Abstract:
This invention is about a method and apparatus for fabricating large scale stationery parabolic solar collector. The method involves use of a robot like apparatus for sculpting shape of underlying base material to define contour of a parabolic collector. The reflective panels installed on the base material form a parabolic shaped mirror which reflects the sun&#39;s rays to a well defined focal point. The method of construction is scalable and can be used for fabricating small size parabolic collectors as well as large scale ones.

Description:
FIELD 
       [0001]    This invention relates to solar heat collectors, more specifically to components, parts and details of solar heat collectors. It is also related to arrangements of mountings and supports of solar heat collectors. The invention is also related to means for cleaning solar heat collectors. The invention is also related to mirrors with curved surfaces and more specifically to multifaceted mirrors. 
       BACKGROUND 
       [0002]    Parabolic solar collectors are among the best known types of solar collectors for concentrating solar radiation to a focal point. Parabolic collectors are also used for concentrating electromagnetic rays to a spot and used onboard satellites as well as in ground stations for concentrating radiation to a sensor. Database indicates there are numerous patents related to parabolic collectors. 
         [0003]    Literature survey indicated several documents that summarize state of the art in parabolic solar collectors. One of the documents is authored by H. E. Imadojemu and published in Vol. 36, No. 4 issue in pages pp. 225-237, 1995 of Energy Conversion Management Journal. This document is titled “Concentrating Parabolic Collectors: a Patent Survey”. Another article published is by S. Kalogirou and published in the Journal of Recent Patents on Engineering in 2007, issue #1, pages 23-33. The publication by Kalogirou is titled as “Recent Patents in Solar Energy Collectors and Applications”. The state of the art in solar collectors is summarized in these articles. 
         [0004]    Parabolic solar collectors are type of mirrors which have parabolic shape and installed facing towards sun. The solar rays coming from the mirror are focused to a specific spot where an energy absorber or a secondary mirror is placed. These types of solar collectors are known as “SRTA” which stands for Stationary Reflector/Tracking Absorber configuration. In this particular configuration the solar concentrator is stationary, but due to changing position of sun, the point where the solar radiation is focused changes continuously in a well-defined path. This is where the tracking absorber is used; by placing the absorber always in the focal point of the solar collector, it is possible to receive energy no matter what time of the day is. 
         [0005]    The main problem with parabolic type SRTA&#39;s is the construction of the parabolic mirror. Although small size parabolic mirrors can be cast efficiently using carefully designed molds, making oversized parabolic mirrors has been difficult and challenging. There have been patents toward methods of making parabolic mirrors such as U.S. Pat. No. 4,124,277 “Parabolic Mirror Construction” and U.S. Pat. No. 4,860,023, “Parabolic Reflector Antennas and Method of Making Same”. Another recent patent application; US 2010/010-8057A1, “Inflatable Solar Concentrator Baldwin Method and Apparatus” are among different attempts for making parabolic solar collectors. 
       SUMMARY 
       [0006]    The invention declares a method of making stationary parabolic concave mirror and an apparatus specifically designed for this purpose. The primary goal of this invention is to make large scale parabolic mirrors quickly and accurately in an automated fashion by the aid of designed apparatus. One of the most difficult aspects of parabolic mirror construction is formation of the specific parabolic shape accurately. The invention teaches a practical method of shaping underlying base structure of the parabolic so that the mirror sheets placed over the sculpted base end up forming parabolic shaped mirror. 
         [0007]    Another important problem addressed by this invention is the cleaning aspect of parabolic solar collector. Solar collectors require regular cleaning to keep the performance level high. Dust and dirt accumulates on reflective mirrors and degrades the reflective quality of the mirror surface. Unless the mirrors are cleaned regularly, the efficiency of the solar collector gets reduced drastically. The cleaning approach used in most contemporary solar collectors is manual cleaning using high pressure washers. If the mirror surface is large, the manual cleaning process can be very slow and tedious. The invention also teaches a method of cleaning mirror surface in an automated way utilizing the same apparatus used for construction of the parabolic structure. 
         [0008]    The invention utilizes a robot like apparatus for construction of the parabolic mirror. The robot like apparatus has an end effecter which a suitable sculpting apparatus can be attached to suit the base material that needs to be shaped. During the construction phase different sculpting tools are used as end effecters for shaping the underlying base structure. After the construction phase is over, the end effecter is attached with cleaning apparatus which can be used for automatic cleaning of the mirror face of the solar collector. Due to the automatic nature of cleaning process which requires no manual intervention, the cleaning can be performed during night time when solar collector is not utilized. 
         [0009]    The construction apparatus of the parabolic mirror comprises a central post and a horizontal jib connected to the post which can rotate around the central post. The horizontal jib has a trolley which can travel along the length of the jib from center position to the tip. The trolley has a vertical working arm which is positioned perpendicularly with respect to the trolley. The vertical arm mounted perpendicularly to the trolley is free to move up or down under gravity or by external means. A braking apparatus installed on the vertical working arm can stop the free movement of the arm in case it is activated. The up or down movement of the vertical working arm is further restrained by a system of pulley and inelastic string which is strategically connected to different parts of the apparatus. The system of pulley and string determines the tip position of the vertical arm as the trolley on the horizontal jib moves back and forth. The arrangement is done such that, as the trolley moves along the jib, the tip of the vertical arm is forced to follow path of a perfect parabola. 
         [0010]    The tip of the vertical arm is used for defining the contour of the parabolic mirror. By using appropriate attachments and continuously rotating the tip of the vertical arm around the central post, the base material underneath can be sculpted in parabolic shape. Just like a lathe that shapes a metal piece by rotating and removing miniscule pieces of the base material in each rotation, the jib and the vertical arm removes material from the base material underneath to sculpt parabolic shape on the base material. By rotating the jib arm and moving the trolley along the length of the jib arm, the tip of the vertical arm can be used to shape the sand, soil or material underneath the apparatus in parabolic shape. The tip of the vertical arm needs to be equipped with appropriate tools for excavating or shaping the filler material forming the base of the collector. The brake apparatus installed on the vertical working arm is used selectively during the carving phase of the base material to limit the amount of material removed during the operation. Just as a lathe is made to remove the base material gradually in each turn, the brake mechanism used in the apparatus ensures only manageable amount is removed in each turn. 
         [0011]    Base material can also be shaped by depositing additional material to the base material. In a different embodiment of the invention, the sculpting apparatus is made to deposit material to the base material rather than remove from it. By depositing varying amounts of filler material by means of an appropriate applicator, the base is sculpted to have parabolic shape. 
         [0012]    Once the base material is shaped in the form of parabolic shape, the surface of the base material can be covered by concrete, foam, adhesive material, metal mesh or any other agent to make the surface firm and solid. The tip of the vertical arm can be equipped with appropriate dispensing tool to apply concrete, foam, adhesive material or any other agent to the base material in a controlled manner so that dispensed material conforms to parabolic shape. 
         [0013]    The speed of the construction process can be increased by installing more than one trolley on the horizontal jib. Each trolley has an independent vertical working arm installed on it. The movement of the vertical arm is controlled by the string and pulley arrangement mentioned before. By fixing the focal point of each arrangement the same, the tips of all vertical arms can be made to follow the same parabolic path. By attaching appropriate tools to each one of the tips, the shaping or the cleaning process can be sped up. 
         [0014]    Using the method and the apparatus declared in this invention, the parabolic shape construction can also be made partial rather than a complete parabolic shape. This can be achieved by limiting the rotation angle of jib arm and the moving trolley within certain limits. This way a structure with partial parabolic shape can be constructed. Partial parabolic construction is valuable in case the physical space for construction is limited or the terrain is not suitable. 
         [0015]    Using the method and the apparatus declared in this invention, the parabolic shape construction can also be tilted toward one side so that final constructed parabolic surface can be used as offset-parabola. The advantages and proper use of offset-parabola is known to people who knows this particular art. Partial offset-parabola construction can be achieved by placing central post at a tilted angle and restricting rotation angle of the jib. 
         [0016]    The parabolic shape can also be constructed on top of plastic, wood or metal studs using the method and the apparatus described in this invention. In such cases, the tip of the vertical arm which defines a parabolic path which can be used for adjusting the length of the braces or studs where the tips define a parabolic base for mirror. 
         [0017]    When the construction process is over, the construction apparatus is modified with different end effecter apparatus which will be used for cleaning purposes. Since mirror needs frequent cleaning to keep performance level high, the vertical working arm of the apparatus is converted to cleaning use by installing cleaning brushes on tip of the vertical arm. The pulley and string set up which is used to determine the height of the piles is now used for determining the height of the brushes that clean the parabolic mirror. By rotating the horizontal jib incrementally and moving the vertical trolley along the horizontal jib, the mirror can be cleaned mechanically by moving brushes or cleaning apparatus following contour of the mirror surface precisely. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0018]      FIG. 1  shows the main parts of the parabolic mirror construction apparatus, 
           [0019]      FIG. 2  shows the construction process of the outer rim of the parabolic mirror, 
           [0020]      FIG. 3  shows the details of the tip of the horizontal jib while resting on the outer rim, 
           [0021]      FIG. 4A  shows the basic principle of parabolic mirror, 
           [0022]      FIG. 4B  shows the working principle of construction apparatus, 
           [0023]      FIG. 4C  shows the general outlook of the pulley and string arrangement with respect to the whole apparatus for determining the outlines of the parabolic shape, 
           [0024]      FIG. 4D  shows the details of the string and pulley arrangement and the vertical working arm set up, 
           [0025]      FIG. 4E  shows an alternative embodiment of the horizontal jib with more than one trolley and vertical working arm which can be used for speeding up construction and cleaning process, 
           [0026]      FIG. 5A  shows the initial stage of process of shaping underlying base material, 
           [0027]      FIG. 5B  shows the interim stage of process of shaping underlying base material with brake assembly activated to inhibit movement of vertical working arm, 
           [0028]      FIG. 5C  shows the final stage of process of shaping underlying base material, 
           [0029]      FIG. 6  shows the block diagram of the elements that make up the apparatus, 
           [0030]      FIG. 7  shows the flowchart of construction process, 
           [0031]      FIG. 8  shows the flowchart of cleaning process. 
       
    
    
     DESCRIPTION 
       [0032]    Now the method of operation and the details of the construction apparatus will be described in detail with the aid of figures. 
         [0033]    In the following text, the terms parabolic mirror, parabolic collector all refer to the parabolic solar collector. 
         [0034]      FIG. 1  shows the major parts that comprise the apparatus for making parabolic mirror. The item labeled as  10  is the central post of the parabolic mirror. Item  20  is the horizontal jib which rotates around central post  10 . Item labeled as  30  is the focal mast of the apparatus. Item labeled as  70  is the focal point of the parabolic mirror which is the end point of the focal mast  30 . Item  40  is the trolley which can move along horizontal jib  20 . Item  45  is the vertical working arm of the horizontal jib. Item  50  defines the end point of the vertical working arm  45 . Item  60  is an inelastic string which is connected to point  70  on one side and to point  85  on the other side which is located on trolley  40 . While going from point  70  to point  85 , the inelastic string goes over the roller at point  50  and makes a triangular shape. Item labeled as  80  is the end point of the horizontal jib  20  and it is equipped with rollers that rest on rim support  90 . The rim support  90  is a platform raised on supports and partially carries the weight of horizontal jib  20 . 
         [0035]    Central post  10  defines the geometric center of the parabolic solar collector and the axis where the focal point of the parabolic solar collector is located on. Since the focal point parabolic collector will be determined by the central post  10 , the location and the orientation of it is of crucial importance to the performance of the solar collector. Declination angle of sun at a specific location on earth is very much related to geographical latitude of that particular place. In a preferred embodiment, the central post  10  should be slanted from perpendicular position for the amount of declination angle to receive the solar radiation in the most optimum manner. The determination of declination angle is well known to the people who are skilled in the solar collector art. By way of slanting the central post for the amount of declination angle, the parabolic collector constructed around the central post can be made to receive radiation of sun at or near perpendicular angle for maximum duration of the day. 
         [0036]    The steps of the construction process are now explained with the aid of  FIG. 2 . As a first step, the central post  10  is placed in the ground where the parabolic mirror is to be constructed. As a second step the horizontal jib  20  is attached to the central post  10  in such a way that horizontal jib can rotate around the central post freely. The height of central post  10  and the attachment point of horizontal jib  20  are high enough for the horizontal jib  20  to rotate around central post  10  in an unobstructed manner In a preferred embodiment the horizontal jib  20  is constructed using triangulated lattice structure which makes the horizontal jib lightweight yet mechanically strong. 
         [0037]    The third step in the process parabolic mirror construction is construction of the rim support  90 . In this particular step, the horizontal jib  20  is fixed at the starting point of the rim and support pile  101  is driven into ground right underneath the tip of the horizontal jib  20 . The height of the pile  101  is adjusted such that, the end point of the pile is at the same height with the end point of horizontal jib  20 . 
         [0038]    The process is repeated after horizontal jib  20  is rotated slightly and pile  102  is driven into the ground ending up at the same height as the end of jib  20 . After driving more support piles into the ground and repeating the process, rim runner platform  92  is installed on piles  100 ,  101 ,  102  and  103 . After completion of the process, rim runner platform  92  will be supporting the end of horizontal jib  20  in the plane of movement of the horizontal jib  20 . 
         [0039]      FIG. 3  shows the detailed view of the horizontal jib  20  resting on rim support  90  with the aid of rollers  21  installed at the tip of the horizontal jib  20 . The rim support  90  defines the rim of the parabolic mirror and provides a support structure for the horizontal jib  20  to rest and move on. 
         [0040]    The fourth step of the construction method is explained by the aid of  FIG. 4  series.  FIGS. 4A and 4B  are used for explaining the basic operational principle. 
         [0041]      FIG. 4A  shows cross section of a perfect parabolic mirror. X represents the parabolic mirror with reflective face facing upwards toward sun. Rays A and B are coming from the sun which is situated directly overhead and parallel to axis MF of the parabolic mirror. F represents the focal point of the parabolic mirror. In a condition like this, the rays A and B will be reflected to point F of the mirror which is the focal point of the mirror X. 
         [0042]      FIG. 4B  shows a geometrical property of parabolic mirror. Assuming D is a line that crosses over parabolic mirror X, crossing line MF perpendicularly over center point of parabola designated as M, and C is a specific length line that is drawn perpendicularly to line D, the following equations hold true: 
         [0000]      distance  GE+ distance  EF=K;  where  K  is a constant, 
         [0043]    as we move over line D. 
         [0044]    The same formula is also valid for another instance, 
         [0000]      distance  HL+ distance  LF=K;  where  K  is a constant, 
         [0045]    In the drawing G and H are points where line C crosses line D perpendicularly. 
         [0046]    The apparatus makes use of this geometrical property to define outline of the parabolic mirror which is further explained in  FIG. 4C . Focal mast  30  is erected on top of the central post  10  in such a way that as horizontal jib  20  turns around central post  10 , the focal mast  30  also turns with the horizontal jib assembly. The end point of the focal mast  30  is marked as  70  which define the focal point of the parabolic mirror being constructed. Item  35  is a trolley that runs freely along the horizontal jib  20  in the direction of  31 - 32 . Item  40  is a vertical working arm installed on trolley  35  which can move up or down freely along the direction  38 - 39  which is parallel to central post  10  and perpendicular to horizontal jib  20 . The end point of the vertical working arm  40  is designated as  50 . An inelastic string  60  is connected to focal point  70  on one end and passes over a roller placed at point  50  and connected to point  46  on the other end. In this particular arrangement, as the trolley  35  moves along the horizontal jib  20  in the direction of  31 - 32 , the tip  50  of the vertical working arm  40  defines a parabolic path. Being inelastic, the length of string  60  remains constant but height of the vertical working arm  40  changes in accordance to the principle explained in  FIG. 4B . The focal point of the parabola defined through this process is designated as  70 . As the position of trolley  35  and the rotational angle of horizontal jib  20  are changed, many points describing the outline of the parabolic profile are obtained. 
         [0047]    The principle behind defining parabola is further explained as follows: Points  70 ,  50  and  46  in  FIG. 4C  define a triangle. In this triangle the following geometrical relation exist due to inelastic nature of the string:
       (the distance from  70  to  50 )+(the distance from  50  to  46 )=constant   Vertical working arm  40  is perpendicular to horizontal jib  20  and can move up or down freely,       
 
         [0050]    As a result of these constraints, the height of the point  50  which is the end point of vertical working arm  40  varies as the trolley  35  is moved on horizontal jib  20  from point  80  toward the center post  10 . 
         [0051]    The relationship between vertical working arm and inelastic string as well as further details of the structure of trolley is explained with the aid of  FIG. 4D . Trolley  35  is installed on horizontal jib  20  and can travel freely along the jib in the direction of  31 - 32 . Vertical working arm  40  has a roller attached at the end of vertical working arm which is designated as  50 . Vertical working arm  40  is free to move in the direction of  38 - 39 . Inelastic string  60  which is connected to focal point on one end passes under the roller  50  and gets attached to the point  58  on the other end of the string. Point  58  is on trolley  35  and in close proximity to where vertical working arm  40  is connected to trolley  35 . Item  95  indicates the sculpting apparatus connected to the end of vertical working arm  40 . Item  99  indicates a brake assembly which can prevent movement of vertical working arm  40  in the direction of  38 - 39  when activated. 
         [0052]    A different embodiment of the horizontal jib arm is shown in  FIG. 4E  where more than one trolley is installed on the horizontal jib arm. Horizontal jib  20  has two trolleys  35  and  36  installed on horizontal jib  20 . Each trolley  35 ,  36  have their own inelastic string  60  which connects the trolley to focal point  70  using the arrangement explained in  FIG. 4D . 
         [0053]      FIGS. 5A ,  5 B and  5 C explains the operation of the apparatus for shaping the underlying structure.  FIG. 5A  shows initial state of the operation before the underlying base material is sculpted. In  FIG. 5A  the Item  72  indicates the underlying base material which needs to be sculpted. Item  35  is the vertical working arm with sculpting apparatus  95  attached to the end. By rotating the horizontal jib  20  around central post  10  and moving trolley  35  incrementally along the direction  38 - 39 , the vertical working arm  40  shapes the underlying base material  72  with sculpting apparatus  95 .  FIG. 5B  indicates the interim stage of the fabrication process where the depth of the vertical working arm  40  is fixed by the braking assembly  99  to enable removal of reasonable amount of base material  72  during the sculpting process by the sculpting apparatus  95 .  FIG. 5C  indicates the final stage of the operation where the underlying base material  72  is sculpted in parabolic shape by sculpting apparatus  95  after making repeated number of turns around central post  10  and repeatedly moving trolley  35  along horizontal jib  20 . Once the underlying base structure is sculpted in parabolic form, the underlying base structure  72  is covered by mirror material to finish the construction of the parabolic mirror. 
         [0054]      FIG. 6  shows the block diagram of the control system of the apparatus where the computer block  1  is interfaced to azimuth motor  2  which rotates the horizontal jib along central post axis, trolley motor  3  controlling position of the trolley travelling along horizontal jib, brake assembly  4  which inhibits movement of vertical working arm selectively and sculpting apparatus control  5  which turns sculpting apparatus on or off selectively. Computer  1  also runs algorithm for construction and cleaning process. 
         [0055]    The flowchart of the operation is illustrated in  FIG. 7  which shows the flow chart of the operation during construction phase. The flowchart describes the steps for constructing the rim runner and sculpting of the base material. 
         [0056]    The rim construction process ( 90 ) starts by bringing the azimuth motor to starting position ( 91 ). Starting position is an arbitrary position, but once selected, it should be used as the starting position throughout the construction process. In this position the trolley is moved until the far end of the horizontal jib opposite to the central post ( 92 ). At this stage,the rim runner platform support piles are driven into the ground and part of rim runner platform is placed on top of them ( 93 ). The height of the support piles should be as high as the tip of the horizontal jib. After the pile driving process is over, azimuth motor is activated and azimuth angle is increased for a predetermined amount ( 94 ). Horizontal jib is now pointing to a different support pile location. The process of driving support pile to ground is repeated for this new position ( 93 ). The process is repeated over and over again until the starting position is reached again ( 95 ). At the end of this process, the rim support piles are all driven into ground and the rim runner platform is installed completely on these support piles. Now the tip of the horizontal jib can be supported by the rim runner platform underneath. 
         [0057]    The next process starts from the starting position of the horizontal jib. In the flowchart Y and X indicate distance of the trolley from the center. Y=100 means trolley is at the farthest point from the central post on the horizontal jib. The same notation is used for X, where X=100 means farthest location from the central post. The trolley is moved to the far end of the horizontal jib ( 96 ). The sculpting apparatus is turned on( 98 ),and azimuth motor is started ( 99 ) to start the sculpting process. Horizontal jib makes a full turn while sculpting apparatus is on ( 100 ). This way, the outermost circumference of the parabolic shape is constructed. The process is repeated by moving trolley closer to center gradually ( 79 ). While doing this, the vertical working arm is allowed to adjust its height for parabolic shape with brakes in de-activated state ( 81 ,  82 ). Once the trolley is in the new position with the vertical working arm height is adjusted to the new position, the brake assembly is activated and height of the vertical working arm is fixed ( 83 ). Now the azimuth motor is turned on ( 84 ) and the horizontal jib is allowed to make a full turn ( 85 ). After rotation is completed, the trolley is further moved closer to central post with the height of vertical working arm is in fixed position ( 86 ). This process of turning azimuth motor and moving trolley closer is repeated until trolley reaches the center position ( 80 ). This process scrapes the surface of the underlying base material and sculpts a surface with even height. (This is an intermediate stage of the process which is indicated in  FIG. 5B .) After this process, the trolley is moved into the previous position which is indicated as process ( 79 ) in  FIG. 7 . While doing this the brake is deactivated ( 81 ) to adjust to the new height level ( 82 ) dictated by the parabolic shape and the process is repeated until the parabolic shape sculpting process is finished ( 88 ). 
         [0058]    The cleaning process is also controlled by the computer system using the same control system shown in  FIG. 6 . 
         [0059]      FIG. 8  shows the flowchart of the cleaning process which essentially moves the motors in an organized way to clean the mirror. Before the cleaning process end effecter of the vertical working arm is equipped with cleaning brushes. The cleaning process ( 70 ) starts by moving azimuth motor to the starting position ( 71 ). The starting position is an arbitrary position selected, but once selected, it should be maintained as the starting position throughout the cleaning process. The trolley is moved into the far end of the horizontal jib near the rim ( 72 ). The cleaning brush motor is turned on ( 73 ) and azimuth motor is rotated gradually ( 74 ) which causes the horizontal jib to rotate. Every time starting position is reached the vertical trolley is pulled toward the center ( 76 ) and process is repeated. Eventually every part of the mirror is cleaned through this process. At the end of the cleaning process the trolley is parked at a position near the central post and cleaning process is finished ( 78 ).