Electroplating of Selective Surfaces for Concentrating Solar Collectors
Abstract
A spectrally selective surface is produced by electrodeposition of a Co-Cr coating on a stainless steel substrate. The plating bath consisted of CrCl3·6H2O and CoCl2·6H2O dissolved in a deep eutectic solvent (DES) of choline chloride and ethylene glycol. This DES enables the use of trivalent (Cr(III)) instead of hexavalent chromium (Cr(XI)) which significantly reducing the health risk associated with chromium electroplating. The selective surface exhibits an absorptance (α) of 0.96 and an emittance (ϵ) of 0.13 at 100°C making it well adapted for mid- to low-temperature concentrating solar collectors. The coating is ≈ 1.8 μm thick and exhibits a porous structure on the surface as well as throughout the entire coating. The surface of the coating is covered in Co oxides and hydroxides, while the bulk of the coating consists of Co in the form of both metallic and oxidized Co and only a small fraction of Cr compounds. Initial tests of the thermal stability indicate that the coating does not maintain optical properties at an acceptable level over a lifetime of 25 years and that a large part of the coating is oxidized at approximately 438°C in an oxygen rich environment.