Patent Abstract:
A fuel cell assembly having a flow distribution subassembly that comprises four sets of flow channels, the first set facing an anode for distribution of a fuel reactant to said anode, the second set facing a cathode for distribution of an oxidant to said cathode, the third set in flow communication with said second set and in heat transfer relation with at least one of said anode and said cathode, and the fourth set receiving a coolant different from said oxidant.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 11/112,102 filed on Apr. 25, 2004. The entire disclosures of the above application is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the humidification and cooling of a fuel cell power system, and, in particular, to an apparatus and method for the integrated humidification and cooling of a fuel cell. 
     BACKGROUND OF THE INVENTION 
     Fuel cell power systems convert a fuel and an oxidant into electricity. One such fuel cell power system has a proton exchange membrane (hereinafter also referred to as “PEM”) to catalytically facilitate the reaction of fuels (such as hydrogen) and oxidants (such as oxygen or air) into 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 the stack of fuel cells present in a fuel cell power system. 
     In a typical fuel cell assembly, or stack, each fuel cell has flow fields in flow communication with manifolds that provide channels for the various reactant gases to flow into each cell. Gas diffusion assemblies then distribute the reactants from the flow fields to the reactive anode and cathode of a membrane electrode assembly (hereinafter also referred to “MEA”). 
     Effective operation of a PEM fuel cell requires proper humidification of the PEM to maintain its proton conductivity. At the same time, the flow field channels and gas diffusion assemblies must be maintained in non-flooded operational states. In operation, the oxidant is supplied to the cathode where it reacts with hydrogen cations that have crossed the PEM and electrons from an external circuit. The fuel cell generates both electricity and water through the electrochemical reaction. The water is typically removed with the cathode effluent, which may dehydrate the PEM unless the water is otherwise replaced. It should be noted that the rate of evaporation to the cathode is generally greater than the rate of water generation. 
     When hydrated, the polymeric PEM possesses “acidic” properties that provide a medium for conducting protons from the anode to the cathode of the fuel cell. However, if the PEM is not sufficiently hydrated, the “acidic” character diminishes, and may impede the desired electrochemical reaction of the cell. Hydration of the PEM also assists in temperature control within the fuel cell, insofar as the heat capacity of water provides a heat sink. In addition to issues of water balance and cell hydration, another issue in fuel cell design is the efficient use of space. For example, space in a vehicle is precious and designs that minimize the ongoing use of space in the vehicle clearly benefit the utility of the vehicle; this leads toward integration of the humidifying system into each of the fuel cells. 
     The need for efficiency in operation and greater integration of cooling and humidification to achieve efficient space utilization in fuel cell systems continues to be strongly felt. What is needed is a fuel cell power system which provides integrated humidification of the feed gases (especially the oxidant) and cooling of the MEA. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a fuel cell having a membrane electrode assembly in reactive interface with (1) a plurality of oxidant reactant flow channels receiving and carrying an oxidant reactant, and (2) a plurality of fuel reactant flow channels receiving and carrying a fuel reactant. The fuel cell includes a plurality of oxidant coolant channels, each in thermal interface with an MEA, preferably for the length of the reactive interface. Preferably each oxidant coolant channel is also in flow communication with a respective cathode reactant flow channel. Two-phase air feed, which may include nebulized water and air, is provided to each oxidant coolant channel. The nebulized water humidifies the air using heat from the fuel cell. Humidified air is discharged from the oxidant coolant channel outlet to provide humidified oxidant to the cathode reactant channel. 
     In a further aspect, the present invention provides a plurality of coolant flow channels adjacent to the reactant flow channels and the MEA. The coolant flow channels are positioned providing a thermal interface surface adjacent the MEA. Preferably, each coolant flow channel has an elongated axis in parallel alignment with the elongated axis of the adjacent reactant flow channel for the length of the reactive interface of the fuel cell. In one embodiment, the plurality of coolant flow channels transports a dielectric liquid coolant. 
     In another variation, the oxidant coolant and the liquid dielectric coolant are used together in separate coolant channels. 
     In yet another aspect of the present invention, an oxidant cooling channel cools the fuel cell while receiving water from the liquid coolant channel via a water transport media. The water humidifies the oxidant prior to entering the cathode reactant channel. 
     In still another aspect of the present invention, the fuel cell system includes a fuel processor making a reformate gas for the fuel cell from a hydrocarbon fuel feed, a reformer water feed, and a reformer air feed. The fuel processor and fuel cell are controlled by a computer which balances water flows to hydrate the fuel cell. 
     A further aspect of the present invention discloses a method for cooling an electrochemical fuel cell. The method includes conducting an electrochemical reaction by oxidizing a fuel reactant with an oxidant reactant at an MEA. In addition to water, the reaction produces electricity and thermal energy. The MEA is cooled by transferring heat to at least one of the reactants in a first flow path, thereby heating the reactant. The heated reactant is subsequently directed to a second flow path leading to the MEA in a reactant capacity. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  illustrates a fuel cell power system with water management instrumentation and control; 
         FIG. 2  is a schematic, exploded, isometric illustration of an exemplary liquid-cooled PEM fuel cell stack (only two cells shown); 
         FIG. 3  is an exploded, partial cross-sectional view of a prior art PEM fuel cell assembly; 
         FIG. 4  is an exploded, partial cross-sectional view of a PEM fuel cell assembly according to a first preferred embodiment of the present invention; 
         FIGS. 5A and 5B  are exploded, partial cross-sectional views of a PEM fuel cell assembly according to a second preferred embodiment of the present invention; and 
         FIG. 6  is a partial isometric view of  FIG. 5B  illustrating a support member according to the third preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     As shown in  FIG. 1 , a fuel cell power system  10  includes a fuel cell stack  22 . Hydrogen or hydrogen-containing reformate from a hydrogen source  20  which feeds hydrogen through the anode chamber of the fuel cell stack  22 . At the same time, oxygen in the form of air in oxidant stream  24  is fed from a compressor, or a blower  12 , into the cathode chamber of fuel cell stack system  22  through passage  18 . The hydrogen from hydrogen stream  20  and the oxygen from oxidant stream  24  react in fuel cell stack  22  to produce electricity. The hydrogen or hydrogen-containing reformate can be supplied from a hydrogen storage vessel or produced by a fuel processor, as is known in the art. 
     Anode exhaust (or effluent)  26  from the anode side of fuel cell stack  22  contains some unreacted hydrogen. Cathode exhaust (or effluent)  28  from the cathode side of fuel cell stack system  22  may contain some unreacted oxygen. These unreacted gases represent additional energy that can optionally be recovered in combustor  30 , in the form of thermal energy, for various heat requirements within power system  10 . 
     Specifically, a hydrocarbon fuel  32  and/or anode effluent  26  are combusted, catalytically or thermally, in combustor  30  with oxygen either from air in stream  34  or from cathode effluent stream  28 , depending on power system  10  operating conditions. Combustor  30  discharges exhaust stream  36  to the environment, and the heat generated thereby may be directed to a fuel processor or other system components as needed. 
     Coolant is supplied to the fuel cell stack  22  at inlet  88  and exits through passage  90 . A pump  89  and heat exchanger  91  are provided for providing continuous flow of coolant through the fuel cell stack. 
     As shown in  FIG. 2 , a partial PEM fuel cell stack  50  is depicted having a pair of membrane electrode assemblies (MEAS)  52 ,  54  separated from each other by a non-porous, electrically-conductive bipolar plate  56 . Each MEA  52 ,  54  has a cathode face  52   c ,  54   c  and an anode face  52   a ,  54   a . The MEAs  52 ,  54  and bipolar plate  56  are stacked together between non-porous, electrically-conductive, liquid-cooled end plates  58 ,  60 . Each plate  56 ,  58 ,  60  includes a respective flow field  62 ,  64 ,  66  established from a plurality of flow channels formed in the faces of the plates for the distribution of fuel and oxidant gases to the reactive faces of the MEAs  52 ,  54 . Nonconductive gaskets or seals  68 ,  70 ,  72 ,  74  provide sealing and electrical insulation between the several plates of fuel cell stack  50 . 
     Porous, gas-permeable, electrically conductive sheets  76 ,  78 ,  80 ,  82  press up against the electrode faces of MEAs  52 ,  54  and serve as primary current collectors for the respective electrodes. The electrically conductive sheets  76 ,  78 ,  80 ,  82  additionally provide mechanical support for the respective MEAs. Bipolar plate  58  presses up against primary current collector  76  on the cathode face  52   c  of the MEA  52 ; likewise, bipolar plate  60  presses up against the primary current collector  82  on the anode face  54   a  of the MEA  54 , and bipolar plate  56  presses up against primary current collector  78  on the anode face  52   a  of one MEA  52 , and against the primary current collector  80  on the cathode face  54   c  of another MEA  54 . 
     An oxidant  18  is supplied to the cathode side of the fuel cell stack  50  from an appropriate supply plumbing  84 . Similarly, hydrogen  20  is supplied to the anode side of the fuel cell  50  from an appropriate supply plumbing  86 . Exhaust plumbing (not shown) is also provided for removing anode effluent from the anode flow fields and the cathode effluent from the cathode flow fields. Coolant plumbing  88 ,  90  is provided for supplying and exhausting liquid coolant to the bipolar plates  56 ,  58 ,  60 , as needed. 
     It should be noted that, for simplicity, the fuel cell stack  50  shows two fuel cells with plate  56  being shared between the two fuel cells and plates  58 ,  60  being shared between one of the shown fuel cells. In this regard, a “fuel cell” within a fuel cell stack is not physically fully separable insofar as any particular fuel cell in the stack will share at least one side of a bipolar plate with another cell. 
       FIG. 3  is an exploded, partial cross-sectional view, illustrating a prior art PEM fuel cell stack  100 . The fuel cell stack  100  includes a bipolar plate  102  consisting of an anode distribution plate  104  and a cathode distribution plate  106 . The distribution plates  104 ,  106  are patterned with a plurality of alternating lands and grooves defining various sets of flow channels, each having respective anode-confronting and cathode-confronting faces. The anode distribution plate  104  defines a first set of flow channels, or anode reactant channels  110 , in reactive interface with the anode face  109  of MEA  108 . The cathode distribution plate  106 , defines a second set of flow channels, or cathode reactant channels  112 , in reactive interface with the cathode face  111  of MEA  108 . The anode and cathode distribution plates  104 ,  106  are joined and electrically coupled together at a plurality of lands, or interfaces  116 , using conventional methods such as brazing, welding or adhesive bonding. In this regard, the respective anode and cathode reactant channels  110 ,  112  are axially aligned and directly oppose one another. A first set of coolant channels  114  is defined by the back sides, or heat exchange faces  113 , of the gas distribution plates  104 ,  106 . The flow channels  110 ,  112 ,  114  are preferably arranged in the anode and cathode distribution plates  104 ,  106  such that the first set of coolant channels  114  extends the entire height of the bipolar plate  102  and is located adjacent the anode and cathode channels  110 ,  112  which extend approximately one-half the height of the bipolar plate  102 . The coolant channels  114  have thermal interfaces with two MEAs  108 ,  108 ′. 
     The fuel cell stack  100  also includes a second bipolar plate  102 ″ consisting of an anode distribution plate  104 ′ and a cathode distribution plate  106 ′ joined together. The anode distribution plate  104 ′ defines a similar set of anode reactant channels  110 ′ in reactive interface with the anode face  109 ′ of MEA  108 . Likewise, the cathode distribution plate  106 ′ defines a similar set of cathode reactant channels  112 ′ in reactive interface with the cathode face  111 ′ of MEA  108 ′. The anode and cathode distribution plates  104 ′,  106 ′ are joined and electrically coupled together at a plurality of lands, or interfaces  116 ′, using conventional methods such as brazing, welding or adhesive bonding. Similar to bipolar plate  102 , the distribution plates  102 ′,  104 ′ are axially aligned and the reactant channels  110 ′,  112 ′ directly oppose one another. A second set of coolant channels  114 ′ is defined by the back sides, or heat exchange faces  113 ′, of the distribution plates  104 ′,  106 ′. The flow channels  110 ′,  112 ′,  118  are preferably arranged in the anode and cathode distribution plates  104 ′,  106 ′ such that the second set of coolant channels  114 ′ extends the entire height of the bipolar plate  102 ′ and is located adjacent the anode and cathode channels  110 ′,  112 ′ which extend approximately one-half the height of the bipolar plate  102 ′. The second set of coolant channels  114 ′ have thermal interfaces with two MEAs  108 ,  108 ′. For completeness, anode plate  104 ′ and cathode plate  106 ′ are also shown. 
     The present invention involves providing a fourth flow field to the stack design for providing integrated humidification and cooling of a fuel cell. Insofar as the preferred embodiments of  FIGS. 4-6  share many similar aspects and considerations with those of  FIG. 3 , considerations discussed in  FIG. 3  respective to channel layout, materials, fluids, and thermodynamics are equally applicable to the embodiments illustrated in  FIGS. 4-6  in view of the foregoing discussion. Accordingly, only those features of differentiating interest in  FIGS. 4-6  will be discussed hereinafter. 
       FIG. 4  illustrates a partial cross-sectional view of a PEM fuel cell stack  200  according to a first preferred embodiment of the present invention. The fuel cell stack  200  includes a bipolar plate  202  consisting of an anode distribution plate  204  and a cathode distribution plate  206 . The anode distribution plate  204  defines a set of anode reactant channels  210  in reactive interface with the anode face  209  of MEA  208 . The cathode distribution plate  206  defines a set of cathode reactant channels  212  in reactive interface with the cathode face  211  of MEA  208 . The anode and cathode distribution plates  204 ,  206  are joined and electrically coupled together at a plurality of interfaces  216  using conventional methods such as brazing, welding or adhesive bonding. In this regard, the respective anode reactant channels  210  and cathode reactant channels  212  are offset from one another and are surrounded by a coolant on three sides. A first set of coolant channels  214  is defined by the back sides, or anode heat exchange faces  213 , of the anode distribution plate  204 . A second set of coolant channels  218  is defined by the back sides, or cathode heat exchange faces  217 , of the cathode distribution plate  206 . 
     The flow channels  210 ,  212 ,  214 ,  218  are arranged in the anode and cathode distribution plates  204 ,  206  such that the first coolant channel  214  extends approximately one-half the height of the bipolar plate  202  and is directly opposed the cathode reactant channel  212  which also extends approximately one-half the height of bipolar plate  202 . The second coolant channel  218  extends approximately one-half the height of bipolar plate  202  and is directly opposed the anode reactant channel  210  which also extends approximately one-half the height of bipolar plate  202 . 
     The cross-sectional areas of the plurality of flow channels  210 ,  212 ,  214 ,  218  are determined by the shape or pattern of the respective distribution plates  204 ,  206 . The distribution plates  204 ,  206  can be designed to have the specific cross-sectional area corresponding to a desired volumetric flow rate. In this regard, while  FIG. 4  depicts the reactant channels  210 ,  212  and the cooling channels  214 ,  218  having similar respective cross-sectional areas, each can be designed having a different cross-sectional area. 
     In various embodiments, oxidant, or compressed air, is provided to the second set of coolant channels  218  wherein heat is extracted from the fuel cell stack  200  and used to heat the oxidant which is subsequently directed to the cathode reactant channels  212  as the cathode reactant. In this manner, the second set of coolant channels is used to heat the air prior to entering the cathode reactant flow channels. Alternatively, in various other embodiments, the compressed air may need to be cooled prior to its entry in the flow channels. The determination of whether the air needs to be heated or cooled depends, in part, on the source of the air supply (a compressor or a blower) and the discharge pressure. In either embodiment, the air should be adjusted (by heating or cooling) to the stack temperature prior to entering the cathode flow field. Hydrogen is provided to the anode reactant channels  210  as the anode reactant. A dielectric liquid coolant, such as water, is provided to channels  214  as a coolant to further extract heat from the fuel cell stack  200 . 
     In accordance with the present invention, the oxidant coolant channels  218  are in flow communication with the cathode reactant channels  212 . In one embodiment, the fluid interface is direct, such that an outlet of coolant channel  218  is essentially a continuous passage leading to an inlet of cathode reactant channels  212 . In an alternate embodiment, a group of outputs from the coolant channels  218  fluidly interface with a manifold (not shown) which also fluidly interfaces with a group of inlets from the reactant channels  212 . A forwarding pump (not shown) may be utilized as a part of the fluid communication interface. In a preferred operation, a two-phase feed stream consisting of nebulized water (atomized water, water mist, water particles, fog) and air is provided to the second set of coolant channels  218 . The nebulized water proceeds through the oxidant coolant channels  218  and humidifies the air using heat from the electrochemical reactions as the latent heat of vaporization to fully vaporize the nebulized water. The humidification process thus provides a degree of heat extraction, thereby further cooling the fuel cell stack  200 . The humidified air is discharged therefrom providing a humidified oxidant reactant to the cathode reactant channels  212 . 
     Water is preferably added to the oxidant coolant channels  218  by nebulizing the water so that it is essentially entrained in air, providing a two-phase fluid of air and nebulized water. Water may be nebulized using spray nozzles, sonic misters, vaporizers or other means to adequately disperse water into the air stream. 
     As used herein, “water” means water that, in a compositional nature, is useful for operation of a fuel cell power system. While certain particulates are acceptable in generally available water, they might cause plugging in addition to plugging caused by particulates in the oxidant gas; therefore, as should be apparent, the water used must be appropriately filtered before being introduced into the fuel cell. 
       FIG. 5A  illustrates a PEM fuel cell stack  300  according to a second preferred embodiment of the present invention. The fuel cell stack  300  includes a bipolar plate  302  consisting of an anode distribution plate  304  and a cathode distribution plate  306 . A set of anode reactant channels  310  formed in the anode distribution plate  304  is in reactive interface with the anode face  309  of MEA  308 . A set of cathode reactant channels  312  formed in the cathode distribution plate  306  is in reactive interface with the cathode face  311  of MEA  308 . A separation layer, or separator plate  320 , is interposed between the anode distribution plate  304  and the cathode distribution plate  306 . The separator plate  320  and anode heat exchange faces  313  of the anode distribution plate  304  define a first set of coolant channels  314 . Likewise, the separator plate  320  and cathode heat exchange faces  317  of the cathode distribution plate  306  defines a second set of coolant channels  318 . The separation layer  320  is preferably a non-porous electrically conductive material, and provides heat transfer control between the coolant channels  314 ,  318 . 
     The anode distribution plate  304  is patterned and arranged such that the first set of coolant channels  314  extend approximately one-half the height of the bipolar plate  302  and alternates with the set of anode channels  310 . The cathode distribution plate  306  is patterned and arranged such that the second set of coolant channels  318  extends approximately one-half the height of the bipolar plate  302  and alternates with the set of cathode channels  312 . The anode and cathode distribution plates  304 ,  306  and the separation layer  320  are joined and electrically coupled together at a plurality of interfaces  316  using conventional methods such as brazing, welding or adhesive bonding. 
     As presently preferred, oxidant is provided to the second set of coolant channels  318  wherein heat is extracted from the fuel cell stack  300  and used to heat the oxidant which is subsequently directed to the cathode reactant channels  312  as the cathode reactant. Hydrogen is provided to the anode reactant channels  310  as the anode reactant. A dielectric liquid coolant such as water is provided to the first set of coolant channels  314  further extracting heat from the fuel cell stack  300 . 
       FIG. 5B  illustrates an alternate design of the second preferred embodiment having a water transport media, or water permeable membrane  322 , separating the distribution plates  304 ,  306 . Preferably, the coolant in the first set of coolant channels  314  contains liquid water. The water travels through the first set of coolant channels  314  thereby cooling the MEA. It also permeates through the water transport media and humidifies the oxidant coolant flowing in the adjacent coolant channels  318 . 
     Depending on the material used, the water transport media  322  may require additional structural support to maintain the integrity of the bipolar plate  302 .  FIG. 6  illustrates one embodiment using one or more support members  324  disposed adjacent the water transport media  322 . Preferably, the support members  324  extend in a direction perpendicular to the channels  310 ,  312 ,  314 ,  318  and continue across the entire width of the fuel cell stack  300 . 
     A further aspect of the present invention discloses a method for cooling an electrochemical fuel cell. With renewed reference to  FIG. 4 , the method includes introducing an oxidant stream to a plurality of cathode reactant channels  212  adjacent a cathode  211 , and a fuel stream to a plurality of anode reactant channels  210  adjacent an anode  209 . An electrochemical reaction occurs at the MEA  208  wherein a fuel reactant is oxidized with an oxidant reactant producing water, electricity and thermal energy. The MEA  208  is subsequently cooled by transferring heat from the assembly to at least one of the reactants flowing through a first flow path in thermal communication with the MEA  208 , thereby heating the reactant. This heated reactant is then directed to a second flow path leading to the MEA  208  in a reactant capacity. A preferred embodiment of the present invention uses the oxidant as the heat transfer reactant. Preferably, the oxidant is humidified as it cools the MEA. The humidification process further cools the fuel cell by using thermal energy as the latent heat of vaporization necessary to vaporize liquid water. The method may also include a second coolant which is transported through a plurality of coolant flow channels in thermal communication with the MEA providing an additional degree of heat extraction and cooling of the fuel cell stack. 
       FIG. 1  shows the fuel cell power system  10  with instrumentation and control for balancing water flow to hydrate the fuel cell. A real-time control computer  38  is symbolically indicated as an element with signal line interfaces (providing signal or data communication with said real-time control computer) to a differential pressure transmitter  42 , hydration measurement  43 , control valve  44 , flow measurement  45 , and combustor emissions measurement  46 . 
     Differential pressure transmitter  42  is shown in a single instance in system  10 , representing at least one such transmitter  42  for a fuel cell stack  22 ; in alternative embodiments, a larger number of instances of differential pressure transmitter  42  are deployed for measuring fluid flow pressure drop between the inlet and outlet of the air feed coolant channels. 
     In an embodiment using a membrane in a fuel cell with bipolar plates, the flow rate of water  47  to the membrane is further controlled in one embodiment responsive to a measurement signal from differential pressure transmitter  42  by use of control computer  38 , control valve  44 , and flow measurement  45 . In an embodiment using a nebulized water feed to air stream  24  in a fuel cell with bipolar plate, the flow rate of water  47  to the nebulizing system (e.g., spray nozzles—not shown, but which should be apparent) is further controlled in one embodiment responsive to a measurement signal from differential pressure transmitter  42  by use of control computer  38 , control valve  44 , and flow measurement  45 . 
     Further in a consideration of a fully balanced fuel cell system, even as the above description has focused on the humidification of air feed streams to the cathodes of MEAs, it should be apparent from the above discussion that fuel streams delivered to anode reactant (fuel reactant) channels are, in some embodiments, substituted for a portion of the set of air feed stream humidification channels so that humidification/moisturization along with appropriate cooling of the fuel reactant stream to the fuel cell is also appropriately achieved. 
     As should also be apparent to one skilled in the art, an oxygen gas stream having about 25 weight percent oxygen or greater may, in some embodiments, be fed or provided to the oxidant coolant channels in the place of air. 
     It should be appreciated that while the present invention discloses the use of two-plate member bipolar plates, the concept of unitized bipolar plates, or bipolar plates manufactured from a single piece of material are also suitable for use herein. Unitized bipolar plates may be formed by the extrusion of a conductive material as is known in the art. The use of a unitized bipolar plate would simplify the fuel cell manufacturing process by eliminating the need to pattern, shape, align and electrically bond two plate members together. Additionally, a variety of combinations or configurations alternating the liquid coolant channels and oxidant coolant channels in the fuel cell assemblies is feasible and contemplated within the scope of the present invention. 
     Thus, the description of the invention is merely exemplary in nature and variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.

Technology Classification (CPC): 7