Improved ashrae model to predict hourly and daily solar radiation components in Botswana, Namibia, and Zimbabwe

ASHRAE model empirical coefficients A, B and C are obtained for clear days in Botswana from analysis of different solar radiation components recorded at the University of Botswana, Botswana Technology Centre and some synoptic stations. Investigations show that the direct normal radiation in Botswana usually reveals a specific diurnal profile. For example 30 minutes before sunset, Ibn-radiation could be above 600 W m−2 while at sunset and sunrise (half of the solar disc is under the horizon) it can be as high as 100 Wm−2. Such big values of direct normal solar radiation at sunset and sunrise are not only due to low humidity and turbidity but also due to small values of the relative air mass, m, which can be obtained with the help of a new formula developed by Nijegorodov et al. (1995). By using the ASHRAE empirical coefficients obtained, isotropic and anisotropic sky models and the new formula for the relative air mass a computer program to predict hourly and daily beam, diffuse and ground-reflected radiation on tilted, variously oriented surfaces is developed. This computer program can be used in Botswana, Namibia and Zimbabwe.