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E.M. Vazifeh, M. Schuss, A. Mahdavi:
"A Comparative Assessment of Diffuse Fraction Models";
Applied Mechanics and Materials, 861 (2017), 509 - 515.

Many building performance applications (energy use, solar gains, thermal comfort,
renewable energy systems, daylight, etc.) require information about both direct and diffuse
components of the incident solar radiation. However, most meteorological stations only monitor
global horizontal irradiance. Consequently, multiple methods have been proposed in the past to
derive from measured global horizontal irradiance data the diffuse fraction. Thereby, additional data
regarding other parameters such as clearness index, solar altitude, air mass, and turbidity are used.
Given the importance of this procedure for the down the line tool, its reliability represents a critical
issue. To address this point, we pursued an empirical approach. A number of existing methods for
the computation of the diffuse fraction were selected. Actual measurements of global and diffuse
irradiance were obtained for seven locations in USA and one location in Austria. The measured
global irradiance data for these locations were fed to the aforementioned diffuse fraction models.
The calculation results were then compared with the corresponding empirical data. The comparative
assessment yielded a number of findings. The relative performance ("ranking") of the models was
found to be more or less consistent across the different locations. However, none of the models can
be said to be performing wholly satisfactory. For instance, the best performing model displayed
only in 45 to 65% of the cases relative errors less than 20%. In case of the worst performing model,
the percentage of the cases for which relative errors were less than 20% was even smaller, namely
30% to 60%.

(no english version) Many building performance applications (energy use, solar gains, thermal comfort,
renewable energy systems, daylight, etc.) require information about both direct and diffuse
components of the incident solar radiation. However, most meteorological stations only monitor
global horizontal irradiance. Consequently, multiple methods have been proposed in the past to
derive from measured global horizontal irradiance data the diffuse fraction. Thereby, additional data
regarding other parameters such as clearness index, solar altitude, air mass, and turbidity are used.
Given the importance of this procedure for the down the line tool, its reliability represents a critical
issue. To address this point, we pursued an empirical approach. A number of existing methods for
the computation of the diffuse fraction were selected. Actual measurements of global and diffuse
irradiance were obtained for seven locations in USA and one location in Austria. The measured
global irradiance data for these locations were fed to the aforementioned diffuse fraction models.
The calculation results were then compared with the corresponding empirical data. The comparative
assessment yielded a number of findings. The relative performance ("ranking") of the models was
found to be more or less consistent across the different locations. However, none of the models can
be said to be performing wholly satisfactory. For instance, the best performing model displayed
only in 45 to 65% of the cases relative errors less than 20%. In case of the worst performing model,
the percentage of the cases for which relative errors were less than 20% was even smaller, namely
30% to 60%.

Solar Radiation, Diffuse Fraction Models, Performance Simulation.

http://dx.doi.org/10.4028/www.scientific.net/AMM.861.509