JP2012248493A - Fuel cell system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a system that manages a fuel concentration and a liquid level in a mixing tank in order to continue a stable power generation at a desired output for which DMFC is required.SOLUTION: A fuel cell system includes: a mixing tank for storing fuel; high concentration fuel supply means for supplying high concentration fuel to the mixing tank; water supply means for supplying water to the mixing tank; concentration detection means for detecting a concentration of fuel supplied to a fuel cell; liquid surface detection means for detecting a liquid surface position of the mixing tank; and control means. When the liquid surface position detected by the liquid surface detection means is located above a predetermined liquid surface position, the control means outputs a supply command signal only to the high concentration fuel supply means on the basis of the concentration detected by the concentration detection means. When the liquid surface position detected by the liquid surface detection means is located below the predetermined liquid surface position, the control means outputs the supply command signal to only one of the high concentration fuel supply means and the water supply means on the basis of the concentration detected by the concentration detection means.

Description

  The present invention relates to a fuel cell using a liquid organic compound as a fuel, and to a method for controlling the amount and concentration of fuel circulated and supplied to the fuel cell.

  In a fuel cell using a liquid organic compound as a fuel, a solid polymer fuel cell using a liquid organic compound such as methanol, ethanol, dimethyl ether as a fuel has low noise and a low operating temperature (about 70 to 80 ° C.). It has features such as easy refueling. Therefore, a wide range of uses are expected as a portable power source, a power source for an electric vehicle, or a power source for an electric motorcycle, an assist type bicycle, a light vehicle such as a medical care wheelchair or a senior car.

  Among these applications, a direct methanol fuel cell (hereinafter referred to as DMFC) using methanol as a fuel can omit a reformer, can supply fuel at room temperature, and has a fuel cost relative to output such as gasoline. Since it can generate power at a relatively low temperature of 50 to 70 ° C., it has advantages such as a short start-up time. In particular, an active DMFC that forcibly circulates fuel by a pump or the like can obtain a high output of several tens of watts to several hundred watts, and is suitable for feeding relatively low power devices such as electronic devices and lighting fixtures. In addition, if a DMFC of 1 kW or more is used due to an increase in cell size and an increase in the number of stacked cells, it can also be applied to a moving body.

  In the active DMFC, an aqueous fuel solution is circulated and supplied to the fuel cell by a circulation means. In such a circulation type fuel cell system, since fuel and water are consumed by the power generation reaction, the fuel concentration of the aqueous fuel solution supplied to the anode electrode is maintained within an appropriate range, and the fuel is sent to the anode electrode. It is important to maintain an appropriate amount of liquid in the mixing tank. Conventionally, as a system for controlling the fuel concentration and the liquid amount, for example, in Patent Document 1, the concentration of the mixed liquid is controlled by operating the fuel supply amount from the high concentration fuel tank and the water supply amount from the water tank. A system to do this is proposed.

JP 2005-11633 A

  The present invention overcomes the following technical problems. In the following description, methanol is used as an example of the liquid organic compound, but the present invention can also be applied to fuel cells using other liquid organic fuels such as ethanol.

  The problem to be solved by the present invention is to appropriately control the amount of methanol or water supplied to the mixing tank in accordance with the required power generation output of the fuel cell, changes in the fuel cell temperature and the outside air temperature.

The DMFC battery reaction (formula 1) is the sum of the half-cell reaction formula of the oxidation reaction (formula 2) on the anode (fuel electrode) and the reduction reaction (formula 3) on the cathode (air electrode). It means that 1 mol of CO ₂ is generated by giving and receiving 6 electrons per hit.
CH ₃ OH + 3 / 2O ₂ → CO ₂ + 2H ₂ O (Formula 1)
CH ₃ OH + H ₂ O → CO ₂ + 6H ⁺ + 6e ⁻ (Formula 2)
3 / 2O ₂ + 6H ⁺ + 6e ⁻ → 3H ₂ O (Formula 3)
Therefore, when the DMFC current is changed to obtain the required output, the amount of methanol and water consumed in power generation and the amount of CO ₂ produced are changed. Theoretically, the amount of methanol and water consumed by power generation is determined from the above reaction formula. Based on the relationship between the amount of methanol and water consumed and the methanol concentration in the mixing tank, methanol and water are supplied to the mixing tank. The amount of liquid and the methanol concentration can be maintained within a predetermined range.

  However, in practice, it is difficult to estimate the amount of methanol and water consumed by power generation due to the following factors.

In DMFC, it is known that methanol present in the anode permeates the electrolyte membrane, moves to the cathode, and a direct oxidation reaction of oxygen and methanol occurs on the cathode.
This is a so-called methanol crossover phenomenon. When this occurs, the potential of the cathode is remarkably lowered, and the voltage across terminals and the output are lowered in the whole battery, which is a problem.

  In this phenomenon, if the concentration of methanol supplied to the DMFC is lowered, the amount of methanol crossover can be relatively reduced. However, under the condition where high output is required, the output decreases due to fuel shortage. Further, when the methanol concentration is high, the output decrease due to methanol crossover becomes a large ratio to the output under low output conditions. That is, it becomes a system with low fuel utilization efficiency.

  Similarly, the water present in the anode also permeates the electrolyte membrane, and the amount varies depending on the current and temperature during power generation. The temperature is affected by changes in current value (or output), outside air temperature, or cell resistance over time. In particular, since the temperature of the DMFC changes transiently before and after the power generation output changes, the amount of methanol crossover and the amount of water permeation until reaching a stable temperature are likely to change.

  In this way, the amount of methanol crossover and the amount of water permeation vary depending on the operating conditions during power generation, such as changes in output and temperature, and the concentration and amount of methanol in the methanol aqueous solution in the mixing tank vary significantly from theoretical values. It will be. Therefore, for example, when the liquid volume is managed based on the estimated value, it takes time to converge the methanol concentration to the desired concentration with the fluctuation of the liquid volume, or to converge to the desired concentration If methanol or water continues to be supplied, the amount of liquid in the mixing tank increases significantly, and the system may not be established. This also reduces the power generation efficiency of the fuel cell.

  In particular, when the output area of the DMFC is increased to 100 W or more, the consumption of methanol and water also increases. Therefore, the fuel in the mixing tank having a low methanol concentration causes a problem that the fluctuation of the liquid level and concentration becomes large. This problem can be dealt with by increasing the volume of the mixing tank and increasing the volume of fuel to be supplied. However, in a DMFC system that is portable rather than stationary, it is not preferable to increase the size of the system.

  In conventional systems such as Patent Document 1, sufficient studies have not been made on the influence of fluctuations in methanol crossover amount and water permeation amount in the control of the methanol concentration and the liquid amount in the mixing tank.

  An object of the present invention is to provide a system for managing the fuel concentration and the liquid level in the mixing tank so that the DMFC can continue the stable power generation at the desired output required.

  Accordingly, the inventors have intensively studied a method for managing the methanol concentration and the liquid level in the mixing tank that enables the DMFC to stably generate power at a desired output, and established a fuel cell system equipped with a novel control system.

  According to the present invention, a polymer electrolyte fuel cell using a liquid organic compound as a fuel, a mixing tank for storing fuel to be supplied to the fuel cell, and a main component of the organic compound than the fuel to be supplied to the fuel cell A high-concentration fuel supply means for supplying a high-concentration fuel having a high concentration to the mixing tank; a water supply means for supplying water to the mixing tank; and a concentration of the fuel supplied from the mixing tank to the fuel cell. Concentration detecting means, liquid level detecting means for detecting the position of the liquid level in the mixing tank, and the high concentration fuel supply means and the water based on information input from the concentration detecting means and the liquid level detecting means. And a control means for outputting a supply command signal to the supply means, wherein the control means has a position of the liquid level detected by the liquid level detection means exceeding a predetermined position of the liquid level. In this case, a supply command signal is output only to the high-concentration fuel supply means based on the concentration detected by the concentration detection means, and the position of the liquid level detected by the liquid level detection means falls below a predetermined liquid level position. In this case, a fuel cell system is provided in which a supply command signal is output to only one of the high-concentration fuel supply unit and the water supply unit based on the concentration detected by the concentration detection unit.

  ADVANTAGE OF THE INVENTION According to this invention, the control method which can continue a driving | operation of a fuel cell stably with a desired output can be provided.

The structure of the fuel cell system of this invention is shown. The flowchart in the 1st Embodiment of this invention is shown. An example of the driving | running method by the 1st Embodiment of this invention is shown. An example of the driving | running method by the 1st Embodiment of this invention is shown. The flowchart in the 2nd Embodiment of this invention is shown. An example of the driving | running method by the 2nd Embodiment of this invention is shown.

  Embodiments according to the present invention will be described below, but the present invention is not limited to the following embodiments.

  FIG. 1 shows the configuration of the DMFC system 101. There is a DMFC main body 102 at the center of the system 101. A part of the methanol aqueous solution stored in the mixing tank 108 is supplied to the anode of the DMFC main body 102 via the fuel circulation line 105 by the fuel circulation pump 109. At the anode, methanol is oxidized (formula 2), and then the unreacted aqueous methanol solution is returned to the mixing tank 108 again. Air is supplied to the cathode of the DMFC main body 102 from a fan or other air supply means 111 to generate water (Formula 3). The hydrogen ions are the hydrogen ions generated when methanol is oxidized at the anode and have passed through the electrolyte membrane (Formula 2). In this manner, the DMFC main body 102 generates power by supplying the methanol aqueous solution to the anode and the air to the cathode.

  In addition, the methanol aqueous solution stored in the mixing tank 108 decreases in storage amount and concentration with power generation. Therefore, the mixing tank 108 includes a methanol container 103 and a water container 107 for supplementing methanol and water. The methanol stored in the methanol container 103 may be 100% methanol, but generally a higher concentration aqueous methanol solution is used than the fuel used for power generation of the DMFC main body 102 diluted with water. Its concentration is approximately 40% or more. A required amount of methanol is introduced into the mixing tank 108 by the fuel supply means 104 including a valve and a pump. Further, the water stored in the water container 106 is introduced into the mixing tank 108 by the water supply means 107 including a valve and a pump.

  In the mixing tank 108, it is important to maintain the methanol concentration of the aqueous methanol solution supplied to the main body of the DMFC 102 within an appropriate range and to maintain the liquid amount in the mixing tank 108 at an appropriate level. In order to control the methanol concentration and the liquid amount, a concentration detecting means 112 and a liquid level detecting means 113 for detecting the methanol concentration are provided. Concentration detection means for detecting the concentration of methanol is to measure the concentration, temperature, voltage, current, etc. of various concentration detection elements such as optical, ultrasonic, and surface acoustic wave types, DMFC itself, and use the measured values to determine the concentration. And a desired one can be selected. Further, the liquid level detection means 113 has a function of determining whether the fuel level in the mixing tank 108 is greater than or less than a predetermined position. For example, the position of the liquid is determined by outputting to the automatic control mechanism 120 a signal such as “HIGH” if it is greater than or equal to a predetermined position and “LOW” if it is less than the predetermined position.

  The fuel supply unit 104 and the water supply unit 107 operate according to the methanol concentration detected by the concentration detection unit 112 and the liquid level detected by the liquid level detection unit 113. The fuel supply unit 104 and the water supply unit 107 are controlled by an automatic control mechanism 120 such as a microcomputer based on information input from the fuel supply unit 104 and the water supply unit 107. For example, as an operation method of the fuel supply means, taking a case where a concentration detection element is used as the concentration detection means 112 as an example, a method of operating when the methanol concentration detected by the concentration detection element 112 becomes a predetermined concentration or less, There is a method of determining the supply amount according to the difference between the methanol concentration detected by the concentration detection element 112 and a predetermined concentration. Details of the method for controlling the methanol concentration and the liquid level in the mixing tank 108 will be described later.

  Carbon dioxide generated by the oxidation reaction of methanol (formula 2) exists in the DMFC main body 102 as dissolved or fine bubbles. The carbon dioxide moves to the mixing tank 108 via the fuel circulation line 105, and most of the carbon dioxide is released into the gas phase. Further, when the pressure of the gas phase increases, the gas is released to the outside of the system 101 through the gas-liquid separation membrane 110 installed at the upper part of the mixing tank 108. The gas-liquid separation membrane 110 may be provided with a mechanism for removing a trace amount of organic substances by providing a catalyst treatment reactor.

  The exhaust gas after power generation is released to the outside of the system 101. A method of installing a gas-liquid separation membrane and a cooler at the air exhaust gas outlet, collecting water, and returning it to the cooling water tank 106 can also be adopted.

  The installation location of the concentration detection means 112 used in the present invention is not particularly limited as long as it is on the line through which the aqueous methanol solution circulates. In order to measure the methanol concentration more quickly, it is desirable to install the concentration detection means 112 in the mixing tank 108 or a place close to the mixing tank 108. By providing in the mixing tank or in the vicinity thereof, the measurement time delay can be avoided and the concentration control can be performed quickly. With such a configuration, the fuel whose methanol concentration is adjusted can be supplied from the mixing tank 108 to the DMFC system 102.

  An example of a method for controlling the fuel in the mixing tank according to the first embodiment of the present invention will be described with reference to FIG.

  When the system 101 is activated and the voltage of the DMFC main body 102 reaches a predetermined value, the liquid level signal detected by the liquid level detection means and the concentration signal detected by the concentration detection means 112 are input to the automatic control mechanism 120 ( S1, S2), it is determined whether or not it is above a predetermined liquid level (S3).

  When it is determined that the predetermined liquid level is exceeded, it is determined whether or not the concentration is higher than a predetermined value (S4).

  When it is determined that the concentration is higher than the predetermined value, nothing is performed. When the concentration is lower than the predetermined value, a supply command signal for the high-concentration methanol aqueous solution is output from the fuel control line 122 to the fuel supply means 104 and operated. (S5) After supplying the high concentration methanol aqueous solution to the mixing tank 108 for a certain time, the fuel supply means 104 is stopped (S6).

  If it is determined in S3 that the liquid level of the fuel in the mixing tank 108 is lower than the predetermined liquid level, it is determined whether or not the concentration is higher than the predetermined value (S7).

  In this case, when it is determined that the concentration exceeds a predetermined value, a water supply command signal is output from the water control line 123 to the water supply means 107, the water supply means 107 is operated (S8), and a predetermined time period is reached. After supplying water to the mixing tank 108, the water supply means 107 is stopped (S9).

  If it is determined that the concentration is lower than the predetermined value, the fuel control line 122 outputs a supply command signal for the high-concentration methanol aqueous solution to the fuel supply unit 104 to operate the fuel supply unit 104 (S10), and for a certain period of time. After the high-concentration methanol aqueous solution is supplied to the mixing tank 108, the fuel supply means 104 is stopped (S11).

  An example in which the system is operated by the above-described method is illustrated in FIGS.

Example 1
When 50% aqueous methanol solution was charged into the methanol container 103 and the output of the system was operated at a constant 100 W, the change in concentration and liquid level in the mixing tank 108 showed the behavior as shown in FIG. That is, when the fuel concentration reaches a predetermined concentration, the concentration in the mixing tank 108 is increased by supplying the high-concentration methanol aqueous solution, and then the concentration is decreased due to fuel consumption accompanying the power generation of the DMFC main body 102. When the fuel concentration reached a predetermined concentration again, a periodic behavior of supplying high concentration methanol was shown.

  Further, the liquid level in the mixing tank 108 behaved such that when the fuel concentration is higher than the predetermined concentration, the water supply means 104 operates at a relatively short period and maintains the predetermined liquid level. In addition, when the high concentration fuel was supplied, the liquid level rose relatively large. The liquid level gradually decreases due to a decrease in the amount of fuel accompanying the power generation of the DMFC main body 102, and when the liquid level reaches a predetermined level, the fuel concentration reaches a predetermined value and thus shows a behavior that fluctuates in a relatively short cycle. It was.

(Example 2)
When 50% aqueous methanol solution was charged into the methanol container 103 and the output of the system was changed halfway, the concentration and the liquid level in the mixing tank 108 changed with time as shown in FIG. The point that the high concentration methanol aqueous solution is supplied when the fuel concentration reaches a predetermined concentration is the same as that of the first embodiment. When the output was changed from 100 W to 150 W, an increase in liquid level was observed due to an increase in the amount of carbon dioxide generated by the DMFC main body 102 by power generation. The time interval for supplying the fuel was shortened, and the amplitude of the concentration was also reduced. This is because the consumption of methanol and water has increased due to the increase in output.

  In this example, it was exemplified that the concentration and the liquid level can be controlled even if the output is changed according to the first embodiment of the present invention.

  Next, an example of a method for controlling the fuel in the mixing tank according to the second embodiment of the present invention will be described with reference to FIG.

  When the system 101 is activated and the voltage of the DMFC main body 102 reaches a predetermined value, the liquid level signal detected by the liquid level detection means and the concentration signal detected by the concentration detection means are input to the automatic control mechanism 120 (SS1). SS2), it is determined whether or not the liquid level is higher than a predetermined liquid level (SS3). In this embodiment, the detected density value is density A.

  When it is determined that the predetermined liquid level is higher, it is determined whether or not the concentration A is higher than a predetermined value (SS4). Here, the predetermined density is C.

  When it is determined that the density A exceeds the predetermined value, nothing is done, and when it is determined that the density A does not exceed the predetermined value, it is determined whether the density A exceeds the density C1 (SS5). ). Here, C1 has a relationship of C1 <C. When it is determined that the concentration A is higher than C1, a supply command signal for the high-concentration methanol aqueous solution is output from the fuel control line 122 to the fuel supply means 104, and the fuel supply means 104 is operated for t1 seconds (SS6).

  When it is determined that the density A does not exceed C1, it is determined whether the density A exceeds the density C2 (SS7). Here, C2 has a relationship of C2 <C1. When it is determined that the concentration A is higher than C2, a supply command signal for the high-concentration aqueous methanol solution is output from the fuel control line 122 to the fuel supply means 104, and the fuel supply means 104 is operated for t2 seconds (SS8). Here, the time t2 has a relationship of t1 ≦ t2.

  If it is determined that the concentration A does not exceed C2, it is determined whether the concentration A exceeds the concentration C3 (SS9). Here, C3 has a relationship of C3 <C2. When it is determined that the concentration A exceeds C3, a supply command signal for the high-concentration aqueous methanol solution is output from the fuel control line 122 to the fuel supply unit 104, and the fuel supply unit 104 is operated for t3 seconds (SS10). Here, the time t3 has a relationship of t2 ≦ t3. When it is determined that the concentration A does not exceed C3, a supply command signal for the high-concentration aqueous methanol solution is output from the fuel control line 122 to the fuel supply unit 104, and the fuel supply unit 104 is operated for t4 seconds (SS11). Here, the time t4 has a relationship of t3 ≦ t4.

  If it is determined in SS3 that the liquid level does not exceed the predetermined value, it is determined whether the concentration A is higher than the predetermined value C (SS12).

  If it is determined that the density A is higher than C, it is determined whether the density A is lower than the density C4 (SS13). When it is determined that the concentration A is lower than the predetermined value, a water supply command signal is output from the water control line 123 to the water supply means 107, the water supply means 107 is operated for a certain time (SS14), and fuel control is performed. A supply command signal for the high-concentration aqueous methanol solution is output from the line 122 to the fuel supply means 104, and the fuel supply means 104 is operated for t seconds (SS15). Here, the time t has a relationship of t ≦ t1.

  When it is determined that the concentration A does not exceed the predetermined value, a water supply command signal is output from the water control line 123 to the water supply unit 107, and the water supply unit 107 is operated for a predetermined time (SS16).

  When it is determined that the concentration A does not exceed C, a water supply command signal is output from the water control line 123 to the water supply means 107, the water supply means 107 is operated for a certain time (SS17), and the concentration A is the concentration. It is determined whether or not it exceeds C1 (SS5).

  When it is determined that the concentration A is higher than C1, a supply command signal for the high-concentration methanol aqueous solution is output from the fuel control line 122 to the fuel supply means 104, and the fuel supply means 104 is operated for t1 seconds (SS6).

  When it is determined that the density A does not exceed C1, it is determined whether the density A exceeds the density C2 (SS7). Here, C2 has a relationship of C2 <C1. When it is determined that the concentration A is higher than C2, a supply command signal for the high-concentration aqueous methanol solution is output from the fuel control line 122 to the fuel supply means 104, and the fuel supply means 104 is operated for t2 seconds (SS8).

  If it is determined that the concentration A does not exceed C2, it is determined whether the concentration A exceeds the concentration C3 (SS9). Here, C3 has a relationship of C3 <C2. When it is determined that the concentration A exceeds C3, a supply command signal for the high-concentration aqueous methanol solution is output from the fuel control line 122 to the fuel supply unit 104, and the fuel supply unit 104 is operated for t3 seconds (SS10). When it is determined that the concentration A does not exceed C3, a supply command signal for the high-concentration aqueous methanol solution is output from the fuel control line 122 to the fuel supply unit 104, and the fuel supply unit 104 is operated for t4 seconds (SS11).

  In the embodiment shown here, the method for determining the concentration according to the flowchart as shown in FIG. 2 or FIG. 5 is shown. However, for example, an action according to the difference between the detected concentration and the predetermined concentration, in this case, fuel or water If the conditions for supplying the mixture to the mixing tank 108 are tabulated in advance and control is performed so as to perform processing according to the table, a more detailed control method than that of the present embodiment can be realized.

(Example 3)
In the control method according to the flowchart of FIG. 5, the parameters are C = 3.0%, C1 = 2.9%, C2 = 2.8%, C3 = 2.7%, C4 = 3.2%, t = 1s, The t1 = 4 s, t2 = 7 s, t3 = 10 s, and t4 = 14 s were set, and the flow cycle was set to 30 s and incorporated in the automatic control mechanism 120. The methanol container 103 was charged with a 60% aqueous methanol solution, the system output was operated at 100 W, and was increased to 150 W in the middle. The change in concentration and liquid level in the mixing tank 108 over time is shown in FIG. It showed the following behavior. That is, the fuel concentration fluctuated in the vicinity of a predetermined concentration (3% in this embodiment), and the amplitude thereof was smaller than those in the first and second embodiments.

  In addition, the fluctuation level of the liquid level in the mixing tank 108 was smaller than that in Example 1 or Example 2, and the behavior was such that a predetermined liquid level was maintained.

  These behaviors showed no significant change even when the output changed, and it was found that the fuel concentration and liquid level could be controlled without depending on the output.

  In this example, it was exemplified that the concentration and the liquid level can be controlled even if the output changes according to the second embodiment of the present invention.

DESCRIPTION OF SYMBOLS 101 Fuel cell system 102 Fuel cell 103 Methanol container 104 Fuel supply means 105 Fuel circulation line 106 Cooling water tank 107 Water supply means 108 Mixing tank 109 Fuel circulation pump 110 Gas-liquid separation film 111 Air supply means 112 Concentration detection element 113 Liquid level detection Means 120 Control circuit 121 Concentration signal line 122 Fuel control line 123 Water control line

Claims (8)

A polymer electrolyte fuel cell using a liquid organic compound as a fuel;
A mixing tank for storing fuel to be supplied to the fuel cell;
High-concentration fuel supply means for supplying high-concentration fuel having a higher concentration of the main component of the organic compound than the fuel supplied to the fuel cell to the mixing tank;
Water supply means for supplying water to the mixing tank;
Concentration detecting means for detecting the concentration of the fuel supplied from the mixing tank to the fuel cell;
Liquid level detection means for detecting the position of the liquid level of the mixing tank;
A fuel cell system comprising: control means for outputting a supply command signal to the high concentration fuel supply means and the water supply means based on information input from the concentration detection means and the liquid level detection means;
The control means supplies a supply command only to the high concentration fuel supply means based on the concentration detected by the concentration detection means when the position of the liquid level detected by the liquid level detection means exceeds a predetermined liquid level position. Output signal,
When the liquid level detected by the liquid level detection means falls below a predetermined liquid level, either the high-concentration fuel supply means or the water supply means based on the concentration detected by the concentration detection means A fuel cell system that outputs a supply command signal only to a fuel cell system.   2. The control unit according to claim 1, wherein the liquid level detected by the liquid level detection unit exceeds a predetermined liquid level, or the concentration detected by the concentration detection unit is higher than a predetermined value. Outputs a supply command signal to the fuel supply means, and does not output a supply command signal to the fuel supply means when the concentration detected by the concentration detection means is lower than a predetermined value.   3. The fuel according to claim 2, wherein the control unit controls the supply amount of the high concentration fuel supplied from the high concentration fuel supply unit to the mixing tank based on the concentration detected by the concentration detection unit. Battery system.   4. The fuel cell system according to claim 3, wherein the control unit controls a supply amount by changing a time during which the high concentration fuel is supplied from the high concentration fuel supply unit to the mixing tank.   The control unit according to claim 1, wherein the liquid level detected by the liquid level detection unit falls below a predetermined liquid level, or the concentration detected by the concentration detection unit is higher than a predetermined value. Outputs a supply command signal to the water supply means, and outputs a supply command signal to the high-concentration fuel supply means when the concentration detected by the concentration detection means is lower than a predetermined value. system.   6. The control unit according to claim 5, wherein the control unit controls a supply amount of the high concentration fuel or water supplied from the high concentration fuel supply unit or the water supply unit to the mixing tank based on the concentration detected by the concentration detection unit. A fuel cell system.   7. The fuel according to claim 6, wherein the control means controls a supply amount by changing a time for supplying the high concentration fuel or water from the high concentration fuel supply means or the water supply means to the mixing tank. Battery system.   2. The fuel cell system according to claim 1, wherein the fuel is an aqueous solution mainly composed of methanol.