Reflective coatings for large-area solar concentrators

Abstract

Reflective/protective coatings were applied to pre-formed 2.5-m-long solar concentrator panels by the magnetron sputtering process. Low-cost manufacturing processes such as hydrostatic forming of aluminum solar concentrator panels are needed to keep the costs of domestic power generation low. Without treatment, the specular reflectance of the aluminum panels was less than 20%. As expected, silver (Ag) and aluminum (Al) coatings applied directly over the untreated panels did not significantly increase specular reflectance. To provide a specular base surface, approximately 100-μm-thick urethane layers were applied to the panels before deposition of the reflective coating. This smoothing layer filled in scratches and defects. Reflective Ag and Al layers, with protective overcoats of Al2O3 and Si3N4, were deposited onto the urethane-coated panels by reactive magnetron sputtering with ion assist in Pacific Northwest National Laboratory’s 3 m coating chamber. The specular reflectance of the panels increased to an average of 92% at visible wavelengths, very near the reflectance of Al coatings on highly polished glass. Panels with Ag metal layers displayed similar increases. The optical performance uniformity of the coating was better than ±5%. The measured primary concentration ratio of the panels was 800:1, which far exceeded the design ratio of 500:1. The total concentration ratio was 2400:1. The coatings passed MIL-spec environmental and durability tests. Field testing of the concentrator dishes and coatings is continuing, and test results will be reported.