experimental study of phase change materials japs … · experimental study of phase change...
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1
Experimentalstudyofphasechangematerialsforphotovoltaicmodules:
energyperformanceandeconomicyieldfortheEPEXspotmarket
EwaldJapsa,GerritSonnenreinb,StefanKrautera,*,JadranVrabecb
aUniversityofPaderborn,ElectricalEnergyTechnology–SustainableEnergyConcepts,
33098Paderborn,Germany,[email protected],[email protected]
bUniversityofPaderborn,ThermodynamicsandEnergyTechnology,33098Paderborn,
Germany,[email protected],[email protected]
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
2.1. C
The ap
polyme
THERM
ingexp
nenrein
mal con
conduc
asdesc
specific
mentsw
Bauer,
withDi
soor,20
Pricedevel
rFriday,Au
measuredi
Measurem
Characteriz
pplied PCM
ercomposit
M®RT28H
pandedgrap
netal.,201
nductivity
tivityofcribedin(M
c enthalpy
werecondu
2014) spe
ifferentialS
016).Comp
lopmenton
ugust2nd,2
inPaderbo
mentsetup
zationofPC
M was herm
tefilmwith
HCalonewa
phite(THE
15).Aftera
ofPCM+ in
=2.4W/(m
Mehlinget
of PCM+ c
uctedwith
cifically de
ScanningC
paredtoco
ntheEPEX
2013.Thes
rn,German
CMandPCM
metically en
h500gof
as=0.19ERMOPHIT®
addinggrap
ncreasedb
m∙K)hasb
al.,2000).
comparedw
heatflow3
eveloped fo
alorimetry
ommonDSC
spotmark
solarirradi
nyissuperi
M+
ncapsulate
PCMeach.
W/(m∙K);
®GFG,SG
phitewitha
bymore th
eenmeasu
Fig.2show
with that o
3‐layer‐cal
or analyzin
y(DSC,TG‐
Cdevices,W
et(EPEXSp
anceontha
imposedon
ed in bags
.Thetherm
thiswassi
LGROUP),
amassfrac
hana factor
uredbyapp
wsthetem
ofpurePC
orimeter(W
ng PCM an
DSC111,S
WOTKAall
potSE,201
atnearlype
nthegraph
consisting
malconduc
gnificantly
following
ctionofω≈
r of 12.Th
plyingthes
mperatured
M.Theund
WOTKA,W
d for valid
SETARAM)
lowsincrea
13),showin
erfectsum
h.
g of an alu
ctivityofth
yincreased
theworkb
≈0.2g/g,t
herefore a
stationary
dependenc
derlyingm
W&A)(Kenf
dation addi
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asedsampl
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ngreal
mer
minum‐
heRUBI‐
byadd‐
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hether‐
thermal
method
ceofthe
measure‐
fackand
itionally
ndMan‐
lequan‐
tities o
temper
mental
Fig.2Sp
Thehea
duetoi
specific
to185
should
commo
heatpe
kJforP
2.2. M
Two o
equippe
f up to 10
rature of c
resultsoft
pecificenth
atstorage
itssignifica
cheatstora
kJ/kg.The
be noted
onsalthydr
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CMandap
Modulesan
f the thre
edwiththr
0 gwhat i
composites
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capacityo
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agecapacit
emeltingte
that the p
rates.Base
uleoverthe
proximatel
ndmeasure
ee technica
reePCMb
is of partic
. Therefore
r‐calorimet
(hysteresis
f thecomp
tecontent.
tywasthus
emperatur
araffin use
edonthem
etemperatu
ly275kJfo
ementsetu
ally identi
agseach.F
cular impo
e, the stud
ermeasure
s)function
poundedm
Overthete
sreducedb
eremained
ed here sh
measuredP
urerangeo
orPCM+(cf
up
ical multi‐
First,oneP
rtance to d
dy shows i
ements.
oftempera
material is l
emperatur
byapproxi
dunaffecte
hows no sig
PCMprope
of20°Cto3
f.Fig.2and
‐crystalline
Pt100foilr
determine
in the follo
atureforPC
lower than
erangeof
mately28%
edatappro
gnificant s
rties,thea
35°Cwasa
dTable1).
e silicon P
resistancet
the phase
owing the
CMandPCM
nthatofpu
20°Cto35
%,from26
oximately2
sub‐cooling
amountofs
approximat
PV module
thermomet
6
change
experi‐
M+.
urePCM
5°C,the
60kJ/kg
28°C. It
g, unlike
storable
tely391
es were
terwith
anaccu
therma
eachPV
attache
areaon
placed
nally,a
to tight
fixed b
thethre
itednat
equaliz
back si
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Fig.3 E
B.
Thethr
derborn
izontal
the sho
and log
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uracyof ab
ally conduc
Vmodule,
ed next to
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ontheair‐
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tly press th
elow the P
eePVmodu
turalconve
e convectiv
deof them
rementswe
Experimen
reemodule
n,Germany
andanazi
ort‐circuit c
gged simul
cal loadwi
ofthePVm
bout±0.5K
ctivepaste,
as indicate
each other
sideofthe
‐facingsur
mgridwas
hePCMba
PV outdoor
ulesatafr
ectionalon
ve heat tra
module.M
ereconduc
ntalsetupin
eswereins
y(51°45’2
muthof0°
current, op
ltaneously
ith aMPP‐
oduleswas
(classB a
positioned
edinFig.3
r,where th
PVmodule
faceofthe
sfixedwith
ags to theP
r laborator
eedistance
ng thewho
ansfer i.e.,
oreover, th
tedfromea
nPaderbor
stalledinth
3.87”N,8°
°,asshown
pen‐circuit
every 10 s
error of ab
smeasured
according t
dat thece
.Next,thre
hey covere
e.Another
ePCMbelow
hscrewsat
PVmodule
ry framewo
eof8cm.H
olemodule
to reduce
his conditio
arlyJulyto
rn,German
hePVoutd
°38’38.43”
ninFig.3.I
voltage an
s for each
bout ±1%.
dusingaca
to IEC6075
enterofa s
eemacroe
d approxim
Pt100foil
wthefirst
ttheback
es. In addit
ork, coveri
However,h
, cf.Fig.4.
the impac
on simulat
mid‐Augu
y,cf.Fig.4
doorlabora
E)withan
naddition
ndMPP po
PVmodul
Furtherm
alibratedpy
51:2008)w
solar cell, t
encapsulate
mately 80%
resistance
resistance
sideofthe
tion, an alu
ng the pro
orizontalf
Thismeas
t ofwind b
ed aPV ro
stof2013.
forcrosss
atoryofthe
inclination
tothesurf
ower outpu
e by an in
ore, global
yranomete
wasattache
to theback
edPCMba
% of the so
thermome
ethermom
ePVmodul
uminumpl
ojected are
fixationbar
surewasa
blowing al
ooftop inst
ectionsA‐A
eUniversit
nof30°to
facetempe
utwerem
n‐house de
l irradianc
er(CMP21
7
edviaa
k sideof
gswere
olar cell
eterwas
eter.Fi‐
leframe
atewas
a under
rsinhib‐
added to
long the
allation.
AandB‐
tyofPa‐
thehor‐
eratures,
easured
veloped
e in the
,Kipp&
Zonen)
Fig.4 C
ingthe
Prior to
tions (S
ramete
Proper
Specific
Averag
Specific
Therma
Melting
Electri
Maximu
Conver
.
Crosssecti
positionso
o their inst
STC)at the
rsofthein
rty
centhalpyo
especificis
cdensity
alconducti
gtemperatu
Table
calparam
umpower
sionefficie
onschema
ofthefoilr
tallation, th
e Institute
nvestigated
Table
offusion(2
sobarichea
vity
ure
e2:Parame
meter
output
ency
aticofthem
esistanceth
he PVmod
forSolarE
PCMandm
1:Properti
20°C‐30°
atcapacity
etersofthe
Sym
P
measureme
hermomet
duleswere
EnergyRes
modulesar
iesofthein
Symb
°C) h
cp
ρ
λ
einvestigat
mbol Re
Pmax
entsetup,a
ers.
emeasured
searchHam
relistedinT
nvestigated
bol PC
23
p 2.
76
0.1
2
tedPVmod
eference
28.3
12.1
sillustrate
d under sta
melin (ISFH
Tables1an
dPCMs.
CM PC
35 15
3 2
60 89
19 2
8 2
dulesunder
PCM
26.6
11.4
edinFig.3,
andard tes
H).All relev
nd2.
CM+ U
58 k
2.2 kJ/
90 k
2.4 W/
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
13.
useof thed
d over the
rprofittha
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rison value
instead
,
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nergyand
edwithPCM
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ingmorep
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ing period,
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ig.5.
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orenoon,ov
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9. Although
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15
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aseline)
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Fig.9 A
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fectontheo
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nddiscussi
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summarize
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onAugust2
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wereevalu
esults for e
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and thedai
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graph.
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• R
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i
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ainfindings
Regarding
nificantint
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PCM+yield
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mostthee
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Higherene
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additionalla
al insulatio
dabovethe
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MandPCM
cumulative
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thecumula
terdepende
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nergyyield
dedmoree
nductivity
e in econom
ntiremeas
neconomic
ergyprices
don16ofth
ayeronthe
on to the e
etemperatu
yandecono
M+module
daily sola
mparisona
ativedaily
encyhasbe
yyieldwas
ment period
d.
energythan
producing
mic yieldw
surementp
yield.
beforeno
he32days
ebackofth
environme
ureofther
omicyieldd
es during t
r insolatio
areasfollow
solarinso
eenobserve
observed
d. For two
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period.For
onleadto
s.Thesame
hePVmodu
ent. Theref
referenceP
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theperiod
n is illustr
ws:
lationand
ed.
forbothPC
days only,
28ofthe3
mperature
ved forbot
twodayso
abetterec
eholdsforP
ule,bothPC
fore, their
PVmodule
scomparin
of July 14t
rated with
theyieldd
CMandPC
PCM+ ind
32daysbec
.
thPCMand
only,PCM+
conomicyi
PCM+for1
CMandPC
operating
aftertheP
ngthereferth, 2013 to
an additio
differences
CM+during
dicated a sl
causeofits
dPCM+du
+indicated
ieldofPCM
14ofthe32
M+actasa
temperatu
PCMmelted
17
encePV
August
onal bar
,nosig‐
galmost
light in‐
shigher
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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