Entropy Generation and Ideal Work Limit in Near-field Radiative Transfer between Dielectric Plates

Abstract

Due to the presence of evanescent electromagnetic modes, near-field radiation can enhance energy and entropy transfer, thus offering broad application prospects in radiative energy conversion systems. In this study, the second law of thermodynamics is applied to investigate entropy generation in a parallel plate configuration, considering variable vacuum gaps that span from the near field to the far field. Lossless dielectric materials and silicon carbide (SiC) are chosen as representative materials for the semi-infinite media. It is determined that the contribution to entropy generation from an individual plate depends on the temperature, material properties, and the vacuum gap spacing. Nevertheless, under coherent thermal radiation conditions, the ratio of spectral entropy generated by the emitter to that by the receiver is independent of the vacuum gap or material properties, unlike the case of incoherent far-field radiation. The endoreversible assumption is applied to either the emitter (hot side) or the receiver (cold side) to determine the upper bounds of ideal work and efficiency when the power is produced by the emitter or receiver, respectively. The maximum achievable efficiency for the hot-side converter is a function of the vacuum gap distance and material properties. On the other hand, cold-side converters may achieve the highest efficiency in the far-field case when the incoming radiation is incoherent. This work establishes a framework for employing entropy generation as a tool to optimize thermal radiative systems and thermoelectric conversion devices in both the near and the far fields.