Page 81 - Energize October 2022

TECHNICAL

For a tilted panel the angle of incidence at any time will depend
on the azimuth, the elevation and the tilt angle. Tilted panels
receives diffuse radiation from more than half of the sky dome.

(a) Cape town

Figure 3: Tilted panel
For a horizontally mounted panel the angle of incidence at any
time depends only on the elevation at that time. A horizontal
panel is insensitive to azimuth, i.e., is omnidirectional, and the
facing of the panel has no effect on the angle of incidence. The
horizontal panel receives diffuse radiation from the full sky
dome. As the tilt angle decreases, the effect of orientation also
decreases, to the point that at a tilt angle of zero degrees the (b) Upington
panel becomes orientation independent. Panels with low tilt
angles (<10%) behave nearly the same as horizontal panels.

(c) Pretoria

Figure 4: Horizontally mounted panel

The most important property of the horizontally mounted panel
is that radiation reaching the panel is independent of azimuth,
and the panel can be orientated in any direction. The panel gains
from the fact that it is independent of the azimuth, and receives
diffuse radiation from the total sky dome, but loses the benefit
of an optimum tilt angle. The effect will depend on the latitude
of the site, which affects the elevation at the solar zenith.
Figure 5 shows how monthly total radiation figures vary
with tilt angle for four Southern African sites. It can be seen that (d) Windhoek
although the annual radiation is higher for tilted surfaces, the
difference in total solar radiation between the optimal tilt angle Figure 5: Annual total global radiation on tilted surfaces (kWh/m2/
and a horizontal panel is of the order of 5 to 10%, depending on year) for Cape Town. (latitude 33° 59’,Upington (latitude 28° 24’),
Pretoria (latitude 25° 44’ ) and Windhoek (latitude 22° 34’) (Derived
the latitude of the site. from data in )
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