The diffuse-global correlation: Its application to estimating solar radiation on tilted surfaces in Marrakesh, Morocco

This investigation has two main objectives. The first is to develop a diffuse-global correlation using data available in Marrakesh for diffuse component calculation purposes. The second objective is to carry out a statistical comparison of three specific models for estimating daily global radiation on tilted surfaces and to recommend the most accurate for the Marrakesh location. The isotropic model and two anisotropic models, Hay's model and Tadili's model, are chosen for discussion in this paper. Due to the lack of the diffuse measurements on a horizontal surface needed to perform the comparison of the models, a simple quadratic regression model has been developed for the estimation of this component using measurements taken in Marrakesh over one year. Compared to the Liu and Jordan correlation developed from 10 years of data, the proposed correlation gives satisfactory results for diffuse component prediction. Both correlations were used for the parameterization of tilted surface models to test the utilizability of the proposed correlation in the prediction of tilted surface radiation. Four statistical indicators were used to compare the accuracy of the models for each month. The results show that for the period November–February, the anisotropic model of Tadili parameterized with the proposed correlation performed best. For July and October, the daily tilted radiation is better estimated using anisotropic models parameterized with the Liu and Jordan correlation; whereas for the months March, April, June and August the isotropic model parameterized with the Liu and Jordan correlation is the best model for estimating radiation on tilted surfaces in the Marrakesh location.