Global and diffuse solar irradiance in Yemen (Y.A.R.)

Abstract

Measurements of global and diffuse solar irradiance on a horizontal surface were made at Sana'a University during 1980/81 and the results are presented. Use is made of these results and other meteorological data to examine some of the methods of prediction of global solar irradiance H and of diffuse solar irradiance H d . The regression coefficients of the Angstrom correlation relation and the zone parameter of an empirical relation due to Barbaro et al. are determined for Sana'a. The results are extended to compute H at five other stations and comparison is made with the available data. The computed and measured values of H are in good agreement. The monthly variation of H and the effects of latitude and altitude on the regression coefficients and on the clearliness index H H 0 are discussed. The abundance of solar energy in Yemen is evident from the annual total global solar irradiance which ranges between 7500 and 7910 MJm −2 in the six towns. Correlations between the diffuse fraction H d H and the clearliness index H H 0 and between the diffuse fraction and the relative duration of sunshine n N are determined for Sana'a and compared with the Page and Liu and Jordan correlations. Agreement is found only with Page's correlation during the months March to September. The relation between H d H and n N is used to compute H d at the other towns making use of the available sunshine data. The effect of altitude is found to be more distinct on the fraction H d H than on other parameters.