Abstract:
A fuel cell stack comprising a plurality of serially-connected fuel cell stages, each stage comprising a plurality of fuel cells arranged electrically in parallel such that each stage has the voltage drop of a single fuel cell but current output defined by the total cell area. The assembled stack thus comprises essentially a plurality of internal fuel cell stacks arranged in parallel, each stack having the same voltage, and the stack currents being additive. The total voltage is the same as for a prior art stack of the same number of stages, but the current and hence the power output is multiplied over that of a single-cell stack by the number of internal fuel cell stacks. Preferably, each stage is a cassette including a plurality of windows for receiving the individual fuel cell units; a plurality of anode and cathode interconnects; and a single separator plate.

Description:
RELATIONSHIP TO GOVERNMENT CONTRACTS 
       [0001]    The present invention was supported in part by a US Government Contract, No. DE-FC26-02NT41246. The United States Government may have rights in the present invention. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to fuel cell stacks; more particularly, to a fuel cell stack having multiple parallel fuel cells; and most particularly to a solid oxide fuel cell stack comprising a plurality of fuel cell cassettes arranged in series electric flow wherein each cassette includes at least two fuel cells arranged in parallel electric flow. 
       BACKGROUND OF THE INVENTION 
       [0003]    In practical fuel cell systems, the output of a single fuel cell is typically less than one volt, so connecting multiple cells in series is required to achieve useful operating voltages. Typically, a plurality of fuel cell stages, each stage comprising a single fuel cell unit, are mechanically stacked up in a “stack” and are electrically connected in series electric flow from the anode of one cell to the cathode of an adjacent cell via intermediate stack elements known in the art as interconnects and separator plates. 
         [0004]    A solid oxide fuel cell (SOFC) comprises a cathode layer, an electrolyte layer formed of a solid oxide bonded to the cathode layer, and an anode layer bonded to the electrolyte layer on a side opposite from the cathode layer. In use of the cell, air is passed over the surface of the cathode layer, and oxygen from the air migrates through the electrolyte layer and reacts in the anode with hydrogen being passed over the anode surface, forming water and thereby creating an electrical potential between the anode and the cathode of about 1 volt. Typically, each individual fuel cell is mounted, for handling, protection, and assembly into a stack, within a metal frame referred to in the art as a “picture frame”, to form a “cell-picture frame assembly”. 
         [0005]    To facilitate formation of a prior art stack of fuel stages wherein the voltage formed is a function of the number of fuel cells in the stack, connected in series, a known intermediate process for forming an individual fuel cell stage joins together a cell-picture frame assembly with an anode interconnect and a metal separator plate to form an intermediate structure known in the art as a fuel cell cassette (“cassette”). The thin sheet metal separator plate is stamped and formed to provide, when joined to the mating cell frame and anode spacers, a flow space for the anode gas. Typically, the separator plate is formed of ferritic stainless steel for low cost. 
         [0006]    In forming the stack, the cell-picture frame assembly of each cassette is sealed to the perimeter of the metal separator plate of the adjacent cassette to form a cathode air flow space and to seal the feed and exhaust passages for air and hydrogen against cross-leaking or leaking to the outside of the stack. 
         [0007]    The power output P of a fuel cell stack is the product of the voltage V and current I, 
         [0000]        P=IV   (Eq. 1) 
         [0000]    The voltage is a function of the number of fuel cells connected in series in the stack, while the current is a function of the active area of the individual fuel cells. Thus, in designing a fuel cell system, to increase the power output requires an increase in either the number of fuel cells, or the individual fuel cell area, or both. 
         [0008]    There are tradeoffs in the number of cells and the surface area of the cells to achieve a desired power level. 
         [0009]    Adding more cells in series to increase stack voltage is relatively straightforward, but the reliability of each cell-to-cell connection becomes more critical since the overall reliability of a stack of N cells is a function of the reliability of each connection raised to the Nth power. Also, the resistive losses at the cell-to-cell junctures increase with each connection, and the proportion of system volume required for manifolding of the inlet and return gases increases. 
         [0010]    On the other hand, increasing the cell active area to increase the stack amperage by increasing the areal extent of each cell presents many challenges. The cell is a planar ceramic structure, so as the size increases the thickness must also increase to preserve the same level of mechanical strength (that is, resistance to breakage) which significantly increases the cost and size (volume) of the cell per unit area of electric generating capacity. In addition, the manufacturing defect rate is determined by the number of defects per cell, not per unit area, so as the area of a cell increases the number of defects per cell will increase, which adversely affects the overall manufacturing rejection rate in both cell manufacturing and stack manufacturing. Also, as the surface area increases at a constant length-to-width ratio (currently preferred aspect ratio of a prior art fuel cell is about 3:2), the thermal differences across the cell will increase, or the pressure drop will increase, or the gas channel height (and thus overall stack height) will increase, or some intermediate combination of all of these effects must occur. Alternatively, the width or length may be increased while maintaining the same length or width, but this departure from a prior art near-square cell shape makes firing of the ceramic cell very difficult while maintaining acceptable flatness and uniform shrinkage. 
         [0011]    What is needed in the art is a means to increase the power output of a fuel cell stack without increasing either the number of cell-to-cell connections or the size of individual fuel cells. 
         [0012]    It is a principal object of the present invention to increase the power output of a fuel cell stack. 
       SUMMARY OF THE INVENTION 
       [0013]    Briefly described, a fuel cell stack in accordance with the invention comprises a plurality of serially-assembled fuel cell stages preferably formed as individual cassette units. Each stage comprises a plurality of fuel cell units arranged electrically in parallel, such that each stage has the voltage drop of a single fuel cell unit. The assembled stack thus comprises a plurality of internal fuel cell stacks arranged in parallel. The voltage of the plurality of internal stacks is the same as for a prior art single-cell stack of the same number of stages, but the current and hence the power output is multiplied over that of a single-cell stack by the number of internal fuel cell stacks. 
         [0014]    Preferably, each cassette includes a plurality of windows for receiving a plurality of individual fuel cell units; a plurality of anode and cathode interconnects; and a single separator plate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0016]      FIG. 1  is a schematic drawing of a prior art SOFC mounted in a frame; 
           [0017]      FIG. 2  is an exploded isometric drawing of a prior art fuel cell stack employing a plurality of single-cell cassettes; 
           [0018]      FIG. 3  is a plan view of a first embodiment of a multiple-cell fuel cell cassette for use in a stack of multiple-cell cassettes, showing two cells in a single cassette; 
           [0019]      FIG. 4  is a plan view of an assembly stage of the two-cell cassette shown in  FIG. 3 , showing the placement of the anode interconnects; 
           [0020]      FIG. 5  is a plan view of the side of the cassette shown in  FIG. 3 , showing a single separator plate; 
           [0021]      FIG. 6  is a plan view of a second embodiment of a multiple-cell fuel cell cassette for use in a stack of multiple-cell cassettes, showing an alternative arrangement of two cells in a single cassette; and 
           [0022]      FIG. 7  is a plan view of a third embodiment of a multiple-cell fuel cell cassette for use in a stack of multiple-cell cassettes, showing four cells in a single cassette. 
       
    
    
       [0023]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate currently preferred embodiments of the invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    Referring to  FIGS. 1 and 2 , an exemplary prior art SOFC fuel cell module  10  comprises a cathode layer  12 , an electrolyte layer  14  formed of a solid oxide and bonded to the cathode layer  12 , and an anode layer  16  bonded to the electrolyte layer  14  on a side opposite from the cathode layer. Air  18  is passed over the surface  34  of the cathode layer  12 , and oxygen from the air migrates through the electrolyte layer  14  and reacts in the anode layer  16  with hydrogen anode gas  20  being passed over the anode surface  31  to form water, thereby creating an electrical potential between the anode and the cathode of about 1 volt. Each individual fuel cell module  10  is mounted, for handling, protection, and assembly into a stack, within a metal frame  22  referred to in the art as a “picture frame”, to form a “cell-picture frame assembly”  24 . 
         [0025]    To facilitate formation of a prior art stack  26  of individual fuel cells connected in series wherein the voltage formed is a function of the number of individual fuel cell modules in the stack, an intermediate process joins together each cell-picture frame assembly  24  with a separator plate  28  and a first solid (anode) interconnect  30  to form an intermediate structure known as a fuel cell cassette  32 . The thin sheet metal separator plate  28  is stamped and formed to provide, when joined to the mating cell frame  22  and anode spacers  29 , a flow space for the anode gas  20 . Preferably, the separator plate  28  is formed of ferritic stainless steel for low cost. Anode interconnect  30  is placed between the separator plate  28  and the anode surface  31  of the cell within the cassette  32 . The solid anode interconnect  30  is typically a woven wire mesh of uniform thickness and is solid in the direction perpendicular to the cell surface in a multitude of points. 
         [0026]    A second solid (cathode) interconnect  35 , installed during final assembly against cathode surface  34 , provides a cathode air flow space. Interconnect  35  also is typically a woven wire mesh of uniform thickness and solid in the direction perpendicular to the cell surface in a multitude of points. 
         [0027]    During the final prior art stack assembly process, a glass perimeter seal  42  is disposed between adjacent of the cassettes  32 , and the stack under pressure is brought to operating temperature and allowed to settle to its final form. The separator plate and cell frame may deform slightly, providing a compliant assembly, until the cells and interconnects are resting on one another, under load, which prevents further motion. 
         [0028]    The present invention provides the capability to increase the active fuel cell area in a cassette without increasing the size of an individual fuel cell element. Alternatively, each fuel cell element can be sized for an optimum combination of cost, manufacturability, and mechanical robustness, largely independent of the cassette active area requirement. Multiple cells are then arranged into a single cassette to achieve the desired active area per cassette. 
         [0029]    The cell picture frame has a plurality of openings also referred to herein as “windows”, to accept a plurality of fuel cells in a single frame which is then assembled to a single separator plate, with interconnects and anode spacers, to form a multiple-cell cassette having the desired active area per cassette. The invention thus provides an optimum combination of cost per unit power, volume per unit power, manufacturability, and mechanical robustness. 
         [0030]    Cell cost is driven largely by surface area and thickness: for a given thickness and manufacturing discard rate, two cells would cost approximately the same as one cell of the same area. However, an increase in active area requires the thickness of a single cell to be increased for the required mechanical strength, and there would be a higher discard rate as well. Multiple cells in a single frame have slightly less active area than comparable single cells, due to the need for divider bars (in effect, window “mullions”), and more components to assemble than stacks having large single cells, but the additional cost is more than offset by savings in thickness and discard rate. 
         [0031]    On the other hand, cassette and stack cost are driven largely by the number of components. The cost of stamping one larger cassette is only slightly more than the cost of stamping one smaller cassette and is much less than the cost of stamping two smaller cassettes. The total number of components, and therefore the assembly cost, is much less for a large stack with multiple cells in a cassette than for multiple stacks of single-cell cassettes having the same power capability. A single stack with more single-cell cassettes is also less reliable and manufacturable due to the large number of electrical and mechanical connections as discussed above. 
         [0032]    Regarding mechanical robustness, the picture frames and separator plates preferably are fabricated of ferritic stainless steel which has very little strength at the elevated operating temperature of an SOFC stack. Therefore, the multiple cells are relatively independent of each other mechanically although they reside in a single cassette. In this way, stresses induced by the operating environment (such as thermal cycling, vibration, and the like) are absorbed independently by smaller, more robust cells. 
         [0033]    Referring to  FIGS. 3 through 5 , a first embodiment  132  of a fuel cell cassette having a plurality of fuel cell modules in accordance with the invention comprises a picture frame  122  having first and second windows  123   a , 123   b  for receiving first and second fuel cell modules  110   a , 110   b , respectively. The fuel cell modules preferably are slightly larger than the windows and are surface bonded on either their cathode sides or their anode sides to the periphery of the windows in a face seal joint. As in the prior art, the picture frame  122  has a raised edge surrounding the windows to accommodate during stack assembly cathode interconnects (not shown) analogous to prior art cathode interconnect  35  shown in  FIG. 2 . First and second anode interconnects  130   a , 130   b  are arranged within cassette  132  in contact with first and second fuel cell modules  110   a , 110   b , respectively, and with separator plate  128  as in the prior art. Anode spacers (not visible) are also provided as in the prior art, configured for use in cassette  132  to provide ports  170 , 172  for flow of anode gas into and out of both first and second fuel cell modules  110   a , 110   b . Similarly, raised rims  174 , 176  define ports  178 , 180  for flow of cathode air into and out of both first and second fuel cell modules  110   a , 110   b . Thus first and second fuel cell modules  110   a , 110   b  are arranged in parallel for independent electricity generation within a single picture frame  122 . Of course, their individual electric contributions to a fuel cell stack are averaged by mutual connection of the first and second fuel cell elements with separator plate  128  in the shown cassette  132  and the separator plate of the next adjacent cassette in the stack. 
         [0034]    In first embodiment  132 , the fuel cell elements, having a length-to-width aspect ratio of about 3:2, are arranged with their short sides adjacent in the adjacent windows  123   a , 123   b.    
         [0035]    Referring now to  FIG. 6 , a second embodiment  232  of a fuel cell cassette in accordance with the invention includes first and second fuel cell elements  210   a , 210   b , which may or may not be identical with first and second fuel cell elements  110   a , 110   b , arranged in first and second windows  223   a , 223   b , respectively, such that the first and second fuel cell elements are arranged with their long sides adjacent. Thus, the only difference between embodiments  132  and  232  is the arrangement of the windows and fuel cell elements, and thus the aspect ratio of the resulting cassettes and fuel cell stacks (not shown) formed from the cassettes. 
         [0036]    It will be seen that a fuel cell stack formed in accordance with either first embodiment  132  or second embodiment  232  has two internal parallel electric generating stacks and thus has twice the surface area of a prior art stack having the same number of cassettes, and thus has twice the current and hence power generation capability at the same stack voltage. 
         [0037]    Higher pluralities of fuel cell elements in each cassette are possible within the scope of the present invention, to generate even more power at the same stack voltage. Referring to  FIG. 7 , a third embodiment, four-element cassette  332 , has four windows  323   a , 323   b , 323   c , 323   d  and four independent fuel cell elements  310   a , 310   b , 310   c , 310   d . The cassette has four anode interconnects (not visible), one of each being arranged adjacent each of the four fuel cell cassettes, analogous to the two interconnects  130   a , 130   b  in embodiment  132 . A common separator plate (also not visible) completes the cassette  332 , analogous to common separator plate  128  in embodiment  132 . Thus, a fuel cell stack comprising a plurality of embodiment  332  cassettes is able to provide four times the electric power of prior art stack  26  at the same output voltage. 
         [0038]    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.