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All of the Earth’s plant and animal material, or‘biomass’, exists in a thin layer around thesurface of the planet called the biosphere.The biosphere is really a huge store of solarenergy, which is continually replenishedthrough photosynthesis by green plants.

Energy obtained from biomass is called bio-energy. At the beginning of the 21st century,over 10 per cent of the total energy used bypeople on the planet comes from biomass.Most of this comes from simply burningwood, rice husks or other plant and animalresidues on a small scale for heat production.

However, in many LEDCs (less economicallydeveloped countries), biomass is not used ina sustainable way. If trees aren’t grownspecifically for burning, using them can causedeforestation. Wood is only a renewableenergy if it is grown specifically for thepurpose.

Bio-energywhat is bio-energy?

A farmer harvests trees for fuel. Photo courtesy of NRE Slide Library/DTI

How does bio-energy work? Over the last few decades new, large-scaleways of generating bio-energy have beendeveloped. These can be split into two maintypes.

n Energy crops are any plants that arespecifically grown as fuel, such as wood,oilseed rape and sugar beet. A form ofbio-diesel can be produced fromvegetable oils, and other crops can befermented to produce ethanol, a high-energy substance similar to petrol.

n Waste is an important source of bio-energy.

<< Farm waste, sewage plants andlandfill sites give off methane as thematerial rots. Methane can be burnt toproduce steam, which can turn aturbine and generator to produceelectricity, or produce heat.

<< Food waste can be treated and usedto produce methane. The end-productis a dry material that can be used as asafe fertiliser. This produces energyand saves on waste disposal costs.

<< Plant waste can also be burnt toproduce electricity, or it can be usedto produce gases, which can then beburnt to power a gas turbine or even acar engine.

At some stage in the process, most forms ofbio-energy involve combustion (i.e. burningsomething) and the release of carbon dioxideinto the atmosphere. However, this is offsetby energy crops, which absorb carbondioxide from the atmosphere. Even allowingfor carbon dioxide emissions in planting,harvesting, processing and transporting thefuel, replacing fossil fuel energy with bio-energy will typically reduce net carbondioxide emissions by over 90 per cent.

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How much does electricity from bio-energy cost? Currently, the cost of producing electricity in a coal-firedpower station can range from 2.5p to 3.2p per kWh, in anuclear power station 2.2p per kWh and bio-energy from apoultry-litter-fired power station 6.8p per kWh. However, aslarge-scale bio-energy production becomes more established,these production costs will fall.

Advantages

n The fuel is cheap.

n Using waste materials to generate energy makes goodenvironmental and economic sense.

Disadvantages

n Burning the fuel creates carbon dioxide, a greenhousegas. This can be balanced by growing energy crops,which absorb the same amount of carbon dioxide that iscreated by burning them.

n Collecting waste in big enough quantities can bedifficult, and the vehicles used may create moregreenhouse gases.

household wastedecays and producesmethane gas

steam turnsturbines

turbines rotatethe generator tomake electricity

water is turned intosteam by burning

methane gas

electric cables

to the National Grid

sy

gas wells collect methane

Using landfill gas to generate electricity

See www.dti.gov.uk/renewables/schools for a bio-energy case study

The UK produces 28 milliontonnes of household waste everyyear. Although we can use someof it to produce energy, weshould also do more to minimisewaste by recycling. Only 11 percent of the UK’s waste is used tomake electricity at the moment. Courtesy of NRE Slide Library/DTI

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Coal miners know that the deeper you dig, the hotter it gets.This is because of the heat that radiates from the Earth’smolten core. Geothermal energy is the natural heat that existswithin the Earth.

In some areas, heat from deep within the Earth can rise up tothe surface naturally, as hot liquids or gases. For example,water that is either already present deep underground (or hasflowed underground through cracks and fissures) can becomeheated and re-surface naturally as superheated steam or a hotspring. If these heated fluids do not rise to the surface,boreholes can be drilled to reach them.

A geothermal energy plant uses this heat.

How does geothermal energy work? The method for using geothermal energy is quite simple: oneor more boreholes are drilled into an underground reservoir;hot water then flows or is pumped to the surface and is usedin a steam turbine to generate electricity, or for direct heating.Places that are near a tectonic plate boundary (i.e. arevolcanic), like Iceland and Japan, have many places whereboreholes can reach reservoirs of water at high temperaturesand pressures.

Geothermal energywhat is geothermal energy?

Mammoth Hot Springs,Yellowstone, USA. Photo courtesy of www.pdphoto.org

Exploiting geothermal energy using Hot Dry Rock technology

power station

cool water ispumped down

the injection well

hot water returnsup the recoverywell

hot rocks (or geothermal hotwater reservoir)

cooling towers

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See www.dti.gov.uk/renewables/schools for a geothermal energy case study

Southampton Civic Centre usesheat from a geothermal well.Photo by kind permission of Southampton

City Council

Because of the cost of drilling deep boreholes, only the top 6km of the earth’s crust is generally considered for geothermalenergy projects.

We can also extract thermal energy from hot rocks wherewater is not naturally present. Hot Dry Rock (HDR) technologyinvolves finding a site where there are rocks at a suitabletemperature. The rocks are fractured and water is injected intoa borehole and circulated through the cracked rock severalkilometres below the surface. The water is heated throughcontact with the rock and is then returned to the surfacethrough another borehole, where it is used to generateelectricity. The water is then re-injected into the first boreholeto be reheated by the rocks and used again.

The amount of energy that can be extracted in using HDR ispotentially huge, with one cubic kilometre of rock providing theenergy equivalent of 70,000 tonnes of coal if cooled by 1°C.

heat exchanger

cold waterre-injectedinto reservoir

pump

hot water

cold water in

hot water out

Lower-temperature reservoirs may not besuitable for producing electricity, but caninstead be used for direct heating. Anexample is the Southampton GeothermalDistrict Heating Scheme, where water at70°C is pumped up from a depth of 1800mand provides heat to a number of buildingswithin 2km of the borehole.

How much does geothermalenergy cost? There are currently no geothermal powerstations in the United Kingdom producingelectricity for commercial use.

Advantages

n Geothermal energy doesn’t produce anypollution.

n Running costs for a geothermal power stationare very low.

Disadvantages

n It is difficult to find suitable sites forgeothermal power stations.

n The hot rocks have to be of a suitable typeand depth, and the rocks above them have tobe soft enough to drill through.

n If not carefully managed, a borehole can ‘runout of steam’ and may not be useable againfor several decades.

n Occasionally, dangerous gases and mineralscan come out of a borehole and it maydifficult to dispose of them. An extremeexample happened at a borehole drilled atKrafla in Iceland in 1977. Rising magmaentered the borehole at 1138m below theground and, within 20 minutes, 3 tonnes ofmagma had erupted through the hole.

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Hydroelectricitywhat is hydroelectricity?

Hydroelectricity is electricity generated by theflow of water. Like most other renewableenergy sources, it is actually indirect solarpower. Each year the Earth receives 1.5 billion TWh (terawatt hours) of solarenergy. Nearly a quarter of this is consumedin the evaporation of water. This means thatthe water vapour in the atmosphere isactually a massive store of solar energy.Unfortunately, only a fraction of this energy isavailable for us to use: when the watervapour in the atmosphere condenses, mostof the energy it holds is released into theatmosphere as heat. However, a tiny fraction,about 0.06 per cent, is held as gravitationalpotential energy by the rain that falls onmountains and hills. Hydroelectric power(HEP) stations can transfer this energy intoelectrical energy for use in homes andbusinesses.

Hydroelectricity is a well-establishedtechnology and is already a major contributorto world energy supplies. Hydroelectricityprovides about one sixth of the world’sannual electrical output and over 90 per centof the total global output from renewableenergy sources. In the UK it provides about0.8 per cent of the total electrical demandper year. It is also very efficient, with large-scale hydroelectric plants reaching energyconversion levels of 90 per cent.

Britain had over 1500MW (megawatts) ofHEP capacity in 2003, just under 1 per centof the UK’s total energy needs. More recentlyin the UK there have been more small-scaleHEP projects based on weirs or locks. Thesemay produce less than 1MW each, butBritain has hundreds of rivers, and if all thepotential sites were used it could produce10,000GWh of electricity per year.

How does hydroelectricitywork? A hydroelectric plant works uses water flowto turn a water wheel or turbine, which thenturns a generator, which produces electricity.Most commercial hydroelectric plants use adam in a river to create a powerful flow ofwater through a turbine. There are severalways in which this can be done.

n In a diversion scheme, water ischannelled from a dammed river or laketo a powerhouse containing the turbineand generator some distance away.

n River weirs and river and canal lockscan be used for smaller scale HEPschemes, generating up to 1MW each.

This picture shows water beingreleased under pressure, after ithas turned the turbines insidethe dam to generate electricity(Laggan Dam). Photo courtesy of Roy Dyckhoff

generator

water turnsturbine

tosubstation

dam

reservoir

sluice gate isopened to allowwater into intake

sy

Hydroelectric dam

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See www.dti.gov.uk/renewables/schools for a hydroelectricity case study

n In run of river schemes, the turbine and generator arelocated in or alongside a dam. For example, in the ElanValley in Wales, four dams hold 199 million tonnes ofwater. This is the main water supply for Birmingham. Inthe 1990s, turbines were fitted to the dam outlet pipes.When water is released, the turbines produce enoughelectricity to power nearly 11,000 homes.

n Pumped storage is not really a renewable energy sourcebut a way of storing energy. This system uses tworeservoirs, one above the turbine and one below. At timesof low demand, electricity powers the turbine to pumpwater from the lower reservoir back up to the higher one.This water can then be released when electricity isneeded. An example is the Dinorwig Power station inWales, where water from a mountain lake is used togenerate 288MW.

Large-scale hydroelectric plants can produce over 200 timesmore energy during their lifetimes than the energy needed tobuild and run them in the first place. This is ten times morethan oil-fired power stations.

How much doeshydroelectricity cost? Currently, the cost of producingelectricity in a coal-fired powerstation can range from 2.5p to3.2p per kWh, and in a nuclearpower station 2.2p per kWh.Electricity from a large-scalehydroelectric plant costs only0.016p per kWh to produce butcan rise to 0.07p per kWh formicro-hydro schemes.

Advantages

n Once construction iscompleted, operating costsare very low.

n No waste or pollution isproduced.

n The technology is veryreliable.

n Water can be stored behindthe dam to deal with peaks indemand.

n Power output can beincreased very quickly tomeet sudden demand.

n Electricity can be generatedconstantly.

Disadvantages

n Dams are very expensive tobuild.

n Large-scale flooding maycause environmentalproblems.

n Suitable sites are limited.

n HEP sites are often remote,which leads to higherdistribution costs.

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Hydrogen fuel cellswhat is a hydrogen fuel cell?

Hydrogen fuel cells use hydrogen gas to produce electricity. They areextremely efficient and can potentially be made in any size to power mobilephones, cars, or even to power large projects.

Until recently, they were difficult and costly to build and were only used invery specialised areas, such as on spacecraft and satellites.

Advances in technology in the last decade mean production costs have comedown and power output has greatly improved. However, if we were to usefuel cells for mass transport or other large scale applications, the costs wouldhave to come down a lot more.

How does a hydrogen fuel cell work? Water is a compound made from hydrogen and oxygen. If anelectric current is passed through water, it is possibleto split the water molecules back into separatehydrogen and oxygen molecules. Theoxygen and hydrogen are released asgas and can be collected in separatecontainers. This process is known aselectrolysis. A fuel cell is a devicethat uses electrolysis in reverse. Itconverts hydrogen and oxygen intowater, producing electricity and heatin the process.

Although there are a number ofdifferent kinds of fuel cells, theyall work in the same way andhave a similar basic set-up. Thisconsists of two electrodes separatedby an electrolyte.

The proton exchange membrane(PEM) cell is one of the more promising variations of fuel cell. The PEM cell’selectrolyte is a thin membrane made from Teflon. This membrane is treatedso that protons may pass through it, but it does not conduct electrons. Themembrane is sandwiched between the two electrodes. The fuel, hydrogen,enters at the anode and the oxidant, oxygen, enters at the cathode. Both theanode and cathode are coated with particles of platinum, which catalyses thereaction.

On the anode, hydrogen molecules split into protons and electrons. Theprotons travel through the membrane. This leaves the anode with a negativecharge and the cathode with a positive charge. This forces the electrons onthe anode to travel around an external circuit to reach the cathode, wherethey combine with the protons and oxygen to form water.

hydrogen

heat

water

oxygenfrom air

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See www.dti.gov.uk/renewables/schools for a fuel cell case study

This is all done without the need to burn the hydrogen.Energy is converted in a fuel cell much more efficiently thanin conventional power sources, such as internal combustionengines, and they produce almost no pollution.

Fuel cells also have no moving parts so they make no noiseand are less prone to wear and tear. Fuel cells, in themselves,are not renewable energy sources as they require hydrogenfuel. As the use of fuel cells increases and renewable energyfrom other sources becomes more available, it should bepossible to produce the hydrogen fuel directly fromrenewable sources, such as biomass. In Iceland there areplans to produce hydrogen using geothermal andhydroelectric power, which are both renewable sources ofenergy. The hydrogen will be used to power fuel cells to drivethe country's cars.

Advantages

n There’s no pollution: the onlywaste product is water andsome heat, some of whichcan also be reclaimed.

n There’s no noise.

n Fuel cells can be built in ahuge range of sizes to suitalmost any use.

Disadvantages

n Cost: fuel cells are moreexpensive than existingtechnologies.

n Fuel cells rely on hydrogen,which has to be made. Mostof the methods we use toproduce hydrogen today usenon-renewable energy,resulting in emissions of CO2

and pollutants.

n It will be some time beforehydrogen is as freelyavailable as petrol is now.

n Hydrogen, like petrol, isflammable and potentiallyexplosive, though systemscan be designed to allowsafe use.

London is one of several European cities testing how wellelectric buses powered by fuel cells work. The buses arequiet and produce almost no pollution. They are refuelledwith bottles of liquid hydrogen, which are stored on the roof of the bus.Photo courtesy of Matthew Wooll

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Solar energywhat is solar energy?Solar energy is energy from the sun. The sun is incrediblypowerful, delivering enough energy to the earth each minuteof the day to meet global demand for a whole year.

The sun actually gives us almost all the energy we use, eitherdirectly or indirectly. Fossil fuels release solar energycaptured by plants and trees millions of years ago. Even windenergy and tidal energy depend on the heat from the sun.

How does solar energy work? Solar energy can be captured and used in a number ofdifferent ways.

n Active solar water heating is a simple way of providing

hot water. A series of pipes run through a flat box with a

transparent cover and a black, insulated base. This is the

collector panel and it needs to be tilted to face the sun – in

Britain, it must face south. Water is pumped through the

pipes and heated by the sun. It is then circulated through a

heat exchanger to heat water for use in the home. It’s

surprisingly efficient, even in the chilly UK. Some systems

do not use a pump to circulate the water and rely on

convection currents to carry the hot water up to a storage

tank. These are known as thermosyphon systems.

transparent cover

black plate

insulation

hot water

hot water tank

heat exchanger

cold main supply

temperature sensorcontroller

pump

temperaturesensor

coldwaterin hot

waterout

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Active solar water heating can produce water

temperatures of 80–90°C because the glass on the front

of the collector panel allows visible light and short-wave

infrared radiation into the collector. This warms the

collector pipes and is re-radiated as long-wave infrared

radiation. However, long-wave infrared radiation cannot

pass back out through glass, so this thermal energy is

trapped in the collector and causes the temperature to

rise significantly. The same effect can be felt in a car on a

sunny day.

n Passive solar design involves making new buildings – or

adapting old ones – in a way that saves energy. An

energy-saving building keeps heat loss to a minimum by

using lots of insulation and a good design that allows the

sun’s heat to warm rooms directly. This can reduce

heating bills by up to 20 per cent.

n Solar thermal engines use mirrors to concentrate the

sun’s energy. Depending on the exact design of the

mirrors, temperatures in excess of 1500°C can be

reached. It is possible to use these systems to produce

steam for running a turbine and generator to produce

electricity.

n Photovoltaic cells (PV cells) use semi-conductors to

convert sunlight directly to electricity. PV cells can be

arranged in panels and can be fitted to almost anything,

even a backpack. Experimental cars have been built where

the body of the car is covered in PV cells which generate

electricity to power the vehicle.

How much does electricity from solarenergy cost? Currently, the cost of producing electricity in a coal-firedpower station can range from 2.5p to 3.2p per kWh, and in anuclear power station 2.2p per kWh. At present there are nosolar power stations in the UK producing electricity on acommercial basis. For individual users of PV systems, thecost of electricity has been calculated at 41–57p per kWh.This is due to the high cost of the initial installation. As themarket for this kind of technology expands, the cost willcome down and it is estimated that by 2020, PV users will bepaying about 10–16p per kWh.

Advantages

n Once the solar power plant isconstructed, running costsare very low.

n Solar energy can be used togenerate electricity in remoteplaces.

n No waste or pollution isproduced.

n Energy is usually generatedat or near where it will beused, keeping transmissionand distribution costs to aminimum.

Disadvantages

n It doesn’t work at night.

n Photovoltaic cells are veryexpensive.

See www.dti.gov.uk/renewables/schools for a solar energy case study

How does wind energy work? The force of the wind spins a large propeller(rotor). The propeller is connected to a generator:this is a machine that contains coils of wire andpowerful magnets. When the wire coils arespun quickly inside the magnets, theyproduce electricity.

To produce power, you need a wind speed of atleast four metres per second (around 16KPH).To reach maximum output, you need windspeeds of around 15 metres per second(53KPH). Wind speed increases as you go higher above ground level, so wind turbines are usually mounted on tall towers. Wind speedalso increases if there are no hills or buildings to slow it down. It is normally windier at seathan on land.

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Wind energywhat is wind energy?

Wind is created because the atmosphere is heated unevenlyby the sun – the equator gets more heat than Britain, and weget more heat than the Polar regions. These uneventemperatures cause convection currents where warm air risesand cooler air moves in to replace it. This movement of air isthe wind.

Thanks to its position on the edge of the continent, Britain isthe windiest place in Europe. We receive around 40 per centof the wind energy in the region.

Inside a wind turbine

shaft gear

generator

rotor

tower

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How much does electricityfrom wind energy cost? Since the 1980s, the technology has beenavailable to allow wind power to generatelarge enough amounts of electricity to makeit commercially viable. Wind energy is nowone of the most cost-effective methods ofelectricity generation available. Forcommercial wind power stations, largenumbers of turbines may be groupedtogether in wind farms.

There are over 1000 wind turbines at 89 sitesin Britain. There are offshore wind farms atBlythe, North Hoyle and Scroby Sands. Mostwind farms are located in the west of Britain,with the exception of Scroby Sands, off theeast coast near Great Yarmouth.

Currently, the cost of producing electricity ina coal-fired power station can range from2.5p to 3.2p per kWh, and in a nuclear powerstation 2.2p per kWh. Electricity from anonshore wind farm costs between 3.7p and5.4p per kWh, and from an offshore windfarm between 5.5p and 7.2p per kWh toproduce.

This offshore wind farm is at Blythe,England. Photo © E.ON UK

Novar Wind Farm, Ross-shire, ScotlandPhoto © npower renewables 2005

Advantages

n Once the wind turbine is constructed, runningcosts are very low.

n No waste or pollution is produced.

n The land occupied by a wind farm can still beused for farming.

n Wind farms can become tourist attractions.

Disadvantages

n No wind, no power

n Some people object because they feel windfarms spoil the view.

n Wind farms create a constant low-level noise.

n They can interfere with television receptionand radar.

See www.dti.gov.uk/renewables/schools for a wind energy case study

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Wave energywhat is wave energy?

Anyone who has seen the sea on a rough daywill realise the energy that it contains. For over200 years, people have been trying to harnessthis energy but it is only since the 1970s that ithas been possible to do this in any meaningfulway.

The Atlantic seaboard of the British Isles has oneof the best wave energy climates on the planet,with an energy potential of 60–70 kilowatts permetre (kW/m) in deep water off the WesternIsles, falling to 15–20kW at the shoreline.

How does wave energy work? There are many methods of getting energy fromwaves, but they fall into two main types.

Fixed devices are those that are fixed to the seabed or the seashore. Most of these are of theoscillating water column (OWC) type, such asthe LIMPET (Land Installed Marine PoweredEnergy Transformer) in Islay, Scotland. In itssimplest form, this is a tube set half in and halfout of the water. As waves go past it, the waterinside the column moves up and down, makingthe air in the column rush in and out. As the airmoves, it spins a specially designed turbine

LIMPET at Islay, Scotland Photo courtesy of Wavegen

trapped columnof air

sea wall

generator

turbine

The LIMPET

called a Wells turbine, which turns in the samedirection regardless of which way the airmoves so it can be turned by both incomingand retreating waves. This turbine is thenconnected to a generator, which produceselectricity.

The LIMPET was the world’s first large wavepower machine and can supply electricity toabout 350 homes.

wave direction

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See www.dti.gov.uk/renewables/schools for a wave energy case study

Floating devices or barrages are generally found offshore.There are currently a huge variety of different floating waveenergy conversion devices including the Duck, Clam andPelamis from the UK, the Whale and Backward Bent DuctBuoy (BBDB) from Japan, Floating Wave Power Vessels(FWPV) from Sweden, and Swan DK3 and Wave Dragon fromDenmark.

Pelamis is a wave-power project off the Scottish coast. It is aseries of giant metal tubes joined together with flexiblehinges. The movement of the waves causes sections of thesnake to move up and down. Each hinge is connected to apump, which pumps oil through a hydraulic motor as itmoves. The motor generates electricity as it spins.

Pelamis first began generating electricity in 2004. Its locationwas chosen for ideal waves, the depth of sea, and to avoidpassing ships. It is fixed to the sea bed by strong cables.Pelamis is Greek for ‘sea snake’.

How much does electricity from waveenergy cost? Currently, the cost of producing electricity in a coal-firedpower station can range from 2.5p to 3.2p per kWh, and in anuclear power station 2.2p per kWh. At present it is difficultto calculate a cost for wave-generated electricity as there isstill no overall agreement on the best type of generator to useand the best places to locate them. Best current estimatesshow electricity generated by wave devices costing 5p–7pper kWh.

Pelamis near Orkney, Scotland Photo courtesy of Ocean Power Delivery

Advantages

n Once constructed, runningcosts are very low.

n No waste or pollution isproduced.

n Wave-energy generators canproduce large amounts ofenergy.

Disadvantages

n Wave-energy generators areonly effective where they areexposed to strong waves.

n They can be noisy.

n Wave-energy generators maybe a hazard to ships andleisure craft.

Pelamis

mooring cable

generators areinside the body

wave

wave direction

sea bed

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Tidal energywhat is tidal energy?

Tidal energy uses the natural ebb and flow ofthe tides to generate electricity. Likegeothermal energy, tidal energy is not a formof indirect solar power. The action of thetides is largely due to the gravitationalinteraction between the earth and the moon.Around the coast of the UK, the sea levelrises and falls twice daily. Electricity can begenerated from tidal energy using barrages or dams in those places where the tidemoves in and out of a bay or inlet, and thedifference between high and low tide is atleast 5 metres. Today there is only one majortidal power station in operation in Europe.This is the 240MW (megawatt) barrage on LaRance river in France.

How does tidal energy work? n In a barrage scheme, a bay or inlet is

dammed with gates and turbines fittedalong the length of the dam. The gatesare opened to allow the tide to flow in,and turn the turbines, which turns thegenerators to produce electricity. At hightide the gates are closed and the water isheld until the tide level outside the gateshas dropped. The water is then released,running through the turbines and turningthe generators as it escapes. Soelectricity is generated by water flowingboth in and out of the dam.

The use of barrage-type tidal energyschemes is restricted by the number ofavailable sites. In the UK, about 30potential sites have been identified. Thebiggest of these would be the proposedSevern Barrage, which would span theRiver Severn. If such a scheme ever goesahead it would generate nearly five percent of the electricity consumed in the UKin 2002. As of 2004, there are no barrage-type schemes operating in the UK.

Tidal barrage at La Rance, France Photo courtesy of EDF Médiathèque / G. Halary

Tidal barrage

tide coming in

tide going out

sea estuary

sea estuary

turbine andgenerator

turbine andgenerator

road

road

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n A tidal stream generator is one alternative beingdeveloped, which avoids the damage to the environmentthat barrage schemes can cause. It works like anunderwater wind turbine. The tide-flow turns a propellerblade, which drives a turbine. These can be used in tideraces: places where the tide speeds up to 4–6 knots as itpasses through a narrow gap. Tide races such as theMenai Straits and Ramsey Strait off the Welsh coastwould be ideal locations for a seabed turbine, along withother places with strong tidal currents such asMorecambe Bay.

How much does electricity from tidalenergy cost? Currently, the cost of producing electricity in a coal-firedpower station can range from 2.5p to 3.2p per kWh, and in anuclear power station 2.2p per kWh. At present there are notidal power stations in the UK producing electricity on acommercial basis.

Initial estimates for the cost of electricity from a farm ofhydroplane-type devices (see case study on website)producing 5MW of electrical power are between 5p and 19pper kWh.

Advantages

n Running costs are very low.

n No waste or pollution is produced.

n The technology is very reliable.

n No fuel is required.

n The amount of electricity and the time it is produced istotally predictable.

Disadvantages

n There are only a few suitable places for tidal energyprojects.

n Tidal stream technology is at a very early stage ofdevelopment.

n Tidal schemes are expensive to install compared withother renewable energies.

n Damming bays or inlets can affect the environment overa large area.

n Barrage schemes will only provide energy for about 10hours each day, as the tide moves in and out.

Marine turbineImage courtesy of Marine Current Turbines

See www.dti.gov.uk/renewables/schools for a tidal energy case study