Abstract:
A fuel cell system may include a cathode loop having an operating pressure during fuel cell system operation. The cathode loop may include a normally open mechanical check valve disposed at a water pooling location within the loop and having a cracking pressure approximately equal to the operating pressure.

Description:
BACKGROUND 
     Fuel cell systems are increasingly being used as power sources in a wide variety of applications. Fuel cell systems, for example, may be used as replacements for vehicle internal combustion engines (ICE). 
     A proton exchange membrane (PEM) fuel cell includes a membrane electrode assembly (MEA) that is sandwiched between conductive anode and cathode plates. This membrane functions as a proton conductive electrolyte membrane in a water containing state. In a dry state, however, its proton conductivity decreases, thus causing a decrease in power output. Therefore, a fuel cell system equipped with this type of fuel cell is often designed to humidify reaction gases (anode gas and cathode gas) supplied to an anode and cathode of the fuel cell by a humidifier so that the membrane can maintain proper humidity. 
     To produce electricity through an electrochemical reaction, hydrogen (H 2 ) is supplied to the anode and oxygen (O 2 ) is supplied to the cathode (via 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 while the electrons flow through an electrical load 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. 
     Because fuel cells have become viable for widespread commercial use, cold weather performance characteristics of fuel cells have become more important. The ambient temperature tolerance specification for certain vehicles, for example, typically includes temperatures between −40° C. to 52° C. Liquid and vapor water within the fuel cell system, however, may present issues for cold weather operation of the fuel cell. The fuel cell stack humidification systems and water generation at the cathode during operation generally ensure that water in a liquid or vapor state will exist in almost all parts of the fuel cell stack during dwell times. At one atmosphere and temperatures below 0° C., water freezes and may block the flow passages of the fuel cell stack and the fuel cell system balance of plant. These blockages may hinder fuel cell system operation. 
     SUMMARY 
     A fuel cell system may include a cathode loop and a mechanical check valve. The check valve may be configured to close when a pressure within the loop is greater than a predetermined threshold and to open when the pressure is less than the threshold. The check valve may be disposed within the loop such that water within the loop and in a vicinity of the check valve drains from the loop if the check valve is open. 
     A vehicle may include a fuel cell system configured to provide motive power for the vehicle. The fuel cell system may include a cathode loop being arranged such that a portion of the loop forms a water pooling location where water normally pools during stack operation or after stack shut-down, and a normally open mechanical check valve disposed within a vicinity of the water pooling location. 
     A vehicle may include a fuel cell system configured to provide motive power for the vehicle. The fuel cell system may include a cathode loop having an operating pressure during fuel cell system operation. The cathode loop may include a normally open mechanical check valve disposed at a water pooling location within the loop and having a cracking pressure approximately equal to the operating pressure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an example fuel cell system for an automotive vehicle. 
         FIG. 2  is a schematic diagram of a portion of the fuel cell system of  FIG. 1 . 
         FIG. 3  is a schematic diagram of another portion of the fuel cell system of  FIG. 1 . 
         FIG. 4  is a schematic diagram of yet another portion of the fuel cell system of  FIG. 1 . 
         FIG. 5  is a schematic diagram of an embodiment of a cathode subsystem. 
     
    
    
     Like numbered elements of the Figures may have similar, although not necessarily identical, descriptions. As an example, elements  24 ,  124  may share similar descriptions. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , in an example PEM based fuel cell system  10 , an anode subsystem  11  may provide the necessary hydrogen fuel at correct pressure, flow and humidity to a fuel cell stack  12 . Likewise, a cathode subsystem (loop)  13  may provide the necessary oxygen (air) at correct pressure, flow and humidity to the stack  12 . 
     As known in the art, electrical energy may be generated by the fuel cell stack  12  as the hydrogen and oxygen react. This electrical energy may be used to power various electrical devices and/or stored within an energy storage unit (not shown). 
     The fuel cell stack  12  of  FIG. 1 , for example, is configured to provide motive power for a vehicle  14 . That is, the stack  12  is electrically connected in a known fashion with an electric machine (not shown) that converts electrical energy generated by the stack  12  to mechanical energy used to move the vehicle  14 . Alternatively, the stack  12  may be electrically connected with a battery (not shown) to store electrical energy generated by the stack  12 . An electric machine, in this alternative arrangement, may be configured in a known fashion to draw electrical energy from the battery to produce mechanical energy used to move the vehicle  14 . Other arrangements are, of course, also possible. 
     Fuel supply from a hydrogen storage tank system  15  is enabled through a system isolation valve  16 . The supply pressure to the fuel cell stack  12  is regulated by a pressure control device  17 . The pressure control device  17  takes input from a pressure sensor  18  at the inlet of the fuel cell stack&#39;s anode  20  to regulate the hydrogen fuel pressure to the stack  12 . 
     An air compressor  22  increases the ambient pressure of air filtered by air filter  23  based on input from an air pressure sensor  24  at the inlet of the fuel cell stack&#39;s cathode  26 . 
     Controls are established in such a way that the pressure on either side of the fuel cell membrane (not shown) is maintained within a certain tolerance, for example around 600 mbar. The tolerance may vary depending upon the fuel cell stack design. Any overpressure or under pressure may result in system shut down to protect the fuel cell stack membrane from malfunction. 
     For effective power generation, the PEM type fuel cell stack  12  may require humidified gases. Anode gas humidity may be maintained by re-circulating the anode gas mixture from the fuel cell stack&#39;s outlet using a blower  28  to mix feed gas from the hydrogen storage tank system  15  with the re-circulated hydrogen. Cathode gas (air) humidity is maintained by passing air through a humidifier  32 . The humidifier  32  may be by-passed via valve  34 . 
     At the anode side of the fuel cell stack&#39;s outlet, a water knock-out  36 , purge/drain filter  38 , and purge/drain valve  40  are provided to remove water from the anode outlet. This removed water is passed to an exhaust system  42  of the vehicle  14 . At the cathode side of the fuel cell stack&#39;s outlet, a back pressure throttle valve  44  fluidly connects the humidifier  32  and the exhaust system  42 . 
     The humidified gases along with the generated water (which is a by product of the chemical process during power generation), may present issues during fuel cell system start at or below freezing temperatures. The water from the humidified gases, for example, may condense and pool in low spots (due to gravity) within the cathode subsystem  13  during normal operation and/or during soak—the period between system shutdown and restart. These pools may freeze if ambient temperatures are at or below freezing. 
     Referring to  FIGS. 2 and 3 , water has condensed and pooled, for example, in low spots of the inlet and outlet tubing fluidly connecting the fuel cell stack  12  and humidifier  32 . Referring to  FIG. 4 , water has condensed and pooled in a low spot of the tubing fluidly connecting the humidifier  32  and back pressure throttle valve  44 . These low spots result from the manner in which the tubing connecting these components is routed within the vehicle  14  ( FIG. 1 ). Of course, low spots may occur elsewhere depending on fuel cell system design and layout, vehicle inclination, etc. As an example, the humidifier  32  may be a low spot if it is positioned lower than other components of the cathode subsystem  13  ( FIG. 1 ) in the vicinity. As another example, a reservoir (not shown) that is associated with (or is a part of) the cathode subsystem  13  may be a low spot if it is positioned lower than other components of the cathode subsystem  13  in the vicinity. 
     Referring to  FIG. 5 , a mechanical check valve  146  is disposed within a low spot of the tubing fluidly connecting the outlet of the fuel cell stack&#39;s cathode  126  and the humidifier  132 . As known in the art, a mechanical check valve (e.g., non-return valve, one-way valve, etc.) normally allows fluid to flow through it in only one direction. The check valve  146  is configured, in a known fashion, to be normally open. That is, when there is no pressure or pressures below the cracking pressure within the cathode subsystem  113 , the check valve  146  is open to allow water pooled in its vicinity to exit the cathode subsystem  113 . When the cathode subsystem  113  is pressurized for operation, the check valve  146  is closed so as to prevent air from escaping the cathode subsystem  113 . That is, the cracking pressure for the check valve  146  may be set at, for example, a pressure that is less than or equal to the pressure within the cathode subsystem  113  during operation. The cracking pressure may also account for any head resulting from water pooled above the check valve  146 . 
     In the embodiment of  FIG. 5 , the check valve  146  is positioned within a bottom portion of the tubing (as opposed to the top portion for example) so as to permit maximum water drain via gravity. The check valve  146 , however, may be positioned in any suitable location within the tubing, etc. 
     In other embodiments, one or more mechanical check valves  146  may be disposed, for example, in the tubing fluidly connecting the humidifier  132  and inlet of the fuel cell stack&#39;s cathode  126  (as indicated by phantom line), in the tubing fluidly connecting the humidifier  132  and back pressure throttle valve  144  (as indicated by phantom line), within the fuel cell stack&#39;s cathode  126 , within the humidifier  132 , etc. depending on where low spots occur in the cathode subsystem  113 . Such low spots may be identified via testing, simulation, etc. Low spots may also located by design. 
     Any suitable/known check valve type may be used. For example, a ball check valve, diaphragm check valve, swing check valve, clapper valve, stop-check valve, lift-check valve, etc. may be used. 
     While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. The words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.