JP2021128907A - Fuel cell system - Google Patents

Abstract

To provide a fuel cell system that allows a water content of a fuel cell to reach a target range in a certain period of time when power generation is stopped without constantly controlling the water balance during power generation of the fuel cell.SOLUTION: A fuel cell system includes a fuel cell, a measuring unit that measures a water content of the fuel cell, a temperature raising unit that raises the temperature of scavenging gas of the fuel cell, and a control unit that controls the scavenging of the fuel cell. When stop processing of stopping the power generation by the fuel cell is started, the control unit determines a target temperature of the scavenging gas on the basis of the water content measured by the measuring unit such that the water content reaches a target range within a preset scavenging time, raises the scavenging gas to the target temperature at the temperature rising unit, and performs scavenging with the heated scavenging gas.SELECTED DRAWING: Figure 1

Description

The present invention relates to a fuel cell system.

In recent years, as part of efforts to address environmental issues, the development of low-emission vehicles has been promoted, and one of them is a fuel cell vehicle that uses a fuel cell system as an in-vehicle power source. A fuel cell system causes an electrochemical reaction by supplying a reaction gas to a membrane-electrode assembly in which an anode electrode is arranged on one surface of an electrolyte membrane and a cathode electrode is arranged on the other surface, thereby causing a chemical energy. Is an energy conversion system that converts Among them, the polymer electrolyte electrolyte fuel cell system, which uses a polymer electrolyte membrane as an electrolyte, is expected to be used as an in-vehicle power source because it is low in cost, easy to be compact, and has a high output density. There is.

Inside the gas channel of the fuel cell stack, the generated water generated by the electrochemical reaction of the reaction gas and the humidified water for humidifying the reaction gas remain, and if the power generation is stopped while this residual water is left unattended, In a low temperature environment, the residual water freezes, hindering the diffusion of the reaction gas into the membrane-electrode assembly, and lowering the low temperature startability.

I want to adjust the amount of water remaining inside the fuel cell stack (water content) to an appropriate range, but the water content varies greatly depending on the operating output, temperature, humidity, and so on. Then, when excess water is discharged by scavenging, the scavenging time differs depending on the original water content, which gives a feeling of strangeness to the fuel cell user. The fuel cell system disclosed in Patent Document 1 constantly controls the water balance that flows in and out of the fuel cell stack so that the scavenging time for scavenging water when the fuel cell is stopped is constant.

Japanese Unexamined Patent Publication No. 2008-103169

According to the fuel cell system disclosed in Patent Document 1, in order to constantly control the water balance during power generation of the fuel cell, for example, when a time-consuming control such as lowering the temperature of the fuel cell is required, In some cases, the process shifted to stopping power generation before reaching the target water content. In addition, if priority is given to keeping the water balance following the target value during power generation, the power generation efficiency may be significantly reduced.

The present invention has been made to solve such a problem, and the water content of the fuel cell is set within a target range in a fixed time when the power generation is stopped without constantly controlling the water balance during the power generation of the fuel cell. It provides a fuel cell system to reach.

The fuel cell system according to the first aspect of the present invention controls the fuel cell, the measuring unit for measuring the water content of the fuel cell, the temperature raising unit for raising the temperature of the scavenging gas of the fuel cell, and the scavenging of the fuel cell. A control unit is provided, and the control unit includes a control unit measured by the measurement unit so that the water content reaches the target range within a preset scavenging time when starting the stop process for stopping the power generation by the fuel cell. The target temperature of the scavenging gas is determined based on the amount of water, the scavenging gas is raised to the target temperature by the temperature raising unit, and the scavenging is executed by the raised scavenging gas. In this way, how much the scavenging gas should be warmed according to the water content at the start of the power generation stop treatment and the set scavenging time is determined, and the excess water is discharged by the scavenging gas that has been adjusted to the appropriate temperature. Therefore, the water content can reach the target range in the scavenging time.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a fuel cell system that allows the water content of a fuel cell to reach a target range in a fixed time when power generation is stopped, without constantly controlling the water balance during power generation of the fuel cell.

It is a system block diagram of the fuel cell system which concerns on this embodiment. It is a figure which shows the change of the water content by the difference in the temperature of the scavenging gas. It is a figure which shows the relationship between water content and high frequency impedance. It is a figure which shows the relationship between the scavenging time and the water content. It is a flow chart which shows the processing procedure of a scavenging process. It is a figure which shows the relationship between the cooling water temperature and the water content. It is a figure which shows the relationship between the pressure loss of an air electrode flow path, and the water content.

Hereinafter, the present invention will be described through embodiments of the invention, but the invention according to the claims is not limited to the following embodiments. Moreover, not all of the configurations described in the embodiments are indispensable as means for solving the problem.

FIG. 1 is a system configuration diagram of a fuel cell system according to the present embodiment. In the following description, a fuel cell hybrid vehicle (FCHV) is assumed as an example of a vehicle, but it can also be applied to an electric vehicle or a hybrid vehicle. Further, it can be applied not only to vehicles but also to various moving objects (for example, ships, airplanes, robots, etc.), stationary power supplies, and portable fuel cell systems.

The fuel cell system 10 functions as an in-vehicle power supply system mounted on a fuel cell vehicle, and is a fuel cell stack 20 that receives a supply of reaction gas (fuel gas, oxide gas) to generate power, and air as an oxide gas. The oxide gas supply system 30 for supplying the fuel to the fuel cell stack 20, the fuel gas supply system 40 for supplying hydrogen gas as the fuel gas to the fuel cell stack 20, and the electric power for controlling the charging and discharging of the electric power. It includes a system 50, a cooling system 60 for cooling the fuel cell stack 20, and a controller (ECU) 70 that controls the entire system.

The fuel cell stack 20 is a solid polymer electrolyte cell stack in which a plurality of cells are laminated in series. In the fuel cell stack 20, the oxidation reaction of equation (1) occurs at the anode electrode, and the reduction reaction of equation (2) occurs at the cathode electrode. The electromotive reaction of Eq. (3) occurs in the fuel cell stack 20 as a whole.
_{^{H 2 → 2H + + 2e -}} ... (1)
^{_{(1/2) O 2 + 2H +}} + 2e - → H 2 O ... (2)
H ₂ + (1/2) O ₂ → H ₂ O ... (3)

A voltage sensor 71 for detecting the output voltage of the fuel cell stack 20 and a current sensor 72 for detecting the generated current are attached to the fuel cell stack 20.

The oxidation gas supply system 30 has an oxidation gas passage 34 through which the oxidation gas supplied to the cathode electrode of the fuel cell stack 20 flows, and an oxidation off gas passage 36 through which the oxidation off gas discharged from the fuel cell stack 20 flows. .. In the oxidation gas passage 34, an air compressor 32 that takes in the oxidation gas from the atmosphere through the filter 31, a humidifier 33 for humidifying the oxidation gas supplied to the cathode electrode of the fuel cell stack 20, and an oxidation gas supply A throttle valve 35 for adjusting the amount is provided. In the oxidation off gas passage 36, a back pressure adjusting valve 37 for adjusting the oxidation gas supply pressure and a humidifier 33 for exchanging water between the oxidation gas (dry gas) and the oxidation off gas (wet gas) are provided. It is provided.

The fuel gas supply system 40 includes a fuel gas supply source 41, a fuel gas passage 45 through which fuel gas supplied from the fuel gas supply source 41 to the anode pole of the fuel cell stack 20 flows, and fuel discharged from the fuel cell stack 20. A circulation passage 46 for returning the off-gas to the fuel gas passage 45, a circulation pump 47 for pumping the fuel off-gas in the circulation passage 46 to the fuel gas passage 45, and an exhaust / drainage passage 48 branched and connected to the circulation passage 47. Have.

The fuel gas supply source 41 is composed of, for example, a high-pressure hydrogen tank or a hydrogen storage alloy, and stores high-pressure (for example, 35 MPa to 70 MPa) hydrogen gas. When the shutoff valve 42 is opened, fuel gas flows out from the fuel gas supply source 41 to the fuel gas passage 45. The fuel gas is decompressed to, for example, about 200 kPa by the regulator 43 and the injector 44, and is supplied to the fuel cell stack 20. The fuel gas supply source 41 is composed of a reformer that generates hydrogen-rich reformed gas from a hydrocarbon-based fuel and a high-pressure gas tank that stores the reformed gas generated by the reformer in a high-pressure state. It may be configured.

The regulator 43 is a device that regulates the upstream pressure (primary pressure) to a preset secondary pressure, and is composed of, for example, a mechanical pressure reducing valve that reduces the primary pressure. The mechanical pressure reducing valve has a housing in which a back pressure chamber and a pressure adjusting chamber are separated by a diaphragm, and the back pressure in the back pressure chamber reduces the primary pressure to a predetermined pressure in the back pressure chamber. It has a structure to be the next pressure.

The injector 44 is an electromagnetically driven on-off valve capable of adjusting the gas flow rate and gas pressure by directly driving the valve body with an electromagnetic driving force at a predetermined driving cycle and separating it from the valve seat. The injector 44 includes a valve seat having an injection hole for injecting a gaseous fuel such as fuel gas, a nozzle body for supplying and guiding the gaseous fuel to the injection hole, and an axial direction (gas flow direction) with respect to the nozzle body. It is equipped with a valve body that is movably housed and held and opens and closes the injection hole.

An exhaust drain valve 49 is provided in the exhaust drain passage 48. The exhaust / drain valve 49 operates according to a command from the controller 70 to discharge the fuel off gas containing impurities and the water in the circulation passage 46 to the outside. By opening the exhaust / drain valve 49, the concentration of impurities in the fuel-off gas in the circulation passage 46 can be lowered, and the hydrogen concentration in the fuel-off gas circulating in the circulation system can be increased.

The fuel-off gas discharged through the exhaust-drain valve 49 is mixed with the oxidation-off gas flowing through the oxidation-off gas passage 36 and diluted by a diluter (not shown). The circulation pump 47 circulates and supplies the fuel off gas in the circulation system to the fuel cell stack 20 by driving the motor.

The power system 50 includes a DC / DC converter 51, a battery 52, a traction inverter 53, a traction motor 54, accessories 55, and an impedance measuring device 56. The DC / DC converter 51 has a function of boosting the DC voltage supplied from the battery 52 and outputting it to the traction inverter 53, DC power generated by the fuel cell stack 20, or regenerative power recovered by the traction motor 54 by regenerative braking. Has a function of lowering the pressure and charging the battery 52. These functions of the DC / DC converter 51 control the charging / discharging of the battery 52. Further, the operating points (output voltage, output current) of the fuel cell stack 20 are controlled by the voltage conversion control by the DC / DC converter 51.

The battery 52 functions as a storage source for surplus electricity, a regenerative energy storage source during regenerative braking, and an energy buffer when the load fluctuates due to acceleration or deceleration of the fuel cell vehicle. As the battery 5, for example, a secondary battery such as a nickel-cadmium storage battery, a nickel-hydrogen storage battery, or a lithium secondary battery is suitable. Further, the battery 52 supplies electric power for supplying electric power to each detector and starting the fuel cell while the power generation of the fuel cell stack 20 is stopped.

The traction inverter 53 is, for example, a PWM inverter driven by a pulse width modulation method, and converts a DC voltage output from the fuel cell stack 20 or the battery 52 into a three-phase AC voltage according to a control command from the controller 70. , Controls the rotational torque of the traction motor 54. The traction motor 54 is, for example, a three-phase AC motor and constitutes a power source for a fuel cell vehicle.

Auxiliary equipment 55 includes motors (for example, power sources such as pumps) arranged in each part of the fuel cell system 10, inverters for driving these motors, and various in-vehicle auxiliary equipment. Classes (for example, air compressors, injectors, cooling water circulation pumps, radiators, etc.) are collectively referred to.

The impedance measuring device 56 is a device that measures high-frequency impedance by superimposing a high-frequency AC signal on a power signal to the fuel cell stack 20 and detecting the voltage response thereof.

The cooling system 60 includes refrigerant passages 61, 62, 63, 64 for flowing the refrigerant circulating inside the fuel cell stack 20, a circulation pump 65 for pumping the refrigerant, and heat exchange between the refrigerant and the outside air. It includes a radiator 66, a three-way valve 67 for switching the circulation path of the refrigerant, and a temperature sensor 74 for detecting the refrigerant temperature. During normal operation after the warm-up operation is completed, the refrigerant flowing out of the fuel cell stack 20 flows through the refrigerant passages 61 and 64 and is cooled by the radiator 66, then flows through the refrigerant passage 63 and flows into the fuel cell stack 20 again. The three-way valve 67 is controlled to open and close as described above. On the other hand, during the warm-up operation immediately after the system is started, the three-way valve 67 is controlled to open and close so that the refrigerant flowing out of the fuel cell stack 20 flows through the refrigerant passages 61, 62, 63 and flows into the fuel cell stack 20 again. In this embodiment, water is used as the refrigerant and the circulation path is circulated as the cooling water.

The controller 70 is a computer system including a CPU, ROM, RAM, an input / output interface, and the like, and each part of the fuel cell system 10 (oxidation gas supply system 30, fuel gas supply system 40, power system 50, and cooling system 60). Functions as a control means for controlling. For example, when the controller 70 receives the start signal IG output from the ignition switch, the operation of the fuel cell system 10 is started, the accelerator opening signal ACC output from the accelerator sensor, and the vehicle speed signal output from the vehicle speed sensor. Obtain the required power of the entire system based on VC and the like.

The required power of the entire system is the total value of the vehicle running power and the auxiliary power. Auxiliary power includes power consumed by in-vehicle auxiliary equipment (humidifier, air compressor, hydrogen pump, cooling water circulation pump, etc.), equipment necessary for vehicle running (transmission, wheel control device, steering device, and suspension). This includes electric power consumed by equipment (devices, etc.), electric power consumed by equipment (air conditioning equipment, lighting equipment, audio, etc.) arranged in the occupant space, and the like.

Then, the controller 70 determines the distribution of the output powers of the fuel cell stack 20 and the battery 52, calculates the power generation command value, and oxidizes the fuel cell stack 20 so that the amount of power generated matches the target power. The gas supply system 30 and the fuel gas supply system 40 are controlled. Further, the controller 70 controls the DC / DC converter 51 to adjust the output voltage of the fuel cell stack 20 to control the operating points (output voltage, output current) of the fuel cell stack 20. The controller 70 outputs each AC voltage command value of U phase, V phase, and W phase to the traction inverter 53 as a switching command so that the target torque corresponding to the accelerator opening can be obtained, and the traction motor 54 Controls the output torque and the number of revolutions.

The air compressor 73 executes scavenging and discharges the water accumulated inside the fuel cell stack 20 from the oxidation off-gas passage 36. At this time, the back pressure adjusting valve 37 is released. The temperature sensor 75 detects the temperature in the fuel cell stack 20. More specifically, the temperature sensor 75 detects the temperature of the gas present in the fuel cell stack 20.

FIG. 2 is a diagram showing a change in water content due to a difference in the temperature of the scavenging gas. The horizontal axis represents the time point before scavenging and the time point after scavenging, and the vertical axis represents the water content of the fuel cell stack 20. The dotted line graph shows the change when the scavenging gas is left at a low temperature, and the solid line graph shows the change when the scavenging gas is once warmed to a high temperature. That is, if the water content of the fuel cell stack 20 is the same at the time before scavenging, the water content is significantly reduced when the scavenging gas is heated to a high temperature by scavenging for the same time. By paying attention to this phenomenon and repeating experiments, the inventor of the present application has obtained a finding on how many times the scavenging gas should be raised in advance in order to reduce the water content to the target range in a predetermined scavenging time. rice field.

In order to reduce the water content to the target range within the scavenging time determined when starting the stop treatment for stopping power generation, first, the water content at that time is measured. In the present embodiment, the high frequency impedance is measured, and the water content of the fuel cell stack 20 is estimated from the result.

FIG. 3 is a diagram showing the relationship between the water content and the high frequency impedance. The horizontal axis represents the water content and the vertical axis represents the high frequency impedance. The high frequency impedance is equivalent to the hydrogen ion transfer resistance. Specifically, in order for hydrogen and oxygen to react in the fuel cell stack 20, hydrogen must move as protons in the electrolyte membrane, but when the water content decreases and the electrolyte membrane begins to dry, the protons move with water. Is difficult to move, so the movement resistance of protons increases. Since the measured high-frequency impedance represents this movement resistance, as a result, there is a one-to-one correspondence with the dryness of the electrolyte membrane, that is, the water content of the fuel cell stack 20. The relationship is represented by a curve function as shown. Therefore, if the measured high frequency impedance is converted by this function, the water content at the time of measurement can be estimated.

FIG. 4 is a diagram showing the relationship between the scavenging time and the water content. The horizontal axis represents the scavenging time, and the vertical axis represents the water content. As shown in the figure, the fuel cell stack 20 is set with an appropriate range of water content during non-power generation. That is, if the water content is high, the fuel cell stack 20 freezes when placed in an environment below the freezing point, and the rise in output is significantly reduced when power generation is started. If the water content is too low, the transfer resistance of the protons will increase, and the rise of output will also decrease significantly. Therefore, an appropriate range of water content is set according to the characteristics of the fuel cell stack 20 so that a certain degree of rising characteristics can be obtained even in a sub-zero environment.

No matter how much the water content exceeds the appropriate range at the time of starting the stop treatment for stopping the power generation, it is possible to converge the water content to the appropriate range by adjusting the scavenging time. However, the user who uses the fuel cell system 10 may feel uncomfortable if the scavenging time at the time of stop processing is different each time the fuel cell system 10 is used. Therefore, in the present embodiment, the scavenging time is set to a fixed time so as not to cause such a feeling of strangeness. Here, the fixed time is 50 seconds.

At the time when the stop processing is started, the controller 70 that functions as a control unit that controls scavenging is set even if the water contents are different from each other (in the case of a in the figure, in the case of b, in the case of c). The water content is converged to an appropriate range by scavenging for 50 seconds. As shown in the figure, since a>b> c, the amount of water to be drained is also large in this order. Therefore, to each of the water content to reach the proper range in scavenging time in the same 50 seconds, the target temperature T _a of each of the scavenging gas, T _b, when the _{T c, T a> T b} > T c Relationship is established. The specific values of T _a , T _b , and T _c are determined in advance for each fuel cell stack through experiments and simulations. The controller 70 determines the target temperature T of the scavenging gas with respect to the water content estimated by using the impedance measuring device 56 by referring to the lookup table thus predetermined.

As a method for raising the temperature of the scavenging gas in the fuel cell stack 20, various methods can be adopted. For example, a method of lowering the power generation efficiency of the fuel cell stack 20 to generate a large amount of reaction heat of the reaction gas can be adopted. The controller 70 adjusts the amount of the reaction gas introduced to reduce the power generation efficiency, and monitors the temperature rise due to the reaction heat with the temperature sensor 74 to bring the scavenging gas to the determined target temperature T. In this case, the component that adjusts the power generation efficiency functions as a temperature raising unit.

Alternatively, the scavenging gas in the fuel cell stack 20 may be raised in temperature by reducing the circulation amount of the cooling water circulating in the cooling system 60. In this case, the controller 70 adjusts the circulation amount of the cooling water to reduce the amount of heat taken from the fuel cell stack 20, and monitors the temperature rise due to this by the temperature sensor 74 to reach the determined target temperature T of the scavenging gas. Reach. In this case, the component that adjusts the circulation amount of the cooling water functions as a temperature raising unit. If the water content at the time of starting the stop treatment is less than the appropriate range, the controller 70 increases the target power generation amount to increase the water generated by the reaction of the reaction gas.

FIG. 5 is a flow chart showing a processing procedure of the scavenging process. The controller 70 executes the stop process when the power generation is stopped. The figure shows the procedure of the scavenging process executed by the controller 70 among the stop processes.

The controller 70 measures the water content of the fuel cell stack 20 in step S101. Specifically, as described above, the high frequency impedance is measured via the impedance measuring device 56, and the measurement result is converted into the water content. In step S102, the controller 70 determines whether or not the measured water content is within the appropriate range. If YES (if included), the scavenging process is terminated as it is. In this case, scavenging may be performed for a short time for the purpose of discharging impurities.

If NO in step S102 (if not included), the process proceeds to step S103 to determine whether or not the water content is greater than the appropriate range. If NO (not many), the process proceeds to step S104, and if YES (not many), the process proceeds to step S105. When the process proceeds to step S104, the process of increasing the water content is performed as described above, and the scavenging process is completed. In this case, scavenging may be performed for a short time for the purpose of discharging impurities.

When the process proceeds to step S105, the controller 70 determines the target temperature of the scavenging gas according to the water content measured in step S101, and warms the scavenging gas to that temperature. When the target temperature is reached, the process proceeds to step S106, and the controller 70 performs scavenging of the warmed scavenging gas for a preset scavenging time. When the scavenging time has elapsed, the series of processes is completed.

With the fuel cell system 10 controlled in this way, the water content of the fuel cell can reach the target range in a certain time when the power generation is stopped, even if the water balance is not constantly controlled during the power generation of the fuel cell. Can be done. Further, when scavenging is executed, the scavenging time is constant at the end of any use, so that the user does not feel a sense of discomfort.

Next, another method for estimating the water content of the fuel cell stack 20 will be described. FIG. 6 is a diagram showing the relationship between the cooling water temperature and the water content. The horizontal axis represents the temperature of the cooling water, and the vertical axis represents the water content of the fuel cell stack 20. As shown in the figure, there is a one-to-one correspondence between the temperature and the water content of the cooling water. Therefore, if the controller 70 acquires the detection result of the temperature sensor 74, the controller 70 can convert the detection result into the water content. By using such a method, the water content of the fuel cell stack 20 at the start of the stop processing can be measured without measuring the high frequency impedance as described above.

FIG. 7 is a diagram showing the relationship between the pressure loss in the air electrode flow path and the water content. The horizontal axis represents the pressure loss in the air electrode flow path, and the vertical axis represents the water content of the fuel cell stack 20. As shown in the figure, there is a one-to-one correspondence between the pressure loss and the water content of the air electrode flow path. Therefore, if the controller 70 acquires the pressure loss in the air electrode flow path, the controller 70 can convert the pressure loss into the water content. By using such a method, the water content of the fuel cell stack 20 at the start of the stop processing can be measured without measuring the high frequency impedance as described above.

10 ... Fuel cell system, 20 ... Fuel cell stack, 30 ... Oxidation gas supply system, 40 ... Fuel gas supply system, 50 ... Electric power system, 60 ... Cooling system, 70 ... Controller

Claims (1)

With fuel cells
A measuring unit that measures the water content of the fuel cell,
A temperature raising unit that raises the temperature of the scavenging gas of the fuel cell,
A control unit that controls scavenging of the fuel cell is provided.
When the control unit starts the stop process for stopping the power generation by the fuel cell, the water content is adjusted to the water content measured by the measurement unit so that the water content reaches the target range within a preset scavenging time. A fuel cell system that determines a target temperature of the scavenging gas based on the above, raises the scavenging gas to the target temperature by the temperature raising unit, and executes scavenging with the raised scavenging gas.

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