Abstract:
A method of manufacturing a fuel cell flow field plate is disclosed in which a generally even flow distribution across the flow field is provided. The method includes providing an inlet manifold in fluid communication with the flow field. The flow field includes multiple channels for which some of the channels receive restricted flow from the inlet manifold as compared to other channels. A relative pressure drop between the channels is altered with a pressure drop feature to encourage fluid flow from the inlet manifold to the channels with restricted flow, which results in a generally even flow distribution across the flow field.

Description:
BACKGROUND 
       [0001]    This disclosure relates to a fuel cell plate flow field configuration. 
         [0002]    A fuel cell includes an anode and a cathode arranged on either side of a membrane electrode assembly. The anode and the cathode are provided by a plate, which includes a flow field. The anode plate flow field delivers fuel to the membrane electrode assembly, and the cathode plate flow field delivers a reactant to the membrane electrode assembly. 
         [0003]    The flow fields are provided by multiple channels that are provided fluid from an inlet manifold. The channels have been arranged in a variety of configurations depending upon a variety of factors, such as packaging constraints. Typically, it is desirable to provide a manifold that is wider than inlets to the channels to ensure a generally even distribution of flow across the channels. Occasionally, it is not possible to supply each of the channel inlets with unobstructed flow from the inlet manifold. As a result, some of the channels receive a somewhat limited flow, which results in an uneven distribution of flow across the flow field. Uneven flow distribution can create temperature gradients across the plate and reduce the efficiency of the chemical reactions within the fuel cell. In the case of anode flow fields, insufficient hydrogen at a location can create carbon corrosion of the anode plates. In the case of cathode flow fields, insufficient oxygen at a location can cause high temperatures and cell voltage dropoff. 
         [0004]    What is needed is a fuel cell plate having a flow field with a generally even flow distribution in configurations where it is not possible to supply an uninhibited flow to at least some of the channels. 
       SUMMARY 
       [0005]    A method of manufacturing a fuel cell plate flow field is disclosed in which a generally even flow distribution across the flow field is provided. The method includes providing an inlet manifold in fluid communication with the flow field. The flow field includes multiple channels for which some of the channels receive restricted flow from the inlet manifold as compared to other channels. A relative pressure drop between the channels is altered with a pressure drop feature to encourage fluid flow from the inlet manifold to the channels with restricted flow, which results in a generally even flow distribution across the flow field. 
         [0006]    In one example, first and second sets of channels are arranged in alternating relationship. Inlet passages from the inlet manifold are misaligned with the first channels to encourage fluid flow from across first set of channels in a balanced manner. In another example, unobstructed channels include a shallow channel portion to increase the pressure drop along those channels. Cross-cuts can be used from the unobstructed channels to the obstructed channels to reduce the pressure drop along the obstructed channels. 
         [0007]    What is needed is a fuel cell plate having a flow field with a generally even flow distribution in configurations where it is not possible to supply an uninhibited flow to at least some of the channels. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a highly schematic view of a fuel cell. 
           [0009]      FIG. 2  is a plan view of an example fuel cell plate having a flow field. 
           [0010]      FIG. 3  is an enlarged view of a portion of the plate shown in  FIG. 2 . 
           [0011]      FIG. 4  is an enlarged view of another portion of the plate shown in  FIG. 2 . 
           [0012]      FIG. 5  is a plan view of another example fuel cell plate. 
           [0013]      FIG. 6  is an enlarged perspective view of a portion of the fuel cell plate shown in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    A fuel cell  10  is shown in a highly schematic fashion in  FIG. 1 . The fuel cell  10  includes a membrane electrode assembly  16  arranged between an anode  12  and a cathode  14 . The membrane electrode assembly  16  comprises a proton exchange membrane arranged between gas diffusion layers, for example. The anode  12  and the cathode  14  respectively provide fuel and reactant flow fields provided by channels in a solid or porous plate. The flow fields are fluidly connected to flow field inlets and exhausts using either internal or external manifolds that are in fluid communication with their respective fluid flow component. 
         [0015]    A plate  18  is illustrated in  FIGS. 2-4  having internal inlet and exhaust manifolds  20 ,  22 . A flow field  24  is fluidly interconnected between the inlet and exhaust manifolds  20 ,  22 . In the example, the inlet and exhaust manifolds are arranged on opposite sides of the plate  18 . Parallel channels  26  arranged between risers  28  provide the flow field  24 . In the example, the channels  26  extend a length L and are parallel with one another along the length without any significant bends. That is, there are no right angle turns and a given channel does not double back on itself as is typical with some flow fields. The flow field  24  has a width W 2  that is greater than the width of the inlet manifold  20 . This configuration presents a challenge of evenly distributing fluid across the flow field  24 . Specifically, the channels outboard of the inlet manifold  20  are typically starved of fluid, resulting in an uneven chemical reaction at the proton exchange membrane and hot-cold spots on the plate  18  or carbon corrosion on the anode side. 
         [0016]    In one example, the channels  26  are divided into first and second sets of channels  34 ,  36  arranged in alternating relationship with one another to provide an interdigitated flow field. The first set of channels  34  are fluidly interconnected by a lateral inlet passage  32 , extending a width W 2 , that is supplied fluid from the inlet manifold  20  through discrete, spaced apart inlet passages  30 . In the example, the inlet passages  30  are generally evenly spaced laterally from one another and misaligned with the channels in the first set of channels  34 . This misalignment encourages even fluid distribution across the first set of channels  34 . Each channel of the first set of channels  34  extends from the lateral inlet passage  32  to a first terminal end  38 , best shown in  FIG. 4 . 
         [0017]    Each channel of the second set of channels  36  extend from a second terminal end  40 , which is arranged near the lateral inlet passage  32  (best shown in  FIG. 3 ), to a lateral exhaust passage  42  that fluidly interconnects the second set of channels  36  with one another. In the example, there is a pair of lateral exhaust passages  42  interconnected to and parallel with one another, extending the width W 2 , as best shown in  FIG. 4 . The first terminal ends  38  are arranged near the lateral exhaust passages  42 . Discrete exhaust passages  44  fluidly connect the lateral exhaust passages  42  to the exhaust manifold  22 . 
         [0018]    In operation, fluid is supplied to the first set of channels  34  by the inlet manifold  20  via the inlet passages  30 . Since the first set of channels  34  is dead-ended at the first terminal ends  38 , fluid will flow into the gas diffusion layer of the membrane electrode assembly  16 , for example, and into the second set of channels  36 . This interdigitated arrangement of channels provides a pressure drop feature between the first and second sets of channels  34 ,  36  that evenly distributes flow across the flow field  24 . Fluid from the gas diffusion layer is provided to the proton exchange membrane for chemical reaction. From the second set of channels  36 , fluid is returned to the exhaust manifold  22 . 
         [0019]    Another plate  118 , which has an external inlet manifold  46 , is shown in  FIG. 5 . Fluid is supplied to a header within the plate  118 , which provides the lateral inlet passage  132 , through inlet passages  48 . Flow from the inlet passages  48  encounters baffles  50  that distribute the flow within the header. The flow field  124  has a width W 2  that is wider than the width of the manifold  46 , W 1 . Flow to the first set of channels  134  is generally unobstructed. In the configuration shown in  FIG. 5 , the flow becomes choked at the extremities within the header at a restricted flow region  52  such that flow to the second set of channels  136  is obstructed. Risers  128  separate the first and second sets of channels  134 ,  136 . 
         [0020]    Obstructed flow to the second set of channels  136  would create a pressure drop across a length L of the second set of channels  136 . To counter this pressure drop and provide an even flow distribution across the flow field  124 , cross-cuts or cross passages  54  are arranged from some of the first set of channels  134  near the header and extending at an angle and away from the header into the second set of channels  136  beneath the restricted flow region  52 . The cross passages  54  can also be arranged perpendicular to the channels. As a result, flow will be evenly distributed across the flow field  124  from the inlet manifold  46  to the exhaust manifold through passages  56 . 
         [0021]    Referring to  FIG. 6 , another pressure drop feature is shown that can be used instead of or in addition to the cross passages  54  in the plate  118 . The first set of channels  134 , which would otherwise be unobstructed, include shallow channel portions  58  providing a smaller cross-sectional area that create a pressure drop across the length L of the first set of channels  134 . The second set of channels  136  include a channel depth D 1  that is greater than the channel depth D 2  associated with the shallow channel portion  58 , which is arranged near the header. The first set of channels  134  may transition from the depth D 2  at the shallow channel portion  58  to the depth D 1  further downstream. The length of the shallow channel portion  58  and its depth are selected to achieve a desired pressure drop that results in an even flow distribution across the flow field  124 . The term “depth” is also intended to include width. 
         [0022]    Although example embodiments have been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of the claims. For that reason, the following claims should be studied to determine their true scope and content.