Fuel cell system

Abstract

Suppresses overshoot in fuel cell systems. [Solution] The fuel cell system comprises a fuel cell, an oxidizing gas supply system that supplies oxidizing gas to the fuel cell, and a control unit configured to control the oxidizing gas supply system and adjust the stoichiometric ratio of the oxidizing gas. In a low-load state where the stack current of the fuel cell is below a predetermined current threshold, the control unit changes the stoichiometric ratio between at least two values ​​according to the stack voltage of the fuel cell. In a high-load state where the stack current is greater than the current threshold, the control unit maintains a constant stoichiometric ratio regardless of the stack voltage.

Description

Technical Field

[0003]

[0001] This specification discloses a fuel cell system.

Background Art

[0002] Patent Document 1 discloses a fuel cell that can suppress a decrease in stack voltage by increasing the air stoichiometry ratio.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

Means for Solving the Problems

[0006] With the above configuration, the stoichiometric ratio of the oxidizing gas can be changed under low load conditions, while the stoichiometric ratio is kept constant under high load conditions. This makes it possible to control the stack voltage using the stoichiometric ratio under low load conditions. Furthermore, overshoot can be suppressed under high load conditions. This allows for control of the stack voltage while suppressing overshoot.

[0007] The control unit may be configured to change the stoichiometric ratio between a first stoichiometric ratio and a second stoichiometric ratio greater than the first stoichiometric ratio under low load conditions. If the stack voltage drops to a first voltage while the stoichiometric ratio is set to the first stoichiometric ratio under low load conditions, the control unit may change the stoichiometric ratio from the first stoichiometric ratio to the second stoichiometric ratio.

[0008] With this configuration, if the stack voltage drops to the first voltage under low load conditions, the stack voltage can be increased by increasing the stoichiometric ratio. This prevents the stack voltage from falling below the first voltage.

[0009] The control unit may change the stoichiometric ratio from the second stoichiometric ratio to the first stoichiometric ratio if the stack voltage rises to a second voltage higher than the first voltage while the stoichiometric ratio is set to the second stoichiometric ratio under low load conditions.

[0010] With this configuration, if the stoichiometric ratio is set high under low load conditions, and the stack voltage rises to the second voltage, the stack voltage can be reduced by decreasing the stoichiometric ratio. This prevents the stack voltage from exceeding the second voltage.

[0011] The control unit may maintain the stoichiometric ratio at a second stoichiometric ratio under high load conditions.

[0012] The control unit may change at least one of the first voltage and the second voltage in accordance with the requested output to the fuel cell.

[0013] This configuration makes it possible to control the stack voltage while suppressing overshoot, even when the required output changes.

[0014] Details of the technology disclosed herein and further improvements are described in the following "Modes for Carrying Out the Invention". [Brief explanation of the drawing]

[0015] [Figure 1] This is a schematic diagram of the fuel cell system 1. [Figure 2] This graph shows the first map M1 and the second map M2. [Figure 3] This graph shows the IV characteristics of fuel cell stack 10. [Figure 4] This is a flowchart illustrating the operation of fuel cell system 1. [Figure 5] This graph shows the third map M3 of the comparative example. [Figure 6] This graph shows the IV characteristics of the comparative example. [Modes for carrying out the invention] [Examples]

[0016] (Configuration of fuel cell system 1) The fuel cell system 1 will be described with reference to Figure 1. The fuel cell system 1 can be applied to various mobile devices such as stationary power sources, ships, trains, and fuel cell vehicles. The fuel cell system 1 mainly consists of 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. In Figure 1, signal lines are shown as dotted lines.

[0017] The fuel cell stack 10 is a device that generates electric power through the chemical reaction of hydrogen and oxygen. When hydrogen and oxygen chemically react, water is produced. The fuel cell stack 10 includes a plurality of single cells (not shown). Each single cell includes a fuel electrode and an air electrode. Power is generated by supplying fuel gas (hydrogen gas) to the fuel electrode and oxidizing gas (air containing oxygen) to the air electrode.

[0018] The output terminals of the fuel cell stack 10 are connected to the load 50. The electric power generated by the fuel cell stack 10 is supplied to the load 50. The type of the load 50 can be various, for example, it may be a motor for a moving body. Also, the load 50 may include various power conversion devices such as a DC / DC converter.

[0019] The output path of the fuel cell stack 10 is provided with a current sensor 41 and a voltage sensor 42. 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.

[0020] The air supply system 2 includes an air supply path 20a and an air discharge path 20b. The air supply path 20a is connected to the air inlet portion (oxidizing gas inlet portion) 11a of the fuel cell stack 10. The air supply path 20a is provided with an air compressor 21. The air compressor 21 is a device for pumping the air inhaled from the atmosphere to the fuel cell stack 10. The air compressor 21 adjusts the air stoichiometric ratio supplied to the fuel cell stack 10 based on the first map M1 and the second map M2 described later.

[0021] The air discharge passage 20b is connected to the air outlet (oxidizing gas outlet) 11b of the fuel cell stack 10. The air discharge passage 20b is a path for discharging unreacted air and generated water produced by electrochemical reactions inside the fuel cell stack 10 to the outside. The air discharge passage 20b is equipped with a pressure regulating valve 23. The pressure regulating valve 23 is a valve that controls the stack pressure of the oxidizing gas (air) based on a control signal CS2 input from the control unit 40. Various valve structures can be used for the pressure regulating valve 23.

[0022] The hydrogen supply system 3 includes a hydrogen supply passage 30a and a hydrogen discharge passage 30b. The hydrogen supply passage 30a is a path for introducing hydrogen, which is the fuel gas, into the fuel cell stack 10. The hydrogen supply passage 30a is connected to the hydrogen inlet (fuel gas inlet) 12a of the fuel cell stack 10. The hydrogen discharge passage 30b is a path for discharging unreacted hydrogen and generated water emitted from the fuel cell stack 10. The hydrogen discharge passage 30b is connected to the hydrogen outlet (fuel gas outlet) 12b of the fuel cell stack 10. A detailed explanation of the hydrogen supply passage 30a and the hydrogen discharge passage 30b is omitted.

[0023] The control unit 40 is a control means for controlling various devices of the fuel cell system 1. The control unit 40 includes a CPU, memory, and peripheral circuits (not shown). The stack current OC and stack voltage OV are input to the control unit 40 from the current sensor 41 and the load 50. The control unit 40 is also input to the target power TP 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 then controls the air supply system 2 so that the amount of power generated by the fuel cell stack 10 matches the target power TP.

[0024] (Control details of air compressor 21) Figure 2 shows the first map M1 (solid line) and the second map M2 (dotted line). The first map M1 and the second map M2 are maps for adjusting the air stoichiometric ratio 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 shows the ratio of the stack current when the maximum load is 100%. The vertical axis represents the air stoichiometric ratio. The air stoichiometric ratio is the excess rate of the actual air supply amount compared to the theoretical air supply amount required to generate the stack current OC. In other words, an air stoichiometric ratio of 1.0 corresponds to the theoretical air supply amount. Normally, when operating the fuel cell stack 10, the air stoichiometric ratio is set to 1.0 or higher (theoretical value) in order to suppress power loss and obtain high power generation efficiency.

[0025] As shown in Figure 2, a current threshold IT, a low-load state LL, and a high-load state HL are predetermined for the stack current OC. The current threshold IT is a value that can be appropriately determined depending on the configuration of the fuel cell system 1 and the type of load 50. In this embodiment, the current threshold IT was set to 67% of the maximum load. The low-load state LL is the state in which the stack current OC is less than or equal to the current threshold IT. The high-load state HL is the state in which the stack current OC is greater than the current threshold IT. Generally, the low-load state LL has higher power generation efficiency than the high-load state HL. Therefore, the control unit 40 controls the fuel cell stack 10 to the low-load state LL during steady-state operation.

[0026] The first map M1 will now be explained. The first map M1 is a map in which the air stoichiometric ratio is set to the first stoichiometric ratio ST1 in the low load state LL, and to the second stoichiometric ratio ST2 in the high load state HL. The second stoichiometric ratio ST2 is a larger value than the first stoichiometric ratio ST1. In this embodiment, the first stoichiometric ratio ST1 = 1.2 and the second stoichiometric ratio ST2 = 1.5. The first map M1 includes 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 low load state LL. In the first region R1, the air stoichiometric ratio is constant at the first stoichiometric ratio ST1. In the second region R2, the air-stoichiometric ratio decreases from the second stoichiometric ratio ST2 to the first stoichiometric ratio ST1. In the third region R3, the air-stoichiometric ratio increases from the first stoichiometric ratio ST1 to the second stoichiometric ratio ST2.

[0027] Let's explain the second map, M2. The second map, M2, sets the air stoichiometric ratio to the second stoichiometric ratio ST2 in both the low-load state LL and the high-load state HL.

[0028] The first map M1 and the second map M2 are stored in the memory of the control unit 40. The control unit 40 is capable of switching between the two maps. This makes it possible to change the stoichiometric ratio between the first stoichiometric ratio ST1 and the second stoichiometric ratio ST2 in the low-load state LL. In the high-load state HL, it is possible to maintain the stoichiometric ratio at a constant value of the second stoichiometric ratio ST2.

[0029] This explains the reasons for using the first stoichiometric ratio ST1 and the second stoichiometric ratio ST2. The first stoichiometric ratio ST1 has a lower airflow rate than the second stoichiometric ratio ST2. Therefore, cell drying during power generation is less likely to occur with the first stoichiometric ratio ST1 than with the second stoichiometric ratio ST2. However, flooding during continuous power generation and a decrease in oxygen concentration near the air outlet are more likely to occur with the first stoichiometric ratio ST1 than with the second stoichiometric ratio ST2. Therefore, from the viewpoint of suppressing cell drying, it is preferable to use the first stoichiometric ratio ST1. On the other hand, from the viewpoint of eliminating flooding, it is preferable to use the second stoichiometric ratio ST2. Flooding is a phenomenon in which the supply of gas to the electrode is hindered by water generated by the power generation reaction at the oxidizing electrode. When flooding occurs, the power generation function deteriorates.

[0030] (Specific examples of actions) A specific example of operation will be explained using the current-voltage characteristics (IV characteristics) of the fuel cell stack 10 shown in Figure 3. The horizontal axis represents the stack current OC (%), showing the percentage of the stack current when the maximum load is set to 100%. The vertical axis represents the stack voltage OV. The IV curves G1 and G2 are curves that can be determined as appropriate.

[0031] The equipower line L1 is the trajectory of the operating point when the fuel cell system 1 generates power at a constant net output power. In other words, the operating point of the fuel cell system 1 is ideally located on the equipower line L1. The intersection of the equipower line L1 and the IV curve G1 is defined as the operating point P1. The operating point P1 is the point where the system switches from the first map M1 to the second map M2. The stack voltage value at the operating point P1 is defined as the first voltage VT1, and the stack current value is defined as the first current CT1. The intersection of the equipower line L1 and the IV curve G2 is defined as the operating point P2. The operating point P2 is the point where the system switches from the second map M2 to the first map M1. The stack voltage value at the operating point P2 is defined as the second voltage VT2, and the stack current value is defined as the second current CT2.

[0032] 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. Preferably, the first current CT1 is lower than the current threshold IT (67%). This is because the switching from the first map M1 to the second map M2, which occurs at the operating point P1, can be performed under low load conditions LL. In the example in Figure 3, the first current CT1 is 63%.

[0033] (Operation details of fuel cell system 1) The operation of the fuel cell system 1 will be explained using the flowchart in Figure 4. Hereafter, "Step 10" will be abbreviated as "S10". The flow in Figure 4 begins when the fuel cell system 1 is turned on.

[0034] In S10, the control unit 40 sets the first map M1 as the map to be used as an initial setting. In the initial operation, the operating point is operating point P0 (see Figure 3). The operating point P0 in the initial operation is one of the positions on the equipower line L1 between operating points P1 and P2.

[0035] In S30, the control unit 40 controls the air compressor 21 based on the first map M1. Specifically, the first stoichiometric ratio ST1 (1.2) is used in the low-load state LL, and the second stoichiometric ratio ST2 (1.5) is used in the high-load state HL. As mentioned above, during steady-state operation, the fuel cell stack 10 is in the low-load state LL, so the first stoichiometric ratio ST1 is used.

[0036] As power generation using the first stoichiometric ratio ST1 (e.g., steady-state operation) continues, the amount of water remaining on the oxidizer electrode of the fuel cell stack 10 increases. Consequently, the voltage gradually decreases due to flooding. Therefore, the operating point moves along the equipower line L1 in the direction of decreasing stack voltage OV (see arrow A1).

[0037] In S40, the control unit 40 determines whether the stack voltage OV has dropped to the first voltage VT1 (operating point P1). If it has not dropped (S40: NO), the process returns to S30 and continues.

[0038] On the other hand, when the stack voltage OV drops to the operating point P1 (S40: YES), the process proceeds to S50. In S50, the control unit 40 switches the map to be used from the first map M1 to the second map M2. In S70, the control unit 40 controls the air compressor 21 based on the second map 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.

[0039] In the example shown in Figure 3, the first current CT1 (63%) at the operating point P1 is smaller than the current threshold IT (67%). Therefore, the fuel cell stack 10 is in a low-load state LL, and the stoichiometric ratio can be switched from the first stoichiometric ratio ST1 to the second stoichiometric ratio ST2 by switching the map. By using the second stoichiometric ratio ST2, which has a larger airflow rate, water remaining inside the fuel cell stack 10 can be scavenged to the outside. This makes it possible to recover from flooding. It also makes it possible to increase the oxygen concentration near the air outlet. Therefore, the operating point moves along the equipower line L1 in the direction in which the stack voltage OV increases (see arrow A2).

[0040] Furthermore, switching from the first stoichiometric ratio ST1 to the second stoichiometric ratio ST2 may cause a rapid increase in the output of the fuel cell stack 10. As a result, the output power of the fuel cell stack 10 may overshoot the target power TP. This overshoot is more pronounced in high-load conditions HL than in low-load conditions LL. Therefore, in the technology described herein, as mentioned above, the switching of the stoichiometric ratio is controlled to be performed in the low-load condition LL. This makes it possible to suppress the overshoot caused by the switching of the stoichiometric ratio.

[0041] In S80, the control unit 40 determines whether the stack voltage OV has risen to the second voltage VT2 (operating point P2). If it has not risen (S80: NO), the unit returns to S70 and continues operation.

[0042] On the other hand, when the stack voltage OV rises to the operating point P2 (S80: YES), the process proceeds to S90. In S90, the control unit 40 switches the map being used from the second map M2 to the first map M1. Then the process returns to S30.

[0043] In S30, the control unit 40 controls the air compressor 21 based on the first map M1. As a result, as described above, the voltage gradually decreases due to flooding. Therefore, the operating point moves along the equipower line L1 from operating point P2 to operating point P0 (see arrow A3). The subsequent operation is a loop, so the explanation is omitted.

[0044] (assignment) The problem will be explained using a comparative example. Figure 5 shows the third map M3 (solid line) of the comparative example. The third map M3 is a map that sets the air stoichiometric ratio to the first stoichiometric ratio ST1 in both the low-load state LL and the high-load state HL. Figure 6 shows the IV characteristic graph of the comparative example. In the comparative example (Figure 6), the net output power is larger than in this embodiment (Figure 3), and the equipower line L1a has moved to the right side of the figure. Therefore, at the operating point P1a of the comparative example, the first current CT1a (70%) exceeds the current threshold IT (67%). In other words, at the operating point P1a of the comparative example, it is the high-load state HL.

[0045] In the comparative example, in the flow shown in Figure 4, the third map M3 is set as the initial setting (S10). Then, based on the third map M3, the air compressor 21 is controlled using the first stoichiometric ratio ST1 (S30). The operating point moves along the equipower line L1a in the direction in which the stack voltage OV decreases (see arrow A1a). When the moving operating point exceeds the operating point Pt corresponding to the current threshold IT, a high load state HL is entered. However, in the third map M3 of the comparative example, the first stoichiometric ratio ST1 is used even in the high load state HL, so power generation continues using the first stoichiometric ratio ST1.

[0046] When it is determined that the stack voltage OV has dropped to the first voltage VT1a (operating point P1a) (S40: YES), the system switches from the third map M3 to the second map M2 (S50). In the comparative example, since the third map M3 is used, the first stoichiometric ratio ST1 is used when the operating point P1a is reached. Therefore, under high load conditions HL, the stoichiometric ratio is switched 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 (see region R11 and arrow A2a). The output power from the fuel cell system 1 momentarily exceeds the predetermined net output power by a large margin. Therefore, there is a risk that the receiving equipment that is supplied with power from the fuel cell system 1 may experience malfunctions or other problems.

[0047] (effect) In the technology described herein, under low-load conditions LL, the stoichiometric ratio can be changed between a first stoichiometric ratio ST1 and a second stoichiometric ratio ST2. This allows for the suppression of cell drying during power generation by using a lower first stoichiometric ratio ST1 under low-load conditions LL. Furthermore, if flooding occurs under low-load conditions LL (S40: YES), the flooding can be resolved by switching to the second stoichiometric ratio ST2 (S50). Under high-load conditions HL, the stoichiometric ratio is maintained at a constant value of the second stoichiometric ratio ST2, thereby preventing the stoichiometric ratio from being switched. This suppresses overshoot caused by switching the stoichiometric ratio.

[0048] In the technology described herein, 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 is lower than the current threshold IT (67%). In other words, the operating point P1 is adjusted to a low-load state LL. This allows the switching from the first stoichiometric ratio ST1 to the second stoichiometric ratio ST2 (S50) at the operating point P1 to be performed under the low-load state LL. Thus, it is possible to suppress overshoot caused by the switching of the stoichiometric ratio.

[0049] Generally, fuel cells experience significant performance degradation due to cell drying during continuous power generation for 12 hours or more. Furthermore, stationary and marine fuel cells are more prone to cell drying than fuel cells for electric vehicles because the power generation time per trip is longer. The technology described herein is suitable for stationary and marine fuel cells because it can suppress cell drying during power generation and eliminate flooding.

[0050] Although specific examples of the present invention have been described in detail above, these are merely illustrative and do not limit the scope of the claims. The technologies described in the claims include various modifications and changes to the specific examples illustrated above. The technical elements described in this specification or drawings exhibit technical usefulness individually or in various combinations, and are not limited to the combinations described in the claims at the time of filing. Furthermore, the technologies illustrated in this specification or drawings can achieve multiple objectives simultaneously, and achieving even one of these objectives itself constitutes technical usefulness.

[0051] (modified version) The values ​​for the current threshold IT, the first stoichiometric ratio ST1, and the second stoichiometric ratio ST2 are examples only and can be set to various values. The IV curves G1 and G2, and the equipower line L1 are examples only and can be set to various curves. [Explanation of Symbols]

[0052] 1: Fuel cell system 2: Air supply system 10: Fuel cell stack 21: Air compressor 40: Control unit ST1: First stoichiometric ratio ST2: Second stoichiometric ratio

Claims

1. Fuel cells and An oxidizing gas supply system that supplies oxidizing gas to the fuel cell, A control unit configured to control the oxidizing gas supply system and adjust the stoichiometric ratio of the oxidizing gas, Equipped with, The control unit, In a low-load state where the stack current of the fuel cell is below a predetermined current threshold, the stoichiometric ratio is changed between at least two values ​​according to the stack voltage of the fuel cell. In a high-load state where the stack current is greater than the current threshold, the stoichiometric ratio is kept constant regardless of the stack voltage. Fuel cell system.

2. The control unit is configured to change the stoichiometric ratio between a first stoichiometric ratio and a second stoichiometric ratio greater than the first stoichiometric ratio in the low-load state. The fuel cell system according to claim 1, wherein the control unit changes the stoichiometric ratio from the first stoichiometric ratio to the second stoichiometric ratio when the stack voltage drops to a first voltage while the stoichiometric ratio is set to the first stoichiometric ratio in the low-load state.

3. The fuel cell system according to claim 2, wherein the control unit changes the stoichiometric ratio from the second stoichiometric ratio to the first stoichiometric ratio when the stack voltage rises to a second voltage higher than the first voltage while the stoichiometric ratio is set to the second stoichiometric ratio in the low-load state.

4. The fuel cell system according to claim 2 or 3, wherein the control unit maintains the stoichiometric ratio at the second stoichiometric ratio in the high-load state.

5. The fuel cell system according to claim 2 or 3, wherein the control unit changes at least one of the first voltage and the second voltage in accordance with the requested output to the fuel cell.

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