Page 36 - Applied Photovoltaics

Consequently, we get

S sin( Į ȕ)
S (1.21)
ȕ
sin Į
where Į is the altitude of the sun (i.e. the angle between the sun and the horizontal) at
noon, and is given by:
Į 90 $ ș į (1.22)

where ș is the southern latitude.
The above is for solar modules facing north in the southern hemisphere. If facing
south in the northern hemisphere, use Į = 90 – ș + į, where ș is, in this case, the
northern latitude.

Eqn. (1.21) is only strictly correct at midday, although it is often used in system
sizing to convert the direct component of mean daily solar radiation on a horizontal
surface for solar panels at angle ȕ, which introduces a small error.

Fig. 1.16 gives typical daily sunlight intensity profiles for a sunny and a cloudy
winter’s day. The cloudy day has a light intensity of only about 10% of that of the
sunny day, owing to the boosting of the direct component relative to the diffuse
component by tilting the array at 60° to the horizontal.

Figure 1.16. Relative output current from a photovoltaic array on a sunny and a
cloudy winter’s day in Melbourne (38°S) with an array tilt angle of 60° (after Mack,
1979).

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