Development of models for hourly solar radiation prediction

In this paper several solar models based on Zhang et al., Zhang and Huang, and Watanabe models are developed and tested against measured solar data obtained for several locations throughout the world. The solar models can predict hourly global, direct normal, and diffuse solar radiation from cloud cover information and non-solar data including dry-bulb temperature, wind speed, and relative humidity. Refinement of the Zhang and Huang model is also carried out to improve its prediction accuracy for tropical climates. Specifically, new coefficients of the model were determined based on regression analysis using measured hourly solar data from various tropical sites. Variations of the Zhang and Huang models requiring fewer input variables were also developed. These model variations were tested for both tropical and nontropical climates. The analysis results indicated that while only two variables (solar angle and cloud cover) are required to predict hourly global solar radiation for high latitude locations, dry-bulb temperature is required to accurately predict solar radiation for the tropical locations.