Patent Publication Number: US-2007122670-A1

Title: Fuel cell unit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2005-345909, filed Nov. 30, 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, it relates to a fuel cell unit that generates exhaust gas during generation of electricity.  
      2. Description of the Related Art  
      Attention has recently been paid to the use, as power supplies for electronic devices, such as portable computers, of compact, high-output fuel cell units that are not necessary to be charged. As fuel cell units of this type, direct methanol fuel cells (DMFCs) are known which use an aqueous solution of methanol as fuel.  
      The electromotive section of a DMFC generates electricity by causing an aqueous solution of methanol to chemically react with oxygen in the air. As a result of the chemical reaction, water vapor and carbon dioxide are produced in the electromotive section. To exhaust gas containing carbon dioxide and water vapor, the DMFC has an exhaust section for exhausting the gas to the outside.  
      For instance, a power supply system equipped with a fuel cell and heating unit is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2005-32585. The disclosed heating unit includes heating means, such as a heater, for heating a part of the system or the whole system when the system is stopped. As a result, the system is protected from freezing even when it does not operate.  
      Many DMFCs incorporate a cooling unit for cooling the exhaust gas. The cooling unit cools the exhaust gas to condense part of the water vapor contained in the exhaust gas to collect water. The collected water is circulated in the DMFC and used to adjust the concentration of the methanol aqueous solution.  
      The remaining exhaust gas, from which a necessary amount of water has been collected, is exhausted to the outside through the exhaust pipe and outlet. Since the exhaust gas is cooled by the cooling unit to the degree at which water vapor is condensed, saturated water vapor with a humidity of almost 100% is guided to the outlet. Further, in the exhaust pipe leading to the outlet, the temperature of the water vapor gradually reduces.  
      When the temperature of the exhaust gas is reduced, the water vapor is further condensed, and water droplets may well occur in the exhaust pipe. The droplets may scatter to the outside of the DMFC, or may remain in the DMFC and cause the DMFC to malfunction.  
      The power supply system disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2005-32585 does not prevent the droplets. Namely, the heating unit employed in the system is developed to protect the system from freezing when the system is stopped. 
    
    
     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 illustrating a fuel cell unit according to a first embodiment of the invention;  
       FIG. 2  is an exemplary perspective view illustrating a state in which a portable computer is mounted on the fuel cell unit of the first embodiment;  
       FIG. 3  is an exemplary perspective view illustrating a DMFC unit incorporated in the fuel cell unit of the first embodiment;  
       FIG. 4  is an exemplary schematic view illustrating the configuration of the fuel cell unit of the first embodiment;  
       FIG. 5  is an exemplary sectional view illustrating an exhaust section and its vicinity incorporated in the fuel cell unit of the first embodiment;  
       FIG. 6  is an exemplary sectional view illustrating the exhaust section and its vicinity taken along line F 6 -F 6  in  FIG. 5 ;  
       FIG. 7  is an exemplary sectional view illustrating a heat exchange section incorporated in the fuel cell unit of the first embodiment;  
       FIG. 8  is an exemplary sectional view illustrating a fuel cell unit according to a second embodiment of the invention;  
       FIG. 9  is an exemplary perspective view partly in section, illustrating a heat exchange section incorporated in the second embodiment;  
       FIG. 10  is an exemplary schematic view illustrating the configuration of a fuel cell unit according to a third embodiment of the invention;  
       FIG. 11  is an exemplary sectional view illustrating an exhaust section and its vicinity incorporated in the fuel cell unit of the third embodiment;  
       FIG. 12  is an exemplary sectional view illustrating a modification of the fuel cell unit of the third embodiment;  
       FIG. 13  is an exemplary schematic view illustrating the configuration of a fuel cell unit according to a fourth embodiment of the invention;  
       FIG. 14  is an exemplary schematic view illustrating the configuration of a fuel cell unit according to a fifth embodiment of the invention;  
       FIG. 15  is an exemplary schematic view illustrating the configuration of a fuel cell unit according to a sixth embodiment of the invention; and  
       FIG. 16  is an exemplary schematic view illustrating the configuration of a modification of the fuel cell unit according to the sixth embodiment of the invention. 
    
    
     DETAILED DESCRIPTION  
      Various embodiments according to the invention will be described hereinafter with reference to the accompanying drawings. In general, according to one embodiment of the invention, a fuel cell unit is provided with a housing; an electromotive section contained in the housing and including an anode and a cathode; a cooling section which cools a fluid having passed through the cathode and separates the fluid into a gas and a liquid; an exhaust pipe which guides, to an outside of the housing, the gas separated by the cooling section; and a heat transfer mechanism which transfers, to the exhaust pipe, part of heat generated by the electromotive section.  
      Fuel cell units according to embodiments of the invention will 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 the entire structure of the fuel cell unit  1 . The fuel cell unit  1  of the first embodiment is, for example, a DMFC fuel cell. As shown in  FIG. 2 , the fuel cell unit  1  has a size that enables the unit  1  to be usable as a power supply for, for example, a portable computer  2 .  
      As can be seen from  FIG. 1 , the fuel cell unit  1  includes a main body  3  and mount section  4 . The main body  3  has a long and narrow shape and extends in the width direction of the portable computer  2 . The mount section  4  horizontally projects from the front end of the main body  3 , and allows the rear end of the portable computer  2  to be mounted. A power supply connector  5  is provided on the upper surface of the mount section  4 . The power supply connector  5  is electrically connected to the portable computer  2  when the computer  2  is mounted on the mount section  4 .  
      Also as can be seen from  FIG. 1 , the main body  3  includes a housing  6 . The housing  6  contains such a DMFC unit  7  as shown in  FIG. 3 . The DMFC unit  7  includes a holder  10 , fuel cartridge  11 , mixing section  12 , air inlet section  13 , DMFC stack  14 , cathode cooling section  15 , anode cooling section  16 , exhaust section  17  and control section  18 .  
      As shown in  FIG. 3 , the holder  10  detachably holds the fuel cartridge  11 . The fuel cartridge  11  contains methanol of high density as a liquid fuel used to generate electricity.  
      As shown in  FIG. 4 , the fuel cartridge  11  held by the holder  10  communicates with the mixing section  12  via a first fuel supply pipe  21 . A fuel pump  22  is provided across the first fuel supply pipe  21 , and used to supply methanol in the fuel cartridge  11  to the mixing section  12 .  
      The mixing section  12  includes a mixing tank  24  and gas/solution separation section  25 . The mixing tank  24  communicates with the first fuel supply pipe  21  and receives methanol from the fuel cartridge  11 . The mixing tank  24  dilutes the received methanol of high density by adding water, thereby forming methanol aqueous solution with a density of several to several tens percents.  
      As can be seen from  FIG. 4 , the mixing tank  24  communicates with the DMFC stack  14  via a second fuel supply pipe  26 . A filter  27  and solution feed pump  28  are provided across the second fuel supply pipe  26 . The solution feed pump  28  supplies the DMFC stack  14  with the methanol aqueous solution formed by the mixing tank  24 .  
      The gas/solution separation section  25  includes a gas/solution separation chamber  31  and first exhaust pipe  32 . The gas/solution separation chamber  31  is formed as a single body with the mixing tank  24 , and communicates with the interior of the tank  24 . The gas/solution separation chamber  31  has a gas/solution separation membrane  33 . The mixing tank  24  and gas/solution separation chamber  31  are partitioned by the gas/solution separation membrane  33 . The first exhaust pipe  32  connects the gas/solution separation chamber  31  to the cathode cooling section  15  to guide the gas in the chamber  31  to the exhaust section  17  via the cathode cooling section  15 .  
      As shown in  FIG. 3 , the air inlet section  13  includes an air inlet  13   a,  air supply pipe  35  and air feed pump  36 . The air inlet  13   a  opens to the outside of the DMFC unit  7 . An air filter  37  is attached to the air inlet  13   a.  The air inlet section  13  guides the outside air into the DMFC unit  7  through the air inlet  13   a.  As shown in  FIG. 4 , the air inlet  13   a  communicates with the DMFC stack  14  via the air supply pipe  35 . The air feed pump  36  is provided across the air supply pipe  35  to supply the DMFC stack  14  with the air guided through the air inlet  13   a.    
      The DMFC stack  14  is an example of an electricity generation section. As shown in  FIG. 4 , the DMFC stack  14  includes a cathode  41 , anode  42  and electrolytic membrane  43 . The electrolytic membrane  43  is interposed between the cathode  41  and anode  42  to partition them. The cathode  41  is supplied with an oxidizing agent, i.e., air, through the air inlet section  13 . The anode  42  is supplied with the methanol aqueous solution from the mixing tank  24 .  
      The DMFC stack  14  causes the methanol aqueous solution to react with oxygen contained in the air to thereby generate electricity. As a result, water vapor and carbon dioxide are generated as accessory products in the cathode  41  and anode  42 , respectively.  
      Again as shown in  FIG. 4 , the cathode  41  of the DMFC stack  14  is connected to an end of the second exhaust pipe  45 . The other end of the second exhaust pipe  45  communicates with the first exhaust pipe  32  and cathode cooling section  15 . The water vapor generated by the cathode  41  and the air having passed through the cathode  41  are sent to the cathode cooling section  15  via the second exhaust pipe  45 .  
      The cathode cooling section  15  includes a first condenser  47 , first cooling fan  48  and water collection tank  49 . As shown in  FIG. 6 , the first condenser  47  includes branching piping  50  including a plurality of branch pipes. The piping  50  branches into, for example, four branches, which extend vertically and parallel. The upper ends of the branches are again joined into one that communicates with the exhaust section  17 . As a result, the fluid having passed through the cathode  41  is guided to the exhaust section  17  via the branching piping  50 .  
      A plurality of radiator fins  51 , for example, are attached to the branching piping  50 . The first cooling fan  48  sends air to the radiator fins  51  to cool them. As a result, the gas in the branching piping  50 , which contains water vapor, is cooled, and the amount of saturated water vapor of the gas is reduced. When the amount of saturated water vapor of the gas is reduced and the humidity reaches 100%, part of the water vapor is condensed into water. In the description, “humidity” means so-called relative humidity and indicates the ratio of the amount of vapor in the gas to the amount of saturated water vapor at each temperature.  
      As can be seen from  FIG. 6 , the water collection tank  49  is located below the first condenser  47 . The water obtained by the first condenser  47  is collected by the water collection tank  49 . The fluid having passed through the cathode  41  is separated into gas and water, and a required amount of water is collected. As shown in  FIG. 4 , the water collection tank  49  communicates with the mixing tank  24  via a collection pipe  53  and collection pump  54 .  
      The exhaust section  17  includes an exhaust outlet  56  and exhaust pipe  57 . As shown in  FIG. 5 , the exhaust outlet  56  opens to the outside of the housing  6  through an opening  6   a  formed in the housing  6 . The exhaust pipe  57  communicates with the branching piping  50  of the first condenser  47 , and guides, to the exhaust outlet  56 , the gas passing through the cathode cooling section  15 . A filter  58  and valve  59  are provided across the exhaust pipe  57 .  
      On the other hand, the anode  42  of the DMFC stack  14  is connected to one end of a fuel return pipe  61 , as shown in  FIG. 4 . The other end of the fuel return pipe  61  is connected to the mixing tank  24  via the anode cooling section  16 . As a result, the carbon dioxide gas generated by the anode  42 , and the non-reacted methanol aqueous solution are returned to the mixing tank  24  and reused for generating methanol aqueous solution.  
      As shown in  FIG. 4 , a heat exchange pipe  62  diverges from the middle portion of the fuel return pipe  61 . The heat exchange pipe  62  is an example of an anode pipe. As can be seen from  FIG. 5 , the heat exchange pipe  62  is extended along a side of the exhaust pipe  57  in contact therewith. Namely, the heat exchange pipe  62  and exhaust pipe  57  are thermally coupled to each other via their pipe walls.  
      Thus, the contact portions of the heat exchange pipe  62  and exhaust pipe  57  cooperate to serve as a heat exchange section  63  for transfer of heat. The heat exchange section  63  is an example of a heat transfer mechanism for transfer, to the exhaust pipe  57 , part of the heat generated by the DMFC stack  14 .  
      More specifically, the heat exchange pipe  62  directly contacts the exhaust pipe  57  between the downstream end  57   a  and upstream end  57   b  of the exhaust pipe  57 . The heat exchange pipe  62  is designed to cause the fluid contained therein to flow in the direction from the downstream end  57   a  of the pipe  57  to the upstream end  57   b.  Namely, the fluid in the heat exchange pipe  62  flows in the direction opposite to that of the gas in the exhaust pipe  57 . In other words, the heat exchange section  63  is a so-called counter-flow-type heat exchange section.  
      Although in the first embodiment, the heat exchange pipe  62  directly contacts the exhaust pipe  57 , a member of a heat transfer, such as a metal, may be interposed between the heat exchange pipe  62  and exhaust pipe  57 . Further, although in the first embodiment, the heat exchange pipe  62  is thermally coupled to the exhaust pipe  57  between the filter  58  and valve  59 , they may be thermally coupled, for example, upstream of the filter  58 , or downstream of the valve  59 .  
      As shown in  FIG. 4 , the downstream end of the heat exchange pipe  62  communicates with the fuel return pipe  61 . As a result, the fluid having passed through the heat exchange pipe  62  is returned to the mixing tank  24  via the fuel return pipe  61 .  
      The anode cooling section  16  is provided across the fuel return pipe  61 . More specifically, the anode cooling section  16  is located across the pipe  61  downstream of the confluence of the pipes  61  and  62 . The anode cooling section  16  includes a second condenser  65  and second cooling fan  66 . The second condenser  65  includes radiator fins  67  thermally coupled to the fuel return pipe  61 . The second cooling fan  66  sends air to the radiator fins  67  to cool them, whereby the fluid flowing through the fuel return pipe  61  is cooled.  
      The control section  18  is contained in the mount section  4 . The control section  18  monitors the states the mixing section  12 , air inlet section  13 , DMFC stack  14 , cathode cooling section  15 , anode cooling section  16  and exhaust section  17 , and controls the operations of the sections  12  to  17 . Further, the control section  18  supplies the power supply connector  5  with the electricity generated by the DMFC stack  14 .  
      The operation of the fuel cell unit  1  constructed as the above will now be described. Referring first to  FIG. 4 , the entire operation of the DMFC unit  7  will be described.  
      Methanol contained in the fuel cartridge  11  is sent to the mixing tank  24  via the first fuel supply pipe  21 , where it is diluted with water. The resultant methanol aqueous solution is sent to the anode  42 . On the other hand, the cathode  41  receives air from the air inlet section  13 . Thus, the DMFC stack  14  causes the methanol aqueous solution to react with oxygen in the air, thereby generating electricity. During generation of electricity, carbon dioxide and water vapor are produced in the anode  42  and cathode  41 , respectively.  
      The carbon dioxide gas having passed through the anode  42 , and the non-reacted methanol aqueous solution are cooled by the anode cooling section  16  and returned to the mixing tank  24 . The methanol aqueous solution returned to the mixing tank  24  is subjected to density adjustment, and used as a new methanol aqueous solution. This new solution is again sent to the anode  42  and reused for generating electricity.  
      The carbon dioxide gas returned to the mixing tank  24  is separated from the methanol aqueous solution when passing through the gas/solution separation membrane  33 , and is temporarily received in the gas/solution separation chamber  31 . The carbon dioxide gas in the gas/solution separation chamber  31  is sent to the cathode cooling section  15  via the first exhaust pipe  32 . The carbon dioxide gas in the cathode cooling section  15  is sent to the exhaust section  17 , where it is exhausted to the outside of the DMFC unit  7 .  
      In contrast, the water vapor and air having passed through the cathode  41  are cooled by the cathode cooling section  15 , whereby water is separated from the gas as a result of condensation of water vapor. The gas, from which a necessary amount of water is collected, is exhausted to the outside of the DMFC unit  7  along with the vapor remaining therein. The collected water is returned to the mixing tank  24  and reused to dilute methanol.  
      The operation of the heat exchange section  63  will be descried.  
      Part of the fluid (hereinafter referred to as the “anode circulation solution”) passing through the anode  42  is guided from the fuel return pipe  61  to the heat exchange pipe  62 . During generating electricity, the DMFC stack  14  also generates heat. While flowing through the anode  42 , the anode circulation solution is heated to about 50 to 60° C. The hot solution of about 50 to 60° C. flows through the heat exchange pipe  62 .  
      In contrast, the fluid (hereinafter referred to as the “exhaust gas”) passing through the cathode  41  is cooled to about 30 to 40° C. by the cathode cooling section  15 . The cool exhaust gas of about 30 to 40° C. flows through the exhaust pipe  57 . At this time, the exhaust gas is in the saturated state in which its relative humidity is substantially 100%.  
      In the heat exchange section  63 , the anode circulation solution and exhaust gas are thermally coupled via the walls of the heat exchange pipe  62  and exhaust pipe  57 . As a result, in the heat exchange section  63 , the part of heat of the anode circulation solution is transferred to the exhaust gas by, for example, heat conduction and convection, as shown in  FIG. 7 , whereby the temperature of the exhaust gas is increased to increase the amount of saturated water vapor of the gas.  
      When the amount of saturated water vapor of the exhaust gas is increased, the relative humidity of the gas is reduced even if the absolute amount of vapor contained in the exhaust gas does not change. When the relative humidity of the exhaust gas is reduced, the vapor contained therein does not easily condense. Accordingly, the exhaust gas, which is not condensed, is exhausted to the outside of the housing  6  through the exhaust outlet  56 .  
      The anode circulation solution, from which heat is transferred to the exhaust gas, flows into the fuel return pipe  61  from the downstream end of the heat exchange pipe  62 , and is cooled by the anode cooling section  16 .  
      The fuel cell unit  1  constructed as the above is substantially prevented from condensation. Namely, as described above, part of the heat generated by the DMFC stack  14  during electricity generation is transmitted to the exhaust pipe  57 , thereby reducing the relative humidity of the exhaust gas. This substantially prevents vapor contained in the exhaust gas from condensing in the exhaust pipe  57 .  
      The heat generated by the DMFC stack  14  during electricity generation is actually waste heat that should be exhausted to the outside of the fuel cell unit  1 . By effectively utilizing the waste heat to heat the exhaust gas, no particular heating devices are necessary.  
      The fuel cell unit  1  may be used as a power supply for electronic devices, such as the portable computer  2 . Accordingly, the prevention of condensation in the fuel cell unit  1  also contributes to the prevention of malfunction or failure of the electronic devices.  
      It is efficient to use the pipe for circulating the anode circulation solution as the mechanism for heating the exhaust gas utilizing the waste heat generated by the DMFC stack  14 . The fuel cell unit  1  of the first embodiment can be realized simply by attaching, for example, the fuel exchange pipe. This contributes to the cost reduction of the fuel cell unit  1 .  
      Further, the supply of the heat of the anode circulation solution to the exhaust gas means the absorption of part of the heat of the anode circulation solution by the exhaust gas. Since the anode circulation solution is later cooled by the anode cooling section  16 , the absorption of the heat of the anode circulation solution by the heat exchange section  63  assists the cooling operation of the anode cooling section  16 .  
      Since the heat exchange pipe  62  is located in contact with the exhaust pipe  57 , the heat exchange section  63  can be made simplest in structure, and the fuel cell unit  1  can be made compact.  
      The structure of the heat exchange section  63  is not limited to the above-described counter-flow type. For instance, a heat exchange section of a parallel-flow type, in which the anode circulation solution and exhaust gas flow in the same direction, may be employed. Alternatively, a heat exchange section of a perpendicular-flow type, in which the anode circulation solution and exhaust gas flow at right angles to each other, may be employed.  
      In the first embodiment, part of the fuel return pipe  61  branches as the heat exchange pipe  62  incorporated in the heat exchange section  63 . However, the fuel return pipe  61  may be directly guided to the heat exchange section  63 , without separating the heat exchange pipe  62  from the fuel return pipe  61 , thereby transferring heat to the exhaust pipe  57 .  
      It is one example to transmit heat to the exhaust gas to much increase the temperature of the exhaust gas at the heat exchange section  63 . However, it is sufficient even if the temperature of the exhaust gas does not increase. If the temperature of the exhaust gas does not reduce so much until it reaches the exhaust outlet  56 , condensation in the exhaust pipe  57  can be avoided.  
      Referring then to  FIGS. 8 and 9 , a fuel cell unit  71  according to a second embodiment of the invention will be described. In the second embodiment, elements similar to those of the fuel cell unit  1  of the first embodiment are denoted by corresponding reference numbers, and no description is given thereof.  
      As can be seen from  FIG. 8 , the fuel cell unit  71  includes a heat exchange section  72 . The heat exchange section  72  has a double piping structure as shown in  FIG. 9 . Specifically, the heat exchange pipe  62  of the heat exchange section  72  has a large-diameter portion  73  larger than the other portions. The large-diameter portion  73  extends along the exhaust pipe  57 , with the exhaust pipe  57  contained therein. The large-diameter portion  73  permits the anode circulation solution to flow between the outer peripheral surface  57   c  of the exhaust pipe  57  and the inner peripheral surface  73   a  of the large-diameter portion  73 .  
      More specifically, the large-diameter portion  73  permits the anode circulation solution to flow from the downstream end  57   a  of the exhaust pipe  57  to the upstream end  57   b  of the same. Namely, the anode circulation solution in the large-diameter portion  73  flows in a direction opposite to that of the exhaust gas in the exhaust pipe  57 . In other words, the heat exchange section  72  is a so-called counter-flow type heat exchange section. However, the heat exchange section  72  is not limited to this structure, but may be of the parallel-flow type or perpendicular-flow type.  
      The fuel cell unit  71  constructed as the above is substantially prevented from condensation therein. That is, in the fuel cell unit  71 , heat is transferred to the exhaust gas to reduce the relative humidity of the exhaust gas, as in the first embodiment. As a result, the vapor contained in the exhaust gas can be prevented from condensing in the exhaust pipe  57 .  
      Further, in the fuel cell unit  71 , the heat exchange section  72  has a double piping structure, therefore the heat exchange pipe  62  is effectively thermally coupled with the exhaust pipe  57 . Namely, since the exhaust pipe  57  can receive heat from the entire peripheral surface as shown in  FIG. 9 , the heat exchange efficiency of the heat exchange section  72  may be higher than that of the heat exchange section  63  of the first embodiment, which further reliably prevents condensation in the fuel cell unit  71 .  
      Referring then to  FIGS. 10 and 11 , a fuel cell unit  81  according to a third embodiment of the invention will be described. In the third embodiment, elements similar to those of the fuel cell unit  1  of the first embodiment are denoted by corresponding reference numbers, and no description is given thereof.  
      As can be seen from  FIGS. 10 and 11 , the fuel cell unit  81  includes a gas supply mechanism  82 . The gas supply mechanism  82  includes a gas supply pipe  83 . The upstream end of the gas supply pipe  83  diverges from the middle portion of the air supply pipe  35 . The downstream end of the gas supply pipe  83  communicates with the exhaust pipe  57 . Further, as shown in  FIG. 11 , the gas supply pipe  83  extends near the DMFC stack  14 . Part of the gas supply pipe  83  is adjacent to the DMFC stack  14 .  
      In the third embodiment, the gas supply pipe  83  is coupled to the exhaust pipe  57  upstream of the filter  58 . However, the gas supply pipe  83  may be coupled to the exhaust pipe  57  downstream of the filter  58 .  
      The operation of the fuel cell unit  81  will be described.  
      The air guided into the DMFC unit  7  through the air inlet  13   a  is fed by the air feed pump  36  to the DMFC stack  14  via the air supply pipe  36 . Part of the air fed to the DMFC stack  14  is guided to the gas supply pipe  83  diverging from the air supply pipe  35 .  
      The gas supply pipe  83  extends near the DMFC stack  14 . Accordingly, the air flowing through the gas supply pipe  83  receives heat from the DMFC stack  14  when it passes near the DMFC stack  14 .  
      The air in the gas supply pipe  83  is directly guided into the exhaust pipe  57  without passing through the cathode  41 . The relative humidity of the air in the gas supply pipe  83  is substantially the same as that of the atmosphere, since the air does not pass through the cathode cooling section  15 . Namely, the air guided from the gas supply pipe  83  has a lower humidity than the exhaust gas in the exhaust pipe  57 .  
      When air of a lower humidity is guided from the gas supply pipe  83  to the exhaust pipe  57 , the relative humidity of gas in the exhaust pipe  57  is reduced. If, for example, air with a humidity of 50% is mixed into an exhaust gas with a humidity of 100%, the humidity of the exhaust gas in the exhaust pipe  57  is reduced to, for example, 70%. When the humidity of the exhaust gas is reduced, the exhaust gas does not easily condense, and may be exhausted without condensation to the outside of the housing  6  through the exhaust outlet  56 .  
      Furthermore, the air in the gas supply pipe  83  is heated by the DMFC stack  14  when it passes near the DMFC stack  14 . Accordingly, when the air in the gas supply pipe  83  is guided into the exhaust pipe  57 , the temperature of the exhaust gas in the exhaust pipe  57  is increased. At this time, the amount of saturated water vapor of the exhaust gas is increased, and hence the relative humidity in the exhaust pipe  57  is further reduced.  
      The fuel cell unit  81  constructed as the above is substantially prevented from condensation. Namely, air of a lower humidity is mixed into the exhaust gas in the exhaust pipe  57  to dilute the same, thereby reducing the humidity in the exhaust pipe  57 . This substantially prevents condensation in the exhaust pipe  57  even when the temperature of the exhaust gas is somewhat reduced in the exhaust pipe  57 .  
      The heat exchange sections  63  and  72  employed in the first and second embodiments, respectively, differ from the gas supply mechanism  82  in that whether the amount of saturated vapor of the exhaust gas is increased, or the exhaust gas in the exhaust pipe  57  is diluted with dry air. However, the heat exchange sections  63  and  72  and gas supply mechanism  82  are similar in the function of reducing the relative humidity of the exhaust gas, and realize the substantial prevention of condensation in the exhaust pipe  57 , utilizing this function.  
      As one example of the gas supply mechanism, it includes the gas supply pipe  83 . The fuel cell unit  81  of the third embodiment can be realized simply by attaching, for example, the gas supply pipe  83 . This contributes to the cost reduction of the fuel cell unit  81 .  
      Furthermore, since the gas supply pipe  83  extends near the DMFC stack  14 , the exhaust gas is heated by the air guided from the gas supply pipe  83 , thereby more reliably preventing condensation in the fuel cell unit  81 . However, it is not always necessary to locate the gas supply pipe  83  near the DMFC stack  14 . Even if air of the room temperature is mixed into the exhaust gas in the exhaust pipe  57 , the fuel cell unit  81  may be prevented from condensation.  
       FIG. 12  shows a fuel cell unit  85  according to a modification of the third embodiment. As shown, the anode cooling section  16  of the fuel cell unit  85  includes the second cooling fan  66 . One end of a pipe  86  is coupled to the exhaust hole of the second cooling fan  66 . The other end of the pipe  86  extends and opens toward the lateral portion of the exhaust pipe  57  of the DMFC unit  7 . The pipe  86  discharges, to the exhaust pipe  57 , the air exhausted by the second cooling fan  66 .  
      The fuel cell unit  85  constructed as the above can more reliably prevent condensation in the unit. Namely, the radiator fins  67  of the anode cooling section  16  are heated by the anode circulation solution passing through the second condenser  65 . Accordingly, the air around the radiator fins  67  is also heated by the fins  67 .  
      The second cooling fan  66  draws the heated air from around the radiator fins  67 , and sends it to the periphery of the exhaust pipe  57  via the pipe  86 . Namely, the second cooling fan  66  heats the exhaust pipe  57  using the air that has cooled the radiator fins  67 .  
      Thus, the exhaust pipe  57  is heated to thereby increase the temperature of the exhaust gas in the pipe  57 . When the temperature of the exhaust gas is increased, the relative humidity of the exhaust gas is reduced as described above, with the result that condensation is less likely to occur. Note that the pipe  86  can be provided for the second cooling fan  66  regardless of whether the fuel cell unit employs the gas supply mechanism  82 .  
      Referring to  FIG. 13 , a fuel cell unit  91  according to a fourth embodiment of the invention will be described. In the fourth embodiment, elements similar to those of the fuel cell units  1  and  81  of the first and third embodiments are denoted by corresponding reference numbers, and no description is given thereof.  
      As can be seen from  FIG. 13 , the fuel cell unit  91  includes the heat exchange section  63  and gas supply mechanism  82 . That is, the fuel cell unit  91  is the combination of the fuel cell units  1  and  81  of the first and third embodiments.  
      The fuel cell unit  91  constructed as the above is substantially prevented from condensation therein. Namely, in the fuel cell unit  91 , part of the heat generated by the DMFC stack  14  during generation of electricity is transferred to the exhaust pipe  57  to reduce the relative humidity of the exhaust gas, as in the fuel cell unit  1  of the first embodiment.  
      Further, a gas of a lower relative humidity is mixed into the exhaust gas, using the gas supply mechanism  82 , thereby further reducing the relative humidity of the exhaust gas. Consequently, the moisture in the exhaust gas can be effectively prevented from condensing in the exhaust pipe  57 , compared to the first and third embodiments.  
      A fuel cell unit  101  according to a fifth embodiment of the invention will be described with reference to  FIG. 14 . In the fifth embodiment, elements similar to those of the fuel cell units  1 ,  71  and  81  of the first, second and third embodiments are denoted by corresponding reference numbers, and no description is given thereof.  
      As can be seen from  FIG. 14 , the fuel cell unit  101  includes the heat exchange section  72  and gas supply mechanism  82 . That is, the fuel cell unit  101  is the combination of the fuel cell units  71  and  81  of the second and third embodiments.  
      The fuel cell unit  101  constructed as the above is substantially prevented from condensation therein. Namely, in the fuel cell unit  101 , part of the heat generated by the DMFC stack  14  during generation of electricity is transferred to the exhaust pipe  57  to reduce the relative humidity of the exhaust gas, as in the fuel cell unit  71  of the second embodiment.  
      Further, a gas of a lower relative humidity is mixed into the exhaust gas, using the gas supply mechanism  82 , thereby further reducing the relative humidity of the exhaust gas. Consequently, the moisture in the exhaust gas can be effectively prevented from condensing in the exhaust pipe  57 , compared to the second and third embodiments.  
      A fuel cell unit  111  according to a sixth embodiment of the invention will be described with reference to  FIG. 15 . In the sixth embodiment, elements similar to those of the fuel cell units  1 ,  71  and  81  of the first, second and third embodiments are denoted by corresponding reference numbers, and no description is given thereof.  
      As can be seen from  FIG. 15 , the fuel cell unit  111  includes the heat exchange section  63  and a gas supply mechanism  112 . The gas supply mechanism  112  includes a gas supply pipe  113 . The upstream end of the gas supply pipe  113  diverges from the middle portion of the air supply pipe  35 . The downstream end of the gas supply pipe  113  communicates with the exhaust pipe  57 .  
      Further, the gas supply pipe  113  is thermally coupled to the radiator fins  67  of the anode cooling section  16 . Namely, part of the heat of the cathode circulation solution is transferred to the gas supply pipe  113  via the radiator fins  67  to heat the air passing through the gas supply pipe  113 .  
      In the sixth embodiment, the gas supply pipe  113  communicates with the exhaust pipe  57  upstream of the filter  58 . However, the gas supply pipe  113  may communicate with the exhaust pipe  57  downstream of the filter  58 .  
      The fuel cell unit  111  constructed as the above is substantially prevented from condensation therein. Namely, in the fuel cell unit  111 , a gas of a lower relative humidity is mixed into the exhaust gas, using the gas supply mechanism  112 , thereby reducing the relative humidity of the exhaust gas, as in the fuel cell unit  81  of the third embodiment. Consequently, the moisture in the exhaust gas can be prevented from condensing in the exhaust pipe  57 .  
      Further, since the gas supply pipe  113  is thermally coupled to the radiator fins  67 , the exhaust gas is heated by the air mixed therein through the gas supply pipe  113 . As a result, condensation in the fuel cell unit  111  can be further effectively prevented.  
      Although the sixth embodiment employs the heat exchange section  63 , it may employ the heat exchange section  72  shown in  FIG. 16 , instead of the heat exchange section  63 . In addition, the gas supply mechanism  112  may be employed solely without the heat exchange section  63  or  72 .  
      The present invention is not limited to the above-described fuel cell units  1 ,  71 ,  81 ,  91 ,  101  and  111  of the first to sixth embodiments. For instance, the components employed in the first to sixth embodiments may be selectively combined in accordance with the size and/or purpose of the fuel cell unit.  
      Specifically, a gas supply pipe with a dedicated gas inlet and gas feed pump may be employed, instead of the gas supply pipe  83  or  113  diverging from the air supply pipe  35 .  
      The fuel cell unit, to which an embodiment of the invention is applied, is not limited to a DMFC, but may be a fuel cell unit using another alcohol, such as ethanol, or other liquid fuels. The invention is not limited to fuel cell units for portable computers, but is also applicable to those for electronic devices, such as cellular phones or digital cameras, or for vehicles.  
      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.