8/2/2019 Manuscript PVSEC17th

1/2

CORRELATION BETWEEN ELECTRICAL PARAMETERS OF SOLAR CELLS ANDTEMPERATURE CHARACTERISITCS OBTAINED FROM INFRARED

THERMOGRAPHY

Panom Parinya, Buntoon Wiengmoon, Dhirayut Chenvidhya,Chaya Jivacate, Krissanapong Kirtikara, and Chamnan Limsakul.

Clean Energy System Group(CES), King Mongkuts University of Technology Thonburi,126 Pracha Utit Rd., Bangmod, Toongkru, Bangkok 10140 Thailand.

ABSTRACT

Forty non-encapsulated polycrystalline silicon solarcells, size12.5 x 12.5cm 2, with a nominal rating of 2.5 W,are obtained from a local supplier with their electricalparameters at the Standard Test Condition (STC).

Thermal characteristics of non-encapsulated cells,encapsulated cells in the module, and the module aredetermined from IR thermography. They are comparedand their correlations are obtained using the SpearmanRank Correlation Coefficients. Later on, 36 out of 40cells are encapsulated into a module. From our work, wecan conclude that IR thermography with statisticalmethod can be used to differentiate cells havingabnormal electrical performance.

1. INTRODUCTION

A non-contact temperature measurement methodcalled IR Thermography was used in this research.Testing are done with 40 non-encapsulated and 36encapsulated polycrystalline silicon solar cells in amodule. For 40 non-encapsulated solar cells, electricaltesting at STC (irradiance 1000W/m2, AM1.5, 25C)results are provided by the manufacturer.

For IR Thermography, non-encapsulated cells weretested under dark with 5A forward bias condition. Afterencapsulation 36 cells in a module, the module and cellswere tested under dark with 5A forward bias. After that,testing of module was done under illumination withoutdoor sunlight.

For the statistical analysis, the Spearman Rank Correlation Coefficient method was used to analyzerelationship between electrical and temperatureparameters of solar cells. For a hypothesis testing of significance, we use P-value method. This process wasdone using a MINITAB program.

2. RESULTS AND ANALYSIS

2.1 Electrical parameters under the STC of non-encapsulated silicon solar cells

Measured electrical parameters of 40 non-encapsulated cells under Standard Test Condition (STC)were obtained from the manufacturer. These parametersare Open-Circuit Voltage(V oc),Short-Circuit Current(I sc),Fill Factor(FF), Power at maximum power point(P mp),Voltage and Current at Maximum power point(V mp and

Imp), Efficiency (Eff), Series Resistance(R s) and ShuntResistance(R sh). Cell No.17 is interesting due to itspoorest performance and follows by cells No.1,14,16,and 38.

2.2 Temperature characteristics and correlationbetween temperature and electrical parameters of non-encapsulated silicon solar cells

The 40 non-encapsulated cells were tested underdark condition with 5A forward bias. IR images wererecorded every about 10 seconds. The cells No. 1, 14,16, 17, 26 and 34 (replace damaged cell No.38) are

presented. It can be noticed that high temperature occursroughly in 4 areas, namely; corner, edge, center andmiddle area of cell. It can be noted that cell No.17which has poor electrical performance revealing thewhite color or higher temperature in most areas exceptthe center (Fig. 2.1).

Fig. 2.1 IR image of cell No.1, 14,16,17,26 and 34 after 100second of 5A forward bias under dark condition.

Therefore, by using IR thermography we canroughly screen low quality solar cell, at leastqualitatively.

CellNo.1 Cell No.14 Cell No.16

Cell No.17 Cell No.26 Cell No.34

8/2/2019 Manuscript PVSEC17th

2/2

2.3 Temperature characteristics and correlationbetween temperature and electrical parameters of encapsulated silicon solar cells in a module

Thirty six cells were encapsulated in a module withglass, laminated sheet and back sheet. This module wastested under dark and 5A forward bias both front sideand backside. The IR images reveal the highertemperature over busbars of solar cells from I 2R loss oncontact resistance. Under illumination and short-circuitcondition, it is clearly that there are some cells hotterthan the others especially cells have lower I sc will verymuch hotter that the others (Fig 2.2). For example cellthe No.17 is the hottest. It also has poor electricalperformance. Well-known localized heating pointsoccur in modules are called Hot Spot [1].

Fig. 2.2 IR image of encapsulated solar cells in a moduleunder illumination and short-circuit condition.

When connect to battery load under illumination, hotspots occurred again on cell No.17 similar to when short

circuit but is of lower temperature partly due to theenergy consumption of load.

2.4 Correlation between temperature and electricalparameters of before and after encapsulation siliconsolar cells

By using Spearman Rank Correlation Coefficientmethod, the results are presented in Table 2.1. For 40non-encapsulated cells under dark and forward bias,there is no significantly relationship betweentemperature and electrical parameters. For 36 solar cellsafter encapsulation, A cell which has larger FF and R sh will have the larger Tavg while cell which has larger I

sc

will have the lower Tavg . For under illumination andopen-circuit condition, cells which have small R s willhave higher dTmin/dt . This result is agreed withO.Breitensteins report [2]. For under illumination andshort-circuit, cells have lower I sc will hotter that theothers, these hot cells always have the negative voltageand the lower voltage will result the higher temperature.For testing under illumination and connecting with load,cells have larger I sc results the lower Tavg due to lowerheat generation from recombination. For temperaturedistribution ( Stdev and SdMin ), the effect of R s is morenoticeable when cell is operating than when cell isopened circuit or shorted circuit.

Table2.1 Spearman Rank Correlation Coefficientbetween temperature and electrical parameters of siliconsolar cells after encapsulation

Pmp - - - - Tavg -0.457 - -

Isc TavgTmin

-0.466-0.371

- -

Tavg

dTavg/dtdTmin/dtdTmax/dt

-0.469

-0.529-0.470-0.505

Tav g - 0. 44 2

Voc - - - - - - - -

Imp Tmin -0.355 dTavg/dt -0.426 - - - -

Vmp Tmax 0.364 dTmin/dt 0.443 - - - -

FF Tavg 0.434 - - dTavg/dt 0.447 - -

Rs - - dTmin/dt -0.438 - -Stdev

SdMin0.4960.497

Rsh Tavg 0.33 - - - - - -

Eff - - - - Tavg -0.457 - -Significance

level / r0.01 0.421 0.01 0.421 0.01 0.421 0.01 0.421

**Remark: r = correlation coefficient,

ElectricalParameters

36 encapsulated solar cells

Dark Forward(Front)

Light&Open Light&ShortLight&

Operating

The temperature parameters are as follow: T is

Temperature, dT is Change in temperature, dT/dt is Rateof change of temperature, Stdev is Standard deviationof temperature, SdMin is Minimum Standard deviationof temperature, avg is Average, max is Maximum andmin is Minimum.

3. CONCLUSIONS

From our work with limited numbers of solar cellsavailable, certain temperature characteristics of non-encapsulated cells and encapsulated cells can be used toscreen electrical parameters, without resorting to directelectrical measurement. This would facilitate solar cellsscreening with an IR thermography technique.

ACKNOWLEDGEMENTS

We would like to thank Assoc.Prof.Dr. Koarakot,Prof.Dr.R.H.B.Exel, Dr.Cherdchai and Asst.Prof.Dr.Sirichai for their precious suggestion. We would like tothank the Electricity Generating Authority of Thailand(EGAT) for the supporting instrument. We would liketo thank the Department of Alternative EnergyDevelopment and Efficiency (DEDE) for direct andindirect supports. We would like also to thank The JointGraduate School of Energy and Environment (JGSEE)

for all the facilities and supports. We also thank theentire CES staff for their kind help and encouragement.

REFERENCES

[1] E. Molenbroek, D.W. Waddington, K.A. Emery,HOT SPOT SUSCEPTIBILITY AND TESTING OF PVMODULES ,National Renewable Energy Laboratory,Golden, Colorado.

[2] O.Breitenstein et al., Lock-in thermography - Auniversal tool for local analysis of solar cells,15th International Photovoltaic Science &Engineering Conference (PVSEC-15) ShanghaiChaina (2005).

9 8 7 6 5 4 3 2 1

18 17 16 15 14 13 12 11 10

28 27 26 24 23 22 21 20 19

40 39 38 35 34 33 31 30 29

Cell No.1 Cell No.14 Cell No.16

Cell No.38Cell No.17 Cell No.26

Real Back

IR Back