Calibration of the modified Angstrom global solar radiation models for different seasons in South of Iran

Abstract

The Angstrom equation is one of the most accurate and widely used sunshine based models to estimate global solar radiation. The key in its application is the calibration of the local specific coefficients. The global solar radiation in fall and winter is affected by cloudiness conditions and the lower ratios of actual to maximum possible sunshine hours, resulted in receiving the lower global solar radiation during these seasons compared with the received radiation in spring and summer. Therefore, in this study, due to differences in sky-cloudiness, and atmosphere transmissivity in different seasons, the coefficients of the Angstrom modified models, namely Angstrom-Prescott, Modified Angstrom-Prescott and Reitveld, calibrated for the fall, winter, spring and summer seasons in Bajgah, Iran, independently. The seasonal based calibrated equations were compared based on statistical error tests such as the mean bias error (MBE), root mean square error (RMSE), coefficient of determination (R 2 ) and Nash–Sutcliffe equation (NSE) indexes. Good agreement was found between measured values and estimated data by the mentioned models. Statistical analysis is indicated that three seasonal based calibrated models are more accurate and recommendable to use in study region, when sunshine hours data are available in comparison with Angstrom-Prescott equation using the recommended coefficients for northern hemisphere. * Corresponding Author: A. Majnooni-Heris  majnooni@tabrizu.ac.ir