Advancements in thermoelectric generators for enhanced

Nomenclature

Fillfactor

Temperature,K

Area,m

Advancementsinthermoelectricgeneratorsforenhancedhybridphotovoltaicsystemperformance

SamsonShittua

GuiqiangLia,∗

Guiqiang.Li@hull.ac.uk

YousefGolizadehAkhlaghia

XiaoliMaa

XudongZhaoa,∗∗

Xudong.Zhao@hull.ac.uk

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.

Concentrationratio

Solarirradiance,W/m2

Opencircuitvoltage,V

Shortcircuitcurrent,A

Incidentpower,W

Figureofmerit

Greeksymbols

Seebeckcoefficient,V/K

Thermalconductivity,W/m/K

Electricalconductivity,S/m

Efficiency,%

Abbreviations

Photovoltaic

Thermoelectric

PV/TEG

Photovoltaic/thermoelectricgenerator

TPV/TEG

Thermophotovoltaic/thermoelectricgenerator

Cadmiumtelluride

Copperindiumgalliumselenide

Dye-sensitizedsolarcell

Galliumarsenide

Galliumantimonide

Copperantimony

Indiumphosphide

Bi2Te3

Bismuthtelluride

Leadtelluride

Germaniumtelluride

Tintelluride

Current-Voltage

Thermoelectricgenerator

Amorphoussilicon

Crystallinesilicon

Copperzinctinsulfide

Phasechangematerial

Solarselectiveabsorber

Perovskitesolarcell

Electroencephalography

AirMass

Lock-Onmechanism

Subscripts

Hotside

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].

D Tab q εG k A

Rs Ns Np Iph

(Isc) Vg=0 Isc=Iph (Voc) Ig=0

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

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

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

I R TH TC

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

Ppv Pteg Apv Pin

ηopt ηot ηdiss

C Aopt τopt τopt

Pdiss ηdiss=1

Th=Tpv

Tpv=Tenc

αotconv τenc τhm ε′pv Aabs

Ta Rpv

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.

Rhm(λ)

ηpv ηteg ηot

ηpv ηteg

Fig.6Variationofhybridsystemandindividualcomponentsefficiencywithtemperature[11].

alt-text:Fig.6

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

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

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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