It has been found that the values of ratio of hourly global illuminance to daily global illuminance, rvt, are very close to the corresponding values for global solar radiation, rt, examined from the measured data of six IDMP  (International Day light Measurement Programme) locations. This has been further confirmed by examining the values for rvt and rt as calculated from TMY2  (Typical Metrological Year) data base for primary locations. Based on this, it has been proposed that the correlations available in literature to predict rt can be employed to predict rvt. Adequacy of the correlation due to Collares-Pereira and Rabl  available for rt has been examined to predict rvt as obtained from TMY2 data base for the 56 primary locations. It has been found that the values of rvt obtained from measured illuminance data of six locations have been predicted within a rms difference of 2.71% and within a rms difference of 4.3% for the 56 primary locations of TMY2 data when the correlation due to Collares-Pereira and Rabl for rt has been employed.
Interrelations of Hourly and Daily Global Illuminance
Ravikumar, P. "Interrelations of Hourly and Daily Global Illuminance." Proceedings of the ASME 2006 International Solar Energy Conference. Solar Energy. Denver, Colorado, USA. July 8–13, 2006. pp. 457-465. ASME. https://doi.org/10.1115/ISEC2006-99070
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