Abstract:
A method of controlling pressure of a fuel gas in a fuel cell stack is disclosed. The method includes establishing a specification for a fuel gas/oxidant gas delta pressure vs. time value, operating the fuel cell stack, monitoring the fuel gas/oxidant gas delta pressure vs. time value and reducing stress resulting from excessive pressure of the fuel gas by implementing at least one of the following: (1) inducing the fuel cell stack to convert excess fuel gas into electrical current; (2) shutting off supply of the fuel gas to the fuel cell stack; and (3) and raising an operating pressure of the fuel cell stack when the fuel gas/oxidant gas delta pressure vs. time value strays beyond the specification.

Description:
This is a Divisional of a application Ser. No. 11/285,543, filed on Nov. 21, 2005 now U.S. Pat. No. 7,855,025. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to fuel cells suitable for generating electricity for automotive or other applications. More particularly, the present invention relates to an anode loop pressure control method in which excess hydrogen pressure in an anode loop of a fuel cell system is mitigated by: (1) increasing the current demand on the fuel cell; (2) periodically shutting off the fuel supply; and (3) raising the operating pressure of the fuel cell system. These methods can be implemented independently or in any combination. 
     BACKGROUND OF THE INVENTION 
     In recent years, much research has been devoted to the development of fuel cell systems to generate energy for automotive and other applications. A fuel cell system produces electricity by harvesting electrons from hydrogen gas. Oxygen is reduced by the harvested electrons and combined with hydrogen protons to produce water as a by-product. Fuel cell vehicles are highly efficient and environmentally-friendly. 
     A typical conventional PEM (polymer electrolyte membrane) fuel cell system includes multiple fuel cells, each of which includes a polymer electrolyte membrane interposed between an anode catalyst layer and a cathode catalyst layer to form a membrane electrode assembly (MEA). A gas diffusion medium (GDM) layer engages each catalyst layer, and a bipolar plate engages each GDM layer. The anode side bipolar plate is provided with flowfield channels which distribute a reactant gas, which contains hydrogen rich gas or may be pure hydrogen gas, to the anode catalyst layer through the anode side GDM layer. The cathode side bipolar plate is likewise provided with flowfield channels which distribute an oxidant gas, which may be air and contains oxygen or may be pure oxygen, to and reactant water vapor away from the cathode catalyst layer through the cathode side GDM layer. 
     During operation of the fuel cell system, hydrogen gas is split into electrons and protons at the anode catalyst layer. The protons are passed from the anode catalyst layer, through the electrolyte membrane and to the cathode catalyst layer. The electrons are distributed as electrical current from the anode catalyst layer, through an external circuit to drive an electric load, and then to the cathode catalyst layer. At the cathode catalyst layer, molecular oxygen is split into oxygen atoms, which combine with the electrons and hydrogen protons to form water. The water is distributed from the fuel cell system through the flowfield plates of the cathode side bipolar plate. In the fuel cell system, multiple individual fuel cells are typically stacked in series to form a fuel cell stack in which voltages and quantities of electricity proportional to the number of fuel cells are generated. 
     In a typical PEM fuel cell system, fuel cell stack modules are constrained by a specification which maintains a certain hydrogen/air delta pressure vs. time value, defined as changes in the hydrogen pressure relative to the air pressure at the fuel cell stack inlet over time. Normally, a pressure control device in the fuel cell system monitors the hydrogen and air pressures at the stack inlet and supplies hydrogen and air to the stack in the proper hydrogen/air pressure gradient in such a manner that the hydrogen/air delta pressure vs. time value is constrained within the specification. However, as the pressure control device ages over time, the pressure regulation capability of the pressure control device typically degrades, and consequently, its ability to control hydrogen pressure in the anode loop of the stack, particularly at idle flowrates, decreases. If the hydrogen/air delta pressure vs. time value drifts beyond the specification, particularly by the introduction of excess quantities of hydrogen into the stack via the defective or degrading pressure control device, then the stack membrane may rupture and cause the excess hydrogen to leak from the anode side into the cathode side of the stack. A common solution to this problem is to purge the excess hydrogen from the anode side of the fuel cell stack to the atmosphere. However, this method adversely affects fuel economy and hydrogen emissions. 
     Accordingly, an anode loop pressure control method is needed in which excess hydrogen in an anode loop of a fuel cell stack in a fuel cell system is consumed (1) by increasing the current demand on the stack, (2) periodically shutting off the fuel supply to the stack and/or (3) raising the operating pressure of the system. This adaptation of the system operating characteristics eliminates excessive quantities of hydrogen from the anode loop of the stack, thus mitigating fuel cell degradation while conserving fuel economy and hydrogen emissions. 
     SUMMARY OF THE INVENTION 
     The present invention is generally directed to a novel anode loop pressure control method in which the system operating characteristics of a fuel cell system are adapted to (1) consume excess hydrogen in an anode loop of a fuel cell stack; (2) reduce the quantity of hydrogen delivered to the anode loop; and/or (3) raise the air pressure in the cathode loop. Consequently, excessive accumulation of hydrogen in the stack is prevented, or adjusted for, thus preventing rupture of the stack membrane and leakage of hydrogen from the anode side to the cathode side of the stack. According to the method, when the fuel cell stack is operated typically at an idle state or low power level, a delta pressure vs. time value, which indicates the hydrogen pressure relative to the air pressure at the fuel cell stack inlet over time, is monitored. When excess hydrogen is present in the anode side of the fuel cell stack relative to the quantity of air in the cathode side of the stack, a pre-determined hydrogen/air delta pressure specification is exceeded and a hydrogen/air alarm may be activated. If the hydrogen/air alarm is activated once or a predetermined number of times, a signal is transmitted to a vehicle controller, causing the vehicle controller to (1) increase the output current demand on the fuel cell stack. Additionally or alternatively, the (2) supply of hydrogen to the fuel cell stack may be periodically shut off and/or (3) the operating pressure of the system increased. Consequently, either (1) the fuel cell stack consumes the excess hydrogen to generate electrical current in order to comply with the increased current demand, (2) the excess hydrogen is removed by shutting off the hydrogen supply, or (3) the excess hydrogen pressure is balanced by increasing the air pressure on the cathode. In all three cases, the pressure exerted by the hydrogen against the stack membrane is relieved, preventing rupturing of the membrane and leakage of hydrogen into the cathode side of the stack. Furthermore, continued occurances of the hydrogen/air alarm are used to adjust or adapt the minimum current demand on the stack, the frequency and duration of the shutoff, and/or the air pressure on the cathode. Using this method, the vehicle controller is able to compensate for continued degradation of the pressure control device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described, by way of example, with reference to the accompanying drawing, in which: 
         FIG. 1  is a schematic diagram of a fuel cell system in implementation of the method of the present invention; and 
         FIG. 2  is a flow diagram illustrating sequential steps carried out according to an anode loop pressure control method of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A schematic view of a fuel cell system having an electronically controlled pressure control device  26  in implementation of the present invention is generally indicated by reference numeral  10  in  FIG. 1 . The fuel cell system  10  shown in  FIG. 1  is one example of a system which is suitable for implementation of the present invention and may be used in a fuel cell vehicle (not shown) or in any other application in which electrical power is required. The fuel cell system  10  generally includes a fuel cell stack  12  having multiple, stacked fuel cells (not shown). A fuel source  22  is connected to the fuel cell stack  12  through a fuel inlet conduit  14 , and an oxidant source  24  is connected to the fuel cell stack  12  through an oxidant inlet conduit  18 . Accordingly, the fuel inlet conduit  14  is adapted to distribute a fuel gas, which hereinafter will be referred to as hydrogen  34 , from the fuel source  22  to the anode loop of the fuel cell stack  12 , whereas the oxidant inlet conduit  18  is adapted to distribute an oxidant gas, which hereinafter will be referred to as air  38  that contains oxygen, from the oxidant source  24  to the cathode loop of the fuel cell stack  12 . A fuel exhaust outlet  16  extends from the fuel cell stack  12  for distributing fuel exhaust  36  from the fuel cell stack  12 , and an oxidant exhaust outlet  20  extends from the fuel cell stack  12  for distributing oxidant exhaust and water  40  from the fuel cell stack  12 . Each of the fuel source  22  and the oxidant source  24  typically includes a gas delivery subsystem (not shown) having a mechanical device such as a compressor, fan, pump, rotary piston blower or equivalent mechanical device that forces the fuel gas  34  through the fuel inlet conduit  14  and the oxidant gas  38  through the oxidant inlet conduit  18 , respectively. In one configuration of the fuel cell system  10 , the fuel exhaust outlet  16  is routed back to the fuel inlet conduit  14  in a closed loop. In an alternative configuration of the fuel cell system  10 , the fuel cell stack  12  is dead-ended, in which case the fuel exhaust outlet  16  may be capped, for example. 
     A fuel gas sensor  42  is provided at the inlet of the fuel inlet conduit  14  with the fuel cell stack  12  to sense the pressure of the hydrogen  34  in the anode loop of the fuel cell stack  12 . An oxidant gas sensor  46  is likewise provided at the inlet of the oxidant inlet conduit  18  with the fuel cell stack  12  to sense the pressure of the air  38  in the cathode loop of the fuel cell stack  12 . Alternatively, the fuel gas sensor  42  and the oxidant gas sensor  46  may be provided in the fuel exhaust outlet  16  and oxidant exhaust outlet  20 , respectively. A pressure control device  26  is provided in the fuel inlet conduit  14 , and thus, is connected to the fuel gas sensor  42  and the fuel source  22  through the fuel inlet conduit  14 . Accordingly, the pressure control device  26  is designed to control the fuel source  22  in such a manner that the hydrogen  34  and air  38 , respectively, are supplied to the fuel cell stack  12  in the proper ratios to maintain optimum operation of the fuel cell stack  12 . 
     A signal controller  54  is connected to the fuel gas sensor  42 , such as via controller electrical connection. The signal controller  54  is further connected to the oxidant gas sensor  46 , such as via controller electrical connection. A hydrogen/air alarm  56 , the purpose of which will be hereinafter described, is connected to the signal controller  54  such as via alarm electrical connection  57 . The signal controller  54  is further connected to the pressure control device  26  through electrical connection  50  and to the fuel source  22  and the oxygen source  24 , as shown. During operation of the fuel cell stack  12 , the fuel gas sensor  42  transmits to the signal controller  54  signals which indicate the pressure of the hydrogen  34  in the anode side of the fuel cell stack  12 , whereas the oxidant gas sensor  46  transmits to the signal controller  54  signals which indicate the pressure of the air  38  in the cathode side of the fuel cell stack  12 . As will be hereinafter described, the signal controller  54  is programmed to constantly monitor the delta pressure vs. time value of the hydrogen  34  relative to the air  38 . In the event that the delta pressure vs. time value exceeds a predetermined specification, indicating an excessive quantity of hydrogen  34  in the anode loop of the fuel cell stack  12 , the signal controller  54  activates the hydrogen/air alarm  56  to indicate the need to replace and/or service the pressure control device  26 . 
     The signal controller  54  is connected to a vehicle controller  60  such as via vehicle controller electrical connection (not shown). The vehicle controller  60  is connected to electrical loads (not shown) on the fuel cell vehicle, which are, in turn, connected to the fuel cell stack  12  The vehicle controller  60  controls the current demand of the fuel cell vehicle, for example, or other application which is supplied with electrical power by the fuel cell stack  12 , typically in conventional fashion. The vehicle controller  60  is typically further connected to the fuel source  22  and may be capable of periodically shutting off the supply of hydrogen  34  to the fuel cell  12 . The signal controller  54  may further be connected to a back pressure valve  70  which is provided in the oxidant exhaust conduit  20  to regulate the oxidant pressure inside the fuel cell stack  12 . Additionally or alternatively, the signal controller  54  may regulate the oxidant pressure inside the fuel cell stack  12  through the oxidant source  24 . According to the method of the present invention, in the event that the monitored delta pressure vs. time value of the hydrogen  34  relative to the air  38  exceeds the specification once or a predetermined number of times, the signal controller  54  transmits an activation signal to the vehicle controller  60 . In turn, the vehicle controller  60  may (1) increase the demand for electrical current from the fuel cell stack  12 , thus consuming the excess hydrogen  34  in the anode side of the fuel cell stack  12 , as will be hereinafter further described. In addition or alternatively, the signal controller  54  may (2) periodically cause the fuel source  22  to shut off the supply of hydrogen  34  to the fuel cell stack  12  and/or (3) raise the operating pressure of the fuel cell system  10  by increasing the air pressure at the cathode side of the fuel cell stack  12 . 
     The present invention is suitable for the pressure control device  26  which was heretofore described and illustrated with respect to  FIG. 1  and is known as an electrically controlled pressure control device. However, the invention is equally adaptable to a mechanical pressure control device which is known to those skilled in the art, the inlet side of which is connected to the fuel source  22  through the fuel inlet conduit  14  and the outlet side of which is mechanically or confluently connected to the fuel inlet conduit and oxidant inlet conduit through separate sense lines or small tubes (not shown). 
     Under normal operation of the fuel cell system  10 , the signal controller  54  operates the gas delivery sub-systems to provide the appropriate reactants to the fuel cell stack  12  to meet a demanded electrical power output for operation of the fuel cell vehicle or other application responsive to input from a vehicle driver or other operator of the fuel cell system  10 . Therefore, the pressure control device  26  causes the fuel source  22  to distribute hydrogen  34  through the fuel inlet conduit  14  and into the anode loop of the fuel cell stack  12  and the oxidant source  24  to distribute air  38  through the oxidant inlet conduit  18  and into the cathode loop of the fuel cell stack  12 . 
     In the fuel cell stack  12 , the individual fuel cells generate electrical power by harvesting electrons from the hydrogen  34 ; passing the electrons as electrical current to an external circuit, which powers an electric motor (not shown); splitting molecular oxygen in the air  38  into oxygen atoms; and combining protons from the oxidized hydrogen with the electrons and oxygen atoms to form water. Excess hydrogen  34  is discharged as fuel exhaust  36  from the fuel cell stack  12  through the fuel exhaust outlet  16 . Exhaust air and water vapor is discharged as oxidant exhaust  40  from the fuel cell stack  12  through the oxidant exhaust outlet  20 . 
     Normally, as the hydrogen  34  and air  38  are delivered to the fuel cell stack  12 , the pressure control device  26  constantly monitors the hydrogen pressure in the anode loop of the fuel cell stack  12  via input from the fuel gas sensor  42  and the air pressure in the cathode loop of the fuel cell stack  12  via input from the oxidant gas sensor  46 . Responsive to this input, the pressure control device  26  controls the pressure of hydrogen  34  and oxygen  38  flowing into the fuel cell stack  12  to ensure that the hydrogen/air delta pressure vs. time value stays within the specification. Over time, however, the pressure control device  26  gradually loses the capability to properly control the pressure of hydrogen  34  relative to the pressure of air  38  which enters the fuel cell stack  12 . This may cause the actual hydrogen/air delta pressure value over time value to stray beyond the specification, resulting in flow of excessive quantities of hydrogen into the anode loop of the fuel cell stack  12 . Therefore, the stack membrane (not shown) of the fuel cell stack  12  is vulnerable to being ruptured, resulting in leaking of the hydrogen  34  from the anode loop to the cathode loop of the fuel cell stack  12 . 
     Throughout operation of the fuel cell system  10 , the signal controller  54  receives input from the fuel gas sensor  42  and the oxidant gas sensor  46  regarding the pressure of hydrogen  34  and air  38  at the respective inlets of the fuel cell stack  12 . Using this input, the signal controller  54  constantly monitors the hydrogen/air delta pressure vs. time value to determine whether it falls within the specification. In the event that the monitored hydrogen/air delta pressure vs. time value exceeds the specification, the signal controller  54  typically activates the hydrogen/air alarm  56 , thus signaling the need to replace and/or repair the pressure control device  26 . If the monitored hydrogen/air delta pressure vs. time value exceeds the specification once or a predetermined number of times (the number of which was previously programmed into the signal controller  54 ), the signal controller  54  transmits a signal to the vehicle controller  60 . In response, the vehicle controller  60  may (1) increase the current demand from the fuel cell stack  12  by increasing the electrical load. As a result, in order to supply the increased electrical load, the fuel cell stack  12  consumes the excess hydrogen in the anode loop of the fuel cell stack  12  to generate the increased current. Additionally or alternatively, the signal controller  54  may (2) periodically shut off flow of hydrogen  34  from the fuel source  22  to the fuel cell stack  12 . Additionally or alternatively, the signal controller  54  may raise the operating pressure of the fuel cell system  10  through the back pressure valve  70  and/or oxidant source  24 . Consequently, the pressure of the hydrogen in the anode loop of the fuel cell stack  12  decreases, thereby preventing rupturing of the stack membrane and leakage of excess hydrogen from the anode loop to the cathode loop of the fuel cell stack  12 . Furthermore, the pressure control device  26  can remain in service until replacement without significantly compromising the structural or functional integrity of the fuel cell stack  12 . 
     Sequential steps carried out according to the method of the present invention are summarized in the flow diagram of  FIG. 2 . In step  1 , a fuel cell stack is operated at an idle state (such as about &lt;10 amps current output, for example). In step  2 , the hydrogen/air delta pressure vs. time value of the fuel cell stack is constantly monitored. In step  3 , the monitored hydrogen/air delta pressure vs. time value is simultaneously compared with a predetermined specification for the hydrogen/air delta pressure vs. time value. In step  4 , an alarm is typically activated when the monitored hydrogen/air delta pressure vs. time value exceeds the specification, thus indicating the presence of excessive quantities of hydrogen in the anode loop of the fuel cell and signaling the need to replace or repair the pressure control device  26 . In step  5 , the fuel cell may be induced to convert excess hydrogen into electrical current by increasing the current demand on the fuel cell stack. Additionally or alternatively, supply of hydrogen to the fuel cell stack may be periodically shut off. Additionally or alternatively, the operating pressure of the system may be increased. Consequently, the stress resulting from excessive pressure of the hydrogen in the anode loop of the fuel cell stack decreases, thereby preventing rupturing of the stack membrane and leakage of excess hydrogen from the anode loop to the cathode loop of the fuel cell stack. 
     It is to be understood that the invention is not limited to the exact construction and method which has been previously delineated, but that various changes and modifications may be made without departing from the spirit and scope of the invention as delineated in the following claims.