Thermal treatment of stainless steel substrates along with a thin silicon layer for high temperature solar thermal applications

Abstract

Solar energy harvesting in solar thermal systems using different solar absorber coatings on collectors has been widely studied. Here, we incorporate a single layer Si (∼250 nm) onto one of the most common and cost-effective collector materials, i.e., stainless steel (SS304) to improve its optical performance by thermal treatment. During the heating cycle, the samples were heated at 900 °C for short durations and suddenly quenched to room temperature in air. A high solar absorptance of 0.92 along with an emittance of 0.37 were obtained after heat-treatment. The reflectance studies showed reduced uniform reflectance (<10%) over the entire UV–Vis-NIR region and high reflectance in the far IR region. The morphology, composition, and crystal structures were studied in detail. The Si layer and the quenching process aided in the formation of a highly irregular microstructure consisting islands of Cr-Fe-Mn-Si oxides and silicides (with trace amount of Ni) which in turn increases surface roughness. This irregular array of islands of varying sizes and varying gaps between them helps in increasing absorptance due to multiple reflections, scattering and diffraction effects. Similar studies carried out on SS202 substrates resulted in different surface morphology and hence different absorptance (0.84), suggesting that the surface morphology and absorptance are highly dependent on the substrate composition. The initial thermal stability data on heat-treated Si/SS304 suggests stability up to 700 °C in air for 1000 hrs. Our studies indicate that thermal treatment of Si/SS304 for a short duration is a plausible solution to develop cost effective high temperature solar thermal collectors.