The upper limit to solar energy conversion

Petela carried out research to determine the maximum ideal theoretical work output obtainable from BR independent of any conversion device. However, omnicolor conversion is considered by many to be the ideal theoretical process for solar energy conversion. Petela's result for blackbody radiation (BR) exergy is often thought to be of little importance because it appears to neglect fundamental theoretical issues that are specific to the conversion of thermal radiation (TR) fluxes. In this paper, it is shown that Petela's BR exergy result does represent the upper limit for the production of work from solar radiation (SR) fluxes approximated as BR. This result means that reversible conversion of BR fluxes is theoretically possible. It is shown that the definition of the environment in conventional exergy analysis completely suffices for defining TR exergy. Inherent emission of TR by the conversion device is indicated to be a fundamental issue that cannot be avoided. It is explained why, in contrast to nonideal conversion, inherent emission has a beneficial effect on the maximum work output for an ideal device. Various approaches used in solar energy conversion, including omnicolor conversion, are compared using second law efficiencies. In addition to being irreversible, the emission of TR by these devices generally has nonzero exergy which is usually considered lost.