The solar ventilation system described above consists of four main components: the slates, the PV module, the DC motor/fan combination and the duct. The flow rate in the system is governed by the interaction of the three latter components. Furthermore, since maximising the daily volume of air necessitates using a system with minimal resistance (i. e. smallest length of duct), the optimisation methodology depends primarily on the coupling between
the PV module and DC motor/fan combination. For a fixed length of duct, the maximum flow rate of air is obtained at the motor/fan maximum possible speed.

1.1 Photovoltaic module model

The I-V characteristic of the PV module can be described by the following equation, which is derived from the equivalent circuit described by Applebaum[3]

V + I ■ R S

I = I G — I о ( Є " — 1) (1)

where I is the PV module output current (A), IG is the light-generated current (A), I0 is the diode reverse saturation current (A), V is the PV module’s output voltage (V), RS is the series resistance of the solar cell (Q) and A is a curve fitting parameter (V).

This equation and the parameters included in it (i. e. IG, I0 and A) are valid at a given irradiance and temperature. Many methods are available so that the I-V characteristic of the PV module can be adapted to different levels of irradiance and module temperature. This work makes use of a new method which has been previously investigated by our research group at Napier. For a given irradiance (G, W/m2) and PV module temperature (Tmod, °C), the module short circuit current, its maximum power, its voltage and current at maximum power and its open circuit voltage are calculated. The parameters I0, A and IG are then calculated and substituted in Eq. 1 for an I-V characteristic which is valid at these conditions of G and Tmod.