Abstract:
A fuel cell system to rapidly increase the temperature of unit cells. The fuel cell system includes; a plurality of current generating unit cells; a load circuit to supply the current to a load; a short circuit to connect the cells to an electrically closed loop without passing through the load; a thermo sensor to measure the temperature of the cells, and a controller that controls the delivery of the current to the load circuit and the short circuit, according to the temperature measured by the thermo sensor. The fuel cell system can rapidly increase the temperature of the unit cells when the temperature of the unit cells is below an operating temperature, thereby reducing the time required for the fuel cell to generate a stable output voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of Korean Patent Application No. 2006-101046, filed on Oct. 17, 2006, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    Aspects of the present invention relate to a fuel cell system and a method of operating the same. 
         [0004]    2. Description of the Related Art 
         [0005]    A fuel cell is an electric generator that converts the chemical energy of a fuel into electrical energy through a chemical reaction. A fuel cell can continuously generate electricity for as long as fuel is supplied thereto.  FIG. 1  is a schematic drawing illustrating the energy transformation structure of a fuel cell. Referring to  FIG. 1 , when air that includes oxygen is supplied to a cathode  1 , and a fuel containing hydrogen is supplied to an anode  3 , electricity is generated by the reverse electrolysis of water through an electrolyte membrane  2 . However, conventionally, a unit cell  10  does not generate a useable high voltage. Therefore, electricity is generated by a stack of unit cells  10  connected in series. 
         [0006]    A fuel cell can perform a normal operation when the temperature of the unit cells  10  is maintained at or above an appropriate operating temperature. Therefore, when the unit cells  10  are not preheated before beginning fuel cell operation, the normal power output of the unit cells is not generally produced. Accordingly, at the beginning of the operation, an electrochemical reaction is generated in the unit cells  10  without applying a load. When the temperature of the unit cells  10  reaches an appropriate temperature, the fuel cell may be operated with a load. In a direct methanol fuel cell (DMFC) that uses methanol as a hydrogen source, a fuel supplied to the anode  3  reacts with a catalyst in the cathode  1 , by passing through the electrolyte membrane  2 . This cross-over reaction occurs regardless of the connection of a load. Since the cross-over reaction is an exothermic reaction, the temperature of the unit cells  10  increases as the fuel is supplied. Accordingly, a desirable method of operating the fuel cell is that, after starting up the fuel cell, an electrical apparatus to be supplied with power is not connected to the fuel cell until the temperature of the unit cells  10  reaches a desired operating temperature. When the temperature reaches a desired level, that is, when an output current is stable enough to be supplied to an electrical apparatus due to the increase in the temperature, a load is connected. 
         [0007]    However, when a fuel cell system is configured and operated as described above, the preheating time is long.  FIG. 2  is a graph showing the measurement results of temperature vs. voltage of each unit cell when a normal operation begins. The normal operation includes connecting a load after the temperature of the fuel cell reaches a normal operating temperature, for example, 50° C., after starting a passive-type DMFC by supplying fuel to the unit cells  10 . When fuel is supplied to the unit cells  10 , an electrochemical reaction occurs, and thus, a voltage is generated. Assuming that a voltage V th , that is, an open circuit voltage (OCV) (hereinafter, an operating voltage) of each of the unit cells  10  that enables the fuel cell to operate in normal operation, is approximately 0.5V, the time to reach the operating voltage is approximately 5 minutes. Therefore, the time needed for the unit cells  10  to reach the operating voltage V th  is not a significant loss of time for operating the fuel cell. However, as depicted in  FIG. 2 , the increase in temperature is very slow. It takes almost 50 minutes to reach a temperature T th  (hereinafter, an operating temperature) at which a load can be applied. That is, a load can only be applied to the fuel cell almost one hour after start up of the fuel cell. 
         [0008]    In addition, there is a possibility that the temperature of the unit cells, during operation in very cold conditions, may be reduced below the normal operating temperature. In this case, if the temperature is not increased rapidly, the power supplied to an electrical apparatus can be intermittently stopped. 
         [0009]    Therefore, in order to solve these and/or other problems, there is a need to develop a fuel cell system and a method of heating a fuel cell that can rapidly heat the unit cells when the temperature of the unit cells is below the normal operating temperature. 
       SUMMARY OF THE INVENTION 
       [0010]    Aspects of the present invention provide a fuel cell system that can rapidly increase the temperature of unit cells when necessary, and a method of operating the fuel cell. 
         [0011]    According to an aspect of the present invention, there is provided a fuel cell system comprising: a plurality of cells in which a power generation reaction occurs; a load circuit that forms a load path to supply a current to a load by electrically connecting the cells to a load; a short circuit that connects the cells to an electrically closed loop, without passing through the load; a thermo sensor that measures the temperature of the cells; and a controller that controls the delivery of a current generated from the cells to one of the load circuit and the short circuit, according to the temperature measurement of the thermo sensor. 
         [0012]    The plurality of cells may be connected in series. The short circuit may comprise a unit cell short circuit that connects an anode and a cathode of each of the cells in a closed loop, and a stack short circuit that connects a terminal of an end of an uppermost cell and a terminal of an end of a lowermost cell, of the cells connected in series. 
         [0013]    According to an aspect of the present invention, there is provided a method of operating a fuel cell system comprising: performing a normal operation in which a current is supplied to a load, by electrically connecting cells to the load when a measured temperature of the cells is higher than an appropriate temperature; and rapidly heating the cells using a short circuit that bypasses the load when the measured temperature of the cells is lower than the appropriate temperature. 
         [0014]    The rapid heating of the cells may be performed by repeatedly turning the short circuit ON and OFF. 
         [0015]    With regard to repeatedly turning the short circuit ON and OFF, each of the cells may comprise an individual short circuit, and the process of repeatedly turning ON and OFF of the short circuit may be sequentially performed by sequentially turning the short circuit for each of the cells ON and OFF. 
         [0016]    The plurality of cells may be connected in series to one short circuit, and the process of repeatedly turning ON and OFF of the short circuit may be performed by turning the entire short circuit ON and OFF. 
         [0017]    Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
           [0019]      FIG. 1  is a schematic drawing illustrating the energy transformation structure of a fuel cell; 
           [0020]      FIG. 2  is a graph showing the measurement result of the variation of temperature and voltage in a conventional fuel cell; 
           [0021]      FIG. 3  is a block diagram showing an overall configuration of a fuel cell system according to an embodiment of the present invention; 
           [0022]      FIG. 4  is a flow chart showing an operation process using the fuel cell system illustrated in  FIG. 3 ; 
           [0023]      FIG. 5  is a timing graph showing an ON and OFF method of a short circuit in the fuel cell system illustrated in  FIG. 3 ; and 
           [0024]      FIG. 6  is a graph showing the variation of temperature and voltage of the fuel cell system illustrated in  FIG. 3 , compared to that of the conventional fuel cell system. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0025]    Reference will now be made in detail to the present embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures. 
         [0026]      FIG. 3  is a block diagram showing an overall configuration of a fuel cell system  11  according to an embodiment of the present invention. 
         [0027]    The fuel cell system  11  comprises: a plurality of unit cells  10  disposed in a stack  15 ; a load  20 ; a controller  30 ; a temperature sensor  40 ; and a DC-DC converter  50 . The stack  15  comprises a stack anode and a stack cathode. The unit cells  10  can be connected to one another in series. Each of the unit cells  10  comprises a cell anode and a cell cathode. The load  20  can comprise any electrical device that provides resistance to an electrical current, for example, a motor or any other electrically operated device that provides a resistance, other than a switch. 
         [0028]    The fuel cell system  11  has a basic structure in which the current generated in the stack  15 , is selectively supplied to the load  20 , under the control of the controller  30 . In  FIG. 3 , the stack  15  includes 5 unit cells  10  connected in series, but the number of the unit cells  10  in the stack  15  can be increased or decreased according to a desired electrical output. 
         [0029]    The fuel cell system  11  can comprise short circuits C 0 -C 5  and a load circuit C L . The short circuits C 1 -C 5  can be referred to as unit cell short circuits C 1 -C 5 , and the short circuit C 0  can be referred to as a stack short circuit C 0 . Each of the unit cell short circuits C 1 -C 5  respectively comprises a switch S 1 -S 5 . The stack short circuit C 0  comprises a switch S 0 . The load circuit C L  comprises a switch S L . Each of the unit cell short circuits C 0 -C 5  comprises a direct connection between the anode and the cathode of a respective unit cell  10 . The stack short circuit C 0  comprises a direct connection between the anode and cathode of the stack  15 . Herein, a direct connection and/or directly connecting can refer to an electrical connection that does not pass through a load. 
         [0030]    The unit cell short circuits C 1 -C 5  and/or the stack short circuit C 0  can rapidly increase the temperature of the unit cells  10 , The unit cell short circuits C 1 -C 5  are completed by closing the switches S 1 -S 5 . The stack short circuit C 0  is completed by closing the switch S 0 . When the switches S 0 -S 5  are closed, the current generated in the unit cells  10  flows through the short circuits C 0 -C 5 , without passing through the load  20  (a no load state). When the current flows through the short circuits C 0 -C 5  in a no load state, the temperature of the unit cells  10  increases faster than when the current passes through the load circuit C L  is connected to the load  20 , or when the circuit is completely open. This is because, as described above, the electrochemical reaction in the unit cells  10  is an exothermic reaction, and the short circuit itself is an extremely exothermic circuit, that is, all or nearly all the electric energy generated in the unit cells  10  is transformed into heat. The temperature of the unit cells  10  is measured using a thermo sensor  40 , mounted on the stack  15 . When an increase in temperature is needed, the controller  30  closes the switches S 0 -S 5  of the short circuits C 0 -C 5 , to increase the temperature of the unit cells  10 . In this way, a selective temperature control can be performed. The DC-DC converter  50  reduces the fluctuation of voltage applied to the load  20  by the load circuit C L . 
         [0031]    Rapidly raising the temperature of the unit cells  10  is often useful at an initial start up operation, when warming the unit cells  10  is necessary.  FIG. 4  is a flow chart showing a method of rapidly increasing the temperature of the unit cells  10  at an initial start up. 
         [0032]    Referring to  FIG. 4 , the method comprises an operation P 1  where fuel is supplied to the unit cells  10 , in order to generate a power generation reaction in the unit cells  10 . At this point, the switches S 0 -S 5  and S L , are in an open state. In an operation P 2 , the voltage of the unit cells  10  is detected. When the voltage generated by the unit cells  10  reaches an operating voltage V th , an operation P 3  begins a rapid temperature increase by turning ON the short circuits C 0 -C 5  by closing stitches S 0 -S 5 . 
         [0033]    The operation P 2  can further comprise detecting the temperature of the unit cells  10 . If the temperature of the unit cells is less than an operating temperature, the method will proceed to operation P 3 . If the temperature is greater than or equal to an operating temperature, the method will proceed to an operation P 5 , discussed below. 
         [0034]    The operation P 3  can comprise using the controller  30  to control the actuation of the switches S 0 -S 5 , of the short circuits C 0 -C 5 . The current generated in the unit cells  10  flows through the short circuits C 0 -C 5 . Accordingly, the temperature in the unit cells  10  rapidly increases due to the transformation of electrical energy into heat, in addition to an exothermic reaction for power generation. Whether to establish the electrical connections through the short circuits C 0 -C 5  is determined by measuring the temperature of the unit cells  10  with the thermo sensor  40 . That is, the temperature measured by the thermo sensor  40  is compared to a set value, and when the temperature of the unit cells  10  is lower than the set value, the controller  30  closes the switches S 0 -S 5  to connect the short circuits C 0 -C 5 . The switches S 0 -S 5  may be periodically opened and closed instead of being maintained in a closed (ON) state for a long period of time, e.g., an hour. This is because, as described above, the short circuits C 0 -C 5  are extremely exothermic circuits, and when the ON state is maintained for a long period of time, the unit cells  10  may be damaged by overheating. A method of repeatedly closing and opening (turning ON and OFF) the short circuits C 0 -C 5  can include a variable duty method. The variable duty method can comprise maintaining a constant ON and OFF frequency. The ON time and OFF time of the short circuits C 0 -C 5  can be varied, within a unit frequency, or within a variable frequency method in which the ON and OFF frequency is varied. 
         [0035]    The repeated turning ON and OFF of the short circuits C 0 -C 5  can be performed by using the stack short circuit C 0 , by closing and opening (turning ON and OFF) the switch S 0 . In addition, as shown in  FIG. 5 , each of the short circuits C 1 -C 5  can be independently cycled ON and OFF by sequentially turning the switches S 1 -S 5  ON and OFF. 
         [0036]    When the short circuits C 0 -C 5  are repeatedly turned ON and OFF, the temperature of the unit cells  10  rapidly increases, due to the transformation of electrical energy into heat, in the short circuits C 0 -C 5  in addition to an exothermic reaction for power generation. In an operation P 4 , the temperature of the unit cells  10  is detected as the short circuits C 0 -C 5  are cycled ON and OFF. When the temperature in the unit cells  10  reaches an operating temperature T th , the controller  30  shuts off all the short circuits C 0 -C 5 , and closes the switch S L  of the load circuit CL. Closing the switch SL allows the current generated from the unit cells  10  to be supplied to a load, thereby performing an operation P 5  (normal operation). 
         [0037]      FIG. 6  is a graph showing the comparison of the temperature increase at the initial start up of a conventional fuel cell system, and the temperature increase of a fuel cell system having a short circuit according to an embodiment of the present invention. The graph shows an assumed operating temperature of 34° C. In the case of the present embodiment, for a rapid increase in the temperature of the unit cells, an ON state of the short circuit is maintained for 0.1 seconds per second. The conventional fuel cell system took approximately 40 minutes for its unit cells to reach the operating temperature. However, the present embodiment took only approximately 20 minutes, which is about half of the warm up time of the conventional fuel cell system. This time difference is due to the additional heat from the short circuits, and the exothermic power generation reaction. Thus, the warm up time at the initial start up of a fuel cell system can be greatly reduced. 
         [0038]    In the present teachings, the use of a short circuit for warm up at the initial start up of a fuel cell system has been described. However, a rapid temperature increasing process can also be performed during normal operation, by connecting the unit cell short circuits C 1 -C 5 . For example, the temperature of a fuel cell may drop below the operating temperature if the fuel cell is operated in cold conditions. The temperature of the unit cells  10  can be rapidly increased by connecting the unit cell short circuits C 1 -C 5 . If the stack short circuit C 0  is turned ON and OFF, the voltage generated from the stack  15  can fluctuate to a large degree. However, when the unit cell short circuits C 1 -C 5  of the unit cells  10  are alternately and/or sequentially turned ON and OFF, the fluctuation of the voltage may not affect the load since the fluctuation can be sufficiently compensated for by the DC-DC converter  50 . For example, the converter  50  can mitigate the instant voltage fluctuation of the load circuit C L . 
         [0039]    Accordingly, a fuel cell system that can rapidly increase the temperature of unit cells as necessary, and a method of operating the fuel cell system can be realized. 
         [0040]    As described above, a fuel cell system and a method of operating the fuel cell system according to aspects of the present invention has the following advantages. First, a warm up time at the initial start up of the fuel cell system can be greatly reduced since the unit cells can be rapidly heated using short circuits. Second, the fuel cell system is useful in cold weather since the temperature of unit cells can be rapidly increased if the temperature falls during normal operation due to the cold weather. 
         [0041]    Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.