Patent Publication Number: US-2007111049-A1

Title: Fuel cell unit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is based upon and claims the benefit of priortiy from Japanese Patent Application No. 2005-332979, filed Nov. 17, 2005, the entire contents of which are incorporated herein by reference.  
     BACKGROUND  
      1. Field  
      One embodiment of the invention relates to a fuel cell unit, for example, a fuel cell unit with a circulation section in which a fluid is circulated.  
      2. Description of the Related Art  
      In recent years, small-sized, high-power fuel cell units that require no charging have been notice as power sources for electronic devices, such as portable computers. A direct methanol fuel cell (DMFC) unit that uses, for example, an aqueous methanol solution as its fuel is proposed as a fuel cell unit of this type.  
      An electromotive section of a DMFC unit performs operation for power generation by causing an aqueous methanol solution and oxygen in air to react chemically with each other. As the power generation advances, by-products in the electromotive section. The DMFC unit includes a circulation section through which the fuel, an oxidant, and the by-products flow into and from the electromotive section.  
      Afuel cell system with pressure valves is described in Jpn. Pat. Appln. KOKAI Publication No. 2004-127905. This fuel cell system includes a casing, a fuel tank, and an air tank, the tanks being located in the casing. The fuel and air tanks are provided with pressure valves, individually. The pressure valves are controlled by a control section through a converter. The control section operates the pressure valve in the fuel tank to lower pressure in the fuel tank. Thereupon, an aqueous methanol solution is fed from a fuel pack into the fuel tank. Further, the control section operates the pressure valve in the air tank to raise pressure in the air tank. Thus, water that collects in the air tank is discharged to the outside of the air tank.  
      A part of the circulation section in which a fluid is circulated sometimes may be clogged during operation of the fuel cell unit. If the operation is continued with the circulation section partially clogged, pressure in the circulation section increases so that the interior of the circulation section is highly pressurized. If the circulation section is thus internally pressurized, there is a possibility of a fluid, such as the fuel, water, or steam, leaking out through a structurally weak part of the circulation section. In some cases, moreover, a part of the circulation section may undergo deformation or breakdown, such as cracking. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
      A general architecture that implements the various feature of the invention will now be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate embodiments of the invention and not to limit the scope of the invention.  
       FIG. 1  is an exemplary perspective view of a fuel cell unit according to a first embodiment of the invention;  
       FIG. 2  is an exemplary perspective view showing a portable computer set on the fuel cell unit according to the first embodiment;  
       FIG. 3  is an exemplary perspective view of the DMFC unit according to the first embodiment;  
       FIG. 4  is an exemplary sectional view typically showing the fuel cell unit according to the first embodiment;  
       FIG. 5  is an exemplary sectional view of a safety valve according to the first embodiment;  
       FIG. 6  is an exemplary sectional view showing the safety valve of the first embodiment in an on state;  
       FIG. 7  is an exemplary diagram showing a relationship between temperature and pressure in a circulation section according to a first embodiment;  
       FIG. 8  is an exemplary sectional view typically showing a fuel cell unit according to a second embodiment of the invention;  
       FIG. 9  is an exemplary sectional view typically showing a fuel cell unit according to a third embodiment of the invention;  
       FIG. 10  is an exemplary sectional view of a safety valve according to the third embodiment; and  
       FIG. 11  is an exemplary sectional view showing a modification of the fuel cell unit according to the third embodiment. 
    
    
     DETAILED DESCRIPTION  
      Various embodiments according to the invention will be described hereinafter with reference to the accompanying drawings. In general, according to one embodiment, a fuel cell unit is provided with a housing, a circulation section which is housed in the housing and in which a fluid is circulated, and a valve which is provided for the circulation section and releases a pressure more than predetermined value produced in the circulation section.  
      Embodiments of the present invention applied to a fuel cell unit will now be described with reference to the accompanying drawings.  
      FIGS.  1  to  7  show a fuel cell unit  1  according to a first embodiment of the invention.  FIG. 1  shows an outline of the unit  1 . The fuel cell unit  1  according to the present embodiment is of the DMFC type. As shown in  FIG. 2 , the unit  1  has a size such that it can be used as a power source for, e.g., a portable computer  2 .  
      As shown in  FIG. 1 , the fuel cell unit  1  includes a unit body  3  and a setting section  4 . The unit body  3  has an elongate shape extending along the longitudinal direction of the portable computer  2 . The setting section  4  protrudes horizontally from the front end of the unit body  3 . The rear end portion of the computer  2  is set on the setting section  4 . A power supply connector  5  is located on the upper surface of the setting section  4 . The connector  5  is connected electrically to the computer  2  when it is placed on the setting section  4 .  
      As shown in  FIG. 1 , the unit body  3  is provided with a housing  6 . The housing  6  contains a DMFC unit  7  therein, as shown in  FIG. 3 . The DMFC unit  7  includes a holder  11 , fuel cartridge  12 , mixing section  13 , intake section  14 , DMFC stack  15 , cooling section  16 , exhaust section  17 , control section  18 , and safety valve  19 .  
      The mixing section  13 , intake section  14 , DMFC stack  15 , cooling section  16 , exhaust section  17 , and pipes that connect those sections cooperate with one another to form a part of a circulation section  21  in which a fluid is circulated. The fuel cartridge  12  is an example of a fuel container.  
      As shown in  FIG. 3 , the fuel cartridge  12  is removably attached to the holder  11 . The fuel cartridge  12  contains therein high-concentration methanol as a liquid fuel to be used for power generation, for example. A methanol-soluble odorant, for example, is added to this methanol. An example of the odorant that is mixed into the fuel for fuel cells may be one that can produce a sufficient odor despite its scantiness and has low adsorptivity to pipes, containers, etc. The odorant used may be one that contains, for example, a pyridine derivative and a solid-state compound.  
      As shown in  FIG. 4 , the fuel cartridge  12  that is attached to the holder  11  is connected to the mixing section  13  by a first fuel supply pipe  23 . The first fuel supply pipe  23  is an example of a first pipe. A first valve  24  and a fuel pump  25  are provided in the middle of the first pipe  23 . The first valve  24  can be switched between a position in which it allows a passage of the pipe  23  to open and a position in which it closes the passage. The fuel pump  25  feeds the methanol from the fuel cartridge  12  into the mixing section  13 .  
      As shown in  FIG. 4 , the mixing section  13  is provided with a mixing tank  31  and a gas-liquid separator section  32 . The tank  31  communicates with the first fuel supply pipe  23 . The tank  13  is supplied with the methanol from the fuel cartridge  12 . In the mixing tank  31 , the supplied high-concentration methanol is diluted to form an aqueous solution of methanol with a concentration of several to tens of percent.  
      As shown in  FIG. 4 , the mixing tank  31  is connected to the DMFC stack  15  by a second fuel supply pipe  34 . A filter  35  and a liquid pump  36  are provided in the middle of the supply pipe  34 . The liquid pump  36  feeds the aqueous methanol solution generated in the mixing tank  31  into the DMFC stack  15 .  
      The mixing tank  31  is provided with a sensor section  37 . The sensor section  37  includes a liquid quantity sensor, a temperature sensor, and a concentration sensor. The sensor section  37  detects some pieces of information, such as a liquid quantity, temperature, and the concentration of the aqueous methanol solution in the mixing tank  31 , and delivers them to the control section  18 .  
      The gas-liquid separator section  32  is provided with a gas-liquid separation chamber  41  and a first exhaust pipe  42 . The separation chamber  41  is formed integrally with the mixing tank  31  and communicates internally with the tank  31 . The chamber  41  has a gas-liquid separation membrane  43 . The membrane  43  is situated on a boundary between the chamber  41  and the tank  31  and divides the chamber  41  and the tank  31 . The first exhaust pipe  42  connects the gas-liquid separation chamber  41  to the cooling section  16  and guides a gas in the chamber  41  into the exhaust section  17  through the cooling section  16 .  
      As shown in  FIG. 4 , the safety valve  19  is provided on, for example, a ceiling of the mixing tank  31 . Much of a gas that is contained in the fluid in the tank  31  gathers in an upper part of the tank  31 . The safety valve  19  is an example of a valve that releases a pressure of a predetermined or higher value produced in the circulation section  21 . As shown in  FIG. 5 , the valve  19  includes a case  45 , valve body  46 , leaf spring  47 , and bearing  48 . The case  45  is mounted on a top surface of the mixing tank  31 . The case  45  has a first opening portion  45   a  that opens into the mixing tank  31  and a second opening portion  45 b that opens outward from the tank  31 . The valve body  46  is housed in the case  45 . The valve body  46  is supported by the bearing  48  in such a manner that the valve body  46  is movable between a position in which it closes the first opening portion  45   a  and a position in which it allows the opening portion  45   a  to open. The leaf spring  47  is an example of an urging mechanism. It urges the valve body  46  toward the position in which the first opening portion  45   a  is closed. The urging mechanism is not limited to the leaf spring  47  but may be any other spring, such as a coil spring, or an elastic member such as rubber.  
      As shown in  FIG. 3 , the intake section  14  is provided with an intake port  51  that opens outward from the DMFC unit  7 . An air filter  52  is attached to the port  51 . The intake section  14  introduces the outside air into the DMFC unit  7  through the intake port  51 . As shown in  FIG. 4 , the intake section  14  is connected to the DMFC stack  15  by an air supply pipe  53 . The pipe  53  is an example of a second pipe.  
      An air pump  54  and a second valve  55  are provided in the middle of the air supply pipe  53 . The pump  54  supplies the DMFC stack  15  with air that is introduced through the intake port  51 . The second valve  55  can be switched between a position in which it allows a passage of the pipe  53  to open and a position in which it closes the passage.  
      The DMFC stack  15  is an example of an electromotive section. It includes an anode  57 , cathode  58 , and electrolyte membrane  59 . The electrolyte membrane  59  is interposed between the anode  57  and the cathode  58  and divides these electrodes. The anode  57  is supplied with the aqueous methanol solution from the mixing tank  31 . The cathode  58  is supplied with an oxidant, i.e., air, from the intake section  14 .  
      The DMFC stack  15  causes the aqueous methanol solution and oxygen in the air to react chemically with each other, thereby generating electric power. This operation for power generation produces, as by-products, carbon dioxide at the anode  57  and steam at the cathode  58 .  
      The cooling section  16  is provided with a first cooling mechanism  61  and a second cooling mechanism  62 . The first cooling mechanism  61  has a first condenser  63  and a first cooling fan  64 . The fan  64  is driven to cool the condenser  63 .  
      As shown in  FIG. 4 , the anode  57  of the DMFC stack  15  is connected to the mixing tank  31  by a fuel return pipe  65 . The first cooling mechanism  61  is provided in the middle of the pipe  65 . The carbon dioxide generated at the anode  57  and unreacted methanol are cooled as they pass through the first condenser  63  after having passed through the anode  57 , and are supplied back to the mixing tank  31 .  
      The second cooling mechanism  62  has a second condenser  66 , second cooling fan  67 , and water recovery tank  68 . The second cooling fan  67  is driven to cool the condenser  66 . As shown in  FIG. 4 , the cathode  58  of the DMFC stack  15  is connected to one end of a second exhaust pipe  71 . The other end of the pipe  71  joins the first exhaust pipe  42  and is connected to the second condenser  66 . The steam produced at the cathode  58  and air are delivered to the second condenser  66  after having passed through the cathode  58 . The steam delivered to the second condenser  66  is cooled to be condensed and is recovered as water in the water recovery tank  68 .  
      As shown in  FIG. 4 , the water recovery tank  68  is connected to the mixing tank  31  by a recovery pipe  72 . A recovery pump  73  is provided in the middle of the pipe  72 . It supplies the water recovered in the water tank  68  back to the tank  31 .  
      As shown in  FIG. 4 , the exhaust section  17  is provided with an exhaust port  75  that opens outward from the circulation section  21 . It has a third exhaust pipe  76  that connects the second condenser  66  to the exhaust port  75 . The third exhaust pipe  76  is an example of a third pipe. A filter  77  and a third valve  78  are provided in the middle of the pipe  76 . The third valve  78  can be switched between a position in which it allows a passage of the pipe  76  to open and a position in which it closes the passage. When the third valve  78  is closed, the exhaust section  17  is closed externally.  
      The first, second, and third valves  24 ,  55  and  78  cooperate with one another to serve as a valve mechanism  81  that hermetically closes most divisions of the DMFC unit  7 . Further, the intake section  14 , cathode  58  of the DMFC stack  15 , second cooling mechanism  62 , mixing tank  31 , vapor-liquid separator section  32 , exhaust section  17 , and pipes that connect those sections cooperate with one another to form a part of a gas circulation section  82  in which a gas is circulated.  
      The control section  18  is housed in the setting section  4 . It serves to monitor the states of the mixing section  13 , intake section  14 , DMFC stack  15 , cooling section  16 , exhaust section  17 , etc. and control the operations of these units  13  to  17 . Further, the control section  18  supplies the power supply connector  5  with electric power that is generated in the DMFC stack  15 .  
      The following is a description of the function of the fuel cell unit  1 . The general operation of the DMFC unit  7  will first be described with reference to  FIG. 4 .  
      The methanol that is stored in the fuel cartridge  12  is fed into the mixing tank  31  through the first fuel supply pipe  23 , whereupon it is diluted in the tank  31 . The resulting aqueous methanol solution diluted in the mixing tank  31  is delivered to the anode  57 . On the other hand, the cathode  58  is supplied with air through the intake section  14 . The DMFC stack  15  performs operation for power generation by causing the aqueous methanol solution and oxygen in the air to react chemically with each other. As this operation advances, carbon dioxide and steam are produced at the anode  57  and the cathode  58 , respectively.  
      The carbon dioxide and unreacted methanol having passed through the anode  57  are cooled by the first cooling mechanism  61  and supplied back to the mixing tank  31 . The aqueous methanol solution supplied back to the tank  31  has its concentration adjusted as a fresh aqueous methanol solution, and is supplied again to the anode  57  to be used for power generation.  
      The carbon dioxide supplied back to the mixing tank  31  is separated from the aqueous methanol solution by being passed through the gas-liquid separation membrane  43 , and is temporarily stored in the gas-liquid separation chamber  41 . The carbon dioxide in the chamber  41  is delivered to the second cooling mechanism  62  through the first exhaust pipe  42 . The carbon dioxide delivered to the cooling mechanism  62  is further fed to the exhaust section  17  and discharged to the outside of the circulation section  21 .  
      On the other hand, the steam and air having passed through the cathode  58  are cooled by the second cooling mechanism  62 , whereupon the steam condenses. Thus, air is separated from water. The separated air is discharged to the outside of the circulation section  21  through the exhaust section  17 . The separated water is supplied back to the mixing tank  31  and used to dilute the aqueous methanol solution.  
      The function of the safety valve  19  will now be described with reference to FIGS.  5  to  7 .  
      When the fuel cell unit  1  is not in use, the first, second, and third valves  24 ,  55  and  78  are closed. When the valves  24 ,  55  and  78  are closed, most divisions of the circulation section  21 , exclusive of the upstream end portion of the first fuel supply pipe  23 , the upstream end portion of the air supply pipe  53 , and the downstream end portion of the third exhaust pipe  76 , are hermetically closed. If most divisions of the circulation section  21  are hermetically closed, a substance, such as a liquid fuel, in the circulation section  21  can be prevented from leaking out of it.  
      If the fuel cell unit  1  is left with any divisions of the circulation section  21  hermetically closed, the temperature of the fluid in the circulation section  21  increases when the outside air temperature is high, for example. In  FIG. 7 , a broken line shows a relationship between temperature and pressure in the circulation section  21  without the safety valve  19 . If the temperature in the circulation section  21  increases, as shown in  FIG. 7 , the pressure in the circulation section  21  also increases correspondingly.  
      If a passage or a container in the circulation section  21  is partially clogged during the operation of the fuel cell unit  1 , the pressure in the circulation section  21  also increases.  
      The safety valve  19  according to the present embodiment is actuated when a predetermined value (e.g., value S in  FIG. 7 ) is exceeded by the pressure in the circulation section  21 . Thus, the valve body  46  moves so as to allow the first opening portion  45   a  to open when the force of the leaf spring  47  is surpassed by the pressure in the mixing tank  31  that acts on the valve body  46 , as shown in  FIG. 6 . When the first opening portion  45   a  is open, the gas in the mixing tank  31  flows out of it through the safety valve  19 , as indicated by hollow arrows. Thus, the safety valve  19  releases a pressure of a predetermined or higher value in the mixing tank  31 . By doing this, the pressure in the mixing tank  31  can be restrained from increasing above the predetermined value, as indicated by a solid line in  FIG. 7 .  
      The mixing tank  31  is connected directly or indirectly to all of the other units that constitute the circulation section  21 , that is, the vapor-liquid separator section  32 , intake section  14 , DMFC stack  15 , cooling section  16 , exhaust section  17 , first and second fuel supply pipes  23  and  34 , air supply pipe  53 , first, second, and third exhaust pipes  42 ,  71  and  76 , fuel return pipe  65 , and recovery pipe  72 . If the pressure in the mixing tank  31  is released, pressures in those units are also restrained from increasing.  
      Thus, the pressure in the circulation section  21  cannot be increased above a predetermined level even if the temperature in the circulation section  21  continues to increase, as shown in  FIG. 7 .  
      In the present embodiment, the odorant is previously added to the liquid fuel. When the safety valve  19  is actuated to open the first opening portion  45   a , therefore, the odor of the odorant gets out of the housing  6  of the fuel cell unit  1 . A user can recognize the actuation of the safety valve  19  by smelling the odor of the odorant and take measures such as to stop the operation of the fuel cell unit  1  or ventilate the room.  
      According to the fuel cell unit  1  constructed in this manner, a trouble that is attributable to the internal pressure of the circulation section  21  can be prevented. More specifically, if the pressure in the circulation section  21  is expected to exceed an allowable value, the safety valve  19  is actuated before the allowable value is exceeded. Therefore, the pressure in the circulation section  21  can always be kept at or below the allowable value. Thus, leakage of the fuel or water from the circulation section  21  that is attributable to high pressure can be prevented. Further, any part of the circulation section  21  can be prevented from being damaged due to the internal pressure.  
      Since the fuel cell unit  1  is provided with the safety valve  19 , moreover, it can be reduced in size. If the fuel cell unit  1  has no safety valve, the strength of the members of the circulation section  21  should be made increased to ensure that the circulation section  21  is not damaged when the pressure therein is increased. Since the fuel cell unit  1  has the safety valve  19 , however, it is necessary only that the members of the circulation section  21  have a minimum strength such that they can comply with an optionally set allowable value. Thus, the individual members can be reduced in size.  
      Since the fuel cell unit  1  has the valve mechanism  81  that hermetically closes at least some divisions of the circulation section  21 , leakage of the fluid from the circulation section  21  can be minimized when the unit  1  is nonoperating.  
      The circulation section  21  has a region in which a liquid, such as the aqueous methanol solution or water, is circulated and a region in which a gas, such as air or carbon dioxide, is circulated. The safety valve  19  may be located in either of these regions.  
      If the safety valve  19  is located in the region in which the liquid is circulated, something like a tank is provided so as to recover the liquid that leaks out of the circulation section  21  through the valve  19  when the valve  19  is actuated. If the safety valve  19  is located in the region in which the gas is circulated, on the other hand, it is unnecessary to provide any special tank or the like. This is efficient to reduction of the fuel cell unit  1  in size and in cost.  
      The safety valve  19  may be attached to the mixing tank  31 . The circulation section  21  roughly includes an anode system passage  83  and a cathode system passage  84 . The anode system passage  83  is a passage through which the aqueous methanol solution or some other liquid fuel flows. The passage  83  includes, for example, the second fuel supply pipe  34 , anode  57 , first cooling mechanism  61 , fuel return pipe  65 , etc. On the other hand, the cathode system passage  84  is a passage through which the oxidant, i.e., air, or the product at the cathode  58  flows. The passage  84  includes, for example, the air supply pipe  53 , cathode  58 , second exhaust pipe  71 , second cooling mechanism  62 , recovery pipe  72 , etc.  
      The safety valve  19  may be provided in either the anode system passage  83  or the cathode system passage  84 . If the circulation section  21  has a complicated piping arrangement, a pressure increase in the cathode system passage  84  may possibly fail to be fully restrained even when the safety valve  19  in, for example, the anode system passage  83  is actuated. Therefore, at least one safety valve  19  may be provided in each of the passages  83  and  84 , as shown in  FIG. 8 .  
      On the other hand, the mixing tank  31  is a region at which the anode system passage  83  and the cathode system passage  84  join each other. Specifically, the tank  31  communicates intimately with each of the passages  83  and  84 . When the safety valve  19  that is provided on the mixing tank  31  is actuated, therefore, it can restrain the pressure in the passages  83  and  84  from increasing. Thus, by providing the mixing tank  31  with the one safety valve  19 , the same effect can be obtained as in the case where two safety valves  19  are provided separately in the anode system passage  83  and the cathode system passage  84 .  
      The safety valve  19  according to the present embodiment need not be provided with any special attachment member, such as a pressure sensor or control section. However, the object that is to restrain the occurrence of trouble in the circulation section  21  can be also achieved if a pressure sensor is located in the circulation section  21  with use of a safety valve that is controlled by a control section, for example.  
      A fuel cell unit  85  according to a second embodiment of the invention will now be described with reference to  FIG. 8 . Like numerals are used to designate like portions of the fuel cell unit  1  of the first embodiment with the same functions, and a description of those portions is omitted.  
      The fuel cell unit  85  has two safety valves  19 , first and second. The first safety valve  19  is provided in the middle of an air supply pipe  53 . The second safety valve  19  is provided in a second fuel supply pipe  34 . Thus, the two safety valves  19  are provided on the anode system passage  83  and the cathode system passage  84 , individually.  
      The fuel cell unit  85  constructed in this manner, like the fuel cell unit  1  according to the first embodiment, is configured so that occurrence of trouble that is attributable to the internal pressure of the circulation section  21  can be restrained.  
      The circulation section  21  has a complicated piping arrangement. Even when the safety valve  19  in, for example, the anode system passage  83  is actuated, therefore, a pressure increase in the cathode system passage  84  may possibly fail to be fully restrained. If at least one safety valve  19  is provided in each of the passages  83  and  84 , however, the possibility of such a failure is reduced.  
      A fuel cell unit  91  according to a third embodiment of the invention will now be described with reference to FIGS.  9  to  11 . Like numerals are used to designate like portions of the fuel cell unit  1 ,  85  of the first and second embodiments with the same functions, and a description of those portions is omitted.  
      As shown in  FIG. 9 , the fuel cell unit  91  has a circulation section  21  and a safety valve  92 . As shown in  FIG. 10 , the safety valve  92  includes a case  45 , valve body  46 , leaf spring  47 , bearing  48 , liquid absorbing sheet  93 , and filter  94 . The liquid absorbing sheet  93  is an example of a liquid absorbing member. It is mounted so as to cover a second opening portion  45   b . The sheet  93  is formed of a liquid absorbing material, such as sponge or paper, and absorbs a liquid that adheres to its surface.  
      The filter  94  is mounted so as to cover the second opening portion  45   b . An example of the filter  94  can remove vaporized methanol that is contained in a gas.  
      The fuel cell unit  91  constructed in this manner, like the fuel cell unit  1  according to the first embodiment, is configured so that occurrence of trouble that is attributable to the internal pressure of the circulation section  21  can be restrained.  
      According to the fuel cell unit  91  of the present embodiment, moreover, leakage of substance, such as methanol vapor, can be restrained from flowing out of a DMFC unit  7 . That is, some of a gas in a mixing tank  31  flows out of the DMFC unit  7  when the safety valve  92  is actuated. Some of the gas in the tank  31  contains methanol vapor.  
      The safety valve  92  according to the present embodiment has the filter  94 , which removes a substance, such as methanol vapor, contained in the gas as the gas passes through the valve  92 .  
      According to the fuel cell unit  91  of the present embodiment, moreover, leakage of liquid can be restrained from leaking out of the DMFC unit  7 . That is, when the safety valve  92  is actuated, some of an aqueous methanol solution in the mixing tank  31  is apt to leak out of the DMFC unit  7  through the safety valve  92 . The safety valve  92  according to the present embodiment has the liquid absorbing sheet  93 , whereby the liquid having penetrated into the valve  92  is absorbed and prevented from leaking out of the DMFC unit  7 .  
      The safety valve  92  need not hold both the filter  94  and the liquid absorbing sheet  93 , but may alternatively use only one of these elements, depending on the characteristics of the fuel cell unit to which the embodiment of the invention is applied. Further, the attachment of the filter  94  and the liquid absorbing sheet  93  are not limited to the position according to the present embodiment. For example, the filter  94  may be located nearer to the valve body  46  than the liquid absorbing sheet  93  is. Some of conventional, commercially available filters cannot fulfill their essential function if a liquid adheres to their surface. If one such filter is used, the liquid absorbing sheet  93  is suitably located on the side nearer to the mixing tank  31 , as in the present embodiment.  
      Furthermore, the liquid absorbing sheet  93 , for example, need not cover the second opening portion  45   b , but may be located along the inner surface of the case  45  of the safety valve  92  without failing to produce its effect as a liquid absorbing member. As shown in  FIG. 11 , the safety valve  92  may be provided in each of passages, an anode system passage  83  and a cathode system passage  84 .  
      Although the fuel cell units  1 ,  85  and  91  according to the first, second, and third embodiments have been described herein, it is to be understood that the present invention is not limited to these embodiments. The components of the fuel cell units according to these three embodiments may be suitably combined according to the purpose.  
      For example, the location of the safety valves  19  and  92  is not limited to the positions described in connection with the first and second embodiments, but may be in any region in which the fluid can be circulated without failing to fulfill their function. For example, the construction of each safety valve is not limited to those of the embodiments. The valve may be of any shape or configuration as long as it is designed to release the pressure in the circulation section  21  when it reaches a predetermined or higher value. Further, a plurality of safety valves may be provided depending on the size, construction, and function of the fuel cell unit.  
      Instead of adding the odorant to the fuel, for example, odorizors  105  may be located in the safety valve  19  or  92 , as indicated by two-dot chain lines in  FIG. 5  or  10 . Each odorizor  105  may be one that produces an odor by reacting directly with the fuel or water or one formed of an inherently odoriferous material contained in a capsule that is soluble in the fuel or water. Further, the odorizors  105  may be located outside the case  45  instead of being located inside.  
      The embodiments of the present invention are not limited to DMFC units, but may be also applied to fuel cell units that use alcohols, such as ethanol, or some other fluid fuels. Further, the embodiments of the invention are not limited to fuel cell units for portable computers, but may be also applied to fuel cell units for electronic devices, such as cell phones, digital cameras, etc., or vehicles such as automobiles.  
      While certain embodiments of the inventions have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.