Estimating the diffuse component from daily and monthly measurements of global radiation

Ln this study approximate to 83 000 measurements of daily global (R-s) and diffuse (Rd) solar irradiance from 25 sites in Australia and Antarctica (12 degrees S-67 degrees S, 1971-1995) are used to develop correlations between the diffuse fraction (R-d/R-S) and the clearness index (R-s/R-o) where R-o is the extra-terrestrial solar irradiance. A single set of parameters which are slightly dependent on latitude are derived for use with daily irradiance data (typical R-2 approximate to 0.90). Comparisons with previously published regressions from the northern hemisphere indicate that the model and parameters are also suitable for use in that hemisphere. A theoretical approach is used to scale the daily model to a monthly basis and subsequent testing established the validity of the theoretical approach. However, for practical applications, empirical relationships are necessary to estimate the variances and covariances which are part of the theory but which are not normally available in databases. A final empirical scaling model was developed to estimate the monthly average daily diffuse radiation (RMS = 0.91 MJ m(-2) day(-1), R-2 = 0.86, N = 2780). The methods are intended for use in broad scale carbon balance models but should be equally applicable to a broad range of solar engineering tasks. (C) 1999 Elsevier Science B.V. All rights reserved.