Waste heat harvesting from thin-film solid oxide fuel cells via multi-junction near-field thermophotovoltaic integration

A thin-film solid oxide fuel cell (TF-SOFC) is an electrochemical device that generates electricity at medium temperatures through redox reactions between fuel (e.g., hydrogen or hydrocarbons) and oxygen, offering high energy conversion efficiency. However, waste heat generated during TF-SOFC operation reduces overall system efficiency. To address this issue, we propose an integrated energy system that combines a TF-SOFC operating at 873 K with a multi-junction-based near-field thermophotovoltaic (NFTPV) converter for improved power output via effective waste heat recovery. Multi-junction NFTPV converters were optimized to minimize performance degradation caused by additional losses associated with scalable photovoltaic designs. Among the evaluated configurations (single-, tandem-, and triple-junction), the tandem-junction NFTPV achieved the highest power density. We further investigated the effect of surface emissivity, which governs radiative losses arising from the area mismatch between the TF-SOFC and NFTPV components. A critical emissivity threshold was identified, beyond which the area ratio required for achieving peak power density could no longer compensate for radiation losses. Additionally, the influence of the fuel stoichiometric factor on system performance was investigated, revealing a trade-off between power density and overall efficiency. Under optimized conditions, the integrated system achieved a maximum power density of 1.50 W/cm2 and an overall energy conversion efficiency of 34.5%, representing a 1.34-fold improvement in power density over a standalone TF-SOFC.