Abstract:
A solar tracker operates on a single axis, but partially simulates a dual-axis tracker by adjusting tilt angle as the tracker rotates. The tracker is disclosed in particular embodiments which fit efficiently within a hemispherical transparent dome.

Description:
REFERENCE TO PROVISIONAL APPLICATION 
     This application claims the benefit of Provisional Application Ser. No. 60/104,214, filed Oct. 14, 1998 
    
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
     This invention concerns solar trackers, and particularly the invention relates to several embodiments of efficient single-axis tracking systems which partially emulate multiple-axis trackers. 
     Solar panels, i.e. arrays of photovoltaic cells arranged in panels, are in increasing use today. The use of such photovoltaic cells is expected to accelerate as the cost of the cells decreases. 
     Various forms of solar trackers are also well known, for use with arrays or panels of photovoltaic cells. However, the most efficient trackers, for absorbing maximum sunlight in a given day, have been multiple-axis trackers, which rotate about more than one axis so as to follow both the azimuth variation (progression of the sun&#39;s bearing angle, i.e. east to south to west), and the sun&#39;s change in elevation angle from the horizon. 
     A simplified solar tracker apparatus is disclosed pursuant to this invention, in several embodiments. In each embodiment the basic rotation of the tracker is about a single axis, but the mechanism of the tracker partially emulates a two-axis tracking mechanism by making additional adjustment to more accurately track the sun, as the apparatus rotates about a single axis. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic perspective view showing one embodiment of a single-axis solar tracker for a photovoltaic (PV) array. 
     FIG. 2 is a schematic detail view partially in section, showing a preferred retention scheme for the panel and mechanism shown in FIG.  1 . 
     FIGS. 3,  4  and  5  show clamp configurations for use with the retention arrangement indicated in FIG.  2 . 
     FIG. 6 is a schematic perspective view showing mounting features of an individual solar panel with multiple PV cells. 
     FIGS. 7A,  7 B and  7 C show various size panels which fit in a hemisphere dome. 
     FIG. 8 is a diagram supporting a calculation of area for one of the panel configurations depicted. 
     FIG. 9 is a schematic perspective view showing another single axis solar tracker, with a correction guide track. 
     FIG. 10 shows a horizontal circular guide ring for another embodiment of a single axis solar tracker. 
     FIGS. 11 and 12 are schematic side elevation and front elevation views indicating components and a subassembly of a solar tracker with horizontal guide. 
     FIG. 13 is a schematic perspective view showing the assembled solar tracker with horizontal guide. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 1-6 show a single, vertical axis solar tracker which accommodates rectangular PV panels in a semi-circular array. The various figures schematically indicate a mounting for solar panels, the tracking rotation axis and associated hardware. 
     In FIG. 1 a semi-circular PV array  10  can rotate about a vertical axis  12  for daily solar tracking. Conventional, rectangular PV panels (not shown in detail in FIG. 1) are attached in a semi-circular array to a trellis type mounting  14 , which is comprised of multiple, horizontal rods  14   a  with a backbone rod  16  and a semi-circular perimeter  18 . In this embodiment, the solar array is tilted at a fixed angle of preferably 45° from the horizontal (or equivalently, 135° from the opposite horizon). The 135° angle is shown at  20 . 
     The apparatus  10  also includes a frame  22  which is rigidly secured to the trellis  14  and which carries counterweights  24 , which balance the weight of the inclined PV panels and trellis  14 . 
     In FIG. 6, a typical rectangular PV panel  28  is indicated. The panel has feet  30 , each of which has a bolt hole for securing to a mounting structure. 
     FIGS. 4 and 5 show flat plate and “C” shaped clamps  32  and  34 , each with slots  36 . These are used for formatting (positioning and securing) the solar panels  28  on the trellis device  14 . A cross section of this assembly is shown in FIG.  2 . In that figure, two panels have been placed side by side, the panels not being fully shown but the L-shaped feet  30  of each panel being shown, back to back. Below the feet  30  is a single plate  32 , below which is a trellis rod  14   a,  from the trellis  14  in FIG. 1, and a “C” clamp  34 . Bolts  38  and nuts  40  retained this assembly together, preferably including a washer (not shown). This assembly retains the solar panel  28  securely to the trellis  14 . 
     FIG. 3 shows an alternative form of “C” clamp  34   a,  with simple bores rather than slotted holes. The slotted holes are preferred for allowing some latitude in positioning of the solar panels into the entire assembly. 
     In FIGS. 7A-7C, different solar panels  42 ,  44  and  46  are shown inside a hemispherical dome  48 . FIG. 7A shows a half semi-circle array, for rotation about a vertical axis  50 . Thus, FIG. 7A is consistent with the arrangement shown in FIG.  1 . 
     Variations are shown in FIGS. 7B and 7C. In FIG. 7B an array comprising a complete circle  44  rotates about the point where the vertical axis  50  intersects the top of the dome  48 . The circle of the array  44  has the same area as the half circle shown in FIG. 7A (and in FIG.  1 ). This is shown in the following calculations where r s  is the radius of the hemisphere and thus also of the half circle in FIG. 7A, and r c  is the radius of the circle  44 ; 
     
       
           A= ½ πr   s   2   
       
     
     (for area of half circle in  7 A) 
     
       
           r   c   =r   s ×{square root over ( )}2/2 
       
     
     
       
           A (circle)=π r   s   2 /2 
       
     
     A truncated circle  46  within the dome  48 , as shown in FIG. 7C, has a larger area than either of the panel configurations shown in FIGS. 7A and 7B. This is shown in FIG.  8  and the following calculations associated with FIG.  8 : 
     parameterize in displacement d 
     ⊖=angle formed by panel and sphere radius (perpendicular to sphere surface at point of panel &amp; sphere contact) 
     
       
         0≦ d≦{square root over ( )}s d/r   s (sphere)=sin ⊖ r   c (circle)= r   s  cos ⊖ 
       
     
     
       
           d/r   c =tan ⊖(=sin ⊖/cos ⊖= d/r   s   /r   c   /r   s   =d/r   c )  r   c   2   +d   2   =r   s   2   
       
     
     
       
           A   c   =πr   c   2 =π( r   s   2   −d   2 ) 
       
     
     (area un-truncated circle) 
     All of FIGS. 7A-7C show the solar arrays at 45° to the horizontal, and the calculations are based on this assumption. The illustrated panel configurations are of interest when the solar array is to be contained and is to rotate within a protective transparent dome. 
     FIG. 9 depicts a semi-circular PV panel array mount for rotation about a vertical solar tracking axis  50 , similar to what is shown in FIG.  1 . However, in this embodiment the angle which the solar panel array  52  makes relative to the horizon is not fixed at 45°. Instead, this tilting is now varied throughout the day, to more closely approximate a two axis tracker. 
     As in the embodiment shown in FIG. 1, the solar panel array  52  rotates about a single vertical axis  50 , during daily solar tracking. The individual solar panels are attached to the trellis mount frame  14 , which may be similar to that shown in FIG.  1 . However, this embodiment differs from FIG. 1 in having a correction mechanism. It is desired that the solar panel array  52  be tilted to more nearly vertical for sunrise and sunset, and closer to horizontal at high noon. The amount of desired tilt correction varies throughout the day, and with time of year (season) and installation latitude. 
     A polar axis correction guide track or ring  54  is tilted relative to horizontal, and parallels the sun&#39;s relative motion. The guide track  54  is adjustable for height, i.e. height relative to the remainder of the apparatus, being raised for summer (to move the panel array  14  closer to horizontal) and lowered for spring and fall, lower still for winter. Its tilt angle is also adjustable to accommodate the installation latitude. This tilt angle is set once, by setting the guide track ring  54  permanently parallel to earth&#39;s equator. Thus, the angle which the ring  54  makes relative to local horizontal ground is equal to (90° minus latitude), and this tilt angle is around an east/west axis. 
     In all cases, the axis  50  of rotation of the array device remains vertical. 
     In this particular implementation of the principle of this embodiment, a forked follower arm (or an equivalent device)  56  engages the correction guide track  54 , imparting tilt correction on the panel mount  14 . The fork follower arm  56  is rigidly secured to the trellis frame  14  so that the trellis frame and panel  52  follow the tilt motion of the arm  56 . An altitude pivot point  58  allows the panel frame  14  to tilt relative to the horizon and relative to an optional, fixed horizontal guide ring  60 . 
     FIGS. 10-13 show a variation of the embodiment of FIG. 9, based on similar principles. In this embodiment, axes are exchanged as compared to the embodiment of FIG.  9 . This form of the invention accomplishes the same goal, of tilting the solar panel to more nearly approximate a dual axis tracker. 
     FIGS. 10-12 schematically show components and subassemblies of the solar tracking apparatus, while FIG. 13 schematically illustrates the assembled apparatus. 
     In FIG. 10, a horizontal guide ring or guide slot  70  maintains the bottom of the solar panel mount parallel to the horizon, and this ring is fixed in position. Its function is similar to that of the tilted correction guide track  54  shown in FIG.  9 . 
     In FIG. 11 a polar axis shaft  72  is mounted with its lower section  72   a  parallel to earth&#39;s axis. A motor  74  turns the polar axis shaft  72   a  for daily rotation. Seasonal tilt angle, the angle  76  shown in FIG. 11, may be fixed or adjustable. A pivot pin  78  is perpendicular to the upper portion  72   b  of the polar axis shaft, as is a sliding pin  80 . 
     A solar panel mount  82  is shown in FIG. 12, attached to the polar axis shaft  72 . The panel mount  82  has a pivot pin bushing  84 , which fits over the pivot pin  78  shown in FIG. 11, and the panel mount also has a sliding pin guide slot  86 , which fits over the sliding pin  80  shown in FIG.  11 . 
     In FIG. 13 the solar panel mount  82  and polar axis shaft  72  are shown assembled to the horizontal guide ring  70 . Horizontal follower forks  88  (or equivalent structure) secured to a lower edge of the solar panel mount  82  engage the horizontal guide ring  70 . 
     Thus, it is seen that the polar axis shaft  72  imparts true polar tracking to the solar panel mount  82 . Meanwhile, the horizontal guide ring  70 , followed by the forks  88 , causes the bottom edge of the solar panel mount  82  to remain horizontal. 
     The several embodiments illustrated above show different ways of fitting a solar panel within a hemispherical dome. Variations in panel shape and rotational axes allow more area to fit under the dome, or tilt correction to occur for maximum solar collection efficiency with a single axis tracker. 
     The above described preferred embodiments are intended to illustrate the principles of the invention, but not to limit its scope. Other embodiments and variations to this preferred embodiment will be apparent to those skilled in the art and may be made without departing from the spirit and scope of the invention.