Electricity Without Carbon

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HydropowerThe world has a lot of dams 45,000 largeones, according to the World Energy Council,and many more at small scales. Its hydroelec-tric power plants have a generating capacityof 800 gigawatts, and they currently supplyalmost one-fifth of the electricity consumedworldwide. As a source of electricity,

dams are second only to fossil fuels,and generate 10 times more powerthan geothermal, solar and windpower combined. With a claimed fullcapacity of 18 gigawatts, the ThreeGorges dam in China can generatemore or less twice as much power asall the worlds solar cells. An addi-tional 120 gigawatts of capacity isunder development.

One reason for hydropowers suc-cess is that it is a widespread resource 160 countries use hydropowerto some extent. In several countries

hydropower is the largest contributorto grid electricity it is not uncom-mon in developing countries for a largedam to be the main generating source.Nevertheless, it is in large industrial-ized nations that have big rivers thathydroelectricity is shown in its mostdramatic aspect. Brazil, C anada,China, Russia and the United Statescurrently produce more than half ofthe worlds hydropower.

Cost: According to the InternationalHydropower Association (IHA),

installation costs are usually in therange of US$1 million to more than

ELECTRICITY WITHOUT CARBON

E

lectricity generation provides 18,000

terawatt-hours of energy a year, around 40%

of humanitys total energy use. In doing so itproduces more than 10 gigatonnes of carbon dioxide

every year, the largest sectoral contribution of

humanitys fossil-fuel derived emissions. Yet there

is a wide range of technologies from solar and

wind to nuclear and geothermal that can generate

electricity without net carbon emissions from fuel.

The easiest way to cut the carbon released by

electricity generation is to increase efficiency.

But there are limits to such gains, and there is the

familiar paradox that greater efficiency can lead

to greater consumption. So a global response to

climate change must involve a move to carbon-freesources of electricity. This requires fresh thinking

about the price of carbon, and in some cases new

technologies; it also means new transmission

systems and smarter grids. But above all, the various

sources of carbon-free generation need to be scaled

up to power an increasingly demanding world. In

this special feature,Natures News team looks at

how much carbon-free energy might ultimately be

available and which sources make most sense.

$5 million per megawatt of capacity, depend-ing on the site and size of the plant. Dams inlowlands and those with only a short dropbetween the water level and the turbine tendto be more expensive; large dams are cheaperper watt of capacity than small dams in similar

settings. Annual operating costs are low 0.82% of capital costs; electricity costs$0.030.10 per kilowatt-hour, which makesdams competitive with coal and gas.

Capacity: The absolute limit on hydropoweris the rate at which water flows downhillthrough the worlds rivers, turning potential

energy into kinetic energy as it goes.

The amount of power that could theo-retically be generated if all the worldsrun-off were turbined down to sea levelis more than 10 terawatts. However, itis rare for 50% of a rivers power to beexploitable, and in many cases the figureis below 30%.

Those figures still offer considerableopportunity for new capacity, accord-ing to the IHA. Europe currently sets abenchmark for hydropower use, with75% of what is deemed feasible alreadyexploited. For Africa to reach the samelevel, it would need to increase its hydro-

power capacity by a factor of 10to morethan 100 gigawatts. Asia, which alreadyhas the greatest installed capacity, also hasthe greatest growth potential. If it wereto triple its generating capacity, thusharnessing a near-European fraction ofits potential, it would double the worldsoverall hydroelectric capacity. The IHAsays that capacity could triple worldwidewith enough investment.

Advantages: The fact that hydro-electric systems require no fuel meansthat they also require no fuel-extracting

infrastructure and no fuel transport. Thismeans that a gigawatt of hydropower

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saves the world not just a gigawatts worth ofcoal burned at a fossil-fuel plant, but also thecarbon costs of mining and transporting that

coal. As turning on a tap is easy, dams canrespond almost instantaneously to changingelectricity demand independent of the time ofday or the weather. This ease of turn-on makesthem a useful back-up to less reliable renewablesources. That said, variations in use accordingto need and season mean that dams produceabout half of their rated power capacity.

Hydroelectric systems are unique amonggenerating systems in that they can, if cor-rectly engineered, store the energy gener-ated elsewhere, pumping water uphill whenenergy is abundant. The reservoirs they createcan also provide water for irrigation, a way to

control floods and create amenities for recrea-tional use.

Disadvantages: Not all regions have largehydropower resources the Middle East, forexample, is relatively deficient. And reservoirstake up a lot of space; today the area under man-made lakes is as large as two Italys. The largedams and reservoirs that account for most ofthat area and for more than 90% of hydro-gen-erated electricity worldwide require lengthy andcostly planning and construction, as well as therelocation of people from the reservoir area. Inthe past few decades, millions of people have

been relocated in India and China. Dams haveecological effects on the ecosystems upstreamand downstream, and present a barrier tomigrating fish. Sediment build-up can shortentheir operating life, and sediment trapped by thedam is denied to those downstream. Biomassthat decomposes in reservoirs releases methaneand carbon dioxide, and in some cases theseemissions can be of a similar order of magni-tude to those avoided by not burning fossil fuels.Climate change could itself limit the capacity ofdams in some areas by altering the amount andpattern of annual run-off from sources such asthe glaciers of Tibet.

Because hydro is a mature technology, thereis little room for improvement in the efficiencyof generation. Also, the more obvious and easylocations have been used, and so the remain-ing potential can be expected to be harder toexploit. Small (less than 10 megawatts) run-of-river schemes that produce power from thenatural flow of water as millers have beendoing for four millennia are appealing, asthey have naturally lower impacts. However,they are about five times more expensive andharder to scale than larger schemes.

Verdict: A cheap and mature technology, but

with substantial environmental costs; roughlya terawatt of capacity could be added.

Nuclear fissionWhen reactor 4 at the Cher-

nobyl nuclear power plantin Ukraine melted down on26 April 1986, the falloutcontaminated large parts ofEurope. That disaster, andthe earlier incident at ThreeMile Island in Pennsylvania,blighted the nuclear industryin the West for a generation.Worldwide, though, the pic-ture did not change quite asdramatically.

In 2007, 35 nuclear plants wereunder construction, almost all in

Asia. The 439 reactors already in opera-tion had an overall capacity of 370 gigawatts,and contributed around 15% of the electricitygenerated worldwide, according to the mostrecent figures from the International AtomicEnergy Agency (IAEA), which serves as theworlds nuclear inspectorate.

Costs: Depending on the design of the reac-tor, the site requirements and the rate of capitaldepreciation, the light-water reactors that makeup most of the worlds nuclear capacity pro-duce electricity at costs of between US$0.025and $0.07 per kilowatt-hour. The technology

that makes this possible has benefited fromdecades of expensive research, developmentand purchases subsidized by governments;without that boost it is hard to imagine thatnuclear power would currently be in use.

Capacity: Because nuclear power requiresfuel, it is constrained by fuel stocks. There aresome 5.5 million tonnes of uranium in knownreserves that could profitably be extracted at acost of US$130 per kilogram or less, according

to the latest edition of the Red Book, in whichthe IAEA and the Organisation for EconomicCo-operation and Development (OECD)assess uranium resources. At the current use of66,500 tonnes per year, that is about 80 yearsworth of fuel. The current price of uranium isover that $130 threshold.

Geologically similar ore deposits that areas yet unproven undiscovered reserves are thought to amount to roughly double

the proven reserves, and lower-grade oresoffer considerably more. Uranium is nota particularly rare element it is about ascommon a constituent of Earths crust as zinc.Estimates of the ultimate recoverable resourcevary greatly, but 35 million tonnes might beconsidered available. Nor is uranium the onlynaturally occurring element that can be madeinto nuclear fuel. Although they have not yetbeen developed, thorium-fuelled reactorsare a possibility; bringing thorium into play

In 2005, 18,000 terawatt-hours of electricity weregenerated. With almost 9,000hours in a year, that averagesout at a constant 2 TW orso. Generating capacity is alot higher than that, becausethere are peaks and troughsand no plants operate at theirfull output all of the time.

No analogy makes it easyto picture a terawatt. Athousandth of a terawatt,a gigawatt, is more

comprehensible. It is the

output of a fairly large powerstation: Sizewell B, one ofBritains largest nuclear powerstations, has an output ofabout 1.2 GW; the HooverDam on the Colorado Rivercan produce about 1.8 GW.

A megawatt is a thousandthof a gigawatt. It takes 35 MWto power most modern trains(or, if you feel flash, you canthink of one as the power oftwo Formula One cars). Akilowatt is easily thought of as

an electric fan heater.

Domestic energyconsumption is measuredin kilowatt-hours. In 2004,the highest per capita useof electricity was in Iceland,where it reached 28,200 kWhper year. In the United Statesit is about 13,300 kWh ayear; 300 million Americansthus use about 400 GW ofpower. In Chile the per capitalevel is 3,100 kWh, in China1,600 kWh, in India 460 kWh.The lowest level, in Haiti, is

30 kWh.

By the numbers

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to the extent that these assumptions proveviable, electricity is not the only potential usefor such plantations. By storing solar energy in

the form of chemical bonds, biomass lends itselfbetter than other renewable energy resourcesto the production of fuel for transportation (seepage 841). Although turning biomass to biofuelis not as efficient as just burning the stuff, itcan produce a higher-value product. Biofuelsmight easily beat electricity generation as a usefor biomass in most settings.

Advantages: Plants are by nature carbon-neutral and renewable, although agriculturedoes use up resources, especially if it requireslarge amounts of fertilizer. The technologiesneeded to burn biomass are mature and effi-

cient, especially in the case of co-generation.Small systems using crop residues can mini-mize transportation costs.

If burned in power plants fitted with carbon-capture-and-storage hardware, biomass goesfrom being carbon neutral to carbon negative,effectively sucking carbon dioxide out of theatmosphere and storing it in the ground (seeCarbon capture and storage, page 822). Thismakes it the only energy technology that canactually reduce carbon dioxide levels in theatmosphere. As with coal, however, there arecosts involved in carbon capture, both in termsof capital set-up and in terms of efficiency.

Disadvantages: There is only so much landin the world, and much of it will be needed toprovide food for the growing global population.It is not clear whether letting market mecha-nisms drive the allocation of land between fueland food is desirable or politically feasible.Changing climate could itself alter the availabil-ity of suitable land. There is likely to be oppo-sition to increased and increasingly intensecultivation of energy crops. Use of waste andresidues may remove carbon from the land thatwould otherwise have enriched the soil; long-term sustainability may not be achievable.

Bioenergy dependence could also open thedoors to energy crises caused by drought orpestilence, and land-use changes can haveclimate effects of their own: clearing land forenergy crops may produce emissions at a ratethe crops themselves are hard put to offset.

Verdict: If a large increase in energy cropsproves acceptable and sustainable, much of itmay be used up in the fuel sector. However,small-scale systems may be desirable in anincreasing number of settings, and the pos-sibility of carbon-negative systems whichare plausible for electricity generation but

not for biofuels is a unique and attractivecapability.

WindWind power is expanding faster than evenits fiercest advocates could have wisheda few years ago. The United States added5.3 gigawatts of wind capacity in 2007 35%of the countrys new generating capacity and has another 225 gigawatts in the planningstages. There is more wind-generating capac-

ity being planned in the United States thanfor coal and gas plants combined. Globally,capacity has risen by nearly 25% in each of thepast five years, according to the Global WindEnergy Council.

Wind Power Monthly estimates that theworlds installed capacity for wind as of January2008 was 94 gigawatts. If growth continued at21%, that figure would triple over six years.

Despite this, the numbers remain small ona global scale, especially given that wind farmshave historically generated just 20% of theircapacity.

Costs: Installation costs for wind powerare around US$1.8 million per megawattfor onshore developments and between $2.4million and $3 million for offshore projects.That translates to $0.050.09 per kilowatt-hour, making wind competitive with coal atthe lower end of the range. With subsidies, asenjoyed in many countries, the costs come inwell below those for coal hence the boom.The main limit on wind-power installation atthe moment is how fast manufacturers canmake turbines.

These costs represent significant improve-ments in the technology. In 1981, a wind farm

might have consisted of an array of 50-kilowattturbines that produced power for roughly

$0.40 per kilowatt-hour. Todays turbines canproduce 30 times as much power at one-fifththe price with much less down time.

Capacity: The amount of energy generatedby the movement of Earths atmosphere is vast hundreds of terawatts. In a 2005 paper, a pair

of researchers from Stanford University calcu-lated that at least 72 terawatts could be effec-tively generated using 2.5 million of todayslarger turbines placed at the 13% of locationsaround the world that have wind speeds of atleast 6.9 metres per second and are thus prac-tical sites (C. L. Archer and M. Z. JacobsonJ. Geophys. Res. 110, D12110; 2005).

Advantages: The main advantage of windis that, like hydropower, it doesnt need fuel.The only costs therefore come from buildingand maintaining the turbines and power lines.Turbines are getting bigger and more reliable.

The development of technologies for capturingwind at high altitudes could provide sourceswith small footprints capable of generatingpower in a much more sustained way.

Disadvantages: Winds ultimate limita-tion might be its intermittency. Providing upto 20% of a grids capacity from wind is nottoo difficult. Beyond that, utilities and gridoperators need to take extra steps to deal withthe variability. Another grid issue, and one thatis definitely limiting in the near term, is thatthe windiest places are seldom the most popu-lous, and so electricity from the wind needs

infrastructure development especially foroffshore settings.

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As well as being intermittent, wind power is,like other renewable energy sources, inherentlyquite low density. A large wind farm typicallygenerates a few watts per square metre 10 isvery high. Wind power thus depends on cheapland, or on land being used for other things atthe same time, or both. It is also hard to deploy

in an area where the population sets great storeby the value of a turbine-free landscape.

Wind power is also unequally distributed: itfavours nations with access to windy seas andtheir onshore breezes or great empty plains.Germany has covered much of its windiest landwith turbines, but despite these pioneering

efforts, its combined capacity of 22 GW sup-plies less than 7% of the countrys electricityneeds. Britain, which has been much slower

to adopt wind power, has by far the largest off-shore potential in Europe enough to meetits electricity needs three times over, accordingto the British Wind Energy Association. Indus-try estimates suggest that the European Unioncould meet 25% of its current electricity needsby developing less than 5% of the North Sea.

Such truly large-scale deployment of wind-power schemes could affect local, and poten-tially global, climate by altering wind patterns,according to research by David Keith, head ofthe Energy and Environmental Systems Groupat the University of Calgary in Canada. Windtends to cool things down, so temperatures

around a very large wind farm could rise as tur-bines slow the wind to extract its energy. Keithand his team suggest that 2 TW of wind capacitycould affect temperatures by about 0.5 C, withwarming at mid-latitudes and cooling at thepoles perhaps in that respect offsetting theeffect of global warming (D. W. Keithet al. Proc.Natl Acad. Sci. USA101, 1611516120; 2004).

Verdict: With large deployments on theplains of the United States and China, andcheaper access to offshore, a wind-powercapacity of a terawatt or more is plausible.

Geothermal

Earths interior contains vast amounts of heat,some of it left over from the planets originalcoalescence, some of it generated by the decayof radioactive elements. Because rock conductsheat poorly, the rate at which this heat flowsto the surface is very slow; if it were quicker,Earths core would have frozen and its conti-nents ceased to drift long ago.

The slow flow of Earths heat makes it a hardresource to use for electricity generation exceptin a few specific places, such as those with

abundant hot springs. Only a couple of dozencountries produce geothermal electricity, andonly five of those Costa Rica, El Salvador,Iceland, Kenya and the Philippines generatemore than 15% of their electricity this way. Theworlds installed geothermal electricity capac-ity is about 10 gigawatts, and is growing onlyslowly about 3% per year in the first half ofthis decade. A decade ago, geothermal capacitywas greater than wind capacity; now it is almosta factor of ten less.

Earths heat can also be used directly. Indeed,small geothermal heat pumps that warmhouses and businesses directly may represent

the greatest contribution that Earths warmthcan make to the worlds energy budget.

Costs: The cost of a geothermal systemdepends on the geological setting. JeffersonTester, a chemical engineer who was part of ateam that produced an influential Massachu-setts Institute of Technology (MIT) report ongeothermal technology in 2006, explains thesituation as being similar to mineral resources.There is a continuum of resource grades from shallow, high-temperature regionsof high-porosity rock, to deeper low-porosityregions that are more challenging to exploit.

That report put the cost of exploiting the bestsites those with a lot of hot water circulat-ing close to the surface at about US$0.05per kilowatt-hour. Much more abundant low-grade resources are exploitable with currenttechnology only at much higher prices.

Absolute capacity: Earth loses heat atbetween 40 TW and 50 TW a year, whichworks out at an average of a bit less than atenth of a watt per square metre. For compari-son, sunlight comes in at an average of 200watts per square metre. With todays technol-ogy, 70 GW of the global heat flux is seen as

exploitable. With more advanced technolo-gies, at least twice that could be used. The MIT

Average power of the worlds winds during the boreal winter (top) and summer. The recoupable energy

is some two orders of magnitude lower because of turbine spacing and engineering constraints.

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study suggested that using enhanced systemsthat inject water at depth using sophisticateddrilling systems, it would be possible to set up

100 GW of geothermal electricity in the UnitedStates alone. With similar assumptions a glo-bal figure of a terawatt or so can be reached,suggesting that geothermal could, with a greatdeal of investment, provide as much electricityas dams do today.

Advantages: Geothermal resources requireno fuel. They are ideally suited to supplyingbase-load electricity, because they are drivenby a very regular energy supply. At 75%,geothermal sources boast a higher capacityfactor than any other renewable. Low-gradeheat left over after generation can be used for

domestic heating or for industrial processes.Surveying and drilling previously unexploited

geothermal resources has become much easierthanks to mapping technology and drillingequipment designed by the oil industry. A sig-nificant technology development programme Tester suggests $1 billion over 10 years could greatly expand the achievable capacity aslower-grade resources are opened up.

Disadvantages: High-grade resources arequite rare, and even low-grade resources arenot evenly distributed. Carbon dioxide can leakout of some geothermal fields, and there can

be contamination issues; the water that bringsthe heat to the surface can carry compoundsthat shouldnt be released into aquifers. In dryregions, water availability can be a constraint.Large-scale exploitation requires technologiesthat, although plausible, have not been demon-strated in the form of robust, working systems.

Verdict: Capacity might be increased by morethan an order of magnitude. Without spec-tacular improvements, it is unlikely to outstriphydro and wind and reach a terawatt.

SolarNot to take anything away from the miracle ofphotosynthesis, but even under the best condi-tions plants can only turn about 1% of the solarradiation that hits their surfaces into energythat anyone else can use. For comparison, astandard commercial solar photovoltaic panel

can convert 1218% of the energy of sunlightinto useable electricity; high-end models comein above 20% efficiency. Increasing manufac-turing capacity and decreasing costs have ledto remarkable growth in the industry over thepast five years: in 2002, 550 MWof cells wereshipped worldwide; in 2007 the figure was sixtimes that. Total installed solar-cell capacity isestimated at 9 GW or so. The actual amount ofelectricity generated, though, is considerablyless, as night and clouds decrease the poweravailable. Of all renewables, solar currently hasthe lowest capacity factor, at about 14%.

Solar cells are not the only technology by

which sunlight can be turned into electricity.Concentrated solar thermal systems use mir-rors to focus the Suns heat, typically heatingup a working fluid that in turn drives a turbine.The mirrors can be set in troughs, in parabolasthat track the Sun, or in arrays that focus theheat on a central tower. As yet, the installedcapacity is quite small, and the technology willalways remain limited to places where there area lot of cloud-free days it needs direct sun,whereas photovoltaics can make do with morediffuse light.

Costs: The manufacturing cost of solar cells

is currently US$1.502.50 for a watts worthof generating capacity,and prices are in the

$2.503.50 per watt range. Installation costsare extra; the price of a full system is normallyabout twice the price of the cells. What thismeans in terms of cost per kilowatt-hour overthe life of an installation varies according tothe location, but it comes out at around $0.25

0.40. Manufacturing costs are dropping, andinstallation costs will also fall as photovoltaiccells integrated into building materials replacefree-standing panels for domestic applications.Current technologies should be manufactur-ing at less than $1 per watt within a few years(see Nature454, 558559; 2008).

The cost per kilowatt-hour of concentratedsolar thermal power is estimated by the USNational Renewable Energy Laboratory(NREL) in Golden, Colorado, at about $0.17.

Capacity: Earth receives about 100,000 TWof solar power at its surface enough energy

every hour to supply humanitys energy needsfor a year. There are parts of the Sahara Desert,the Gobi Desert in central Asia, the Atacamain Peru or the Great Basin in the United Stateswhere a gigawatt of electricity could be gener-ated using todays photovoltaic cells in an array 7or 8 kilometres across. Theoretically, the worldsentire primary energy needs could be served byless than a tenth of the area of the Sahara.

Advocates of solar cells point to a calcula-tion by the NREL claiming that solar panelson all usable residential and commercial roofsurfaces could provide the United States withas much electricity per annum as the country

used in 2004. In more temperate climes thingsare not so promising: in Britain one might

Fusion power could meet all Earths energyneeds. It just requires two heavy isotopes ofhydrogen and the technology to use them.The reactors would produce some low-levelradioactive waste, but only a minor amountcompared with nuclear fission. The problemis the necessary technology commercialreactors are unlikely before the 2040s.

Another far-off dream is the space-basedsolar power satellite. In orbit, solar panelscould soak up sunshine 24/7, beaming it toEarth as microwaves. This requires reallycheap space travel to lift thousands of tonnesof solar cells into orbit. At the moment,

unfortunately, space travel is really expensive.

Farther out

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expect an annual insolation of about 1,000 kilo-watt-hours per metre on a south-facing paneltilted to take account of latitude: at 10% effi-

ciency, that means more than 60 square metresper person would be needed to meet currentUK electricity consumption.

Advantages: The Sun represents an effec-tively unlimited supply of fuel at no cost, whichis widely distributed and leaves no residue. Thepublic accepts solar technology and in mostplaces approves of it it is subject to less geo-political, environmental and aesthetic concernthan nuclear, wind or hydro, although extremelylarge desert installations might elicit protests.

Photovoltaics can often be installed piece-meal house by house and business by busi-

ness. In these settings, the cost of generationhas to compete with the retail price of electric-ity, rather than the cost of generating it by othermeans, which gives solar a considerable boost.The technology is also obviously well suited tooff-grid generationand thus to areas withoutwell developed infrastructure.

Both photovoltaic and concentrated solarthermal technologies have clear room forimprovement. It is not unreasonable to imag-

ine that in a decade or two new technologiescould lower the cost per watt for photovolta-ics by a factor of ten, something that is almostunimaginable for any other non-carbon elec-tricity source.

Disadvantages: The ultimate limitationon solar power is darkness. Solar cells do notgenerate electricity at night, and in places withfrequent and extensive cloud cover, generationfluctuates unpredictably during the day. Someconcentrated solar thermal systems get aroundthis by storing up heat during the day for useat night (molten salt is one possible storage

medium), which is one of the reasons theymight be preferred over photovoltaics for largeinstallations. Another possibility is distributedstorage, perhaps in the batteries of electric andhybrid cars (see page 810).

Another problem is that large installationswill usually be in deserts, and so the distribution

of the electricity generated will pose problems.A 2006 study by the German Aerospace Centreproposed that by 2050 Europe could be import-

ing 100 GW from an assortment of photovoltaicand solar thermal plants across the Middle Eastand North Africa. But the report also noted thatthis would require new direct-current high-voltage electricity distribution systems.

A possible drawback of some advancedphotovoltaic cells is that they use rare elementsthat might be subject to increases in cost andrestriction in supply. It is not clear, however,whether any of these elements is either trulyconstrained more reserves might be madeeconomically viable if demand were higher or irreplaceable.

Verdict: In the middle to long run, the sizeof the resource and the potential for furthertechnological development make it hard notto see solar power as the most promising car-bon-free technology. But without significantlyenhanced storage options it cannot solve theproblem in its entirety.

An alternative to renouncing fossilfuels is not to release their CO2 intothe atmosphere. Carbon capture

and storage (CCS) technologystrips CO2 out of exhaust gasesand stores it underground. Thetechnology could reduce carbonemissions from power stations by8090%, although after takinglife-cycle factors into account, thatnumber could drop to as little as67%. Estimates of the extra costof CCS vary widely depending ontechnology and location, but itcould add US$0.010.05 to thecost of a kilowatt-hour. On coal-fired power plants the technologycould be competitive if CO2 were

priced at around $50 per tonne.Part of the extra cost of CCS

is the capital invested in newplant; part is due to decreasedefficiency because of the energycosts of removing the carbon.For a conventional coal plant, theefficiency loss could be as much as40%. In more modern integratedgasification combined-cycle (IGCC)power plants, the capital costs ofwhich are higher, the gasificationstep produces a CO2 stream thatis more easily handled. CCS thusreduces the efficiency of IGCC

plants by less than 20% and their

efficiency is higher to start with. Asyet there are very few IGCC plants,but the possibility of carbon taxes,

or indeed more expensive coal, maytip the market their way.Although early implementations

of CCS will probably concentrate onpumping CO2 into depleted oil fields(where it is used already to helpextract the dregs), the technologyis likely ultimately to be targetedat saline aquifers, which representby far the largest CO2 storagecapacity. Estimates of global aquifercapacity range from 2,000 Gt CO2to nearly 11,000 Gt CO2, althoughthis resource is not evenlydistributed around the world. The

Global Energy Technology StrategyProgram, led by researchers at theUniversity of Maryland in CollegePark, estimates that the 8,100 majorfacilities worldwide that might becandidates for CCS currently emitabout 15 Gt CO2 annually. Aquiferscould thus offer centuries of storageat current levels of CO2, and alsoallow the use of coal to continuewhile work progresses on makinga less dirty baseload technologypossible.

The task is enormous, and seriousindustrial proof-of-concept studies

of the feasibility of CCS have barely

Carbon capture and storage

begun. The probability of CCSbeing widespread in 10 or even20 years is very low unless thetechnology is promoted much moreaggressively. The biggest problemis scale. Capturing 60% of the CO2from US coal-fired power stations

would mean handling a volume of

CO2 daily that rivals the 20 millionbarrels of oil moved around bythe oil industry, according to a2007 Massachusetts Institute ofTechnology study. Creating such aninfrastructure is not impossible, butsetting it up in a decade or two is a

tall order.

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Ocean energy

The oceans offer two sorts of available kinetic

energy that of the tides and that of thewaves. Neither currently makes a significantcontribution to world electricity generation,but this has not stopped enthusiasts fromdeveloping schemes to make use of them.There are undoubtedly some places where,thanks to peculiarities of geography, tidesoffer a powerful resource. In some situationsthat potential would best be harnessed bya barrage that creates a reservoir not unlikethat of a hydroelectric dam, except that it isrefilled regularly by the pull of the Moon andthe Sun, rather than being topped up slowlyby the runoff of falling rain. But although

there are various schemes for tidal barragesunder discussion most notably the SevernBarrage between England and Wales, whichproponents claim could offer as much as 8 GW the plant on the Rance estuary in Brittany,rated at 240 MW, remains the worlds largesttidal-power plant more than 40 years after itcame into use.

There are also locations well suited to tidal-stream systems submerged turbines that spinin the flowing tide like windmills in the air. The1.2 MW turbine installed this summer in themouth of Strangford Lough, Northern Ireland,is the largest such system so far installed.

Most technologies for capturing wave powerremain firmly in the testing phase. Individualcompanies are working through an array ofpotential designs, including machines thatundulate on waves like a snake, bob up anddown as water passes over them, or nestle onthe coastline to be regularly overtopped bywaves that power turbines as the water drainsoff. The European Marine Energy Centres testbed off the United Kingdoms Orkney Islands,where manufacturers can hook up prototypesto a marine electricity grid and test how wellthey withstand the pounding waves, is a lead-ing centre of research. Pelamis Wave Power, a

company based in Edinburgh, UK, for instance,has moved from testing there to installingthree machines off the coast of Portugal, whichtogether will eventually generate 2.25 MW.

Costs: Barrage costs differ markedly from siteto site, but are broadly comparable to costs forhydropower. At an estimated cost of 15 billion(US$30 billion) or more, the capital costs ofthe Severn Barrage would be about $4 millionper megawatt. A 2006 report from the BritishCarbon Trust, which spurs investment in non-carbon energy, puts the costs of tidal-streamelectricity in the $0.200.40 per kilowatt-hour

range, with wave systems running up to $0.90per kilowatt-hour. Neither technology is any-

where close to the large-scale productionneeded to significantly drive such costs down.

Capacity: The interaction of Earths masswith the gravitational fields of the Moon andthe Sun is estimated to produce about 3 TWof tidal energy rather modest for such an

astronomical source (although enough to playa key role in keeping the oceans mixed seeNature447, 522524; 2007). Of this, perhaps1 TW is in shallow enough waters to be easilyexploited, and only a small part of that is realis-tically available. EDF, a French power companydeveloping tidal power off Brittany, says thatthe tidal-stream potential off France is 80% ofthat available all round Europe, and yet it is stilllittle more than a gigawatt.

The power of ocean waves is estimated atmore than 100 TW. The European OceanEnergy Association estimates that the acces-sible global resource is between 1 and 10

terawatts, but sees much less than that as recov-erable with current technologies. An analysis intheMRS Bulletin in April 2008 holds that about2% of the worlds coastline has waves with anenergy density of 30 kW m1, which wouldoffer a technical potential of about 500 GWfor devices working at 40% efficiency. Thuseven with a huge amount of development, wavepower would be unlikely to get close to the cur-rent installed hydroelectric capacity.

Advantages: Tides are eminently predict-able, and in some places barrages really dooffer the potential for large-scale generation

that would be significant on a countrywidescale. Barrages also offer some built-in stor-

age potential. Waves are not constant butthey are more reliable than winds.

Disadvantages: The available resource var-ies wildly with geography; not every countryhas a coastline, and not every coastline hasstrong tides or tidal streams, or particularly

impressive waves. The particularly hot wavesites include Australias west coast, SouthAfrica, the western coast of North Americaand western European coastlines. Buildingturbines that can survive for decades at seain violent conditions is tough. Barrages haveenvironmental impacts, typically flooding pre-viously intertidal wetlands, and wave systemsthat flank long stretches of dramatic coastlinemight be hard for the public to accept. Tidesand waves tend by their nature to be found atthe far end of electricity grids, so bringing backthe energy represents an extra difficulty. Surf-ers have also been known to object

Verdict: Marginal on the global scale. I

Reported and written by Quirin Schiermeier,

Jeff Tollefson, Tony Scully, Alexandra Witze

and Oliver Morton.

See Editorial, page 805.

SOURCE MATERIAL AND FURTHER READINGKey World Energy Statistics 2007(International Energy Agency,2007).Hohmeyer, O. & Trittin , T. (eds) Proc. IPCC Scoping Meetingon Renewable Energy Sources 2025 January 2008, Lbeck,Germany (Intergovernmental Panel on Climate Change, 2008).Smil, V. Energy in Nature and Society: General Energetics ofComplex Systems (MIT Press, 2008).Metz, B., Davidson, O., Bosch, P., Dave, R. & Meyer, L. (eds)

Climate Change 2007: Mitigation of Climate Change (CambridgeUniv. Press, 2007).

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