JP2003151592A - Fuel cell system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stabilize operation of a fuel cell even when output of a pump provided in a circulation path in a fuel cell system that an anode offgas is circulated is lowered. SOLUTION: The system is constructed in such a manner that an anode offgas of a fuel cell 10 is circulated by a pump 26. A shut-off valve 28 for controlling the displacement of the anode offgas by opening or closing the valve is provided at a downstream of the pump 26. The open duty of the shut- off valve 28 is increased when the output of the pump 26 is lowered, whereby increase in the concentration of nitrogen and steam in the anode offgas can be avoided to avoid the lowering of generating efficiency in the fuel cell.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for operating a fuel cell system having a circulation passage for circulating exhaust gas from a hydrogen electrode.

[0002]

2. Description of the Related Art In recent years, fuel cells, which generate electricity by an electrochemical reaction of hydrogen and oxygen, have attracted attention as an energy source. In the fuel cell, part of the hydrogen supplied to the hydrogen electrode may be discharged unreacted. In order to effectively utilize the residual hydrogen contained in the exhaust gas of the hydrogen electrode (hereinafter referred to as the anode off gas), a system for circulating the anode off gas has been proposed.

In such a system, a circulation flow path connecting the outlet of the hydrogen electrode to the inlet is provided, and the anode off-gas is forcedly circulated by a pump. A shut-off valve for discharging a part of the anode off gas to the outside air is provided in the circulation path. The concentration of hydrogen supplied to the hydrogen electrode can be maintained in an appropriate state by periodically opening this shut-off valve when the concentration of components other than hydrogen contained in the anode off-gas becomes high. .

[0004]

However, it has been found that in such a system, when the output of the pump is reduced due to a failure or the like, the operating efficiency of the fuel cell is rapidly reduced. Generally, the pressure of the anode off gas is relatively low due to the pressure loss in the fuel cell and the consumption of hydrogen. Therefore, if the anode off-gas cannot be forcibly circulated due to the reduction in the output of the pump, the circulation efficiency of the anode off-gas, and thus the exhaust efficiency from the fuel cell, deteriorates. As a result, the concentration of components other than hydrogen, such as water vapor and nitrogen, contained in the anode off gas increases. Nitrogen is a component contained in the anode off-gas when a part of the air supplied to the oxygen electrode leaks to the hydrogen electrode side. An increase in the concentration of components other than hydrogen leads to a relative decrease in hydrogen concentration, and thus a decrease in power generation efficiency. Further, the increase of water vapor causes dew condensation.

The present invention has been made in view of the above problems, and it is an object of the present invention to stabilize the operation of a fuel cell in a system for circulating the exhaust gas of a hydrogen electrode even when the output of the pump is reduced.

[0006]

A fuel cell system according to the present invention includes a fuel cell having a hydrogen electrode and an oxygen electrode, a circulation passage for circulating exhaust gas of the hydrogen electrode to the hydrogen electrode, and a circulation flow. A circulation pump provided in the passage and a communication portion for discharging the exhaust gas to the outside of the circulation flow path are provided. The open / closed state of the communication unit is controlled according to the output of the circulation pump. By doing so, it is possible to open and close the communication portion according to the circulation efficiency that varies according to the output of the circulation pump, and it is possible to avoid an increase in the concentration of components other than hydrogen in the exhaust gas. As a result, the fuel cell system can be stably operated even when the output of the circulation pump is reduced.

It is preferable that the communicating portion be controlled so as to increase the time density (hereinafter referred to as duty) in which the communicating portion is in the open state when the output of the circulation pump is reduced. By doing so, the exhaust gas can be easily discharged to the outside of the circulation flow channel when the circulation efficiency is lowered, and the fuel cell system can be stably operated. The duty may be increased stepwise in accordance with the output reduction of the circulation pump, or may be continuously increased. Further, the duty may be 100%, that is, the state may be completely opened. However, in consideration of effective utilization of residual hydrogen in the exhaust gas, it is preferable to set the duty to the minimum necessary value.

It is preferable that the opening / closing control of the communication portion is performed in consideration of the output of the fuel cell in addition to the output of the circulation pump. The amount of components other than hydrogen contained in the exhaust gas is
Because of the correlation with the output of the fuel cell,
It is possible to further stabilize the operation of the fuel cell. As an example, it is preferable to increase the duty of the communication part when the output of the fuel cell increases. The duty may be increased stepwise or continuously.

When the output of the fuel cell is taken into consideration, the output itself is used as a parameter, the required output value to the fuel cell,
The pressure or flow rate of the gas supplied to the fuel cell, the pressure or flow rate of the gas discharged from the fuel cell, or the like can be used.

In the fuel cell system of the present invention, with respect to the output of the circulation pump and the time density at which the communication portion is in the open state, the operation is performed when the maximum achievable values are below predetermined preset values. It is preferable to stop. For example, this corresponds to a case where the output of the circulation pump is reduced due to a failure or the like, and the communication portion cannot be opened with a sufficient duty. In such a case, the exhaust gas of the hydrogen electrode cannot be efficiently discharged to the outside of the circulation flow path. Under such circumstances, by stopping the operation of the fuel cell, damage to the fuel cell due to dew condensation or the like can be suppressed.

The predetermined value serving as a criterion for stopping the operation may be set independently for each of the output of the circulation pump and the duty of the communicating portion, or may be set in association with each other. In the latter mode, for example, a mode in which the predetermined value of the duty of the communication unit changes according to the output of the circulation pump can be considered. By thus setting the both in association with each other, it is possible to suppress the adverse effects such as dew condensation while ensuring a wide operation of the fuel cell.

The present invention is highly useful in a configuration in which hydrogen in exhaust gas is released into the atmosphere from the communication section without being treated. In such a case, by controlling the duty of the communication part, it is possible to stabilize the operation of the fuel cell while suppressing the amount of untreated hydrogen discharged into the atmosphere.

The present invention can be configured in various modes such as a fuel cell system and a control method of the fuel cell system. The above-mentioned characteristic points can be appropriately combined or partly omitted.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION A. System configuration: B. Fuel cell operation control process: C. effect: D. Modification:

A. System Configuration: FIG. 1 is an explanatory diagram showing a schematic configuration of a fuel cell system as an embodiment. The fuel cell 10 generates electricity by an electrochemical reaction between hydrogen and oxygen in the air. At the oxygen electrode (hereinafter referred to as the cathode), air is supplied from the supply port 11a by the pump 30 and exhausted from the discharge port 11b. At the hydrogen electrode (hereinafter referred to as the anode), hydrogen is supplied from the supply port 12a and discharged from the discharge port 12b.

Although various types of fuel cells 10 can be applied, a solid polymer membrane type is used in this embodiment.
1 schematically shows an internal state of the fuel cell 10. The hydrogen and oxygen flow paths are separated by the electrolyte membrane 13. Protons are carried from the hydrogen electrode to the cathode side inside the electrolyte membrane 13, and are combined with oxygen in the air to generate water, thereby generating power. The flow paths for hydrogen and oxygen can have various configurations, but in principle they are the same as in this schematic diagram.

Since hydrogen is consumed in the anode as described above, the pressure of the gas discharged from the discharge port 12b (hereinafter referred to as the anode off gas) is relatively low. The anode offgas contains undigested residual hydrogen, nitrogen in the air leaking from the cathode to the anode side, water vapor used for humidifying the electrolyte membrane 13, and the like.

The piping structure on the anode side will be described.
Hydrogen is supplied from the hydrogen tank 20 to the supply port 12a of the anode. The supply pipe for supplying hydrogen is provided with a valve 21 for adjusting the supply amount and regulators 22, 23 for adjusting the pressure.

The anode off-gas discharged from the discharge port 12b is separated into gas and liquid by the condenser 27 and then circulated to the supply port 12a by the pump 26. A check valve 25 is provided at the connecting portion with the supplier to prevent the reverse flow of hydrogen from the hydrogen tank 20. By circulating the anode off-gas, the residual hydrogen can be effectively utilized.

A supply port 12 is provided in the flow path of the anode off gas.
In addition to the circulation flow path to a, a flow path for discharging to the outside is also provided. A shut valve 28 that can be opened and closed electromagnetically is provided in the discharge flow path. When the shut valve 28 is open, the anode off gas is exhausted to the outside, and when it is closed, it is circulated to the supply port 12a. The anode off gas contains not only residual hydrogen but also nitrogen and water vapor. Since nitrogen and water vapor are not consumed at the time of power generation, if the anode off-gas is continuously circulated for a long period of time, the concentration of these components becomes high and the hydrogen concentration relatively decreases. By opening the shut valve 28, it is possible to reduce the concentrations of nitrogen and water vapor, and to avoid a decrease in power generation efficiency due to a decrease in hydrogen concentration. While the shutoff valve 28 is opened, residual hydrogen in the anode offgas is also discharged. Therefore, from the viewpoint of reducing the hydrogen consumption, it is preferable to control the opening degree of the shutoff valve 28.

The operation of the fuel cell system is controlled by the control unit 50. The control unit 50 is configured as a microcomputer including a CPU, a memory and the like inside. The control unit 50 receives and outputs various inputs and outputs required for control. In the figure, a part thereof is illustrated. In order to realize the control, the signals input to the control unit 50 include, for example, a power generation request from the outside, an operating state of the pump 26, and flow rate sensors 31 and 24 that detect the supply amounts of air and hydrogen. Flow rate sensor 31, 24
May be replaced by a pressure sensor. The output signal from the control unit 50 includes, for example, a signal for controlling opening / closing of the valve 21 and the shutoff valve 28.

B. Fuel Cell Operation Control Process: FIG. 2 is a flowchart of the fuel cell operation control process. This is a process that the control unit 50 repeatedly executes at a predetermined timing. When this process is started, the control unit 50 inputs a power generation request from the outside (step S10), and accordingly sets the supply amounts of air and hydrogen to be supplied to the fuel cell 10 (step S11). ). This setting can be performed, for example, by referring to a map in which the power generation request and the supply amount are associated with each other.

Next, the control unit 50 confirms the operating state of the pump 26 and, if there is an abnormality, limits its output (steps S12 and S13). The abnormality is the pump 26
It can be determined by monitoring the output from the device and the temperature. It is not always necessary to positively limit the output, and the processes of steps S12 and S13 may be omitted.

The control unit 50 detects the output of the pump 26 and the air flow rate (step S14), and sets the open duty of the shut valve 28 (step S15). The open duty means the time density during which the shut valve 28 is open.

FIG. 3 is an explanatory diagram showing an example of a map giving an open duty. The open duty is set by referring to this map. The open duty of the shutoff valve 28 changes depending on the air flow rate and the pump 26. The open duty increases as the air flow rate increases. Further, the open duty increases as the pump output decreases. For example,
When the air flow rate is constant at Af, the open duty has a relatively small value Da when the pump 26 is normal, and has a relatively large value Db when the pump 26 is stopped.

First, the relationship between the open duty and the air flow rate will be described. As described above, the shut valve 28
Are opened to vent the anode offgas to the outside and reduce the concentration of nitrogen and water vapor. Since the amounts of these components have a correlation with the flow rate of the air supplied to the fuel cell 10, it is preferable to increase the open duty and increase the discharge efficiency of the anode off gas in accordance with the increase of the air flow rate.

The relationship between the open duty and the pump output will be described. The pump 26 has a function of efficiently circulating and discharging a relatively low pressure anode off-gas. When the output of the pump 26 decreases, the efficiency of circulation and discharge decreases, and the vicinity of the discharge port 21b of the fuel cell 10 (area A in FIG. 1).
At higher concentrations of nitrogen and water vapor. In order to avoid such a state, when the output of the pump 26 is reduced, it is preferable to increase the open duty to increase the discharge amount of the anode off gas to the outside.

The open duty map in this embodiment is set based on the above concept. here,
Although the map in which the open duty changes in five steps according to the pump output is illustrated, it may change continuously. On the contrary, although a map that continuously changes with respect to the air flow rate is illustrated, the map may be changed stepwise. It is also possible to use a map that does not depend on the air flow rate. Moreover, although the case where it changes linearly with respect to the air flow rate and the pump output is illustrated, it may be non-linear. In the present embodiment, the case where the map is used has been illustrated, but the set value corresponding to the map may be given as a function of the air flow rate and the pump output.

The control unit 50 determines whether or not the shut valve 28 can realize the open duty set by the process of step S15 (step S1).
6). When the set value of the open duty can be realized, the opening / closing of the shut valve 28 is controlled based on the set value (step S17). If the set value cannot be realized, the operation of the fuel cell 10 is stopped (step S18). In such a case, the concentration of nitrogen and water vapor becomes high, and the fuel cell 1
This is because the operating efficiency of 0 decreases and the life of the fuel cell 10 may be shortened due to dew condensation.

C. Effect: According to the fuel cell system of the present embodiment described above, by controlling the open duty of the shut valve 28 according to the output of the pump 26 for circulation, the fuel cell 10 of the fuel cell 10 is controlled even when the output of the pump 26 is reduced. It is possible to suppress a decrease in operating efficiency.

D. Modification: The fuel cell system of the present invention is applicable to the power source of various devices. For example, the present invention can be configured as a power source of a mobile body.

The present invention exemplifies the case where the open duty is controlled according to the air flow rate in addition to the pump output. For this control, various parameters related to the amounts of nitrogen and water vapor contained in the anode offgas can be used instead of the air flow rate. Available parameters are:
For example, the power generation requirement for the fuel cell, the pressure of the gas supplied to the cathode or the anode, the flow rate or the pressure of the gas discharged from the cathode or the anode, and the like can be mentioned.

In this embodiment, the case where the anode off gas is discharged from the shut valve 28 to the outside air has been illustrated. In the present invention, since hydrogen has flammability, a mechanism for oxidizing or burning hydrogen in the anode off-gas before discharging may be provided.

The present invention can be applied not only to a system for supplying hydrogen from a hydrogen tank, but also to a system for producing hydrogen by reforming.

Although various embodiments of the present invention have been described above, it is needless to say that the present invention is not limited to these embodiments and that various configurations can be adopted without departing from the spirit of the invention. For example, the above control processing may be realized by hardware as well as software.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a schematic configuration of a fuel cell system as an example.

FIG. 2 is a flowchart of a fuel cell operation control process.

FIG. 3 is an explanatory diagram showing an example of a map that gives an open duty.

[Explanation of symbols]

10 ... Fuel cell 11a ... Supply port 11b ... outlet 12a ... Supply port 12b ... outlet 13 ... Electrolyte membrane 20 ... Hydrogen tank 21 ... Valve 22, 23 ... Regulator 25 ... Check valve 26 ... Pump 27 ... Condenser 28 ... Shut valve 30 ... Pump 24, 31 ... Flow rate sensor 50 ... Control unit

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Takashi Furudo             1 Toyota Town, Toyota City, Aichi Prefecture Toyota Auto             Car Co., Ltd. (72) Inventor Hideaki Mizuno             1 Toyota Town, Toyota City, Aichi Prefecture Toyota Auto             Car Co., Ltd. F-term (reference) 5H026 AA06                 5H027 AA06 BA01 BA13 BA19 KK05                       KK25 KK26 KK52 MM08

Claims (8)

[Claims] 1. A fuel cell system, comprising: a fuel cell having a hydrogen electrode and an oxygen electrode; a circulation passage for circulating exhaust gas of the hydrogen electrode to the hydrogen electrode; and a circulation passage provided in the circulation passage. A circulation pump, a communication section for discharging the exhaust gas to the outside of the circulation flow path, and a communication control section for controlling opening / closing of the communication section are provided, and the communication control section controls the open / closed state of the communication section. A fuel cell system that controls according to the output of the circulation pump. 2. The fuel cell system according to claim 1, wherein the communication control unit increases a time density in which the communication unit is in an open state when the output of the circulation pump decreases. 3. The fuel cell system according to claim 1, wherein the communication control unit further performs the control based on an output of the fuel cell. 4. The fuel cell system according to claim 3, wherein the communication control unit increases the time density in which the communication unit is open when the output of the fuel cell increases. 5. The fuel cell system according to claim 3, wherein the communication control unit has a required output value for the fuel cell and a gas supplied to the fuel cell as parameters indicating the output of the fuel cell. And the flow rate of the gas discharged from the fuel cell, the fuel cell system performing the control using at least one. 6. The fuel cell system according to claim 1, wherein maximum achievable maximum values of the output of the circulation pump and the time density at which the communication section is in an open state are equal to or less than predetermined values. A fuel cell system including stop control means for stopping the operation of the fuel cell in the case of: 7. The fuel cell system according to claim 1, wherein the communication section releases the exhaust gas to the atmosphere without processing hydrogen. 8. A method of controlling a fuel cell system, the fuel cell system comprising: a fuel cell having a hydrogen electrode and an oxygen electrode; and a circulation channel for circulating exhaust gas of the hydrogen electrode to the hydrogen electrode. A circulation pump provided in the circulation flow passage, and a communication unit for discharging the exhaust gas to the outside of the circulation flow passage, the control method including a step of detecting an output of the circulation pump. Controlling the open / closed state of the communication unit according to the output of the circulation pump.