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Indirect benefits of rooftop photovoltaic (PV) systems for building insulation are quantified through measurements and modeling. Measurements of the thermal conditions throughout a roof profile on a building partially covered by solar photovoltaic (PV) panels were conducted in San Diego, California. Thermal infrared imagery on a clear April day demonstrated that daytime ceiling temperatures under the PV arrays were up to 2.5K cooler than under the exposed roof. Heat flux modeling showed a significant reduction in daytime roof heat flux under the PV array. At night the conditions reversed and the ceiling under the PV arrays was warmer than for the exposed roof indicating insulating properties of PV. Simulations showed no benefit (but also no disadvantage) of the PV covered roof for the annual heating load, but a 5.9kWhm−2 (or 38%) reduction in annual cooling load. The reduced daily variability in rooftop surface temperature under the PV array reduces thermal stresses on the roof and leads to energy savings and/or human comfort benefits especially for rooftop PV on older warehouse buildings.
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1
EffectsofSolarPhotovoltaicPanelsonRoofHeatTransfer
AnthonyDomingueza,JanKleissla,andJeffreyC.Luvallb
aUniversityofCalifornia,SanDiego,DepartmentofMechanicalandAerospaceEngineering
bNASA,MarshallSpaceFlightCenter,AL35812,USA
Correspondingauthor
JanKleissl,jkleissl@ucsd.edu
Office:(858)5348087;Fax:(858)5347599;
Address:9500GilmanDr,EBUII580,UniversityofCalifornia,SanDiego,LaJolla,CA,92093
0411
Abstract
Indirectbenefitsofrooftopphotovoltaic(PV)systemsforbuildinginsulationarequantified
throughmeasurementsandmodeling.Measurementsofthethermalconditionsthroughouta
roofprofileonabuildingpartiallycoveredbysolarphotovoltaic(PV)panelswereconductedin
SanDiego,California.ThermalinfraredimageryonaclearAprildaydemonstratedthatdaytime
ceilingtemperaturesunderthePVarrayswereupto2.5Kcoolerthanundertheexposedroof.
HeatfluxmodelingshowedasignificantreductionindaytimeroofheatfluxunderthePVarray.
AtnighttheconditionsreversedandtheceilingunderthePVarrayswaswarmerthanforthe
exposedroofindicatinginsulatingpropertiesofPV.Simulationsshowednobenefit(butalsono
disadvantage)ofthePVcoveredrooffortheannualheatingload,buta5.9kWhm2(or38%)
reductioninannualcoolingload.Thereduceddailyvariabilityinrooftopsurfacetemperature
underthePVarrayreducesthermalstressesontheroofandleadstoenergysavingsand/or
humancomfortbenefitsespeciallyforrooftopPVonolderwarehousebuildings.
Keywords:Buildingenergyuse;coolingload;photovoltaic;roofheatflux;thermalinfrared
camera
2
1. Introduction
BuildingHeating,VentilationandAirConditioning(HVAC)isamajorcontributortourban
energyuse.Especiallyinpoorlyinsulated,singlestorybuildingswithlargesurfaceareasuchas
warehouses,mostoftheheatentersthroughtheroof.Increasingroofalbedo(orsolar
reflectance)reducescoolingloadinsunnyandhotclimates.Installingreflectiveroof
membranesresultedinseasonalairconditioningenergysavingsof57%onaCaliforniahome
(Akbarietal.,1997a),49%onabungalow,2%to43%inFlorida(ParkerandParkaszi,1997),
and30Whm2d1onaregenerationbuilding(AkbariandRainer,2000).However,theenergy
savingsdependontheroofinsulatingproperties.Increasingtheroofalbedofrom0.09to0.75
onabuildingwithoutinsulationresultedinenergysavingsof28%,whileincreasingthealbedo
from0.30to0.75onabuildingwithR30insulation(anadditionof5.28Km2W1inthermal
resistance)resultedinsavingsofonly5%(SimpsonandMcPherson,1997).
Shadetreesplantednearresidentialbuildingsresultedinaseasonalcoolingenergysavingsof
30%andpeakdemandsavingsof27%and42%intwoSacramento,CAresidences(Akbarietal.,
1997b).
Arooftop‘modification’whoseimpactoncoolingloadshasnotbeenexaminedexperimentally
issolarphotovoltaic(PV)arrays.InCaliforniaalone,overaGW(109Watts,correspondingto
about1km2ofrooftopspace)inresidentialandcommercialrooftopPVareapprovedorinthe
planningstages.AnindicationoftheinfluenceofPVoncoolingcomesfromanevaluationof
residentialbuildingenergyuseat260sitesinsouthernCaliforniapreandpostinstallationofa
PVsystem,whichindicatedthatACenergyuseinhighcoolingdegreedayconditionsdecreased
comparedtoareferencesample(ITRONInc.,2010).A1degreeincreaseindailyaverage
3
temperatureinSanDiegoGas&Electric(SDG&E)territorycausedpostPVhouseholdswithair
conditioningtouse0.501kWhlessenergyperdaythanhouseholdswithairconditioningthat
didnothavePVinstalled.
ModelingtheeffectsofbuildingintegratedPV(BIPV)onthemicroclimateoftheurbancanopy
layershowedasignificantreductioninBIPVroofsurfacetemperaturescomparedtoa
conventionalroofwithalbedo0.30andathermalresistanceof1.33Km2W1(Tianetal.,2007).
Aonedimensionaltransientheattransfermodelshowedapeakcoolingloadsavingsof52%
withventilatedBIPVcomparedtotraditionalroofingwithasolarabsorbanceof0.9anda
thermalresistanceof1.33Km2W1(Wangetal.,2006).Aconductionmodelshoweda65%
reductionincoolingloadcomponentthroughaPVroofcomparedtoaconventionalroofwitha
thermalresistanceof2.8Km2W1(Yangetal.,2001).
Inthisstudy,weinvestigateabuildingpartiallycoveredbyaflushandhorizontalsolarPVarray
andanoffsetandtiltedsolarPVarray(Section2).Meteorologicalandrooftemperature
measurements(includingthermalimagery)wereconducted(Section3).Section4describesa
roofconductionmodeltoestimateaverageandpeakcoolingenergydifferencesfortheroof
sectionswithandwithoutPV.InSection5wepresentafullroofenergybalancemodelto
calculateannualroofheatingandcoolingloadswithandwithoutPV.Conclusionsarepresented
insection6.
2. Experimentalsetup
2.1. Buildingandlocation
4
ThebuildingusedinthisstudyisthePowellStructuralLaboratory(PoSL)attheUniversityof
California,SanDiego(Fig.1;Tables1,2).ItisahollowconcretecubewithoutHVACsystem.
Therearenowindowsexceptforasmallpartiallyshadedrowofwindowsontheeastandwest
sidesneartheroof.Onworkdaysthebuildingiscooledbynaturalventilationthroughagateon
thesouthfaceofthebuilding(manycoastalbuildingsinSanDiegolackHVACsystemsasthe
seabreezeskeeptheindoorenvironmentcomfortableformostoftheyear).Duetosafetyand
productivityconsiderations,theexperimenthadtobeconductedonaweekendday,whenall
doorswereclosedandthebuildingwas‘ventilated’onlythroughinfiltration.PoSLhas2solar
PVarraysontherooftop;oneistiltedsouthat4.4degreesandelevated0.10moffoftheroof
surfaceandtheotherishorizontalandflushwiththeroofsurface(Fig.2).Theroofis0.20m
thickandcomposedofmeshreinforcedinsulatingconcreteand24GAcorrugatedsteelontop
oftrusses.ThisresultsinasignificantlylowerRValuethannewconstruction,butistypicalof
olderwarehousebuildingswithlarge,flatrooftopsforwhichPVisanattractiveoption.Table1
showsthesiteandbuildingcharacteristics.
Table1:PowellStructuralLaboratory(PoSL)buildingcharacteristics.Thebuildingislocatedat
32o52’50”N,117o14’06”WintheWorldGeodeticSystem1984coordinatesystem(WGS84)and
is1.7kmfromthePacificOceancoastline.
Length(northsouth)Width(eastwest) HeightRooftopareaMeasuredRoofAlbedo
36.6m19.2m18.0m703m20.218
Table2:PoSLflushPVarraycharacteristics(seealsoFig.1c).ThetiltedPVarrayisidenticalto
theflusharraywiththeexceptionofthe4.4degreesouthtilt(measuredaverageof14panels
onthee
d
panel).
Lengt
6.01
m
Fig.1.a.
TIRcam
e
Google
E
d
geofthea
r
h
Widt
m
7.80
Photograph
e
racircled,f
i
E
arthimage
o
r
ray)andal
h
#ofpa
m
8X4=
ofPoSLfac
i
i
eldofview
o
fPoSL(No
r
engthof6.
9
nelsPan
32K
y
i
ngSouth;b
shownbyb
l
r
thisup)wi
t
9
9m.Theto
t
elType
y
ocera
.Photograp
l
uelines,an
t
htiltedarr
a
t
alratedD
C
Solar
Reflectanc
e
0.178
hofPoSLfr
o
dPVlocati
o
a
yontheN
o
C
outputis1
3
e
Sol
a
o
minsidefa
o
noutlinedi
n
o
rthsideof
t
3
kW(200
W
a
rConversi
o
Efficiency
0.08
cingNorth
w
n
green;c.
t
heroofan
d
5
per
o
n
w
ith
d
flusharr
a
larger(T
a
Fig.2.S
o
disconti
n
variable
d
allowing
ceilingt
e
image(F
2.2. Equ
i
2.2.1. I
n
Datawe
r
Insideth
pointed
a
viewwa
s
partially
a
yinthece
n
a
ble2).
o
uth(left)to
n
uitiesatth
e
d
efinitions
s
airflowbet
w
e
mperature
s
ig.3).
i
pment
n
teriormea
s
r
etakenfro
m
ebuilding,
a
a
ttheceilin
g
s
notsuffici
e
theceiling
u
n
ter.Thelar
g
North(righ
t
e
verticalda
s
s
eeTable3).
w
eenpanel
a
s
T
c
,T
cs
and
s
urements
m
1500PST
a
FLIRA320
t
g
torecord
t
e
nttocaptu
r
u
nderthefl
u
g
erNorthS
o
t
)verticalcr
s
hedlines)
w
ThePVpa
n
a
ndroof,w
h
T
cflat
arere
p
Friday,Apri
l
t
hermalinfr
a
t
helongwav
r
etheentir
e
u
sharrayan
d
o
uthpanel
s
osssection
w
ithschem
a
n
elsare1.4
m
h
iletheflat
p
p
resentative
l
17,2009t
o
a
red(TIR)c
a
eirradiance
e
ceiling,th
e
d
theexpos
e
s
pacingmak
ofthePoSL
a
ticofallex
t
m
long.The
t
panelsaref
l
ofdifferen
t
o
0600PST
M
a
merawith
a
(Fig.1b).Si
e
TIRcamer
a
e
dpartoft
h
esthetilte
d
roof(tosca
t
eriormeas
u
tiltedPVpa
n
l
ushtothe
r
t
areaswithi
M
onday,Ap
r
a
45
o
wide
a
nceeventh
e
a
waspositi
o
h
eroof,and
d
arrayappe
a
leexceptfo
r
u
rements(f
o
n
elsarerais
r
oofsurface
.
ntheTIRca
r
il20,2009.
a
nglelensw
a
e
wideangl
e
o
nedtoima
g
thefullare
a
6
a
r
r
o
r
ed
.
The
mera
a
s
e
g
e
a
underth
through
o
Stefan’s
tempera
t
conducti
Fig.3.a.
denotes
trussest
h
isvisible
represe
n
tempera
t
theceili
n
etiltedarra
y
o
utthestud
y
lawassumi
n
t
urestotha
t
ngepoxy.
ThermalIR
c
temperatur
e
h
atarebel
o
asacoolar
e
n
tativeofth
e
t
ure.b.Pho
t
n
gprovidel
a
y
(Fig.3).TI
R
y
period.Th
n
ganemissi
v
t
ofaconta
c
c
ameraima
g
e
inK.Horiz
wtheceilin
g
e
ainthece
n
e
ceilingun
d
t
ographoft
a
ndmarksto
R
imagesco
n
esurfacete
v
ityof0.95.
c
tthermoco
g
eofceiling
ontalandv
e
g
atadiffer
e
n
ter.Pixels
w
d
erneathea
c
heceilingfr
visuallyge
o
n
taining32
0
mperature
o
Thisassum
p
uplesensor
at1710PS
T
e
rticallines
t
e
nttemper
a
w
ithinblack
c
hrooftype
omsamea
n
o
referencet
h
0
x240pixels
o
feachpixe
p
tionwasv
e
affixedtot
h
T
onApril1
9
t
hroughthe
a
ture.Thef
o
rectangles
w
andwereu
n
gle.Theco
o
heimages.
weretaken
lwascomp
u
e
rifiedbyco
m
h
eceilingw
i
9
,2009.The
imageresu
l
o
otprintoft
h
w
erechose
n
sedtoobta
i
o
ler(turned
every5mi
n
u
tedusing
m
paringTIR
i
thheat
colorbar
l
tfromsteel
h
etiltedPV
n
as
i
nitsaverag
off)lamps
o
7
utes
R
array
e
o
n
8
2.2.2. Exteriormeasurements
SurfacetemperaturewasmeasuredbyaffixingHOBOProV2externaltemperaturesensors
usingheatconductingepoxytoboththeundersideofthetiltedsolarpanelsandthesurfaceof
theroofunderthesolarpanel(Fig.2).Anairtemperatureprobewasmounted0.1mabovethe
roofsurfaceunderthetiltedarray.Thespaceundertheflusharraywasinaccessiblesono
measurementscouldbetakenthere.Thesamplingfrequencywas1minand5minaverages
werestored.
ApermanentmeteorologicalstationontherooftopofPoSLmonitoredexposedroofsurface
temperature,airtemperature,totalanddiffusesolarirradiance,andwindspeed(Table3).Data
weresampledeverysecondandstoredas5minaverages.TherooftopalbedoandsolarPV
albedoweremeasuredusingaKipp&ZonenCMP3albedometer.Afterthestudy,theTIR
camera,HOBOs,andTN9TIRsensorswerecrosscalibratedontheexposedroofandalinear
regressionwasappliedtoforceagreementbetweenthesensors.AppendixAshowsananalysis
ofthesensitivityofourresultstotemperatureoffsetsinthesensors.
Table3:Sensortype,make,model,andheightaboverooflevel(ARL)formeasurementsonthe
PoSLroof(Fig.2).TheTN9accuracyisgivenforthe1535oCtemperaturerange.Fullrange
accuracyis2oC.
MeasurementVariable SensorHeightAccuracy
RoofsurfacetemperatureTrZyTempTN9TIR 1.41m0.6oC
AirtemperatureTaSensirionSHT75T/RH1.93m0.4oC
GlobalhorizontalincidentsolarGHILiCorSZ200pyranometer 2.26m3%
9
irradiance
DiffusehorizontalsolarirradianceDIFFDynamaxSPN1
pyranometer
2.27m5%
WindspeeduDaviswindsensor2.00m5%
TemperaturesundertiltedPVarray:
RoofsurfacetemperatureTrsHoboProV20.0m0.2oC
AirtemperatureundertiltedarrayTasHoboProV20.1m0.2oC
PVbackpaneltemperatureTpHoboProV20.17m0.2oC
3. Measurementresults
3.1. Solarradiation,windspeed,andoutsideairtemperature
OnlydataforSunday,April19,2009areanalyzed,becauseitwastheclearestdaywithafew
cloudsfrom0730to1000PST(Fig.4a).Thedailyglobalhorizontalsolarirradiationwas7.72
kWhm2,whichwaslargerthanatypicalAprilday.Figure4bshowsthewindspeed,which
followstypicalseabreezepatterns(cf.annualaverageuinFig.4b)withcalmwindsuntil0800
PST,increasesto5ms1at1400PSTanddecreasestolessthan1ms1by2030PST.Theair
temperaturecycle(Fig.5a)hasasmalldiurnalamplitude.Overallthemeteorologicalconditions
onApril19wererepresentativeforcoastalsouthernCaliforniawheremuchofthegrowthinPV
isexpectedtooccur.
10
Fig.4.a.Diffuseandglobalhorizontalincidentsolarirradianceandb.windspeedonApril19,
2009(seeTable3fordetailsonsensorsandmeasurementlocations).
3.2. Roofandceilingtemperatures
Figure5ashowsthetemperaturesofoutsideair,roof,andceilingfortheexposedroof.The
roofsurfacetemperaturepeaksatnoonandishigherthantheairandceilingsurface
temperaturesduringdaylighthours,asitisheateddirectlybysolarradiation.Theceiling
surfacetemperaturepeaksat1737PSTduetoatimelaginthetransportofheatfromthe
exteriorroofsurfacetotheinteriorceilingsurface.
Figure5bshowsthetemperaturesunderthetiltedPVarray.Thebackpaneltemperatureofthe
solarpanelissimilartotherooftemperaturefortheexposedroof.However,sincetheroof
surfaceunderneaththePVpanelisshadeditstemperatureissignificantlylowerthanforthe
exposedroof.Theairtemperatureinthegapbetweenthepanelandtheroofislowerthanthe
backpaneltemperatureandrooftemperatureunderthepanels,buthigherthantheair
temperatureat1.93mabovetheroof.Toconcludetheroofunderthesolarpanelsisheatedby
0
200
400
600
800
1000
Solar Radiation [Wm
-2]
a)
GHI
DIFF
April average GHI
00:00 04:00 08:00 12:00 16:00 20:00 00:00
0
1
2
3
4
5
6
Time [P ST]
wind speed [ms-1]
b)
u
April average u
Annual average u
11
longwaveradiationfromthepanelundersideanddiffuseradiationfromthesky(whichissmall
giventhesmalltiltangle),thesumofwhichislessthanthesolarirradiancetotheexposedroof.
Convectionofairthroughtheairspacebelowthepanelresultsinheatremoval.Atnight,the
roofsurfaceunderthesolarpanelsremainswarmer,duetothereductioninradiativecooling
tothesky.
Fig.5.a.Ceilingsurface(Tc),roofsurface(Tr)andoutsideairtemperature(Ta)measurements
fortheexposedroof.b.Ceilingsurface(Tcs),roofsurface(Trs),outsideair(Tas),andbackpanel
temperature(Tp),measurementsunderthetiltedPVarray.c.Interiorceilingsurface
10
20
30
40
50
60
Temperature [oC]
a)
Tc
Tr
Ta
10
20
30
40
50
60
Temperature [oC]
b)
Tcs
Trs
Tas
Tp
00:00 04:00 08:00 12:00 16:00 20:00
24
26
28
30
32
34
36
Time [PST]
Temperature [oC]
c)
Exposed Roof
Tilted PV array
Flat PV array
12
temperaturesunderexposedroof,tiltedPVarray,andflatPVarrayaveragedoverareas
identifiedinFig.3.
Theinteriorceilingsurfacetemperatures(belowtheexposedroof,tiltedPVarray,andflushPV
array)areredrawnforclarityinFigure5c.From0900to2100PSTtheceilingundertheexposed
roofiswarmerthantheceilingunderneaththeflushpanels,whichinturniswarmerthanthe
ceilingunderneaththetiltedpanels.Themaximumtemperaturedifferencebetweenexposed
roofandtiltedPVis2.5oCat1700PST.ThetemperatureoftheceilingunderneaththeflushPV
arrayisbetweentheothercasesduringthedaytime,asitprovidesshadingtotheroof,butthe
enclosedairspacebetweenthepanelsandtherooflimitshorizontaladvectionofheat.The
ceilingcoveredbytheflushPVarrayhasthehighesttemperatureatnightduetotheinsulating
propertiesoftheenclosedairbetweenroofandsolararrayandtheincreaseinincident
longwaveradiationfromthepanelcomparedtothesky.
4. Simulationofroofheatflux
Theresultsinsection3haveshownmarkeddifferencesinthethermalresponseofaroof
underneathasolarpanelcomparedtothatofanexposedroof.However,todeterminethe
potentialHVACenergysavingsassociatedwithsolarPVpanelstheroofheatfluxintotheair
conditionedspace(orroofcoolingload)isthemostrelevantvariable.Quantifyingthisheatflux
independentlyforeachsurfaceisdifficult,sinceconvectiveandradiative(andtoalesserextent
conductive)exchangeofheatbetweenceilingareasthroughtheroomairandwallandfloor
surfacesbluntsthetruedifferencesbetweentheceilingtemperaturesundereachrooftype.
13
TheconductionheatfluxthroughtheroofcanbemodeledusingaCrankNicholsonmethod
(UnderwoodandYik,2004)appliedtotheonedimensionaltransientheatconductionequation

 
(1)
usingadiscretizationof32layers.Theboundaryconditionsarethemeasuredrooftopsurface
andinteriorceilingsurfacetemperatures.Theeffectsoftheupperroofmembraneandlower
corrugatedsteel(Fig.2)werenegligible,astheyarethinandhavehighthermalconductivityin
comparisontotheinsulatingconcrete,sothemodelwassimplifiedbydefiningtheentireroof
asmeshreinforcedinsulatingconcrete.Thethermalpropertiesusedinthemodelwerethermal
conductivityk=0.38Wm1K1,densityρ=1200kgm3,andheatcapacitycp=1000Jkg1K1
(ThermalPropertiesofConcrete,2010),resultinginathermaldiffusivityof3.2x107m2s1and
thermalresistanceof0.526Km2W1(abouttheequivalentofR3insulation).Themodelwas
validatedagainstananalyticalsolutionforablockwithuniforminitialtemperaturebeing
submergedintoaliquidwithconstantuniformtemperature(UnderwoodandYik,2004).Since
thesystemisquasiperiodiconcleardays,theinitialtemperatureprofileintheroofwas
estimatedbytakingthetemperatureprofileattheendofthedayandaddingalinearfitsothat
thesurfacetemperaturesmatchtheinitialmeasuredsurfacetemperatures.
Fromthemodeledrooftemperatureprofileatimelaginheatpenetrationfromrooftopto
ceilingisapparent(Fig.6).Fortheexposedroof,thecoolingandheatingoftheuppersurface
drivestheheatconductionthroughtheroof(Fig.6a).UnderthetiltedPVarrayhowever,while
heatingfromthetopisdominant,heatingfromthebottomalsooccurs(Fig.6b).Thoughthe
interiorairtemperaturewasnotmeasured,theundersideofaconcreteblocksurfacewithin
thebuildingcapturedbytheTIRcamera(bottomrightofFig.3a)wassignificantlylargerthan
14
theoutsideairtemperatureandtypicaltemperatureforairconditionedbuildings.Thefactthat
buildingisnotventilatedorairconditionedleadstoanincreaseinindoorairtemperatureand
reductioninroofheatflux.
Fig.6.Timeseriesofsimulated1Dtemperatureprofilethrougha.theexposedroofandb.the
roofunderthetiltedPVarray.ThecolorbarshowstemperatureinoC.c.Conductiveheatflux
frombottomrooflayertoceilingsurfacefortheexposedroofandthetiltedPVarray(negative
meansupwardfluxasperEq.2).
Giventheenergybalanceattheceiling,theheatfluxintotheindoorbuildingair(orroofcooling
load)shouldbeequaltotheconductiveheatfluxintothebottomrooflayer(layer32)

3231 TT
x
k
q
,(2)
Depth [m]
Time [PST]
a)
00:00 06:00 12:00 18:00
0
.05
.10
.15
.20
Time [PST]
b)
00:00 06:00 12:00 18:00
0
.05
.10
.15
.20 15
20
25
30
35
40
00:00 04:00 08:00 12:00 16:00 20:00 00:00
-40
-30
-20
-10
0
10
20
Time [PST]
Heat Flux [Wm
-2
]
c)
exposed roof
tilted PV array
15
whereΔxisthenumericaldiscretizationdistancebetweenrooflayers.Thisanalysiscannotbe
conductedfortheflushPVarrayastheexteriorrooftemperaturewasnotavailabletodrivethe
conductionmodel.AppendixBshowsanattempttoestimatetheflusharrayheatflux.
Figure6cshowstheresultingheatfluxthroughtheexposedroofandtheroofunderthetilted
array.Asexpectedtheheatfluxisnegative(upward)intheearlymorning.Theminimum
between09001000PSTisaresultoftheheatlossattheroofsurfacethroughthenightbut
likelyamplifiedbysolarradiativeheatingoftheinteriorandceilingthroughthewindowsinthe
earlymorningaswellasincreasedconvectiveheatingfromtheinteriorairtemperature.At
1300PST(exposedroof)and1900PST(tiltedPV)theheatfluxbecomespositive(downward)
withalargerpeakfortheexposedroofthanthePVcoveredroofat1930PST.Theheatfluxes
remainpositivethroughthenightconsistentwiththetimelagofheattransferthroughthe
roof.Generally,therelativelysmallthermaldiffusivityoftheroofunderconsiderationcauses
secondaryeffects(ceilingbuildinginteraction,windowshortwavetransmission)tohavea
significantinfluenceonthemodeledconductiveheatfluxesneartheceiling.
5. Modelingannualroofheatflux
5.1.Roofheatflux
Table4:Variablesusedintheannualroofheatfluxmodelandtheirsources.
TermDescriptionSource
aVelocitycoefficientinhcforexposed
roof/PVcoveredroof
18.65/14.82(Palyvos,2008)
bVelocityexponentinhcforexposedroof/PV
coveredroof
0.605/0.420(Palyvos,2008)
hcExteriorconvectiveheattransfercoefficient DOE‐2model(Eq.9)
h
i
Interiorconvectiveheattransfercoefficient1.25Wm2K1(ASHRAE2005b)
SVFSkyviewfactorofroofunderPVpanel0.0817(Calculatedfrompanelangle)
16
uWindspeedMeasured2mARL
DIFFDiffuseirradiationMeasured
GHIGlobalHorizontalIrradiationMeasured
R
f
Surfaceroughnessmultiplierinhc2.17forstucco
RHRelativehumidityMeasured2mARL
TaAirtemperatureMeasured2mARL
TiInteriorairtemperaturePrescribedas23.3oC(coolingday)or
21.7oC(heatingday)
T
p
PVpaneltemperatureModeled(JonesandUnderwood,2001)
Tr
n1Rooftemperatureatprevioustimestepn1
T1TemperatureatfirstrooflayerFromCrankNicolsonmodel
α RoofsurfacealbedoMeasured0.218
εrRoofemissivityAssumed0.95
εpPVemissivityAssumed0.95
Themodeldescribedinsection4wasexpandedtosimulatetheenvelopeheatfluxoverayear
forcedusingcontinuousmeteorologicalobservationsfromthePoSLroof(topofTable3).Table
4showsalistofallvariablesusedintheannualroofheatfluxmodel.Fortheexposedroof,
theseare(inthisorderinEqs.3and4)netshortwave(solar)radiation,incominglongwave
radiation,outgoinglongwaveradiation,convection,conductionintotheroof,andchangein
internalenergy(storage).ForthePVcoveredroof,globalsolarradiationisreplacedbydiffuse,
andincominglongwaveradiationcomesfromboththesolarpanelandtheskyweightedby
theirrelativeskyviewfactors(SVF)(Eq.4).
0  1
  
,

∆ 
∆

 

(3)
0
1
  
1

∗
, 
 
∆  
∆
 
 
(4)
Thedownwellinglongwaveradiationfromtheskywascalculatedusing(CIMIS2010)
, 

.(5)
17
Thecloudfraction,f,iscalculatedbasedontheratioofmeasuredGHItoclearskyGHIand
variesfrom0.595(verycloudy)to1(clear).Thenetemissivityisbasedonthevaporpressure
(ea,calculatedfromRHandT)as  0.34  0.14.ThePVpaneltemperature(Tp)is
neededtocalculate,andwasmodeledusinganenergybalancemethod(Jonesand
Underwood,2001).
Modelresultsaremostsensitivetotheconvectiveheattransfercoefficient(hc)thatwas
obtainedusingtheDOE2convectionmodel(Eqs.68,ASHRAE,2005b)asacombinationofthe
coefficientsfornaturalconvection(hn)andforcedconvectionoverasmoothsurface(hc,glass).
 1.520|
|
 (upwardheatflow)or(6a)
 0.4958|
|
 (downwardheatflow)(6b)
, 
(7)


, 
(8)
Theconstantsaandbwerechosenbasedonwindtunnelmeasurementsoverasmoothsurface
fortheexposedroofandmeasurementsona6thfloorrecessedsurface,whichexperiencesa
similardropinwindspeedsfromthefreestream,forthePVcoveredroof(Palyvos,2008).hi=
1.25Wm2K1wasusedfortheinteriorheattransfercoefficient(ASHRAE,2005b).
Equations3and4werecoupledtotheheatconductionmodel(Section4)tocalculatetheheat
fluxintothebuildingfromexteriorboundaryconditionsofTa,u,RH,GHI,andDIFFandinterior
boundaryconditionofconstantairtemperature(Ti).SinceEqs.3and4arenonlinear,
Newton’smethod(Eq.9)isusedtoiterativelysolvefortheroofsurfacetemperature.In
Newton’smethod,Trattheprevioustimestepisusedastheinitialguess(xi)andEqs.3and4
18
andtheirderivatives(FandF’)areusediterativelyuntilTratthecurrenttimestep(xi+1)
converges.
 

(9)
Theroofheatfluxintothebuildingisthencalculatedfromtheinteriorceilingsurface
temperatureandtheassignedinteriorairtemperatureby 
.
5.2Validation
Themodelwasvalidatedagainstthedataobtainedintheintensivestudyperiod.Fig.7shows
themodeledandmeasuredroofsurfacetemperatures,andFig.8showsthemodeledheatflux
intothebuildingagainsttheheatfluxcalculatedinSection4basedonthemeasuredroof
temperature.Theexposedroofheatfluxhadarootmeansquareerror(RMSE)of5.48Wm2
andameanbiaserror(MBE)of4.40Wm2.ThePVcoveredroofhadanRMSEof2.18Wm2
andaMBEof1.72Wm2.Consideringthecomplexenvironmentofabuildingrooftopthe
agreementwasconsideredsufficientforvalidation.
19
Fig.7.MeasuredandmodeledroofsurfacetemperatureusingEqs.3and4fora.theexposed
roofandb.PVcoveredroofforApril19,2009.
Fig.8.Heatfluxfromtheceilingtobuildingair(positivevalueequalsfluxintobuilding)
computedusingtheconductionmodelofSection4usingthemeasuredTr(Fig.6,Section4)and
themodeledTr(Eqs.3and4)duringApril19,2009fora)exposedroofandb)PVcoveredroof.
10
20
30
40
50
60
Temperature [
o
C]
a)
T
r
modeled T
r
00:00 06:00 12:00 18:00
10
20
30
40
50
60
Time [PST]
Temperature [
o
C]
b)
T
rs
modeled T
rs
-40
-20
0
20
40
Heat Flux [Wm
-2
]
a) from measured T
r
from modeled T
r
00:00 06:00 12:00 18:00 00:00
-40
-20
0
20
40
Time [PST]
Heat Flux [Wm
-2
]
b) from measured T
rs
from modeled T
rs
20
5.3Roofcoolingload
Theroofheatfluxcontributiontocoolingandheatingloadsisestimatedbydividingthedaysof
theyearintocooling(averageofdailymaximumandminimumoutsideairtemperaturegreater
than18.3oC(65oF))andheating(averageofdailymaximumandminimumtemperatureless
than18.3oC(65oF))days.Forcooling(heating)daysthetotaldailynetincoming(outgoing)
heatfluxisassumedtoconstitutethedailyHVACloadrelatedtotheroofenvelopeflux.Based
onHVACsetpointsandhoursofoperationusedinotherairconditionedcommercialbuildings
oncampusforcoolingdays,Ti=23.3oC(74oF),andabuildingscheduleof08002000PSTare
assumed.FortheheatingdaysTi=21.7oC(71oF)anda24hourbuildingscheduleareassumed.
ThejustificationsforthebuildingschedulesarediscussedattheendofSection5.
Theroofcoolingandheatingloadanalysiswasrunbymonth(Table5)forallof2009which
included155coolingdaysand210heatingdays.
Table5:Meanmonthlyroofheatfluxcontributionstocoolingandheatingloadsfor2009.
Coolingloadisaverageloadduring08002000PSToncoolingdays.Heatingloadisaverageload
overtheentireheatingday.Negativeheatingloadmeansthattheroofheatflowsintothe
buildingonaheatingday.Numbersinitalicsrepresentmonthswith2orlessdaysofcoolingor
heating.CDD:coolingdegreedays.HDD:heatingdegreedays.
MonthCDD/HDDmeancoolingload[Wm2]meanheatingload[Wm2]
[oCd]Exposed
roof
PVcoveredExposed
roof
PVcovered
133.4/79.4 2.330.933.403.38
25.33/1202.701.152.062.78
35.56/1185.114.080.750.70
417.4/96.112.85.79‐1.99‐0.77
53.05/38.211.74.223.52‐3.43
64.44/13.38.066.48‐4.09‐4.07
21
780.6/0.5911.46.622.70‐5.83
8106/010.417.27N/AN/A
9100/09.736.31N/AN/A
1030.4/20.66.133.27‐1.41‐1.16
1110.1/60.64.101.710.701.22
123.69/1352.252.683.513.12
Total400/6838.385.21‐0.270.16
Themaximumcoolingloadreduction(4.8Wm2)occurredinJuly,whichwaswarmandmostly
clear.Onclearcoolingdaysthebenefitsofshadingaremaximizedcausingalargereductionin
coolingloadcomparedtotheexposedroof(e.g.Fig.9).Onovercastcoolingdaystheexposed
roofisalsoshadedbycloudssothePVcoolingloadissimilartotheexposedroofcoolingload.
InJune2009(acloudymonth)averagecoolingloadsdifferedbylessthan1.6Wm2.
Onheatingdaystheincreasedlongwaveradiationfromthepanelbecomesabenefitonall
nights(especiallyclearnights)and(toalesserextent)oncloudydays.Forexample,twocloudy
heatingdaysinDecemberhadalowerheatingloadforPVduringdaytimeandatnight(Fig.10).
Onclearheatingdaystheexposedroofhasalowerdaytimeheatingloadduetotheincreased
solarirradiation.Sinceonlytheroofcontributiontotheheatingloadwascalculated,thesolar
radiationcontributionleadstoanegativeheatingloadformanymonthlyaverages,i.e.theroof
heatfluxisactingtoreducetheheatingloadcausedbywallheatfluxesandinfiltration.Dueto
themoderateairtemperaturesandsignificantsolarirradiationinSanDiegowinterstheannual
meanroofheatingloadisnearlyzeroforbothcases;theheatlossesdrivenbytheinsideto
outsidetemperaturegradientareovercomebyheatgainthroughabsorptionofsolarradiation.

22
Fig.9.a.Modeledroofcoolingloadandb.measuredGHIonJuly1213,2009.Hot,cloudfreeconditions
resultinthegreatestcoolingloadsavingsunderthePVarraywithameandaily(08002000)coolingload
of13.9Wm2fortheexposedroofand6.19Wm2forthePVcoveredroof.
Fig.10.a.Modeledroofheatingloadandb.measuredGHIonDecember1112,2009.Coldandcloudy
daysresultinthegreatestreductioninheatingloadunderthePVarray,withameanheatingloadof
5.96Wm2fortheexposedroofand4.03Wm2forthePVcoveredroof.
-10
0
10
20
30
roof cooling load [W m
-2
]
a) exposed roof
PV covered roof
00:00 06:00 12:00 18:00 00:00 06:00 12:00 18:00 00:00
0
200
400
600
800
1000
Time [PST]
GHI [W m
-2
]
b)
-5
0
5
10
roof heating load [W m
-2
]
a)
exposed roof
PV covered roof
00:00 06:00 12:00 18:00 00:00 06:00 12:00 18:00 00:00
0
100
200
300
400
500
Time [PST]
GHI [W m
-2
]
b)
23
Theannualcoolingorheatingloadmodelhasseveralshortcomings.Thecoolingloads
representonlytheloadnecessarytomaintainTi=23.3oCduringtheday.The‘startup’loadto
cooltothedesiredTiinthemorningatthebeginningofHVACoperationisnotconsidered.For
coolingload,theeffectofignoringthestartuploadissmall,sincethegenerallycoolnights
wouldresultinTi<23.3oCatthebeginningofHVACoperation.However,fortheheatingload
the‘startup’loadcanbesignificant,andwasaccountedforbyusinga24hbuildingschedulefor
theheatingload.Theinteriorboundaryconditiondoesnotaccountforlongwaveradiativeheat
exchangebetweenbuildingenvelopesurfaces.Whiletheabsoluteresultsmaynotbeaccurate
orrepresentative,thedifferencesbetweentheexposedroofandthePVcoveredroofsurface
temperatureandheatfluxesareconsideratecorrect.
6. DiscussionandConclusions
Carefulmeasurementsofthethermalconditionsthroughoutaroofprofileonabuilding
partiallycoveredbysolarphotovoltaic(PV)panelswereconducted.Thermalinfrared(TIR)
imagerydemonstratedthatceilingtemperaturesunderthePVarrayswereupto2.5Klower
thanundertheexposedroofat1700PST,atimethatlieswithintheintervalofpeakenergy
demand,definedbySDG&Eas12001800PST.Thedailyvariabilityinrooftopsurface
temperatureunderthePVarraywashalfthatoftheexposedroof,indicatingareductionin
thermalstressesoftheroofstructure.TheceilingtemperaturesunderatiltedPVarrayoffset
fromtheroofallowingheatadvectionwerecoolerthanunderaflatarraywhichwasmounted
flushwiththeroof.Atnightwithcalmwindstheconditionsreversedandtheceilingunderthe
PVarrayswaswarmerthantheceilingundertheexposedroof,especiallyfortheflushPVarray.
24
Largeindoorairandsurfacetemperaturescausedtheroofheatflux(definedasconductive
heatfluxfromthebottomrooflayertotheceiling)underthetiltedPVarraytobeupwardfor
mostoftheday.Themeandaytimeheatflux(12002000PST)undertheexposedroofinthe
modelwas14.0Wm2largerthanunderthetiltedPVarray.Themaximumdownwardheatflux
was18.7Wm2fortheexposedroofand7.0Wm2underthetiltedPVarray,a63%reduction
duetothePVarray.Thereductioninheatfluxiscomparabletothe65%reductionincooling
loadshownbythemodelin(Yangetal.,2001)andthe52%reductionofsummertimepeak
coolingloadfromthesimulationin(Wangetal.,2006).Asensitivityanalysisofourresultsto
temperaturemeasurementuncertaintyconfirmstherobustnessofthedifferenceinroofheat
fluxunderexposedandPVcoveredroof(AppendixA).
Expandingthemodeltoutilizeinternalairtemperature(Ti)andoutsidemeteorological
conditions(GHI,DIFF,Ta,RH,andu)asboundaryconditionsallowedfortheroofheatfluxtobe
modeledfortheyear2009toestimateheatingandcoolingloads.Totalannualcoolingloadof
thePVcoveredroofdecreased38%to9.69kWhm2from15.6kWhm2fortheexposedroof.
ConsideringthetotalannualPVenergyproductionof148kWhm2,theannualcoolingload
reductionof5.91kWhm2enhancestheannualnetenergybalanceofPVby4%.Thebenefits
weregreatestinJuly(awarmandsunnymonth),withadifferenceindailycoolingloadof57.36
Whm2andanaveragedailyPVarrayelectricityproductionof570Whm2,resultingina10%
enhancementofthenetenergybalanceofPV.ThedifferencebetweenPVandexposedroof
wasdependentoncloudcover,ascloudydaysincreasedthebenefitsofPVonheatingdaysand
decreasedthebenefitsoncoolingdays.ThedaytimeshadingprovidedbyPVbecomesa
disadvantageonclearheatingdaysaspassiveheatingisreducedcomparedtotheexposed
25
roof.However,thislossinpassiveheatingisaboutbalancedbyareductioninnighttime
radiativeheatlossresultinginoverallsimilarheatingloadforPVcoveredandexposedroof.The
reductioninnighttimeheatlossisexpectedtoreduceheatingenergyuseinmostregionsofthe
world,especiallyforresidentialapplicationswheremostheatingoccursatnight.Thetools
developedinthisstudycanbeusedwithdifferentmeteorologicalforcingdataandroofthermal
propertiestoestimateheatingandcoolingbenefitsofPV.
ThepresentstudyisuniqueastheimpactoftiltedandflushPVarrayscouldbecompared
againstatypicalexposedroofatthesameroofforacommercialuninhabitedbuildingwith
exposedceilingandconsistingonlyofthebuildingenvelope.Consequently,otherfactorsthat
oftenmakeitdifficulttocomparedifferentroofingmodifications,suchasmicrometeorological
conditionsatthesite,influenceofthesurroundingurbanenvironment,occupantbehavior,and
differencesinbuildingthermalpropertiescouldbeexcludedascontributingfactorsinour
study.NeverthelessthisisacasestudyandtheresultsfortheimpactofPVonthebuilding
coolingloadmaynotbegenerallyapplicable.Shortcomingsoftheanalysisoftheintensive
measurementsinSection4arethat(i)thebuildingwasunventilatedreducingroofcoolingload;
(ii)theceilingareasunderthedifferentroofsectionswerethermallycoupledthroughradiative
andconvectiveexchangeviathebuildinginteriorandgroundslabbluntingthethermal
differencesbetweenexposedandPVcoveredceiling;(iii)thecorrugatedsteelontheceiling
mayhaveconductedheathorizontallywhichisnotaccountedforinour1dmodelagain
bluntingthethermaldifferencesbetweenexposedandPVcoveredceiling.However,allof
theseshortcomingsacttoreducethebenefitofshadingfromthePVpanel,soourresultscould
beconsideredaconservativeestimateofPVbenefitsforthisparticularbuilding.Forafuture
26
studywerecommendcomparingclimatecontrolled,identicalbuildingsinaneighborhood,one
withandonewithoutaPVarray.
Inaddition,theresultsserveasapotentialexplanationofthereductioninenergyuseonhot
daysfoundbytheCaliforniaSolarInitiativeimpactevaluation(ITRONInc.,2010),which
presumablyalsohavethehighestroofradiationloadingandmaximumshadingbenefitofPV
systems.WiththeexponentialgrowthinrooftopPV,itbecomesmoreimportanttoconsider
theeffectofrooftopPVsystemsonbuildingHVACcosts.ThemodelsforindirectPVeffectson
coolingloadcouldbeusedsimilartoexistingroofcalculatorsforinsulationimprovementsand
roofalbedoincreases(e.g.ASHRAE,2005a;DOECoolRoofCalculator2009;EPARoof
Calculator,2009).
Nomenclature
TermDescription
a Velocitycoefficientinhcforexposed
roof/PVcoveredroof
bVelocityexponentinhcforexposedroof/PV
coveredroof
c
p
Heatcapacityofroofmaterial
eaVaporpressure
fCloudfraction
qConductiveheatflux
hcExteriorconvectiveheattransfercoefficient
hc,glassHeattransfercoefficientoverasmooth
surface
h
i
Interiorconvectiveheattransfercoefficient
hnNaturalconvectionheattransfercoefficient
kThermalconductivityofroofmaterial
uWindspeed
ARLAboveRoofLevel
BIPVBuildingIntegratedPV
CDDCoolingDegreeDays
27
DIFFDiffuseirradiation
GHIGlobalHorizontalIrradiation
HConvective(sensible)heatflux
HDDHeatingDegreeDays
HVACHeating,Ventilating,andAirConditioning
Ld,sk
y
Downwellinglongwaveradiationfromsky
MBEMeanBiasError
PoSLPowellStructuralLaboratory
PSTPacificStandardTime
PVPhotovoltaic
R
f
Surfaceroughnessmultiplierinhc
RHRelativehumidity
RMSERootMeanSquareError
SDG&ESanDiegoGasandElectric
SVFSkyviewfactorofroofunderPVpanel
TaAirtemperature
TasAirtemperatureundertiltedarray
TcCeilingtemperature
TcsCeilingtemperatureundertiltedarray
Tcfla
t
Ceilingtemperatureunderflatarray
T
i
Interiorairtemperature
T
x
Temperatureatxthrooflayer
T
p
PVpaneltemperature
Tr
Roofsurfacetemperature
Trs
Roofsurfacetemperatureundertilted
array
Tr
n1Rooftemperatureatprevioustimestep
TIRThermalInfrared
α Roofsurfacealbedo
εnetNetemissivityofair
εrRoofemissivity
εpPVemissivity
ρ Densityofroofmaterial
σ StefanBoltzmannconstant
ΔxDiscretizedspacingofrooflayers
AppendixA:SensitivityAnalysis
Thesensitivityoftheheatfluxcalculationtothesurfacetemperatureswasanalyzedbyvarying
theoffsetbetweenroofsurfacetemperaturesTrsandTrandtheceilingtemperaturesacquired
28
bytheTIRcamera(Tc).Figure12showsthatoffsettingexposedroofandPVcoveredroof
measurementshasasmalleffectonreductioninpeakheatfluxanddifferenceinmeandaytime
heatflux.OffsettingTrsandTrwithina+‐0.5oCwindow(atypicaluncertaintyofathermistor)
resultsinarangeof55%‐69%reductioninpeakheatfluxandameandaytimeheatflux
differenceof12.215.9Wm2.Thesenarrowrangesshowthatourheatfluxresultsarerobust.
Fig.11:Sensitivityofa.reductionofpeakroofheatflux[%/100]andb.differenceinmean
daytime(1200‐2000PST)heatflux[Wm2]tooffsetsinexposedroof(Tr)andPVroof(Trs)
surfacetemperatures(seeFig.6cforaplotofthisheatfluxfortheexposedroofandPVroof).
AppendixB:ExtensionofconductiveheatfluxanalysistoincludeflushPVarray
Inamannersimilartothederivationofsolairorenvironmentaltemperaturesoutdoors,the
heatfluxintothebuildingaircanbesimplifiedintosomeoverallindoorheattransfer
coefficienthitimesthedifferenceintheceilingsurfacetemperatureTcandsomeinternal
environmentaltemperatureTie


(A.1)
PV offset [
o
C]
Exposed offset [
o
C]
a)
-2.0 -1.0 0.0 1.0 2.0
-2.0
-1.0
0.0
1.0
2.0 0.3
0.4
0.5
0.6
0.7
PV offset [
o
C]
Exposed offset [
o
C]
b)
-2.0 -1.0 0.0 1.0 2.0
-2.0
-1.0
0.0
1.0
2.0 8
10
12
14
16
18
20
29
KnowingqandTcforboththeexposedroofcaseandthetiltedPVarray(section4)ateachtime
stepallowssolvingforhiandTieforeachtimestepusingthelinearsystemofEq.A.1.Thesecan
inturnbeusedtosolvefortheheatfluxundertheflusharraygiventheceilingtemperature.
Thedaytimeresultsofthisanalysis(Fig.12)showthatthemeandaytimeheatflux(12002000
PST)is7.25Wm2lessthanundertheexposedroofand6.78Wm2greaterthanthatunderthe
tiltedarray.
Fig.12.Heatfluxthroughthebottomrooflayer(section4.,Eq.2)forallthreerooftypes.The
solutiongoestoinfinity,however,whentheTc,exposedequalsTc,pvorqPVequalsqexposedwhich
occursaround0100and0900PST.
Acknowledgements
AnthonyDominguezwasfundedbytheNASAgraduatestudentresearchersprogram.Kleissl
acknowledgesfundingfromaNSFCAREERawardandtheHellmanfoundation.Thefollowing
UCSDundergraduatestudentswereinstrumentalinthedatacollection:AvneetSingh,Kevin
04:00 08:00 12:00 16:00 20:00
-30
-20
-10
0
10
20
Time [PST]
Heat Flux [Wm
-2
]
exposed roof
tilted PV array
flush PV array
30
Chivatakarn,ThomasMinor,JeremiahFarinella.WethankRonnenLevinsonforcontributinghis
expertiseandthePowellStructuresLaboratorybuildingstaff,especiallyAndrewGunthardt,for
beingsupportiveofourworkandallowingaccesstothebuildingforthemeasurements.
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... BIPV can help reduce thermal summer load of roofs due to direct solar irradiance [34]. BIPV has significant influence on the heat transfer through the building envelope and its consequent effect on the buildings cooling and heating loads [33]. ...
... This system also was the most efficient solution in term of the produced output power and comfortability. Table 1 Simulation Results of thermal analysis for rooftop shaded by PV modules Another study was conducted by Dominguez et al. [34] to assess the impact of BIPV on rooftop heat transfer. They shaded the rooftop by air-gap tilted PV and flat PV without air-gap. ...
... 3. Improving PV conversion efficiency: The distance between the surface of the rooftop and the back surface of the PV is sufficient to prevent the heat generated from the solar panels from reaching the roof surface, and it facilitates the natural cooling of the solar panels and the rooftops. Combination of PV and ventilated roof improve PV conversion efficiency and reduce cooling load [34,38]. 4. Improving the air-conditioning efficiency: Improving the air-conditioning efficiency by creating a cooler microclimate zone beneath the PV arrays and by preventing the effect of direct normal irradiance (GDNI) on the outdoor unit of the ac. ...
Article
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The main objective of this study is to develop strategies which aim to improve the energy saving and the sustainability at the University of Jeddah (UJ) - Khulais branch. The energy saving can be improved by reducing -indirectly- the cooling load, while the sustainability can be improved by recycling the sewage water, generating energy from the solar irradiance and improving land-use efficiency. Initially, the main reasons for increasing energy consumption in the buildings were identified. The most important reasons were large heat gain through the flat rooftops and sidewalls, infiltration and exfiltration, partial damage to the insulation materials of walls and ceilings, and thermal mass. To reduce the heat gained through buildings' rooftops, it is proposed to shade the rooftops by tilted and ventilated airgap Building Integrated Photovoltaic (BIPV). BIPV also help to improve the shading efficiency, land use efficiency, natural cooling of the rooftops and PV modules, Energy Efficiency Rating (EER) of the air-cooled ac system, and efficiency of night ventilation system. To reduce heat gained through the sidewalls, it is proposed to shade the walls by planting long-stemmed trees (Eucalyptus) around the buildings. Irrigating the trees by gray water will improve the sustainability and improve Leed Credit Point. Previous studies and current measurements of solar radiation under shaded surfaces have shown that shading the building is an appropriate strategy to reduce solar heat gain. Startup load of air conditioning system is another source of increasing energy use and it forms around 10-20% of the total cooling load due to high indoor temperature at the morning. As the buildings are not occupied after 3:00 PM, and during weekends, it is proposed to use mechanical night ventilation to reduce the indoor temperature and to improve the air quality. Connecting the ventilation system with a separate control system and the BMS at the buildings help to reach to the possible minimum indoor temperature.
... Increasing the solar reflectance results in a lower surface temperature, since solar radiation is reflected, not absorbed. Therefore, less heat penetrates the building, and the output power and energy efficiency consequently increase [6][7][8]. ...
... 1-White Shingle conductive heat flux using Crank-Nicholson method [6]: is the roof testing area. The roof area around the Middle modules was chosen due to their central location on the roof. ...
Conference Paper
Full-text available
Cool-roofing systems with a high solar reflective index offer numerous benefits to buildings, occupants, and solar electric power generation systems. The proposed experimental cool roof-mounted solar project demonstrates how a cooler roof turbocharges solar photovoltaic system by not only boosting power and energy, but also extending the lifespan of solar modules and other rooftop electrical equipment. The project was approached by viewing the energy production of grid-tie solar photovoltaic system, mounted on the ENERGY STAR® certified cool roof. Upon project completion, produced DC-AC voltages, along with generated power, were monitored daily/monthly/yearly from 2015 to 2019 and authenticated by thermal modeling calculations. As result, obtaining an average of 78.54% experimental output efficiency, calculating a cooling load of 0.18 Wm-2, and decreasing total energy usage by 20% over 25 years proved that cool-roof application is more advantageous than a conventional hot-roof.
... where Δx is the numerical discretization distance between roof layers, k is the Thermal Conductivity, T₃₁ is the hot temperature and T₃₂ is the cold temperature [43]. ...
... Most of the building's thermal simulation software is unable to predict the actual energy consumption of the building accurately and precisely due to many factors, such as different run engines for each software [38,39], the accuracy of the input, or default data such as the R-value, weather data and the sky temperature [42]. Many retrofitting approaches were used to improve the building's thermal performance and decrease energy consumption, even though, in some cases, a photovoltaic system will be a cheaper option than the retrofitting [43][44][45].The inside temperature was set and assumed to be 20 °C in winter, and 26 °C in summer, while the outside temperature is not fixed and varies all the time, as the weather station that is mentioned and used in the simulation has its own data for the temperature in the studied location. ...
Article
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Solar energy is one of the most abundant and available forms of renewable energy. Reliance on the electricity network can be decreased and net-zero energy achieved by mounting photo-voltaic power on the tops of houses. Photovoltaic arrays can also change how the roof's surface reacts to its environment. The influence of the structural system of a roof and weather on the energy consumption of a building is important. This research is concerned with focusing on the indirect effect of solar photovoltaic rooftop panels (shading effect) on the roof surface to see whether this effect is worth studying and calculating the total electrical load in the residential sector. Photovoltaic panels were modeled as a shading device, and the Integrated Environmental Solution-Virtual Environment Software was used to anticipate the monthly decline and growth in heating and cooling loads associated with the roof level. The influence of a photovoltaic system on a building's roof-related energy load was measured concerning low-rise residential buildings in Mafraq city, which belongs to a mild dry-warm temperature zone. The findings indicated that a solar roof structure decreased heat loss by 4.85% in the summer and boosted heat transfer by 5.54% in the winter. The results highlight that renewable energy is very important in our times due to climate change and the increased demand for electricity by the residential sector, which is stimulated to find multiple ways to decrease and adapt to this change, and the aim of this paper helps to encourage to use solar energy by identifying the indirect effect of solar panels on building's rooftops. This investigation also focuses on the value of offering essential instructions to who is concerned to the utilization of alternative energy to heat and cool structures, also will educate the public on a building's total energy requirements, which is critical for future green structure design.
... Although the low albedo of RPVPs may cause excessive rooftop radiation load, the incident energy can be partially harvested into electricity, leading to a reduction in radiation heating. Dominguez et al. [21], Taha [22], Masson et al. [23], Salamanca et al. [24] and Zhang et al. [25] have demonstrated the significant interceptive impacts of large-scale implementation of RPVPs through numerically modeling. The financial effects of RPVPs were also highlighted by Ma et al. [26], who priced the air cooling benefits (between A$230,000 and US$3,380,000 net, depending on the coverage of the RPVPs implemented in Sydney). ...
... Based on the understanding of the physical mechanisms driving the modifications induced by RPVPs, several RPVPs models with different complexities have been developed to evaluate RPVPs impact on thermal condition and energy demand for the cooling of buildings. Dominguez et al. [21] quantified the impact of RPVPs on rooftop thermal condition at the building scale through numerical simulations, but their results could not be upscaled to the city scale, since the used model did not consider the influence of a wide range of urban geometry, thermal properties of building materials and climatic conditions. Some studies further developed the RPVPs models to couple with urban parameterizations in the mesoscale meteorological models, which have been used to investigate the city-wide impact of RPVPs. ...
Article
Full-text available
Rooftop photovoltaic panels (RPVPs) implementation is one of the effective strategies to mitigate urban heat island and relieve urban energy demand with renewable energy resources, which are in need, especially during extreme heatwave events. However, the effects of RPVPs on cooling the urban thermal environments and saving energy have not been fully investigated in terms of RPVPs coverage (CV) and conversion efficiency (CE). To address this question, this study conducted a numerical evaluation of RPVPs on the thermal environment, cooling energy consumption (EC) in buildings, and electricity production (EP) during an extreme heatwave event in a semi-arid Chinese city using the WRF model coupled with building effects parameterization and building energy model. Simulations of twelve scenarios with four CVs and three CEs were conducted. The results indicated that RPVPs implementation could generally lower the 2-m air temperature, due to RPVPs’ higher effective albedo and higher emissivity. An increase in RPVPs CV and CE resulted in stronger cooling effects and larger cooling areas, where the RPVPs with a CV of 100% and a CE of 0.3 could lower the 2-m air temperature by 0.4 – 0.7°C, and significantly contributed to a decrease of cooling EC by 14.74%. The EP could offset and even exceed the cooling EC, with the EP/EC reaching 182.61%, 135.77%, and 123.37% in 100% coverage (CE=0.3), 75% coverage (CE=0.3) and 100% coverage (CE=0.2) scenarios, respectively, indicating that RPVPs could potentially generate enough electricity to compensate cooling EC in semi arid cities. Our findings have practical implications for the RPVPs implementation and the necessity of improving conversion efficiency for better thermal and energy benefits.
... panels (PV) are a form of renewable energy. They are used in roofs because of their ability to provide electricity and reducing of fossil fuels. They can have an indirect impact on the energy efficiency of the building because they provide shade, absorb solar radiation, and minimize heat gains.(Dominguez, Kleissl, Luvall, 2011). They can reduce heat flux by 60-63% and save energy by 6-7% in a tropical zone but their effect is low if roof insulation is installed(Chenvidhya, Seapan, Parinya, 2015). ...
Thesis
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People face a critical problem with the gradual increase in temperature due to climate change as well as being in Egypt, a hot arid climate zone. Most of the construction companies supply their buildings with manufactured heating and cooling systems like air conditioners with different types and variations. Although these systems are well designed to achieve their goal which is internal thermal comfort, they heavily rely on electricity to function which results in huge power consumption. So, green roofing is considered as a solution that can solve these problems and incorporate sustainable development principles in building features. Green roofs have a lot of other environmental advantages such as improving the management of stormwater, reducing the effects of urban heat islands, decreasing air and noise pollution, and increasing wildlife ecosystems as well as social and economic benefits. The aim of this thesis is to investigate how green roof technology can improve the thermal behavior and energy usage of educational buildings in Egypt as compared to a traditional roof. The research adopts a quantitative approach towards investigating and comparing the green roof with the conventional roof. The simulation working mechanism is by building a digital model, measure and compare the effect of this alternative roof design on temperature and energy demand, to determine if the green roof minimizing heat gain through the roof. This method is applied in building A in the British university in Egypt. The research results proved that green roofs could enhance thermal performance and reduce energy demand in buildings in summer and winter by decreasing indoor air temperature to be able to reach thermal comfort.
... However, cool roofs and green roofs are not the only roofing options that can mitigate heat and reduce electricity consumption. Recently, the use of solar photovoltaic panels has surged in the cities. Dominguez et al. (2011) showed that in San Diego, California, solar photovoltaic panels that partially cover building rooftops can reduce not only greenhouse gas emissions but also the annual cooling load. Salamanca et al. (2016) found that solar photovoltaic panels could lower the 2-m temperature over Phoenix and Tucson, Arizona. ...
Article
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This study examined the impact of cool roofs, green roofs, and solar panel roofs on near-surface temperature and cooling energy demand through regional modeling in the Chicago metropolitan area (CMA). The new parameterization of green roofs and solar panel roofs based on model physics has recently been developed, updated, and coupled to a multilayer building energy model that is fully integrated with the Weather Research and Forecasting model. We evaluate the model performance against with observation measurements to show that our model is capable of being a suited tool to simulate the heatwave event. Next, we examine the impact by characterizing the near-surface air temperature and its diurnal cycle from experiments with and without the different rooftops. We also estimate the impact of the rooftop on the urban island intensity (UHII), surface heat flux, and the boundary layer. Finally, we measure the impact of the different rooftops on citywide air-conditioning consumption. Results show that the deployment of the cool roof can reduce the near-surface temperature most over urban areas, followed by green roof and solar panel roof. The cool roof experiment was the only one where the near-surface temperature trended down as the urban fraction increased, indicating the cool roof is the most effective mitigation strategy among these three rooftop options. For cooling energy consumption, it can be reduced by 16.6 %, 14.0 %, and 7.6 %, when cool roofs, green roofs, and solar panel roofs are deployed, respectively. Although solar panel roofs show the smallest reduction in energy consumption, if we assume that all electricity production can be applied to cooling demand, we can expect almost a savings of almost half (46.7 %) on cooling energy demand.
... Similarly, in hot climates, the potential loss in shading from stand-off PV arrays (which reduces cooling loads) must be compensated by added insulation from the BIPV element. The beneficial impact of the shading of stand-off PV arrays on reducing building cooling needs is well known [204][205][206]. Thus, BIPV applications need to achieve superior insulation for roofing elements to provide equivalent performance to conventional PV systems. ...
Article
Despite the technical maturity and substantial potential cost reduction of BIPV technologies, there are still challenges to overcome for the expansion of BIPV applications and their wider adaptation at global level. Among these, the alignment of PV integration with particular climate and environmental conditions of the local solar architecture is crucial. This will facilitate the transition to sustainable buildings and the mitigation of climate change. In this context, this study proposes for the first time, a novel BIPV climatic design framework for PV buildings positioning and adaptation to local climate towards the minimization of energy expenditure and use of resources. With the review and analysis of a large numbers of BIPV studies globally for seventy parameters grouped in eight main categories of an open-access database, the global horizontal irradiation (GHI) value is selected as an additional index to the Köppen-Geiger classification scheme. The extension accounts for the urban suitability and vulnerability and prioritize the building integration of photovoltaics. Four zones of cold (low GHI), moderate (medium GHI), warm (high GHI) and hot (very high GHI) climatic regions are considered and applied for 127 cities globally. In this framework, the sequence of PV building component integration is proposed according to local climate of each zone and the energy performance of buildings is maximized towards their positive energy contributions and sharing in local, district and city grids. Barriers and limitations of the BIPV implementation at a larger scale are discussed and the emerging research needs are revealed.
... The rooftops of warehouses provide an ideal site for the installation of PV systems, which deploy solar panels on the warehouse rooftop to convert solar energy to electricity. Photovoltaic panels not only provide power to the warehouse system, but also lower the temperature of rooftops, which reduce the energy consumption required for cooling on the refrigerated warehouse roof (Dominguez et al., 2011;Luerssen et al., 2019). MSR can provide essential electricity supplies by on-site generation and the discharge of energy storage devices and reduce the consumption of grid-purchased electricity. ...
Article
Full-text available
The rapid development of intelligent warehouse systems is resulting in the realization of automation in warehouse activities and raising awareness of decarbonization, particularly the need to reduce carbon emissions from electricity consumption. Driven by the decarbonization trend, microgrid systems with rooftop photovoltaic panels are becoming more popular in warehouses and are providing zero-carbon electricity for warehouse operations. How to make better use of microgrid systems and reduce the consumption of electricity generated from traditional energy sources is becoming increasingly important in warehouse systems. This paper investigates an operational problem in a warehouse system equipped with a shuttle-based storage and retrieval system, in which a microgrid system acts as the main electricity source. Power-load management is applied to avoid peaks of energy consumption, and a mixed linear programming model is developed to optimize task sequencing and scheduling with decarbonization awareness. To solve the proposed problem, a data-driven variable neighbourhood search algorithm is built. Numerical experiments are conducted to validate the model and algorithm. Sensitivity analysis shows the effectiveness of power-load management and the influence of system configuration on energy consumption.
Article
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บทความวิจัยนี้นำเสนอผลการศึกษาบางส่วนของการประยุกต์ใช้แผงเซลล์แสงอาทิตย์กับระบบบำบัดน้าเสียสำหรับ โครงการอาคารพักอาศัยของการเคหะแห่งชาติ การศึกษาถึงประโยชน์ด้านอื่น ๆ ของการประยุกต์ใช้แผงเซลล์แสงอาทิตย์ ในอาคารนอกเหนือจากการผลิตไฟฟ้า ได้แก่ ด้านการลดความร้อนผ่านกรอบอาคาร ด้านการลดการปลดปล่อยก๊าซคาร์บอนไดออกไซด์ ด้านการใช้งานเมื่อยามเกิดภัยพิบัติและด้านความคุ้มค่าเชิงเศรษฐศาสตร์ ผลการวิจัยสรุปได้ว่า ระบบบำบัดน้ำเสียสำหรับอาคารพักอาศัยการเคหะฯ มีความต้องการใช้พลังงานไฟฟ้าเฉลี่ยอยู่ที่ 2.10 กิโลวัตต์-ชั่วโมง/วัน โดยกำลังการผลิตไฟฟ้าของแผงเซลล์แสงอาทิตย์ 2.22 กิโลวัตต์ (0.185 กิโลวัตต์/แผง จำนวน 12 แผง) จะสามารถผลิตพลังงาน ไฟฟ้าได้ 5.48 กิโลวัตต์-ชั่วโมง/วัน ซึ่งรองรับการใช้พลังงานไฟฟ้าในระบบบำบัดน้ำเสีย ส่วนผลประโยชน์ด้านอื่น ๆ นั้น รวมถึงด้านการลดความร้อนผ่านกรอบอาคาร พบว่าการติดตั้งแผงเซลล์แสงอาทิตย์ครอบคลุมร้อยละ 81 ของพื้นที่หลังคา สามารถช่วยลดอุณหภูมิอากาศภายในห้องพักอาศัยที่อยู่ในชั้นใต้หลังคาได้สูงสุด 4 องศาเซลเซียส ( ํC) ในด้านการลดการ ปลดปล่อยก๊าซคาร์บอนไดออกไซด์ พบว่า สามารถลดได้ 44.91-718.61 ตันคาร์บอนไดออกไซด์ ตลอดช่วงอายุการใช้งาน ซึ่งเทียบได้กับการใช้รถยนต์ 6-107 คัน ทั้งนี้ ขึ้นกับรูปแบบการบริหารจัดการระบบบำบัดน้ำเสียที่มีรูปแบบเปิด 15 นาที แบบเปิด 3 ชั่วโมงและแบบเปิด 24 ชั่วโมง สำหรับด้านการใช้งานเมื่อยามเกิดภัยพิบัติ พบว่า การติดตั้งเซลล์แสงอาทิตย์ ให้สามารถรองรับการใช้พลังงานในระบบบำบัดน้ำเสียรูปแบบเปิด 24 ชั่วโมง จะสามารถผลิตไฟฟ้าได้ 33.60 กิโลวัตต์- ชั่วโมง/วัน ซึ่งมากกว่าการใช้พลังงานไฟฟ้าในพื้นที่ส่วนกลางยามปกติถึง 1.4 เท่า จึงมีความเป็นไปได้สูงในการใช้งานเมื่อ ยามเกิดภัยพิบัติ ในด้านความคุ้มค่าเชิงเศรษฐศาสตร์ พบว่า การติดตั้งเซลล์แสงอาทิตย์เพื่อขายไฟฟ้าคืนมีระยะเวลาการ คืนทุนอยู่ที่ 21 ปี เร็วกว่าการผลิตไฟฟ้าเพื่อใช้ในอาคารเพียงอย่างเดียว อีกทั้งเมื่อเปรียบเทียบระหว่างการติดตั้งแผงเซลล์ แสงอาทิตย์รูปแบบหลังคา (โซลาร์รูฟ) กับแบบเพื่อการผลิตไฟฟ้าอย่างเดียวบนที่ดินขนาดใหญ่ (โซลาร์ฟาร์ม) สำหรับพื้นที่โครงการบ้านเอื้ออาทรบึงกุ่ม พบว่าระบบโซลาร์รูฟมีความคุ้มค่าในเชิงเศรษฐศาสตร์โดยมีระยะการคืนทุนที่เร็วกว่าการ ลงทุนในระบบโซลาร์ฟาร์มบนเนื้อที่เดียวกันถึง 5.82 เท่า