Near-field enhancement of thermoradiative devices

Thermoradiative (TR) device has recently been proposed for noncontact direct photon-electricity energy conversion. We investigate how the near-field effect can boost the performance of a TR device. For a near-field TR device, a heat sink is placed close to the TR cell, with the separation being small compared to the characteristic photon wavelength. It is demonstrated that the TR device, like the thermophotovoltaic (TPV) device, can be formulated using the transmissivity and the generalized Planck distribution. We quantitatively show that -function transmissivity is a very good approximation (capturing up to 90% of total radiative energy transfer) when the radiative energy transfer is governed by resonances. Three practical types of heat sink are considered, a metallic material described by the Drude model, a polar dielectric material described by the Lorentz oscillator model, and a semiconductor material that is identical to the TR cell. The blackbody heat sink serves as the far-field reference. By properly choosing the resonant frequencies supported by the heat sink, we show the heat sink made of a Drude or Lorentz material can enhance the output power by about 60 and 20 times respectively, as compared to the blackbody reference. Even with a heat sink made of the same material as the TR-cell, which does not support any resonant modes, the output power can be enhanced by about 10 times. The mechanisms can be elucidated from the impedance matching condition derived from coupled-mode theory.