It is clear from the plots shown in Figs. 5-9, that circulating water through the front surface of a PV module will lead to cooling of the module. The cooling effect was at maximum at noon, and appeared to be uniform between 6-8 °C, as shown in Fig. 5, at the flow rate (36 lt/hr) used. The drop in the operating temperature of the PV cells may also affect the electrical conversion efficiency that will be revealed in another publication.

The mass of circulating water and the glass jacket both reduced the intensity of insolation reaching the PV cells. This was reflected as a drop in the electrical energy collected by the hybrid module and plotted in Fig. 8. The loss in electrical energy, EL= Ec — Eh, was however well offset by a large gain, Qw, in thermal energy that was collected by the circulating water, as shown in Fig. 9.

Regarding energy extraction from solar insolation, off course the hybrid unit showed a much better performance. The advantage of using the hybrid system may be better displayed by plotting a factor, which may be termed as Energy Gain factor, EGF (Fig.10). The overall energy gain, EG, may be defined as

Eg = Qw — El (3)

And Energy Gain factor defined as

EGF = Eg /Ec (4)

The hybrid system has an advantage of increasing the energy collection, however it also suffers a disadvantage in terms of extra cost incurred by having the jacket. Nevertheless, the extra cost forms only a fraction of the module cost.

Many researchers have studied a similar experimental set-ups [12-15]. All however with a difference that the cooling operation was applied at the rear of the PV module. In one case [12], a forced water cooling of the modules from 60 oC down to 25 oC increased the output power by 23%, while the open-circuit voltage was reported to increase by 18%. Similarly the PV conversion efficiency improved by 3%. Some researchers [13], accept the new structures as an important surplus value in terms of an enhanced architectural aestetics. However the pay-back period was difficult to calculate since the thermal energy yield could not be directly utilized.

CONCLUSION

Partial absorption of solar insolation by the glass jacket and mass of water in circulation, lead to some drop in the total electrical energy conversion. However, measurements based on electrical characteristics and the rate of heat exchange in a hybrid module showed that the overall energy extraction from solar insolation could be improved.

The improvement was in terms of thermal energy gained by the circulating water which can be used as preheat for another application. The Energy Gain Factor, EGF, which reflects the ratio of total energy gain over the Control module reached the value of about 2.0 showing a 100% improvement in energy extraction, over the PV system alone. In addition, more modification to the system are needed to increase the temperature difference of the circulating water and to improve the efficiency of the hybrid system.

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