The exergy flux of radiative heat transfer for the special case of blackbody radiation

Abstract

Exergy analysis is a highly effective method of analysis for thermal processes because it provides insight that cannot be obtained from energy analysis alone. In general the field of exergy analysis is both well formulated and well understood. However, the exergy flux, or the maximum work obtainable, from thermal radiation (TR) heat transfer has not been clearly formulated. Several researchers claim that Petela's thermodynamic approach for determining the maximum work obtainable from radiation is irrelevant to the conversion of fluxes because it appears to neglect a number of fundamental issues—issues that are unusual in the context of exergy analysis. In this paper it is shown that Petela's result gives the exergy flux of blackbody radiation (BR) and represents the upper limit to the conversion of solar radiation (SR) approximated as BR. This conclusion is obtained by resolving a number of fundamental questions including that of: inherent irreversibility, definition of the environment, the effect of inherent emission and the effect of concentrating source radiation. Correctly identifying the exergy flux of TR allows the general exergy balance equation for a control volume to be re-stated so that it correctly applies to TR heat transfer. An ideal (reversible) thermal conversion process for BR fluxes is also presented. Finally, exergetic (second-law) efficiencies are presented for common solar energy conversion processes such as single-cell photovoltaics