The use of leached gradient index anti-reflection surfaces on borosilicate glass solar collector cover tubes

Abstract

When alkali-borosilicate glasses are heat treated, they can be caused to phase separate. The silica and non-silica constituents are separated on a sub-microscopic scale. This enables small amounts of the non-silica phase to be chemically leached from the surface, which results in a graded reduction in the index of refraction near the surface. The index gradient can be controlled so that it leads to the most effective broad band, antireflection (A.R.) films known. At normal incidence, average reflection per surface can be kept well below 1% in the spectral region from 0.35 to 2.5 μm. Further, films produced by this method maintain their effectiveness even at incident angles up to 70°. All these aspects; i.e. manufacturing method, spectral response and angular response, combine to make the leached gradient index A.R. treatment ideally suited to enhance the efficiency of linear trough solar collector systems. Such systems typically use borosilicate glass cover tubes of the absorber tubes to cut heat losses, but in absence of A.R. treatment, reflective losses from the cover tubes can cut system output by about 7–8%. The leached A.R. surface eliminates reflective losses almost entirely. Exposure to the elements in an industrial environment for five (5) years has resulted in no degradation of the effectiveness of the leache A.R. surfaces. While the surface is somewhat sensitive to contamination by body oils because of its porous nature, once installed, the inherent chemical durability of the nearly pure silica leads to excellent long-term performance.