Abstract:
A fuel cell stack compression system in which a spring assembly and mechanical linkage assembly are used in conjunction with tie rods and tie bars to apply a compressive load to a fuel cell stack. The linkage assembly includes a lever and three pins to redirect the force or movement generated by the spring assembly into a movement or force for the tie rods. The tie rods, in turn, connect to the tie bars which span the top end plate of the fuel cell stack and transfer the load to the stack. The linkage also includes a slotted bearing which compensates for the circular arc formed by the lever and allows the spring assembly to be rigidly mounted under the bottom end plate. The spring assembly comprises multiple springs arranged in parallel and designed to provide a non-linearly decreasing load as the stack compresses due to cell consolidation. This load profile reduces overall stack shrinkage while providing high pressure at the beginning of life to insure proper cell-to-cell contact.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to fuel cells and, in particular, to methods and apparatus for maintaining a compressive load on stacks of such fuel cells. More specifically, the invention relates to methods and apparatus for maintaining a compressive load in high temperature systems such as molten carbonate and solid oxide fuel cell stacks. 
     A fuel cell is a device which directly converts chemical energy stored in a fuel such as hydrogen or methane into electrical energy by means of an electrochemical reaction. This differs from traditional electric power generating methods which must first combust the fuel to produce heat and then convert the heat into mechanical energy and finally into electricity. The more direct conversion process employed by a fuel cell has significant advantages over traditional means in both increased efficiency and reduced pollutant emissions. 
     In general, a fuel cell, similar to a battery includes a negative (anode) electrode separated by an electrolyte which serves to conduct electrically charged ions between them. In contrast to a battery, however, a fuel cell will continue to produce electric power as long as fuel and oxidant are supplied to the anode and cathode, respectively. To achieve this, gas flow fields are produced adjacent to the anode and cathode through which fuel and oxidant gas are supplied. In order to produce a useful power level, a number of individual fuel cells must be stacked in series with an electrically conductive separator plate in between each cell. 
     In the present state of the art, a fuel cell stack may have several hundred cells in series. In order to work properly, intimate contact must be maintained between all cells in the stack. Also this contact must continue during all stack operating conditions for the duration of the stack&#39;s life. Factors to be considered in achieving this requirement include manufacturing tolerances of the cell components, non-uniform thermal expansion of the cell components during operation and long term consolidation of the cell components resulting in shrinkage of the stack. 
     Accordingly, a variety of requirements are placed on the system used to compress the fuel cell stack. The system must apply enough load to overcome the manufacturing tolerances early in life to bring the cell components into intimate contact. It also must be great enough during operation to prevent the cells from delaminating due to the inevitable thermal gradients within the stack. At the same time, the compression system load should not be so great as to cause excessive shrinkage of the stack during its life as this places undue demands on auxiliary stack hardware and on the required follow-up of the compression system itself An additional requirement is that the system does not completely relax over time to insure that adequate stack pressure is maintained through the end of life. 
     Conventional fuel cell stack designs use one of a number of mechanisms for applying compressive load to the stack. U.S. Pat. No. 4,430,390 describes spring members which run within the manifolds of the fuel cell stack attaching to the endplates and forcing them toward each other. This design is not desirable for high temperature systems such as molten carbonate and solid oxide stacks because the spring members would need to be excessively large and be constructed of exotic, corrosion resistant materials to withstand the high temperature environment. U.S. Pat. No. 4,692,391 describes a design where the end plates are directly connected by rigid tensile members such as bolts or threaded rods. However, this system provides practically no load following capability to maintain stack compression as it shrinks. 
     U.S. Pat. No. 5,686,200 describes small, twisted wire or ribbon springs which may be used to apply load to individual cells within a stack. This design is inappropriate for large area fuel cells as the separator plates to which the springs are attached could not be constructed stiff enough to insure adequate load was delivered to the central area of the cells. U.S. Pat. No. 5,789,091 describes the use of continuous compression bands which are wrapped around the stack and placed in tension. Again, this method suffers from inadequate follow-up for stacks with significant long term creep. 
     Other methods of stack compression commonly used in the field include placing coil or belleville disk springs in compression at the end of a set of stiff tie rods which tie the opposing stack end plates together. Another design utilizes flexible bars to span the opposing end plates which are again connected by rigid rods to form leaf springs. These designs suffer from either inadequate follow-up or inadequate load capability for present state of the art, large scale, high temperature fuel cell stacks. 
     A more complex system, previously employed by the assignee of the subject application, uses rigid tie bars to span the top end plate. Rigid tie rods are connected to the tie bars and to a mechanical linkage near the bottom of the stack. This linkage connects the tie rods to a spring assembly in the form of a belleville disk pack located under the bottom end plate. Insulating layers are used to protect the spring assembly from excessive temperature during service. In this system the spring and linkage are designed so as to apply a fairly constant load to the stack during its life. Additionally, to accommodate the geometry of the mechanical linkage, the spring assembly is designed to rotate about one of its ends during operation. 
     This design has the limitations of causing excessive stack shrinkage and requiring extra space under the stack to accommodate rotation of the spring assembly. Another limitation of this design lies in the high cost associated with the numerous, large belleville disks which make up the spring assembly. Finally, the design presents safety concerns, since the disks of the spring assembly must be preloaded prior to installation in order to generate a relatively constant load profile. 
     It is therefore an object of the present invention to provide a fuel cell stack compression system which overcomes the above disadvantages of the prior art systems. 
     It is a further object of the present invention to provide a fuel cell stack compression system with desirable load-deflection characteristics applicable to large, high temperature fuel cell stacks; 
     It is also an object of the present invention to provide fuel cell stack compression system with minimized space requirements under the fuel cell stack; 
     It is yet a further object of the present invention to provide a fuel cell stack compression system having reduced costs; and 
     It is also an object of the present invention to provide fuel cell stack compression system which is safe to use. 
     SUMMARY OF THE INVENTION 
     In accordance with the principles of the present invention, the above and other objectives are realized in a compression system for providing a compressive force to a fuel cell stack in which the compression system includes one or more members each connected to a first end of the stack and extending to the opposite second end of the stack and a coupling mechanism situated adjacent the second end of the stack and including for each of the one or more members a spring assembly and a linkage assembly together adapted to cause translation movement provided by the spring assembly to be converted into a movement of the associated member in a direction between the first and second ends of the stack without rotation of the spring assembly. In this way, a desired compressive force can be maintained between the ends of the stack by the one or more members as the stack geometry changes. 
     In the embodiment of the invention to be disclosed hereinafter, the spring assembly is horizontally situated under the second end of the fuel cell stack and includes a fixed base plate and a translatable captivating plate between which the one or more springs of the assembly are situated. A shaft extends horizontally through the center of the springs and a first end of the shaft is attached to the captivating plate and a second end engages the linkage assembly. The latter assembly includes a lever arm, first, second and third pins and a slotted bearing . The first pin is rotationally mounted at one end of the lever arm and is connected to the end of the associated member. The second pin serves as the pivot point for the lever arm and is rotationally mounted to a base frame. The third pin is rotationally mounted to the second end of the lever arm and is situated in the slot of the slotted bearing. The bearing, in turn, is engaged by the second end of the shaft of the spring assembly. 
     With this configuration, the horizontal translation of the one or more springs of the spring assembly is carried to the captivating plate and from the captivating plate to the shaft of the spring assembly. The horizontal translation of the shaft is then carried by the shaft to the bearing and from the bearing to the third pin. This causes the pin to undergo both horizontal translation and translation in the orthogonal direction, i.e. vertical translation, due to rotation of the lever arm about the second pin. These translations are then imparted to the first pin at the first end of the lever. This, in turn, causes the associated member to undergo a vertical translation, which translation maintains the desired compressive force on the fuel cell stack 
     In a further aspect of the invention, a plurality of concentric springs of different length are used in the spring assembly to realize a desired non-linear, decreasing compressive load on the fuel cell stack. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features and aspects of the present invention will become more apparent upon reading the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1 is an elevation view of a fuel cell stack utilizing a fuel cell stack compression system in accordance with the principles of the present invention; 
     FIG. 2 is a cross-sectional, isometric view of the spring assembly and mechanical linkage assembly of the fuel cell stack compression system shown in FIG. 1; 
     FIGS. 3A-3C are schematic diagrams of the mechanical linkage assembly of the fuel cell stack compression system of FIG. 1 in the fully compressed, half compressed and non compressed states, respectively; 
     FIGS. 4A-4C are schematic illustrations of different configurations which can be used for the slotted bearing and the pin of the mechanical linkage assembly of the fuel cell stack compression system of FIG. 1; and 
     FIG. 5 is a graph illustrating one possible stack loading profile achievable with the fuel cell stack compression system of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a fuel cell stack  1  having end plates  2  and  3 , between which are stacked the fuel cells  4  of the stack. One or more manifolds  5  are provided for supplying and extracting gases from the stack The stack  1  might typically be a high temperature system such as, for example, a molten carbonate or solid oxide type system 
     In order to maintain the fuel cells  4  of the stack  1  in intimate contact, a fuel cell stack compression system  11  in accordance with the principles of the present invention is provided. More particularly, the compression system  11  causes compressive forces to be supplied to the stack between the end plates  2  and  3  over the life of the stack in such a way as to accommodate for changes in the stack geometry and, in particular, the stack vertical height or length. 
     As shown, the compression system  11  includes rigid tie bars  12  and  13  spanning opposite ends of the top end plate  2 . In FIG. 1, only the tie bar  12  is observable as the tie bar  13  is situated behind it in the view shown. Connected to each end of each tie bar is a connecting member in the form of a rigid tie rod which extends along the face of the stack from the first end plate  2  to the second end plate  3 . In FIG. 1, only tie rods  14  and  15  connected to the ends of tie bar  12  are visible with the tie rods  16  and  17  connected to the ends of the tie bar  13  again being invisible in the view shown. 
     In accordance with the principles of the invention, the compression system  11  further includes coupling mechanisms  21  situated at the ends of the tie rods  14 - 17  adjacent the second end plate  3  of the stack  1 . The coupling mechanisms  21  are of like configuration and are adapted to maintain a desired compression force on the stack  1  through forces applied to the stack via the tie rods  14 - 17  and tie bars  12 - 13 . 
     As shown in FIG.  1  and in more detail in FIG. 2, each coupling mechanism includes a spring assembly  22  situated under the second plate  3  of the stack and a mechanical linkage assembly  23  linking the spring assembly  22  to the end of the associated tie rod. In accordance with the invention, the combination of each spring assembly  22  and its associated mechanical linkage assembly  23  is adapted to cause translation movement provided by the spring assembly to be converted into a movement of the associated tie rod between the first and second end plates  2  and  3  of the stack  1  without rotation of the spring assembly. In particular, horizontal movement by the spring assembly is converted in this way into a vertical movement of the associated tie rod, to thereby maintain a desired compressive force between the end plates of the stack. 
     As seen more clearly in FIG. 2, each spring assembly  22  includes a base plate  31  which is rigidly mounted under the fuel cell stack  1 . One or more springs, shown in FIG. 2 as a series of concentric parallel springs  32 ,  33  and  34 , are disposed so that a first end of each spring is adjacent the base plate  31 . These springs then extend toward a second captivating plate  35  and have their second ends adjacent this plate. A shaft  36  is routed along the central axis of the springs  32 - 34 . One end  36   a  of the shaft passes through an aperture  35   a  in the captivating plate  35  and is held to the plate by a nut  37 . The other end  36   b  of the shaft passes through an aperture  31   a  in the base plate  31  and connects to the mechanical linkage assembly  23 . With this configuration for the spring assembly  22 , the one or more springs of the assembly can be placed in compression at an initial state of the fuel cell stack and then allowed to expand as the stack geometry changes to provide the necessary horizontal translation movement and, in turn, the necessary vertical translation movement of the associated tie rod to accommodate for these changes. 
     In the present embodiment of FIG. 2, the two outermost springs  33 - 34  of the spring assembly  22  are coil springs. The innermost spring  32 , in turn comprises a stack of belleville spaced from the base plate  31  by a spacer disk  38 . The thickness of this disk and the corresponding spacer disk  39  situated at the captivating plate  35  are selected so that the belleville disks begin to be compressed at a predetermined point in the stroke of the shaft  36 . The disks also serve to properly position the coil springs  33 - 34  along the axis of the shaft. The combination of spacers and belleville disks additionally provide a positive stop for the shaft  36  internal to the spring assembly once the belleville disks have been completely flattened. This feature is important during initial compression of the fuel cell stack so that a very high pressure can be applied to the stack without the need for external stop mechanisms and without yielding of the springs. 
     As above-described, the horizontal movement of each spring assembly  22  is coupled as a vertical movement to an associated tie rod via the associated linkage assembly  23 . As shown in FIG. 2, each linkage assembly  23  includes a rigid lever arm  41  and three pins  42 ,  43  and  44 . A first pin  42  is rotatably mounted at a first end  41   a  of the lever  41  and connects to a block  45  having a seating area  45   a  for seating an associated tie rod. A second pin  43  is mounted to a base frame  46  which is connected to the base plate  31 . The lever arm  41  is rotatably mounted at its central area  41   b  to the pin  43  which acts as the pivot point of the lever arm. A third pin  44  is rotatably mounted at a second end  41   c  of the lever arm  41  and is used to transfer the translational movement of the shaft  36  of the spring assembly  22  to the lever arm  41 . 
     As can be appreciated, the pins  42  and  44  are arranged with a finite radius from pin  43 . As a result, the pin  42  and the pin  44  will move in a circular arcuate path about the pin  43 . To accommodate the horizontal and vertical translation resulting from this arcuate path, the aforementioned block  45  connecting the lever arm  41  to the associated tie rod is allowed to rotate. This results in a slight angle change of the tie rod relative to the stack face. As the tie rod tends to be long, the angular change of the rod is typically small and results in very little impact on the achieved stack load. 
     As is apparent, the movement of the shaft  36  of the spring assembly  22  is coupled to the lever arm  41  via the pin  44 . Since the shaft movement is confined to horizontal translation due to the base plate  31  being fixed and since the pin  44  undergoes both horizontal and vertical translation due to its arcuate rotation as the lever arm  41  rotates, in order to accommodate these different types of movement, the mechanical linkage assembly  23  is further provided with a sliding bearing  47  to couple the shaft  36  to the pin  44 . 
     More particularly, as shown in FIG. 2, the bearing  47  has a threaded hole  47   a  which receives the second end  36   b  of the shaft  36  so that the bearing is urged horizontally with the horizontal movement of the shaft. The bearing also includes a central slot  47   b  in which the pin  44  of the mechanical linkage assembly is received. As a result, as the bearing  47  moves horizontally, the pin  44  moves horizontally with the bearing, while also sliding vertically in the slot  47   b . The pin  44  is thus allowed to undergo horizontal and vertical movement to accommodate the arcuate path followed by the pin as a result of rotation of the pin with the lever arm  41 . 
     In order to facilitate smooth sliding of the pin  44  within the slot  47   b , the pin and slot surfaces may be polished and hardened to reduce friction between them. Alternately, a ball or roller bearing may be used to mount pin  44  in the slot  47   b , thereby allowing the pin to rotate within the lever arm and roll with respect to the slot. Another pin configuration would be a pin with a flat portion in contact with the slot so as to provide less contact pressure on the slot and have less likelihood of binding. Further reduction in sliding friction between pin  44  and the slot  47   b  could also be easily achieved through the use of a lubricant or lubricating coating on the pin and/or slot 
     FIGS. 4A-4C illustrate some of the above and other pin configurations usable for the pin  44 . In FIG. 4A, the pin  44  is of round configuration. In FIG. 4B, the pin  44  has a square configuration and in FIG. 4C the pin  44  has a semi-square configuration. 
     The operation of the compression system  11  can be understood by reference to FIGS. 3A-3C which show various positions of the linkage assemblies  23  during operation of the fuel cell stack  1 . FIG. 3A illustrates the position of the linkage assemblies when the stack is placed in operation. At this time, the spring assemblies  22  of the compression system  11  are fully compressed so that the compression system is under maximum compression. The lever arms  41  have thus been fully rotated so that their ends  41   a  and pins  42  are at the highest vertical position. This places the blocks  45  and the tie rods  1417  also in their highest upward positions, thereby creating the necessary compressive force on the end plates  2  and  3  of the stack via the tie bars  12  and  13 . At this time, the pins  44  are at their most outward positions horizontally and at their highest positions vertically in their arcuate paths  51 . 
     FIG. 3B illustrates the linkage after the stack has been in operation and the geometry, i.e. the height, of the stack has reduced to a point where the compression system  11  is under half compression. At this time, the lever arms  41  are at half rotation so that the ends  41   a  and the pins  42  are at half their highest vertical position. This places the blocks  45  and the tie rods  14 - 17  at half their maximum vertical positions, thereby maintaining the necessary compressive force on the stack end with the reduced height of the stack. In this case, the pins  44  have been moved horizontally to their mid horizontal positions and to their lowest vertical positions in their arcuate paths  51 . 
     Finally, FIG. 3C shows the linkage assemblies after the stack has reached an operating condition where little if any further shrinkage of the stack is expected and the compression system is under no compression. At this time, the lever arms  41  have moved to a position where their ends  41   a  and the pins  42  are at their lowest vertical position. This places the blocks  45  and the tie rods  14 - 17  at their lowest vertical positions, thereby again maintaining the necessary compressive force on the stack end plates with the reduced height of the stack. At this time, the pins  44  have moved to their most inward positions horizontally and to their highest positions vertically in their arcuate paths  51 . 
     As can be appreciated, with the compression system  11  of the invention, the shrinkage of the stack  1  is accommodated while maintaining the desired compressive force on the stack. This is accomplished with spring assemblies which can be located under the end plate  2  of the stack, but which do not have to be rotated. As a result, the space requirements for these assemblies and the complexity of the assemblies is significantly reduced. This, in turn, provides an overall more compact and less expensive system. 
     While the spring assemblies  22  of the compression system  11  of FIGS. 1 and 2 have been shown as using-three concentric springs, any number of one or more springs may be used to generate the desired load versus deflection characteristic for each spring assembly. In addition, while the invention has been illustrated with coil springs and belleville disk packs for the spring assemblies, any other resilient member which can be placed in compression may be used as the one or more springs. 
     It should also be noted that the mechanical linkage assemblies  23  can be other than the assemblies illustrated. Alternate configurations include rack and pinion gear sets, cable pulley systems or any other mechanisms capable of redirecting the horizontal movement of the spring assembly to a vertical movement without rotation of the spring assembly. 
     In a further aspect of the present invention, and as shown in FIG. 2, the concentric springs  32 - 34  of the spring assemblies are formed so that one or more of the springs have varying or different lengths so as to provide a non-linearly, decreasing load or compressive force on the fuel cell stack  1  as it shrinks. In the case shown, the belleville disk pack  32  is of shorter length than the coil springs  33  and  34 . This is advantageous in providing good consolidation pressure early in stack life while minimizing the long term, total stack shrinkage. 
     FIG. 5 is a graph displaying one possible stack loading profile achievable with the concentric springs  32 - 34 . In the load profile of FIG. 5, a high load is applied at the beginning of stack life and this load decreases non-linearly as the stack shrinks. 
     More particularly, section a of the load curve represents the point at which the belleville disk packs  32  of the spring assemblies of the compression system of FIG. 1 are fully flattened or compressed. Section b of the load curve is achieved when both coil springs  33 - 34  and the belleville disk packs  32  of the spring assemblies are working together in parallel and in partial compression. Section c of the load curve is achieved when the belleville disk packs are no longer compressed and only the coil springs  33 - 34  are partially compressed and applying a force or load. 
     In all cases it is understood that the above-described arrangements are merely illustrative of the many possible specific embodiments which represent applications of the present invention. Numerous and varied other arrangements can be readily devised in accordance with the principles of the present invention without departing from the spirit and scope of the invention.