A comparative analysis is presented of the conversion of radiant energy to useful work by thermal and quantum processes. The operation of thermal and quantum converters and the thermodynamic conversion efficiency of each are developed in terms of the mechanism of radiation-matter interaction in thermal and quantum systems. From the analysis the maximum conversion efficiency of a single-collector thermal converter with unconcentrated solar radiation and an ambient (reservoir) temperature of 300 K is 0.540; for the same conditions the maximum conversion efficiency of a single-collector quantum system is 0.309. The analysis is extended to consider the effects on the conversion efficiency of heat reject temperature, cascaded operation, in which the reject heat of the quantum converter is used as the input to a thermal bottoming cycle, and of concentration of the solar radiation. The results obtained represent the thermodynamic limits for radiant energy conversion by thermal and quantum processes, and calculations with solar input serve as a reference against which to judge the performance and capabilities of prospective solar energy conversion systems.

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