High contrast grating based thermal emitters for portable thermophotovoltaic systems

Abstract

The quest for the development of portable thermophotovoltaic (TPV) systems has been a growing interest due to the ability to achieve high power and energy densities using hydrocarbon based fuels. Recent studies based on intermediate filters and photonic crystals have shown significant improvement in system efficiencies for combustion driven and solar-based TPV systems. The key goal is to engineer directionally and spectrally selective thermal emitters ideally matched to the solar cell. Here, a high contrast grating based thermal emitter using silicon as a grating material on a quartz substrate is proposed which is suitable for integrating to GaSb solar cell based thermophotovoltaic systems powered by microcombustor. The intrinsic properties of quartz substrate filter the below bandgap (greater than 4.5 μm) radiation in the infrared region. The silicon gratings are optimized (period = 2.4 μm, duty cycle = 40 % and thickness = 0.55 μm), to provide transmission only for photons with wavelengths lower than 1.8 μm thus inhibiting below bandgap radiation of GaSb cell. The spectrally tuned emitter structure shows transmission of more than 70% of convertible photons (above the bandgap) and reflection of 80% of unconvertible photons (below bandgap) back to the combustor thus reducing the heat losses in the photovoltaic conversion and increasing the combustion system temperature there by contributing to overall increase in TPV system efficiency.