Numerical computations of diffuse fraction of global irradiance on an hourly basis
Date Issued
April 2014
Abstract
Solar Energy is the feedstock for various applications of renewable energy sources, thus, the necessity of using
global tilted irradiance is acknowledged for the computations of the performance and monitoring of PV Parks
and solar energy applications. In general, global tilted irradiance is computed as the sum of the beam component
of direct irradiation on the tilted surface, diffuse tilted and reflected irradiance. These three components can be
computed using only the values of Global horizontal and diffuse irradiance. However, although for some locations
both global and diffuse irradiance are measured, in most locations, the data comprise measurements of only global
horizontal irradiance, either measured on-site or determined from satellite data.
This research is based on a numerical analysis and the development of empirical correlations for the computation
of the hourly diffuse fraction, based on the measurements of the clearness index. The solar altitude is
included as a parameter in the computations in order to reduce the error in the computations, since it embraces the
effect of the different time and date in the computations.
The derived numerical equations are presented in terms of the solar altitude in steps of 5 degrees and are
validated using data from the meteorological station of Athalassa, Cyprus, for a ten year period (2001-2010).
The statistical analysis from the comparison (in terms of R-squared and RMSE) showed better results for higher
elevation angles, compared to the lower elevation angles that represent the early morning or late afternoon times.
global tilted irradiance is acknowledged for the computations of the performance and monitoring of PV Parks
and solar energy applications. In general, global tilted irradiance is computed as the sum of the beam component
of direct irradiation on the tilted surface, diffuse tilted and reflected irradiance. These three components can be
computed using only the values of Global horizontal and diffuse irradiance. However, although for some locations
both global and diffuse irradiance are measured, in most locations, the data comprise measurements of only global
horizontal irradiance, either measured on-site or determined from satellite data.
This research is based on a numerical analysis and the development of empirical correlations for the computation
of the hourly diffuse fraction, based on the measurements of the clearness index. The solar altitude is
included as a parameter in the computations in order to reduce the error in the computations, since it embraces the
effect of the different time and date in the computations.
The derived numerical equations are presented in terms of the solar altitude in steps of 5 degrees and are
validated using data from the meteorological station of Athalassa, Cyprus, for a ten year period (2001-2010).
The statistical analysis from the comparison (in terms of R-squared and RMSE) showed better results for higher
elevation angles, compared to the lower elevation angles that represent the early morning or late afternoon times.
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