Patent Publication Number: US-8110311-B2

Title: Fuel cell system

Description:
This is a 371 national phase application of PCT/JP2006/323532 filed 20 Nov. 2006, which claims priority of Japanese Patent Application No. 2005-344948 filed 30 Nov. 2005, the contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a fuel cell system. 
     BACKGROUND ART 
     In general, a fuel cell has a poor starting property at a low temperature as compared with another power source. A power generation efficiency of such a fuel cell decreases with lowering of a temperature. When the temperature is low, a desired voltage/current cannot be supplied, and a device sometimes cannot be started. 
     In view of such a situation, a technology is disclosed in which a short circuit is constituted between an input terminal and an output terminal (input/output terminals) of the fuel cell connected to a system load, and during starting at a low temperature, the fuel cell is connected to the short circuit by use of a relay or the like, to supply a short-circuit current to a fuel cell main body, whereby heat is generated by the fuel cell itself to warm up the fuel cell (e.g., see Patent Document 1). 
     [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-93143 
     DISCLOSURE OF THE INVENTION 
     However, in a case where a gas (an oxidizing gas and a fuel gas; hereinafter generically referred to as a “reactive gas”) which contributes to power generation remains in a fuel cell even during starting at a low temperature, a rush current is generated at a time when the fuel cell is connected to a short circuit, and flows through a fuel cell main body, whereby a problem that the fuel cell breaks and the like occur. 
     In view of the above-mentioned situation, the present invention has been developed, and an object thereof is to provide a fuel cell system capable of warming up a fuel cell while inhibiting generation of a rush current. 
     To solve the above-mentioned problem, a fuel cell system according to the present invention is characterized by comprising: a fuel cell; a load connected to the fuel cell; a short circuit provided between an input terminal and an output terminal from the fuel cell to the load; and control means for reducing a reactive gas remaining in the fuel cell, and then connecting the fuel cell to the short circuit. 
     According to such a constitution, the reactive gas remaining in the fuel cell is reduced, and then the fuel cell is connected to the short circuit, so that it is possible to suppress a problem generated in a case where the fuel cell in which the reactive gas remains is connected, that is, a problem that a rush current is generated to break the fuel cell. 
     Here, in the above constitution, a configuration is preferable in which the control means allows the fuel cell to generate a power and consume the reactive gas remaining in the fuel cell, thereby reducing the reactive gas. Moreover, a configuration is preferable which further comprises a sensor to detect an output voltage of the fuel cell, wherein the control means stops the power generation of the fuel cell based on a detection result of the sensor, and then connects the fuel cell to the short circuit. Furthermore, a configuration is further preferable in which the reactive gas includes a fuel gas to be supplied to an anode of the fuel cell and an oxidizing gas to be supplied to a cathode, and the control means reduces at least the oxidizing gas. 
     In addition, in the above constitution, a configuration is preferable which further comprises inactive gas supply means for supplying an inactive gas to the fuel cell, wherein the control means supplies the inactive gas to the fuel cell to reduce the reactive gas remaining in the fuel cell. Moreover, a configuration is preferable which further comprises a sensor to detect an output voltage of the fuel cell, wherein the control means stops the supply of the inactive gas based on a detection result of the sensor, and then connects the fuel cell to the short circuit. 
     Furthermore, in the above constitution, a configuration is preferable in which the reactive gas cathode, thereby reducing the oxidizing gas. Moreover, a configuration is preferable which further comprises a sensor to detect an output voltage of the fuel cell, wherein the control means stops the supply of the fuel gas based on a detection result of the sensor, and then connects the fuel cell to the short circuit. Furthermore, a configuration is preferable which further comprises adjustment means for connecting the fuel cell to the short circuit, and then adjusting the supply of the oxidizing gas in accordance with a targeted short-circuit current. 
     As described above, according to the present invention, it is possible to warm up the fuel cell while inhibiting generation of a rush current. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a constitution of a main part of a fuel cell system according to a first embodiment; 
         FIG. 2A  is a diagram showing a method for connecting a conventional shorting relay; 
         FIG. 2B  is a diagram showing a method for connecting the conventional shorting relay; 
         FIG. 2C  is a diagram showing a method for connecting the conventional shorting relay; 
         FIG. 3A  is a diagram showing a method for connecting a shorting relay according to the present invention; 
         FIG. 3B  is a diagram showing a method for connecting the shorting relay according to the present invention; 
         FIG. 3C  is a diagram showing a method for connecting the shorting relay according to the present invention; 
         FIG. 4  is a flow chart showing preprocessing according to a first embodiment; 
         FIG. 5  is a diagram showing a constitution of a main part of the fuel cell system according to a second embodiment; 
         FIG. 6  is a flow chart showing preprocessing according to a second embodiment; and 
         FIG. 7  is a flow chart showing preprocessing according to a third embodiment. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     An embodiment according to the present invention will hereinafter be described with reference to the drawings. 
     A. First Embodiment 
       FIG. 1  is a diagram showing a constitution of a main part of a fuel cell system  100  according to a first embodiment. In the present embodiment, a fuel cell system to be mounted on a vehicle such as a fuel cell hybrid vehicle (FCHV), an electric car or a hybrid car is assumed, but the system may be applied to not only the vehicle but also any type of mobile body (e.g., a ship, an airplane, a robot or the like) or a stationary power source. 
     A fuel cell  40  is means for generating a power from a supplied reactive gas (a fuel gas and an oxidizing gas), and a fuel cell of any type such as a solid polymer type, a phosphate type or a dissolving carbonate type may be used. The fuel cell  40  has a stack structure in which a plurality of unitary cells including an MEA and the like are laminated in series, and the fuel cell  40  is provided with a cell voltage monitor (a sensor)  50  for detecting each cell voltage. 
     A fuel gas such as a hydrogen gas is supplied from a fuel gas supply source  10  to a fuel pole (an anode) of the fuel cell  40 , whereas an oxidizing gas such as air is supplied from an oxidizing gas supply source  70  to an oxygen pole (a cathode). 
     The fuel gas supply source  10  is constituted of, for example, a hydrogen tank, various valves and the like, and a valve open degree, an ON/OFF time and the like are adjusted to control an amount of the fuel gas to be supplied to the fuel cell  40 . 
     The oxidizing gas supply source  70  is constituted of, for example, an air compressor, a motor for driving the air compressor, an inverter and the like, and a rotation number of the motor and the like are adjusted to adjust an amount of the oxidizing gas to be supplied to the fuel cell  40 . 
     A system load  60  includes a vehicle auxiliary device and an FC auxiliary device driven with a power supplied from the fuel cell or any type of secondary cell (e.g., a nickel hydrogen battery or the like; not shown). The system load  60  is connected between an input terminal  41  and an output terminal  42  of the fuel cell  40  via FC relays  61 . The FC relays  61  are controlled by a control device  90  to switch connection/disconnection between the fuel cell  40  and the system load  60 . It is to be noted that the vehicle auxiliary device is any type of power device (an illuminative device, an air conditioning device, a hydraulic pump or the like) for use in running the vehicle or the like, and the FC auxiliary device is any type of power device (a pump for supplying the fuel gas or the oxidizing gas or the like) for use in operating the fuel cell  40 . 
     A short circuit  80  is a circuit for supplying a short-circuit current to the fuel cell  40 , and is provided between the input terminal  41  and the output terminal  42  (between the input terminal and the output terminal) of the fuel cell  40 . The short circuit  80  is constituted by connecting a shorting relay  81 , a fuse  82  and a current sensor  83  in series. The shorting relay  81  is controlled by the control device  90  to switch the connection/disconnection between the fuel cell  40  and the short circuit  80 . The fuse  82  realizes fail safe during failure of the shorting relay  81 . When the current (the short-circuit current) excessively flows through the short circuit  80  owing to generation of the rush current or the like, the fuse itself fuses to block the current, thereby protecting the fuel cell  40 . The current sensor  83  detects a current flowing through the short circuit  80 , and outputs a detection result to the control device  90 . 
     The control device (control means)  90  is constituted of a CPU, an ROM, an RAM and the like, and centrally controls units of the system based on each input sensor signal. Moreover, for example, in a case where a starting instruction of the system is input and a temperature measured with a temperature sensor  555  is lower than a reference temperature, the control device  90  performs preprocessing as described later, and then connects the shorting relay  81  to supply the short-circuit current to the fuel cell  40 , whereby control is performed so as to raise the temperature in the fuel cell  40 . 
       FIGS. 2A to 2C  are diagrams showing a method for connecting a conventional shorting relay, and  FIGS. 3A to 3C  are diagrams showing a method for connecting a shorting relay according to the present invention.  FIGS. 2A and 3A  show an amount of a remaining oxidizing gas,  FIGS. 2B and 3B  show a switch timing of the connection/disconnection of the shorting relay, and  FIGS. 2C and 3C  are diagrams showing a current (an FC current) flowing through the fuel cell  40 . 
     As shown in  FIGS. 2A to 2C , in a case where the shorting relay is switched from disconnection to connection (from OFF to ON) in a state in which the oxidizing gas remains in the cathode of the fuel cell  40 , a rush current Cr is generated during relay connection, and the current flows through the fuel cell  40  to cause breakage or the like. 
     To solve the problem, in the present invention, as shown in  FIGS. 3A to 3C , auxiliary devices are driven before switching the shorting relay from the disconnection to the connection, whereby the oxidizing gas remaining in the cathode of the fuel cell  40  is consumed to form an oxidizing gas deficient state (hereinafter referred to as the preprocessing). Thus, the oxidizing gas deficient state is formed, and then the shorting relay is connected to supply the short-circuit current through the fuel cell  40 , whereby the fuel cell can be warmed up while inhibiting the generation of the rush current. 
     The preprocessing according to the present invention will hereinafter be described in detail with reference to the drawings. 
       FIG. 4  is a flow chart showing the preprocessing. 
     On receiving a starting instruction of the system from an operation switch (not shown), the control device  90  judges whether or not a temperature (hereinafter referred to as an FC temperature) of the fuel cell  40  measured with the temperature sensor  55  is below a set reference temperature (step S 1 →step S 2 ). When the FC temperature exceeds the reference temperature (step S 2 ; NO), the control device  90  ends processing without executing steps. On the other hand, when the FC temperature is below the reference temperature (step S 2 ; YES), the control device  90  allows the fuel cell  40  to generate the power, and starts driving the auxiliary devices so as to consume (reduce) the oxidizing gas remaining in the cathode of the fuel cell  40 . Specifically, the supply of the oxidizing gas to the cathode of the fuel cell  40  is stopped, whereas a sufficient amount of the fuel gas is supplied to the anode of the fuel cell  40 , whereby the fuel cell  40  generates the power, and the driving of the auxiliary devices is started. Needless to say, instead of driving the auxiliary devices, the generated power may be accumulated in a secondary cell such as a battery or a capacitor (either is not shown). 
     The control device  90  advances to step S 4  to judge whether or not the oxidizing gas deficient state has been formed. Specifically, it is detected whether or not each cell voltage detected by the cell voltage monitor (the sensor)  50  lowers to a predetermined value (e.g., around 0 V). In a case where each cell voltage lowers to the predetermined value, it is judged that the oxidizing gas deficient state has been formed. On the other hand, when each cell voltage does not lower to the predetermined value, it is judged that the oxidizing gas deficient state is not formed. 
     In a case where it is judged that the oxidizing gas deficient state is not formed (step S 4 ; NO), the control device  90  returns to the step S 3  to continue driving the auxiliary devices. On the other hand, in a case where it is judged that the oxidizing gas deficient state has been formed (step S 4 ; YES), the control device  90  stops the driving of the auxiliary devices, and switches the FC relays  61  from ON to OFF, whereby the fuel cell  40  is electrically disconnected from the system load  60  (step S 5 →step S 6 ). Furthermore, the control device  90  switches the shorting relay  81  from OFF to ON to electrically connect the fuel cell  40  to the short circuit  80  (step S 7 ), and prepares for the supply of the short-circuit current, thereby ending the processing. 
     When such preprocessing is performed, the control device  90  supplies the fuel gas and the oxidizing gas, and allows the fuel cell  40  to start the power generation. As a result, the short-circuit current flows through the fuel cell  40  which generates heat from itself, and the fuel cell  40  is warmed up. It is to be noted that a current value of the short-circuit current, a time to supply the short-circuit current and the like may appropriately be set in accordance with a warm-up temperature of the fuel cell  40 . Moreover, the current value of the short-circuit current may be controlled so that the current value detected by the current sensor  83  is a predetermined value (α targeted current value or the like; hereinafter referred to as a target short-circuit current value). Specifically, the control device (adjustment means)  90  obtains a required amount of the oxidizing gas from the current value detected by the current sensor  83  and the target short-circuit current value, and may control driving of an air compressor or the like so that the obtained amount of the oxidizing gas is supplied to the fuel cell  40 . 
     Furthermore, the short circuit  80  is provided with the fuse  82 , so that even when abnormality occurs in the current sensor  83  or the like and an excessively large current flows through the short circuit  80 , it can be prevented that the current flows through the fuel cell  40 . 
     As described above, according to the present embodiment, the oxidizing gas remaining in the cathode of the fuel cell is consumed to form the oxidizing gas deficient state, and then the shorting relay is connected to supply the short-circuit current to the fuel cell, whereby the fuel cell can be warmed up while inhibiting the generation of the rush current. 
     In the above example, a case where the oxidizing gas deficient state is formed and then the short-circuit current is supplied to the fuel cell has been described, but instead (or in addition), the oxidizing gas deficient state is formed, and then the short-circuit current may be supplied to the fuel cell. However, when the fuel cell  40  generates the power in a fuel gas deficient state, the MEA and the like of the fuel cell  40  are damaged. On the other hand, it is experimentally confirmed that when the fuel cell  40  generates the power in the oxidizing gas deficient state, the MEA and the like are not damaged. Therefore, it is preferable that the oxidizing gas deficient state is formed and then the short-circuit current is supplied to the fuel cell. 
     B. Second Embodiment 
     In the above first embodiment, a case where the oxidizing gas remaining in the cathode is consumed to form the oxidizing gas deficient state has been described. In a second embodiment, a case where an inactive gas is supplied and an oxidizing gas remaining in a cathode is pushed out to form an oxidizing gas deficient state will be described. 
       FIG. 5  is a diagram showing a constitution of a fuel cell system  100 ′ according to the second embodiment. 
     In the fuel cell system  100 ′, the fuel cell system  100  shown in  FIG. 1  is provided with an inactive gas supply source  110  and a three-way valve  120 . Therefore, a part corresponding to  FIG. 1  is denoted with the same reference numerals, and detailed description thereof is omitted. 
     The inactive gas supply source (inactive gas supply means)  110  is means for supplying an inactive gas such as an N 2  gas, and is constituted of a high-pressure tank filled with the inactive gas, a pump and the like. 
     The three-way valve  120  is provided in an oxidizing gas supply path, and is controlled by a control device  90  to switch a gas to be supplied to a fuel cell  40 . Specifically, the three-way valve  120  is controlled to switch supply of the oxidizing gas or the inactive gas to the anode of the fuel cell  40 . 
       FIG. 6  is a flow chart showing preprocessing according to the second embodiment. It is to be noted that in a flow shown in  FIG. 6 , instead of the steps S 3 , S 5  shown in  FIG. 4 , steps S 13 , S 15  are provided. Therefore, the other steps are denoted with the same reference numerals, and detailed description thereof is omitted. 
     In a case where the control device  90  judges that an FC temperature is below a reference temperature (step S 2 ; YES), the three-way valve  120  is switched to start supply of the inactive gas to the cathode of the fuel cell  40  so as to push out (reduce) the oxidizing gas remaining in the cathode of the fuel cell  40  (step S 13 ). 
     When the control device  90  advances to step S 4 , it is judged whether or not an oxidizing gas deficient state has been formed by pushing out the oxidizing gas remaining in the cathode of the fuel cell  40  with the supplied inactive gas. Specifically, it is judged whether or not each cell voltage detected by a cell voltage monitor  50  lowers to a predetermined value (e.g., around 0 V). When each cell voltage lowers to the predetermined value, it is judged that the oxidizing gas deficient state has been formed. On the other hand, when each cell voltage does not appropriately lower, it is judged that the oxidizing gas deficient state is not formed. 
     In a case where the control device  90  judges that the oxidizing gas deficient state is not formed (step S 4 ; NO), the device returns to the step S 13  to continue the supply of the inactive gas. On the other hand, in a case where it is judged that the oxidizing gas deficient state has been formed (step S 4 ; YES), the control device  90  stops the supply of the inactive gas, and switches FC relays  61  from ON to OFF to electrically disconnect the fuel cell  40  from a system load  60  (step S 15 →step S 6 ). It is to be noted that the subsequent operation is similar to that of the first embodiment, and hence further description is omitted. 
     Thus, the inactive gas may be supplied to the cathode of the fuel cell to reduce the oxidizing gas and form the oxidizing gas deficient state. 
     It is to be noted that in the above example, the inactive gas is supplied to the cathode of the fuel cell to form the oxidizing gas deficient state, but instead (or in addition), the inactive gas may be supplied to the anode of the fuel cell to form a fuel gas deficient state. 
     Thus, in a case where the fuel gas remaining in the anode is reduced without generating any power from the fuel cell, a problem that an MEA and the like of the fuel cell are damaged does not occur (see the first embodiment), and hence the inactive gas may be supplied to either the anode or the cathode. 
     C. Third Embodiment 
     In the above first embodiment, the oxidizing gas remaining in the cathode is consumed to form the oxidizing gas deficient state, but in a third embodiment, the oxidizing gas deficient state is formed by cross leak of a fuel gas from an anode to the cathode. 
       FIG. 7  is a flow chart showing preprocessing according to the third embodiment. It is to be noted that in a flow shown in  FIG. 7 , instead of the steps S 3 , S 5  shown in  FIG. 4 , steps S 23 , S 25  are provided. Therefore, the other steps are denoted with the same reference numerals, and detailed description thereof is omitted. 
     In a case where a control device  90  judges that an FC temperature is below a reference temperature (step S 2 ; YES), supply of the oxidizing gas to the cathode is stopped, whereas a sufficient amount of the fuel gas is supplied to the anode of a fuel cell  40 . As well known, a molecular diameter of the fuel gas (e.g., a hydrogen gas) to be supplied to the anode is smaller than that of the oxidizing gas to be supplied to the cathode, so that the cross leak of the fuel gas from the anode to the cathode is started (step S 23 ). 
     When the control device  90  advances to step S 4 , it is judged whether or not an oxidizing gas deficient state has been formed by pushing out the oxidizing gas remaining in the cathode of the fuel cell  40  with the fuel gas generated by the cross leak. Specifically, it is judged whether or not each cell voltage detected by a cell voltage monitor  50  lowers to a predetermined value (e.g., around 0 V). When each cell voltage lowers to the predetermined value, it is judged that the oxidizing gas deficient state has been formed. On the other hand, when each cell voltage does not appropriately lower, it is judged that the oxidizing gas deficient state is not formed. 
     In a case where the control device  90  judges that the oxidizing gas deficient state is not formed (step S 4 ; NO), the device returns to the step S 23  to continue the cross leak of the fuel gas (e.g., for 30 seconds). On the other hand, in a case where it is judged that the oxidizing gas deficient state has been formed (step S 4 ; YES), the control device  90  stops the supply of the fuel gas to the anode, and switches FC relays  61  from ON to OFF to electrically disconnect the fuel cell  40  from a system load  60  (step S 25 →step S 6 ). It is to be noted that the subsequent operation is similar to that of the first embodiment, and hence further description is omitted. 
     Thus, the cross leak of the fuel gas from the anode to the cathode may be generated to push out the oxidizing gas remaining in the cathode with the fuel gas generated by the cross leak, whereby the oxidizing gas is reduced to form the oxidizing gas deficient state. 
     D. Modification 
     In the above embodiments, warm-up during starting at a low temperature has been assumed, but the present invention is applicable in any case where the warm-up is required, for example, a case where rapid warm-up is performed before stopping a system operation. 
     Moreover, in the above embodiments, as means for switching connection/disconnection between a fuel cell  40  and a short circuit  80 , a shorting relay  81  has been illustrated, but a semiconductor switch constituted of IGBT, FET and the like may be used. It is to be noted that the short circuit  80  may be provided with an LCR load for limiting a current during short-circuit. The short circuit  80  may not be provided with a fuse  82  or a current sensor  83 .