Estimation of monthly mean diffuse solar radiation over India: Performance of two variable models under different climatic zones

In the present work, empirical models are proposed to estimate monthly-mean diffuse solar radiation on a horizontal surface under different climatic zones of India. Long-term meteorological data consisting of monthly average global and diffuse solar radiation together with sunshine duration for five major locations under prominent climatic zones was obtained from Indian Meteorological Department (IMD), Pune. Models with two inputs predictors (clearness index and sunshine ratio) under two categories i.e. diffuse fraction and diffusion coefficient were developed and compared (with each category consisting of eight models). Proposed models were also compared with well-established models from literature and were statistically analyzed to assess the performance. Global Performance Indicator (GPI) was used to organize models in order of best estimation capability within the two categories defined and among the overall group of sixteen models. Further, the developed models were also validated against subsets of measured data for five representative locations considered. Good agreement was found between estimated values from the developed models and the measured data. It was concluded that the developed models can conveniently be utilized to estimate diffuse solar radiation under varying climates within reasonable accuracy.