Patent Publication Number: US-2012040260-A1

Title: Fuel cell system and water draining method for fuel cell system

Description:
TECHNICAL FIELD 
     The present invention relates to a fuel cell system including, on a water passage, a replaceable device, such as a water purifier, which needs to be replaced periodically, and a device, such as a temperature sensor, which is replaced when it breaks, and also relates to a method for removing water of the fuel cell system including the above devices. 
     BACKGROUND ART 
     Regarding conventional fuel cell systems, proposed is to remove water from a cooling water passage of a fuel cell when the system requires maintenance (see PTL 1, for example).
     PTL 1: Japanese Laid-Open Patent Application Publication No. 2007-134206 (paragraph [0033])   

     SUMMARY OF INVENTION 
     Technical Problem 
     Regarding the fuel cell system described in PTL 1, removing the water in the cooling water passage at the time of maintenance is disclosed. However, the fuel cell system includes water passages in addition to the cooling water passage, and how to remove the water from these water passages depending on the situations at the time of maintenance is not considered. 
     The present invention was made to solve the conventional problem, and an object of the present invention is to provide a fuel cell system capable of removing water depending on the situations at the time of maintenance and a method for removing the water of the fuel cell system. 
     Solution to Problem 
     To solve the above conventional problem, a fuel cell system of the present invention includes: a fuel cell; a water circulation passage through which water circulates, the water being necessary for an operation of the fuel cell system; a separation mechanism configured to divide the water circulation passage into a plurality of blocks when removing the water and to divide the water in the water circulation passage among the blocks; water discharge passages respectively connected to the blocks; and water discharge valves respectively provided on the water discharge passages. 
     Moreover, a method for removing water of a fuel cell system of the present invention is a method for removing water of a fuel cell system, the fuel cell system including: a fuel cell; a water circulation passage through which water circulates, the water being necessary for an operation of the fuel cell system; a separation mechanism configured to divide the water circulation passage into a plurality of blocks when removing the water and to divide the water in the water circulation passage among the blocks; water discharge passages respectively connected to the blocks; and water discharge valves respectively provided on the water discharge passages, the method including the step of removing the water from only the block in which a part requiring maintenance is provided or the block whose abnormality is detected, by opening the water discharge valve of the water discharge passage connected to the block. 
     The above object, other objects, features and advantages of the present invention will be made clear by the following detailed explanation of preferred embodiments with reference to the attached drawings. 
     Advantageous Effects of Invention 
     In accordance with a fuel cell system of the present invention and a method for removing water of the fuel cell system, only the water in a block, from which the water needs to be removed, of a water circulation passage is discharged to the outside of the water circulation passage. Therefore, the amount of time for a water filling operation performed after the water removing operation is reduced. Thus, the maintenance can be completed more quickly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram schematically showing a schematic configuration of a fuel cell system according to Embodiment 1 of the present invention. 
         FIG. 2  is a schematic diagram showing a schematic configuration of an operating unit of the fuel cell system shown in  FIG. 1 . 
         FIG. 3  is a schematic diagram showing a schematic configuration of the operating unit of the fuel cell system shown in  FIG. 1 . 
         FIG. 4  is a flow chart schematically showing a water removing process of a first water circulation passage of the fuel cell system shown in  FIG. 1 . 
         FIG. 5  is a flow chart schematically showing a water removal abnormality detecting sequence of the first water circulation passage in the water removing process of the fuel cell system shown in  FIG. 4 . 
         FIG. 6  is a flow chart schematically showing the water removing process of a second water circulation passage of the fuel cell system shown in  FIG. 1 . 
         FIG. 7  is a flow chart schematically showing the water removal abnormality detecting sequence of the second water circulation passage in the water removing process of the fuel cell system shown in  FIG. 6 . 
         FIG. 8  is a flow chart schematically showing the water removing process of all blocks of a water circulation passage of the fuel cell system shown in  FIG. 1 . 
         FIG. 9  is a flow chart schematically showing the water removal abnormality detecting sequence of the water circulation passage in the water removing process of the fuel cell system shown in  FIG. 8 . 
         FIG. 10  is a block diagram schematically showing a schematic configuration of the fuel cell system according to Embodiment 2 of the present invention. 
         FIG. 11  is a flow chart schematically showing the water removing process of an exhaust heat recovered water passage (water passage) of the fuel cell system shown in  FIG. 10 . 
         FIG. 12  is a flow chart schematically showing the water removing process of a hot water tank of the fuel cell system shown in  FIG. 10 . 
         FIG. 13  is a block diagram schematically showing a schematic configuration of the fuel cell system according to Embodiment 3 of the present invention. 
         FIG. 14  is a flow chart showing a series of steps including the water removing operation of the fuel cell system shown in  FIG. 13 . 
         FIG. 15  is a block diagram schematically showing a schematic configuration of the fuel cell system of Modification Example 4. 
         FIG. 16  is a flow chart showing a series of steps including the water removing operation of the fuel cell system shown in  FIG. 15 . 
         FIG. 17  is a block diagram schematically showing a schematic configuration of the fuel cell system according to Embodiment 4 of the present invention. 
         FIG. 18  is a flow chart schematically showing the water removing process of all the blocks of the water circulation passage of the fuel cell system shown in  FIG. 17 . 
         FIG. 19  is a flow chart schematically showing the water removal abnormality detecting sequence of the water circulation passage in the water removing process of the fuel cell system shown in  FIG. 18 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A fuel cell system according to embodiments of the present invention and a method for removing water of the fuel cell system will be specifically explained. 
     A fuel cell system of a first aspect includes: a fuel cell; a water circulation passage through which water circulates, the water being necessary for an operation of the fuel cell system; a separation mechanism configured to divide the water circulation passage into a plurality of blocks when removing the water and to divide the water in the water circulation passage among the blocks; water discharge passages respectively connected to the blocks; and water discharge valves respectively provided on the water discharge passages. 
     With this, the water is removed from only the block, from which the water needs to be removed, of the water circulation passage. Therefore, the amount of time for a water filling operation performed after the water removing operation is reduced. Thus, the maintenance can be completed more quickly. 
     Here, the above invention includes both a case where the water removing operation of only the block, from which the water needs to be removed, of the water circulation passage is executed in accordance with the water removal mode selected by the operator through the command acquiring unit and a case where the water removing operation of only the block, from which the water needs to be removed, is executed automatically without the selection of the water removal mode through the command acquiring unit. 
     Moreover, in the present invention, the “water removal” denotes that the water in a predetermined block of the water circulation passage is discharged from the water discharge passage to the outside of the fuel cell system by opening the water discharge valve. 
     Here, at least one “water discharge valve” is provided on each block. 
     Moreover, the fuel cell system of a second aspect is configured such that in the fuel cell system of the first aspect, the water circulation passage includes a first water circulation passage through which cooling water for cooling the fuel cell circulates and a second water circulation passage through which the cooling water discharged from the first water circulation passage is purified and returns to the first water circulation passage, and the separation mechanism is constituted by a first water tank to which the first water circulation passage and the second water circulation passage are connected and in which the cooling water is stored. 
     Moreover, the fuel cell system of a third aspect is configured such that the fuel cell system of the second aspect further includes: a first water discharge passage provided at the first water circulation passage; a first water discharge valve provided on the first water discharge passage; a second water discharge passage provided at the second water circulation passage; and a second water discharge valve provided on the second water discharge passage. 
     Moreover, the fuel cell system of a fourth aspect is configured such that the fuel cell system of the second aspect further includes: at least one of a first maintenance part provided on the first water circulation passage and a first abnormality detector provided on the first water circulation passage to detect an abnormality of the first water circulation passage; and at least one of a second maintenance part provided on the second water circulation passage and a second abnormality detector provided on the second water circulation passage to detect an abnormality of the second water circulation passage. 
     Here, the “first maintenance part” is a part which is periodically maintained (checked/replaced). Examples of the “first maintenance part” are a filter provided in a second water pump configured to cause the cooling water to flow and a filter configured to capture impurities in the first water tank. The “second maintenance part” is a part which is periodically maintained (checked/replaced). Examples of the “second maintenance part” are a purifier configured to purify recovered water recovered from an exhaust gas of the fuel cell, a filter configured to capture impurities in a recovered water tank configured to store the recovered water, and a filter provided in a first water pump configured to cause the purified water to flow. 
     Examples of the first abnormality detector are a temperature detector configured to detect the temperature of the cooling water, a water level detector configured to detect the water level in the first water tank, and a rotation detector of the second water pump. Similarly, examples of the second abnormality detector are a temperature detector configured to detect the temperature of the recovered water, a water level detector of the recovered water tank, and a rotation detector of the first water pump. 
     Moreover, the fuel cell system of a fifth aspect is configured such that in the fuel cell system of the first aspect, the water circulation passage includes: a hot water tank configured to store the water having recovered exhaust heat of the fuel cell system; and a water passage through which water flows, the water being taken out from the hot water tank, recovering the exhaust heat of the fuel cell system, and returning to the hot water tank, and the separation mechanism includes: a first on-off valve provided on a portion, through which the water having not yet recovered the exhaust heat of the fuel cell system flows, of the water passage; and a second on-off valve provided on a portion, through which the water having recovered the exhaust heat of the fuel cell system flows, of the water passage. 
     Moreover, the fuel cell system of a sixth aspect is configured such that the fuel cell system of the fifth aspect further includes: a third water discharge passage provided at the water passage; a third water discharge valve provided on the third water discharge passage; a fourth water discharge passage provided at the hot water tank; and a fourth water discharge valve provided on the fourth water discharge passage. 
     Moreover, the fuel cell system of a seventh aspect is configured such that the fuel cell system of the fifth aspect further includes: a third abnormality detector provided on the water passage to detect an abnormality of the water passage; and a fourth abnormality detector provided on the hot water tank. 
     Here, examples of the “third abnormality detector” are a temperature detector configured to detect the temperature of the water flowing through the water passage and a rotation detector of the third water pump configured to cause the water in the water passage to flow. Examples of the “fourth abnormality detector” are a temperature detector configured to detect the temperature of the water in the hot water tank and a water level detector configured to detect the water level in the hot water tank. 
     Moreover, the fuel cell system of an eighth aspect is configured such that in the fuel cell system of the first aspect, the water circulation passage includes: a second water tank configured to store recovered water recovered from an exhaust gas of the fuel cell; a first water tank configured to store, as cooling water for cooling the fuel cell, purified water obtained by purifying the recovered water; a first connection passage connecting the first water tank and the second water tank; a first branched passage branching from the first connection passage; and a fifth water discharge valve provided on the first branched passage, a purifier is provided on the first connection passage, and the first branched passage is configured such that the water in the passage including the purifier is removed by opening the fifth water discharge valve. 
     Moreover, the fuel cell system of a ninth aspect is configured such that in the fuel cell system of the eighth aspect, the second water tank is provided above the purifier, and the fuel cell system of the eighth aspect further includes: a water level detector configured to detect a water level in the second water tank; and a controller configured to open the fifth water discharge valve to remove the water in the block including the purifier and the second water tank, and in a water filling operation performed after completion of the removal of the water in the above block, detect completion of water filling of the purifier by an increase in the water level detected by the water level detector. 
     The fuel cell system of a tenth aspect is configured such that the fuel cell system of the eighth aspect further includes: a water level detector configured to detect a water level in the first water tank; and a controller configured to open the fifth water discharge valve to remove the water in the block including the purifier and the second water tank, and in a water filling operation performed after completion of the removal of the water in the above block, detect completion of water filling of the purifier by an increase in the water level detected by the water level detector. 
     With this, in the water filling operation performed after the execution of the water removal of the block including the purifier, the completion of the water filling of this block can be confirmed. 
     Here, the “controller” may be constituted by a single controller which performs centralized control or may be constituted by a plurality of controllers which cooperate to perform distributed control. Examples of the “controller” are a microprocessor and a CPU. 
     Moreover, the fuel cell system of an eleventh aspect is configured such that the fuel cell system of the eighth aspect further includes a water pump provided on the first connection passage to cause the recovered water in the second water tank to flow to the first water tank, wherein the first branched passage branches from a portion of the first connection passage, the portion being located on the purifier side of the water pump. 
     Moreover, the fuel cell system of a twelfth aspect is configured such that the fuel cell system of any one of the eighth to eleventh aspects further includes a second connection passage through which the water having overflowed from the first water tank returns to the second water tank, wherein: the second water tank is open to the atmosphere; and the first water tank is open to the atmosphere through the second connection passage and the second water tank. 
     Moreover, the fuel cell system of a thirteenth aspect is configured such that the fuel cell system of any one of the first to twelfth aspects further includes: a command acquiring unit configured to receive a set command of the block of the water circulation passage by a manual operation of an operator, the block being subjected to a water removing process; and a controller configured to open the water discharge valve provided on the water discharge passage (hereinafter referred to as “water removal water discharge passage”) connected to the block, which is subjected to the water removing process based on the command of the command acquiring unit, such that the water is discharged from the water removal water discharge passage. 
     Here, the “command acquiring unit” is constituted by a remote controller configured to control the operation of the fuel cell system by the operation of the operator. Then, the operator operates buttons provided on the remote controller to input a signal indicating the selected water removal mode (water discharge valve control mode) to the controller. 
     Moreover, the fuel cell system of a fourteenth aspect is configured such that the fuel cell system of any one of the first to fourth and eight to twelfth aspects further includes: a command acquiring unit configured to receive a set command of the block of the water circulation passage by a manual operation of an operator, the block being subjected to a water removing process; and a controller configured to start detecting an abnormality of the water removing process corresponding to the block which is subjected to the water removing process based on the command of the command acquiring unit. 
     Moreover, the fuel cell system of a fifteenth aspect is configured such that the fuel cell system of the fourteenth aspect further includes: a water tank provided on the block which is subjected to the water removing operation based on the command of the command acquiring unit; and a water level detector configured to detect a water level in the water tank, wherein the controller detects the abnormality based on the water level detected by the water level detector. 
     Further, the fuel cell system of a sixteenth aspect is configured such that the fuel cell system of the second aspect further includes: a hydrogen generator configured to generate a hydrogen-containing gas by using a raw material and water; a water supply unit configured to supply the water to the hydrogen generator; a water supply passage through which the water supplied from the water supply unit to the hydrogen generator flows; a second water tank provided on the second water circulation passage; a second branched passage which branches from the water supply passage and through which the water supplied to the second water tank flows; a switching unit configured to switch a destination to which the water supplied from the water supply unit flows, between the hydrogen generator and the second water tank; and a controller configured to activate the water supply unit and causes the switching unit to switch the destination, to which the water supplied from the water supply unit flows, to the second water tank in the water removing process of the second water circulation passage or in the water removing process of all the passages of the water circulation passage. 
     Moreover, a method for removing water of the fuel cell system of the first aspect is a method for removing water of a fuel cell system, the fuel cell system including: a fuel cell; a water circulation passage through which water circulates, the water being necessary for an operation of the fuel cell system; a separation mechanism configured to divide the water circulation passage into a plurality of blocks when removing the water and to divide the water in the water circulation passage among the blocks; water discharge passages respectively connected to the blocks; and water discharge valves respectively provided on the water discharge passages, the method including the step of removing the water from only the block in which a part requiring maintenance is provided or the block whose abnormality is detected, by opening the water discharge valve of the water discharge passage connected to the block. 
     With this, the water is removed from only the block, from which the water needs to be removed, of the water circulation passage. Therefore, the amount of time for the water filling operation performed after the water removing operation is reduced. Thus, the maintenance can be completed more quickly. 
     Moreover, the method for removing the water of the fuel cell system of the second aspect is designed such that the method for removing the water of the fuel cell system of the first aspect further includes the step of opening all the water discharge valves to remove the water from all the blocks of the water circulation passage in at least one of a case where there is a possibility of freezing of the water circulation passage and a case where the fuel cell system is set to a resting state. 
     With this, in a case where there is a possibility of freezing of the water circulation passage, the possibility of freezing of the water in the water circulation passage can be reduced. When the user is away for a long period of time, and the fuel cell system is set to the resting state, the present mode is selected. With this, the water is removed from all the blocks of the water circulation passage. Therefore, it is possible to avoid a case where the water goes rotten and clogs in the water circulation passage since it is not used for a long period of time or a case where the water freezes in the water circulation passage since it is not used for a long period of time. 
     Moreover, the method for removing the water of the fuel cell system of the third aspect is designed such that in the method for removing the water of the fuel cell system of the first aspect, the water circulation passage includes a first water circulation passage through which cooling water for cooling the fuel cell circulates and a second water circulation passage through which the cooling water discharged from the first water circulation passage is purified and returns to the first water circulation passage; the separation mechanism is constituted by a cooling water tank to which the first water circulation passage and the second water circulation passage are connected and in which the cooling water is stored; the water discharge passages include a first water discharge passage provided at the first water circulation passage and a second water discharge passage provided at the second water circulation passage; the water discharge valves include a first water discharge valve provided on the first water discharge passage and a second water discharge valve provided on the second water discharge passage; and the fuel cell system further includes at least one of a first maintenance part provided on the first water circulation passage and a first abnormality detector provided on the first water circulation passage to detect an abnormality of the first water circulation passage and at least one of a second maintenance part provided on the second water circulation passage and a second abnormality detector provided on the second water circulation passage to detect an abnormality of the second water circulation passage, and the method includes the steps of: removing the water from only the first water circulation passage at least one of when maintenance of the first maintenance part is performed and when the abnormality of the first water circulation passage is detected; and removing the water from only the second water circulation passage at least one of when maintenance of the second maintenance part is performed and when the abnormality of the second water circulation passage is detected. 
     Further, the method for removing the water of the fuel cell system of the fourth aspect is designed such that in the method for removing the water of the fuel cell system of the first aspect, the water circulation passage includes a hot water tank configured to store the water having recovered exhaust heat of the fuel cell and a water passage through which water flows, the water being taken out from the hot water tank, recovering the exhaust heat of the fuel cell, and returning to the hot water tank; the separation mechanism includes a first on-off valve provided on a portion, through which the water having not yet recovered the exhaust heat of the fuel cell flows, of the water passage, and a second on-off valve provided on a portion, through which the water having recovered the exhaust heat of the fuel cell flows, of the water passage; the water discharge passages include a third water discharge passage provided at the water passage and a fourth water discharge passage provided at the hot water tank; the water discharge valves includes a third water discharge valve provided on the third water discharge passage and a fourth water discharge valve provided on the fourth water discharge passage; and the fuel cell system further includes a third abnormality detector provided on the water passage to detect an abnormality of the water passage and a fourth abnormality detector provided on the hot water tank to detect an abnormality of the hot water tank, and the method further includes the steps of: when the abnormality of the water passage is detected, closing the first on-off valve and the second on-off valve and opening the third water discharge valve to remove only the water in the water passage; and when the abnormality of the hot water tank is detected, closing the first on-off valve and the second on-off valve and opening the fourth water discharge valve to remove only the water in the hot water tank. 
     Hereinafter, embodiments of the present invention will be specifically exemplified in reference to the drawings. In the drawings, the same reference signs are used for the same or corresponding components, and a repetition of the same explanation is avoided. In the drawings, only the components necessary to explain the present invention are shown, and the other components are omitted. Further, the present invention is not limited to the embodiments below. 
     Embodiment 1 
     In a fuel cell system according to Embodiment 1 of the present invention, a water circulation passage includes a first water circulation passage and a second water circulation passage. 
     Configuration of Fuel Cell System 
       FIG. 1  is a block diagram schematically showing a schematic configuration of the fuel cell system according to Embodiment 1 of the present invention.  FIGS. 2 and 3  are schematic diagrams each showing a schematic configuration of an operating unit of the fuel cell system shown in  FIG. 1 . 
     As shown in  FIG. 1 , a fuel cell system  100  according to Embodiment 1 of the present invention includes a casing  70  and a hot water tank  9 . The casing  70  includes an intake port and an exhaust port. Arranged in the casing  70  are: a fuel cell  1 ; an oxidizing gas supply unit  2  configured to supply an oxidizing gas (air); a hydrogen generator  101  configured to cause a reforming reaction between a raw material and water to generate a fuel gas; a heat exchanger  6 ; a cooling water tank  12 ; a first recovered water tank  14 A; and a controller  30 . 
     The fuel gas (hydrogen gas) generated by the hydrogen generator  101  is supplied to the fuel cell  1 , and the oxidizing gas is supplied from the oxidizing gas supply unit  2  to the fuel cell  1 . In the fuel cell  1 , the supplied fuel gas and oxidizing gas electrochemically react with each other to generate electricity and heat. The fuel gas (off fuel gas) unconsumed in the fuel cell  1  flows through a fuel gas discharge passage  32  to the outside of the fuel cell system  100 , and the oxidizing gas (off oxidizing gas) unconsumed in the fuel cell  1  flows through an oxidizing gas discharge passage  31  to the outside of the fuel cell system  100 . Water generated from steam of the off oxidizing gas while the off oxidizing gas is flowing through the oxidizing gas discharge passage  31  is recovered by the first recovered water tank  14 A. 
     Moreover, cooling water for recovering heat generated in the fuel cell  1  to cool the fuel cell  1  is supplied through a cooling water passage  11  to the fuel cell  1 . The cooling water tank  12  is provided on the cooling water passage  11  and stores the cooling water. A second water pump  10 , a first temperature detector  33 , and a heater  36  are provided on the cooling water passage  11 . The second water pump  10  is configured to cause the cooling water in the cooling water passage  11  to flow. A filter and a rotation detector configured to detect the number of rotations of the second water pump  10  are provided at the second water pump  10 . The first temperature detector  33  is configured to detect the temperature of the cooling water in the cooling water passage  11  and transmit the detected temperature to the controller  30 . The heater  36  is configured to heat the cooling water flowing through the cooling water passage  11 . An electric heater may be used as the heater  36 . It is preferable that the heater  36  be configured to consume surplus electricity of the fuel cell  1 . 
     Moreover, the heat exchanger  6  is provided on the cooling water passage  11 . The heat exchanger  6  is configured to perform heat exchange between the cooling water having exhaust heat recovered from the fuel cell  1  and water (hot water) taken out from the hot water tank  9  and flowing through an exhaust heat recovered water passage  8 . The water (hot water) having recovered the heat of the cooling water in the heat exchanger  6  flows through the exhaust heat recovered water passage  8  to be supplied to the hot water tank  9 . A third water pump  7  is provided on the exhaust heat recovered water passage  8 . The third water pump  7  is configured to cause the water (hot water) in the exhaust heat recovered water passage  8  to flow. A filter and a rotation detector configured to detect the number of rotations of the third water pump  7  are provided at the third water pump  7 . 
     Further, a first water discharge passage  18  is connected to the cooling water passage  11 , and a downstream end of the first water discharge passage  18  is open to the outside of the fuel cell system  100  (casing  70 ). A first water discharge valve  20  is provided on a portion of the first water discharge passage  18 , the portion being located outside the casing  70 . The first water discharge valve  20  is configured such that by opening a valve body thereof, the water in the cooling water passage  11  and the cooling water tank  12  is discharged through the first water discharge passage  18  to the outside of the fuel cell system  100 . The first water discharge valve  20  may be constituted by an automatic on-off valve whose open and close states are controlled by the controller  30  or may be constituted by a manual on-off valve which is opened and closed by a user or a maintenance worker. 
     A reforming water passage  50  through which the water is supplied to the hydrogen generator  101  is connected to the cooling water passage  11 . A fourth water pump  45  configured to cause the water in the reforming water passage  50  to flow is provided on the reforming water passage  50 . An upstream end of a branched passage  48  is connected to a portion of the reforming water passage  50 , the portion being located downstream of the fourth water pump  45 . A downstream end of the branched passage  48  is connected to the first recovered water tank  14 A. A fourth on-off valve  47  is provided on the branched passage  48 . Further, a third on-off valve  46  is provided on the reforming water passage  50  to be located downstream of a portion to which the branched passage  48  is connected. The third on-off valve  46  and the fourth on-off valve  47  constitute a switching unit. The controller  30  can control the open and close states of the third on-off valve  46  and the open and close states of the fourth on-off valve  47  to switch a destination to which the cooling water from the cooling water tank  12  flows, between the hydrogen generator  101  and the first recovered water tank  14 A. In Embodiment 1, the third on-off valve  46  and the fourth on-off valve  47  constitute the switching unit. However, the present embodiment is not limited to this. For example, a three-way valve may be provided at a branch point where the branched passage  48  branches from the reforming water passage  50 , and the controller  30  may control switching ports of the three-way valve to switch the destination to which the cooling water from the cooling water tank  12  flows, between the hydrogen generator  101  and the first recovered water tank  14 A. A portion to which the reforming water passage  50  is connected is not limited to the present example. For example, the reforming water passage  50  may be connected to the water tank  12 . To be specific, the reforming water passage  50  may be connected to any portion on the cooling water passage  11 . 
     A water level detector  13  configured to detect a water level in the cooling water tank  12  is provided at the cooling water tank  12 . The water level detector  13  may be any device as long as it can detect the water level in the cooling water tank  12  and output the detected water level to the controller  30 . Examples of the water level detector  13  are a float type water level detector and a hydraulic type water level detector. Further, a second connection passage  28  through which the water having overflowed from the cooling water tank  12  returns to the first recovered water tank  14 A is connected to the cooling water tank  12 . 
     A water level detector  15 A and a filter  51  are provided at the first recovered water tank  14 A. The water level detector  15 A may be any device as long as it can detect the water level in the first recovered water tank  14 A and output the detected water level to the controller  30 . Examples of the water level detector  15 A are a float type water level detector and a hydraulic type water level detector. The filter  51  is configured to capture impurities in the first recovered water tank  14 A. 
     An upstream end of a first connection passage  21  is connected to the first recovered water tank  14 A, and a downstream end thereof is connected to the cooling water tank  12 . A first water pump  16  and a purifier  17  are provided on portions of the first connection passage  21 . The first water pump  16  is configured to cause the water in the first connection passage  21  to flow. A filter and a rotation detector configured to detect the number of rotations of the first water pump  16  are provided at the first water pump  16 . The purifier  17  is configured to purify the water in the first recovered water tank  14 A. For example, a container filled with ion-exchange resin is used as the purifier  17 . The purifier  17  is not limited to the present example and may be any member, such as activated carbon, which purifies the recovered water supplied to the cooling water tank  12 . 
     A first branched passage  24  branching from the first connection passage  21  is connected to a portion of the first connection passage  21 . A downstream end of the first branched passage  24  is open to the outside of the fuel cell system  100  (casing  70 ). A second water discharge valve  26  is provided on a portion of the first branched passage  24 , the portion being located downstream of the casing  70 . 
     When the second water discharge valve  26  opens its valve body, the water in the purifier  17  and the first recovered water tank  14 A flows through the first branched passage  24  to be discharged to the outside of the fuel cell system  100 . Thus, the water is removed from the purifier  17  and the first recovered water tank  14 A. After the completion of this water removal, water filling of the purifier  17  and the first recovered water tank  14 A is executed. The purifier  17  and the first recovered water tank  14 A are configured such that when the water level detector  15 A detects the completion of the water filling of the first recovered water tank  14 A, the water filling of the purifier  17  has already been completed. To be specific, an upper end of the purifier  17 A is lower than the water level at the time of the completion of the water filling of the first recovered water tank  14 A. In Embodiment 1, the first branched passage  24  serves as both a “second water discharge passage” and a “first branched passage”, and the second water discharge valve  26  serves as both a “second water discharge valve” and a “fifth water discharge valve”. This is because in Embodiment 1, since the first connection passage  21  is configured to be connected to a bottom surface of the first recovered water tank  14 A, the water can be removed from both the purifier  17  and the first recovered water tank  14 A only by providing the first branched passage  24  which is open to the outside of the fuel cell system  100  and the second water discharge valve  26  which opens and closes the first branched passage  24 . 
     The above arrangement of the first recovered water tank  14 A, the purifier  17 , and the first branched passage  24  is just one example, and any arrangement may be adopted as long as the water in the purifier  17  and the first recovered water tank  14 A is removed through the first branched passage  24 , and the water filling of the purifier  17  is completed when the water level detector  15 A detects the completion of the water filling of the first recovered water tank  14 A after the completion of the water removal. The second water discharge valve  26  may be constituted by an automatic on-off valve whose open and close states are controlled by the controller  30  or may be constituted by a manual on-off valve which is opened and closed by a user or a maintenance worker. Further, the water filling of the purifier  17  and the first recovered water tank  14 A may be performed by supplying the water (for example, tap water) from the outside of the fuel cell system  100  to the first recovered water tank  14 A or by supplying the water from the hot water tank  9  to the first recovered water tank  14 A. 
     Further, the first recovered water tank  14 A is open to the atmosphere, and the cooling water tank  12  is open to the atmosphere through the second connection passage  28  and the first recovered water tank  14 A. 
     An exhaust unit  19  configured to discharge gases (the air, the fuel gas, and the like) in the casing  70  to the outside is provided in the vicinity of the exhaust port of the casing  70 . Examples of the exhaust unit  19  are fans, such as a blower and a sirocco fan. 
     The controller  30  may be any device as long as it controls respective devices, such as the second water pump  10 , constituting the fuel cell system  100 . For example, the controller  30  may be constituted by a microprocessor, a CPU, or the like. The controller  30  may be constituted by a single controller or may be constituted by a group of a plurality of controllers which cooperate to execute control operations of the fuel cell system  100 . The controller  30  may include not only a calculation processing portion, such as a microprocessor or a CPU, but also a storage portion constituted by a memory, and a timer portion. 
     As shown in  FIGS. 2 and 3 , an operating unit (command acquiring unit)  29  includes a control portion (not shown) constituted by a microprocessor, a communication portion (not shown), a display portion  29   a,  and an input portion  29   b.  The control portion controls the communication portion and the like. Herein, the operating unit  29  is constituted by a panel computer. In the operating unit  29 , the communication portion receives a control signal, and the control portion processes the control signal and transmits the control signal to the display portion  29   a.  The input portion  29   b  of the operating unit  29  receives a set command of a block of the water circulation passage by a manual operation of an operator (the user or maintenance worker of the fuel cell system  100 ), the block being subjected to the water removing process. An operation signal input to the input portion  29   b  is transmitted through the control portion and communication portion of the operating unit  29  to the controller  30  and is received by the communication portion of the controller  30 . In the following explanation, for ease of explanation, regarding the transmission and reception of signals between the controller  30  and the operating unit  29 , the communication between respective communication portions of the controller  30  and the operating unit  29  and the process of the control portion of the operating unit  29  are omitted. 
     Moreover, in Embodiment 1, examples of a “first maintenance part” are a filter (not shown) provided in the second water pump  10  and a filter (not shown) provided at an exit, through which the cooling water is supplied to the cooling water passage  11 , of the cooling water tank  12  to capture impurities in the cooling water tank  12 . Examples of a “second maintenance part” are the purifier  17  and the filter  51 . Examples of a “first abnormality detector” are the water level detector  13 , the first temperature detector  33 , and the rotation detector (not shown) configured to detect the number of rotations of the second water pump  10 . Examples of a “second abnormality detector” are a temperature detector (not shown) configured to detect the temperature of the recovered water, the water level detector  15 A, and the rotation detector (not shown) configured to detect the number of rotations of the first water pump  16 . 
     Further, in Embodiment 1, the “first water circulation passage” is constituted by the cooling water passage  11  and the cooling water tank  12 , and the “second water circulation passage” is constituted by the cooling water tank  12 , the first connection passage  21 , the second connection passage  28 , and the first recovered water tank  14 A. The cooling water tank  12  serves as a separation mechanism. 
     Operations of Fuel Cell System 
     Next, the operations of the fuel cell system  100  according to Embodiment 1 will be explained. Herein, the water removing operation of the fuel cell system  100  will be explained. Since the electric power generating operation of the fuel cell system  100  according to Embodiment 1 is the same as the electric power generating operation of a common fuel cell system  100 , an explanation thereof is omitted. 
     Water Removing Process of First Water Circulation Passage 
       FIG. 4  is a flow chart schematically showing the water removing process of the first water circulation passage of the fuel cell system  100  shown in  FIG. 1 . 
     First, for example, during the electric power generating operation or operation stop of the fuel cell system  100 , if the controller  30  detects that the abnormality (failure) of the first temperature detector  33  that is the first abnormality detector has occurred or if the controller  30  detects that the time for replacement of the filter (not shown) that is the first maintenance part and is provided at the cooling water tank  12  has come, the controller  30  informs the display portion  29   a  of the operating unit  29  by using an error code that the failure of the first temperature detector  33  has occurred or the time for replacement of the filter has come, and also informs a maintenance company that the failure has occurred or the time for replacement of the filter has come. Next, in a case where the fuel cell system  100  is performing the electric power generating operation, the controller  30  performs abnormality stop of the electric power generating operation of the fuel cell system  100 . 
     Then, the maintenance worker of the maintenance company which has received the information of the failure performs the below-described water removing process of the first water circulation passage. 
     As shown in  FIG. 4 , the maintenance worker manually opens the first water discharge valve  20  (Step S 101 ) and selects a water removal mode of the first water circulation passage (Step S 102 ). Specifically, the maintenance worker operates the input portion  29   b  of the operating unit  29  to select a water removal mode  1  shown in  FIG. 2  and press a set button. When the water removal mode  1  is selected, the controller  30  keeps the stop states of the first water pump  16 , the second water pump  10 , and the fourth water pump  45  and keeps the close states of the third on-off valve  46  and the fourth on-off valve  47 . With this, the cooling water tank  12  serves as the separation mechanism, separates a block of the first water circulation passage and a block of the second water circulation passage from each other, and divides the water between these blocks. The water in the first water circulation passage including the cooling water passage  11  and the cooling water tank  12  is discharged through the first water discharge passage  18  to the outside of the fuel cell system  100 . Thus, the water is removed from the first water circulation passage. 
     When the water removal mode  1  is selected in Step S 102 , the controller  30  executes a water removal abnormality detecting sequence of the first water circulation passage (Step S 103 ). Here, the water removal abnormality detecting sequence of the first water circulation passage will be explained in reference to  FIG. 5 . 
       FIG. 5  is a flow chart schematically showing the water removal abnormality detecting sequence of the first water circulation passage in the water removing process of the fuel cell system  100  shown in  FIG. 4 . 
     As shown in  FIG. 5 , when an elapsed time since the selection of the water removal mode  1  in Step S 102  becomes equal to or longer than T 1  (Step S 11 ), the controller  30  determines by the water level detector  13  in the cooling water tank  12  whether or not the water is remaining in the cooling water tank  12  (Step S 12 ). Here, T 1  is a time equal to or longer than a time necessary to discharge the cooling water remaining in the cooling water passage  11  and the cooling water tank  12  to the outside of the fuel cell system  100  and is determined in advance by experiments or the like. In the present embodiment, whether or not the water is remaining in the cooling water tank  12  is determined based on whether or not the water level equal to or higher than a lower detection limit of the water level detector  13  is detected by the water level detector  13 . Specifically, when the water level equal to or higher than the lower detection limit of the water level detector  13  is detected, it is determined that the water is remaining in the cooling water tank  12 . When the water level equal to or higher than the lower detection limit of the water level detector  13  is not detected, it is determined that the water is not remaining in the cooling water tank  12 . 
     When the water level detector  13  detects that the water is remaining in the cooling water tank  12  (Yes in Step S 12 ), the controller  30  causes the display portion  29   a  of the operating unit  29  to inform that the abnormality has occurred (Step S 13 ) and terminates the present sequence. In contrast, when the water level detector  13  does not detect that the water is remaining in the cooling water tank  12  (No in Step S 12 ), the controller  30  does not cause the display portion  29   a  to inform that the abnormality has occurred (Step S 14 ) and terminates the present sequence. 
     Next, when the controller  30  confirms that there is no abnormality of the water removal of the first water circulation passage in the water removal abnormality detecting sequence (Step S 104 ) and confirms that the water removal is terminated, the controller  30  causes the input portion  29   b  of the operating unit  29  to inform that the water removal mode of the first water circulation passage has been terminated (see  FIG. 3 ). Thus, the maintenance worker presses the set button of the input portion  29   b  to set the termination of the water removal mode (Step S 105 ). 
     Next, the maintenance worker manually closes the first water discharge valve  20  (Step S 106 ) and replaces the first temperature detector  33  or the filter. After the replacement of the first temperature detector  33  or the filter, the water filling of the first water circulation passage is executed. 
     Water Removing Process of Second Water Circulation Passage 
       FIG. 6  is a flow chart schematically showing the water removing process of the second water circulation passage of the fuel cell system  100  shown in  FIG. 1 . 
     First, as with the water removing process of the first water circulation passage in the fuel cell system  100  according to Embodiment 1, for example, during the electric power generating operation or operation stop of the fuel cell system  100 , if the controller  30  detects that the abnormality (failure) of a temperature detector (not shown) that is a second abnormality detector and is configured to detect the temperature of the recovered water has occurred or if the controller  30  detects that the time for replacement of the filter  51  that is the second maintenance part and is provided at the first recovered water tank  14 A has come, the controller  30  informs the display portion  29   a  of the operating unit  29  by using an error code that the failure of the temperature detector has occurred or the time for replacement of the filter  51  has come, and also informs the maintenance company that the failure has occurred or the time for replacement of the filter has come. Next, in a case where the fuel cell system  100  is performing the electric power generating operation, the controller  30  performs the abnormality stop of the electric power generating operation of the fuel cell system  100 . 
     Then, the maintenance worker of the maintenance company which has received the information of the failure performs the below-described water removing process of the second water circulation passage. 
     As shown in  FIG. 6 , the maintenance worker manually opens the second water discharge valve  26  (Step S 201 ) and selects the water removal mode of the second water circulation passage (Step S 202 ). Specifically, the maintenance worker operates the input portion  29   b  of the operating unit  29  to select a water removal mode  2  shown in  FIG. 2  and press the set button. When the water removal mode  2  is selected, the controller  30  keeps the stop states of the first water pump  16 , the second water pump  10 , and the fourth water pump  45  and keeps the close states of the third on-off valve  46  and the fourth on-off valve  47 . With this, the cooling water tank  12  serves as the separation mechanism, separates the block of the second water circulation passage and the block of the first water circulation passage from each other, and divides the water between these blocks. The water in the second water circulation passage including the first connection passage  21 , the second connection passage  28 , and the first recovered water tank  14 A is discharged through the first branched passage  24  to the outside of the fuel cell system  100 . Thus, the water is removed from the second water circulation passage. At this time, the water in the purifier  17  is also removed. 
     When the water removal mode of the second water circulation passage is selected in Step S 202 , the controller  30  executes a water removal abnormality detecting sequence of the second water circulation passage (Step S 203 ). Here, the water removal abnormality detecting sequence of the second water circulation passage will be explained in reference to  FIG. 7 . 
       FIG. 7  is a flow chart schematically showing the water removal abnormality detecting sequence of the second water circulation passage in the water removing process of the fuel cell system  100  shown in  FIG. 6 . 
     As shown in  FIG. 7 , when an elapsed time since the selection of the water removal mode  2  in Step S 202  becomes equal to or longer than T 2  (Step S 21 ), the controller  30  determines by the water level detector  15 A in the first recovered water tank  14 A whether or not the water is remaining in the first recovered water tank  14 A (Step S 22 ). Here, T 2  is a time equal to or longer than a time necessary to discharge the recovered water remaining in the second water circulation passage to the outside of the fuel cell system  100  and is determined in advance by experiments or the like. In the present embodiment, whether or not the water is remaining in the first recovered water tank  14 A is determined based on whether or not the water level equal to or higher than a lower detection limit of the water level detector  15 A is detected by the water level detector  15 A. Specifically, when the water level equal to or higher than the lower detection limit of the water level detector  15 A is detected, it is determined that the water is remaining in the first recovered water tank  14 A. When the water level equal to or higher than the lower detection limit of the water level detector  15  is not detected, it is determined that the water is not remaining in the first recovered water tank  14 A. 
     When the water level detector  15 A detects that the water is remaining in the first recovered water tank  14 A (Yes in Step S 22 ), the controller  30  causes the display portion  29   a  of the operating unit  29  to inform that the abnormality has occurred (Step S 23 ) and terminates the present sequence. In contrast, when the water level detector  15 A detects that the water remains in the first recovered water tank  14 A, the controller  30  does not cause the display portion  29   a  to inform that the abnormality has occurred (Step S 24 ) and terminates the present sequence. 
     Next, when the controller  30  confirms that there is no abnormality of the water removal of the second water circulation passage in the water removal abnormality detecting sequence (Step S 204 ) and confirms that the water removal is terminated, the controller  30  causes the input portion  29   b  of the operating unit  29  to inform that the water removal mode of the second water circulation passage has been terminated (see  FIG. 3 ). Thus, the maintenance worker presses the set button of the input portion  29   b  to set the termination of the water removal mode (Step S 205 ). 
     Next, the maintenance worker manually closes the second water discharge valve  26  (Step S 206 ) and replaces the temperature detector or the filter  51 . After the replacement of the temperature detector or the filter, the water filling of the second water circulation passage is executed. At this time, as described above, by detecting the water level by the water level detector  15 A, the controller  30  can detect the completion of the water filling of the purifier  17 . 
     As above, in accordance with the fuel cell system  100  of Embodiment 1, the water is removed from only the block (the first water circulation passage or the second water circulation passage), from which the water needs to be removed, of the water circulation passage. With this, the water removing operation can be performed more quickly than a case where all the water in the water circulation passage is removed. In addition, the amount of time for the water filling operation performed after the water removing operation is reduced. Thus, the maintenance is completed more quickly. 
     Water Removing Process of All Blocks of Water Circulation Passage 
       FIG. 8  is a flow chart schematically showing the water removing process of all the blocks of the water circulation passage of the fuel cell system  100  shown in  FIG. 1 . 
     First, for example, the below-described water removing process of all the blocks of the water circulation passage is executed in a case where the user is aware of the possibility of freezing of the water in the water circulation passage in advance based on information, such as weather forecast or in a case where there is a possibility of the rottenness or freezing of the water in the water circulation passage since the user is away for a long period of time and the fuel cell system  100  is not used for a long period of time. 
     As shown in  FIG. 8 , the user manually opens the first water discharge valve  20  and the second water discharge valve  26  (Step S 301 ) and selects the water removal mode of all the blocks of the water circulation passage (Step S 302 ). Specifically, the user operates the input portion  29   b  of the operating unit  29  to select an all water removal mode shown in  FIG. 2  and press the set button. When the all water removal mode is selected, the controller  30  keeps the stop states of the first water pump  16  and the second water pump  10 , keeps the close state of the third on-off valve  46 , opens the fourth on-off valve  47 , and activates the fourth water pump  45  (Step S 303 ). With this, the water in the first water circulation passage and the second water circulation passage is discharged to the outside of the fuel cell system  100 . Thus, the water is removed from all the blocks of the water circulation passage, and the water in the reforming water passage  50  and the branched passage  48  is also removed. Specifically, the water in the first water circulation passage including the cooling water passage  11  and the cooling water tank  12  is discharged through the first water discharge passage  18  to the outside of the fuel cell system  100 . Thus, the water is removed from the first water circulation passage. In addition, the controller  30  activates the fourth water pump  45 , keeps the close state of the third on-off valve  46 , and opens the fourth on-off valve  47 . With this, the water is supplied from the reforming water passage  50  through the branched passage  48  to the first recovered water tank  14 A. Then, the cooling water supplied to the first recovered water tank  14 A is discharged through the first branched passage  24  to the outside of the fuel cell system  100 . To be specific, by the operations in Step S 303 , the water is removed from the reforming water passage  50  and the branched passage  48 . Further, the water in the second water circulation passage including the first connection passage  21 , the second connection passage  28 , and the first recovered water tank  14 A is discharged through the first branched passage  24  to the outside of the fuel cell system  100 . Thus, the water is removed from the second water circulation passage. At this time, the water in the purifier  17  is also removed. 
     When the all water removal mode is selected in Step S 302 , the controller  30  executes a water removal abnormality detecting sequence of all the blocks of the water circulation passage (Step S 304 ). Here, the water removal abnormality detecting sequence of all the blocks of the water circulation passage will be explained in reference to  FIG. 9 . 
       FIG. 9  is a flow chart schematically showing the water removal abnormality detecting sequence of all the blocks of the water circulation passage in the water removing process of the fuel cell system  100  shown in  FIG. 8 . 
     As shown in  FIG. 9 , when an elapsed time since the selection of the all water removal mode in Step S 302  becomes equal to or longer than T 3  (Step S 31 ), the controller  30  determines by the water level detector  13  whether or not the water is remaining in the cooling water tank  12  and determines by the water level detector  15 A whether or not the water is remaining in the first recovered water tank  14 A (Step S 32 ). Here, T 3  is a time equal to or longer than a time necessary to discharge the water remaining in all the blocks of the water circulation passage to the outside of the fuel cell system  100  and is determined in advance by experiments or the like. A method for determining by the water level detector  13  whether or not the water is remaining in the cooling water tank  12  and a method for determining by the water level detector  15 A whether or not the water is remaining in the first recovered water tank  14 A are the same as those of the above-described water removing process of the first water circulation passage and the above-described water removing process of the second water circulation passage, so that explanations thereof are omitted. 
     Then, when the water level detector  13  detects that the water is remaining in the cooling water tank  12  or the water level detector  15 A detects that the water is remaining in the first recovered water tank  14 A (Yes in Step S 32 ), the controller  30  causes the display portion  29   a  of the operating unit  29  to inform that the abnormality has occurred (Step S 33 ) and terminates the present sequence. In contrast, when the water level detector  13  detects that the water is not remaining in the cooling water tank  13  or the water level detector  15 A detects that the water is not remaining in the first recovered water tank, the controller  30  does not cause the display portion  29   a  to inform that the abnormality has occurred (Step S 34 ) and terminates the present sequence. 
     Next, when the controller  30  confirms that there is no abnormality in all the blocks of the water circulation passage in the water removal abnormality detecting sequence (Step S 305 ) and confirms that the water removal is terminated, the controller  30  causes the input portion  29   b  of the operating unit  29  to inform that the water removal mode of all the blocks of the water circulation passage has been terminated (see  FIG. 3 ). Thus, the maintenance worker presses the set button of the input portion  29   b  to set the termination of the water removal mode (Step S 306 ). With this, the controller  30  stops the fourth water pump  45  and closes the fourth on-off valve  47 . 
     Next, the maintenance worker manually closes the first water discharge valve  20  and the second water discharge valve  26  (Step S 307 ). 
     As above, by performing the all water removing process, the freezing and rottenness of the water in all the blocks of the water circulation passage can be avoided in advance. 
     Embodiment 1 adopts a mode in which the heat exchanger  6  configured to recover the heat of the cooling water by the hot water is provided to recover the exhaust heat from the fuel cell  1 . However, the present embodiment is not limited to this and may adopt a mode in which a first circulation passage through which cooling water circulates is provided, the cooling water recovering the heat of the unconsumed reactant gas flowing through the fuel gas discharge passage  32  or the oxidizing gas discharge passage  31 , and the first water circulation passage and the second water circulation passage are separated from each other by the cooling water tank. 
     Moreover, Embodiment 1 adopts a mode in which the water generated from the steam of the unconsumed oxidizing gas is recovered by the first recovered water tank  14 A. However, the present embodiment is not limited to this and may adopt a mode in which the water generated from the steam of the unconsumed fuel gas is recovered by the first recovered water tank  14 A or a mode in which the water generated from the steam of both the unconsumed oxidizing gas and the unconsumed fuel gas is recovered by the first recovered water tank  14 A. 
     Further, Embodiment 1 adopts a mode in which in the water removing process, the maintenance worker manually opens and closes the first water discharge valve  20  and the second water discharge valve  26 . However, the present embodiment is not limited to this and may adopt a mode in which in the water removing process, the controller  30  opens and closes at least one of the first water discharge valve  20  and the second water discharge valve  26 . In this case, after a predetermined water removal mode is selected by the maintenance worker (Step S 102 , S 202 , or S 302 ), the controller  30  executes the operation (Step S 101 , S 201 , or S 301 ) of opening at least one of the first water discharge valve  20  and the second water discharge valve  26 . 
     Embodiment 2 
     In the fuel cell system according to Embodiment 2 of the present invention, the water circulation passage is constituted by a hot water tank and a water passage. 
     Configuration of Fuel Cell System 
       FIG. 10  is a block diagram schematically showing a schematic configuration of the fuel cell system according to Embodiment 2 of the present invention. 
     As shown in  FIG. 10 , the fuel cell system  100  according to Embodiment 2 of the present invention is the same in basic configuration as the fuel cell system  100  according to Embodiment 1 of the present invention but is different from the fuel cell system  100  according to Embodiment 1 of the present invention in that the exhaust heat recovered water passage (water passage)  8  and the hot water tank  9  constitute the water circulation passage. 
     Specifically, a second temperature detector  35  configured to detect the temperature of the water in the exhaust heat recovered water passage  8  is provided on the exhaust heat recovered water passage  8 . In addition, a first on-off valve  52  is provided on a portion, through which the water having not yet recovered the exhaust heat of the fuel cell  1  flows, of the exhaust heat recovered water passage  8 , and a second on-off valve  53  is provided on a portion, through which the water having recovered the exhaust heat of the fuel cell  1  flows, of the exhaust heat recovered water passage  8 . To be specific, the first on-off valve  52  is provided on the exhaust heat recovered water passage  8  to be located upstream of the heat exchanger  6  configured to recover the exhaust heat of the fuel cell  1 , and the second on-off valve  53  is provided on the exhaust heat recovered water passage  8  to be located downstream of the heat exchanger  6 . Then, the first on-off valve  52  and the second on-off valve  53  serve as the separation mechanism configured to separate the block including the hot water tank  9  (to be precise, the block including the hot water tank  9 , a portion, located upstream of the first on-off valve  52 , of the exhaust heat recovered water passage  8 , and a portion, located downstream of the second on-off valve  53 , of the exhaust heat recovered water passage  8 ) and the block including the exhaust heat recovered water passage  8  (to be precise, the block including a portion of the exhaust heat recovered water passage  8 , the portion being located downstream of the first on-off valve  52  and upstream of the second on-off valve  53 ) from each other by closing valve bodies of the first on-off valve  52  and the second on-off valve  53  and to divide the water between these blocks. 
     Moreover, a third water discharge passage  41  is connected to a portion of the exhaust heat recovered water passage  8 , the portion being located between the first on-off valve  52  and the second on-off valve  53 . A third water discharge valve  27  is provided on the third water discharge passage  41 . The first on-off valve  52  and the second on-off valve  53  are configured to allow the water to flow through the exhaust heat recovered water passage  8  and block the water in the exhaust heat recovered water passage  8 . The third water discharge valve  27  is configured such that by opening a valve body of the third water discharge valve  27 , the water in the exhaust heat recovered water passage  8  is discharged through the third water discharge passage  41  to the outside of the fuel cell system  100 . Each of the first on-off valve  52 , the second on-off valve  53 , and the third water discharge valve  27  may be constituted by an automatic on-off valve whose open and close states are controlled by the controller  30  or may be constituted by a manual on-off valve which is opened and closed by a user or a maintenance worker. 
     A gas venting unit  37  is provided on the exhaust heat recovered water passage  8  to be located between the first on-off valve  52  and the second on-off valve  53 . The gas venting unit  37  is configured to allow the exhaust heat recovered water passage  8  to communicate with the atmosphere by activating the gas venting unit  37 . For example, the gas venting unit  37  may be constituted by a gas venting pipe connected to a pipe constituting the exhaust heat recovered water passage  8 , and an on-off valve provided on this gas venting pipe. This on-off valve may be constituted by an automatic on-off valve whose open and close states are controlled by the controller  30  or may be constituted by a manual on-off valve which is opened and closed by a user or a maintenance worker. 
     A gas venting unit  38  configured to allow the hot water tank  9  to communicate with the atmosphere is provided on an upper portion (herein, an upper end) of the hot water tank  9 , and a fourth water discharge passage  42  is connected to a lower portion (herein, a lower end) of the hot water tank  9 . A fourth water discharge valve  39  is provided on the fourth water discharge passage  42 . The fourth water discharge valve  39  is configured such that by opening a valve body of the fourth water discharge valve  39 , the water in the hot water tank  9  is discharged through the fourth water discharge passage  42  to the outside of the fuel cell system  100 . The fourth water discharge valve  39  may be constituted by an automatic on-off valve whose open and close states are controlled by the controller  30  or may be constituted by a manual on-off valve which is opened and closed by a user or a maintenance worker. 
     In Embodiment 2, examples of a “third abnormality detector” are the second temperature detector  35  configured to detect the temperature of the water flowing through the exhaust heat recovered water passage  8  and the rotation detector (not shown) configured to detect the number of rotations of the third water pump  7 . Examples of a “fourth abnormality detector” are a temperature detector (not shown) configured to detect the temperature of the water in the hot water tank  9  and a water level detector (not shown) configured to detect the water level in the hot water tank  9 . 
     The fuel cell system  100  of Embodiment 2 is configured such that the water flowing through the exhaust heat recovered water passage  8  recovers heat from the cooling water, which cools the fuel cell  1 , by the heat exchanger  6 . Instead of this, a heat exchanger configured to perform heat exchange between the fuel gas discharge passage  32  and the exhaust heat recovered water passage  8  may be provided, a heat exchanger configured to perform heat exchange between the oxidizing gas discharge passage  31  and the exhaust heat recovered water passage  8  may be provided, or a heat exchanger configured to perform heat exchange between a flue gas passage (not shown), through which a flue gas having heated the hydrogen generator  101  flows, and the exhaust heat recovered water passage  8  may be provided. To be specific, the heat exchanger may be any device as long as it recovers the exhaust heat of the fuel cell system  100  by using the water in the exhaust heat recovered water passage  8 . 
     Operations of Fuel Cell System 
     Next, the water removing operation of the fuel cell system  100  according to Embodiment 2 will be explained. 
     Water Removing Process of Water Passage 
       FIG. 11  is a flow chart schematically showing the water removing process of the exhaust heat recovered water passage (water passage)  8  of the fuel cell system  100  shown in  FIG. 10 . 
     First, for example, during the electric power generating operation or operation stop of the fuel cell system  100 , if the controller  30  detects that the abnormality (failure) of the second temperature detector  35  that is the third abnormality detector has occurred, the controller  30  informs the display portion  29   a  of the operating unit  29  that the failure of the second temperature detector  35  has occurred and also informs the maintenance company that the failure has occurred. Next, in a case where the fuel cell system  100  is performing the electric power generating operation, the controller  30  performs the abnormality stop of the electric power generating operation of the fuel cell system  100 . 
     Then, the maintenance worker of the maintenance company which has received the information of the failure performs the below-described water removing process of the exhaust heat recovered water passage (water passage)  8 . 
     As shown in  FIG. 11 , the maintenance worker manually closes the first on-off valve  52  and the second on-off valve  53  (Step S 401 ) and manually opens the third water discharge valve  27  and the gas venting unit  37  (Step S 402 ). With this, the first on-off valve  52  and the second on-off valve  53  serve as the separation mechanism, separates the block including the exhaust heat recovered water passage  8  (to be precise, the block including a portion of the exhaust heat recovered water passage  8 , the portion being located downstream of the first on-off valve  52  and upstream of the second on-off valve  53 ) and the block including the hot water tank  9  (to be precise, the block including the hot water tank  9 , a portion, located upstream of the first on-off valve  52 , of the exhaust heat recovered water passage  8 , and a portion, located downstream of the second on-off valve  53 , of the exhaust heat recovered water passage  8 ) from each other, and divides the water between these blocks. 
     After that, the maintenance worker selects the water removal mode of the above water passage (Step S 403 ). Specifically, the maintenance worker operates the input portion  29   b  of the operating unit  29  to select a water removal mode  3  shown in  FIG. 2  and press the set button. When the water removal mode  3  is selected, the controller  30  starts measuring a processing time of the water removing process. 
     By the above operation, the water in the exhaust heat recovered water passage  8  is discharged through the third water discharge passage  41  to the outside of the fuel cell system  100 . Thus, the water is removed from the exhaust heat recovered water passage  8 . Then, when the measured processing time has passed a time at which the water removing process may be completed, the controller  30  causes the input portion  29   b  of the operating unit  29  to inform that the water removal mode of the exhaust heat recovered water passage  8  has been terminated (see  FIG. 3 ). When an image showing that the water removal mode has been terminated is displayed on the display portion  29   a  of the operating unit  29 , the maintenance worker visually confirms that the water removing process through the third water discharge passage  41  has been completed and manually closes the third water discharge valve  27  and the gas venting unit  37  (Step S 404 ). Then, the maintenance worker presses the set button of the input portion  29   b  to set the termination of the water removal mode (Step S 405 ). The termination of the water removal of the exhaust heat recovered water passage  8  is determined based on whether or not a time it takes to remove the water from the exhaust heat recovered water passage  8  has elapsed, the time being obtained in advance by experiments or the like. 
     Next, the maintenance worker replaces the second temperature detector  35 . When the replacement of the second temperature detector  35  is terminated, the water filling in the exhaust heat recovered water passage (water passage)  8  is performed. 
     Water Removal of Hot Water Tank 
       FIG. 12  is a flow chart schematically showing the water removing process of the hot water tank  9  of the fuel cell system  100  shown in  FIG. 10 . 
     First, for example, during the electric power generating operation or operation stop of the fuel cell system  100 , if the controller  30  detects that the abnormality (failure) of the temperature detector (not shown) that is the fourth abnormality detector and provided at the hot water tank  9  has occurred, the controller  30  informs the display portion  29   a  of the operating unit  29  that the failure of the temperature detector has occurred and also informs the maintenance company that the failure has occurred. Next, in a case where the fuel cell system  100  is performing the electric power generating operation, the controller  30  performs the abnormality stop of the electric power generating operation of the fuel cell system  100 . 
     Then, the maintenance worker of the maintenance company which has received the information of the failure performs the below-described water removing process of the hot water tank  9 . 
     As shown in  FIG. 12 , the maintenance worker manually closes the first on-off valve  52  and the second on-off valve  53  (Step S 501 ) and manually opens the fourth water discharge valve  39  and the gas venting unit  38  (Step S 502 ). With this, the first on-off valve  52  and the second on-off valve  53  serve as the separation mechanism, separates the block including the exhaust heat recovered water passage  8  (to be precise, the block including a portion of the exhaust heat recovered water passage  8 , the portion being located downstream of the first on-off valve  52  and upstream of the second on-off valve  53 ) and the block including the hot water tank  9  (to be precise, the block including the hot water tank  9 , a portion, located upstream of the first on-off valve  52 , of the exhaust heat recovered water passage  8 , and a portion, located downstream of the second on-off valve  53 , of the exhaust heat recovered water passage  8 ) from each other, and divides the water between these blocks. 
     After that, the maintenance worker selects the water removal mode of the hot water tank  9  (Step S 503 ). Specifically, the maintenance worker operates the input portion  29   b  of the operating unit  29  to select a water removal mode  4  shown in  FIG. 2  and press the set button. When the water removal mode  4  is selected, the controller  30  starts measuring the processing time of the water removing process. 
     By the above operation, the water in the hot water tank  9  is discharged through the fourth water discharge passage  42  to the outside of the fuel cell system  100 . Thus, the water is removed from the hot water tank  9 . Then, when the measured processing time has passed a time at which the water removing process may be completed, the controller  30  causes the input portion  29   b  of the operating unit  29  to inform that the water removal mode of the hot water tank  9  has been terminated (see  FIG. 3 ). When an image showing that the water removal mode has been terminated is displayed on the display portion  29   a  of the operating unit  29 , the maintenance worker visually confirms that the water removing process through the fourth water discharge passage  42  has been completed and manually closes the fourth water discharge valve  39  and the gas venting unit  38  (Step S 504 ). Then, the maintenance worker presses the set button of the input portion  29   b  to set the termination of the water removal mode (Step S 505 ). The termination of the water removal of the hot water tank  9  is determined based on whether or not a time it takes to remove the water from the hot water tank  9  has elapsed, the time being obtained in advance by experiments or the like. 
     Next, the maintenance worker replaces the temperature detector. When the replacement of the temperature detector is terminated, the water filling in the hot water tank  9  is performed. 
     As above, in accordance with the fuel cell system  100  of Embodiment 2, the water is removed from only the block (the exhaust heat recovered water passage  8  or the hot water tank  9 ), from which the water needs to be removed, of the water circulation passage. With this, the water removing operation can be performed more quickly than a case where the water in all the blocks of the water circulation passage is removed. In addition, the amount of time for the water filling operation performed after the water removing operation is reduced. Thus, the maintenance is completed more quickly. 
     In the case of performing the water removing process of all the blocks of the water circulation passage in the fuel cell system  100  according to Embodiment 2, as with the fuel cell system  100  according to Embodiment 1, the third water discharge valve  27  and the fourth water discharge valve  39  may be opened, and the gas venting unit  37  and the gas venting unit  38  may be activated (opened). 
     In the above water removing process, the maintenance worker opens and closes the first on-off valve  52 , the second on-off valve  53 , the third water discharge valve  27 , the fourth water discharge valve  39 , the gas venting unit  37 , and the gas venting unit  38 . However, the present embodiment is not limited to this, and the controller  30  may automatically open and close these components. In this case, after a predetermined water removal mode is selected by the maintenance worker (Step S 403 , S 503 ), the controller  30  executes the operation of closing the first on-off valve  52  and the second on-off valve  53  (Step S 401 , S 501 ) and opening the third water discharge valve  27  and the gas venting unit  37  (Step S 402 , S 502 ). 
     Embodiment 3 
     The fuel cell system of Embodiment 3 of the present invention will be explained in detail. 
     Configuration 
       FIG. 13  is a block diagram schematically showing a schematic configuration of the fuel cell system according to Embodiment 3 of the present invention. 
     As shown in  FIG. 13 , the fuel cell system  100  of Embodiment 3 includes: the fuel cell  1  configured to generate electric power using the fuel gas and the oxidizing gas; the oxidizing gas supply unit  2  configured to supply the oxidizing gas to the fuel cell  1 ; a fuel gas supply unit  3  configured to supply the fuel gas to the fuel cell  1 ; a first condenser  4  configured to condense moisture contained in the off oxidizing gas discharged from a cathode gas channel in the fuel cell  1 ; a second condenser  5  configured to condense moisture contained in the off fuel gas discharged from an anode gas channel in the fuel cell  1 ; the first recovered water tank  14 A configured to store recovered water including recovered water condensed and recovered by the first condenser  4 ; the water level detector  15 A configured to detect the water level in the first recovered water tank  14 A; the filter  51  configured to capture impurities in the first recovered water tank  14 A; a second recovered water tank  14 B configured to store recovered water condensed and recovered by the second condenser  5 ; a water level detector  15 B configured to detect the water level in the second recovered water tank  14 B; the cooling water passage  11  through which the cooling water for cooling the fuel cell  1  flows; the cooling water tank  12  configured to store the cooling water; the water level detector  13  configured to detect the water level in the cooling water tank  12 ; the first connection passage  21  through which the water supplied from the first recovered water tank  14 A to the cooling water tank  12  flows; the first water pump  16  configured to cause the water to flow through the first connection passage  21  to the cooling water tank  12 ; the second connection passage  28  through which the water having overflowed from the cooling water tank  12  returns to the first recovered water tank  14 A; the purifier  17  provided on the first connection passage  21  to purify the water in the first recovered water tank  14 A; the second water pump  10  configured to cause the cooling water in the cooling water passage  11  to flow; the heater  36  configured to heat the cooling water in the cooling water passage  11 ; the first temperature detector  33  configured to detect the temperature of the cooling water in the cooling water passage  11 ; the exhaust heat recovered water passage (water passage)  8  through which water flows, the water recovering potential heat from the cooling water having cooled the fuel cell  1 ; the heat exchanger  6  configured to perform heat exchange between the cooling water in the cooling water passage  11  and the water in the exhaust heat recovered water passage  8 ; the second temperature detector  35  configured to detect the temperature of the water in the exhaust heat recovered water passage  8 ; the third water pump  7  configured to cause the water in the exhaust heat recovered water passage  8  to flow; and the hot water tank  9  configured to store hot water heated by the heat exchanger  6 . 
     Here, in the fuel cell system of the present embodiment, a “first water tank” that is one example of a water supply source corresponds to the cooling water tank. A “second water tank” that is one example of the water supply source is constituted by the first recovered water tank  14 A and the second recovered water tank  14 B. However, the “second water tank” is not limited to the present example and may be constituted by one of the first recovered water tank  14 A and the second recovered water tank  14 B. To be specific, one of the recovered water recovered from the off fuel gas and the recovered water recovered from the off oxidizing gas may be supplied to a water utilizing device (for example, the cooling water tank  12 ). However, to prevent the off fuel gas from being discharged to the atmosphere, the second recovered water tank  14 B needs to be configured not to be open to the atmosphere. Therefore, in the case of supplying the water to the water utilizing device, the water may be moved to a water tank, such as the first recovered water tank  14 A, which is open to the atmosphere, to be indirectly supplied to the water utilizing device. A “hot water tank” that is one example of the water supply source corresponds to the hot water tank  9 . 
     Moreover, examples of a “first maintenance part” are a filter (not shown) provided in the second water pump  10  and a filter (not shown) provided at an exit, through which the cooling water is supplied to the cooling water passage  11 , of the cooling water tank  12  to capture impurities in the cooling water tank  12 . Examples of a “second maintenance part” are the purifier  17  and the filter  51 . For example, a container filled with ion-exchange resin is used as the purifier  17 . The purifier  17  is not limited to the present example and may be any member, such as activated carbon, which purifies the recovered water supplied to the water utilizing device. The filter  51  is provided in the second recovered water tank  14 B but may be provided at anywhere as long as it can suppress the inflow of the impurities into the first water pump  16 . For example, the filter  51  may be provided on the first connection passage  21  to be located between the first water pump  16  and the second recovered water tank  14 B. 
     Moreover, a “heater” corresponds to the heater  36 . An electric heater or the like is used as the heater  36 . Especially, a surplus electric power heater configured to consume surplus electric power of the fuel cell  1  is more preferable. The “heater” is provided on the cooling water passage  11 . However, the “heater” is not limited to the present example and may be provided on the exhaust heat recovered water passage  8 . 
     Further, examples of the “first abnormality detector” are the water level detector  13  configured to detect the water level in the cooling water tank  12 , the first temperature detector  33  configured to detect the temperature of the cooling water flowing through the cooling water passage  11 , and the rotation detector (not shown) configured to detect the number of rotations of the second water pump  10 . Examples of the “second abnormality detector” are the temperature detector (not shown) configured to detect the temperature of the recovered water, the water level detector  15 A configured to detect the water level in the first recovered water tank  14 A, and the rotation detector (not shown) configured to detect the number of rotations of the first water pump  16 . Examples of the “third abnormality detector” are the second temperature detector  35  configured to detect the temperature of the water flowing through the exhaust heat recovered water passage  8  and the rotation detector (not shown) configured to detect the number of rotations of the third water pump  7 . Further, examples of the “fourth abnormality detector” are the temperature detector (not shown) configured to detect the temperature of the water in the hot water tank  9  and the water level detector (not shown) configured to detect the water level in the hot water tank  9 . 
     The fuel cell system  100  of Embodiment 3 further includes: the first branched passage  24  through which the water in the first connection passage  21  is discharged to the outside of the passage; a fifth water discharge valve  23  configured to open the first branched passage  24  to discharge the water in the first connection passage  21  to the outside of the passage; the first water discharge passage  18  through which the water in the cooling water passage  11  is discharged to the outside of the passage; the first water discharge valve  20  configured to open the first water discharge passage  18  to discharge the water in the cooling water passage  11  to the outside of the passage; the third water discharge passage  41  through which the water in the exhaust heat recovered water passage  8  is discharged to the outside of the passage; the third water discharge valve  27  configured to open the third water discharge passage  41  to discharge the water in the exhaust heat recovered water passage  8  to the outside of the passage; a second water discharge passage  22  and a second water discharge passage  25  through which the water in the first recovered water tank  14 A and the water in the second recovered water tank  14 B are discharged to the outside of the tanks; the second water discharge valve  26  configured to open the second water discharge passage  22  and the second water discharge passage  25  to discharge the water in the first recovered water tank  14 A and the water in the second recovered water tank  14 B to the outside of the tanks; the fourth water discharge passage  42  through which the water in the hot water tank  9  is discharged to the outside of the tank; and the fourth water discharge valve  39  configured to open the fourth water discharge passage  42  to discharge the water in the hot water tank  9  to the outside of the tank. The water discharged by opening the first to fifth water discharge valves is discharged to the outside of the fuel cell system  100  and flows to, for example, a sewer infrastructure. 
     The first on-off valve  52  and the second on-off valve  53  are provided on the exhaust heat recovered water passage  8 . Specifically, the first on-off valve  52  is provided on a portion, through which the water having not yet recovered the exhaust heat of the fuel cell  1  flows, of the exhaust heat recovered water passage  8 . The second on-off valve  53  is provided on a portion, through which the water having recovered the exhaust heat of the fuel cell  1  flows, of the exhaust heat recovered water passage  8 . To be specific, the first on-off valve  52  is provided on the exhaust heat recovered water passage  8  to be located upstream of the heat exchanger and the condenser, such as the first condenser  4 , configured to recover the exhaust heat of the fuel cell  1 . The second on-off valve  53  is provided on the exhaust heat recovered water passage  8  to be located downstream of the first condenser  4  and the like. 
     In Embodiment 1, the cooling water passage  11  and the cooling water tank  12  constitute the first water circulation passage, and the first connection passage  21 , the second connection passage  28 , the cooling water tank  12 , and the first recovered water tank  14 A constitute the second water circulation passage. The first and second water circulation passages constitute one water circulation passage. 
     As described above, the cooling water tank  12  constitutes the separation mechanism. Specifically, the water in the first recovered water tank  14 A is supplied through the first connection passage  21  to the cooling water tank  12  by the operation of the first water pump  16 , and the water having overflowed from the cooling water tank  12  is supplied through the second connection passage  28  to the first recovered water tank  14 A. The cooling water tank  12  serves as the separation mechanism by the stop of the first water pump  16 . 
     Further, the exhaust heat recovered water passage  8  constitutes another water circulation passage, and the first on-off valve  52  and the second on-off valve  53  constitute the separation mechanism. To be specific, by closing the first on-off valve  52  and the second on-off valve  53 , the block including the hot water tank  9  (to be precise, the block including the hot water tank  9 , a portion, located upstream of the first on-off valve  52 , of the exhaust heat recovered water passage  8 , and a portion, located downstream of the second on-off valve  53 , of the exhaust heat recovered water passage  8 ) and the block including the exhaust heat recovered water passage  8  (to be precise, the block including a portion of the exhaust heat recovered water passage  8 , the portion being located downstream of the first on-off valve  52  and upstream of the second on-off valve  53 ) are separated from each other. Thus, the water is divided between these blocks. 
     To remove the water from the above water passages, the water passage from which the water is removed is opened to the atmosphere. The first connection passage  21  is opened to the atmosphere through the cooling water tank  12 , the second connection passage  28 , and the first recovered water tank  14 A which is open to the atmosphere. As with the first connection passage  21 , the cooling water passage  11  is opened to the atmosphere through the cooling water tank  12 , the second connection passage  28 , and the first recovered water tank  14 A which is open to the atmosphere. Regarding the water removal of the exhaust heat recovered water passage  8 , the exhaust heat recovered water passage  8  is opened to the atmosphere by opening the gas venting unit  37 . The first recovered water tank  14 A and the second recovered water tank  14 B are opened to the atmosphere by the first recovered water tank  14 A configured to be open to the atmosphere. The hot water tank  9  is opened to the atmosphere by opening the gas venting unit  38 . 
     The cooling water tank  12  is provided above the first recovered water tank  14 A. The cooling water tank  12  and the first recovered water tank  14 A are provided to form a rising slope from the first recovered water tank  14 A toward the cooling water tank  12 . By opening the fifth water discharge valve  23 , the water in the first connection passage  21  and thus the water in the purifier  17  are discharged by their own weight through the first branched passage  24 . 
     The fuel cell system of the present embodiment includes the casing  70  having the intake port and the exhaust port, and the casing  70  accommodates the fuel cell  1 , the above water passages, and the water tanks. The exhaust unit  19  configured to discharge gases (the air, the fuel gas, and the like) in the casing  70  to the outside is provided in the vicinity of the exhaust port of the casing  70 . Examples of the exhaust unit  19  are fans, such as a blower and a sirocco fan. The first to fifth water discharge valves and the gas venting units  37  and  38  are provided outside the casing  70 . 
     The controller  30  is configured to determine based on signals from the first temperature detector  33  and the second temperature detector  35  whether or not the failures of the first temperature detector  33  and the second temperature detector  35  and the failure of the heater  36  have occurred. Moreover, the controller  30  is configured to inform the outside by a display portion (not shown in  FIG. 12 ) of the operating unit (command acquiring unit)  29  that the failures of respective abnormality detectors have occurred. Further, the controller  30  is configured to also determine whether or not the time for replacement of the purifier  17  or the filter  51  has come. Specifically, the controller  30  includes a timer device, not shown, and is configured to inform the outside by the display portion (not shown in  FIG. 12 ) of the operating unit  29  that the replacement of the purifier  17  or the filter  51  is necessary, in a case where an accumulated value of the time, measured by the timer device, of the electric power generating operation of the fuel cell system is equal to or longer than a time threshold which requires the replacement of the purifier  17  or the filter  51 . 
     Operations 
     The following will explain a series of operations including the water removing operation performed when the first maintenance part and/or the second maintenance part provided on the water circulation passage that is one feature of the fuel cell system of the present embodiment are replaced or when the controller  30  has detected the abnormalities of the first to fourth abnormality detectors.  FIG. 14  is a flow chart showing a series of steps including the water removing operation of the fuel cell system  100  shown in  FIG. 13 . 
     Here, when the controller  30  detects the abnormality by any one of the first to fourth abnormality detectors as shown in  FIG. 14  (Step S 601 ), the controller  30  informs the display portion (not shown) of the operating unit  29  by using an error code or the like that the abnormality has been detected and also informs the maintenance company that the abnormality has occurred, and performs the abnormality stop of the electric power generating operation of the fuel cell system  100  (Step S 602 ). After the abnormality stop of the electric power generating operation of the fuel cell system  100 , the maintenance worker specifies the block, from which the water needs to be removed, of the water circulation passage based on an abnormality message displayed on the display portion (not shown). Then, to remove the water from the specified block, the maintenance worker suitably, manually opens and closes the first to fifth water discharge valves, the gas venting units, and the like. In addition, the maintenance worker operates the operating unit  29  to select the water removal mode corresponding to the block, from which the water needs to be removed, by using an up-down key and press the set button, thereby setting the water removal mode (Step S 603 ). When a signal indicating the water removal mode selected through the operating unit  29  is output to the controller  30 , the controller  30  controls the first water pump  16 , the second water pump  10 , and the like to execute the water removing operation corresponding to the selected water removal mode (Step S 604 ). 
     Specifically, in a case where the failure (breaking of wire, short, or the like) of the first temperature detector  33  is detected based on the signal transmitted from the first temperature detector  33 , the controller  30  causes the display portion  29   a  of the operating unit  29  to display an error display image corresponding to this failure and informs the maintenance company that the failure of the first temperature detector  33  has occurred. Then, based on this error display image, the maintenance worker of the maintenance company which has received the information of the failure manually opens the first water discharge valve  20  and selects and sets the water removal mode (for example, the water removal mode  1  shown in  FIG. 2 ). Then, the controller  30  keeps the stop states of the first water pump  16  and the second water pump  10 . 
     With this, the cooling water tank  12  serves as the separation mechanism configured to divide the water circulation passage into a first block including the cooling water passage  11  and the cooling water tank  12  and a second block including the first connection passage  21 , the second connection passage  28 , and the first recovered water tank  14 A and to divide the water between these blocks. Only the cooling water in the cooling water passage  11  including the first temperature detector  33  which needs to be replaced is discharged through the first water discharge passage  18  to the outside of the fuel cell system. 
     Next, after the completion of the water removing operation, the maintenance worker replaces the broken device (Step S 605 ). Then, the maintenance worker operates the operating unit  29  to cancel the abnormality stop state (Step S 606 ) and further operates the operating unit  29  to execute the water filling operation of the water passage from which the water has been discharged by the above water removing operation (Step S 607 ). Here, in Step S 604 , the water is removed from only the water passage including the broken device. Therefore, the amount of time for the water filling operation becomes shorter than that in a case where the water is removed from all the blocks of the water circulation passage. Thus, the maintenance is completed more quickly. Next, when the water filling operation is completed, the fuel cell system shifts to a stand-by state for the next start-up. 
     Here, the above “abnormality stop state” is a state in which the controller  30  does not allow the start-up of the fuel cell system  100  even when a start-up request is made, and the above “stand-by state” is a state in which the controller  30  starts the start-up process of the fuel cell system  100  when the start-up request is made. The above expression “the start-up request is made” denotes a case where an operation start command is input by operating the operating unit  29 , a case where a time set as a start-up time of the fuel cell system  100  has come, a case where an electric power demand of an electric power load is equal to or more than a predetermined electric power threshold which requires the electric power generating operation of the fuel cell system  100 , or the like. 
     Next, the following will explain the water removing operation performed when the purifier  17  that is the second maintenance part is replaced due to its operating life, not the water removing operation performed at the time of the replacement of the broken device. Other than the device to be replaced, the water removal mode to be selected, and the water removing operation corresponding to the selected water removal mode, the operations herein are the same as the above series of operations including the water removing operation performed at the time of replacement of the broken first temperature detector  33 , so that explanations thereof are omitted. 
     As explained above, when the accumulated value of the time of the electric power generating operation of the fuel cell system  100  is equal to or longer than a predetermined time threshold which requires the replacement of the purifier  17 , the controller  30  causes the display portion of the operating unit  29  to show that the purifier  17  needs to be replaced and informs the maintenance company that the purifier  17  needs to be replaced. The controller performs the abnormality stop of the fuel cell system  100 . After the abnormality stop of the fuel cell system  100 , the maintenance worker manually opens the fifth water discharge valve  23  and selects and sets the water removal mode (for example, the water removal mode  2 ) corresponding to the replacement of the purifier  17 . 
     Then, the controller  30  keeps the stop state of the first water pump  16 . In the water removing process, the first connection passage  21  and the first branched passage  24  that are rising slopes serve as the separation mechanism configured to divide the second block of the water circulation passage into a block including the first connection passage  21  and a block including passages other than the first connection passage  21  and to divide the water between these blocks. Thus, only the water in the block (the first connection passage  21 ) including the purifier  17  which needs to be replaced is discharged to the outside of the fuel cell system  100 . Therefore, the amount of time for the water filling operation performed after the replacement of the purifier  17  becomes shorter than that in a case where the water in the water tanks (the cooling water tank  12 , the first recovered water tank  14 A, and the second recovered water tank  14 B) communicating with the first connection passage  21  is simultaneously discharged. Thus, the maintenance is completed more quickly. 
     Next, in a case where the water circulation passage is constituted by the exhaust heat recovered water passage  8  and the hot water tank  9 , and the failure (breaking of wire, short, or the like) of the second temperature detector  35  provided on the exhaust heat recovered water passage  8  is detected as the abnormality of the water circulation passage (the abnormality of the third abnormality detector is detected), the maintenance worker manually closes the first on-off valve  52  and the second on-off valve  53 , manually opens the third water discharge valve  27 , and opens the gas venting unit  37 . Then, the maintenance worker selects and sets the water removal mode (for example, the water removal mode  3 ) of the exhaust heat recovered water passage  8  through the operating unit  29 . Then, the controller  30  keeps the stop state of the third water pump  7 . With this, the first on-off valve  52  and the second on-off valve  53  serve as the separation mechanism configured to divide the water circulation passage into the block including the hot water tank  9  (to be precise, the block including the hot water tank  9 , a portion, located upstream of the first on-off valve  52 , of the exhaust heat recovered water passage  8 , and a portion, located downstream of the second on-off valve  53 , of the exhaust heat recovered water passage  8 ) and the block including the exhaust heat recovered water passage  8  (to be precise, the block including a portion of the exhaust heat recovered water passage  8 , the portion being located downstream of the first on-off valve  52  and upstream of the second on-off valve  53 ) and to divide the water between these blocks. Thus, only the water in the exhaust heat recovered water passage  8  including the second temperature detector  35  which needs to be replaced is discharged to the outside of the fuel cell system  100 . Therefore, the amount of time for the water filling operation performed after the replacement of the second temperature detector  35  becomes much shorter than that in a case where the water in the water tank (hot water tank  9 ) communicating with the exhaust heat recovered water passage  8  is simultaneously discharged. Thus, the maintenance is completed more quickly. 
     At the time of the replacement of the device provided on the water circulation passage, the fuel cell system  100  of Embodiment 3 selects the corresponding water removal mode to execute the water removing operation. In addition, the fuel cell system  100  of Embodiment 3 includes the water removal mode in which the water in all the blocks of the water circulation passage is discharged in advance to the outside of the fuel cell system  100  in a case where the user is aware of the possibility of freezing of the water in the water circulation passage in advance based on information, such as weather forecast, or in a case where there is a possibility of the rottenness or freezing of the water in the water circulation passage since the user is away for a long period of time and the fuel cell system  100  is not used for a long period of time. 
     Specifically, in a case where there is the above possibility, the user manually opens the first to fifth water discharge valves and the gas venting units  37  and  38  and operates the operating unit  29  to select and set the all water removal mode. With this, the water is removed from all the blocks of the water circulation passage. Thus, the freezing and rottenness of the water in the water passage can be avoided in advance. 
     MODIFICATION EXAMPLE 1 
     The fuel cell system  100  of Embodiment 3 is configured such that when replacing the replaceable device (the filter  51 , the purifier  17 , the temperature detector  33  or  35 , or the heater  36 ) provided on the water circulation passage, the maintenance worker operates the operating unit  29  to select the water removal mode corresponding to the device to be replaced. However, the fuel cell system  100  of Modification Example 1 is configured such that without the selection of the water removal mode by the maintenance worker through the operating unit  29 , the controller  30  automatically executes the water removing operation corresponding to the device to be replaced. Specifically, a program which executes the water removing operation corresponding to the device whose failure has been detected or the device whose time for replacement has come is stored in a storage device, not shown. When the failure of the device is detected or when the time for replacement has come, the water removing operation corresponding to the device to be replaced is executed in accordance with this program. Specific steps of the water removing operation executed in accordance with the device to be replaced are the same as those in Embodiment 3, so that explanations thereof are omitted. With this, in accordance with the fuel cell system  100  of Modification Example 1, some water has already been removed before the arrival of the maintenance worker, so that the standby time of the maintenance worker until the completion of the water removing operation is reduced. Thus, the maintenance is completed more quickly. 
     MODIFICATION EXAMPLE 2 
     In accordance with the fuel cell system  100  of each of Embodiment 3 and Modification Example 1, when the purifier  17  needs to be replaced, the fifth water discharge valve  23  is opened, and only the water in the water passage including the purifier  17  is discharged. However, the fuel cell system of Modification Example 2 is configured such that the water in the cooling water tank  12  is also discharged to the outside of the fuel cell system  100 . To be specific, in Modification Example 2, the fifth water discharge valve  23  is opened, and in addition, the first water discharge valve  20  is also opened. In Modification Example 2, the first water discharge valve  20  is opened to discharge the water in the cooling water tank  12 . However, Modification Example 2 is not limited to the present example. Modification Example 2 may be configured such that a water discharge passage (not shown) and a water discharge valve (not shown), by which only the water in the cooling water tank  12  can be discharged, are provided, and the water in the cooling water tank  12  is removed by opening this water discharge valve. 
     The reason why the water in not only the first connection passage  21  but also the cooling water tank  12  is removed when replacing the purifier  17  is because when the water filling operation of the first connection passage  21  is executed after the replacement of the purifier  17  by operating the first water pump  16  to supply the water in the first recovered water tank  14 A to the first connection passage  21 , whether or not the water filling operation of the first connection passage  21  is properly executed can be determined by the water level detected by the water level detector  13 . To be specific, when the higher water level than the water level at the time of the completion of the water removing operation is detected by the water level detector  13  at the time of the water filling operation of the first connection passage  21 , it is possible to determine that the water filling of the first connection passage  21  has been properly executed. 
     MODIFICATION EXAMPLE 3 
     The fuel cell system of Modification Example 3 will be explained in detail. 
     Configuration 
       FIG. 15  is a block diagram schematically showing a schematic configuration of the fuel cell system of Modification Example 3. As shown in  FIG. 15 , the fuel cell system  100  of Modification Example 3 includes: the hydrogen generator  101  configured to generate hydrogen by a reforming reaction using a raw material and steam, instead of the fuel gas supply unit  3  of the fuel cell system  100  of Embodiment 3; the reforming water passage  50  through which the water (reforming water) used as the above steam is supplied from the cooling water tank  12  to the hydrogen generator  101 ; the branched passage  48  branching from the reforming water passage  50  and connected to the first recovered water tank  14 A; the fourth water pump  45  provided on the reforming water passage  50 ; the third on-off valve  46  and the fourth on-off valve  47  which are provided to switch a destination to which the water from the cooling water tank  12  flows, between the hydrogen generator  101  and the first recovered water tank  14 A; and a fifth on-off valve  43  provided on a communication passage  49  extending between the first recovered water tank  14 A and the second recovered water tank  14 B. As shown in  FIG. 15 , the third on-off valve  46  is provided on the reforming water passage  50  to be located downstream of a branch point where the branched passage  48  branches from the reforming water passage  50 , and the fourth on-off valve  47  is provided on the branched passage  48 . The first branched passage  24  meets a portion of the second water discharge passage  22 , the portion being located upstream of the second water discharge valve  26 . Instead of the third on-off valve  46  and the fourth on-off valve  47 , a three-way valve may be provided on the branch point where the branched passage  48  branches from the reforming water passage  50 . 
     The other configurations are the same as those of the fuel cell system of Embodiment 3, so that explanations thereof are omitted. 
     Operations 
     Next, a series of operations including the water removing operation performed when replacing the purifier  17  in the fuel cell system of Modification Example 3 will be explained.  FIG. 16  is a flow chart showing a series of steps including the water removing operation of the fuel cell system  100  shown in  FIG. 15 . 
     When the controller  30  determines that the operating life of the purifier  17  is close to the end, as with Embodiment 3, the controller  30  causes the display portion of the operating unit  29  to show that the purifier  17  needs to be replaced (Step S 701 ), informs the maintenance company that the failure has occurred, and performs the abnormality stop of the fuel cell system  100 . Then, the maintenance worker manually opens the second water discharge valve  26  (Step S 702 ). When the maintenance worker operates the operating unit  29  to select and set the water removal mode (for example, the water removal mode  2 ) corresponding to the replacement of the purifier  17  (Step S 703 ), the controller  30  closes the third on-off valve  46  and the fifth on-off valve  43 , opens the fourth on-off valve  47 , and activates the fourth water pump  45  (Step S 704 ). Then, the recovered water in the first recovered water tank  14 A is discharged through the second water discharge passage  22  to the outside of the fuel cell system  100 , and the cooling water in the cooling water tank  12  is discharged through the reforming water passage  50 , the branched passage  48 , the first recovered water tank  14 A, and the second water discharge passage  22  to the outside of the fuel cell system  100 . When it is determined based on the signals of the water level detectors  13  and  15 A that the water removal of the first recovered water tank  14 A and the cooling water tank  12  is completed, the controller  30  closes the fourth on-off valve  47  and stops the operation of the fourth water pump  45  to complete the water removing operation of the first recovered water tank  14 A and the cooling water tank  12  (Step S 705 ). Next, the controller  30  opens the fifth water discharge valve  23  to remove the water in the water passage (first connection passage  21 ) including the purifier  17  (Step S 706 ). When the water removal of the first connection passage  21  is completed, the controller  30  closes the fifth water discharge valve  23  (Step S 707 ), and the maintenance worker replaces the purifier  17  (Step S 708 ). When the replacement of the purifier  17  is completed, the maintenance worker manually closes the second water discharge valve  26  (Step S 709 ) and operates the operating unit  29  to cancel the abnormality stop state (Step S 710 ). Then, the maintenance worker operates the operating unit  29  to execute the water filling operation (Step S 711 ). When the water filling operation is completed, the fuel cell system shifts to the stand-by state. 
     The reason why the water in the cooling water tank  12  is discharged in the above series of operations is because as with the fuel cell system of Modification Example 2, the completion of the water filling operation of the first connection passage  21  is confirmed by the water level detected by the water level detector  13 . 
     MODIFICATION EXAMPLE 4 
     The fuel cell system  100  of each of Embodiment 3 and Modification Examples 1 to 3 is configured such that the maintenance worker manually opens and closes the first to fifth water discharge valves and the gas venting units  37  and  38 . However, the fuel cell system  100  of Modification Example 4 is configured such that each of the first to fifth water discharge valves and the gas venting units  37  and  38  is configured to be openable and closeable by the controller  30 . Therefore, when the failure of the device has occurred or when the time for replacement of the purifier  17  has come, the controller  30  opens the appropriate water discharge valve(s) and/or gas venting unit(s) among the first to fifth water discharge valves and the gas venting units  37  and  38  in accordance with the water removal mode selected and set by the maintenance worker such that only the water of the block of the water circulation passage from which the water needs to be removed is discharged to the outside of the fuel cell system  100 . The water discharge valve(s) and/or the gas venting unit(s) which are opened by the controller  30  for the replacement of the device are the same as those in the fuel cell system  100  of each of Embodiment 3 and Modification Examples 1 to 3, so that explanations thereof are omitted. 
     In a case where the user is aware of the possibility of freezing of the water in the water circulation passage in advance or in a case where the user is away for a long period of time, the user selects and sets the all water removal mode of the operating unit  29 . With this, the controller  30  opens all of the first to fifth water discharge valves and the gas venting units  37  and  38 , so that the water is removed from all the blocks of the entire water circulation passage. Thus, the freezing and rottenness of the water in the water circulation passage can be avoided in advance. 
     Embodiment 4 
     In the fuel cell system according to Embodiment 4 of the present invention, the water circulation passage includes the first water circulation passage, the second water circulation passage, the hot water tank, and the water passage. 
     Configuration of Fuel Cell System 
       FIG. 17  is a block diagram schematically showing a schematic configuration of the fuel cell system according to Embodiment 4 of the present invention. 
     As shown in  FIG. 17 , the fuel cell system  100  according to Embodiment 4 of the present invention is the same in basic configuration as the fuel cell system  100  according to Embodiment 1 of the present invention but is different from the fuel cell system  100  according to Embodiment 1 of the present invention in that: the fuel cell system  100  according to Embodiment 4 includes the first on-off valve  52 , the second on-off valve  53 , the gas venting unit  37 , the gas venting unit  38 , the third water discharge valve  27 , the third water discharge passage  41 , the fourth water discharge valve  39 , and the fourth water discharge passage  42  such that the exhaust heat recovered water passage  8  and the hot water tank  9  constitute a water circulation passage; the second water circulation passage further includes the second recovered water tank  14 B and the communication passage  49 ; and the fuel cell system  100  further includes the first condenser  4  and the second condenser  5 . The first on-off valve  52 , the second on-off valve  53 , the gas venting unit  37 , the gas venting unit  38 , the third water discharge valve  27 , the third water discharge passage  41 , the fourth water discharge valve  39 , and the fourth water discharge passage  42  are the same in configuration as those in the fuel cell system  100  according to Embodiment 2, so that explanations thereof are omitted. In addition, as with Embodiment 2, the fuel cell system  100  according to Embodiment 4 includes the third abnormality detector (for example, the second temperature detector  35  provided on the exhaust heat recovered water passage  8 ) and the fourth abnormality detector (for example, the temperature detector (not shown) provided in the hot water tank  9 ). 
     Specifically, the first condenser  4  is configured to perform heat exchange between the water flowing through the exhaust heat recovered water passage  8  and the off oxidizing gas flowing through the oxidizing gas discharge passage  31  to condense the moisture contained in the off oxidizing gas. The second condenser  5  is configured to perform heat exchange between the water flowing through the exhaust heat recovered water passage  8  and the off fuel gas flowing through the fuel gas discharge passage  32  to condense the moisture contained in the off fuel gas. The water (recovered water) condensed by the first condenser  4  is stored in the first recovered water tank  14 A, and the water (recovered water) condensed by the second condenser  5  is stored in the second recovered water tank  14 B. The second recovered water tank  14 B is provided to be located above the first recovered water tank  14 A. The recovered water stored in the second recovered water tank  14 B is supplied through the communication passage  49  to the first recovered water tank  14 A. 
     The water level detector  15 B configured to detect the water level in the second recovered water tank  14 B is provided at the second recovered water tank  14 B. The water level detector  15 B may be any device as long as it can detect the water level in the second recovered water tank  14 B and output the detected water level to the controller  30 . Examples of the water level detector  15 B are a float type water level detector and a hydraulic type water level detector. 
     The fifth on-off valve  43  configured to allow the water to flow through the communication passage  49  and block the water in the communication passage  49  is provided on the communication passage  49 . The fifth on-off valve  43  may be constituted by an automatic on-off valve whose open and close states are controlled by the controller  30  or may be constituted by a manual on-off valve which is opened and closed by a user or a maintenance worker. In Embodiment 4, a downstream end of the branched passage  48  is connected to a portion of the communication passage  49 , the portion being located upstream of the fifth on-off valve  43  (located on a side where the second recovered water tank  14 B is provided). 
     Operations of Fuel Cell System 
     Next, the water removing operation of the fuel cell system  100  according to Embodiment 4 will be explained. 
     First, the water removing process of the first water circulation passage of the fuel cell system  100  according to Embodiment 4 is the same as that of the fuel cell system  100  according to Embodiment 1, so that an explanation thereof is omitted. 
     The water removing operation of the second water circulation passage of the fuel cell system  100  according to Embodiment 4 is the same as that of the fuel cell system  100  according to Embodiment 1. However, the water removing operation of the second water circulation passage is different between a case where the water in the second recovered water tank  14 B is removed and a case where the water in the second recovered water tank  14 B is not removed. Specifically, in a case where the water in the second recovered water tank  14 B is removed, the controller  30  opens the fifth on-off valve  43  to discharge the water in the second recovered water tank  14 B through the communication passage  49 , the first recovered water tank  14 A, the first connection passage  21 , and the first branched passage  24  to the outside of the fuel cell system  100 . In contrast, in a case where the water in the second recovered water tank  14 B is not removed, the controller  30  keeps the close state of the fifth on-off valve  43 . 
     The water removal of the water passage (exhaust heat recovered water passage  8 ) and the water removal of the hot water tank  9  in the fuel cell system  100  according to Embodiment 4 are the same as those in the fuel cell system  100  according to Embodiment 2, so that explanations thereof are omitted. 
     Water Removing Process of All Blocks of Water Circulation Passage 
       FIG. 18  is a flow chart schematically showing the water removing process of all the blocks of the water circulation passage of the fuel cell system  100  shown in  FIG. 17 . 
     As described above, the fuel cell system  100  according to Embodiment 4 includes a water circulation passage constituted by the first water circulation passage and the second water circulation passage and a water circulation passage constituted by the exhaust heat recovered water passage  8  and the hot water tank  9 . Therefore, in the case of removing the water in all the blocks of the water circulation passage, the water in all the blocks of these two water circulation passages is removed. Hereinafter, the water removing process of all the blocks of the water circulation passage of the fuel cell system  100  according to Embodiment 4 will be explained in reference to  FIG. 18 . 
     As shown in  FIG. 18 , the user manually opens the first water discharge valve  20 , the second water discharge valve  26 , the third water discharge valve  27 , and the fourth water discharge valve  39  (Step S 801 ) and selects the water removal mode of all the blocks of the water circulation passage (Step S 802 ). Specifically, the user operates the input portion  29   b  of the operating unit  29  to select the all water removal mode shown in  FIG. 2  and press the set button. When the all water removal mode is selected, the controller  30  keeps the stop states of the first water pump  16  and the second water pump  10 , keeps the close state of the third on-off valve  46 , opens the fourth on-off valve  47  and the fifth on-off valve  43 , and activates the fourth water pump  45  (Step S 803 ). With this, the water in the first water circulation passage is discharged through the first water discharge passage  18 , and the water in the second water circulation passage is discharged through the first branched passage  24 . Moreover, the water in the exhaust heat recovered water passage  8  is discharged through the third water discharge passage  41 , and the water in the hot water tank  9  is discharged through the fourth water discharge passage  42 . Further, by the operation of the fourth water pump  45 , the water in the branched passage  48  is also removed through the first recovered water tank  14 A and the first branched passage  24 . Thus, the water in all the blocks of the water circulation passage is removed. 
     When the all water removal mode is selected in Step S 802 , the controller  30  executes the water removal abnormality detecting sequence of all the blocks of the water circulation passage (Step S 804 ). Here, the water removal abnormality detecting sequence of all the blocks of the water circulation passage will be explained in reference to  FIG. 19 . 
       FIG. 19  is a flow chart schematically showing the water removal abnormality detecting sequence of all the blocks of the water circulation passage in the water removing process of the fuel cell system  100  shown in  FIG. 18 . 
     As shown in  FIG. 9 , when an elapsed time since the selection of the all water removal mode in Step S 802  becomes equal to or longer than T 4  (Step S 81 ), the controller  30  determines by the water level detector  13  whether or not the water is remaining in the cooling water tank  12 , determines by the water level detector  15 A whether or not the water is remaining in the first recovered water tank  14 A, and determines by the water level detector  15 B whether or not the water is remaining in the second recovered water tank  14 B (Step S 82 ). Here, T 4  is a time equal to or longer than a time necessary to discharge the water remaining in all the blocks of the water circulation passage to the outside of the fuel cell system  100  and is determined in advance by experiments or the like. A method for determining by the water level detector  13  whether or not the water is remaining in the cooling water tank  12  and a method for determining by the water level detector  15 A whether or not the water is remaining in the first recovered water tank  14 A are the same as those of the water removing process of all the blocks in Embodiment 1, so that explanations thereof are omitted. Whether or not the water is remaining in the second recovered water tank  14 B is determined by the water level detector  13 B in the same manner as above. That is, when the water level equal to or higher than the lower detection limit of the water level detector  13 B is detected, it is determined that the water is remaining in the second recovered water tank  14 B. When the water level equal to or higher than the lower detection limit of the water level detector  13 B is not detected, it is determined that the water is not remaining in the second recovered water tank  14 B. 
     Then, when the controller  30  determines by at least one of the water level detector  13 , the water level detector  15 A, and the water level detector  15 B that the water is remaining in the cooling water tank  12 , the first recovered water tank  14 A, or the second recovered water tank  14 B (Yes in Step S 82 ), the controller  30  causes the display portion  29   a  of the operating unit  29  to inform that the abnormality has occurred (Step S 83 ) and terminates the present sequence. In contrast, when the controller  30  determines by the water level detector  13 , the water level detector  15 A, and the water level detector  15 B that the water is not remaining in the cooling water tank  12 , the first recovered water tank  14 A, and the second recovered water tank  14 B, the controller  30  does not cause the display portion  29   a  to inform that the abnormality has occurred (Step S 84 ) and terminates the present sequence. 
     Next, when the controller  30  confirms that there is no abnormality in all the blocks of the water circulation passage in the water removal abnormality detecting sequence (Step S 805 ) and confirms that the water removal is terminated, the controller  30  causes the input portion  29   b  of the operating unit  29  to inform that the water removal mode of all the blocks of the water circulation passage has been terminated (see  FIG. 3 ), and the maintenance worker presses the set button of the input portion  29   b  to set the termination of the water removal mode (Step S 806 ). With this, the controller  30  stops the fourth water pump  45  and closes the fourth on-off valve  47  and the fifth on-off valve  43 . Here, the fourth on-off valve  47  and the fifth on-off valve  43  may not be closed. 
     Next, the maintenance worker manually closes the first water discharge valve  20 , the second water discharge valve  26 , the third water discharge valve  27 , and the fourth water discharge valve  39  (Step S 807 ). 
     The fuel cell system  100  of Embodiment 4 configured as above obtains the same operational advantages as that of Embodiment 1. 
     From the foregoing explanation, many modifications and other embodiments of the present invention are obvious to one skilled in the art. Therefore, the foregoing explanation should be interpreted only as an example and is provided for the purpose of teaching the best mode for carrying out the present invention to one skilled in the art. The structures and/or functional details may be substantially modified within the spirit of the present invention. In addition, various inventions can be made by suitable combinations of a plurality of components disclosed in the above embodiments. 
     INDUSTRIAL APPLICABILITY 
     In accordance with the fuel cell system of the present invention and the method for removing the water of the fuel cell system, the water is removed from the block, from which the water needs to be removed, of the water circulation passage. Therefore, the amount of time for the water filling operation performed after the water removing operation can be reduced. Thus, the maintenance can be completed more quickly. On this account, the fuel cell system of the present invention is useful as, for example, a fuel cell system of home use or professional use. 
     REFERENCE SIGNS LIST 
       1  fuel cell 
       2  oxidizing gas supply unit 
       3  fuel gas supply unit 
       4  first condenser 
       5  second condenser 
       6  heat exchanger 
       7  third water pump 
       8  exhaust heat recovered water passage 
       9  hot water tank 
       10  second water pump 
       11  cooling water passage 
       12  cooling water tank 
       13  water level detector 
       14 A first recovered water tank 
       14 B second recovered water tank 
       15 A water level detector 
       15 B water level detector 
       16  first water pump 
       17  purifier 
       18  first water discharge passage 
       19  exhaust unit 
       20  first water discharge valve 
       21  first connection passage 
       21   a  upstream portion 
       21   b  midstream portion 
       21   c  downstream portion 
       22  second water discharge passage 
       23  fifth water discharge valve 
       24  first branched passage 
       25  second water discharge passage 
       26  second water discharge valve 
       27  third water discharge valve 
       28  second connection passage 
       29  command acquiring unit 
       29   a  display portion 
       29   b  input portion 
       30  controller 
       31  oxidizing gas discharge passage 
       32  fuel gas discharge passage 
       33  first temperature detector 
       35  second temperature detector 
       36  heater 
       37  gas venting unit 
       38  gas venting unit 
       39  fourth water discharge valve 
       41  third water discharge passage 
       42  fourth water discharge passage 
       43  fifth on-off valve 
       45  fourth water pump 
       46  third on-off valve 
       47  fourth on-off valve 
       48  branched passage 
       49  communication passage 
       50  reforming water passage 
       51  filter 
       52  first on-off valve 
       53  second on-off valve 
       70  casing 
       100  fuel cell system 
       101  hydrogen generator