Roofmountedclosecoupledthermosiphonsolarwaterheater.

SolarwaterheatingFromWikipedia,thefreeencyclopedia

Solarwaterheating(SWH)istheconversionofsunlightintorenewableenergyforwaterheatingusingasolarthermalcollector.Solarwaterheatingsystemscomprisevarioustechnologiesthatareusedworldwideincreasingly.

Ina"closecoupled"SWHsystemthestoragetankishorizontallymountedimmediatelyabovethesolarcollectorsontheroof.Nopumpingisrequiredasthehotwaternaturallyrisesintothetankthroughthermosiphonflow.Ina"pumpcirculated"systemthestoragetankisgroundorfloormountedandisbelowthelevelofthecollectorsacirculatingpumpmoveswaterorheattransferfluidbetweenthetankandthecollectors.

SWHsystemsaredesignedtodeliverhotwaterformostoftheyear.However,inwintertheresometimesmaynotbesufficientsolarheatgaintodeliversufficienthotwater.Inthiscaseagasorelectricboosterisusedtoheatthewater.

Contents

1Overview2History

2.1Mediterranean2.2AsiaPacific

3Systemdesignrequirements3.1Freezeprotection3.2Overheatprotection

4Typesofsolarwaterheatingsystems4.1Directandindirectsystems4.2Passiveandactivesystems4.3Passivedirectsystems4.4Activeindirectsystems:drainbackandantifreeze4.5PoweringaheatpumphotwaterheaterviaSolarPVpanels4.6Aroughcomparisonofsolarhotwatersystems

5CollectorsusedinmoderndomesticSWHsystems6Heatingofswimmingpools7Economics,energy,environment,andsystemcosts

7.1Energyproduction7.2Systemcost7.3Operationalcarbon/energyfootprintandlifecycleassessment

7.3.1Terminology7.3.2Carbon/energyfootprint7.3.3Lifecyclecarbon/energyassessment

8Doityourself(DIY)systems9Systemspecificationandinstallation10Standards

AsolarwaterheaterinstalledonahouseinBelgium

10Standards10.1Europe10.2UnitedStates10.3Australia

11APPENDIX1.Worldwideuse11.1Topcountriesworldwide11.2SolarheatinginEuropeanUnion+Switzerland

12Seealso13References14Externallinks

Overview

Waterheatedbythesunisusedinvariousways.Whileperhapsbestknowninaresidentialsettingtoprovidedomestichotwater,solarhotwateralsohasindustrialapplications,e.g.togenerateelectricity.[1]Designssuitableforhotclimatescanbemuchsimplerandcheaper,andcanbeconsideredanappropriatetechnologyfortheseplaces.TheglobalsolarthermalmarketisdominatedbyChina,Europe,JapanandIndia.

Inordertoheatwaterusingsolarenergy,acollector,oftenfastenedtoarooforawallfacingthesun,heatsaworkingfluidthatiseitherpumped(activesystem)ordrivenbynaturalconvection(passivesystem)throughit.[2]Thecollectorcouldbemadeofasimpleglasstoppedinsulatedboxwithaflatsolarabsorbermadeofsheetmetal,attachedtocopperheatexchangerpipesanddarkcolored,orasetofmetaltubessurroundedbyanevacuated(nearvacuum)glasscylinder.Inindustrialcasesaparabolicmirrorcanconcentratesunlightonthetube.Heatisstoredinahotwaterstoragetank.Thevolumeofthistankneedstobelargerwithsolarheatingsystemsinordertoallowforbadweather,andbecausetheoptimumfinal

temperatureforthesolarcollectorislowerthanatypicalimmersionorcombustionheater.Theheattransferfluid(HTF)fortheabsorbermaybethehotwaterfromthetank,butmorecommonly(atleastinactivesystems)isaseparateloopoffluidcontainingantifreezeandacorrosioninhibitorwhichdeliversheattothetankthroughaheatexchanger(commonlyacoilofcopperheatexchangertubingwithinthetank).Copperisanimportantcomponentinsolarthermalheatingandcoolingsystemsbecauseofitshighheatconductivity,resistancetoatmosphericandwatercorrosion,sealingandjoiningbysoldering,andmechanicalstrength.Copperisusedbothinreceiversandprimarycircuits(pipesandheatexchangersforwatertanks).[3]

Anotherlowermaintenanceconceptisthe'drainback':noantifreezeisrequiredinstead,allthepipingisslopedtocausewatertodrainbacktothetank.Thetankisnotpressurizedandisopentoatmosphericpressure.Assoonasthepumpshutsoff,flowreversesandthepipesareemptybeforefreezingcouldoccur.

Residentialsolarthermalinstallationsfallintotwogroups:passive(sometimescalled"compact")andactive(sometimescalled"pumped")systems.Bothtypicallyincludeanauxiliaryenergysource(electricheatingelementorconnectiontoagasorfueloilcentralheatingsystem)whichisactivatedwhenthewaterinthetankfallsbelowaminimumtemperaturesettingsuchas55C.Hence,hotwaterisalwaysavailable.The

HowaSolarHotWatersystemworks

AnadvertisementforaSolarWaterHeaterdatingto1902

combinationofsolarwaterheatingandusingthebackupheatfromawoodstovechimneytoheatwater[4]canenableahotwatersystemtoworkallyearroundincoolerclimates,withoutthesupplementalheatrequirementofasolarwaterheatingsystembeingmetwithfossilfuelsorelectricity.

Whenasolarwaterheatingandhotwatercentralheatingsystemareusedinconjunction,solarheatwilleitherbeconcentratedinapreheatingtankthatfeedsintothetankheatedbythecentralheating,orthesolarheatexchangerwillreplacethelowerheatingelementandtheupperelementwillremaininplacetoprovideforanyheatingthatsolarcannotprovide.However,theprimaryneedforcentralheatingisatnightandinwinterwhensolargainislower.Therefore,solarwaterheatingforwashingandbathingisoftenabetterapplicationthancentralheatingbecausesupplyanddemandarebettermatched.Inmanyclimates,asolarhotwatersystemcanprovideupto85%ofdomestichotwaterenergy.Thiscanincludedomesticnonelectricconcentratingsolarthermalsystems.InmanynorthernEuropeancountries,combinedhotwaterandspaceheatingsystems(solarcombisystems)areusedtoprovide15to25%ofhomeheatingenergy.

History

TherearerecordsofsolarcollectorsintheUnitedStatesdatingbacktobefore1900,[5]comprisingablackpaintedtankmountedonaroof.In1896ClarenceKempofBaltimore,USAenclosedatankinawoodenbox,thuscreatingthefirst'batchwaterheater'astheyareknowntoday.AlthoughflatplatecollectorsforsolarwaterheatingwereusedinFloridaandSouthernCaliforniainthe1920stherewasasurgeofinterestinsolarheatinginNorthAmericaafter1960,butespeciallyafterthe1973oilcrisis.

SeeAppendix1atthebottomofthisarticleforanumberofcountryspecificstatisticsonthe"Useofsolarwaterheatingworldwide".Wikipediaalsohascountryspecificarticlesaboutsolarenergyuse(thermalaswellasphotovoltaic)inAustralia,Canada,China,Germany,India,Israel,Japan,Portugal,Romania,Spain,theUnitedKingdomandtheUnitedStates.

Mediterranean

Israel,CyprusandGreecearethepercapitaleadersintheuseofsolarwaterheatingsystemswithover30%40%ofhomesusingthem.[6]

FlatplatesolarsystemswereperfectedandusedonaverylargescaleinIsrael.Inthe1950stherewasafuelshortageinthenewIsraelistate,andthegovernmentforbadeheatingwaterbetween10pmand6am.LeviYissarbuiltthefirstprototypeIsraelisolarwaterheaterandin1953helaunchedtheNerYahCompany,Israel'sfirstcommercialmanufacturerofsolarwaterheating.[7]DespitetheabundanceofsunlightinIsrael,solarwaterheaterswereusedbyonly20%ofthepopulationby1967.Followingtheenergycrisisinthe1970s,in1980theIsraeliKnessetpassedalawrequiringtheinstallationofsolarwaterheatersinallnewhomes(excepthightowerswithinsufficientroofarea).[8]Asaresult,Israelisnowtheworldleaderinthe

Passive(thermosiphon)solarwaterheatersonarooftopinJerusalem

Newsolarhotwaterinstallationsduring2007,worldwide.

useofsolarenergypercapitawith85%ofthehouseholdstodayusingsolarthermalsystems(3%oftheprimarynationalenergyconsumption),[9]estimatedtosavethecountry2millionbarrels(320,000m3)ofoilayear,thehighestpercapitauseofsolarenergyintheworld.[10]

In2005,Spainbecamethefirstcountryintheworldtorequiretheinstallationofphotovoltaicelectricitygenerationinnewbuildings,andthesecond(afterIsrael)torequiretheinstallationofsolarwaterheatingsystemsin2006.[11]

AsiaPacific

Theworldsawarapidgrowthoftheuseofsolarwarmwaterafter1960,withsystemsbeingmarketedinJapanandAustralia.[5]Technicalinnovationhasimprovedperformance,lifeexpectancyandeaseofuseofthesesystems.Installationofsolarwaterheatinghasbecomethenormincountrieswithanabundanceofsolarradiation,liketheMediterranean,[12]Japan,andAustralia.

ColombiadevelopedalocalsolarwaterheatingindustrythankstothedesignsofLasGaviotas,directedbyPaoloLugari.Drivenbyadesiretoreducecostsinsocialhousing,theteamofGaviotasstudiedthebestsystemsfromIsraelandmadeadaptationsastomeetthespecificationssetbytheBancoCentralHipotecario(BCH)whichprescribedthatthesystemmustbeoperationalincitieslikeBogotwheretherearemorethan200daysovercast.TheultimatedesignsweresosuccessfulthatLasGaviotasoffereda25yearwarrantyonanyofitsinstallationsin1984.Over40,000wereinstalledandstillfunctionaquarterofacenturylater.

Australiahasavarietyofincentives(nationalandstate)andregulations(state)forsolarthermalintroducedstartingwithMRETin1997.[13][14][15]

SolarwaterheatingsystemshavebecomepopularinChina,wherebasicmodelsstartataround1,500yuan(US$235),muchcheaperthaninWesterncountries(around80%cheaperforagivensizeofcollector).Itissaidthatatleast30millionChinesehouseholdsnowhaveoneandthatthepopularityisduetotheefficientevacuatedtubeswhichallowtheheaterstofunctionevenundergrayskiesandattemperatureswellbelowfreezing.[16]

Systemdesignrequirements

Thetype,complexity,andsizeofasolarwaterheatingsystemismostlydeterminedby:

Changesinambienttemperatureandsolarradiationbetweensummerandwinter.Thechangesinambienttemperatureduringthedaynightcycle.Thepossibilityofthepotablewaterorcollectorfluidoverheating.

Thepossibilityofthepotablewaterorcollectorfluidfreezing.

Theminimumrequirementsofthesystemaretypicallydeterminedbytheamountortemperatureofhotwaterrequiredduringwinter,whenasystem'soutputandincomingwatertemperaturearetypicallyattheirlowest.Themaximumoutputofthesystemisdeterminedbytheneedtopreventthewaterinthesystemfrombecomingtoohot.

Freezeprotection

Freezeprotectionmeasurespreventdamagetothesystemduetotheexpansionoffreezingtransferfluid.Drainbacksystemsdrainthetransferfluidfromthesystemwhenthepumpstops.Manyindirectsystemsuseantifreeze(e.g.Propyleneglycol)intheheattransferfluid.

Insomedirectsystems,thecollectorscanbemanuallydrainedwhenfreezingisexpected.Thisapproachiscommoninclimateswherefreezingtemperaturesdonotoccuroften,butissomewhatunreliablesincetheoperatorcanforgettodrainthesystem.Otherdirectsystemsusefreezetolerantcollectorsmadewithflexiblepolymerssuchassiliconerubber.

Athirdtypeoffreezeprotectionisfreezetolerance,wherelowpressurepolymerwaterchannelsmadeofsiliconerubbersimplyexpandsonfreezing.OnesuchcollectornowhasEuropeanSolarKeymarkaccreditation,followingextradurabilitytesting.

Overheatprotection

Whennohotwaterhasbeenusedforadayortwo,thefluidinthecollectorsandstoragecanreachveryhightemperaturesinallsystemsexceptforthoseofthedrainbackvariety.Whenthestoragetankinadrainbacksystemreachesitsdesiredtemperature,thepumpsareshutoff,puttinganendtotheheatingprocessandthuspreventingthestoragetankfromoverheating.

Onemethodofprovidingoverheatprotectionistodumptheheatintoahottub.

Someactivesystemsdeliberatelycoolthewaterinthestoragetankbycirculatinghotwaterthroughthecollectorattimeswhenthereislittlesunlightoratnight,causingincreasedheatloss.Thisismosteffectiveindirectorthermalstoreplumbingandisvirtuallyineffectiveinsystemsthatuseevacuatedtubecollectors,duetotheirsuperiorinsulation.Nomatterthecollectortype,however,theymaystilloverheat.Highpressuredsealedsolarthermalsystemsversionsultimatelyrelyontheoperationoftemperatureandpressurereliefvalves.Lowpressure,openventedoneshavesimpler,morereliablesafetycontrols,typicallyanopenvent.

Typesofsolarwaterheatingsystems

Directandindirectsystems

Directoropenloopsystemscirculatepotablewaterthroughthecollectors.Theyarerelativelycheapbutcanhavethefollowingdrawbacks:

Theyofferlittleornooverheatprotectionunlesstheyhaveaheatexportpump.

Directsystems:(A)PassiveCHSsystemwithtankabovecollector.(B)Activesystemwithpumpandcontrollerdrivenbyaphotovoltaicpanel

Theyofferlittleornofreezeprotection,unlessthecollectorsarefreezetolerant.Collectorsaccumulatescaleinhardwaterareas,unlessanionexchangesoftenerisused.

Untiltheadventoffreezetolerantsolarcollectors,theywerenotconsideredsuitableforcoldclimatessince,intheeventofthecollectorbeingdamagedbyafreeze,pressurizedwaterlineswillforcewatertogushfromthefreezedamagedcollectoruntiltheproblemisnoticedandrectified.

Indirectorclosedloopsystemsuseaheatexchangerthatseparatesthepotablewaterfromthefluid,knownasthe"heattransferfluid"(HTF),thatcirculatesthroughthecollector.ThetwomostcommonHTFsarewaterandanantifreeze/watermixthattypicallyusesnontoxicpropyleneglycol.Afterbeingheatedinthepanels,theHTFtravelstotheheatexchanger,whereitsheatistransferredtothepotablewater.Thoughslightlymoreexpensive,indirectsystemsofferfreezeprotectionandtypicallyofferoverheatprotectionaswell.

Passiveandactivesystems

Passivesystemsrelyonheatdrivenconvectionorheatpipestocirculatewaterorheatingfluidinthesystem.Passivesolarwaterheatingsystemscostlessandhaveextremelylowornomaintenance,buttheefficiencyofapassivesystemissignificantlylowerthanthatofanactivesystem.Overheatingandfreezingaremajorconcerns.

Activesystemsuseoneormorepumpstocirculatewaterand/orheatingfluidinthesystem.

Thoughslightlymoreexpensive,activesystemsofferseveraladvantages:

Thestoragetankcanbesituatedlowerthanthecollectors,allowingincreasedfreedominsystemdesignandallowingpreexistingstoragetankstobeused.Thestoragetankcanbehiddenfromview.Thestoragetankcanbeplacedinconditionedorsemiconditionedspace,reducingheatloss.Drainbacktankscanbeused.Superiorefficiency.Increasedcontroloverthesystem.

Modernactivesolarwatersystemshaveelectroniccontrollersthatofferawiderangeoffunctionality,suchasthemodificationofsettingsthatcontrolthesystem,interactionwithabackupelectricorgasdrivenwaterheater,calculationandloggingoftheenergysavedbyaSWHsystem,safetyfunctions,remoteaccess,and

Thebubbleseparatorofabubblepumpsystem

informativedisplays,suchastemperaturereadings.

Themostpopularpumpcontrollerisadifferentialcontrollerthatsensestemperaturedifferencesbetweenwaterleavingthesolarcollectorandthewaterinthestoragetankneartheheatexchanger.Inatypicalactivesystem,thecontrollerturnsthepumponwhenthewaterinthecollectorisabout810Cwarmerthanthewaterinthetank,anditturnsthepumpoffwhenthetemperaturedifferenceapproaches35C.Thisensuresthewateralwaysgainsheatfromthecollectorwhenthepumpoperatesandpreventsthepumpfromcyclingonandofftoooften.(Indirectsystemsthis"ondifferential"canbereducedtoaround4Cbecausethereisnoheatexchangerimpediment.)

SomeactiveSWHsystemsuseenergyobtainedbyasmallphotovoltaic(PV)paneltopoweroneormorevariablespeedDCpump(s).Toensureproperperformanceandlongevityofthepump(s),theDCpumpandPVpanelmustbesuitablymatched.SomePVpumpedsolarthermalsystemsareoftheantifreezevarietyandsomeusefreezetolerantsolarcollectors.Thesolarcollectorswillalmostalwaysbehotwhenthepump(s)areoperating(i.e.,whenthesunisbright),andsomedonotusesolarcontrollers.Sometimes,however,adifferentialcontroller(thatcanalsobepoweredbytheDCoutputofaPVpanel)isusedtopreventtheoperationofthepumpswhenthereissunlighttopowerthepumpbutthecollectorsarestillcoolerthanthewaterinstorage.OneadvantageofaPVdrivensystemisthatsolarhotwatercanstillbecollectedduringapoweroutageifthesunisshining.Anotheradvantageisthattheoperationalcarbonclawbackofusingmainspumpedsolarthermal(whichtypicallynegatesupto23%ofitscarbonsavings)iscompletelyavoided.

Anactivesolarwaterheatingsystemcanbeequippedwithabubblepump(alsoknownasgeyserpump)insteadofanelectricpump.Abubblepumpcirculatestheheattransferfluid(HTF)betweencollectorandstoragetankusingsolarpower,withoutanyexternalenergysource,andissuitableforflatpanelaswellasvacuumtubesystems.Inabubblepumpsystem,theclosedHTFcircuitisunderreducedpressure,whichcausestheliquidtoboilatlowtemperatureasitisheatedbythesun.Thesteambubblesformageyserpump,causinganupwardflow.Thesystemisdesignedsuchthatthebubblesareseparatedfromthehotfluidandcondensedatthehighestpointinthecircuit,afterwhichthefluidflowsdownwardtowardtheheatexchangercausedbythedifferenceinfluidlevels.[17][18][19]TheHTFtypicallyarrivesattheheatexchangerat

70Candreturnstothecirculatingpumpat50C.InfrostproneclimatestheHTFiswaterwithpropyleneglycolantifreezeadded,usuallyintheratioof60to40.Pumpingtypicallystartsatabout50Candincreasesasthesunrisesuntilequilibriumisreached,whichdependsontheefficiencyoftheheatexchanger,thetemperatureofthewaterbeingheated,andthetotalsolarenergyavailable.

Passivedirectsystems

Anintegratedcollectorstorage(ICSorBatchHeater)systemusesatankthatactsasbothstorageandsolarcollector.Batchheatersarebasicallythinrectilineartankswithaglasssidefacingthepositionofthesunatnoon.Theyaresimpleandlesscostlythanplateandtubecollectors,buttheysometimesrequireextra

Anintegratedcollectorstorage(ICS)system

bracingifinstalledonaroof(sincetheyareheavywhenfilledwithwater[400700lbs],)sufferfromsignificantheatlossatnightsincethesidefacingthesunislargelyuninsulated,andareonlysuitableinmoderateclimates.

Aconvectionheatstorageunit(CHS)systemissimilartoanICSsystem,exceptthestoragetankandcollectorarephysicallyseparatedandtransferbetweenthetwoisdrivenbyconvection.CHSsystemstypicallyusestandardflatplatetypeorevacuatedtubecollectors,andthestoragetankmustbelocatedabovethecollectorsforconvectiontoworkproperly.ThemainbenefitofaCHSsystemsoveranICSsystemisthatheatlossislargelyavoidedsince(1)thestoragetankcanbebetterinsulated,and(2)sincethepanelsarelocatedbelowthestoragetank,heatlossinthepanelswillnotcauseconvection,asthecoldwaterwillprefertostayatthelowestpartofthesystem.

Activeindirectsystems:drainbackandantifreeze

Pressurizedantifreezeorpressurizedglycolsystemsuseamixofantifreeze(almostalwaysnontoxicpropyleneglycol)andwatermixforHTFinordertopreventfreezedamage.

Thougheffectiveatpreventingfreezedamage,antifreezesystemshavemanydrawbacks:

IftheHTFgetstoohot(forexample,whenthehomeownerisonvacation,)theglycoldegradesintoacid.Afterdegradation,theglycolnotonlyfailstoprovidefreezeprotection,butalsobeginstoeatawayatthesolarloop'scomponents:thecollectors,thepipes,thepump,etc.Duetotheacidandexcessiveheat,thelongevityofpartswithinthesolarloopisgreatlyreduced.Mostdonotfeaturedrainbacktanks,sothesystemmustcirculatetheHTFregardlessofthetemperatureofthestoragetankinordertopreventtheHTFfromdegrading.Excessivetemperaturesinthetankcauseincreasedscaleandsedimentbuildup,possiblesevereburnsifatemperingvalveisnotinstalled,and,ifawaterheaterisbeingusedforstorage,possiblefailureofthewaterheater'sthermostat.Theglycol/waterHTFmustbereplacedevery38years,dependingonthetemperaturesithasexperienced.Somejurisdictionsrequiredoublewalledheatexchangerseventhoughpropyleneglycolisnontoxic.EventhoughtheHTFcontainsglycoltopreventfreezing,itwillstillcirculatehotwaterfromthestoragetankintothecollectorsatlowtemperatures(e.g.below40F(4C)),causingsubstantialheatloss.

AdrainbacksystemisanindirectactivesystemwheretheHTF(almostalwayspurewater)circulatesthroughthecollector,beingdrivenbyapump.Thecollectorpipingisnotpressurizedandincludesanopendrainbackreservoirthatiscontainedinconditionedorsemiconditionedspace.Ifthepumpisswitchedoff,theHTFdrainsintothedrainbackreservoirandnoneremainsinthecollector.Sincethesystemreliesuponbeingabletodrainproperly,allpipingabovethedrainbacktank,includingthecollectors,mustslope

downwardinthedirectionofthedrainbacktank.Installedproperly,thecollectorcannotbedamagedbyfreezingoroverheating.[20]Drainbacksystemsrequirenomaintenanceotherthanthereplacementoffailedsystemcomponents.

PoweringaheatpumphotwaterheaterviaSolarPVpanels

WiththedrasticdropinthepricesofPhotovoltaicscirca2010itbecameincreasinglypopularinresidentialsettingswithlowhotwaterdemandstoconsiderheatingwaterviaanelectricheatpumphotwaterheaterpoweredbyasolarPVarray.Thishasthefollowingadvantages:1)simpler/cheaperinstallationandmaintenance,2)excessenergycollectedcanbeusedforhouseholdelectricityuseorputbackintothegrid,and3)theheatpumpdehumidifiesthelivingspace.Seeforexample:GettingintoHotWaterPart1MarcRosenbaum(http://www.greenbuildingadvisor.com/blogs/dept/guestblogs/gettinghotwaterpart1)

Aroughcomparisonofsolarhotwatersystems

ComparisonofSWHsystems[21]

Characteristic ICS(Batch) ThermosiphonActivedirect

Activeindirect Drainback

BubblePump

LowprofileunobtrusiveLightweightcollector

SurvivesfreezingweatherLowmaintenance

Simple:noancillarycontrolRetrofitpotentialtoexisting

storeSpacesaving:noextra

storagetank

CollectorsusedinmoderndomesticSWHsystems

Solarthermalcollectorscaptureandretainheatfromthesunanduseittoheataliquid.[22]Twoimportantphysicalprinciplesgovernthetechnologyofsolarthermalcollectors:

Anyhotobjectultimatelyreturnstothermalequilibriumwithitsenvironment,duetoheatlossfromthehotobject.Theprocessesthatresultinthisheatlossareconduction,convectionandradiation.[23]Theefficiencyofasolarthermalcollectorisdirectlyrelatedtoheatlossesfromthecollectorsurface(efficiencybeingdefinedastheproportionofheatenergythatcanberetainedforapredefinedperiodoftime).Withinthecontextofasolarcollector,convectionandradiationarethemostimportantsourcesofheatloss.Thermalinsulationisusedtoslowdownheatlossfromahotobjecttoitsenvironment.ThisisactuallyadirectmanifestationoftheSecondlawofthermodynamicsbutwemaytermthisthe'equilibriumeffect'.Heatislostmorerapidlyifthetemperaturedifferencebetweenahotobjectanditsenvironmentislarger.Heatlossispredominantlygovernedbythethermalgradientbetweenthetemperatureofthecollectorsurfaceandtheambienttemperature.Conduction,convection,andradiationalloccurmorerapidlyoverlargethermalgradients.[23]Wemaytermthisthe'deltateffect'.

Flatplatesolarthermalcollector,viewedfromrooflevel

Themostsimpleapproachtosolarheatingofwateristosimplymountametaltankfilledwithwaterinasunnyplace.Theheatfromthesunwouldthenheatthemetaltankandthewaterinside.Indeed,thiswashowtheveryfirstSWHsystemsworkedmorethanacenturyago.[5]However,thissetupwouldbeinefficientduetoanoversightoftheequilibriumeffect,above:assoonasheatingofthetankandwaterbegins,theheatgainedstartstobelostbackintotheenvironment,andthiscontinuesuntilthewaterinthetankreachestheambienttemperature.Thechallengeisthereforetolimittheheatlossfromthetank,thusdelayingthetimewhenthermalequilibriumisregained.

ICSorbatchcollectorsreduceheatlossbyplacingthewatertankinathermallyinsulatedbox.[1][24]Thisisachievedbyencasingthewatertankinaglasstoppedboxthatallowsheatfromthesuntoreachthewatertank.[25]However,theotherwallsoftheboxarethermallyinsulated,reducingconvectionaswellasradiationtotheenvironment.[26]Inaddition,theboxcanalsohaveareflectivesurfaceontheinside.Thisreflectsheatlostfromthetankbacktowardsthetank.InasimplewayonecouldconsideranICSsolarwaterheaterasawatertankthathasbeenenclosedinatypeof'oven'thatretainsheatfromthesunaswellasheatofthewaterinthetank.Usingaboxdoesnoteliminateheatlossfromthetanktotheenvironment,butitlargelyreducesthisloss.

StandardICScollectorshaveacharacteristicthatstronglylimitstheefficiencyofthecollector:asmallsurfacetovolumeratio.[27]Sincetheamountofheatthatatankcanabsorbfromthesunislargelydependentonthesurfaceofthetankdirectlyexposedtothesun,itfollowsthatasmallsurfacewouldlimitthedegreetowhichthewatercanbeheatedbythesun.CylindricalobjectssuchasthetankinanICScollectorinherentlyhaveasmallsurfacetovolumeratioandmostmoderncollectorsattempttoincreasethisratioforefficientwarmingofthewaterinthetank.Therearemanyvariationsonthisbasicdesign,withsomeICScollectorscomprisingseveralsmallerwatercontainersandevenincludingevacuatedglasstubetechnology,atypeofICSsystemknownasanEvacuatedTubeBatch(ETB)collector.[1]

Flatplatecollectorsareanextensionofthebasicideatoplaceacollectorinan'oven'likeboxwithglassinthedirectionoftheSun.[1]Mostflatplatecollectorshavetwohorizontalpipesatthetopandbottom,calledheaders,andmanysmallerverticalpipesconnectingthem,calledrisers.Therisersarewelded(orsimilarlyconnected)tothinabsorberfins.Heattransferfluid(waterorwater/antifreezemix)ispumpedfromthehotwaterstoragetank(directsystem)orheatexchanger(indirectsystem)intothecollectors'bottomheader,andittravelsuptherisers,collectingheatfromtheabsorberfins,andthenexitsthecollectoroutofthetopheader.Serpentineflatplatecollectorsdifferslightlyfromthis"harp"design,andinsteaduseasinglepipethattravelsupanddownthecollector.However,sincetheycannotbeproperlydrainedofwater,serpentineflatplatecollectorscannotbeusedindrainbacksystems.

Thetypeofglassusedinflatplatecollectorsisalmostalwayslowiron,temperedglass.Beingtempered,theglasscanwithstandsignificanthailwithoutbreaking,whichisoneofthereasonsthatflatplatecollectorsareconsideredthemostdurablecollectortype.

Unglazedorformedcollectorsaresimilartoflatplatecollectors,excepttheyarenotthermallyinsulatednorphysicallyprotectedbyaglasspanel.Consequentlythesetypesofcollectorsaremuchlessefficientfordomesticwaterheating.Forpoolheatingapplications,however,thewaterbeingheatedisoftencolderthan

theambientrooftemperature,atwhichpointthelackofthermalinsulationallowsadditionalheattobedrawnfromthesurroundingenvironment.[28]

Evacuatedtubecollectors(ETC)areawayinwhichheatlosstotheenvironment,[1]inherentinflatplates,hasbeenreduced.Sinceheatlossduetoconvectioncannotcrossavacuum,itformsanefficientisolationmechanismtokeepheatinsidethecollectorpipes.[29]Sincetwoflatsheetsofglassarenormallynotstrongenoughtowithstandavacuum,thevacuumisrathercreatedbetweentwoconcentrictubes.Typically,thewaterpipinginanETCisthereforesurroundedbytwoconcentrictubesofglasswithavacuuminbetweenthatadmitsheatfromthesun(toheatthepipe)butwhichlimitsheatlossbacktotheenvironment.Theinnertubeiscoatedwithathermalabsorbent.[30]Lifeofthevacuumvariesfromcollectortocollector,anywherefrom5yearsto15years.

FlatplatecollectorsaregenerallymoreefficientthanETCinfullsunshineconditions.However,theenergyoutputofflatplatecollectorsisreducedslightlymorethanevacuatedtubecollectorsincloudyorextremelycoldconditions.[1]MostETCsaremadeoutofannealedglass,whichissusceptibletohail,breakinginroughlygolfballsizedhail.ETCsmadefrom"cokeglass,"whichhasagreentint,arestrongerandlesslikelytolosetheirvacuum,butefficiencyisslightlyreducedduetoreducedtransparency.

Heatingofswimmingpools

Bothpoolcoveringsystemsfloatingatopthewaterandseparatesolarthermalcollectorsmaybeusedforpoolheating.

Poolcoveringsystems,whethersolidsheetsorfloatingdisks,actasinsulationandreduceheatloss.Muchofapool'sheatlossoccursthroughevaporation,andusingacoverprovidesabarrieragainstevaporation.Usingapoolcoverwillsupplementthesolarthermalcollectorsdiscussedbelow.SeeSwimmingPoolCoversforadetaileddiscussion.

Solarthermalcollectorsfornonpotablepoolwateruseareoftenmadeofplastic.Poolwater,mildlycorrosiveduetochlorine,iscirculatedthroughthepanelsusingtheexistingpoolfilterorsupplementalpump.Inmildenvironments,unglazedplasticcollectorsaremoreefficientasadirectsystem.Incoldorwindyenvironmentsevacuatedtubesorflatplatesinanindirectconfigurationdonothavepoolwaterpumpedthroughthem,theyareusedinconjunctionwithaheatexchangerthattransferstheheattopoolwater.Thiscauseslesscorrosion.Afairlysimpledifferentialtemperaturecontrollerisusedtodirectthewatertothepanelsorheatexchangereitherbyturningavalveoroperatingthepump.[31]Oncethepoolwaterhasreachedtherequiredtemperature,adivertervalveisusedtoreturnpoolwaterdirectlytothepoolwithoutheating.[32]Manysystemsareconfiguredasdrainbacksystemswherethewaterdrainsintothepoolwhenthewaterpumpisswitchedoff.

Thecollectorpanelsareusuallymountedonanearbyroof,orgroundmountedonatiltedrack.Duetothelowtemperaturedifferencebetweentheairandthewater,thepanelsareoftenformedcollectorsorunglazedflatplatecollectors.Asimpleruleofthumbfortherequiredpanelareaneededis50%ofthepool'ssurfacearea.[32]Thisisforareaswherepoolsareusedinthesummerseasononly,notyear'round.Addingsolarcollectorstoaconventionaloutdoorpool,inacoldclimate,cantypicallyextendthepool'scomfortableusagebysomemonthsormoreifaninsulatingpoolcoverisalsoused.[28]Anactivesolarenergysystemanalysisprogrammaybeusedtooptimizethesolarpoolheatingsystembeforeitisbuilt.

AlaundromatinCaliforniawithpanelsontheroofprovidinghotwashingwater.

Economics,energy,environment,andsystemcosts

Energyproduction

Theamountofheatdeliveredbyasolarwaterheatingsystemdependsprimarilyontheamountofheatdeliveredbythesunataparticularplace(theinsolation).Intropicalplacestheinsolationcanberelativelyhigh,e.g.7kW.h/m2perday,whereastheinsolationcanbemuchlowerintemperateareaswherethedaysareshorterinwinter,e.g.3.2kW.h/m2perday.Evenatthesamelatitudetheaverageinsolationcanvaryagreatdealfromlocationtolocationduetodifferencesinlocalweatherpatternsandtheamountofovercast.UsefulcalculatorsforestimatinginsolationatasitecanbefoundwiththeJointResearchLaboratoryoftheEuropeanCommission[33]andtheAmericanNationalRenewableEnergyLaboratory.[34][35]

Belowisatablethatgivesaroughindicationofthespecificationsandenergythatcouldbeexpectedfromasolarwaterheatingsysteminvolvingsome2m2ofabsorberareaofthecollector,demonstratingtwoevacuatedtubeandthreeflatplatesolarwaterheatingsystems.Certificationinformationorfigurescalculatedfromthosedataareused.Thebottomtworowsgiveestimatesfordailyenergyproduction(kW.h/day)foratropicalandatemperatescenario.Theseestimatesareforheatingwaterto50Caboveambienttemperature.

Withmostsolarwaterheatingsystems,theenergyoutputscaleslinearlywiththesurfaceareaoftheabsorbers.Therefore,whencomparingfigures,takeintoaccounttheabsorberareaofthecollectorbecausecollectorswithlessabsorberareayieldlessheat,evenwithinthe2m2range.SpecificationsformanycompletesolarwaterheatingsystemsandseparatesolarcollectorscanbefoundatInternetsiteoftheSRCC.[36]

Dailyenergyproduction(kWth.h)offivesolarthermalsystems.Theevactubesystemsusedbelowbothhave20tubesTechnology Flatplate Flatplate Flatplate Evactube Evactube

Configuration Directactive ThermosiphonIndirectactive

Indirectactive

Directactive

Overallsize(m2) 2.49 1.98 1.87 2.85 2.97

Absorbersize(m2) 2.21 1.98 1.72 2.85 2.96

Maximumefficiency 0.68 0.74 0.61 0.57 0.46Energyproduction(kW.h/day):Insolation3.2kW.h/m2/day(temperate)e.g.Zurich,Switzerland

5.3 3.9 3.3 4.8 4.0

Insolation6.5kW.h/m2/day(tropical)e.g.Phoenix,USA

11.2 8.8 7.1 9.9 8.4

Thefiguresarefairlysimilarbetweentheabovecollectors,yieldingsome4kW.h/dayinatemperateclimateandsome8kW.h/dayinamoretropicalclimatewhenusingacollectorwithanabsorberareaofabout2m2insize.Inthetemperatescenariothisissufficienttoheat200litresofwaterbysome17C.Inthetropicalscenariotheequivalentheatingwouldbebysome33C.Manythermosiphonsystemsarequiteefficientandhavecomparableenergyoutputtoequivalentactivesystems.Theefficiencyofevacuatedtubecollectorsissomewhatlowerthanforflatplatecollectorsbecausetheabsorbersarenarrowerthanthetubesandthetubeshavespacebetweenthem,resultinginasignificantlylargerpercentageofinactiveoverallcollectorarea.Somemethodsofcomparison[37]calculatetheefficiencyofevacuatedtubecollectorsbasedontheactualabsorberareaandnotonthe'roofarea'ofthesystemashasbeendoneintheabovetable.Theefficiencyofthecollectorsbecomeslowerifonedemandswaterwithaveryhightemperature.

Systemcost

Insunny,warmlocations,wherefreezeprotectionisnotnecessary,anICS(batchtype)solarwaterheatercanbeextremelycosteffective.[26]Inhigherlatitudes,thereareoftenadditionaldesignrequirementsforcoldweather,whichaddtosystemcomplexity.Thishastheeffectofincreasingtheinitialcost(butnotthelifecyclecost)ofasolarwaterheatingsystem,toalevelmuchhigherthanacomparablewaterheateroftheconventionaltype.Thebiggestsingleconsiderationisthereforethelargeinitialfinancialoutlayofsolarwaterheatingsystems.[38]Offsettingthisexpensecantakeseveralyears[39]andthepaybackperiodislongerintemperateenvironmentswheretheinsolationislessintense.[40]Whencalculatingthetotalcosttoownandoperate,aproperanalysiswillconsiderthatsolarenergyisfree,thusgreatlyreducingtheoperatingcosts,whereasotherenergysources,suchasgasandelectricity,canbequiteexpensiveovertime.Thus,whentheinitialcostsofasolarsystemareproperlyfinancedandcomparedwithenergycosts,theninmanycasesthetotalmonthlycostofsolarheatcanbelessthanothermoreconventionaltypesofwaterheaters(alsoinconjunctionwithanexistingwaterheater).Athigherlatitudes,solarheatersmaybelesseffectiveduetolowersolarenergy,possiblyrequiringlargerand/ordualheatingsystems.[40]Inaddition,governmentincentivescanbesignificant.

CostsandpaybackperiodsforresidentialSWHsystemswithsavingsof200kW.h/month(using2010data)

Country Currency Systemcost Subsidy(%)Effective

costElectricitycost/kW.h

Electricitysavings/month

Paybackperiod(y)

Brazil BRL 2500[42] 0 2500 0.25 50 4.2

SouthAfrica ZAR 14000 15

[43] 11900 0.9 180 5.5

Australia AUD 5000[44] 40[45] 3000 0.18[46] 36 6.9

Belgium EUR 4000[47] 50[48] 2000 0.1[49] 20 8.3

UnitedStates USD 5000

[50] 30[51] 3500 0.1158[52] 23.16 12.6

UnitedKingdom GBP 4800

[53] 0 4800 0.11[54] 22 18.2

ThecalculationoflongtermcostandpaybackperiodforahouseholdSWHsystemdependsonanumberoffactors.Someoftheseare:

Priceofpurchasingsolarwaterheater(morecomplexsystemsaremoreexpensive)EfficiencyofSWHsystempurchasedInstallationcostPriceofelectricityuseformainspumping(ifthisisused)Priceofwaterheatingfuel(e.g.gasorelectricity)savedperkW.hAmountofwaterheatingfuelusedpermonthbyahouseholdUpfrontstateorgovernmentsubsidyforinstallationofasolarwaterheaterRecurrentorannualtaxrebatesorsubsidyforoperatingrenewableenergyAnnualmaintenancecostofSWHsystem(e.g.antifreezeorpumpreplacements)Savingsinannualmaintenanceofconventional(electric/gas/oil)waterheatingsystem

Thefollowingtablegivessomeideaofthecostandpaybackperiodtorecoverthecosts.Itdoesnottakeintoaccountannualmaintenancecosts,annualtaxrebatesandinstallationcosts.However,thetabledoesgiveanindicationofthetotalcostandtheorderofmagnitudeofthepaybackperiod.Thetableassumesanenergysavingsof200kW.hpermonth(about6.57kW.h/day)duetoSWH.Unfortunatelypaybacktimescanvarygreatlyduetoregionalsun,extracostduetofrostprotectionneedsofcollectors,householdhotwateruseetc.somoreinformationmaybeneededtogetaccurateestimatesforindividualhouseholdsandregions.ForinstanceincentralandsouthernFloridathepaybackperiodcouldeasilybe7yearsorlessratherthanthe12.6yearsindicatedonthechartfortheUS.[41]

Twopointsareclearfromtheabovetable.Firstly,thepaybackperiodisshorterincountrieswithalargeamountofinsolationandeveninpartsofthesamecountrywithmoreinsolation.Thisisevidentfromthepaybackperiodlessthan10yearsinmostsouthernhemispherecountries,listedabove.Thisispartlybecauseofgoodsunshine,allowingusersinthosecountriestoneedsmallersystemsthanintemperateareas.Secondly,eveninthenorthernhemispherecountrieswherepaybackperiodsareoftenlongerthan10years,solarwaterheatingisfinanciallyextremelyefficient.ThisispartlybecausetheSWHtechnologyisefficientincapturingirradiation.Thepaybackperiodforphotovoltaicsystemsismuchlonger.[40]Inmany

casesthepaybackperiodforaSWHsystemisshortenedifitsuppliesallornearlyallofthewarmwaterrequirementsusedbyahousehold.ManySWHsystemssupplyonlyafractionofwarmwaterneedsandareaugmentedbygasorelectricheatingonadailybasis,[39]thusextendingthepaybackperiodofsuchasystem.

SolarleasingisnowavailableinSpainforsolarwaterheatingsystemsfromPretasol[55]withatypicalsystemcostingaround59eurosandrisingto99eurospermonthforasystemthatwouldprovidesufficienthotwaterforatypicalfamilyhomeofsixpersons.Thepaybackperiodwouldbefiveyears.

AustraliahasinstitutedasystemofRenewableEnergyCredits,basedonnationalrenewableenergytargets.Thisexpandsanoldersystembasedonlyonrebates.[45]

Operationalcarbon/energyfootprintandlifecycleassessment

Terminology

Operationalenergyfootprint(OEF)isalsocalledenergyparasiticsratio(EPR)orcoefficientofperformance(CoP).Operationalcarbonfootprint(OCF)isalsocalledcarbonclawbackratio(CCR).LifecycleassessmentisusuallyreferredtoasLCA.

Carbon/energyfootprint

ThesourceofelectricityinanactiveSWHsystemdeterminestheextenttowhichasystemcontributestoatmosphericcarbonduringoperation.Activesolarthermalsystemsthatusemainselectricitytopumpthefluidthroughthepanelsarecalled'lowcarbonsolar'.Inmostsystemsthepumpingcancelstheenergysavingsbyabout8%andthecarbonsavingsofthesolarbyabout20%.[56]However,somenewlowpowerpumpswillstartoperationwith1Wanduseamaximumof20W.[57][58]Assumingasolarcollectorpaneldelivering4kW.h/dayandapumprunningintermittentlyfrommainselectricityforatotalof6hoursduringa12hoursunnyday,thepotentiallynegativeeffectofsuchapumpcanbereducedtoabout3%ofthetotalpowerproduced.

Thecarbonfootprintofsuchhouseholdsystemsvariessubstantially,dependingonwhetherelectricityorotherfuelssuchasnaturalgasarebeingdisplacedbytheuseofsolar.Exceptwhereahighproportionofelectricityisalreadygeneratedbynonfossilfuelmeans,naturalgas,acommonwaterheatingfuel,inmanycountries,hastypicallyonlyabout40%ofthecarbonintensityofmainselectricityperunitofenergydelivered.Thereforethe3%or8%energyclawbackinagashomereferredtoabovecouldthereforebeconsidered8%to20%carbonclawback,averylowfigurecomparedtotechnologiessuchasheatpumps.

However,PVpoweredactivesolarthermalsystemstypicallyusea530WPVpanelwhichfacesinthesamedirectionasthemainsolarheatingpanelandasmall,lowpowerdiaphragmpumporcentrifugalpumptocirculatethewater.Thisreducestheoperationalcarbonandenergyfootprint:agrowingdesigngoalforsolarthermalsystems.

Workisalsotakingplaceinanumberofpartsoftheworldondevelopingalternativenonelectricalpumpingsystems.Thesearegenerallybasedonthermalexpansionandphasechangesofliquidsandgases,avarietyofwhichareunderdevelopment.

Lifecyclecarbon/energyassessment

Nowlookingatawiderpicturethanjusttheoperationalenvironmentalimpacts,recognisedstandardscanbeusedtodeliverrobustandquantitativelifecycleassessment(LCA).LCAtakesintoaccountthetotalenvironmentalcostofacquisitionofrawmaterials,manufacturing,transport,using,servicinganddisposingoftheequipment.Thereareseveralaspectstosuchanassessment,including:

Thefinancialcostsandgainsincurredduringthelifeoftheequipment.Theenergyusedduringeachoftheabovestages.TheCO2emissionsduetoeachoftheabovestages.

EachoftheseaspectsmaypresentdifferenttrendswithrespecttoaspecificSWHdevice.

Financialassessment.Thetableintheprevioussectionaswellasseveralotherstudiessuggestthatthecostofproductionisgainedduringthefirst512yearsofuseoftheequipment,dependingontheinsolation,withcostefficiencyincreasingastheinsolationdoes.[39]

Intermsofenergy,some60%ofthematerialsofaSWHsystemgoesintothetank,withsome30%towardsthecollector[59](thermosiphonflatplateinthiscase)(Tsiligiridisetal.).InItaly,[60]some11GJofelectricityareusedinproducingtheequipment,withabout35%oftheenergygoingtowardsthemanufacturingthetank,withanother35%towardsthecollectorandthemainenergyrelatedimpactbeingemissions.TheenergyusedinmanufacturingisrecoveredwithinthefirsttwotothreeyearsofuseoftheSWHsystemthroughheatcapturedbytheequipmentaccordingtothissouthernEuropeanstudy.

Movingfurthernorthintocolder,lesssunnyclimates,theenergypaybacktimeofasolarwaterheatingsysteminaUKclimateisreportedasonly2years.[61]Thisfigurewasderivedfromthestudiedsolarwaterheatingsystembeing:direct,retrofittedtoanexistingwaterstore,PVpumped,freezetolerantandof2.8sqmaperture.Forcomparison,asolarelectric(PV)installationtookaround5yearstoreachenergypayback,accordingtothesamecomparativestudy.

IntermsofCO2emissions,alargedegreeoftheemissionssavingtraitsofaSWHsystemisdependentonthedegreetowhichwaterheatingbygasorelectricityisusedtosupplementsolarheatingofwater.UsingtheEcoindicator99pointssystemasayardstick(i.e.theyearlyenvironmentalloadofanaverageEuropeaninhabitant)inGreece,[59]apurelygasdrivensystemmaybecheaperintermsofemissionsthanasolarsystem.Thiscalculationassumesthatthesolarsystemproducesabouthalfofthehotwaterrequirementsofahousehold.TheproductionofatestSWHsysteminItaly[60]producedabout700kgofCO2,withallthecomponentsofmanufacture,useanddisposalcontributingsmallpartstowardsthis.Maintenancewasidentifiedasanemissionscostlyactivitywhentheheattransferfluid(glycolbased)wasperiodicallyreplaced.However,theemissionscostwasrecoveredwithinabouttwoyearsofuseoftheequipmentthroughtheemissionssavedbysolarwaterheating.InAustralia,[39]thelifecycleemissionsofaSWHsystemarealsorecoveredfairlyrapidly,whereaSWHsystemhasabout20%oftheimpactofanelectricalwaterheaterandhalfoftheemissionsimpactofagaswaterheater.

Analysingtheirlowerimpactretrofitfreezetolerantsolarwaterheatingsystem,Allenetal.(qv)reportaproductionCO2impactof337kg,whichisaroundhalftheenvironmentalimpactreportedintheArdenteetal.(qv)study.

Whereinformationbasedonestablishedstandardsareavailable,theenvironmentaltransparencyaffordedbylifecycleanalysisallowsconsumers(ofallproducts)tomakeincreasinglywellinformedproductselectiondecisions.Asforidentifyingsectorswherethisinformationislikelytoappearfirst,environmentaltechnologysuppliersinthemicrogenerationandrenewableenergytechnologyarenaareincreasinglybeingpressedbyconsumerstoreporttypicalCoPandLCAfiguresfortheirproducts.

Insummary,theenergyandemissionscostofaSWHsystemformsasmallpartofthelifecyclecostandcanberecoveredfairlyrapidlyduringuseoftheequipment.Theirenvironmentalimpactscanbereducedfurtherbysustainablematerialssourcing,usingnonmainscirculation,byreusingexistinghotwaterstoresand,incoldclimates,byeliminatingantifreezereplacementvisits.

Doityourself(DIY)systems

Peoplehavebegunbuildingtheirown(smallscale)solarwaterheatingsystemsfromscratchorbuyingkits.PlansforsolarwaterheatingsystemsareavailableontheInternet.[62]andpeoplehavesetaboutbuildingthemfortheirowndomesticrequirements.DIYSWHsystemsareusuallycheaperthancommercialones,andtheyareusedbothinthedevelopedanddevelopingworld.[63]

Systemspecificationandinstallation

ExceptinrareinstancesitwillbeinsufficienttoinstallaSWHsystemwithnoelectricalorgasorotherfuelbackup.ManySWHsystemshaveabackupelectricheatingelementintheintegratedtank,theoperationofwhichmaybenecessaryoncloudydaystoensureareliablesupplyofhotwater.Thetemperaturestabilityofasystemisdependentontheratioofthevolumeofwarmwaterusedperdayasafractionofthesizeofthewaterreservoir/tankthatstoresthehotwater.Ifalargeproportionofhotwaterinthereservoirisusedeachday,alargefractionofthewaterinthereservoirneedstobeheated.Thisbringsaboutsignificantfluctuationsinwatertemperatureeveryday,withpossiblerisksofoverheatingorunderheating,dependingonthedesignofthesystem.Sincetheamountofheatingthatneedstotakeplaceeverydayisproportionaltohotwaterusageandnottothesizeofthereservoir,itisdesirabletohaveafairlylargereservoir(i.e.equaltoorgreaterthandailyusage,)whichwillhelppreventfluctuationsinwatertemperature.Ifamplestorageispreexistingorcanotherwisebereasonablyacquired,alargeSWHsystemismoreefficienteconomicallythanasmallsystem.[59]Thisisbecausethepriceofasystemisnotlinearlyproportionaltothesizeofthecollectorarray,sothepricepersquaremeterofcollectorischeaperinalargersystem.Ifthisisthecase,itpaystouseasystemthatcoversnearlyallofthedomestichotwaterneeds,andnotonlyasmallfractionoftheneeds.Thisfacilitatesmorerapidcostrecovery.Notallinstallationsrequirenewreplacementsolarhotwaterstores.Existingstoresmaybelargeenoughandinsuitablecondition.Directsystemscanberetrofittedtoexistingstoreswhileindirectsystemscanbealsosometimesberetrofittedusinginternalandexternalheatexchangers.TheinstallationofaSWHsystemneedstobecomplementedwithefficientinsulationofallthewaterpipesconnectingthecollectorandthewaterstoragetank,aswellasthestoragetank(or"geyser")andthemostimportantwarmwateroutlets.Theinstallationofefficientlaggingsignificantlyreducestheheatlossfromthehotwatersystem.Theinstallationoflaggingonatleasttwometersofpipeonthecoldwaterinletofthestoragetankreducesheatloss,asdoestheinstallationofa"geyserblanket"aroundthestoragetank(ifinsidearoof).IncoldclimatestheinstallationoflaggingandinsulationisoftenperformedevenintheabsenceofaSWHsystem.ThemostefficientPVpumpsaredesignedtostartveryslowlyinverylowlightlevels,soifconnecteduncontrolled,theymaycauseasmallamountofunwantedcirculationearlyinthemorning

forexamplewhenthereisenoughlighttodrivethepumpbutwhilethecollectorisstillcold.Toeliminatetheriskofhotwaterinthestoragetankfrombeingcooledthatwaythisisveryimportant.solarcontrollermayberequired.Themodularityofanevacuatedtubecollectorarrayallowstheadjustmentofthecollectorsizebyremovingsometubesortheirheatpipes.Budgetingforalargerthanrequiredarrayoftubesthereforeallowsforthecustomisationofcollectorsizetotheneedsofaparticularapplication,especiallyinwarmerclimates.Particularlyinlocationsfurthertowardsthepolesthan45degreesfromtheequator,roofmountedsunfacingcollectorstendtooutperformwallmountedcollectorsintermsoftotalenergyoutput.However,itistotalusefulenergyoutputwhichusuallymattersmosttoconsumers.Soarraysofsunnywallmountedsteepcollectorscansometimesproducemoreusefulenergybecausetherecanbeasmallincreaseinwintergainattheexpenseofalargeunusedsummersurplus.

Standards

Europe

EN806:Specificationsforinstallationsinsidebuildingsconveyingwaterforhumanconsumption.General.EN1717:Protectionagainstpollutionofpotablewaterinwaterinstallationsandgeneralrequerementsofdevicestopreventpollutionbybackflow.EN60335:Specificationforsafetyofhouseholdandsimilarelectricalappliances.(221)UNE94002:2005Thermalsolarsystemsfordomestichotwaterproduction.Calculationmethodforheatdemand.

UnitedStates

OG300:OG300CertificationofSolarWaterHeatingSystems.[64]

Australia

RenewableEnergy(Electricity)Act2000RenewableEnergy(Electricity)(LargescaleGenerationShortfallCharge)Act2000RenewableEnergy(Electricity)(SmallscaleTechnologyShortfallCharge)Act2010RenewableEnergy(Electricity)Regulations2001RenewableEnergy(Electricity)Regulations2001STCCalculationMethodologyforSolarWaterHeatersandAirSourceHeatPumpWaterHeatersRenewableEnergy(Electricity)Amendment(TransitionalProvision)Regulations2010RenewableEnergy(Electricity)Amendment(TransitionalProvisions)Regulations2009

AllrelevantparticipantsoftheLargescaleRenewableEnergyTargetandSmallscaleRenewableEnergySchememustcomplywiththeaboveActs.[65]

APPENDIX1.Worldwideuse

Topcountriesworldwide

SolarhotwatersysteminstalledonlowcosthousingintheKougaLocalMunicipality,SouthAfrica

Topcountriesusingsolarthermalpower,worldwide:GWth[11][66][67][68][69][70][71]

# Country 2005 2006 2007 2008 2009 2010 2011 2012 20131 China 55.5 67.9 84.0 105.0 101.5 117.6 EU 11.2 13.5 15.5 20.0 22.8 23.5 25.6 29.7 31.42 UnitedStates 1.6 1.8 1.7 2.0 14.4 15.3 3 Germany 7.8 8.9 9.8 10.5 11.4 12.14 Turkey 5.7 6.6 7.1 7.5 8.4 9.3 5 Australia 1.2 1.3 1.2 1.3 5.0 5.8 6 Brazil 1.6 2.2 2.5 2.4 3.7 4.3 7 Japan 5.0 4.7 4.9 4.1 4.3 4.0 8 Austria 2.5 3.0 3.2 2.8 3.4 3.59 Greece 2.7 2.9 2.9 2.9 2.9 2.9

10 Israel 3.3 3.8 3.5 2.6 2.8 2.9 World(GWth) 88 105 126 149 172 196

SolarheatinginEuropeanUnion+Switzerland

SolarthermalheatinginEuropeanUnion(MWth)[72][73][74]

# Country 2008 2009 2010[68] 2011 2012 2013

1 Germany 7,766 9,036 9,831 10,496 11,416 12,0552 Austria 2,268 3,031 3,227 2,792 3,448 3,5383 Greece 2,708 2,853 2,855 2,861 2,885 2,9154 Italy 1,124 1,410 1,753 2,152 2,380 2,5905 Spain 988 1,306 1,543 1,659 2,075 2,2386 France 1,137 1,287 1,470 1,277 1,691 1,8027 Poland 254 357 459 637 848 1,0408 Portugal 223 395 526 547 677 7179 CzechRepublic 116 148 216 265 625 681

10 Switzerland 416 538 627 11 Netherlands 254 285 313 332 605 61612 Denmark 293 339 379 409 499 55013 Cyprus 485 490 491 499 486 47614 UK 270 333 374 460 455 47515 Belgium 188 204 230 226 334 37416 Sweden 202 217 227 236 337 34217 Ireland 52 85 106 111 177 19618 Slovenia 96 111 116 123 142 14819 Hungary 18 59 105 120 125 13720 Slovakia 67 73 84 100 108 11321 Romania* 66 80 73 74 93 11022 Bulgaria* 22 56 74 81 58 5923 Malta* 25 29 32 36 34 3524 Finland* 18 20 23 23 30 3325 Luxembourg* 16 19 22 25 23 2726 Estonia* 1 1 1 3 10 1227 Latvia* 1 1 1 3 10 1228 Lithuania* 1 2 2 3 6 8

Total EU27+Sw(MWth) 19,08 21,60 23.49 25.55 29.66 31.39

*=estimation,F=Franceasawhole

Seealso

WikimediaCommonshasmediarelatedtoSolarwaterheating.

Australia:SolarhotwaterinAustralia

SolarthermalcollectorSolarairheatingSolarairconditioningConcentratingsolarpowerPassivesolarRenewableheatSolarcombisystemSolarenergySolarthermalenergyRenewableenergycommercializationSustainabledesign

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