Abstract:
A fuel cell assembly having manifold means for providing fuel and air to, and removing spent fuel and air from, flow passageways across the anodes and cathodes in a fuel cell stack. The sizes and proportions of the supply and return manifolds are optimized, and the total cross-sectional area of the return manifold is about twice the cross-sectional area of the supply manifold. The pressure drop in the manifolds is less than about one-quarter of the total pressure drop across the anode and cathode passageways in the stack, which ratio may be attained by adjusting the thickness of the anode and cathode spacers and/or the size of the chimneys. Widthwise uniformity of flow across the anodes and cathodes is improved by forming each of the manifolds as a plurality of smaller, parallel flow conduits.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to hydrogen/oxygen fuel cells; more particularly, to fuel cell stacks comprising a plurality of individual cells connected by interconnect elements; and most particularly, to a fuel cell assembly or system wherein manifolds for supplying fuel and air to and from fuel cells are sized and configured for optimal flow through the cells and stacks.  
         BACKGROUND OF THE INVENTION  
         [0002]    Fuel cells which generate electric current by the electrochemical combination of hydrogen and oxygen are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are deposited on opposite surfaces of an electrolyte formed of a ceramic solid oxide. Such a fuel cell is known in the art as a “solid oxide fuel cell” (SOFC). Hydrogen, either pure or reformed from hydrocarbons, is flowed along the outer surface of the anode and diffuses into the anode. Oxygen, typically from air, is flowed along the outer surface of the cathode and diffuses into the cathode where it is ionized. The oxygen anions transport through the electrolyte and combine with hydrogen ions to form water. The cathode and the anode are connected externally through a load to complete the circuit whereby electrons are transferred from the anode to the cathode. When hydrogen is derived from “reformed” hydrocarbons, the reformate gas includes CO which is converted to CO 2  at the anode via an oxidation process similar to that performed on the hydrogen. Reformed gasoline is a commonly used fuel in automotive fuel cell applications.  
           [0003]    A single cell is capable of generating a relatively small voltage and wattage, typically between about 0.5 volt and about 1.0 volt, depending upon load, and less than about 2 watts per cm 2  of cell surface. Therefore, in practice it is usual to stack together, in electrical series, a plurality of cells. Because each anode and cathode must have a free space for passage of gas over its surface, the cells are separated by perimeter spacers which are vented to permit flow of gas to the anodes and cathodes as desired but which form seals on their axial surfaces to prevent gas leakage from the sides of the stack. The perimeter spacers include dielectric layers to insulate the interconnects from each other. Adjacent cells are connected electrically by “interconnect” elements in the stack, the outer surfaces of the anodes and cathodes being electrically connected to their respective interconnects by electrical contacts disposed within the gas-flow space, typically by a metallic foam which is readily gas-permeable or by conductive filaments. The outermost, or end, interconnects of the stack define electric terminals, or “current collectors,” which may be connected across a load.  
           [0004]    A complete SOFC system typically includes auxiliary subsystems for, among other requirements, generating fuel by reforming hydrocarbons; tempering the reformate fuel and air entering the stack; providing air to the hydrocarbon reformer; providing air to the cathodes for reaction with hydrogen in the fuel cell stack; providing air for cooling the fuel cell stack; providing combustion air to an afterburner for unspent fuel exiting the stack; and providing cooling air to the afterburner and the stack. A complete SOFC assembly also includes appropriate piping and valving, as well as a programmable electronic control unit (ECU) for managing the activities of the subsystems simultaneously.  
           [0005]    The various components of a fuel cell stack, including the fuel cells themselves, the anode and cathode spacers which create the flow passageways across the anodes and cathodes, the perimeter seals, and the electrical interconnects, are rectangular and are perforated along all four edges. When the components are stacked up, the perforations define fuel and air distribution manifolds, known as “chimneys,” within the fuel cell stack perpendicular to the planes of the stacked fuel cells, through which fuel and air may be supplied to and removed from the individual fuel cells. The heights of the air and fuel passageways across the fuel cells and the size and shape of the chimneys are not optimized in prior art assemblies for the most nearly uniform flow of fuel and air through the stack at the optimal pressure drop.  
           [0006]    It is a principal object of the present invention to provide an improved fuel cell assembly wherein fuel cell output is increased through more uniform flow of fuel and air through the stack.  
           [0007]    It is a further object of the invention to provide such a fuel cell stack wherein flow restriction through the stack is minimized.  
         SUMMARY OF THE INVENTION  
         [0008]    Briefly described, a fuel cell assembly in accordance with the invention has manifold means for providing fuel and air to, and removing spent fuel and air from, flow passageways across the anodes and cathodes in a fuel cell stack. The sizes and proportions of the supply and return manifolds (“chimneys”) are optimized by Computational Fluid Dynamics analysis. The total cross-sectional area of the return chimney is preferably about twice the cross-sectional area of the supply chimney. The pressure drop in the chimneys is preferably less than about one-quarter of the total pressure drop across the anode and cathode passageways in the stack, which ratio may be attained by adjusting the thickness of the anode and cathode spacers and/or the size of the chimneys. Widthwise uniformity of flow across the anodes and cathodes is improved by forming each of the chimneys as a plurality of smaller, parallel flow passages. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    These and other features and advantages of the invention will be more fully understood and appreciated from the following description of certain exemplary embodiments of the invention taken together with the accompanying drawings, in which:  
         [0010]    [0010]FIG. 1 is an exploded isometric view of a single solid oxide fuel cell, showing the various elements and the flow paths of fuel and oxygen through the cell;  
         [0011]    [0011]FIG. 2 is an isometric view of a fuel-cell stack comprising five cells like the cell shown in FIG. 1;  
         [0012]    [0012]FIG. 3 is an isometric view like that shown in FIG. 2, partially exploded, showing the addition of current collectors, end plates, and bolts to form a complete fuel cell stack ready for use;  
         [0013]    [0013]FIG. 4 is an exploded isometric view of a separator subassembly in accordance with the invention;  
         [0014]    [0014]FIG. 5 is an isometric view of the subassembly shown in FIG. 4;  
         [0015]    [0015]FIG. 6 is an exploded isometric view, showing the separator subassembly ready for joining to a framed PEN for forming a modular fuel cell unit;  
         [0016]    [0016]FIG. 7 is an isometric view of the cathode air flow field; and  
         [0017]    [0017]FIG. 8 is an isometric view of the anode gas flow field. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    [0018]FIGS. 1 through 3 are useful in describing generally the structure of a multiple fuel cell stack assembly. FIGS. 4 through 8 are illustrative of improved fuel cell assemblies in accordance with the invention.  
         [0019]    Referring to FIGS. 1 and 2, an individual fuel cell  11  includes a multilayer fuel cell element  09  comprising an electrolyte  14  (E) having an anode  16  or positive element (P) deposited on a first surface thereof and a cathode  18  or negative element (N) deposited on a second surface thereof. Thus, element  09 , which is the actual “fuel cell,” is known in the art by the acronym PEN. Passage  24  for flow of fuel  21  across the free surface  20  of anode  16  is provided by first cut-out spacers  36  sealed to anode  16  by peripheral seal  37 , and passage  26  for flow of air  31  across the free surface of cathode  18  is provided by second cut-out spacers  38  sealed to cathode  18  by another peripheral seal  37 . Fuel  21 , typically in the form of hydrogen or reformate gas, is provided at a first edge  25  of anode surface  20  via supply conduits  23  formed in each element and is removed via exhaust conduits  27  provided at a second and opposite edge  29  of anode surface  20 . Oxygen, typically in the form of air, is provided via supply conduits  33  to passages  26  at a first edge  39  of cathode  18  and is removed via exhaust conduits  41  at a second and opposite edge  43  of cathode  18 .  
         [0020]    Referring to FIG. 3, a plurality of fuel cells  11  may be stacked together to form a stack  12 , five such cells being shown in FIG. 2. In a complete working fuel cell  13 , stack  12  is sandwiched between an anodic current collector  34  and a cathodic current collector  32  which in turn are sandwiched between a top plate  15  and a gas-manifold base  17 , the entire assembly being bound together by bolts  19  extending through bores in top plate  15  and threadedly received in bores in base  17 . Air is provided to base  17  for supply to conduits  33  via a first connector  44 . In the stack, the cathode air and anode fuel conduits define distribution manifolds, or “chimneys”, as described above.  
         [0021]    Referring to FIGS. 4 through 6, an improved structure for a modular fuel cell  50  (FIG. 6) includes four sheet metal parts stamped from flat stock which do not require any forming operations such as folding or dishing. Each part has a different thickness to suit its function. Because the parts are flat, stacked on top of one another, and metallurgically joinable, the cell active area is maximized because there is no wasted space in radial corners.  
         [0022]    The first sheet is the cell mounting frame  52 . It needs to be thick enough only to provide sufficient structure to allow the fuel cell element (PEN)  09  (FIG. 6) to be mounted to it. Frame  52  has a central opening  54  smaller than the external dimensions of the PEN such that the surface of electrolyte  14  outboard of cathode layer  18  can be bonded and sealed to mounting frame  52 . Opening  54  in frame  52  allows the surface of cathode  18  to be exposed to cathode air flow in the fuel cell assembly. The anode side of the PEN could be mounted to the PEN frame  52  as an alternate configuration, if desired. Frame  52  also has openings  33 ′,  41 ′ for passage of cathode air and openings  23 ′,  27 ′ for passage of fuel to supply all the cells in a stack. None of these openings is open in the plane of frame  52  and therefore gas cannot flow out of the manifold chimneys in this part. The PEN does not extend to the outer edges of frame  52  nor to the air or fuel openings. Thus, the PEN does not require openings formed in it which are difficult to fabricate and which increase the chance of PEN cracking. Moreover, the edges of the PEN do not require sealing as they otherwise would because of the porous anode material.  
         [0023]    The second sheet is a cathode spacer  38 ′, analogous to spacer  38  in FIG. 1. The thickness of spacer  38 ′ determines the height  56  of the cathode air flow field  58  (FIG. 7). Anode gas openings  23 ′,  27 ′ are not open in the plane of spacer  38 ′ so that anode gas is contained in the manifold chimneys. Cathode openings  33 ′,  41 ′, however, are open in the plane of the spacer to allow cathode air to flow across the surface of the cathode from supply opening  33 ′ to return opening  41 ′. Spacer  38 ′ is positioned on the cathode side of cell mounting frame  52 .  
         [0024]    The third sheet is an anode spacer  36 ′, analogous to spacer  36  in FIG. 1. The thickness of spacer  36 ′ determines the height  60  of the anode flow field  62  (FIG. 8). Cathode gas openings  33 ′,  41 ′ are not open in the plane of spacer  36 ′ so that cathode gas is contained in the manifold chimneys. Anode openings  23 ′,  27 ′, however, are open in the plane of the spacer to allow anode gas to flow across the surface of the anode from supply opening  23 ′ to return opening  27 ′. Spacer  36 ′ is positioned on the anode side of cell mounting frame  52 .  
         [0025]    The fourth sheet is a separator plate  64 , which needs to be thick enough only to provide sufficient structure for separating the anode gas flow in one cell from the cathode air flow in an adjacent cell in a fuel cell stack. The only openings in separator plate  64  are anode manifold openings  23 ′,  27 ′ and cathode manifold openings  33 ′,  41 ′ which form part of the supply and return chimneys. None of these openings is open in the plane of the plate and therefore no gas is allowed to flow out of separator plate  64 .  
         [0026]    The four plates  36 ′,  38 ′,  52 ,  64  may be joined by any combination of welding or brazing which suits the stack assembly. Also, they may be assembled (joined) in any order or combination which suits the assembly process. In a preferred assembly sequence, cathode spacer  38 ′ and anode spacer  36 ′ are attached to opposite sides of separator plate  64 , as shown in FIG. 4, to provide a separator subassembly  65 , shown in FIG. 5. Subassembly  65  is then attached to cell mounting frame  52  carrying PEN  09 , as shown in FIG. 6, to form fuel cell module  50 . Any desired number of modules  50  may be stacked together to form a fuel cell stack  12  (FIG. 3).  
         [0027]    The sheet thicknesses of the four plates may be readily changed as required to adjust the structural properties or field flow heights. Since these parts may be formed by a blanking process, the tooling can be simple and inexpensive. Alternatively, the plates may be formed by other processes, for example, water-jet cutting. All of these factors make fuel cells in accordance with the invention well-suited to high-volume manufacturing as well as low volume or prototype manufacturing.  
         [0028]    Referring to FIGS.  4 - 8 , for a fuel cell assembly having a plurality of cells, for example,  60  cells divided into two stacks of  30  cells each, the optimal sizes for the fuel conduits  23 ′,  27 ′ and air conduits  33 ′,  41 ′, as well as the heights  56 ,  60  of the cathode and anode flow fields  58 ,  62  have been determined by computer analysis using Computational Fluid Dynamics. “Optimal” means providing non-stagnating, essentially uniform flow across the surface of the anodes and the cathode and minimal back pressure in the anode and cathode exhaust chimneys.  
         [0029]    The cathode air flow field  58  is shown in FIG. 7, having an entry width  66  divided among six supply conduits  33 ′ of equal width  68  and length  70 , each having a cross-sectional flow area  71 , an exit width  72  divided among six return conduits  41 ′ of equal width  74  and length  76 , each having a cross-sectional flow area  77 , an overall flow field length extent  78 , and an average flow field length extent  80  between the centerlines of the supply and return conduits.  
         [0030]    The anode fuel flow field  62  is shown in FIG. 8, having an entry width  82  divided among six supply conduits  23 ′ of equal width  84  and length  86 , each having a cross-sectional flow area  87 , an exit width  88  divided among six return conduits  27 ′ of equal width  90  and length  92 , each having a cross-sectional flow area  93 , an overall flow field length extent  94 , an average flow field length extent  96 , and an overall flow field width  98  across the anode.  
         [0031]    Pressure losses along the manifold chimneys were also considered as a function of the conduit cross-sectional dimensions, conduit length, and gas flow rate.  
         [0032]    It was found that counterflow between the supply chimneys and the return chimneys, wherein gas is returned in the reverse direction from its supply, is superior to co-flow. It was also found that the return chimneys should have a cross-sectional area at least 1.5 times, and preferably about 2 times, that of the supply chimneys, and that the pressure drop across the cells, between the supply and return chimneys, should be more than about 4 times the total pressure drop in the chimneys. The pressure drop across the cells can be changed by changing the thickness of the anode spacer  36 ′ or the cathode spacer  38 ′, as required to change the flow field height  62 ,  58 , respectively. Alternatively, the chimneys can be re-sized to adjust the chimney pressure drops.  
         [0033]    It is known that flow distribution across the width  72 ,  98  of the flow field is highly important to the efficiency and lifetime of a fuel cell, and it was found that breaking the chimneys into a series of shorter, parallel mini-chimneys having optimized lengths and widths, as shown in FIGS.  4 - 8 , substantially improved side-to-side gas distribution.  
         [0034]    In addition, the material left between the chimneys improves the structural integrity of the stack and increases the transfer of waste heat from the interior of the stack to the exterior of the stack.  
         [0035]    A fuel cell assembly in accordance with the invention is especially useful as an auxiliary power unit (APU) for vehicles  132  on which the APU may be mounted as shown in FIG. 3, such as cars and trucks, boats and ships, and airplanes, wherein motive power is supplied by a conventional engine and the auxiliary electrical power needs are met by the SOFC assembly.  
         [0036]    A fuel cell assembly in accordance with the invention is also useful as a stationary power plant such as, for example, in a household or for commercial usage.  
         [0037]    While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.