Photovoltaics systems
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| Fig. 1 - Mounting types: on sloped rooftop (left), and on flat rooftop (right) |
Introduction
The first step in designing a system for solar radiation conversion should be the analysis of the optimal configuration of the collector surface. In this case, the photovoltaic panels are the collector surface. The total electrical energy yield depends on the position of that surface.
Panels can be mounted on a tracking system, on sloped rooftops and on arrays. The latter is for flat rooftops and for land.
How should one install the panels? The solar radiation varies along the day, because of the movement of the sun in the sky sphere. Therefore, the incidence angle of sun rays on the panels surface change along the day. The optimal position can be achieved by facing the solar panels plane to the SOUTH. In Fig. 1, a configuration like that in B is effective only in the afternoon.
Besides, the sun ray angle changes along the seasons: in summer, sun rays hit the earth with a lower average incidence angle than that in winter. For Italy, the optimal inclination for the panel surface is in the range 30o+/-5o.
In terms of architectural integration and aesthetic, sloped rooftop mounting are more advantageous. On the other hand, rack or array mounting offers easier installation procedure, but the required area is bigger (roughly, twice the area required for a sloped roof installation). This is due to the self-shadowing of adjacent rows of panels.
Moreover, the thermal losses for sloped rooftop mounting are in general higher than those of array mounting, because the surface for the thermal convection is reduced (the back side of the panels is in contact with the rooftop). Thermal losses can impact the total electrical energy yield.