This work presents the first practical selective emitter for high performance thermophotovoltaics (TPV) that offers high optical performance, high temperature stability, and 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. 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 polycrystalline tantalum to fabricate large-area samples; by developing a HfO2 passivation coating for improved thermo-chemical stability; and by developing a HfO2 cavity filling process for improved omnidirectional performance.
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 1000C with no detectable degradation, due to the passivation by HfO2. With our original design, we demonstrated the highest heat-to-electricity efficiency in a hydrocarbon TPV experiment to-date, exceeding 4% and greater than previous 2–3% efficiencies thought to be the practical limit. Furthermore, we expect from simulations that our filled photonic crystal design will enable over 12% efficiency with only engineering optimization. For reference, a 1.5% efficiency corresponds to the energy density of lithium ion batteries.
Dr. Ivan Celanovic (supervisor)
Prof. Marin Soljacic
Prof. John Joannopoulos
Prof. Leslie Kolodziejski
Prof. Karl Berggren