An important parameter in the analysis provided by the second law of thermodynamics for the evaluation of solar energy conversion systems is the radiation temperature. The solar radiation is diluted due to atmospheric scattering and absorption, and reflection at the absorber surface. This dilution reduces the temperature of the sun (as a blackbody source) to an effective radiation temperature. In the present study, spectral dilution functions were derived for the three components of the absorbed global terrestrial solar radiation (direct beam component + forward Mie component + diffuse component). The energy and entropy fluxes and effective temperature for each component were expressed in terms of these dilution functions. They were calculated for different atmospheric conditions using numerical integration over the wavelength. The effects of the air mass and the atmospheric parameters on these thermodynamic quantities and the maximum conversion efficiency of solar energy were investigated. An apparent temperature assigned for the sun as a high temperature reservoir for Carnot engine was calculated and found to vary between about 3600 K for clear sky and about 2000 K for highly turbid sky under the given atmospheric conditions.

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