Abstract:
A solar heat receiver system includes multiple vertically extending solar heat receiving tubes for transferring solar heat to a heat transfer medium flowing through the solar heat receiving tubes, a rigid support structure, two first connectors to connect two spaced-apart portions of the solar heat receiving tubes to the rigid support structure so as to prevent horizontal movement of the two spaced-apart portions of the solar heat receiving tubes, and a second connector for connecting at least one intermediate portion of the solar heat receiving tubes to the rigid support structure, which intermediate portion is located between the spaced-apart portions. The second connector allows a limited amount of horizontal movement of the at least one intermediate portion of the solar heat receiving tubes due to a thermally induced bow of the solar heat receiving tubes.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a solar heat receiver system. More particularly, the present invention relates to a solar heat receiver system that is mounted on top of a tower so that sunlight is concentrated onto the receiver system. 
         [0003]    2. Description of the Related Art 
         [0004]    Generally speaking, a solar thermal power plant with a central receiver system has a solar heat receiver system mounted on the top of a tower for receiving sunlight concentrated onto the receiver system by means of a field of mirrors, so-called heliostats, that track the movement of the sun from dawn to dusk. The receiver system can be an arrangement having receiver panels configured in numerous ways, for example, in a cylindrical, a square, a square with beveled corners, a rectangular, or a hexagonal configuration. The tube panels of a solar heat receiver system may form different steam generating stages, such as economizers, evaporators, superheaters and reheaters. The tube panels are subject to high heat flux on their exposed surfaces due to the incident radiation directed from the heliostats. Heat is transferred to a heat transfer medium, usually steam or water, flowing inside the tubes of the tube panels. 
         [0005]    The tube panels generally comprise vertical heat receiving tubes, which, typically, have a length on the order of five to ten meters, but they can be as long as thirty meters, or even more. Solar heat receiving tubes may be subject to strong wind forces and seismic forces, which may cause horizontal movements or bowing of the tubes. Such movements or bowing may cause large stresses and eventual breakage of the tubes. 
         [0006]    The solar heat receiving tubes of a tube panel, especially, of a superheater or a reheater panel, every day undergo large temperature changes, up to 500° C., from the night-time conditions to the time of maximum sunlight. Because an intense heat flux is generally directed only on the exposed outer surface of the individual tubes, there is a large temperature gradient between the outer and the inner tube surfaces, which tends to bow the tubes outwards. 
         [0007]    U.S. Pat. No. 4,384,550 discloses a solar heat receiver system comprising a heat collecting system that includes a plurality of vertically extending receiver tubes and a plurality of substantially horizontally extending and vertically flexible attachment members having an end affixed to a support structure and another end affixed to individual tubes, to permit vertical movement of the tubes, while restraining the tubes from any substantial horizontal movement. 
         [0008]    U.S. Pat. No. 4,653,470 discloses a support structure for multiple adjacent parallel vertical tubes of a solar heat receiver panel. Each tube is attached by a lug located at an intermediate point along the length of the tube to a horizontal steel support bar. The support bar is attached to tie links, which are pivotally attached at their other ends to a vertical support frame structure. The support structure accommodates differences in thermal expansion and contraction of the tubes, and resists transverse wind and seismic loads for the vertically-oriented solar heated panel. 
         [0009]    According to conventional practice, as disclosed in the U.S. Pat. No. 4,384,550 and No. 4,653,470, mentioned above, the tubes are supported horizontally at one or more locations along their length, so that the tubes are not overstressed by the wind or seismic loads. While this horizontal restriction limits the wind and seismic stresses on the tube, it may create high thermal stresses due to the aforementioned temperature gradient across the tube. Therefore, there still exists a need for a solar heat receiver system, in which the problems discussed above are minimized. 
       SUMMARY OF THE INVENTION 
       [0010]    An object of the present invention is to provide a solar heat receiver system in which the problems discussed above are minimized. 
         [0011]    The present invention provides a solar heat receiver system that includes multiple vertically extending solar heat receiving tubes having a length for transferring solar heat to a heat transfer medium flowing through the solar heat receiving tubes, a rigid support structure, two first connectors to connect two spaced-apart portions of the solar heat receiving tubes to the rigid support structure, so as to prevent horizontal movement of the two spaced-apart portions of the solar heat receiving tubes, and a second connector for connecting at least one intermediate portion of the solar heat receiving tubes to the rigid support structure, which intermediate portion is located between the spaced-apart portions, wherein the second connector allows a limited amount of horizontal movement of the at least one intermediate portion of the solar heat receiving tubes, due to thermally induced bow of the solar heat receiving tubes. 
         [0012]    The second connector is thus advantageously designed so as to allow the at least one intermediate portion of the solar heat receiving tubes to bow a predefined amount in the horizontal direction, after which it is substantially prevented from further horizontal movement. According to a preferred embodiment of the present invention, the second connector is made, for example, by bolts or other fixing means in a horizontal slot or by multiple parallel plates with gaps, so as to allow a certain amount of relatively free horizontal movement of the intermediate portion of the solar heat receiving tubes. When the horizontal movement or bow of a solar heat receiving tube extends to a limit, as defined, for example, by the horizontal length of a slot, further horizontal movement is prevented. 
         [0013]    The second connector preferably allows more horizontal movement outwards than inwards. A movement either outwards or inwards is here meant to be movement of an at least one intermediate portion of a solar heat receiving tube from its straight starting position, or night-time position, towards the exterior or interior, respectively, of the tower. Even more preferably, the second connector allows horizontal movement of the at least one intermediate portion of the solar heat receiving tubes only outwards. 
         [0014]    When intermediate portions of a solar heat receiving tube are not fixed, the sunlight causes a temperature gradient across the tube, which bows the solar heat receiving tube into the shape of an arch. According to the present invention, a limited amount of bowing of the tube is allowed. The exact value of the allowed bow is based on not allowing an overstress of tubes due to wind and seismic forces, and minimizing the thermal stresses. In order to avoid excessive bowing, the horizontal movement of intermediate portions of the heat receiving tube is preferably limited to 1%, even more preferably, to 0.5% of the length of the tube. 
         [0015]    The use of the second connector that allows limited horizontal movement or bow of an intermediate portion of the solar heat receiving tubes significantly reduces thermal stresses of the solar heat receiving tubes. Thus, the present invention allows absorption of higher incident thermal radiation than the use of solar heat receiving tubes that are fixed at multiple intermediate points. Therefore, by the present invention, it is possible to transfer higher heat fluxes than by using conventional fixing of the solar heat receiving tubes, and thus, to obtain a more efficient central receiver. 
         [0016]    An advantage of the present invention is that a bowed solar heat receiving tube, even a slightly bowed tube, is stronger than a straight tube in resisting the wind and seismic forces. Thus, a solar heat receiving tube allowed to bow is more rigid and less prone to overstresses caused by wind forces or seismic forces. Wind forces will move the central portions of a fully bowed tube only inwards. The system is designed so that the stresses caused by wind or seismic forces moving the panels from their bowed outward position to their cold position are within Code allowable limits. 
         [0017]    The second connector can, according to another preferred embodiment of the present invention, be arranged to be flexible, so as to allow a restrained horizontal movement of the at least one intermediate portion of the solar heat receiving tubes. A restrained movement here means a movement that is not free, but is resisted by a progressively increasing force. Such a restrained movement can be achieved by various mechanical devices, for example, by a horizontal spring or bellows. A flexible second connector, as defined above, is preferably more flexible, i.e., less resisting, to horizontal movement of the solar heat receiving tubes outwards than inwards. Alternatively, the flexibility or resistance inward and outward of the spring or bellows may be the same, but additional devices such as pin connected slotted holes further limit inward movement of the solar heat receiving tubes. 
         [0018]    The solar heat receiving tubes can be in their cold position either substantially straight between the two first connectors, or the first connectors can be arranged on a bent portion of mainly straight tubes. In other words, the two horizontally fixed spaced-apart portions and the at least one intermediate portion of the solar heat receiving tubes are either on the same linear portion of the solar heat receiving tubes, or the horizontally fixed spaced-apart portions are horizontally displaced from the at least one intermediate portion by bent portions of the solar heat receiving tubes. When using the first mentioned simple arrangement, the straight tube simply bows to an arch. 
         [0019]    A solar heat receiver system according to the present invention is especially useful when the temperature changes of the solar heat receiving tubes are high, i.e., especially when the solar heat receiving tubes are part of a superheater or a reheater. 
         [0020]    The above brief description, as well as further objects, features, and advantages of the present invention will be more fully appreciated by reference to the following detailed description of the currently preferred, but nonetheless illustrative, embodiments of the present invention, taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a schematic diagram of a solar thermal power plant with a central receiving system. 
           [0022]      FIG. 2  is a schematic diagram of a solar heat receiving tube in accordance with a first embodiment of the present invention. 
           [0023]      FIG. 3  is a schematic diagram of a solar heat receiving tube in accordance with a second embodiment of the present invention. 
           [0024]      FIGS. 4   a,    4   b  and  4   c  are schematic diagrams of exemplary connections of a solar heat receiving tube in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]      FIG. 1  schematically shows a solar thermal power plant  10  comprising a central heat receiving system  12  mounted on top of a tower  14  for receiving sunlight concentrated onto the receiving system  12  by means of a field of mirrors  16 , so-called heliostats, that track the sun&#39;s movement from dawn to dusk. The receiving system  12  comprises an arrangement of receiver panels  18  configured in suitable way, for example, in a cylindrical, a square, a square with beveled corners, a rectangular, or a hexagonal configuration. The receiver panels  18  may be tube panels that may function as economizers  18 ′, evaporators  18 ″, superheaters  18 ″ and reheaters, usually disposed in different vertical levels. The tube panels are subject to high heat flux on their exposed surfaces, due to the incident radiation directed from the heliostats. Heat is transferred to a heat transfer medium, usually steam or water, flowing inside the tubes of the tube panels. 
         [0026]      FIG. 2  schematically shows the shape of a vertical solar heat receiving tube  20 , which is part of a heat receiving panel (not shown), such as a superheater panel, arranged into a tower construction (not shown) of a solar thermal power plant. The solar heat receiving tube  20  is shown in a cold starting position  22 , with a solid line, and in a bowed position  24 , with a dashed line, corresponding to a hot heat receiving condition. The actual length of the tube  20  may be, for example, ten meters, and the allowed horizontal displacement of the center portion of the tube, for example, five centimeters. Thus, the relative size of the bow is here highly exaggerated. 
         [0027]    The upper  26  and lower  28  end portions of the tube  20  are fixed against horizontal movement to a rigid support structure  30  of the tower. Here the upper  26  and lower  28  end portions are connected by welding to upper and lower headers  32 ,  32 ′ of a heat receiving panel. The headers  32 ,  32 ′ are usually connected by connection means  34  to the rigid support structure  30 , so as to prevent any horizontal movement. At least one of the headers, here, the lower header  32 ′, is generally arranged vertically movable by connecting it to a spring  36 , so as to accommodate vertical thermal expansion of the tube  20 . A central portion  38  of the tube is attached, in accordance with the present invention, to connection means  40 , which is not firmly fixed to the support structure, but allows a limited bow of the central portion  38 . 
         [0028]      FIG. 3  is correspondingly a schematic illustration of another solar heat receiving tube  42  in its cold starting shape  44 , shown by a solid line, and in a hot heat receiving shape  46 , shown by a dashed line. The tube  42  is again fixed against horizontal movements at upper  48  and lower  50  end portions to a support structure  52  by connection means  34 . The tube  42  differs from that shown in  FIG. 2 , by having bent portions  54  adjacent to the upper  48  and lower  50  end portions. The actual shape of the bent portions  54  may differ from those shown in  FIG. 3 , for example, by having a second bend to a direction opposed to the vertical main direction of the tube  42 . Due to the bent portions  54 , the shape  46  of the tube in the hot heat receiving conditions may be distorted, as shown by the dashed line, also, in the bent portions  54 . A central portion  56  of the tube  42  again comprises connection means  58 , which are not firmly fixed to the support structure  52 , but instead, allow a limited bow of the central portion  56 . 
         [0029]      FIGS. 4   a,    4   b  and  4   c  are schematic diagrams of exemplary connectors (connection means) of an intermediate portion of a solar heat receiving tube, in accordance with the present invention.  FIG. 4   a  shows a connection means  60  having an inwards extending bar  62  connected, for example, by welding to a solar heat receiving tube  64 . The inwards extending bar  62  is connected to a transverse bar  66  that extends to a horizontally elongated slot  68  in a slab  70 , which is connected to the rigid support structure  72 . In practice, there may be several heat receiving tubes connected to a common transverse bar. 
         [0030]    Horizontal distances d 1  and d 2  from the outward and inward ends of the slot  68  to the transverse bar  66  naturally define the limits of allowed horizontal movement outwards and inwards, respectively, of the solar heat receiving tube  64 . According to a preferred embodiment of the present invention, the outward distance d 1  is, in a cold starting position of the solar heat receiving tube  64 , greater than the inward distance d 2 . According to an even more preferred embodiment of the present invention, the inward distance d 2  is zero in a cold starting position of the solar heat receiving tube  64 . 
         [0031]    If the inwards extending bar  62  is not connected to the center of the tube  64 , which is the case necessarily for some of the intermediate connections  60 , if there are more than one intermediate connections, or if only one of the end portions of a solar heat receiving tube is connected so as to allow vertical movement, there may also be vertical movement of the bar  62  due to thermal expansion of the tube  64 . In this case, the slot  68  cannot be entirely horizontal, but it has to be slanted, so as to allow the thermally induced vertical movement. Alternatively, the slot  68  can be horizontal if the slab  70 , instead of being welded to the rigid support structure  72 , could itself also move vertically by being inserted into a vertical slot in the rigid support structure  72 . 
         [0032]      FIG. 4   b  shows another exemplary connector (connection means)  60 ′, which comprises an inwards extending bar  74  connected to each heat receiving tube  76  and a transverse plate  78  connected to the other end of the inwards extending bars of a solar heat receiving panel. The transverse plate  78  can move horizontally a distance that is limited by stopping plates  80 . The stopping plates  80  are connected to the rigid support structure by suitable means, not shown in  FIG. 4   b.    
         [0033]    Horizontal distances d 1  and d 2  from the transverse plate  78  to the outward and inward stopping plates  80  define here the limits of allowed horizontal movement outwards and inwards, respectively, of the solar heat receiving tubes  76 . According to a preferred embodiment of the present invention, the outward distance d 1  is, in a cold starting position of the solar heat receiving tubes  76 , greater than the inward distance d 2 . According to an even more preferred embodiment of the present invention, the inward distance d 2  is zero in a cold starting position of the solar heat receiving tubes  76 . An advantage of the connector (connection means)  60 ′ is that vertical movement of the transverse plate  78  is not limited. 
         [0034]      FIG. 4   c  shows still another exemplary connector (connection means)  60 ″ in accordance with the present invention. This connection means  60 ″ differs from those of  FIGS. 4   a  and  4   b  in that horizontal movement of intermediate portions of the heat receiving tubes  82  is not free, up to certain limits, but, instead, the connection means  60 ″ allow only a restrained horizontal movement. The restraining is here brought about by bellows  84  connected by connecting rods  88  to the solar heat receiving tubes  82  and to a rigid support structure  86 . A moving portion of the bellows  84  may advantageously contain transverse extensions  90 , by which final limits to the horizontal movement of the heat receiving tubes can be set, for example, by suitable stopping plates, as shown in  FIG. 4   b.  The connection means  60 ″ can advantageously be constructed by using non-linear bellows or spring(s), in order to restrain an outward movement of the tubes  82 , less than a corresponding inward movement or by combining features shown in  FIG. 4   a  or  FIG. 4   b  to further limit the inward movement. 
         [0035]    While the invention has been described herein by way of examples in connection with what are at present considered to be the most preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of its features and several other applications included within the scope of the invention as defined in the appended claims.