Stretched membrane solar collector

An improved solar reflector utilizing a tensioned reflective membrane. The reflector structure includes a membrane attached on the outer surface of a metal strap that is positioned on an end form by means of a curved-face tensioning block.

BACKGROUND OF THE INVENTION

The invention relates to a new method of design and construction of linear tensioned membrane solar reflectors for solar parabolic trough concentrators, solar linear reflectors, and linear heliostats for solar Fresnel reflecting systems, in particular those that utilize thin flexible films for the membrane substrate.

Linear tensioned membrane reflectors have many advantages over more traditional designs incorporating ridged frame structures. They are relatively light and easy to assemble. In part because of the light weight, multiple reflectors can be mounted on a single frame structure which can be balanced on pillow block bearings allowing for tilting adjustments to be made with minimal energy expended.

Trough-shaped linear tensioned membrane reflectors, such as those shown in U.S. Pat. No. 4,293,192, issued Oct. 6, 1981, to Allen I. Bronstein and U.S. Pat. No. 4,510,923, issued Apr. 16, 1985 to Allen I. Bronstein, usually comprises a frame structure with parallel-facing identical end form members, each describing the desired cross-sectional shape of the reflector. A membrane of highly reflecting material, such as metalized reflective plastic film, is wrapped tightly around the edges of the form members and the membrane. The membrane is then placed under 1000 to 7000 pounds per square inch (PSI) of tension in one direction, usually by moving one of the end form members away from the other.

However, linear tensioned membrane reflector technology presents certain problems that do not exist for linear solar reflector technologies constructed with a rigid structural frame structures, especially when the device utilizes certain materials or laminates, such as plastic films, as the membrane's substrate. For example, Mylar (Biaxially-oriented polyethylene terephthalate boPET polyester film) is a dimensionally stable material that reacts in undesirable ways when the film is placed under compression. A typical means of mounting the membrane is to adhere it to the underside of a metal strap with a structural adhesive, such as epoxy. The strap is then wrapped around the end form and clamped in place. However, as the strap is bent around the end form the strap's inward facing surface and the membrane are placed in compression, wrinkles are produced; they are then crushed and locked in place as the strap is tightened on the end form. These distortions in the film are magnified by the film and transmitted into the membrane as large longitudinal wrinkles and ripples that span across the entire membrane's surface, distorting its shape.

It is an objective of this invention to reduce the wrinkles and other shape distortions that may occur when thin films are used as a membrane substrate in tensioned membrane solar reflectors.

BRIEF SUMMARY OF THE INVENTION

The present invention teaches attaching the membrane to the outside of the strap so that when the strap is bent around the end form the membrane is placed under lateral tension rather than lateral compression. A curved attachment block is preferably used to eliminate improper curvatures and edge distortions in the membrane that could be caused by using a flat-faced attachment block. Further, the potential loss of structural integrity caused by not having the membrane sandwiched between the end form and the strap can be ameliorated by wrapping the end of the membrane longitudinally around the strap and attaching the membrane to the inner surface of the strap as well. Slits, notches or other cut-outs along the edge of the membrane that is attached to the inner surface of the strap eliminate bunching and/or wrinkles that could affect the seating of the strap and membrane on the end form.

DETAILED DESCRIPTION OF THE INVENTION

With reference toFIGS. 1,2and3, the trough-like solar reflector100shown uses only the metal strap110to tension the membrane120around the end form130peripheral edge140. In conventional attachment means, the membrane is adhered to the inside of the metal strap using epoxy or similar adhesive. In the embodiment shown, unlike conventional means, the membrane is adhered to the top or outside surface115of the metal strap, so that when the membrane and strap are mounted on the end form and held in place, the bending of the strap around the end form puts the strap in tension, thus laterally tensioning the membrane. The strap material and geometry must be such that the strap has sufficient flexibility to bend elastically along its length around the end form but sufficient rigidity so as to not bend significantly along the its width. It will be noted that although a metal strap, for example 1/16 inch aluminum or 1/16 inch steel, has been conventionally used in trough reflectors, other materials, such as carbon composites, might be utilized.

When the membrane is attached to the inside surface of the strap, as in a conventional reflector design, the peripheral edge140of the end form130can describe ideal cross-sectional profile of the solar reflector100. In the current invention, however, the cross-sectional profile of the end forms must be adjusted to compensate for the thickness of the metal strap110, and any cushioning pads150that are under it, so that the outside strap surface115becomes the correct cross-sectional profile.

With specific reference toFIG. 2, a cross-section view of the end form130with a flexible rubber, plastic or similar resilient pad150between the end form130and the adjacent metal strap110is shown. The pad150acts as a cushion so that the metal strap110can be seated smoothly on the end form130allowing the strap to make a larger bend radius, which aids the strap in tensioning the membrane120which is on the outside surface115. It also smoothes out any imperfections on the periphery of the end form130. As described in U.S. Pat. No. 4,510,923, incorporated by reference herein, a series of pins141inserted through slots (not shown) in the strap110and into the end form130can be used to position the strap on the end form periphery.

The strap can be held to the periphery of the end form by a number of fastening means. A preferred method is shown inFIGS. 3 and 4in which a unique strap tensioning block170attaches to the strap110with two pins160that are inserted into two matching holes113in the strap110. The pins160are used to hold the block170securely in place on the strap110, allowing the tensioning block170to tighten the strap110on the end form130. Modifications are possible. For example, rivets could replace the pins. A fastener, such as the attachment bolt190shown, and load transfer spacer180are then passed through the slots197,198in the block and strap, respectively, and the bolt190is tightened to the end form130, holding the tensioning block170in place. The tensioning screw195is then tightened down on the load transfer spacer180thereby pulling the strap110tightly around the end form130.

The load transfer spacer's180function is to evenly distribute the force of the tensioning screw195on the attachment bolt190, preventing the block170from twisting or riding up. The height of the load transfer spacer180is slightly thicker than the tensioning block170, but not as thick as the block and strap together. It has the same inside diameter as the attachment bolt190and the same outside diameter as the tensioning block's slot197.

FIG. 4is an exploded view of the attachment block and strap. An important aspect of the tensioning block170is the curve on the inwardly facing surface175. This curve matches the desired cross-sectional profile of the membrane120at that specific location and insures that the ends of the strap110and the longitudinal edges of the membrane120follow the contour of that profile. The curved surface175on the tensioning block170is a significant improvement over a conventional flat block which does not duplicate the desired profile at the ends of the end form, thus creating flat spots and misalignments at the longitudinal edges of the membrane120and corresponding loss of focal integrity.

Two tensioning blocks would normally be employed for each strap, one on each side of the reflector, although it would be possible to have an alternative form of attaching the strap to the end form at one end of the strap, including bolts or other fasteners. It is also possible to have different types of tensioning blocks which still include some of the same advantageous features described above. For example, with reference toFIG. 6, a fixed block270, including a curved facing surface275, may be held to a strap210with pins260and holes213in the manner described above but be fastened fixedly to the end form with a fastener, such as the attachment bolt290, shown. The fixed block and associated strap end would have holes297,298for the bolt290rather than slots197,198. The strap210could have a slot at the opposite end to accommodate a tensioning block design such as that shown inFIG. 4.

With reference toFIG. 5, attachment of the membrane120to the outside surface of the strap110, while reducing the wrinkles and potential shape distortions that occur with attachment to the inner surface of the strap, presents a challenge in effective attachment of the membrane to the strap. In a conventional design, the membrane is sandwiched between the end form and the metal strap, which provides additional security and structural integrity. When the membrane is attached to the outside of the strap, however, it is of benefit to attach the membrane120to the outside surface115of the strap110near but not at the edge of the membrane so as to allow the edge of the membrane120to be wrapped around the strap110longitudinally. The end of the membrane120can then be attached to the inside surface116of the strap110providing a second plane of attachment to the strap as well as the structural stability of having the membrane sandwiched between the strap110and the end form130.

However, since the strap110is bent around the end form130wrinkles in the membrane120on the inner surface116of the strap can still present a problem. While the tensioning effect of having the membrane attached to the outer surface115of the strap110reduces or eliminates wrinkles along the trough, wrinkles between the strap and the end form can cause the membrane and strap to be misaligned and may make it difficult to seat the strap smoothly on the end form. To overcome this problem, slots or cutouts125are placed along the edge of the membrane120to eliminate such wrinkles. For example, it is possible to use a series of symmetrical shaped cutouts positioned so that a vertex just touches the outer surface115of the strap110. It would be clear to those of ordinary skill in the art that the angle of the vertices and the width of the cutouts necessary to eliminate wrinkles would be dependent on the radius of the end form and the width of the strap. Other shapes and configurations of cutouts could also be used.

It will be noted that even in reflector designs where the membrane is on the inner surface of the strap and is between the strap and the end form, wrapping the edge of the membrane around the strap and having a second plane of attachment on the outside of the strap can add structural stability. In such a design, cutouts along the edge of the membrane would still be used, not to prevent wrinkles, but to allow the edge of the membrane to expand as the strap is bent around the end form.