Abstract:
A system and method for correcting an estimation of nitrogen in an anode side of a fuel cell stack. The system includes a fuel cell stack and a pressure sensor for measuring pressure in an anode sub-system. The system also includes a controller configured to control the estimation of nitrogen permeation from the cathode side to the anode side of the stack, where the controller determines if the pressure in the anode sub-system equilibrates with atmospheric pressure in a shorter period of time after shutdown compared to the time necessary for the anode sub-system to reach approximately atmospheric pressure after a previous shutdown or calibrated time value, and corrects the estimation of nitrogen in the anode side of the stack if the pressure equilibrates in a shorter period of time.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to a system and method for determining a change in the permeation rate of a membrane in a fuel cell and, more particularly, to a system and method for determining a change in the permeation rate of the membranes in a fuel cell stack and correcting an estimation of nitrogen in an anode sub-system based on the change in the permeation rate of the membranes. 
         [0003]    2. Discussion of the Related Art 
         [0004]    Hydrogen is a very attractive fuel because it is renewable and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. 
         [0005]    Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell type for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs require adequate fuel supply and humidification for effective operation. 
         [0006]    Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack. 
         [0007]    A fuel cell stack typically includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows. 
         [0008]    The MEAs are permeable and thus allow nitrogen in the air from the cathode side of the stack to permeate therethrough and collect in the anode side of the stack, often referred to as nitrogen cross-over. Even though the anode side pressure may be slightly higher than the cathode side pressure, cathode side partial pressures will cause oxygen and nitrogen to permeate through the membrane. The permeated oxygen combusts in the presence of the anode catalyst, but the permeated nitrogen in the anode side of the fuel cell stack dilutes the hydrogen. If the nitrogen concentration increases above a certain percentage, such as 50%, fuel cells in the stack may become starved of hydrogen. If the anode becomes hydrogen starved, the fuel cell stack will fail to produce adequate electrical power and may suffer damage to the electrodes in the fuel cell stack. 
         [0009]    It is known in the art to provide a bleed valve at the anode exhaust gas output of a fuel cell stack to remove nitrogen from the anode side of the stack. It is also known in the art to estimate the molar fraction of nitrogen in the anode side using a model to determine when to perform the bleed of the anode side or anode sub-system. However, the model estimation may contain errors, particularly as degradation of the components in the fuel cell system occurs over time. If the anode nitrogen molar fraction estimation is significantly higher than the actual nitrogen molar fraction, the fuel cell system will vent more anode gas than is necessary, i.e., will waste fuel. If the anode nitrogen molar fraction estimation is significantly lower than the actual nitrogen molar fraction, the system will not vent enough anode gas and may starve the fuel cells of reactants, which may damage the electrodes in the fuel cell stack. 
         [0010]    Therefore, there is a need in the art to determine changes in fuel cell membrane permeation and incorporate the detected change in membrane permeation into a model for estimating the nitrogen flow rate from the cathode to the anode side of the stack and the concentration of nitrogen in the anode side to efficiently utilize anode fuel and to avoid anode reactant starvation events from increasing in frequency as the membranes age. 
       SUMMARY OF THE INVENTION 
       [0011]    In accordance with the teachings of the present invention, a system and method for correcting an estimation of nitrogen in an anode side of a fuel cell stack are disclosed. The system includes a fuel cell stack and a pressure sensor for measuring pressure in an anode sub-system. The system also includes a controller configured to control the estimation of nitrogen in the anode side of the stack by means of nitrogen permeation adjustment, where the controller determines if the pressure in the anode sub-system equilibrates with atmospheric pressure in a shorter period of time after shutdown compared to the time necessary for the anode sub-system to reach approximately atmospheric pressure after a previous shutdown or calibrated time value, and corrects the estimation of nitrogen in the anode side if the pressure equilibrates in a shorter period of time. 
         [0012]    Additional features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a cross-sectional view of a fuel cell; 
           [0014]      FIG. 2  is a simplified block diagram of a fuel cell system; 
           [0015]      FIG. 3  is a graph with time on the x-axis and anode pressure on the y-axis, illustrating how membrane thickness impacts the anode pressure trajectory over time; and 
           [0016]      FIG. 4  is a flow chart diagram of an algorithm for determining changes in anode pressure and adjusting a model estimation of nitrogen in the anode side of a fuel cell stack. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0017]    The following discussion of the embodiments of the invention directed to a system and method for preventing anode starvation by detecting changes in membrane permeation of the membranes in a fuel cell stack is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
         [0018]      FIG. 1  is a cross-sectional view of a fuel cell  10  that is part of a fuel cell stack of the type discussed below. The fuel cell  10  includes a cathode side  12  and an anode side  14  separated by a perfluorosulfonic acid membrane  16 . A cathode side diffusion media layer  20  is provided on the cathode side  12 , and a cathode side catalyst layer  22  is provided between the membrane  16  and the diffusion media layer  20 . Likewise, an anode side diffusion media layer  24  is provided on the anode side  14 , and an anode side catalyst layer  26  is provided between the membrane  16  and the diffusion media layer  24 . The catalyst layers  22  and  26  and the membrane  16  define an MEA. The diffusion media layers  20  and  24  are porous layers that provide for input gas transport to and water transport from the MEA. A cathode side flow field plate or bipolar plate  28  is provided on the cathode side  12 , and an anode side flow field plate or bipolar plate  30  is provided on the anode side  14 . 
         [0019]      FIG. 2  is a simplified block diagram of a fuel cell system  40  including a fuel cell stack  42 . The fuel cell system  40  is intended to generally represent any type of fuel cell system that requires an anode exhaust gas bleed to remove nitrogen from the anode side of the stack  42 . Examples of such fuel cell systems include fuel cell systems that re-circulate the anode exhaust gas back to the anode inlet and fuel cell systems that employ a split stack design with anode flow-shifting, both of which are referred to herein as an “anode sub-system.” Hydrogen gas from a hydrogen source  44  is provided to the anode side of the fuel cell stack  42  on line  48 . An anode exhaust gas is output from the fuel cell stack  42  on line  50  and is sent to a bleed valve  56 . 
         [0020]    As discussed above, nitrogen cross-over from the cathode side of the fuel cell stack  42  dilutes the hydrogen in the anode side of the stack  42 , thereby affecting fuel cell stack performance. Therefore, it is necessary to periodically bleed the anode exhaust gas from the anode sub-system using the bleed valve  56  to reduce the amount of nitrogen in the anode sub-system, i.e., in the anode side of the fuel cell stack  42 . When the bleed valve  56  is open, the bled anode exhaust gas flows through a bleed line  48 . A pressure sensor  60  is also provided in the line  50  to measure the pressure of the anode sub-system of the fuel cell system  40 . 
         [0021]    Air from a compressor  62  is provided to the cathode side of the fuel cell stack  42  on line  64 . A cathode exhaust gas is output from the fuel cell stack  42  on a cathode exhaust gas line  66 . A mixing device  68  is provided in the line  66  for mixing the cathode exhaust gas and the bled anode exhaust gas on the line  48 . 
         [0022]    A controller  54  monitors the pressure of the anode sub-system of the fuel cell system  40 , as measured by the pressure sensor  60 , controls the speed of the compressor  62 , controls the injection of hydrogen from the hydrogen source  44  to the anode side of the stack  42 , and controls the position of the anode bleed valve  56 , as is discussed in more detail below. The controller  54  also utilizes a model to estimate the permeation flow rate of nitrogen from the cathode side to the anode side and the concentration of nitrogen in the anode side of the stack  42 , and to determine when to bleed nitrogen from the anode side of the stack  42 . In addition, the controller  54  measures the length of time required for the anode sub-system to reach atmospheric pressure after the fuel cell system  40  has been shutdown. The controller  54  may adjust the cathode to anode nitrogen permeation estimation through the stack  42  based on the time necessary for the anode side to reach atmospheric pressure, as discussed in more detail below. 
         [0023]    During normal operation of the fuel cell system  40 , nitrogen from the cathode side of the stack  42  permeates through the membranes in the fuel cells to the anode side of the stack  42 , which dilutes the fuel concentration in the anode side of the stack  42 . Thus, to achieve stable operation of the fuel cells, the nitrogen concentration in the anode side of the fuel cell stack  42  needs to be estimated and controlled. Over time, the permeation rate of the membrane  16  changes due to thinning and other physical changes of the membrane  16 , causing the rate of permeation of nitrogen through the membrane  16  to change. Therefore, the change in the rate of permeation of nitrogen through the membrane  16  needs to be periodically determined and accounted for in the estimation of nitrogen concentration in the anode side of the stack  42  to avoid estimating an incorrect level of anode concentration, as is described in more detail below. 
         [0024]    To determine changes in the rate of permeation of the membrane  16 , the pressure profile of the anode side of the stack  42  after shutdown of the fuel cell system  40  can be used. During a normal shutdown, the cathode side of the stack  42  is depleted of oxygen, causing high levels of nitrogen and low levels of hydrogen at atmospheric pressure. The cathode side is at atmospheric pressure because the cathode side of the stack  42  is not sealed, as is known to those skilled in the art. The anode side of the stack  42 , however, is sealed and is left sealed at system shutdown, thus the anode side of the fuel cell stack  42  has a pressurized mixture of a known amount of hydrogen and nitrogen. When the fuel cell system  40  enters the off state, or is shutdown, hydrogen in the anode side of the stack  42  will rapidly diffuse through the membrane  16  into the cathode side of the stack  42  until the hydrogen partial pressure has equilibrated across the membrane  16 . This will cause a decrease in pressure in the anode side of the fuel cell stack  42 , as measured by the pressure sensor  60 . The decrease in pressure in the anode side of the stack  42  will typically create a vacuum as hydrogen rapidly permeates to the cathode side of the stack  42 . 
         [0025]    Nitrogen has a lower permeation rate than hydrogen through the membrane  16  due to the larger size of the nitrogen molecules compared to the hydrogen molecules. Thus, nitrogen will permeate through the membrane  16  at a slower rate than the hydrogen, causing the pressure in the anode side of the fuel cell stack  42  to increase until the nitrogen partial pressures equilibrate across the membrane  16 . Because of the difference in permeation rate between hydrogen and nitrogen, the resulting pressure trajectory of the anode side of the stack  42  after each shutdown, as measured by the pressure sensor  60 , can be correlated to changes in the permeation rate of the membranes of the fuel cells in the stack  42 . 
         [0026]    The faster the hydrogen pressure and the nitrogen pressure in the anode side of the stack  42  equilibrate, as measured by the pressure sensor  60 , the higher the rate of permeation of the membranes.  FIG. 3  is a graph with shutdown time in minutes on the x-axis and anode pressure in kPa on the y-axis, which illustrates the impact of membrane thickness on anode pressure at shutdown of the fuel cell system  40 . As shown in  FIG. 3 , line  70  illustrates the change in pressure over time of a membrane that is 6 μm thick, line  72  illustrates the change in pressure over time of a membrane that is 12 μm thick, and line  74  illustrates the change in pressure over time of a membrane that is 18 μm thick.  FIG. 3  illustrates that the pressure equilibrium over time after shutdown, i.e., how long the anode side takes of the stack  42  takes to reach atmospheric pressure, is directly related to the thickness of the membrane  16 , indicating that membrane permeation can be estimated using the anode pressure trajectory of a fuel cell system  40  after a typical shutdown of the system  40 . The correlation between the thickness of the membrane  16  and the pressure changes in the anode sub-system may be used by an adaptive controller, such as the controller  54 , to adapt the estimation of nitrogen concentration in the anode side to changes in membrane permeation, allowing for the anode fuel concentration to be accurately estimated over the life of the fuel cell stack  42 . A better estimation of fuel concentration allows for improved efficiency, performance and durability. 
         [0027]      FIG. 4  is a flow chart diagram  80  for an algorithm for detecting changes in the permeation rate of the membranes  16  in the stack  42 . At box  82 , the fuel cell system  40  transitions from run mode to off mode, i.e., is shutdown. After the fuel cell system  40  transitions to off mode at the box  82 , the algorithm records the amount of time it takes for the pressure in the anode sub-system to equilibrate with atmospheric pressure, or the pressure of the cathode sub-system, at box  84 . Next, the algorithm determines if the amount of time necessary for the pressure in the anode sub-system to equilibrate has changed since the last shutdown, or has changed from a calibrated value, at decision diamond  86 . If no change in time has occurred, then the algorithm does not take any action at box  88 . The fuel cell system  40  will operate without a permeation correction at box  90  and will return to the box  82  when the fuel cell system  40  transitions from run mode to off mode. 
         [0028]    If a change in the amount of time it takes for the pressure in the anode sub-system to equilibrate with atmospheric pressure is detected at the decision diamond  86 , then the algorithm will modify the estimation of anode fuel concentration and/or modify the nitrogen bleed schedule in proportion to the change in the permeation rate of the membrane  16 , as determined by the change in time, at box  92 , and the fuel cell system  40  will run with the new permeation correction to the fuel estimation and/or nitrogen bleed schedule at box  94 . For example, the frequency of performing anode bleeds to remove nitrogen from the anode side using the bleed valve  56  may be increased, the duration of the anode bleeds using the bleed valve  56  may be increased, and/or the flow of anode fuel to the stack may be increased. After making the appropriate modification at the box  92  and operating with the new modification at the box  94 , the algorithm will return to the box  82  when the fuel cell system  40  transitions from run mode to off mode. 
         [0029]    The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.