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Measuring diffuse irradiance with a
shadowband
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Diffuse radiation is often measured with a
shadowband that blocks the direct beam sunlight. While
shadowbands need to be
adjusted twice a week, the adjustment is quick and one doesn't need sunlight to
make it.
The problem with shadowbands is that they block
light from other parts of the sky as well that coming directly from the
sun. Many papers have been written on models to correct for the additional sunlight
blocked by the shadowband: the matrix model developed by Brock LeBaron and Joe
Michalsky [DSR1] is among the best.
The shadowband data in our data set have not yet
been corrected, but will be in the future. However, two factors need to be addressed
before such corrections are made. First, the accuracy of the correction process needs
to be tested. The data set now available should suffice to make such tests. Second,
the responsivity of solar cell based LI-COR
pyranometers change significantly
depending on whether the sky is clear or cloudy (see
Diffuse
Responsivity of Solar Cell Based Pyranometers). Work is under way to understand
and correct this spectral sensitivity of the LI-COR pyranometer. Once these two problems
have been tackled, the models for correcting our diffuse data can be applied.
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References
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DSR1 |
|
B.A. LeBaron, J.J. Michalsky, & R. Perez,
"A simple procedure for correcting shadowband data for all sky conditions,
Solar Energy, 44, pp. 249-256, 1990. |
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© 2022, UO Solar Radiation Monitoring
Laboratory.
Last
revised: April 7, 2022.
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Home page URL: solardatz.uoregon.edu |