Abstract:
A method for determining a low performing cell in a fuel cell stack. The method measures the voltage of each cell in the fuel cell stack and calculates an average cell voltage of all of the cell voltages from the fuel cell stack at a plurality of stack current densities. The method also identifies a minimum cell voltage from all of the cell voltages from the fuel cell stack at the plurality of stack current densities that the average cell voltages are calculated and determines a relative delta voltage relationship between the average cell voltage and the minimum cell voltage at each of the plurality of stack current densities. The relative delta voltage relationships are used to determine whether the minimum cell voltage indicates a persistent stack problem.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a method for detecting minimum cell voltage degradation in a fuel cell stack and, more particularly, to a method for detecting minimum cell voltage degradation in a fuel cell stack that calculates a relative average voltage between an average cell voltage and a minimum cell voltage over a range of stack current densities. 
     2. Discussion of the Related Art 
     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. 
     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. 
     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. 
     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. 
     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 the 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. 
     Monitoring the voltage of the fuel cells to ensure that the voltage of the minimum performing cell does not fall below a predetermined threshold requires that the current draw from the cell does not exceed a predetermined limit. Different techniques are known in the art for monitoring the cell voltage and improvements can be made. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a method for determining a low performing cell in a fuel cell stack is disclosed. The method measures the voltage of each cell in the fuel cell stack and calculates an average cell voltage of all of the cell voltages from the fuel cell stack at a plurality of stack current densities. The method also identifies a minimum cell voltage from all of the cell voltages from the fuel cell stack at the plurality of stack current densities that the average cell voltages are calculated and determines a relative delta voltage relationship between the average cell voltage and the minimum cell voltage at each of the plurality of stack current densities. The relative delta voltage relationships are used to determine whether the minimum cell voltage indicates a persistent stack problem. 
     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 
         FIG. 1  is a simplified block diagram of a fuel cell stack; 
         FIG. 2  is a graph with current on the horizontal axis and voltage on the vertical axis showing polarization curves for an average cell voltage and a minimum cell voltage; 
         FIG. 3  is a graph with current on the horizontal axis and voltage on the vertical axis showing raw data points for a sample period for a stack polarization curve; 
         FIG. 4  is a graph with a number of data points on the horizontal axis and relative delta voltage on the vertical axis showing an average voltage for the raw sample points shown in  FIG. 3 ; and 
         FIG. 5  is a graph with time on the horizontal axis and relative delta voltage on the vertical axis showing the relative delta voltage for several sample periods to identify a low performing cell trend. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the invention directed to a method for identifying a low performing cell in a fuel cell stack by calculating a relative delta voltage between an average cell voltage and a minimum cell voltage is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
       FIG. 1  is a plan view of a fuel cell stack  10  including a plurality of fuel cells  12  electrically coupled in series. The fuel cell stack  10  also includes a positive terminal  14  and a negative terminal  16  that are electrically coupled to the fuel cells  12 . A system load  18  is electrically coupled to the terminals  14  and  16 . A voltage monitoring circuit  20  is electrically coupled to the fuel cells  12  and monitors the voltage of each of the fuel cells  12 . As will be discussed in detail below, the fuel cell monitoring circuit  20  monitors the voltage of the fuel cells  12  to determine the average cell voltage and to identify the minimum performing cell. 
       FIG. 2  is a graph with stack current on the horizontal axis and stack voltage on the vertical axis showing typical polarization curves for the fuel cell stack  10 . The graph shows a polarization curve  30  for the average voltage of the fuel cells  12  over the stack current density range of operation, a polarization curve  32  for the cell with the minimum voltage after 1 hour of stack life operation over the stack current density range of operation and a polarization curve  34  for the cell with the minimum voltage after a significant portion of the stack life has occurred over the stack current density range of operation. As is apparent, the center portion of the polarization curves  30 ,  32  and  34  is relatively linear, also known as ohmic polarization region. However, the slope of the linear portion is different for the average cell voltage and the minimum cell voltage. Particularly, the difference between the voltages of the polarization curves  30  and  32  for low current densities is about 20 mV and the difference between the voltages of the polarization curves  30  and  32  for higher current densities is about 30 mV, thus giving the minimum cell voltages a higher negative slope. 
     According to the invention, a relative delta voltage value rel. delta U is calculated using the average cell voltages and the minimum cell voltages at several sample locations along the polarization curves. The relative delta voltage value rel. delta U can then be compared to a predetermined threshold to determine whether the minimum cell voltage is too low. The relative delta voltage value rel. delta U is calculated as:
 
rel.delta  U =( U   avg   −U   min )· U   avg /1000
 
Where U avg  is the average voltage of the fuel cells  12 , U min  is the minimum voltage of the fuel cells  12  and  1000  is a scaling factor that is not essential for the calculation but is used to get integers and prevent decimals to simplify the visualization and the usage of the rel. delta U values.
 
     The present invention calculates the relative delta voltage value rel. delta U for a plurality of sample points between the average cell voltage and the minimum cell voltage during the life of the stack  10  to monitor the minimum cell voltage and determine when the minimum cell voltage may affect stack performance. Because every voltage difference between the average cell voltage and the minimum cell voltage is multiplied with, i.e., related to, the average cell voltage, it is independent of the current stack power level and has a higher impact the higher the voltage is. Thus, there is a difference at low current/high voltages than at high current/low voltages. Therefore, even low voltage differences between the average cell voltage and the minimum cell voltage can have a high impact due to weighting with the average cell voltage. 
     This weighting is done because the minimal voltage usually has a higher ohmic loss, i.e., a higher negative slope in the center portion of the polarization curve, than the average cell voltage so that the voltage difference becomes higher the lower the voltage is, and hence, the higher the current density. So, the relation is shifting the severity of a voltage difference towards lower current densities. Due to the independence from the current density, there is no need for a well controlled fuel cell stack test platform that keeps the current density constant so that the present invention allows detection of a permanent single cell voltage degradation even in dynamic operated systems, such as fleet vehicles. By definition of a maximum allowed rel. delta U and specific alarm thresholds, one can use the present invention for an early detection of minimum cell voltage degradation. 
     The above discussion can be shown by the values in  FIG. 2 . It should be noted that the values shown in  FIG. 2  are not real world data, but are artificial values to illustrate the principal idea of the rel. delta U. For the polarization curves  30  and  32 , calculating the relative delta voltage value rel. delta U between points  36  and  38  gives 20·850/1000=17 and calculating the relative delta voltage value rel. delta U between points  40  and  42  gives 30·700/1000=21. For the polarization curves  30  and  34 , the relative delta voltage value rel. delta U between points  36  and  44  is 30·850/1000=25.5, which provides a difference between 17 and 25.5 of 8.5. The relative delta voltage value rel. delta U between points  40  and  46  is 40·700/1000=28, which provides a difference between the lines  32  and  34  of 7. This example shows that a performance loss of 10 mV over time of the minimum cell voltage over the whole current density range results in a higher increase of rel. delta U at higher voltages compared to the increase at lower voltages. Thus, the severity of a voltage difference is shifted towards lower current densities due to the relation to the average cell voltage. 
       FIG. 3  is a graph with current on the horizontal axis and voltage on the vertical axis showing example data points  54  that could have been used to calculate the polarization curve  50  for the average cell voltage and example data points  56  that could have been used to calculate the polarization curve  52  for the minimum cell voltage. The data points  54  and  56  are data for one raw data file over a predetermined time period. It is clear from the data points  54  and  56  that the slope of the polarization curves  50  and  52  are not the same. Thus, as discussed above, the relative delta voltage values rel. delta U at lower stack current densities will be lower than the relative delta voltage values rel. delta U at higher stack current densities. 
       FIG. 4  is a graph with a raw data file sample number n on the horizontal axis and rel. delta U on the vertical axis showing data points  60  for all of the relative delta voltage value rel. delta U calculated from the sample points  54  and  56  in  FIG. 3 . Line  62  represents the average of the relative delta voltage values rel. delta U for the data points  60  where the data points  60  are collected within one raw data file or within a certain and frequent time frame. An averaging reduces the number of data points without decreasing the information about the minimum cell voltage performance. Point  64  represents the relative delta voltage value rel. delta U for the low stack current density line between the curves  50  and  52  in  FIG. 3 , point  66  represents the relative delta voltage value rel. delta U for the high stack current density line between the polarization curves  50  and  52  in  FIG. 3  and point  68  represents the relative delta voltage value rel. delta U for the medium stack current density line between the polarization curves  50  and  52  in  FIG. 3 . 
       FIG. 5  is a graph with time on the horizontal axis and the average relative delta voltage value rel. delta U for each separate raw data file where each sample point  70  is an average taken from the relative delta voltage values rel. delta U from  FIG. 4 . Therefore, for each group of data points available from each data file, a point is placed in the graph on  FIG. 5  and a trend line  72  is observed. Thus, the trend of the low performing cell over time can be watched and a calculation can be made as to when that cell may require attention. The trend line  72  is represented by a polynomial 5 th  grade mathematical relationship. 
     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.