Patent Publication Number: US-2010126499-A1

Title: Solar Thermal Energy Absorber Tube

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to absorber tubes used in the collection and use of solar thermal energy. More particularly, the present application involves an absorber tube with an improved design to handle thermal expansion thereof and a getter material to aid in the generation of a vacuum within the absorber tube. 
     BACKGROUND 
     A parabolic trough is a solar thermal energy collector that functions to convert thermal energy into mechanical energy. The parabolic trough includes a series of parabolic mirrors that are arranged into parallel rows. The mirrors may be made from polished aluminum and function to focus solar energy directed thereon to an absorber tube that runs the lengths of the rows of mirrors. The parabolic mirrors may be arranged so as to be capable of moving in order to be oriented towards the sun for maximum solar thermal energy collection. 
     A substance, such as oil or water, flows through the absorber tube and is heated by the parabolic mirrors. The heated substance is transported to a heat engine that in turn converts thermal energy stored in the heated substance into mechanical energy. The substance is located and transported within an inner tube of the absorber tube that is surrounded by a concentric outer tube. The outer tube is made of glass so that solar thermal energy directed by the parabolic mirrors is more easily directed therethrough and onto the inner tube that includes the substance. The space between the inner tube and the outer tube can be evacuated so that a vacuum is formed. The vacuum functions to reduce heat transfer from the heated substance and inner tube back to the outer tube. Preventing temperature rise of the outer tube increases the efficiency of the parabolic trough and also enhances safety as the outer tube may be cool to the touch even though the inner tube is extremely hot. 
     Temperature differences between the inner tube and outer tube present certain challenges in the design of absorber tubes. For example, the inner tube and outer tube may expand at different rates or amounts due to differences in the thermal properties of the material making up the inner and outer tubes. Thermal expansion compensation devices may be located at various points along the concentric tubes in order to compensate for thermal expansion between the tubes. Thermal expansion compensation devices may be arranged so that the inner and outer tubes are in sliding engagement with one another and may include bellows that function to control the movement between the expanding components. These components may include moving parts that can ultimately fail and cause a loss of vacuum within the absorber tube that can in turn lead to a decrease in the efficiency of the parabolic trough. As such, there remains room for variation and improvement in the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, which makes reference to the appended Figs. in which: 
         FIG. 1  is a perspective view of a parabolic trough in accordance with one exemplary embodiment. 
         FIG. 2  is a cross-sectional view of an absorber tube in accordance with one exemplary embodiment. 
         FIG. 3  is a cross-sectional view of the absorber tube of  FIG. 2  in a thermally expanded condition. 
         FIG. 4  is a cross-sectional view of the absorber tube of  FIG. 2  with a getter evaporated and forming a coating on the inner wall of the outer tube. 
     
    
    
     Repeat use of reference characters in the present specification and drawings is intended to represent the same or analogous features or elements of the invention. 
     DETAILED DESCRIPTION OF REPRESENTATIVE EMBODIMENTS 
     Reference will now be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, and not meant as a limitation of the invention. For example, features illustrated or described as part of one embodiment can be used with another embodiment to yield still a third embodiment. It is intended that the present invention include these and other modifications and variations. 
     It is to be understood that the ranges mentioned herein include all ranges located within the prescribed range. As such, all ranges mentioned herein include all sub-ranges included in the mentioned ranges. For instance, a range from 100-200 also includes ranges from 110-150, 170-190, and 153-162. Further, all limits mentioned herein include all other limits included in the mentioned limits. For instance, a limit of up to 7 also includes a limit of up to 5, up to 3, and up to 4.5. 
     The present invention provides for an absorber tube  14  for use with a parabolic trough  10 . The absorber tube  14  features an expansion element  34  that is used to minimizes stress imparted to the absorber tube  14  through thermal expansion. The absorber tube  14  may also include a getter  40  located between the outer tube  16  and inner tube  22  for use in maintaining a vacuum formed in the space formed between tubes  16  and  22 . Additionally, the getter  40  may be provided in order to act as an indicating device to alert personnel of the loss of vacuum within the absorber tube  14  so that repair or replacement can then be conducted. 
     One exemplary embodiment of an absorber tube  14  as incorporated into a parabolic trough  10  is illustrated in  FIG. 1 . The parabolic trough  10  includes a parabolic mirror  12  that functions to focus sunlight imparted thereon onto an absorber tube  14 . The parabolic trough  10  and absorber tube  14  may extend any length in the longitudinal direction  30 . Additional parabolic troughs  10  and absorber tubes  14  can be located next to one another in rows to provide for additional solar energy collection. Further, the parabolic mirrors  12  and/or absorber tubes  14  can be made mobile so that their orientation with respect to the sky can be modified. In this manner, the parabolic trough  10  can be oriented with respect to the sun so as to achieve maximum solar collection. Heat collected by the absorber tube  14  is transported to a downstream location for utilization or storage. In accordance with certain embodiments the collected heat energy can be used in a single effect or in a double effect absorption chiller. The absorber tube  14  can be used in a variety of applications in accordance with various exemplary embodiments and it is to be understood that the absorber tube  14  is not limited to a specific application or limited in use with a parabolic trough  10  and/or parabolic mirror  12 . 
       FIG. 2  shows an absorber tube  14  in accordance with one exemplary embodiment. The absorber tube  14  includes an outer tube  16  that has an outer wall  18  and an inner wall  20 . The outer tube  16  may be made of borosilicate glass in accordance with one embodiment. The material making up the outer tube  16  may be selected so that it is capable of allowing light energy to pass therethrough with little or no resistance or reflection. An inner tube  22  is provided and includes an outer wall  24  and an inner wall  26 . The outer tube  16  and inner tube  22  are concentric with one another about axis  28 . A substance such as oil or water is present within the inner tube  22  and absorbs heat transferred from the parabolic mirror  12  though the outer tube  16  and into the inner tube  22 . The substance is then transferred through the inner tube  22  to a desired location in which the heat contained therein can be utilized or stored. The outer wall  24  of the inner tube  22  may be coated with a metal or paint that affords enhanced solar radiation absorption and minimal reflection so that solar energy absorption by the inner tube  22  is encouraged. For example, in one arrangement the outer wall  24  may be coated with flat black paint. The inner tube  22  may be made from a variety of materials. In accordance with one exemplary embodiment inner tube  22  is made of stainless steel. 
     An expansion element  34  is located beyond an end  48  of the outer tube  16  in the longitudinal direction  30 . In this regard, the expansion element  34  is located completely beyond the end  48  in the longitudinal direction  30  so that no portion of the expansion element  34  is located between the inner tube  22  and the outer tube  16 . As shown, the expansion element  34  is located outward of the inner tube  22  in the radial direction  32  and surrounds a portion of the inner tube  22 , but the expansion element  34  does not surround and is not surrounded by any portion of the outer tube  16  in the radial direction  32 . The expansion element  34  includes a series of radially extending portions  36  that are connected by a series of curved portions  38 . As shown, the radially extending portions  36  extend in the radial direction  32  but do not extend in the longitudinal direction  30 , with the exception of their thickness. The curved portions  38  extend both in the longitudinal direction  30  and the radial direction  32 . It is to be understood that the arrangement of the expansion element  34  shown is but one example and that others are possible in accordance with other exemplary embodiments. The expansion element  34  may be made from a variety of materials. In accordance with certain exemplary embodiments the expansion element  34  is made from stainless steel. 
     A connecting member  52  is present and functions to attach the expansion element  34  to the end  48  of the outer tube  16 . The connecting member  52  may be made of a material different than that of the outer tube  16  or may be made of the same material. The connecting member  52  may be made out of steel, aluminum or glass in accordance with various exemplary embodiments. The connecting member  52  may be attached to the inner wall  20  so that a portion of the connecting member  52  does not extend beyond the end  48 . In other arrangements, the connecting member  52  is attached to the end  48  of the outer tube  16  so that the entire connecting member  52  extends beyond the outer tube  16  in the longitudinal direction  30 . In this regard, no portion of the connecting member  52  contacts or is connected to the outer wall  18  or the inner wall  20  of the outer tube  16 . Connection between the connecting member  52  and end  48  may be effected though welding, soldering or mechanical fasteners. Further, these two components may be attached by being integrally formed with one another in accordance with other embodiments. In accordance with one exemplary embodiment the connection is a metal to glass joint in which the end  48  is melted glass and the connecting member  52  is oxidized metal. The connecting member  52  extends away from the end  48  in the longitudinal direction  30  such that it moves in the radial direction  32  closer to the axis  28 . The connecting member  52  is attached to the expansion element  34  such that the entire expansion element  34  is located longitudinally beyond the entire connecting member  52  in the longitudinal direction  30  with respect to the outer tube  16 . 
     The absorber tube  14  also includes a cap  54  that is attached to the outer wall  24  of the inner tube  22 . The attachment may be effected through welding, soldering, mechanical fasteners, or through integral formation in accordance with various exemplary embodiments. Cap  54  has a longitudinally extending portion  56  that extends completely in the longitudinal direction  30  with the exception of the thickness of the longitudinally extending portion  56 . The longitudinally extending portion  56  is attached to the expansion element  34  such that the entire longitudinally extending portion  56  is located beyond the entire expansion element  34  in the longitudinal direction  30  with respect to the outer tube  16 . In accordance with certain exemplary embodiments, the entire cap  54  is located beyond the entire expansion element  34  in the longitudinal direction  30  with respect to the outer tube  16 . The cap  54  and connecting member  52  may be made out of material that is different than the expansion element  34 . In this regard, the cap  54  and connecting member  52  may thermally expand at a different rate than the expansion element  34 . However, the three components  54 ,  34  and  52  may be made out of the same material and may thermally expand at the same rate in accordance with certain exemplary embodiments. Both of the ends of the absorber tube  14  may be constructed in an identical manner. However, it is to be understood that the two ends of absorber tube  14  can be configured differently in accordance with various exemplary embodiments. 
       FIG. 3  illustrates thermal expansion of the absorber tube  14  shown in  FIG. 2 . Here, solar energy transferred through the outer tube  16  is directed into the inner tube  22  that increases in temperature and thus expands thermally in the longitudinal direction  30 . The outer tube  16  is not heated to the extent of inner tube  22  and thus does not expand thermally to the same degree as inner tube  22 . Expansion of the inner tube  22  causes the attached expansion element  34  to expand as shown in  FIG. 3 . The expansion element  34  thus allows the inner tube  22  and outer tube  16  to slide relative to one another to compensate for differences in thermal expansion rates. The expansion element  34  is made of a material and provided in a certain thickness and orientation that affords some degree of flexing so that the expansion element  34  can extend and contract in the longitudinal direction  30 . Once the temperature difference between the outer tube  16  and the inner tube  22  is minimized, the two tubes  16  and  22  will contract to their non-heated position. The expansion element  34  is capable of contracting so as to allow the tubes  16  and  22  to slide relative to one another. The expansion element  34  may be arranged so that is biased into a contracted position. In this regard, the expansion element  34  may expand but it will be inherently mechanically biased so as to resist expansion and act to pull the tubes  16  and  22  back into their initial position. However, the expansion element  34  need not be biased towards one position or another in accordance with various exemplary embodiments. 
     The space  50  present between the outer wall  24  of the inner tube  22  and the inner wall  20  of the outer tube  16  is evacuated so that a vacuum is formed therein. Space  50  may also include the area between the outer wall  24  and the longitudinally extending portion  56 , connecting member  52  and expansion element  34 . Provision of a vacuum increases insulation properties of the absorber tube  14 . In this regard, thermal energy transferred into the inner tube  22  causes the inner tube  22  to increase in temperature. The vacuum within space  50  functions to prevent this heat from being transferred through space  50  and into the outer tube  16  and out of the absorber tube  14 . As such, the inner tube  22  may be extremely hot while the outer tube  16  is cool to the touch. The vacuum thus increases the efficiency of the absorber tube  14 . During manufacture of the absorber tube  14 , a hole can be present in the outer tube  16  through which gases in the space  50  can be removed to achieve vacuum. The hole can be closed with a melted glass seal  58  once the gases have been removed. 
     A getter  40  may be provided in order to absorb or reduce gas within the space  50  so that the vacuum is maintained as strong as possible to increase efficiency of the absorber tube  14 . A plate  42  can extend from a longitudinally extending portion  56  of the expansion element  34 . A getter trough  44  can be defined on the plate  42  for use in carrying the getter  40 . Getter  40  may be made of barium in accordance with certain exemplary embodiments. However, it is to be understood that getter  40  can be made of various materials in accordance with other embodiments. For example, getter  40  may be made of aluminum, magnesium, calcium, zirconium, phosphorus, and/or sodium in accordance with various exemplary embodiments. After evacuation of gases from the space  50 , the melted glass seal  58  may be formed and the getter  40  can be heated or otherwise treated so as to effect gas absorption. For example, the getter  40  can be heated by way of radio frequency induction heating. In other embodiments, getter  40  can be heated by way of introduction into a high frequency magnesia field. The absorber tube  14  may be treated so that the getter  40  is heated or treated while the majority of the other portions of the absorber tube  14  are not treated. 
     Treatment of the getter  40  may cause the getter  40  to completely evaporate from the getter trough  44 . The treatment process may cause the getter  40  to absorb or react with any gases remaining in the space  50  so that a more perfect vacuum is achieved. Treatment of the getter  40  may cause a coating  46  to be formed on a portion of the inner wall  20  of the outer tube  16  as shown in  FIG. 4 . The coating  46  may be a silver-colored metallic deposit in accordance with certain exemplary embodiments. The getter  40  and/or coating  46  will have a working life to absorb gases present within the space  50  so that a more perfect vacuum is achieved. The getter  40  may be present in getter tough  44  in addition to the presence of coating  46 . However, in other embodiments, the getter  40  will be completely evaporated so that only coating  46  is present. In yet other embodiments, getter  40  will be present in getter trough  44  and coating  46  will not be present. Gases absorbed by the getter  40  may include air, carbon dioxide, carbon monoxide, nitrogen, oxygen, water and/or hydrogen. 
     Coating  46  may be reactive with oxygen. As such, should the absorber tube  14  leak such that air is introduced into space  50  oxygen present within the air will cause coating  46  to oxidize. Oxidization of coating  46  may cause coating  46  to turn white thus providing an indication to personnel that the absorber tube  14  has lost vacuum and is in need of repair or replacement in order to reestablish desired operating efficiency. Barium getter  40  may result in a coating  46  that becomes white when exposed to air. Barium getter  40  may continue to function to absorb gases once exposed to air. Other types of getters  40  can be used as discussed and may cause coatings  46  that turn colors other than white when the vacuum is compromised. 
     Getter trough  44  may be arranged so that the getter  40  is located completely within the space  50  defined between the outer wall  24  of the inner tube  22  and the inner wall  18  of the outer tube  16 . In this regard, the getter  40  may be positioned so that it does not extend beyond the end  48  of the outer tube  16  in the longitudinal direction  30 . However, other embodiments are possible in which the getter  40  extends partially or is completely located beyond the end  48  of the outer tube  16  in the longitudinal direction  30 . Although described as being connected to expansion element  34 , getter trough  44  can extend from other portions of the absorber tube  14  for example the outer tube  16 , cap  54 , inner tube  22 , or connecting member  52  in other embodiments. Further, getter  40  need not be carried by a getter trough  44  in other arrangements and this component is thus not present in other versions of the absorber tube  14 . 
     Getter  40  may be a solar barrier and could thus hinder the absorption of solar energy into the inner tube  22 . In this regard, the getter  40  may decrease the efficiency of the parabolic trough  10 . Getter  40  can thus be arranged within the absorber tube  14  so that its impact on preventing solar absorption is minimized or reduced. In this regard, getter  40  can be provided in two pieces that are oriented at 180° with respect to one another about axis  28 . The two part getter  40  may thus be arranged to minimize interference with the absorption of solar energy by the inner tube  22 . In other embodiments, pieces of getter  40  can be located 90°-180° from one another about axis  28 . In yet other arrangements, pieces of getter  40  can be located up to 45° from one another. In other versions of the absorber tube  14 , getter  40  may be a single piece and can be located 360° about the axis  28 . Further, getter  40  can be placed proximate to the expansion element  34  so that it covers a minimum amount of the inner tube  22  that is exposed to solar energy though the outer tube  16 . Although shown in  FIG. 2  as being located at both ends of the absorber tube  14 , it is to be understood that the getter  40  need not be located at both ends but may be located only at a single end of the absorber tube  14  in certain exemplary embodiments. Such an arrangement may act to minimize the surface area of getter  40  and thus minimize the resulting decrease in efficiency of the absorber tube  14 . The coating  46  may be limited to an end of the outer tube  16  so that interference of thermal energy transmission into the inner tube  22  is minimized. Coating  46  may be arranged so that it is generally two sections that are located 180° from one another about axis  28 . However, it is to be understood that coating  46  may extend across any portion of the outer tube  16  in other embodiments. 
     While the present invention has been described in connection with certain preferred embodiments, it is to be understood that the subject matter encompassed by way of the present invention is not to be limited to those specific embodiments. On the contrary, it is intended for the subject matter of the invention to include all alternatives, modifications and equivalents as can be included within the spirit and scope of the following claims.