Estimation of horizontal diffuse solar radiation in Egypt

The measured values of monthly average daily global (H) over bar and diffuse (H) over bar (d) solar radiation on a horizontal surface, as well as the number of bright sunshine hours (n) over bar, for four Egyptian locations are analyzed. The selected locations include Matruh, Al-Arish, Rafah and Aswan to represent the weather conditions of the north and south of Egypt. First, second and third order correlations between the diffuse fraction (H) over bar (d)/(H) over bar and the clearness index (K) over bar (t) and also between the diffuse transmittance (H) over bar (d)/(H) over bar (0) and the maximum relative number of bright sunshine hours (n) over bar/(N) over bar have been proposed for the selected locations using the method of regression analysis. Comparisons between the measured and calculated values of (H) over bar (d) show that a first order correlation between (H) over bar (d)/(H) over bar and (K) over bar (t) can be used for estimating (H) over bar (d) at the present locations with good accuracy. However, second order correlations between (H) over bar (d)/(H) over bar or (H) over bar (d)/(H) over bar (0) and (n) over bar/(N) over bar are recommended for estimating (H) over bar (d) at these locations. For all locations, the relative percentage errors for a single month rarely exceed +/-10%. Furthermore, the data available from the four locations are combined and employed to develop correlations that may be applied for any Egyptian location. All Egypt correlations are found to be able to predict the annual averages of horizontal diffuse radiation with excellent accuracy. Therefore, the long term performance of solar energy devices can be estimated. (C) 2003 Elsevier Science Ltd. All rights reserved.