Patent Publication Number: US-2005142409-A1

Title: Fuel cell system and control method thereof

Description:
TECHNICAL FIELD  
      The present invention relates to a fuel cell system, and more particularly, to a fuel cell system capable of increasing a reliability of a fuel cell by making a temperature of a fuel cell stack reach a goal temperature within the shortest time and capable of enhancing a stability of the fuel cell by fast dropping a temperature of the fuel cell stack at the time of stopping the fuel cell, and a control method thereof.  
     BACKGROUND ART  
      In general, a fuel cell system has been proposed as a substitution of fossil fuel and differently from a general cell (a second cell), it supplies fuel (hydrogen or hydrocarbon) to an anode and supplies oxygen to a cathode. Thus, the fuel cell system undergoes an electrochemical reaction between hydrogen and oxygen without a combustion reaction (oxidation reaction) of fuel and thereby directly converts an energy difference between before and after a reaction into electric energy.  
      As shown in  FIG. 1 , a fuel cell system in accordance with the conventional art comprises: a fuel cell stack  106  where an anode  102  having an electrolyte membrane (not shown) therein in order to generate electric energy by an electrochemical reaction between hydrogen and oxygen and a cathode  104  are stacked with the plural number; a fuel tank  108  for storing fuel including hydrogen to be supplied to the anode  102 ; and an air supplying unit  110  for supplying air including oxygen to the cathode  104 .  
      A fuel pump  112  for pumping fuel stored in the fuel tank  108  is installed between the fuel tank  108  and the anode  102  of the fuel cell stack  106 .  
      The air supplying unit  110  includes: an air pump  114  for supplying air in the atmosphere to the cathode  104  of the fuel cell stack  106 ; an air filter  116  for filtering air supplied to the fuel cell stack  106 ; and a humidifier  118  for humidifying air supplied to the fuel cell stack  106 . Herein, the humidifier  118  is provided with a water tank  120  for supplying water to the humidifier  118 .  
      Processes for generating electric energy by supplying fuel to the conventional fuel cell will be explained as follows.  
      If the fuel pump  112  is operated by a control signal of a control unit (not shown), fuel stored in the fuel tank  108  is pumped thus to be supplied to the anode  102  of the fuel cell stack  106 . Also, if the air pump  114  is operated, air filtered by the air filter  116  passes through the humidifier  118  thus to be humidified and is supplied to the cathode  104  of the fuel cell stack  106 .  
      Once fuel and air are supplied to the fuel cell stack  106 , an electrochemical oxidation of hydrogen is performed in the anode  102  and an electrochemical deoxidation of oxygen is performed in the cathode  104  in a state that the electrolyte membrane (not shown) is positioned between the anode  102  and the cathode  104 . At this time, generated electron moves and thereby electricity is generated. The generated electricity is supplied to a load  126 .  
      In the conventional fuel cell system, it takes a lot of time to make a temperature of the fuel cell stack reach a goal temperature, so that a reliability and a function of the fuel cell are degraded.  
      Also, a temperature of the fuel cell stack is maintained to be high even after stopping the fuel cell, so that a stability of the fuel cell is lowered.  
     DISCLOSURE OF THE INVENTION  
      Therefore, it is an object of the present invention to provide a fuel cell system capable of increasing a reliability and a function of a fuel cell by making a temperature of a fuel cell stack reach a goal temperature within the shortest time by heating fuel by using heat generated when fuel powder is mixed with water and heat generated when hydrogen generated from an anode of the fuel cell stack is ignited, and a control method thereof.  
      It is another object of the present invention to provide a fuel cell system capable of increasing a stability of a fuel cell by fast dropping a temperature of a fuel cell stack when the fuel cell system is stopped and capable of increasing a performance by returning fuel remaining at each system to a fuel tank, and a control method thereof.  
      To achieve these objects, there is provided a fuel cell system comprising: a fuel cell stack that an anode and a cathode are arranged in a state that an electrolyte membrane is interposed therebetween; a fuel tank connected to the anode of the fuel cell stack by a fuel supplying line for supplying hydrogen-including fuel to the anode; an air supplying unit connected to the cathode of the fuel cell stack by an air supplying line for supplying oxygen-including air to the cathode; a heating unit for heating air and fuel supplied to the fuel cell stack; and a purge unit for returning fuel remaining at each system to the fuel tank when a system driving is stopped.  
      A cooling fan for cooling the fuel cell stack when the system driving is stopped is installed at the fuel cell stack.  
      The heating unit is composed of a hydrogen combustor installed at the fuel supplying line and the air supplying line for heating fuel and air supplied to the fuel cell stack by using hydrogen generated from the fuel cell stack as a heating source.  
      The purge unit is composed of a fuel recollecting line connected between the fuel cell stack and the fuel tank for recollecting fuel discharged from the fuel cell stack into the fuel tank, and a recycling pump installed at the fuel recollecting line for returning fuel remaining at each system to the fuel tank through the fuel recollecting line when a system driving is stopped.  
      To achieve these objects, there is also provided a method for controlling a fuel cell system comprising: a heating step for heating fuel; an electricity generating step for supplying the heated fuel and air to a fuel cell stack and thus generating electric energy; and a purge step for returning fuel remaining at each system to a fuel tank when a system driving is stopped while performing the first and second steps.  
      The heating step further comprises a step for driving a system by using a power source of a battery after heating fuel.  
      In the heating step, fuel is heated by using heat generated when fuel is mixed with water.  
      In the heating step, a fuel kit where fuel powder (NaOH and BH 4  powder) is stored is mounted to a fuel tank where water is stored and thereby fuel powder is mixed with water.  
      In the purge step, a recycle pump is driven to recollect fuel remaining at the fuel cell stack and each line to the fuel tank through a recollecting line when a system driving is stopped. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a construction view of a fuel cell system in accordance with the conventional art;  
       FIG. 2  is a construction view of a fuel cell system according to one embodiment of the present invention;  
       FIG. 3  is a sectional view of a fuel tank of a fuel cell system according to the present invention;  
       FIG. 4  is a block diagram showing a control means of a fuel cell system according to one embodiment of the present invention;  
       FIG. 5  is a construction view of a fuel cell system according to another embodiment of the present invention; and  
       FIG. 6  is a flow chart showing a control method of a fuel cell system according to one embodiment of the present invention.  
    
    
     MODES FOR CARRYING OUT THE PREFERRED EMBODIMENTS  
      Hereinafter, a control method of a fuel cell system according to the present invention will be explained with reference to attached drawings.  
      Even if a plurality of embodiments can exist in the control method of a fuel cell system according to the present invention, the most preferable embodiment will be explained.  
       FIG. 2  is a construction view of a fuel cell system according to one embodiment of the present invention.  
      A fuel cell system according to the present invention comprises: a fuel cell stack  14  where an anode  10  having an electrolyte membrane (not shown) therein in order to generate electric energy by an electrochemical reaction between hydrogen and oxygen and a cathode  12  are stacked with the plural number; a fuel tank  16  for storing fuel to be supplied to the anode  10 ; an air supplying unit  18  for supplying oxygen including air to the cathode  12 ; a hydrogen combustor  22  for heating fuel and air supplied to the fuel cell stack  14  by using hydrogen generated from the anode  10  after a reaction; a purge unit for recollecting fuel remaining at each system to the fuel tank  16  when the system is stopped; and a control means for controlling each component.  
      The fuel cell stack  14  is provided with a cooling fan  20  for cooling the fuel cell stack  14 .  
      The fuel tank  16  is connected to the anode  10  of the fuel cell stack  14  by a fuel supplying line  26 , and a fuel pump  28  for pumping fuel stored in the fuel tank  16  is installed at one side of the fuel supplying line  26 .  
      Also, as shown in  FIG. 3 , the fuel tank  16  includes a fuel kit  30  for increasing a temperature of fuel by using reaction heat generated when fuel powder is mixed with water stored in the fuel tank  16  before operating the fuel cell system; and a blade  32  for making fuel power be mixed with water well when the fuel power is supplied to the fuel tank  16  from the fuel kit  30 .  
      The fuel power stored in the fuel kit  30  is composed of NaOH and BH 4 . If the NaOH is mixed with water, a reaction is performed as a following reaction formula and heat is generated. 
 
NaOH+H 2 O-&gt;NaOH (H 2 O)+9˜13 Kcal/mol  Reaction formula: 
 
      The air supplying unit  18  includes: an air supplying line  34  for introducing air in the atmosphere to the cathode  12  of the fuel cell stack  14 ; an air filter  36  installed at an entrance of the air supplying line and filtering air sucked into the air supplying line  34 ; an air pump  42  installed at one side of the air supplying line  34  and generating a suction power for sucking external air; and a humidifier  38  for humidifying air sucked by the air pump  42 . The humidifier  38  is provided with a water tank  40  for supplying water.  
      The purge unit can be implemented by various methods, which will be explained as follows.  
      The purge unit according to one embodiment includes: a gas/liquid separator  44  for separating fuel discharged from the anode  10  of the fuel cell stack  14  after reaction into gas and liquid; a recycling line  48  for recollecting liquid fuel discharged from the gas/liquid separator  44  into the fuel tank  16 ; 
          and a recycling pump  46  installed at the recycling line  48  and pumping recycling liquid fuel to the fuel tank  16 .        

      The purge unit according to one embodiment recollects fuel remaining at the fuel cell stack  14  to the fuel tank  16  through the recycling line  48  by driving the recycling pump  46  for a certain time after the system is stopped.  
      NaBO 2  and 4H 2  generated in the anode  10  of the fuel cell stack  14  after reaction are separated into gas and liquid. Herein, water and NaBO 2  of liquid are recollected into the fuel tank  16  through the fuel recycling line  48  and the hydrogen gas is exhausted outside.  
      The hydrogen combustor  22  is connected with the fuel supplying line  26  and the air supplying line  34  and connected with the gas/liquid separator by a hydrogen supplying line  50 , thereby heating fuel and air which pass through the fuel supplying line  26  and the air supplying line  34  by using heat generated when hydrogen supplied from the gas/liquid separator  44  is ignited.  
      The purge unit according to a second embodiment reversely drives the fuel pump  28  when the system is stopped and thereby returns fuel remaining at the fuel cell stack  14  and each line to the fuel tank  16 . That is, when the system is stopped, the purge unit drives the fuel pump  28  in a reverse direction by a control unit  80  for a certain time.  
      The purge unit according to a third embodiment, as shown in  FIG. 4 , is composed of a purge line connected between the fuel supplying line and the air supplying line, and a three-way valve installed at a part where the purge line and the fuel supplying line are connected to each other.  
      In the purge unit according to the third embodiment, the three-way valve is operated to connect the air supplying line and the anode each other when the system is stopped, and the air pump  42  is operated to supply air to the anode and thereby fuel remaining at the anode is returned to the fuel tank through the recycling line.  
       FIG. 5  is a block diagram showing a control means for controlling the fuel cell system according to the present invention.  
      The control means includes: a temperature sensor  64  installed at the fuel cell stack  14  and detecting a temperature of the fuel cell stack  14 ; an on/off switch  66  for turning on/off a fuel cell; and a control unit  80  for controlling an operation of each component according to signals applied from the temperature sensor  64  and the on/off switch  66 .  
      A control method of the fuel cell system according to the present invention will be explained as follows.  
       FIG. 6  is a flow chart showing a control method of the fuel cell system according to the present invention.  
      First, the fuel kit  30  is mounted at the fuel tank  16  thus to mix water stored in the fuel tank  16  with fuel powder stored in the fuel kit  30 , thereby fabricating fuel solution. At this time, as said water and fuel powder are mixed with each other in the fuel tank  16 , heat is generated (S 10 ).  
      Also, if a temperature of the fuel solution reaches a proper level, the fuel cell system is operated by a power of a battery (not shown) (S 20 ).  
      That is, by a power of the battery, the fuel pump  28  is operated and thereby fuel of which temperature is increased by a mixture in the fuel tank  16  is supplied to the anode  10  of the fuel cell stack  14 . At the same time, by a power of the battery, the air pump  42  is operated and thereby air is supplied to the cathode  12  from the air supplying unit  18 . According to this, fuel and air react with the electrolyte membrane thus to form ions. In the process that the ions form water by an electrochemical reaction, electron is generated from the anode  10  and moves to the cathode  12 , thereby generating electricity.  
      Also, hydrogen generated from the anode  10  of the fuel cell stack  14  after reaction is obtained by the gas/liquid separator  44  thus to be supplied to the hydrogen supplying line  50 .  
      The hydrogen exhausted from the gas/liquid separator  44  is supplied to the hydrogen combustor  22  through the hydrogen supplying line  50 . Then, the hydrogen is ignited in the hydrogen combustor  22  thus to generate heat, and fuel and air supplied to the fuel cell stack  14  are heated by passing through the hydrogen combustor  22  (S 30 ).  
      Like this, at the first stage, fuel is heated by using heat generated from a mixture between fuel and water in the fuel tank  16 , and after the fuel cell system is operated, fuel is heated by the hydrogen combustor  22 . According to this, a temperature of the fuel cell stack  14  can reach a goal temperature within the shortest time.  
      While the fuel cell system is operated, it is judged that a temperature of the fuel cell stack  14  is higher than a set temperature a or not (S 40 ).  
      That is, if the temperature sensor  64  mounted at the fuel cell stack  14  detects a temperature of the fuel cell stack  14  and thus applies to the control unit  80 , the control unit  80  compares a temperature of the fuel cell stack  14  with the set temperature a and thereby judges that a temperature of the fuel cell stack  14  is more than the set temperature a. Herein, the set temperature a is preferably set as 60° C.  
      In said process, if a temperature of the fuel cell stack  14  is judged to be more than the set temperature α, the battery is charged, the system is operated by using electric current generated from the fuel cell stack  14 , and current is supplied to a load (S 50 ).  
      While the fuel cell system is operated, it is judged that the system is a purge mode state (S 60 ). That is, it is judged that the user stops the system by adjusting the on/off switch  66  in order to stop the fuel cell system or not.  
      Herein, if it is judged that the system is not the purge mode state, it is judged that a temperature of the fuel cell stack  14  is higher than a set temperature β (S 70 ). That is, if the temperature sensor  64  detects a temperature of the fuel cell stack  14  and thereby applies to the control unit  80 , the control unit  80  compares a temperature of the fuel cell stack  14  with the set temperature β. Herein, the set temperature β is preferably set as approximately 80° C.  
      In said process, if a temperature of the fuel cell stack  14  is judged to be higher than the set temperature, the cooling fan  20  is operated thus to prevent a temperature of the fuel cell stack  14  from being increased more than the set temperature β.  
      Again, it is judged that the system is a purge mode state (S 90 ).  
      If the system is not a purge mode state, it is judged that a temperature of the fuel cell stack  14  becomes lower than the set temperature a (S 100 ).  
      Also, if it is judged that a temperature of the fuel cell stack  14  becomes lower than the set temperature α, the control unit  80  stops an operation of the cooling fan  20  (S 110 ).  
      In the steps of S 60  and S 90 , if it is judged that the system is a purge mode state, that is, if the user adjusts the on/off switch  66  into off, the control unit  80  operates the cooling fan  20  by electric signals applied from the on/off switch  66  thus to perform a cooling of the fuel cell stack  14  and to perform a purge operation of the system (S 120  and S 130 ).  
      Herein, the purge operation is an operation for recollecting fuel remaining at each line of the system or the fuel cell stack  14  into the fuel tank  16  before stopping the system.  
      Various embodiments of the purge operation will be explained as follows.  
      By the purge operation according to one embodiment, the control unit  80  drives the recycling pump  46  for a certain time when the system is stopped and thereby recollects fuel remaining at the fuel cell stack  14  and each line to the fuel tank  16  through the recycling line  48 .  
      By the purge operation according to the second embodiment, the control unit  80  reversely drives the fuel pump  28  when the system is stopped, and thereby returns fuel remaining at the fuel supplying line  26  and the fuel cell stack  14  to the fuel tank  16 .  
      By the purge operation according to the third embodiment, the control unit  80  operates the three-valve thus to connect the air supplying line and the anode of the fuel cell stack each other and the air pump is driven thus to inject air into the anode, thereby returning fuel remaining at the anode to the fuel tank  16  through the recycling line  48 .  
      Then, if the purge operation is completed, the system is stopped (S 140 ).  
      The purge mode can be applied to any step since the user can stop the fuel cell system if necessary while the system is operated. Also, if the on/off switch  66  is adjusted into on in order to re-operate the system by the user after the system is stopped, a power of the battery is transmitted to each part of the system thus to repeat said processes (S 150 ).  
      According to the fuel cell system and the control method thereof, at the first stage, fuel is heated by using heat generated when fuel power is mixed with water, and after the system is operated, fuel is heated by using hydrogen generated at the anode after reaction. Therefore, a temperature of the fuel cell stack can reach a goal temperature within the shortest time thus to enhance a function and a reliability of the fuel cell.  
      Also, when the fuel cell system is temporarily stopped or an operation of the fuel cell system is finished, the cooling fan is operated thus to cool the fuel cell system within a short time, thereby enhancing a stability of the system.  
      Besides, when the system is stopped, fuel remaining at the fuel cell stack and each system is returned to the fuel tank thus to increase a performance.  
      It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.