Revisiting the theoretical and practical limits of solar thermophotovoltaics

Solar thermophotovoltaic (STPV) systems offer a modular pathway to convert solar energy into dispatchable electricity by coupling high-temperature thermal storage with radiative energy conversion. Despite rapid progress in thermophotovoltaic (TPV) device efficiency in recent years, experimentally demonstrated STPV system efficiencies remain comparatively low, highlighting a lack of understanding of system-level performance constraints, with prior modeling efforts largely focused on the TPV sub-system. In this work, we present a unified thermodynamic framework to analyze STPV performance in terms of three key system-level design parameters: solar concentration, absorber spectral selectivity, and emitter-to-absorber area ratio. The TPV cell is treated as a radiative heat engine characterized by two temperature-dependent figures of merit — conversion efficiency and electrical power density — enabling a system-level analysis independent of specific device implementations. Using this framework, we show that the electrical power density of TPV devices emerges as a dominant constraint on STPV performance. As a result, realistic TPV behavior necessitates substantially larger emitter-to-absorber area ratios than predicted by idealized models, which are challenging to realize in storage-free STPV configurations but more readily compatible with storage-integrated architectures. These findings provide quantitative guidance for the design of next-generation STPV systems and clarify the conditions under which high-efficiency, storage-compatible solar-to-electricity conversion can be practically achieved. More broadly, this work establishes general system-level design rules linking ideal thermodynamic limits to realistic device constraints. • Unified thermodynamic framework for system-level STPV performance analysis. • General efficiency bounds established across the full concentration range. • Framework applied to record-efficiency TPV cell performance. • Electrical power density of TPV cells limits system-level STPV efficiency. • Large area ratios favor storage-integrated STPV architectures.