Fuel cell system

Abstract

The fuel cell system disclosed in the present specification is provided with: a fuel cell stack in which a plurality of single cells are stacked; and a controller that executes an idling operation that maintains an output voltage of the fuel cell stack within a predetermined idling voltage range. The controller is configured to perform a reset process if a prescribed reset condition is satisfied, and to return to the idling operation again. By performing the reset process, it is possible to suppress exacerbation of degradation of the fuel cell stack. One example of the reset process is a process of making the oxygen flow rate supplied to the fuel cell stack greater than a maximum value of the oxygen flow rate supplied to the fuel cell stack in the idling operation. One example of the reset condition is that a prescribed idling permission time elapses from the start of the idling operation.

Description

Technical Field

[0001] The technology disclosed in this specification relates to fuel cell systems. Background Technology

[0002] A fuel cell system comprises a fuel cell stack with multiple individual cells and a compressor that supplies air to the fuel cell stack. Japanese Patent Application Publication No. 2010-244937 describes intermittent operation of fuel cells. According to this document, intermittent operation refers to maintaining the compressor's speed, which supplies oxygen (air) to the fuel cell stack, at a predetermined low speed to keep the fuel cell stack's output voltage within a specified range, thereby reducing the processing of oxygen supplied to the fuel cell stack. A voltage converter is connected between the output of the fuel cell stack and the device to which the power is supplied, and a battery is connected in parallel to the output of the voltage converter. When the power demand of the device to which the power is supplied is low, the fuel cell system controller causes the fuel cell stack to operate intermittently and adjusts the voltage ratio of the voltage converter to substantially suppress the current output from the fuel cell stack. Power is supplied to the device to which the power is supplied from the battery. Intermittent operation is implemented when the fuel cell stack's power generation efficiency is low. By operating intermittently, power generation in areas of low efficiency is avoided.

[0003] During intermittent operation, the output voltage range of the fuel cell is set to a range where the degradation of a single cell is relatively slow. However, if intermittent operation with a continuously reduced supply of oxygen is maintained for an extended period, oxygen inhomogeneity occurs within the fuel cell stack, resulting in deviations in the output voltage of each single cell. If the output voltages of multiple single cells deviate from the range suitable for degradation suppression, the overall degradation of the fuel cell stack accelerates. This specification provides a technique for suppressing the accelerated degradation of an FC stack during intermittent operation. Furthermore, in this specification, the situation of maintaining the output voltage of the fuel cell stack within a specified range (idle voltage range) instead of the term "intermittent operation" is referred to as "idle operation." Summary of the Invention

[0004] The fuel cell system disclosed in this specification includes: a fuel cell stack having multiple individual cells stacked on top of each other; and a controller that performs idle operation to maintain the output voltage of the fuel cell stack within a predetermined idle voltage range. The controller is configured to perform a reset process and return to idle operation if a predetermined reset condition is met. By performing the reset process, the accelerated degradation of the fuel cell stack can be suppressed.

[0005] One method of reset is to ensure that the oxygen flow rate supplied to the fuel cell stack is greater than the maximum oxygen flow rate supplied during idling. Another method is to ensure that the oxygen pressure supplied to the fuel cell stack is lower than the minimum oxygen pressure supplied during idling. Furthermore, another method is to maintain the fuel cell stack's output voltage at a level exceeding the idling voltage range for a specified reset time. Alternatively, the reset process could involve ensuring that the fuel cell stack's output current is greater than the maximum output current during idling. Any of these reset processes mitigates oxygen unevenness within the fuel cell stack, thereby suppressing subsequent degradation of individual cells.

[0006] An example of a reset condition is that a specified idle time has elapsed since the start of idling operation. Another example of a reset condition is that the number of single cells whose output voltage exceeds the specified upper limit voltage for a single cell exceeds the specified upper limit number of cells. Alternatively, a reset condition is that the number of single cells whose output voltage is below the specified lower limit voltage for a single cell exceeds the specified lower limit number of cells. Yet another example of a reset condition is that the number of single cells whose output voltage deviates from the specified single cell voltage range is below the specified allowable number of cells.

[0007] The detailed description of the technology disclosed in this specification and further improvements are described in the following "Detailed Description". Attached Figure Description

[0008] The features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, in which the same reference numerals denote the same elements, wherein,

[0009] Figure 1 This is a block diagram of the fuel cell system in an embodiment. Detailed Implementation

[0010] The fuel cell system 2 of the embodiment will be described with reference to the accompanying drawings. Hereinafter, for ease of explanation, "fuel cell" will be abbreviated as "FC". "Fuel cell system" will be referred to as "FC system", and "fuel cell stack" will be referred to as "FC stack".

[0011] Figure 1 This is a block diagram of FC system 2. FC system 2 supplies power to electrical equipment 100. FC system 2 includes an FC assembly 10 with multiple individual cells 11 stacked on top of each other, an oxygen supply pipe 12, an oxygen exhaust pipe 13, a hydrogen supply pipe 14, an exhaust pipe 15, a return pipe 16, a hydrogen tank 17, and a compressor 21. Oxygen is supplied to the cathode of FC assembly 10, and hydrogen is supplied to the anode. Here, the term "oxygen" includes pure oxygen and oxygen-containing gases (typically air).

[0012] A "single cell" is the smallest unit of construction that generates electricity through the reaction of hydrogen and oxygen. In the case of solid polymer fuel cells, the membrane electrode assembly (MEA) is equivalent to a "single cell".

[0013] Oxygen supply pipe 12 is connected to the cathode inlet of FC group 10. Oxygen discharge pipe 13 is connected to the cathode outlet of FC group 10. A compressor 21, a pressure regulating valve 22, a flow meter 23, and a pressure sensor 29 are connected to the oxygen supply pipe 12. The compressor 21 compresses oxygen (external air) and delivers the compressed oxygen to the cathode inlet of FC group 10 through the oxygen supply pipe 12. The flow meter 23 measures the amount of oxygen supplied to the cathode inlet of FC group 10. The pressure regulating valve 22 adjusts the pressure of the oxygen supplied to the cathode inlet of FC group 10. The pressure sensor 29 measures the pressure of the oxygen supplied to the cathode inlet of FC group 10.

[0014] Hydrogen supply pipe 14 is connected to the anode inlet of FC group 10. Exhaust pipe 15 is connected to the anode outlet of FC group 10. Hydrogen tank 17, main shut-off valve 25, injector 24, and pressure sensor 30 are connected to hydrogen supply pipe 14. Hydrogen in hydrogen tank 17 is supplied to the anode inlet of FC group 10 through hydrogen supply pipe 14. Injector 24 adjusts the amount of hydrogen supplied to the anode inlet of FC group 10. Pressure sensor 30 measures the pressure of the hydrogen supplied to the anode inlet of FC group 10.

[0015] As is well known, in FC group 10 (in each individual cell 11), oxygen reacts with hydrogen to generate electrons and water. Residual oxygen not used in the reaction is discharged to the outside from the cathode outlet through oxygen discharge pipe 13. Oxygen discharge pipe 13 is equipped with a pressure regulating valve 26 to adjust the pressure of the discharged residual oxygen.

[0016] The water generated in the reaction and the remaining hydrogen are conveyed from the anode outlet to the gas-liquid separator 27 through exhaust pipe 15. In the gas-liquid separator 27, the exhaust gas is separated into remaining hydrogen and water. The remaining hydrogen is returned to FC group 10 through return pipe 16. The water is mixed with the remaining oxygen and conveyed to silencer 28, from which it is discharged to the outside.

[0017] The input terminal 31a of voltage converter 31 is connected to the power output terminal of FC group 10. The output terminal 31b of voltage converter 31 is connected to the power supply terminal of electrical equipment 100. A storage battery 32 is connected in parallel to the output terminal of voltage converter 31.

[0018] The compressor 21, pressure regulating valves 22 and 26, ejector 24, and voltage converter 31 are controlled by the controller 40. The controller 40 uses the compressor 21 and pressure regulating valves 22 and 26 to adjust the amount of oxygen supplied to the FC group 10. Additionally, the controller 40 uses the ejector 24 to adjust the amount of hydrogen supplied to the FC group 10. Furthermore, a pressure regulating valve may be attached upstream of the ejector 24, and the controller 40 may also use the upstream pressure regulating valve with the ejector 24 to adjust the amount of hydrogen supplied to the FC group 10.

[0019] The controller 40 uses the voltage converter 31 to adjust the output power of the FC group 10. To increase the output power of the FC group 10, the output voltage of the voltage converter 31 is increased. To decrease the output power of the FC group 10, the output voltage of the voltage converter 31 is decreased. In particular, if the output voltage of the voltage converter 31 is lower than the voltage of the battery 32, the output of the voltage converter 31, i.e., the output of the FC group 10, becomes essentially zero.

[0020] When the required power (required power) for electrical equipment 100 exists in a region where the power generation efficiency of FC group 10 is poor, controller 40 causes FC group 10 to idle. Here, idling refers to the following process: controller 40 adjusts the amount of oxygen supplied to FC group 10 so that the output voltage of FC group 10 is maintained within the idle voltage range and the output current of FC group 10 is suppressed to a minimum.

[0021] The allowable idle voltage range is determined to be the range within which the degradation progression of FC group 10 slows down. The degree of degradation depends on the physical and electrical characteristics of FC group 10 (single cell 11). That is, the allowable idle voltage range depends on the physical and electrical characteristics of FC group 10 (single cell 11). The allowable idle voltage range is predetermined through experiments, simulations, etc.

[0022] The controller 40 primarily controls the compressor 21 to maintain the output voltage of the FC group 10 within the idle voltage range and to keep the output current of the FC group 10 at a minimum. To reduce the amount of oxygen supplied to the FC group 10, the controller 40 can, for example, operate the compressor 21 intermittently (repeatedly switching the compressor 21 on and off in short cycles). If a compressor 21 capable of precise speed control is used, the controller 40 can keep the compressor at an extremely low rotational speed.

[0023] The minimum value at which the output current of FC group 10 is not zero is determined by the configuration of FC group 10. That is, the minimum value of the output current of FC group 10 (>0) is predetermined. At the same time, controller 40 controls voltage converter 31 to allow power from battery 32 to flow to electrical equipment 100, and not to allow current to flow from FC group 10 to electrical equipment 100.

[0024] During idling, oxygen and hydrogen are supplied to FC group 10, albeit in small quantities. That is, an oxygen-hydrogen reaction also occurs in FC group 10 during idling. By implementing idling, the deterioration of FC group 10 can be suppressed, and its inefficient power generation can be prevented. Furthermore, the reaction continues in FC group 10 during idling, thus enabling a rapid increase in the output of FC group 10 even when the power demand on it increases dramatically.

[0025] However, during idling, the amount of oxygen supplied to the FC group 10 is low in order to keep the output current of the FC group 10 at a minimum. If the low oxygen supply to the FC group 10 continues, the oxygen distribution within the FC group 10 becomes uneven, resulting in occasional deviations in the output voltage of multiple individual cells. If idling continues for an extended period, the output voltage of the FC group 10 is kept within the idling voltage range, but the output voltage of each individual cell 11 may deviate from the allowable voltage range for individual cells to suppress further degradation. If more individual cells deviate from the allowable voltage range, the degradation of the FC group 10 intensifies. Therefore, in the FC system 2, if the specified idling conditions are met during idling, the idling operation is temporarily reset to reduce the deviation in the output voltage of individual cells. After performing the reset process, the controller 40 returns to idling operation.

[0026] The reset process includes several candidates. One reset process is to increase the oxygen flow rate supplied to FC group 10 to a value greater than the maximum oxygen flow rate supplied to FC group 10 during idling. Another reset process is to decrease the oxygen pressure supplied to FC group 10 to a value lower than the minimum oxygen pressure supplied to FC group 10 during idling. Either reset process will agitate the oxygen distribution in FC group 10, thereby mitigating oxygen inhomogeneity.

[0027] Another method of reset is to maintain the output voltage of FC group 10 at a voltage exceeding the idle voltage range for a specified reset time. Alternatively, the reset process can also involve making the output current of FC group 10 greater than the maximum output current of FC group 10 during idle operation. Even if the output voltage (target value) or output current (target value) of the FC group changes, the oxygen flow rate supplied to FC group 10 will change as a result. Consequently, the unevenness of oxygen in FC group 10 is mitigated.

[0028] An example of a reset condition that triggers the reset process is that a specified idle time has elapsed since the start of idling operation. Another example of a reset condition is that the number of single cells whose output voltage exceeds the specified upper limit voltage for a single cell exceeds the specified upper limit number of cells. A reset condition could also be that the number of single cells whose output voltage is below the specified lower limit voltage for a single cell exceeds the specified lower limit number of cells. Alternatively, a reset condition could be that the number of single cells whose output voltage deviates from the specified permissible voltage range for a single cell is below the specified permissible number of cells.

[0029] The upper limit voltage, lower limit voltage, and allowable voltage range of a single cell depend on the physical and electrical characteristics of the single cell 11. The upper limit voltage of a single cell is predetermined through experimentation or simulation. When the reset condition depends on the voltage of a single cell, the FC system 2 includes a unit for measuring the individual voltages of multiple single cells.

[0030] The specific examples of the present invention have been described in detail above, but these are merely illustrative and do not limit the technical solutions. The technology described in the technical solutions includes various modifications and variations of the specific examples described above. The technical elements described in this specification or drawings exert their technical usefulness individually or in various combinations, and are not limited to the combinations described in the technical solutions at the time of application. Furthermore, the technology illustrated in this specification or drawings can achieve multiple objectives simultaneously, and achieving one of these objectives is itself technically useful.

Claims

1. A fuel cell system, characterized in that, The fuel cell system comprises: Fuel cell stacks consist of multiple individual cells stacked together; and The controller performs idle operation to maintain the output voltage of the fuel cell stack within a specified idle voltage range. The controller is configured such that if a specified reset condition is met during the idling operation, a specified reset process is performed, and the system returns to idling operation.

2. The fuel cell system according to claim 1, characterized in that, The reset process is a process that causes the oxygen flow rate supplied to the fuel cell stack to be greater than the maximum oxygen flow rate supplied to the fuel cell stack during the idling operation.

3. The fuel cell system according to claim 1, characterized in that, The reset process is a process that causes the pressure of oxygen supplied to the fuel cell stack to be lower than the lowest value of oxygen pressure supplied to the fuel cell stack during the idling operation.

4. The fuel cell system according to claim 1, characterized in that, The reset process is a process of maintaining the output voltage of the fuel cell stack at a voltage exceeding the idle voltage range during a specified reset time.

5. The fuel cell system according to claim 1, characterized in that, The reset process is a process that makes the output current of the fuel cell stack greater than the maximum output current of the fuel cell stack during idling.

6. The fuel cell system according to any one of claims 1 to 5, characterized in that, The reset condition is that a predetermined allowable idling time has elapsed since the start of the idling operation.

7. The fuel cell system according to any one of claims 1 to 5, characterized in that, The reset condition is that the number of single cells whose output voltage exceeds the specified upper limit voltage for a single cell exceeds the specified upper limit number of cells.

8. The fuel cell system according to any one of claims 1 to 5, characterized in that, The reset condition is that the number of single cells whose output voltage is lower than the specified lower limit voltage exceeds the specified lower limit number of cells.

9. The fuel cell system according to any one of claims 1 to 5, characterized in that, The reset condition is that the number of single cells whose output voltage deviates from the specified allowable voltage range for a single cell is less than the specified allowable number of cells.

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