Chapter 1
Principles of renewable energy
1.1 Introduction
The aim of this text is to analyse the full range of renewable energy supplies
available for modern economies. Such renewables are recognised as
vital inputs for sustainability and so encouraging their growth is significant.
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 environmental
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 uneconomic
operation. Frequently there will be a marked contrast between the
methods developed for renewable supplies and those used for the nonrenewable
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 compromising
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 improvement
in quality of life, and, especially, standard of living in the less developed
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 following
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, especially
regarding production and consumption, will eventually be forced on
populations by ecological and economic pressures. Nevertheless, the economic
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, industrial
equipment, transport, etc. Purchases of energy account for 5–10% of
gross national product in developed economies. However, in some developing
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 indicates.
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 significantly
raised the concentration of CO2 in the Atmosphere. The balance of
scientific opinion is that if this continues, it will enhance the greenhouse
4 Principles of renewable energy
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 sustainable
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: ≤300W m−2
Released at ≥100kW m−2
Lifetime of supply Infinite Finite
Cost at source Free Increasingly expensive.
Equipment capital cost per
kW capacity
Expensive, commonly ≈US$1000kW−1
Moderate, perhaps $500kW−1
without emissions control;
yet expensive >US$1000kW−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
6 Principles of renewable energy
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 equipment,
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 technical
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’. Examples
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 purposes.
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.
8 Principles of renewable energy
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 1 2 × 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.
10 Principles of renewable energy
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 useful
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
RefiningCrude 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 primary
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 frequently
result. For instance, if a dominant domestic energy requirement
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 electricity
in lossy cables and then dissipate this electricity as heat. Sadly
12 Principles of renewable energy
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 generation
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 process.
Consider electric lighting produced from ‘conventional’ thermally
generated electricity and lamps. Successive energy efficiencies are: electricity
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
1.4 Scientific principles of renewable energy 13
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 thermodynamic
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 Pmax = 1kW m−2
Diffuse sunshine 24 h, 1 y Cloud cover, perhaps air
pollution
P ≪ G
P ≤ 300W m−2
Significant energy over time (4.3)
Biofuels 1 y Soil condition, insolation,
water, plant species, wastes
Stored energy
∼10MJkg−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 u0 P ∝ u3
0 Most variable (9.2)
Height nacelle above ground z;
height of anemometer mast h
uz/uh = z/h b
b ∼ 0 15 (9.54)
Wave 1 y ‘Significant wave height’ Hs
wave period T
P ∝ H2
s T Relatively large power density
∼50kW 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 ∝ R2
A Enhanced tidal range if
L/
√
h = 36000m1/2
(13.35) and
(13.28)
Tidal stream/current; peak
current u0, density sea water
∼1000×air
P ∝ u3
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)
1.4 Scientific principles of renewable energy 15
Renewable energy supply systems divide into three broad divisions:
1 Mechanical supplies, such as hydro, wind, wave and tidal power. The
mechanical source of power is usually transformed into electricity 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 proportion
of heat energy extractable as mechanical work, and hence electricity,
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 mechanical
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.
16 Principles of renewable energy
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 control
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 production
and nutrient cycling for optimum agricultural yields.
1.4.6 Situation dependence
No single renewable energy system is universally applicable, since the ability
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 formations
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 northern
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, particularly
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 monitoring
stations is as a base for comparison with local site variations. Thus
1.5 Technical implications 17
wind velocity may be monitored for several months at a prospective generating
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 environmental
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 thermal
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.
1.5 Technical implications 19
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 different
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 network
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 feedforward
control.
1.5.4 Control options
Good matching of renewable energy supply to end-use demand is accomplished
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 illustrated
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
20 Principles of renewable energy
Figure 1.5 Examples of control. (a) Control by spilling excess energy: constant pressure
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
1.5 Technical implications 21
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 hydrogen
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 example
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 rotational
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.
22 Principles of renewable energy
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 industrialised
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 relationships
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 significant
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 technology
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
24 Principles of renewable energy
systems become widespread. The influence of modern science and technology
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 illumination
at 22% efficiency, life of 10 000 h, price E3.0. Use a fixed
electricity price of E0 10kW 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 sophisticated
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 journal
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 R&D 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 Sustainability:
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, Cambridge
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.
26 Principles of renewable energy
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 greenhouse
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, classified
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 Technologies:
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 continually
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. Electrical
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 Renewable
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 collaborative
international R&D, 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.
28 Principles of renewable energy
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 technologies
– for use in rural areas of developing countries. They have an extensive publication
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.