Lecture 24: Renewable Energy April 23, 2009petra/phys3150/Lecture24.pdfRenewable Energy Sources •...
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Energy & Sustainability Lecture 24: Renewable Energy April 23, 2009
Transcript of Lecture 24: Renewable Energy April 23, 2009petra/phys3150/Lecture24.pdfRenewable Energy Sources •...
Energy&Sustainability
Lecture24:RenewableEnergy
April23,2009
RenewableEnergySources• Solar–themotherofallrenewables
• Directsolarpluswindandwavepower,hydroelectricityandbiomass(indirectsolar),accountforover99.9%oftheavailableflowofrenewableenergyonEarth
• Geothermal–theonlyrenewablenotfromtheSun
174 petawatts
SolarEnergy• 174PetaWatts(1015)ofincomingsolarradiation(insolation)attheupperatmosphere
• Approximatelyhalfofitmakestothesurface
• 100x1015Wx3x107s/yr=3x1024J/yr=3000ZetaJoules• Includingatmosphericabsorptionweget:
– Thetotalsolarenergyavailabletotheearthisapproximately3850zettajoules(ZJ)peryear.
– Oceansabsorbapproximately2850ZJ/yr– Windscantheoreticallysupply6ZJ/yr
– Biomasscapturesapproximately1.8ZJ/yr• Worldwideenergyconsumptionwas0.471ZJin2004.
Insolation
Top of the atmosphere!
Surface of the earth!
Photovoltaics• ThePhotoelectriceffect
Metal
Ele
ctro
n E
nerg
y
Light Frequency
ExplainedbyEinsteinin1905–thisiswhathewonhisNobelprizefor!
Photovolatics• Lightcomesinphotons
– TheenergyofaphotonisE=hf(wherehisPlanck’sconstant)
Photovolatics• Lightcomesinphotons
– TheenergyofaphotonisE=hf(wherehisPlanck’sconstant)
– Iftheenergyishighenoughitcanknockoutanelectronoritcan’t
– Electronsarefreetomoveinmetals
– Ifyouhitthemwith3eVtheewillcomeout.
– Insemiconductorsthereisabandgapwhereelectronscan’ttravel
Photovoltaics• Insemiconductors,thefilledband(valenceband)andthebandinwhichelectronsarefreetomove(conductionband)areseparatedbyapotentialdifferenceofabout1volt.
• iflighthasanenergyofabout1eVitcanpushanelectronfromthevalencebandintotheconductionband.
• Theelectronintheconductionbandisfreetomove.Ifitiskeptfromrecombining,itcangiveupitsenergyinanexternalcircuitbeforecomingbacktothematerial.
Photovoltaics• PureSiliconisdopedwithimpuritiestogivethematerial
eitherextramobileelectrons(n‐type)orextraholeswheretherearenoelectrons(p‐type).
Photovoltaics• PureSiliconisdopedwithimpuritiestogivethematerial
eitherextramobileelectrons(n‐type)orextraholeswheretherearenoelectrons(p‐type).
Photovoltaics• PureSiliconisdopedwithimpuritiestogivethematerial
eitherextramobileelectrons(n‐type)orextraholeswheretherearenoelectrons(p‐type).
• Ifyoucombinetheseinlayers
Photovoltaics• PureSiliconisdopedwithimpuritiestogivethematerialeither
extramobileelectrons(n‐type)orextraholeswheretherearenoelectrons(p‐type).
• Ifyoucombinetheseinlayers• Thismakesadiode(adeviceinwhichcurrentcanonlyflowin
onedirection)
• Ifyoushinelightonthisitcanproduceanelectric
current
• ThisisaPhoto(light)volatic(voltageproducing)device
Photovoltaics
• Severalmaterialsavailable–– Silicon– GalliumArsenide– AluminumGalliumArsenide
PhotovoltaicEfficiency• AbouthalftheenergyinsunlightisunusablebymostPVcells
becausethisenergyisbelowthebandgap,andsocan’tfreeanelectronfromthevalencetotheconductionband,orbecauseitcarriesexcessenergy,whichmustbetransferredtothecellasthermalenergy,heatingupthecell.
40%EfficiencySolarCells
• Mostconventionalsolarcellsusedintoday’sapplications,suchasforsupplementalpowerforhomesandbuildings,areone‐sun,single‐junctionsiliconcellsthatuseonlythelightintensitythatthesunproducesnaturally,andhaveoptimalefficiencyforarelativelynarrowrangeofphotonenergies.
• Spectrolabgrouphasexperimentedwithconcentratormultijunctionsolarcellsthatusehighintensitiesofsunlight,theequivalentof100sofsuns,concentratedbylensesormirrors.
• Significantly,themultijunctioncellscanalsousethebroadrangeofwavelengthsinsunlightmuchmoreefficientlythansingle‐junctioncells.
• StandardtestconditionsforPVcellsandmodules– 25oC,1000W/m2,AM1.5
(canbereachedearlyafternoononasunnysummerday)
• Inpracticepowerratinginpeakwatts(Wp)ofcellsisdeterminedbymeasuringthemaximumpowerwhenexposedtoradiationoflampsdesignedtoreproducethespectrumatstandardconditions
Air Mass ≈ 1/cos θ
Australia
Norway
GridParity
• Gridparity,thepointatwhichphotovoltaicelectricityisequaltoorcheaperthangridpower,isachievedfirstinareaswithabundantsunandhighcostsforelectricitysuchasinCaliforniaandJapan.
• GridparityhasbeenreachedinHawaiiandotherislandsthatotherwiseusedieselfueltoproduceelectricity.
• GeorgeW.Bushhasset2015asthedateforgridparityintheUSA
EnergyusedtomakePhotovoltaics• StudiesofPhotovoltaic(PV)materialsthatmakeupthe
majorityoftheactivesolarmarketfoundthatsolarcellspayforthemselvesintermsofenergyinafewyears(1‐5years):– CrystallinesiliconPVsystemspresentlyhaveenergypay‐backtimesof1.5‐2yearsforSouth‐Europeanlocationsand2.7‐3.5yearsforMiddle‐Europeanlocations.
• Forsilicontechnologyclearprospectsforareductionofenergyinputexist,andanenergypay‐backof1yearmaybepossiblewithinafewyears.Thinfilmtechnologiesnowhaveenergypay‐backtimesintherangeof1‐1.5years(S.Europe).
• Withlifetimesofsuchsystemsofatleast30years,theEROEIisintherangeof10to30.
• Theythusgenerateenoughenergyovertheirlifetimestoreproducethemselvesmanytimes.
Photovoltaics
• EnergyStorageisstillanissue– Daily– Seasonal– weather
PVSummary • Advantages
– The89petawattsofsunlightreachingtheearth'ssurfaceisplentiful‐almost6,000timesmore‐comparedtothe15terawattsofaveragepowerconsumedbyhumans.
– Solarelectricgenerationhasthehighestpowerdensity(globalmeanof170W/m2)amongrenewableenergies.
– Solarpowerispollutionfreeduringuse.Productionendwastesandemissionsaremanageableusingexistingpollutioncontrols.End‐of‐userecyclingtechnologiesareunderdevelopment.
– Facilitiescanoperatewithlittlemaintenanceorinterventionafterinitialsetup.Solarelectricgenerationiseconomicallysuperiorwheregridconnectionorfueltransportisdifficult,costlyorimpossible.
PVSummary
• Advantages(cont).– Whengrid‐connected,solarelectricgenerationcandisplacethehighestcostelectricityduringtimesofpeakdemand(inmostclimaticregions),canreducegridloading
– Oncetheinitialcapitalcostofbuildingasolarpowerplanthasbeenspent,operatingcostsareextremelylowcomparedtoexistingpowertechnologies.
PVSummary
• Disadvantages– Solarelectricityisalmostalwaysmoreexpensivethanelectricitygeneratedbyothersources.
– Solarelectricityisnotavailableatnightandislessavailableincloudyweatherconditions.Therefore,astorageorcomplementarypowersystemisrequired.
– Limitedpowerdensity:AveragedailyinsolationinthecontiguousU.S.is3‐7kW∙h/m2andonaveragelowerinEurope.
– SolarcellsproduceDCwhichmustbeconvertedtoAC(usingagridtieinverter)whenusedincurrentlyexistingdistributiongrids.Thisincursanenergylossof4‐12%
WindPower• Inthe14thcenturytheDutchstartedusingwindmills
• Bythe1800stheyhad10,000windmills
• Todaytheyhave1,000modernwindmills
• Duringthewinterof1887‐88CharlesBrushbuiltwhatistodaybelievedtobethefirstautomaticallyoperatingwindturbineforelectricitygeneration.
• Itwasagiant‐theWorld'slargest‐witharotordiameterof17m(50ft.)and144rotorbladesmadeofcedarwood.
•
• Theturbineranfor20yearsandchargedthebatteriesinthecellarofhismansion.
• Brush’sWindmillonlygenerated12kw
• Itturnedslowlywithlotsofblades• Itwaslaterdiscoveredthatfasterwindmillswithlessbladesgeneratedmorepower
Danish scientist Poul la Cour (1846-1908)
CopenhagenHarbor‐Danesget20%ofelec.fromwind
Windpower• WindEnergygoeslikewindspeedtothe3rdpower– Consideramassofairthatpassawindmillin1sec–ithasamass,mandspeedv.
– Theenergyrequiredtoslowthisdowngoes½mv2
– Howeverthemassoftheairpassingthewindmillin1secdepends(linearly)onthewindspeed(m=ρAv)
– sotheenergygoeslikev3
Measurementin2002attheLeeRanchfacilityinColorado
• Thecartoonshowshowacylindricalsliceofair1meterthickmovesthroughthe2,300m2rotorofatypical1,000kilowattwindturbine.
• Witha54meterrotordiametereachcylinderweighs2.8tonnes
• Rotorareagoeslikethesquareofthebladelength
• 750kWNEGMiconTurbineinMoorheadMinnesota.‐>
• halfoftheenergyavailablearrivedinjust15%oftheoperatingtime.
• windenergydoesnothaveasconsistentanoutputasfuel‐firedpowerplants;
• Makingwindpowermoreconsistentrequiresthatstoragetechnologiesmustbeusedtoretainthelargeamountofpowergeneratedintheburstsforlateruse.
FromAmericanWindAssociation
• Theratioofactualproductivityinayeartothistheoreticalmaximumiscalledthecapacityfactor.
• Typicalcapacityfactorsare20‐40%,withvaluesattheupperendoftherangeinparticularlyfavorablesites
• A1megawattturbinewithacapacityfactorof35%willnotproduce8,760megawatt‐hoursinayear,– butonly0.35x24x365=3,066MWh,averagingto0.35MW
Otherrenewables
• Biomass• TidalPower• WaveEnergy
• GeothermalEnergy
• LastHomeworkwillbepostedonthewebtonight
• Nextweek:– Tuesday:CO2emissionsandgreenhousegaseffect– Thursday:Review– Final:Tue,May5
