Patent Publication Number: US-2007104989-A1

Title: Apparatus and method for heat exchange of liquid fuel type fuel cell system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefit of Korean Patent Application No. 2005-90741, filed Sep. 28, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      Aspects of the present invention relate to a method of controlling a heat exchange which can decrease a warming-up time of a fuel cell and over-heating of the fuel cell, and a liquid fuel type fuel cell system using the same.  
      2. Description of the Related Art  
      A fuel cell is a power generation system to generate electric energy by electrochemically reacting hydrogen and oxygen. According to various sorts of electrolyte used, fuel cells can be categorized as phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, polymer electrolyte fuel cells and alkaline fuel cells, etc. These respective fuel cells are basically operated based on the same principle, but are different in view of operating temperatures, and sorts of fuels, catalyzers and electrolytes, used, etc.  
      Among other advantages over other fuel cells, the polymer electrolyte membrane fuel cell (PEMFC) has a high output feature, a low operating temperature feature, and rapid starting and answering features, and is widely applicable to a mobile power source, such as portable electronic equipment or a transportable power source, such as a power source for automobiles as well as a distributed power source, such as a stationary power plant used in a house or a public building, etc.  
      Also, as a category of polymer electrolyte membrane fuel cells, there is a liquid fuel type fuel cell, which directly supplies a liquid methanol fuel to a fuel cell. The liquid fuel type fuel cell is termed a direct methanol fuel cell (DMFC). The liquid fuel type fuel cell is advantageous in view of miniaturization because it does not use a reformer, unlike other polymer electrolyte membrane fuel cells.  
      Generally, as shown in  FIG. 1 , the liquid fuel type fuel cell system includes a membrane electrode assembly (MEA) configured of a polymer electrolyte membrane  11  to selectively pass hydrogen ions; and an anode electrode  13  and a cathode electrode  15  closely adhered respectively to both sides of the polymer electrolyte membrane  11 .  
      The liquid fuel type fuel cell system has a stack structure of a plurality of membrane electrode assemblies each functioning as a general fuel cell; and a plurality of separators  17  and  19  to supply fuel and air to the anode electrode  13  and the cathode electrode  15 , respectively, and to collect electricity, and to allow liquid fuel such as methanol supplied by a liquid pump  30  from a fuel tank  20  to be electrochemically reacted with oxygen in the air supplied by a fan  40 , etc., to generate electric energy.  
      Also, some of the conventional liquid fuel type fuel cell systems can use a mixing tank and a recycler. In this case, the mixing tank mixes and circulates non-reacted liquid fuel and water discharged from the stack to provide the mixture to the stack again, and the recycler, for example, is disposed so that it condenses a high temperature steam discharged from an outlet on a cathode side to provide the condensed steam to the mixing tank as liquid. According to such a construction as described above, the conventional liquid fuel type fuel cell system exhibits an improved efficiency.  
      However, such a conventional liquid fuel type fuel cell system as described above has disadvantages such as a warming-up time is long and the temperature of the stack rises excessively when operated for a long time. When the temperature of the stack rises excessively, the components of the stack such as the polymer electrolyte membrane may be damaged.  
     SUMMARY OF THE INVENTION  
      It is an aspect of the present invention to provide a method of controlling a heat exchanger disposed to exchange heat between an outlet on a cathode side of a fuel cell and a fuel supply route on an anode side thereof.  
      It is another aspect of the present invention to provide a liquid fuel type fuel cell which can decrease a warming-up time of a fuel cell and an excessive temperature rising of the fuel cell by controlling a heat exchanger disposed to exchange heat between an outlet on a cathode side and a fuel supply route on an anode side using the method as above, and/or realize additional advantages.  
      In order to accomplish the above, according to one aspect of the present invention, there is provided a heat exchanger of a liquid fuel type fuel cell system including an electricity generating unit to generate electricity by an electrochemical reaction between fuel supplied to an anode electrode and oxidizer supplied to a cathode electrode, including: a first flow path unit connected to an outlet on the cathode electrode; a second flow path unit connected to an inlet on the anode electrode; a heat exchanging unit to exchange thermal energy between fluids passing through the first flow path unit and the second flow path unit; and a third flow path unit connected to the inlet on the anode electrode and bypassing the heat exchanging unit.  
      While not required in all aspects, preferably, the heat exchanger may further include a first valve to control a flow ratio of fluids passing through the second flow path unit and the third flow path unit. Also, the first valve is a valve to open/close a flow path of the third flow path unit. Also, the heat exchanger may further include a second valve to open/close a flow path of the second flow path unit. Also, the heat exchanger may further include a control device to control the opening or closing the flow path of the flow path units in response to the temperature sensed from a sensor.  
      According to a second aspect of the present invention, there is provided a method of heat exchange of a liquid fuel type fuel cell system in a heat exchanger including a first flow path unit connected to an outlet on a cathode electrode of a fuel cell; a second flow path unit connected to an inlet on an anode electrode of a fuel cell; and a third flow path unit connected to the inlet on the anode electrode and bypassing a heat exchanging unit, where the heat exchanging unit exchanges thermal energy between fluids passing through the first flow path unit and the second flow path unit, the method comprising: sensing the temperature of the electricity generating unit; controlling a flow path of the third flow path unit to be in a closed state, when the temperature of the electricity generating unit is below a reference temperature; and controlling a flow path of the third flow path unit to be in an open state, when the temperature of the electricity generating unit is the reference temperature or higher and below a critical temperature.  
      While not required in all aspects, preferably, when the temperature of the electricity generating unit is the critical temperature or higher, the method of heat exchange of the liquid fuel type fuel cell system further includes transmitting a control signal to limit the power required by a load connected to the electricity generating unit to a predetermined value or less to the load.  
      Also, when the temperature of the electricity generating unit is below the reference temperature, the method of heat exchange of the liquid fuel type fuel cell system further includes controlling a flow path of the second flow path unit to be in an open state; and when the temperature of the electricity generating unit is the reference temperature or higher, and below the critical temperature, the method of heat exchange of the liquid fuel type fuel cell system further includes controlling the flow path of the second flow path unit to be in an open state. Furthermore, when the temperature of the electricity generating unit is the critical temperature or higher, the method of heat exchange of the liquid fuel type fuel cell system further includes controlling the flow path of the second flow path unit to be in a closed state and controlling the flow path of the third flow path unit to be in an open state.  
      According to a third aspect of the present invention, there is provided a liquid fuel type fuel cell system including: an electricity generating unit including an electrolyte membrane and an anode electrode and a cathode electrode positioned on both sides of the electrolyte membrane to generate electricity by an electrochemical reaction between liquid fuel supplied to the anode electrode and oxidizer supplied to the cathode electrode; a sensor to sense the temperature of the electricity generating unit; and a heat exchanging unit including a first flow path unit connected to an outlet on the cathode electrode of the electricity generating unit and a second flow path unit connected to an inlet on the anode electrode of the electricity generating unit; a third flow path unit connected to the inlet on the anode electrode and bypassing the heat exchanging unit to exchange thermal energy between the first flow path unit and the second flow path unit; and a first control member to select a main flow path of fluids passing through the second flow path unit and the third flow path unit.  
      While not required in all aspects, preferably, the liquid fuel type fuel cell system further includes a second control member to control a flow path of the second flow path unit. Also, the liquid fuel type fuel cell system further includes a control device to control the first and second control members to control the flow paths of the first and second flow path units respectively, in response to the temperature sensed from the sensor.  
      Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:  
       FIG. 1  is a diagram showing a conventional direct methanol fuel cell system;  
       FIG. 2  is a block diagram showing a liquid fuel type fuel cell system including a heat exchanger according to a first embodiment of the present invention;  
       FIG. 3A  to  FIG. 3C  are block diagrams showing three operation modes of the heat exchanger as shown in  FIG. 2 ;  
       FIG. 4  is a flow chart showing the method of heat exchange of the liquid fuel type fuel cell system as shown in  FIG. 2 ;  
       FIG. 5  is a block diagram showing a liquid fuel type fuel cell system including a heat exchanger according to a second embodiment of the present invention;  
       FIG. 6A  to  FIG. 6C  are block diagrams showing three operation modes of the heat exchanger as shown in  FIG. 5 ; and  
       FIG. 7  is a flow chart showing the method of heat exchange of the liquid fuel type fuel cell system as shown in  FIG. 5 . 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
      Reference will now be made in detail to the present embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures.  
      In the following description, a liquid fuel type fuel cell includes a direct methanol fuel cell (DMFC).  FIG. 2  is a block diagram showing a liquid fuel type fuel cell system including a heat exchanger according to a first embodiment of the present invention. Referring to  FIG. 2 , the liquid fuel type fuel cell system includes an electricity generating unit  110 , a sensor  120 , a first flow path unit  131 , a second flow path unit  132 , a third flow path unit  133 , a heat exchanging unit  134 , a flow path control valve  135 , a recycler  137 , a control device  140  and a mixing tank  150 . Here, the heat exchanger comprises a sensor  120 , a first flow path unit  131 , a second flow path unit  132 , a third flow path unit  133 , a heat exchanging unit  134 , a flow path control valve  135  and a control device  140 .  
      Describing the liquid fuel type fuel cell system in more detail, the electricity generating unit  110  is a power system to generate electric energy by electrochemically reacting fuel and oxidizer. The electricity generating unit  110  includes an anode inlet  112   a  to flow in fuel, an anode outlet  112   b  to discharge unreacted fuel and reaction products, a cathode inlet  114   a  to flow in oxidizer and a cathode outlet  114   b  to discharge unreacted oxidizer and reaction products. The fuel includes fuel in a liquid state such as methanol, ethanol and petroleum, and the oxidizer includes air or oxygen gas, etc. The electricity generating unit  110  can be a stack structure where a plurality of fuel cells, each comprise a membrane electrode assembly (MEA), are stacked, with a separator therebetween. Herein, the membrane electrode assembly has a structure where an anode electrode (termed, “fuel electrode” or “oxidation electrode”) and a cathode electrode (termed, “air electrode” or “reduction electrode”) are respectively attached to both sides of a polymer electrolyte membrane.  
      The sensor  120  is a device to measure the temperature of the electricity generating unit  110 . The sensor is properly disposed on the surface or the inside of the electricity generating unit  110 . The sensor  120  can be a temperature detecting device such as a conventional thermistor, a thermocouple, an infrared detector, a semiconductor bandgap temperature sensor or a shape memory alloy, etc.  
      The first flow path unit  131  is heated by heat generated when the electricity generating unit  110  is electrochemically reacted and includes a flow path to transmit fluid of a high temperature, that is, fluid of the electrochemical reaction temperature, discharged on the cathode side to the mixing tank  150 . One end of the first flow path unit  131  is connected to the cathode outlet  114   b  of the electricity generating unit  110 , and the other end of the first flow path unit  131  is connected to the mixing tank  150  through the recycler  137 .  
      The second flow path unit  132  includes a flow path to supply liquid fuel to the anode side of the electricity generating unit  110 . One end of the second flow path unit  132  is connected to the anode inlet  112   a  of the electricity generating unit  110 , and the other end of the second flow path unit  132  is connected to the mixing tank  150  through a first pump  152 .  
      The heat exchanging unit  134  exchanges thermal energy between the fluid of the high reaction temperature passing through the first flow path unit  131  and the liquid fuel passing through the second flow path unit  132 . The heat exchanging unit  134  may be any one of various conventional heat exchangers to absorb heat of the fluid of the reaction temperature passing through the first flow path unit  131  and transmit the absorbed heat to the liquid fuel passing through the second flow path unit  132 . Such conventional heat exchangers include a tube and shell heat exchanger, a plate and frame heat exchanger, a microchannel heat exchanger, etc.  
      The third flow path unit  133  does not pass through the heat exchanging unit  134  and includes a flow path to supply liquid fuel to an anode side of the electricity generating unit  110 . One end of the third flow path unit  133  is connected to the anode inlet  112   a  of the electricity generating unit  110 , and the other end of the third flow path unit  133  is connected to the mixing tank  150  through the first pump  152 . The first, second and third flow path units  131 ,  132  and  133  may be pipes, and in particular, the first and second flow path units  131  and  132  may be pipes having a good thermal conductivity.  
      The flow path control valve  135  is disposed in the third flow path unit  133 , to open and close a flow path of the third flow path unit  133 . The flow path control valve  135  is operated in response to a control signal CS 1  of the control device  140 . The flow path control valve  135  may be a valve to control flow and pressure of the fluids passing through the third flow path unit  133 . Also, the flow path control valve  135  may be a thermostat valve to control flow and pressure of the fluids automatically passing through the third flow path unit  133  depending on the temperature sensed from the sensor  120 . In the liquid fuel type fuel cell system the control device  140  may be omitted, when the thermostat valve is used.  
      The recycler  137  forcibly condenses a predetermined amount of the fluid of the reaction temperature passing through the first flow path unit  131 . For example, the recycler  137  condenses steam to generate water and to discharge undesired gas outward. The recycler  137  may be one of various conventional devices such as a condenser, a cooler, a radiator, etc.  
      The control device  140  senses a temperature signal TS measured in the sensor  120 , generates a first control signal CS 1  to control a state of the flow path control valve  135  in response to the sensed signal, and applies the generated signal CS 1  to the flow path control valve  135 . Also, the control device  140  generates a second control signal CS 2  to decrease the power required by a load  160  to a predetermined value or less depending on the temperature level sensed from the sensor  120 , and to transmit the generated control signal CS 2  to the load  160 . The control device  140  may be a valve control device, a fuel cell control device to control the electricity generating unit  110  or a valve control device connected to the fuel cell control device  140 .  
      The mixing tank  150  recovers unreacted fuel discharged from the electricity generating unit  110  and dilutes a high concentration of the liquid fuel stored in a fuel tank  154 . The mixing tank  150  is connected to the electricity generating unit  110  by the first, second and third flow path units  131 ,  132  and  133 . Also, the mixing tank  150  is connected to the anode outlet  112   b  of the electricity generating unit  110  by a fourth flow path unit  138  to recover unreacted fuel on the anode side of the electricity generating unit  110 . Here, the fuel stored in the mixing tank  150  is supplied to the electricity generating unit  110  by a pumping force of the first pump  152 , and the liquid fuel stored in the fuel tank  154  is supplied to the mixing tank  150  by a pumping force of a second pump  156 .  
      Meanwhile, the liquid fuel type fuel cell system may include an auxiliary power supplier (not shown) to supply power required by the peripherals of the fuel cell such as the control device  140  and the first and second pumps  152 ,  156 , etc., when starting. The auxiliary power supplier may be a battery, a capacitor, or a supercapacitor, etc.  
      An operation of the liquid fuel type fuel cell system as described above is described as follows. When fuel is supplied to the anode electrode of the electricity generating unit  110 , the fuel is ionized and oxidized as hydrogen ions (proton, H + ) and electrons (e − ) by making an electrochemical reaction in the catalyst layer. The ionized hydrogen ion is moved to a catalyst layer on the cathode through the polymer electrolytic film from the catalyst layer on the anode, and the electron is moved to the catalyst layer on the cathode side through an outside conducting wire. The hydrogen ion moved to the catalyst layer on the cathode is electrochemically reduced with oxygen in air supplied to the cathode electrode by the fan or the air pump  158  to produce heat of reaction and water. The electric energy is generated by movement of the electrons. The reactions of the electricity generating unit  110  that occur at the anode electrode with a fuel plus water mixture and at the cathode electrode can be represented by the following reaction equations: 
 
Anode electrode: CH 3 OH+H 2 O→CO 2 +6H + +6e − 
 
Cathode electrode: 6H + +3/2O 2 +6e − →3H 2 O 
 
Overall: CH 3 OH+3/2O 2 →3H 2 O+CO 2   [Reaction equation 1]
 
       FIG. 3A  to  FIG. 3C  are block diagrams showing three operation modes of the heat exchanger as shown in  FIG. 2 . Referring to  FIG. 3A  to  FIG. 3C , the heat exchanger adopted in the liquid fuel type fuel cell system includes the second flow path unit  132  passing through the heat exchanging unit  134  and the third flow path unit  133  bypassing the heat exchanging unit  134 , to supply fuel to the anode and further includes the flow path control valve  135  to control a flow path of the third flow path unit  133 , which is a bypass flow path unit. Here, it is described that the control device  140  controls the flow path control valve  135  disposed on the bypass third flow path unit  133  based on a temperature of the electricity generating unit  110  detected by the sensor  120 .  
      A reference temperature T 1  is the temperature of the electricity generating unit  110  where the electricity generating unit  110  begins to show good efficiency, a critical temperature T 2  is the temperature of the high temperature side of the electricity generating unit  110  where the electricity generating unit  110  begins to show bad efficiency, and the temperature detected in the electricity generating unit  110  is a sensed temperature Tf. Although the temperature of the electricity generating unit  110  is preferably measured at the inside thereof, the temperature may be measured at the anode outlet  112 , etc., and used by correcting for a temperature difference between the temperature at the measured location and the temperature at the inside of the electricity generating unit  110 .  
      First, the operation mode when the sensed temperature Tf is less than the reference temperature T 1  will be described with reference to  FIG. 3A . In the case of the operation mode when the sensed temperature Tf is less than the reference temperature T 1 , the flow path of the third flow path unit  133  is closed by the flow path control valve  135  so that most of the thermal energy of the fluid of the reaction temperature discharged from the cathode outlet  114   b  and passing through the first flow path unit  131  is transmitted, through the heat exchanging unit  134 , to the fuel passing through the second flow path unit  132 . In the case of the operation mode as above, the heated fuel is applied to the electricity generating unit  110 , thereby having the effect to promptly raise the temperature of the electricity generating unit  110 . Therefore, it is suitable that the operation mode as above is applied when the system is operated.  
      Next, the operation mode when the sensed temperature Tf is the reference temperature T 1  or higher, and less than the critical temperature T 2 , will be described with reference to  FIG. 3B . In the case of the operation mode when the sensed temperature Tf is the reference temperature T 1  or higher and less than the critical temperature T 2 , the flow path of the third flow path unit  133  is opened by the flow path control valve  135  so that the fuel of the mixing tank  150  is divided into the second flow path unit  132  passing through the heat exchanging unit  134  and the third flow path unit  133  bypassing the heat exchanging unit  134 , and then supplied to the electricity generating unit  110 . Therefore, only some of the fuel supplied to the electricity generating unit  110  is heated by receiving the thermal energy from the fluid of the reaction temperature discharged from the cathode outlet  114   b  and passing through the first flow path unit  131  by the heat exchanging unit  134 . In the case of the operation mode as above, the properly preheated fuel is supplied to the electricity generating unit  110  so that the temperature of the electricity generating unit  110  is prevented from dropping below the reference temperature T 1 , which is the temperature suitable for a high efficient operation, because of cold fuel during the operation of the electricity generating unit  110 .  
      Next, the operation mode when the sensed temperature Tf is the critical temperature T 2  or higher will be described with reference to  FIG. 3C . In the case of the operation mode when the sensed temperature Tf is the critical temperature T 2  or higher, the power required by the load  160  is limited by the control device  140  (refer to  FIG. 2 ) in a state where the flow path of the third flow path unit  133  is opened by the flow path control valve  135 . Then, the thermal energy generated in the electricity generating unit  110  is decreased by the decrease of the power required by the load, and the temperature of fluid discharged from the cathode outlet  114   b  is correspondingly lowered. In the case of the operation mode as described above, some fuel of low thermal energy from fluid having a lowered temperature, that is, the fuel substantially not heated, is supplied to the electricity generating unit  110 , so that the temperature of the electricity generating unit  110  can easily be lowered to the temperature suitable for a high efficient operation.  
       FIG. 4  is a flow chart showing the method of heat exchange of the liquid fuel type fuel cell system as shown in  FIG. 2 . Referring to  FIG. 4 , the liquid fuel type fuel cell system monitors the temperature of the electricity generating unit and periodically senses the electricity generating unit, that is, the temperature of the fuel cell (S 10 ).  
      Next, whether the sensed temperature Tf is less than the reference temperature T 1  is judged (S 12 ). According to the judged result, when the sensed temperature Tf is lower than the reference temperature T 1 , the flow path of the third flow path unit  133  closes by controlling the flow path control valve  135  connected to the third flow path unit  133  (S 14 ). The operation (S 14 ) maximally exchanges thermal energy between the fluid of the reaction temperature discharged from the cathode of the electricity generating unit  110  and the fuel supplied to the anode. According to the process, the temperature of the electricity generating unit  110  is rapidly raised.  
      Next, according to the judged result, when the sensed temperature Tf is not less than the reference temperature T 1 , whether the sensed temperature Tf is the reference temperature T 1  or higher and less than the critical temperature T 2  is judged (S 16 ). According to the judged result, when the sensed temperature Tf is the reference temperature T 1  or higher and less than the critical temperature T 2 , the flow path of the third flow path unit  133  closes by controlling the flow path control valve  135  (S 18 ). The operation is made to exchange thermal energy between the fluid of the reaction temperature discharged from the cathode of the electricity generating unit and only some of the fuel supplied to the anode. According to the process, the temperature of the electricity generating unit is maintained.  
      Next, according to the judged result of the operation (S 16 ), when the sensed temperature Tf is the critical temperature T 2  or higher, the control device  140  opens the flow path of the third flow path unit  133  by controlling the flow path control valve  135 , and generates a control signal to limit the power required by the load  160  to a predetermined value or less and then applies the generated control signal to the load  160  (S 20 ). The operation is made to limit exchange of thermal energy between the fluid of the reaction temperature discharged from the cathode of the electricity generating unit  110  to only some of the fuel supplied to the anode and to limit the power required by the load  160 . According to such a process, the temperature of the electricity generating unit is rapidly lowered. Meanwhile, when the temperature of the electricity generating unit  110  is lower than the critical temperature T 2  again, the limitation on the power required by the load  160  is released, enhancing the output of the electricity generating unit to optimum efficiency. After this, when the temperature of the electricity generating unit is lower than the reference temperature, the operation S 14  is repeated.  
      According to the foregoing heat exchange method, advantages are that the electricity generating unit  110  can rapidly be preheated when starting and the temperature of the electricity generating unit  110  can easily be maintained at the temperature showing high efficiency during operation.  
       FIG. 5  is a block diagram showing a liquid fuel type fuel cell system including a heat exchanger according to a second embodiment of the present invention. Referring to  FIG. 5 , a liquid fuel type fuel cell system includes an electricity generating unit  110 , a first flow path unit  131 , a second flow path unit  132 , a third flow path unit  133 , a heat exchanging unit  134 , a first flow path control valve  135   a , a second flow path control valve  136 , a recycler  137 , a fourth flow path unit  138 , a control device  140   a  and a mixing tank  150 . Here, the heat exchanger is configured of a first flow path unit  131 , a second flow path unit  132 , a third flow path unit  133 , a heat exchanging unit  134 , a first flow path control valve  135   a  and a second flow path control valve  136 .  
      The foregoing liquid fuel type fuel cell system is very similar to the liquid fuel type fuel cell system according to the first embodiment. However, the liquid fuel type fuel cell system according to the second embodiment differs from the liquid fuel type fuel cell system according to the first embodiment in that the second flow path control valve  136  and the control device  140   a  connected to the second flow path unit  132  are electrically connected to each other and the first and second flow path control valves  135 ,  136  may be thermostat valves, which automatically operate in response to a temperature.  
      Describing the second embodiment of aspects of the present invention in more detail, the first flow path control valve  135   a  is installed on the third flow path unit  133  to open and close the flow path of the third flow path unit  133 . The first flow path control valve  135   a  may be the thermostat valve to automatically operate at a predetermined temperature considering a difference between a temperature of a mounting position on the third flow path unit  133  and a temperature of the electricity generating unit  110 . Here, the first flow path control valve  135   a  as the thermostat valve is set to be closed at normal temperature and to be opened at a preset higher reference temperature. The reference temperature is the temperature of the electricity generating unit  110  where the electricity generating unit  110  begins to show good efficiency.  
      The second flow path control valve  136  is installed on the second flow path unit  132  to open and close the flow path of the second flow path unit  132 . The second flow path control valve  136  may be the thermostat valve to automatically operate at a predetermined temperature considering a difference between a temperature of a mounting position on the second flow path unit  132  passing through the heat exchanging unit  134  and the temperature of the electricity generating unit  110 . Here, the second flow path control valve  136  as the thermostat valve is set to be opened at normal temperature and to be closed at a higher preset critical temperature. The critical temperature is the temperature of a high temperature side of the electricity generating unit  110  where the electricity generating unit  110  begins to show bad efficiency.  
      On the other hand, the control device  140   a  of the present invention is not limited to a particular device, but may be any control devices capable of controlling the electricity generating unit. For example, the control device  140   a  may be a digital arithmetic processing unit, but is not limited thereto.  
      With the foregoing construction, by using the thermostat valves as the flow path control valves  135   a ,  136 , the sensor to detect the temperature of the electricity generating unit  110  and the control device  140   a  to control the flow path control valves  135   a ,  136  may be omitted, simplifying the construction.  
       FIG. 6A  to  FIG. 6C  are block diagrams showing three operation modes of the heat exchanger as shown in  FIG. 5 . Referring to  FIG. 6A  to  FIG. 6C , a heat exchanger adopted in a liquid fuel type fuel cell system includes the second flow path unit  132  passing through the heat exchanging unit  134  and a third flow path unit  133  bypassing the heat exchanging unit  134 , as a flow path unit supplying fuel to an anode and further includes the first and second flow path controlling valves  135   a ,  136  to control respective flow path units  132 ,  133 . Here, the first and the second flow path control valves  135   a ,  136  may be thermostat valves.  
      First, the operation mode when the sensed temperature Tf is less than the reference temperature T 1  will be described with reference to  FIG. 6A . In the case of the operation mode when the sensed temperature Tf is less than the reference temperature T 1 , the flow path of the third flow path unit  133  is closed by the operation of the first flow path control valve  135   a  and the flow path of the second flow path unit  132  is opened by the operation of the second flow path control valve  136  so that most of the thermal energy of the fluid of the reaction temperature discharged from the cathode outlet  114   b  and passing through the flow path of the first flow path unit  131  is transferred, through the heat exchanging unit  134 , to the fuel passing through the flow path of the second flow path unit  132 . In the case of the operation mode as above, the heated fuel is applied to the electricity generating unit  110 , thereby having the effect to promptly raise the temperature of the electricity generating unit  110 .  
      Next, the operation mode when the sensed temperature Tf is the reference temperature T 1  and higher, and less than the critical temperature T 2 , will be described with reference to  FIG. 6B . In the case of the operation mode when the sensed temperature Tf is the reference temperature T 1  or higher, and less than the critical temperature T 2 , in the state where the flow path of the second flow path unit  132  is opened, the flow path of the third flow path unit  133  is opened by the flow path control valve  135   a  so that the fuel of the mixing tank  150  is divided into the second flow path unit  132  passing through the heat exchanging unit  134  and the third flow path unit  133  bypassing the heat exchanging unit  134 , and then supplied to the electricity generating unit  110 . Therefore, only some of the fuel supplied to the electricity generating unit  110  is heated by receiving the thermal energy from the fluid of the reaction temperature discharged from the cathode outlet  114   b  and passing through the first flow path unit  131  by the heat exchanging unit  134 . In the case of the operation mode as above, the properly preheated fuel is supplied to the electricity generating unit  110  so that the temperature of the electricity generating unit  110  can be maintained to be suitable for a high efficient operation during the operation of the electricity generating unit  110 .  
      Next, the operation mode when the sensed temperature Tf is the critical temperature T 2  or higher will be described with reference to  FIG. 6C . In the case of the operation mode when the sensed temperature Tf is the critical temperature T 2  or higher, the flow path of the third flow path unit  133  is opened by the first flow path control valve  135   a  and the flow path of the second flow path unit  132  is closed by the operation of the second flow path control valve  136 , to supply non-heated fuel to the electricity generating unit  110 . In the case of the operation as described above, the non-heated fuel is supplied to the electricity generating unit  110  so that the temperature of the electricity generating unit  110  can easily be lowered to the temperature suitable for a high efficient operation.  
       FIG. 7  is a flow chart showing the method of heat exchange of the liquid fuel type fuel cell system as shown in  FIG. 5 . Referring to  FIG. 7 , in the liquid fuel type fuel cell system, two thermostat valves respectively installed on the third flow path unit  133  and the second flow path unit  132  as the first and second flow path control valves  135   a ,  136  automatically sense the temperature of their respective mounting positions (S 30 ). A temperature preset to operate each thermostat valve is preset considering a difference between a temperature of each respective mounting position and the temperature of the electricity generating unit.  
      Next, whether the sensed temperature Tf is less than the reference temperature T 1  is judged (S 32 ). According to the judged result, when the sensed temperature Tf is less than the reference temperature T 1 , the flow path of the third flow path unit  133  closes by controlling the first flow path control valve  135   a  connected to the third flow path unit  133  and the flow path of the second flow path unit  132  opens by controlling the second flow path control valve  136  connected to the second flow path unit  132  (S 14 ). According to the process, the fuel supplied to the electricity generating unit  110  passes through the heat exchanging unit  134  to make heat exchange between fluid of the reaction temperature discharged from the cathode and fuel supplied to the anode of the electricity generating unit  110  so that the electricity generating unit  110  can naturally be preheated and operated at high efficiency.  
      Next, according to the judged result, when the sensed temperature Tf is not less than the reference temperature T 1 , whether the sensed temperature Tf is the reference temperature T 1  or higher and less than the critical temperature T 2  is judged (S 36 ). According to the judged result, when the sensed temperature Tf is the reference temperature T 1  or higher and less than the critical temperature T 2 , the flow path of the third flow path unit  133  and the flow path of the second flow path unit  132  are opened, by controlling the flow path control valves  135   a ,  136  (S 38 ). According to such a process, a temperature rising of the electricity generating unit  110 , caused by a high load operation and a long time operation, is suppressed due to some fuel supplied to the electricity generating unit  110  through the bypass flow path unit, i.e., the third flow path unit  133 . Therefore, the temperature of the electricity generating unit  110  can be maintained so the electricity generating unit  110  can perform at optimum efficiency. On the other hand, when the temperature of the electricity generating unit  110  is lower than the reference temperature T 1  again, the operation (S 34 ) may be repeated.  
      Next, according to the judged result of the operation (S 36 ), when the sensed temperature Tf is the critical temperature T 2  or higher, the control device  140   a  opens the flow path of the third flow path unit  133  by controlling the first flow path control valve  135   a  and closes the flow path of the second flow path unit  132  by the second flow path control valve  136  (S 20 ). According to the process, the temperature of the electricity generating unit can rapidly be lowered from the critical temperature T 2  or higher to the temperature showing optimum efficiency.  
      According to the foregoing heat exchange method, advantages are that the electricity generating unit  110  can rapidly be preheated when starting and the temperature of the electricity generating unit  110  can easily be maintained at the temperature showing high efficiency during operation, while rapidly lowering the temperature of the electricity generating unit  110  to a proper temperature when the temperature of the electricity generating unit  110  rises excessively.  
      Meanwhile, a type of a membrane structure of the fuel cell using liquid fuel as the electricity generating unit described above can properly be selected in accordance with an environment of the fuel cell system. Further, the first and/or second valves can be implemented as one valve for controlling flow ratio of fluids passing through the second flow path unit and the third flow path unit.  
      As described above, when a fuel cell system adopts the heat exchanger and a method of controlling heat exchange according to aspects of the present invention, it is possible to rapidly preheat a fuel cell when starting and maintain the fuel cell at a preferred temperature during operation. Therefore, the internal temperature and operation environment of a liquid fuel type fuel cell system can be optimized. Further, the fuel cell system can stably and continuously be operated.  
      Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.