Consideration of temperature-dependent emissivity of selective emitters in thermophotovoltaic systems

Spectral emissivity control is paramount for designing a high-efficiency selective emitter surface required for thermophotovoltaic (TPV) applications. Owing to the temperature dependency of materials optical constants, the spectral properties of a selective emitter surface changes with the emitter temperature. This paper presents the fabrication of a multilayer metal-dielectric (Si_3N_4/W/Si_3N_4) coated tungsten selective emitter aimed for GaSb-based TPV systems and studies the dependence of its surface spectral emissivity, ε(λ), upon a temperature ranging from 300 K to 1500 K. Both the simulation and experimental methods were used to characterize ε(λ) as a function of temperature. For wavelengths less than 1.4 µm, ε(λ) was found to have a minimal dependence on temperature. Beyond 1.4 µm, ε(λ) increases with the temperature. At 1.55 µm, the simulation and experimental data estimated a ∼4% greater emissivity at 1500 K than at room temperature. At 1500 K, the increased ε(λ) at longer wavelengths lowered the spectral conversion efficiency of the selective emitter from 58% to 47%. The output power density, sub-bandgap loss, and TPV conversion efficiency (η_TPV) for a GaSb cell illuminated by the selective thermal emitter at 1500 K were estimated. η_TPV drops from 13.7% to 11% due to the increased sub-bandgap emission at 1500 K. Essential approaches for mitigating the sub-bandgap losses to further improve η_TPV are also discussed.