Patent Publication Number: US-2013240473-A1

Title: Uniform tension distribution mechanism for stretched membrane solar collectors

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to an improved design for a key component of linear tensioned membrane reflectors for solar parabolic trough concentrators, solar linear reflectors, and linear heliostats for solar Fresnel reflecting systems, specifically, those that utilize thin flexible films for the membrane substrate. 
     A thin membrane of highly reflecting material, such as metalized reflective plastic film, is securely attached at the peripheral edges of parallel-facing end form members for uniformly tensioning and stretching the membranes into long, semi-rigid and accurately formed trough surfaces that create the critical focusing mirror component of the collector. Each end form member has a cross-sectional curved shape selected to produce the desired cross-sectional shape, usually parabolic, of the reflector. The end forms, which control the accuracy of the curved membrane surface, are manufactured to very close tolerances to last the life of the product. The membrane is then placed under 1000 to 7000 pounds per square inch (PSI) of tension in the longitudinal direction, usually by carefully moving one of the end form members away from the other. 
     Linear tensioned membrane reflectors have many advantages over more traditional designs incorporating rigid frame structures. They are generally less expensive, relatively light weight, and easy to assemble and replace. Because of their light weight, and corresponding ability to be stretched over long spans, the membranes are able to be firmly attached at the ends of their spans to simple light weight frame structures which are capable of being easily rotated about their longitudinal axis to precisely track the sun as it moves across the sky. This in turn produces a very efficient and economical method for solar energy conversion using a minimum of tracking force. 
     However, linear tensioned membrane reflector technology presents certain problems that do not exist for linear solar reflector technologies constructed with a rigid reflector structure. A single die spring centered on the end form and attached to the outer supporting frame can provide the tensile force sufficient to place the entire membrane of the reflector in a state of longitudinal tension. But, variations in longitudinal tension may produce wrinkles and other shape distortions that reduce the effectiveness of the collector. 
     Thus, while is desirable to reduce the weight of the end form in order to reduce the weight of the collector, before the improvements of this current invention, end forms have generally been “D” shaped and either solid or ribbed and have been constructed of die or sand cast aluminum, a complicated and expensive manufacturing process. Moreover, even with standard “D” shaped end forms, careful measurements have shown that slight bending of the end forms can still occur which can produce undesirable variations in longitudinal tension in the resulting stretched membrane. Generally, these variations would be overcome by increasing the thickness of the end form at the cost of increasing the weight of the collector. 
     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, thus producing undistorted reflective surfaces which precisely focus solar energy reflections on the longitudinal collector receiving pipe. 
     It is a further objective of this invention to reduce the materials used to create the end forms. 
     It is a further objective of this invention to simplify the manufacturing process for the end forms. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention teaches that by using a single or double whiffle-tree mechanism (also referred to as a whiffle-tree) to distribute the tensile force uniformly along the peripheral edge of the end form, the bending of the end form, and thus the possible distortion of the membrane, can be significantly reduced, for practical purposes, or eliminated. Further, the uniform distribution of the overall tensile force reduces the load transfer stresses sufficiently to allow for a “C” shaped end form to be developed which uses significantly less material than the solid or ribbed design, while delivering better and more reliable performance. Finally, this design allows for the same shape to be used in cutting the peripheral edge of the end form and the upper edge of the end form, simplifying the manufacturing process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a ribbed end form as taught by in the prior art. 
         FIG. 2  shows the “C” shaped curved member of the present invention. 
         FIG. 3  shows an embodiment of the present invention including an eight-point, single-pull whiffle-tree. 
         FIG. 4  shows a second embodiment of the present invention including a sixteen-point, single-pull whiffle-tree. 
         FIG. 5  shows a perspective view of the embodiment of  FIG. 3 . 
         FIG. 6  shows a perspective view of the embodiment of  FIG. 4   
         FIG. 7  shows an embodiment of the present invention including an eight-point, dual-pull whiffle-tree. 
         FIG. 8  shows a second embodiment of the present invention including a sixteen-point, dual-pull whiffle-tree. 
         FIG. 9  shows a perspective view of the embodiment of  FIG. 7   
         FIG. 10  shows a perspective view of the embodiment of  FIG. 8 . 
         FIG. 11  shows an alternative embodiment of the present invention. 
         FIG. 12  shows a first alternative bar shape for use in the present invention. 
         FIG. 13  shows a second alternative bar shape for use in the present invention. 
         FIG. 14  shows a third alternative bar shape for use in the present invention. 
         FIG. 15  shows a fourth alternative bar shape for use in the present invention. 
         FIG. 16  shows a perspective view of the embodiment of  FIG. 7  in use in a solar collector. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows the “D” shaped, ribbed end form  100  of the prior art. The ribbed end form  100  includes a peripheral edge  110  which holds the flexible reflective membrane of the collector into the appropriate, usually parabolic, shape, but also includes a cross-piece  120 . A plurality of ribs  130  extend from points near the center of the “D” form toward the peripheral edge  110  and the cross-piece  120 . Tensile force, which can be effected through a die spring (not shown), can be applied to the ribbed end form  100  at a point  140  near the center of the form. The ribbed end form  100  is also adapted to receive a single die spring or similar device for applying tensile force or holding the end plate in place on a support frame, thus allowing two parallel plates to appropriately stretch and shape the flexible reflective membrane. A second opening  150 , is positioned in the ribbed end form  100  at an appropriate location, generally the focal point of the parabola, and is adapted to accept a pipe or similar device carrying liquid to be heated or a material that can otherwise utilize the concentrated solar energy collected. 
     With reference to  FIGS. 2 through 10 , the end form of the present invention includes a curved member  205  that forms the peripheral edge  110 . Generally, the peripheral edge  110  will be designed so as to cause the reflective membrane of the solar collector to form a parabolic cross-section when the end form is in use. The curved member  205  also has an upper edge  240 . Preferably, for ease of manufacture, the curve of the upper edge  240  is identical to the curve of the peripheral edge  110  in that the same curved shape is used to cut both edges. The curved member is preferably cut from 1 inch thick aluminum and ranges from approximately 2 inches wide at the upper points  250  of the curved member  205  to approximately 3 inches wide at the bottom  260  of the curved member  205 . It will be understood that the exact measurements could vary depending on other design factors, including the amount of force to be applied to the end form, the physical properties of the material from which the curved member is created, and the thickness of that material. However, as will be evident to those of ordinary skill in the art, use of the same curve to create the upper edge  240  and peripheral edge  110  will result in the upper points  250  of the curved member  205  to be narrower than the bottom  260  of the curved member  205 . It should also be noted that while there are significant advantages to using a narrow curved member with similar upper and lower curve shapes, the curved member could be wider, and even retain a “D” shape, and still obtain advantages by using the features described below. 
     The end form of the current invention also includes a whiffle-tree, which is sometimes also known as a whiffle-tree. A whiffle-tree consists of a bar with a pivot or similar connector point, usually located at or near the center of the bar, with a normal force applied from one direction at the pivot. The force is divided or “split” by the mechanism and transmitted to the ends of the bar proportionally to the distances of the ends to the pivot point. Whiffle-trees may be used in series to distribute the force further. Placement of the pivot at a location other than at, or very near, the center of the bar, for example at a point one-third the length of the bar from one end, may be appropriate in certain unusual applications depending on the geometry of the end form. 
     With reference to  FIGS. 3 and 5 , the embodiment of the improved end form shown employs an 8-point whiffle-tree  210  consisting of seven bars at different “levels” away from the curved member. Each end of four fixed bar members  220  is attached near the peripheral edge  110  of the curved member  205  with eight fixed attachment points  221 , which are preferably equally and symmetrically spaced at set intervals along the curve of the peripheral edge  110  or otherwise to distribute the tensile force evenly. Thus, the distance between the ends of each of the fixed bar members  220  is close, although slightly shorter, to the overall length of the fixed bar members  220  themselves. 
     A first pivot  222  is positioned near the center of each of the fixed bar members  220 . The first pivots  222  are attached to the ends of the two second level bar members  225 , each of which has a second pivot  226  near the center of the member. A top bar member  230  is attached at each end to the second pivots  226 . Tensile force  500  is applied at the center  235  of the top bar member  230  in order to allow the end form stretch and shape the flexible membrane of the solar collector. 
     With reference to  FIGS. 4 and 6 , the embodiment of the end form shown incorporates a sixteen point whiffle-tree, attached to the curved member  205  in a manner similar to that described with respect to  FIGS. 3 and 5 . In this embodiment, there are eight fixed bar members  220  attached directly to the curved member  205 , and the ends of the fixed bar members  220  fixedly attached at sixteen fixed attachment points  221  to the curved member  205 . In addition to four second level bar members  225 , the ends of which are attached to the first pivots  222  in the approximate center of the fixed bar members  220 , there are additionally two third level bar members  620  with central pivot points  621  to which the top bar member  230  is attached. As with the 8-point whiffle-tree, a single tensile force  500  is applied at or near the center  235  of the top bar member  230 , but is distributed to sixteen rather than eight points near the peripheral edge  110  of the curved member  205 . 
     With reference to  FIGS. 7 through 10 , it is possible to eliminate the single top bar member  230  of the whiffle-tree, and, instead have two substantially equal tensile forces applied at each second pivot  226 . Essentially, then, as shown in  FIGS. 7 and 9 , two 4-point whiffle-trees are positioned symmetrically on the curved member  205  to form an 8-point dual pull whiffle-tree and in  FIGS. 8 and 10  two 8-point whiffle-trees are positioned symmetrically on curved member  205  to form a 16-point dual pull whiffle-tree. The tensile force  900  is applied at the second pivots  226  for the 8-point dual pull whiffle-tree and at the central pivot points  621  of the third level bar members  620  for the 16-point dual pull whiffle-tree. The “two-pull” approach uses fewer components, and thus may be less expensive to construct. Further, the two-pull design may use less longitudinal space in the solar collector frame, since there fewer levels of bars that need to be accommodated, thus increasing the area of the reflector available to collect sunlight. Finally, when a single tensile force  500  is applied at a single top bar member  230 , the end form has a tendency to rotate about the point  235 , which may necessitate the use of an additional device attached to the frame of the solar collector to prevent undue rotation. The two-pull approach eliminates the need to have an additional device to prevent internal rotation. However, the two-pull approach does require, for most applications, that the tensile force of each pull point be equal. This may be difficult given variations in the frame or other structure from which the force is applied to the pulls. For example, beams may be bowed. Mechanical springs, such as die-springs, may be used to ameliorate this problem. 
     It is preferable that the first bar members  220 , although fixedly attached to the peripheral edge  110 , not be fastened too tightly to the curved member  205  to allow for slight play or movement and to eliminate torqueing. Thus, if the first bar members  220  are bolted to the curved member  205 , the holes (not shown) in the curved member  205  are preferably slightly oversized and the bolts (not shown) would preferably not be fully tightened with a gap between each bolt and the surface of the curved member  205  on the order of ⅛ inch. Locknut, or double nuts, may be used on loose bolts. 
     Although the embodiments of  FIGS. 3 through 10  show the ends of each second level bar members  225  having approximately the same length and connected to adjacent fixed bar members  220  distributed at approximately equal intervals along the peripheral edge  110 , other connection schemes are possible. For example, as shown in  FIG. 11 , additional bar members  1125  and  1126 , performing the same function as the second bar members of  FIGS. 2 through 9 , may be of different lengths and pivotally connected to first bar members  220  on opposite sides of the curved member  205 . The additional bar members  1125  and  1126  are additionally pivotally connected at their centers in a substantially perpendicular arrangement at the mid-points  1127  and  1128  to a final bar member  1130 , which is ultimately attached to a source of tensile force and performs the same function as the top bar member  230  shown in  FIGS. 3 through 7 . 
     The bars may be of different geometries. In addition to bars having a square or rectangular cross-section with or without rounded ends  1210 , as shown in  FIGS. 2 through 10  and in  FIGS. 12 and 14 , bars or rods with circular cross-sections  1510  as shown in  FIG. 15  could be used, either as solid bars or hollow bars. Triangular or diamond shaped bars, which may also have varying thickness, as shown in  FIGS. 11 and 13  could also be employed and may have advantages in some configurations. 
     With reference to  FIG. 16 , an end form of the current invention is shown in use in a solar collector. The 8-point dual pull whiffle-tree embodiment of  FIGS. 7 and 9  is shown held in the frame  1600  and stretching the thin film reflective membrane  1610 . Holes  1620  in the frame  1600  allow for a source of tensile force to be applied to the second pivots  226 . 
     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.