Electroplating of selective surfaces for concentrating solar collectors

Abstract: The optical properties of the absorber pipe in a parabolic trough collector isessential for the performance of the solar collector. The desirable propertiesare high absorptance (α) of solar radiation and low emittance (ε) of thermalradiation. A surface with these properties is known as a solar selective surface. There are several techniques used to produce selective surfaces, but one of the most common ones is electroplating. Research done by Vargas, indicates that optical properties of α = 0.98 and ε = 0.03 [1], which is superior to the best commercial alternatives (α = 0.95 and ε = 0.04 [2]), can be achieved by electroplating a Co-Cr coating on a stainless steel substrate. Additionally, Vargas used an electrolyte of trivalent chromium dissolved in a deep eutectic solvent, as opposed to the traditionally used aqueous electrolytes containing hexavalent chromium, which is toxic and carcinogenic. In this project, a coating of Co-Cr was electroplated on a stainless steel substratewith a method similar to that of Vargas in order to obtain a solar selective surface. The electrolyte was composed of ethylene glycol, choline chloride, CrCl3•6H2O and CoCl2•6H2O in a molar ratio of 16:1:0.4:0.2. The plating process was conducted using chronoamperometric electrodeposition with an applied potential of -1.2 V (against an Ag/AgCl reference electrode) for 15 min. The system was investigated using Cyclic Voltammetry (CV). The total absorptance was measured using UV-Vis spectroscopy, while the emittance was measured using an IR-thermometer. The microstructure and chemical composition was investigated using Scanning ElectronMicroscopy (SEM), Focused Ion Beam (FIB), Energy-Dispersive X-ray Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS) and Raman spectroscopy. The thermal stability of the coating was investigated by exposingit to 400°C in air for 24 h. The electroplated coating is approximately 2.8 μm thick and exhibits a porousstructure with a surface of fine fiber-like flakes. The coating consists largely of Co hydroxides with low concentrations of Cr compounds, Co oxides and metallic Co. Hence, a satisfactory co-deposition was not accomplished, as the Cr concentration is low. The coating is not thermally stable up to 400°C, exhibiting signs of at least partially melting in the annealing process. The compounds in the coating were largely oxidized in the process. The electroplated surface does however exhibits strong selectivity, with a total solar absorptance of α = 0.952 and total emittance of ε = 0.32 at 160°C.