Estimation of global solar radiation using sunshine-based models in Ethiopia

Abstract

Solar energy is one of the renewable energy sources that can be used to solve Ethiopia’s current energy problems. However, global solar radiation data for the country are either not available at all levels or recorded for only a few years at some locations. In this study, monthly mean daily global solar radiation (H) over the horizontal surface at 15 sites in Ethiopia was calculated using sunshine hour-based models, such as the Angstrom-Prescott model (AP), the Louche model (LO), and the Glover-McCulloch Estimation Model (GM). For the performance of the proposed model, statistical error analysis and geospatial results were performed to ensure the validity of the model used. The validation results show that the H estimates from all stations agree well with the measured data from all models. Therefore, the proposed model can be used to predict global solar radiance. However, among the other models, the GM model performed the best H estimates for all sites, with an average r of 0.9574, an average RMSE of 0.6017, an average MPE of 0.00335, and an average MBE of −0.7555, respectively. Therefore, the GM model is suitable for estimating global insolation for the entire country. The highest measured solar radiation value (from NASA) was recorded in February (6.89 kWh/m2/day). Likewise, the highest estimated global solar irradiance in February was observed using the three empirical models, with values of 7.55, 7.12, and 7.47 kWh/m2/day for the AP, LO, and GM models, respectively. The lowest measured and calculated radiation values were recorded in July due to the country’s highest cloud cover and rainy season. The results also show that the estimated H in Ethiopia ranges from 3.45 6.89 kWh/m2/day (July) to 7.47 6.89 kWh/m2/day (February) based on the GM model calculations, with an annual average of 5.83 kWh/m2/day (average across all sites). This study can be utilized in the design, analysis and performance evaluation of solar energy potential, which is gaining significant attention from the Ethiopian government in the advocacy of a green economy and reducing greenhouse gas emissions as one of its political agendas to meet the Sustainable Development Goal (SDG).