EFFECT OF FLAT REFLECTORS ON THE PERFORMANCE OF PHOTOVOLTAIC.pdf

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AIAA-2000-2826

EFFECT OF FL AT REFLEC TORSON THE PERFORMANCE OF PHOTOVOLTAICMODULES

Dr. A. Md. Aziz-ul Huq*Professor, Mechanical Engineering Department

Bangladesh University of Ehgineering and T echnologyDhaka 1O00, Bangladesh

Mohammad IrtezaHowinPost Graduate Student, KTH

Stockholm, Sweden

MuhammadMustaflzur RahmanAssociate Professor, Deparlment of Mechanical Engineering

University of South FloridaTampa, Florida 33620. U.S.A.

ABSTRACT

To convert solar energy directly intoelectrical energy, photovoltaic modules are widelyW i g used, Different techniques such as retrofittingof reflectors and concentrators are adopted toincrease the amount of radiation flux falling on themodule and the output of the module therebyincreases. The present work investiga tes the effect offlat reflecting materials such as flat mirrors, shinnyaluminum, and white tiles on the performance of aPV module. The pexfonnance was compared with an

identical module without reflector; both studiedunder the same environmental conditions. It wasfound that the performance depends on the skycondition and has no advantage on a rainy or cloudyday. The reflector fitted module output based onMaximum Power Point showed a significant increase,and there was also an increase in temperature of themodule.

NOMENCLATURE

b PV module length (m)C PV module width (m )

Fr-c

I Current (A)

View Eactor from reflec tor to co llector

* Corresponding AuthorCopyright 0 2000 by the American Institute ofAeronautics and Astronautics, Inc. All rightsreserved.

P Power0r Reflecto r width (m)V Voltage (V)

W Angle

INTRODUCTION

In many remote and isolated areas far fromthe grid line and in many island areas, a photovoltaicsystem is a very promising option. High initialinstallation costs of a complete PV system in manycases becomes a deterrent for such options,

overriding the advantages. In addition to this, thereare two major handicaps of a PV system. Solarenergy is very much seasonal and energy is no tavailable a t night. An electrical storage system like a

battery is required to maintain power supply duringsuch periods when solar energy is not available or no tenough to meet th e demand. Moreover, solar energy

flux is dilute and rarely exceeds lkW /m2. T he use of

some form of reflector or concentrator will increasethe intensity of solar radiation flux falling on the PVmodule. As cell output is also dependent on celltemperature, this intensified flux will increase theoutput depending on he cell temperature. Any formof retrofit involves cost, and all these analyses enter

into the finalchoice of options.

A numbex of studies, such as those byStacey and McCormick’, Nann2, Peters andKarlsson3, Gordon4 have been done to study theeffect of concentrators on the performance of PVmodules. Mills et al.” performed a relative

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comparison of certain truncated symmetrical andasymmetrical fixed reflector designs for solarcollectors. Bollentin and Wilk6 presented ananalytical model to determine solar irradiation on flatcollectors augmented with planar reflectors. Kumaret al.' analytically studied the general case of a

collector with four reflectors. An alternative to glassmirrors coated with an aluminum or silver reflectingmaterial is a s ilvered polymer reflector developed bySchissel et a18. The silver acrylic film has rhe

advantages of being low cost and lightweight. ?hefocus of these studie s was to investigate reflectors for

large systems with potential comm ercial application.

&queg studied the performance of a sFaal l

PV (40 watt peak) module using flat mirrors as

reflectors and found that on typical days, moduleoutput increased around 11-13% with a moduletemperature rise of 2-10 "C. The present work

extended this work and checked the effect of otherreflecting materials such as flat shinny aluminum, flattiles and also flat mirrors. All of th ese are spectral innature. The findings were compared with an identicalmodu le having no reflector placed side by s ide so as

to expose them to s imilar environmental conditions.

EXPERIMENTAL SET -U P

The set-up was installed on th e roof of a six-story building on the campus of BangladeshUniversity of Engineering and Technology(longitude 90.0"E and latitude 23.5" N). A structurewas specially designed to mount th e reflectors on all

four sides of th e PV mo dule. The angle between eachreflector and the PV module could be variedindependently so as to change the shape factor. Theexperimental set-up is shown in Fig.1.

For the experiment, 40-watt TATA BP solarmodules were used. During the experiment, solarradiation was measured, module temperature wasmeasured by thermocouples, and wind speed andpsychrometric data were recorded. Loads on the PVmodules were controlled by rheostats, andcorresponding volts and amperes were recorded bydigital meters. The data was used for plotting thecorresponding I-V an d Power-Voltage curves.

Three types of reflecting materials wereused during this experimental investigation. Thesewere flat mirrors, shinny aluminum, and white tiles.All of these are spectral in nature. PV modules wereplaced facing south at the latitu de angle, 23.5"N.

'Ihe view factor is an important parameterthat gives the effect of reflectors on the PV panel,

and it is defined as how much of the reflector couldbe seen by the PV panel. The view factor between themodu le and the reflector varied throughout the seasonand also over the day, depending on the position ofthe sun in the sky. The theoretical value of the viewfactor for a particular day and for a particular settingwas calculated using the following relation given byDuffie and Beckman".

where ,

F R - p v= ( c + r - s ) / 2 r

s =[ c 2+ r - 2 c*r cosy 1"

Table 1 gives the angles subtended by thereflector with respect to the module. In determiningthese angles, the sh ading effect of different reflectorsat different times of the day was considered. Theangle between the module and the reflector is

schematically shown in Fig. 2. This paper presentsexperimen tal results in volving all four reflectors, and

the performances were compared with respect to themaximum power point; the percentage increase wascalculated by using the following relationship:

Percentage increase of output

={@reflector -W') "100

RESULTS ANDDISCUSSION

The tests were performed by puttingreflectors on all sides of the PV module (i.e. top,bottom, left, and right). A number of tests wereperformed at different times of the day. For eachtest, the load current and voltage readings of the

module fitted with the reflector and for the modulewithout the reflector were recorded; then, the poweroutput at each loading condition was calculated bymultiplying the current and voltage readings.Analyses were done on the basis of maximum powerpoint.

Figu re 3 shows the performan ce of mirroredreflectors in a clear sky environment. It can be

noticed that the output power increased by 33 % at

1 5 1 5 hrs. The output power dropped to nearly 10%at 1 5 2 5 hours based on the maximum power point.The temperature of the panel fitted with fourreflectors increased by 6-10 "C compared to the

module without reflec tors.

For Shinny Aluminum reflectors, the outputpower increased by 30% (Fig. 4) at 15:40 hours; atother times, it was a round 15% (based on maximumpower point). T he tem perature rise with shinnyaluminum sheet reflectors was around 7 "C morecompared to the module without reflector.

16 2



For White Tile reflectors, the power fromthe PV module increased by 16.12 % (Fig. 5) andwas around 8% on other days. The temperature ofthe PV module having White Tile reflectors wasincreased by 2-3 “C compared to module withoutreflector.

-

Reflector Mirror y~ Shinny reflector y~ White tiles y~

TOP 133.5 123.5 123.5

Bottom 128.5 128.5 128.5

Two sides 152 152 152

Figure 6 shows the effect of cloud cover onthe performance of the two modules. There ispractically no difference between the two outputs.During tests with mirrored reflectors or with tiles, acomplete cloudy condition was not present, so similarresults could not be obtained. Similar experimentswere done with pairs of two reflectors and a singlereflector. All showed some degree of augmentationcompared to the module without any reflectordepending on the time of the day and the skycondition.

CONCLUSIONS

From the above findings, it is clear that theuse of reflectors augments energy gain per metersquare of module area. The percentage gain is verymuch dependent on the sky conditions. Theexperiment was done for a limited period of time. Amore extensive study is required with differentreflecting materials, including the diffuse type (suchas white paint) and under different sky co nditions, toidentify some inexpensive reflective materials thatcan be economically viable. The optimum angles ofthe reflectors with the module for different months inthe year is needed to design systems for bestperformance.

ACKNOWLEDGEMENT

’Ihe authors gratefully acknowledge thefinancial support provided by the Department of

Energy Technology, Royal Institute of Technology,Stockholm, Sweden.

REFERENCES

1. Stacey, R.W. and M ccormick, P.G., “Effect of

Concentration on the Performance of Flat Plate

2.

3.

4.

5.

6.

7.

8.

9.

Photovoltaic M odules,” Solar Energy, Vol. 33,1984, pp. 565-569.

Nann, S., “Potential for Tracking PhotovoltaicSystem s and V-Troughs in M oderate Climates,”Solar Energy, Vol. 45, No. 6, 1990, pp. 385-393.

Peters, B. and Karlsson, B., “External Reflectorsfor Large Solar Collector Arrays, SimulationModel and Experimental Results,” Solar Energy,

Vol. 51, 1993, pp. 327-337.

Gordon, J., “A 100-Sun Liner Photovoltaic SolarConcentrator Design from InexpensiveC o m m e r d Components,” Solar Energy, Vol.57, NO.4, 1996, pp. 301-305.

Mills, D.R., Monger, A., and Morrison, G.L.,“Comparison of Fixed Asymmetrical andSymmetrical Reflectors for Evacuated TubeSolar Receivers,” Solar Energy, Vol. 53, 1994,pp. 91-104.

Bollentin, J.W. and Wilk, R.D., “Modeling ofSolar Irradiation on Flat Plate CollectorsAugmented with Planar Reflectors,” SolarEnergy, Vol. 55, 1995, pp. 343-354.

Kumar, R., Kaushik, S.C., and Garg, H.P.,

Enhancement by Multiple Reflectors, Energy,“Analytical Study of Collector Solar-Gain

Vol. 20, 1995, pp. 511-522.

Schissel, P., Neidlinger, H., and Czanderna, A.,“Silvered Polymer Reflectors,” SERvTp-255-2670, Solar Energy Research Institute, Golden,Colorado, 1985.

Haque, M.J., “Study of the Effect of Reflectorson the Performance of PV Module”, M.S.Thesis, KTH, weden, 1998.

10. Duffie, J.A. and Beckman, W.A., SolarEngineering of Thermal Processes, SecondEdition, Wiley- Intersc ience, 1991.



Fig. 1 Schematic diagram of th e set-up

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k-----+b

Fig. 2 Angle between the module and the reflector

CV Curve +-without reflector

1.4

1.2

1

0.8

2 0.6

0 0.4

0.2

0

-c;

0 5 10 15 20

Volage, V

Amtaw insolationon

he PV Panel is 504.5 W/m2

18

16

14

n 12

5 0

B 8a 6

4

20

1 1 1 1

0 5 10 15 20

voltage, vi d a t i o n on he PV is 504.5 W/m2

Hg. I-V and P-V urve for modules without andwith reflector (fourmirror reflectors, clearsky),3d August 1999, 15:15 hours

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2

1.6

1.2

0.8

0.4

0

II II

II

II

1

I I I I

0 5 10 15

Voltage, V

20

Average insolation on the s d a r Panel is 662w/m2

Power VS Voltage Curve

32

24

3 16

8

0

L"Q)

0a

0 5 10 15 20

Voltage, V

Average insolation on he PV panel is 662w/m2

Fig. 4 I-V andP-V curve for modules without and with reflector (fourshinny aluminum reflectors,white cloud and sun), 7'hSeptember 1999,15:40 hours

1'66



I-V Curve

1 i

+with reflector(White Tiles)

+without reflector

1.6

- 1.2a=

C

E 0.8

0 0.4

0

s

0 5 10 15 20

Voltage, VAverage insolation on the solar Panel s 556W h 2

1with reflector

+without reflector(Whit e Tiles)ower vs Voltage curve 1

20

16

L" 12

n

l aO 4

0

0 5 10 15 20

Voltage, VAverage insolationon the solar Panel is 556 W/m2

Fig. 5 I-V and P-V curve formodules without and with reflector (four white tile reflectors,

white cloud and sun), September 1999,1530 hours

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I-V Curvei I

0.5

0.4

0.3

g!3 0.20

0.1

0

IL

0 5 10 15 20

Voltage, VAverage insohtion on the PV Panel is 135.5W/m2

Power vs Voltage Curve +Fghymior..m)c

0 5 10 15 20

Voltage,vAveraae insolation onthePV Panel is 135.5W/m2

Fig. 6 I-V and P-V curve for modules without and with reflector (four shinny aluminum reflectors,very cloudy day), Mm ugust 1999,10:45hours

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EFFECT OF FLAT REFLECTORS ON THE PERFORMANCE OF PHOTOVOLTAIC.pdf