Page 315 - Applied Photovoltaics

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where the units are kW/m , Į is the noon-time altitude of the sun for month i, given
by Eqn. (1.22), ȕ is the angle of inclination of the array on which I si is incident, AM is
the air mass (1/sinĮ) and the factor 1.10 allows for the inclusion of the diffuse
component for a cloudless day.

Figure H.1: Procedure for determining light intensity incident on solar panels at
angle ȕ at noon.

Similarly, for typical cloudy days, the maximum light intensity I ci for month i is
estimated from

I ci . 1 353u 7 . 0 AM . 0 678 u . 0 20 (H.2)

where the factor 0.20 represents the assumed diffuse light intensity, which is assumed
to be independent of b.
1. From the values for I si and I ci for each month i, use Fig. H.2 to estimate the
light intensity incident on the array at inclination angle ȕ throughout each day
for sunny and cloudy days, respectively. This facilitates the calculation of the
2
kWh/m /day for both sunny and cloudy days by using appropriate values for
the ‘time’ axis, as given by N in Fig. H.2.

2. Determine the percentage of sunny days given by Fig. H.2 with I equated to
I si for each month. This can be found from Eqn. (H.3), which gives the
average daily global radiation (G i ) falling on the array at inclination ȕ for
each month (i) in the design period as the sum of the sunny-day and cloudy-
day components of insolation

G X u 76 u N u I Y u 76 u N u I (H.3)
.
6
.
6
i i i si i i ci
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