Fuel cell system

Abstract

The present invention addresses the problem of suppressing overshoot in a fuel cell system. A fuel cell system is provided with: a fuel cell; an oxidizing gas supply system for supplying an oxidizing gas to the fuel cell; and a control unit configured so as to be able to control the oxidizing gas supply system and adjust the stoichiometric ratio of the oxidizing gas. The control unit changes the stoichiometric ratio between at least two values in accordance with the stack voltage of the fuel cell in a low load state in which the stack current of the fuel cell is equal to or less than a preset current threshold value. The control unit maintains the stoichiometric ratio constant regardless of the stack voltage in a high-load state in which the stack current is greater than a current threshold value.

Description

Technical Field

[0001] This specification discloses a fuel cell system. Background Technology

[0002] Patent document 1 discloses a fuel cell that can suppress stack voltage drop by increasing the air stoichiometry.

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Application Publication No. 2024-37203

[0006] Increasing the air stoichiometry can sometimes cause overshoot of the fuel cell's output power relative to the target power. This overshoot is more pronounced under high load conditions with high stack current than under low load conditions with low stack current. Summary of the Invention

[0007] The fuel cell system disclosed in this specification includes: a fuel cell; an oxidant gas supply system that supplies oxidant gas to the fuel cell; and a control unit configured to control the oxidant gas supply system to adjust the stoichiometry of the oxidant gas. Under low load conditions where the fuel cell stack current is below a preset current threshold, the control unit changes the stoichiometry between at least two values ​​based on the fuel cell stack voltage. Under high load conditions where the stack current is above the current threshold, the control unit maintains the stoichiometry constant regardless of the stack voltage.

[0008] Based on the above structure, the stoichiometric ratio of the oxidizing gas can be varied under low load conditions, while it remains constant under high load conditions. Therefore, the stoichiometric ratio can be used to control the stack voltage under low load conditions. Furthermore, overshoot can be suppressed under high load conditions. Thus, the stack voltage can be controlled while suppressing overshoot.

[0009] The control unit can be configured to change the stoichiometry between a first stoichiometry and a second stoichiometry greater than the first stoichiometry under low load conditions. The control unit can change the stoichiometry from the first stoichiometry to the second stoichiometry when the stack voltage drops to the first voltage while the stoichiometry is set to the first stoichiometry under low load conditions.

[0010] According to this structure, when the stack voltage drops to the first voltage under low load conditions, the stack voltage can be increased by increasing the stoichiometry. This prevents the stack voltage from falling below the first voltage.

[0011] The control unit can change the stoichiometric ratio from the second stoichiometric ratio back to the first stoichiometric ratio when the stoichiometric ratio is set to the second stoichiometric ratio under low load conditions and the stack voltage rises to the second voltage, which is higher than the first voltage.

[0012] According to this structure, when the stoichiometry is set relatively high under low load conditions, and the stack voltage rises to the second voltage, the stack voltage can be reduced by decreasing the stoichiometry. This prevents the stack voltage from becoming higher than the second voltage.

[0013] The control unit can maintain the stoichiometry at the second stoichiometry under high load conditions.

[0014] The control unit can change at least one of the first voltage and the second voltage according to the required output of the fuel cell.

[0015] Invention Effects

[0016] According to this structure, even when the required output changes, the stack voltage can be controlled while suppressing overshoot.

[0017] The detailed description of the technology disclosed in this specification and further improvements are described in the following "Detailed Description". Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the fuel cell system 1.

[0019] Figure 2 It is a diagram representing the first mapping M1 and the second mapping M2.

[0020] Figure 3 This is a graph representing the IV characteristics of fuel cell stack 10.

[0021] Figure 4 This is a flowchart illustrating the operation of fuel cell system 1.

[0022] Figure 5 This is a diagram representing the third mapping M3 of the comparative example.

[0023] Figure 6 This is a graph representing the IV characteristics of the comparative example.

[0024] Explanation of reference numerals in the attached figures:

[0025] 1: Fuel cell system; 2: Air supply system; 10: Fuel cell stack; 21: Air compressor; 40: Control unit; ST1: First stoichiometry; ST2: Second stoichiometry. Detailed Implementation

[0026] Example

[0027] (Structure of fuel cell system 1)

[0028] refer to Figure 1 The fuel cell system 1 will be described below. The fuel cell system 1 is applicable to various mobile devices, such as stationary power sources, ships, trams, and fuel cell vehicles. The fuel cell system 1 mainly includes an air supply system 2, a hydrogen supply system 3, a fuel cell stack 10, a control unit 40, a current sensor 41, and a voltage sensor 42. Furthermore, in... Figure 1 In the diagram, signal lines are represented by dashed lines.

[0029] The fuel cell stack 10 is a device that generates electricity through the chemical reaction of hydrogen and oxygen. Water is produced through the chemical reaction of hydrogen and oxygen. The fuel cell stack 10 comprises multiple individual cell units (not shown). Each individual cell unit has a fuel electrode and an air electrode. Electricity is generated by supplying fuel gas (hydrogen) to the fuel electrode and oxidizing gas (air containing oxygen) to the air electrode.

[0030] The output terminals of the fuel cell stack 10 are connected to the load 50. The power generated in the fuel cell stack 10 is supplied to the load 50. The load 50 can be of various types, such as a motor for moving parts. Furthermore, the load 50 can include various power conversion devices such as DC / DC converters.

[0031] A current sensor 41 and a voltage sensor 42 are provided in the output path of the fuel cell stack 10. The current sensor 41 is a sensor that detects the stack current OC of the fuel cell stack 10. The voltage sensor 42 is a sensor that detects the stack voltage OV of the fuel cell stack 10. The detected stack current OC and stack voltage OV are input to the control unit 40.

[0032] The air supply system 2 includes an air supply path 20a and an air exhaust path 20b. The air supply path 20a is connected to the air inlet (oxidizing gas inlet) 11a of the fuel cell stack 10. An air compressor 21 is provided in the air supply path 20a. The air compressor 21 is a device for compressing air drawn from the atmosphere into the fuel cell stack 10. The air compressor 21 adjusts the stoichiometric ratio of the air supplied to the fuel cell stack 10 according to the first mapping M1 and the second mapping M2 described later.

[0033] Air exhaust path 20b is connected to the air outlet (oxidizing gas outlet) 11b of the fuel cell stack 10. Air exhaust path 20b is a path for discharging unreacted air or generated water produced through the electrochemical reaction from inside the fuel cell stack 10 to the outside. A pressure regulating valve 23 is provided in air exhaust path 20b. Pressure regulating valve 23 is a valve that controls the internal pressure of the oxidizing gas (air) in the fuel cell stack according to a control signal CS2 input from the control unit 40. Various valve structures can be used in pressure regulating valve 23.

[0034] The hydrogen supply system 3 includes a hydrogen supply path 30a and a hydrogen discharge path 30b. The hydrogen supply path 30a is the path for introducing hydrogen, as fuel gas, into the fuel cell stack 10. The hydrogen supply path 30a is connected to the hydrogen inlet (fuel gas inlet) 12a of the fuel cell stack 10. The hydrogen discharge path 30b is the path for discharging unreacted hydrogen and generated water from the fuel cell stack 10. The hydrogen discharge path 30b is connected to the hydrogen outlet (fuel gas outlet) 12b of the fuel cell stack 10. Further detailed descriptions of the hydrogen supply path 30a and the hydrogen discharge path 30b are omitted.

[0035] The control unit 40 is a control mechanism for controlling various devices in the fuel cell system 1. The control unit 40 includes a CPU (not shown), a memory, and peripheral circuitry. In the control unit 40, the stack current OC and stack voltage OV are input from the current sensor 41 and voltage sensor 42, respectively. Furthermore, the target power TP is input to the control unit 40 from the load 50. The control signal CS1 output from the control unit 40 is input to the air compressor 21. The control signal CS2 output from the control unit 40 is input to the pressure regulating valve 23. The control unit 40 also controls the air supply system 2 to ensure that the power generation of the fuel cell stack 10 matches the target power TP.

[0036] (Control functions of air compressor 21)

[0037] exist Figure 2 The diagram shows the first mapping M1 (solid line) and the second mapping M2 (dashed line). Mappings M1 and M2 are used to adjust the air stoichiometry supplied to the fuel cell stack 10 by controlling the air compressor 21. The horizontal axis represents the stack current OC (%) of the fuel cell stack 10. The horizontal axis indicates the proportion of the stack current when the maximum load is set to 100%. The vertical axis represents the air stoichiometry. The air stoichiometry refers to the excess rate of the actual air supply relative to the theoretical air supply required to generate electricity from the stack current OC. That is, an air stoichiometry of 1.0 corresponds to the theoretical air supply. Typically, during the operation of the fuel cell stack 10, the air stoichiometry is set to 1.0 or higher (theoretical value) to suppress power loss and achieve high power generation efficiency.

[0038] like Figure 2 As shown, relative to the stack current OC, a current threshold IT, a low-load state LL, and a high-load state HL are preset. The current threshold IT is a value that can be appropriately determined according to the structure of the fuel cell system 1 or the type of load 50. In this embodiment, the current threshold IT is set to 67% of the highest load. The low-load state LL is the state where the stack current OC is below the current threshold IT. The high-load state HL is the state where the stack current OC is greater than the current threshold IT. Generally, the power generation efficiency of the low-load state LL is higher than that of the high-load state HL. Therefore, during stable operation, the control unit 40 controls the fuel cell stack 10 to the low-load state LL.

[0039] The first mapping M1 will be explained. The first mapping M1 is a mapping where the air stoichiometry is set to the first stoichiometry ST1 in the low-load state LL, and the air stoichiometry is set to the second stoichiometry ST2 in the high-load state HL. The second stoichiometry ST2 is a value larger than the first stoichiometry ST1. In this embodiment, the first stoichiometry ST1 is set to 1.2, and the second stoichiometry ST2 is set to 1.5. In the first mapping M1, in the low-load state LL, there are a first region R1 (the region where the stack current OC is 20% to 58%), a second region R2 (the region where the stack current OC is 5% to 20%), and a third region R3 (the region where the stack current OC is 58% to 67%). In the first region R1, the air stoichiometry remains constant at the first stoichiometry ST1. In the second region R2, the air stoichiometry decreases from the second stoichiometry ST2 to the first stoichiometry ST1. In region 3 R3, the air stoichiometry increases from the first stoichiometry ST1 to the second stoichiometry ST2.

[0040] The second mapping M2 is explained. The second mapping M2 is a mapping in which the air stoichiometry is set to the second stoichiometry ST2 in either the low load state LL or the high load state HL.

[0041] The first mapping M1 and the second mapping M2 are stored in the memory of the control unit 40. Furthermore, the control unit 40 can switch the used mapping between the two mappings. Thus, in the low-load state LL, the stoichiometric ratio can be changed between the first stoichiometric ratio ST1 and the second stoichiometric ratio ST2. And in the high-load state HL, the stoichiometric ratio can be maintained at a constant value of the second stoichiometric ratio ST2.

[0042] The reasons for distinguishing between using the first stoichiometric ratio ST1 and the second stoichiometric ratio ST2 are explained. The air flow rate of the first stoichiometric ratio ST1 is less than that of the second stoichiometric ratio ST2. Therefore, compared to the second stoichiometric ratio ST2, the first stoichiometric ratio ST1 is less likely to cause cell drying during power generation. However, compared to the second stoichiometric ratio ST2, the first stoichiometric ratio ST1 is more likely to cause flooding during continuous power generation or a decrease in oxygen concentration near the air outlet. Therefore, from the viewpoint of suppressing cell drying, the first stoichiometric ratio ST1 is preferred. Furthermore, from the viewpoint of eliminating flooding, the second stoichiometric ratio ST2 is preferred. Flooding is a phenomenon where water generated in the oxidant electrode by the power generation reaction hinders the supply of gas to the electrode. If flooding occurs, the power generation performance decreases.

[0043] (Specific action examples)

[0044] use Figure 3 The current-voltage characteristic (IV characteristic) of the fuel cell stack 10 shown is illustrated with a specific operating example. The horizontal axis represents the stack current OC (%), indicating the proportion of the stack current when the maximum load is set to 100%. The vertical axis represents the stack voltage 0V. Furthermore, IV curves G1 and G2 are curves that can be appropriately determined.

[0045] The isopower line L1 represents the trajectory of the operating point when fuel cell system 1 generates electricity with a constant net output. Ideally, the operating point of fuel cell system 1 lies on the isopower line L1. The intersection of the isopower line L1 and the IV curve G1 is designated as the operating point P1. Operating point P1 is the point where the system switches from the first mapping M1 to the second mapping M2. The stack voltage value at operating point P1 is designated as the first voltage VT1, and the stack current value is designated as the first current CT1. The intersection of the isopower line L1 and the IV curve G2 is designated as the operating point P2. Operating point P2 is the point where the system switches from the second mapping M2 to the first mapping M1. The stack voltage value at operating point P2 is designated as the second voltage VT2, and the stack current value is designated as the second current CT2.

[0046] The first current CT1 at the operating point P1 can be appropriately set by adjusting at least one of the net output power and the IV curve G1. Furthermore, the first current CT1 is preferably lower than the current threshold IT (67%). This is because the switching from the first mapping M1 to the second mapping M2 at the operating point P1 can be performed under low load conditions LL. Figure 3 In the example, the first current CT1 is 63%.

[0047] (Action details of fuel cell system 1)

[0048] use Figure 4The flowchart below illustrates the operation of fuel cell system 1. Hereinafter, "Step 10" will be referred to as "S10". Figure 4 The process begins when the fuel cell system 1 is turned on.

[0049] In S10, the control unit 40 sets the first mapping M1 to use the initial settings in the mapping. In the initial operation, the operation point becomes the operation point P0 (reference). Figure 3 In addition, the action point P0 in the initial action becomes any position on the equipower line L1 between action points P1 and P2.

[0050] In S30, the control unit 40 controls the air compressor 21 according to the first mapping M1. Specifically, in the low load state LL, the first stoichiometric ratio ST1 (1.2) is used, and in the high load state HL, the second stoichiometric ratio ST2 (1.5) is used. In addition, as mentioned above, during stable operation, since the fuel cell stack 10 is in the low load state LL, the first stoichiometric ratio ST1 is used.

[0051] If power generation continues using the first stoichiometric ratio ST1 (e.g., stable operation), the amount of water remaining at the oxidant electrode of the fuel cell stack 10 increases. Therefore, due to flooding, the voltage gradually decreases. Consequently, the operating point moves along the isopower line L1 in the direction of decreasing stack voltage OV (refer to arrow A1).

[0052] In S40, the control unit 40 determines whether the fuel cell voltage OV has dropped to the first voltage VT1 (operation point P1). If it has not dropped (S40: No), it returns to S30 and continues the operation.

[0053] On the other hand, if the fuel cell voltage OV drops to the operating point P1 (S40: Yes), then proceed to S50. In S50, the control unit 40 switches the mapping used from the first mapping M1 to the second mapping M2. In S70, the control unit 40 controls the air compressor 21 according to the second mapping M2. Specifically, the second stoichiometric ratio ST2 is used regardless of whether it is a low load state LL or a high load state HL.

[0054] exist Figure 3In the example, the first current CT1 (63%) at the operating point P1 is less than the current threshold IT (67%). Therefore, since the fuel cell stack 10 is in a low-load state LL, the stoichiometric ratio can be switched from the first stoichiometric ratio ST1 to the second stoichiometric ratio ST2 by switching the mapping. By using the second stoichiometric ratio ST2 with a large airflow, the water remaining inside the fuel cell stack 10 can be discharged to the outside. Thus, recovery from flooding can be achieved. Furthermore, the oxygen concentration near the air outlet can be increased. Therefore, the operating point moves on the isopower line L1 in the direction of increasing stack voltage OV (refer to arrow A2).

[0055] Furthermore, by switching from the first stoichiometric ratio ST1 to the second stoichiometric ratio ST2, the output of the fuel cell stack 10 sometimes increases sharply. Therefore, the output power of the fuel cell stack 10 sometimes overshoots the target power TP. This overshoot is more pronounced under high load conditions HL than under low load conditions LL. Therefore, in the technology of this specification, as described above, the control is to perform the stoichiometric ratio switching under low load conditions LL. This allows overshoot caused by the stoichiometric ratio switching to be suppressed.

[0056] In S80, the control unit 40 determines whether the fuel cell voltage OV has risen to the second voltage VT2 (operation point P2). If it has not risen (S80: No), it returns to S70 and continues the operation.

[0057] On the other hand, if the fuel cell voltage 0V rises to the operating point P2 (S80: Yes), then proceed to S90. In S90, the control unit 40 switches the used mapping from the second mapping M2 to the first mapping M1. Then, it returns to S30.

[0058] In S30, the control unit 40 controls the air compressor 21 according to the first mapping M1. Therefore, as described above, the voltage gradually decreases due to the flooding phenomenon. Consequently, the operating point moves from operating point P2 to operating point P0 on the equal power line L1 (refer to arrow A3). Since the operation is cyclical thereafter, the explanation is omitted.

[0059] (Topic)

[0060] Use comparative examples to illustrate the topic. Figure 5 The third mapping M3 (solid line) of the comparative example is shown. The third mapping M3 is a mapping that sets the air stoichiometry to the first stoichiometry ST1 in both the low-load state LL and the high-load state HL. Furthermore, Figure 6 The diagram shows the IV characteristic graphs of the comparative example. Compared with this embodiment (…),… Figure 3 Compared to the example () Figure 6The net output power of the current is relatively large, and the equal power line L1a shifts to the right of the graph. Therefore, at the operating point P1a of the comparative example, the first current CT1a (70%) exceeds the current threshold IT (67%). That is, the operating point P1a of the comparative example is a high load state HL.

[0061] In the comparative example, Figure 4 In the process, the third mapping M3 is set as the initial setting (S10). Then, according to the third mapping M3, the air compressor 21 is controlled using the first stoichiometric ratio ST1 (S30). The operating point moves along the direction of decreasing stack voltage OV on the equal power line L1a (refer to arrow A1a). Then, if the moving operating point exceeds the operating point Pt corresponding to the current threshold IT, it becomes a high load state HL. However, in the third mapping M3 of the comparative example, the first stoichiometric ratio ST1 is used even in the high load state HL, so power generation continues to use the first stoichiometric ratio ST1.

[0062] If it is determined that the stack voltage OV has dropped to the first voltage VT1a (operation point P1a) (S40: Yes), then the system switches from the third mapping M3 to the second mapping M2 (S50). In the comparative example, since the third mapping M3 is used, the first stoichiometric ratio ST1 is used when the operation point P1a is reached. Therefore, under the high load condition HL, the stoichiometric ratio switches from the first stoichiometric ratio ST1 to the second stoichiometric ratio ST2. As a result, the output power of the fuel cell stack 10 overshoots (reference area R11 and arrow A2a). The output power from the fuel cell system 1 momentarily exceeds the preset net output power significantly. Therefore, the power supply destination device from the fuel cell system 1 may malfunction, etc.

[0063] (Effect)

[0064] In the technology described in this specification, under low load condition LL, the stoichiometry can be changed between a first stoichiometry ST1 and a second stoichiometry ST2. Therefore, under low load condition LL, by using a lower first stoichiometry ST1, cell drying during power generation can be suppressed. Furthermore, if flooding occurs under low load condition LL (S40: Yes), the flooding can be eliminated by switching to the second stoichiometry ST2 (S50). Furthermore, under high load condition HL, by maintaining the stoichiometry at a constant value of the second stoichiometry ST2, switching of the stoichiometry is avoided. Therefore, overshoot caused by stoichiometry switching can be suppressed.

[0065] In the technique described in this specification, at least one of the net output power and the IV curve G1 is appropriately adjusted so that the first current CT1 at the operating point P1 becomes lower than the current threshold IT (67%). That is, the operating point P1 is adjusted to a low load state LL. As a result, switching from the first stoichiometric ratio ST1 to the second stoichiometric ratio ST2 at the operating point P1 can be performed in the low load state LL (S50). Therefore, overshoot caused by the switching of stoichiometric ratios can be suppressed.

[0066] Typically, fuel cells experience significant performance degradation due to cell drying after continuous power generation for more than 12 hours. Furthermore, in stationary or marine fuel cells, the power generation time per trip is longer compared to that in electric vehicle fuel cells, making cell drying more likely. The technology described in this specification addresses both suppressing cell drying during power generation and eliminating flooding, thus making it suitable for stationary or marine fuel cells.

[0067] The specific examples of the present invention have been described in detail above, but these are merely illustrative and do not limit the scope of the claims. The technology described within the scope of the claims includes technologies obtained by various modifications and alterations to the specific examples described above. The technical elements illustrated in this specification or drawings exert their technical usefulness individually or in various combinations, and are not limited to the combinations described in the claims at the time of application. Furthermore, the technologies illustrated in this specification or drawings can achieve multiple objectives simultaneously, wherein achieving only one objective is itself technically useful.

[0068] (Modified Example)

[0069] The values ​​of the current threshold IT, the first stoichiometric ratio ST1, and the second stoichiometric ratio ST2 are examples, and can be set to various values. The IV curve G1, IV curve G2, and isopower line L1 are examples, and can be set to various curves.

Claims

1. A fuel cell system characterized by comprising: Possessing: a fuel cell; an oxidizing gas supply system that supplies an oxidizing gas to the fuel cell; and a control section configured to be able to control the oxidizing gas supply system to adjust a stoichiometric ratio of the oxidizing gas, the control section performs the following processing: in a low load state in which a stack current of the fuel cell is below a current threshold value set in advance, changing the stoichiometric ratio between at least two values in accordance with a stack voltage of the fuel cell; and in a high load state in which the stack current is greater than the current threshold value, maintaining the stoichiometric ratio as constant regardless of the stack voltage.

2. The fuel cell system according to claim 1, characterized in that the control section is configured to be able to change the stoichiometric ratio between a first stoichiometric ratio and a second stoichiometric ratio that is greater than the first stoichiometric ratio in the low load state, the control section changes the stoichiometric ratio from the first stoichiometric ratio to the second stoichiometric ratio when the stack voltage drops to a first voltage during the period in which the control section sets the stoichiometric ratio to the first stoichiometric ratio in the low load state.

3. The fuel cell system according to claim 2, characterized in that the control section changes the stoichiometric ratio from the second stoichiometric ratio to the first stoichiometric ratio when the stack voltage rises to a second voltage that is higher than the first voltage during the period in which the control section sets the stoichiometric ratio to the second stoichiometric ratio in the low load state.

4. The fuel cell system according to claim 2 or 3, characterized in that the control section maintains the stoichiometric ratio as the second stoichiometric ratio in the high load state.

5. The fuel cell system according to claim 2 or 3, characterized in that the control section changes at least one of the first voltage and the second voltage in accordance with a required output to the fuel cell.

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