JP7479691B2 - Fuel cell system and fuel cell power generation method - Google Patents

Description

The present invention relates to a fuel cell system.

As shown in FIG. 4, when power generation by the fuel cell stops, some fuel cell systems stop supplying fuel gas and oxidant gas to the fuel cell if the fuel cell voltage exceeds an upper limit, and resume supplying fuel gas and oxidant gas to the fuel cell if the fuel cell voltage falls below a lower limit. In the example of fuel cell voltage fluctuation shown in FIG. 4, a current is temporarily passed from the fuel cell to a load so that the fuel cell voltage does not exceed the upper limit. Related technology is disclosed in Patent Document 1.

JP 2007-048517 A

However, in the above fuel cell system, if the difference between the upper and lower limits is relatively large, there is a concern that the fuel cell may deteriorate due to an increase in the fluctuation range of the fuel cell voltage.

Therefore, an object of one aspect of the present invention is to provide a fuel cell system and a fuel cell power generation method that can suppress the fluctuation range of the fuel cell voltage when power generation by the fuel cell is stopped.

The fuel cell system according to one embodiment of the present invention includes a fuel cell that generates electricity through an electrochemical reaction between hydrogen contained in a fuel gas and oxygen contained in an oxidant gas, a DC-DC converter provided downstream of the fuel cell, an injector that adjusts the amount of fuel gas supplied to the fuel cell, an air compressor having a motor that adjusts the amount of oxidant gas supplied to the fuel cell, and a control unit that controls the operation of the DC-DC converter, the injector, and the motor of the air compressor.

The control unit calculates a ratio between the unit time oxygen supply amount, which is the amount of oxygen supplied to the fuel cell per unit time when the motor is driven at the minimum rotation speed, and the unit time reaction oxygen amount, which is the amount of oxygen that reacts with the amount of hydrogen permeating the electrolyte membrane of the fuel cell per unit time, and obtains a first period of the unit time and a second period of the unit time other than the first period from the ratio. When power generation by the fuel cell is stopped while the DCDC converter is stopped, the control unit controls the operation of the injector so that the pressure of the fuel gas in the fuel cell is constant, and alternately repeats a process of driving the motor at the minimum rotation speed during the first period and a process of stopping the motor during the second period.

As a result, when the amount of hydrogen that permeates the electrolyte membrane of the fuel cell per unit time is relatively small, the first period can be made relatively short to suppress the rise in fuel cell voltage during the first period. In addition, the amount of oxygen fuel that permeates through the electrolyte membrane to the hydrogen electrode during the second period can be suppressed, so the drop in fuel cell voltage during the second period can be suppressed. Therefore, when power generation by the fuel cell is stopped while the DCDC converter is stopped, the fluctuation range of the fuel cell voltage can be suppressed.

The control unit may also be configured to control the operation of the injector so that the fuel gas is not supplied to the fuel cell during the second period when power generation by the fuel cell is stopped while the DCDC converter is stopped.

This reduces the amount of fuel gas consumed, improving the fuel efficiency of vehicles that run on electricity supplied by the fuel cell system.

The fuel cell system may also include a power storage device provided downstream of the DCDC converter, and the DCDC converter may include an inductor, a switch, a diode having an anode terminal connected to the positive terminal of the fuel cell via the inductor and connected to the negative terminal of the fuel cell via the switch and a cathode terminal connected to the positive terminal of the power storage device, and a capacitor having one terminal connected to the cathode terminal of the diode and the positive terminal of the power storage device and the other terminal connected to the negative terminal of the fuel cell and the negative terminal of the power storage device.

As a result, when the switch is off and the fuel cell voltage is higher than the voltage of the power storage device, current flows from the fuel cell to the power storage device via the diode, causing the fuel cell voltage to drop, preventing the fuel cell from deteriorating due to the relatively high fuel cell voltage.

The control unit may also be configured to control the operation of the air compressor motor so that the output power of the fuel cell changes stepwise depending on the voltage or charging rate of the power storage device when the fuel cell is generating power while the DCDC converter is operating.

In addition, a fuel cell power generation method according to one embodiment of the present invention is a method of generating power for the fuel cell in the above fuel cell system, in which the control unit calculates a ratio between a unit time oxygen supply amount, which is the amount of oxygen supplied to the fuel cell per unit time when the motor is driven at the minimum rotation speed, and a unit time reaction oxygen amount, which is the amount of oxygen reacting with the amount of hydrogen permeating the electrolyte membrane of the fuel cell per unit time, and obtains a first period of the unit time and a second period of the unit time other than the first period from the ratio. When power generation by the fuel cell is stopped when the DCDC converter is stopped, the operation of the injector is controlled so that the pressure of the fuel gas in the fuel cell is constant, and the process of driving the motor at the minimum rotation speed during the first period and the process of stopping the motor during the second period are alternately repeated.

This makes it possible to suppress the rise in the fuel cell voltage during the first period and the fall in the fuel cell voltage during the second period, thereby suppressing the fluctuation range of the fuel cell voltage when the fuel cell stops generating electricity when no current is flowing from the fuel cell to the load.

According to the present invention, it is possible to suppress the fluctuation range of the fuel cell voltage when the fuel cell stops generating electricity.

1 is a diagram illustrating an example of a fuel cell system according to an embodiment. 10 is a flowchart showing an example of an operation of a control unit. 5 is a diagram showing an example of fluctuation in the voltage of a fuel cell in the fuel cell system according to the embodiment. FIG. FIG. 1 is a diagram showing an example of fluctuation in the voltage of a fuel cell in an existing fuel cell system.

The following describes the embodiment in detail with reference to the drawings.

Figure 1 shows an example of a fuel cell system according to an embodiment.

The fuel cell system 1 shown in FIG. 1 is mounted on a vehicle Ve, such as an industrial vehicle such as a forklift or an automobile, and supplies power to a load Lo, etc. The load Lo is an inverter and electrical components that drive a traction motor mounted on the vehicle Ve.

The fuel cell system 1 also includes a fuel cell FC, a hydrogen tank HT, an injector INJ, an air compressor ACP, an air pressure regulating valve ARV, a DCDC converter CNV, a power storage device B, voltmeters SV1 and SV2, an ammeter SI, a memory unit 2, and a control unit 3.

The fuel cell FC is a fuel cell stack consisting of multiple fuel cell cells connected in series with each other, and generates electricity through an electrochemical reaction between the hydrogen contained in the fuel gas (hydrogen gas) and the oxygen contained in the oxidant gas (air).

The hydrogen tank HT is a storage container for fuel gas. The fuel gas stored in the hydrogen tank HT is supplied to the fuel cell FC via the injector INJ.

The injector INJ adjusts the amount (flow rate) of fuel gas per unit time T supplied to the fuel cell FC.

The air compressor ACP supplies oxidant gas to the fuel cell FC via the air pressure regulating valve ARV. For example, the higher the voltage for driving the motor of the air compressor ACP, the higher the motor rotation speed, and the larger the amount (flow rate) of oxidant gas output from the air compressor ACP per unit time T. Also, the lower the voltage for driving the motor of the air compressor ACP, the lower the motor rotation speed, and the smaller the amount of oxidant gas output from the air compressor ACP per unit time T.

The air pressure regulator valve ARV adjusts the pressure of the oxidant gas supplied to the fuel cell FC.

The DCDC converter CNV is provided between the fuel cell FC and the power storage device B, and increases the voltage of the fuel cell FC and supplies the power output from the fuel cell FC to the power storage device B.

The DC-DC converter CNV includes an inductor L, a switch SW (such as a MOSFET (Metal Oxide Semiconductor Field Effect Transistor)), a diode D, and a capacitor C. That is, the anode terminal of the diode D is connected to the positive terminal of the fuel cell FC via the inductor L and to the negative terminal of the fuel cell FC via the switch SW. The cathode terminal of the diode D is connected to the positive terminal of the power storage device B. One terminal of the capacitor C is connected to the cathode terminal of the diode D and the positive terminal of the power storage device B, and the other terminal of the capacitor C is connected to the negative terminal of the fuel cell FC and the negative terminal of the power storage device B. When the switch SW is off (open state) and the voltage of the fuel cell FC is higher than the voltage of the power storage device B, a current flows from the fuel cell FC to the power storage device B via the diode D.

The power storage device B is composed of a lithium ion capacitor or the like, and is provided after the DCDC converter CNV. The power storage device B also supplies power to the load Lo.

The voltmeter SV1 measures the voltage of the fuel cell FC and sends the measured voltage to the control unit 3.

The voltmeter SV2 measures the voltage of the storage device B and sends the measured voltage to the control unit 3.

The ammeter SI measures the current flowing through the storage device B and sends the measured current to the control unit 3.

The storage unit 2 is composed of memory such as RAM (Random Access Memory) or ROM (Read Only Memory).

The control unit 3 is composed of a CPU (Central Processing Unit) or a programmable device (such as an FPGA (Field Programmable Gate Array) or a PLD (Programmable Logic Device)), and controls the operation of the injector INJ, the air pressure regulating valve ARV, the DCDC converter CNV, and the motor of the air compressor ACP.

<Operation control of injector INJ>
The control unit 3 controls the operation of the injector INJ so that the pressure of the fuel gas inside the fuel cell FC remains constant when the fuel cell FC is generating electricity.

For example, when the fuel cell FC is generating electricity, the control unit 3 controls the operation of the injector INJ so that the supply of fuel gas from the hydrogen tank HT to the fuel cell FC is stopped when the pressure of the hydrogen electrode (anode) of the fuel cell FC reaches or exceeds a predetermined pressure, and controls the operation of the injector INJ so that the supply of fuel gas from the hydrogen tank HT to the fuel cell FC is resumed when the pressure of the hydrogen electrode of the fuel cell FC falls below the predetermined pressure.

<Operation control of air pressure regulating valve ARV>
The control unit 3 controls the operation of the air pressure regulating valve ARV so that the pressure of the oxidant gas supplied to the fuel cell FC is constant when the fuel cell FC is generating electricity, thereby making it possible to suppress fluctuations in the flow rate of the oxidant gas supplied from the air compressor ACP to the oxygen electrode of the fuel cell FC.

<Determination of Fully Charged State of Storage Device B>
The control unit 3 determines that the storage device B is fully charged when the voltage of the storage device B (the voltage measured by the voltmeter SV2) is equal to or higher than the voltage threshold value or when the charging rate of the storage device B is equal to or higher than the charging rate threshold value, and determines that the storage device B is not fully charged when the voltage of the storage device B is lower than the voltage threshold value or when the charging rate of the storage device B is lower than the charging rate threshold. The control unit 3 determines the charging rate of the storage device B from the integrated value of the voltage measured by the voltmeter SV2 or the current measured by the ammeter SI.

<Operation Control of DCDC Converter CNV When Power Storage Device B is Not in a Fully Charged State>
When the control unit 3 determines that the storage device B is not fully charged, it drives the DCDC converter CNV according to the voltage or charging rate of the storage device B (the ratio of the current charging capacity to the full charging capacity of the storage device B).

For example, the control unit 3 increases the target output voltage sent to the DCDC converter CNV as the voltage or charging rate of the storage device B increases, and decreases the target output voltage sent to the DCDC converter CNV as the voltage or charging rate of the storage device B decreases. The DCDC converter CNV increases the output voltage of the DCDC converter CNV by increasing the duty ratio of the control signal that controls the on/off of the switch SW as the target output voltage increases. Also, the DCDC converter CNV decreases the duty ratio of the control signal as the target output voltage decreases, thereby decreasing the output voltage of the DCDC converter CNV.

The control unit 3 performs high potential avoidance processing to lower the voltage of the fuel cell FC by drawing current from the fuel cell FC to the power storage device B when the voltage of the fuel cell FC is relatively high. Specifically, when the voltage of the fuel cell FC reaches or exceeds an upper limit, the control unit 3 controls the operation of the DCDC converter CNV so as to increase the current flowing from the fuel cell FC to the power storage device B via the DCDC converter CNV, and changes the voltage of the fuel cell FC to a voltage lower than the upper limit. This reduces the state in which the voltage of the fuel cell FC exceeds the upper limit, thereby preventing the fuel cell FC from deteriorating due to the voltage of the fuel cell FC becoming relatively high.

<Operation Control of the Motor of the Air Compressor ACP When the Storage Device B is Not in a Fully Charged State>
When the control unit 3 determines that the power storage device B is not fully charged, it controls the operation of the motor of the air compressor ACP so that the power required to charge the power storage device B is output from the fuel cell FC.

For example, when the control unit 3 determines that the power storage device B is not fully charged, that is, when the fuel cell FC is generating power while the DCDC converter CNV is being driven, the control unit 3 controls the operation of the motor of the air compressor ACP so that the power output from the fuel cell FC changes stepwise according to the voltage or charging rate of the power storage device B. That is, when the voltage or charging rate of the power storage device B is equal to or lower than the threshold th1, the control unit 3 controls the operation of the motor of the air compressor ACP so that the fuel cell FC outputs power P3. When the voltage or charging rate of the power storage device B is greater than the threshold th1 and equal to or lower than the threshold th2, the control unit 3 controls the operation of the motor of the air compressor ACP so that the fuel cell FC outputs power P2. When the voltage or charging rate of the power storage device B is greater than the threshold th2, the control unit 3 controls the operation of the motor of the air compressor ACP so that the fuel cell FC outputs power P1. Note that the threshold th1 is set to be less than the threshold th2. Also, the power P1 is set to be less than the threshold th2.

<Operation Control of DCDC Converter CNV When Power Storage Device B is in Fully Charged State>
When the control unit 3 determines that the power storage device B is fully charged, it stops the DCDC converter CNV. That is, when the control unit 3 determines that the power storage device B is fully charged, it keeps the switch SW of the DCDC converter CNV off at all times. When the DCDC converter CNV is stopped and the voltage of the power storage device B is higher than the voltage of the fuel cell FC, no current flows from the fuel cell FC to the power storage device B via the diode of the DCDC converter CNV.

<Operation Control of Motor of Air Compressor ACP When Power Storage Device B is in Fully Charged State>
When the control unit 3 determines that the storage device B is fully charged, it causes the fuel cell FC to generate electricity by controlling the operation of the motor of the air compressor ACP so that the voltage of the fuel cell FC (the voltage measured by the voltmeter SV1) is constant between an upper limit value and a lower limit value.

For example, when the control unit 3 determines that the power storage device B is in a fully charged state, that is, when the DCDC converter CNV is stopped, the control unit 3 stops the power generation of the fuel cell FC. Note that stopping the power generation of the fuel cell FC includes not only the case where the power generation amount of the fuel cell FC becomes completely zero, but also the case where the power generation by the fuel cell FC is suppressed and the power generation amount of the fuel cell FC becomes infinitesimally close to zero. At that time, the process of operating the motor of the air compressor ACP at the minimum rotation speed during the period T1 (first period) and the process of stopping the motor of the air compressor ACP during the period T2 (second period) are alternately repeated. Note that the minimum rotation speed is the rotation speed of the motor of the air compressor ACP when the smallest flow rate of oxidizing gas is output from the air compressor ACP.

The control unit 3 may be configured to control the operation of the injector INJ so that, when power generation by the fuel cell FC is stopped while the DCDC converter CNV is stopped, fuel gas is supplied from the hydrogen tank HT to the fuel cell FC during period T1, and fuel gas is not supplied from the hydrogen tank HT to the fuel cell FC during period T2. When configured in this manner, the amount of fuel gas consumed can be reduced compared to when the operation of the injector INJ is controlled so that the pressure of the fuel gas in the fuel cell FC is constant, thereby improving the fuel efficiency of the vehicle Ve.

<Example of acquiring periods T1 and T2>
When the motor of the air compressor ACP is driven at the minimum rotation speed, the control unit 3 calculates the ratio between the amount of oxygen Vo1 per unit time T supplied from the air compressor ACP to the fuel cell FC (i.e., the amount of oxygen supplied per unit time) and the amount of oxygen Vo2 (i.e., the amount of oxygen reacted per unit time) that reacts with the amount of hydrogen that permeates (cross leaks) from the hydrogen electrode through the electrolyte membrane in the fuel cell FC per unit time T, and obtains a period T1 within the unit time T and a period T2 within the unit time T other than period T1 from the period T1.

That is, first, the control unit 3 calculates the amount of oxygen Vo1 by calculating the following formula 1. Note that the amount [mol] of oxidant gas per unit time T supplied from the air compressor ACP to the fuel cell FC: the amount [mol] of oxygen contained in the oxidant gas = 5:1.

Amount of oxygen Vo1 = amount of oxidant gas per unit time T supplied from air compressor ACP to fuel cell FC / 5 ... formula 1

Next, the control unit 3 calculates the amount of oxygen Vo2 by calculating the following formula 2. Note that the amount of hydrogen [mol] that permeates from the hydrogen electrode to the oxygen electrode through the electrolyte membrane in the fuel cell FC per unit time T: the amount of oxygen [mol] that reacts with that amount of hydrogen = 2:1.

Amount of oxygen Vo2 = Amount of hydrogen that permeates from the hydrogen electrode through the electrolyte membrane to the oxygen electrode in the fuel cell FC per unit time T / 2 ... Equation 2

Next, the control unit 3 calculates the period T1 by calculating the following formula 3.

Time period T1 = (amount of oxygen Vo2 / amount of oxygen Vo1) x unit time T ... formula 3

Then, the control unit 3 calculates the period T2 by calculating the following formula 4.

Period T2 = unit time T - period T1 ... formula 4

The control unit 3 may be configured to acquire the periods T1 and T2 by reading the periods T1 and T2 from the storage unit 2. In this case, the periods T1 and T2 are determined in advance by an experiment, a simulation, or the like, and are stored in the storage unit 2.

Figure 2 is a flowchart showing an example of the operation of the control unit 3. Note that the flowchart shown in Figure 2 is performed for each operation cycle of the control unit 3.

First, when the control unit 3 determines that the power storage device B is not fully charged, i.e., when the fuel cell FC is generating electricity while the DCDC converter CNV is operating (step S1: No), the control unit 3 controls the operation of the injector INJ, the air pressure regulating valve ARV, and the motor of the air compressor ACP according to the voltage or charging rate of the power storage device B to cause the fuel cell FC to generate electricity, and controls the operation of the DCDC converter CNV to increase the voltage of the fuel cell FC and charge the power storage device B (step S2). Note that the control unit 3 may be configured to control the operation of the injector INJ, the air pressure regulating valve ARV, the motor of the air compressor ACP, and the DCDC converter CNV in step S2, taking into account the power required by the load Lo.

On the other hand, when the control unit 3 determines that the storage device B is in a fully charged state, that is, when power generation by the fuel cell FC is stopped while the DCDC converter CNV is stopped (step S1: Yes), it acquires periods T1 and T2 (step S3).

Next, the control unit 3 drives the motor of the air compressor ACP at the minimum rotation speed until the period T1 has elapsed (step S4, step S5: No).

Next, when the period T1 has elapsed (step S5: Yes), the control unit 3 stops the motor of the air compressor ACP until the period T2 has elapsed (step S6, step S7: No).

Then, when the period T2 has elapsed (step S7: Yes), the control unit 3 waits until the next operating cycle.

Here, FIG. 3(a) is a diagram showing an example of fluctuations in the voltage of the fuel cell FC when power generation by the fuel cell FC is stopped with the DCDC converter CNV stopped. Note that the horizontal axis of the two-dimensional coordinate system shown in FIG. 3(a) indicates time, and the vertical axis indicates the voltage of the fuel cell FC. In addition, the upper and lower limits of the voltage of the fuel cell FC are set based on the withstand voltage and rated voltage of the fuel cell FC.

As described above, when power generation by the fuel cell FC is stopped while the DCDC converter CNV is stopped, the control unit 3 alternately performs a process of driving the motor of the air compressor ACP at the minimum rotation speed during period T1 and a process of stopping the motor of the air compressor ACP during period T2.

In general, the amount of hydrogen that permeates the electrolyte membrane of the fuel cell FC per unit time T is relatively small, so as shown in FIG. 3(a), by making the period T1 relatively short, the increase in the voltage of the fuel cell FC during the period T1 can be suppressed. Also, when the period T1 is relatively short and the amount of oxygen supplied to the fuel cell FC is relatively small, the amount of oxygen that permeates from the oxygen electrode through the electrolyte membrane to the hydrogen electrode is also relatively small, so the combustion of hydrogen and oxygen at the hydrogen electrode can be suppressed. Therefore, as shown in FIG. 3(a), the decrease in the voltage of the fuel cell FC during the period T2 can be suppressed. This makes it possible to suppress the fluctuation range of the voltage of the fuel cell FC when the power generation of the fuel cell FC is stopped when the DCDC converter CNV is stopped. And, because the fluctuation range of the voltage of the fuel cell FC can be suppressed, the deterioration of the fuel cell FC can be suppressed.

In addition, according to the fuel cell system 1 of the embodiment, when power generation by the fuel cell FC is stopped because the DCDC converter CNV is stopped, there is no need to prepare an air compressor capable of supplying a minute amount of oxidant gas to the fuel cell FC, so an increase in the manufacturing costs of the fuel cell system 1 can be suppressed.

In addition, according to the fuel cell system 1 of the embodiment, when power generation by the fuel cell FC is stopped while the DCDC converter CNV is stopped, the voltage of the fuel cell FC can be prevented from exceeding an upper limit value, so that the number of times the DCDC converter CNV is driven can be reduced, and unnecessary increases in the voltage of the storage device B can be prevented.

In addition, according to the fuel cell system 1 of the embodiment, when power generation by the fuel cell FC is stopped while the DCDC converter CNV is stopped, the fuel gas consumption can be reduced, thereby improving the fuel efficiency of the vehicle Ve. In addition, since the fuel gas consumption can be reduced, the number of times fuel gas is supplied to the fuel cell FC can be reduced, and water clogging can be suppressed. Therefore, there is no need to perform complex control to eliminate water clogging, and the load on the control unit 3 can be reduced.

In addition, according to the embodiment of the fuel cell system 1, when the switch SW is stopped and the voltage of the fuel cell FC is higher than the voltage of the power storage device B, a current flows from the fuel cell FC to the power storage device B via the diode D, causing the voltage of the fuel cell FC to drop, and deterioration of the fuel cell FC due to the relatively high voltage of the fuel cell FC can be suppressed.

The present invention is not limited to the above-described embodiment, and various improvements and modifications are possible without departing from the spirit of the present invention.

<Modification>
In the above embodiment, when power generation of the fuel cell FC is stopped while the DCDC converter CNV is stopped, the control unit 3 configures the unit time T to be composed of periods T1 and T2. However, the period T1 may be equally divided into a plurality of periods T1' and the period T2 may be equally divided into a plurality of periods T2', so that the unit time T is composed of a plurality of periods T1' and a plurality of periods T2'.

For example, as shown in FIG. 3(b), the control unit 3 divides the period T1 into periods T11 and T12, and divides the period T2 into periods T21 and T22, so that the unit time T is composed of periods T11, T21, T12, and T22. Note that period T11=period T12. Also, period T21=period T22.

According to the modified fuel cell system 1, when the power generation of the fuel cell FC is stopped while the DCDC converter CNV is stopped, the periods T11 and T12 during which oxygen is supplied to the fuel cell FC can be further shortened, so that the rise and fall of the voltage of the fuel cell FC can be further suppressed. As a result, when the power generation of the fuel cell FC is stopped while the DCDC converter CNV is stopped, the fluctuation range of the voltage of the fuel cell FC can be further suppressed.

1 Fuel Cell System 2 Memory Unit 3 Control Unit Ve Vehicle HT Hydrogen Tank INJ Injector ACP Air Compressor ARV Air Pressure Regulating Valve FC Fuel Cell CNV DCDC Converter L Inductor SW Switch D Diode C Capacitor B Power Storage Devices SV1, SV2 Voltmeter SI Ammeter Lo Load

Claims (5)

a fuel cell that generates electricity through an electrochemical reaction between hydrogen contained in a fuel gas and oxygen contained in an oxidant gas;
A DC-DC converter provided downstream of the fuel cell;
an injector for adjusting the amount of the fuel gas supplied to the fuel cell;
an air compressor having a motor for adjusting the amount of the oxidant gas supplied to the fuel cell;
A control unit that controls operations of the DC-DC converter, the injector, and the motor of the air compressor;
Equipped with
The control unit is
a unit time oxygen supply amount, which is the amount of oxygen supplied to the fuel cell per unit time when the motor is driven at a minimum rotation speed, and a unit time reaction oxygen amount, which is the amount of oxygen reacting with the amount of hydrogen permeating the electrolyte membrane of the fuel cell per unit time, are calculated as a ratio;
obtaining a first period of the unit time and a second period of the unit time other than the first period from the ratio;
a control circuit for controlling the operation of the injector so that the pressure of the fuel gas in the fuel cell is constant when power generation of the fuel cell is stopped while the DC-DC converter is stopped, and a process for driving the motor at a minimum rotation speed during the first period and a process for stopping the motor during the second period are alternately performed. 2. The fuel cell system according to claim 1,
The control unit controls the operation of the injector so that the fuel gas is not supplied to the fuel cell during the second period when power generation of the fuel cell is stopped while the DC-DC converter is stopped. 3. The fuel cell system according to claim 1,
A power storage device is provided downstream of the DC-DC converter,
The DC-DC converter comprises:
An inductor;
Switch,
a diode having an anode terminal connected to the positive electrode terminal of the fuel cell via the inductor and to the negative electrode terminal of the fuel cell via the switch, and a cathode terminal connected to the positive electrode terminal of the power storage device;
a capacitor having one terminal connected to the cathode terminal of the diode and the positive terminal of the power storage device, and having the other terminal connected to the negative terminal of the fuel cell and the negative terminal of the power storage device;
A fuel cell system comprising: 4. The fuel cell system according to claim 3 ,
The control unit controls the operation of the motor of the air compressor so that the output power of the fuel cell changes stepwise depending on the voltage or charging rate of the storage device when the fuel cell is generating power while the DCDC converter is operating. A power generation method for a fuel cell in a fuel cell system including: a fuel cell that generates electricity by an electrochemical reaction between hydrogen contained in a fuel gas and oxygen contained in an oxidant gas; a DC-DC converter provided downstream of the fuel cell; an injector that adjusts the amount of the fuel gas supplied to the fuel cell; an air compressor having a motor that adjusts the amount of the oxidant gas supplied to the fuel cell; and a control unit that controls operation of the DC-DC converter, the injector, and the motor of the air compressor, the method comprising:
The control unit is
a unit time oxygen supply amount, which is the amount of oxygen supplied to the fuel cell per unit time when the motor is driven at a minimum rotation speed, and a unit time reaction oxygen amount, which is the amount of oxygen reacting with the amount of hydrogen permeating the electrolyte membrane of the fuel cell per unit time, are calculated as a ratio;
obtaining a first period of the unit time and a second period of the unit time other than the first period from the ratio;
A method for generating electricity from a fuel cell, characterized in that when power generation by the fuel cell is stopped while the DCDC converter is stopped, the operation of the injector is controlled so that the pressure of the fuel gas in the fuel cell is constant, and a process of driving the motor at a minimum rotation speed during the first period and a process of stopping the motor during the second period are alternately repeated.

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