Abstract:
A fuel cell system is disclosed that employs a thermal sensor for measuring an amount of heat generated in the fuel cell system, wherein a sensor signal from the thermal sensor is used to adjust operation of the fuel cell system when the fuel cell system is operating outside of desired thermal operating conditions.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to a fuel cell system and, more particularly, to a method of operating a fuel cell system outside of desired thermal operating conditions. 
       BACKGROUND OF THE INVENTION 
       [0002]    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. 
         [0003]    A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte disposed therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is disassociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen 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. 
         [0004]    Many fuel cells are typically combined in a fuel cell stack to generate the desired power for the vehicle. The fuel cell stack receives a cathode input gas as a flow of air, typically forced through the stack by a compressor. Not all of the oxygen in the air 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. 
         [0005]    The components of the fuel cell system in the vehicle, such as a compressor motor and a compressor motor power inverter module, generate heat during operation of the fuel cell system. The heat energy must be removed from the fuel cell system to keep the internal temperatures of the components and fuel cell system within desired operating conditions to militate against damage to the components. Heat energy is typically removed from the fuel cell system by a coolant caused to flow through the fuel cell system by a recirculation pump. If the recirculation pump fails or the coolant leaks from the fuel cell system, the fuel cell system may overheat, shutdown, and cause the components to perform below an optimal level. Further, if the fuel cell system is shutdown or damaged, an operator of the vehicle incorporating the fuel cell system may not be able to operate the vehicle, resulting in a walk home incident 
         [0006]    It would be desirable to provide a method for operating a fuel cell system when the fuel cell system is operating outside of desired thermal operating conditions. 
       SUMMARY OF THE INVENTION 
       [0007]    Concordant and congruous with the present invention, a method for operating a fuel cell system when the fuel cell system is operating outside of desired thermal operating conditions, has surprisingly been discovered. 
         [0008]    In one embodiment, the fuel cell system comprises a fuel cell stack including at least a cathode inlet and a cathode outlet, wherein a fluid is caused to flow through the cathode inlet and the cathode outlet of said fuel cell stack; a compressor in fluid communication with said fuel cell stack; a first thermal sensor in communication with said compressor, said first thermal sensor adapted to generate a sensor signal indicative of a temperature of said compressor; and a control system adapted to receive the sensor signal from said first thermal sensor and cause a change in operation of said compressor to minimize an amount of heat generated by said compressor. 
         [0009]    In another embodiment, the fuel cell system comprises a fuel cell stack including at least a cathode inlet and a cathode outlet, wherein a fluid is caused to flow through the cathode inlet and the cathode outlet of said fuel cell stack; a compressor including a compressor power inverter and in fluid communication with said fuel cell stack; a first thermal sensor in communication with said compressor, said first thermal sensor adapted to generate a sensor signal indicative of a temperature of one of said compressor and the compressor power inverter; and a control system adapted to receive the sensor signal from said first thermal sensor and cause a change in operation of said compressor to minimize an amount of heat generated by said compressor. 
         [0010]    In another embodiment, the method of operating a fuel cell system comprises the steps of: providing a fuel cell stack including at least a cathode inlet and a cathode outlet; providing a compressor in fluid communication with the fuel cell stack; providing a first thermal sensor in fluid communication with the compressor, the sensor adapted to generate a sensor signal indicative of a temperature of the compressor; providing a control system adapted to receive the sensor signal from the first thermal sensor; causing a fluid comprising oxygen to flow through the compressor of the fuel cell system, wherein the sensor signal is indicative of a temperature of the compressor; and adjusting the operation of the compressor with the control system based on the sensor signal to minimize an amount of heat generated by the compressor. 
       BRIEF DESCRIPTION OF THE DRAWINGS 
       [0011]    The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawing in which a schematic flow diagram of a fuel cell system is shown according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0012]    The following detailed description and appended drawings describe and illustrate various exemplary embodiments of the invention. The description and drawings serve to enable one skilled in the art to make and use the invention, and are not intended to limit the scope of the invention in any manner. In respect of the methods disclosed, the steps presented are exemplary in nature, and thus, the order of the steps is not necessary or critical. 
         [0013]    The drawing shows a fuel cell system  10  according to an embodiment of the invention. The fuel cell system  10  includes a compressor  12  in fluid communication with a fuel cell stack  14 . 
         [0014]    In the embodiment shown, the compressor  12  is a centrifugal air compressor including a compressor motor power inverter  18 . An inlet  12   a  of the compressor  12  is in fluid communication with an oxygen source  20  through a conduit  38 . An outlet  12   b  of the compressor  12  is in fluid communication with an inlet  22   a  of a heat exchanger  22 . The compressor  12  may be any conventional means for compressing a fluid such as a turbomachine, a centrifugal compressor, a mixed flow compressor, a blower or a fan, for example. The oxygen source  20  is typically a source of air. It is understood that the oxygen source  20  may be a fuel tank or the atmosphere, for example. 
         [0015]    An outlet  22   b  of the heat exchanger  22  is in fluid communication with a first inlet  24   a  of a humidifier  24 . In the embodiment shown, the heat exchanger  22  is a low-temperature core. It is understood that any conventional heat exchanger may be used such as a shell and tube heat exchanger, a plate heat exchanger, an air-cooled heat exchanger, or other heat exchanger known in the art. 
         [0016]    The humidifier  24  is a water vapor transfer unit adapted to humidify an oxidant such as air prior to entering into the fuel cell stack  14 . The water vapor transfer unit includes a dry side and a wet side separated by a water vapor permeable membrane (not shown) or the like. The dry side has the first inlet  24   a  and a first outlet  24   b,  and the wet side has a second inlet  24   c  and a second outlet  24   d.  The first outlet  24   b  is in fluid communication with a cathode inlet  14   a  of the fuel cell stack  14 . The second inlet  24   c  is in fluid communication with a cathode outlet  14   b  of the fuel cell stack  14 , and the second outlet  24   d  is in fluid communication with an exhaust system  30  that is in fluid communication with the atmosphere. 
         [0017]    The fuel cell stack  14  includes a stack of cathodes, anodes, and membranes (fuel cells), as previously discussed herein. It is understood that the number of fuel cells in the fuel cell stack  14  may vary. Each fuel cell of the fuel cell stack  14  has a pair of MEAs (not shown) separated by an electrically conductive bipolar plate (not shown). The MEAs and bipolar plates are stacked together between clamping plates or end plates (not shown) and end contact elements (not shown). The end contact elements and bipolar plates contain a plurality of grooves or channels for distributing fuel and oxidant gases (i.e. hydrogen and oxygen) to the MEAs. 
         [0018]    The fuel cell stack  14  further includes the cathode inlet  14   a,  the cathode outlet  14   b,  an anode inlet  14   c,  an anode outlet  14   d,  and an anode purge outlet  14   e.  The anode inlet  14   c  is in fluid communication with a hydrogen source  28 . The anode outlet  14   d  is in fluid communication with the second inlet  24   c  of the humidifier  24 . The anode purge outlet  14   e  is in fluid communication with another exhaust system  30  which is in fluid communication with the atmosphere. The number of inlets and outlets in the fuel cell stack  14  may vary based on the size of the stack in use, an amount of outlet energy required from the stack  14 , and other design considerations. It is understood that the hydrogen source  28  may be a fuel tank or other system component, for example. It is also understood that the anode outlet  14   b  may be in fluid communication with the atmosphere, another fuel cell stack (not shown), or other system component, as desired. 
         [0019]    In the embodiment shown, the fuel cell system  10  includes a first thermal sensor  32  in communication with the compressor  12  and the compressor motor power inverter  18 . A second thermal sensor  34  is in communication with the first outlet  24   b  of the humidifier  24 . The first thermal sensor  32  is adapted to provide a signal indicative of the temperature of the compressor  12  and the compressor motor power inverter  18 . The second thermal sensor  34  is adapted to provide a signal indicative of the temperature of the fluid caused to flow from the humidifier  24  to the fuel cell stack  14 . The first thermal sensor  32  and second thermal sensor  34  are in electrical communication with a control system  36 . It is understood that the thermal sensors  32 ,  34  may be any conventional thermal sensor known in the art. It is also understood that the fuel cell system  10  may include additional thermal sensors in communication with the heat exchanger  22 , a fuel cell system pump, the humidifier  24 , and other system components, as desired. 
         [0020]    In use, the air including oxygen is caused to flow from the oxygen source  20  and through the fuel cell system  10 . The air is caused to flow through conduit  38  to the inlet  12   a  of the compressor  12 . In the compressor  12 , the volume of the air is reduced, thereby increasing the pressure and the temperature thereof. The air is then caused to flow through the conduit  38  to the inlet  22   a  of the heat exchanger  22 . In the heat exchanger  22 , the air is cooled to a desired temperature. The air from the outlet  22   b  of the heat exchanger  22  flows through the conduit  38  to the first inlet  24   a  of the humidifier  24  and through the dry side of the humidifier  24 . In the humidifier  24 , a fluid such as air having a higher moisture content than the air flowing through the dry side is caused to flow through the wet side. Moisture is transferred through the membrane to the air flowing through the dry side. The fluid in the wet side is caused to flow through the second outlet  24   d  of the humidifier  24 , through the exhaust system  30 , and to the atmosphere. The air in the dry side is caused to flow through the first outlet  24   b  to the cathode inlet  14   a  of the fuel cell stack  14 . 
         [0021]    Simultaneous to the air being caused to flow to the stack  14 , hydrogen gas is caused to flow from the hydrogen source  28  to the anode inlet  14   c  of the fuel cell stack  14 . In the fuel cell stack  14 , the oxygen in the air electrochemically reacts with the hydrogen to generate power to drive a vehicle or other system as is known in the art. A portion of unreacted hydrogen may be caused to flow out of the fuel cell stack  14  through the anode outlet  14   d,  and through the conduit  38  to the second inlet  24   c  of the humidifier  24 . Another portion of unreacted hydrogen may be caused to flow out of the fuel cell stack  14  through the anode purge outlet  14   e,  through the conduit  38 , through the exhaust system  30 , and to the atmosphere. Unreacted oxygen is caused to flow through the cathode outlet  14   b,  through the conduit  38 , to the second inlet  24   c  of the humidifier  24 . 
         [0022]    While the fuel cell system  10  is in operation, the compressor  12  and the compressor motor power inverter  18  generate heat. If the temperature is outside a desired range due to the amount of heat generated, the first thermal sensor  32  sends a sensor signal to the control system  36 . The control system  36  adjusts the operation of the compressor  12  and compressor motor power inverter  18 . A maximum allowable compressor power based on the thermal sensor  32  signal is calculated by the control system  36 . As the temperature in the fuel cell system  10  increases, the power generated by the compressor  12  and compressor motor power inverter  18  is minimized by the control system  36 , thereby minimizing the heat generated by the compressor  12  and compressor motor power inverter  18 . To minimize the power generated by the compressor  12  and the compressor motor power inverter  18 , the rotational speed of the compressor  18  is minimized, thereby causing a reduction in the flow of fluid through the compressor  12 . A reduction in the flow of fluid also minimizes the current produced by the fuel cell stack  14  and the power output of the fuel cell system  10 . The power reduction of the compressor  12  and compressor motor power inverter  18  is typically gradual, continuous, and proportional to the temperature state of the fuel cell system  10 . The fuel cell system  10  continues to operate at a minimized power output and reduced speed of the compressor  12 , until the fuel cell system  10  is within the desired temperature range or until the fuel cell system  10  can be serviced. 
         [0023]    Additionally, if a coolant flowing through the fuel cell system  10  leaks or if a coolant recirculation pump ceases operation, system components would generate heat and introduce the heat into the fuel cell system  10 . If the temperature is outside a desired range due to the amount of heat generated, the first thermal sensor  32  sends a sensor signal to the control system  36 . The control system  36  adjusts the operation of the compressor  12  and compressor motor power inverter  18 . A maximum allowable compressor power based on the temperature sensor  32  signal is calculated by the control system  36 . As the temperature in the fuel cell system  10  increases, the power generated by the compressor  12  and compressor motor power inverter  18  is minimized by the control system  36 , thereby minimizing the heat generated by the compressor  12  and compressor motor power inverter  18 . To minimize the power generated by the compressor  12  and the compressor motor power inverter  18 , the rotational speed of the compressor  18  is minimize, thereby causing a reduction in the flow of fluid through the compressor  12 . A reduction in the flow of fluid also minimizes the current produced by the fuel cell stack  14  and the power output of the fuel cell system  10 . The power reduction of the compressor  12  and compressor motor power inverter  18  is typically gradual, continuous, and proportional to the temperature state of the fuel cell system  10 . The fuel cell system  10  continues to operate at a minimized power output and reduced speed of the compressor  12  until the fuel cell system  10  is within the desired temperature range or until the fuel cell system  10  can be serviced. It is understood that the control system  36  may also adjust the operation of the heat exchanger  22 , the humidifier  24 , or other system component to minimize the heat generated and introduced to the fuel cell system  10 , as desired. 
         [0024]    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.