Correlations of direct solar luminous efficacy for all sky, clear sky and intermediate sky conditions and comparisons with experimental data of five localities

Many correlations of mean hourly direct luminous efficacy for all sky, clear sky and intermediate sky conditions have been compared with experimental data measured in Arcavacata di Rende (Italy), Geneva (Switzerland), Vaulx-en-Velin (France), Bratislava (Slovakia) and Osaka (Japan). The comparisons show that, for all sky conditions, the correlations developed for one locality predict the luminous efficacy in the other localities with mean errors between −13% and 20% and root mean square errors between 15% and 35%; for clear sky conditions, the correlations developed for one locality predict the luminous efficacy in the other localities with mean errors between −7% and 14% and root mean square errors between 7% and 19%; for intermediate sky conditions, the correlations developed for one locality predict the luminous efficacy in the other localities with mean errors between −17% and 26% and root mean square errors between 20% and 41%.If the correlations with local coefficients are applied in each locality, the errors become considerably shorter. The paper gives indications on the values of the measured mean luminous efficacies and on the best models.