat content of fluids that will be discharged by the wells in the geothermal field. 8. Compilation of a body of data against which the results of future monitoring can be viewed. 9. Assessment of the exploitation values collected on environmentally sensitive parameters. 10. Determination of any characteristics that might cause problems during field development.

220

4 Geothermal Energy

Fig. 4.3 The ring of fire. United Sates Geological Survey [19]

11. Assessing the geothermal system (i.e. water or vapor-dominated). 12. Determining the homogeneity of the water supply. 13. Determining the source of recharge water. Most of the geothermal activities in the world occur in the Pacific Ocean rim, known as the Ring-of-Fire (Fig. 4.3). A number of active volcanoes exist around the ring. It also contains many high-temperature hydrothermal-convection systems. Several countries around this Ring-of-Fire are utilizing the geothermal energy. The Philippines, Indonesia, and several countries in Central America (Costa Rica, El Salvador, Guatemala, Nicaragua, and Honduras) are using geothermal energy for generating electricity. The use of geothermal energy is already contributing to the economic development of industrialized nations along the circum-Pacific Ring of Fire, such as the United States, Japan, New Zealand, and Mexico. The geothermal activities have also been found in other areas and are shown in Fig. 4.4. For the practical use of geothermal energy, large geothermal reservoirs must be found. Drilling a well and testing the temperature deep underground is the only way to ensure that a geothermal reservoir exists. However, a number of other studies prior to drilling should be performed and these include [21]: • Satellite imagery and aerial photography • Volcanological studies

4.2

Resource Identification

221

Eurasian Plate

North American Plate

60º

Eurasian Plate 45º 30º Mi

d-

E a s t Pacific Ris

Indo-Australian Plate

A tlan tic R

0º

e idg

Pacific Plate

e

African Plate

Nazca Plate

South American Plate

30º

60º

Antarctic Plate 90º

120º

150º

180º

150º

120º

High.Temperature Geothermal Provines (Schematicalty Shown)

90º

60º

30º

0º

30º

60º

90º

Fig. 4.4 Geothermal province (Adapted from Hulen and Wright [20])

• • • •

Geologic and structural mapping Geochemical surveys Geophysical surveys Temperature gradient hole drilling

Satellite images and aerial survey provide the initial indications of existence of geothermal activities. Geologic landform and rock analysis can provide further information on the presence of geothermal energy. Once the existence of a geothermal source is identified, drilling and various temperature measurements are carried out to determine the size of the source and economic feasibility. The International Heat Flow Commission has analyzed the temperature profile data of the earth at various depths and noted that there are a number of areas around the world with high geothermal activities. This data is shown in Fig. 4.5. In the USA, most of the geothermal resources are located in the western region of the country. As a result, the geothermal power plants are located in four states: California, Nevada, Utah and Hawaii. At present, Idaho and New Mexico are considering construction of several plants. However, as shown in Fig. 4.6, geothermal energy can be used by other states too for other purposes such as district heating, heat and heat-pump. An additional 23 states in the USA are using the geothermal direct heat. Since the 1980s, Europe is very active in identifying and quantifying its geothermal resources. Various regional mapping projects, such as the Geothermal Atlas of Europe were published in 1992. Another study entitled, Atlas of Geothermal Resources in Europe, was released in 2002 based on the data obtained from drilling of boreholes. The locations of main geothermal basins in Europe are shown in

222

4 Geothermal Energy

Fig. 4.5 Worldwide geothermal temperature levels (Adapted from Czisch [22])

Fig. 4.6 Estimated subterranean temperatures at a depth of 6 km (Courtesy of US Department of Energy [23])

Fig. 4.7 and the temperature distribution at 5,000 m depth is shown in Fig. 4.8. Areas of high enthalpy are located in Iceland, Italy, Greece, parts of France, Germany and Austria. Countries such as Ireland, Norway, Sweden, UK, and Poland contain low enthalpy regions and may not be economical for geothermal development.

4.3

Geothermal Systems

223

Fig. 4.7 Major geothermal basins in Europe (Courtesy of Antics and Sanner [24])

4.3 Geothermal Systems A geothermal system is consists of three major elements: a heat source, a reservoir and a fluid, as schematically shown in Fig. 4.9. The heat source can be from the Magma having a temperature greater than 600ı C at depths of 5–10 km. Additionally, a heat source can also be low-temperature systems using the earth’s normal temperature, which increases with depth. The reservoir that is naturally formed by hot permeable rocks can heat a circulating fluid, which generally is the water. However, water loss occurs when it escapes from the reservoir through springs or extracted by boreholes. Therefore, the water must be replenished, if the meteoric water is not sufficient. The heat is transferred to the fluid mainly by convection. The hot water rises upward due to its lower density and is replaced by cold dense water. The mechanism is shown in Fig. 4.10. If the geothermal fluid is used for steam generation or direct heating, it must be replenished continuously to maintain the hydrostatic pressure

224

4 Geothermal Energy

Fig. 4.8 Temperature distribution at a depth of 5,000 m in Europe (Source: Shell International as reported by Antics and Sanner [24])

and the fluid mass. One practice is to inject it back into the reservoir through specific injection wells. This also reduces the impact on the environment from operation of geothermal plants.

4.4 Applications The use of the geothermal energy depends on the temperature of the resources. For electricity generation, the temperature of the resources must be above 150ıC. If the temperature of geothermal resources is below 150ı C, it can be still used for a number of other applications. The main applications of geothermal energy may be divided into following categories: • • • •

Electricity generation Direct district heating Heat pump Industrial applications

Lindal [26] suggested a number of uses of geothermal energy based on the temperature of the resources. Various uses of the geothermal energy corresponding to a particular temperature were illustrated in a diagram, which is currently known as the Lindal diagram. The original Lindal Diagram has been modified by a number

4.4

Applications

225

Recharge area

Hot spring or steam vent

Geothermal well Impermeable caprock (thermal conduction)

Cold Meteoric waters

Hot fluidsi

Reservoir (thermal convection)

Flow of heat (conduction)

ble rock Impermea nduction) co al m er (th

Magmatic intrusion

Fig. 4.9 Schematic representation of a geothermal system (Courtesy of Dickson and Fanelli [2])

Fig. 4.10 Model of a geothermal system. Curve 1 is the reference curve for the boiling point of pure water. Curve 2 shows the temperature profile along a typical circulation route from recharge at point A to discharge at point E (From White, 1973). White D E (1973). Characteristics of geothermal resources. In: Kruger P and Otte C (Eds.) Geothermal energy, Stanford University Press, Stanford, pp. 69–94 (Courtesy of Dickson and Fanelli [25])

226

4 Geothermal Energy

Fig. 4.11 Various uses of geothermal energy based on temperature of the geo-resources (Printed with permission from Hunter and Schellschmidt [27])

of researchers as more applications of the geothermal energy were found. These applications are summarized in Fig. 4.11. From this figure, the minimum useful temperature for any application should be above 20ı C. The Lindal diagram is now widely used in the geothermal community to depict temperature as the yardstick of applications. In the USA, few areas have temperatures above 150ıC. As can be seen from Fig. 4.12, most of the areas have temperatures below 20ı C; however, these areas are good for geothermal heat pumps, which are discussed later in this chapter. Most of the western states have temperatures greater than 150ıC and are suitable for both district heating and electrical power generation. In Europe, Iceland is the leader in utilization of the geothermal energy. It is further discussed in the following section. The Paris area in France has the largest geothermal district heating systems. Other countries including Austria, Germany,

4.4

Applications

227

Fig. 4.12 US geothermal provinces used for various activities suggested in Fig. 4.11 (Adapted from Hulen and Wright [20])

Hungary, Italy, Poland, and Slovakia are developing a substantial number of geothermal district heating systems. Sweden, Switzerland, Germany and Austria are the leading countries in terms of market for geothermal heat pumps in Europe.

4.4.1 Electricity Generation Worldwide about 60,877 million kilowatt-hours of electricity were produced in 2007 from 9,968 MW of installed geothermal power plants. It is estimated that the average capacity factor of geothermal power plants is around 73%. At present 24 countries are utilizing the geothermal energy. The installed electricity generation capacity of top 20 countries is shown in Fig. 4.13. However, as can be noted from Table 4.1, the growth of geothermal energy in these top 20 countries is rather mixed. The installed capacity of these countries as of 2007, and their percent share of the world total electricity generation from the geothermal energy is given in Table 4.2. Potentially another 22 countries are expected to start using geothermal energy for electricity production by 2010. These new countries include Armenia, Canada, Chile, Djibouti, Dominica, Greece, Honduras, Hungary, India, Iran, Korea, Nevis, Rwanda, Slovakia, Solomon Islands, St. Lucia, Switzerland, Taiwan, Tanzania, Uganda, Vietnam, and Yemen.

228

4 Geothermal Energy

3500

3.09 103

4000

2500

1.5

France

29

Portugal

24

52

Guatemala

6.6

56

Papua New Guine

Germany

82

China

82

166

Costa Rica

Russia

167

Kenya

Turkey

204

EI salvador

88

575

536

Japan

Italy

Mexico

Indonesia

Philippines

0

United states

500

Nicaragua

628

Iceland

1000

New Zealand

1500

958

1.18 103

2000

843

1.9 103

Megawatt (MW)

3000

Fig. 4.13 Installed geothermal electricity generation capacity of various countries around the world. In 2010 (Source of data: Holm [28])

In Europe, Italy is expected to nearly double its installed capacity by 2020. Germany is planning 150 new plants with most of the activities centered in Bavaria. In Asia, the Philippines is also planning to increase its installed geothermal capacity from about 2,000 MW to 3,130 MW. Indonesia is planning for 6,870 MW of new geothermal capacity to be developed over the next 10 years. The geothermal development potential of the Great Rift Valley in Africa is enormous. Kenya has announced a plan to install about 1,700 MW of new geothermal capacity within 10 years. The total projected growth of the installed capacity in the world is shown in Fig. 4.14. Electrical Power Generation in the USA As shown in Fig. 4.13, the USA has the highest installed capacity in the world. As of March 2008, geothermal electric power generation is occurring in eight U.S. states: Alaska, California, Hawaii, Idaho, Nevada, New Mexico, Utah and Wyoming. The installed capacities of these states are given in Fig. 4.15. The highest installed capacity is in California, providing about 4.5% of California’s electric energy generation in 2007, amounting to a net-total of 13,439 GWh. Hawaii has one power plant operating in the big island of Hawaii, called the Puna Geothermal Venture (PGV). PGV delivers an average of 25–35 MW on a continuous basis, supplying approximately 20% of the total electricity needs of the Big Island.

4.4

Applications

229

Table 4.1 A comparison of installed geothermal power production capacity between 2007 and 2010 Installed capacity Installed capacity Country in 2010, MW in 2007, MW % change United States 3;087 2923:5 5:59 Philippines 1;904 1969:7 3:33 Indonesia 1;179 992:0 18:85 Mexico 958 953:0 0:52 Italy 843 810:5 4:00 New Zealand 628 471:6 33:16 Iceland 575 421:2 36:51 Japan 536 535:2 0:15 El Salvador 204 204:2 0:09 Kenya 167 128:8 29:65 Costa Rica 166 162:5 2:15 Nicaragua 88 87:4 0:68 Russia 82 79:0 3:79 Turkey 82 38:0 115:79 Papua New Guinea 56 56:0 0:00 Guatemala 52 53:0 1:88 Portugal 29 23:0 26:08 China 24 27:8 13:66 Germany 6:6 8:4 21:42 France 1:5 14:7 89:79 Former USSR with total potential of 768–1,902 MW and Yugoslavia with a potential of 50–100 MW The GEA 2007 report considered France and Guadeloupe as one entity. Also development interest identified in six countries is not identified in 2010 – Korea, Solomon Islands, St Lucia, Tanzania, Uganda and Vietnam. Guadeloupe’s 15 MW installed capacity is included in the 2010 IGA report as part of France’s total installed capacity. Mainland France has 1.5 MW installed geothermal capacity in 2010 2007 source: Installed capacity from Ruggero Bertani, “World Geothermal Generation in 2007,” GHC Bulletin, September 2007, p. 9; United States from Geothermal Energy Association, Update on US Geothermal Power Production and Development (Washington, DC: 16 January 2008); capacity factor from Ingvar B. Fridleifsson et al., “The Possible Role and Contribution of Geothermal Energy to the Mitigation of Climate Change,” in Hohmeyer and Trittin [29]

According to the U.S. Bureau of Land Management, the Western states could generate 5,500 MW of geothermal energy from 110 plants by 2015, and projected to rise by another 6,600 MW by 2025. Geothermal resources that can be used to generate electricity may be divided into the following four categories: • • • •

Hydrothermal fluids Geopressurized brines Enhanced (Engineered) geothermal systems or Hot dry rock systems Magma

230

4 Geothermal Energy

Table 4.2 Contribution to total electricity generating capacity by geothermal sources Electricity generated by geothermal energy in 2009, TWh/year 16:60 10:31 9:60 7:04 5:52 4:99 4:05 3:06 1:43 1:42 1:13 0:49 0:45 0:44 0:31

Country United States Philippines Indonesia Mexico Italy Iceland New Zealand Japan Kenya El salvador Costa Rica Turkey Papua New Guinea Russia Nicaragua

Total electricity generated from all sources in 2009 TWh/year 4149:6 60:6 151:7 258:0 289:2 16:8 43:5 1115:1 8:4 6:5 8:7 194:1 2:9 993:1 3:3

% of electricity from geothermal source 0:4 17:0 6:3 2:7 1:9 29:7 9:3 0:2 16:9 22:0 12:9 0:2 15:5 0:0 9:4

[Source: International Energy Agencyand BP Statistical Review of World Energy [30, 31]]

15000.0

Installed Capacity (MW)

Geothermal Energy Association Data

10000.0

5000.0

International Geothermal Association Data 0.0 1970

1980

1990

2000

2010

Year Fig. 4.14 Projected trend of the use of geothermal energy (Adapted from Gawell and Greenberg [33])

Of these four resources, only hydrothermal fluids have been developed commercially for power generation. Geopressurized reservoirs containing brine are generally saturated with natural gas and are under high pressure. The extraction of this fluid is technologically challenging. In the dry rock, the typical thermal

4.4

Applications

231

3500.0 3152.7 3000.0

2000.0 1500.0

New Mexico 0.24

Wyoming 0.25

Alaska 0.73

Idaho 15.8

Hawaii 35.0

California

0.0

Total Capacity

500.0

Utah 47.0

448.4

1000.0

Nevada

Installed Capacity (MW)

2605.3 2500.0

Fig. 4.15 Installed electricity generation capacity of various states in the USA as of August 2009 (Adapted from Geothermal Energy Association [32])

gradient is 30ı C=km. To attend a temperature of 190ı C, holes must be drilled to a depth of about 6,096 m (20,000 ft). The potential of this resource is enormous, but so far an economically feasible technology is not available to extract this energy in a commercially useable way. The temperature gradient in hot dry rock is little higher, 40ı C=km. Although the temperature of molten Magma is above 2;000ıC, there is no technology to take advantage of this energy. The ambient ground heat is generally too low for most of the applications. 4.4.1.1 Hydrothermal Fluid Three methods can be used for the generation of electricity from hot hydrothermal fluids [34–43]. Methods suitable for the electricity generation depend on the state of the fluid (whether it is steam or hot water) as determined by its temperature. These methods are called: dry steam, flash, and binary cycle hydrothermal systems. Dry Steam Power Plants If the geothermal energy is available in the form of steam, it can be used directly to run a conventional steam turbine. Therefore, fossil fuels and boilers that are essential for conventional power plants are not necessary. The dry steam geothermal system

232

4 Geothermal Energy

Fig. 4.16 Dry steam power plant (Courtesy of Idaho National Laboratory [44])

is the oldest type of geothermal power plant. It was first used at Lardarello in Italy in 1904, and is still in operation. The Geysers in northern California, the world’s largest single source of geothermal power, also uses the same technology to generate electricity. These plants emit excess steam and very small amounts of other gases to the atmosphere. A schematic diagram of a dry steam power plant is shown in Fig. 4.16. Flash Steam Hydrothermal Power Plants Flash steam power plants use the hydrothermal fluid, which is primarily water, for electricity generation. Water is available at temperatures above 200ıC and at a high pressure. The basic arrangement of this system is shown in Fig. 4.17. Water is sprayed into a flash-tank that operates at a lower pressure than the inlet water, causing some of the fluid to rapidly vaporize, or flash, to steam. The steam is used to drive a turbine and a generator. Depending on the temperature of the water collected in the first flash-tank, a second tank may be used to further generate steam. However, this depends on the temperature and pressure of the steam and economics of the process. Binary Cycle Hydrothermal Power Plant If the water temperature is less than 200ıC, a binary cycle method may be most suitable and cost effective for the generation of electricity. In this method

4.4

Applications

233

Fig. 4.17 Flash steam production (Courtesy of Idaho National Laboratory [44])

Fig. 4.18 Binary plant (Courtesy of Idaho National Laboratory [44])

(See Fig. 4.18), the geothermal energy is used to vaporize another working fluid, which then drives a turbine and a generator. Generally, hydrocarbons are preferred as the working fluid. A 1;000-kW binary cycle geothermal power plant with isobutane as the working medium was successfully run for the first time at Otake, Kyushu,

234

4 Geothermal Energy

Japan, in 1978. The working liquid mostly used in U.S. Geothermal power plants is isopentane, which vaporizes at a lower temperature and higher pressure than water. In a closed loop cycle, the vapor produced from the binary liquid drives the turbine-generator unit, and then it is condensed back to liquid before being reused in the heat exchanger. After a portion of the heat is used from the geothermal water, it exits the binary plant and is injected back into the reservoir. Exergy/AmeriCulture, USA, has designed and constructed a binary cycle geothermal plant near Cotton City, New Mexico, that has a gross electricity generation capacity of 1,420 kW (approximately 1,000 kW net). An ammonia-water working fluid is used in a Kalina cycle. The Kalina cycle boosted the geothermal plant efficiency by 20–40% and reduced plant construction costs by 20–30%, thereby lowering the cost of geothermal power generation. The Kalina cycle is discussed in Chap. 3 of Volume 1 of this book series. Maghiar and Antal [45] discussed a binary power plant at the University of Oradea, Romania that used low enthalpy geothermal source for power generation. The working fluid used at this binary power plant was carbon dioxide. There are some advantages of using CO2 , such as no explosion danger, it is non-flammable and non-toxic and is available at low cost. Moderate hot water plants, using high- or moderate-temperature geothermal fluids, have been developed recently. Hot water resources are much more common than steam. Hot water plants are now the major source of geothermal power in both the United States and the world. In the United States, hot water plants are operating in California, Hawaii, Nevada, and Utah. 4.4.1.2 Geopressurized Brines Geopressurized reservoirs exist at depths of 3,000–6,000 m (10,000–20,000ft) below the earth’s surface that contain brine and dissolved methane. These type of reservoirs is located throughout the world. In the USA, the best known geopressurized reservoirs are along the Texas and Louisiana Gulf Coast. The brine is typically saturated with methane, containing in the range of 30–80 cft of methane per barrel of fluid. These brines are hot with temperature in the range of 149–204ıC .300–400ıF/. The geopressurized brine can provide: (1) thermal energy from the temperature of the fluid; (2) mechanical energy from the fluid pressure; and (3) chemical energy from the methane that is dissolved within the fluid. The operation of geothermal power plants using geopressurized brine has been discussed by a number of researchers [46–53]. The US Department of Energy [54] has evaluated several reservoirs along the Texas and Louisiana Gulf Coast and noted that recoverable methane gas varied from 22 to 28 standard cubic feet per barrel and brine content was 70,000–131,000ppm. The analyses from test wells are provided in Table 4.3. Goldsberry [54] also provided a conceptual schematic diagram of a geopressurized geothermal system for both recovery of methane gas and electricity generation. This diagram is shown in Fig. 4.19.

4.4

Applications

235

Table 4.3 Test results geopressured geothermal well

Well location Pleasant Bayou No-2, TX Amoco Fee No-1, LA L. R. Sweezy No-1, LA Gladys McCall No-1, LA

Production depth (ft)

Interval thickness Temperature Pressure Permeability (ft) (ı F) (psia) (MD)

Recoverable Brine gas content TDS (Scf/bbl) (ppm)

14,700

60

309

11,050

157

22

131,000

15,400

254

310

12,000

296

22

128,000

13,400

49

235

12,500

1,250

23

103,000

14,412

1,072

288

13,000

170

28

70,000

[Source: Goldsberry [54]]

Fig. 4.19 A schematic diagram of a geopressurized system for methane recovery and power generation (Adapted from Goldsberry [54])

A commercial plant of 10 MW using geopressurized brine from the Salton Sea geothermal field in southern California, USA, was developed by Unocal Corporation in 1989. Two more units were added to boost the total power production to 47.5 MWe [55]. A schematic diagram of the Unocal system is shown in Fig. 4.20. The brine from these reservoirs is extremely corrosive, and scale formation in various parts of the plant must be avoided for commercial development. The brine can also contain traces of oxidizing metals enhancing the corrosion of the materials. Therefore, the brine chemistry should be properly understood and implemented for economical operation of power plants [57]. Corrosion-resistant alloys and cement linings have allowed reduction of corrosions of the system. Various measures are suggested by researchers to address the prevention of corrosion, scale formation, fouling, and minimization of environmental effects [58–76].

236

4 Geothermal Energy Production

Cyclone Separator Steam

Power Plant

Cyclone Scrubber

Steam Turbine

Electrical Generator

Transformer

Electrical Power to Grid

Cooling Tower

Condenser

Pump Pump Brine

Pump

Pump Condensate

Flashing to Steam-Brine in Borehole Reservoir

Brine Carry Over

Brine Injection Well

Condensate Injection Well

Fig. 4.20 Use of geopressurized brine for electricity production (Printed with permission from Gallup [56])

4.4.1.3 Enhanced (Engineered) Geothermal Systems A significant amount of heat is stored in hot rocks underneath the earth’s surface. This heat can be harvested through Enhanced or Engineered Geothermal systems (EGS). EGS are engineered reservoirs below the earth’s surface to extract energy from engineered geothermal resources (mainly from hot rocks) [77–91]. These resources are otherwise not economical due to the lack of a body of water and/or permeability through the rock formation. The USDOE estimates that the application of EGS technology is capable of providing at least 100,000 MW of electricity within the next 50 years. The technology to mine the heat from the hot rocks is available, but only up to a certain depth beneath the surface of the earth. Los Alamos National Laboratory, New Mexico, USA developed a conceptual design of the hot dry rock mining technology. As shown in the Fig. 4.21, water is pumped into hot, crystalline rock via injection wells, becomes superheated as it flows through open joints in the hot rock reservoir, and is returned through production wells. At the surface, the heat is extracted by conventional processes. Any unused water or the remaining water is recirculated back to the reservoir for reuse. The amount of thermal energy available at 10 km depth in the USA has been estimated by various agencies. The results from these studies are summarized in Table 4.4. Although the amount of heat that is projected to be extractable using EGS is enormous, the recoverable fraction from any underground resource is inherently speculative. Areas within national parks and monuments, and other recreational areas

4.4

Applications

237 injection pump

makeup water

Power Plant Sediments and/or Volcanics

Injection Well Production Well

Low Permeability Crystalline Basement Rocks

3-10km Depth

10,000-30,000ft Depth

Fig. 4.21 Schematic of a conceptual two-well enhanced geothermal system in hot rock in a low-permeability crystalline basement formation (Adapted from Renewable Energy and Power Department [89]) Table 4.4 Estimated U.S. geothermal resource base to 10 km depth by category

Category of resource Conduction-dominated EGS Sedimentary rock formations Crystalline basement rock formations Supercritical Volcanic EGSa Hydrothermal Coproduced fluids Geopressured systems

Thermal energy, in exajoules (1 EJ D 1; 018 J)

Reference

100,000

This study

13,300,000

This study

74,100

USGS Circular 790

2,400–9,600 0.0944–0.4510 71,000–170,000b

USGS Circulars 726 and 790 McKenna et al. (2005) USGS Circulars 726 and 790

[Source: Renewable Energy and Power Department [89]] a Excludes Yellowstone National Park and Hawaii b Includes methane content.

would permanently be excluded from development. When all these constrained are taken into account, the total thermal energy available from the EGS is shown in Fig. 4.22.

4 Geothermal Energy

Resource Base

238

Stored Thermal Energy in Place (3 to 10 km) 14 × 106 EJ

Estimated Recoverable EGS Resource

40% Upper Limit 5.6 × 106 EJ

20% Midrange 2.8 × 106 EJ

2% Conservative 2.8 × 105 EJ 0.0

5.0

10.0

15.0

Thermal Energy (106 EJ)

Fig. 4.22 Estimated total geothermal resource base in the USA and its recoverable amount in EJ .1018 J/ (Renewable Energy and Power Department [89])

4.4.1.4 Magma Magma is potentially a huge energy source [92–96]. According to Dunn et al. [94], in the USA an estimated 50,000–500,000 quads of energy is contained in magma at temperatures above 600ı C at depths shallower than 10 km. The greatest challenge with magma system is the extraction of energy. An open heat exchanger can be formed for the extraction of heat by solidifying magma around a cooled borehole and the resulting mass will be extensively fractured by thermally-induced stresses (See Fig. 4.23). However, the construction materials must be chosen carefully. Westrich [95] indicates that consideration of corrosion resistance, high-temperature strength, and cost suggest that Ni-base superalloys offer the most promise for use as construction material of a heat exchanger in rhyolite magma.

4.4.2 Direct Use of Geothermal Energy Direct uses of geothermal energy involve the use of hot waters from geothermal resources directly for bathing and cooking, heating of homes and buildings (better known as district heating), heating of greenhouses for growing vegetables and flowers, fish farming (aquaculture), drying of foods and lumber, and the use of heat pumps [97–104]. Spent fluids from geothermal electric plants still contain a significant amount of heat and can be subsequently used for direct use applications in a so-called “cascaded” operation. As shown in Fig. 4.24, in Europe, most of

4.4

Applications

239

Fig. 4.23 Conceptual representation of open heat exchanger with fluid flow through fractured, solidified magma (Courtesy of Dunn [94])

Casing

Overburden Transition Zone (Plastic)

Solidified Fracture Region

Convecting Magma

Plastic Region Tubing Injection Flow

Convecting Magma

Fluid Flow Through Fractures

Plastic Region Open Heat Exchanger Injection Tubing

the geothermal energy is used for direct heating. In the North, Central, and South America, the use of geothermal energy for electricity production is almost double of that used in direct use applications. In other continents, the geothermal energy is used evenly for direct use and electricity production. According to the US Department of Energy’s Energy Efficiency and Renewable Energy office, the direct use of geothermal energy in homes and commercial operations is significantly less expensive than using traditional fuels. Savings can be as much as 80% over fossil fuels. In the USA, there are approximately 120 facilities, each using hundreds of individual systems. In an individual facility, geothermal energy is used for district heating and space heating. There are also 38 greenhouse complexes, 28 aquaculture

240

4 Geothermal Energy

Fig. 4.24 Global distribution of geothermal production (Adapted from Oldmeadow [105])

Fig. 4.25 Direct use of geothermal energy in the US (Source of data: Geothermal Energy [20])

operations, 12 industrial plants, and more than 218 spas that use geothermal hot waters to provide heat. However, as shown in Fig. 4.25, percentage wise, aquaculture facilities utilizes the maximum amount of geothermal energy. The district heating systems are extremely reliable [106–123]. One such system in Boise, Idaho has been operating since the 1890s, and continues to provide

4.4

Applications

241 User Application Peaking/Backup Unit Heat Exchanger 5°C)

170°F (7

180°F (80°C)

130°F

(55°C)

TO Injection Well From Production Well

140°F (60°C)

Geothermal Water Working Fluid

Geothermal Reservoir

Fig. 4.26 Geothermal direct-use for direct heating (Adapted from US Department of Energy [124])

heating needs today. Another successful implementation of geothermal district heating is in Philips, South Dakota, USA. Direct-use systems typically include three components: • A production facility – usually a well to bring the hot water to the surface. • A mechanical system – piping, heat exchanger, and controls to deliver the heat to the space or processes. • A disposal system – injection well or storage pond to receive the cooled geothermal fluid. A schematic diagram of a direct-use system is shown in Fig. 4.26. The city Philips installed a district heating system to provide all of the energy necessary for heating purposes. A 1,300 m (4,266 ft) deep well, drilled in 1980, provides a maximum artesian flow of 0:0214 m3=s (340 gpm) at 69:4ı C .157ı F/. The geothermal fluid is first used by two schools located next to each other. The fluid, which is at around 60ı C (140ıF), is transported from the schools in a single pipe and is circulated through the downtown area providing heat to commercial and residential buildings. The schools and the fire station house the control points for the system. The geothermal fluid is discharged to the river after removing radium. Different types of heat exchange systems are used in different buildings. The buildings connected to the system used either Modine heaters, unit heaters, or piping in the floor. The bank building uses plate heat exchangers to isolate the geothermal fluid. This direct heating system in Philips, South Dakota, USA is shown in Fig. 4.27. According to Energy Efficiency and Renewable Energy, US Department of Energy, geothermal district heating systems in the USA can save consumers 30–50% of the cost of natural gas heating. The tremendous potential for district heating in the western U.S. was illustrated in a 1980s inventory which identified 1,277 geothermal sites within 5 miles of 373 cities in eight states.

Fig. 4.27 A schematic diagram of the Philip, South Dakota, district heating system using geothermal energy (Courtesy of Geo-Heat Center [126])

242 4 Geothermal Energy

4.4

Applications

243

Thorsteinsson [125] collected the data for US geothermal district heating systems that utilize a geothermal resource as a heat source and distributes heat through a distribution network to five or more buildings. His data are given in Table 4.5 along with their location, start up year, number of customers, capacity, annual energy use and temperature of the system. Although direct heat from a geothermal source can be used in a variety of applications, as shown in Fig. 4.28a,b, the main worldwide uses primarily involve heat pumps, balneology, and space heating. As shown in Fig. 4.29, a number of countries in Europe are using geothermal energy for district heating. Among these countries, Iceland is the leader, not only in Europe, but also in the world in utilization of the geothermal energy. In Iceland, most of the homes and buildings are connected to geothermal district-heating systems. Some district heating and cooling data for Iceland is given in Table 4.6. Direct heating was initiated in Iceland 1930s. At present, it serves about 99% of Reykjavik or about 190,000 people. Iceland utilizes about 62 geothermal wells and uses large storage tanks to meet the peak-load demands. As back-up, Iceland has several oil-fired stations. As shown in Fig. 4.30, several geothermal fields are interconnected with the city’s heating and cooling systems to provide a better reliable system year around. Iceland has both low and high enthalpy sites. Low-temperature fields that are located in the vicinity of Reykjavik, produce water at a temperature of below 150ıC. The hot water from these fields are used directly for space heating and washing. Iceland’s high-temperature fields are only found on the active volcanic rift zone that runs across the country, and yield water at temperatures in excess of 200ıC. However, these fields are rich in gases and minerals, and hot water can not be used directly in the distribution system. Its high pressure and high thermal energy, however, make it well suited to heating fresh cold water, which then can be used for space heating, and also for generation of electricity. The percent of the total geothermal energy used by various sectors in Iceland is shown in Fig. 4.31 and the layout of the district heating system is given in Fig. 4.32. Paris basin in France is utilized to heat many homes by bringing thermal water to the surface. Geothermal greenhouses are prominent in Italy and in the western U.S. As of early 2000s, the percentage of district heated houses in various countries in Europe is given in Table 4.7. Geothermal district heating represents about 35% of the European installed geothermal systems that are dedicated to direct uses, totaling about 5,000 MWt. Major geothermal district heating sites (over 35 exceeding 5 MWt capacity) are shown in Fig. 4.29.

4.4.3 Ambient Ground Heat/Geothermal Heat Pump Geothermal Heat Pumps (GHP) are extremely efficient for home heating and airconditioning. Heat pumps are electrical devices that transfer heat from a cool space

244

4 Geothermal Energy

Table 4.5 U.S. geothermal district heating systems 2007

System Susanville District Heating San Bernardino District Heating I’SOT District Heating System (Canby) Pagosa Springs District Heating Boise City Geothermal District Heating Fort Boise Veteran’s Hospital (Boise) Idaho Capital Mall (Boise) Warm Springs Water District (Boise) College of Southern Idaho (Twin Falls) Kanaka Rapids Ranch (north of Buhl) Gila Hot Springs New Mexico State University (Las Cruces) Warren Estates (Reno) Manzanita Estates (Reno) Elko County School District Elko District Heat City of Klamath Falls District Heating Oregon Institute of Technology (Klamath Falls) Lakeview Midland District Heating Philip District Heating Bluffdale

Number of customers 7

Capacity, MWt 5:60

Annual energy, GWh/year 3:4

System temp. (ı F) 168

22

128

State CA

Start up year 1982

CA

1984

77

12:80

CA

2003

1

0:50

1:2

185

CO

1982

22

5:10

4:8

146

ID

1983

58

31:20

19:4

170

ID

1988

1

1:80

3:5

161

ID

1982

1

3:30

18:7

150

ID

1892

275

3:60

8:8

175

ID

1980

1

6:34c

ID

1989

42

1:10c;d

NM NM

1987 1982

<15a 1

NV NV

1983 1986

NV

14

100

2:4

98

0:30 2:70

0:9 10:5

140 143

110b

1:10 3:60

2:3 21:2

204 204

1986

4

4:30

4:6

190

NV OR

1982 1984

18 20

3:80 4:7

6:5 10:3

176 210

OR

1964

1

6:20

13:7

192

OR SD

2005 1969

1 12

2:44 0:09

3:8 0:2

206 152

SD UT

1980 2003

7 1

2:50 1:98

5:2 4:3

151 175

CA California, CO Colorado, ID Idaho, NM New Mexico, NV Nevada, OR Oregon, SD South Dakota, UT Utah [127] a There are 15 buildings on the system, the number of customers is probably a little smaller b The combined number of customers for the Warren and Manzanita Estates c Only includes geothermal capacity of the system and ignores capacity added by heat pumps d Assumes a T of 10ı F

4.4

Applications

245

a

Others 0.3%

Cooling / snowmelting 1.3%

Balneology 19.1%

Industrial uses 1.7% Agriculture 0.6% Aquaculture 2.2% Geothermal heat pumps 54.4%

Greenhouse heating 5.0%

Space heating 15.4%

b

Others 0.4%

Balneology 30.4%

Cooling / snowmelting 0.6% Industrial uses 4.0% Agriculture 0.7% Aquaculture 4.0% Greenhouse heating 7.6%

Geothermal heat pumps 32.0 %

Space heating 20.2%

Fig. 4.28 Geothermal direct applications worldwide in 2005, distributed by percentage of total installed capacity (a) and percentage of total energy use (b) (Printed with permission from Lund et al. [103])

into a warm space, making the cool space cooler and the warm space warmer. During the heating season (i.e., during winter), heat pumps move heat from the cool outdoors into the house making it warm. The process is reversed during cooling season (i.e., during summer). During the summer months, heat pumps move heat from the house into the warm outdoors. The basic operation of a heat pump in heating and cooling mode is shown in Figs. 4.33 and 4.34, respectively. Because they move heat rather than generate heat, heat pumps can provide up to four times

246

4 Geothermal Energy

Fig. 4.29 Main geothermal district heating sites in Europe (Courtesy of European Geothermal Energy Council [128])

the amount of energy than is consumed. GHPs reduce electricity use by 30–60% compared with traditional electrical heating and cooling systems. GHP systems can be used for a variety of applications, including heating and cooling of homes, schools and other buildings, as well as for commercial and industrial heating and refrigeration [131–188]. Geothermal heat pumps are making a big impact on energy efficiency in the U.S. and Europe.

4.4

Applications

Table 4.6 District heating (DH) system statistics for Iceland

247

Number of DH utilities Total installed DH capacity Total length of DH pipeline system Annual turnover in the DH sector Number of households connected District heated floor space New connections to DH Heat sales volume for new connections to DH Total district heat delivered

22 2,012 MWth 6,738 km 9 bln. ISK 314,000 13,300,000 m2 283,038 m2 600 GJ 24,516 TJ

Krafla

Reykjavik

Reykjanes Svartsengi Nesjavellir Hellisheioi

High temperature field Low temperature field

Bedrock < 0,8 M. years 0,8 - 3,3 M. years 3,3 - 15 M. years

Fig. 4.30 Volcanic zones and geothermal areas in Iceland (Courtesy of National Energy Authority and Ministries of Industry and Commerce [129])

Approximately 50,000 geothermal heat pumps are installed annually in the United States. The life time of the heat pump components is about 25 years, but the ground loop can be functional up to 50 years. An estimate of the installed GHP units in Canada was 35,000 in 2004 and 37,000 in 2005. A recent estimate of Canadian installations of GHPs is believed to be about 10,000 units annually. The number of heat pumps shipped by the model types within the USA between 1999 and 2008 are given in Table 4.8. A comparison between the use of heat pumps and direct use of the geothermal energy in the USA is given in Table 4.9. Ground-source heat pumps (GSHP) play a key role in the development of the geothermal energy in Central and Northern Europe. Europe has seen significant growth in the GSHP industry in the past 10 years. GSHPs represent 25% of all heat pumps sold in Europe. A combination of a heat pump and a water heater unit

248

4 Geothermal Energy

Industry 4.7%

Swimming pools 3.7%

Snow melting 5.4%

Electricity generation 15.9%

Fish farming 10.4%

Greenhouses 2.6% Space heating 57.4%

Fig. 4.31 Utilization of geothermal energy by various sectors in Iceland in 2005 (Courtesy of National Energy Authority and Ministries of Industry and Commerce [129])

Oil fired peak power station 100 MW(thermal)

Deaerator

Snow melting

80°C

Drain

85-90°C Pumping station Deep well pumps

Heating

Storage Tanks

90°C

Mixing

Reykir geothermal field (1700 kg/s) Ellidaar geothermal field (220 kg/s)

35°C

80°C

90°C

80°C 127°C

Heating

Laugarnes geothermal field (330 kg/s) 80°C

35°C Drain

35°C

83°C 200 MW (thermal)

Drain

Heating

127°C

Nesjavellir

35°C

Drain Heating

Drain Geothermal wells

Cold water wells

Fig. 4.32 District heating in Reykjavik, Iceland using geothermal energy (Courtesy of Lund [130])

4.4

Applications

Table 4.7 The percentages of houses heated by the geothermal energy in various countries in Europe

249

Country Iceland Poland Sweden Finland Austria Netherlands Estonia Denmark Slovakia Hungary Germany UK

% of houses using geothermal energy for heating 95 52 50 49 12:5 3 52 51 40 16 12 1

[Source: European Geothermal Energy Council 2009 [128]]

Fig. 4.33 Sketch of a heat pump in heating mode (Courtesy of the Geo-Heat Center [126])

has been deployed by many European countries such as Austria, Finland, Germany, and France. The total sales volume of the heat pump system was about 520,000 pieces in 2009, which increased by about 110% when compared to that in 2005 (See Fig. 4.35).

250

4 Geothermal Energy HEAT EXCHANGER REFRIGERANT / AIR (EVAPORATOR) COOL SUPPLY AIR TO CONDITIONED SPACE

WARM RETURN AIR FROM CONDITIONED SPACE

EXPANSION VALVE DOMESTIC HOT WATER EXCHANGER (DESUPERHEATER)

REFRIGERANT REVERSING VALVE HEAT EXCHANGER REFRIGERANT / WATER (CONDENSER)

IN OUT DOMESTIC WATER REFRIGERANT COMPRESSOR

TO / FROM GROUND HEAT EXCHANGER (GEOTHERMAL)

Fig. 4.34 Heat pump in cooling mode (Courtesy of the Geo-Heat Center [126]) Table 4.8 Geothermal heat pump shipments by model type, 1999–2008 (number of units) Model type Year 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008

ARI-320 7,910 7,808 NA 6,445 10,306 9,130 9,411 10,968 8,112 23,204

ARI-325/330 31,631 26,219 NA 26,802 25,211 31,855 34,861 47,440 66,863 91,402

ARI-870 – – NA – – – – – 809 783

Other non-ARI rated 2,138 1,554 NA 3,892 922 2,821 3,558 5,274 10,612 5,854

Total 41,679 35,581 NA 37,139 36,439 43,806 47,830 63,682 86,396 121,243

[Source: Energy Information Administration, Form EIA-902, “annual geothermal heat pump manufacturers survey”] [184] NA not available. No survey was conducted for 2001, – no data reported

The implementation of the Renewable Energy Sources (RES) directives in the member states of Europe will further enhance the market because of the heat pump’s contribution to the renewable energy portfolio. Heat pumps can be a significant source of the renewable energy portfolio of a country. European Heat

4.4

Applications

251

Table 4.9 Geothermal energy consumption by direct use of energy and from heat pumps, 1990–2008 (Quadrillion Btu)

Year 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008

Direct use 0.0048 0.0050 0.0051 0.0053 0.0056 0.0058 0.0059 0.0061 0.0063 0.0079 0.0084 0.0090 0.0090 0.0086 0.0086 0.0088 0.0091 0.0094 0.0097

Heat pumps 0.0054 0.0060 0.0067 0.0072 0.0076 0.0083 0.0093 0.0101 0.0115 0.0114 0.0122 0.0135 0.0147 0.0188 0.0212 0.0240 0.0276 0.0317 0.0365

Total 0.0102 0.0110 0.0118 0.0125 0.0132 0.0141 0.0152 0.0162 0.0178 0.0193 0.0206 0.0225 0.0237 0.0274 0.0298 0.0328 0.0367 0.0411 0.0462

Source: Lund [182]

160000 Total 2005

140000

Total 2006 Total 2007

Number of Unit Sold

120000

Total 2008 Total 2009

100000 80000 60000 40000 20000

UK

Switzerland

Sweden

Norway

Italy

Germany

France

Finland

Austria

0

Fig. 4.35 Growth in heat pump sales in some selected countries in Europe (Source: European Heat Pump Association) [183]

4 Geothermal Energy

8 7 6 5 4 3 2

UK

Switzerland

Sweden

Norway

Italy

Germany

France

0

Finland

1 Austria

Renewable Energy from Installed Heat Pump (TWh)

252

Fig. 4.36 Contribution of ground source heat pumps to the total renewable energy portfolio in several European countries (Printed with permission from Rees et al. [168])

Pump Association estimates that the supply of renewable energy from all heat pumps installed between 2005 and 2009 would be about 27.2 TWh. The contribution of renewable energy from heat pumps to several European countries is shown in Fig. 4.36. The Asian heat pump market is currently much less established than that of Europe and North America. However, China, Japan, and South Korea are becoming very aggressive in installing geothermal heat pumps. In China, the GHP growth rate in 2007 is reported to have tripled over the previous year’s value. China has approximately 630 MWt of installed GSHP capacity and are used in residences, office buildings, schools, hotels, commercial buildings, hospitals, and banks. In Beijing, China, over 3530 m2 .38;000 ft2 / of the Olympic Village was air-conditioned by GSHPs. In South Korea, the capacity of shipped GSHP equipment is reported to have increased by a factor of 55 from 2005 to 2007. This growth has been fueled by the legislation passed in 2005 by the Korean government that required new public buildings to incorporate alternative and renewable energy sources. The basic installation of GSHP in a house is shown in Fig. 4.37. The GSHP unit is generally installed inside the building. A loop of plastic or metal pipe is placed in a vertical hole bored several hundred meter deep, and the hole is then backfilled. An antifreeze solution is circulated through the loop and through the heat pump for removing heat from or transferring heat to the ground. The heat transfer process takes place through the pipe wall and between pipe and the earth or ground water. Therefore, no physical contact takes place between the antifreeze and the

4.4

Applications

253

Fig. 4.37 Basic arrangement of geothermal heat pump

ground. The type of antifreeze that can be used in the loop depends on the ground temperature. A general variation of the soil temperature during a year is shown in Fig. 4.38. The loops of plastic pipe can be placed in the ground in various orientations. They can be also placed in a nearby pond provided the pond does not freeze during winter months. There are four basic types of ground loop systems. These are horizontal, vertical, and pond/lake, which are closed-loop systems, and an open-loop type system. The choice of the loop-system depends on the climate, soil conditions, available land, and local installation costs at the site. All of these approaches can be used for both residential and commercial buildings.

4.4.3.1 Closed-Loop Systems Horizontal Closed loop systems are generally used for residential installations. They can be most cost effective, particularly for new construction if sufficient land is available. A closed loop horizontal system is shown in Fig. 4.39. Trenches of 1.2–1.8 m (four to

254

4 Geothermal Energy

A

M

82 72

J

J

A

S

O

N

D

Oct 29

M

Ground Surface X = 2 ft X = 5 ft X = 12 ft

62

32

0

40

80

Apr 8

Mar 11

42

Feb 18

52

Feb 4

Ground Temperature, °F

F

Sept 10

J

92

Aug 6 Aug 20

Month

120 160 200 240

280 320

360

Day of the year Fig. 4.38 Seasonal soil temperature change as a function of depth below ground surface for an average moist soil (Adapted from Virginia Department of Mines, Minerals, and Energy [185]) Fig. 4.39 A closed loop geothermal heat pump system (Courtesy of US Department of Energy [193])

six feet) deep are required depending on the design. Either two loops, one buried at 1.8 m (six feet), and the other at 1.2 m (four feet) can be used, if there is no restriction on land use. Other design configurations include two pipes placed side-by-side at 1.5 m (five feet) in the ground in a 0.6 m (two-foot) wide trench. The SlinkyTM

4.4

Applications

255

Fig. 4.40 A vertical closed loop geothermal heat pump system (Courtesy of US Department of Energy [193])

method of looping pipes allows more pipes to be used in a shorter trench, which cuts down on installation costs and makes horizontal installation possible in areas where conventional horizontal methods may not have been feasible otherwise.

Vertical The vertical systems are more convenient for large commercial buildings and schools which often have limited availability of land. Vertical loops can also be used where the soil is too shallow for trenching, minimizing the disturbance to the existing landscaping. For installation of a vertical system, holes (approximately 4 in. in diameter) are drilled about 6 m (20 ft) apart and 30–120 m (100–400 ft) deep. The arrangements of pipes are shown in Fig. 4.40.

Pond/Lake If the house has a large pond nearby, a Slinky type loop can be used for the heat pump (See Fig. 4.41). This type of installation is affordable for the residential customers costing the least of all the loop systems. The underground pipe lines connect a heat pump inside the building to the loop in the pond. The coils should only be placed in a water source that meets a minimum volume, depth, and quality criteria.

256

4 Geothermal Energy

Fig. 4.41 A closed loop geothermal heat pump system using the nearby pond or lake (Courtesy of US Department of Energy [193])

4.4.3.2 Open-Loop System The open loop systems take advantage of the groundwater to operate the heat pump. This type of system uses wells or a surface body water as the heat exchange fluid that is circulated directly through the GHP system. The water circulates in a closed loop and is returned to the ground either through a recharge well or by discharging to the surface. This option is practical only where there is an adequate water supply with relatively clean water for surface discharge. All federal and local codes and regulations regarding groundwater discharge must be met. An open loop system is shown in Fig. 4.42. The advantages of open loop systems are less heat loss and lower costs. However, some of the disadvantages can drive the cost up, which are problems associated with the disposal of water from the once-through heat pump onto the surface if the codes and regulations are too strict, construction of disposal wells, requirement of a large supply of clean water. This often limits their use to coastal areas, and areas adjacent to lakes, rivers, and streams. Also, secondary or backup heat sources may be required in cooler climates.

4.5 Summary The electricity generated from the geothermal energy is clean, reliable, and cost effective. Geothermal resources are abundant, and may be considered as a secure source of energy. Geothermal power plants are very reliable when compared to

4.5

Summary

257

Fig. 4.42 A open loop geothermal heat pump system (Courtesy of US Department of Energy [193])

conventional power plants. A geothermal power plant can be operated about 99% of the time. The capacity factor of geothermal power plants is highest among all types of power plants. Also, it can provide an abundant source of energy with minimal environmental impact. A geothermal power plant emits no nitrogen oxides, very few sulfur dioxides and between 1,000 and 2,000 times less carbon dioxide than a fossil fuel plant and can be operated 24 h a day. It can reduce dependence on imported fuel. Geothermal plants may be attractive to developing countries since these require less investment in infrastructure and equipment. However, geothermal energy is not without some disadvantages. The impact on groundwater and other water resources is not fully understood.

Problems 1. What does geothermal mean? 2. Define geothermal energy. 3. Why is geothermal energy considered a renewable energy source? 4. What is the source of geothermal energy and how it gets up to the earth’s surface? 5. How is a location for geothermal energy determined? 6. Is there a way to determine the energy capacity of a geothermal source?

258

4 Geothermal Energy

7. What are the steps involved in the development of geothermal energy? 8. Describe various types of geothermal energy. Can all the sources be used for the same purposes? 9. How is the geothermal energy extracted from rocks? 10. Describe various uses of geothermal energy. 11. Can geothermal energy be used for generation of electricity and what are the limitations? 12. What are heat pumps and explain their operations during summer and winter? 13. What is the mechanism of heating water in the earth’s interior? 14. Is geothermal energy inexhaustible? 15. Can earthquake be related to the use of geothermal energy? 16. Can too much use of geothermal energy imbalance the earth’s thermal equilibrium? 17. Estimate the cost of installation of a geothermal heat pump and its payback time. 18. What strategies may be adopted to promote heat pumps in the world. 19. Can heat pumps be used in hotter climate, such as middle east, of the world or extreme cold places, such as Alaska, USA. 20. Does Iceland use heat pumps? 21. Can a geothermal power plant, under the best case scenario, compete with an equivalent solar, wind, or hydrothermal power plant? 22. Can a geothermal power plant be used as a base load system? 23. Is there a relationship between the depth and the temperature of the soil? How will this relationship be derived? 24. What is enhanced geothermal system? Is it feasible? What are the associated costs for its commercialization?

References 1. Braile L (2000) Earth’s interior structure. http://web.ics.purdue.edu/braile. Accessed 16 Nov 2010 2. Dickson MH, Fanelli M (2004) What is geothermal energy? Istituto di Geoscienze e Georisorse, CNR, Pisa 3. Kruger P, Otte C (1973) Geothermal energy resources, production, stimulation. Stanford University Press, Stanford

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Chapter 5

Ocean Energy

Abstract Oceans are the largest collector of solar energy on the earth’s surface. Considering oceans cover more than 70% of the earth’s surface, the amount of energy stored by the oceans is enormous. The energy can be harvested from the ocean by taking advantage of waves, tidal current, and the thermal gradients that exist within the body of water. The gravitational pull of the moon primarily drives the tides, and the wind powers the ocean waves. In theory, these ocean-based renewable resources could meet the world’s energy requirements many times over, but they are extremely difficult to harvest economically for large scale production. In this chapter, various methods including three main techniques; wave power, tide power and ocean thermal energy conversion, are discussed for harvesting energy from oceans.

5.1 Introduction Ocean energy refers to the energy derived from oceans or seas. Oceans cover about 70% of the Earth’s surface, making it the world’s largest solar energy collector and energy storage system. Theoretically, 60 million square kilometers (23 million square miles) of tropical seas absorb an amount of solar radiation that is equivalent to 250 billion barrels of oil per day in terms of energy content. This stored energy of the ocean is manifested in various forms and may be harvested for various applications [1–16]. These can be potentially utilized to generate electricity. Several technologies have been developed to harvest energy or electricity from oceans that include: • • • •

Wave Power Tidal Power and Tidal Current Energy Ocean Thermal Energy Conversion (OTEC) Salinity Gradient Energy

T.K. Ghosh and M.A. Prelas, Energy Resources and Systems: Volume 2: Renewable Resources, DOI 10.1007/978-94-007-1402-1 5, © Springer Science+Business Media B.V. 2011

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Fig. 5.1 Ocean energy potential at the outer continental shelf (Adapted from Robinson [17])

Among these methods, wave, tidal, and OTEC are technologically most advanced. The energy extraction potential by these methods in the USA is compared in Fig. 5.1. The use of energy from the ocean was proposed as early as 1881 by a French physicist, Jacques Arsene d’Arsonval. He suggested the tapping of the thermal energy of the ocean using the OTEC method. In 1930, Georges Claude, d’Arsonval’s student, built the first OTEC plant in Cuba. The system produced 22 kW of electricity with a low-pressure turbine. In 1935, Claude constructed another plant aboard a 10,000-ton cargo vessel moored off the coast of Brazil. Weather and waves destroyed both plants before they became fully functional. In 1956, French scientists designed a 3-MW OTEC plant for Abidjan, Ivory Coast, West Africa, but the construction cost prevented its completion. The United States became involved in OTEC research in 1974 with the establishment of the Natural Energy Laboratory of Hawaii Authority. As of the mid-1990s, about 12 types of wave energy systems were proposed to extract energy from surface waves, pressure fluctuations below the water surface, and from the full wave utilization. As of 2002, one MW of grid-connected generating capacity using wave energy is operating worldwide. This one MW capacity comes from demonstration plants ranging in size from 20 to 350 kW.

5.2 Wave Power Wind blowing over the surface of the ocean create waves that can be harvested for energy [18–22]. In several parts of the world, ocean waves are fairly consistent to produce energy on a continuous basis. It is estimated that using the current

5.2

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269

Fig. 5.2 Wave energy potential around the world (Adapted from Thorpe [18])

Fig. 5.3 Wave energy in the USA coastal line (Adapted from Robinson [17])

technologies, about 140–750 TWh/year could be generated economically from wave energy worldwide [23]. Annual average wave power levels at various parts of the world are shown in Fig. 5.2. As shown in Fig. 5.3, the total annual average wave energy off the U.S. coastlines (including Alaska and Hawaii) at a water depth of 60 m has been estimated to be 2,100 Terawatt-hours (TWh). With projected longterm technical improvements, wave energy could be increased by a factor of 2 to 3. The fraction of the total wave power that is economically recoverable in the U.S.

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5 Ocean Energy

Fig. 5.4 Wave energy potential in Europe given as wave power level in kW/m of crest length (Adapted from Center for Renewable Energy Sources [19])

offshore regions has not been estimated. Wave energy conversion devices have the greatest potential for applications at islands, such as Hawaii, because of the combination of the available shoreline, availability of wave energies due to trade winds, and the relatively high costs of other local energy sources. Various countries in Europe also have significant wave energy resources as shown in Fig. 5.4. Some of the best sites are around England, and they are aggressively developing these sites for generating electricity. In the area of the north-eastern Atlantic (including the North Sea) available wave power resource is about 290 GW. According to a report by the Centre for Renewable Energy Resource (CRES), “The long-term annual wave power level increases from about 25 kW/m off the southernmost part of Europe’s Atlantic coastline (Canary Islands) up to 75 kW/m off Ireland and Scotland (Fig. 5.4). In the North Sea, the resource changes significantly, varying from 21 kW/m in the most exposed (northern) area to about the half of that value in the more sheltered (southern) area. In the Mediterranean basin, the annual power level off the coasts of the European countries varies between 4 and 11 kW/m, the highest values occurring for the area of the south-western Aegean Sea. The entire annual deep-water resources along the European coasts in the Mediterranean are of the order of 30 GW, the total wave energy resources for Europe resulting thus to 320 GW”. The Pacific-facing coastline of Asia has a modest wave, 10 to 15 kW/m of wave crest length off the coast of Japan. Several island countries including Indonesia, Philippines, and Sri Lanka are developing wave energy systems for power production.

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China has more than 14484 km (9,000 miles) of coastline, providing enormous potential for ocean generated power. Also, many of China’s largest cities are on its coastline; therefore, using ocean power for these cities, China could reduce the energy transportation cost. China is planning to generate 10 GW of power from ocean energy in the near future. Japan has a total coastline of 32,000 km, with estimated wave energy of 1.4 billion kW at peak times. However, only about 160 km of coastline may be useful for power production, which can provide an annual average wave energy of 3.9 million kilowatts however, Japan is generating an annual average of 1.3 million kilowatts electricity.

5.3 Theory Calculations of the energy carried by ocean waves require an understanding of various aspects of the wave. Notations used in various equations are explained in Fig. 5.5. .x; t/ t u; v ®.x; y; t/

D the free water surface D time D velocity component in the x and y directions, respectively D the two-dimensional velocity potential

z h

λ

H

SWL (Still Water Level) c x

a

H/2

h

Fig. 5.5 Various notations used to explain ocean energy theory (Reprinted with permission from McCormick [20])

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5 Ocean Energy

Fig. 5.6 Dependence of wave profile on sea floor characteristic (Reprinted with permission from McCormick [20])

¡ g a H k ! T h C MWL

D the fluid density D gravitational acceleration D wave amplitude D H=2 D wave height D wave number D 2 =L D wave length D wave frequency D 2 =T D wave period D mean water depth D wave celerity D L=T D mean water level

The water wave motion is periodic and is non-linear in nature. The wave motion is affected by the seafloor characteristics. The wave profile is not necessarily a perfect sinusoidal curve as shown in Fig. 5.5. The profile changes to one with a narrow crest and broad trough as shown in Fig. 5.6. Several researchers tried to model this nonlinear wave motion, however, these models still involved various approximations in order to obtain a solution. In order to simplify the mathematical treatment, often the wave motion is described by the Airy’s theory or the linear wave theory. As suggested by McCormick [20], for a wave height to wavelength ratio H= of 1/50 or less, the linear wave theory can provide excellent approximations of various aspects of a wave.

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273

5.3.1 Linear Wave Theory The mass and momentum balance equations provide the basis for the linear wave theory. The mass balance equation for a volume element ( x y z) can be written as: @uy @ux @uz @ C C C D SP (5.1) @t @x @y @z Assuming that there is no generation of water (i.e., Sp = 0) and it is incompressible and inviscid, the mass balance equation is reduced to the following form which is generally known as the continuity equation. @uy @uz @ux C C D0 @x @y @z

(5.2)

Equation 5.2 is often written as: ruD0

(5.3)

where, the operator, del .r/, in the Cartesian coordinate system is given by: rDi

@ @ @ Cj Ck @x @y @z

(5.4)

where (i; j; k) are unit vectors and u represents the velocity vector. If we assume that waves are two dimensional (the coordinate system is shown in Fig. 5.5) and that properties are constant along lines parallel to the wave crests, then Eq. 5.3 reduces to the following two-dimensional form: @ux @uz C D0 @x @z

(5.5)

The momentum balance equation for water particles under the influence of the waves can be derived as follows by starting with the Navier-Stokes equation which is given by: DEu rP (5.6) D FE C r 2 u Dt where,

DEu Dt

is called the total derivative and is given by @Eu DEu D C .Eu r/ uE Dt @t

(5.7)

*

F is the external force, P is the pressure, and is viscosity. Assuming that the viscosity of water can be ignored, one obtains rP @Eu C .Eu r/ uE D FE @t

(5.8)

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Since the only external force acting on the water particle is gravity, FE becomes g, the acceleration due to gravity. Therefore, 1 .Eu r/ uE D r.Eu uE/ uE .r uE / (5.9) 2 It may be assumed that the condition of irrotationality of water particle is valid. Since the original motion of the particle is generated from rest by normal force and the viscosity of water is ignored, therefore, the following condition is applicable: .r uE/ D 0

(5.10)

Using Eqs. 5.9 and 5.10 the following expression for momentum balance is obtained: 1 rP @Eu C r.Eu uE / D g (5.11) @t 2 Equation 5.11 is called the Euler equation in which P uE ; t is the fluid pressure and g D k g, where k is the unit vector in the positive z-direction. Therefore, Eq. 5.11 becomes: @Eu 1 rP C r.Eu uE / D r.gz/ (5.12) @t 2 The propagation speed of the wave and the wave-induced pressure in the water are found by solving the mass and momentum balance equations with proper boundary conditions, which are of kinematic (due to the motions of a water particle) and dynamic nature (due to forces acting on water particles). The boundary conditions for the system can be defined as follows. At the water surface, the velocity of water particles normal to the surface is equal to the speed of the surface in that direction, which is expressed by Eq. 5.13 uz D

@ ; @t

at z D 0

(5.13)

where, is the surface elevation as shown in Fig. 5.5. The boundary condition at the bottom (seafloor) may be defined as follows: uz D 0;

at z D h

(5.14)

The third boundary condition is obtained by assuming that the wave is subjected to the gravitational force only and that the atmospheric pressure at the water surface is constant and may be assumed to be zero. This is known as the dynamic surface boundary condition and is given as: P D 0;

at z D 0

(5.15)

An analytical solution of the mass balance equation coupled with momentum balance equation with the above boundary conditions is challenging. The researchers

5.3

Theory

275

have proposed solutions by introducing a function, known as the velocity potential function; .x; y; z; t /, which is defined as follows: ux D

@ ; @x

uy D

@ ; @y

uz D

@ @z

(5.16)

These conditions exist only if the motion of the water particle is irrotational. The continuity equation in terms of the velocity potential function becomes: @2 ' @2 ' @2 ' C C D 0; dx2 dy2 dz2

(5.17)

Equation 5.17 is also known as the Laplace equation. By substituting uE D r into Eq. 5.17, the following expression is obtained: r

@ 1 P C r r C C gz D 0 @t 2

(5.18)

However, since rF .x; t / D 0 is equivalent to F .x; t/ D C , where C is a constant, Eq. 5.18 becomes: @ 1 P C r r C C gz D C (5.19) @t 2 Equation 5.19 is known as the Bernoulli’s equation. The kinematic boundary conditions at the surface and at the bottom can be expressed in terms of velocity potential function as follows: @' @ D ; @z @t @' D 0; @z

at z D 0; and

(5.20)

at z D h

(5.21)

The dynamical boundary condition derived from the Bernoulli equation in terms of the velocity potential function becomes: @ P C C gz D 0 @t

(5.22)

The contribution of the term 12 r r can be neglected since the magnitude of the wave velocity is generally very small. C is a function of time only and generally is assumed zero. This is called the linearized Bernoulli equation. This linearized equation at the surface z D , with P D 0, gives: @' C g D 0; @t

at z D

(5.23)

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5 Ocean Energy

The Laplace equation and the kinematic boundary conditions provide the solution for the velocity potential and other kinematic expressions for the waves. The linearized Bernoulli equation and linearized dynamic boundary conditions, along with the velocity potential function provide the expressions for dynamic aspects of the waves. One of the analytical solutions of the Laplace equation is a long crested harmonic wave propagating in the positive x-direction and can be written as: .x; t / D a sin.!t kx/

(5.24)

The corresponding velocity potential function becomes: D

!a cosh Œk.h C z/ cos.!t kx/ k sinh.kh/

(5.25)

The above solution to the Laplace equation is based on a cylindrical wave with constant wave height, propagating in the x-direction. Although a sine wave (Eq. 5.24) is used to represent the wave, a cosine function could have been chosen as well. It would have made no difference in the results, except that everywhere sine function could be replaced with cosine function. This means a 90ı phase difference, which is rather immaterial here. Equation 5.24 in terms of the wave height (H ), the wave period (T ), and the wave length () may be expressed as: H 2x 2 t .x; t / D (5.26) sin 2 T T is the period and is given by: s T D 2

1 2 2g 2h D tanh D f !

(5.27)

where, f is the wave frequency and ! is the radian frequency, which is given by ! D 2 =T . The wave number is defined as, k D 2 =. Rearranging Eq. 5.27, an expression for wavelength () can be obtained as: gT 2 2h D (5.28) tanh 2

5.3.2 Energy Transport and Power As the waves move over the ocean surface, they carry both kinetic and potential energy with them. The knowledge of this energy content is important to estimate power production potential of devices that are used to convert this energy to mechanical and electrical energy.

5.3

Theory

277

Wave Amplitude (m)

200 100 0 –100 –200 0

20

40

60 Time (S)

80

100

120

Fig. 5.7 Representation of ocean wave (Adapted from Open University Course Team [24])

In practice, a wave cannot be represented by a simple sinusoidal wave. It is, rather, characterized by many waves with different periods and phases as shown in Fig. 5.7. Therefore, a time averaged kinetic energy over one period for the wave from the surface to the bottom is calculated. The time-averaged kinetic energy (Ek ) per unit surface area can be written as: Z Ek D

h

1 2 Eu dz 2

(5.29)

Integrating Eq. 5.29 for a harmonic wave with amplitude a, using the expression for ux and uz (obtained from the linear wave theory), the following expression is obtained: Ek D

1 ga2 4

(5.30)

The approximation of the linear wave theory also suggests that the potential energy is equal to the kinetic energy. Therefore, the total time-averaged energy density of the wave is: E D E k C EP D

1 ga2 2

(5.31)

The energy flux or the power (P ) contained in the wave can be calculated from the energy density by the following expression: P D E cg

(5.32)

where cg is called the group velocity and is related to the phase velocity by the following expression 4h= c cg D 1C (5.33) 2 sinh 4h=

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5 Ocean Energy

Fig. 5.8 Illustration of group velocity of waves (Reprinted with permission from McCormick [20])

Group velocity is defined as the velocity at which wave energy is transmitted. It is further explained in Fig. 5.8. The physical concept of group velocity is that the leading wave will disappear while travelling in x-direction but a new wave will be created at the end of the group, so there will always be the same number of waves in that particular group. Thus, the group travels at a slower speed than the individual waves within it. Both the phase velocity and the group velocity depend on the particle motion and water pressure, which, in turn, depends on the depth of the water. Equation 5.16 may be used to calculate the velocity components of the water particle and is given by: @ @ !a coshŒk .h C z/ ux D D cos.!t kx/ (5.34) @x @x k sinh.kh/ @ @ !a coshŒk .h C z/ D cos.!t kx/ uz D @z @z k sinh.kh/

(5.35)

The phase velocity may be calculated from Eq. 5.23. The differentiation of Eq. 5.23 with respect time (t) provides @2 @ Cg D0 2 @t @t

(5.36)

Since, @ @ D D uz @t @z Substitution of Eq. 5.37 into Eq. 5.36 provides @ @2 D0 Cg @t 2 @z

(5.37)

(5.38)

5.3

Theory

279

Using Eq. 5.28 and c D =T , the following expression for the phase velocity is obtained in terms of wave number k: c2 D

g tanh.kh/ k

(5.39)

5.3.2.1 Deep-Water Waves For deep-water, it may be assumed that x ! 1, and tanh x ! 1. For engineering calculations, for deep water, h= 0:5 may be assumed. Therefore, the wavelength and phase velocity become gT 2 Deepwater D (5.40) 2 and, gT cDeepwater D (5.41) 2 Using these two expressions, in terms of the length of the wave (kW/m), power may be calculated from the following equation: P D

g2 a2 T 8

(5.42)

5.3.2.2 Shallow-Water Waves Shallow-water waves are defined when h= 0:05. This suggests that for x ! 0; tanh x ! x. The shallow-water wavelength and phase velocity (or celerity) are given by: p Shallowwater D T gh (5.43) and cShallowwater D

p gh

(5.44)

5.3.2.3 Intermediate Depths The intermediate depth is defined when 0:5 h= 0:05. Approximation for wavelength and celerity does not work, rather Eq. 5.28 should be solved to obtain wavelength (). A trial and error approach is needed since Eq. 5.28 is implicit in . An alternative approximate solution has been suggested in the Shore Protection Manual (1984) [25], which is given below: Intermediatedepth D Deepwater

q tanh 2h=Deepwater

(5.45)

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5 Ocean Energy

Fig. 5.9 Range of applicability of linear wave theory (Adapted from Shaw [26])

5.3.3 Applicability of Linear Wave Theory The linear wave theory is generally applicable to waves with small amplitudes compared to wavelength. The range of applicability of the theory is shown in Fig. 5.9. In the figure, breaking limit suggests the limit of wave steepness above which the waves would break. The breaking limit may be expressed by the following expression: 2h H D 0:140 tanh (5.46)

5.3.4 Significant Wave Height As shown in Fig. 5.7, ocean waves are rather random. However, often time the ocean condition or sea state is described by the wave height and wave length, which is called the significant wave height. The significant wave height H1=3 is the average height of the highest 1/3 of all waves observed in a given period of time. This average height is generally used for various navigational applications. Beaufort [21] related the significant wave height to the wind speed and is called the Beaufort scale. The Beaufort scale for various conditions is given in Table 5.1.

5.3

Theory

281

Table 5.1 The Beaufort scale for various wave conditions Appearance of wind effects WMOa Force Wind (knots) classification On the water 0 Less than 1 Calm Sea surface smooth and mirror-like 1 1–3 Light air Scaly ripples, no foam crests 2

4–6

Light breeze

3

7–10

Gentle breeze

Small wavelets, crests glassy, no breaking

Large wavelets, crests begin to break, scattered whitecaps 4 11–16 Moderate Small waves 1–4 ft. breeze becoming longer, numerous whitecaps 5 17–21 Fresh breeze Moderate waves 4–8 ft taking longer form, many whitecaps, some spray 6 22–27 Strong Larger waves 8–13 ft, breeze whitecaps common, more spray 7 28–33 Near gale Sea heaps up, waves 13–20 ft, white foam streaks off breakers 8 34–40 Gale Moderately high (13–20 ft) waves of greater length, edges of crests begin to break into spindrift, foam blown in streaks 9 41–47 Strong gale High waves (20 ft), sea begins to roll, dense streaks of foam, spray may reduce visibility 10 48–55 Storm Very high waves (20–30 ft) with overhanging crests, sea white with densely blown foam, heavy rolling, lowered visibility 11 56–63 Violent Exceptionally high storm (30–45 ft) waves, foam patches cover sea, visibility more reduced 12 64+ Hurricane Air filled with foam, waves over 45 ft, sea completely white with driving spray, visibility greatly reduced Source: National Oceanic and Atmospheric Administrations [21] a World Meteorological Organization

On land Calm, smoke rises vertically Smoke drift indicates wind direction, still wind vanes Wind felt on face, leaves rustle, vanes begin to move Leaves and small twigs constantly moving, light flags extended Dust, leaves, and loose paper lifted, small tree branches move Small trees in leaf begin to sway

Larger tree branches moving, whistling in wires Whole trees moving, resistance felt walking against wind Whole trees in motion, resistance felt walking against wind

Slight structural damage occurs, slate blows off roofs Seldom experienced on land, trees broken or uprooted, “considerable structural damage”

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5.4 Wave Power Wave power systems can be installed nearshore, offshore, or far offshore. The design of the energy extraction systems depends on these locations. In nearshore systems, all the devices are within 19.3 km (12 miles) of the shore. Nearshore operations have to consider how the plant will affect the aesthetic influence of the area. The impact on shipping lane and marine life also requires assessment. Various aspects of power generation from the wave energy are discussed by a number of researchers and organizations [3, 22–46]. A depth greater than 40–50 m will constitute an offshore operation. Even for an offshore operation, the preference is to install all the equipment and control systems at or near the water’s surface. When installing energy extraction systems, the orientation of the turbines to the waves with which they are interacting should be taken into account. The electricity production depends heavily on these factors. Five basic devices have been proposed or employed for electricity generation from wave energy, and they are: • • • • •

Tapered Channel (TAPCHAN) [47–53] Terminator Devices [54, 55] Point Absorber [56–71] Attenuators [72–78] Overtopping Devices [53, 79–82].

5.4.1 Tapered Channel (TAPCHAN) A tapered channel (TAPCHAN) is used to divert water into a reservoir as shown in Fig. 5.10. These systems, generally, are constructed on a cliff providing a higher water head. The electricity is generated using the same basic concept of a hydroelectric system. The reservoir stores the water that is fed through a Kaplan turbine. The narrowing of the channel causes the wave amplitude (wave height) to increase as waves move towards the cliff face, and spill over the walls of the channel and into the reservoir. Depending on the wave height, the reservoir can be constructed at higher elevation above mean sea level. Several meters of water head can be achieved based on the location of such a system. The advantages of TAPCHAN system include adaptation of instruments, turbine of well established hydroelectric power systems, low maintenance costs, reliability, and use for the peak load supply. The disadvantages are the need for locations with consistent waves, a water head of at least 1 m, and a suitable location for a reservoir.

5.4

Wave Power

283

Fig. 5.10 Harnessing of wave energy by TAPCHAN systems for power production (Printed with permission Boyle [53])

5.4.2 Terminator Device/Oscillating Water Column Terminator devices also known as oscillating water column (OWC) devices operate perpendicular to the direction of wave travel to capture the power of the wave. These devices are installed typically onshore or nearshore. Floating types have also been designed for offshore applications. In the oscillating water column devices, water enters through a subsurface opening into a chamber that contains a column of air on the top of a column of water. The wave action causes the captured water column to rise and fall like a piston which in turn compresses and decompresses the air column. This trapped air flows to and from the atmosphere via a turbine causing it to rotate and generate electricity. A schematic diagram of an oscillating water column system is shown in Fig. 5.11. The turbine can also be installed in horizontal direction as shown in Fig. 5.12. Various stages of operation of the oscillating water column system are described in these figures. The wave pushes the air column through the turbine generating the power (Fig. 5.12b). When waves recede, the air column again moves in the opposite direction (Fig. 5.12c). The turbine works in both directions, and, therefore increases the power production. One of the major challenges is the design of the turbine. To maximize the power generation, the turbine must rotate in the same direction irrespective of the air flow direction. A prototype oscillating water column operating at Port Kembla, Australia is shown in Fig. 5.13.

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Fig. 5.11 The working principle of the oscillating water column system for harnessing wave energy (Source: Travassos et al. [81])

5.4.3 Point Absorber A point absorber is a floating device in which a floating buoy moves inside a fixed cylinder due to wave action. The relative motion is used to drive electromechanical or hydraulic energy converters. Currently, the point absorber is most widely used as a commercial device in the USA. Other challenges with the point absorber are maintaining the structural integrity of the device in contact with moorings, foundations, and under extreme wave conditions. The buoy or piston movement must be controlled so that its motion is in resonance with the waves to maximize energy capture. Also, the piston movement must be limited to keep it within the

5.4

Wave Power

285

Fig. 5.12 Different stages of an OWC system (Courtesy of Fujita Research [82])

fixed cylinder. A point absorber can also utilize a bidirectional turbine to enhance the power output. The basic function of a point absorber is shown in Fig. 5.14. Figure 5.15 is the conceptual design of a wave energy farm using point absorbers.

5.4.4 Attenuator An attenuator is a long multi-segment floating device which works in parallel to the wave direction and effectively rides the waves. As shown in Fig. 5.16, movements along its length are selectively constrained to produce energy. The flexing along the length of the device where the segment connects cause connected hydraulic pumps or other converters to generate electricity. It has a lower area perpendicular to the waves in comparison to a terminator, so the device experiences lower forces. An example of this type of device is the Pelamis WEC that is being developed by the Ocean Power Delivery. A Pelamis device is shown in Fig. 5.17. The power

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Fig. 5.13 A prototype oscillating water column system by Energetech at Port Kembla, Australia (Courtesy of National Oceanic and Atmospheric Administration [83])

Fig. 5.14 A point absorber wave energy farm (Courtesy of Argonne National Laboratory [84])

5.4

Wave Power

287

Fig. 5.15 Rendition of a wave farm made up of permanent magnet linear generator buoys (Courtesy of Oregon State University [85])

Fig. 5.16 The working mechanism of the Pelamis wave power conversion system (Source: Wave energy: Technology transfer and generic R C D recommendations [86])

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Fig. 5.17 A Pelamis unit floating in the sea (Source: Cruz [87])

Fig. 5.18 The cutaway of the power conversion module of the Pelamis system (Source: Cruz [87])

conversion module is located inside the device protecting it from water and other harsh environmental conditions (Fig. 5.18). It is moored slackly to hold it in place. As the Pelamis moves with the waves, the motion causes the hydraulic rams to move, which then act as a pump circulating high pressure oil through hydraulic

5.4

Wave Power

289

Fig. 5.19 An artist’s impression of a wave farm in the sea using Pelamis wave conversion systems (Source: Cruz [87])

motors. These motors drive several generators to produce power. A 750 kW Pelamis system would be 150 m long and 3.5 m in diameter and comprises of five sections. An artist’s impression of a Pelamis wave power generation farm is illustrated in Fig. 5.19.

5.4.5 Overtopping Devices Overtopping devices have reservoirs that are filled with water by incoming waves to levels above the average surrounding ocean. The water is then released and is allowed to fall under gravity through low head hydro-turbines toward the ocean surface. The energy of the falling water turns hydro-turbines generating electricity. Specially built seagoing vessels can also capture the energy of offshore waves. These floating platforms create electricity by funneling waves through internal turbines and then back into the sea. The basic operating principle is shown in Figs. 5.20 and 5.21.

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Fig. 5.20 Working principle of an overtopping system (Courtesy of Wave Dragon APS [88])

Fig. 5.21 Construction details of the overtopping unit (Source: Kofoed et al. [89])

Overtopping devices have been designed for both onshore and floating offshore applications. The most common offshore devices called the Wave DragonTM. This system has been tested for a 7 MW demonstration project off the coast of Wales (See Fig. 5.22). A commercial scale prototype system is tested for hydraulic behavior, turbine strategy, and power production to the grid in Denmark. Another wave dragon design has been scaled to 11 MW, and even larger systems are feasible. A number of parameters influence the power generation from an overtopping device. These parameters are listed below. • Overtopping: It is determined by; – Free-board (adjustable in wave dragons), – Actual wave height, and – Physical dimension of the converter (ramps, reflectors etc).

5.4

Wave Power

291

Fig. 5.22 Prototype wave dragon overtopping system (Courtesy of Wave Dragon APS [88])

• Outlet: This is determined by; – Size of reservoir – Turbine design – Turbine on/off strategy • Mooring system, free or restricted orientation toward waves • Size of the energy converter • Wave climate – Energy in wave front (kW/m) – Distribution of wave heights • Availability – Theoretical availability; Reliability, maintainability, serviceability – Accessibility on the site – Maintenance strategy A single wave dragon unit can be designed in the range of 24–72 kW/m providing annual power output in the range of 12–52 GWh/year, respectively. The key design parameters of a wave dragon system are listed in Table 5.2.

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Table 5.2 Design parameters of wave dragon systems of various power output Nissum Bredning Wave power Wave dragon key prototype, 24 kW/m 36 kW/m 48 kW/m figures 0.4 kW/m Weight, a combination of re-inforced concrete, ballast and steel Total width and length Wave reflector length Height Reservoir Number of low-head Kaplan turbines Permanent magnet generators Rated power/unit Annual power production/unit Water depth

237 t

22,000 t

33,000 t

54,000 t

58 33 m

260 150 m

300 170 m

390 220 m

28 m

126 m

145 m

190 m

3.6 m 55 m3 7

16 m 5;000 m3 16

17.5 m 8;000 m3 16–20

19 m 14;000 m3 16–24

7 2:3 kW

16 250 kW

20 kW –

4 MW 12 GWh/y

16–20 16–24 350–440 kW 460–700 kW 7 MW 11 MW 20 GWh/y 35 GWh/y

6m

>20 m

>25 m

>30 m

Source: Wave Dragon [88]

5.5 Tidal Current Energy The tides are cyclic variations in the level of the seas and oceans. The cyclic variation is predictable, since it is dependent on the position of the earth and the moon in their respective orbits. The kinetic energy of sea or ocean current can be harvested to generate power [90–97]. Various terminologies associated with tide and tidal currents are discussed below. Range: The difference in the height between consecutive high and low tides occurring at a given place. The range is reported in meter or feet. Syzygy: The locations of the moon when it is at new phase and full phase. During this time the gravitational attractions of the moon and sun act to reinforce each other, and, therefore, highest on the earth surface. The tidal range is greater at all locations which display a consecutive high and low water. Spring Tide: The tidal effect of the sun and the moon acting in concert twice a month, when the sun, earth and moon are all in a straight line (full moon or new moon). The range of tide is larger than average. Neap Tide: This is opposite of the spring tide, which occurs when the moon is at right angles to the earth-sun line (first or last quarter). The range of tide is smaller than average. Parallax Effects: The distance between the earth and moon vary throughout the month by about 49,879 km (31;000 miles). As a result the gravitational pull that

5.5

Tidal Current Energy

293

Fig. 5.23 The lunar parallax and solar parallax inequalities (Courtesy of National Oceanic and Atmospheric Administration [98])

causes the tide also vary. During perigee, when the moon is closest to the earth, above-average ranges in the tides occur. When the moon is at apogee, which is farthest from the earth, the tidal ranges will be less than average. The distance between the earth and the sun also varies causing different ranges in the tide. When the earth is closest to the sun, called perihelion, the tidal ranges increase, and when the earth is farthest from the sun (aphelion), the tidal ranges will be reduced. The parallax effects, both for the earth-moon, and the earth-sun systems are shown in Fig. 5.23. The monthly cycle (tropical month of 27.32 days) of lunar declination contributes to the overall tidal effects. The closer the moon comes to being overhead, the more powerful is its effect. Since, both the moon and the earth revolve in elliptical orbits, the distances from their centers of attraction vary. Increased gravitational influences and tide-raising forces are produced when the moon is at position of perigee, its closest approach to the earth (once each month) or the earth is at perihelion, its closest approach to the sun (once each year). Figure 5.24 also shows the possible coincidence of perigee with perihelion to produce tides of augmented range. The gravitational pull is enhanced when the plane of the moon’s orbit is inclined only about 5ı to the plane of the earth’s orbit (the ecliptic). The latitude of the moon reaches its maximum value at this inclination. Thus, the total declination of the moon can reach some 23:5ı above and below the ecliptic orbit. In Fig. 5.24, this condition is shown by the dashed position of the moon. The corresponding tidal force envelope due to the moon is depicted, in profile, by the dashed ellipse.

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Fig. 5.24 Moon’s declination effect (Change in angle with respect to the equator) and the diurnal inequality; semidiurnal, mixed, and diurnal tides (Courtesy of National Oceanic and Atmospheric Administration [98])

Moon’s declination affects the types of tide in any particular location. There are three daily cycles of tide, called diurnal, semidiurnal, and mixed. These tides are shown in Fig. 5.25. • Semidiurnal tide – Having a period of approximately one-half of a tidal day. The predominant type of tide throughout the world is semidiurnal, with two high waters and two low waters each tidal day. • Mixed Diurnal – Type of tide characterized by a conspicuous diurnal inequality in the higher high and lower high waters and/or higher low and lower low waters. • Diurnal tide – Having a period of one tidal day. The tide is said to be diurnal when only one high water and one low water occur during a tidal day (tidal day=24 h and 50 min). There are two fundamentally different approaches for using tidal energy to generate electricity. The first approach exploits the cyclic rise and fall of the sea level and the second approach use the local tidal currents to run turbines. The World Offshore Renewable Energy Report 2002–2007, released by the DTI Renewable Energy Consulting, CO, USA [99], estimated that 3000 GW of tidal energy is available worldwide, however less than 3% is located in areas suitable for power generation. The total worldwide power in ocean currents has been estimated to be about 5,000 GW, with power densities of up to 15 kW=m2 . The tide can increase dramatically when it reaches continental shelves, bringing huge masses of water into narrow bays and river estuaries along a coastline. The tides in the Bay of Fundy, Canada are the greatest in the world, with amplitude between 16 and 17 m near shore. The other locations around the world where high tides are observed are listed in Table 5.3.

5.5

Tidal Current Energy

295 Tidal Day

Tidal Height (in feet above or below the standard datum)

Tidal Period 3 2 1 0 −1 −2 −3 −4

Tidal Period

Datum

SEMIDIURNAL TIDE

Tidal Period Higher Lower 3 High Water High Water 2 Tidal 1 Rise Datum 0 −1 Higher −2 Low Water −3 −4

Lower Low Water

Tidal Day

Tidal Range Tidal Range

MIXED TIDE

Tidal Amplitude = 1/2 Range

Tidal Day Tidal Period 2 1 0 −1 −2

Datum DIURNAL TIDE

Fig. 5.25 Different types of tides (Courtesy of National Oceanic and Atmospheric Administration [98]) Table 5.3 Locations for high tides

Country Canada England France France Argentina Russia Russia

Site Bay of Fundy Severn Estuary Port of Ganville La Rance Puerto Rio Gallegos Bay of Mezen (White Sea) Penzhinskaya Guba (Sea of Okhotsk)

Source: Wave Dragon APS [88]

Tide range (m) 16.2 14.5 14.7 13.5 13.3 10.0 13.4

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As mentioned earlier, the tidal current energy resource of the world is rather large, particularly in countries such as the UK, Ireland, Italy, the Philippines, Japan and parts of the United States. In some parts of the world, water currents can be intense. The general surface current pattern around the world is shown in Fig. 5.26. Some of these locations are the Pentland Firth to the north of the Scottish mainland, around the British Islands and Ireland, between the Channel Islands and France, in the Straits of Messina between Italy and Sicily and in various channels between the Greek Islands in the Aegean. However, a good estimation of this resource has not been carried out by most of the countries. The potential for installation of marine current turbines in Europe is estimated to exceed 12,000 MW. The UK has the major component of the EU resource at approximately 4.3 GW. Ocean current in the vicinity of the USA coast line is shown in Fig. 5.27. Electric Power Research Institute (EPRI) has studied the North America tidal energy potential at selected sites and is shown in Fig. 5.28. EPRI estimates that the total tidal and river in stream potential is on the order of 140 TWh/year. Tidal power technologies may be categorized into two groups as follows: • Tidal Barrage or Dam Method • Tidal Turbine Method

5.5.1 Tidal Barrage Method A barrage or dam is typically used to force the water during high tide into a reservoir. When the tides produce an adequate difference in the level of the water on the opposite side of the dam, the gates are opened. The water is allowed to flow through a low head turbine, in a similar manner as a hydroelectric system. Gates and turbines are installed along the dam. The turbines turn an electric generator to produce electricity. There are currently two commercial barrages in operations. One is located in La Rance, France and the other one is in Annapolis Royal, Nova Scotia, Canada.

5.5.2 Principles of Operation Tidal barrage systems can be operated in two modes: • Single-Basin Tidal Barrage Mode • Multiple-Basin Mode

Fig. 5.26 Surface ocean current (Source: Windows to the universe [100])

5.5 Tidal Current Energy 297

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5 Ocean Energy

Fig. 5.27 Tidal current in the vicinity of the USA coast line (Courtesy of the University of Texas [101])

Fig. 5.28 Electricity generation capacity from the tidal energy from selected sites in the USA (Source: Bedard [102])

5.5

Tidal Current Energy

299

Fig. 5.29 A hypothetical single-basin tidal barrage system (Printed with permission from Bryden et al. [103])

5.5.2.1 Single-Basin Tidal Barrage Mode The operating principle of a tidal barrage in a single basin mode is shown in Fig. 5.29. A single barrage across the estuary is used for power generation. Three different methods of operation of barrages may be employed with a single basin for electricity generation. All of the options allow water to flow relatively freely through the barrage, and gated turbines. Ebb Generation Mode During the flood tide, incoming water flows freely through sluices in the barrage. During low tide, when the water height outside the barrage has fallen sufficiently, the required water head is reached between the basin and the ocean water. At that point, the basin water is allowed to flow out though low-head turbines to generate electricity.

Flood Generation Mode The sluices and turbine gates are kept closed during the flood tide to allow the water level to build up outside the barrage. Once a sufficient head has been established the turbine gates are opened and water flows into the basin through turbines generating electricity. The water level during various stages is critical and must be maintained at the appropriate level as shown in Fig. 5.30. The energy produced by this method is generally lower than the ebb method, as the surface area of a basin would be larger at high tide than at low tide, which would result in rapid reductions in the head during the early stages in the generating cycle.

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Fig. 5.30 Water levels in a flood generation scheme (Printed with permission from Bryden et al. [103])

Fig. 5.31 Hypothetical two-basin system (Printed with permission from Bryden et al. [103])

Two-Way Generation Mode With bi-directional turbines, it is possible to generate electricity in both ebb and flood methods. However, this would increase the operating and maintenance costs. The main advantage of this method is a reduced period with no generation and the peak power would be lower, allowing a reduction in the cost of the generators.

5.5.2.2 Double-Basin Systems Some of the problems associated with single basin systems may be overcome using double basin systems. Power could be generated almost on a continuous basis using double basin systems. A double-basin system, as shown schematically in Fig. 5.31, allows storage and provides control over power output levels.

5.5

Tidal Current Energy

Table 5.4 Country France Russia Canada China

301

Existing tidal power plants Site Installed power (MW) La Rance 240 Kislaya Guba 0.4 Annapolis 18 Jiangxia 3.9

Basin area (km2 ) 22 1.1 15 1.4

Mean tide (m) 8.55 2.3 6.4 5.08

Source: Gorlov [104] Table 5.5 Large scale tidal barrage plants under consideration

Site Seven Estuary (UK) Solway Firth (UK) Bay of Fundy (Canada) Gulf of Khambat (India) White Sea (Russia)

Mean tidal range (m) 7.0 5.5 11.7 6.1 –

Barrage length (m) 17,000 30,000 8,000 25,000 –

Estimated annual energy production (GWh) 8,600 10,050 11,700 7,000 15,000

The main basin used in the ebb generation mode is utilized the same way in the double basin as is used in a single-basin system. A proportion of the electricity generated during the ebb phase would be used to pump water to and from the second basin to ensure that there would always be a generation capability. The electricity generation efficiency of such a low-head storage system is in the range of 30%. The overall efficiency of these systems, when considered the use of the second basin for a pumped storage, can exceed 70%. Four large-scale tidal power plants currently exist. They are the La Rance Plant (France, 1967), the Kislaya Guba Plant (Russia, 1968), the Annapolis Plant (Canada, 1984), and the Jiangxia Plant (China 1985). Some of the features of these plants are given in Table 5.4. Worldwide there are several sites where tidal basins can be developed economically and are given in Table 5.5.

5.5.3 Tidal Lagoons Tidal lagoons are more environmental friendly. Both the Severn Estuary, which lies between England and Wales, and the mouth of the Yalu River, China have been suggested as potential locations for the lagoon-style development.

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Fig. 5.32 An artist’s impression of a tidal fence system (Source: Blue Energy Ltd [105])

5.5.4 Tidal Fence A tidal fence has vertical axis turbines mounted in a fence. They can be used in areas where there are channels between two landmasses. The tide water is forced through the turbines. As shown in Fig. 5.32, tidal fences sometimes look like giant turnstiles. They can reach across channels between small islands or across straits between the mainland and an island. This can be used as roadways too. The tidal currents can be at 5–8 knots (5.6–9 miles/h) and generate as much energy as winds of much higher velocity. Because seawater has a much higher density than air, ocean currents carry significantly more energy than air currents (wind).

5.5.5 Tidal Turbine Method Among various technological options, the use of tidal currents to run the turbines appears to be the best option. A number of countries are interested in pursuing the use of ocean current energy technologies. These countries include the European Union, Japan, and China. However, the technology is at an early stage of development, with only a small number of prototypes and demonstration units are tested to date. The main issue with the tidal current system is the design of the turbine. Most of the designs involve submerged systems. Although the tide turbines look the same as wind turbines, their working principle is different. Submerged tide turbines capture

5.5

Tidal Current Energy

303

Fig. 5.33 A submerged turbine working on tidal current (Courtesy of World Energy Council [106])

energy by the processes of hydrodynamic, rather than aerodynamic lift or drag. These turbines have rotor blades, a generator for converting the rotational energy into electricity, and a means for transporting the electrical current to shore for incorporation into the electrical grid. The basic operating concept of a submerged turbine working by the water current is shown in Fig. 5.33. Four types of turbine have been explored for generating electricity from the tidal current, and these include: • • • •

Horizontal Axis Turbines Vertical Axis Turbines Linear Lift Mechanism or Oscillating Hydroplane Systems Venturi Based Systems

5.5.5.1 Horizontal Axis Turbines (HAT) HATs are similar to wind turbines. The working principle is shown in Fig. 5.34. The power output depends on the stream flow rate and can be controlled using pitch controlled blades. The amount of power that can be harvested from the water current also depends on the rotor diameter (See Fig. 5.35). Similar to a wind turbine, the efficiency of water turbines depends on the water flow rate. Also, a minimum water velocity, called cut-in speed, is required before the turbine can start producing power. The power production increases exponentially with the increase in the velocity up to its rated power. The power output will not increase beyond this rated power even if the water velocity increases. These three regions of operations are shown in Fig. 5.36.

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Fig. 5.34 Basic orientation of horizontal axis turbines

Fig. 5.35 Power production as a function of rotor diameter (Adapted from Electric Power Research Institute [107])

Tidal current turbines can be either seabed-mounted or hanged from floating platforms. The sea-bed mounting is preferred for shallow water, whereas the floating platform is preferred for deep water installations. Various installation methods are shown in Fig. 5.37. Various designs of tidal horizontal axis turbines that are commercially available are shown in Fig. 5.38.

5.5.5.2 Vertical Axis Turbines In vertical axis turbines, water stream flow is perpendicular to the rotational axis of the turbine as shown in Fig. 5.39. Among vertical axis turbines, Gorlov turbines have the best performance, which are approximately 35% efficient in extracting energy from the water current. The Gorlov turbine always turns in the same direction, regardless of the stream direction, providing a higher energy conversion.

Power (kW)

5.5

Tidal Current Energy

305

2500

Region I: Velocity below cut-in speed

2000

Electric power = 0 (rotor cannot turn power train)

1500

Flow power Electric power

Region II: Velocity above cut-in speed Electric power = fluid power x power train efficiency

1000

500 I

Region III: Velocity above rated speed Electric power = rated power II

III

0 0.0

0.5

1.0

1.5

2.0 2.5 3.0 Flow speed (m/s)

3.5

4.0

4.5

Fig. 5.36 Dependence of turbine output power on the water speed (Adapted from Electric Power Research Institute [107])

Fig. 5.37 Various sea-bed mounting methods for turbines (Source: U.S. Department of the Interior [108])

Several vertical axis turbines have been designed and tested for their feasibility in commercial applications. Some of these turbines are shown in Fig. 5.40. Blue Energy Canada Inc. is involved in developing multiple vertical axis hydroturbines for generating electricity from ocean currents and tides. One such turbine is shown in Fig. 5.41. Current Power Sweden AB has also developed a vertical axis turbine (See Fig. 5.42) for extracting energy from streaming water, such as ocean current.

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5 Ocean Energy

Fig. 5.38 Horizontal axis turbines for power generation from the tidal current. (a) Lunar Energy RTT Turbine (Rotor dia. 21 m, Rated Power: 2 MW) (b) MCT Experimental Sea Flow (Rotor dia. 18 m, Rated Power: 1.5 MW) (c) Open Hydro Ream Drive Turbine (Rotor dia. 15 m, Rated Power: 1.5 MW) (d) UEK Shrouded Turbine (Rotor dia. 4 m, Rated Power: 0.4 MW) (e) Verdant Power RITE Turbines (Rotor dia. 5 m, Rated Power: 0.034 MW) (f) SMD Hydrovision (Rotor dia. 8 m, Rated Power: 1 MW)

5.5

Tidal Current Energy

307

Fig. 5.39 Working principle of vertical axis turbines

Fig. 5.40 Vertical axis turbines. (a) Gorlov Helical Turbine (GCK Technology) (Rotor dia. 1 m, Rated Power: 7 kW) (b) Seapower Vertical Axis Turbine (Rotor dia. 1 m, Rated Power: 44 kW)

5.5.6 Linear Lift Mechanism or Oscillating Hydroplane Systems In this method, a large wing-like hydroplane moves up and down in a linear motion and compresses the oil in a hydraulic ram to run an hydraulic power converter (See Fig. 5.43). These systems are generally mounted on the sea bed and prototypes are found in the range of 200–250 kW.

5.5.7 Venture Based Systems A venturi tube is used to accelerate the water flow. As the water is accelerated through the tube due to the reduction in cross-sectional area, a pressure drop is

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Machinery Enclosure Generator Gearbox Coupling Chamber

Turbine Power Duct

Water Flow

A

Rotor Shaft Support Arms Working Blades (Hydrofoil) A Bearings

Fig. 5.41 Blue energy vertical axis turbine (Nameplate capacity: 0.25 MW) (Source: Blue Energy Ltd [105]) Fig. 5.42 Current power AB vertical axis turbines (Nameplate capacity: 0.012-3 MW) (Source: Blue Energy Ltd [105])

5.7

Ocean Thermal Energy Conversion (OTEC)

309

Fig. 5.43 BioPower system oscillating hydroplane systems for power production (Nameplate capacity: 0.25-1 MW) (Source: Blue Energy Ltd [105])

generated in the tube as shown in Fig. 5.44. The pressure gradient is the driving force for the turbine which is connected to the constricted point in the tube.

5.6 Tidal Farm Coastal currents between 3.6 and 4.9 knots (4 and 5.5 mph) is ideal for the best performance of tidal turbines. In this range of water current, a 15-m (49.2-ft) diameter tidal turbine can generate as much energy as that of a 60-m (197-ft) diameter wind turbine. In large areas with powerful currents, it would be possible to develop tidal energy farm (See Fig. 5.45). These are arrayed underwater in rows, as in some wind farms. Turbine spacing would be determined based on wake interactions and maintenance needs. A 30-MW demonstration array of vertical turbines in a tidal fence is being investigated in the Philippines (WEC 2001). Ideal locations for tidal turbine farms are close to shore in water depths of 20–30 m (65.5–98.5 ft).

5.7 Ocean Thermal Energy Conversion (OTEC) Solar energy stored in the ocean water is converted to electric power by using the OTEC technology which utilizes the ocean’s natural thermal gradient [110–131]. The ocean’s layers of water have different temperatures. A thermodynamic cycle

Fig. 5.44 Venturi based system for power generation from tidal current (Courtesy of VerdErg) [109]

310 5 Ocean Energy

5.7

Ocean Thermal Energy Conversion (OTEC)

311

Fig. 5.45 An artists impression of a tidal farm with two different types of turbines. As reported in U.S. Department of the Interior [108]

Fig. 5.46 Thermal gradient in different water bodies around the world (Courtesy of Ocean Thermal Energy Conversion, NREL [130])

could be operated between this temperature difference to drive a power-producing cycle. A temperature difference of about 20ı C .68ı F/ between the warm surface water and the cold deep water is desirable for an OTEC system to produce a significant amount of power. The temperature gradient at various parts of the world is shown in Fig. 5.46. As can be seen from the figure, a temperature gradient of 20ı C exists in tropical coastal areas, roughly between the Tropic of Capricorn and the Tropic of Cancer. Three basic OTEC system designs have been demonstrated to generate electricity. These are: • Closed cycle, • Open cycle, and • Hybrid cycle. The basic design and various components of an OTEC system are shown in Fig. 5.47.

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Fig. 5.47 Basic components and output from an ocean thermal energy conversion system (Courtesy of Ocean Thermal Energy Conversion, NREL [130])

5.7.1 Closed-Cycle OTEC System In a closed-cycle OTEC system, warm seawater vaporizes a working fluid, such as ammonia, flowing through a heat exchanger (evaporator). The vapor expands at a moderate pressure and turns a turbine coupled to a generator that produces electricity. The vapor is then condensed in an another heat exchanger (condenser) using cold seawater pumped from a certain depth of the ocean through a cold-water pipe. The condensed working fluid is pumped back to the evaporator to repeat the cycle. The working fluid remains in a closed system and circulates continuously. A schematic diagram of the cycle is shown in Fig. 5.48.

5.7.1.1 Work Done in the Closed/Anderson Cycle The Anderson closed cycle is mainly used in closed cycle OTEC systems. It is a Rankine-type cycle. In the Anderson cycle, the working fluid is superheated only a few degrees Fahrenheit above the saturation temperature of the working fluid. The P-V diagram of the cycle is shown in Fig. 5.49. In Fig. 5.49, the starting point of the cycle may be considered is at a where heat is added to the working fluid until it is pressurized to point b. The working fluid is allowed to vaporize at a constant temperature by continuous addition of heat. The volume increases and when it reaches point c, the fluid is expanded adiabatically to the point d to obtain the work. The low pressure vapor from the turbine is cooled

5.7

Ocean Thermal Energy Conversion (OTEC)

313

Fig. 5.48 A schematic diagram of a closed cycle OTEC process (Courtesy of Ocean Thermal Energy Conversion, NREL [130]) Fig. 5.49 The P–V diagram of the closed OTEC Rankine cycle (Printed with permission from Avery and Wu [131])

to its original state given by point a. In this cycle, QH is the heat transferred to the evaporator from the warm sea water that vaporizes the working fluid. The working fluid exits from the evaporator as a gas near its dew point. The high-pressure, hightemperature gas is expanded in the turbine to yield turbine work, WT . The working fluid is slightly superheated at the turbine exit and the turbine typically has an efficiency of 90% based on reversible, adiabatic expansion. Various parameters of the cycle are shown in Fig. 5.50. From the turbine exit, the working fluid enters the condenser where it rejects heat, QC , to the cold sea water. The condensate is then compressed to the highest pressure in the cycle, requiring condensate pump work, WC . The major additional

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5 Ocean Energy

Fig. 5.50 Work and heat flow in an closed cycle (Printed with permission from Iqbal and Starling [132])

energy requirements in the OTEC plant are the cold water pump work, WC T , and the warm water pump work, WH T . If other energy requirement is WA , the net work from the OTEC plant, WNP is WNP D WT C WC C WC T C WH T C WA

(5.47)

A simple energy balance for the working fluid of the system may be written as: WN D QH C QC

(5.48)

WN D WT C WC

(5.49)

where, WN is the net work for the thermodynamic cycle. The other works are rather small and can be neglected. The heat transferred to the evaporating fluid (QH ) and removed in the condenser .QC / can be expressed by following expressions if there is no pressure drop in the heat exchangers, Z TH ds (5.50) QH D H

5.7

Ocean Thermal Energy Conversion (OTEC)

and,

315

Z QC D

TC ds

(5.51)

C

where, TH is the temperature at the evaporator and TC is the temperature at the condenser and s is the entropy. The net thermodynamic cycle work becomes: Z

Z

WN D

TH ds C H

TC ds

(5.52)

C

The net thermal efficiency of the cycle is calculated as: D

WN QH

(5.53)

5.7.2 Open-Cycle OTEC System In an open-cycle OTEC system, warm seawater is the working fluid. The warm seawater is “flash” evaporated in a vacuum chamber to produce steam at an absolute pressure of about 2.4 kilopascals (kPa). The steam expands through a low-pressure turbine that is coupled to a generator to produce electricity. The steam exiting the turbine is condensed by cold seawater pumped from the ocean’s depths through a cold-water pipe. If a surface condenser is used in the system, the condensed steam remains separated from the cold seawater and provides a supply of desalinated water. A schematic diagram of the cycle is shown in Fig. 5.51. The T-s diagram of an open-cycle OTEC cycle is shown in Fig. 5.52. It may be assumed that the process starts at State 1, which defines the condition of the warm surface seawater. The sea water is then introduced into the deaerator at Warm seawater in

Non-condensable Desalinated gases water vapor Desalinated (saturated) water vapor (unsaturated) Vacuum Deaeration chamber Turbo(optional) flash generator evaporator

Non-condensable Warm seawater discharge to sea gases

Cold seawater discharge to sea

Condenser

Desalinated water (optional)

Cold seawater in

Fig. 5.51 A schematic diagram of an open-cycle OTEC process (Courtesy of Ocean Thermal Energy Conversion, NREL [130])

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5 Ocean Energy

Fig. 5.52 T s diagram of an open cycle OTEC system (Printed with permission from Avery and Wu) [131]

State 2, where the pressure is reduced suddenly allowing the water to flash evaporate. The majority of the dissolved gases are removed this way. The thermodynamic state of water in the flash evaporator is the same as that in the deaerator (State 2). The ambient pressure is dropped to the saturation pressure at this point. The steam produced at this point is referred to State 3. A two-phase system exists at this state. The vapor phase is designated as 3g , the mass flow rate of which can be written as x m, P where x is the vapor mass fraction and m P is the total warm water mass flow rate. The vapor expands through a turbine at this point converting thermal energy to mechanical energy. The liquid from State 3 having a flow rate of .1x/m P is pumped back to the sea at State 7. The turbine exhaust which is at State 4 is condensed using cold sea water to State 5. The condensate can be pumped back to sea water under the condition given by State 6. The condensate is considered fresh water and can be used for other purposes such as irrigation. However, it is released to the sea; its thermodynamic state will follow State 6 to State 7 to State 1.

5.7.2.1 Work Done by Open Cycle OTEC The efficiency of the open cycle system can be calculated as follows. QP D m P H CP .TH TW /

(5.54)

5.7

Ocean Thermal Energy Conversion (OTEC)

317

TC P Pg D T Qin 1 TH

(5.55)

QP out D QP in Pg

(5.56)

Pn D Pg PCSW PWSW Pmisc

(5.57)

D

Pn QP i n

(5.58)

Where, m P H is the warm water mass flow rate, TH is the warm water inlet temperature, TW is the outlet temperature of the warm water, Tc is the temperature of condensate, CP is the specific heat, QP in is the heat input to the system, QP out is the heat rejected from the system, is the overall system efficiency, T is the turbine efficiency, Pn is the net power, Pg is final power output after taking into account various losses given by PCWS (parasitic loss in cold water loop), PWSW (warm water loop loss), and Pmisc (other losses).

5.7.3 Hybrid OTEC System A hybrid cycle combines the features of both the closed-cycle and open-cycle systems. In a hybrid OTEC system, warm seawater enters a vacuum chamber where it is flash-evaporated into steam, which is similar to the open-cycle evaporation process. The steam vaporizes the working fluid of a closed-cycle loop on the other side of an ammonia vaporizer. The vaporized fluid then drives a turbine to generate electricity. The steam condenses within the heat exchanger and provides desalinated water. A schematic diagram of the hybrid system in shown in Fig. 5.53.

5.7.4 Components of an OTEC System The major components of OTEC systems are heat exchangers, evaporators, turbines, and condensers. The design of various components of OTEC systems depends on the working fluid. Various fluids have been proposed over the past decades for use in a closed OTEC cycle. However, the best working fluid appears to be ammonia, due to its superior transport properties, easy availability, and low cost. Ammonia, however, is toxic and flammable. The power plant size is dependent upon the vapor pressure of the working fluid. For fluids with high vapor pressure, the size of the turbine and heat exchangers decreases while the wall thickness of the pipe and heat exchangers should increase to endure high pressure especially on the evaporator side. The materials for construction of these components are a major issue and costly. The materials must be resistant to corrosion from sea water.

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Fig. 5.53 A schematic diagram of the hybrid OTEC system (Courtesy of Ocean Thermal Energy conversion, NREL [130])

5.7.5 Byproducts of OTEC System OTEC systems have many other applications other than electricity generation. OTEC can be used to produce desalinated water, support deep-water aquaculture (mariculture), and provide refrigeration and air-conditioning. These applications may make OTEC systems attractive to industry and island communities even if the price of oil remains low. Various uses are summarized in Fig. 5.54.

5.8 Summary The energy generation from the ocean is a developing technology. Although many energy devices have been invented, only a small number have been tested and evaluated, and very few of these have been tested in oceans – testing has usually been undertaken in a tank. In general terms ocean generation has the following advantages and disadvantages. Advantages: • the energy is free – no fuel is needed and no waste is produced • not expensive to operate and maintain • can produce a significant amount of energy. Disadvantages: • variable energy supply but more consistent than wind or solar energy • needs a suitable site, where waves or currents are consistently strong

Problems

319

Fig. 5.54 OTEC by products and their applications (Courtesy of Ocean Thermal Energy conversion, NREL [130])

• • • •

must be able to withstand very rough weather costly to develop visual impact if turbines are above water or on shore can disturb or disrupt marine life – including changes in the distribution and types of marine life near the shore • poses a possible threat to navigation from collisions • may interfere with mooring and anchorage lines with commercial and sportfishing • may degrade scenic ocean front views from wave energy devices located near or on the shore, and from onshore overhead electric transmission lines.

Problems 1. Why is ocean energy considered renewable? 2. What is the outer continental shelf (OCS)? 3. How does ocean thermal energy work? 4. What are various technologies for harvesting ocean energy? 5. What are the advantages and disadvantages of ocean energy? 6. What energy sources can be utilized from the ocean for electricity generation?

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7. What are the uses of ocean wave’s energy resource? 8. What are the advantages of ocean tidal energy? 9. Ocean waves of wavelength 100 m reach a maximum height of 20 m in 10 s with a velocity of 20 m/s. What is the power density of the wave? 10. How does ocean wave or tidal energy work? 11. What are the obstacles for mass scale utilization of ocean energy? 12. Is there a limitation on how much energy can be produced from ocean energy in a particular location? 13. Can ocean energy be used as a base load power? 14. How close to shore do some of the wave energy systems have to be for transmission of power to onshore or customers? 15. Is there a difference between marine current energy and tidal energy? 16. Can you propose any new concept designs for utilization of ocean power? 17. How would you design, build, and test a ocean energy device for power generation. 18. How would you advance wave energy conversion technologies – including point absorbers, oscillating water column devices, over-topping devices, and wave attenuators – along the commercial development path by demonstrating the economic and technical viability of their inventions? 19. How would you advance critical hydrokinetic turbine technologies – including horizontal and vertical axis water turbines, either tethered or fixed-mounted in a moving water stream – along the commercial development path by demonstrating the economic and technical viability of their systems? 20. Discuss a technical and integrated operational description of an ocean thermal energy conversion system including a detailed engineering analysis of the concept at full scale, a detailed engineering-economic analysis of the system at full scale based on the system and component capital costs, O&M costs (including refurbishment, levelized replacement, etc.), labor, fuel, and other costs such as decommissioning, and the expected energy and/or other (ammonia, hydrogen, water, etc.) production returns; and analysis of the critical design factors of the approach.

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Chapter 6

Bioenergy

Abstract Bioenergy is derived from various biological sources, called biomass, and is considered a renewable energy source, since biomass can be replenished on a regular basis. Biomass offers opportunity in every part of the world to develop sustainable resources including fuel, power, and chemicals. Biomass can be used to generate heat, electricity, and transportation fuel (called biofuel). In this chapter, the energy content of various types of biomass and their conversion to useful energy sources are discussed. Both the cellulosic and lignocellulosic based biomass can be used for energy generation or for biofuel synthesis. Various methods developed to process biomass are also discussed in this chapter.

6.1 Introduction Energy derived from the biomass is called bioenergy. Biomass can be vegetationtrees, grasses, plants parts such as leaves, stems and twigs, sea weeds, and waste products from various industries—including agriculture, forest products, transportation, and construction—that dispose of large quantities of wood and plant products. All of these materials can be used for generation of energy. Since some biomass, such as trees and plants, can be cultivated on a regular basis and replenished, bioenergy is considered a renewable energy source [1–19]. Biomass can be also used to produce biofuels, which is short for biomass fuel. It can be in the form of both liquid and gas. Another term, “biopower” refers to biomass power systems that produce electricity. The major use of “biofuels” is in the transportation sector. Bioenergy may be considered as a carbon neutral system. Carbon dioxide is released back into the atmosphere when burning biomass. However, it is hypothesized that there is little or no net addition of carbon to the atmosphere. If the growing of bioenergy crops is optimized to add humus to the soil, there may even be some net sequestration or long-term fixation of carbon dioxide into soil organic matter. Therefore, it is assumed that the bioenergy cycle, shown in Fig. 6.1, is not only renewable, but it can also provide a stable energy source without harming the environment. T.K. Ghosh and M.A. Prelas, Energy Resources and Systems: Volume 2: Renewable Resources, DOI 10.1007/978-94-007-1402-1 6, © Springer Science+Business Media B.V. 2011

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Fig. 6.1 The bioenergy cycle (Courtesy of Oakridge National Laboratory [20])

Carbon dioxide is a naturally occurring gas. Plants collect and store carbon dioxide to aid in the photosynthesis process. As plants or other matters decompose, or natural fires occur, CO2 is released. Before the anthropomorphic discovery of fossil fuels, the carbon dioxide cycle was stable; the same amount that was released was sequestered, but it has since been disrupted. In the past 150 years, the period since the industrial revolution, the carbon dioxide level in the atmosphere has risen from around 150 to 330 ppm, and is expected to double before the year 2050. The burning of any type of fossil fuel: coal, natural gas, or petroleum, releases CO2 into the atmosphere and is considered a major contributor to the current increased level. Carbon dioxide is released into the atmosphere in various ways and its estimated quantities is shown in Fig. 6.2. Worldwide, biomass is the fourth largest energy resource after coal, oil, and natural gas. It is used for heating (for example, wood stoves in homes and for

6.1

Introduction

329

Fig. 6.2 Amount of CO2 stored in the earth and its annual release from various sources (Courtesy of NASA [21]) Fig. 6.3 Fuel share of total primary energy supply of 11741 Mtoe in 2006 (Courtesy of International Energy Agency [22])

Hydro 2.2% Nuclear 6.2%

Combustible renewables & waste 10.1%

Other∗∗ 0.6% Coal/peat 26.0%

Gas 20.5% Oil 34.4%

process heat in bioprocessing industries), cooking (especially in many developing countries), as transportation fuels, such as ethanol, and increasingly, for electric power production. As shown in Fig. 6.3, combustible renewables and wastes contribute almost 10.1% of the world’s total energy supply. Out of this 10.1%, more than 90% is from biomass.

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Fig. 6.4 Share of electricity production by fuel types worldwide in 2006 (Courtesy of International Energy Agency [22])

Hydro 16.0%

Other∗∗ 2.3%

Coal/peat 41.0%

Nuclear 14.8% Gas 20.1%

Oil 5.8%

Fig. 6.5 Renewable energy consumption by regions and contribution of biomass (Adapted from FAO [23])

Although biomass contributes about 1,067 million tons oil equivalent (Mtoe) of energy worldwide, the electricity generation from biomass is less than 2% (Fig. 6.4). Developing countries are dominating on the use of bioenergy, although a number of developed countries such as the US, Canada, and several European countries have significant bioenergy resources. It is also expected that the biomass will continue to be a significant source of energy for these developing countries in the near future. In Fig. 6.5 is shown the current and projected use of bioenergy by different regions of the world.

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331

Fig. 6.6 Bioenergy consumption by G8 C 5 countries. Food and Agriculture Organization (Adapted from FAO [23])

Among G-8 countries, the USA is well ahead of other countries in utilizing biomass for energy production. Other countries are using only a small amount of biomass. In comparison, India and China together generate more than six times of bioenergy than that of combined G-8 countries. These data are shown in Fig. 6.6. In terms of percent contribution of bioenergy to the total energy consumption, most of the G-8 countries are still below 5%, whereas it is more than 20% for Brazil, China, and India (see Fig. 6.7). Among all the continents, Africa is the world’s largest consumer of biomass energy when calculated as a percentage of overall energy consumption. Biomass used in Africa includes firewood, agricultural residues, animal wastes, and charcoal. Biomass accounts for almost two-thirds of the total African energy consumption, which is equivalent to 205 Mtoe of biomass and 136 Mtoe of conventional energy in 1995, according to the International Energy Agency. Most of Africa’s biomass

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Fig. 6.7 Bioenergy consumption in G8 C 5 countries. Percent of biofuels considering all fuels (Adapted from FAO [23])

energy use is in sub-Saharan Africa. Biomass accounts for 5% of North African, 15% of South African, and 86% of sub-Saharan (minus South Africa) consumption. In the USA, about half of the biomass used today comes from burning wood and wood scraps such as saw dust. Another third is from biofuels, principally from ethanol that is used as a gasoline additive. The rest comes from crops, garbage, and landfill gas. Industries are the biggest user of biomass. Biomass use by various sectors in the USA is given in Fig. 6.8. Homes are the next biggest users of biomass; about one-fifth of homes in the USA burn wood for heating. Most of these homes

Energy Source of Biomass 60 52.3 50

40

30

12.6

20

10.9

21.4

Fig. 6.8 Biomass consumption by various sectors in the USA in 2008 (Courtesy of National Energy Education Development Project [24])

333

Biomass Consumption (% of total)

6.2

Commercial

Electricity

Residential

Transportation

0

Industry

2.8

10

burn wood in fireplaces and wood stoves for additional heat. A detailed breakdown of the use of biomass by various sectors and fuel type is given in Table 6.1. A general overview of the potential contribution of renewable energy to the world’s energy resources is given in Table 6.2. The contribution of bioenergy to the world’s future energy demand under two scenarios (technical and theoretical) is given in Table 6.2. The United Nations conducted a study on the cost of electricity generation from biomass. The study showed that the biomass is competitive with other renewable energy sources, and just like other sources, the price is expected to go down in the near future (see Table 6.3).

6.2 Energy Source of Biomass Plants store energy as carbohydrates or sugar, lignin and cellulose. During photosynthesis, plants use sunlight to combine carbon dioxide from air and water from the soil to form carbohydrates, which are the building blocks of biomass. The structure of the biomass is shown in Fig. 6.9. While the actual ratio of components varies among species, the main components are carbohydrates or sugars and lignin.

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Table 6.1 Biomass consumption by various sectors in the USA (in Quadrillion BTU) Sector and energy source 2000 2001 2002 2003 2004 2005 2006 Total 6.264 5.316 5.893 6.150 6.261 6.444 6.922 Biomass 3.013 2.627 2.706 2.817 3.023 3.154 3.374 Biofuelsa 0.241 0.258 0.309 0.414 0.513 0.595 0.795 0.511 0.364 0.402 0.401 0.389 0.403 0.407 Wasteb Wood and derived fuels 2.262 2.006 1.995 2.002 2.121 2.156 2.172 Residential 0.490 0.439 0.449 0.471 0.483 0.527 0.495 Biomass 0.420 0.370 0.380 0.400 0.410 0.450 0.410 0.420 0.370 0.380 0.400 0.410 0.450 0.410 Wood and derived fuelsc 0.128 0.101 0.104 0.113 0.118 0.119 0.117 Commerciald Biomass 0.119 0.092 0.095 0.101 0.105 0.105 0.102 Biofuelse NA NA NA 0.001 0.001 0.001 0.001 0.047 0.025 0.026 0.029 0.034 0.034 0.036 Wasteb 0.071 0.067 0.069 0.071 0.070 0.070 0.065 Wood and derived fuelsf Industriald 1.930 1.721 1.723 1.731 1.861 1.884 1.999 Biomass 1.884 1.684 1.679 1.684 1.824 1.848 1.966 0.102 0.112 0.136 0.178 0.217 0.248 0.311 Biofuelsg 0.145 0.129 0.146 0.142 0.132 0.148 0.140 Wasteb Wood and derived fuelsf 1.636 1.443 1.396 1.363 1.476 1.452 1.515 Transportation 0.138 0.145 0.172 0.235 0.296 0.346 0.483 0.138 0.145 0.172 0.235 0.296 0.346 0.483 Biofuelsh 3.579 2.910 3.445 3.601 3.503 3.568 3.827 Electric power sectori Electric Utilitiesd 2.607 2.063 2.529 2.615 2.522 2.530 2.688 Biomass 0.021 0.014 0.033 0.029 0.031 0.040 0.042 0.014 0.008 0.022 0.012 0.011 0.013 0.015 Wasteb 0.007 0.006 0.011 0.017 0.020 0.027 0.027 Wood and derived fuelsf Independent power producer 0.972 0.847 0.916 0.986 0.981 1.038 1.139 Biomass 0.432 0.323 0.347 0.368 0.357 0.365 0.370 Wasteb 0.305 0.202 0.208 0.218 0.212 0.208 0.216 0.127 0.121 0.140 0.151 0.145 0.158 0.154 Wood and derived fuelsf Source: Energy Information Administration [25] Revised data are in italics. Totals may not equal sum of components due to independent rounding a Biofuels and biofuel losses and coproducts b Municipal solid waste biogenic, landfill gases, agriculture byproducts/crops, sludge waste, and other biomass solids, liquids and gases. Includes municipal solid waste nonbiogenic and tires for 1989–2000 c Wood and wood pellet fuel d Includes small amounts of distributed solar thermal and photovoltaic energy used in the commercial, industrial and electric power sectors e Ethanol primarily derived from corn f Black liquor, and wood/woodwaste solids and liquids g Ethanol primarily derived from corn and losses and coproducts from production of biodiesel and ethanol h Biodiesel primarily derived from soy bean oil and ethanol primarily derived from corn i The electric power sector comprises electricity-only and combined-heat-power (CHP) plants within North American Industry Classification System (NAICS) 22 category whose primary business is to sell electricity, or electricity and heat, to the public

6.3

Composition of Biomass

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Table 6.2 Overview of current use, and the technical and theoretical potentials of different renewable energy options (EJ Exajoule, 1 EJ D1018 J) Source Current use (EJ) Biomass energy 50 Hydropower 9 Solar energy 0.1 Wind energy 0.12 Geothermal energy 0.6 Ocean energy NA Source: World Energy Assessment [26]

Technical potential (EJ) 200–400 .C/ 50 >1;500 640 5,000 NA

Theoretical potential (EJ) 2,900 147 3,900,000 6,000 140,000,000 >140,000,000

Table 6.3 Cost ranges (US-cents per unit) for production of electricity, heat, and fuel from various renewable energy options in 2004 and projection for 2050 Potential long-term future Technology Current energy cost energy cost (2050) Biomass energy (based on energy crops as feedstock) • Electricity USc= 7–21/kWh electricity USc= 5.6–14/kWh electricity • Heat USc= 1.4–7/kWh fuel USc= 1.4–7/kWh • Biofuels USc= 11.22–35/GJ fuel USc= 8.4–14/GJ Wind electricity USc= 7–18.2/kWh USc= 4.2–14/kWh Solar PV electricity USc= 35–175/kWh USc= 7–35/kWh Solar thermal electricity USc= 17–25/kWh USc= 5.6–14/kWh Low temperature solar heat USc= 4.2–28/kWh USc= 2.8–28/kWh Hydroelectricity USc= 2.4–14/kWh USc= 2.8–14/kWh Geothermal energy • Electricity USc= 2.8–14/kWh USc= 1.4–14/kWh • Heat USc= 0.7–7/kWh USc= 0.7–7/kWh Source: World Energy Assessment [26]

Most species also contain a smaller molecular fragments called extractives. When biomass is burned, oxygen in the air reacts with the carbon in plants to produce carbon dioxide and water.

6.3 Composition of Biomass The composition of biomass determines their use. Biomass that is rich in carbohydrates, which is essentially glucose, is suitable for generation of biofuels. The carbohydrate fraction consists of many sugar molecules linked together in long chains or polymers. In a number of plants, biomass is present in the form of starch. Starch is composed of glucose, but it is a mixture of ’-amylose and amylopectin (see Fig. 6.10). Starches found in nature are 10–30% ’-amylose and 70–90% amylopectin. Starch is soluble in water and relatively easy to break down into utilizable sugar units.

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Fig. 6.9 Plant cell structure. US Department of Energy (2008) An overview of science (Adapted from Bioenergy Research Center [27])

6.3.1 Lignocellulosic Biomass The non-grain portion of biomass (e.g., cobs, stalks), often referred to as agricultural stover or residues, and energy crops such as switchgrass, contains biomass in the form of lignin or cellulose. These lignocellulosic biomass resources are not as

6.3

Composition of Biomass

337

CH2OH

a

CH2OH

CH2OH

O

O

O

OH

OH

OH O

O

O OH

OH

OH

b

O

OH

O

O

CH2OH

OH

CH2OH O

OH

CH2OH

O

HO

OH

O

HO

CH2OH

O

O

O

O

OH

O

OH

OH O

O

CH2OH

CH2OH

CH2O

OH

OH

O

O

O OH

OH

OH

Fig. 6.10 Chemical structure of (a) alpha-amylose and (b) amylopectin (Courtesy of Garrett and Grisham [28]) OH

HO

HO

O

HO

H

OH O OH

H

H

O HO

O

O

H

O

H

O

O

OH

O

O OH

O

HO

HO

OH

O

O

O

H

H

OH

O O

OH O

O

O

H

O

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HO HO

HO

HO O

O

H

H

OH

O

O

O

OH

HO

O

O

H

H

O

O

O

O

O

O

O

Fig. 6.11 Chemical structure of cellulose (Printed with permission from Sierra et al. [29])

readily accessible as starch. They are also called cellulosic and are comprised of cellulose, hemicellulose, and lignin. Generally, lignocellulosic material contains 30–50% cellulose, 20–30% hemicellulose, and 20–30% lignin. Some exceptions to this are cotton that contains 98% cellulose and flax that has 80% cellulose. Cellulose is also composed of glucose. However, in cellulose, glucose molecules are joined by “-1,4-glycosidic linkages (see Fig. 6.11) making them very stable chemically and insoluble. They serve as a structural component in plant walls.

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Fig. 6.12 Hemicellulose structure (Printed with permission from Sierra et al. [29])

6.3.2 Hemicellulose Hemicellulose is also a polymer containing primarily 5-carbon sugars such as xylose and arabinose (see Fig. 6.12). Glucose and mannose molecules are dispersed throughout within the structure. It forms a shortchain polymer that interacts with cellulose and lignin to form a matrix in the plant wall, strengthening it. Hemicellulose is more easily hydrolyzed than cellulose. Much of the hemicellulose in lignocellulosic materials is solubilized and hydrolyzed to pentose and hexose sugars.

6.3.3 Lignin Lignin helps bind the cellulose/hemicelluloses matrix while adding flexibility to the mix. The molecular structure of lignin polymers is very random and disorganized and consists primarily of carbon ring structures containing benzene rings with methoxyl, hydroxyl, and propyl groups. They are interconnected by polysaccharides (sugar polymers) (see Fig. 6.13). The ring structures of lignin have great potential as valuable chemical intermediates. However, the separation and recovery of lignin from plants is difficult. Lignin can be burned to produce the electricity required for the ethanol production process. Burning lignin directly can provide more energy than needed and selling extra electricity may help the process economics.

6.4 Types of Biomass Biomass resources may be divided into following categories: • Biomass Processing Residues – Pulp and Paper Industry Residues – Forest Residues – Agricultural or Crop Residues

Types of Biomass

339

O

C H 3

6.4

O

O

OCH3OCH3 OCH3 O HO

O O HO OH H3CO OCH3

OCH3

OCH3 OH

OCH3OCH3 O OH O O HO OH H3CO OCH3

OCH3 OH

Fig. 6.13 Lignin structure (Printed with permission from Sierra et al. [29])

• Municipal Solid Wastes – Landfill Gas • Urban Wastes • Animal Wastes • Energy Crops – – – – –

Herbaceous Energy Crops Woody Energy Crops Industrial Crops Agricultural Crops Aquatic Crops

These resources are discussed in the following section. Biomass Processing Residues: All industrial processes that use biomass produce byproducts and waste streams called residues, which have significant energy content. Some of these residues can be used to generate electricity. Others may be recycled back to the soil as a source of fertilizer. Pulp and Paper Industry Residues: Paper industry generates significant amount of biomass residues from the processing of plants where cellulose fiber is separated

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from the plants by making the pulp. Pulping process may be described as the separation and breaking down of the lignin fibers of a plant. The cellulose fibers is extracted to create paper. Leftover pulp, residues of logging and processing of wood create wastes that cannot be used in paper production. These wastes along with sawdust, barks, branches, leaves/needles, and chipped wood are used for power generation in paper mills. This power actually contributes a significant percent of the overall power consumption of paper mills. It may be noted that paper mills are an energy intensive industry. Forest Residues: These include wood from forest thinning operations, biomass from logging sites of commercial hardwood and softwood processing operations, and removal of dead and dying trees. Agricultural or Crop Residues: These are the biomass discarded during harvesting. They can be collected and prepared as pellets, chips, stacks or bales. Agriculture crop residues include corn stover (stalks and leaves), wheat straw, rice straw and nut hulls. Corn stover is expected to become a major biomass resource for bioenergy applications. Municipal Solid Wastes: These are waste paper, cardboard, wood waste and yard wastes. Landfill Gas: Biomass in various landfills is decomposed using bacteria to produce methane, which can be captured and used to create energy, most often through anaerobic digestion (AD). Urban Wastes: The construction industry generates significant amount of wood wastes. According to an estimate by the California Integrated Waste Management Board, CA, USA, about 4 million tons of wood wastes are available in California alone. Urban wood wastes generally consist of lawn and tree trimmings, tree trunks, wood pallets and other construction and demolition wastes made from lumber. Animal Wastes: These include cattle, chicken and pig manure. These can be converted to gas or burned directly for heat and power generation. The wastes may be processed to generate methane, which can be burned further to generate electricity. Generally, anaerobic digestion methods are used for conversion of animal manure to methane Energy Crops: These are fast-growing plants, trees, and other herbaceous biomass, which are harvested specifically for energy production. These crops can be grown, cut and replaced quickly. A list of potential plants which may be used as energy crops is given in the Handbook of Energy Crops [30]. Energy crops may be divided into several categories and are discussed below. Herbaceous energy crops are perennials, but takes 2–3 years before they can be harvested. These include grasses such as switchgrass, miscanthus (Elephant grass), bamboo, sweet sorghum, tall fescue, kochia, and wheatgrass.

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341

Woody Energy Crops include hybrid poplar, hybrid willow, silver maple, eastern cottonwood, green ash, black walnut, sweetgum, and sycamore. Generally, they are fast growing hardwood trees and have short-rotation time between harvesting; these trees may be used within 5–8 years after planting. Industrial Crops include plants such as kenaf and straws. They are more fibrous than others and are considered as industrial crops. These plants are grown specifically to produce industrial chemicals. For example, castor plants can be used for ricin or oleic acid. Agricultural Crops include cornstarch, corn oil, soybean oil and meal, wheat starch, and other vegetable oils. They generally yield sugars, oils, and extracts. Soybeans and sunflowers seeds are used to produce oil, which can be used to make fuels. These plants are also called oil-plants. Aquatic Crops are a wide variety of aquatic biomass such as algae, giant kelp, other seaweed, and marine microflora. They can be used for bioenergy generation. Giant kelp extracts are already used for thickeners and food additives. Global biomass resources vary widely from one country to another country. Also, it is extremely difficult to make an annual estimate of these resources. A number of variables such as rainfall, use of fertilizer and pesticides, and the availability of irrigation system can significantly affect the yield of the energy crops, and, thereby, the estimation. International Energy Agency (IEA) has compiled data from various researchers and projected the use of bioenergy by 2050 under various biomass resource categories. Their data are shown in Table 6.4. An intensive farming may be necessary to produce energy crops on a large-scale, which may result in loss of biodiversity. A balance between conventional crops, such as cereals and seeds, and energy crops may be necessary for optimal utilization of land. Also, it may be necessary to free up grassland currently used for grazing. These issues are discussed in more details in Volume 4 of this book series.

6.5 Biomass Resources, Land Requirement, and Production The energy contents of all of the biomass that are available today is estimated to be 2,740 Quads. At present, the world population uses only about 7% of the annual production of biomass. The large-scale use of existing forest resources for bioenergy generation is not feasible and should not be encouraged. The large scale supply of biomass should be from two sources: • Residues associated with current agricultural commodity production and processing • Energy crops grown on available land The USA uses three types of biomass: wood and wood wastes, solid wastes (garbage and landfill wastes), and biofuels. The percent contribution of these resources is shown in Fig. 6.14.

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Table 6.4 Overview of the global potential of biomass for energy (EJ (Exajoules) per year) generation to 2050 under various scenarios Energy potential in Biomass category Main assumptions and remarks biomass up to 2050 Potential land surplus: 0–4 Gha (average: Energy farming 0–700 EJ (more 1–2 Gha). A large surplus requires structural on current average deadaptation towards more efficient agricultural agricultural velopment: production systems. When this is not feasible, land 100–300 EJ) the bioenergy potential could be reduced to zero. On average higher yields are likely because of better soil quality: 8–12 dry ton/ha/yeara is assumed <60–110 EJ On a global scale a maximum land surface of Biomass 1.7 Gha could be involved. Low productivity production on of 2–5 dry ton/ha/year.a The net supplies could marginal be low due to poor economics or competition lands with food production 15–70 EJ Residues from Potential depends on yield/product ratios and the agriculture total agricultural land area as well as type of production system. Extensive production systems require re-use of residues for maintaining soil fertility. Intensive systems allow for higher utilization rates of residues 30–150 EJ Forest residues The sustainable energy potential of the world’s forests is unclear – some natural forests are protected. Low value: includes limitations with respect to logistics and strict standards for removal of forest material. High value: technical potential. Figures include processing residues 5–55 EJ Dung Use of dried dung. Low estimate based on global current use. High estimate: technical potential. Utilization (collection) in the longer term is uncertain 5–50 EJ Organic wastes Estimate on basis of literature values. Strongly dependent on economic development, consumption and the use of bio-materials. Figures include the organic fraction of MSW and waste wood. Higher values possible by more intensive use of bio-materials 40–1100 EJ Combined Most pessimistic scenario: no land available for (200–400 EJ) potential energy farming; only utilization of residues. Most optimistic scenario: intensive agriculture concentrated on the better quality soils. In parentheses: average potential in a world aiming for large-scale deployment of bioenergy Sources: International Energy Agency (IEA) [31] based on data presented by Berndes et al. [19], Smeets et al. [18], and Hoogwijk et al. [32] EJ Exajoule, Gha Global hectare, ha hectare, MSW Municipal solid waste a Heating value: 19 GJ/ton dry matter

6.5

Biomass Resources, Land Requirement, and Production

Fig. 6.14 US sources of biomass, 2006 (Courtesy of The NEED Project [24])

343

Garbage and Landfills Waste 11.1%

Biofuels 36.4%

Wood and Wood Waste 52.5%

According to an estimate by the US Department of Energy (USDOE) and the US Department of Agriculture (USDA) [24], 333 million tons (mt) of biomass feedstocks are available annually from forestlands (Forestlands account for an estimated 33% of the US land area). This includes: 47 mt from harvesting of fuelwood, 131 mt from wood processing and pulp and paper mills, 43 mt from urban wood residues, 58 mt from logging and site-clearing operations, and 55 mt from forest fire-hazard reduction efforts. Agricultural lands are estimated to account for approximately 46% of the entire US land base with 26% consisting of grassland pasture and range, and 20% consisting of cropland. The biomass feedstock available from agricultural lands, while still meeting food, feed and export demands, could be 910 mt annually. This includes the following: 390 mt from crop residues, 343 mt from perennial energy crops (both tree and herbaceous), 80 mt of grains from biofuels, and 96 mt from animal manure, process residue, and miscellaneous feedstocks on a dry basis. To sustain biomass based energy generation systems, a continuous supply of biomass is necessary. A number of assessments have been carried out in the USA to determine the sustainability of bioenergy systems [4, 9, 15, 33]. A variety of sources are examined including energy crops. These studies noted that switchgrass, alfalfa, willow, poplar and eucalyptus are examples of plants that can be grown as energy crops. In the USA, switchgrass has been the subject of considerable research and development. In Europe, miscanthus is considered as the most promising energy crop. Availability of other types of biomass in Europe has been studied by Grassi [34], McCormick and Kaberger [6], and IEA [35, 36]. Several countries are also engaged in careful assessment of the use of bioenergy and its sustainability without using the agricultural land. These countries include: Bangladesh [37], Canada [33, 36–39], China [40–42], Southeast Asia [43, 44], India [12, 45], Poland [46],

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Finland [47–50], UK [51, 52], Denmark [53, 54], Jamaica [55], Sweden [56, 65], Mexico [57], Brazil [58, 59], Australia [60], Argentina [61], Turkey [62], New Zealand [63], Netherlands [64]. The application of genetic engineering to increase drought tolerance and improve nutrient-use efficiencies also promises the availability of lands for growing improved crops that would not have been possible otherwise [66–70].

6.5.1 Energy Crops Production Area The regions in the USA that are suitable for growing energy crops are shown in Fig. 6.15. The US is focusing more on the switchgrass as an energy crop due to better growing conditions in several regions. One of the main concerns of using biomass is its sustainable availability. Various natural factors such as weather and rainfall can dramatically change the annual production. Several studies in the USA noted that the physical near-term sustainable biomass potential is over 1 Gt of biomass (18 EJ of primary energy equivalent), which should be sufficient to sustain a large scale bioenergy production system. A GIS-based atlas of the biomass resources of the US is available from the NREL.

Fig. 6.15 Energy crops production area in the USA (Source: US Department of Energy [72])

6.5

Biomass Resources, Land Requirement, and Production

Table 6.5 Yield of various perennial grasses in the USA and Europe Common English Photo-synthesis Yields reported name Latin name pathway (t DM ha1 a1 ) Meadow foxtail Alopecurus C3 6–13 pratensis L. Big bluestem Andropogon gerardii C4 8–15 Vitman Giant reed Arundo donax L. C3 3–37 Cypergras, galingale Cyperus longus L. C4 4–19 Cocksfoot grass Dactylis C3 8–10 glomerata L. Tall fescue Festuca arundinacea C3 8–14 Schreb. Raygras Lolium ssp. C3 9–12 Miscanthus Miscanthus spp. C4 5–44 Switchgrass Panicum virgatum L. C4 5–23 Napier grass Pennisetum C4 27 purpureum Schum Reed canary grass Phalaris C3 7–13 arundinacea L. Timothy Phleum pratense L. C3 9–18 Common reed Phragmites C3 9–13 communis Trin. C4 27 Energy cane Saccharum officinarum L. Giant cordgrass Spartina C4 9 cynosuroides L. Salt reedgrass – – 5–20 Prairie cordgrass Spartina pectinata C4 4–18 Bosc.

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References [73]

[74] [75] [76] [77] [77] [78] [79] [80]

[81] [73] [82] [80] [73, 83] – [73]

Source: Lewandowski et al. [71]

Europe evaluated about 20 perennial grasses. Four perennial rhizomatous grasses (PRG), namely miscanthus (Miscanthus spp.), reed canarygrass (Phalaris arundinacea), giant reed (Arundo donax) and switchgrass (Panicum virgatum) were chosen for further study. Reed canarygrass and giant reed are native to Europe. Lewandowski et al. [71] compiled the information on the yield of various grasses from the literature. Their findings are given in Table 6.5. The amount of land involved in the production of various energy crops in Europe is shown in Table 6.6.

6.5.2 Lignocellulosic Based Biomass According to Alan Greenspan, former Chairman of the Federal Reserve of the USA, in a testimony to Senate Foreign Relations Committee in 2006, “Corn ethanol, though valuable, can play only a limited role, because its ability to displace gasoline

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6 Bioenergy Table 6.6 Energy crop species in Europe (areas are based on the national report) Latin name Common name Area (hectares) Brassica sp. Rape seed 800,000 Eucaliptus sp. Eucalyptus 500,000 Helianthus annuus Sunflower 91,000 Salix sp. Willow 18,000 Triticum aestivum Winter wheat (GWC) – Secale cereale Winter rye (GWC) – Triticale Triticale (GWC) – Hordeum vulgare Spring barley (GWC) – – Total for GWC (Grain Whole Crop) 9,370 – – Ethanol use (grain only) 8,850 – – Combustion of GWC 520 Beta vulgaris Sugar beet 6,250 Phalaris arundinacea Reed canary grass 4,050 Sorghum bicolor Sweet Sorghum 1,500 – Fiber Sorghum 20 Populus sp. Poplar 550 Cannabis sativa Hemp 350 Miscanthus sp. Miscanthus 170 Hibiscus cannabinus Kenaf 65 Cynara cardunculus Cardoon 55 Alnus sp. Alders 15 Arundo donax Giant reed 3 Helianthus tuberosus Jerusalem artichoke 2 Camelina sativa False flax 2 Robinia pseudoaccacia Black locust 2 Source: Luger et al. [84]

is modest at best. But cellulosic ethanol, should it fulfill its promise, would help to wean us of our petroleum dependence”. Therefore, lignocellulosic biomass can be an important feedstock for ethanol or other biofuels production. A number of energy crops are considered to be expensive feedstock mainly because of their low productivity. However, as can be seen from Fig. 6.16, lignocellulosic crops have much higher productivities, and also they are renewable and abundant. Lignocellulosic biomass is the most abundant material in the world. Its sources range from trees to agricultural residues. Lignocellulosic biomass is the nonstarch, fibrous part of plant material. Lignocellulosic feedstocks are composed primarily of carbohydrate polymers (cellulose and hemicellulose) and phenolic polymers (lignin). Lower concentrations of various other compounds, such as proteins, acids, salts, and minerals, are also present. The percentage of three main components of lignocellulosic biomass is given in Table 6.7. However, it should be noted that the percentage of these components can vary widely from one species to another. The composition of lignocellulosic feedstocks is a key factor affecting the efficiency of biofuel production during conversion processes. A detailed analysis

Biomass Resources, Land Requirement, and Production Crop Productivity, dry m.t./(ha-yr)

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Fig. 6.16 Productivity of various energy crops and lignocellulosic biomass (Printed with permission from Sierra et al. [29]) Table 6.7 Composition of lignocellulosic biomass

Component Cellulose Hemicellulose Lignin Other

Percentage 30–50% 20–40% 15–25% 5–35%

of various feedstocks is necessary to address several questions, for example, is it the right kind of tree or energy crop for the intended purpose. The composition of various biomass, trees, and crop residues are given in Tables 6.8–6.11.

6.5.3 Land for Biomass The availability of land for biomass production in any country will determine the sustainability of bioenergy systems in that country. Although various wastes could be significant resources for bioenergy, a reliable energy system cannot depend on them. Therefore, biomass or energy crops must be grown specifically for the energy production. One of the questions regarding the use of land for biomass production is whether regular farming land should be used for this purpose rather than food production. The use of farming land for biomass production can create competition for the land for food production. This can have significant impact globally on the food price and supply. Research is now underway to develop energy crops and plants that can be grown on marginal land [131–137], but with the increase in the global population, this land may be required for food production in the near future.

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6 Bioenergy Table 6.8 Composition of switchgrass, corn stover, and wheat straw (% of dry matter basis)a Minimum

Maximum

Mean

SD

Corn stover Celluloseb Structural glucan Hemicellulose Xylan Arabinan Galactan Mannan Total lignin Acid soluble lignin Acid insoluble lignin Acid detergent lignin Crude protein Ash Soil

31:3 33:8 20 19:8 1:7 0:7 0:3 15:8 1:9 13:6 3:1 3:5 4:2 –

41 41 34:4 25:8 6:1 3 1:8 23:1 3:6 19:8 5 8:7 7:5 –

37:5 37:5 26:1 21:7 2:7 1:6 0:6 18:9 2:9 16:4 4:1 4:7 6:3 1:3

2.8 2.2 4.8 2.1 1.6 1 1.1 2.6 0.9 3.1 1.3 2.2 1.2 –

Wheat straw Cellulose Structural glucan Hemicellulose Xylan Arabinan Galactan Mannan Total lignin Acid soluble lignin Acid insoluble lignin Acid detergent lignin Crude protein Ash Soil

31:5 31:5 22:6 19:2 2:4 0:8 0:3 5:3 – 5:3 7:6 1:9 1:4 –

48:6 32:6 38:8 19:7 3:2 1:5 0:9 19 – 16:5 11:2 5:7 10:2 –

37:6 32:1 28:8 19:5 2:8 1:1 0:6 14:5 2:5 10:9 9:2 3:8 6:4 –

5.7 4.5 5.7 0.3 0.6 0.5 0.4 6.2 – 7.9 1.6 1.9 3.4 –

Switchgrass Cellulose Structural glucan Hemicellulose Xylan Arabinan Galactan Mannan Total lignin Acid soluble lignin Acid insoluble lignin Acid detergent lignin Crude protein Ash Soil

31:4 31:4 22 20:2 2:7 0:7 0:3 17:7 3:3 15:8 4 1:6 4:4 –

45 38 35:1 24 3:8 1:9 0:4 22 3:7 16:5 12 3:8 8:5 –

37:3 34:2 28:5 22:8 3:1 1:4 0:3 19:1 3:5 16:2 6:4 3:1 5:9 –

4.4 2.1 3.5 1 0.5 0.5 0 1.7 0.3 0.5 2.7 0.7 1 –

Source: South Dakota State University [85] a Values in table are compiled from references listed in Table 6.1 b Cellulose and hemicellulose values may not equal the sum of individual sugars because of limited data for individual sugars

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349

Table 6.9 Analysis of carbohydrate fraction of several biomass Tree species Latin name Angiosperms Betula papryrifera Platanus occidentalis Populus tremuloides Populus deltoides Quercus falcata Eucalyptus grndis

Common name

Xylan (wt%)

Glucan (wt%)

Other sugar (wt%)

Lignin (wt%)

White birch

26

43

2.9

19

American sycamore Quaking aspen

18

44

4.9

20

17

49

4.6

21

19–14.5b

42–43b

4.7–3.2b

24–26.8b

19

41

3.6

24

19

54

Bagasse Corn stalks Soybean stalks and leaves

21.8b 18.9 11.8

39.7b 35.9 32.1

2.8b 4.1 6.7

Wheat straw

16.5

31.4– 37.0b

4.0–3.7b

23.4– 22.1b

Red pine Eastern white pine Loblolly pine Douglas fir

9.3 6.0

42 45

11.6c 14.4c

29 29

Eastern cottonwood Southern red oak Eucalyptus

26

Agricultural residues

Gymnosperms Pinus resinosa Pinus strobus

25.2b

Pinus taeda 6.8 45 15.0c 28 32 Pseudotsuga 2.8 44 18.4c menziesii Thuja Northern white 10 43 10.6c 31 occidentalis cedar Pinus radiata Radiata pine 6.4b 43b 14.0b;c 27.1b Source: Bob Dawson of the Iowa State University Biomass project [89], Pettersen [86], Krull and Inglett [87], Anon [88] nd not determined Given as the anhydropolymer b data from reference 3 c Chiefly mannan

A summary of the land use in the USA is shown in Fig. 6.17. The amount of available land with potential for biomass production and land not presently in food crop production are also shown in this figure. In the USA, a significant amount of land is not available for biomass production. The U.S. Department of Energy’s (DOE) Bioenergy Feedstock Development Program (BFDP) at ORNL has been conducting research for DOE since 1978 to

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6 Bioenergy

Table 6.10 Composition of potential lignocellulosic biomass resourcesa (data are reported as percent of dry matter basis)

Cellulosec Hemicellulosec Lignind

Acid detergent Crude proteinf Ash References lignine

Crop residues Corn stover Soybean Wheat straw

38 33 38

26 14 29

19 – 15

4 14 9

5 5 4

6 6 6

Rye straw Barley straw

31 42

25 28

– –

3 7

3 7

6 11

Warm-season grasses Switchgrass

37

29

19

6

3

6

37 39 35 41

28 29 31 33

18 – – –

6 6 – 6

6 3 – 3

6 8 7 6

[90, 91, 95, 101, 112, 114– 122] [117, 118, 122] [117, 122] [122] [123]

43

24

19

–

3

2

[71, 124–126]

35

29

–

6

3

6

[118]

24

36

–

2

10

8

[105, 127]

32

36

–

6

14

8

[105, 127]

28 25

30 25

– 14

5 –

7 13

6 11

[128, 129] [91]

27 34

12 17

– 16

8 –

17 –

9 5

23 25 33

14 35 27

11 – –

– 10 12

5 9 12

Big bluestem Indiangrass Little bluestem Prairie cordgrass Miscanthus Cool-season grasses Intermediate wheatgrass Reed canarygrass b Smooth bromegrassb Timothyb Tall fescue Other crops Alfalfab Forage sorghum Sweet sorghum Pearl milletb Sudangrass

– 3 8

[90–104] [105, 106] [91, 93, 95, 104, 107–111] [112, 113] [95]

[128] [91] [91] [105, 125] [105, 125]

Source: South Dakota State University [85] a Values in table are means or values obtained from cited references b Data for these species were available for hay only. All other species harvested for biomass c Reported directly from literature, calculated from individual sugars (cellulose D glucan; hemicellulose D xylan C arabinan C galactan C mannose), or based on fiber analysis (cellulose D ADF – ADL; hemicellulose D NDF – ADF) d Lignin is total lignin (acid soluble lignin C acid insoluble lignin), which is measured using ASTM standard method e Acid detergent lignin (ADL) is commonly used for forage. In general, acid insoluble lignin is 30% higher than ADL for legumes and two to four times greater than ADL for grasses [93] f Reported directly from literature or calculated from total nitrogen (crude protein D % total N 6:25)

25.80 33.90

17.82 19.58 16.35 18.60

Bark Douglas fir bark Loblolly pine bark

Energy crops Eucalyptus camaldulensis Casuarina Poplar Sudan Grass 81.42 78.58 82.32 72.75

73.00 54.70 0.76 1.83 1.33 8.65

1.20 0.40

0.65 0.80 0.80 0.73 1.35 0.29 0.65 0.40 0.40 2.20 1.31 0.25 1.52 0.20 2.98

– 18.26 17.70 – – 17.17 – 12.50 16.10 15.20 – 16.58 17.20 12.00 11.36

Wood Beech Black locust Douglas fir Hickory Maple Ponderosa pine Poplar Red alder Redwood Western hemlock Yellow pine White fir White oak Madrone Mango wood – 80.94 81.50 – – 82.54 – 87.10 83.50 84.80 – 83.17 81.28 87.80 85.64

Ash

Table 6.11 Elemental analysis of various biomass Name Fixed carbon % Volatiles

49.00 48.50 48.45 44.58

56.20 56.30

51.64 50.73 52.30 47.67 50.64 49.25 51.64 49.55 53.50 50.40 52.60 49.00 49.48 48.94 46.24

C

5.87 6.04 5.85 5.35

5.90 5.60

6.26 5.71 6.30 6.49 6.02 5.99 6.26 6.06 5.90 5.80 7.00 5.98 5.38 6.03 6.08

H

43.97 43.32 43.69 39.18

36.70 37.70

41.45 41.93 40.50 43.11 41.74 44.36 41.45 43.78 40.30 41.10 40.10 44.75 43.13 44.75 44.42

O

0.30 0.31 0.47 1.21

0.00 0.00

0.00 0.57 0.10 0.00 0.25 0.06 0.00 0.13 0.10 0.10 0.00 0.05 0.35 0.05 0.28

N

0.01 0.00 0.01 0.01

0.00 0.00

0.00 0.01 0.00 0.00 0.00 0.03 0.00 0.07 0.00 0.10 0.00 0.01 0.01 0.02

S

19.42 18.77 19.38 17.39

22.10 21.78

20.38 19.71 21.05 20.17 19.96 20.02 20.75 19.30 21.03 20.05 22.30 19.95 19.42 19.51 19.17

HHV, measured, kJ/g

19.46 19.53 19.26 17.62

22.75 22.35

21.10 20.12 21.48 19.82 20.42 19.66 21.10 19.91 21.45 20.14 22.44 19.52 19.12 19.56 18.65

(continued)

HHV, calculated, kJ/g

6.5 Biomass Resources, Land Requirement, and Production 351

19.85 21.16 21.54 18.56 18.54 19.80 22.43 19.25 14.95 15.80 26.12 15.10

Agricultural Peach pits Walnut shells Almond prunings Black walnut prunings Corncobs Wheat straw Cotton stalk Corn stover Sugarcane bagasse Rice hulls Pine needles Cotton gin trash

Aquatic biomass Water hyacinth (Florida) – – Brown kelp,giant, soquel point Average Source: Gaur and Reed [130]

82.14

15.77

80.40 57.90

79.12 78.28 76.83 80.69 80.10 71.30 70.89 75.17 73.78 63.60 72.38 67.30

Volatiles

Fixed carbon %

Table 6.11 (continued) Name Processed biomass Plywood

19.60 42.10

1.03 0.56 1.63 0.78 1.36 8.90 6.68 5.58 11.27 20.60 1.50 17.60

2.09

Ash

4.60 3.77 5.74

47.91

5.90 5.71 5.29 5.82 5.87 5.00 5.81 5.56 5.35 4.36 6.57 5.26

5.87

H

40.30 27.80

53.00 49.98 51.30 49.80 46.58 43.20 43.64 43.65 44.80 38.30 48.21 39.59

48.13

C

40.98

33.99 23.69

39.14 43.35 40.90 43.25 45.46 39.40 43.87 43.31 39.55 35.45 43.72 36.38

42.46

O

0.52

0.05

0.00 1.05

0.00

2.09 1.51 4.63

0.05 0.01 0.01 0.01 0.01 0.11 0.00 0.01 0.01 0.06

0.00

S

0.32 0.21 0.66 0.22 0.47 0.61 0.00 0.61 0.38 0.83

1.45

N

19.11

14.86 10.75

20.82 20.18 20.01 19.83 18.77 17.51 18.26 17.65 17.33 14.89 20.12 16.42

18.96

HHV, measured, kJ/g

19.15

15.54 10.85

21.39 19.68 19.87 19.75 18.44 16.71 17.40 17.19 17.61 14.40 20.02 15.85

19.26

HHV, calculated, kJ/g

352 6 Bioenergy

6.5

Biomass Resources, Land Requirement, and Production

353

Fig. 6.17 Major uses of land in the USA (Source: Oak Ridge National Laboratory [138])

identify and develop fast-growing trees and herbaceous crops [91, 138]. Under this program more than 100 woody species and 35 herbaceous species were evaluated for their potential to produce in large quantities over broad geographical areas. The evaluation was based on a combination of newly established species comparison trials and results of previously established trials. Maximum utilization of the available land for biomass production requires proper selection of the energy crops. In the US, the Herbaceous Energy Crops Program (HECP) evaluated 35 potential herbaceous crops which included 18 perennial grasses as potential energy crops. Switchgrass, which is the native perennial grass, showed the greatest potential [139–141]. Switchgrass is a sod-forming, warm season grass, which combines good forage attributes and soil conservation benefits typical of perennial grasses. Switchgrass tolerates diverse growing conditions, ranging from arid sites in the shortgrass prairie to brackish marshes and open woods. Two major ecotypes of switchgrass occur, a thicker stemmed lowland type better adapted to warmer, more moist habitats of its southern range, and a finer stemmed upland type, more typical of mid to northern areas. The ecological diversity of switchgrass can be attributed to three principal characteristics, genetic diversity associated with its open pollinated reproductive mode, a very deep, well-developed rooting system, and an efficient physiological metabolism. However, as shown in the US map, only a small region is suitable for growing switchgrass (Fig. 6.18). An analysis in 2007 by the Energy Information Administration (EIA) suggested that producing 25% of US electricity and transportation fuel with renewable energy by 2025 (EIA 2007) would require a significant contribution from biomass energy. To meet this target, about 67% more biomass from energy crops have to be harvested than the previous estimate used by the EIA. Most of the increase is due to assumed increases in energy crop yields over time and increases in the amount of land (mostly pasture lands) available for energy crop development.

354

6 Bioenergy

Fig. 6.18 Switchgrass plating areas in agricultural statistical districts for a price of $50.00/dry ton (Source: Walsh et al. [142]) 400

Area, million ha

350

Agriculture Pasture and Unused Land

300 250 200 150 100

France

Mexico

Venezuela

Ukraine

Colombia

Indonesia

Argentina

Canada

Australia

China

India

Russia

U.S.

0

Brazil

50

Fig. 6.19 Available arable land for selected countries (Printed with permission from Sierra et al. [29])

Many countries around the world are seriously considering biomass energy as a significant part of their energy supply in the near future. With the world’s population increasing, and the total amount of land fixed, food production must be taken into serious consideration. Certainly, the best farmland in any country will not be used for growing biomass. Therefore, less fertile land would be used, increasing the

6.5

Biomass Resources, Land Requirement, and Production

355

production cost. There is also an assumption that if there is no water shortage and increased agriculture yields in the coming decades, partly due to genetically modified crops, some agricultural land could be freed for biomass production. According to an estimate, 20–50% of current arable land would be available for biomass production. However, it is clear that the availability of land for the growth of dedicated energy crops will depend on the food requirements for the growing population. Food production will, therefore, determine which and how much of the land is available for biomass production. An estimate of the amount of land currently used for agriculture and the amount used as pasture or unused at this time by various countries are given in Fig. 6.19. Table 6.12 provides the similar data for several European countries. Table 6.12 Current area and potential area for biomass production for 27 European countries Total area (106 Ha) Austria 8:4 Belgium 3:1 Bulgaria 11:1 Cyprus 0:9 Czech Republic 7:9 Denmark 4:3 Estonia 4:5 Finland 33:8 France 55:2 Germany 35:7 Greece 13:2 Hungary 9:3 Ireland 7:0 Italy 30:1 Latvia 6:5 Lithuania 6:5 Luxemburg 0:3 Malta 0:03 Netherlands 4:2 Poland 31:3 Portugal 9:2 Romania 23:8 Slovakia 4:9 Slovenia 2:0 Spain 50:5 Sweden 45:0 U. K. 24:4 EU-27 433:1 Source: Nielsen et al. [143]

Agricultural area (106 Ha) 3:4 1:4 5:3 0:1 4:3 2:7 0:8 2:2 29:7 17:0 8:4 5:9 4:4 15:1 2:5 3:5 0:1 0:01 1:9 16:2 3:7 14:7 2:4 0:5 30:2 3:2 17:0 196:6

Arable land .106 Ha/ 1:4 0:8 3:3 0:1 3:1 2:3 0:5 2:2 18:5 11:8 2:7 4:6 1:2 8:0 1:8 2:9 0:06 0:01 0:9 12:6 1:6 9:4 1:4 0:2 13:7 2:7 5:7 113:5

(% of total area) 17 27 30 11 39 53 12 7 33 33 20 50 17 26 28 45 24 31 22 40 17 39 29 9 27 6 23 26

Hectares of agricultural land per capita 0.42 0.13 0.68 0.18 0.42 0.49 0.63 0.43 0.49 0.21 0.77 0.60 1.09 0.26 1.08 1.02 0.28 0.03 0.12 0.42 0.37 0.66 0.45 0.26 0.73 0.36 0.28 0.41

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6 Bioenergy

6.6 Wood Fuel Wood fuels currently make up about 50% of the combustible biomass. This amounts to about 24 EJ. Wood as fuels is used by both OECD member countries and a large population in various non-developing countries [144–154]. Wood fuels are used mainly for home heating in OECD member countries, whereas in nondeveloping countries it is used mainly for cooking or making charcoal for later use as fuel. Generally, highly-efficient combustion technologies under tight regulations on emissions are used in OECD countries. However, emissions from burning wood fuel are a major source of indoor air pollution and are of significant concerns. An estimated three billion people in various non-developing countries in Asia and Africa use wood fuels in small-scale appliances, such as three-stone fires and cooking stoves. The use of wood fuel in this manner is not only inefficient in terms of energy utilization, but it is also highly polluting. Pollutions from wood burning are discussed in Volume 4 of this book series. As shown in Table 6.13, Asia and Africa use most of the wood fuels. Accordingly, these two regions produce most of the wood fuels (Fig. 6.20). Wood Fuel may be classified into three categories: • Fuelwood • Charcoal • Black liquor Fuelwood and charcoal are the traditional wood products. About half is used directly for energy production, and the remainder for industrial use (lumber, veneer and paper). The waste products from the industrial use of wood result in products such Table 6.13 Wood fuel consumption in 2005 in peta joules Region Fuelwood Charcoal Africa 5;633 688 North America 852 40 Latin American Countries 2;378 485 Asia 7;795 135 Europe 1;173 14 Oceania 90 1 Total

17;921

1;361

Black liquor 33 1;284 288 463 644 22

Total 6,354 2,176 3,150 8,393 1,831 113

2;734

22,017

Source: World Energy Council [155] All FAOSTAT-forestry data revised to the version of March 2007 for the reporting year of 2005. Fuelwood data were available expressed volumetrically i.e. cubic meters, converted (after density conversion) at 10 Gj/ton. Charcoal data were available, expressed in terms of mass, converted to 30 Gj/ton. Black liquor is not reported; however, bleached and unbleached sulphate pulp are reported in terms of mass. Based on average mass yields of pulp for the two processes (bleached D0.45, unbleachedD0.55) the pulping liquor energy was calculated on the basis that the majority of its energy content is lignin with a heating value of 24 Gj/ton. North America is defined as the NFTA region and includes Canada, Mexico, and the United States. LAC represents the Latin American Countries together with the Caribbean

6.6

Wood Fuel

357

Fig. 6.20 Global woodfuel production as of 1998 (Source: Matthews [156])

as bark, trim ends, and sawdust, which are further used for energy production. In the USA and Europe, fuelwood is used primarily for home heating. Wood and its wastes (bark, sawdust, wood chips, and wood scrap) provide only about 2% of the total energy in the USA and about 16% of it is used in homes for heating and cooking. The rest of the wood and wood waste fuels are consumed by the industry, electric power producers, and commercial businesses. Charcoal is produced by pyrolysis of wood and other biomass. The wood based charcoals often are of superior quality than coal based charcoal. Coconut based charcoals (produced by pyrolysis of hard shell of coconut) are used in variety of specialized applications. The energy efficiency of charcoal production ranges from 25% in Africa to 48% in Brazil, who uses industrial kilns with extensive energy and materials recovery. The Food and Agricultural Organization Statistical database (FAOSTAT) of forestry uses a single conversion of 6 m3 roundwood solid volume equivalents for 1 tonne of charcoal, corresponding to nearly 50% efficiency. The use of fuelwood and industrial roundwood by various regions of the world over the last several decades is presented in Figs. 6.21–6.25. The above figures show the pattern of wood usage by various countries. Industrialized countries mainly use industrial roundwood, whereas both developing and non-developing countries are emphasizing on fuelwoods. Black liquor is the spent pulping chemicals and the lignin component of wood after chemical pulping. It is fired in a chemical recovery boiler to produce process steam and electricity. Wood products (fuelwood and industrial roundwood) for bioenergy generation are generally consumed locally. In some countries, there is a growing interest in

6 Bioenergy

2000

1990

2000

2005

1990

2005

1990

100

2000

150

1990

Wood Removed (Million m3)

200

2005

2000

250

2005

358

0

Central Africa

East Africa

2005

2000

1990

50

Northern Africa

Industrial roundwood

Southern Africa

West Africa

Fuel wood

Fig. 6.21 Wood removed in Africa (Source: Killmann [157]) 250

Wood Removed (Million m3)

200

150

100

50

0 East Asia

South Asia

Industrial roundwood

South East Asia Fuel wood

Fig. 6.22 Wood removed in Asia and Oceania (Source: Killmann [157])

Oceania

6.6

Wood Fuel

359

500

Wood Removed (million m3)

400

300

200

100

0 Caribbean Industrial roundwood

Central America

South America

Fuelwood

200

2000

300

2005

400

1990

Wood Removed (million m3)

500

2005

2000

1990

100

0 Canada Industrial roundwood

Mexico Fuelwood

Fig. 6.24 Wood removed in North America (Source: Killmann [157])

USA

2005

1990

600

2000

Fig. 6.23 Wood removed in Caribbean, Central and South America (Source: Killmann [157])

360

6 Bioenergy

1990

2005

1990

200

2005

300

2000

Wood Removed (million m3)

400

2000

500

100

0 Europe Excluding Russian Federation Industrial roundwood

Russian Federation Fuelwood

Fig. 6.25 Wood removed in European region (Source: Killmann [157])

the international trade, because the trade can provide biofuels at lower prices, larger quantities and better quality than domestic alternatives. For example, in Europe it is traded heavily among European countries. Several OECD countries have developed new markets for different types of biofuels such as charcoal, woodfuels and wood pellets and the market is growing rapidly. The international trade of woodfuels and wood pellets is also growing, as some countries can produce biofuels at lower prices and in larger quantities [36, 158–163]. The trade routes for woodfuel in Europe are shown in Fig. 6.26. It has been estimated that cross-border trade of solid biofuels (including wood-fuels) in Europe has reached a level of almost 50 petajoules (PJ)/annum.

6.6.1 Unit of Wood In the USA, wood is sold by the unit cord. The unit cord and its conversion to other equivalent units are discussed in Chap. 3. Currently, there is a trend towards selling firewood by weight and small bundle. There are two ways wood is sold in the USA;

6.6

Wood Fuel

361

Fig. 6.26 Biomass trade routes in Europe (Source: Killmann [157])

Fig. 6.27 Various types of cord used for selling woods in the USA. (a) Standard cord (b) Face cord (c) Bundle (1/64 cord) (Source: Slusher [164])

(1) standard cord which is defined as a volume of wood given by 8 ft 4 ft 4 ft and (2) face cord, which is a volume of wood given by 8 ft 4 ft 2 ft. Wood is also sold as a bundle, which is a fraction of a cord. These units are shown in Fig. 6.27. In Canada and Europe, wood is sold by the unit stere, which is the metric version of the cord. One stere is 1 cubic meter (m3 / or about 0.276 cord.

362

6 Bioenergy

Fig. 6.28 Different stages of wood burning (Source: Hirayama [165])

6.6.2 Wood Burning Wood burning is a leading cause of pollution in both indoors and outdoors. Significant advances have been made in burning wood more efficiently by designing better fire places and wood burning furnaces. Three major stages are involved during wood burning: (1) The first stage involves drying; water within the wood is removed by evaporation; (2) In the second stage, wood breaks down chemically. The cellulose, lignocelluloses, and hemi-cellulosic components convert into charcoal, gas and volatile liquids. (3) In the third and final stage, charcoal burns. Various stages of the wood burning process are shown in Fig. 6.28. Water content of the wood is a major factor when burning it, which should be below 40%. For some wood, the water content could be more than 50%. Generally, wood is air dried to reduce its water content to 20%. The water content of several types of trees when they are green and after air drying is given in Table 6.14 along with their heat content.

6.7 Use of Biomass The biomass may be used for three applications as shown below. • Process Heat and Steam Generation • Electrical Power Generation

6.7

Use of Biomass

363

Table 6.14 Approximate weight per standard cord (80 cubic feet of solid wood content) of various woods (green and air-dried to 20% moisture content) and potential heat of air-dried wood Trees Pounds greena Pounds air-driedb Available energy (million BTU)c Ash 3,940 3,370 23.6 Basswood 3,360 2,100 14.7 Box elder 3,500 2,500 17.5 Cottonwood 3,920 2,304 16.1 Elm (American) 4,293 2,868 20.1 Elm (red) 4,480 3,056 21.4 Hackberry 4,000 3,080 21.6 Hickory (shagbark) 4,980 4,160 29.1 Locust (black) 4,640 4,010 28.1 Maple (silver) 3,783 2,970 20.8 Maple (sugar) 4,386 3,577 25.0 Oak (red) 4,988 3,609 25.3 Oak (white) 4,942 3,863 27.0 Osage orange 5,480 4,380 30.7 Pine (shortleaf) 4,120 2,713 19.0 Red cedar 3,260 2,700 18.9 Sycamore 4,160 2,956 20.7 Walnut (black) 4,640 3,120 21.8 Source: Slusher [164] a Approximate weight of standard cord (occupying 128 cubic feet of space and containing 80 cubic feet of solid wood), for the first two columns of figures b To 20% moisture content c Potential available heat from standard cord with 100% unit efficiency. Heat at 20% moisture content

• Liquid Fuel – Biofuels – Biodiesels The heat content of various biomass based fuel types other than fuelwood that can be used for heat and power generation is given in Table 6.15. Liquid fuels from biomass can be produced by a number of routes, which depends on the characteristics of the biomass and methods employed. Depending on the intended applications, various biomass processing routes must be employed. This is shown in Fig. 6.29.

6.7.1 Process Heat and Steam Generation The main use of biomass in the industrial sector is to produce heat and electricity. The combined heat and power (CHP) plants generate both electricity and useful heat and steam [167–191]. CHP plants can achieve efficiencies greater than 35% by using biomass. Paper, chemical, and food-processing industries are the main users

364

6 Bioenergy Table 6.15 Heat content of various biomass Fuel type Heat content Units Agricultural byproducts 8:248 Million btu/short ton Biodiesel 5:359 Million btu/barrel Black liquor 11:758 Million btu/short ton Digester gas 0:619 Million btu/1,000 cubic feet Ethanol 3:539 Million btu/barrel Landfill gas 0:490 Million btu/1,000 cubic feet MSW biogenic 9:696 Million btu/short ton Methane 0:841 Million btu/1,000 cubic feet Paper pellets 13:029 Million btu/short ton Peat 8:000 Million btu/short ton Railroad ties 12:618 Million btu/short ton Sludge waste 7:512 Million btu/short ton Sludge wood 10:071 Million btu/short ton Solid byproducts 25:830 Million btu/short ton Spent sulfite liquor 12:720 Million btu/short ton Utility poles 12:500 Million btu/short ton Waste alcohol 3:800 Million btu/barrel Sources: Biodiesel and ethanol: Energy Information Administration, Monthly Energy Review December 2008, DOE/EIA-0035 (2008/12) (Washington, DC, October 2007); MSW Biogenic: Energy Information Administration, Methodology for Allocating Municipal Solid Waste to Biogenic and NonBiogenic Energy (Washington, DC, May 2007); and all other fuel types: Energy Information Administration, Form EIA-860B (1999) Annual Electric Generator Report – Nonutility 1999 [166] For detailed characteristics of biomass feedstocks, see the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, website here: http://www1.eere.energy.gov/biomass/for researchers.html MSW municipal solid waste

of CHP plants. In the US, the forest products industry consumes about 85% of all wood waste for energy production. They are able to generate over half of their own energy by the CHP technology. Although most of the electricity, heat and steam produced by the industry are consumed on-site, some of them also sell excess power to the grid. In biomass powered CHP facilities, steam turbines are used to generate electricity. The steam condition, temperature and pressure, depends on the type of biomass used as boiler fuel. Chemically untreated wood-like biomass can generate steam at 540ı C, whereas the steam produced using waste wood is at approximately 450ıC. The limitation on the steam temperature is to avoid increased deposition and corrosion attack in the boiler. The steam pressure is in the range of 20–200 bar. The electrical efficiency of such a biomass CHP plant, in the capacity range between 2 and 25 MWe , is in the range of 18–30%. A summary of various methods for producing power and heat is provided in Table 6.16. Total electricity generation from biomass worldwide in 2005 was about 180 TWh from an installed capacity of about 40 GW. The overall rate of growth has been greater than 5% in the last decade as shown in Table 6.17. Countries leading

Fig. 6.29 Various conversion paths for biomass to secondary energy carriers (Source: International Energy Agency [31])

6.7 Use of Biomass 365

Generally several 100 kWe

Residential: 5–50 kWth Industrial: 1–5 MWth

0.1–1 MWe 1–20 MWe

Combustion for heat

Combined heat and power

Typical capacity Up to several MWe

Landfill gas production

Conversion option Biogas production via anaerobic digestion

60–90% (overall)

Low for classic fireplaces, up to 70–90% for modern furnaces

As above

Net efficiency (LHV basis) 10–15% electrical (assuming on-site production of electricity)

100/kWth for logwood stoves, 300–800/kWth for automatic furnaces, 300–700/kWth for larger furnaces 3,500 (Stirling) 2,700 (ORC) 2,500–3,000 (Steam turbine)

Investment cost ranges (E/kW)

Status and deployment Well established technology. Widely applied for homogeneous wet organic waste streams and waste water. To a lesser extent used for heterogeneous wet wastes such as organic domestic wastes Very attractive GHG mitigation option. Widely applied and, in general, part of waste treatment policies of many countries Classic firewood use still widely deployed, but not growing. Replacement by modern heating systems (i.e., automated, flue gas cleaning, pellet firing) in e.g., Austria, Sweden, Germany ongoing for years Stirling engines, steam screw type engines, steam engines, and organic rankine cycle (ORC) processes are in demonstration for small-scale applications between 10 kW and 1 MWe. Steam turbine based systems 1–10 MWe are widely deployed throughout the world (continued)

Table 6.16 A summary of various conversion methods of biomass for generation of power and heat. Investment cost is approximate value

366 6 Bioenergy

Typically 5–100 MWe at existing coalfired stations. Higher for new multifuel power plants

Typically hundreds kWth

0.1–1 MWe

Gasification for heat production

Gasification/ CHP using gas engines

Typical capacity 20->100 MWe

Co-combustion of biomass with coal

Table 6.16 (continued) Conversion option Combustion for power generation

15–30% (electrical) 60–80% (overall)

Several hundred/ kWth, depending on capacity 1.000–3.000 (depends on configuration)

100–1,000 C costs of existing power station (depending on biomass fuelCco-firing configuration)

30–40% (electrical)

80–90% (overall)

Investment cost ranges (E/kW) 2.500 –1,600

Net efficiency (LHV basis) 20–40% (electrical)

Status and deployment Well established technology, especially deployed in Scandinavia and North America; various advanced concepts using fluid bed technology giving high efficiency, low costs and high flexibility. Commercially deployed waste to energy (incineration) has higher capital costs and lower (average) efficiency Widely deployed in various countries, now mainly using direct combustion in combination with biomass fuels that are relatively clean. Biomass that is more contaminated and/or difficult to grind can be indirectly co-fired, e.g., using gasification processes. Interest in larger biomass co-firing shares and utilisation of more advanced options is increasing Commercially available and deployed; but total contribution to energy production to date limited Various systems on the market. Deployment limited due to relatively high costs, critical operational demands, and fuel quality (continued)

6.7 Use of Biomass 367

10 tons/h in the shorter term, up to 100 tons/h in the longer term

Typical capacity 30–200 MWe

Source: International Energy Agency [31]

Pyrolysis for production of bio-oil

Table 6.16 (continued) Conversion option Gasification using combined cycles for electricity (BIG/CC) Investment cost ranges (E/kW) 5.000 – 3.500 (demos) 2.000 – 1.000 (longer term, larger scale) Scale and biomass supply dependent; Approx 700/kWth input for a 10 MWth input unit

Net efficiency (LHV basis) 40–50% (or higher; electrical)

60–70% biooil/feedstock and 85% for oil C char

Status and deployment Demonstration phase at 5–10 MWe range obtained. Rapid development in the nineties has stalled in recent years. First generation concepts prove capital intensive Commercial technology available. Bio-oil is used for power production in gas turbines, gas engines, for chemicals and precursors, direct production of transport fuels, as well as for transporting energy over longer distances

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6.7

Use of Biomass

369

Table 6.17 Electricity production from biomass in TWh Types 1995 2002 Solid biomass 85:3 110:0 Biogas 6:0 16:9 Liquid biomass Municipal solid waste (MSW) 13:4 21:3 Total 104:8 Source: World Energy Council [192]

148:2

2003 118:2 18:3 0:8 25:0

2004 131:4 20:7 0:6 24:0

2005 134:9 24:8 0:9 22:8

162:2

176:6

183:4

Table 6.18 Top biopower producing countries in 2005 Total electricity Percentage Country produced (TWh) of world USA 56:3 30.7 Germany 13:4 7.3 Brazil 13:4 7.3 Japan 9:4 5.1 Finland 8:9 4.9 UK 8:5 4.7 Canada 8:5 4.6 Spain 7:8 4.3 Rest of World 57:1 31.1 Source: World Energy Council [192]

the biomass-based electricity production are given in Table 6.18. Among these countries, Brazil is unique in that at present almost all of its biomass is bagasse from the expanding sugar cane-based alcohol fuel industry. Although the biopower generation is increasing, a key issue for the biopower sector is the efficiency of the system. In the United States about 9,733 MW of electricity was generated in 2002 using biomass. Of this 9,733 MW of capacity, about 5,886 MW was generated using forest product and agricultural residues, 3,308 MW from municipal solid waste, and 539 MW from other resources such as landfill gas. The majority of electricity production from biomass is used as the base load power for existing electrical distribution systems. Also, more than 200 companies outside the wood products and food industries generate biomass power in the United States.

6.7.2 Electric Power Generation There are four primary classes of biomass power systems: • • • •

Direct-fired System [193, 194] Co-fired Biopower Plants [46, 195–198] Gasification Process [199–214] Modular Systems [215]

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6.7.2.1 Direct-Fired System In direct-fired biomass power plants, the biomass fuel is burned in a boiler, similar to a coal power plant, to produce high-pressure steam. The steam is introduced into a steam turbine to generate electricity. Direct-fired biomass power boilers are typically in the range of 20–50 MW compared to a range of 100–1,500 MW for pure coalfired plants. Because of the small capacity, its efficiency is in the low 20% range. The small capacity plants tend to be lower in efficiency because of economic tradeoffs; efficiency-enhancing equipment cannot pay for itself in small plants. Although techniques exist to enhance biomass steam generation efficiency over 40%, it may not be economical. 6.7.2.2 Co-Fired Biopower Plants Co-firing involves substituting biomass for a portion of coal in an existing power plant furnace. Up to about 15% biomass can be mixed with coal for an existing fuel feed, thus, burning these together. In some plants, a separate boiler feed for the biomass is used. Most of the existing power plant equipment can be used without major modifications. The preparation of biomass for co-firing employs well known, commercially available technologies. Boiler technologies where co-firing have been successfully practiced, tested, or evaluated, include the following: pulverized coal (PC) boilers of both wall fired and tangentially fired designs, coal-fired cyclone boilers, fluidized-bed boilers, and spreader stokers. As a result, co-firing offers the most economic alternative to building a new biomass power plant. The replacement of biomass can also reduce sulfur dioxide (SO2 /, nitrogen oxides (NOx /, and other air emissions. After “tuning” the boiler for peak performance, there is little or no loss of efficiency from adding biomass. The efficiency can be in the range of 33– 37%. Currently, six power plants in the U.S. are co-firing coal and wood residue products on a commercial basis. A schematic diagram of a coal boiler retrofitted for biomass co-firing is shown in Fig. 6.30. Types of biomass that can be used in cofiring, include: (1) wood wastes from pallets, telephone poles, sawdust, and manufacturing scraps; (2) agricultural remnants from peach pits, rice hulls, wheat straw, alfalfa, barley, soybeans, sunflowers, bagasse, and other grains; (3) residues from logging, orchards, and forest management; (4) fast-growing energy crops such as hybrid poplar, willow, black locust, eucalyptus trees, and switchgrass; and (5) municipal wastes including plastic, paper, and cardboard. According to an estimate by the Electric Power Research Institute the payback time for retrofitting a coal power plant for co-firing of biomass can be 3.3 years. 6.7.2.3 Gasification Process The working principle of biomass gasifiers is the same as that of other gasifiers, such as coal gasifiers. The solid biomass particles break down when heated at a

Use of Biomass

Fig. 6.30 A schematic diagram of a pulverized coal boiler system retrofitted for biomass cofiring. (Source: Energy Efficiency and Renewable Energy [216])

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high temperature in the absence of oxygen to form a flammable gaseous product, called biogas. The biogas is next cleaned and filtered for use in more efficient power generation systems called combined-cycles, which combine a gas turbine and a steam turbine to produce electricity (See Fig. 6.31). The efficiency of these systems can reach 60%. Three types of gasifiers are under development by the USDOE. These are: fluidized-bed, fixed-bed, and entrained-flow gasifiers. The gasifiers can be used in a direct-fired mode in which air or oxygen is fed directly to the gasifier, or in an indirect mode in which externally supplied heat is used to gasify the biomass. The heating value of the biogas is generally considerably lower than natural gas and also depends on the gasification method. Gasification with air produces a low-Btu gas, with a heating value about one-fifth that of natural gas. Both indirectly heated gasification and oxygen-blown gasification produce a medium-Btu gas, with heating values as much as one-half that of natural gas. Gasification is a two-step process. The first step is called pyrolysis. During pyrolysis, the volatile components of the fuel are vaporized at temperatures below 600ıC (1;100ıF). The vapor produced in this step includes various hydrocarbons, hydrogen, carbon monoxide, carbon dioxide, tar, and water vapor. Because biomass fuels tend to have more volatile components (70–86% on a dry basis) than coal which is 30%, pyrolysis plays a larger role in biomass gasification than in coal gasification. Char (fixed carbon) and ash are the by-products, which are not vaporized. Depending on the desired heating value of the gaseous stream, either an air-blown or an oxygen blown gasifier is used. Currently, the preferred equipment for biomass integrated gasifier power systems is the air-blown gasifier, since the cost is much lower than oxygen blown system. Gasifiers are also designed for dry-ash or slag handling. The slagging gasifier requires substantially less blast steam injection for the gasification process, since it operates at higher temperatures. In the second step, char is further burned. In direct combustion gasifiers, the char is burned in another vessel. Indirect gasifier systems also use a separate reactor for char combustion from which heat is transferred to the gasifier reactor. A biomass gasification burner is shown in Fig. 6.32. The design of the gasifier is extremely crucial for high efficiency and the success of the process. Generally, all three stages of burning: pyrolysis, oxidation, and reduction are accommodated in a single burner to reduce the installation and operating costs and to increase the efficiency. The TK Energi (TKE) of Denmark has developed a 3-stage, down-draft gasifier, which includes pyrolysis and partial oxidation zones, and a reformer based char gasification zone with a rocking grate for ash discharge. The TKE gasifier is shown in Fig. 6.33. In a fluidized-bed reactor, the biomass is first pyrolyzed in the absence of oxygen using staged steam reformation. The vapors from the pyrolysis stage subsequently are reformed to synthesis gas with steam providing added hydrogen, as well as the proper amount of oxygen. The heat necessary for this stage comes from burning of the char.

Steam

PRESSURIZED FLUIDIZED-BED GASIFIER Oxygen/Air Char / Coke

Synthesis Gas

Process Use

Methanol, Petrochemicals, Fuels, Ammonia

Electricity Generation

Steam

Medium-BTU Gas, Substitute Natural Gas

Direct Combustion

Combined Cycle / Fuel Cell Power Generation

Use of Biomass

R Fig. 6.31 A schematic diagram of a typical biomass gasification system. The above system is called Renugas developed by IGT/GTI and Carbona. (Source: Babu [217])

FEED HOPPER

LOCKHOPPER

ASH CYCLONE

GAS PURIFICATION AND UPGRADING

Low-BTU Gas

GAS PURIFICATION

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Fig. 6.32 Schematic of a biomass gasification burner (Source: Babu [217])

Fig. 6.33 Three stage TKE gasifier (Source: Babu [217])

In an another fluidized-bed gasifier design, a continuous feed of biomass and an inert heat-distributing material (i.e., sand) are “fluidized” by an oxidant and/or steam. Fluidized bed gasifiers can be operated in two modes: direct heating and

6.7

Use of Biomass

375

Fig. 6.34 Biomass gasification via staged steam reformation with a fluidized bed gasifier (Source: U.S. Department of Energy [218])

Fig. 6.35 Biomass gasification via staged steam reformation with a screw Auger gasifier (Source: U.S. Department of Energy [218])

indirect heating. In a directly heated fluidized-bed gasifier, char is burned in the gasifier that supplies the required heat. In an indirectly heated fluidized bed gasifier, char is removed from the gasifier and is burned in a separate vessel. The advantage of the indirect heating of the gasifier is that the gasification product is not diluted with the char combustion by-products such as CO2 and N2 , if air is used. Product gas composition, efficiency, and hot gas utilization of the fluidized-bed process are comparable to those found in a fixed-bed gasifier design. Fluidized-bed gasifiers are capable of handling much smaller, less dense, and less uniform feedstocks. The process is shown in Fig. 6.34. Fixed bed gasifier utilizes a screw Auger reactor (See Fig. 6.35). Biomass is introduced at the pyrolysis stage that operates at about 700ıC. The process heat comes from burning some of the gas produced in the latter stage. The most commercial fixed-bed designs are an updraft gasifier type. The biomass is fed from the top of the gasifier and successively undergoes drying, pyrolysis, char

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Fig. 6.36 Biomass gasification via entrained flow steam reformation (Source: U.S. Department of Energy [218])

gasification, and char combustion as it settles to the bottom of the gasifier. The vapor from the gasifier is removed from the top and is introduced into a steam reforming unit. The ash is collected from the bottom. Blast air and steam are injected into the gasifier to keep the ash below its melting temperature (in a dry-ash gasifier) and to facilitate char conversion. Carbon conversion efficiency is typically 99%; the hot gas efficiency is in the range of 90–95%. The fixed-bed gasifier, however, requires large, dense, uniformly sized fuels. Thus, agricultural residues would generally require densification, thereby increasing fuel handling costs. A schematic diagram of the process is shown in Fig. 6.36. In entrained flow reformation, both external steam and air are introduced in a single-stage gasification reactor. Partial oxidation gasifiers use pure oxygen, with no steam, to provide the proper amount of oxygen. The use of air instead of oxygen, as in small modular system, yields producer gas (including nitrogen oxides) rather than synthesis gas. A partial oxidation gasification system is shown in Fig. 6.37.

Advantages of Gasification Biogasification offers several advantages including reduced emissions, increased efficiencies, and flexibility with respect to biomass feedstocks. Emissions from a biogasifier could be extremely low compared with conventional power plants. Furthermore, these systems can achieve high efficiencies. The use of advanced biomass gasifier and gas turbines can increase the electricity generation from the biomass by 50% or more. Various combinations of thermal cycles may further enhance the efficiency.

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Use of Biomass

377

Fig. 6.37 Biomass gasification via partial oxidation (auto thermal) (Source: U.S. Department of Energy [218])

There are several types of biomass that cannot be used directly in a furnace. The inorganic portion of the biomass tends to stick to the furnace wall and reduce the heat transfer through the wall. Many fast-growing, desirable energy crops and residues have high proportions of these inorganic compounds. These inorganic compounds can be removed during gasification as a part of the cleanup process.

6.7.2.4 Small, Modular Systems Modular systems employ some of the same technologies mentioned above, but on a smaller scale that is more applicable for use in remote areas, villages, farms, and small industries. There are many opportunities to use these systems in developing countries. Large amounts of biomass are available in these areas for fuel that a small, modular system can utilize. Small systems, those with rated capacities of 5 MW and smaller, could potentially provide power to a small community. By adopting a standardized modular design, 5 kW-to-5 MW, systems can be designed at a lower cost. The hot gas from the gasification unit is cleaned and used for electricity generation. Waste heat from the turbine or engine can also be captured and directed to other applications. Small modular systems lend themselves to such combined heat and power operations much better than large central facilities. The basic operating principles of a small, modular system is shown in Fig. 6.38.

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Fig. 6.38 Biomass gasification via partial oxidation (auto thermal) for small modular applications (Source: U.S. Department of Energy [218])

Benefits of Small Modular Systems For a small community, small modular systems can have added economic benefit, since the biomass waste stream can be a source of energy. Otherwise, a landfill needs to be designed and operated. The flexibility to use more than one fuel is another advantage.

6.8 Biomethane Livestock manures contain fats, carbohydrates, and proteins along with other components that can be converted to methane and carbon dioxide using anaerobic bacteria [219–224]. The process mechanism is shown in Fig. 6.39. This is a two-stage process. In the first stage, most of the components in the manure break down in to a series of fatty acids by a particular group of bacteria, called acidogenic bacteria. In the second stage, methane producing bacteria (methanogenic bacteria) converts the acids to methane gas and carbon dioxide. Methanogenic bacteria is very sensitive to the oxygen content and pH of the solution and functions best at 95ı F (35ıC) and pH between 6.8 and 7.4. Methane concentration in the final product is between 60% and 70% and the rest is carbon dioxide and a trace quantity of hydrogen sulfide. The operation of a digester is challenging. A variety of materials

6.8

Biomethane

379

Fig. 6.39 Process for conversion of manure to methane (Source: Fulhage et al.[225])

can become toxic to anaerobic bacteria including salts, heavy metals, ammonia and antibiotics. Their concentrations must be monitored carefully; a minimum amount of these components is required for their growth, but in greater content they can be toxic to both acidogenic and methanogenic bacteria. Ammonia toxicity is a major concern in the anaerobic digestion of livestock manures. To avoid the problem, loading rates must be carefully controlled. Methane production rates from various types of animal manure are given in Table 6.19. There are several types of anaerobic digesters [226–234]. The main differences among these digesters are in the operating temperature and design of the unit. A digester can be designed to carry out acidogenesis and methanogenesis reactions separately or together in the same unit. The operating temperature ranges are identified as psychrophilic (68ı F or 20ı C), mesophilic (95–105ıF or 35–41ı C) and thermophilic (125–135ı F or 52–57ı C). There are a number of process designs currently used to digest livestock manures and are listed below. 1. Covered lagoons [235, 236] 2. Plug-flow digesters [237, 238] 3. Mixed plug-flow digesters [239, 240]

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Table 6.19 Potential gas production of swine, dairy, poultry and beef manure (20ı C or 68ı F, atmospheric pressure) Swine (150 lb) 12

Gas yield, cubic feet per pound volatile solids destroyed 0:7 Volatile solids voided, pounds per day Percent reduction 49 of volatile solids 4:1 Potential gas production cubic feet per animal unit per day 103 Energy production rate, Btu per hour per animal 70 Available energy Btu per hour (after heating digester) Source: Fulhage et al. [225]

4. 5. 6. 7. 8.

Dairy (1,200 lb) 7:7

9:5

31

22:7

Poultry (4 lb bird) 8:6

0:044

56

Beef (1,000 lb) 15

5

41

0:21

31

568

5:25

775

380

3:5

520

Complete-mixed digesters [241, 242] Fixed-film digesters [243–245] Temperature-phased anaerobic digesters [246–251] Anaerobic sequencing-batch reactor (ASBR) [252–266] Upflow anaerobic sludge bed (UASB) [267–271]

Among these technologies, covered lagoons, plug-flow digesters, complete mixed digesters and fixed-film digesters are most common. A comparison of these four technologies is provided by the US Environmental Protection Agency and is given in Table 6.20. Basic components of a manure digester are shown in Fig. 6.40. Various components required for operating a continuous-mix, heated anaerobic digester are expensive and require a substantial investment. Biomethane can be also produced from a number of other wastes. In the USA, California is the leader of utilization of biomethane. Krich et al. [273] analyzed the potential for biomethane production in California from diary and other wastes. Their findings are shown in Table 6.21.

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BioFuels

381

Table 6.20 Summary characteristics of digester technologies Covered lagoon Deep lagoon

Characteristics Digestion vessel

Level of technology Supplemental heat Total solids Solids Characteristics HRTa (days) Farm type Optimum location

Low No 0.5–3% Fine

Complete mix digester Round/Square in/ aboveground tank Medium Yes 3–10% Coarse

Plug flow digester Rectangular in-ground tank Low Yes 11–13% Coarse

Fixed film Above ground tank Medium No 3% Very fine

40–60 15C 15C 2–3 Dairy, hog Dairy, hog Dairy only Dairy, hog Temperate and All climates All climates Temperate warm and warm climates Source: US Environmental Protection agency [272] a Hydraulic Retention Time (HRT) is the average number of days a volume of manure remains in the digester Mixing device Other uses

Gas Collection pressure regulation

To heat exchanger

Gas Collector

Heated liquid

Air seal Liquid removal

Heat exchanger Digested liquid Pump

sludge removal

Pump

Digester Raw manure Holding dilution mixing

Sludge storage and disposal

Fig. 6.40 Basic components of a manure digester for methane production (Source: Fulhage et al.[225])

6.9 BioFuels Biomass can be converted directly into liquid fuels, called “biofuels,” which can be used as transportation fuel replacing petroleum [274–287]. The two most common types of biofuels are ethanol and biodiesel. Various process concepts for producing transportation fuels from biomass are shown in Table 6.22.

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Table 6.21 Potential methane generation from biomass sources, California Annual methane productiona (million ft3 /y) Biomass waste material Gross methane potential Technical methane potential Swine manureb Poultry layer manurec Poultry broiler manured Turkey manured Dairy manure Cattle feedlot manured Crop residues Vegetable residue Meat processing Rendering (wastewater)e Cheese whey (lactose permeate) Food processing waste Processed green wastef Landfilled manuref Landfilled composite organic waste Landfilled food wastef Landfilled green wastef Total

320 1;700 1;800 1;300 21;100 4;100 10;700 11;300 660 120 250 720 18;000 220 15;200 19;900 16;500

160 850 0 0 14,300 0 5,220 940 530 120 250 360 0 0 0 0 0

123;890

22,730

Krich et al. [273] Ft3 =y cubic feet per year a Unless otherwise indicated, these figures calculated based on Buswell AM., Hatfield WD (1936) Anaerobic fermentations. State of Illinois Dept. of Registration & Education, Div. of the State Water Survey, Urbana, IL, Bull No 32: 1–193 b American Society of Agricultural Engineers (ASAE). Standards 1990. 37th edition. American Society of Agricultural Engineers, St. Joseph, MI: 464 c RCM Digesters (1985) Study on poultry layer manure. February 1985 d California Biomass Collaborative (CBC) (2004) An assessment of biomass resources in California. http://faculty.engineering.ucdavis.edu/jenkins/CBC/Resource.html accessed 11/16/2010 e Metcalf A, Eddy E (1979) Wastewter engineering: treatment, disposal and reuse. 2 edition McGraw-Hill: 614 f Al Seadi T (11/20/2010) Good practice in quality management of AD residues in biogas production. IEA Bioenergy Task 24, Energy from Biological Conversion of Organic Waste. Available at . Accessed 20 Nov 2010

Ethanol, also known as ethyl alcohol or grain alcohol, can be used either as an alternative fuel or as an octane-boosting, pollution-reducing additive to gasoline. There are four basic steps for converting biomass to bioethanol: • Production of biomass such as corn or sugar cane. • Conversion of biomass to a useable fermentation feedstock (typically some form of sugar). • Fermentation of the biomass intermediates using suitable microorganisms including yeast and bacteria for production of ethanol. • Processing of the fermentation product into fuel-grade ethanol and byproducts that can be used to produce other fuels, chemicals, heat and/or electricity.

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BioFuels

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Table 6.22 Main conversion processes for biomass to transportation fuel Energy efficiency (HHV) Estimated production cost C energy inputs (euro cent/GJ fuel) Concept Hydrogen: via biomass gasification and subsequent syngas processing. Combined fuel and power production possible; for production of liquid hydrogen additional electricity use should be taken into account Methanol: via biomass gasification and subsequent syngas processing. Combined fuel and power production possible

Short term Long term Short term 60% (fuel only) 55% (fuel) 9–12 (C energy input 6% (power) of 0.19 GJe/GJ (C 0.19 GJe/GJ H2 for H2 for liquid liquid hydrogen) hydrogen)

Long term 5–8

55% (fuel only)

48% (fuel) 12% (power)

10–15

6–8

45% (fuel only) Fischer-Tropsch liquids: via biomass gasification and subsequent syngas processing. Combined fuel and power production possible

45% (fuel) 10% (power

12–17

7–9

46% (fuel) Ethanol from wood: production takes 4% (power) place via hydrolysis techniques and subsequent fermentation and includes integrated electricity production of unprocessed components

53% (fuel) 8% (power)

12–17

5–7

(continued)

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Table 6.22 (continued)

Concept Ethanol from sugar beet: production via fermentation; some additional energy inputs are needed for distillation

Energy efficiency (HHV) C energy inputs

Estimated production cost (euro cent/GJ fuel)

Short term 43% (fuel only) 0.065 GJeC 0.24 GJth/GJ EtOH

Short term Long term 20–30 20–30

Long term 25–35%

85 l EtOH per ton 95 l EtOH per ton of Ethanol from sugar wet cane. of wet cane, cane: production Electricity generally via cane crushing surpluses depend energy neutral and fermentation on plant lay-out and with respect to and power power generation power and heat generation from technology the bagasse. Mill size, advanced power generation and optimized energy efficiency and distillation can reduce costs further in the longer term Biodiesel RME: takes place via extraction (pressing) and subsequent esterification. Methanol is an energy input. For the total system it is assumed that surpluses of straw are used for power production

8–12

88%; 0.01 GJe C 0.04 GJ MeOH per GJ output 25–40 Efficiency of power generation in the shorter term, 45%; in the longer term, 55%

7–8

20–30

Source: International Energy Agency [22] •

•

•

Assumed biomass price of clean wood: E2/GJ. RME cost figures varied from E20/GJ (shortterm) to E12/GJ (longer term), for sugar beet a range of E8–E12/GJ is assumed. All figures exclude distribution of the fuels to fuelling stations For equipment costs, an interest rate of 10%, economic lifetime of 15 years is assumed. Capacities of conversion unit are normalised on 400 MWth input in the shorter term and >1,000 MWth input using advanced technologies and optimised systems in the longer term Diesel and gasoline production costs vary strongly depending on the oil prices, but for indication: recent cost ranges (1990s till 2006) are between E4 and E9/GJ. Longer term projections give estimates of roughly E6–E10/GJ. Note that the transportation fuel retail prices are usually dominated by taxation and can vary between Ect50 and Ect130 /l depending on the country in question

6.9

BioFuels

Table 6.23 Typical oil extraction from 100 kg of oil seeds

385

Crop Oil/100 kg Castor seed 50 kg Copra 62 kg Cotton seed 13 kg Groundnut kernel 42 kg Mustard 35 kg Palm kernel 36 kg Palm Fruit 20 kg Rapeseed 37 kg Sesame 50 kg Soybean 14 kg Sunflower 32 kg Source: Petroleum Club (with permission) [310]

The production of ethanol from biomass is discussed in details in Chap. 7. In the USA, the goal of ethanol production is about 28:4 hm3 by 2012. The drastic increase in the price of crude oil is encouraging worldwide increase of the ethanol production. The potential of biofuels for transportation may be limited since it may come at the expense of global food production. This is discussed in more details in Volume 4 of this book series. The expansion of biofuels has various other limitations. A major issue is the subsidies provided by various governments to their agricultural sectors. Although Brazil has foregone most agricultural subsidy to its sugar industry, the agricultural sectors in both the USA and EU countries are provided with subsidies particularly for ethanol and other biofuels production. The subsidy for ethanol production in the USA costs US$ 5 billion per year in 2009. Agricultural subsidies have been challenged during the Doha round of World Trade Organization negotiations as being bad for the environment (by encouraging intensive agriculture) and for their negative effects on the development of agriculture in third-world countries.

6.9.1 Biodiesel Biodiesel is a form of diesel fuel manufactured from vegetable oils, animal fats, or recycled restaurant greases by esterification [288–309]. It is considered safe, biodegradable, and produces less air pollutants than petroleum-based diesel. The energy content of biodiesel is about 90% that of petroleum diesel. There is a variety of vegetable oil that can be used for production of biodiesel [310–332]. Rapeseed and soybean oils are most commonly used; soybean oil alone accounts for about 90% of all feed stocks. Other oils such as mustard, flax, sunflower, palm oil have been also explored. Several researchers have also proposed the use of algae for biofuel production [333–336]. A list of vegetable oils that are promising feedstock for biodiesel is given in Table 6.23.

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Although biodiesel may be considered as renewable energy source, significant agricultural land must be dedicated for continuous and reliable supply of feedstock. Table 6.24 shows the land requirements for production of various types of vegetable oil yielding crops. Animal fats including tallow, lard, yellow grease, chicken fat and the by-products of the production of Omega-3 fatty acids from fish oil can also be used as the feedstock. For commercial production of vegetable oils, about 20 different species are used with soybean oil, palm/palm kernel oil, sunflower, rapeseed (Colza), and coconut oils. Although the worldwide annual production of oils and fats was about 142 mt in 2004–2005, the consumption was at around 138 mt. A 22% annual reserve stock to consumption ratio was maintained (According to the FAO Food Outlook series reports the 2004/2005 production [337]). Biodiesel production is increasing rapidly. Palm oil and soy oil comprise 50% of the annual vegetable oil production. The mandates by several governments are likely to increase the production of biodiesel. Brazil has a nationwide mandate for B2 in 2008 resulting in an estimated 1.1 hm3 demand for biodiesel (935 kt). The EU mandates the use of 5.75% biofuels in the transportation sector by 2010. The biodiesel production by several countries is given in Table 6.25. Europe, the largest producer and user of biodiesel, produces most of it from rapeseed (canola) oil. The USA, the second largest producer and user of biodiesel, makes it from soybean oil and recycled restaurant grease.

6.9.2 Biofuel Production Method Vegetable oils generally contains 16 and 22 carbon atoms that are generally in the form of triacyl glycerides (TAG), which on transesterification with methanol produce glycerol as a by-product and fatty acid methyl ester (FAME) as the precursor to biodiesel. After FAME purification and testing for compliance with either EN 14214 or ASTM D6751 standards, the product can be sold as biodiesel and as blends – typically B5 (5% biodiesel) to B20, depending on the engine warranties. There are three basic routes for synthesis of biodiesel or alkyl esters from oils and fats: • Base catalyzed transesterification of the oil with alcohol. • Acid catalyzed esterification of the oil with alcohol. • Lipase catalyzed transesterification. Among these methods, base catalyzed transesterification is mostly used for production of biodiesel [341–416]. This method has several advantages over the other two methods; it requires a low temperature 65ı C (150ıF) and pressure 1.4 bar (20 psi), a conversion in the range of 98% is achievable, by-products have various uses and values, and methyl ester is produced directly without any intermediate reactions.

6.9

BioFuels

387

Table 6.24 Yields of common crops in different measurement units Crop Kg oil/ha Liters oil/ha lbs oil/acre US gal/acre Corn (maize) 145 172 129 18 Cashew nut 148 176 132 19 Oats 183 217 163 23 Lupine 195 232 175 25 Kenaf 230 273 205 29 Calendula 256 305 229 33 Cotton 273 325 244 35 Hemp 305 363 272 39 Soybean 375 446 335 48 Coffee 386 459 345 49 Linseed (flax) 402 478 359 51 Hazelnuts 405 482 362 51 Euphorbia 440 524 393 56 Pumpkin seed 449 534 401 57 Coriander 450 536 402 57 Mustard seed 481 572 430 61 Camelina 490 583 438 62 Sesame 585 696 522 74 Safflower 655 779 585 83 Rice 696 828 622 88 Tung oil tree 790 940 705 100 Sunflowers 800 952 714 102 Cocoa (cacao) 863 1;026 771 110 Peanuts 890 1;059 795 113 Opium poppy 978 1;163 873 124 Rapeseed (Canola) 1;000 1;190 893 127 Olives 1;019 1;212 910 129 Castor beans 1;188 1;413 1;061 151 Pecan nuts 1;505 1;791 1;344 191 Jojoba 1;528 1;818 1;365 194 Jatropha 1;590 1;892 1;420 202 Macadamia nuts 1;887 2;246 1;685 240 Brazil nuts 2;010 2;392 1;795 255 Avocado 2;217 2;638 1;980 282 Coconut 2;260 2;689 2;018 287 Oil palm 5;000 5;950 4;465 635 Chinese tallow 5;500 6;545 4;912 699 6;894 7;660 6;151 819 Algae (actual yield)a 39;916 47;500 35;613 5;000 Algae (theoretical yield)b Source: Chinese tallow data, Mississippi State University. Used with permission from The Global Petroleum Club [310] Chinese tallow (Triadica sebifera, or Sapium sebiferum) is also known as the “Popcorn Tree” or Florida Aspen a Actual biomass algae yields from field trials conducted during the NREL’s aquatic species program, converted using the actual oil content of the algae species grown in the specific trials b Algae yields are projected based on the sustainable average biomass yields of the NREL’s aquatic species program, and an assumed oil content of 60%. Actual oil content was much less

388 Table 6.25 Biodiesel production by various countries in 1,000 tons

6 Bioenergy

Country Germany France Italy Malaysia USA Czech Republic Poland Austria Slovakia Spain Denmark UK Other EU Total

2004 1;035 348 320 83 60 57 15 13 70 9 6 2;016

2005 11;669 492 396 260 250 133 100 85 78 73 71 51 36 3;694

2006 2;681 775 857 600 826 203 150 134 89 224 81 445 430 7;495

Source: European Biodiesel Board [338], Malaysian Palm Oil Board [339], National Biodiesel Board, USA [340]

Fig. 6.41 Schematic diagram of a biodiesel production system (Source: National Biodiesel Board [340])

A schematic diagram of the process is shown in Fig. 6.41. A fat or oil is reacted with an alcohol, generally methanol, in the presence of a catalyst to produce glycerine and methyl esters or biodiesel. The unused methanol is recovered and recycled back to the system. The catalyst is usually sodium or potassium hydroxide, which is mixed with methanol before feeding to the reactor. The feed generally contains 12% alcohol, 1% catalyst, and rest oil. The product contains 86% methyl ester along with 9% glycerine, 4% unreacted alcohol, and 1% other components. An acid catalyzed esterification method is preferred if feedstock has a high free fatty acid (FFA) content (as is common with rendered fats and spent restaurant oils) [417–449]. Excess of alkali used in alkali catalyzed processes causes loss of the free fatty acids as their insoluble soaps. However, the acid-catalyzed reaction requires higher reaction temperatures (100ıC) and longer reaction times than alkali-catalyzed transesterification. The transesterification process is catalyzed by BrØnsted acids, preferably by sulfonic and sulfuric acids.

6.10

Biofeedstock for Industrial Chemicals

389

In the lipase catalyzed transesterification process, lipase is used as a biocatalyst for synthesis of biodiesel from oil and FFA [450–499]. Lipase-catalyzed transesterification reactions offer several advantages over chemically catalyzed reactions. Both alkaline and acid transesterification reactions have several backdraws: they are energy intensive, recovery of glycerol is difficult, the catalyst has to be removed from the product, wastewater requires treatment, and free fatty acids and water interfere with the reaction. In addition, they have low selectivity, as a result various undesirable side reactions take place. Lipase-catalyzed transesterification reactions can be carried out at much milder operating conditions and can overcome the problems of conventional chemical processes discussed above. The second-generation biodiesel is often called ‘renewable diesel’ and is produced by treating vegetable oil with hydrogen over catalysts in oil refineries. This is intended for use as a blend or co-processed with ‘fossil diesel’. The resultant product can be used in the range of B5 – B50 [500, 501]. As a fuel, the FAME biodiesel has about 90–95% of the volumetric energy content of regular diesel.

6.10 Biofeedstock for Industrial Chemicals Biofuels can be used as a feedstock to produce a number of industrial chemicals in a similar fashion as produced from the crude petroleum feed. The process of producing various industrial chemicals from biofuels is called biorefining [502–508]. There are several advantages for biorefining that include: • Lower feedstock costs: This may allow access to markets for bioproducts whose volume is too high and/or price is too low to be accessed when using corn as a raw material. • New markets: There is a potential to create new markets for products, such as polylactic acid and 1,3-propanediol. • Tax incentives: Biorefinery may qualify for various tax breaks. • Sustainable resource supply: Biomass refining has the potential to significantly reduce both greenhouse gas emissions and to slow down the non-renewable resource depletion. • Energy security: By reducing the dependence on foreign oil and the military investment associated with this dependence, large-scale biomass refining would enhance a nation’s energy security. • Rural economic development: By creating a large market for energy crops, various economic developments in rural areas are possible. Industrial and consumer products that can be manufactured wholly or in part from renewable biomass (plant-based resources) are listed in Table 6.26. Kamm and Kamm [509] provided an excellent review of various industrial products that can be derived from biomass through biorefining processes, which are shown in Figs. 6.42–6.47.

390 Table 6.26 Chemicals from renewable biomass

6 Bioenergy

Biomass resources Corn

Vegetable oils

Wood

Uses Solvents, pharmaceuticals, adhesives, starch, resins, binders, polymers, cleaners, ethanol Surfactants in soaps and detergents, pharmaceuticals (inactive ingredients), inks, paints, resins, cosmetics, fatty acids, lubricants, biodiesel Paper, building materials, cellulose for fibers and polymers, resins, binders, adhesives, coatings, paints, inks, fatty acids, road and roofing pitch

Fig. 6.42 Products from biological raw materials (Printed with permission from Kamm and Kamm [509])

6.10

Biofeedstock for Industrial Chemicals

391

Fig. 6.43 Possibility of various industrial chemicals from different types of biomass (Printed with permission from Kamm and Kamm [509]) Ligno-Cellulosic Feedstock Biorefinery [LCF-Biorefinery] LC-Feedstock (LCF) e.g. Cereals (Straw, Chaff); Ligno-Cellulosic Biomass (e.g. Reed, Reed Grass); Forest Biomass (Underwood, Wood); Paper-and Cellulosic Municipal Solid Waste

Lignocellulose (LC) Lignin

Hemicellulose

Cellulose

“Phenol-polymer”

Pentoses, Hexoses

“Glucose-polymer”

Hydrolysis

Natural Binder and Adhesives Sub-bituminous Coal Sulphur-free Solid Fuel

Xylose (Pentose)

Plant Gum Thickeners, Adhesives, Protective Colloids, Emulsifiers, Stabilizers

Celluloseapplicants

Xylite Sugar-Substitute

Hydrolysis (E/C)

Glucose HMF

(Hexose)

(5-Hydroxymethyl-furfural),

Furfural

Levulinic Acid

Furan Resins

Softener + Solvents

Chemical Products

Lubricants

Nylon 6; Nylon 6,6

Chemicals and Polymers

FermentationProducts ● Fuels e.g. Ethanol ● Organic Acids e.g. Lactic Acid ● Solvents Acetone, Butanol

Fig. 6.44 Various routes for producing consumer products from lignocelluloses (Printed with permission from Kamm and Kamm [509])

392

6 Bioenergy

Combustion Electricity

Grain [ Cereals, Corn, Maize ]

Energy

Straw Decomposition

Elevatedpressure Gasification Biotechnological Conversion

Fields

Seed

Grinding

Meal

Strach

Extrusion

Directly Use Decomposition to paste/ pasting

Co-Extrusion

Chemical Conversion / Modification

Plasticization

Sorbitol

PHB

Methanol

Ether formation

Esterification

Ethanol

Syngas

Red. Amination

Fermentation

AcetateStarch

Glucosamine

Hydrogenation

Cellulose

Hemicellulose

Lignin

Glucose

Carboxymethyl Starch

Binder Bio-Plastic Adhesive

Co- and Mixpolymerisate

Cement

Fig. 6.45 Products from various grains (Printed with permission from Kamm and Kamm [509]) Precursors-contained Biomass Wood / Soft wood

BiomassPrecursors Straw Bagasse Leaf Lignin

Cereals / Maize

Carbohydrates

Lignin

Cellulose

Starch

Levulinic acid

Soya/Rape

Fats

Saccharose

Alfalfa / Grass/Clover

Proteins Aminoacids

Oil

enzymatic

chemical

Energy MaterialPrecursors

Sugar-beet /-cane

Feed

Enzymes

Glucose bacterial Acetic acid

Syngas

2,3-Pentanedion

Lactic acid Acrylic acid

Ethanol Dilactide Methanol Ethene Gasoline

Ethyllactate

Polymers

Fig. 6.46 Production routes for various industrial chemicals from a variety of biomass (Printed with permission from Kamm and Kamm [509])

6.11

Summary

393

Green Crop Drying Plant Wet Fractionation

Energy

Green (Wet) Raw Material

Press

Press Cake drying to Pellets + Bales

Power Station Heat, Electricity

Press Juice

Valuable Products

Biogas

Fermentation Fermenter

Carbohydrate Sources

Separation Decanter Lactic Acid + Derivatives

Green Pellets for Fodder Pellets or Bales for Solid Fuel

Enzymes Flavourings

Raw Material for Syngas

Dyes

Raw Material for Hydrocarbons

Amino Acids Carbohydrates

Proteins Pre-Treatment

Raw Material for Biogas

Proteins

Enzymes Whole Crops, Straw, Seeds, Starch, Hydrolyzate, Molasse, a.o.

Fields

Grass, Lucerne, Alfalfa, Herb a.o.

Organic Acids Ethanol

Raw Material for Fibres + Fleece Raw Material for Chemicals e.g. Levulinic Acid

Fig. 6.47 Production routes for various industrial chemicals from green crops (Printed with permission from Kamm and Kamm [509])

6.11 Summary Bioenergy is a renewable energy source as it is derived from biological sources, which can be replenished on a regular basis. Bioresources include dedicated energy crops and trees, agricultural crop wastes and residues, wood wastes and residues, and aquatic plants as well as animal, municipal, and other wastes. Bioenergy can be used for the generation of heat, electricity or biofuel for vehicles. Biofuel derived from plant materials is among the most rapidly growing renewable energy technologies. Recent legislations by various countries suggest further growth of both corn-based and advanced biofuels from other sources. In the United States, cornbased ethanol is currently the largest source of ethanol as a gasoline substitute or additive. Biomass resources can also be used to produce varieties of chemicals such as glues, cleaners, solvents, and plastics. The generation of heat or electricity using biomass requires continuous and reliable supply of biomass. To ensure the continuous supply, energy crops must be planted and harvested on a regular basis requiring additional lands. This may have significant implications for world agriculture. The impacts of bioenergy on food security have been highly debated. A set of criteria, indicators, good practices and policy options must be developed for sustainable bioenergy production that safeguards and, if possible, fosters food security.

394

6 Bioenergy

Problems 1. Define biomass. 2. Describe bioenergy. 3. What types of feedstock development are necessary to sustain the bioenergy? 4. What are energy crops? 5. Describe the mechanisms by which plants store energy. 6. What are the sources of biomass? 7. How much of biomass resources are being used as a fuel source? 8. Where are biomass resources located? 9. Is municipal solid waste (MSW) considered biomass? 10. How is biomass used for electric power generation? 11. Explain biomass gasifiers. 12. What are the best biomass fuels for electric power generation? 13. What are different techniques for producing electric power from biomass? 14. What are the advantages of biomass co-firing power generating systems? 15. Describe liquid fuel production processes from biomass. 16. What is the likely delivered cost of switchgrass in the U.S., per dry ton? 17. How many acres of land would be necessary in order to have a steady supply of bioenergy feedstock for a 1 MW power plant? 18. Compare the cost of electricity from biomass with conventional power plants. 19. What kind of biomass is grown for bioenergy systems? 20. Can energy crops be used for home use? 21. How does woody bioenergy crop used by wildlife compare to that of natural forests? 22. Will energy crops put pressure on forests in the United States, including National Forests? 23. Can roadside land and interstate medians be used for energy crops, and how tall are they? 24. Would use of animal wastes as energy sources encourage more large animalraising operations? 25. How can trees and forests act as a carbon sink?

6.11

Summary

395

26. Does tree harvesting cancel out the carbon sink? 27. How is the area for crop production calculated? 28. What area of land is needed to supply bioenergy to a 1 MW power station? 29. What area of forest is needed to offset the CO2 emissions from a 1 MW power station or from running a car for 100 km? 30. Can land be managed simultaneously as a carbon sink and for bioenergy and fiber production? 31. What are your thoughts on feedstock supply in the developing world? Does the food vs. fuel debate not hold up there as well? 32. What is green crude and its outlook for production? 33. What are the five top candidate biomass materials in the US? 34. In the wild, fires consume massive cellulosic mass. Can this be harvested and reduce fire hazards? 35. How do you produce microalgae in large scale for energy production? 36. How do you get the oil out of microalgae? 37. What is the current perspective of the necessary breakthroughs in algal biofuels development; genetic engineering, yield improvements, processing (harvesting, extraction, refining), or the biorefinery business model? 38. Will cap and trade and/or tax subsidies affect bioenergy? 39. Can biofuel operations also produce energy? 40. Do tropical areas have an advantage for feedstock production? 41. Is oil an attractive feedstock for biofuel? 42. What is a vertically integrated operation? 43. What does the dairy farmer need to do to turn the manure into profit? 44. How much manure is necessary on a continuous basis to operate a 1 MW electricity generating plant? 45. What is biodiesel? 46. How is biodiesel made? 47. Does special types of engine necessary to use biodiesel? 48. What is the mileage of biodiesel vehicles? 49. What types of chemicals can be derived from biofuel? 50. What are the issues surrounding lignocellulosic production of ethanol?

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Chapter 7

Ethanol

Abstract Ethanol is considered to be the best alternative to gasoline as a liquid fuel for use in automobiles. Although ethanol can be produced from a variety of biomass, currently the main focus is on the use of corn and sugarcane as the feedstock. Corn is the main feedstock in the USA, whereas it is sugarcane in Brazil for ethanol production. However, corn is not considered to be an ideal feedstock for ethanol. Not only it is one of the major food grains of the world, but also the energy balance for ethanol production is not very attractive. Alternative to corn are lignocelluloses based plants, such as switchgrass, that are rich in cellulose or lignin. Since, the biomass can be harvested on an annual basis, ethanol produced from these biomasses may be considered as a renewable energy source. Production methods of ethanol from corn, sugarcane, and lignocelluloses and its uses as energy sources are discussed in this chapter.

7.1 Introduction The best known use of ethanol is as an alcoholic beverage. It is also used extensively in various industrial applications that include the use as a solvent in the manufacture of varnishes and perfumes, as a preservative for biological specimens, in the preparation of essences and flavorings, in many medicines and drugs, and as a disinfectant. The use of ethanol in automobiles goes back to the days of Henry Ford, 1908, who envisioned ethanol as the primary fuel for his Model T car [1]. Although Henry Ford eventually used gasoline for Model T, currently the use of ethanol as fuel is increasing rapidly worldwide [2–10]. Various fuel properties of ethanol are compared with that of gasoline and No. 2 diesel in Table 7.1. As can be seen from Fig. 7.1, the use of ethanol as alcoholic beverage and industrial chemical remains rather flat, but its use as fuel increasing rapidly. In the USA, ethanol received a major boost in its use and production with the passage of the Clean Air Act Amendments (CAAA) of 1990. Various events that helped to boost the ethanol production are shown in Fig. 7.2. T.K. Ghosh and M.A. Prelas, Energy Resources and Systems: Volume 2: Renewable Resources, DOI 10.1007/978-94-007-1402-1 7, © Springer Science+Business Media B.V. 2011

419

420 Table 7.1 Fuel properties of common transportation fuels Property Ethanol Chemical formula C2 H5 OH Molecular weight 46.07 Carbon 52.2 Hydrogen 13.1 Oxygen 34.7 0.796 Specific gravity, 60ı F=60ı F Density, lb/gal @ 60ı F 6.61 172 Boiling temperature, ı F Reid vapor pressure, psi 2.3 Research octane no. 108 Motor octane no. 92 (R C M)/2 100 Cetane no.(1) – Fuel in water, volume % 100 Water in fuel, volume % 100 173.2 Freezing point, ı F Viscosity, centipoise @ 60ı F 1.19 Flash point, closed cup, ı F 55 793 Autoignition temperature, ı F Lower 4.3 Higher 19 Btu/gal @ 60ı F 2,378 396 Btu/lb @ 60ı F 44 Btu/lb air for stoichiometric mixture @ 60ı F Higher (liquid fuel-liquid water) Btu/lb 12,800 Lower (liquid fuel-water vapor) Btu/lb 11,500 Higher (liquid fuel-liquid water) 84,100 Btu/gal 76;000a Lower (liquid fuel-water vapor) ı Btu/gal @ 60 F Mixture in vapor state, Btu/cubic foot 92.9 @ 68ı F Fuel in liquid state, Btu/lb or air 1,280 Specific heat, Btu=lbı F 0.57 Stoichiometric air/fuel, weight 9 Volume % fuel in vaporized 6.5 stoichiometric mixture Source: U.S. Department of Energy [11] a Calculated value b Pour point ASTM D97 c Based on cetane

7 Ethanol

Gasoline C4–C12 100–105 85–88 12–15 0 0.72–0.78 6.0–6.5 80–437 8–15 90–100 81–90 86–94 5–20 Negligible Negligible 40 0.37–0.44a –45 495 1.4 7.6 900 150 10

No. 2 Diesel C3–C25 200 84–87 33–16 0 0.81–0.89 6.7–7.4 370–650 0.2 – – N/A 40–55 Negligible Negligible 40–30b 2.6–4.1 165 600 1 6 700 100 8

18,800–20,400 18,000–19,000 124,800

19,200–20,000 18,000–19,000 138,700

115,000

128,400

95.2

96:9c

1,290 0.48 14:7a 2

– 0.43 14.7 –

To meet the oxygen requirements in the fuel mandated by the CAAA, blending of ethanol with gasoline became a popular method. One of the objectives of the CAAA was to reduce carbon monoxide (CO) and ground-level ozone concentration in the

7.1

Introduction

421

Fig. 7.1 World ethanol use by various sectors (Source: Berg [12])

20000

60 RFS (?)

15000

50

30

10000 Clean Air Act

20

Cents/gallon

40 California

5000 10 0

0 1980 1984 1988 1992 1996 2000 2004 2008 2012 1978 1982 1986 1990 1994 1998 2002 2006 2010 Tax incentive (Y2)

Output (Y1)

Fig. 7.2 Ethanol production in the USA. RFS Renewable Fuel Standard (Source: Berg [12])

air. Oxygen level in gasoline of about 2.7% by weight for oxygenated fuel and 2.0% by weight for reformulated gasoline would be necessary to achieve the goal stated in the CAAA. As stated by the Energy Information Administration, “The Energy Policy Act of 1992 (EPACT) allowed two additional gasoline blends (7.7% and 5.7% ethanol). It also defined ethanol blends with at least 85% ethanol as “alternative transportation fuels.” It also required specified car fleets to begin purchasing alternative fuel vehicles, such as vehicles capable of operating on E-85 (a blend of 85% ethanol and 15% gasoline). EPACT also provided tax deductions for purchasing (or converting) a vehicle so that it could use an alternative fuel, such as E-85, and for installing equipment to dispense alternative fuels.

422

7 Ethanol

The Clean Air Act Amendments mandated the winter-time use of oxygenated fuels in 39 major carbon monoxide non attainment areas (areas where the US Environmental Protection Agency (USEPA) emissions standards for carbon monoxide had not been met) and required year-round use of oxygenates in nine severe ozone non attainment areas in 1995. During 1999, some states began to pass legislation banning the use of Methyl Tertiary Butyl Ether (MTBE) in motor gasoline, because traces of it were showing up in drinking water sources, presumably from leaking gasoline storage tanks. MTBE was the primary oxygenate used in gasoline in the USA prior to 1999. Because ethanol and Ethyl Tertiary Butyl Ether (ETBE) are the main alternatives to MTBE as an oxygenate in gasoline, these bans increased the need for ethanol as the MTBE ban went into effect. In 2000, the USEPA recommended that MTBE should be phased out nationally. California began switching from MTBE to ethanol to make reformulated gasoline, resulting in a significant increase in ethanol demand by mid-year, even though the California MTBE ban did not officially go into effect until 2004. The Energy Policy Act of 2005 mandated that the gasoline sold in the USA must contain a minimum volume of renewable fuel, called the Renewable Fuels Standard. The regulations aimed to double the use of renewable fuel, mainly ethanol made from corn, by 2012. The Energy Independence and Security Act of 2007 expanded the Renewable Fuels Standard to require that 36 billion gallons (136,275 thousands cubic meter) of ethanol and other (27,254 thousands cubic meter) fuels be blended with gasoline, diesel, and jet fuel by 2022. The United States consumed 6.8 billion gallons (25,740 thousands cubic meter) of ethanol and 0.5 billions gallons (1,892 thousands cubic meter) of biodiesel in 2007. As of March 2008 (23,469 thousands cubic meter), United States ethanol production capacity was at 7.2 billion gallons (27,354 thousands cubic meter), with an additional 6.2 billion gallons (23,469 thousands cubic meter) of capacity under construction. Brazil is the second largest producer of ethanol and a leading exporter. As shown in Fig. 7.3, the ethanol production in Brazil is increasing slowly over the last decade. Brazil is consuming a significant percentage of their own ethanol for transportation. The fuel consumption in Brazil by fuel type is shown in Table 7.2. In 2008, Brazil used about 19,600 thousands cubic meter of ethanol fuel. The ethanol consumption in Brazil is increasing due to the introduction of flex-fuel cars. Table 7.3 shows the increase in the number of flex-fuel cars in Brazil, which increased from just over 48 thousands in 2003 to nearly 2 millions in 2007. Currently (as of May 2009) ethanol use in light vehicles in Brazil is more than 50% of the use of gasoline. When trucks and other diesel vehicles are included, ethanol represents about 20% of the road transportation usage. Ethanol represents 15% of the total supply of liquid fuels in Brazil. In 2003, the European Commission issued directives that will govern European Union (EU) biofuels policy through 2010. The commission adopted a target of 5.75% biofuels consumption in the transportation sector by 2010. However, the directive also had a target of 2% biofuel use by 2005, since there were little biofuel productions in Europe outside of the biodiesel production in Germany. The directive was designed to promote ethanol demand and supply and provided

7.1

Introduction

423

22.478

Anhydrous Hydrous

20

17.720 14.808

15 10.592

11.535

10 5.62

6.465

12.622

15.416 15.946 8.302 8.304

7.838

7.015 9.418 8.108

2006-07

2005-06

2002-03

2004-05

5.07

14.300

5.896

2003-04

4.972

2001-02

7.112 5.607

2007-08

5

0

8.178

8.912

2000-01

Ethanol Production (Billion Liters)

25

Crop Year

Fig. 7.3 Ethanol production in Brazil (Source: Global Agriculture Information Network (GAIN) [13])

Table 7.2 Various types of fuel consumption in Brazil Yearly fuel consumption (in thousands m3 ) Fuel type 2003 2004 2005 Diesel 36,853 39,219 39,052 21,791 23,165 23,542 Gasoline Ca Hybrid ethanol 3,245 4,355 4,654 Source: ANP[14] a Includes 20–25% anhydrous ethanol

2006 39,854 23,979 6,010

2007 41,559 24,326 9,367

Table 7.3 Brazil’s sales of light vehicles Year Gasoline Ethanol Flex-fuel 2000 1,310,479 10,292 2001 1,412,420 18,335 2002 1,283,963 55,961 2003 1,152,463 36,380 48,178 2004 1,077,945 50,950 328,379 2005 697,033 32,357 812,104 2006 316,561 1,863 1,430,334 2007 245,660 107 1,995,090 Source: National Association of Automotive Vehicle Manufacturers and Associac¸a˜ o Nacional dos Fabricantes de Ve´ıculos Automotores (ANFAVEA) [15]

2008 44,764 25,175 13,290

424 Table 7.4 Bioethanol production in Europe (million liters)

Table 7.5 World production of fuel ethanol in 2008

7 Ethanol

Country 2004 2005 2006 Germany 25 165 431 Spain 254 303 396 France 101 144 293 Poland 48 64 161 Sweden 71 153 140 Italy 0 8 78 Hungary 0 35 34 Lithuania 0 8 18 Netherlands 14 8 15 Czech Republic 0 0 15 Latvia 12 12 12 Finland 3 13 0 Total 528 913 1,592 Source: European Biomass Industry Association [16]

Country

Millions of gallons

USA 9000.0 Brazil 6472.2 European 733.6 Union China 501.9 Canada 237.7 Total 17,335.2 Data source: Licht [17]

Country

Millions of gallons

Other Thailand Colombia

128.4 89.8 79.29

India Australia

66.0 26.4

tax benefits and exemptions to facilitate growth. The target of 2% biofuel use by 2005 was non-binding. The biofuel use in the EU’s 25 member states in 2006, before Romania and Bulgaria joined the trade bloc in January, reached about 1.4%. However, in March 2006, the EU unveiled its Renewable Energy Roadmap that created a binding target of 10% biofuel use by 2020 for all of its 27 member states. Bioethanol productions by various European countries are given in Table 7.4. The leading ethanol producing countries in the world are given in Table 7.5. Ethanol is becoming an attractive fuel additive in many countries. The feedstock for ethanol can be home grown and produced locally since the technology is fairly well known. The use of ethanol can reduce the import of petroleum by most of the countries around the world. As a result, a number of countries are mandating the use of ethanol in the gasoline. The ethanol blending requirements mandated by various countries are given in Table 7.6.

7.2 Ethanol Production from Corn Ethanol can be produced from petroleum products, starches, sugar and lignocelluloses. Depending on the starting feedstock, production methods also differ. Various

7.2

Ethanol Production from Corn

425

Table 7.6 Ethanol blending requirements by various countries Country Current ethanol blending requirement Brazil All gasoline must contain between 20% and 25% anhydrous ethanol. Currently, the mandate is 23% Canada By 2010, 5% of all motor vehicle fuel must be ethanol or biodiesel France Set target rates for incorporation of biofuels into fossil fuels (by energy content). Calls for 5.75% in 2008, increasing to 10% in 2010 Germany 8% energy content in motor fuels by 2015, 3.6% coming from ethanol Lithuania Gasoline must contain 7–15% ETBE. The ETBE must be 47% ethanol Poland Mandatory “National Biofuel Goal Indicators” calling for biofuels to represent a set percentage of total transportation fuel use. 2008’s standard is 3.45%, on an energy content basis Argentina Requires the use of 5% ethanol blends by 2010 Thailand Gasoline in Bangkok must be blended with 10% ethanol India Requires 5% ethanol in all gasoline China Five Chinese provinces require 10% ethanol blends – Heilongjian, Jilin, Liaoning, Anhui, and Henan The Philippines Requires 5% ethanol blends in gasoline beginning in 2008. The requirement expands to 10% in 2010 Bolivia Expanding ethanol blends to 25% over the next 5 years. Current blend levels are at 10% Colombia Requires 10% ethanol blends in cities with populations over 500,000 Venezuela Phasing in 10% ethanol blending requirement Source: Renewable Fuels Associations [18] ETBE ethyl tertiary butyl ether

routes for ethanol productions are given below and the basic steps involved in the production of ethanol from these feedstocks are shown in Fig. 7.4. • Direct hydration of ethylene • Indirect hydration of ethylene • Ethanol from corn – Dry milling and fermentation – Wet milling and fermentation • Sugar crop fermentation • Ethanol from lignocellulose Among these methods, direct hydration of ethylene [19–24] and indirect hydration of ethylene [25–27] processes use a catalyst to convert ethylene, a petroleum product from the refinery, to ethanol. The rest of the methods are bio-based and are discussed in details later in this chapter.

7.2.1 Structure, Types, and Composition of Corn Corn and sugarcane are considered to be the two main feedstocks for production of ethanol. The use of other feedstocks, such as switchgrass, is still in the evaluation stage. Beside its use for the production of ethanol, corn has a number of other uses. These are summarized in Fig. 7.5.

Fig. 7.4 Processes for ethanol production from various feedstocks [28]

426 7 Ethanol

7.2

Ethanol Production from Corn

427

Fig. 7.5 Various components of a corn kernel and their potential use (Source: Food and Agriculture Industry [29])

Several varieties of corn are grown worldwide; some of them are for specific purposes and are discussed below. Dent corn (Zea mays indentata) is also called field corn. Its kernels contain both hard and soft starch and become indented at maturity. The major use of dent corn is for food production, animal feed, and industrial products, such as ethanol. This is the only variety used for cornstarch manufacturing. Flint corn (Zea mays indurate) has a hard, horny, rounded or short and flat kernel with the soft and starchy endosperm completely enclosed by a hard outer layer. This is grown mostly in South America and used for the same purposes as dent corns. Waxy corn has a waxy appearance when cut and contains only a branched-chain starch. Waxy corn is processed in a wet milling process to produce waxy cornstarch which slowly retrogrades back to the crystalline form of starch. It is grown to make special starches for thickening foods, particularly those that undergo large temperature changes in processing and preparation. Sweet or green corn is eaten fresh, canned, or frozen. It can be grown in many horticultural varieties. Its kernels contain a high percentage of sugar in the milk stage when they are eaten fresh. Popcorn (Zea mays everta) has small ears and small pointed or rounded kernels with very hard corneous endosperm. When exposed to high dry heat, they pops

428

7 Ethanol

or everted by the expulsion of the contained moisture, and form a white starchy mass many times the size of the original kernel. It got its name from this popping characteristic. Indian (Zea mays) corn has white, red, purple, brown, or multicolored kernels. It was the original corn grown by the Indians. It is a attractive Halloween decorations in the USA. High-Amylose corn is a specialty corn, whose kernels have amylose content higher than 50%. This starch is used in textiles, candies, and adhesives. High-Oil corn contains 7–8% oil, 2–3% more than dent corn. High-oil corn also has enhanced protein quality and quantity. High-Lysine corn contains increased levels of two amino acids that are essential in the diet of non-ruminant animals, such as swine. The two amino acids are lysine and tryptophane. Flour corn (Zea mays amylacea) also called soft corn or squaw corn. Its kernels are shaped like those of flint corn and composed mostly of soft starch. The USA grows small amounts of blue flour corn to make tortillas, chips, and baked goods. In South America, this corn is grown in various colors to make food and beer. The appearance of different types of corn kernels is shown in Fig. 7.6. Not only the appearance of the corn kernels varies from one corn to another corn, but they are also different in their chemical compositions. The chemical analysis of three types of corn is given in Table 7.7. Among these corns, dent corn is most suitable for ethanol production. Most of the corn crops in the United States are yellow dent corn. Its vitamin A content,

Fig. 7.6 Kernels of most common corns (Source: Dickerson [30]) Table 7.7 Chemical composition of some common corns

Chemical compounds Flint in 100 g of grain corn Protein 10.7 Fat 4.7 Fiber 2.7 Starch 69.6 Sugars 1.9 Source: Domin and Kluza [31]

Popping corn 10.4–14.6 3.8–5.3 – 62.2–71.8 –

Sweet corn 2.1–4.5 1.1–2.7 0.9–1.9 3.0–20.0 2.5–8.5

7.2

Ethanol Production from Corn

429

Fig. 7.7 A mature dent corn kernel. (1 and 2 vertical sections in two planes of a mature kernel of dent corn, showing arrangement of organs and tissues). a silk scar, b pericap, c aleurone, d endosperm, e scutellum, f glandular layer of scutellum, g coleoptile, h plumule with stem and leaves, i first internode, j lateral seminal root, k scutellar node, l primary root, m coleorhiza, n basal conducting cells of endosperm, o brown abscission layer, p pedicel or flower stalk. 3 enlarged section through pericap and endosperm. a pericap, b nucellar membrane, c aleurone, d marginal cells of endosperm, e interior cells of endosperm. 4 enlarged section of scutellum. a glandular layer, b interiror cells. 5 vertical sction of the basal region of endosperm. a ordinary endosperm cells, b thick walled conducting cells of endosperm, c abscission layer (Source: Kiesselbach [32])

high feed value and availability in various hybrid forms account for its extensive use. Of the cereal grains, it contains the highest amount of carotene (vitamin A). Dent corn originated from crosses of flint and floury corns. Dent hybrids vary in the proportion of hard and soft endosperm. A careful analysis of dent corn is necessary before its processing and fermentation. A detailed structure of a dent corn kernel is shown in Fig. 7.7.

430

7 Ethanol

Table 7.8 Composition of various components of yellow dent corn on percent of dry basis Kernel Component Percent (%) Starch (%) Protein (%) Oil (%) Ash (%) Sugars (%) Fiber (%) Endosperm 82:9 88:4 Germ 11:0 11:9 Bran coat 5:3 7:3 Tip cap 0:8 5:3 Whole kernel 100:0 75:0 Source: Bunge Milling [33]

8:0 18:4 3:7 9:1 8:9

0:8 29:6 1:0 3:8 4:0

0:3 10:5 0:8 1:6 1:5

Table 7.9 Proximate analysis of yellow dent corn grain Characteristic Moisture (% wet basis) Starch (% dry basis) Protein (% dry basis) Fat (% dry basis) Ash (oxide) (% dry basis) Pentosans (as xylose) (% dry basis) Fiber (neutral detergent residue) (% dry basis) Cellulose C Lignin (acid detergent residue) (% dry basis) Sugars, total (as glucose) (% dry basis) Total carotenoids (mg/kg) Source: White and Johnson [34]

0:6 10:8 0:3 1:6 1:7

Range 7–23 61–78 6–12 3.1–5.7 1.1–3.9 5.8–6.6 8.3–11.9 3.3–4.3 1.0–3.0 12–36

1:9 18:8 86:9 78:6 8:9

Average 16:0 71:7 9:5 4:3 1:4 6:2 9:5 3:3 2:6 26:0

The chemical composition of a dent corn kernel is given in Table 7.8. The composition can vary signficantly from one batch to another and even from one kernel to another kernel. The range of various contituents of corn kernels obtained from proximate analysis in given in Table 7.9.

7.2.2 Processing of Corn There are two approaches for converting corn to ethanol [35–41]: • Dry Milling Process • Wet Milling Process In the USA, the wet milling process was preferable in the 1990s. At present, dry milling processes are the most common type for ethanol production. In 2008, dry milling processes represented almost 90% of the ethanol production from corn (See Fig. 7.8). As discussed later in this chapter, the dry milling process requires fewer steps and equipment than the wet milling process providing better economy for ethanol production. 7.2.2.1 Dry Milling Process for Ethanol Production In dry milling, the entire corn kernel is first grounded into flour, and the starch in the flour is converted to ethanol via fermentation [42–55]. The other products are

7.2

Ethanol Production from Corn

431

5000 Forecast

Wet Mill Process

4000

3000

2000

2015

2014

2013

2012

2011

2010

2009

2008

2007

2006

2005

2004

2003

2002

2001

2000

0

1999

1000

1998

Corn Used (Million Bushels)

Dry Mill Process

Year

Fig. 7.8 A comparison of corn use between dry mill and wet mill processes (Source: Food and Agricultural Policy Research Institute [41])

carbon dioxide (used in the carbonated beverage industry) and animal feeds called distillers dried grain with soluble. The dry milling process for production of ethanol may be divided into following five steps: 1. 2. 3. 4. 5.

Milling (Biomass Handling) Liquefaction Hydrolysis (Saccharification) Fermentation Distillation and Recovery

The ground corn flour, which is referred to as meal, is processed without separating out the various components of the grain kernel. Various processing steps are shown in Fig. 7.9. The liquefaction process involves making slurry of the meal with water to form a mash. Enzymes are added to the mash to convert the starch to sugar (dextrose). Ammonia is added for pH control and as a nutrient to the yeast. The mash is next cooked at a high-temperature to reduce bacteria levels prior to fermentation. The mash is cooled and transferred to fermenters where yeast is added for the conversion of sugars to ethanol. Carbon dioxide .CO2 / is also produced during the fermentation process. The fermentation process generally takes about 40–50 h. During fermentation, the mash is agitated continuously for uniform distribution of yeast and is kept cool to maintain high activity of the yeast. After fermentation, the resulting beer is transferred to distillation columns where ethanol is separated from the remaining stillage. The ethanol is concentrated to 190 proof using conventional distillation method and then is dehydrated to approximately 200 proof using a molecular sieve desiccant system. (The proof of an alcohol beverage is equal to twice the

432

7 Ethanol

Fig. 7.9 Dry milling process for ethanol production (Courtesy of Singh et al. [43])

percentage of ethyl alcohol contained therein, Therefore, pure ethanol is 200 proof).The anhydrous ethanol is then blended with about 5% denaturant (such as natural gasoline) to make it a non-beverage, and thus allows its exemption from alcohol tax. The stillage is sent through a centrifuge that separates the coarse grain from the solubles, and several co-products are produced. The co-products are Corn Distillers Dried Grains (DDG), Corn Condensed Distillers Solubles (CDS), Corn Distillers Dried Grains/Solubles (DDGS), and Wet Distillers Grains with Solubles (WDGS). These products are described below. • Distillers Dried Grains (DDG) is obtained after the removal of ethanol by distillation from the yeast fermentation of grains or grain mixtures by separating the resultant coarse grain fraction of the whole stillage and drying it by methods employed in the grain distilling industry. • Distillers Dried Grains with Solubles (DDGS) is the product obtained after the removal of ethanol by distillation from the yeast fermentation of grains or grain mixtures by condensing and drying at least 3/4 of the solids by methods employed in the grain distilling industry. • Condensed Distillers Solubles (CDS) is the product obtained after the removal of ethanol by distillation from the yeast fermentation of grains or grain mixtures by condensing the thin stillage fraction to a semi-solid.

7.2

Ethanol Production from Corn

433

• Wet Distillers Grains with Solubles (WDGS): Wet distillers grains (WDG) is the main co-product with the remaining volume after fermentation of corn starch to ethanol. Soluble nutrient-rich syrup is separated during the fermentation process which can be sold for feeding purposes as such or added back to the final product to obtain wet distillers grains plus solubles. The solubles from the dry milling processes are concentrated to about 30% solids by evaporation, resulting in CDS or “syrup”. The coarse grain and the syrup are then dried together to produce DDGS. This is a high quality feed for livestock. The CO2 released during fermentation is typically used for carbonating soft drinks and beverages.

7.2.2.2 Wet Milling In the wet milling process, the corn kernel is separated into three parts: (1) the hull, (2) the germ, and (3) the endosperm in an aqueous medium prior to fermentation [56–59]. The primary products of wet milling include starch and starch-derived products (e.g. high fructose corn syrup and ethanol), corn oil, and corn gluten. As shown schematically in Fig. 7.10, the process consists of several steps. A wet mill generally receives shelled corns, which pass through mechanical cleaners designed to remove unwanted material, such as pieces of cobs, sticks, husks, meal and stones. The cleaned corns are next fed into “steep” tanks, where these are soaked in dilute sulfuric acid from 24 to 48 h at a temperature of 125ı F .52ı C/. Steeping softens the kernel, helps to break down the protein holding the starch particles, and removes various soluble constituents. A number of tanks are used in series. Corn that has steeped for the desired length of time is discharged from the tank for further processing, and the tank is filled with fresh corn. Generally, water drained from the first steep tank is discharged to evaporators, called “light steepwater” that contains about 6% of the original dry weight of the grains. The solids from steepwater are rich in protein and are concentrated to 30–55% solids in multiple-effect evaporators. The resulting steeping liquor can be sold as animal feeds. The germ is removed from the steeped corn in the degerminating mills, which break the kernel apart to free both the germ and about half of the starch and gluten. The germ is separated in liquid cyclones from the mixture of fiber, starch, and gluten. It is subsequently washed, dewatered, and dried, and further processed to extract corn oil. The starch and gluten from the product slurry are removed from the rest of the fibrous material by further washing, grinding, and screening operations. The discarded hulls are dried for use in animal feeds. The starch is separated from gluten by centrifugation. A number of stages may be necessary. The separated starch is fed to the fermenter for production of ethanol.

434

Fig. 7.10 Corn wet milling process flow diagram (Source: Watson and Ramstad [60])

7 Ethanol

7.2

Ethanol Production from Corn

435

7.2.3 Fermentation Process The fermentation step for the ethanol production is the controlling step of the process. It is a biological process in which complex organic materials are converted by microorganisms mainly to sugars. Sugars and similar compounds are further fermented by microorganisms to produce ethanol and CO2 . The selection of microorganisms for the fermentation is very critical. Several bacteria, yeasts, and fungi have been reportedly used for the production of ethanol. Among various microorganisms, yeasts are preferred for ethanol production, and among the yeasts, Saccharomyces cerevisiae also known as Bakers’ yeast, is most widely used. Saccharomyces cerevisiae can produce ethanol providing a concentration as high as 18% of the fermentation broth. Other types of bacteria explored by various researchers are given in Table 7.10. Another type of yeast that has been studied for ethanol production is Zymomonas mobilis. It can tolerate higher ethanol loading, up to 120 g/l ethanol and can yield 5–10% more ethanol per fermented glucose. However, Z. mobilis ferments only glucose, fructose, and sucrose and is not as hardy as Saccharomyces cerevisiae. A variety of microorganisms have been engineered to selectively produce ethanol. Several gram-negative bacteria have been engineered for this purpose that include: Escherichia coli, Klebsiella oxytoca, and Zymomonas mobilis. Engineered E. coli was able to ferment a wide spectrum of sugars. However, E. coli works in a

Table 7.10 Bacterial species used by various researchers for production of ethanol as main fermentation product Mesophilic organisms Clostridium sporogenes Clostridium Indoli (pathogenic) Clostridium sphenoides Clostridium sordelli (pathogenic) Zymomonas mobilis (syn. Anaerobica) Zymomonas mobilis subsp. Pomaceas Spirochaeta aurantia Spirochaeta stenostrepta Spirochaeta litoralis Erwinia amylovora Escherichia coli KO11 Escherichia coli LY01 Leuconostoc mesenteroides Streptococcus lactis Klebsiella oxytoca Klebsiella aerogenes Mucor sp. M105 Source: Lin and Tanaka [66]

mmol ethanol produced per mmol glucose metabolized up to 4.15a 1.96a 1.8a (1.8)b 1.7 1.9 1.7 1.5 (0.8) 0.84 (1.46) 1.1 (1.4) 1.2 0.7–0.1 40–50 g ethanol produced/l 1.1 1.0 0.94–0.98 24 g ethanol produced/l –

References [62] [62] [62] [62] [62] [62] [62] [62] [62] [62] [63, 64] [63] [62] [64] [65] [63]

436

7 Ethanol

narrow and neutral pH range (6.0–8.0). It is a less hardy culture compared to yeast. Also, by-products may be tainted with E. coli reducing their value. Fermentation of starch is more challenging than fermentation of sugars because starch must first be converted into sugar and then into ethanol. Although, yeasts discussed above are capable of fermenting starch, the process is rather slow. According to Abouzied and Reddy [61], “Synergistic coculture of an amylolytic yeast (Saccharomycopsis fibuligera) and S. cerevisiae, a non-amylolytic yeast, fermented unhydrolyzed starch to ethanol with conversion efficiencies over 90% of the theoretical maximum. Fermentation was optimal between pH 5.0 and 6.0. Using a starch concentration of 10% (w/v) and a 5% (v/v) inoculum of S. fibuligera, increasing S. cerevisiae inoculum from 4% to 12% (w/v) resulted in 35–40% (w/v) increase in ethanol yields. Anaerobic or “limited aerobic” incubation almost doubled ethanol yields”. Conversion of cellulose to ethanol is some what complex. Saccharomyces cerevisiae and other engineered bacteria are not effective in fermenting cellulose. Various anaerobic thermophilic bacteria have been explored by a number of researchers and are given in Table 7.11. However, not only the fermentation process is found to be very slow (3–12 days), the yield was also poor (0.8–60 g/l of ethanol). Another disadvantage of using bacteria for fermentation is the production of various by-products, primarily acetic and lactic acids.

7.2.4 Byproducts from Corn Processing By-products from both dry and wet milling processes play an important role in determining economic viability of these two processes for ethanol production [81–87, 89, 90]. As discussed later in this chapter, byproducts are crucial in determining the energy balance of the fuel ethanol production from corns. Currently, nearly 3.8 million tons of distillers dry grains are produced during domestic dry grind ethanol production. For every bushel of corn made into ethanol, 18 pounds of DDGS are generated and must maintain the product value to contribute to plant profitability. The capacity for ethanol production is set to double in the near future and assuming that dry grind production will double as well, the potential supply of DDGS will be almost 7 million tons. The dry grind ethanol process uses most of the starch present in the corn kernel during ethanol fermentation, leaving protein, fat, minerals and vitamins behind in a concentrated form.

7.2.5 Comparison Between Dry Mill and Wet Mill Processes The amount of ethanol produced per bushel of corn in the USA by dry mill process is slightly higher than that by the wet mill process. A comparison of various byproducts produced by these two processes is shown in Fig. 7.11.

5.0

28

30

4.5

30

A3-S. cerevisiae

– 4.5

24 30

5.0

5.0

5.0

30

30

6.0

–

30–35

27

pH value 5.5

Temp .ı C/ 30

Fiso-S. cerevisiae

181-S. cerevisiae (aerobic) UO-1-S. cerevisiae (aerobic) V5-S. cerevisiae ATCC 24860-S. cerevisiae Bakers’ yeast-S. cerevisiae Bakers’ yeast-S. cerevisiae

Strain-species 27817Saccharomyces cerevisiae L-041-S. cerevisiae

Galactose (20–150)

Galactose (20–150)

Sucrose (220)

Glucose (250) Molasses (1.6–5.0) Sugar (150–300)

Sucrose (20)

Glucose (10)

Sucrose (100)

Carbon source and concentration (g/l) Glucose (50–200)

Peptone(5) and ammonium dihydrogen phosphate (1.5) Peptone, ammonium sulfate and casamino acid (10) Peptone, ammonium sulfate and casamino acid (10)

Ammonium sulfate (1) – Ammonium sulfate (0.72–2.0) –

Nitrogen source and concentration (g/l) Peptone (2) and ammonium sulfate (4) Urea (1) or ammonium sulfate (1–2) Peptone (5.0)

60

4.8–36.8

4.8–40

96.71

96

60

53 (max)

– 5–18.4

–

–

25–50

Concentration of ethanol produced (g/l) 5.1–91.8

192

36 24

60–96

40–160

24

Incubation time (h) 18–94

Table 7.11 Yeast species employed by various researchers for production of ethanol as the main fermentation product

(continued)

[75]

[75]

[74]

[73]

[71] [72]

[70]

[69]

[68]

References [67]

7.2 Ethanol Production from Corn 437

5.5

30

5.5

5.5

4.5

30

30

ATCC-32691-Pachysolen 30 tannophilus Source: Lin and Tanaka [66]

30016-Kluyveromyces marxianus 30091-Candida utilis

5.5

30

27774-Kluyveromyces fragilis 30017-K. fragilis

6.0 6.0 6.0 4.5 5.5 –

30 30 30 30 30 –

GCB-K5-S. cerevisiae GCA-II-S. cerevisiae KR18-S. cerevisiae CMI237-S. cerevisiae 2.399-S. cerevisiae 24860-S. cerevisiae

pH value 5.0

Temp .ı C/ 30

Strain-species L52-S. cerevisiae

Table 7.11 (continued)

Glucose (0–25) and xylose (0–25)

Glucose (100)

Glucose (100)

Glucose (20–120)

Glucose (20–120)

Sucrose (30) Sucrose (30) Sucrose (30) Sugar (160) Glucose (31.6) Glucose (150)

Carbon source and concentration (g/l) Galactose (20–150)

Nitrogen source and concentration (g/l) Peptone, ammonium sulfate and casamino acid (10) Peptone (5) Peptone (5) Peptone (5) Ammonium sulfate (0.5) Urea (6.4) Ammonium dihydrogen phosphate (2.25) Peptone (2) and ammonium sulfate (4) Peptone (2) and ammonium sulfate (4) Peptone (2) and ammonium sulfate (4) Peptone (2) and ammonium sulfate (4) Peptone (3.6) and ammonium sulfate (3) 100

18–94

18–94

18–94

18–94

72 72 72 30 30 27

7.8 (max)

44.4 (max)

44.4 (max)

48.96 (max)

48.96 (max)

27 42 22.5 70 (max) 13.7 (max) 48 (max)

Incubation Concentration of time (h) ethanol produced (g/l) 60 2.4–32.0

[80]

[67]

[67]

[67]

[67]

[76] [76] [76] [77] [78] [79]

References [75]

438 7 Ethanol

7.2

Ethanol Production from Corn

439

THE WET MILLING PROCESS 12.4 lbs of 21% protein feed 2.5 gallons of ethanol (or)

3.0 lbs of 60% gluten meal

33 lbs of sweetener 1.5 lbs of corn oil

(or) 31.5 lbs of starch

17 lbs of carbon dioxide THE DRY MILLING PROCESS

2.7 gallons of ethanol

10 one-lb boxes of cereal

22 lbs of hominy feed for livestock

15 lbs of brewer grits

0.7 lbs of corn oil

10 eight-oz packages of cheese curls

17 lbs of carbon dioxide

1 lb of pancake mix

Fig. 7.11 Various products from a bushel of corn by dry mill and wet mill processes (Source: Module 2 Ethanol Science and Technology [91]) Table 7.12 Potential byproducts from dry mill ethanol process Grits are used in Corn meal goes into Corn cones/flour is used in Brewing beer Corn flakes Other breakfast cereals Snack foods

Bakery mixes Cereals Corn bread Corn meal mixes Corn muffins Fritters Hush puppies Pancake mixes Snacks Spoon bread

Baby foods Bakery mixes Breadings, coatings and batters Cereals Dusting for pizzas English muffins Fermentation processes Meat products Pancakes, muffins, doughnuts

Source: Iowa Corn [92]

Although both the processes produce almost the same amount of ethanol, the main difference between these two processes is in the production of various byproducts that have very high product-value. By-products from a wet mill process are more versatile than from a dry mill process and are compared in Tables 7.12 and 7.13.

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7 Ethanol

Table 7.13 Potential byproducts from wet mill ethanol process Industrial starch uses

Industrial sweetener uses

Industrial fermentation products

Paper, recycled paper Cardboard Textiles Glues and adhesives Batteries Bookbinding Cleaners, detergents

Acetic acid Charcoal briquettes Dyes and inks Enzymes Insecticides Laminated building materials Matches

Coatings on paper, wood and metal Color carrier for printing Crayons and chalk, dyes Fireworks Industrial filters and water recovery Lubricants Ore and oil refining Paints, Plastics Rubber tires Surgical dressings Wallboard and wallpaper

Metal plating Organic solvents Paper Plasticizing agents Rayon Shampoo Shoe polish Textiles Theatrical makeup

Acetic and amino acids Blankets and bedding Carpet tile Cosmetics Electroplating and galvanizing Food packaging Disposable cold drink cups, plates and cutlery Industrial chemicals Leather tanning Mannitol Organic solvents Paper Plastics Plasticizers Soaps and cleaners Sports and active wear Textiles

Food and drug starch uses Aspirin Baby food Baked goods Baking powder Cake, cookie, dessert mixes Candies Cereals Coffee whitener Dried soups Drugs Gravy mixes Instant breakfast foods Instant pudding mix Instant tea Salad dressings Spray cooking oil

Food and drug sweetener uses Alcoholic beverages and brewing Baby foods Bacon, sausage Cereals, baked goods Caramel color Carbonated and fruit beverages Canned fruits, fruit fillings Cheese spreads Chewing gum, condiments Confections, chocolate Drugs Food coloring Frosting and icing Frozen and dried eggs Hams, Hot dogs, bologna Ice cream, sherbets, and frozen puddings Jams, jellies, preserves Marshmallows, peanut butter Pet food Pickles and relishes Snack foods (pretzels, potato chips, corn chips) Soups, spices Tomato sauces, vegetables Vinegar, yeast

Powdered mixes Powdered sugar Precooked frozen foods Salt Seasoning mixes Yeast

Source: Iowa Corn [92]

Food and drug fermentation products Antibiotics Bakery products Citric acid Drugs Enzymes Food acids Pharmaceuticals Wine

7.2

Ethanol Production from Corn

441

Fig. 7.12 A comparison between dry mill and wet mill processes (Source: Module 2 Ethanol Science and Technology [91])

As discussed earlier, the dry mill processes are used more and more in the USA for the production of ethanol from corn. As shown in Fig. 7.12, one of the main reasons for using the dry mill process is the elimination of a number of steps that can reduce both the capital investment and the operating costs compared to the wet mill process.

442

7 Ethanol 0.6 Dry mill

$ 0.53 0.5

Wet mill

$ 0.46

$ 0.46

Cost ($ per gallon)

$ 0.41

$ 0.44 $ 0.39

0.4

0.3

0.2

0.1

0.0 15

30

40

(million gallons per year)

(million gallons per year)

(million gallons per year)

Plant Size Fig. 7.13 A comparison of ethanol production cost between dry mill and wet mill by plant size (Source: Kansas Energy Chart Book [93])

A comparison of ethanol production cost between these two processes is shown in Fig. 7.13. Although the cost decreased for both processes with the increase in the plant size, the production cost by the dry mill process always remained lower compared to the wet mill process.

7.3 Sugar Crop Fermentation Various sugar crops that include sweet sorghum, sugar cane, and sugar beats are also used for ethanol production [94–111]. Among these feedstocks, sugarcane is used most widely. During ethanol production from sugarcane, cane stocks are washed, crushed and milled to extract the juice and produce bagasse. The cane juice is sent to a clarification process for removal of impurities. The clear juice is sterilized and directed toward a fermentation tank. S. cerevisiae is used for the fermentation, which is continuously separated by centrifugation and recycled back to the fermenter. The stillage is further treated for generation of by-products that can be used as a fertilizer for cane plantations. A schematic of the process is shown in Fig. 7.14.

7.5

Production of Ethanol from Cellulosic Biomass

Sugarcane

Purge

443

Water

7

1 Yeast

2

9

8

10

Sulfuric acid

3

5

Sugarcane juice

Fermentation gases Yeast recycle

Bagasse

4 Cachaze

6 Culture broth

13 Flue gases

Regenerate

Condensed 11 water

Wastewater Stillage

Water

Ethanol

Steam

12

Very low Low pressure High pressure steam pressure steam steam

Concentrated stillage (Fertilizer)

Fig. 7.14 Simplified flowsheet of fuel ethanol production from sugarcane. 1 Washing tank, 2 Mill, 3 Clarifier, 4 Rotary drum, 5 Fermenter, 6 Centrifuge, 7 Ethanol absorber, 8 Concentration column, 9 Rectification column, 10 Molecular sieves, 11 Evaporator train, 12 Combustor, 13 Turbogenerator (Printed with permission from Quintero et al. [112])

7.4 Corn Versus Sugarcane Corn and sugarcane are the two main feedstocks for commercial production of ethanol. Several researchers compared between corn and sugarcane as the feedstock for ethanol production and noted that a variety of factors determine their use by a country [113–115]. The USA and Brazil are the two top producers of ethanol. As mentioned earlier, the USA uses corn as the feedstock, whereas sugarcane is used as the main feedstock in Brazil. An analysis of these two countries with respect to the ethanol production can provide some insight regarding the factors associated with the choice of a feedstock. This is provided in Table 7.14. The main factors in the choice of the feedstock are climate, growing season, and availability of land and water.

7.5 Production of Ethanol from Cellulosic Biomass Cellulosic ethanol is attractive because feedstocks that include the agricultural wastes such as wheat straw, corn stover, grass, paper, cardboard, wood chips, and other fibrous plant material, are cheap and abundant [117–139]. The feedstock is

444

7 Ethanol

Table 7.14 Comparison of Brazil and the US ethanol industries Brazil – sugarcane United States – corn The sugar (sucrose) in sugarcane can be The starch in corn is first converted into sugar. converted directly into ethanol Then the sugar is converted into ethanol Sugarcane is planted every 6 years using Corn is planted every year using seeds cuttings Sugarcane provides five cuttings over 6 Corn is harvested once each year years and then is replanted Sugarcane yields about 35 t per acre (entire Corn yields about 8.4 t per acre (entire plant) per plant) per harvested acre harvested acre Sugarcane yields about 4.2 t of sucrose per Corn yields 4.2 t of corn grain per acre (150 acre (10–15% of sugarcane yield) bushels) or 2.4 t of starch An acre of sugarcane produces about 560 An acre of corn produces about 420 gallons of gallons of ethanol (35 t yield) ethanol (150 bushels yield) Sugarcane feedstock is cheaper to grower Corn feedstock is more expensive to grow than than corn per gallon of ethanol sugarcane per gallon of ethanol Sugarcane-ethanol can be produced Corn-ethanol is more expensive to produce than cheaper than corn-ethanol sugarcane-ethanol The by-product of ethanol production is The byproduct of ethanol production is distillers bagasse grains with soluble that is used as livestock feed Currently about 9 million acres are used for The energy source for ethanol production is ethanol production natural gas, coal, and diesel Brazil has great potential for expanding Currently about 28 million acres are used for sugarcane acreage without limiting the ethanol production acreage of other crops No subsidies for ethanol US expansion of corn acreage will come at the expense of reduced soybean and other crop acres No import tariffs on ethanol Subsidy reduced from $0.51 per gallon to $0.45 A $0.54 per gallon import tariff Source: Hofstrand [116]

outside the human food chain; therefore, does not raise moral or ethical issues like the use of corn. Converting cellulosic feedstocks into ethanol requires less fossil fuel compared to corn, so it can have a bigger effect than corn ethanol on reducing greenhouse-gas emissions. Potentially, an acre of grasses or other energy crops could produce more than two times the number of gallons of ethanol as an acre of corn. When using cellulosic biomass, the whole plant can be used instead of just the grain. Cellulosic biomass contains three main groups; cellulose, lignin, and hemicelluloses. Their percentage distribution is shown in Fig. 7.15. Cellulosic biomass also contains sugars, but they are much harder to extract than those in corn, sugarcane, and other starchy biomass. Therefore, special pretreatments are necessary to release the sugars [141–156]. Three major steps are involved in production of cellulosic ethanol: Pretreatment, Hydrolysis and Fermentation and process integration. Several by-products formed during the pretreatment process can inhibit fermentation, and also some of the sugars from cellulosic biomass are difficult to ferment. A process flow diagram showing the basic steps of production of ethanol from cellulosic biomass is given in Fig. 7.16.

7.5

Production of Ethanol from Cellulosic Biomass

445

Fig. 7.15 Composition of lignocellulose portion of biomass (Source: US Department of Energy [140])

Fig. 7.16 Processing of cellulosic biomass for ethanol production

7.5.1 Pretreatment The main objective of the pretreatment of cellulosic biomass is to make cellulose more accessible to enzymatic hydrolysis and solubilize hemicllulosic sugars. As shown in Fig. 7.17, the plant cell starts to disintegrate during the pretreatment stage. The tight bonding among lignin, cellulose, and hemicelluloses are broken by the process. The benefits of pretreatment include: no need for size reduction of biomass particles and the preservation of the pentose (hemicelluloses) fractions (See Fig. 7.18). It also limits formation of degradation products that inhibit growth of fermentative microorganism, minimizes energy demands, and limits cost.

446

7 Ethanol

Fig. 7.17 The effect of pretreatment of biomass (Courtesy of US Department of Energy [140])

Fig. 7.18 Objectives of pretreatment process (Printed with permission from Mosier et al. [155])

A variety of processes for the pretreatment of cellulosic biomass have been explored. These processes may be divided into following four categories [158]. • Biological Pretreatments [159–161] • Physical Pretreatments – Mechanical treatments [162–164] – Extrusion [165, 166] • Chemical Pretreatments – Acid pretreatment [167–176] – Alkali (Lime) pretreatment [177–182]

7.5

Production of Ethanol from Cellulosic Biomass

447

– Ozone pretreatment [183–185] – Organosolv [186–189] – Ionic liquid pretreatment [190–194] • Physico-Chemical Pretreatments – – – – – – – –

Uncatalyzed steam explosion [195–203] Liquid hot water [204–209] Ammonia fiber/freeze explosion (AFEX) pretreatment [210–220] Ammonia recycled percolation (ARP) pretreatment [221–227] Wet oxidation [228–233] Microwave pretreatment [234–238] Ultrasound pretreatment [239–245] Carbon dioxide explosion [246–248]

Among these processes, following processes are most widely used. Other processes are still at the experimental stage and require further evaluation before commercialization.

7.5.1.1 Uncatalyzed Steam Explosion Process Uncatalyzed steam explosion is used commercially to hydrolyze hemicelluloses for manufacture of fiberboard and other products by the Masonite process. In this process, high-pressure steam is applied for a few minutes. No other additives or chemical is necessary for removal of hemicelluloses. Following pressure treatment, a portion of steam is rapidly released from the treatment vessel into another vessel allowing rapid (flash) cooling of the biomass.

7.5.1.2 Liquid Hot Water Process In this process, pressurized water at a temperature of 200–230ıC is used to disrupt the plant structure within 15 min. The high pressure maintains the liquid state of water. About 40–60% of the total biomass is dissolved in the method. All of the hemicelluloses could be removed by this method, followed by 35–60% of the lignin. Only a small amount of cellulose (4–22%) is recovered. Both co-current and counter-current arrangement have been used with similar results. Using a proper design, a continuous flow through system can be employed.

7.5.1.3 Acid Pretreatment Process Dilute sulfuric acid is used to convert hemicelluloses to xylose and other sugar. However, sulfuric acid can continue to breakdown xylose to form furfural. A mixture of acid and biomass is heated either indirectly or by direct steam injection, at

448

7 Ethanol

temperatures of 160–220ı C for few seconds to few minutes. The acid concentration is typically in the range of 0.7–3.0%. However, a concentration as low as 0.07%, has also been tried in a flow through reactor. Although, sulfuric acid appears to be the best acid for pretreatment of the biomass, other acids including nitric, hydrochloric, and phosphoric acid have also been employed with a limited success.

7.5.1.4 Lime Pretreatment A lower temperature and pressure can be used during the alkali pretreatment compared to the acid pretreatment method. Generally, lime (CaO or Ca.OH/2 / is used to remove lignin and improve cellulose digestion by enzymes through opening up the structure. Lime pretreatment conditions are: 100ı C for 1–2 h at a lime loading of 0.1 g Ca.OH/2 =g biomass with 5–15 g water /g biomass. This process has been used to pretreat wheat straw, poplar wood, switchgrass, and corn stover. However, for these resources, the temperature range was 85–150ıC and the treatment time varied between 2 and 13 h.

7.5.1.5 Ammonia Fiber Expansion (AFEX) Pretreatment The AFEX process is operated in the same manner as the steam explosion process. Lignocellulosic biomass is first soaked with liquid ammonia under pressure and then the pressure is released rapidly. This causes the fiber to expand resulting in decrystalization of cellulose, and hydrolysis of hemicelluloses. The fiber structure is greatly disrupted altering the lignin that is present in the plant. This allows hydrolysis of cellulose to glucose with high yields at low enzyme loadings. Herbaceous and agricultural residues are well suited for AFEX process. However, this method works only moderately on hardwoods, and not so well on softwoods. Although AFEX pretreatment is a batch process, it can be modified to a continuous process called FIBEX (fiber extrusion) that can significantly reduce both the time required for the treatment, and the amount of ammonia required for the same level of hydrolysis, as for the batch AFEX process.

7.5.1.6 Ammonia Recycle Percolation (ARP) Pretreatment An aqueous solution containing 5–10% ammonia is fed through a column packed with biomass at a temperature in the range of 80–180ıC. Ammonia is separated from the outlet stream and recycled back to the column. As noted by Yang and Wyman [249], when incorporated with a biomass saccharification process, ARP technology almost completely fractionates biomass into three major constituents (pentose/pentosans, cellulose, and lignin). The solid residue is a low-lignin, shortchained cellulosic material with a high glucan content. Ammonia reacts primarily with lignin, not cellulose, and causes depolymerization of lignin and does not

7.5

Production of Ethanol from Cellulosic Biomass

449

react at all with carbohydrate linkages. The removal of lignin increases cellulose accessibility to cellulase. ARP pretreatment process works rather well with hardwoods as it enhances the enzymatic digestibility of hardwoods. ARP technology was subsequently employed to herbaceous biomass, corn stover, and switchgrass with good success. ARP method generally provides a high degree of delignification while keeping most of the cellulosic components in biomass intact. However, a substantial amount of xylan is also removed along with lignin. A two-stage process, in which another pretreatment method such as dilute acid or hot water treatment is used followed by the ARP, can provide better results. Wu and Lee [226] suggested a two-stage diluteacid percolation (DA) process as a pretreatment method for switchgrass. Extremely low acid concentration (0.078 wt% sulfuric acid) under moderate temperature .145–170ıC/ completely solubilized hemicellulose in switchgrass showing no sugar decomposition. The treated switchgrass contained about 70% glucan and 30% lignin. The DA pretreatment, when combined with the ARP process, was found to be more effective in delignification. Kim and Lee [250] noted from their laboratory study that with optimized operating conditions, in a two-stage processhot water treatment followed by ARP- the xylan fraction was hydrolyzed with 92–95% conversion, and was recovered with 83–86% yields; and the lignin removal was 75–81%. The remaining solid after two-stage treatment contained 78–85% cellulose. The two-stage treatments enhanced the enzymatic digestibility to 90–96% with 60 Filter-Paper-Unit (FPU)/g of glucan, and 87–89% with 15 FPU/g of glucan. A comparison of these pretreatment processes is given in Tables 7.15 and 7.16. Among these processes, the lime and ARP treatment processes are found to be most effective in breaking down the cellulose structure. Both the processes removed hemicelluloses and lignin from the plant wall. The lime treatment process requires the lowest temperature, but the treatment time of 4 weeks is extremely high compared to other processes, which require 10–24 min of treatment time, but also need a higher temperature.

7.5.2 Hydrolysis After the pretreatment process, two processes may be used to hydrolyze the pretreated feedstocks for fermentation into ethanol. The hydrolysis methods most commonly used are: • Acid hydrolysis [251–268] – dilute acid hydrolysis – concentrated acid hydrolysis • Enzymatic hydrolysis [269–292]

450

7 Ethanol

Table 7.15 Composition of solids from pretreatment of corn stover (percent of dry weight) by CAFI leading technologies and their digestibilities after 72 h for an enzyme loading of 15 FPU/g cellulose in the original feedstock

Percent lignin 17.2

Percent conversion at 72 h (15 FPU/g cellulose) 23.3

9.5

22.5

91.1

76.1

4.8

7.1

95.5

52.7 36.1

16.2 21.4

25.2 17.2

85.2 96.0

61.9

17.9

8.7

90.1

52.70

16.20

25.20

93.0

Pretreatment system Untreated corn stover Dilute acid

Temp ı C –

Reaction time, minutes –

Percent chemical used –

160

20

59.3

Flow through Control pH AFEX

200

24

0.49 of sulfuric acid Water only

190 90

15 15

ARP

170

10

Lime

55

None 100 of anhydrous ammonia 15 of ammonia 0.08 g CaO/g biomass

4 weeks

Percent glucan 36.1

Percent xylan 21.4

Source: Yang and Wyman [249] Table 7.16 Effect of various pretreatment methods on the chemical composition and chemical/physical structure of lignocellulosic biomass Increases accessible Removes Alters surface Decrystalizes hemicelRemoves lignin area cellulose lulose lignin structure Uncatalyzed Major effect steam explosion Liquid hot Major effect water pH controlled Major effect hot water Flow-through Major effect liquid hot water Dilute acid Major effect Flow-through Major effect acid AFEX Major effect ARP Major effect Lime Major effect Source: Mosier et al. [155]

–

Major effect

–

Minor effect

ND

Major effect

–

Minor effect

ND

Major effect

–

ND

ND

Major effect

Minor effect

Minor effect

– –

Major effect Major effect

– Minor effect

Major effect Major effect

Major effect Major effect ND

Minor effect Minor effect Minor effect

Major effect Major effect Major effect

Major effect Major effect Major effect

7.5

Production of Ethanol from Cellulosic Biomass

451

7.5.2.1 Acid Hydrolysis Dilute Acid Hydrolysis In this process, aqueous solution containing 1% sulfuric acid is used in a continuous flow reactor at a high temperature of about 215ıC to hydrolyze lignocellulosic biomass. The sugar conversion efficiency of this method is about 50%. The operating parameters of the dilute acid hydrolysis process require proper controls. The cellulosic materials that are converted to sugar could be further converted to other chemicals if the reaction continues. The conditions under which the first reaction takes place are also the right conditions for the subsequent reactions to occur. The second reaction proceeds rapidly to convert sugars into other products, such as furfural (See Fig. 7.19). The sugar degradation not only reduces the sugar yield, but furfural and other by-products can inhibit the fermentation process. Since,

Spruce wood

Hemicellulose

Cellulose

Lignin

CH3COOH

Acetic acid (3) CHO H HO H

CHO

CHO

OH

HO

H

H

H

HO

H

HO

H

OH

HO

H

OH

H

OH CH2OH

O

H

HO

H

H

H

OH

OH

H

OH

Galactose (4)

HOH2C

HCOOH

Furfural (6)

OH

Formic acid (8)

O

Phenolic compounds

CH2OH

CH2OH

Mannose (2)

CHO

H

OH

CH2OH

Xylose (1)

CHO

Glucose (5)

CHO

O H3C

Hydroxymethylfurfural (7)

C

CH2

CH2

COOH

Levulinic acid (9)

Fig. 7.19 Reactions products during hydrolysis of lignocellulosic materials (Printed with permission from Palmqvist and Hahn-Hagerdal [293])

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7 Ethanol

hemicelluloses (5-carbon) sugars degrade more rapidly than cellulose (6-carbon) sugars, one way to decrease sugar degradation is to implement a two-stage process. The two-stage dilute acid process was developed by the NREL for softwood. The process proceeds as follows: • Stage 1: 0.7% sulfuric acid, 190ıC, and a 3 min residence time • Stage 2: 0.4% sulfuric acid, 215ıC, and a 3 min residence time The first stage maximizes the yield from the more readily hydrolyzed hemicelluloses. The second stage is optimized for hydrolysis of the more resistant cellulose fraction. The liquid hydrolyzed products are recovered from each stage and fermented to alcohol. Bench scale tests confirmed the potential to achieve yields of 89% for mannose, 82% for galactose and 50% for glucose. Fermentation with Saccharomyces cerevisiae provided ethanol conversion of 90% of the theoretical yield. Lime is used to neutralize residual acids before the fermentation stage. Sugar degradation still occurs. In the two-stage method about 80 gallons of ethanol per ton of dry wood could be produced. The residual cellulose and lignin are used as boiler fuel for electricity or steam production.

Concentrated Acid Hydrolysis In this method, the decrystallization followed by dilute acid hydrolysis is the controlling step of the process. Lignocellulosic biomass that has been dried to 10% moisture content is contacted with a 70–77% sulfuric acid solution at about 50ı C for 2–6 h in a reactor. Acid is added at a ratio of 1.25:1 (acid: cellulose C hemicellulose). The low temperature and pressure minimize the degradation of sugars. The sugar formed at this stage is recovered through repeated water wash. Adding water to dilute the acid to 20–30% and heating at 100ı C for an hour results in further release of sugars from hemicelluloses. The residual solid that is mainly cellulose is hydrolyzed in the next step. The residue is dewatered and soaked in a 30–40% concentrated sulfuric acid for 1–4 h. The acid concentration in the solution is increased to 70% through dewatering and evaporation. After reacting for another 1–4 h at low temperature, the sugar and acid are recovered and the acid is concentrated and recycled to the first stage to provide the acid needed for the first stage of hydrolysis. A complete and rapid conversion of cellulose to glucose and hemicelluloses to 5-carbon sugars can be achieved with little degradation. Sugar recovery efficiency of about 90% of both hemicelluloses and cellulose sugars is possible. The acid and sugar are separated via an ion exchange column and acid is reconcentrated via multiple effect evaporators. The low temperature and pressure employed in this process allow the use of relatively low cost materials such as fiberglass tanks and piping.

7.5

Production of Ethanol from Cellulosic Biomass

453

7.5.2.2 Enzymatic Hydrolysis Enzymatic hydrolysis involves splitting the constituents of cellulose and hemicelluloses using enzymes. The cellulose produces glucose as the main hydrolysis product, whereas hemicelluloses breaks down into several pentoses and hexoses during hydrolysis from mannan, xylan, glucan, galactan, and arabinan. One of the main issues with enzymatic hydrolysis is the blocking of the enzyme accessibility by lignin. This has several detrimental effects: end-product inhibition, reduced reaction rate, and lower yield. Cellobiose and glucose also act as strong inhibitors of cellulases. Enzymatic hydrolysis requires mild conditions and long periods of time. Maximum cellulase and “-glucosidase activities occur at 40–60ı C and pH of 4.0–5.0. However, optimal conditions may change depending on the hydrolysis time. Combining a pretreatment process, such as high temperature with dilute acid or enzymatic hydrolysis, could increase the efficiency of hydrolysis of cellulosic materials. Enzymatic hydrolysis of lignocelluloses to glucose occurs by the synergistic actions of three distinct classes of enzymes: the endo-1, 4-“-glucanases, which act randomly on soluble and insoluble 1, 4-“-glucan substrates, but in the regions of low crystallinity of the cellulosic fiber; the exo-1, 4-“-D-glucanases, which release D-glucose from 1, 4-“-D-glucans and hydrolyze D-cellobiose slowly and liberate D-cellobiose from 1, 4-“-glucan, which comes from the non-reducing end of cellulose chains.; and the “-D-glucosidases, which release D-glucose units from cellobiose, soluble cellodextrins, and a group of glycosides. The hydrolysis process sequence is shown in Fig. 7.20. “-D-glucosidases not only generate glucose from cellobiose but also reduce cellobiose inhibition, allowing the cellulolytic enzymes to function more efficiently. The cellulases and “-glucosidase are inhibited by cellobiose and glucose, respectively. For a complete hydrolysis of cellulose to glucose, all these three enzymes must be present in the solution and in proper proportions. endo–glucanase Cellulose chain Shorter cellulose chains

exo–glucanase

Cellobiose fragments beta–glucosidase

Glucose Yeast Ethanol

Fig. 7.20 Mechanism for ethanol synthesis (Source: Process Description and Overview of Enzymatic Hydrolysis Technology, SERI, TP-3161 [294])

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7 Ethanol

Ethanol production from cellulosic biomass is challenging. To produce it cheap and to make it competitive with gasoline and other ethanol producing methods, the enzymatic hydrolysis process needs to address the following issues. • The pretreatment processes should be further optimized and new processes may be needed that do not require expensive and hazardous chemicals and/or high pressure expensive equipment. • A high density of cells within the reactor should be maintained and sugars must be converted to ethanol quickly to avoid product inhibition. • A process should be developed to combine enzymatic conversion of cellulose and hemicelluloses with the fermentation process to keep sugar levels low that can enhance the enzymatic conversion rates. • For a better product yield, both the cellulose (glucose) and hemicelluloses (xylose) should be utilized fully. • A process to remove ethanol from the reactor vessel continuously should be designed to maintain a high fermentation reaction rate.

7.5.3 Fermentation and Process Integration After hydrolysis, the product sugar is fermented to ethanol. Although the fermentation process is the same as that described earlier, to increase the ethanol yield, the fermentation process is often integrated with the hydrolysis step. For process integration, generally, the enzymatic hydrolysis method is preferred. Different types of process integration at different level are possible when enzymatic hydrolysis is used for ethanol production. In all cases, pretreatment of the biomass is required to make the cellulose more accessible to the enzymes, and to hydrolyze the hemicellulose. Various process integration schemes have been explored to reduce the product inhibition associated with enzyme, eliminate a number of steps and associated reactors, reduce costs, and enhance the ethanol yield [295,296]. Successful integration of all the unit operations of biomass conversion is the key element to enabling commercialization. Various unit operations should be linked together in a mini-pilot and full scale pilot plants to demonstrate functionality of integrated parameters for industrial scale applications. The general integration scheme for a lignocelluloses based ethanol plant is shown in Fig. 7.21. For lignocelluloses based ethanol plants, integration of pretreatment, hydrolysis, and fermentation units are critical for the success of the process. Four integration schemes have been suggested: • • • •

Separate or Sequential Hydrolysis and Fermentation (SHF) Simultaneous Saccharification and Fermentation (SSF) Simultaneous Saccharification and Co-Fermentation (SSCF) Consolidated BioProcessing (CBP)

7.5

Production of Ethanol from Cellulosic Biomass

455

Fig. 7.21 A general scheme for a lignocelluloses based ethanol plant

7.5.3.1 Sequential Hydrolysis and Fermentation (SHF) Traditionally, hydrolysis and fermentation processes are carried out in separate steps. In the SHF configuration, the hydrolyzed stream from the reactor first enters the glucose fermentation reactor [297–312]. The mixture is distilled to remove ethanol. The unconverted xylose is then fed to a second reactor where xylose is further fermented to ethanol. The product from the second reactor is again distilled in another column to separate ethanol. As shown in Fig. 7.22 a,b, both glucose and xylose could be fermented simultaneously, and a single distillation column would be sufficient for separation of ethanol.

7.5.3.2 Simultaneous Saccharification and Fermentation (SSF) The SSF process, shown in Fig. 7.23, consolidates the hydrolyses of cellulose with the direct fermentation of the produced glucose, which reduces the number of reactors [313–377]. This approach also avoids the problem of product inhibition associated with enzymes. The presence of glucose inhibits the hydrolysis reaction. In SSF, there is a trade-off between the cost of cellulase production and the cost of hydrolysis/fermentation. The optimum conditions should be determined from process conditions. The SSF process was also shown to be better than the saccharification and subsequent fermentation due to the rapid assimilation of sugars by yeast during SSF. Other advantages of the SSF process are listed below. • SSF requires lower amounts of enzyme. • The contamination can be minimized.

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7 Ethanol

Fig. 7.22 Schematic diagrams of sequential hydrolysis (SSF) and fermentation process for ethanol production. (a) most common type of configuration, (b) more compact version of SSF (Source: Hamelinck et al. [163])

• The inhibition effects of cellobiose and glucose to enzyme are lower. • SSF process requires lower capital cost. • SSF process has higher ethanol yield. The main disadvantage of SSF process is that different temperatures are required for saccharification .50ı C/ and fermentation .35ı C/. Besides, ethanol itself exerts some inhibition. Therefore, a thermotolerant yeast capable of fermenting glucose to ethanol at temperatures above 40ı C, which can also promote saccharification in the range of 40–45ı C is desirable. The conventional yeast, Saccharomyces cerevisiae, appears to be the best microbe for the SSF process. A number of researchers are exploring various yeast strains to improve the performance of the SSF process.

7.5

Production of Ethanol from Cellulosic Biomass

457

Fig. 7.23 Block diagram for conversion of biomass to ethanol by Simultaneous Saccharification and Fermentation process (SSF) (Printed with permission from Wyman [378])

Fig. 7.24 Process flow diagram for simultaneous Saccharification and co-fermentation of biomass for ethanol production (Source: Aden [392])

7.5.3.3 Simultaneous Saccharification and Co-Fermentation (SSCF) The co-fermentation of hexoses and pentoses sugars is the focus of SSCF system. A schematic flow diagram of the system is shown in Fig. 7.24. The objective is to ferment both these sugars in a single reactor using a single microorganism. The pretreated hemicelluloses and solid celluloses are not separated after pretreatment. Hemicelluloses sugars are converted to sugar along with SSF of the cellulose. The main challenge of the SSCF process is to engineer or identify a microorganism that can co-ferment glucose and xylose [315, 327, 348, 379–388].

458

7 Ethanol

Fig. 7.25 A consolidated bioprocessing system for conversion of biomass to ethanol (Printed with permission from Hamelinck et al. [163])

A metabolically engineered strain of Zymomonas mobilis was developed by NREL for the co-fermentation of glucose and xylose by the SSCF process from a synthetic pre-hydrolyzed hardwood and glucose. Later McMillan et al. [389] from NREL used a variant of Z. mobilis for ethanol production from dilute-acidpretreated yellow poplar by the SSCF. Kim et al. [390] used a recombinant E. coli for corn stover that was pretreated by ammonia. Teixeira et al. [391] used a recombinant strain of Z. mobilis to convert hybrid poplar wood and sugarcane bagasse. 7.5.3.4 Consolidated BioProcessing (CBP) In this process, ethanol and all required enzymes are produced by a single class of microorganisms, in a single reactor [393–414]. The capital and operating costs could be reduced significantly by this approach, since dedicated enzyme production (or purchase), reduced diversion of substrate for enzyme production, and compatible enzyme and fermentation systems are not necessary. However, currently such organisms or compatible combinations of microorganisms are not available that can produce both cellulase and other enzymes at the required high levels and also produce ethanol at the required high concentrations and yields. The conceptual process is shown in Fig. 7.25.

7.6 Energy Balance A discussion of fuel ethanol production from corn is not complete without addressing the energy balance of the system. There is a great deal of controversy regarding whether ethanol from corn provides a net energy gain [415–419]. The energy balance from various studies is shown in Fig. 7.26. For any energy resources to be economically viable, the amount of energy necessary to produce the fuel (energy input) must be lower than the useful energy that can be extracted (energy output) from the same fuel.

Fig. 7.26 Energy balance reported by various researchers for ethanol production from corn (Adapted from Aden 2007) [392]

7.6 Energy Balance 459

460

7 Ethanol

The research team led by Shapouri [420] at the US Department of Agriculture claims that ethanol production provides a net energy return. However, a number of other studies contradicted the findings by the Shapouri team. Other major concern of using corn for ethanol is that a large cropland will be necessary for growing corn that is currently used for food production and raises serious ethical issues. The difference in the net energy return among various studies is mainly due to the use of different values of input parameters and associated costs. Values of a set of input parameters used by Pimentel [421] are given in Table 7.17. These values are extremely variable and depend from one region to another region. The numbers cited in Table 7.17 are estimates and can change from one year to another year. For example, in 2002, Shapouri et al. [432] reported a net energy gain of 34%, however, in 2004 Shapouri group [439] claimed almost twice in the net positive energy (67%). During the same time period, Pimentel group [421] reported a negative 29% deficit. The Pimentel group [421] pointed out several issues that made the difference in the energy balance. According to the Pimentel group [421]: (1) Shapouri omitted several inputs, for instance, all the energy required to produce and repair farm machinery, as well as the fermentation-distillation equipment. All the corn production in the U.S. is carried out with an abundance of farm machinery, including tractors, planters, sprayers, harvesters, and other equipment. These are large energy inputs in corn ethanol production, even when allocated on a life cycle basis. (2) Shapouri used corn data from only nine states, whereas Pimentel group used corn data from 50 states. (3) Shapouri reported a net energy return of 67% for the co-products, primarily dried-distillers grain (DDG) that were used to feed cattle. (4) Although Pimentel group did not allocate any energy related to the impacts that the production of ethanol has on the environment, they are significant in the US for corn production. (5) Ferguson [457] made an astute observation about the USDA data. The proportion of the sun’s energy that is converted into useful ethanol, using the USDA’s positive data, only amounts to five parts per 10,000. If the figure of 50 million ha were to be devoted to growing corn for ethanol, then this acreage would supply only about 11% of U.S. liquid fuel needs. (6) Many other investigators supported Pimentel group’s assessment of ethanol production. As noted earlier, coproducts play an important role in the energy balance as demonstrated through Tables 7.18 and 7.19. Differences among these studies are related to various assumptions about corn yields, ethanol conversion technologies, fertilizer manufacturing efficiency, fertilizer application rates, co-product evaluation, and the number of energy inputs included in the calculations. For example, there is about a 64,000 Btu/gal difference between the results of Pimentel [421] and Lorenz and Morris [83]. With respect to growing the corn, Pimentel reports that it requires 56,720 Btu/gal (LHV) compared with Lorenz and Morris’s 27,134 Btu/gal (HHV). Both studies used the same basic inputs, such as fertilizer, pesticides, and fuel, but Pimentel also included the energy value embodied in farm machinery, though he did not present any details on how he derived embodied energy in farm machinery. Another factor that makes Pimentel’s estimates higher is the use of a national average corn yield of only 110 bu/ac, which is characteristic of corn yields seen in U.S. agriculture in the early 1980s. Lorenz and Morris [83] used 120 bu/ac, which is based on data from more recent years.

Table 7.17 Energy inputs and associated costs of corn production per hectare in the US Inputs

Quantity

kcal1,000

Costs $

Labor Machinery Diesel Gasoline Nitrogen Phosphorus Potassium Lime Seeds Irrigation Herbicides Insecticides Electricity Transport Total Corn yield 8;655 kg=ha ii

11:4 ha 55 kgd 188 Lg 40 Li 153 kgk 65 kgn 77 kgq 1;120 kgt 21 kgv 8:1 cmy 6:2 kgbb 2:8 kg cc 13:2 kWhdd 204 kggg

462b 1;018e 1;003h 405j 2;448l 270o 251r 315u 520w 320z 620ee 280ee 34ff 169hh 8,115 31,158 kcal

148:20c 103:21f 34:76 20:80 94:86m 40:30p 23:87s 11:00 74:81x 123:00aa 124:00 56:00 0:92 61:20 $916:93 Input:output 1:3.84

Source: Pimentel and Patzek [421] a NASS [443] b It is assumed that a person works 2,000 h per year and utilizes an average of 8,000 l of oil equivalents per year c It is assumed that labor is paid $13 an hour d Pimentel and Pimentel 1996 [444] e Prorated per ha and 10 year life of the machinery. Tractors weigh from 6 to 7 t and harvesters 8–10 t, plus plows, sprayers, and other equipment f Hoffman et al. [445] g Wilcke and Chaplin [446] h Input 11,400 kcal per l i Estimated j Input 10,125 kcal per l k USDA [447] l Patzek [438] m Cost 62 c/kg n USDA [447] o Input 4,154 kcal/kg p Cost $62/ kg q USDA [447] r Input 3,260 kcal/kg s Cost 31c/kg t Brees [448] u Input 281 kcal/kg v Pimentel and Pimentel [444] w Pimentel [449] x USDA [450] y USDA [451] z Batty and Keller [452] aa Irrigation for 100 cm of water per ha costs $1,000 (Larsen et al. [453]) bb Larson and Cardwell [454] cc USDA [447] dd USDA [455] ee Input 100,000 kcal/kg of herbicide and insecticide ff Input 860 kcal/kWh and requires 3 kWh thermal energy to produce 1 kWh electricity gg Goods transported include machinery, fuels, and seeds that were shipped an estimated 1,000 km hh Input 0.83 kcal/kg/km transported ii USDA [456]

462

7 Ethanol Table 7.18 Energy use and net energy value per gallon without co-products energy credits Production process Corn production Corn transport Ethanol conversion Ethanol distribution Total energy used Net energy value Energy ratio Source: Shapouri [458]

Milling process Dry

Wet

Weighted average

18,875 2,138 47,116 1,487 69,616 6,714 1.10

18,551 2,101 52,349 1,487 74,488 1,842 1.02

18,713 2,120 49,733 1,487 72,052 4,278 1.06

Table 7.19 Energy use and net energy value per gallon with co-products energy credits, 2001 Production process Corn production Corn transport Ethanol conversion Ethanol distribution Total energy used Net energy value Energy ratio Source: Shapouri [458]

Milling process Dry 12,457 1,411 27,799 1,467 43,134 33,196 1.77

Wet 12,244 1,387 33,503 1,467 48,601 27,729 1.57

Weighted average 12,350 1,399 30,586 1,467 45,802 30,528 1.67

Although Pimentel [431] increased corn yield in his 2001 report, the net energy value remained about the same as reported in the 1991 study. The co-products values play an important role in determining net energy value. When corn is converted into ethanol, the material that remains is a high-protein animal feed. One assumption is that the availability of that feed will enable traditional feed manufacturers to produce less, saving energy; ethanol producers should, therefore, get to subtract those energy savings from their energy consumption. When Groode and Heywood [459] put co-product credits into their calculations, ethanol’s life-cycle energy use became lower than gasoline’s. The choice of system boundary influences the outcome of the energy balance. To determine the importance of the system boundary, Groode and Heywood [459] compared their analysis with the study by Pimentel [421] and three other studies [458, 460, 461], considering the same energy-consuming inputs and no co-product credits in each case. Based on the results, Groode and Heywood [459] concluded that everybody is basically correct, and the energy balance is so close that the outcome depends on exactly how one defines the problem. The results also validated the methodology and results from the other studies and were within the range of probable results (See Fig. 7.27). However, among all feedstocks, the net energy balance for corn-ethanol system, even if it is positive, is considered to be lowest. According to Bourne [462],

1.7

9.0 2.8

6.8

lowa Corn Grain Ethanol

NEV (MJ/Ethanol Liter)

3.7

7.2

15.8

−7.6

−1.1

7.2

Fig. 7.27 A comparison of net energy balance for corn-ethanol production reported by several researchers with a Monte-Carlo model (Groode and Heywood 2008) [459]

−10

−3.2

1.7

Farrell

0

Pimentel

5.8

Shapourl

10

lowa Corn Grain Ethanol With 20% Co Product Credits

20

Wang,et.al.

Monte Carlo LCA Results

lowa Corn Grain Ethanol Plus DDGS

(Corn Grain Ethanol)

Georgia Corn Grain Ethanol

Previous Results

lowa Coal Powered Corn Grain Ethanol

30

2025 lowa Corn Grain Ethanol

7.6 Energy Balance 463

464

7 Ethanol

fossil-fuel energy used to make fuel (input) compared with the energy in the fuel (output) is 1:1.3 (input: output) for corn-ethanol, 1:8 for sugarcane ethanol, 1:2.5 for biodiesel, and 1:2–1:36 for cellulosic ethanol. For cellulosic ethanol, the output energy depends on production method.

7.7 DDGS Market The co-products from corm processing plants play a key role in determining the energy balance of the corn to ethanol production system [462–470]. Among various co-products, distiller’s dried grains with solubles (DDGS) has the best market value and also marketed internationally for use in dairy, beef, swine, poultry and aquaculture feeds. Nutrient values of high quality DDGS produced by modern ethanol plants in the US are generally higher than those required for various feeds, such as for the swine, poultry, dairy and beef. Wet mills produce corn gluten feed, corn gluten meal and corn germ meal that also have significant market value.

7.8 Water Requirements for Corn Growing Recent increases in oil prices in conjunction with the farm subsidy policies in the US have led to a dramatic expansion in corn based ethanol production. If the crude oil price remains high, which is most likely, further expansion is expected. A National Research Council committee was convened to look at how shifts in the nation’s agriculture to include more energy crops, and potentially more crops overall, could affect water management and long-term sustainability of biofuel production. The committee concluded: “In terms of water quantity, the committee found that agricultural shifts to growing corn and expanding biofuel crops into regions with little agriculture, especially dry areas, could change current irrigation practices and greatly increase pressure on water resources in many parts of the United States. The amount of rainfall and other hydroclimate conditions from region to region causes significant variations in the water requirement for the same crop, the report says. For example, in the Northern and Southern Plains, corn generally uses more water than soybeans and cotton, while the reverse is true in the Pacific and mountain regions of the country. Water demands for drinking, industry and such uses as hydropower, fish habitat, and recreation could compete with, and in some cases, constrain the use of water for biofuel crops in some regions. Consequently, growing biofuel crops requiring additional irrigation in areas with limited water supplies is a major concern, the report says.” [472] Several studies also addressed the use of water for corm growing and its impact [473–476]. There is a significant shortage of fresh water throughout the world, as well as in the USA. The effect of water usage for growing corn on the society is further discussed in Volume 4 of this book series.

7.9

Fuel Ethanol Quality Comparison

465

7.9 Fuel Ethanol Quality Comparison In the USA, fuel ethanol is blended in over 70% of the nation’s gasoline, and up to 10 vol% ethanol in gasoline (called E10) is allowed by federal gasoline regulations. All U.S. conventional vehicles are designed, certified and warranted for the use of blends up to E10 (10% by volume, approx 3.5% by mass). However, ethanol is also used in reformulated gasoline (RFG), and in much higher concentration in flexible fuel vehicles (FFVs). An ethanol concentration of up to 85% (called E85 fuel) can be used in FFVs. E85 is currently restricted to use in FFVs, but can be used at any level (E0–E85) of ethanol. E85 composition actually ranges form 70% ethanol/30% hydrocarbons to 79% ethanol/21% hydrocarbons. It is estimated that there are over 7 million FFVs on the roads today, and based on the U.S. automakers’ commitment, about 50% of their vehicles sold by 2012 could be FFVs. Fuel volatility (The vapor pressure of the blended fuel) is an important parameter for E85, since it determines the cold start and warm up performance. Therefore, there are three classes of E85 fuel. • Class 1 E85, which would typically contain 85% denatured ethanol, is required to meet a 79% minimum ethanol content. This is called summer grade E85 and requires a vapor pressure of 38–59 kPa (5.5–8.5 psi) at 37:8ı C. • Class 2 E85 may contain up to 74% by volume denatured ethanol. It requires a vapor pressure of 48–65 kPa (7.0–9.5 psi) at 37:8ı C, and is called inter-seasonal (spring/fall) blend. • Class 3 is formulated for winter that requires a vapor pressure of 66–83 kPa (9.5–12.0 psi) at 37:8ı C. Various aspects of E-85 blends that include handling, storing, dispensing are discussed in Handbook for Handling, Storing, and Dispensing E85 [476]. The vapor pressure of gasoline-ethanol blend for various compositions is given in Fig. 7.28. GASOLINE-ETHANOL MIXTURES VAPOR PRESSURE (kPa)

80 70 60 50 40 30 20 10 0 0

10

20

30

40

50

60

70

80

90

100

ETHANOL CONTENT (% V/V)

Fig. 7.28 Vapor pressure data for different gasoline-ethanol mixtures (Source: Renewable Fuels Association [478])

466

7 Ethanol

Table 7.20 ASTM D 5798–99 standard specification for fuel ethanol for automotive sparkignition engines Property ASTM volatility class Ethanol, plus higher alcohols (minimum volume %) Hydrocarbons (including denaturant) (volume %) Vapor pressure at 37:8ı C kPa psi Lead (maximum, mg/L) Phosphorus (maximum, mg/L) Sulfur (maximum, mg/kg)

Methanol (maximum, volume %) Higher aliphatic alcohols, C3–C8 (maximum volume %) Water (maximum, mass %) Acidity as acetic acid (maximum, mg/kg) Inorganic chloride (maximum, mg/kg) Total chlorine as chlorides (maximum, mg/kg) Gum, unwashed (maximum, mg/100 mL) Gum, solvent-washed (maximum, mg/100 mL) Copper (maximum, mg/100 mL) Appearance

Value of class 1 2 79 74

3 70

Test method N/A ASTM D 5501

17–21

17–26

17–30

ASTM D 4815

38–59 5.5–8.5

48–65 7.0–9.5

66–83 9.5–12.0

2.6 0.3

2.6 0.3

3.9 0.4

ASTM D 4953 D 5190 D 5191 ASTM D 5059 ASTM D 3231

210

260

300

0.5

N/A

ASTM D 3120 D 1266 D 2622

2

N/A

1.0 50

ASTM E 203 ASTM D 1613

1

ASTM D 512 D 7988

2

ASTM D 4929

20

ASTM D 381

5.0

ASTM D 381

0.07

ASTM D 1688

Product shall be visibly free of suspended or precipitated contaminants (shall be clear and bright) Source: US Department of Energy [477] N=A not applicable

Appearance determined at ambient temperature or 21 ı C .70ı F/, whichever is higher

Various properties and their standard test methods as recommended by the ASTM is given in Table 7.20 for fuel ethanol (Ed75–Ed85) for automotive spark-ignition engines, and in Table 7.21 for denatured fuel ethanol for blending with gasoline for use in automotive spark-ignition engine fuel. The lower case “d” in “Ed” stands for “denatured” ethanol.

7.9

Fuel Ethanol Quality Comparison

467

Table 7.21 Specifications contained in ASTM D 4806 standard specification for denatured fuel ethanol for blending with gasoline Property Ethanol volume %, min Methanol, volume %. max Solvent-washed gum, mg/100 mL max Water content, volume %, max Denaturant content, volume %, min volume %, max Inorganic Chloride content, mass ppm (mg/L) max Copper content, mg/kg, max Acidity (as acetic acid CH3COOH), mass percent (mg/L), max pHe Appearance

Specification 92.1

ASTM test method D 5501

0.5

5

D 381

1

E 203

1.96

4.76 40

D 512

0.1

D 1688

0.007

D 1613

6.5–9.0 Visibly free of suspended or precipitated contaminants (clear and bright) Source: Renewable Fuels Association [478]

D 6423

The Energy Efficiency and Renewable Energy, Alternative Fuels Data Center of the USDOE compared various properties of three fuels that are used for transportation: ethanol, gasoline, and No.2 Diesel. These data provide a basis for comparison among these fuels and are earlier given in Table 7.21. The Table 7.22 shows how an 2007 Chevrolet Tahoe performed while running on E85 and gasoline in three fuel-economy tests, in four acceleration tests, and in three emissions tests [479].

468 Table 7.22 Testing of a 2007 Chevrolet Tahoe with E85 fuel and its comparison with gasoline

7 Ethanol

Fuel economy, mpg City Highway 150-mile trip Overall Acceleration 0–30 mph, s 0–60 mph, s 45–65 mph, s Quarter-mile, s/mph Emissions, parts per million Nitrogen oxide Hydrocarbons Carbon monoxide Source: Consumer report [479] a Blended with 10% ethanol

E85 7 15 13 10

Gasolinea 9 21 18 14

3.4 8.9 5.7 16.8/84.6

3.5 9.1 5.8 16.9/84.5

1 1 0

9 1 0

7.10 E-Diesel E-Diesel is a diesel fuel that uses conventional diesel blend stock(s) with 7.7–15 vol% anhydrous ethanol (ASTM D 4806) and from 0:6 to 5:0 vol% of special proprietary additive(s) to prevent ethanol and diesel from separating at very low temperatures and water contamination [480–497]. In some cases, a cetane enhancement, if required, is added. E-Diesel is currently an experimental fuel and is being developed by many companies, who can receive federal ethanol tax credit when blending ethanol with diesel. E-diesel is targeted for use in heavy-duty trucks, busses, and farm machinery. Although there are many environmental benefits of using e-diesel, 7–10% decreased in mileage is expected. The use of e-diesel may reduce emissions of particulate matter from 27% to 41%, carbon monoxide by 20–27%, and nitrogen oxides by 4–5%.

7.11 Summary Ethanol is currently blended with gasoline into more than 50% of the USA’s fuel supply. Ethanol is also increasingly available in E85, an alternative fuel that can be used in flexible fuel vehicles. Studies have estimated that ethanol and other biofuels could replace 30% or more of U.S. gasoline demand by 2030. Corn is the primary feedstock for ethanol production in the USA, whereas Brazil, the second largest producer of ethanol, uses sugarcane. About 20% of the corn supply in the USA is used for ethanol production. Although ethanol can also be made from other grains such as sorghum as well as from cellulosic biomass such as corn cobs, cornstalks, wheat straw, rice straw, and switchgrass, further technological advancements are needed to make it competitive with corn or sugar based ethanol.

Problems

469

Ethanol production methods depend mainly on the feedstock. In the USA, ethanol is primarily produced from corn crops by dry-mill or wet-mill processing. Although cellulosic ethanol has not yet been produced commercially, several commercial cellulosic ethanol production plants are under construction. Mainly two processes, biochemical and thermochemical processes, are currently explored for cellulosic ethanol production. The use of corn-ethanol raises several concerns including worldwide food shortages, increase in food price, and the consumptive use of water, which is defined as any use of water that reduces the supply from which it is withdrawn or diverted. These concerns must be addressed before expanding corn based ethanol production.

Problems 1. What is the difference between ethanol and E85? 2. What are distillers dried grains (DDG) or even distillers dried grains with solubles (DDGS)? Do they have any market value? 3. Describe ethanol production processes and their advantages and disadvantages. 4. Calculate the amount of Carbon Dioxide .CO2 / emitted during production of a gallon of ethanol? 5. What feedstock types can I use for ethanol production? Discuss various advantages and disadvantages associated with these feedstock. 6. What are the advantages to using ethanol blended fuels? 7. Discuss environmental benefits of using ethanol or ethanol blended fuel. 8. Does ethanol blended fuel burn cleaner than premium gasoline? If so, why? 9. What are ETBE and MTBE? 10. Can 100% ethanol be used in automobile? 11. Does ethanol blended with gasoline require special handling? 12. How much ethanol can I get from one ton of wheat? 13. What is the difference between a wet and a dry mill ethanol plant? 14. What is cellulosic ethanol? 15. How is cellulosic ethanol made? 16. What is switchgrass and why is it a good potential source for ethanol? 17. How close is cellulosic ethanol to being commercialized? 18. Can ethanol blends be used in small engines, such as boats, lawnmowers, or chainsaws?

470

7 Ethanol

19. What is E85? 20. Can my vehicle run on E85 even if it’s not an FFV? 21. What is the ethanol “subsidy”? 22. How many bushels of corn are needed for a typical ethanol plant? 23. How many gallons are in a barrel of oil / ethanol? 24. What is ethanol’s “net energy balance”? 25. Discuss the issues surrounding the use of E15. 26. Why corn is favored for ethanol production in the US? 27. Is the subsidy for corn ethanol in the US necessary for its competition with other Fuel? 28. What is the energy balance of Brazil sugarcane-produced ethanol? 29. What are the byproducts when making cellulosic ethanol? 30. How much water does it take to produce a gallon of cellulosic ethanol? 31. What kinds of ethanol-blended fuels are available? Discuss their merits and demerits. 32. What type of storage and dispensing conversion procedures are required for offering E85 at a gas station? 33. How many bushels of corn are needed for a typical ethanol plant? How many acres of corn would be needed to satisfy that demand? 34. What is switchgrass? Why is it considered to be a good potential source for ethanol in the USA? 35. Discusses issues for commercialization of cellulosic ethanol? 36. What is the Renewable Fuels Standard? 37. What is the ethanol incentive? 38. How does the production and use of ethanol impact the USA, Europe, and world economy? 39. How much oil can ethanol displace? 40. Can corn- ethanol drive up prices at the grocery store? 41. What does “net energy balance” mean? What is ethanol’s energy balance? Compare corn-ethanol energy balance with other feedstocks? 42. What about ethanol’s impact on fuel economy?

References

471

References 1. Ford fleet (2009). https://www.fleet.ford.com/Showroom/environmental vehicles/ethnol vehicles.asp. Accessed 20 Nov 2010 2. Belcher A (2005) The world looks to higher-tech to advance fuel ethanol production into the 21st Century. Int Sugar J 107(1275):196–199 3. Karpov SA (2008) Current aspects of fuel ethanol production in Russia and abroad. Chem Technol Fuels Oils 44(1):1–4 4. Li SZ, Chan-Halbrendt C (2009) Ethanol production in China: potential and technologies. Appl Energy 86(suppl 1):S162–S169 5. Mostert O (2004) Small scale ethanol production for farms and villages. World renewable energy congress VIII: linking the world with renewable energy, 8th, Denver, CO, United States, pp 89–91 6. Yang B, Lu Y (2007) Perspective: the promise of cellulosic ethanol production in China. J Chem Technol Biotechnol 82(1):6–10 7. Goh CS, Tan KT, Lee KT, Bhatia S (2010) Bio-ethanol from lignocellulose: status, perspectives and challenges in Malaysia. Bioresour Technol 101(13):4834–4841 8. Keeney D (2009) Ethanol USA. Environ Sci Technol 43(1):8–11 9. van Thuijl E, Deurwaarder EP (2006) European biofuel policies in retrospect. Energy research center of Netherlands. Report No ECN-V-06-016 10. Sapp M (2007) Europe’s ethanol affair. Biofuel Bioprod Bioref 1:88–91 11. U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Alternative Fuels Data Center. http://www.eere.energy.gov/afdc/altfuel/fuel properties.html 12. Berg C (2009) World fuel ethanol analysis and outlook. Licht FO, Agra Informa Ltd, Tunbridge Wells 13. Global Agriculture Information Network (GAIN) (2008) Brazil bio-fuels annual – ethabol 2008. Report BR8013, USDA Foreign Agricultural Service 14. Agˆencia Nacional do Petr´oleo (ANP) Brazil (2007) USDA foreign agricultural service, Gain report. Brazil biofuels annual ethanol 2007. Global Agriculture Information Network. GAIN Report number: BR7011 15. National Association of Automotive Vehicle Manufacturers and Associac¸a˜ o Nacional dos Fabricantes de Veıculos Automotores (ANFAVEA), Brazil 16. European Biomass Industry Association. www.eubia.org. Accessed 20 Nov 2010 17. Licht FO (2009) Cited in Renewable Fuels Association, Ethanol industry outlook 2008 and 2009: 16, 29. www.ethanolrfa.org/. Accessed 21 Nov 2010 18. Renewable Fuels Associations (2008) Changing the climate. Ethanol industry outlook 2008. www.ethanolRFA.org. Accessed 21 Nov 2010 19. Britton RA (1972) Direct hydration of ethylene to ethanol. US patent 3,686,334 20. Genon G, Onofrio M (1990) Ethanol by direct hydration: model selection of process parameters. ICP 18(9):37–42 21. Kuribayashi H, Kugo M (1967) Studies on hydration of ethylene to ethanol: cold-shot fixed bed reactor. Kagaku Kogaku 31(9):928–9 22. Nakano M, Sekizawa K, Fujii S, Tsutsumi Y (1989) Vapor-phase hydration of ethylene over zeolite catalysts under high pressure. Nippon Kagaku Kaishi 3:535–341 23. Thomson RC, Greenhalgh RK (1953) Catalytic hydration of ethylene to ethanol. British Patent 691,360, 13 May 1953 24. Ishihara T, Matsuo J, Ito M, Nishiguchi H, Takita Y (1997) H-ZSM-5 zeolite tube as a novel application of catalyst for the synthesis of ethanol by hydration of ethylene. Ind Eng Chem Res 36:4427–4429 25. John JA (1969) Hydration [of ethylene]. 2. indirect or sulfuric acid process. In: Miller SA (ed) Ethylene Its Ind Deriv, Ernest Benn, London: 692–709 26. Klima A, Matejicek J, Svrsek V, Sedliak J (1957) New results on indirect hydration of ethylene. Chemicky Prumysl 7:119–122

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475. Pfromm PH (2008) The minimum water consumption of ethanol production via biomass fermentation. Open Chem Eng J 2:1–5 476. US Department of Energy, Energy Efficiency and Renewable Energy (2008) Handbook for handling, storing, and dispensing E85. DOE/GO-102008-2450, April 2008 477. US Department of Energy, Alternative fuels and advanced vehicles data center (2010) http:// www.afdc.energy.gov/afdc/ethanol/e85 specs.html. Accessed 20 Nov 2010 478. Renewable Fuels Association, Industry Guidelines, Specifications, and Procedures. http:// www.ethanolrfa.org/industry/resources/guidelines/ 479. Consumer report. http://editorial.autos.msn.com/article.aspx?cp-documentid=435631 480. The European Union (2010) Short report from emission test using E-diesel fuel. Bioethanol for sustainable transport, Deliverable No. 3.14 481. Lofvenberg U (2002) E-diesel in Europe, a new available fuel technology. ISAF XIV Paper No. 2002-ET-31, Nov 2002 482. Ahamed I (2002) E-Diesel: A US experience on the development and commercialization of ethanol-blend engineered fuels. The 14th international symposium on alcohol fuels. ISAF XIV Paper No. 2002-SM-06, Nov 2002 483. Rickeard DJ, Thompson ND (1993) A review of the potential for bio-fuels as transportation fuels. SAE Paper No. 932778 SAE, Warrendale, PA 484. Ecklund EE, Bechtold RL, Timbario TJ, McCallum PW (1984) State-of-the-art report on the use of alcohols in diesel engines. SAE Paper No. 840118 SAE, Warrendale, PA 485. Likos B, Callahan TJ, Moses CA (1982) Performance and emissions of ethanol and ethanol– diesel blends in direct-injected and pre-chamber diesel engines. SAE Paper No. 821039 SAE, Warrendale, PA 486. Li D, Zhen H, Xingcai L, Wugao Z, Jianguang Y (2005) Physico-chemical properties of ethanol–diesel blend fuel and its effect on performance and emissions of diesel engines. Renew Energy 30:67–76 487. Simonsen H, Chomiak J (1995) Testing and evaluation of ignition improvers for ethanol in a DI diesel engine. SAE Paper No. 952512 SAE, Warrendale, PA 488. Yoshimoto Y, Onodera M (2002) Performance of a diesel engine fueled by rapeseed oil blended with oxygenated organic compounds. SAE Paper No. 2002-01-2854 SAE, Warrendale, PA 489. SD Minteer (2006) Ethanol blends: E10 and E-Diesel. Chemical industries Marcel Dekker AG, New York 490. Hansen AC, Zhang Q, Lyne PWL (2005) Ethanol-diesel fuel blends – a review. Bioresour Technol 96(3):277–285 491. Lapuerta M, Armas O, Garc´ıa-Contreras R (2007) Stability of diesel-bioethanol blends for use in diesel engines. Fuel 86(10–11):1351–1357 492. Lapuerta M, Armas O, Herreros JM (2008) Emissions from a diesel-bioethanol blend in an automotive diesel engine. Fuel 87(1):25–31 493. Pinto AC, Guarieiro LLN, Rezende MJC, Ribeiro NM, Torres EA, Lopes WA, Pereira PAdP, Andrade JBd (2005) Biodiesel: an overview. J Br Chem Soc 16:1313–1330 494. Rakopoulos DC, Rakopoulos CD, Hountalas DT, Kakaras EC, Giakoumis EG, Papagiannakis RG (2010) Investigation of the performance and emissions of bus engine operating on butanol/diesel fuel blends. Fuel 89(10):2781–2790 495. Rakopoulos DC, Rakopoulos CD, Kakaras EC, Giakoumis EG (2008) Effects of ethanoldiesel fuel blends on the performance and exhaust emissions of heavy duty DI diesel engine. Energy Convers Manag 49(11):3155–3162 496. Ribeiro NbM, Pinto AC, Quintella CM, da Rocha GO, Teixeira LSG, Guarieiro LlLN, do Carmo Rangel M, Veloso MrCC, Rezende MJC, Serpa da Cruz R, de Oliveira AM, Torres EA, de Andrade JB (2007) The Role of additives for diesel and diesel blended (Ethanol or Biodiesel) fuels: a review. Energy Fuels 21(4):2433–2445 497. Souza CDRd, Chaar JdS, Souza RCR, Jeffreys MF, Souza KdSd, Costa EJC, Santos JCd (2009) Caracterizac¸ a˜ o f´ısico-qu´ımica das misturas bin´arias de biodiesel e diesel comercializados no Amazonas. Acta Amazon 39:383–387

Chapter 8

Hydrogen Energy

Abstract Hydrogen may be considered as a secondary energy source, since it is not available as a pure hydrogen gas. Pure hydrogen must be produced from its compound using another energy source prior to its use. For example, the electricity that is produced from a primary energy source can be used to produce hydrogen from water by electrolysis. The supply of hydrogen on demand also requires a storage system. Hydrogen production, storage, and distribution methods are discussed in this chapter.

8.1 Introduction Hydrogen is abundant on the earth’s surface, however, it is mainly bound with other chemical compounds, such as water (H2 O) and organic compounds. Hydrogen is an energy carrier; it can store and deliver usable energy. Hydrogen must be first dissociated from bound chemical compounds using a primary energy source. Hydrogen is an environmentally attractive fuel as it produces only water during its combustion and use as a fuel. Various physical and chemical properties of hydrogen are given in Table 8.1. Before using hydrogen as a fuel, its combustion properties should be known and compared with other common fuels such as methane and gasoline to determine its effectiveness. Combustion related properties of hydrogen are compared with that of methane and gasoline in Table 8.2. Hydrogen can be used in any application in which fossil fuels are being used today, except where carbon is specifically needed [1–4]. Hydrogen can be used as a fuel in furnaces, internal combustion engines, turbines and jet engines, more efficiently than fossil fuels, i.e., coal, petroleum and natural gas. Automobiles, buses, trains, ships, submarines, airplanes and rockets can run on hydrogen. Hydrogen can also be converted directly to electricity by using fuel cells, which can be fed directly to the grid or to operate automobiles. Combustion of hydrogen with oxygen results in pure steam, which has applications in industrial processes and space heating. T.K. Ghosh and M.A. Prelas, Energy Resources and Systems: Volume 2: Renewable Resources, DOI 10.1007/978-94-007-1402-1 8, © Springer Science+Business Media B.V. 2011

495

496

8 Hydrogen Energy Table 8.1 Various physical and chemical properties of hydrogen Properties Values Atomic hydrogen Atomic number 1 Atomic weight 1.0080 Ionization potential 13.595 eV Electron affinity 0.7542 eV Nuclear spin 1/2 Nuclear magnetic moment 2.7927 (nuclear magnetons) Nuclear quadrupole moment 0 Electronegativity (Pauling) 2.1 Molecular hydrogen Bond distance Dissociation energy (25ı C) Ionization potential Density of solid Melting point Heat of fusion Density of liquid Boiling point Heat of vaporization Critical temperature Critical pressure Critical density Heat of combustion to water (g)

0.7416 104.19 kcal/mol 15.427 eV 0:08671 g=cm 3 259:20ı C 28 cal/mol 0.07099 at 252:78ı C 252:77ı C 216 cal/mol 240:0ı C 13.0 atm 0:0310 g=cm 3 57:796 kcal=mol

Table 8.2 A comparison of fuel properties of hydrogen with that of methane and gasoline Properties Lower heating value, kW h/kg Self ignition temperature, ı C Flame temperature, ı C Ignition limits in air, vol% Minimum ignition energy, mWs Flame propagation in air, m/s Detonation limits, vol% Detonation velocity, km/s Explosion energy, kg TNT=m3 Diffusion coefficient in air, cm2 =s

Hydrogen 33.33 585 2,045 4–75 0.02 2.65 13–65 1.48–2.15 2.02 0.61

Methane 13.9 540 1,875 5.3–15 0.29 0.4 6.3–13.5 1.39–1.64 7.03 0.16

Gasoline 12.4 228–501 2,200 1.0–7.6 0.24 0.4 1.1–3.3 1.4–1.7 44.22 0.05

Moreover, hydrogen is an important industrial gas and raw material for numerous industries, such as the computer, metallurgical, chemical, pharmaceutical, fertilizer and food industries. The major use of hydrogen is listed below. • Ammonia (NH3 ) production for use in fertilizer • Oil industry • Semi conductor production

8.3

• • • • • • •

Hydrogen Demand

497

Glass industry (shielding gas) Hydrogenation of fats and oils Methanol production Production of HCl Plastics recycling Rocket fuel Welding and cutting

8.2 Hydrogen Economy A hydrogen economy is envisioned as an economy in which hydrogen will be the main energy source [5–13]. The current global economy is called the carbon economy since most of the primary energy resources are derived from carbon or fossil fuels: Coal, Petroleum, and Natural gas. Hydrogen energy based economy is attractive as hydrogen can be produced from water; a resource that every country on the earth has. The regional dominance of an energy source can be eliminated leading to a better political environment. Various components of the hydrogen economy are discussed in Volume 4 of this book series.

8.3 Hydrogen Demand Total hydrogen production worldwide is about 550 billion Nm3 =year. Approximately 50% of it is used for ammonia based fertilizer production. The consumption of hydrogen in refineries per year is around 200 billion Nm3 . Other major uses of hydrogen include methanol production (8%) and the use as fuel in space programs (1%). Ninety-five percent of hydrogen production is captive, i.e., produced at the site where it is used. Other 5% is called merchant produced, which is sold for industrial and chemical uses. Hydrogen demand in the United States in 2006 was 11 million tons/year and accounted for 5% of the natural gas consumed in the US for its production. Centrally produced merchant hydrogen, which is sold for industrial and chemical uses, amounts to 1.5 million tons. Hydrogen use in the USA in 2006 is shown in Fig. 8.1. The usage is expected to double by 2010. However, this represents only a small increase in the production capacity in the USA from 2003 and 2008 (see Table 8.3). The production capacity must be increased to meet the future demand. The supply of hydrogen could be a significant issue, if hydrogen fuel cell cars are introduced in the market in large numbers. The main objective of a hydrogen economy is to use hydrogen as a fuel source. Its major use as fuel will be in fuel-cell cars. The working principle of fuel cells is discussed in Volume 3 of this book series. It is assumed that fuel cell cars will go

498

100

8 Hydrogen Energy Percent of Regional Total

80 Ammonia Producers Refineries Methanol Producers Other Captive Use Pipeline or On Site Cylinder and Bulk

60

40

20

0 North America

Central/South America

Europe

CIS

Africa/ Middle East

Japan

Other Asia

Fig. 8.1 Hydrogen use by end users in 2006 (Courtesy of Suresh et al. [14])

Table 8.3 Hydrogen production capacity in the US in 2003 and 2008 Production capacity (thousands metric tons per year) Capacity type 2003 2008 On-purpose capacitya Oil refinery 2,870 2,723 Ammonia 2,592 2,271 Methanol 393 189 Other 18 19 On-purpose merchanta Off-site refinery Non-refinery compressed gas (cylinders and bulk) Compressed gas (pipeline) Liquid hydrogen Small reformers and electrolyzers Total on-purpose Byproduct Catalytic reforming at oil refineries Other off-gas recoveryb Chloro-alkali production

976 2 201 43 <1

1,264 2 313 56 <1

7,095

6,839

2,977 462 –

2,977 478 389

Total byproducts 3,439 3,844 Total hydrogen production capacity 10,534 10,683 Source: Energy Information Administration (EIA) [15] a On-purpose are those units where hydrogen is the main product as opposed to byproduct units where hydrogen is produced as a result of processes dedicated to producing other products b From membrane, cryogenic and pressure swing adsorption (PSA) units at refineries and other process plants

8.3

Hydrogen Demand

499

Fig. 8.2 U.S. Hydrogen demand assuming commercialization of the fuel cell vehicles in 2015 (Courtesy of An “optimally plausible” solution based on NRC report.3 3 [16])

Fig. 8.3 Projected hydrogen demand in various regions of the world (Source: Argonne National Laboratory [17])

into mass scale commercial production in the US around 2015, and consequently the demand for hydrogen will also increase. The increase in demand for hydrogen in the US is shown in Fig. 8.2.

500

8 Hydrogen Energy

It is anticipated that by 2040, the use of hydrogen in fuel cell powered cars and light trucks could replace consumption of petroleum by about 18.3 million barrels per day. If it is assumed that hydrogen powered vehicles will have 2.5 times the energy efficiency of improved gasoline vehicles, this reduction in petroleum use would require the annual production of approximately 150 million tons of hydrogen by 2040. The use of hydrogen is yet to take off, but the anticipation is that its use by most of the countries will start around 2020. An exponential growth following the initial period of slow growth is expected as shown in Fig. 8.3.

8.4 Hydrogen Internal Combustion Engine The use of hydrogen as a fuel for internal combustion engines (ICE) has been explored in the last several decades [18–23]. Hydrogen use in ICE has several advantages including wide range of flammability, low ignition energy, small quenching distance, high autoignition temperature, high flame speed at stoichiometric ratios, high diffusivity, and very low density. The ability for H2 -ICEs to burn cleanly and operate efficiently is due to the unique combustion characteristics of hydrogen that allow ultra-lean combustion with significantly reduced NOx production and efficient low-engine load operation. Some of these properties are given in Table 8.4. Most of the cars use an Otto cycle engine. The theoretical thermodynamic efficiency of an Otto cycle engine is based on the compression ratio of the engine and the specific-heat ratio of the fuel as shown in Eq. 8.1. 1 th D 1 1

(8.1)

V1 V2

where, V1 =V2 D the compression ratio D ratio of specific heats t h D theoretical thermodynamic efficiency Table 8.4 Fuel properties at 298 K and 1 atm pressure relevant to internal combustion engines Property Hydrogen CNG Gasoline Density (kg=m3 ) Flammability limits (') Quenching distance (mm)b Stoichiometric fuel/air mass ratio Stoichiometric volume fraction % Heat of combustion .MJ=kgair /b Source: White et al. [24] a Liquid at 0ı C b At stoichiometry c Methane d Vapor

0.0824 0.1–7.1 0.64 0.029 29.53 3.37

0.72 0.4–1.6 2:1c 0.069 9.48 2.9

730a 0.7–4 2 0.068 2d 2.83

8.5

Hydrogen Production Methods

501

Equation 8.1 suggests that higher is the compression ratio and/or the specific-heat ratio, the higher is the thermodynamic efficiency of the engine. The compression ratio limit of an engine is based on the fuel’s resistance to knock. A lean hydrogen mixture is less susceptible to knock than conventional gasoline, and, therefore, can tolerate higher compression ratios. The high RON (Research Octane Number) and low lean-flammability limit of hydrogen provide the necessary combination to attain high thermal efficiencies in an ICE. The stoichiometric air/fuel (A/F) ratio for the complete combustion of hydrogen in air is about 34:1 by mass. This is much higher than the 14.7:1 A/F ratio required for gasoline. However, because of hydrogen’s wide range of flammability, hydrogen engines can run on A/F ratios of anywhere from 34:1 (stoichiometric) to 180: 1. The specific heat ratio is related to the fuel’s molecular structure. The less complex is the molecular structure, the higher is the specific-heat ratio. Hydrogen ( D 1:4) has a much simpler molecular structure than gasoline, and, therefore, its specific-heat ratio is higher than that of conventional gasoline ( D 1:1). Hydrogen fueled ICEs have demonstrated efficiencies in excess of 20% compared to conventional gasoline-fueled ICEs. Auto mobile companies including BMW [25] and Ford [26] are investing significant amount of their resources to develop hydrogen fueled ICEs. Berckm¨uller et al. [25] supercharged a single-cylinder engine up to 1.8 bar and reported 30% increase in specific power output compared to a naturally aspirated gasoline engine. Natkin et al. [27] also reported increase in specific power while investigating a supercharged 4-cylinder 2.0-l Ford Zetec engine and a 4-cylinder 2.3-l Ford Duratec engine that was used in conventional and hybrid vehicles [26]. Musashi Institute of Technology showed a 35% increase in power for two Nissan hybrid engines when using hydrogen fuel. NOx emissions were 10 ppm, same as other gasoline engines [28].

8.5 Hydrogen Production Methods There are three aspects of hydrogen production and these are: 1. Identification of a source (chemical compound) for hydrogen. 2. Identification of a primary energy source. 3. A method for using primary energy source to extract hydrogen from its sources. Hydrogen can be produced from a number of sources such as water, natural gas, coal, various hydrocarbons, and biomass. Processes for extraction of hydrogen from these sources are source specific [29–36]. For example, biomass may have to be converted to methane or an alcohol, such as ethanol, before hydrogen can be produced. Hydrocarbons must be reformed to produce hydrogen, which is a very energy intensive process. This could result in a higher price than the direct use of gasoline. Hydrogen produced from natural gas may cost twice as much compared to the use of gasoline directly in automobiles. A primary energy source is necessary to extract hydrogen from its compounds (see Fig. 8.4). Although all kinds of energy sources can be used, the cost would vary

502

8 Hydrogen Energy

Fig. 8.4 The use of various energy sources and pathways for hydrogen production (Source: Yacobucci and Curtright [36])

significantly depending on the methods selected for hydrogen extraction from its compounds. For example, to generate hydrogen from water, the process may need the heat from a primary energy source, electricity from a power plant, or sunlight to split water into hydrogen and oxygen. Methods for production of hydrogen can be divided into several categories and are given in Table 8.5. One of the most common processes for hydrogen generation is called the thermal process. In thermal processes, feed stocks are natural gas, petroleum based liquid fuels (propane or higher hydrocarbons), biomass, coal (hydrogen present in coal), and water (direct thermal decomposition). The heat from a primary source is used directly to dissociate hydrogen containing compounds into hydrogen and other constituents. The specific chemical processes used in the thermal process are given below: • Reforming of natural gas • Gasification of coal • Gasification of biomass

8.5

Hydrogen Production Methods

503

Table 8.5 Processes for hydrogen production Primary method Thermal

Process Steam reformation Thermochemical water splitting

Feedstock Natural gas Water

Gasification

Coal, Biomass

Pyrolysis

Biomass

Electrochemical

Electrolysis

Water

Biological

Photoelectrochemical Photobiological

Water Water and algae strains Biomass Biomass

Anaerobic digestion Fermentative microorganisms Source: National Hydrogen Association [37]

Energy High temperature steam High temperature heat from advanced gas-cooled nuclear reactors Steam and oxygen at high temperature and pressure Moderately high temperature steam Electricity from wind, solar, hydro, nuclear, coal or natural gas Direct sunlight Direct sunlight High temperature heat High temperature heat

• Reforming of renewable liquid fuels • Nuclear energy • Solar energy

8.5.1 Reforming of Natural Gas About 95% of hydrogen produced in 2008 in the USA was generated via steam methane reforming. Worldwide about 50% of the total hydrogen is produced by this method. In this process, methane (CH4 ) is reacted with either steam (called steam reforming reaction), oxygen (called partial oxidation), or both in sequence (called autothermal reforming). A steam-methane reforming process [38–58] consists of essentially four steps: Feed purification, Steam reforming reactions, Water gas shift reaction, and Product purification (CO2 and trace impurities removal). The process flow diagram is shown in Fig. 8.5. The source for methane is natural gas, which contains a variety of impurities that must be removed prior to feeding it to the catalytic reactor. Natural gas purification involves removal of sulfur (S) and chlorine (Cl), which act as poisons for the catalyst, to increase the life of the downstream steam reforming and other catalysts. Organic sulfur compounds are first hydrogenated converting sulfur to H2 S, which next reacts with zinc oxide in a column to form zinc sulfide. It is a solid and is removed from the bed as a waste. For high or variable sulfur loadings, more

504

8 Hydrogen Energy

Fig. 8.5 Main components of a hydrogen production plant using natural gas (Source: Spath and Mann [58])

complicated systems using separate reactors for conversion and adsorption are necessary. Often several conversion and adsorption vessels are used in series. Steam methane reforming reactions producing hydrogen consist of two reactions. The first reaction involves methane reacting with steam at 750–800ıC (1; 380–1; 470ıF) under 3–25 bar pressure in the presence of a catalyst to produce hydrogen, carbon monoxide, and a relatively small amount of carbon dioxide. The steam reforming reaction can be written as: Catalyst

CH 4 C H2 O ! CO C 3H2 ;

0

H298 D 206 kJ=mol

(8.2)

The gas mixture of CO and H2 (three-part H2 and one-part CO) is called the synthetic gas (or syngas). The positive H suggests that it is an endothermic reaction. Carbon monoxide (CO) further reacts with steam, which is known as “water-gas shift reaction,” over a catalyst to produce carbon dioxide and more hydrogen. This process occurs in two stages, consisting of a high temperature shift (HTS) at 350ıC (662ıF) and a low temperature shift (LTS) at 190–210ıC (374–410ıF). The overall reaction can be written as: Catalyst

CO C H2 O ! CO2 C H2 ;

0

H298 D 41 kJ=mol

(8.3)

8.5

Hydrogen Production Methods

505

This is an exothermic reaction, but the heat released in the reaction is much smaller compared to the amount consumed by first reaction (Eq. 8.2). A number of other hydrocarbons can be used as a feedstock for hydrogen production using the same steam reforming reaction: Propane: Ethanol:

Heat

C3 H8 C 3H2 O ! 3CO C 7H2 Heat

C2 H5 OH C H2 O ! 2CO C 4H2

(8.4) (8.5)

Gasoline (using iso-octane and toluene as example compounds from the hundred or more compounds present in gasoline): Heat

C8 H18 C 8H2 O ! 8CO C 17H2 Heat

C7 H8 C 7H2 O ! 7CO C 11H2

(8.6)

One of the problems of using higher hydrocarbon is the formation of carbon via following reactions. Also, if higher hydrocarbons are present with methane, carbon may also form, which will deposit on the catalyst reducing its activity, and the product yield. The methane decomposition reaction is given below: CH 4 ! C.s/ C 2H2 ; H D 75 kJ=mol

(8.7)

A similar reaction (Eq. 8.7) is more favorable for higher hydrocarbons. Carbon deposition can also occur via the Boudouard reaction given below: 2CO ! C.s/ C CO2 ; H D 172 kJ=mol

(8.8)

In the product purification unit, CO2 and other impurities are removed from the gas stream by the pressure-swing adsorption (PSA) process leaving essentially pure hydrogen. Prior to feeding the gas stream to a PSA unit, entrained liquids (water and condensed hydrocarbons) are removed in a condenser. The PSA unit that contains carbon and zeolite based adsorbents removes unreacted CH4 , CO, CO2 , and unrecovered hydrogen from the gas stream. The PSA off-gas (regenerated stream from the adsorption bed) is used to fuel the reformer; 80–90% of the required heat can be supplied by burning this stream. The remaining heat is provided by supplemental natural gas. As shown in Fig. 8.6, the process, when using other nonmethane feed, is not significantly different from the process shown in Fig. 8.5 for the natural gas feed.

8.5.1.1 Steam Methane Reforming Catalyst Catalysts are required for both the steam methane reforming and the water gas shift reactions. The key component of the reformer system is the catalyst. A variety

506

8 Hydrogen Energy Purification Steam

Steam reforming

High T shift

Low T shift Product hydrogen Carbon dioxide removal

Hydrocarbon feed Condensation

Fig. 8.6 Process for producing hydrogen from hydrocarbon feed (Source: Elder [59])

of catalysts have been explored for the reforming reaction [60–101]. However, the main steam reformer catalyst is supplied as a 10–33 wt% NiO supported on alumina, cement, or magnesia substrate. During startup, the catalyst is first heated in a stream of inert gas and then by steam. Once the catalyst attends the operating temperature, hydrogen or a light hydrocarbon is introduced to the stream to reduce NiO to metallic Ni. Although Ni-catalysts are widely used in the industry, its deactivation, poisoning by sulfur and other impurities, and carbon deposition are of great concerns. A number of other active metals that include iron, cobalt, rhodium, ruthenium, platinum, and palladium, have been tried for replacement of Ni-catalysts. However, all of them were found unacceptable by the industry: Iron based catalysts oxidize rapidly, cobalt cannot withstand steam pressure, and others are too expensive. For the high temperature water shift (HTS; 300–450ı C) reaction, an iron oxide-chromium oxide (Fe3 O4 Cr2 O3 ) based catalyst is used, whereas the major component of the low temperature shift catalyst (LTS; 180–270ı C) is copper oxide, usually in a mixture with zinc oxide (H¨aussinger, 2000). The LTS catalyst is sensitive to changes in operating conditions. Typical lifetime for both HTS and LTS catalysts is 3–5 years. The HTS catalyst is supplied in the form of ferric oxide (F2 O3 ) and chromium oxide (CrO3 ), which is reduced by hydrogen and carbon monoxide in the feed gas as part of the start-up procedure to obtain the desired form of the catalyst. The LTS catalyst is supplied as CuO on a ZnO support. The copper is reduced by heating it in a stream of inert gas containing hydrogen. The coke formation, when using heavy feedstock, may be reduced by using promoters, such as potassium, lanthanum, ruthenium, and cerium during the reforming reaction. Promoters can increase the steam gasification of solid carbons reducing the coke formation. Nickel-free catalysts containing mostly strontium, aluminum and calcium oxides have been successfully tested on feedstocks heavier than naphtha, however, the product gas contains high concentrations of methane which requires a secondary reformer for its conversion to H2 . LTS catalysts are highly sensitive to sulfur, but HTS catalysts can tolerate sulfur concentrations up to several hundred parts per million, although their catalytic activity declines. Several researchers are trying to develop sulfur tolerant catalysts, but

8.5

Hydrogen Production Methods

507

Fig. 8.7 A flow diagram of partial oxidation system for hydrogen production (Printed with permission from Kothari et al. [115])

with a limited success. ICI Katalco, USA, makes sulfur tolerant catalysts consisting of cobalt and molybdenum oxides, which operate at temperatures between 230ıC and 500ıC. The ratio of steam to sulfur in the feed gas and the catalyst temperature are the controlling factors.

8.5.1.2 Partial Oxidation In this method, methane is reacted with less than a stoichiometric amount of oxygen (typically, from air) to partially oxidize certain amount of methane [102–115]. Reaction products contain primarily hydrogen and carbon monoxide. However, nitrogen from the air and a relatively small amount of carbon dioxide remain in the product gas stream. Higher hydrocarbons react in a similar manner; only the oxygen content will vary depending on the molecular weight of the hydrocarbon. The reaction is followed by the water-gas shift reaction, where the carbon monoxide reacts with water to form carbon dioxide and more hydrogen. A schematic of the process is shown in Fig. 8.7. The partial oxidation process has several advantages. It is an exothermic process and also a much faster process than steam reforming and requires a smaller reactor vessel. The main drawback of this approach is that it initially produces less hydrogen per unit of input fuel than is obtained by steam reforming of the same fuel. Partial oxidation reactions of various hydrocarbons are given below. Methane:

CH 4 C 1=2 O2 ! CO C 2H2 C Heat

(8.9)

508

8 Hydrogen Energy

Propane:

C3 H8 C 1 1=2O2 ! 3CO C 4H2 C Heat

(8.10)

Ethanol:

C2 H5 OH C 1=2O2 ! 2CO C 3H2 C Heat

(8.11)

Gasoline (using iso-octane and toluene as example compounds from the hundred or more compounds present in gasoline): C8 H18 C 4O2 ! 8CO C 9H2 C Heat

(8.12)

C7 H8 C 3 1=2O2 ! 7CO C 4H2 C Heat

(8.13)

8.5.1.3 Autothermal Reforming for Hydrogen Production Autothermal reforming combines steam reforming and partial oxidation into a onestep process [116–130]. Methane or a hydrocarbon is reacted with both steam and oxygen from air to produce the syngas. A reaction temperature of 950–1;100ıC and a pressure up to about 10 MPa are maintained in the reactor. Both the steam reforming and partial oxidation reactions occur concurrently. The process can be optimized to supply all the heat from the partial oxidation reaction. An efficiency of 80–90% is possible. The syngas is then subjected to the water gas shift reaction to produce more hydrogen. The process can be compact, since a number of heat exchangers can be eliminated. Additional heat is not required making the process more attractive than the steam reforming process.

8.5.2 Biomass Gasification Biomass can be converted to the syngas mixture, which then can be subjected to the water-gas shift reaction for production of hydrogen. Biomass gasification processes have been discussed in Chap. 6.

8.5.3 Reforming of Biofuel Another route of hydrogen production from biomass is to convert it to a liquid fuel such as ethanol, bio-oils, and biodiesel. This is discussed later in this chapter. The advantage of doing this is that liquid fuels can be transported at relatively low cost to a reforming facility to produce hydrogen. Reforming bioliquids to hydrogen is very similar to reforming natural gas. The liquid fuel is first reacted with the steam at high temperatures in the presence of a catalyst to produce the syngas, followed by the water-gas shift reaction. Finally, the hydrogen is separated out and purified. The steam reforming reaction of ethanol is given below. Heat

C2 H5 OH C H2 O ! 2CO C 4H2

(8.14)

8.5

Hydrogen Production Methods

509

8.5.4 Hydrogen from Coal Coal gasification is the oldest method for hydrogen production. This type of plant has long been operating in Europe, South Africa, and the U.S. There are large supplies of coal in the U.S., other North American countries, and the world. Coal is mainly carbon, which can be converted to syngas by reacting it with steam. Coal gasification processes have been discussed in Chap. 6 of Volume 1 of this book series.

8.5.5 High-Temperature Water Splitting 8.5.5.1 One Step Reaction The direct decomposition of water into hydrogen and oxygen is a very challenging chemical process [131–134]. Complete decomposition of water (100% dissociation) requires a temperature of about 4027ıC at a pressure of 1 bar. At a temperature of 2227ıC and a pressure of 1 bar, only 9% water dissociates. A partial decomposition of 25% may be achieved at 2227ıC if pressure is reduced to about 0.05 bar. The one step reaction occurs as follows: H2 O D H2 C 1=2 O2

(8.15)

Reaction given in Eq. 8.15 is a reversible reaction, and the recombination of hydrogen and oxygen upon cooling must be prevented otherwise no net production would result. Solar energy may be used to generate such a temperature by using a solar concentrator. Quartz windows, using dish-type mirrors, focus the solar radiation into a cavity to achieve the high temperature that is necessary for the decomposition of water. However, materials of construction for such a cavity and large temperature swings (heating and cooling cycles) would make the process very expensive. 8.5.5.2 Two or Multiple Step Reactions The reaction temperature can be reduced significantly by using two or more chemical steps. A Temperature in the range of 14272727ıC would be sufficient to achieve an efficiency in the range of 40–50%. For multi-step thermochemical water splitting, two energy sources that can provide temperatures in this range are: • Nuclear energy • Solar energy Processes that are proposed for production of hydrogen, using these energy sources are listed below. Although these processes are technically viable, these may not be economically suitable.

510

• • • • • •

8 Hydrogen Energy

High temperature electrolysis Metal/metal oxide based systems Sulfur iodine cycle Hybrid sulfur cycle Steam methane reforming cycle Direct methane cracking cycle

The current process configurations (without the use of solar or nuclear energy) for direct steam methane reforming and direct methane cracking cycles are most economical and provide excellent positive energy balance. In direct processes, a portion of methane is burned to provide all the heat making the process economical. The use of solar or nuclear energy will add additional instability and increase operating costs. Both solar and nuclear energy can be used for high temperature electrolysis, but solar energy is most suitable for metal/metal oxide based water decomposition process, whereas nuclear energy provides the best economics for sulfur iodine and hybrid sulfur cycles.

8.6 Nuclear Energy for Hydrogen Production Nuclear energy is a prime candidate to supply heat for the large scale production of hydrogen [135–141]. Three options are available where nuclear energy can be used for commercial hydrogen production. These options are: • The use of the electricity generated from nuclear power plants for conventional water electrolysis. • The use of both high-temperature heat and electricity from the nuclear power plant for high-temperature steam electrolysis (HTSE) or the hybrid processes. • The use of the heat from nuclear plant for thermochemical processes. Although a high operating temperature is necessary for high efficiency and efficient operation of the plant, the advantages of using nuclear energy are: (1) no emission of greenhouse gases (GHG) and CO2 during the operation of nuclear reactors, and (2) nuclear energy can contribute to large-scale hydrogen production. The demand for energy is growing in all sectors worldwide, particularly in the transportation sector. Large scale hydrogen production systems will be essential to address this issue, which cannot yet be sufficiently addressed by the renewable energy resources. One of the major disadvantages of solar energy or other renewable energy sources is their low power density and intermittency. Technologies available for hydrogen production using nuclear energy are shown in Fig. 8.8.

8.6.1 Water Electrolysis The electrolysis of water at room temperature can lead to the decomposition of water into hydrogen and oxygen. Although electrolysis process is more than 85% efficient,

Fig. 8.8 Hydrogen production technologies for a specific nuclear reactor. VHTR Very High Temperature Reactor, S-CO2-AGR Supercritical-Carbon Dioxide Advanced Gas Reactor, SCWR SuperCritical Water Reactor, ALWR Advanced Light Water Reactor, STAR Secure Transportable Autonomous Reactor, SFR Sodium-cooled Fast Reactor (Source: Yildiz and Kazimi [160])

8.6 Nuclear Energy for Hydrogen Production 511

512

8 Hydrogen Energy

it requires high electrical energy consumption that makes the overall process not only inefficient, but also expensive. The current electricity generating processes (steam turbine based nuclear or coal power plants) have only 37–40% thermal efficiency. Therefore, combination of such a power plant and water electrolysis will make the process only 30–32% efficient. As a result, a high-temperature steam electrolysis process that is more energy efficient for hydrogen production is being developed.

8.6.2 High Temperature Electrolysis (HTE) of Steam The decomposition of water is accomplished by using both thermal and electrical energies, which can be provided by nuclear energy [142–159]. Water is heated by an outer heat source, in the present case by the heat from a nuclear plant, before feeding it into an electrolysis cell as steam, which is supplied to the cathode of the electrolysis cell that decomposes water into hydrogen and oxygen according to Eq. 8.16. H2 O C 2e ! H2 C O 2 (8.16) Hydrogen is removed as the product. Oxygen ions move through the electrolyte that is conductive only to oxygen ions towards the anode, where they release electrons producing molecular oxygen. The reaction at the anode is given by: O 2 !

1 O2 C 2e 2

(8.17)

This overall reaction can be described as: 1 H2 O ! H2 C O2 2

(8.18)

The process is shown schematically in Fig. 8.9. A major component of the high temperature electrolysis system is the electrolyte, which is a solid oxide capable of oxygen-ion-conduction. Most common electrolyte is Yttria-Stabilized Zirconia (YSZ) with porous, electrically conducting electrodes deposited on either side of the electrolyte. The construction of the electrolysis cell using YSZ is shown in Fig. 8.10. A mixture of steam and hydrogen at 750–950ı C is supplied to the cathode side of the electrolyte. The oxygen ions are drawn through the electrolyte by the applied electrochemical potential, liberating their electrons and recombining to form molecular O2 on the anode side. The entering steamhydrogen mixture may contain as much as 90% steam. Similarly, the exiting mixture may contain as much as 90% H2 . The product steam and hydrogen gas mixture is passed through a condenser or membrane separator to produce pure hydrogen. In a commercial unit, several electrolytic cells are combined to form a stack, separated by electronically conducting interconnects.

8.6

Nuclear Energy for Hydrogen Production

513

Fig. 8.9 A schematic representation of high temperature steam electrolysis

90 v/o H2O + 10 v/o H2

Porous Cathode, Nickel-Zirconia cermet

H2O

→

4 e−→

10 v/o H2O + 90 v/o H2

→

2 H2O + 4e− → 2 H2 + 2 O− →

2 O−

H2

Gastight Electrolyte, Yttria-Stabilized Zirconia

2 O− → O2 + 4 e− →

O2

Porous Anode, Strontium-doped Lanthanum Manganite ← O2

Interconnection

→

H2O

Next Nickel-Zirconia Cermet Cathode

→

H2O + H2 → H2

Fig. 8.10 Components of a high temperature electrolysis cell (Source: O’Brien et al. [161])

Although the construction of the electrolysis cell is similar to a fuel cell, operation of the cell in the electrolysis mode is fundamentally different than the operation in the fuel cell mode. In the fuel cell mode, hydrogen ions combine with oxygen ions releasing heat, whereas in the electrolysis mode, the steam reduction

514

8 Hydrogen Energy

Fig. 8.11 Thermodynamic system

reaction is endothermic. Therefore, heat must be supplied continuously to maintain the high reaction rate. Depending on the current density, the net heat generation in the stack may be negative, zero, or positive. The conceptual design of a plant that combines a nuclear reactor with a hydrogen production unit is described in Chap. 9 of Volume 1 of this book series. In this concept, a nuclear reactor generates both electrical power and supplies heat to the high temperature electrolysis unit for hydrogen production.

8.6.2.1 Efficiency of High Temperature Electrolysis Cycle For dissociation of steam into hydrogen and oxygen by HTE, both thermal and electrical energies are required. The total energy input to the system may be described by Eq. 8.19.

H D G C T S (8.19) In this equation, H is the enthalpy change which may be viewed as negative of the hydrogen combustion heat, G is the Gibbs free energy change, S is the entropy change in the reaction, and T is the reaction temperature in Kelvin. The term T S represents the thermal energy input, and G is the electrical energy input. The thermodynamic process may be represented by the simple Carnot system as shown in Fig. 8.11. The overall thermal efficiency can be expressed in terms of the net enthalpy change of the working fluid divided by the heat input at high-temperature to the system:

H D (8.20) QH

8.6

Nuclear Energy for Hydrogen Production

515

where H can be written from the first law of thermodynamics as:

H D QH QL

(8.21)

where QH is the heat input and QL is the heat rejected by the system. The second law provides: QH QL (8.22)

SR TH TL Substitution of Eqs. 8.20 and 8.21 into Eq. 8.22 provides the following expression for efficiency: 1 TTHL D (8.23) L S 1 T H The entropy S can be replaced by the Gibbs free energy of formation of water at a reference temperature by using Eq. 8.24.

S D

0

Gf;H

H 2O TL TL

(8.24)

Substitution of Eq. 8.24 into Eq. 8.23 provides the following expression for efficiency: !

H TL (8.25) D 1 0 TH Gf; H2 O 0 Since, Gf; H2 O for water is negative, the maximum theoretical efficiency may be calculated assuming that H is equal to the high heating value. The efficiency as a function of temperature is shown in Fig. 8.12. In this plot, the value of TL is assumed to be 20ı C. A maximum theoretically possible efficiency of around 87.5% is possible at 1;000ıC. This is the Carnot efficiency. For a practical cycle, about 65% of the Carnot efficiency is achievable. A high temperature electrolysis system can attend an overall efficiency of around 60%.

8.6.3 Thermochemical Water Splitting Thermochemical water-splitting refers to the decomposition of water into hydrogen and oxygen by a series of chemical reactions at high temperatures [162–177]. The same overall result is obtained, but using a much lower temperature compared to the direct thermal dissociation. Brown et al. [167] conducted an extensive review of about 115 thermochemical cycles. They identified 25 cycles that have good commercial potential and merit further study. These 25 cycles are listed in Table 8.6. Among these 25 cycles, three cycles: sulfur-iodine, Ca–Fe–Br, and Cu–Cl cycles were explored further. These three cycles are discussed below.

516

8 Hydrogen Energy

Fig. 8.12 Theoretical efficiency for high temperature hydrogen splitting (Source: O’Brien et al. [161])

8.6.3.1 Sulfur-Iodine Cycle (S-I Cycle) The sulfur–iodine (S-I) cycle was developed by the General Atomics, USA, in the mid-1970s. The cycle involves three chemical reactions that lead to the dissociation of water and produce hydrogen [178–198]. The chemical reactions involved in the S-I cycle are shown in Fig. 8.13. As shown in Fig. 8.13, Equations 8.1 and 8.3 are endothermic, whereas Equation 8.2 is exothermic. All reactions occur in the fluid phase and no solid phase is involved or formed during these reactions. All reagents, sulfuric acid, HI, and byproducts such as SO2 , and I2 are recycled, resulting in no effluents from the system. Only water is added to the system, and H2 SO4 or HI is replenished from time to time. The sulfur-iodine cycle is very advanced; each of the chemical reactions in this process was demonstrated in the laboratory by the US Department of Energy Laboratories, and General Atomics. Japan Atomic Energy Research Institute has developed a demonstration unit. The thermodynamic properties of the cycle have been determined and evaluated and are shown in Fig. 8.14 in a very basic flow diagram. Based on the three reactions shown in Fig. 8.13, the S-I cycle may be divided into three sections: Section I: Section II: Section III:

Sulfuric Acid Concentration and Decomposition Section Bunsen Reaction Section Hydrogen Iodide Decomposition Section

Name Westinghouse [183]

Ispra Mark 13 [184]

UT-3 Univ. of Tokyo [185]

GA Sulfur-Iodine [186]

Julich Center EOS [187]

Tokyo Inst. Tech. Ferrite [188]

Hallett Air Products 1965 [187]

Cycle 1

2

3

4

5

6

7

T/Ea T E T E T T T T T T T T T T T T T T E

Tı C 850 77 850 77 77 600 600 750 300 850 300 100 800 700 200 1,000 600 800 25

Reaction Fb 2H2 SO4 .g/ D 2SO2 .g/ C 2H2 O.g/ C O2 .g/ 1/2 SO2 .g/ C 2H2 O.a/ D H2 SO4 .a/ C H2 (g) 1 1/2 2H2 SO4 .g/ D 2SO2 .g/ C 2H2 O.g/ C O2 .g/ 1 2HBr.a/ D Br2 .a/ C H2 .g/ Br2 .l/ C SO2 .g/ C 2H2 O.l/ D 2HBr.g/ C H2 SO4 .a/ 1 1/2 2Br2 .g/ C 2CaO D 2CaBr2 C O2 .g/ 1 3FeBr2 C 4H2 O D Fe3 O4 C 6HBr C H2 .g/ 1 CaBr2 C H2 O D CaO C 2HBr Fe3 O4 C 8HBr D Br2 C 3FeBr2 C 4H2 O 1 1/2 2H2 SO4 .g/ D 2SO2 .g/ C 2H2 O.g/ C O2 .g/ 1 2HI D I2 .g/ C H2 .g/ 1 I2 C SO2 .a/ C 2H2 O D 2HI.a/ C H2 SO4 .a/ 2Fe3 O4 C 6FeSO4 D 6Fe2 O3 C 6SO2 C O2 .g/ 1/2 1 3FeO C H2 O D Fe3 O4 C H2 .g/ 6 Fe2 O3 C SO2 D FeO C FeSO4 1 2MnFe2 O4 C 3Na2 CO3 C H2 O D 2Na3 MnFe2 O6 C 3CO2 .g/ C H2 (g) 4Na3 MnFe2 O6 C 6CO2 .g/ D 4MnFe2 O4 C 6Na2 CO3 C O2 (g) 1/2 1/2 2Cl2 .g/ C 2H2 O.g/ D 4HCl.g/ C O2 (g) 1 2HCl D Cl2 .g/ C H2 (g) (continued)

Table 8.6 Most promising thermochemical cycles for hydrogen production from water using nuclear heat

8.6 Nuclear Energy for Hydrogen Production 517

Nickel Ferrite [189]

Aachen Univ Julich 1972 [187]

Ispra Mark 1C [184]

LASL-U [187]

Ispra Mark 8 [184]

Ispra Mark 6 [184]

9

10

11

12

13

14

Table 8.6 (continued) Cycle Name 8 Gaz de France [187] T/Ea T T T T T T T T T T T T T T T T T T T T T T

Tı C 725 825 125 800 800 850 170 800 100 900 730 100 25 250 700 700 900 100 850 170 700 420 Reaction 2K C 2KOH D 2K2 O C H2 (g) 2K2 O D 2K C K2 O2 2K2 O2 C 2H2 O D 4KOH C O2 (g) NiMnFe4 O6 C 2H2 O D NiMnFe4 O8 C 2H2 (g) NiMnFe4 O8 D NiMnFe4 O6 C O2 (g) 2Cl2 .g/ C 2H2 O.g/ D 4HCl.g/ C O2 (g) 2CrCl 2 C 2HCl D 2CrCl3 C H2 (g) 2CrCl3 D 2CrCl2 C Cl2 (g) 2CuBr2 C Ca.OH/2 D 2CuO C 2CaBr2 C H2 O 4CuO.s/ D 2Cu2 O.s/ C O2 (g) CaBr2 C 2H2 O D Ca.OH/2 C 2HBr Cu2 O C 4HBr D 2CuBr2 C H2 .g/ C H2 O 3CO2 C U3 O8 C H2 O D 3UO2 CO3 C H2 (g) 3UO2 CO3 D 3CO2 .g/ C 3UO3 6UO3 .s/ D 2U3 O8 .s/ C O2 (g) 3MnCl2 C 4H2 O D Mn3 O4 C 6HCl C H2 (g) 3MnO2 D Mn3 O4 C O2 (g) 4HCl C Mn3 O4 D 2MnCl2 .a/ C MnO2 C 2H2 O 2Cl2 .g/ C 2H2 O.g/ D 4HCl.g/ C O2 (g) 2CrCl 2 C 2HCl D 2CrCl3 C H2 (g) 2CrCl3 C 2FeCl2 D 2CrCl2 C 2FeCl3 2FeCl3 D Cl2 .g/ C 2FeCl2

Fb 1 1 1/2 1 1/2 1/2 1 1 1 1/2 2 1 1 1 1/2 1 1/2 3/2 1/2 1 1 1

518 8 Hydrogen Energy

Ispra Mark 3 [184]

Ispra Mark 2 (1972) [184]

Ispra CO/Mn3 O4 [190]

Ispra Mark 7B [184]

Vanadium Chloride [191]

16

17

18

19

20

Table 8.6 (continued) Cycle Name 15 Ispra Mark 4 [184] T/Ea T T T T T T T T T T T T T T T T T T T T T T

Tı C 850 100 420 800 850 170 200 100 487 800 977 700 700 1;000 420 650 350 400 850 25 700 25 Reaction 2Cl2 .g/ C 2H2 O.g/ D 4HCl.g/ C O2 (g) 2FeCl2 C 2HCl C S D 2FeCl3 C H2 S 2FeCl3 D Cl2 .g/ C 2FeCl2 H2 S D S C H2 (g) 2Cl2 .g/ C 2H2 O.g/ D 4HCl.g/ C O2 (g) 2VOCl2 C 2HCl D 2VOCl3 C H2 (g) 2VOCl3 D Cl2 .g/ C 2VOCl2 Na2 O:MnO2 C H2 O D 2NaOH.a/ C MnO2 4MnO2 .s/ D 2Mn2 O3 .s/ C O2 (g) Mn2 O3 C 4NaOH D 2Na2 O:MnO2 C H2 .g/ C H2 O 6Mn2 O3 D 4Mn3 O4 C O2 (g) C.s/ C H2 O.g/ D CO.g/ C H2 (g) CO.g/ C 2Mn3 O4 D C C 3Mn2 O3 2Fe2 O3 C 6Cl2 .g/ D 4FeCl3 C 3O2 .g/ 2FeCl3 D Cl2 .g/ C 2FeCl2 3FeCl2 C 4H2 O D Fe3 O4 C 6HCl C H2 (g) 4Fe3 O4 C O2 .g/ D 6Fe2 O3 4HCl C O2 .g/ D 2Cl2 .g/ C 2H2 O 2Cl2 .g/ C 2H2 O.g/ D 4HCl.g/ C O2 (g) 2HCl C 2VCl2 D 2VCl3 C H2 (g) 2VCl3 D VCl4 C VCl2 2VCl4 D Cl2 .g/ C 2VCl3

Fb 1/2 1 1 1 1/2 1 1 2 1/2 1 1/2 1 1 3/4 3/2 1 1/4 3/2 1/2 1 2 1 (continued)

8.6 Nuclear Energy for Hydrogen Production 519

US-Chlorine [187]

Ispra Mark 9 [184]

Ispra Mark 6C [184]

23

24

25

T/Ea T T T T T T T T T T T T T T T T T T T T

Tı C 420 650 350 1;000 120 800 850 700 25 25 850 200 500 420 150 650 850 170 700 500 Reaction 2FeCl3 .l/ D Cl2 .g/ C 2FeCl2 3FeCl2 C 4H2 O.g/ D Fe3 O4 C 6HCl.g/ C H2 (g) 4Fe3 O4 C O2 .g/ D 6Fe2 O3 6Cl2 .g/ C 2Fe2 O3 D 4FeCl3 .g/ C 3O2 (g) Fe2 O3 C 6HCl.a/ D 2FeCl3 .a/ C 3H2 O(l) H2 S.g/ D S.g/ C H2 (g) 2H2 SO4 .g/ D 2SO2 .g/ C 2H2 O.g/ C O2 (g) 3S C 2H2 O.g/ D 2H2 S.g/ C SO2 .g/ 3SO2 .g/ C 2H2 O.l/ D 2H2 SO4 .a/ C S S.g/ C O2 .g/ D SO2 .g/ 2Cl2 .g/ C 2H2 O.g/ D 4HCl.g/ C O2 (g) 2CuCl C 2HCl D 2CuCl2 C H2 (g) 2CuCl2 D 2CuCl C Cl2 .g/ 2FeCl3 D Cl2 .g/ C 2FeCl2 3Cl2 .g/ C 2Fe3 O4 C 12HCl D 6FeCl3 C 6H2 O C O2 (g) 3FeCl2 C 4H2 O D Fe3 O4 C 6HCl C H2 (g) 2Cl2 .g/ C 2H2 O.g/ D 4HCl.g/ C O2 (g) 2CrCl2 C 2HCl D 2CrCl3 C H2 (g) 2CrCl3 C 2FeCl2 D 2CrCl2 C 2FeCl3 2CuCl2 D 2CuCl C Cl2.g/

1/2 1 1 3/2 1/2 1 1/2 1 1 1

1/4 1/4 1 1 1/2 1/2 1/2

Fb 3/2

Source: Brown et al. [167] a T Thermochemical, E Electrochemical b Reactions are stored in database with minimum integer coefficients. Multiplier from reaction junction table converts the results to the basis of one mole of water decomposed

GA Cycle 23 [192]

22

Table 8.6 (continued) Cycle Name 21 Mark 7A [184]

520 8 Hydrogen Energy

8.6

Nuclear Energy for Hydrogen Production

521

Fig. 8.13 Sulfur-iodine process (Source: Brown [199])

Section I: Sulfuric Acid Concentration and Decomposition Section This section of the S-I cycle is the most critical and also determines the efficiency of the process. In this section, H2 SO4 is decomposed to SO2 ; O2 , and H2 O. A temperature greater than 850ı C is required to decompose H2 SO4 . Sulfuric acid stream from this section that contains approximately 57% acid is concentrated to about 86% acid in the first step. The concentration of acid is generally carried out under a vacuum of 0.1 bar. The acid stream is then pressurized to about 70 bars. A schematic flow diagram of this section is shown in Fig. 8.15. The decomposition reaction consists of the two following reactions: H2 SO4 D SO3 C H2 O

(8.26)

SO3 D SO2 C 1=2 O2

(8.27)

The first reaction (Eq. 8.26) takes place at around 350ı C with or without catalysts. The second reaction (Eq. 8.27) occurs above 750ıC in the presence of a catalyst. Both the reactions are endothermic and the required heat could be supplied by Generation IV-Very High Temperature Reactors (VHTR), which use helium as a

Fig. 8.14 Schematic representation of sulfur-iodine cycle (Brown et al. [200])

522 8 Hydrogen Energy

Nuclear Energy for Hydrogen Production

Fig. 8.15 A schematic flow diagram of the sulfuric acid concentration and decomposition section (Brown et al. [200])

8.6 523

524

8 Hydrogen Energy

Fig. 8.16 Effect of pressure on SO2 production during H2 SO4 decomposition in the S-I cycle (Source: Pickard [201])

cooling gas that is pressurized over 40 bars. The VHTR systems have been described in detail in Chap. 9 of Volume I of this book series. For efficient operation of the heat exchanger, H2 SO4 decomposition reactions should be carried out under a pressure as high as the heat exchanger wall would permit, although, the equilibrium conversion to SO3 and H2 O, and SO2 and O2 are unfavorable according to the Le Chatelier’s principle. As can be seen from Fig. 8.16, at higher pressures, decomposition of H2 SO4 to SO2 decreases. Various types of catalysts have been explored for H2 SO4 decomposition reaction and high SO2 yield [202–216]. Platinum on porous metal oxides as the support material is found to be the most promising than other catalyst in terms of SO2 yield, stability, and resistance to corrosion [217]. However, Pickard [201] noted that Pt based catalysts are not stable in the high-temperature reaction environment and deactivate due to sintering of Pt and supports. As much as 30% of Pt was lost during 10 days of testing. Both sintering and volatilization of Pt were noted. Test results using Pt catalysts with different loading are shown in Fig. 8.17. Although Pt based catalysts were found to be the most promising, their cost is a major issue in determining the overall economics of the process. A number of other catalysts were explored under the US DOE hydrogen program. The SO2 yield by various non-Pt based catalysts is summarized in Fig. 8.18. Among these catalysts CuCr2 O4 ; NiCr2 O4 ; and FeTiO3 had leaching problems. Activity of FeTiO3 and NiFe2 O4 decreased at the highest temperature, and CuFe2 O4 -spinel was found to be most promising at high temperatures. Both reactants and products of these two reactions are highly corrosive, and only a few materials may be used at these severe reaction conditions of high temperature and pressure [217–220]. Sandia National Laboratory (SNL), USA, has designed a H2 SO4 decomposer in which both reactions could take place.

8.6

Nuclear Energy for Hydrogen Production

Fig. 8.17 SO2 yield by Pt based catalysts (Source: Pickard [221])

Fig. 8.18 SO2 yield by non-Pt based catalysts (Source: Pickard [221])

525

526

8 Hydrogen Energy

Fig. 8.19 SiC bayonet heat exchanger (Source: Evans [222])

The decomposition reaction takes place within the individual tube of a multi-tube reactor that is fed with liquid H2 SO4 at 50–150ı C. SNL proposed to use silicon carbide (SiC) for the reactor, which has excellent resistance to sulfuric acid and SO3 at high temperatures, but it possesses ceramic-like properties. Various problems such as the lack of flexible connection between parts and high pressure sealing still need to be resolved (see Fig. 8.19).

Section II: Bunsen Reaction Section Water reacts with iodine and SO2 spontaneously at a temperature between 80ı C and 120ıC and 10 bar pressure [223–232]. The Bunsen reaction is conducted with an excess of iodine to promote the separation of the two phases. Two liquid immiscible phases, one containing a mixture of H2 SO4 and water, and the other containing HI, I2 and water, are formed. These two phases are separated from each other by utilizing the difference in their specific gravity. There are four steps, including the Bunsen reaction in this section. A schematic flow diagram of this Bunsen section is shown in Fig. 8.20. Oxygen that is formed from the water decomposition reaction is removed from the stream to avoid any complex formation later in the process. Sulfur dioxide is also removed from the product stream to prevent any side reaction. Finally, water

Nuclear Energy for Hydrogen Production

Fig. 8.20 A schematic flow diagram of a Bunsen reaction (Adapted from Brown et al. [200])

8.6 527

528 Fig. 8.21 Reactor design for a Bunsen reaction of the sulfur-iodine cycle (a) countercurrent reactor (b) co-current reactor (Source: Brown [199])

8 Hydrogen Energy

a M

b

SO2 O2

H2SO4 H2O

I2

3-Phase

Static mixer assy.

34 cm

Counter H2O

Current Air cooling jacket

Reactor

SO2 O2

H2O, l2, Hl 1 cm ID

present in the HI–I2 product stream is removed before it is sent to Section III: Hydrogen Iodide Decomposition Section. This helps in the energy balance of the system since water does not have to be evaporated again in Section III: Hydrogen Iodide Decomposition Section. The H2 SO4 H2 O phase contains about 50–57%w/w H2 SO4 and is sent back to Section II: Bunsen Reaction Section to further concentrate H2 SO4 to 90–98%w/w. The reactor for the Bunsen reaction may play a critical role in the process evaluation. Both counter-current and co-current configurations have been studied. A co-current Bunsen reactor has the following characteristics: • • • •

Short residence time Efficient heat exchanger No side reactions Reactor previously demonstrated

The characteristics of the counter-current reactor are as follows: • • • •

Fewer pieces of equipment Fewer streams Less recycle Scale-up concerns

Both reactor designs are shown in Fig. 8.21. The co-current design features do not provide any significant advantages over the counter-current configurations. Further study is necessary before selecting a configuration.

8.6

Nuclear Energy for Hydrogen Production

529

Section III: Hydrogen Iodide Decomposition Section This section produces H2 by decomposing hydrogen iodide (HI) that is present in the HIx feed stream [233–238]. Hydrogen iodide is separated from the feed stream followed by decomposition into H2 and I2 . Two approaches have been suggested for this section: extractive and reactive distillation. Extractive distillation is more advanced than the reactive distillation, and all processing steps are well studied and demonstrated in laboratories. The reactive distillation is a fairly new concept and various processing steps and corresponding data are yet to be obtained. Although the reactive distillation needs fewer pieces of equipment with less streams, it is based on extrapolated phase equilibria data. These two distillation systems are described next. Extractive Distillation A schematic diagram of the extractive distillation process is shown in Fig. 8.22. One of the main issues of separating HI, I2 , and H2 O from their mixture is that the mixture forms an azeotrope (i.e., the whole mixture starts boiling at a certain temperature resulting in a composition of gas phase that is the same as that of the liquid phase near the boiling point). The extractive distillation process takes advantage of the solubility of HI and H2 O and the insolubility of I2 in phosphoric acid (H3 PO4 ). Additionally, H3 PO4 breaks down the azeotrope mixture. About 96 wt% H3 PO4 is added to the feed stream of HIx from Section I: Sulfuric Acid Concentration and Decomposition Section. This results in a formation of two phases that are separated from each other by gravity. The denser phase, I2 , is returned to the Bunsen reaction section and the lighter phase containing HI and H2 O is fed to the distillation column. Hydrogen iodide is boiled off in the column and the gaseous HI stream is fed to the decomposition vessel where it breaks down to H2 and I2 in the presence of a carbon based catalyst. The HI decomposition reaction can be carried out either in the gas phase at 350–450ıC or in the liquid phase at 150–300ı C. The hydrogen and unreacted HI are separated from I2 , which is returned to the Bunsen reactor. The gaseous H2 product is separated from HI by using a membrane. Hydrogen iodide is recycled back to the reactor, and pure hydrogen is stored in an appropriate storage medium. At 450ı C, the equilibrium decomposition of HI is about 22%, therefore, the reaction products must be removed from the reaction chamber to maintain this equilibrium conversion rate. Phosphoric acid is concentrated to 96% in the distillation column and is added to the incoming stream from Section I: Sulfuric Acid Concentration and Decomposition Section. Reactive Distillation In the reactive distillation method, no attempt is made to break the azeotrope point of the mixture. The mixture from Section I: Sulfuric Acid Concentration and Decomposition Section is distilled under pressure in the distillation column. HI is decomposed within the column by a catalyst in the gas phase. A schematic diagram of the process is shown in Fig. 8.23. HIx feed from Section I: Sulfuric Acid Concentration and Decomposition Section is heated to about 260–265ı C from 120ı C before feeding it to the reactive distillation column. The temperature at the bottom of the column is maintained at about 300ıC at which

Fig. 8.22 Extractive distillation flowsheet (Source: Brown [199])

530 8 Hydrogen Energy

Fig. 8.23 A schematic flow diagram of reactive distillation method for a Bunsen reaction (Adapted from Brown et al. [199])

8.6 Nuclear Energy for Hydrogen Production 531

532

8 Hydrogen Energy

HIx boils and maintains a vapor pressure of about 52 bar. The mixture of HI, I2 , and H2 O flows upward through a bed of activated carbon catalyst placed at the upper half of the column. HI is decomposed to H2 and I2 within the column in the presence of the catalyst, which is maintained at a temperature of 300ı C. A condenser removes unreacted HI, I2 and water from the stream, and the liquid stream from the condenser is recycled back to the column. Brown et al. [200] noted that about 16% of the HI is decomposed in a single pass in the reactive distillation configuration. The recycling, together with approximately five times of excesses iodine, is required in the product flow stream from the Bunsen reaction section to the sulfuric acid concentration and decomposition section. Also, the equal amount of water is necessary, which means that for each mole of hydrogen produced, approximately 6 moles of HI, 30 moles of iodine and 30 moles of water must flow from the Bunsen Reaction Section to the Sulfuric Acid Concentration and Decomposition Section, and 5 moles of HI, 30 moles of iodine and 30 moles of water must flow back from the sulfuric acid concentration and decomposition section to the Bunsen Reaction Section.

Efficiency of the S-I Cycle The efficiency of the S-I cycle depends on the mode of operation and the flow sheet followed for the cycle [239,240]. If high temperature helium from the reactor is split into two streams to provide the energy for Sections II: Bunsen Reaction Section and Section III: Hydrogen Iodide Decomposition Section in a parallel configuration, an efficiency of 42% may be expected. The efficiency may be increased to 48% (when both hydrogen and electricity production are considered) by heating Section III: Hydrogen Iodide Decomposition Section with the waste heat from a Brayton cycle. Even a higher efficiency could be achieved by raising the process temperature. The graph shown in Fig. 8.24 indicates that a hydrogen production efficiency of 51% could be obtained using a peak process temperature of 900ı C. The efficiency of an S-I cycle may be expressed by the following expression: H;SI D

QH;out HHV H D Qi n;SI Qi n;SI

(8.28)

where QH;out is the High Heating Value (HHV) carried away by hydrogen per unit basis, and Qin;SI is the total thermal energy necessary to produce a unit amount of hydrogen. A more effective heat recuperation configuration, better heat exchanger materials that can withstand high pressure differential and materials that can perform at higher temperatures can further increase the efficiency. Efficiencies of various processes under different operating conditions are shown in Fig. 8.25.

8.6

Nuclear Energy for Hydrogen Production

533

Fig. 8.24 Estimated sulfur iodine hydrogen production efficiency vs process temperature (Source: Brown et al. [199])

Fig. 8.25 Efficiency comparison among SI, HTSE, and WSP cycles for hydrogen production (Printed with permission from Yildiz and Kazimi [160])

8.6.3.2 Ca–Br–Fe (UT-3) Cycle The UT-3 cycle was first proposed by the University of Tokyo, Japan. This cycle involves solid–gas interactions and requires a lower temperature compared to the S-I cycle. Various aspects of an UT-3 cycle are discussed in the following references [241–270]. The solid-gas system allows better separation of products. The UT-3

534

8 Hydrogen Energy

cycle involves four reactions that take place in four adiabatic fixed packed bed chemical reactors that contain the solid reactants and products. 730ı C

CaBr2 C H2 O ! CaO C 2HBr 550ı C

CaO C Br 2 ! CaBr2 C 1=2 O2 220ı C

Fe3 O4 C 8HBr ! 3FeBr2 C 4H2 O C Br2 650ı C

3FeBr2 C 4H2 O ! Fe3 O4 C 6HBr C H2

(8.29)

A schematic diagram of the cycle is presented in Fig. 8.26. As shown in the figure, two reactors involve reactions with calcium compounds, and the other two reactors involve reactions with the iron compounds. The thermodynamic calculations using Eq. 8.29 showed these reactions to be favorable under normal operating temperatures. Free energies of these reactions were studied at the Argonne National Laboratory (ANL), USA, which showed the viability of the process. Free energies of these reactions are given in Fig. 8.27. The cycle efficiency is found to be limited by the temperature of the first reaction, since CaBr2 melts at 760ıC. Because of this limitation on the temperature, the hydrogen production efficiency of the process is limited to about 40%. ANL is working on coupling a UT-3 cycle to a Secure Transportable Autonomous Reactor (STAR). However, the operation of the STAR, which is a liquid-metal reactor, requires a temperature above 600ıC. The thermal energy from a nuclear reactor can be used directly to heat the gaseous stream, which flows through the four reactors (one for each reaction) and other process equipment before being recycled back to the nuclear reactor. Solid reactants in each reactor go through a regeneration cycle. The gaseous reactant passes through the bed of solid reactant until it is all consumed. For example, the first reaction continues until all of CaBr2 is converted to CaO by the reaction. At this point, the flow paths are switched and chemical reactors, in each pair, switch functions. Now the second reaction takes place in the first reactor. A similar reaction sequence occurs in Fe-reactors. The gaseous reactants/products by themselves are not capable of carrying all the thermal energy necessary for these reactions. A large quantity of steam is used as the carrier of the thermal energy. A steam pressure of about 20 bar is used in the cycle, which also helps to remove the products from the reactors by shifting the reaction equilibrium towards completion. This is necessary since the Gibbs free energy is positive for some of the reactions. 8.6.3.3 Cu–Cl Cycle The copper–chlorine thermochemical cycle is expected to operate at 500ıC [272–292], allowing the use of a number of Generation IV reactors, such as the sodium-cooled fast reactor (SFR). Generation IV reactors are described in Chap. 9

Fig. 8.26 Process flow diagram of the adiabatic UT-3 system (Source: Brown et al. [167])

8.6 Nuclear Energy for Hydrogen Production 535

Free Energy (keal/gm-mole)

536

8 Hydrogen Energy

60.00

CaBr2 + H2O

50.00

CaO + Br2

40.00

CaO + 2HBr CaBr2 + 0.5 O2

30.00

Fe3O4 + 8 HBr

20.00

Water Electrolysis

3 FeBr2 + 4H2O + Br2

10.00 0.00 –10.00 0

500

1000

1500

–20.00 –30.00

Temperature (K)

Fig. 8.27 Gibbs free energies of the reactions involved in UT-3 cycles (Adapted from Doctor et al. [271])

of Volume 1 of this book series. The materials of construction may be cheaper due to the lower temperature and less corrosion. The energy efficiency of the process is projected to be 40–45%. Both thermal and electrical energy are necessary in this cycle. The cycle involves the following reactions: 450ı C

2Cu C 2HCl ! 2CuCl C H2 30ı C

4CuCl C 4Cl ! 4CuCl2 30ı C

4CuCl2 ! 2CuCl2 C 2Cu C 4Cl 100ı C

2CuCl2 .aq/ ! 2CuCl2 .s/ 400ı C

2CuCl2 C H2 O ! CuO C CuCl2 C 2HCl 1 500ı C CuO C CuCl2 ! 2CuCl C O2 2

(8.30)

The general concept of the cycle is shown in Fig. 8.28 and a proposed schematic diagram of the cycle along with various process parameters are given in Fig. 8.29. The first reaction of the Cu–Cl cycle occurs at 430–475ı C producing H2 . The reactions of the cycle proceed as follow. Copper particles enter the reaction chamber, flow downward and react with incoming HCl vapor producing H2 and liquid CuCl. The second reaction is carried out in an electrochemical cell where CuCl is decomposed to Cu and CuCl2 . The Cu particles are fed back to first reaction chamber. CuCl2 goes into the aqueous phase. The aqueous CuCl2 stream exiting from the electrochemical cell is preheated to 150ıC before feeding into a flash dryer to produce solid CuCl2 .s/ that reacts with water vapor to produce gaseous HCl(g) in a fluidized bed reactor. The product is HCl(g) and CuO CuCl2 solid particles. The HCl(g) is recycled back for the hydrogen production, while the CuO CuCl2 particles are decomposed to produce oxygen and complete the cycle.

8.6

Nuclear Energy for Hydrogen Production

537

Fig. 8.28 Basic concept of Cu–Cl cycle for hydrogen production (Adapted from Lewis [292]) H2 gas (20 C)

O2 gas (20 C)

H2O 400 C

150 C flash dryer (evaporator / spray dryer) Step 3

valve

heat

steam (400 C) H2 gas (430 - 475 C)

copper particles enter / descend (or moving ion exchange bed)

20 C

HCI(g) production (step 4; fluidized or packed bed) HCI gas (400 C) H2 Production (step 1)

CuO*CuCl2 (s)

CuCl2 solid

CuCl2 (s) 400 C 400 C heat

heat

water

500 C

CuCl solid (430 - 475 C) heat

CuCl2 + water

O2 production (step 5) CuCl (l)

500 C

heat recovery heat recovery 20 C

CuCl (s) electrochemical cell (Cu production; step 2)

Cu solid falls (20 C)

solid copper conveyors (screw propeller)

Fig. 8.29 Conceptual layout of a copper-chlorine (Cu–Cl) cycle (Adapted from Rosen et al. [293])

538

8 Hydrogen Energy

A detailed thermodynamic analysis of the process was carried out by ANL. The free energies of all the reaction steps were determined. It is concluded that all of these reactions are thermodynamically favorable based on the values of the free energies. Thermal Efficiency of the Cu–Cl Cycle The efficiency of the Cu–Cl can be calculated from the following expression: D

o H25 ı C .H2 O/

Qhot C

W 0:5

(8.31)

The thermal efficiency of the process is obtained by dividing the lower heating value (LHV) of the hydrogen products from the reaction by the sum of thermal heat and electrical energy input to the system (converted to thermal equivalent with a 50% factor). The efficiency is found to be: ˜ D 44% with thermal C electrolysis energy, or ˜ D 41% including all the energy loss C shaft work 8.6.3.4 Comparison of the Processes Yildiz and Kazimi [167] compared various cycles that are proposed for hydrogen production using nuclear energy. Various aspects of the processes were considered in the analysis. Their analysis is given in Table 8.6. Wang et al. [285] compared the S–I and Cu–Cl cycles. They noted that the overall heat requirements of the two cycles are similar to each other, and the overall efficiencies are also in the same range; between 37% and 54%. The higher efficiencies depend on the heat recovery. However, the copper–chlorine cycle has the advantage of a lower maximum temperature of 530ı C, which is 27ı C lower than the maximum temperature of 850ıC in the sulfur–iodine cycle. 8.6.3.5 Reactor Types for Hydrogen Production One of the design objectives of Generation IV nuclear reactors is to facilitate hydrogen production. The plan is to use the thermal energy generated from the nuclear reactor directly to supply all the energy needs of the hydrogen production plant [294–302]. The requirements and criteria for the selection of a reactor type have been discussed by Schultz et al. [302] and are given below: Basic requirements 1. Chemical compatibility of coolant with primary loop materials and fuel. 2. Coolant molecular stability at operating temperatures in a radiation environment.

8.6

Nuclear Energy for Hydrogen Production

539

Fig. 8.30 Temperature output from various nuclear reactors and the maximum temperature requirements for thermochemical cycles (Adapted from Evans [222])

3. Pressure requirements for primary loop. 4. Nuclear requirements: parasitic neutron capture, neutron activation, fission product effects, gas buildup, etc. 5. Basic feasibility, general development requirements, and development risk. Important Criteria 1. 2. 3. 4. 5.

Safety Operational issues Capital costs Intermediate loop compatibility Other merits and issues

Based on these criteria, Schultz et al. [302] proposed several reactors that are suitable for hydrogen production which are shown in Fig. 8.30. These reactors are described in Chap. 9 of Volume 1 of this book series. Nuclear reactors provide a number of options regarding hydrogen plant configurations [303–313]. A reactor can be dedicated for either H2 production only or both for hydrogen production and electricity generation. The plant size can be large, modular, or distributed. These reactors also provide the operational flexibility of either electricity generation depending on the load, or hydrogen generation without changing the reactor power. Hydrogen can be produced by using either direct heat

540

8 Hydrogen Energy

Fig. 8.31 Interface configurations for a nuclear hydrogen production plant (Adapted from Petri [314])

through thermochemical cycles or indirect heating via electricity production (only if electricity is produced by the reactor). The system configurations, as shown in Fig. 8.31, can be either in parallel or in series based on heat loads. Also, heat from a reactor can be transferred to a distant H2 production facility (see Fig. 8.32) from the exit of the reactor or elsewhere in the plant, such as the exit stream from the turbine.

8.6

Nuclear Energy for Hydrogen Production

541

Fig. 8.32 A conceptual design of a nuclear plant-hydrogen production facility with hydrogen plant located outside the nuclear plant building (Adapted from Schultz et al. [302])

Materials of Construction One of the biggest challenges of using thermochemical cycles is the material compatibility at high temperatures. These cycles employ corrosive chemicals. The corrosion of materials, sealing, valves, numerous fittings and their compatibility with other materials, make the material selection of construction materials extremely challenging. As noted earlier, the S-I cycle is the most advanced with significant efforts being made at identifying materials for the various section of the process. The maximum temperature for the two acids (HIx and H2 SO4 ) in Section I: Sulfuric Acid Concentration and Decomposition Section is 120ıC. Iodine present in this section can also be corrosive, since it is a strong oxidizer. The material requirements for the Bunsen section are less demanding. A glass lined steel and Nb-alloys have been found to be acceptable. Pickard [201] tested Incoloy 800 H, Saramet 23, and Hastelloy C276 materials with sulfuric acid at 850ıC and ambient pressure. The results are given in Table 8.7. These materials are found to be vulnerable to acid attack under the operating conditions mentioned above.

542

8 Hydrogen Energy

Table 8.7 Corrosion properties of several alloys when exposed to sulfuric acid at 850ı C Species Cr Ni Fe Si Mo Au

Corrosion products (g/L) 10.0 8.0 6.6 0.042 0.26 0.0000046

Incoloy 800H (%) 21 32 40 <5 – 0

Saramet 23 (%) 18 17 55 5 0.06 0

Hastelloy C276 (%) 16 57 5 <0.08 16 0

Source: Adapted from Pickard [201] Fig. 8.33 Sulfuric acid decomposer made from SiC (Adapted from Pickard [201])

SO3 Decomposer

HEAT

Catalyst Boiler/Vaporizer /Superheater Section with Recuperation H2SO4

HEAT

Outer SiC tube Inner SiC tube

SO2 + O2 + H2O

Sandia National Laboratory used a H2 SO4 decomposer made from SiC for both SO3 decomposer and H2 SO4 boiler, vaporizer, and the superheating section. The design of the decomposer is shown in Fig. 8.33. According to Pickard [201], the best materials for the HI decomposition section are found to be metals and ceramics. The performance of some selected materials is shown in Table 8.8. The University of Nevada, Las Vegas, USA and General Atomics, USA, tested 22 coupons from four classes of materials: refractory, reactive metals, superalloys and ceramics. The conclusion based on short term results are also presented in Fig. 8.8. Wong et al. [315] noted that Hastelloy B2 and B3 have the lowest general corrosion rate after more than 1,000 h of testing. Their results are presented in Table 8.9. Both liquid I2 and HI acid are extremely corrosive. Wong et al. [315] noted that Ta and Ta-W alloys have the best corrosion resistance properties for use in the HI decomposition section of the S-I system. The corrosion rate of Ta and Ta-W alloys were less than 0.05 mils per year (mpy) penetration. For the high temperature operation with gaseous HI and iodine, Hastelloy compounds were found to be the most suitable as construction materials.

8.7

Solar Energy for Hydrogen Production

543

Table 8.8 Evaluation of various materials for construction of a HI decomposer Excellent Ta-40Nb Nb-1Zr Nb-10Hf

Good Ta Ta-10W Nb-7.5Ta

SiC (CVD) SiC (Ceramatec sintered) Mullite

SiC (sintered) Si-SiC (3 kinds)

Fair Mo-47Re Alumina

Poor Mo C-276 Haynes 188 Graphitea Zr702 Zr705

Source: Pickard [201] a Structurally good but adsorbs HI Table 8.9 Corrosion rates of Hastelloy B2 and B3 for exposure to HI decomposition reaction Material Hastelloy B2 Hastelloy B3 Hastelloy C22 Hastelloy C276 Monel Ta-2.5W Ti grade 2 Porous SiC Hastelloy B2 (condenser) Hastelloy B3 (condenser) Monel

Hours of exposure 1,172 1,022 1,570 1,220 970 250 430 966 1,172 1,022 1,570

Mils per year (mpy) 2.55 0.14 10.70 13.50 67.8 76.67 Disintegrated 24.0 (gained weight) 0.82 1.72 3,43

Source: Adapted from Wong et al. [315]

8.7 Solar Energy for Hydrogen Production Solar energy can potentially be used to generate hydrogen via a number of processes. These processes include: thermolysis, thermochemical process, solar generated electricity and electrolysis, solar reforming, solar cracking, and solar gasification. Among these processes, thermochemical processes have the greatest potential for industrial scale production. The very high temperature necessary for thermochemical processes can be obtained using solar concentrators [316–326]. In the USA, the National Renewable Energy Laboratory (NREL), USA, assessed the hydrogen production capability from the solar energy driven electrolysis process. Solar irradiance received by various parts of the USA has been discussed in Chap. 2. The hydrogen generation capability via solar electrolysis assessed by NREL for various regions in the USA is presented in Fig. 8.34. The Southwest region is shown to have the highest potential. The electricity requirement of the electrolysis system was assumed to be 58.8 kWh/kg hydrogen. Counties with very

544

8 Hydrogen Energy

Fig. 8.34 Solar energy resource for hydrogen production (Source: Milbrandt and Mann [327])

good solar resources and low population count (such as the Rocky MountainGreat Plains region) clearly show high potential for producing hydrogen from solar resources, per person.

8.7.1 High-Temperature Water Splitting-Solar Concentrators Water starts to dissociate directly into hydrogen and oxygen at temperatures higher than 1;700ı C [317]. A solar concentrator using mirrors and reflective or refractive lens to capture and focus sunlight can produce temperatures up to 2;000ıC. This high temperature heat can be used to drive the chemical reaction for splitting water to produce hydrogen. However, as shown in Fig. 8.35, the yield at 2;000ı C is just 0.01 kmol. A much higher temperature is necessary to have any appreciable amount of hydrogen production. The materials that can withstand such a high temperature are very expensive making such a process uneconomical. Potentially, solar energy may be used, instead of nuclear energy, in most of the processes described earlier for hydrogen production. The harvesting of the solar energy is challenging, which is mainly due to the complexity in designing equipment for capturing the solar energy economically, and for providing high temperature heat, required for most of the processes. Among these processes, the solar reforming

8.7

Solar Energy for Hydrogen Production

545

0.30

0.25 H2(g)

kmol

0.20 HO(g) H(g)

0.15

0.10 O2(g) O(g) 0.05

0.00 1500

1700

1900

2100

2300

2500

2700

2900

Temperature ⬚C Fig. 8.35 Hydrogen production rate at different temperatures by high temperature water decomposition (Adapted from Evans [222])

of natural gas and other hydrocarbons, using either steam or CO2 as partial oxidant, and 2-step thermochemical cycles using metal oxide redox reactions, have been extensively studied by various researchers. The processes where solar energy can be used for hydrogen production are given below and shown in Fig. 8.36. (a) (b) (c) (d) (e) (f)

Solar oxide based redox pair cycle Sulfur-iodine cycle Hybrid-sulfur cycle Natural gas reforming Thermal splitting of methane High temperature electrolysis

8.7.2 Solar Reforming of Natural Gas The energy necessary for reforming methane or other hydrocarbons for hydrogen or syngas production can be accomplished by using solar energy [329–336]. The decomposition of various hydrocarbons (methane, propane, gasoline) over carbon catalysts has been reported by the NERL, USA in a bench-scale fluidized bed at 850ıC. A solar tubular quartz reactor was designed and tested for the decomposition of natural gas using carbon black particles as catalysts that were suspended in the feed gas stream. About 90% of the natural gas was decomposed in the reactor [337,

546

8 Hydrogen Energy

a step 1

MO = metal oxide

step 2 solar heat

solar heat N2

receiver reactor 800 8C

H2O

H2

regeneration:

thermal splitting:

flush receiver reactor 1200 8C

O2

MOoxidised → MOdeoxidised + O2

MOdeoxidised + H2O → MOoxidised + H2

Solar oxide based redox pair cycle

b solar heat

solar heat H2O

SO2 receiver reactor 850 8C O2

thermal splitting:

l2

2Hl →H2 + l2 receiver reactor 360 8C

Bunsen section 120 8C H2SO4

three-stage process

thermal splitting: H2SO4 → SO2 + H2O + ½ O2

Bunsen reaction:

H2

2HI

9 l2 + SO2 + 16 H2O → (2 Hl + 10 H2O + 8 l2) + (H2SO4 + 4 H2O)

Sulfur-iodine cycle

c H 2O

solar heat

receiver reactor 1200 8C

SO2

two-stage process solar electricity

electrolysis 80 8C

thermal splitting: H2SO4 → H2O + SO3 → SO2 + ½ O2 + H2O

H2

electrolysis: 2 H2O + SO2 → H2SO4 + H2

O2 H2SO4

Hybrid-Sulfur cycle Fig. 8.36 Various processes where solar energy can be used for hydrogen production (Printed with permission from Pregger et al. [328])

338]. This reactor, called the “aerosol” solar reactor, is shown in Fig. 8.37. Two concentric graphite cylinders were used in the design. The outer tube was solid and absorbed the solar energy to provide the necessary heat, whereas the reactants flowed through the porous inner tube. A schematic diagram of the complete system is shown in Fig. 8.38. The gaseous feed is introduced from the top along with the recycled carbon particles that are separated from the gas stream in a bag house filter, located at the outlet of the reactor. A pressure swing adsorption (PSA) system is used to separate product H2 from the

8.7

Solar Energy for Hydrogen Production

547

d

reforming: CH2 + H2O → CO + 3H2 solar heat

watergas shift reaction:

H2

CO + H2O → CO2 + H2

fuelgas

PSA H2O

receiver reactor 850 8C

CH4

syngas

pressure swing adsorption (PSA): H2 separation, use of fuel gas or optional CO2/methane separation

shift reactor

optional methane separation and recirculation

MDEA gas washer

CO2

Natural gas reforming

e solar heat

thermal splitting: CH4 → C + 2H2 H2

receiver reactor 1800 8C

CH4

H2 separation: separation of elemental carbon by cyclone and a filter device, methane separation

C

Thermal splitting of methane

f solar heat solar electricity e− +

650⬚C − 2− O

O2

electrolysis: 2 H 2 O → O2 + 2 H 2 heat exchanging

H2 H2O O2

H2 H2 H2 O

H2O

O2

O2

High temperature electrolysis Fig. 8.36 (continued)

undecomposed feed, which is recycled to the reactor. Carbon particles escaping the baghouse can be burned and the gaseous stream can be used in a fuel cell to generate electricity for an electrolyzer to produce more H2 from water electrolysis. Solar reforming of the natural gas using either steam or CO2 as a partial oxidant is accomplished in the presence of an Rh-based catalyst. The solar tower concept, using two solar reforming reactors: an indirect irradiation tubular reactor [340] and a direct-irradiation volumetric reactor [341], has been studied for generating 300–500 kW of power at 727ı C and 8–10 bars pressure. The indirect-irradiated solar

548 Fig. 8.37 Design concept of an aerosol flow reactor for methane decomposition using solar energy (Source: Lewandowski and Weimer [339])

8 Hydrogen Energy Particle Feed H2 Sweep

Feed NG

Porous C(gr)-tube Solid C(gr)-absorber Quartz Envelope

H2, C, CHx

reactor proposed by Epstein et al. [342] consists of a ceramic-insulated pentagonal cavity-receiver containing a set of vertical Inconel tubes filled with a packed bed of catalyst (2% Rh on Al2 O3 support). The Compound Parabolic Concentrator (CPC) system provides uniform irradiation to all the tubes. The direct irradiated solar reactor, also referred to as the “volumetric” reactor, is shown in Fig. 8.39. A porous ceramic absorber was coated with Rh catalyst and directly exposed to the concentrated solar radiation. Methane (CH4 ) and CO2 conversions of 70% and 65%, respectively, were achieved in the absence of any added catalysts with a residence time of 10 ms at 1727ıC [343].

8.7.3 Thermochemical Solar Cycle Solar thermochemical water splitting is a two-step process [345–354]. In the first step, a metal oxide is decomposed to metal and oxygen using solar energy. In the second step, pure metal reacts with water producing hydrogen and metal oxide. These two reactions may be written as follows: First step W Second step W

Mx Oy D xM C 1=2 yO2 xM C yH2 O D Mx Oy C yH2

(8.32) (8.33)

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Solar Energy for Hydrogen Production

549

Fig. 8.38 A schematic diagram of solar natural gas reforming system (Adapted from Lewandowski and Weimer [339]) CPC

Window

Catalytic absorber

Reactants inlet

Products outlet

Fig. 8.39 Scheme of a “volumetric” solar reactor concept for the reforming of NG. CPC Compound Parabolic Concentrators (Source: Deutsches Zentrum fur Luft- und Raumfahrt e.V., Germany) (Printed with permission from Steinfeld [344])

550

8 Hydrogen Energy

In these reactions, M denotes metal and Mx Oy is the corresponding metal oxide. The first reaction is an endothermic reaction and for most metal oxides a temperature greater than 1727ıC is required to dissociate the metal oxide to metal or the lowervalence metal oxide. The second reaction is an exothermic reaction and does not need the solar energy. The U.S. Department of Energy’s Solar ThermoChemical Hydrogen (STCH) program evaluated over 400 potential cycles. From these cycles, nine of them were selected for the detailed study. The criteria used in the selection included: the chemical reactions, suitability to various solar thermal collectors (e.g., troughs, dishes, power towers) for the reaction, use of corrosive or environmentally harmful chemicals, and the projected operating efficiency. The nine cycles given in Table 8.10 can be further divided into following two categories: • High-temperature Cycles – Two-step reaction cycles – Multi-step reaction cycles • Low-temperature cycles. Although Zn/ZnO and Fe=Fe2 O3 cycles involve two reaction steps, the oxide decomposition temperature is high, requiring expensive materials of construction, innovative designs and higher operating costs. There are several metal oxides that decompose at lower temperatures compared to ZnO and Fe2 O3 , however, these oxides, when reduced, will not directly split water. Additional steps, which require relatively high temperature, are necessary. Also, various side reactions and byproducts introduce more complexity in the separation steps.

8.7.3.1 Zinc/Zinc Oxide Cycle Zinc/zinc oxide cycle is the most studied reaction for the solar decomposition of water. Zinc oxide powder is passed through a reactor heated by a solar concentrator to about 1900ıC. At this temperature, zinc oxide dissociates to zinc and oxygen. Zinc is cooled, separated, and reacted with water to form hydrogen gas and solid zinc oxide. The net result is the production of hydrogen and oxygen from water. The hydrogen is purified and stored for future use. The zinc oxide can be recycled and reused to create more hydrogen through this process. The reactions are given below: 2ZnO C heat ! 2Zn C O2 2Zn C 2H2 O ! 2ZnO C 2H2

(8.34)

Several designs of the solar reactor have been proposed in the literature. However, the rotating cavity concept proposed by Haueter et al. [349] appears to be most promising. This solar reactor is shown in Fig. 8.40. In this design, ZnO particles are deposited on a rotating window and are held there by the centrifugal force. In a

8.7

Solar Energy for Hydrogen Production

551

Table 8.10 Various potential thermochemical cycles for hydrogen production using solar energy Cycle High temperature cycles

Reactions

Zn/ZnO [347, 355–362]

ZnO

16001800ıC

!

400ı C

Zn C 1=2O2

Zn C H2 O ! ZnO C H2 FeO=Fe3 O4 [353, 363, 364]

F e3 O4

20002300ıC

!

3F eO C 1=2O2

400ı C

3F eO C H2 O ! F e3 O4 C H2 Cd=CdCO3 [349]

CdO

14501500ı C

!

Cd C 1=2O2 350ı C

Cd C H2 O C CO2 ! CdCO3 C H2 500ı C

CdCO3 ! CO2 C CdO Cd=CdO=Cd.OH/2 [349]

CdO

14501500ı C

!

Cd C 1=2O2

25ı C;Electrochemical

Cd C 2H2 O

!

375ı C

Cd.OH /2 C H2

Cd.OH /2 ! CdO C H2 O Sodium manganese [349]

M n2 O3

14001600ıC

!

2M nO C 1=2O2 627ı C

2M nO C 2NaOH ! 2NaM nO2 C H2 25ı C

2NaM nO2 C H2 O ! M n2 O3 C 2NaOH M-Ferrite (M D Co, Ni, Zn) [352, 365–387]

F e3x Mx O4

12001400ı C

!

F e3x Mx O4ı C ı2 O2 10001200ıC

F e3x Mx O4ı C ıH2 O

!

F e3x Mx O4 C ıH2

1500ı 1900ı C

Mg/MgO [388]

M gO C C ! M g C CO or; M gO C CH4 ! M g C CO C 2H2 M g C H2 O ! M gO C H2

SnO2 /SnO [389, 390]

SnO2 .s/ ! SnO.s/ C 12 .O2 /

1600ı C

550ı C

SnO.s/ C H2 O.g/ ! SnO2 .s/ C H2 CeO2/Ce2O3 [391]

2000ı C

2CeO2 .s/ ! Ce2 O3 .s/ C 12 O2 Ce2 O3 .s/ C H2 O.g/

400ı 600ı C

!

2CeO2 .s/ C H2 .g/

Low temperature cycles Sulfur-iodine [392–398]

850ı C

H2 SO4 ! SO2 C H2 O C 1=2 O2 100ı C

I2 C SO2 C 2H2 O ! 2HI C H2 SO4 300ı C

2HI ! I2 C H2 Hybrid sulfur

850ı C

H2 SO4 ! SO2 C H2 O C 1=2O2 SO2 C 2H2 O

Hybrid copper chloride [399]

77ı C;Electrochemical

!

H2 SO4 C H2

550ı C

C u2 OC l2 ! 2C uC l C 1=2O2 425ı C

2C u C 2H C l ! H2 C 2C uC l 4C uC l

25ı C;Electrochemical

!

325ı C

2C u C 2C uC l2

2C uC l2 C H2 O ! C u2 OC l2 C 2H C l Source: Adapted from Perkins and Weimer [350]

552

8 Hydrogen Energy

Fig. 8.40 Schematic of the “rotating-cavity” solar reactor concept for the thermal dissociation of ZnO to Zn and O2 at 2,300 K (Printed with permission from Paul Scherrer Institute, Switzerland; Steinfeld [344])

10 kW prototype unit, ZnO was directly exposed to high-flux solar irradiation, and a temperature of 1727ıC was achieved in 2 s and the ZnO coating did withstand the thermal shocks. Steinfeld [347] studied the efficiency of the cycle by evaluating the exergy of the system and noted that an efficiency of 29% is possible without any heat recovery. The theoretical upper limit in the exergy efficiency, with complete heat recovery during the quenching and hydrolysis, was calculated to be 82%. The unique feature of the cycle is that in the absence of nucleation sites, Zn(g) and O2 can coexist in a meta-stable state. Otherwise, these need to be quenched to avoid their recombination. Fletcher and his coworkers [400, 401] demonstrated that electrothermal separation of Zn(g) and O2 at high temperatures is possible in situ. The sensible and latent heat of the products (e.g., 116 kJ/mol during Zn condensation) can be recovered to further enhance the system efficiency. Stienfeld and co-workers [402] proposed a hybrid system to decompose metal oxides such as ZnO by CH4 according to the following reactions: Mx Oy C yC.gr/ D xM C yCO Mx Oy C yCH 4 D xM C y.2H2 C CO/

(8.35)

Reduction of the oxides can be achieved at a moderate temperature. The reduction of Fe3 O4 , MgO, and ZnO with C(gr) and CH4 , according to above reactions, has been demonstrated in a solar furnace using packed/fluidized beds and vortextype

8.7

Solar Energy for Hydrogen Production

553

Fig. 8.41 Schematic of the “two-cavity” solar reactor concept for the carbothermal reduction of ZnO. It features two cavities in series, with the inner one functioning as the solar absorber and the outer one as the reaction chamber. The inner cavity (#1) is made of graphite and contains a windowed aperture (#2) to let in concentrated solar radiation. A CPC (#3) is implemented at the reactor’s aperture. The outer cavity (#4) is well insulated and contains the ZnO/carbon mixture that is subjected to irradiation by the graphite absorber separating the two cavities. With this arrangement, the inner cavity protects the window against particles and condensable gases coming from the reaction chamber. Uniform distribution of continuously fed reactants is achieved by rotating the outer cavity (#5). The reactor is specifically designed for beam-down incident radiation, as obtained through a Cassegrain optical configuration that makes use of a hyperbolical reflector at the top of the tower to re-direct sunlight to a receiver located on the ground level (Source: Paul Scherrer Institute, Switzerland) (Printed with permission from Steinfeld [344])

reactors [403]. Two designs of the reactor have been investigated. The first reaction given in Eq. 8.35 is carried out in a two-cavity solar reactor that is operated based on the indirect irradiation of metal oxide and carbon. This design is shown in Fig. 8.41 when using ZnO. As described by Steinfeld [347], “It consists of a rotating conical cavity-receiver (#1) that contains an aperture (#2) for access of concentrated solar radiation through a quartz window (#2). The solar flux concentration is further augmented by incorporating a CPC (#3) in front of the aperture. Both the window mount and the CPC are watercooled and integrated into a concentric (non-rotating) conical shell (#4). ZnO particles are continuously fed by means of a screw powder feeder located at the rear of the reactor. The centripetal acceleration forces the ZnO powder to the wall where it forms a thick layer of ZnO that insulates and reduces the thermal load on the inner cavity walls. A purge gas flow enters the cavity-receiver tangentially at the front and keeps the window cool and clear of particles or condensable gases. The gaseous products Zn and O2 continuously exit via an outlet port to a quench device”. For the second reaction, a vortex reactor as shown in Fig. 8.42 may be used. The calculation of the equilibrium composition for various metal oxides of interest

554

8 Hydrogen Energy

Fig. 8.42 Schematic of a “vortex” solar reactor concept for the combined ZnO reduction and CH4 reforming. It consists of a cylindrical cavity (#1) that contains a windowed aperture (#2) to let in concentrated solar energy. Particles of ZnO, conveyed in a flow of NG, are continuously injected into the reactor’s cavity via a tangential inlet port (#3). Inside the reactor’s cavity, the gas-particle stream forms a vortex flow that progresses towards the front following a helical path. The chemical products, Zn vapor and syngas, continuously exit the cavity via a tangential outlet port (#4) located at the front of the cavity, behind the aperture. The window ( #5) is actively cooled and kept clear of particles by means of an auxiliary flow of gas (#6) that is injected tangentially and radially at the window and aperture planes, respectively. Energy absorbed by the reactants is used to raise their temperature to above about 1,300 K and to drive reaction (12) (Source: Paul Scherrer Institute, Switzerland) (Printed with permission from Steinfeld [344])

shows that only the carbothermic reduction of Fe2 O3 , MgO, and ZnO will result in a significant free metal formation [404]. However, various other carbides may also form. Stienfield [344] noted that these carbides have high value as byproducts. 8.7.3.2 Fe3 O4 =FeO Cycle A two-step water-splitting cycle using iron oxide (or ferrite) redox pair was developed in the early 1977. This cycle is generally called “iron oxide process” or “ferrite process”. In this cycle, Fe3 O4 is reduced to FeO by thermal decomposition. FeO then reacts with H2 O to produce hydrogen. The two reactions can be written as: Fe3 O4 ! 3FeO C 1=2 O2 H2 O C 3FeO ! Fe3 O4 C H2

(8.36)

8.9

Thermochemical Hybrid Cycles

555

The thermal reduction of Fe3 O4 to FeO proceeds at temperatures above 2227ıC under 1 bar. The second reaction thermodynamically proceeds at temperatures below 727ı C.

8.8 Electrolytic Process Water electrolysis takes place in an electrolyzer that contains an electrolyte, an anode, and a cathode. Several electrolytic processes have been demonstrated commercially. These processes are: • • • •

Solid Oxide Electrolyzers High-Temperature Electrolysis Polymer Electrolyte Membrane (PEM) Electrolysis Alkaline Electrolyzers

The first two processes have been discussed in detail earlier in this chapter. The PEM electrolysis process will be discussed in Volume 3 of this book series. The alkaline electrolysis process is described below.

8.8.1 Alkaline Electrolysis Alkaline electrolysis is a very mature technology and has been commercially available for many years. Alkaline electrolyzers use an aqueous KOH solution (caustic) as an electrolyte that typically circulates through the electrolytic cells. Alkaline electrolysers are suited for stationary applications and are available at operating pressures up to 25 bar. The following reactions take place inside an alkaline electrolysis cell: ElectrolyteW CathodeW AnodeW Overall Reaction W

4H2 O ! 4H C C 4OH 4H

C

(8.37)

C 4e ! 2H2

4OH ! O2 C 2H2 O C 4e 2H2 O ! O2 C 2H2

(8.38)

(8.39) (8.40)

Commercial electrolyzers consist of a number of electrolytic cells arranged in a cell stack. The main components of alkaline electrolyzers are shown in Fig. 8.43.

8.9 Thermochemical Hybrid Cycles A thermochemical hybrid process is a combined cycle in which both thermochemical and electrolytic reactions for water splitting have been incorporated. The hybrid process offers the possibility to run low-temperature reactions using electricity.

556

8 Hydrogen Energy

Fig. 8.43 An Alkaline electrolysis cell arrangement for hydrogen production (Adapted from International Energy Agency [405])

Fig. 8.44 Schematic diagram of the Westinghouse Sulfur Process (Adapted with permission from Yildiz and Kazimi [160])

One such cycle was developed by Westinghouse Electric Co., USA in 1975, called sulfuric acid hybrid cycle or the Westinghouse Sulfur Process (WSP). The reactions that take place in this cycle are given below: 1 H2 SO4 ! SO2 C H2 O C O2 .800ı C/ 2 2H2 O C SO2 ! H2 SO4 C H2 Electrolytic reaction .80ı C/

(8.41)

A schematic diagram of the WSP cycle is given in Fig. 8.44. As can be seen from this figure, the hydrogen is generated by electrolysis. The heat for the sulfuric acid

8.10

Hydrogen from Wind Energy

557

Fig. 8.45 Energy efficiency of Westinghouse Sulfur Process coupled with a GT-MHR nuclear reactor (Adapted with permission from Yildiz and Kazimi [160])

decomposition unit can be supplied from a nuclear reactor. The hydrogen generation section of the plant can be located away from the nuclear plant for safety reasons. At the same time, the part of the process that requires the high-temperature heat from the nuclear reactor can be kept close to the reactor. This type of plant configuration can also reduce the heat losses and the overall costs. The energy efficiency of the WSP cycle is shown in Fig. 8.45 as a function of process temperature. The nuclear reactor used in the analysis of the efficiency was a Gas Turbine-Modular Helium Reactor (GT-MHR). It is assumed that a helium gas turbine would be used for electricity generation. Helium gas turbines have significant advantages over steam turbines. Not only gas turbines offer higher efficiency, they are also smaller in size, as shown in Fig. 8.46. The smaller size requires a smaller footprint and is also easy to install. The use of supercritical carbon dioxide may offer even better economics, but it is still in the research stage.

8.10 Hydrogen from Wind Energy The electricity generated by wind energy may be used to produce hydrogen via water electrolysis [406–443]. The National Renewable Energy laboratory (NREL) performed an analysis for potential hydrogen production from wind energy via electrolysis process in the USA (see Fig. 8.47). In their calculation for the hydrogen production capacity, the capacity factor for different classes of wind was taken into account. The capacity factors are given in Table 8.11. The capacity factor is discussed in detail in Chap. 1.

558

8 Hydrogen Energy

Fig. 8.46 Size comparison of various turbine systems (Adapted with permission from Yildiz and Kazimi [160])

Fig. 8.47 Hydrogen production potential from wind energy in the USA (Source: Milbrandt and Mann [444])

8.10

Hydrogen from Wind Energy

559

Table 8.11 Wind class capacity factors based on year 2000 technology

Capacity factor 0.2 0.251 0.3225 0.394 0.394

Commercial System Efficiencies (54-67 kWh/kg)

H2 cost $/kg

$8 $7 $6 $5 $4 $3 $2 $1 $0 0 .0 $0

Class 3 4 5 6 7

Ideal System (HHV of Hydrogen 39 kWh/kg)

1

.0

$0

2

.0

$0

3

.0

$0

4

.0

$0

7

6

5

.0

$0

.0

$0

.0

$0

8

.0

$0

0

9

.0

$0

.1

$0

Electricity costs $/kWh Fig. 8.48 The cost of hydrogen production via electrolysis considering only electricity contribution (no capital, operating, or maintenance costs are included). This analysis demonstrates that regardless of any additional cost elements, electricity costs will be a major price contributor to the price of hydrogen produced via electrolysis (Printed with permission from Levene et al. [430])

The other assumptions are as follows: • 100% excluded are all national park service areas, fish and wildlife service lands, all federal lands with special designations (parks, wilderness and study areas, wildlife refuges, wildlife areas, recreational areas, battlefields, monuments, conservation areas, recreational areas, and wild and scenic rivers), conservation areas, water, wetlands, urban areas, and airports/airfields. The land areas also exclude a 3 km surrounding perimeter. • 50% exclusions were applied to the remaining forest service lands, Department of Defense lands, and non-ridge crest forest. • This study also excluded areas with slopes greater than 20% for the highresolution data. These areas are considered too steep for siting wind turbines. The main route for the hydrogen production using renewable energy sources is via electrolysis. H2 cost is dependent on the cost of electricity that is generated by renewable energy sources. Levene et al. [430] estimated H2 production costs as a function of the cost of electricity, and, as expected, the H2 production cost increased with the increase of the electricity cost (see Fig. 8.48). Electricity generated from renewable energy sources, except from hydropower, costs more than from conventional sources such as coal and nuclear. Although water electrolysis

560

8 Hydrogen Energy

process is 80–85% efficient for H2 production, when combined with the efficiency of electricity generation, which is around 30–40%, the overall efficiency becomes only 25–32%.

8.11 Hydrogen from Biomass Various routes explored for hydrogen production from biomass [445,446] are shown in Fig. 8.49. These processes are described in Chap. 6. There is an inherent problem associated with hydrogen production from biomass. The yield of hydrogen is low due to the low hydrogen content in biomass, which is approximately 6% versus 25% for methane. The energy content is also low due to the 40% oxygen content of biomass. Biomass has long been considered a leading near term source for renewable hydrogen. A conservative estimate shows that the near-term economic potential of the annual hydrogen production from biomass in the USA is about 40 million tons, which would supply fuel for 150 million fuel cell vehicles. The distribution of biomass resources for H2 production in the United States is shown in Fig. 8.50. The cost for growing, harvesting and transporting biomass is high. As a result, at the present time, the biomass route for hydrogen production is uneconomical even with reasonable energy efficiencies. Unless the by-products have high commercial value, it may not be competitive with the natural gas steam reforming process.

8.12 Photolytic Processes Photolytic processes are defined as those processes in which the solar energy is used indirectly to split water into H2 and O2 [448–457]. Two photolytic processes that are currently investigated show promise for the future. These processes are: • Photobiological Water Splitting • Photoelectrochemical Water Splitting

8.12.1 Photobiological Water Splitting In this process, hydrogen is produced from water using sunlight and specialized microorganisms, such as green algae and cyanobacteria. The process may be divided into two main steps: photosynthesis and biophotolysis. During photosynthesis, plants convert solar energy to biochemical energy by a photochemical reaction that utilizes CO2 and H2 O according to the following reaction: CO2 C H2 O C light ! 6.CH 2 O/ C O2 (8.42)

Fig. 8.49 Various methods for hydrogen production from biomass (Milne et al. [447])

8.12 Photolytic Processes 561

562

8 Hydrogen Energy

Fig. 8.50 Hydrogen production potential from biomass at different region of the USA (Source: Milbrandt and Mann [444])

The reaction may be referred to as reduction or fixation of CO2 to organic compounds such as sugar phosphates. Approximately, 114 kcal of free energy are stored in plant biomass for every mole of CO2 fixed during photosynthesis. Under certain conditions, some microalgae and cyanobacteria can utilize biochemical energy to produce molecular hydrogen directly. Both hydrogenase and nitrogenase enzymes are capable of hydrogen production [458–500]. 8.12.1.1 Hydrogenase Enzyme-Catalyzed Hydrogen Production Gaffron and Rubin [500] noted that Green alga, Scenedesmus, can produce molecular hydrogen under light conditions after being kept under anaerobic and dark conditions. The mechanism for hydrogen production via hydrogenase-pathway may be described by the scheme shown in Fig. 8.51. The reaction involved in the H2 production may be written as shown below. An electron carrier is necessary for the reaction to proceed. 2H C C 2Xred uced ! 6H2 C 2Xoxid i zed

(8.43)

where X is the electron carrier. It is often not clear how the electron transfer occurs. The ferredoxin (fd) is considered to be the electron carrier. Since ferredoxin is

8.12

Photolytic Processes

563

Fig. 8.51 Hydrogenase-mediated hydrogen production (Source: Miyamoto [501])

reduced with water as an electron donor by the photochemical reaction, green algae are theoretically water-splitting microorganisms. The process is very slow, and the challenge is to increase the hydrogen yield through development of sophisticated engineering processes before commercial scale production. One of the main issues is how to reduce the reactor area. A schematic of such a process is shown in Fig. 8.52.

8.12.1.2 Nitrogenase-Enzyme Catalyzed Hydrogen Production Nitrogen-fixing cyanobacterium, Anabaena cylindrica, is capable of producing hydrogen and oxygen gas simultaneously under an inert atmosphere. Hydrogen production occurs as a side reaction at a rate of one-third to one-fourth that of nitrogen-fixation, even in a 100% nitrogen gas atmosphere. The reactions can be described as follows: N2 C 6H1C C 6e ! 2HN 3

(8.44)

12ATP • 12.ADP C P i /

(8.45)

2H

C

C 2e ! H2

4ATP • 4.ADP C P i /

(8.46) (8.47)

A schematic diagram showing the H2 production mechanism is given in Fig. 8.53.

8.12.2 Photocatalytical Processes Photocatalytic splitting of water into H2 and O2 using sunlight was first reported by Fujishima and Honda in 1971 [502]. The mechanism for photocatalysis of water is shown in Fig. 8.54.

Fig. 8.52 Photobiological method for hydrogen production (International Energy Agency [405])

564 8 Hydrogen Energy

8.12

Photolytic Processes

565

Fig. 8.53 Nitrogenase-mediated hydrogen production in heterocystous cyanobacteria (Source: Miyamoto [501]) Fig. 8.54 Working principle of photocatalysis for hydrogen production from water

In this process, the catalyst, which is a semiconductor material, absorbs sunlight leading to creation of electrons and holes in the conduction band and valance band, respectively [502–552]. The process is described in Chap. 2. The photo-generated electrons and holes cause redox reactions similar to electrolysis. Water molecules are reduced by electrons to form hydrogen and are oxidized by the holes to form oxygen for the overall reaction. Almost half of the energy that reaches us from the sun arrives as visible light. Therefore, recent research focus is to develop photocatalyst capable of direct splitting of water using visible light. The quantum yield of the process is 0.66%, and a better photocatalyst is necessary for commercial viability of this process. Titanium oxide is found to exhibit high activity as a photocatalyst for the decomposition of water. Modification of TiO2 by doping it with various compounds was investigated by a number of researchers to enhance its activity. A list of various photocatalysts has been compiled by Kudo [449] and is presented in Table 8.12.

566

8 Hydrogen Energy

Table 8.12 Various types of photocatalysts used by researchers for hydrogen production UV responsive photocatalysts Overall water splitting ZnNb2 O6 Sr2 Nb2 O7 Cs2 Nb4 O11 Ba5 Nb4 O15 ATaO3 (A: Li, Na, K) NaTaO3 : A (ADLn, Ca, Sr, Ba) ATa2 O6 (ADMg, Ca, Sr, Ba) Sr2 Ta2 O7 K3 Ta3 Si2 O13 K3 Ta3 B2 O12 K2 LnTa5 O15 AgTaO3

Visible light responsive photocatalysts H2 -evolution O2 -evolution (sacrificial) (sacrificial) SrTiO3 : Cr,Sb TiO2 : Cr,Sb SrTiO3 : Cr,Ta TiO2 : Ni,Nb SrTiO3 : Rh PbMoO4 : Cr SnNb2 O6 BiVO4 ZnS: Cu Bi2 MoO6 Bi2 WO6 ZnS: Ni AgNbO3 ZnS: Pb,Cl

Overall water splitting SrTiO3 : RhBiVO4 SrTiO3 : RhBi2 MoO6 SrTiO3 : RhWO3

Ag3 VO4 In2 O3 .ZnO/3

NaInS2 AgGaS2 CuInS2 AgInS2 -ZnS In2 O3 .ZnO/3

Source: Kudo [449]

8.13 Cost of Hydrogen Production Although hydrogen can be produced from a variety of sources, its production cost will determine the use of a particular source. Hydrogen produced by steam reformation costs approximately three times more than the cost of natural gas per unit of energy produced. If natural gas costs $6/million BTU, then the cost of hydrogen will be $18/million BTU. Electrolysis processes with electricity at 5 cents/kWh will cost $28/million BTU. The cost of hydrogen production from electricity is a linear function of electricity costs. The cost and performance characteristics of various hydrogen production processes are given in Table 8.13.

8.14 Hydrogen Storage Hydrogen can be stored both is gaseous and liquid states. However, for its use in automobiles, an onboard storage system that can store hydrogen safely is necessary. Although a number of technologies have been explored for onboard storage of hydrogen for automobile applications, a number of issues that include safety, cost, storage capacity, storage volume, and mass of the storage system still need to be

8.14

Hydrogen Storage

567

Table 8.13 A listing of the cost and performance characteristics of various hydrogen production processes

Process Steam methane reforming (SMR) Methane/ NG pyrolysis H2 S methane reforming Landfill gas dry reformation Partial oxidation of heavy oil Naphtha reforming Steam reforming of waste oil Coal gasification (TEXACO) Partial oxidation of coal Steam-iron process Grid electrolysis of water Solar & PV-electrolysis of water High-temp. electrolysis of water Thermochemical water splitting Biomass gasification Photobiological Photolysis of water Photoelectrochemical decomp. of water Photocatalytic decomp. of water

Energy required (kWh=Nm3 ) Ideal Practical

Status of tech.

Efficiency (%)

Costs relative to SMR

0.78

2–2.5

Mature

70–80

1

1.5

–

R&D to mature R&D R&D

72–54 50 47–58

0.9 <1 1

0.94

4.9

Mature

70

1.8

Mature R&D

75

<1

8.6

Mature

60

1.4–2.6

4.9

Mature R&D R&D

55 46 27

1.9 3–10

R&D to mature

10

>3

R&D

48

2.2

Early R&D

35–45

6

R&D Early R&D Early R&D Early R&D

45–50 <1 <10

2.0–2.4

1.01

3.54

Early R&D

Source: T-Raissi and Block [553]

resolved before its commercial use. Among these concerns, safety is the main issue. Various storage systems are listed below: • Gaseous Hydrogen Storage System – High pressure cylinder – Glass microspheres • Liquid Hydrogen Storage System – Cryogenic liquid hydrogen – NaBH4 solutions – Rechargeable organic liquids

568

8 Hydrogen Energy

• Carbon and Other High surface Area Materials – – – – –

Activated charcoals Carbon nanotubes Graphite nanofibers MOFs, Zeolites, etc. Clathrate hydrates

• Hydrides – Encapsulated NaH – LiH and MgH2 slurries – CaH2 , LiAlH4 , etc The amount of hydrogen that can be stored in these materials per volume or per weight basis is critical in choosing a storage medium. The volumetric and gravimetric H2 densities of some of the most common storage options are shown in Fig. 8.55. There are other system requirements that need to be fulfilled and they are discussed in the following section.

8.14.1 High Pressure Cylinder The most common method for storing hydrogen in gaseous form is in high pressure cylinders, which are classified into four categories. The classification is based mainly on construction materials and is given below: Type I: Type II: Type III: Type IV:

Metal tanks. Metal tanks wrapped with filament, such as glass fiber, around the cylindrical part. Composite material tanks with a metal liner. Composite tanks (mainly made of carbon fiber) with a polymer liner.

Although metal tanks are suitable for stationary storage, Type III and Type IV are more suitable for onboard storage. Lightweight composite tanks are now designed to endure higher pressures, up to 700 bar (10,000 psi). The density of compressed H2 under cryogenic conditions is shown in Fig. 8.56. A comparison of these four types of cylinders is provided in Fig. 8.57.

8.14.1.1 Composite Tanks Type IV composite tanks offer several advantages over other types of tanks. These are light weight, less expensive, and exhibit longer life span. A schematic diagram of a typical high-pressure, carbon fiber-wrapped H2 storage composite tank designed by Quantum Inc [557] is shown in Fig. 8.58. These tanks are designed for 350 bar (5000 psi) and are already commercially available. The design of tanks that can

0

20

40

60

80

100

120

140

160

0

20000

300 K, 1.5 bar

FeTiH1.7

620 K, 1 bar

5000

dec. 680 K

NaBH4

DOE 2010 System Goal

2000 800

15

500

bp. 272 K

Gravimetric H2 density [mass%]

10

1000

dec. 580 K dec. 400 K

liq.

liq.

C4H10

C8H18

dec.373 K m.p. 208 K

b.p. 112 K

20

Gas

H2 physisorbed on carbon

liq. H2 20.3 K

liq.

CH4

H2 chemisorbed on carbon

0.7 g cm−3

25

Pressurized H2 (Composit material) p [bar] 200 130

LiBH4 dec. 553 K

1 g cm−3 A1(BH4)3

dec. 650 K

LiH

α-A1H3

KBH4 LiA1H4

620 K, 5 bar

dec. >520 K

5

H0.95

nano

2 g cm−3

MgH2

NaA1H4

Mg2NiH4 550 K, 4 bar

300 K, 2 bar

LaNi5H6

<373 K, 1 bar

C

Mg2FeH6

5 g cm−3

BaReH9

density:

Hydrogen Storage

Fig. 8.55 Hydrogen storage capacity of various materials and systems (Adapted from Sandrock [554])

Volumetric H2 density [kg H2 m−3]

8.14 569

8 Hydrogen Energy 50

100 3.25

3.00 2.50 kWh/kg

2.75 kWh/kg

90

2000

H2 density (kg/m3)

80 100

70

0a

70 0a

0a

25

tm

tm

35

50

2.25

tm

0a

50

60

atm

tm

0a

tm

100

2.00

40 30

1.75

200

20 1.50 kWh/kg

10 0 20

500

1.25 1.00

60

100

140

180

220

260

Volume (liters) occupied by 5 kg of hydrogen

570

300

Temperature, K Fig. 8.56 Density of cryo-compressed hydrogen (Adapted from Aceves et al. [555])

Fig. 8.57 Various types of tanks for high pressure hydrogen storage (Printed with permission from Mori and Hirose [556])

Fig. 8.58 A typical composite tank for storing compressed hydrogen (International Energy Agency [405])

8.14 Hydrogen Storage 571

572

8 Hydrogen Energy

be pressurized up to 700 bar (10,000 psi) is underway. These tanks must meet international codes that are accepted in several countries for pressures in the range of 350–700 bar. Composite tanks require no internal heat exchanger and may be usable for cryogas. The compression of hydrogen is rather energy intensive. The work necessary to compress a gas depends on the thermodynamic process that is followed to compress it. The isothermal compression requires the least energy; however, it is difficult to maintain the isothermal condition. The work necessary during isothermal compression may be expressed by the following equation: W D Pi Vi `n

Pf Pi

(8.48)

where, W D Specific compression work (J/kg) Pi D Initial pressure (Pa) Pf D Final pressure (Pa) Vi D Initial specific volume (m3 =kg) Since, it is extremely difficult to maintain isothermal conditions, generally, an adiabatic compression process is followed. The work input necessary during the adiabatic compression of an ideal gas is expressed by: ! 1 Pf W D 1 Pi Vi 1 Pi

(8.49)

where, is the ratio of specific heats. For hydrogen, is 1.41. As shown in Fig. 8.59, the energy consumed during the adiabatic compression of hydrogen is significantly higher than that during isothermal compression. In order to reduce the energy consumption, multistage compression with cooling between each compression stage is used. Various aspects of gas storage systems are discussed by several researchers [558–562]. The disadvantages of hydrogen storage in high pressure cylinders include: the requirement of large physical volume, difficult to fit the system in the available space, and high cost .500–600 USD=kg H2 /. Safety issues still have not been resolved, particularly the problem of rapid loss of H2 in an accident. The longterm effect of hydrogen on materials under cyclic or cold conditions is also not fully understood. 8.14.1.2 Glass Microspheres Teitel [563] at the Brookhaven National Laboratory, USA, suggested that hollow glass micro spheres could be filled with hydrogen gas at high pressures [563, 564]. Following loading of H2 at high pressure (350–700 bar) and high temperature

Hydrogen Storage

Compression Energy to HHV

8.14

573

20% 15% 10% 5% 0% 0

200

400

600

800

Final Pressure [bar]

Fig. 8.59 Energy consumption during compression of hydrogen (Source: Eliasson and Bossel [562])

(300ıC) by the permeation process, microspheres are next cooled to room temperature for storage. Hydrogen is released from microspheres by heating at 200–300ıC. The main challenge to this process is the preparation of hollow glass spheres that can withstand high pressure. A thicker wall glass sphere may be necessary. However, this may require higher temperatures for the release of hydrogen from the glass spheres.

8.14.2 Liquid Hydrogen Storage System 8.14.2.1 Cryogenic Liquid Hydrogen (LH2) The most common method to store hydrogen in a liquid form is to cool it down to a cryogenic temperature .253ıC/ [565–574]. Other storage options include dissolving hydrogen as a constituent in other liquids, such as NaBH4 solutions, rechargeable organic liquids, or anhydrous ammonia (NH3 ). Hydrogen can be liquefied at 253ı C (normal boiling point of hydrogen at atmospheric pressure) and stored in specially designed cylinders. Liquid hydrogen (LH2) has a density of 70:8 kg=m3 at normal boiling point (253ıC). Liquid hydrogen has a much better energy density than the pressurized gas. The liquefaction of hydrogen at this cryogenic temperature requires about 30–40% of its heating value. The other main disadvantage with LH2 is the boil-off loss during dormancy. The structure of a cryogenic, liquid hydrogen storage container is shown in Fig. 8.60. Several methods are available for cryogenic compression of gaseous hydrogen to liquid hydrogen. One such compression process is shown in Fig. 8.61. In this process, liquid nitrogen is used for cooling hydrogen streams between compressors. Several compression stages are necessary to liquefy hydrogen. The process is also very energy intensive. The majority of the cost is associated with the capital investment. Although the operating and maintenance costs of a hydrogen liquefaction plant are about 12% of the total plant cost, the power costs are about 30% of the total cost.

574

Fig. 8.60 Liquid hydrogen storage tank (Courtesy of Linde group [575])

Fig. 8.61 Hydrogen liquefaction process (Source: Shimko [576])

8 Hydrogen Energy

8.14

Hydrogen Storage

575

8.14.2.2 NaBH4 Solutions Borohydride .NaBH4 / solutions can be used as a liquid storage medium for hydrogen. The release of hydrogen takes place by reacting it with water in the presence of a catalyst. The catalytic hydrolysis reaction is given below: NaBH4 .l/ C 2H2 O.l/ ! 4H2 .g/ C NaBO2 .s/

(8.50)

The theoretical maximum hydrogen energy storage density for NaBH4 is 10.9 wt% H2 . The NaBH4 solutions can be stored safely onboard and controllable onboard generation of H2 is possible. The main disadvantage is that the reaction product NaBO2 must be regenerated back to NaBH4 off-board. Currently, the use of NaBH4 solutions in vehicles is expensive due to the regeneration cost. Millennium Cell in the U.S. and MERIT in Japan, however, are promoting this technology, particularly for specialized applications.

8.14.2.3 Rechargeable Organic Liquids Several organic liquids can also be used to indirectly store hydrogen in the liquid form. The basic concept is as follows. An organic liquid can be dehydrogenated by a catalytic process to produce H2 gas onboard. The dehydrogenated organic compound will be next rehydrogenated in a central processing plant. For example, methylcyclohexane (C7 H14 ) can be used as the carrier of hydrogen. The dehydrogenation reaction produces toluene (C7 H8 ) and proceeds as follows: C7 H14 .l/ , C7 H8 .l/ C 3H2 ; .Tdehyd D 300 400ı C/

(8.51)

8.14.3 Carbon and Other High Surface Area Materials Storage of hydrogen in porous solid materials (called adsorbents) can be a safe and efficient way of storing hydrogen, particularly for onboard storage. Among various porous solids, carbon-based adsorbents including carbon nanotubes and graphite nanofibers are considered to have the best capacity per mass basis [577– 612]. A variety of carbonaceous adsorbents that include activated carbon, carbon fibers, carbon nanotubes, fullerenes, and graphite nanofibers have been investigated for their hydrogen adsorption capacity. Although some initial studies reported the adsorption capacity in the range of 30–60% on per unit mass basis, later studies, both theoretical and experimental, concluded that the adsorption capacity of carbon based adsorbents could be only in the 2–4% range. The adsorption capacity depends mainly on the surface area and temperature. Several studies indicated that a new bonding mechanism with energies between physisorption and strong covalent chemisorption may be necessary to increase adsorption capacity at room

576

8 Hydrogen Energy

temperature. The surface and bulk properties needed to achieve practical room temperature storage are not clearly understood, and it is far from certain that useful carbon can be economically and consistently synthesized. A number of studies focused on the modification of carbon or carbon nanotubes for enhancement of hydrogen storage [613–641]. Although it was assumed that carbon based materials might be the best adsorbents for hydrogen due to their large surface area, several studies were focused on the development of non-carbon based porous adsorbents [642–656]. However, the hydrogen storage capacity remained in the same range as that of carbon based adsorbents. In Table 8.14, the hydrogen storage capacity of carbon based materials reported by various researchers is summarized. Thomas [657] compiled data on the hydrogen storage capacity of various non-carbon adsorbents and plotted it as a function of the surface area of adsorbents (see Fig. 8.62).

8.14.4 Clathrates Clathrates are compounds in which guest molecules are incorporated inside the cage of clathrates through a hydrogen bonded water network [706–728]. Rovetto et al. [729] noted that a binary H2 /Tetrahydrofuran binary clathrate can store hydrogen at pressures nearly two orders of magnitude lower than that in pure hydrogen hydrates. Fundamental understanding of the structures of the clathrates and the hydrogen formation and release mechanism is underway. Lee et al. [730] reported that hydrogen storage capacities in THF-containing binary-clathrate hydrates can be increased to about 4 wt% at modest pressures by tuning their composition to allow the hydrogen guests to enter both the larger and the smaller cages, while retaining low-pressure stability.

8.14.5 Hydrides Metal hydrides have the potential for on-board hydrogen storage. Hydrogen can be loaded to a number of metals (i.e., formation of hydride) at a relatively low temperature, and released at higher temperatures and pressures [647,656,731–741]. Sandrock [554] developed a hydride family tree incorporating various types of hydrides compounds that have potential for use as a hydrogen storage medium. However, hydrogen loading and release mechanisms of all the hydrides are not the same, and these may be divided into following categories based on how hydrogen is released from them. • • • • •

Rechargeable Hydrides Alanates Borohydrates Water Reactive Hydrides Thermally Activated Hydrides

8.14

Hydrogen Storage

577

Table 8.14 Summary of literature on adsorbtion of hydrogen by carbon based adsorbents Adsorbent type SWNT SWNT SWNT SWNT SWNT SWNT SWNT MWNT MWNT Li doped MWNT K doped MWNT Li doped MWNT K doped MWNT GNF GNF GNF Purified SWNT As Prepared MWNT SWNT SWNT SWNT MWNT MWNT MWNT MWNT AC K3a AC K5b AX 21.AC/c KUA 6.AC/d Maxsorb-3000 .AC/e CB850h.TC/f TCg Pure OMC Pd-OMC (1% Pd) Pd-OMC (10% Pd) Pt-OMC (1% Pt) Pt-OMC (10% Pt) GNF Graphite nano-fiber Surface area a 2009 m2 =g b 3190 m2 =g c 2800 m2 =g d 3808 m2 =g e 3178 m2 =g f 3150 m2 =g g 2535 m2 =g

Hydrogen storage capacity (wt%) 11 2 6.5 5–10 8 10 4 5 0.25 20 14 2.5 1.8 6.5 6.5 10 1.7 0.2 3.5–4.5 1–15 1.2 56 0.68 13.8 0.7–0.8 5.63 7.08 4.6 5.6 5.2 6.9 5.3 0.024 0.064 0.083 0.069 0.099

Temperature (K) 80 80 300 300 80 300 300 300 300 200–400 300 200–400 300 300 300 300 292 293 Ambient 77–300 Ambient 298 Ambient 300 300 77 77 77 77 77 77 77 298 298 298 298 298

Pressure (MPa) 10 10 16 0.04 8 0.04 12 10 0.1 0.1 0.1 0.1 0.1 12 12 12 12.2 12.04 0.067 8 4.8 12.16 10 1.0 7 20bar 20bar 10bar 40bar 40bar 20bar 20bar 0.011 0.011 0.011 0.011 0.011

Reference [658] [659] [660] [661] [662] [663] [664] [665] [666] [667] [667] [668] [669] [670] [671] [672] [673] [673] [674] [675] [676] [677] [678] [679] [680] [681] [681] [682] [683] [683] [684] [685] [686] [686] [686] [686] [686]

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8 Hydrogen Energy

Fig. 8.62 Hydrogen storage capacity of several adsorbents. Carbon materials (solid symbols): (“) Nijkamp et al. [687], (•) Pang et al. [688], () Parra et al. [689], (F) Takagi et al. [690, 691], (N) Zhao et al. [692,693], (‘) Schimmel et al. [694,695], (H) Texier-Mandoki et al. [696], ( ) Gadiou et al. [697], (I) Gogotsi et al. [698]. Silicas, alumina, zeolites (cross symbols): (C) Nijkamp et al. [687], (x) Takagi et al. [691]; MOFs (open symbols): () Rowsell and Yaghi [699], () Chapman et al. [700], (4) Chun et al. [701], (˙) Kaye and Long [702], Dybtsev et al. [703], () Chen et al. [704], ( ) Dietzel et al. [705]. Surface areas obtained mainly from the BET method [657]

8.14.5.1 Rechargeable Hydrides A number of metals and their alloys are capable of reversibly absorbing large amounts of hydrogen [742–791]. Charging can be done using either molecular hydrogen gas or hydrogen atoms from an electrolyte. Two methods may be used for charging/loading H2 onto metals: Gas phase reaction with molecular hydrogen: M C 1=2 xH 2 , MH x C heat

(8.52)

M C xH 2 O C xe , MH x C xOH

(8.53)

Electrochemical reaction:

8.14

Hydrogen Storage

579

Fig. 8.63 Pressure-composition-isotherms (left) of a typical hydrogen absorption or desorption process and corresponding van’t Hoff plot (Printed with permission from Principi et al. [792])

It is generally assumed that hydrogen is present in the form of atoms, never as molecules, on interstitial sites of the host metal lattice. Molecular hydrogen that is used as feed dissociates at the metal surface before absorption. Two hydrogen atoms recombine to H2 during the desorption process. The formation of hydride from gaseous hydrogen can be described by using pressure-composition isotherms of the hydride system (see Fig. 8.63). The stability of the hydrides and their decomposition temperature can be studied more accurately using this type of diagram. A flat plateau region in the isotherm is the indication of the coexistence of the solid solution and hydride phase. The amount of hydrogen that can be stored in the metal can be estimated from this plateau region. The hydrides generally exist as ˛ and ˇ phase. In the pure ˇ-phase, the H2 pressure rises steeply with the concentration. The two-phase region ends in a critical point .TC /, above which the transition from ˛ to ˇ-phase is continuous. The change of enthalpy that is critical in determining the energy release during charging and the heat input necessary during desorption from metal may be calculated from the von’t Hoff equation and is given below: ! Peq

H 1

S `n D (8.54) 0 Peq R T R 0 and Peq are equilibrium pressures at some standard state and at the where Peq system conditions, respectively, H is the enthalpy, S is the entropy, R is the universal gas constant, and T is the system temperature. The entropy change corresponds mostly to the change from molecular hydrogen gas to dissolved solid hydrogen and can be approximated as the entropy of H2 at the standard state and is generally used for all metal hydride systems. The heat released during hydride

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8 Hydrogen Energy

Fig. 8.64 Possible metal hydrides and their classification for hydrogen storage (Source: Sandrock [554])

formation is given by Q D T S . Theoretically, the same amount of heat would be necessary during decomposition of the hydride for releasing H2 . For a stable hydride like MgH2 , the heat necessary for the desorption of hydrogen at 300ıC and 1 bar is approximately 25% of the higher heating value of hydrogen. A part of hydrogen is burned to provide the necessary energy during decomposition. The Energy Information Administration-Hydrogen Implementing Agreement has developed a large database for hydrides with information about their properties (IEA HIA Annex 17; http://hydpark.ca.sandia.gov). A summary of this database (the metal hydride “family tree”) is provided in Fig. 8.64.

8.14.5.2 Alanates Alanates are hydrides containing AlH 4 ions [793–803]. There are about 18 alanates reported in the literature that have potential for use as a H2 storage medium. Hydrogen storage density of some of the alanates ranges from 7% to 10.5% by weight and potentially can meet the US Department of Energy’s requirement for onboard hydrogen storage. The hydrogen storage capacity and desorption temperature of potential alanates are provided in Table 8.15.

8.14

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581

Table 8.15 Storage density and desorption temperature of common alanates Type Storage densitya , wt% H2 Desorption temperature, ı C LiAlH4 10.6 190 NaAlH4 7.5 100 9.3 140 Mg.AlH4 /2 7.8 >230 Ca.AlH4 /2 Source: International Energy Agency [405] a Theoretical maximum

Among these alanates, NaAlH4 has been studied most to get a better understanding of the properties of alanates. Alanates can be synthesized by a number of processes. The synthesis reactions for some alanates are shown below: et her

4LiH C AlCl3 ! LiAlH 4 C 3LiCl 140ı C;5000psiH2

3 NaH C Al C H2 ! NaAlH 4 2 140ı C;5000psiH2

Na C Al C 2H2 ! NaAlH 4 270280ı C;2539psiH2

Na C Al C 2H2 ! NaAlH4 Bal l M i l li ng

MH C AlH 3 ! MAlH 4

(8.55) (8.56) (8.57) (8.58) (8.59)

The decomposition reactions are shown below: 1 2 Na3 AlH 6 C Al C H2 3 3 3 Na3 AlH 6 ! 3NaH C Al C H2 2

NaAlH 4 !

(8.60) (8.61)

The kinetics and reversibility of the decomposition reaction are not favorable for the onboard hydrogen source. The low-temperature kinetics and reversibility of these alanates are improved by adding a catalyst, mainly metal titanium or a titanium based compound. The addition of titanium, or any other catalyst reduces the per mass basis hydrogen capacity of the material. Other issues are pyrophoricity and high cost.

8.14.5.3 Borohydrides Borohydrides may be expressed by the general formula: Mx .BH4 /y . Potentially, borohydrides have much higher hydrogen storage capacity than alanates [804–812]. However, these are not inherently reversible and have high stability. They can also

582

8 Hydrogen Energy Table 8.16 Hydrogen storage capacity and desorption temperature of some borohydrides Borohydride LiBH4 NaBH4 KBH4 Be.BH4 /2 Al.BH4 /3 Mg.BH4 /2 Ca.BH4 /2

Storage capacity, wt% H2 18.5 10.6 7.4 20.8 16.7 14.9 11.6

Desorption temperature, ı C 300 350 125 125 200 320 260

Source: Sandrock and International Energy Agency [405, 554]

produce diborane with H2 . The storage capacity and desorption temperature of several promising borohydrides are given in Table 8.16. Hydrogen can be released from borohydrides either through thermal decomposition or via hydrolysis reaction. The thermal decomposition steps can be expressed by the following reactions: 2MBH 4 $ 2MH C B2 H6 2MH $ 2M C H2 B2 H6 $ 2.BH/n C 2H2

(8.62)

A high temperature is necessary for thermal decomposition. Some borohydrides would melt before the start of the decomposition step. This may cause significant problems in designing the storage system. Hydrogen release from borohydrides via the hydrolysis reaction may be a better route. The hydrolysis reactions may be expressed as follows: MBH 4 C 2H2 O ! MBO2 C 4H2 MBH 4 C 4H2 O ! MB.OH/4 C 4H2

(8.63)

Several alkaline borohydrides can form aqueous or alcoholic solutions that can be stored in a tank in automobiles, just like a regular gasoline storage system. Hydrogen can be released from the solution on demand by using a catalyst. One of the issues of hydrolysis reaction is that anhydrous borate is never produced. As a result, the total amount of hydrogen released in the reaction is much lower than the theoretical amount. For example, 10.8% by mass of hydrogen could be obtained from NaBH4 if anhydrous borate .NaBO2 / is formed. This amount drops to 7.3% by mass if dihydro-borate (NaBO2 2H2 O) is formed. It drops further to 5.5% by mass if tetrahydrate is formed (NaBO2 4H2 O).

8.14

Hydrogen Storage

583

Table 8.17 Hydrogen release by hydrolysis of chemical hydrides Hydrolysis reaction Storage densitya , wt% H2 LiH C H2 O ) H2 C LiOH 7.8 NaH C H2 O ) H2 C NaOH 4.8 6.5 M gH2 C 2H2 O ) 2H2 C M g.OH /2 5.2 CaH2 C 2H2 O ) 2H2 C Ca.OH /2 Source: International Energy Agency [405] a Theoretical maximum Table 8.18 Thermal decomposition reaction of chemical hydride Decomposition reaction NH4 BH4 ) NH3 BH3 C H2 NH3 BH3 ) NH2 BH2 C H2 NH2 BH2 ) NHBH C H2 NHBH ) BN C H2

Storage densitya , wt% H2 6.1 6.5 6.9 7.3

Decomposition temperature, ı C <25 <120 >120 >500

Source: Autrey (2004) DOE EERE Program Review. International Energy Agency [405] a Theoretical maximum

8.14.5.4 Chemical Hydrides (H2 O-Reactive) A number of metal hydrides readily react with water, releasing hydrogen. For onboard storage applications, the solid hydride must be stored separately and mixed with water on demand for hydrogen production. The feeding of solid hydrides into the reaction chamber is challenging. However, attempts have been made to form a semi-liquid (i.e., slurry) by mixing the hydrides with mineral oil. In this form, hydrides can be pumped and safely handled. Controlled injection of H2 O during vehicle operation can be used to generate H2 via hydrolysis reactions. The hydrolysis reaction is exothermic and does not require heat from the vehicle’s power source. However, the reaction product is an alkaline solution that is extremely corrosive. Disposing of the alkaline solution in a refueling station would require special handling and a storage facility. An overview of the hydrolysis reactions for the most common chemical hydrides is presented in Table 8.17. The theoretical storage density of these hydrides is around 5–8 wt% H2 .

8.14.5.5 Chemical Hydrides (Thermal) Several hydrides can be decomposed thermally to generate hydrogen. Ammonia borane is one such hydride that can be used to store hydrogen in a solid state. The decomposition takes place in four steps and at different temperatures. The hydrogen release at each step is different. These data are summarized in Table 8.18. The reactions are not reversible, and onboard regeneration is not possible.

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8 Hydrogen Energy

Temperature, ⬚C 1000

10000

600

400

300

200

100

Dissociation Pressure, atm

1000

50

25

NiH

100 VH2 10 PdH0.6

TiH2

1

ZrH2 CaH2

MgH2

0.1 Th4H15 0.01 LaH2 0.001

UH3

YH2 0.5

NaH

ThH2 1

1.5

2

2.5

3

3.5

1000/T, K−1 Fig. 8.65 van’t Hoff lines for desorption of elemental hydride (Source: Sandrock [554])

8.14.5.6 Desorption of Hydrogen from Hydrides Hydrogen take-up by metals and formation of metal hydrides generally occur at a low temperature and pressure, even at room temperature and at atmospheric pressure (1 bar). Some metals absorb hydrogen only at elevated pressures and temperatures. However, a much higher temperature is necessary for the desorption (release) of hydrogen. The temperature and pressure required for dissociation of hydride for the release of hydrogen can be calculated from the van’t Hoff plots. Such plots for several types of metals hydrides are shown in Figs. 8.65 and 8.66. A summary of hydrogen uptake capacity of various hydrides as a function of temperature is given in Fig. 8.67.

8.14.5.7 Borane Boranes are chemical compounds of boron and hydrogen [813–825]. There are several borane compounds with a general formula of Bx Hy . The simplest borane, B2 H6 , is spontaneously flammable in air. Higher boranes, such as B10 H14 , are very stable in air, water, and heat. However, pure boranes do not have the required capacity for hydrogen on a weight basis. A number of researchers have been

8.14

Hydrogen Storage

585

Temperature, ⬚C 200

150

100

50

25

−25

0

100 MmNi4.15Fe0.85

LaNi5

Dissociation Pressure, atm

−50

CaNi5 10 MmNi5

1 LaNi4.25Al0.75 MmNi4.5Al0.5

MmNi3.5Co0.7Al0.8

LaNi4.8Sn0.2

0.1 2

2.5

3

3.5

4

4.5

1000/T, K−1 Fig. 8.66 van’t Hoff lines for desorption of representative AB5 hydrides (Source: Sandrock [554])

Fig. 8.67 Hydrogen storage capacity of various materials at different temperatures (Source: Sandrock [554])

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8 Hydrogen Energy

Table 8.19 Comparison of 3 kg (or 215 km range) hydrogen storage by various medium Technology 5,000 psi compressed H2 tanks 10,000 psi compressed H2 tanks Metal hydrides Liquid H2 Source: Sirosh [827]

Storage system volume 145 L 100 L 55 L 90 L

Storage system weight 45 kg 50 kg 215 kg 40 kg

Table 8.20 Comparison of 7 kg (or 700 km range) hydrogen storage by various medium Technology 5,000 psi compressed H2 tanks 10,000 psi compressed H2 tanks Alanate hydrides Carbon nanotubes Source: Sirosh [827]

Storage system volume 320 L 220 L 200 L 130 L

Storage system weight 90 kg 100 kg 222 kg 120 kg

evaluating borane based compounds to increase the capacity on weight basis. Ammonia borane, which is a solid at room temperature, is composed of 19 wt% hydrogen and could be an ideal solid medium for hydrogen storage. However, ammonia borane releases hydrogen too slowly for its use as a hydrogen storage media for an automobile. A temperature higher than 170ı C may be required to release hydrogen at a rate acceptable for automobile applications. Researchers are working on developing a catalyst that can increase the rate of extraction of hydrogen at a lower temperature. Blhum et al. [826] noted that both the extent and rate of hydrogen release from ammonia borane by dehydrogenation are significantly increased at 85ı ; 90ı , and 95ı C when the release reactions are carried out in 1-butyl-3-methylimidazolium chloride compared to analogous solid-state reactions.

8.15 Comparison of Hydrogen Storage Capacity A comparison of various leading technologies for storage of 3 kg and 7 kg hydrogen is given in Tables 8.19 and 8.20, respectively.

8.16 Hydrogen Delivery Methods The delivery and distribution of hydrogen are critical components to the cost and its wide range use as an energy source. Both a central production facility and distributed production systems are possible for hydrogen. The choice of the lowest-cost delivery mode (compressed gas trucks, cryogenic liquid trucks or gas pipelines) will depend

8.17

Summary

587

Fig. 8.68 Hydrogen distribution pipeline in Louisiana, USA (Source: Joseph [828])

upon specific geographic and market characteristics (e.g., city population and radius, population density, size and number of refueling stations and market penetration of fuel cell vehicles). Generally, compressed gas truck delivery is ideal for small stations with very low demand. Liquid delivery is ideal for long distance delivery and moderate demand. Pipeline delivery is ideal for dense areas with large hydrogen demand. In the USA, Air Products, PA delivers H2 through two pipelines to several facilities in Lousiana and the Gulf Coast. These two pipelines are shown in Figs. 8.68 and 8.69. Joseph [828] prepared a chart (see Fig. 8.70) showing when a distribution method is most effective.

8.17 Summary Hydrogen is a secondary source of energy. It stores and carries energy produced from other resources (fossil fuels, water, and biomass). Hydrogen is not currently widely used, but it has potential as an energy carrier in the future. Hydrogen can be produced from a variety of resources (water, fossil fuels, or biomass) and is a byproduct of other chemical processes.

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8 Hydrogen Energy

Fig. 8.69 Hydrogen distribution pipeline in Texas, USA (Source: Joseph [828])

Fig. 8.70 Various hydrogen distribution method depending on demand (Source: Joseph [828])

8.17

Summary

589

Hydrogen has the highest energy content of any common fuel by weight (about three times more than gasoline), but the lowest energy content by volume (about four times less than gasoline). Steam reforming is currently the least expensive method for producing hydrogen and accounts for about 95% of the hydrogen produced in the United States. Hydrogen storage is a key enabling technology. None of the current technologies satisfy all of the hydrogen storage attributes sought by manufacturers and end users. Government, industry and academia are working together to lower costs, improve performance, and develop advanced materials for hydrogen storage. Efforts are also underway on improving existing commercial technologies, including compressed hydrogen gas and liquid hydrogen. Researchers are also exploring higher-risk storage technologies involving advanced materials (such as lightweight metal hydrides and carbon nanotubes). An energy economy based on hydrogen could resolve the growing concerns about America’s energy supply, security, air pollution, and greenhouse gas emissions.

Problems 1. What is the role of hydrogen as a fuel? 2. Why use hydrogen? 3. What is a hydrogen economy? 4. What are the components of a hydrogen economy and its associated issues and challenges? 5. What are the major obstacles for implementation of hydrogen economy? 6. Is hydrogen safe to use as a fuel? 7. Assume that in the future all cars will be run on hydrogen and oxygen. The only byproduct will be water. Since oxygen from atmosphere will be used for oxygen source, will this upset the oxygen balance of air? 8. How much hydrogen is available today? 9. How do we produce hydrogen today? 10. How do you get the hydrogen to the customer? 11. What is the cost of hydrogen production by various methods? 12. Can hydrogen be put into natural gas pipelines? 13. How much will it cost to develop a hydrogen infrastructure? 14. Can diesel engines burn hydrogen instead of diesel? 15. What are the safety concerns for hydrogen use?

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16. Explain how hydrogen power can help other renewable energy sources. 17. Can hydrogen energy be reliable and efficient as other conventional sources? 18. What kind of purity of water is necessary for hydrogen production? 19. Can seawater be used directly for hydrogen production? 20. Discuss the merits and demerits of various hydrogen storage systems. 21. What are the technological barriers for hydrogen production using solar energy? 22. Discuss the various potential uses of nuclear hydrogen. Can nuclear heat or electricity generated from nuclear energy be used directly in same applications? Which could be more efficient? 23. Discuss if the hydrogen generation by the hydropower system during off peak time via water electrolysis could be more efficient than a pumped storage system. 24. What kind of infrastructure is necessary for the large scale distribution of hydrogen? 25. How much hydrogen would be the lost during its storage from hydrogen refueling stations?

References 1. Bennaceur K, Clark B, Orr FM Jr, Ramakrishnan TS, Roulet C, Stout E (2005) Hydrogen: a future energy carrier? Oilfield Rev 17(1):30–41 2. Ohi J (2005) Hydrogen energy cycle: an overview. J Mater Res 20(12):3180–3187 3. Kruger P (2004) Electric power required in the world by 2050 for electric power and hydrogen fuel. World nuclear association annual symposium, 8–10 Sept 2004, London 4. Momirlan M, Veziroglu TN (2002) Current status of hydrogen energy. Renew Sustain Energy Rev 6:141–179 5. Christen K (2005) NRC finds hydrogen economy on track. Environ Sci Technol 39(19):398A 6. Penner SS (2005) Steps toward the hydrogen economy. Energy (Amsterdam, Neth) 31(1):33–43 7. Turner JA, Williams MC, Rajeshwar K (2004) Hydrogen economy based on renewable energy sources. Electrochem Soc Interface 13(3):24–30 8. Anon (2008) Event review: the potential for hydrogen as an energy source: the hydrogen economy. Chem Ind (London, UK), 21 April 2008 (8):30 9. Chapman PK, Haynes WE (2005) Power from space and the hydrogen economy. Acta Astronaut 57(2–8):372–383 10. Cooper HW (2007) Fuel cells, the hydrogen economy and you. Chem Eng Prog 103(11):34–43 11. Crabtree GW, Dresselhaus MS, Buchanan MV (2004) The hydrogen economy. Phys Today 57(12):39–44 12. Eikerling M, Kornyshev A, Kucernak A (2007) Driving the hydrogen economy. Phys World 20(7):32–36

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Appendices

Appendix 1: Wind Energy

Table A1.1 Wind turbine standards Standard no. Description AGMA 6006-A03 Standard for Design and Specification of Gearboxes for Wind Turbines – Supersedes AGMA 921-A97 BSI BS EN 45510-5-3 Guide for Procurement of Power Station Equipment – Part 5–3: Wind Turbines BSI BS EN 50308 Wind turbines Protective measures Requirements for design, operation and maintenance BSI PD CLC/TR 50373 Wind turbines Electromagnetic compatibility BSI BS EN 61400-12 Wind Turbine Generator Systems – Part 12: Wind Turbine Power Perfomance Testing – IEC 61400-12: 1998; BSI PD IEC WT 01 IEC System for Conformity Testing and Certification of Wind Turbines – Rules and Procedures CSA F417-M91 CAN/CSA Wind Energy Conversion Systems (WECS) – Performance – General Instruction No 1 DIN EN 61400-25-4 (DRAFT) Wind turbines – Part 25-4: Communications for monitoring and control of wind power plants – Mapping to XML based communication profile (IEC 88/241/CDV:2005); German version prEN 61400-25-4:2005, text in English DS DS/EN 61400-12-1 Wind turbines – Part 12-1: Power performance measurements of electricity producing wind turbines DNV DNV-OS-J101 Design of Offshore Wind Turbine Structures – Incorporates Amendment: 10/2007 GOST R 51237 Nontraditional power engineering. Wind power engineering. Terms and definitions (continued)

T.K. Ghosh and M.A. Prelas, Energy Resources and Systems: Volume 2: Renewable Resources, DOI 10.1007/978-94-007-1402-1, © Springer Science+Business Media B.V. 2011

631

632 Table A1.1 (continued) Standard no. IEC 60050-415 IEC 61400-1 IEC 61400-1 Ed2 IEC 61400-2 IEC 61400-12 IEC 61400-11 IEC 61400-13 IEC 61400-22 IEC 61400-23 IEC 61400-21 IEEE 1547

Appendices

Description International Electrotechnical Vocabulary – Part 415: Wind Turbine Generator Systems Wind Turbine Safety and Design Wind Turbine Safety and Design Revision Small Wind Turbine Safety Power Performance Noise Measurement Mechanical Load Measurements Wind Turbine Certification Blade Structural Testing Power Quality IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems

AGMA American Gear Manufacturers Association, BSI British Standards Institution, CSA Canadian Standards Association, DIN Deutsches Institut f¨ur Normung e.V, DS Danish Standards Association, IEC International Electrotechnical Commission, IEEE Institute of Electrical and Electronics Engineers Inc.

Appendices

Fig. A1.1 Wind power density in California at 50 m

633

634

Appendices 100°

99°

97°

98°

96°

3

34°

34°

Wichita Falls

3

3 2

3

33°

3

3

Abilene

3

3

2

Dallas

2

2

2

1

1

1

32° 2

2 31°

2

2

2

2

2

2

2 2

2

Waco

1 1

2

1

1

2

Austin

1

1

1 1

1 1

1

1 1

31°

1 1

1

Port Arthur

30° 1

1

1

1

1

1

2

2

1

2

2

2

2

2

2

2

2

2

2

2

3

MATAGORDA ISLAND

2

GULF OF MEXICO

3

PADRE 2

3 3

2

28°

3

Laredo 2

3

2

Corpus Christi 2

27°

ISLAND

2

100

0 3

0

100

26°

Miles

Kilometers

Brownsville 100°

99°

98°

29°

3

3 3

2

Galveston

2

Victoria

2

30°

Houston

San Antonio

27°

33°

1

1

Fort Worth 2

28°

1

1

3

32°

29°

94°

95°

97°

Fig. A1.2 Average wind power map of East Texas

96°

95°

94°

26°

Appendices

635 106°

105°

104°

103°

102°

3

4

101°

4

4

36°

100° 4 36° 4

4 4

4

4

4 Amarillo

4

35°

4

3

3

3 3

3

34°

34° 3

3 3 0 33°

0

50 50

1 4 El Paso

6

2

3 Odessa

3 2 2

2 DAVIS MTNS

2

3 1

31° 2

2

2

2 BIG BEND NAT’L PARK 2

2 San Angelo

2

2

4

32°

3

2

2

3

2 1

2

3

2

2

5 30°

2

Midland 3

2

2

4 2 5

3

3

2

31°

3

3

2 4

33° 3

3

3 1

3 3

3

100 Kilometers

Ridge Crest Estimates 32°

Lubbock 3

100 Miles

35°

4

4

2

30° 2

2 2

Del Rio 5

29°

29°

2

2 28°

28° 106°

105°

104°

103°

Fig. A1.3 Average wind power map of West Texas

102°

101°

100°

Fig. A1.4 Maps of mean 80-m wind speeds for year 2000 for North America (Printed with permission from Archer CL, Jacobson MZ (2005) Evaluation of global wind power. J Geophys Res 110:D12110. doi: 10.1029/2004JDD005462)

636 Appendices

Fig. A1.5 Maps of mean 80-m wind speeds for year 2000 for South America (Printed with permission from Archer CL, Jacobson MZ (2005) Evaluation of global wind power. J Geophys Res 110:D12110. doi: 10.1029/2004JDD005462)

Appendices 637

Fig. A1.6 Maps of mean 80-m wind speeds for year 2000 for Europe (Printed with permission from Archer CL, Jacobson MZ (2005) Evaluation of global wind power. J Geophys Res 110:D12110. doi: 10.1029/2004JDD005462)

638 Appendices

Fig. A1.7 Maps of mean 80-m wind speeds for year 2000 for Australia (Printed with permission from Archer CL, Jacobson MZ (2005) Evaluation of global wind power. J Geophys Res 110:D12110. doi: 10.1029/2004JDD005462)

Appendices 639

Fig. A1.8 Maps of mean 80-m wind speeds for year 2000 for Asia (Printed with permission from Archer CL, Jacobson MZ (2005) Evaluation of global wind power. J Geophys Res 110:D12110. doi: 10.1029/2004JDD005462)

640 Appendices

Appendices

641

Fig. A1.9 Maps of mean 80-m wind speeds for year 2000 for Africa (Printed with permission from Archer CL, Jacobson MZ (2005) Evaluation of global wind power. J Geophys Res 110:D12110. doi: 10.1029/2004JDD005462)

Fig. A1.10 Wind resource map of Russia at 50 m (With permission from Technical University of Denmark Nils Koppels All´e Building-403 Dk-2800 Kgs. Lyngby Denmark)

642 Appendices

Appendices

643

Fig. A1.11 Wind resource map of France at 50 m (With permission from Technical University of Denmark Nils Koppels All´e Building-403 Dk-2800 Kgs. Lyngby Denmark)

644

Appendices

Fig. A1.12 Wind resource map of Germany at 10 m (With permission from Technical University of Denmark Nils Koppels All´e Building-403 Dk-2800 Kgs. Lyngby Denmark)

Appendices

645

Fig. A1.13 Wind resource map of India at 50 m. Most of states are not mapped yet (With permission from Technical University of Denmark Nils Koppels All´e Building-403 Dk-2800 Kgs. Lyngby Denmark)

646

Appendices

Appendix 2: Solar Energy

kWh / sq. m 6.6 - 6.4 6.4 - 6.2 6.2 - 6.0 6.0 - 5.8 5.8 - 5.6 5.6 - 5.4 5.4 - 5.2 5.2 - 5.0 5.0 - 4.8 4.8 - 4.6 4.6 - 4.4

Fig. A2.1 Solar radiation on India (Courtesy of Sun@Home). http://www.sunathome.in/2010/06/ overview-of-technologies-opportunities-and-challenges/. Accessed 25 Nov 2010

Appendices

647

Fig. A2.2 Solar resource map for China for 40 km solar concentrator (kWh/m2 /day) Global Energy Network Institute, San Diego, CA, USA

648

Appendices

Table A2.1 Use of solar energy in water and air collectors by various countries Water collectors

Air collectors

Country

Unglazeda

Glazed

Albania Australia Austria Barbados Belgium Brazil Bulgaria Canada China Cyprus Czech Republic Denmark Estonia Finland Francec Germany Greece Hungary India Ireland Israel Italy Japan Jordan Latvia Lithuania Luxembourg Macedonia Malta Mexico Namibia Netherlands New Zealand Norway Poland Portugal Romania Slovak Republic Slovenia South Africa Spain Sweden

– 2,849.00 426.22 – 34.18 68.21 – 466.14 – – 10.66

34.95 1,162.00 2,064.69 57.96 93.63 2,511.25 19.32 57.23 7,280.00 556.32 67.96

14.96

275.70 1.03 10.91 991.55 5,448.87 2,496.34 28.94 1,505.00 19.36 3,455.83 611.46 4,777.20 588.23 3.75 2.42 13.23 13.35 20.55 310.72 4.19 230.65 72.04 7.85 138.51 193.23 48.72 61.81

– 0.35 73.15 525.00 – 1.96 – – 16.94 18.39 – – – – – – – 327.31 – 240.47 4.35 1.12 0.91 0.42 – – – 440.03 2.10 56.00

81.07 173.38 814.92 156.10

Evacuated tube 0.17 16.10 30.09 – 8.66 0.25 – 3.32 72,618.00 0.67 10.84 2.38 – 0.91 23.10 604.79 4.76 1.79 – 5.54 – 72.00 88.95 5.04 – – – 0.14 – – 0.13 – 7.03 0.11 25.71 3.83 – 6.94 0.81 – 31.92 20.30

Unglazeda

Glazeda

Total [MWth]

– – – – – – – 90.97 – – –

35.12 – – – – – – 0.13 – – –

4,027.10 2,521.00 57.96 136.46 2,579.70 19.32 617.80 79,898.00 557.00 89.47

2.38 – – – – – – – – – – 304.06 – – – – – – – – – – – 2.10 – – –

13.13 – – – – – – 11.90 – – – 8.76 – – – – – – – – – – 0.84 1.75 – – –

308.55 1.03 12.17 1,087.80 6,578.65 2,501.10 32.69 1,516.90 24.90 3,472.77 701.86 5,178.96 593.27 3.75 2.42 13.23 13.49 20.55 638.03 4.32 471.12 83.42 9.92 168.98 197.48 48.72 68.75

– – – –

– – – –

81.88 613.40 848.93 232.40 (continued)

Appendices

649

Table A2.1 (continued) Water collectors Country Switzerlandb Taiwan Thailand Tunisia Turkey United Kingdom United States Total

Unglazeda 148.68 – – – – – 19,347.55 25,074.11

Glazed 303.44 795.84 49.00 151.57 7,105.00 194.54 1,329.19 46,390.78

Air collectors Unglazeda

Glazeda

18.90

586.60 – – – – –

– – – – – –

404.86 74,119.76

0.07 986.18

160.82 197.33

Evacuated tube 17.79 82.89 – 1.03 –

Total [MWth] 1,056.52 878.74 49.00 152.60 7,105.00 213.44 21,242.49 146,768.15

Source of data: Weiss W, Bergmann I, Stelzer R (2009) Solar heat worldwide: markets and contribution to the energy supply 2007. International Energy Agency Solar Heating and Cooling Program a If no data is given: no reliable data base for this collector type available b Unglazed air collectors in Switzerland: this is a very simple site-built system for hay drying purposes c France: includes Overseas Departments

650

Appendices Table A2.2 Bandgap energy of several common semiconductors Material Symbol Band gap (eV) @ 302K Silicon Si 1.11 Selenium Se 1.74 Germanium Ge 0.67 Silicon carbide SiC 2.86 Aluminium phosphide AlP 2.45 Aluminium arsenide AlAs 2.16 Aluminium antimonide AlSb 1.6 Aluminium nitride AlN 6.3 Diamond C 5.5 Gallium(III) phosphide GaP 2.26 Gallium(III) arsenide GaAs 1.43 Gallium(III) nitride GaN 3.4 Gallium(II) sulfide GaS 2.5 Gallium antimonide GaSb 0.7 Indium (III) antimonide InSb 0.17 Indium(III) nitride InN 0.7 Indium(III) phosphide InP 1.35 Indium(III) arsenide InAs 0.36 Zinc oxide ZnO 3.37 Zinc sulfide ZnS 3.6 Zinc selenide ZnSe 2.7 Zinc telluride ZnTe 2.25 Cadmium sulfide CdS 2.42 Cadmium selenide CdSe 1.73 Cadmium telluride CdTe 1.49 Lead(II) sulfide PbS 0.37 Lead(II) selenide PbSe 0.27 Lead(II) telluride PbTe 0.29 Copper(II) oxide CuO 1.2 2.1 Copper(I) oxide Cu2 O Sources of data: Streetman BG, Banerjee S (2000) Solid state electronic devices, 5th edn. Prentice Hall, Upper Saddle River; Wu J (2002) Unusual properties of the fundamental band gap of InN. Appl Phys Lett 80:3967; Otfried M (1996) Semiconductor: basic data. Springer, New York; Elliot RJ (1961) Symmetry of excitons in Cu2 O. Phys Rev 124:340; Baumeister PW (1961) Optical absorption of cuprous oxide. Phys Rev 121:359–362; Kittel C (1986) Introduction to solid state physics, 6th edn. John Wiley, New York, p 185

Appendices

651

Table A2.3 Physical characteristics of Si and the major WBG semiconductors Property Si GaAs Bandgap, Eg(eV) 1:12 1:43 Dielectric constant, ©ar 11:9 13:1 300 400 Electric breakdown field, Ec .kV=cm/ 1;500 8;500 Electron mobility, n .cm2 =Vs/ Hole mobility, 600 400 p .cm2 =Vs/ Thermal conductivity, 1:5 0:46 (W/cm K) 1 1 Saturated electron drift velocity, vsat .107 cm=s/ a © D ©r ©o , where ©o D 8:85 1014 F/cm

6H-SiC 3:03 9:66 2;500 500 80 101

4H-SiC 3:26 10:1 2;200

GaN 3:45 9 2;000

1;000

1;250

2;200

115

850

850

4:9

4:9

1:3

2

2

2:2

Diamond 5:45 5:5 10;000

22 2:7

Table A2.4 Main figures of merit for WBG semiconductors compared with Si JFM BFM FSFM BSFM FPFM FTFM BPFM BTFM

Si 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

GaAs 1.8 14.8 11.4 1.6 3.6 40.7 0.9 1.4

6H-SiC 277.8 125.3 30.5 13.1 48.3 1,470.5 57.3 748.9

4H-SiC 215.1 223.1 61.2 12.9 56.0 3,424.8 35.4 458.1

GaN 215.1 186.7 65.0 52.5 30.4 1,973.6 10.7 560.5

Diamond 81,000 25,106 3,595 2,402 1,476 5,304,459 594 1,426,711

Source of data: Ozpineci B, Tolbert LM (2003) Comparison of wide-bandgap semiconductors for power electronics applications. Oak Ridge National Laboratory. Report No. ORNL/TM-2003/257 JFM Johnson’s figure of merit, a measure of the ultimate high-frequency capability of the material, BFM Baliga’s figure of merit, a measure of the specific on-resistance of the drift region of a vertical field effect transistor (FET), FSFM FET switching speed figure of merit, BSFM Bipolar switching speed figure of merit, FPFM FET power-handling-capacity figure of merit, FTFM FET powerswitching product, BPFM Bipolar power handling capacity figure of merit, BTFM Bipolar power switching product

Material property Melting point (K) Energy gap Eg at 300K(eV)(ZB*/WZ*) dEg =dT .104 eV/K) ZB/WZ Structure Bond length ( m) Lattice constant (ZB) a0 at 300 K (nm) ZB nearest-neighbor dist. at 300 K (nm) ZB density at 300 K (g/cm3 ) Lattice constant (WZ) at 300 K (nm) a0 D b0 c0 c0 =a0 WZ density at 300 K (g/cm3 ) Symmetry ZB=WZ Electron affinity, (eV) Stable phase(s) at 300 K Solid–solid phase transition temperature (K) Heat of crystallization HLS (kJ/mol)

ZnO 2248 –=3.4 –=9.5 WZ 1.977 (WZ) – – –

0.32495 0.52069 1.602 5.606 –=C6me WZ –

62

ZnS 2038 (WZ, 150 atm) 3.68=3.911

4.6=8.5

ZB=WZ 2.342 (WZ) 0.541

0.234

4.11

0.3811 0.6234 1.636 3.98

C6me=F43m

ZB & WZ 1293

44

52

–=F43m 4.09 ZB 1698

0.398 0.653 1.641 –

5.26

0.246

ZB=WZ 2.454 (ZB) 0.567

4.0=–

ZnSe 1797 2.71=–

Table A2.5 Properties of some wide-bandgap II–VI compound semiconductors

56

–=F43m 3.53 ZB –

0.427 0.699 1.637 –

5.65

0.264

ZB 2.636 (ZB) 0.610

5.5=–

ZnTe 1513 2.394

58

C6me=F43m 4.79 ZB & WZ –

0.4135 0.6749 1.632 4.82

4.87

0.252

WZ 2.530 (ZB) 0.582

–=5.2

CdS 2023 (WZ, 100 atm) 2.50=2.50

45

C6me=F43m 4.95 ZB & WZ 403

0.430 0.702 1.633 5.81

5.655

0.263

WZ 2.630 (ZB) 0.608

–=4.6

CdSe 1623 –=1.751

(continued)

57

–=– 4.28 ZB 1273(?)

– – – –

5.86

0.281

ZB 2.806 (ZB) 0.648

5.4=–

CdTe 1370 (ZB) 1.475

652 Appendices

Material property Heat capacity CP (cal/mol K) Ionicity (%) Equilibrium pressure at c.m.p. (atm) Minimum pressure at m.p. (atm) Specific heat capacity (J/gK) Thermal conductivity (W cm1 K1 ) Thermo-optical cofficient (dn=dT )( D 10:6 m) Electrooptical coefficient r41 (m/V) ( D 10:6 m) Linear expansion coefficient, (106 K1 ) ZB/WZ Poisson ratio Dielectric constant, eo D e1 Refractive index ZB/WZ Absorption coeff. (including two surfaces) . D 10:6 m/.cm1 / Electron effective mass (m =m0 ) Hole effective mass m dos =m0 Electron Hall mobility (300) K for n D lowish (cm2 =Vs) Hole Hall mobility at 300 K for p D lowish (cm2 /Vs) Exciton binding energy (meV)

Table A2.5 (continued) ZnO 9.6 62 – 7.82 – 0.6 – – 2.9/7.2

8.65/4.0 –/2.029 –

–0.27 – 125 – 60

ZnS 11.0 62 3.7

2.8 0.469 0.27

4.7

2 1012

–/6.9

0.27 8.6/5.2 2.368/2.378

0.15

–0.40 – 165

5

36

21

30

0.21 0.6 500

0.28 9.2/5.8 2.5/– 1–2 103

7.6/–

2:2 1012

6.1

0.53 0.339 0.19

ZnSe 12.4 63 1.0

10

100

0.2 circa 0.2 340

9.3/6.9 2.72/– –

4:0 1012 .r41 D r52 D r63 / 8.0/–

–

0.64 0.16 0.18

ZnTe 11.9 61 1.9

30.5

340

0.21 0.8 340

8.6/5.3 –/2.529

0.007

3.0/4.5

–

–

2.2 0.47 0.2

CdS 13.2 69 3.8

15

–

0.13 0.45 650

9.5/6.2 2.5/–

0.0015

3.0/7.3

–

–

0.4–0.5 0.49 0.09

CdSe 11.8 70 1.0

12 (continued)

100

0.11 0.35 1050

0.41 2.27/– 2.72/–

0.003

5.1/–

6.8 10–12

11.0

0.23 0.21 0.01

CdTe – 72 0.7

Appendices 653

ZnO –

– – – 0.5 –

ZnS 16.1/17.1

1.01 ˙ 0.05 0.64 ˙ 0.05 0.42 ˙ 0.04 0.18 10.8 Mpsi

8.10 ˙ 0.52 4.88 ˙ 0.49 4.41 ˙ 0.13 0.15 10.2 Mpsi

ZnSe 15.1/–

Source: 16. Wide-bandgap II-VI semiconductors: Growth and properties. www.springer.com/cda/content/document/. . . /9780387260594-c3.pdf?. . .

Table A2.5 (continued) Material property Average phonon energy (meV) ZB/WZ Elastic constant (1010 N/m2 ) C11 C12 C44 Knoop hardness (N/cm2 ) Young’s modulus 0.72 ˙ 0.01 0.48 ˙ 0.002 0.31 ˙ 0.002 0.13 –

ZnTe 10.8/–

– – – – 45 GPa

CdS –/13.9

– – – – 5 1011 dyne/cm2

CdSe 18.9/25.4

5.57 3.84 2.095 0.10 3.7 1011 dyne/cm2

CdTe 5.8/–

654 Appendices

Effic.b (%) 25.0 ˙ 0.5 20.4 ˙ 0.5 16.7 ˙ 0.4 10.5 ˙ 0.3

26.4 ˙ 0.8 26.1 ˙ 0.8 18.4 ˙ 0.5 22.1 ˙ 0.7

19.4 ˙ 0.6h 16.7 ˙ 0.4 16.7 ˙ 0.5h

10.1 ˙ 0.3i 10.1 ˙ 0.2j

10.4 ˙ 0.3k 9.2 ˙ 0.4k

5.15 ˙ 0.3k 3.5 ˙ 0.3k

Silicon Classificationa Si (crystalline) Si (multicrystalline) Si (thin film transfer) Si (thin film submodule)

III-V Cells GaAs (crystalline) GaAs (thin film) GaAs (multicrystalline) InP (crystalline)

Thin film chalcogenide CIGS (cell) CIGS (submodule) CdTe (cell)

Amorphous/nanocrystalline Si Si (amorphous) Si (nanocrystalline)

Photochemical Dye sensitized Dye sensitized (submodule)

Organic Organic polymer Organic (submodule) 1.021 (ap) 208.4 (ap)

1.004 (ap) 17.19 (ap)

1.036 (ap) 1.199 (ap)

0.994 (ap) 16.0 (ap) 1.032 (ap)

1.006 (t) 1.001 (ap) 4.011 (t) 4.02 (t)

Areac (cm2 ) 4.00 (da) 1.002 (ap) 4.017 (ap) 94.0 (ap)

0.876 8.620

0.729 0.712g

0.886 0.539

0.716 0.661g 0.845

1.030 1.045 0.994 0.878

Voc (V) 0.706 0.664 0.645 0.492g

9.39 0.847

22.0 19.4g

16.75 24.4

33.7 33.6g 26.1

29.8 29.6 23.2 29.5

Jsc (mA/cm2 ) 42.7 38.0 33.0 29.7g

62.5 48.3

65.2 66.4

67.0 76.6

80.3 75.1 75.5

86.0 84.6 79.7 85.4

FFd (%) 82.8 80.9 78.2 72.1

NREL (12/06)f NREL (7/09)

AIST (8/05)f AIST (2/10)

NREL (7/09) JQA (12/97)

NREL (1/08)f FhG-ISE (3/00)f NREL (9/01)f

FhG-ISE (3/10) FhG-ISE (7/08)f NREL (11/95)f NREL (4/90)f

Test centere (and date) Sandia (3/99)f NREL (5/04)f FhG-ISE (7/01)f FhG-ISE (8/07)f

(continued)

[15] [16]

[13] [14]

[11] [12]

[8] [9] [10]

Fraunhofer ISE [5] [6] [7]

Reference [1] [2] [3] [4]

Table A2.6 Confirmed terrestrial cell and submodule efficiencies measured under the global AM1.5 spectrum (1,000 W/m2 ) at 25ı C (IEC 60904-3: 2008, ASTM G-173-03 global)

Appendices 655

32.0 ˙ 1.5j 30.3j 25.8 ˙ 1.3j 11.7 ˙ 0.4jl 6.1 ˙ 0.2k

Multijunction devices GaInP/GaAs/Ge GaInP/GaAs GaAs/CIS (thin film) a-Si/ c-Si (thin submodule)jl Organic (2-cell tandem) 3.989 (t) 4.0 (t) 4.00 (t) 14.23 (ap) 1.989

Areac (cm2 ) 2.622 2.488 – 5.462 1.589

Voc (V) 14.37 14.22 – 2.99 6.18

Jsc (mA/cm2 ) 85.0 85.6 – 71.3 61.9

FFd (%) NREL (1/03) JQA (4/96) NREL (11/89) AIST (9/04) FhG-ISE (7/09)

Test centere (and date)

Spectrolab (monolithic) [17] [18] [19] [20]

Reference

(continued)

1. Zhao J, Wang A, Green MA, Ferrazza F (1998) Novel 19.8% efficient ‘honeycomb’ textured multicrystalline and 24.4% monocrystalline silicon solar cells. Appl Phys Lett 73:1991–1993 2. Schultz O, Glunz SW, Willeke GP (2004) Multicrystalline silicon solar cells exceeding 20% efficiency. Prog Photovolt Res Appl 12:553–558 3. Bergmann RB, Rinke TJ, Berge C, Schmidt J, Werner JH (2001) Advances in monocrystalline Si thin-film solar cells by layer transfer. In: Technical Digest, PVSEC-12, June 2001, Chefju Island, Korea, pp 11–15 4. Keevers MJ, Young TL, Schubert U, Green MA (2007) 10% efficient CSG minimodules. In: 22nd European Photovoltaic Solar Energy Conference, Milan, Sept 2007 5. Bauhuis GJ, Mulder P, HaverKamp EJ, Huijben JCCM, Schermer JJ (2009) 26.1% thin-film GaAs solar cell using epitaxial lift-off. Solar Energy Mater Solar Cells 93: 1488–1491.

b

CIGS CuInGaSe2, a-Si amorphous silicon/hydrogen alloy Effic. D efficiency c (ap) D aperture area; (t) D total area; (da) D designated illumination area d FF fill factor e FhG-ISE Fraunhofer Institut f¨ur Solare Energiesysteme, JQA Japan Quality Assurance, AIST Japanese National Institute of Advanced Industrial Science and Technology f Recalibrated from original measurement g Reported on a ‘per cell’ basis h Not measured at an external laboratory i Light soaked at Oerliken prior to testing at NREL (1000 h, 1 sun, 508C) j Measured under IEC 60904-3 Ed. 1: 1989 reference spectrum k Stability not investigated. References [28] and [29] review the stability of similar devices l Stabilized by 174 h, 1 sun illumination after 20 h, 5 sun illumination at a sample temperature of 508C

a

Effic.b (%)

Silicon Classificationa

Table A2.6 (continued)

656 Appendices

6. Venkatasubramanian R, O’Quinn BC, Hills JS, Sharps PR, Timmons ML, Hutchby JA, Field H, Ahrenkiel A, Keyes B (1997) 18.2% (AM1.5) efficient GaAs solar cell on optical-grade polycrystalline Ge substrate. In: Conference Record, 25th IEEE Photovoltaic Specialists Conference, Washington, May 1997, pp 31–36 7. Keavney CJ, Haven VE, Vernon SM (1990) Emitter structures in MOCVD InP solar cells. In: Conference Record, 21st IEEE Photovoltaic Specialists Conference, Kissimimee, May 1990, pp 141–144 8. Repins I, Contreras M, Romero Y, Yan Y, Metzger W, Li J, Johnston S, Egaas B, DeHart C, Scharf J, McCandless BE, Noufi R (2008) Characterization of 19.9%-efficient CIGS absorbers. In: 33th IEEE Photovoltaics Specialists Conference Record, 2008 9. Kessler J, Bodegard M, Hedstrom J, Stolt L (2000) New world record Cu (In,Ga) Se2 based mini-module: 16.6%. In: Proceedings of 16th European Photovoltaic Solar Energy Conference, Glasgow, 2000, pp 2057–2060 10. Wu X, Keane JC, Dhere RG, DeHart C, Duda A, Gessert TA, Asher S, Levi DH, Sheldon P (2001) 16.5%- efficient CdS/CdTe polycrystalline thin-film solar cell. In: Proceedings of 17th European Photovoltaic Solar Energy Conference, Munich, 22–26 Oct 2001, pp 995–1000 11. Benagli S, Borrello D, Vallat-Sauvain E, Meier J, Kroll U, H¨otzel J et al. (2009) High-efficiency amorphous silicon devices on LPCVD-ZNO TCO prepared in industrial KAI-M R&D reactor. In: 24th European Photovoltaic Solar Energy Conference, Hamburg, Sept 2009 12. Yamamoto K, Toshimi M, Suzuki T, Tawada Y, Okamoto T, Nakajima A (1998) Thin film poly-Si solar cell on glass substrate fabricated at low temperature. In: MRS Spring Meeting, April 1998, San Francisco 13. Chiba Y, Islam A, Kakutani K, Komiya R, Koide N, Han L (2005) High efficiency dye sensitized solar cells. In: Technical Digest, 15th International Photovoltaic Science and Engineering Conference, Shanghai, Oct 2005, pp 665–666 14. Morooka M, Noda K (2008) Development of dye-sensitized solar cells and next generation energy devices. In: 88th Spring Meeting of The Chemical Society of Japan, Tokyo, 26 Mar 2008 15. See http://www.konarka.com 16. See http://www.solarmer.com 17. Ohmori M, Takamoto T, Ikeda E, Kurita H (1996) High efficiency InGaP/GaAs tandem solar cells. In: Techical Digest, International PVSEC-9, Myasaki, Japan, Nov 1996, pp 525–528 18. Mitchell K, Eberspacher C, Ermer J, Pier D (1988) Single and tandem junction CuInSe2 cell and module technology. In: Conference Record, 20th IEEE Photovoltaic Specialists Conference, Las Vegas, Sept 1988, pp 1384–1389 19. Yoshimi M, Sasaki T, Sawada T, Suezaki T, Meguro T, Matsuda T, Santo K, Wadano K, Ichikawa M, Nakajima A, Yamamoto K (2003) High efficiency thin film silicon hybrid solar cell module on Im2-class large area substrate. In: Conference Record, 3rd World Conference on Photovoltaic Energy Conversion, Osaka, May 2003, pp 1566–1569 20. See http://www.heliatek.com

Table A2.6 (continued)

Appendices 657

22.9 ˙ 0.6 21.4 ˙ 0.6 17.3 ˙ 0.5 8.2 ˙ 0.2 13.8 ˙ 0.5 13.5 ˙ 0.7 10.9 ˙ 0.5 10.4 ˙ 0.5g 778 (da) 15780 (ap) 12753 (ap) 661 (ap) 9762 (ap) 3459 (ap) 4874 (ap) 905 (ap)

5.60 68.6 33.6 25.0 26.34 31.2 26.21 4.353

3.97 6.293 8.63 0.320 7.167 2.18 3.24 3.285

80.3 78.4 76.1 68.0 71.2 68.9 62.3 66.0

Sandia (9/96)e NREL (10/09) AIST (x/10) Sandia (7/02)e NREL (4/10) NREL (8/02)e NREL (4/00)e NREL (10/98)e

[1] Kyocera [2] [3] [4] [5] [6] [7]

1. Zhao J, Wang A, Yun F, Zhang G, Roche DM, Wenham SR, Green MA (1997) 20,000 PERL silicon cells for the ‘1996 World Solar Challenge’ solar car race. Prog Photovolt Res Appl 5:269–276 2. Swanson RM (2009) Solar cells at the cusp. In: Presented at 19th International Photovoltaic Science and Engineering Conference, Korea, Nov 2009 3. Basore PA (2002) Pilot production of thin-film crystalline silicon on glass modules. In: Conference Record, 29th IEEE Photovoltaic Specialists Conference, New Orleans, May 2002, pp 49–52 4. See: http://www.miasole.com 5. Tanaka Y, Akema N, Morishita T, Okumura D, Kushiya K (2001) Improvement of Voc upward of 600mV/cell with CIGS-based absorber prepared by Selenization/Sulfurization. In: Conference Proceedings, 17th EC Photovoltaic Solar Energy Conference, Munich, Oct 2001, pp 989–994 6. Cunningham D, Davies K, Grammond L, Mopas E, O’Connor N, Rubcich M, Sadeghi M, Skinner D, Trumbly T (2000) Large area ApolloTM module performance and reliability. In: Conference Record, 28th IEEE Photovoltaic Specialists Conference, Alaska, Sept 2000, pp 13–18 7. Yang J, Banerjee A, Glatfelter T, Hoffman K, Xu X, Guha S (1994) Progress in triple-junction amorphous silicon based alloy solar cells and modules using hydrogen dilution. In: Conference Record, 1st World Conference on Photovoltaic Energy Conversion, Hawaii, Dec 1994, pp 380–385

b

CIGSS1=4CuInGaSSe; a-Si1=4amorphous silicon/hydrogen alloy; a-SiGe1=4amorphous silicon/germanium/hydrogen alloy Effic.1=4efficiency c (ap)1=4aperture area; (da)1=4designated illumination area d FF1=4fill facto e Recalibrated from original measurement f Light soaked at NREL for 1000 h at 508C, nominally 1-sun illumination g Measured under IEC 60904-3 Ed. 1: 1989 reference spectrum

a

Si (crystalline) Si (large crystalline) Si (multicrystalline) Si (thin-film polycrystalline) CIGS CIGSS (Cd free) CdTe a-Si/a-SiGe/a-SiGe (tandem)f

Table A2.7 Confirmed terrestrial module efficiencies measured under the global AM1.5 spectrum (1,000 W/m2 ) at a cell temperature of 25ı C (IEC 60904-3: 2008, ASTM G-173-03 global) Classificationa Effic.b (%) Areac (cm2 ) Voc (V) Jsc (mA/cm2 ) FFd (%) Test centere (and date) Reference

658 Appendices

Appendices

659

Appendix 3: Hydropower

Table A3.1 Entrance loss coefficient (k1 ) for various types of entrances

Entrance type Protruding Sharp edge Well rounded

k1 0.75 0.50 0.01

Table A3.2 Abrupt contraction loss coefficient (k2 )

Ratio of downstream to upstream flow area, A2 /A1 0.60 0.40 0.20 0.05

k2 0.13 0.28 0.38 0.45

A1 upstream flow area, A2 downstream flow area

Table A3.3 Values of bend loss coefficient (kB )

Table A3.4 Values of valve loss coefficient (kv )

Smooth bends r/D 1.0 2.0 3.0

Gate valve Position Fully open Half open Quarter open

kB 0.40 0.27 0.20

kv 2.3 4.3 10.0

Mitred bends 20ı 40ı 60ı 90ı

Butterfly valve t/D 0.1 0.2 0.3

kB 0.06 0.21 0.50 1.10

kv 0.1 0.3 0.75

t thickness of the butterfly, D pipe diameter Table A3.5 Gradual expansion loss coefficient (kge )

kge A2 =A1 3.0 2.5 2.0 1.5

™ 20ı 0.4 0.3 0.2 0.15

15ı 0.30 0.25 0.15 0.1

10ı 0.2 0.15 0.12 0.08

Fig. A3.1 The Moody chart

660 Appendices

Appendices

Appendix 4: Geothermal Energy

Fig. A4.1 Temperature range for geothermal energy for use in industry

661

USA Nevada

Iceland

Chemicals Salt plant

Iceland Namafjall

Heap leaching

Mining Diatomaceous earth plant

Taiwan Tatun

Timber drying

Recrystallizing of coarse salt into five grain mineral salt used in bathing (Saga Salt). It previously produced 8,000 tons/year of salt for the fish Processing industry

Production of 28,000 tons/year dried diatomaceous earth recovered by wet mining techniques. Dredging of Lake Myvatn is done only in the summer while plant runs throughout the year Two gold mining operations use geothermal fluids in heat exchangers to heat cyanide solutions

The capacity of the kiln is 1,400 ft3 and can produce 8,500 ft3 of kiln dried lumber/month

Processing and a small amount of electric power generation. Kraft process used. Geothermal energy delivered to mills by 0.60 million lb/h of 200 and 100 psig steam, which are obtained by flashing the wet steam at a central flash plant The facility consists of a vacuum dryer and a bark boiler

New Zealand Kawerau

Japan Yuzawa

Description

Country

Timber drying

Application Wood and paper industry Pulp and paper

Table A4.1 A list of industries where geothermal energy is currently used

356ı F at 145 psi

1,100 gpm of hot water at 180 To 240ı F

24 ton/h of steam at 356ı F

25

17.5

16

1.0

(continued)

100–125

245 ton/h of wet steam 529ı F reservoir temp

47.6 ton/h hot water with 200ı F inlet and 176ı F outlet temp 0.5 ton/h 140ı F in kiln

Associated power (MW)

Production steam or water flow rate

662 Appendices

Country Italy Larderello

Alfalfa drying

USA San Emidio Desert, Nevada New Zealand, Broadlands

USA – San Bernardino, California Agriculture product drying Vegetable USA Brady Hot Springs, Nevada

Water waste

Application Boric acid

Table A4.1 (continued)

Geothermal Food Processors produce dried onions using hot water coils from an 85 to 5% moisture content using a continuous throughcirculation conveyor dryer. Production rate is 10,000 lb/h of fresh onions, resulting in 1,800 lb/h of dried product for 6 mo/year Integrated Ingredients produce dried onions and garlic. Production rate is 14 millions lbs. Per year Taupo Lucerne limited (NZ) uses geothermal steam and hot water as the heat source for the drying of alfalfa (lucerne) into “De-HI”, produced from the fibrous part of the plant, and “LPC” (lucerne protein concentrate) which is a high-protein Product produced from extracted juice. It produces 3,000 tons/year of dried De-Hi and 200 tons/year of LPC. In addition, 35,000 cubic feet of dried fence posts, Poles and swan timber products are produced per month

Description Geothermal steam used for processing imported ore Sludge digester heating

40 ton/h of steam at 347ı F

12

14

6

500 gpm of hot water at 325ı F

900 gpm 266ı F

0.5

(continued)

Associated power (MW) 15–19

155 gpm of hot water at 145ı F

Production steam or water flow rate 30 ton/h of steam

Appendices 663

Country USA Vale, Oregon

Description Oregon Trail Mushrooms produces 2,500 ton of white button mushrooms annually. Geothermal fluids are used for soil composting and space heating and cooling

Production steam or water flow rate 275 gpm of hot water at 235ı F

Associated power (MW) 4.0

1. Wilson RD (1974) Use of geothermal energy at Tasman Pulp and Paper Company Limited, New Zealand. In: Lienau PJ, IW Lund (eds) Multipurpose use of geothermal energy. Oregon Institute of Technology, Klamath Falls 2. Carter AC, Hotson GW (1992) Industrial use of geothermal energy at the Tasman Pulp & Paper Co., Ltd’s Mill, Kaweran, New Zealand. In: Geothermics, vol 21, No 5/6. Pergamon Press, NY, pp 689–700 Hotson GW (1995) Utilization of geothermal energy in a Pulp and Paper Mill. In: Proceedings of the World Geothermal Congress, Florence, Italy, International Geothermal Association, pp 2357–2360 3. Horii S (1985) Direct heat update of Japan. In: International Symposium on Geothermal Energy, International Volume, Geothermal Resources Council, Davis, pp 107–112 4. Chin (1976) Geothermal energy in Taiwan, Republic of China. Mining Research & Service Organization, ITRI, Taipei, Taiwan 5. Ragnarsson A (1996) Geothermal energy in Iceland. Geo-Heat Center Quarterly Bulletin Klamath Falls, OR 17(4): 1–6 6. Trexler DT, Flynn T, Hendrix JL (1990) Preliminary results of column leach experiments at two gold mines using geothermal fluids. In: 1990 International Symposium on Geothermal Energy, GRC Transactions, Hawaii vol 14, pp 351–358 7. Kristjansson I (1992) Commercial production of salt from geothermal brine at Reykjanes, Iceland. Geothermics 21(5/6):765–771 8. Lindal B (1973) Industrial and other applications of geothermal energy. In: Geothermal energy: review of research and development (LC No. 72-97138, UNESCO), pp 135–148 9. Racine WC (1981) Feasibility of geothermal heat use in the san bernardino municipal waste water treatment plant. Municipal Water Department, San Bernardino 10. Lund JW (1994) Geothermal vegetable dehydration at Brady’s Hot Springs, Nevada. Geo-Heat Center Quarterly Bulletin, Klamath Falls, OR 15(4):22–23 11. Lund JW, Lienau PJ (1994) Onion dehydration. Geo-Heat Center Quarterly Bulletin, Klamath Falls, OR 15(4):15–18 12. Pirrit N, Dunstall M (1995) Drying of fibrous crops using geothermal steam and hot water at the Taupo Lucerne Company. In: Proceeding of the World Geothermal Congress, Florence, Italy, International Geothermal Association, pp 2239–2344 13. Rutten P (1986) Summary of process – mushroom production. Oregon Trail Mushroom Company, Vale, OR

Application Mushroom growing

Table A4.1 (continued)

664 Appendices

Table A5.1 Various ocean energy projects around the world Company Technology Pelamis wave power Attenuator Wave star energy Attenuator AWS ocean energy Point absorber Wave dragon Overtopper WaveGen Oscillating water column Oceanlinx Oscillating water column SyncWave energy Point absorber WAVEenergy Overtopper Seabased Point absorber Offshore Wave Energy Oscillating water column ORECon Oscillating water column SeaPower Pacific Oscillating wave surge converter Ocean power technologies Point absorber Finavera renewables Point absorber Ocean wave master Attenuator Wave energy technologies Point absorber WaveBob Point absorber Fred. Olsen Point absorber C-Wave Attenuator S.D.E. energy Terminator

Appendix 5: Ocean Energy

Country U.K. Denmark U.K. Denmark U.K. Australia Canada Norway Sweden U.K. U.K. Australia U.S Canada U.K. Canada Ireland Norway U.K. Israel

Year started 1998 2000 2004 1987 1990 1997 2004 2004 2003 2001 2002 1999 1994 2006 2002 2004 1999 2004 (1848)a 2002 1998

Stage Commercial Pilot Pre-commercial Commercial Commercial Commercial Prototype Pilot Pilot Prototype Prototype Pilot Commercial Pre-commercial Prototype Pilot Pre-commercial Pre-commercial Prototype Commercial (continued)

Appendices 665

Technology Point absorber Point absorber Oscillating wave surge converter Oscillating wave surge converter

Country U.K. U.K. U.K. Australia

Source: Greentech Media and the Prometheus Institute for Sustainable Development a Fred Olsen is a shipping company started in 1848. It entered into wave energy business in 2004

Table A5.1 (continued) Company Trident energy Ocean Navitas Aquamarine power BioPower systems

Year started 2003 2006 2007 2006

Stage Prototype Prototype Prototype Pre-pilot

666 Appendices

Appendices

667

Tidal Energy Projects & Companies

Table A5.2 Existing large tidal power plants Country Site Installed power (MW) France La Rance 240 Russia Kislaya Guba 0.4 Canada Annapolis 18 China Jiangxia 3.9

Basin area (km2 ) 22 1.1 15 1.4

Mean tide (m) 8.55 2.3 6.4 5.08

Source: www.gcktechnology.com

Table A5.3 Tidal stream resources Location UK Europe (excluding UK) Others worldwide

Total (TWh/year) 90 90 600

Extractable (TWh/year) 18 17 120?

Economic (TWh/year) 12 ? ?

Source: Black & Vetch-for Carbon Trust-2004–5

Table A5.4 Country France Canada China Russia Korea India Australia Argentina UK

Tidal barrage projects and proposals Location Power MW La Rance 240 Bay of Fundy – Cumberland Basin 1400 Various 1000 Mezan Bay and Tugur 28000 Siwha and Garolim 740 Kambhat 1800 Secure Bay and Cape Keraudren 600 San Jose/Nuevo 600 Severn and Mersey 9300

Energy TWh/year 0.5 3.3 2.5 31.0 1.4 3.9 1.1 1.8 18.5

Source: http://www.raeng.org.uk/policy/reports/pdf/energy 2100/David Lindley.pdf

Fig. A5.1 Ocean energy potential around the world (Courtesy of Emerging energy research (October 2010) Global ocean energy markets and strategies: 2010–2030. www.emerging-energy.com. Accessed 11/20/2010)

668 Appendices

Fig. A5.2 Ocean energy development roadmap for Europe

Appendices 669

670

Appendices

Appendix 6: Bioenergy

Table A6.1 Estimated Bagasse potential (Data at 2005) Bagasse potential availability Benin Burkina Faso Burundi Cameroon Chad Congo (Brazzaville) Congo (Democratic Rep.) Cˆote d’Ivoire Egypt (Arab Rep.) Ethiopia Gabon Guinea Kenya Madagascar Malawi Mali Mauritius Morocco Mozambique Niger Senegal Sierra Leone Somalia South Africa Sudan Swaziland Tanzania Uganda Zambia Zimbabwe Total Africa Barbados Belize Costa Rica Cuba Dominican Republic El Salvador

At 50% humidity (thousand tonnes)

Dry matter (thousand tonnes)

16 130 75 388 114 206 196 473 3,912 1,125 68 82 1,733 89 864 114 1,708 191 865 33 293 20 49 8,174 2,373 2,128 908 636 808 1.401 29,169

8 65 37 194 57 103 98 236 1,956 562 34 41 866 44 432 57 854 95 433 16 147 10 24 4,087 1,186 1,064 454 318 404 700 14,584

130 331 1,299 4,238 1,549 2,062

65 166 649 2,119 774 1,031 (continued)

Appendices

671

Table A6.1 (continued) Bagasse potential availability At 50% humidity (thousand tonnes) Honduras Jamaica Mexico Nicaragua Panama St. Christopher-Nevis Trinidad & Tobago United States of America Total North America

Dry matter (thousand tonnes)

1,174 411 18,319 1,532 513 65 108 9,029 47,330

587 205 9,159 766 256 33 54 4,514 23,665

Argentina Bolivia Brazil Colombia Ecuador Guyana Paraguay Peru Suriname Uruguay Venezuela Total South America

7,058 1,304 91,720 8,747 1,532 802 381 2,264 16 20 2,249 116,095

3,529 652 45,860 4,374 766 401 191 1,132 8 10 1,125 58,047

Azerbaijan Bangladesh China India Indonesia Japan Malaysia Myanmar (Burma) Nepal Pakistan Philippines Sri Lanka Taiwan, China Thailand Vietnam Total Asia Unspecified Total Europe Iran (Islamic Rep.) Total Middle East

6 391 29,839 49,604 7,938 423 261 489 424 9,215 7,119 196 147 14,960 2,851 123,861 968 968 1,206 1,206

3 196 14,920 24,802 3,969 212 130 245 212 4,607 3,559 98 73 7,480 1,426 61,931 484 484 603 603 (continued)

672

Appendices

Table A6.1 (continued) Bagasse potential availability Australia Fiji Papua New Guinea Western Samoa Total Oceania Total world

At 50% humidity (thousand tonnes)

Dry matter (thousand tonnes)

17,581 997 143 7 18,729 337,357

8,791 499 72 3 9,364 168,679

Notes: 1. Bagasse potential availability based on production of cane sugar published in the I.S.O. Sugar Yearbook 2005 , International Sugar Organization 2. Bagasse potential availability conversion factor from United Nations Energy Statistics Yearbook 2004 (assumes a yield of 3.26 tonnes of fuel bagasse at 50% humidity per tonne of cane sugar produced)

Table A6.2 ASTM D6751 specification for B100 biodiesel fuel Flash point, closed cup Water and sediment Kinematic viscosity 40ı C Sulfated ash

Test method ASTM D93 ASTM D2709 ASTM D445 ASTM D874

Limits 93 min 0.05 max 1.9–6.0 0.02 max

Units ı C % volume mm2 /s % mass

Sulfur S 15 grade S 500 grade Copper strip corrosion

ASTM D5453 ASTM D5453 ASTM D130

0.0015 max 0.05 max No 3 max

% mass % mass

Alcohol content (one of the following must be met) Methanol content EN 14110 Flash point, closed cup D93 Cetane number ASTM D613 Cloud point ASTM D2500 Carbon residue ASTM D4530 Acid number ASTM D664 Free glycerin ASTM D6584 Total glycerin ASTM D6584 Phosphorus ASTM D4951 Vacuum distillation end point ASTM D1160 Oxidatione stability EN 14112 3 Calcium and Magnesium EN 14538 (combined)

0.20 max 130 min 47 min Report to customer 0.05 max 0.50 max 0.02 0.24 10 max 360ı C max min 5 max

% volume C

ı ı

C % mass mg KOH/g % mass % mass ppm ı C hours ppm

Source: American Society for Testing and Materials, Standard Specification for Biodiesel Fuel (B100) Blend Stock for Distillate Fuels, Designation D6751-07 (2007)

Appendices

673

Table A6.3 Comparison of certain key parameters for B100 biodiesel fuel with conventional petroleum based diesel fuel Fuel property Fuel standard Lower heating value Kinematic viscosity @ 40ı C Specific gravity @ 60ı C Density Water and sediment Carbon Hydrogen Oxygen Sulfur Boiling point Flash point Cloud point Pour point Cetane number Lubricity SLBOCLE Lubricity HFRR

Diesel ASTM D975 129,050 1.3–4.1 0.85 7.079 0.05 max 87 13 0 0.0015 180–340 60–80 15 to 5 35 to 15 40–55 2,000–5,000 300–600

Biodiesel ASTM D6751 118,170 1.9–6.0 0.88 7.328 0.05 max 77 12 11 0.0–0.0024 315–350 130–170 3 to 12 15 to 10 47–65 >7,000 <300

Units Btu/gal mm2 /s kg/l lb/gal % volume wt. % wt. % wt. % C ı C ı C ı C ı

grams microns

Table A6.4 Comparison of biofuel properties with the standard diesel Property Heat value (MJ kg1 ) Cloud point (ı C) Density @ 15ı C (kg litre1 ) Total sulfur (mass %) Viscosity @ 40ı C (cSt) Carbon residue (wt. %) Ash content (mass %) Flash point (ı C) Pour point (ı C) Initial boiling point (ı C) 10% vol. recovered (ı C) 50% vol. recovered (ı C) 70% vol. recovered (ı C) 90% vol. recovered (ı C) Final boiling point (ı C) Final recovery (vol. %) Residue (vol. %) Loss (vol. %)

Diesel 45.91 – 0.84 – 3.6 <0.1 0.001 98.0 15.0 228 258 298 325 376 400 – – –

B2 45.165 13.4 0.8441 0.2 4.0 <0.1 0.003 81.1 9.0 197.3 242.2 290.5 317.6 360.8 379.4 99.1 0.5 0.4

B5 45.135 13.7 0.8452 0.194 4.1 <0.1 0.004 81.1 9.0 197.3 243.2 293. 322.3 364.3 367.5 98.5 1.0 0.5

B10 44.78 13.6 0.8497 0.178 4.9 <0.1 0.004 83.1 9.0 200.0 246.2 1,298.7 331.0 356.8 357.5 98.5 1.0 0.5

Source: U.S. Department of Energy, Biodiesel Handling and Use Guidelines (2nd Edition, March 2006)

674

Appendices

Table A6.5 Final blend fuel requirements (at point of delivery) Requirements Performance characteristics Flash point, ı C, min. Water and sediment, vol %, max. Physical distillation, T90, ı C, max. Kinematic viscosity, cSt at 40ı C Ash, mass%, max. Sulfur, wt%, max. Copper strip corrosion rating, max. Cetane number, min. Cloud pointa Ramsbottom carbon residue on 10% distillation residue, wt%, max. Lubricity, HFRR at 60ı C, micron, max. Acid number, mg KOH/g, max. Phosphorus, wt%, max. Total Glycerin Alkali metals (Na C K), ppm, max. Alkaline metals (Mg C Ca), ppm max. Blend fraction, vol. %b Thermo-oxidative Stability, insolubles, mg/100 mL, max. Oxidation stability, induction time, hours, minimum

D1 38 0.05

D2 52 0.05

Test procedure ASTM D93 ASTM D2709 or D1796

343

343

ASTM D86

1.3 4.1

1.9 4.1

ASTM D445

0.01 Perregulation No. 3

0.01 Perregulation No. 3

ASTM D482 ASTM D130

43 Per footnote 0.15

43 Per footnote 0.35

ASTM D613 ASTM D2500 ASTM D524

460

460

ASTM D6079

0.3 0.001

0.3 0.001

Nd

Nd

ASTM D664 ASTM D4951 N/A EN14108

Nd

Nd

EN14108

C=2% 10

C=2% 10

EN14078 Modified ASTM D2274c

6

6

EN14112 (Rancimat)

Source: Engine Manufacturing Association (2006) Test specification for biodiesel fuel. www.enginemanufacturers.com Notes: A blend of petroleum diesel fuel meeting ASTM D975 and 100% (neat) biodiesel fuel meeting either ASTM 6751 or EN 14214, where the biodiesel content of the blended fuel is no more than 20% biodiesel by volume (B20), shall meet the requirements identified in Table A6.5 at the point of delivery of the fuel to the end user D1 and D2 Blends – Both Number 1 and Number 2 petroleum diesel fuel (“D1” and “D2”) may be blended with biodiesel for a variety of reasons, including the need for lower temperature operation. D1 and D2 may be blended a The maximum cloud point temperature shall be equal to or lower than the tenth percentile minimum ambient temperature in the geographical area and seasonal timeframe as defined by ASTM D975 b Blend fractions refers to the variation in volume percent of B100 in diesel fuel claimed c Use glass fiber filter

Appendices

675

Table A6.6 European Norm pr EN 15376/Bio-ethanol (low blends E5, E10, E15, E20 etc) Properties Units Min. Max. Test methods Ethanol (incl. higher % wt. 98, 70 – EC/2870/2000 saturated alcohols) method I Higher saturated % wt. – 2.0 EC/2870/2000 monoalcohols (C3-C5) method III Methanol % wt. – 1.0 EC/2870/2000 method III Water content % wt. – 0.3 EN 15489 Inorganic chloride content mg/l – 20.0 EN 15484 Copper mg/kg – 0.1 EN 15488 % wt. – 0.007 EN 15491 Total acidity (as acetic acid CH3 COOH) Appearance (to be – Clear and Clear and Visual inspection determined at ambient bright bright temperature or 15ı C whichever is higher) Phosphorous mg/l – 0.5 EN 15487 Involatile material mg/100 ml – 10.0 EC/2870/2000 method II Sulfur content mg/kg – 10.0 EN 15485, EN 15486 Source: http://www.fuel4life-biofuels.com

Table A6.7 European Norm CWA (EN) 15293/Bioethanol (high blends E75, E85, and up)a Properties Units Min. Max. Test methods Research octane 95.0 – EN ISO 5164 number RON Motor octane number 85.0 – EN ISO 5163 MON Oxidation stability Minutes 360 – EN ISO 7536 pHe 6.5 9.0 ASTM D 6423 Higher saturated % wt. – 2.0 EN 1601 monoalcohols EN 13132 (C3-C8) Methanol % wt. – 1.0 EN 1601 EN 13132 Ethers (5 or more C % wt. – 5.2 EN 1601 atoms) EN 13132 Water content % wt. 0.3 0.3 ASTM E 1064 Inorganic chloride mg/l 1.0 1.0 EN ISO 6227 content rating Class 1 Class 1 EN ISO 2160 Copper strip corrosion (3 h at 50ı ) (continued)

676 Table A6.7 (continued) Properties Total acidity (as acetic acid CH3 COOH) Appearance (to be determined at ambient temperature or 15ı whichever is higher) Phosphorous Involatile material Sulfur content

Appendices

Units % wt mg/l –

Min. 0.005 (40) Clear and bright

Max. 0.005 (40) Clear and bright

Test methods ASTM D 1613

mg/l mg/100 ml mg/kg

Not detectable – –

Not detectable 5.0 20ı

ASTM D 3231 EN ISO 6246 EN ISO 20846 EN ISO 20884

Visual inspection

Source: http://www.fuel4life-biofuels.com a In concept (January 2010)

Table A6.8 Approximate heat values for common firewood species in Virginia. Based on air-dried, standard (40 40 80 ) cords

Species Black locust Hickory Hophornbeam Beech Hard maple Red oak Yellow birch Yellow pine White ash White oak Soft maple Black cherry White birch Sweetgum Elm Yellow poplar Hemlock Red spruce Fir White pine Basswood

Btu/cord 26,500,000 25,400,000 24,700,000 21,800,000 21,800,000 21,700,000 21,300,000 20,500,000 20,000,000 19,200,000 19,100,000 18,500,000 18,200,000 18,100,000 17,700,000 15,900,000 15,000,000 15,000,000 13,500,000 13,300,000 12,600,000

Country/area Angola Botswana Comoros Kenya Lesotho Madagascar Malawi Mauritius Mayotte Mozambique Namibia Seychelles South Africa Swaziland Uganda United Republic of Tanzania Zambia Zimbabwe Algeria

Total (Mt) 10,734 – 2 743 – 6,641 – 9 – 1,382 527 – 1,878 – 315 5,140

1,821 843 183

492 226 44 –

324 150

2,636 1,219 –

– –

61

Other wooded land Dead wood (Mt) 1,076 – n.s. 74 – 382 – 1 – 170 65 – 231 – 39 631

Above-ground biomass (Mt) – 665 – – n.s. – – 2 – 1,728 231 – 1,242 7 12 248

Below-ground biomass (Mt) 2,053 55 n.s. 133 – 1,481 62 2 – 235 98 1 346 9 59 873

Forest

Above-ground biomass (Mt) 7,605 228 1 536 – 4,778 260 6 – 978 364 6 1,302 38 218 3,636

Table A6.9 Biomass stock in forest and other wooded land 2005

16 – –

Below-ground biomass (Mt) – 159 – – n.s. – – n.s. – 829 62 – 596 2 6 119 11 – –

Dead wood (Mt) – – – – n.s. – – n.s. – 358 41 – 257 – 2 51

88 – – (continued)

Total (Mt) – – – – 1 – – 2 – 2,915 335 – 2,095 – 19 418

Appendices 677

Country/area Burkina Faso Chad Djibouti Egypt Ethiopia Libyan Arab Jamahiriya Mali Mauritania Morocco Niger Somalia Sudan Tunisia Western Sahara Cameroon Cape Verde Central African Republic Congo Cˆote d’Ivoire Democratic Republic of the Congo

94 3 112 5 150 827 5 22 1;125 4 1;085

2;005 365 8;970

8;356 3;649 37;376

Below-ground biomass (Mt) 127 100 n.s. 3 107 2

390 10 368 20 624 2;235 15 29 2;679 12 4;519

Above-ground biomass (Mt) 469 371 1 11 396 11

Table A6.9 (continued) Forest

414

387

108 337

442 2,432

–

–

– –

– – – –

Dead wood (Mt) 108 – – 2 70 2

– 4,456 48,778

881 3,398 – – 4,191 – 6,018

– – – –

Total (Mt) 704 – – 16 573 15

Other wooded land

882 – –

514 – n.s. 19 – – 0 – – – –

Above-ground biomass (Mt) – 114 – n.s. 365 3

212 – –

123 – n.s. 5 – – n.s. – – – –

Below-ground biomass (Mt) – 55 – n.s. 175 1

– – –

– – – – – – – – – – –

Dead wood (Mt) – – – n.s. 76 1

(continued)

– – –

– – – – – – – – – – –

Total (Mt) – – – 1 616 4

678 Appendices

Country/area Equatorial Guinea Gabon Gambia Ghana Guinea Guinea-Bissau Liberia Nigeria Rwanda Saint Helena Sao Tome and Principe Senegal China Democratic People’s Republic of Korea Japan Mongolia Republic of Korea Bangladesh Bhutan

175 2;920 125

733 278 132

12 187

3;052 870 383

51 503

–

7 76

230 57

28 1;836 68

70 766

– 1;378 572

769 14;027 532

– –

–

– 1

38 577 –

– 10 – – – – – 17 – –

566 9;271 340

7;677 76 1;132 1;394 160 1;033 3;195 88 – 10

1;314 13 267 246 24 175 543 13 – 3

5;971 53 726 1;026 98 731 2;261 75 – 6

392 9 139 122 38 127 392 n.s. – 0

Above-ground biomass (Mt) –

Total (Mt) 261

Other wooded land

Above-ground biomass (Mt) 186

Dead wood (Mt) 30

Below-ground biomass (Mt) 45

Forest

Table A6.9 (continued)

3

5

– –

– n.s. –

12 219 –

– –

– – – – –

–

Below-ground biomass (Mt) –

1

2

– –

– n.s. –

2 138 –

– –

– – – – –

–

Dead wood (Mt) –

1

– – (continued)

–

–

52 934 –

– 17 – – – – – 21 – –

Total (Mt) –

Appendices 679

Country/area Brunei Darussalam Cambodia India Indonesia Lao People’s Democratic Republic Malaysia Maldives Myanmar Nepal Pakistan Philippines Singapore Sri Lanka Thailand Viet Nam Afghanistan Armenia Azerbaijan Cyprus Georgia 1,053 – 697 145 57 214 – 9 158 258 1 5 – – 59

8,073 – 7,032 1,114 573 2,156 – 88 1,592 2,606 14 41 – – 479

– – – 63 – – – – – – – 1 – – –

– – – 78

1;359 – 1;226 251 135 376 – 15 305 455 4 9 17 1 86

2,811 5,748 13,090 3,301

5;661 – 5;109 718 381 1;566 – 64 1;129 1;893 9 27 99 4 334

279 570 1,297 327

628 1;085 2;926 632

1;904 4;093 8;867 2;342

Total (Mt) 87

Other wooded land Above-ground biomass (Mt) –

Dead wood (Mt) 9

Below-ground biomass (Mt) 15

Forest

Above-ground biomass (Mt) 63

Table A6.9 (continued)

– – – 22 – – – – – – – n.s. – – –

– – – 21

Below-ground biomass (Mt) –

– – – 13 – – – – – – – n.s. – – –

– – – 11

Dead wood (Mt) –

– – – 98 – – – – – – – 1 – – – (continued)

– – – 110

Total (Mt) –

680 Appendices

Country/area Iran (Islamic Republic of) Jordan Kazakhstan Kyrgyzstan Lebanon Saudi Arabia Tajikistan Turkey Turkmenistan United Arab Emirates Uzbekistan Yemen Albania Austria Belarus Belgium Bosnia and Herzegovina Bulgaria Croatia Czech Republic Denmark Estonia 4 1 29 – 216 3 –

– 251 26 76

132 80 113 12 75

395 304 612 40 259 – 54 36 – 16 –

–

351

439 761

– 1,295 133 –

28 12 132

40 38

40 6

–

– –

3

0

– 18 8 – – – –

– 10 – n.s. 205 1 – – n.s.

7 3 25

–

5 n.s. – 5 5

5 312

17 7 78 773 828 104 275

– –

– –

1 38

1 63 8 1 7 2 233 17 10

3 211 17 3 28 4 1;400 17 23

Total (Mt) 763

Other wooded land Above-ground biomass (Mt) –

Dead wood (Mt) 94

Below-ground biomass (Mt) 153

Forest

Above-ground biomass (Mt) 516

Table A6.9 (continued)

– – 0 – 1

– 8 23 – – – –

– 3 – n.s. 55 n.s. – – n.s.

Below-ground biomass (Mt) –

– – 0 – n.s.

– 4 10 – – – –

– 2 – – 36 – – – n.s.

Dead wood (Mt) –

0 4 (continued)

–

– –

– 30 41 – – – –

– 14 – – 297 – – – n.s.

Total (Mt) –

Appendices 681

Country/area Finland France Germany Greece Hungary Iceland Ireland Italy Latvia Liechtenstein Lithuania Luxembourg Malta Moldova, Republic of Netherlands Norway Poland Portugal Romania Russian Federation Serbia 44

356

–

0 37 – 8 – 450

70

– 1,314 88,815

54 727 1,804

242

181 24,396

–

2 37 13

9 103 412 82 229 12;846

43 587 1;379 146 904 51;574

Total (Mt) 1,666 – 2,659 – 340 3 40 1,431 471 – 279 – – –

Other wooded land Above-ground biomass (Mt) 3 – – – – 1 – 88 – – 1 – – 1

Dead wood (Mt) 35 – 54 – n.s. n.s. n.s. 159 9 – 21 – – –

Below-ground biomass (Mt) 281 602 585 19 83 n.s. 7 230 105 n.s. 48 2 n.s. 2

Forest

Above-ground biomass (Mt) 1;351 1;850 2;020 98 257 3 33 1;043 357 1 210 17 n.s. 24

Table A6.9 (continued)

5

–

– 300

–

0 4

Below-ground biomass (Mt) 1 – – – – n.s. – 37 – – n.s. – – 1

–

– – – 750

0 5

Dead wood (Mt) n.s. – – – – n.s. – 12 – – n.s. – – –

– (continued)

– – – 1,500

0 46

Total (Mt) 4 – – – – 2 – 137 – – 2 – – –

682 Appendices

Country/area Slovakia Slovenia Spain Sweden Switzerland The former Yugoslav Republic of Macedonia Ukraine United Kingdom Cuba Dominican Republic Haiti Jamaica Puerto Rico Trinidad and Tobago Belize Costa Rica Guatemala Nicaragua

5 13 7 9

24 161 241 278

94 224 755 1;154

4 18 42 149 158

–

2 8 51 137 427 1;145 1;590

–

19 76

1 – – –

– 34 – –

– 1 – –

12 55 35 38

1;698 227 789 188

290 34 171 32

1;199 190 569 132

208 3 49 24

Above-ground biomass (Mt) – 2 1 32 – –

Total (Mt) 438 342 – 3;010 316 –

Below-ground biomass (Mt) 73 65 210 530 60 8

Above-ground biomass (Mt) 334 229 661 1;810 248 33

Dead wood (Mt) 32 48 – 670 8 –

Other wooded land

Forest

Table A6.9 (continued)

n.s. – – –

– 8 – –

– 0 – –

Below-ground biomass (Mt) – n.s. n.s. 9 – –

n.s. – – –

– 5 – –

– 0 – –

Dead wood (Mt) – n.s. – 14 – –

1 – – – (continued)

– 47 – –

– 1 – –

Total (Mt) – 3 – 55 – –

Appendices 683

Country/area Panama United States of America American Samoa Australia Nauru New Caledonia Argentina Bolivia Brazil Chile Colombia Guyana Suriname

Below-ground biomass (Mt) 258 6,276

1 5,581 – 28 993 2,740 22,017 649 4,180 619 3,367

Above-ground biomass (Mt) 980 31,653

3 12,929 – 118 3,824 7,828 79,219 3,243 11,945 2,824 8,016

Table A6.9 (continued) Forest

– 4,909 – 13 516 1,163 6,359 739 2,419 378 1,252

Dead wood (Mt) 136 5,350 – 23,419 – 160 5,333 11,731 107,595 4,631 18,544 3,821 12,635

Total (Mt) 1,374 43,279

Other wooded land

– – – – – – – – 3,453 – –

Above-ground biomass (Mt) 91 – – – – – – – – – 1,209 – –

Below-ground biomass (Mt) 38 –

– – – – – – – – 699 – –

Dead wood (Mt) 14 –

– – – – – – – – 5,361 – –

Total (Mt) 144 –

684 Appendices

Fat Soybean Sunflower Sunflower Sunflower Soybean Soybean Ricebran Soybean Soybean Soybean Corn Cottonseed Opium poppy Rapeseed Soybean Sunflower Castor Honne Jatropha Karanja Mahua Neem Olive Palm

Crude Crude

Crude

Degummed Salad oil Once-refined

Variety or processing Alkali-refined Crude Degummed Dewaxed Degummed Crude

51.7

Viscosity, ı mm2 /s, 40 C ASTM-D 445, ASTM-D 88

0.915 0.912 0.921 0.914 0.914 0.918

0.935

Density, kg/L, 15.6ı C ASTM-D 1298 Density, kg/L 25ı C 0.922 0.918 0.92 0.92 0.918 0.921

Surface tension, dynes/cm ASTM-D 971

Table A6.10 Various chemical and physical properties of different oils as potential feedstock for biofuel

42 (continued)

40–45

37.6 37.9 37.1

36.553

37.6 48.1

30

Cetane ASTM-D 613, ASTM-D 976

45 47

Gross calorific value, MJ/kg ASTM-D 270 38.9 39.6 39.3 38.8 39 38.8

39.648 38.92 37.62 39.623 39.525 39.5 33.058 39.774 37.1 30.248 39.399

38.952

Net calorific value, MJ/kg

Appendices 685

Restaurant grease Restaurant grease Restaurant grease Restaurant grease Restaurant grease

Fat Canola Cottonseed Neem Peanut Rapeseed Safflower Safflower Soybean Stillingia Sunflower Tallow seed Restaurant grease

Unprocessed

Unprocessed

Unprocessed

Boiled

Processed mixed with unprocessed Processed

Linoleic Oleic

Variety or processing

Table A6.10 (continued)

Viscosity, ı mm2 /s, 40 C ASTM-D 445, ASTM-D 88 39 71.9 140.3 56.8 51 32.3 42.1 44.6 40 34.9 35 Density, kg/L, 15.6ı C ASTM-D 1298 0.92 0.923 0.922 0.918 0.91 0.93 0.92 0.923 0.91 0.92 0.882 Density, kg/L 25ı C

Surface tension, dynes/cm ASTM-D 971 Net calorific value, MJ/kg

Gross calorific value, MJ/kg ASTM-D 270 39.721 43.957 41.64 38.79 39.923 39.542 39.165 40.9 39.124 39.486 39.193

(continued)

35.4

40.1

Cetane ASTM-D 613, ASTM-D 976

686 Appendices

Palm oil Cottonseed Crambe Linseed Safflower Walnut Sunflower Beechnut Corn Soybean Poppy Rapeseed Hazelnut kernel Peanut Beech Castor

Restaurant grease Restaurant grease Restaurant grease

Fat

Solids and free water removed

Unprocessed

Unprocessed

Variety or processing

Table A6.10 (continued)

40 34.6 29.7

38.23 33.7 53.2 28 31.6 36.8 34.4 38 35.1 33.1 42.4 37.3 24

Viscosity, ı mm2 /s, 40 C ASTM-D 445, ASTM-D 88

0.9102 @ 20

Density, kg/L, 15.6ı C ASTM-D 1298 Density, kg/L 25ı C

Surface tension, dynes/cm ASTM-D 971

36.543

Net calorific value, MJ/kg

39.45 39.59 37.41

39.047 39.44 40.62 39.33 39.52 39.56 39.57 39.82 39.64 39.63 39.59 39.73 39.83

Gross calorific value, MJ/kg ASTM-D 270

34.6 36.2 42.3 (continued)

42 33.6 36.7 38.2 37.5 38.1 36.7 37.5 35.8

42 33.7

Cetane ASTM-D 613, ASTM-D 976

Appendices 687

Ailanthus Safflower Spruce Sesame Bay laurel Corn marrow Olive Almond Walnut Wheat grain Cottonseed Soybean Castor Corn Cottonseed Crambe Linseed Peanut Rapeseed Safflower Safflower Sesame Soybean

Fat

High oleic

High oleic

Variety or processing

Table A6.10 (continued)

30.2 40.8 35.6 36 23.2 35.1 29.4 34.2 24 32.6 33.7 33.1 297 34.9 33.5 53.6 27.2 39.6 37 41.2 31.3 35.5 32.6

Viscosity, ı mm2 /s, 40 C ASTM-D 445, ASTM-D 88

0.9537 0.9095 0.9148 0.9044 0.9236 0.9026 0.9115 0.9021 0.9144 0.9133 0.9138

Density, kg/L, 15.6ı C ASTM-D 1298 Density, kg/L 25ı C

Surface tension, dynes/cm ASTM-D 971

37.274 39.5 39.468 40.482 39.307 39.782 39.709 39.516 39.519 39.349 39.623

Net calorific value, MJ/kg 39.38 39.61 39.44 39.42 38.32 39.6 39.7 39.8 39.8 39.3 39.4 39.6

Gross calorific value, MJ/kg ASTM-D 270

35.1 48.8 34.2 40.4 33.6 37.5 49.3 34.5 52.9 35.2 33.7 38.1 7 37.6 41.8 44.6 34.6 41.8 37.6 49.1 41.3 40.2 37.9 (continued)

Cetane ASTM-D 613, ASTM-D 976

688 Appendices

Sunflower Cottonseed Cottonseed

Fat Sunflower Peanut Deccan hemp Cottonseed Sunflower Rapeseed Rapeseed Rapeseed Canola Rapeseed Safflower Safflower Sunflower Jatropha Soybean Soybean Sunflower

Crude Crude Degummed Used cooking oil Crude Once-refined Bleached and deodorized

High erucic Linoleic Oleic

Heated Unheated

Crude

Variety or processing

Table A6.10 (continued)

32.56 34.25

72.6

39 51 32.3 42.1 34.9

35.8 34.2 38

Viscosity, ı mm2 /s, 40 C ASTM-D 445, ASTM-D 88 33.9 42.3

0.91001 0.9104

0.923 0.922 0.915

0.925 @15 0.9231 0.916 0.884 0.918 0.92 0.91 0.93 0.92 0.92

Density, kg/L, 15.6ı C ASTM-D 1298 0.9161 0.915

0.92

Density, kg/L 25ı C

32.1 32.6 32.55

35

Surface tension, dynes/cm ASTM-D 971

37.188 37.418 37.517

36.87 39.08 39.08 36.543 36.33 36.379 36.032 36.327 36 39.558 39.591 36.94

39.575

Net calorific value, MJ/kg

39.849 39.993

40.3 38.72 63.5 39.5

Gross calorific value, MJ/kg ASTM-D 270

(continued)

59.5 41.8 44–48

37.1

Cetane ASTM-D 613, ASTM-D 976

Appendices 689

Sunflower Sunflower Sunflower Sunflower Babassu Palm Canola Palm Rapeseed Sunflower Corn Cottonseed Rapeseed

Soybean Soybean Soybean Soybean

Cottonseed Peanut Peanut Peanut

Fat

Crude Crude Canola

Crude Crude Once-refined Bleached and deodorized Once-refined Degummed Crude Light hydrogenated Dewaxed Once-refined Crude Deodorized

Variety or processing

Table A6.10 (continued)

30.61 30.69 30.96 31.67 30.3 39.6 37.82 40.85 51 33.9

30.55 31.28 32.23 37.54

34.89 36.33 37.16 37.38

Viscosity, ı mm2 /s, 40 C ASTM-D 445, ASTM-D 88

0.9153 0.921 0.9174

0.90747 0.90785 0.90867 0.90747 0.946 0.918

0.90878 0.90182 0.90198 0.90288

0.90899 0.90591 0.91292 0.90159

Density, kg/L, 15.6ı C ASTM-D 1298 Density, kg/L 25ı C

33.2 32.82 32.01 32.82

32.13 32.13 32.01

31.95 32.22 31.89 32.28

Surface tension, dynes/cm ASTM-D 971

36.75

39.5

37.626 37.616 37.528 37.688

37.76 36.952 36.981 37.332

36.958 37.102 37.241 37.433

Net calorific value, MJ/kg

40.075 40.06 39.956 40.135

40.204 39.388 39.388 39.82

39.4 39.614 39.749 39.929

Gross calorific value, MJ/kg ASTM-D 270

(continued)

38 42 33.5 52

Cetane ASTM-D 613, ASTM-D 976

690 Appendices

Table A6.10 (continued) Cold filter clogging pointı C ASTM-EN 116, IP 309 Fat Soybean Sunflower Sunflower Sunflower Soybean Soybean Ricebran Soybean Soybean Soybean Corn Cottonseed Rapeseed Soybean Sunflower Castor Mahua Neem Palm Palm oil Cottonseed Crambe Linseed

6

31.7

Pour pointı C ASTM-D 97

27 14

Cloud pointı C ASTM-D 2500

235 274 240

Flash pointı C ASTM-D 93 269 255 257 262 153 229 200 247 340 279 270–295 210 275–290 230 220 260 99 180 344

Fire pointı C Auto-ignition point

Initial boiling point, 0.1% evaporatedı C ASTM-D 86

(continued)

Final boiling point, 99.5% evaporatedı C ASTM-D 86

Appendices 691

Table A6.10 (continued) Cold filter clogging pointı C ASTM-EN 116, IP 309 Fat Safflower Walnut Beechnut Corn Soybean Poppy Rapeseed Hazelnut kernel Peanut Beech Castor Ailanthus Safflower Spruce Sesame Bay laurel Corn Cottonseed Crambe Linseed Peanut Rapeseed Safflower Cloud pointı C ASTM-D 2500

1.1 1.7 10 1.7 12.8 3.9 12.2

Pour pointı C ASTM-D 97

40 15 12 15 6.7 31.7 20.6

Flash pointı C ASTM-D 93 260 232 260 276 255 265 245 230 270 242 260 238 292 238 262 226 277 234 274 241 271 246 293 Fire pointı C Auto-ignition point

Initial boiling point, 0.1% evaporatedı C ASTM-D 86

(continued)

Final boiling point, 99.5% evaporatedı C ASTM-D 86

692 Appendices

Table A6.10 (continued) Cold filter clogging pointı C ASTM-EN Fat 116, IP 309 Safflower Sesame Soybean Sunflower Deccan Hemp Cottonseed 12 Sunflower 18 Rapeseed 15 Jatropha Sunflower Sunflower Sunflower Cottonseed Cottonseed Cottonseed Peanut Peanut Peanut Soybean Cloud pointı C ASTM-D 2500 18.3 3.9 3.9 7.2 8.7 6.7

0 1 0 10 10 10 1

Pour pointı C ASTM-D 97 6.7 9.4 12.2 15

10.2 12.2 20

4 4 3 2 0 1 9

Flash pointı C ASTM-D 93 260 260 254 274 255 242 232 220–300 225 290 290 275 325 323 300 296 235 330 320 340

270

Fire pointı C

Auto-ignition point

358.7 355.5 339.9 345.3 359.4 361.3 360.7

Initial boiling point, 0.1% evaporatedı C ASTM-D 86

681.4 681.9 683.3 680.5 680 679.5 681.4 (continued)

Final boiling point, 99.5% evaporatedı C ASTM-D 86

Appendices 693

Soybean Soybean Soybean Sunflower Sunflower Sunflower Sunflower Babassu Palm Canola Palm Sunflower Peanut Olive Soybean Corn Cottonseed Rapeseed

17

Table A6.10 (continued) Cold filter clogging pointı C ASTM-EN Fat 116, IP 309

10 10 2 9 11 9 9

Pour pointı C ASTM-D 97 Flash pointı C ASTM-D 93 315 312 325 322 328 314 323 150 267 240 266 274 282 225 220

Cloud pointı C ASTM-D 2500 7 6 32–37 16 16 10 7 20 31

Fire pointı C

445 343 445 393 343 315

Auto-ignition point 360.7 344.2 359.4 358.7 358.7 358.1 360

Initial boiling point, 0.1% evaporatedı C ASTM-D 86

(continued)

352

679.1 679.5 681.4 681.4 682.3 682.8 681.9

Final boiling point, 99.5% evaporatedı C ASTM-D 86

694 Appendices

Fat Soybean Sunflower Sunflower Sunflower Soybean Soybean Ricebran Animal fat Restaurant grease Yellow grease Restaurant grease Restaurant grease Restaurant grease Restaurant grease Restaurant grease Restaurant grease

Carbon residue, % weight ASTM-D 189, ASTM-D 524

Table A6.10 (continued)

0.002 0.01 0.01 0.01 0.01 0.2

Ash, % in weight ASTM-D 482 Particulate matter, mg/100 ml

Sediment content by the extraction method, % volume ASTM-D 473

2.63 0.99 2.53 0.34 0.43 0.42 0.38 0.47 4.83

3.83 0.09 <0.1 0.03 0.03 0.08 1.22

Unsaponifiable matter, % weight AOCS Method Ca 6a-40

0.11 1.03

Insoluble impurities, % weight AOCS Method Ca 3a-46 Water by Karl-Fischer, % weight ASTM-D 1744

(continued)

0.3

Water and sediment content, % weight ASTM-D 1796

Appendices 695

Restaurant grease Restaurant grease Soybean Soybean Palm oil Cottonseed Crambe Linseed Safflower Walnut Sunflower Beechnut Corn Soybean Poppy Rapeseed Hazelnut kernel

Fat

0.25 0.23 0.24 0.26 0.24 0.28 0.23 0.22 0.24 0.25 0.31 0.21

Carbon residue, % weight ASTM-D 189, ASTM-D 524

Table A6.10 (continued)

0.003 0.02 0.04 0.01 0.007 0.02 0.01 0.03 0.01 0.006 0.02 0.006 0.01

Ash, % in weight ASTM-D 482 Particulate matter, mg/100 ml

Sediment content by the extraction method, % volume ASTM-D 473 0.52 0.25 0.43 0.41

2.51 <0.1 <0.1

Unsaponifiable matter, % weight AOCS Method Ca 6a-40

0.11

Insoluble impurities, % weight AOCS Method Ca 3a-46 Water by Karl-Fischer, % weight ASTM-D 1744

(continued)

0

Water and sediment content, % weight ASTM-D 1796

696 Appendices

Fat Peanut Beech Castor Ailanthus Safflower Spruce Sesame Bay laurel Corn marrow Olive Almond Walnut Wheat grain Cottonseed Soybean Castor Corn Cottonseed

Carbon residue, % weight ASTM-D 189, ASTM-D 524 0.22 0.24 0.21 0.22 0.24 0.26 0.25 0.2 0.22 0.23 0.22 0.21 0.23 0.25 0.24 0.22 0.24 0.24

Table A6.10 (continued)

Ash, % in weight ASTM-D 482 0.02 0.04 0.01 0.02 0.01 0.01 0.002 0.03 0.01 0.02 0.01 0.01 0.02 0.02 0.006 <0.01 0.01 0.01 Particulate matter, mg/100 ml

Sediment content by the extraction method, % volume ASTM-D 473

Insoluble impurities, % weight AOCS Method Ca 3a-46 Unsaponifiable matter, % weight AOCS Method Ca 6a-40

Water by Karl-Fischer, % weight ASTM-D 1744

Trace Trace 0.04 (continued)

Water and sediment content, % weight ASTM-D 1796

Appendices 697

Fat Crambe Linseed Peanut Rapeseed Safflower Safflower Sesame Soybean Sunflower Peanut Cottonseed Sunflower Rapeseed Canola Safflower Safflower Jatropha

305 <0.01 0.005 0.054 <0.001 0.006 <0.01 <0.01 <0.01

0.23 0.22 0.24 0.3 0.24 0.25 0.25 0.27 0.23

0.002 Absent 0.01 0.0043 0.0046 0.0074 0.00984

Ash, % in weight ASTM-D 482

Carbon residue, % weight ASTM-D 189, ASTM-D 524

Table A6.10 (continued)

Particulate matter, mg/100 ml

Absent 0.002

Sediment content by the extraction method, % volume ASTM-D 473

0.02

Insoluble impurities, % weight AOCS Method Ca 3a-46

0.61

Unsaponifiable matter, % weight AOCS Method Ca 6a-40

Water by Karl-Fischer, % weight ASTM-D 1744

(continued)

Absent Absent

0.2 Trace Trace Trace Trace Trace Trace Trace Trace

Water and sediment content, % weight ASTM-D 1796

698 Appendices

Fat Cottonseed Cottonseed Cottonseed Peanut Peanut Peanut Soybean Soybean Soybean Soybean Sunflower Sunflower Sunflower Sunflower Palm

7.15

Carbon residue, % weight ASTM-D 189, ASTM-D 524

Table A6.10 (continued)

Ash, % in weight ASTM-D 482 0.01 <0.01 0.23 <0.01 <0.01 <0.01 <0.01 0.04 0.08 <0.01 <0.01 <0.01 0.03 <0.01 <0.01 Particulate matter, mg/100 ml 15.6 17 385 20.2 3 27.8 1 14.7 101 2.4 2 1.6 187 1.6

Sediment content by the extraction method, % volume ASTM-D 473

Insoluble impurities, % weight AOCS Method Ca 3a-46 Unsaponifiable matter, % weight AOCS Method Ca 6a-40

Water by Karl-Fischer, % weight ASTM-D 1744 0.64 0.012 0.112 0.122 0.059 0.02 0.066 0.068 0.036 0.03 0.089 0.104 0.066 0.02 0.037

(continued)

Water and sediment content, % weight ASTM-D 1796

Appendices 699

2.85

Soybean Sunflower Sunflower Sunflower Soybean Soybean Ricebran Castor Coconut Corn Cottonseed Crambe Linseed Olive Palm Peanut Rapeseed Safflower Sesame Sunflower Animal fat

0.11

Moisture, volatiles, insolubles, and unsaponifiables % weight

Table A6.10 (continued) Moisture and volatiles by hotplate % weight AOCS Method Ca 2b-38 Fat

25.7

0.2 1.3 1.5 1.5 6.1 6.2 7

Free fatty acid, % weight as oleic acid Acid value, mg KOH/g

82–88 6/12/2009 10–140 90–140 93 168–204 75–94 35–61 80–106 94–120 126–152 104–120 110–143

109 130 128 130 109 110

Iodine value, g I/100g oil

Saturation level

<0.2

6 12 14 17 4 11

(continued)

Peroxide number, ppm Oxygen

700 Appendices

Brown grease Restaurant grease Lard Yellow grease Restaurant grease Restaurant grease Restaurant grease Restaurant grease Restaurant grease Restaurant grease Restaurant grease Soybean Soybean Soybean Tallow

2.37

6.42 3.54

0.74

1.71

1.06

24.11

0.98

58.14

0.44 0.41

0.26 3.11

0.31

1.26

0.65

18.06

0.35

55.38

0.01 <0.10

Moisture, volatiles, insolubles, and unsaponifiables % weight

0.35

Table A6.10 (continued) Moisture and volatiles by hotplate % weight AOCS Method Ca 2b-38 Fat

0.02 0.01

14.8

1.1

41.8

1.3

0.7

9.7

25.5 2.6

10.5

Free fatty acid, % weight as oleic acid Acid value, mg KOH/g

Iodine value, g I/100g oil

47–63

15.34

41–50

37.03

Saturation level

66 7.3

0.6

3.4

0.8

3.1

4.6

4

<0.2 3.7

1

(continued)

Peroxide number, ppm Oxygen

Appendices 701

Yellow grease Cottonseed Crambe Linseed Safflower Walnut Sunflower Beechnut Corn Soybean Poppy Rapeseed Hazelnut kernel Peanut Beech Castor Ailanthus Safflower Spruce Sesame

Table A6.10 (continued) Moisture and volatiles by hotplate % weight AOCS Method Ca 2b-38 Fat

Moisture, volatiles, insolubles, and unsaponifiables % weight Free fatty acid, % weight as oleic acid Acid value, mg KOH/g

119.35 105.15 88.72 107.18 88.57 96.08 91.76

113.2 99.83 156.74 139.83 135.24 132.32 110.64 119.41 120.52 116.83 108.05 98.62

Iodine value, g I/100g oil 38.63

Saturation level

(continued)

Peroxide number, ppm Oxygen

702 Appendices

Bay laurel Corn marrow Olive Almond Walnut Wheat grain Cottonseed Soybean Castor Corn Cottonseed Crambe Linseed Peanut Rapeseed Safflower Safflower Sesame Soybean Sunflower Peanut

0.08

Table A6.10 (continued) Moisture and volatiles by hotplate % weight AOCS Method Ca 2b-38 Fat

Moisture, volatiles, insolubles, and unsaponifiables % weight

0.131

Free fatty acid, % weight as oleic acid

0.21 0.11 0.07 0.36 0.2 0.2 1.14 0.26 0.7 4.96 0.2 0.15

Acid value, mg KOH/g

90.2

69.82 119.41 100.16 102.35 98.62 120.96 113.2 69.82

Iodine value, g I/100g oil

Saturation level

9.6 18.4 64.8 26.5 33.7 82.7 30.2 13.6 56.4 22.4 44.5 10.7 (continued)

Peroxide number, ppm Oxygen

Appendices 703

Table A6.10 (continued) Moisture and volatiles by hotplate % weight AOCS Method Ca 2b-38 Fat Deccan hemp Cottonseed Sunflower Rapeseed Cottonseed Cottonseed Cottonseed Peanut Peanut Peanut Soybean Soybean Soybean Soybean Sunflower Sunflower Sunflower Sunflower Sunflower 0.03

Moisture, volatiles, insolubles, and unsaponifiables % weight

0.081 0.027 0.0585 0.171 0.062 0.0555

0.073 0.0629 0.022

0.0192 0.034

Free fatty acid, % weight as oleic acid

111 109 108:94 109:18 92:36 95:09 95:32 130:81 128:57 129:57 102:49 132:01 134:5 131:93 132:99 130 2

0.1

65:48

Iodine value, g I/100g oil

0.24

Acid value, mg KOH/g

Saturation level

149 137 2:46 48:5 240 21:3 162 53:8 43:2 8:41 304 246 262 72:5 14 (continued)

Peroxide number, ppm Oxygen

704 Appendices

Fat Soybean Sunflower Sunflower Sunflower Soybean Soybean Palm oil Cottonseed Crambe Linseed Safflower Walnut Sunflower Beechnut Corn Soybean Poppy Rapeseed Hazelnut kernel Peanut Castor Ailanthus

15

Oxidation stability, mg/100 ml ASTM-D 2274

Table A6.10 (continued)

Jet fuel thermal oxidation test rating ASTM-D 3241 Induction period

Copper corrosion rating ASTM-D 130

178.59 178.59 188.71 190.23 190.82 191.7 193.52 194.14 194.61 196.82 197.07 197.63 199.8 202.71 206.34

Saponification value, mg KOH/g oil 189 197 201 208 190 192

Carbon, % weight

(continued)

Hydrogen, % weight

Appendices 705

Table A6.10 (continued) Oxidation stability, mg/100 ml Fat ASTM-D 2274 Beech Safflower Spruce Sesame Bay laurel Corn marrow Olive Almond Walnut Wheat grain Cottonseed Soybean Castor Corn Cottonseed Crambe Linseed Peanut Rapeseed Safflower Safflower Sesame Soybean

Jet fuel thermal oxidation test rating ASTM-D 3241 Copper corrosion rating ASTM-D 130

1A 1A 1A 1A 1A 1A 1A 1A 1A 1A 1A

Induction period

95 9.3 7.3 9 2.9 6.4 10 9.8 3.1 8.7 7.4

Saponification value, mg KOH/g oil 202.16 206.82 207.79 210.34 220.62 194.14 196.83 197.56 197.63 205.68 207.71 220.78

Carbon, % weight

(continued)

Hydrogen, % weight

706 Appendices

Fat Sunflower Cottonseed Sunflower Rapeseed Sunflower Cottonseed Cottonseed Cottonseed Peanut Peanut Peanut Soybean Soybean Soybean Soybean Sunflower Sunflower Sunflower Sunflower Babassu Palm Canola Palm Rapeseed Sunflower

0.1

Oxidation stability, mg/100 ml ASTM-D 2274

Table A6.10 (continued)

4 1 4C 1 1 2 0 4 4C 1 1 2 4C 3

Jet fuel thermal oxidation test rating ASTM-D 3241 Induction period 5.4

1A

1A FP 1A FP 1B 1A FP FP FP FP 1B 1B FP 1A

Copper corrosion rating ASTM-D 130 1A 1A 1A

206.34

195.3

Saponification value, mg KOH/g oil

77.6

77.3 76.61 77.2 77.5 76.8 77.4 77.3 77.3 77.4 77.3 76.9 77.4 77.7 77.6 77.5 77.7

Carbon, % weight

(continued)

11.7

11.9 12.09 11.5 11.7 11.5 11.8 11.8 11.8 11.5 11.4 11.3 11.7 11.5 11.5 11.4 11.5

Hydrogen, % weight

Appendices 707

Sunflower Sunflower Sunflower Soybean Soybean Palm oil Cottonseed Crambe Linseed Safflower Walnut Sunflower Beechnut Corn Soybean Poppy Rapeseed Hazelnut kernel Peanut Beech Castor Ailanthus Safflower

Fat

Oxygen, % weight

Table A6.10 (continued)

0.01 0.01 0.01 0.01 0.02 0.01 0.008 0.01 0.01 0.01 0.01 0.02 0.01 0.006 0.01 0.01 0.02

Sulfur % weight ASTM-D 1266, ASTM-D 129, IP 336 Nitrogen, % weight 0.46 0.07 0.05 0.14 0.97

Phosphorus % weight AOCS Method Ca 12–55 Phosphorus ppm

(continued)

12.987

Stochiometric air:fuel

708 Appendices

Fat Spruce Sesame Bay laurel Corn marrow Olive Almond Walnut Wheat grain Cottonseed Soybean Castor Corn Cottonseed Crambe Linseed Peanut Rapeseed Safflower Safflower Sesame Soybean Sunflower Peanut Cottonseed

Oxygen, % weight

Table A6.10 (continued)

Sulfur % weight ASTM-D 1266, ASTM-D 129, IP 336 0.01 0.01 0.02 0.01 0.02 0.01 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.01 Nitrogen, % weight

0.015

Phosphorus % weight AOCS Method Ca 12–55

3 7 8 12 6 9 18 0.42 20 10 32 15

Phosphorus ppm

(continued)

Stochiometric air:fuel

Appendices 709

10.5

10.8 10.6 10.7 11 10.2 10.3 10.3 10.5 10.7 10.7 10.7 10.5 10.6 10.5 10.4

Sulfur % weight ASTM-D 1266, ASTM-D 129, IP 336 Absent 0.0002 Not detected Not detected Not detected Not detected Not detected Not detected Not detected Not detected Not detected Not detected Not detected Not detected Not detected Not detected 0.044 <0.0001 0.002 0.006 0.035 0.003 0.001 Not detected 0.001 0.004 0.009 Not detected 0.002 0.001 0.004 0.004

Nitrogen, % weight

0.01

Not detected Not detected 0.062 Not detected Not detected Not detected Not detected 0.015 0.027 Not detected Not detected Not detected 0.007 Not detected

Phosphorus % weight AOCS Method Ca 12–55

Source of data: Courtesy of Creative commons. C:nUsersnOwnernDesktopnFuel Properties of Various Oils and Fats.mht

Fat Sunflower Rapeseed Cottonseed Cottonseed Cottonseed Peanut Peanut Peanut Soybean Soybean Soybean Soybean Sunflower Sunflower Sunflower Sunflower Palm Rapeseed Sunflower

Oxygen, % weight

Table A6.10 (continued) Phosphorus ppm

12.63

Stochiometric air:fuel

710 Appendices

Appendices

711

Appendix 7: Ethanol

Fig. A7.1 Effect of different amount of ethanol on octane number of gasoline-ethanol blended fuel

Fig. A7.2 Effect of different amount of ethanol on lower heating value of gasoline-ethanol blended fuel

712

Appendices

Fig. A7.3 Effect of different amount of ethanol on stoichiometric air/fuel ratio of gasoline-ethanol blended fuel

Fig. A7.4 Effect of different amount of ethanol on Reid vapor pressure of gasoline-ethanol blended fuel

Appendices

713

Fig. A7.5 Effect of different amount of ethanol on specific gravity of gasoline-ethanol blended fuel

Table A7.1 E10 fuel properties Form Appearance Odor Flash point Auto ignition temperature Thermal decomposition Lower explosive limit Upper explosive limit pH Freezing point Boiling point Vapor Pressure Relative Vapor Density Density Water solubility Viscosity, dynamic Viscosity, kinematic Percent volatiles Conductivity

Liquid Clear, straw colored Characteristic hydrocarbon-like 45ı C (49ı F) 257.22ı C (495.00ı F) No decomposition if stored and applied as directed 0.3% (V) 7.6% (V) Not applicable No data available 85–437ı F (39–200ı C) 345–1,034 hPa at 37.8ı C (100.0ı F) Approximately 3–4 0.8 g/cm3 Negligible No data available No data available 100% (Conductivity can be reduced by environmental factors such as a decrease in temperature)

Hydrocarbon liquids without static dissipater additive may have conductivity below 1 picoSiemens per meter (pS/m). The highest electro-static ignition risks are associated with “ultra-low conductivities” below 5 pS/m

E-15 15 (2.2) 3.3 13 (55) 19 42 (108) 13 (55) 230 (445) 0.851 (7.10) 1.6 40.4 (17,400) 34.4 (123,000) 44.2 (19,000) 37.7 (135,000) 350 (150) 0.10 (0.39)

Diesel <3 (<0.4) 0.6 64 (145) 5.6 150 (300) 64 (145) 230 (445) 0.863 (7.20) 5.5 42.6 (18,300) 36.7 (132,000) 46.5 (20,000) 40.2 (144,000) 270 (120) 0.046 (0.18)

29.8 (12,800) 23.4 (83,900) 840 (360) 0.10 (0.39)

27.0 (11,600) 21.2 (76,000)

19 42 (108) 13 (55) 366 (691) 0.785 (6.55) 1.6

3.3 13 (55)

Ethanol 15 (2.2)a

46.7 (20,100) 34.8 (125,000) 350 (150) 0.064 (0.25)

43.9 (18,900) 32.7 (117,000)

7.6 20 (4) 43 (45) 300 (570) 0.791 (6.6) 3.5

1.4 45 (49)

Gasoline 62 (9.0)

Waterland LR, Venkatesh S, Unnasch S (2003) Safety and performance assessment of ethanol/diesel blends (E-Diesel). Report No. NREL/SR-540-34817 a Ethanol has blending Reid vapor pressure in hydrocarbon fuels of 120–125 kPa (17–18 psi)

Property Reid vapor pressure, kPa (psi) Lower flammability limit Concentration, vol% Temperature, ı C (ı F) Upper flammability limit Concentration, vol% Temperature, ı C (ı F) Flash point, ı C (ı F) Autoignition temperature, ı C (ı F) Density, kg/L (lb/gal) Vapor specific gravity, (air D 1) Lower heating value Mass, MJ/kg (Btu/lb) Volume, MJ/L (Btu/gal) Higher heating value Mass, MJ/kg (Btu/lb) Volume, MJ/L (Btu/gal) Latent heat of vaporization, kJ/kg (Btu/lb) Diffusivity, cm2 /s (ft2 /hr)

Table A7.2 Comparison of fuel properties of E-Diesel (E-15) with other fuels

714 Appendices

Appendices

715

Appendix 8: Hydrogen Liquid Hydrogen Facts Form: Para

Ortho -

H+

-

-

H+

H+

H+

-

Normal Hydrogen: 75% Ortho, 25% Para Liquid Hydrogen: 0.2% Ortho, 99.8% Para Heat of Conversion from Normal to Para: 0.146 kWhth /kg Heat of Liquefaction: 0.123 kWhth /kg

Table A8.1 Properties of ortho and para hydrogen Physical properties Triple point Temperature (K) Pressure (KPa) Density (solid) (kg/m3 ) Density (liquid) (kg/m3 ) Density (vapor) (kg/m3 ) Boiling point at 101.3 Kpa (K) Heat of vaporization (J/mol)

Para-hydrogen

Ortho (normal)-hydrogen

13.803 7.04 86.48 77.03 0.126 20.268 898.30

13.957 7.2 86.71 77.21 0.130 20.39 899.1

Liquid phase Density (kg/m3 ) Cp (J/mol/K) Cv (J/mol/K) Enthalpy (J mol) Entropy (J/mol/K) Viscosity (m Pa s) Velocity of sound (m/s) Thermal conductivity (W/m/K) Compressibility factor

70.78 19.70 11.60 516:6 16.08 13:2 103 1089 98:92 103 0.01712

70.96 19.7 11.6 548.3 34.92 13:3 103 1101 100 103 0.01698

Gaseous phase Density (kg/m3 ) Cp (J/mol/K) Cv (J/mol/K)

1.338 24.49 13.10

1.331 24.60 13.2 (continued)

716

Appendices

Table A8.1 (continued) Physical properties Enthalpy (J/mol) Entropy (J/mol/K) Viscosity (m Pa s) Velocity of sound (m/s) Thermal conductivity (W/m/K) Compressibility factor

Para-hydrogen 381.61 60.41 1:13 103 355 60:49 103 0.906

Ortho (normal)-hydrogen 1447.4 78.94 1:11 103 357 16:5 103 0.906

Critical point Temperature (K) Pressure (MPa) Density (kg/m3 )

32.976 1.29 31.43

33.19 1.325 30.12

Properties at STP (273.15 K, 101.3Kpa) 0.0899 0. Density (kg/m3 ) Cp (J/mol/K) 30.35 Cv (J/mol/K) 21.87 Viscosity (m Pa s) 8:34 103 Velocity of sound (m/s) 1246 Thermal conductivity (W/m/K) 182:6 103 Compressibility factor 1.0005 Dielectric constant 1.00027 Prandtl number 0.6873

0899 28.59 20.3 8:34 103 1246 173:9 103 1.00042 1.000271 0.680

Table A8.2 Hydrogen standards and codes already published ISO Description Stage ISO 13984:1999 Liquid hydrogen – Land 90.93 vehicle fueling system interface ISO 13985:2006 ISO 14687-1:1999/Cor 1:2001 ISO 14687-1:1999/Cor 2:2008 ISO 14687-1:1999

Liquid hydrogen – Land vehicle fuel tanks

Hydrogen fuel – Product specification – Part 1: All applications except proton exchange membrane (PEM) fuel cell for road vehicles

ICS No 71.100.20 43.060.40

TC TC 197

90.60

43.060.40 71.100.20

TC 197

60.60

71.100.20

TC 197

60.60

71.100.20

TC 197

90.92

71.100.20

TC 197

(continued)

Appendices Table A8.2 (continued) ISO ISO/TS 14687-2:2008

ISO/PAS 15594:2004 ISO/TS 15869:2009

ISO/TR 15916:2004

ISO 16110-1:2007

ISO 16110-2:2010

ISO 16111:2008

ISO 17268:2006

ISO/TS 20100:2008 ISO 22734-1:2008

ISO 26142:2010

717

Description Hydrogen fuel – Product specification – Part 2: Proton exchange membrane (PEM) fuel cell applications for road vehicles Airport hydrogen fueling facility operations Gaseous hydrogen and hydrogen blends – Land vehicle fuel tanks Basic considerations for the safety of hydrogen systems Hydrogen generators using fuel processing technologies – Part 1: Safety Hydrogen generators using fuel processing technologies – Part 2: Test methods for performance Transportable gas storage devices – Hydrogen absorbed in reversible metal hydride Compressed hydrogen surface vehicle refueling connection devices Gaseous hydrogen – fueling stations Hydrogen generators using water electrolysis process – Part 1: Industrial and commercial applications Hydrogen detection apparatus – Stationary applications

Stage 90.92

ICS No 71.100.20 43.060.40

TC TC 197

90.93

49.100

TC 197

90.92

43.060.40

TC 197

90.92

71.020 71.100.20

TC 197

90.60

71.100.20 71.020

TC 197

60.60

71.100.2071.020

TC 197

60.60

71.100.20

TC 197

90.92

43.180 71.100.20

TC 197

90.92

71.100.20 43.060.40 71.120.99 71.100.20

TC 197

60.60

60.60

71.020 71.100.20

TC 197

TC 197

718 Table A8.3 Hydrogen standards and codes under development Standard and or project Description ISO/CD 14687-2 Hydrogen fuel – Product specification – Part 2: Proton exchange membrane (PEM) fuel cell applications for road vehicles ISO/NP 14687-3 Hydrogen fuel – Product specification – Part 3: Proton exchange membrane (PEM) fuel cell applications for stationary appliances ISO/NP 15399 Gaseous hydrogen – Cylinders and tubes for stationary storage ISO/NP 15869 Gaseous hydrogen and hydrogen blends – Land vehicle fuel tanks ISO/NP TR 15916 Basic considerations for the safety of hydrogen systems ISO/DIS 17268 Gaseous hydrogen land vehicle refueling connection devices ISO/CD 20100 Gaseous hydrogen – fueling stations ISO/DIS 22734-2 Hydrogen generators using water electrolysis process – Part 2: Residential applications

Appendices

Stage 30.60

ISO 71.100.20 43.060.40

TC TC 197

10.99

71.100.20

TC 197

10.99

23.020.30 71.100.20 43.060.40 71.100.20

TC 197

10.99

10.99 40.60 30.60 40.60

71.020 71.100.20 71.100.20 43.180 71.100.20 43.060.40 71.120.99 71.100.20

TC 197

TC 197 TC 197 TC 197 TC 197

Index

A ˛-Amylose, 335 Absorber plate, 94, 95 Absorption coefficients of several semiconductor materials, 133 Absorption cooling, 101 Acid catalyzed esterification, 386 Acid hydrolysis, 451–452 Acidogenesis, 379 Acidogenic bacteria, 378 Acid pretreatment process, 447–448 Activation energy, 120 Aerial view of the power tower plant at USA, 106 Agricultural crops, 339, 341 Air flat-plate collectors, 95–96 used for space heating, 96 Airfoil designs, 46 Airy’s theory, 272 Alanates, 576, 580–581 Alders, 346 Alfalfa, 343, 370 Alkaline electrolysis, 555 Alkaline electrolyzers, 555 American sycamore, 349 Ammonia fiber expansion (AFEX) pretreatment, 448 Ammonia recycle percolation (ARP) pretreatment, 448–449 Amorphous silicon, 138–139 Amylolytic yeast, 436 Amylopectin, 335 Anaerobic bacteria, 378 Anderson cycle, 312 Anemometer, 31 Animal wastes, 339, 340

Annual average daily peak sun hours in various region of USA, 88 Aquaculture, 238–240 Aquatic crops, 339, 341 Arrangements for micro-hydropower system, 191 of small hydropower system, 188 Arrays, 136 Ash, 348, 363 Astronomical Unit (AU), 130 Attenuator, 285 AU. See Astronomical Unit (AU) Autothermal reforming for hydrogen production, 508

B Bacterial species used by various researchers for production of ethanol, 435 Bagasse, 349 Bakers’ yeast, 435 Band-bending in p-n junction, 121 Band-gap (Eg), 128 Band structures and Fermi levels for n-type and p-type doped semiconductors, 121 Base catalyzed transesterification, 386 Batch systems. See Integral collector storage systems Beaufort scale, 281 Bends, 202 Bernard Forest de B´elidor, 157 Bernoulli’s equation, 275 Betz limit, 21 Betz’s law. See Betz limit Big bluestem, 345

719

720 Binary cycle hydrothermal power plant, 232 Biodiesel, 334, 363, 364, 385, 386, 388 Bioenergy, 327, 331, 332, 336, 349, 382, 393 Biofeedstock, 389 Biofuel, 334, 346, 363, 381, 385, 389, 393 Biomass, 327, 331–336, 338–343, 345, 347, 349, 361, 362, 375–378, 381, 382, 389, 390, 393 gasification, 508 Biomethane, 378, 380 Black liquor, 334, 356, 357 Black locust, 346 Borane, 584–586 Borohydrides, 581–583 Boundary conditions, 274 Brayton cycle, 532 British Wind Energy Association (BWEA), 18 Broad band, 123 Bulb turbine, 177, 178 Bunsen reaction section, 516, 526, 528

C CAAA. 419, 422 Ca-Br-Fe (UT-3) cycle, 533–534 Calculation of power from water flow, 194 Capacity factor of wind turbine, 23 Carbohydrates, 333, 335, 378 Carbon and other high surface area materials, 568, 575–576 Carbon dioxide, 327, 328, 333, 372, 378 Cardoon, 346 Carnot system, 514 Cascade cells. See Multijunction cells Cashew nut, 387 Castor seed, 385 Catalysts, 505, 612 CBP, 454, 458 CAAA, 420 CDS. 432, 433 Cellulose, 333, 336–339, 346, 362, 390 Cellulosic, 346, 362 Cellulosic biomass, 337 Cellulosic ethanol, 443 Centre for Renewable Energy Resource (CRES), 270 Char, 372, 375 Charcoal, 356, 357 Chemical composition of some common corns, 428 Clathrates, 576 Clean Air Act Amendments (CAAA). See CAAA Closed-cycle OTEC, 312

Index Closed-loop systems, 253 Cocksfoot grass, 345 Coefficient of performance for wind turbines, 21, 35 Co-fired biopower plants, 369, 370 Collector storage systems, 92 Common reed, 345 Comparison of Brazil and US ethanol industries, 444 Composite tanks, 568 Composition of corn, 425, 427–430 Compound parabolic concentrator, 548 Concentrated acid hydrolysis, 452 Concentrating solar power (CSP) systems. See CSP Concrete Arch Dam. See Concrete dams Concrete dams, 167, 168 Conduction band, 114, 120, 134 Consolidated bioprocessing (CBP). See CBP Continuity equation, 273 Contraction, 204 Contraction loss, 200 Contribution of hydropower to total world energy mix, 160 to world electricity generation, 161 Copper-chlorine thermochemical cycle, 534 Copra, 385 Cord, 360, 361, 363 Coriolis force, 6 Coriolis parameter, 7 Corn, 334, 340, 341, 348, 382, 387, 389, 393, 422, 425, 427–431, 433, 436, 439–441, 443, 444, 448, 449, 458, 460–462, 464, 468, 469 Corn condensed distillers solubles (CDS). See CDS Corn distillers dried grains (DDG). See DDG Corn distillers dried grains/soluble (DDGS). See DDGS Corn stalks, 349 Cost of wind energy, 38 of hydrogen production, 566 Cotton seed, 385 Covalent bonds, 112–114 Crossflow turbine, 170, 192 Cross section of gravity dam, 169 Crude protein, 348 Cryogenic liquid hydrogen, 573–574 CSP. 102 Cu-Cl cycle, 534–538 Cycloturbine, 35 Cypergras, Galingale, 345

Index D Dam, 166, 168, 169, 186, 206 Darrieus wind turbine, 33 DDG, 432, 460 DDGS, 432, 433, 436, 464 Defects, 119, 122, 138 Dent corn, 429 Desiccant cooling, 101, 102 Desorption of hydrogen from hydrides, 584 Diffusion of carriers, 116 Digester, 378–381 Dilute acid hydrolysis, 451–452 Direct circulation systems, 92 Direct-fired biomass power plants, 370 Direct-fired system, 369 Direct gain, 98 Direct hydration of ethylene, 425 Direct methane cracking cycle, 510 Dish engine solar system, 107 Distribution, 553, 560, 586–588 District heating in Reykjavik Iceland, 248 District heating systems, 226, 227, 240, 241, 243, 244 Diurnal tide, 295 Diversion or run of river, 161 Dopant, 112, 116, 120, 134 Double-basin systems, 300 Douglas fir, 349 Drift of carriers, 116 Dry mill(ing), 436–442 and fermentation, 425 processes, 430–433, 441 Dry steam power plants, 231 Dynamical boundary condition, 275 Dynamic and kinematic viscosities, 198

E Eastern cottonwood, 349 Eastern white pine, 349 Ebb generation mode, 299 E. coli, 435, 458 E-diesel, 468 Effect of dopant concentration, 134, 135 Effect of thickness of cell, 1131 Efficiency of high temperature electrolysis cycle, 514–515 Efficiency of PV, 131 of S-I cycle, 532–533 E85 fuel, 465, 467, 468

721 EGS, 229, 236, 237 Electrical generator, 31 Electricity generation using solar concentrators, 86 Electrolysis, 510, 512–515, 538, 543, 547, 555–557, 559, 565, 567 Electrolytic process, 555 Electron/hole pair, 121, 123–125 Electronic controller, 31 Embankment dams, 167 Energy and the Sun, 83–85 Energy balance, 458–464 Energy cane, 345 Energy crops, 339–341, 344, 353 Energy Policy Act (EPACT) of 1992. See EPACT Energy Policy Act of 2005, 422 Energy transport and power, 276 Enhanced or engineered geothermal systems (EGS). See EGS Entrance loss, 200 Enzymatic hydrolysis, 449, 453–454 EPACT, 421 Equivalent circuit model for photovoltaic cell, 125 Escherichia coli. See E. coli Ethanol, 420–434, 439, 441, 443–445, 454, 456, 458, 462, 465–469 Ethyl alcohol, 382 Eucalyptus, 343, 346, 349, 370 Euler equation, 274 European Wind Energy Association (EWEA), 4 Evacuated-tube collectors, 93, 96–97 for heating water or air, 96 Expansion loss, 202

F Failure of various components of offshore turbine, 57 False flax, 346 Far offshore, 282 Fatty acids, 378, 386, 388–390 methyl ester, 386 Feeder canal, 188 Fe3 O4 /FeO cycle, 554–555 Fermentation, 382, 429–433, 435, 436, 442, 444, 449, 451, 452, 454–458, 460 and process integration, 454–458 Fermi level, 120 Fiber sorghum, 346 Fill factor, 118, 119 First hydroelectric plant, 157

722 Fish ladder, 189 and fish passage, 205 Fixed bed gasifier, 375 Flash steam hydrothermal power plants, 232 Flat-plate collectors, 93, 94 for heating water, 94 Flint corn, 427 Flood generation mode, 299 Flour corn, 428 Fluidized-bed gasifier design, 374 Fluid pressure, 274 Forebay, 188 Francis turbine, 177, 182 Frequency and photon energy, 85 Friction against the pipe wall, 197 Fuel properties of common transportation fuels, 420 Fuelwood, 356

G Gallium arsenide, 139 Gaseous hydrogen storage, 567 Gasification of biomass, 502 of coal, 502 process, 369, 370 Gasoline-ethanol mixtures, 465 Gas turbine-modular helium reactor, 557 Gates and valves, 202 Gearbox, 30 Geopressurized brines, 229, 234 Geothermal direct-use for direct heating, 241 Geothermal energy, 218 Geothermal heat pumps (GHP). See GHP Geothermal power plants, 221, 227, 234, 257 Geothermal systems, 223, 236 GHP, 243, 246, 247, 252, 256 Giant cordgrass, 345 Giant reed, 345, 346 Gibbs free energy, 514, 515, 534 Giromill wind turbine, 35 Glass microspheres, 572–573 Glucose, 335, 337 Glycerol, 386, 389 Gradual expansion, 202 Grain alcohol, 382 Greenhouses, 98 Groundnut kernel, 385 Ground-source heat pumps. See GSHP Group velocity, 278 GSHP, 247, 252 Gustave-Gaspard Coriolis, 7

Index H Hastelloy B2 and B3, 542, 543 HAT, 303 HAWT, 26 Head loss, 196 equations for closed channels, 200 in open channels, 202 Heat engine/vapor compression cooling (Rankine-cycle), 101 Heating using air, 99 using liquid, 99 Height of sun in sky, 83 Heliostats, 104 Hemicellulose, 338, 347, 348 Hemp, 346 Herbaceous energy crops, 339 Hexose sugars, 338 HI decomposer, 543 High-amylose corn, 428 High-lysine corn, 428 High-oil corn, 428 High pressure cylinders, 568–573 High speed shaft with its mechanical brake, 31 High-temperature cycles, 550 High temperature electrolysis (HTE) of steam, 512–515 High-temperature hydrothermal-convection systems, 220 High-temperature water splitting, 509–510, 544–545 Horizontal axis turbines. See HAT Horizontal axis wind turbines (HAWTs), 25, 26 Hot air systems, 92 Hot box. See Solar collector Hot dry rock systems. See EGS Hub, 30 Hybrid OTEC, 317 Hybrid photovoltaic and thermal (PVT) collector. See PVT collector Hybrid power systems, 110 Hybrid solar lighting, 141 Hybrid sulfur cycle, 510 Hybrid vehicles, 501 Hydrides, 568, 576–586 Hydrogen from biomass, 560 from coal, 509 produced from natural gas, 501 from wind energy, 557–560 Hydrogenase enzyme-catalyzed hydrogen production, 562–563

Index Hydrogen delivery methods, 586–587 Hydrogen demand, 497–500 Hydrogen economy, 497, 589 Hydrogen internal combustion engine, 500–501 Hydrogen iodide decomposition section, 516, 528, 529, 532 Hydrogen production capacity in US, 498 Hydrogen storage, 566–586 Hydrogen use by end users, 498 Hydrolysis, 449–454 Hydronic collector, 100 Hydropower generating plants, 157 construction methods, 164 efficiency, 205 Hydrothermal fluids, 229, 231 Hydroturbine, 168

I ICE, 500, 501 Iceland’s high-temperature fields, 243 ICS, 93, 97–98 Illustration of group velocity of waves, 278 Impoundment, 161, 164, 165 Impulse turbine, 169 Indian (Zea mays) corn, 428 Indirect gain, 99 Indirect heating or circulation systems, 92 Indirect hydration of ethylene, 425 Industrial crops, 339, 341 Insolation, 80–83 Installed wind power capacity of various countries, 3 Insulators, 114, 115 Intake, 189 Integral collector storage systems (ICS). See ICS Interaction of photons with semiconductor material, 112 Internal combustion engines (ICE). See ICE International Heat Flow Commission, 221 Intrinsic efficiency, 122 Intrinsic efficiency (i /, 122 for photovoltaic conversion, 124 Isolated gain, 99 Isopentane, 234

J Jerusalem artichoke, 346

723 K Kalina cycle, 234 Kaplan turbine, 177, 180–182 Kenaf, 341, 346, 387 Kinematic boundary conditions, 276 Kinetic energy of wind, 20 turbine, 177, 184 Klebsiella oxytoca, 435

L Land area requirement, 16–20 Landfill gas, 339, 340, 364 Land requirement, 341 Land use in USA, 349 Laplace equation, 275 Large hydropower, 162 Leakage loss around runner, 196 LH2. 573–574 Lift-type vertical axis configurations, 34–35 Lignin, 333, 336, 338–340, 346–349, 356, 357 Lignocelluloses, 424, 453–455 Lignocellulosic biomass, 336, 338, 346, 347 Lime pretreatment, 448 Lindal diagram, 224 Linear lift mechanism, 307 Linear wave theory, 272, 273, 277, 280 Lipase catalyzed transesterification, 386 Liquid flat-plate collectors, 94–95 Liquid hot water process, 447 Liquid hydrogen (LH2). See LH2 Liquid hydrogen storage, 567 Load variation of pumped storage facility, 193 Loblolly pine, 349 Locations for high tides, 295 Longitudinal section of underground hydropower plant, 166 Losses at bends, 203 in semiconductor photovoltaic cells, 122 Low frequency noise form wind turbines, 61–66 Low speed shaft, 30 Low-temperature cycles, 550 Lupine, 387

M Magma, 223, 229, 231, 238, 239 Maintenance strategy, 291 Mantle, 217 Maximum power, 118

724 Maximum PV efficiency as function of band-gap energy, 128 Maximum PV power, 127 Meadow foxtail, 345 Meal; 431, 433, 439, 464 Mean annual wind speed, 48 Mechanical loss in the turbine, 196 Metal/metal oxide based systems, 510 Methane, 340, 378, 379, 381 Methanogenes, 379 Methanogenic bacteria, 378 Methyl tertiary butyl ether (MTBE). See MTBE Micro-head hydropower systems, 190 Micro hydropower, 162 Million ton oil equivalent. See Mtoe Miscanthus, 340, 343, 345, 346 MIS silicon solar cell, 122 Mixed tide, 295 Model of geothermal system, 225 Modine heaters, 241 Modular systems, 369, 377 Molten nitrate salt, 104 Momentum balance equation, 273 Monthly isolation at equator, 82 Moody chart, 199, 200 Mooring system, 291 MTBE, 422 Mtoe, 329–331 Multijunction and high-efficiency solar cells, 139 Multijunction cells, 140–141 Multiple-basin, 296 Multi-step reaction cycles, 550 Municipal solid wastes, 339, 340 Mustard, 385

N NaBH4 solutions, 575 Nacelle, 29 Napier grass, 345 Natural water cycle, 158 Neap tide, 292 Nearshore, 282, 283 Nitrogenase-enzyme catalyzed hydrogen production, 563 No. 2 Diesel, 420 Non-amylolytic yeast, 436 Northern white cedar, 349 N-type semiconductor, 112 Nuclear energy, 503, 509, 510 for hydrogen production, 510–543

Index O Oats, 387 Ocean energy, 267, 268 Ocean thermal energy conversion. See OTEC Ocean waves, 268, 271, 273, 276–285, 288, 289, 291, 318 theory, 271, 273, 280 Offshore, 270, 282, 283, 289, 290 wind farm, 53–58 One step reaction, 509 One sun, 129–131 Open circuit voltage (Voc); 118, 128 Open-cycle OTEC, 315 Open-loop system, 256 Operating cost for large onshore turbines, 42 Operating costs of hydropower generating systems, 160–161 Operation of photovoltaic device, 122 Oscillating hydroplane, 307 Oscillating water column. See OWC Ossberger turbine, 170, 174, 177, 178 OTEC, 268, 309, 311–319 Overall efficiency, 135 Overtopping devices, 282, 289, 290 OWC, 283, 285 Oxidation, 372, 376–378 Oxygen blown gasifier, 372

P Palm fruit, 385 Palm kernel, 385 Palm oil, 386 Parabolic reflectors, 102 Parallax effects, 292 Paris basin in France, 243 Partial oxidation, 507–508 Passive, batch type solar heater, 98 Payback time for wind energy, 43–45 Pelamis WEC, 285 Pelton turbine, 170, 191 Penstock, 189 Pentose, 338 Perspective view of Earth cross section, 218 Phase velocity, 277–279 Photobiological water splitting, 560–563 Photocatalytical processes, 563–566 Photoelectrochemical water splitting, 560 Photolytic processes, 560–566 Photon energy, 116, 121–124, 131 Photosynthesis, 328, 333 Photovoltaic (PV) cells. See PV cells Planck’s equation, 84 Plant cell structure, 336

Index Platinum on porous metal oxides, 524 Point absorber, 282, 284 Point absorber wave energy farm, 286 Polycrystalline silicon thin film, 138 Polycrystalline thin films, 139 Polylactic acid, 389 Polymer electrolyte membrane (PEM) electrolysis, 555 Popcorn, 427–428 Poplar, 341, 343, 346, 370 Port Kembla, Australia, 283 Potential byproducts from dry mill ethanol process, 439, 440 Potential energy, 194 Power coefficient for wind turbine, 37 Power house, 189 Power production equation for wind energy, 24 Power tower systems, 104 Prairie cordgrass, 345 Processes for hydrogen production, 503 Process heat, 362, 363 Product purification, 503 Projected increase in world wind power installed capacity, 4 1,3-Propanediol, 389 Propeller turbine, 177 Pt catalysts, 524 P-type semiconductor, 112–114, 116, 120, 121 Pumped storage, 161, 193 Pumped storage hydropower system, 192 PV cells, 86, 110–131, 136–141 PVT, 141 Pyrolysis, 357, 372, 375

Q Quaking aspen, 349 Quantum efficiency, 135

R Radiant heating, 100 Radiata pine, 349 Range, 292 of applicability of linear wave theory, 280 Rapeseed, 346, 385 Rated power for wind energy, 24 Raygras, 345 Rayleigh distribution, 24. See also Weibull distribution Reaction turbine, 169, 174 Rechargeable organic liquids, 575 Red pine, 349

725 Reduction, 343, 372, 380 Reed canary grass, 345, 346 Reforming of biofuel, 508 of natural gas, 502 of renewable liquid fuels, 503 Representation of ocean wave, 277 Research octane number. See RON Reserve power, 189 Reservoir, 158, 164–167, 169, 185, 186, 192, 193 Residues, 338–341, 349 Reynolds number, 197, 198, 200 Rh-based catalyst, 547 Ring-of-Fire, 220 RON, 501 Rotor blades, 29 Run-of-the-river hydropower systems, 185

S Saccharomyces cerevisiae, 435, 436, 442, 452, 456 Salt reedgrass, 345 Saturation current, 117 Savonius wind turbine, 33–34 S. cerevisiae. See Saccharomyces cerevisiae Schematic diagram of pumped storage plant, 194 Seasonal variation, 81 Selection of hydroturbines, 185 of turbine for small or micro head systems, 191 Semiconducting materials for PV, 112, 114, 136, 139 Semidiurnal tide, 295 Separate/sequential hydrolysis and fermentation (SHF). See SHF Sequential hydrolysis and fermentation (SHF), 455, 456 Series resistance, 120 Sesame, 385 Shallow-water waves, 279 SHF, 454–456 Shockley diode equation, 117 Short circuit current, 118 Shunt current, 117 Shunt resistance, 119 SiC, 526, 542–544 S-I cycle, 516, 532, 533, 541 Silicon, 112, 113, 123, 129, 134, 138–139 Silicon carbide (SiC). See SiC

726 Simultaneous saccharification and co-fermentation (SSCF). See SSCF Simultaneous saccharification and fermentation (SSF). See SSF Single-basin, 296 Single crystal, 138, 139 Slagging gasifier, 372 SlinkyTM method of looping pipes, 254–255 Small hydroelectric power system, 186 Small hydropower, 162, 174, 187, 188, 190 Small wind systems, 58–61 Soil, 348 Solar cell basic components of, 133 equivalent electrical circuit, 116 materials, 136, 137 Solar collector, 85, 91, 93 Solar energy, 503, 509, 543 for hydrogen production, 543–555 Solar irradiance measurement unit. See One sun Solar lighting, 86, 141 Solar pool heating, 86 Solar power plant at Andulusia, Spain, 102 Solar radiation, 79–81, 91, 93, 94, 102, 105, 139 on USA landmass, 87 Solar reforming of natural gas, 545–548 Solar space cooling, 88 heating, 86, 98–100 in buildings, 88 Solar spectrum in space and on the earth’s surface, 84 Solar thermal concentrators, 104, 107 Solar thermal energy, 86–88 Solar thermal molten salt technology, 107–109 Solar water and pool heaters, 88 Solar water heating, 86, 88, 91–98 Solid oxide electrolyzers, 555 Sound intensity as applied to wind turbines, 63–64 Sound pressure as applied to wind turbines, 64 Southern red oak, 349 Soybean, 349, 385 stalks, 349 Soy oil, 386 Space cooling, 101–102 Space heating, 98–100 Spectral irradiance of solar spectrum (Air Mass 2–AM2), 124 Spillways, 166, 168

Index Spring barley, 346 Spring tide, 292 SSCF, 454, 457–458 SSF, 454–457 Starches, 335, 424, 427 Steam generation, 362, 363 Steam methane reforming catalyst, 505–507 cycle, 510 reactions, 504 Steam reforming reactions, 503 Stoichiometric air/fuel (A/F) ratio, 501 Storage system, 566, 582 Straflo turbine, 177–179 Structure of doped silicon, 116 Sugar, 333, 335, 338, 363, 369, 382, 385, 424, 427, 431, 436, 440, 442, 444, 447, 449, 451, 452, 454, 457 Sugar beet, 346 Sugarcane, 425, 442–444, 458, 464, 468 Sugar crop fermentation, 425, 442–443 Sulfuric acid hybrid cycle, 556 Sulfur-iodine (S-I) cycle, 510, 516, 521, 524 Sunflower, 346, 385 Sunrooms, 98 Sun-tracking mirrors. See Heliostats Sweet sorghum, 346 Switchgrass, 336, 340, 343, 345, 348, 353, 370, 425, 448, 449, 468 Syzygy, 292

T Tail race, 189 Tall fescue, 345 Tandem cells. See Multijunction cells TAPCHAN, 282, 283 Tapered channel, 282 Terminator device. See OWC Theoretical efficiency of photovoltaic cells, 120 Theoretical maximum efficiency of wind turbine. See Betz limit Theoretical photovoltaic efficiency, 122 Theoretical thermodynamic efficiency of Otto cycle engine, 500 Theoretical wind power equation, 21 Thermal receiver, 105 Thermochemical hybrid cycles, 555–557 Thermochemical solar cycle, 548–555 Thermochemical water splitting, 503, 515–543 Thermosyphon, 92 Tidal barrage, 296, 299

Index Tidal current, 267, 296, 302, 303, 306 energy, 292 turbines, 304 Tidal farm, 309 Tidal fence, 302 Tidal horizontal axis turbines, 304 Tidal lagoons, 301 Tidal power and tidal current energy, 267 Tidal turbine, 302 Time-averaged kinetic energy, 277 Timothy, 345 Tip speed ratio, 36 TKE gasifier, 372, 374 Total derivative, 273 Tower, 31 Trash rack losses, 202 Triticale, 346 Trough systems, 102–104 Tube turbine, 177, 179, 181 Turgo wheel turbine, 170, 171 Two or multiple step reactions, 509–510 Two-step reaction cycles, 550 Two-way generation mode, 300

U Uncatalyzed steam explosion process, 447 Urban wastes, 339, 340 Use of solar energy, 85–102 UT-3 cycle, 533, 534 Utility-scale power production, 110

V Valence electrons, 114 VAWT, 32–35 Velocity potential function, 276 Vertical axis turbines, 303, 304 Vertical axis wind turbines (VAWTs). See VAWT Volcanic zones and geothermal areas in Iceland, 247

W Water electrolysis, 510, 512 Water gas shift reaction, 503 Water head for power production, 189 Water requirements for corn growing, 464 Wave climate, 291 Wave dragon design, 290 Wave energy in USA, 269 Wave energy potential around the world, 269 Wave heights, 291

727 Wave-induced pressure, 274 Wavelength, 272, 276, 279, 280 Wave power, 267, 268, 279, 282, 292 Waxy corn, 427 WDGS, 432, 433 Weibull distribution, 24. See also Rayleigh distribution Westinghouse sulfur process. See WSP Wet distillers grains with solubles (WDGS). See WDGS Wet mill(ing), 433–442 and fermentation, 425 process, 430 Wheat straw, 349 White birch, 349 Willow, 341, 343, 346, 370 Wind class, 8, 9 Wind energy, 1–69 economics, 52 and intermittency, 66–67 software packages, 16 Wind farms, 2, 51–58 Wind generated electricity, 4, 41 Windmills, 1 Wind power density, 8, 48 Wind resource map, 8–16 Wind turbines, 2, 6–8, 10, 16–18, 20, 24–28, 32, 34, 39, 40, 42, 46–48, 51, 53, 54, 57, 61, 62, 65–68 components, 29–31 Wind vane, 31 Winter rye, 346 Winter wheat, 346 Wood burning, 362 Wood fuels, 356 Wood pellets, 360 Wood usage by various countries, 357 Woody energy crops, 339 Working principle of overtopping system, 290 Worldwide generation of hydroelectric power, 158 WSP, 533, 556

Y Yaw mechanism, 31 Yeasts, 435, 436 YSZ, 512 Yttria-stabilized zirconia (YSZ). See YSZ

Z Zinc/zinc oxide cycle, 550 Zymomonas mobilis, 435, 458