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Nomenclature
FF
Fillfactor
T
Temperature,K
A
Area,m
Advancementsinthermoelectricgeneratorsforenhancedhybridphotovoltaicsystemperformance
SamsonShittua
GuiqiangLia,∗
YousefGolizadehAkhlaghia
XiaoliMaa
XudongZhaoa,∗∗
EmmanuelAyodeleb
aSchoolofEngineeringandComputerScience,UniversityofHull,HU67RX,UK
bInstituteofRobotics,AutonomousSystemsandSensing,UniversityofLeeds,LS29JT,UK
∗Correspondingauthor.
∗∗Correspondingauthor.
Abstract
Effectivethermalmanagementofphotovoltaiccellsisessentialforimprovingitsconversionefficiencyandincreasingitslifespan.Solarcelltemperatureandefficiencyhaveaninverserelationshiptherefore,coolingofsolarcellsisacriticalresearch
objectivewhichnumerousresearchershavepaidattentionto.Amongthewidelyadoptedthermalmanagementtechniquesistheuseofthermoelectricgeneratorstoenhancetheperformanceofphotovoltaics.Photovoltaiccellscanconverttheultra-violentand
visibleregionsofthesolarspectrumintoelectricalenergydirectlywhilethermoelectricmodulesutilizetheinfraredregiontogenerateelectricalenergy.Consequently,thecombinationofphotovoltaicandthermoelectricgeneratorswouldenabletheutilizationof
awidersolarspectrum.Inaddition,thecombinationofbothsystemshasthepotentialtoprovideenhancedperformanceduetothecompensatingeffectsofbothsystems.Thewasteheatproducedfromthephotovoltaiccanbeusedbythethermoelectric
generatortoproduceadditionalenergytherebyincreasingtheoverallpoweroutputandefficiencyofthehybridsystem.However,theintegrationofbothsystemsiscomplexbecauseoftheiropposingcharacteristicsthus,effectivecouplingofbothsystemsis
essential.Thisreviewpresentstheconceptsofphotovoltaicsandthermoelectricenergyconversion,researchfocusareasinthehybridsystems,applicationsofsuchsystems,discussionofthemostrecentresearchaccomplishmentsandrecommendationsfor
futureresearch.Alltheessentialelementsandresearchareasinhybridphotovoltaic/thermoelectricgeneratorarediscussedindetailedtherefore,thisreviewwouldserveasavaluablereferenceliterature.
Keywords:Hybridphotovoltaicsystem;Thermoelectricgenerator;Solarenergy;Thermalmanagement;Photovoltaic-thermoelectric
This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
C
Concentrationratio
G
Solarirradiance,W/m2
Voc
Opencircuitvoltage,V
Isc
Shortcircuitcurrent,A
Pin
Incidentpower,W
ZT
Figureofmerit
Greeksymbols
Seebeckcoefficient,V/K
Thermalconductivity,W/m/K
Electricalconductivity,S/m
Efficiency,%
Abbreviations
PV
Photovoltaic
TE
Thermoelectric
PV/TEG
Photovoltaic/thermoelectricgenerator
TPV/TEG
α
κ
σ
η
Thermophotovoltaic/thermoelectricgenerator
CdTE
Cadmiumtelluride
CIGS
Copperindiumgalliumselenide
DSSC
Dye-sensitizedsolarcell
GaAs
Galliumarsenide
GaSb
Galliumantimonide
CoSb3
Copperantimony
InP
Indiumphosphide
Bi2Te3
Bismuthtelluride
PbTe
Leadtelluride
GeTe
Germaniumtelluride
SnTe
Tintelluride
I-V
Current-Voltage
TEG
Thermoelectricgenerator
a-Si
Amorphoussilicon
c-Si
Crystallinesilicon
CZTS
Copperzinctinsulfide
PCM
Phasechangematerial
SSA
Solarselectiveabsorber
PSC
Perovskitesolarcell
EEG
Electroencephalography
AM
AirMass
LOM
Lock-Onmechanism
Subscripts
H
Hotside
C
Coldside
1IntroductionAconsequenceofindustrializationandtheexponentialrateatwhichtheworld'spopulationisgrowingistheunprecedentedincreaseinenergydemand.Currently,fossilfuelsupplies80%oftheworld'senergyduetothehighinitialcostofrenewableenergy
systems.However,conventionalenergysourceslikefossilfuelarelimitedenergysourcesthatcauseseriousenvironmentalissueswhichaffecttheclimateandhealthofthepeople.Eventually,theglobalenergydemandwouldoutgrowtheavailableenergysupplyfrom
conventionalsources[1,2].Itisthereforeimperativetoconsiderrenewableenergysourcesbecauseoftheiruniqueadvantagessuchas;inexhaustibilityandlowpollution.Solarenergyisthemostabundantrenewableenergyeasilyaccessibleglobally.Theglobalenergy
demandcanbemetbysolarenergyduetoitsvastenergycapacity.Infact,thetotalsolarradiationimpingedontheenergysurfaceismorethan7500timestheworld'stotalannualprimaryenergyconsumptionof450 EJ[3].
Solarphotovoltaicsisoneofthetwomajorsolarenergytechnologiesincluding,solarthermal(Fig.1).Photovoltaic(PV)cellsconvertsolarradiationintoelectricitydirectlyhowever,onlyabout10–15%oftheabsorbedsolarradiationisconvertedintoelectricity
whiletheremainderiseitherreflectedtotheambientenvironment(heatloss)orabsorbedasheatthus,increasingtheoperatingtemperatureofthePVcellanddecreasingitsconversionefficiency[4].Althoughphotovoltaicsystemshavebeencommerciallyavailablefor
severalyears,someofthebarrierstotheirwidespreadapplicationare:elevatedtemperatureinthePV,limitedconversionefficiencyanddustaccumulation.Consequently,severalcoolingtechniquesforPVsystemshavebeenproposedandamongthem,airandliquid
basedcoolingofPVsystemsarethemostmaturetechnologieswithpracticalapplicationsworldwide[5].Asidesthesetwomaturecoolingtechniques,thermoelectric,heatpipeandnanofluidcoolingareviablealternativeswhichhavebeeninvestigated.Thesignificanceof
photovoltaiccoolingcannotbeoveremphasisedasitgreatlyaffectstheperformanceofthesystem.Dependingonthecellmaterialused,thePVefficiencydecreasesbyarangeof0.25%–0.5%perdegreeCelsius[6].ThismeansthateventheslightlydecreaseinPV
temperaturecansignificantlyincreaseitsefficiencytherefore,coolingtechniquesareveryessentialtoPVsystems.AneffectivePVwasteheatextractioncanenhancetheconversionefficiencyofthePVandprovideadditionalenergy(thermalorelectrical)simultaneously.
Thermoelectric(TE)devicesarebi-directionalenergyconverterscapableofoperatingasageneratororcooler.Dependingontheoperatingmode,theTEdevicecanconvertelectricalenergytothermalenergyandviceversa.Theadvantagesofthermoelectric
energyconvertersare;solid-stateoperation,gas-freeemission,maintenancefreeoperation,vastscalability,zeropollutionand long-timeoperational reliability [7,8].However, lowconversionefficiencyhas limited thewidespreadapplicationof thermoelectricdevices.
Geometryoptimizationandmaterialoptimizationarethetwomajorsolutionsbeingresearchedfortheenhancementofthermoelectricconversionefficiency[9,10].Inaddition,theincorporationofTEdevicesintophotovoltaicmoduleswouldresultinahybriddevicewith
enhancedoverallperformance.Thisisbecausethetwodevices(PVandTE)havecomplimentarycharacteristicsthus,theadvantageoftheTEcanbeusedtocompensateforthedisadvantageofthePV.Thethermoelectricdevicecanprovidedualfunctionofcoolingthe
PVandproducingadditionalenergy.
Inthisstudy, themostsignificantadvancementsmadeintheefficient thermalmanagementofPVsystemsusingthermoelectricgeneratorsarediscussed.Theaimof thestudyistoprovideaconcisereviewof theroleof thermoelectricgenerators inhybrid
photovoltaicperformanceenhancement.Aplethoraofliteratureexistsdescribingthetwoindividualtechnologiesseparately(PVandTE)andthefieldofhybridPhotovoltaic/Thermoelectric(PV/TE)isgrowingrapidlyespecially,theintegrationofthermoelectricgenerators
intoPhotovoltaic(PV/TEG).Recently,abookonhybridPV/TEGwaswrittenbyNarduccietal.[11]whichexplainsthefundamentalsofsolarharvestingusingphotovoltaicandthermoelectricgenerators.Thisstudyontheotherhand,offersamorecondensedreviewofthe
mainconceptsandunderlingprinciplesofhybridPV/TEG.Theobjectiveofthisstudyistoprovideadetailedoverviewofthecurrentstateofartinhybridphotovoltaic-thermoelectricgeneration.Inparticular,themainresearchfocusareasinhybridPV/TEGwillbeexplored
therebyprovidingvaluableinformationonthemajorissuesbeingtackledinthehybridsystemresearch.Furthermore,themostrecentworkspublishedonhybridPV/TEGareanalysedindetailandthesignificanceofeachstudyisexplored.Inaddition,thisreviewpresents
nicheapplicationsofhybridPV/TEG.Finally,recommendationsforfutureresearcharepresentedtoprovideaguideforinterestedresearchersonhybridPV/TEG.Itisenvisagedthatreadersinterestedinaquickandfundamentalunderstandingofhybridphotovoltaic-
thermoelectricgeneratorswould find this reviewveryusefulwhile theaforementionedbook [11] is recommended foramore in-depthunderstandingofall issues relating tohybridPV/TEG.Consequently, this reviewalongwith thebookwill serveasessential and
indispensablereferenceliteraturesonhybridPV/TEG.
2PhotovoltaicsystemsThephotovoltaiceffectwasfirstdiscoveredbyFrenchphysicist,EdmondBecquerelin1839.However,thefirstsiliconsolarcellwithap-njunctionwasdevelopedin1954byagroupofresearchersledbyChapinD.MattheBelltelephonelaboratories[12].A
timelineoftheprogressonphotovoltaicsolarenergyconversionisshowninTable1[13].Photovoltaicsystemscanbegroupedintothreegenerationsandtheyare[14]:
1) Firstgenerationsystems:Thesearefullycommercialsystemsbasedoncrystallinesilicontechnology.Theyincludemonocrystallineandpolycrystallinesiliconcells.
2) Secondgenerationsystems:Thesearebasedonthephotovoltaicthinfilmandtheyinclude,amorphoussilicon,cadmiumtellurideandindiumcopperselenide,indiumandgallium-diselenide.
3) Thirdgenerationsystems:Theseincludeorganicphotovoltaiccellsthatarestillindevelopmentalphaseandusedinnicheapplications.Inaddition,Dye-sensitizedsolarcellandIII-Vcompound(e.g.GaAs,GaSbandInP)arepartofthisgenerationofphotovoltaicsystems.
Table1Relevantdatestophotovoltaicsolarenergyconversion[13].
alt-text:Table1
Scientist Innovation Year
Fig.1Solarenergyconversiontechnologies.
alt-text:Fig.1
EdmondBecquerel Discoveredthephotovoltaiceffect 1839
WilliamAdamsandRichardDay Noticedthephotovoltaiceffectinselenium 1876
MaxPlanck Predictedthequantumnatureoflight 1900
AlanWilson Proposedthequantumtheoryofsolids 1930
NevillMottandWalterSchottky Developedthetheoryofsolid-staterectifier(diode) 1940
JohnBardeen,WalterBrattainandWilliamShockley Inventedthetransistor 1949
DarylChapin,CalvinFullerandGeraldPearson Developeda6%efficientsiliconsolarcell 1954
D.CReynold,G.Leies,L.L.AntesandR.E.Marburger Developedsolarcellbasedoncadmiumsulphide 1954
SolarcellswereusedforthefirsttimeonanorbitingsatelliteVanguard1 1958
Aphotovoltaiccellismadeupofp-typeandn-typesemiconductorsthatabsorbincomingphotonsandconvertthemintoelectron-holepairs.Basically,electronsarepromotedfromthevalencebandtoconductionbandwhentheabsorbedenergyisequaltoor
greaterthanthebandgapenergy.Thisprocessgenerateselectron-holepairswhichdiffuseandseparatesatthep-njunctionofthesemiconductorsduetogeneratedelectricfield.Subsequently,electronsareattractedtothenegativesidewhiletheholesmovetothe
positiveside.Finally,theelectronsflowintheexternalcircuitandcurrentisgeneratedasshowninFig.2a[15].SomeofthebestreportedmeasuredefficiencyfordifferentsolarcellmaterialsareshowninTable2[16].Itcanbeseenthatmonocrystallinesiliconcellstillhas
thebestconversionefficiencyhowever,PVmaterialoptimizationresearchisstillon-goingandbetterefficiencyvaluescouldbeachievedinthefuture.
Table2ConfirmedcellefficiencymeasuredundertheglobalAirMass(AM)1.5,spectrum(1000 W/m2)at25 °C[16].alt-text:Table2
Celltype Efficiency(%) Description Reference
Silicon,(monocrystallinecell) 26.7 ± 0.5 Kaneka,n-typerearIBC [133]
Silicon,(polycrystallinecell) 22.3 ± 0.4′ FhG-ISE,n-type [134]
III-V,GaAs 28.8 ± 0.9 Altadevices [135]
III-V,InP 24.2 ± 0.5 NREL [136]
Thinfilm,CIGS 22.9 ± 0.5 Solarfrontier [137,138]
Thinfilm,CdTe 21.0 ± 0.4 Firstsolar,onglass [139]
Amorphoussilicon 10.2 ± 0.3 AIST [140]
Perovskite 20.9 ± 0.7 KRICT [141]
Dyesensitized 11.9 ± 0.4 Sharp [142]
Fig.2Photovoltaiccell(a)p-njunctionstructure[15]and(b)simplifiedequivalentcircuit[13].
alt-text:Fig.2
Organic 11.2 ± 0.3 Toshiba [143]
2.1ModellingofPVcellAphotovoltaiccellcanbemodelledasacurrentsourcewithaparalleldiodeasshowninFig.2b.ThediodecurrentcanbeobtainedfromtheShockleyequationas[13]:
Reversesaturationcurrentisobtainedas,
where isthediodediffusionfactor, isabsolutetemperature, iselectroncharge, ismaterialbandgapenergy, isBoltzmannconstantand iscrosssectionalarea.
Dependingonrequiredvoltageandcurrentlevels,solarcellsareconnectedinseriesandparallelrespectively.Thesolarcellgeneratorvoltageandcurrentcanbeobtainedas,
where istheseriesresistance, isnumberofcellsinseries, isnumberofcellsinparalleland isthecellphotocurrentproportionaltosolarirradiance.
where isthecelltemperature.
ThePVcellshortcircuitcurrent canbeobtainedbysetting and .ThisvaluevarieswithcellirradianceandthePVcellopencircuitvoltage canbeobtainedbysetting thus,
ThemaximumoutputpowerofthePVisexpressedas,
Thefillfactor(FF)canbeexpressedas,
TheefficiencyofthePVcanbeexpressedas,
where istheincidentpoweronthePVcell.
2.2InfluenceoftemperatureonphotovoltaiccellsMajorityoftheresearchonPVsystemhasbeenonefficiencyenhancementbyapplicationofeffectivethermalmanagementtechniques.TheconversionefficiencyofthePVislargelydependentonthesolarcelltemperaturetherefore,coolingofthePVisof
utmostimportance.ItisobviousfromFig.3thatthesolarcelltemperatureaffectsthecellefficiency,opencircuitvoltageandshortcircuitcurrent[17].Inaddition,Fig.4showstheinfluenceofcelltemperatureonthecurrent-voltage(I-V)characteristicsofthePVcell[18].
(1)
(2)
D Tab q εG k A
(3)
Rs Ns Np Iph
(4)
T
(Isc) Vg=0 Isc=Iph (Voc) Ig=0
(5)
(6)
(7)
(8)
(9)
Pin
Generally,thePVperformsbetteratlowercelltemperaturevalues.ThetemperaturedependenceofPV'sefficiencyisoftencharacterizedbyapropertyknownasTemperaturecoefficient.ItisusedinquantifyingthetemperaturesensitivitiesofthePVcellperformance.To
comparedifferentPVcells,thetemperaturecoefficientareusuallygivenatanormalizedvalueof25 °Cor298.15 K[19].
ThePVelectricalefficiencycanbe increasedby removing theaccumulatedheat from theconcealedPVsurfaceandusing thisheatappropriately [20].Different technologiessuchasPhotovoltaic/Thermal (PV/T)andPhotovoltaic/ThermoelectricGenerator
(PV/TEG)havebeendevelopedforthispurpose.However,thePV/TEGcanonlyfulfilthispurposeiftheTEGisinphysicalcontactwiththePV(i.e.directcouplingmethod).Nevertheless,theTEGwillhavetooperateatatemperaturehigherthantheambienttemperature
toproducesomeelectricalpoweranditmostlikelywillheatupthesolarcellifnotproperlycooled.IftheTEGisnotinphysicalcontactwiththePV(i.e.spectrumsplittingmethod),itcannotcooldownthePVcell.
3ThermoelectricdevicesThermoelectricdevicescangenerateelectricalpowerfromthermalenergyviatheSeebeckeffectasshowninFig.5a.Whenatemperaturegradient isappliedtoathermoelectriccouplemadeupofn-typeandp-typesemiconductormaterials,themobile
chargecarrierslocatedatthehotend(heatsource)diffusetothecoldend(heatsink)thus,anelectrostaticpotential isproduced.ThisprocessisknownastheSeebeckeffectwhichwasdiscoveredin1821byThomasSeebeck.TheSeebeckcoefficientisanintrinsic
thermoelectricmaterialpropertywhichisexpressedas,
Fig.3Effectofcelltemperatureonefficiency,opencircuitvoltageandshortcircuitcurrentofamonocrystallinesiliconcell[17].
alt-text:Fig.3
Fig.4InfluenceoftemperatureontheI-Vcharacteristicsofaphotovoltaiccell[18].
alt-text:Fig.4
(ΔT)
(ΔV)
(10)
Onthecontrary,theapplicationofacurrentonathermoelectriccouplecausesthechargecarrierstoattempttoreturntotheelectronequilibriumthatexistedbeforethecurrentwasappliedbyabsorbingorreleasingenergyattheinterfaceoftwodissimilar
materials.ThisprocessisknownasthePeltiereffectwhichwasdiscoveredin1834byJean-CharlesPeltieranditisshowninFig.5b[21].
TheThomsoneffectwhichwasdiscoveredin1852byWilliamThomsonisthelastofthethermoelectriceffectsanditisrelatedtotherateofreversibleheat generation.Thereversibleheatresultsfromcurrentpassingthroughaportionofasingleconductor
alongwhichthereisatemperaturedifference.AlthoughThomsoneffectisnotofprimaryimportanceinthermoelectricdevices,itisstillessentialfordetailedcalculationsasitinfluencesthedeviceperformance[22].
3.1ThermoelectricmaterialsThequalityofthermoelectricmaterialsusedforgeneratingelectricpowerviatheSeebeckeffectorcooling(refrigeration)viathePeltiereffectismainlydeterminedbythreeintrinsicmaterialpropertiesincluding,electricalconductivity,seebeckcoefficientand
thermalconductivity.Materialswithhighelectricalconductivityarefavourablebecauseelectricalcurrentispassedinboththepowergenerationandcoolingmode.Inaddition,alargeseebeckcoefficientisessentialbecausealargegeneratedvoltageperunittemperature
gradient is desired. Lastly, a low thermal conductivity is essential forTEmaterialsbecause temperaturedifferencemustbemaintainedacross thematerial [23].Adimensionlessparameter knownas thermoelectric figureofmerit (ZT) isusuallyused toobtain the
thermoelectricefficiencyanditisexpressedas[24],
where istheSeebeckcoefficient, istheelectricalconductivity, isthethermalconductivityand istheabsolutetemperature.
Generally,materialswithhighZTarepreferablehowever,optimizingall the intrinsicmaterialproperties that influence theZT togetheratonce isverydifficultbecause theyare interdependentand reciprocal.Thischallengecaused themaximumZTofany
thermoelectricmaterial to remainat for almost fifty years [23].However, due to the extensivematerial research been undertaken, there has been reported progress in overcoming the limitations and increasing the thermoelectric figure ofmerit greatly. Two
approacheshavebeeninvestigatedincluding,modifyingthematerialmicrostructuretoincreasephotonscatteringthus,decreasingthethermalconductivity.Materialssuchasskutterudites,clathratesandchalcogenideshavebeenoptimizedusingthisapproach.Theother
approachistoreducethematerialdimensionalityinorderforquantumsize-effectstoaltertherationbetweentheelectricalandthermalconductivity[25].Inaddition,effortshavebeenmadetoincreasetheZTofmaterialsbyaddingothersemiconductorornanostructure
materials.Itwasfoundthatthefigureofmeritfornanostructurematerialsis3 at300 °Canditvariesfrom0.4to1.1 atlowtemperaturedifferencevaluelike27 °C[26,27].ThisisamuchhighervaluethanthetypicalZTvalue(0.8)ofcommercialmaterialsliken-typeBi2Te3andp-typeSb2Te3attemperaturesbelow150 °C[24].Super-latticestructure,plasmatreatmentandmaterialsegmentationaretheothermethodsusedindevelopingthermoelectricmaterialswithhighefficiency[28].
Classifyingthermoelectricmaterialsbasedonoperatingtemperaturerange,Bismuthtelluride(Bi2Te3)isusedforlowtemperature(<500 K)powergeneration.Materialsbasedongroup-IVtelluridessuchasPbTe,GeTeandSnTeareusedformid-temperature
(500–900 K)powergeneration.Lastly,silicon-germaniumalloysareusedforhightemperature(>900 K)powergeneration[29].Forthermoelectricdevicestogainwiderapplication,materialswithhighZTandlowpricemustbedeveloped,andthisisanachievablefuture
goalduetotheextensiveresearchbeingcarriedoutinthisarea.
Fig.5Schematicofathermoelectric(a)generatorand(b)cooler[21].
alt-text:Fig.5
q
(11)
α σ κ T
ZT≈1
3.2ModellingofthermoelectricgeneratorandcoolerTheefficiencyofathermoelectricgeneratorisexpressedas[22],
Assumingconstantthermoelectricmaterialpropertiesandnegligiblecontactresistances,theefficiencycanbeexpressedas,
where iscurrent, isseriesresistance, ishotsidetemperatureand iscoldsidetemperature.
Themaximumconversionefficiencyisgivenas,
where istheCarnotefficiencyanditisexpressedas,
Theenergyefficiencyofthethermoelectriccoolerismeasuredintermsofitscoefficientofperformance(COP)anditisexpressedas[22],
Thecurrent formaximumcoolingpowerisexpressedas,
Themaximumcoefficientofperformanceisgivenas,
Asinthecaseofthethermoelectricgenerator,thefigureofmerit(ZT)alsodeterminesthemaximumcoefficientofperformancethatcanbeachieved.
3.3ApplicationsofthermoelectricgeneratorandcoolerThermoelectricgeneratorshaveawiderangeofapplicationssuchasinwasteheatrecoveryforautomobiles[30–33],wearablesensors[34–37],micropowergeneration[38],wirelesssensornetwork[39],spacepower[40]andbuildings[7].Thermoelectriccoolers
areusedincoolingelectronicdevices[41],refrigeratorsandairconditioners[29]andforspecificapplicationsinmilitary,aerospace,instrument,biology,medicineandindustrialproducts[41].Detailedexplanationoftheapplicationofthethermoelectricgeneratorandcooler
intheaforementionedsectorscanbefoundinthereferencedliteratures.Forthesakeofthisreview,morefocusisplacedontheapplicationofhybridPV/TEGinthelatersections.
4Hybridphotovoltaic/thermoelectricgenerator(PV/TEG)Integrating thermoelectricdevices intophotovoltaic systemscanenable theefficient thermalmanagementofPV thus,enhancing itsoverall performance.When thermoelectricgeneratorsarecombinedwithPV,dependingon the integrationmethodof the
PV/TEG,theTEGcanutilizethewasteheatfromthePVtogeneratesomeelectricalenergyifitisproperlycooledandthereissufficienttemperaturedifferenceacrossit.Inaddition,theoverallhybridsystemperformancecouldpotentiallybeenhancedbytheintegrationof
thermoelectricgeneratorsintoPVifthesystemisproperlydesignedalthoughtheisapossibilityofreducedperformanceduetothecomplexrelationshipbetweenPVandTEG.
Similarly,thermoelectriccoolerscanbeusedtoremovethewasteheatfromthePVandenhancetheoverallhybridsystemperformance.ResearchonhybridPV/TEGdatesbackto1988whenMooreandPeterson[42]designedahybridsystemwiththeintention
(12)
(13)
I R TH TC
(14)
ηc
(15)
(16)
(17)
(18)
ofusingthePVtosupplypowerduringhighsolarirradianceperiodswhileusingtheTEGtoprovidepowerandheatduringperiodsoflowsolarirradiance.TheauthorsexaminedtwoexistingsitesinNorthernCanada,andtheyconcludedthatatlocationswheredelivered
fuelcostswassignificantandconventionalPVwasn'tavailable,thehybridPV/TEGcouldprovideunmatchedreliabilityandeconomics.
4.1ModellingofhybridPV/TEGGenerally,theoverallefficiencyofthehybridPV/TEGisasumoftheindividualefficienciesofthePV( )andTEG( )anditcanbeexpressedas[11]:
where isthepoweroutputofthesolarcell, istheTEGpoweroutput, isthesolarcellareaand istheinputsolarpower.
TheaboveEq.(19)isapplicableforasimplifieddesignofahybridPV/TEGwherethePVandTEGarethermallycoupledbutelectricallyseparated.
ConsideringtheefficienciesofallthemaincomponentsofthePV/TEGincluding,opticalcollector,opto-thermalconverter,thermalcollector,thermoelectricconvertandthermaldissipater,thehybridsystemefficiencycanalsobedefinedas[11]:
where istheopticalcollectorefficiency, istheopto-thermalefficiencyand isthethermaldissipaterefficiency.
Theopticalcollectorefficiencyisgivenas:
where is theoptical concentration, is the optical collector aperture area and is either the optical collector transmittance or reflectivity, depending on the optical component used (i.e. lens ormirror).Normally, is expected to be ≥ 0.9 therefore, the
opticalcollectorcanbeassumedtonotabsorbpowerthus,itdoesnotheatup.Consequently, canbeconsideredastemperatureindependent.
Thethermaldissipaterefficiencyisgivenas:
where is theelectricalpowerneededtocirculate thecooling fluidand is theelectricalpoweroutputof thesolarTEG.Whenpassivedissipation isconsidered(i.e. , whilewhenactivedissipation is considered, thedetails of theheat
dissipatergeometryhastobeconsidered.
Theopto-thermalefficiencyofthehybridsystemisgivenas:
wheretheassumption is that theTEGhotsidetemperature isequal tothesolarcell temperature(i.e. ).Thisassumption isonlyvalid for thecaseofdirectcouplingmethod.For thespectrumsplittingmethod, it isassumedthat thesolarcell temperature is
equaltothetemperatureoftheencapsulation(i.e. ).Therefore,itisimportanttounderstandthattheopto-thermalefficiencyisdifferentforthespectrumsplittinganddirectcouplingmethods.Infact,theopto-thermalefficiencyissmallerforthecaseofspectrum
splittingmethodcomparedtodirectcouplingmethod[43].Thereasonforthisbehaviouristhatinsolarcells,themajorityofthethermallossesisactuallyintherangeofenergieshigherthantheenergygap(notintheinfraredregion),forwhichthespectrumsplitting
methodcannotactasarecoverystrategy[44].Furthermore, isopto-thermalconvertabsorbance, istransmittanceofthepossibleencapsulation, istheheatmirrortransmittance, encompassestheheatreflectionpropertiesoftheheatmirror, is
opto-thermalconverterarea, isambienttemperatureand isthereflectedfractionofsolarpowerbythesolarcell.
TheresultantemittanceoftheTEGparallelsurfacescanbedefinedas[45]:
where isthethermalcollectoremittanceand isemittance.
ηpv ηteg
(19)
Ppv Pteg Apv Pin
(20)
ηopt ηot ηdiss
(21)
C Aopt τopt τopt
ηopt
(22)
Pdiss ηdiss=1
(23)
Th=Tpv
Tpv=Tenc
αotconv τenc τhm ε′pv Aabs
Ta Rpv
(24)
εthcol εc
Whenheatmirrorsareused,thesolarcellemittance isreplacebyaneffectiveemissivitywhichisdefinedas[11]:
where istheback-reflectingefficiencyfortheheatcomingfromthesolarcellanditisdefinedas:
where isthereflectanceoftheheatmirrorandthesubscript‘ir’representstheevaluationoftheintegraloverarangeofwavelengthsfrom2500to30,000 nm.
FromEq.(20),theefficienciesofsomeofthehybridsystemcomponents(i.e. , and )aretemperaturedependentandtheirvariationwithtemperatureisshowninFig.6[11].ItisobviousfromFig.6thatthePVandTEGhaveanoppositetendencywith
temperaturevariation.Asexpected,theefficiencyofthePV( )decreasesasthetemperatureincreaseswhiletheefficiencyoftheTEG( )increasesasthetemperatureincreases.ThisisbecausethePVismoreefficientatlowertemperaturevalueswhiletheTEG
operatesbetterwhenalargetemperaturedifferenceispresentacrossitshotandcoldsideterminals.ThisopposingtrendshowthecomplexrelationshipbetweenaPVandaTEG.Inaddition,alltheheatproducedinthePVcannotbeconvertedbytheTEGbecausethere
isusuallyheatlosstotheambientfromthePVduetoconvectionandradiation.Furthermore,whenthesurfaceareaoftheTEGissmallerthanthatofthePVitisattachedtodirectly,alargequantityoftheheatproducedbythePVmaybedissipateddirectlytotheambient
withoutpassingthroughtheTEG.
AsidesthethermalcouplingofhybridPV/TEGwhichEq.(19)describes,thePVandTEGcanalsobeelectricallyconnectedtoformahybridsystem.Inthethermalcoupledsystem,thePVandTEGgenerateelectricalpowerindependentlywhileintheelectrically
coupledsystem,theTEGcanbeconnectedinseriesorparallelwiththePVasshowninFig.7[11].Theoverallefficiencyofthehybridsystemwhenitiselectricallycoupledisdifferentfromthatofthethermalcouplingtherefore,itisgivenas[11]:
where isthesolarTEGefficiency, istheelectricalhybridizationefficiencyand istheelectricalpowerloss.
εpv
(25)
(26)
Rhm(λ)
ηpv ηteg ηot
ηpv ηteg
Fig.6Variationofhybridsystemandindividualcomponentsefficiencywithtemperature[11].
alt-text:Fig.6
(27)
ηsteg ηel Pel−loss
4.2HybridsystemintegrationmethodsThecombinationofphotovoltaicand thermoelectricallows for thewideruseof thesolarspectrum.This isbecausePVconverts theultra-violentandvisible regions (200–800 nm)of thesolarspectrum intoelectricitywhileTEGconverts the infrared region
(800–3000 nm)intoelectricity[46].ThetwomainPV/TEGintegrationmethodsarethespectrumsplittinganddirectcouplingintegration.Thedifferenceisthepresenceorabsenceofareflectivecomponent(e.g.spectrum-splitterorprism).Determiningthebestintegration
methodforthehybridsystembycomparingthetwointegrationmethods(spectrumsplittinganddirectcoupling)isnotstraightforward.Thisisbecause,whenconversionefficiencyisusedastheonlycomparisonparameter,thedirectcouplinghybridsystemcanperform
betterthanthespectrumsplittinghybridsystem[43].However,thespectrumsplittinghybridsystemhasanadvantageoverthedirectcouplinghybridsystembecauseitrequiresasmallerquantityofactivethermoelectricmaterialperunitareaduetothesmallerhybrid
system fill factors atmaximumefficiency. In addition, the spectrum splitting system requires a smaller areawhichmust be covered by cooling devices thus, the costs of the spectrum splitting system should be lower than that of the direct coupling system [47].
Nevertheless, the largersizeof thesystemalongwith theadditionalcostof thesplittingdevicemight result inabalancebetweentheprosandconsof thespectrumsplittinganddirectcoupling integrationmethods [43].Consequently, the finaldecisionon thebest
integrationmethodforthehybridsystemcanonlybereacheduponcompletionofadetailedcomparisonbetweentheprosandcons(includingacostevaluation)ofthetwointegrationmethods[47].Integratingthermoelectricgeneratorsintosolarpanelscouldprovidean
additionalenergyof2–10%dependingonthethermoelectricmaterial,connectionandconfiguration[48].Therefore,researchonPV/TEGisincreasingexpeditiouslyduetoitshugepotentialtoprovideenhancedperformancecomparedtostandalonePVorTEGsystems.
4.2.1SpectrumsplittingmethodBasically,inthespectrumsplittingsystem,thesolarradiationisreflectedbyasplitterataspecificwavelength(cut-offwavelength)andthisseparatestheradiationusedbythePVandTEGforenergyconversionasshowninFig.8.ThePVandTEGareusuallyplacedperpendicularly
whenthespectrumsplittingintegrationmethodisusedandtheradiationthatislongerthanthecut-offwavelengthisreflectedbytheTEGwhilethoseshorterthanthecut-offwavelengthtransmitthroughthespectrumsplitterandareabsorbedbythePV[15].Itisimportanttonotethatwhenthis
integrationmethodisused,thePVandTEGworkindependentlyonconvertingsolarenergyintoelectricitythus,theTEGdoesn'tcooldownthePVorusethePV'swasteheatforenergyconversion.
Kraemeretal.[49]presentedageneraloptimizationmethodologyforahybridPV/TEGsystemusingthespectrumsplittingmethod.ThreedifferentPVtypeswerestudiedexperimentally,anditwasfoundthattheamorphoussiliconcellprovidedthebesthybridsystemefficiencyof
Fig.7EquivalentcircuitofanelectricallyconnectedPV/TEG[11].
alt-text:Fig.7
Fig.8SchematicofspectrumsplittingPV/TEGintegration[46].
alt-text:Fig.8
13.26%whenaTEGwithefficiencyof8%wasused.Inaddition,theauthorsarguedthatthehybridsystemmaximumefficiencywashighlydependentonboththesolarcellspectralefficiencyandthesolarTEGefficiency.Furthermore,theauthorsarguedthattheshortwavelengthregiondirected
tothesolarTEGisusuallyonlyasmallportionofthetotalsolarspectrumthus,itcouldbeneglected.Regardless,itisworthnotingthattheauthorsusedanassumedefficiencyvaluefortheTEGwhichmaynotbepractical.
AcomprehensivestudyofaspectrumsplittingconcentratedPV/TEGsystemwasperformedbyJuetal.[50].Theinfluenceofcut-offwavelength,concentrationratioandheattransfercoefficientontheperformanceofthehybridsystemwerestudiedandoptimizationofthehybridsystem
wasperformed.Theinvestigatedsystemconsistedofasolarconcentrator,PVcell,TEG,spectralbeamsplitterandwater-coolingsystem.Twosignificantwaystoimprovetheefficiencyofhybridsystemswereproposedbytheauthorsincluding,increasingtheconcentrationratioandimprovingthe
coolingsystemperformance.Inaddition,theyfoundthattheTEGcontributedabout10%ofthetotalhybridsystempowerandtheoptimizedhybridsystemefficiencywasabout27.49%.Furthermore,itwasfoundthattheconcentrationratiohasanalmostlinearrelationshipwiththecoolingsystem
heattransfercoefficient.Also,theauthorsconcludedthatthesolarcellbandgapessentiallydeterminedtheoptimizedcut-offwavelengthofthehybridsystem.Finally,acomparisonofthehybridsystemwiththeconventionalPVsystemwasmadeanditwasfoundthatthehybridsystemisbetter
suitedforhighconcentrationconditionsduetoitsenhancedperformance.Althoughtheresultsfromthisresearcharesignificant,anexperimentalvalidationofthesimulationresultswasn'tperformed.
Furthermore,theoptimumdesignforaconcentratedspectrumsplittingPV/TEGwasproposedbyYinetal.[51]tooptimizethedistributionofsolarenergyinaspectrumsplittingCPV/TEGwithoutcompromisingtheoptimumdesignstateoftheindividualsystems.Theoptimumhybrid
systemoperatingtemperatureandcutoffwavelengthwerepresented.Inaddition,theeffectofthermoelectricfigureofmeritandcoolingsystemconvectiveheattransfercoefficientwerediscussed.Theauthorsarguedthatthethermoelectricstructurefactorinfluencestheoptimumtemperature
distributionintheTEG.Itwasalsofoundthatthespectralsplitteroptimumcut-offwavelengthandthermoelectricfigureofmerithaveaninverserelationship.Yangetal.[52]studiedtheperformanceofaspectrumsplittingPV/TEGsystemusingnumericalsimulation.Optimizationofthehybrid
systemcut-offenergy,cellvoltageandTEGdimensionlesscurrentwasdoneandtheinfluenceofthearearatioofthecollectortoPVonthehybridsystemperformancewasstudied.Itwasfoundthattheefficiencyofthehybridsystemincreasedby2.67%and2.19%comparedtothatofthePV
onlysystematconcentrationfactorsof30and100respectively.
Bjørketal.[53]studiedthemaximumtheoreticalperformanceofaPV/TEGsystemwithoutconcentration.Theauthorsusedananalyticalmodeltostudytheperformanceofthesystemandfoundthatthehybridsystemusingspectrumsplittingcouldachieveamaximumefficiency
increaseof1.8%pointcompared to thePVonlysystem.Furthermore,Liangetal. [54]performedanexperimentalandnumerical investigationon theperformanceofaspectrumsplittingconcentratedhybridPV/TEGsystem.Tooptimize theperformanceof the thermoelectricgenerator in
convertingthehigherwavelength(infrared)intoelectricity,acascadedthermoelectricgeneratorconfigurationwasutilizedbytheauthors.ThesystemconsistedoftwoindividualTEGstagesinwhichmiddletemperaturethermoelectricmaterial(CoSb3)andlowtemperaturethermoelectricmaterial
(Bi2Te3)wereusedforthetwostages.Thenumericalstudyinvolvedtheoptimizationofthethermoelectricgeometryandopticalconcentrationratio.Resultsshowedthatthedirectnormalirradiation(DNI),opticalconcentrationratioandheightratioofthetwoTEGstagescouldsignificantlyaffect
theperformanceofthehybridPV/TEGsystem.Inaddition,resultsshowedthattheTEGsubsystem,PVsubsystemandoverallhybridsystemefficiencieswere8%,44%and35%respectivelywhenDNI = 1000 W/m2,opticalconcentrationratiowas1000andoptimizedheightratioofthetwostage
TEGwas0.6.TheperformancedataforsomeofthespectrumsplittingPV/TEGsystemsreviewedcanbefoundinTable4.
Table3Poweroutputandefficiencyenhancementofhybridsystemusingdifferentcoolingsystems[103].
alt-text:Table3
Naturalcooling Forcedcooling Watercooling SiO2/waternanofluidcooling Fe3O4/waternanofluidcooling
Totalpowerincrease(%) Base 4.885 5.776 8.26 6.284
Totalefficiencyincrease(%) Base 1.865 3.051 3.355 3.131
Table4SummaryofsomeselectedspectrumsplittingPV/TEGsystemsreviewed.
alt-text:Table4
Reference Material Studytype Efficiency Remarks
PV TE PV/TEG PV
Kraemeretal.[49] Monocrystallinesilicon
N/A Simulation 11.45% 9.09% TEGefficiencyof8%correspondingtofigureofmerit(ZT = 1.7)wasused.
Amorphoussilicon N/A Simulation 13.26% 9.40%
Polymerthinfilm N/A Simulation 8.32% 3.41%
Juetal.[50] GaAs SkutteruditeCoSb3
Simulation 27.49% N/A Figureofmerit(ZT = 1.4)at800 K,heattransfercoefficientof4500 W/m2/Kwereused,andtheoptimizedresultsweregiven.
Mizoshirietal.[144] Amorphoussilicon Thin-filmBismuth Experiment N/A N/A Opencircuitvoltageofhybridsystemincreasedby1.3%comparedtoPVonlysystem.
Lietal.[95] N/A N/A Simulation 31–34% N/A Figureofmerit(ZT = 1)wasusedand30%poweroutputenhancementwasobtained.
Elsarragetal.[145] Monocrystallinesilicon
Bismuthtelluride Experimentandsimulation
N/A N/A HybridsystemperformedbetterthanPVonlysystem.
Skjølstrupetal.[146]
Amorphoussilicon N/A Simulation 19.1% 15.8% Beamsplitterlayerwas114andTEefficiencywas8%.
Microcrystallinesilicon
N/A Simulation 19.8% 17.5% Beamsplitterlayerwas128andTEefficiencywas8%.
Sibinetal.[147] N/A N/A Experiment N/A N/A ITO/Ag/ITOspectralbeamsplittercoatingwasdeveloped,andithadahighvisibletransmittanceof88%.
Yinetal.[51] GaAs N/A Simulation 30% N/A Figureofmeritwas1andcut-offwavelengthwasequaltomaximumwavelengthofPV.
Yangetal.[52] Silicon N/A Simulation 40.2% 39.32% Concentrationfactorwas100.
Bjørketal.[53] N/A N/A Simulation 1.8%pointsincrease
N/A Maximumhybridsystemefficiencywithoutconcentrationwasstudied.
Djafaretal.[148] N/A Bismuthtelluride Experiment N/A N/A Longwavelengthsofaround800 nmwereemittedbythehalogenlampsfortheTEG.Shouetal.[149] Crystallinesilicon N/A Simulation 3.24%increase N/A Hybridsystemhadafilterat150suns.
4.2.2DirectcouplingmethodInthedirectcouplingsystem,nosplitterisusedthus,thePVandTEGaredirectlycoupledandplacedinaparallelarrangement.ThePVisplaceddirectlyabovetheTEGandaheatsinkisattachedtothebottomoftheTEGjustasinthecaseofthespectrumsplittingasshowninFig.
9.ThereasonforplacingthePVabovetheTEGisbecausethePVabsorbstheshorterwavelengthswhiletheTEGabsorbsthelongerwavelength[15].Inaddition,whenthedirectcouplingmethodisused,theunabsorbedsolarradiationfromthePVtransmitthroughthePVtotheTEGbelow
andthisservesastheinputheatfluxfortheTEGtogeneratesomeelectricalpower.
VanSark[55]proposedaneffectivethermalmanagementtechniqueforphotovoltaiccellsbyintegratingthermoelectricmodulesintothePVusingthedirectcouplingmethodtoformahybridPV/TEGsystemwithenhancedelectricalperformance.Twocasestudieswerepresentedfor
Malaga,SpainandUtrecht,Netherlands.Inaddition,theannualenergyyieldwascalculatedusingtheweatherdatafromthetwolocationsandtheauthorsalsopresentedefficiencyenhancementpredictionsbasedonfuturethermoelectricmaterialstobedeveloped.Aseriesofthermoelectric
moduleswereattachedtothebacksideofthePVwhileaheatsinkwasusedtomaintainatemperaturedifferenceofabout50–60 °C.ThevariationofthehybridsystemgeneratedpowerwithsolarirradiancefordifferentfigureofmeritvaluesandPVconditionsisshowninFig.10a.Asexpected,
Fig.9SchematicofdirectcouplingPV/TEGintegration[18].
alt-text:Fig.9
thehighestgeneratedpowerwasfromthehybridsystemwiththehighestfigureofmeritvalue(Z = 0.01/K).Inaddition,Fig.10bshowsthegeneratedenergyfromthehybridsystemfor10daysinAugustforMalaga,Spain.TheadvantageofthePV/TEGovertheindividualPVandTEsystemscan
beseenclearlyfromthisfigureforallthedaysconsidered.Usingtypicalfigureofmeritvalueof0.004/Kat300 K,theauthorsobservedanefficiencyincreaseof23%fortheroofintegratedPV-TEG.Theresultsobtainedalsoshowedthatbyusingtheannualirradianceandtemperatureprofilesof
MalagaandUtrecht,theannualenergyofthesecitiescouldincreasebyabout14.7%and11%respectively.However,whenanassumedhighfigureofmeritvalue(Z = 0.01/K)fromfuturedevelopmentsinTEmaterialwasused,efficiencyincreaseofabout50%andannualenergyincreaseof
24.9%werepredictedbytheauthors.Notwithstanding,thisresearchignoredradiationlossonthefrontcoverandtheidealizedmodeldeveloped,overestimatedtheresultsbyabout10%forpracticalPV/TEGsystems.
Yinetal.[56,57]performedacoupleofdetailedinvestigationontheoptimumdesignofhybridPV/TEGsystemusingdirectcouplingmethod.TheactualperformanceofaPV/TEGsystemthroughoutasingledaywasstudiedtoseetheinfluenceofsolarradiationvariationwithtimeon
thehybridsystemperformance[56].ThechangesinthehybridsystemtemperatureandpowerefficiencywithinadaywerepresentedandtheinfluenceofPVtemperaturecoefficient,thermoelectricfigureofmerit,watercoolingmassandvelocitywerediscussed.Theresultsobtainedshowedthat
thehybridsystemperformedbetterthanthePVonlysystemwithinaone-dayperiodandahighefficiencyof16.7%wasachievedbythehybridsystem[56].Furthermore,anoptimumdesignmethodandselectionprincipleforaconcentrateddirectcouplingPV/TEGsystemwaspresentedinRef.
[57].Firstly,thetemperaturedistributioncorrespondingtotheoptimumhybridsystemefficiencywascalculatedandthentheoptimumTEGthermalresistancecorrespondingtotheoptimumhybridsystemtemperaturedistributionwascalculated.Lastly,theoptimumTEGstructurewasdetermined
afterthetwopreviousstepsandtheinfluenceofthermoelectricfigureofmeritandcoolingsystemconvectiveheattransfercoefficientonthehybridsystemperformancewereinvestigated.ItwasfoundthattheminimumTEGfigureofmeritvaluecanbeusedtoperformafeasibilitystudyforthe
CPV/TEGandselectthecouplingdevices.TheauthorsalsofoundthattheoptimumtemperatureandthermoelectricthermalresistancebothhaveaninverserelationshipwiththePVtemperaturecoefficient.Recently,Lietal.[58]performedacomparativestudyoftheoptimumgeometryfor
thermoelectricelementsinadirectcouplingphotovoltaic-thermoelectricandsolarthermoelectricsystem.TheoptimumgeometryforenhancedPV-TEandSTEGperformancewasinvestigatedusingfiniteelementmethod.Twophotovoltaiccellsanddifferentthermoelectricleggeometrieswere
studiedandcomparedwithasolarthermoelectricgenerator.TheeffectsofenvironmentalconditionslikesolarradiationandconcentrationratiowerealsostudiedandtheresultsobtainedshowedthattheoptimumthermoelectricelementgeometryinahybridPV/TEGsystemwasdependenton
thecharacteristicsofthesolarcellusedandthisoptimumgeometryisdifferentfromthatofthesolarthermoelectricgeneratorundersimilarconditions.TheperformancedataforsomeofthedirectlycouplingPV/TEGsystemsreviewedcanbefoundinTable5.
Table5SummaryofsomeselecteddirectcouplingPV/TEGsystemsreviewed.
alt-text:Table5
Reference Material Studytype Efficiency Remarks
PV TE PV/TEG PV
Guoetal.[104] Dye-sensitizedsolarcell(DSSC)
N/A Experiment 10%increase N/A HybridefficiencywascomparedwithasingleDSSC.
Wangetal.[105] Dye-sensitizedsolarcell N/A Experiment 13.8% 9.26% Solarselectiveabsorberwasused.
Sark[55] Polycrystallinesilicon Bismuthtelluride(Bi2Te3) Simulation 13.98% 10.78% Typicalfigureofmeritvalueof1.2andcoefficientc = 0.058wereused.Daudetal.[150] Polycrystallinesilicon Bismuthtelluride Experiment 9.064% 5.970% Solarradiationof868 W/m2andliquidcoolingwasused.
Parketal.[79] Crystallinesilicon Bismuthtelluride Experimentandsimulation
16.30% 12.5% 30%optimizedefficiencyincreaseat15 °CTEtemperaturedifference.
Fig.10HybridPV/TEGsystem(a)generatedpowerforfourfigureofmeritZvaluesand(b)totalenergyfor10-dayperiodinAugustforMalaga,Spain[55].
alt-text:Fig.10
Zhangetal.[151] Polymer Bismuthtelluride Experiment N/A N/A Hybridsystempoweroutputincreaseof46.6%comparedtoPVonlysystemwasobserved.
Lietal.[81] Crystallinesilicon N/A Simulation 11.07% 9.5% TEloadresistancewas0.75Ωandfigureofmeritwas0.0085/K.
GaAs N/A Simulation 22.94% 21.91% TEloadresistancewas1.60Ωandfigureofmeritwas0.0022/K
Zhangetal.[152] Crystallinesilicon Nanostructuredbismuth-antimony-telluride
Simulation 18.6% 18.4% Concentrationratiowas16.
Thin-filmsilicon Simulation 14% 11% Concentrationratiowas12.
Polymer Simulation 12% 4% Concentrationratiowas5.
CIGS Simulation 23.5% 21.5% Concentrationratiowas30.
Cuietal.[96] Crystallinesilicon Bismuthtelluride Simulation 20.1% N/A Operatingtemperaturewas300 K,opticalconcentrationwas100andPCMwasused.CIGS Bismuthtelluride Simulation 20.5% N/A Operatingtemperaturewas300 Kandopticalconcentrationwas0.Single-junctionGaAs Bismuthtelluride Simulation 28.09% N/A Operatingtemperaturewas425 K,figureofmeritwas1.5,andopticalconcentrationwas500.GaInP/InGaAs/Ge(III-V) Bismuthtelluride Simulation 38.90% N/A Operatingtemperaturewas300 Kandopticalconcentrationwas500.
Liaoetal.[153] Polycrystalline Bismuthtelluride Simulation 15% N/A CG(ConcentrationratioxSolarirradiance)was875 W/m2.
Chenetal.[154] DSSC N/A Simulation 24.60% N/A Maximumpoweroutputof1.389 mWwasobtained.
Linetal.[82] Crystallinesilicon Bismuthtelluride Simulation 13% 10.24% Powerandefficiencyenhancementofabout27%wasobserved.
Beerietal.[155] Multijunction Bismuthtelluride Experimentandsimulation
32.09% 32.08% Concentrationfactorwas20andhybridpoweroutputwas0.190 W.
Daetal.[123] GaAs N/A Simulation 18.51% N/A Figureofmeritwas2.5andAirMasswas1.5.
Douetal.[106] DSSC Bi2Te3/ZnO Simulation 4.27% N/A Hybridefficiencywas44.3%higherthanefficiencyofZnOphotoanode.
Attivissimoetal.[156]
Polycrystalline Bismuthtelluride Simulation N/A N/A TEGcontributesabout12.2%tothehybridsystemenergyinPachino.
Luoetal.[157] Heterojunction Bismuthtelluride Experiment 23.30%increase
N/A Efficiencyincreasewasachievedafter1 minillumination.
Pangetal.[158] Monocrystallinesilicon Bismuthtelluride Simulation 5.9% 5.7% Efficiencyincreaseof3.9%wasobserved.
Cotfasetal.[159] Monocrystallinesilicon Bismuthtelluride Simulation N/A 18.93% Solarirradiancewas920 W/m2.
Polycrystallinesilicon Bismuthtelluride Simulation N/A 16.71% Solarirradiancewas1020 W/m2.
Amorphoussilicon Bismuthtelluride Simulation N/A 2.88% Solarirradiancewas720 W/m2.
Lambaetal.[75] Monocrystallinesilicon Bismuthtelluride Simulation 5.8% 5.2% NumberofTEGwas127andconcentrationratiowas3.
Zhuetal.[74] Monocrystallinesilicon N/A Experimentandsimulation
23% 19% TEGcontributedextraelectricalenergyof648 Jduringzerosolarradiationperiod.
Hashimetal.[90] Amorphoussilicon Bismuthtelluride Simulation 10.2% N/A Hybridsystempoweroutputincreasedto163 mW.
Kossyvakisetal.[92]
Polycrystallinesilicon Bismuthtelluride Experimentandsimulation
22.5%increase
N/A Hybridsystemefficiencywasobtainedtheoretically.
DSSC Bismuthtelluride Experimentandsimulation
30.2%increase
N/A Hybridsystemefficiencywasobtainedtheoretically.
Zhangetal.[111] Perovskite Bismuthtelluride Simulation 18.6% 17.8% Solarselectiveabsorberwasused.
Cuietal.[97] Single-junctionGaAs Bismuthtelluride Experiment 13.45% 13.43% Phasechangematerial(PCM)wasused.
Zhouetal.[160] DSSC p-typeBi0.4Sb1.6Te3,n-type Experiment 9.08% 7.21% HybridefficiencywasgreaterthanTEGefficiencyby725.5%.
Bi2.85Se0.15Te3
Lambaetal.[76] Monocrystallinesilicon Bismuthtelluride Simulation 7.44% 7.068% Maximumpoweroutputofthehybridsystemwas595.5 mW.
Dallanetal.[114] Monocrystallinesilicon Bismuthtelluride Experiment 13.2% 8.052% PVandTEpoweroutputwere60.5 W/m2and0.01 W/m2respectively.
Kiletal.[161] SinglejunctionGaAs Bismuthtelluride Experiment 23.2% 22.5% Solarconcentrationwas50suns.
Soltanietal.[103] Crystallinesilicon Bismuthtelluride Experiment 3.355%increase
N/A SiO2/waternanofluidcoolingwasusedandpoweroutputwasincreaseby8.26%comparedtonaturalcooling.
Lietal.[162] CIGS Bismuthtelluride Simulation 21.6% 20.71% Concentrationratiowas200.
Thinfilmsilicon Bismuthtelluride Simulation 13.1% 12.89% Concentrationratiowas200.
Polymer Bismuthtelluride Simulation 8% 7.47% Concentrationratiowas180.
Contentoetal.[43] Amorphoussilicon NanostructuredBi2Te3 Simulation ≈57%increase N/A ≈57%increaseand≈42%fordirectlyandindirectlycoupledsystemsrespectively.
HeterojunctionCZTS NanostructuredBi2Te3 Simulation ≈35%increase N/A ≈35%increaseand≈24%fordirectlyandindirectlycoupledsystemsrespectively.
Liuetal.[163] Perovskite Bismuthtelluride Experiment 22.2% 9.88% IcebathwasusedforTEcoolingandAirmasswas1.5.
Zhangetal.[164] Silicon N/A Experiment N/A N/A Hybridsystemachievedhighabsorptionforwavelengthsof0.3–1.1 μm.Machrafietal.[165]
Monocrystallinesilicon p-Sb2Te3n-Bi2Se3 Simulation 25% N/A Thermoelectricnanoparticleswereused,andoptimumcoolingvelocitywas10 m/s.
Jeyashreeetal.[166]
Polycrystallinesilicon Bismuthtelluride Experiment N/A N/A IceblockwasusedforTEGcoolingandhybridsystempoweroutputwas10.772 W.
Nishijimaetal.[167]
Blacksilicon N/A Simulation N/A N/A Ge Snlayerwasaddedtothesolarcellandvoltageincreaseof7%wasobserved.
Babuetal.[68] Polycrystalline Bismuthtelluride Simulation 6%increase N/A TEGcontributedenergyof1–3%ofPVrating.
Lietal.[168] InGaP/InGaAs/Getriple-junction
Bismuthtelluride Experiment 33.53% 32.86 PCMandwatercoolingwereused.Averageefficiencywasconsidered.
4.3PV/TEGstudytypeRecently,therehasbeenanincreasingnumberofresearchworkspublishedrelatingtoPV/TEGduetothehighlevelofinterestinsuchhybridsystemsanditshugepotentialforenhancedperformancecomparedtoPVonlysystems.Someofthemostrecently
publishedworksonPV/TEGasatthetimeofwritingthisreviewarediscussedinthissectionbasedonthetypeofstudyconducted.Generally,hybridPV/TEGisusuallystudiedexperimentallyortheoretically.Thetheoreticalstudyalsoinvolvescomputational/numerical
study.
4.3.1ExperimentalstudyMahmoudinezhadetal.[59]presentedanexperimentalstudyofthetransientbehaviourofahybridconcentratingtriplejunctionsolarcell-thermoelectricgeneratorsystem.Asolarsimulatorwasusedintheexperimentalstudytovarytheconcentratedsolarradiationsbetween0and39
suns.Themainobjectiveoftheresearchwastoobtainthetransienttemperaturesoftheconcentratingtriple-junctionsolarcellandthehotandcoldsidesofthethermoelectricgeneratorinadditiontotheshortcircuitcurrent,opencircuitvoltageandmaximumpoweroutputs.Resultsobtained
showedthattheuseofthermoelectricgenerator inahybridsystemisaneffectivewaytostabilizetheoverallpoweroutputofthehybridsystem.Inaddition,theauthorsarguedthatgeometrycanmaterialoptimizationaretwoeffectivewaystoenhancethecontributionofthethermoelectric
generatortotheoverallhybridsystempoweroutput.
Yinetal.[60]performedanexperimentalinvestigationonthefeasibilityofaconcentratedphotovoltaic-thermoelectricsystemwithphasechangematerial(PCM)andthethermalresistanceanalysisofsuchhybridsystem.Anexperimentalcomparisonoftheperformanceofconcentrated
photovoltaicsystem,conventionalconcentratedphotovoltaic-thermoelectricsystemandaconcentratedphotovoltaic-phasechangematerial-thermoelectric(CPV-PCM-TE)hybridsystemwaspresented.Furthermore,theperformanceofthreehybridCPV-PCM-TEsystemsusingdifferentPCM
platesincludingparaffinwithfins,paraffin/expandedgraphitecompositewithfinsandparaffin/copperfoamcompositewerecomparedtostudyandanalysethePCMthermalresistanceeffect.Resultsobtainedshowedthatthephasechangematerialefficientlymaintainedthetemperatureofthe
PVcellinthehybridCPV-PCM-TEtoabout50 °CwhilethePVtemperatureinthehybridCPV-TEsystemattainedahighvalueof80 °C.Inaddition,theresultsshowedthattheaveragepoweroutputofthehybridCPV-PCM-TEsystemincreasedby23.52%comparedtothatofthehybridCPV-TE
system.Furthermore,theauthorsarguedthatthecoolingsystemthermalresistancehadalittleinfluenceonthePVcellduetothepresenceofthePCMwhichcontrolledthetemperature.
ThefeasibilityandoptimizationofahybridPV/TEGsystemwasstudiedexperimentallybyLekbiretal.[61].Theexperimentalstudywascarriedoutinalaboratoryandamonocrystallinesiliconsolarcellwasused.Resultsfromtheexperimentcarriedoutshowedthatthemaximum
poweroutputofthehybridsystemwas0.12 WandthiswasgreaterthanthatofthePVcellandTEGtherefore,thehybridsystemperformedbetterunderthesameenvironmentalconditions.Marandietal.[62]performedanexperimentalinvestigationofahybridPV/TEGsystemwithasolarcavity
receiver.Anovelmethodtoreducere-radiationfromPVpanelsbyusingcavityreceiverwaspresentedandthedevelopedcavityhybridPV/TEGsystemachievedapeakefficiencyof21.9%.Zhangetal.[63]presentedauniquestructuralarrangementforenhancedperformanceofhybridPV/TEG
system.Inthedesign,ceramicplatesontheTEmodulewereeliminatedtoenhanceheattransferbyreducingthermalresistanceandaV-typegroovewasusedtoenhanceabsorptionofsolarenergybykeepingeachPVcell inaperpendicularpositiontoitsadjacentPVcells.Theauthors
performedanexperimentalinvestigationandfoundthatthenewTEstructureenhancedtheperformanceofthehybridsystem.SomeoftheotherexperimentalpapersonhybridPV/TEGsystemcanbeseenunderthecolumn‘studytype’inTables4and5withtheircorrespondingperformance
data.
4.3.2Theoretical/computationalstudyRodrigoetal.[64]presentedatheoreticalstudyontheperformanceandeconomiclimitsofpassivelycooledhybridPV/TEGsystems.Thenoveltyofthisstudywasthedevelopmentofanelectric/thermal/economicmodeltoinvestigatetheperformanceandeconomiclimitsobtainable
withpassivecoolingofaconcentratedhybridPV/TEGsystem. Inaddition, thedevelopedmodelallowed theareaof the thermoelectricgenerator tobeadjusted.Resultsobtainedshowed that theoptimizationof the thermoelectricgeneratorarea isessential forkeeping thecelloperating
temperaturewithinacceptable limits.Furthermore,anavant-gardemathematicalmodelwaspresented in thestudyanda three-dimensionalmodelof theconcentratorphotovoltaic-thermoelectric receiverwasdevelopedbasedon finiteelementmethod.Theauthorsargued that thenewly
proposedpassivecoolingconceptwouldinfluencethedevelopmentoffuturepassivelycooledconcentratedPV/TEGprototypes.
AdetailedparametricstudyontheperformanceofahybridPV/TEGsystemusingnumericalsimulationwasperformedbyLakehetal.[65].Inthestudy,theeffectofambientconditions,coldsidetemperatureandloadresistanceofthethermoelectricgeneratorontheperformanceofthe
hybridsystemwasconsidered.Furthermore,thematerialproperties,numberofthermoelectricgeneratorcouple,crosssectionalareaandlengthofthesystemweredeterminedintheoptimumrangesoastooptimizetheperformanceofthehybridsystem.Resultsobtainedshowedthatthe
electricalperformanceofthehybridsystemintermsofmaximumpoweroutputwashighlydependentonthegeometricalcharacteristicsofthedevice.Also,thenumericalsimulationresultsshowedthatthehybridPV/TEGsystemhadahigherefficiencycomparedtothesinglesolarcell.
Lekbiretal.[66]performedanumericalinvestigationofananofluidbasedconcentratedphotovoltaic/thermal-thermoelectricgenerator(CPV/T-TEG)hybridsystemwithacoolingchannel.ComparedtothenanofluidbasedCPV/T,CPVandCPV/TEGwithheatsink,theproposedhybrid
systemelectricenergywashigherby10%,47/7%and49.5%respectively.Lorenzietal. [67]presentedamodel fordeterminingthetheoreticalefficiencyofahybridPV/TEGsystemfor terrestrialapplication.Theauthorsarguedthat there isanoptimumoperatingtemperatureforobtaining
maximumhybridsystemefficiencyandthistemperatureisnotinfluencedbytheTEGgeometricaldimensionsandnumberoflegs.Efficiencyincreaseof4–5%comparedtoPVonlysystemwasobservedforthehybridsystem.Babuetal.[68]alsoperformedatheoreticalinvestigationofan
unconcentratedhybridPV/TEGsystemusingtheMATLAB/Simulinkenvironment.Itwasfoundthatthehybridsystemhadanoverallefficiencyincreaseof6%andadditionalenergyprojectionof5%.
Motieietal.[69]performedanumericalsimulationofahybridPV/TEGsystemusinganunsteady,twodimensionalnumericalmodel.TheperformanceofthehybridsystemwascomparedwiththatofaPVonlysystemanditwasfoundthatinthehybridsystem,thePVconversion
efficiencyandelectricalpoweroutput increasedby0.59%and5.06%respectivelycompared to thePVonlysystem.However, it isworthnoting that theThomsoneffectwasnotconsidered in thisstudyand temperature independent thermoelectricmaterialpropertieswereused.Similarly,
Mahmoudinezhadetal.[70]studiedthetransientresponseofahybridCPV/TEGsystem.Anumericalinvestigationwascarriedoutusingfinitevolumealgorithm.TheinfluenceofthermalcontactresistanceattheinterfacesbetweentheCPV/TEGandTEGheatsinkwasstudiedandtheinfluence
ofvaryingweatherconditionsontheperformanceofthehybridsystemwasinvestigated.TheresultsshowedthatincreaseinthermalcontactresistanceleadstoadecreaseinefficiencyoftheTEGandCPV.Furthermore,theauthorsarguedthattheuseofTEGinthehybridsystemenabledthe
stablegenerationofpowerundervaryingweathercondition.
Ararethree-dimensionalnumericalsimulationofaPV/TEGwasperformedbyFallanKohanetal.[71].Theinnerstructuralcomplexitieswereignoredinthenumericalmodelandthehybridsystemwasconsideredasahomogeneousmedium.Electricpoweroutputinthehybridsystem
wasmodelledasaninternalenergysinkandfinitevolumemethodwasusedforthenumericalsimulation.Itwasfoundthatundercertainenvironmentalconditions,thehybridPV/TEGsystemgeneratedmorepowerthanthePVonlysystem.TheauthorsarguedthatthepresenceoftheTEGcould
haveanegativeeffectonthecoolingofthePV.Zhouetal.[72]developedaMultiphysicscouplingmathematicalmodelforstudyingtheperformanceofahybridPV/TEGsystemwhichconsistedofanall-back-contactsiliconPVwithnanostructuredsurface,athermoelectricdeviceandaplanefin
heatsink.ItwasfoundthatthepoweroutputdensityofthePV/TEGincreasedby9.1%duetotheoptimizationofthehybridsystemheattransferstructure.Underthecolumn‘studytype’inTables4and5,someoftheothertheoreticalpapersonhybridPV/TEGsystemcanbeseenwiththeir
correspondingperformancedata.
5CurrentresearchfocusareasinhybridPV/TEGAplethora of studies on hybridPV/TEGhave been performed recentlywith each study addressing a particular area in the hybrid system research. This section presents some of themain research focus areas being explored by researchers on hybrid
photovoltaic-thermoelectricgeneratorandsomenicheapplicationsofhybridPV/TEG.
5.1ConcentratedhybridsystemVorobievetal.[73]presentedatheoreticalstudyoftwodifferentapproachesforthethermalmanagementofPV(Fig.11).Inthefirstapproach,theunabsorbedsolarradiationsfromthesemiconductormaterialofthePVwasconcentratedonathermoelectric
generator for furtherconversion intoelectricalenergythus, thePVoperatedata lowtemperature.ThesecondapproachseesthePVcelloperatingatelevatedtemperatureswhilethethermoelectricgenerator isusedtoconvert theexcessheat.Theonlydifference
betweenbothapproachesisthepositionoftheconcentratorandPV.ThebasicelementsofthesystemsshowninFig.11are;Concentrator(CONC),PVcell(PVC),ThermoelectricGenerator/HighTemperatureStage(HTS)andthe2-axisSolarTrackingSystem(STS2).
Theauthorsfoundthatusingthefirstapproach,thehybridsystemobtainedenhancedefficiencyof5–10%whilethesecondapproachdidn'tsignificantlyimprovetheoverallhybridsystemefficiency.Adrawbackfromthisresearchisthat,anassumedhighZTvaluewas
usedwhichisnotcurrentlypractical.
Zhuetal.[74]performedadetailedexperimentalandnumericalinvestigationoftheperformanceofathermalconcentratedhybridPV/TEGsystem(Fig.12).AcopperplateoperatingasthethermalconcentratorandconductorwassandwichedbetweenthePV
andTEGanditincreasedthetemperaturedifferenceacrosstheTEG.Finiteelementsimulationsoftware,ANSYSwasusedtostudythetemperaturedistributionandwatercoolingwasappliedtothehybridsystem.Itwasobservedthatalargepercentageofthermalloss
iscausebyairconvectionthus,transparentenclosure(Polymethylmethacrylate)andoptimalthermalconcentrationwereintroducedintothedesignedsystem.Resultsobtainedshowedthattheuseofthecopperplateenhancedtemperatureuniformityandtheefficiencyof
thehybridsystemwasabout23%.
Lambaetal.[75,76]performedacoupleof investigationsonconcentratedPV/TEGsystems.TheinfluenceofThomsoneffect,concentrationratio,numberofTEG,solar irradiance,PVcurrentandTEcurrentontheefficiencyandpoweroutputof thehybrid
system,PVandTEGonlysystemswerestudied[75].Resultsobtained(Fig.13)showedthatthepoweroutputofthehybridPV/TEGsystemdecreasesduetoThomsonheatingwhentheThomsoneffectisconsideredintheTEGanalysis.ItwasfoundthatThomsoneffect
significantlyreducesthehybridsystempoweroutputespeciallyforhighlyconcentratedsystems[75].AnothertheoreticalinvestigationwascarriedoutontheconcentratedPV/TEGandtheinfluenceofTEGleglength,loadresistanceratio,concentrationratioandfillfactor
onperformanceofthehybridsystemwerestudied[76].ItwasobservedthattheTEGcontributesmoretothetotalhybridsystempoweroutputwhenhigherconcentrationratiosareused.Inaddition,thehybridsystemobtainedamaximumefficiencyof7.44%whichwas
5%higherthanthatofthePVonlysystem[76].
Fig.11SchematicdiagramofPV/TEGwithPVoperatingat(A)lowand(B)hightemperatures[73].
alt-text:Fig.11
Fig.12HybridPV/TEG(a)cross-sectionalview,(b)bottomview,(c)globalviewand(d)physicaldiagram[74].
alt-text:Fig.12
Anadditional studyonhybridconcentratedPV/TEGsystemwascarriedoutbyRezaniaandRosendahl [77].Theeffectof themost importantdesignparameterson thehybridsystemperformancewerestudiedand itwasobserved that theconcentrated
photovoltaic-thermoelectric (CPV/TEG)systemusing thecurrentavailable thermoelectricmaterials (ZT≈1)hadabetterconversionefficiencycompared to theCPVonlysystem(Fig.14).Recently,Mahmoudinezhadetal. [78]performeda feasibilitystudyofahybrid
CPV/TEGsystemforlowsolarconcentrations(Fig.15).Anexperimentalandnumericalinvestigationwascarriedouttostudytheperformanceofthehybridsystem.Theexperimentwascarriedoutusingconcentratedradiationfromasolarsimulatorwhilefinitevolume
methodwasusedtoperformthenumericalsimulation.Theauthorsarguedthattheexperimentalresultswereinagreementwiththesimulationresults.ItwasfoundviatheexperimentthatthemaximumandminimumefficiencyoftheCPVwere35.33%and23.02%
respectively.
Fig.13VariationofhybridPV/TEGandTEGsystempoweroutputwithsolarirradiance[75].
alt-text:Fig.13
Fig.14Variationofsystemefficiencywithsunconcentration[77].
alt-text:Fig.14
5.2HybridsystemcouplingEffectivecouplingofthephotovoltaicandthermoelectricsystemscanenhancetheperformanceofthehybridsystemandreducelosses.Infact,itisessentialtoperformalosslessmatchingofthetwodifferentsystems(PVandTE)toobtainoptimizedefficiency
results.Inaddition,aTEGpossessesaninternalresistancewhichmustbeadequatelymatchedwiththatofthePVsoastheensurethehybridsystemperformanceisnotworsethanthatoftheindividualsystems.
Parketal.[79]performedtheoptimizationofahybridPV-TEGsystemvialosslesscouplingandobservedanefficiencyenhancementofabout30%inthehybridsystemcomparedtotheconventionalPVsystem.Theachievedlosslesscouplingenabledthehybrid
systempoweroutputtobeequaltothesumofthepoweroutputfromtheindividualsystems(PVandTE).Inaddition,theauthorsarguedthatproperselectionoftheinternalresistanceandTEelementsnumberwasessentialtoobtainingfillfactor(FF)andvoltagegains.
Inaddition,Lorenzietal.[80]analysedtheeffectofseveralparametersonthepoweroutputofanelectricalcoupledPV/TEGsystem.Resultsobtainedshowedthatforsolarcellswithasmallseriesresistance,thevoltageneededforelectrical losslesscouplingwas
smaller.Inaddition,itwasfoundthatlosslessconditionsstrongdependontemperature.
Loadresistancematchingisanotheroptimizationtechniquetoenhancetheperformanceofthehybridsystem.Successfullymatchingtheinternalresistanceofthehybridsystemwiththeexternalloadresistancewouldensuremaximumpoweroutputisobtained.
Lietal.[81]studiedtheinconsistentphenomenonofthethermoelectricloadresistanceinPV/TEGsystems.TwodifferentPVcellswereusedintheanalysisandthethermoelectricloadresistancesforobtainingmaximumpoweroutputintheTEonlysystemandPV/TEG
systemweredetermined.ThemainargumentoftheauthorswasthattheloadresistanceatwhichmaximumpoweroutputcouldbeobtainedfromathermoelectricgeneratormightbedifferentfromtheoptimumthermoelectricloadresistanceinahybridPV/TEGsystem.
Therefore,theyperformedaseriesofinvestigationswiththeaimofstudyingthisinconsistentloadresistanceeffect.Inaddition,thermalcontactresistancebetweenthePVandTEG,TEGandheatsinkwereconsideredinthesimulations.Theeffectofconcentrationratio,
windspeedandambienttemperaturewereinvestigated.ResultsobtainedshowedthatthethermoelectricloadresistanceformaximumpoweroutputfromtheTEGalone,TEGinPV/TEGandPV/TEGareentirelydifferent.Therefore,theauthorsconcludedthatusingthe
optimumTEloadresistanceinaTEGonlysystemfortheanalysisofaPV/TEGsystemwouldcauseerrorsandpreventtheattainmentofhybridsystemmaximumpoweroutput.
Linetal.[82]performedasimilar research toRef. [81]onhybridPV/TEG loadresistancematching.Thermodynamicmethodwasused to investigate theoptimumperformancecharacteristicsof thehybridsystemandproblemsrelating to loadmatching in
practicalhybridsystemdesignwerediscussed.Resultsobtainedshowedthatthemaximumpoweroutputandmaximumefficiencyofthehybridsystemcouldbeobtainedatthesameoperatingcurrent(Fig.16).Inaddition,hybridsystemefficiencyandpoweroutputof
about27%comparedtothePVonlysystemwereobserved.Finally,theauthorsarguedthattheuseofthermoelectricmaterialwithafigureofmerit(Z = 0.004/K)couldleadtothehybridsystemefficiencyenhancementofabout30%.
Fig.15SchematicdiagramofexperimentalPV/TEGsetup[78].
alt-text:Fig.15
ThepotentialnegativeeffectofPV/TEGcouplingwaspresentedbyLinetal.[83].Theauthorsplacedemphasisonthecouplingbetweendiscretizednodaltemperaturesandthehybridsystemelectricalpoweroutput.AmodifiedNewton-Raphsonmethodwhich
involvedtwocomputationalstageswasusedtosolvethemodellingequations.Theauthorsarguedthattheirmethodreducedcomputationaleffortsintermsofsolvingalgebraicmanipulationsandcodingaswellas,makingdebuggingofthecodeeasierwhendivergence
occurs.Resultsobtainedshowedthatforthespecificparametricvalueschosen,theefficiencyofthehybridsystemwaslowerthanthatofthePVonlysystem(Fig.17).ItisthereforeimperativetoproperlycouplethePV/TEGforachievementofenhancedoverallefficiency
ratherthanreducedefficiencywhencomparedtothePVonlysystem.
SimilartoRef.[83],Bjørketal.[84]observedanegativeeffectofhybridsystemcouplingduetothereducedperformanceofthehybridsystemcomparedtothePVonlysystem.AnanalyticalinvestigationwascarriedoutusingfourdifferentPVcelltypesincluding,
crystallinesilicon(c-Si),amorphoussilicon(a-Si),copperindiumgalliumselenide(CIGS)andcadmiumtelluride(CdTe).Bismuthtelluridethermoelectricmaterialwithamaximumoperatingtemperatureof200 °Cwasusedintheanalysis.Theauthorsfoundthatonlythe
Fig.16HybridsystemandPVonlysystem(a)poweroutputand(b)efficiency[82].
alt-text:Fig.16
Fig.17VariationofhybridsystemandPVonlysystemefficiencywithsolarradiation[83].
alt-text:Fig.17
hybridsystemwithamorphoussiliconhadanenhancedefficiencyandpoweroutputcomparedtothePVonlysystem.Contrarily,thehybridsystemwiththeothertypesofPVconsideredhadaworseperformancethanthePVonlysystem(Fig.18).Theexplanationforthis
trendwasthatthePVperformancedegradationwithincreasedtemperaturewasmuchgreaterthantheTEGpowerproductionduetothelowefficiencyoftheTEG.Consequently,theauthorsarguedthatcouplingofphotovoltaicandthermoelectricisnotaviableoptionfor
powerproductionduetothedecreaseinPVperformanceastemperatureincreased.
Hajjietal.[85]deviatedfromthenormbypresentinganindirectcouplingofahybridPV/TEGsystem(Fig.19).AcombinationoffiniteelementmethodandMATLAB/Simulinksoftwarewasusedtoperformthenumericalsimulationsandacomparisonbetweenthe
directlycoupledand indirectlycoupledsystemswaspresented.Basically, in thedirectlycoupledsystem,all thecomponentswerephysicallyconnectedwhilefor the indirectcoupling, theopticalconcentratorhadnodirectphysicalcontactwiththePVandTEG.The
developedsystemwasproperlyinsulatedtoreduceheatlossanditwasobservedthattheindirectcouplingmethodsignificantlyimprovedthehybridsystemoverallefficiency.
Fig.18HybridsystemefficiencyvariationwithtemperaturefordifferentPVcells[84].
alt-text:Fig.18
Fig.19SchematicofPV/TEGsystemfor(a)directand(b)indirectcoupling[85].
alt-text:Fig.19
Adoptingasimilarapproachas[85],Contentoetal.[43] investigatedtheperformanceofanopticallycoupled(indirect)PV/TEGsystemusingavacuum-sealedcompoundparabolicconcentratorandathermallycoupled(direct)PV/TEGsystem(Fig.20).Two
typesofPVcellswereconsideredincluding,singlejunctionamorphoussiliconandheterojunctionCu2ZnSnS4(CZTS).ResultsobtainedshowedthatdirectcouplingofhybridPV/TEGsystemenablestheachievementoflargeconversionefficiencywhileindirectcoupling
reducesthetemperatureofthePVthus,improvingitsreliabilityandlifespan.Inaddition,theauthorsobservedtheefficiencyimprovementinthedirectlycoupledhybridsystemtobe ≈ 57%and≈35%fortheamorphoussiliconandCZTSrespectively.Whilefortheindirectly
coupledhybridsystem,theefficiencyimprovementwas≈42%and≈24%fortheamorphoussiliconandCZTSrespectively.
5.3ThermoelectricgeometryoptimizationThe geometry of a thermoelectric generator significantly affects its performance therefore, several research [86–89] on optimization of TEG geometry has been carried out. However, the integration of thermoelectric generators into PV necessitates the
investigationoftheoptimumgeometryoftheTEGinthehybridPV/TEGsystemasthismaydifferfromtheoptimumgeometryintheTEGonlysystem.
Hashimetal.[90]developedanumericalmodelfortheoptimizationofthermoelectricgeneratorsinahybridPV/TEGsystem.ThemodelcouldefficientlydeterminetheoptimumTEGgeometryformaximumhybridsystempoweroutput.Inthisstudy,passivewater
andcopperplatewereusedasthecoolingsystemandthermalconcentratorrespectively.TheauthorsarguedthatthegeometryoftheTEGinahybridsysteminfluencestheoverallpoweroutputofthesystemasitdeterminesthesolarcelloperatingtemperatureandTEG
temperaturedifference.ItwasalsofoundthatthepoweroutputofTEGmoduleswithasmallercross-sectionalareathanthatofthePVwashigherthanthosewithalargercross-sectionalarea.Finally,itwasobservedthatoperatingthehybridsysteminvacuumcould
enhanceitsperformance.
Recently,Shittuetal. [91]performedasignificantstudyon theoptimum thermoelectricgeneratorgeometry inahybridPV/TEGsystem forenhancedoverallefficiency.Theauthorsoptimized the twogeometryarea ratios and of a
thermoelectricgenerator.TwodifferentPVtypes(CellAandCellB)withdifferentreferenceefficiencyandtemperaturecoefficientvaluewerestudiedandtheinfluenceofTEGgeometricparameters,solarirradiationandconcentrationratioonthehybridsystemefficiency
wereinvestigated.COMSOLMultiphysicswasusedtoperformthefiniteelementnumericalsimulationsandtemperaturedependentthermoelectricpropertieswereused.Resultsobtained(Fig.21)showedthattheoptimumgeometryoftheTEGinahybridsystemishighly
dependentonthetypeofsolarcellused.
Fig.20Schematicofdevelopedhybridsystemwith(a)indirectand(b)directcoupling[43].
alt-text:Fig.20
(RA =AH/AC) (RS = An/Ap)
Kossyvakisetal.[92]investigatedtheinfluenceofTEGgeometryontheperformanceofahybridPV/TEGsystemusingtwodifferentPVcelltypes(polycrystallineanddye-sensitized).Anexperimentalandnumericalstudywascarriedoutonfour-wireandtwo-
wirehybridPV/TEGsystemsandwatercoolingwasused.Resultsobtainedshowedthattheuseofthermoelectricgeneratorwithshortleglengthisfavourableforhybridsystempoweroutputenhancementundersufficientilluminationbecauseitcanreducetheoperating
temperatureofthePVcell.Thisobservationissignificantbecauseitcanreducethehybridsystemmanufacturingcostsbecauselessthermoelectricmaterialwillbeusedtoachievehighoverallpoweroutput.
MoreTEGgeometryoptimizationefforts inhybridPV/TEGwerecarriedoutbyLietal. [93,94].Theauthorsarguedthat thePVandTEsystemshaveacomplexrelationship in thehybridconfiguration therefore, theoptimizationofTEGalonewouldnotbe
sufficientforobtainingmaximumpoweroutputandefficiencyfromthehybridsystem[93].Furthermore,theprimaryconstraintsofahybridPV/TEGsystemwereinvestigatedandtheinfluenceofTEGgeometryparameters,coldsidetemperatureandsolarirradianceonthe
performanceofthehybridsystemwerestudied.Itwasfoundthattheoverallefficiencyofthehybridsystemincreasedasthecross-sectionalareaoftheTEGincreasedandshorterTEGleglengthisfavourable[94].
5.4EnergystorageDuetotheintermittentnatureofsolarradiations,itissometimesnecessarytoaddanenergystorageunittothehybridPV/TEGsystem.Thestorageunitcanhelpstorethermalenergyforuseduringperiodsoflowsolarradiation.Lietal.[95]investigatedthe
performofahybridPV/TEGsystemwithanenergystorageunit(Fig.22).ThethermalenergywhichwasstoredintheenergystorageunitwasusedastheheatsourcefortheTEGhotside.Theauthorsarguedthatitisessentialtostorethermalenergyforbothheating
andcoolingreservoirsusingphasechangematerials(PCMs)soastomaintainstablePVandTEoperatingtemperatures.Inaddition,theyrecommendedtheuseofinorganiceutecticsaltmixturesasPCMsformoderatetohightemperaturethermalenergystorage.While
forhighgradecoldstorageapplications,PCMeutecticsolutionswererecommendeddue to theirability tostorage thermalenergydownto159 K.The resultsobtainedshowedanoverallhybridsystemefficiencyenhancementofabout31–34%usingthermoelectric
materialswithZT = 1(seeFig.24)(seeFig.23).
Fig.21Hybridsystemefficiencyvariationwithgeometryarearatiofor(a)solarcellAand(b)solarcell[91].
alt-text:Fig.21
Similarly,Cuietal.[96]studiedtheperformanceofanovelPV-PCM-TEsysteminwhichthePCMwasusedtomitigatethetemperaturefluctuationsinthehybridsystemthus,enablingthehybridsystemtooperateatfixedconditions.Theadvantageofusinga
PCM is that it canabsorba largeamountofenergyas latentheatataconstantphase transition.FourPV typeswere investigated for thePVonly,PV/TEandPV-PCM-TEsystemsand thesignificanceof incorporating thePCM into thehybridPV/TEsystemwas
investigatedunderfluctuatingsolarradiation.ThePCMtankwasattacheddirectlybelowthePVcelltoreducethethermalcontactresistanceanddifferenceintemperaturebetweenthePVandPCM.TheauthorsrecommendedtheuseofparaffinPCMwhenthePV-PCM-
Fig.22SchematicofPV/TEGsystemwithheatstorageunit[95].
alt-text:Fig.22
Fig.23SchematicdiagramandenergyflowofthehybridPV-PCM-TEsystem[97].
alt-text:Fig.23
Fig.24(a)HybridPV-PCM-TEsystemandPVonlysystem,(b)PVonlysystem,(c)topviewofhybridPV-PCM-TE,(d)bottomviewofhybridsystemwithwaterheatsinkand(3)bottomviewofhybridsystemwithairheatsink[97].
alt-text:Fig.24
TEisoperatedatatemperaturebelow375 K.However,fortemperaturesabove420 K,NaOH
KOHPCMwasrecommended.ResultsobtainedfromthisresearchshowedthatthePV-PCM-TEhadabetterperformancecomparedtoPVonlyand/orPV/TEsystems.
SubsequenttothetheoreticalinvestigationcarriedoutbyRef.[96],thesameauthors(Cuietal.)carriedouttheexperimentalinvestigationoftheproposednovelhybridPV-PCM-TEsystem[97].Theschematicdiagramandexperimentalrealisationofthesystem
areshown inFigs.21and22 (Pleasechange these toFigs. 23and24) respectively.Themainnoveltyof thisstudy is the introductionofphasechangematerial (PCM) into thehybridPV/TEGsystem tomaintain thesystemoperating temperature. Influenceofoptical
concentrationratioandcoolingsystemontheperformanceofthehybridsystemwerestudied.Inaddition,acomparisonbetweenthenovelhybridsystemandPVonlysystemwasmade.Itwasobservedthatthenovelhybridsystemhadahigherefficiencycomparedto
thePVonlysystemduetotheuseofphasechangematerial.
5.5ThermoelectricgeneratorcoolingAdoptingasimilarapproachusedbyRef.[73],Willars-Rodríguezetal.[98]performedanexperimentalinvestigationofahybridPV/TEG(Fig.25).ThePVcellswereoperatedinacoldarea(≤310 K)whilethecoolingunitandtheTEGwereoperatedinahigh
temperaturearea(≤500 K).ThereasonforthisarrangementisbecausethePVperformsbetteratlowertemperatureswhiletheTEGrequireshightemperatureforhighperformancethus,theirseparationintotwoareas(coldandhot)wouldenhancetheperformance.In
addition,theTEGwascooledusingthethermosiphoneffectofrunningwaterwhilethehybridsystemgeneratedthermalenergywasstoredinthewatertank.Theauthorsperformedfiniteelementsimulationofthethermoelectricgeneratorcoolingunitandtheresult
obtainedisshowninFig.26.Theassumptionconsideredwasthatsolarradiationisapplieddirectlytothecopperplateanditwasfoundthatthemaximumtemperatureoftheplatewas350 °Cwhiletheinletandoutletwatertemperatureswere25 °Cand45 °Crespectively.
Resultsobtainedshowedthatthehybridsystemgeneratedanelectricpowerof7 Wandthermalpowerof30 W.
Yinetal.[99]investigatedtheperformanceofahybridPV/TEGsystemusingthreedifferentcoolingmethods(Fig.27).Naturalcooling,forcedaircoolingandwatercoolingwerecomparedandtheinfluenceofopticalconcentrationratio,watervelocityandthermal
contactresistancewerestudied.Inaddition,thehybridsystemsusingfourdifferentPVtypesincluding,monocrystallinesilicon,polycrystallinesilicon,amorphoussiliconandpolymercellwereinvestigatedtodeterminethebestperformingPV.Resultsobtainedshowed
Fig.25(a)Hybridsystemexperimentaltestrig,(b)hybridsystemschematicrepresentationand(c)integratedthermoelectricgeneratorwithcoolingunit.Where:(1)PVmountedoncoolingsystem,(2)Fresnellens,(3)solartracker,(4)structuralelements,(5)TEG,(6)storagetank,(7)pipe,(8)heatsinkand(9)thermalincident
spot[98].
alt-text:Fig.25
Fig.26Temperaturedistributioninthermoelectricgeneratorcoolingunit(a)runningwatertubeand(b)copperplate[98].
alt-text:Fig.26
thatareductioninthecoolingsystemthermalresistancecouldleadtoenhancedheatfluxtotheTEGthus,improvingitstotalpoweroutput.TheeffectofthecoolingsystemontheperformanceofthehybridsystemwithdifferentPVcellsisshowninFig.28.Itisobvious
thatnaturalcooling(freecooling)isnotsuitableforconcentratedhybridsystemsduetoitsinferiorperformancecomparedtotheothercoolingmethods.Inaddition,Fig.28showsthatwatercoolingismoreeffectiveforhybridsystemsthannaturalcoolingandforcedair
cooling.Therefore,theauthorsconcludedthatwatercoolingisthemostsuitablecoolingmethodforhybridPV/TEGsystemsespeciallyhighlyconcentratedsystems.Inaddition,theyarguedthatcrystallinesiliconandpolycrystallinesiliconPVarenotsuitableforhybrid
PV/TEGsystemsbecauseoftheirreducedoverallefficiencycomparedtothePVonlysystem.
Adoptingasimilarresearchobjectiveas[99],Zhangetal.[100]carriedoutathermalresistanceanalysisofaconcentratedPV/TEGsystem.Athree-dimensionalnumericalsimulationwasusedtostudythetemperaturedistributionandpoweroutputofthehybrid
system.Inaddition, the influenceofcoolingsystem,thermalresistanceandconcentrationwerestudied.Waterandaircoolingwereappliedto thehybridsystemandtheperformanceof thesystemwasobserved. Itwasobservedthat the insertionofacopperplate
betweenthePVandTEcandecreasethethermalresistancebetweenthesystemsasthecopperplateimprovesthetemperatureuniformityandthisisinagreementwith[74].Furthermore,theauthorsarguedthatthenaturalconvectionandradiationdonotaffectthe
performanceofhighlyconcentratedPV/TEGsystems.Inaddition,itwasobservedthatadecreaseinthePVcellarealeadstoanincreaseinthehybridsystemefficiency.Finally,watercoolingwasobservedtobemoresuitableforhighlyconcentratedPV/TEGsystems
comparedtoaircooling.Thisfindingisinagreementwith[99].
Pangetal.[101]investigatedexperimentally,thesignificanceofheatsinksinahybridPV/TEGsystem.Twoheatsinkstructures(PinfinandPlatefin)wereinvestigatedtodeterminewhichonehadabettercoolingeffectonthehybridsystem.Also,thehybrid
Fig.27Differenttypesofthermoelectricgeneratorcoolingsystems[99].
alt-text:Fig.27
Fig.28Hybridsystemoverallefficiencyvariationwithconcentrationratiofor(a)crystallinesiliconcell,(b)polycrystallinesiliconcell,(c)amorphoussiliconcelland(d)polymercell[99].(Forinterpretationofthereferencestocolourinthisfigurelegend,thereaderisreferredtotheWebversionofthisarticle.)
alt-text:Fig.28
systemwithandwithoutheatsinkswerecomparedtoshowthesignificanceoftheheatsink.ResultsobtainedshowedthattheheatsinkwithnaturalconvectioncooledthePV/TEsystemby8.29 °Cwhichwas1.8 °CgreaterthanthatofthePVonlysystem.However,the
authorsarguedthattheintegrationofthermoelectricintoPV,amplifiedthefluctuationofthecoolingperformanceofthehybridsystemwithheatsink.
Comparedtotheconventionalcoolingmethodslikewaterandair,nanofluidwasproposedasamoreefficientcoolingmethodforhybridPV/TEGbyWuetal.[102].Theauthorsusedatheoreticalapproachtoinvestigatetheperformanceofglazedandunglazed
PV/TEGsystems.Theeffectof thermoelectric figureofmerit,concentrationratio,windvelocityandnanofluid flowratewerestudied. Itwasfoundthat theefficiencyofunglazedPV/TEGishigher thanthatof theglazedPV/TEGwhenfigureofmeritZ = 0.0021/K.In
addition,theauthorsarguedthatsomelargefigureofmeritvaluesmaynotimprovetheperformanceofthehybridsystem.SimilartotheobservationfromRef.[81],itwasfoundthattheoptimumloadresistanceformaximumTEGefficiencyandPV/TEGefficiencyare
different.Finally,theresultsobtainedshowedthatnanofluidcoolingenhancestheperformanceofPV/TEGsystemscomparedtowatercoolingespeciallyfortheglazedsystem(Fig.29).
Soltanietal.[103]investigatedanewnanofluid-basedcoolingsystemforenhancingtheperformanceofhybridPV/TEGsystemsandanexperimentalcomparisonwithconventionalcoolingsystemswaspresented.Fivedifferentcoolingmethodswereinvestigated
including,naturalcooling,forcedaircooling,watercooling,SiO2/waternanofluidcoolingandFe3O4/waternanofluidcooling.TheresultsobtainedintermsofefficiencyandpoweroutputenhancementareshowninTable3.Meanrelativeerror(MRE)statisticalerroranalysis
wasusedtocalculatetheperformanceenhancementrelativetothenaturalcoolingsystem.ItwasfoundthattheperformanceoftheTEGwasmainlyaffectedbythecoolingsystemwhilethePVcell'stemperaturewasalsoinfluencedbythecoolingsystem.Theauthors
arguedthatnanofluidcoolingperformedbetterespeciallySiO2/waternanofluidcoolingwhichenhancedtheefficiencyandpoweroutputofthehybridsystemby3355%and8.26%respectivelycomparedtothenaturalcooling.
5.6NewgenerationPVandTEmaterialsGuoetal.[104]comparedtheperformanceofafour-wireandtwo-wirehybridPV/TEGsystemwithdye-sensitizedsolarcell(DSSC).ThisstudyissimilartoRef.[92]whereafour-wireandtwo-wirePV/TEGsystemwerealsoinvestigated.Consideringthetwo-
wiresystem,theauthorsfoundthatpropermatchingtheTEandPVoutputcurrentscanenhancetheefficiencyofthehybridsystemby10%comparedtoasingledye-sensitizedsolarcell.Similarly,Wangetal.[105]integratedaseries-connecteddye-sensitizedsolarcell
withasolarselectiveabsorber(SSA)andathermoelectricgeneratorasaproofofconceptforahybridsystem.Thesystemobtainedanoverallconversionefficiencyof13.8%andmaximumpoweroutputof13.8 mW/cm2underasolarirradianceof100 mW/cm2andAir
Massof1.5G.Theauthorsarguedthattheconversionefficiencyofthehybridsystemcouldbeenhancedfurtherbyoptimizingthesystem.
Douetal.[106]fabricatedanovelphotoanodearchitecturebyintegratingaBi2Te3nanotubeswithZincoxide(ZnO)nanoparticles(Fig.30)toimprovetheperformanceofadye-sensitizedsolarcell.TheBi2Te3nanotubesweresynthesizedusingatwo-stepsolution
phasereactionanditwasobservedthatthenanotubesacceleratedtheelectrontransportprocessinphotoanode,increasedthelifespanoftheelectronsandenhancedtheelectroncollectionefficiency.Theauthorsfoundthehighestefficiencyofthenovelhybridsystem
composedofDSCCandBi2Te3(1.5 wt%)tobe4.27%whichwas44.3%higherthanthatofasingleZnOphotoanode.
Fig.29Variationofhybridsystemefficiencywithconcentrationratiofordifferentcoolingsystems[102].
alt-text:Fig.29
Increasingthesolarradiationabsorptivityofahybridsystemcansignificantlyenhanceitsefficiency.Raietal.[107]developedamethodologytoenhancetheperformanceofahybridPV/TEGsystembyusingafishnetmetamaterialstructurewhichcouldincrease
the solar radiationabsorptivity of thehybrid system.Aplanar fishnet structurewas included in theback-passivation layer of the solar cell for enhancedabsorptivity of radiations close to the infraredbandof the solar spectrum.The fishnet structure is capable of
accumulatingelectromagneticradiationbyusingitsrectangularcellsasaresonantcircuitanditwasfabricatedbye-beamlithographyassistedlift-offtechnique.Inaddition,nanoporousBi2Te3andSb2Te3particlesweresynthesizedandusedasthep-typeandn-type
thermoelectricmaterials respectively.Thefabricationprocess for the fishnetmetamaterialstructureusingnanoimprintinganddie transfer forp-typeandn-type thermoelectricmaterials isshown inFig.31.Resultsobtainedshowedthat thehybridsystemwith fishnet
structurehadanenhancedefficiencyaround11folds.
Fig.30SchematicdiagramofBi2Te3/ZnOcompositephotoanode(I,II)electrontransportprocess,(III,IV)energydiagramofsystemoperation[106].
alt-text:Fig.30
Fig.31Fabricationprocessforfishnetmetamaterialstructure[107].
alt-text:Fig.31
Adoptingthesamemethodas[107],Ohetal.[108] investigatedtheperformanceofahybridPV/TEGsystemwithfishnetstructure(Fig.32).Theauthorsarguedthat the fishnetgeometryandmaterialpropertiesare importantparameters thatdeterminethe
performanceofthesystemtherefore,optimizationofthefishnetstructurewascarriedoutusingparametricsimulation.Thepitch,widthandthicknessofthefishnetstructurewereoptimizedusingnumericalsimulationapproach.Aslightimprovementintheresultsobtained
fromRef.[107]wasobservedbytheauthors[108]astheoptimizedhybridsystemwithfishnetstructurehadanenhancedefficiencyaround12foldscomparedtothesystemwithoutfishnet.
Xuetal.[109]presentedauniquemethodforphotonmanagementinfullspectrumforPV/TEapplication.Thismethodisthecombinationofanti-reflectionandlight-trappingforultra-broadbandphotonmanagement.Theauthorsusedcrystallinesiliconthin-film
solarcellsandnanostructureswereusedonthefrontandbackside.FiniteDifferenceTimeDomainmethodwasusedforthenumericalsimulation.Anewcompositesurfacestructuremadeupofmoth-eyeandinvertedparabolicsurfacewhichcouldbeusedforanti-
reflection inultra-broadbandwavelengthswasproposedby theauthors(Fig.33).Resultsobtainedshowedthat theuseofanti-reflectionand light-trappingmethodcould improve theabsorptivityof thesolarcellwhilealso improving the transmissionofunusedsolar
radiationtothethermoelectricgenerator.Therefore,theperformanceofthehybridPV/TEGcouldbeenhanced.
Another researchonadvancedphotonmanagement forhybridPV/TEGsystemwascarriedoutbyZhangandXuan [110].Theabsorptanceofphotonswithwavelengthsof0.3–11 μmcapableofgeneratingelectron-holepairswasenhancedby theuseof
biomimeticparabolic-shapedstructureonthesiliconfilmandathinSiO2filmatthebottomofthesiliconfilmastheback-antireflectioncoating.Therefore,photonswithwavelengthsof1.1–2.5 μmcouldpassthroughthestructuredsurfacedandwereabsorbedbytheTEG
thus,theoverallefficiencyofthehybridsystemwasincreased.Finally,theauthorsarguedthattheuseofomnidirectionalbroadbandphotonmanagementcanenhancetheperformanceofhybridsystems.
Zhangetal.[111]performedafeasibilitystudyontheperformanceofahybridPV/TEGsystememployingperovskitesolarcells.Theinfluenceofthermalconcentration,solarselectiveabsorber(SSA)andopticalconcentrationratioontheperformanceofthe
hybridsystemwasstudiedusingthree-dimensionalnumericalsimulation.Theadvantageofusingtheperovskitesolarcellisitslargebandgapandlowertemperaturecoefficient.Resultsobtainedshowedthatthetemperaturecoefficientoftheperovskitesolarcellwas
lowerthan0.02/Kthus,theauthorsarguedthattheperovskitesolarcellisabetterchoiceforhybridPV/TEGsystemcomparedtosiliconsolaranddye-sensitizedsolarcell.Recently,Xuetal.[112]presentedanexperimentalstudyofaperovskitePV/TEGsystem.The
TEG(Bi2Te3)wasattachedtothecarbonsideoftheperovskitesolarcell(PSC)usingthermalsiliconpastewhilethePSCwasfabricatedusingthemesoscopicTiO2/ZrO2/carbonstructure.Resultsobtainedshowedthatthehybridsystemachievedamaximumpower
Fig.32Schematicdiagramofhybridstemwithfishnet.(a)3Dschematicofthinfilmsolarcell,(b)fishnetstructuralparametersand(c)topviewofsolarcell[108].
alt-text:Fig.32
Fig.33(a)Siliconsolarcellstructure,(b)topviewofsolarcellstructureand(c)cross-viewofbottomantireflectioncoating[109].(Forinterpretationofthereferencestocolourinthisfigurelegend,thereaderisreferredtotheWebversionofthisarticle.)
alt-text:Fig.33
outputof20.3%andopencircuitvoltageof1.29 Vunder100 mW/cm2irradiance.
TheuseofsolarcellandnanostructuredthermoelectricmaterialinhybridPV/TEGsystemwasinvestigatedbyRabarietal.[113].NanostructuredBismuthAntinomyTelluride(BiSbTe)thermoelectricmaterialwasusedanditspowergenerationcapabilitywas
comparedwiththetraditionalBiSbTe.Temperaturedependentnanostructuredthermoelectricmaterialpropertieswereconsideredandtheinfluenceofsolarradiationandconvectiveheattransfercoefficientonthehybridsystemperformancewerestudied.Theauthors
arguedthatthenanostructuredthermoelectricmaterialenhancedtheperformanceoftheTEG.
5.7MaximumpowertrackingandcontrolsystemOneofthedisadvantagesofthehybridPV/TEGsystemisthegreaterneedforaccuratemaximumpowerpointtracking(MPPT)systembecausethehybridconfigurationresultsinareducedfillfactorvaluethus,thePVonlysystemandthehybridsystemhave
differentmaximumpowerpointasshowninFig.34[114].Infact,Babuetal.[48]identifiedtheneedforeffectiveMPPTalgorithmasoneoftheresearchgapsinhybridPV/TEG.Maximumpowerpointtrackingissimplyanalgorithmusedinchargecontrollersforaccurately
determiningthemaximumavailablepowerfromaPVorTEGundercertainconditions.Voltageatwhichmaximumpowerisproducediscalledmaximumpowerpointorpeakpowervoltage.Factorsuchassolarradiation,ambienttemperatureandoperatingtemperature
influencethemaximumpowerpoint.
Zhanget al. [115] presented a hybridPV/TEG systemwith aMPPT control strategywhich enabled themaximumpower output of thePV andTEG to be obtained simultaneously. The developed system could be used in hybrid electric vehicles and an
experimentalvalidationoftheproposedsystemwasdone.ThedevelopedsystemconsistedofPV,TEG,powerconditioningcircuit,digitalsignalprocessor(DSP)controllerandabatteryservingastheload.Thepowerconditioningcircuitwasusedtotrackthemaximum
powerpointofthehybridsystembytuningthedutycycleofthepulsewidthmodulator(PWM)switchingsignaltoensuretheinputresistancewasequaltotheinternalresistanceofthehybridsystem.Inthisstudy,thesingle-endedprimary-inductorconverter(SEPIC)
circuitwasused.
Animprovedresearch[116]wascarriedoutbythesameauthorsinRef.[115]usingasimilarapproach.ThehybridPV/TEGsystem(Fig.35)wasdevelopedforuseinautomobilesandapowerconditioningcircuitusingMPPTwasdevelopedforenhancedhybrid
systempoweroutput.Ćuk–Ćukmultipleinputconvert(MIC)wasimplementedonthehybridsystembecauseitcouldoffernon-pulsatinginputandoutputcurrentswhichcouldreducethelevelofdisturbanceonthesystemoperationsignificantlythus,increasingthebattery
lifespan.TheauthorsstatedthatthedifferencebetweentheĆuk–ĆukMICandtheSEPIC-SEPICMICisthattheformerneedsonlythreeinductorswhilethelaterneedsfourinductorsthus,Ćuk–ĆukMICoffersalowercost,lighterweightandsmallervolume.Results
obtainedshowedthattheoverallhybridsystempoweroutputwasenhancedbytheĆuk–ĆukMICusingtheMPPT.
Fig.34PV/TEGandPVcurrent-voltagecharacteristics[114].
alt-text:Fig.34
Fig.35HybridPV/TEGsystemwiththeĆuk–ĆukMIC[116].
AdoptingasimilarapproachtoRef.[116],Fathimaetal.[117]presentedtheuseofĆukconverterstoenhancetheperformanceofhybridPV/TEGsystemandreducethesystemnoise/disturbance.PulsegeneratorcircuitwereusedindesigningtheMPPTsystem
andamicrocontrollerwasusedtochangethedutycycleoftheĆukconverter.ResultsobtainedshowedthatthedesignedMPPTsystemusingĆukconvertercouldfunctioninBoost,Buck-BoostorBuckmodetherefore,thehybridsystempoweroutputcouldbeenhanced.
Jungetal.[118]presentedtheuseofasingle-inductor,dual-inputanddual-outputboostconvertwithanoveltime-multiplexingMPPTalgorithmtoenhancetheperformanceofahybridPV/TEGsystem.Theauthorsarguedthatthedevelopedtime-multiplexing
MPPTalgorithmenabledtheboostconvertertoachieveMPPTforthehybridsystemusingasingleclockfrequency.Therefore,thehybridsystemefficiencywasincreased,andtheboostconvertedwasdesignedusing65 nmcomplementarymetal-oxide-semiconductor
(CMOS)technology.Finally,apeakefficiencyof78%wasobtainedbythehybridsystem.
Vermaetal.[119]developedadynamicmodelinMATLAB/SimulinkenvironmenttostudytheperformanceofahybridPV/TEGsystem(Fig.36).TheeffectivenessofMPPTsystemundervaryingloadandsolarradiationwasstudiedanditwasfoundthatthe
controlschemeusedinthisstudyensuredoptimumhybridsystemvalueswereobtainedwhilepreventingthePVandTEGfromoperatingattemperaturesabovetheprescribedlimits.ADC-DCboostconverterwasusedtoensuretheeffectiveoftheMPPTsystem.In
addition,thehybridsystemwascontrolledusingamaster-slaveconfigurationwherethebatterysourceoperatedastheMasterforboththePVandTEG.Resultsobtainedshowedthattheusedmaster-slaveconfigurationensuredthesmoothoperationofthehybrid
system.
KwanandWu[120]presentedtheapplicationofLock-OnmechanismMPPTalgorithmtohybridPV/TEGsystems.AdetailedexperimentalandnumericalinvestigationwascarriedoutonthecapabilityofLock-OnMechanism(LOM)algorithmtoachievemaximum
powerpointtrackingforhybridPV/TEGsystem.TheLOMalgorithmwasappliedtoadoubleSEPICconverterandthenumericalsimulationwasperformedusingtheMATLAB/Simulinkenvironment(Fig.37).ThedoubleSEPICconverterusedwasmadeupoftwosingle
SEPICconvertersandtheiroutputcapacitorswereconnectedinparalleltogether.ResultsobtainedfromtheexperimentalandnumericalinvestigationcarriedoutshowedthattheLock-OnMechanismMPPTalgorithmperformsbetterthantheconventionalhillclimbing
algorithm.
alt-text:Fig.35
Fig.36ProposedhybridsystemwithMPPTforwasteheatrecovery[119].
alt-text:Fig.36
5.8NicheapplicationsYuetal.[121]investigatedaPV/TEGforpoweringwirelesssensornetworks.ThedevelopedsystemconsistedofaPV,TEGandheatsinkwithaircooling.ComparedtoasinglePV,thePVinthehybridsystemhadanefficiencyincreaseof5.2%duetoasolar
celltemperaturereductionof13 °C.Energystoragedeviceswereincorporatedintothehybridsystemtostoreenergyforuseduringperiodsoflowsolarirradiance.Alithiumionbatterywithstoragecapacityof1400 mAhandanultra-capacitorwithstoragecapacityof30 F
wereusedtostoreenergyfromthePVandTEGrespectively.Thedevelopedhybridsystemhadthecapacitytorenewenergybyitselfthusitcouldprovidereliableandlong-timepowertothesensornode.
Leonovetal.[122]investigatedtheuseofaPV/TEGtopoweranautonomousmedicaldevice:electroencephalography(EEG)inashirt.ThedevicewasbatteryfreeandthePVwaspositionedabouttheradiatorsusedtoheatuptheTEG(Fig.38).Theauthors
developedanultralowpowerbiopotential readout integratedcircuitwhichhadapowerconsumptionof60 perchannel.Thesignalqualityprovidedby the readoutwas thesamewith thatofmodernambulatorysystemsand thedevelopedsystemhadanextra
advantageofbeingwirelesscomparedtowiredcommercialsystemsthus,thebiopotentialsignalscouldbetransmittedtoadoctorinrealtime.
Fig.37HybridsystemwithMPPTSimulinkmodelusinghillclimbingalgorithm[120].
alt-text:Fig.37
μW
ThefeasibilityofPV/TEGforterrestrialandspaceapplicationswasinvestigatedbyDaetal.[123].AsimilarapproachusedbyRef.[109]wasemployedinthisstudywhichistheuseofphotonandthermalmanagementtoenhancetheperformanceofthehybrid
PV/TEGsystem.Theauthorsusedbio-inspiredmoth-eyenanostructuredsurfacetosuppressthefullsolarspectrumphotonsreflectionandanenhancedtransmissionfilmwasusedtoimprovethesystemperformance.ResultsobtainedshowedthatforhybridPV/TEG
systems, lowconcentration ratio isbetterespeciallywhenused in terrestrialandspaceapplications. Inaddition, the resultsshowed that thehybridsystemwithoutopticalconcentrationperformedbetter than thePVonlysystem. Itwasalso found that for terrestrial
application(correspondingtoAirMass1.5),theoverallhybridsystemefficiencyincreasedfrom13.79to18.51%duetotheuseofthemoth-eyestructuredsurface.Whileforspaceapplication(AirMass0),theuseofthemoth-eyestructuredinthehybridsystemincreased
theefficiencyto16.84%(Fig.39).
Kwanetal.[124]studiedtheperformanceofahybridPV/TEGsystemforouterspaceapplication.Amulti-objectiveNon-dominatedSortingGeneticAlgorithm(NSGA-II)wasusedtooptimizethethermoelectricgeneratordesignforobtainingmaximumpower
output.Inaddition,theauthorscomparedtheperformanceofasinglestageandtwostagethermoelectricgenerator.Resultsobtainedfromthisstudyshowedthatforspaceapplications,thepowergenerationcontributionofthethermoelectricgeneratorinahybridPV/TEG
systemisnegligible.Furthermore,itwasobservedthatsinglestageTEGisforhybridPV/TEGsystemscomparedtotwostageTEG.Finally,theauthorsalsoarguedthattheoptimizedPV/TEGsystemhadalowerefficiencycomparedtothePVonlysystem.Thisfindingis
inagreementwithothersimilarfindingslike[83,84].
ConsideringtheactualweatherconditionsofthreedifferentsamplecitiesinEurope,Rezaniaetal.[125]developedamodeltopredicttheperformanceofhybridPV/TEGsystemundersuchconditions.Theinfluenceofsolar irradiation,windspeed,ambient
temperature,convectiveandradiatedheatlosseswereallaccountedforinthemodel.ItwasfoundthatradiatedheatlossonthesurfaceofthePVandconvectiveheatlossduetowindspeedarethemostimportantparametersinfluencingthehybridsystemperformance.
Comparingtheperformanceofthehybridsysteminthesamplelocations,itwasfoundthatmaximumpoweroutputwasobtainedduringspringperiodinNorthernEuropewhileforSouthernEurope,themaximumpoweroutputwasobtainedduringsummerperiod.
Ariffinetal.[126]presentedaconceptualdesignofahybridPV/TEGsystemforapplicationinanautomatedgreenhousesystemproject(Fig.40).ThedevelopedsystemwascomparedtoconventionalPVonlysystemandtheauthorsprovidedrecommendations
forfutureworks.Theyrecommendedtheuseofautomatedsemi-transparentthinfilmsolarpaneltoefficientlyabsorbsolarradiations.AnexperimentalandnumericalinvestigationwascarriedoutandtheeffectivenessofusinghybridPV/TEGinanautomatedgreenhouse
systemwasdemonstrated.
Fig.38(a)Electroencephalographydiadem:(1)right-sidehybridmodule,and(2)electronicmoduleand2.4 GHzwirelesslink.(b)Schematiccross-sectionofhybridmodule:(1)thermophile,(2)radiator,(3)PVcells,and(4)thermalshunts[122].
alt-text:Fig.38
Fig.39Performancecomparisonbetweenmoth-eyestructuredsurfaceandplanarsurface.(a)Reflectionspectrum,(b)solarcellcharacteristicsunderAM1.5,(c)solarcellcharacteristicsunderAM0and(d)hybridsystemefficiency[123].
alt-text:Fig.39
6DiscussionandrecommendationsPassivecoolingofphotovoltaic-thermoelectricgeneratorisaninterestingresearchareabeingexploredduetotheeffectivenessofpassivecoolingdeviceslikeheatpipeinsignificantlyreducingthetemperatureofphotovoltaiccells.Heatpipesareefficientheat
transferdevicesthatcantransportheatoveralongdistancewithsmalltemperaturegradient[127].Therefore,theuseofheatpipesinahybridPV/TEGsystemcouldreducethequantityofTEGusedinthethermalmanagementofphotovoltaiccellswhilealsoprovidingan
enhancedoverallperformance[128].FlatplatemicrochannelheatpipesaremoreefficientthancylindricalheatpipebecauseofthereducedthermalcontactresistancebetweenthesurfaceofthePVandheatpipeduetotheshapeoftheheatpipe.Therefore,more
researchontheintegrationofhybridPV/TEGwithflatplatemicrochannelheatpipesarestronglyrecommendedespeciallybecauseoftheencouragingresultsreportedfromsuchheatpipehybridsystemsbyRefs.[127,128].
Severalresearchers[9,58,59,91]haveagreedthattheperformanceofathermoelectricgeneratorinahybridsystemcanbeenhancedbymaterialandstructuraloptimization.Alotofmaterialoptimizationeffortsarebeingcarriedoutonimprovingtheefficiencyof
thePVandTE.SomeofthisresearchhaveobtainedsignificantresultsaspresentedinSection5.6.ImprovingthethermoelectricfigureofmeritisthemajorresearchtaskforincreasingtheefficiencyoftheTE.A50%increaseinhybridsystemefficiencycouldbeachieved
simplybyusingTEGwithhigherfigureofmeritcomparedtothecurrentlyavailableones[55].Theuseofnanostructuredmaterialshasalsobeengainingmomentumrecentlyduetothegoodresultsobtainedthus,moreresearchisencouragedonhybridPV/TEGsystems
withnanostructurematerials.Inaddition,moreresearchisencouragedintheareaofPVsurfaceabsorptivity.IncreasingtheabsorptivityofPVcouldsignificantlyincreaseitsefficiencythus,moreresearchonphotonmanagementofhybridPV/TEGishighlyrecommended.
Furthermore,theoptimizationoftheTEstructureisnecessarytoenhancetheperformanceofthehybridsystem.Effortsmadeonthestructural/geometryoptimizationoftheTEhavebeenpresentedinSection5.3.InspiteoftheplethoraofresearchavailableonTEG
optimization,itsintegrationwithPVnecessitatesnewinvestigationsbemadeduetothecomplexrelationshipbetweenthePVandTEG.Whileonerequiresmoretemperatureforhigherperformance(TEG),theothersystem(PV)preferstheopposite.Thus,moreresearch
ontheefficientcouplingofPV/TEGisneededespeciallyconsideringthethermalcontactresistanceinthehybridsystem.Inaddition,alosslesscouplingwouldbeverygoodforthehybridsystemperformanceanditisveryimportanttorememberthatresultsobtainedfrom
TEGonlyoptimizationisnotsufficientforthehybridsystemoptimization.Therefore,theoptimizedloadresistanceandgeometryoftheTEGinaTEGonlysystemisdifferentfromtheoneinahybridsystemduetotheinfluenceofthePV.
Onlya fewresearchhasbeenconductedonspectrumsplittingPV/TEG(Table4)however,sufficientworkshavebeendoneondirectcouplingPV/TEGsystem(Table5). It is therefore recommended thatmoreattentionbepaid tospectrumsplittinghybrid
systemsduetotheirpotentiallyhighperformancewhenproperlyoptimized.Furthermore,CoolingisanintegralpartofanyTEGsystemasitsdirectlyaffectsthesystemperformancesignificantly.Therefore,thehybridPV/TEGsystemneedsefficientcoolingsystems
capableofcreatinga largertemperaturedifferenceacrosstheTEwhilealsoreducingthetemperatureof thePV.Fromthedetailedreviewcarriedout inSection5.4, theconsensus is thatwatercooling ismoreeffectivethanaircooling.However, the introductionof
nanofluidcoolingintohybridPV/TEGsystemshasresultedinsignificantlylowertemperatureontheTEGcoldsidecomparedtowatercoolingtherefore,moreresearchonPV/TEGusingnanofluidissuggested.However,theextracostofnanofluidmustbetakeninto
considerationandajustificationmustbemadeintermsofoverallperformancecomparedtohybridsystemswithcheapconventionalcooling.
Duetotheintermittentnatureofsolarenergy,storagesystemshavebeenincorporatedintothehybridPV/TEGtostoreenergyforuseduringperiodsoflowirradiance.Theuseofphasechangematerial(PCM)seemstobethebestoptionduetoitsunique
capabilitytostoreasignificantamountofheatandthusmitigatethetemperaturefluctuationsinthehybridsystem.MoreresearchonthehybridsystemswithPCMissuggestedhowever,againtheextracostmustbeconsidered.Alimitingfactortotheenhancementof
hybridsystemperformanceistheneedforopportunitycostanalysis.Whilethereareobviouswaystoeasilyimprovetheperformanceofthesystem,atrade-offmustbemadeduetothehighcostofsuchoptimization.
Theuseofconcentratedsolarenergyisaneasywaytoimprovethehybridsystemefficiencyhowever,caremustbetakennottodamagethePVbyoverapplyinghighconcentration.ItiswidelyknownthattheperformanceofthePVreduceswithtemperature
Fig.40ConceptualdesignofahybridPV/TEGforautomatedgreenhousesystem[126].
alt-text:Fig.40
however,whenhighconcentrationisproperlyapplied,theoverallperformanceofthesystemcouldbeincreased.Thus,thereisaneedtoproperlydeterminetheconcentrationratioforoptimizedperformance.Someresearchhasbeenconductedusingtheconceptof
changingthepositionoftheconcentratorinthehybridsystem.RatherthanapplyingthehighconcentrationonthePVsurfacetraditionally, it isappliedtotheTEsurfaceinstead.Thisway,thePVcanoperateatalowtemperaturewhiletheTEcanoperateatahigh
temperature.Notwithstanding,thistechniqueseemstoplaceemphasisontheuseofTEGforadditionalenergygenerationhowever,itiswidelyknownthatthePVcontributesmoretotheoverallhybridsystemperformance(poweroutputandefficiency)thus,itmightbe
worthconsideringplacingmoreemphasisonoptimizingthePVforhighperformancebyusingtheTEGmorehasacoolingdevicethanapowergenerator.TheTEGisaneffectivewasteheatrecoverydevicethus,reducingthetemperatureofthePVshouldbethefocusof
hybridPV/TEGsystemoptimization.
Aprofusionof literatureexistson thesteadystateperformanceofhybridPV/TEGsystemshowever, theactualperformanceof thehybridsystemisaffectedby thedailyvariations inweathercondition.Thus,moreresearch isneededon thehybridsystem
performance under transient conditions. In addition, very few researchworks on hybridPV/TEG systems have been conductedwith the use of finite elementmethod (FEM).Contrarily, there is an abundance of research on the one-dimensional simulation using
MATLAB/Simulink.TheadvantageofusingFEMis that, itcanbeused for three-dimensionalstudyof theactualsystem.Thus, itprovidesmorerealistic resultsandbetteroptimizationeffortscanbemadeusing thismethod.FiniteelementmethodhasMultiphysics
capabilitythusitishighlysuggestedfordeepresearchonhybridPV/TEG.Inaddition,FEMallowstheThomsoneffectsandtemperaturedependentthermoelectricpropertiestobeeasilycoupledanditprovidesauser-friendlyinterfaceforeasilyvisualizationofresults[91].
Moreresearchon thermophotovoltaic/thermoelectric (TPV/TE)systems is recommendeddue to itsuniqueadvantages.Thermophotovoltaic (TPV)cellsarecapableofconverting infraredradiation thus,whenthermalemitterareusedas thesource forTPV
systems,theycanoperatealldayastheywouldn'tbeaffectedbytheintermittentnatureofthesolarenergy[129].Recently,afirstofitskindstudyonhybridTPV/TEwasconductedbyRef.[130].ResultsobtainedshowedthattheTPV/TEsystemperformedbetterthanthe
TEandTPVonlysystems.MoreresearchonsuchsystemsishighlyrecommendedasanalternativetotheconventionalhybridPV/TEGsystems.Furthermore,moreresearchonthecombinationofTEG,TECandPVisrecommendedduetotheencouragingresults
obtainedbyRefs.[131,132].Thebasicideaistocombinetheadvantagesofeachindividualsystemsandobtainabetterperforminghybridsystem.Insuchsystems,thethermoelectriccoolerisusedtocoolthePVwhiletheTEGisusedastheheatsinkfortheTEC,thus
theoverallperformanceofthehybridsystemisincreased,andadditionalenergyisgenerated.However,moreresearchneedstobeconductedonthefeasibilityofsuchsystemsasitcouldpotentiallyprovideabetterperformancecomparedtothehybridPV/TEGsystems.
7ConclusionOwingtothefastrateatwhichthefieldofPV/TEGisgrowingandthenumeroussignificantresearchbeingcarried,thisreviewwaswrittentopresentanddiscussthestateofartinthefieldofPV/TEG.Thermoelectricgeneratorsofferuniqueadvantageswhich
whencombinedwiththephotovoltaiccanresultinanenhancedhybridsystemperformanceandwiderutilizationofthesolarspectrum.PV/TEGoffersanalternativetotheverywellresearchedandwidelyusedPV/Tsystems.Thesameideaisusedinbothsystemswhich
istoenhancetheperformanceofthePVbyreducingitstemperatureandproducingadditionalenergy.ThisreviewpresentedadetailedoverviewofallresearchareasandoptimizationeffortsrelatingtohybridPV/TEG.Theuseofconcentratedsolarenergyforhybrid
PV/TEGwasdiscussed,keyfocusareasinthehybridsystemresearchsuchas:TEGgeometryoptimization,Energystorage,TEGcooling,PVandTEmaterialoptimizationwerealldiscussedindetail.NicheapplicationofPV/TEGwerealsopresentedtoshowitswide
applicabilityinvariousfieldsandnotjustelectricitygeneration.ThesignificanceofmaximumpowerpointtrackinginhybridPV/TEGwasalsodiscussedandasummaryofthemostrecentlypublishedworksinhybridPV/TEGwaspresented.Finally,adeepdiscussionofall
theresultsobtainedfromtheextensionliteraturesreviewedwaspresentedandmoreimportantly,recommendationsforfutureworkswereprovided.AthoroughinvestigationofhybridPV/TEGsystemshasbeenperformedinthisresearchandit isenvisagedthatthis
reviewwouldserveasanindispensableliteratureonhybridPV/TEG.
AcknowledgementThisstudywassponsoredbytheProjectofEUMarieCurieInternationalincomingFellowshipsProgram(745614).TheauthorswouldalsoliketoexpressourappreciationforthefinancialsupportsfromEPSRC(EP/R004684/1)andInnovateUK(TSB
70507-481546)fortheNewtonFund-China-UKResearchandInnovationBridgesCompetition2015Project‘AHighEfficiency,LowCostandBuildingIntegrate-ableSolarPhotovoltaic/Thermal(PV/T)systemforSpaceHeating,HotWaterandPower
Supply’andDongGuanInnovationResearchTeamProgram(No.2014607101008).
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Highlights
• Adetailedreviewofphotovoltaicthermalmanagementispresented.
• Theroleofthermoelectricgeneratorinhybridphotovoltaicenhancementisexplained.
• Researchfocusareasinhybridphotovoltaic/thermoelectricgeneratorarediscussed.
• RecommendationsforfutureresearchonhybridPV/TEGarepresented.
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