1

Experimentalstudyofphasechangematerialsforphotovoltaicmodules:

energyperformanceandeconomicyieldfortheEPEXspotmarket

EwaldJapsa,GerritSonnenreinb,StefanKrautera,*,JadranVrabecb

aUniversityofPaderborn,ElectricalEnergyTechnology–SustainableEnergyConcepts,

33098Paderborn,Germany,Ewald.Japs@upb.de,1Stefan.Krauter@upb.de

bUniversityofPaderborn,ThermodynamicsandEnergyTechnology,33098Paderborn,

Germany,Gerrit.Sonnenrein@upb.de,Jadran.Vrabec@upb.de

Keywords

phasechangematerial,electricityspotmarket,photovoltaicmodule,coolingmeasure

Abstract

Coolingofphotovoltaic(PV)devicesincreasesvoltageandpoweroutput,but instand‐

ardapplications,coolingmeasuresareonlybeneficial if theassociatedcostsare lower

thanthecumulativeprofit.Atechnicalandeconomicanalysisofapassivecoolingmeas‐

urebasedonphasechangematerials(PCMs)isconductedhere.ThreePVmodules,one

standard referencemodule and two equippedwith PCMs, are studied experimentally.

Althoughbothhavethesamemeltingtemperature,oneofthePCMshasasignificantly

higherthermalconductivityandalowerheatstoragecapacitythantheother.Theanaly‐

sisofthepresentexperimentaldataconsiderstheenergypricevariationattheEurope‐

anPowerExchange(EPEX)spotmarketduringthedaywithoutconsideringanycosts.

Becauseadditionalpower is suppliedbeforenoon forPCMcharging, favorable results

are observed during this period. However, higher operating temperatures of the PV

modulesoccurlaterinthedayduetothethermalinsulationeffectofthePCMlayerat‐

tachedtothebacksideofthemodules.Intotal,thisresultsinanegativeeconomicyield

onmostdays.ThePCMwithahigherthermalconductivityhadsignificantlylowertem‐

peraturesafterchargingandacorrespondinghigheryield.

*Correspondingauthor

2

Nomenclature

Symbols

cp Averagespecificisobaricheatcapacity kJ/(kg∙K)

Energyyield Wh

e Relativeenergyyielddifference %

Specificenthalpyoffusion(20°C‐30°C) kJ/kg

Electricalpoweroutput Wp

Energyprice €/kWh

Time s

Y Economicyield €

y Relativeeconomicyielddifference %

TemperaturecoefficientofPmax %/K

Conversionefficiency %

Temperature °C

Thermalconductivity W/(m∙K)

Specificdensity kg/m³

  Massfraction g/g

∆ Difference

Index

Basedonadailytradingperiod

EquippedwithPCMorPCM+

Countingvariable

15minor1htimeblockenergyprice

Meltingpoint

Maximumpoint

Summationlimit

Reference

Unequipped(noPCMorPCM+)

Abbreviation

EPEX EuropeanPowerExchange

ISFH InstituteforSolarEnergyResearchHamelin

MPP Maximumpowerpoint

PV Photovoltaic

PCM Phasechangematerial

PCM+ Phasechangematerialwithimprovedthermalconductivity

3

STC Standardtestconditions

1. Introduction

Thenegativeeffectofelevatedoperating temperatureson theconversionefficiencyof

crystallinesiliconsolarcellsiswellknown(Radziemska,2006).Interventionstoreduce

operating temperatures are usually considered to increase the electrical energy yield.

Lowoperatingtemperaturesofphotovoltaic(PV)modulesalsohaveapositiveeffecton

degradation(MeyerandvanDyk,2004;JunsangsriandLombardi,2010).Furthermore,a

damping of short‐term temperature fluctuations may increase PV module lifetimes

(Köntgesetal.,2014).

Bothactivecoolingmeasures,suchaswatercoolingonthemoduleback(Bahaidarahet

al., 2013; Moharram et al., 2013) or front side (Krauter, 2004), and passive cooling

measuresadaptedfromlatentheatstorageconsistingofselectedphasechangemateri‐

als(PCMs)havebeeninvestigated(Nortonetal.,2011;Hasanetal.,2014).Thescientific

interestinusingPCMforthethermalmanagementofPVmoduleshasincreasedrapidly

overthelastdecade.Numerousexperimentalandcomputationalstudieshavebeencon‐

ductedfortheuseofPCMtomanagethetemperatureissuesofelectronicdevicessuch

as PVmodules (Browne et al., 2015).Meanwhile, it iswell known that the operating

temperatureofPVmodulescanbedecreasedsignificantlyduetothemeltingoftheat‐

tachedPCM.However,athightemperatures,PCMlayersmaycauseunwantedandsig‐

nificantthermalinsulationduetotheirtypicallylowthermalconductivity.Onepossibil‐

ityofcombatingthisproblemis tomixthePCMwithexpandedgraphite(Mehlingand

Cabeza,2008).

The first investigationinto integratingPCMwithaPVmodulewasconducted in1978;

thisstudyshowedthatthebeneficialcoolingeffectofPCMcanbeenhancedbyincreas‐

ingitsthermalconductivityandincreasingtheheattransferfromthePVmoduletothe

PCMatthethermalinterface(StultzandWen,1977).Recently,oneofthemainresearch

objectivesofHuangetal.(Huangetal.,2011)wastopromotetheheattransferintoand

out of PCM using finswithin the aluminum container encapsulation. They also devel‐

opedavalidatednumericalmodelforaPV‐PCMmodule.Hasanetal.(Hasanetal.,2015)

comparedtheeffectsoftwodifferentPCMsencapsulatedinanaluminumcontainerwith

internalfinsfortwodifferentclimateconditions(Dublin,IrelandandVehari,Pakistan)

usingoutdoormeasurementsandsimulationsusingthenumericalmodel from(Huang

etal.,2011).Twomainconclusionsweredrawn:First,thedeviationbetweenthesimula‐

4

tionandexperimentresultswasvery low in termsof theaveragetemperaturesof the

front surfaceofbothPV‐PCMmodules. Second, thehighest temperaturedropwasap‐

proximately21K compared to the referencePVmodulewasobserved inVehari fora

PV‐PCMmodulebasedonasalthydrate.

Laboratoryexperimentscombinedwithacomputationalstudywereconductedby(Jay

et al., 2010).TwoPV‐PCMsystems, aPVmodulewitha thermally insulatedback side

anda referencePVmodule,weresimultaneouslyexposed to threedifferent insolation

intensities(600,800and1000W/m²)usingasolarsimulator.Bothparaffin‐basedPCMs

(withmeltingtemperaturesof27°Cand45°C)werefilledintoahoneycombaluminum

structure,whichwasclosedonbothsidesbyanaluminumplatetopromoteheattrans‐

ferfromthePVmodule.A15‐25%increaseinenergyyieldcomparedtothereference

PVmoduleduetotemperatureregulationwasmeasured.

The combination of PCM‐infused graphite and finnedheat sinks for the thermalman‐

agementofPVmodules(AtkinandFarid,2015)achieveda13%increaseinenergyyield

throughreducedpeaktemperaturesandatemporarytimeshiftinthetemperaturerise.

In thepresentwork,acommercialparaffinRUBITHERM®RT28HCwithan improved

thermalconductivityof=2.4W/(m·K)andthesamePCMcompoundwiththestand‐ardthermalconductivityof=0.19W/(m·K)werestudied.Thehighthermalconductiv‐itywasachievedbyaddingexpandedgraphitetothePCMcompound.Althoughtheim‐

proved PCM (hereafter referred to as PCM+) has a reduced heat storage capacity be‐

causeof adecreasedmass fractionof theeffectivephase changematerial, itpromises

better performance and applicability. Therefore, one PV module was equipped with

PCM+ and another PV module was equipped with the conventional PCM; both were

comparedwith a standard referencePVmodule, simultaneouslymeasured at outdoor

summerconditionsduringtheyear2013inPaderborn,Germany.Atechnicalcompari‐

sonofthetwoPV‐PCMmoduleswasconductedtoassessthetemperaturedevelopment

andenergyyield.

Ingeneral,duetotherapiddecreaseofPVmodulecosts,coolinginterventionsareoften

less cost‐effective in termsofdirectpowergain.On theotherhand, the typicalpower

generationshifttothemorninghourswithPV‐PCMmodulesmaynonethelessbefavor‐

able, considering thehigherelectricitypricesbeforenoonon theEuropeanPowerEx‐

change(EPEX)spotmarket,cf.Fig.1.Inthisinvestigation,nocostswereconsidered,and

theeconomicanalysisfocusedexclusivelyontheyielddifferencesduetotheuseofPCM

andPCM+.

Fig.1 P

datafor

dayasm

2. M

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dulesunder

PCM

26.6

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edinFig.3,

andard tes

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nd2.

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28

rSTC.

PCM+

28.8

12.4

8

indicat‐

t condi‐

vantpa‐

Units

kJ/kg

/(kg∙K)

g/m³

/(m∙K)

°C

Units

Wp

%

9

TemperaturecoefficientofPmax ‐0.41 ‐0.40 ‐0.42 %/K

Significantdifferences in the temperature coefficientof thepoweroutput and conver‐

sion efficiency were detected among the three PV modules, so the measured energy

yield of the PV‐PCM modules cannot be directly compared. Therefore, the following

basicassumptionsweremadetoensureasatisfactorycomparabilityoftheresults:

AllthreePVmoduleshavethesameoperatingtemperatureatidenticaloperating

conditions.

Differencesbetweenthemeasuredtemperaturevaluesarecausedexclusivelyby

theattachedPCMorPCM+bags.

The temperaturevaluesmeasuredby the thermometersRef, PCMIn andPCM+In

representtheoperatingtemperatureofthecorrespondingmodule.

Themeasured temperatureof theRef thermometerwasusedasa referencevalue for

comparison.Thus,themeasuredelectricalpoweroutputofthePCMmoduleaswellas

thatof thePCM+modulewasadjustedaccording to themeasured temperatureof the

RefthermometerandthetemperaturecoefficientofthePCMandPCM+modulesgiven

inTable2.AdetaileddescriptionoftheseadjustmentsisgiveninSection3.2.

3. Energyperformanceandeconomicyield

3.1. InfluenceofPCMontemperature

DuetothestronginfluenceofthePCMonthetemperatureofthePVmodules,adetailed

analysisofthemeasureddiurnaltemperaturevariationsisnecessary.Forthispurpose,

twodayswithhighinsolationandveryclearskyconditionswereselected.Bothcondi‐

tionswerefulfilledonthe1stand2ndofAugust2013.Thesurfacetemperaturesmeas‐

uredonthesedaysareshowninFig.5asafunctionoftime.

Fig.5 S

tom)su

Fig.5sh

tionsw

the ope

Surfacetem

urfacetemp

howsthat

with signific

erating tem

mperatures

peratureso

themodule

cantly lowe

mperatures

softherefe

ofthePCM

esequippe

er fluctuati

decreased

erencemod

andPCM+

edwithPCM

ions than t

d during th

duleandth

moduleso

MandPCM

the referen

hemelting

heinner(to

nAugust1

M+exhibitt

ncePVmo

of the PCM

op)andout

1stand2nd,2

temperatur

dule.As ex

M, as indic

10

ter(bot‐

2013.

revaria‐

xpected,

cated by

11

“charging”inFig.5.Coincidentally,themeltingprocessesofPCMandPCM+terminated

atapproximatelythesametime.Aftercompletemelting,thePCMtemperatureincreased

more rapidly than that of PCM+due to thehigher thermal resistance to heat transfer

through thePCMpackage; both temperatures increased above the temperatureof the

reference PV module. During the saturation stage, a smaller difference occurred be‐

tweenPCM+InandthePCM+Outsurfacetemperaturescomparedtothetemperaturedif‐

ferencebetweenPCMInandPCMOut, cf. Fig.5. Inaddition, it canbe concluded that the

increasedthermalconductivityofPCM+leadstosignificantlyloweroperatingtempera‐

turesaftercompletemelting.Finally,itwasobservedthatPCMneededapproximately4

h for complete solidification, whereas PCM+ needed only approximately 3 h, which

means that approximately 25%moremass of PCM+ can be solidified overnight com‐

paredtoPCM.

3.2. Temperaturedependentenergyyield

AsmentionedinSection2.2,thesurfacetemperature ofRefwasusedasareference

valueforcalculatingthepowerdifferencescausedbyPCMandPCM+.Explicitlyconsid‐

eringthedifferencesbetweentheelectricalparametersofthestudiedmodules(cf.Table

2),thehypotheticalelectricalpoweroutput ofthePCMandPCM+moduleswithout

theattachedphasechangematerialwasdeterminedaccordingtoequation(1).Thetem‐

perature variation due to the presence of the phase changematerial ( )was

measuredbythethermometersPCMInandReforPCM+InandRef.Themeasuredelectri‐

calpoweroutputofthemodulesisdenotedby

∙ ∙ (1)

where isthetemperaturecoefficientofmaximumpoweroutput.Basedonthesepower

outputvalues, alldayswithin themeasurementperiodwereanalyzedbyapplying the

followingthreesteps:

First,thetemperaturedependentenergydifferencesweredetermined.

Second,theywereweightedwiththeEPEXspotmarketprice.

Third,fourenergyeconomicdifferencevaluesforthecomparisonofthePV‐PCM

modulesweredefinedforeachday.

Because themeasureddatawere recordedeveryΔt=10 s, itwasnecessary to assume

constantvaluesuntilthenextsampling.Thus,thepoweroutputvalues and were

multipliedby∆ toobtainthegeneratedenergy and over∆ .Subsequently,the

and

energy

Tocom

Illustra

anexam

PCMan

shown

Fig.6 E

equippe

Thecum

d value

valueseac

mparetheP

atingtheap

mple comp

ndPCM+on

inFig.6.

Example d

edwithPC

mulativeso

eswerethe

h

CM,therel

ppliedproce

parison.Th

ntheenerg

daytime de

MorPCM+

olarinsolat

naddedin

ativeenerg

edures,the

he resulting

gyyieldas

ependent r

+vs.theref

tionvalues

ntothe15m

∑

gyyielddif

.

emeasured

gdiurnalv

wellasof

relative en

ferencePV

over15m

minblocks

, w

fference w

ddataofAu

variationof

theinsolat

ergy yield

module(b

inareplott

and

ith

wasdeterm

ugust2nd,2

f the relativ

tionattheP

difference

aseline)on

tedinsupe

,consistin

or .

minedby

2013wereu

ve contrib

PVmodule

es of PV m

nAugust2n

erposition.

12

ngof90

(2)

(3)

usedfor

utionof

elevelis

modulesnd,2013.

13

3.3. Economicevaluation

Forthesecondstep,thefollowingboundaryconditionswereconsideredinthepresent

evaluation:

Electrical energy is traded on the EPEX intraday spotmarket of Germany and

Austria.

Electricalenergyispreferablytradedin15‐minperiods;1‐hperiodtradesareal‐

soallowed.

Thegeneratedphotovoltaicelectricalpowerequalstheactualtradedpoweratall

times.

Thedailytradingperiodrangesfrom6a.m.to9p.m.

Moreover,thestudiedmoduleswerelinearlyscaledupto10MWpplantstocomplywith

theprequalificationrequirements forEPEXtradingsuchthat theminimumtrading in‐

crementvolumesof0.1MWcouldbeoffered.Therefore,the15‐minenergyblocks

and weremultipliedbytheratiooftheassumedpeakpowerofthePVplantandthe

maximumpower of thePVmodule. Inaddition, theupscaledenergyblockswere

monetarilyweightedbyrealaverageenergypricesretrievedfromtheEPEXonlinedata‐

base(EPEXSpotSE,2013)

, , ∙

∙ with or , (4)

wheretheindexjindicateswhethera15‐minora1‐hperiodwaschosen.Theresulting

economicyield , ofthePVpowerplantwassubtractedfromtheresultingeconomic

yield , ofthePV‐PCMpowerplant.Finally,theeconomicyielddifferencevalues∆

werecalculatedby

∆ , , , (5)

toisolatethecontributionofPCMandPCM+intermsofeconomicprofitorloss.

By considering the boundary conditionsmentioned above, the example diurnal varia‐

tionsofenergyeconomiccomparisonsofthePCMplantandthePCM+plantareshown

inFig.7.DuetothelowenergyyieldsinthemorningandeveningonAugust2nd,thepe‐

riodsfrom6a.m.to7a.m.andfrom8p.m.to9p.m.weretradedin1‐hblocks;there‐

mainderwastradedin15‐minblocks.

Fig.7 E

plantof

onAug

posedo

3.4. D

Thesey

configu

yields

Thus,th

yieldof

ticulard

Exemplary

fPVmodul

ust2nd,20

onthegrap

Dailyenerg

yielddiffer

urationand

, and

hedailyen

f thePV‐PC

difference

ydaytimed

lesequippe

13.Theso

ph.

gyeconomi

renceswer

dday.There

, weresu

nergyyield

CMpower

wasdivide

dependent

edwithPCM

larinsolati

iccomparis

reused to

efore,thee

ummedfort

, ∑

, ∑

ofthePVp

plant.Too

edbytheda

,

economic

MorPCM+

ionvalues

son

determine

energyyiel

thedaytim

∑ , . . . .

, . . . .

powerplan

obtain the

ailyenergy

,

,

yielddiffe

+vs.theref

accumulat

twocomp

lds and

meperiodfr

w

w

ntwassubt

relativeco

yyieldofth

rences for

ferencePV

edover15

parisonvalu

d aswe

rom6a.m.t

ith

ith

tractedfrom

omparison

ePVpowe

a10MWp

module(b

5minares

ues foreac

ellastheec

to9p.m.

or ,

or .

mthedaily

value , t

rplant

14

ppower

aseline)

uperim‐

chplant

conomic

(6)

(7)

yenergy

thepar‐

(8)

The sec

dailyec

Inaddit

ly for t

should

yieldso

Fig.8 E

plantof

onAugu

Tocom

was ac

achieve

thePCM

cond relati

conomicyie

tiontothes

the chargin

benotedt

ofthecorre

Exemplary

fPVmodul

ust2nd,201

mpare theu

ccumulated

edasmalle

M+plantw

ive compar

eldvaluesi

sedailyval

ng and sat

hattherela

espondings

yrelativee

lesequippe

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ieldofPCM

14ofthe32

M+actasa

temperatu

PCMmelted

17

encePV

August

onal bar

,nosig‐

galmost

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uring al‐

aslight

Mduring

2days.

apartial

ures in‐

d.Inthis

18

light,anincreaseinthermalmasscouldbeconsidered.However,furtherimprovements

ofthethermalconductivityoroptimizationoftheheattransferonthebacksideofthe

PV‐PCMmoduleseemstobemoreappropriate.Ashortstudy,exemplifiedforbothdays

discussed above, is showing that: The minimum mass increase of PCM, respectively

PCM+,toreachtheeconomicthresholdhasbeenassessed:Todoso,themeasuredoper‐

ating temperature curvesof the charging stagesofPCMandPCM+havebeen linearly

interpolated.Asaresult,aminimummassincreaseofabout50%hasbeenfoundforthe

PCM+. In addition, it was found out that under those circumstances PCM never may

reach an economic feasible threshold. Therefore, a maximummass increase of about

70%hasbeenidentified;thatlimitisgivenbytheinterceptpointofthereferencetem‐

peratureandthetrendlineoftheoperatingtemperature.Using

thosenumbers,thefollowingimprovementsinrelativedailyeconomicyieldoccur:

01.08.2013:PCMfrom‐1.55%to‐1.07%;PCM+from‐0.41%to0.001%.

02.08.2013:PCMfrom‐1.46%to‐0.87%;PCM+from‐0.1%to0.278%.

5. Conclusion

TheassumptionthataPVmoduleequippedwithalayerofphasechangematerialonits

backsidecanachieveabettereconomicyieldthanastandardPVmodulewasevaluated

for a specific configuration.Despite promising results beforenoon, the resultingdaily

energyandeconomicyieldswerealmostallnegative.Inthestudiedconfiguration,high‐

erpricesforelectricityonthespotmarketinthemorningcombinedwiththehigheren‐

ergy yield during that period are not sufficient for profitability. However, the phase

changematerialwithahigherthermalconductivityismoreappropriateforaPVmodule

applicationthanaconventionalphasechangematerial.Next,higherenergypricesonthe

EPEX spotmarket in themorning provide better results than considering the sum of

dailygeneratedelectricityonly.

To further investigate the economic feasibility of different configurations, the energy‐

economic tradeoff between thermal conductivity, heat storage capacity and resulting

layerthicknessaswellasthebacksideheattransferofthePCMwithrespecttothere‐

lated costs should be investigated and optimized via computational simulations and

evaluatedbasedonenhancedprototypes.

19

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