Molecularly Wired Nanostructured Assemblies for Solar Energy Conversion. After Oil

Transcription

1 Molcularly Wird Nanostructurd Assmblis for Solar Enrgy Convrsion Thrmal a. Hating b. Elctricity Photoconvrsion a. Photosynthsis b. Solar H 2 c. Photovoltaics d. Emrging aras Aftr Oil Byond Fossil Fuls Farming Solar Graph: World Oil Production Sourc: Dr. C.J. Campbll "Undrstanding dpltion is simpl. Think of an Irish pub. Th glass starts full and nds mpty. Thr ar only so many mor drinks to closing tim. It s th sam with oil. W hav to find th bar bfor w can drink what is in it." Campbll 1

2 Global warming ovr th past millnnium Vry rapidly w hav ntrd unchartd trritory - what som call th anthropocn climat rgim. Ovr th 20 th cntury, human population quadrupld and nrgy consumption incrasd sixtnfold. Nar th nd of th last cntury, w crossd a critical thrshold, and global warming from th fossil ful grnhous bcam a major, and incrasingly dominant, factor in climat chang. Global man surfac tmpratur is highr today than it s bn for at last a millnnium. Marty Hoffrt NYU Th Unitd Nations Framwork Convntion on Climat Chang calls for stabilization of grnhous-gas concntrations in th atmosphr at a lvl that would prvnt dangrous anthropognic intrfrnc with th climat systm.... A standard baslin scnario that assums no policy intrvntion to limit grnhous-gas missions has 10 TW (10 x watts) of carbon-mission-fr powr bing producd by th yar 2050, quivalnt to th powr providd by all today s nrgy sourcs combind..nature, VOL 395, 881,1998 2

3 Thr possibl options for mting th 10 TW- Challng by 2050 Carbon Nutral Enrgy (fossil ful inconjunction with carbon squstration) -Nd to find scur storag for 25 billion mtric tons of CO 2 producd annually (qual to th volum of km 3 or volum of lak suprior!) Nuclar Powr -Rquirs construction of a nw on-gigawatt-lctric (1-GW) nuclar fission plant vryday for th nxt 50 yars Rnwabl Enrgy Sourcs - hydrolctric rsourc 0.5 TW - from all tids & ocan currnts 2 TW - gothrmal intgratd ovr all th land ara 12 TW - globally xtractabl wind powr 2-4 TW - solar nrgy striking th arth 120,000 TW!!! Th Silvr Lining. Th arth rcivs mor nrgy from th sun in just on hour than th world uss in a whol yar. Cumulativ solar nrgy production accounts for lss than 0.01% of total Global Primary Enrgy dmand. Solar Enrgy dmand has grown at about 25% pr annum ovr th past 15 yars (hydrocarbon nrgy dmand typically grows btwn 0-2% pr annum). Worldwid photovoltaic installations incrasd by 927 MW in 2004, up from 574 MW installd during th prvious yar. An avrag crystallin silicon cll solar modul has an fficincy of 15%, an avrag thin film cll solar modul has an fficincy of 6%. (Thin film manufacturing costs potntially ar lowr, though.) Solar Enrgy (photovoltaic) prics hav dclind on avrag 4% pr annum ovr th past 15 yars. For th Fiscal Yar 2002, th Japans solar roof top program rcivd applications from 42,838 housholds. Without incntiv programs, solar nrgy costs (in an avrag sunny climat) rang btwn cnts/kwh for vry larg PV systms. (installation costs $8-$10 with no govrnmnt incntivs) Japan has takn ovr from th Unitd Stats as th largst nt xportr of PV clls and moduls. Around 50 % of th world's solar cll production was manufacturd in Japan in Unitd Stats accountd for 12%. 3

4 Solar Enrgy Rflction 30% 173, 000 TW Long Wavlngth Radiation Winds Wavs Convction <1% (370 TW) Convrsion to Hat 47% Solar Enrgy Cycl Storag in plants <1% (90TW) EARTH Fossil Fuls Evaporation Storag in watr/ic 23% Tidal Volcanic 3 TW 30TW Thrmal, Nuclar Solar Enrgy Potntial Thortical: 1.2x10 5 TW solar nrgy potntial (1.76 x10 5 TW striking Earth; 0.30 Global man albdo) Practical: 600 TW solar nrgy potntial (50 TW TW dpnding on land fraction tc.; WEA 2000) Onshor lctricity gnration potntial of 60 TW (10% convrsion fficincy): Photosynthsis: 90 TW 4

5 PV Land Ara Rquirmnts U.S. Land Ara: 9.1x10 12 m 2 (incl. Alaska) Avrag Insolation: 200 W/m U.S. Primary Powr Consumption: 99 Quads=3.3 TW 1999 U.S. Elctricity Consumption = 0.4 TW Hnc: 3.3x10 12 W/(2x10 2 W/m 2 x 10% Efficincy) = 1.6x10 11 m 2 Rquirs 1.6x10 11 m 2 / 9.1x10 12 m 2 = 1.7% of Land PV Land Ara Rquirmnts 1.2x10 5 TW of solar nrgy potntial globally Gnrating 2x10 1 TW with 10% fficint solar farms rquirs 2x10 2 /1.2x10 5 = 0.16% of Glob = 8x10 11 m 2 (i.., 8.8 % of U.S.A) Gnrating 1.2x10 1 TW (1998 Global Primary Powr) rquirs 1.2x10 2 /1.2x10 5 = 0.10% of Glob = 5x10 11 m 2 (i.., 5.5% of U.S.A.) 5

6 PV Land Ara Rquirmnts 3 TW 20 TW PV Land Ara Rquirmnts 6 Boxs at 3.3 TW Each 6

7 U.S. Singl Family Housing Roof Ara 7x10 7 dtachd singl family homs in U.S sq ft/roof = 44ft x 44 ft = 13 m x 13 m = 180 m 2 /hom = 1.2x10 10 m 2 total roof ara 2000 Hnc can (only) supply 0.25 TW, or 1/10 th of U.S. Primary Enrgy Consumption (U.S. Elctricity Consumption = 0.4 TW) Solar Enrgy Thrmal Convrsion Infrard Photons E=hν Photoconvrsion Enrgtic Photons in th visibl rgion Hating Elctricity Gnration Photosynthsis Ful Production Photovoltaics /Elctricity Gnration E = hc/λ = /λ (kj/mol) Or E= 1240/λ 7

8 Optimum Absorption Thrshold Irradianc, mw/(cm 2 -nm) 0.12 AM 1.5 Solar Spctrum Wavlngth, nm Too Larg Band Gap Too Small Band Gap Ia. Thrmal Convrsion: Solar Hating Roughly qual global nrgy us in ach major sctor: transportation, rsidntial, transformation, industrial World markt: 1.6 TW spac hating; 0.3 TW hot watr; 1.3 TW procss hat (solar crop drying: 0.05 TW) Tmporal mismatch btwn sourc and dmand rquirs storag ( S) yilds high hat production costs: ($0.03-$0.20)/kW-hr High-T solar thrmal: currntly lowst cost solar lctric sourc ($ /kW-hr); potntial to b comptitiv with fossil nrgy in long trm, but nds larg aras in sunblt Solar-to-lctric fficincy 18-20% (rsarch in thrmochmical fuls: hydrogn, syn gas, mtals) 8

9 1b. Thrmal Convrsion: Elctricity Gnration Concntrating solar powr (CSP) focuss on thr typs of CSP tchnologis: trough systms, dish/ngin systms, and powr towrs. Ths tchnologis ar usd in CSP plants that us diffrnt kinds of mirror configurations to convrt th sun's nrgy into high-tmpratur hat. Th hat nrgy is thn usd to gnrat lctricity in a stam gnrator. CSP's rlativly low cost and ability to dlivr powr during priods of pak dmand whn and whr w nd it man that CSP can b a major contributor to th nation's futur nds for distributd sourcs of nrgy. At th National Solar Thrmal Tst Facility (NSTTF), built at Sandia National Laboratoris in Albuqurqu in 1976, th 63-mtr tall towr has 222 computr-controlld hliostats that can dirct th sun into any of four tst bays to produc a total thrmal capacity of 5 mgawatts. Solar thrmal powr plant. (Mojav Dsrt in Kramr Junction, California) This is on of nin such plants built in th 1980s. During opration, oil in th rcivr tubs collcts th concntratd solar nrgy as hat and is pumpd to a powr block (in background) for gnrating lctricity. Solar dish-ngin systm. Th dish, a concntrator, collcts th nrgy coming dirctly from th sun and concntrats it on a small ara. A thrmal rcivr absorbs th concntratd bam of solar nrgy, convrts it to hat, and transfrs th hat to th ngin/gnrator. (Crdit: Sandia National Laboratoris) 9

10 Solar Two, a 10-mgawatt cntral rcivr powr towr that opratd in Daggtt, CA. This facility dmonstratd th fasibility of powr-towr systms, a solar-thrmal lctric or concntrating solar powr tchnology. In 1988, th final yar of opration, th systm could b dispatchd 96% of th tim. Moltn-Salt Powr Towr Tchnology In a moltn-salt powr towr, sun-tracking hliostats can concntrat solar nrgy up to 1000 tims onto a cntral, towr-mountd rcivr. Moltn nitrat salt, which is a clar liquid with proprtis lik watr at tmpraturs abov its 240 C mlting point, is pumpd from a larg storag tank to th rcivr, whr it is hatd in tubs to tmpraturs of 565 C. Th salt is thn rturnd to a scond larg storag tank, whr it rmains until ndd by th utility for powr gnration. At that tim, th salt is pumpd through a stam gnrator to produc th stam to powr a convntional, high-fficincy stam turbin to produc lctricity. Th salt at 285ƒC thn rturns to th first storag tank to b usd in th cycl again. 10

11 10-mgawatt Solar Two powr towr pilot plant nar Barstow, California Prdictd (rd) and actual (blu) solar nrgy collction and powr gnration. Ovr on 30-day priod, Solar Two producd 1633 mgawatthours, xcding its on-month prformanc goal of 1500 mgawatt-hours of powr production. Th plant also producd a rcord gross turbin powr output of 11.6 mgawatts Succssfully compltd oprations in April a. Photosynthsis Photosynthsis is th procss by which plants, som bactria, and som protistans us th nrgy from sunlight to produc sugar, which cllular rspiration convrts into ATP, th "ful" usd by all living things. Th convrsion of unusabl sunlight nrgy into usabl chmical nrgy, is associatd with th actions of th grn pigmnt chlorophyll. Most of th tim, th photosynthtic procss uss watr and rlass th oxygn that w absolutly must hav to stay aliv. 6H 2 O + 6CO > C 6 H 12 O 6 + 6O

12 2a. Photosynthsis Photosynthsis is a two stag procss. Th first procss is th Light Dpndnt Procss (Light Ractions), rquirs th dirct nrgy of light to mak nrgy carrir molculs that ar usd in th scond procss. Th Light Indpndnt Procss (or Dark Ractions) occurs whn th products of th Light Raction ar usd to form C-C covalnt bonds of carbohydrats. Th Dark Ractions can usually occur in th dark, if th nrgy carrirs from th light procss ar prsnt. 2a. Photosynthsis adnosin diphosphat (ADP) adnosin triphosphat (ATP) nicotin adnin dinuclotid phosphat (NADP+) NADPH 12

13 Hydrogn Production Watr molculs can b split into hydrogn and oxygn atoms using alga, on-clld organisms that thriv in watr. Th grn alga Chlamydomonas rinhardtii can produc hydrogn and oxygn from watr undr crtain conditions. Ths alga normally grow nw clls by photosynthsis, using carbon dioxid from th air in th prsnc of sunlight. But aftr placing th aquatic organisms in a larg flask of watr illuminatd by lamps, th ORNL rsarchrs "trick" th alga by dpriving thm of carbon dioxid and oxygn. As a rsult, a normally dormant gn bcoms activatd, lading to th synthsis of th nzym hydrognas. Th alga us this nzym to produc both hydrogn and oxygn from watr. Th rlativ amounts of oxygn and hydrogn that volv in th flask ar masurd by swping th gass ovr hydrogn and oxygn snsors, whos lctrical conductivity incrass with rising gas concntration. Ful Altrnativs Altrnativ Fuls Biodisl ful us is on th ris. Mad from natural, rnwabl sourcs (vg oils, animal fats). Can b usd as pur ful or blndd with ptrolum Ethanol is rnwabl, but currntly mor xpnsiv than gasolin. Critics argu that it taks mor nrgy to produc a gallon of thanol than you will obtain from burning it. Conflict of Intrst: National Corn Growrs vs. Amrican Ptrolum Institut 13

14 Two opposing viws Cornll cologist, David Pimntl study finds that producing thanol and biodisl from corn and othr crops is not worth th nrgy -corn rquirs 29 prcnt mor fossil nrgy than th ful producd. Ethanol can rplac gasolin with significant nrgy savings, comparabl impact on grnhous gass (Jan 26, 2006) Danil M. Kammn, UC Brkly Dspit th uncrtainty, it appars that thanol mad from corn is a littl bttr may b 10 or 15 prcnt - than gasolin in trms of grnhous gas production SCIENCE, 27 JANUARY 2006 VOL This analysis taks into account nrgy gain from coproducts Nt Enrgy Summary * - adjustd for comparability Patzk Pimntl Shapouri Graboski d Olivira Wang Farrll Scinc papr 2006 Input nrgy (kj/ml) Rportd HV of thanol (MJ/L) Coproduct crdits (kj/ml) Coproducts as % of total nrgy 15% 7% 36% 19% 20% 21% 20% Output Enrgy (MJ/L) Nt nrgy valu, NEV (kj/ml) Nt nrgy ratio Sourc: Farrll t al SCIENCE, 2006, 311,

15 2b. Photolctrochmical Hydrogn Production Photolctrochmical Production of Hydrogn SrTiO 3 KTaO 3 TiO 2 SnO 2 O2 H2O h + F 2 O 3 Solid Liquid mtal H2 H2O Low surfac ara Highr cost Highr fficincy Light is Convrtd to Elctrical + Chmical Enrgy Singl Crystal Smiconductor Solar-Drivn Photolctrochmical Watr Splitting H 2 O 2 Glass or plxiglass Aquous lctrolyt High surfac ara Low cost Low fficincy Porous mmbran Photolctrochmical clls H 2 O 2 Stainlss stl or conducting plastic Polycrystallin or nanostructurd films Photolctrochmical Cll - O H 2 H 2 O SrTiO 3 h + mtal H 2 O Solid Liquid Light is Convrtd to Elctrical+Chmical Enrgy 15

16 Band Edg and Enrgtic Considrations Bandgap Band dg ovrlap Fast charg transfr All thr nrgtic conditions must b satisfid SIMULTANEOUSLY + Stability H 2 O / H 2 V vs. NHE SIC CB GaAsCdS CdS Eg > 1.7 V p-typ smiconductor 1.23 V H 2 O / H 2 Countr Elctrod VB TiO 2 MoS 2 F 2 O 2 In 2 O 3 WO H + /H 2 CdO V E g +3.0 Smiconductor (.g., TiO 2 ) nanoparticls for hydrogn production H 2 2H + 2H + hν Pt t + CB h t + VB 4OH 2H 2 O+ O 2 Hydrogn Evolution rats for various Photocatalysts (µl/hr) Pt/TiO Pd/TiO Rh/TiO Ru/TiO Sn/TiO Ni/TiO TiO 2 <0.1 Toshima, J. Phys. Chm. 1985, 89,

17 2c Photolctric Effct Th nrgy of th absorbd light is transfrrd to lctrons in th atoms of th PV cll. With thir nwfound nrgy, ths lctrons scap from thir normal positions in th atoms of th smiconductor PV matrial and bcom part of th lctrical flow, or currnt, in an lctrical circuit. Timlin 1839 Edmond Bcqurl discovrd th procss of using sunlight to produc an lctric currnt in a solid matrial. But it took mor than anothr cntury to truly undrstand this procss. Scintists vntually larnd that th photolctric or photovoltaic (PV) ffct causd crtain matrials to convrt light nrgy into lctrical nrgy at th atomic lvl Albrt Einstin publishs his papr on th photolctric ffct, along with a papr on his thory of rlativity. -Nobl Priz was awardd for this discovry in Photovoltaic tchnology is born in th Unitd Stats whn Daryl Chapin, Calvin Fullr, and Grald Parson dvlop th silicon photovoltaic (or PV) cll at Bll Labs th first solar cll capabl of gnrating nough powr from th sun to run vryday lctrical quipmnt. Bll Tlphon Laboratoris thn producs a silicon solar cll with 6% fficincy and latr, 11% fficincy. S th California Solar Cntr for mor information. 17

18 Timlin 1964 NASA launchs th first Nimbus spaccraft a satllit powrd by a 470- watt photovoltaic array. S NASA's Nimbus Program for mor information Exxon Corporation & Dr. Elliot Brman dsigns a significantly lss costly solar cll, bringing th pric down from $100 pr watt to $20 pr watt. Solar clls bgin powring navigation warning lights and horns on offshor gas and oil rigs, and railroad crossings ARCO Solar bcoms th first company to produc mor than 1 mgawatt (a thousand kilowatts) of photovoltaic moduls in on yar Pacific Gas & Elctric installs th first gridsupportd photovoltaic systm in Krman, California. Th 500-kilowatt systm is th first "distributd powr" PV installation Hom Dpot bgins slling rsidntial solar powr systms in thr stors in San Digo, California. A yar latr it xpands sals to 61 stors nationwid. In Spring 2002, largst solar lctric systm in th US bgan oprating a top th Santa Rita Jail in Dublin, California. This solar installation, hlps Alamda County rduc and stabiliz nrgy costs. Efficincy of Solar Clls Efficincy Compard with Cost Pr Unit Ara of PV Dvics (Th diagonal lins show installd 2001 pric of moduls pr pak-watt. Th thortical limit for Shockly-Quissr dvics [prsnt limit] is 32 Third gnration dvics [shown in rd] may xcd this limit by using multipl absorbrs, hot carrir ffcts, or photocurrnt doubling via impact ionization. Th lattr two phnomna ar associatd with quantum siz ffcts in smiconductors and ar bing studid in smiconductor nanocrystals). L. Kazmrski, Solar-Elctric Powr: A 2001 Dvic Ovrviw, National Cntr for Photovoltaics, National Rnwabl Enrgy Laboratory, Goldn, CO (2001). M. Grn, Annual Rport, Third Gnration Photovoltaics, Univrsity of Nw South Wals, Sydny, Australia (2000). 18

19 n-and p-typ Silicon Doping with B or Ga lavs xcss hols Doping with P or As lavs xcss lctrons Introduction of an atom of anothr lmnt into th silicon crystal to altr its lctrical proprtis. Th "dopant," which is th introducd lmnt, has ithr thr or fiv valnc lctrons In a crystallin silicon cll, w nd to contact p-typ silicon with n-typ silicon to crat th built-in lctrical fild. Whn photons of sun light strik a PV cll, only th photons with a crtain lvl of nrgy ar abl to fr lctrons from thir atomic bonds to produc an lctric currnt. This lvl of nrgy, known as th band-gap nrgy, is th amount of nrgy rquird to mov an outr-shll lctron from th valnc band (or lvl) to th conduction band (or lvl). It is diffrnt for ach matrial and for diffrnt atomic structurs of th sam matrial. For crystallin silicon, th band-gap nrgy is 1.1 lctron-volts (V). An lctron-volt is qual to th nrgy an lctron acquirs whn it passs through a potntial of 1 volt in a vacuum. Othr PV cll matrials hav band-gap nrgis ranging from 1 to 3.3 V. Dtrmin th wavlngth of light rquird for bandgap xcitation. 19

20 Principl of PV opration Whn n- and p-typ silicon com into contact, xcss lctrons mov from th n-typ sid to th p-typ sid. Th rsult is a buildup of positiv charg along th n-typ sid of th intrfac and a buildup of ngativ charg along th p-typ sid. Bcaus of th flow of lctrons and hols, th two smiconductors bhav lik a battry, crating an lctric fild at th surfac whr thy mt what w call th p/n junction. Th lctrical fild causs th lctrons to mov from th smiconductor toward th ngativ surfac, whr thy bcom availabl to th lctrical circuit. At th sam tim, th hols mov in th opposit dirction, toward th positiv surfac, whr thy await incoming lctrons. Grid contacts on th top surfac of a typical cll ar dsignd to hav many thin, conductiv fingrs sprading to vry part of th cll's surfac. 20

21 Th powr from an lctrical dvic such as a PV cll is qual to th product of th voltag (V) and th th currnt (I). Low-band-gap clls hav high currnt but low voltag; high-band-gap clls hav high voltag and low currnt. A compromis is ncssary in th dsign of PV clls. Clls mad of matrials with band gaps btwn 1 V and 1.8 V can b usd fficintly in PV dvics. P max Powr (W)= Currnt (A) x Voltag (V) Voltag Currnt 21

22 Emrging Aras THIN Film Solar Clls Organic Solar Clls Dy Snsitizd Solar Clls Quantum Dot Solar Clls Cu(In,Ga)S 2 (CIGS) solar clls CIGS solar clls ar formd by fiv vry thin films on a plat of glass. Th pictur abov shows, schmatically, th structur of a solar cll. First th molybdnum back contact (th plus sid) is dpositd onto th glass plat. On top of that layr, th CIGS film, which absorbs th light, is vaporatd. Two vry thin films of cadmium sulfid (CdS) and zinc oxid (ZnO) ar thn put ontop of th CIGS, and th solar cll is compltd by a layr of aluminium dopd zinc oxid (ZnO:Al). 22

23 Organic Solar Clls Th activ polymr layr is sandwichd btwn two conducting lctrods. On of th lctrods is transparnt to lt th light in or out dpnding on th application. An additional conducting polymr layr, calld PEDOT, is somtims usd to flattn th transport contact and hlp injct / carry positiv chargs in or out of th dvic. 23

24 Nanocrystallin Titanium Dioxid Particl Siz nm 35 nm Bandgap 3.2 V (absorbs only in UV) Nanostructurd TiO 2 film 200 nm Surfac Ara is largr than singl crystal ~1000 tims Commonly prpard in rutil and anatas forms Diffrnt Elctrochmistry from singl crystal smiconductors nanocrystallin solar cll hν conducting glass - - LOAD S* S Rd i mtal film S + Ox dy-snsitizd nanocrystallin TiO 2 B. O Rgan, M. Grätzl Natur 1991, 353,

25 Dy Snsitizd Photochmical Solar Clls Dvlopmnt of SC nanoclustr basd clls with mor than 10% powr convrsion fficincy. Photon-to-photocurrnt fficincy up to 100% has bn claimd! Sourc: Principl of Dy-snsitizd Photochmical Solar Cll M. Grätzl, Natur 2001, 414, B. O Rgan, M. Grätzl, Natur 1991, 353,

26 Quantum Dot Solar Clls Tunabl band dg Offrs th possibility to harvst light nrgy ovr a wid rang of visibl-ir light with slctivity Hot carrir injction from highr xcitd stat (minimizing nrgy loss during thrmalization of xcitd stat) Multipl xciton gnration solar clls. Impact ionization allows singl high nrgy photons to multipl lctron-hol pairs CdS Quantum Dot Solar Clls a b c d hν OTE TiO 2 h CdS h h h h h h h h Rd Ox IPCE, % d c b a TiO 2 b TiO 2 /TAA/CdS c TiO 2 /MDA/CdS d TiO 2 /MPA/CdS 5 0 a Wavlngth, nm Robl, Bunkr, Kamat J. Am. Chm. Soc. 2006,128,

27 Photochmical Solar Clls Each modul 24 cm x24 cm AISIN & TOYOTA Konarka Wall-intgratd DSC panls (Prototyp) Total ara: 2.25 m 2.5 m 4 = 22 m 2 27