Empirical Models for Estimating Global Solar Radiation over the Ashanti Region of Ghana

The performances of both sunshine and air temperature dependent models for the estimation of global solar radiation (GSR) over Ghana and other tropical regions were evaluated and a comparison assessment of the models was carried out using measured GSR at Owabi (6°45′0′′N, 1°43′0′′W) in the Ashanti region of Ghana. Furthermore, an empirical model which also uses sunshine hours and air temperature measurements from the study site and its environs was proposed. The results showed that all the models could predict very well the pattern of the measured monthly daily mean GSR for the entire period of the study. However, most of the selected models overestimated the measured GSR, except in April and November, where the empirical model using air temperature measurements underestimated the measured GSR. Nevertheless, a very good agreement was found between the measured radiations and the proposed models with a coefficient of determination within the range 0.88–0.96. The results revealed that the proposed models using sunshine hours and air temperature had the smallest values of MBE, MPE, and RMSE of ？0.0102, 0.0585, and 0.0338 and ？0.2973, 1.7075, and 0.9859, respectively. 1. Introduction Solar radiation is the primary source of the Earth’s energy, providing about 99.97% of the heat energy required for chemophysical processes in the atmosphere, ocean, land, and other water bodies [1]. Solar radiation plays an important role as a renewable energy source as solar radiation measurements could be used to estimate potential power levels that can be generated from photovoltaic cells and also necessary for determining cooling loads for buildings [2]. Solar radiation thus has many useful applications in architectural design, evapotranspiration estimates, agriculture, and atmospheric, land, ocean, and hydrologic models [3, 4]. The acquisition and the development of database on the long term solar radiation will facilitate the evaluation of solar energy potential as an input to the country’s energy budget and other modeling applications mentioned earlier. The development of solar technology in the country will minimise its overreliance on wood fuel consumption, estimated at 18 million tons per annum, especially in the rural communities ([5] and references therein). Studies carried out on solar irradiance measurements in some parts of the country suggest that there is a potential for solar energy to be used on commercial scale and in this vein the Ghana Grid Company (GRIDco) has begun producing electrical energy from a solar farm established in the northern part of Ghana