Review, Evaluation, and Improvement of Direct Irradiance Models

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Abstract
A study was performed to: (1) define the relative significance of the various atmospheric constituents to the depletion of the direct solar beam irradiance; (2) compare and evaluate several simple models versus a complex SOLTRAN-model; and (3) develop an improved, easy to use model. The results indicate that for a reasonable range of atmospheric conditions the broadband direct beam energy is attenuated by atmospheric constituents in the following order: aerosols attenuate the most; molecular scattering is next in importance; and water vapor absorption is third. Attenuation by O3, O2, and CO2 is minor. The total irradiance from various models agree to within ±10 percent at small zenith angles, but diverge significantly at large zenith angles.

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