The second equation of balance (2) can be used to obtain the generation of entropy in the converter: By taking into account (21) and (26) we see that <7 is always non-negative. Consequently, from (24) we obtain This result was previously obtained by Landsberg & Tonge [7] within a different theoretical framework. In the case of black-body radiation, gives a better approximation for r] than the two other simple formulae we quoted in the Introduction [13]. In order to derive a more accurate upper bound for the conversion efficiency, we will consider a black-body converter at temperature T which receives radiation from a hemispherical black-body at a temperature Th eq given by: The absorbed and emitted fluxes of energy and entropy are now: The conversion efficiency will be given by (6): where the function a is given by:
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