Patent Publication Number: US-2017350621-A1

Title: Secondary solar concentrator

Description:
CROSS REFERENCE 
     The present application claims the benefit of the filing date of U.S. Provisional Application No. 62/346,020 having a filing date of Jun. 6, 2016, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure is directed to improving the performance of parabolic trough solar collectors. More specifically, the present disclosure is directed to a secondary solar concentrator that improves concentration of beam radiation onto tubular receivers or heat collection elements (HCE&#39;s) of parabolic trough solar collectors. 
     BACKGROUND 
     A parabolic trough power plant generates electricity using concentrated sunlight as the heat source for its power cycle. Most commonly rows of single-axis-tracking, linear parabolic mirrors form a solar field that concentrate beam radiation onto tubular receivers which are also known as heat collection elements (HCE&#39;s). See, e.g.,  FIG. 1A . The HCE&#39;s are located along the focal line of each parabolic trough. Heat-transfer fluid pumped through the HCE&#39;s is heated by the sun heated receiver walls on which the parabolic mirrors focus solar radiation. See, e.g.,  FIG. 1B . After being heated by the solar field, the heat-transfer fluid is typically used generate high-pressure superheated steam in a series of heat exchangers. Most commonly, the energy in the steam is converted to electricity in a Rankine steam turbine power cycle. After passing through the heat exchangers, the cooled heat transfer fluid is recirculated through the solar HCE&#39;s. 
     SUMMARY 
     Aspects of the presented inventions are based on the recognition by the inventor that the focal point of linear parabolic reflectors/mirrors is often not exact. That is, the consistency of the actual foci of the parabolic mirrors as it focuses light onto the HCE&#39;s is somewhat loose in tolerance. Along these lines, a portion of the beam radiation reflected by the mirrors may not contact the heat collection elements mounted along the foci of the linear parabolic reflectors. Stated otherwise, some of the reflected beam radiation is lost via spillage. The reflected beam radiation which never contacts an HCE is lost energy, which could be utilized to further heat the heat-transfer fluid and further improve overall efficiency of the system. To reduce such spillage, the presented inventions are directed to a secondary solar concentrator that may be affixed about an external surface of an existing HCE to capture and redirect reflected beam radiation that would otherwise bypass the HCE. 
     In one aspect, an external concentrator includes a plurality of ribs that are adapted to extend radially outward from the outside surface of an HCE and along the linear length of the HCE to help redirect stray/spilled light into the absorber tube of the HCE. The number and spacing of the ribs may be varied. In any arrangement, the ribs form a reflective surface that allows for redirecting stray light into the HCE. 
     In a further arrangement, the external concentrator includes two sets of ribs that are disposed on different radial sections of the outside surface of the HCE. In such an arrangement, the different sets of ribs may be separated by a reflective shield that covers a portion of the HCE tube. Most commonly, when applied to an HCE tube, the reflective shield is disposed outside of the tube opposite of the vertex of a parabolic reflector that focuses light onto the tube. 
     In another arrangement, the external concentrator includes two sets of ribs that are disposed on different radial sections of the outside surface of the HCE. In such an arrangement, individual ribs may be disposed in non-radial orientations relative to the surface of the HCE, and different orientations relative to other ribs. 
     In yet another arrangement, the external concentrator may include ribs or brims that are disposed adjacent to one or both lateral edges of a reflective shield applied to the outside surface of a HCE tube. In such an arrangement the ribs/brims may connect to the reflective shield at a pivot point and be disposed in various angular orientations relative to the surface of the HCE. Such ribs/brims may extend above and outward from the surface of the HCE to collect additional stray light. Certain embodiments also contemplate spacing the reflective shield and/or the ribs at a distance from the surface of the HCE. 
     In another aspect, a method is provided for retrofitting existing parabolic trough power plants to increase efficiency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates a solar field formed of a plurality of parabolic reflectors. 
         FIG. 1B  illustrates a cross-sectional view of parabolic reflector focusing solar energy on a focal point. 
         FIG. 2  illustrates one embodiment of a heat collecting element. 
         FIG. 3A  illustrates spillage of reflected light at a heat collecting element. 
         FIG. 3B  illustrates an end view of an external concentrator. 
         FIG. 3C  illustrates an end view of an external concentrator as applied to the heat collecting element. 
         FIG. 3D  illustrates the external concentrator of  FIG. 3C  redirecting spilled light the  FIG. 3A  onto the heat collecting element. 
         FIGS. 4A and 4B  illustrate first and second perspective views of an external concentrator as applied to a heat collecting element. 
         FIGS. 5A and 5B  illustrate first and second and views of an external concentrator. 
         FIG. 6  illustrates an external concentrator embodiment with ribs disposed in non-radial orientations. 
         FIG. 7  illustrates an external concentrator with ribs disposed at end portions of the reflective shield. 
         FIGS. 8A and 8B  illustrate first and second embodiments of an external collector oriented in a spaced relationship relative to the heat collecting element. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made to the accompanying drawings, which assist in illustrating the various pertinent features of the presented inventions. The following description is presented for purposes of illustration and description and is not intended to limit the inventions to the forms disclosed herein. Consequently, variations and modifications commensurate with the following teachings, and skill and knowledge of the relevant art, are within the scope of the presented inventions. The embodiments described herein are further intended to explain the best modes known of practicing the inventions and to enable others skilled in the art to utilize the inventions in such, or other embodiments and with various modifications required by the particular application(s) or use(s) of the presented inventions. 
       FIG. 1A  illustrates an exemplary parabolic trough solar assembly having a plurality of linear parabolic mirrors/reflectors  10 . As shown, the linear parabolic reflectors  10  are disposed in rows and each row of reflectors is operative to concentrate or focus solar radiation onto a receiver tube(s) or heat collection element(s)  20  (HCE) linearly disposed along the linear focal lines/points of the reflectors  10 . See  FIG. 1B  showing a cross-section of a parabolic trough  10  with an ideal focal point at the HCE  20 . That is, the reflectors are oriented such that sunlight which they reflect concentrates on the HCE, which contains a circulating heat transfer fluid that is heated to a high temperature by the energy of the concentrated sunlight. The heat transfer fluid (often thermal oil) runs through the HCE to absorb the concentrated sunlight. This increases the temperature of the fluid to, in some cases, 400° C. The heat transfer fluid is then most commonly used to heat steam in a turbine generator. Other thermal uses are possible. 
       FIG. 2  illustrates one non-limiting embodiment of an HCE  20 . The illustrated embodiment of the HCE  20  is shortened for purposes of illustration. However, it will be appreciated that such HCE&#39;s may be of considerable length (e.g., 4 m and more) and the illustrated embodiment is for purposes of discussion only. The HCE  20  includes a steel absorber tube  22  through which the heat transfer fluid flows. A common outside diameter for such an absorber tube is around 70 mm; however this is not a limitation. The outside surface of the absorber tube  22  typically includes a solar selective absorber surface. Further, the HCE  20  includes an annular glass envelope  24  concentrically disposed about the absorber tube  22 . A common outside diameter for such a glass envelope is 115 mm; however this is not a limitation. The increased diameter of the glass envelope defines an annulus  26  between the inside surface of the glass envelope  24  and the outside surface of the absorber tube  22 . The annulus  26  is evacuated to reduce heat losses at high operating temperatures and to protect the solar selective absorber surface from oxidation. As further shown, the ends of the illustrated HCE  20  include bellows  28 , which accommodate thermal expansion differences between the steel absorber tube and the glass envelope. 
       FIG. 3A  illustrates the concentration of sunlight energy/rays on the absorber tube  22  to heat the tube and the heat-transfer fluid therein. The parabolic reflector (not shown) reflects sunlight rays onto the HCE  20 , which is disposed at the focal point of the reflector. As shown, a majority the reflected sunlight rays impinge on the absorber tube  22 . However, due to mirror surface imperfections, sunlight tracking misalignments and/or other optical-mechanical phenomena, some of the reflected sunlight rays bypass the HCE without contacting the absorber tube  22 . That is, some spillage occurs due to the imperfection of the focal point of the parabolic reflector. Such spillage reduces the overall efficiency of the HCE. Accordingly, the presented inventions are directed to an external or secondary reflector/concentrator that is attachable to the outside surface of an HCE (e.g., glass envelope  24 ), which captures reflected sunlight rays from the parabolic or primary reflector/concentrator that would normally be lost via spillage. 
       FIGS. 4A and 4B  illustrate first and second perspective views of an external concentrator  40  as applied to an HCE  20 . More specifically, these figures illustrate the external concentrator  40  as disposed about an outside surface of the glass envelope  24  of the HCE.  FIGS. 5A and 5B  illustrate a cross-sectional end view of the external concentrator  50  and the external concentrator as applied to the HCE  20 , respectively. As shown, the external concentrator  40  includes a series of reflective ribs  50  that extend substantially parallel to the parabolic trough and its focal line once the external concentrator  40  is attached to the HCE  20 . In the illustrated embodiment, the external concentrator  40  includes two sets of reflective ribs  50 , which are separated by a reflective shield  60 . 
     As shown, each of the ribs is an elongated element that is substantially rectangular in cross-section having two ends/edges and two opposing side surfaces. However, it will be appreciated that in further embodiments the ribs  50  may be shaped (e.g., curved, parabolic, cusp etc.). In any arrangement, the ribs will typically each have an end/edge surface that may be disposed along the length of the HCE  20 . However, in various embodiments the ribs may be spaced above the surface of the HCE  20 . The cross-sectional height of each rib, extending radially outward from the surface of the HCE  20 , permit gathering of stray and misaligned reflected light rays while allowing properly directed light rays to pass into the HCE. In this regard, one or both side surfaces of each of the ribs  50  forms a reflector that allows for capturing stray and misaligned reflected light rays, which may then be re-directed onto the absorber tube  22  within the evacuated glass envelope  24 . To redirect the stray reflected light rays, one or both side surfaces of the ribs is a partially reflective surface, which may be formed of, for example, reflective polished aluminum or specially coated reflective metal. Alternatively, a reflective film may be applied to the ribs  50 . 
     The redirection of the stray and misaligned light rays by the ribs  50  is at least partially illustrated in  FIGS. 3A-3D . As noted above,  FIG. 3A  illustrates the spillage of light rays that are reflected by the parabolic reflector but fail to contact the absorber tube  22 .  FIG. 3B  illustrates an end-view of the external concentrator  40  and  FIG. 3C  illustrates the external concentrator  40  as applied to the HCE  20 .  FIG. 3D  illustrates the reflected light rays of  FIG. 3A  as redirected upon the attachment of the external concentrator  40  the outside surface of the HCE  20 . As shown, the ribs  50  which extend radially outward allow for capturing and redirecting a portion of stray light rays back onto the absorber tube  22 . Further, due to the substantially radial alignment of the ribs  50 , the ribs do not interfere with incoming light rays, which are properly focused on the absorber tube  20 . That is, even if properly reflected incoming light rays contact the ribs, they are most commonly redirected to another point on the absorber tube  22 . 
     While the ribs  50  provide the ability to capture some additional light rays which would otherwise spill past the HCE  20 , it is been recognized that additional spilled light rays may be recaptured by the use of the external reflective shield  60 . The illustrated embodiment of the reflective shield  60  is a corrugated element that is adapted for disposition on a radial outside portion of the HCE  20 . More specifically, the reflective shield is disposed on the outside surface of the glass envelope  24  on the side of the glass envelope that is opposite of the vertex of the parabolic reflector. Referring again to  FIG. 3D , it is shown that the inclusion of the reflective shield  60  significantly improves the redirection of spilt light rays back onto the absorber tube  22 . 
     As noted above, the disclosed embodiment of the external concentrator  40  utilizes a pair of rib sets  50  that are separated by a reflective shield  60 . The size and orientation of each of these elements may be varied. For instance, the number of the rib reflectors of each rib set may be varied based on physical parameters of the system with which they are used. Commonly, a height of the ribs in the radial direction will be between about ½ cm and about 3 cm. However, other sizes are possible and considered within the scope of the presented inventions. For instance, the height of the ribs will vary based on the diameter of the HCE. Along these lines, the height of the ribs may be between about 1% and 40% of the diameter of the HCE. Further, it will be appreciated that the axial length of the ribs may be varied based on, for example, the length of an HCE on which the ribs will be placed. Likewise, the number and placement of the radial ribs about the outside surface of the HCE  20  may likewise be varied. Currently, it is believed that the location of the reflectors should extend from approximately 30° (i.e.,  0 ) on either side of a reference line between the vertex of the parabolic receiver and a central axis of the HCE  20  to about 90° (i.e., φ) on either side of the reference line. However, these angles may be increased plus or minus 30°. See  FIG. 5B . 
     To correctly position the ribs and reflective shield, the present embodiment of the external concentrator  40  utilizes wire cables  62  that are spaced along the length of the concentrator  40 . See  FIGS. 4A and 4B . As shown, the wires  62  extend through apertures  54  in the base of each of the ribs. To provide appropriate spacing between each of the ribs, an annular spacer  56  may be disposed between each adjacent pair of ribs. In this regard, the wire passes through the annular spacer which maintain a desired spacing between the bases of adjacent ribs. Further will be appreciated that the ribs may be equally spaced or different sets of ribs may utilize different spacing. In the present embodiment, the wires  62  also extend around the outside surface of the shield  60  to maintain its position on the HCE. Various spacers may be incorporated between the shield and the ribs and or that the wire may extend through one or more apertures may be formed within the shield. In any arrangement, the wires allow for conveniently attaching and detaching the device with the outside surface of the HCE. 
       FIG. 6  illustrates a non-limiting embodiment of an external concentrator  40  as applied to an HCE  20 . More specifically, this figure illustrates the external concentrator  40  with reflective ribs  50  disposed in non-radial orientations. That is, the ribs (i.e., in cross-section) need not extend outward from the central axis of the HCE. As noted above, each of the ribs is an elongated element that is substantially rectangular in cross-section having two ends/edges and two opposite side surfaces. However, it will also be appreciated that in further embodiments the ribs  50  may be shaped (e.g., curved, parabolic cusp, etc.). In any arrangement the ribs  50  will each have an end/edge surface that may be disposed generally along the length of the HCE  20  or spaced above the surface of the HCE  20 . In the illustrated embodiment, the cross-sections of at least some of the ribs  50 , extend outward from the surface of the HCE  20  in a direction that is non-radial with the centerline axis of the HCE  20 . For example, the orientation of the ribs  50  may be described by a rib axis  32  extending from a cross-section of the rib  50  and intersecting a center line  30  extending through the central axis of a cross-section of the HCE  20  and the vertex of the collector (not shown). The ribs  50  may be oriented such that the rib axis  32  intersects the HCE  20  at a point on the center line  30  other than the central axis of the cross-section of the HCE  20 . 
     Accordingly, each rib  50  may have a rib axis  32  that intersects the center line  30  of the HCE  20  at the same point, wherein this point is located at some distance from the central axis of the HCE  20 . In other embodiments, individual ribs  50  within a rib set, located on one side of the collector may be oriented a different angles relative to each other. For example a first rib  50   a  or subset of ribs located on one side of the HCE  20  may have a rib axis  32   a  that intersects center line  30  at a first location  100   a,  and a second rib  50   b  or subset of ribs located on the same side of the HCE  20  may have a rib axis  32   b  that intersects center line  30  at a second location  100   b.  Further, ribs  50  located on opposite sides of the HCE  20  may be oriented independently of each other. In such an embodiment (not shown) a first rib  50  located on a first side of the HCE may have rib axis  32  that intersects a center line  30  at a first point, and a corresponding second rib  50  located on the opposite side of the HCE may have a rib axis  32  that intersects the center line  30  at a second point. 
     In addition to varying the orientation of the ribs  50  in relation to the HCE  20 ,  FIG. 6  also illustrates that individual ribs  50  may have different lengths. In certain embodiments, the length of any rib  50  may be 1% to 40% of the HCE  20  diameter, and different ribs  50  may have different lengths within this range. For example a first rib  50   a  may have a first length that is 20% of the HCE  20  diameter and a second rib  50   b  may have a second length that is 15% of the HCE  20  diameter. In certain embodiment each rib  50  may be chosen to have a different length. It would be understood that one skilled in the art could vary the length and orientation of each rib  50  independently of the other ribs to increase radiation received by the HCE  20 . Moreover, as described above, various ribs  50  may also take on different shapes (e.g., a first rib may be rectangular and another rib  50  may be curved). The shape may also be varied with the length and the orientation to increase the radiation received by the HCE  20 . 
       FIG. 7  illustrates an embodiment of an external concentrator  40  with ribs or brims  150  located proximate to the end portions (e.g., lateral edges) of the reflective shield  60 . In this embodiment, the brims  150  are attached to the reflective shield at an attachment point axis  34 . In these arrangements, the brims  150  have an end/edge surface connected to reflective shield at attachment point axis  34  that may be disposed along the length of the HCE  20 . The brims may have different orientations in relation to the surface of the HCE  20 . The orientations of the brims  150  may vary in relation to a cross-sectional radial line  36  extending from the central axis of the HCE  20  through the attachment point  34 . In the embodiment shown, the brims  150  are rotated about the attachment point  34  running along the length of the HCE  20  and oriented at angle β from the cross-sectional radial line  36 . Currently, it is believed that the orientation of the brims  150  could range from approximately parallel to the cross-sectional radial line  36  to about 90° (i.e., β) on either side of the radial line  36 . 
     To further increase the amount of radiation received by the HCE  20 , the brims  150  illustrated in  FIG. 7  may be shaped (e.g., curved, parabolic cusps, etc.). Moreover, as noted above in relation to embodiments describing the ribs  50  in a spaced relation from the reflective shield (see  FIG. 6 ), the brims  150  connected to the reflective shield  60  may also vary in length. It is further understood, that each brim  150  shown in the embodiment of  FIG. 7  may be varied independent of the other brims. For example, a first brim may be oriented at a first angle (e.g., 20°), have a first length (e.g., 30% of the diameter of the HCE  20 ) and have a rectangular cross-section, and a second brim may be oriented at a second angle (e.g., 30°), have a second length (e.g., 20% of the diameter of the HCE  20 ) and have a curved shape. 
     As shown, the reflective shield  60  in  FIG. 7  comprises a continuous surface and is adapted for disposition on the outside portion of the HCE  20 . In this illustrative embodiment, the reflective shield  60  has a corrugated surface consisting of a plurality of rectangular cross-sectional sections (e.g., flat cross-sections) disposed to form alternating ridges and grooves. However, in other embodiments the reflective shield  60  may have a corrugated surface consisting of a plurality of shaped (e.g., curved surfaces, parabolic cusps, etc.) cross-sectional sections disposed to form alternating ridges and grooves. 
       FIGS. 8A and 8B  illustrate first and second embodiments of the external collector  40  located in a spaced relationship to the HCE  20 . More specifically, the external collector  40  is oriented at a distance Δ from the surface of the HCE  20 , such that the surface portions of the reflective shield  60  are not in direct contact with the HCE  20 . Currently it is believed that the external collector  40  should be located at approximately a distance of 0-40% of the HCE  20  diameter away from the surface of the HCE  20  (i.e., Δ=0-40% the diameter of the HCE  20 ). Further as noted above and shown in  FIGS. 8A and 8B , brims  150  may be located on the ends of the spaced reflective shield  60 . These brims  150  may vary in shape, length and orientation as described in relation to the previous figures. In addition, when utilizing a spaced reflective shield, it will be appreciated that ribs or rib sets may be applied to the surface of the HCE and/or spaced above the surface of the HCE. 
     The foregoing description has been presented for purposes of illustration and description. Furthermore, the description is not intended to limit the inventions and/or aspects of the inventions to the forms disclosed herein. Consequently, variations and modifications commensurate with the above teachings, and skill and knowledge of the relevant art, are within the scope of the presented inventions. Along these lines, different aspects of the inventions shown in different figures may be utilized in various combinations including combinations not explicitly shown. The embodiments described hereinabove are further intended to explain best modes known of practicing the inventions and to enable others skilled in the art to utilize the inventions in such, or other embodiments and with various modifications required by the particular application(s) or use(s) of the presented inventions. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art.