A practical high temperature photonic crystal for high performance thermophotovoltaics

Abstract

This work presents the first practical selective emitter for high performance thermophotovoltaics (TPV) that offers high optical performance, high temperature stability, and the ability to be fabricated in large area samples. In a TPV system, a heat source brings the photonic crystal emitter to incandescence, and the resulting thermal radiation drives a low-bandgap photovoltaic cell. The photonic crystal enables high eciency by enhancing the radiation from the heat source in the wavelength range that can be converted by the photovoltaic cell and suppressing the radiation outside of that range. Our photonic crystal, composed of a square array of cylindrical cavities etched into a metallic substrate, enables unprecedented efficiencies in solar, radioisotope, and hydrocarbon TPV systems. We overcome multiple technical challenges previously limiting selective emitters by developing new fabrication processes to improve optical performance; by adopting commercial poly-crystalline tantalum to fabricate large-area samples; by developing a HfO₂ passivation coating for improved thermo-chemical stability; and by developing a HfO₂ cavity filing process for improved omnidirectional performance. More specically, we developed a process for fabrication of uniformly patterned 50 mm diameter photonic crystals, integratable with virtually any heat source by brazing. Furthermore, we fabricated a photonic crystal in a sputtered tantalum coating, which can be directly sputtered onto a heat source. Our photonic crystal design reaches 67% of the performance of an ideal emitter. To further improve the omnidirectional performance, we fabricated a filled-cavity emitter, which experimentally demonstrated the theoretical prediction that HfO2-filled photonic crystals would have superior hemispherical in-band emissivity. Both fabricated photonic crystal designs were tested for 300 hours at 100°C with no detectable degradation due to the passivation by HfO₂. With our original design, we demonstrated the highest heat-to-electricity eciency in a hydrocarbon TPV experiment to-date, exceeding 4% and greater than previous 2-3% effissences thought to be the practical limit. Furthermore, we expect from simulations that our filled photonic crystal design will enable over 12% eciency with only engineering optimization. For reference, a 1.5% eciency corresponds to the energy density of lithium ion batteries.