To predict the performance of solar collectors, it is necessary to evaluate the radiation exchange between a surface and the sky. The sky can be considered as a blackbody at some equivalent sky temperature Ts so that the actual net radiation between a horizontal flat plate and the sky is

Q = zAa(T4 — Ts4) (2.1)

The equivalent blackbody sky temperature, equation 2.2, accounts for the fact that the atmosphere is not at a uniform temperature and that the atmosphere radiates only in certain wavelength bands. Several relations have been proposed to relate Ts for clear skies to measured meteorological variables. Berdahl and Martin (1984) determined such a relationship by relating the effective sky temperature to the dew point temperature, dry bulb temperature, and the number of hours from midnight t by the following equation

1

Ts =7p.711+0.0056^ +0.0000737p2 +0.013cos(15f)]4

The prediction of solar systems performance requires knowledge of the amount of solar energy absorbed by the collector absorber plate. The solar energy incident on a tilted collector consists of three different distributions: beam radiation; diffuse

radiation, and ground-reflected radiation. In this study the absorbed radiation is calculated by isotropic sky model [1]:

(2.3)

Assuming no directional dependence of є and a, the following relationships are valid. The total emittance is found by integrating over wavelengths from zero to infinity [2]:

(2.4)

The total absorptance for a surface for a given incident spectrum is found by integrating over wavelengths from zero to infinity [2]:

(2.5)