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 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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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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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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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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2 Solar Energy

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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2 Solar Energy

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])

132 2 Solar Energy

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133

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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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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