Antireflection Pyrex Envelopes for Parabolic Solar Collectors

Abstract

Parabolic trough solar collectors utilize glass envelopes around the receiver tube in order to reduce thermal losses. Antireflective (AR) coatings applied to the envelope can potentially increase the solar transmittance by 7 percent. An excellent AR surface can be formed on Pyrex (Corning Code 7740 glass) by first heat treating the glass to cause a compositional phase separation. After heat treating, a surface layer is removed using a pre-etch solution of aqueous ammonium bifluoride. Finally, the AR layer is formed by etching in a solution containing hydrofluorosilic and ammonium bifluoride acid. Processing parameters studied included the phase separation temperature and heat treatment time, the pre-etch time, and the etching bath temperature and time. AR-coated samples with solar transmittance values > 0.97, as compared to a value of 0.91 in untreated samples, were obtained for a range of heat treatment temperatures from 560–630°C. The phase separation time and temperature interact so that at 630°C short times are required (3 hrs) while at 560°C longer times are necessary (24 hrs). Optimum values for the other processing parameters are 12–18 min in the pre-etching bath, and 5–10 min in the film forming bath when maintained between 35–45°C. Application of this process to full scale 3-m-long × 6-cm dia Pyrex envelopes was successful in producing solar transmittance values ≥ 0.97.