Abstract:
A method and apparatus for thermal control of air flow in a fuel cell system, capable of accurately controlling the temperature of the air stream entering the water vapor transfer unit, maintaining a desired temperature set-point, and minimizing the time required for the air stream to reach the optimum operating temperature.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to a method of operation of a fuel cell system. More particularly, this invention is directed to a method and apparatus for thermal control of cathode inlet airflow in a hydrogen fuel cell. 
       BACKGROUND OF THE INVENTION 
       [0002]    In most modern fuel cell systems, a compressor provides compressed air to a fuel cell stack, and a water vapor transfer unit humidifies the compressed air before it enters the fuel cell stack. The temperature of the air stream exiting the compressor, at the air mass flow rates required for fuel cell operation, are typically beyond the desirable thermal limit of the water vapor transfer unit. 
         [0003]    A control system typically employs a heat exchanger to keep the temperature of the air stream exiting the compressor below the thermal limit of the water vapor transfer unit. 
         [0004]    In such fuel cell systems coolant flow to the heat exchanger cannot be stopped. There are situations where it may be desirable to heat the air stream entering the cathode, such as starting the fuel cell, but the coolant entering the heat exchanger is at a lower temperature and cools the air. Therefore, the time required to reach a desired operating temperature is unnecessarily extended. Additionally, such fuel cell systems do not maintain active control of the air stream entering the water vapor transfer unit, and are not capable of maintaining a desired temperature. This prohibits the water vapor transfer unit from performing at an optimum level. 
         [0005]    It would be desirable to develop a method and apparatus for accurately controlling the temperature of the air stream entering the water vapor transfer unit, which would maintain a desired temperature and minimize the time required for the air stream to reach a desired operating temperature. 
       SUMMARY OF THE INVENTION 
       [0006]    According to the present invention a method and apparatus for controlling the temperature of the air stream entering the water vapor transfer unit in a fuel cell system has surprisingly been discovered. This method maintains a desired temperature of the air stream entering the water vapor transfer unit, and minimizes the time required for the air stream to reach an optimum operating temperature. 
         [0007]    In one embodiment, the method for thermal control of air flow in a fuel cell system includes the steps of: providing a system coolant loop fluidly connected to a heat exchanger, the heat exchanger fluidly connected to a fuel cell stack, and the system coolant loop containing a first fluid; determining an actual temperature of the air flow at a predetermined point in the fuel cell system; determining a desired temperature of the air flow at the predetermined point; and controlling the flow of at least a portion of the first fluid from the system coolant loop to the heat exchanger as a function of the actual temperature of the air flow and the desired temperature of the air flow at the predetermined point to achieve the desired temperature. 
         [0008]    In another embodiment, the method for thermal control of air flow in a fuel cell system includes the steps of: providing a fuel cell stack, a first fluid, a system coolant loop, a heat exchanger, at least one water vapor transfer unit, a first valve, a second valve, and a secondary cooling heat exchanger; determining an actual temperature of an air flow at an inlet of the water vapor transfer units; determining a desired temperature of the air flow as a function of the temperature at which the water vapor transfer units operate with maximum efficiency; and controlling the flow of at least a portion of the first fluid from the system coolant loop to the heat exchanger with the first valve and the second valve to achieve the desired temperature, wherein the first valve when open directs a portion of the first fluid from the system coolant loop into the heat exchanger, and the second valve when open directs the coolant from the system coolant loop into the secondary cooling heat exchanger before the coolant enters the heat exchange. 
         [0009]    In another embodiment, the apparatus for thermal control of air flow in a fuel cell system includes an air compressor fluidly connected to a cathode side of a fuel cell stack via a first conduit, a heat exchanger disposed in the first conduit between the air compressor and cathode side of the fuel cell stack, at least one water vapor transfer unit disposed in the first conduit between the heat exchanger and the cathode side of the fuel cell stack, a temperature sensor disposed in the conduit between the heat exchanger and water vapor transfer units, a valve array fluidly connected to the heat exchanger via a second conduit, and a system coolant loop via a third conduit, a secondary cooling heat exchanger fluidly connected to the valve array via a fourth conduit and the heat exchanger via the second conduit, a control system in electrical communication with the valve array via a first connection, and in electrical communication with the temperature sensor via a second connection, wherein the control system is responsive to an electrical signal received from the temperature sensor and selectively influences the valve array to direct a first fluid from the system coolant loop into at least one of the heat exchanger and secondary cooling heat exchanger to achieve a desired temperature of air leaving the conduit and entering the water vapor transfer units. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0010]    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 drawings in which: 
           [0011]      FIG. 1  is a schematic diagram of a fuel cell system according to an embodiment of the invention; 
           [0012]      FIG. 2  is a graph showing a plot of valve commands as a percentage of the total duty cycle for the fuel cell system illustrated in  FIG. 1 ; 
           [0013]      FIG. 3  is a graph showing a plot of the desired cathode air set-point of the fuel cell system illustrated in  FIG. 1  compared to the actual temperature control in degrees Celsius versus time; and 
           [0014]      FIG. 4 . is a fragmentary perspective view of a water vapor transfer unit and shows associated components schematically according to the embodiment of the invention shown in  FIG. 1 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0015]    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. 
         [0016]    Referring now to  FIG. 1 , a basic layout of a fuel cell system with associated components is shown; in practice many variants are possible. A schematic representation of a fuel cell stack  10  integrated into a fuel cell system and consisting of a plurality of individual fuel cells which are connected electrically in series is shown. It is further understood that the individual fuel cells can be connected electrically in parallel without departing from the scope of this invention. The anode sides of all individual fuel cells of the fuel cell stack  10  are connected together in a manner commonly known in the art, with the resulting anode side of the stack being designated with the reference numeral  12 . In a similar manner, the cathode sides of the fuel cells of the fuel cell stack  10  are connected together in a manner commonly known in the art with the resulting cathode side of the stack being designated with the reference numeral  14 . The operation of various types of fuel cell systems are commonly known in the art; one embodiment can be found in commonly owned U.S. Pat. No. 6,849,352, hereby incorporated herein by reference in its entirety. Therefore, only the operation of a fuel cell system as pertinent to this invention will be explained in the description. 
         [0017]    In the exemplary embodiment described herein, the fuel cell system includes a control system  16 . The control system  16  is electrically linked via a connection  18  to a motor  20 . The connection  18  may be any conventional means of electrical communication. The motor  20  is coupled with a compressor  22 . The compressor  22  is in fluid communication with a cathode inlet  24  of the fuel cell stack  10  via an air supply conduit  26 . The conduit  26  can be any conventional conduit providing a sealed passageway. 
         [0018]    A humidifier  28  is disposed in the conduit  26  between the compressor  22  and the cathode inlet  24 . The humidifier  28  includes at least one water vapor transfer unit  23 , and an inlet temperature sensor  25 . The inlet temperature sensor  25  is in electrical communication with the control system  16  via a connection  27 . The water vapor transfer unit  23  is in fluid communication with a cathode exhaust  15 , and a system exhaust, as shown in  FIG. 4 . 
         [0019]    A heat exchanger  30  is disposed in the conduit  26  between the compressor  22  and the humidifier  28 . The heat exchanger  30  is in fluid communication with a valve array  35  via a conduit  32 . The valve array  35  includes a bypass valve  36  and a cooling valve  38 . The bypass valve  36  is in fluid communication with a fuel cell system coolant loop  58 , and the cooling valve  38  via a conduit  40 . An exhaust conduit  31  fluidly connects the heat exchanger  30  and the fuel cell system coolant loop  58 . 
         [0020]    The control system  16  is in electrical communication with the bypass valve  36  via a connection  56 , and is in electrical communication with the cooling valve  38  via a connection  54 . 
         [0021]    The cooling valve  38  is in fluid communication with a secondary cooling heat exchanger  51  via a conduit  50 . The secondary cooling heat exchanger  51  is in fluid communication with the heat exchanger  30  via a conduit  52 . The secondary cooling heat exchanger  51  can be any properly sized cooling heat exchanger, such as a wheel house radiator. 
         [0022]    In operation, air is supplied to the compressor  22  via a conduit  42 . The compressor  22  is driven by the motor  20 . The air compressed by the compressor  22  is supplied via the conduit  26  through the heat exchanger  30  and the humidifier  28  to the cathode inlet  24  of the fuel cell stack  10 . 
         [0023]    A temperature of the compressed air exiting the compressor  22  is typically above a desired temperature for efficient operation of the water vapor transfer unit  23 . At flow rates normally necessary for fuel cell operation, air leaving the. compressor  22  is typically about  120  degrees Celsius. The acceptable thermal limit of the water vapor transfer unit  23  in the embodiment shown is approximately 90 degrees Celsius. 
         [0024]    The heat exchanger  30  influences the temperature of the air traveling through the heat exchanger  30  in a manner commonly known in the art. Coolant (not shown) enters the heat exchanger  30  via the conduit  32 . The temperature and flow of coolant cycling through the heat exchanger  30  is controlled to reach the desired temperature of the air. Typically, during normal operation when the ambient temperature is approximately 20 degrees Celsius, the coolant is maintained at about 60 to 80 degrees Celsius. 
         [0025]    The water vapor transfer unit  23  humidifies the air prior to delivery to the fuel cell stack  10 , by transferring moisture from a portion of air in the cathode exhaust  15  to the air stream in the conduit  26 . It is desirable to control the humidity of the air to optimize the operation of the fuel cell stack  10 . Additionally, it is desirable to maintain an air temperature entering the humidifier  28  and the water vapor transfer unit  23  in order to aid in the humidification process. The temperature of the air entering the humidifier  28  is measured by the inlet temperature sensor  25 , and communicated to the control system  16  via the connection  27 . 
         [0026]    In the embodiment shown herein, the valve array  35 , including the valves  36 , 38 , maintains a desired air temperature entering the humidifier  28  and the water vapor transfer unit  23 . A portion of the coolant that is used to cool the entire fuel cell system enters the valve array  35  from the fuel cell system coolant loop  58 . The control system  16  positions the bypass valve  36  via the connection  56 , and positions the cooling valve  38  via the connection  54 . The bypass valve  36  selectively causes the coolant to bypass the secondary cooling heat exchanger  51  and to enter the heat exchanger  30 . The cooling valve  38  selectively causes the coolant to flow to the secondary cooling heat exchanger  51  via the conduit  50 , before the coolant enters the heat exchanger  30  via the conduit  52 . The coolant is returned to the fuel cell system coolant loop  58  via the exhaust conduit  31  after traversing the heat exchanger  30 . A desired temperature of the air stream entering the humidifier  28  is maintained by balancing the coolant flow through the valves  36 ,  38  with the controller  16 . It is further understood that the valve array  35  can contain different quantities and arrangements of valves without departing from the scope of this invention. 
         [0027]    In this embodiment, it is first necessary to select how long the bypass valve  36  and the cooling valve  38  are open, similar to a slow duty cycle approach, in order to balance the coolant flow through the valves  36 , 38  and maintain a desired temperature of the air stream entering the humidifier  28 . For example if the maximum valve open time is four seconds, a 75% duty cycle on the valve is equivalent to three seconds open and one second closed. For the purpose of explaining the current invention it is assumed a maximum open time has been chosen and the controls will only be discussed in terms of duty cycles. 
         [0028]    In  FIG. 2 , an extreme limit of valve command method  70  is shown, wherein the system controller  16  does not maintain a constant coolant flow through the heat exchanger  30 . It may be desirable not to maintain a constant coolant flow in some situations such as starting the fuel cell stack  10  from a cold condition. 
         [0029]    For example, as shown in Table 1, if the temperature of the air entering the heat exchanger  30  is higher than the temperature of the coolant in the system coolant loop  58  the control system  16  implements Max Heating (1) in Table 1. A 0% duty cycle for the bypass valve  38 , and a 0% duty cycle for the cooling valve  36  restrict the flow of coolant from the fuel cell system coolant loop  58  to the heat exchanger  30 , allowing the warmer air to enter the humidifier  28  without being cooled by the lower temperature coolant. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 PID Control 
                 Bypass 
                 Cooling 
               
               
                   
                 Output 
                 Valve % 
                 Valve % 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Max Heating (1)  
                 0% 
                 0% 
                 0% 
               
               
                   
                   
                 8% 
                 25% 
                 0% 
               
               
                   
                   
                 17% 
                 50% 
                 0% 
               
               
                   
                   
                 25% 
                 100% 
                 0% 
               
               
                   
                 Max Heating (2) 
                 33% 
                 100% 
                 0% 
               
               
                   
                   
                 42% 
                 100% 
                 25% 
               
               
                   
                   
                 50% 
                 100% 
                 50% 
               
               
                   
                   
                 58% 
                 100% 
                 75% 
               
               
                   
                   
                 67% 
                 100% 
                 100% 
               
               
                   
                   
                 75% 
                 75% 
                 100% 
               
               
                   
                   
                 83% 
                 50% 
                 100% 
               
               
                   
                   
                 92% 
                 25% 
                 100% 
               
               
                   
                 Max Cooling 
                 100% 
                 0% 
                 100% 
               
               
                   
                   
               
             
          
         
       
     
         [0030]    It may be further desirable for the control system  16  to implement Max Heating (1) in Table 1, to improve efficiency and fuel consumption when the fuel cell system is operating at low power or idling. The temperature of the air compressed by the compressor  22  typically does not increase substantially over the ambient temperature, and remains lower than the temperature of the coolant in the heat exchanger  30  when the fuel cell system is operating at low power or idling. Typically, the fuel cell system is required to use extra fuel to maintain fuel cell stack  10  temperature during idling and low power operation, because the cooler air entering the heat exchanger  30  decreases the temperature of the coolant in the fuel cell system coolant loop  58 . Implementing Max Heating (1) restricts the flow of coolant from the fuel cell system coolant loop  58  to the heat exchanger  30 , allowing the cooler air to enter the humidifier  28  without reducing the temperature of the coolant in the fuel cell system coolant loop  58 . The temperature of the air in the conduit  26  will be increased when the moisture from the gas in the cathode exhaust  15  is transferred to the air in the water vapor transfer unit  23 . 
         [0031]    If the temperature of the coolant within the system coolant loop  58  is greater than the temperature of the air entering the heat exchanger  30  then the control system  16  controls the valve array  35  using Max Heating (2) in Table 1. A 100% duty cycle for the bypass valve  38 , and a 0% duty cycle for the cooling valve  36 , allows the coolant from the system coolant loop  58  to enter the heat exchanger  30  without passing through the secondary cooling heat exchanger  51 , and thus bring the temperature of the air up before it enters the humidifier  28 .  FIG. 2  illustrates the valve duty cycles for the extreme limit of valve command method  70 . 
         [0032]    In alternative embodiments the extreme limit of valve command method  70  of controlling the valve array  35  may be implemented using a proportional-integral-derivative controller, commonly referred to as a PID controller (not shown). Table 1, shows the conversion of the control output of the PID controller to the duty cycle of the bypass valve  36  and the cooling valve  38  that the control system  16  implements. The PID controller output is a saturation type control between Max Heating (1) and Max Cooling. When the coolant temperature of the system coolant loop  58  is greater than the air temperature out of the compressor  22 , the PID controller begins with Max Heating (2). 
         [0033]      FIG. 3  shows the results of the extreme limit of valve command control methodology when a fuel cell system current density  80  is changed randomly from 0.5 A/cm 2  to 1 A/cm 2 .  FIG. 3  demonstrates that a desired cathode air temperature  84  closely tracks a temperature control  82 , and the extreme limit of valve command method  70  produces desirable results. 
         [0034]      FIG. 2  also shows a graph of the cooling valve  38  and the bypass valve  36  duty cycles for an alternative control method, a synchronous actuation control method  72 , wherein the control system  16  maintains a constant coolant flow through the heat exchanger  30 . Synchronous actuation of the cooling valve  38  and the bypass valve  36  is desirable in order to maintain a constant flow of coolant through the heat exchanger  30 . 
         [0035]    For example, if the desired control is a  25 % duty cycle for the bypass valve  36  and a 75% duty cycle for the cooling valve  38 , then the bypass valve  36  is only open for the first 25% of the total duty cycle, and the cooling valve  38  is only open for the remaining 75% of the total duty cycle in order to ensure there is a constant flow of coolant to the heat exchanger  30 . 
         [0036]    The synchronous actuation control method  72  is application dependent, and may be desirable if the heat exchanger  30  requires constant coolant flow for optimal effectiveness. 
         [0037]    From the foregoing description, one ordinarily skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, make various changes and modifications to the invention to adapt it to various usages and conditions.