Derivation of Cloud Index from Geostationary Satellites and Application to the Production of Solar Irradiance and Daylight Illuminance Data |
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Authors: | P Ineichen R Perez |
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Institution: | (1) Group of Applied Physics-Energy, University of Geneva, Switzerland, CH |
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Abstract: | Summary We investigate in the present paper the relationship between satellite count, global irradiance and other solar and illumination
resource components, bringing a particular attention to low solar elevation situations (below 20 °) which are very important
in northern latitudes. Our investigation is based on data from two geostationary satellites, METEOSAT and GOES, backed by
ground measurements in Switzerland and the northeastern USA.
The study of different clear sky normalizations lead to the conclusion that a linear correlation between the global clearness
index and the irradiance (like the heliosat method) would be inaccurate for low solar elevations, and therefore for high latitude
regions. We developed a model that directly relates an elevation dependent clearness index to the could index. This methodology
presents a definite advantage because it can be generalized to address the clearness index of other solar radiation components,
besides global irradiance, such as direct irradiance, diffuse illuminance, etc.
The correlations described in this paper were developed on the data from Geneva (in the frame of the EC program “Satellight”)
and evaluated on two other independent data sets (Albany, USA and Lausanne, Switzerland). Their precisions, on a hourly basis,
are respectively 30%, 40% and 60% for the global, diffuse and beam components) (90,55 and 95 W/m2). The use of independent data for thederivation and the validation of the models shows thatthose can be used in a wide range
of locations, even if the applicability has to be assessed for very different climates.
Received June 27, 1998 Revised February 26, 1999 |
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