Abstract:
An array of Fresnel reflectors is presented with an adjustable focus. The focus of the array is controlled by only a few mechanical parts and simple inputs. One preferred embodiment of this device is to collect sunlight and focus it onto a photovoltaic (PV) cell or a boiler, for the purpose of collecting solar energy. This solar collector, or heliostat, is easily adjusted so that it always directs the sun&#39;s rays onto the PV cell or boiler.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not Applicable 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable 
       NAMES OF PARTIES TO A JOINT RESEARCH AGREEMENT 
       [0003]    Not Applicable 
       REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISK APPENDIX 
       [0004]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0005]    1. 
       Field of the Invention 
       [0006]    This invention relates to a mirror array with an adjustable focus. A preferred embodiment of the invention is in the field of solar energy collection. Such devices are commonly called heliostats, and often—as in this case—can adjust throughout the year, according to the season. Furthermore, many heliostats track the sun daily from east to west 
         [0007]    2. Description of Related Art 
         [0008]    The prior art contains many examples of adjustable mirror arrays, many of which are primarily or exclusively designed as solar collectors. These heliostats employ a great variety of methods to attain the goal of focusing sunlight on a common point. Some of these heliostat arrays contain many individually controlled mirrors, such as U.S. Pat. Nos. 4,122,827 and 7,192,146. Other arrays, including U.S. Pat. Nos. 4,102,326, 4,425,904, 4,466,423, and 4,832,002 contain mirrors that are linked together mechanically, but with cables or belts. The arrangement of cables requires, however, that each mirror moves the same amount under a single input, but the present invention moves each mirror a different amount in order to preserve the parabolic geometry of the reflector array. Furthermore, it is believed that cables may pose a liability, as they can break. Related U.S. Pat. Nos. 4,110,010, 7,677,241 and 7,905,227, are mechanically linked with configurations that appear more robust than the arrays using cables. 
         [0009]    U.S. Pat. No. 6,349,718 looks similar in some respects to the present invention, but is actually substantially different, particularly regarding the movement of the mirrors. It appears that they can be independent or linked together, but if linked, they are to be forced to move the same amount. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    The present invention comprises an array of mirrors, designed with an adjustable focus. In its present embodiment, it is configured to reflect parallel incident rays onto a common point. The focal point can be moved in multiple dimensions, and the array can also be adjusted to account for the direction of the incident rays. The preferred embodiment of the invention is to reflect sunlight and focus it onto a solar collector; such a device is commonly known as a heliostat. The contributing theory that inspired the invention comes from Fresnel; concentric rings can be used in lieu of a large spherical or parabolic mirror, reducing expense and material, and in this case allowing for a simple method to adjust the focus. In placing the mirror arrays around a solar collector, it is possible but not required that the mirrors be arranged concentrically. 
         [0011]    The mirrors in this array are linked together; they need not be controlled individually. This is believed to be especially beneficial, given the large number of mirrors expected in a large-scale solar collector facility. Because of this linkage, all the adjustments enumerated above can be made with only two mechanical inputs on each control axis. 
         [0012]    One particular feature of this array is the use of an expanding linkage, in the form of a lattice, between mirrors to rotate the mirrors. This allows each mirror to be rotated a different amount, proportional to its location in order to focus all the mirrors exactly on the desired point. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1 . Single mirror motion element 
           [0014]      FIG. 2 . Alternate single mirror motion element 
           [0015]      FIG. 3 . Idealized Control element 
           [0016]      FIG. 4 . Mirror control structure 
           [0017]      FIG. 5 . One-dimensional array of mirrors 
           [0018]      FIG. 6 . Several side views of array 
           [0019]      FIG. 7 . Two-dimensional array of mirrors 
           [0020]      FIG. 8 . Solar collector facility 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]      FIG. 1  shows a single mirror motion element ( 1 ). The full array requires one of these per mirror. The first part of the element is a frame ( 2 ) containing two grooves, top ( 3 ) and bottom ( 4 ). The center of the frame contains a hole ( 5 ), in which a rotating axle ( 6 ) is held. At one end of the rotating axle ( 6 ), an arm ( 7 ) is affixed to facilitate the rotation. 
         [0022]    A top extensible shaft ( 8 ) extends from the top groove ( 3 ) to the rotating axle ( 6 ). The extensible shaft ( 8 ) is designed for one end to slide horizontally through the top groove ( 3 ) and the other end to rotate freely around the rotating axle ( 6 ) without directly rotating it. Likewise, a bottom extensible shaft ( 9 ) comprises one end that slides along the bottom groove ( 4 ), and the other end that rotates freely about the rotating axle ( 6 ). The top ( 8 ) and bottom ( 9 ) extensible shafts connect to the arm ( 7 ) via connecting rods ( 10 ). One end of each rod ( 10 ) is allowed to rotate freely where it is mounted to the extensible shafts ( 8 - 9 ). The other end of each rod ( 10 ) is constrained to pivot within a collar ( 11 ) which is free to slide along the arm ( 7 ). As the rods ( 10 ) slide, they cause the collar ( 11 ) and arm ( 7 )—and by extension, the rotating axle ( 6 )—to rotate in the hole ( 5 ). The rotating axle ( 6 ) will support a mirror ( 16 ), which will rotate with the axle ( 6 ), as is shown in  FIG. 5 . 
         [0023]      FIG. 2  shows an alternate embodiment of single mirror motion element ( 1 ). The full array requires one of these per mirror. The first part of the element is a frame ( 2 ) containing two grooves, top ( 3 ) and bottom ( 4 ). The center of the frame contains a hole ( 5 ), in which a rotating axle ( 6 ) is held. At one end of the rotating axle ( 6 ), an arm ( 7 ) is affixed to facilitate the rotation. 
         [0024]    A top extensible shaft ( 8 ) extends from the top groove ( 3 ) to the rotating axle ( 6 ). The extensible shaft ( 8 ) is designed for one end to slide horizontally through the top groove ( 3 ) and the other end to rotate freely around the rotating axle ( 6 ) without directly rotating it. Likewise, a bottom extensible shaft ( 9 ) comprises one end that slides along the bottom groove ( 4 ), and the other end that rotates freely about the rotating axle ( 6 ). The top ( 8 ) and bottom ( 9 ) extensible shafts support large bevel gears ( 28 ). Each large bevel gear ( 28 ) is allowed to rotate freely where it is mounted to the extensible shafts ( 8 - 9 ). A small bevel gear ( 29 ) is allowed to rotate freely around arm ( 7 ). The toothed surface of the large bevel gear ( 28 ) meshes with the small bevel gear ( 29 ). As the large bevel gears ( 28 ) rotate, they cause the small bevel gear ( 29 ) on arm ( 7 )—and by extension, the rotating axle ( 6 )—to rotate in the hole ( 5 ). The single mirror motion element ( 1 ) described and shown in  FIG. 1  will be used for farther illustration, though the mirror motion element ( 1 ) of  FIG. 2  could be substituted with no alteration to farther description of mechanism motion. 
         [0025]    Referring to  FIG. 3 , in  FIG. 3A  a detailed illustration of the control element ( 13 ) is shown. In  FIG. 3B  it is shown in context within the mirror control structure ( 12 ). The top ( 3 ) and bottom ( 4 ) grooves present in the mirror motion elements ( 1 ) extend through the control element ( 13 ). The top lattice ( 14 ) contains a top slide ( 17 ) at its top corner; the top slide ( 17 ) fits in the top groove ( 3 ), and as it slides it translates the entire top lattice ( 14 ) from side to side. Likewise, the bottom lattice ( 15 ) contains a bottom slide ( 18 ) at its bottom corner, which slides within the bottom groove ( 4 ) and translates the bottom lattice ( 15 ). Vertical slides ( 19 - 20 ) span across the end of each lattice ( 14 - 15 ), causing expansion or contraction with the results described in the previous paragraph. 
         [0026]    Looking further at  FIG. 3A , a wide variety of configurations can be achieved using only the slides shown ( 17 - 20 ). First, the top slide ( 17 ) allows the array to track the sun throughout the day. The bottom slide ( 18 ) adjusts the x coordinate of the array&#39;s focal point. The vertical slides ( 19 - 20 ) control the z coordinate of the focus. As a result, only four slides ( 17 - 20 ) will move or rotate all the mirrors in the array; the number of mirrors that can be moved will be limited only by the manufacturing, specifically the weight and compliance of the moving parts. The slides themselves will be moved by any desired actuating means, including motors, hydraulics or pneumatics, cables, belts, chains, or other means; there is no preferred mode, but the actuating means should be equivalent to one another for the purpose of this invention. 
         [0027]    Referring now to  FIG. 3B , the mirror control structure ( 12 ) is shown, which is comprised of multiple mirror motion elements ( 1 ) in addition to a control element ( 13 ). The control structure ( 12 ) may be manufactured as one piece; alternatively, several mirror motion elements ( 1 ) may be produced separately and placed together sequentially with a control element ( 13 ) added. The control structure ( 12 ) further includes two expanding lattices, top ( 14 ) and bottom ( 15 ). The purpose of these is to slide the ends of the extensible shafts ( 8 - 9 ) along the grooves ( 4 - 5 ), causing the rotation described above. If an entire lattice ( 14  or  15 ) is moved, all the mirror motion elements ( 1 ) will be adjusted similarly, as required to maintain the focus. If a lattice ( 14  or  15 ) is made to expand or contract by using the vertical slides ( 19  or  20 ), the mirror motion elements ( 1 ) will be adjusted proportionally to their distance from the end of said lattice ( 14  or  15 ). As an alternative to the expanding lattice framework, it may be desirable, and would be comparably achieved, to replace the lattice with threaded rods or springs, as shown in  FIG. 3C . 
         [0028]    A representative example of control motion is illustrated in  FIG. 4 . The control structure is shown first, in  FIG. 4A , maintaining a position in which all mirror control elements are held in an original position; level and at equal angles. It is seen that neither the top ( 17 ) nor bottom ( 18 ) slide, neither the top ( 19 ) nor bottom ( 20 ) vertical slide, has been displaced from the level and equal condition shown in  FIG. 4A . In  FIG. 4B , the control structure is shown again in a representative new position. It is seen in  FIG. 4B  that the top slide ( 17 ) has been displaced laterally, while the top vertical slide ( 19 ) has not been displaced from its original position. Thus the upper portion of the control elements in this configuration is set to receive parallel light from a location laterally displaced from the controller. It is also seen in  FIG. 4B  that the bottom slide ( 18 ) and the bottom vertical slide ( 20 ) have both been displaced from the level and equal condition shown in  FIG. 4A . In  FIG. 4B  it is seen that the bottom slide ( 18 ) has been displaced laterally, and the bottom vertical slide ( 20 ) vertically, from the original position. Thus the lower portion of the control elements in this configuration is set to reflect light to a location slightly laterally displaced and vertically displaced from the controller. 
         [0029]      FIG. 5  shows the mirror control structure ( 12 ) on the opposite side. The control element ( 13 ) is now on the left, with the mirror control elements ( 1 ) on the right. Several of the rotating axles ( 6 ) are shown, each one attached to a mirror ( 16 ) attached. Depending on the state of the control element ( 13 ), each rotating axle ( 6 ) may be rotated at a different angle. The result of this differential rotation is that the mirrors ( 16 ) are rotated differently as well, which is seen in the figure. In order to achieve the desired mirror configuration, the control element ( 13 ) is used to account for the angular position of the sun, as well as the horizontal and vertical distance to the desired focus. It is likely that the position of the focus will be the location of an energy collector, and may be set at installation, as well as whenever needed. The position of the sun will change throughout each day, and the adjustment for that parameter will undergo continual change, directed by the control element ( 13 ). 
         [0030]    Looking now at  FIG. 6 , it is noted that the mirrors ( 16 ) will be systematically rotated so that light from the sun will reflect to a common focal point. As shown in the  FIGS. 6A-6C , it is possible to program a range of (x,z) coordinates for the incident light as well as the reflected light. In the preferred embodiment, the incident light will come from the sun, very far away, and will arrive as parallel rays. This will be the standard operating mode, as shown in  FIGS. 6D-6E , though the device is capable of focusing non-parallel light as seen in the other figures. 
         [0031]    Looking at  FIG. 7 , it is further possible to create a 2-dimensional array of mirrors ( 16 ), which will allow a 3-dimensional focus adjustment and also provide a greater efficiency for energy collection or storage. The first consideration to this end is to use two mirror control structures ( 12 )—one for the x-direction and one for the y-direction.  FIG. 7A  shows a 2-dimensional array of mirrors ( 16 ) rotated about 2 axes, which may be adjusted to focus in 3 dimensions.  FIG. 6A  also shows one embodiment, to arrange the control structures ( 12 ) at right angles. This embodiment would require support structures on the remaining 2 sides of the array (not shown in the figure). 
         [0032]    Looking further at  FIG. 7 , some additional consideration is required to attach both control structures ( 12 ) to each mirror.  FIG. 7B  shows one of many solutions: a gimbal ( 21 ), which allows the mirror ( 16 ) to rotate about two axes. The gimbal ( 21 ) allows rotation using the following mechanism, which will be repeated along both axes. First, a rotating cross ( 22 ) will be attached to the rotating axle ( 6 ), as well as potentially to the rotating crosses ( 22 ) of adjacent gimbals ( 21 ), to rotate as the control structure ( 12 ) is adjusted. At opposite ends of each cross ( 22 ), a vertical beam ( 23 ) is attached to translate the rotational motion of the cross ( 22 ). The vertical shafts ( 23 ) then connect to cross-members ( 24 ) on the underside of each mirror ( 16 ), so that as the cross rotates, the mirror ( 16 ) rotates equally in the same direction. Note that the cross-members ( 24 ), capable of swiveling such that the angle between the two is not fixed, are necessary such that rotation of one axis ( 22 A) does not affect the accurate positioning of the other axis ( 22 B) as the mirror ( 16 ) is inclined. By including the described mechanism in two directions, independent rotation is made possible in two axes.  FIG. 7C  shows an example of a gimbal ( 21 ) attached to the rotating axles ( 6 ) of two perpendicular control structures ( 12 ). 
         [0033]    Looking now at  FIG. 8 , a solar collector facility is shown. The entire array ( 25 ), as described in detail above, is placed near an energy collector ( 26 ), such as a boiler or photovoltaic cell. As shown in the figure, one array ( 25 ) may be arranged around a collector. In this case, it is still considered a single array because it may be controlled by a single set of four inputs for each axis. However, it will be appreciated that alternative arrangements are possible, without changing the novelty of the invention. For example, the same arrangement of mirrors could possibly be separated into a plurality of arrays, whereby more inputs, controllers, motors, etc. are used to control the motion of the mirrors in different locations. Furthermore, the invention as shown in  FIG. 8  can be expanded within a solar collecting facility, with multiple collectors. In this expansion, each collector may be used in conjunction with a single array, or with multiple arrays as well. In this embodiment, it is quite conceivable that the controller and the power may be shared by multiple arrays.