Abstract:
A fuel cell system comprising: an anode gas circulation path comprising an anode gas supplying path and an anode gas ejection path; a fuel cell; a blowdown valve; and a control unit controlling the blowdown valve, wherein the control unit comprises a freeze estimation unit estimating whether the anode gas circulation path is likely to freeze, a pressure reduction unit reducing a pressure of the anode gas circulation path, and a pressure condition confirmation unit confirming a pressure of the anode gas circulation path; and when the freeze estimation unit estimates a freezing when the fuel cell stops generating electricity, the blowdown valve is closed, a pressure of the anode gas circulation path is reduced by the pressure reduction unit, and, the blowdown valve is opened after the pressure condition confirmation unit confirms that a pressure of the anode gas circulation path reaches a predetermined pressure less than or equal to an atmospheric pressure, thereby moving a moisture inside the anode gas circulation path in a direction towards the fuel cell.

Description:
[0001]    The present application claims priority on Japanese Patent Application No. 2009-191785, filed Aug. 21, 2009, the content of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a fuel cell system. 
         [0004]    2. Description of the Related Art 
         [0005]    According to a well known fuel cell equipped on vehicles, for example, a membrane electrode assembly is created by flanking a solid polymer electrolyte membrane from both sides with an anode electrode and a cathode electrode, a tabular unitary fuel cell (hereinafter referred to as a unit cell) is created by placing a pair of separators on both sides of the membrane electrode assembly, and a fuel cell stack (hereinafter referred to as a fuel cell) is created by stacking a plurality of unit cells. According to such a fuel cell, a hydrogen gas is supplied as an anode gas (fuel gas) between the anode electrode and the separator. At the same time, air is supplied as a cathode gas (oxidant gas) between the cathode electrode and the separator. As a result, a hydrogen ion, created by a catalytic reaction at the anode electrode, moves to the cathode electrode by passing through the solid polymer electrolyte membrane, conducts an electrochemical reaction at the cathode electrode with oxygen in the air, and thereby generates electricity. In accordance with this electric power generation, water is created inside the fuel cell. 
         [0006]    When a fuel cell system comprising such a fuel cell is used, for example, at an environment below freezing point, water that was generated and is still remaining inside the fuel cell system may freeze while the fuel cell system is not operating. In this case, technology for removing the water inside the fuel cell system has been suggested (see, for example, Japanese Unexamined Patent Application, First Publication, No. 2004-335240 (hereinafter referred to as “Patent Document 1”) and Japanese Unexamined Patent Application, First Publication, No. 2007-242326 (hereinafter referred to as “Patent Document 2”)). 
         [0007]    According to a fuel cell system disclosed in Patent Document 1, when the fuel cell system is not operating, a control is performed so that an air, which was introduced through an air introduction path in a direction opposite to a direction of pure water when the fuel cell system is operating normally, is supplied into a pure water circulation path. As a result, according to this fuel cell system, it is possible to remove the pure water remaining inside the fuel cell stack, pure water pump, foreign object filter, ion filter, and the pure water circulation path with more reliability, compared to a case in which pure water is removed by driving a pump, for instance. In addition, it is possible to prevent damage such as operational malfunctions and breakage due to the freezing and expansion of the pure water, with more reliability. 
         [0008]    According to a fuel cell system disclosed in Patent Document  2 , a sewer valve is provided to a water supplying system connected with a water tank. When water is discharged from the sewer valve, the pressure inside the water tank is increased by a tank pressure elevating unit. As a result of the increased pressure inside the water tank, the water inside the water supplying system is swiftly and reliably discharged from the sewer valve. Consequently, the water supplying system is prevented from freezing in cold climates while the fuel cell system is not operating. 
         [0009]    The fuel cell systems disclosed in Patent Documents 1 and 2 relate to technology removing pure water used for adding moisture to the solid polymer electrolyte membrane included in the fuel cell and to perform an adjustment of the temperature. In other words, the fuel cell systems disclosed in Patent Documents 1 and 2 do not relate to technology for preventing generated water remaining inside the fuel cell system from freezing while the fuel cell system is not operating. When such generated water freezes, there is a possibility that an anode gas, remaining inside a flow path in the anode side while the fuel cell system is not operating, cannot be substituted with fresh air (cathode gas) in a scavenging processing. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention is made considering the problems described above. Accordingly, an object of the present invention is to provide a fuel cell system which can prevent the anode gas flow path from freezing, and can perform a scavenging process with reliability. 
         [0000]    (1) Namely, a fuel cell system according to an aspect of the present invention comprises: an anode gas circulation path comprising an anode gas supplying path and an anode gas ejection path, an anode gas flowing through the anode gas supplying path and the anode gas ejection path; a fuel cell generating electricity by receiving a supply of the anode gas through the anode gas supplying path; a blowdown valve provided in the anode gas ejection path; and a control unit controlling the blowdown valve. Here, the control unit comprises a freeze estimation unit estimating whether the anode gas circulation path is likely to freeze, a pressure reduction unit reducing a pressure of the anode gas circulation path, and a pressure condition confirmation unit confirming a pressure of the anode gas circulation path. When the freeze estimation unit estimates a freezing when the fuel cell stops generating electricity, the blowdown valve is closed, a pressure of the anode gas circulation path is reduced by the pressure reduction unit, and, the blowdown valve is opened after the pressure condition confirmation unit confirms that a pressure of the anode gas circulation path reaches a predetermined pressure less than or equal to an atmospheric pressure. Thus, a moisture is moved inside the anode gas circulation path in a direction towards the fuel cell.
 
(2) In addition, the fuel cell system may be configured as follows: the pressure reduction unit uses an electric power acquisition equipment, the electric power acquisition equipment acquiring an electric output from the fuel cell.
 
         [0011]    (3) In addition, the fuel cell system may be configured as follows: the pressure reduction unit drives a cathode gas supplying pump in a backspin direction, the cathode gas supplying pump being connected to the anode gas circulation path. 
         [0000]    (4) In addition, the fuel cell system may be configured as follows: a fuel cell system according to an aspect of the present invention comprises: an anode gas circulation path comprising an anode gas supplying path and an anode gas ejection path, an anode gas flowing through the anode gas supplying path and the anode gas ejection path; a fuel cell generating electricity by receiving a supply of the anode gas through the anode gas supplying path; a blowdown valve provided in the anode gas ejection path; and a control unit controlling the blowdown valve. Here, the control unit comprises a freeze estimation unit estimating whether the anode gas circulation path is likely to freeze. When the freeze estimation unit estimates a freezing when the fuel cell stops generating electricity, the blowdown valve is closed, and the blowdown valve is opened after a predetermined amount of time has passed since the blowdown valve closed. As a result, a moisture is moved inside the anode gas circulation path in a direction towards the fuel cell. 
         [0012]    According to the fuel cell system described in (1) above, when the fuel cell stops generating electricity, the interior of the anode gas flow path is maintained to be at a negative pressure state. In addition, a sewer valve is opened before water such as generated water remaining inside the anode gas flow path is frozen. Thus, air is pulled into the anode gas flow path from outside the fuel cell system. Due to this air flow, water inside the anode gas flow path can be moved toward a side of the fuel cell. In other words, since water that was remaining near the sewer valve is removed, water can be prevented from freezing near the sewer valve. Hence, a subsequent scavenging process, through which the interior of the anode gas flow path is replaced with air, can be performed reliably. 
         [0013]    According to the fuel cell system described in (2) above, an electric power acquisition equipment is used according to an instruction from a pressure reduction unit. As a result, it is possible to consume the anode gas remaining inside the anode gas flow path. Therefore, the pressure inside the anode gas flow path can be reliably reduced. Thus, a preparation for preventing the freezing of water, for instance, may be made appropriately. 
         [0014]    According to the fuel cell system described in (3) above, a cathode gas supplying pump is generally branch connected to the anode gas flow path in order to perform a scavenging process of the anode gas flow path. Therefore, by operating the cathode gas supplying pump in a backspin direction according to an instruction by the pressure reduction unit, anode gas and the like inside the anode gas flow path may be ejected outside the fuel cell system. Therefore, the pressure inside the anode gas flow path may be reduced reliably. Thus, a preparation for preventing the freezing of water, for instance, may be made appropriately. 
         [0015]    According to the fuel cell system described in (4) above, a sewer valve is closed when the fuel cell system stops generating electricity. As a result, the pressure inside the anode gas flow path naturally decreases as time passes. Therefore, by opening the sewer valve after a predetermined amount of time has passed before water such as generated water remaining inside the anode gas flow path freezes, air can be pulled into the anode gas flow path from outside the fuel cell system. Due to this air flow, it is possible to move the moisture inside the anode gas flow path towards a side of the fuel cell. In other words, water that was remaining near the sewer valve is removed. Therefore, it is possible to prevent the moisture from freezing near the sewer valve. Therefore, a subsequent scavenging process, through which the interior of the anode gas flow path is replaced with air, can be performed reliably. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a configuration diagram of a fuel cell system according to an aspect of the present invention. 
           [0017]      FIG. 2  is a block diagram of a control device according to an aspect of the present invention. 
           [0018]      FIG. 3  is a flow chart indicating a method preventing a freezing of a fuel cell system according to an aspect of the present invention. 
           [0019]      FIG. 4  is a time chart illustrating a timing of an opening and closing operating of each valve while preventing a freezing of a fuel cell system according to an aspect of the present invention. 
           [0020]      FIG. 5  is a flow chart indicating a method preventing a freezing of a fuel cell system according to a second embodiment of the present invention. 
           [0021]      FIG. 6  is a graph showing a relation between a pressure inside a circulation pipe and time according to a second embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       [0022]    Hereunder, a first embodiment of the present invention is described with reference to  FIGS. 1-4 . The present embodiment is described under the assumption that a fuel cell system is equipped on a vehicle. 
         [0023]      FIG. 1  is a configuration diagram of a fuel cell system according to the present invention. As shown in  FIG. 1 , a fuel cell  11  of a fuel cell system  10  is a solid polymer membrane type fuel cell which generates electricity based on an electrochemical reaction between an anode gas such as hydrogen and a cathode gas such as air. An anode gas supplying tube  23  is connected town anode gas supplying communication hole  13  formed on the fuel cell  11  (at an entrance side of an anode gas flow path  21 ). A hydrogen tank  30  is connected to an end part of an upper stream of the anode gas supplying tube  23 . Further, a cathode gas supplying tube  24  is connected to a cathode gas supplying communication hole  15  formed on the fuel cell  11  (at an entrance side of a cathode gas flow path  22 ). An air compressor  33  is connected to an end part of an upper stream of the cathode gas supplying tube  24 . Further, an anode offgas discharge pipe  35  is connected to an anode offgas discharging communication hole  14  formed on the fuel cell  11  (at an outlet side of the anode gas flow path  21 ). In addition, a cathode offgas discharge pipe  38  is connected to a cathode offgas discharging communication hole  16  (at an outlet side of the cathode gas flow path  22 ). Here, an anode gas circulation pipe  46  comprises the anode gas supplying tube  23  and the anode offgas discharge pipe  35 . In addition, a cathode gas flow path  47  comprises a cathode gas supplying tube  24  and a cathode offgas discharge pipe  38 . 
         [0024]    Further, hydrogen gas provided from the hydrogen tank  30  to the anode gas supplying tube  23  is depressurized by a regulator (not diagramed), then passes through an ejector  26 , and is supplied to the anode gas flow path  21  of the fuel cell  11 . Further, an electromagnetic-driving type electromagnetic valve  25  is provided near a lower stream side of the hydrogen tank  30 . In this way, a configuration is made to shield a supply of hydrogen gas from the hydrogen tank  30 . 
         [0025]    In addition, the anode offgas discharge pipe  35  is connected to the ejector  26 . Thus, a configuration is made so that an anode offgas which passed through the fuel cell  11  and was ejected may be reused as an anode gas of the fuel cell  11 . Furthermore, three pipes are provided partway through the anode offgas discharge pipe  35  while being branched out. Namely, a drain ejection pipe  36 , a purge gas ejection pipe  37 , and a scavenging gas discharge pipe  32  are provided. The drain ejection pipe  36  ejects the water (i.e., drain) generated by the fuel cell  11 . The purge gas ejection pipe  37  ejects gas (i.e., offgas) inside the pipe in order to maintain the hydrogen concentration inside the anode pipe at an appropriate level. The scavenging gas discharge pipe  32  discharges a scavenging gas (i.e., air) when a scavenging process is performed while the fuel cell  11  is not operating. 
         [0026]    The drain ejection pipe  36 , the purge gas ejection pipe  37 , and the scavenging gas discharge pipe  32  are all connected to a dilution box  31  at a lower stream of each of the drain ejection pipe  36 , the purge gas ejection pipe  37 , and the scavenging gas discharge pipe  32 . An electromagnetic-driving type drain valve  51  is provided on the drain ejection pipe  36 . An electromagnetic-driving type purge valve  52  is provided on the purge gas ejection pipe  37 . Further, an electromagnetic-driving type air discharge valve  48  is provided on the scavenging gas discharge pipe  32 . Further, the diameter of the purge gas ejection pipe  37  is larger than the diameter of the drain ejection pipe  36 . In addition, the diameter of the scavenging gas discharge pipe  32  is larger than the diameter of the purge gas ejection pipe  37 . Further, a catch tank  53  is provided at a branching point between the anode offgas discharge pipe  35  and the drain ejection pipe  36  as a gas-liquid separator. 
         [0027]    Next, air (cathode gas) is compressed by the air compressor  33 . After the air passes through the cathode gas supplying tube  24 , the air is supplied to the cathode gas flow path  22  of the fuel cell  11 . After oxygen in the air is used in electric power generation as an oxidant, the oxygen is discharged to the cathode offgas discharge pipe  38  from the fuel cell  11  as a cathode offgas. The cathode offgas discharge pipe  38  is connected to a dilution box  31 . Thereafter, a discharge is made outside the vehicle. Incidentally, the cathode offgas discharge pipe  38  comprises a back pressure valve  34 . The air compressor  33  according to the present embodiment is configured so that a backspin is possible. In other words, by backspinning the air compressor  33 , gas in each of the pipes can be discharged to an upper stream side. Thus, the pressure inside the pipe can be reduced. In addition, a configuration is possible in which a humidifier (not diagrammed) is bridged between the cathode gas supplying tube  24  and the cathode offgas discharge pipe  38  so that the cathode gas is humidified by the movement of moisture included in the cathode offgas. 
         [0028]    Further, the cathode gas supplying tube  24  connecting the air compressor  33  and the fuel cell  11  is configured so that the pipe is branched, and an end of a scavenging gas injection pipe  54  is connected. The other end of the scavenging gas injection pipe  54  is connected between the fuel cell  11  and the ejector  26  of the anode gas supplying tube  23 . In other words, the air supercharged by the air compressor  33  can be supplied to the anode gas circulation pipe  46  and the anode gas flow path  21  of the fuel cell  11 . Incidentally, an electromagnetic driving type electromagnetic valve  55  is provided on the scavenging gas injection pipe  54 . Thus, a configuration is made so that the supply of air from the air compressor  33  can be shielded. 
         [0029]    Here, a temperature sensor  41  is provided immediately adjacent to (the lower stream side of) the anode offgas discharging communication hole  14  of the anode offgas discharge pipe  35 . In addition, a temperature sensor  42  is provided to the fuel cell  11  as well. For example, by averaging the temperature detected by the temperature sensor  41  and the temperature sensor  42 , it is possible to detect a temperature which is approximately the same as the temperature of the interior of the fuel cell  11 . The detection result (i.e., a sensor output) of the temperature sensor  41  and the temperature sensor  42  is transmitted to a control device (i.e., an ECU)  45 . Thus, a configuration is made so that it is determined whether or not each type of control (described later) can be carried out, based on the detection result. 
         [0030]    Furthermore, a pressure sensor  43  is provided in the anode gas supplying tube  23 . The pressure sensor  43  detects an in-tube gas pressure. The pressure sensor  43  is provided between the anode gas supplying communication hole  13  of the fuel cell  11  and a converging point of the anode gas supplying tube  23  and the scavenging gas injection pipe  54 . A pressure sensor  44  is provided in the cathode gas supplying tube  24 . The pressure sensor  44  also detects an in-tube gas pressure. The pressure sensor  44  is provided between the anode gas circulation pipe  46  and the cathode gas supplying communication hole  15  of the fuel cell  11 . 
         [0031]    Further, a configuration is made so that the electric power (electric current) generated by the fuel cell  11  is supplied to an electric power consumption device  50  such as a motor and an electric heater. In other words, the electric power consumption device  50  is configured as an electric power acquisition equipment. 
         [0032]      FIG. 2  is a block diagram of the control device  45 . As indicated in  FIG. 2 , the control device  45  comprises a freeze estimation unit  61  which determines whether or not the fuel cell system  10  may freeze, a pressure reduction unit  62  which reduces the pressure inside the anode gas circulation pipe  46 , a pressure condition confirmation unit  63  which confirms the pressure inside the anode gas circulation pipe  46 , and a scavenging processing unit  64  which performs a scavenging process while the fuel cell  11  is not operating. 
         [0033]    In addition, a control device  45  is configured so that, according to an output required by the fuel cell  11 , the electromagnetic valve  25  is controlled, and a predetermined amount of hydrogen gas is supplied to the fuel cell  11  from the hydrogen tank  30 . Further, according to an output required by the fuel cell  11 , the control device  45  drives the air compressor  33 , thus supplying a predetermined amount of air to the fuel cell  11 , and thereby controlling the back pressure valve  34  so as to adjust the supplying pressure of the air supplied to the cathode gas flow path  22 . 
         [0034]    [Method Preventing a Fuel Cell System from Freezing] 
         [0035]    Next, a method preventing a freezing of a fuel cell system  10  according to the present embodiment is described. 
         [0036]      FIG. 3  is a flow chart illustrating a method preventing a freezing of a fuel cell system  10 . 
         [0037]    As indicated in  FIG. 3 , the flow chart starts from a condition in which an ignition switch (not diagrammed) is turned off. The ignition switch is a seizing signal of the fuel cell system  10 . 
         [0038]    In step S 11 , it is determined whether or not the ignition switch has been turned on. When the ignition switch is turned on, the procedure ends. When the ignition switch is still turned off, the procedure moves on to step S 12 . 
         [0039]    In step S 12 , the freeze estimation unit  61  of the control device  45  determines whether or not the fuel cell system  10  may freeze. When it is determined that the fuel cell system  10  may freeze, the procedure moves on to step S 13 . When it is determined that there is no possibility of the cell system  10  freezing, the procedure moves back to step S 11 . In particular, the freeze estimation unit  61  is configured so that a temperature is detected from the temperature sensor  41  and the temperature sensor  42 , and when the temperature is less than or equal to a predetermined threshold temperature, the freeze estimation unit  61  determines that there is a possibility of freezing. 
         [0040]    In step S 13 , a procedure is performed to reduce the in-pipe pressure of the fuel cell system  10  and the pressure inside the fuel cell  11 . In particular, based on an instruction by the pressure reduction unit  62  of the control device  45 , the electromagnetic valve  25 , the air discharge valve  48 , the drain valve  51 , and the purge valve  52  are closed. At the same time, the electromagnetic valve  55  is opened. In this condition, the air compressor  33  is driven in a direction of a backspin. Thus, gas inside the anode gas circulation pipe  46  is discharged, and a pressure inside the anode gas circulation pipe  46  is reduced. 
         [0041]    In step S 14 , the pressure of the anode gas circulation pipe  46  is confirmed at the pressure sensor  43  according to an instruction by the pressure condition confirmation unit  63  of the control device  45 . At this time, the air compressor  33  remains driven, and the pressure of the anode gas circulation pipe  46  is gradually reduced. Then, at a stage in which the pressure of the anode gas circulation pipe  46  is lower than a predetermined pressure less than or equal to an atmospheric pressure, the electromagnetic valve  55  is closed, and a setting is made so that the anode gas circulation pipe  46  is sealed under a negative pressure condition. This predetermined pressure is determined in advance based on experiments and the like as a value at which a movement of water, as described later, is made possible. Further, immediately after the electromagnetic valve  55  is closed, a blowdown valve (such as a purge valve  52 ) provided on the anode gas circulation pipe  46  is opened. Then, air is blown into the anode gas circulation pipe  46  from outside the fuel cell system  10 . As a result of this air flow, moisture such as generated water that was remaining near the purge valve  52  inside the anode gas circulation pipe  46  can be moved towards the fuel cell  11  side. In other words, it is possible to remove water that was remaining near the purge valve  52 . 
         [0042]    In step S 15 , it is determined whether or not the ignition switch is turned on. When the ignition switch has been turned on, the procedure is terminated. When the ignition switch is still turned off, the procedure moves on to step S 16 . 
         [0043]    In step S 16 , it is determined whether or not a temperature of an area surrounding the fuel cell system  10  is less than or equal to a predetermined temperature. When the temperature is less than or equal to the predetermined temperature, the procedure moves on to step S 17 . When the temperature is higher than the predetermined temperature, the procedure returns to step S 15 . The predetermined temperature refers to a temperature at which moisture inside the gas remaining in the pipe undergoes a dew formation and freezes. The predetermined temperature is determined in advance based on actual experiments. An example of the predetermined temperature is −10° C. Here, a setting may be made using the temperature sensor  41  and the temperature sensor  42 . In addition, a temperature sensor (not diagrammed) may be provided immediately adjacent to the fuel cell system  10 , and a determination may be made based on the detection value of the temperature sensor. 
         [0044]    In step S 17 , the scavenging process of the fuel cell system  10  is executed based on an instruction by the scavenging processing unit  64  of the control device  45 . In particular, the electromagnetic valve  25 , the drain valve  51 , and the purge valve  52  are closed. At the same time, the air ejection valve  48  and the electromagnetic valve  55  are opened. Under this condition, the air compressor  33  is driven, and air is provided to the anode gas circulation pipe  46 . After air is supplied to the anode gas circulation pipe  46  for a predetermined amount of time, the driving of the air compressor  33  is halted, and the scavenging process is terminated. Thus, the method preventing the freezing of the fuel cell system  10  is terminated. By performing the scavenging process, the interior of the anode gas circulation pipe  46  and the anode gas flow path  21  may be replaced by air with low moisture such that freezing can be prevented. 
         [0045]      FIG. 4  is a time chart corresponding to the processing flow of the flow chart described above. As indicated in  FIG. 4 , the anode pressure becomes lower than the atmospheric pressure since a pressure reduction procedure is executed in step S 13 . The anode pressure increases up to the atmospheric pressure since the purge valve  52  (or the drain valve  51 ) is opened in step S 14 . At this time, the purge valve  52  (or the drain valve  51 ) is opened. As a result, the generated water that was remaining near the purge valve  52  moves towards the fuel cell  11  side together with gas (air). 
         [0046]    According to the present embodiment, when the fuel cell  11  stops generating electricity, the interior of the anode gas circulation pipe  46  is maintained at a negative pressure state. Further, the purge valve  52  (or the drain valve  51 ) is opened before moisture such as generated water remaining inside the anode gas circulation pipe  46  is frozen. As a result, air is pulled into the anode gas circulation pipe  46  from outside the fuel cell system  10 . Due to this air flow, moisture inside the anode gas circulation pipe  46  may be moved towards the fuel cell  11  side. In other words, moisture that was remaining near the purge valve  52  (drain valve  51 ) is removed. As a result, it is possible to prevent moisture from freezing near the purge valve  52  (the drain valve  51 ). Hence, it is possible to perform the subsequent scavenging process, in which the interior of the anode gas circulation pipe  46  is replaced with air, in a more reliable manner. 
         [0047]    In addition, by providing an air compressor  33  which can be backspinned, and by driving the air compressor in the direction of a backspin according to an instruction by the pressure reduction unit  62 , the anode gas and the like inside the anode gas circulation pipe  46  can be ejected outside of the fuel cell system  10 . Therefore, according to a simple configuration in which an air compressor  33  designed partially differently is employed, the pressure inside the anode gas circulation pipe  46  may be reduced with reliability. 
         [0048]    Further, in step S 14  of the present embodiment, a configuration is made in which the purge valve  52  is opened, and gas (air) is brought in from the purge gas ejection pipe  37 . Meanwhile, another configuration is also possible in which the drain valve  51  is opened, or gas and the generated water may be moved towards the fuel cell  11  side by opening both of the valves  51  and  52 . 
       Second Embodiment 
       [0049]    Next, a second embodiment of a fuel cell system according to the present invention is described based on  FIGS. 5 and 6 . The present second embodiment differs from the first embodiment in that a method preventing the freezing of a fuel cell system is configured differently. The configuration of the fuel cell system is approximately the same as what was described in the first embodiment. Therefore, the same reference numeral is used when describing the same components, and detailed descriptions regarding the overlapping components are omitted. 
       (Method Preventing a Freezing of a Fuel Cell System) 
       [0050]    Hereinafter, a method preventing a freezing of a fuel cell system  10  according to the present embodiment is described. 
         [0051]      FIG. 5  is a flow chart indicating a method preventing a freezing of a fuel cell system  10 . 
         [0052]    As shown in  FIG. 5 , the flow chart starts from a condition in which the ignition switch (not diagrammed), which is a seizing signal of the fuel cell system  10 , is turned off. 
         [0053]    In step S 21 , it is determined whether or not an ignition switch has been turned on. When the ignition switch is turned on, the procedure is terminated. When the ignition switch is still turned off, the procedure moves on to step S 22 . 
         [0054]    In step S 22 , the freeze estimation unit  61  of the control device  45  determines whether or not this is a possibility that the fuel cell system  10  will freeze. When it is estimated that the fuel cell system  10  may freeze, the procedure moves on to step S 23 . When it is determined that there is no possibility of the fuel cell system  10  freezing, the procedure moves back to step S 21 . In particular, the freeze estimation unit  61  is configured so that a temperature is detected from the temperature sensor  41  and the temperature sensor  42 , and when the temperature is less than or equal to a predetermined threshold temperature, the freeze estimation unit  61  determines that there is a possibility of freezing. 
         [0055]    In step S 23 , a procedure is performed to reduce the in-pipe pressure of the fuel cell system  10  and the pressure inside the fuel cell  11 . In particular, based on an instruction by the pressure reduction unit  62  of the control device  45 , the electromagnetic valve  25 , the air discharge valve  48 , the drain valve  51 , the purge valve  52 , and the electromagnetic valve  55  are closed. In other words, a setting is made so that the anode gas circulation pipe  46  is maintained in a sealed condition. 
         [0056]    In step S 24 , it is determined whether or not a predetermined amount of time has passed since the anode gas circulation pipe  46  became a sealed condition. When a predetermined amount of time has passed, the procedure moves on to step S 25 . When a predetermined amount of time has not yet passed, the step S 24  is repeated. Incidentally, the pressure of the anode gas circulation pipe  46  and the elapsed time are related as shown in  FIG. 6 . In other words, when the anode gas circulation pipe  46  is in a sealed condition, it is observed that the pressure inside the anode gas circulation pipe  46  decreases as time elapses. Therefore, a time T 1  required to reach a pressure P 1  (P 1  is less than the atmospheric pressure) which is suitable for removing generated water from the anode gas circulation pipe  46  is set to be a predetermined amount of time. Here, the pressure of the anode gas circulation pipe  46  is confirmed by the pressure sensor  43  based on an instruction by the pressure condition confirmation unit. 
         [0057]    In step S 25 , since a predetermined amount of time has passed in step S 24 , the blowdown valve (for example, the purge valve  52 ) provided on the anode gas circulation pipe  46  is opened. Then, since the anode gas circulation pipe  46  is in a state of negative pressure, air is pulled into the anode gas circulation pipe  46  from outside the fuel cell system  10 . Due to this air flow, moisture such as generated water that has remained near the purge valve  52  of the anode gas circulation pipe  46  may be moved towards the fuel cell  11  side. In other words, it is possible to remove the moisture that has been remaining near the purge valve  52 . 
         [0058]    In step S 26 , it is determined whether or not the ignition switch has been turned on. When the ignition switch has been turned on, the process is terminated. When the ignition switch remains to be turned off, the process moves on to step S 27 . 
         [0059]    In step S 27 , it is determined whether or not a temperature of an area surrounding the fuel cell system  10  is less than or equal to a predetermined temperature. When the temperature is less than or equal to the predetermined temperature, the procedure moves on to step S 28 . When the temperature is higher than the predetermined temperature, the procedure returns to step S 26 . The predetermined temperature refers to a temperature at which moisture inside the gas remaining in the pipe undergoes a dew formation and freezes. The predetermined temperature is determined in advance based on actual experiments. An example of the predetermined temperature is −10° C. Here, a setting may be made using the temperature sensor  41  and the temperature sensor  42 . In addition, a temperature sensor (not diagrammed) may be provided immediately adjacent to the fuel cell system  10 , and a determination may be made based on the detection value of the temperature sensor. 
         [0060]    In step S 28 , the scavenging process of the fuel cell system  10  is executed based on an instruction by the scavenging processing unit  64  of the control device  45 . In particular, the electromagnetic valve  25 , the drain valve  51 , and the purge valve  52  are closed. At the same time, the air ejection valve  48  and the electromagnetic valve  55  are opened. Under this condition, the air compressor  33  is driven, and air is provided to the anode gas circulation pipe  46 . After air is supplied to the anode gas circulation pipe  46  for a predetermined amount of time, the driving of the air compressor  33  is halted, and the scavenging process is terminated. Thus, the method preventing the freezing of the fuel cell system  10  is terminated. By performing the scavenging process, the interior of the anode gas circulation pipe  46  and the anode gas flow path  21  may he replaced by air with low moisture such that freezing can be prevented. 
         [0061]    According to the present invention, a blowdown valve such as a drain valve  51  and a purge valve  52  are closed when the fuel cell  11  stops generating electricity. As a result, the pressure inside the anode gas circulation pipe  46  decreases naturally as time passes. Therefore, by opening the purge valve  52  (or the drain valve  51 ) after a predetermined amount of time has passed before water such as generated water remaining inside the anode gas circulation pipe  46  freezes, air can be pulled into the anode gas circulation pipe  46  from outside the fuel cell system  10 . Due to this air flow, it is possible to move the moisture inside the anode gas circulation pipe  46  towards the fuel cell  11  side. In other words, water that was remaining near the purge valve  52  (drain valve  51 ) is removed. Therefore, it is possible to prevent the moisture from freezing near the purge valve  52  (drain valve  51 ). Therefore, a subsequent scavenging process, through which the interior of the anode gas circulation pipe  46  is replaced with air, can be performed reliably. 
         [0062]    While a preferred embodiment of the present invention has been described above, it should be understood that these are exemplary of the invention and are not to be considered as limiting the present invention. Additions, omissions, substitutions, and other modifications can be made without departing from the scope of the present invention. The invention is not to be considered as being limited by the foregoing description, and is only limited by the scope of the appended claims. 
         [0063]    For example, in the present embodiment, a configuration has been described in which the gas inside the anode gas circulation pipe  46  is forcibly exhausted by backspinning the air compressor  33  when the pressure inside the anode gas circulation pipe  46  is reduced, thereby making the pressure inside the anode gas circulation pipe  46  less than or equal to the atmospheric pressure. However, another configuration is possible in which, for example, an electric power consumption device  50  is used for the fuel cell  11  to generate electricity, then an anode gas remaining inside the anode gas circulation pipe  46  is forcibly consumed, thereby reducing the pressure inside the anode gas circulation pipe  46 .