Abstract
Hourly pyrheliometer and pyranometer data from four U.S. locations
are used to establish a relationship between the hourly diffuse fraction
and the hourly clearness index kr. This relationship is compared
to the relationship established by Orglll and Hollands and to a set
of data from Highett, Australia, and agreement is within a few percent
in both cases. The transient simulation program TRNSYS is used to
calculate the annual performance of solar energy systems using several
correlations. For the systems investigated, the effect of simulating
the random distribution of the hourly diffuse fraction is negligible.
A seasonally dependent daily diffuse correlation is developed from
the data, and this daily relationship is used to derive a correlation
for the monthly-average diffuse fraction.
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