Determining Diffuse Solar Irradiance: A Comparison of Single and Multiparameter Separation Models for Nepal

Abstract Increasing the use of solar energy technologies, such as photovoltaic and solar thermal systems, presents Nepal with a significant opportunity to realize its potential for sustainable development. Despite being located in the global sunbelt, Nepal's understanding of its solar resource remains limited and has not been adequately explored in the existing literature. To address this gap, this study developed a model to separate hourly global solar irradiance into its direct and diffuse components using a single predictor variable (clearness index), specifically tailored for Nepal, as shown in kd=1−0.04kt+0.09kt2−5.63kt3+4.60kt4. The results showed that the model's performance, measured by root mean squared error and mean bias error, was comparable to that of the leading multiparameter separation method (Engerer2). Given the elegant simplicity of the model, it offers a promising starting point for those wanting to calculate the diffuse and direct solar irradiance from measurements of global solar irradiance on a horizontal surface which can then be used to assess the performance of solar energy systems in Nepal.