Page 37 - Applied Photovoltaics
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Fig. 1.17 shows the effect of array tilting on the daily solar energy incidence for a
location at latitude 23.5°N with a clear sky. Meinel and Meinel (1976, p. 108)
tabulate the theoretical, clear sky daily energy interception for a range of fixed and
tracking orientations at two representative latitudes for summer and winter solstices
and the equinoxes.
Figure 1.17. The effect of array tilting on the total insolation received each day for
a location at latitude 23.4°N (Used with permission of McGraw-Hill Companies,
Hu, C. & White, R.M. (1983), Solar Cells: From Basic to Advanced Systems,
McGraw-Hill, New York.).
Tilt towards equator
Lorenzo (2003) outlines the general method for converting monthly average daily
radiation on the horizontal to monthly average daily radiation on an arbitrarily tilted
surface. It requires estimation of hourly horizontal global, direct and diffuse
components, their transposition to the tilted surface, and integration over a day. This
procedure is computationally intensive and is done by some available PV system
sizing computer programs.
However, as noted by Duffie and Beckman (1991, Section 2.19), a method has been
devised by Liu and Jordan (1962) and extended by Klein (1977) for the special case
of a flat surface tilted towards the horizon, for which a simple approximation may be
used; that is
§ R d · 1 cos ȕ 1 cos ȕ
¨
R( ȕ) R 1 ¸ R R ȡ (1.23)
b ¨ ¸ d
© R ¹ 2 2
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