Empirical Relationships Between Global and Diffuse Radiation and Sunshine Duration in Chad: Polynomial Regression Approach

Chad, in regards to its geographical position is considered as one of the countries in the world that receives an important annual solar radiation over its entire territory. The average sunshine duration is about 10 hours in the North, 8 hours in the Center and 7 hours in the South. This can be used to quantify the solar radiation over the whole territory and evaluate the solar potential. In this paper, some new correlations between sunshine duration and solar radiation have proposed in four cities (Abeche, Bitkine, Mongo and Ndjamena) located in the Sahelian climate in order to have tools for quantifying all components of solar radiation. The polynomial regression model approach is used and their parameters are estimated by the ordinary least squares method. A statistical evaluation was carried out to compare the different correlations obtained. The results show that the third order polynomial approach is the most appropriate for an empirical relationship between global, diffuse and relative sunshine duration in each of the cities. Considering the long dry seasons characterized by high temperatures in this region, the consideration of the temperature variable in the functional form could improve the quality and accuracy of the models in their estimation.