Abstract:
A method of operating a fuel cell includes the step of selectively connecting and disconnecting the fuel cell to at least one electrical load dependent at least in part upon at least one of a fuel cell voltage, a fuel cell current and a fuel cell temperature.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to fuel cells.  
       BACKGROUND OF THE INVENTION  
       [0002]     Automobiles emit hydrocarbons, nitrogen oxides, carbon monoxide and carbon dioxide as a result of the combustion process. Automobile emissions are said to be a significant contributor to pollution. In order to reduce and/or eliminate such emissions automobile manufacturers have attempted to utilize alternative transportation fuels and/or alternative sources of power, such as, for example, fuel cells. Generally, fuel cells generate electricity by electrochemically combining across an ion-conducting electrolyte a fuel, such as hydrogen, carbon monoxide, or a hydrocarbon, and an oxidant, such as air or oxygen.  
         [0003]     A fuel cell system typically includes a “stack” of individual fuel cells that are electrically interconnected in a series configuration. Thus, the number of cells in the stack, i.e., the number of cells connected together in series, determines the voltage that is produced by the stack. Each of the individual fuel cells within the stack produces a voltage that varies dependent at least in part upon the current being drawn from that cell and/or the stack. The voltage produced by a typical single cell varies from an open circuit voltage, such as, for example, approximately 1.0 Volts (V) at low or zero current loads to a lower limit, such as, for example, approximately 0.7 V, under high current loads. If the voltage produced by a cell drops below a minimum threshold, such as, for example, 0.6 V, an undervoltage condition exists that may result in damage to the cell, such as, for example, cell oxidation.  
         [0004]     Since the voltage produced by each cell varies dependent at least in part upon the current load upon the cell, the voltage produced by the stack also varies dependent at least in part upon the current load. More particularly, due to the series interconnection of the cells in the stack, the variation in the voltage produced by the cells is cumulative, i.e., the stack voltage will vary in a manner that reflects the sum of the voltage variations of the individual cells within the stack. This cumulative effect on the stack voltage can be relatively substantial. For example, the voltage produced by a fuel cell having sixty cells may vary from approximately sixty volts to approximately forty-two volts.  
         [0005]     Most electrical systems are designed to operate with a supply voltage that falls within a predetermined range. As described above, the voltage produced by a fuel cell stack may vary substantially. Thus, if a fuel cell system is to be used as a power source for such an electrical system the stack voltage must typically be regulated by a voltage regulating device or devices to ensure the stack voltage supplied to the electrical system remains within the voltage range required by the electrical system, independent of the voltage produced by the stack. As the amount of variation in the voltage produced by the stack increases a correspondingly greater amount of regulation is required in order to provide a supply voltage to the electrical system that is within the specified range. In order to provide adequate regulation of such a widely-varying voltage, voltage regulation or control devices that are relatively complex, costly, sizeable, and power consuming are required.  
         [0006]     Therefore, what is needed in the art is a fuel cell system that substantially reduces damage and/or oxidation to the cells, such as, for example, due to an under voltage condition.  
         [0007]     Furthermore, what is needed in the art is a method and apparatus that controls the output voltage of a fuel cell system while also controlling the operation of the fuel cell such that the fuel cell operates with improved efficiency relative to unregulated operation.  
         [0008]     Moreover, what is needed in the art is a fuel cell system that generates a controlled and/or regulated output voltage.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention provides a method and apparatus for controlling the operation of a fuel cell system.  
         [0010]     The present invention comprises, in one form thereof, the step of selectively connecting and disconnecting the fuel cell to at least one electrical load dependent at least in part upon at least one of a fuel cell voltage, a fuel cell current and a fuel cell temperature. The invention further comprises, in one form thereof, a fuel cell unit having a fuel cell stack producing a fuel cell voltage and a fuel cell current. A power conditioner electrically connected to the fuel cell unit includes a power switching device. The power switching device selectively connects and disconnects the fuel cell voltage to at least one load dependent at least in part upon an operating temperature of the fuel cell stack, the fuel cell voltage, and the fuel cell current to thereby produce an output voltage.  
         [0011]     An advantage of the present invention is that the potential of damage and/or oxidation of the cells, such as, for example, due to an under voltage condition, is substantially reduced.  
         [0012]     Another advantage of the present invention is the output voltage of the fuel cell system is controlled while the operation of the fuel cell is also controlled such that the fuel cell operates with improved efficiency relative to unregulated operation.  
         [0013]     A further advantage of the present invention is the output voltage generated is substantially controlled and/or regulated. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become apparent and be more completely understood by reference to the following description of one embodiment of the invention when read in conjunction with the accompanying drawings, wherein:  
         [0015]      FIG. 1  is a schematic block diagram of one embodiment of a fuel cell system of the present invention; and  
         [0016]      FIG. 2  is a schematic diagram of the power converter of  FIG. 1 .  
     
    
       [0017]     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner.  
       DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]     Referring now to the drawings, and particularly to  FIG. 1 , there is shown one embodiment of a fuel cell system of the present invention. Fuel cell system  10  includes fuel cell unit  12 , power conditioner  14  and fuel cell controller  16 .  
         [0019]     Fuel cell unit  12  includes a conventional fuel cell stack  18  constructed of a plurality of individual fuel cells (not shown), such as, for example, solid oxide fuel cells (SOFC), that are electrically interconnected in series. Fuel cell unit  12  also includes associated components, such as, for example, at least one reformer, waste energy recovery system and conduits interconnecting the components to each other and with supplies of fuel and/or air, etc (none of which are shown). Fuel cell unit  12  generates a substantially unregulated output voltage V STACK  and output current I STACK .  
         [0020]     Power conditioner  14 , in general, controls and/or conditions the voltage produced by fuel cell unit  12  to remain within a desired or predetermined voltage, and ensures fuel cell unit  12  is operated in a relatively efficient manner in each of the several operating modes thereof. Power conditioner  14  includes power converter circuitry  22 , gate drive circuitry  24 , control logic  26 , and mode controller  28 .  
         [0021]     Converter circuitry  22  is electrically interconnected with fuel cell unit  12 , and receives therefrom V STACK  and I STACK  Converter circuitry  22  is also electrically interconnected with external load  32 , such as, for example, one or more forty-two Volt loads. Converter circuitry  22  is further electrically connected, via DC/DC converter  34 , to external load  36 , such as, for example, one or more twelve Volt loads. Generally, converter circuitry  22  supplies voltage V OUT  to external load  32  and DC/DC converter  34 . A schematic of an exemplary converter circuit  22  is shown in  FIG. 2 .  
         [0022]     Converter circuitry  22  includes at least one power switching device  42  ( FIG. 2 ), such as, for example, one or more power metal oxide semiconductor field effect transistors (MOSFETs), integrated gate bipolar transistors (IGBTs) or other suitable power switching devices. Power switching device  42  is electrically connected between fuel cell unit  12  and each of load  32  and DC/DC converter  34 . Generally, power switching device  42  controls the current flowing from fuel cell unit  12  to load  32  and to DC/DC converter  34 . Power switching device  42  is operated in one of three modes dependent at least in part upon the signal applied to control terminal  42   a , such as, for example, the gate, thereof. In a current blocking mode, such as, for example, an open-circuit mode, power switching device  42  disallows substantially all current flow from fuel cell unit  12 , thereby enabling fuel cell unit  12  to operate in a substantially unloaded condition. In another or a first mode of operation, such as, for example, a pulse-width modulated mode, power switching device  42  is operated in such a manner that the value of V OUT  is maintained within a predetermined voltage range. In yet another or second mode of operation, such as, for example, a linear mode, power switching device  42  is operated as a series pass through device to thereby maintain the value of V OUT  within a predetermined voltage range.  
         [0023]     Converter circuitry  22  may be configured as a single power switching device  42  interconnected between fuel cell unit  12  and issuing V OUT  to external loads. Preferably, however, converter circuitry  22  is configured as a conventional linear regulator integrated circuit, such as, for example, model number 1802 manufactured by Unitrode Corporation of Merrimack, N.H., model number MC78BC30 manufactured by ON Semiconductor Corporation of Phoenix, Ariz., or model number LM1723 manufactured from ON Semiconductor Corporation, that integrates onto a single chip/integrated circuit the unreferenced components, such as the diodes, capacitors, inductors, etc., shown in  FIG. 2 .  
         [0024]     Gate drive circuitry  24 , in general, interfaces control logic  26  with power converter circuitry  22  thereby enabling signals from control logic  26  to drive power converter circuitry  22 . More particularly, drive circuitry  24  is electrically connected to and receives control signal  50  from control logic circuitry  26 , and is electrically connected and issues drive signal  52  to power converter circuitry  22 . Drive signal  52  is dependent at least in part upon control signal  50 . Drive signal  52  is electrically connected to and received by control terminal  42   a  of power switching device  42 . Thus, the mode in which power switching device  42  is operating is dependent at least in part upon drive signal  52 . Gate drive circuitry  24  is configured as a conventional gate drive circuit, such as, for example, model numbers IR2110 or IR2125 manufactured by International Rectifier Corporation of El Segundo, California.  
         [0025]     Control logic  26  is electrically connected to gate drive circuit  24  and to mode controller  28 . Control logic  26  issues control signal  50  to drive circuitry  24 , and receives converter mode signal  54  from mode controller  28 . Control logic  26  also receives current signal  62  and voltage error signal  64 . Current signal  62  is indicative of the current being supplied by fuel cell unit  12 , i.e., I STACK , and voltage error signal  64  is indicative of the difference between V OUT  and a reference voltage V REF , as determined by, for example, a comparator (not referenced). Dependent at least in part upon converter mode signal  54 , current signal  62  and voltage error signal  64 , control logic circuitry  26  issues control signal  50  to drive circuitry  24 . Control logic circuitry  26  is configured as a conventional pulse-width modulation switching and control logic circuit, such as, for example, model numbers 1802 or 1526A manufactured by Unitrode Corporation of Merrimack, N.H.  
         [0026]     Mode controller  28 , as is described more particularly hereinafter, determines the mode in which power conditioner  14  and fuel cell unit  12  operate in order to maintain efficient operation and/or increase the efficiency thereof. Mode controller  28  receives and monitors the output voltage V OUT  of power conditioner  14 . Mode controller  28  also receives V STACK  and current signal  62 , which is indicative of I STACK . Mode controller  28  issues converter mode signal  54  which is indicative of the operational mode that is most efficient given the operating conditions and parameters of fuel cell unit  12  and power conditioner  14 . Mode controller  28  issues to fuel cell controller  16  a cell operational control signal  70 , which is indicative of any adjustments necessary to the output, such as, for example, I STACK  and V STACK , of fuel cell unit  12  in light of instantaneous operating conditions and parameters. Mode controller  28  is configured as one or more logic gates, such as, for example, AND, OR and/or NAND gates. Preferably, mode controller  28  is configured as a microprocessor executing mode control software  72 .  
         [0027]     Fuel cell controller  16 , such as, for example, a microprocessor-based control unit, controls the operation of fuel cell unit  12  dependent at least in part upon stack signals  74 , such as, for example, sensor signals, indicative of the operating conditions and parameters, such as, for example, the amount of reformate flow, operating temperature, etc, of fuel cell unit  12 . Fuel cell controller  16  also receives cell operational control signal  70  from, mode controller  28 , receives V OUT  and I STACK  signal  62 . Fuel cell controller  16  controls the operation of fuel cell unit  12  by issuing stack control signals  76  that are dependent at least in part upon stack signals  74 , cell operational control signal  70 , V OUT  and I STACK  signal  62  to adjust the operational parameters, such as, for example, reformate and air flow, to thereby adjust and/or control the operation of fuel cell unit  12 .  
         [0028]     In use, fuel cell system  10  supplies output voltage V OUT  to loads  32  and  36 . More particularly, power conditioner  14  in conjunction with fuel cell controller  16  control the operation of fuel cell unit  12  and maintain V OUT  within a predetermined and desired voltage range, thereby rendering fuel cell system  10  suitable for use as a power source for a variety of electrical systems, such as, for example, an electrical system of a motor vehicle.  
         [0029]     Fuel cell unit  12  has three general modes of operation, i.e., start-up, operating, and cool down modes. During the start-up mode of operation, the fuel cell unit  12  has not reached its intended operational temperature. Accordingly, current I STACK  is substantially lower than a predetermined or nominal value. The difference between the start-up value of I STACK  and the nominal value of I STACK  is detected by mode controller  28 , which, in turn, issues mode signal  54  to control logic  26 . Control logic  26  decodes mode signal  54  and, dependent at least in part thereon, issues control signal  50  to gate drive circuitry  24 . Gate drive circuitry  24 , dependent at least in part upon control signal  50 , issues drive signal  52 . Drive signal  52  is received by power converter circuitry  22  and, more particularly, the control terminal of power switching device  42 .  
         [0030]     Drive signal  52 , when fuel cell unit  12  is operating in the start-up mode, places power switching device  42  into a corresponding start-up mode, such as, for example, substantially an open circuit, wherein current flow from fuel cell unit  12  to loads  32  and  36  is substantially disallowed or precluded. By disallowing current flow from fuel cell unit  12 , power conditioner  14  enables fuel cell  12  to operate at the open circuit voltage, thereby reducing the duration of time fuel cell unit  12  operates in the start-up mode. Thus, power conditioner  14  expedites fuel cell unit  12  reaching its operating temperature and entering the operating mode.  
         [0031]     When fuel cell unit  12  reaches a predetermined minimum start-up or warm-up temperature, the value of I STACK  has increased and reached a predetermined start-up value. This increase in I STACK  is detected and recognized by mode controller  28  of power conditioner  14  which, in response to I STACK  exceeding the predetermined threshold, issues an updated mode control signal  54 . Control logic circuitry  26  decodes the revised mode control signal  54  and, in turn, issues control signal  50  to gate drive circuitry  24 . In response to the revised control signal  50 , gate drive circuitry  24  issues drive signal  52  that places power switching device  42  in a condition that allows a predetermined and relatively small amount of current I STACK  to flow from fuel cell unit  12  through to loads  32  and  36 . This relatively small flow of I STACK  enhances the pre-heating of fuel cell unit  12  and fuel cell stack  18  due to the chemical conversion therein of reformate and air to electricity, and thereby reduces the amount of time required for fuel cell unit  12  to reach its operating or use temperature.  
         [0032]     Once fuel cell unit  12  reaches its operating or use temperature, fuel cell unit  12  exits the start-up mode and enters the operating mode. The readiness of fuel cell unit  12  to enter the operating mode is detected by mode controller  28 , through the monitoring of I STACK  and V STACK , which alters mode signal  54  accordingly. Control logic circuitry  26  decodes mode signal  54  and issues a corresponding control signal  50  to gate drive circuitry  24 . Gate drive circuitry  24  issues a corresponding drive signal  52  to power converter  22  thereby causing power switching device  42  to operate in an appropriate one of the first or second modes of operation (i.e., the pulse-width modulated mode or the linear mode), typically the first or PWM mode of operation, as described above.  
         [0033]     During shut down of fuel cell system  10 , the values of V STACK  and I STACK  being drawn from fuel cell unit  12  are substantially reduced relative to the warm-up and operating modes. Mode controller  28  detects this shut down condition and issues a corresponding mode signal  54  to control logic  26 . Control logic  26  decodes mode signal  54  and, dependent at least in part thereon, issues control signal  50  to gate drive circuitry  24 . Gate drive circuitry  24 , in turn, issues drive signal  52 . Drive signal  52  is received by power converter circuitry  22  thereby causing power switching device to enter the current blocking or open-circuit operating mode. With power switching device  42  in the current blocking mode, substantially no current flows from fuel cell unit  12  to loads  32  or  36  thereby ceasing the heat-emitting reaction within fuel cell stack  18  and expediting the cooling and/or shut down process thereof.  
         [0034]     With fuel cell unit  12  in the operating or use mode, i.e., fuel cell unit  12  has reached its operating or use temperature, mode controller  28  monitors the difference between V STACK  and V OUT  in order to determine the most efficient operating mode of power converter  14  and fuel cell unit  12 . As described above, with fuel cell unit  12  in the operating mode power conditioner  14  operates in a first or pulse-width modulated mode when the difference between stack voltage V STACK  and V OUT  is relatively large, such as, for example, greater than approximately 3.0 V. Power switching device  42  is placed into the first mode of operation through the application of a corresponding drive signal  52 , such as, for example, a pulse-width modulated (PWM) signal. V STACK  is controlled by controlling and/or adjusting the duty cycle of the pulse-width modulated drive signal  52 . Conversely, power conditioner  14  operates in a second or linear mode when the difference between stack voltage V STACK  and V OUT  is relatively small, such as, for example, less than approximately 3.0 V, and when V STACK  is less than the desired nominal output voltage, such as, for example, approximately 42 V. Power switching device  42  is placed into the second mode through a corresponding drive signal  52 , such as, for example, a voltage level sufficient to bias power switching device  42  into the linear region of operation. In the linear region, power switching device  42  dissipates a relatively low amount of power and therefore operates in a relatively efficient manner.  
         [0035]     It should be particularly noted that mode controller  28  monitors V STACK , I STACK  and V OUT  to detect start-up, over load and short circuit conditions. When fuel cell unit  12  is operating under any one of those conditions, mode controller  28  controls I STACK  via power switching device  42 . By controlling the amount of current I STACK  being drawn from fuel cell unit  12 , mode controller  28  indirectly controls the reformate flow through fuel cell unit  12 , and thereby substantially protects fuel cell unit  12  from damage.  
         [0036]     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the present invention using the general principles disclosed herein. Further, this application is intended to cover such departures from the present disclosure as come within the known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.