M. Cucumo — A. De Rosa — V. Ferraro — D. Kaliakatsos — V. Marinelli

Department of Mechanical Engineering — University of Calabria 87030 Arcavacata di Rende (CS) — Italy Phone +390984494603 — Fax +390984494673 E-mail: m. cucumo@unical. it

The analysis of numerous experimental data recorded on a 2.85 kW photovoltaic plant, suitably monitored and installed at the Department of Mechanical Engineering at the University of Calabria, has shown that the cell temperature significantly influences the performances of the photovoltaic modules, with noticeable diminution of efficiency at temperature increase. To check this parameter, a cooling system was researched, to be mounted at the back of the photovoltaic module, which, by means of a suitable air flow conducted through a system of channels, attenuates the effects of the cell temperature. Finally, the economic availability of using air heated by the modules to heat the buildings in the winter period was considered.

1. TECHNICAL CHARACTERISTICS AND PERFORMANCES OF THE PHOTOVOLTAIC FIELD.

The photovoltaic monitored plant, shown in fig. 1 (Cucumo et al., 2003), is a grid — connected type, with a theoretical peak power of 2.85 kW and it was designed and created at the Building Energy Laboratory of the University of Calabria.

— 30 Modules;

— 72 cells connected in series, 100 cm2 Eurosil cells;

— Upper covering: tempered glass with high transmittance;

— Sheathing material: EVA (Ethylene Vinyl Acetate);

— Rear covering: Tedlar/polyester/Tedlar;

— Frame: Anodised aluminium;

— Junction box: weatherproof (8IP500), glass — fibre reinforced;

— N° 1 inverter

— N° 1 instrument board and protections;

— N° 1 interface to network board;

— N° 1 datalogger,

Tab. 1 — Components of the photovoltaic plant

The photovoltaic field, mounted on a variably inclinable trellis, has a gross surface of 27 m2 and a useful surface of 25.7 m2; it is subdivided into 3 strings of 10 modules, connected in series, with a maximum string voltage of 340 V (420 V at open circuit).

The photovoltaic modules used, each contain 72, 103 x103 mm polycrystalline silicon cells in series, with a reference efficiency of t|r = 11,11 %.

Table 1 shows the components of the field and the technical specifications of the modules, each 0.996 x 0.904 m, while tab. 2 shows the electrical specifications in the reference conditions.

The manufacturing company of the photovoltaic modules supplied for each module the experimental values of the maximum power, the short circuit current (Icc), open circuit voltage (Vca), maximum power current (Imp) and the maximum power voltage (Vmp). These values, were certified by JRC (Joint Research Centre) of Ispra (Italy).

Once the characteristic curves of the single modules are known, taking into account their series-parallel connections, the characteristic curve (I, V) of the photovoltaic field was obtained. In the reference conditions (TR= 298 K, GR= 1000 W/m2) and of maximum power, the reference efficiency of the field was 10.45 % while the maximum power supplied was 2.687 kW.

Figure 2 shows the characteristic curve (I, V) and the power curve (P, V) of the photovoltaic field, evaluated in the reference conditions.

V (volt)

Fig. 2 — Characteristic curve (I, V) and curve (P, V) of the field.

Figures 3 and 4 show the experimental data of the photovoltaic field efficiency in February and April 2003. A marked oscillation in efficiency can be noted, owing both to the effect of radiation and the temperature of the modules.

The Evans correlation (Evans, 1981) reported in literature

Л = hR [1_P(tc _tr) + Y log G] (1)

shows how efficiency decreases with cell temperature increase and with a decrease in solar radiation. According to this equation, if the cell temperature doubles in value from 25 to 50 °C, efficiency decreases by about 12%, whereas if solar radiation is halved from 1000 to 500 W/m2, efficiency decreases by 4%. Therefore the influence of temperature is greater than that of solar radiation.

The study of an air-cooling system for photovoltaic modules to keep their temperature low to improve their performances is considered interesting.