renewable energy resources (book)

Renewable Energy Resources is a numerate and quantitative text covering subjects of proven technical and economic importance worldwide. Energy supplies from renewables (such as solar, thermal, photovoltaic, wind, hydro, biofuels, wave, tidal, ocean and geothermal sources) are essential components of every nation’s energy strategy, not least because of concerns for the environment and for sustainability. In the years between the first and this second edition, renewable en

Renewable Energy Resources

Renewable Energy Resources is a numerate and quantitative text covering subjects

of 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 nation’s 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 come

of age: it makes good sense, good government and good business

This second edition maintains the book’s basis on fundamentals, whilst 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 added

includ-to 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 the

interface 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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Second edition published 2006

by Taylor & Francis

2 Park Square, Milton Park, Abingdon, Oxon OX14 4RN

Simultaneously published in the USA and Canada

by Taylor & Francis

270 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 or

reproduced or utilised in any form or by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying and recording, or in any information storage or retrieval system, without permission in writing from the publishers.

The publisher makes no representation, express or implied, with regard

to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for any errors or

omissions that may be made.

British Library Cataloguing in Publication Data

A catalogue record for this book is available

from the British Library

Library of Congress Cataloging in Publication Data

Twidell, John.

Renewable energy resources / John Twidell and

Anthony Weir — 2nd ed.

p cm.

Includes bibliographical references and index.

ISBN 0–419–25320–3 (hardback) — ISBN 0–419–25330–0 (pbk.)

1 Renewable energy sources I Weir, Anthony D II Title.

TJ808.T95 2005

621.042—dc22

2005015300 ISBN10: 0–419–25320–3 ISBN13: 9–78–0–419–25320–4 Hardback ISBN10: 0–419–25330–0 ISBN13: 9–78–0–419–25330–3 Paperback

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

“To purchase your own copy of this or any of Taylor & Francis or Routledge’s

collection of thousands of eBooks please go to www.eBookstore.tandf.co.uk.”

2.6 Friction in pipe flow 35

2.7 Lift and drag forces: fluid and turbine machinery 39

3.4 Convection 51

3.5 Radiative heat transfer 61

3.6 Properties of ‘transparent’ materials 73

3.7 Heat transfer by mass transport 74

3.8 Multimode transfer and circuit analysis 77

4.4 Geometry of the Earth and Sun 89

4.5 Geometry of collector and the solar beam 93

4.6 Effects of the Earth’s atmosphere 98

4.7 Measurements of solar radiation 104

4.8 Estimation of solar radiation 107

Problems 110

Bibliography 112

5.1 Introduction 115

5.2 Calculation of heat balance: general remarks 118

5.3 Uncovered solar water heaters – progressive analysis 119

5.4 Improved solar water heaters 123

5.5 Systems with separate storage 129

Contents vii

6.7 Solar ponds 164

6.8 Solar concentrators 166

6.9 Solar thermal electric power systems 170

6.10 Social and environmental aspects 173

Problems 175

Bibliography 179

7.1 Introduction 182

7.2 The silicon p–n junction 184

7.3 Photon absorption at the junction 193

7.4 Solar radiation absorption 197

7.5 Maximising cell efficiency 200

7.6 Solar cell construction 208

7.7 Types and adaptations of photovoltaics 210

7.8 Photovoltaic circuit properties 220

7.9 Applications and systems 224

7.10 Social and environmental aspects 229

8.7 The hydraulic ram pump 255

8.8 Social and environmental aspects 257

Problems 258

Bibliography 261

9.1 Introduction 263

9.2 Turbine types and terms 268

9.3 Linear momentum and basic theory 273

9.5 Blade element theory 288

9.6 Characteristics of the wind 290

9.7 Power extraction by a turbine 305

10.2 Trophic level photosynthesis 326

10.3 Photosynthesis at the plant level 330

11.3 Biomass production for energy farming 357

11.4 Direct combustion for heat 365

11.5 Pyrolysis (destructive distillation) 370

11.6 Further thermochemical processes 374

11.7 Alcoholic fermentation 375

11.8 Anaerobic digestion for biogas 379

11.9 Wastes and residues 387

11.10 Vegetable oils and biodiesel 388

11.11 Social and environmental aspects 389

13.4 Tidal current/stream power 442

13.5 Tidal range power 443

13.6 World range power sites 447

13.7 Social and environmental aspects of tidal range power 449 Problems 450

15.3 Dry rock and hot aquifer analysis 475

15.4 Harnessing Geothermal Resources 481

15.5 Social and environmental aspects 483

Problems 487

Bibliography 487

16.1 The importance of energy storage and distribution 489

Our aim

Renewable Energy Resources is a numerate and quantitative text covering

subjects of proven technical and economic importance worldwide Energysupply from renewables is an essential component of every nation’s strat-egy, especially when there is responsibility for the environment and forsustainability

This book considers the timeless principles of renewable energy nologies, yet seeks to demonstrate modern application and case studies

tech-Renewable Energy Resources supports multi-disciplinary master degrees in

science and engineering, and also specialist modules in science and 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 material

engineer-is indicated for further study Therefore the book engineer-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 nomic importance in all countries A wide range of renewable energy tech-nologies are established commercially and recognised as growth industries

eco-by 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 aim

to unify the approach within one book

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 operated

by 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 tinue to drive this change, this new edition includes a new final chapter

con-on instituticon-onal and eccon-onomic 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 section

in Chapter 1 showing how renewable energy is a key component of 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

sus-The book maintains the same general format as the first edition, butmany improvements and updates have been made In particular we wish

to 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 and

in sophistication of the industries In all aspects of renewable energy, posite materials and microelectronic control have transformed traditionaltechnologies, including hydro-power and the use of biomass

com-Extra problems have been added at the end of each chapter, with hintsand 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

if 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 with

a 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, as

we do, and to be stimulated to apply the energy sources for the benefit

of 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 Strathclyde

in Glasgow, Scotland, and then accepted the Chair in Renewable Energy

at the AMSET Centre, De Montfort University, Leicester, England He is

editor of the academic journal Wind Engineering, has been a Council and

Board 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

Union–funded research programmes TW was for several years the SeniorEnergy Officer of the South Pacific Forum Secretariat, where he managed

a substantial program of renewable energy pilot projects He then workedfor the Australian Government as an adviser on climate change, and later

on new economy issues

We do not see the world as divided sharply between developed 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 buildings

industri-to the outside This is no great hardship however; the natural environment

is 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 University

of 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 on

‘renewable 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 inone specialist course This enables the instructor and readers to concentrate

on those particular energy technologies appropriate in their situation Toassist in this selection, each chapter starts with a preliminary outline andestimate of each technology’s 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, and

in 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 4–7 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 errors

in this work However, many have helped us, and we express our gratitude

to 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 TW’s work on this second edition was done while he was

on leave at the International Global Change Institute of the University

of Waikato, New Zealand, in 2004 He gratefully acknowledges the demic hospitality of Neil Ericksen and colleagues, and the continuing sup-port of the [Australian Government] Department of Industry Tourism and

aca-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 SponPress 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

EMF Electromotive force (V)

G Solar irradiance (W m−2) Gravitational constant (N m2kg−2);

Temperature gradient (K m−1);Gibbs energy

Gb Gd Gh Irradiance (beam, diffuse, on

horizontal)

wave crest height (m); insolation( J m−2day−1); heat of reaction (H)

I Electric current (A) Moment of inertia (kg m2)

J Current density (A m−2)

K Extinction coefficient (m−1) Clearness index (KT); constant

L Distance, length (m) Diffusion length (m); litre (10−3m3)

Symbol Main use Other use or comment

Q Volume flow rate (m3s−1)

R Thermal resistance (K W−1) Radius (m); electrical resistance ();

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

Rm Thermal resistance (mass

RFD Radiant flux density (W m−2)

U Potential energy ( J) Heat loss coefficient (W m−2K−1)

X Characteristic dimension (m) Concentration ratio

Script capitals (Non-dimensional numbers

characterising fluid flow)

( J kg−1K−1)

Speed of light (m s−1); phase velocity

of wave (m s−1); chord length (m);Weibull speed factor (m s−1)

Symbol Main use Other use or comment

bright sunshine, of wind-turbineblades; electron concentration(m−3)

area (‘R-value’= RA)(m2K W−1)

Radius (m); distance (m)

u Velocity along stream (m s−1) Group velocity (m s−1)

(m s−1)

moisture content (wet basis, %)(w)

 (delta) Increment of    (other

symbol) (lambda) Latent heat ( J kg−1)

Summation sign

u Probability distribution of

wind speed ( m s−1−1)

 (omega) Solid angle (steradian) Phonon frequency (s−1); angular

velocity of blade (rad s−1)Greek lower case

(K−1)

 (delta) Boundary layer thickness (m) Angle of declination (deg)

dielectric constant

Symbol Main use Other use or comment

 (theta) Angle of incidence (deg) Temperature difference (C)

 (kappa) Thermal diffusivity (m2s−1)

 (lambda) Wavelength (m) Tip speed ratio of wind-turbine

 (mu) Dynamic viscosity (N m−2s)

 (nu) Kinematic viscosity (m2s−1)

 (xi) Electrode potential (V) Roughness height (m)

 (rho) Density (kg m−3) Reflectance; electrical resistivity

( m)

 Monochromatic reflectance

 (sigma) Stefan–Boltzmann constant

 (chi) Absolute humidity (kg m−3)

 (omega) Angular frequency (= 2f )

Symbol Main use Other use or comment

net Heat flow across surface

(holes)

room; resonant; rock

dry matter; saturated;ground-level

sup-a globsup-al resource, four questions sup-are sup-asked for prsup-acticsup-al sup-applicsup-ation:

the resource?

The first two are technical questions considered in the central chapters bythe type of renewables technology The third question relates to broad issues

of planning, social responsibility and sustainable development; these areconsidered in this chapter and in Chapter 17 The environmental impacts

of specific renewable energy technologies are summarised in the last section

of 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:

(Section 1.4)

both minimising losses and maximising economic, social and mental benefits

nuclear power

When these conditions have been met, it is possible to calculate the costsand benefits of a particular scheme and compare these with alternatives for

an economic and environmental assessment

Failure to understand the distinctive scientific principles for harnessingrenewable 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 and

consuming in a manner that meets the needs of the present without 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!

com-In the international context, the word ‘development’ refers to 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 a

improve-positive triple bottom line, i.e a improve-positive contribution to the economic, social and environmental well-being of the community in which they operate.

The concept of sustainable development became widely accepted 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 planet’slands, 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

fol-Energy resources exemplify these issues Reliable energy supply is essential

in all economies for lighting, heating, communications, computers, trial equipment, transport, etc Purchases of energy account for 5–10% ofgross national product in developed economies However, in some devel-oping countries, energy imports may have cost over half the value of total

indus-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 use

of energy

Fossil fuels are not being newly formed at any significant rate, and thuspresent 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 Increasing

an ecological understanding of our Earth’s 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, it

is 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 likewiseimply that current patterns of use are unsustainable in the longer term In

scientific opinion is that if this continues, it will enhance the greenhouse

effect1and lead to significant climate change within a century or less, which

could have major adverse impact on food production, water supply andhuman, e.g through floods and cyclones (IPCC) Recognising that this is

a 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 governments

in industrialised countries to disturb the lifestyle of their voters However,potential climate change, and related sustainability issues, is now established

as one of the major drivers of energy policy

In short, renewable energy supplies are much more compatible with tainable development than are fossil and nuclear fuels, in regard to bothresource limitations and environmental impacts (see Table 1.1)

sus-Consequently almost all national energy plans include four vital factorsfor improving or maintaining social benefit from energy:

1.2.2 A simple numerical model

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

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 to provision of food and manufactured goods The unit of E is energy per

unit 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 of

total commercial energy use, the average per capita value of E worldwide

is 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 atmosphere keeps the Earth some 30 degrees warmer than it would otherwise be By analogy with horticultural greenhouses, this is called the ‘greenhouse effect’.

all these figures and has the major proportional benefit in countries where

the 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:

Here f is a complex and non-linear coefficient that is itself a function of

many factors It may be considered an efficiency for transforming energyinto wealth and, by traditional economics, is expected to be as large as

possible However, S does not increase uniformly as E increases Indeed

S may even decrease for large E (e.g because of pollution or technical

inefficiency) Obviously unnecessary waste of energy leads to a lower value

of f than would otherwise be possible Substituting for E in (1.1), the

national requirement for energy becomes:

Now consider substituting global values for the parameters in (1.4) In

50 years the world population N increased from 2500 million in 1950 to

over 6000 million in 2000 It is now increasing at approximately 2–3% peryear so as to double every 20–30 years Tragically, high infant mortality andlow life expectancy tend to hide the intrinsic pressures of population growth

in many countries Conventional economists seek exponential growth of S

at 2–5% per year Thus in (1.4), at constant efficiency f , the growth of

total world energy supply is effectively the sum of population and economicgrowth, i.e 4–8% per year Without new supplies such growth cannot

be 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 is

to increase renewable energy supplies Moreover, from (1.3) and (1.4), it is

most beneficial to increase the parameter f , i.e to have a positive value of

f Consequently there is a growth rate in energy efficiency, so that S can increase, while R decreases.

1.2.3 Global resources

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

equip-1.3 Fundamentals 7

a 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 earth’s habitable surface is crossed by, or accessible

to, an average energy flux from all renewable sources of about 500 W (seeProblem 1.1) This includes solar, wind or other renewable energy forms

in an overall estimate If this flux is harnessed at just 4% efficiency, 2 kW

methods Suburban areas of residential towns have population densities

of about 500 people per square kilometre At 2 kW per person, the total

just 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 and

a summary of the institutional factors involved Implementation is theneveryone’s 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 flows

of energy occurring in the immediate environment’ An obvious example

is solar (sunshine) energy, where ‘repetitive’ refers to the 24-hour majorperiod Note that the energy is already passing through the environment

as a current or flow, irrespective of there being a device to intercept and harness this power Such energy may also be called Green Energy

or Sustainable Energy.

2 Non-renewable energy ‘Energy obtained from static stores of energy

that remain underground unless released by human interaction’ ples are nuclear fuels and fossil fuels of coal, oil and natural gas Note

Exam-that the energy is initially an isolated energy potential, and external

action is required to initiate the supply of energy for practical poses To avoid using the ungainly word ‘non-renewable’, such energy

pur-supplies are called finite pur-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 continuous

energy flow A

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:

decay in the Earth

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 at

40 000

80 000 Sensible heating Latent heat and potential energy 300

Kinetic energy

Photon processes

Geothermal

30 100

Wind and wave turbines

Biomass and biofuels Photovoltaics

Geothermal heat Geothermal power

Tidal range power Tidal current power

Figure 1.2 Natural energy currents on earth, showing renewable energy system Note

the great range of energy flux 1 105

and heat Units terawatts 1012

diesel electric generators, enough to supply all the energy needs of a town

of about 50 000 people The maximum solar flux density (irradiance)

easy number to remember In general terms, a human being is able tointercept such an energy flux without harm, but any increase begins to

begin 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 practicalengineering 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 diagram’because of its appearance) Sections across such a diagram can be drawn

as pie charts showing primary energy supply and energy supply to end-use

Thermal electricity generation

Refining Crude oil

District heating 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 Austria’s energy use has grown by over 50% since

1970, 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 2000–2001.]

1.3 Fundamentals 11

(b)

Energy End-Use (total: 970 PJ)

Industry 30%

Transport 30%

pri-1.3.5 Energy planning

1 Complete energy systems must be analysed, and supply should not be considered separately from end-use Unfortunately precise needs for

energy are too frequently forgotten, and supplies are not well matched

to end-use Energy losses and uneconomic operation therefore 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 energy

fre-as thermal emission from the boiler and turbine, distribute the tricity in lossy cables and then dissipate this electricity as heat Sadly

elec-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 pinpoint

unnecessary 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-

(energy in visible radiation, usually with a light-shade) 4–5% The totalefficiency is 1–1.5% Contrast this with cogeneration of useful heat

The total efficiency is now 14–18%; 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 efficiency

and 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 forms

of 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 the

local environment It is not good practice to try to create this energy currentespecially for a particular system Renewable energy was once ridiculed

by calculating the number of pigs required to produce dung for sufficientmethane generation to power a whole city It is obvious, however, that

biogas (methane) production should only be contemplated as a by-product

of 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 E

of 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 usually

remains undefined We define quality as the proportion of an energy source

that can be converted to mechanical work Thus electricity has high quality

because when consumed in an electric motor >95% of the input energy

may be converted to mechanical work, say to lift a weight; the heat losses

are correspondingly small, <5% The quality of nuclear, fossil or biomass

fuel in a single stage thermal power station is moderately low, becauseonly about 33% of the calorific value of the fuel can be made to appear

as mechanical work and about 67% is lost as heat to the environment

If the fuel is used in a combined cycle power station (e.g methane gasturbine stage followed by steam turbine), then the quality is increased to

∼50% It is possible to analyse such factors in terms of the namic variable energy, defined here as ‘the theoretical maximum amount ofwork obtainable, at a particular environmental temperature, from an energysource’

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 The

mechanical source of power is usually transformed into electricity athigh efficiency The proportion of power in the environment extracted

by the devices is determined by the mechanics of the process, linked

to the variability of the source, as explained in later chapters Theproportions are, commonly, wind 35%, hydro 70–90%, wave 50%and tidal 75%

2 Heat supplies, such as biomass combustion and solar collectors These

sources provide heat at high efficiency However, the maximum 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 process

pro-is 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 20–30% 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 energy

densities that are orders of magnitude greater For instance, boiler tubes

distribution, however, supplies from finite sources must be greatly reduced

in flux density Thus apart from major exceptions such as metal refining,end-use loads for both renewable and finite supplies are similar In summary

finite energy is most easily produced centrally and is expensive to distribute Renewable energy is most easily produced in dispersed locations and is expensive to concentrate With an electrical grid, the renewable generators

are 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 renewable

energy favours rural development and not urbanisation

1.4.6 Situation dependence

No single renewable energy system is universally applicable, since the ity of the local environment to supply the energy and the suitability ofsociety to accept the energy vary greatly It is as necessary to ‘prospect’the 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 energy

abil-is 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 might

be 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 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

gen-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-use

requirement is called the load, and the size and dynamic characteristics of

the 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:

within 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 since

the 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 negative

feedback control is a consequence of renewable energy being flow or

or Sustainable Energy.

2 Non -renewable energy ? ?Energy obtained from static stores of energy< /i>

that...

energy flow A

Renewable energy Finite energy< /small>

Figure 1.1 Contrast between renewable (green) and finite (brown) energy supplies

Environmental energy. .. versus centralised energy< /b>

A pronounced difference between renewable and finite energy supplies isthe energy flux density at the initial transformation Renewable energy

densities