Independent models for estimation of daily global solar radiation: A review and a case study

Independent models are models that estimate daily horizontal global solar radiation without need for prior records of any solar or meteorological parameters such as sunshine duration, cloud cover or temperature. These models are based on the day number, which means that they are very simple, enabling the user to estimate global radiation using only a pocket calculator. Though, they are relatively newer and much fewer as compared to other dependent models such as sunshine-based models. In this study, a first comprehensive review of all such models is provided. Besides, daily radiation data integrated from highly accurate minutely irradiance data measured at Cairo (Egypt) is used to recalibrate and compare independent models statistically. All models are trained and cross-validated using a detailed novel approach. The results show that, despite their simplicity, independent models have very good estimations of solar irradiation with coefficients of determination no less than 82%.