Empirical models for estimation of diffuse solar radiation based on measured data for humid-subtropical climatic region of India

Solar radiation information at the site of solar energy systems is required for proper planning and execution of energy projects. In this work, global and diffuse solar radiations were measured for a period of two years (September, 2013 to August, 2015) in the Humid Subtropical Climatic Region of India in the city of Aligarh (27.89 degrees N, 78.08 degrees E). Secondary standard pyranometers were used to observe solar radiation at the ground level. The measured data for Aligarh were compared with the available data for the neighboring Capital city, New Delhi (28.61 degrees N, 77.21 degrees E). It was observed that Aligarh has a better availability of solar energy and solar energy utilization potential. Annual average global and diffuse solar radiations were observed to be 22.12 and 7.92 MJ/m(2)-day, respectively, for Aligarh. Using measured solar radiation data for Aligarh, six empirical models were established to assess monthly mean diffuse solar radiation (by correlating the diffuse fraction with the sky-clearness index). Models were analyzed in terms of several statistical indicators and exhibited reasonable agreement with correlation coefficients in the range of 0.7610 to 0.8188. Models were subsequently ranked based on the Global Performance Indicator. The Global Performance Indicator (GPI) of the developed models was in the range of -1.86 to 5.84, and the highest GPI was achieved for the cubic model. Published by AIP Publishing.