Abstract:
A fuel cell system enables time required for purging to be reduced without a major increase in discharge gas concentration at a time of purging. It comprises a fuel cell; a fuel gas supply path for supplying the fuel gas to an anode; an oxidizing gas supply path for supplying an oxidizing gas to a cathode; a fuel gas circulating path for returning an unreacted fuel gas to an anode inlet side; a dilution box for diluting the fuel gas by the oxidizing gas and for discharging it to outside; and a fuel gas discharge path connecting the fuel gas circulating path and a dilution box discharge gas inlet. A drain valve, a purge valve and an air discharge valve are provided, opening areas of which are different from one another. The drain valve with a smallest opening area is initially opened.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fuel cell system which is used in fuel cell-powered vehicles or the like, and more specifically to a fuel cell system which is provided with a means for purging an anode electrode side which is supplied with a fuel gas. Further, the present invention relates to a method of discharging a reaction gas from the fuel cell system. 
     Priority is claimed on Japanese Patent Application No. 2004-325120, filed Nov. 9, 2004, the content of which is incorporated herein by reference. 
     2. Description of the Related Art 
     In such a fuel cell mounted in a fuel cell-powered vehicle or the like, a fuel gas and a oxidizing agent gas are chemically reacted, and at the time of this reaction, energy is obtained and then directly channeled out to external circuits as electrical power. This type of fuel cell has a typical structure which includes an anode electrode and a cathode electrode, which together sandwich a solid polymer electrolyte membrane therebetween, in which a hydrogen gas as a fuel gas is supplied to the anode electrode side on one hand, and on the other hand, air containing oxygen as an oxidizing agent gas is supplied to the cathode electrode side. 
     Further, there is one type of fuel cell system, which uses such a fuel cell, and in which an unreacted fuel gas, which has passed through the anode electrode, is returned to an inlet side of the anode electrode to thereby promote the efficient use of the fuel gas. This fuel cell system is provided with a fuel gas circulating path, which connects an inlet side and an outlet side of the anode electrode and through which a fuel gas containing a reacted gas is repeatedly circulated, so that water (or moisture) and other impurities are gradually and increasingly deposited in a path (including a circulating path portion) at the outlet side of the anode electrode. Consequently, a mechanism for appropriately purging an internal gas is provided on the path at the outlet side of the anode electrode. 
     In a fuel cell system of this type, when operation of a fuel cell is stopped, it is necessary to reliably discharge water and the like, which remain in an outlet side of the anode electrode, to outside in order to avoid freezing and clogging in a path at a time of resumption of operation. As a countermeasure, a new type of fuel cell system has been developed, wherein a dilution gas that is different from a fuel gas is introduced in the anode electrode side and wherein water and other impurities in a path are purged by this dilution gas (see, for example, Japanese Unexamined Patent Application, First Publication No. 2003-331893). 
     This fuel cell system is structured such that a feed path for an oxidizing agent gas is connected to the anode electrode side and that a gas feed valve of open/close type is provided on the feed path and is opened at a time of purging of the anode electrode side. It is also structured such that a plurality of discharge ports are provided at an outlet side of the anode electrode and that these ports are opened at a time of purging to thereby discharge residual gas of the anode electrode side together with a dilution gas (oxidizing agent gas) to outside. 
     However, in this conventional fuel cell system, since all of the discharge ports are suddenly opened at a time of purging of the anode electrode side, a fuel gas emission concentration of a discharged gas is temporarily and totally raised. 
     Therefore, in the above-described conventional fuel cell system, provision is made in which power generation is continued within a predetermined period of time while continuing stoppage of the supply of a fuel gas and in which purging is carried out after a fuel gas in a fuel gas circulating path has sufficiently been consumed. As a result, in this conventional fuel cell system, time required for the completion of purging is inevitably elongated. Therefore, at the present time, improvements with respect thereto are earnestly desired. 
     SUMMARY OF THE INVENTION 
     In consideration of the above circumstances, according to the present invention, a fuel cell system is provided, wherein the time required for the completion of purging can be reduced without any extensive increase in a fuel gas emission concentration at a time of purging in an anode electrode side. 
     According to a first aspect of the present invention, a fuel cell system is provided including: a fuel cell which includes an anode electrode and a cathode electrode, and which is adapted to perform power generation by a chemical reaction of a fuel gas which is supplied to the anode electrode and an oxidizing agent gas which is supplied to the cathode electrode; a fuel gas supply path for supplying the fuel gas to the anode electrode; an oxidizing agent gas supply path for supplying the oxidizing agent gas to the cathode electrode; a fuel gas circulating path for returning an unreacted fuel gas, which has passed through the anode electrode, to an inlet side of the anode electrode; a discharge port element for discharging the gas from an outlet side of the anode electrode to outside; fuel gas dilution means for diluting the fuel gas, which has passed through the discharge port element, by the oxidizing agent gas and for discharging it to outside; and anode purging means for introducing a dilution gas, which is different from the fuel gas, to the anode electrode side and for purging residual gases, which remain in the anode electrode and the fuel gas circulating path, through the discharge port element; wherein when purging by the anode purging means, the flow volume of a gas flowing through the discharge port element is controlled so as to be gradually increased. 
     Preferably, the discharge port element comprises a plurality of discharge ports, and wherein when purging by the anode purging means, the discharge ports are separately opened with time differences thereamong in such a manner that a total of opening areas of the discharge ports is stepwisely increased. Further, preferably, the opening areas of the discharge ports are different from one another, and wherein when purging by the anode purging means, of all the plurality of discharge ports one discharge port whose opening area is the smallest is opened prior to openings of the remaining discharge ports. Still further, preferably, the system further comprises a flow control valve which controls the flow volume of a gas flowing through the discharge port element, and wherein when purging by the anode purging means, the flow control valve is controlled in such a manner that the flow volume of a gas flowing through the discharge port element is gradually increased. Yet further, preferably, the anode purging means comprises a feed path and a gas feed valve, the feed path is provided in a branching manner on the oxidizing agent gas supply path for introducing the oxidizing agent gas to the anode electrode side, and the gas feed valve is provided on the feed path and capable of an opening and closing motion. 
     According to a second aspect of the present invention, a method for discharging a reaction gas from a fuel cell system is provided, in which the system includes an anode electrode and a cathode electrode and is adapted to perform power generation by a chemical reaction of a fuel gas which is supplied to the anode electrode and an oxidizing agent gas which is supplied to the cathode electrode, said method comprising: providing a discharge port element for discharging the gas from an outlet side of the anode electrode to outside; introducing a dilution gas, which is different from the fuel gas, to the anode electrode side; and carrying out purging by controlling the discharge port element such that the flow volume of a gas flowing through the discharge port element is gradually increased. 
     Preferably, the method further comprises: providing a plurality of discharge ports as the discharge port element; and carrying out purging by controlling the discharge ports such that the discharge ports are separately opened with time differences thereamong in such a manner that a total of opening areas of the discharge ports is stepwisely increased. Further, preferably, the method further comprises: making the opening areas of the discharge ports different from one another; and carrying out purging by controlling the discharge ports such that, of all the plurality of discharge ports, one discharge port whose opening area is the smallest is opened prior to openings of the remaining discharge ports. Still further, preferably, the method further comprises: providing a flow control valve which controls the flow volume of a gas flowing through the discharge port element; and carrying out purging by controlling the flow control valve such that the flow volume of a gas flowing through the discharge port element is gradually increased. Yet further, preferably, the method further comprises: providing a feed path in a branching manner on the oxidizing agent gas supply path so as to introduce the oxidizing agent gas to an anode electrode side; and providing on the feed path a gas feed valve, which is capable of an opening and closing motion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a general structural view showing a fuel cell system according to a first embodiment of the present invention. 
         FIG. 2  is a flowchart partly showing the process in the first embodiment. 
         FIG. 3  is a time chart showing a change in a system state including an operating condition of valves used, a power generating condition, and hydrogen gas concentration, of the first embodiment 
         FIG. 4  is a general structural view showing a fuel cell system according to a second embodiment of the present invention. 
         FIG. 5  is a flowchart partly showing the process in the second embodiment. 
         FIG. 6  is a time chart showing a change in a system state including an operating condition of valves used, a power generating condition, and hydrogen gas concentration, of the second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 1 through 3 , a first embodiment of the present invention will be described hereinafter. Note that the following descriptive embodiment is an example of a fuel cell system mounted in a fuel cell-powered vehicle. 
       FIG. 1  is an overall structural view showing a fuel cell system according to the present invention. 
     As shown in  FIG. 1 , a fuel cell  1  includes a stack, which is formed by stacking a plurality of cells, each of which includes a solid polymer electrolyte  1   a  (e.g., a solid polymer ion-changing membrane, which is referred to as an “electrolyte membrane  1   a ”), an anode electrode (negative pole)  1   b , and a cathode electrode (positive pole)  1   c , said anode electrode  1   b  and cathode electrode  1   c  sandwiching the electrolyte membrane  1   a . For the sake of simplicity, only a single cell is shown in the figure. 
     In the fuel cell  1 , the anode electrode  1   b  is supplied with hydrogen as a fuel gas, and the cathode electrode  1   c  is supplied with air containing oxygen as an oxidant gas. When the anode electrode  1   b  is supplied with hydrogen, hydrogen ions are produced in the anode electrode  1   b  by a catalytic reaction, which reach the cathode electrode  1   c  through the electrolyte membrane  1   a . In the cathode electrode  1   c , the hydrogen ions electrochemically react with oxygen contained in the air, such that electrical power is generated. 
     Note that, at a time of power generating, water is formed in the cathode electrode  1   c  side, and that part of the formed water back-diffuses to the anode electrode  1   b  side through the electrolyte membrane  1   a , as a phenomenon. 
     A fuel gas supply path  2  is connected to the anode electrode  1   b  of the fuel cell  1  to supply a hydrogen gas as a fuel gas. An oxidizing agent gas supply path  3  is connected to the cathode electrode  1   c  of the fuel cell  1  to supply air as an oxidizing agent gas. 
     A hydrogen tank  4 , which receives a hydrogen gas, is connected to the upstream side of the fuel gas supply path  2  via an on-off valve  5 . The on-off valve  5  is controlled by an unillustrated controller such that the hydrogen gas can be supplied from the hydrogen tank  4  to the anode electrode  1   b  side in response to an opening motion of the on-off valve  5 . 
     To the anode electrode  1   b  of the fuel cell  1  is connected a fuel gas circulating path  6 , which returns an unreacted hydrogen gas that has passed through the inside of the anode electrode  1   b , to an inlet side of the anode electrode  1   b . The fuel gas circulating path  6  is connected and merged with the fuel gas supply path  2  through an ejector  7  at the inlet side of the anode electrode  1   b , so that the unreacted hydrogen gas is mixed with fresh hydrogen and then resupplied to the anode electrode  1   b.    
     Further, a fuel gas discharge path  9  is provided in a branching manner at a portion of an outlet side of the anode electrode  1   b  of the fuel gas circulating path  6 . On the fuel gas discharge path  9  are provided three branch paths  9   a ,  9   b , and  9   c , which have respective valves, i.e., a drain valve  10 , a purge valve  11 , and an air discharge valve  12 . 
     The drain valve  10  is intended mainly to discharge residual water in the fuel gas circulating path  6  to a catching tank  13 . Of the three valves  10 ,  11 , and  12 , the drain valve  10  has a smallest opening area. 
     The purge valve  11  is intended mainly to appropriately discharge water and other impurities which reside in combination in the fuel gas circulating path  6  when the fuel cell system is in operation. Of the three valves  10 ,  11 , and  12 , the purge valve  11  has an intermediate opening area. 
     The air discharge valve  12  is intended to discharge a gas to purge the inside of the fuel gas circulating path  6 . Of the three valves  10 ,  11 , and  12 , the air discharge valve  12  has a largest opening area. 
     The three branching paths  9   a ,  9   b , and  9   c  are merged at the downstream sides of the respective valves  10 ,  11 , and  12 , and then connected to a discharge gas inlet  14   a  of a dilution box  14 , which will be described hereinafter, and which corresponds to an oxidizing agent gas dilution means. Note that, in this embodiment, openings  10   a ,  11   a , and  12   a  of the valves  10 ,  11 , and  12 , respectively, through which gases flow, constitute “a discharge port element” or “discharge ports” of the present invention. 
     In the mean time, a compressor  8  to pressurize and convey a gas is connected to the upstream side of the oxidizing agent gas supply path  3 . Further, to the cathode electrode  1   c  of the fuel cell  1  is connected an oxidizing agent gas discharge path  16 , which discharges air that has passed through the cathode electrode  1   c . On the oxidizing agent gas discharge path  16  is provided a back-pressure control valve  17 , which regulates an internal pressure of the cathode electrode  1   c . The downstream side of the oxidizing agent gas discharge path  16  is connected to a diluent gas inlet  14   b  of the above-described dilution box  14 . 
     As described, the dilution box  14  has the discharge gas inlet  14   a  connected to the fuel gas discharge path  9 , and the diluent gas inlet  14   b  connected to the oxidizing agent gas discharge path  16 . In addition thereto, it has a discharge outlet  14   c , which is connected to a dilution gas discharge path  18  that opens outside the system. In the dilution box  14 , a hydrogen gas, which enters via the discharge gas inlet  14   a  from the fuel gas discharge path  9 , is mixed with air, which flows from a diluent gas inlet  14   b  side toward a discharge outlet  14   c  side. The hydrogen gas diluted thereby is discharged through the dilution gas discharge path  18  to the outside of the system. Note that a hydrogen gas concentration sensor  19  is provided on the dilution gas discharge path  18  at the downstream side of the dilution box  14 , and detection signals emitted therefrom are inputted to the unillustrated controller. 
     On the oxidizing agent gas supply path  3  is provided in a branching manner a feed path  20 , which introduces pressurized air (i.e., an oxidizing agent gas) from the compressor  8  to an inlet side of the anode electrode  1   b . On the feed path  20  is provided an air feed valve  21 , opening and closing of which the controller controls. The air feed valve  21 , in a normal position thereof, closes the feed path  20 . It opens the feed path  20 , at a time of purging of the anode electrode  1   b , under control of the controller. Note that, in this embodiment, the feed path  20  and the air feed valve  21  together with the compressor  8  constitute “anode purging means” of the present invention. 
     Hereinafter, control of the fuel cell system when the fuel cell-powered vehicle is stopped will be described with reference to a flowchart of  FIG. 2 , while referring to a time chart of  FIG. 3 . Note that, when an ignition switch of the fuel cell-powered vehicle is turned ON, the air feed valve  21 , the drain valve  10 , the purge valve  11 , and the air discharge valve  12  are normally closed, whereas the on-off valve  5  is open. 
     When the ignition switch is turned OFF, in step S 100 , the controller initially closes the on-off valve  5  and increases a flow rate of the compressor  8 . Step S 101  judges whether or not a time T that elapses after the on-off valve  5  is closed reaches a predetermined time T 1 . After it reaches the predetermined time T 1 , the routine proceeds to next step S 102 . Note that, until it proceeds to step S 102 , the fuel cell system continues consumption of power generation due to a burden. 
     In step S 102 , such consumption of power generation is stopped, and the air feed valve  21  and the drain valve  10  are opened. After the air feed valve  21  is thus opened, pressurized air flows from the compressor  8  through the feed path  20  to the anode electrode  1   b  and hence to the fuel gas circulating path  6 . The air, which has thus flowed in the fuel gas circulating path  6 , dilutes residual hydrogen gas in the path  6  and together flows through the drain valve  10  into the dilution box  14 . This process is continued until an elapsed time T reaches a predetermined time T 2  (step S 103 ). After a time in which it has reached T 2 , the routine proceeds to next step S 104 . 
     In step S 102 , as mentioned above, when the drain valve  10  is opened, the gas in the fuel gas circulating path  6  flows in the dilution box  14  through the discharge gas inlet  14   a . At this time, since hydrogen gas gradually flows in the diluent box  14  through the drain valve  10  due to the small opening area of the drain valve  10 , the fuel gas is sufficiently diluted in the dilution box  14  by air which is introduced through the diluent gas inlet  14   b . Thus, peak concentration of a discharge gas, which is discharged outside through the dilution gas discharge path  18 , is controlled below a predefined value, as shown in  FIG. 3 . 
     Then, after a certain time has passed, the routine proceeds to step S 104 , where the purge valve  11  and the air discharge valve  12  are simultaneously opened by the controller, such that a residual hydrogen gas in the fuel gas circulating path  6  is rendered to flow at once in the dilution box  14 . At this time, large quantities of gas from the fuel gas circulating path  6  flow in the dilution box  14 . In the fuel gas circulating path  6 , dilution to a certain level is already realized by air fed from the compressor  8  in previous steps (i.e., S 102 , S 103 ). Therefore, in the dilution box  14 , satisfactory dilution is realized due to further mixing with a lot of air. As a result, emission concentration of hydrogen, which is discharged from the dilution gas discharge path  18 , is also controlled below a predetermined level (see  FIG. 3 ). At the same time, a gas in the fuel gas circulating path  6  is discharged therefrom at once through the purge valve  11  and the air discharge valve  12 , and therefore, water and other impurities, which reside in the fuel gas circulating path  6 , can be reliably outwardly discharged. 
     As described above, in the present fuel cell system, it is structured such that, the drain valve whose opening area is smallest (of all related valves) is firstly opened for (at least) a predetermined period, and thereafter, the purge valve  11  and the air discharge valve  12 , both of which have lager opening areas, are opened. The problems in which a hydrogen gas, which has a high concentration beyond a level of the diluting ability of the dilution box  14 , simultaneously enters the dilution box  14  can be avoided. Further, by changing an opening area of the drain valve  10  and an opening time of the same, hydrogen gas concentration in the fuel gas circulating path  6  and the dilution box  14  can be regulated. It is thus possible to reduce the time required for purging, while maintaining the hydrogen concentration below a predetermined level. It is also possible to reduce the time required for power generation just after the ignition switch (of the fuel cell-powered vehicle) is turned OFF. 
     Note that, although, in the present embodiment, the oxidizing agent gas supply path  3  has the feed path  20  provided thereon in a branching manner through which air as a purging/scavenging gas is introduced to the anode electrode  1   b  side, instead, a tank or the like dedicated to purging a gas can be separately provided. Nevertheless, in cases such as the embodiment in which the feed path  20  and the air feed valve  21  are provided and in which the air in the oxidizing agent gas supply path  3  is used as a purging gas, there is no need to separately provide a purge gas supply unit to be used exclusively. As a result, a reduction in size and manufacturing cost reductions of the whole system can be achieved. 
     Further, in the above-mentioned embodiment, it is structured such that the timing of opening of the purge valve  11  and the air discharge valve  12  after the drain valve  10  is opened is controlled and determined based on an elapsed time (see step S 103  of  FIG. 2 ). However, a structure is possible in which the hydrogen gas concentration at a downstream side of the dilution box  14  is controlled based on detection signals emitted from the hydrogen gas concentration sensor  19  and in which the purge valve  11  and the air discharge valve  12  are opened when the hydrogen gas concentration is below a predetermined level. 
     Next, a second embodiment depicted in  FIGS. 4 through 6  of the present invention will be described. Although a basic structure of a fuel cell system of the second embodiment is almost the same as that of the first embodiment, they are distinctly different in that a flow control valve  30  with continuous (or stepless) opening-area settings is provided on the fuel gas discharge path  9  connecting the anode electrode  1   b  of the fuel cell  1  and the discharge gas inlet  14   a  of the dilution box  14 . Note that, for convenience of explanation, parts and portions of the second embodiment which are common to the first embodiment or which can be used in common are denoted by the same reference numerals, and a repeated description will be appropriately omitted. 
     Hereinafter, control of the fuel cell system when the fuel cell-powered vehicle is stopped will be described with reference to a flowchart of  FIG. 5  and a time chart of  FIG. 6 . 
     When the ignition switch is turned OFF, in step S 200 , the controller closes the on-off valve  5  and increases a flow rate of the compressor  8 . Step S 201  judges whether or not a time T that elapses after the on-off valve  5  is closed reaches a predetermined time T 1 , and after it reaches the predetermined time T 1 , the routine proceeds to next step S 202 . Note that the fuel cell system continues consumption of power generation due to a burden. 
     In step S 202 , such consumption of power generation comes to a stop, and the air feed valve  21  opens, and further, control of the flow control valve  30  starts in such a manner that the flow volume of the gas flowing through the fuel gas discharge path  9  gradually increases. At an initial or earlier stage of control in which the hydrogen gas concentration in the fuel gas circulating path  6  is high, the volume of a hydrogen gas, which flows in the dilution box  14  from the fuel gas circulating path  6 , is low, and therefore, the hydrogen gas in the dilution box  14  is reliably diluted below a predetermined value. Further, as time goes by after control of the flow control valve  30  starts, the hydrogen gas in the fuel gas circulating path  6  is gradually diluted by air. Therefore, even if the volume of a gas, which flows from the fuel gas circulating path  6  into the dilution box  14 , gradually increases, it is possible to control a hydrogen gas emission concentration of the gas, which was diluted in the dilution box  14 , below a predetermined value. 
     The present invention is by no means limited to the above-mentioned preferred embodiments. It will be obvious to those skilled in the art that various modifications may be made without departing from the true spirit and scope of the present invention. For example, although the aforementioned embodiments are ones in which a fuel cell system according to the present invention is applied to a fuel cell-powered vehicle, it is possible for the present invention to be applied to an apparatus other than a fuel cell-powered vehicle. 
     Again, as is appreciated from the foregoing description, while preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. Additions, omissions, substitutions, and other modifications can be made without departing from the spirit or scope of the present invention. Accordingly, the invention is not to be considered as being limited by the foregoing description, and is only limited by the scope of the appended claims.