Design of selective coatings for solar thermal applications using sub-wavelength metal-dielectric structures

Abstract

ABSTRACT Spectral control of the emissivity of surfaces is essential in applications such as solar thermal energy and thermophotovoltaic energy conversion in order to achieve the highest conversion efficiencies possible. We investigated surfaces consisting of periodic, nanoscale V-grooves coated with aperiodic metal-dielectric stacks. This approach combines impedance matching using tapered metallic features with the excellent spectral selectivity of aperiodic metal-dielectric stacks. We expl ain how changes in the angle of the V-grooves can be used to tailor the spectral selectivity over a wide angular range to significantly in crease the efficiency of thermophotovoltaic and solar thermal systems. Optimal coatings for concentrated solar power are predicted to have thermal emissivity below 5% at 450qC while absorbing >90% of the incident light. Keywords: solar thermal, thermophotovoltaics, solar selective coatings, metal-dielectric, V-groove, grating 1. INTRODUCTION The efficiency of concentrated solar thermal (CST) system s is thermodynamically limite d by the Carnot efficiency and is thus strongly dependent on the maximal achievable temperature of the working fluid, often synthetic oil. Parabolic troughs have concentration ratios of ~80, and operate at temperatures up to 660K