Fig. 1: Photovoltaic system on the roof of the Physics Dept, Bologna University, installed by SEI, Solar Energy Italia, in October 2000.

As part of their "10000 photovoltaic roofs" programme, ENEA has installed an experimental photovoltaic system on the roof of the Physics Department of the University of Bologna. This system has supplied electric energy in parallel to the Enel distribution network since 2001.

The system (Fig. 1) also regularly records data on solar radiation and electric energy generated.

There is regular monitoring of insulation, electric energy generated and the system's efficiency, in order to establish its performance and its overall efficiency over relatively long periods.

The system consists of six units, made up of 22 photovoltaic modules and support structures. It should be noted that the solar panels are facing south and are inclined at 30°; this optimizes the insulation of the panels at the latitude of Bologna (~44° latitude North).

The system is visited frequently by high school students as part of the Physics Department's "Open Laboratories" and "Scientific Degrees" projects.

Fig. 2: Dimensions of a cell and a photovoltaic module installed on the roof of the Physics Department.

Each of the 22 panels is made up in turn of 36 pseudo-square cells; the total surface area of one panel is 0.63 m² (Fig. 2).

The photovoltaic system occupies a total surface area of 13.86 m². The peak power output of the project in direct current (d.c.), is 1.87 kW maximum [kW_peak], the tension is 198 V d.c.

The direct current is converted into alternate current at 220 V by means of an inverter, equipped with an automatic control system.

The electric energy generated by the system is used in tandem with the Enel network to provide underground parking lights; the power is comparable to that needed by a small family.

The overall cost of a flat-panelled photovoltaic system connected to the electricity grid is made up of:

• cost of system (investment), approx. 8000 Euros/kW_peak (approx. 15000 Euros for the Physics Department's system);
• cost of protection, e.g. railings, if needed ;
• cost of maintenance, personnel, changing the inverter (approx. 5000 Euros in 20 years);
• other costs (insurance and taxes).

For the moment, no mention has been made of the cost of closing down the systems in 20 or 30 years' time: this too will be expensive