Abstract:
A method for managing fuel cell power increases in a fuel cell system using an air flow feedback delay. The method comprises the steps of determining a required air mass flow rate at a predetermined point in the fuel cell system, determining an actual air mass flow at a predetermined point in the fuel cell system, calculating an air flow feedback delay as a function of the required air mass flow rate and the actual air mass flow, and delaying an external circuit from increasing current draw from the fuel cell stack by the magnitude of the air flow feedback delay.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 11/536,914 filed on Sep. 29, 2006. The entire disclosure of the above application is hereby incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a method of operation for a fuel cell system. More particularly, the invention is directed to managing power increases in a hydrogen fuel cell system using an air mass flow feedback delay. 
     BACKGROUND OF THE INVENTION 
     In most modern fuel cell systems, a compressor provides compressed air to the fuel cell stack. Having sufficient air for the fuel cell reaction is extremely important and is characterized as “Cathode Stoichiometry” wherein a higher value (e.g. 5) is typically needed at low current densities and a lower value (e.g. 1.8) is typical at high current densities. In such systems it is necessary to have a means for sensing the air mass flow rate leaving the compressor and entering the fuel cell stack, such as an air mass flow sensor. 
     A control system will typically take this flow information and change the speed of the compressor along with the position of a back pressure valve to achieve a desired air mass flow and gas pressure entering the fuel cell stack. The desired air mass flow and gas pressure are generally calculated using known factors such as the fuel cell stack current, number of cells in the fuel cell stack, and the desired cathode stoichiometry at that stack current. 
     In such fuel cell systems the control system typically allows an external circuit to draw current out of the fuel cell system immediately upon detection of the desired air mass flow rate by the air mass flow sensor. The volume and distance between the location where the air mass flow sensor is taking the measurement and the location where the air is required at the reaction site of the fuel cell stack are not taken into account. Therefore, the current is drawn out of the fuel cell stack before the desired air mass flow is actually present at the reaction site. The lack of air at the reaction site can cause the cathode stoichiometry at the reaction site to drop enormously, and lead to significant voltage drops in cells that are sensitive to low cathode stoichiometries. The lowered cell voltages can at least cause the power management circuit to limit power output and could reverse (i.e. negative voltage) causing massive degradation. The lack of air is particularly harmful on current draw up-transients. The prior art systems do not take into account the distance and volume between where the air mass flow meter is taking the measurement and where the air-H 2  reaction actually takes place. 
     It would be desirable to develop a method of managing fuel cell power increases which would account for the volume and distance between the air mass flow sensor and the reaction site insuring the required air mass flow rate had reached the reaction site before the current is drawn from the fuel cell stack. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a method of managing fuel cell power increases which would account for the volume and distance between the air mass flow sensor and the reaction site insuring the required air mass flow rate had reached the reaction site before the current is drawn from the fuel cell stack, has surprisingly been discovered. This method ensures that throughout an up-transient, the cathode stoichiometric requirement is always met at the site of the reaction. By ensuring this, stack stability is improved by preventing any one cell that is cathode stoichiometrically sensitive from losing voltage as a result of not having sufficient air. A secondary, but equally important, effect is the prevention of cathode starvation that leads to accelerated voltage degradation. 
     In one embodiment, the method for managing fuel cell power increases using air flow feedback delay comprises the steps of determining gas flow effecting characteristics of the fuel cell system between the compressor and the cathode outlet; determining an air mass flow rate between the compressor and the cathode outlet; determining a gas pressure of the fuel cell system between the compressor and the cathode outlet; calculating the air flow feedback delay as a function of said gas flow effecting characteristics, said air mass flow rate, and said gas pressure; and delaying the external circuit from drawing current out of the fuel cell stack by the magnitude of the air flow feedback delay. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a schematic diagram of a fuel cell system according to an embodiment of the invention; and 
         FIG. 2  is a graph with time on the x-axis and showing actual air mass flow on a cathode side of the fuel cell system illustrated in  FIG. 1  compared to a required air mass flow on the cathode side based on a current drawn by an external circuit. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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. 
     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 and/or in parallel is shown. 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 similar manner the cathode sides of all fuel cells of the stack 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  14 . The operations 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. Therefore, only the operation of a fuel cell system as pertinent to this invention will be explained in the description. 
     In the exemplary embodiment described herein, the fuel cell system includes a control system  16 . The control system  16  is connected via a line  18  to a motor  20 . 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 line  26 . The line  26  is a sealed passageway having known gas flow effecting characteristics such as static volume, distance, internal roughness, laminar and/or turbulent flow effects. 
     An air mass flow sensor  30  is disposed in the line  26  between the compressor  22  and a humidifier  28 . The air mass flow sensor  30  is linked to the control system  16  via a line  32 . The air mass flow sensor  30  is an electromechanical device having known gas flow effecting characteristics such as roughness, laminar and/or turbulent flow effects. 
     A temperature sensor  31  is connected to the line  26  between the compressor  22  and the cathode outlet  48  of the fuel cell stack  10 . The temperature sensor  31  is linked to the control system  16  via a line  52 . The temperature sensor  31  can be an electrical or electromechanical device having a known gas flow effecting characteristics such as roughness, laminar and/or turbulent flow effects. 
     The humidifier  28  is disposed in the line  26  between the air mass flow sensor  30  and the cathode inlet  24 . The humidifier unit  28  is composed of a plurality of individual components all having known gas flow effecting characteristics such as roughness, laminar and/or turbulent flow effects. 
     Additionally, other components may be disposed in or connected to the line  26  between the air mass flow sensor  30  and the cathode inlet  24  in other embodiments. 
     The cathode side  14  of the fuel cell stack  10  comprises a plurality of cathodes of individual fuel cells connected in a manner commonly known in the art. Each individual fuel cell has a plurality of channels between the cathode inlet  24  and the cathode outlet  48  all having known gas flow effecting characteristics such as static volume, distance, internal roughness, laminar and/or turbulent flow effects. 
     A back pressure valve  29  is connected to the cathode outlet  48  of the fuel cell stack. It may also be desirable for the back pressure valve  29  to be connected to the line  26  between the compressor  22  and cathode side  14  of the fuel cell stack  10 . The back pressure valve  29  is linked to the control system  16  via a line  54 . The back pressure valve  29  is an electromechanical device having known gas flow effecting characteristics such as roughness, laminar and I or turbulent flow effects. 
     A gas pressure sensor  33  is connected to the cathode outlet  48  of the fuel cell stack  10 . It may also be desirable for the gas pressure sensor  33  to be connected to the line  26  between the compressor  22  and cathode side  14  of the fuel cell stack  10 . The gas pressure sensor  33  is linked to the control system  16  via a line  50 . The gas pressure sensor  30  is an electromechanical device having known gas flow effecting characteristics such as roughness, laminar and/or turbulent flow effects. 
     An external circuit  34  is electrically linked to the cathode side  14  of the fuel cell stack  10  via a line  36  and electrically linked to the anode side  12  of the fuel cell stack  10  via a line  38 . The external circuit  34  is linked to the control system  16  via a line  40 , 
     In operation, air is pulled in via a line  42  and compressed by the compressor  22  driven by the motor  20  and is supplied via the line  26  through the cathode inlet  24  of the fuel cell stack  10  to the cathode outlet  48 . The amount of time required for the air to reach the cathode inlet  24  of the fuel cell stack  10  is influenced by the gas flow effecting characteristics of the line  26  such as the static volume, distance, internal roughness, laminar and/or turbulent flow effects of the line  26 . The amount of time required for the air to reach the cathode inlet  24  of the fuel cell stack  20  is also further influenced by the gas flow effecting characteristics of the components disposed in and connected to the line  26  such as roughness, laminar and/or turbulent flow effects of including but not limited to the air mass flow sensor  30 , the gas temperature sensor  31 , and the humidifier  28 . The time required for the air to travel from the cathode inlet  24  to the cathode outlet  48  is influenced by gas flow effecting characteristics of the static volume, distance, internal roughness, laminar and/or turbulent flow effects of the plurality of channels on the cathode side  14  of the fuel cell stack  10 . 
     The air mass flow can be measured by the air mass flow sensor  30  and communicated to the control system  16  via the line  32 . 
     The gas temperature can be measured by the gas temperature sensor  31  and communicated to the control system  16  via the line  52 . 
     The gas pressure is measured by the gas pressure sensor  33  and communicated to the control system  16  via the line  50 . 
     The control system  16  can influence the speed of rotation of the air compressor  22  by controlling the motor  20  via the line  18  and thus the air mass flow delivered by the air compressor  22 . The control system can further influence the position of the back pressure valve  29  via the line  54  and thus the gas pressure in the cathode side  14  of the fuel cell system. By influencing the air mass flow delivered and the gas pressure on the cathode side  14  of the fuel cell system the control system  16  can achieve a desired air mass flow and pressure in the cathode side  14  of the fuel cell system. The desired air mass flow and pressure in the cathode side  14  of the fuel cell system are calculated using known variables such as the stack current, number of cells, and desired cathode stoichiometry at that stack current. 
     Hydrogen gas is delivered to the anode side  12  in a manner commonly known in the art via a line  44 . A reaction known per se in the art occurs between the air in the cathode side  14  and the hydrogen in the anode side  12  of the fuel cell stack  10  that releases electrons which can be drawn by the external circuit  34  via the line  38 . 
     The pressure and air mass flow rate of the gas into the cathode side  14  of the fuel cell stack  10  influence the rate of the electron releasing reaction between the air in the cathode side  14  and the hydrogen in the anode side  12  thus influencing the voltage and current available to be drawn from the fuel cell stack  10  by the external circuit  34 . 
     The control system  16  will calculate a feedback delay  46  ( FIG. 2 ), taking air mass flow feedback received via the line  32 , gas pressure feedback received via the line  50 , gas temperature feedback received via the line  52 , and the known influence of the static volume and distance of the line  26  on air flow, and the known influence on air flow of the static volume and distance of the plurality of channels between the cathode inlet  24  and the cathode outlet  48  of the fuel cell stack  10 . 
     Furthermore, the control system may use supplemental factors in calculating the feedback delay. Additional gas flow effecting characteristics of the line  26  and the plurality of channels between the cathode inlet  24  and the cathode outlet  48  such as the internal roughness, geometry, laminar and/or turbulent flow effects on the gas can be used as factors in calculating the feedback delay. The gas flow effecting characteristics of components disposed in or connected to the line  26  such as the air mass flow sensor, gas temperature sensor, back pressure valve, and humidifier may also be used as inputs in calculating the feedback delay  46 . 
     The feedback delay  46  is an amount of time that the control system  16  will delay the external circuit  34  from drawing current out of the fuel cell stack  10 . The control system  16  can influence the external circuit  34  via the line  40  to draw current from the fuel cell stack  10  when a desired air mass flow rate is achieved after the feedback delay  46 . The feedback delay calculation is in real time so that the control system instantaneously adjusts the current draw. 
     The feedback delay  46  is implemented in order to compensate for the distance and volume between the air mass flow sensor  30  and the cathode outlet  48  of the fuel cell stack  10  and to ensure that the desired air mass flow is actually present at the cathode outlet  48  of the fuel cell stack  10  when the external circuit  34  draws current from the fuel cell stack  10 . The feedback delay  46  may also compensate for the air flow restricting characteristics of the components disposed in or connected to the line  26 . 
     Without departing from the scope of this invention the control system  16  also, or additionally can use the gas pressure signal on the line  50  and the gas temperature signal on the line  52  as inputs in determining the delay  46 . The control system  16  can further factor laminar and/or turbulent flow effects on the air mass and take into account the internal roughness of each component between the compressor  22  and the cathode outlet  48  without departing from the scope of this invention. 
       FIG. 2  illustrates the actual air mass flow at the cathode side  14  of the fuel cell stack  10  in comparison to the required air mass flow at the cathode side  14  based on the stack current actually being drawn by the external circuit  34 . A value in amperes (y-axis) of the stack current being drawn during the up-transient versus time (x-axis) is indicated by a line  56 . A value of the actual air flow at the reaction site without delay (y-axis) versus time is indicated by a line  58 . A value of the required air mass flow at the reaction site based upon the stack current (y-axis) is indicated by a line  60 . The delay  46  will ensure sufficient air at the cathode side  14  of the fuel cell stack  10 . 
     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.