High-throughput, non-imaging, secondary concentrating optics that utilize refraction and total internal reflection are undergoing development for use in conjunction with advanced primary solar concentrators to provide solar thermal energy for space applications. This development is prompted by (1) a need to concentrate sunlight by factors of as much as 104 to satisfy design and operating requirements for some advanced solar thermal systems and (2) the impracticality of fabricating primary concentrators with sufficient precision to afford such high concentration ratios by themselves. Figure 1 illustrates the operation of a refractive secondary concentrator.

Figure 1. A Reflective Secondary Concentrator efficiently concentrates solar energy from a primary concentrator then allows the energy to escape onto a receiver cavity.

The innovative refractive secondary concentrator offers significant advantages over all other types of secondary concentrators. The refractive secondary offers the highest throughput efficiency, provides for flux tailoring, requires no active cooling, relaxes the pointing and tracking requirements of the primary concentrator, and enables very high system concentration ratios. This technology has broad applicability to any system that requires the conversion of solar energy to heat, including electric power generation, thermal propulsion and high temperature furnaces. NASA Glenn initiated the development of the refractive secondary concentrator in support of Shooting Star, a solar thermal propulsion flight experiment, and continued the development in support of Space Solar Power.

Figure 2. A Prototype Sapphire Refractive Secondary Concentrator is shown.

A prototype sapphire refractive secondary concentrator (see Figure 2), has completed solar vacuum performance testing using a liquid-cooled calorimeter in NASA Glenn’s Tank 6 facility. The effort involved the design and fabrication of a sapphire refractive secondary concentrator, design and fabrication of a calorimeter and its support systems, calibration of the calorimeter, on-sun vacuum testing of the refractive secondary, and comparing the test results with modeling predictions.

The prototype refractive secondary concentrator, measuring 3.5 in. (8.9 cm) in diameter and 11.2 in. (28.5 cm) long, was designed for the Tank 6 facility and the existing primary concentrator/ solar simulator system. Ray trace optics software was used to model the secondary concentrator resulting in predicted efficiency without an antireflective coating of 90 percent. The solar vacuum test results indicate an average throughput efficiency of 87 percent, which agrees well with the modeling predictions. It is anticipated that reduction of a known reflection loss off of the inlet surface by applying an antireflective coating to that surface would result in a secondary concentrator throughput efficiency of approximately 93 percent. Potential future activities to further develop the technology include high temperature, high power throughput tests, antireflective coating tests, and additional material characterization and interaction tests.

This work was done by Wayne A. Wong and Steven M. Geng of Glenn Research Center and Robert P. Macosko and Charles H. Castle of the Analex Corporation. Under the Physical Sciences category.

Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Glenn Research Center
Commercial Technology Office
Attn: Steve Fedor
Mail Stop 4–8
21000 Brookpark Road
Ohio 44135.

Refer to LEW-17149.

NASA Tech Briefs Magazine

This article first appeared in the April, 2002 issue of NASA Tech Briefs Magazine.

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