Thermodynamic Limitations on the Conversion of Radiant Energy into Work

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Abstract
The fraction of radiant energy incident on an absorber which may appear as work is limited by the radiation entropy and entropy gained in irreversible transfer from the radiation field to an absorber. Irreversibility may result from directionality of the radiation field, and some irreversibility is necessary to cause a net flow of energy from a radiation absorber into work or free‐energy storage. Impedance in the conversion apparatus may further limit the efficiency. Maximization of power storage under these constraints is discussed, and the general arguments are then applied to photoelectrical and photochemical systems; in these systems nonresonant decay of the excited state represents a major source of inefficiency, which may be minimized by appropriate choices for the Boltzmann temperature and optical density of the absorber. The relationships are developed for narrow‐band absorption, and application to broad‐band systems is discussed only briefly.