Chapter 17 SOLAR ENERGY

Transcription

1 Chaper 17 SOLAR ENERGY Renewable echnologies represen an imporan opporuniy, bu no a panacea for he U.S. energy economy. Their long-erm conribuion is predicaed on overcoming remaining echnical and cos barriers, mainly hrough inensified R&D. The Naional Energy Sraegy's renewable energy iniiaives are based on hese conclusions and on a clear undersanding of he conribuions ha renewable energy can and canno be expeced o make. For example, given policies o address exising regulaory barriers and marke imperfecions, solar hermal or phoovolaic elecriciy echnologies can compee oday o provide elecriciy generaion in remoe locaions and for peaking purposes. (Naional Energy Sraegy, Execuive Summary, 1991/1992) The Adminisraion suppors fundamenal and applied research ha helps he renewable indusry develop echnologically advanced producs. [...] Applied research ino hin reflecive membrane deposiion, airfoil design, and solar module fabricaion has reduced coss and increased produciviy from solar hermal power plans, wind urbines, and fla plae phoovolaic arrays. [...] Programs supporing renewable elecric supply will conribue 0.6 quads of primary energy in he year 2000, saving $4 billion in annual fuel coss and reducing 7 million meric ons of carbon-equivalen emissions. (Susainable Energy Sraegy, 1995)

2 314 CHAPTER 17 In Chaper 16 we discussed he following nondepleable energy sources: geohermal, wind, idal, wave, hydroelecric and biomass energy. We saw ha hey will no solve he world's energy problems. Bu hey are, hey can be and hey will be imporan on a local scale. In Chaper 14 we discussed nuclear fusion; i could solve all our energy problems, bu many echnical problems need o be overcome before i can be harnessed and commercialized. The producion of elecriciy using fusion mus go hrough he boleneck of hermal-omechanical energy conversion, which is inherenly inefficien. The las energy source ha we need o discuss, direc solar energy, will solve all sociey's energy problems, bu no ye. Is efficien large-scale uilizaion is expeced o become a realiy some ime in he 21s cenury (probably in he second half). Is greaes virue apar from being free, inexhausible, universally available and polluion-free is ha i can be convered direcly ino elecriciy, unlike any oher energy source. Is poenial, is curren saus and he challenges lying ahead are discussed nex. Solar Energy Balance More han 99.9% of he energy flow on he earh's surface is due o incoming solar radiaion. The res is from geohermal, graviaional (idal) and nuclear sources. The sun is an average-size sar, wih a diameer of 864,000 miles and 93 million miles away from our plane. I is a gian nuclear fusion reacor whose inerior and surface emperaures are 35,000,000 and 10,000 F, respecively. Each second 657 million ons of hydrogen isoopes are convered ino 653 million ons of helium. The residual mass of 4 million ons is convered o energy, according o he Einsein equaion, E = mc 2 : Power from he sun = (4x10 9 kg s ) (3x108 m s )2 = 3.6x10 26 W To place his number ino perspecive, if gasoline were pouring from Niagara Falls, a a rae of 5 billion gallons per hour, and if we had begun collecing i 3.5 million years ago, he combusion of all his accumulaed gasoline would liberae he amoun of energy equivalen o one minue of he sun's producion. The reader is urged o verify his. Being quie far away from he sun, he earh receives only abou half a billionh of his radiaion. Bu i receives i more or less coninuously. Abou 30% of his energy does no reach he surface of he earh because i is refleced from he amosphere (as ulraviole radiaion, see Figures 3-1 and 11-7). Sill, he radiaion ha does reach he surface is four orders of magniude larger han he oal world's energy consumpion (see Illusraion 5-1 and Figure 5-2). In fac, only 40 minues of sunshine would be sufficien if available in adequae forms o supply he enire annual energy demand on earh. The if menioned in he previous senence is a big one, however. Because solar energy spreads ou more or less evenly hrough space, i reaches he surface of he earh in quie

3 SOLAR ENERGY 315 dilued form, a a rae of abou 220 W/m 2 (see Figure 3-1). In oher words, if one square meer were available for conversion of solar energy o elecriciy (a 100% efficiency), he energy produced would be sufficien for jus wo or hree ligh bulbs. The challenge of solar energy uilizaion is o concenrae i. Pracical ways o achieve his are discussed below. They include direc solar heaing, indirec producion of elecriciy and direc producion of elecriciy. Direc Solar Heaing The use of solar rays o achieve effecive heaing has been praciced since ancien imes. In 213 B.C., he Greek savan Archimedes used mirrors o direc sunligh ono he flee of Marcellus, he Roman general who ried o capure Syracuse (Sicily), and se his ships on fire. Today's devices are no necessarily more sophisicaed han he ones used by Archimedes. They are called collecors. A collecor is hus a device ha collecs solar radiaion and convers i o hermal energy. Figure 17-1 shows he saisics of mos recen shipmens of solar collecors in he U.S. The low-emperaure collecors are used primarily for less demanding residenial consumpion (o hea swimming pools, for example); i is good o see ha heir sales are up again. The medium-emperaure collecors are used primarily for residenial ho waer. Boh kinds became popular in he decade of he oil crises (1970s). However, consumer ineres in hem decreased sharply when he price of oil decreased in he 1980s (see Chaper 20) and when Federal solar energy ax credis expired in Million square fee s s s s s s s s s ss ss s s s s s s s FIGURE Shipmen of solar collecors in he U.S. [Source: Energy Informaion Adminisraion.] s Lowemperaure collecors Mediumemperaure collecors

4 316 CHAPTER 17 Figure 17-2 is a schemaic represenaion of a ypical fla-plae collecor used for domesic heaing. A working fluid (such as air, waer, oil or anifreeze) circulaes hrough he ubes. The enclosure, wih is black meal surface beween he ubes and insulaion a he boom, is designed o maximize he absorpion of solar radiaion and is conversion o hea: he glass cover provides he greenhouse effec. The efficiency of conversion of solar radiaion o hea sored in he working fluid is a complex issue; i is quie dependen on collecor design. Man-made collecors are much less efficien han naural collecors, ha is, animal furs. The fur of polar bears, for example, has been repored o have an efficiency of abou 95%; no wonder hey enjoy swimming in he icy Arcic waers! The mos sophisicaed, and mos expensive, solar collecors have maximum efficiencies in he 65-70% range. Typical values on a cold winer day, when hey are mos used, are around 20%. FIGURE Schemaic represenaion of a fla-plae collecor. [From Energy and Problems of a Technical Sociey, by J.J. Kraushaar and R.A. Risinen. Copyrigh 1988 by John Wiley & Sons. Reproduced wih permission.]

5 SOLAR ENERGY 317 In addiion o he collecor and he working fluid, a complee acive solar sysem mus have an energy sorage faciliy and/or a backup sysem, because he sun does no shine all he ime and i may no shine every day. Such a sysem is illusraed in Figure The ho working fluid (such as anifreeze) exchanges hea wih waer in he primary loop, similar o he primary loop of a pressurized waer nuclear reacor (see Figure 13-8). In he secondary loop, his ho waer is used o hea he sorage ank, from which he ho waer is disribued o he various consumers (for example, a shower in a home or a dishwasher). The size of he sorage sysem depends on he amoun of solar energy inciden on he collecor and on he efficiency of he collecor. This is shown in Illusraion 17-1, based on he informaion given in Table In addiion o he acive solar energy sysem, passive solar heaing sysem can be used effecively o reduce he heaing (and cooling) requiremens of houses and buildings. A passive sysem conains no acive componens, such as collecors and pumps; i relies on boh regular and special feaures of building design. Walls, ceilings and floors consiue boh he collecion and he sorage sysem. Hea is disribued by naural convecion. Building design is opimized o le he sun in and keep i in in he winer, and o do he opposie in he summer. There are wo ways o accomplish his: using he so-called direc gain and indirec gain. FIGURE Schemaic represenaion of a solar energy sorage sysem.

6 318 CHAPTER 17 Illusraion A home in Phoenix (Arizona) requires 62 kwh of hea on a winer day o mainain a consan indoor emperaure of 20 C. (a) How much collecor surface area does i need for an all-solar heaing sysem ha has a 20% efficiency? (b) How large does he sorage ank have o be o provide his much energy? Soluion. Phoenix is locaed a abou 33 N, so we can use he daa for 32 N given in Table The average solar radiaion in winer is abou 6.5 kwh/m 2 /day. Hence, he daily quaniy of hermal energy obained using collecors will be: Thermal energy = 6.5 [kwh (solar)] m 2 day [0.20 kwh (hermal)] [1 kwh (solar)] = 1.3 kwh m 2 day This means ha for every square meer of collecor surface area, 1.3 kwh of hea are produced every day. Therefore, he required collecor surface area is obained as follows: 62 kwh day Collecor surface area = = 48 m kwh m 2 day So a collecor 6 m long and 8 m wide would do he job. Obviously, i can be placed on he roof. The size of he sorage ank can be obained by remembering he quaniaive definiion of hea (Chaper 3): Hea = [Mass] [Hea capaciy] [Temperaure difference] Here he mass is ha of he sorage medium, waer, which needs o be deermined. The hea capaciy of waer is 1 kcal/kg/ C (see Table 3-2), and he emperaure difference is ha beween he ho fluid in he secondary loop and he cold waer going ino he sorage ank (say, = 40 C); see Figure Therefore, he required mass of waer for a day's worh of hea is obained as follows: Mass = Hea [Hea capaciy] [Temperaure difference] = = 62 kwh (1 kcal kg C ) (40 C) (1.16x10-3 kwh 1 kcal ) = 1336 kg H 2 O This is equivalen o a volume of 1336 liers (or abou 350 gallons), because he densiy of waer is 1 kg/l.

7 SOLAR ENERGY 319 TABLE 17-1 Variaion of solar radiaion (in W h/m 2 ) wih ime and laiude Dae Perpendicular Horizonal Verical Souh 60 Souh Ocober N 8,498 5, N 7,735 4,249 5,212 6, N 6,789 3,221 November N 7,584 4, N 6,707 2,969 5,314 6, N 5,257 1,879 December N 7,401 3, N 6,235 2,465 5,188 5, N 4,551 1,406 January N 7,748 4, N 6,878 2,988 5,440 6, N 5,390 1,879 February N 9,053 5, N 8,321 4,457 5,452 6, N 7,344 3,404 March N 9,494 6, N 9,191 5,838 4,677 6, N 8,763 4,974 [Sources: Kraushaar and Risinen, op. ci.; A.W. Culp, Jr., Principles of Energy Conversion, McGraw-Hill, 1991.] Direc gain refers o sysems ha admi sunligh direcly ino he space requiring hea. The maximum recepion of sunligh is obained hrough windows facing souh, as shown in Figure 17-4 (see also Table 17-1). The sunligh received during he day while i lass mus be absorbed by a high-hea-capaciy maerial on he floor or he walls. As shown in Table 3-2, dense subsances such as concree, brick, sone, adobe and waer can sore relaively large quaniies of hea in a reasonable amoun of space. When he sun ceases o

8 320 CHAPTER 17 shine, he warm floor and walls ransfer he accumulaed hea o he cold space in he house. Indirec-gain passive sysems are hose ha also absorb solar radiaion in a high-heacapaciy maerial, such as an ouside concree wall. The accumulaed hermal energy is hen ransferred o he space needing hea. The souh-facing Trombe wall, named afer he French engineer Félix Trombe, is he mos commonly used passive solar srucure. I is buil of concree, brick or sone; i can even be filled wih waer. I is ofen pained black for maximum absorpion of radiaion. This increases is efficiency bu does no help o embellish he neighborhood; here are regulaions in some residenial areas ha do no allow hese srucures on he sree-facing side of he house. An alernaive sysem is a roof pond: waer is conained in large, shallow bags beween he ceiling and he roof. Movable insulaing maerial separaes i from he roof. During he day, insulaion is removed o allow sunligh o srike he pond; during he nigh, i is placed back in is posiion so ha i allows he hea o be ransferred mosly oward he inside of he house. The mos familiar example of a passive solar sysem is he greenhouse, also called sunspace or sunporch. I is used boh for growing plans and for residenial comfor in winer. I combines direc and indirec gain, by leing he sunshine ino he room hrough souh-facing glass and absorbing he radiaion on a brick wall inside he room. (This is he origin of he erm greenhouse effec, discussed in Chaper 11.) FIGURE Solar hea gain for differen window orienaions. [Source: G. Aubrech, op. ci.]

9 SOLAR ENERGY 321 In he ruins of ancien civilizaions, here are numerous examples of very effecive passive solar heaing and cooling sysems. The ancien cliff dwellings in he Mesa Verde Naional Park in Colorado are a familiar example. As he cos of heaing and cooling increases in our days, archiecs, home owners and builders are wise in remembering hese examples and adaping hem o curren building maerials and lifesyles. I is well documened ha hey can provide significan energy savings. Indirec Producion of Elecriciy The use of a more sophisicaed collecor sysem compared o he one represened in Figures 17-3 and 17-4 allows he working fluid o achieve a higher emperaure. Such a sysem can hen be used o produce elecriciy. This is illusraed in Figure A solar hermal power plan is essenially idenical o an ordinary seam-urbine power plan, excep ha i ges he hea from solar radiaion raher han from combusion or nuclear fission. FIGURE Schemaic represenaion of a solar hermal power generaion plan.

10 322 CHAPTER 17 A fla-plae collecor ypically raises he emperaure of he working fluid o abou 100 C. A number of fla-plae collecors placed in series can raise he emperaure of he working fluid o levels ha can provide economically compeiive elecriciy. For example, he maximum efficiency of he urbine of a power plan whose enering seam emperaure is 200 C would be E max = T H T L T H = ( ) ( ) ( ) = 0.21 This is a low efficiency, bu he fuel (solar energy) is free. A higher emperaure can be achieved by using concenraing, or focusing, collecors as Archimedes did o burn Roman ships. Wih he use of parabolic rough collecors, for example, seam emperaures of up o C can be reached. This echnology is currenly cos-compeiive in cerain markes. In he Mojave deser (Kramer Juncion, CA), Luz Inernaional has buil a plan ha delivers 354 MW of elecriciy o Souhern California Edison's power grid (see Invesigaion 17-1). Temperaures as high as hose in convenional power plans can be achieved easily wih solar ower echnology. Here, a sysem of compuer-conrolled mirrors (called heliosas) racks he sun across he sky so ha he refleced sunligh from all he mirrors falls on a cenral ower conaining waer or oil or, in more recen designs, a molen sal. A Barsow, California, some 1900 heliosas were used o raise he emperaure of waer o 510 C and he 10 MW(e) Solar One plan had an overall efficiency comparable o ha of convenional power plans. And he new 10-MW(e) Solar Two a he same locaion is adverised oday as he world's mos echnically advanced solar power plan. I uses a molen sal as he hea ransfer fluid. The advanage is ha he hermal energy colleced during sunny hours can be sored in he molen sal (see Illusraion 17-2) and used on cloudy days or a nigh. If successful, i will pave he way for a new generaion of commercial power plans. Illusraion How much less hea sorage medium would be needed in Illusraion 17-1 if a molen sal were used insead of waer? Because i undergoes a phase change (from solid o liquid), he amoun of hea ha can be sored in he sal is larger, say 120 BTU/lb, han he amoun ha can be sored in waer. Soluion. The mass of molen sal required is 3414 BTU Hea (62 kwh) ( Mass = [Hea capaciy] [Temperaure difference] = 1 kwh ) 120 BTU = 1764 lb lb Compare his o almos 3000 pounds of waer needed for he same hea sorage ask.

11 SOLAR ENERGY 323 The Deparmen of Energy repors ha annual shipmens of hese high-emperaure collecors were less ha 5 housand square fee in 1994, down from 5.24 million square fee in Clearly, non-elecric use of low- and medium-emperaure solar collecors is more popular a he presen ime (see Figure 17-1). Direc Producion of Elecriciy Indirec producion of elecriciy from solar energy, while quie promising because of recen significan progress in echnology and in economic compeiiveness (see Chaper 18), has wo major drawbacks. Because of is relaively low efficiency (especially using fla-plaecollecor sysems), he size of he proposed solar farms can be very large. This is illusraed below. Illusraion How much collecor area would a 1000-MW(e) solar farm require if he individual efficiencies of he collecor sysem, urbine and generaor are 30, 25 and 90%, respecively? Soluion. Le us assume ha he average inciden solar radiaion a he proposed sie of he plan is 200 W(solar)/m 2. This means ha 1 m 2 of earh's surface receives 200 W of solar radiaion. If a collecor is placed on his surface, i will conver 30% of his energy ino hea; herefore for every square meer of collecor, 60 W of hermal energy will be available. Now, aking ino accoun he efficiencies of he urbine and he generaor, we have ha he collecor area required is: 1 m Collecor area = ( 2 60 W(h) ) ( 1 W(h) 0.25 W(m) ) ( 1 W(m) 0.9 W(e) ) (109 W(e)) = 7.4x10 7 m 2 Thus, wih he efficiencies given above, his solar farm would occupy an area of abou 75 square kilomeers. If land is expensive, his would represen a significan capial invesmen. Direc conversion of solar energy o elecriciy would no only avoid his problem, bu would avoid he hermodynamic boleneck illusraed in Figure 17-6, which none of he echnologies menioned so far are capable of doing. In mos of our discussion of elecriciy generaion so far, he energy conversion pah has been he one shown in he upper porion of Figure The direc conversion of chemical energy o elecriciy is possible in devices called fuel cells. These are a ype of large-scale baeries ha have been used for decades in he NASA

12 324 CHAPTER 17 space programs. In conras o ordinary baeries, however, fuel cells require a coninuous supply of chemical energy (from naural gas, for example) and he elecrode maerial is no depleed as i supplies elecriciy. Their commercial use in power plans and elecric ransporaion is increasingly being considered, paricularly in densely populaed areas (because of heir very low environmenal emissions and silen operaion). A 4.8 MW(e) demonsraion plan had been operaional in downown Manhaan since he early eighies. A number of uiliy companies across he U.S. have purchased 2 MW(e) plans, one of hem o power he New York Ciy subway sysem (The New York Times, June 30, 1991, p. F6). More recenly, a 2 MW(e) plan ha is considered o be simpler and more efficien han many oher ypes of fuel cell power plans was conneced o he grid of he Sana Clara municipal elecric sysem. (For an updae on his Sana Clara demonsraion projec, see For an updae on fuel cell echnology in general, visi he Web sie of he Morganown Energy Technology Cener of he Deparmen of Energy, Chemical/ Nuclear Energy Thermal Energy Mechanical Energy Elecriciy Fuel Cells (e.g., baeries) Solar Cells Solar Energy Thermal Energy Mechanical Energy Elecriciy FIGURE Energy conversion pahways of fuel cells and solar cells. Like fuel cells, solar cells produce elecriciy direcly, wihou going hrough he hermodynamically unfavorable conversion of high-enropy hermal energy ino lowenropy mechanical energy (remember Chaper 3). Therefore, in heory a leas, he efficiency of his conversion could be as high as 100% and his ogeher wih he fac ha solar energy is free, inexhausible and nonpolluing provides grea incenive o develop his new echnology.

13 SOLAR ENERGY 325 A solar cell, also called phoovolaic cell, is hus a device ha direcly convers solar radiaion ino elecriciy. I is based on he phooelecric (or phoovolaic) effec, which was known since he early 19h cenury, bu which was ranslaed ino a useful device only in he 1950s, in response o he needs of he U.S. space program. This effec, exhibied by maerials called semiconducors (such as silicon), is illusraed in Figure Transisors and compuer chips, which have revoluionized he elecronics indusry since he 1940s, are also made from semiconducing maerials. (phoons) Elecron Energy Conducion band Band gap Valence band Band-gap energy FIGURE Phoovolaic effec in semiconducors. The elecrons ha have he poenial o creae an elecric curren are normally ied up in heir valence band, ha is, a a low energy level. An energy barrier (called he band-gap energy) mus be overcome before hey can become carriers of elecriciy in his maerial, by jumping ino he so-called conducion band. Solar radiaion, in he form of elemenary paricles called phoons, provides he needed energy; he phoons srike he surface of he semiconducor and some of he valence elecrons are ejeced ino he conducion band. They are hus made free or available for conducion of elecriciy. Bu for he producion of elecriciy, he acual solar cell device mus be made from wo differen ypes of so-called doped semiconducors. This is shown in Figures 17-8 and 17-9 and described below.

14 326 CHAPTER 17 n-ype p/n p-ype Conducion band Elecron energy Valence band n-ype p/n p-ype FIGURE The charge disribuion in he p/n juncion region of a solar cell: (a) wihou solar radiaion; (b) wih solar radiaion. In a normal silicon crysal, here are four valence elecrons in every aom. They are held in place by he posiive charge from he nuclei of he silicon aoms. They easily come back o he valence band before hey can give up heir energy in an exernal elecric circui. However, if he silicon is doped wih a small quaniy of an elemen ha has five valence

15 SOLAR ENERGY 327 elecrons and can fi ino he silicon crysal srucure (such as phosphorus or arsenic), some exra elecrons are creaed. Such a doped maerial is called an n-ype semiconducor, because he exra elecrons carry a negaive charge. Alernaively, if he semiconducor is doped wih an elemen ha has only hree valence elecrons (such as boron or gallium), insead of creaing exra elecrons, exra missing elecrons, or posiive holes, are creaed. This is a p-ype semiconducor. Sill, boh maerials are elecrically neural when hey are separaed: in he n-ype maerial he negaive charge of he exra elecrons is balanced by he higher posiive charge of he dopan nuclei (e.g., phosphorus), and in he p-ype maerial he exra elecron holes are balanced by he lower posiive charge of he dopan nuclei (e.g., boron). When hese wo ypes of maerial are combined, a p/n juncion is formed. This is wha makes possible he producion of elecriciy, as opposed o simple conducion of elecriciy in a semiconducor illuminaed by solar radiaion. Because of he high concenraion of elecrons in he n-ype semiconducor, some of he exra elecrons spill over ino he holes of he p-ype semiconducor. This makes he n-ype maerial posiively charged in he viciniy of he juncion. Conversely, he p-ype maerial becomes negaively charged in he viciniy of he juncion. An (inernal) elecric field across he juncion is hus creaed. Normally, however, here is equal flow of elecrons in boh direcions across he juncion (Figure 17-8a) and no elecriciy can be produced. (flow of elecrons) n-ype semiconducor p-n juncion p-ype semiconducor resisance load FIGURE Schemaic represenaion of a solar cell.

16 328 CHAPTER 17 When solar radiaion srikes he solar cell (Figure 17-8b), excess elecrons flow from he n- ype maerial o he p-ype maerial and excess holes flow in he opposie direcion. This, ogeher wih he exisence of he elecric field across he juncion, makes possible he flow of elecrons away from he (charge-separaing) juncion and hrough an exernal circui (Figure 17-9). Thus, solar energy is convered ino elecriciy. Figures and summarize he growh of he phoovolaic-cell marke in he U.S. and he world. The conribuion o he overall energy supply is sill low (see Chapers 5 and 18) bu he growh has been phenomenal. I has occurred mosly in he developed naions (U.S., Japan, Europe). The growh in he U.S. has been mos significan in he residenial secor. Developing naions (such as Brazil, India, and China) have also conribued o he worldwide growh, because one of he key advanages of he phoovolaic echnology is is rural applicabiliy, in remoe areas lacking access o cenral power supplies. The cumulaive world capaciy now approaches 600 MW. I is mosly used for on-peak consumpion (see Chaper 18). I mus be concluded, however, ha boh economic and efficiency problems sill sand in he way of large-scale commercial uilizaion of his echnology. Solar cell shipmens (megawas) s s s s s ss s sss sssss ss s s s s World Unied Saes FIGURE Use of solar cells in he U.S. and he world in he pas wo decades. [Source: Vial Signs 1996, Worldwach Insiue, and Energy Informaion Adminisraion.] Every new incremenal increase in he efficiency of a solar cell aracs a grea deal of aenion in he popular press. While here are no hermodynamic limiaions, as menioned above, here are inheren energy losses ha severely limi he performance of currenly available cells. These include opical losses (for example, reflecion of he radiaion from he cell's surface, before reaching he p-n juncion) and he (more serious) inabiliy of he

17 SOLAR ENERGY 329 currenly designed cells o provide for he conversion of he enire sunligh specrum (see Figure 2-2) ino elecriciy. Despie hese limiaions, he efficiency of an individual solar cell has increased from 5% in he early designs o abou 35% in he mos advanced designs Oher Elecric Uiliy Transporaion Indusrial Governmen Commercial Residenial FIGURE U. S. phoovolaic energy conribuion by economic secor (in kilowas purchased). [Source: Energy Informaion Adminisraion.] More imporan han he issue of efficiency is he cos issue afer all, solar energy comes for free and here dramaic changes have aken place. This is illusraed in Figure In Vial Signs 1996, he Worldwach Insiue repors no addiional decrease in he price of solar elecriciy; oday solar cells cos $ per wa. In a recen NYT aricle ( 70's Dreams, 90's Realiies. Renewable Energy: A Luxury Now. A Necessiy Laer?, 4/11/95), he following coss for a kilowahour of elecriciy are given: Naural gas 3 cens Wind 5 cens Geohermal 5.5 cens Solar (hermal) 14 cens A similar summary was published in a May 1994 issue of Business Week ( The sun shines brigher on alernaive energy ): Coal 4-5 cens Naural gas 4-5 cens Wind 5-9 cens Geohermal 5-8 cens

18 330 CHAPTER 17 Hydropower 4-7 cens Biomass 6-8 cens Solar (hermal) cens Phoovolaic cens (Environmenal benefis of solar energy may no have been facored ino hese prices; see Chaper 21.) In hese circumsances, some companies in he U.S. have abandoned solar energy, unsure of when hey will be able o conver i ino elecriciy wih a profi (see U.S. Companies Losing Ineres in Solar Energy, in he NYT of 3/7/89; see also Invesigaion 17-1). Ohers are using cheaper bu lower-efficiency maerials (hin films of amorphous silicon) and are sill working on efficiency improvemens. During he Bush Adminisraion, he Deparmen of Energy had anicipaed (in is Naional Energy Sraegy) ha uiliy-scale applicaions of phoovolaics will reach commercial level around he year Curren official projecions do no seem o be as opimisic. In he Susainable Energy Sraegy, he saemen abou DOE's Phoovolaics Sysem Program is more vague when i comes o commercial-scale uilizaion: [This] Program suppors privae secor research o develop roofing maerials and windows ha incorporae phoovolaics and can produce elecriciy. These effors are pursued in close collaboraion wih indusry in programs wih he Uiliy PV Group. Finally, phoovolaic echnology is expeced o bring closer o realiy he use of hydrogen as an energy source. The concep is illusraed in Figure I is a sunassised waer cycle. Solar radiaion is convered o elecriciy, which is hen used o break up waer ino hydrogen (H 2 ) and oxygen (O 2 ). Hydrogen is hen used as a clean gaseous fuel, whose combusion regeneraes waer, produces a lo of energy (274 BTU/f 3 ) and causes no polluion. Bu don' expec o see his wonderful echnology a your local elecric uiliy any ime soon (in your lifeime, I mean). INTERNET INFO For he mos recen developmens in solar energy and oher renewable energy sources, visi he following Inerne sies: solsice.cres.org

19 SOLAR ENERGY 331 FIGURE Hisory and projecions of solar elecriciy coss. [Sources: C.J. Weinberg and R.H. Williams, Energy from he Sun, Scienific American, Sepember 1990, p. 154.] FIGURE Concepual scheme of he use of solar cells o produce hydrogen.

20 332 CHAPTER 17 REVIEW QUESTIONS Show he origin of he numbers quoed a he beginning of his chaper from he Susainable Energy Sraegy On p. 314 i is saed ha only 40 minues of sunshine would be sufficien o supply he enire annual energy demand on earh. Show where his number comes from I has been repored ha he laes module of he SEGS (solar elecric generaing sysem) plan in Kramer Juncion, CA generaes 80 MW of elecriciy by using 483,360 square meers of collecors o achieve an annual oupu of 260 gigawahours. Check wheher hese numbers make sense. Wha is he efficiency of he collecors? In 1994 he Spanish El País has repored on he larges phoovolaic power plan in Europe, near he ciy of Toledo. Is elecric capaciy is 1 MW (enough for 2000 people, he paper says), and he oal area of he solar panels is repored o be 16,700 square meers. Do hese numbers make sense? Wha is he efficiency of hese solar cells? Indicae wheher he following saemens are rue or false: (a) A solar cells convers solar radiaion direcly ino elecriciy. (b) A window in New York Ciy ha is facing souh receives as much as hree imes more solar radiaion han a window facing norh. (c) From 1980 o 1983 he sales of medium-emperaure solar collecors exceeded he sales of low-emperaure solar collecors. (d) In 1995 U.S. sales of phoovolaic cells represened less han 50% of world sales. (e) In 1983 U.S. sales of phoovolaic cells represened less han 50% of world sales. (f) The principal cusomers for he phoovolaics indusry are he elecric uiliies. (g) In 1990 more han 50% of he phoovolaics have been sold o he residenial and commercial secors. INVESTIGATIONS I may be difficul o keep up wih all he echnical, economic and poliical developmens in solar energy. The Inerne is he ideal ool o ry o accomplish his, bu disinguishing facs from boh ficion and propaganda may be ime-consuming. Spend some ime surfing he Inerne o find ou abou he curren saus of he SEGS plan in Kramer Juncion, CA and abou he company ha has pioneered his echnology, Luz Inernaional. Use a leas wo search engines and check a leas hree differen Web sies Use of phoovolaics in rural areas may already be cos-compeiive wih convenional echnologies. See why i makes sense in Souh Africa ( Solar power: Nigh and day, Economis of 9/9/95) and elsewhere in he developing world ( Here Comes he Sun, Time

21 SOLAR ENERGY 333 of 10/18/93, and A Sunny Forecas, 11/7/94; and America Unplugged, Newsweek of 10/18/93) The following companies have been repored o be involved (in May 1994) in developing phoovolaic echnology: Siemens Solar Indusries, Solar Engineering Applicaions, Unied Solar Sysems, Canon USA, Alpha Solarco, Solarex, Bechel Power, Mobil Solar Energy, Amonix, Ascension Technology, Cummins Engineering, Scienific Analysis, Fresnel Opics, Texas Insrumens, Inegraed Power and SunPower. See wheher hey have Web sies. How many of hem are sill in he business? Maybe you'll even wan o inves in some of hem The Enron Corporaion has been known as a naural gas company (see Invesigaion 9-6). Bu hey were (and sill are?) also ineresed in phoovolaics ( Solar Power, for Earhly Prices, NYT of 11/15/94). Find ou abou he fae of heir pledge o deliver he elecriciy a 5.5 cens a kilowahour in abou wo years. See also NYT of 12/4/94 ( Thirsy New Solar Cells Drink In he Sun's Energy ) Find ou more abou he direc conversion of chemical energy o elecriciy accomplished by fuel cells. See BW of 5/27/96 ( How To Build a Clean Machine: Fuel cells are powering hospials. Cars are down he road. ) and Economis of 2/5/94 ( The differen engine: Fuel cells are efficien, clean and quie. So why are hey also rare? ) Find ou more abou he possibiliies of using gaseous hydrogen (H 2 ) as a fuel. See NYT of 4/16/95 ( Use of Hydrogen as Fuel Is Moving Closer o Realiy ). See also Popular Science of 10/93 ( The Oulook for Hydrogen ).