High-performance metallo-dielectric photonic crystals: Design, fabrication, and testing of a practical emitter for portable thermophotovoltaic generators

Abstract

Hydrocarbon thermophotovoltaic (TPV) systems, a concept first proposed in the 1950s, are emerging as a viable power source for small, portable generators for a spectrum of applications such as UAVs and robotic platforms. In a TPV system, an emitter is heated to above 1000 K, producing thermal radiation that is then converted to electricity by a low-band-gap photovoltaic cell (in hydrocarbon TPV, the heat source is fuel combustion). Unfortunately, state of the art TPV systems still have low efficiencies (<10%).

One approach to increase both the efficiency and power density of the system is to use a selective emitter (one which preferentially emits in the wavelength range that can be converted by the photovoltaic cell). A promising class of broadband selective emitters is two-dimensional photonic crystals, which consist of a square array of cavities etched into a refractory metal substrate, and whose emission spectrum can be tuned by adjusting the geometry of the cavities. In particular, previous work has shown that photonic crystals made of tantalum and conformally coated with hafnium oxide can achieve in-band emissivities up to 0.6, allowing for prototype systems with 4.4% fuel-to-electricity efficiency. Even higher in-band emissivities of 0.8-0.9 are theoretically possible using a metallo-dielectric, or filled, photonic crystal: a tantalum photonic crystal both filled and capped with hafnium oxide.

This thesis presents a metallo-dielectric photonic crystal with close to full theoretical performance. Using a combination of numerical simulations and cross section images, I identified a number of major geometric imperfections in previous prototypes: a hollow air core within the cavity, a thick and uneven capping layer of hafnium oxide, and the recession of hafnium oxide from the top of cavity. I then developed and implemented a fabrication process to achieve a better-filled cavity and a thin capping layer of hafnium oxide, enabling in-band emissivities of 0.7-0.9. Full system simulations predict an up to 37.5% increase in system output power: 6.0W for 100W fuel input, compared to 4.4W system output power for my group’s previous prototype system. This selective emitter paves the way towards efficient, practical, and portable mesoscale generators.