Stretched-membrane Heliostat Research Articles

Wind Effects on a Stretched Membrane Heliostat

Wind effects on stretched membrane heliostat were investigated in a boundary layer wind tunnel. The membrane response was measured at stow and representative operational conditions. It was found that both at the stow and operational conditions the mean response was much higher than the rms response. At stow conditions the largest response occurred near the leading edge of the membrane, while the rms response was the largest at the membrane centerpoint. For the operational conditions, the largest mean and rms responses were found at the membrane centerpoint. The membrane response was significantly reduced by the membrane focusing induced through the internal underpressure.

Performance of a CRS with stretched membrane heliostats for steam reforming of methane

This paper compiles some conclusions after modelling a solar CRS for a steam reforming of methane application. An amount of 24 different plants are optimized for different plant sizes, heliostat beam dispersion errors, number of receiver apertures and focusing or flat heliostats. All of them were conceived with 120 m 2 stretched membrane heliostats and convective reformers for a receiver/reactor separated solution. For such a system, commercial competitiveness can be expected.

Analysis and Design of Two Stretched-Membrane Parabolic Dish Concentrators

The state-of-the-art of parabolic dish solar concentrators is the faceted, glass-metal dish. The mass production costs of glass-metal dishes may be high because they do not incorporate the innovations of design and materials developed over the last eight years. Therefore, Sandia National Laboratories has undertaken to develop two stretched-membrane parabolic dish concentrators for the Department of Energy’s Solar Thermal Program. These solar concentrators are being designed for integration with an advanced solar receiver and a Stirling engine/generator in a 25-kWe power production unit. The first dish, which builds on the successful design of the stretched-membrane heliostats, is to be a low risk, near-term commercial solar concentrator. This solar concentrator comprises 12 large, 3.6-m diameter, stretched-membrane facets that are formed into parabolic shapes either with a large vacuum or by performing the thin membranes plastically. The focal length-to-diameter ratios (f/Ds) for the facets are about 3.0, relatively large for a dish but much lower than heliostats where they typically range from 50 to 100. Two contractors are currently fabricating facets for this dish, and a third contractor is designing the facet support structure and pedestal for the dish. The second stretched-membrane concentrator is a single-element monolithic dish with an f/D of 0.6. The dish is shaped into a parabola by plastically yielding the membrane using a combination of uniform and nonuniform loading. Initial measurements of the dish indicate that it has a slope error of 2.6 milliradians (one standard deviation) relative to a perfect parabola. In this paper, the designs of the two stretched-membrane dishes are analyzed using the computer code CIRCE to model the optical performance of the concentrators and a thermal model, which includes conduction, convection, and radiation heat transfer, to calculate the thermal losses from the cavity solar receivers. The solar collector efficiency, defined as the product of the optical efficiency of the collector and the thermal efficiency of the receiver, is optimized for comparing the performance of several solar concentrator configurations. Ten facet arrangements for the faceted stretched-membrane dish and the single-element stretched-membrane dish are modeled and their performances compared with that of a state-of-the-art glass-metal dish. Last, the initial designs of these two stretched-membrane dishes are described along with the results of preliminary performance measurements on their respective optical elements.

The development of stretched-membrane heliostats in the United States

For more than 10 years, the US 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. In the last six years, Sandia National Laboratories has been developing and transferring to private industry a technology that uses stretched-membrane reflectors in place of the more familiar glass mirrors. Because of their simplicity and lighter weight, stretched-membrane heliostats have the potential to cost significantly less than current glass-mirror designs. Two generations of 50 m 2 prototype stretched-membrane mirror modules were evaluated at Sandia's National Solar Thermal Test Facility in Albuquerque, NM. They demonstrated an optical performance rivaling that of the more familiar glass-mirror heliostats. Sandia recently initiated the final phase of development: the design of fully integrated, market-ready heliostats. Field tests are planned for late 1990.

Dynamic characteristics of streched-membrane heliostats

Stretched-membrane heliostats provide efficient two-axis tracking mirrors for use in solar thermal applications. The solution of the two-dimensional wave equation for a circular stretched membrane provided lower and upper bounds of the natural frequencies of the heliostat. The effects of the boundary condition on the mode shapes of the membrane were shown in the finite element analysis as well as in the vibration tests. Different natural modes were excited depending upon the number of supports around the circumference of the membrane. Damping in the heliostat was small. Damping ratios estimated from vibration decay curves were less than 3% of critical when the membrane was not stretched and less than 1% when stretched. The bending stiffness of the membrane became significant when the membrane tension was low. The anisotropic effect causing non-uniform membrane tension due to the weldments was negligible.

Analysis of Composite Stretched-Membrane Heliostats

The stretched-membrane concept has the potential for reducing the cost of heliostats used for solar thermal energy generation in central receiver systems. In this concept, a high strength/structural membrane carrying the reflective film is stretched uniformly on a toroidal frame. The resulting structure has high stiffness and is structurally efficient. It is practically suitable for composite materials, since the loading is primarily in-plane. In this paper, the application of composite materials to stretched-membrane heliostat design is investigated. The results of the analysis show that composite materials for both the membrane and the frame exhibit significant advantages over metals. These include high strength-to-weight design, higher resistance to wind-induced deformation, the possibility of independently tailoring bending and torsional stiffness, and better transportability.

Moderate Axisymmetric Deformations of Optical Membrane Surfaces

Large deformable membranes are currently used as optical reflector surfaces for solar concentrators. In this paper, we study moderate axisymmetric deformations of optical membrane surfaces and their impact on the macroscopic optical quality of the deformed surface. A uniform pressure loading applied to an initially flat membrane surface is assumed in attaining the final concentrator surface shape. The problem is important in the design of focusing stretched-membrane heliostat modules. Insights into the design of stretched-membrane parabolic dish reflectors also result from this work. This paper presents several simple design and sizing relationships, based on a nonlinear variational principle, in terms of common engineering parameters. Good approximate predictions of the nonlinear load-deformation response and the stress state in the membrane are derived; simple but accurate predictions for membrane surface slope and nominal focal length also result. The phenomena that lead to deviation of the surface shape from the desired parabolic reflector shape are identified and design procedures to minimize those deviations are deduced. Results show that the macroscopic optical surface quality, for a given nominal f/D (concentrator focal length/diameter), is highly dependent on the non-dimensionalized membrane stiffness parameter, Et/To (membrane modulus × thickness/initial tension). Further, for the heliostat cases of interest, the focal-length variations relative to the desired parabolic shape were shown to be about ±5 percent about the nominal value.