The exergy flux of radiative heat transfer with an arbitrary spectrum

Abstract

In general the field of exergy analysis is both well formulated and well understood. However, the exergy flux, or maximum work obtainable, from thermal radiation (TR) heat transfer has not been clearly formulated. In a previous article it was shown that Petela's result, from his thermodynamic approach, does in fact represent the exergy flux of blackbody radiation (BR) and the upper limit to the conversion of solar radiation (SR) fluxes approximated as BR. This conclusion was obtained by resolving a number of fundamental issues including questions relating to: inherent irreversibility, definition of the environment, the effect of inherent emission and the effect of concentrating source radiation. In this paper, a new expression based on inherent irreversibility is presented for the exergy flux of TR with an arbitrary spectrum. It is shown that previous approaches by Petela and Karlsson are equivalent and assume that reversible conversion of non-blackbody radiation (NBR) is theoretically possible. However, evidence is presented indicating that the conversion of NBR is inherently irreversible. The analysis in this paper emphasizes the proper formulation for TR exergy by re-stating the general exergy balance equation for thermodynamic systems so that it correctly applies to NBR heat transfer. Finally, it is shown that the exergy flux of NBR, or the maximum work obtainable from NBR conversion, can be a small fraction of the energy flux.