Stretched-Membrane Heliostats For Solar Central-Receiver Power Plants

Abstract

For more than 10 years, the U.S. Department of Energy's Solar Thermal Program has pioneered the development of heliostats, mirrors that track the sun, for solar central-receiver power plants. The field of heliostats is the single most expensive part of such plants, so their cost must be as low as possible for the technology to be commercially successful. Recent efforts have focused on the development of heliostats that use stretched-membrane reflectors in place of the more familiar glass mirrors. In a stretched-membrane heliostat, metal foils are stretched over both sides of a large-diameter metal ring. The reflective surface is a silvered-polymer film glued to the front membrane. A slight vacuum in the space between the two membranes is actively controlled to provide a concave, focused contour to the mirror, and in an emergency this space can be rapidly pressurized to defocus it. Because of their simplicity and lighter weight, stretched-membrane heliostats have the potential to cost significantly less than current glass-mirror designs. The first 50-m 2 mirror modules, built under contract to Sandia in 1986, demonstrated that the optical performance of membrane heliostats would be at least as good as heliostats using glass mirrors. Insights gained from Sandia's testing and evaluation of the first-generation units were incorporated into the designs of two improved 50-m 2 mirror modules that were recently completed. Preliminary measurements show significant improvement in optical performance over the first-generation designs, especially in windy conditions.