Photovoltaic--Power--Generation

PHOTOVOLTAICPOWERGENERATIONABSTRACTThisreportisanoverviewofphotovoltaicpowergeneration.ThepurposeofthereportistoprovidethereaderwithageneralunderstandingofphotovoltaicpowergenerationandhowPVtechnologycanbepracticallyapplied.Thereisabriefdiscussionofearlyresearchandadescriptionofhowphotovoltaiccellsconvertsunlighttoelectricity.Thereportcoversconcentratingcollectors,flat-platecollectors,thin-filmtechnology,andbuilding-integratedsystems.Thediscussionofphotovoltaiccelltypesincludessingle-crystal,poly-crystalline,andthin-filmmaterials.Thereportcoversprogressinimprovingcellefficiencies,reducingmanufacturingcost,andfindingeconomicapplicationsofphotovoltaictechnology.Listsofmajormanufacturersandorganizationsareincluded,alongwithadiscussionofmarkettrendsandprojections.Theconclusionisthatphotovoltaicpowergenerationisstillmorecostlythanconventionalsystemsingeneral.However,largevariationsincostofconventionalelectricalpower,andotherfactors,suchascostofdistribution,createsituationsinwhichtheuseofPVpoweriseconomicallysound.PVpowerisusedinremoteapplicationssuchascommunications,homesandvillagesindevelopingcountries,waterpumping,camping,andboating.Grid-connectedapplicationssuchaselectricutilitygeneratingfacilitiesandresidentialrooftopinstallationsmakeupasmallerbutmorerapidlyexpandingsegmentofPVuse.Furthermore,astechnologicaladvancesnarrowthecostgap,moreapplicationsarebecomingeconomicallyfeasibleatanacceleratingrate.INTRODUCTIONThisreportistheresultofGaleGreenleaf’sOctober19,1998requestforproposal.BillLoukandTomPenickrespondedtoherrequestwithaproposal,datedOctober30,1998,tocontinueearlierresearchonphotovoltaicpowergeneration.TheproposalwasapprovedandresultedincontinuedresearchfollowedbyapresentationonNovember30,1998andthisfinalreportonphotovoltaicpowergeneration.PHOTOVOLTAICTECHNOLOGYScientistshaveknownofthephotovoltaiceffectformorethan150years.Photovoltaicpowergenerationwasnotconsideredpracticaluntilthearrivalofthespaceprogram.Earlysatellitesneededasourceofelectricalpowerandanysolutionwasexpensive.Thedevelopmentofsolarcellsforthispurposeledtotheireventualuseinotherapplications.DISCOVERYANDDEVELOPMENTOFPHOTOVOLTAICPOWERThephotovoltaiceffecthasbeenknownsince1839,butcellefficienciesremainedaround1%untilthe1950swhenU.S.researcherswereessentiallygivenablankchecktodevelopameansofgeneratingelectricityonboardspacevehicles.BellLaboratoriesquicklyachieved11%efficiency,andin1958,theVanguardsatelliteemployedthefirstpracticalphotovoltaicgeneratorproducingamodestonewatt.Inthe1960s,thespaceprogramcontinuedtodemandimprovedphotovoltaicpowergenerationtechnology.Scientistsneededtogetasmuchelectricalpoweraspossiblefromphotovoltaiccollectors,andcostwasofsecondaryimportance.Withoutthistremendousdevelopmenteffort,photovoltaicpowerwouldbeoflittleusetoday.POWEROUTPUTANDEFFICIENCYRATINGSThefiguresgivenforpoweroutputandefficiencyofphotovoltaiccells,modules,andsystemscanbemisleading.Itisimportanttounderstandwhatthesefiguresmeanandhowtheyrelatetothepoweravailablefrominstalledphotovoltaicgeneratingsystems.PowerRatingsPhotovoltaicpowergenerationsystemsareratedinpeakkilowatts(kWp).Thisistheamountofelectricalpowerthatanew,cleansystemisexpectedtodeliverwhenthesunisdirectlyoverheadonaclearday.Wecansafelyassumethattheactualoutputwillneverquitereachthisvalue.Systemoutputwillbecompromisedbytheangleofthesun,atmosphericconditions,dustonthecollectors,anddeteriorationofthecomponents.Whencomparingphotovoltaicsystemstoconventionalpowergenerationsystems,oneshouldbearinmindthatthePVsystemsareonlyproductiveduringthedaytime.Therefore,a100kWphotovoltaicsystemcanproduceonlyafractionofthedailyoutputofaconventional100kWgenerator.EfficiencyRatingsTheefficiencyofaphotovoltaicsystemisthepercentageofsunlightenergyconvertedtoelectricalenergy.Theefficiencyfiguresmostoftenreportedarelaboratoryresultsusingsmallcells.Asmallcellhasalowerinternalresistanceandwillyieldahigherefficiencythanthelargercellsusedinpracticalapplications.Additionally,photovoltaicmodulesaremadeupofnumerouscellsconnectedinseriestodeliverausablevoltage.Duetotheinternalresistanceofeachcell,thetotalresistanceincreasesandtheefficiencydropstoabout70%ofthesingle-cellvalue.Efficiencyishigheratlowertemperatures.Temperaturesusedinlaboratorymeasurementsmaybelowerthanthoseinapracticalinstallation.CONVERTINGSUNLIGHTTOELECTRICITYAtypicalphotovoltaiccellconsistsofsemiconductormaterial(usuallysilicon)havingap-njunctionasshowninFigure1.Sunlightstrikingthecellraisestheenergylevelofelectronsandfreesthemfromtheiratomicshells.Theelectricfieldatthep-njunctiondrivestheelectronsintothenregionwhilepositivechargesaredriventothepregion.Ametalgridonthesurfaceofthecellcollectstheelectronswhileametalback-platecollectsthepositivecharges.Figure1.HowsolarcellsworkThinFilmTechnologyThin-filmsolarcellsaremanufacturedbyapplyingthinlayersofsemiconductormaterialstoasolidbackingmaterial.Thecompositionofatypicalthin-filmcellisshowninFigure2.Sunlightenteringtheintrinsiclayergeneratesfreeelectrons.Thep-typeandn-typelayerscreateanelectricfieldacrosstheint