Mapping global, diffuse and beam solar radiation over Zimbabwe

A database for long-term monthly radiation over Zimbabwe is developed. The meteorological raw data inputs are long-term monthly average records of pyranometer-measured hemispherical radiation, monthly average sunshine records, and satellite-measured hemispherical records over a 2-year period. The sunshine records are incorporated into the database by use of Angstrom-type correlations developed for Zimbabwe, and the shortterm satellite data are 'cultured to long-term ground-measurement basis by means of an empirically derived correlation' and a 'time series factor'. Diffuse radiation values an generated from the resulting hemispherical radiation database by a locally developed correlation of the monthly average diffuse fraction of hemispherical radiation with monthly average dearness index. Normal beam radiation is computed from the hemispherical and diffuse radiation using two different methods. The two methods are found to agree generally to within 7%. The results are presented as country-border-contained isolines of radiation. The sensitivity of beam radiation to the accuracy of estimating diffuse radiation is inspected. The sensitivity is quite high, about 1:1 for months and locations with very low clearness. This translates to an almost equal sensitivity of insolation available to tracking tilted apertures, underscoring the importance of developing a local diffuse fraction-clearness index correlation rather than relying on a correlation developed elsewhere. (C) 1999 Elsevier Science Ltd. All rights reserved.