renewable-energy-resources-by-john-twidell-tony-weir.pdf

Renewable Energy Resources
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 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 includ-
ing experience gained from the rapid growth of renewable energy technologies as
secure national resources and for climate change mitigation, more extensively illus-
trated with case studies and worked problems. The presentation has been improved
throughout, along with a new chapter on economics and institutional factors. Each
chapter begins with fundamental theory from a scientific perspective, then considers
applied engineering examples and developments, and includes a set of problems and
solutions 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
to each technology chapter.
Renewable Energy Resources supports multi-disciplinary master degrees in sci-
ence and engineering, and specialist modules in first degrees. Practising scientists
and engineers who have not had a comprehensive training in renewable energy will
find 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 editor
and 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 supply
and demand, climate change and innovation in business. He was formerly Senior
Energy Officer at the South Pacific Forum Secretariat in Fiji, and has lectured and
researched in physics and policy studies at universities of the UK, Australia and the
Pacific.

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Evaluation of the Built Environment for
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Renewable Energy
Resources
Second edition
John Twidell and Tony Weir

First published 1986
by E&FN Spon Ltd
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.”

Contents
Preface xi
List of symbols xvii
1 Principles of renewable energy 1
1.1 Introduction 1
1.2 Energy and sustainable development 2
1.3 Fundamentals 7
1.4 Scientific principles of renewable energy 12
1.5 Technical implications 16
1.6 Social implications 22
Problems 24
Bibliography 25
2 Essentials of fluid dynamics 29
2.1 Introduction 29
2.2 Conservation of energy: Bernoulli’s equation 30
2.3 Conservation of momentum 32
2.4 Viscosity 33
2.5 Turbulence 34
2.6 Friction in pipe flow 35
2.7 Lift and drag forces: fluid and turbine machinery 39
Problems 41
Bibliography 44
3 Heat transfer 45
3.1 Introduction 45
3.2 Heat circuit analysis and terminology 46
3.3 Conduction 49

vi Contents
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
Problems 80
Bibliography 82
4 Solar radiation 85
4.1 Introduction 85
4.2 Extraterrestrial solar radiation 86
4.3 Components of radiation 87
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 Solar water heating 115
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
5.6 Selective surfaces 134
5.7 Evacuated collectors 137
5.8 Social and environmental aspects 140
Problems 141
Bibliography 145
6 Buildings and other solar thermal applications 146
6.2 Air heaters 147
6.3 Energy-efficient buildings 149
6.4 Crop driers 157
6.5 Space cooling 161
6.6 Water desalination 162

Contents vii
6.7 Solar ponds 164
6.8 Solar concentrators 166
6.9 Solar thermal electric power systems 170
Problems 175
Bibliography 179
7 Photovoltaic generation 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
Problems 233
Bibliography 234
8 Hydro-power 237
8.2 Principles 240
8.3 Assessing the resource for small installations 240
8.4 An impulse turbine 244
8.5 Reaction turbines 249
8.6 Hydroelectric systems 252
8.7 The hydraulic ram pump 255
Problems 258
Bibliography 261
9 Power from the wind 263
9.2 Turbine types and terms 268
9.3 Linear momentum and basic theory 273
9.4 Dynamic matching 283
9.5 Blade element theory 288

viii Contents
9.6 Characteristics of the wind 290
9.7 Power extraction by a turbine 305
9.8 Electricity generation 307
9.9 Mechanical power 316
9.10 Social and environmental considerations 318
Problems 319
Bibliography 322
10 The photosynthetic process 324
10.2 Trophic level photosynthesis 326
10.3 Photosynthesis at the plant level 330
10.4 Thermodynamic considerations 336
10.5 Photophysics 338
10.6 Molecular level photosynthesis 343
10.7 Applied photosynthesis 348
Problems 349
Bibliography 350
11 Biomass and biofuels 351
11.2 Biofuel classification 354
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
Problems 395
Bibliography 397
12 Wave power 400
12.2 Wave motion 402
12.3 Wave energy and power 406
12.4 Wave patterns 412
12.5 Devices 418

Contents ix
Problems 424
Bibliography 427
13 Tidal power 429
13.2 The cause of tides 431
13.3 Enhancement of tides 438
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
Bibliography 451
14 Ocean thermal energy conversion (OTEC) 453
14.2 Principles 454
14.3 Heat exchangers 458
14.4 Pumping requirements 464
14.5 Other practical considerations 465
14.6 Environmental impact 468
Problems 469
Bibliography 469
15 Geothermal energy 471
15.2 Geophysics 472
15.3 Dry rock and hot aquifer analysis 475
15.4 Harnessing Geothermal Resources 481
Problems 487
Bibliography 487
16 Energy systems, storage and transmission 489
16.1 The importance of energy storage and distribution 489
16.2 Biological storage 490
16.3 Chemical storage 490
16.4 Heat storage 495
16.5 Electrical storage: batteries and accumulators 499
16.6 Fuel cells 506

x Contents
16.7 Mechanical storage 507
16.8 Distribution of energy 509
16.9 Electrical power 513
Problems 521
Bibliography 524
17 Institutional and economic factors 526
17.2 Socio-political factors 526
17.3 Economics 530
17.4 Some policy tools 534
17.5 Quantifying choice 536
17.6 The way ahead 545
Problems 550
Bibliography 550
Appendix A Units and conversions 553
Appendix B Data 558
Appendix C Some heat transfer formulas 564
Solution guide to problems 568
Index 581

Preface
Our aim
Renewable Energy Resources is a numerate and quantitative text covering
subjects of proven technical and economic importance worldwide. Energy
supply from renewables is an essential component of every nation’s strat-
egy, especially when there is responsibility for the environment and for
sustainability.
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 in
science and engineering, and also specialist modules in science and engineer-
ing first degrees. Moreover, since many practising scientists and engineers
will not have had a general training in renewable energy, the book has wider
use beyond colleges and universities. Each chapter begins with fundamental
theory from a physical science perspective, then considers applied exam-
ples and developments, and finally concludes with a set of problems and
solutions. The whole book is structured to share common material and to
relate aspects together. After each chapter, reading and web-based material
is indicated for further study. Therefore the book is intended both for basic
study and for application. Throughout the book and in the appendices, we
include 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 industries
by most governments. World agencies, such as the United Nations, have
large programmes to encourage the technology. In this book we stress the
scientific understanding and analysis of renewable energy, since we believe
these 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.

xii Preface
This book bridges the gap between descriptive reviews and specialised
engineering treatises on particular aspects. It centres on demonstrating how
fundamental physical processes govern renewable energy resources and their
application. Although the applications are being updated continually, the
fundamental principles remain the same and we are confident that this new
edition will continue to provide a useful platform for those advancing the
subject and its industries. We have been encouraged in this approach by the
ever increasing commercial importance of renewable energy technologies.
Why a second edition?
In the relatively few years between the first edition, with five reprinted
revisions, and this second edition, renewable energy has come of age; its
use 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 consumers
and 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 chapter
on institutional and economic factors. The new chapter also discusses and
demonstrates 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 sus-
tainable development, an ideal which has become much more explicit since
the first edition. Each technology chapter now includes a brief concluding
section 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 wish
to relate to the vibrant developments in the individual renewable energy
technologies, and to the related commercial growth. We have improved the
presentation of the fundamentals throughout, in the light of our teaching
experience. Although the book continues to focus on fundamental physi-
cal principles, which have not changed, we have updated the technological
applications and their relative emphases to reflect market experience. For
electricity generation, wind-power and photovoltaics have had dramatic
growth over the last two decades, both in terms of installed capacity and
in sophistication of the industries. In all aspects of renewable energy, com-
posite materials and microelectronic control have transformed traditional
technologies, 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 emphasise
simplified, order-of-magnitude, calculations of the potential outputs of the
various technologies. Such calculations are especially useful in indicating

Preface xiii
the potential applicability of a technology for a particular site. However we
appreciate that specialists increasingly use computer modelling of whole,
complex systems; in our view such modelling is essential but only after
initial 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 necessary
to read or refer to chapters consecutively, as each aspect of the subject is
treated, in the main, as independent of the other aspects. However, some
common elements, especially heat transfer, will have to be studied seriously
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 science
and agricultural science, but we stress the importance of these subjects with
obvious 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 the
work in both hemispheres and in a range of countries. We first taught,
and therefore learnt, renewable energy at the University of Strathclyde in
Glasgow (JWT) and the University of the South Pacific in Fiji (ADW and
JWT). So teaching, together with research and application in Scotland and
the South Pacific, has been a strong influence for this book. Since the first
edition we have made separate careers in universities and in government
service, whilst experiencing the remarkable, but predicable, growth in rele-
vance of renewable energy. One of us (JWT) became Director of the Energy
Studies 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 Solar
Energy Society, and has supervised many postgraduates for their disserta-
tions. The AMSET Centre is now a private company, for research, education
and training in renewables; support is given to MSc courses at Reading
University, Oxford University and City University, and there are European

xiv Preface
Union–funded research programmes. TW was for several years the Senior
Energy Officer of the South Pacific Forum Secretariat, where he managed
a substantial program of renewable energy pilot projects. He then worked
for 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 industri-
alised countries and developing countries of the Third World. Renewables
are essential for both, and indeed provide one way for the separating con-
cepts to become irrelevant. This is meaningful to us personally, since we
wish our own energies to be directed for a just and sustainable society,
increasingly free of poverty and the threat of cataclysmic war. We sincerely
believe the development and application of renewable energy technology
will favour these aspirations. Our readers may not share these views, and
this fortunately does not affect the content of the book. One thing they will
have to share, however, is contact with the outdoors. Renewable energy is
drawn from the environment, and practitioners must put on their rubber
boots or their sun hat and move from the closed environment of buildings
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 is
devoted to its subject. For example, the book originated from short and
one-semester courses to senior undergraduates in Physics at the University
of the South Pacific and the University of Strathclyde, namely ‘Energy
Resources and Distribution’, ‘Renewable Energy’ and ‘Physics and Ecology’.
When completed and with regular revisions, the book has been mostly used
worldwide for MSc degrees in engineering and science, including those on
‘renewable energy’ and on ‘energy and the environment’. We have also
taught other lecture and laboratory courses, and have found many of the
subjects and technologies in renewable energy can be incorporated with
great 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 concentrate
on those particular energy technologies appropriate in their situation. To
assist in this selection, each chapter starts with a preliminary outline and
estimate of each technology’s resource and geographical variation, and ends
with 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 that
for fossil or nuclear fuels. Chapter 2 (Fluid Mechanics) and Chapter 3 (Heat
Transfer) 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. Readers
interested in this area are advised to start with the early sections of Chapter 5
(Solar Water Heating) or Chapter 7 (Photovoltaics), and review Chapters 3
and 4 as required. Chapters 8 (Hydro), 9 (Wind), 12 (Waves) and 13 (Tides)
present applications of fluid mechanics. Again the reader is advised to start
with an applications chapter, and review the elements from Chapter 2 as
required. Chapters 10 and 11 deal with biomass as an energy source and
how 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, like
Chapter 1, treats matters of importance to all renewable energy sources,
namely the storage and distribution of energy and the integration of energy
sources into energy systems. Chapter 17, on institutional and economic
factors bearing on renewable energy, recognises that science and engineering
are not the only factors for implementing technologies and developments.
Appendices A (units), B (data) and C (heat transfer formulas) are referred to
either implicitly or explicitly throughout the book. We keep to a common
set of symbols throughout, as listed in the front. Bibliographies include both
specific and general references of conventional publications and of websites;
the internet is particularly valuable for seeking applications. Suggestions
for further reading and problems (mostly numerical in nature) are included
with most chapters. Answer guidance is provided at the end of the book
for 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 and
contacts that had helped and encouraged us at that stage. For this second
edition, 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. We
acknowledge the help and information we have gained from many such
sources, with specific acknowledgement indicated by conventional referenc-
ing and listing in the bibliographies. We welcome communications from our
readers, 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 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 from
students 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 left
home 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 Canberra
Leicestershire, LE16 8DH, UK Australia
and
Visiting Professor in Renewable Energy
University of Reading, UK
email <info.renewable@tandf.co.uk>
see <www.amset.com>

List of symbols
Symbol Main use Other use or comment
Capitals
A Area (m2
) Acceptor; ideality factor
AM Air-mass-ratio
C Thermal capacitance (J K−1
) Electrical capacitance (F); constant
CP Power coefficient
Cr Concentration ratio
C Torque coefficient
D Distance (m) Diameter (of pipe or blade)
E Energy (J)
EF Fermi level
Eg Band gap (eV)
EK Kinetic energy (J)
EMF Electromotive force (V)
F Force (N) Faraday constant (Cmol−1
)
F
ij Radiation exchange factor (i to j)
G Solar irradiance (W m−2
) Gravitational constant (Nm2
kg−2
);
Temperature gradient (Km−1
);
Gibbs energy
GbGdGh Irradiance (beam, diffuse, on
horizontal)
H Enthalpy (J) Head (pressure height) of fluid (m);
wave crest height (m); insolation
(Jm−2
day−1
); heat of reaction (H)
I Electric current (A) Moment of inertia (kg m2
)
J Current density (Am−2
)
K Extinction coefficient (m−1
) Clearness index (KT); constant
L Distance, length (m) Diffusion length (m); litre (10−3
m3
)
M Mass (kg) Molecular weight
N Concentration (m−3
) Hours of daylight
N0 Avogadro number
P Power (W)
P
Power per unit length
(W m−1
)
PS Photosystem

(Continued)
Q Volume flow rate (m3
s−1
)
R Thermal resistance (KW−1
) Radius (m); electrical resistance ();
reduction level; tidal range (m); gas
constant (R0);
Rm Thermal resistance (mass
transfer)
Rn Thermal resistance
(conduction)
Rr Thermal resistance (radiation)
Rv Thermal resistance
(convection)
RFD Radiant flux density (W m−2
)
S Surface area (m2
) entropy
Sv Surface recombination
velocity (ms−1
)
STP Standard temperature and
pressure
T Temperature (K) Period (s−1
)
U Potential energy (J) Heat loss coefficient (W m−2
K−1
)
V Volume (m3
) Electrical potential (V)
W Width (m) Energy density (Jm−3
)
X Characteristic dimension (m) Concentration ratio
Script capitals (Non-dimensional numbers
characterising fluid flow)
Rayleigh number
Grashof number
Nusselt number
Prandtl number
Reynolds number
Shape number (of turbine)
Lower case
a Amplitude (m) Wind interference factor; radius (m)
b Wind profile exponent Width (m)
c Specific heat capacity
(Jkg−1
K−1
)
Speed of light (ms−1
); phase velocity
of wave (ms−1
); chord length (m);
Weibull speed factor (ms−1
)
d Distance (m) Diameter (m); depth (m); zero plane
displacement (wind) (m)
e Electron charge (C) Base of natural logarithms (2.718)
f Frequency of cycles
(Hz = s−1
)
Pipe friction coefficient; fraction;
force per unit length (Nm−1
)
g Acceleration due to gravity
(ms−2
)
h Heat transfer coefficient
(W m−2
K−1
)
Vertical displacement (m); Planck
constant (Js)

i
√
−1
k Thermal conductivity
(W m−1
K−1
)
Wave vector (=2/); Boltzmann
constant (=138×10−23
JK−1
)
l Distance (m)
m Mass (kg) Air-mass-ratio
n Number Number of nozzles, of hours of
bright sunshine, of wind-turbine
blades; electron concentration
(m−3
)
p Pressure (Nm−2
= Pa) Hole concentration (m−3
)
q Power per unit area (W m−2
)
r Thermal resistivity of unit
area (‘R-value’ = RA)
(m2
KW−1
)
Radius (m); distance (m)
s Angle of slope (degrees)
t Time (s) Thickness (m)
u Velocity along stream (ms−1
) Group velocity (ms−1
)
v Velocity (not along stream)
(ms−1
)
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 (N m) Gamma function
(delta) Increment of (other
symbol)
(lambda) Latent heat (Jkg−1
)
(sigma) Summation sign
(phi) Radiant flux (W) Probability function
u Probability distribution of
wind speed ( ms−1 −1
)
(omega) Solid angle (steradian) Phonon frequency (s−1
); angular
velocity of blade (rads−1
)
Greek lower case
(alpha) Absorptance Angle of attack (deg)
Monochromatic absorptance
(beta) Angle (deg) Volumetric Expansion coefficient
(K−1
)
(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
s−1
)
(lambda) Wavelength (m) Tip speed ratio of wind-turbine
(mu) Dynamic viscosity (Nm−2
s)
(nu) Kinematic viscosity (m2
s−1
)
(xi) Electrode potential (V) Roughness height (m)
(pi) 3.1416
(rho) Density (kg m−3
) Reflectance; electrical resistivity
( m)
Monochromatic reflectance
(sigma) Stefan–Boltzmann constant
(tau) Transmittance Relaxation time (s); duration (s);
shear stress (Nm−2
)
Monochromatic transmittance
(phi) Radiant flux density (RFD)
(W m−2
)
Wind-blade angle (deg); potential
difference (V); latitude (deg)
Spectral distribution of RFD
(W m−3
)
(chi) Absolute humidity (kg m−3
)
(psi) Longitude (deg) Angle (deg)
(omega) Angular frequency (= 2f )
(rads−1
)
Hour angle (deg); solid angle
(steradian)
Subscripts
B Black body Band
D Drag Dark
E Earth
F Force
G Generator
L Lift
M Moon
P Power
R Rated
S Sun
T Tangential Turbine
a Ambient Aperture; available (head); aquifer
abs Absorbed
b Beam Blade; bottom; base; biogas
c Collector Cold
ci Cut-in
co Cut-out
cov Cover
d Diffuse Dopant; digester
e Electrical Equilibrium; energy
f Fluid Forced; friction; flow
g Glass Generation current; band gap
h Horizontal Hot

i Integer Intrinsic
in Incident (incoming)
int Internal
j Integer
m mass transfer Mean (average); methane
max Maximum
n conduction
net Heat flow across surface
o (read as numeral zero)
oc Open circuit
p Plate Peak; positive charge carriers
(holes)
r radiation Relative; recombination;
room; resonant; rock
rad Radiated
refl Reflected
rms Root mean square
s Surface Significant; saturated; Sun
sc Short circuit
t Tip Total
th Thermal
trans Transmitted
u Useful
v convection Vapour
w Wind Water
z Zenith
Monochromatic, e.g.
0 Distant approach Ambient; extra-terrestrial;
dry matter; saturated;
ground-level
1 Entry to device First
2 Exit from device Second
3 Output Third
Superscript
m or max Maximum
∗
Measured perpendicular to direction
of propagation (e.g. Gb
∗
)

(dot) Rate of, e.g. ṁ
Other symbols
Bold face Vector, e.g. F
= Mathematical equality
≈ Approximate equality (within a
few %)
∼ Equality in order of magnitude
(within a factor of 2–10)
≡ 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 as
vital inputs for sustainability and so encouraging their growth is signifi-
cant. Subjects will include power from wind, water, biomass, sunshine and
other such continuing sources, including wastes. Although the scale of local
application ranges from tens to many millions of watts, and the totality is
a global resource, four questions are asked for practical application:
1 How much energy is available in the immediate environment – what is
the 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 by
the type of renewables technology. The third question relates to broad issues
of planning, social responsibility and sustainable development; these are
considered 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 other
institutional factors in the last chapter, may dominate for consumers and
usually 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 of
both minimising losses and maximising economic, social and environ-
mental benefits.
c Like-for-like comparisons, including externalities, with fossil fuel and
nuclear power.

2 Principles of renewable energy
When these conditions have been met, it is possible to calculate the costs
and benefits of a particular scheme and compare these with alternatives for
an 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 the
methods 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 com-
promising the ability of future generations to meet their own needs. It has
become a key guiding principle for policy in the 21st century. Worldwide,
politicians, industrialists, environmentalists, economists and theologians
affirm that the principle must be applied at international, national and local
level. 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 the
improvement to be achieved whilst maintaining the ecological processes on
which life depends. At a local level, progressive businesses aim to report a
positive triple bottom line, i.e. a 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 fol-
lowing the seminal report of the World Commission on Environment and
Development (1987). The commission was set up by the United Nations
because the scale and unevenness of economic development and population
growth were, and still are, placing unprecedented pressures on our planet’s
lands, waters and other natural resources. Some of these pressures are severe
enough to threaten the very survival of some regional populations and, in
the longer term, to lead to global catastrophes. Changes in lifestyle, espe-
cially regarding production and consumption, will eventually be forced on
populations by ecological and economic pressures. Nevertheless, the eco-
nomic and social pain of such changes can be eased by foresight, planning
and 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 5–10% of
gross 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 for
sustainable development. World energy use increased more than tenfold
over the 20th century, predominantly from fossil fuels (i.e. coal, oil and
gas) and with the addition of electricity from nuclear power. In the 21st
century, further increases in world energy consumption can be expected,
much for rising industrialisation and demand in previously less developed
countries, aggravated by gross inefficiencies in all countries. Whatever the
energy 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 thus
present stocks are ultimately finite. The location and the amount of such
stocks depend on the latest surveys. Clearly the dominant fossil fuel type by
mass is coal, with oil and gas much less. The reserve lifetime of a resource
may be defined as the known accessible amount divided by the rate of
present use. By this definition, the lifetime of oil and gas resources is usually
only a few decades; whereas lifetime for coal is a few centuries. Economics
predicts that as the lifetime of a fuel reserve shortens, so the fuel price
increases; consequently demand for that fuel reduces and previously more
expensive sources and alternatives enter the market. This process tends to
make the original source last longer than an immediate calculation indi-
cates. In practice, many other factors are involved, especially governmental
policy and international relations. Nevertheless, the basic geological fact
remains: fossil fuel reserves are limited and so the present patterns of energy
consumption 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
concentration of CO2 in the Atmosphere is such an example. Indeed, from
an ecological understanding of our Earth’s long-term history over billions of
years, carbon was in excess in the Atmosphere originally and needed to be
sequestered below ground to provide our present oxygen-rich atmosphere.
Therefore from arguments of: (i) the finite nature of fossil and nuclear fuel
materials, (ii) the harm of emissions and (iii) ecological sustainability, it
is essential to expand renewable energy supplies and to use energy more
efficiently. Such conclusions are supported in economics if the full external
costs of both obtaining the fuels and paying for the damage from emissions
are internalised in the price. Such fundamental analyses may conclude that
renewable energy and the efficient use of energy are cheaper for society
than 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. In
particular, CO2 emissions from the combustion of fossil fuels have signifi-
cantly raised the concentration of CO2 in the Atmosphere. The balance of
scientific opinion is that if this continues, it will enhance the greenhouse

effect1
and lead to significant climate change within a century or less, which
could have major adverse impact on food production, water supply and
human, e.g. through floods and cyclones (IPCC). Recognising that this is
a global problem, which no single country can avert on its own, over 150
national governments signed the UN Framework Convention on Climate
Change, 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 sus-
tainable development than are fossil and nuclear fuels, in regard to both
resource 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 supplies
2 increased efficiency of supply and end-use
3 reduction in pollution
4 consideration of lifestyle.
1.2.2 A simple numerical model
Consider the following simple model describing the need for commercial
and 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 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 is
from commercial sources, especially fossil fuels; however, significant use of
non-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 North
America 9 kW, Europe as a whole 4 kW, and several regions of Central
Africa 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’.

Table
1.1
Comparison
of
renewable
and
conventional
energy
systems
Renewable
energy
supplies
(green)
Conventional
energy
supplies
(brown)
Examples
Wind,
solar,
biomass,
tidal
Coal,
oil,
gas,
radioactive
ore
Source
Natural
local
environment
Concentrated
stock
Normal
state
A
current
or
flow
of
energy.
An
income
Static
store
of
energy.
Capital
Initial
average
intensity
Low
intensity,
dispersed:
≤300
W
m
−2
Released
at
≥100
kW
m
−2
Lifetime
of
supply
Infinite
Finite
Cost
at
source
Free
Increasingly
expensive.
Equipment
capital
cost
per
kW
capacity
Expensive,
commonly
≈US$1000
kW
−1
Moderate,
perhaps
$500
kW
−1
without
emissions
control;
yet
expensive
US$1000
kW
−1
with
emissions
reduction
Variation
and
control
Fluctuating;
best
controlled
by
change
of
load
using
positive
feedforward
control
Steady,
best
controlled
by
adjusting
source
with
negative
feedback
control
Location
for
use
Site-
and
society-specific
General
and
invariant
use
Scale
Small
and
moderate
scale
often
economic,
large
scale
may
present
difficulties
Increased
scale
often
improves
supply
costs,
large
scale
frequently
favoured
Skills
Interdisciplinary
and
varied.
Wide
range
of
skills.
Importance
of
bioscience
and
agriculture
Strong
links
with
electrical
and
mechanical
engineering.
Narrow
range
of
personal
skills
Context
Bias
to
rural,
decentralised
industry
Bias
to
urban,
centralised
industry
Dependence
Self-sufficient
and
‘islanded’
systems
supported
Systems
dependent
on
outside
inputs
Safety
Local
hazards
possible
in
operation:
usually
safe
when
out
of
action
May
be
shielded
and
enclosed
to
lessen
great
potential
dangers;
most
dangerous
when
faulty
Pollution
and
environmental
damage
Usually
little
environmental
harm,
especially
at
moderate
scale
Environmental
pollution
intrinsic
and
common,
especially
of
air
and
water
Hazards
from
excess
biomass
burning
Soil
erosion
from
excessive
biofuel
use
Large
hydro
reservoirs
disruptive
Compatible
with
natural
ecology
Permanent
damage
common
from
mining
and
radioactive
elements
entering
water
table.
Deforestation
and
ecological
sterilisation
from
excessive
air
pollution
Climate
change
emissions
Aesthetics,
visual
impact
Local
perturbations
may
be
unpopular,
but
usually
acceptable
if
local
need
perceived
Usually
utilitarian,
with
centralisation
and
economy
of
large
scale

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:
S = f E (1.2)
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 energy
into 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:
R =
SN
f
(1.3)
so
R
R
=
S
S
+
N
N
−
f
f
(1.4)
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% per
year so as to double every 20–30 years. Tragically, high infant mortality and
low 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 economic
growth, i.e. 4–8% per year. Without new supplies such growth cannot
be maintained. Yet at the same time as more energy is required, fossil
and nuclear fuels are being depleted and debilitating pollution and climate
change 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 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. Such
a target is consistent with an energy policy of ‘contract and converge’ for
global equity, since worldwide energy supply would total approximately
the present global average usage, but would be consumed for a far higher
standard 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 (see
Problem 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
of power can be drawn from an area of 10m × 10m, assuming suitable
methods. Suburban areas of residential towns have population densities
of about 500 people per square kilometre. At 2 kW per person, the total
energy demand of 1000kW km
−2
could be obtained in principle by using
just 5% of the local land area for energy production. Thus renewable energy
supplies can provide a satisfactory standard of living, but only if the tech-
nical methods and institutional frameworks exist to extract, use and store
the energy in an appropriate form at realistic costs. This book considers
both the technical background of a great variety of possible methods and
a summary of the institutional factors involved. Implementation is then
everyone’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 major
period. 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’. Exam-
ples are nuclear fuels and fossil fuels of coal, oil and natural gas. Note
that the energy is initially an isolated energy potential, and external
action is required to initiate the supply of energy for practical pur-
poses. To avoid using the ungainly word ‘non-renewable’, such energy
supplies are called finite supplies or Brown Energy.
These two definitions are portrayed in Figure 1.1. Table 1.1 provides a
comparison of renewable and conventional energy systems.

Natural Environment:
green Mined resource: brown
Current source of continuous
energy flow
A
C
D
E
F
B
Device
Use
D
E
F
Device
Use
Environment Sink Environment Sink
Finite source of
energy 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. The
fifth category is relatively minor, but useful for primary batteries, e.g. dry
cells.
1.3.3 Environmental energy
The flows of energy passing continuously as renewable energy through the
Earth are shown in Figure 1.2. For instance, total solar flux absorbed at
sea level is about 12 × 1017
W. Thus the solar flux reaching the Earth’s
surface is ∼20MW per person; 20 MW is the power of ten very large

1.3 Fundamentals 9
Reflected
to space
50 000
Solar
radiation
From
Sun
From
Earth
From
planetary
motion
120 000
Absorbed on
Earth
40 000
80 000
Sensible
heating
Latent heat
and potential
energy
300
Kinetic energy
Photon
processes
Geothermal
30
100
Heat
Gravitation,
orbital motion Tidal motion
3
Infrared
radiation
Solar water heaters
Solar buildings
Solar dryers
Ocean thermal energy
Hydropower
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 the dominance of solar radiation
and heat. Units terawatts 1012
W .
diesel electric generators, enough to supply all the energy needs of a town
of about 50 000 people. The maximum solar flux density (irradiance)
perpendicular to the solar beam is about 1kW m−2
; a very useful and
easy number to remember. In general terms, a human being is able to
intercept such an energy flux without harm, but any increase begins to
cause stress and difficulty. Interestingly, power flux densities of ∼1kW m−2
begin to cause physical difficulty to an adult in wind, water currents or
waves.
However, the global data of Figure 1.2 are of little value for practical
engineering applications, since particular sites can have remarkably different
environments and possibilities for harnessing renewable energy. Obviously
flat regions, such as Denmark, have little opportunity for hydro-power but
may have wind power. Yet neighbouring regions, for example Norway, may
have vast hydro potential. Tropical rain forests may have biomass energy
sources, but deserts at the same latitude have none (moreover, forests must
not be destroyed so making more deserts). Thus practical renewable energy
systems have to be matched to particular local environmental energy flows
occurring 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 through
which 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 flows
through 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
PRIMARY
ENERGY
SUPPLIES
Coal
Fossil gas
Biomass
Hydro
Oil products
Non-energy use
ENERGY
END-USE
Transport
Industry
Residential
and other
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 shown
dashed. (b)–(c) Pie diagrams. The contribution of hydropower and biomass
(wood and waste) is greater than in most industrialised countries, as is the
use of heat produced from thermal generation of electricity (‘combined heat
and power’). Energy use for transport is substantial and very dependent on
(imported) oil and oil products, therefore the Austrian government encourages
increased 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 need
for greater efficiency of energy use. [Data source: simplified from International
Energy Agency, Energy Balances of OECD countries 2000–2001.]

1.3 Fundamentals 11
(b)
Energy End-Use
(total: 970 PJ)
Industry
30%
Transport
30%
Residential
28%
Other
12%
(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 the
generation of electricity from fossil fuels.
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 fre-
quently result. For instance, if a dominant domestic energy require-
ment is heat for warmth and hot water, it is irresponsible to generate
grid quality electricity from a fuel, waste the majority of the energy
as 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. Heating
would be more efficient and cost-effective from direct heat production
with 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 useful
energy output from a process to the total energy input to that pro-
cess. Consider electric lighting produced from ‘conventional’ thermally
generated 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) 4–5%. The total
efficiency is 1–1.5%. Contrast this with cogeneration of useful heat
and electricity (efficiency ∼85%), distribution ∼90% and lighting in
modern low consumption compact fluorescent lamps (CFL) ∼22%.
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 less
than for the conventional, despite higher per unit capital costs, because
(i) less generating capacity and fuel are needed, (ii) less per unit emission
costs 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 renewable
supplies 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 renewables
reduces 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 requires
the 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 current
especially for a particular system. Renewable energy was once ridiculed
by calculating the number of pigs required to produce dung for sufficient
methane 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

for a biomass energy station, the biomass resource must exist locally to
avoid large inefficiencies in transportation. The practical implication of this
principle is that the local environment has to be monitored and analysed
over a long period to establish precisely what energy flows are present. In
Figure 1.1 the energy current ABC must be assessed before the diverted
flow through DEF is established.
1.4.2 Dynamic characteristics
End-use requirements for energy vary with time. For example, electricity
demand on a power network often peaks in the morning and evening,
and reaches a minimum through the night. If power is provided from a
finite 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 uncontrollably
with time but so too does the natural supply in the environment. Thus a
renewable energy device must be matched dynamically at both D and E
of Figure 1.1; the characteristics will probably be quite different at both
interfaces. Examples of these dynamic effects will appear in most of the
following 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 accurately
predictable (e.g. tidal power). Solar energy may be very predicable in some
regions (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, because
only 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 gas
turbine stage followed by steam turbine), then the quality is increased to
∼50%. It is possible to analyse such factors in terms of the thermody-
namic variable energy, defined here as ‘the theoretical maximum amount of
work obtainable, at a particular environmental temperature, from an energy
source’.

Table
1.2
Intensity
and
periodical
properties
of
renewable
sources
System
Major
periods
Major
variables
Power
relationship
Comment
Text
reference
(equation)
Direct
sunshine
24
h,
1
y
Solar
beam
irradiance
G
∗
b
W
m
−2
;
P
∝
G
∗
b
cos

z
Daytime
only
(4.2)
Angle
of
beam
from
vertical

z
P
max
=
1
kW
m
−2
Diffuse
sunshine
24
h,
1
y
Cloud
cover,
perhaps
air
pollution
P
G!
P
≤
300
W
m
−2
Significant
energy
over
time
(4.3)
Biofuels
1
y
Soil
condition,
insolation,
water,
plant
species,
wastes
Stored
energy
∼10
MJ
kg
−1
Very
many
chemical
types
and
sources.
Linked
to
agriculture
and
forestry.
Stored
energy
Table
11.1
Table
11.4
Wind
1
y
Wind
speed
u
0
P
∝
u
3
0
Most
variable
(9.2)
Height
nacelle
above
ground
z;
height
of
anemometer
mast
h
u
z
/u
h
=
z/h
b
b
∼
015
(9.54)
Wave
1
y
‘Significant
wave
height’
H
s
wave
period
T
P
∝
H
2
s
T
Relatively
large
power
density
∼50
kW
m
−1
across
wave
front
(12.47)
Hydro
1
y
Reservoir
height
H
water
volume
flow
rate
Q
P
∝
HQ
Established
resource
(8.1)
Tidal
12
h
25
min
Tidal
range
R;
contained
area
A;
estuary
length
L,
depth
h
P
∝
R
2
A
Enhanced
tidal
range
if
L/
√
h
=
36
000
m
1/2
(13.35)
and
(13.28)
Tidal
stream/current;
peak
current
u
0
,
density
sea
water

∼1000
×
air
P
∝
u
3
0
Enhanced
tidal
currents
between
certain
islands
(13.30)
Ocean
thermal
energy
conversion
Constant
Temperature
difference
between
sea
surface
and
deep
water,
T
P
∝
T
2
Some
tropical
locations
have
T
∼
20

C,
so
potentially
harnessable,
but
small
efficiency
(14.5)

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 at
high 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. The
proportions 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 pro-
portion of heat energy extractable as mechanical work, and hence elec-
tricity, is given by the second law of thermodynamics and the Carnot
Theorem, which assumes reversible, infinitely long transformations. In
practice, maximum mechanical power produced in a dynamic process
is about half that predicted by the Carnot criteria. For thermal boiler
heat 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 makes
matching difficult and photon conversion efficiencies of 20–30% are
considered good.
1.4.4 Dispersed versus centralised energy
A pronounced difference between renewable and finite energy supplies is
the energy flux density at the initial transformation. Renewable energy
commonly arrives at about 1kW m−2
(e.g. solar beam irradiance, energy in
the wind at 10ms−1
), whereas finite centralised sources have energy flux
densities that are orders of magnitude greater. For instance, boiler tubes
in gas furnaces easily transfer 100kW m−2
, and in a nuclear reactor the
first wall heat exchanger must transmit several MW m−2
. At end-use after
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.

1.4.5 Complex systems
Renewable energy supplies are intimately linked to the natural environment,
which is not the preserve of just one academic discipline such as physics
or electrical engineering. Frequently it is necessary to cross disciplinary
boundaries 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 of
society 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-use
needs and local renewable energy supplies can then be matched, subject to
economic and environmental constraints. In this respect renewable energy
is similar to agriculture. Particular environments and soils are suitable for
some crops and not others, and the market pull for selling the produce
will depend on particular needs. The main consequence of this ‘situation
dependence’ of renewable energy is the impossibility of making simplistic
international or national energy plans. Solar energy systems in southern
Italy should be quite different from those in Belgium or indeed in north-
ern Italy. Corn alcohol fuels might be suitable for farmers in Missouri but
not in New England. A suitable scale for renewable energy planning might
be 250 km, but certainly not 2500 km. Unfortunately present-day large
urban and industrialised societies are not well suited for such flexibility and
variation.
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 of
official stations are often different from the energy generating sites, and
the 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

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 to
obtain. In particular, flows of biomass and waste materials will often not
have been previously assessed, and will not have been considered for energy
generation. In general, prospecting for supplies of renewable energy requires
specialised methods and equipment that demand significant resources of
finance and manpower. Fortunately the links with meteorology, agriculture
and 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 follow
quantitative 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 with
good 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 on
energy conservation and improvements in end-use efficiency usually gives
better long-term benefit than money spent on increased generation and
supply capacity. The largest energy requirements are usually for heat and
transport. Both uses are associated with energy storage capacity in ther-
mal mass, batteries or fuel tanks, and the inclusion of these uses in energy
systems can greatly improve overall efficiency.
1.5.3 Matching supply and demand
After quantification and analysis of the separate dynamic characteristics of
end-use demands and environmental supply options, the total demand and
supply have to be brought together. This may be explained as follows:
1 The maximum amount of environmental energy must be utilised
within the capability of the renewable energy devices and systems. In
Figure 1.4(a), the resistance to energy flow at D, E and F should be
small. The main benefit of this is to reduce the size and amount of
generating 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 effect
the capital value of the equipment is not fully utilised. Such control
should only be used at times of emergency or when all conceivable
end-uses have been satisfied. Note that the disadvantage of negative
feedback control is a consequence of renewable energy being flow or

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

current sources that can never be stopped. With finite energy sources,
negative feedback control to the energy source is beneficial, since less
fuel is used.
3 The natural periods and dynamic properties of end-use are most
unlikely to be the same as those of the renewable supply, as discussed in
Section 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 is
expensive (see Chapter 16), especially if not incorporated at the earliest
stages of planning.
4 The difficulties of matching renewable energy supplies to end-use in
stand-alone systems are so great that one common approach is to
decouple supply from local demand by connection to an energy net-
work or grid (Figure 1.4(d)). Here the renewable supply is embedded
in an energy grid network having input from finite sources having
feedback control. Such systems imply relatively large scale operation
and include electricity grids for transmission and distribution. As in (3)
the addition of substantial energy storage in the system, say pumped
hydro or thermal capacity for heating, can improve efficiency and allow
the proportion of renewable supply to increase. By using the grid for
both the export and the import of energy, the grid becomes a ‘virtual
store’.
5 The most efficient way to use renewable energy is shown in
Figure 1.4(e). Here a range of end-uses is available and can be switched
or 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. variable
voltage water heating, pumped water storage). Such systems require
feedforward control (see Section 1.5.4). Since the end-use load increases
with 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 in
Section 1.5.3 shows that there are three possible categories of control:
(1) spill the excess energy, (2) incorporate storage and (3) operate load
management 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 flow
sources, 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 in
hydroelectric catchment dam. (c) Control by load variation: feedforward
control. Load controller automatically shunts power between end-uses,
maintaining constant generator load at E. Turbine also has constant load
and hence constant frequency: only rudimentary mechanical control of
turbine is needed.
energy provides easy control and may be the cheapest option. Examples
occur with run-of-the-river hydroelectric systems (Figure 1.5(a)), shades
and blinds with passive solar heating of buildings, and wind turbines
with adjustable blade pitch.
2 Incorporate storage. Storage before transformation allows a maximum
amount of energy to be trapped from the environment and eventually

harnessed or used. Control methods are then similar to conventional
methods with finite sources, with the store equivalent to fuel. The main
disadvantages are the large relative capital costs of storage, and the
difficulty of reducing conventional control methods to small-scale and
remote operation. In the example of Figure 1.5(b), hydro storage is
usually only contemplated for generation at more than ∼10MW. The
mechanical flow control devices become unwieldy and expensive at a
microhydro scale of ∼10kW. A disadvantage of hydro storage may be
the environmental damage caused by reservoirs.
Storage after energy transformation, e.g. battery charging or hydro-
gen production, is also possible and may become increasingly important
especially in small systems. Thermal storage is already common.
3 Load control. Parallel arrangements of end-uses may be switched and
controlled so as to present optimum total load to the supply. An exam-
ple of a microhydro load controller for household power is shown in
Figure 1.5(c) (see also Section 8.6). The principle may be applied on
a small or large scale, but is perhaps most advantageous when many
varied end-uses are available locally. There are considerable advantages
if load control is applied to renewable energy systems:
a No environmental energy need be wasted if parallel outputs are
opened 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 be
incorporated in many varied control modes (e.g. low priority uses
can receive energy at low cost, provided that they can be switched
off by feedforward control; electrical resistive heaters may receive
variable voltage and hence variable power).
c End-uses having storage capability (e.g. thermal capacity of water
heating and building space conditioning) can be switched to give
the benefits of storage in the system at no extra cost.
d Electronic and microprocessor-based control may be used with
benefits of low cost, reliability, and extremely fast and accurate
operation.
Feedforward load control may be particularly advantageous for
autonomous 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 is
necessary without adding greatly to the cost or mechanical complexity, and
so electronically based feedforward control into several parallel electrical
loads is most useful. An example is shown in Figure 1.6.

Figure 1.6 Wind energy conversion system for Fair Isle, Scotland. Electrical loads
are switched by small changes in the supply frequency, so presenting a
matched load to the generator over a wide range of wind speeds.
1.6 Social implications
The Industrial Revolution in Europe and North America and industrial
development in all countries have profoundly affected social structures
and patterns of living. The influence of changing and new energy sources
has been the driving function for much of this change. Thus there is a
historic relationship between coal mining and the development of indus-
trialised countries, which will continue for several hundred years. In the
non-industrialised countries, relatively cheap oil supplies became available
in the 1950s at the same time as many countries obtained independence
from colonialism. Thus in all countries the use of fossil fuels has led to
profound changes in lifestyle.
1.6.1 Dispersed living
In Sections 1.1 and 1.4.4 the dispersed and small energy flux density of
renewable sources was discussed. Renewable energy arrives dispersed in the
environment and is difficult and expensive to concentrate. By contrast finite
energy sources are energy stores that are easily concentrated at source and
expensive to disperse. Thus electrical distribution grids from fossil fuel and
nuclear 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 of
industry and commerce. Similar effects have occurred with the relation-
ships between coal mining and steel production, oil refining and chemical
engineering 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 will
favour dispersed, rather than concentrated, communities. Electricity grids
in such situations are powered by smaller-scale, embedded, generation, with
power flows moving intermittently in both directions according to local
generation and local demand. In Section 1.2.2 an approximate estimate
of 500 people per square kilometre was made of maximum population
density for communities relying on renewable sources. This is considerably
greater than for rural communities (∼100 people per square kilometre) and
corresponds with the population densities of the main administration and
commercial towns of rural regions. Thus the gradual acceptance of signifi-
cant supplies of renewable energy could allow relief from the concentrated
metropolises of excessive urbanisation, yet would not require unacceptably
low population densities. A further advantage is the increased security for
a nation having its energy supplies from such indigenous and dispersed
sources.
1.6.2 Pollution and environmental impact
Harmful emissions can be classified as chemical (as from fossil fuel and
nuclear power plant), physical (including acoustic noise and radioactivity)
and biological (including pathogens); such pollution from energy generation
is overwhelmingly a result of using ‘brown’ fuels, fossil and nuclear. In
contrast, renewable energy is always extracted from flows of energy already
compatible with the environment (Figure 1.1). The energy is then returned to
the environment, so no thermal pollution can occur on anything but a small
scale. Likewise material and chemical pollution in air, water and refuse tend
to be minimal. An exception is air pollution from incomplete combustion
of biomass or refuses (see Chapter 11). Environmental pollution does occur
if brown energy is used for the materials and manufacture of renewable
energy 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 each
technology chapter that follows. General institutional factors, often related
to 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 energy
supplies. 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 in
rural and previously less developed sectors. It is impossible to predict exactly
the long-term effect of such changes in energy supply, but the sustainable
nature of renewable energy should produce greater socio-economic stability
than has been the case with fossil fuels and nuclear power. In particular
we expect the great diversity of renewable energy supplies to be associated
with a similar diversity in local economic and social characteristics.
Problems
1.1 a Show that the average solar irradiance absorbed during 24 h over
the whole Earth’s surface is about 230 W (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 Earth’s
habitable surface is crossed or accessible to an average flux of about
500 W’?
1.2 a Compare the direct costs to the consumer of using:
i a succession of ten 100 W incandescent light bulbs with an
efficiency 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 fixed
electricity price of E010kW h
−1
;
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 local
lamps and electricity. Both the price of CFL’s in local shops and of
electricity vary markedly, so your answers may differ significantly.
Nevertheless it is highly likely the significant lifetime savings will still
occur.
1.4 Economists argue that as oil reserves become smaller, the price will
increase, so demand will decrease and previously uneconomic supplies
will come into production. This tends to make the resource last longer
than would be suggested by a simple calculation (based on ‘today’s
reserves’ divided by ‘today’s use’) . On the other hand, demand increases
driven by increased economic development in developing countries tend
to shorten the life of the reserve. Discuss.

Bibliography 25
Bibliography
Refer to the bibliographies at the end of each chapter for particular subjects and
technologies.
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) Renewable
Energy: Sources for fuels and electricity, produced for the UN Solar Energy
Group for Environment and Development, Earthscan, London, and Island Press,
Washington DC, 1000pp. An authoritative study; but does not attempt to include
the built environment.
Sørensen, B. (2004, 3rd edn) Renewable Energy, Academic Press, London.
Outstandingly the best theoretical text at postgraduate level, considering energy
from the environment to final use.
US Department of Energy (1997) Renewable Energy Characterisations, US-DOE
Topical Report TR-109496. [Available on website www.eere.gov] Emphasis on
prospects for electricity generation and RD requirements.
Energy, society and the environment (including ‘sustainable
development’)
[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 Open
University. 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 account
for ‘science and society’ courses.
Elliot, D. (2003, 2nd edn) Energy, Society and the Environment, Routledge. Brief
survey of technologies, but more extensive discussion of institutional and societal
aspects.
Goldemberg, J. (1996) Energy, Environment and Development, Earthscan (with
James and James), London. Wide-ranging and readable exposition of the links
between energy and social and economic development and sustainability, with
consideration 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 (Sir
John Houghton was co-chair of IPCC).
Intergovernmental Panel on Climate Change (IPCC) (2001) Third Assessment
Report – 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 of
the Twentieth Century, Penguin, London. The growth of fossil-fuel-fired cities
and 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 contrasting
views. Illustrates that many of the issues (including some of the funding issues
concerning renewable energy) have not changed from the 1970s, when the policy
motivation 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, Pergamon, Oxford.
Descriptions of some of the early circumstances outlined in Chapter 1.
Von Weizsacker, E., Lovins, A.B. and Lovins, H. (2000) Factor Four: Doubling
Wealth, Halving Resource Use, Penguin, London. Explores the wider social and
political issues of energy supply, especially those associated with renewable and
nuclear supplies.
World Commission on Environment and Development (1987) Our Common Future,
Oxford University Press (the ‘Bruntland report’). A seminal work, warning about
the key issues in plain language for politicians.
Official publications (including energy statistics and
projections)
See also below under journals and websites, as many official publications, especially
those of a statistical nature, are updated every year or two. United Nations agencies
produce a wide range of essential publications regarding energy. These are especially
important for data. For instance we recommend:
United Nations World Energy Supplies, UN document no. ST/ESA/STAT/
SER.J/l9, annual. Gives statistics of energy consumption around the world, classi-
fied by source, country, continent, etc., but counts only ‘commercial energy’ (i.e.
excludes firewood, etc.).
Government publications are always important. For instance, UK Department of
Energy Series of Energy Papers. Such publications are usually clearly written and
include economic factors at the time of writing. Basic principles are covered, but
usually without the details required for serious study. Annual updates of many
of government and UN publications are also available through the corresponding
websites.
World Energy Council (2001) Survey of world energy resources. Compiled every
5 years or so by the WEC, which comprises mainly energy utility companies from
around the world; covers both renewable and non-renewable resources.
International Energy Agency, World Energy Outlook (annual), Paris. Focus is on
fossil fuel resources and use, based on detailed projections for each member country,
and for those non-member countries which are significant in world energy markets,
e.g. OPEC and China.

Bibliography 27
Do-it-yourself publications
There are many publications for the general public and for enthusiasts. Do not
despise these, but take care if the tasks are made to look easy. Many of these
publications give stimulating ideas and are attractive to read, e.g. Merrill, R. and
Gage, T. (eds) Energy Primer, Dell, New York (several editions).
Berrill, T. et al. (eds) (2003, 4th edn) Introduction to Renewable Energy Technolo-
gies: Resource book, Australian Centre for Renewable Energy, Perth. Written with
tradespeople in mind; clearly describes available equipment with basic account of
principles; emphasis on practicalities of installation and operation.
Journals, trade indexes and websites
Renewable energy and more generally energy technology and policy are continu-
ally advancing. Use a web search engine for general information and for technical
explanations and surveys. For serious study it is necessary to refer to the periodical
literature (journals and magazines), which is increasingly available on the web, with
much more available by payment. Websites of key organisations, such as those listed
below and including governmental sites with statistical surveys, also carry updates
of our information.
We urge readers to scan the serious scientific and engineering journals, e.g. New
Scientist, Annual Review of Energy and the Environment, and magazines, e.g. Elec-
trical Review, Modern Power Systems. These publications regularly cover renewable
energy projects among the general articles. The magazine Refocus, published for
the International Solar Energy Society, carries numerous well-illustrated articles on
all aspects of renewable energy. The series Advances in Solar Energy, published
by the American Solar Energy Society, comprises annual volumes of high level
reviews, including all solar technologies and some solar-derived technologies (e.g.
wind power and biomass). There are also many specialist journals, such as Renew-
able and Sustainable Energy Reviews, Solar Energy, Wind Engineering, and Biomass
and Bioenergy, referred to in the relevant chapters.
As renewable energy has developed commercially, many indexes of companies and
products have been produced; most are updated annually, e.g. European Directory
of Renewable Energy Supplies and Services, annual, ed. B. Cross, James and James,
London.
www.iea.org
The International Energy Agency (IEA) comprises the governments of about 20
industrialised countries; its publications cover policies, energy statistics and trends,
and to a lesser extent technologies; it also co-ordinates and publishes much collab-
orative international RD, including clearly written appraisals of the state of the
art of numerous renewable energy technologies. Its publications draw on detailed
inputs from member countries.
www.wec.org
The World Energy Council comprises mainly energy utility companies for around the
world, who cooperate to produce surveys and projections of resources, technologies
and prices.

www.ipcc.ch
The Intergovernmental Panel on Climate Change (IPCC) is a panel of some 2000
scientists convened by the United Nations to report on the science, economics and
mitigation of greenhouse gases and climate change; their reports, issued every five
years or so, are regarded as authoritative. Summaries are available on the website.
www.itdg.org
ITDG (the Intermediate Technology Development Group) develops and promotes
simple and cheap but effective technology – including renewable energy technolo-
gies – for use in rural areas of developing countries. They have an extensive publi-
cation list plus on-line ‘technical briefs’.
www.caddet.org
The acronym CADDET stands for Centre for the Analysis and Dissemination of
Demonstrated Energy Technologies. The site gives information and contact details
about renewable energy and energy efficiency projects from many countries.
www.ewea.org
European Wind Energy Association is one of many renewable energy associations,
all of which have useful websites. Most such associations are ‘trade associations’,
as funded by members in the named renewable energy industry. However, they are
aware of the public and educational interest, so will have information and give
connections for specialist information.

Chapter 2
Essentials of fluid dynamics
2.1 Introduction
Several renewable energy resources derive from the natural movement of
air and water. Therefore the transfer of energy to and from a moving
fluid is the basis of meteorology and of hydro, wind, wave and some
solar power systems. Examples of such applications include hydropower
turbines (Figures 8.3, 8.5 and 8.6), wind turbines (picture on front cover
and Figure 9.4), solar air heaters (Figure 6.1) and wave energy systems
(Figure 12.14).
To understand such systems, we must start with the basic laws of mechan-
ics as they apply to fluids, notably the laws of conservation of mass, energy
and momentum. The term fluid includes both liquids and gases, which,
unlike solids, do not remain in equilibrium when subjected to shearing
forces. The hydrodynamic distinction between liquids and gases is that
gases are easily compressed, whereas liquids have volumes varying only
slightly with temperature and pressure. Gaseous volumes vary directly with
temperature and inversely with pressure, approximately as the perfect-gas
law pV = nRT. Nevertheless, for air, flowing at speeds 100ms−1
and
not subject to large imposed variations in pressure or temperature, density
change is negligible; this is the situation for the renewable energy sys-
tems analysed quantitatively in this book. It does not apply to the analysis
of gas turbines, for which specialist texts should be consulted. Therefore,
throughout this text, moving air is considered to have the fluid dynamics
of an incompressible fluid. This considerably simplifies the analysis of most
renewable energy systems.
Many important fluid flows are also steady, i.e. the particular type of flow
pattern at a location does not vary with time. So it is useful to picture a set
of lines, called streamlines, parallel with the velocity vectors at each point.
A further distinction is between laminar and turbulent flow (Section 2.5).
For example, watch the smoke rising from a smouldering taper in still air.
Near the taper, the smoke rises in an orderly, laminar, stream, with the
paths of neighbouring smoke particles parallel. Further from the taper, the

30 Essentials of fluid dynamics
flow becomes chaotic, turbulent, with individual smoke particles intermin-
gling in three dimensions. Turbulent flow approximates to a steady mean
flow, subject to internal friction caused by the velocity fluctuations. How-
ever, even in turbulence, the airflow remains within well-defined (though
imaginary) streamtubes, as bounded by streamlines.
2.2 Conservation of energy: Bernoulli’s equation
Consider the most important case of steady, incompressible flow. At first,
we assume no work is done by the moving fluid on, say, a hydro turbine.
Figure 2.1 shows a section of a streamtube between heights z1 and z2.
The tube is narrow in comparison with other dimensions, so z is considered
constant over each cross-section of the tube. A mass m = A1u1t enters
the control volume at 1, and an equal mass m = A2u2t leaves at 2 (where
is the density of the fluid, treated as constant). Then the energy balance
on the fluid within the control volume is
potential energy lost+work done by pressure forces
= gain in kinetic energy+heat losses due to friction
and may be written as
mgz1 −z2+p1A1u1t−p2A2u2t =
1
2
mu2
2 −u2
1+Ef (2.1)
where the pressure force p1A1 acts through a distance u1t, and similarly
for p2A2, and Ef is the heat generated by friction.
Figure 2.1 Illustrating conservation of energy: a streamtube rises from height z1
to z2.

2.2 Conservation of energy: Bernoulli’s equation 31
We neglect fluid friction, Ef, now, but we will examine some of its effects
in Section 2.5. In this ideal, frictionless case, (2.1) reduces to
p1

+gz1 +
1
2
u2
1 =
p2

+gz2 +
1
2
u2
2 (2.2)
or, equivalently, so each term has the dimension of height (m)
p
g
+z+
u2
2g
= constant along a streamline, with no loss of energy (2.3)
Either of these forms of the equation is called Bernoulli’s equation. Equa-
tions (8.10) and (9.19) are examples of its application in hydro and wind
power respectively.
The sum of the terms on the left of (2.3) is called the total head of
fluid (H). It relates to the total energy of a unit mass of fluid, however the
constant in (2.3) may vary from streamline to streamline. Moreover, for
many situations, the friction losses, Ef, have to be included. Head has the
dimensions of length. For hydropower, head is the effective height of the
moving water column incident on the turbine – see Section 8.3.
The main limitation of (2.2) and (2.3) is that they apply only to flu-
ids treated as ideal, i.e. with zero viscosity, zero compressibility and zero
thermal conductivity. However, this is applicable to wind and hydro tur-
bines with their relatively low-speed movement of air and water, and with
no internal heat sources. The energy equation can however be modified
to include non-ideal characteristics (see Bibliography), as for combus-
tion engines and many other thermal devices, e.g. high temperature solar
collectors.
In solar heating systems and heat exchangers, power Pth is added to the
fluid from heat sources (Figure 2.2). Heat E = Ptht is added to the energy
inputs on the left hand side of (2.1). The mass m coming into the control
volume at temperature T1 has heat content mcT1 (where c is the specific
heat capacity of the fluid), and that going out has heat content mcT2.
Thus we add to the right hand side of (2.1) the net heat carried out of
the control volume in time t, namely mcT2 −T1. This gives an equation
corresponding to (2.2), namely
p1

+gz1 +
1
2
u2
1 +cT1 +
Pth
Q
=

p2

+gz2 +
1
2
u2
2 +cT2 (2.4)
where the volume flow rate is
Q = Au (2.5)

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