Patent Publication Number: US-2007104986-A1

Title: Diagnostic method for detecting a coolant pump failure in a fuel cell system by temperature measurement

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention relates generally to a method for detecting cooling fluid pump failure in a fuel cell system and, more particularly, to a method for detecting cooling fluid pump failure in a fuel cell system that includes measuring one or both of the temperature of the cooling fluid at the outlet from the fuel cell stack and the temperature of the cathode exhaust at the outlet from the fuel cell stack, and comparing the measured temperature to a temperature that would be expected based on the operating conditions of the fuel, cell system to determine whether the cooling fluid is flowing through the stack.  
      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. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today&#39;s vehicles employing internal combustion engines.  
      A hydrogen fuel cell is an electrochemical 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. The work acts to operate the vehicle.  
      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 to generate the desired power. For the automotive fuel cell stack mentioned above, the stack may include about two hundred or more fuel cells. The fuel cell stack receives a cathode 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 fuel cell stack includes a series of flow field or bipolar plates positioned between the several MEAs in the stack. 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 gas to flow to the anode side of the MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode gas to flow to the cathode side of the MEA. The bipolar plates also include flow channels through which a cooling fluid flows.  
      The cooling fluid is pumped through the cooling fluid flow channels in the stack by a pump to maintain the stack at a desirable operating temperature, such as 60°-80° C., for efficient stack operations. However, if the cooling fluid pump fails, then the stack may overheat depending on the output load of the stack, possibly damaging the fuel cell components, such as the membranes. Therefore, it is necessary to monitor whether the cooling fluid pump is pumping the cooling fluid through the cooling fluid flow channels to prevent fuel cell stack failure.  
      One known technique for determining if the cooling fluid pump is operating is to provide a flow sensor at a suitable location in the cooling fluid flow line outside of the fuel cell stack to measure the flow rate of the cooling fluid. However, such flow sensors are typically expensive devices that add significant cost to the fuel cell system. It would be desirable to eliminate the flow sensor in the fuel cell system used for this purpose.  
     SUMMARY OF THE INVENTION  
      In accordance with the teachings of the present invention, a technique for determining whether a cooling fluid pump used for pumping a cooling fluid through a fuel cell stack has failed. The technique includes measuring the temperature of the cooling fluid at the output from the stack and/or measuring the cathode exhaust gas temperature as close as possible to the cathode outlet of the stack. The measured temperature is compared to a stack temperature that would be expected under the current operating conditions of the fuel cell system. If the difference between the measuring temperature and the expected temperature is large enough, then the controller provides a warning signal of pump failure, and also possibly reduces the stack outlet power.  
      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 DRAWING  
       FIG. 1  is a block diagram of a fuel cell system that uses temperature sensors for determining whether a cooling fluid pump has failed, according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
      The following discussion of the embodiments of the invention directed to a technique for determining whether a cooling fluid pump has failed in a fuel cell system is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses.  
       FIG. 1  is a block diagram of a fuel cell system  10  including a fuel cell stack  12 . A cooling fluid pump  14  pumps a cooling fluid through a pipe  16  external to the stack  12  and through cooling fluid flow channels between the several fuel cells in the stack  12 , as is well understood in the art. The cooling fluid is also pumped through a radiator  18  external to the stack  12  to dissipate heat from the cooling fluid before it is returned to the stack  12 . A fan (not shown) could also be provided to-force air through the radiator to remove the waste heat. The speed of the pump  14  and the speed of the fan provide the desired cooling and are determined from the output load of the stack  12  and other operating conditions by a controller  34  so that the temperature of the stack  12  is maintained at a desirable operating temperature for efficient stack operation.  
      According to the invention, a temperature sensor  20  is positioned in the line  16  as close as possible to the outlet from the fuel cell stack  12 . Additionally, a temperature sensor  22  is positioned in a cathode exhaust line  24 , also as close as possible to the stack  12 . Although two temperature sensors  20  and  22  are used in the system  10 , it is within the scope of the present invention that only one of the temperature sensors  20  or  22  be used to determine if the pump  14  has failed. The temperature sensors  20  and  22  could also be positioned within the stack  12 , where the sensor  20  measures the temperature of the cooling fluid and the sensor  22  measures the temperature of the cathode exhaust. For example, the sensor  20  could be positioned within the cooling fluid outlet header and the sensor  22  could be positioned within the cathode exhaust outlet header.  
      The temperature sensor  20  measures the temperature of the cooling fluid leaving the stack  12  and provides a signal indicative of same to a look-up table  26  within the controller  34 . Likewise, the temperature sensor  22  measures the temperature of the cathode exhaust in the exhaust line  24  and provides a temperature signal indicative of same to the look-up table  26 . The look-up table  26  also receives signals from a sub-system  28  identifying the current operating conditions of the fuel cell system  10 , such as ambient temperature, output load of the stack  12 , etc.  
      The look-up table  26  determines what the temperature of the cooling fluid and/or the cathode exhaust gas should be based on the current operating conditions of the fuel cell system  10  and outputs the temperature signals to a deviation device  30  to determine the difference between the two temperature signals for the cathode exhaust and/or the two temperature signals for the cooling fluid. Particularly, the look-up table  26  provides the measured temperature signal of the cathode exhaust and the expected temperature of the cathode exhaust if the system  10  only uses the temperature sensor  22  to determine if the pump  14  has failed. Or, the look-up table  26  provides the measured temperature signal of the cooling fluid and the expected temperature of the cooling fluid if the system  10  only uses the temperature sensor  20  to determine if the pump  14  has failed. Both sensors  20  and  22  can be used, where the look-up table  26  would send the four temperature signals to the deviation device  30 .  
      The difference between the two temperature signals is then applied to a comparison device  32  that compares the difference to a predetermined value. If the difference between the measured temperature from either of the temperature sensors  20  and  22  and the calculated temperature is greater than the predetermined value, it is an indication that the cooling fluid is not cooling the stack  12 . Therefore, the pump  14  has either completely failed or partially failed and is not providing the desired cooling.  
      It is desirable that the sensors  20  and  22  be positioned as close as possible to the active area of the fuel cell stack  12 , possibly within the stack  12  itself, so that they respond quickly enough to a rise in temperature. As discussed above, either of the temperature sensors  20  or  22  can be used to determine if the pump  14  has failed. The sensor  22  may provide a better indication of the stack temperature because if the cooling fluid is not flowing, then the temperature of the cooling fluid within the stack  12  may increase significantly before the temperature of the cooling fluid outside of the stack  12  where the sensor  20  is located increases significantly. However, if there are water droplets in the cathode exhaust gas, water on the sensor  22  could provide evaporative cooling, possibly giving an inaccurate temperature reading.  
      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.