




已阅读5页,还剩3页未读, 继续免费阅读
版权说明:本文档由用户提供并上传,收益归属内容提供方,若内容存在侵权,请进行举报或认领
文档简介
英文资料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
温馨提示
- 1. 本站所有资源如无特殊说明,都需要本地电脑安装OFFICE2007和PDF阅读器。图纸软件为CAD,CAXA,PROE,UG,SolidWorks等.压缩文件请下载最新的WinRAR软件解压。
- 2. 本站的文档不包含任何第三方提供的附件图纸等,如果需要附件,请联系上传者。文件的所有权益归上传用户所有。
- 3. 本站RAR压缩包中若带图纸,网页内容里面会有图纸预览,若没有图纸预览就没有图纸。
- 4. 未经权益所有人同意不得将文件中的内容挪作商业或盈利用途。
- 5. 人人文库网仅提供信息存储空间,仅对用户上传内容的表现方式做保护处理,对用户上传分享的文档内容本身不做任何修改或编辑,并不能对任何下载内容负责。
- 6. 下载文件中如有侵权或不适当内容,请与我们联系,我们立即纠正。
- 7. 本站不保证下载资源的准确性、安全性和完整性, 同时也不承担用户因使用这些下载资源对自己和他人造成任何形式的伤害或损失。
最新文档
- 福建艺考生高考数学试卷
- 高一联考期末数学试卷
- 肌理画课件儿童创意
- 肉羊的饲养管理技术
- 肝病的症状与诊断方法课件
- 2025年03月浙江嘉兴市海宁市人民医院招聘1人笔试历年专业考点(难、易错点)附带答案详解
- 2025年浙江医疗卫生招聘温州医科大学附属第二医院招聘特殊专业技术岗位106人笔试历年专业考点(难、易错点)附带答案详解
- 食品采购加工培训课件
- 智能城市运输创投项目计划书
- 2025至2030轨道轴箱行业市场深度研究与战略咨询分析报告
- T/CADBM 63-2022建筑室内窗饰产品百叶帘
- 2025年贵州省公务员录用考试《行测》真题及答案解析
- 第一单元:三视图的判断、确认与绘制专项练习-2023-2024学年五年级数学下册典型例题系列(原卷版)人教版
- 杨梅承包合同协议书
- 物理光学知到智慧树期末考试答案题库2025年山东理工大学
- 2024年新沪科版七年级上册数学教学课件 第1章 有理数 1.2 数轴、相反数和绝对值 第1课时 数轴
- 《高一物理力学进阶:动力学与能量学教学教案》
- 总监理工程师作业指导书
- GA/T 2160-2024法庭科学资金数据检验规程
- 2025年全国高压电工证(复审)理论考试试题(1000题)附答案
- 2024北京西城区五年级(下)期末语文试题及答案
评论
0/150
提交评论