Patent Publication Number: US-2015059827-A1

Title: Torque Tube for Solar Panel System

Description:
FIELD OF THE DISCLOSURE 
     This invention relates in general to torque tubes on which solar or photo-voltaic panels are mounted for rotation with the torque tube. 
     BACKGROUND 
     Solar or photo-voltaic panels may be installed with a tracking system to pivot and track the sun during the day. One type of system has parallel rows of panels, each row extending north and south. The solar panels or modules in each row are mounted on a torque or torsion tube for rotation with the tube. Each row has a separate torque tube, which may have a length of 200 feet or more. A drive shaft extends perpendicular to the torque tubes and has mechanical devices that convert movement of the drive shaft into rotation of the torque tube. A controller programmed to track the sun operates the drive shaft. 
     The torque tube is supported on several posts, each of which has a bearing on its upper end. Because of the length, the drive system has to apply significant torque to rotate the torque tubes. Also, wind blowing against the solar panels generates torque along the lengths of the torque tubes that is transferred to the drive system. 
     The solar panels in each row must be installed on the torque tube in the same plane, and they must stay in the same plane during operation. Consequently, the torque tubes must be very stiff in torsion. A stiffer torque tube allows less twist out at the free ends of the torque tube as compared to a more compliant torque tube, which would allow more twist. 
     Since the solar panels must be mounted to the torque tube in a single plane, preferably the torque tube has a datum surface or reference for orienting the panels while they are being attached. Having a fixed datum surface extending along the length of the torque tube facilitates the installer installing the solar panels in the same plane. For example, it is difficult to mount flat solar panels on a cylindrical torque tube in a single plane and so as to be able to transmit torque due to wind. Normally, brackets would have to be welded to a cylindrical torque tube to provide the datum surface. As a result most solar array systems employ torque tubes with a square cross-sectional configuration. One of the flat sides becomes the datum surface, and clamping a panel to the flat side so as to be able to withstand torque is not difficult. 
     SUMMARY 
     A solar panel array system has plurality of vertical posts. A torque tube is supported on upper ends of the vertical posts. The torque tube has a cross-sectional configuration having an exterior surface with more than four flat sides. A photo-voltaic module clamp fastens to one of the flat sides. 
     The flat side to which the photo-voltaic mount is fastened preferably has a width that is no greater than 40 percent a cross-sectional dimension of the exterior surface of the torque tube measured along a line passing through the longitudinal axis. In the preferred embodiment, each of the flat sides of the exterior surface of the torque tube has an interior surface that is also flat and of the same dimensions as on the exterior surface. Preferably, the torque tube has a uniform wall thickness. In one embodiment, a cross-sectional dimension of the exterior surface of the torque tube measuring along a line passing through the axis is at least 30 times the wall thickness. 
     In the preferred embodiment, the torque tube is roll formed from a flat sheet. A weld seam extends along one of the flat sides parallel with the longitudinal axis. 
     The exterior surface of the torque tube may be an octagon with eight flat sides. Each of the flat sides joins two other of the flat sides and has an identical width. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a photo-voltaic panel tracking system in accordance with this disclosure. 
         FIG. 2  is a schematic isometric view of a torque tube rotatably mounted by a bearing on a post of the tracking system of  FIG. 1 . 
         FIG. 3  is an sectional view of the torque tube of  FIG. 2  with part of a solar module clamped on it. 
         FIG. 4  is a graph of shear stress due to torsion for torque tubes of circular, octagonal and square cross-section. 
         FIG. 5  is a graph of angle of twist due to torsion for torque tubes of circular, octagonal and square cross-section. 
         FIG. 6  is a graph of deflection due to bending for torque tubes of circular, octagonal and square cross-section. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     Referring to  FIG. 1 , solar array system  11  is of a type having the ability to track the sun during the day. Solar array system  11  has several parallel rows  13  (three shown) aligned in a north-south direction. In each row  13 , solar panels  15 , also called photo-voltaic panels or modules, are mounted on a torque tube  17 , which extends in a north-south direction. Each torque tube  17  may extend from one end to the other end of one of the rows  13 , or each torque tube  17  may have sections coupled together with flexible joints or field-welded to each other. Torque tubes  17  rotate incrementally, causing solar panels  15  to tilt and remain in better exposure to the sun. 
     Torque tubes  17  are mounted by bearings  19  to vertical posts  21 . Posts  21  are embedded in the earth or a provided foundation at selected distances apart from each other. A drive shaft  23  driven by a drive unit  25  extends perpendicular to rows  13  and engages each torque tube  17  to cause pivotal rotation of each torque tube  17 . Drive shaft  23  may rotate or may move linearly. Drive shaft  23  is illustrated as engaging torque tubes  17  midway along the lengths of each row  13 . Each torque tube  17 , may for example be 100 to 200 feet in length or more, and posts  21  may be about 14-19 feet apart from each other. Torque tube  17  is typically made up of sections about 30 feet in length that are joined in the field by articulating joints or welding. 
     Referring to  FIG. 2 , bearing  19  is shown schematically and may be a variety of configurations. Bearing  19  is mounted on a bracket  27  that secures to the upper end of one of the posts  21 . Bearing  19  has a cavity  29  through which toque tube  17  extends. Cavity  29  is polygonal, having more than four flat sides, and preferably eight. 
     Torque tube  17  has an exterior  31  that mates with bearing cavity  29 . Torque tube  17  has an interior  33  that has the same configuration as exterior  31 , defining a uniform wall thickness. Referring to  FIG. 3 , exterior  31  has more than four flat sides  35  and is preferably octagonal. Flat sides  35  are identical in width and join each other at a 45 degree angle  37 . Each flat side  35  is in a single plane that extends the full length of torque tube  17 . Each flat side  35  is parallel to another one of the flat sides  35 . 
     The wall thickness of torque tube  17  between exterior  31  and interior  33  is quite thin compared to the cross-sectional flat-to-flat dimension  41  of torque tube  17  measured along a line passing through a longitudinal axis  38  from one flat exterior side  35  to an opposite exterior flat side  35  and normal to those flat sides  35 . In one embodiment, the wall thickness is in the range from about 0.060 to 0.1 inch and the cross-sectional flat-to-flat dimension  41  between exterior flat sides  35  is about six inches. This results in the cross-sectional flat-to-flat dimension  41  being in a range from about 60 to 100 times greater than the wall thickness, and preferably it is at least 30 times greater. Having a wall thickness of 0.060 inch and a flat-to-flat dimension of 6 inches results in weight of about 4.0 pounds per foot. Torque tube  17  is preferably formed of a galvanized steel alloy, but composite fiber or plastic materials are also feasible. 
     Torque tube  17  is preferably roll formed. A flat sheet of metal is drawn through an array of rollers (not shown) that gradually bend and form flat sides  35  into an octagonal configuration. The edges of the flat sheet abut and are welded to each other as the octagon shape is achieved, creating a weld seam  39  that is parallel with longitudinal axis  38  of torque tube  17 . Weld seam  39  is centered in one of the flat sides  35 . 
     One of the flat sides  35  serves as a datum surface  43  to attach a photo-voltaic panel mount or module clamp  45 . Module clamp  45  is illustrated schematically and may have various configurations for mounting one of the photo-voltaic panels  15  ( FIG. 1 ) to torque tube  17  for rotation with torque tube  17 . Module clamp  45  may comprise two clamps (only one shown) that are spaced apart from each other along longitudinal axis  38 . One of the photo-voltaic panels  15  is retained between the two module clamps  45 . Each module clamp  45  may be hat-shaped in cross-section, having a flange that overlies and secures one of the photo-voltaic panels  15  on datum surface  43 . Each module clamp  45  fits flush on datum surface  43  and extends laterally past datum surface  43  in opposite directions from axis  38 . In this example, datum surface  43  is illustrated facing upward and is opposite the flat side  35  containing weld seam  39 . 
     Each flat side  35  has the same width measured from one edge to the other, and that width is significantly less than the cross-sectional flat-to-flat dimension  41  of torque tube  17  from one flat side  35  to the opposite flat side  35 . Being an octagon, the circumferential width of each flat side  35 , as well as datum surface  43 , is about 41.6% the cross-sectional dimension  41  of torque tube  17 . Thus in the preferred embodiment, datum surface  43  is about 2.5 inches wide. Photo-voltaic panels  15  are typically about one meter or 6.5 feet measured perpendicular to torque tube axis  38 . Consequently, in one embodiment, the ratio of the width of datum surface  43  to the dimension of panel  15  perpendicular to torque tube axis  38  is about 0.032, and the ratio is preferably not greater than about 0.04. Regardless of the dimensions of panel  15 , preferably the width of each flat side  35  is at least one inch. 
     Module clamp  45  is secured to torque tube  17  in various manners. For example, a generally U-shaped strap  47  with five flat portions is shown extending around the lower portion of torque tube  17 . Strap  47  has three lower flat sections  48  that are at the same angle  37  relative to each other as flat sides  35  and fit flush against the three lower flat sides  35  of torque tube  17 . A strap leg  50  extends upward from opposite sides of the three lower flat sections  48 . Each strap leg  50  joins the three lower flat sections  48  at angle  37  and extends flush along the flat sides  35  that are illustrated in a vertical position in  FIG. 3 . The upper ends of strap legs  50  are bent 90 degrees into tabs, which are secured to module clamp  45  by fasteners  49 . Torque is thus readily transmitted between photo-voltaic panels  15  ( FIG. 1 ) and torque tube  17  via from module clamp  45  and strap  47 . The engagement of module clamp  45  and strap  47  is similar to that of a wrench engaging a polygonal nut, because module clamp  45  and strap  47  engage six of the eight flat sides  35  of torque tube  17 . 
     During operation, a control system moves drive shaft  23 , which rotates torque tube  17  incrementally, causing solar panels  15  to remain more normal to the sun during the day. Alternately, torque tube  17  could be oriented east-west and remain fixed and non rotating except for seasonal manual rotational adjustments. Further, torque tube  17  could be employed with a completely fixed solar panel array system and never rotate. Even if fixed, torque tube  17  still encounters torque when wind blows across solar panels  15 . Torque tube  17  thus must be able to transmit torque and have a high torsional stiffness. Further, because of the distance between posts  21 , torque tube  17  must be able to withstand bending due to its own weight as well as the weight of solar panels  15  and snow load. 
     The configuration of torque tube  17  was selected by analytically comparing an octagonal cross-sectional shape to a cylindrical or circular cross-sectional shape and a square cross-sectional shape. Referring to  FIGS. 4-6 , analytical studies were made of circular, square and octagonal tubes. In the study, the cross-sectional area of each shape was held equal. Thus a 30 foot long piece of a circular tube or a square tube would weigh the same as a 30 foot long piece of octagonal tube. Also, the wall thickness was held constant for all three shapes. Each tube was analytically subjected to the same amount of torsion. As shown in  FIG. 4 , the shear stress for a circular tube is less than for an octagonal tube when undergoing the same amount of torsion. The octagonal tube had less shear stress than a square tube when undergoing the same amount of torsion. The graph of  FIG. 4  shows the octagonal tube developing about five percent more shear stress than the circular tube. The square tube develops about 28 percent more shear stress than the circular tube. 
       FIG. 5  illustrates the amount of angular twist occurring when the circular, octagonal and square tubes are undergoing the same amount of torsion. The octagonal shaped tube developed about 11 percent more angular twist than the circular tube. The square tube developed about 62 percent more angular twist than the circular tube experiencing the same torsion. 
       FIG. 6  illustrates the effect of bending forces on the circular, octagonal and square tubes. When subjected to the same bending moments, the octagon tube developed about five percent more deflection than the circular tube. The square tube developed about 22 percent more deflection than the circular tube experiencing the same bending moment. 
     The study shows clearly that a circular tube is stiffer than both an octagonal tube and a square tube under both torsion and bending moments. The circular tube also develops a lower shear stress under a torsion load than the octagonal tube and the square tube. However, it is difficult to utilize a circular or cylindrical tube as a torque tube in solar panel installations because of the need to mount the solar panels on a common, flat datum surface. An octagonal tube performs better than a square tube for torsion and bending loads. Also, an octagonal tube has a natural datum surface due to its eight flat sides. When considering optimum torque tube shapes, the study shows that increasing the number of flat sides over a square tube causes the tube to perform more like a tube of circular cross-section. Less flat sides or faces cause the tube to behave more like a square tube. The octagon profile is well suited for solar panel array torque tubes because it behaves much like a circular profile while also providing datum faces or surfaces for alignment and transmitting torque. An improved stiffness allows greater spans between posts. Reducing the number of posts lowers system cost. 
     When compared to a square tube, the octagon tube provides a higher torsional stiffness and lower shear stress; therefore, less material is required in the octagon tube to perform the same task. For example, one commercially available torque tube is a square tube with 4 inch flats and a wall thickness of 0.125 inch. That prior art torque tube has a weight of 6.5 pounds per foot, requiring more material than the preferred embodiment of an octagonal tube, described above, which has a weight of 4.0 pounds per foot. Less material required in the torque tube lowers the system cost. Moreover, when compared to the prior art square tube mentioned, the preferred embodiment octagon tube provides less shear stress, less angle of twist, and higher stiffness due to bending. 
     The round tube typically requires gussets or brackets to be welded in place to establish a datum for mounting modules. Welding gussets or brackets to the torque tube is not required for the octagon tube, lowering the cost of the system. 
     While the disclosure has been shown in only one of its forms, it should be apparent that various modifications are possible. For example, the exterior could have a different number of flat sides, such as ten sides.