Abstract:
A concentrating solar collector that includes a target, a plurality of mirror assemblies, and a master tracking assembly that controls the mirror assemblies is disclosed. The mirror assemblies are fixed at a plurality of distinct locations with respect to the target, each mirror assembly includes a moveable mirror and a tracking assembly. Each tracking assembly includes a target member, a sun member, and a tracking actuator. The target member is fixed relative to the target such that the target member points at the target. The sun member is moveable relative to the target such that when the sun member points to the sun, light from the sun is reflected onto the target. The master tracking assembly moves each of the sun members such that the sun members simultaneously point at the sun. The tracking actuators are mechanical devices that are coupled to the master tracking assembly by a mechanical linkage.

Description:
BACKGROUND 
     Power generation via solar systems that utilize heliostats or similar arrangements provide substantial advantages for very large scale power plants and for applications that require heat rather than electricity. A common form of heliostat utilizes a large number of planar mirrors that direct sunlight onto a common target. The heat can then be used to operate heat engines that generate electricity or the energy can be stored for later use when the sun is not shining. 
     A substantial fraction of the costs of a heliostat resides in the cost of the mirrors. There is a tradeoff between the size of a mirror and the cost of the mirror. Each mirror must be individually and continuously positioned relative to the target as the sun moves across the sky. The positioning mechanisms contribute substantially to the cost of each mirror; hence, large mirrors are preferred, since fewer positioning mechanisms are required to provide a predetermined amount of heat at the target. However, as the size of a mirror is increased, the mechanical stresses on the mirror and the positioning mechanism resulting from wind loading also increase. These stresses also result in higher structural costs for the mirror frames as well as higher costs for the positioning mechanism, which must now operate against the wind loading. As a result, heliostats are limited to mirrors that are less than 50 ft 2 . 
     SUMMARY 
     The present invention includes a concentrating solar collector that includes a target, a plurality of mirror assemblies, and a master tracking assembly that controls the mirror assemblies. The mirror assemblies are fixed at a plurality of distinct locations with respect to the target, each mirror assembly includes a moveable mirror and a tracking assembly. Each tracking assembly includes a target member, a sun member, and a tracking actuator. The target member is fixed relative to the target such that the target member points at the target. The sun member is moveable relative to the target such that when the sun member points to the sun, light from the sun is reflected onto the target. The master tracking assembly moves each of the sun members such that the sun members simultaneously point at the sun. The tracking actuators are mechanical devices that are coupled to the master tracking assembly by a mechanical linkage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates two mirrors in a heliostat. 
         FIG. 2  illustrates one embodiment of a mirror assembly according to the present invention. 
         FIGS. 3A-3C  illustrate the tracking of the sun by one mirror assembly. 
         FIG. 4  illustrates two mirror assemblies according to the present invention in a solar concentrating array. 
         FIG. 5  illustrates one embodiment of a positioning actuator according to the present invention. 
         FIG. 6  illustrates another embodiment of a positioning assembly according to the present invention. 
         FIG. 7  illustrates another embodiment of a mirror assembly according to the present invention. 
         FIG. 8  illustrates a mirror assembly according to another embodiment of the present invention. 
         FIG. 9  illustrates another embodiment of a solar concentrating system according to the present invention. 
         FIG. 10  illustrates another embodiment of a mirror assembly according to the present invention. 
         FIG. 11  illustrates yet another embodiment of a mirror assembly according to the present invention. 
         FIG. 12  illustrates another embodiment of a mirror assembly according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention is based on the observation that the cost of a heliostat mirror could be substantially reduced if a common controller could be used to position a plurality of mirrors without the need for electromechanical actuators that can accurately position each mirror individually. The manner in which the present invention provides its advantages can be more easily understood with reference to  FIG. 1 , which illustrates two mirrors in a heliostat. Mirrors  23  and  24  are positioned to image the sun shown at  22  onto target  21 . Each mirror includes an actuator  25  that positions the mirror such that the light striking that mirror from sun  22  is reflected onto target  21 . The actuator is connected to a structure  26  that is anchored in the ground  27 . 
     To simplify the following discussion, it will be assumed that the mirrors are planar. However, the present invention could also be utilized with other types of mirrors. Consider the normal to the mirror&#39;s surface. The mirror will be correctly positioned when the target, sun, and mirror normal lie in the same plane and when the angle between the sun and the normal to the mirror is equal to the angle between the target and the normal. It should be noted that this plane changes as the sun moves across the sky, and hence, actuator  25  must position the mirror in two dimensions. 
     Refer now to  FIG. 2 , which illustrates one embodiment of a mirror assembly according to the present invention. Mirror assembly  30  includes a mirror  31  and a positioning actuator  34  having a mounting surface  32  on which mirror  31  is attached. Positioning actuator  34  includes a fixed member  33  that is fixed relative to the earth and points in the direction of the target. Member  33  will be referred to as the target member in the following discussion. Mirror assembly also includes a member  36  that is moveable with respect to member  33 . This member will be referred to as the sun member in the following discussion as this member points to the sun. Positioning actuator  34  is constructed such that when sun member  36  is pointing toward the sun, the angle between sun member  36  and the normal to the surface of mirror  31 , which is shown at  35 , is equal to the angle between target member  33  and normal  35 . Hence, if a mechanism is provided for moving sun member  36  so that sun member  36  remains pointed at the sun as the sun moves during the day, mirror  31  will direct the sunlight onto the target. 
     Refer now to  FIGS. 3A-3C , which illustrate the tracking of the sun by one mirror assembly.  FIGS. 3A-3B  illustrate configurations in which the sun  22  is on one side of target  21 .  FIG. 3C  illustrates a configuration in which the sun  22  is on the other side of target  21 . It should be noted that the sun member must move past the target member as the sun moves from one side of the target to the other. This requirement poses structural challenges with respect to the position actuator. These challenges will be discussed in more detail below with respect to various embodiments of the position actuator. 
     Refer now to  FIG. 4 , which illustrates two mirror assemblies according to the present invention in a solar concentrating array. Mirror assembly  41  includes a target member  42  that points toward target  21  and is fixed in space. Mirror assembly  41  also includes a sun member  43  and a position actuator  44 . Sun member  43  is connected to a linkage  51  which moves in a manner that causes sun member  43  to point to the sun, which is omitted from this drawing for reasons discussed below. Similarly, mirror assembly  45  includes a target member  46  that points toward target  21  and is fixed in space. Mirror assembly  45  also includes a sun member  47  and a position actuator  48 . Sun member  43  is also connected to linkage  51 . Linkage  51  is controlled from sun tracker  52  that moves linkage  51  in a manner that causes all of the sun members attached to linkage  51  to move together and point toward the sun. 
     It should be noted that the position of the sun in the figures discussed above is greatly exaggerated relative to the position of the target. In reality, the sun is so far from the mirror assemblies and target, that the radiation from the sun over the entire area of the concentrating array can be treated as a collimated light beam in which all of the light rays are parallel with respect to one another. As a result, the sun member in each mirror assembly points in the same direction as the sun member in the other mirror assemblies. Hence, the sun members can be rigidly connected together. As a result, the only electromechanical actuator is that in sun tracker  52  that moves linkage  51 . 
     Refer now to  FIG. 5 , which illustrates one embodiment of a positioning actuator according to the present invention. Mirror assembly  60  includes a mirror  61  and a position actuator  70  that adjusts the position of mirror  61 . Position actuator  70  is constructed from the five members shown at  64 - 68  that are joined by the four joints shown at  71 - 74 . The five members lie in a plane that will be referred to as the actuator plane in the following discussion. Members  64 - 67  being connected to member  68  at one end of each of member. Joints  71 - 72  are hinge joints. Joint  74  includes two hinge points attached to a sliding joint that can move along member  68 . Mirror  61  is mounted such that the normal to mirror  61  shown at  69  is parallel to the axis of member  68 . Member  66  is the same length as member  67 . Similarly, members  64  and  65  have the same length, and angles  75  and  76  are equal to one another. 
     In the following discussion the structure defined by members  64 - 67  and joints  71 - 74  will be defined to be a “kite” structure. Members  64 - 67  will be referred to as the first, second, third, and fourth members of the kite structure, respectively. In a kite structure, members  66  and  67  have the same length, and members  64  and  65  have the same length. In this embodiment, members  64 - 67  remain in the same plane. Joints  73  and  74  are constrained to move toward or away from one another along a line, referred to as the center line of the kite structure in the following discussion. In the case of the kite structure shown in  FIG. 5 , this constraint is provided by member  68  which defines the kite center line. However, as will be explained in more detail, other arrangements for confining the motion can be utilized. 
     Member  64  includes target member  62 , which is an extension of member  64 . Target member  62  is fixed to a location that is determined by the respective positions of the target and the mirror assembly in the mirror array. Member  65  includes sun member  63 , which is an extension of member  65 . Sun member  63  is attached to the linkage that moves to maintain the position of the mirror such that the sun&#39;s light is reflected onto the target. 
     The target and the sun must also lie in the actuator plane of position actuator  70 . Hence, some mechanism is needed to rotate the plane of position actuator  70  as the sun moves across the sky if position actuator  70  is to provide ideal tracking during the entire day. Refer now to  FIG. 6 , which illustrates another embodiment of a positioning assembly according to the present invention. To provide the extra degree of freedom needed to track the sun, target member  62  is mounted in a rotational coupling  81  which allows the actuator plane to be rotated while maintaining the direction in which target member  62  points so that target member  62  remains pointed at the target while the actuator plane is rotated. The rotation of target member  62  is set such that the plane defined by the target, the sun, and target member  62  also includes member  68 . 
     The above-described embodiments of a positioning actuator, however, have limitations that render the embodiments less than ideal. First, the range of sun positions that can be accommodated by moving the sun pointer is limited by the allowable range of motion of sun member  63 . To attain all of the desired positions, angle  82  must be varied from 0 to 90 degrees. However, the finite dimensions of the members and limitations of the joints will still not allow angle  82  to attain all angles between 0 and 90 degrees, particularly for angles near zero. Hence, there will be some positions in which the tracking mechanism will not provide the desired reflections of the sun. The extent to which these limitations provide problems will depend on the details of the mirror array. In addition, rotational coupling  81  increases the cost of the positioning actuator. Finally, the linkages to the mechanism for moving the sun pointers in unison present challenges if the sun pointers are under the mirror. 
     Refer now to  FIG. 7 , which illustrates another embodiment of a mirror assembly  90  according to the present invention. This embodiment can be viewed as a kite structure consisting of members  95 ,  96 ,  103 , and  104 . The motion of joints  94  and  98  is constrained by slot  101  in mirror  91 . In this case, slot  101  is the kite center line. In this embodiment, mirror  91  also acts as the motion constraining member of the kite structure. The actuator mechanism includes member  92  that functions as the sun member and member  93  which functions as the target member. Member  93  is fixed in a rotating adapter  102  in a manner analogous to that discussed above. Members  95  and  96  are analogous to members  66  and  67  discussed above with reference to  FIG. 5 . Members  95  and  96  connect to a sliding joint  94  which moves in slot  101  as the position of the sun member is changed. Joints  97 ,  98 , and  99  are analogous to joints  71 - 73  shown in  FIG. 5 . Since the sun member is now above the mirror, the problems discussed above with respect to connecting the sun member to a common linkage are substantially reduced. 
     As noted above, embodiments in which the target member does not need to rotate would be advantageous. Refer now to  FIG. 8 , which illustrates a mirror assembly  110  according to another embodiment of the present invention. The position actuator in mirror assembly  110  can be viewed as consisting of two kite structures that share two members and a mirror  111 . The first kite structure includes members  120 ,  116 ,  117 , and  119 . The second kite structure includes members  114 ,  118 ,  117 , and  119 . The slideable joint of the first kite structure is shown at  126  and moves in slot  127  that is cut in mirror  111 . The slideable joint of the second kite structure is shown at  125  and moves in slot  121 . Slots  121  and  127  are the center lines of the two kite structures, respectively. In this case, the mirror lies in the plane defined by the center lines of the two kite structures. An extension  113  of member  117  is fixed so that member  117  points at the target. An extension  112  of member  119  is attached to the positioning mechanism, which moves member  119  such that member  119  points at the sun. It should be noted that member  117  does not rotate while the sun member tracks the sun. It should be noted that joints  122 ,  123 , and  124  are hinge joints. Since hinge joints are relatively inexpensive, the cost of mirror assembly  110  can be significantly less than mirror assembly  90  discussed above. 
     As noted above, the sun members move in parallel such that each sun member points towards the sun. Refer now to  FIG. 9 , which illustrates another embodiment of a solar concentrating system according to the present invention. Concentrator  200  utilizes mirror assemblies in which the sun member is on the same side of the mirror as the sun. Exemplary mirror assemblies are shown at  201  and  202 . The sun members corresponding to these mirror assemblies are shown at  205  and  206 , respectively. The sun members are attached to a mechanical linkage  211  via joints  213  and  214 . Sun tracker  210  moves the sun members together so that each of the sun members points towards the sun. 
     In this embodiment, sun tracker  210  tracks the sun continuously using solar position detectors  215  and  216 . Such solar position detectors are well known in the art and hence will not be described in detail here. It is sufficient to note that solar position detectors typically include a lens for imaging the sun onto a detector in which the position of the sun on the detector can be determined. Sun tracker  210  moves mechanical linkage  211  such that the position of the sun on the detector remains constant. In principle, a single solar position detector would be sufficient to determine the position of the sun. However, there will be positions of the sun in which target  21  eclipses the sun as seen from any single solar position detector. Hence a second solar position detector is provided at a location such that at least one of the solar trackers can view the sun at all times. 
     Additional solar position detectors also provide a means for detecting errors in the alignment of the arrays. If a solar position detector is provided for each mirror assembly, and the output of that solar position detector is provided to sun tracker  210 , any significant discrepancy in the outputs of the solar position detectors can be used to identify mirror assemblies that are not functioning properly. 
     In the embodiment shown in  FIG. 9 , the solar position detectors are used to provide position information to sun tracker  210  so that sun tracker  210  can continuously follow the sun&#39;s position in the sky. However, it should be noted that once the mirror assemblies are fixed relative to target  21  such that the target members are properly aligned, the position of the sun can then be tabulated and stored in sun tracker  210 . In this case, the issues related to the target eclipsing the sun are no longer relevant. In addition, the tracking system will continue to operate properly even if the sun is obscured by clouds during part of the day. However, it still may be useful to include the solar position trackers on a plurality of mirror assemblies to provide diagnostic information with respect to the functioning sun tracker  210 , and the linkages connecting the mirror assemblies. 
     In the above-described embodiments, the mirror was connected to the center member of the kite structure such that the center line of the kite structure(s) is in the plane of the mirror. However, other arrangements could be utilized. Refer now to  FIG. 10 , which illustrates another embodiment of a mirror assembly according to the present invention. Mirror assembly  300  includes a mirror  301  that is connected to a target member  304  of a kite structure  302  by a hinge  311 . Mirror  301  is also connected to the center member of kite structure  302  by a link  306  that is connected to a second location on mirror  301  by a hinge  312 . It should be noted that target members  304 - 307  form a parallelogram that moves when sun member  303  moves to track the sun. To provide two-dimensional tracking, the kite structure and parallelogram are rotated about the axis defined by target member  304 . In this case, the plane of the mirror is parallel to the center line of the kite structure but does not include the center line of the kite structure. 
     Mirror assembly  300  provides two improvements over mirror assembly  90  discussed above. First, mirror assembly  300  does not require a slot in the mirror. This reduces the cost of the mirror and strengthens the mirror. Second, the sun pointer and associated linkages are on the opposite side of the mirror from the sun, and hence, optical losses from the shadows of these elements are avoided. 
     Refer now to  FIG. 11 , which illustrates another embodiment of a mirror assembly according to the present invention. Mirror assembly  400  also utilizes an arrangement in which the mirror is mounted on an extension of the target pointer. Mirror assembly  400  is similar to mirror assembly  100  discussed above with reference to  FIG. 8  in that mirror assembly  400  utilizes two kite structures so that two dimensional tracking can be provided without requiring that the target member rotate about an axis through the target member. 
     Referring to  FIG. 11 , mirror assembly  400  includes a first kite structure having arms  414 ,  419 ,  417 , and  418 . Arms  417  and  419  are connected by hinge  424 . The second kite structure shares arms  417  and  419 . Arms  416 , and  420 , complete the second kite structure. Arms  416  and  420  are connected by a hinge  426  that moves in a slot in disk  411 . Similarly, arms  414  and  418  are connected by a hinge that moves in another slot in disk  411 . In this embodiment, the target pointer is arm  419  that is hingedly connected to the mirror  461  by an extension  412  and hinge  462 . Two additional arms shown at  471  and  451  are connected by joints  472  and  463  such that the plane of mirror  461  remains parallel to the plane defined by the center lines of the two kite structures as the mirror assembly is moved to track the sun. 
     The mirror connections operate in a manner analogous to that shown in  FIG. 10  and form a similar parallelogram. The plane of disk  411  is parallel to the plane of mirror  461 . The distance between hinges  462  and  463  is the same as the distance between hinges  452  and  424 . Arm  451  has a length equal to the distance between hinge  424  and hinge  462 . 
     The sun member in this embodiment is arm  417 , which is moved by moving extension  413  in a manner analogous to that discussed above. The linkages for moving arm  417  can be positioned on the opposite side of the mirror from the sun, and hence, shading of the mirror by the mechanism that moves extension  413  as well as shading of the mirror by extension  413  itself is avoided. 
     The above-described embodiments of the invention utilize planar mirrors; however, other forms of mirrors could be advantageously utilized. Planar mirrors are less expensive to manufacture; however, such mirrors require a target that is at least as large as the largest planar mirror in the array. Since the target is required to operate at very high temperatures in many applications, the cost savings achieved by utilizing planar mirrors can be substantially reduced by the increased costs of a large target. 
     Arrays based on parabolic mirrors having long focal lengths have the advantage of requiring substantially smaller targets. The present invention facilitates designs in which a large number of mirrors can be utilized. The individual mirrors can be mass-produced using conventional plastic molding techniques, and hence, the increased costs of the parabolic mirrors relative to planar mirrors is significantly reduced. 
     The manner in which the present invention operates with non-planar mirrors can be more easily understood in terms of the relationship of the axis of the mirror relative to the center line of the kite structures in the tracking assemblies. For the purpose of this discussion, the axis of a mirror is defined to be the normal to the surface of the mirror at the center of the mirror. 
     Refer now to  FIG. 12 , which illustrates an embodiment of a mirror assembly according to the present invention that utilizes a parabolic mirror. For the purpose of this discussion, a parabolic mirror is defined to be a non-planar mirror having a shape determined by a parabola that is rotated about the axis of the mirror. To simplify the discussion, those elements of mirror assembly  500  that serve functions analogous to elements of mirror assembly  60  shown in  FIG. 5  have been given the same numeric designations and will not be discussed in detail here. Mirror assembly  500  utilizes a parabolic mirror  510  whose axis is shown at  501 . Mirror axis  501  is perpendicular to a tangent  503  to the mirror surface at the center of the mirror. The center line of the kite structure is shown at  69 . It will be apparent from the drawing that the axis of the mirror is parallel to the center line of the kite structure, or perpendicular to the tangent to the mirror surface at the center of the mirror, in this embodiment and displaced from the plane defined by the center lines of the kite structures. 
     In embodiments in which the two kite structures are utilized such as the embodiments shown in  FIGS. 8 and 11 , the axis of the mirror is perpendicular to the plane defined by the center lines of the two kite structures. It should be noted the axis of a planar mirror can also be defined in this manner and the embodiments shown with planar mirrors also satisfy similar constraints. 
     The above-described embodiments of the present invention have been provided to illustrate various aspects of the invention. However, it is to be understood that different aspects of the present invention that are shown in different specific embodiments can be combined to provide other embodiments of the present invention. In addition, various modifications to the present invention will become apparent from the foregoing description and accompanying drawings. Accordingly, the present invention is to be limited solely by the scope of the following claims.