Abstract:
A bottom supported solar receiver tube/header assembly having a bottom clip for supporting the entire load of a plurality of tubes carrying a heat absorbing fluid. The tubes are allowed to expand vertically upwardly under thermal flux conditions created when the fluid absorbs heat from the solar receiver panel. A receiver panel assembly incorporating a plurality of the bottom supported receiver tubes requires less piping than a comparably sized, conventional receiver panel assembly with top supported tubes and even better facilitates access and maintenance of valves associated with the receiver panel assembly. The invention further allows a solar receiver panel assembly to be constructed with significantly fewer drain and vent valves than previously developed, top supported receiver panel assemblies.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to solar receiver panels, and more particularly to bottom supported solar receiver panels that enable the upward thermal expansion of tubes attached to the solar receiver panels. 
     BACKGROUND OF THE INVENTION 
     In the backdrop of ever dwindling fossil fuel resources, researchers are exploring ways of harnessing alternate sources of energy. Solar energy is a promising alternate source of energy. Engineers are facing several challenges in capturing, storing and converting solar energy. The problem of converting solar energy into electricity in a cost-effective manner and on a large scale still poses challenges. 
     Several approaches are being followed to reach the goal of generating a large-scale and reliable flow of electricity from solar energy. One such method is through the use of a solar central receiver mounted on top of a tower. The solar receiver is basically a heat exchanger that absorbs concentrated solar energy. The receiver absorbs the sun&#39;s energy in a concentrated form from an array of mirrors called heliostats. The receiver comprises a number of panels. Mounted on the panels is a connected set of tubes carrying a heat absorbing fluid. The fluid inside the tubes traces a serpentine path from panel to panel when circulating inside the tubes. The receiver functions as a heat exchanger to transfer the solar energy received from the heliostats to the heat absorbing fluid carried by the tubes. For example, in one design molten salt is pumped up to the receiver and circulated inside the receiver panel tubes. The molten salt is heated by the solar energy absorbed by the receiver tubes. The heated molten salt flows into a ground based hot thermal storage tank(s). Hot molten salt is then pumped from the hot thermal storage tank as needed to create steam that powers a steam turbine for generating electricity. 
     Panels are comprised primarily of a strongback, insulation, receiver tubes, headers and tube guide/supports. Tubes are connected at the top and bottom of the panel by the headers. The tube-header assembly is connected to the guide/supports by clips. The guide/supports are rigidly attached (welded or bolted) to the strongback, which in turn is attached to the receiver tower super-structure. In known receiver designs, though multiple clips may be used to hold the tubes, it is the topmost clip which bears the vertical deadweight of the tubes and header assembly. Other clips along the tube length carry the horizontal and bowing loads on the tubes and also maintain the alignment of the tubes. Thus, the known receiver designs use a top supported panel design where the vertical load of each tube is carried solely by the topmost clip at an upper end of each tube. 
     The top supported receiver panel design, while having proven to be effective, could nevertheless be improved in several ways. Since the tubes are supported at the top, as the tubes thermally expand, they expand in a downward direction when thermal flux is applied. Top supported receiver panels generally also require relatively large cold supply and hot return pipelines for molten salt to be attached near the top of the receiver panel that is stationary during changes in temperature. Consequently, the top supported receiver panels all require relatively large pipelines to be run through the congested center of the cylindrical receiver and to the tops of the panels. This arrangement also requires lengthy pipe runs. Therefore, construction can often become complex due to the routing difficulties and lengthier pipes used within the cylindrical receiver. Even a flat billboard shape receiver requires long runs of large pipe if the panels are top supported. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a solar receiver panel having bottom supported tubes that carry heat absorbing fluid. In a preferred embodiment, the tubes are attached to the receiver panels with clips spaced apart vertically along the lengths of the tubes. A bottommost clip of each tube carries the entire vertical weight of its associated tube and header assembly. The clips allow portions of the tubes to move up and down when the tubes undergo thermal expansion and contraction. The tubes undergo thermal expansion in a vertically upward direction when the thermal fluid (e.g., molten salt or others) inside the tubes is heated by absorbing heat from the receiver panels. The bottom supported receiver panel reduces the lengths of piping required, as well as the routing complexity of the piping, within the receiver. Further, the number of valves required in comparison to the top supported receiver can be reduced. Further, the valves can be placed at a convenient location, for example, on the deck of the receiver for easy maintenance. The individual receiver panels are connected by jumper lines. Jumper lines connecting the bottoms of adjacent panels typically have drain valves located on each servicing two receiver panels. Similarly, jumper lines connecting the top of adjacent panels have vent valves. The bottom supported receiver requires a fewer number of vent and drain valves as compared to a top supported receiver. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  shows a perspective view of a solar absorption panel in accordance with a preferred embodiment of the present invention; 
         FIG. 2  shows a perspective view of a clip assembly used with the receiver; 
         FIG. 3  shows a cross section of the clip arrangement of the present invention in a representative form; 
         FIG. 4A  shows a prior art receiver constructed using top supported receiver panels; 
         FIG. 4B  shows the reduced piping requirements of the bottom supported solar receiver of the present invention; 
         FIG. 5A  shows the drain valve arrangement in a conventional, top supported receiver; and 
         FIG. 5B  shows the drain valve arrangement for the bottom supported receiver panel of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     Initially, a general description of the construction and operation of a solar power tower will be provided. A solar power tower is used to collect solar thermal energy and convert it into electricity. A large number of sun-tracking mirrors called heliostats collect the solar energy. The collected solar energy from the heliostats is redirected and concentrated onto a solar receiver mounted on top of the solar power tower. The solar receiver can be constructed by various methods. The present invention is adapted for a solar receiver that functions as a heat exchanger. The receiver functioning as a heat exchanger transfers the concentrated solar energy redirected from the heliostats to a fluid circulated through a piping system inside the receiver. 
     The fluid inside the receiver functions as an energy transfer medium. The fluid is preferably a molten salt coolant, but the present invention is not limited to use with specific type of fluid. Molten salt is used only for explanatory purposes in the following description. A first set of ground based cold thermal tank(s) store the molten salt coolant at around 550° F. (287° C.). A pump(s) is used to transfer the molten salt from the cold thermal storage tank to the receiver located on top of the solar power tower. The molten salt is heated up to around 1050° F. (565° C.) as the fluid circulates through the receiver panels in a serpentine manner. 
     The heated molten salt then exits the receiver through a down comer hot return pipe and flows into ground based hot thermal storage tank(s). The stored hot salt in the hot thermal tank(s) is drawn out as needed to generate steam to power a steam turbine. The steam turns the turbine which is connected to a generator to produce electricity, for example, via a standard Rankine cycle. The receiver panels are described next in detail. 
     Referring now to  FIG. 1 , an absorption panel  10  in accordance with a preferred embodiment of the present invention is shown. A solar receiver typically comprises a plurality of the solar absorption panels  10  positioned adjacent one another. There are several arrangements of panels, the most common of which forms either a cylindrical shape receiver or a flat “billboard” shape receiver. Each panel  10  comprises a plurality of solar absorption tubes  12 . A plurality of independent clip guide/support assemblies  16  secure tubes  12  to a panel strongback structure  14 . Typically, each clip guide/support assembly  16  is welded to the panel strongback structure  14 . Insulation  18  thermally isolates the tubes  12  from the panel strongback structure  14 . Insulation  18  is provided behind absorption tubes  12  and in front of the panel strongback structure  14 . As will be explained in greater detail in the following paragraphs, the fluid first enters through header  20 A at the bottom of panel  1 . 
       FIG. 2  shows an enlarged view of the clip  22  and guide/support assembly  16 . The clip  22  and guide/support assembly  16  consists of slidable clips  22  disposed on guide rods  24 . The guide rods  24  are fixed on a bracket  26  in parallel, spaced apart in relation to one another. The bracket  26  is joined to a strongback structure  29  of the panel  10  by a support  28  member. 
     Tubes  12  are each firmly welded to the slidable clips  22 . Molten salt inside the tubes  12  is heated as it absorbs the thermal energy collected by the receiver. The heated molten salt causes the tubes to undergo thermal expansion. The clip  22  and guide/support assembly  16  is designed to allow unrestrained axial expansion of the tubes  12  along the Y-axis (i.e., vertically), or rather along the length of the tubes  12  from the bottoms toward the tops. Clip  22  and guide/support assembly  16  effectively restrains any bowing or motion of the tubes  12  in either the X-axis or the Z-axis of a given plane. 
       FIG. 3  shows a side view portion of the solar panel receiver  10  of the present invention incorporating a plurality of supporting clips  22  and guide/support assemblies  16 . The clip  22 A and guide/support assembly  16 A is the topmost clip  22  and guide/support assembly  16  of a given panel  10  (shown in FIG.  1 ). The middle clip  22 B and guide/support assembly  16 B is positioned between the topmost clip  22 A and guide/support assembly  16 A and a bottom clip  22 C and guide/support assembly  16 C. It will be appreciated that the panel  10  may include additional clip  22  and guide/support assemblies  16 , depending on the overall length of the panel  10 . Nevertheless, the vertical load of the tube  12  and header  20 A/ 20 B assembly is carried entirely by the bottom clip  22 C and guide/support  16 C, while any additional clips  22  along the vertical length of the tube  12  restrain the tubes  12  from bowing in the other two normal directions along the X and Z axis. Hence, the tubes  12  are allowed to thermally expand upward in relation to the bottom clip  22 C and guide/support assembly  16 C. The unidirectional arrow  30  indicates the direction of thermal expansion of tube  12 . 
       FIG. 4A  shows a prior art receiver  32 A constructed using top supported receiver panels.  FIG. 4B  shows a receiver  32 B constructed using the bottom supported receiver panels of the present invention, and highlights the reduced piping requirements of the bottom supported solar receiver. The top supported receiver  32 A uses a relatively complex routing of internal pipes since the hot molten salt is supplied to, and collected from, the top of the receiver. To further complicate pipe routing, the supply riser  36  is typically split into two supply lines  36   a  and  36 B that supply two parallel flow circuits through the receiver. Similarly, two return lines  38 A and  38 B exit each of these circuits before being combined into a single return downcomer  38 . In the top supported receiver  32 A, a cold salt riser pipe  36  and a hot salt down comer pipe  38  have to extend well up into the receiver to be able to connect to the first and last panels within the receiver. 
     The cold salt riser pipe  36  and the hot salt down comer pipe  38  have relatively large dimensions because they are used to transport a large quantity of salt to and from the receiver  32 A. Accommodating large dimensioned pipes in the receiver  32 A requires either a physically larger receiver or much more complex and congested piping, both of which complicate and increase construction cost. Further, cold salt control valves  40  for the cold salt riser pipe  36  also are typically positioned within the receiver panels. Thus, the relatively complex piping and valving needed within a receiver having top supported tubes can significantly increase the overall cost of constructing the receiver. 
     With the present invention, a bottom supported receiver assembly  32 B of  FIG. 4B , which includes one or more receiver panels  10 , simplifies the piping required within the receiver assembly by enabling a substantial degree of the piping to be included within the tower  34 . Space is limited within the receiver assembly  32 B, but the tower  34  typically has a significantly greater internal area to accommodate the cold salt riser pipe  36  and supply lines  36 A and  36 B and the hot salt down comer pipe  38  and return lines  38 A and  38 B. For the bottom supported receiver panels  10  of the present invention, the hot and cold salt pipelines need to be routed only to a deck  33  of the receiver  32 B. This arrangement saves a significant length of pipe. In one instance, the savings has been found to be several hundred feet of hot and cold salt piping. Further, the movement and loads of the hot salt down comer pipe  38  are significantly reduced, leading to a less complex interface between the receiver assembly  32 B and the hot down comer pipe  38 . Hence, it will be appreciated that it is desirable, both from a cost standpoint and an overall system complexity standpoint, to have the piping  36 ,  36 A,  36 B,  38 ,  38 A,  38 B contained within the tower  34  as much as possible rather than in the receiver assembly  32 B. 
     As should be clear from  FIG. 4B , for the bottom supported receiver panels  10  used in the receiver panel assembly  32 B, the cold salt riser pipe  36  does not need to be routed within the receiver panel assembly  32 B up to the top area of the panel thereof; it only needs to be connected near the bottom of the receiver panel assembly  32 B, and preferably placed near the deck  33 . Similar simplification is achieved for the hot salt down comer pipe  38 . Bottom supported receiver panel  10  allows placement of the cold salt control valves  40  on or below the deck  33  of the receiver  32 B. Such an arrangement of salt control valves  40  further facilitates access and maintenance. 
       FIG. 5A  shows the drain and vent valve arrangement in a top supported, prior art receiver  42 . Purely for illustration purposes, the receiver  42  is shown as having eight receiver panels  42   1 - 42   8 . The fluid enters the chain of connected receiver panels  42   1 - 42   8  from an input conduit  44 . The panels are interconnected by means of jumper lines  46   a  and  46   b . The low level jumper lines  46   a  have drain valves  48  for draining the fluid. The drain valves  48  are normally closed during operation and are opened only when the receiver is drained or filled. Similarly, vent valves  50  are shown that are normally closed during operation and are opened only when the receiver is drained or filled. 
       FIG. 5B  shows a drain and vent valve arrangement for a plurality of bottom supported receiver panels  10  of the present invention. As shown, the bottom supported receiver panels  10  require fewer drain and vent valves  48  and  50 , respectively, in comparison to the drain and fill valves required by the top supported receiver panel assembly  42  illustrated in FIG.  5 A. Both examples use the same number of receiver panels, i.e., eight independent panels, with  FIG. 5B  denoting the receiver panels via reference numerals  10   1 - 10   8 . Hence, the bottom supported receiver panels  10   1 - 10   8  require fewer drain and vent valves  48  and  50  respectively than the top supported receiver panel assembly  42 . 
     It will be appreciated then that the bottom supported receiver panels  10  have several advantages over the top supported receiver panels. A solar panel system constructed using the bottom supported receiver panels  10  requires less piping and a reduced number of drain and vent valves that help to lower the overall cost of a solar panel system. Further, the placement of flow control valves on or below the receiver deck  33  permits easy access and maintenance of the valves and pipes. Such positioning of the valves also can reduce the complexity of the piping inside the receiver panel assembly. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.