~ Pergamon Renewable Eneryy, Vol. II , No. 4, pp. 421~426, 1997

© 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain

P I I : S 0 9 6 0 - 1 4 8 1 (97)00010----4 0960-1481/97 $17.00 + 0.00

P E R F O R M A N C E OF A M O R P H O U S SILICON SOLAR CELL M O D U L E A N D SOLAR

L A N T E R N

O. P. SINGH, S. K. SINGH* and A. G A U R Energy Laboratory, Department of Applied Sciences and Humanities,

Institute of Engineering and Technology, Lucknow-226021, India

(Received 10 October 1996; accepted 6 February 1997)

Abstract--The solar lantern (manufactured by BHEL) could regularly be lit for 5-6 h up to a maximum of 7 h, if the battery was fully charged. It is desirable, for regular use, that the solar lantern should be lit for not more than 5 h a day if the clear sky condition exists. If the weather is partially cloudy, use of the lantern should be reduced accordingly. A performance study of the amorphous silicon (a-Si) module shows that the maximum power transfer voltage (Vmp) and corresponding current is ca. 65 and 75% of the open circuit voltage (Voc) and short circuit current (/so), respectively. Efficiency of the module is 3-4% under field conditions and is slightly greater for a higher ambient temperature. © 1997 Elsevier Science Ltd.

INTRODUCTION

Solar cells, which convert sunlight directly into electricity through the photovoltaic effect of semiconductors, are a key technology toward the development of new energy sources that are both abundant and safe as a substitute for fossil fuels. Solar cells are very reliable, perfectly static and maintenance free, and can supply energy to systems with power from milliwatts to megawatts. Solar cells are interconnected to develop the PV module which can produce the desired power. Single crystalline silicon (C-Si) solar cells were first to be developed but amorphous silicon (a-Si) solar cells, due to low cost, are now coming into practical use.

In the past, solar cells have served as power supplies for various kinds of electrical equipment. Recently, stand-alone systems, e.g. solar street lights, solar domestic lights, solar lanterns, a pumping system, a solar refrigerator for storing vaccines, etc. have come into practical use. For the optimization and effective operation of the solar PV power generating systems the dependence of PV outputs on insolation parameters needs to be studied. The volt-ampere curve of the solar cell is to be examined for the operating conditions. This curve varies in shape and value as cell temperature and illumination vary.

In the present study, performance evaluation of the solar lantern, manufactured by

* Author to whom correspondence should be addressed.

421

422 O.P. SINGH et al.

BHEL, has been carried out and the results are presented here. Performance evaluation of the amorphous silicon (a-Si) PV module, supplied with the solar lantern, was also one of the main objectives of the present study.

METHOD OF STUDY

The solar lantern taken for the present study was manufactured by Bharat Heavy Electrical Limited (BHEL). The main parts of the solar lantern are (i) an a-Si PV module (manufactured by BHEL), (ii) storage battery, (iii) charge controller and (iv) tube light.

Three a-Si PV modules, having the same number of cells and equal area, were connected in series to form a single PV module [details of the PV module: Sun-100 mW/cm 2, tem- perature 30°C, Voc = 12.17 V, I~c = 2.05 A, Pmax = 13.51 W, Vmp = 8.70 V, Imp = 1.55 A, FF = 0.54, Rs = 1.39 A, efficiency (cell) = 5.6%, efficiency (module) = 5%, area of the module = 0.27 m2]. The storage battery was a sealed lead acid battery with power rating 6 V, 10 Ah. The battery was manufactured by the Shine-Kobe Electric Machinery Co. Ltd, Japan. The tube light supplied with the lantern was made in the USA (5 W/21). A per- formance study of the PV module and solar lantern was done under field conditions at Lucknow during different seasons of the year having different ambient temperatures and insolations. Output of the module and battery was measured by a digital multimeter while Suryamapi, manufactured by Central Electronics Limited (Sahibabad, India), was used to measure the irradiance incident on the module. Suryamapi is an instrument used to measure irradiance in which a calibrated reference solar cell has been used.

RESULTS AND DISCUSSION

Performance o f the a-Si P V module A performance study of the amorphous silicon (a-Si) module was made in different

months of the year. Results for the months of September 1994, December 1994, June 1995 and January 1996 are presented here. Variation of the open-circuit voltage (Voc) and short- circuit current (Isc) with irradiance (H) for two days (15 December 1994 and 16 December 1994) has been listed in Table 1. The combined open-circuit voltage and short circuit current of the three series connected sections of the modules is generally slightly different than the sum of the open-circuit voltages of the individual modules and the short circuit current of the lowest section of the modules, respectively. This may be due to any small variation in irradiance and cell temperature during the measurements. Characteristic curves of the a-Si module for different days at different ambient temperature and irradiance were drawn and the results for several days are presented in Table 2. Characteristic curves of the module for only three days having different climatic conditions are given in Figs 1-3. The irradiance (H), ambient temperature (Ta) and conversion efficiency 01) along with the maximum power point have been shown on each curve. It has been observed that the efficiency of the module is ca. 3-4% under field conditions.

Measurements show that the efficiency of the module is slightly greater for higher ambient temperature and lower irradiance in comparison to its value at higher irradiance and lower temperature. It has also been observed that the maximum power transfer voltage (Vmp) is ca. 65% of the open-circuit voltage and the current at maximum power transfer is ca. 75% of the short circuit current (Table 2).

Solar cell module and lantern

Table 1. Module output at different irradiance

423

Date : 15 December 1994

Vo~ for module (V) /~c for module (A) Irradiance

Time (W/m 2) I II III Combined I II III Combined

10.15 AM 740 3.88 3.91 3.92 11.84 1.17 1 .12 1.16 1.15 11.15 AM 840 3.82 3.81 3.83 11.46 1.36 1.31 1.35 1.34 12.15PM 800 3.80 3.77 3.82 11.46 1.23 1.16 1.19 1.21 2.15PM 680 3.81 3.79 3.83 11.47 1.03 0.96 1.02 1.01 3.15PM 380 3.71 3.72 3.78 11.25 0.50 0.48 0.48 0.48

Date : 16 December 1994

10.00 AM 840 3.94 3.93 3.79 11.88 1.37 1.32 1.36 1.34 l l . 00AM 860 3.82 3.78 3.81 11.49 1.82 1 .73 1.57 1.67 12.00AM 800 3.87 3.86 3.86 11.47 1.64 1.86 1.47 1.32

1.00PM 860 3.80 3.78 3.82 11.38 1.39 1 .32 1.19 1.27 2.00 PM 880 3.85 3.84 3.86 11.60 1.42 1 .36 1.40 1.36 3.00PM 880 3.79 3.81 3.86 11.58 1.08 1.01 1.02 1.11

Table 2. Efficiency of the a-Si module under different field conditions

I r r ad i ance T a Vmp Imp Efficiency Date Time (W/m z) (°C) (V) (A) (%)

30.9.94 11.35 AM 920 38 6.75 1.19 3.23 (66.1)* (74.4)*

30.9.94 11.50 AM 820 38 6.50 1.24 3.64 (65.7) (76.1)

13.6.95 11.20 AM 800 40 7.70 1.08 3.85 (65.4) (74.5)

13.6.95 11.45 AM 760 40 7.10 1.10 3.81 (65.6) (74.8)

15.6.95 11.15 AM 820 46 7.30 1.19 3.92 (62.1) (78.3)

15.6.95 11.30 AM 800 46 7.00 1.14 3.69 (65.2) (75.5)

19.1.96 12.10 PM 960 19 7.80 1.04 3.13 (63.6) (72.7)

19.1.96 12.30 PM 960 20 8.10 0.97 3.03 (64.2) (66.2)

20.1.96 1.05 PM 980 22 7.70 1.00 2.91 (67.4) (66.4)

*Bracketed quantities in the above table under Vmp and Imp values are (Vmp/Voc)X 100 and (Imp//~c) x 100, respectively.

424 O. P. S I N G H et al.

1 •8 - 1.6 - 920 W/m 2 Date-30/9/94

Time-I 1.35 A.M. 1•4 - - - ° ' " ° " ~ ° ~ o ~ Ta-380C

• ° ~ o " q - 3 23% 1.2 - Maximum power----'eib " o point %Ox

= i . o - 0\ ,~ 0.8 - 820W/m 2 •

\ r..) 0.6

0 . 4 - k

0.2

I i J t i 0 2 4 6 8 10 12

Voltage (V)

Fig. 1. V - I character is t ic curve o f an a-Si PV module•

1 . 8 -

920 W/m 2 Date-30/9/94 1.6 = " ° % ° ~ o Time-I 1.35 A.M.

1.4 - ~ o ~ T -38°C ~'o rla3.23%^

- Maximum p o w e r - 2 ~ o _ point %

\ - •

: \ \

- ~

0 2 8 10

1.2 < ~" 1.0

.o 0.S i .

r..) 0.6

0.4

0.2

I I I 4 6 12

Voltage (V)

Fig. 2. V-I characteristic curve of an a-Si PV module.

1.6

1.4

1.2 ,<

1.0 = • P" 0.8 = rO 0.6

0.4

0.2

1 . 8 - 980 W/m 2 Date-20/l/96

Time- 1.05 P.M. ? ° " ' ° ~ o ~ o ~ o ~ o Ta---4220C rt-2.91%

- Maximum power -~o _ point \ - .,,

I I I I I 0 2 4 6 8 10 12

Voltage (V)

Fig. 3. V - I character is t ic curve o f the a-Si module•

I 10

Performance of the solar lantern To test the performance of the solar lantern, charging and discharging of the battery

were studied for different days of the year. Charging of the battery was made using the a- Si PV module under the sun. The study shows that the battery could be charged fully in a day having clear sky if the battery was discharged for an approximate maximum of 6 h.

Solar cell module and lantern

Table 3. Discharging of the storage battery of the solar lantern

425

Date : 28.6.95 Date : 26.6.95

VB (no load) = 6.18 V Va (no load) = 6.13 V Lantern ON = 8.48 AM Lantern ON = 9.50 AM

VB (on load) VB (on load) Time (V) Time (V)

9.00 AM 6.06 10.00 AM 6.02 9.30 AM 6.03 11.30 AM 5.93

10.00 AM 5.99 12.00 noon 5.89 10.30 AM 5.96 12.30 PM 5.85 11.00 AM 5.92 11.30 AM 5.89 1.00 PM 5.80 12.00 noon 5.85 12.30 PM 5.81 1.30 PM 5.77

1.00 PM 5.76 3.30 PM 5.53 1.55 PM 5.67 4.00 PM 5.39 2.35 PM 5.56 4.30 PM 4.86 3.00 PM 5.48 4.50 PM 4.44 3.30 PM 5.28 4.10 PM 4.44

(Lantern OFF) (Lantern OFF)

VB (no load) = 4.95 V VB (no load) = 5.25 V on 29.6.95 at 8.30 AM on 27.6.95 at 8.30 AM

For proper charging of the battery in a clear sky day the battery should not be discharged for more than 5 h. I f there is cloudy/partial cloudy sky, duration of use of the lantern should be reduced accordingly.

Discharging of the battery of the solar lantern through a 5 W tube, supplied with the lantern, shows that the solar lantern could be lit for 5-6 h daily (Fig. 4) in its normal operation if the battery is fully charged (6.1~6.2 V). It will be better if the lantern is used for not more than 5 h. The fully charged battery could be used for a maximum of 7 h. The discharging rate of the battery was observed to be 0.07 V/h when the charge state of the battery is sufficient.

The discharging rate of the battery remains nearly constant (0.07 V/h) up to the battery voltage of 5.55 V (on load) and after that the discharging rate sharply increases resulting in the lantern being in the O F F position at a battery voltage of c a . 4.44 V (on load). This position (OFF) of the lantern was observed after 7 h of operation of the lantern. This shows that the lantern could be used for a maximum of 7 h provided the battery is fully charged. The tube light draws a current of 0.93-0.94 A from the battery which is c a . 0.98 A during the starting time. Details regarding the discharging of the battery are given in Table 3.

CONCLUSION

Performance evaluation of the a-Si module shows that the efficiency of the module is c a .

3-4% under field conditions. Efficiency of the module is slightly greater for lower irradiance

426 O. P. SINGH et al. 6 . 4 -

o%o.I ON Date-21 •6•95

6.0 ~ o ---......o....~ o ~ O ~ o ~ o ~

4,4

5.6

"" 5.2 o

4.8 t~

4.0 I I I I I I I I I I 0 11 12 13 14 15 16 17 18 19 20

Time (h)

Fig. 4. Discharging of the storage battery of the solar lantern.

and higher ambient temperature in comparison to its value at higher irradiance, but lower temperature. For some irradiance values, efficiency of the module is slightly greater at higher temperature. Maximum power transfer voltage and maximum power transfer current is ca. 65 and 75% of the open-circuit voltage and short circuit current, respectively.

A performance study of the solar lantern shows that the solar lantern could be lit for 5- 6 h, regularly, provided that the clear sky condition exists. It is desirable, for better operation of the lantern, that the lantern should be lit for not more than 5 h. The solar lantern could be operated for a maximum of 7 h continuously if the battery is fully charged (6.1~.2 V). The discharging rate of the battery is ca. 0.07 V/h for a battery voltage up to 5.55 V (on load) and it increases sharply after this voltage until the lantern became self OFF at 4.44 V (on load).

A c k n o w l e d g e m e n t s - - T h e authors are thankful to Prof. G. N. Pandey, Director, Institute of Engineering and Technology, Sitapur Road, Lucknow, for providing laboratory and other facilities, as well as for his valuable guidance. One of the authors (O.P.S) is also thankful to CSIR, New Delhi for providing financial assistance.