Abstract:
A system and method for identifying leaks in a cathode sub-system of a fuel cell system. An air flow meter is provided up-stream of a compressor and monitors the air flowing into the compressor. When an air leakage diagnostic is commanded, a fuel cell stack by-pass valve and back-pressure valve are closed so that no air flows through or around the stack, and the recirculation valve is opened so that the air flows around the compressor. By knowing the leakage through the by-pass valve and the back-pressure valve, any flow above those values measured by the air flow meter gives an indication of air leakage out of the cathode sub-system components.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to a system and method for identifying leaks in a cathode sub-system of a fuel cell system and, more particularly, to a system and method for identifying air leaks in a cathode sub-system of a fuel cell system that includes monitoring the air flow into a compressor when valves are positioned so that air flows only around the compressor. 
         [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 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]    There are many components, devices and elements in a fuel cell system through which the reactant gases flow both upstream and downstream of the fuel cell stack. For example, in the cathode sub-system, the compressor provides air flow to the cathode side of the stack typically through a charge air cooler that cools the compressed air heated as a result of the compression and a water vapor transfer (WVT) unit that humidifies the cooled air, generally using the cathode exhaust, before the air is sent to the stack. The cathode sub-system also typically includes a by-pass valve for by-passing air around the stack and a back-pressure valve in the cathode exhaust line that controls the cathode side pressure. Any of these devices and components can develop leaks over time where air may be dumped overboard before it reaches the fuel cell stack, which reduces the amount of reactant air provided to the fuel cell stack, thus causing performance issues. In other words, the control algorithms for the fuel cell stack may command the compressor to a certain speed for a desired stack output current, but that amount of air may not reach the stack because air leaks occur through one or more of the components before the stack. Therefore, it would be desirable to be able to determine that a significant leak is occurring in the cathode sub-system as a diagnostic tool. 
       SUMMARY OF THE INVENTION 
       [0009]    In accordance with the teachings of the present invention, a system and method are disclosed for identifying leaks in a cathode sub-system of a fuel cell system. The cathode sub-system includes a compressor that provides cathode air to the cathode side of a fuel cell stack at a desired flow rate and pressure. A recirculation line is provided around the compressor that includes a recirculation valve so that for certain operating conditions some or all of the compressor air can be directed around the compressor instead of flowing through the stack. The cathode sub-system also includes a by-pass valve that allows the cathode air to flow around the fuel cell stack and a back-pressure valve provided in a cathode exhaust line for controlling the pressure within the cathode side of the stack. An air flow meter is provided up-stream of the compressor and monitors the air flowing into the compressor. When the air leakage diagnostic is commanded, the by-pass valve and the back-pressure valve are closed so that no air flows through or around the stack, and the recirculation valve is opened so that the air flows around the compressor. By knowing the leakage through the by-pass valve and the back-pressure valve, any flow above those values measured by the air flow meter gives an indication of air leakage out of the cathode sub-system components. 
         [0010]    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 
         [0011]      FIG. 1  is a schematic block diagram of a cathode sub-system of a fuel cell system; and 
           [0012]      FIG. 2  is a flow chart diagram showing a process for determining leaks in the cathode sub-system shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0013]    The following discussion of the embodiments of the invention directed to a system and method for determining overboard leakage in a cathode sub-system of a fuel cell system is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. For example, the present invention has particular application for a fuel cell system on a vehicle. However, as will be appreciated by those skilled in the art, the present invention has application for other fuel cell systems. 
         [0014]      FIG. 1  is a schematic block diagram of a cathode sub-system in a fuel cell system  10  that includes a fuel cell stack  12 . The system  10  includes a compressor  14  that is operated by a motor  16  to draw in and compress air on line  18  to be provided to the cathode side of the fuel cell stack  12  on a cathode input line  20 . The air from the compressor  14  is heated as a result of the compression process and therefore is cooled by a charge air cooler  22  in a process that is well understood by those skilled in the art. The charge air cooler  22  can be any heat exchanger suitable for this purpose, such as a liquid-gas heat exchanger. The cathode exhaust is output from the fuel cell stack  12  on a cathode exhaust gas line  26 . The cooled air from the charge air cooler  22  is humidified in a WVT unit  24  to increase the relative humidity of the air to be more suitable for the fuel cell stack reaction process. The humidity and water vapor in the exhaust gas on the line  26  is used to provide the moisture to increase the relative humidity of the cathode input air in the WVT unit  24 . A back-pressure valve  28  is provided in the exhaust gas line  26 , and is a proportional valve whose position can be controlled to control the pressure within the cathode side of the fuel cell stack  12 , including closing the line  26  to prevent flow through the cathode side of the stack  12 . A by-pass valve  30  is provided in a by-pass line  32  so that the air can be selectively directed around the stack  12 , where the valve  30  is also a proportional valve so that the amount of air flowing through the stack  12  and flowing around the stack  12  can be selectively controlled. 
         [0015]    The system  10  also includes a recirculation line  34  around the compressor  14  having a recirculation valve  36 . There may be certain operating conditions where the power demanded from the stack  12  is so low that the speed of the compressor  14  cannot be set low enough to only provide the necessary air, i.e., the compressor  14  has a minimum speed that provides more air than is desired. In this situation, the recirculation valve  36  is selectively controlled so that at least a portion of the air flows back to the input line  18  and not to the stack  12 . Further, for low transient demands where the power output from the stack  12  is quickly reduced, the valve  36  may be controlled to prevent air from flowing to the stack  12  until the compressor  14  has had a chance to reduce its speed. An air flow meter  38  is provided in the input line  18  to measure the flow to the line  18  at a location that is upstream from the junction where the recirculated air is reintroduced back to the input of the compressor  14 . A temperature sensor  40  measures the temperature of the air between the compressor  14  and the charge air cooler  22 . The system  10  includes a controller  42  that controls the operation of the system  10  consistent with the discussion herein, including controlling the position of the valves  28 ,  30  and  36 , setting the speed of the compressor  14 , receiving an air flow measurement from the air flow meter  38 , etc., and receiving any input and providing any control for determining the overboard leakage from the cathode sub-system as discussed herein. 
         [0016]    The present invention proposes a technique for determining if any of the components in the cathode sub-system of the fuel cell system  10  is leaking enough air to significantly effect the operation of the system  10 . These components include, but are not limited to, the charge air cooler  22 , the WVT unit  24 , valves, fittings, pipes, etc. As is well understood by those skilled in the art, models are known in the art that use the position of a valve as feedback to determine leakage through that particular valve. Therefore, leakage through the valves  28 ,  30  and  36  is not determined by the overboard leakage process described herein, where such models are used to determine that leakage, and that leakage is then subtracted from the flow, discussed in more detail below. 
         [0017]    When the system control determines that an air overboard leakage diagnostic is to be performed, the valves  28  and  30  are closed so that there is no air flow through or around the stack  12 . The valve  36  is opened so that all, or most, of the air that flows out of the compressor  14  is returned to the input of the compressor  14 . The compressor  14  is set to a predetermined diagnostic speed, which would likely be at or near the minimum compressor speed. Some of the air will leak through the valves  28  and  30 , which can be modeled for a particular compressor speed using position feedback, as discussed above. This minimal amount of air that is lost and is not recirculated back to the input of the compressor  14  is drawn in through the line  18  and allows a measurement by the air flow meter  38 . Since this leakage is known, it can be subtracted from the air flow measurement, providing a zero value if no other leakage is occurring in the cathode sub-system. 
         [0018]    If there is leakage through a component or device in the cathode sub-system other than the valves  28  and  30 , then that leakage will also be measured by the air flow meter  38 . Thus, once the calculated valve leakage is subtracted from the air flow meter measurement, the additional air flow that may be measured by the air flow meter  38  is an indication of the amount of leakage. If that leakage value exceeds some predefined threshold, indicating that the cathode sub-system has too large of a leak, a diagnostic can be set that indicates an overboard leakage is occurring and that the system  10  should be serviced. The temperature sensor  40  can measure the temperature during the diagnostic, and if a maximum temperature threshold is reached, the diagnostic can be aborted. However, if the speed of the compressor  14  is set at or near a minimal speed, then the amount of heating of the cathode air should not be significant. 
         [0019]    In certain fuel cell systems, the compressor  14  may be of the type that can be run at a slow enough speed to provide the minimum flow rate under all applicable system operating conditions so that the recirculation valve  36  and the recirculation line  34  can be eliminated. For this type of system, the back-pressure valve  28  may need to be opened at least slightly to overcome compressor surge concerns. In this system, leak detection can be performed by modeling the low flow through the slightly open back-pressure valve  28 , and subtracting that flow value from the flow measured through the air flow meter  38 . 
         [0020]    It is noted that in certain fuel cell systems, the back-pressure valve  28  can be replaced with a cathode inlet valve  44 , and a leak through the valve  44  can be determined consistent with the discussion herein. It is further noted that the valves  28 ,  30 ,  36  and  44  can be either proportional valves or discrete valves depending on the particular system. 
         [0021]    The diagnostic for determining cathode sub-system overboard leakage can be performed at any suitable time and at any suitable rate. For example, the diagnostic may be performed at system shut-down once every fifty shut down operations. Also, there are various operating modes where the stack  12  may not be producing power to operate the vehicle, such as a stand-by mode when the vehicle is at a stop light. The diagnostic discussed above may be performed during those operating conditions where no stack power is required. 
         [0022]      FIG. 2  is a flow chart diagram  50  showing the process discussed above for the overboard leakage diagnostic. The diagnostic is initiated at box  52  and the compressor  14  is set to the desired compressor speed at box  54 . The valves are then set at box  56 , where the valves  28  and  30  are closed and the valve  36  is opened. The air flow measurement is determined at box  58  and the valves leakage model value is obtained and subtracted from the measurement at box  60 . The modified measurement value is then compared to a threshold at box  62  and a diagnostic is set at box  64  if the modified measurement value exceeds the threshold. 
         [0023]    As will be well understood by those skilled in the art, the several and various steps and processes discussed herein to describe the invention may be referring to operations performed by a computer, a processor or other electronic calculating device that manipulate and/or transform data using electrical phenomenon. Those computers and electronic devices may employ various volatile and/or non-volatile memories including non-transitory computer-readable medium with an executable program stored thereon including various code or executable instructions able to be performed by the computer or processor, where the memory and/or computer-readable medium may include all forms and types of memory and other computer-readable media. 
         [0024]    The foregoing discussion disclosed 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.