Abstract:
A fuel cell assembly is disclosed that utilizes a water transport structure extending from fuel cell plates of the assembly into fuel cell assembly manifolds, wherein the water transport structure facilitates the transport of liquid water from the fuel cell plates thereby minimizing the accumulation of liquid water and ice in the fuel cell stack.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional application of U.S. patent application Ser. No. 11/851,401 filed Sep. 7, 2007, hereby incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a fuel cell assembly, and more particularly to a fuel cell assembly utilizing water transport structures partially disposed in a manifold of the fuel cell stack to facilitate the transport of liquid water from the fuel cell assembly. 
     BACKGROUND OF THE INVENTION 
     Fuel cell power systems convert a fuel and an oxidant to electricity. One type of fuel cell power system employs a proton exchange membrane (hereinafter “PEM”) to catalytically facilitate reaction of fuels (such as hydrogen) and oxidants (such as air or oxygen) to generate electricity. The PEM is a solid polymer electrolyte that facilitates transfer of protons from the anode to the cathode in each individual fuel cell of a stack of fuel cells normally deployed in a fuel cell power system. 
     In a typical fuel cell stack of a fuel cell power system, individual fuel cells provide channels through which various reactants and cooling fluids flow. Fuel cell plates may be unipolar, or a bipolar plate may be formed by combining a plurality of unipolar plates. Fuel cell plates may be designed with serpentine flow channels. Serpentine flow channels are desirable as they effectively distribute reactants over the active area of an operating fuel cell, thereby maximizing performance and stability. Movement of water from the flow channels to outlet manifolds of the fuel cell plates is caused by the flow of the reactants through the fuel cell. Drag forces cause the liquid water to flow through the channels until the liquid water exits the fuel cell through the outlet manifolds. However, when the fuel cell is operating at a lower power output, the velocity of the gas flow is too low to produce an effective drag force to transport the liquid water, and the liquid water accumulates in the flow channels. 
     A further limitation of relying on gas flow drag forces to remove the liquid water is that the drag forces may not be strong enough to effectively transport the liquid water creating pinning points that may cause the water to accumulate and pool, thereby stopping the water flow. Such pinning points are those commonly located where the channel outlets meet the fuel cell stack manifold. 
     Some current fuel cell assemblies utilize plates having hydrophilic surfaces. Water has been observed to form a film on the surface of the material and accumulate at the outlet of the flow channels and the perimeter of the plates. The water film can block the gas flow, which in turn reduces the driving force for removing liquid water and prevents the removal of the liquid water from the fuel cell stack. The accumulation of water can cause gas flow blockages or flow imbalances that can have negative impacts on the performance of the stack. 
     Further, the accumulated water may form ice in the fuel cell assembly. The presence of water and ice may affect the performance of the fuel cell assembly. During typical operation of the fuel cell assembly, waste heat from the fuel cell reaction heats the assembly and militates against vapor condensation and ice formation in the assembly. During a starting operation or low power operation of the fuel cell assembly in subzero temperatures, the condensed water in the flow channels of the fuel cell plates and at edges of the outlet manifolds may form ice within the fuel cell assembly. The ice formation may restrict reactant flow, resulting in a voltage loss. 
     It would be desirable to develop a fuel cell assembly with an improved means for removing liquid water from fuel cell gas flow channels of the fuel cell stack to minimize the accumulation of liquid water and ice in the fuel cell assembly. 
     SUMMARY OF THE INVENTION 
     Concordant and congruous with the present invention, a fuel cell assembly with an improved means for removing liquid water from fuel cell gas flow channels of the fuel cell assembly to minimize the accumulation of liquid water and ice in the fuel cell assembly, has surprisingly been discovered. 
     In one embodiment, the fuel cell plate comprises a plate having a first aperture formed therein; a plurality of flow channels formed on said plate; and a water transport structure disposed between at least one of said flow channels and the aperture of said plate to facilitate a transport of water from the at least one of said flow channels to the aperture. 
     In another embodiment, the fuel cell plate comprises a bipolar plate; a plurality of flow channels formed on each face of said bipolar plate; at least one aperture formed through said bipolar plate; and a water transport structure, wherein said water transport structure includes a first end disposed through an aperture formed in a face of said bipolar plate between the flow channels and the aperture, an intermediate portion disposed between the faces of said bipolar plate, and a second end extending from the intermediate portion into the aperture. 
     In another embodiment, the fuel cell assembly comprises a fuel cell stack including a plurality of fuel cell plates, each fuel cell plate having a plurality of flow channels and a plurality of faces, wherein each fuel cell plate includes at least one aperture formed therein, the apertures of the fuel cell plates substantially aligned to form a manifold; and a water transport structure extending into the manifold from an inner edge of the aperture of each fuel cell plate, wherein water is caused to flow from the fuel cell plate, through said water transport structure, and through the manifold. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which: 
         FIG. 1  is a cross-sectional view of a fuel cell stack incorporating a water transport structure in a fuel cell stack manifold according to an embodiment of the invention; 
         FIG. 2  is a schematic cross-sectional view of the water transport structure of the fuel cell stack illustrated in  FIG. 1 ; 
         FIG. 3  is a top plan view of a fuel cell plate of the fuel cell stack illustrated in  FIG. 1 ; and 
         FIG. 4  is an enlarged fragmentary top plan view of the fuel cell plate illustrated in  FIG. 3 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following detailed description and appended drawings describe and illustrate various exemplary embodiments of the invention. The description and drawings serve to enable one skilled in the art to make and use the invention, and are not intended to limit the scope of the invention in any manner. 
       FIGS. 1 and 2  show a fuel cell assembly  10  according to an embodiment of the invention. The fuel cell assembly includes a plurality of stacked fuel cell plates  12 . Each of the plates  12  includes an inlet aperture, an outlet aperture, and a plurality of water transport structures  18 . The inlet apertures of each of the plates  12  cooperate to form an inlet manifold  14  and the outlet apertures of each of the plates  12  cooperate to form an outlet manifold  16 . The inlet manifold  14  is in fluid communication with an inlet  28  and the outlet manifold  16  is in fluid communication with an outlet  30 . It is understood that the fuel cell assembly  10  shown in  FIGS. 1 and 2  may be a cross-section of either an anode side or a cathode side. 
       FIGS. 3 and 4  show a top plan view of a bipolar fuel cell plate  12  formed from a pair of unipolar plates. The bipolar plate  12  includes two inlet apertures  20 , two outlet apertures  22 , and a plurality of flow channels  24 . It is understood that the flow channels  24  may include the channels disposed on an external face of the fuel cell plate  12 , as well as the passages disposed intermediate internal faces of the fuel cell plate  12 . It is also understood that the material of construction, size, shape, quantity, and type of plates  12  in the fuel cell assembly  10 , and the configuration of the fuel cell plates  12  within the assembly  10 , may vary based on design parameters such as the amount of electricity to be generated, the size of the machine to be powered by the fuel cell assembly  10 , the volumetric flow rate of gases through the fuel cell assembly  10 , and other similar factors, for example. The fuel cell plates  12  may be formed from any conventional material such as graphite, a carbon composite, or a stamped metal, for example. The fuel cell plate  12  shown in  FIG. 3  may be used for an anode side or for a cathode side (not shown) of the fuel cell assembly  10 . Further, it is understood that the plate  12  may have any number of inlet apertures  20  and outlet apertures  22 , as desired. As shown, the flow channels  24  are undulated. However, the flow channels  24  may be substantially linear, serpentine, or have other configurations, as desired. 
     Water transport structures  18  are disposed on the fuel cell plate  12  at the inlet apertures  20  and the outlet apertures  22 , as shown in  FIGS. 3 and 4 . It is understood that more or fewer water transport structures  18  can be used as desired. The water transport structures  18  include a first end  18   a , a second end  18   c , and an intermediate portion  18   b  formed between the first end  18   a  and the second end  18   c.    
     The first ends  18   a  of the water transport structures  18  extend into apertures  26  formed in the fuel cell plate  12  intermediate the flow channels  24  and the inlets  20  and intermediate the flow channels  24  and the outlets  22 . Typically, the apertures  26  are formed intermediate a gasket  32  and the flow channels  24 , as shown in  FIG. 2 , although other configurations can be used if desired. 
     The intermediate portions  18   b  of the water transport structures  18  are disposed between the uniploar plates of the fuel cell plate  12 . In the embodiment shown, the intermediate portions  18   b  of the water transport structures  18  circumvent the gasket  32 . Accordingly, a flow path is provided adjacent the gasket  32 , as shown in  FIG. 2 . 
     The second ends  18   c  of the water transport structures  18  extend from between the fuel cell plates  12  and into the inlet apertures  20  and outlet apertures  22 . In the embodiment shown, the water transport structures  18  have a substantially rectangular shape. However, the water transport structures  18  may have any shape as desired such as a triangular shape, a curvilinear shape, and an irregular shape, for example. As illustrated in  FIGS. 1 and 2 , the second ends  18   c  of the water transport structures  18  depend downwardly due to gravity, thereby causing adjacent second ends  18   c  to substantially abut. However, it is understood that the second ends  18   c  can hang individually and in other configurations as desired. 
     The water transport structures  18  may be formed from any non-conductive porous material such as a foam, cotton, wool, glass fibers, felt, flocked fibers, paper, and paper and polymer fiber composites, for example. The water transport structure  18  may also include a hydrophilic coating such as a silicon oxide (SiO x ), another metal oxide, or other chemical coating, for example, a hydrophobic coating, or be formed from a hydrophilic or hydrophobic material. 
     The inlet manifold  14  includes the inlet  28  in fluid communication with the inlet manifold  14  formed in the fuel cell assembly  10  by the inlet apertures  20  of the fuel cell plates  12 . The plates  12  are stacked with the inlet aperture  20  of each plate  12  substantially aligned with the inlet aperture  20  of an adjacent plate or fuel cell plates  12 . It is understood that the diameter, quantity, and length of the inlet manifold  14  will depend on the size and quantity of inlet apertures  20  in the fuel cell plates  12  and the number of fuel cell plates  12  stacked in the fuel cell assembly  10 . 
     The outlet manifold  16  includes the outlet  30  in fluid communication with the outlet manifold  16  formed in the fuel cell assembly  10  by the outlet apertures  22  of the fuel cell plates  12 . The plates  12  are stacked with the outlet aperture  22  of each plate  12  substantially aligned with the outlet aperture  22  of an adjacent plate or plates  12 . It is understood that the diameter, quantity, and length of the outlet manifold  16  will depend on the size and quantity of outlet apertures  22  in the plates  12  and the number of plates  12  stacked together in the fuel cell assembly  10 . 
     Generally, during operation of a fuel cell power system, a hydrogen reactant is fed into the anode side of the fuel cell assembly  10 . Concurrently, an oxygen reactant is fed into the cathode side of the fuel cell assembly  10 . On the anode side, the hydrogen is catalytically split into protons and electrons. The oxidation half-cell reaction is represented by: H 2             2H + +2e − . In a polymer electrolyte membrane fuel cell, the protons permeate through the membrane to the cathode side. The electrons travel along an external load circuit to the cathode side creating the current of electricity of the fuel cell assembly  10 . On the cathode side, the oxygen reacts with the protons permeating through the membrane and the electrons from the external circuit to form water molecules. This reduction half-cell reaction is represented by: 4H + +4e − +O 2           2H 2 O. Anode exhaust from the anode side flows through a backpressure control valve to a combustor, or is alternatively recycled back to the anode inlet manifold. Cathode exhaust from the cathode side flows through a second backpressure control valve to the combustor or to the ambient environment. A control module typically regulates the conditions of the hydrogen stream, oxygen stream, and exhaust streams by operating various control valves, backpressure control valves, and compressors in response to signals from pressure sensors and electrical power sensors connected to the fuel cell assembly  10 .
     During operation of the fuel cell assembly  10 , droplets of liquid water are formed in the channels  24  of the fuel cell plates  12  on the cathode sides of the fuel cell assembly  10 . Some water also may be transported into the anode flow channels, or may form in the anode channels via condensation resulting from consumption of the hydrogen. It is understood that the operation as described herein for the cathode side is similar to operation for the anode side of the fuel cell assembly  10 . The air stream flowing through the cathode side causes the water droplets to flow through the channels  24 , toward the outlet manifold  16 . Water vapor also flows towards the outlet manifold  16 . Once the water droplets contact the first ends  18   a  of the water transport structures  18 , the water is wicked away from the channels  24  by the water transport structures  18 , through the intermediate portions  18   c , and into the manifolds  14 ,  16  from the second ends  18   b . Because the apertures  26  are formed intermediate the gasket  32  and the flow channels  24 , the water and vapor may be removed from the assembly  10  while also facilitating proper sealing by the gasket  32 . If the water transport structures  18  are spaced apart as shown in  FIG. 3 , water and water vapor will also be transported past the water transport structures  18  through the manifolds  14 ,  16  and from the fuel cell assembly  10  in the known methods of water removal. If the water transport structures  18  include a hydrophilic coating, or are produced from a hydrophilic material, this will provide additional capillary force to attract the water droplets and the condensed water vapor. The exhaust gas streams also pass through the water transport structures  18 , and through the outlet manifold  16 . If the water transport structures  18  include a hydrophobic coating or are produced from a hydrophobic material, capillary action is aided by the repulsive nature of the coating or material. 
     It is expected that three different water transport mechanisms may be utilized to remove the water from the channels  24 , depending on the material used for the water transport structures  18 . First, the porous materials  18  may form a network of open, continuous pores that are capable of utilizing capillary forces to transport the water therethrough. Second, because the second ends  18   c  of the water transport structures  18  abut and form continuous paths through the inlet manifold  14  and the outlet manifold  16  of the fuel cell assembly  10 , the water absorbed by the water transport structures  18  will create a static pressure head to facilitate removal of the water from the manifolds  14 ,  16 . It is desirable, though not necessary, for a portion of the water transport structures  18  to be saturated to create a sufficient head to cause the water to drain from the water transport structures  18 . If a saturated portion is not created in the water transport structures  18 , a peristaltic pump (not shown) may be used with the fuel cell assembly  10  to cause the water to flow through the assembly  10  and out of the water transport structures  18 . The peristaltic pump may be a peristaltic pump such as the one disclosed by Anonymous, Pump to Remove Water from a Wick, Pub. No. 494084, O. G. June 2005. Third, during operation of the fuel cell assembly  10 , it is anticipated that a portion of an operational cycle will result in the outlet reactants streams being less than saturated, wherein evaporation will aid water removal from the water transport structures  18 . 
     From the foregoing description, one ordinarily skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications to the invention to adapt it to various usages and conditions.