Page 33 - Applied Photovoltaics

component, R , from the more commonly measured R if K and K can
d d T
be correlated. There are several such correlations in the literature, with that
due to Page (1961) being considered the most reliable for latitudes less than
40° (Lorenzo, 2003)

K 1 . 1 13 K (1.18)
d T
2. Estimate K for each month using Eqn. (1.16).
T
3. Estimate K for each month using Eqn. (1.18).
d
4. Estimate R far each month using Eqn. (1.17).
d
Correlation models, such as that of Eqn. (1.18) are available in the literature for
different averaging times from one month down to less than one hour. These models
depend strongly on the averaging times and should not be applied for different
averaging periods (Perez et al., 2001).

Telecom model

If the separate components for diffuse and direct insolation are not known, a
reasonable approximation for both (for most locations) may be obtained by equating
the total monthly global insolation with the total insolation theoretically calculated for
an appropriate number of ‘sunny’ and ‘cloudy’ days. The calculations proceed as
follows:
1. ‘Sunny’ days—The intensity of the direct component of the sunlight
throughout each day can be determined as a function of the air mass from the
experimentally-based equation (Meinel & Meinel, 1976)
AM . 0 678 2
I . 1 3661u 7 . 0 kW/m (1.19)

where the currently accepted value of the solar constant has been inserted in
place of the original value, I is the intensity of the direct component incident
on a plane perpendicular to the sun’s rays and air mass (AM) values are a
function of the latitude, the time of year and the time of day, which can be
calculated using the algorithms in Appendix B.
By determining I values throughout a typical day, the daily direct insolation
can be calculated. This value is then increased by 10% to account for the
diffuse component, the origin of which is indicated in Fig. 1.14. This then
gives the expected daily insolation on a sunny day for the given location and
time of year.
2. ‘Cloudy’ days—All incident light is assumed to be diffuse, with an intensity
on a horizontal surface typically 20% of that determined by Eqn. (1.19).
Consequently, an approximation to the daily insolation (all diffuse) for a
‘cloudy’ day can be estimated.

By assuming that the known average global insolation data can be represented
by the sum of an appropriate number of ‘sunny’ days, estimated as described
in Section 1.8.2.1 above with insolation given by (i), and ‘cloudy’ days with
insolation given by (ii), the direct and diffuse components can then be

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