Patent Publication Number: US-2005130003-A1

Title: Cooling system for a fuel cell stack

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is a divisional of U.S. patent application Ser. No. 10/152,858 filed on May 22, 2002, the disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION  
      This invention relates generally to fuel cells and more particularly to a cooling system for a fuel cell stack that allows a liquid coolant to change phase inside the fuel cell stack in order to reduce the pumping power required to circulate the coolant and provide more uniform temperatures within each cell of a fuel cell stack.  
     BACKGROUND OF THE INVENTION  
      Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. The oxygen can be either a pure form (O 2 ) or air (a mixture of O 2  and N2). PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive, gas impermeable, solid polymer electrolyte membrane having the anode catalyst on one face and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of non-porous, electrically conductive elements or plates which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings formed therein for distributing the fuel cell&#39;s gaseous reactants over the surfaces of the respective anode and cathode catalysts.  
      The term “fuel cell” is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A plurality of individual cells are typically bundled together to form a fuel cell stack and are commonly arranged in electrical series. Each cell within the stack includes the membrane electrode assembly (MEA) described earlier, and each such MEA provides its increment of voltage. By way of example, some typical arrangements for multiple cells in a stack are shown and described in U.S. Pat. No. 5,663,113.  
      The electrically conductive plates sandwiching the MEAs may contain an array of grooves in the faces thereof that define a reactant flow field for distributing the fuel cell&#39;s gaseous reactants (i.e., hydrogen and oxygen in the form of air) over the surfaces of the respective cathode and anode. These reactant flow fields generally include a plurality of lands that define a plurality of flow channels therebetween through which the gaseous reactants flow from a supply header at one end of the flow channels to an exhaust header at the opposite end of the flow channels.  
      In a fuel cell stack, a plurality of cells are stacked together in electrical series while being separated by a gas impermeable, electrically conductive bipolar plate. In some instances, the bipolar plate is an assembly formed by securing a pair of thin metal sheets having reactant flow fields formed on their external face surfaces. Typically, an internal coolant flow field is provided between the metal plates of the bipolar plate assembly. Various examples of a bipolar plate assembly of the type used in PEM fuel cells are shown and described in commonly-owned U.S. Pat. No. 5,766,624.  
      Fuel cell stacks produce electrical energy efficiently and reliably. However, as they produce electrical energy, losses in the electrochemical reactions and electrical resistance in the components that make up the stack produce waste thermal energy (heat) that must be removed for the stack to maintain a constant optimal temperature. Typically, the cooling system associated with a fuel cell stack includes a circulation pump for circulating a single-phase liquid coolant through the fuel cell stack to a heat exchanger where the waste thermal energy (i.e., heat) is transferred to the environment. The two most common coolants used are de-ionized water and a mixture of ethylene glycol and de-ionized water. The thermal properties of these typical liquid coolants require that a relatively large volume be circulated through the system to reject sufficient waste heat in order to maintain a constant stack operating temperature, particularly under maximum power conditions. Large amounts of electrical energy are required to circulate the coolant, which reduces the overall efficiency of the fuel cell power system. To this end, it is desirable to reduce the amount of coolant needed to cool a fuel cell stack and thereby reduce the amount of pumping power required.  
     SUMMARY OF THE INVENTION  
      Accordingly, the present invention provides a cooling system for channeling coolant through a fuel cell. The cooling system includes a plate having a first side defining a reactant flow field and a second side defining a coolant flow field. The coolant flow field has inlet and outlet passages with a source of liquid coolant connected to the inlet passage. A pressure control mechanism is provided for maintaining a pressure at the outlet passage of the coolant flow field at a pressure that causes the liquid coolant to boil within the coolant flow field. Allowing the coolant to change phase to a gas inside the stack reduces the amount of coolant needed to cool the fuel cell stack. The energy needed to change a liquid to a gas is much greater than the heat carrying capacity of the liquid. As a result, the amount of coolant needed to cool a stack, and thereby the amount of coolant that must be pumped through the system is reduced as is the parasitic load on the system.  
      The present invention enables improved temperature uniformity and increased radiator/condenser efficiency.  
      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 embodiment 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  is an exploded isometric view of a PEM fuel stack;  
       FIG. 2  is a schematic view of the cooling system according to the principles of the present invention for use with the PEM fuel cell stack shown in  FIG. 1 ;  
       FIG. 3  graphically illustrates the boiling curve relationship between temperature and pressure for a water/methanol coolant mixture; and  
       FIG. 4  is a perspective view of the coolant channels within the stack. 
    
    
     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.  
      Before further describing the invention, it is useful to understand an exemplary fuel cell system within which the invention operates. Specifically,  FIG. 1  schematically depicts a PEM fuel cell stack having a pair of membrane-electrode assemblies (MEAs)  4  and  6  separated from each other by a non-porous, electrically-conductive, liquid-cooled bipolar plate assembly  8 . Each MEA  4  and  6  has a corresponding cathode face  4   a ,  6   a  and an anode face  4   b  and  6   b . MEAs  4  and  6  and bipolar plate assembly  8  are stacked together between non-porous, electrically-conductive, liquid-cooled monopolar end plate assembly  14  and  16 . Steel clamping plates  10  and  12  are provided for enclosing the exemplary fuel cell stack. Connectors (not shown) are attached to clamping plates  10  and  12  to provide positive and negative terminals for the fuel cell stack. Bipolar plate assembly  8  and end plate assemblies  14  and  16  include corresponding flow fields  20 ,  22 ,  18  and  24 , each having a plurality of flow channels formed in the faces thereof for distributing fuel and oxidant gases (i.e., H 2  and O 2 ) to the reactive faces of MEAs  4  and  6 . Nonconductive gaskets or seals  26 ,  28 ,  30 , and  32  provide a seal and electrical insulation between the several plates of the fuel cell stack.  
      With continued reference to  FIG. 1 , porous, gas permeable, electrically conductive sheets  34 ,  36 ,  38  and  40  are shown to be pressed up against the electrode faces of MEAs  4  and  6  and serve as primary current collectors for the electrodes. Primary current collectors  34 ,  36 ,  38  and  40  also provide mechanical supports for MEAs  4  and  6 , especially at locations where the MEAs are otherwise unsupported in the flow fields.  
      End plates  14  and  16  press up against primary current collector  34  on cathode face  4   a  of MEA  4  and primary current collector  40  on anode face  6   b  of MEA  6  while bipolar plate assembly  8  presses up against primary current collector  36  on anode face  4   b  of MEA  4  and against primary current collector  38  on cathode face  6   a  of MEA  6 . An oxidant gas, such as oxygen or air, is supplied to the cathode side of the fuel cell stack from a storage tank  46  via appropriate supply plumbing  42 . Similarly, a fuel, such as hydrogen, is supplied to the anode side of the fuel cell from a storage tank  48  via appropriate supply plumbing  44 . In a preferred embodiment, oxygen tank  46  may be eliminated, such that ambient air is supplied to the cathode side from the environment. Likewise, hydrogen tank  48  may be eliminated and hydrogen supplied to the anode side from a reformer which catalytically generates hydrogen from methanol or a liquid hydrocarbon (e.g., gasoline). While not shown, exhaust plumbing for both the H 2  and O 2 /air sides of MEAs  4  and  6  is also provided for removing H 2 —depleted anode gas from the anode reactant flow field and O 2 —depleted cathode gas from the cathode reactant flow field.  
      Coolant supply plumbing  50 ,  52 , and  54  is provided for supplying a liquid coolant from an inlet header (not shown) of the fuel cell stack to the coolant flow fields of bipolar plate assembly  8  and end plates  14  and  16 . The coolant flow fields of the bipolar plate assembly  8  and end plates  14  and  16  include long narrow channels  56  defining coolant passages within the plates  8 ,  14 , and  16 . As shown in  FIG. 1 , coolant exhaust plumbing  58 ,  60 , and  62  is provided for exhausting the heated coolant discharged from bipolar plate assembly  8  and end plates  14  and  16  of the fuel cell stack.  
       FIG. 2  is a schematic diagram of a phase-change cooling system according to the principles of the present invention. As shown in  FIG. 2 , a fuel cell stack  70 , such as the one shown in  FIG. 1 , is provided. A cooling system  72  includes a pump  74  which provides liquid coolant to the fuel cell stack  70  through coolant passage  76 . A pressure control valve  78  is provided at the exhaust end of the fuel cell stack  70  and a radiator/condenser  80  is provided downstream of the pressure control valve  78  for cooling the coolant (in liquid and vapor mixture form) and condensing it back to a liquid form for return to the pump  74 . An accumulator  86  is provided upstream of the pump  74  to remove bubbles from the coolant fluid before the pump  74 . The accumulator  86  may optionally be provided with a dehydrator functionality which would remove water if the chosen fluid reacts adversely to water (i.e., the needed properties change when mixed with water). A controller  82  is provided for controlling the pressure control valve  78  in response to a temperature of the stack  70  as determined by a temperature sensor  84 . The pressure is controlled such that for the measured temperature level, a pressure is maintained that causes the coolant to boil within the stack. The control  82  may include a processor (CPU) or dedicated circuitry for carrying out this function.  
      Liquid coolant exits the coolant pump  74  at an elevated pressure and enters the fuel cell stack  70 . While in the stack, a fraction of the coolant boils at a temperature determined by the pressure of the coolant. As best illustrated in  FIG. 4 , the coolant channels  90  within the stack are designed to accommodate the expansion of some of the liquid to a gaseous state minimizing the increased pressure drop brought on by the increase in volumetric flow of two phase fluid. The coolant channels  90  include a narrow liquid inlet  92  and a series of alternating direction serpentine channel segments  94   a - e  that progressively widen from the inlet  92  to the outlet  96 . The widening channel design ensures somewhat uniform coolant distribution which in turn avoids regions where the coolant boils away completely causing hot spots. The coolant emerges from the stack as a two-phase mixture of liquid and vapor coolant. The mixture enters the pressure control valve  78  that is used to control the system pressure. Next, the coolant enters the radiator/condenser  80  where the coolant changes back to a liquid. Upon exiting the radiator/condenser, the coolant returns to the coolant pump  74 .  
      The present invention reduces the amount of coolant needed to cool a fuel cell stack by allowing a liquid coolant to change phase to a gas inside the stack. The energy needed to change a liquid to a gas is much greater than the heat carrying capacity of the liquid. As a result, the amount of coolant needed to cool a stack, and thereby the amount of coolant that must be pumped through the system, is reduced. Thus, the parasitic load on the system that is typically required to pump the large amounts of coolant is also reduced. Test results have shown that a mixture of 40 percent of methanol in water which was allowed to change phase in a fuel cell stack resulted in a pumping power reduction from 1,000 watts to 200 watts in an 85 kilowatt fuel cell power system. In other words, one-fifth of the pumping power was required when the pressure of the coolant was regulated such that some of the coolant was allowed to change phase within the fuel cell stack according to the principles of the present invention.  
      Using a cooling system where the coolant boils has benefits in addition to reduced system parasitic losses. These benefits include improved temperature control reduced, improved temperature uniformity, and increased radiator/condenser efficiency. When a liquid changes to a gas; i.e., boils, it does so at a single temperature. The local pressure of the liquid determines the temperature at which a liquid boils. Therefore, controlling the pressure control valve to maintain a predetermined pressure drop in a stack, specifies the temperature gradient, and changing the pressure of the cooling loop changes the stack temperature. Increasing the pressure will increase the stack temperature while lowering the pressure reduces the stack temperature, all the while the temperature gradient across the stack remains the same. The relationship between pressure and boiling temperature for a water-methanol mixture is shown in  FIG. 3 . It should be understood that other liquids with similar boiling characteristics could be used.  
      Using a coolant that changes phase has the added benefit that it increases the efficiency of the radiator/condenser used to reject the waste thermal-energy to the environment. The increase in efficiency is due to the constant temperature relationship of condensing fluids. The constant temperature in the radiator/condenser means the temperature difference between the coolant and the air used to remove the heat is maintained instead of reduced, as it is in single phase heat transfer. The temperature difference between the coolant and the air is one of the major factors determining radiator/condenser efficiency.  
      The description of the invention is merely exemplary in nature and, thus, 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.