Field tests of these heliostats are planned to begin in FY90. Sandia recently initiated the final phase of development: the design of fully integrated, market-ready heliostats. The durability of the silvered-acrylic reflective film has improved so that a lifetime of at least 5 years is likely methods of replacing the film in the field are being investigated. They demonstrated that the optical performance of membrane heliostats rivals that of glass-mirror heliostats.
Two generations of 50 sq meter prototype stretched-membrane mirror modules have been built and evaluated at Sandia's Central Receiver Test Facility in Albuquerque, NM. Because of their simplicity and light weight, stretched-membrane heliostats are expected to cost up to one-third less than conventional glass-mirror designs. Heliostats with single 14 m diameter (150 sq meter) stretched-membrane reflectors have been designed. The reflective surface is provided by either a silvered-acrylic film or thin glass mirrors attached to the front membrane. They differ from conventional glass-mirror heliostats in that the optical surface is a stretched membrane - a thin metal foil stretched over both sides of a large diameter ring. Since the early 1980s, Sandia National Laboratories has been developing stretched-membrane heliostats for solar central receiver power plants. The instrument functionality is illustrated through the preliminary experimental results obtained on the dish recently installed in ENEA-Casaccia in the framework of the E.U. With respect to prior methods, VISdish offers several advantages: (i) simpler data processing, because light point-source and its reflections are univocally related, (ii) higher accuracy.
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The shape data are entered in a simulation software for evaluating the flux profile and concentration quality. The instrument is based on the visual inspection system (VIS) approach and can work in two functionalities: canting and shape-measurement. All these tasks can be achieved by the new tool we developed at ENEA, named VISdish. Finally, flux profile should be measured or evaluated to verify the concentration quality. After the dish assembly, facet-canting is an important task for improving the concentration of solar radiation around the focus-point, as well as the capture ratio at the receiver placed there. For practical reasons, the reflecting surface is composed by a number of facets. Solar dishes allow us to obtain highly concentrated solar fluxes used to produce electricity or feed thermal processes/storage.
Montecchi, Marco Cara, Giuseppe Benedetti, Arcangelo VISdish: A new tool for canting and shape-measuring solar- dish facets. The prototype optical element is a significant step in the development of the complete, full-sized dish. This effort indicates that the stretch membrane dish concept is a promising approach for solar concentration. This development further reduces manufacturing and installation costs of the completed dish. This allows the critical forming of the membrane to be performed in a controlled factory environment, and the membrane then to be shipped using standard dimension shipping containers. (4) A technique was developed and demonstrated to ship the formed membranes from the factory to the dish-installation site. This approach allows for the practical field replacement of the reflective membrane when it has degraded due to weathering. (3) The use of field-replaceable, unattached polymer reflective membrane was demonstrated. Optical effects from ring distortion were not apparent. This innovative structure, like a bicycle wheel, was shown to be very stiff. (2) A tensioned hub-and-spoke structure was demonstrated to be practical to fabricate. Three 7 m membranes were formed without any contoured tooling. Four major technical issues were successfully addressed in this work: (1) The technique of large-scale membrane forming was shown to be predictable, accurate, and repeatable. The slope error, as measured with Solar Kinetics' laser-ray-trace system, was within 3.6 mrad of a perfect parabola. In Task 2, the membrane forming process was successfully scaled to 7 m in diameter, and an innovative hub-and-spoke structure optical element was fabricated. Earlier work on this project defined the configuration of the optical element and demonstrated the membrane forming process on 1.4- and 3.7 m diameter membranes. Solar Kinetics, Inc., successfully designed and constructed the optical element of a 7 m diameter stretched membrane dish as Task 2 of the second phase of a contract directed by Sandia National Laboratories.
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