A semi-empirical model for estimating diffuse solar irradiance under a clear sky condition for a tropical environment

Diffuse irradiance under clear sky condition is of importance for many solar energy applications. In this work, a semi-empirical model for the estimation of clear sky diffuse irradiance in a tropical environment was developed. In formulating the model, clear sky diffuse irradiance data from 3 sites in Thailand, namely Chiang Mai (18.78° N 98.98° E), Nakhon Pathom (13.82°N, 100.04° E) and Songkhla (7.2° N, 100.6° E) were collected. Aerosol optical depth from AERONET of NASA and total column ozone from OMI/AURA satellite at the positions of these sites were also collected. The model expresses diffuse irradiance as an semi-empirical function of precipitable water, total column ozone, aerosol optical depth and solar zenith angle. The empirical constants of the model were obtained by using a non-linear multi-variable regression technique. To evaluate its performance, the model was used to calculate clear sky diffuse irradiance using an independent data set. It was found that diffuse irradiance calculated from this model reasonably agreed with that obtained from measurements.