Solar-thermal-radiation concentrators in the form of transmissive diffractive optical elements (DOEs) have been proposed as alternatives to mirror- type solar concentrators now in use. In comparison with functionally equivalent mirror-type solar concentrators, the transmissive, diffractive solar concentrators would weigh and cost less, and would be subject to relaxed mechanical tolerances.
A DOE concentrator would be made from a thin, flat disk or membrane of a transmissive material having a suitable index of refraction. By virtue of its thinness, the DOE concentrator would have an areal mass density significantly less than that of a functionally equivalent conventional mirror.
The DOE concentrator would have a relatively wide aperture — characterized by a focal-length/ aperture-diameter ratio ("f number") on the order of 1. A kinoform (a surface-relief phase hologram) of high diffractive order would be microfabricated onto one face of the disk. The kinoform (see figure) would be designed to both diffract and refract incident solar radiation onto a desired focal region, without concern for forming an image of the Sun. The high diffractive order of this kinoform (in contradistinction to the low diffractive orders of some other kinoforms) would be necessary to obtain the desired f number of 1, which, in turn, would be necessary for obtaining a desired concentration ratio of 2,500 or greater.
The design process of optimizing the concentration ratio of a proposed DOE solar concentrator includes computing convolutions of the optical bandwidth of the Sun with the optical transmission of the diffractive medium. Because, as in the cases of other non-imaging, light-concentrating optics, image quality is not a design requirement, the process also includes trading image quality against concentration ratio.
A baseline design for one example calls for an aperture diameter of 1 m. This baseline design would be scalable to a diameter as large as 10 m, or to a smaller diameter for a laboratory test article. Initial calculations have indicated that the characteristics of the test article would be readily scalable to a full-size unit.
This work was done by Richard Baron, Philip Moynihan, and Douglas Price of Caltech for NASA's Jet Propulsion Laboratory.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Patent Counsel
NASA Management Office–JPL.
Refer to NPO-43801.
This Brief includes a Technical Support Package (TSP).
Transmissive Diffractive Optical Element Solar Concentrators
(reference NPO-43801) is currently available for download from the TSP library.
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Overview
The document discusses NASA's innovative approach to solar concentrators using Transmissive Diffractive Optical Elements (DOEs). Developed by the Jet Propulsion Laboratory (JPL), this technology aims to meet the challenges posed by the Defense Advanced Research Projects Agency (DARPA) for lightweight solar concentrators capable of achieving high power concentration ratios exceeding 1900.
The key innovation lies in the use of a thin, flat transmissive optic, which is micro-fabricated with a specially designed kinoform pattern. This high-diffractive-order design allows the optical element to diffract and refract sunlight into a desired focal region, significantly enhancing the concentration ratio while maintaining a low f-number (F/1). The design is a departure from conventional mirrors, offering a paradigm shift in solar concentration methodology. The resulting concentrator is not only lightweight, with an areal density of approximately 1 kg/m², but also cost-effective, making it suitable for space power systems where minimizing launch mass is critical.
The document outlines the technical aspects of the DOE, emphasizing its scalability from one-meter diameter apertures to larger sizes of five to ten meters. This flexibility enables the design to be adapted for various applications, from laboratory testing to full-scale deployment in space missions. The non-imaging nature of the system allows for optimization focused on solar concentration rather than image quality, which is a significant advantage in achieving high concentration ratios.
Furthermore, the document highlights the potential for this technology to revolutionize solar power systems by providing high optical efficiency with relaxed mechanical tolerances. The use of high-diffractive-order design rules capitalizes on advancements in optics technology, positioning JPL's approach as a leading solution in the field of solar concentrators.
In summary, the document presents a comprehensive overview of the development and advantages of transmissive diffractive optical element solar concentrators, showcasing their potential to enhance solar power systems for space applications while reducing costs and mass. This innovative technology represents a significant step forward in the quest for efficient and effective solar energy solutions.
