Correlations for the estimation of hourly global solar radiation

Empirical correlations for estimating the ratios of monthly mean hourly to daily global solar radiation received on horizontal surfaces have been compared and their validities tested. The correlation of Collares-Pereira and Rabl [Solar Energy, 22, (1979) 155] yields the best results, while the correlations of Liu and Jordan [Solar Energy, 4 (1960) 19], and Garg and Garg [Solar & Wind Technology 4 (1987) 113] give satisfactory values. Calculated ratios show small deviations from observed ratios around the solar noon. Measured ratios are generally lower than the calculated ratios in the morning and evening hours, while the reverse is observed for the hours around the solar noon.