Abstract:
A high temperature proton exchange medium (PEM) fuel stack system includes features for enhancing the thermal management of the fuel cell. The fuel cell can include a plurality of membrane-electrode-assemblies (MEA) separated by bipolar plates. The upper and lower edges of the bipolar plates are configured such that a plurality of fins is formed therein. Air can be passed along the fins in the upper edges of the plates and along the fins in the lower edges in opposite directions. A plurality of channels is formed on one or both surfaces of the bipolar plates. The channels extend along a serpentine path. Except for the end plates, hydrogen is supplied to the channels on one side of each plate and air is supplied to the channels on the channels on the opposite side of each plate. Such features keep the fuel cell within acceptable temperature limits during operation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of Provisional Application Ser. No. 61/266,480 entitled “HIGH TEMPERATURE PEM FUEL CELL WITH THERMAL MANAGEMENT SYSTEM”, filed Dec. 3, 2009, which is herein incorporated by reference in its entirety 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates in general to fuel cells and, more particularly, to high temperature proton exchange medium fuel cells. 
       BACKGROUND OF THE INVENTION 
       [0003]    A fuel cell electrochemically combines hydrogen and oxygen to produce electricity. Byproducts of the energy-generating electrochemical reaction in a fuel cell include water vapor and carbon dioxide. The electrochemical reaction also generates heat. In a stack plate fuel cell where numerous plates are stacked together and sandwich multiple electrochemical layers, heat dissipation from internal portions of the stack remains a challenge. Current heat management techniques rely on thermal cooling layers disposed adjacent to each electrochemical layer and between each set of plates. For a fuel cell having a stack of numerous plates and electrochemical layers, conventional heat removal techniques for each layer would significantly increase the fuel cell package thickness, volume, and size, thereby rendering the fuel cell impractical or infeasible for many applications. Historically, some of the most difficult operations in high temperature fuel cells are temperature control and temperature spread across the membrane-electrode-assembly (MEA) of the fuel cell. Thus, there is a need for a system that can effectively manage heat within a fuel cell. 
       SUMMARY 
       [0004]    Aspects of the invention are directed to a high temperature proton exchange medium (PEM) fuel stack system with enhanced thermal management features. The fuel cell can include a plurality of membrane-electrode-assemblies (MEA) separated by bipolar plates. The bipolar plates can comprise a plurality of repeating units and two non-repeating units, one on each end of the stack of repeating units. The upper and lower edges of the repeating units and non-repeating units are configured such that a plurality of fins is formed therein. A coolant, such as air, can be passed along the fins in the upper edges of the units in a first direction. A coolant, such as air, can be passed along the fins in the lower edges of the units in a second direction that is opposite the first direction. 
         [0005]    Alternatively or in addition, a plurality of channels can be formed on both major surfaces of the repeating units and on one surface of each of the non-repeating units. The channels can extend along a serpentine path. Fuel, such as hydrogen, can be supplied to the channels on one side of each repeat unit, and on one side of one of the non-repeat units. Oxidant, such as air, can be supplied to the channels on the channels on the opposite side of each repeat unit and on one side of the other one of the non-repeat units. 
         [0006]    Such features can keep the temperature of the fuel cell within acceptable limits. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is an exploded view of one cell of PEM fuel cell stack configured in accordance with aspects of the invention. 
           [0008]      FIG. 2  is a perspective view of a non-repeat unit of a fuel cell stack configured in accordance with aspects of the invention. 
           [0009]      FIG. 3  shows portions of a fuel cell assembly configured in accordance with aspects of the invention. 
           [0010]      FIG. 4  shows one possible coolant flow system in accordance with aspects of the invention. 
           [0011]      FIG. 5A  shows a computational flow dynamics thermal analysis of a repeat unit configured in accordance with aspects of the invention. 
           [0012]      FIG. 5B  is a chart showing the temperature profile across the MEA from corner to corner of a repeat unit configured in accordance with aspects of the invention. 
           [0013]      FIG. 6  shows a computational flow dynamics thermal analysis of the pressure drop across the edge protrusions of a bipolar plate configured in accordance with aspects of the invention. 
           [0014]      FIG. 7  shows a perspective view of a high temperature fuel cell assembly in accordance with aspects of the invention, showing heaters and knife blowers mounted on the fuel cell assembly. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Embodiments of the invention are directed to a thermal management system for a high temperature PEM fuel cell. The term “high temperature PEM fuel cell” means a fuel cell that operates at a temperature of at least about 120° C. In some instances, a high temperature fuel cell can operate in a temperature range of about 120° C. to about 200° C. Various possible aspects of the invention will be explained herein, but the detailed description is intended only as exemplary. Embodiments of the invention are shown in  FIGS. 1-7 , but the invention is not limited to the illustrated structure or application. 
         [0016]    Referring to  FIG. 1 , a one cell bipolar proton exchange medium (PEM) fuel cell stack  10  is shown. The fuel cell  10  can include a membrane-electrode-assembly (MEA)  12  between two bipolar plates  14 , which can be, for example, an electrically conductive graphite bipolar plate. In one embodiment, the bipolar plates  14  can be made of graphite TG-728. The bipolar plates  14  can have very high thermal conductivity in the x-y plane and good thermal conductivity in the through or z plane. Between the MEA  12  and each of the bipolar plates  14 , there can be a non-conductive gasket  16 , which can provide a seal for distributing the fuel (i.e., hydrogen) and the oxidant (i.e., air) to the MEA  12 . The bipolar plates  14  can be repeat units  14 ′; that is, a plurality of substantially identical bipolar plates that is used in the PEM fuel stack  10 . In operation, there are flow fields on both sides of a repeat unit  14 ′—one side for the fuel and the other one for the oxidant. Each repeat unit  14 ′ can have a central portion  18 , an upper end  20  and an opposite lower end  22 . Further, each repeat unit  14 ′ can have opposing lateral ends  24 . 
         [0017]    A fuel cell assembly  10  can also include two non-repeat units  14 ″. A “non-repeat unit” is a bipolar plate with a flow field on only one side of the bipolar plate  14 . An example of a non-repeat  14 ″ unit is shown in  FIG. 2 . The non-repeat units  14 ″ are the first and last plates in the stack of plates forming the fuel cell. As a result, one of the non-repeat units  14 ″ has a fuel flow field on one side of the plate  14 , and the other non-repeat unit  14 ″ has an oxidant flow field on one side of the plate  14 . Each non-repeat unit  14 ″ can have a central portion  18 , an upper end  20 , and an opposite lower end  22 . Further, each non-repeat unit  14 ″ can have opposing lateral ends  24 . 
         [0018]    When the components are assembled, a plurality of cells  28  is formed, as shown in  FIG. 3 . One example of such a fuel cell assembly  30  is shown in  FIG. 3 . Generally, there can be X cells in the assembly, X−1 repeat units and 2 non-repeat units. In one embodiment, there can be a 32 cells assembly with 31 repeat units and 2 non-repeat units. However, aspects of the invention are not limited to such a construction and can readily be used in connection with greater or fewer cells in the assembly. The plurality of cells  28  can be sandwiched between two current collector plates  32 , coupled to positive and negative electrical terminals  70 , and two thick insulating end-plates  34 , such as shown in  FIG. 3 . 
         [0019]    The upper and lower edges  20 ,  22  of the bipolar plates  14 , both for repeating units  14 ′ and non-repeating units  14 ″, can be configured so that a portion of the material of the plate  14  is removed, thereby leaving a protrusion or fin  40 . That is, The fin  40  can be thin relative to the thickness of the rest of the plate  14 , i.e., thinner than central portion  18 . In one embodiment, material can be removed from the front and back side of the plate  14  in the edge region such that the fin  40  is centrally located along the respective edge of the plate. However, in other embodiments, the fin  40  can be closer to or at one of the sides of the plate  14 . In one embodiment, the non-repeat units  14 ″ can have material removed on only one face of the plate, as shown in  FIG. 2 , and the repeat units  14 ′ can have material removed from both sides of each plate, as shown in  FIG. 1 . The fins  40  can have any suitable size, shape. In one embodiment, the fins  40  can be about 0.10 inches thick and about 1.25 inches tall. The fins  40  can extend along at least a portion of the respective edge of the bipolar plate  14 . 
         [0020]    When the plurality of plates  14  is stacked together in the fuel cell assembly  30 , a plurality of fins is formed along the top  42  and bottom  44  of the fuel cell assembly  30 . There can be any suitable spacing between the fins  40 . In one embodiment, the fins  40  can be spaced about 0.20 inches apart. The spacing between each neighboring pair of fins  40  can be the same or the spacing can be different between at least one pair of fins  40  of neighboring plates. Any suitable coolant, such as air, can be supplied by at least one coolant source  60  to the space between the fins  40  and flow laterally along the fins  40 . Any suitable structure for coupling the coolant source(s)  60  to the fuel cell assembly  30  can be used in the various embodiments of the invention. For example, a coolant source can be a blower, a gas cylinder, or any other source of gas in fluid connection with the fins  40  in fuel cell assembly. 
         [0021]      FIG. 4  shows the one example of fluid flow into the fuel cell in which a counter flow cooling scheme can be employed. For instance, as shown in  FIG. 4 , coolant flow in the fins  40  in the top  42  of the fuel cell  30  can flow in a first direction, and coolant flow in the fins  40  in the bottom  44  of the fuel cell assembly  30  can flow in a second direction that is opposite the first direction. In the scheme illustrated in  FIG. 4 , the flow of coolant can be provided by one or more coolant sources  60 , as shown in  FIG. 4 . However, in some embodiments, a single coolant source can be provided for cooling fins  40  in the top  42  and bottom  44  of fuel cell assembly  30 . 
         [0022]    On one side of each repeat unit  14 ′ and on one side of only one of the non-repeat units  14 ″, hydrogen can enter into an individual cell by way of slot  1 . The slot  1  can have any suitable configuration. The flow can then split into a plurality of channels  50 . In one embodiment, there can be eight channels. In one embodiment, the channels can have a depth of about 0.040 inches. The channels can be generally parallel to each other over their entire path. The channels can have any suitable size, shape and configuration. The channels  50  can be formed by recesses in the plate  14  or by raised structures formed on the face of the plate  14 . The channels  50  can be substantially identical to each other or at least one of the channels  50  can be different from the other channels  50  in one or more respects. The channels  50  can extend across each bipolar plate  14  in a direction from one lateral end  24  to the opposite lateral end  24 . The channels  50  can be generally serpentine. In one embodiment, the channels  50  can turn on itself five times before exiting through the slot  3 , as is shown in  FIG. 4 . 
         [0023]    On the other side of each repeat unit  14 ′ and on one side of the second of the non-repeat units  14 ″, air can enters into an individual cell by way of slot  2 . The air can be transported from the slot  2  to the surface of the bipolar plate  14  using angled channels (not shown). The flow can then split into a plurality of channels (not shown). There can be any suitable quantity of channels. In one embodiment, there can be eight channels. In one embodiment, the channels can have a depth of about 0.040 inches. 
         [0024]    The channels can be generally parallel to each other over their entire path. The channels can have any suitable configuration. The channels can be substantially identical to each other or at least one of the channels can be different from the other channels in one or more respects. The channels can extend across each bipolar plate  14  from one lateral end  24  to the opposite lateral end  24 . The channels can be generally serpentine. In one embodiment, the channels can turn on itself five times before exiting through the slot, as is shown in  FIG. 4 . As a result, the flow in one laterally extending segment of the channels can be flowing in an opposite direction of the airflow in a neighboring one of the laterally extending segments of the channels. 
         [0025]    The channels for the hydrogen can be substantially identical to the channels for the air. As they are situated on the opposite side of a repeating plate, the direction of flow of the hydrogen can be opposite to the direction of flow of air. In some instances, the channels for the hydrogen can be different from the channels for the air in one or more respects. 
         [0026]    This combination of fins, slots and serpentine channels can reduce the risk of a MEA impingement. 
         [0027]    Because of the exothermic reaction that occurs in the MEA, a maximum of 55 W of heat can be generated per cell. In some cases, the MEA manufacturer recommends operating the fuel cell at a temperature of 140° C. to 180° C.; however, the temperature spread across the MEA should be as low as possible. To maintain a small temperature spread, the cells can be cooled by supplying air to the space between fins and passing air along the fins. Air can be introduced into the fins using a counter flow strategy, as shown in  FIG. 4 . The best advantage in high temperature fuel cell systems is the amount of component used. Low temperature fuel cell systems requires humidifiers, compressors, heat exchanger and recycle streams to be efficient; whereas high temperature fuel cells only need heaters before starting up the fuel cell. 
         [0028]    Before operating the high temperature fuel cell assembly  30 , heaters  75  can be used to heat-up the stack to 140° C., as is shown in  FIG. 7 . Any suitable type of heater can be used. In one embodiment, the heaters  75  can be surface mounted heaters. 
         [0029]    To minimize temperature spread, the coolant flow must be even across each fuel cell. Accordingly, two coolant sources  60 , such as knife blowers, can be used to create substantially even flow and can be positioned on both side of the stack to provide a counter flow strategy. The knife blowers can be used to direct air or any other type of coolant gas into the fuel cell assembly  30 . This counter flow strategy will minimize the temperature spread across the MEA. 
         [0030]      FIG. 5  shows the computational flow dynamics (CFD) thermal analysis of one cell for a heat generation of 55 W per cell and an air coolant temperature of 25° C. Various analyses on the fin thickness were made because the air knife blowers only can sustain a pressure drop of 0.3 in H 2 O.  FIG. 6  depicts the CFD analysis of the pressure drop across the fins. Flowing a total of 18 L/min of air per cell or 9 L/min on each wing resulted in a temperature difference of 8° C. and a maximum temperature of 180.7° C. across the MEA. The analysis also resulted in a pressure drop of 0.074 in H 2 O across the wings, as shown in  FIG. 6 . Analysis on extreme temperature conditions (−40° C. and 50° C.) was also performed. It was proven that by increasing or decreasing the inlet coolant air temperature, the temperature conditions and the pressure drop across the wings could be managed. Finally, insulating foam can be mounted to the stack to remain as efficient as possible and prevent any heat loss to the environment. 
         [0031]    While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Thus, it will be understood that the invention is not limited to the specific details described herein, which are given by way of example only, and that various modifications and alterations are possible within the scope of the invention.