Comparison of empirical models to estimate monthly mean diffuse solar radiation from measured data: Case study for humid-subtropical climatic region of India

This study demonstrates the performance comparison of the models to estimate monthly mean diffuse solar radiation on a horizontal surface for Humid-Subtropical Climatic Region of India. Meteorological data (global and diffuse solar radiation) were observed for the city of Aligarh (27.89 degrees N, 78.08 degrees E) at the Heat Transfer and Solar Energy Laboratory, Department of Mechanical Engineering, Aligarh Muslim University, Aligarh. The mean global, beam and diffuse solar radiation values for Aligarh were observed as 22.12 MJ/m(2)-day, 14.20 MJ/m(2)-day and 7.92 MJ/m(2)-day respectively. The mean values of sky-clearness index, diffuse fraction and diffusion coefficient were found to be 0.69, 0.37 and 0.25, respectively. The region exhibits abundant potential for solar energy utilization with favorable sky conditions. Diffuse solar radiation models with single input parameter (in the form of global solar radiation) were analyzed for suitability of estimation. 100 typical models were selected from the literature which correlated diffuse fraction (ratio of horizontal diffuse to global solar radiation) to sky clearness index (ratio of global to extraterrestrial solar radiation). The objective here was to find the most precise model which is suitable to assess diffuse solar radiation in the region considered. Distinctive statistical evaluation of the models was performed in terms of well-known statistical indicators. Further, the scaled values of statistical indicators were used to calculate Global Performance Indicator (GPI). GPI of the selected models lied in the range of from -4.2987 to 2.6606 with the highest value representing the best performing model. Models were subsequently ranked in order of descending GPI. This study helps to recognize the significance of diffuse solar radiation models where the absence of required equipment as well as high cost of operation and maintenance represents a critical barrier for measurement. The outcomes of the present study are valuable for locations within the developing countries and remote areas with similar climatic conditions.