Abstract:
A fuel cell power plant ( 10 ) includes an oxidant stream controlled to enter a fuel cell ( 12 ) of the plant at a pressure of between about 0.058 pounds per square inch gas (‘psig’) and about 4.4 psig and the oxidant stream passes through the fuel cell ( 12 ) at an oxidant stoichiometry of between about 120% and about 180%, and preferably between about 150% and 170%. A macro-pore cathode gas diffusion layer ( 36 ) is secured between a cathode catalyst ( 16 ) and a cathode flow field ( 28 ). A porous coolant plate ( 44 ) is secured in fluid communication with and adjacent the cathode flow field ( 28 ). The gas diffusion layer ( 36 ) and coolant plate ( 44 ) facilitate removal of product water to eliminate flooding and to permit operation at low oxidant stoichiometry and high water balance temperature, thereby minimizing need for water capture and heat rejection apparatus.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to fuel cell power plants that are suited for usage in transportation vehicles, portable power plants, or as stationary power plants, and the disclosure especially relates to a fuel cell power plant that operates efficiently at low oxidant stoichiometries and low pressure drop, and that thereby minimizes need for water recovery devices, heat rejection apparatus and complex pressure control valves. 
       BACKGROUND ART 
       [0002]    Fuel cells are well known and are commonly used to produce electrical current from hydrogen containing reducing fluid fuel and oxygen containing oxidant reactant streams to power electrical apparatus such as transportation vehicles. As is well known in the art, a plurality of fuel cells are typically stacked together to form a fuel cell stack assembly which is combined with controllers, thermal management systems, and other components to form a fuel cell power plant. 
         [0003]    In fuel cells of the prior art considerable effort is directed to operating a fuel cell in water balance. Operating in water balance essentially means that product water generated by the fuel cell is adequate to maintain sufficient water content of an electrolyte of a fuel cell, such as a “proton exchange membrane” (“PEM”) electrolyte, and is adequate to properly humidify reactant streams. If the fuel cell operates in water balance, no additional water has to be added to efficiently support the fuel cell. As is well known, fuel cell product water may accumulate within a reactant flow field adjacent a cathode electrode of the fuel cell. Typically, the oxidant stream passing through the reactant flow field will remove most of such product water as water vapor or entrained droplets. However, if a rate of removal of such water is inadequate, accumulated water will restrict flow of the oxidant stream effectively flooding a portion of the fuel cell causing decreased performance of the cell. Additionally, heat generated during operation of the fuel cell increases a temperature of the oxidant stream, thereby increasing the amount of water the oxidant stream may remove as the stream moves through the fuel cell. 
         [0004]    Known efforts to efficiently operate a fuel cell have typically included a high flow rate or high pressure drop of the oxidant stream passing through tortuous or serpentine flow channels adjacent solid flow field plates to remove adequate fuel cell product water to avoid flooding of the flow channels. It is also known to permit the oxidant stream to steadily increase in temperature as the oxidant stream moves through the fuel cell, such as by co-flowing a coolant stream adjacent the oxidant stream. This results in the heated oxidant stream removing increasing amounts of water vapor as the stream moves through the fuel cell. While such operating approaches produce enhanced fuel cell electrical current production, the high oxidant stream flow rate and high temperature of the stream typically result in excess water moving out of the cell, thereby forcing the cell out of water balance. 
         [0005]    An oxidant exhaust stream exiting such a fuel cell is hot and burdened with water, and is typically processed through water capture apparatus, such as a condenser or an enthalpy recovery device, to return water to the fuel cell. Additionally, such fuel cells will also require a relatively large heat rejection device, such as a radiator, to cool down either or both of the oxidant exhaust stream and a circulating coolant stream. Such heat rejection devices are relatively large because fuel cells operate at relatively low temperatures (for example, relative to internal combustion engines). These fuel cells also require complex and costly oxidant compressors or pumps and related pressure valve control apparatus to maintain high pressure and flow rates of reactant streams passing through the fuel cells. 
         [0006]    An example of such a fuel cell is disclosed in U.S. Pat. No. 5,879,826 that issued to Lehman et al. on Mar. 9, 1999. Lehman et al. disclose that efficient operation of their fuel cell requires an air stoichiometry of between 200-300% and specifically states that fuel cell performance falls off significantly at stoichiometries below 200% because the rate of air flow through the fuel cell is insufficient to remove product water, thereby resulting in flooding of the fuel cell. (For purposes herein, the phrase “stoichiometry of ______% (such as 200%) is to mean the stated percentage of a required amount of a compound, wherein the “required amount of the compound” results in a perfectly efficient reaction that consumes all reactants through the reaction. For example, an oxidant stream stoichiometry of 200% is to mean that twice as much oxygen, or 100% more oxygen, is directed through the fuel cell than is needed to react with perfect efficiency with the hydrogen reactant to produce water at a given current. An oxidant stoichiometry of 200% results in one-half of the oxygen not being utilized within the fuel cell.) To maintain an oxidant stream stoichiometry between 200-300%, Lehman et al. must cool down or somehow recapture the water leaving the fuel cell within all of the excess air. This results in use of costly and complex apparatus necessary to maintain the fuel cell in water balance. 
       SUMMARY 
       [0007]    The disclosure includes a fuel cell power plant for generating electrical current from oxidant and hydrogen rich reactant streams, wherein an oxidant stream enters a fuel cell of the plant at a pressure of between about 0.058 pounds per square inch gas (“psig”) and about 4.4 psig and the oxidant stream passes through the fuel cell at an oxidant stoichiometry of between about 120% and about 180%, and preferably between about 150% and about 170%. (For purposes herein, the word “about” is to mean plus or minus 20%.) 
         [0008]    The power plant includes, at least one fuel cell having an anode catalyst and a cathode catalyst secured to opposed sides of an electrolyte. An anode flow field is defined in fluid communication with the anode catalyst and with a source of the hydrogen rich reactant for directing flow of the hydrogen rich reactant from an anode flow field inlet, adjacent the anode catalyst and out of the anode flow field through an anode flow field exit. A cathode flow field is also defined in fluid communication with the cathode catalyst and with a source of the oxidant for directing flow of the oxidant from a cathode flow field inlet, adjacent the cathode catalyst and out of the cathode flow field through a cathode flow field exit. A macro-pore cathode gas diffusion layer is secured adjacent the cathode catalyst and between the cathode catalyst and the cathode flow field. 
         [0009]    The power plant also includes an oxidant pump that is secured to an oxidant inlet line in fluid communication with the oxidant source and with the cathode flow field inlet for selectively varying a flow rate of the oxidant stream into and through the cathode flow field. A thermal management system controls a temperature of the fuel cell and includes a porous coolant plate secured in fluid communication with and adjacent the cathode flow field and the plate is configured to direct a coolant fluid from a coolant plate inlet, through the plate and out of the plate through a coolant plate exit. The coolant plate is also secured in fluid communication with a coolant loop for directing the coolant fluid from the coolant plate exit through the coolant loop, through a coolant pump for circulating the coolant fluid through the coolant loop and plate, through a heat exchanger secured in heat exchange relationship with the coolant loop, through a pressure regulating valve for regulating a pressure of the coolant fluid within the porous coolant plate, and back into the coolant plate inlet. 
         [0010]    A primary load is secured in electrical communication through a load circuit and primary load switch with the anode and cathode catalysts for selectively receiving and utilizing electrical current generated by the fuel cell. 
         [0011]    The disclosure includes the fuel cell, oxidant pump, and thermal management system configured so that whenever the primary load is receiving electrical current from the fuel cell the oxidant is delivered to the cathode flow field inlet at a pressure of between about 0.58 psig and about 4.4 psig, and, so that the oxidant stream passes through the fuel cell at a stoichiometry of between about 120% and about 180%, and preferably between about 150% and about 170%. Additionally, the power plant may be configured so that a temperature of the oxidant stream adjacent the cathode flow field exit is less than a temperature of the coolant fluid adjacent the coolant plate exit, and so that a temperature of the oxidant stream, adjacent the cathode flow field exit is no more than five degrees Celsius (“° C.”) greater than a temperature of the coolant fluid adjacent the coolant plate inlet. 
         [0012]    The porous coolant plate provides a pathway for fuel cell product water to leave the cathode flow field directly into the coolant fluid within the coolant plate instead of into the oxidant stream, thereby facilitating use of such a low oxidant stoichiometry. Additionally, the macro-pore cathode gas diffusion layer produces rapid transport of fuel cell product water away from the cathode catalyst compared to micro pore or micro-pore/macro-pore bi-layers. The macro-pore cathode gas diffusion layer defines pores having an average diameter of between about 15 micrometers to about 40 micrometers. By so efficiently removing product water from the cathode catalyst, the present disclosure provides for an extraordinarily low oxidant stoichiometry, which is also referred to as a very high air or oxygen utilization. (Air or oxygen utilization is the inverse of oxidant stoichiometry.) By providing for a low oxidant stoichiometry and therefore a very low flow rate of the oxidant stream passing through the cathode flow field, a minimal amount of water is removed from the flow field into the oxidant stream. This helps maintain the fuel cell in water balance. This also provides for a very high water balance temperature. A water balance temperature means an air or oxidant exhaust temperature which cannot be exceeded if the fuel cell is to remain in water balance. The present fuel cell power plant, therefore, minimizes requirements for oxidant stream compressors and pumps and related pressure control valves, water recapture apparatus, and/or heat rejection devices, thereby dramatically improving operating efficiencies of the fuel cell power plant. 
         [0013]    In a preferred embodiment of the fuel cell power plant, the oxidant stoichiometry is between about 120% and 150%. In a further embodiment, the cathode flow field defines a cathode exit that is adjacent the coolant inlet. This results in a large amount of water condensation in the cathode flow field. However, this is not a problem for the present fuel cell power plant which has porous coolant plates that can remove the condensed liquid water. 
         [0014]    Accordingly, it is a general purpose of the present disclosure to provide a fuel cell power plant having improved operating efficiencies that overcomes deficiencies of the prior art. 
         [0015]    It is a more specific purpose to provide a fuel cell power plant having improved operating efficiencies that minimizes requirements for oxidant pumps, pressure control valves, water recovery apparatus, heat rejection devices, and related components. 
         [0016]    These and other purposes and advantages of the present fuel cell power plant having improved operating efficiencies will become more readily apparent when the following description is read in conjunction with the accompanying drawing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a simplified schematic representation of a fuel cell power plant having improved operating efficiencies constructed in accordance with the present disclosure. 
           [0018]      FIG. 2  is a simplified, schematic representation of a two-pass cathode flow field showing a flow path of an oxidant stream and a coolant fluid. 
           [0019]      FIG. 3  is a graph showing air utilization (the inverse of stoichiometry) of the fuel cell power plant of the present disclosure compared to a prior art fuel cell power plant. 
           [0020]      FIG. 4  is a graph showing maximum water balance temperature and oxidant stoichiometry of the fuel cell power plant of the present disclosure compared to prior art fuel cell power plants. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    Referring to the drawings in detail, a fuel cell power plant having improved operating efficiencies is shown in  FIG. 1 , and is generally designated by the reference numeral  10 . The power plant includes at least one fuel cell  12  having an anode catalyst  14  and a cathode catalyst  16  secured to opposed sides of an electrolyte  18 , such as a proton exchange membrane electrolyte  18 . An anode flow field  20  is defined in fluid communication with the anode catalyst  14  and with a source  22  of the hydrogen rich reactant for directing flow of the hydrogen rich reactant from an anode flow field inlet  24 , adjacent the anode catalyst  14  and out of the anode flow field  20  through an anode flow field exit  26 . A cathode flow field  28  is also defined in fluid communication with the cathode catalyst  16  and with an oxidant source  30  for directing flow of the oxidant from a cathode flow field inlet  32 , adjacent the cathode catalyst  16  and out of the cathode flow field  28  through a cathode flow field exit  34 . A macro-pore cathode gas diffusion layer  36  is secured adjacent the cathode catalyst  16  and between the cathode catalyst  16  and the cathode flow field  28 . A macro-pore anode gas diffusion layer  38  may also be secured between the anode catalyst  14  and the anode flow field  20 . The cathode and anode macro-pore gas diffusion layers  36 ,  38  include a average pore diameter of between about 10 micrometers and about 40 micrometers, a contact angle of greater than 0 degrees and less than about 80 degrees, and a thickness of between about 50 micrometers and about 200 micrometers. 
         [0022]    The power plant  10  also includes an oxidant pump  40  that is secured to the oxidant inlet line  42  in fluid communication with the oxidant source  30  and with the cathode flow field inlet  32  for selectively varying a flow rate of the oxidant stream into and through the cathode flow field  28 . The “oxidant pump  40 ” may be any apparatus capable of directing flow of the oxidant reactant stream into the fuel cell  12  at the pressures described herein, including for example a compressed oxidant tank with a pressure regulator (not shown), a blower (not shown), a compressor (not shown), the pump  40 , etc. 
         [0023]    A thermal management system  42  controls a temperature of the fuel cell  12  and includes a porous coolant plate  44  secured in fluid communication with and adjacent the cathode flow field  28  and the plate  44  is configured to direct a coolant fluid  46  from a coolant plate inlet  48 , through the plate  44  and out of the plate  44  through a coolant plate exit  50 . The coolant plate  44  is also secured in fluid communication with a coolant loop  52  for directing the coolant fluid from the coolant plate exit  50  through the coolant loop  52 , through a coolant pump  54  for circulating the coolant fluid through the coolant loop  52  and plate  44 , through a heat exchanger  56  secured in heat exchange relationship with the coolant loop  52 , through a pressure regulating valve  58  for regulating a pressure of the coolant fluid within the porous coolant plate  44 , and back into the coolant plate inlet  48 . The thermal management system  42  may also include an accumulator  59  secured in fluid communication through an accumulator feed line  60  with the coolant loop  52  for storing excess coolant fluid  46 . 
         [0024]    A primary load  61  is secured in electrical communication through a load circuit  62  and primary load switch  64  with the anode catalyst  14  and cathode catalysts  16  for selectively receiving and utilizing electrical current generated by the fuel cell  12 . 
         [0025]      FIG. 2  shows a schematic representation of a two-pass cathode flow field  66  and an adjacent porous coolant plate  68  (shown in hatched lines). The two-pass cathode flow field  66  includes a first pass  70  that directs the oxidant stream from a cathode flow field inlet  72  along the first pass  70  to a turn-around header  74 . The two-pass cathode flow field  66  also includes a second pass  76  that directs the oxidant stream from the turn-around header  74  in a direction opposed to the first pass  70  and out of the flow field  66  through a cathode exit  78 . The first pass  70  and second pass  76  may be separated within the two-pass cathode flow field  66  by a pass separator  80 , and the flow of an oxidant stream through the two-pass cathode flow field  66  is represented by oxidant flow directional arrow  82 . 
         [0026]    The  FIG. 2  porous coolant plate  68  is secured adjacent and in fluid communication with the two-pass cathode flow field  66 , such as by pores defined within the plate  68 . The plate also includes a coolant flow pathway  84  for directing flow of the coolant fluid  46  through the coolant plate  68  from a coolant inlet  85  in a direction perpendicular to the flow direction  82  of the oxidant stream flowing through the two-pass cathode flow field  66 , as represented by coolant flow directional arrow  86 . As is apparent from  FIG. 2 , in a preferred embodiment, the cathode exit  78  is adjacent or over the coolant inlet  85 . (The  FIG. 1  porous coolant plate  44  and the  FIG. 2  porous coolant plate  68  are structured and operate in a manner similar to a “water transport plate” disclosed in U.S. Pat. No. 6,911,275 that issued on Jun. 28, 2005 to Michels et al., which patent is owned by the assignee of all rights in the present disclosure.) The coolant fluid  46  enters the porous coolant plate  68  through a coolant plate inlet  85  adjacent the cathode exit  78  and leaves the coolant plate  68  through a coolant plate exit  87 . 
         [0027]      FIG. 3  shows an air utilization (the inverse of oxidant stoichiometry) graph that plots at plot line  88  data showing a rapid decline in cell voltage from about 0.658 volts at 52% air utilization to 0.568 volts about at 78% air utilization. Plot line  88  represents performance of a prior art fuel cell (not shown) having a different cathode macro-pore gas diffusion layer that requires a micro-pore layer. This micro-pore layer retards oxygen transport to the cathode catalyst resulting in diminished fuel cell performance. In contrast, plot line  90  shows dramatically improved performance of a fuel cell  12  constructed in accordance with the present disclosure. In particular, plot line  90  shows that at an air utilization of about 60% cell voltage is about 0.665, and cell voltage only drops off to about 0.622 at an air utilization rate as high as about 90%, which corresponds to an oxidant stoichiometry of about 110%. 
         [0028]    Further data is shown in  FIG. 4  comparing at plot line  92  and  94  performance of a fuel cell  12  constructed in accordance with the present invention. It is noted that the solid plot line  92  represents data associated with the left vertical axis of the graph, namely water balance temperature in degree Celsius, while the hatched plot line  94  represents data associated with the right vertical axis of the graph, namely oxidant stoichiometry. The solid plot line  96  and corresponding hatched plot line  98  represent data resulting from tests of a first prior art fuel cell (not shown). The solid plot line  100  and corresponding hatched plot line  102  represent data resulting from tests of a second prior art fuel cell (not shown). Results from tests of the fuel cell power plant  10  of the present disclosure shown in  FIG. 4  at plot lines  92  and  94  demonstrate that a maximum water balance temperature at 0.6 volts could be maintained above 70° C. as power density increased from 0.0 watts per square centimeter (W/cm 2 ) to 0.6 W/cm 2 , and as oxidant stoichiometry remained below about 120%. As power density was increased to 0.8 W/cm 2  the water balance temperature decreased only to about 68° C. while the oxidant stoichiometry increased only to about 130%. Increasing the power density to about 0.86 W/cm 2  the water balance temperature declined to only about 61° C. while the oxidant stoichiometry increased to only about 175%. In contrast, plot lines  96 ,  98 , and  100 ,  102 , for the two separate prior art fuel cells show dramatically reduced performance. The prior art fuel cells (not shown) included a macro-pore gas diffusion layer that required use of a micro-pore layer (not shown) adjacent to the cathode. 
         [0029]    The present disclosure also includes a method of operating the fuel cell power plant  12  for generating electrical current from oxidant and hydrogen rich reactant streams. The method includes the steps of directing flow of the hydrogen rich reactant stream from the hydrogen source  22  through the anode flow anode flow field cell  20  defined adjacent the anode catalyst  14  of the fuel cell  12  and out of the anode flow field  20  through an anode flow field exit  26 ; directing flow of the oxidant reactant stream from an oxidant source  30  through a cathode flow  28  field defined adjacent the cathode catalyst  16  of the fuel cell  12  and out of the cathode flow field  28  through a cathode flow field exit  34 , wherein the oxidant reactant stream enters the cathode flow field  28  at a pressure of between about 0.58 psig and about 4.4 psig, and wherein the flow of the oxidant reactant stream through the cathode flow field  28  is directed at a stoichiometry of between about 120% and about 180%, and preferably between about 150% and about 170%. 
         [0030]    The method also includes the steps of directing flow of a coolant fluid  46  through a coolant plate inlet  48  of a porous coolant plate  44 , through the plate  44  and directing flow of the coolant fluid out of the plate  44  through a coolant plate exit  50 , the porous coolant plate being secured in fluid communication with the cathode flow field  28  for removing heat from the fuel cell  12  and for removing water generated at the cathode catalyst  16  into the porous coolant plate  44 . The method may also include the steps of controlling the flow of coolant fluid through the porous coolant plate  44  and removal of water from the cathode flow field  28  through the porous coolant plate  44  so that a temperature of the oxidant stream adjacent the cathode flow field exit  50  is less than a temperature of the coolant fluid adjacent the coolant plate exit  50 , and so that a temperature of the oxidant stream adjacent the cathode flow field exit  34  is no more than 5 degrees Celsius greater than a temperature of the coolant fluid adjacent the coolant plate inlet  48 . The method also includes the step of directing electrical current generated by the fuel cell  12  through a load circuit  62  to a primary load  61 . The method may also include the steps of directing flow of the oxidant stream through a two-pass cathode flow field  66 , and securing a macro-pore cathode gas diffusion layer  36  between the cathode flow field  28  and the cathode catalyst  16  and directing the oxidant stream to flow adjacent the macro-pore cathode gas diffusion layer  36 . 
         [0031]    While the present disclosure has been presented with respect to the described and illustrated fuel cell power plant  10  with improved operating efficiencies, it is to be understood that the disclosure is not to be limited to those alternatives and described embodiments. Accordingly, reference should be made primarily to the following claims rather than the forgoing description to determine the scope of the disclosure.