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1、英文资料ENERGYFROMTHESUNThesunhasproducedenergyforbillionsofyears.Solarenergyisthesolarradiationthatreachestheearth.Solarenergycanbeconverteddirectlyorindirectlyintootherformsofenergy,suchasheatandelectricity.Themajordrawbacks(problems,orissuestoovercome)ofsolarenergyare:(1)theintermittentandvariableman
2、nerinwhichitarrivesattheearthssurfaceand,(2)thelargearearequiredtocollectitatausefulrate.Solarenergyisusedforheatingwaterfordomesticuse,spaceheatingofbuildings,dryingagriculturalproducts,andgeneratingelectricalenergy.Inthe1830s,theBritishastronomerJohnHerschelusedasolarcollectorboxtocookfoodduringan
3、expeditiontoAfrica.Now,peoplearetryingtousethesunsenergyforlotsofthings.Electricutilitiesaretryingphotovoltaics,aprocessbywhichsolarenergyisconverteddirectlytoelectricity.Electricitycanbeproduceddirectlyfromsolarenergyusingphotovoltaicdevicesorindirectlyfromsteamgeneratorsusingsolarthermalcollectors
4、toheataworkingfluid.Outofthe14knownsolarelectricgeneratingunitsoperatingintheUSattheendof2004,10oftheseareinCalifornia,and4inArizona.Nostatisticsarebeingcollectedonsolarplantsthatproducelessthan1megawattofelectricity,sotheremaybesmallersolarplantsinanumberofotherstates.PHOTOVOLTAICENERGYPhotovoltaic
5、energyistheconversionofsunlightintoelectricitythroughaphotovoltaic(PVs)cell,commonlycalledasolarcell.Aphotovoltaiccellisanonmechanicaldeviceusuallymadefromsiliconalloys.Sunlightiscomposedofphotons,orparticlesofsolarenergy.Thesephotonscontainvariousamountsofenergycorrespondingtothedifferentwavelength
6、softhesolarspectrum.Whenphotonsstrikeaphotovoltaiccell,theymaybereflected,passrightthrough,orbeabsorbed.Onlytheabsorbedphotonsprovideenergytogenerateelectricity.Whenenoughsunlight(energy)isabsorbedbythematerial(asemiconductor),electronsaredislodgedfromthematerialsatoms.Specialtreatmentofthematerials
7、urfaceduringmanufacturingmakesthefrontsurfaceofthecellmorereceptivetofreeelectrons,sotheelectronsnaturallymigratetothesurface.Whentheelectronsleavetheirposition,holesareformed.Whenmanyelectrons,eachcarryinganegativecharge,traveltowardthefrontsurfaceofthecell,theresultingimbalanceofchargebetweenthece
8、llsfrontandbacksurfacescreatesavoltagepotentiallikethenegativeandpositiveterminalsofabattery.Whenthetwosurfacesareconnectedthroughanexternalload,electricityflows.ThephotovoltaiccellisthebasicbuildingblockofaPVsystem.Individualcellscanvaryinsizefromabout1cm(1/2inch)toabout10cm(4inches)across.However,
9、onecellonlyproduces1or2watts,whichisntenoughpowerformostapplications.Toincreasepoweroutput,cellsareelectricallyconnectedintoapackagedweather-tightmodule.Modulescanbefurtherconnectedtoformanarray.Thetermarrayreferstotheentiregeneratingplant,whetheritismadeupofoneorseveralthousandmodules.Asmanymodules
10、asneededcanbeconnectedtoformthearraysize(poweroutput)needed.Theperformanceofaphotovoltaicarrayisdependentuponsunlight.Climateconditions(e.g.,clouds,fog)haveasignificanteffectontheamountofsolarenergyreceivedbyaPVarrayand,inturn,itsperformance.Mostcurrenttechnologyphotovoltaicmodulesareabout10percente
11、fficientinconvertingsunlightwithfurtherresearchbeingconductedtoraisethisefficiencyto20percent.Thepvcellwasdiscoveredin1954byBellTelephoneresearchersexaminingthesensitivityofaproperlypreparedsiliconwafertosunlight.Beginninginthelate1950s,pvswereusedtopowerU.S.spacesatellites.ThesuccessofPVsinspacegen
12、eratedcommercialapplicationsforpvtechnology.Thesimplestphotovoltaicsystemspowermanyofthesmallcalculatorsandwristwatchesusedeveryday.Morecomplicatedsystemsprovideelectricitytopumpwater,powercommunicationsequipment,andevenprovideelectricitytoourhomes.Photovoltaicconversionisusefulforseveralreasons.Con
13、versionfromsunlighttoelectricityisdirect,sothatbulkymechanicalgeneratorsystemsareunnecessary.Themodularcharacteristicofphotovoltaicenergyallowsarraystobeinstalledquicklyandinanysizerequiredorallowed.Also,theenvironmentalimpactofaphotovoltaicsystemisminimal,requiringnowaterforsystemcoolingandgenerati
14、ngnoby-products.Photovoltaiccells,likebatteries,generatedirectcurrent(DC)whichisgenerallyusedforsmallloads(electronicequipment).WhenDCfromphotovoltaiccellsisusedforcommercialapplicationsorsoldtoelectricutilitiesusingtheelectricgrid,itmustbeconvertedtoalternatingcurrent(AC)usinginverters,solidstatede
15、vicesthatconvertDCpowertoAC.Historically,pvshavebeenusedatremotesitestoprovideelectricity.However,amarketfordistributedgenerationfromPVsmaybedevelopingwiththeunbundlingoftransmissionanddistributioncostsduetoelectricderegulation.Thesitingofnumeroussmallscalegeneratorsinelectricdistributionfeederscoul
16、dimprovetheeconomicsandreliabilityofthedistributionsystem.SOLARTHERMALHEATThemajorapplicationsofsolarthermalenergyatpresentareheatingswimmingpools,heatingwaterfordomesticuse,andspaceheatingofbuildings.Forthesepurposes,thegeneralpracticeistouseflat-platesolar-energycollectorswithafixedorientation(position).Wherespaceheatingisthemainconsideration,thehighestefficiencywithafixedflat-platecollectorisobtainedifitfacesapproximatelysouthandslopesatanangletothehorizone
