On the Thermodynamics of the Conversion of Diluted and Undiluted Black-body Radiation V. BADESCU Summary The maximum conversion efficiency r],^ of thermal radiation is a function of a, e and f, which are the spectrally averaged absorbtance and emittance of the converter and a factor defined by Landsberg & Tonge (Journal of Physics A, 12, p. 551, 1979) respectively. It was shown that has an upper bound which depend on a, e and f. This upper bound is more accurate than the existing formulae, which are not dependent on these factors. 1. Introduction The idea that an intermediate material could be used to absorb solar radiation, turn some of the heat so generated into work via a thermodynamic cycle and re-radiate some for photovoltaic conversion has been discussed from time to time. Such a scheme is especially attractive for large space power systems and where conductive and convective losses can be kept very low. Many authors have studied the thermodynamics of the conversion of diluted and undiluted black-body radiation, because of its implication in solar energy applications (see, e.g., [1-9]; for a good review see [10]). Two main research directions have resulted. The first one focuses on the efficiency of some particular conversion process. The second one is to derive general formulae describing upper bounds for the maximum conversion efficiency of any conversion process. In the second case, different simple formulae are proposed for both undiluted and diluted radiation. We quote the formulae of Spanner [2], Petela [3] and Jeter [8], respectively, in the case of undiluted radiation, and the formula of Landsberg & Tonge [7] for diluted radiation. However, there are situations when all these formulae yield values too high to be useful in application [7,11], In this paper we propose a more accurate upper bound formula, which can be used in case of both black-body and non-black-body converters. 2. Thermodynamics of Conversion We suppose a non-emissive ambient and a selective (non-black-body) converter with negligible conductive and convective thermal losses. Then, the steady-state balance equations per unit area of converter are [7]: V. Badescu, Mecanica, Termotehnica, Polytechnic Institute of Bucarest, Bucarest 79590, Romania.
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