Estimation of Global Solar Radiation in the Eastern Terai Region of Nepal using Meteorological Parameters in RadEst 3.0

Abstract

Understanding the distribution of solar radiation is essential for the development and optimisation of solar energy technologies. This study estimates daily global solar radiation (GSR) in Janakpur, a city in the Eastern Terai Region of  Nepal, using RadEst 3.0 software. The analysis used seven meteorological parameters: rainfall, maximum and minimum temperatures, maximum and minimum humidity, and wind speed, with global solar radiation measurement for model validation.

Four empirical models were employed: Donatelli and Bellocchi (DB), Campbell and Donatelli (CD), Bristow and Campbell (BC), and the combined Donatelli-Campbell-Bristow-Bellocchi (DCBB). Their performance was evaluated using statistical metrics including the coefficient of determination (R²), mean bias error (MBE), mean percentage error (MPE), mean error (ME), root mean square error (RMSE), and correlation coefficient (r). Parameter fitting (PF) was applied to optimize model performance by maximizing R² and minimizing CRM and RMSE. The annual mean GSR was 12.9 ± 0.25, 16.3 ± 0.27, 14.7 ± 0.33, and 15.3 ± 0.33 MJ/m²/day for 2021, 2022, 2023, and 2024, respectively, with maximum daily values of 26.4, 28.3, 27.7, and 26.4 MJ/m²/day.

Among the models, DCBB consistently outperformed others, yielding R² values of 0.37, 0.63, 0.62, and 0.69, and RMSE values of 2.54, 2.13, 2.77, and 2.64 MJ/m²/day for the respective years. Additionally, DCBB showed the lowest MBE and MPE values across all years. Therefore, the DCBB model is identified as the most reliable for accurate GSR estimation in this region of Nepal.