renewable energy sources

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Renewable Energy Resources

Renewable Energy Resources is a numerate and quantitative text covering subjectsof proven technical and economic importance worldwide. Energy supplies fromrenewables (such as solar, thermal, photovoltaic, wind, hydro, biofuels, wave, tidal,ocean and geothermal sources) are essential components of every nations energystrategy, not least because of concerns for the environment and for sustainability.In the years between the first and this second edition, renewable energy has comeof age: it makes good sense, good government and good business.

This second edition maintains the books basis on fundamentals, whilst includ-ing experience gained from the rapid growth of renewable energy technologies assecure national resources and for climate change mitigation, more extensively illus-trated with case studies and worked problems. The presentation has been improvedthroughout, along with a new chapter on economics and institutional factors. Eachchapter begins with fundamental theory from a scientific perspective, then considersapplied engineering examples and developments, and includes a set of problems andsolutions and a bibliography of printed and web-based material for further study.Common symbols and cross referencing apply throughout, essential data are tabu-lated in appendices. Sections on social and environmental aspects have been addedto each technology chapter.

Renewable Energy Resources supports multi-disciplinary master degrees in sci-ence and engineering, and specialist modules in first degrees. Practising scientistsand engineers who have not had a comprehensive training in renewable energy willfind this book a useful introductory text and a reference book.

John Twidell has considerable experience in renewable energy as an academic pro-fessor, a board member of wind and solar professional associations, a journal editorand contractor with the European Commission. As well as holding posts in the UK,he has worked in Sudan and Fiji.

Tony Weir is a policy adviser to the Australian government, specialising in theinterface between technology and policy, covering subjects such as energy supplyand demand, climate change and innovation in business. He was formerly SeniorEnergy Officer at the South Pacific Forum Secretariat in Fiji, and has lectured andresearched in physics and policy studies at universities of the UK, Australia and thePacific.

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Renewable EnergyResources

Second edition

John Twidell and Tony Weir

First published 1986by E&FN Spon LtdSecond edition published 2006by Taylor & Francis2 Park Square, Milton Park, Abingdon, Oxon OX14 4RN

Simultaneously published in the USA and Canadaby Taylor & Francis270 Madison Ave, New York, NY 10016, USA

Taylor & Francis is an imprint of the Taylor & Francis Group

1986, 2006 John W. Twidell and Anthony D. Weir

All rights reserved. No part of this book may be reprinted orreproduced or utilised in any form or by any electronic, mechanical, orother means, now known or hereafter invented, including photocopyingand recording, or in any information storage or retrieval system, withoutpermission in writing from the publishers.

The publisher makes no representation, express or implied, with regardto the accuracy of the information contained in this book and cannotaccept any legal responsibility or liability for any errors oromissions that may be made.

British Library Cataloguing in Publication DataA catalogue record for this book is availablefrom the British Library

Library of Congress Cataloging in Publication DataTwidell, John.Renewable energy resources / John Twidell andAnthony Weir. 2nd ed.p. cm.

Includes bibliographical references and index.ISBN 0419253203 (hardback) ISBN 0419253300 (pbk.)1. Renewable energy sources. I. Weir, Anthony D. II. Title.

TJ808.T95 2005621.042dc22

2005015300

ISBN10: 0419253203 ISBN13: 9780419253204 HardbackISBN10: 0419253300 ISBN13: 9780419253303 Paperback

This edition published in the Taylor & Francis e-Library, 2006.

To purchase your own copy of this or any of Taylor & Francis or Routledgescollection of thousands of eBooks please go to www.eBookstore.tandf.co.uk.

Contents

Preface xiList of symbols xvii

1 Principles of renewable energy 1

1.1 Introduction 11.2 Energy and sustainable development 21.3 Fundamentals 71.4 Scientific principles of renewable energy 121.5 Technical implications 161.6 Social implications 22

Problems 24Bibliography 25

2 Essentials of fluid dynamics 29

2.1 Introduction 292.2 Conservation of energy: Bernoullis equation 302.3 Conservation of momentum 322.4 Viscosity 332.5 Turbulence 342.6 Friction in pipe flow 352.7 Lift and drag forces: fluid and turbine machinery 39


3 Heat transfer 45

3.1 Introduction 453.2 Heat circuit analysis and terminology 463.3 Conduction 49

vi Contents

3.4 Convection 513.5 Radiative heat transfer 613.6 Properties of transparent materials 733.7 Heat transfer by mass transport 743.8 Multimode transfer and circuit analysis 77


4 Solar radiation 85

4.1 Introduction 854.2 Extraterrestrial solar radiation 864.3 Components of radiation 874.4 Geometry of the Earth and Sun 894.5 Geometry of collector and the solar beam 934.6 Effects of the Earths atmosphere 984.7 Measurements of solar radiation 1044.8 Estimation of solar radiation 107


5 Solar water heating 115

5.1 Introduction 1155.2 Calculation of heat balance: general remarks 1185.3 Uncovered solar water heaters progressive analysis 1195.4 Improved solar water heaters 1235.5 Systems with separate storage 1295.6 Selective surfaces 1345.7 Evacuated collectors 1375.8 Social and environmental aspects 140


6 Buildings and other solar thermal applications 146

6.1 Introduction 1466.2 Air heaters 1476.3 Energy-efficient buildings 1496.4 Crop driers 1576.5 Space cooling 1616.6 Water desalination 162

Contents vii

6.7 Solar ponds 1646.8 Solar concentrators 1666.9 Solar thermal electric power systems 1706.10 Social and environmental aspects 173


7 Photovoltaic generation 182

7.1 Introduction 1827.2 The silicon pn junction 1847.3 Photon absorption at the junction 1937.4 Solar radiation absorption 1977.5 Maximising cell efficiency 2007.6 Solar cell construction 2087.7 Types and adaptations of photovoltaics 2107.8 Photovoltaic circuit properties 2207.9 Applications and systems 2247.10 Social and environmental aspects 229


8 Hydro-power 237

8.1 Introduction 2378.2 Principles 2408.3 Assessing the resource for small installations 2408.4 An impulse turbine 2448.5 Reaction turbines 2498.6 Hydroelectric systems 2528.7 The hydraulic ram pump 2558.8 Social and environmental aspects 257


9 Power from the wind 263

9.1 Introduction 2639.2 Turbine types and terms 2689.3 Linear momentum and basic theory 2739.4 Dynamic matching 2839.5 Blade element theory 288

viii Contents

9.6 Characteristics of the wind 2909.7 Power extraction by a turbine 3059.8 Electricity generation 3079.9 Mechanical power 3169.10 Social and environmental considerations 318


10 The photosynthetic process 324

10.1 Introduction 32410.2 Trophic level photosynthesis 32610.3 Photosynthesis at the plant level 33010.4 Thermodynamic considerations 33610.5 Photophysics 33810.6 Molecular level photosynthesis 34310.7 Applied photosynthesis 348


11 Biomass and biofuels 351

11.1 Introduction 35111.2 Biofuel classification 35411.3 Biomass production for energy farming 35711.4 Direct combustion for heat 36511.5 Pyrolysis (destructive distillation) 37011.6 Further thermochemical processes 37411.7 Alcoholic fermentation 37511.8 Anaerobic digestion for biogas 37911.9 Wastes and residues 38711.10 Vegetable oils and biodiesel 38811.11 Social and environmental aspects 389


12 Wave power 400

12.1 Introduction 40012.2 Wave motion 40212.3 Wave energy and power 40612.4 Wave patterns 41212.5 Devices 418

Contents ix

12.6 Social and environmental aspects 422Problems 424Bibliography 427

13 Tidal power 429

13.1 Introduction 42913.2 The cause of tides 43113.3 Enhancement of tides 43813.4 Tidal current/stream power 44213.5 Tidal range power 44313.6 World range power sites 44713.7 Social and environmental aspects of tidal range power 449


14 Ocean thermal energy conversion (OTEC) 453

14.1 Introduction 45314.2 Principles 45414.3 Heat exchangers 45814.4 Pumping requirements 46414.5 Other practical considerations 46514.6 Environmental impact 468


15 Geothermal energy 471

15.1 Introduction 47115.2 Geophysics 47215.3 Dry rock and hot aquifer analysis 47515.4 Harnessing Geothermal Resources 48115.5 Social and environmental aspects 483


16 Energy systems, storage and transmission 489

16.1 The importance of energy storage and distribution 48916.2 Biological storage 49016.3 Chemical storage 49016.4 Heat storage 49516.5 Electrical storage: batteries and accumulators 49916.6 Fuel cells 506

x Contents

16.7 Mechanical storage 50716.8 Distribution of energy 50916.9 Electrical power 51316.10 Social and environmental aspects 520


17 Institutional and economic factors 526

17.1 Introduction 52617.2 Socio-political factors 52617.3 Economics 53017.4 Some policy tools 53417.5 Quantifying choice 53617.6 The way ahead 545


Appendix A Units and conversions 553Appendix B Data 558Appendix C Some heat transfer formulas 564Solution guide to problems 568Index 581

Preface

Our aim

Renewable Energy Resources is a numerate and quantitative text coveringsubjects of proven technical and economic importance worldwide. Energysupply from renewables is an essential component of every nations strat-egy, especially when there is responsibility for the environment and forsustainability.This book considers the timeless principles of renewable energy tech-

nologies, yet seeks to demonstrate modern application and case studies.Renewable Energy Resources supports multi-disciplinary master degrees inscience and engineering, and also specialist modules in science and engineer-ing first degrees. Moreover, since many practising scientists and engineerswill not have had a general training in renewable energy, the book has wideruse beyond colleges and universities. Each chapter begins with fundamentaltheory from a physical science perspective, then considers applied exam-ples and developments, and finally concludes with a set of problems andsolutions. The whole book is structured to share common material and torelate aspects together. After each chapter, reading and web-based materialis indicated for further study. Therefore the book is intended both for basicstudy and for application. Throughout the book and in the appendices, weinclude essential and useful reference material.

The subject

Renewable energy supplies are of ever increasing environmental and eco-nomic importance in all countries. A wide range of renewable energy tech-nologies are established commercially and recognised as growth industriesby most governments. World agencies, such as the United Nations, havelarge programmes to encourage the technology. In this book we stress thescientific understanding and analysis of renewable energy, since we believethese are distinctive and require specialist attention. The subject is not easy,mainly because of the spread of disciplines involved, which is why we aimto unify the approach within one book.

xii Preface

This book bridges the gap between descriptive reviews and specialisedengineering treatises on particular aspects. It centres on demonstrating howfundamental physical processes govern renewable energy resources and theirapplication. Although the applications are being updated continually, thefundamental principles remain the same and we are confident that this newedition will continue to provide a useful platform for those advancing thesubject and its industries. We have been encouraged in this approach by theever increasing commercial importance of renewable energy technologies.

Why a second edition?

In the relatively few years between the first edition, with five reprintedrevisions, and this second edition, renewable energy has come of age; itsuse makes good sense, good government and good business. From being(apart from hydro-power) small-scale curiosities promoted by idealists,renewables have become mainstream technologies, produced and operatedby companies competing in an increasingly open market where consumersand politicians are very conscious of sustainability issues.In recognition of the social, political and institutional factors which con-

tinue to drive this change, this new edition includes a new final chapteron institutional and economic factors. The new chapter also discusses anddemonstrates some tools for evaluating the increasingly favourable eco-nomics of renewable energy systems. There is also a substantial new sectionin Chapter 1 showing how renewable energy is a key component of sus-tainable development, an ideal which has become much more explicit sincethe first edition. Each technology chapter now includes a brief concludingsection on its social and environmental impacts.The book maintains the same general format as the first edition, but

many improvements and updates have been made. In particular we wishto relate to the vibrant developments in the individual renewable energytechnologies, and to the related commercial growth. We have improved thepresentation of the fundamentals throughout, in the light of our teachingexperience. Although the book continues to focus on fundamental physi-cal principles, which have not changed, we have updated the technologicalapplications and their relative emphases to reflect market experience. Forelectricity generation, wind-power and photovoltaics have had dramaticgrowth over the last two decades, both in terms of installed capacity andin sophistication of the industries. In all aspects of renewable energy, com-posite materials and microelectronic control have transformed traditionaltechnologies, including hydro-power and the use of biomass.Extra problems have been added at the end of each chapter, with hints

and guidance for all solutions as an appendix. We continue to emphasisesimplified, order-of-magnitude, calculations of the potential outputs of thevarious technologies. Such calculations are especially useful in indicating

Preface xiii

the potential applicability of a technology for a particular site. However weappreciate that specialists increasingly use computer modelling of whole,complex systems; in our view such modelling is essential but only afterinitial calculation as presented here.

Readership

We expect our readers to have a basic understanding of science and tech-nology, especially of physical science and mathematics. It is not necessaryto read or refer to chapters consecutively, as each aspect of the subject istreated, in the main, as independent of the other aspects. However, somecommon elements, especially heat transfer, will have to be studied seriouslyif the reader is to progress to any depth of understanding in solar energy.The disciplines behind a proper understanding and application of renew-able energy also include environmental science, chemistry and engineering,with social science vital for dissemination. We are aware that readers witha physical science background will usually be unfamiliar with life scienceand agricultural science, but we stress the importance of these subjects withobvious application for biofuels and for developments akin to photosynthe-sis. We ourselves see renewable energy as within human-inclusive ecology,both now and for a sustainable future.

Ourselves

We would like our readers to enjoy the subject of renewable energy, aswe do, and to be stimulated to apply the energy sources for the benefitof their societies. Our own interest and commitment has evolved from thework in both hemispheres and in a range of countries. We first taught,and therefore learnt, renewable energy at the University of Strathclyde inGlasgow (JWT) and the University of the South Pacific in Fiji (ADW andJWT). So teaching, together with research and application in Scotland andthe South Pacific, has been a strong influence for this book. Since the firstedition we have made separate careers in universities and in governmentservice, whilst experiencing the remarkable, but predicable, growth in rele-vance of renewable energy. One of us (JWT) became Director of the EnergyStudies Unit, in the Faculty of Engineering at the University of Strathclydein Glasgow, Scotland, and then accepted the Chair in Renewable Energyat the AMSET Centre, De Montfort University, Leicester, England. He iseditor of the academic journal Wind Engineering, has been a Council andBoard member of the British Wind Energy Association and the UK SolarEnergy Society, and has supervised many postgraduates for their disserta-tions. The AMSET Centre is now a private company, for research, educationand training in renewables; support is given to MSc courses at ReadingUniversity, Oxford University and City University, and there are European

xiv Preface

Unionfunded research programmes. TW was for several years the SeniorEnergy Officer of the South Pacific Forum Secretariat, where he manageda substantial program of renewable energy pilot projects. He then workedfor the Australian Government as an adviser on climate change, and lateron new economy issues.We do not see the world as divided sharply between developed industri-

alised countries and developing countries of the Third World. Renewablesare essential for both, and indeed provide one way for the separating con-cepts to become irrelevant. This is meaningful to us personally, since wewish our own energies to be directed for a just and sustainable society,increasingly free of poverty and the threat of cataclysmic war. We sincerelybelieve the development and application of renewable energy technologywill favour these aspirations. Our readers may not share these views, andthis fortunately does not affect the content of the book. One thing they willhave to share, however, is contact with the outdoors. Renewable energy isdrawn from the environment, and practitioners must put on their rubberboots or their sun hat and move from the closed environment of buildingsto the outside. This is no great hardship however; the natural environmentis the joy and fulfilment of renewables.

Suggestions for using the book in teaching

How a book is used in teaching depends mainly on how much time isdevoted to its subject. For example, the book originated from short andone-semester courses to senior undergraduates in Physics at the Universityof the South Pacific and the University of Strathclyde, namely EnergyResources and Distribution, Renewable Energy and Physics and Ecology.When completed and with regular revisions, the book has been mostly usedworldwide for MSc degrees in engineering and science, including those onrenewable energy and on energy and the environment. We have alsotaught other lecture and laboratory courses, and have found many of thesubjects and technologies in renewable energy can be incorporated withgreat benefit into conventional teaching.This book deliberately contains more material than could be covered in

one specialist course. This enables the instructor and readers to concentrateon those particular energy technologies appropriate in their situation. Toassist in this selection, each chapter starts with a preliminary outline andestimate of each technologys resource and geographical variation, and endswith a discussion of its social and environmental aspects.The chapters are broadly grouped into similar areas. Chapter 1 (Principles

of Renewable Energy) introduces renewable energy supplies in general, andin particular the characteristics that distinguish their application from thatfor fossil or nuclear fuels. Chapter 2 (Fluid Mechanics) and Chapter 3 (HeatTransfer) are background material for later chapters. They contain nothing

Preface xv

that a senior student in mechanical engineering will not already know.Chapters 47 deal with various aspects of direct solar energy. Readersinterested in this area are advised to start with the early sections of Chapter 5(Solar Water Heating) or Chapter 7 (Photovoltaics), and review Chapters 3and 4 as required. Chapters 8 (Hydro), 9 (Wind), 12 (Waves) and 13 (Tides)present applications of fluid mechanics. Again the reader is advised to startwith an applications chapter, and review the elements from Chapter 2 asrequired. Chapters 10 and 11 deal with biomass as an energy source andhow the energy is stored and may be used. Chapters 14 (OTEC) and 15(Geothermal) treat sources that are, like those in Chapters 12 (wave) and 13(tidal), important only in fairly limited geographical areas. Chapter 16, likeChapter 1, treats matters of importance to all renewable energy sources,namely the storage and distribution of energy and the integration of energysources into energy systems. Chapter 17, on institutional and economicfactors bearing on renewable energy, recognises that science and engineeringare not the only factors for implementing technologies and developments.Appendices A (units), B (data) and C (heat transfer formulas) are referred toeither implicitly or explicitly throughout the book. We keep to a commonset of symbols throughout, as listed in the front. Bibliographies include bothspecific and general references of conventional publications and of websites;the internet is particularly valuable for seeking applications. Suggestionsfor further reading and problems (mostly numerical in nature) are includedwith most chapters. Answer guidance is provided at the end of the bookfor most of the problems.

Acknowledgements

As authors we bear responsibility for all interpretations, opinions and errorsin this work. However, many have helped us, and we express our gratitudeto them. The first edition acknowledged the many students, colleagues andcontacts that had helped and encouraged us at that stage. For this secondedition, enormously more information and experience has been available,especially from major international and national R&D and from commer-cial experience, with significant information available on the internet. Weacknowledge the help and information we have gained from many suchsources, with specific acknowledgement indicated by conventional referenc-ing and listing in the bibliographies. We welcome communications from ourreaders, especially when they point out mistakes and possible improvement.Much of TWs work on this second edition was done while he was

on leave at the International Global Change Institute of the Universityof Waikato, New Zealand, in 2004. He gratefully acknowledges the aca-demic hospitality of Neil Ericksen and colleagues, and the continuing sup-port of the [Australian Government] Department of Industry Tourism and

xvi Preface

Resources. JWT is especially grateful for the comments and ideas fromstudents of his courses.And last, but not least, we have to thank a succession of editors at Spon

Press and Taylor & Francis and our families for their patience and encour-agement. Our children were young at the first edition, but had nearly all lefthome at the second; the third edition will be for their future generations.

John Twidell MA DPhil A.D. (Tony) Weir BSc PhD

AMSET Centre, Horninghold CanberraLeicestershire, LE16 8DH, UK Australia

and

Visiting Professor in Renewable EnergyUniversity of Reading, UK

email see

List of symbols

Symbol Main use Other use or comment

CapitalsA Area (m2) Acceptor; ideality factorAM Air-mass-ratioC Thermal capacitance ( J K1) Electrical capacitance (F); constantCP Power coefficientCr Concentration ratioC Torque coefficientD Distance (m) Diameter (of pipe or blade)E Energy ( J)EF Fermi levelEg Band gap (eV)EK Kinetic energy ( J)EMF Electromotive force (V)F Force (N) Faraday constant (Cmol1)F ij Radiation exchange factor (i to j)G Solar irradiance (Wm2) Gravitational constant (Nm2 kg2);

Temperature gradient (Km1);Gibbs energy

Gb Gd Gh Irradiance (beam, diffuse, onhorizontal)

H Enthalpy (J) Head (pressure height) of fluid (m);wave crest height (m); insolation( Jm2 day1); heat of reaction (H)

I Electric current (A) Moment of inertia (kgm2)J Current density (Am2)K Extinction coefficient (m1) Clearness index (KT); constantL Distance, length (m) Diffusion length (m); litre (103 m3)M Mass (kg) Molecular weightN Concentration (m3) Hours of daylightN0 Avogadro numberP Power (W)P Power per unit length

(Wm1)PS Photosystem

(Continued)


Q Volume flow rate (m3 s1)R Thermal resistance (KW1) Radius (m); electrical resistance ();

reduction level; tidal range (m); gasconstant (R0);

Rm Thermal resistance (masstransfer)

Rn Thermal resistance(conduction)

Rr Thermal resistance (radiation)Rv Thermal resistance

(convection)RFD Radiant flux density (Wm2)S Surface area (m2) entropySv Surface recombination

velocity (ms1)STP Standard temperature and

pressureT Temperature (K) Period (s1)U Potential energy ( J) Heat loss coefficient (Wm2 K1)V Volume (m3) Electrical potential (V)W Width (m) Energy density (Jm3)X Characteristic dimension (m) Concentration ratio

Script capitals (Non-dimensional numberscharacterising fluid flow)

Rayleigh number Grashof number Nusselt number Prandtl number Reynolds number Shape number (of turbine)

Lower casea Amplitude (m) Wind interference factor; radius (m)b Wind profile exponent Width (m)c Specific heat capacity

( J kg1 K1)Speed of light (ms1); phase velocityof wave (ms1); chord length (m);Weibull speed factor (ms1)

d Distance (m) Diameter (m); depth (m); zero planedisplacement (wind) (m)

e Electron charge (C) Base of natural logarithms (2.718)f Frequency of cycles

(Hz = s1)Pipe friction coefficient; fraction;force per unit length (Nm1)

g Acceleration due to gravity(ms2)

h Heat transfer coefficient(Wm2 K1)

Vertical displacement (m); Planckconstant ( Js)


i1

k Thermal conductivity(Wm1 K1)

Wave vector (=2/); Boltzmannconstant (=1381023 JK1)

l Distance (m)m Mass (kg) Air-mass-ration Number Number of nozzles, of hours of

bright sunshine, of wind-turbineblades; electron concentration(m3)

p Pressure (Nm2 = Pa) Hole concentration (m3)q Power per unit area (Wm2)r Thermal resistivity of unit

area (R-value = RA)(m2 KW1)

Radius (m); distance (m)

s Angle of slope (degrees)t Time (s) Thickness (m)u Velocity along stream (ms1) Group velocity (ms1)v Velocity (not along stream)

(ms1)w Distance (m) Moisture content (dry basis, %);

moisture content (wet basis, %)(w )

x Co-ordinate (along stream)(m)

y Co-ordinate (across stream)(m)

z Co-ordinate (vertical) (m)

Greek capitals (gamma) Torque (Nm) Gamma function (delta) Increment of (other

symbol) (lambda) Latent heat ( J kg1)

(sigma) Summation sign (phi) Radiant flux (W) Probability functionu Probability distribution of

wind speed (ms11) (omega) Solid angle (steradian) Phonon frequency (s1); angular

velocity of blade (rad s1)Greek lower case (alpha) Absorptance Angle of attack (deg) Monochromatic absorptance (beta) Angle (deg) Volumetric Expansion coefficient

(K1) (gamma) Angle (deg) Blade setting angle (deg) (delta) Boundary layer thickness (m) Angle of declination (deg) epsilon Emittance Wave spectral width; permittivity;

dielectric constant Monochromatic emittance (eta) Efficiency

(Continued)


(theta) Angle of incidence (deg) Temperature difference (C) (kappa) Thermal diffusivity (m2 s1) (lambda) Wavelength (m) Tip speed ratio of wind-turbine (mu) Dynamic viscosity (Nm2 s) (nu) Kinematic viscosity (m2 s1) (xi) Electrode potential (V) Roughness height (m) (pi) 3.1416 (rho) Density (kgm3) Reflectance; electrical resistivity

( m) Monochromatic reflectance (sigma) StefanBoltzmann constant (tau) Transmittance Relaxation time (s); duration (s);

shear stress (Nm2) Monochromatic transmittance (phi) Radiant flux density (RFD)

(Wm2)Wind-blade angle (deg); potentialdifference (V); latitude (deg)

Spectral distribution of RFD(Wm3)

(chi) Absolute humidity (kgm3) (psi) Longitude (deg) Angle (deg) (omega) Angular frequency (= 2f )

(rad s1)Hour angle (deg); solid angle(steradian)

SubscriptsB Black body BandD Drag DarkE EarthF ForceG GeneratorL LiftM MoonP PowerR RatedS SunT Tangential Turbinea Ambient Aperture; available (head); aquiferabs Absorbedb Beam Blade; bottom; base; biogasc Collector Coldci Cut-inco Cut-outcov Coverd Diffuse Dopant; digestere Electrical Equilibrium; energyf Fluid Forced; friction; flowg Glass Generation current; band gaph Horizontal Hot


i Integer Intrinsicin Incident (incoming)int Internalj Integerm mass transfer Mean (average); methanemax Maximumn conductionnet Heat flow across surfaceo (read as numeral zero)oc Open circuitp Plate Peak; positive charge carriers

(holes)r radiation Relative; recombination;

room; resonant; rockrad Radiatedrefl Reflectedrms Root mean squares Surface Significant; saturated; Sunsc Short circuitt Tip Totalth Thermaltrans Transmittedu Usefulv convection Vapourw Wind Waterz Zenith Monochromatic, e.g. 0 Distant approach Ambient; extra-terrestrial;

dry matter; saturated;ground-level

1 Entry to device First2 Exit from device Second3 Output Third

Superscriptm or max Maximum Measured perpendicular to direction

of propagation (e.g. Gb)

(dot) Rate of, e.g. m

Other symbolsBold face Vector, e.g. F= Mathematical equality Approximate equality (within a

few %) Equality in order of magnitude

(within a factor of 210) Mathematical identity (or definition),

equivalent

Chapter 1

Principles of renewable energy

1.1 Introduction

The aim of this text is to analyse the full range of renewable energy sup-plies available for modern economies. Such renewables are recognised asvital inputs for sustainability and so encouraging their growth is signifi-cant. Subjects will include power from wind, water, biomass, sunshine andother such continuing sources, including wastes. Although the scale of localapplication ranges from tens to many millions of watts, and the totality isa global resource, four questions are asked for practical application:

1 How much energy is available in the immediate environment what isthe resource?

2 For what purposes can this energy be used what is the end-use?3 What is the environmental impact of the technology is it sustainable?4 What is the cost of the energy is it cost-effective?

The first two are technical questions considered in the central chapters bythe type of renewables technology. The third question relates to broad issuesof planning, social responsibility and sustainable development; these areconsidered in this chapter and in Chapter 17. The environmental impactsof specific renewable energy technologies are summarised in the last sectionof each technology chapter. The fourth question, considered with otherinstitutional factors in the last chapter, may dominate for consumers andusually becomes the major criterion for commercial installations. However,cost-effectiveness depends significantly on:

a Appreciating the distinctive scientific principles of renewable energy(Section 1.4).

b Making each stage of the energy supply process efficient in terms ofboth minimising losses and maximising economic, social and environ-mental benefits.

c Like-for-like comparisons, including externalities, with fossil fuel andnuclear power.

2 Principles of renewable energy

When these conditions have been met, it is possible to calculate the costsand benefits of a particular scheme and compare these with alternatives foran economic and environmental assessment.Failure to understand the distinctive scientific principles for harnessing

renewable energy will almost certainly lead to poor engineering and uneco-nomic operation. Frequently there will be a marked contrast between themethods developed for renewable supplies and those used for the non-renewable fossil fuel and nuclear supplies.

1.2 Energy and sustainable development

1.2.1 Principles and major issues

Sustainable development can be broadly defined as living, producing andconsuming in a manner that meets the needs of the present without com-promising the ability of future generations to meet their own needs. It hasbecome a key guiding principle for policy in the 21st century. Worldwide,politicians, industrialists, environmentalists, economists and theologiansaffirm that the principle must be applied at international, national and locallevel. Actually applying it in practice and in detail is of course much harder!In the international context, the word development refers to improve-

ment in quality of life, and, especially, standard of living in the less devel-oped countries of the world. The aim of sustainable development is for theimprovement to be achieved whilst maintaining the ecological processes onwhich life depends. At a local level, progressive businesses aim to report apositive triple bottom line, i.e. a positive contribution to the economic, socialand environmental well-being of the community in which they operate.The concept of sustainable development became widely accepted fol-

lowing the seminal report of the World Commission on Environment andDevelopment (1987). The commission was set up by the United Nationsbecause the scale and unevenness of economic development and populationgrowth were, and still are, placing unprecedented pressures on our planetslands, waters and other natural resources. Some of these pressures are severeenough to threaten the very survival of some regional populations and, inthe longer term, to lead to global catastrophes. Changes in lifestyle, espe-cially regarding production and consumption, will eventually be forced onpopulations by ecological and economic pressures. Nevertheless, the eco-nomic and social pain of such changes can be eased by foresight, planningand political (i.e. community) will.Energy resources exemplify these issues. Reliable energy supply is essential

in all economies for lighting, heating, communications, computers, indus-trial equipment, transport, etc. Purchases of energy account for 510% ofgross national product in developed economies. However, in some devel-oping countries, energy imports may have cost over half the value of total

1.2 Energy and sustainable development 3

exports; such economies are unsustainable and an economic challenge forsustainable development. World energy use increased more than tenfoldover the 20th century, predominantly from fossil fuels (i.e. coal, oil andgas) and with the addition of electricity from nuclear power. In the 21stcentury, further increases in world energy consumption can be expected,much for rising industrialisation and demand in previously less developedcountries, aggravated by gross inefficiencies in all countries. Whatever theenergy source, there is an overriding need for efficient generation and useof energy.Fossil fuels are not being newly formed at any significant rate, and thus

present stocks are ultimately finite. The location and the amount of suchstocks depend on the latest surveys. Clearly the dominant fossil fuel type bymass is coal, with oil and gas much less. The reserve lifetime of a resourcemay be defined as the known accessible amount divided by the rate ofpresent use. By this definition, the lifetime of oil and gas resources is usuallyonly a few decades; whereas lifetime for coal is a few centuries. Economicspredicts that as the lifetime of a fuel reserve shortens, so the fuel priceincreases; consequently demand for that fuel reduces and previously moreexpensive sources and alternatives enter the market. This process tends tomake the original source last longer than an immediate calculation indi-cates. In practice, many other factors are involved, especially governmentalpolicy and international relations. Nevertheless, the basic geological factremains: fossil fuel reserves are limited and so the present patterns of energyconsumption and growth are not sustainable in the longer term.Moreover, it is the emissions from fossil fuel use (and indeed nuclear

power) that increasingly determine the fundamental limitations. Increasingconcentration of CO2 in the Atmosphere is such an example. Indeed, froman ecological understanding of our Earths long-term history over billions ofyears, carbon was in excess in the Atmosphere originally and needed to besequestered below ground to provide our present oxygen-rich atmosphere.Therefore from arguments of: (i) the finite nature of fossil and nuclear fuelmaterials, (ii) the harm of emissions and (iii) ecological sustainability, itis essential to expand renewable energy supplies and to use energy moreefficiently. Such conclusions are supported in economics if the full externalcosts of both obtaining the fuels and paying for the damage from emissionsare internalised in the price. Such fundamental analyses may conclude thatrenewable energy and the efficient use of energy are cheaper for societythan the traditional use of fossil and nuclear fuels.The detrimental environmental effects of burning the fossil fuels likewise

imply that current patterns of use are unsustainable in the longer term. Inparticular, CO2 emissions from the combustion of fossil fuels have signifi-cantly raised the concentration of CO2 in the Atmosphere. The balance ofscientific opinion is that if this continues, it will enhance the greenhouse


effect1 and lead to significant climate change within a century or less, whichcould have major adverse impact on food production, water supply andhuman, e.g. through floods and cyclones (IPCC). Recognising that this isa global problem, which no single country can avert on its own, over 150national governments signed the UN Framework Convention on ClimateChange, which set up a framework for concerted action on the issue. Sadly,concrete action is slow, not least because of the reluctance of governmentsin industrialised countries to disturb the lifestyle of their voters. However,potential climate change, and related sustainability issues, is now establishedas one of the major drivers of energy policy.In short, renewable energy supplies are much more compatible with sus-

tainable development than are fossil and nuclear fuels, in regard to bothresource limitations and environmental impacts (see Table 1.1).Consequently almost all national energy plans include four vital factors

for improving or maintaining social benefit from energy:

1 increased harnessing of renewable supplies2 increased efficiency of supply and end-use3 reduction in pollution4 consideration of lifestyle.

1.2.2 A simple numerical model

Consider the following simple model describing the need for commercialand non-commercial energy resources:

R= EN (1.1)

Here R is the total yearly energy requirement for a population of N people.E is the per capita energy-use averaged over one year, related closely toprovision of food and manufactured goods. The unit of E is energy perunit time, i.e. power. On a world scale, the dominant supply of energy isfrom commercial sources, especially fossil fuels; however, significant use ofnon-commercial energy may occur (e.g. fuel wood, passive solar heating),which is often absent from most official and company statistics. In terms oftotal commercial energy use, the average per capita value of E worldwideis about 2 kW; however, regional average values range widely, with NorthAmerica 9 kW, Europe as a whole 4 kW, and several regions of CentralAfrica as small as 0.1 kW. The inclusion of non-commercial energy increases

1 As described in Chapter 4, the presence of CO2 (and certain other gases) in the atmospherekeeps the Earth some 30 degrees warmer than it would otherwise be. By analogy withhorticultural greenhouses, this is called the greenhouse effect.

Table1.1Com

parisonof

renewable

andconventio

nale

nergysystem

s

Renewableenergy

supp

lies(green)

Conventionalenergysupp

lies(brown)

Exam

ples

Wind,

solar,biom

ass,tid

alCoal,oil,gas,radioactiveore

Source

Naturallocale

nviron

ment

Con

centratedstock

Normalstate

Acurrentor

flow

ofenergy.A

nincome

Staticstoreof

energy.C

apital

Initialaverageintensity

Low

intensity,d

ispersed:

300W

m2

Releasedat

100

kWm

2Lifetim

eof

supply

Infinite

Finite

Costat

source

Free

Increasinglyexpensive.

Equipm

entcapitalcostper

kWcapacity

Expensive,

common

lyU

S$1000

kW1

Mod

erate,

perhaps$5

00kW

1with

outem

ission

scontrol;

yetexpensive>US$1000

kW1

with

emission

sredu

ction

Variatio

nandcontrol

Fluctuating;best

controlledby

change

ofload

using

positivefeedforw

ardcontrol

Steady,b

estcontrolledby

adjustingsource

with

negative

feedback

control

Locatio

nforuse

Site-andsociety-specific

Generalandinvariantuse

Scale

Smalland

mod

eratescaleoftenecon

omic,large

scalemay

presentdifficulties

Increasedscaleoftenimproves

supplycosts,largescale

frequentlyfavoured

Skills

Interdisciplinaryandvaried.W

iderangeof

skills.

Impo

rtance

ofbioscience

andagricultu

reStrong

links

with

electricalandmechanicale

ngineering.

Narrow

rangeof

person

alskills

Con

text

Bias

torural,decentralised

indu

stry

Bias

tourban,

centralised

indu

stry

Dependence

Self-sufficientandisland

edsystemssupported

System

sdepend

enton

outsideinputs

Safety

Localh

azards

possible

inop

eration:

usually

safe

whenou

tof

actio

nMay

beshielded

andenclosed

tolessen

greatpo

tential

dangers;mostdangerou

swhenfaulty

Pollutio

nand

environm

entald

amage

Usuallylittle

environm

entalh

arm,e

specially

atmod

erate

scale

Environm

entalp

ollutio

nintrinsicandcommon

,especially

ofairandwater

Hazards

from

excess

biom

assburning

Soilerosionfrom

excessivebiofuelu

seLargehydroreservoirs

disruptive

Com

patib

lewith

naturale

cology

Perm

anentdamagecommon

from

miningandradioactive

elem

ents

entering

water

table.

Deforestatio

nand

ecologicalsterilisatio

nfrom

excessiveairpo

llutio

nClim

atechange

emission

sAesthetics,visualimpact

Localp

erturbations

may

beunpo

pular,butusually

acceptable

iflocaln

eedperceived

Usuallyutilitarian,w

ithcentralisationandecon

omyof

largescale


all these figures and has the major proportional benefit in countries wherethe value of E is small.Standard of living relates in a complex and an ill-defined way to E. Thus

per capita gross national product S (a crude measure of standard of living)may be related to E by:

S = f E (1.2)Here f is a complex and non-linear coefficient that is itself a function ofmany factors. It may be considered an efficiency for transforming energyinto wealth and, by traditional economics, is expected to be as large aspossible. However, S does not increase uniformly as E increases. IndeedS may even decrease for large E (e.g. because of pollution or technicalinefficiency). Obviously unnecessary waste of energy leads to a lower valueof f than would otherwise be possible. Substituting for E in (1.1), thenational requirement for energy becomes:

R= SNf

(1.3)

so

R

R= S

S+ N

N f

f(1.4)

Now consider substituting global values for the parameters in (1.4). In50 years the world population N increased from 2500 million in 1950 toover 6000 million in 2000. It is now increasing at approximately 23% peryear so as to double every 2030 years. Tragically, high infant mortality andlow life expectancy tend to hide the intrinsic pressures of population growthin many countries. Conventional economists seek exponential growth of Sat 25% per year. Thus in (1.4), at constant efficiency f , the growth oftotal world energy supply is effectively the sum of population and economicgrowth, i.e. 48% per year. Without new supplies such growth cannotbe maintained. Yet at the same time as more energy is required, fossiland nuclear fuels are being depleted and debilitating pollution and climatechange increase; so an obvious conclusion to overcome such constraints isto increase renewable energy supplies. Moreover, from (1.3) and (1.4), it ismost beneficial to increase the parameter f , i.e. to have a positive value off . Consequently there is a growth rate in energy efficiency, so that S canincrease, while R decreases.

1.2.3 Global resources

Considering these aims, and with the most energy-efficient modern equip-ment, buildings and transportation, a justifiable target for energy use in a

1.3 Fundamentals 7

modern society with an appropriate lifestyle is E = 2kW per person. Sucha target is consistent with an energy policy of contract and converge forglobal equity, since worldwide energy supply would total approximatelythe present global average usage, but would be consumed for a far higherstandard of living. Is this possible, even in principle, from renewable energy?Each square metre of the earths habitable surface is crossed by, or accessibleto, an average energy flux from all renewable sources of about 500W (seeProblem 1.1). This includes solar, wind or other renewable energy formsin an overall estimate. If this flux is harnessed at just 4% efficiency, 2 kWof power can be drawn from an area of 10m 10m, assuming suitablemethods. Suburban areas of residential towns have population densitiesof about 500 people per square kilometre. At 2 kW per person, the totalenergy demand of 1000kWkm2 could be obtained in principle by usingjust 5% of the local land area for energy production. Thus renewable energysupplies can provide a satisfactory standard of living, but only if the tech-nical methods and institutional frameworks exist to extract, use and storethe energy in an appropriate form at realistic costs. This book considersboth the technical background of a great variety of possible methods anda summary of the institutional factors involved. Implementation is theneveryones responsibility.

1.3 Fundamentals

1.3.1 Definitions

For all practical purposes energy supplies can be divided into two classes:

1 Renewable energy. Energy obtained from natural and persistent flowsof energy occurring in the immediate environment. An obvious exampleis solar (sunshine) energy, where repetitive refers to the 24-hour majorperiod. Note that the energy is already passing through the environmentas a current or flow, irrespective of there being a device to interceptand harness this power. Such energy may also be called Green Energyor Sustainable Energy.

2 Non-renewable energy. Energy obtained from static stores of energythat remain underground unless released by human interaction. Exam-ples are nuclear fuels and fossil fuels of coal, oil and natural gas. Notethat the energy is initially an isolated energy potential, and externalaction is required to initiate the supply of energy for practical pur-poses. To avoid using the ungainly word non-renewable, such energysupplies are called finite supplies or Brown Energy.

These two definitions are portrayed in Figure 1.1. Table 1.1 provides acomparison of renewable and conventional energy systems.


Natural Environment:green Mined resource: brown

Current source of continuousenergy flow

A

C

D

E

F

B

Device

Use

D

E

F

Device

Use

Environment Sink Environment Sink

Finite source ofenergy potential

Renewable energy Finite energy

Figure 1.1 Contrast between renewable (green) and finite (brown) energy supplies.Environmental energy flow ABC, harnessed energy flow DEF.

1.3.2 Energy sources

There are five ultimate primary sources of useful energy:

1 The Sun.2 The motion and gravitational potential of the Sun, Moon and Earth.3 Geothermal energy from cooling, chemical reactions and radioactive

decay in the Earth.4 Human-induced nuclear reactions.5 Chemical reactions from mineral sources.

Renewable energy derives continuously from sources 1, 2 and 3 (aquifers).Finite energy derives from sources 1 (fossil fuels), 3 (hot rocks), 4 and 5.The sources of most significance for global energy supplies are 1 and 4. Thefifth category is relatively minor, but useful for primary batteries, e.g. drycells.

1.3.3 Environmental energy

The flows of energy passing continuously as renewable energy through theEarth are shown in Figure 1.2. For instance, total solar flux absorbed atsea level is about 12 1017W. Thus the solar flux reaching the Earthssurface is 20MW per person; 20MW is the power of ten very large

1.3 Fundamentals 9

Reflectedto space50 000

Solarradiation

FromSun

FromEarth

Fromplanetarymotion

120 000Absorbed on

Earth

40 000

80 000 Sensibleheating

Latent heatand potentialenergy

300Kinetic energy

Photonprocesses

Geothermal

30

100

Heat

Gravitation,orbital motion Tidal motion

3

Infraredradiation

Solar water heatersSolar buildingsSolar dryersOcean thermal energy

Hydropower

Wind and wave turbines

Biomass and biofuelsPhotovoltaics

Geothermal heatGeothermal power

Tidal range powerTidal current power

Figure 1.2 Natural energy currents on earth, showing renewable energy system. Notethe great range of energy flux 1 105 and the dominance of solar radiationand heat. Units terawatts 1012W.

diesel electric generators, enough to supply all the energy needs of a townof about 50 000 people. The maximum solar flux density (irradiance)perpendicular to the solar beam is about 1kWm2; a very useful andeasy number to remember. In general terms, a human being is able tointercept such an energy flux without harm, but any increase begins tocause stress and difficulty. Interestingly, power flux densities of 1kWm2begin to cause physical difficulty to an adult in wind, water currents orwaves.However, the global data of Figure 1.2 are of little value for practical

engineering applications, since particular sites can have remarkably differentenvironments and possibilities for harnessing renewable energy. Obviouslyflat regions, such as Denmark, have little opportunity for hydro-power butmay have wind power. Yet neighbouring regions, for example Norway, mayhave vast hydro potential. Tropical rain forests may have biomass energysources, but deserts at the same latitude have none (moreover, forests mustnot be destroyed so making more deserts). Thus practical renewable energysystems have to be matched to particular local environmental energy flowsoccurring in a particular region.


1.3.4 Primary supply to end-use

All energy systems can be visualised as a series of pipes or circuits throughwhich the energy currents are channelled and transformed to become use-ful in domestic, industrial and agricultural circumstances. Figure 1.3(a)is a Sankey diagram of energy supply, which shows the energy flowsthrough a national energy system (sometimes called a spaghetti diagrambecause of its appearance). Sections across such a diagram can be drawnas pie charts showing primary energy supply and energy supply to end-use

Thermalelectricitygeneration

RefiningCrude oil

PRIMARYENERGYSUPPLIES

Coal

Fossil gas

Biomass

Hydro

Oil products

Non-energy useENERGYEND-USE

Transport

Industry

Residentialand other

Districtheating

Waste heat

Electricity

(a)

300 PJ

Figure 1.3 Energy flow diagrams for Austria in 2000, with a population of 8.1 million.(a) Sankey (spaghetti) diagram, with flows involving thermal electricity showndashed. (b)(c) Pie diagrams. The contribution of hydropower and biomass(wood and waste) is greater than in most industrialised countries, as is theuse of heat produced from thermal generation of electricity (combined heatand power). Energy use for transport is substantial and very dependent on(imported) oil and oil products, therefore the Austrian government encouragesincreased use of biofuels. Austrias energy use has grown by over 50% since1970, although the population has grown by less than 10%, indicating the needfor greater efficiency of energy use. [Data source: simplified from InternationalEnergy Agency, Energy Balances of OECD countries 20002001.]

1.3 Fundamentals 11

(b)

Energy End-Use(total: 970 PJ)

Industry30%

Transport30%

Residential28%

Other12%

(c)

Figure 1.3 (Continued).

(Figure 1.3(b)). Note how the total energy end-use is less than the pri-mary supply because of losses in the transformation processes, notably thegeneration of electricity from fossil fuels.

1.3.5 Energy planning

1 Complete energy systems must be analysed, and supply should not beconsidered separately from end-use. Unfortunately precise needs forenergy are too frequently forgotten, and supplies are not well matchedto end-use. Energy losses and uneconomic operation therefore fre-quently result. For instance, if a dominant domestic energy require-ment is heat for warmth and hot water, it is irresponsible to generategrid quality electricity from a fuel, waste the majority of the energyas thermal emission from the boiler and turbine, distribute the elec-tricity in lossy cables and then dissipate this electricity as heat. Sadly


such inefficiency and disregard for resources often occurs. Heatingwould be more efficient and cost-effective from direct heat productionwith local distribution. Even better is to combine electricity genera-tion with the heat production using CHP combined heat and power(electricity).

2 System efficiency calculations can be most revealing and can pinpointunnecessary losses. Here we define efficiency as the ratio of the usefulenergy output from a process to the total energy input to that pro-cess. Consider electric lighting produced from conventional thermallygenerated electricity and lamps. Successive energy efficiencies are: elec-tricity generation 30%, distribution 90% and incandescent lighting(energy in visible radiation, usually with a light-shade) 45%. The totalefficiency is 11.5%. Contrast this with cogeneration of useful heatand electricity (efficiency 85%), distribution 90% and lighting inmodern low consumption compact fluorescent lamps (CFL) 22%.The total efficiency is now 1418%; a more than tenfold improvement!The total life cycle cost of the more efficient system will be much lessthan for the conventional, despite higher per unit capital costs, because(i) less generating capacity and fuel are needed, (ii) less per unit emissioncosts are charged, and (iii) equipment (especially lamps) lasts longer(see Problems 1.2 and 1.3).

3 Energy management is always important to improve overall efficiencyand reduce economic losses. No energy supply is free, and renewablesupplies are usually more expensive in practice than might be assumed.Thus there is no excuse for wasting energy of any form unnecessarily.Efficiency with finite fuels reduces pollution; efficiency with renewablesreduces capital costs.

1.4 Scientific principles of renewable energy

The definitions of renewable (green) and finite (brown) energy supplies(Section 1.3.1) indicate the fundamental differences between the two formsof supply. As a consequence the efficient use of renewable energy requiresthe correct application of certain principles.

1.4.1 Energy currents

It is essential that a sufficient renewable current is already present in thelocal environment. It is not good practice to try to create this energy currentespecially for a particular system. Renewable energy was once ridiculedby calculating the number of pigs required to produce dung for sufficientmethane generation to power a whole city. It is obvious, however, thatbiogas (methane) production should only be contemplated as a by-productof an animal industry already established, and not vice versa. Likewise

1.4 Scientific principles of renewable energy 13

for a biomass energy station, the biomass resource must exist locally toavoid large inefficiencies in transportation. The practical implication of thisprinciple is that the local environment has to be monitored and analysedover a long period to establish precisely what energy flows are present. InFigure 1.1 the energy current ABC must be assessed before the divertedflow through DEF is established.

1.4.2 Dynamic characteristics

End-use requirements for energy vary with time. For example, electricitydemand on a power network often peaks in the morning and evening,and reaches a minimum through the night. If power is provided from afinite source, such as oil, the input can be adjusted in response to demand.Unused energy is not wasted, but remains with the source fuel. However,with renewable energy systems, not only does end-use vary uncontrollablywith time but so too does the natural supply in the environment. Thus arenewable energy device must be matched dynamically at both D and Eof Figure 1.1; the characteristics will probably be quite different at bothinterfaces. Examples of these dynamic effects will appear in most of thefollowing chapters.The major periodic variations of renewable sources are listed in Table 1.2,

but precise dynamic behaviour may well be greatly affected by irregularities.Systems range from the very variable (e.g. wind power) to the accuratelypredictable (e.g. tidal power). Solar energy may be very predicable in someregions (e.g. Khartoum) but somewhat random in others (e.g. Glasgow).

1.4.3 Quality of supply

The quality of an energy supply or store is often discussed, but usuallyremains undefined. We define quality as the proportion of an energy sourcethat can be converted to mechanical work. Thus electricity has high qualitybecause when consumed in an electric motor >95% of the input energymay be converted to mechanical work, say to lift a weight; the heat lossesare correspondingly small,

Table1.2Intensity

andperiod

icalprop

ertie

sof

renewable

sources

System

Major

perio

dsMajor

varia

bles

Power

relationship

Comment

Text

reference

(equation)

Directsunshine

24h,

1y

Solarbeam

irradiance

G bW

m2

;

PG bcos

z

Daytim

eon

ly(4.2)

Angle

ofbeam

from

verticalz

P max=1kW

m2

Diffusesunshine

24h,

1y

Cloud

cover,perhapsair

pollutio

nP

G!

P300W

m2

Significant

energy

over

time

(4.3)

Biofuels

1y

Soilcond

ition

,insolation,

water,p

lant

species,wastes

Stored

energy

10MJkg

1Verymanychem

icaltypesand

sources.Linked

toagricultu

reandforestry.Storedenergy

Table

11.1

Table

11.4

Wind

1y

Windspeedu 0

Pu3 0

Mostvariable

(9.2)

Heightnacelle

abovegrou

ndz;

height

ofanem

ometer

masth

u z/u

h=z/h

b

b015

(9.54)

Wave

1y

SignificantwaveheightH

swaveperiod

TPH

2 sT

Relativelylargepo

wer

density

50kW

m1

across

wavefron

t(12.47)

Hydro

1y

Reservoirheight

Hwater

volumeflo

wrate

QPHQ

Establishedresource

(8.1)

Tidal

12h25

min

TidalrangeR;con

tained

area

A;e

stuary

length

L,depthh

PR2A

Enhanced

tidalrangeif

L/ h

=36

000m

1/2

(13.35)and

(13.28)

Tidalstream

/current;p

eak

currentu 0,d

ensity

seawater

1000

air

Pu3 0

Enhanced

tidalcurrents

betw

eencertainisland

s(13.30)

Ocean

thermal

energy

conversion

Con

stant

Tem

perature

difference

betw

eenseasurfaceanddeep

water,

T

P

T

2

Sometrop

icallocatio

nshave

T

20 C

,sopo

tentially

harnessable,

butsm

alle

fficiency

(14.5)

1.4 Scientific principles of renewable energy 15

Renewable energy supply systems divide into three broad divisions:

1 Mechanical supplies, such as hydro, wind, wave and tidal power. Themechanical source of power is usually transformed into electricity athigh efficiency. The proportion of power in the environment extractedby the devices is determined by the mechanics of the process, linkedto the variability of the source, as explained in later chapters. Theproportions are, commonly, wind 35%, hydro 7090%, wave 50%and tidal 75%.

2 Heat supplies, such as biomass combustion and solar collectors. Thesesources provide heat at high efficiency. However, the maximum pro-portion of heat energy extractable as mechanical work, and hence elec-tricity, is given by the second law of thermodynamics and the CarnotTheorem, which assumes reversible, infinitely long transformations. Inpractice, maximum mechanical power produced in a dynamic processis about half that predicted by the Carnot criteria. For thermal boilerheat engines, maximum realisable quality is about 35%.

3 Photon processes, such as photosynthesis and photochemistry(Chapter 10) and photovoltaic conversion (Chapter 7). For example,solar photons of a single frequency may be transformed into mechani-cal work via electricity with high efficiency using a matched solar cell.In practice, the broad band of frequencies in the solar spectrum makesmatching difficult and photon conversion efficiencies of 2030% areconsidered good.

1.4.4 Dispersed versus centralised energy

A pronounced difference between renewable and finite energy supplies isthe energy flux density at the initial transformation. Renewable energycommonly arrives at about 1kWm2 (e.g. solar beam irradiance, energy inthe wind at 10ms1), whereas finite centralised sources have energy fluxdensities that are orders of magnitude greater. For instance, boiler tubesin gas furnaces easily transfer 100kWm2, and in a nuclear reactor thefirst wall heat exchanger must transmit several MWm2. At end-use afterdistribution, however, supplies from finite sources must be greatly reducedin flux density. Thus apart from major exceptions such as metal refining,end-use loads for both renewable and finite supplies are similar. In summaryfinite energy is most easily produced centrally and is expensive to distribute.Renewable energy is most easily produced in dispersed locations and isexpensive to concentrate. With an electrical grid, the renewable generatorsare said to be embedded within the (dispersed) system.A practical consequence of renewable energy application is development

and increased cash flow in the rural economy. Thus the use of renewableenergy favours rural development and not urbanisation.


1.4.5 Complex systems

Renewable energy supplies are intimately linked to the natural environment,which is not the preserve of just one academic discipline such as physicsor electrical engineering. Frequently it is necessary to cross disciplinaryboundaries from as far apart as, say, plant physiology to electronic con-trol engineering. An example is the energy planning of integrated farming(Section 11.8.1). Animal and plant wastes may be used to generate methane,liquid and solid fuels, and the whole system integrated with fertilizer pro-duction and nutrient cycling for optimum agricultural yields.

1.4.6 Situation dependence

No single renewable energy system is universally applicable, since the abil-ity of the local environment to supply the energy and the suitability ofsociety to accept the energy vary greatly. It is as necessary to prospectthe environment for renewable energy as it is to prospect geological forma-tions for oil. It is also necessary to conduct energy surveys of the domestic,agricultural and industrial needs of the local community. Particular end-useneeds and local renewable energy supplies can then be matched, subject toeconomic and environmental constraints. In this respect renewable energyis similar to agriculture. Particular environments and soils are suitable forsome crops and not others, and the market pull for selling the producewill depend on particular needs. The main consequence of this situationdependence of renewable energy is the impossibility of making simplisticinternational or national energy plans. Solar energy systems in southernItaly should be quite different from those in Belgium or indeed in north-ern Italy. Corn alcohol fuels might be suitable for farmers in Missouri butnot in New England. A suitable scale for renewable energy planning mightbe 250 km, but certainly not 2500 km. Unfortunately present-day largeurban and industrialised societies are not well suited for such flexibility andvariation.

1.5 Technical implications

1.5.1 Prospecting the environment

Normally, monitoring is needed for several years at the site in question.Ongoing analysis must insure that useful data are being recorded, particu-larly with respect to dynamic characteristics of the energy systems planned.Meteorological data are always important, but unfortunately the sites ofofficial stations are often different from the energy generating sites, andthe methods of recording and analysis are not ideal for energy prospecting.However, an important use of the long-term data from official monitor-ing stations is as a base for comparison with local site variations. Thus

1.5 Technical implications 17

wind velocity may be monitored for several months at a prospective gen-erating site and compared with data from the nearest official base station.Extrapolation using many years of base station data may then be possible.Data unrelated to normal meteorological measurements may be difficult toobtain. In particular, flows of biomass and waste materials will often nothave been previously assessed, and will not have been considered for energygeneration. In general, prospecting for supplies of renewable energy requiresspecialised methods and equipment that demand significant resources offinance and manpower. Fortunately the links with meteorology, agricultureand marine science give rise to much basic information.

1.5.2 End-use requirements and efficiency

As explained in Section 1.3.5, energy generation should always followquantitative and comprehensive assessment of energy end-use requirements.Since no energy supply is cheap or occurs without some form of environ-mental disruption, it is also important to use the energy efficiently withgood methods of energy conservation. With electrical systems, the end-userequirement is called the load, and the size and dynamic characteristics ofthe load will greatly affect the type of generating supply. Money spent onenergy conservation and improvements in end-use efficiency usually givesbetter long-term benefit than money spent on increased generation andsupply capacity. The largest energy requirements are usually for heat andtransport. Both uses are associated with energy storage capacity in ther-mal mass, batteries or fuel tanks, and the inclusion of these uses in energysystems can greatly improve overall efficiency.

1.5.3 Matching supply and demand

After quantification and analysis of the separate dynamic characteristics ofend-use demands and environmental supply options, the total demand andsupply have to be brought together. This may be explained as follows:

1 The maximum amount of environmental energy must be utilisedwithin the capability of the renewable energy devices and systems. InFigure 1.4(a), the resistance to energy flow at D, E and F should besmall. The main benefit of this is to reduce the size and amount ofgenerating equipment.

2 Negative feedback control from demand to supply is not beneficial sincethe result is to waste or spill harnessable energy (Figure 1.4(b)); in effectthe capital value of the equipment is not fully utilised. Such controlshould only be used at times of emergency or when all conceivableend-uses have been satisfied. Note that the disadvantage of negativefeedback control is a consequence of renewable energy being flow or

Figure 1.4 Matching renewable energy supply to end-use. (a) Maximum energy flowfor minimum size of device or system requires low resistance to flowat D, E and F. (b) Negative feedback control wastes energy opportunityand capital value. (c) Energy storage allows the dynamic characteristics ofend-use to be decoupled from the supply characteristics. (d) Decouplingwith a large grid system. (e) Feedforward load management control of thesupply; arguably the most efficient way to use renewable energy. Totalload at E may be matched to the available supply at D at all times and socontrol the supply device.


current sources that can never be stopped. With finite energy sources,negative feedback control to the energy source is beneficial, since lessfuel is used.

3 The natural periods and dynamic properties of end-use are mostunlikely to be the same as those of the renewable supply, as discussed inSection 1.4.2. The only way to match supply and demand that have dif-ferent dynamic characteristics, and yet not to waste harnessable energy,is to incorporate storage (Figure 1.4(c)). Satisfactory energy storage isexpensive (see Chapter 16), especially if not incorporated at the earlieststages of planning.

4 The difficulties of matching renewable energy supplies to end-use instand-alone systems are so great that one common approach is todecouple supply from local demand by connection to an energy net-work or grid (Figure 1.4(d)). Here the renewable supply is embeddedin an energy grid network having input from finite sources havingfeedback control. Such systems imply relatively large scale operationand include electricity grids for transmission and distribution. As in (3)the addition of substantial energy storage in the system, say pumpedhydro or thermal capacity for heating, can improve efficiency and allowthe proportion of renewable supply to increase. By using the grid forboth the export and the import of energy, the grid becomes a virtualstore.

5 The most efficient way to use renewable energy is shown inFigure 1.4(e). Here a range of end-uses is available and can be switchedor adjusted so that the total load equals the supply at any one time.Some of the end-use blocks could themselves be adjustable (e.g. variablevoltage water heating, pumped water storage). Such systems requirefeedforward control (see Section 1.5.4). Since the end-use load increaseswith increase in the renewable energy supply, this is positive feedfor-ward control.

1.5.4 Control options

Good matching of renewable energy supply to end-use demand is accom-plished by control of machines, devices and systems. The discussion inSection 1.5.3 shows that there are three possible categories of control:(1) spill the excess energy, (2) incorporate storage and (3) operate loadmanagement control. These categories may be applied in different ways,separately or together, to all renewable energy systems, and will be illus-trated here with a few examples (Figure 1.5).

1 Spill excess energy. Since renewable energy derives from energy flowsources, energy not used is energy wasted. Nevertheless spilling excess


Figure 1.5 Examples of control. (a) Control by spilling excess energy: constant pres-sure maintained for the turbine. (b) Control incorporating storage inhydroelectric catchment dam. (c) Control by load variation: feedforwardcontrol. Load controller automatically shunts power between end-uses,maintaining constant generator load at E. Turbine also has constant loadand hence constant frequency: only rudimentary mechanical control ofturbine is needed.

energy provides easy control and may be the cheapest option. Examplesoccur with run-of-the-river hydroelectric systems (Figure 1.5(a)), shadesand blinds with passive solar heating of buildings, and wind turbineswith adjustable blade pitch.

2 Incorporate storage. Storage before transformation allows a maximumamount of energy to be trapped from the environment and eventually


harnessed or used. Control methods are then similar to conventionalmethods with finite sources, with the store equivalent to fuel. The maindisadvantages are the large relative capital costs of storage, and thedifficulty of reducing conventional control methods to small-scale andremote operation. In the example of Figure 1.5(b), hydro storage isusually only contemplated for generation at more than 10MW. Themechanical flow control devices become unwieldy and expensive at amicrohydro scale of 10kW. A disadvantage of hydro storage may bethe environmental damage caused by reservoirs.Storage after energy transformation, e.g. battery charging or hydro-

gen production, is also possible and may become increasingly importantespecially in small systems. Thermal storage is already common.

3 Load control. Parallel arrangements of end-uses may be switched andcontrolled so as to present optimum total load to the supply. An exam-ple of a microhydro load controller for household power is shown inFigure 1.5(c) (see also Section 8.6). The principle may be applied ona small or large scale, but is perhaps most advantageous when manyvaried end-uses are available locally. There are considerable advantagesif load control is applied to renewable energy systems:

a No environmental energy need be wasted if parallel outputs areopened and closed to take whatever input energy flow is available.Likewise, the capital-intensive equipment is well used.

b Priorities and requirements for different types of end-use can beincorporated in many varied control modes (e.g. low priority usescan receive energy at low cost, provided that they can be switchedoff by feedforward control; electrical resistive heaters may receivevariable voltage and hence variable power).

c End-uses having storage capability (e.g. thermal capacity of waterheating and building space conditioning) can be switched to givethe benefits of storage in the system at no extra cost.

d Electronic and microprocessor-based control may be used withbenefits of low cost, reliability, and extremely fast and accurateoperation.

Feedforward load control may be particularly advantageous forautonomous wind energy systems (see Chapter 9, especially Section 9.8.2).Wind fluctuates greatly in speed and the wind turbine should change rota-tional frequency to maintain optimum output. Rapid accurate control isnecessary without adding greatly to the cost or mechanical complexity, andso electronically based feedforward control into several parallel electricalloads is most useful. An example is shown in Figure 1.6.


Figure 1.6 Wind energy conversion system for Fair Isle, Scotland. Electrical loadsare switched by small changes in the supply frequency, so presenting amatched load to the generator over a wide range of wind speeds.

1.6 Social implications

The Industrial Revolution in Europe and North America and industrialdevelopment in all countries have profoundly affected social structuresand patterns of living. The influence of changing and new energy sourceshas been the driving function for much of this change. Thus there is ahistoric relationship between coal mining and the development of indus-trialised countries, which will continue for several hundred years. In thenon-industrialised countries, relatively cheap oil supplies became availablein the 1950s at the same time as many countries obtained independencefrom colonialism. Thus in all countries the use of fossil fuels has led toprofound changes in lifestyle.

1.6.1 Dispersed living

In Sections 1.1 and 1.4.4 the dispersed and small energy flux density ofrenewable sources was discussed. Renewable energy arrives dispersed in theenvironment and is difficult and expensive to concentrate. By contrast finiteenergy sources are energy stores that are easily concentrated at source andexpensive to disperse. Thus electrical distribution grids from fossil fuel andnuclear sources tended to radiate from central, intensive distribution points,typically with 1000MWe capacity. Industry has developed on these grids,with heavy industry closest to the points of intensive supply. Domestic

1.6 Social implications 23

populations have grown in response to the employment opportunities ofindustry and commerce. Similar effects have occurred with the relation-ships between coal mining and steel production, oil refining and chemicalengineering and the availability of gas supplies and urban complexes.This physical review of the effect of the primary flux density of energy

sources suggests that widespread application of renewable energy willfavour dispersed, rather than concentrated, communities. Electricity gridsin such situations are powered by smaller-scale, embedded, generation, withpower flows moving intermittently in both directions according to localgeneration and local demand. In Section 1.2.2 an approximate estimateof 500 people per square kilometre was made of maximum populationdensity for communities relying on renewable sources. This is considerablygreater than for rural communities (100 people per square kilometre) andcorresponds with the population densities of the main administration andcommercial towns of rural regions. Thus the gradual acceptance of signifi-cant supplies of renewable energy could allow relief from the concentratedmetropolises of excessive urbanisation, yet would not require unacceptablylow population densities. A further advantage is the increased security fora nation having its energy supplies from such indigenous and dispersedsources.

1.6.2 Pollution and environmental impact

Harmful emissions can be classified as chemical (as from fossil fuel andnuclear power plant), physical (including acoustic noise and radioactivity)and biological (including pathogens); such pollution from energy generationis overwhelmingly a result of using brown fuels, fossil and nuclear. Incontrast, renewable energy is always extracted from flows of energy alreadycompatible with the environment (Figure 1.1). The energy is then returned tothe environment, so no thermal pollution can occur on anything but a smallscale. Likewise material and chemical pollution in air, water and refuse tendto be minimal. An exception is air pollution from incomplete combustionof biomass or refuses (see Chapter 11). Environmental pollution does occurif brown energy is used for the materials and manufacture of renewableenergy devices, but this is small over the lifetime of the equipment.The environmental impact of renewables depends on the particular tech-

nology and circumstances. We consider these in the last section of eachtechnology chapter that follows. General institutional factors, often relatedto the abatement of pollution, are considered in the last chapter.

1.6.3 The future

In short, we see that many changes in social patterns are related to energysupplies. We can expect further changes to occur as renewable energy


systems become widespread. The influence of modern science and technol-ogy ensures that there are considerable improvements to older technologies,and subsequently standards of living can be expected to rise, especially inrural and previously less developed sectors. It is impossible to predict exactlythe long-term effect of such changes in energy supply, but the sustainablenature of renewable energy should produce greater socio-economic stabilitythan has been the case with fossil fuels and nuclear power. In particularwe expect the great diversity of renewable energy supplies to be associatedwith a similar diversity in local economic and social characteristics.

Problems

1.1 a Show that the average solar irradiance absorbed during 24 h overthe whole Earths surface is about 230W (see Figure 1.2)

b Using devices, the average local power accessible can be increased,e.g. by tilting solar devices towards the Sun, by intercepting winds.Is it reasonable to state that each square metre of the Earthshabitable surface is crossed or accessible to an average flux of about500W?

1.2 a Compare the direct costs to the consumer of using:

i a succession of ten 100W incandescent light bulbs with anefficiency for electricity to visible light of 5%, life of 1 000 h,price E0.5;

ii one compact fluorescent lamp (CFL) giving the same illumina-tion at 22% efficiency, life of 10 000 h, price E3.0. Use a fixedelectricity price of E010kWh1;

b what is the approximate payback time in lighting-hours of (b)against (a). [See also Problem 17.1 that allows for the more sophis-ticated discounted costs.]

1.3 Repeat the calculation of Problem 1.2, with tariff prices of your locallamps and electricity. Both the price of CFLs in local shops and ofelectricity vary markedly, so your answers may differ significantly.Nevertheless it is highly likely the significant lifetime savings will stilloccur.

1.4 Economists argue that as oil reserves become smaller, the price willincrease, so demand will decrease and previously uneconomic supplieswill come into production. This tends to make the resource last longerthan would be suggested by a simple calculation (based on todaysreserves divided by todays use) . On the other hand, demand increasesdriven by increased economic development in developing countries tendto shorten the life of the reserve. Discuss.

Bibliography 25

Bibliography

Refer to the bibliographies at the end of each chapter for particular subjects andtechnologies.

Surveys of renewable energy technology and resources

Boyle, G. (ed.) (2004, 2nd edn) Renewable Energy, Oxford University Press.Excellent introduction for both scientific and non-scientific readers.

Jackson, T. (ed.) (1993) Renewable Energy: Prospects for implementation,Butterworth-Heinemann, Oxford. Collection of a series of articles from the jour-nal Energy Policy, with focus on implementation rather than technical detail.

Johansson, T.J., Kelly, H., Reddy, A.K.N., Williams, R.H. (eds) (1993) RenewableEnergy: Sources for fuels and electricity, produced for the UN Solar EnergyGroup for Environment and Development, Earthscan, London, and Island Press,Washington DC, 1000pp. An authoritative study; but does not attempt to includethe built environment.

Srensen, B. (2004, 3rd edn) Renewable Energy, Academic Press, London.Outstandingly the best theoretical text at postgraduate level, considering energyfrom the environment to final use.

US Department of Energy (1997) Renewable Energy Characterisations, US-DOETopical Report TR-109496. [Available on website www.eere.gov] Emphasis onprospects for electricity generation and R&D requirements.

Energy, society and the environment (including sustainabledevelopment)

[see also the bibliography for chapter 17]

Boyle, G., Everett, R. and Ramage, J. (eds) (2003) Energy Systems and Sustain-ability: Power for a Sustainable Future, Oxford UP in association with the OpenUniversity. Good non-technical account for science and society courses.

Cassedy, E.S. and Grossman, P.G. (2002, 2nd edn) Introduction to Energy:Resources, Technology and Society, Cambridge UP. Good non-technical accountfor science and society courses.

Elliot, D. (2003, 2nd edn) Energy, Society and the Environment, Routledge. Briefsurvey of technologies, but more extensive discussion of institutional and societalaspects.

Goldemberg, J. (1996) Energy, Environment and Development, Earthscan (withJames and James), London. Wide-ranging and readable exposition of the linksbetween energy and social and economic development and sustainability, withconsideration of equity within and between countries by a Brazilian expert.

Houghton, J.T. (1997, 2nd edn) Global Warming: The Complete Briefing, Cam-bridge UP. Less technical and more committed than the official IPCC report (SirJohn Houghton was co-chair of IPCC).

Intergovernmental Panel on Climate Change (IPCC) (2001) Third AssessmentReport 3 vols see especially Summary for Policy Makers: synthesis report(see IPCC website listed below). The full report is 3 large volumes.


International Energy Agency (IEA) (2001) Toward a Sustainable Energy Future,Paris. Emphasises the economic dimension of sustainable development.

McNeill, J. R. (2000) Something New Under the Sun: An Environmental History ofthe Twentieth Century, Penguin, London. The growth of fossil-fuel-fired citiesand their impacts on water, air and the biosphere.

Ruedisili, L.C. and Firebaugh, M.W. (eds) (1978, 2nd edn) Perspectives on Energy,Oxford University Press. Well-chosen collection of reprints, often of contrastingviews. Illustrates that many of the issues (including some of the funding issuesconcerning renewable energy) have not changed from the 1970s, when the policymotivation for alternative energy sources was oil shortages rather than green-house gas emissions.

Twidell, J., Hounam, I. and Lewis, C. (1986) Energy for Highlands and Islands,IV proceedings of fourth annual conference on this subject, Pe

renewable energy sources

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