Abstract
Nearly all diffuse fraction correlations have been based on integrated
hourly or daily data. Solar collectors, however, see instantaneous
solar radiation. Measurements of the terrestrial instantaneous diffuse
fraction are shown to depend on both clearness index and air mass.
The resulting instantaneous diffuse fraction correlation differs
markedly from the correlations obtained for integrated diffuse fractions.
A model for the instantaneous diffuse fraction is developed, which
describes the generation and transmission of diffuse solar radiation
in the atmosphere. The measurements of instantaneous diffuse fraction
are used to explain the scatter inherent in daily diffuse fraction
data. It is shown that measurements of the instantaneous diffuse
fraction together with the frequency distribution of instantaneous
radiation may be used to determine monthly average daily diffuse
fractions.
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