Black solar collector coatings from a trivalent chrome electrolyte

Selective coatings play a major role in the harnessing of abundant solar energy, as they improve the efficiency of the flat-plate collector. Among various plateable selective coatings, black-chrome processes have been adopted industrially because of good optical properties coupled with long term durability. However, the conventional black chrome process suffers because of its toxic effluent, containing hexavalent chromium, which needs a rigorous wastewater treatment. Hence, the authors have worked out a pollution-free trivalent chromium electrolyte for black chrome deposition. A typical electrolyte and conditions consist of: chromium III chloride, 200 g/l; ammonium chloride, 30 g/l; oxalic acid, 3 g/l; pH 2.5-3.0; temperature 23 +-2 deg.C; and current density, 1200-2200 A/m2. Coatings applied for a duration of 45 seconds in this electrolyte reached a stagnation temperature of 104 deg.C, which is comparable to that of a standard black chrome panel, with high absorptance (alpha=0.96) and low thermal emittance(e=0.10). The cycling tests made on these panels indicate satisfactory thermal stability towards long term exposure to solar radiation. This paper deals with the various characteristics of the electrolyte, apart from its suitability in solar thermal device