week3 renewable energy sources

20
1 Introducing renewable energy & Sustainable Development Week 3 Renewable energy sources, derived principally from the enormous power of the Sun's radiation, are simultaneously the most ancient and the most modern forms of energy used by humanity. Solar power, both in the form of direct solar radiation and in indirect forms such as bioenergy, water or wind power, was the energy source upon which early human societies were based. When our ancestors first used fire, they were harnessing the power of photosynthesis, the solar-driven process by which plants are created from water and atmospheric carbon dioxide. Societies went on to develop ways of harnessing the movements of water and wind, both caused by solar heating of the oceans and atmosphere, to grind corn, irrigate crops and propel ships. As civilizations became more sophisticated, architects began to design buildings to take advantage of the Sun's energy by enhancing their natural use of its heat and light, so reducing the need for artificial sources of warmth and illumination.

Upload: georgiadisg

Post on 04-Dec-2015

15 views

Category:

Documents


3 download

DESCRIPTION

Renewable energy

TRANSCRIPT

Page 1: Week3 Renewable Energy Sources

1

Introducing renewable energy&

Sustainable Development

Week 3

Renewable energy sources, derived principally from theenormous power of the Sun's radiation, aresimultaneously the most ancient and the most modernforms of energy used by humanity.Solar power, both in the form of direct solar radiationand in indirect forms such as bioenergy, water or windpower, was the energy source upon which early humansocieties were based.

When our ancestors first used fire, they were harnessingthe power of photosynthesis, the solar-driven process bywhich plants are created from water and atmosphericcarbon dioxide.Societies went on to develop ways of harnessing themovements of water and wind, both caused by solarheating of the oceans and atmosphere, to grind corn,irrigate crops and propel ships.

As civilizations became more sophisticated, architectsbegan to design buildings to take advantage of the Sun'senergy by enhancing their natural use of its heat andlight, so reducing the need for artificial sources ofwarmth and illumination.

Page 2: Week3 Renewable Energy Sources

2

Technologies for harnessing the power of Sun,firewood, water and wind continued to improveright up to the early years of the industrialrevolution.

However, by then the advantages of coal, thefirst of the fossil fuels to be exploited on a largescale, had become apparent.

These highly-concentrated energy sources soondisplaced wood, wind and water in the homes,industries and transport systems of theindustrial nations.

Today the fossil fuel trio of coal, oil and naturalgas provides over 80% of the world's energy.

Environmental concerns of fossil fuel use,such as air pollution and about the finite natureof supplies, have been voiced for centuries.

In 1970s, with the 'oil crisis' and the steepprice rises the environmental movementemerged

Humanity began to take more seriously theprospect of fossil fuels 'running out', and thepossibility that their continued use could bedestabilizing the planet's natural ecosystemsand the global climate.

After the Second World War nuclear energy raisedhopes of an alternative to fossil fuels believed to be

• cheap,• plentiful and• clean

However, nuclear power development has stalled insome countries in recent years, due to increasingconcerns about safety, cost, waste disposal and weaponsproliferation,Note that in some countries nuclear expansion iscontinuing.

Continuing concerns about the `sustainability' of bothfossil and nuclear fuel use have renewed interest in therenewable energy sources.Ideally, a sustainable energy source is one that:

• is not substantially depleted by continued use• does not entail significant pollutant emissions or other

environmental problems• does not involve the perpetuation of substantial health hazards

or social injustices.

Page 3: Week3 Renewable Energy Sources

3

In practice, only a few energy sources come close tothis ideal, but `renewables' appear generally moresustainable than fossil or nuclear fuels:• they are essentially inexhaustible and their use

entails fewer health hazards and much loweremissions of greenhouse gases or other pollutants.

The word energy is derived from the Greek en (in) and ergon(work) i.e.

'the capacity to do work'.

1. such as thermal energy (heat),

2. chemical energy (in fuels or batteries),

3. kinetic energy (in moving substances),

4. electrical energy,

5. gravitational potential energy,

6. and various others.

Energy conservation:The First Law of Thermodynamics

The renewable energy technologies transform one form ofenergy into another (in many cases being electricity).

In any such transformation of energy, the total quantity ofenergy remains unchanged.

This principle, that energy is always conserved, is expressed bythe First Law of Thermodynamics.So if the electrical energy output of a power station, forexample, is less than the energy content of the fuel input, thensome of the energy must have been converted to another form(usually waste heat).

Energy is neither created nor destroyed

First Law of Thermodynamics is a version of the lawof conservation of energy, adapted for thermodynamicsystems.The law of conservation of energy states that the totalenergy of an isolated system is constant;energy can be transformed from one form to another,but cannot be created or destroyed.

Page 4: Week3 Renewable Energy Sources

4

If the total quantity of energy is always the same, how can wetalk of consuming it?

Wedon't:

we just convert it from one form into other forms.

We consume fuels, which are sources of readily availableenergy.

We may burn fuel in a vehicle engine, converting its storedchemical energy into heat and then into the kinetic energy of themoving vehicle.

By using a wind turbine we can extract kinetic energyfrom moving air and convert it into electrical energy,which can in turn be used to heat the filament of anincandescent lamp causing it to radiate light energy.

Forms of energy

At the most basic level, the diversity of energy formscan be reduced to four:

• kinetic• gravitational• electrical• nuclear.

Kinetic energy

The kinetic energy possessed by any moving object isequal to half the mass (m) of the object times the squareof its velocity (v), i.e.:

kinetic energy = 1/2MV2

• where energy is in joules (J),• mass in kilograms (kg)• and velocity in metres per second (m s-1).

Page 5: Week3 Renewable Energy Sources

5

Less obviously, the kinetic energy within a materialdetermines its temperature.All matter consists of atoms, or combinations of atomscalled molecules.In a gas, such as the air that surrounds us, these movefreely.In a solid or a liquid, they form a more or less looselylinked network in which every particle is constantlyvibrating.

Thermal energyThermal energy, or heat, is the name given to the energyassociated with this rapid random motion.

The higher the temperature of a body, the faster its moleculesare moving.

In the temperature scale that is most natural to scientific theory,the Kelvin (K) scale, zero corresponds to zero molecularmotion. In the more commonly used Celsius scale oftemperature (written as °C), the size of one degree is the sameas 1 kelvin, but zero corresponds to the freezing point of wa and100 °C to the boiling point of water at atmospheric pressure.

The t, scales are therefore related by a simple formula:

temperature (K) = temperature (°C) + 273.

Gravitational energyA second fundamental form of energy is gravitationalenergy.On Earth, input of energy is required to lift an objectbecause the gravitational pull the Earth opposes thatmovement. If an object, such as an apple, is lift aboveyour head, the input energy is stored in a form calledgravitation potential energy (often just 'potential energy'or 'gravitational energy').That this stored energy exists is obvious if you releasethe apple and observe the subsequent conversion tokinetic energy.

Figure 1.1 The amount of energy requiredto raise a 100 g apple vertically through I mis approximately one joule (I J)

Page 6: Week3 Renewable Energy Sources

6

Electrical energy

Gravity is not the only force influencing the objects around us.

On a scale far too small for the eye to see, electrical forces holdtogether the atoms and molecules of all materials; gravity is aninsignificant force at the molecular level.

The electrical energy associated with these forces is the third ofthe basic forms.

Every atom can be considered to consist of a cloud ofelectrically charged particles, electrons, moving incessantlyaround a central nucleus.

When atoms bond with other atoms to form molecules,the distribution of electrons is changed, often withdramatic effect.Thus chemical energy, viewed at the atomic level, canbe considered to be a form of electrical energy.When a fuel is burned, the energy liberated (thechemical energy) is converted into heat energy.Essentially, the electrical energy released as theelectrons are rearranged (that is, the net release ofenergy from the breaking and forming of bonds) isconverted to the kinetic energy of the molecules of thecombustion products.

Nuclear energy

The fourth and final basic form of energy, bound up in thecentral nuclei of atoms, is called nuclear energy.

The technology for releasing it was developed during theSecond World War for military purposes, and subsequently in amore controlled version for the commercial production ofelectricity.

Nuclear power stations operate on much the same principles asfossil fuel plants, except that the furnace in which the fuel burnsis replaced by a nuclear reactor in which atoms of uranium aresplit apart in a 'fission' process that generates large amounts ofheat

The energy source of the Sun is also of nuclear origin.Here the process is not nuclear fission but nuclearfusion, in which hydrogen atoms fuse to form heliumatoms — such enormous numbers of these reactionstake place that massive amounts of solar radiation aregenerated in the process.Attempts to imitate the Sun by creating power-producing nuclear fusion reactors have been the subjectof many decades of research and development effort buthave yet to come to fruition.

Page 7: Week3 Renewable Energy Sources

7

Energy sources

There are five ultimate primary sources of useful energy:

1 The Sun.

2 The motion and gravitational potential of the Sun, Moon andEarth.

3 Geothermal energy from cooling, chemical reactions andradioactive decay in the Earth.

4 Human-induced nuclear reactions.

5 Chemical reactions from mineral sources.

Renewable energy

Renewable energy derives continuously from sources 1,2 and 3.Finite energy derives from fossil fuels, 3 (hot rocks), 4and 5.The sources of most significance for global energysupplies are 1 and 4.The fifth category is relatively minor, but useful forprimary batteries, e.g. dry cells.

For all practical purposes energy supplies can bedivided into two classes:1 Renewable energy.2 Non-renewable energy.

1 Renewable energy.

‘Energy obtained from natural and persistent flows ofenergy 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 theenvironment as a current or flow, irrespective of therebeing a device to intercept and harness this power.Such energy may also be called Green Energy orSustainable Energy.

Page 8: Week3 Renewable Energy Sources

8

2 Non-renewable energy.‘Energy obtained from static stores of energy thatremain underground unless released by humaninteraction’.Examples are nuclear fuels and fossil fuels of coal, oiland natural gas.Note that the energy is initially an isolated energypotential, and external action is required to initiate thesupply of energy for practical purposes.To avoid using the ungainly word ‘non-renewable’,such energy supplies are called finite supplies or BrownEnergy.

Environmental energyThe flows of energy passing continuously as renewableenergy through the Earth are shown in Figure 1.2. Forinstance, total solar flux absorbed at sea level is about1.2 ×1017 W. Thus the solar flux reaching the Earth’ssurface is ∼20 MW per person; 20 MW is the power often very large diesel electric generators, enough tosupply all the energy needs of a town of about 50 000people.

The maximum solar flux density (irradiance)perpendicular to the solar beam is about 1 kWm −2; avery useful and easy number to remember. In generalterms, a human being is able to intercept such an energyflux without harm, but any increase begins to causestress and difficulty. Interestingly, power flux densitiesof ∼1kWm begin to cause physical difficulty to an adultin wind, water currents or waves.

Figure 1.2 Natural energy currents on earth, showing renewable energy system. Note the greatrange of energy flux 1:105 and the dominance of solar radiation and heat. Units terawatts 1012W.

Page 9: Week3 Renewable Energy Sources

9

Sankey (‘spaghetti’) diagram, Present-day energy useWorld energy supplies

The energy used by a final consumer is usually the end result ofa series of energy conversions.

For example, energy from burning coal may be converted in apower station to electricity, which is then distributed tohouseholds and used in immersion heaters to heat water indomestic hot water tanks.

The energy released when the coal is burned is called theprimary energy required for that use.

Page 10: Week3 Renewable Energy Sources

10

The amount of electricity reaching the consumer, afterconversion losses in the power station and transmissionlosses in the electricity grid, is the delivered energy.After further losses in the tank and pipes, a finalquantity, called the useful energy, comes out of the hottap.

World total annual consumption of all forms of primaryenergy increased more than tenfold during the twentiethcentury, and by the year 2009 had reached an estimated502 EJ (exajoules 1018), or some 12 000 million tonnesof oil equivalent (Mtoe) (Figure 1.2).As the figure reveals, fossil fuels provided more thanfour fifths of the total.The world population in 2009 was some 6.8 billion, sothe annual average energy consumption per person wasabout 74 GJ (gigajoules), equivalent to the energycontent of approximately 5.5 litres of oil per day forevery man, woman and child.

Figure 1.2 Percentage contributions to world primary energy consumptionin 2009. `Other sources' are `new' biomass, solar and geothermal energy,and energy from wind, wave, tide and wastes (sources: authors' estimatesbased on BP, 2010; lEA, 2009; WWEA, 2010) But these figures conceal major differences.

The average North American consumes more than 250GJ per year, most people in Europe use roughly half thisamount, and many of those in the poorer countries ofthe world less than one fifth — much of it in the form oflocal 'biofuels'.

Page 11: Week3 Renewable Energy Sources

11

How much do renewables contribute to world energysupplies?As Figure 1.2 shows, traditional biomass, hydro powerand a range of other renewable sources contributed anestimated 13% of world primary energy in 2009.Figure 1.3 gives a more detailed breakdown, for theyear 2008.

Figure 1.3 Chart showing percentage breakdown of individualrenewable energy sources' contributions to world primary energysupplies 2008 (sources authors estimates on EIA 2009 BP 2010).

The largest contribution is an estimated value of 30 EJfrom 'traditional biomass' (wood, straw, dung, etc.)mainly used in developing countries.Since most of this isn't traded, it often doesn't enter intonational economic statistics and its true magnitude isonly known approximately.The next largest category is 'new biomass'.This includes wood and other crops specifically grownfor energy purposes, biogas, and biofuels such asethanol and biodiesel.

This is a commodity that is likely to be traded and so itsmagnitude is more certain.Hydro power is the next largest category, supplyingover 2% of the world's primary energy.'New biomass', together with energy from wastes,geothermal energy, solar energy and energy from wind,wave and tidal power make up the 'other sources' shownin Figure 1.2.

Page 12: Week3 Renewable Energy Sources

12

In practice, many electricity-generating fossil fuelledand renewable energy technologies produce largeamounts of unused 'waste' heat.Renewable energy proportions based on primary energymay thus give a misleading picture.Proportions of renewable energy in national (andglobal) statistics are now often quoted in terms of grossfinal energy consumption (see Box 1.1).

How long will the world's fossil fuel reserves last?At current consumption rates, it is estimated that worldcoal reserves could last for about 120 years, oil forapproximately 45 years and natural gas for around 60years (BP, 2010).However, in the more immediate future there are likelyto be serious constraints on the rate at which fossil fuelscan be produced, particularly oil.Existing oilfields have a limited life; once exhaustedthey have to be replaced with new ones.

In order just to maintain the world's oil production at itscurrent level, a large number of new oil fields will haveto be developed.Even more challenging is the need for new fields to becontinuously discovered (Figure 1.4).According to the International Energy Agency, the 'easyoil' has been largely used up.What remains is likely to be more expensive and indifficult areas such as the Arctic or in deep offshorewells (IEA, 2010a).

Figure 1.4 An International Energy Agency chart indicating thechallenges involved in maintaining current levels ofconventional oil production (source: IEA, 2010a)

Page 13: Week3 Renewable Energy Sources

13

According to a study of global oil depletion by the UKEnergy Research Centre:

• a peak of conventional oil production before 2030 appears likely andthere is a significant risk of a peak before 2020. (Sorrell et al., 2009)

So although large worldwide reserves of oil will remain,the overall production of conventional oil seems likelyto 'plateau' or even decline.This has serious implications for the UK, where oilproduction peaked in 1999 and gas production peakedin 2000.

Fossil fuels and climate change

Society's current use of fossil and nuclear fuels hasmany adverse consequences.These include air pollution, acid rain, the depletion ofnatural resources and the dangers of nuclear radiation.This brief introduction concentrates on one of theseproblems: global climate change caused by emissions ofgreenhouse gases from fossil fuel combustion.

The surface temperature of the Earth establishes itself atan equilibrium level where the incoming energy fromthe Sun balances the outgoing infrared energy re-radiated from the surface back into space.If the Earth had no atmosphere its surface temperaturewould be -18 °C; but its atmosphere, which includes'greenhouse gases' –principally, water vapour, carbondioxide and methane – acts like the panes of agreenhouse, allowing solar radiation (which lies in therange from ultra-violet to short wave infrared) to enterbut inhibiting the outflow of long-wave infraredradiation.

The natural 'greenhouseeffect' that these gases causeis essential in maintaining theEarth's surface temperature ata level suitable for life ataround 15 °C.

Figure 1.6 (a) Primary energy contributions fromrenewable energy in the UK, 2009. The total, 6875Mtoe, is equivalent to 288 PJ.The main contributorswere wind, biomass in various forms, and hydropower (b) Growth in electricity generation fromrenewable sources in the UK 1990-2009- In 2009renewables contributed 6.7% of UK electricitySource —.DECC 2010b)

Page 14: Week3 Renewable Energy Sources

14

Since the Industrial Revolution, however, humanactivities have been adding extra greenhouse gases tothe atmosphere.The principal contributor to these increased emissions iscarbon dioxide from the combustion of fossil fuels.Humanity's rate of emission of CO2 from these fuels hasincreased enormously since 1950 (see Figure 1.7).

Scientists estimate (IPCC, 2007a) that these'anthropogenic' (human-induced) emissions caused arise in the Earth's global mean surface temperature ofapproximately 0.7 °C between 1950 and 2005 (Figure1.8).If emissions are not curbed, the IntergovernmentalPanel on Climate Change (IPCC) estimates that theEarth's surface temperature could rise by between 1.4and 5.8 °C (depending on the assumptions made) by theend of the twenty-first century.

Such rises would probably be associated with anincreased frequency of climatic extremes, such as floodsor droughts, and serious disruptions to agriculture andnatural ecosystems.The thermal expansion of the world's oceans couldmean that sea levels would rise by around 0.5 m by theend of the century, which could inundate some low-lying areas.Beyond 2100, or perhaps before, much greater sea levelrises could occur if major Antarctic ice sheets were tomelt.

Figure 1.8 Observed changes in global average surface temperature1860-2005, relative to corresponding averages for the period 1961-1990.The term '95% confidence range' indicates that there is only a one in20 chance of a measurement lying outside this range (source: IPCC,2007b)

Page 15: Week3 Renewable Energy Sources

15

The threat of global climate change, mainly caused by carbondioxide emissions from fossil fuel combustion, is one of themain reasons why there is a growing consensus on the need toreduce such emissions.

In order to ensure that global mean temperature rises do notexceed 2 °C above pre-industrial levels by 2050, studies showthat global carbon emissions will need to be reduced byapproximately 80% by that date.

This implies that global CO, emissions need to peak almostimmediately and then fall sharply over the course of the rest ofthis century (Allen et al., 2009).

Emission reductions on this scale will inevitably involve aswitch to low-or zero-carbon energy sources such asrenewables.

Renewable energy sources

Renewable energy can be defined as:energy obtained from the continuous or repetitivecurrents of energy recurring in the natural environmentOr energy flows which are replenished at the same rateas they are 'used'From Figure 1.9, which summarizes the origins andmagnitudes of the Earth's renewable energy sources, itis clear that their principal source is solar radiation.

Approximately 30% of the 5.4 million EJ per yeararriving at the Earth is reflected back into space.The remaining 70% is, in principle, available for use onEarth, and amounts to approximately 3.8 million EJ,more than 8200 times the rate of consumption of fossiland nuclear fuels in 2009, some 462 EJ.

(If biofuels and hydro power are included, total worldenergy consumption was 502 EJ, as stated earlier.)Two non-solar, renewable energy sources are alsoshown on the figure: the motion of the ocean tides andgeothermal heat from the Earth's interior, whichmanifests itself in convection in volcanoes and hotsprings, and in conduction in rocks.

Page 16: Week3 Renewable Energy Sources

16

Solar energy: direct uses

Solar radiation can be converted into useful energydirectly, using various technologies.Absorbed in solar 'collectors', it can provide hot wateror space heating.Buildings can also be designed with 'passive solar'features that enhance the contribution of solar energy totheir space heating and lighting requirements.

Solar energy can also be concentrated by mirrors to providehigh-temperature heat for generating electricity.

Such 'solar thermal-electric' power stations are in commercialoperation in countries like the USA and Spain. Solar thermalenergy conversion is described in Chapter 2.

Solar radiation can also be converted directly into electricityusing photovoltaic (PV) modules, normally mounted on theroofs or facades of buildings.

At the time of writing, electricity from photovoltaics is moreexpensive than that from conventional sources, but prices arefalling fast and the industry is expanding rapidly.

Figure 1.9 The various forms of renewable energy depend primarilyon incoming solar radiation, which totals some 5.4 million EJ per year

Solar energy: Indirect uses

Solar radiation can be converted to useful energyindirectly, via other energy forms.A large fraction of the radiation reaching the Earth'ssurface is absorbed by the oceans, warming them andadding water vapour to the air.The water vapour condenses as rain to feed rivers, intowhich dams and turbines can be located to extract theenergy of the flowing water.

Page 17: Week3 Renewable Energy Sources

17

Hydropower, has steadily grown during the twentieth century,and at the time of writing provides about a sixth of the world'selectricity.

Sunlight falls in a more perpendicular direction in tropicalregions and more obliquely at high latitudes, heating the tropicsto a greater degree than the polar regions.

The result is a massive heat flow towards the poles, carried bycurrents in the oceans and the atmosphere. The energy in suchcurrents can be harnessed, for example by wind turbines.

Wind power & Wave power

Wind power, has developed on a large scale only in thepast few decades, but is one of the fastest-growing ofthe 'new' renewable sources of electricity.Where winds blow over long stretches of ocean, theycreate waves, and a variety of devices can be used toextract that energy.Wave power, is attracting new funding for research,development and demonstration in several countries.

BioenergyBioenergy, is another indirect manifestation of solar energy.

Through photosynthesis in plants, solar radiation converts waterand atmospheric carbon dioxide into carbohydrates, which formthe basis of more complex molecules.

Biomass, in the form of wood or other `biofuels', is a majorworld energy source, especially in the developing world.

Gaseous and liquid fuels derived from biological sources makesignificant contributions to the energy supplies of somecountries.

Biofuels can also be derived from wastes, many of which arebiological in origin.

Biofuels are a renewable resource if the rate at whichthey are consumed is no greater than the rate at whichnew plants are re-grown – which, unfortunately, is oftennot the case.Although the combustion of biofuels generatesatmospheric CO2 emissions, these should be offset by theCO2 absorbed when the plants were growing, butsignificant emissions of other greenhouse gases canresult if the combustion is inefficient.

Page 18: Week3 Renewable Energy Sources

18

Non-solar renewablesTwo other sources of renewable energy do not dependon solar radiation:

tidal and geothermal energy.

Tidal energy, is often confused with wave energy, butits origins are quite different.

Ocean tides are caused by the gravitational pull of theMoon (with a small contribution from the Sun) on theworld's oceans, causing a regular rise and fall in waterlevels as the Earth rotates.The power of the tides can be harnessed by building alow dam or 'barrage' behind which the rising waters arecaptured and then allowed to flow back throughelectricity-generating turbines.

It is also possible to harness the power of strongunderwater currents, which are mainly tidal in origin.Various devices for exploiting this energy source, suchas marine current turbines (rather like underwater windturbines) are at the demonstration stage.

Heat from within the Earth is the source of geothermalenergy.The high temperature of the interior was originallycaused by gravitational contraction of the planet as itwas formed, but has since been enhanced by the heatfrom the decay of radioactive materials deep within theEarth.

Page 19: Week3 Renewable Energy Sources

19

In some places where hot rocks are very near to thesurface, water is heated in underground aquifers.These have been used for centuries to provide hot wateror steam.In some countries, geothermal steam is used to produceelectricity and, in others, hot water from geothermalwells is used for heating.

If steam or hot water is extracted at a greater rate thanheat is replenished from surrounding rocks, ageothermal site will cool down and new holes will haveto be drilled nearby.When operated in this way, geothermal energy is notstrictly renewable.However, it is possible to operate in a renewable modeby keeping the rate of extraction below the rate ofrenewal.

Renewable energy in a sustainablefuture

The EU in its '20:20:20' Directive, passed in 2009, set atarget for Europe to achieve by 2020 a 20% reduction incarbon emissions combined with a 20% contribution togross final energy consumption from renewablesources.Within this overall target, individual member stateshave been given different specific targets suited to theirclimates and circumstances:

Summary

each of the principal renewable energy sources areexamined in turn: in each case their physical principles,the main technologies involved, their costs andenvironmental impact, the size of the potential resourceand their future prospects are discussed. To start, wediscuss the renewable source that is the basis of most ofthe others: solar energy.

Page 20: Week3 Renewable Energy Sources

20

Figure 1.10 Scenarios from the UK Committee on Climate Changeillustrating the potential contribution of renewables to UK heat (H),electricity (E) and transport energy (T), and to overall gross final energyconsumption, by 2020 and 2030, compared with the contributions in 2009(adapted from CCC, 2011). Note: `gross final consumption' shown aboveis approximately equivalent to `delivered energy', excluding losses inconversion and delivery, as shown in the last three bars of Figure 1.5above

Renewable energy supply systemsdivide into three broad divisions:

1 Mechanical supplies, such as hydro, wind, wave andtidal power. The mechanical source of power is usuallytransformed into electricity at high efficiency. Theproportion of power in the environment extracted by thedevices is determined by the mechanics of the process,linked to the variability of the source, as explained inlater chapters. The proportions are, commonly, wind35%, hydro 70–90%, wave 50%and tidal 75%.

2 Heat supplies

2 Heat supplies, such as biomass combustion and solarcollectors. These sources provide heat at highefficiency. However, the maximum proportion of heatenergy extractable as mechanical work, and henceelectricity, is given by the second law ofthermodynamics and the Carnot Theorem, whichassumes reversible, infinitely long transformations. Inpractice, maximum mechanical power produced in adynamic process is about half that predicted by theCarnot criteria. For thermal boiler heat engines,maximum realisable quality is about 35%.

3 Photon processes

3 Photon processes, such as photosynthesis andphotochemistry (Chapter 10) and photovoltaicconversion (Chapter 7). For example, solar photons of asingle frequency may be transformed into mechanicalwork via electricity with high efficiency using amatched solar cell.In practice, the broad band offrequencies in the solar spectrum makes matchingdifficult and photon conversion efficiencies of 20–30%are considered good.1.4.4 Dispersed versus centralisedenergy