Estimation of global, direct and diffuse solar radiation in Athens under clear sky conditions

Abstract

In this paper the theoretical models proposed by Davies-Hay, Hoyt and Bird for the estimation of the various components of solar radiation under clear sky conditions are applied to Athens. The results produced by the models have been compared to measured hourly values of global irradiation and to instantaneous values of direct and diffuse irradiance. From the comparison it has been found that global solar irradiance estimated by all three models is higher than that of the actual measurements. From the examined three models Davies-Hay model gives the highest hourly values of global solar irradiance. Bird model gives very good results for global solar irradiance and for direct irradiance for most of the months of the year while the estimation of the diffuse irradiance is considered satisfactory. Hoyt model underestimates the direct solar irradiance and over-estimates the diffuse. Davies-Hay estimates of diffuse irradiance are comparable to those produced by Bird model. It is concluded that Bird model produces the best estimates of solar irradiance in Athens.