A comparative study of total, direct and diffuse solar irradiance by using different models on horizontal and inclined surfaces for Cairo, Egypt

The measured hourly daily data of total, direct and diffuse solar irradiation incident on a horizontal and an inclined surface for Cairo, Egypt (Lat. 30°05′N and Long. 31°15′E), during the period (1990–2010) are analyzed. The regression equations between (G/Go) and meteorological variables along with the values of MBE, RMSE, MPE, R2 and the t-test statistics are summarized in this research. The values of correlation coefficients (R2) are higher than 0.95 and the values of the RMSE are found in the range 3.13–6.34, thus indicating a good agreement between measured and calculated values of the total solar radiation (G). The models of Eqs. (10), (11) and (14) have well estimated the total solar irradiation in the selected location during the time period in the present study. For all models, the absolute values of the MPE indicate very good agreement between measured and calculated values of the diffuse solar fraction (Gd/G) or the diffuse solar transmittance (Gd/Go) and clearness index Kt, relative number of sunshine hours (S/So) and their combination. The models of Hay (Ha), Skartveit and Olseth (SO) and Perez et al. (P9) give the most accurate predictions for the south-facing surface, and Hay (Ha) and Perez et al. (P9) models performs better as estimated for the west-facing surface.