Performance Improvement for Germanium-Based Near-Field Thermophotovoltaic Converter

Abstract

Near-field Thermophotovoltaic (NF-TPV) converter utilizes the tunnelling of an evanescent wave to surpass the blackbody limit, enhancing the radiative heat transfer of the TPV converter by several orders of magnitude. One of key challenges for commercial NF-TPV converter is cost reduction. Germanium is known to be a cost-effective material with bandgap energy compatible with a TPV application. In this study, the germanium-based NF-TPV converter is introduced with an addition of air-bridge gap (ABG) at the interface between a TPV cell substrate and a metal back surface reflector (BSR) as a strategy to improve sub-bandgap photon utilization. Effects of air-bridge gap and germanium substrate thickness on radiative heat fluxes and converter performance are investigated. At the radiator temperature of 1400 K and the optimum air-bridge gap thickness of 1300 nm, system efficiency of NF-TPV converter increases from 17.16% to 24.98% for a 175  thick TPV cell, and from 16.73% to 31.39% for a 40  thick TPV cell under moderate surface passivation. In addition, the converter's performance under varying radiator temperature is analysed. This study demonstrates the potential of the air-bridge gap to optimize NF-TPV converter performance.