Page 25 - Applied Photovoltaics

Figure 1.11. Polar chart showing the apparent motion of the sun for an observer
at 35°S. ("Copyright © CSIRO 1992, Reproduced by permission of CSIRO
PUBLISHING, Melbourne Australia from Sunshine and Shade in Australasia 6th
edition (R.O. Phillips) http://www.publish.csiro.au/pid/147.htm)

1.8 SOLAR INSOLATION DATA AND ESTIMATION
Good reviews of this field have been done by, for example, Duffie and Beckman
(1991), Iqbal (1983), Reddy (1987), Perez et al. (2001) and Lorenzo (1989, 2003).
Extraterrestrial irradiation is known from geometry and the solar constant (see
Eqn. 1.5) but terrestrial intensities are less well defined.
Photovoltaic system designers often need estimates of the insolation expected to fall
on arbitrarily-tilted surfaces. For most purposes, monthly average daily insolation
values are sufficient (Lorenzo, 2003) and ‘characteristic’ days near the middle of each
month are often used to define average monthly values (Appendix C). Asterisks are
used in this book to denote variables based on characteristic days and overbars
indicate monthly averages. Separated direct and diffuse components are usually
required for estimation of the effects of module tilt, but these need to be estimated
from global values if not separately measured. Hence, there are three basic problems:
1. Evaluating the global radiation on a horizontal surface for a given site from
available measured quantities.

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