Abstract:
A method for starting a cold or frozen fuel cell stack as efficiently and quickly as possible in a vehicle application is based upon a state of charge of a first power source such as a high voltage battery. Power flow between the first power source and fuel cell system is coordinated in conjunction with a specific load schedule and parallel control algorithms to minimize the start time required and optimize system warm-up.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a divisional application of U.S. patent application Ser. No. 11/536,759 filed on Sep. 29, 2006, the entire disclosure of which is hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to a method of operation for a fuel cell system. More particularly, the invention is directed to a method for starting a cold or frozen fuel cell system. 
       BACKGROUND SUMMARY 
       [0003]    In vehicle applications, starting a frozen or cold fuel cell system has several challenges. Starting a cold or frozen fuel cell stack and related components requires a very specific and carefully coordinated procedure. In order to maximize operating performance, the fuel cell system must be started and warmed up as quickly as possible. 
         [0004]    The startup method must be able to handle many different beginning use scenarios. For example, an operator may immediately subject the system to heavy demand before the fuel cell system reaches normal operating temperatures and is capable of fulfilling the power requirements to meet such a demand. Conversely, the operator may place little or no demand on the fuel cell system for an extended period of time, causing components of the fuel cell system to freeze since the fuel cell system is producing water but is not producing sufficient heat. 
         [0005]    Another concern facing current fuel cell startup methods is adaptability to specific fuel cell system components and the specific condition of those components. As a stack degrades it may not tolerate the same loading schedule that a new stack can handle, and the variations betweens components in different systems may change the operating requirements of the startup method. 
         [0006]    It would be desirable to have an adaptive method of starting a cold or frozen fuel cell that is able to balance increasing a temperature ramp-up rate of the fuel cell system by loading the fuel cell stack quickly without overloading the fuel cell stack or forcing it shut down due to low cell voltage, and account for different beginning use scenarios that an operator may place on a cold or frozen fuel cell system. 
       SUMMARY OF THE INVENTION 
       [0007]    In agreement with the present invention, an adaptive method of starting a cold or frozen fuel cell that is able to balance increasing a fuel cell system&#39;s temperature ramp-up rate by loading the fuel cell stack quickly, without overloading the stack or forcing it shut down due to low cell voltage, and account for different beginning use scenarios that an operator may place on a cold or frozen fuel cell system has surprisingly been discovered. This method also optimizes the warm-up time for fuel cell systems and high voltage batteries, and minimizes the time required for the fuel cell system to run after a user causes the fuel cell system to shut down. 
         [0008]    In one embodiment, the method for starting a fuel cell system having a fuel cell stack, a high voltage power source, and at least one startup component, comprises the steps of determining a state of charge of the high voltage power source; selecting a power source for starting the essential components as a function of the state of charge of the high voltage power source; controlling power flow between the fuel cell system and the high voltage power source; and maintaining a desired fuel cell stack voltage using a load schedule. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0009]    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: 
           [0010]      FIG. 1  is a schematic diagram of a fuel cell system according to an embodiment of the invention; 
           [0011]      FIG. 2  is a block diagram showing a subsystem of the fuel cell system of  FIG. 1 , and 
           [0012]      FIGS. 3A and 3B  combine to show a flow diagram illustrating a method of controlling the fuel cell of  FIG. 1  with the flow diagram connecting at point A. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0013]    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. 
         [0014]    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 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 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. 
         [0015]    In the embodiment shown 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. 
         [0016]    A humidifier  28  is disposed in the conduit  26  between the compressor  22  and the cathode inlet  24 . Additionally, other components (not shown) may be provided between the compressor  22  and the cathode inlet  24  in other embodiments without departing from the scope of the invention. 
         [0017]    The cathode side  14  of the fuel cell stack  10  includes 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 a cathode outlet  48 . 
         [0018]    A plurality of loads  34  is electrically linked to the fuel cell stack  10  via a high voltage bus  36 . The loads  34  are linked to the control system  16  via a connection  40 . The loads  34  may include a high temperature coolant pump  30 , a propulsion motor  70 , a plurality of fuel cell stack end plate heaters  72 , at least one electric supplemental cabin heater  76 , an electric supplemental coolant heaters  74 , and a plurality of valve heaters  78 , for example as illustrated in  FIG. 2 . 
         [0019]    The supplemental heaters  74  are typically disposed in a radiator bypass portion of the coolant loop (not shown). It is desirable for the heaters to have local closed-loop controls. Also, another load that may require electrical power at startup is the compressor motor  20  electrically linked to the high voltage bus  36 . 
         [0020]    A first power source  50  is electrically linked to the loads  34  via the high voltage bus  36 , and linked to the control system  16  via a connection  60 . It is desirable that the first power source  50  is a battery capable of generating a relatively high voltage of about  200  to  300  volts. A second power source  52  is linked to the control system  16  via a connection  58 . It is desirable that the second power source  52  is a battery capable of generating about  12  volts. The battery  52  is also electrically linked to the high voltage bus  36  through a DC/DC boost circuit  56 . The voltage bus  36  links the fuel cell stack  10  to the first power source  50  through a contactor  62 . 
         [0021]    A method of controlling the fuel cell system is illustrated in  FIGS. 3A and 3B . In operation, the control system  16  receives a start request  82  that may come from an operator of a vehicle (not shown). Upon receiving the start request  82  the control system  16  will determine the state of charge of the first power source  50  for sufficient energy, and determines a preferred startup option from a plurality of startup options at  84 . Methods for checking the state of charge of a power source are commonly known in the art. 
         [0022]    If sufficient energy is detected in the first power source  50  (Y at  84 ), then the control system  16  will command a fast start  88 . Sufficient energy for this embodiment would be around 10 kW and 80W-h. During the fast start  88  the control system  16  supplies full power to startup components of the fuel cell stack from the first power source  50 . The startup components may include the motor  20  for the compressor  22 , the valve heaters  78 , and the high temperature coolant pump  30 , for example. However, it is understood that power can be supplied to other components as desired. The control system  16  can also provide power to a plurality of the loads  34 , which may include the fuel cell stack end plate heaters  72 , and electric supplemental coolant heaters  74 . The control system  16  provides power to the stack heaters  72 , and the supplemental heaters  74 , based on the system starting temperature. Real-time temperature monitoring is typically used to avoid undesirable localized hot spots adjacent to the heater. The discharging of the first power source  50  during a fast start  88  also helps to warm the first power source  50 , increasing the available power and energy that can be drawn from the first power source  50  as the temperature increases, and thus indirectly decreasing the overall start time. 
         [0023]    If insufficient energy is detected in the first power source  50  (N at  84 ), the control system  16  will command a slow start  86 . During the slow start  86  the control system will supply less power than during a fast start  88  to the startup components from the second power source  52  through the DC/DC boost circuit  56 . During the slow start  86  the control system  16  typically will not provide power to the loads  34  and will provide low power to the compressor motor  20  and the valve heaters  78 . 
         [0024]    The compressor  22 , the valve heaters  78 , and the high temperature coolant pump  30  are included in the startup components because air is pulled into the fuel cell system and compressed by the compressor  22  driven by the motor  20 , and supplied to the cathode inlet  24  of the fuel cell stack  10 . A speed of rotation of the air compressor  22  may be changed by the control system  16  by controlling the motor  20  thereby changing the air flow delivered by the air compressor  22 . The control system  16  can also control the temperature of the valve heaters  78 . The control system can control the speed of rotation of the high temperature coolant pump  30  via a connection, and thus, the coolant delivered to the fuel cell stack  10 . However, the startup of the pump  30  may be delayed to maintain heat in the fuel cell stack. 
         [0025]    Hydrogen gas is delivered to the anode side  12  in a manner commonly known in the art by fuel injectors  43 . The valve heaters  78  maintain the fuel injectors  43  at a desired operating temperature. A reaction 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 external circuits (not shown) and the loads  34 . 
         [0026]    The control system  16  typically controls the air flow into the cathode side  24  of the fuel cell stack  10  as efficiently as possible in order to maximize the release of electrons. However, it may be desirable for the control system  16  to implement a set of inefficient cathode controls in order to increase the load on the fuel cell stack  10  and thus increase the rate of warmup of the fuel cell stack  10 . 
         [0027]    When the fuel cell stack  10  has reached open circuit voltage, the control system  16  ramps down power at  90  to the compressor  22 , and the valve heaters  78  and opens the fuel injectors  43 . When the fuel cell stack  10  achieves open circuit voltage at  92  the control system  16  closes the contactors  62  connecting the fuel cell stack  10  to the voltage bus  36  at  94 . The control system  16  then begins loading the fuel cell stack  10 . The control system  16  continuously monitors the temperature of the fuel cell stack  10  at  96 . 
         [0028]    Power is provided to the startup components and the loads  34  with a closed loop on the fuel cell stack  10  voltages according to a default load schedule at  108 . The control system  16  may determine from specific operating conditions that the priority of the loads  34  be changed. It has been found desirable if the control system  16  loads the fuel cell stack to maintain an average cell voltage of 0.5 volts, or a minimum cell voltage of 0.2 volts at  104 . If cell voltages are higher than desired, the control system  12  will add a load until either the desired average cell voltage or minimum cell voltage is achieved. 
         [0029]    If the fuel cell stack  10  goes below either the desired average cell voltage or minimum cell voltage, the control system  16  reduces the load on the fuel cell stack  10  at  106  by removing the loads in the reverse order in which the loads were added. The loads are added and removed with ramp functions to avoid drastic changes to the fuel cell stack  10  under cold conditions. 
         [0030]    The control system  16  supplies power from the fuel cell stack  10  according to the default load schedule at  108 . First, power is supplied to the startup components of the fuel cell system such as the air compressor motor  20 , the high temperature coolant pump  30 , and the valve heaters  78 . Second, power is supplied to the propulsion motor  70  if the vehicle is not idling. Third, power is supplied to the fuel cell stack end plate heaters  72 . Fourth, power is supplied to the electric supplement coolant heaters  74 . Fifth, power is supplied to the electric cabin heaters  76 . Sixth, power is supplied to the recharging of the first power source  50 . Seventh, the control system will implement inefficient cathode controls to further load the compressor  22 . 
         [0031]    The power supplied to the startup loads and loads  34  from the default load schedule  108  may change subject to the requirements of the startup loads and loads  34  with time. For example, if the first power source  50  is fully recharged during the sixth step of the default load schedule  108 , then power to the first power source  50  will be zero. 
         [0032]    The load schedule at  108  runs in parallel with the local control loops (not shown) such as temperature control loops for heaters, or vehicle operator requirements such as windshield defrost requirements. The local control loops may override the load schedule  108 . 
         [0033]    The operational order of supplying power from the fuel cell stack  10  to the startup components and loads  34  may change from the default load schedule based on specific circumstances. For example, the essential components, the supplement heaters  74 , and the electric cabin heaters  76 , can all be active without the fuel cell stack endplate heaters  72 , if the local fuel cell stack end plate temperatures override the default loading schedule. Also, a request for windshield defroster may move the electric cabin heater  76  to a higher priority in the loading schedule. Further, if the first power source  50  has been drained enough to hurt drive-away performance it could be given priority over supplying power to the electric cabin heaters  76 . 
         [0034]    Increasing the load on the fuel cell stack  10  at  108  in order to achieve the desired average and minimum cell voltages at  104  should be maintained until the target temperature is achieved at  96 . The automobile idle may now be up to or above 30% of the full system power, e.g., 30 kW for a system capable of 90 kW. 
         [0035]    If the system receives a stop-request  98  prior to the threshold temperature being reached, it may be desirable to complete the normal warm-up procedure at  100  until the target temperature at  96  is reached. This could also ensure that the first power source  50  is returned to a threshold state of charge before shutdown. If the warm-up has been sufficiently completed then the control system  16  will exit the start-up and shutdown the system at  102 . 
         [0036]    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.