Abstract:
A solar tracker device continuously captures sun rays to be redirected towards a target The device includes a mirror defining a center point and fixedly mounted to a heliostat, an imaging device having an optical axis passing through the mirror center point, an electronic board, and a partly transparent dome extending between the imaging device and the target When sun rays penetrate said dome, the mirror reflects rays toward the dome and a portion are reflected back by the dome to the imaging device to form an image of the mirror center point An image of the fixed target is formed on the imaging device through the dome and defines an image of the target Whenever the images of the mirror and the target center are not in coincidence, the electronic board is activated to rotate the heliostat reflecting surface toward an orientation for which coincidence is obtained

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a device and a system for automatic electronic sun tracking, and more particularly, to a device and a system for heliostat control. 
       BACKGROUND OF THE INVENTION 
       [0002]    Heliostats are typically used to orient mirrors in systems to convert solar energy into useful energy, for example, electrical power, by reflecting sunlight onto a fixed target. Their orientation is adjusted by means of a mechanical system allowing the rotation of the mirror generally around two axes of rotation. The rotation is typically activated by motors and gears. These motors are controlled by solar trackers that calculate the required position of the heliostat in order to provide adjustment information needed by the system to assure constant reflection of the sunlight onto the fixed target while the relative position of the sun changes during the course of a day. 
         [0003]    Whenever a number of such heliostats are used to reflect the sunlight onto the same target, then a considerable amount of energy is concentrated onto a relative small area of the target, and this energy can be efficiently used to provide usable energy. For example, a steam engine coupled to a generator or an array of photocells can be used to produce electrical energy. The high concentration of energy on the target allows the higher efficiency of energy conversion devices, thus reducing the cost of the system. However, this cost reduction is compensated for by the cost of the heliostats. Thus it is advantageous to reduce the cost of heliostats. 
         [0004]    The cost of the motors, gears and motion controllers constitutes a significant part of the total heliostat cost. Motors should be controllable, like adding steppers, but this requires an associated costly controller. Gears should be without backlash and linear, i.e., should provide a linear relation between the rotation of a particular motor driveshaft and the amount and angle of rotation of an associated heliostat around a corresponding rotation axis. 
         [0005]    Many prior art heliostat systems using stepper motors need adjustments during set-up, since their rotation angle is commanded incrementally, and not by absolute position. This means that each heliostat must be provided with an individual reference point mechanism. This requirement generally entails additional expense, especially, additional calibration costs at installation. 
         [0006]    Furthermore, this calibration often needs to be re-done periodically in order to avoid an eventual accumulation of small errors. Such small errors can be due to command noise, imprecision of gears, slight movement of the heliostat base, or some rare failure of the stepper motor to execute a commanded increment. Such a periodic re-calibration also adds operating costs to a solar plant. 
         [0007]    Thus there is a need for a solar tracker device and system that is accurate, simple to operate, needs no adjustment, and that is cost-effective. 
       SUMMARY OF THE INVENTION 
       [0008]    Accordingly, it is a broad object of the present invention to overcome the above disadvantages of the prior art and to provide a solar tracker which is able to control a simpler heliostat. The motors do not need to be position-controlled and can be used in simple forward, stop and reverse command modes. The gear can be of any type, and does not need to be linear. The solar tracker device of the present invention does not need precise adjustment, and no periodical re-adjustment is required. 
         [0009]    It is convenient to define a center point of the fixed target, which is a virtual point in space toward which the heliostat is oriented to reflect the sun rays so that a maximum of them will hit the fixed target. 
         [0010]    It is an object of the present invention to provide a solar tracker whose accuracy is substantially not affected by changes in the position of the heliostat rotation axes. 
         [0011]    It is another object of the present invention to provide a device that allows for the manufacture of a lower-cost heliostat system. 
         [0012]    According to an aspect of the present invention, there is provided a solar tracker device associated to a heliostat having a reflecting surface to reflect sun rays onto a fixed target, the solar tracker device comprising: 
         [0013]    a mirror with a pre-defined shape fixedly mounted parallel to the heliostat reflecting surface, the pre-defined shape defining a mirror center point; 
         [0014]    an imaging device having an optical axis passing through the mirror center point; and 
         [0015]    a dome extending between the imaging device and the mirror on one side and the sun and the fixed target on a second side, 
         [0016]    such that when sun rays penetrate the dome, the mirror substantially reflects back these sun rays toward the dome and a portion of these sun rays are reflected back by the dome to the imaging device forming an image of the mirror, the shape of this image defining a) an image of the mirror center point; and an image of the fixed target is formed on the imaging device through the partly transparent dome, defining b) an image of the target center point, 
         [0017]    such that the coincidence of a) the image of the mirror center point and b) the image of the target center point corresponds to the desired orientation of the heliostat reflecting surface in order to reflect the sun rays onto the fixed target. 
         [0018]    According to another aspect of the present invention, there is provided a solar tracker system for controlling the orientation of a heliostat, the system comprising: 
         [0019]    a heliostat having a reflective surface, the reflective surface being movable along at least two axes of rotation to define an orientation with respect to the sun, and 
         [0020]    a tracking device having a mirror mounted parallel to said reflective surface, an imaging device, and a diffracting element positioned between the imaging device and mirror on one side and the sun and a fixed target on a second side, 
         [0021]    wherein images formed on the imaging device from reflections off the diffracting element and from light emanating from the fixed target are used by the tracking device to control the orientation of the reflective surface of the heliostat. 
         [0022]    According to yet another aspect of the present invention, there is provided a method for positioning a heliostat for tracking the sun, the method comprising: 
         [0023]    obtaining an image of a mirror reflecting sunlight at a fixed target, 
         [0024]    calculating a position of a center of the image of the mirror; 
         [0025]    obtaining an image of the fixed target; 
         [0026]    calculating a position of a center of the image of the fixed target; 
         [0027]    calculating a length and orientation of a vector joining the mirror center image to the fixed target center image; 
         [0028]    calculating an amount of rotation and a direction of at least one motor to change the orientation of the mirror so that the length of the vector is reduced; and 
         [0029]    driving the at least one motor in the calculated direction and by the calculated amount of rotation; 
         [0030]    wherein the length of the vector is substantially reduced and the images of the mirror center and fixed target center are substantially coincident. 
         [0031]    The solar tracker device of the present invention is mounted on or connected to an electronic board which comprises a frame grabber to capture images from the imaging device. There is also provided a central processing unit for calculating the relative positions of the image of the mirror center point and the image of the target center point from captured images of both the mirror and the fixed target in an imaging area of the imaging device. In a preferred embodiment of the present invention, there are two output units for activating motors to orient the heliostat reflecting surface in two axes in relation to the continuously changing position of the sun. 
         [0032]    The frame grabber captures both the fixed target image and the mirror image, the central processing unit calculates the relative positions of the images of the target center and the image of the mirror center point for coincidence and directs the at least a pair of output units to periodically activate the output units to orient the heliostat reflecting surface so as to reflect the sun rays onto the fixed target. 
         [0033]    The solar tracker device of the present invention includes an imaging system and a mirror surface parallel to the reflective heliostat surface. Also included is a transparent dome covering both the imaging system and the mirror and a central processing unit (CPU). The covering dome, in a preferred embodiment of the present invention, is mainly made of transparent material having a small concentration of diffracting particles. Sun rays entering the dome are reflected by the mirror back to the dome surface, and a small amount of rays are diffracted. Some of the diffracted rays reach the imaging system, forming an image of the mirror surface. 
         [0034]    The CPU executes an image processing algorithm and then identifies a) the image of the fixed target seen through the dome, and b) the reflected image of the mirror. The CPU image-processing algorithm also calculates the position on the imaging area of the imaging device of a pre-defined reference point of the image of the fixed target (hereinafter called “target center point”) and also the position on the imaging area of the imaging device of a pre-defined reference point of the mirror image (hereinafter called “mirror center point”). These reference points of the target and the mirror are pre-defined so that the coincidence of their images on the imaging device corresponds to the desired orientation of the heliostat. 
         [0035]    The CPU then activates the motor output units according to the relative positions of the images of a) the fixed target center and b) the mirror center point in the appropriate direction in order to reach a coincidence between them. Whenever coincidence is obtained, the motor output units are set to stop the motors, and the heliostat is set in the desired position where sun rays are reflected onto the fixed target. 
         [0036]    In a preferred embodiment of the present invention, the mirror has a ring shape, and the center of the ring is the mirror center point and the target reference point in the image of the fixed target is the target center point. 
         [0037]    It is an advantage that the accuracy of the solar tracker is substantially not affected by changes in the position of the heliostat axes. For example, if the heliostat is mounted on a base anchored to the ground, movements of the ground due to heat or humidity and rain will have little or no affect on the tracking accuracy. As mentioned heretofore, many conventional heliostats must be adjusted periodically in order to compensate for small deviations, whereas the present invention dispenses with any need for adjustments of this type. Furthermore, the foundation needed to fix a heliostat can be simplified since stability is not an essential requirement, thus foundation costs can be substantially reduced. 
         [0038]    Another advantage is that the solar tracker of the present invention does not need tracking calibration. By way of example, before installation, an image of the fixed target is loaded into the CPU memory, and this image is used by a pattern recognition algorithm of the CPU. This eliminates the costly calibration needed with conventional heliostats. 
         [0039]    A further advantage is that heliostat motors do not need to be position-controlled. They only need Forward, Reverse and Standstill activation. This eliminates the need for expensive motor motion controllers, and simple output units, like bidirectional switches, can be used with DC motors to connect them to a DC power supply in the appropriate direction. For example, it allows the use of a small photovoltaic panel fixed to the heliostat in order to provide power to the solar tracker and the motors. In such a case, no electrical cables are needed for power or control of the heliostat, resulting in further, significant cost reduction of the power plant. 
         [0040]    Yet another advantage of the present invention is that it is not sensitive to dust, scratches and other defects on the dome. Heliostats are generally exposed to dust, wind and rain, but in a preferred embodiment of the present invention, sensitivity and precision in orientation are not affected by these factors. This is due to the fact that the image of the mirror is returned by the diffracting particles of the dome, and the dust deposed on the dome would simply play the same role as these diffracting particles. 
         [0041]    Still another advantage of the present invention is that it is not sensitive to the accuracy of the image sensing device. This is due to the fact that both images of mirror and target are formed, whenever coincidence is obtained, on the same area of the imaging device, so that optical distortions and/or image sensor inaccuracy do not generate offset errors. 
         [0042]    These, additional, and/or other aspects and/or advantages of the present invention are: set forth in the detailed description which follows; possibly inferable from the detailed description; and/or learnable by practice of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0043]    For a better understanding of the invention with regard to the embodiments thereof, reference is made to the accompanying drawings, in which like numerals designate corresponding elements or sections throughout, and in which: 
           [0044]      FIG. 1  is a general view of a preferred embodiment of the system of the present invention; 
           [0045]      FIG. 2  is a block diagram showing the main electronic components of the solar tracker in accordance with the principles of a preferred embodiment of the present invention; 
           [0046]      FIG. 3  schematically shows the shape of the images formed on the imaging device for a given position of the sun; 
           [0047]      FIG. 4  is a block diagram of a method of positioning a heliostat, consistent with an embodiment of the present invention; and 
           [0048]      FIG. 5  shows an example of another embodiment of a mirror shape usable in a solar tracker in accordance with the principles of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0049]      FIG. 1  is a general view of a preferred embodiment of the system of the present invention. Referring now to  FIG. 1 , there is depicted an embodiment of a solar tracker  10  mounted on a reflective surface P of a heliostat. In this figure, for clarity of the drawing, the reflective surface P is shown only slightly larger than the solar tracker, however in a real system it will be considerably larger. The solar tracker  10  includes a planar ring-shaped mirror  16  fixedly positioned parallel to the reflecting surface P of a heliostat, an imaging device  30 , and a diffracting element or dome  24  covering the imaging device and mirror. Imaging device  30  is mounted on an electronic board  20  (see also  FIG. 2 ). The optical axis of the imaging device is set perpendicular to the mirror surface plane and intersects that plane at the mirror center point coincident with rotation axis Z. Also shown is a fixed target  12  having a reference center point  12   a.    
         [0050]    In  FIG. 1 , the reflective surface P of the heliostat is schematically shown being rotatable around two rotation axes X and Z utilizing two motors  26 ,  28 , respectively. Rotation axis Z is a primary axis and axis X is secondary, i.e., when a heliostat is rotated around axis Z, axis X is also rotated around axis Z. In the particular arrangement of  FIG. 1 , axis Z is substantially vertical, and axis X is substantially horizontal. It will be understood that the heliostat may have different arrangements of rotation axes. In some embodiments, an arrangement of three axes of rotation controlled by three motors may be used. 
         [0051]    In operation, by way of example, a sun ray  14  from sun S is shown going through dome  24  and hitting mirror  16 . The reflected ray  14   a  hits dome  24  at an intersection point  22  and part of its energy continues along the same path outside dome  24 , as indicated by ray  14   b.    
         [0052]    In another aspect, at the intersection point  22  of the reflected ray  14   a  with dome  24 , a small amount of the optical energy of the ray is diffracted by the diffracting particles included in the dome material, generating light rays  18   a  to  18   e , in all directions (shown by short arrows). Some of these rays  18   a  to  18   e , such as ray  18  reach imaging device  30 . A plurality of rays, like ray  18 , form on the imaging device  30  an image of the area of dome  24  which is illuminated by rays reflected by mirror  16 . Since rays emitted by the sun S are approximately parallel, the shape of the illuminated area of dome  24  forms a projection of the shape of mirror  16  in the direction of the reflected rays. Whenever focal distance of imaging device  30  is small compared to the radius of dome  24 , then it can be shown that the image on image device  30  has the form of a projection of the mirror shape on a plane parallel to the focal plane of imaging device  30 . In this way, a first image of mirror  16  is captured by imaging device  30 . 
         [0053]    In another aspect, fixed target  12  and surrounding scene is illuminated by ambient daylight and emits rays that traverse dome  24  and reach imaging device  30  to form an image of fixed target  12  on imaging device  30  captured through the partly transparent dome  24 . 
         [0054]    In a preferred embodiment of the present invention, dome  24  is made to diffract part of the rays hitting it by means of small particles included in the dome material, such as, for example, titanium dioxide particles included in a polymer plastic. According to some embodiments of the present invention, the material of dome  24  includes a small concentration of such diffracting particles. The concentration of these particles is set sufficiently low in order to make the dome  24  partly transparent. It will be understood that other ways of making dome  24  partly transparent and partly diffracting can be used. For example, a mesh grid pattern could be applied on the dome. In another example, the dome could be a grid made of woven metal wires. 
         [0055]    In a preferred embodiment of the present invention, dome  24  is shown having a hemispherical shape. In will be understood that other shapes can be used, as long as dome  24  extends between imaging device  30  on one side, and the sun S and fixed target  12  on the other side. 
         [0056]    It will be understood that mirror  16  can be formed with various contour shapes as long as the shape provides a way of calculating a center point. In the embodiment of the present invention illustrated in  FIG. 1 , the mirror surface is planar, although a non-planar surface could also be used. For example, mirror  16  could be concave, resulting in light concentration. In this case, the dome area illuminated by rays reflected by such a concave mirror would be smaller than the area of the mirror itself. This would result in higher luminosity of this illuminated area, and have an added advantage of increasing the contrast of the mirror image on imaging device  30  and thus increasing the precision of the pattern recognition algorithms used by the system of the present invention. 
         [0057]      FIG. 2  is a block diagram showing the main electronic components of the solar tracker in accordance with the principles of a preferred embodiment of the present invention. 
         [0058]    Referring now to  FIG. 2  in detail, there is shown the main electronic components of solar tracker  10  comprising: a frame grabber  32  used to store in memory images captured by image sensor  30 , a central processing unit (CPU)  34 , and two output units  36 ,  38  which are controlled by CPU  34  in order to activate the motors  26 ,  28  respectively to operate solar tracker  10 . Standard cables (shown by dark lines), as are known to those skilled in the art, are used to connect image sensor  30  to frame grabber  32  and to connect output units  36 , 38  with their respective motors  26 ,  28 . 
         [0059]    In  FIG. 2 , the two output units  36  and  38  are shown as simple relay switches, thus illustrating the low cost of implementation of the present invention. As mentioned hereinbefore, most known heliostat tracking controllers in the prior art require a costly position controller to drive the motors. 
         [0060]    Both images of mirror  16  and fixed target  12 , formed on imaging device  30  (see  FIG. 1 ), are captured by frame grabber  32 . CPU  34  then executes an image processing algorithm, as is known to those skilled in the art, which determines the positions of a) an image of fixed target  12 , and b) an image of mirror  16  (see  FIG. 1 ). According to their relative position, a CPU program (described hereinafter) calculates the direction in which the motors should be activated in order to reduce the distance between images of mirror center point and target center point. The two output units  36  and  38  are then activated in order to run motors  26  and  28  in the calculated direction. This program is run cyclically, so that the distance (D) between images of target center point  48  and mirror center point  40  (see  FIG. 3 ) is progressively reduced until coincidence is obtained. Whenever coincidence is obtained, output units  36  and  38  are set by the CPU program to stop motors  26 ,  28 . As mentioned hereinbefore, the coincidence of these two center point images corresponds to the desired position in which sun rays  14  (see  FIG. 1 ) are reflected by heliostat reflecting surface P so as to be redirected toward fixed target  12  to illuminate its surface. 
         [0061]      FIG. 3  schematically shows the shape of the images formed on the imaging device for a given position of the sun. 
         [0062]    An imaging area  50  of imaging device  30  has two axes XX and YY. In a preferred embodiment of the present invention, axis XX is set parallel to the X axis of rotation of the heliostat. It will be understood that this particular orientation of axis XX has been chosen in order to simplify the description of the orientation algorithm, but any orientation could be chosen. 
         [0063]    Image of the target center point  48  is not necessarily a real image, but it may be a virtual point whose coordinates are calculated by the image processing algorithm applied to the data captured from fixed target image  46 . Hereinafter, the term “image of the target center point” refers to such a virtual point. 
         [0064]    So too, image of the mirror center point  40  is not a real image, since mirror  16  (see  FIG. 1 ) has a ring shape and its center is not a reflecting surface, but it is a virtual point whose coordinates are calculated by the image processing algorithm applied to the data captured from mirror image  44 . Hereinafter, the term “image of the mirror center point” refers to such a virtual point. 
         [0065]    In the example of the images shown in  FIG. 3 , the shape of mirror image  44 , is depicted as an elliptical ring. The position of imaged mirror center point  40  of this elliptical image is calculated by CPU  34  (see  FIG. 2 ). Fixed target image  46  is also shown. The position of the imaged target center point  48  is also calculated by CPU  34 . In the preferred embodiment of the present invention illustrated in  FIG. 3 , the symmetrical features of the mirror shape simplify the pattern recognition algorithm executed by CPU  34 . 
         [0066]    An image processing algorithm processes the data from mirror image  44  and calculates the position of the imaged mirror center point  40 . Another image processing algorithm processes the data from fixed target image  46  and calculates the position of the imaged target center point  48 . A distance “d” (shown in  FIG. 3 ) can be defined as the distance between the calculated center points  40  and  48 . These calculations are done using pattern recognition and machine vision algorithms as are known to those skilled in the art. 
         [0067]    The calculated positions coordinates of the image of the mirror center point and the image of the target center point are processed to define the movement direction of the motors required to reduce the distance “d” between these two reference points. 
         [0068]    It will be understood that various algorithms as are known to those skilled in the art can be defined to select the appropriate directions of rotation needed to move the heliostat toward the desired orientation. 
         [0069]    The program run by the CPU (see  FIGS. 2 and 4 ) thus selects Forward, Backward, or Standstill commands for both motors  26 ,  28  (see  FIG. 2 ) and activates or de-activates them accordingly by regulating the respective output units  36 ,  38  (see  FIG. 2 ). The CPU program algorithm selects the proper activation of motors  26 ,  28  in order to reduce the distance between a) image of the mirror center point  40  and b) image of the target center point  48 . 
         [0070]    The CPU program runs in a cyclical way, so that the distance between the imaged mirror center point  40  and imaged target center point  48  progressively reduces until it is minimized, at or close to zero, and the two points substantially coincide. Whenever the sun S position in the sky slowly changes, the coincidence condition is lost, and the CPU program immediately communicates a correcting action to output units  36 ,  38  (see  FIG. 2 ), which activate their respective motors  26 ,  28  and thus restores this coincidence. 
         [0071]    As explained hereinbefore, coincidence corresponds to the desired orientation of the heliostat surface P. Solar tracker  10 , in a preferred embodiment of the present invention, is thus capable of continuous control of the heliostat motors  26 ,  28  (see  FIG. 2 ) in order to reflect the sun rays  14  (see  FIG. 1 ) onto fixed target  12  (see  FIG. 1 ). 
         [0072]      FIG. 4  is a block diagram that illustrates the method of positioning a heliostat, according to some embodiments of the present invention. The method can be implemented by a computer program such as that discussed above. Upon starting at module  100 , in module  102  the system locates the position of image  44  of mirror  12 . In module  104  the position of mirror center point image  40  is calculated. Subsequently, in module  106  fixed target image  46  is determined, and in module  108  fixed target center point image  48  is calculated. 
         [0073]    A vector “D” having distance “d” and an orientation in space is then determined in module  110 . In some embodiments, the length and orientation of the vector is that which connects mirror center point image  40  to fixed target center point image  48 . In module  112  the direction and extent of motor rotation that would have the effect of decreasing the length of vector D is calculated. This information is then used in module  114  to activate output units  36  and  38 , to cause the motors to rotate for an appropriate time and in an appropriate direction to obtain coincidence of center points  40  and  48 . Control then returns to module  102 , to repeat the process so that accurate tracking is maintained as the position of the sun changes. 
         [0074]    It is particularly advantageous that the tracking accuracy is not affected by the dome shape precision. In the embodiment of the present invention shown in  FIG. 1 , the coincidence, when the heliostat surface P is oriented in the desired direction, of imaged target center point  48  and imaged mirror center point  40  (see  FIG. 3 ) is determined by the precision of the centering of mirror  16  relative to the optical axis of imaging device  30 , and not by the dome shape. Ensuring this centering is easily done during manufacturing, and thus does not require high-cost manufacturing. The dome can be made of low cost polymer, as explained heretofore (see  FIG. 1 ), since its shape does not require high precision. 
         [0075]      FIG. 5  shows an example of another embodiment of a mirror shape usable in a solar tracker in accordance with the principles of the present invention. In this example, mirror  60  is composed of four rectangular mirrors ( 61  to  64 ) symmetrically disposed around a central point  65 . 
         [0076]    The fixed target of a solar plant may have various shapes; in some cases it may be that the pattern recognition algorithm needed to locate the target image on the imaging device sensing area becomes too complex. According to some embodiments of the present invention CPU  34  may be low-cost. However, if the required pattern recognition becomes too complex, a low-cost CPU may not be sufficient to implement the required pattern recognition algorithm. In that case, it is possible to associate to the fixed target  12  (see  FIG. 1 ) known objects fixed at pre-defined distances from fixed target  12 , these objects being designed to ease the computational load on the pattern recognition algorithm used. For example, flashing lights (not shown), blinking at a regular rate, can be placed substantially above and under fixed target  12 . These almost punctual light sources can be easily located by imaging device  30 , and the data collected easily processed by an image processing algorithm. The imaged target center point  48  can then be calculated by its relative position with these flashing lights. 
         [0077]    Having described the present invention with regard to certain specific embodiments thereof, it is to be understood that the description is not meant as a limitation, since further modifications may now suggest themselves to those skilled in the art, and it is intended to cover such modifications as fall within the scope of the appended claims.