Abstract:
A system for modulating a temperature of one or more fuel cells in a fuel cell stack includes a catalytic combustor in heat exchange relationship with the fuel cell stack. The catalytic combustor promotes an exothermic reaction. A hydrogen source selectively supplies hydrogen (H 2 ) to the catalytic combustor. The H 2  reacts with oxygen (O 2 ) in the exothermic reaction. In one feature, the catalytic combustor lies adjacent to the fuel cell stack and includes a series of catalyst coated flow channels. In another feature, the catalytic combustor includes a plate having a catalyst layer and that is offset from the fuel cell stack. Heat to radiates from the catalytic combustor to the fuel cell stack. In still another feature, a jacket encloses the fuel cell stack to form a gap between the jacket and the fuel cell stack through which hot exhaust from the catalytic combustor circulates.

Description:
FIELD OF THE INVENTION 
     The present invention relates to fuel cells, and more particularly to a device to reduce fuel cell stack start-up time and maintain fuel cell stack temperature above 0° C. 
     BACKGROUND OF THE INVENTION 
     Fuel cell systems are increasingly being used as a power source in a wide variety of applications. Fuel cell propulsion systems have also been proposed for use in vehicles as a replacement for internal combustion engines. The fuel cells generate electricity that is used to charge batteries and/or to power an electric motor. A solid-polymer-electrolyte fuel cell includes a membrane that is sandwiched between an anode and a cathode. To produce electricity through an electrochemical reaction, a fuel, commonly hydrogen (H 2 ), is supplied to the anode and an oxidant, such as oxygen (O 2 ) is supplied to the cathode. The source of the oxygen is commonly air. 
     In a first half-cell reaction, dissociation of the hydrogen (H 2 ) at the anode generates hydrogen protons (H + ) and electrons (e − ). The membrane is proton conductive and dielectric. As a result, the protons are transported through the membrane. The electrons flow through an electrical load (such as the batteries or the electric motor) that is connected across the membrane. In a second half-cell reaction, oxygen (O 2 ) at the cathode reacts with protons (H + ), and electrons (e − ) are taken up to form water (H 2 O). 
     For optimum operation, defined as high power output and quick power delivery, fuel cells need a certain operating temperature. Heat generated through the electrochemical reaction increases the operating temperature of the fuel cell. Excess heat is dissipated through a cooling system. 
     At sub-freezing temperatures (e.g. below 0° C. or 273K), however, starting the fuel cell quickly is more difficult due to frozen water in the fuel cell and the fact that the electrochemical reaction rate in the fuel cell is significantly reduced. This limits current flow and further heating of the fuel cell to the optimum operating temperature. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a system for modulating a temperature of one or more fuel cells in a fuel cell stack. The system includes a catalytic combustor in heat exchange relationship with the fuel cell stack. The catalytic combustor promotes an exothermic reaction. A hydrogen source selectively supplies hydrogen (H 2 ) to the catalytic combustor. The H 2  reacts with oxygen (O 2 ) in the exothermic reaction. 
     In one feature, the hydrogen source supplies the H 2  based on a temperature of the fuel cell stack. 
     In another feature, the system further includes a flow regulator selectively supplying the H 2  from the hydrogen source to the catalytic combustor. The flow regulator is modulated based on a pressure of the hydrogen source. A heater heats the hydrogen source to increase the pressure thereby increasing flow of the H 2  through the flow regulator. 
     In another feature, the catalytic combustor lies adjacent to the fuel cell stack and includes a series of catalyst coated flow channels through which the H 2  and the O 2  flow. 
     In still another feature, the catalytic combustor includes a plate having a catalyst layer and that is offset from the fuel cell stack. The H 2  and the O 2  flow over the catalyst layer to induce the exothermic reaction causing heat to radiate from the catalytic combustor to the fuel cell stack. 
     In yet another feature, a jacket encloses the fuel cell stack to form a gap between the jacket and the fuel cell stack. Hot exhaust from the catalytic combustor circulates through the gap to heat the fuel cell stack. 
     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 a schematic illustration of a fuel cell system including a fuel cell stack according to the principles of the present invention; 
         FIG. 2  is a schematic illustration of the fuel cell stack having an adjacent catalytic combustor according to the principles of the present invention; 
         FIG. 3  is a schematic illustration of the fuel cell stack having a diffused radiant catalytic combustor according to the principles of the present invention; and 
         FIG. 4  is a schematic illustration of the fuel cell stack in an enclosure heated by a catalytic combustor according to the principles 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. 
     Referring now to  FIG. 1 , a fuel cell system  10  is shown. The fuel cell system includes a fuel cell stack  12  that is supplied with hydrogen (H 2 ) from a hydrogen source  14 . An injector  16  facilitates supply of H 2  from the hydrogen source  14  to the fuel cell stack  12 . A compressor  18  facilitates supply of oxygen (O 2 ) containing air to the fuel cell stack  12 . H 2  is dissociated at an anode side of the fuel cell stack  12  to generate hydrogen protons (H + ) and electrons (e − ). The protons are transported through to a cathode side of the fuel cell stack  12  and the electrons flow through an electrical load (not shown). O 2  at the cathode side reacts with protons (H + ) and electrons (e − ) are taken up to form water (H 2 O). H 2 O is exhausted from the fuel cell stack  12 . 
     The reaction at the cathode side is exothermic. The heat generated by the exothermic reaction warms the fuel cell stack  12  to a desired operating temperature. The operating temperature is preferably 80° C. However, at 20° C sufficient current is immediately available from the fuel cell stack  12  to power the load. 
     Coolant is circulated through the fuel cell stack  12  to maintain the operating temperature of the fuel cell stack  12 . Initially, in the start-up mode during which the fuel cell stack  12  is warming up to a desired operating temperature, the coolant circulates the heat to uniformly warm the fuel cell stack  12 . Once the fuel cell stack  12  achieves the desired operating temperature, the coolant maintains the temperature. A pump  20  pumps coolant through the fuel cell stack  12  from a coolant source  22 . The coolant is in heat exchange relationship with the various components of the fuel cell stack  12 . The coolant exiting the fuel cell stack  12  flows through a heat exchanger  24  where heat from the fuel cell stack  12  is discharged to a heat sink, such as atmosphere. 
     A catalytic combustor  26  is associated with the fuel cell stack  12 . As explained in further detail below, exothermic reactions within the catalytic combustor  26  generate heat to warm the fuel cell stack  12 . The heat generated by the catalytic combustor  26  is used during a park mode to maintain the temperature of the fuel cell stack  12  above freezing (0° C.). The catalytic combustor  26  can also be used during the start-up mode to assist in raising the fuel cell stack temperature to the desired operating temperature. 
     The fuel cell system  10  further includes an exemplary flow regulator  28  associated with the hydrogen source  14 . The flow regulator  28  can be a pressure relief valve. As pressure within the hydrogen source  14  exceeds a threshold pressure, H 2  is exhausted through the flow regulator  28  to reduce the pressure within the hydrogen source  14 . A heater  30  is associated with the hydrogen source  14  and is operable to heat the hydrogen source  14 . Heating of the hydrogen source  14  induces an increased pressure condition therein. The exhausted H 2  is fed into the fuel cell stack  12  through a flow control device  32 . In one example, the flow control device  32  includes a venturi nozzle that concurrently draws in O 2  containing air from atmosphere. The O 2  containing air mixes with the gaseous H 2  and is fed into the fuel cell stack  12 . As discussed in further detail below, an exothermic oxidization reaction occurs within the catalytic combustor  26  to heat the fuel cell stack. 
     A controller  34  is in electrical communication with various components and sensors of the fuel cell system  10 . The controller  34  controls operation of the compressors  16 , 18  and the pump  20  to regulate operation of the fuel cell stack  12 . A temperature sensor  36  generates a temperature signal indicating the temperature of the fuel cell stack  12 . A pressure sensor  38  generates a pressure signal indicating a pressure within the hydrogen source  14 . The controller  34  communicates with the flow regulator  28  to exhaust H 2  when the pressure within the hydrogen source  14  exceeds the threshold pressure. The controller  34  regulates operation of the heater  30  to selectively induce an increased pressure condition within the hydrogen source  14 , as discussed in further detail below. 
     Referring now to  FIG. 2 , a first configuration of the catalytic combustor  26  is shown and is indicated as  26 ′. The catalytic combustor  26 ′ includes a series of flow channels  40  that are covered by a catalyst layer (not shown) and lies adjacent to the fuel cell stack  12 . The H 2  and O 2  mix from the flow control device  32  flows into the flow channels  40  where the catalyst induces the exothermic oxidization reaction. Because the catalytic combustor  26 ′ is in heat exchange relationship with the fuel cell stack  12 , heat transfer (Q) occurs, warming the fuel cell stack  12 . 
     Referring now to  FIG. 3 , a second configuration of the catalytic combustor  26  is shown and is indicated as  26 ″. The catalytic combustor  26 ″ functions as a diffused radiant heater and includes a substrate  42  that is offset by a gap  44  from the fuel cell stack  12 . A housing  46  seals the gap  44  between the substrate  42  and the fuel cell stack  12 . A face of the substrate  42  is coated with a catalyst layer  48 . Gaseous H 2  and O 2  are fed into the gap  44  through an inlet  50  and contact the catalyst layer  48 . The catalyst layer  48  induces the exothermic oxidization reaction. Heat transfer (Q) occurs across the gap  44  to warm the fuel cell stack  12 . Cooled exhaust gas is exhausted from the gap through an outlet  52 . 
     Although the illustration of  FIG. 3  includes the catalyst layer  48  on the fuel cell stack side of the substrate  42 , it is anticipated that other configurations are conceivable. For example, the catalyst layer  48  could be on the face of the substrate  42  facing away from the fuel cell stack  12 . Heat transfer to the stack would then occur through the substrate  42  and across the gap  44  to the fuel cell stack  12 . Although the heat transfer performance of such a configuration is not optimal, such a configuration is possible. Further, the illustration of  FIG. 3  includes the catalytic combustor  26 ″ positioned adjacent to one face of the fuel cell stack  12 . It is anticipated, however, that the catalytic combustor  26 ″ could be configured so as to include a substrate  42  with a catalyst layer  48  opposed to one ore more faces of the fuel cell stack  12  or even encompassing the entire fuel cell stack  12 . 
     Referring now to  FIG. 4 , a third configuration of the catalytic combustor  26  is shown and is indicated as  26 ′″. The fuel cell stack  12  is covered by an insulated covering or enclosure  56 . The insulated covering  56  is formed of a synthetic cover or wrapping. There is a gap  58  between the insulated covering  56  and the fuel cell stack  12 . It is anticipated however, that the insulated covering  56  could be defined by walls of a fuel cell stack compartment within which the fuel cell stack  12  is retained. 
     An exhaust end of the catalytic combustor  26 ′″ extends into the gap  58  through the insulated covering. An H 2  and O 2  gaseous mixture are fed into the catalytic combustor  26 ′″ through the flow control device  32 . An exothermic oxidization reaction occurs generating hot exhaust gas including residual O 2 , N 2  and H 2 O. The exhaust gas flows about the fuel cell stack  12  in the gap  58  between the fuel cell stack  12  and the insulated covering  56 , warming the fuel cell stack  12 . 
     As the exhaust gas flows through the gap  58  and heat transfer to the fuel cell stack  12  occurs, the exhaust gas is cooled and the H 2 O vapor condenses. The gap  58  is configured to enable sufficient dwell time of the exhaust gas within the gap  58  so adequate heat transfer occurs. The cooled exhaust gas and the condensed H 2 O are exhausted from the gap  58  by a vent  60  disposed through the bottom of the insulated covering  56 . 
     The catalytic combustor  26  is constantly supplied with H 2  and O 2 . In this manner, costly regulation and monitoring components and algorithms are avoided. The catalytic combustor  26  provides a steady stream of hot exhaust gases and thus heat transfer. The exhaust gas temperature, however, is limited to 100° C. (373K). This can be controlled using increased air flow provided by a fan blower (not shown). The fan blower, operates cyclically to lower its energy consumption. Local over-heating resulting from temperature spikes are avoided by sufficient gas distribution within the gap  58 . High temperature spikes are balanced as a result of the rapid and sufficient heat distribution within the gap  58  and through the high heat capacity of the fuel cell stack  12 . 
     The fuel cell system  10  is operable in three main modes: park, start-up and normal operation. Operation of the fuel cell system  10  during each of these modes will be discussed in turn. Park mode is a cool-down period generally occurring after normal operation of the fuel cell system  10 . As the fuel cell system  10  initially enters the park mode, boil off H 2  is exhausted through the flow regulator  28  and through the flow control device  32  where it is mixed with O 2 . The H 2 /O 2  mixture flows into the catalytic combustor  26  and exothermically reacts to generate heat. The heat initially maintains the temperature of the fuel cell stack  12  as the temperature of fuel cell system  10  drops to ambient. 
     As discussed above, the fuel cell stack  12  is maintained at a temperature above 0° C. (273K) to avoid freezing of residual H 2 O. As the effectiveness of the original heat wears off and the temperature of the fuel cell stack  12  drops toward 0° C., the controller  34  switches on the heater  30  to heat the hydrogen source  14 . As the hydrogen source  14  is heated, an increased pressure condition results and is detected by the pressure sensor  38 . The flow regulator  28  again exhausts H 2  to the fuel cell stack  12  to induce a subsequent exothermic reaction. In this manner, as the temperature of the fuel cell stack  12  periodically dips toward 0° C. the fuel cell system  10  initiates the exothermic reaction in the catalytic combustor  26  to avoid sub-freezing temperatures. Although the freezing temperature of water at nominal conditions is 0° C., liquid water in the stack will typically have solids dissolved therein or be subject to pressure variation, resulting in the freezing temperature of water in the stack varying from the nominal value. Thus, the invention is exemplified based on the 0° C. reference for convenience, but a range around same is contemplated. Further, the method of the invention contemplates corrective measures as the temperature of the stack declines toward 0° C., and initiation of corrective measures near and slightly above the freezing temperature of water. 
     During the start-up mode, the initial temperature of the fuel cell stack  12  is presumably lower than the desired operating temperature. Although operation of the fuel cell stack  12  increases the temperature to the desired operating temperature, the fuel cell system  10  assists the temperature increase by feeding H 2  and O 2  into the catalytic combustor  26 . As similarly described above, an exothermic reaction occurs within the catalytic combustor  26  resulting in a more rapid temperature increase. Because the catalytic combustor  26  is also in heat exchange relation with the coolant flow of the fuel cell stack  12 , the heat generated by the reaction warms the coolant. The warmed coolant evenly distributes the heat through the fuel cell stack  12  to warm the fuel cell stack  12  to the desired operating temperature. 
     Once the fuel cell stack  12  is warmed to the desired operating temperature, as sensed by the temperature sensor  36 , normal operation of the fuel cell system  10  ensues. That is to say, the flow regulator  28  is closed to inhibit H 2  flow into the catalytic combustor  26  through the flow control device  32 . The controller  34  regulates operation of the compressors  16 , 18  and pump  20  to generate current from the fuel cell stack  12  and to maintain the fuel cell stack  12  at the desired operating temperature. 
     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.