Prisms with total internal reflection as solar reflectors

Abstract

For the sake of good optical efficiency, the reflectivity ϱ of mirror surfaces in solar collectors should be as high as possible. Until now, most solar reflectors have used aluminum with values of ϱ in the 80–90 per cent range. Even the best material, silver, allows reflectivities only up to 95 per cent. With total internal reflection (TIR), on the other hand, the effective reflectivity is limited only by absorption in the transparent medium, and absorption losses can as easily be kept below 5 per cent. In certain solar collectors, conventional mirrors can be replaced by an array of small rectangular glass prisms, an optical trick well known from binoculars. The only problem is that TIR occurs for a restricted range of incidence angles, limited by the low value of the refractive index n ⋍ 1.5 of commonly available glass or acrylic. However, an additional degree of freedom is gained in the design of solar collectors because only concentration, not imaging, is relevant. The suitability of TIR prismatic reflectors for solar energy collection is investigated systematically, and the following applications are found to be promising: (i) heliostats for central receiver; (ii) parabolic reflectors with point focus; (iii) line focus systems (both parabolic and Fresnel reflectors) tracking around north-south axis, provided the tilt of the system is adjusted seasonally; (iv) under some conditions, V- and Compound Parabolic Concentrators, in trough or cone geometry. (This is important for second stage concentrators which are to maintain high reflectivity when exposed to air at high temperatures.) Of course, reflection at the front surface of a prism will split any incident ray into separate rays which may leave the prism in two different directions. However, in all the designs considered here, all these rays will reach the absorber, and thus the effective reflectivity is indeed 100 per cent apart from absorption losses. Even if TIR fails at certain times of the day, prismatic reflectors may be advantageous in some applications; one can place a metallic reflector behind the prism to avoid leakage of radiation, and thus one can obtain an effective reflectivity which surpasses that of a conventional reflector.