Design and development of a faceted secondary concentrator for a fiber-optic hybrid solar lighting system

Fiber-optic solar hybrid lighting for mobile application such as military shelters in remote areas is appealing since high initial costs of such systems appear to be justified. This paper addresses the aspect of two-stage optics for a fiber-optic solar lighting system for the mobile application. More specifically, the focus of this paper is on the design and development of a second stage or secondary concentrator. Two-stage optics offers a distinct advantage of higher sun angle tolerance and hence a low-accuracy tracker can be used for the mobile application. The objective of this study was to design and develop a secondary concentrator that yielded a minimized peak illuminance on the fiber-optic inlet and achieved uniformity, to avoid localized heat damage to the fiber inlet. In addition, the overall goal was to increase the lumen output from the two-stage optics. Specifications for the design of the two-stage concentrator were: (1) the primary stage concentrator was a 10-in. diameter Fresnel lens; (2) solar tolerance angle or the half-angle of acceptance was 1.75'; (3) the fiber half-acceptance angle was 40% and (4) the fiber optic cable diameter was 0.5-in. Raytracing was used to determine the optimum geometry for the secondary lens. Among various geometries simulated included a conical secondary, a Compound Parabolic Concentrator (CPC), and a combination of conical and Compound Elliptical Concentrator (CEC). The analysis showed that a combination design comprising conical/CEC geometry was better than the conical and CPC geometries in terms of greatest overall light output and most reduced peak illuminance on the fiber optic cable. Prototype secondary lenses based on the design were fabricated from acrylic and optical silicones. Testing of the prototype lenses revealed that acrylic secondary performed better than the silicone lenses in terms of the measured lumen output. Testing also showed that the fiber-optic cable temperature was within the maximum operating temperature of acrylic and didn't suffer any high temperature damage during the five month long outdoor testing.