Abstract
Southern Africa is a location abundant with solar irradiance. With the increase of the solar energy sector, the importance of the evaluation of solar irradiance models has grown. This study examines the performance of 15 basic clear sky solar irradiance models that merely depend on solar zenith angle and require only two scaling parameters. It utilises 1-minute resolution data taken from 5 SAURAN measuring stations within the Southern African region: Vanrhynsdorp, Richtersveld, Graaff-Reinet (all located in South Africa), Gaborone (located in Botswana) and Windhoek (located in Namibia). Five models each of global horizontal (GHI), direct normal (DNI) and diffuse horizontal (DHI) irradiance were investigated. Three of the models do not appear to have been proposed and tested previously. This study reworks and later linearises the various models to a standard mathematical form, apparently the first time this has been done systematically for such a wide set of models. Using linear regression and statistical indicators such as the RMSE and MAE values, it was found that the Meinel model performed the best for both GHI and DNI, while the Power Law model performed worst for those irradiance types. The newly introduced Logarithmic model performed the best for DHI and the Daneshyar model performed the worst for the same irradiance type.
The influence of site altitude and aerosol concentration levels on the scaling parameters κ(2) for GHI, ψ(2) for DNI and δ(2) for DHI, and the generic q, was also investigated. The amplitude of the former three clearly depended on site altitude, and the q value also varied in most cases, except for the Meinel GHI and DNI models. The overall values of the scaling parameters indicated that the atmosphere found above the 5 stations investigated is normally more transparent than those of the original locations assumed for the models. From the aerosol concentration levels, for all models of GHI and DNI, κ(2) and ψ(2) decrease with increasing aerosols while q also varies. δ(2) increases for all models with increasing aerosol concentrations while q showed no correlation with the increase of aerosol concentrations for the DHI models.