The Design of Novel Transparent Heat Mirrors for Enhanced Solar Thermal Applications

Herein the design of transparent heat mirrors for enhanced solar thermal applications is presented. Dielectric mirrors are considered to function as infrared mirrors that reduce heat losses in the receiver of solar concentrators. The alternating layers have been designed to selectively reflect light strongly over the infra-red spectral region while being highly transmissive towards solar irradiance. This is achieved by designing the interfaces between the high- and low- index films within the dielectric mirror to have a graded refractive index profile. Three different graded index profiles are considered including: 1. a linear index variation, 2. a cubic index variation and 3. a quintic index variation. The effects of altering the dielectric mirror parameters on its performance are investigated. Accordingly, the sensitivity of the transmittance and reflectance spectra of the dielectric mirror towards its number of layers, resolution of the refractive index profiles, refractive index, number of stacked dielectric mirrors, absorption, and incident angle of the incoming light is analyzed. Results show that a single stack infrared dielectric mirror coated on top of the receiver of a solar concentrator system operating at 500 K can increase its power electric generation by more than 60 % and using a triple stack design can increase the power by more than 100 %. Furthermore, considering a typical Dish-Sterling system operating at a temperature of T=1000 K, the electricity generated can be increased by ~ 30 % by coating the receiver within the system with the transparent heat mirrors designed in this work.