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Measuring diffuse irradiance with a shadowband

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.

References

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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© 2003, UO Solar Radiation Monitoring Laboratory.
Last revised: August 26, 2003.


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