Here and are the rates of energy and entropy absorbed from the source of radiation (diluted or undiluted), Q' is the unbalanced flux of thermal energy, 0S and % are the rates of energy and entropy emission by the absorber to a sink; Sg is the rate of entropy generation in the absorber and T is absorber temperature. Subsequently, the flux Q is partially converted into work: where W is the rate of mechanical, chemical or other type of work performed and Q is the flux of thermal energy finally reaching the surroundings. In the most favourable situation there is no further increase of entropy during work production. Then: where To is the sunk temperature. Let us call the energy flux of the incoming radiation. The conversion efficiency can now be defined as: The derivation of (6) was performed without taking into consideration the nature of the incoming radiation. Consequently, this equation can be used for both diluted and undiluted black-body radiation. Before applying (6) we must remember some important results on diluted blackbody radiation (see [7]). The effective temperature Tc of the diluted radiation depends on the black-body temperature TR by: where £ < 1 is the so-called dilution factor and /(£) is a monotonic function exactly calculated for the first time in the quoted paper, that can be approximated for small £ by where a is Stefan’s constant and the function A(a>) refers to a geometrical factor given by: a) being the solid angle subtended by the source of radiation. We can easily verify that in the case of a hemispherical source (cd = 2n) of undiluted radiation (5=1) the equations (7), (9) and (10) reduce to the usual black-body quantities.
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