The isotropic sky model is the simplest model that assumes that all diffuse radiation is uniformly distributed over the sky dome and that reflection on the ground is diffuse. A number of other models have been developed by a number of researchers. Three of these models are summarized in this section: the Klucher model, the Hay-Davies model, and the Reindl model. The latter proved to give very good results in the Mediterranean region.
Klucher (1979) found that the isotopic model gives good results for overcast skies but underestimates irradiance under clear and partly overcast conditions, when there is increased intensity near the horizon and in the circumsolar region of the sky. The model developed by Klucher gives the total irradiation on a tilted plane:
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The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.