Thermodynamics of light management in near-field thermophotovoltaics

TL;DR

This paper develops an analytical model for near-field thermophotovoltaic systems, revealing how near-field effects enhance voltage and efficiency by enabling thinner cells and reducing non-radiative losses, with potential for high efficiency at moderate temperatures.

Contribution

The paper introduces an analytic model for near-field TPV systems, highlighting physical insights and advantages over previous numerical approaches, including improved voltage and efficiency predictions.

Findings

Near-field operation enhances radiative recombination and voltage.

Thinner PV cells are feasible due to photon recycling and low radiation leakage.

Achieves >50% efficiency at 1100 K emitter temperature.

Abstract

We evaluate near-field thermophotovoltaic (TPV) energy conversion systems focusing in particular on their open-circuit voltage (Voc). Unlike previous analyses based largely on numerical simulations with fluctuational electrodynamics, here, we develop an analytic model that captures the physics of near-field TPV systems and can predict their performance metrics. Using our model, we identify two important opportunities of TPV systems operating in the near-field. First, we show analytically that enhancement of radiative recombination is a natural consequence of operating in the near-field. Second, we note that, owing to photon recycling and minimal radiation leakage in near-field operation, the PV cell used in near-field TPV systems can be much thinner compared to those used in solar PV systems. Since non-radiative recombination is a volumetric effect, use of a thinner cell reduces non-radiative losses per unit area. The combination of these two opportunities leads to increasingly large values of Voc as the TPV vacuum gap decreases. Hence, although operation in the near-field was previously perceived to be beneficial for electrical power density enhancement, here, we emphasize that thin-film near-field TPVs are also significantly advantageous in terms of Voc and consequently conversion efficiency as well as power density. We provide numerical results for an InAs-based thin-film TPV that exhibits efficiency > 50% at an emitter temperature as low as 1100 K.