Modified And Multivariate Models for Estimation of Diffuse Solar Radiation in Ogoja, Cross River State, Nigeria

This research utilized a 22-year dataset (2001-2022) of monthly average climatic data for Ogoja, Nigeria. The dataset includes global solar radiation, all-sky surface shortwave diffuse irradiance, mean temperatures, wind speed, relative humidity, and surface pressure, obtained from NASA. The data were analyzed using multiple linear regression, with atmospheric pressure, relative humidity, mean temperature, and wind speed as independent variables. Nineteen models were developed and categorized into modified Page, Liu and Jordan models, clearness index and single variable models, two-variable models, three-variable models, and four-variable models. The effectiveness of each model was evaluated using statistical indices: Mean Bias Error (MBE), Root Mean Square Error (RMSE), Mean Percentage Error (MPE), t-test, and coefficient of determination (R²). Among the models, the quadratic regression expression model (equation 28b) was found most suitable in the modified Page, Liu, and Jordan models category. In the clearness index and single variable models category, the model relating diffuse solar radiation with clearness index and mean temperature (equation 28h) was the best. For the two-variable models category, the model relating diffuse solar radiation with wind speed and mean temperature (equation 28j) was the most found best. In the three-variable models, the model relating diffuse solar radiation with wind speed, relative humidity, and mean temperature (equation 28o) performed best. Overall, the model relating diffuse solar radiation with clearness index and temperature (equation 28h) was found to be the most effective for estimating diffuse solar radiation in Ogoja, which indicates that this model can be used for accurate estimation of diffuse solar radiation in this climatic location via simple temperature measurements, without the use of advanced instruments like pyrheliometers, which makes it a reliable method for estimating diffuse solar radiation, beneficial for solar energy planning and climate studies in similar regions.