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 determining locations and height of support piles to be staked to the ground such that the tips of the piles define contour and shape of a parabolic collector. The reflective panels installed on the support piles 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. The construction apparatus and method of operation are also used for periodic cleaning of solar collector in a practical and fast manner.

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 stationery, 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 stationery 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. 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 determining the exact location and height of the structural elements of the parabolic mirror so that the mirror constructed over the structural elements end up having the required parabolic concave shape. 
         [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. Due to the large surface area of solar collector, dust and dirt builds up on reflective mirrors which degrades the reflective quality. 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 Often high-pressure washers are used manually for regular cleaning of mirrors. Due to the large surface area of parabolic mirror declared in this invention, manual cleaning would be very slow and tedious. The invention utilizes a robot like apparatus for construction of the parabolic mirror. The apparatus goes thorough some modification after construction is over and later used for automatic cleaning of the solar collector. Due to the automatic nature of cleaning process which requires no manual intervention, the cleaning can be performed during night when solar collector is not utilized. 
         [0008]    The construction apparatus of the parabolic mirror comprises a central post and a horizontal jib connected to the post which can rotate around it 360 degrees. 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. The up or down movement of the vertical arm is 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. 
         [0009]    The construction apparatus has pile driver mounted on the trolley right next to the vertical arm. The pile driver is a type of equipment used in construction industry for driving stakes or piles into the ground. There are essentially two different kinds of pile drivers used in industry which are known as vibratory and hammer type pile drivers. Hammer type uses a hydraulically or pneumatically activated hammer to drive piles to ground. Vibratory type pile driver uses eccentrically generated vibrations to drive piles to ground. Both types of pile drivers as well as screw driver type arrangements are suitable for this application. 
         [0010]    The pile driver and the vertical working arm are both mounted on the trolley in close proximity. The arrangement is done such that pile driver drives piles to the ground down to the height indicated by the vertical arm of the apparatus. A proximity sensor installed at the tip of the vertical arm senses the height of the support piles being driven into the ground. The piles are driven into the ground until the tip of the pile is exactly at the same level indicated by the proximity sensor arrangement attached to the tip of the vertical arm. The pile driver stops as soon as a pile is driven up to the height indicated by the apparatus. 
         [0011]    By rotating the horizontal jib incrementally and changing the position of the vertical trolley in regular distances, multiple numbers of piles are driven into the ground. As a result of string and pulley arrangement of the vertical arm, the depth of each pile is different. At the end of pile driving process, the tips of the support piles define the outline of a parabolic mirror. The piles are later covered by reflective material which forms reflective part of the parabolic mirror. 
         [0012]    After the construction process is over, the pile driver is removed from the trolley that transverses the horizontal jib since it is no longer needed. 
         [0013]    When the construction process is over, the construction apparatus is modified and used this time for cleaning purposes. Since mirror needs frequent cleaning to keep the performance levels high, the vertical working arm of the apparatus is converted to cleaning use by installing cleaning brushes on the vertical arm instead of the pile driver. 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 under computer control, the mirror can be cleaned mechanically during the night. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]      FIG. 1  shows the main parts of the parabolic mirror construction apparatus, 
           [0015]      FIG. 2A  shows the position of the central post in a slanted mirror application, 
           [0016]      FIG. 2B  shows the construction apparatus placed on the central post during a slanted mirror construction process, 
           [0017]      FIG. 3  shows the construction process of the outer rim of the parabolic mirror, 
           [0018]      FIG. 4  shows the details of the tip of the horizontal jib while resting on the outer rim, 
           [0019]      FIG. 5A  shows the basic principle of parabolic mirror, 
           [0020]      FIG. 5B  shows the working principle of construction apparatus, 
           [0021]      FIG. 5C  shows the details of the pulley and string arrangement for determining the outlines of the parabolic shape, 
           [0022]      FIG. 5D  shows the profile of an alternative embodiment of horizontal jib, 
           [0023]      FIG. 6  shows the details of the string and pulley arrangement and the pile driver arrangement on the trolley including the proximity sensor set up, 
           [0024]      FIG. 7  shows the tip of the vertical arm and the pile driver with proximity sensor arrangement while a pile is driven to perfect depth indicated by the proximity sensor, 
           [0025]      FIG. 8  shows series of piles driven to ground which outlines parabolic cross section, 
           [0026]      FIG. 9  shows a rib element mounted on set of piles which defines a parabolic outline, 
           [0027]      FIG. 10A  shows the method of mounting mirror tiles on the rib, 
           [0028]      FIG. 10B  shows an alternative way of covering set of parabolic ribs with continuous reflective sheets, 
           [0029]      FIG. 11  shows the completed state of parabolic mirror construction, 
           [0030]      FIG. 12  shows the completed state of slanted parabolic mirror construction, 
           [0031]      FIG. 13  shows the block diagram of electronics and the control elements that make up the apparatus, 
           [0032]      FIG. 14  shows the flowchart of construction process, 
           [0033]      FIG. 15  shows the tip of the vertical arm and the trolley equipped with cleaning brush in cleaning configuration, 
           [0034]      FIG. 16  shows the flowchart of cleaning process. 
       
    
    
     DESCRIPTION 
       [0035]    Now the method of operation and the details of the construction apparatus will be described in detail with the aid of figures. 
         [0036]    In the following text, the terms parabolic mirror, parabolic solar collector, parabolic collector all refer to the parabolic solar collector. 
         [0037]      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  and works as the working arm of the apparatus. Item labeled as  30  is the focal mast of the apparatus. Item labeled as  70  is the end point of the focal mast which defines the focal point of the parabolic mirror. Item  40  is the trolley and vertical working arm of the horizontal jib. Item  50  defines the tip point of the vertical working arm  40  which is equipped with a roller at the end. Item  60  is an inelastic string which is connected to point  70  on one side and to point  85  on the other side. 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 tip of the horizontal jib and it is equipped with rollers that rest on rim support  90 . The rim support  90  is a circularly shaped platform raised on supports that makes a complete circle around the central post  10 . 
         [0038]    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 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. 
         [0039]    The concept explained above is further explained by  FIGS. 2A and 2B .  FIG. 2A  shows the position of the central post with respect to position of the sun in a location other than equator.  FIG. 2A  shows the central post  10  making angle D with line L which is positioned perpendicular to earth. Angle D in this figure represents the declination angle of the particular location.  FIG. 2B  shows the central post  10  and the horizontal jib  20  slanted toward sun  5  for the amount of declination angle. When central post is slanted toward one side, the parabolic mirror constructed around will also be slanted toward the sun. The determination of exact orientation of the parabolic collector requires careful planning and optimization decision and it is well know by those who are skilled in this art. Information necessary for determining the appropriate slant angle of the central post is available from open sources on World Wide Web for those who are skilled in this art. 
         [0040]    The steps of the construction process are now explained with the aid of  FIG. 2A . The first step in the process of parabolic mirror construction is the placement of the central post  10  to the ground at an appropriate angle D. Central post is driven into the ground with the required slant angle using commercially available pile drivers. In a preferred embodiment, the diameter of the central post is 1 meter and the pile should be driven into the depth of at least 6 meters. The diameter of the central post depends on the size of the parabolic mirror and can be adjusted accordingly. The actual depth of the pile is also dependent on the soil properties and it should be deep enough to support the weight of the horizontal jib and associated weights without buckling or getting loose. In another embodiment of the design, concrete may be poured into ground at the base location of the central post  10  and central post can be placed or erected over the concrete slab. Yet in another embodiment, the central post may be placed on a portable yet sturdy base which may be moved or relocated. 
         [0041]    The second step of the construction method is explained with the aid of  FIG. 2B . In this 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 360 degrees. The height of central post  10  should be high enough for the horizontal jib to rotate in 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. Horizontal jib has to be stable, sturdy and straight. 
         [0042]    The third step in the process parabolic mirror construction is construction of the support rim and it is explained with the aid of  FIG. 3 . 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 . 
         [0043]    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 repeating the same process, rim runner platform  90  is installed on piles  100 ,  101 ,  102  and  103 . After completion of the process, a circular rim runner  92  will be spanning 360 degrees around the central post  10 . Rim runner  92  is positioned at the same height as the tip of the horizontal jib  20 .  FIG. 4  shows the detailed view of the horizontal jib  20  resting on rim runner  90  with the aid of rollers  21  installed at the tip of the jib  20 . The rim runner  90  defines the rim of the parabolic mirror and provides a support surface for the horizontal jib to rest and move all around the rim edge. 
         [0044]    The fourth step of the construction method is explained by the aid of  FIG. 5  series.  FIG. 5A  shows cross section of a perfect parabolic mirror. X represents the parabolic mirror with reflective face facing toward sun. Rays A and B are coming from the sun which is situated directly overhead and perpendicular to axis of 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.  FIG. 5B  shows a geometrical property of parabolic mirror. Assuming D is a line that crosses parabolic mirror right at the center while passing over point center point of parabola designated as M, and C is a specific length line that is drawn perpendicular to line D, the following equations hold true: 
         [0000]      distance GE+distance EF=constant, 
         [0000]    The same formula is also valid for another instance, 
         [0000]      distance HL+distance LF=constant 
         [0000]    In the drawing G and H are points where line C crosses line D. In the drawing G and H are shown slightly apart from the crossing point for clarity. 
         [0045]    The apparatus makes use of this geometrical property to define outline of the parabolic mirror which is further explained in  FIG. 5C . 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  39  which is parallel to central post  10  and perpendicular to horizontal jib  20 . The tip of the vertical working arm  40  is designated as  50 . An inelastic string  60  is connected to 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  to  32 , the tip  50  of the vertical working arm  40  defines a parabolic profile. Being inelastic, the length of string  60  remains constant but height of vertical arm  40  changes in accordance to the principle explained in  FIG. 5B . 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  is changed, many points describing the outline of the parabolic mirror are obtained. 
         [0046]    The principle behind defining parabola is further explained as follows: Points  70 ,  50  and  46  in  FIG. 5C  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,       
 
         [0049]    As a result of these constraints, the height of vertical arm (from the ground to the tip point  50  which is shown as  39 ) varies as the trolley  35  is moved on horizontal jib  20  from point  80  toward the center post  10 . 
         [0050]      FIG. 5D  shows another embodiment of horizontal jib  20  which is made up of two parallel arms  47  and  48 . The trolley  40  is attached to both arms  47 ,  48  and there is a gap between two arms indicated as  49 . In this embodiment, the trolley  40  can run along the gap  49 . 
         [0051]    This principle of defining outline of a parabola is used for determining location and height of the support piles of the parabolic mirror. This is explained using  FIG. 6 . Trolley  35  is installed on horizontal jib  20  and can travel freely along the jib in the direction of  31  to  32 . The movement of the trolley is controlled by a motor assembly which is installed on the trolley  35 . In another embodiment the trolley motor can be installed on either side of the horizontal jib  20 . Item  33  is a pile driver which is installed on the trolley  35  right next to the vertical working arm  40 . Vertical working arm  40  has a roller  41  attached at the tip of working arm which is designated as  50 . Inelastic string  60  which is connected to focal point on one end of the string and passes under the roller  41  and gets attached to the point  58  on the other end of the string. Item  47  is a proximity sensor which looks sideways toward the pile being driven labeled as  43 . As the pile driver  33  drives the pile  43  to the ground by hammering, the proximity sensor  47  continuously checks the height of pile  43  and detects the condition when pile  43  is at the same height with roller  41 .  FIG. 7  shows the condition where the pile  43  is at the same height with the roller  41 . At this point, the height of the support pile  43  is driven to the ground enough and hammering of pile  43  can be stopped. 
         [0052]    In another embodiment of the invention the support piles are screwed to the ground rather than being hammered. In this case the pile driver  33  is replaced by a rotary motor which screws the pile  43  to the ground until proximity sensor  41  indicates appropriate height. In this particular embodiment support pile  33  is shaped like a screw. 
         [0053]      FIG. 8  shows the case where a series of support piles are driven to the ground and the height of piles  120 ,  121 ,  122 ,  123 ,  124 ,  125  and  126  define outline of a parabola. The tips of the support piles are covered by a cap  129  which the thickness of the cap can be adjusted so that the overall height of the pile can be accurately adjusted. Even though the heights of the piles are controlled by the pile driver and the proximity sensor arrangement, due to coarse stroke size of the pile driver, the height of the support pile may not be accurate. In an embodiment of parabolic mirror where the focal length is 10 meters, the height of the support piles should be accurate to +/−2.5 mm. This is achieved by installing cap  145  with variable thickness so that height can be adjusted accurately. 
         [0054]      FIG. 9  shows the next stage of the construction method where a rib labeled as  129  is installed on the piles  120 ,  121 ,  122 ,  123 ,  124 ,  125 . Since the heights of these support piles already describe a parabolic mirror outline, the flat rib installed on theses piles also describe a parabolic contour. 
         [0055]      FIG. 10  shows the next stage of the construction method where reflective mirror tiles like  133  are installed on the rib  129  forming reflective surface of parabolic mirror. The rib  129  forms a continuous and suitable platform for installing reflective tiles or sheets. 
         [0056]      FIG. 10B  shows an alternative embodiment of construction process where reflective mirror tiles are replaced by continuous reflective sheets like  111  and  112 . 
         [0057]      FIG. 11  shows the completed parabolic mirror  135  with all mirror tiles installed. 
         [0058]      FIG. 12  shows the slanted version of the parabolic mirror arrangement  135  after the installation is completed. 
         [0059]      FIG. 13  shows the block diagram of control system of the invention where the computer block  3  is interfaced to azimuth motor  4  which rotates the horizontal jib along central post axis, trolley motor  5  controlling position of the trolley travelling along horizontal jib, pile driver control  6  which controls the hammer or the screw motor that drives the piles to the ground and the proximity sensor  7  that determines the height of the piles. Computer  3  also runs algorithm for construction and cleaning process. 
         [0060]    The flowchart of the operation is illustrated in  FIG. 14  which shows the flow chart of the operation during the construction phase. The flowchart describes the steps for constructing the rim runner and support piles of the parabolic mirror. 
         [0061]    The rim construction process ( 90 ) starts by bringing the azimuth motor to starting position ( 91 ). 
         [0062]    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 ( 92 ). At this stage the pile driver is turned on and the rim support piles are driven into ground ( 93 ). 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 a different location. The process of pile driving 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 rim runner should be installed on the rim support piles. Now the tip of the horizontal jib can be supported by the rim runner underneath. 
         [0063]    The next process starts from the starting position of the horizontal jib. The trolley is moved toward the center for a predetermined distance ( 96 ). The pile driver is turned on and support pile is driven into ground ( 97 ) until the proximity sensor indicates the limit position is reached ( 98 ). 
         [0064]    The process is repeated after moving the trolley toward center for another predetermined distance ( 99 ). The new piles are driven into the ground until proximity sensor indicates the limit position is reached. This process is repeated until trolley comes near the center post. The process is repeated by increasing azimuth angle ( 80 ) and repeating the process. Process finishes upon reaching the end azimuth position which is also the starting position ( 83 ). 
         [0065]    After construction of the parabolic mirror is over, the pile driver is removed from the trolley that transverses along the horizontal jib. Since construction process is over, the pile driver is no longer is needed. Instead of the pile driver, cleaning brush arrangement is installed on the trolley. This is shown in  FIG. 15  where a rotating brush arrangement  81  is installed instead of the pile driver. Now the cleaning brush  81  and the vertical arm  41  can work in unison and clean the mirror tiles. In a preferred embodiment series of brushes are installed on the trolley in such a way that one of the brushes applies polishing compound, another brush buffs and another one cleans the mirror surface. Yet in another embodiment of the invention the brushes are replaced by high pressure water nozzles which spray water or cleaning liquid onto the mirror surface for cleaning purpose. 
         [0066]    The cleaning process is also controlled by the computer system using the same control system shown in  FIG. 13 .  FIG. 16  shows the flowchart of the cleaning process which essentially moves the motors in an organized way to clean the mirror. 
         [0067]    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 the center position and process is finished ( 78 ).