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Abstract
Monthly means of daily solar irradiation retrieved from the HelioClim-3 version 3 database (HC3v3), elaborated from Meteosat satellite images, were tested at 14 Egyptian stations along with the model of Yang, Koike, and Ye (YKY) and 10 empirical models (EMs) for the period 2004 to 2009. YKY and EMs were fitted to measurements from the period 1980 to 1989. Overall, HC3v3 exhibits a bias of 0.4 MJ m−2 (i.e. 2% of the mean of the observations – similar to the best EMs). The root mean square error (RMSE) was 1.8 MJ m−2 (9%) for HC3v3, which is lower than for most EMs. Coefficients of determination (R2) were greater than 0.9 for most models. The regression line between estimates and observations exhibits a slope of 1.01 and an intercept of 0.09 MJ m−2 for HC3v3, reflecting a better performance than other models. HC3v3 shows remarkably constant performance as a function of irradiation or cloudiness compared with EMs and YKY. In general, HC3v3 is preferred to EMs when estimating monthly means of daily solar irradiation in Egypt. It is suggested that more effort is needed towards the validation and promotion of HC3v3 before researchers and practitioners use it rather than EMs.
Acknowledgements
The authors are grateful to the Egyptian Meteorological Authority and the World Radiation Data Centre (WRDC) for the provision of ground measurements. The authors thank the company Transvalor, which maintains the SoDa Service, thereby allowing efficient access to HelioClim databases. The authors also thank Prof. Kun Yang for providing the source code of the YKY model. The authors acknowledge the help of the anonymous reviewers whose fruitful comments contributed to enhancing the quality of the article.