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

Abstract

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 (Si3N4/W/Si3N4) 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 (𝜂TPV) for a GaSb cell illuminated by the selective thermal emitter at 1500 K were estimated. 𝜂TPV 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 𝜂TPV are also discussed.