fuel cell system

Abstract

This invention relates to a fuel cell system comprising a first flow path and a second flow path branching from a common flow path, a first compressor that supplies gas from the first flow path to a first fuel cell, a second compressor that supplies gas from the second flow path to a second fuel cell, and a control device for controlling the first and second compressors. When the power generation demand of the first fuel cell is higher than the power generation demand of the second fuel cell, the control device performs an unbalanced operation, causing the first compressor to operate and the second compressor to operate at a speed lower than the speed of the first compressor but higher than a lower limit speed. In this unbalanced operation, the higher the gas flow rate in the first flow path, the higher the lower limit speed is set by the control device.

Description

Technical Field

[0001] The technology disclosed in this specification relates to fuel cell systems. Background Technology

[0002] The fuel cell system disclosed in Japanese Patent Application Publication No. 2020-057460 has two fuel cells, two compressors, and a gas supply line. Multiple valves are installed in the gas supply line. These valves allow for modification of the gas flow path within the gas supply line.

[0003] In the fuel cell system described in Japanese Patent Application Publication No. 2020-057460, the gas flow path is altered by installing multiple valves in the gas supply path. However, installing valves in the gas supply path increases the pressure loss. This specification proposes a technique for appropriately altering the gas flow path without using valves. Summary of the Invention

[0004] The fuel cell system disclosed in this specification includes: a common flow path for gas flow; a first flow path and a second flow path branching from the common flow path; a first fuel cell connected to the first flow path; a first compressor disposed in the first flow path and supplying gas from the first flow path to the first fuel cell; a second fuel cell connected to the second flow path; a second compressor disposed in the second flow path and supplying gas from the second flow path to the second fuel cell; and a control device for controlling the first and second compressors. When the power generation demand of the first fuel cell is higher than the power generation demand of the second fuel cell, the control device performs an unbalanced operation, causing the first compressor to operate and the second compressor to operate at a speed lower than the speed of the first compressor but higher than a lower limit speed. In the unbalanced operation, the higher the gas flow rate in the first flow path, the higher the lower limit speed is set by the control device.

[0005] In this fuel cell system, power generation can be achieved in the first fuel cell by activating the first compressor to flow gas into the first flow path. Additionally, power generation can be achieved in the second fuel cell by activating the second compressor to flow gas into the second flow path. Furthermore, power generation can be achieved in both the first and second fuel cells by activating both the first and second compressors. Thus, according to this fuel cell system, the flow path for gas supply can be changed by changing the operating compressor. Furthermore, when the power generation demand of the first fuel cell is higher than that of the second fuel cell, the control device performs an unbalanced operation, activating the first compressor and operating the second compressor at a lower speed than the first compressor. During this unbalanced operation, the speed of the first compressor (i.e., the gas flow rate in the first flow path) varies according to the power generation demand of the first fuel cell. If the gas flow rate in the first flow path becomes higher during the unbalanced operation, there is a concern that the pressure upstream of the second flow path may decrease, leading to gas backflow in the second flow path. Therefore, during unbalanced operation, the higher the gas flow rate in the first flow path, the higher the lower limit speed of the second compressor set by the control device. Thus, even when the gas flow rate in the first flow path increases during unbalanced operation, backflow of gas in the second flow path can be prevented. In this way, according to this fuel cell system, the gas flow path can be changed without using valves, and backflow of gas in the second flow path can be appropriately prevented during unbalanced operation.

[0006] The features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, in which the same reference numerals denote the same elements. Attached Figure Description

[0007] Figure 1 This is a block diagram of the fuel cell system according to the implementation method.

[0008] Figure 2 This is a 3D view of the part of the fuel cell system upstream of the compressor.

[0009] Figure 3 This is a graph showing the simulation results of the airflow in branch path 34b without implementing backflow prevention control.

[0010] Figure 4 This is a block diagram showing the backflow prevention control for compressor 46b.

[0011] Figure 5 It is a diagram showing a mapping table for calculating the lower limit speed based on airflow, atmospheric pressure, and air temperature.

[0012] Figure 6 This is a graph showing the simulation results of the airflow in branch flow path 34b under the condition of implementing backflow prevention control.

[0013] Figure 7 This is a block diagram showing the backflow prevention control for compressor 46a. Detailed Implementation

[0014] Alternatively, the fuel cell system disclosed in this specification may also be configured such that it further includes an atmospheric pressure sensor for detecting atmospheric pressure. Alternatively, the upstream end of the aforementioned common flow path may be connected to the atmosphere. Alternatively, the aforementioned gas may be air. Alternatively, in the aforementioned unbalanced operation, the aforementioned control device may change the aforementioned lower speed limit based on the atmospheric pressure detected by the aforementioned atmospheric pressure sensor.

[0015] When the upstream end of the shared flow path is connected to the atmosphere, the pressure in the upstream section of the second flow path during unbalanced operation is affected by atmospheric pressure. Therefore, the rotational speed of the second compressor, which can prevent backflow of gas in the second flow path, varies according to atmospheric pressure. According to this fuel cell system, the lower limit speed of the second compressor is changed according to the atmospheric pressure detected by an atmospheric pressure sensor, thus more appropriately preventing backflow of gas in the second flow path.

[0016] It can also be configured such that, based on the fuel cell system of one example disclosed in this specification, during the aforementioned unbalanced operation, the lower the atmospheric pressure detected by the atmospheric pressure sensor, the higher the lower limit speed is set by the control device.

[0017] According to this structure, the lower limit speed can be appropriately changed based on the atmospheric pressure detected by the atmospheric pressure sensor.

[0018] Alternatively, the fuel cell system disclosed in this specification may also be configured such that it further includes a temperature sensor for detecting the temperature of the gas within the fuel cell system. Alternatively, during the aforementioned unbalanced operation, the control device may adjust the lower speed limit based on the temperature detected by the temperature sensor.

[0019] Since the density of a gas varies with its temperature, the pressure upstream of the second flow path during unbalanced operation is affected by the gas temperature. Therefore, the rotational speed of the second compressor, which prevents backflow of gas in the second flow path, varies according to the gas temperature. According to this fuel cell system, the lower limit speed of the second compressor is changed based on the gas temperature detected by a temperature sensor, thus more appropriately preventing backflow of gas in the second flow path.

[0020] It can also be configured such that, based on the fuel cell system of one example disclosed in this specification, during the aforementioned unbalanced operation, the higher the temperature detected by the aforementioned temperature sensor, the higher the aforementioned lower limit speed is caused by the aforementioned control device.

[0021] According to this structure, the lower limit speed can be appropriately changed based on the temperature detected by the temperature sensor.

[0022] It can also be configured such that, based on the fuel cell system of an example disclosed in this specification, the lower limit speed is the speed at which the gas does not flow backward in the second flow path.

[0023] Figure 1 The fuel cell system 10 of the illustrated embodiment is mounted in a vehicle. The fuel cell system 10 has two fuel cells 20a and 20b, which generate electricity respectively. The electricity generated by the fuel cells 20a and 20b is supplied to the vehicle's motor. The fuel cell system 10 has an air supply device 30 that supplies air to the fuel cells 20a and 20b respectively. Additionally, although not shown, the fuel cell system 10 has a hydrogen supply device that supplies hydrogen to the fuel cells 20a and 20b respectively. The fuel cells 20a and 20b generate electricity by reacting hydrogen with the supplied air.

[0024] like Figure 1 , 2 As shown, the air supply device 30 has a common flow path 32, a branch flow path 34a, and a branch flow path 34b. An air intake 33 is provided at the upstream end of the common flow path 32. The common flow path 32 communicates with the atmosphere at the air intake 33. Branch flow paths 34a and 34b branch off from the common flow path 32. The upstream ends of branch flow paths 34a and 34b are connected to the downstream end of the common flow path 32. A fuel cell 20a is connected to the downstream end of branch flow path 34a. An exhaust flow path 36a is connected to the fuel cell 20a. Air flowing from the common flow path 32 into the branch flow path 34a is supplied to the fuel cell 20a. Air passing through the fuel cell 20a is discharged to the outside of the fuel cell system 10 via the exhaust flow path 36a. A fuel cell 20b is connected to the downstream end of branch flow path 34b. An exhaust flow path 36b is connected to the fuel cell 20b. Air flowing from the common flow path 32 into the branch flow path 34b is supplied to the fuel cell 20b. The air that has passed through fuel cell 20b is discharged to the outside of fuel cell system 10 via exhaust flow path 36b.

[0025] An air purifier 40 is provided in the common flow path 32. The air purifier 40 removes foreign objects from the air flowing in the common flow path 32.

[0026] A flow sensor 42a, a temperature sensor 44a, and a compressor 46a are provided in the branch flow path 34a. The flow sensor 42a detects the flow rate Fa of the air flowing in the branch flow path 34a. The temperature sensor 44a detects the temperature Ta of the air flowing in the branch flow path 34a. The compressor 46a is located in the branch flow path 34a downstream of the flow sensor 42a and the temperature sensor 44a. The compressor 46a pressurizes the air in the branch flow path 34a and delivers it downstream. When the compressor 46a operates, air flows from the air intake 33 into the common flow path 32, and the air flowing into the common flow path 32 flows through the branch flow path 34a and the fuel cell 20a to the discharge flow path 36a.

[0027] A flow sensor 42b, a temperature sensor 44b, and a compressor 46b are provided in the branch flow path 34b. The flow sensor 42b detects the flow rate Fb of the air flowing in the branch flow path 34b. The temperature sensor 44b detects the temperature Tb of the air flowing in the branch flow path 34b. The compressor 46b is located downstream of the flow sensor 42b and the temperature sensor 44b in the branch flow path 34b. The compressor 46b pressurizes the air in the branch flow path 34b and delivers it downstream. When the compressor 46b is activated, air flows from the air intake 33 into the common flow path 32, and the air flowing into the common flow path 32 flows through the branch flow path 34b and the fuel cell 20b to the discharge flow path 36b.

[0028] The fuel cell system 10 has an atmospheric pressure sensor 52. The atmospheric pressure sensor 52 detects the atmospheric pressure P outside the vehicle.

[0029] The fuel cell system 10 has a control unit 50. The control unit 50 is electrically connected to flow sensors 42a and 42b, temperature sensors 44a and 44b, compressors 46a and 46b, and atmospheric pressure sensor 52. The control unit 50 controls compressors 46a and 46b.

[0030] The power generation requirements for fuel cell 20a and fuel cell 20b are input to the control device 50 via an arithmetic circuit (not shown). For the control device 50, a higher power generation requirement for fuel cell 20a results in a higher rotational speed for compressor 46a, and vice versa. If the rotational speed of compressor 46a is higher than that of compressor 46b, air flows into branch path 34a at a higher flow rate than branch path 34b. If the rotational speed of compressor 46b is higher than that of compressor 46a, air flows into branch path 34b at a higher flow rate than branch path 34a. If the rotational speeds of compressor 46a and 46b are equal, air flows almost equally into branch paths 34a and 34b. Thus, by controlling the rotational speeds of compressors 46a and 46b, the control device 50 can change the airflow path.

[0031] The control device 50 performs backflow prevention control on the compressors 46a and 46b to prevent air backflow within the fuel cell system 10, as detailed below. In other words, without backflow prevention control, air backflow within the fuel cell system 10 can occur. The following describes the air backflow that occurs without backflow prevention control. When the power generation requirement for fuel cell 20a is high and the power generation requirement for fuel cell 20b is low, the control device 50 operates compressor 46a at a higher speed and compressor 46b at a lower speed. In this case, because compressor 46a operates at a higher speed, the airflow from the common flow path 32 to the branch flow path 34a becomes higher. As the air flows in this way, the air pressure in the branch flow path 34b decreases near the upstream end of the branch flow path 34b. As described above, the speed of compressor 46a varies according to the power generation requirement of fuel cell 20a. When the compressor 46a speed increases due to the increased demand for power generation from the fuel cell 20a, the airflow in the branch flow path 34a increases, resulting in very low air pressure near the upstream end of the branch flow path 34b. If very low air pressure is generated near the upstream end of the branch flow path 34b when the compressor 46b speed is low, the air in the branch flow path 34b will flow backward.

[0032] For example, Figure 3 The results of simulating the airflow in branch path 34b without implementing backflow prevention control are shown. Figure 3During period A1, the vehicle's accelerator opening decreases, and the vehicle speed decreases. Therefore, although not illustrated, the power generation requirement of fuel cell 20b decreases during period A1. Furthermore, although not illustrated, the power generation requirement of fuel cell 20a is maintained at a high value during period A1, and the airflow in branch path 34a is maintained at a high value. During period A1, due to the reduced power generation requirement of fuel cell 20b, control device 50 reduces the speed of compressor 46b. For example, in the initial stage of period A1, control device 50 reduces the speed of compressor 46b to 748 rpm. As a result, in the initial stage of period A1, the airflow in branch path 34b decreases to a negative value (i.e., -202 NL / min). That is, air backflow occurs in branch path 34b. Furthermore, in... Figure 3 During period A1, the minimum speed of compressor 46b is 748 rpm. However, when the speed of compressor 46b is further reduced during period A1, air backflow is more likely to occur within branch flow path 34b. For example, when the speed of compressor 46b is reduced to zero during period A1 (i.e., compressor 46b is stopped), compared to... Figure 3 It is easier to create air backflow.

[0033] If air flows backward in branch flow path 34b, fuel cell 20b will not operate properly. Furthermore, if air flows backward in branch flow path 34b, foreign matter present in fuel cell 20b and exhaust flow path 36b may flow into fuel cell 20a via branch flow path 34b and branch flow path 34a, raising concerns about potential adverse effects. Additionally, when the power generation requirement of fuel cell 20b is high and the power generation requirement of fuel cell 20a is low (i.e., when compressor 46b operates at a higher speed and compressor 46a operates at a lower speed), air backward inflow may occur in branch flow path 34a.

[0034] Next, we will explain the backflow prevention and control measures to prevent air backflow. Figure 4 This is a block diagram illustrating the process by which the control device 50 controls the compressor 46b. During the operation of the fuel cell system 10, the control device 50 repeatedly executes... Figure 4The control device 50 calculates the target speed Rbt of the compressor 46b in control block Bb1 based on factors such as the power generation requirement of the fuel cell 20b. Here, the higher the power generation requirement of the fuel cell 20b, the higher the target speed Rbt calculated by the control device 50. Additionally, the control device 50 calculates the lower limit speed Rbmin of the compressor 46b in control block Bb2. Details about control block Bb2 will be described later. Furthermore, the control device 50 obtains the upper limit speed Rbmax of the compressor 46b. The control device 50 can store the upper limit speed Rbmax as a fixed value, or it can calculate it based on the state of the fuel cell system 10. In control block Bb3, the control device 50 calculates the command speed Rbi. When the target speed Rbt is lower than the upper limit speed Rbmax but higher than the lower limit speed Rbmin, the control device 50 calculates the target speed Rbt as the command speed Rbi. Furthermore, when the target speed Rbt is higher than the upper limit speed Rbmax, the control device 50 calculates the upper limit speed Rbmax as the command speed Rbi. Furthermore, when the target speed Rbt is below the lower limit speed Rbmin, the control device 50 calculates the lower limit speed Rbmin as the command speed Rbi. The control device 50 controls the compressor 46b so that its speed matches the command speed Rbi. In this way, even when the power generation requirement of the fuel cell 20b is low, the control device 50 keeps the compressor 46b operating at a speed above the lower limit speed Rbmin.

[0035] When the power generation requirement of fuel cell 20a is higher than that of fuel cell 20b, control device 50 performs an unbalanced operation, causing compressor 46b to operate at a lower speed than compressor 46a. During this unbalanced operation, control device 50 also adjusts according to... Figure 4 The compressor is operated at a speed above the limit speed Rbmin of the compressor 46b. For example, even if the power generation requirement of the fuel cell 20b becomes zero during unbalanced operation, the control device 50 will still operate the compressor at a speed above the limit speed Rbmin of the compressor 46b.

[0036] like Figure 4 As shown, the control device 50 calculates the lower limit speed Rbmin of the compressor 46b based on the air flow rate Fa, atmospheric pressure P, and air temperature T. Furthermore, the air flow rate Fa is the air flow rate of the branch flow path 34a detected by the flow sensor 42a. Additionally, the atmospheric pressure P is the atmospheric pressure outside the vehicle detected by the atmospheric pressure sensor 52. Furthermore, the air temperature T is the air temperature Ta detected by the temperature sensor 44a or the air temperature Tb detected by the temperature sensor 44b. The control device 50 stores... Figure 5The control device 50 uses the mapping table shown to calculate the lower limit speed Rbmin. For example, when the air flow rate Fa is 2000 NL / min, the air temperature T is 20°C, and the atmospheric pressure P is 90 kPa, the control device 50 calculates 921 rpm as the lower limit speed Rbmin. Alternatively, for example, when the air flow rate Fa is 4000 NL / min, the air temperature T is 50°C, and the atmospheric pressure P is 80 kPa, the control device 50 calculates 2188 rpm as the lower limit speed Rbmin.

[0037] like Figure 5 As shown, the higher the airflow rate Fa, the higher the lower limit speed Rbmin set by the control device 50. Additionally, the lower the atmospheric pressure P, the higher the lower limit speed Rbmin set by the control device 50. Furthermore, the higher the air temperature T, the higher the lower limit speed Rbmin set by the control device 50.

[0038] As mentioned above, when the compressor 46a operates at a higher speed and the compressor 46b operates at a lower speed, a lower air pressure is generated near the upstream end of the branch flow path 34b. Furthermore, as mentioned above, the higher the airflow rate Fa of the branch flow path 34a, the lower the resulting lower air pressure near the upstream end of the branch flow path 34b. Conversely, as... Figure 5 As shown, the higher the airflow Fa of branch flow path 34a, the higher the lower limit speed Rbmin of compressor 46b set by control device 50. According to this structure, the higher the airflow Fa of branch flow path 34a (i.e., the lower the air pressure near the upstream end of branch flow path 34b), the higher the lower limit speed Rbmin of compressor 46b, and the higher the lower limit of compressor 46b's ability to deliver air downstream. Therefore, even if the air pressure near the upstream end of branch flow path 34b decreases due to the increase in airflow Fa of branch flow path 34a during unbalanced operation, backflow of air in branch flow path 34b can be prevented.

[0039] Furthermore, as mentioned above, the common flow path 32 is connected to the atmosphere at the air intake 33. Therefore, the air pressure near the upstream end of the branch flow path 34b during unbalanced operation is affected by atmospheric pressure. That is, the lower the atmospheric pressure, the easier it is for the air pressure near the upstream end of the branch flow path 34b to decrease. Therefore, when the compressor 46a operates at a higher speed and the compressor 46b operates at a lower speed, the lower the atmospheric pressure, the easier it is for the air pressure near the upstream end of the branch flow path 34b to decrease. Conversely, as... Figure 5As shown, the lower the atmospheric pressure P, the higher the lower limit speed Rbmin of the compressor 46b set by the control device 50. According to this structure, the lower the atmospheric pressure P (i.e., the lower the air pressure near the upstream end of the branch flow path 34b), the higher the lower limit speed Rbmin of the compressor 46b, and the higher the lower limit of the compressor 46b's ability to deliver air downstream. Therefore, even when performing unbalanced operation at low atmospheric pressure P, backflow of air in the branch flow path 34b can be prevented.

[0040] Furthermore, air density varies with air temperature; therefore, the air pressure near the upstream end of branch flow path 34b during unbalanced operation is affected by the air temperature T within the fuel cell system 10. Simulations show that the higher the air temperature T, the easier it is for the air pressure near the upstream end of branch flow path 34b to decrease. Conversely, as... Figure 5 As shown, the higher the air temperature T, the higher the lower limit speed Rbmin of the compressor 46b set by the control device 50. According to this structure, the higher the air temperature T (i.e., the lower the air pressure near the upstream end of the branch flow path 34b), the higher the lower limit speed Rbmin of the compressor 46b, and the higher the lower limit of the compressor 46b's ability to deliver air downstream. Therefore, even when performing unbalanced operation at a high air temperature T, backflow of air in the branch flow path 34b can be prevented.

[0041] Figure 6 Indicates in relation to Figure 3 The change in airflow in branch path 34b under the same conditions where backflow prevention control was implemented. Figure 6 During this period, the lowest speed of compressor 46b in A1 was 1191 rpm, which was higher than... Figure 3 The lowest value in the range (i.e., 748 rpm). The result is that, at... Figure 6 During period A1, the minimum airflow rate in branch flow path 34b is 49.1 NL / min, which is a positive value. This means that during period A1, no backflow of air occurs in branch flow path 34b. Thus, according to the backflow prevention control, backflow of air in branch flow path 34b can be prevented.

[0042] in addition, Figure 7This is a block diagram illustrating the process by which the control device 50 controls the compressor 46a. The compressor 46a is controlled in roughly the same manner as the compressor 46b. Specifically, in control block Ba1, the control device 50 calculates the target rotational speed Rat of the compressor 46a based on factors such as the power generation requirement of the fuel cell 20a. Here, the higher the power generation requirement of the fuel cell 20a, the higher the target rotational speed Rat calculated by the control device 50. Additionally, in control block Ba2, the control device 50 calculates the lower limit rotational speed Ramin of the compressor 46a. The control device 50 calculates the lower limit rotational speed Ramin of the compressor 46a based on the airflow rate Fb, atmospheric pressure P, and air temperature T. Furthermore, the airflow rate Fb is the airflow rate of the branch flow path 34b detected by the flow sensor 42b. The control device 50 calculates the lower limit rotational speed Ramin based on factors such as the airflow rate Fb, atmospheric pressure P, and air temperature T. Figure 5 The same mapping table is used to calculate the lower speed limit Ramin. Here, for control device 50, a higher airflow rate Fb results in a higher lower speed limit Ramin, a lower atmospheric pressure P results in a higher lower speed limit Ramin, and a higher air temperature T results in a higher lower speed limit Ramin. Additionally, as... Figure 7 As shown, the control device 50 obtains the upper limit speed Ramax of the compressor 46a. The control device 50 can store the upper limit speed Ramax as a fixed value, or it can calculate it based on the state of the fuel cell system 10. In control block Ba3, the control device 50 calculates the command speed Rai. When the target speed Rat is lower than the upper limit speed Ramax but higher than the lower limit speed Ramin, the control device 50 calculates the target speed Rat as the command speed Rai. Furthermore, when the target speed Rat is higher than the upper limit speed Ramax, the control device 50 calculates the upper limit speed Ramax as the command speed Rai. Additionally, when the target speed Rat is lower than the lower limit speed Ramin, the control device 50 calculates the lower limit speed Ramin as the command speed Rai. The control device 50 controls the compressor 46a so that its speed matches the command speed Rai. Because the control device 50 controls the compressor 46a in this way, when an unbalanced operation occurs where the airflow in branch flow path 34b is higher than the airflow in branch flow path 34a, backflow of air in branch flow path 34a can be prevented.

[0043] As explained above, the fuel cell system 10 according to the embodiment can prevent air backflow and change the air flow path by controlling the two compressors 46a and 46b. That is, air backflow can be prevented and the air flow path can be changed without using valves. According to this structure, the number of valves provided in the common flow path 32 and the branch flow paths 34a and 34b can be reduced, thus reducing the pressure loss generated in the air flow path. Therefore, the fuel cell system 10 can generate electricity with high efficiency.

[0044] Furthermore, in the above embodiments, a fuel cell system 10 having two branch flow paths 34a and 34b has been described. However, the technology disclosed in this specification can also be applied to fuel cell systems having three or more branch flow paths. In this case, compressors can be installed in each branch flow path. That is, the fuel cell system can also have three or more compressors. In this case, backflow prevention control can be implemented in each compressor. In this case, the lower limit speed of the compressor installed in one branch flow path can also be changed according to the air flow rate of one or more of the other branch flow paths.

[0045] Furthermore, the fuel cell system described above is mounted in a vehicle. However, the technology disclosed in this specification can also be applied to other fuel cell systems (e.g., stationary fuel cell systems).

[0046] Furthermore, in the above embodiments, an air supply device for supplying air to a fuel cell has been described. However, the technology disclosed in this specification can also be applied to gas supply devices that supply other gases to a fuel cell (e.g., an oxidizing gas supply device that supplies an oxidizing gas other than air to a fuel cell, a hydrogen supply device that supplies hydrogen to a fuel cell, etc.).

[0047] The embodiments have been described in detail above, but these are merely illustrative and do not limit the scope of the claims. The technology described in the claims includes technologies modified or altered from the specific examples described above. The technical elements described 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 technology illustrated in this specification or drawings achieves multiple objectives simultaneously, and achieving one of these objectives is itself technically useful.

Claims

1. A fuel cell system, wherein, The fuel cell system has the following features: Shared flow path for gas flow; The first flow path and the second flow path branch from the shared flow path; The first fuel cell is connected to the first flow path; A first compressor is disposed in the first flow path and delivers the gas from the first flow path to the first fuel cell; The second fuel cell is connected to the second flow path; A second compressor is disposed in the second flow path and delivers the gas from the second flow path to the second fuel cell; as well as The control device controls the first compressor and the second compressor. When the power generation requirement of the first fuel cell is higher than that of the second fuel cell, the control device performs an unbalanced operation, causing the first compressor to operate and the second compressor to operate at a speed lower than that of the first compressor but higher than the lower limit speed. In the aforementioned unbalanced operation, the higher the gas flow rate in the first flow path, the higher the lower limit rotational speed set by the control device. The lower limit speed is the speed at which the gas does not flow backward in the second flow path.

2. The fuel cell system according to claim 1, wherein, It also has an atmospheric pressure sensor to detect atmospheric pressure. The upstream end of the shared flow path is connected to the atmosphere. The gas is air. During the unbalanced operation, the control device changes the lower limit speed based on the atmospheric pressure detected by the atmospheric pressure sensor.

3. The fuel cell system according to claim 2, wherein, In the unbalanced operation, the lower the atmospheric pressure detected by the atmospheric pressure sensor, the higher the lower limit speed is set by the control device.

4. The fuel cell system according to any one of claims 1 to 3, wherein, It also includes a temperature sensor that detects the temperature of the gas within the fuel cell system. During the unbalanced operation, the control device changes the lower limit speed based on the temperature detected by the temperature sensor.

5. The fuel cell system according to claim 4, wherein, In the unbalanced operation, the higher the temperature detected by the temperature sensor, the higher the lower limit speed is set by the control device.

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