The refractive secondary has numerous design advantages over the conventional reflective secondary concentrator, typically envisioned as a hollow cone with a reflective internal surface. The below figure illustrates the significant advantages of the refractive secondary concentrator compared to the reflective secondary concentrator that make it attractive to a broad variety of solar applications.

Benefits of the refractive secondary:

Higher Throughput Efficiency

Solar energy entering the secondary is refocused by refraction and total internal reflection both of which are essentially loss-less mechanisms. The only throughput losses associated with the refractive secondary are due to reflection at the inlet surface (which can be minimized with coatings), absorption in the crystal (which for candidate materials like sapphire is very low), and back reflection within the crystal (which is minimized through design). The throughput efficiency of the refractive secondary has been estimated to be in excess of 90% for the designs and materials under consideration. In comparison, the reflective secondary efficiency is reduced due to energy absorption at the reflective surface which is as high as 35% depending on the reflective coating used and the operating temperature.

High Concentration Ratio

The refractive secondary produces concentration ratios many times larger than an equivalent reflective secondary for the same inlet condition. A typical design using sapphire for the refractive secondary has a concentration ratio of 20:1 compared with 7:1 for a reflective secondary. Therefore the refractive secondary can lessen the concentration ratio and/or pointing accuracy requirement of the primary concentrator or allow higher operating temperatures.

Tailored Energy Distribution

The flux extractor, a faceted extended tip, is an essential part of the refractive secondary design. It is needed to efficiently pass the concentrated solar energy into the receiver cavity and allows for flux tailoring via the adjustment of the facet geometry. Unlike the reflective secondary, which directs most of the energy at the front of the receiver cavity, the refractive secondary and extractor can be designed to allow more uniform energy distribution further into the cavity. The flux extractor serves as a light pipe to efficiently deliver the energy to the point of extraction. Variations of extractor designs are envisioned for eliminating hot spots, customized for each application (i.e. solar dynamic conversion heat receiver, thermal photovoltaic array (TPV), thermionic diodes, thermal propulsion, furnaces, etc.)

Requires No Active Cooling

A refractive secondary does not require active cooling because little incident energy is absorbed, and misalignment can be tolerated with minimal impact on performance. In comparison, reflective secondaries are limited in application due to the reduction in reflectivity of the surface material at temperatures above 1500 K and their need for active cooling in high temperature applications. In addition, high power applications may introduce troublesome thermal management issues for reflective secondaries, especially if off-pointing or mis-alignment occurs which can cause uneven heating and distortion of the reflective secondary surface.

Improved Performance Using Optical Coatings

An anti-reflective coating on the refractive secondary inlet surface minimizes incident reflection losses. Of even greater significance for high temperature applications is the use of the refractive secondary with an infrared (IR) retainment element at the inlet of the secondary. At temperatures in excess of 1000 K, typical of many solar thermal applications, the IR radiation loss out of the open aperture of the heat receiver becomes substantial. In order to reduce the radiation loss, and thereby improve the overall system efficiency, a coated element, reflective in the IR spectrum, can reflect back the infrared radiation leaving the cavity, reducing the net radiation loss. Analysis has shown that this coated element can potentially reflect as much as 80% of the radiant energy that would have otherwise been lost through the receiver aperture while only blocking a small portion of the incident solar energy.

Last Revision: January 4, 2000

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