To evaluate cell temperature, to use in the Evans equation, it was necessary to develop a finite difference computing model suitable to carry out the transitory thermal analysis of the layers of which the photovoltaic module is made up: the glass covering, a layer of EVA, the silicon layer, a layer of EVA, a layer of Tedlar, layer of PET and a layer of Tedlar (Green, 1992, Duffie et al., 1991, Clark et al. 1984).

In the model, as boundary conditions, the experimental values of the external surface temperatures of the module were considered, acquired by means of two thermal resistors as well as the value of the solar irradiance on the plane of the modules acquired by means of a pyranometer.

A thermal analysis by means of finite element softwares PRO-E and PRO-Thermal was also carried out as a comparison.

The temperature profiles obtained with the two models proved to be perfectly overlapping with small differences only in the silicon layer.

Fig. 5 (a, b, c, d, e, f, g,) shows the temperature profiles for the seven layers composing the module, obtained with the finite differences model, for July 2003.

As the figure shows, the temperature of the silicon layer (cell) is always greater than the reference one and reaches temperatures even greater than 80 °C.

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