Pumping Water For Irrigation Using Solar Energy

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Pumping Water For Irrigation Using Solar Energy
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Helikson, H.J.
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University of Florida Cooperative Extension Service, Institute of Food and Agriculture Sciences, EDIS
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FactSheetEES-63 November1991 PumpingWaterforIrrigationUsingSolarEnergy1 H.J.Helikson,D.Z.HamanandC.D.Baird2Thispublicationdiscussesphotovoltaictechnologyand thecostofphotovoltaicpowerforwaterpumping. Theinformationpresentedincludes: anoverviewofhowelectricityisgeneratedfrom solarradiationusingphotovoltaiccells, adescriptionofademonstrationalphotovoltaic poweredwaterpumpingsystem,and adiscussionofthepresentdaypriceofsucha systemandthepotentialfutureeffectsofcurrent trendswhichcontinuetodecreasethecostof photovoltaicpower.FROMSUNSHINETOELECTRICAL CURRENTPhotovoltaiccellsareabletoturntheenergyinsolar radiationintoelectricityduetoanenergytransfer thatoccursatthesub-atomiclevel.Solarenergy comesinsmallpackagescalledphotons.These photonshittheouterlevelelectronsinthe photovoltaiccellsliketheflappershitthemetalball inthepinballmachine.Thedislocatedelectrons formtheelectricalcurrent. Siliconisoneoftheelementsusedasabasematerial fortheproductionofphotovoltaiccells.Asilicon atomhasfourvalenceelectronswhicharesharedwith adjacentsiliconatomsincovalentbonding(Figure 1a).Toproducethepositive-chargedsideofa photovoltaiccell,boronatomswhichhaveonlythree valenceelectronsareintroducedintothelattice structureofpuresilicon.Theboronatomsoccupya latticepositionwithinthesiliconstructure,anda positive-chargedholeformsinplaceofthemissing fourthelectron(Figure1b).Siliconmaterialwith boronimpuritiesiscalledapositiveorp-type semiconductor.Toproducethenegative-chargedside ofaphotovoltaiccell,phosphorusatomswhichhave fivevalenceelectronsareintroducedintothepure siliconstructure.Thephosphorusatomsoccupya latticepositionwithinthesiliconstructureandform anegativeorn-typesemiconductor(Figure1c). Photovoltaiccellsaremadebyputtingalayerofntypeandalayerofp-typesemiconductormaterial together.Whenthephotonsinsolarradiationstrike aphotovoltaiccell,thekineticenergyofthephotons istransferredtothevalencelevelofelectrons.The freedelectronsandpositive-chargedholesattracteach otherandcreatepositive-negativepairs.The formationofthesepairscreateselectricity(Garg, 1987). 1.ThisdocumentisFactSheetEES-63,aseriesoftheFloridaEnergyExtensionService,FloridaCooperativeExtensionService,InstituteofFood andAgriculturalSciences,UniversityofFlorida.Publicationdate:November1991. 2.H.J.Helikson,FormerAgriculturalEnergySpecialist;D.Z.Haman,AssistantProfessor,AgriculturalEngineeringDept.;C.D.Baird,Professor, AgriculturalEngineeringDept.,CooperativeExtensionService,InstituteofFoodandAgriculturalSciences,UniversityofFlorida,Gainesville FL32611. TheFloridaEnergyExtensionServicereceivesfundingfromtheFloridaEnergyOffice,DepartmentofCommunityAffairsandisoperated bytheUniversityofFlorida'sInstituteofFoodandAgriculturalSciencesthroughtheCooperativeExtensionService.Theinformation containedhereinistheproductoftheFloridaEnergyExtensionServiceanddoesnotnecessarilyreflecttheviewsoftheFloridaEnergyOffice. TheInstituteofFoodandAgriculturalSciencesisanequalopportunity/affirmativeactionemployerauthorizedtoprovideresearch,educational informationandotherservicesonlytoindividualsandinstitutionsthatfunctionwithoutregardtorace,color,sex,age,handicap,ornational origin.Forinformationonobtainingotherextensionpublications,contactyourcountyCooperativeExtensionServiceoffice. FloridaCooperativeExtensionService/InstituteofFoodandAgriculturalSciences/UniversityofFlorida/ChristineTaylorStephens,Dean

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PumpingWaterforIrrigationUsingSolarEnergy Page2Aphotovoltaiccellisanalyzedbyitsopencircuit voltageandshortcircuitcurrentcapabilities(Figure 2a).Opencircuitvoltageisthevoltageoutputfrom Figure1. Siliconlatticeschematicwith4valenceelectrons (top);withboronimpurities(center);withphosphorous impurity,theextraelectrongivesanegativecharge(bottom)aphotovoltaiccellwhennocurrentisflowingthrough thecircuit.Itisthemaximumpossiblevoltagethata photovoltaiccellcanproduceinsunlight.Short circuitcurrentisthecurrentflowingfreelyfroma photovoltaiccellthroughanexternalcircuitthathas noloadorresistance.Itisthemaximumpossible currentthatthephotovoltaiccellcanproduceata givenlevelofirradiance2(FloridaSolarEnergy Center,1988). Toincreasevoltageoutput,photovoltaiccellsare wiredinseries;toincreaseamperageoutput, photovoltaiccellsarewiredinparallel.A photovoltaicmoduleisacombinationofphotovoltaic cellswiredtogetherinseriesandparallelwiththe purposeofgeneratingaspecificcurrentandvoltage atagivenlevelofirradiance.Aphotovoltaicarrayis composedoftwoormorephotovoltaicmodules. Themaximumpowerpointisthepointonagiven photovoltaicI-Vgraphwhichgivesthehighest amperageandvoltageproductatagivenlevelof irradiance(Figure2b).Thisisthedesiredpointof operationforaphotovoltaicarray. Thedirectcurrent(DC)powerreceivedbyan electricalloadfromaphotovoltaicarrayismainly controlledbytwoparameters3:thesolarirradiance availabletothemodule,andthecurrent-and-voltage demandoftheload.Thevoltageproductionofa photovoltaiccellremainspracticallyconstantunderall levelsofirradiance,butthecurrentproducedis directlyproportionaltothelevelofirradiance availableatanygivenpointintime.Sincethepower producedbyaphotovoltaiccellistheproductofthe currentandvoltagebeingproducedatanygiventime, photovoltaicpowerisdirectlyproportionaltothe levelofirradianceavailableatanygiventime(Figure 2c). Thecurrent-and-voltagecomponentsofaDC electricalloadformastraightlineonanI-Vgraph, aftertheinitialstart-uppowersurge,whichrisesata constantcurrent-to-voltageratio(Figure3a).In comparison,therepresentativeI-Vgraphofa photovoltaicpowersupplyshowsaconstantamperage whilethevoltageincreasesuntiltheamperagefalls sharplytozeroattheopencircuitvoltage.Ifa photovoltaicarrayisdesignedtoproduce24voltsbut theloadonlyrequires12volts,theloadwillonlydraw fromthephotovoltaicarraythepowerwhich correspondsto12voltsontheI-Vcurveeventhough thephotovoltaicarrayisabletoproducemorepower. Figure3ashowsanelectricloadlineanda photovoltaicpowersupplylinewhicharenotproperly matched. InadditiontothedifferenceinI-Vcurveformation betweenanelectricloadandaphotovoltaicelectric supply,thereisacontinualvariationintheamperage levelofthephotovoltaicpowersupplyduetochanges inthelevelofirradianceavailablethroughouttheday. Amperagefluctuationscontinuallychangethelocation ofthemaximumpowerpointontheI-Vgraphand

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PumpingWaterforIrrigationUsingSolarEnergy Page3hinderthematchingofthemaximumpowerpointsof Figure2. Amperage-voltage(I-V)curveforphotovoltaic module(top);I-Vcurveshowingmaximumpowerpoint (center);I-Vcurveshowingvariouslevelsofsolarirradiance (bottom).aphotovoltaicmoduletopointsalongthestraight loadlineoftheDCelectricload(Figure3b).Care mustbetakenwhendesigningaphotovoltaicsystem tomatchtheI-Vloadcurveandmaximumpower pointsoverthewidestpossiblerangetocreatea systemwithhighoverallefficiency.SYSTEMDESCRIPTIONAphotovoltaicarraycomprisedoftwounitsofthree moduleseachwasusedtopowerthewaterpumping systemusedinthisdemonstration(Figure3c).The sixphotovoltaicmoduleshadaphotonresponsive surfaceareaof3.17m2.Thethreemodulesofeach unitwereconnectedend-to-endandreflectors, constructedfromsheetmetalandaluminumfoiltape, wereattachedtothetwolongsidesofeachunit.The reflectorsdoubledtheareaofthearraystructure normaltothesunandincreasedtheshortcircuit amperageoftheunitsupto33percentoverall. Thephotovoltaicarraywasattachedtoaone-axis trackingmechanism.Thissystemenabledthearray toremainessentiallynormaltothesunthroughout thedaysothatthephotovoltaicmoduleswereableto utilizealargerportionoftheavailablesunlight.The trackingmechanismwaspoweredandcontrolledby two,smallphotovoltaicmoduleswhichfunctioned independentlyfromthesixprimarymodules(Dinh, 1988). Photovoltaiccellshaveminimalcurrentresistance whenexposedtolight,butwhentheyareshaded,all currentflowthroughthemisblocked.Thetrackingcontrolphotovoltaicmodulesonthephotovoltaic systemusedinthisdemonstrationwereplacedonthe eastandwestsidesofthearray.Whenbothtracking moduleswereinequalsunlight,theelectricity producedbythemflowedbetweenthetwomodules andthearrayremainedstationary.Whenoneofthe moduleswasshaded,theelectricityproducedbythe moduleremaininginsunlightflowedtothetracking motorwhichturnedthearrayuntilbothtracking moduleswereagaininequalsunlight(Figure4a). Tomatchthemaximumpowerpointsofthe photovoltaicarraywiththeI-VloadlineoftheDC electricmotor,thephotovoltaicsystemusedinthis demonstrationincludedanelectronicarray reconfigurationcontroller(EARC)(Salamehetal., 1989).AnEARCisanelectronicallycontrolled circuitwhichmonitorstheamperagebeinggenerated byamoduleandconnectsthemodulesinseriesor paralleltomatchthemaximumpowerpointsofa photovoltaicsystemtotheI-Vcurveoftheconnected loadoverthewidestpossiblerange.Todescribethe

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PumpingWaterforIrrigationUsingSolarEnergy Page4functionoftheEARC,assumeaphotovoltaicmodule Figure3. GraphshowingI-Vcurveofarrayatgivenlevelof irradianceandaresistiveload(top);variancesinamperage component(center);photovoltaic-poweredwaterpumping systemusedinhere(bottom).ofseven,10cm2photovoltaiccellswiredtogetherin parallelsothatatmaximumsunlight(1000W/m2)the shortcircuitcurrentequals1.00ampandtheopen circuitvoltageis0.6volts.Atlowerlevelsof irradiance,theamperagefallsproportionally,butthe voltageremainspracticallyconstant.Iftwo,seven-cell modulesareconnectedpermanentlyinparallel,their maximumpowerpointatlowirradiancevalueswill matchtheDCmotorloadlineatlowoperational voltages(Figure4b).Inthesamemanner,ifthese twomodulesareconnectedpermanentlyinseries,the maximumpowerpointathighirradiancevalueswill matchtheloadlineathighoperationalvoltages (Figure4c). TheEARCusedinthisdemonstrationsystem alternatedtheconfigurationofthetwounitsofthe photovoltaicarraybetweenparallelandseriesin referencetotheirradiancelevel.Whenthe irradiancelevelwaslow,thetwounitswere electronicallyconnectedinparalleltomaintainan adequateamperagelevelforcontinuedoperationof themotoratalowvoltage.Whentheirradiancelevel washigh,thetwounitswereelectronicallyconnected inserieswhichincreasedthevoltageoutputofthe arrayandproducedanamperageandvoltagepower supplywhichcloselymatchedtheamperageand voltagepowerdemandoftheattachedloadatthe higherlevel.Inthismanner,astheirradiancelevel varied,themotorutilizedthepoweravailablefrom thephotovoltaicarraymoreefficientlythanifthetwo unitshadbeenstaticallyconfiguredinaparallelor seriesconnection. Thepumpingsystemusedinthisdemonstration includeda0.5Hp,DC,permanentmagnetmotorand asingle-stagecentrifugalpump.Thewaterwas pumpedfromasurfacepondthrough5-cm(2-in) diameterPVCpipeanddischargedataheightof 2.44m(8ft).Thephotovoltaicsystemhadanaverage SOC4peakwatt(Wp)5ratingof374wattsatthesolar irradiancelevelof1000watts/m2.6SUBSYSTEMEFFICIENCIESThephotovoltaicsystemusedinthisdemonstration wasmathematicallyanalyzedasthreeseparate subsystems:1)thephotovoltaicmoduleswiththe reflectors,2)theEARC,and3)themotorandpump subsystem.Theindividualefficienciesofthese subsystemswere11percent,96percent,and44 percentrespectively.Figure5showstheflowof energyconvertedfromsolarradiationtofluidpower. Theoverallefficiencyoftheentiresystemwas4.6 percent.

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PumpingWaterforIrrigationUsingSolarEnergy Page5 Figure4. 2photovoltaicmodules,onecellshaded(l),bothinfullsun(r)(topl.);27-cellmodules-irradiancelevel.25kw/m2 (bottom);27-cellmodules-irradiancelevel.10kw/m2(topr.)WATERVOLUMESPUMPEDPERDAYThevolumeofwaterwhichaphotovoltaic-powered pumpisabletoproduceisrelatedtotheirradiance levelwhichitreceivesthroughouttheday.On October7,1989,highlevelsofirradiancewere availablethroughouttheentiredayandthe photovoltaicsystempumped20,180gallonsofwater againstastaticheadof2.44m(8ft)(Figure6).On January6,1990,cloudsblockedthesun'sraysovera largepartoftheday,andthesystempumpedonly 1,655gallonsofwater(Figure7). Anadvantageofusingdirectsolarradiationasa powersourceforirrigationisthatitisavailableatthe siteofapplicationwithouttheemploymentofa distributionsystem(HalcrowandPartners,1981). Plantwaterdemandandthequantityofwater pumpedbyaphotovoltaicpoweredwaterpumping systemarebothdirectlycorrelatedtodailysolar insolation.7Toascertaintheareaforwhichthesystemusedin thisdemonstrationcouldreplacedailypotential evapotranspiration(ETp),tenyearsofhistorical weatherdatawereusedtocalculatethetheoretical dailyfluidpoweroutputofthesystemandthedaily

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PumpingWaterforIrrigationUsingSolarEnergy Page6ETp(Smajstrlaetal.,1984).Thevolumeofwater Figure5.Thephotovoltaicsystemanditspointsofpower loss.Thephotovoltaicarraywithreflectorsconvertedonly 11%ofthesolarenergyitreceivedintoelectricalenergy.whichcouldbepumpedthroughanirrigationsystem withatotaldynamicheadof5m(16ft)usingthefluid powergeneratedbythephotovoltaicsystemwas dividedbythedailyETp.Thesecalculationsshowed thatthephotovoltaicsystemusedinthis demonstrationcouldreplacethedailyETpthrough thewintervegetablegrowingseasonbetween SeptemberandMayfor1.42hectares(3.5acres)on asoilabletostoreaseven-dayvolumeofdailyETpat an80percent8probabilityofsuccess.COMPARATIVEECONOMICANALYSISAcomparisonwasmadeofthenetpresentvalueof thecostsofirrigationpowerovera20-yearperiod usingdifferentpowersources.Thefivepowersources studiedwere: 1.0.3-horsepowerelectricmotorpoweredbyelectric gridwithamainlineavailable, 2.0.3-horsepowerelectricmotorpoweredbyelectric Figure6.Onanalmostcloudlessday,thearrayreceived almost900watt/m2for7straighthours.Thesystem pumped20,180gallonsofwatertoaheightof2.44m(8ft.). Figure7.Averycloudydayinwhichthesystempumpedonly 1,655gallonsofwater.gridwhena2-milemainlinemustbeinstalled, 3.3-horsepowergasolineengine(smallestavailable), 4.5-horsepowerdieselengine(smallestavailable), 5.0.5-horsepowerphotovoltaicpowersystem. Figure8aisagraphicalrepresentationofthechange inthe20-yearnetpresentcostofeachpowersupply asthesizeoftheirrigatedlandareaincreases.It showsthatat1989prices,photovoltaicpoweris competitiveforirrigatedlandareasoflessthan1.5ha (3.7ac)withdieselpowerandelectricgridpower whenmainlineinstallationisrequiredofmorethan twomilestoreachthesiteofenergyapplication.For irrigatedlandareasgreaterthanthis,thecurrentcost ofphotovoltaicpowerisprohibitive.

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PumpingWaterforIrrigationUsingSolarEnergy Page7 Figure8a. Netcostcomparisonforirrigationpowersourcesassizeoflandtobeirrigatedincreases.Thesystemat1989prices iscompetitiveforirrigatedlandareaof1.5haorless.ECONOMICSANDTHEFUTUREThecostofphotovoltaic-poweredwaterpumping systemsisdecreasing.Thecotofphotovoltaic moduleshasfallen400percentinthelast30years andthistrendcontinues.Photovoltaictechnology alsocontinuestoimprovethepowerconversion efficiencyofthephotovoltaiccell.Increasesin photovoltaiccellefficiencydecreasethecostof photovoltaicpower,becausefewermodulesare requiredtoproducethesameamountofpower. Whilethecostofphotovoltaicpowerisdecreasing, thecostofpowerderivedfromfossilfuelsis increasing.Inanefforttounderstandthecombined effectofthesetrendsontheeconomicviabilityof photovoltaicpowerforirrigation,atheoretical scenariooffuturepowercostswasdeveloped.Figure 8bshowstheresultsofthesecalculations. Atthepresenttime(1989),thecostofasemicrystallinesiliconmoduleis$9.00/Wp.9Figure8b showsthenetpresentcostofaphotovoltaicpower supplyforanirrigatedlandareaof5ha(12.4ac)at amodulecostof$7.00/Wp,$3.00/Wp,and$1.00/Wp. Italsoshowstheeffectofincreasesinfuelcostonthe netpresentcostofthedieselsystem,gasolinesystem, andelectricgridwithtwo-milemainlineinstallation system. The$7.00/Wpphotovoltaicsystembecame competitivewiththegasolinesystemforan irrigatedlandareaof5ha(12.4ac)atagasoline fuelpriceof$2.70/gallonwhichwas170percent higherthanthe1989price. The$3.00/Wpphotovoltaicsystembecame competitivewiththegasolinesystematafuel priceof$1.90/gallonwhichwas90percenthigher thanthe1989price. The$1.00/Wppricemadethephotovoltaicsystem competitivewithgasolinewithanincreaseinthe gasolinefuelpriceof50percent,to$1.50/gallon. The$3.00/Wpphotovoltaicsystemwascompetitive withanelectricgridsystem(withtwo-mile mainlineinstallation)whenthecostofelectricity wasincreasedby300percentfromthecurrent priceof$0.80/kWhto$0.32/kWh.

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PumpingWaterforIrrigationUsingSolarEnergy Page8 The$1.00/Wpphotovoltaicsystemwascompetitive Figure8b. Netpresentcostcomparisonforpowersourcesforirrigationforlandareaof5ha(12.4ac)asprice ofphotovoltaicsystemisdecreasedandpriceoffuelsisincreased. withadieselsystemata270percentincreaseof the1989priceofdieselto$4.63/gallon.CLOSINGREMARKSTheuseofphotovoltaicpowerforirrigationisawellmatchedapplicationofsolarenergysupplytoenergy need,becauseboththeplantwaterneededandthe availabilityofwatersuppliedbyaphotovoltaicsystem dependuponthesolarirradianceavailable.Complete utilizationofallofthesolarpoweravailable,however, withoutanenergy(battery)orfluidpower(elevated waterstoragetank)storageunitisimpossiblefortwo reasons:1)Solarenergywillbeavailableondays whennoadditionalwaterisrequiredbytheplants, i.e.,asunnydayafterarainstorm.2)Sincetheratio ofETptofluidpowerisgreaterinthesummer monthsthanthewintermonthsinFlorida,the photovoltaicsystemmustbeoversizedforthewinter monthstoguaranteeanadequatewatersupplyforthe cropduringthesummermonths. ThevegetablewintergrowingseasonofFlorida (September-May)wasusedfortheirrigatedarea exampleinthispaper.Theefficientapplicationof photovoltaicpowertoanirrigationsystemrequires thatthesolarpoweravailabilitypatternatthe applicationsitematchtheannualcropwaterneed closelyandthatthecropgrowingseasonbelengthyto utilizethephotovoltaicsystemasmuchaspossible throughouttheyear. Sincetheincreaseinpriceperincreaseinunitpower outputofaphotovoltaicsystemisgreaterthanthat foradiesel,gasoline,orelectricsystem,photovoltaic powerismorecostcompetitivewhentheirrigation systemwithwhichitoperateshasalowtotaldynamic head.Forthisreason,photovoltaicpowerismore costcompetitivewhenusedtopoweramicroirrigationsystemascomparedtoanoverheadsprinklersystem. Inconclusion,photovoltaicpowerforirrigationiscost competitivewithtraditionalenergysourcesforsmall, remoteapplications,ifthetotalsystemdesignand utilizationtimingiscarefullyconsideredand organizedtousethesolarenergyasefficientlyas possible.Inthefuture,whenthepricesoffossilfuels riseandtheeconomicadvantagesofmassproduction reducethepeakwattcostofthephotovoltaiccell,

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PumpingWaterforIrrigationUsingSolarEnergy Page9photovoltaicpowerwillbecomemorecostcompetitive andmorecommon.ENDNOTES2.Irradiance:Solarpowerperunitarea. 3.Celltemperatureandcelltypealsoinfluence poweroutput. 4.Standardoperatingconditions(SOC):Irradiance -1000watts/m2,Celltemperature-45degreesC. 5.Peakwatt(Wp):Maximumpoweroutputof photovoltaicsystematSOC. 6.1000watts/m2isconsideredtobethepractical maximumamountofsolarirradiancewhichcan reachtheearth'ssurfaceafterpassingthroughthe atmosphere. 7.Insolation:Solarenergyreceivedduringa specifictimeinterval. 8.An80percentchanceofsuccessmeansthaton anygivendaythechancethatthepowersupply willbeunabletoreplacethedailywaterloss throughevapotranspirationis20percent.Eighty percentisconsideredtobeanappropriatesuccess percentageforirrigationsystemdesignbythe FloridaWaterManagementDistricts. 9.Priceofmoduledividedbytheactualwattage outputofthemoduleunder1000watts/m2of solarirradianceastestedinthefield.REFERENCESDinh,K.1988.Apassivephotovoltaic-poweredsolar tracker. TechnicalBulletin .DinhCompany, Alachua,FL. FloridaSolarEnergyCenter.1988. Photovoltaic design:coursemanual.CapeCanaveral,FL. Garg,H.P.1987. Advancesinsolarenergytechnology, Volume3.ReidelPublishing,Boston,MA. Halcrow,S.W.andPartners.1981. Small-scalesolarpoweredirrigationpumpingsystems:technicaland economicreview .UNDPProjectGLO/78/004. IntermediateTechnologyPower,London,UK. Salameh,Z.,A.K.Milpur,andF.Dagher.1989. Controllergetslastdropforphotovoltaicsystem. SolarToday14 (1):26-27. Smajstrla,A.G.,G.A.Clark,S.F.Shih,F.S.Zazueta, andD.S.Harrison.1984. Characteristicsof potentialevapotranspirationinFlorida .Soiland CropScienceSociety.Fla.Proc.43:40-46.