Abstract:
A trough collector for solar energy, with multiple parallel troughs preferably being contained within a single unit. The collector does not use conventional azimuth tracking in order to keep the sun&#39;s rays directed toward the parabola&#39;s focus as the sun moves across the sky. Instead, the relative position between the collecting device (preferably a conductive tube containing a circulating working fluid) and the plane of symmetry for each collector is adjusted so that the collecting device remains within the focal zone of the collector as the sun traverses the sky.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     Pursuant to the provisions of 37 C.F.R. section 1.53(c), this non-provisional patent application claims the benefit of an earlier-filed provisional application. The provisional application was assigned Ser. No. 61/673,389 and it listed the same inventors. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     MICROFICHE APPENDIX 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of solar energy. More specifically, the invention comprises a solar collector incorporating multiple parabolic troughs and multiple collector pipes running through the troughs, in which the position of the pipes relative to the troughs is varied in order to keep the collector pipes in the focus of the troughs as the sun moves across the sky. 
     2. Description of the Related Art 
     Solar energy collecting devices frequently use focusing lenses or reflectors to intensify the energy of the sun. Some collecting devices directly convert the solar energy to electrical energy using a photovoltaic array. Other collecting devices use the solar energy to heat a circulating working fluid. The present invention may be adapted to either type of collecting devices, as well as other types. 
       FIG. 1  shows an elevation view of a prior art solar collector suitable for heating a circulating working fluid. Parabolic trough  10  receives incoming rays  12 . Because the sun may be considered to be an infinite distance away, the incoming rays are effectively parallel. The parabola that is used to define parabolic trough  10  is selected to bring the parallel incoming rays to the same point central focal point  14 . Meeting this limitation produces the maximum collection efficiency. 
     The reflecting trough shown extends for any suitable distance in a direction that is perpendicular to the orientation of the view. For this type of collector, a conductive pipe containing the circulating working fluid is run through central focal point  14 , with the pipe running in a direction that is also perpendicular to the view of  FIG. 1 . 
     Those skilled in the art will quickly realize that focal point  14  lies along the parabola&#39;s axis of symmetry  15 , so long as the incoming rays are parallel to the axis of symmetry. Because the reflecting trough actually extends for some distance in a direction that is perpendicular to the view of  FIG. 1 , axis of symmetry  15  actually defines a “plane of symmetry” that extends along the length of the trough (the plane of symmetry being perpendicular to the orientation of the view). 
     In  FIG. 1 , the sun is located directly above the reflector and the rays are coming straight down. Of course, the sun moves across the sky during the course of the day. Parabolic trough  10  must generally be tilted so that the plane of symmetry running through axis of symmetry  15  remains parallel with the incoming rays. This tilting action is generally referred to as an adjustment in “elevation.” It is indicated by the reciprocating arrow labeled tracking pivot  16 . 
     Prior art parabolic trough collectors typically include a suitable tilting mechanism in order to adjust the elevation of the collector. This mechanism regulates the elevation of the collector throughout the course of the day. An azimuth tracking mechanism is also frequently included. Such mechanisms tend to be complex and relatively expensive. 
       FIG. 5  shows a simplified representation of a prior art solar collector that includes both elevation and azimuth tracking. Three parallel parabolic trough reflectors  10  are contained within housing  46 . The parabolic troughs each have a finite length. The focus of each trough is therefore not a single point but rather an axis that runs through the focal point existing at each section taken through the trough. Thus, as seen in the view, the three trough reflectors have three parallel focus axes  44 . 
     In order to keep the housing perfectly perpendicular to the incoming solar rays it must be adjusted in both elevation and azimuth. Elevation adjustment  48  tilts housing  46  as indicated. Azimuth adjustment  50  rotates the housing so that it tracks the sun crossing the sky. 
     One may simply set the elevation adjustment to match the latitude of the location and gain a good approximation of the optimum elevation through the middle of the day. Azimuth, however, is not so easy to approximate. A simple visualization exercise demonstrates this fact: If one sets the azimuth of a device such as shown in  FIG. 5  so that housing  46  directly faces the sun at sunrise, it is immediately apparent that the trough reflectors will not receive any direct rays by sunset. For this reason, a housing that lacks azimuth tracking is most often set so that the azimuth is correct when the sun is directly overhead. 
     This static approach works fairly well for photovoltaic cells but it does not work well for parabolic trough reflectors.  FIG. 2  graphically illustrates the focusing error that occurs in the absence of a tracking mechanism. This figure shows a static parabolic trough  10  after the sun has moved away from its zenith position. Incoming rays  12  are no longer parallel to the parabola&#39;s axis of symmetry  15  (and plane of symmetry). The result is that the incoming rays are no longer reflected toward central focal point  14 . Instead, they have shifted to the right toward shifted focal zone  42 . The shift is primarily in a direction that is perpendicular to the plane of symmetry running through axis of symmetry  15 . The term “focal zone” is used because a parabola only brings incoming rays into sharp focus when the rays are parallel to the parabola&#39;s axis of symmetry. Once the incoming rays are angularly offset from the axis of symmetry, the parabola is no longer able to create a perfect focus. However, over a reasonable range of angular displacement, the parabola is still able to create a good concentration of solar energy and the region of this concentration is therefore referred to as a “focal zone.” 
     The reader will thereby appreciate that it is desirable to position a collector pipe within the focal zone even when the focal zone moves away from the trough collector&#39;s plane of symmetry. The present invention presents such a solution. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention comprises a trough collector for solar energy, with multiple parallel troughs preferably being contained within a single unit. The collector does not use conventional azimuth tracking in order to keep the sun&#39;s rays directed toward the parabola&#39;s focus as the sun moves across the sky. Instead, the relative position between the collecting device (preferably a conductive tube containing a circulating working fluid) and the plane of symmetry for each collector is adjusted so that the collecting device remains within the focal zone of the collector as the sun traverses the sky. 
     A trough reflector has only one true focal axis. As the incoming rays become misaligned with the parabola&#39;s plane of symmetry, the focus shifts laterally and blurs somewhat. However—over a reasonable range of travel—the blurring does not significantly degrade the collection efficiency. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is an elevation view, showing the operation of a prior art parabolic trough reflector. 
         FIG. 2  is an elevation view, showing the operation of the trough reflector of  FIG. 1  as the incoming rays are angularly offset from the parabola&#39;s axis of symmetry. 
         FIG. 3  is an elevation view, showing how the focus of the trough reflector moves laterally as the sun transits the sky. 
         FIG. 4  is a perspective view, showing a depiction of a simple example of the present invention. 
         FIG. 5  is a perspective view, showing a prior art collector that is adjustable in both azimuth and elevation. 
     
    
    
     
       
         
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 REFERENCE NUMERALS IN THE DRAWINGS 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 10 
                 parabolic trough 
                 12 
                 incoming ray 
               
               
                   
                 14 
                 central focal point 
                 15 
                 axis of symmetry 
               
               
                   
                 16 
                 tracking pivot 
                 18 
                 movable focal point 
               
               
                   
                 20 
                 receiver pipe 
                 22 
                 inlet 
               
               
                   
                 24 
                 outlet 
                 26 
                 serpentine connector pipe 
               
               
                   
                 28 
                 movable frame 
                 30 
                 mounting bracket 
               
               
                   
                 32 
                 gear rack 
                 34 
                 drive pinion 
               
               
                   
                 36 
                 motor 
                 38 
                 chassis 
               
               
                   
                 40 
                 mounting rail 
                 42 
                 shifted focal zone 
               
               
                   
                 44 
                 focal axis 
                 46 
                 housing 
               
               
                   
                 48 
                 elevation adjustment 
                 50 
                 azimuth adjustment 
               
               
                   
                 52 
                 frame notch 
               
               
                   
                   
               
             
          
         
       
     
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 3  shows a parabolic trough  10  with a depiction of a movable focal point  18 . The three positions shown in  FIG. 3  represent three of the varying positions of the focal zone as the angle of incidence of the sun&#39;s rays on the trough reflector changes. The reader will observe how the translation of the focal zone occurs primarily in a direction that is perpendicular to the trough collector&#39;s plane of symmetry. A goal of the present invention is to position a suitable energy collection device in the focal zone and alter the relative position between the collecting device and the trough reflector as necessary to keep the collection device in the focal zone. 
     Many different devices could be employed to achieve this goal and the invention is by no means limited to any particular method or device. However, as the description of a few embodiments may aid the reader&#39;s understanding, such a description is provided. 
       FIG. 4  shows a simplified embodiment of the present invention that is able to achieve this goal. The reader should appreciate that the embodiment of  FIG. 4  illustrates the important components of the invention but should not by any means be viewed as an optimized embodiment. An optimized embodiment would likely contain many more parallel reflector troughs and other components. However, the embodiment of  FIG. 4  illustrates the important concepts. 
     Chassis  38  serves to mount the other components. It is depicted as a flat plate, though it will likely be a more complex structure for most embodiments. Four parallel parabolic troughs  10  are attached to the chassis using one or more supports  40 . The parabolic troughs are fixedly attached to the chassis. That is, in this embodiment the troughs do not move. The positioning of the collecting devices in the focal zones of the troughs is accomplished by moving the collecting devices relative to the stationary troughs. In other embodiments, just the opposite will be done. 
     A receiver pipe  20  is provided for each parabolic trough  10 . This receiver pipe is moved laterally with respect to the parabolic trough it lies within so that it remains within the focal zone of the parabolic trough as the sun transverses the sky. In the embodiment of  FIG. 4 , the four receiver pipes  20  are joined by serpentine connector pipes  26  to form a serpentine flow path from inlet  22  to outlet  24 . In other words, a given control volume within the working fluid travels through every receiver pipe in the array. Such a flow path is generally referred to as a “series path.” One could also create a parallel flow path by using an inlet manifold, an outlet manifold, and a plurality of receiver pipes flowing therebetween. In such a parallel scheme, a given control volume would flow through only one of the receiver pipes as it travels from the inlet manifold to the outlet manifold. 
     In the series-flow scheme of  FIG. 4 , all the pipes in the serpentine flow path are connected to movable frame  28 . The connection may assume a wide variety of forms. In this embodiment five mounting brackets  30  are used. The use of these mounting brackets unites movable frame  28  and all the pipes in the serpentine flow path into a single moving unit. The entire assembly therefore moves as one piece. 
     Movable frame  28  moves on a pair of mounting rails  40 . Frame notches  52  in movable frame  28  allow movable frame  28  (and the attached piping) to translate in the directions indicated by the reciprocating arrow. Linear bearings may be employed to allow a smooth movement. A low-friction sliding block may also be placed in each frame notch. Such blocks could be made of NYLON, DELRIN, or any other suitable material. The translation required will be quite slow, so a fairly crude sliding connection will suffice in many applications. 
     It is, however, preferable to move the frame in a controlled fashion so that the four receiver pipes  20  in this embodiment are accurately maintained in the focal zone of the four parabolic troughs  10  (as the sun transits the sky). In the embodiment of  FIG. 4 , gear rack  32  is provided somewhere on the movable frame. Drive pinion  34  engages gear rack  32  on movable frame  28 . The drive pinion is driven by motor  36  (preferably through a set of reduction gears). Motor  36  is attached to chassis  38 . 
     Thus, by selectively energizing motor  36 , movable frame  28  can be moved with respect to chassis  38 . The motion may be controlled in any number of suitable ways. One simple approach is to use an open-loop “timetable” that moves the receiver pipes  20  to a predetermined position according to the time of day. One could also employ a closed-loop control system. In this arrangement an energy sensor could be placed at a suitable location on one of the receiver pipes. The control system would then be activated every few minutes during daylight hours and the closed-loop motion control system would adjust the position of the movable frame in order to maximize the energy received by the energy sensor. The energy sensor could be a simple temperature probe or some type of light intensity sensor. Of course, one could also employ a timetable motion controller that is relined by the application of a closed-loop energy sensing function. 
     Solar tracking is thus performed by the motion of movable frame  28  along with its attached components. Returning briefly to  FIG. 1 , the reader will recall that each parabolic trough reflector includes an axis of symmetry  15  and a plane of symmetry (the plane of symmetry simply being the axis of symmetry projected along the length of the trough). As explained with respect to  FIGS. 2 and 3 , the focal zone translates laterally as the sun&#39;s angle of incidence on the trough reflector varies. This translation is almost exclusively in a direction that is perpendicular to the axis of symmetry/plane of symmetry. It is therefore apparent that if one can vary the position of the collection device in a direction that is perpendicular to the axis of symmetry/plane of symmetry, one may continually position the collection device in the focal zone. 
     The embodiment of  FIG. 4  achieves the desired positioning of the collection device(s). By activating motor  36  in a controlled fashion, movable frame  28  and its associated receiver pipes  20  is translated in a direction that is perpendicular to the planes of symmetry of the four parabolic trough collectors. This movable feature allows the device to “track” the azimuth of the sun as it transits the sky. The displacement of the center of each receiver pipe from the axis of symmetry/plane of symmetry is referred to as the “receiver pipe displacement distance.” Using this nomenclature, those skilled in the art will immediately recognize that a value for the “receiver pipe displacement distance” may be positive, negative, or zero. In other words, although the displacement distance is defined, the actual value of that displacement may be zero at a particular point in time. 
     Chassis  38  may be set to a fixed elevation setting or elevation tracking may be provided using a conventional mechanism that tilts the entire assembly with respect to the horizontal. If the chassis is static, it is preferable to set the elevation of chassis  38  in order to maximize the efficiency of the collector. The elevation may be set according to latitude. On the equator, the elevation would be zero and chassis  38  would simply be parallel to the ground. At fifteen degrees north latitude, chassis  38  would preferably be set to an elevation of 15 degrees or something slightly less than this to achieve the best approximation of true elevation tracking. For example, the chassis could be mounted so that the side of the chassis proximate motor  36  in the embodiment of  FIG. 4  would be lower than the distal side of the chassis (with the upward facing surface of the chassis lying at an angle of 15 degrees to the horizontal). 
     Those skilled in the art will realize that many other components beyond those depicted in  FIG. 4  are preferred in the creation of an efficient collector. For example, it is preferable to enclose many of the components in an enclosure in order to elevate the internal temperatures. Thus, the chassis would typically include opaque side walls extending up and beyond movable frame  28 . A clear cover would then be placed over the movable frame and joined to the side walls in order to create a “greenhouse effect.” It is also preferable to include mounting bracketry to facilitate the attachment of the chassis to some other structure. All these components are well known to those skilled in the art and they have not been illustrated. 
     Those skilled in the art will also realize that many other embodiments are possible within the inventive scope of the present invention. As one example, one could design a collector where the receiver pipes remain fixed but the parabolic troughs move laterally to track the sun. Returning to  FIG. 4 , such an embodiment would involve connecting the reflector troughs to movable frame  28  and connecting the receiver pipes to the chassis so that they remain stationary. One could also choose to move both the reflector troughs and the receiver pipes in order to maintain the desired relative position between the two. 
     It is also possible to combine the azimuth-accommodating features of the present invention with conventional azimuth-tracking devices. For example, a crude azimuth turntable could be provided that sets the device of  FIG. 4  in one of three azimuth positions—morning, noon, and evening. The motion between the receiver pipes and the reflector troughs could then be used to optimize the performance of the device in each of these three stationary azimuth positions. 
     Thus, although the preceding descriptions contain significant detail, they should properly be viewed as disclosing examples of the inventions&#39; many possible embodiments rather than disclosing the full scope of the invention itself. The scope of the invention will properly be determined by the claims to follow rather than any specific example provided.