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英文资料ENERGYFROMTHESUNThesunhasproducedenergyforbillionsofyears.Solarenergyisthesolarradiationthatreachestheearth.Solarenergycanbeconverteddirectlyorindirectlyintootherformsofenergy,suchasheatandelectricity.Themajordrawbacksproblems,orissuestoovercomeofsolarenergyare1theintermittentandvariablemannerinwhichitarrivesattheearthssurfaceand,2thelargearearequiredtocollectitatausefulrate.Solarenergyisusedforheatingwaterfordomesticuse,spaceheatingofbuildings,dryingagriculturalproducts,andgeneratingelectricalenergy.Inthe1830s,theBritishastronomerJohnHerschelusedasolarcollectorboxtocookfoodduringanexpeditiontoAfrica.Now,peoplearetryingtousethesunsenergyforlotsofthings.Electricutilitiesaretryingphotovoltaics,aprocessbywhichsolarenergyisconverteddirectlytoelectricity.Electricitycanbeproduceddirectlyfromsolarenergyusingphotovoltaicdevicesorindirectlyfromsteamgeneratorsusingsolarthermalcollectorstoheataworkingfluid.Outofthe14knownsolarelectricgeneratingunitsoperatingintheUSattheendof2004,10oftheseareinCalifornia,and4inArizona.Nostatisticsarebeingcollectedonsolarplantsthatproducelessthan1megawattofelectricity,sotheremaybesmallersolarplantsinanumberofotherstates.PHOTOVOLTAICENERGYPhotovoltaicenergyistheconversionofsunlightintoelectricitythroughaphotovoltaicPVscell,commonlycalledasolarcell.Aphotovoltaiccellisanonmechanicaldeviceusuallymadefromsiliconalloys.Sunlightiscomposedofphotons,orparticlesofsolarenergy.Thesephotonscontainvariousamountsofenergycorrespondingtothedifferentwavelengthsofthesolarspectrum.Whenphotonsstrikeaphotovoltaiccell,theymaybereflected,passrightthrough,orbeabsorbed.Onlytheabsorbedphotonsprovideenergytogenerateelectricity.Whenenoughsunlightenergyisabsorbedbythematerialasemiconductor,electronsaredislodgedfromthematerialsatoms.Specialtreatmentofthematerialsurfaceduringmanufacturingmakesthefrontsurfaceofthecellmorereceptivetofreeelectrons,sotheelectronsnaturallymigratetothesurface.Whentheelectronsleavetheirposition,holesareformed.Whenmanyelectrons,eachcarryinganegativecharge,traveltowardthefrontsurfaceofthecell,theresultingimbalanceofchargebetweenthecellsfrontandbacksurfacescreatesavoltagepotentiallikethenegativeandpositiveterminalsofabattery.Whenthetwosurfacesareconnectedthroughanexternalload,electricityflows.ThephotovoltaiccellisthebasicbuildingblockofaPVsystem.Individualcellscanvaryinsizefromabout1cm1/2inchtoabout10cm4inchesacross.However,onecellonlyproduces1or2watts,whichisntenoughpowerformostapplications.Toincreasepoweroutput,cellsareelectricallyconnectedintoapackagedweathertightmodule.Modulescanbefurtherconnectedtoformanarray.Thetermarrayreferstotheentiregeneratingplant,whetheritismadeupofoneorseveralthousandmodules.Asmanymodulesasneededcanbeconnectedtoformthearraysizepoweroutputneeded.Theperformanceofaphotovoltaicarrayisdependentuponsunlight.Climateconditionse.g.,clouds,foghaveasignificanteffectontheamountofsolarenergyreceivedbyaPVarrayand,inturn,itsperformance.Mostcurrenttechnologyphotovoltaicmodulesareabout10percentefficientinconvertingsunlightwithfurtherresearchbeingconductedtoraisethisefficiencyto20percent.Thepvcellwasdiscoveredin1954byBellTelephoneresearchersexaminingthesensitivityofaproperlypreparedsiliconwafertosunlight.Beginninginthelate1950s,pvswereusedtopowerU.S.spacesatellites.ThesuccessofPVsinspacegeneratedcommercialapplicationsforpvtechnology.Thesimplestphotovoltaicsystemspowermanyofthesmallcalculatorsandwristwatchesusedeveryday.Morecomplicatedsystemsprovideelectricitytopumpwater,powercommunicationsequipment,andevenprovideelectricitytoourhomes.Photovoltaicconversionisusefulforseveralreasons.Conversionfromsunlighttoelectricityisdirect,sothatbulkymechanicalgeneratorsystemsareunnecessary.Themodularcharacteristicofphotovoltaicenergyallowsarraystobeinstalledquicklyandinanysizerequiredorallowed.Also,theenvironmentalimpactofaphotovoltaicsystemisminimal,requiringnowaterforsystemcoolingandgeneratingnobyproducts.Photovoltaiccells,likebatteries,generatedirectcurrentDCwhichisgenerallyusedforsmallloadselectronicequipment.WhenDCfromphotovoltaiccellsisusedforcommercialapplicationsorsoldtoelectricutilitiesusingtheelectricgrid,itmustbeconvertedtoalternatingcurrentACusinginverters,solidstatedevicesthatconvertDCpowertoAC.Historically,pvshavebeenusedatremotesitestoprovideelectricity.However,amarketfordistributedgenerationfromPVsmaybedevelopingwiththeunbundlingoftransmissionanddistributioncostsduetoelectricderegulation.Thesitingofnumeroussmallscalegeneratorsinelectricdistributionfeederscouldimprovetheeconomicsandreliabilityofthedistributionsystem.SOLARTHERMALHEATThemajorapplicationsofsolarthermalenergyatpresentareheatingswimmingpools,heatingwaterfordomesticuse,andspaceheatingofbuildings.Forthesepurposes,thegeneralpracticeistouseflatplatesolarenergycollectorswithafixedorientationposition.Wherespaceheatingisthemainconsideration,thehighestefficiencywithafixedflatplatecollectorisobtainedifitfacesapproximatelysouthandslopesatanangletothehorizonequaltothelatitudeplusabout15degrees.Solarcollectorsfallintotwogeneralcategoriesnonconcentratingandconcentrating.Inthenonconcentratingtype,thecollectorareai.e.theareathatinterceptsthesolarradiationisthesameastheabsorberareai.e.,theareaabsorbingtheradiation.Inconcentratingcollectors,theareainterceptingthesolarradiationisgreater,sometimeshundredsoftimesgreater,thantheabsorberarea.Wheretemperaturesbelowabout200oFaresufficient,suchasforspaceheating,flatplatecollectorsofthenonconcentratingtypearegenerallyused.Therearemanyflatplatecollectordesignsbutgenerallyallconsistof1aflatplateabsorber,whichinterceptsandabsorbsthesolarenergy,2atransparentcoversthatallowssolarenergytopassthroughbutreducesheatlossfromtheabsorber,3aheattransportfluidairorwaterflowingthroughtubestoremoveheatfromtheabsorber,and4aheatinsulatingbacking.Solarspaceheatingsystemscanbeclassifiedaspassiveoractive.Inpassiveheatingsystems,theairiscirculatedpastasolarheatsurfacesandthroughthebuildingbyconvectioni.e.lessdensewarmairtendstorisewhilemoredensecoolerairmovesdownwardwithouttheuseofmechanicalequipment.Inactiveheatingsystems,fansandpumpsareusedtocirculatetheairortheheatabsorbingfluid.SOLARTHERMALPOWERPLANTSSolarthermalpowerplantsusethesunsraystoheatafluid,fromwhichheattransfersystemsmaybeusedtoproducesteam.Thesteam,inturn,isconvertedintomechanicalenergyinaturbineandintoelectricityfromaconventionalgeneratorcoupledtotheturbine.Solarthermalpowergenerationisessentiallythesameasconventionaltechnologiesexceptthatinconventionaltechnologiestheenergysourceisfromthestoredenergyinfossilfuelsreleasedbycombustion.Solarthermaltechnologiesuseconcentratorsystemsduetothehightemperaturesneededfortheworkingfluid.PARABOLICTROUGHTheparabolictroughisusedinthelargestsolarpowerfacilityintheworldlocatedintheMojaveDesertatKramerJunction,California.Thisfacilityhasoperatedsincethe1980andaccountedforthemajorityofsolarelectricityproducedbytheelectricpowersectorin2004.Aparabolictroughcollectorhasalinearparabolicshapedreflectorthatfocusesthesunsradiationonalinearreceiverlocatedatthefocusoftheparabola.Thecollectortracksthesunalongoneaxisfromeasttowestduringthedaytoensurethatthesuniscontinuouslyfocusedonthereceiver.Becauseofitsparabolicshape,atroughcanfocusthesunat30to100timesitsnormalintensityconcentrationratioonareceiverpipelocatedalongthefocallineofthetrough,achievingoperatingtemperaturesover400degreesCelcius.Acollectorfieldconsistsofalargefieldofsingleaxistrackingparabolictroughcollectors.Thesolarfieldismodularinnatureandiscomposedofmanyparallelrowsofsolarcollectorsalignedonanorthsouthhorizontalaxis.Aworkingheattransferfluidisheatedasitcirculatesthroughthereceiversandreturnstoaseriesofheatexchangersatacentrallocationwherethefluidisusedtogeneratehighpressuresuperheatedsteam.Thesteamisthenfedtoaconventionalsteamturbine/generatortoproduceelectricity.Aftertheworkingfluidpassesthroughtheheatexchangers,thecooledfluidisrecirculatedthroughthesolarfield.Theplantisusuallydesignedtooperateatfullratedpowerusingsolarenergyalone,givensufficientsolarenergy.However,allplantsarehybridsolar/fossilplantsthathaveafossilfiredcapabilitythatcanbeusedtosupplementthesolaroutputduringperiodsoflowsolarenergy.TheLuzplantisanaturalgashybrid.SOLARDISHAsolardish/enginesystemutilizesconcentratingsolarcollectorsthattrackthesunontwoaxes,concentratingtheenergyatthefocalpointofthedishbecauseitisalwayspointedatthesun.Thesolardishsconcentrationratioismuchhigherthatthesolartrough,typicallyover2,000,withaworkingfluidtemperatureover750oC.Thepowergeneratingequipmentusedwithasolardishcanbemountedatthefocalpointofthedish,makingitwellsuitedforremoteoperationsor,aswiththesolartrough,theenergymaybecollectedfromanumberofinstallationsandconvertedtoelectricityatacentralpoint.Theengineinasolardish/enginesystemconvertsheattomechanicalpowerbycompressingtheworkingfluidwhenitiscold,heatingthecompressedworkingfluid,andthenexpandingthefluidthroughaturbineorwithapistontoproducework.Theengineiscoupledtoanelectricgeneratortoconvertthemechanicalpowertoelectricpower.SOLARPOWERTOWERAsolarpowertowerorcentralreceivergenerateselectricityfromsunlightbyfocusingconcentratedsolarenergyonatowermountedheatexchangerreceiver.Thissystemuseshundredstothousandsofflatsuntrackingmirrorscalledheliostatstoreflectandconcentratethesunsenergyontoacentralreceivertower.Theenergycanbeconcentratedasmuchas1,500
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