英文資料ENERGYFROMTHESUNThesunhasproducedenergyforbillionsofyears.Solarenergyisthesolarradiationthatreachestheearth.Solarenergycanbeconverteddirectlyorindirectlyintootherformsofenergy,suchasheatandelectricity.Themajordrawbacks(problems,orissuestoovercome)ofsolarenergyare:(1)theintermittentandvariablemannerinwhichitarrivesattheearthssurfaceand,(2)thelargearearequiredtocollectitatausefulrate.Solarenergyisusedforheatingwaterfordomesticuse,spaceheatingofbuildings,dryingagriculturalproducts,andgeneratingelectricalenergy.Inthe1830s,theBritishastronomerJohnHerschelusedasolarcollectorboxtocookfoodduringanexpeditiontoAfrica.Now,peoplearetryingtousethesunsenergyforlotsofthings.Electricutilitiesaretryingphotovoltaics,aprocessbywhichsolarenergyisconverteddirectlytoelectricity.Electricitycanbeproduceddirectlyfromsolarenergyusingphotovoltaicdevicesorindirectlyfromsteamgeneratorsusingsolarthermalcollectorstoheataworkingfluid.Outofthe14knownsolarelectricgeneratingunitsoperatingintheUSattheendof2004,10oftheseareinCalifornia,and4inArizona.Nostatisticsarebeingcollectedonsolarplantsthatproducelessthan1megawattofelectricity,sotheremaybesmallersolarplantsinanumberofotherstates.PHOTOVOLTAICENERGYPhotovoltaicenergyistheconversionofsunlightintoelectricitythroughaphotovoltaic(PVs)cell,commonlycalledasolarcell.Aphotovoltaiccellisanonmechanicaldeviceusuallymadefromsiliconalloys.Sunlightiscomposedofphotons,orparticlesofsolarenergy.Thesephotonscontainvariousamountsofenergycorrespondingtothedifferentwavelengthsofthesolarspectrum.Whenphotonsstrikeaphotovoltaiccell,theymaybereflected,passrightthrough,orbeabsorbed.Onlytheabsorbedphotonsprovideenergytogenerateelectricity.Whenenoughsunlight(energy)isabsorbedbythematerial(asemiconductor),electronsaredislodgedfromthematerialsatoms.Specialtreatmentofthematerialsurfaceduringmanufacturingmakesthefrontsurfaceofthecellmorereceptivetofreeelectrons,sotheelectronsnaturallymigratetothesurface.Whentheelectronsleavetheirposition,holesareformed.Whenmanyelectrons,eachcarryinganegativecharge,traveltowardthefrontsurfaceofthecell,theresultingimbalanceofchargebetweenthecellsfrontandbacksurfacescreatesavoltagepotentiallikethenegativeandpositiveterminalsofabattery.Whenthetwosurfacesareconnectedthroughanexternalload,electricityflows.ThephotovoltaiccellisthebasicbuildingblockofaPVsystem.Individualcellscanvaryinsizefromabout1cm(1/2inch)toabout10cm(4inches)across.However,onecellonlyproduces1or2watts,whichisntenoughpowerformostapplications.Toincreasepoweroutput,cellsareelectricallyconnectedintoapackagedweather-tightmodule.Modulescanbefurtherconnectedtoformanarray.Thetermarrayreferstotheentiregeneratingplant,whetheritismadeupofoneorseveralthousandmodules.Asmanymodulesasneededcanbeconnectedtoformthearraysize(poweroutput)needed.Theperformanceofaphotovoltaicarrayisdependentuponsunlight.Climateconditions(e.g.,clouds,fog)haveasignificanteffectontheamountofsolarenergyreceivedbyaPVarrayand,inturn,itsperformance.Mostcurrenttechnologyphotovoltaicmodulesareabout10percentefficientinconvertingsunlightwithfurtherresearchbeingconductedtoraisethisefficiencyto20percent.Thepvcellwasdiscoveredin1954byBellTelephoneresearchersexaminingthesensitivityofaproperlypreparedsiliconwafertosunlight.Beginninginthelate1950s,pvswereusedtopowerU.S.spacesatellites.ThesuccessofPVsinspacegeneratedcommercialapplicationsforpvtechnology.Thesimplestphotovoltaicsystemspowermanyofthesmallcalculatorsandwristwatchesusedeveryday.Morecomplicatedsystemsprovideelectricitytopumpwater,powercommunicationsequipment,andevenprovideelectricitytoourhomes.Photovoltaicconversionisusefulforseveralreasons.Conversionfromsunlighttoelectricityisdirect,sothatbulkymechanicalgeneratorsystemsareunnecessary.Themodularcharacteristicofphotovoltaicenergyallowsarraystobeinstalledquicklyandinanysizerequiredorallowed.Also,theenvironmentalimpactofaphotovoltaicsystemisminimal,requiringnowaterforsystemcoolingandgeneratingnoby-products.Photovoltaiccells,likebatteries,generatedirectcurrent(DC)whichisgenerallyusedforsmallloads(electronicequipment).WhenDCfromphotovoltaiccellsisusedforcommercialapplicationsorsoldtoelectricutilitiesusingtheelectricgrid,itmustbeconvertedtoalternatingcurrent(AC)usinginverters,solidstatedevicesthatconvertDCpowertoAC.Historically,pvshavebeenusedatremotesitestoprovideelectricity.However,amarketfordistributedgenerationfromPVsmaybedevelopingwiththeunbundlingoftransmissionanddistributioncostsduetoelectricderegulation.Thesitingofnumeroussmall-scalegeneratorsinelectricdistributionfeederscouldimprovetheeconomicsandreliabilityofthedistributionsystem.SOLARTHERMALHEATThemajorapplicationsofsolarthermalenergyatpresentareheatingswimmingpools,heatingwaterfordomesticuse,andspaceheatingofbuildings.Forthesepurposes,thegeneralpracticeistouseflat-platesolar-energycollectorswithafixedorientation(position).Wherespaceheatingisthemainconsideration,thehighestefficiencywithafixedflat-platecollectorisobtainedifitfacesapproximatelysouthandslopesatanangletothehorizonequaltothelatitudeplusabout15degrees.Solarcollectorsfallintotwogeneralcategories:nonconcentratingandconcentrating.Inthenonconcentratingtype,thecollectorarea(i.e.theareathatinterceptsthesolarradiation)isthesameastheabsorberarea(i.e.,theareaabsorbingtheradiation).Inconcentratingcollectors,theareainterceptingthesolarradiationisgreater,sometimeshundredsoftimesgreater,thantheabsorberarea.Wheretemperaturesbelowabout200oFaresufficient,suchasforspaceheating,flat-platecollectorsofthenonconcentratingtypearegenerallyused.Therearemanyflat-platecollectordesignsbutgenerallyallconsistof(1)aflat-plateabsorber,whichinterceptsandabsorbsthesolarenergy,(2)atransparentcover(s)thatallowssolarenergytopassthroughbutreducesheatlossfromtheabsorber,(3)aheat-transportfluid(airorwater)flowingthroughtubestoremoveheatfromtheabsorber,and(4)aheatinsulatingbacking.Solarspaceheatingsystemscanbeclassifiedaspassiveoractive.Inpassiveheatingsystems,theairiscirculatedpastasolarheatsurface(s)andthroughthebuildingbyconvection(i.e.lessdensewarmairtendstorisewhilemoredensecoolerairmovesdownward)withouttheuseofmechanicalequipment.Inactiveheatingsystems,fansandpumpsareusedtocirculatetheairortheheatabsorbingfluid.SOLARTHERMALPOWERPLANTSSolarthermalpowerplantsusethesunsraystoheatafluid,fromwhichheattransfersystemsmaybeusedtoproducesteam.Thesteam,inturn,isconvertedintomechanicalenergyinaturbineandintoelectricityfromaconventionalgeneratorcoupledtotheturbine.Solarthermalpowergenerationisessentiallythesameasconventionaltechnologiesexceptthatinconventionaltechnologiestheenergysourceisfromthestoredenergyinfossilfuelsreleasedbycombustion.Solarthermaltechnologiesuseconcentratorsystemsduetothehightemperaturesneededfortheworkingfluid.PARABOLICTROUGHTheparabolictroughisusedinthelargestsolarpowerfacilityintheworldlocatedintheMojaveDesertatKramerJunction,California.Thisfacilityhasoperatedsincethe1980andaccountedforthemajorityofsolarelectricityproducedbytheelectricpowersectorin2004.Aparabolictroughcollectorhasalinearparabolic-shapedreflectorthatfocusesthesunsradiationonalinearreceiverlocatedatthefocusoftheparabola.Thecollectortracksthesunalongoneaxisfromeasttowestduringthedaytoensurethatthesuniscontinuouslyfocusedonthereceiver.Becauseofitsparabolicshape,atroughcanfocusthesunat30to100timesitsnormalintensity(concentrationratio)onareceiverpipelocatedalongthefocallineofthetrough,achievingoperatingtemperaturesover400degreesCelcius.Acollectorfieldconsistsofalargefieldofsingle-axistrackingparabolictroughcollectors.Thesolarfieldismodularinnatureandiscomposedofmanyparallelrowsofsolarcollectorsalignedonanorth-southhorizontalaxis.Aworking(heattransfer)fluidisheatedasitcirculatesthroughthereceiversandreturnstoaseriesofheatexchangersatacentrallocationwherethefluidisusedtogeneratehigh-pressuresuperheatedsteam.Thesteamisthenfedtoaconventionalsteamturbine/generatortoproduceelectricity.Aftertheworkingfluidpassesthroughtheheatexchangers,thecooledfluidisrecirculatedthroughthesolarfield.Theplantisusuallydesignedtooperateatfullratedpowerusingsolarenergyalone,givensufficientsolarenergy.However,allplantsarehybridsolar/fossilplantsthathaveafossil-firedcapabilitythatcanbeusedtosupplementthesolaroutputduringperiodsoflowsolarenergy.TheLuzplantisanaturalgashybrid.SOLARDISHAsolardish/enginesystemutilizesconcentratingsolarcollectorsthattrackthesunontwoaxes,concentratingtheenergyatthefocalpointofthedishbecauseitisalwayspointedatthesun.Thesolardishsconcentrationratiois