Fuel cell system

Abstract

To provide a fuel cell system that can suppress the decrease in power generation efficiency of fuel cells. [Solution] A fuel cell system comprising a fuel cell and a control unit, wherein the control unit calculates a current deviation between the initial current value of the fuel cell and the current value of the fuel cell when the fuel cell is generating power, the control unit calculates the amount of pressure control of the oxygen electrode of the fuel cell necessary to reduce the current deviation from the calculated current deviation, and controls the pressure of the oxygen electrode based on the amount of pressure control, and the control unit compares a first system efficiency of the oxygen electrode pressure control with a second system efficiency of the fuel cell current control based on the pressure of the hydrogen electrode of the fuel cell, and switches from the pressure control of the oxygen electrode to the current control of the fuel cell when the second system efficiency is greater than the first system efficiency.

Description

Technical Field

[0001] This disclosure relates to a fuel cell system.

Background Art

[0002] Various technologies have been proposed regarding fuel cells (FCs) as disclosed in Patent Document 1.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In Patent Document 1, a protection control device during battery performance deterioration that adjusts the output by increasing or decreasing the current value according to an output command based on the result of periodically measuring the voltage V and current I of a fuel cell is disclosed. In order to maintain the same power generation output after battery performance deterioration, it is necessary to instruct the fuel cell to generate electricity at a high current. However, the heat loss of the fuel cell increases compared to before the battery performance deterioration, and the power generation efficiency of the fuel cell decreases. When performing oxygen electrode pressurization control to suppress heat loss, it is necessary to increase the hydrogen electrode pressure as well. Therefore, the hydrogen permeation amount in the fuel cell increases easily, and the system efficiency decreases.

[0005] This disclosure has been made in view of the above circumstances, and the main object is to provide a fuel cell system that can suppress a decrease in the power generation efficiency of a fuel cell and also suppress a decrease in the system efficiency.

Means for Solving the Problems

[0006] That is, the present disclosure includes the following aspects. <1> A fuel cell system, The fuel cell system comprises a fuel cell and a control unit. The control unit calculates the current difference between the current value of the fuel cell in its initial state and the current value of the fuel cell in its current state when the fuel cell is generating power. The control unit calculates the amount of pressure control for the oxygen electrode of the fuel cell necessary to reduce the current deviation from the calculated current deviation, and controls the pressure of the oxygen electrode based on the pressure control amount. A fuel cell system characterized in that the control unit compares a first system efficiency of pressure control of the oxygen electrode and a second system efficiency of current control of the fuel cell based on the pressure of the hydrogen electrode of the fuel cell, and switches from the pressure control of the oxygen electrode to the current control of the fuel cell when the second system efficiency is greater than the first system efficiency. [Effects of the Invention]

[0007] The fuel cell system of this disclosure can suppress the decrease in the power generation efficiency of the fuel cell. [Brief explanation of the drawing]

[0008] [Figure 1] Figure 1 is a system configuration diagram showing an example of the fuel cell system of this disclosure. [Figure 2] Figure 2 is a flowchart showing an example of switching control between oxygen electrode pressure control and fuel cell current control. [Figure 3] Figure 3 shows a sequence illustrating an example of switching control between oxygen electrode pressure control and fuel cell current control. [Modes for carrying out the invention]

[0009] In this disclosure, the reaction gas supplied to the anode of the fuel cell is the fuel gas (anode gas), and the reaction gas supplied to the cathode of the fuel cell is the oxidizing gas (cathode gas). The fuel gas is mainly a gas containing hydrogen, but may also be hydrogen. The oxidizing gas is a gas containing oxygen, but may also be oxygen, air, etc.

[0010] In this disclosure, a fuel cell system, The fuel cell system comprises a fuel cell and a control unit. The control unit calculates the current difference between the current value of the fuel cell in its initial state and the current value of the fuel cell in its current state when the fuel cell is generating power. The control unit calculates the amount of pressure control for the oxygen electrode of the fuel cell necessary to reduce the current deviation from the calculated current deviation, and controls the pressure of the oxygen electrode based on the pressure control amount. The control unit compares a first system efficiency of pressure control of the oxygen electrode with a second system efficiency of current control of the fuel cell based on the pressure of the hydrogen electrode of the fuel cell, and switches from the pressure control of the oxygen electrode to the current control of the fuel cell when the second system efficiency is greater than the first system efficiency, thereby providing a fuel cell system.

[0011] According to this disclosure, even if the current-voltage characteristics of a fuel cell deteriorate over time, the deterioration of the current-voltage characteristics can be determined from the current deviation between the target current value calculated based on the target power generation value and the current value measured by the sensor. The oxygen electrode pressure required to reduce the current deviation can be calculated, and the current deviation can be reduced by controlling the oxygen electrode pressure (pressurizing the oxygen electrode), thereby restoring the current-voltage characteristics of the fuel cell and bringing it closer to the state at the beginning of fuel cell power generation. This suppresses the increase in heat loss of the fuel cell due to deterioration over time, and as a result, it is possible to suppress the decrease in the power generation efficiency of the fuel cell due to deterioration over time. On the other hand, when the oxygen electrode pressure is changed, the hydrogen electrode pressure is controlled in accordance with that oxygen electrode pressure. However, increasing the hydrogen electrode pressure also increases the amount of hydrogen permeate into the cell that does not contribute to power generation, and if the oxygen electrode pressure is continued to be pressurized, the FC system efficiency will decrease as a result. Furthermore, if the oxygen electrode pressure is continued to be pressurized, there is an upper limit to the pressure resistance of the cell and system components, and it becomes impossible to continue pressurizing due to the upper limit of the oxygen electrode pressure resistance. In this disclosure, while suppressing the decrease in fuel cell power generation efficiency due to the aging degradation of the fuel cell's current-voltage characteristics by controlling the oxygen electrode pressure, if a decrease in fuel cell system efficiency due to oxygen electrode pressure pressurization or reaching the system's upper pressure limit is estimated, the system switches to fuel cell current control to continue fuel cell power generation. By switching from oxygen electrode pressurization control to fuel cell current control according to the hydrogen electrode pressure, it is possible to achieve both heat loss suppression and system efficiency.

[0012] The fuel cell system of this disclosure may be used mounted on a mobile body such as a vehicle, or it may be used mounted on a vehicle. Furthermore, the fuel cell system of this disclosure may be used mounted on a stationary power generation system such as a generator that supplies power to the outside of the fuel cell system. The vehicle may be a fuel cell vehicle or the like. Other means of transport include, for example, trains, ships, and aircraft. Furthermore, the fuel cell system of this disclosure may be used mounted on a mobile device such as a vehicle that can also run on the power of a secondary battery. The mobile unit and the stationary power generation system may include the fuel cell system of this disclosure. The mobile unit may have drive units such as a motor, inverter, and hybrid control system. The hybrid control system may be capable of propelling the vehicle by using both the output of the fuel cell and the power of a secondary battery.

[0013] Figure 1 is a system configuration diagram showing an example of the fuel cell system of this disclosure. The fuel cell system comprises a fuel cell 1, a control unit 2, a current sensor 3, a voltage sensor 4, a fuel gas system 10, and an oxygen system 20. The fuel gas system 10 and the oxygen system 20 are controlled by the control unit 2. The cooling system is omitted in Figure 1 for convenience. The fuel gas system 10 includes a regulator 13, which controls the hydrogen stored in the hydrogen tank 11 to an appropriate supply pressure and supplies it to the fuel cell stack 1. The fuel gas system 10 also includes a shut-off valve 12, an ejector 14, a hydrogen pole pressure sensor 15, a gas-liquid separator 17, a drain valve 18, and a muffler 19. The oxygen system 20 includes a regulator 23, controls the oxygen stored in the oxygen tank 21 to an appropriate supply pressure, and supplies it to the fuel cell stack 1. The oxygen system 20 includes a shut-off valve 22, an ejector 24, a cathode outlet pressure sensor 25, a gas-liquid separator 27, and a drain valve 28. In FIG. 1, an oxygen circulation system using an ejector that utilizes the pressure of a high-pressure oxygen tank 21 is assumed as the oxygen electrode pressure control means. Different from a system using an air compressor, since the oxygen electrode pressure can be controlled such as pressurization using the tank pressure, the auxiliary device loss becomes extremely small, and an improvement in the FC system efficiency can be expected together with the effect of improving the FC power generation efficiency.

[0014] The fuel cell system of the present disclosure includes a fuel cell and a control unit. The fuel cell system may include a fuel gas system, an oxygen system, a cooling system, and the like.

[0015] The fuel cell may be a fuel cell stack (stack) that is a laminate in which a plurality of single cells (cells) of the fuel cell are laminated. In the present disclosure, both the cell and the fuel cell stack may be referred to as a fuel cell. The number of cells laminated in the fuel cell stack is not particularly limited, and may be, for example, 2 to several hundred. The fuel cell stack may have a current collector plate, a pressure plate, etc. at the ends in the lamination direction.

[0016] The cell may have a power generation part. The shape of the power generation part may be rectangular in plan view. The power generation part may be a membrane electrode assembly (MEA) including an electrolyte membrane and two electrodes. The electrolyte membrane may be a solid polymer electrolyte membrane. Examples of the solid polymer electrolyte membrane include fluorine-based electrolyte membranes such as thin films of perfluorosulfonic acid containing moisture, and hydrocarbon-based electrolyte membranes. The electrolyte membrane may be, for example, a Nafion membrane (manufactured by DuPont). One of the two electrodes is an anode (hydrogen electrode), and the other is a cathode (oxygen electrode). The electrode includes a catalyst layer and may optionally include a gas diffusion layer, and the power generation section may be a membrane electrode gas diffusion layer assembly (MEGA). The catalyst layer includes a catalyst, which may comprise a catalytic metal that promotes an electrochemical reaction, a proton-conducting electrolyte, and an electron-conducting support, etc. Examples of catalyst metals that can be used include platinum (Pt) and alloys of Pt with other metals (for example, Pt alloys mixed with cobalt and nickel). The catalyst metal used as the cathode catalyst and the catalyst metal used as the anode catalyst may be the same or different. The electrolyte may be a fluororesin or the like. For example, a Nafion solution may be used as the fluororesin. The catalyst metal is supported on a support, and in each catalyst layer, the support on which the catalyst metal is supported (catalyst support) and the electrolyte may be mixed. Examples of carriers for supporting the catalytic metal include commercially available carbon materials such as carbon. The gas diffusion layer may be a conductive material having pores or the like. Examples of conductive materials include carbon porous materials such as carbon cloth and carbon paper, as well as metal porous materials such as metal mesh and foamed metal. A cell may include a separator. Separators collect the current generated by power generation and function as partitions. In a cell, separators are usually placed on both sides of the power generation section in the stacking direction, with a pair of separators sandwiching the power generation section. One of the separators is the anode separator and the other is the cathode separator. The anode separator may have a groove on the side facing the power generation section that serves as a fuel gas passage, or a groove on the side opposite to the side facing the power generation section that serves as a cooling medium passage. The cathode separator may have grooves on the side facing the power generation section that serve as oxidizing agent gas channels, and may have grooves on the side opposite to the side facing the power generation section that serve as cooling medium channels. The separator may have holes that constitute a manifold, such as supply holes and discharge holes, for circulating fluid in the stacking direction of the cells. The separator may be, for example, dense carbon that has been compressed to be gas-impermeable, or press-formed metal (for example, iron, titanium, and stainless steel). The cell may include an insulating resin frame positioned on the outer (circumferential) side in the planar direction of the membrane electrode assembly between the anode separator and the cathode separator. The resin frame is molded from a thermoplastic resin to form a plate-like or frame-like structure, and seals the space between the anode separator and the cathode separator while holding the membrane electrode assembly in its central region. For example, resins such as PE, PP, PET, and PEN can be used for the resin frame. The resin frame may also be a three-layer sheet composed of three layers with an adhesive layer on the surface.

[0017] Figure 2 is a flowchart showing an example of switching control between oxygen electrode pressure control and fuel cell current control. In S101, after power generation starts, the current deviation between the initial current value of the fuel cell and the current value of the fuel cell is calculated and the current deviation is detected. In S102, it is determined whether the detected current deviation is 0A. In S103, if the current deviation detected in S102 is 0A, the oxygen pole pressure is maintained and the process returns to S101. Instead of returning to S101, the control may be terminated. In S104, the required oxygen pole pressure is calculated if the current deviation detected in S102 is not 0A. In S105, oxygen electrode pressure control is performed based on the calculated required oxygen electrode pressure, and the hydrogen electrode pressure is controlled in accordance with the control of the oxygen electrode pressure. Based on the hydrogen electrode pressure, the first system efficiency of the oxygen electrode pressure control (η oxygen electrode pressure control) and the second system efficiency of the fuel cell current control (η FDC current control) are compared. In S106, if the first system efficiency (η oxygen pole pressure control) is greater than the second system efficiency (η FDC current control) in S105, the oxygen pole pressure indication value is compared with the oxygen pole pressure upper limit. In S107, if the oxygen pole pressure indication value in S106 is below the upper limit of the oxygen pole pressure, the oxygen pole pressure control is continued and the process returns to S101, or the control is terminated. In S108, if the second system efficiency is greater than the first system efficiency in S105, the control switches from oxygen electrode pressure control to fuel cell current control and terminates. Also in S108, if the oxygen electrode pressure reading in S106 is greater than or equal to the upper limit of the oxygen electrode pressure, the control switches from oxygen electrode pressure control to fuel cell current control and terminates.

[0018] The control unit calculates the current deviation between the current value of the fuel cell in its initial state and the current value of the fuel cell in its current state when the fuel cell is generating power. The current value of the fuel cell in its initial state may be prepared in advance as data. The current value of the fuel cell in its current state may be measured by a current sensor. The control unit may calculate a target current value (corresponding to the initial current value of the fuel cell) and a target voltage value based on the target power generation value required for the fuel cell system, and may also calculate the current deviation between the target current value at the target voltage value and the current value of the current state measured by the current sensor. The control unit may determine whether or not the current-voltage characteristics of the fuel cell have deteriorated over time based on the current deviation.

[0019] The control unit calculates the amount of pressure control for the oxygen electrode of the fuel cell necessary to reduce the current deviation from the calculated current deviation, and controls the pressure of the oxygen electrode based on the pressure control amount. The pressure of the oxygen electrode may be measured by an oxygen electrode pressure sensor. The control unit calculates the amount of oxygen electrode pressure control required to reduce the current deviation calculated by the current deviation calculation unit, and controls the oxygen electrode pressure to reduce the current deviation and restore the current-voltage characteristics of the fuel cell. The control unit controls the pressure of the oxygen electrode to increase when the current deviation is positive, and controls the pressure of the oxygen electrode to decrease when the current deviation is negative.

[0020] When the oxygen electrode pressure is changed, the control unit may control the hydrogen electrode pressure in accordance with the oxygen electrode pressure, maintaining the relationship oxygen electrode pressure < hydrogen electrode pressure. This prevents back pressure from the oxygen electrode to the hydrogen electrode in fuel cell cells, which are normally designed so that oxygen electrode pressure < hydrogen electrode pressure, and prevents excessive differential pressure between the cell electrodes. Specifically, the target hydrogen electrode pressure can be calculated by adding the pressure deviation in the form of oxygen electrode pressure sensor value + 10 kPa. Alternatively, back pressure can be prevented by setting the upper limit of the target oxygen electrode pressure to the hydrogen electrode pressure. The hydrogen electrode pressure may also be measured by a hydrogen electrode pressure sensor. The control unit may control the oxygen electrode pressure to a level above the lower limit of the oxygen electrode pressure. However, if the oxygen electrode pressure is lowered too drastically, there is a risk of power generation becoming impossible due to oxygen deficiency. Therefore, the minimum required oxygen pressure is set to the lower limit to prevent oxygen deficiency.

[0021] The control unit compares a first system efficiency of oxygen electrode pressure control with a second system efficiency of fuel cell current control based on the pressure of the hydrogen electrode of the fuel cell. If the second system efficiency is greater than the first system efficiency, the control unit switches from oxygen electrode pressure control to fuel cell current control.

[0022] Figure 3 shows a sequence illustrating an example of switching control between oxygen electrode pressure control and fuel cell current control. The control unit may calculate two system efficiencies as the FC system efficiency, taking into account the amount of hydrogen permeation within the cell estimated from the hydrogen electrode pressure: a first system efficiency (η oxygen electrode pressure control), which is the FC system efficiency when oxygen electrode pressure control is continued, and a second system efficiency (η FDC current control), which is the FC system efficiency assuming current-voltage control using an FDC. As shown in Figure 3, the control unit may compare the first system efficiency and the second system efficiency, and select the control method that results in higher FC system efficiency from among oxygen electrode pressure control and fuel cell current control, and switch the control method. Even if the first system efficiency is greater than the second system efficiency, the control unit may compare the oxygen electrode pressure indication value or the hydrogen electrode pressure indication value with the pre-set pressure upper limit for each electrode. If the indication value for each electrode is greater than or equal to the pressure upper limit for at least one of the electrodes, the control unit may switch from oxygen electrode pressure control to fuel cell current control. This allows the FC system efficiency effect of oxygen electrode pressure control to be maximized within the system pressure tolerance range. By switching to the above control method, the decrease in power generation efficiency due to the aging of the fuel cell is suppressed to the greatest extent possible by controlling the oxygen electrode pressure, and then by switching to current control of the fuel cell, the FC system efficiency at EOL can be improved compared to when the aging degradation of the fuel cell's current-voltage characteristics is addressed solely by current control of the fuel cell.

[0023] The control unit may control the oxygen system, fuel gas system, cooling system, etc., and control the entire fuel cell system. Physically, the control unit includes, for example, a processing unit such as a CPU (Central Processing Unit), a ROM (Read-Only Memory) for storing control programs and control data processed by the CPU, a storage device such as a RAM (Random Access Memory) used primarily as various work areas for control processing, and an input / output interface. It may also be an ECU (Electronic Control Unit).

[0024] The cooling system supplies cooling water to the fuel cell as a cooling medium. Cooling water may include water and ethylene glycol, or a mixture thereof. The cooling system may include a coolant pump, reserve tank, cooling passages, radiator, bypass passages, rotary valve, ion exchanger, intercooler, etc. [Explanation of Symbols]

[0025] 1: Fuel cell stack, 2: Control unit, 3: Current sensor, 4: Voltage sensor, 10: Fuel gas system, 11: Hydrogen tank, 12: Shut-off valve, 13: Regulator, 14: Ejector, 15: Hydrogen electrode pressure sensor, 17: Gas-liquid separator, 18: Drain valve, 19: Muffler, 20: Oxygen system, 21: Oxygen tank, 22: Shut-off valve, 23: Regulator, 24: Ejector, 25: Cathode outlet pressure sensor (oxygen electrode pressure sensor), 27: Gas-liquid separator, 28: Drain valve

Claims

[Claim 1] A fuel cell system, The fuel cell system comprises a fuel cell and a control unit. The control unit calculates the current difference between the current value of the fuel cell in its initial state and the current value of the fuel cell in its current state when the fuel cell is generating power. The control unit calculates the amount of pressure control for the oxygen electrode of the fuel cell necessary to reduce the current deviation from the calculated current deviation, and controls the pressure of the oxygen electrode based on the pressure control amount. A fuel cell system characterized in that the control unit compares a first system efficiency of pressure control of the oxygen electrode and a second system efficiency of current control of the fuel cell based on the pressure of the hydrogen electrode of the fuel cell, and switches from the pressure control of the oxygen electrode to the current control of the fuel cell when the second system efficiency is greater than the first system efficiency.

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