Abstract:
An end unit for a fuel cell stack is described that contains an integrated heat exchanger for superheating the fuel gas before delivery to the stack. Heat is transferred from the hot cathode outlet stream to the cool fuel inlet stream in a space adjacent the stack&#39;s end plate. The end unit is designed as a hollow box forming the shell of the exchanger with the heat exchanger inside. The end unit has openings that allow fuel cell process gas to be taken directly from the stack without requiring piping or ductwork to be attached to thin manifolds. Separate chambers are provided for both the cathode outlet and anode outlet gas, thereby allowing all process connections to be made at one end of the stack. The end unit also features a current collection post that is separated from the end cell of the stack by a multitude of members which provide structural support for the end unit and act to more uniformly collect electrical current than would a single, large current post.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to fuel cells and, in particular, to end plates and heat exchangers for fuel cell systems. More specifically, this invention relates to highly integrated, compact heat exchangers for use in superheating fuel gas for high temperature fuel cells.  
           [0002]    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.  
           [0003]    In general, a fuel cell, similar to a battery, includes a negative (anode) electrode and a positive (cathode) 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 provided 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 between each cell.  
           [0004]    In a conventional fuel cell stack for stationary power applications, the active area of the fuel cells is large, typically between {fraction (1/2)} and 1 m 2 . In order to apply a reasonable interface pressure on the cells, a large compressive load must be applied to the cells through the end plates. As the end plates must remain flat to insure intimate contact is maintained with the cells, the end plates are typically thick relative to their length and width. This thickness adds to the overall length of the fuel cell stack and size of the fuel cell power plant.  
           [0005]    In addition, for high temperature fuel cell systems, a heat exchanger is required to heat the fuel gas to near the temperature of the stack prior to delivery to the stack. In one type of fuel cell system, this heat exchanger is placed external to the fuel cell stack as part of the balance of the plant. This requires additional space to accommodate the fairly thick insulation (2-3 inches) used to encase the heat exchanger. Also, in this type of system, process gas must be piped to and from the heat exchanger, adding to both the size and cost of the system.  
           [0006]    As described in U.S. Pat. No. 5,856,034, insulation for the heat exchanger can be eliminated by placing the heat exchanger inside the already insulated fuel cell module enclosing the fuel cell stack. Specifically, the heat exchanger is placed upstream and adjacent the cathode inlet face of the stack, making it necessary to construct the exchanger large enough so as to completely cover the cathode inlet face. Also, in this system, due to the inherent non-uniform temperature distribution at the outlet of the heat exchanger, the stack inlet temperature distribution is also non-uniform. This condition is undesirable as non-uniform cathode inlet temperature not only creates a potential performance variation in the stack but also creates the risk of cell-to-cell wet seal leaks due to thermal expansion differences of the stack face.  
           [0007]    U.S. Pat. No. 5,009,968 describes an end plate structure in which a thin membrane is used to maintain good electrical contact with the end cells of the fuel cell stack. The thin membrane structure is not specifically adapted to uniformly collect electrical current from the stack. U.S. Pat. No. 4,719,157 describes a thin end plate with multiple current collecting terminals used to inhibit deformation of the plate. Again, this arrangement is not specifically adapted to provide uniform collection of electrical current.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention provides an end unit of a fuel cell stack having an assembly adapted to receive and convey gases in a heat exchange relationship, and/or to restrict electrical current flow from the fuel cell stack to a current collection post. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    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:  
         [0010]    [0010]FIGS. 1 and 2 are isometric views of the end unit in accordance with the principles of the present invention;  
         [0011]    [0011]FIG. 3 is a cross-sectional top plan view of the end unit of FIG. 1 taken along the line  3 - 3  of FIG. 2;  
         [0012]    [0012]FIG. 4 is a front cross-sectional view of a fuel cell stack utilizing the end unit of FIG. 1;  
         [0013]    [0013]FIG. 5 is a cross-sectional side view of a conventional fuel cell stack; and  
         [0014]    [0014]FIG. 6 is a side cross-sectional view of the fuel cell stack of FIG. 4. 
     
    
     DETAILED DESCRIPTION  
       [0015]    Referring to FIGS. 1, 2 and  3 , the illustrative embodiment of the invention has an end unit  1  adapted to be attached to one end of a fuel cell stack. The end unit  1  houses an assembly  2  including first and second units  2 A,  2 B associated with the flow of a first and second gas, respectively, through the assembly and which act together as a heat exchanger. In the case shown, the first and second gases are fuel cell stack anode or fuel inlet gas and fuel cell stack cathode exhaust gas, respectively.  
         [0016]    Particularly, the first unit  2 A has an inlet  3  through which fuel gas passes (depicted by arrow  7 ) into an inlet chamber  18 . Fuel gas collects in inlet chamber  18 , flows in a direction  8  through a first set of tubes  19 A and is delivered to a turn plenum  20 . Fuel gas flows in a direction  9  through the turn plenum  20  and from the plenum  20  flows in a direction  10  through the tubes  19 B. The gas is delivered by tubes  19 B to an outlet chamber  21  (shown in FIGS. 2 and 3). Fuel gas exits the outlet chamber  21  through an outlet pipe  4  in a direction indicated by arrow  11  and, as described in further detail below with respect to FIG. 6, flows through the fuel cell stack.  
         [0017]    The second unit  2 B forms an enclosure for the first unit  2 A. In the illustrative embodiment, the second unit  2 B has a first (or top) plate  14 , opposing side walls  15 A and  15 B, front and back walls  25 A and  25 B, and second (or bottom) plate  16  so as to create a generally hollow box structure of the appropriate length and width to match the fuel cell stack and the appropriate depth so as to remain flat within a desired tolerance upon compressive loading of the stack. As shown in FIGS. 1 and 2, the first unit  2 A is contained within the hollow interior of the second unit  2 B. In addition, an inlet port  5  (shown in FIG. 1) and an outlet port  6  (shown in FIG. 2) are formed in opposing side walls  15 A,  15 B of the unit  2 B. Also shown in the second unit  2 B is a plurality of members  17  extending between the first plate  14  and second plate  16  to provide structural support for the second unit  2 B. Members  17  will be described in further detail below with respect to the current collection characteristics of the end unit.  
         [0018]    Fuel cell stack cathode gas enters the second unit  2 B through inlet port  5  (depicted by arrow  12  in FIG. 1) and flows in a direction substantially transverse to the plurality of tubes  19 A,  19 B. As described above with respect to the first unit  2 A, fuel gas flows along paths  8  and  10  in the first and second sets of the multitude of tubes  19 A,  19 B. Collectively, the tubes  19 A,  19 B have the required heat transfer surface area to adequately transfer heat from the hot cathode gas to the fuel gas, thereby raising the temperature of the fuel gas to the desired temperature for delivery to the stack. The cathode outlet gas exits the end plate through opening  6  (as shown by direction  13  in FIG. 2).  
         [0019]    The tubes  19 A,  19 B of the first unit  2 A are designed to be mechanically separated from the first (or top) plate  14  forming the end of the stack, second (bottom) plate  16  and side walls  15 A,  15 B of the second unit  2 B. This configuration prevents both excessive stress on the joints of the unit  2 A and thermal distortions from affecting the flatness of the top and bottom plates  14 ,  16  of the second unit  2 B.  
         [0020]    Also depicted in FIGS. 1, 2 and  3  is a separate chamber  23  in the second unit  2 B adapted to collect the anode outlet gas from the fuel cell stack by way of an anode outlet manifold (not shown). Fuel cell stack anode outlet gas is delivered to the chamber  23  through an inlet opening  22  formed in a rear wall  25 B of the second unit  2 B and exits the chamber  23  through an outlet opening  24  formed in a side wall  15 A. With the above configuration for the end unit, all gas connections (ducts, pipes and bellows) for delivering and removing process gases to and from the stack are made through the end unit  1  at one end of the stack.  
         [0021]    The path of cathode gas flow through a fuel cell stack employing the end unit  1  of the invention is shown in the fuel cell stack cross-sectional view of FIG. 4. First, cathode inlet gas enters the fuel cell stack  104  along a first face  104 A of the stack in a direction depicted by arrows  101 . The cathode gas flows through the stack and exits the stack from a second stack face  104 B opposite the first (cathode gas inlet) stack face  104 A. Attached to the face  104 B is a cathode outlet gas manifold  106 . Cathode outlet or exhaust gas is collected in the cathode outlet gas manifold  106  and flows through the cathode outlet gas manifold  106  in a direction shown by arrow  102 . The cathode outlet gas manifold  106  delivers cathode outlet gas to the end unit  1  through opening  5 . Cathode outlet gas then flows through the end unit in a direction represented by arrow  103  and as described above with respect to FIGS.  1 - 3 , and exits the end unit through opening  6 . In this configuration, as previously stated, heat is transferred from the fuel cell stack cathode exhaust gas to the anode inlet gas by heat exchange in the end unit  1 .  
         [0022]    In a conventional system, as shown in FIG. 5, heat is taken from an inlet stream (depicted by arrows  201 ) of fuel cell stack cathode gas, requiring an assembly  207  for heat exchange between cathode and anode gases to be disposed along an entire stack face. After flowing through the heat exchanger  207 , cathode inlet gas flows through the stack  204  and exits the stack into a cathode outlet gas manifold  206 . By providing heat exchange in the end unit attached to the end of a fuel cell stack in accord with the invention, rather than along an entire stack face as in the conventional structure of FIG. 5, large space requirements, non-uniformity in stack inlet temperature distribution and the risk of cell-to-cell wet seal leaks, as discussed above, are obviated.  
         [0023]    [0023]FIG. 6 shows the path of anode or fuel gas flow through a fuel cell stack employing the end unit  1  of the invention. Fuel gas enters the end unit  1  at inlet  309 , fills the inlet plenum  18  (not shown in FIG. 6) and flows through the tubes  19 A, turn plenum  20  (not shown in FIG. 6), and tubes  19 B in a substantially U-shaped path as depicted by arrows  310 . As described above, anode gas is superheated by the transfer of heat from cathode outlet gas flowing transverse to the tubes  19 A,  19 B. Next, the heated anode gas exits the tubes  19 B and flows from outlet plenum  21  (not shown in FIG. 6) of the end unit  1  into a fuel header  308  in a direction depicted by arrow  311 . The fuel header  308  is disposed within a fuel gas inlet manifold  307  and extends along the length of the manifold  307 . The fuel gas header  308  and manifold  307  permit the heated fuel gas to exit the header and manifold at points along the length of the manifold and flow into the fuel cell stack in a direction depicted by arrows  312 . The flow of fuel gas through the fuel cell stack  104  as shown in FIG. 6 is in a direction  312  perpendicular to the direction of the flow of cathode gas through the fuel cell stack, but the anode and cathode gas flow paths do not intersect. After flowing through the stack  104 , the fuel gas enters an anode outlet gas manifold  306  and flows in a direction depicted by arrow  313 . The anode outlet gas manifold  306  then delivers the gas to the anode outlet gas chamber  23  of the end unit  1  as it flows in a direction shown by arrow  314 . In the anode outlet gas chamber  23  of the end unit  1 , the anode outlet stream is collected by the necessary ductwork and piping to be delivered to the balance of the fuel cell power plant.  
         [0024]    With the end unit  1  of the invention, any fuel gas leaks that may develop over the life of the unit are immediately swept away from the stack by the cathode outlet gas. This is unlike the case of a heat exchanger placed upstream of the stack, in which a leak must first pass through the stack and cathode outlet manifold before leaving the fuel cell module. The risk of a build-up of the mixture of gases within the fuel cell module is reduced.  
         [0025]    Turning back to FIG. 4, current collection posts  105 ,  107  are disposed at the positive and negative ends of the fuel cell stack. The current collection post  107  at the positive end of the stack is spaced from the first plate  14  by a plurality of members  17  (also shown in FIGS.  1 - 3 ). In the case shown, the members  17  are formed from electrically conductive material and are shaped as cylindrical columns. As shown in detail in FIGS.  1 - 3 , the members  17  extend between the first plate  14  and second plate  16  and are disposed at uniformly spaced intervals among the first and second sets of said tubes  19 A,  19 B and in the anode outlet gas chamber  23 . In the illustrative embodiment, the first plate  14  of the end unit  1  is in electrical contact with the stack, and the second plate  16  (see FIG. 1) is in electrical contact with the current collection post  107 . The members  17  thus provide an electrical connection between the first and second plates  14 ,  16  of the end unit and additionally provide structural support to the end unit, distributing mechanical and thermal stresses in the end unit that develop during operation of the stack.  
         [0026]    [0026]FIG. 5 shows a cross-sectional side view of a conventional fuel cell stack. As shown in FIG. 5, current collection posts  205  at each end of the stack are disposed adjacent to the positive and negative ends of the stack and collect current directly from the stack. In the present invention shown in FIG. 4, the separation of the current collection post  107  from the first plate  14  of the end unit  1  by the plurality of members  17  is advantageous in that the members  17  act to restrict electrical current flow slightly, allowing more uniform current collection from the stack through the uniformly spaced members  17 .  
         [0027]    In all cases it is understood that the above-described apparatus, method and arrangements are merely illustrative of the many possible specific embodiments that 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 the scope of the invention. For example, while shown in FIG. 4 at the positive end of the stack, the end unit may be disposed at either the positive or negative ends of the fuel cell stack. Also, designs using a plate fin, compact heat exchanger, could also be configured.