A qualitative review of empirical models for estimating diffuse solar radiation from experimental data in Africa

A good working knowledge of diffuse solar radiation (Hd) is of vital requirement for determining the gross primary productivity, net ecosystem, exchange of carbon dioxide, light use efficiency and changing colour of the sky. However, routine measurement of Hd is not available in most location across Africa. During the past 36 years in order to estimate Hd in the horizontal surface on hourly, daily and monthly mean basis, several empirical models have been developed for numerous location in Africa. As a result, numerous input parameters have been utilized and different functional forms applied. The empirical models so far utilized were classified into six main categories and presented based on the input parameters applied. The models were further reclassified into numerous main sub-classes (groups) and finally represented according to their developing year. In general, 301 empirical models, 58 functional forms and 28 groups were reported in literature for estimating Hd in Africa. The empirical and soft computing models developed within Africa and across the globe were examined in order to determine the best technique of estimation. The result revealed that soft computing models are more suitable for estimating Hd in Africa and other climatic zones across the globe. The authors equally identified and compared several photovoltaic (PV) stimulation tools and technologies often employed by PV system researchers for electrification purposes, as a direct application of the reviewed empirical models in this study. Thus, this review would provide energy researchers in terms of identifying the input parameters, functional forms, algorithms, PV stimulation tools and technologies which are widely applied up till now as well as recognizing their importance for estimating Hd and modelling PV systems respectively in numerous stations distributed in Africa and across the globe; so as to robust assess to smart energy that is eco-friendly in developed and developing countries.