Solar_Energy.pdf

ENERGY AND ENVIRONMENT

S I solar energy technology will have a ben-

eficial impact on air pollution and glob-

.0 ar al climatic change. In developing coun-

tries, it can alleviate the environmental

...damage caused by the often inefficient

Technology "VIII a 110 "V radIatIon practice of burning plant material for

cooking and heating. Advanced solar

(140m the sun to provide nonpolluting technologies have the potential to use

less land than does biomass cultivation:

and cheap fuels, as well as electricity photosynthesis typically captures less

than 1 percent of the available sunlight,

but modern solar technologies can, at

by William Hoagland least in the laboratory, achieve efficien-

cies of 20 to 30 percent. With such effi-

ciencies, the u.s. could meet its current

.demand for energy by devoting less

E very year the earth's surface re- France obtained the first pat~nt for a than 2 percent of its land area to ener-

ceives about 10 times as much en- solar-powered motor. Other. pioneers collection.

ergy from sunlight as is contained. also investigated using the sun's ener- 5It is uluikely that a single solar tech-

in all the known reserves of coal, oil, gy, but the convenience of coal and oil nology will predominate. Regional van- i-

natural gas and uranium combined. This was overwhelming. As a result, solar ations in economics and the availability ..

energy equals 15,000 times the world's power was mostly forgotten until the of sunlight will naturally favor some ap-

annual consumption by humans. Peo- energy crisis of the 1970s threatened proaches over others. Electricity may

pIe have been burning wood and other many major economies. be generated by burning biomass, erect-

forms of biomass for thousands of :3 Economic groWth depends on energy ing wind turbines, building solar-pow-

years, and that is one way of tapping use. By 2025 the worldwide demand ered heat engines, laying out photovol-

solar energy. But the sun also provides for fuel is projected to increase by 30 taic cells or harnessing the energy in

hydropower, wind power and fossil fu- percent and that for electricity by 265 rivers with dams. Hydrogen fuel can be:'~:'.{.tf;j,

~,--.els-in fact, all forms of energy other percent. Even with more efficient use produced by electrochemical cells or bi':~-:-;:,:

~?~1;.;~~r;~; ':;;; nuclear, geothermal and tidal. and conservati,;>D, new sources of:~er~1 olo~ical processes-involving microor;'",~:..~~;;~~,~~

.'~.',,;:Attempts to coll~~",*edire~t,~nergy, gy will be reqwred. Solar energyco~4~~,.gamsms or enz~~!'::-that are driven;,,~, .~~. of the sun are not n~h1:;18g1!a:mathe- provide 60 percent of the electridty,ijI by sunlight. -." -.." .-

".::~: nam~a"Afi~gtlii'~Ber'": -methanol may-!.. ..C mass or other solar

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DIVERSE DEVICES aid in capturing solarenergy. Wind turbines (a) draw out theenergy stored in the atmosphere throughdifferential heating by the sun. A solarfurnace (b) uses radiation reflected ontoa central tower to drive an engine. Solarpanels (cand background) employpho-tovoltaic celIs to create electricity. Andcrops;; such. as sugarcane (d) tap sun-light b~'photo'synthesis..' "..

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oughly O.2S:percent of the sun'senergy reaching the'Iower atmo-

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(, Solar energy also exists in the oceans crops or from wood (by converting cel- nificant source of energy. By one esti-as waves and gradients of temperature lulose), are fermented. mate, 80 percent of the electrical con-and salinity, and they, too, are poten- <1 Alcohols are now being blended with sumption in the U.S. could be met bytial reservoirs to tap. Unfortunately, al- gasoline to enhance the efficiency of the wind energy of North and Souththough the energy stored is enormous, combustion in car engines and to re- Dakota alone. The early problems sur-it is diffuse and expensive to extract. duce harmful tail-pipe emissions. But rounding the reliability of "wind farms"

.ethanol can be an effective fuel in its have now been by and large resolved,Growing Energy own right, as researchers in Brazil have and in certain locations the electricity

demonstrated. It may be cost-competi- produced is already cost-competitive1- A gricultural or industrial wastes such tive with gasoline by 2000. In the future, with conventional generation.

1-\ as wood chips can be burned to biomass plantations could allow such tZIn areas of strong wind-an averagegenerate steam for turbines. Such facil- energy to be "grown" on degraded land of more than 7.5 meters per second-ities are competitive with conventional in developing nations. Energy crops electricity from wind farms costs as lit-electridty production wherever biomass could also allow for better land manage- tIe as $0.04 per kilowatt-hour. The costis cheap. Many such plants already ex- ment and higher profits. But much re- should drop to below $0.03 per kilo-ist, arid more are being commissioned. search is needed to achieve consistent- watt-hour by the year 2000. In Califor-Recently in Viirnamo, Sweden, a mod- ly high crop yields in diverse climates. nia and Denmark more than 17,000em power plant using gasified wood to 10 Questions do remain as to how use- wind turbines have been completely in-fuel a jet engine was completed. The fa- ful biomass can be, even with techno- tegrated into the utility grid. Wind nowcility converts 80 percent of the energy logical innovations. Photosynthesis is supplies about 1 percent of California'sin the wood to provide six megawatts inherently ineffident and requires large electricity.of power and nine megawatts of heat supplies of water. A 1992 study com- 12 One reason for the reduction is thatfor the town. Although biomass com- missioned by the United Nations con- ~bustion can be polluting, such technol- cluded that 55 percent of the world'sogy makes it extremely clean. energy needs could be met by biomass

q., Progress in combustion engineering by 2050. But the reality will hinge on~d biotechnology has also made it eco- what other options are available.nomical to convert plant material into .'.liquid or gaseous fuels. Forest products,"energy crops," agricultural residues IIand 9ther; wastes can be gasified and -

used ta,s~thes1ze methanol.'.\ is released. when sugars, 1---0- ~ "- various kinds of

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ENERGY AND ENVlltONMENT

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stronger and lighter matcrials for the ing it, a thcrmal storagc devicc and a ponds has been widcly investigated inblades have allowcd wind machincs to convertcr for changing the hcat to elec- countrics with hot, dry climates, suchbecome substantially larger. The tur- tricity. The collectors come in tltree ba- as in Israel.bines now provide as much as 0.5 sic configurations: a parabolic dish thatmegawatt apiece. Advances in variable- focuses light to a point, a parabolic Solar Cellsspeed turbines have reduced stress trough that focuses light to a line and .;1..0and fatigue in the moving parts, thus an array of flat mirrors spread over sev- T he conversion of light directly toimproving reliability. Over the next 20 eraI acres that reflect light onto a single electricity, by the photovoltaic ef-years better materials for air foils and central tower. fect, was first observed by the Frenchtransmissions and smoother controls '~These devices convert bet\veen 10 physicist Edmond Becquerel in 1839.and electronics for handling high levels and 30 percent of the direct sunlight to When photons shine on a photovoltaicof electrical power should become electricity. But uncertainties remain re- device, commonly made of silicon, theyavailable. garding their life span and reliability. A eject electrons from their stable posi-

Ill.! One early use of wind encrgy will particular technical challenge is to de- tions, allowing them to move freely.most likcly be for islands or other areas vclop a Stirling engine that pcrforms through the material. A voltage canthat are far from an electrical grid. Many well at low cost. (A Stirling engine is then be generated using a semiconduc-such commuriities currently import die- onc in which hcat is added continuous- tor junction. A method of producing ex-sel for gcnerating power, and some are ly from the outside to a gas containcd tremcly pure cl1'stalline silicon for pho-actively seeking alternatives. By the in a closed system.) tovoltaic cells with high voltages andmiddle of the next ccntury, wind power Ig Solar ponds, another solar-thcrmal efficiencics was developed in the 1940s.}-could meet 10 to 20 percent of the source, contain highly saline water near It proved to bc a tremendous boost for..world's demand for electrical energy. their bottom. Typically, hot \vater rises the industry. In 1958 photovoltaics

15 The major limitation of wind energy to the surface, where it cools off. But were first used by the American spaceis that it is intermittent. If wind power salinity makes the watcr densc, so that program to po\ver the radio of the u.s.constitutes more than 25 to 45 perccnt hot water can stay at the bottom and Va/1guard I space satellite with lessof the total power supply, any shortfall thus retain its heat. The pond traps the than one watt of electricity.causes severe economic penalties. Bet- sun's radiant heat, creating a high tem- Zl Although significant advances haveter means of energy storage would al- perature gradient. Hot, salty fluid is been made in the past 20 years-thelow the percentage of wind power used drawn out from the bottom of the pond currcnt record for photovoltaic efficien-in the grid to increase substantially. (I and allowed to evaporate; thc vapor is cy is more than 30 percent-cost re-will return to this question presently.) used to drive a Rankine-cycle engine mains a barrier to widespread use.

similar to that installed in cars. The cool There are two approaches to reducingHeat Engines liquid at the top of the pond can also the high price: producing cheap materi-

be used, for air-conditioning." als for so-called flat-plate systems, andIbO ne way of generating electricity is \~ A by-product of this process-is fresh- using lenses or reflectors to concen-

:0 drive an engine with the sun's water from the steam. Solar ponds are trate sunlight onto smaller areas of (ex-radiant heat and light. Such solar-ther- limited by the large amounts of water pensive) solar cells. Concentrating sys-mal electric devices have four basic they need and are more suited to re- terns must track the sun and do not usecomponents, namely, a system for col- mote communities that require fresh- the diffuse light caused by cloud coverlecting sunlight, a receiver for absorb- water as well as energy. Use of solar as efficiently as flat-plate systems. They

do, however, capture more light early.and late in the day.

SOURCE 22 Virtually all photovoltaic devices op-HYDRO- SOLAR SOLAR erating today are flat-plate systems.POWER OCEANS WIND CELLS THERMAL BIOMASS Some rotate to track the sun, but most

have no moving parts. One may be op-timistic about the future of these de-vices because commercially availableefficiencies are well below theoreticallimits and because modem manufac-turing techniques are only now beingapplied. Photovoltaic electricity pro-duced by either means should soon costless than $0.10 cents per kilowatt-hour,becoming competitive with convention-al gene~ation early in the next century.

Storing Energy23

S unlight, wind and hydropower allvary intermittently, seasonally and

even daily. Demand for energy fluctu-ates as well; matching supply and de-mand can be accompljshed only withstorage. A study by the Department of

UTILITIES RESIDENTIAL AGRICULTURAL TRANSPOR-AND COMMERCIAL AND INDUSTRIAL TATION

END USE

DISTRIB1JfION of renewable solar energy projected for the year 2000 shows thatmany dilferent means of tapping the resource will playa role.

138

SCIENnFIC AMERICAN September 1995

Energy estimated that by 2030 in the " '" ' ;;' " ...;:;,;:';~,:, :~::;;;:;;~.~~;;~i:,8J

U.S., the availability of appropriate stor- A New Chance for ,Solar ;,~ergy,,~*~~;:~l~~,¥:..:.~: 1age could enhance the contribution of , " ,:::~:;!;,;:., :'J.t,,~;."i;,rli1;"T';:i:~';,!~;,i;jrenewable energy by about 18 quadril- : S olar power is getting cheaper-in fact, the cost of,~lc,hlng ~~~:;~~~'s rays~::;'jlion .British theml~ units p~r year. :' has fallen more than 6 5 perce~t in the, past., 10 years. 't,has,c~~~(b~co~e ;,\1

.lJf With the ~ceptIon of biomass, the" Inexpensive enough, though, to rival fo~SII fuels,so ~olarenerg~..\~Talns ~;;1more promismg long-term solar sys- promising, not yet fully mature alternative. Sales run onlyabo~t~,!.\bllllon,:;;,jterns are designed to produce only el~c- annually, as opposed to roughly $800 billion for standard sourc~~ran.~,~p'I~~ci;'Jtricity. Electricity is the, energy ca~Ier customers still generally reside in isolated a~~s! far,from po~erf,gryd~:~,'~!::~:::.;s.~~f choice for mos~ statIon~ry applI.ca- :~ But a new proposal from an American utility may well make"'~i91~~p~~er~1inons, such as heating, co~lmg, lIght~g ;, conventional-o-orat least_more competitlve..EnronCorporatlo~,,~h~J~rges~;t;~and m~c~ery. But it. IS not easily; U.S. supplier of natural ga's,recently joined forc,es w!th A~ocoC~rp~ratl,o_I:1;;\1}stored m s~tabl~ quan.tIties. ~or use in :." owner of the photovoltaic cell prod~cer Solarex. The two compa~)es:'n,tend,,&~Jtransportation, lightweight, high-capac- i; to build a 1 DO-megawatt solar plant In the Nevada ~~sert.bY th~"e".~,C?f,;.1~96'..;fq

J.ty energy storage is needed. ~ ..The facility, which could supply a city ofl 00,000, ~lIllnltl~'lys~'!;'!~~~lfo~~~~15 Sunlight can also be used to p~oduce i 5.5 cents a kilowatt-hour-:about three centscheaperon:~ver~.9~'"t~~n,!~e!1'ii

hydrogen fuel. !he tec~ologles re- ~,electricity generated by 011, coal or gas. "If they can ~ull this off, I~,!=a~~vo.~:iQU}red to d~ s.o ~ectly (Wlt~OUt gener- , lutionize the whole industry, ~ comments Robert H.Wllllams of prl~~~~o~~..~!!I~i):~atmg eleCtriCIty first) are m the. very i: versity. "If they fall, it is going to set ~ack the technologY..1,9yea~.!4f~};i'~~i~Jearly stages of development but m,the ;' Despite its magnitude, the $1 50-million plan,d.oes npt~~:an,t~,a~.~~e.s~'~r.j~;,ilong teml may prove the best. SunlIght ;:, age has finally dawned: Enron's low price is predicated, o,'l,tax.~x~~Pti~~S~~~jfalling on an electrod~ can pro?uce an f,' from the Department o,f Energy and on guaranteed purchases bYc th,!"f~d~~I~!~:1electric current to splIt water mto hy- ~' government. Nor does It mark a sudden technological breakthrouy~~ Solar~~i:jdrogen and oxygen, by a pr~~ess cal~ed : manufactures a conventional thlncfilm, silicon-based photovolta,ic:~el'~ha~!S~~photoelectrolysis. The term photoblol- ;' able to transform Into,electriclty about 8 perc,entofth,e sunllgh~.~~~tf~~ches :~~!ogy" is used to describe a whole class ,: It. Rather the significance of Enron's ventur~houldt~e.b!d~e;~£~~pted,..by:,~~::jof biological systems that produce hy- :' the aovernment-is that it paves the way for other,co~panl~,~,~~(.~~,~~;'a~ge,~,:,j

~.""'~"'.""."'~-""-.Adrogen. Even longer-term research may; scare investments In solar pQwer.lead to photocatalysts that allow sun- ~-' Such investments could bring the price of solar.po~er,te~~nolo:g'y,a~~",~e~i~:'light to split water directly into its com- ~..';'lIvery down even further-for both large;'grid-:based,~ar~et,s"'an~~,r

1t~~,TP~.;;;!jpcnent substances. } dispersed, off-the-grld markets that arethe,norm,ln, manyc~evelop,?g,co~n;:~,.1£" When the resulting hydrogen is ': tries. 'This marks a shift in approach," explains Nicholas Lenssen~~to.rl:n~r1yat;):{3~urned as a fuel or is used to produce :'.,. the World watch Institute in Washington; 9,C;;:andno,,!!,at E s,our~i,Jni,~~ulder,~{:l

electIidty in a fuel cell, the O,nly by-prod- \ Colo. "It allows them to attract 10wer-rlsk;long-termc,apltal~ no~Ju~!:yent~~ ~::~1uct is water. Apart from bemg en.viron- :~, capital, which is ,,-:ery costly." W~lch all means~heNevada de;~,~.~ay,~OOn.,.be,~

~:i mentally benign. hydrogen proVIde~ a ~: home to a very different, but stili veryh~t, kl~~,~,f!~~s~"s!te,:~~~:;,~:,~r!!~.~i!~rSj~::.way to alleVI'ate the proble m of stonng " '! .':0,:":':"""'::';:"',,;-,,;',,-,:,,"]; \,,;'"J;",",;::,:v,...'.i1'::~,;.;~"

, .'" , ,,_.,.:J,. "...,..,~,="._,.",-~I;,..~(:.-.;,csolarenergy,.It can be held efficiently.

for as long as required. Over distances .' ..of more than 1 000 kilometers, it costs my will require alterations m the infra-less to transport hydrogen than to trans- structure. When the decisio? to change

.mit electricity. Residents of the Aleutian is made will depe~d on the Importance.; Islands have developed plans to make placed on the enVIr°?lnen~, energy se-

electricity from \vind turbines, convert- curlty or other consIderatIons. In. theing it to hydrogen for storage. In addi- U.S" federal programs for research mtotion, improvements in fuel cells have renewable energy have been on a r9"er-allowed a number of highly efficient, coaster ride. Even the fate of the De-nonpolluting uses of hydrogen to be de- partment of Energy is uncerta,in.veloped such as electric vehicles pow- :28At present, developed natIons con-ered by hydrogen. sume at lea~t 10 ti~es the e~ergy per

1:t- A radical shift in our energy econo- person than IS used m developmg coun-

tries. But the demand for energy is ris.ing fast everywhere. Solar technologiescould enable the developing world toskip a generation of infrastructure andmove directly to a source of energy thatdoes not contribute to global warmingor otherwise degrade the environment.Developed countries could also benefitby exporting these technologies-if ad.ditional incentives are at all necessaryfor investing in the future of energyfrom the sun. .

The Author Furthel:- ReadingWIlliAM HOAGLAND received an M.S. degree in chemi- BASIC PHOTOVOLTAIC PRINCIPLES AND ME'nIODS. Kenneth Zweibel, Paul

cal engineering from the Massachusetts Institute of Tech- Hersch and Solar Energy Research Institute. Van Nostrand Reinhold, 1984.nology. After working for Syntex, Inc., and the Procter & STFERING A NEW COURSE: TRANSPORTAnON, ENERGY AND THE ENVIRONMENr.Gamble Company, he joined the National Renewable Ener- Deborah Gordon and Union of Concerned Scientists. Island Press, 1991.gy Laboratory (formerly the Solar Energy Research Insti- RENEWABLE ENERGY: SOURCES FOR FUELS AND ELECTRICITY. Edited by Tho-tute) In Golden, Colo., where he managed programs in solar mas B. Johansson, Henry Kelly, Amulya K. N. Reddy, Robert Williams andmaterials, alcohol fuels, biofuels and hydrogen. Hoagland Laurie Burnham. Island Press, 1993.Is currently president of W. Hoagland & Associates, Inc., in PROGRESS IN SOLAR ENERGY TECHNOLOGIES AND APpuCAnONS. Harold M.Boulder. The editors would like to acknowledge the assis- Hubba~d, Paul Notari, Satyen Deb and Shimon Awerbach. American Solartance of Allan Hoffman of the Department of Energy. Energy Society, January 1994.

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SCIEN11FIC AMERICAN September 1995 139