The invention relates to a solar reflector in the nature of an arcuate, generally parabolic, surface which concentrates solar radiation upon an energy absorbing target which is located at the focus of the parabola. Specifically, the present invention relates to improvements to the design of linear tensioned membrane solar reflectors that significantly improve the optical properties of the design based upon the inventor's understanding of how linear tensioned membranes behave and what modifications to the design are required to improve their optical accuracy.
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 lightweight, multiple reflectors can be mounted on a single frame structure which can be balanced on a single pillow block allowing for tilting adjustments to be made with minimal energy expended.
However, linear tensioned membrane reflector technology presents certain problems that don't exist for linear solar reflector technologies constructed with a rigid structural frame structures. For reflectors with a rigid structural frame, the mirrored reflector surface, either glass or reflective film, is adhered to a rigid metal substrate supported by torque tube and ribs or a space frame-type structure without concern for the mirror element's structural strength or mechanical properties.
In contract, a trough-shaped linear tensioned membrane reflector usually comprises a frame structure with parallel-facing identical form members, each describing the desired cross-sectional shape of the reflector. A membrane of highly reflecting material is wrapped tightly around the edges of the form members and 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 members away from the other.
For example, U.S. Pat. No. 4,293,192, issued Oct. 6, 1981, to Allen I. Bronstein, sets forth a solar reflector which is collapsible and portable and which will maintain its true configuration without the requirement of supporting ribs. The invention of this patent includes the use of a slideway on which two form members are supported, the forms members having identical surfaces around a portion of their peripheries, which identical surfaces conform precisely to the desired configuration of the reflecting surface. A reflective membrane is wrapped tightly around the surfaces and secured in place, and at least one of the forms is mounted on a slide which is moved away from the other form until the flexible sheet is in tension. Thereby, the flexible sheet is intended to conform precisely to the curvature of the form surfaces over its full length. The slideway is pivoted on support legs so that it may be tilted sideways at a selected angle, depending on the angle of the sun. Strips of tape may be adhered to the outer or convex surface of the material to dampen it against wind vibration.
Similarly, U.S. Pat. No. 4,510,923, issued Apr. 16, 1985 to Allen I. Bronstein, sets forth a tensioned solar reflector which comprises a longitudinally extending frame structure having first and second frame ends and a second end closure. A first form member is inboard of the first end. A second form member is parallel to the first form member and inboard of the second end closure. The form members have peripheries having identical form surfaces along portions thereof. A support member is attached to either the second end closure or the second form member and is adapted for transferring the weight of the second form member to the second end closure. A reflective membrane has its opposite edges secured to the identical form surfaces. Stretching means stretch the membrane between the first and second form members and into the desired, generally parabolic, shape. Further, after the membrane is placed under tension, stiffening strips are preferably attached to the lateral edges of the membrane by being bonded thereto by an appropriate adhesive.
Notwithstanding the improvements the above inventions made over the prior art, the inventor noticed that all such flexible membrane-type reflectors sometimes deviate from the desired, usually parabolic, cross-sectional shape because of distortion at the free lateral edges of the membrane. This deviation can cause significant loss in optical accuracy and thus the performance of the reflector, particularly in large reflectors where the weight and easy construction advantages of the membrane-type reflector are especially significant.
The distortion is caused primarily by two factors. First, when tension is placed on a membrane in only one direction, there is often a non-uniform the distribution of the forces, especially near the unattached lateral edges of the membrane. This non-uniform distribution of force across the surface of the membrane causes the membrane to vary from the desired cross-sectional shape described by the end form of the reflector. Second, the unsupported edges of the membrane have a tendency to return to a flat shape in a manner similar to that of the edge of a rolled piece of paper held at both ends: towards the center, where the paper is not held, the edge of the paper will attempt to flatten, gapping and distorting from the shape at the ends. Placing the membrane under tension in one direction does not affect this distortion.
The inventor noticed that the behavior of the change in the cross-sectional shape of the reflector is very consistent: it occurs almost immediately, typically within only a few inches from an end form, and remains constant until the membrane reaches another end form.
The invention comprises correcting the cross-sectional profile of the end forms from the selected ideal cross-sectional shape. The correction may be effected by a number of methods. Additionally, a free edge support structure for the membrane is provided. By combining an end form profile/periphery correction with a free edge support structure, a very large, strong and accurate solar reflector can be made without significantly increasing weight or cost. For example, in a parabolic trough measuring 40″ wide×24 feet long, the inventor found that the distortion for that particular trough measured about 4 inches deep from each free edge. Thus, a total of about 20% of the light was not hitting the receiver tube. By adjusting the end forms' cross-sectional shape in the manner proposed by the invention, there was an improvement of almost 20% in the reflector's ability to focus light onto the receiver tube.
It should be noted that many of the reflective films utilized in solar reflector technologies, such as 3M's ECP-305+ acrylic silver film, do not posses the dimensional stability and tensile strength that are required of a membrane under tension. By creating special laminates made by laminating these reflective films to a metal foil or polymer film, like Polyester (PET), the required mechanical attributes are obtained.
In accordance with the current invention, the primary correction to the design of the reflector in order to achieve improved efficiency must be to modify the shape of the end forms 556, 510. Generally, such correction is effected by measuring the distortion along the reflector when the end forms have the selected ideal cross sectional shape at their peripheries and using those measurements to correct the shape of the end forms.
A series of measurements are taken between the ideal cross sectional profile 600 and the actual, distorted profile 610 and checked at various points along the trough's cross-section. All measurements taken are well away from the end forms and the transitional sections 125. In
Although it is possible to similarly adjust the shape of the ribs 530, in practice adjustment of the end form 556, 510 profiles are generally sufficient to achieve the desired optical accuracy. This avoids the necessity of spacers and similar devices where the membrane is attached to the ribs 530. Thus, the ribs 530 may have the desired optical profile shape without any correction.
Those of ordinary skill in the art will appreciate that the modifications to the end forms peripheries will result in slight distortions in optical accuracy near the end forms themselves. However, these slight distortions in optical accuracy occurring at the junctions of end forms represented only a 1% to 2% loss of optical efficiency, significantly less than the usual improvement gained by making the modifications to the end form cross sectional shape.
It should be noted that the behavior of the membrane, and thus the necessary corrections, will vary depending upon the membrane's construction, thickness, and composition, and upon the cross-sectional profile of the desired shape (i.e., parabola, circle, straight line, or any other geometric or compound shape that can be generated into a linear reflector.)
In practice, a single reflective metal foil or metalized polymer film as the reflector's membrane presents many limitations when used in a tensioned membrane reflector. These include the limited tensile strength and dimensional stability of the polymer reflectors, the available thickness and/or width of metal foils or polymer films, and the longevity of the materials available. Laminate constructions utilizing metal foils and high strength dimensionally stable polymer films, such as polyester (PET) as the structural element can overcome these limitations, for example: a 5 mil aluminum foil adhered to a 2 mil metalized aluminum polyester reflector; or alternatively a 5 mil to 7 mil polyester (PET) film laminated to a 2 mil metalized silver acrylic (PMMA) reflector film, such as 3M's ECP 305+. Both metal foil and polyester film offer an excellent structural element (substrate) that is dimensionally stable and can support the metalized polymer reflector element of the laminate when under tension. These laminates can be created using standard converting manufacturing processes that simplify membrane manufacture and assembly.
Although the corrections to the end plates are necessary to create the desired cross-section in the reflector, if the membrane's lateral free edges are not protected by a cover, such as the one described in U.S. Pat. No. 4,510,923, the free edges are vulnerable to vibration, cross-sectional deformation, and damage by wind loading which can also affect the efficiency of the reflector. To prevent this requires an increase in the structural rigidity and strength if a cover is not used.
One method of creating such additional structural integrity is to fold 710 or roll 810 the edge of the membrane 705, 805 as shown in
With reference to
Where the structural element 1550 makes contact with the membrane 1505 the cross-sectional profile must match the desired optical shape of the reflector. The free lateral edges of the membrane 1505 are attached to a sliding stiffening element 1515 either by adhesive 1520 as shown in
With reference to
With reference to
Thus, by combining the cross-sectional end form profile correction with a free lateral edge support means, a very large, strong, and accurate low cost solar reflector can be made. While the present invention has been shown and described with reference to the foregoing preferred embodiments, it will be apparent to those skilled in the art that changes in form, connection, and detail may be made therein without departing from the spirit and scope of the invention as defined in the appended claims.
This invention is a continuation-in-part of provisional patent application 60/910,076, which is incorporated by reference herein.
Number | Date | Country | |
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Parent | 60910076 | Apr 2007 | US |
Child | 12062410 | US |