Abstract:
A stable air supply system in a fuel cell system produces heat and electricity by combining hydrogen modified from a major raw material selected from various hydrogen compounds, with oxygen existing in air. An air supply device and an air supply method in a fuel cell system, automate a lamda control, and achieve a fuel cell system operation efficiently and stably coping with pressure loss and pulsation occurring within the system, while using a minimum number of balance-of-plant (BOP) units. A method efficiently and precisely supplies air during the start-up and operation of the fuel cell system, while preventing flame failure of a burner and maintaining an appropriate carbon monoxide concentration even when an abrupt flow rate variation occurs. In this regard, an operating method is economical and stable in terms of the costs and configuration of the fuel cell system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of Korean Patent Application No. 10-2008-88953, filed on Sep. 9, 2008 in the Korean Intellectual Property Office, the disclosures of which are incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Field 
         [0003]    The present invention relates to a fuel cell system and an air supply method thereof, and, more particularly, to a fuel cell system and an air supply method thereof, which stably supply air to a stack and a burner. 
         [0004]    2. Description of the Related Art 
         [0005]    Generally, a fuel cell is an electricity generating device to directly convert chemical energy of hydrogen and oxygen contained in hydrocarbon-based fuel such as methanol, ethanol, or natural gas into electrical energy in accordance with an electrochemical reaction.  FIG. 1  is a schematic view to illustrate the electricity generating principle of such a fuel cell. When air containing oxygen is supplied to a cathode  1 , and fuel containing hydrogen is supplied to an anode  3 , a reaction reverse to electrolysis of water occurs through an electrolyte membrane  2 , so that electricity is generated. Typically, the electricity generated from such a unit cell  4 , has a voltage too low to enable the electricity to be useful. To this end, several unit cells  4  are connected in series in the form of a stack. 
         [0006]    To produce hydrogen to be supplied to such a fuel cell stack (hereinafter, simply referred to as a “stack”), a fuel treating unit is used. The fuel treating unit includes a burner to heat a reformer functioning to modify fuel (for example, city gas for domestic use), and thus to produce high-quality hydrogen, such that the reaction temperature of the reformer is maintained at an appropriate temperature. Air for the stack is supplied to the stack, together with high-quality hydrogen, for generation of electricity. Air for the burner is supplied to the burner, together with fuel, for adjustment of the temperature of the reformer. Such stack air and burner air have considerable influence on the production of high-quality hydrogen and the generation of electricity in the stack. That is, the operation performance of the fuel cell system is considerably influenced by a stack air control to determine the durability of the stack and the output power of the stack, and a lamda (the ratio of burner fuel to air) control to control the ratio of the burner air to fuel, at which the burner air and fuel are supplied to the burner, and thus to enable fuel supplied to the reformer, namely, reformer fuel, to efficiently react in the reformer while maintaining a flame at the burner. 
         [0007]    Generally, blowers or pumps are used to supply stack air and burner air, respectively. However, this is non-economical in terms of costs. In the following description, a term “air pump” will be used in common for terms “blower” and “pump”. 
         [0008]    The stack air may be used in the stack at an actual usage rate of 50% during operation of the fuel cell system. Accordingly, there may be no problem even when the supply of the stack air is roughly controlled, as compared to the supply of the burner air. However, the burner air used to maintain the temperature for the modification reaction of the reformer has considerable influence on the ignition and flame failure of the burner. Accordingly, an appropriate temperature for a modification reaction and a prevention of soot formation in the burner may be maintained only when a control for the supply amount of the burner air, namely, a lamda control, is stable. To this end, it is necessary to precisely control the supply amount of the burner air. Therefore, the configurations of balance-of-plant (BOP) units, such as pumps, blowers, valves, etc., and control methods thereof, which are used to provide an economical and effective supply system for stack air and burner air, taking into consideration the above-mentioned conditions, play an important role in the fuel cell system. 
       SUMMARY 
       [0009]    The present invention has been made in view of the above-mentioned problems, and an aspect of the present invention is to provide an air supply device in a fuel cell system and a control method thereof, which are capable of automating a lamda control, and achieving a fuel cell system operation efficiently and stably coping with pressure loss and pulsation occurring within the system, while using a minimum number of balance-of-plant (BOP) units for the supply of stack air and burner air. 
         [0010]    Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the invention. 
         [0011]    In accordance with an aspect, a fuel cell system includes: a fuel treating unit including a burner, to produce modified gas; a stack to receive the modified gas from the fuel treating unit and to generate energy; an air supplying unit to supply air to the stack and the burner; and a controller to determine an air supply amount required for a real-time operation of the stack and a real-time operation of the burner, and to control the air supplying unit to supply the determined air supply amount. 
         [0012]    The air supplying unit may include a first air supplier to control a total supply amount of the air supplied to the stack and the burner, and a second air supplier to control air supply amounts respectively supplied to the stack and the burner. 
         [0013]    The first air supplier may include an air pump to supply the air to the stack and the burner, and may be controlled, wherein the air pump supplies an air amount equal to a sum of the air supply amount supplied to the stack as a supply amount of stack air and the air supply amount supplied to the burner as a supply amount of burner air. 
         [0014]    The first air supplier may control the total supply amount of the air so that the air pump supplies the air in an amount that is greater than the sum of the stack and burner air supply amounts, to cause the air to be pressurized. 
         [0015]    The second air supplier may include a first valve to supply the stack air to the stack, and may be controlled, wherein the first valve supplies the stack air supply amount required for the stack. 
         [0016]    The second air supplier may include a second valve to supply the burner air to the burner, and may be controlled, wherein the second valve supplies the burner air supply amount required for the burner. 
         [0017]    In accordance with another aspect, a fuel cell system includes: a stack to generate energy through an electrochemical reaction of hydrogen with oxygen; a fuel treating unit including a burner, to produce hydrogen to be supplied to the stack; an air supplying unit to supply air containing the oxygen to the stack and the burner; and a main controller to control the air supplying unit, wherein the air supplying unit supplies a supply amount of stack air required in real time for the stack, and a supply amount of burner air required in real time for the burner. 
         [0018]    The air supplying unit may include a first air supplier to control a total supply amount of the air supplied to the stack and the burner, and a second air supplier to control supply amounts of the air respectively supplied to the stack and the burner. 
         [0019]    The first air supplier may include an air pump to supply the air to the stack and the burner, wherein the amount of the supplied air is equal to a sum of the air supply amount supplied to the stack as a supply amount of stack air and the air supply amount supplied to the burner as a supply amount of burner air. 
         [0020]    The air pump may control the total supply amount of the air so that the amount of the supplied air is greater than the sum of the stack and burner air supply amounts, to cause the air to be pressurized. 
         [0021]    The first air supplier may further include a first flow meter to detect the total air amount. 
         [0022]    The first flow meter may send, to the main controller and a first controller, an air supply amount varying based on a variation in the internal pressure of the fuel cell system. 
         [0023]    The first air supplier may further include the first controller to compare the actual air supply amount received from the first flow meter with the sum of the set point of the stack air supply amount and the set point of the burner air supply amount, and to determine an operation of the air pump based on a result of the comparison. Here, the “set points” mean target air supply amounts determined by the main controller and first controller, respectively. 
         [0024]    The second air supplier may include a stack air supply line to control the supply amount of the stack air supplied to the stack, and a burner air supply line to control the supply amount of the burner air supplied to the burner. 
         [0025]    The stack air supply line may include a first valve to control whether the stack air should be supplied to the stack. 
         [0026]    The first valve may be a solenoid valve (or a proportional valve) connected to the air pump, to allow and prevent the supply of the stack air. 
         [0027]    The burner air supply line may include a second valve to adjust a supply ratio of the burner air supplied to the burner. 
         [0028]    The second valve may be a proportional valve connected to the air pump, to precisely control a flow rate of the burner air. 
         [0029]    The burner air supply line may further include a second flow meter to detect a flow rate of air passing through the second valve. 
         [0030]    The air cell system may further include a main controller to determine the stack air supply amount and the burner air supply amount, and to send values obtained based on the determination to the air supplying unit. 
         [0031]    The main controller may determine the air supply amount of the air pump based on a set point of the stack air supply amount and a set point of the burner air supply amount, may compare the determined air supply amount of the air pump with an actual air supply amount of the air pump detected by the first flow meter, and may adjust a power of the air pump based on a result of the comparison. 
         [0032]    The main controller may execute a comparison between the set point of the stack air supply amount and an actual stack air supply amount and a comparison between the set point of the burner air supply amount and an actual burner air supply amount, to control the opening/closing of the first valve and the opening degree of the second valve, respectively. 
         [0033]    After the comparison between the actual burner air supply amount detected by the second flow meter and the set point of the burner air supply amount, the main controller may execute a proportional-integral (PI) control for the second valve, to adjust the burner air supply amount. 
         [0034]    The main controller may calculate the difference between the actual air supply amount detected by the first flow meter and the actual air supply amount detected by the second flow meter, and may compare the calculated difference with the set point of the burner air supply amount, to control the power of the air pump. 
         [0035]    In accordance with another aspect, an air supply method of a fuel cell system to generate energy through an electrochemical reaction of modified hydrogen with oxygen, includes: determining a supply amount of stack air supplied to a stack to perform electrochemical reaction of the hydrogen and the oxygen; determining a supply amount of burner air supplied to a burner to heat a reformer to perform a modification of the hydrogen; determining an air supply amount of an air pump based on the determined stack air supply amount and the determined burner air supply amount, and controlling an operation of the air pump in accordance with the determined air supply amount of the air pump; and controlling a flow rate of air supplied to the stack and a flow rate of air supplied to the burner, wherein the flow rates supply, in real time, the determined stack and burner air supply amounts. 
         [0036]    The controlling the operation of the air pump may include operating the air pump wherein the air supply amount of the air pump is equal to a sum of the stack air supply amount and the burner air supply amount. 
         [0037]    The controlling the operation of the air pump may include operating the air pump wherein the air pump supplies the air in an amount greater than a sum of the stack air supply amount and the burner air supply amount, in a pressurized state, to adjust a total supply amount of the air supplied to the stack and the burner. 
         [0038]    The controlling the operation of the air pump may include stopping the air pump when an air amount equal to a sum of the stack air supply amount and the burner air supply amount is supplied by an external supply pressure of the air. 
         [0039]    The controlling, in real time, the flow rate of air supplied to the stack and the flow rate of air supplied to the burner may include providing a second valve and a flow meter in a burner air supply line, and controlling the flow rate of air supplied to the burner to supply the determined burner air supply amount in accordance with an adjustment of an opening degree of the second valve and a flow rate detection of the flow meter, and providing a first valve in a stack air supply line, and controlling the flow rate of air supplied to the stack to supply the determined stack air supply amount in accordance with opening/closing of the first valve and a detection of a difference between an air supply amount detected by the flow meter and an actual air supply amount of the air pump. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0040]    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 in which: 
           [0041]      FIG. 1  is a schematic view to illustrate an electricity generating principle of a general fuel cell; 
           [0042]      FIG. 2  is a block diagram illustrating a fuel cell system according to an embodiment of the present invention; 
           [0043]      FIG. 3  is a block diagram illustrating a control configuration of the fuel cell system according to an embodiment of the present invention; 
           [0044]      FIG. 4  is a block diagram illustrating a part of an air supplying unit according to an embodiment of the present invention; 
           [0045]      FIG. 5  is a block diagram illustrating a main part of the fuel cell system according to an embodiment of the present invention; 
           [0046]      FIG. 6  is a flow chart illustrating a method to control the total air supply amount of the fuel cell system in accordance with an embodiment of the present invention; 
           [0047]      FIG. 7  is a flow chart illustrating a method to supply air to a stack and a burner via a plurality of valves in the fuel cell system in accordance with an embodiment of the present invention; and 
           [0048]      FIG. 8  is a flow chart illustrating an operation to control the opening degree of a second valve in the fuel cell system in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0049]    Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings. The embodiments are described below to explain the present invention by referring to the figures. 
         [0050]      FIG. 2  is a block diagram illustrating a fuel cell system according to an embodiment of the present invention. 
         [0051]    Referring to  FIG. 2 , the fuel cell system  10  according to the illustrated embodiment of the present invention includes a stack  20  to generate electricity, a fuel treating unit  30  to produce hydrogen to be supplied to the stack  20 , an air supplying unit  40  to supply fuel  80  to the fuel treating unit  30 , a cooling unit  50  to cool the stack  20 , and balance-of-plant (BOP) units. 
         [0052]    The stack  20  is an electricity generating device to directly convert chemical energy of hydrogen and oxygen contained in the fuel  80 , namely, hydrocarbon-based fuel such as methanol, ethanol, or natural gas into electrical energy in accordance with an electrochemical reaction. The stack  20  has a laminated structure of several unit cells or several ten unit cells each including a membrane electrode assembly (MEA) and a separator. The stack  20  constitutes a fuel cell body. In  FIG. 2 , fuel electrodes  21 , air electrodes  22 , and cooling plates  24  are conceptually shown as being included in the stack  20  according to the illustrated embodiment of the present invention. Practically, the stack  20  has a structure wherein one cooling plate  23  is installed for every laminated structure of several unit cells each including an MEA in which one fuel electrode  21  and one air electrode  22  are respectively arranged at opposite sides of one electrolyte membrane. 
         [0053]    In an electrochemical reaction, not only electricity, but also heat is generated. For this reason, it is necessary to continuously dissipate the generated heat to achieve a smooth operation of the stack  20 . To this end, one cooling plate is installed in the stack  20  for every 5 to 6 unit cells to provide a channel through which heat-exchanging cooling water passes. 
         [0054]    High-quality hydrogen is supplied to each fuel electrode  12 , for an electrochemical reaction. The hydrogen is separated into hydrogen ions and electrons by a catalyst to generate electricity. Air  60  containing oxygen is supplied to each air electrode  22 , for an electrochemical reaction. The oxygen is combined with the hydrogen ions and electrons generated from the corresponding fuel electrode  21 , thereby producing water. Each cooling plate  23 , which is arranged between adjacent unit cell groups each including several unit cells, provides a flow passage, through which cooling water for heat exchange flows, to dissipate heat generated together with electricity during the electrochemical reaction, and thus to appropriately control the temperature of the stack  20 . 
         [0055]    The fuel treating unit  30  is a modifier which produces hydrogen by modifying and purifying fuel, and supplies the hydrogen to the fuel electrodes  21  of the stack  20 . The fuel treating unit  30  includes a reformer  31  to modify the hydrocarbon-based fuel  80 , namely, modifier fuel, and thus, to produce high-quality hydrogen (modified gas), and a burner  32  to heat the reformer  31 , and thus, to maintain the reaction temperature of the reformer  31  at an appropriate temperature. The fuel treating unit  30  also includes a carbon monoxide shifter  33  and a carbon monoxide remover  34  to reduce the amount of carbon monoxide produced as a byproduct during the production of hydrogen in the reformer  31 , and first and second heat exchangers  35  and  36  to heat water  70  supplied to the reformer  31 . 
         [0056]    The reformer  31  is a section which is heated by the burner  32 , to generate a substantial modification reaction. The reformer  31  modifies the hydrocarbon-based fuel  80 , namely, reformer fuel, using a catalyst. For this function, the reformer fuel (for example, city gas for domestic use) for production of high-quality hydrogen is supplied to the reformer  31 , together with water  70  (DI-water). The steam to carbon ratio (S/C) control to control the ratio of the reformer fuel to water, at which the reformer fuel and water are supplied to the reformer  31 , is important for the control of the concentration of carbon monoxide fatally adversely affecting the stack  20 . That is, the production of high-quality hydrogen is considerably influenced by the S/C control. 
         [0057]    Hydrocarbon contains a sulfur compound. The catalyst may be easily poisoned by the sulfur compound. For this reason, it is necessary to remove the sulfur compound from the hydrocarbon-based fuel  80  before the hydrocarbon-based fuel  80  is supplied to the reformer  31 . To this end, a desulfurizer is arranged at an inlet of the reformer  31 , to cause the hydrocarbon-based fuel  80  to be subjected to a modification process. 
         [0058]    The burner  32  generates heat while burning the hydrocarbon-based fuel  80 , namely, burner fuel, such as methanol, ethanol, or natural gas. The heat generated from the burner  32  is transferred to the reformer  31 , to promote a modification reaction while maintaining the reaction temperature of the reformer  31  at an appropriate temperature. For this function, the burner fuel for the control of the temperature of the reformer  31  is supplied to the burner  32 , together with the oxygen-containing air  60 . The lamda (the ratio of burner fuel to air) control to control the ratio of the burner fuel to air, at which the burner fuel and air are supplied to the burner, enables the reformer fuel to efficiently react in the reformer while maintaining a flame at the burner  32 . That is, the production of high-quality hydrogen is considerably influenced by the lamda control. 
         [0059]    The carbon monoxide shifter  33  is a section to reduce, to 5,000 ppm (parts per million) or less, the amount of carbon monoxide produced as a by-product during the production of hydrogen in the reformer  31 . In the carbon monoxide shifter  33 , the following reaction occurs. 
         [0000]      CO+H 2 O         CO 2 +H 2    
         [0060]    Since carbon monoxide acts as a catalyst poison on the catalyst used for the fuel electrodes  21  of the stack  20 , the modified fuel  80  should not be directly supplied to the stack  20 . The modified fuel  80  may be subjected to a shift process for removal of carbon monoxide, wherein the amount of carbon monoxide produced from the modified fuel  80  may be reduced to 5,000 ppm or less. 
         [0061]    The modification section of the fuel treating unit  30 , which may require high-temperature heat of 500 to 700° C. or more, namely, the reformer  31 , receives required heat from the burner  32  using a hydrocarbon-based material as the fuel thereof. The monoxide shifter  33 , to which hydrogen-containing gas emerging from the reformer  31  is supplied, removes carbon monoxide contained in hydrogen produced in the reformer  31 , and additionally produces hydrogen. Thus, high-quality hydrogen is supplied to the stack  20  which, in turn, generates electricity and heat. 
         [0062]    The carbon monoxide remover  34  is a section of the fuel treating unit  30  to reduce the amount of carbon monoxide within an allowable range through a preferential oxidation (PROX). In the carbon monoxide remover  34 , the following reaction is carried out. 
         [0000]      CO+0.50 2           CO 2    
         [0063]    Through a forward reaction in the carbon monoxide remover  34 , the amount of carbon monoxide is reduced because carbon monoxide is transformed into carbon dioxide. As the hydrogen produced from the reformer  31  passes through the carbon monoxide remover  34 , the content of carbon monoxide in the hydrogen is reduced to 10 ppm or less. The carbon monoxide remover  34  may be unnecessary where the stack  20  is of a high temperature type having a tolerance to carbon monoxide. However, where the stack  20  is of a low temperature type, the carbon monoxide remover  34  may be required. High-quality hydrogen discharged from the carbon monoxide remover  34  is supplied to the stack  20 . 
         [0064]    The first and second heat exchangers  35  and  36  heat the water  70  supplied to the fuel treating unit  30  through a water supply pump  71 , to supply the water  70  in a steam state to the reformer  31 . The water steam reacts with carbon monoxide in the reformer  31  heated by the burner  32 , thereby producing hydrogen. 
         [0065]    The air supplying unit  40  functions to supply the oxygen-containing air  60  to the stack  20  and burner  32 . The air supplying unit  40  includes a pump or air blower (hereinafter, referred to as an “air pump”)  41  to pressurize the air  60  supplied from the external of the air supplying unit  40 , and to supply the pressurized air, a first flow meter  42  to detect the actual air supply amount of the air pump  41 , and a first valve  43  to control the supply start and end of the air  60  supplied, as stack air, to the stack  20 . The air supplying unit  40  also includes a second valve  44  to control the supply ratio of the air  60  supplied to the burner  32 , namely, the supply ratio of the burner air, and a second flow meter  45  to detect the actual supply amount of the burner air. 
         [0066]    The air pump  41  is used to supply the total amount of air  60  required in the stack  20  and burner  32  based on a set point of the supply amount of stack air and a set point of the supply amount of burner air, which are determined in a main controller  92  ( FIG. 3 ) for the performance and lamda control of the stack  20 . The main controller  92  will be described later. The RPM of the air pump  41  is automatically controlled based on an environment (for example, the air supply pressure). That is, once the set point of the stack air supply amount and the set point of the burner air supply amount are determined, the total of the set stack air supply amount and burner air supply amount is supplied based on operation of the air pump  41 . During this operation, the flow rate of the air  60  is controlled, taking into consideration an internal pressure loss, to prevent the air  60  from being unnecessarily pressurized with a positive pressure or a negative pressure. It may be possible to reduce the amount of energy consumed by the air supplying unit  40  by adjusting the power of the air pump  41 . In this case, it may also be possible to secure the durability of the air pump  41 . The operation of the air pump  41  may be controlled in accordance with a feed forward control through the main controller  92  using a hardware configuration of the air supplying unit  40  integrated with the air pump  41  ( FIG. 4 ), namely, a first controller  46  (printed circuit board (PCB)), together with the first flow meter  42 , and a feedback control through the first controller  46  to reduce the power of the air pump  41  based on the supply pressure of the air  60 , and thus, to reduce the consumption of power. Control logics for the feed forward control and feedback control are disclosed in a book by Phillips Harbor entitled “Feedback Control Systems”. 
         [0067]    The first valve  43  is a solenoid valve connected to an outlet of the air pump  41  to constitute a supply line for the stack air, together with the first flow meter  42 . The set point of the supply amount of the stack air, namely, the supply amount of the air  60  required for a desired durability of the stack  20  and a stable operation of the stack  20  through a system output stabilization control, is determined by the main controller  92 . Practically, the supply amount of the stack air is varied in real time due to various factors such as a variation in the internal pressure of the system itself and environmental factors. The first valve  43  receives, from the main controller  92 , a set point representing the supply amount of the stack air varying in real time to operate based on the received set point, to supply the required amount of the stack air to the reformer  31 . Where the first valve  43  is a solenoid valve, the main operation of the first valve  43  is divided into an operation to start the supply of air to the stack  20  and an operation to stop the supply of air to the stack  20 . On the other hand, where the first valve  43  is a proportional valve, the main operation of the first valve  43  is controlled to follow the set point of the supply amount of the stack air. To enable the stack  20  to generate an electrochemical reaction, air and hydrogen are supplied to the stack  20 . That is, high-quality hydrogen gas is supplied to the fuel electrodes  21  of the stack  20 , and the oxygen-containing air  60  is supplied to the air electrodes  22  of the stack  20 . In this case, the supply of air and fuel to the stack  20  may cause a variation in the internal pressure of the stack  20 . Consequently, the air supply amount may be influenced by the internal pressure variation. For a stable air supply control, accordingly, a stable supply of the air  60  to the stack  20  is achieved using the first valve  43  and first flow meter  42  arranged in the supply line for the stack air, together with the second flow meter  45 . 
         [0068]    The second valve  44  is a proportional valve connected to an outlet of the air pump  41  to constitute a supply line for the burner air, together with the second flow meter  45 . The set point of the supply amount of the burner air, namely, the supply amount of the air  60  required for a stable operation of the fuel cell system through the lamda control is determined by the main controller  92 . Practically, the supply amount of the burner air is varied in real time due to various factors such as a variation in the internal pressure of the system itself and a variation in air pressure. The second valve  44  receives, from the main controller  92 , a set point representing the supply amount of the burner air varying in real time to operate to supply, in real time, a required supply amount of the burner air, and thus, to supply the required amount of the burner air to the burner  32 . Thus, the second valve  44 , which is a proportional valve, is used to control the supply amount of air supplied by the air pump  41 . To enable the fuel treating unit  30  to generate a modification reaction, the fuel treating unit  30  is heated to a high temperature (about 500 to 700° C.). To heat the fuel treating unit  30 , the air  60  and fuel  80  are supplied to the burner  32 . When appropriate amounts of air  60  and fuel  80  are supplied to the burner  32 , it may be possible to prevent soot from being formed in the fuel treating unit  30 , and to achieve an enhancement in the durability of the fuel treating unit  30 . Satisfactory conditions of exhaust gas may be obtained only when the supply of the burner air is stable. In particular, it may be possible to avoid a failure of ignition of the burner  32  due to the supply of the burner air, and to avoid a failure of a flame upon a variation in the supply amount of the burner air made to control the power of the burner  32 . To enable a temperature control for the reformer  31  through a control for the supply of the burner air, a set point control logic to control the temperature of the reformer  31  and a set point control logic to control the flow rate of the burner air may be connected to function as main/sub control logics. 
         [0069]    Such a design of the air supplying unit  40  simplifies the BOP units, and thus, greatly contributes to providing economical effects. In particular, the internal feedback control of the air supplying unit  40  may reduce the mechanical influence on the air supplying unit  40 . 
         [0070]    The cooling unit  50  dissipates heat generated, together with electricity, during the electrochemical reaction process of the stack  20 , to cool the stack  20 . For this function, the cooling unit  50  includes a cooling water storage tank  51  and a third heat exchanger  52 . Heat-exchange cooling water stored in the cooling water storage tank  51  absorbs heat from the stack  20  while passing through flow passages defined in the cooling plates  23  of the stack  20 . The heat-absorbed cooling water is cooled by secondary cooling water in the third heat exchanger  52 , and then is again circulated to the cooling plates  23  of the stack  20 . 
         [0071]    Reference numeral “ 53 ” designates a hot water storage tank which stores the secondary cooling water heat-exchanged with the cooling water circulating the stack  20 , to allow the stored secondary cooling water to be used as hot water for domestic use or the like. Reference numeral “ 54 ” designates a fourth heat exchanger to heat water emerging from the air electrodes of the stack  20 , and to recover the heated water to the water supply pump  71 . 
         [0072]      FIG. 3  is a block diagram illustrating a control configuration of the fuel cell system according to an embodiment of the present invention. For the control configuration, the fuel cell system includes a temperature sensor  90  and a driver  94 , together with the main controller  92 . 
         [0073]    The temperature sensor  90  detects the reaction temperature of the reformer  31 , and inputs a value representing the detected reaction temperature to the main controller  92 . 
         [0074]    The main controller  92  controls the overall operation of the fuel cell system  10  including the supply of the stack air and the supply of the burner air. The main controller  92  determines a set point of the supply amount of stack air and a set point of the supply amount of burner air for a stable system output and lamda control. The main controller  92  then determines a set point of the air supply amount of the air pump  41  based on the determined set point of the stack air supply amount and the determined set point of the burner air supply amount. The determined set point of the air supply amount of the air pump  41  is sent from the main controller  92  to the first controller  46 . The first controller  46  compares the set point of the air supply amount of the air pump  41  with the actual air supply amount of the air pump  41  detected by the first flow meter  42 , and adjusts the power of the air pump  41  based on the result of the comparison. 
         [0075]    The main controller  92  may be configured to control the set point of the air supply amount of the air pump  41  such that the air supply amount of the air pump  41  is greater than the sum of the supply amount of stack air and the supply amount of burner air. In this case, a certain amount of air  60  remains because the stack air and burner air are supplied in amounts based on the set points thereof to the first and second valves  43  and  44 , respectively. Due to the remaining fuel amount, the internal pressure of the fuel supply line increases. As a result, it is possible to reduce the supply pulsation of the air  60  occurring due to the air pump  41 . 
         [0076]    For the control of the air supply amount of the stack  20 , the main controller  92  controls the opening/closing of the first valve  43  such that the stack air is supplied to the stack  20  via the first valve  43  in an amount based on the set point of the stack air supply amount. The main controller  92  derives a difference between the air supply amount of the air pump  41  detected by the first flow meter  42  and an actual burner air supply amount detected by the second flow meter  45 , and determines whether the stack air is supplied in a target air supply amount. In accordance with the result of the determination, the main controller  92  controls the opening/closing of the first valve  43 , and controls the set point of the air supply amount of the air pump  41 . 
         [0077]    In addition, the main controller  92  compares a set point of the temperature of the reformer  31  determined, for the lamda control, in accordance with a load, with the actual temperature of the reformer  31  detected by the temperature sensor  90 . In accordance with the result of the comparison, the main controller  92  determines the set point of the burner air supply amount. Thereafter, the main controller  92  compares the determined set point of the burner air supply amount with the actual burner air supply amount detected by the second flow meter  45 . Based on the result of the comparison, the main controller  92  controls the opening degree of the second valve  44 , using the PI control system, such that the burner air is supplied to the burner  32  via the second valve  44  in an amount corresponding to the determined set point of the burner air supply amount. 
         [0078]    The driver  94  operates the air pump  41 , first valve  43 , and second valve  44  in accordance with drive control signals from the main controller  92 . 
         [0079]      FIG. 4  is a block diagram illustrating a part of the air supplying unit according to an embodiment of the present invention. 
         [0080]    Referring to  FIG. 4 , the first controller  46  is a PCB having a hardware configuration integrated with the air pump  41 . The first controller  46  receives the set point of the air supply amount of the air pump  41  from the main controller  92 , compares the received set point with the actual air supply amount of the air pump  41  detected by the first flow meter  42 , and adjusts the power of the air pump  41  based on the result of the comparison. 
         [0081]    Hereinafter, the fuel cell system having the above-described configuration, operations of an air supply method thereof, and functions and effects thereof will be described. 
         [0082]      FIG. 5  is a block diagram illustrating a main part of the fuel cell system according to an embodiment of the present invention.  FIG. 5  illustrates a configuration of the air supplying unit  40  enabling the fuel cell system  10  to operate efficiently and stably against pressure loss and pulsation. 
         [0083]    Referring to  FIG. 5 , the air  60  is supplied, as stack air, to the stack  20  via the air pump  41  and first valve  43 , together with high-quality hydrogen. The air  60  is also supplied, as burner air, to the burner  32  via the air pump  41  and second valve  44 , together with the fuel  80 . 
         [0084]    Now, the control method to supply the air  60 , as stack air and burner air, to the stack  20  and burner  32 , using the configuration of the air supplying unit  40  shown in  FIG. 5  will be described with reference to  FIGS. 6 to 9 . 
         [0085]      FIG. 6  is a flow chart illustrating a method to control the total air supply amount of the fuel cell system in accordance with an embodiment of the present invention. The illustrated method controls the supply of the air  60 , using an air supply line (air pump and first flow meter) designated by “{circle around (1)}” in  FIG. 5 . 
         [0086]    Referring to  FIG. 6 , the main controller  92  determines a set point of the supply amount of stack air for a stable system output control of the fuel cell system  10  and a set point of the supply amount of burner air for a stable lamda control of the fuel cell system  10 . Thereafter, the main controller  92  determines a set point of the air supply amount of the air pump  41  based on the determined set point of the stack air supply amount and the determined set point of the burner air supply amount, and then sends the determined set point of the air supply amount of the air pump  41  to the first controller  46  ( 100  to  102 ). 
         [0087]    The first controller  46  compares the set point of the air supply amount of the air pump  41  received from the main controller  92  with an actual air supply amount of the air pump  41 , F/M_ 1 , detected by the first flow meter  42 . When the received set point of the air supply amount of the air pump  41  is greater than the detected actual air supply amount of the air pump  41 , F/M_ 1  ( 104 ), the first controller  46  increases the power of the air pump  41 , to enable the air  60  to be supplied to the stack  20  and burner  32  in amounts corresponding to the set points of reformer and burner air supply amounts, respectively ( 106 ). 
         [0088]    On the other hand, when the received set point of the air supply amount of the air pump  41  is less than the detected actual air supply amount of the air pump  41 , F/M_ 1  ( 108 ), the first controller  46  decreases the power of the air pump  41 , to enable the air  60  to be supplied to the stack  20  and burner  32  in amounts corresponding to the set points of reformer and burner air supply amounts, respectively ( 110 ). 
         [0089]    Meanwhile, when the received set point of the air supply amount of the air pump  41  is equal to the detected actual air supply amount of the air pump  41 , F/_ 1 , the first controller  46  feeds back to operation  100 , to receive an actual air supply amount of the air pump  41 , F/M_ 1 , newly detected by the first flow meter  42 , and to compare the received actual air supply amount with the received set point of the air supply amount of the air pump  41 . Based on the result of the comparison, the first controller  46  executes the associated operations following operation  100 . 
         [0090]    In accordance with the fuel supply method of the present invention shown in  FIG. 6 , the influence of the internal pressure on the operation of the fuel cell system  10  is reduced. Accordingly, the influence of the environment on the operation of the fuel cell system  10  is reduced. 
         [0091]      FIG. 7  is a flow chart illustrating a method to supply air to the burner and reformer via a plurality of valves in the fuel cell system in accordance with an embodiment of the present invention. In accordance with this method, the main controller  92  determines the set points of the reformer and burner air supply amounts through a performance control of the stack  20  and a lamda control, respectively, to control the opening/closing of the first valve  43  and to adjust the opening degree of the second valve  44 . 
         [0092]    Referring to  FIG. 7 , the main controller  92  compares the set point of the stack air supply amount with a predetermined minimum value of the set point of the stack air supply amount ( 200 ). When the set point of the stack air supply amount is greater than the predetermined minimum value of the set point of the stack air supply amount, the main controller  92  opens the first valve  43  such that the air  60  is supplied in real time. Thereafter, the main controller  92  controls the air pump  41  while comparing the set point of the stack air supply amount with the actual air supply amount of the stack  20  determined through the first and second flow meters  42  and  45  ( 202 ). 
         [0093]    When it is determined at operation  200  that the determined set point of the stack air supply amount is not greater than the predetermined minimum value of the set point of the stack air supply amount, the main controller  92  closes the first valve  43  because it is unnecessary to supply the fuel to the stack  20  ( 204 ). 
         [0094]    Subsequently, the main controller  92  compares the set point of the burner air supply amount determined through the lamda control with the predetermined minimum value of the set point of the burner air supply amount ( 206 ). When the determined set point of the burner air supply amount is greater than the predetermined minimum value of the set point of the burner air supply amount, the main controller  92  adjusts the opening degree of the second valve  44  to be  20 %, such that the air  60  is supplied in real time. Thereafter, the main controller  92  controls the second valve  44  such that the actual air supply amount of the burner  32  determined through the second flow meter  45  is the determined set point (target value), while comparing the actual fuel supply amount of the burner  32  with the determined set point ( 208 ). The opening degree of  20 % is an opening degree of the second valve  44  set when the second valve  44  is initially opened. The second valve  44  may be initially opened at an opening degree of 10%. 
         [0095]    On the other hand, when it is determined at operation  206  that the determined set point of the burner air supply amount is not greater than the predetermined minimum value of the set point of the burner air supply amount, the main controller  92  closes the second valve  44  because it is unnecessary to supply the air to the burner  32  ( 210 ). If the first valve  43  is also in a closed state in this case ( 212 ), the main controller  92  stops the air pump  41  because it is unnecessary to supply the stack air and burner air ( 214 ). 
         [0096]    Thereafter, the main controller  92  returns to a loop to precisely control the flow rates of the first and second valves  43  and  44  ( 216 ). 
         [0097]    In accordance with the fuel supply method of the present invention shown in  FIG. 7 , it is possible to enhance the durability and efficiency of the fuel cell system  10  because the supply of the air  60  is more efficiently and precisely achieved during the start-up and operation of the stack  20 . 
         [0098]      FIG. 8  is a flow chart illustrating an operation to control the opening degree of the second valve in the fuel cell system in accordance with an embodiment of the present invention. The illustrated method controls the supply of the air  60 , using a burner air supply line (second valve and second flow meter) designated by “{circle around (2)}” in  FIG. 5 . 
         [0099]    Referring to  FIG. 8 , the main controller  92  sets a reformer temperature value in accordance with a load, to achieve a stable lamda control, and determines a set point of a burner air supply amount required in accordance with the set reformer temperature ( 400 ). 
         [0100]    Thereafter, the main controller  92  compares the reformer temperature value set in accordance with the load, with the reformer temperature value (TC value) detected by the temperature sensor  90  ( 402 ). When the set reformer temperature value is greater than the reformer temperature value (TC value), the main controller  92  increases the set point of the burner air supply amount, to increase the supply amount of the burner air ( 404 ). This is because the supply amount of fuel to the burner  32  is increased for an increase in reformer temperature value, and the burner air supply amount is increased based on the increased burner fuel supply amount for a lamda control. 
         [0101]    When it is determined at operation  402  that the set reformer temperature value is not greater than the reformer temperature value (TC value), the main controller  92  determines whether the set reformer temperature value is less than the reformer temperature value (TC value) ( 406 ). When the set reformer temperature value is not less than the reformer temperature value (TC value), the main controller  92  proceeds to operation  410 , and executes operation  410  and subsequent operation. 
         [0102]    On the other hand, when it is determined at operation  406  that the set reformer temperature value is less than the reformer temperature value (TC value), the main controller decreases the set point of the burner air supply amount, to correspondingly control the supply of the burner air ( 408 ). 
         [0103]    Thereafter, the main controller  92  controls the opening degree of the second valve  44  to supply the set point of the burner fuel supply amount adjusted in accordance with the reformer temperature value ( 410 ). The main controller  92  compares the set point of the burner air supply amount with the actual burner air supply amount F/M_ 2  detected by the second flow meter  45  ( 412 ). 
         [0104]    When it is determined at operation  412  that the set point of the burner air supply amount is greater than the actual burner air supply amount F/M_ 2 , the main controller  92  executes a PI control to increase the opening degree of the second valve  44 , to precisely control the supply of the burner air ( 414 ). 
         [0105]    When it is determined at operation  412  that the set point of the burner air supply amount is not greater than the actual burner air supply amount F/M_ 2 , the main controller  92  determines whether the set point of the burner air supply amount is less than the actual burner air supply amount F/M_ 2  ( 416 ). When the set point of the burner air supply amount is not less than the actual burner air supply amount F/M_ 2 , the main controller  92  is fed back to operation  400 , and executes operation  400  and subsequent operations. 
         [0106]    On the other hand, when it is determined at operation  416  that the set point of the burner air supply amount is less than the actual burner air supply amount F/M_ 2 , the main controller  92  executes a PI control to decrease the opening degree of the second valve  44 , to precisely control the supply of the burner air ( 418 ). 
         [0107]    In accordance with the air supply method of the present invention illustrated in  FIG. 8 , air  60  is constantly supplied in response to a variation in pressure occurring in the fuel cell system  10 . 
         [0108]    Although a few embodiments of the present general inventive concept have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.