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A clear-sky model is a shortcut for a model providing estimates of the surface solar irradiance / irradiation under clear sky. The ESRA clear-sky model has been developed for the European Solar Radiation Atlas (ESRA, 2000). The ESRA model is described in this book but more accurately in Rigollier, Bauer, Wald (2000).
A library in C is available for implementing the ESRA model. The ESRA model is named model5 in this library. The library "csmodels_lib.c" comprises the following items:
An execution of the code is possible through the SoDa Service to check the correct implementation (search for "Simulation of Hourly Irradiation for One Clear-Sky Day"). The ESRA clear-sky model is used in the method Heliosat-2 for computing the solar surface irradiance for any type of sky starting from Meteosat satellite images. The document on the method Heliosat-2 may be helpful for implementing the ESRA model.
The ESRA clear-sky model requests the knowledge at each geographical site of the elevation and the Linke turbidity factor. There are several sources of the Linke turbidity factor. One is available at the SoDa Service, either in the form of 12 monthly values for a given site in the world (search for "Monthly Values of Linke Turbidity Factor") or in the form of 12 maps covering the world (search for "Linke Turbidity Factor: Monthly maps").
Note that a call to the HelioClim-1 or -3 database in the SoDa Service provides a time-series of surface solar irradiance including an estimate of the irradiance that would be observed if the sky were clear. This estimate is made by the ESRA model.
This software code is available under a Creative Commons license (CC-BY).