Abstract:
A flow field forming one wall of a channel in a flow field plate of a solid oxide fuel cell, the flow field includes a flat substrate having a patterned array of differently-shaped flow barriers projecting from the substrate into the channel, the flow field channel decreases in cross-sectional area in a flow direction.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates to high performance fuel cells and, more specifically, to optimized flow field and channel designs for promoting uniform performance and improved efficiency of the fuel cell system.  
         [0002]     Fuel cells convert reactants, namely fuel and oxidants, to generate electric power and reaction products. Fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. Preferred fuel cell types include solid oxide fuel cells (SOFCs) that comprise a solid oxide electrolyte and operate at relatively high temperatures. Generally, the SOFC employs an oxygen-ion conductor (such as stabilized zirconia, doped ceria, and doped lanthanum gallate) or proton conductors (such as doped perovskite Ba(Sr)CeO 3 , Ba(Sr)ZrO 3 , and mixed perovskites A 3 (B′B″)O 9 ) as the electrolyte. Currently, SOFCs use almost exclusively oxygen-ion conducting yttria-stabilized zirconia (YSZ) as the electrolyte.  
         [0003]     During normal operation of a solid oxide fuel cell with an oxygen-ion electrolyte, oxygen in oxidants is electrochemically reduced at the cathode, typically resulting in the generation of oxygen-ions and electrons. The oxygen-ions are conducted from the reaction sites through the electrolyte, to electrochemically react with the fuel at the anode to form H 2 O, CO 2  and possibly other species depending on the fuel employed.  
         [0004]     Flow field uniformity is a critical issue for high performance fuel cells. Adequate anode and cathode flows must reach over the entire electrode surfaces in a cell. Flow field design should therefore insure that the flow over a cell plate is as uniform as possible, and provide the flexibility to increase or decrease the flow pressure drop in the cell. Typically, a manifold design in a fuel cell stack determines the required pressure drop in the cell based on the number of cells in the stack.  
         [0005]     Uniform current density across a fuel cell is also required to optimize fuel cell performance. Uniform current density eliminates undesired temperature gradients in the cell. Current density is also directly related to the partial pressure of the active fuel (such as hydrogen) and oxygen in the anode and cathode flows, respectively. Along a fuel cell from reactant inlet to outlet, partial pressures of active reactants are reduced as reactions take place and as the reactants are consumed. The reduction in partial pressures can be drastic, causing the Nernst potential across the cell to drop and the reaction rate at the electrodes to decrease significantly along the flow, resulting in an uneven current density across the fuel cell.  
         [0006]     Representative fuel cell designs including flow channel and flow field configurations may be found in, for example, U.S. Pat. Nos. 6,586,128; 6,099,984; 6,093,502; 5,840,438; 5,686,199; and 4,988,583.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0007]     This invention seeks to improve overall fuel cell performance by new flow field and flow channel designs. To this end, the invention addresses two requirements of the fuel cell flow field: 1) uniform flow resistance to enhance flow uniformity in the cell; and 2) flexibility to increase or decrease the flow pressure drop in the cell.  
         [0008]     The invention also addresses flow field plate channel designs that permit increase in the flow velocity to help alleviate the reduction rate in the partial pressures of active reactants along the flow, and consequently enhance the uniformity of the cell current density and performance.  
         [0009]     In the exemplary embodiments, a series of alternative flow fields are disclosed that have been designed to enhance and thus increase fuel utilization in the fuel cell system. In these flow field designs, stamped or machined flow fields are formed with a plurality of dimples or protrusions in selected patterns that serve as flow barriers and thus provide uniform flow resistance along the various paths of flow.  
         [0010]     In one embodiment, fuel flow is introduced to the flow field from an opening at the center of one side of the fuel cell. A “center aisle” is arranged in the direction of flow through the opening, and is comprised of two rows of flow barriers that allow the flow to turn to both sides of the center aisle. The center aisle&#39;s width may vary (i.e., decrease) along the direction of flow since the amount of flow is progressively smaller as flow reaches the opposite side of the fuel cell. The flow field on each side of the center aisle comprises several rows of flow barriers (i.e., dimples) of circular or elliptical shapes. These barriers may be aligned or staggered, the latter providing better mixing of the flow which enhances the diffusion of fuel into the electrodes and thus promotes better cell performance. As mentioned above, the flow is free to turn in opposite directions from the center aisle, and the flow exits the cell through a series of small holes in two opposite ends of the cell. The diameter of these holes may vary along the sides to provide more or less flow resistance and consequently, provide adequate overall flow resistance to ensure flow uniformity. In a variation of the above described flow field design, flow exits from only one end of the fuel cell.  
         [0011]     In another exemplary embodiment, the anode or cathode flow enters one end of the fuel cell and exits at the opposite end of the fuel cell, with opposite sides of the cell blocked. The flow barriers along the direction of flow may be in-line or staggered as described above. The flow exits the opposite side of the cell through a series of small holes as also described above.  
         [0012]     In still another embodiment, the cathode or anode flow is introduced to the flow field through a first manifold at one end of the cell, and in a variation of that design, the flow out of the cell is collected via a second manifold at the opposite end of the cell.  
         [0013]     With respect to the design of the fuel cell flow channels (the flow fields described above are formed on one surface of the otherwise tubular channel), the channel height or width may be reduced gradually in the direction of flow to thereby increase the flow velocity downstream. In one embodiment, variable width channels are incorporated in a serpentine flow.  
         [0014]     Accordingly, in one aspect, the present invention relates to a flow field forming one wall of a channel in a flow field plate of a solid oxide fuel cell, the flow field comprising a flat substrate having a patterned array of differently-shaped flow barriers projecting from the substrate into the channel.  
         [0015]     In another aspect, the invention relates to a flow field for use in a solid oxide fuel cell, the flow field plate comprising a plurality of flow channels, each including a flat substrate having a patterned array of differently-shaped flow barriers projecting from the substrate into the channel; wherein the differently-shaped flow barriers include round and elliptical flow barriers arranged in staggered rows in the direction of flow.  
         [0016]     In still another aspect, the invention relates to a flow field plate for a solid oxide fuel cell, the plate formed with a plurality of flow channels, each flow channel decreasing in cross-sectional area in a flow direction, at least one of the channel walls provided with a patterned array of differently-shaped flow barriers projecting into the channel.  
         [0017]     In still another aspect, the invention relates to a solid oxide fuel cell comprising a solid oxide electrolyte sandwiched between a cathode and an anode and a pair of opposing flow field plates in operative association with the cathode and anode, respectively; the flow field plates each formed with a plurality of flow channels therein, at least one wall of which is formed with a patterned array of differently-shaped flow barriers projecting into the flow channel.  
         [0018]     The invention will now be described in detail in connection with the drawings identified below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIG. 1  is a schematic diagram of a typical fuel cell;  
         [0020]      FIG. 2  is a schematic diagram of a fuel cell incorporating a flow field design in accordance with an exemplary embodiment of the invention;  
         [0021]      FIG. 3  is a schematic diagram of a flow field design in accordance with an alternative embodiment of the invention;  
         [0022]      FIG. 4  is a schematic diagram of a flow field design in accordance with a third exemplary embodiment of the invention;  
         [0023]      FIG. 5  is a schematic diagram of a flow field design in accordance with a fourth exemplary embodiment of the invention;  
         [0024]      FIG. 6  is a schematic diagram of a flow field design in accordance with a fifth exemplary embodiment of the invention;  
         [0025]      FIG. 7  is a schematic diagram of a known fuel cell design incorporating uniform cross section flow channels;  
         [0026]      FIG. 8  is a schematic diagram showing a fuel cell with variable cross section flow channels in accordance with an exemplary embodiment of this invention;  
         [0027]      FIG. 9  is a schematic diagram of a tubular fuel cell with variable cross section flow channels in accordance with another exemplary embodiment of the invention;  
         [0028]      FIG. 10  is a variation of the tubular fuel cell shown in  FIG. 9 ;  
         [0029]      FIG. 11  is a schematic diagram of a flow channel where the channel width is reduced gradually along the flow direction in accordance with another exemplary embodiment of the invention; and  
         [0030]      FIG. 12  is a schematic diagram of a serpentine flow field that incorporates variable width channels in accordance with another exemplary embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]     A schematic diagram of a typical solid oxide fuel cell stack is depicted in  FIG. 1 . For simplicity, however,  FIG. 1  shows only one cell in the stack. The cell  10  comprises an electrolyte-electrode assembly that includes a solid oxide electrolyte  12  sandwiched between a cathode  14  and an anode  16 . During operation, oxidant (typically air) and fuel (typically hydrogen) are supplied to flow field plates  18 ,  20  respectively at inlets  22 ,  24 . The oxidant and fuel streams exhaust from stack  10  at outlets  26 ,  28 . During operation, power is delivered to a load depicted as resistor  30 .  
         [0032]     Flow fields are incorporated into distribution or flow channels  32 ,  34  that are formed in the flow field plates  18 ,  20  for delivery of reactants directly to surfaces of cathode and anode in the outflow direction.  
         [0033]     Referring to  FIG. 2 , a flow field design for a fuel cell flow channel  36  formed in a flow field plate  18  or  20  is illustrated in schematic form. The flow field  38  includes a flat substrate  40  embossed or otherwise suitably formed to include a plurality of flow barriers in the channel, opposite the cathode or anode. Generally, air or fuel flow (or simply, flow) is introduced to the channel  36  from an opening or inlet  42  at the center of side  44  of the channel. A “center aisle”  46  is formed or defined in the flow field plate  38  by a pair of rows  48 ,  50  of spaced, elliptically-shaped flow dimples or barriers  52  that protrude into the channel or flow path. The center (or flow) aisle may have a uniform or varied width along the flow direction. The spaces between the barriers  52  in the two center rows  48 ,  50  allow the flow to turn substantially 90° to both sides of the center aisle. The flow snakes through the spaces between dimples  64  and ellipses  66  and exits through a plurality of outlets in the form of small holes  54 ,  56  provided, respectively, along opposite ends  58 ,  60  of the flow channel. Outlets  54 ,  56  may have the same or varied opening size. Side  62  is closed, and side  44  is closed except for the presence of inlet  42 . Thus, all flow is directed out of the opposite ends of the channel  36 , in directions that are transverse to the direction of flow at the inlet  42 .  
         [0034]     The flow field on each side of the center aisle  46  is made up of several rows of flow barriers  64 ,  66  of circular and elliptical shape, respectively. The flow barriers or dimples  64  that lie adjacent the center aisle  46  are rounded in shape and are staggered in the outflow direction. Larger, elliptical flow barriers (or ellipses)  66  have their major axes oriented parallel to the outflow direction and are also staggered in the outflow direction. Staggered barriers provide better mixing of the flow, which in turn, enhances the flow diffusion into the electrodes and promotes better fuel cell performance. The shape and pattern of the barriers as shown in  FIG. 2  is exemplary only and may be altered to suit requirements.  
         [0035]      FIG. 3  illustrates an alternative flow field design similar to the design in  FIG. 2  but where the channel  68  is essentially configured as half the channel  36 . Thus, flow is permitted to exit from only one end  70  of the channel. The flow enters the channel side  72  via inlet  74  and along a now-closed end  76 . The flow is directed along the end  76  but is permitted to turn and flow in a transverse direction, toward end  70  where the flow exits through a plurality of outlets (small holes)  78 . The flow field barriers are formed in the substrate  80  in a manner similar to the earlier described embodiment in that an inlet aisle  82  is formed by end  76  in combination with spaced elliptical flow barriers  84 . Staggered rows of circular and elliptical dimples  86 ,  88 , respectively, define a plurality of flow paths in a transverse or outflow direction, from the aisle  82  to the outlet holes  78 .  
         [0036]     In another embodiment illustrated in  FIG. 4 , the flow channel  90  is designed to have a substantially straight flow field. Sides  92  and  94  are closed while end  96  is open to inlet flow. Opposed end  98  is closed except for the plurality of holes or outlets  100 . Between ends  96 ,  98 , there are staggered rows of round and elliptical flow barriers  102 ,  104 , respectively, formed in the substrate  106 . Note that the smaller round flow barriers  102  are closest to the inlet while the larger elliptical flow barriers  104  are downstream of the inlet with major axes arranged parallel to the flow direction.  
         [0037]     The diameter of the holes  100 , as well as holes  54  in  FIG. 2  and  78  in  FIG. 3 , may vary to provide more or less flow resistance and, consequently, provide adequate overall flow resistance that ensures flow uniformity. The size, configuration and density of the flow barriers  102  and  104 , as well as barriers  52 ,  64  and  66  in  FIGS. 2 and 84 ,  86  and  88  in  FIG. 3 , may also vary to provide the desired uniform flow for a given flow rate and required fuel cell power.  
         [0038]      FIG. 5  illustrates yet another channel and flow field design that is similar to the channel  90  in  FIG. 4 , but where the cathode or anode flow is introduced through a manifold. Thus, channel  106  includes closed sides  108 ,  110  and one end  112  closed except for the plurality of outlet holes  114 . The inlet  116 , however, is formed by a generally inverted cone-shaped wall with a centered inlet manifold  118  introducing the anode or cathode flow into the flow field. The latter is made up of relatively smaller, round flow barriers  120  and relatively larger elliptically-shaped flow barriers  122  formed in the substrate  124  and arranged substantially identically to the flow field in  FIG. 4 , i.e., in staggered rows in the direction of flow.  
         [0039]     In  FIG. 6 , the channel  130  and flow field design formed in the substrate  131  is similar to the channel  112  in  FIG. 5  but in this case, flow is both introduced and collected by manifolds. Specifically, the channel  130  includes closed sides  132 ,  134  and an inlet  136  formed by a generally inverted cone-shaped end wall with a centered inlet manifold  138  for introducing the anode or cathode flow into the channel. The outlet  140  is formed by a similar, cone-shaped end wall with a centered outlet manifold  142 . Outlets  144  in an internal channel end wall  146  feed the outlet flow to the manifold. This configuration is desirable when the cathode or anode flow is reclaimed at the channel exit. The flow barriers  148 ,  150  are otherwise substantially identical in both shape and pattern to the barriers  120 ,  122  in  FIG. 5 .  
         [0040]     With reference now to  FIGS. 7-10 , a second feature of the invention relates to the configuration of the channels in the flow field plates, and specifically, to the gradual reduction in channel cross section designed to promote uniform performance over the entire cell. By increasing the flow velocity to thereby help alleviate the reduction rate in the partial pressures along the flow and consequently, enhance the uniformity of the cell current density. In  FIG. 7 , a known flow channel configuration is illustrated where the walls  152  and  154  of the channel  156  in combination with an anode/electrolyte/cathode assembly  158 , establish uniform cross section flow paths  160 ,  162  for the respective anode and cathode flows.  
         [0041]     In  FIG. 8 , the flow channel  164  in accordance with an exemplary embodiment of this invention, includes a pair of walls or sides  166  and  168  on either side of a centrally-located anode/electrolyte/cathode assembly  170 . Tapering at least two opposite walls of the channel in the flow direction results in flow paths  172  and  174  for the respective anode and cathode flows that reduce gradually in cross section, and thus increase the flow velocity in a downstream or flow direction. The flow field arrangements of  FIGS. 2-6  may be incorporated into the channel  156 , with the flow barriers formed on the internal side of walls  166 ,  168 .  
         [0042]     In  FIG. 9 , another exemplary embodiment of the invention relates to the channel configuration in a tubular fuel cell  176 . In this embodiment, the anode  178 , electrolyte  180  and cathode  182  are formed in a C-shaped configuration, with internal flow walls  184 ,  186  defining an air inlet passage  188  and a pair of outlet passages  190 ,  192 . The walls  184 ,  186  are sloped to decrease the outlet area passages in the direction of flow. The decrease in cross-sectional area increases flow velocity in the downstream direction. The above-described flow field barrier designs may be provided on the sides of walls  184 ,  186  facing the adjacent cathode.  
         [0043]     In  FIG. 10 , a channel  194  is shown that is similar to channel  176  in  FIG. 9 , but reversed in the sense that the cathode  196 , electrolyte  198  and anode  200  are arranged with internal walls  202 ,  204  such that air flows across the cathode and fuel flows internally through inlet passage  206  and outlet passages  208 ,  210 , the latter decreasing in cross-sectional area in the flow direction. Here again, flow field plate designs may be formed on surfaces of walls  202 ,  204  facing the anode.  
         [0044]     In  FIG. 11 , a cell  212  is illustrated that is reduced gradually in width to thereby also increase flow velocity in a downstream direction. Specifically, the cell  212  includes an anode flow path  224  (formed with sides  220 ,  222  and top  214 ) and a cathode flow path  226  (formed with sides  228 ,  230 , and bottom  216 ) vertically stacked about an anode/electrolyte/cathode assembly  218 . The top  214  and bottom  216  are gradually reduced in width in the direction of flow so that the flow velocity in channels  224  and  226  is increased in the downstream direction and toward a smaller area of the cell. Therefore, the fuel cell performance is expected to be higher than in a constant width channel. Flow field designs as described in connection with  FIGS. 1-6  may be formed on the interior surfaces  214  and  216  facing the anode and cathode, respectively.  
         [0045]      FIG. 12  illustrates yet another exemplary embodiment of the invention. In this case, however, the channel  232  is comprised of parallel sides  234 ,  236  as well as parallel ends  238 ,  240 , and the internal walls  242 ,  244  and  246  are sloped relative to adjacent sides  234 ,  236 . The internal walls thus create a serpentine flow path with an inlet  246  in the upper portion of side  238 . Each section of the serpentine flow path decreases in cross section in the flow direction from one end of the cell to the other. This serpentine flow path may include flow barriers such as dimples or protrusions on the flat substrate  248 . With increased flow velocity at downstream direction, the fuel cell performance is expected to be enhanced.  
         [0046]     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.