Fuel cell system

Abstract

To provide a technique of avoiding surging of a turbo compressor.SOLUTION: A fuel cell system includes: a plurality of fuel cell stacks; an oxidant gas supply unit having a turbo compressor that supplies oxidant gas to each of the fuel cell stacks; and a control device that determines, according to a required output, a required operation number for the fuel cell stacks and a target supply pressure and a target supply flow rate of the oxidant gas to be commanded to the oxidant gas supply unit. The oxidant gas supply unit has an adjustment mechanism that adjusts a supply rate, the supply rate being a ratio of a supply flow rate supplied to the plurality of fuel cell stacks, with respect to a discharge flow rate of the turbo compressor. The control device monitors a pressure ratio in the turbo compressor and controls the adjustment mechanism to reduce the supply rate when the pressure ratio exceeds a predetermined threshold.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The technology disclosed in this specification relates to a fuel cell system. [Background technology]

[0002] Patent Document 1 describes a fuel cell system that includes a plurality of fuel cell stacks, an oxidizing gas supply unit that supplies oxidizing gas to each of the plurality of fuel cell stacks, a cooling unit that supplies a refrigerant that cools each of the plurality of fuel cell stacks, and a control device that determines a target supply flow rate of oxidizing gas to be instructed to the oxidizing gas supply unit and a target supply flow rate of refrigerant to be instructed to the cooling unit. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2022-156906 Summary of the Invention [Problem to be solved by the invention]

[0004] In a fuel cell stack, the supply pressure and supply flow rate of the oxidizing gas to be supplied to the fuel cell stack are determined according to the required output (i.e., the power to be generated) and the temperature of the fuel cell stack. For example, if the temperature of the fuel cell stack is constant, the supply pressure and supply flow rate of the oxidizing gas must be increased as the output required of the fuel cell stack increases. In this case, if a turbo compressor is used to supply the oxidizing gas, surging may occur due to an increase in the pressure ratio of the compressor. In particular, in a fuel cell system equipped with multiple fuel cell stacks, the number of operating fuel cell stacks may be reduced depending on the output required of the entire system. In this case, the required output of the fuel cell stacks that continue to operate increases suddenly, increasing the risk of surging.

[0005] In view of the above circumstances, the present specification provides a technique for avoiding surging in a turbo compressor. [Means for solving the problem]

[0006] As described above, as the output required of the fuel cell stack increases, the supply pressure and supply flow rate of the oxidizing gas to the fuel cell stack must be increased. To achieve this, the turbo compressor must increase the pressure ratio and the discharge flow rate of the oxidizing gas. In this regard, the turbo compressor is more likely to experience surging when the pressure ratio exceeds a certain upper limit. However, this upper limit is not constant; the higher the discharge flow rate of the turbo compressor, the higher the upper limit becomes. Therefore, if the discharge flow rate of the oxidizing gas from the compressor can be increased as the pressure ratio of the turbo compressor increases, compressor surging can be avoided. Furthermore, if the ratio of the supply flow rate to the fuel cell stack to the discharge flow rate of the oxidizing gas from the compressor can be adjusted, an appropriate amount of oxidizing gas can be supplied to the fuel cell stack.

[0007] Based on the above findings, the technology disclosed in this specification is embodied in a fuel cell system. In a first aspect thereof, the fuel cell system includes a plurality of fuel cell stacks, an oxidizing gas supply unit having a turbo compressor that supplies oxidizing gas to each of the plurality of fuel cell stacks, and a control device that determines the required number of operations for the plurality of fuel cell stacks and the target supply pressure and target supply flow rate of the oxidizing gas to be issued to the oxidizing gas supply unit according to the required output. The oxidizing gas supply unit has an adjustment mechanism that adjusts the supply rate, which is the ratio of the supply flow rate supplied to the plurality of fuel cell stacks to the discharge flow rate of the turbo compressor. The control device monitors the pressure ratio in the turbo compressor, and controls the adjustment mechanism to reduce the supply rate when the pressure ratio exceeds a predetermined threshold.

[0008] According to the above configuration, for example, by reducing the number of fuel cell stacks in operation (i.e., generating power), when the pressure ratio in the turbo compressor increases, the discharge flow rate of the oxidizing gas in the turbo compressor can be increased preferentially, thereby preventing surging in the turbo compressor.

[0009] In a second aspect, in the first aspect, the adjustment mechanism may include a reflux path connecting the discharge side and the suction side of the turbo compressor, and a flow rate adjustment valve provided in the reflux path. With this configuration, the oxidizing gas can be sent from the discharge side to the suction side via the reflux path, and the discharge flow rate of the oxidizing gas from the turbo compressor can be preferentially increased.

[0010] In a third aspect, in the first or second aspect, the oxidant gas supply unit may further include an intercooler that cools the oxidant gas discharged from the turbo compressor. In this case, the return path may connect the discharge side and the suction side of the turbo compressor upstream of the intercooler. With this configuration, the oxidant gas sent from the discharge side to the suction side does not pass through the intercooler, so that only the oxidant gas supplied to the fuel cell stack can be cooled by the intercooler.

[0011] In a fourth aspect, in the first or second aspect, the oxidant gas supply unit may further include an intercooler that cools the oxidant gas discharged from the turbo compressor. In this case, the reflux path may connect the discharge side and the suction side of the turbo compressor downstream of the intercooler. With this configuration, the temperature of the oxidant gas taken in by the turbo compressor is lowered, and the energy required to achieve a predetermined pressure ratio in the compressor can be reduced.

[0012] In a fifth aspect, in any of the first to fourth aspects, the oxidant gas supply unit may include an oxidant gas supply path that supplies oxidant gas from the turbo compressor to the multiple fuel cell stacks, and an off-gas exhaust path that exhausts off-gas of the oxidant gas from the multiple fuel cell stacks. In this case, the adjustment mechanism may include a branch path that connects the oxidant gas supply path and the off-gas exhaust path, and a flow rate adjustment valve provided in the branch path. With this configuration, oxidant gas can be exhausted from the oxidant gas supply path to the off-gas exhaust path via the branch path, and the discharge flow rate of the oxidant gas from the turbo compressor can be preferentially increased. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a diagram schematically illustrating the configuration of a fuel cell system 10 according to a first embodiment. [Figure 2] This figure shows the relationship between the target supply pressure and target supply flow rate of oxidizing gas that the control device 36 commands the oxidizing gas supply unit 20 to use in response to the output required for the fuel cell stacks 12, 14. Each of the solid lines U1 to U4 shows the relationship between the supply flow rate and the supply pressure of oxidizing gas for different temperatures of the fuel cell stacks 12, 14. Each of the dashed lines P1 to P5 shows the relationship between the supply flow rate and the supply pressure of oxidizing gas for different currents (i.e., generated power) of the fuel cell stacks 12, 14. Note that larger numbers indicate higher temperatures or higher power. [Figure 3] This shows the surging constraint line SL of the compressor 22. Each of the dotted lines L1 to L5 shows the relationship between the discharge flow rate of the oxidizing gas and the pressure ratio in the compressor 22 for a different rotation speed of the compressor 22. Note that a larger number indicates a higher rotation speed. [Figure 4] 6 is a flowchart showing a first process executed by a control device 36. [Figure 5]Figures 5(A)-(G) show the changes over time of various parameters in the first process shown in Figure 4. In Figure 5(B), curve I1 shows the current of the first fuel cell stack 12, and curve I2 shows the current of the second fuel cell stack 14. In Figure 5(E), curve OV1 shows the aperture of the outlet valve 30 of the first fuel cell stack 12, and curve OV2 shows the aperture of the outlet valve 30 of the second fuel cell stack 14. In Figure 5(F), curve IV1 shows the aperture of the inlet valve 26 of the first fuel cell stack 12, and curve IV2 shows the aperture of the inlet valve 26 of the second fuel cell stack 14. [Figure 6] 10 is a flowchart showing a second process executed by the control device 36. [Figure 7] FIG. 1 is a diagram schematically illustrating the configuration of a fuel cell system 110 according to a second embodiment. [Figure 8] FIG. 10 is a diagram schematically illustrating the configuration of a fuel cell system 210 according to a third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0014] (Example 1) A fuel cell system 10 of this example will be described with reference to the drawings. The fuel cell system 10 is a power generation system that is mounted on a mobile object (e.g., an automobile, a bus, a truck, a train, a ship, or an airplane) or a stationary fuel cell device, and outputs electric power according to an externally required output.

[0015] As shown in FIG. 1, the fuel cell system 10 includes multiple fuel cell stacks 12, 14. Each fuel cell stack 12, 14 has a structure in which multiple fuel cell units are stacked. Each fuel cell stack 12, 14 includes an anode supply port (not shown), a cathode supply port 16a, an anode discharge port (not shown), and a cathode discharge port 16b. The anode supply port and cathode supply port 16a of each fuel cell stack 12, 14 are connected to each of the multiple fuel cell units within the fuel cell stack 12, 14. Each fuel cell stack 12, 14 generates electricity by chemically reacting fuel gas taken in through the anode supply port with oxidizing gas taken in through the cathode supply port 16a within the multiple fuel cell units. Gas that has passed through the multiple fuel cell stacks 12, 14 (i.e., off-gas) is discharged to the outside through the anode discharge port and the cathode discharge port 16b.

[0016] The multiple fuel cell stacks 12, 14 include a first fuel cell stack 12 and a second fuel cell stack 14. In this embodiment, the first fuel cell stack 12 is electrically connected in parallel with the second fuel cell stack 14. However, the number of the multiple fuel cell stacks 12, 14 is not particularly limited as long as it is two or more. In the fuel cell system 10 of this embodiment, hydrogen gas is used as the fuel gas and air is used as the oxidizing gas. The air contains oxygen as an oxidizing agent.

[0017] As shown in Fig. 1, the fuel cell system 10 further includes a plurality of power control units 18. In this embodiment, the power control units 18 have boost converters. The power control units 18 are electrically connected to the fuel cell stacks 12, 14. The power control units 18 can boost the power generated by the fuel cell stacks 12, 14 and output the power to the outside. Although not particularly limited, the power control units 18 may further include inverters in addition to the boost converters.

[0018] 1, the fuel cell system 10 further includes an oxidizing gas supply unit 20 and a control device 36. The oxidizing gas supply unit 20 is a unit for supplying oxidizing gas (air) to each of the multiple fuel cell stacks 12, 14. The oxidizing gas supply unit 20 includes a compressor 22, an intercooler 50, an oxidizing gas supply path 24, multiple inlet valves 26, an off-gas discharge path 28, multiple outlet valves 30, a flow dividing path 32, and a flow dividing valve 34. The control device 36 can start and stop the operation of the fuel cell stacks 12, 14.

[0019] The compressor 22 is a turbo compressor. The compressor 22 is provided in an oxidizing gas supply path 24 and compresses air taken in from the outside and supplies the compressed air to the multiple fuel cell stacks 12, 14. The oxidizing gas supply path 24 is a path for supplying the oxidizing gas discharged by the compressor 22 to the multiple fuel cell stacks 12, 14. The oxidizing gas supply path 24 includes a first oxidizing gas supply path 24a and a second oxidizing gas supply path 24b. The first oxidizing gas supply path 24a is connected to the cathode supply port 16a of the first fuel cell stack 12 and supplies the oxidizing gas from the compressor 22 to the first fuel cell stack 12. The second oxidizing gas supply path 24b is connected to the cathode supply port 16a of the second fuel cell stack 14 and supplies the oxidizing gas from the compressor 22 to the second fuel cell stack 14. The first oxidizing gas supply path 24a and the second oxidizing gas supply path 24b are connected to each other at a branch point B1.

[0020] The oxidizing gas supply path 24 further includes a third oxidizing gas supply path 24c and a fourth oxidizing gas supply path 24d. The third oxidizing gas supply path 24c extends between branch point B1 and the compressor 22 and connects the compressor 22 to the first oxidizing gas supply path 24a and the second oxidizing gas supply path 24b. The third oxidizing gas supply path 24c is located on the discharge side of the compressor 22 and supplies the oxidizing gas discharged from the compressor 22 to the first oxidizing gas supply path 24a and the second oxidizing gas supply path 24b. The fourth oxidizing gas supply path 24d is located on the suction side of the compressor 22 and supplies air taken in from the outside to the compressor 22. Although not particularly limited, the fuel cell system 10 may further include an air cleaner that removes foreign matter such as dust and dirt from the air taken in from the outside.

[0021] The off-gas discharge path 28 is a path for discharging oxidizing off-gas from the plurality of fuel cell stacks 12, 14. The off-gas discharge path 28 is connected to the cathode-side discharge port 16b in each of the plurality of fuel cell stacks 12, 14.

[0022] The intercooler 50 is provided in the third oxidizing gas supply path 24c. The intercooler 50 cools the oxidizing gas discharged from the compressor 22. As a result, the oxidizing gas discharged from the compressor 22 is cooled by the intercooler 50 and then supplied to the multiple fuel cell stacks 12, 14. The intercooler 50 may be a water-cooled type or an air-cooled type.

[0023] The plurality of inlet valves 26 are provided at the cathode supply ports 16a of the plurality of fuel cell stacks 12, 14. The plurality of outlet valves 30 are provided at the cathode discharge ports 16b of the plurality of fuel cell stacks 12, 14.

[0024] The shunt path 32 connects the oxidizing gas supply path 24 and the off-gas exhaust path 28 to each other. As an example, the shunt path 32 in this embodiment branches off from the third oxidizing gas supply path 24c and is connected to the off-gas exhaust path 28. A shunt valve 34 is provided in the shunt path 32. The inlet valve 26, the outlet valve 30, and the shunt valve 34 are control valves with adjustable openings. The operation (openings) of the inlet valve 26, the outlet valve 30, and the shunt valve 34 are controlled by a control device 36. The control device 36 controls the operation of the compressor 22, the inlet valves 26, the outlet valves 30, and the shunt valve 34, thereby adjusting the supply pressure and supply flow rate of the oxidizing gas supplied to each of the multiple fuel cell stacks 12, 14.

[0025] As shown in FIG. 1 , the oxidizing gas supply unit 20 further includes a reflux path 52 and a reflux valve 54. The reflux path 52 connects the fourth oxidizing gas supply path 24d, which is located on the suction side of the compressor 22, to the third oxidizing gas supply path 24c, which is located on the discharge side of the compressor 22. The reflux path 52 is provided with the reflux valve 54. The operation (opening degree) of the reflux valve 54 is controlled by the control device 36. By controlling the operation of the reflux valve 54, the control device 36 can send a portion of the oxidizing gas discharged from the compressor 22 from the third oxidizing gas supply path 24c to the fourth oxidizing gas supply path 24d. This allows the control device 36 to adjust the supply rate (hereinafter sometimes simply referred to as the supply rate), which is the ratio of the supply flow rate supplied to the multiple fuel cell stacks 12, 14 to the discharge flow rate of the compressor 22.

[0026] Although not shown, the fuel cell system 10 further includes a fuel gas supply unit. The fuel gas supply unit is a unit for supplying fuel gas (hydrogen gas) to the fuel cell stacks 12, 14.

[0027] As shown in FIG. 1, the fuel cell system 10 further includes a cooling unit 38. The cooling unit 38 is a unit for cooling each of the multiple fuel cell stacks 12, 14. The cooling unit 38 includes a radiator 40, a circulation path 42, a pump 44, and multiple temperature sensors 46, 48. The radiator 40 releases heat from the refrigerant circulating within the circulation path 42. The circulation path 42 includes an outward path 42a that supplies the refrigerant from the radiator 40 to the fuel cell stacks 12, 14, and a return path 42b that returns the refrigerant from the fuel cell stacks 12, 14 to the radiator 40. The pump 44 is provided in the outward path 42a.

[0028] The multiple temperature sensors 46, 48 include a first temperature sensor 46 and a second temperature sensor 48. The first temperature sensor 46 is provided at the outlet of the first fuel cell stack 12 on the return path 42b and detects the temperature of the coolant after passing through the first fuel cell stack 12. The second temperature sensor 48 is provided at the outlet of the second fuel cell stack 14 on the return path 42b and detects the temperature of the coolant after passing through the second fuel cell stack 14. As the coolant passes through the multiple fuel cells in each fuel cell stack 12, 14, it absorbs heat from the fuel cells. Therefore, the value detected by each temperature sensor 48, i.e., the coolant temperature at the outlet of each fuel cell stack 12, 14, correlates with the actual temperature of the fuel cell stack 12, 14. Here, the actual temperature of the fuel cell stack 12, 14 means the actual temperature of the fuel cell stack 12, 14.

[0029] The control device 36 determines a target cooling temperature for the fuel cell stacks 12, 14 to be instructed to the cooling unit 38. Then, based on the values ​​detected by the temperature sensors 46, 48, the control device 36 controls the pump 44 to adjust the flow rate of the coolant supplied to the fuel cell stacks 12, 14, thereby cooling the fuel cell stacks 12, 14 so that the temperatures of the fuel cell stacks 12, 14 reach the target cooling temperature. In one example, the coolant is water. Note that in other embodiments, the fuel cell system 10 may include a separate cooling unit 38 for each fuel cell stack 12, 14.

[0030] 1, the fuel cell system 10 further includes a first pressure sensor 56 and a second pressure sensor 58. The first pressure sensor 56 is provided in the fourth oxidizing gas supply path 24d and detects the pressure of the oxidizing gas (air) taken into the compressor 22. The second pressure sensor 58 is provided in the third oxidizing gas supply path 24c and detects the pressure of the oxidizing gas (air) discharged from the compressor 22. The values ​​detected by the pressure sensors 56, 58 are acquired by the control device 36. Based on the values ​​detected by the pressure sensors 56, 58, the control device 36 calculates and monitors the pressure ratio in the compressor 22.

[0031] 1, the fuel cell system 10 further includes a third temperature sensor 60 and a flow rate sensor 62. The third temperature sensor 60 is provided in the third oxidizing gas supply path 24c on the outlet side of the intercooler 50. Thus, the third temperature sensor 60 detects the temperature of the oxidizing gas cooled by the intercooler 50. The flow rate sensor 62 is provided in the fourth oxidizing gas supply path 24d and detects the flow rate of the oxidizing gas sucked by the compressor 22. The values ​​detected by these sensors 60, 62 are acquired by the control device 36.

[0032] The control device 36 can change the number of fuel cell stacks 12, 14 to be operated depending on the output required for the fuel cell system 10. For example, when the output required for the fuel cell system 10 is relatively small, the control device 36 stops operation of one of the two fuel cell stacks 12, 14. This makes it possible to avoid a decrease in power generation efficiency.

[0033] Once the number of fuel cell stacks 12, 14 to be operated is determined, the output required for each fuel cell stack 12, 14 is determined. For each fuel cell stack 12, 14, the supply pressure and supply flow rate of the oxidizing gas to be supplied to each fuel cell stack 12, 14 are determined according to the required output (i.e., the power to be generated) and the temperature of the fuel cell stack 12, 14. As shown in FIG. 2, for example, if the temperature of the fuel cell stacks 12, 14 is constant, the supply pressure and supply flow rate of the oxidizing gas must be increased as the output required for the fuel cell stacks 12, 14 increases. In particular, if the number of operating fuel cell stacks 12, 14 decreases, the required output for the fuel cell stack 12 (or 14) that continues to operate will rise sharply, and therefore the supply pressure and supply flow rate of the oxidizing gas must also be increased significantly.

[0034] In this regard, the fuel cell system 10 of this embodiment employs a turbo compressor 22 to supply oxidizing gas. Therefore, if the pressure ratio of the compressor 22 is increased in order to increase the supply pressure of the oxidizing gas to the fuel cell stacks 12, 14, surging of the compressor 22 may occur. For this reason, as shown in FIG. 3 , surging constraint lines SL1, SL2 are defined for the operating point of the compressor 22 (combination of the pressure ratio and the discharge flow rate) in the fuel cell system 10. In other words, if the operating point of the compressor 22 exceeds the surging constraint lines SL1, SL2, surging may occur in the compressor 22. In particular, if the output required of the fuel cell stack 12 (or 14) increases suddenly, such as when the number of operating fuel cell stacks 12, 14 is reduced, the operating point of the compressor 22 may exceed the surging constraint lines SL1, SL2.

[0035] To avoid surging of the compressor 22, it is necessary to maintain the operating point of the compressor 22 at or below the surging constraint line SL1 (or SL2). Here, as the surging constraint line SL1 (or SL2) indicates, the risk of surging occurring in the compressor 22 increases when the pressure ratio exceeds a certain upper limit. However, this upper limit is not constant; the higher the discharge flow rate of the compressor 22, the higher the upper limit becomes. Therefore, if the discharge flow rate of the oxidizing gas from the compressor 22 can be increased when the pressure ratio of the compressor 22 increases, surging of the compressor 22 can be avoided. Even if the discharge flow rate of the oxidizing gas from the compressor 22 is increased, an appropriate amount of oxidizing gas can be supplied to the fuel cell stacks 12, 14 by adjusting the ratio of the supply flow rate to the discharge flow rate.

[0036] Based on this knowledge, the control device 36 of this embodiment is configured to be able to repeatedly execute the first process shown in Fig. 4. By repeatedly executing the first process, the control device 36 can adjust the supply rate, which is the ratio of the supply flow rate supplied to the multiple fuel cell stacks 12, 14 to the discharge flow rate of the compressor 22.

[0037] As shown in FIG. 4, the control device 36 acquires an externally requested output power PR (S10). Based on the output power PR acquired in step S10, the control device 36 calculates a required operation number for the fuel cell stacks 12, 14 (S12). Furthermore, the control device 36 calculates a target current (i.e., a target power generation amount) for each fuel cell stack 12, 14 (S14). Based on the target current determined in S14, the control device 36 calculates a target supply pressure of the oxidizing gas to be commanded to the oxidizing gas supply unit 20 (S16) and calculates a target supply flow rate of the oxidizing gas to be commanded to the oxidizing gas supply unit 20 (S18). The control device 36 of this embodiment pre-stores a map describing the relationship between the target supply pressure and target supply flow rate of the oxidizing gas to be commanded to the oxidizing gas supply unit 20 (see FIG. 2). Therefore, the control device 36 calculates the target supply pressure and target supply flow rate of the oxidizing gas corresponding to the actual operation number based on the relationship described in the pre-stored map. Thereafter, the control device 36 controls each component according to the calculated values.

[0038] The control device 36 acquires the discharge flow rate of the compressor 22 (S20). As described above, the intake flow rate of the oxidizing gas taken into the compressor 22 is detected by the flow rate sensor 62. For example, if the pressure of the oxidizing gas in the third oxidizing gas supply path 24c increases, the discharge flow rate of the oxidizing gas by the compressor 22 decreases. On the other hand, if the pressure of the oxidizing gas in the third oxidizing gas supply path 24c decreases, the discharge flow rate of the oxidizing gas by the compressor 22 increases. The control device 36 can then estimate the discharge flow rate of the compressor 22 from the detection values ​​of the second pressure sensor 58 and the flow rate sensor 62. However, in another embodiment, the fuel cell system 10 may further include a flow rate sensor that detects the discharge flow rate of the compressor 22, and the control device 36 may acquire the detection value of the flow rate sensor.

[0039] The control device 36 acquires the pressure ratio CP in the compressor 22 (S22). As described above, the control device 36 calculates and monitors the pressure ratio in the compressor 22 based on the values ​​detected by the pressure sensors 56, 58.

[0040] Next, the control device 36 acquires the opening degree WR of the reflux valve 54 (S24). As described above, the opening degree WR of the reflux valve 54 is controlled by the control device 36, and therefore the control device 36 can acquire the opening degree WR of the reflux valve 54.

[0041] The control device 36 determines whether or not the number of operating fuel cell stacks 12, 14 is currently being reduced (S26). Here, the term "currently in a transition to reduce the number of operating fuel cell stacks 12, 14" refers to the period from when both fuel cell stacks 12, 14 are in operation to when only one of the fuel cell stacks 12, 14 is in operation (i.e., a standalone operation state). During this period, as shown in the range from time R1 to time R3 in Fig. 5, various parameters such as the discharge flow rate of the oxidizing gas from the compressor 22 fluctuate significantly, making it relatively easy for surging of the compressor 22 to occur.

[0042] If the answer is NO in step S26, the control device 36 sets the first pressure value A1 as the surging constraint pressure SP (S30). In step S30, the control device 36 first adopts the first surging constraint line SL1 shown in Fig. 3 as the surging constraint line SL. The control device 36 determines, as the first pressure value A1, the pressure value on the first surging constraint line SL1 that corresponds to the discharge flow rate of the compressor 22 acquired in S20.

[0043] On the other hand, if the answer to step S26 is YES, the control device 36 sets the second pressure value A2 as the surging constraint pressure SP (S28). In step S28, the control device 36 first adopts the second surging constraint line SL2 as the surging constraint line SL. The control device 36 determines the pressure value on the second surging constraint line SL2 that corresponds to the discharge flow rate of the compressor 22 acquired in S20 as the second pressure value A2. The second pressure value A2 is a value smaller than the first pressure value A1. As shown in FIG. 3, the second surging constraint line SL2 corresponds to the first surging constraint line SL1 shifted downward. Therefore, if the discharge flow rate of the compressor 22 is constant, when the second surging constraint line SL2 is adopted, the surging constraint pressure SP is set smaller than when the first surging constraint line SL1 is adopted. This more reliably avoids surging of the compressor 22.

[0044] The control device 36 determines whether the pressure ratio CP of the compressor 22 obtained in S22 exceeds the surging constraint pressure SP set in S28 or S30 (S32). If the answer is YES in step S32, the control device 36 sets the target opening WT of the reflux valve 54 to be larger than the opening WR of the reflux valve 54 obtained in S24 (S34). This increases the opening of the reflux valve 54, and the amount of oxidizing gas discharged from the compressor 22 that is sent from the third oxidizing gas supply path 24c to the fourth oxidizing gas supply path 24d increases. In other words, the supply rate decreases. If the answer is NO in step S32, the control device 36 sets the target opening WT of the reflux valve 54 to be equal to or smaller than the opening WR of the reflux valve 54 obtained in S24 (S36).

[0045] The control device 36 calculates a target opening to be commanded to the inlet valve 26 of the oxidizing gas supply unit 20 (S38), and also calculates a target opening to be commanded to the outlet valve 30 of the oxidizing gas supply unit 20 (S40). Next, the control device 36 calculates a target rotation speed to be commanded to the compressor 22 (S42). The control device 36 controls each part in accordance with the calculated values, thereby completing one cycle of the first process shown in FIG.

[0046] According to the above-described configuration, by reducing the number of fuel cell stacks 12, 14 that are operated (i.e., that generate power), when the pressure ratio in the compressor 22 increases, the discharge flow rate of the oxidizing gas in the compressor 22 can be preferentially increased. This makes it possible to avoid surging of the compressor 22. In particular, in the first process shown in FIG. 4, the oxidizing gas can be sent from the discharge side to the suction side via the reflux path 52, thereby preferentially increasing the discharge flow rate of the oxidizing gas in the compressor 22 (see FIG. 5). Note that the reflux path 52 and the reflux valve 54 in this specification are examples of an adjustment mechanism in the present technology. The first pressure value A1 and the second pressure value A2 in this specification are examples of predetermined thresholds for the pressure ratio in a turbo compressor in the present technology.

[0047] In this embodiment, the oxidant gas supply unit 20 has an intercooler 50 that cools the oxidant gas discharged from the compressor 22. A return path 52 connects the fourth oxidant gas supply path 24d, which is located on the suction side of the compressor 22, to the third oxidant gas supply path 24c, which is located on the discharge side of the compressor 22. In particular, the return path 52 is connected to the third oxidant gas supply path 24c upstream of the intercooler 50. With this configuration, the oxidant gas sent from the discharge side to the suction side does not pass through the intercooler 50, and therefore only the oxidant gas supplied to the fuel cell stacks 12, 14 can be cooled by the intercooler 50.

[0048] In the first process described above, the threshold value for the pressure ratio in the compressor 22 (here, the second pressure value A2) is set low during a transition to reduce the number of operating fuel cell stacks 12, 14. During a transition to reduce the number of operating fuel cell stacks 12, 14, the discharge flow rate of the oxidizing gas in the compressor 22, the pressure ratio in the compressor 22, and the like fluctuate significantly, making surging more likely to occur. Therefore, with the above-described configuration, surging in the compressor 22 can be more reliably avoided even during a transition to reduce the number of operating fuel cell stacks 12, 14. Note that in the first process shown in FIG. 4, the control device 36 may omit steps S26 to S30. That is, in another embodiment, the control device 36 does not need to set the threshold value for the pressure ratio in the compressor 22 low during a transition to reduce the number of operating fuel cell stacks 12, 14.

[0049] Based on the same findings, the control device 36 of this embodiment is configured to be able to repeatedly execute the second process shown in Fig. 6. In the second process, by using the branch path 32 and the branch valve 34 instead of the return path 52 and the return valve 54 of the first process shown in Fig. 4, it is possible to adjust the supply rate, which is the ratio of the supply flow rate supplied to the multiple fuel cell stacks 12, 14 to the discharge flow rate of the compressor 22. Steps S50 to S82 in the second process correspond to steps S10 to S42 in the first process. In the second process, a third pressure value A3 or a fourth pressure value A4 is set as the surging constraint pressure SP (S78, S80), and the fourth pressure value A4 is a value smaller than the third pressure value A3.

[0050] According to this configuration, by reducing the number of fuel cell stacks 12, 14 that are operated (i.e., that generate power), when the pressure ratio in the compressor 22 increases, the oxidizing gas can be discharged from the oxidizing gas supply path 24 to the off-gas discharge path 28 via the branch path 32. This allows the discharge flow rate of the oxidizing gas in the compressor 22 to be preferentially increased. Here, the branch path 32 and the branch valve 34 in this specification are examples of an adjustment mechanism in the present technology. The third pressure value A3 and the fourth pressure value A4 in this specification are examples of predetermined thresholds for the pressure ratio in a turbo compressor in the present technology.

[0051] Although not particularly limited, in the first process shown in FIG. 4, the control device 36 may preferentially increase the discharge flow rate of the oxidizing gas in the compressor 22 by using the diversion path 32 and the diversion valve 34 in addition to the reflux path 52 and the reflux valve 54.

[0052] (Embodiment 2) A fuel cell system 110 of embodiment 2 will be described with reference to Fig. 7. As shown in Fig. 7, in the fuel cell system 110 of embodiment 2, the position where the return path 152 is provided is changed compared to the fuel cell system 10 of embodiment 1. The other configurations are the same as those of the fuel cell system 10 of embodiment 1, so duplicated explanations will be omitted here.

[0053] 7, the reflux path 152 connects the fourth oxidizing gas supply path 24d located on the suction side of the compressor 22 to the third oxidizing gas supply path 24c located on the discharge side of the compressor 22. In particular, the reflux path 152 is connected to the third oxidizing gas supply path 24c downstream of the intercooler 50.

[0054] Even in the above-described configuration, the control device 36 can repeatedly execute the first process and / or the second process in the first embodiment. This makes it possible to avoid surging of the compressor 22. In particular, as described above, the return path 152 is connected downstream of the intercooler 50, so that the temperature of the oxidizing gas taken in by the compressor 22 is lowered, and the energy required to achieve a predetermined pressure ratio in the compressor 22 can be reduced.

[0055] (Example 3) A fuel cell system 210 of Example 3 will be described with reference to Figure 8. As shown in Figure 8, in the fuel cell system 210 of Example 3, unlike the fuel cell system 10 of Example 1, a plurality of fuel cell stacks 12, 14 are electrically connected in series. The remaining configuration is the same as that of the fuel cell system 10 of Example 1, so duplicated explanations will be omitted here.

[0056] As shown in FIG. 9, the fuel cell system 210 further includes a first relay 64 and a second relay 66. The first relay 64 is provided between one pole of the first fuel cell stack 12 and one pole of the power control unit 18, and the second relay 66 is provided between the other pole of the second fuel cell stack 14 and the other pole of the power control unit 18. As shown in FIG. 9, when the first relay 64 and the second relay 66 are both in a first state, the first fuel cell stack 12 is electrically connected in series with the second fuel cell stack 14. When the first relay 64 is in the first state and the second relay 66 is in the second state (dotted line position), the fuel cell system 10 supplies only the output from the first fuel cell stack 12 to the outside. When the second relay 66 is in the first state and the first relay 64 is in the second state (dotted line position), the fuel cell system 10 supplies only the output from the second fuel cell stack 14 to the outside.

[0057] Even in the above configuration, the control device 36 can repeatedly execute the first process and / or the second process in the first embodiment, thereby making it possible to avoid surging of the compressor 22.

[0058] Furthermore, the position where the return path 52 is provided can be changed in the fuel cell system 210 of Example 3. That is, as another embodiment, a fuel cell system in which a plurality of fuel cell stacks 12, 14 are electrically connected in series may be employed, as compared to the fuel cell system 110 of Example 2. Even in this configuration, the control device 36 can repeatedly execute the first process and / or the second process in Example 1, and surging of the compressor 22 can be avoided.

[0059] Although several specific examples have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above. The technical elements described in this specification or drawings exhibit technical utility either alone or in combination. [Explanation of symbols]

[0060] 10: fuel cell system, 12, 14: fuel cell stack, 16a: cathode side supply port, 16b: cathode side discharge port, 18: power control unit, 20: oxidizing gas supply unit, 22: compressor, 24: oxidizing gas supply path, 26: inlet valve, 28: off-gas discharge path, 30: outlet valve, 32: flow dividing path, 34: flow dividing valve, 36: control device, 38: cooling unit, 40: radiator, 42: circulation path, 44: pump, 46, 48, 60: temperature sensor, 50: intercooler, 52: return path, 54: return valve, 56, 58: pressure sensor, 62: flow rate sensor, 64: first relay, 66: second relay

Claims

1. a plurality of fuel cell stacks; an oxidizing gas supply unit having a turbo compressor that supplies oxidizing gas to each of the plurality of fuel cell stacks; a control device that determines the number of required operations for the plurality of fuel cell stacks and the target supply pressure and target supply flow rate of the oxidizing gas to be instructed to the oxidizing gas supply unit according to the required output; Equipped with the oxidizing gas supply unit has an adjustment mechanism that adjusts a supply rate, which is a ratio of a supply flow rate supplied to the plurality of fuel cell stacks to a discharge flow rate of the turbo compressor; The control device monitors a pressure ratio in the turbo compressor, and controls the regulating mechanism to reduce the supply rate when the pressure ratio exceeds a predetermined threshold.

2. The adjustment mechanism includes: a return path connecting the discharge side and the suction side of the turbo compressor to each other; A flow rate adjustment valve provided in the reflux path. The fuel cell system according to claim 1 .

3. the oxidizing gas supply unit further includes an intercooler that cools the oxidizing gas discharged from the turbo compressor, 3. The fuel cell system according to claim 2, wherein the return path connects the discharge side and the suction side of the turbo compressor to each other upstream of the intercooler.

4. the oxidizing gas supply unit further includes an intercooler that cools the oxidizing gas discharged from the turbo compressor, 3. The fuel cell system according to claim 2, wherein the return path connects the discharge side and the suction side of the turbo compressor to each other downstream of the intercooler.

5. The oxidizing gas supply unit includes: an oxidizing gas supply path that supplies the oxidizing gas from the turbo compressor to the plurality of fuel cell stacks; an off-gas exhaust path for exhausting the oxidizing gas from the plurality of fuel cell stacks; 2. The fuel cell system according to claim 1, wherein the adjustment mechanism comprises: a branch path connecting the oxidizing gas supply path and the off-gas discharge path; and a flow rate adjustment valve provided in the branch path.

Images