Abstract:
A system and method for determining reactant gas flow through a fuel cell stack to determine potential stack problems, such as a possible low performing fuel cell. The method includes applying a perturbation frequency to the fuel cell stack and measuring the stack current and stack voltage in response thereto. The measured voltage and current are used to determine an impedance of the stack fuel cells, which can then be compared to a predetermined fuel cell impedance for normal stack operation. If an abnormal fuel cell impedance is detected, then the fuel cell system can take corrective action that will address the potential problem.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to a system and method for determining reactant gas flows in a fuel cell stack and, more particularly, to a system and method for identifying undesirable reactant gas flows in a fuel cell stack by applying a perturbation frequency to the fuel cell stack, measuring the stack current and stack voltage in response thereto and using the current and voltage measurements to determine the real and complex fuel cell impedance. 
         [0003]    2. Discussion of the Related Art 
         [0004]    Hydrogen is a very attractive fuel because it is clean 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 protons and electrons. The protons pass through the electrolyte to the cathode. The 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 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 are relatively expensive to manufacture and require certain conditions for effective operation. 
         [0006]    Several fuel cells are typically combined in a fuel cell stack by serial coupling to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant 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 reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows. 
         [0007]    The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between the 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]    As a fuel cell stack ages, the performance of the individual cells in the stack degrade differently as a result of various factors. There are different causes of low performing cells, such as cell flooding, loss of catalyst, etc., some temporary and some permanent, some requiring maintenance, and some requiring stack replacement to exchange those low performing cells. Although the fuel cells are electrically coupled in series, the voltage of each cell when a load is coupled across the stack decreases differently where those cells that are low performing have lower voltages. Thus, it is necessary to monitor the cell voltages of the fuel cells in a stack to ensure that the voltages of the cells do not drop below a predetermined threshold voltage to prevent cell voltage polarity reversal, possibly causing permanent damage to the cell. 
         [0009]    Typically, the voltage output of every fuel cell in the fuel cell stack is monitored so that the system knows if a fuel cell voltage is too low, indicating a possible failure. As is understood in the art, because all of the fuel cells are electrically coupled in series, if one fuel cell in the stack fails, then the entire stack will fail. Certain remedial actions can be taken for a failing fuel cell as a temporary solution until the fuel cell vehicle can be serviced, such as increasing the flow of hydrogen and/or increasing the cathode stoichiometry. 
         [0010]    Fuel cell voltages are often measured by a cell voltage monitoring sub-system that includes an electrical connection to each bipolar plate, or some number of bipolar plates, in the stack and end plates of the stack to measure a voltage potential between the positive and negative sides of each cell. Therefore, a  400  cell stack may include  401  wires connected to the stack. Because of the size of the parts, the tolerances of the parts, the number of the parts, etc., it may be impractical to provide a physical connection to every bipolar plate in a stack with this many fuel cells, and the number of parts increases the cost and reduces the reliability of the system. 
         [0011]    A total harmonic distortion (THD) of the fuel cell stack voltage can also be measured and used as a cell voltage detection signal. Typically, however, this method is not reliable as it does not produce a consistent signal, where it may be producing an increasing THD under some conditions, a decreasing THD under other conditions or no change in the THD under other conditions. 
       SUMMARY OF THE INVENTION 
       [0012]    In accordance with the teachings of the present invention, a system and method for determining reactant gas flow through a fuel cell stack are disclosed to determine potential stack problems, such as a possible low performing fuel cell. The method includes applying a perturbation frequency to the fuel cell stack and measuring the stack current and stack voltage in response thereto. The measured voltage and current are used to determine the real and complex impedance of the stack fuel cells, which can then be compared to predetermined fuel cell impedance or ratio of impedances for normal stack operation. If an abnormal fuel cell impedance is detected, then the fuel cell system can take corrective action that will address the potential problem. 
         [0013]    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 
         [0014]      FIG. 1  is a block flow diagram of a fuel cell system that measures reactant gas flow through a fuel cell stack; and 
           [0015]      FIG. 2  is a schematic diagram of a circuit for applying a perturbation frequency to a fuel cell stack and measuring the voltage and current on the stack. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0016]    The following discussion of the embodiments of the invention directed to a system and method for monitoring reactant gas flow in a fuel cell stack to determine stack abnormalities is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
         [0017]      FIG. 1  is a block flow diagram for a fuel cell system  10  including a fuel cell stack  12 . In the system  10 , predetermined desirable spectral measurements, including desired stack voltage, stack current, fuel cell impedance, etc., for optimal stack and system operation is provided on line  16  to a summation junction  18 . These measurements and parameters are sent to a reactant control algorithm at box  20  that also receives a reactant flow request on line  22  for a desired stack output power, such as vehicle throttle position. The reactant control algorithm determines the proper reactant gas flow including both the air flow for the cathode side of the fuel cell stack  12  and the hydrogen gas flow for the anode side of the fuel cell stack  12 . The reactant control algorithm uses the reactant request signal and the desired measurements for the optimal system operation to determine how much reactant flow should be provided to the stack  12  in a manner that is well understood to those skilled in the art. Control signals provided by the algorithm at the box  20  are then sent to a reactant flow box  24  that represents the control for a compressor that provides cathode air to the cathode side of the stack  12  and a hydrogen fuel source that provides hydrogen gas to the anode side of the fuel cell stack, such as an injector or injector bank providing hydrogen gas from a high pressure storage tank. 
         [0018]    As will be discussed in detail below, a perturbation frequency is applied to the stack  12  to determine fuel cell impedance, which can be an indication of proper reactant gas flow for both the cathode side and anode side of the fuel cell stack  12 . A different frequency would be required to detect the flow through the anode and cathode sides of the stack  12 . The reason that a different frequency is needed for the cathode side and the anode side of the fuel cell stack  12  has to do with the catalyst configuration at the electrodes of the MEAs in the fuel cells. The perturbation frequency will be a relatively low frequency, depending on the particular flow being determined. The particular frequency would depend on the stack technology being used, and would typically be determined experimentally. For current stack technologies, a frequency signal in the 2-5 Hz range may be applicable for hydrogen gas flow through the anode side of the fuel cell stack  12  and a frequency signal of about 50 Hz may be applicable for the air flow through the cathode side of the fuel cell stack  12 . 
         [0019]    Spectral measurements of the fuel cell stack  12  are provided at box  26 , which represents a voltage meter that measures the voltage across the stack  12 , or at least a series of fuel cells in the stack  12 , and a current meter that measures the current flow through the stack  12  or the current flow through a series of the fuel cells in the stack  12 . The voltage and the current measurements from the box  26  are provided to an impedance calculation algorithm at box  28  that uses those measurements to calculate the real and complex impedance of the cells in the stack  12  or the group of series connected cells being measured. The impedance calculation algorithm uses the calculated impedance and, depending on whether it is the cathode air or the anode hydrogen gas being monitored, determines whether the calculated impedance is the optimal impedance by a comparison process, or ratio of impedances, for the fuel cells at the current system operating conditions. If the impedance of the fuel cells is not the desired impedance for those operating conditions, then the impedance calculation algorithm sends a signal to the summation junction  18  to adjust the desired spectral measurements on the line  16  so that the reactant control algorithm at the box  20  changes the reactant flow at the box  24 . The reactant control algorithm will know which of the cathode or the anode side of the fuel cell stack  12  is currently being monitored and will for that time adjust only one or the other of the compressor or the hydrogen gas injectors, if necessary. 
         [0020]    In addition, the system controller can take other remedial or corrective actions to improve the cell impedance, such as adjusting the humidification of the cathode inlet air, adjusting the coolant flow through and/or temperature of the fuel cell stack  12 , reducing the stack load current, etc. Thus, in this manner, the system  10  is able to monitor cell voltages to detect abnormal operating conditions with only two connections to the fuel cell stack  12  for the voltage meter and the current meter, instead of the many connections that were typically required to measure fuel cell voltages to detect low performing cells. 
         [0021]    In addition to detecting abnormal or improper system operating conditions, the system and method discussed herein can be used to trim or minimize the cathode air flow and the hydrogen gas flow to the fuel cell stack  12 . Particularly, by identifying the minimum cathode air flow and/or anode gas flow to the stack  12  for the current stack power request or load, determining the cell impedance in the manner as discussed above can be used to ensure that this minimal flow is being achieved for efficient system operation. Thus, the compressor speed can be minimized and the amount of hydrogen provided at the stack  12  can be minimized for efficient operation. 
         [0022]      FIG. 2  is a schematic diagram of a system  40  for applying a perturbation frequency to a fuel cell stack  42  including a plurality of series connected fuel cells  44 , as discussed above. A positive electrical line  46  is coupled to a positive end of the fuel cell stack  42  and a negative electrical line  48  is coupled to a negative end of the fuel cell stack  42 , where the lines  46  and  48  provide the stack power to the particular system being powered. A current meter  50  is provided on the positive line  46  to measure the current flow through the stack  42  and a voltage meter  52  is electrically coupled across the lines  46  and  48  to measure the voltage potential across the stack  42 . 
         [0023]    The present invention contemplates any suitable technique for providing the perturbation frequency to the stack  42  for determining cell impedance in the manner as discussed above. In this non-limiting embodiment, the system  40  includes a load  54  having a certain resonate frequency, such as a suitable resistor, and a MOSFET switch  56  electrically coupled to the lines  46  and  48  across the stack  42 , as shown. When power is being provided by the stack  42 , the switch  56  is opened and closed at the desired frequency, i.e., the resonate frequency of the load  54 , so that an AC frequency signal is applied to the stack  42  on top of the DC power signal provided by the stack  12 . The voltage across the stack  42  and the current through the stack  42  are measured at the frequencies that the switch  56  is opened and closed. These measurements are used to determine both the real and reactive impedance of the cells  44  in the stack  42  in a manner that is well understood to those skilled in the art. The measurement of the voltage and current at the frequencies that the switch  56  is opened and closed to determine cell impedance has to do with the electrodes in the MEAs discharging as a capacitance when the switch  56  is opened. Further, each different catalyst material would provide a different cell impedance. When the cathode airflow is being determined, then the switch  56  is opened and closed at one desirable frequency and when the anode fuel flow is being determined, the switch  56  is opened and closed at a different frequency. In an alternate embodiment, the switch  56  may be some device that is able to provide both the cathode frequency and the anode frequency simultaneously. 
         [0024]    In the discussion above, the perturbation frequency was provided by elements that were added to the system for that particular purpose. In alternate designs, the load  54  may be an existing component in the fuel cell system  10 , such as end cell heaters, power converters, DC/DC boost converters, etc. 
         [0025]    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.