Abstract
The paper presents an improved version of a previously published
model for the diffuse fraction of hourly global irradiance. In addition
to hourly solar elevation and clearness index, an hour-to-hour variability
index and regional surface albedo are included among the input parameters.
Moreover, to prevent excessively high normal incidence beam irradiances
at very low solar elevations, the model does not allow a solar elevation
dependent maximum beam transmittance to be exceeded. This new model
is tuned to 32 years of data from Bergen, Norway. Moreover, a test
against independent data from four European stations showed that
the model performs better than the models of Erbs et al. (1982),
Maxwell (1987) and Perez et al. (1992).
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