FUEL CELL SYSTEM AND CONTROL METHOD FOR FUEL CELL SYSTEM

A fuel cell system including an air discharge valve provided in an air discharge path, through which air discharged from a fuel cell stack flows, a bypass valve provided in a bypass path connecting an air supply path and the air discharge path and bypassing the fuel cell stack, and a controller for controlling the air discharge valve to make an opening degree of the air discharge valve smaller than full opening, controlling the bypass valve to make the opening degree of the bypass valve larger than fully closure, and controlling a compressor to supply air to the air supply path through which the air to be supplied to the fuel cell stack flows.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2022-144968 filed on Sep. 13, 2022, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a fuel cell system and a control method for the fuel cell system.

Description of the Related Art

JP 2007-053015 A discloses a fuel cell system. While the fuel cell system is in operation, air is fed as an oxygen-containing gas from a compressor to a fuel cell stack via a humidity exchanger. An oxygen-containing off-gas from the fuel cell stack is discharged to the outside of the fuel cell system through a first exhaust pipe routed through the humidity exchanger. In the humidity exchanger, the oxygen-containing gas to be supplied to the fuel cell stack is humidified by the water contained in the humid oxygen-containing off-gas from the fuel cell stack.

While the fuel cell system is not in operation, the air is supplied from the compressor to the fuel cell stack to scavenge the fuel cell stack. The air that has passed through the fuel cell stack is discharged to the outside of the fuel cell system through a second exhaust pipe not routed through the humidity exchanger. Thus, water inside the fuel cell stack is removed.

SUMMARY OF THE INVENTION

While the power generation by the fuel cell stack is being stopped, the amount of air supplied from the compressor is set to be relatively small in order to suppress noise, vibrations, and the like. If the compressor discharges a small amount of air, surges may occur in the compressor.

An object of the present invention is to solve the above-described problems.

According to a first aspect of the present invention, there is provided a fuel cell system for generating electrical power by chemical reactions caused in a fuel cell stack by supplying hydrogen as a fuel gas to an anode of the fuel cell stack and supplying air as an oxygen-containing gas to a cathode of the fuel cell stack, the fuel cell system including: an air supply path through which the air to be supplied to the fuel cell stack flows; an air discharge path through which the air discharged from the fuel cell stack flows; a drain path through which water discharged from the fuel cell stack flows; a bypass path connecting the air supply path to the air discharge path while bypassing the fuel cell stack; a bypass valve disposed on the bypass path and configured to adjust a flow rate of the air flowing through the bypass path; an air discharge valve disposed between the fuel cell stack and the bypass path on the air discharge path and configured to adjust a flow rate of the air flowing through the air discharge path; a compressor configured to supply the air to the air supply path; and a controller configured to control the compressor, the bypass valve and the air discharge valve, wherein in scavenging the fuel cell stack, the controller controls the air discharge valve to have an opening degree smaller than a fully opened state at a maximum, controls the bypass valve to have an opening degree greater than a fully closed state at a minimum, and controls the compressor to supply the air to the air supply path.

According to a second aspect of the present invention, there is provided a control method for a fuel cell system for generating electrical power by chemical reactions caused in a fuel cell stack by supplying a fuel gas to an anode of the fuel cell stack and supplying air to a cathode of the fuel cell stack, the fuel cell system including: an air supply path through which the air to be supplied to the fuel cell stack flows; an air discharge path through which the air discharged from the fuel cell stack flows; a drain path through which water discharged from the fuel cell stack flows; a bypass path connecting the air supply path to the air discharge path while bypassing the fuel cell stack; a bypass valve disposed on the bypass path and configured to adjust a flow rate of the air flowing through the bypass path; an air discharge valve disposed on the air discharge path between the fuel cell stack and the bypass path and configured to adjust a flow rate of the air flowing through the air discharge path; and a compressor configured to supply the air to the air supply path, the control method including, in scavenging the inside of the fuel cell stack, setting an opening degree of the air discharge valve to be smaller than a fully opened state at a maximum, setting an opening degree of the bypass valve to be greater than a fully closed state at a minimum, and supplying the air from the compressor to the air supply path.

According to the present invention, surges in the compressor can be suppressed during scavenging of the fuel cell system.

DETAILED DESCRIPTION OF THE INVENTION

First Embodiment

Configuration of Fuel Cell System

FIG.1is a schematic diagram of a fuel cell system10. The fuel cell system10may be mounted in, for example, a fuel cell vehicle or the like.

The fuel cell system10includes a fuel cell stack12, an oxygen-containing gas supply/discharge unit14, a fuel gas supply/discharge unit16, and a controller18. InFIG.1, the configuration of the fuel gas supply/discharge unit16is omitted.

The fuel cell stack12has a plurality of power generation cells20stacked one another. Each of the power generation cells20includes a membrane electrode assembly22and a pair of separators24(separators24aand24b). The membrane electrode assembly22is sandwiched between the pair of separators24.

The membrane electrode assembly22includes an electrolyte membrane26, a cathode28, and an anode30. The cathode28is provided on one surface of the electrolyte membrane26. The anode30is provided on the other surface of the electrolyte membrane26.

An oxygen-containing gas flow field32is formed on the separator24ato allow the oxygen-containing gas to flow on one surface of the membrane electrode assembly22. A fuel gas flow field34is formed on the separator24bto allow the fuel gas to flow on the other surface of the membrane electrode assembly22.

The oxygen-containing gas is supplied to the fuel cell stack12by the oxygen-containing gas supply/discharge unit14. The oxygen-containing gas flows into the oxygen-containing gas flow field32of each of the power generation cells20. The oxygen-containing gas is used for the chemical reactions at the cathodes28. The unconsumed oxygen-containing gas (oxygen-containing off-gas) is discharged from the fuel cell stack12to the oxygen-containing gas supply/discharge unit14.

The fuel gas is supplied to the fuel cell stack12by the fuel gas supply/discharge unit16. The fuel gas flows into the fuel gas flow field34of each of the power generation cells20. The fuel gas is used for the chemical reactions at the anodes30. The unconsumed fuel gas (fuel off-gas) is discharged from the fuel cell stack12to the fuel gas supply/discharge unit16.

The oxygen-containing gas supply/discharge unit14includes a compressor36, a humidifier38, an air supply path40, an air discharge path42, a drain path44, and a bypass path46.

The compressor36supplies air as the oxygen-containing gas to the air supply path40. The oxygen-containing gas supplied to the air supply path40is humidified by the humidifier38and supplied to the fuel cell stack12. The chemical reactions at the cathodes28produces water. The produced water flows from the fuel cell stack12to the drain path44. The produced water having flowed into the drain path44is discharged to the outside of the fuel cell system10. The oxygen-containing off-gas contains some of the produced water. The oxygen-containing off-gas flows from the fuel cell stack12to the air discharge path42. The produced water contained in the oxygen-containing off-gas flowing into the air discharge path42is collected in the humidifier38, and then the oxygen-containing off-gas is discharged to the outside of the fuel cell system10.

In a state where the power generation by the fuel cell stack12is being stopped, scavenging is performed to remove water from the cathode28of each of the power generation cells20. During scavenging, the compressor36supplies air to the air supply path40. The air fed to the air supply path40is supplied to the fuel cell stack12. The water in the cathode28of each of the power generation cells20flows from the fuel cell stack12to the drain path44together with the air supplied to the fuel cell stack12. The water discharged to the drain path44is drained to the outside of the fuel cell system together with the air.

The bypass path46bypasses the fuel cell stack12and connects the air supply path40to the air discharge path42. The bypass path46has a bypass valve48. The bypass valve48adjusts the flow rate of the air flowing through the bypass path46.

The humidifier38absorbs the water contained in the oxygen-containing off-gas discharged from the fuel cell stack12to the air discharge path42, and humidifies the oxygen-containing gas from the air supply path40so that the oxygen-containing gas thus humidified is to be supplied to the fuel cell stack12. The humidifier38is disposed between the bypass path46and the fuel cell stack12on both the air supply path and the air discharge path42.

The air discharge path42has an air discharge valve50. The air discharge valve50is disposed on the air discharge path42between the fuel cell stack12and the bypass path46. The air discharge valve50is provided between the humidifier38and the bypass path46on the air discharge path42. The air discharge valve50adjusts the flow rate of air in the air discharge path42.

The controller18controls the compressor36, the bypass valve48, and the air discharge valve50. The control by the controller18will be described in detail below.

Configuration of Control Unit

FIG.2is a block diagram illustrating a configuration of the controller18. The controller18includes a computation unit52and a storage unit54. The computation unit52is, for example, a processor such as a central processing unit (CPU) or a graphics processing unit (GPU). The computation unit52includes a power generation state determination unit56, a target supply setting unit58, a compressor control unit59, an ambient pressure acquisition unit60, an ambient temperature acquisition unit64, a target opening degree setting unit68, and a valve control unit70. The power generation state determination unit56, the target supply setting unit58, the compressor control unit59, the ambient pressure acquisition unit60, the ambient temperature acquisition unit64, the target opening degree setting unit68, and the valve control unit70can be realized by the computation unit52executing programs which are stored in the storage unit54. At least a part of the power generation state determination unit56, the target supply setting unit58, the compressor control unit59, the ambient pressure acquisition unit60, the ambient temperature acquisition unit64, the target opening degree setting unit68, and the valve control unit70may be realized by an integrated circuit such as an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA) and the like. At least a part of the power generation state determination unit56, the target supply setting unit58, the compressor control unit59, the ambient pressure acquisition unit60, the ambient temperature acquisition unit64, the target opening degree setting unit68, and the valve control unit70may be realized by an electronic circuit including a discrete device.

The storage unit54may be made up of a volatile memory (not shown), and a non-volatile memory (not shown), as a computer-readable storage medium. Examples of the volatile memory include, for example, a RAM (Random Access Memory) or the like. As the non-volatile memory, there may be cited, for example, a ROM (Read Only Memory), a flash memory, or the like. Data, etc. may be stored in the volatile memory, for example. Programs, tables, maps, and the like are stored, for example, in the nonvolatile memory. At least a portion of the storage unit54may be provided in the processor, the integrated circuit, or the like, which were described above. At least a part of the storage unit54may be mounted on a device connected to the fuel cell system10via a network.

The power generation state determination unit56determines whether the fuel cell stack12is generating electrical power or is not generating electrical power.

The target supply setting unit58sets a target value of the volume of the oxygen-containing gas (air) supplied from the compressor36to the air supply path40per unit time. Hereinafter, the volume of the oxygen-containing gas (air) supplied from the compressor36to the air supply path40per unit time may be referred to as a supply amount. Further, the target value of the supply amount may be referred to as a target supply amount.

During scavenging, the target supply amount is set to a predetermined supply amount. In the case where the supply amount of the compressor36is equal to or less than the predetermined supply amount, the compressor36can be rotated at a relatively low speed. As a result, noise, vibrations, and the like caused by the compressor36are suppressed. As a result, for example, while the fuel cell vehicle is stopped, discomfort given to an occupant in the vehicle is reduced. The compressor control unit59controls the compressor36based on the target supply amount.

The ambient pressure acquisition unit60acquires the ambient pressure from the ambient pressure measurement unit62. The ambient pressure measurement unit62is provided in, for example, the fuel cell vehicle or the like. The ambient pressure measurement unit62measures the ambient pressure outside the vehicle.

The ambient temperature acquisition unit64acquires the ambient temperature from the temperature measurement unit66. The temperature measurement unit66is provided in, for example, the fuel cell vehicle or the like. The temperature measurement unit66measures the temperature of air outside the vehicle. The temperature measurement unit66may measure the temperature of the air at the air inlet of the compressor36.

The target opening degree setting unit68sets a target opening degree of the bypass valve48. The target opening degree of the bypass valve48during the scavenging is set based on the ambient pressure acquired by the ambient pressure acquisition unit60and the ambient temperature acquired by the ambient temperature acquisition unit64. The target opening degree of the bypass valve48is set to be larger as the ambient pressure is higher. The target opening degree of the bypass valve48is set to be larger as the ambient temperature is lower. That is, the target opening degree of the bypass valve48is set to be larger as the density (mass per unit volume) of the air taken into the compressor36is larger.

The valve control unit70controls the air discharge valve50and the bypass valve48. During scavenging, the valve control unit70controls the air discharge valve50to close. At the time of scavenging, the valve control unit70controls the bypass valve48to set the opening degree of the bypass valve48to the target opening degree set by the target opening degree setting unit68. By adjusting the opening degree of the bypass valve48, reduction in the amount of the air actually supplied from the compressor36is suppressed. Thus, surges in the compressor36can be suppressed.

Scavenging Control Process

FIG.3is a flowchart illustrating a scavenging control processing procedure executed in the controller18. The scavenging control process is executed once or a plurality of times after power generation by the fuel cell stack12is stopped.

In step S1, the power generation state determination unit56determines whether or not power generation has stopped in the fuel cell stack12. When it is determined that the power generation is stopped in the fuel cell stack12(step S1: YES), the process transitions to step S2. When it is determined that the fuel cell stack12is in power generation operation (step S1: NO), the scavenging control process is brought to an end. When it is determined that the power generation is stopped in the fuel cell stack12, the valve control unit70controls the bypass valve48to open the bypass valve48.

In step S2, the target supply amount setting unit58sets the target supply amount of the compressor36to a predetermined supply amount. The process then transitions to step S3.

In step S3, the valve control unit70determines whether or not the bypass valve48is opened. When the bypass valve48is opened (step S3: YES), the process transitions to step S4. When the bypass valve48is not opened (step S3: NO), the determination of step S3is repeated.

In step S4, the valve control unit70closes the air discharge valve50. The process then transitions to step S5. The valve control unit70may fully close the air discharge valve50or set the opening degree of the air discharge valve50to be smaller than that in the fully-opened state.

In step S5, the target opening degree setting unit68sets the target opening degree of the bypass valve48based on the ambient pressure acquired by the ambient pressure acquisition unit60and the ambient temperature acquired by the ambient temperature acquisition unit64. The process then transitions to step S6.

In step S6, the compressor control unit59drives the compressor36at the target supply amount set at step S2. The process then transitions to step S7.

In step S7, the valve control unit70opens the bypass valve48at the target opening degree set in step S5. The process then transitions to step S8.

In step S8, the compressor control unit59determines whether or not a predetermined time has elapsed from the start of driving the compressor36. In the case where the predetermined time has elapsed (step S8: YES), the process transitions to step S9. In the case where the predetermined time has not elapsed (step S8: NO), the determination of step S8is repeated. When the predetermined time has elapsed from the start of the driving of the compressor36, it is determined that the scavenging inside the fuel cell stack12is completed.

In step S9, the compressor control unit59stops the compressor36. The process then transitions to step S10.

In step S10, the valve control unit70puts the bypass valve48and the air discharge valve50in the open state. Thereafter, the scavenging control process is brought to an end. The valve control unit70may close the bypass valve48and the air discharge valve50. The valve control unit70may open one of the bypass valve48and the air discharge valve50and close the other.

Advantageous Effects

While power generation is stopped in the fuel cell stack12, devices driven by electric power supplied from the fuel cell system10are often stopped. In a state where the other devices are stopped, noise and vibrations caused by the driving of the compressor36are likely to be felt by the user, and the user is likely to feel discomfort.

Therefore, during scavenging in which the power generation is stopped in the fuel cell stack12, the target supply amount of the compressor36is set to a predetermined supply amount. In the case where the supply amount of the compressor36is equal to or less than the predetermined supply amount, the compressor36can be rotated at a relatively low speed. As a result, noise, vibrations, and the like caused by of the compressor36are suppressed. Thus, discomfort given to the user is suppressed.

On the other hand, in scavenging, since the amount of the air supplied from the compressor36is small, the volume of the air supplied to the fuel cell stack12is small, and the pressure in the oxygen-containing gas flow field32in each of the power generation cells20becomes low. As a result, there is a problem that it takes a long time to remove water from the cathodes28.

Therefore, in the fuel cell system10of the present embodiment, in scavenging the fuel cell stack12, the controller18controls the air discharge valve50to make the opening degree of the air discharge valve50smaller than the fully-opened state at a maximum. In this manner, because resistance in the air discharge path42increases, the pressure in the oxygen-containing gas flow field32in each of the power generation cells20can be made higher. As a result, the fuel cell system10can remove water from the cathode28in a short time.

When the pressure in the air supply path40increases as the pressure in the oxygen-containing gas flow field32in each of the power generation cells20increases, the amount of the air discharged from the compressor36decreases. In scavenging, because the target supply amount of the compressor36is set to be relatively small, if the amount of the air discharged from the compressor36decreases in this state, a surge is likely to occur in the compressor36. Therefore, in the fuel cell system of the present embodiment, in scavenging the fuel cell stack12, the controller18controls the bypass valve48to open at an opening degree larger than the fully-closed state at a minimum. As a result, the amount of air supplied to the fuel cell stack12is adjusted while the amount of air discharged from the compressor36is also adjusted to an amount with which a surge in the compressor36can be avoided. As a result, the fuel cell system10can suppress surges in the compressor36.

In the fuel cell system10of the present embodiment, the air discharge valve50is provided between the bypass path46and the humidifier38on the air discharge path42. If the air discharge valve50is opened in a state where the bypass valve48is closed, resistance in the air discharge path42decreases and the flow rate of the air in the air discharge path42can be increased. Therefore, by opening the air discharge valve50, a decrease in the amount of air discharged from the compressor36is suppressed. However, since the humidifier38is provided in the air discharge path42, there is a possibility that the resistance in the air discharge path42cannot be sufficiently reduced as desired even if the air discharge valve50is opened. In the fuel cell system10of the present embodiment, a decrease in the amount of air discharged from the compressor36is suppressed by adjusting the bypass valve48of the bypass path46instead of the air discharge valve50of the air discharge path42in which the humidifier38is provided. Thus, the fuel cell system10can suppress surges in the compressor36.

In the fuel cell system10of the present embodiment, in scavenging the inside of the fuel cell stack12, the controller18controls the air discharge valve50and the bypass valve48so that the flow rate of the oxygen-containing gas in the bypass path46is higher than the flow rate of the oxygen-containing gas in the air discharge path42. Thus, the fuel cell system10can suppress surges in the compressor36.

In the fuel cell system10of the present embodiment, in scavenging the inside of the fuel cell stack12, the controller18controls the air discharge valve50to close. As a result, air is not discharged from the air discharge path42, so that the pressure in the oxygen-containing gas flow field32in each of the power generation cells20can be increased. As a result, the fuel cell system10can remove water from the cathode28in a short time.

In the fuel cell system10of the present embodiment, in scavenging the inside of the fuel cell stack12, the controller18sets the target opening degree of the bypass valve48based on the ambient pressure acquired by the ambient pressure acquisition unit60and the ambient temperature acquired by the ambient temperature acquisition unit64. Thus, it is possible to prevent the mass of the air supplied to the fuel cell stack12from becoming excessive. Therefore, it is possible to prevent the pressure in the oxygen-containing gas flow field32in the fuel cell stack12from becoming excessively high. As a result, the fuel cell system10can suppress surges in the compressor36.

Invention Obtained from Embodiments

The invention understood from the above embodiment will be described below.

The fuel cell system (10) for generating electrical power by chemical reactions caused in the fuel cell stack (12) by supplying hydrogen as the fuel gas to the anode (30) of the fuel cell stack (12) and supplying air as the oxygen-containing gas to the cathode (28) of the fuel cell stack. The fuel cell system includes: the air supply path (40) through which the air to be supplied to the fuel cell stack flows; the air discharge path (42) through which the air discharged from the fuel cell stack flows; a drain path (44) through which water discharged from the fuel cell stack flows; the bypass path (46) connecting the air supply path to the air discharge path while bypassing the fuel cell stack; the bypass valve (48) disposed on the bypass path and configured to adjust the flow rate of the air flowing through the bypass path; the air discharge valve (50) disposed between the fuel cell stack and the bypass path on the air discharge path and configured to adjust the flow rate of the air flowing through the air discharge path; the compressor (36) configured to supply the air to the air supply path; and the controller (18) configured to control the compressor, the bypass valve and the air discharge valve, wherein in scavenging the inside of the fuel cell stack, the controller controls the air discharge valve to have the opening degree smaller than the fully opened state at a maximum, controls the bypass valve to have the opening degree greater than the fully closed state at a minimum, and controls the compressor to supply the air to the air supply path. Thus, the fuel cell system can suppress surges in the compressor.

The above-described fuel cell system may further include a humidifier (38) provided between the bypass path and the fuel cell stack on the air supply path and the air discharge path and configured to humidify the air to be supplied to the fuel cell stack by the water contained in the air discharged from the fuel cell stack, and the air discharge valve may be disposed between the bypass path and the humidifier. Thus, the fuel cell system can suppress surges in the compressor.

In the above-described fuel cell system, in scavenging the inside of the fuel cell stack, the control unit may control the air discharge valve and the bypass valve to make the flow rate of the air in the bypass path higher than the flow rate of the air in the air discharge path. Thus, the fuel cell system can suppress surges in the compressor.

In the above-described fuel cell system, in scavenging the inside of the fuel cell stack, the control unit may control the air discharge valve to close. As a result, the fuel cell system can remove water from the cathode in a short time.

The above-described fuel cell system may further include the ambient pressure acquisition unit (60) configured to acquire the ambient pressure, and the ambient temperature acquisition unit (64) configured to acquire the temperature of the air taken into the compressor. In scavenging the fuel cell stack, the control unit may set the opening degree of the bypass valve based on the ambient pressure and the temperature of the air. Thus, the fuel cell system can suppress surges in the compressor.

In the control method for the fuel cell system for generating electrical power by chemical reactions caused in a fuel cell stack by supplying the fuel gas to an anode of the fuel cell stack and supplying air to the cathode of the fuel cell stack, the fuel cell system including: the air supply path through which the air to be supplied to the fuel cell stack flows; the air discharge path through which the air discharged from the fuel cell stack flows; a drain path through which water discharged from the fuel cell stack flows; the bypass path connecting the air supply path to the air discharge path while bypassing the fuel cell stack; the bypass valve disposed on the bypass path and configured to adjust the flow rate of the air flowing through the bypass path; the air discharge valve disposed between the fuel cell stack and the bypass path on the air discharge path and configured to adjust the flow rate of the air flowing through the air discharge path; and the compressor configured to supply the air to the air supply path. The control method includes, in scavenging the inside of the fuel cell stack, setting the opening degree of the air discharge valve to be smaller than a fully opened state at a maximum, setting the opening degree of the bypass valve to be greater than the fully closed state at a minimum, and supplying the air from the compressor to the air supply path. Thus, the fuel cell system can suppress surges in the compressor.

Moreover, the present invention is not limited to the above-described disclosure, and various configurations can be adopted therein without departing from the essence and gist of the present invention.