Present and Future use of solar energy

Copyright©2005byTheInterAcademyCouncil.Allrightsreserved.THEPRESENTANDFUTUREUSEOFSOLARTHERMALENERGYASAPRIMARYSOURCEOFENERGYByCédricPhilibertInternationalEnergyAgency,Paris,France1.IntroductionTheradiativeenergyfromtheSunthatkeepsourplanetwarmexceedsbyfarthecurrentprimaryenergysupplyusedbymankindforitscomfort,leisureandeconomicactivities.Italsoexceedsvastlyotherenergysourcesatgroundlevelsuchasgeothermicortidalenergy,nuclearpowerandfossilfuelburning.Sunraysalsodrivehydraulics,windandwavepowerandbiomassgrowth.Mankind’stotalprimaryenergysupply(TPES)was433Ejin2002,includingnon-commercialbiomass,equivalenttoacontinuouspowerconsumptionof13.75TW.TheIEAprojectsfor2030aTPESofabout688Ej,equivalentto21.8TWofpower(IEA2004).ThiscomparestothesolarradiationinterceptedbytheEarthof173,000TW,ofwhich120,000TWstriketheEarth’ssurface.Solarenergyisthustheprimaryenergysourceonourplanet’ssurface–andexceeds8,000timesourprimaryenergysupply.Fulfillingglobalenergyneedsasprojectedfor2030wouldrequirecoveringabout0.6%ofemergedlandswith10%netefficientsolarconversionsystems.Anotherindicationoftheabundanceofsolarenergyis,somewhatparadoxically,thethreatofclimatechangeitself.Theincreasesintheatmosphericconcentrationsofwell-mixedgreenhousegasesfromthepre-industrialtopresenttimeresultmainlyfromthecombustionoffossilfuelsforenergypurposes.TheyentailamarginalincreaseintheEarthandatmosphere’scapacitytotrapthesunrays’radiativeenergy,actingasagiganticsolarcollector,calledtheradiativeforcingofclimateandestimatedtobe2.43Wm-2±10%,whichcomparestotheaveragedcontinuousamountofsolarenergyonEarthofabout235Wm-2.Thissuggeststhatsolarenergyhasthepotentialtohelpsolvetheproblemitcreates.Indeed,theprospectsofclimatechangeand,eventually,fossilfueldepletion,triggeragrowinginterestinrenewableenergiesingeneral,solarenergyinparticular.Thebenefitsofrenewableenergysystemswereclearlydefinedinapoliticaldeclarationagreedtobygovernmentrepresentativesof154nationsattheinternational“Renewables2004”conferenceheldinBonn,June2004asafollow-uptothe2001WorldSummitSustainableDevelopment,Johannesburg.Benefitsoutlinedincludedenergysupplysecurity,equityanddevelopment,improvedhealth,overcomingpeakoilpricefluctuations,provisionofcleanwater,closeassociationwithenergyefficiencymeasures,climatechangemitigation,andthecommonbeliefthat“therewillbenoneedforwaroversolarenergy”.Thedrawbacksarewell-known:thesolarradiationreachingtheearthisverydilute(onlyabout1kWthpersquaremeter),intermittent(availableonlyduringday-time),andunequallydistributedoverthesurfaceoftheearth(mostlybetween30°northand30°southlatitude).SolarthermalenergyVarioustechnologies,however,canbeusedtoovercomethedifficultiesinmakingsunlightausableformofenergyforallpurposes.Solar“thermal”energydesignatealltechnologiesthatcollectsolarraysandtransformtheirenergyintousableheat,eitherfordirectlysatisfyingheatingneeds(notablyspaceheating,waterheating–andspacecooling)orforproducingelectricityandfuels.Thelatterincludesconcentratingsolarpowertechnologies,andotherconceptssuchassolarupdrafttowersandoceanthermalenergy.Thispaperthusconsidersalldirectformsofsolarenergyexceptphotovoltaic,assessingresourcepotential,technologystatus,andsupportingpolicies.2.SolarheatAtpresent,solarheatingprovidesbyfarthelargestsolarcontributiontoenergyneeds.Themaintechnologiesbelongtoeither“passive”and“active”solarenergyforms.Passivesolarenergyrelatestothedesignofbuildingscollectingandtransformingsolarenergyusedforpassiveheating,daylightingandnaturalventilation.Activesolarenergyrelatestotheuseofsolarcollectorsforwaterorspaceheatingpurposes,activesolarcooling,heatpumps,desalinisationandindustrialhightemperatureheat.2.1PassivesolararchitecturePassivesolarenergydoesnotshowupinenergystatisticsascollectingdatawouldbehugelyexpensive,requiringclosebuildingbybuildingexamination.Passivesolarenergyisusuallyconsideredfromthedemandsideaspartofenergysavingspotentialratherthanfromthesupplyside.Throughacombinationofahigh-performancethermalenvelope,efficientsystemsanddevices,andfullexploitationoftheopportunitiesforpassivesolarenergy,50to75%oftheenergyneedsofbuildingsasconstructedundernormalpracticecanbeeithereliminatedorsatisfiedthroughpassivesolarmeans.Intheindustrializedworld,buildingsuse35%to40%oftotalprimaryuseofenergy.Lettingthesunheatbuildingsinwinter,andlettingdaylightenterthemtodisplaceelectriclighting,aretheleastcostsolarenergyforms.AscienceandanartPassivesolarheatingcaninvolveextensivesun-facingglazing,variouswall-orroof-mountedsolaraircollectors,double-façadewallconstruction,air-flowwindows,thermallymassivewallsbehindglazing,orpreheatingofventilationairthroughburiedpipes.Lightingandventilationcanbedirectlysuppliedthroughsolarenergy:interiorlightthroughavarietyofsimpledevicesthatconcentrateanddirectsunlightdeepintoabuilding,andventilationthroughthetemperatureandhencepressuredifferencesthatarecreatedbetweendifferentpartsofabuildingwhenthesunshines.Thebuildingfaçadecanbeusedtogenerateandchannelairflowsthatremoveheatthatotherwiseaddtothecoolingload,orwhichcanbeusedtopreheatventila