The ampere-hour efficiency of photovoltaic solar generators

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THE AMPERE-HOUR EFFICIENCY OF PHOTOVOLTAIC SOLAR GENERATORS DONALD G. S. CHUAH School of Physics, Universiti Sains Malaysia, Minden, Penang, Malaysia (Received 9 May 1979)

Abstract The charging power and discharge power of two photovoltaic solar generators are measured simultaneously with the sunshine hours. A comparison of these two powers shows that there is sufficient power under the available local sunshine for lighting purposes. The ampere-hour efficiency of these generators is compared with that of lead-acid and nickel-iron cells. A 10°o difference in the ampere-hour efficiencies for tl~e two generators is observed. Ampere-hour efficiency Photovoltaic solar generators Charging power Sunshine hours Discharge power Electriclighting Encapsulation

INTRODUCTION The charging power and discharge power in amperehours were measured ,over a period of 1 yr for 2 photovoltaic solar generators of electrical powers 25 W, 12 V and 1.4 W, 12 V respectively. Simultaneously, the sunshine hours were recorded. The ampere-hour efficiency is calculated and discussed. The electrical circuit for the operation of a photovoltaic solar generator has been reported elsewhere [1]. A correlation between sunshine hours and solar radiation intensity was also reported therein. Other performance parameters already studied and presented elsewhere are, namely, deterioration of charging current, maximum cell efficiency and photovoltaic conversion efficiency with increase in cell temperature [2, 3].

These measurements are made in Penang, which is situated at EAT. 5°16'N, LONG. 100°16'E. The climate is typically tropical having sunshine throughout the year but interspersed by monsoon rain. The ambient temperature is about 30°C.

PHOTOVOLTAIC SOLAR GENERATORS (a) 25 W 12 V Array The array consists of three modules, each having 64 silicon solar cells, diameter 40ram, being tightly encapsulated within a plastic case. The array charges a 12 V lead-acid battery which operates a fluorescent lamp of 13 W, 12V for 2 hr per night.

25W 12V GENERATOR

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CHUAH: AMPERE-HOUR EFFICIENCY OF PHOTOVOLTAI( SOLAR (;ENERAT()RS CHARGING POWER/AMPERE--HOUR MONTH 1976 t

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(b) 1.4 W, 12 V Module The module consists of twenty silicon cells, each semi-circular in shape with a diameter of 22 mm, and mounted on a printed circuit board (epoxy laminated sheet). The board is enclosed with a heat-resistant acrylic case on top of it and a metallic case at the bottom. There appears to be a marginal air-gap between each of the cases and the board. The module charges a 12 V lead-acid battery which discharges through a lamp of similar power as itself for 2 hr per night. All the panels were mounted horizontally. CHARGING

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Figures 1 and 2 display the monthly charging power with respect to the monthly sunshine hours. Generally, charging power is large where sunshine hours are long as can be seen from these illustrations. The average sunshine hours, charging power and the discharge power are tabulated in Table 1. AMPERE-HOUR EFFICIENCY The ampere-hour efficiency is calculated from the ratio of the ampere-hours output during discharge to the ampere-hours input during charge. The amperehour is integrated hourly. Measurements of I-V characteristics of solar cell arrays under natural lighting to study the photovol-

taic conversion efficiency is limited by the rapid fluctuation of solar radiation intensity. Such measurements are useful for comparison of I-V characteristics with the cell specifications but must be made instantaneously under each typical weather condition. However, daily measurements of ampere-hour give a statistical recording of data over a long period resulting in a more representative performance study. During this period defects may show up.

DISCUSSION The monthly charging and discharge power for each generator are compared in Figs 3 and 4. Generally, these diagrams show that the discharge powers are well within the charging power for the particular application in electrical lighting. However. the discharge power is comparable to the charging power during monsoon rain occurring in the latter half of the year. In the case of the 25W, 12 V generator this is fully stretched from July to September. The average ampere-hour efficiency for the 25 W and 1.4W generators are 71.0"/~, and 82.0~';, respectively. These figures are within the range for lea&acid cells of about 90q.; efficiency but similar to that of nickel iron cells of about 70~757,, efficiency for norreal operation[4]. Nevertheless, the photovoltaic solar generator has an added advantage over these cells in that they are being daily recharged after each night discharge.

Table 1. Performance characteristics Characteristics

25W, 12V Generator

lAW, 12V Generator

Lead acid cells

Nickel iron cells

Sunshine hours Charging power (AH) Discharging power (AH) Ampere-hour efficiency

214.0 (1977) 72.0 51.0 71.0°~,

189 (1976) 8.65 7.11 82.0'~,,

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CItUAH: AMPERE-HOUR EFFICIENCY OF PHOTOVOLTAIC SOLAR GENERATORS

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Fig. 3. Monthly charging and discharge power. From Table l, the ampere-hour efficiency of the 1.4 W generator is higher than that of the 25 W generator by about 10% though the sunshine hours under which the former operated is lower by about the same amount. This is possibly due to the different types of encapsulation a tight binding for the 25 W array

and a non-tight binding for the 1.4W module. The tight encapsulation indicates in previous works [2, 3] an increase of cell temperature as the solar radiation peaks around noon time on a clear sky day. The increase of cell temperature causes a decrease in the charging current [2]. This results in a decrease in eft]-

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Fig. 4. Monthly charging and discharge power.

180

CHUAH: AMPERE-HOUR EFFICIENCY OF PHOTOVOLTAIC SOLAR GENERATORS

ciency which is in general agreement with other workers [5]. Furthermore, the metallization of the fingers of numerous cells in the 25 W array showed significant cracking. This could further reduce collection efficiency of photocurrent generated by the incident sunlight. On the other hand, the contact resistance in the electrodes of the 1.4 W module was reported to be lowered with a special alloy coating [6], A difference of about 10~o in performance is significant when the cost of photovoltaic solar electricity is currently considerably high [7]. It will, therefore, be useful to study suitable methods of encapsulation whereby efficiency is maximised for a high ambient temperature. Work is being pursued in this respect.

CONCLUSION The ampere-hour efficiency of photovoltaic solar generators under local atmospheric conditions shows that these generators are capable of providing low power output electricity. However, a high ambient temperature necessitates a further in-depth study of

improving the encapsulation in order to optimise its charging power. Acknowledgements~This work is supported by a ShortTerm Research Grant from the Universiti Sains Malaysia to which I hereby express my thanks. My appreciation also goes to Professor Y. Marfaing, Director of the CNRS-Solid State Physics Laboratory, Bellevue, France, for the constant interaction between us in this study.

REFERENCES [1] Donald G. S. Chuah et al., Proc. Int. Conf. Solar Elec., pp. 783-803. Toulouse, 1 5 March (1976). [2] Donald G. S. Chuah et al., Proc. 1977 Photovoltaic Solar Energy Conf., pp. 1253 1260. Luxembourg, 27-30 September (1977). [3] Donald G. S. Chuah, Int. Conj. Solar Energy Dev., Varese, Italy, 2(~29 March (1979). [4] L. T. Agger, Introduction to Electricity, p. 180. Oxford University Press, London (1971). [5] H.J. Hovel, Semiconductors and Semi-metals, Vol. 11, Solar Cells, p. 173. Academic Press, New York (1975). [6] Technical Information, Solar Power Battery, Power Supply System, Sharp Corporation, p. 1. [7] Joseph A. Merrigan, Sunlight to Electricity. pp. 92127. M.I.T, Press, Cambridge (1975).