What’s an Ideal PV System...
5
HiQ Solar, Inc. Page 1 of 5 What’s an Ideal PV System Granularity? 15 December 2015, v1.0 Contents Introduction .................................................................................................................................... 1 Native Granularity by Inverter Type ............................................................................................... 2 Competing Factors .......................................................................................................................... 3 Example Commercial 300 kW Installation ...................................................................................... 4 Perspective from a Commercial Installer ........................................................................................ 4 Summary ......................................................................................................................................... 5 Advertising ...................................................................................................................................... 5 Introduction: Aspects of Granularity What is the ideal granularity for a PV installation? Here we’re talking about the addressability of a number of system factors including: • Monitoring (enough data to maximize energy production, but not too much to consume resources unnecessarily) • O&M strategy (lowest cost for maximum energy harvest) • MPPT harvest • Power conversion block size (tolerance for shading, failures, etc) • Arc detection (safety) How much granularity is enough? As with many questions like this, the answer is “it depends….” Granularity impacts initial install cost and O&M differently. • For the install, as an example, the power conversion block size is one factor and this will impact material and labor costs etc. • For later O&M it comes down to how much of the output of an array can underperform before it impacts revenue - and revenue is impacted by energy production and O&M costs. For example, if one module in a residential install of 16 modules dies, a 17% drop in production will be noticeable and result in an urgent need to fix it. If that same module is one of 10,000 at a utility scale install there is less urgency over fixing a 0.01% hit. Granularity has a significant effect on the impact of PV module issues. • As another example, if you have per-string MPPT and a plant with 100 strings of 10 modules each, a module issue will impact less than 1% of plant output. That same plant with per-module power conversion (micro inverter or power optimizer), the impact will be 0.1% of total plant output. So power conversion comes into play. So why not just monitor/ power convert every module? As we’ll see, for small installs that could be the right answer but there’s more to it.
Transcript of What’s an Ideal PV System...
HiQSolar,Inc. Page1of5
What’sanIdealPVSystemGranularity?15December2015,v1.0ContentsIntroduction....................................................................................................................................1NativeGranularitybyInverterType...............................................................................................2CompetingFactors..........................................................................................................................3ExampleCommercial300kWInstallation......................................................................................4PerspectivefromaCommercialInstaller........................................................................................4Summary.........................................................................................................................................5Advertising......................................................................................................................................5
Introduction:AspectsofGranularityWhatistheidealgranularityforaPVinstallation?Herewe’retalkingabouttheaddressabilityofanumberofsystemfactorsincluding:
• Monitoring(enoughdatatomaximizeenergyproduction,butnottoomuchtoconsumeresourcesunnecessarily)
• O&Mstrategy(lowestcostformaximumenergyharvest)• MPPTharvest• Powerconversionblocksize(toleranceforshading,failures,
etc)• Arcdetection(safety)
Howmuchgranularityisenough?Aswithmanyquestionslikethis,theansweris“itdepends….”GranularityimpactsinitialinstallcostandO&Mdifferently.
• Fortheinstall,asanexample,thepowerconversionblocksizeisonefactorandthiswillimpactmaterialandlaborcostsetc.
• ForlaterO&Mitcomesdowntohowmuchoftheoutputofanarraycanunderperformbeforeitimpactsrevenue-andrevenueisimpactedbyenergyproductionandO&Mcosts.Forexample,ifonemoduleinaresidentialinstallof16modulesdies,a17%dropinproductionwillbenoticeableandresultinanurgentneedtofixit.Ifthatsamemoduleisoneof10,000atautilityscaleinstallthereislessurgencyoverfixinga0.01%hit.GranularityhasasignificanteffectontheimpactofPVmoduleissues.
• Asanotherexample,ifyouhaveper-stringMPPTandaplantwith100stringsof10moduleseach,amoduleissuewillimpactlessthan1%ofplantoutput.Thatsameplantwithper-modulepowerconversion(microinverterorpoweroptimizer),theimpactwillbe0.1%oftotalplantoutput.Sopowerconversioncomesintoplay.
Sowhynotjustmonitor/powerconverteverymodule?Aswe’llsee,forsmallinstallsthatcouldbetherightanswerbutthere’smoretoit.
What’stheIdealPVSystemGranularity? 15December2015
HiQSolar,Inc. Page2of5
NativeGranularitybyInverterTypeThechoiceofinverterhasabigimpactonmonitoringandMPPTgranularity,andisusuallydriveneitherbyafocusonthelowestfirstcostorlowestinstalledcost.Figure1givesanexampleofhowthenativegranularityvarieswithinvertertype,highlightedwiththeorangemodules.Granularitycanbeenhancedthroughtheadditionofmodulelevelpowerelectronicssuchasoptimizers,butatextracostandareductioninreliability.Also,theeventualfailureofelectronicsmountedunderthePVmoduleresultsinsignificantlyhigherrepaircost.Ingeneralthelowestinstalledcostformanycommercialprojectscomesfromthelargest3-phasestringinvertersbutthegranularityofbothmonitoringandMPPTgetscoarserasthepowerratingclimbs.
Figure1:Granularitybyinvertertype
Figure2:Competingfactorsaffectingthedegreeofgranularity
Central(with(zone(monitoring( Large(32P(string( TrueString( Micro(
Pushing(for(more(granularity(
Pushing(for(less(granularity(
Large(group(of(strings(
Module(level(
Several(strings(
String(level(
• Be8er(performance(visibility(
• Be8er(pinpoin<ng(for(troubleshoo<ng((
• Be8er(MPPT(granularity/(harvest(
• Visual(and(analy<c(data(overload(
• Customer(visibility(of(minor(issues(
• Cell(modem(data(bandwidth(
• Failure(of(components(under(modules(
• Lower(weight(on(rooCops(
• Cost(
What’stheIdealPVSystemGranularity? 15December2015
HiQSolar,Inc. Page3of5
CompetingFactorsThehigherthegranularity,thebettertheoutcome,right?Figure2showsthatwhilethisistrueuptoapoint,therearefactorspushingintheotherdirectionalso.Someoftheissuesinbothdirectionsare:
• Monitoringisexpensive,andmoredataismoreexpensive–hardwaretodoit,mechanismsforgatheringthedata,datastorage,timeandprocessingtoanalyzeit,etc.YouwanttheminimumthatallowsyoutoperformyourjobandoptimizePVplantenergygeneration.
• Notethattheminimumaccuracyforrevenuegrademetering(RGM)is+/-2%,althoughrelativemeasurementscomparingrelativeoutputsofstringsorinvertersdaytodayareusuallymoreusefulanddonotrequireirradianceandtemperaturedatatobemeaningful,assumingnoshading.And,revenuegrademeteringisusuallyonlyusedfortotalPVplantoutput,sothegranularityofthismeasurementisinsufficienttoidentifyproblemsatamoduleorstringlevel.
• Argumentsaboundonthetopicoffailures–isitbettertohavefewerpointsoffailurethataremoresignificantiftheydie,ormanymorebutwithanyonefailurehavinglessimpactonenergyproduction?Thisiswherethepowerconversionblocksizecomesintoplay–howmuchofthesystemneedstobereplacedwhensomethinggoeswrong,andhowbigajobisittotransportandfitareplacement?Onepieceoffeedbackfrominstallerswehearisthat,eitherway,leveringupindividualPVmodulesinanarraytoreplaceelectronicsistimeconsuming,expensiveandasafetyrisk.Ifyourmodule-levelmonitoringisprimarilyusedtolocatefailedmodulelevelelectronics,ittendstodefeatthepurpose(andinitialexpense).
• Weightonaroofcanbeanissue.Centralinvertersareheavy,butwhenaddedup,thetotalweightofanarrayofmicroinvertersofthesametotalkWisactuallyheavier,justmoredistributed.
• Inplantswhereoutputismeasuredandgovernedbyacontractguaranteeingtheenergyproduction(PPA),thegranularityshouldbesmallenoughsothatanissuecanbenoticedfarenoughinadvanceofacrisisthatmaintenancecanbescheduledratherthanbecominganemergency.Forexample,oftenmodulestaketimetodie,andsoilingcanbequantifiedsowashingofthearraycanbescheduledoptimally.
• Troubleshootingisvastlyeasierwhenitcanbepinpointedtoassmallanareaaspossible.Forlargerplantsthepositionofindividualmodulesisunlikelytohavebeenrecorded,butisolatingtheissuetostringlevelisaveryhelpfulandpracticalplacetostartfrom.Thisresultsinreducedmaintenancetimeandcost.
• WithlowMPPTgranularity(manystringsinparallel)ifonemodulefails,itcanaffecttheoutputofparallelstrings.And,evendiscoveringtheproblemcanbeachallenge,letalonefindingthespecificfailureandfixingit.
• Thereisabigleapintheamountofdatathatisgatheredandprocessedforeachstepmovingbetweenarray,group,stringandmodulelevel.Manyplantsarestartingtousecellularmodemsasthemostreliableformofbackhaulcommunication,buttheeconomicviabilitypartlydependsonkeepingmonthlydatausagelow.
• ThereisalsotheseparateissueaboutthegranularitythatthecompanyresponsibleforO&Mmightwishfor,andthe(usuallymuchlessdetailed)informationtheywishtheendcustomertoseetoavoidexcessivenuisancecallsaboutonePVmoduleoutputpowerbeinglowerthananeighboringmoduleoutput.
• Installedcostisdirectlyaffectedbygranularity.
What’stheIdealPVSystemGranularity? 15December2015
HiQSolar,Inc. Page4of5
ExampleCommercial300kWInstallationFora300kWinstall,300Wmodulesand16modulestrings,we’dhavethefollowing:
• ModuleLevel(microoroptimizersystem)–permodule,0.1%or0.3kWgranularityformonitoringandMPPT
• StringLevel(forexamplesmallstringinverter,HiQTrueString™)–62strings,1.6%or4.8kWgranularityformonitoringandMPPT
• ParalleledStrings(largestringinverter,canbe2-12stringsinparallel)–say21groupswith3stringsparalleledtogether,4.8%or15kWgranularity
• Largegroups(centralinverter,say300kW)–1MPPT,6monitoringzones–17%or50kWmonitoringgranularity
ImplementationsofthesetopologiesaredescribedfurtherinTable1. Power
ConversionBlockSize(Nom.)
MPPT Monitoring ArcDetection
300kWCentral 300kW(100%) 300kW(100%) 50kW(17%) AlmostimpossiblewithoutadditionalAFCIequipment
LargeString 27.6kW(9%) 15kW(5%) 15kW(5%) 15kW(5%)TrueString 8kW(2.7%) 4.8kW(1.6%) 4.8kW(1.6%) 4.8kW(1.6%)Micro/Optimizer
0.25kW(0.08%) 0.3kW(0.1%) 0.3kW(0.1%) Notrequiredfor<80V
Table1:Granularitiesfortheexample300kWsystem
Inthisexample,stringlevelorsub-stringlevelwithmodulesgroupedtogetherwouldmakethemostsenseasacompromisebetweenvisibility,modemchargesandclarity.Thecalculuswouldbeverydifferentfora5MWora5kWsystembuttheideasholdtrue.Thegranularitythatisacceptableinagiveninstallwillvary,startingpointsforeachoftheparametersareshowninTable2. Power
ConversionBlockSize
MPPT Monitoring ArcDetection
SuggestedRanges
1-10% 1-5% 0.5-2% Singlestringlevelorbetter
Table2:Suggestedidealgranularityrangesfordifferentplantparameters
PerspectivefromaCommercialInstallerAsoneexperiencedcommercialinstallerinCaliforniasaidtousrecently:“Ihavenouseformodulelevelmonitoring–ifIwasdoingmostlyresidentialitmightbedifferent.Anythingmodulelevel–Ihavenoneedforit,Idon’twantit…..It’stoomuchdata–Ihatehavingthatmuchdata,Ijustgetburiedinit,ittakestoomuchcellmodembandwidth,andit’sjusttoomuchvisualnoise;Icertainlydon’twantmycustomerslookingatit…..andreliabilitywillbeanissueeventually,too.Stringlevelisgreat.”
What’stheIdealPVSystemGranularity? 15December2015
HiQSolar,Inc. Page5of5
SummaryTherearemanyfactorsthataffecttheidealgranularityofaparticularsystem.Aswithallotheraspectsofsystemdesigntheanswerisacompromisebetweeninstallcost,maintainabilityandmanyotherfactorsaswe’veseen.Theidealgranularitycouldbedifferentforpowerconversionblocksize,MPPT,monitoringandarcdetection,andwe’vesuggestedsomestartingrangesinTable2.
AdvertisingObviouslywebelievethatsinglestringmonitoring,arcdetection,andMPPTforcommercialinstallsmakessomuchsensewedesignedtheHiQSolarTrueString™inverterfamilywiththisinmind.Itiscostcompetitivewithlargestringinverterswhileofferingmuchbettermonitoring,MPPTgranularity(increasedenergyharvest)andsafetyfunctionality.However,theprinciplesdiscussedhereapplybroadly.
Figure3:Individualstringperformancefromacommercialrooftopwitheast/weststrings
CopyrightHiQSolar,Inc.,2015www.hiqsolar.com
