Abstract:
A getter support assembly for supporting getters in a focusing collector type solar collector system comprising a tube radiation absorber (TRA) and a glass enclosure tube, defining therebetween an annular space, and a solar radiation focusing reflector. The getter support assembly comprises a bridge formed with an elongated trough having a getter support portion. The bridge further comprising feet fixedly attached to the TRA supporting the trough so that the trough is spaced apart from the TRA. The trough further comprises a radiation reflecting surface facing the TRA for blocking radiation emitted from the TRA and missed solar radiation reflected from the reflector.

Description:
FIELD OF THE INVENTION 
     This invention is in the field of solar energy collecting systems of the so-called focusing collector type and is particularly concerned with support and thermal protection of getters fixed to the metal tube radiation absorbers of such solar energy systems. 
     The terms getter, getters and getter material will be used interchangeably hereinafter in the specification and claims. 
     BACKGROUND OF THE INVENTION 
     For reasons such as the rapid exploitation of non-renewable energy resources, the desire for environmentally friendly energy sources and the advantages to locally provide energy to remote communities and factories, there is a continued interest in solar energy systems and improvements thereto. Continuous research increases efficiency of solar systems which gradually become more cost effective and render them more favorable. 
     A particular type of solar collector system is the “focusing collector” type which enhances the effect of solar energy by focusing it onto a smaller area using mirrored surfaces or lenses. In this system, a reflector, which is typically parabolic, receives and reflects (focuses) incoming solar radiation onto a metal tube radiation absorber (TRA)—in the form of a tube; the TRA being concentrically surrounded by a treated glass enclosure tube to limit the loss of heat. To further increase the efficiency by increasing the collection of solar radiation, the collector system typically includes means to track the sun. 
     To minimize the loss of heat through convection and conduction and to improve the solar radiation collection efficiency, the space between the tube radiation absorber (TRA) and the glass enclosure tube is evacuated to very low pressure. 
     The TRA is made of metal with a coating having a high solar radiation absorption coefficient to maximize the energy transfer imparted by the solar radiation reflecting off the reflector. A heat transfer medium, which for practical considerations is typically a liquid such as oil, flows within the TRA. 
     At the high temperatures inherent of solar collectors, the heat transfer medium releases some amount of hydrogen. The hydrogen released, being such a small atom, may permeate through the wall of the TRA and enter the space between the TRA and the glass enclosure tube. This situation is very undesirable as it reduces the vacuum in that space thereby allowing heat loss from the TRA via convection and conduction. Accordingly, it is critical to remove this hydrogen to preserve the efficiency of the focusing solar collector. 
     One method for removing hydrogen from the space is by use of a getter located therein, which is a material that may comprise a variety of metal alloys having a high surface area and affinity for adsorbing hydrogen. Other active gaseous impurities such as CO, CO2, N2 and H2O, which may find their way into the space, can also be thereby adsorbed. 
     Data published on the equilibrium of commercial hydrogen getters indicates that hydrogen getters may have an enormously increased capacity and gas adsorption rate with a relatively small reduction in temperature thereby greatly affecting the cost of a solar collector system. In order to maintain the capacity and rate of gas adsorption by the getter, the getter must be supported in the space between the TRA and the glass enclosure tube and be maintained at relatively low temperature. Thus, the getters should be as isolated as possible from heat transfer such as from conduction and radiation. 
     Radiation may come from either of two sources, one being radiation emitted off the TRA. Despite the fact that the TRA has a coating to limit emission, there is still a non-negligible amount of radiation that the TRA emits. The second radiation source potentially heating the getters is so-called missed solar radiation. This radiation can result from situations wherein some solar radiation is not solely reflectedly focused onto the TRA, but rather misses it. This is typically due to reflector mis-alignment or imperfect TRA mounting position. Some of this “missed solar radiation” may directly or indirectly contact the getters on their support and thereby heat them. 
     U.S. Pat. No. 4,306,543, to Doevenspeck et al, discloses a solar collector comprising a getter located within an evacuated transparent envelope defined by an outer glass tube and an inner metal tube. A rigid frame construction, extending between the glass and metal tubes, supports the getter and also supports a heater. The aim of the design is to provide a heating element in the evacuated envelope to crack hydrocarbons that may be present. However, the design does not provide protection for the getters from heating. 
     U.S. Pat. No. 4,455,998, to Kroontje et al, discloses a solar collector comprising a reversibly heatable hydrogen getter located within an evacuated transparent envelope defined by a glass tube. A reservoir formed in the glass tube supports the getter, which is covered by gauze. However, the reservoir is only described in regards to a plate-shaped absorber solar collector. Also, the reservoir requires a specially configured glass tube, the reservoir—being part of the glass tube—and this reservoir is not insulated from the heat absorbed by the glass tube. Further, the described assembly is designed to alternatively heat or not heat the getter in order to help control the temperature of heat transfer medium in the collector; not to maintain the getter at a low temperature. 
     U.S. Pat. No. 4,508,104, to Takeuchi et al, also discloses a solar collector comprising a getter located within an evacuated transparent envelope. A retaining device composed of leaf springs functions to fix the location of the inner tube of the collector with respect to the outer tube. The retaining device also supports a grommet-shaped casing of a “getter metal.” The portion of the retaining device supporting the getter metal is only described as C-shaped and does not appear to protect the getter from solar radiation. 
     Accordingly, it is an object of the present invention to provide an assembly, for a solar energy collector system of the solar radiation focusing type, for supporting getters and maintaining them at relatively low temperature. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided an assembly for supporting getters in an evacuated space between a tube radiation absorber (TRA) and the glass tube of a solar collector system, comprising a bridge for providing a gap between the getters and the tube radiation absorber wherein the bridge is designed and assembled of components to have minimal surface contact therebetween, thereby limiting conductive heat transfer. 
     According to the present invention there is provided such an assembly comprising a radiation protection arrangement for limiting solar radiation from reflecting and/or emitting onto the getters or onto a component (i.e. the bridge) in direct or indirect contact with the getters. 
     The present invention provides a getter support assembly for supporting getters in a solar collector system of the focusing collector type comprising a tube radiation absorber (TRA) and a glass enclosure tube, defining therebetween an annular space, and a solar radiation focusing reflector, said getter support assembly comprising, a bridge formed with an elongated trough having a getter support portion, the bridge further comprising feet fixedly attached to the tube radiation absorber supporting said trough so that the trough is spaced apart from said tube radiation absorber; the trough further comprising a radiation reflecting surface facing the TRA for blocking radiation emitted from the TRA and missed solar radiation reflected from the reflector. 
     According to one embodiment of the present invention, the bridge further comprises a component in addition to the trough, namely a radiation shield, to block and reflect radiation in order to minimize the temperature of the getters. 
     The bridge further comprises slots providing axial freedom of movement to allow for thermal expansion of the TRA and the bridge. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of a non-limiting example only, with reference to the accompanying drawings, in which: 
     FIG. 1 is section of a solar energy absorber portion of a solar collector of the focusing collector type, fitted with a getter support assembly in accordance with the present invention; 
     FIG. 2 is a perspective view of a tube radiation absorber (TRA) with a getter support assembly fixed thereon, according to an embodiment of the present invention; 
     FIG. 3 is an end view of a different embodiment of a getter support assembly according to the present invention; 
     FIG. 4 is a top view of a getter supporting trough showing a trough covering according to a further embodiment of the present invention; and; 
     FIG. 5 is an end view of a modified trough of the getter assembly according to a further embodiment the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Attention is first directed to FIG. 1 of the drawings showing a section of a solar energy absorber, generally designated  10 , composing part of a solar collector system (not shown). Such a solar energy absorber  10 , may be part of a solar field holding hundreds of meters of collector units. The collectors can be flat (plate) or non-flat collectors 
     The solar energy absorber  10  comprises a tube radiation absorber or TRA  12 , a reflector  14 , and a glass enclosure tube  16 . The TRA  12 , which is coated with a solar radiation absorbing coating, is coaxially received within the glass enclosure tube  16  defining a space  18  therebetween, and is fixed at the focus of the reflector  14 , which is typically parabolic. 
     Resting on the TRA  12  is a getter support assembly, generally designated  20 , of an embodiment of the present invention shown in more detail in subsequent figures. It is noted that the assembly  20  is fixed in the so-called one-sun position wherein it faces toward the sun so as to ideally receive solar radiation only from rays directly from the sun and not from the much more concentrated rays reflected off the reflector  14 . 
     Solar radiation rays R 1  and R 2  illustrate how the sun&#39;s rays reflect off the reflector  14 , pass through the glass enclosure tube  16  and onto different locations of the TRA  12 . In contrast, ray R 3  illustrates radiation not reflected onto the TRA  12  rather onto the getter support assembly  20 . This ray R 3  represents so-called missed solar radiation that can heat the assembly  20  thereby reducing the gas adsorption capacity of the getters  30  (shown in FIG.  2 ). 
     It can be understood that other rays, for example ray R 4 , may reflect off the top of TRA  12  and onto the assembly  20 , again, thereby heating the assembly and reducing the gas adsorption capacity of the getters  30 . 
     FIG. 2 shows the assembly  20  atop the TRA  12 . In its simplest form, the assembly  20  merely comprises a bridge  32  composed of a trough  34 , holding the getters  30  (shown with phantom lines), and L-shaped metal feet  36 . The feet  36  are typically made of a thin gauge sheet metal to limit the conduction path—as are all components associated with the assembly  20 . The trough  34  has a getter support portion  37 , upon which the getters  30  rest. 
     Some important features of the design can now be noticed. The feet  36  are attached to the TRA  12  at as limited a contact point(s) as possible, as illustrated by spot welds  38  at lower tabs  39  of feet  36 . Similarly, the feet  36  and the trough  34  are typically in contact only, or mainly, at edges thereof thereby restricting thermal conduction therebetween. 
     Further, trough  34  has slots  40  (indicated by dashed lines) which not only allow the upper portion of feet  36  to pass through to stably support the trough, but also these slots extend in the longitudinal direction of the trough to allow for “play” between the trough and feet. There may also typically “play” in the transverse direction. This “play” eliminates issues involved with the different expansion of the bridge  32  and the TRA  12  which results during the heating and cooling cycles of the collector when alternately exposed and not exposed to the sun on a daily basis or due to passing clouds. At such times, the TRA  12  can heat up more quickly and become considerably hotter than the bridge  32 , resulting in a different expansion and size relative to the bridge. 
     The trough  34  itself can act to reflect solar radiation—and would therefore typically be polished and/or coated, on at least its outer surface  33  (facing the TRA  12  and reflector  14 ); the coating typically consisting of a refractory metal such as silver or gold or other highly reflective materials, and/or the outer surface is polished, to reflect radiation both infrared and visible so that the trough is not heated and will not be a source of conduction to the getters  30 . 
     Infrared radiation is emitted by the TRA  12  and visible radiation is reflected directly off the reflector  14  as illustrated by ray R 3  in FIG.  1 . However, as it will be understood to a person of the art, for protecting the trough  34  from solar radiation—and thus the getters  30  from high temperature—it may be preferable to employ one or more additional components to reflect the aforementioned radiation. 
     In another embodiment of the present invention, FIG. 3 shows an end view of an assembly  50  wherein such an additional component, a radiation shield  52 , is provided. Only one radiation shield  52  is shown, but more than one can be used. The shield  52  is shown attached to feet  54 , which in this embodiment have slots  56  for holding the shield. The shield  52  comprises wings or side-walls  57  and  58  that extend longitudinally the length of trough  60  (which now need not be dimensioned, polished and/or coated to block and reflect solar radiation). Shield  52  also has a base portion  62  (shown with phantom lines), between the side-walls  57  and  58 , and extends under the portion of the trough  60  between the feet  54  (only one seen in FIG.  3 ). Tabs  74  (only one seen in FIG. 3) of feet  54  block radiation emitted from TRA  12  from reaching the ends of trough  60 . 
     The shield  52  is most conveniently made of one piece and discussion of side-walls  57  and  58  and base portion  62  is to clarify the geometry of the shield and to emphasize that it reflects radiation from underneath (off the TRA  12 ) as well from the side (from the TRA  12 , but also missed solar radiation from the reflector  14 ). 
     The shield  52 , as with the trough  34  of the embodiment of FIG. 2, is typically coated and/or polished, on at least its surface  63  facing the TRA  12  and reflector  14 , with an appropriate coating, typically consisting of a refractory metal such as silver or gold or other highly reflective materials, and/or the outer surface is polished, to reflect radiation so that it is not heated and will not be a source of conduction to the getters  30 . 
     Additional important details of the present invention can now be observed. Between the TRA  12  and the feet  54  are wedge-shaped gaps  70  as the assembly  50  can be stably attached to the TRA  12  without need for the tabs  74  to conform to the cylindrical shape of the TRA. Also, outer lower portions  72  of the feet  54  do not extend down to the TRA  12 . Both of these features help keep the temperature of the getters  30  at a minimum. 
     Further, the feet  54  have slots  76  contributing to a more limited heat transfer path from the TRA  12  to the getters  30  via the trough  60 . Each of the feet  54  has a T-shaped extension  77  having side projections  79  to secure the trough  60 , though the fit is not tight so as to allow “play” between the trough  60  and the feet  54 . 
     Also seen in FIG. 3 is a covering  78  that may partly or completely surround the trough  60 . This covering  78 , is convenient for loading the getters  30  onto the trough  60  as they can be rapidly slid thereon, and the covering also prevents the getters from being displaced from the trough during assembly and operation of the solar collector system. The covering  78  may be a sleeve-like mesh (illustrated best in FIG. 4) so as to provide for open area and an easier path for the hydrogen or other gaseous impurities to approach and be adsorbed by the getters  30 . 
     For clarification of certain components, FIG. 4 shows a top view of a trough  80  (which can be of a design similar to the aforementioned troughs  34 ,  60 ) showing an arrangement of a plurality of getters  30  sitting thereon, as well as a trough-covering in the form of a mesh  82 . Also noted is a clearer exemplification of slots  40 , which as understood in conjunction with FIG. 3, provide for freedom of movement or “play” between the feet  36 ,  54  especially in the longitudinal direction, although in the transverse direction as well, including angles in between, in order to allow for variation in thermal expansion between the TRA  12  and the bridge  32 . 
     The trough  80  can further comprise, for example, auxiliary members  84  for purposes such as attaching other components as appropriate such as temperature indicators, etc. 
     FIG. 5 illustrates a modification designed to further limit the conductive heat transfer to the getters  30  wherein a trough  86  comprises a ridge  88  running the length of the trough forming a getter support portion  89 . As is seen, the getters  30  thereby have much more limited physical contact with the trough  86 , thus limiting the conductive heat transfer path originating from the TRA  12 . 
     It is understood that there are numerous patterns that could be produced in the getter support portion  37 ,  89  of the trough  34 ,  60 ,  80 ,  86  upon which the getters rest that would reduce the heat conduction between the trough and the getters—one example being dimples (not shown). 
     It should be noted that various components of the getter support assembly described above, as well as variations thereof are provided merely by way of illustration and are by no means exclusive, and many variations and modifications thereof are possible. 
     For example, the radiation shield  52  could be attached to the TRA  12  or to the trough  60  instead of to feet  54 . 
     In another example, the trough  34 ,  60 ,  80 ,  86  of any of the above embodiments could be formed to curl in on itself to help secure the getters  30  thereon. Or, in the embodiment wherein the getter support assembly  50  has a shield  52 , the trough could be wholly or partly perforated, thereby providing less of a conductive heat transfer path. In a combination of the aforementioned, the trough  34 ,  60 ,  80 ,  86  could be perforated and curled in on itself, or even tube-shaped, thus securing the getters  30  and still allowing hydrogen or other gaseous impurities relatively free access to the getters—serving in a manner analogous to mesh  82 . 
     It will be appreciated that the above descriptions are intended only to serve as examples, and that many other embodiments are possible within the spirit and scope of the present invention.