The cooling system, as already stated, has the purpose of lowering the temperature of the photovoltaic modules, when this becomes excessive, with consequent improvement of performances of the PV field. The system studied provides for the introduction of an adequate air flow, provided by a centrifugal fan from the outside air at its own temperature, in a cavity (15^2 cm gap) obtained by mounting "modular boxes” in aluminium on the back surface of the panel. During the summer the hot air is expelled again to the outside while, in the winter, it can be introduced to the rooms served by the PV plant to integrate with the traditional heating system.

Suitable connecting collectors (diffusers) to the main channel, for the introduction and withdrawal of air, which allow the use of just one fan also when there are more than one string, were studied. The diffusers can be realised in plastic material and are designed to channel the same air flow into each string. Fig. 6a shows the PV field together with details relative to the cooling system, while Fig. 6b shows the assembled PV field.

a) b)

Fig. 6 — PV field and details of the cooling system

Fig. 7 shows the comparison for July 2003 between the efficiency of the PV field, estimated with the Evans formula, with and without cooling system. As the figure shows, in the case when using the cooling system, the efficiency of the field is greater and displays a more even trend: this is due to the intervention of the fan activated by an on-off control system (using a differential thermostat) each time that the difference between the temperature at the back of the module and the outdoor temperature exceeds the At set. The maximum efficiency deviations, sometimes is greater than 31%, whereas the mean deviation is around 18%.

Fig. 8 instead shows the monthly electrical energy trend produced by the PV field with and without the cooling system. The overall energy supplied by the plant, for the 12 months studied, was 4143 kWh for the plant provided with the cooling system and 3441 kWh for the traditional plant. Taking into account the energy absorbed by the fan (112 kWh), the extra energy produced by the PV field is 590 kWh with an increase of 17,2% compared to that produced by the traditional plant.

Figure 9 shows the monthly electric energy absorbed by the fan.

The overall thermal energy released to the air following cooling of the modules, effectively usable and destined to winter heating, is 860 kWh.

The extra cost to be borne for the realisation and installation of the cooling system has been valuated at 37.00 €/m2, 4,8% of the overall plant value.

Considering an electrical energy cost of 15 €cent/kWh and a thermal energy cost of 8 €cent/kWh (natural gas), the simple recovery time to pay off the extra cost of the plant is

6,3 years.

CONCLUSIONS

Using the experimental data obtained from a photovoltaic plant, installed at the Mechanical Engineering Department of the University of Calabria, it has been demonstrated that the temperature increase of the modules significantly reduces the performances of the PV field.

A cooling system to be mounted at the back of photovoltaic modules was studied, which, using a suitable air flow conducted through a set of channels, provides for the attenuation of the effects of the raised temperature of the modules.

Using this system a mean increase has been calculated in the efficiency of the PV field, and therefore of the energy produced, at around 20%.

The economic analysis carried out showed that the simple payback-time to recoup the extra cost of the plant is about 6,3 years.