Abstract:
A fuel cell system is provided which is capable of changing a fuel cartridge while the power generation is active in a fuel cell. For this purpose, the fuel cell system having a fuel cell is provided in a detachable manner, and the system includes: a fuel cartridge which stores fuel supplied to the fuel cell; a fuel sub-tank which stores the fuel delivered from the fuel cartridge; and a buffer tank which stores the fuel which is delivered from the fuel sub-tank and diluted to a predetermined concentration. There is provided a tank communicating passage through which gas freely enters and exits between the top of fuel sub-tank and the buffer tank.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a fuel cell system and, more particularly, to a fuel cell system that generates electric power by supplying the liquid fuel thereto.  
         [0003]     2. Description of the Related Art  
         [0004]     A fuel cell is a device that generates electricity from hydrogen and oxygen so as to obtain highly efficient power generation. A principal feature of a fuel cell is its capacity for direct power generation which does not undergo a stage of thermal energy or kinetic energy as in conventional power generation. This presents such advantages as high power generation efficiency despite the small scale setup, reduced emission of nitrogen compounds and the like, and environmental friendliness on account of minimal noise or vibration. A fuel cell is capable of efficiently utilizing chemical energy in its fuel and as such environmentally friendly. Fuel cells are therefore envisaged as an energy supply system for the twenty-first century and have gained attention as a promising power generation system that can be used in a variety of applications including space applications, automobiles, mobile devices, and large and small scale power generation. Serious technical efforts are being made to develop practical fuel cells.  
         [0005]     Of various types of fuel cells, a polymer electrolyte fuel cell excels in its low operating temperature and high output density. Recently, direct methanol fuel cells (DMFC) are especially attracting the attention as a type of polymer electrolyte fuel cell. In a DMFC, methanol water solution as a fuel is not reformed and is directly supplied to the anode so that electricity is produced by an electrochemical reaction induced between the methanol water solution and oxygen. Discharged as reaction products resulting from the electrochemical reaction are carbon dioxide emitted from the anode and generated water emitted from the cathode. Methanol water solution has a higher energy density per unit volume than hydrogen. Moreover, it is suitable for storage and poses little danger of explosion. Accordingly, it is expected that methanol water solution will be used in power supplies for automobiles, mobile devices (cell phones, notebook personal computers, PDAs, MP3 players, digital cameras, electronic dictionaries and books) and the like.  
       RELATED ART LIST  
       [0000]    
       
          (1) Japanese Patent Application Laid-Open No. 2005-108811.  
       
     
         [0007]     In the fuel cell system, as in Reference (1), containing DMFC, the air serving as an oxidant is supplied to the cathode and therefore the inside of fuel supply passage, including the interior of a buffer tank, at a fuel cell apparatus side is at high pressures. Thus, when the pump stops, there are cases where diluted methanol water solution flows backward into the fuel supply passage. When the methanol water solution flows back into the fuel supply passage, the diluted methanol water solution returns to the buffer tank even if the pump operates next time. This causes a problem where the concentration drops rapidly and the power generation capacity drops. The problem like this also occurs when the air bubble is mixed into the fuel supply passage, and a problem may arise where the air bubble enters the fuel supply passage at the time of replacement of a fuel cartridge and the like.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention has been made in view of the foregoing circumstances and a general purpose thereof is to provide a fuel cell system capable of supplying liquid fuel to a fuel cell. Another advantage of the present invention is that it provides a fuel cell system in which the leakage of fuel from a buffer tank is prevented.  
         [0009]     One embodiment of the present invention relates to a fuel cell system. This fuel cell system is a fuel cell system that supplies liquid fuel to a fuel cell and operates the fuel cell, and the system includes: a first fuel storage, provided detachably, which stores the liquid fuel supplied to the fuel cell; a first fuel supply means which delivers the fuel cell stored in the first fuel storage; a second fuel storage which stores the liquid fuel delivered by the first fuel supply means; a second fuel supply means which delivers the liquid fuel stored in the second fuel storage; and a fuel detection means, provided in the second fuel storage, which detects an amount of the liquid fuel stored in the second fuel storage, wherein a free path through which gas inside the second fuel storage is sucked in and discharged is provided on top of the second fuel storage.  
         [0010]     According to this embodiment, even when the first fuel storage (so-called a fuel cartridge) is removed from the fuel cell system, some fuel is stored in the second fuel storage and therefore the state of power generation can be remained active. Although the air bubble caused when the first fuel storage is mounted or the like may enter the second fuel storage, the air bubble is eliminated in the second fuel storage. Thus such bubble can be prevented from entering the fuel supply means. Since the gas in the second fuel storage flows through the free path, the back flow or the like of the fuel is hardly likely to occur. Hence, the concentration of the fuel supplied to a fuel cell can be kept constant and the fuel cell can be operated stably.  
         [0011]     In the case where the second fuel storage is a container of typical or standard type, a water-level sensor (liquid-level sensor) for detecting the water level inside may serve as the fuel detection means, for example. Also, some mechanism that does not detect the water level constant but one, such as a limiter, capable of detecting whether it is below a predetermined threshold or not may serve as the fuel detection means. Thereby, a case like the imminent situation where the fuel cell system will cease to operate unless the fuel cartridge is replaced anew immediately can be notified to a user.  
         [0012]     In the above embodiment, the free path may be tube-shaped such that one end thereof is open, and the free path may be such that inner volume thereof is greater than or equal to-one-time liquid cell capacity by the first fuel supply means. According to this, the liquid fuel that overflows from the second fuel storage can be retained inside the free path, so that the leakage of liquid to the outside of a system is suppressed.  
         [0013]     In the above embodiment, a vapor-liquid separating structure may be provided in an end of the free path. With the provision of this air-liquid separating structure, the fuel is not leaked out of the free path and the fuel gas of less than or equal to a predetermined concentration is released to the outside, so that the safety of the fuel cell system can be raised. More specifically, the vapor-liquid separating structure may have either an absorber which collects the liquid fuel or a filter which absorbs the fuel in exhaust gas. In the latter case, the filter may be activated carbon.  
         [0014]     In the above embodiment, a fuel cell system may further include a third fuel storage, connected with the second fuel supply means, which mixes emission material from the fuel cell with the liquid fuel delivered by the second fuel supply means, which prepares and stored a fuel to be supplied to the fuel cell, and which discharges unwanted gas component to the outside of the fuel cell system, wherein the end of the free path may be connected with a gaseous layer part of the third fuel storage.  
         [0015]     By employing this structure, when the fuel cell is a fuel cell of the type such as DMFC, in which the liquid fuel is directly supplied to the fuel cell, the liquid fuel, whose concentration of fuel has been reduced, and the carbon dioxide are discharged from the anode of the fuel cell whereas the air, whose concentration of oxygen has been reduced, and the water are discharged from the cathode thereof. In the third fuel storage, the liquid components of this emission material (the liquid fuel whose concentration of fuel has been reduced and the water) and the fuel from the second fuel storage are mixed together, and the liquid gas components of the emission material (the carbon dioxide and the air whose concentration of oxygen has been reduced) are released to the outside. Thus the fuel can be utilized efficiently.  
         [0016]     It is to be noted that any arbitrary combination or rearrangement of the above-described structural components and so forth are all effective as and encompassed by the present embodiments.  
         [0017]     Moreover, this summary of the invention does not necessarily describe all necessary features so that the invention may also be sub-combination of these described features. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     Embodiments will now be described by way of examples only, with reference to the accompanying drawings which are meant to be exemplary, not limiting and wherein like elements are numbered alike in several Figures in which:  
         [0019]      FIG. 1  is a perspective view of a fuel cell system according to a first embodiment of the present invention;  
         [0020]      FIG. 2  schematically illustrates a structure of a fuel cell system according to a first embodiment of the present invention;  
         [0021]      FIG. 3  illustrates in detail a structure of a sub-tank in a fuel cell system according to a first embodiment of the present invention;  
         [0022]      FIG. 4  schematically illustrates a structure of a fuel cell system according to a second embodiment of the present invention;  
         [0023]      FIG. 5  schematically illustrates a gas circulation tube;  
         [0024]      FIG. 6  schematically illustrates a structure of a fuel cell system according to a third embodiment of the present invention;  
         [0025]      FIG. 7  illustrates a configuration, having a vapor-liquid separation structure, in a fuel cell system according to a third embodiment of the present invention; and  
         [0026]      FIG. 8  illustrates another structure of a vapor-liquid separator used in a fuel cell system according to a third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]     The invention will now be described by reference to the preferred embodiments. This does not intend to limit the scope of the present invention, but to exemplify the invention.  
         [0028]     A structure of a fuel cell system  100  according to the present invention will now be described in detail with reference to drawings.  
       First Embodiment  
       [0029]      FIG. 1  is a perspective view of a fuel cell system  100  according to a first embodiment of the present invention.  FIG. 2  illustrates a structure of the fuel cell system  100 . A fuel cell system  100  according to the present embodiment uses methanol as a liquid fuel and generates electric power in a fuel cell by an electrochemical reaction induced between this methanol and air serving as an oxidant. This system is a so-called a direct methanol fuel cell (DMFC), and the overall dimension thereof is configured compactly so that it can be suitably used as a power supply for a portable notebook personal computer or the like.  
         [0030]     The fuel cell system  100  is structured such that inside a casing  10  as shown in  FIG. 1 a  stack of fuel cells  20  is mounted on one side thereof in a longitudinal direction, a fuel cartridge  30  connected detachably with the fuel cell system  100  is mounted on-the other side, and an auxiliary unit  40  is mounted approximately in the center. A control unit and a secondary cell, both not shown in  FIG. 1 , are provided within a cradle  50  on which a notebook personal computer is placed.  
         [0031]     Adjacent to the fuel cartridge  30  there are provided a fuel sub-tank  80 , which is a second fuel storage, and a buffer tank  90 , which is a third fuel storage. Pure methanol or high concentration methanol water solution stored in a fuel bag (first fuel storage) within the fuel cartridge  30  is introduced into the buffer tank  90  via the fuel sub-tank  80  and then diluted, by the buffer tank  90 , into a predetermined concentration of 1 mol/L. That is, the fuel sub-tank  80  has functions of not only detecting that the fuel level has reached the zero but also eliminating the air (gas components) mixed in with the fuel supply passages  34  and  82  at the time when the fuel cartridge  30  is mounted or removed. Also, the buffer tank  90  has not only a function of adjusting the concentration of fuel but also a function of a vapor-liquid separator in which the gas components discharged from the fuel cell  20  is discharged externally from said fuel cell system  100  (the detail will be discussed later).  
         [0032]     The auxiliary unit  40  includes a methanol pump (first liquid pump)  41  which supplies fuel from the fuel bag  32  to the fuel sub-tank  80 , a methanol pump (second liquid pump)  42  which supplies fuel from the buffer tank  90  to the fuel cell  20 , a methanol pump (third liquid pump)  43  which supplies fuel from the buffer tank  90  to the fuel cell  20 , and an air pump  44  which supplies oxygen, which is air in the present embodiment. This auxiliary unit  40  is mounted between the fuel cell  20  and the fuel storage comprised of the fuel cartridge  30 , the fuel sub-tank  80  and the buffer tank  90 . This structure is implemented for the purpose of having minimum length of the fuel supply passages  34 ,  82 ,  84 ,  91  and  92  to save the space and increase the space utilization efficiency and supplying promptly the high concentration methanol supplied intermittently to the fuel cell  20 .  
         [0033]     The auxiliary unit  40  includes: a vapor-liquid separator  45  which mixes an anode discharge (waste methanol+carbon dioxide) composed principally of the liquid discharged from an anode  21  side of a fuel cell  20  with a cathode discharge (exhaust air+generated water) composed principally of gas discharged from a cathode  22  side thereof and separates the mixture into a gas component and a fluid component; and a cooler  46  which distributes the gas component and the fluid component separated by the vapor-liquid separator through different pipings and cools the discharges of the fuel cell  20  by a cooling fan  47  that discharges the air inside the fuel cell system  100 . And the auxiliary unit  40  including the vapor-liquid separator  45  and the cooler  46  is mounted between the fuel cell  20  and the buffer tank  90 . In this manner, the vapor-liquid separator  45  having a function of separating vapor and liquid is installed before the buffer tank  90  as well as the cooler  46  (namely, located upstream of the buffer tank  90 ). Thus, the anode discharge and the cathode discharge in which liquid and gas are mixed are united and then the liquid component and the gas component are distributed respectively to a liquid component passage  46   a  and a gas component passage  46   b  so as to be cooled, so that the heat exchange efficiency can be improved as compared to when the fluid with the gas and the liquid mixed together is cooled.  
         [0034]     The gas component out of the discharges of the fuel cell  20  recovered by the buffer tank  90  is released, outside the fuel cell system  100 , through a gas component exhaust passage  93 . In such a case, the gas component exhaust passage  93  is provided as long as possible so that the gas component is not released outside, and it is preferred that an exhaust filter  94  be provided at an exist. In view of possibility that the amount of generated water produced by the fuel cell  20  is greater than the amount of water vapor discharged from the buffer tank  90  and therefore the fuel (methanol water solution) circulating within the fuel cell system  100  overflows, the buffer tank  90  and the fuel sub-tank  80  are connected with each other through a piping (tank communicating passage  95 ) provided above them. When the buffer tank  90  overflows, the fuel sub-tank  80  plays a role of the buffering in the buffer tank  90  and at the same time the fuel is supplied from the fuel cartridge to the fuel sub-tank  80 . And if the pressure in the fuel sub-tank  80  rises temporarily, the buffer tank  90  plays a role of allowing the pressure of the fuel sub-tank  80  to escape. A check valve  96  is provided between the fuel sub-tank  80  and the buffer tank  90 , and it is so structured that the diluted methanol water solution does not flow backward from the fuel supply passage  91  to the fuel supply passage  84 , namely from the buffer tank  90  to the fuel sub-tank  80 , unless it overflows via the tank communication passage  95 . Cartridge joints  36  and  86  are provided between the fuel bag  32  and the fuel sub-tank  80 , and the fuel supply passage  34  and the fuel supply passage  82  are connected to each other through these cartridge joints  36  and  86 . In order that a safety mechanism for recovering the leakage of fuel, at the time of inserting or removing a cartridge, a locking mechanism for the joint or the like mechanism can be provided in the housing main body side, this joint part is such that the cartridge joint  36  in the fuel cartridge  30  side is a male whereas the cartridge joint  86  in the fuel sub-tank  80  side is a female. In this manner, the female structure allows the assembly of more complicated mechanisms and realizes a simple structure in the fuel cartridge  30  side, thus achieving advantageous aspects in terms of the size and cost.  
         [0035]     To detect the status of whether-the fuel cartridge is being inserted or removed, a limiter LT is provided in the housing main body of the fuel cell system  100  in contact with the fuel cartridge  30 . This structure allows detecting whether the fuel cartridge  30  is normally fit into the fuel cell system  100  or not, so as to make sure that the fuel is not leaked from the cartridge joints  36  and  86  while in use. The means for detecting whether it is inserted or removed is not limited to the limiter LT, and a structure may be such that an IC chip or the like is embedded in a predetermined position of the fuel cartridge  30  so as to detect the position of the IC chip and at the same time the information, on the fuel cartridge, such as volume, concentration, fuel type and serial number is communicated between the fuel cell system  100  and the control unit.  
         [0036]     The fuel supply passage  34  in the fuel cartridge  30  has its inlet positioned in the bottom of the fuel bag  32 , and is so arranged as to move upwards along the side of wall within the fuel cartridge  30  and is then connected with the cartridge joint  36 . A fuel confirmation window is set up in an upper part, namely part of upper hem, of the fuel cartridge so that the fuel supply passage  34  is visible. It is desirable that transparent material such as Teflon (registered trademark) tube be used to form the fuel supply passage to confirm the interior of the fuel supply passage  34  from this fuel confirmation window  38 . The fuel bag  32  is a container such that the volume thereof can be varied and a small amount of gas such as air is presealed inside together with the fuel. Hence, when the remaining fuel stored in the fuel bag  32  gets small, the boundary between liquid phase and gaseous phase can be visibly verified. The confirmation will be further facilitated if the fuel is colored beforehand.  
         [0037]     The above flow of the fuel is summarized as follows. The high concentration methanol or pure methanol in the fuel bag  32  is distributed through the fuel supply passage  34  so as to be supplied to the housing main body of the fuel cell system  100 . The fuel cartridge  30  and the housing main body are connected with each other by way of the cartridge joints  36  and  86 , and the high concentration methanol in the fuel bag  32  is supplied to the fuel sub-tank  80  by the suction force of the methanol pump  41  provided in the fuel supply passage  82  which is connected from the cartridge joint  86  to the fuel sub-tank  80 . If the gas is mixed in with the fuel supply passages  34  and  82  from the cartridge joints  36  and  86  at the time then the fuel cartridge  30  is mounted or removed, such gas is eliminated by this fuel sub-tank  80 . Hence, a structure is such that such gas as air bubbles is not mixed into a buffer tank  90  side from the fuel sub-tank  80 .  
         [0038]     As shown in  FIG. 3 , a liquid-level sensor  81  that detects fuel runout is provided in the fuel sub-tank  80  in a position where it is located ½ or more of the height a of the tank. A structure is such that when it is detected that the water level of fuel in the fuel sub-tank  80  has reached the position of the liquid-level sensor  81  or below, the methanol pump  41  is driven and high-concentration fuel is added to the fuel sub-tank  80  from the fuel bag  32 . If the water level of fuel in the fuel sub-tank  80  is not recovered after the methanol pump  41  has been driven for a predetermined period of time, an alarm will be displayed to a user indicating that the fuel has run out. If the fuel water level of the fuel cartridge  30  is not recovered even after the time for replacement set for the change of the fuel cartridge  30  has passed and the alarm to a user indicating that the fuel has run out was issued, the system will cease to operate. It is necessary that this liquid-level sensor  81  is provided in a position such that a sufficient amount of fuel to operate a system is still stored while the fuel cartridge  30  is replaced anew. In the present embodiment, the time required for the cartridge replacement is set to approximately 5 minutes, and the amount of fuel needed to operate the system while the cartridge is replaced anew is about 5 cc. Also, in the present embodiment the liquid-level sensor  81  is placed in a position where it is located at ½ or more of the height of the tank. The high concentration methanol in the fuel sub-tank  80  is supplied to the buffer tank  90  by the suction force of the methanol pump  42  provided in the fuel supply passage  84 . The fuel supply passage  84  is connected with the fuel supply passage  91  by way of the check valve  96 , and a structure is such that, as a steady state, the diluted methanol water solution in the buffer tank  90  side does not flow back into the fuel sub-tank  80  from the check valve  96 .  
         [0039]     In the sub-tank  80 , a gas intake/exhaust opening  101  communicated with the tank communicating passage  95  is provided on the top surface of a container, thus realizing a structure such that the gas inside the container can freely enter and exit. By employing this structure, the pressure inside the container does not get pressurized nor becomes negative-pressure even when the liquid level varies. As a result, the safety of the fuel sub-tank  80  is raised. Also, abnormal operations where, for example, the liquid fuel flows back into the fuel supply passage  82  and the liquid fuel flows into the fuel supply passage  84  at the timing other than a predetermined timing can be prevented.  
         [0040]     A description will now be given of variation in the liquid level of fuel in the fuel sub-tank  80 . According to the state of power generation in the fuel cell  200 , a predetermined amount v of liquid fuel is supplied intermittently to the fuel cell  20  from a fuel exhaust port  104  located at an end of the fuel supply passage  84 . In what is to follow, assume that the fuel added amount at one time is v and the amount of liquid level of fuel that descends in the container at one time of fuel discharge is y. When it is detected that the water level of fuel in the fuel sub-tank  80  has become the height of the liquid-level sensor  81  or below as a result of the fuel discharge, the methanol pump stops and then the methanol pump  41  starts. As a result, the liquid fuel is added from a fuel filler port  102  located at an end of the fuel supply passage  82  so that the water level of fuel in the fuel sub-tank  80  is in a position of (½)·y above the height of the liquid-level sensor  81 . In this manner, the methanol pump  41  and the methanol pump  42  intermittently operate at asynchronous timing, and the water inside the fuel sub-tank  80  varies within a range of y wherein y indicates a range of variation in the liquid level. The methanol pump  41  and the methanol pump  42  are controlled in an asynchronous manner. Thus, the fuel cell  20  can be operated without disabling the supply of liquid cell even in the case when the quantity accuracy of the methanol pump  41  is low due to the disturbance such as variation in pressure of the methanol pump  41  required of fuel discharge from the fuel cartridge  30  as a result of variation in the remaining amount of the liquid fuel and the time degradation.  
         [0041]     It is also preferred that the gas intake/exhaust opening  101  be provided in a position higher by at least y from the maximum point of the liquid level variation range y. That is, with reference to  FIG. 3 , it is preferable that the length H 1 , which is the distance from the maximum point of the liquid level variation range y to the gas intake/exhaust-opening  101 ), is greater than y. With this configuration, a gas layer can be kept in an upper part of fuel in the fuel sub-tank  80  even when the fuel is added once by the methanol pump  41 . Moreover, when the diameter of the fuel sub-tank  80  is denoted by x, it is preferable that the gas intake/exhaust opening  101  is provided in a position higher than the maximum point of the liquid level variation range y by at least x·tan θ. That is, with reference to  FIG. 3 , the length H 1  is greater than x·tan θ. The angle θ is preferably 45 degrees. By employing this structure, in a case when the fuel sub-tank  80  is tilted by the angle θ and at the same time the gas intake/exhaust opening  101  is soaked in the liquid fuel, the resulting lowered level of the liquid fuel is not detected by the liquid-level sensor  81 . Thus, the operation stability in the fuel cell system  100  can be improved.  
         [0042]     It is also preferred that the fuel filler port  102  be provided in a position higher than the maximum point of the range y of variation in the normal liquid level of the liquid fuel in the container. By implementing this structure, the capacity of fuel which can be stored in the container can be secured and at the same time the adverse effect on the liquid level variation range y can be reduced.  
         [0043]     It is also preferred that the fuel exhaust port  104  be provided in a position as closer to the bottom of the container as possible. According to this structure, the fuel in the container can be discharged in an optimal and useful manner. It is also preferred that the fuel exhaust port  104  be provided in a position lower than the minimum point of the liquid level variation range y by at least y. That is, with reference to  FIG. 3 , it is preferable that the length H 2 , which is the distance from the minimum point of the liquid level variation range y to the fuel exhaust port  104 ), is greater than y.  
         [0044]     By employing this structure, the fuel cell  20  can be operated even in a period during which the fuel cartridge  30  is being replaced anew. It is also preferable that the fuel exhaust port  104  be provided in a position lower than the minimum point of the liquid variation range y by at least 3×y. According to this structure, it is possible to operate the fuel cell  20  during the time required for the replacement of the fuel cartridge  30  three times.  
         [0045]     Example of Positions Where the Liquid-Sensor is Set  
         [0046]     If the gas intake/exhaust opening  101  is in a position higher than the liquid variation range y by y and the fuel exhaust port  104  is in position lower than the liquid variation range y by y, the height of the container of the fuel sub-tank  80  will be about y+y+3y=5y. The height of the liquid-level sensor  81  will be about (½)·y+3y=3.5y. That is, the height of the liquid-level sensor  81  corresponds to the position of (3.5/5)=0.7 if the height of the container is used as a benchmark.  
         [0047]     Similar to the fuel sub-tank  80 , a liquid-level sensor LS 2  that detects water shortage (fuel starvation) is also provided in the buffer tank  90  in a position where it is located ⅓ or more of the height of the tank. When it is detected that the water level of fuel in the buffer tank  90  has reached the position of the liquid-level sensor LS 2  or below, a signal is transmitted to the control unit  60  which in turn controls each device in the fuel cell system  100  in a manner such that the amount of generated water produced from the fuel cell  20  increases, namely, high currents are outputted from the fuel cell  20 . Conversely, in a case where the overflow occurs in the buffer tank  90 , the fuel is introduced into the fuel sub-tank  90  through the tank communicating passage  95 . This tank communicating passage  95  is structured such that, as a steady state, no difference in pressure of gaseous phase part occurs between the fuel sub-tank  80  and the buffer tank  90  in the event that the pressure of gaseous phase part in the fuel sub-tank  80  rises as a result of the insertion/removal of the fuel cartridge  30  and so forth.  
         [0048]     The diluted methanol water solution in the buffer tank  90  is supplied to the fuel cell  20  by the suction force of the methanol pump  43  provided in the fuel supply passage  92 . In the fuel supply passage  92 , a fuel filter  97  is provided anterior to the methanol pump  43 . This fuel filter  97  removes or absorbs the impurities such as contaminants or cations mixed in with the methanol solution water so as to be supplied to the anode of the fuel cell  20 . Although it is acceptable that the fuel filter  97  is installed posterior to the methanol pump  43 , the contaminants or the like in the methanol pump can be avoided if the fuel filter  97  is provided anterior thereto. Different from the other methanol pumps  41  and  42 , this methanol pump  43  runs almost all the while the fuel cell system  100  is in operation. Hence, it is desirable that the methanol filter  97  be installed posterior thereto in consideration of the contaminants or the like.  
         [0049]     On the other hand, the air is supplied to the cathode of the fuel cell  20  by an air pump  44 . The air pump  44  suctions the air inside the fuel cell system  100 . However, since a filter by which to remove particle components such as contaminants is provided in an opening (not shown) that introduces the air outside the fuel cell system  100  into the interior thereof, the organic matters in the air are removed by catalytic combustion in a posterior side of the air pump  44  in an oxidant supply passage  23  or an air filter  24  that absorbs the cations is provided.  
         [0050]     The waste methanol and carbon dioxide discharged from the anode  21  side of a fuel cell  20  are discharged from an anode exhaust passage  25  to the vapor-liquid separator  45 , and at the same time the exhaust air and generated water discharged from the cathode  22  side are discharged from a cathode exhaust passage  26 . They join together at the vapor-liquid separator  45 . The mixture is separated into a liquid component and a gas component. Then, while the liquid component is distributed through the liquid component passage  46   a  and the gas component is distributed through the gas component passage  46   b , the liquid component and the gas component are cooled by the air in the fuel cell system  100  forcibly discharged by the cooling fan  47  and are each introduced into the buffer tank  90 . That is, since the buffer tank  90  recovers the waste methanol and the generated water where the methanol is consumed by the electric power cell reaction, the concentration of methanol in the buffer tank  90  drops. When the methanol concentration in the buffer tank  90  drops, the variation is caused in the voltage of a plurality of cells that constitute the fuel cell  20 . Thus, according to the present embodiment, this variation is detected which in turn serves as a concentration sensor instead. And when it is detected that the voltage of a plurality of cells has become equal to or greater than a predetermined value of variation, a signal is sent to the control unit  60 . As a result, the control unit  60  drives the methanol pump  42  and replenishes the buffer tank  90  with the fuel from the fuel tank  80  so as to adjust the methanol concentration of the buffer tank  90 . In cooperation with the methanol pump  42 , the methanol pump  41  may operate in such a manner that the same amount of high concentration methanol is refilled to the fuel sub-tank  80  from the fuel bag  32  every time the methanol pump  42  is driven. Also, to reduce the energy consumption by the auxiliaries, a structure may, for example, be such that when the methanol pump  42  is driven three times, the three-fold amount of the high concentration methanol is refilled to the fuel sub-tank  80  from the fuel bag  32 .  
       Second Embodiment  
       [0051]      FIG. 4  schematically illustrates a structure of a fuel cell system according to a second embodiment of the present invention. The basic structure of a fuel cell system  100  according to the second embodiment is the same as that according to the first embodiment. Hereinbelow, a description will be given of a structure different from the first embodiment.  
         [0052]     In place of the tank communicating passage  95  in the first embodiment, as a free path there is provided a tube-shaped gas circulation tube  200  whose one end is open.  
         [0053]      FIG. 5  schematically illustrates a gas circulation tube  200 . The gas circulation tube  200  is a small-diameter tube (for instance, the inner diameter φ being about 1.5 mm and the length thereof being about 300 mm) having the length sufficient to store the liquid fuel. For example, it is preferable that the volume inside the gas circulation tube  200  be greater than or equal to a volume v. According to this, the liquid fuel that overflows from the fuel sub-tank  80  can be retained inside the gas circulation tube  200 , so that the leakage of liquid to the outside of a system is suppressed.  
         [0054]     With the material and thickness of the gas circulation tube  200  chosen appropriately, a structure is achieved such that the liquid material evaporated inside the gas circulation tube  200  evaporates gradually from the surface thereof. Examples of material for the tube structured as such include a silicon tube whose thickness is less than or equal to 0.5 mm and a low-density polyethylene tube.  
       Third Embodiment  
       [0055]      FIG. 6  schematically illustrates a structure of a fuel cell system according to a third embodiment of the present invention. The basic structure of a fuel cell system  100  according to the third embodiment is the same as that according to the second embodiment. Hereinbelow, a description will be given of a structure different from the second embodiment.  
         [0056]     In the fuel cell system  100  according to the third embodiment, a vapor-liquid separation structure  210  is provided at the tip of a gas circulation tube  200 . With the provision of the vapor-liquid separation structure  210 , the fuel gas of a predetermined concentration or less is released to the outside without the leakage of the liquid fuel from the gas circulation tube  200 . As a result, the safety of the fuel cell system  100  can be raised.  
         [0057]      FIG. 7  illustrates a configuration of the vapor-liquid separation structure  210 . In the present embodiment, the tip of the gas circulation tube  200  is inserted into a tubular fuel collector  220 . The fuel collector  220  includes a container  222 , absorbent  224  and activated carbon  226 . More specifically, the sponge-like absorbent  224  is provided around the gas circulation tube  200  inserted into the container  222 , and the activated carbon  226  is provided on the top of the absorbent  224 , namely, in the vicinity of an inlet of the container  22 . By implementing this structure, the liquid fuel leaked out of the gas circulation tube  200  stays once in the bottom of the container  22 , thus inhibiting the leakage of the fuel. Also, the fuel left uncollected by the absorbent is acceleratedly evaporated by the activated carbon having a large surface area, and it is evaporated to a degree that the concentration thereof does not exceed a predetermined concentration (for example, 200 ppm).  
         [0058]      FIG. 7  illustrates another configuration of the vapor-liquid separation structure  210 . In this configuration, a fuel collector  220  is placed near heat generating units such as a fuel cell and a heat exchanger. In the fuel collector of this configuration, an activated carbon  226  is provided around a gas circulation tube  200  inserted into a container  222 , and absorbent  224  is further provided around the activated carbon. By employing this structure, the liquid fuel having reached the tip of the gas circulation tube  200  is evaporated, in an accelerated manner, thanks to the heat of the heat generating units. And it is evaporated so that the concentration thereof does not exceed a predetermined concentration (for example, 200 ppm). Also, the fuel left unevaporated is absorbed by the absorbent  224 , so that the leakage of fuel is hardly likely to occur.  
       INDUSTRIAL APPLICABILITY  
       [0059]     In the present embodiments, a description has been given of a fuel cell system formed by DMFC using the methanol as liquid fuel. However, the type of a fuel system, for directly supplying the liquid fuel, to which the present invention can be applied is not limited to the DMFC system. A description has been given of a fuel cell system of such a form that uses a personal computer as a load, but the present invention can be applied to a fuel cell system that can be used in various types of equipment particularly portable equipment.