Mirzaei Et Al-2015-Progress in Photovoltaics- Research and Applications

RESEARCH ARTICLE

Inuence of the underneath cavity on buoyant-forcedcooling of the integrated photovoltaic panels in buildingroof: a thermography studyParham A. Mirzaei1,2,3* and Jan Carmeliet1,2

1 Laboratory for Building Science and Technology, Swiss Federal Laboratories for Materials Testing and Research (Empa),berlandstrasse 129, 8600 Dbendorf, Switzerland2 Swiss Federal Institute of Technology Zurich (ETHZ), Wolfgang-Pauli-Strasse 15, 8093 Zrich, Switzerland3 The University of Nottingham, University Park, Nottingham NG7 2RD, UK

ABSTRACT

Airow around building-integrated photovoltaics (BIPV) has a signicant impact on their hygrothermal behavior anddegradation. The potential of reducing the temperature of BIPV using an underneath cavity is experimentally andnumerically investigated in literature. Most of the models are oversimplied in terms of modeling the impact of 3D owover/underneath of PV modules, which can result in a non-uniform surface temperature and consequently a non-homogenousthermal degradation. Moreover, the simultaneous presence of radiation and convection related to upstream wind, in addition tothe combined impact of back-ventilation and surface convection, is barely addressed in literature. However, these simplica-tions can result in the unrealistic loading climate conditions. This paper aims to present a unique experimental setup to providemore realistic climate conditions for investigating the ventilation potential of the underneath. The setup consists of a solarsimulator and a building prototype with installed PV, placed inside an atmospheric wind tunnel to control upstream wind ve-locity. Thermography is performed using an infrared camera to monitor the surface temperature of the BIPV. The potentialof an underneath cavity with various cavity heights and PV arrangement is further investigated in this paper. The outcomewould be eventually useful in the development of practical guidelines for BIPV installation. Copyright 2013 John Wiley& Sons, Ltd.

KEYWORDS

solar energy; photovoltaic panel; wind tunnel; building; cavity

*Correspondence

Parham A. Mirzaei, The University of Nottingham, University Park, Nottingham NG7 2RD, UK.E-mail: [email protected]

Received 1 September 2012; Revised 28 March 2013; Accepted 28 March 2013

1. INTRODUCTION

The global energy demand from its current amount, 10TW/year, is projected to increase by 300% by 2050 [1]. Withthe present drastic growth of urbanization [2], the energy de-mand is projected to severely increase in the near future.Among major sectors of energy consumption in developedcountries, the building sector is responsible for consumptionof 2040% of the total nal energy [3]. To respond to this in-creasing energy demand and to deal with current issues re-garding global warming, climate changes, and CO2emissions, the widespread implementation of renewable andclean types of energy in all energy consumption sectors in-cluding the building sector is required. In other words, it isnot feasible to pursue the goal of net-zero-energy buildings

without the integration of the renewable energies in the build-ing sector. Despite the extensive benets of the renewable en-ergies, the current share of renewable energies in total primaryenergy of the world is about 13.3% [4]. Among all sources ofrenewable energies, solar energy is known as the most abun-dant, inexhaustible, and clean form of energy [5]. Theintercepted energy from the sun by earth is estimated at about1.8 1011MW [6]. This implies that the energy provided bythe sun is 10 000 times the energy demand of a planet.

Despite the aforementioned advantages and also the sim-plicity of using photovoltaics (PVs), solar energy is still themost expensive choice. At the moment, only 0.05% of thetotal primary energy is reported to be supplied by PV tech-nologies [7]. However, considerable demand and growth inusing PV are projected. The development and installation

PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONSProg. Photovolt: Res. Appl. 2015; 23:1929

Published online 4 July 2013 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/pip.2390

Copyright 2013 John Wiley & Sons, Ltd. 19

of solar PV electricity in various countries are predicted toincrease from 10000 to 140 000MW from 2010 to 2030 [7].

This excessive interest is also observed in the imple-mentation of building-integrated PVs (BIPVs). BIPVs inroof and faade solution and PV thermal hybrid solarcollectors (PV/T) have emerged as a mature technologythat provides signicant advantages on cost and appear-ance. PV/T consists of a solar collector and a PV moduleand is classied into liquid PV/T collectors, air PV/T,ventilated PV with heat recovery, and PV/T concentrators.An advantage of PV/T is mainly reported in considerablethermalelectrical efciency increase of PV technologyfrom 4% to 20% to almost 60%, while PV/T isclaimed to be also a cheaper technology in terms of energyproduction [8].

The crystalline silicon (c-Si)-based PVs are a highlypreferable option, and at the moment, about 90% of totalPV market is reported to be c-Si based [9]. The main draw-back with c-Si PV modules is their temperature-dependentefciency as it is inversely proportional to temperature.This implies that the efciency will fall below the ratedefciency provided by the module manufacturer at temper-atures above 25 C. Many correlations are provided in theliterature to predict this loss, for example, a 0.4%0.65/Kas reported in [4,9]. Moreover, increased surface tempera-tures of building envelopes can have environmentalimpacts such as the creation of urban heat island [10].Furthermore, undesired high surface temperatures on theirvariation in time result in degradation due to thermalstresses [8]. One can add the impact of moisture ingressin the degradation of PV modules mainly at the cell inter-connections and/or in cracked cells.

Back-ventilation or inducing airow underneath thecavity of the PV modules is proposed as an effective strat-egy to reduce their surface temperatures. Computationaluid dynamics has been used as a powerful tool to studythe cavity ventilation potential [1113]. Mei et al. [14]performed an experiment to test PV modules under variousclimate conditions. Back-ventilation was performed byinstalling a fan behind the cavity while different ventilationrates were applied.

However, in the aforementioned investigations, the simul-taneous effect of airow above and underneath the cavitywas barely considered, whereas in reality, a part of theairow passes over the PV modules and a part ushes theunderneath cavity. Therefore, this study intends to under-stand the impact of simultaneous ow above and underneaththe PV modules. For this purpose, a unique experimentalsetup is developed. By employing a thermography tech-nique, the aim of this setup is to monitor the surface temper-ature of PV modules in the presence or absence of anunderneath cavity exposed to various upstream wind veloci-ties and radiation intensities. The result of this study in addi-tion with the future development of a simulation model andthe experiment of the air speed with particle imagevelocimetry technique will result in the development ofrecommendations and guidelines for the installation ofroof-integrated and faade-integrated PV modules, for

example, for promoting cavity ventilation to reduce thehygrothermal loading of the PV modules.

2. EXPERIMENTAL SETUP

As discussed earlier, the main objective of this experimentis to understand the potential of underneath cavity ventila-tion for cooling the PV modules. It is important to induce aradiation ux on the PV module using convective coolingby air that ushes the underneath cavity and ows abovethe PVs surface. Figures 1 and 2 depict the unique setupof the developed experiment, including building prototype,radiation source (solar simulator), and monitoring devices(i.e., infrared camera (IRC), thermocouples, and thermo-piles). The whole setup is placed inside the ETH/Empawind tunnel; the test section of this tunnel is 1300mm inheight and 1900mm in width.

The building prototype consists of an insulated styro-foam structure. The roof is covered with PV modules.The cavity height can be adjusted as well as type: atand stepped installation (Figure 1(c) and (d)). The roofinclination is 45. The building height from the oor tothe highest point of the roof is 582.8mm, whereas theheight of the walls is 300mm. The solar simulator includesa 2 3 array of 250-W infrared lamps (Figure 2(b)). Thesolar simulator is connected to an adjustable power systemto provide different heat ux values. The distance betweenthe solar simulator and the PV surface is 800mm. Asshown in Figure 1(a), an IRC has been installed far awayfrom the prototype and close to the wall of the wind tunnelto monitor the surface temperature of the PV modules. Asurface thermocouple has been attached on the PV surfaceto calibrate the IRC pictures (Figure 2(c)). This techniqueis further explained in the following sections.

As can be seen in Figure 2(c), the installed thermopilemeasures the exact amount of incident radiation to thePV surface. It is noteworthy to mention that the incidentradiation in a xed voltage supplied to the infrared lampssignicantly varies because of the different convectiveuxes using different upstream velocities. Therefore, theimplementation of a thermopile is crucial to control theradiation intensity from the infrared lamps to the PVsurface. The uniformity of the radiation intensity on thePV surface has been monitored by placing the thermopileat various points.

Aluminum sheets and tape have been placed on thebuilding windward wall and the oor close to it to reectthe radiation and avoid a temperature increase of thesesurfaces (Figure 2(a)). The upstream ow has not beenaffected by any other surfaces except the building unit.The performance of this technique has been ensured byIRC observation as the temperature of the windwardwall and neighboring oor was almost equal to theairow temperature.

As illustrated in Figure 1(c), eight thermocouples in twoarrays of four have been attached on the cavity surfaces ofthe underneath buildings roof structure (T1T4) and of the

Influence of the underneath cavity on buoyant-forced cooling of integrated photovoltaic panels P. A. Mirzaei and J. Carmeliet

20 Prog. Photovolt: Res. Appl. 2015; 23:1929 2013 John Wiley & Sons, Ltd.DOI: 10.1002/pip

PV (T5T8). The array of thermocouples was placed180mm from the left side of the roof. The distance of

thermocouples T1, T2, T3, and T4 from the highest pointof the roof is 10, 140, 270, and 390mm, respectively.

(a)

(b)

(c)

Thermopile

ThermocoupleAluminum sheets

Figure 2. (a) Building prototype, (b) infrared radiation source (solar simulator), and (c) surface thermopile and thermocouple.

(a) (b)

(c) (d)

45

300

400

10 mm gap

30 mm gap

600mm400mm T1&T5

T2&T6

T3&T7

T4&T8

180mm

1900m

1300

Figure 1. (a) Sketch of the experiment setup, (b) side views, (c) at photovoltaic (PV) one 590 390mm PV module, and (d) steppedPV three 590 130mm PV modules.

Influence of the underneath cavity on buoyant-forced cooling of integrated photovoltaic panelsP. A. Mirzaei and J. Carmeliet

21Prog. Photovolt: Res. Appl. 2015; 23:1929 2013 John Wiley & Sons, Ltd.DOI: 10.1002/pip

2.1. Measurement scenarios

To investigate the inuence of the underneath cavity on thecooling of the PV modules, four main scenarios weredened to understand the impact of the underneath cavity,its height, and shape on the temperature of the PV mod-ules. These scenarios include the following: (i) at PVwithout an underneath cavity; (ii) at PV with an under-neath cavity with a height of 10, 20, and 30mm; (iii)stepped PV without an underneath cavity; and (iv) steppedPV with an underneath cavity. In the earlier case, threegaps between three PVs are 10mm, and the outlet heightis 30mm (Figure 1(b, d)). Each case was studied underthree wind velocities (i.e., 0.5, 1, and 2m/s) and three radi-ation intensities (i.e., 50, 100, and 200W/m2). The airtemperature and relative humidity are, respectively, keptat about 24.5 0.5 C and 33 1%.

2.2. Infrared camera calibration

The emissivity of PV modules varies with the surface tem-perature. The following correction procedure has beenapplied to ensure the accuracy of the obtained images fromIRC. The recorded temperature at a certain point on the PV(Figure 2(c)) was compared with the temperature observedby IRC at the same point. Then, the emissivity of the imagewas adjusted until similar temperatures are reached forboth IRC and thermocouple. The whole image was auto-matically adjusted by the software according to the newemissivity using the StefanBoltzmann equation. As dem-onstrated in Figure 3, the results of different scenariosshow that the emissivity uctuates between 0.87 and 0.93according to the temperature of the PV surface. The aver-age emissivity is 0.90.

2.3. Thermocouples andthermopiles calibration

The common ice-bath technique was employed to calibratethe thermocouples. In this technique, the T-type thermo-couples were oated within a mixture of ice and waterfor a long time to reach 0 C temperature. The accuracyof thermocouples is 0.3 C.

A setup has been also developed to calibrate the outputvoltage from thermopiles. The incident irradiation on acircular black-painted plate (to represent a black body) at

specic distance has been measured with the thermopile.The generated voltage from the thermopile is multiplied byresponsivity, a constant number, which allows convertingthe irradiation in ux (watt). The distance between the ther-mopile and the plate is 100mm. The diameter of the plateis 100mm. The view factor associated to the incident radia-tion from the circular plate to the thermopile can be analyti-cally calculated as F= (sina)2, where a is the angle betweenthe thermopile and the black plate. The radiation ux (q) isthen given by the StefanBoltzmann equation:

q FAses T4p T4s

(1)

where s=5.67 108 is the StefanBoltzmann constant,As = 0.00785m

2 is the area of the black plate, e is the emissiv-ity of the black plate (here assumed as e =1), Tp is the platetemperature, and Ts is the thermopile temperature.

The incident radiation values on the PV surface measuredby the thermopile (Dexter Company, Huron River Drive,Michigan, USA) and calculated by Eq. 1 are compared byobtaining the responsivity. Figure 4 depicts the responsivityconstant. The maximum and minimum ranges as given bythe producer are also shown in Figure 5. However, the typicalsuggested responsivity is 97.7 as obtained in the measure-ments. The test has been repeated several times, and thesame result has been observed after reaching a stable situa-tion (23min) for the thermopile. Therefore, the numberssuggested by the company are used for the experiment.

0.840.860.88

0.90.920.940.96

290.0 300.0 310.0 320.0 330.0 340.0 350.0 360.0

Emis

sivi

ty

Temperature (K)

Figure 3. Emissivity of photovoltaic modules in different temperatures.

0.0

50.0

100.0

150.0

200.0

250.0

0 60 120

180

240

300

360

420

480

540

600

660

720

780

840

900

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1020

1080

1140

1200

1260

1320

1380

Re

spo

nsi

vity

(V

/W)

Time (s)

Max. Res.Min. Res.

Figure 4. Responsivity calibration of thermopile constant.



2.4. Uniformity test

Prior to the experiments, a uniformity test for the radiationintensity was performed. The conclusion was that the radi-ation intensities vary up to 15% in different points of thePV modules. The radiation was more intense at the middleand less intense at the corners because of the superpositionof infrared lamps. However, the obtained range of unifor-mity was assumed to be acceptable because this studyhad a comparison format, and similar radiation intensitywas emitted in all scenarios. Moreover, the measuredtemperature distribution can be simulated with the currentprole to validate future numerical models.

An anemometer was used to measure the upstreamvelocity of the wind tunnel at 3m from the same heightas the windward wall of the building prototype (300mm).Each experiment case was employed until a steady-statesituation was reached. This means that after a certain time,and under constant wind velocity and radiation intensity,the temperature of PV modules measured by IRC and ther-mocouples did not signicantly uctuate anywhere. A con-siderable change in the airow temperature has not beenalso monitored during the experiments.

3. RESULTS AND DISCUSSION

3.1. Impact of the underneath cavityand height

Figures 68 illustrate the temperature on the surface of thePV module under various upstream velocities and radiationintensities. Mean and maximum surface temperatures ofthe PV module are shown in Tables I and II, respectively.Each upstream velocity and radiation intensity is denedas a specic case study. The highest temperature differenceon the surface is also provided in these tables. The reasonfor this selection of velocities and intensities was to covera range of ow in which both natural and forced convec-tions are contributing. In this study, the correspondingnumber to present the impact of buoyant-forced ow isthe bulk-Richardson number (Rb):

Rb gLb TPVb Ta Ua2

(2)

where g denotes the gravity acceleration, b is the thermalexpansion of air at inow temperature, L represents thelength of PV, TPVb is the bulk surface temperature of thePV module, and Ua and Ta are the air velocity and temper-ature at upstream, respectively.

By using TPVb from Table I in Eq. 2, it is found thatRb varies from Rb = 0.016 (RI = 50W/m2, Ua = 2m/s) toRb = 3.051 (RI = 200W/m2, Ua = 0.5m/s). In Figure 5,the bulk-Richardson number is plotted versus the Reyn-olds number for different radiation intensities andupstream wind velocities.

Figures 6(a, g, m) 8(a, g, m) show the surface tem-perature of the PV module when the underneath cavitiesare closed. In these cases, the ow above the PV surfaceis the only way of convective heat exchange. Aspresented in Table I, the highest PV surface temperature(TPVb = 83.3 C) occurs for the higher radiation RI= 200W/m2 and lower air speed Ua = 0.5m/s. Under constantRI, the TPVb tends to decrease for higher upstream veloc-ities as the forced convection is considerably increased;for example, TPVb is 74.2 C and 60 C in Ua = 1.0m/sand Ua = 2.0m/s, respectively. Surprisingly, a separatedhot region from the main elliptical region can be ob-served in the lower part of the PV module for the highervelocities (Figure 6(a, g, m)). However, for the lower ve-locities, this region is smoothly mixed with the main re-gion. The reason could be related to the fact that theow separates from the edge of the PV module. More-over, corner eddies may contribute to non-uniform tem-peratures on the PV surface. A similar phenomenonalso can be observed for higher radiation intensities(Figures 7(a, g, m) and 8(a, g, m)) where the meanand maximum temperatures increase.

Figures 6(bd), (hj), and (np) demonstrate theimpact of underneath cavity height with RI = 50W/m2

for Ua = 2.0m/s, Ua = 1.0m/s, and Ua = 0.5m/s, respec-tively. The heat removal from the PV module isincreased from the backside as airow penetrates thecavity. When the cavity height is 10mm, the mean sur-face temperature of the PV is respectively reduced by3.9, 4.6, and 6.6 C for Ua = 2.0m/s, Ua = 1.0m/s, andUa = 0.5m/s (Tables I and II), whereas the maximum sur-face temperature is decreased by 5, 5.7, and 8.3 C. Asdiscussed earlier, the reduction of the maximum temper-ature is an essential factor related to an increase of thedurability of PV modules. Only a slight change of themean and maximum temperatures at lower velocitiescan be observed when the cavity height is increased to20mm. This implies that for our study, increasing thecavity height more than 20mm does not have a furtherinuence on the cooling of the PV module. This obser-vation is almost valid for all other radiation intensitiesand implies the existence of an optimum size for thecavity height for the at installation of the PV modules.

0.0E+001.0E+042.0E+043.0E+044.0E+045.0E+046.0E+047.0E+048.0E+049.0E+04

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50

ReH

Rb

Figure 5. Reynolds number (for the height of the prototype)versus bulk-Richardson number in different radiation intensities

and upstream winds.



2 m/s

(a) (b)

(c) (d)

(e) (f)

1 m/s

(g) (h)

(i) (j)

(k) (l)

0.5 m/s

(m) (n)

(o) (p)

(q) (r)Figure 6. Photovoltaic (PV) surface temperature, RI=50W/m2. (a)(f) Ua = 2.0m/s, (g)(l) Ua = 1.0m/s, and (m)(r) Ua = 0.5m/s. (a) FlatPV without gap; (b) at PV gap=10mm; (c) at PV gap=20mm; (d) at PV gap=30mm; (e) stepped PV without gap; (f)stepped PV gap=10mm; (g) at PV without gap; (h) at PV gap=10mm; (i) at PV gap=20mm; (j) at PV gap=30mm;(k) stepped PV without gap; (l) stepped PV gap=10mm; (m) at PV without gap; (n) at PV gap=10mm; (o) at PV gap=20

mm; (p) at PV gap=30mm; (q) stepped PV without gap; and (r) stepped PV gap=10mm.



2 m/s

(a) (b)

(c) (d)

(e) (f)

1 m/s

(g) (h)

(i) (j)

(k) (l)

0.5 m/s

(m) (n)

(o) (p)

(q) (r)Figure 7. Photovoltaic (PV) surface temperature, RI=100W/m2. (a)(f) Ua = 2.0m/s, (g)(l) Ua = 1.0m/s, and (m)(r) Ua = 0.5m/s. (a)Flat PV without gap; (b) at PV gap=10mm; (c) at PV gap=20mm; (d) at PV gap=30mm; (e) stepped PV without gap;(f) stepped PV gap=10mm; (g) at PV without gap; (h) at PV gap=10mm; (i) at PV gap=20mm; (j) at PV gap=30mm;(k) stepped PV without gap; (l) stepped PV gap=10mm; (m) at PV without gap; (n) at PV gap=10mm; (o) at PV gap=20

mm; (p) at PV gap=30mm; (q) stepped PV without gap; and (r) stepped PV gap=10mm.



2 m/s

(a) (b)

(c) (d)

(e) (f)

1 m/s

(g) (h)

(i) (j)

(k) (l)

0.5 m/s

(m) (n)

(o) (p)

(q) (r)Figure 8. Photovoltaic (PV) surface temperature, RI=200W/m2. (a)(f) Ua = 2.0m/s, (g)(l) Ua = 1.0m/s, and (m)(f) Ua = 0.5m/s. (a)Flat PV without gap; (b) at PV gap=10mm; (c) at PV gap=20mm; (d) at PV gap=30mm; (e) stepped PV without gap;(f) stepped PV gap=10mm; (g) at PV without gap; (h) at PV gap=10mm; (i) at PV gap=20mm; (j) at PV gap=30mm;(k) stepped PV without gap; (l) stepped PV gap=10mm; (m) at PV without gap; (n) at PV gap=10mm; (o) at PV gap=20

mm; (p) at PV gap=30mm; (q) stepped PV without gap, and (r) stepped PV gap=10mm.



3.2. Impact of photovoltaicinstallation arrangement

To show the importance of PV arrangement, three PV mod-ules (580 100mm) were placed in a stepped arrangementaccording to Figure 1(b, d) where the size of the cavity inletbeneath each PV module was set to 10mm. Similar to theprevious section, the test was repeated for three air velocitiesand three radiation intensities. Also, the experiment wasrepeated for open-cavity and closed-cavity situations.

Figures 6ef, kl, and qr and 8ef, kl, and qr clearlyshow the considerable change in surface temperature of PVmodules when the underneath cavity is open. The meanand maximum temperatures are given in Tables I and II.For example, the difference of mean surface temperature

between closed-cavity and open-cavity cases exceeds26.8 C when RI=200W/m2 and Ua = 0.5m/s. Surprisingly,the lowest mean of maximum surface temperatures for allcases occurs for the stepped arrangement of the PVmodules.

This observation can be explained by the fact that more airwill penetrate through the three inlets compared with the casewhere only air ushes through the single inlet in the at setup.This means that the shorter length of PV modules in thestepped conguration helps to exchange more fresh air fromabove, resulting in a lower temperature increase of the PVmodules. One should also mention that the stepped arrange-ment causes more turbulence ow around the PVs, which re-sults in higher convective heat exchanges at the PV surfaces.

From Figures 68, it is found that the middle of these PVmodules has the highest temperature in all cases. The highest

Table I. The mean surface temperature of the PV module under various upstream velocities and radiation intensities.

Mean Temperature of the PV Surface (C) Largest T (C)Case Flat PV Stepped PV

Without gap Gap = 10 mm Gap = 20 mm Gap = 30 mm Without gap Gap = 10 mm

123456789Highest temperature Lowest temperature

Ua = 2.0 m/s-RI = 50 W/m2

Ua = 1.0 m/s-RI = 50 W/m2

Ua = 0.5 m/s-RI = 50 W/m2

Ua = 2.0 m/s-RI = 100 W/m2

Ua = 1.0 m/s-RI =100 W/m2

Ua = 0.5 m/s-RI = 100 W/m2

Ua = 2.0 m/s-RI = 200 W/m2

Ua = 1.0 m/s-RI = 200 W/m2

Ua = 1.0 m/s-RI. = 200 W/m2

PV, photovoltaic.

Table II. The maximum surface temperature of the PV module under various upstream velocities and radiation intensities.

Maximum Temperature of the PV Surface (C) Largest T (C) Case Flat PV Stepped PV

Without gap Gap = 10 mm Gap = 20 mm Gap = 30 mm Without gap Gap = 10 mm

123456789

Ua = 2.0 m/s-RI = 50 W/m2

Ua = 1.0 m/s-RI = 50 W/m2

Ua = 0.5 m/s-RI = 50 W/m2

Ua = 2.0 m/s-RI = 100 W/m2

Ua = 1.0 m/s-RI =100 W/m2

Ua = 0.5 m/s-RI = 100 W/m2

Ua = 2.0 m/s-RI = 200 W/m2

Ua = 1.0 m/s-RI = 200 W/m2

Ua = 1.0 m/s-RI. = 200 W/m2

Highest temperature Lowest temperature

PV, photovoltaic.



temperature occurs in a half-ellipse hot region, and it is moreintensied for closed-cavity cases. The half-ellipse hot regionalso exists in the upper and lower PV modules with almostthe same intensity in open-cavity cases. However, the half-ellipse hot region tends to be more slender for higherupstream velocity. In closed-cavity cases and in higherupstream velocities, the half-ellipse hot region in lower PVtends to be slender, similar to the at cases.

On the other hand, the surface temperature of the upper PVmodule is considerably cooled for higher velocities. In gen-eral, it can be concluded that the temperature of the PVs isslightly affected by the 3D ow regime containing complexeddies imposed from the edges and corner of the PVmodules.

3.3. Underneath temperatures

In this section, the potential of airow for removing heatwithin the cavity is investigated in more detail. In additionto surface temperature thermography of the PV modules,the cavity temperature is measured using eight thermocou-ples as shown in Figure 1(c). As illustrated in Figure 9, thetemperature at the back side of the at PV continues toincrease close to the cavity outlet where it is slightly

decreased, which is similar to the results obtained from thethermography gures. The temperature difference betweenthe PV side and the roof side is higher close to the middleand lower at the inlet and outlet of the cavity (Figure 9).Although larger cavity heights result in lower temperatures,the temperature difference between the PV side and the roofside is almost the same for all cavity sizes. It should beemphasized that the enlargement of cavity height does notsignicantly reduce the PV-side and roof-side temperaturesfor cavity heights greater than 20mm. This observation resulthas been also more observed in the previous section.

Evidently, the surface temperatures at both the PV sideand the roof side are reduced with increasing upstreamvelocity. This implies that in the higher upstream veloci-ties, the impact of cavity height is less important, whereasit is inversely more important at the lower upstreamvelocities. Calculation of the bulk-Richardson numberRb gL Tmean Ta =bU2a

for different upstreamvelocities conrms the mentioned results, as it variesbetween 0.05

the length of the PV, g is the gravity acceleration, and b isthe thermal expansion coefcient for the air.

The temperature pattern is signicantly different for thestepped PV arrangement. In general and regardless of theupstream velocity and the cavity height, the PV-side androof-side temperatures are in the lowest range, specicallyin the roof side. This means that the stepped arrangement ismuch more capable of removing heat not only from the PVside but also from the roof side when compared with theat arrangement.

4. CONCLUSION

An experimental setup is developed to provide more realisticclimate conditions for a BIPV prototype. The setup includesa solar simulator that is positioned in an atmospheric windtunnel to provide a range of various radiation intensities overthe BIPV. The approaching upstream wind is controlled inthe wind tunnel. The temperatures within the underneathcavity and at the front side of the PV module are monitoredwith thermocouples and IRC, respectively. To emit thedesired radiation intensity, a control is developed using athermopile on the PV surface.

The measurement is repeated under various upstreamvelocities, radiation intensities, and cavity sizes and arrange-ments. The main results can be summarized as follows:

Higher ventilation can be achieved using stepped PVarrangements with an open cavity behind the modulescompared with a at arrangement.

3D ows (e.g., lateral eddies) contribute in a non-uniform surface temperature distribution over the PVmodules.

Inuence of the cavity height is signicantly greaterfor higher upstream velocities.

Future research will be performed to visualize the owunderneath and above the cavity using a particle imagevelocimetry technique. The results will be used to validatea 3D computational uid dynamics model for simulatingthe annual variation of temperatures of the PV modules.These data can be later used for the development of toolsfor the prediction of PV degradation probability in variousclimates or for dening accelerated aging test conditionsfor enhancing the durability of the PV modules.

ACKNOWLEDGEMENTS

The authors would like to express their gratitude to theSwiss-Electric Research (SER), the Competence CenterEnergy and Mobility (CCEM), the Swiss Federal Ofcefor Energy (SFOE), and the Services Industriels Genevois(SIG) for nancing the Archinsolar project. Also, the nan-cial support byMarie Curie COFUND project, Swiss FederalLaboratories forMaterials Testing and Research (Empa), andETH University is gratefully acknowledged.

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Mirzaei Et Al-2015-Progress in Photovoltaics- Research and Applications

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