Patent Publication Number: US-2023155148-A1

Title: Fuel cell system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. § 119 to Korean Application No. 10-2021-0155853, filed on Nov. 12, 2021, whose entire subject matter is hereby incorporated by reference. 
     BACKGROUND 
     1. Field 
     The present disclosure relates to a fuel cell system, and more particularly, to a fuel cell system that warms up a stack or generates hot water by recovering waste heat of exhaust gas that is discharged from a fuel processing apparatus. 
     2. Background 
     A fuel cell system is a generation system that generates electric energy by electrochemically reacting hydrogen included in a hydrocarbon based material (e.g., methanol, ethanol, natural gas, etc.) with oxygen. 
     A fuel cell system may include a fuel processing apparatus for reforming fuel including a hydrogen atom into hydrogen gas, and a stack for generating the electric energy by using the hydrogen gas supplied from the fuel processing apparatus. The fuel cell system may include a heat exchanger and a cooling water pipe that cool the stack and recovery heat, a power transform apparatus that transforms produced direct current power into alternated current power, etc. 
     Based on a generation operation of the fuel cell system, a speed of an electrochemical reaction of oxygen and hydrogen made in the stack depends on a temperature of the stack, and the operation may be performed while keeping an appropriate temperature according to a type of stack. However, upon an initial generation operation of the fuel cell system, since the temperature of the stack corresponds to a low-temperature state, there may be a problem in that the generation is not smoothly achieved until the temperature of the stack reaches an appropriate predetermined temperature. Further, as a result, there may be a problem in that a significant time is required until normal power generation of the fuel cell system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Arrangements and embodiments may be described in detail with reference to the following drawings in which like reference numerals refer to like elements and wherein: 
         FIG.  1    is a schematic view for a configuration of a fuel processing apparatus according to an embodiment of the present disclosure; 
         FIG.  2    is a configuration diagram of a fuel cell system according to an embodiment of the present disclosure; 
         FIG.  3    is a systematic diagram illustrating a fuel processing apparatus, a stack, a water supply tank, a heat recovery tank, and a waste heat recovery unit according to an embodiment of the present disclosure; 
         FIGS.  4  and  5    are diagrams for describing that in a warm-up mode and/or a reforming mode of a fuel cell system, waste heat of a fuel processing apparatus is recovered to warm up cooling water stored in a water supply tank according to an embodiment of the present disclosure; 
         FIGS.  6  and  7    are diagrams for describing that in a power generation mode of the fuel cell system, the waste heat of the fuel processing apparatus is recovered to heat hot water stored in a heat recovery tank according to an embodiment of the present disclosure; and 
         FIG.  8    is a flowchart for a control method of a fuel cell system according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure will be described in detail with reference to the drawings. Parts not associated with required description are not illustrated to clearly and briefly describe the present disclosure and the same reference numerals are used for the same or extremely similar part throughout the specification. 
     Suffixes “module” and “unit” for components used in the following description are given in consideration of easy preparation of the specification only and do not have their own particularly important meanings or roles. Accordingly, the “module” and “unit” may be used interchangeably. 
     It should be understood that the term “include” or “have” indicates that a feature, a number, a step, an operation, a component, a part or the combination thereof described in the specification is present, but does not exclude a possibility of presence or addition of one or more other features, numbers, steps, operations, components, parts or combinations thereof, in advance. 
     Further, terms such as first, second, etc., may be used in order to describe various elements, but the elements are not limited by the terms. The terms may be used only for distinguishing one element from the other element. 
       FIG.  1    is a schematic view for a configuration of a fuel processing apparatus according to an embodiment of the present disclosure. The fuel processing apparatus may refer to pipes that connect components and/or allow flow of liquid/gas. The pipes (and paths) may also be structures such as channels, conduits, paths, etc. 
     The fuel processing apparatus  10  (or fuel processor) may include a desulfurizer  110 , a burner  120 , a steam generator  130  (or vapor generator), a reformer  140 , a first reactor  150 , and/or a second reactor  160 . The fuel processing apparatus  10  may further include at least one mixer  111  and  112 . Each of the components of the fuel processing device may be a structural component. 
     The desulfurizer  110  may perform a desulfurization process of removing a sulfur compound included in fuel gas. For example, the desulfurizer  110  may have an adsorbent therein. In this example, the sulfur compound included in the fuel gas passing through the inside of the desulfurizer  110  may be adsorbed into the adsorbent. The adsorbent may be composed of a metal oxide, Zeolite, activated carbon, etc. 
     The desulfurizer  110  may further include a foreign substance included in fuel gas. 
     The burner  120  may supply heat to the reformer  140  so as to promote a reforming reaction in the reformer  140 . For example, the fuel gas discharged from the desulfurizer  110  and air introduced from the outside may be mixed by the first mixer  111  and supplied to the burner  120 . In this example, the burner  120  combusts mixed gas in which the fuel gas and the air are mixed to generate combination heat. In this example, an internal temperature of the reformer  140  may be maintained to an appropriate temperature (e.g., 800° C.) by the heat supplied by the burner  120 . 
     The exhaust gas generated by the burner  120  by the combustion of the mixed gas may be discharged to the outside of the fuel processing apparatus  10 . 
     The steam generator  130  (or vapor generator) may vaporize water and discharge the vaporized water as a water vapor. For example, the steam generator  130  absorbs the heat from the exhaust gas generated by the burner  120 , and the first reactor  150  and/or the second reactor  160  to vaporize the water. 
     The steam generator  130  may be disposed adjacent to a pipe in which the exhaust gas discharged from the first reactor  150 , the second reactor  160 , and/or the burner  120  flows. 
     The reformer  140  may perform a reforming process of generating hydrogen gas from fuel gas from which the sulfur compound by using a catalyst. For example, the fuel gas discharged from the desulfurizer  110  and the water vapor discharged from the steam generator  130  may be mixed by the second mixer  112  and supplied to the reformer  140 . In this example, when the fuel gas supplied to the reformer  140  reforming-reacts in the reformer  140 , the hydrogen gas may be generated. 
     The first reactor  150  may reduce carbon monoxide generated by the reforming action among components included in the gas discharged from the reformer  140 . For example, the carbon monoxide included in the gas discharged from the reformer  140  reacts with the water vapor inside the first reactor  150 , and as a result, carbon dioxide and the hydrogen may be generated. In this example, the internal temperature of the first reactor  150  may be a temperature (e.g., 200° C.) lower than the internal temperature of the reformer  140  and higher than a room temperature. 
     The first reactor  150  may be referred to as shift reactor. 
     The second reactor  160  may reduce carbon monoxide which remains among components included in the gas discharged from the first reactor  150 . For example, a preferential oxidation (PROX) reaction may occur in which the carbon monoxide included in the gas discharged from the first reactor  150  reacts with the oxygen inside the second reactor  160 . 
     Meanwhile, in the example of the preferential oxidation reaction, since a large amount of oxygen is required, additional supply of air is required, and the hydrogen is diluted by the additionally supplied air and a concentration of the hydrogen supplied to the stack is reduced. Accordingly, in order to overcome such a disadvantage, a selective methanation reaction in which the carbon monoxide and the hydrogen react may be utilized. 
     The gas discharged from the fuel processing apparatus  10  via the reformer  140 , the first reactor  150 , and/or the second reactor  160  may be referred to as reforming gas. 
     The stack  20  may incur an electrochemical reaction in the reforming gas supplied from the fuel processing apparatus  10  to generate electric energy. 
     The stack  20  may be constituted by stacking a single cell in which the electrochemical reaction occurs. 
     The single cell may be constituted by a membrane electrode assembly (MEA) in which a fuel electrode and an air electrode are disposed around an electrolyte membrane, a separator, etc. In the fuel electrode of the membrane electrode assembly (MEA), the hydrogen is separated into hydrogen ions and electrons by the catalyst to generate electricity and in the air electrode of the membrane electrode assembly (MEA), the hydrogen ions and the electrons are combined with the oxygen to generate the water. 
     The stack  20  may include a stack heat exchanger for dissipating heat generated during an electrochemical reaction process. The stack heat exchanger may be a heat exchanger that uses the water as a refrigerant. For example, cooling water supplied to the stack heat exchanger may absorb the heat generated during the electrochemical reaction process, and cooling water of which temperature rises by the absorbed heat may be discharged to the outside of the stack heat exchanger. 
       FIG.  2    is a configuration diagram of a fuel cell system including a fuel processing apparatus according to an embodiment of the present disclosure. Other embodiments and configurations may also be provided. 
     The fuel cell system  1  may include a fuel processing unit I, a power generation unit II, a cooling water circulation unit III, and/or a heat recovery unit IV. The fuel cell system  1  may include a power transform unit including a power transform apparatus that transforms DC power generated by the power generation unit II into AC power. 
     The fuel processing unit I may include the fuel processing apparatus  10  (or fuel processor), a fuel valve  30  controlling a flow of the fuel gas supplied to the fuel processing apparatus  10 , a first blower  71  that makes the air flow to the fuel processing apparatus  10 , etc. 
     The power generation unit II may include stacks  20   a  and  20   b , a reforming gas heat exchanger  21  in which the heat exchange of the reforming gas discharged from the fuel processing apparatus  10  occurs, an AOG heat exchanger  22  in which heat exchange of gas discharged without reacting in the stacks  20   a  and  20   b  occurs, a humidifying device  23  for supplying moisture to air to be supplied to the stacks  20   a  and  20   b , a second blower  72  that makes (or pumps) the air flow to the stacks  20   a  and  20   b , etc. The gas discharged without reacting in the stacks  20   a  and  20   b  may be referred to as anode off gas (AOG). In an embodiment of the present disclosure, it is described that the fuel cell system  1  includes two stacks  20   a  and  20   b , but the present disclosure is not limited thereto. 
     The cooling water circulation unit III may include a water supply tank  13  for storing the water generated by the fuel cell system  1 , a water pump  38  for making (or pumping) the water flow to the fuel processing apparatus  10 , a water supply valve  39  for controlling the flow of the water to be supplied to the fuel processing apparatus  10 , a cooling water pump  43  for making (or pumping) the water flow to the reforming gas heat exchanger  21 , etc. 
     The heat recovery unit IV may include a heat recovery tank  15  for storing the water used for the heat exchange, a hot water pump  48  for making (or pumping) the water stored in the heat recovery tank  15  to flow to the outside of the heat recovery tank  15 , etc. The water stored in the heat recovery tank  15  may be referred to as hot water. The heat recovery tank  15  is connected to a hot water use place in which the hot water is used (such as a home, etc.) to supply the hot water according to a need of a user. 
     The fuel cell system  1  may include a waste heat recovery unit for heating the water stored in the water supply tank  13  or the hot water stored in the heat recovery tank  15  by recovering the waste heat of the fuel processing apparatus  10 . A detailed description for a configuration and a connection relationship of the waste heat recovery unit  400  may now be provided. 
     The fuel valve  30  may be disposed in (or on) a fuel supply path  101  in which the fuel gas to be supplied to the fuel processing apparatus  10  flows. In response to an opening level of the fuel valve  30 , a flow amount of the fuel gas may be controlled. For example, the fuel valve  30  may block the fuel supply path  101  so as to stop the supply of the fuel gas to the fuel processing apparatus  10 . 
     A first fuel flowmeter  51  for detecting the flow amount of the fuel gas which flows in the fuel supply path  101  may be disposed in (or on) the fuel supply path  101 . 
     The first blower  71  may be connected to a first external air introduction path  201  and a fuel-side air supply path  202 . The first blower  71  may make air introduced from the outside through the first external air introduction path  201  flow to the fuel processing apparatus  10  through the fuel-side air supply path  202 . 
     The air introduced into the fuel processing apparatus  10  through the fuel-side air supply path  202  may be supplied to the burner  120  of the fuel processing apparatus  10 . For example, the air introduced into the fuel processing apparatus  10  may be mixed with the fuel gas discharged from the desulfurizer  110  in the first mixer  111  and supplied to the burner  120 . 
     An air filter  91  for removing a foreign substance (such as dust included in the air) and/or an air-side check valve  81  for limiting a flow direction of the air may be disposed in (or on) the first external air introduction path  201 . 
     The fuel processing unit I may include a first internal gas path  102  through which the fuel gas discharged from the desulfurizer  110  flows to the reformer  140 . In (or on) the first internal gas path  102 , the following components may be provided: a proportional control valve  31 , an internal fuel valve  32  for controlling the flow of the exhaust gas introduced into the reformer  140 , a second fuel flowmeter  52  for detecting a flow amount of the fuel gas which flows in the internal gas path  102 , a fuel-side check valve  83  for limiting a flow direction of the fuel gas which flows in the internal gas path  102 , and/or a sulfur detection device  94 . 
     The proportional control valve  31  may control the flow amount, the pressure, etc., of the fuel gas which is discharged from the desulfurizer  110  and flows to the reformer  140  through an internal/external feedback by an electrical control scheme. 
     The sulfur detection device  94  may detect sulfur included in the fuel gas discharged from the desulfurizer  110 . The sulfur detection device  94  may include an indicator of which color is changed in reaction with a sulfur compound which is not removed by the adsorbent of the desulfurizer  110 . The indicator may include phenolphthalein, a molybdenum compound, etc. 
     The fuel processing unit I may include a second internal gas path  103  through which the fuel gas discharged from the desulfurizer  110  flows to the burner  120 . The burner  120  may use the fuel gas introduced through the second internal gas path  103  for combustion. 
     The first internal gas path  102  and the second internal gas path  103  may be in communication with each other. 
     The fuel processing apparatus  10  may be connected to a water supply path  303  in which the water discharged from the water supply tank  13  flows. In (or on) the water supply path  303  the following components may be provided: a water pump  38 , a water supply valve  39  for controlling the flow of the water, and/or a water flowmeter  54  for detecting the flow amount of the water which flows in the water supply path  303 . 
     The exhaust gas generated by the burner  120  of the fuel processing apparatus  10  may be discharged from the fuel processing apparatus  10  through an exhaust gas discharge path  210 . 
     The fuel processing apparatus  10  may be connected to a reforming gas discharge path  104 . The reforming gas discharged from the fuel processing apparatus  10  may flow through the reforming gas discharge path  104 . 
     The reforming gas discharge path  104  may be connected to the reforming gas heat exchanger  21  in which the heat exchange of the reforming gas occurs. A reforming gas valve  33  for controlling the flow of the reforming gas introduced into the reforming gas heat exchanger  21  may be disposed in (or on) the reforming gas discharge path  104 . 
     The reforming gas discharge path  104  may be in communication with a bypass path  105  through which the reforming gas discharged from the fuel processing apparatus  10  flows to the fuel processing apparatus  10 . The bypass path  105  may be connected to the fuel processing apparatus  10 . The reforming gas introduced into the fuel processing apparatus  10  through the bypass path  105  may be used as fuel for the combustion of the burner  120 . A bypass valve  34  for controlling the flow of the reforming gas introduced into the fuel processing apparatus  10  may be disposed in (or on) the bypass path  105 . 
     The reforming gas heat exchanger  21  may be connected to the reforming gas discharge path  104  in which the reforming gas discharged from the fuel processing apparatus  10  flows. The reforming gas heat exchanger  21  may be connected to a cooling water supply path  304  in which the water discharged from the water supply tank  13  flows. The reforming gas heat exchanger  21  may exchange heat between the reforming gas (introduced through the reforming gas discharge path  104 ) and the water (supplied through the cooling water supply path  304 ). 
     In (or on) the cooling water supply path  304 , a cooling water pump  43  for making (or pumping) the water stored in the water supply tank  13  to flow to the reforming gas heat exchanger  21  and/or a cooling water flowmeter  56  for detecting the flow amount of the water which flows in the cooling water supply path  304  may be disposed. 
     The reforming gas discharge path  104  may be connected to a stack gas supply path  106 . The reforming gas discharged from the reforming gas heat exchanger  21  may flow to the stacks  20   a  and  20   b  through the stack gas supply path  106 . 
     A reforming gas moisture removing device  61  for controlling an amount of moisture included in the reforming gas may be disposed in (or on) the stack gas supply path  106 . The reforming gas introduced into the reforming gas moisture removing device  61  may be discharged from the reforming gas moisture removing device  61  after the moisture is removed. 
     Condensed water generated by the removing gas moisture removing device  61  may be discharged from the removing gas moisture removing device  61  and flow to a first water recovery path  309 . A first water recovery valve  44  for controlling the flow of the water may be disposed in (or on) the first water recovery path  309 . 
     The stacks  20   a  and  20   b  may incur the electrochemical reaction in the reforming gas introduced through the stack gas supply path  106  to generate or provide the electric energy. In an embodiment, when the fuel cell system  1  includes a plurality of stacks  20   a  and  20   b , the reforming gas discharged without reacting in the first stack  20   a  may incur the electrochemical reaction in the second stack  20   b.    
     The second blower  72  may be connected to a second external air introduction path  203  which is in communication with the first external air introduction path  201 , and a stack-side air introduction path  204 . The second external air introduction path  203  may be connected to a rear end of an air filter  91 . The second blower  72  may make the air introduced through the second external air introduction path  203  flow to the stack  20  side through the stack-side air introduction path  204 . 
     A second air-side check valve  82  for limiting the flow direction of the air may be disposed in (or on) the second external air introduction path  203 . 
     An air flowmeter  53  for detecting the flow amount of the air which flows in the stack-side air introduction path  204  may be disposed in (or on) the stack-side air introduction path  204 . 
     The humidifying device  23  may supply the moisture to the air introduced through the stack-side air introduction path  204 , and discharge the air including the moisture through the stack-side air supply path  205 . 
     A stack-side air supply valve  36  for controlling the flow of the air to be supplied to the stack  20  may be disposed in (or on) the stack-side air introduction path  205 . 
     The stack-side air supply path  205  may be connected to individual supply paths  206  and  207  corresponding to the stacks  20   a  and  20   b , respectively. The air which flows through the stack-side air supply path  205  may be supplied to the stacks  20   a  and  20   b  through the individual supply paths  206  and  207 . 
     The plurality of stacks  20   a  and  20   b  may be connected to each other by a gas connection path  107 . The reforming gas discharged without reacting in the first stack  20   a  may be introduced into the second stack  20   b  through the gas connection path  107 . 
     An additional moisture removing device  62  for removing the water condensed and generated while the reforming gas passes through the first stack  20   a  may be disposed in (or on) the gas connection path  107 . 
     The water generated by the additional moisture removing device  62  may be discharged from the additional moisture removing device  62  and flow to a second water recovery path  310 . A second water recovery valve  45  for controlling the flow of the water may be disposed in (or on) the second water recovery path  310 . The second water recovery path  310  may be connected to the first water recovery path  309 . 
     The anode off gas (AOG) discharged without reacting in the stacks  20   a  and  20   b  may flow through the stack gas discharge path  108 . 
     The AOG heat exchanger  22  may be connected to the stack gas discharge path  108  in which the anode off gas (AOG) discharged from the stacks  20   a  and  20   b  flows. The AOG heat exchanger  22  may be connected to a hot water supply path  313  in which the water discharged from the heat recovery tank  15  flows. The AOG heat exchanger  22  may exchange heat between the anode off gas (AOG) (introduced through the stack gas discharge path  108 ) and the water (supplied through the hot water supply path  313 ). 
     In the hot water supply path  313 , a hot water pump  48  for making (or pumping) the water stored in the heat recovery tank  15  flow to the AOG heat exchanger  22  and/or a hot water flowmeter  55  for detecting the flow amount of the water which flows in the hot water supply path  313  may be disposed. 
     The AOG heat exchanger  22  may be connected to an AOG supply path  109 , and discharge the anode off gas (AOG) of which heat is exchanged through the AOG supply path  109 . The AOG discharged from the AOG heat exchanger  22  may flow to the fuel processing apparatus  10  through the AOG supply path  109 . The AOG supplied to the fuel processing apparatus  10  through the AOG supply path  109  may be used as the fuel for the combustion of the burner  120 . 
     An AOG moisture removing device  63  for controlling the amount of moisture included in the AOG and/or an AOG valve  35  for controlling the flow of the AOG supplied to the fuel processing apparatus  10  may be disposed in (or on) the AOG supply path  109 . The AOG introduced into the AOG moisture removing device  63  may be discharged from the AOG moisture removing device  63  after the moisture is removed. 
     Condensed water generated by the AOG moisture removing device  63  may be discharged from the AOG moisture removing device  63  and flow to a third water recovery path  311 . A third water recovery valve  46  for controlling the flow of the water may be disposed in (or on) the third water recovery path  311 . The third water recovery path  311  may be connected to the first water recovery path  309 . 
     The stack-side air discharge path  211  may be connected to individual discharge paths  208  and  209  corresponding to the stacks  20   a  and  20   b , respectively. The air discharged from the stacks  20   a  and  20   b  may flow to the stack-side air discharge path  211  through the individual discharge paths  208  and  209 . In this example, the air which flows through the stack-side air discharge path  211  may include moisture generated by the electrochemical reaction which occurs in the stacks  20   a  and  20   b.    
     The stack-side air discharge path  211  may be connected to the humidifying device  23 . The humidifying device  23  may supply the moisture to the air which flows to the stack  20  by using the moisture include in the air supplied through the stack-side air discharge path  211 . The air supplied to the humidifying device  23  through the stack-side air discharge path  211  may be discharged to a humidifying device discharge path  212  via the humidifying device  23 . 
     A stack-side air discharge valve  37  for controlling the flow of the air discharged from the stacks  20   a  and  20   b  and introduced into the humidifying device  23  may be disposed in (or on) the stack-side air discharge path  211 . 
     The water supply tank  13  may be connected to the water introduction path  301  and store the water supplied through the water introduction path  301 . In the water introduction path  301 , a first liquid filter  92  for removing a foreign substance included in water supplied from the outside and/or a water introduction valve  41  for controlling the flow of the water introduced into the water supply tank  13  may be disposed. 
     The water supply tank  13  may be connected to a water discharge path  302  and discharge at least some of the water stored in the water supply tank  13  through the water discharge path  302 . A water discharge valve  42  for controlling the flow of the water discharged from the water supply tank  13  may be disposed in (or on) the water discharge path  302 . 
     The water supply tank  13  may be connected to a water storage path  308  and store water which flows through the water storage path  308 . For example, the water which is discharged from the reforming gas moisture removing device  61 , the additional moisture removing device  62 , the AOG moisture removing device  63 , and/or the air moisture removing device  64 , and flows through the third water recovery path  311  may be introduced into the water supply tank  13  via the water storage path  308 . A second liquid filter  93  for removing a foreign substance included in the water recovered to the water supply tank  13  may be disposed in (or on) the water storage path  308 . 
     At least some of the water stored in the water supply tank  13  may flow to the reforming gas heat exchanger  21  by a cooling water pump  43 , and exchange heat with the reforming gas in the reforming gas heat exchanger  21 . The water discharged from the reforming gas heat exchanger  21  may be introduced into the stacks  20   a  and  20   b  through a stack water supply path  305 . 
     The water introduced into the stacks  20   a  and  20   b  through the stack water supply path  305  may cool the stacks  20   a  and  20   b . The water introduced into the stacks  20   a  and  20   b  may flow along a stack heat exchanger included in the stacks  20   a  and  20   b , and absorb the heat generated by the electrochemical reaction which occurs in the stacks  20   a  and  20   b.    
     The plurality of stacks  20   a  and  20   b  may be connected to each other by a water connection path  306 . The water discharged from the first stack  20   a  may be introduced into the second stack  20   b  through the water connection path  306 . 
     The water discharged from the stacks  20   a  and  20   b  may be introduced into a cooling water heat exchanger  24  through a stack water discharge path  307 . The cooling water heat exchanger  24  may exchange heat between the water discharged from the stacks  20   a  and  20   b , and the water discharged from the heat recovery tank  15 . The water discharged from the stacks  20   a  and  20   b  may flow to the water storage path  308  via the cooling water heat exchanger  24 . 
     The water discharged from the heat recovery tank  15  by the hot water pump  48  may be introduced into the AOG heat exchanger  22  via the hot water supply path  313 . The water which exchanges heat with the AOG in the AOG heat exchanger  22  may be discharged to a first hot water circulation circuit  314 . 
     An air heat exchanger  25  may be connected to the humidifying device discharge path  212  in which the air discharged from the humidifying device  23  flows. The air heat exchanger  25  may be connected to the first hot water circulation circuit  314  in which the water discharged from the AOG heat exchanger  22  flows. The air heat exchanger  25  may exchange heat between the air (introduced through the humidifying device discharge path  212 ) and the water (introduced through the first hot water circulation circuit  314 ). 
     The air which is heat exchanged in the air heat exchanger  25  may be discharged from the air heat exchanger  25  through an air discharge path  213 . The air discharge path  213  may be in communication with the exhaust gas discharge path  210 , and the exhaust gas which flows in the exhaust gas discharge path  210  and the air which flows in the air discharge path  213  may be mixed. 
     The air moisture removing device  64  may be disposed in (or on) the air discharge path  213 . The air moisture removing device  64  may control the amount of the moisture included in the air discharged to the outside. The air introduced into the air moisture removing device  64  may be discharged from the air moisture removing device  64  after the moisture is removed. 
     Condensed water generated by the air moisture removing device  64  may be discharged from the air moisture removing device  64  and flow to a fourth water recovery path  312 . The fourth water recovery valve  47  for controlling the flow of the water may be disposed in the fourth water recovery path  312 . The fourth water recovery path  312  may be connected to the water storage path  308 . 
     The water of which heat is exchanged in the air heat exchanger  25  may be discharged from the air heat exchanger  25  through a second hot water circulation path  315 . The water discharged from the air heat exchanger  25  may be introduced into the cooling water heat exchanger  24  through the second hot water circulation path  315 . 
     The cooling water heat exchanger  24  may exchange heat between the water introduced through the stack water discharge path  307  and the water introduced through the second hot water circulation circuit  315 . 
     An exhaust heat exchanger  26  may be connected to the exhaust gas discharge path  210  in which the exhaust gas flows. The exhaust heat exchanger  26  may be connected to the third hot water circulation circuit  314  in which the water discharged from the cooling water heat exchanger  24  flows. The exhaust heat exchanger  26  may exchange heat between the exhaust gas (introduced through the exhaust gas discharge path  210 ) and the water (introduced through the third hot water circulation circuit  316 ). 
     The exhaust gas of which heat is exchanged in the exhaust heat exchanger  26  may be discharged to an exhaust path  214 , and the exhaust gas which flows in the exhaust path  214  may be discharged to the outside. 
     The water of which heat is exchanged in the exhaust heat exchanger  26  may be discharged to the hot water recovery path  317 , and the water which flows in the hot water recovery path  317  may be introduced into the heat recovery tank  15 . 
     The fuel cell system  1  may include a thermometer (or temperature sensor) for sensing (or determining) a temperature. For example and as shown in  FIG.  3   , the fuel cell system  1  may include a first thermometer  472  for sensing the temperature of the cooling water stored in the water supply tank  13 , and a second thermometer  474  for sensing the temperature of the water discharged from the stacks  20   a  and  20   b , etc. As an example, the first thermometer  472  may be disposed in the cooling water supply path  304 , and may sense the temperature of the water which is discharged from the water supply tank  13  and that flows in the cooling water supply path  304 . As another example, the first thermometer  472  may be disposed in the water supply tank  13 , and may sense the temperature of the water stored in the water supply tank  13 . The second thermometer  474  may be disposed in (or on) the stack water discharge path  307 , and may sense the temperature of the water which is discharged from the stack  20  and that flows in the stack water discharge path  307 . 
     The fuel cell system  1  may include at least one controller (or control unit). The controller may include at least one processor. The processor may be a general processor such as a central processing unit (CPU). Of course, the processor may be a dedicated device such as ASIC or another hardware based processor. The controller may be a structural device. The controller may control the components of the fuel cell system. 
     The controller may control an overall operation of the fuel cell system  1 . The controller may be connected to each component provided in the fuel cell system  1 , and transmit and/or receive a signal between respective components. 
     The controller may process the signal received from each component provided in the fuel cell system  1 , and transmit a control signal according to a result of processing the signal to each component provided in the fuel cell system  1 . For example, the controller may control an opening level of each valve provided in the fuel cell system  1 . Further, the controller may switch each valve provided in the fuel cell system  1 . The controller may control valves based on temperature (with respect to a set temperature). 
       FIG.  3    is a systematic diagram illustrating the fuel processing apparatus  10 , the stack  20 , the water supply tank  13 , the heat recovery tank  15 , and a waste heat recovery unit  400  in the fuel cell system  1  according to an embodiment of the present disclosure. Other embodiments and configurations may also be provided. 
     The waste heat recovery unit  400  may recover the waste heat of the exhaust gas that is discharged from the fuel processing apparatus  10  through cooling water which flows along a cooling water line. The waste heat recovery unit  400  may be connected to each of the fuel processing apparatus  10 , the water supply tank  13 , and the heat recovery tank  15 . The waste heat recovery unit  400  may form a cooling water line that circulates through the fuel processing apparatus  10 , the water supply tank  13 , and the heat recovery tank  15 . 
     The waste heat recovery unit  400  may include a plurality of heat exchangers  410 ,  412 , and  414  disposed in the fuel processing apparatus  10 , the water supply tank  13 , and the heat recovery tank  15 , respectively. The waste heat recovery unit  400  may include a first heat exchanger  410  disposed in the fuel processing apparatus  10 , a second heat exchanger  412  disposed in the water supply tank  13 , and a third heat exchanger  414  disposed in the heat recovery tank  15 . 
     In the first heat exchanger  410 , the exhaust gas discharged from the fuel processing apparatus  10  and the cooling water supplied from the water supply tank  13  or the heat recovery tank  15  may exchange heat. The first heat exchanger  410  may be disposed to be close to an exhaust outlet of the burner  120 . 
     In the second heat exchanger  412 , the cooling water (of which heat is exchanged in the first heat exchanger  410 ) and the water stored in the water supply tank  13  may exchange heat. 
     In the third heat exchanger  414 , the cooling water (of which heat is exchanged in the first heat exchanger  410 ) and the hot water stored in the heat recovery tank  15  may exchange heat. 
     The waste heat recovery unit  400  may form the cooling water line, and include first heat recovery pipes  422  and  424  connecting the first heat exchanger  410  and the second heat exchanger  412  and second heat recovery pipes  432  and  434  connecting the first heat exchanger  410  and the third heat exchanger  414 . The pipes may also be considered as conduits, channels, paths, etc. 
     The first heat recovery pipes  422  and  424  may include a 1-1 st  heat recovery pipe  422  connecting a discharge end of the second heat exchanger  412  and an introduction end of the first heat exchanger  410 . The cooling water which exchanges heat with the water stored in the water supply tank  13  may flow in the 1-1 st  heat recovery pipe  422 . 
     The first heat recovery pipes  422  and  424  may include a 1-2 nd  heat recovery pipe  424  connecting the introduction end of the second heat exchanger  412  and the discharge end of the first heat exchanger  410 . The cooling water which exchanges heat with the exhaust gas of the fuel processing apparatus  10  may flow in the 1-2 nd  heat recovery pipe  424 . 
     A heat supply pump  440  forming a flow of the cooling water which is circulated in the cooling water line may be disposed in (or on) the 1-2 nd  heat recovery pipe  424 . 
     The waste heat recovery unit  400  may include a bypass pipe  466  in which cooling water bypassing the first heat exchanger  410  flows. The bypass pipe  466  may be disposed at the first heat exchanger  410  side. The bypass pipe  466  may be connected to an introduction end (recovery pipe  422 ) of the first heat exchanger  410  and a discharge end (recovery pipe  424 ) of the first heat exchanger  410 . 
     The second heat recovery pipes  432  and  434  may include a 2-1 st  heat recovery pipe  432  connecting the discharge end of the third heat exchanger  414  and the introduction end of the first heat exchanger  410 . Specifically, the 2-1 st  heat recovery pipe  432  may be joined to the 1-1 st  heat recovery pipe  422  and connected to the introduction end of the first heat exchanger  410 . Accordingly, the cooling water which exchanges heat with the hot water stored in the heat recovery tank  15  may flow in the 2-1 st  heat recovery pipe  432 . 
     The second heat recovery pipes  432  and  434  may include a 2-2 nd  heat recovery pipe  434  connecting the introduction end of the third heat exchanger  414  and the discharge end of the first heat exchanger  410 . Specifically, the 2-2 nd  heat recovery pipe  434  may be branched from the 1-2 nd  heat recovery pipe  424  and connected to the discharge end of the first heat exchanger  410 . Accordingly, the cooling water which exchanges heat with the exhaust gas of the fuel processing apparatus  10  may flow in the 2-2 nd  heat recovery pipe  434 . 
     The waste heat recovery unit  400  may include a heat recovery valve  452  for supplying the cooling water discharged from the water supply tank  13  or the heat recovery tank  15  to the fuel processing apparatus  10 . The heat recovery valve  452  may be controlled by the controller. The heat recovery valve  452  may operate so as for the cooling water discharged from the second heat exchanger  412  to be supplied to the first heat exchanger  410  or bypass the first heat exchanger  410 . The heat recovery valve  452  may operate so as for the cooling water discharged from the third heat exchanger  412  to be supplied to the first heat exchanger  410  or bypass the first heat exchanger  410 . 
     The heat recovery valve  452  may be disposed in (or on) the 1-1 st  heat recovery pipe  422 . The heat recovery valve  452  may be constituted by a 3-way valve. The introduction end of the heat recovery valve  452  may be connected to an upstream of the 1-1 st  heat recovery pipe  422 . A first discharge end of the heat recovery valve  452  may be connected to a downstream of the 1-1 st  heat recovery pipe  422 . A second discharge end of the heat recovery valve  452  may be connected to the bypass pipe  466 . 
     The waste heat recovery unit  400  may include a heat supply valve  462  for supplying the cooling water discharged from the fuel processing apparatus  10  to the water supply tank  13  or the heat recovery tank  15 . The heat supply valve may be controlled by the controller. The heat supply valve  462  may operate to supply the cooling water discharged from the first heat exchanger  410  to the second heat exchanger  412  or the third heat exchanger  414  or supply the cooling water bypassing the first heat exchanger  410  to the second heat exchanger  412  or the third heat exchanger  414 . 
     The heat supply valve  462  may be disposed in the 1-2 nd  heat recovery pipe  424 . The heat recovery valve  452  may be constituted by the 3-way valve. The introduction end of the heat supply valve  462  may be connected to the upstream of the 1-2 nd  heat recovery pipe  424 . The first discharge end of the heat supply valve  462  may be connected to the downstream of the 1-2 nd  heat recovery pipe  424 . The second discharge end of the heat supply valve  462  may be connected to the 2-2 nd  heat recovery pipe  434 . 
     The waste heat recovery unit  400  may recover the waste heat of the exhaust gas by exchanging heat between the cooling water which flows along the cooling water lines  422 ,  424 ,  432 , and  434  with the exhaust gas discharged from the fuel processing apparatus  10 , and heat the water stored in the water supply tank  13  by supplying the heat-exchanged cooling water to the water supply tank  13  or heat the hot water stored in the heat recovery tank  15  by supplying the heat exchanged cooling water to the heat recovery tank  15 . 
       FIGS.  4  and  5    are systematic diagrams for an operation of the waste heat recovery unit  400  in a warm-up mode WM and/or a reforming mode RM of the fuel cell system  1 . 
     The fuel cell system  1  may operate in the warm-up mode WM of warming up the fuel processing apparatus  10  so as to reach a temperature suitable for reforming, and more specifically for warming up the reformer  140  of the fuel processing apparatus  10  with the burner  120 . Alternatively, the fuel cell system  1  may operate in the reforming mode RM of recirculating the reforming gas with the burner and repeating reforming so that concentrations of hydrogen and carbon monoxide of the reforming gas reach concentrations suitable for power generation. 
     In the warm-up mode WM, the fuel cell system  1  may close all of the reforming gas valve  33 , the bypass valve  34 , and the AOG valve  35 . In this example, since the supply of the fuel gas to the reformer  140  is interrupted, the reforming gas is not generated in the reformer  140 . Further, the reforming gas or the AOG does not flow in the reforming gas discharge path  104 , the bypass path  105 , and the AOG supply path  109  (see  FIG.  2   ). 
     In the reforming mode RM, the fuel cell system  1  may close the reforming gas valve  33  and the AOG valve  35 , and open the bypass valve  34 . In this example, the reforming gas discharged from the fuel processing apparatus  10  may be introduced into the fuel processing apparatus  10  again through the reforming gas discharge path  104  and the bypass path  105 , and used as the fuel for the combustion of the burner  120  (see  FIG.  2   ). 
     In the warm-up mode WM and/or the reforming mode RM, the fuel cell system  1  operates the heat supply pump  440  to provide the flow of the cooling water circulated in the cooling water lines  422 ,  424 , and  466 . 
     The flow of the cooling water circulated in the cooling water line in order to recover the waste heat of the exhaust gas discharged from the fuel processing apparatus  10  may now be described. 
     Referring to  FIG.  4   , the cooling water discharged from the second heat exchanger  412  may be supplied to the heat recovery valve  452  while flowing along the upstream of the 1-1 st  heat recovery pipe  422 . In this example, the heat recovery valve  452  may be switched to the first heat exchanger  410  side (or output to the first heat exchanger) so as to be connected to the downstream of the 1-1 st  heat recovery pipe  422 , and the cooling water supplied to the heat recovery valve  452  may be supplied to the first heat exchanger  410  while flowing along the downstream of the 1-1 st  heat recovery pipe  422 . 
     The cooling water supplied to the first heat exchanger  410  may be discharged after exchanging heat with the exhaust gas discharged after the combustion according to the operation of the burner  140 , and supplied to the heat supply valve  462  through the heat supply pump  440  while flowing along the upstream of the 1-2 nd  heat recovery pipe  424 . In this example, the heat supply valve  462  may be switched to the second heat exchanger  412  side (or output to the second heat exchanger) so as to be connected to the downstream of the 1-2 nd  heat recovery pipe  424 , and the cooling water supplied to the heat recovery valve  462  may be supplied to the second heat exchanger  412  while flowing along the downstream of the 1-2 nd  heat recovery pipe  424 . 
     The cooling water supplied to the second heat exchanger  412  may be discharged after exchanging heat with the water stored in the water supply tank  13 , and the cooling water discharged from the second heat exchanger  412  may be circulated in the cooling water line according to the above-described circulation cycle. 
     As a result, in the warm-up mode WM and/or the reforming mode RM, the water stored in the water supply tank  13  may exchange heat with the cooling water recovering the waste heat wasted from the exhaust gas through the first heat exchanger  410 , and may be heated in the second heat exchanger  410 . 
     The flow of the cooling water circulated in the cooling water line when the recovery of the waste heat of the exhaust gas discharged from the fuel processing apparatus  10  is completed may be described. 
     Referring to  FIG.  5   , the cooling water discharged from the second heat exchanger  412  may be supplied (or provided) to the heat recovery valve  452  while flowing along the upstream of the 1-1 st  heat recovery pipe  422 . In this example, the fuel cell system  1  may sense the temperature of the water stored in the water supply tank  13  through the first thermometer  472 , and when a water temperature value (or temperature) sensed by the first thermometer  472  is equal to or more than a first set temperature, the fuel cell system  1  may switch the heat recovery valve  452  to be connected to the bypass pipe  466  (or control the heat recovery valve  452  to open to the bypass pipe  466 ). The first set temperature as a water temperature suitable for sufficiently warming up the stack  20  in a power generation mode PM may be a value prestored in a memory of the controller, or other memory. 
     The cooling water supplied to the heat recovery valve  452  may be supplied to the heat supply valve  462  through the heat supply pump  440  while flowing along the bypass pipe  466  and the downstream of the 1-2 nd  heat recovery pipe  424 . In this example, the heat supply valve  462  may be switched to the second heat exchanger  412  side (or output to the second heat exchanger) so as to be connected to the downstream of the 1-2 nd  heat recovery pipe  424 , and the cooling water supplied to the heat recovery valve  462  may be supplied to the second heat exchanger  412  while flowing along the downstream of the 1-2 nd  heat recovery pipe  424 . 
     The cooling water supplied to the second heat exchanger  412  may be discharged after exchanging heat with the water stored in the water supply tank  13 , and the cooling water discharged from the second heat exchanger  412  may be subjected to the circulation process in the same manner. 
     As a result, when the temperature of the water stored in the water supply tank  13  is sufficiently raised, the recovery of the waste heat of the fuel processing apparatus  10  may be terminated by bypassing the cooling water to be supplied to the fuel processing apparatus  10  and the temperature of the water stored in the water supply tank  13  may be kept to a set temperature suitable for warming up the stack. 
       FIGS.  6  and  7    are systematic diagrams for operation of the fuel cell system  1  in the power generation mode PM of the fuel cell system  1 . 
     The fuel cell system  1  may operate in the power generation mode PM of generating (or providing) electricity in the stacks  20   a  and  20   b  based on the electrochemical reaction of the air and the reforming gas. 
     In the power generation mode PM, the fuel cell system  1  may open the reforming gas valve  33  and close the bypass valve  34  so as to supply the reforming gas discharged from the fuel processing apparatus  10  to the stack  20 . The fuel cell system  1  may supply the air used for the electrochemical reaction of generating the electricity to the stack  20  by driving the second blower  72  (see  FIG.  2   ). 
     In the power generation mode PM, the fuel cell system  1  operates the heat supply pump  440  to provide the flow of the cooling water circulated in the cooling water lines  422 ,  424 ,  432 , and  434 . 
     In the power generation mode PM, the fuel cell system  1  operates the cooling water pump  43  to provide the water flow by supplying the water stored in the water supply tank  13  to the stack  20 . 
     The flow of the cooling water for warming up the stack  20  at an initial stage of the power generation mode PM may now be described. 
     Referring to  FIG.  6   , the cooling water discharged from the second heat exchanger  412  may be supplied to the heat recovery valve  452  while flowing along the upstream of the 1-1 st  heat recovery pipe  422 . In this example, the heat recovery valve  452  may be switched to the first heat exchanger  410  side (or output to the first heat exchanger) so as to be connected to the downstream of the 1-1 st  heat recovery pipe  422 , and the cooling water supplied to the heat recovery valve  452  may be supplied to the first heat exchanger  410  while flowing along the downstream of the 1-1 st  heat recovery pipe  422 . 
     The cooling water supplied to the first heat exchanger  410  may be discharged after exchanging heat with the exhaust gas discharged after the combustion according to the operation of the burner  140 , and supplied to the heat supply valve  462  through the heat supply pump  440  while flowing along the upstream of the 1-2 nd  heat recovery pipe  424 . In this example, the heat supply valve  462  may be switched to the second heat exchanger  412  side (or output to the second heat exchanger) so as to be connected to the downstream of the 1-2 nd  heat recovery pipe  424 , and the cooling water supplied to the heat recovery valve  462  may be supplied to the second heat exchanger  412  while flowing along the downstream of the 1-2 nd  heat recovery pipe  424 . 
     As a result, the water stored in the water supply tank  13  exchanges heat with the cooling water (recovering the waste heat wasted from the exhaust gas) through the first heat exchanger  410  to be heated in the second heat exchanger  412 , and the heated water stored in the water supply tank  13  may be supplied to the stack  20  by operating the cooling water pump  43  to quickly warm up the stack  20  at a temperature suitable for the power generation at the initial stage of the power generation mode PM. As a result, power generation efficiency of the stack  20  may be enhanced. 
     In the power generation mode PM, the fuel cell system  1  may sense the temperature of the water discharged to the stack  20  through the second thermometer  474 . When the water temperature value (or temperature) sensed by the second thermometer  474  is equal to or more than a second set temperature, the fuel cell system  1  may judge that the warm-up of the stack  20  is completed. The second set temperature as a temperature at which a normal power generation operation is enabled as the warm-up of the stack  20  is sufficiently completed in the power generation mode PM may be the value prestored in the memory of the controller, or other memory. 
     The flow of the cooling water at a middle stage of the power generation mode PM in which the warm-up of the stack  20  is completed may now be described. 
     Referring to  FIG.  7   , the cooling water discharged from the third heat exchanger  414  may be supplied to the heat recovery valve  452  while flowing along the 2-1 st  heat recovery pipe  432 . In this example, the heat recovery valve  452  may be switched to the first heat exchanger  410  side (or output to the first heat exchanger) so as to be connected to the downstream of the 1-1 st  heat recovery pipe  422 , and the cooling water supplied to the heat recovery valve  452  may be supplied to the first heat exchanger  410  while flowing along the downstream of the 1-1 st  heat recovery pipe  422 . 
     The cooling water supplied to the first heat exchanger  410  may be discharged after exchanging heat with the exhaust gas discharged after the combustion according to the operation of the burner  140 , and supplied to the heat supply valve  462  through the heat supply pump  440  while flowing along the upstream of the 1-2 nd  heat recovery pipe  424 . In this example, the heat supply valve  462  may be switched to the third heat exchanger  414  side (or output to the third heat exchanger) so as to be connected to the 2-2 nd  heat recovery pipe  434 , and the cooling water supplied to the heat recovery valve  462  may be supplied to the third heat exchanger  414  while flowing along the 2-2 nd  heat recovery pipe  434 . 
     As a result, when the warm-up of the stack  20  is completed, the hot water stored in the heat recovery tank  15  exchanges heat with the cooling water recovering the waste heat wasted from the exhaust gas through the first heat exchanger  410  to be heated in the third heat exchanger  414 , and the hot water stored in the heat recovery tank  15  may be supplied to the hot water use place (such as the home, etc.) according to the need of the user. As a result, a waste heat recovery amount wasted from the fuel processing apparatus  10  may be increased, and total energy efficiency of the fuel cell system  1  may be enhanced. 
     In the power generation mode PM, the fuel cell system  1  may operate the hot water pump  48  so that the water discharged from the stack  20  through the cooling water heat exchanger  24  and the hot water circulated in the heat recovery tank  15  exchange heat with each other. 
       FIG.  8    is a flowchart for a control method of a fuel cell system  1  according to an embodiment of the present disclosure. 
     Referring to  FIG.  8   , the controller may initiate the operation of the fuel cell system  1  (S 100 ). After the operation of the fuel cell system  1  is initiated, the fuel cell system  1  may perform a warm-up operation WM, a reforming operation RM, and/or a power generation operation PM. 
     After S 100 , the controller may judge a current operation mode of the fuel cell system  1  (S 110 ). 
     When the current operation mode of the fuel cell system  1  is the warm-up mode WM or the reforming mode RM, the controller may operate the heat supply pump  440  (S 210 ). As a result, the flow of the cooling water circulated along the cooling water line to the waste heat recovery unit  400  may be formed. 
     After S 210 , the controller may switch the heat supply valve  462  to the water supply valve  13  side so that the cooling water which exchanges heat with the exhaust gas in the first heat exchanger  410  is supplied to the second heat exchanger  412  (S 220 ). In this example, the controller may switch the heat recovery valve  452  to the fuel processing apparatus  10  side so that the cooling water discharged from the second heat exchanger  412  is supplied to the first heat exchanger  410 . Accordingly, the cooling water circulated in the cooling water line recovers the waste heat of the exhaust gas in the first heat exchanger  410 , and then is supplied to the second heat exchanger  412 , and the cooling water supplied to the second heat exchanger  410  and the water stored in the water supply tank  13  exchange heat, and as a result, the water stored in the water supply tank  133  may be heated. 
     After S 220 , the controller may judge whether warm-up of the water stored in the water supply tank  13  is completed (S 230 ). Specifically, the controller may sense the temperature of the water stored in the water supply tank  13  through the first thermometer  472 , and when the water temperature value sensed by the first thermometer  472  is equal to or more than a first set temperature, the controller may judge that the warm-up of the water stored in the water supply tank  13  is completed. The first set temperature may be set to a water temperature suitable for sufficiently warming up the stack  20  in the power generation mode PM. 
     When the warm-up of the water stored in the water supply tank  13  is not completed (No in S 230 ), the controller may switch the heat recovery valve  452  to an inlet end  422  side of the fuel processing apparatus  10  so that the cooling water discharged from the second heat exchanger  412  is supplied to the first heat exchanger  410 . Accordingly, the cooling water circulated in the cooling water line may recover the waste heat of the exhaust gas in the first heat exchanger  410  until the temperature of the water in the water supply tank  13  reaches the first set temperature. 
     When the warm-up of the water stored in the water supply tank  13  is completed (Yes in S 230 ), the controller may switch the heat recovery valve  452  to an outlet end  424  side of the fuel processing apparatus  10  so that the cooling water discharged from the second heat exchanger  412  bypasses the first heat exchanger  410  (S 250 ). Accordingly, the cooling water circulated in the cooling water line may maintain the temperature of the water stored in the water supply tank  13  by not recovering the waste heat of the exhaust gas any longer in the first heat exchanger  410 . 
     After S 240  and S 250 , the controller returns to S 110  to judge the current operation mode of the fuel cell system  1  again. 
     When the current operation mode of the fuel cell system  1  is the power generation mode PM, the controller may judge whether the power generation mode PM is an initial state (S 310 ). As an example, the controller may judge whether the power generation mode PM is in the initial state by considering whether the warm-up of the stack  20  is completed. Specifically, the controller may sense the temperature of the water discharged to the stack  20  through the second thermometer  474 , and when the water temperature value sensed by the second thermometer  474  is equal to or more than a second set temperature, the controller may judge that the warm-up of the stack  20  is completed and judge that the current power generation mode PM corresponds to the middle stage. The second set temperature may be set to a temperature at which the normal power generation operation is enabled as the warm-up of the stack  20  is sufficiently completed in the power generation mode PM. 
     When the power generation mode PM is the initial state (Yes in S 310 ), the controller may judge whether the warm-up of the water stored in the water supply tank  13  is completed (S 320 ). Specifically, the controller may sense the temperature of the water stored in the water supply tank  13  through the first thermometer  472 , and when the water temperature value sensed by the first thermometer  472  is equal to or more than a first set temperature, the controller may judge that the warm-up of the water stored in the water supply tank  13  is completed. 
     When the warm-up of the water stored in the water supply tank  13  is not completed (No in S 320 ), the controller may operate the heat supply pump  440  (S 330 ). As a result, the flow of the cooling water circulated along the cooling water line to the waste heat recovery unit  400  may be formed. 
     After S 330 , the controller may switch the heat supply valve  462  to the water supply tank  13  side so that the cooling water which exchanges heat with the exhaust gas in the first heat exchanger  410  is supplied to the second heat exchanger  412  (S 340 ). In this example, the controller may switch the heat recovery valve  452  to the fuel processing apparatus  10  side so that the cooling water discharged from the second heat exchanger  412  is supplied to the first heat exchanger  410 . Accordingly, the cooling water circulated in the cooling water line recovers the waste heat of the exhaust gas in the first heat exchanger  410 , and then is supplied to the second heat exchanger  412 , and the cooling water supplied to the second heat exchanger  412  and the water stored in the water supply tank  13  exchange heat, and as a result, the water stored in the water supply tank  133  may be heated. 
     After S 340 , the controller may operate the cooling water pump  43  (S 350 ). Further, the controller may stop the operation of the hot water pump  48  or maintain a stop state (S 350 ). Accordingly, at the initial stage of the power generation mode PM, when the water of the water supply tank  13  supplied to the stack  20  is not sufficiently heated, the waste heat of the exhaust gas recovered from the fuel processing apparatus  10  is not supplied to the heat recovery tank  15  side, but concentratedly supplied to the water supply tank  13  side and the water of the water supply tank  13  is supplied to warm up the stack  20 . 
     When the warm-up of the water stored in the water supply tank  13  is completed (Yes in S 320 ), the controller may proceed to an operation of S 360  to be described below. 
     When the power generation mode PM is the middle stage (No in S 310 ), the controller may operate the heat supply pump  440  (S 360 ). As a result, the flow of the cooling water circulated along the cooling water line to the waste heat recovery unit  400  may be formed. 
     After S 360 , the controller may switch the heat supply valve  462  to the heat recovery tank  15  side so that the cooling water which exchanges heat with the exhaust gas in the first heat exchanger  410  is supplied to the third heat exchanger  412  (S 370 ). In this example, the controller may switch the heat recovery valve  452  to the fuel processing apparatus  10  side so that the cooling water discharged from the third heat exchanger  414  is supplied to the first heat exchanger  410 . Accordingly, the cooling water circulated in the cooling water line recovers the waste heat of the exhaust gas in the first heat exchanger  410 , and then is supplied to the third heat exchanger  414 , and the cooling water supplied to the third heat exchanger  414  and the hot water stored in the heat recovery tank  15  exchange heat, and as a result, the hot water stored in the heat recovery tank  15  may be heated. 
     After S 370 , the controller may operate the cooling water pump  43  (S 380 ). Further, the controller may operate the hot water pump  48  or maintain an operation state (S 380 ). Accordingly, at the middle stage of the power generation mode PM, when the stack  20  is sufficiently warmed up, the hot water stored in the heat recovery tank  15  may be heated as the waste heat of the exhaust gas recovered from the fuel processing apparatus  10  is supplied to the heat recovery tank  15  side, and the hot water stored in the heat recovery tank  15  may be supplied to the hot water use place such as the home, etc., according to the need of the user. 
     The present disclosure provides a fuel cell system that warms up a stack by utilizing waste heat of exhaust gas that is discharged from a fuel processing apparatus. 
     The present disclosure also provides a fuel cell system that heats hot water supplied to a hot water use place (such as a home, etc.) by utilizing the waste heat of the exhaust gas that is discharged from the fuel processing apparatus. 
     In an aspect, provided is a fuel cell system which includes: a stack generating power through an electrochemical reaction of reforming gas and air; a fuel processing apparatus generating the reforming gas supplied to the stack; a water supply tank storing the water supplied to the stack; a heat recovery tank storing hot water; a first heat exchanger disposed in the fuel processing apparatus, and exchanging heat between cooling water and exhaust gas discharged from the fuel processing apparatus; and a heat supply valve supplying the cooling water of which heat is exchanged in the first heat exchanger to the water supply tank or the heat recovery tank so as to heat the water stored in the water supply tank or the hot water stored in the heat recovery tank, in which waste heat of the exhaust gas discharged from the fuel processing apparatus is recovered to heat the water stored in the water supply tank or the hot water stored in the heat recovery tank. 
     The fuel cell system may further include a control unit controlling the heat supply valve, and the control unit switches the heat supply valve to the water supply tank side so as to supply the cooling water of which heat is exchanged in the first heat exchanger to the water supply tank in a warm-up mode of warming up the fuel processing apparatus to warm up the water stored in the water supply tank in the warm-up mode. 
     The fuel cell system may further include a heat recovery valve supplying the cooling water discharged from the water supply tank to the first heat exchanger or the heat supply valve. 
     The fuel cell system may further include a control unit controlling the heat supply valve and the heat recovery valve, and the control unit switches the heat recovery valve to the first heat exchanger side so as to supply the cooling water discharged from the water supply tank to the first heat exchanger in the warm-up mode of warming up the fuel processing apparatus to exchange heat between the exhaust gas and the cooling water in the fuel processing apparatus in the warm-up mode. 
     The fuel cell system may further include a first temperature sensor sensing a temperature of the water stored in the water supply tank, and the control unit switches the heat recovery valve to the heat supply valve side when the water temperature sensed by the first temperature sensor is equal to or more than a first set temperature to judge that the warm-up of the water stored in the water supply tank is completed and stop the recovery of the waste heat in the fuel processing apparatus. 
     The fuel cell system may include a first temperature sensor sensing the temperature of the water stored in the water supply tank; a second temperature sensor sensing a temperature of water discharged from the stack; and a control unit controlling the heat supply valve, and the control unit switches the heat supply valve to the water supply valve side so as to supply the cooling water which exchanges heat with the exhaust gas in the first heat exchanger to the water supply tank when the water temperature sensed by the first temperature sensor is equal to or more than the first set temperature and the water temperature sensed by the second temperature sensor is less than a second set temperature in a power generation mode of generating the power through the electrochemical reaction of the reforming gas and the air in the stack to heat the water stored in the water supply tank at an initial stage of the power generation mode. 
     The control unit switches the heat supply valve to the water supply valve side so as to supply the cooling water which exchanges heat with the exhaust gas in the first heat exchanger to the heat recovery tank when the temperature of the cooling water sensed by the second temperature sensor is equal to or more than the second set temperature in the power generation mode of generating the power through the electrochemical reaction of the reforming gas and the air in the stack to heat the hot water stored in the heat recovery tank at a middle stage of the power generation mode. 
     The first set temperature may be higher than the second set temperature. 
     The fuel cell system may further include a heat supply pump forming a flow of the cooling water circulated in the fuel processing apparatus, the water supply tank, and the heat recovery tank. 
     The fuel cell system may further include a cooling water pump disposed between the water supply tank and the stack, and supplying the cooling water stored in the water supply tank to the stack. 
     The fuel cell system may include a control unit controlling the operation of the cooling water pump, and the control unit operates the cooling water pump so as to supply the cooling water to the stack in the power generation mode of generating the power through the electrochemical reaction of the reforming gas and the air in the stack to warm up the stack. 
     The fuel cell system may include: a second heat exchanger disposed in the water supply tank, and exchanging heat between the cooling water which exchanges heat with the exhaust in the first heat exchanger and the cooling water stored in the water supply tank; and a third heat exchanger disposed in the heat recovery tank, and exchanging heat between the cooling water which exchanges heat with the exhaust in the first heat exchanger and the hot water stored in the heat recovery tank. 
     The fuel processing apparatus may further include a burner supplying heat energy required for generating the reforming gas, and the first heat exchanger may be disposed to be close to an outlet side of the burner from which exhaust gas generated after a combustion reaction of fuel is discharged. 
     According to various embodiments of the present disclosure, there is an effect that waste heat of exhaust gas that is discharged from a burner of a fuel processing apparatus is recovered by cooling water circulated in a waste heat recovery unit, cooling water of a water tank supplied to a stack is heated by the cooling water of the waste heat recover unit, and then supplied to the stack, and as a result, a low temperature-state stack is warmed up and power generation efficiency of a fuel cell system is enhanced. 
     Further, according to various embodiments of the present disclosure, there is an effect that the waste heat of the exhaust gas that is discharged from the burner of the fuel processing apparatus is recovered by the cooling water circulated in the waste heat recovery unit, and hot water of a heat recovery tank supplied to a hot water use place (such as a home, etc.) is heated by the cooling water of the waste heat recovery unit and total energy efficiency in the fuel cell system is enhanced. 
     The effects of the present disclosure are not limited to the aforementioned effect, and other effects, which are not mentioned above, will be apparent to a person having ordinary skill in the art from the description of the claims. 
     It is to be understood that the accompanying drawings are just used for easily understanding the embodiments disclosed in the present disclosure and a technical spirit disclosed in the present disclosure is not limited by the accompanying drawings and all changes, equivalents, or substitutes included in the spirit and the technical scope of the present disclosure are included. 
     Similarly, it may not have to be appreciated that operations are depicted in drawings in a specific order, but the operations should be performed in a specific order or a sequentially order illustrated in order to obtain a preferred result or all illustrated operations should be performed. In a specific case, multi-tasking and parallel processing may be advantageous. 
     Further, while the preferred embodiments of the present disclosure have been illustrated and described above, the present disclosure is not limited to the aforementioned specific embodiments, various modifications may be made by a person with ordinary skill in the technical field to which the present disclosure pertains without departing from the subject matters of the present disclosure that are claimed in the claims, and these modifications should not be appreciated individually from the technical spirit or prospect of the present disclosure. 
     It will be understood that when an element or layer is referred to as being “on” another element or layer, the element or layer can be directly on another element or layer or intervening elements or layers. In contrast, when an element is referred to as being “directly on” another element or layer, there are no intervening elements or layers present. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that, although the terms first, second, third, etc., may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section could be termed a second element, component, region, layer or section without departing from the teachings of the present invention. 
     Spatially relative terms, such as “lower”, “upper” and the like, may be used herein for ease of description to describe the relationship of one element or feature to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation, in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “lower” relative to other elements or features would then be oriented “upper” relative to the other elements or features. Thus, the exemplary term “lower” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Embodiments of the disclosure are described herein with reference to cross-section illustrations that are schematic illustrations of idealized embodiments (and intermediate structures) of the disclosure. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments of the disclosure should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     Any reference in this specification to “one embodiment,” “an embodiment,” “example embodiment,” etc., means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of such phrases in various places in the specification are not necessarily all referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with any embodiment, it is submitted that it is within the purview of one skilled in the art to affect such feature, structure, or characteristic in connection with other ones of the embodiments. 
     Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.