Abstract:
A system and method for determining whether valves in a fuel cell system bleed manifold unit (BMU) are blocked with ice or have otherwise failed. The system opens a first bleed valve, closes a second bleed valve and opens an exhaust valve, and then reads a pressure signal to determine whether there is flow through a flow restriction to determine whether the first bleed valve or the exhaust valve is blocked. The system then closes the exhaust valve, leaves the first bleed valve open, and again reads the pressure signal to determine the pressure drop across the flow restriction, which will indicate whether the flow restriction the pressure sensor lines are blocked. The system then closes the first bleed valve and opens the second bleed valve to determine whether the pressure signal indicates a flow through the second bleed valve.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to a system and method for determining whether bleed valves in a fuel cell system are blocked with ice and, more particularly, to a system and method for determining whether various valves in a bleed manifold unit associated with split sub-stacks in a fuel cell system are blocked. 
         [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 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. 
         [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 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, referred to in the industry as nitrogen cross-over. Even though the anode side pressure may be higher than the cathode side pressure, the cathode side partial pressures will cause air to permeate through the membrane. Nitrogen in the anode side of the fuel cell stack dilutes the hydrogen such that if the nitrogen concentration increases beyond a certain percentage, such as 50%, the fuel cell stack becomes unstable and may fail. It is known in the art to provide a bleed valve at the anode exhaust gas output of the fuel cell stack to remove nitrogen from the anode side of the stack. 
         [0009]    An algorithm may be employed to provide an online estimation of the nitrogen concentration in the anode exhaust gas during stack operation to know when to trigger the anode exhaust gas bleed. The algorithm may track the nitrogen concentration over time in the anode side of the stack based on the permeation rate from the cathode side to the anode side, and the periodic bleeds of the anode exhaust gas. When the algorithm calculates an increase in the nitrogen concentration above a predetermined threshold, for example 10%, it may trigger the bleed. The bleed is typically performed for a duration that allows multiple stack anode volumes to be bled, thus reducing the nitrogen concentration below the threshold. 
         [0010]    Some fuel cell systems employ anode flow shifting where the fuel cell stack is split into sub-stacks and the anode reactant gas flows through the split sub-stacks in alternating directions. In these types of designs, a bleed manifold unit (BMU) may be provided that includes valves for providing the anode exhaust gas bleed. Because water is present in the anode exhaust gas, it is likely that the BMU will have water remaining in it at system shut-down regardless of what measures are taken to remove the water. This water may freeze if the outside ambient temperature is low enough for a long enough period of time. On the next start-up, an anode exhaust gas bleed may be required before the BMU is thawed out enough, where ice may block the flow in the BMU. In certain fuel cell system designs, a continuous anode exhaust bleed is performed during the start-up sequence because the fuel cell stack is particularly sensitive to nitrogen collected during that time. 
         [0011]    For a split stack system, the typical location to provide the anode exhaust gas bleed is at the end of the stack flow. Therefore, two bleed values are used to provide the anode bleed depending on the flow direction. Because a BMU is often provided to accommodate this form of bleeding it is typically referred to as the BMU bleed method. However, a center bleed also can be used that bleeds the anode exhaust from a drain valve in a line joining the two sub-stacks. The center bleed is typically less efficient that an end flow or BMU bleed because of the larger size of the drain valve. 
         [0012]    The location for the bleed is one characteristic and the frequency and duration of the bleed is another characteristic. For a frozen stack, the bleed valve should be opened as much as possible to avoid any localized water build-up. This is referred to as a continuous bleed and can be an inefficient method of bleeding because hydrogen is also lost during the anode exhaust gas bleed. Thus, the system should return to a normal bleed schedule once the system is warmed up. The normal bleed method should be providing the bleeds as infrequently as possible to maximize system efficiency, while still maintaining good stack operation. In this mode, the bleed valves can be closed a significant percentage of the time during operation. 
       SUMMARY OF THE INVENTION 
       [0013]    In accordance with the teachings of the present invention, a system and method is disclosed for determining whether valves in a BMU are blocked with ice or have otherwise failed. The BMU is provided in a fuel cell system including first and second split sub-stacks. The BMU includes a first bleed valve, a second bleed valve, an exhaust valve, a flow restriction and a pressure sensor that measures the pressure across the flow restriction. A controller determines whether the valves in the BMU are blocked in stages where the controller opens the first bleed valve, closes the second bleed valve and opens the exhaust valve, and then reads the pressure signal from the pressure sensor to determine whether there is flow through the flow restriction to determine whether the first bleed valve or the exhaust valve is blocked. The controller then closes the exhaust valve and leaves the first bleed valve open, and again reads the pressure signal to determine the pressure drop across the flow restriction, which will indicate whether the flow restriction or the pressure sensor lines are blocked. The controller then closes the first bleed valve and opens the second bleed valve to determine whether the pressure signal indicates a flow through the second bleed valve. 
         [0014]    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 
         [0015]      FIG. 1  is a block diagram of a fuel cell system employing split sub-stacks and a BMU; and 
           [0016]      FIG. 2  is a flow chart diagram showing a process for providing a stack anode exhaust gas bleed in the event of a BMU failure. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0017]    The following discussion of the embodiments of the invention directed to a system and method for determining if fuel cell system BMU is blocked with ice or has otherwise failed 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 block diagram of a fuel cell system  10  including split fuel cell sub-stacks  12  and  14  that operate under anode flow shifting. When the flow is in one direction, an injector bank  16  injects fresh hydrogen into the anode side of the sub-stack  12  on anode input line  24 . Anode gas that is output from the sub-stack  12  is sent to the sub-stack  14  on connecting line  20 . When the flow is in the opposite direction, an injector bank  18  injects fresh hydrogen into the anode side of the sub-stack  14  on anode input line  26  that is output from the sub-stack  14  and sent to the sub-stack  12  on the line  20 . A drain valve  22  is provided in the line  20  and can be used for a center bleed, as will be discussed in more detail below. 
         [0019]    A BMU  30  is provided at an anode input to the split sub-stacks  12  and  14  and provides an anode exhaust gas bleed during certain times to remove nitrogen from the anode side of the sub-stacks  12  and  14  based on any suitable bleed schedule. The BMU  30  includes a line  32  that connects the anode input lines  24  and  26  and an exhaust line  34  that connects the line  32  to the exhaust of the system  10 , typically the cathode side exhaust of the sub-stacks  12  and  14 . A first bleed valve  36  is provided in the line  32  proximate to the sub-stack  12  and a second bleed valve  38  is provided in the line  32  proximate the sub-stack  14 . An exhaust valve  40  is provided in the line  34  that is opened during the anode bleed and other times as may be required. A flow restriction  44  is provided in the line  34  to limit the amount of anode exhaust gas that can be bled into the environment. 
         [0020]    A differential pressure sensor  46  is provided across the flow restriction  44 , and measures a pressure difference across the flow restriction  44  to identify the flow rate through the flow restriction  44 . When flow begins through the flow restriction  44 , the pressure sensor  46  goes up from a zero value, or a near zero value, depending on the resolution of the sensor  46 , to a pressure indicative of a proper flow through the flow restriction  44 . 
         [0021]    When the system  10  is operating under anode flow-shifting and no bleed is commanded, the bleed valves  36  and  38  are both closed, so that depending on the direction of the anode gas flow, the output of the second sub-stack is dead-ended. If a bleed is commanded, and the flow-shifting is in the direction from the sub-stack  12  to the sub-stack  14  through the line  20 , then the bleed valve  38  is opened and the bleed valve  36  is closed. Likewise, if a bleed is commanded and the flow is in the direction from the sub-stack  14  to the sub-stack  12  through the line  20 , then the first bleed valve  36  is opened and the second bleed valve  38  is closed. Thus, the anode exhaust gas is bled out of the exhaust line  34  through the flow restriction  44  and the exhaust valve  40 . 
         [0022]    A controller  48  controls the injector banks  16  and  18 , controls the valves  36 ,  38  and  40  and receives a pressure reading signal from the pressure sensor  46 . 
         [0023]    As discussed above, the BMU  30  may collect water at various locations when the anode exhaust gas flow is flowing therethrough, and may retain water after the system  10  is shut-down. This water may freeze if the system  10  is in a cold enough environment for a long enough period of time. Thus, at the next system start-up water may have frozen in the BMU  30  at locations such as the bleed valves  36  and  38 , the flow restriction  44 , the exhaust valve  40  and the lines to the pressure sensor  46 , which may block the flow of the anode exhaust gas when a bleed is desired. 
         [0024]    The present invention proposes a method for providing a suitable anode exhaust gas bleed during those times when the BMU  30  is in a failure mode, such as from frozen water blocking flow, which may prevent a bleed through the BMU  30 .  FIG. 2  is a flow chart diagram  50  showing a process for providing an anode exhaust gas bleed that includes this consideration. When the system  10  is started at box  52  it will typically immediately perform a BMU diagnostics check at box  54  to determine whether the BMU  30  is operating properly. In certain systems, the BMU diagnostics check can take over five seconds to complete due to complexity. The first BMU diagnostics check should be done at start-up before nitrogen and water have a chance to build up in the sub-stacks  12  and  14 . The BMU diagnostics check may perform a number of diagnostics on the BMU  30 , including determining whether the BMU  30  is frozen or has failed. 
         [0025]    To determine the complete function of the BMU  30 , it can be tested at three stages. For a first stage, one of bleed valves  36  or  38  is opened depending on the anode flow direction and the exhaust valve  40  is opened. If the opened bleed valve  36  or  38  and the exhaust valve  40  allow flow to the system exhaust, then the pressure sensor  46  provides a pressure reading across the flow restriction  44 . If the flow through the BMU  30  is unrestricted, then the pressure sensor  46  should provide a pressure reading that is about the same as the differential pressure between the cathode and anode of the spilt sub-stacks  12  and  14 . 
         [0026]    If a proper flow is detected at the first stage, then a second stage includes closing the exhaust valve  40  to determine if the pressure sensor  46  measures a zero or near zero level, and if it does, the flow restriction  44  is not blocked with ice. If the pressure sensor  46  does provide a high enough pressure reading, the flow restriction  44  or the lines from the pressure sensor  46  to the line  34  maybe restricted, such as by ice. If ice does block the passage downstream of the anode side to the pressure sensor  46 , the flow detection seen in the first stage may actually be just the pressure difference between the anode side pressure and the cathode side pressure that existed if the line  34  was frozen. If only the first stage was used, the diagnostic would fail to detect ice in these parts of the BMU  30 . 
         [0027]    For the third stage of the diagnostics, the bleed valve that was opened in the first stage is closed and the bleed valve that was closed in the first stage is opened to determine whether it is blocked with ice. Particularly, if the bleed valve  36  was opened and the bleed valve  38  was closed during the first stage, the bleed valve  36  is closed and the bleed valve  38  is opened during the third stage, or vice versa. The exhaust valve  40  does not need to be closed because that function has already been tested in the second stage, although other variations of valve closure, such as closing all the valves as a final channel flow, can be included to simplify implementation within the framework of the existing anode flow shift and control. If all of the stages indicate flow, then the BMU  30  is operating properly with no flow restrictions and the BMU  30  passes the diagnostics check. If any of the stages indicate that the BMU  30  is not operating properly, then the system  10  can switch to a center bleed through the drain valve  22  to keep the sub-stacks  12  and  14  operational, or perform some other remedial action. 
         [0028]    An algorithm operating in the system  10  determines whether the diagnostics check has passed at decision diamond  56 . If the BMU diagnostics has passed at the decision diamond  56 , then the algorithm determines whether it is cold enough at decision diamond  58  to be concerned about ice blocking the flow channels in the BMU  30 , even though it may currently be operating properly. The algorithm can use any suitable technique to determine whether the environment is too cold, such as an ambient temperature reading, a stack temperature reading, a cooling fluid temperature reading, etc. The algorithm can employ any suitable temperature threshold for this determination, such as for example 50° C. If the system is not too cold at the decision diamond  58 , then the algorithm uses the normal BMU bleed schedule employing the bleed valves  36  and  38  at box  60 . Any suitable BMU bleed schedule can be employed based on the accumulation of nitrogen on the anode side of the split sub-stacks  12  and  14 . 
         [0029]    The algorithm then determines whether a predetermined time has expired where another BMU diagnostics check should be performed at decision diamond  62 . The system may perform a BMU diagnostics check during normal operation to check for BMU flow that does not have anything to do with ice, but could be for other failures. If it is not time for the next BMU diagnostics check at the decision diamond  62 , then the algorithm returns to providing a normal BMU bleed. If it is time for a BMU diagnostics check, the algorithm prevents the anode exhaust gas bleed at box  64  and returns to the box  54  to perform the BMU diagnostics check. 
         [0030]    If the start-up temperature is cold enough at the decision diamond  58 , it is possible that a no-flow condition could occur due to ice in the BMU  30 . In this situation, the algorithm goes to a continuous BMU bleed at box  66  where one or the other of the bleed valves  36  or  38  is always opened depending on the flow direction through the sub-stacks  12  and  14 . 
         [0031]    If the diagnostics check fails at the decision diamond  56 , the algorithm goes into the less desirable center bleed mode using the drain valve  22 . Typically, the drain valve  22  is larger than the bleed valves  36  and  38 , and thus the amount of hydrogen that is lost during a center bleed is significant. Further, when the drain valve  22  is used for a bleed, little, if any, of the anode exhaust during the flow shifting process is sent to the downstream split stack because most of it is exhausted out of the drain valve  22 . It is possible to perform a parallel flow into both of the split stacks  12  and  14  at the same time during the center bleed. However, such an operation loses even more hydrogen. 
         [0032]    The algorithm first determines whether it is too cold for a normal center bleed at decision diamond  68 . If it is not too cold at the decision diamond  68 , then the algorithm performs the normal center bleed at box  70  where the drain valve  22  is opened and closed on a desirable schedule based on the accumulation of nitrogen in the split sub-stacks  12  and  14 . Periodically, the algorithm will determine whether it is time for a BMU diagnostics check at decision diamond  72 , and if not, returns to the normal center bleed at the box  70 . The BMU bleed diagnostic cannot be done during the center bleed operation because if the BMU  30  is working, the hydrogen flow from the center bleed and the BMU bleed can exceed hydrogen emissions limit, such as for example 4%. The center bleed is suspended temporarily at box  74  for the BMU diagnostics to be run. The time without the bleed could put stack operation at risk. Therefore, the center bleed is maintained and the BMU diagnostics is suspended until the system warms up to a temperature that does not need a continuous bleed, for example 40° C. If it is too cold at the decision diamond  68 , then the algorithm performs a continuous center bleed through the drain valve  22  at box  76  until the temperature is high enough where ice will not be blocking the flow channels. 
         [0033]    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.