Optimized Nanogap Thermophotovoltaic Devices for Waste Heat Recovery

TL;DR

This study optimizes nanogap thermophotovoltaic devices for industrial waste heat recovery, analyzing configurations to maximize power density and efficiency with various design modifications.

Contribution

It identifies optimal nanogap TPV configurations, examines effects of metallic covers and air gaps, and highlights material choices for improved performance.

Findings

Optimal device configuration is highly sensitive to vacuum gap size.

Metallic cover enhances power density below 125nm gap but reduces efficiency.

Air gaps require ultrathin PV cells supported by substrates, which diminish their benefits.

Abstract

Nanogap thermophotovoltaic (TPV) devices can deliver high power densities even with the medium-temperature heat sources. As such, these devices are very promising for recovering industrial waste heat. So far, the demonstrated nanogap TPVs have shown performances far below optimal. The objective of this study is to identify the optimal designs for nanogap TPV devices targeted for industrial waste heat recovery. Optimal configurations for maximal power density, maximal efficiency, and a trade-off between the two are determined as a function of the size of the vacuum gap between the emitter and the photovoltaic (PV) cell. The effects of adding a metallic cover to the PV cell, as well as introducing an air gap between the PV cell and the reflector, are also studied through this optimization framework. Results show that the optimal device configuration is highly sensitive to the vacuum gap size. A metallic cover enhances power density for gaps below 125nm due to surface plasmon-polariton coupling, but significantly reduces efficiency due to its parasitic absorption. To realize the benefits of air gaps, ultrathin PV cells requiring mechanical support by a substrate are needed. The presence of the substrate, however, diminishes the benefits of the air gap rendering them ineffective. ITO and InAs are found as optimal materials for the emitter and PV cell, respectively, owing to tunable plasma frequency of ITO and low bandgap of InAs.