Fuel cell system and operation method of fuel cell system

Abstract

To inhibit performance of a fuel cell stack from being deteriorated by shortage of fuel gas.SOLUTION: A fuel cell system comprises a plurality of fuel cell stacks including a master stack and a slave stack, a recycle line, a recycle blower for supplying fuel gas discharged from the slave stack to the master stack through the recycle line, and a controller. The controller controls the recycle blower to make a flow rate of fuel gas flowing through the recycle line at the time of an output rise of the fuel cell system be larger than a flow rate of fuel gas flowing through the recycle line at the time of rating operation of the fuel cell system so as to make a fuel gas utilization rate of the fuel cell stacks at the time of the output rise be smaller than a fuel gas utilization rate of the fuel cell stacks at the time of the rating operation.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] FIELD OF THE PRESENTLY PREFERRED EMBODIMENTS The present invention relates to a fuel cell system and a method for operating a fuel cell system. [Background technology]

[0002] A fuel cell is a power generating device that generates electricity when fuel gas such as hydrogen and oxidant gas such as air are supplied. In particular, a fixed polymer fuel cell that uses a proton-conductive ion exchange membrane as an electrolyte for the fuel cell that constitutes the fuel cell is known. A fixed polymer fuel cell has the advantage of a short start-up time because it operates at 60°C to 80°C. Taking advantage of this advantage, development is being promoted for a wide range of applications, such as stationary use, general household use, automobile use, mobile phone base station use, railway use, and marine use.

[0003] The fuel cell system includes a fuel cell stack in which multiple fuel cells are stacked, a fuel gas blower that supplies fuel gas to the fuel cell stack, an oxidant gas blower that supplies oxidant gas to the fuel cell stack, a cooling water pump that supplies cooling water to the fuel cell stack, and a control device. Electricity generated by the fuel cell stack is used to power auxiliary equipment other than the fuel cell stack.

[0004] In a fuel cell stack, more fuel gas than the actual consumption is supplied to the fuel cell stack so that the fuel gas is distributed throughout the fuel cell stack. During fuel cell stack operation, the fuel gas supply rate is increased to lower the fuel gas utilization rate (= fuel gas consumption / fuel gas supply rate) in the fuel cell stack.

[0005] Also, a fuel cell system is known that includes multiple fuel cell stacks, each including a primary stack arranged upstream in the direction of fuel gas flow and a secondary stack arranged downstream. This fuel cell system is configured to supply fuel gas to the upstream primary stack, and then supply the fuel gas discharged from the primary stack to the downstream secondary stack. This configuration makes it possible to increase the fuel gas utilization rate of the entire fuel cell system while maintaining a low fuel gas utilization rate of each fuel cell stack.

[0006] In such a fuel cell system, if the surplus fuel gas not consumed in the secondary stack is discharged to the outside, the increased fuel gas consumption will increase the running costs, and discharging the fuel gas into the atmosphere is undesirable from a safety standpoint. Therefore, the fuel cell system can be configured in a recycle configuration in which the fuel gas discharged from the secondary stack is supplied again to the primary stack via a recycle line. A recycle blower is arranged in the recycle line to supply the fuel gas discharged from the secondary stack to the primary stack via the recycle line.

[0007] In a fuel cell system, when the fuel gas is insufficient in the fuel cell, a reaction occurs in which the carbon material of the anode is oxidized to supply protons. This reaction causes the carbon of the catalyst support to be oxidized and disappear as gaseous carbon dioxide, thereby reducing the function of the catalyst layer. In particular, when the fuel cell system is started up, the fuel gas is consumed throughout the fuel cell stack, so if there are any locations where the fuel gas supply is insufficient, the function of the catalyst layer may be reduced. Furthermore, when the load of the fuel cell system increases, the consumption of fuel gas in the fuel cell stack increases, so the function of the catalyst layer may be reduced in a similar manner.

[0008] In particular, in a fuel cell system having multiple fuel cell stacks including the above-mentioned primary and secondary stacks, the length of the fuel gas piping connecting each fuel cell stack is long, which makes it time-consuming to supply fuel gas and increases the possibility of fuel gas shortage in the fuel cells. Therefore, in the fuel cell system, there is a risk of the performance of the fuel cell stack deteriorating due to a shortage of fuel gas. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] JP 2007-128889 A [Patent Document 2] Special Publication No. 2007-536704 [Patent Document 3] Special Publication No. 2007-517370 Summary of the Invention [Problem to be solved by the invention]

[0010] The present invention has been made in consideration of these points, and aims to provide a fuel cell system and an operating method for a fuel cell system that can suppress performance degradation of the fuel cell stack due to a shortage of fuel gas. [Means for solving the problem]

[0011] A fuel cell system according to an embodiment includes a plurality of fuel cell stacks which generate electricity by being supplied with a fuel gas and an oxidant gas, the plurality of fuel cell stacks including a main stack and a secondary stack to which the fuel gas discharged from the main stack is supplied, a recycle line through which the fuel gas discharged from the secondary stack flows upstream of the main stack, a recycle blower disposed in the recycle line and which supplies the fuel gas discharged from the secondary stack to the main stack via the recycle line, and a control device which controls the recycle blower. The control device controls the recycle blower to make the flow rate of the fuel gas flowing through the recycle line greater during output increase than the flow rate of the fuel gas flowing through the recycle line during rated operation, so that the fuel gas utilization rate in the fuel cell stack during output increase of the fuel cell system is smaller than the fuel gas utilization rate in the fuel cell stack during rated operation of the fuel cell system.

[0012] Also, a method of operating a fuel cell system according to an embodiment is a method of operating a fuel cell system having a plurality of fuel cell stacks, the plurality of fuel cell stacks generating electricity by being supplied with fuel gas and oxidant gas, including a main stack and a secondary stack to which fuel gas discharged from the main stack is supplied. The method of operating the fuel cell system includes a supply step of supplying fuel gas to the main stack and the secondary stack, and a recycle step of supplying fuel gas discharged from the secondary stack to the main stack via a recycle line. In the recycle step, the method of operating the fuel cell system increases the flow rate of fuel gas flowing through the recycle line during output increase compared to the flow rate of fuel gas flowing through the recycle line during rated operation, so that the fuel gas utilization rate in the fuel cell stack during output increase of the fuel cell system is reduced compared to the fuel gas utilization rate in the fuel cell stack during rated operation of the fuel cell system. Effect of the Invention

[0013] According to the present invention, it is possible to suppress a decrease in the performance of the fuel cell stack due to a shortage of fuel gas. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic diagram of a fuel cell system according to an embodiment. [Diagram 2] FIG. 2 is a flowchart showing the operation at the start-up of the fuel cell system according to the embodiment. [Diagram 3] FIG. 3 is a flowchart showing the operation of the fuel cell system according to the embodiment when the load increases. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] BEST MODE FOR CARRYING OUT THE PRESENTLY PREFERRED EMBODIMENTS A fuel cell system and a method for operating the fuel cell system according to an embodiment of the present invention will now be described with reference to FIGS.

[0016] First, the schematic configuration of a fuel cell system according to the present embodiment will be described with reference to FIG.

[0017] Fig. 1 is a schematic diagram of a fuel cell system according to the present embodiment. As shown in Fig. 1, the fuel cell system 1 according to the present embodiment includes a plurality of fuel cell stacks 2, a fuel gas supply system 10, an oxidizing gas supply system 20, a cooling water supply system 30, and a control device 40.

[0018] First, the multiple fuel cell stack 2 will be described. The fuel cell stack 2 is configured to generate power by being supplied with fuel gas and oxidant gas. The fuel cell stack 2 is configured by stacking multiple fuel cell cells (not shown). Each fuel cell has an electrolyte, and a fuel electrode and an oxidant electrode arranged with the electrolyte sandwiched between them. The electrolyte is, for example, a solid polymer electrolyte membrane.

[0019] The fuel cell stack 2 has therein a fuel gas flow passage 3, an oxidant gas flow passage 4, and a cooling water flow passage 5. The fuel gas flow passage 3 is a flow passage through which fuel gas flows. The fuel gas flowing through the fuel gas flow passage 3 is supplied to the fuel electrode of each fuel cell. The oxidant gas flow passage 4 is a flow passage through which oxidant gas flows. The oxidant gas flowing through the oxidant gas flow passage 4 is supplied to the oxidant electrode of each fuel cell.

[0020] The fuel cell stack 2 generates power by supplying a fuel gas to the fuel electrode and an oxidizer gas to the oxidizer electrode. More specifically, the fuel cell stack 2 generates power through the reaction shown in the following chemical formula 1. The fuel gas is, for example, a hydrogen-containing gas. The fuel gas flows through the fuel gas flow passage 3 and is supplied to the fuel electrode, where it causes a fuel electrode reaction. The oxidizer gas is, for example, air (atmosphere). The oxidizer gas flows through the oxidizer gas flow passage 4 and is supplied to the oxidizer electrode, where it causes an oxidizer electrode reaction. The fuel cell stack 2 utilizes these electrochemical reactions to extract electrical energy from the electrodes provided on the current collector plates.

[0021] (chemical formula 1) Anode reaction: H2 → 2H + + 2e - Oxidant electrode reaction: 1 / 2O2+ 2H + +2e - → H2O

[0022] The cooling water flow passage 5 is a passage through which the cooling water flows. The cooling water flowing through the cooling water flow passage 5 cools each fuel cell that generates heat as a result of power generation, that is, cools the fuel cell stack 2.

[0023] The multiple fuel cell stacks 2 include primary stacks 2a, 2b and a secondary stack 2c. In the example shown in Fig. 1, the multiple fuel cell stacks 2 include two primary stacks 2a, 2b and one secondary stack 2c. However, this is not limited to this example, and the numbers of the primary stacks 2a, 2b and the secondary stacks 2c are each arbitrary.

[0024] The main stacks 2a and 2b are fuel cell stacks arranged upstream in the flow direction of the fuel gas. In other words, the main stacks 2a and 2b are arranged upstream of the secondary stack 2c in the flow direction of the fuel gas. The secondary stack 2c is a fuel cell stack arranged downstream in the flow direction of the fuel gas. In other words, the secondary stack 2c is arranged downstream of the main stacks 2a and 2b in the flow direction of the fuel gas.

[0025] The fuel gas supplied to the multiple fuel cell stacks 2 is first supplied to the primary stacks 2a and 2b and used to generate power in the primary stacks 2a and 2b. The fuel gas discharged from the primary stacks 2a and 2b is then supplied to the secondary stack 2c and used to generate power in the secondary stack 2c.

[0026] In the fuel cell stack 2, the supplied fuel gas is consumed as power is generated. The ratio of the amount of fuel gas consumed to the amount of fuel gas supplied in the fuel cell stack 2 (fuel gas consumption amount / fuel gas supply amount) is referred to as the fuel gas utilization rate. In this embodiment, the fuel gas utilization rate in each fuel cell stack 2 (primary stacks 2a, 2b and secondary stack 2c) during rated operation of the fuel cell system 1 may be 70% or more and 90% or less. On the other hand, as will be described later, the fuel gas utilization rate in each fuel cell stack 2 (primary stacks 2a, 2b and secondary stack 2c) during output increase (at start-up and load increase) of the fuel cell system 1 may be 40% or more and 60% or less.

[0027] Next, we will explain the fuel gas supply system 10. The fuel gas supply system 10 is configured to supply fuel gas to each fuel cell stack 2. The fuel gas supply system 10 includes a fuel gas blower 11, a fuel gas supply line 12, a fuel gas exhaust supply line 13, a recycle line 14, and a recycle blower 15.

[0028] The fuel gas blower 11 is configured to supply fuel gas to the main stacks 2a, 2b via a fuel gas supply line 12. More specifically, the fuel gas blower 11 supplies fuel gas to each of the fuel gas flow paths 3 of the main stacks 2a, 2b via the fuel gas supply line 12.

[0029] The fuel gas supply line 12 is a pipe through which the fuel gas supplied to the main stacks 2a and 2b flows. The fuel gas supply line 12 is connected to the fuel gas blower 11 and each of the fuel gas flow paths 3 of the main stacks 2a and 2b. The fuel gas supply line 12 extending from the fuel gas blower 11 branches on the upstream side of the main stacks 2a and 2b and is connected to each of the main stacks 2a and 2b. The fuel gas supplied from the fuel gas blower 11 flows through this fuel gas supply line 12 and is supplied to the main stacks 2a and 2b. The fuel gas supply line 12 is also connected to a recycle line 14 described later on the upstream side of the branch point. The fuel gas supplied from a recycle blower 15 described later through the recycle line 14 also merges with the fuel gas supplied from the fuel gas blower 11, flows through the fuel gas supply line 12, and is supplied to the main stacks 2a and 2b.

[0030] The fuel gas exhaust supply line 13 is a pipe through which the fuel gas discharged from the main stacks 2a, 2b and supplied to the secondary stack 2c flows. The fuel gas exhaust supply line 13 is connected to each of the fuel gas flow paths 3 of the main stacks 2a, 2b and the fuel gas flow path 3 of the secondary stack 2c. The fuel gas exhaust supply lines 13 extending from the main stacks 2a, 2b join at a joining point downstream of the main stacks 2a, 2b and upstream of the secondary stack 2c, and are connected to the secondary stack 2c. The fuel gas discharged from each of the main stacks 2a, 2b flows through this fuel gas exhaust supply line 13 and is supplied to the secondary stack 2c.

[0031] The recycle line 14 is a pipe through which the fuel gas discharged from the secondary stack 2c flows to the upstream side of the main stacks 2a, 2b. The recycle line 14 is connected to the fuel gas flow passage 3 of the secondary stack 2c and the junction point of the fuel gas supply line 12 on the upstream side of the main stacks 2a, 2b. The fuel gas discharged from the secondary stack 2c flows through this recycle line 14 to the upstream side of the main stacks 2a, 2b. The fuel gas that flows to the upstream side of the main stacks 2a, 2b via the recycle line 14 merges with the fuel gas supplied from the fuel gas blower 11 as described above, flows through the fuel gas supply line 12, and is supplied to the main stacks 2a, 2b.

[0032] The recycle blower 15 is disposed in the recycle line 14. The recycle blower 15 is configured to supply the fuel gas discharged from the secondary stack 2c to the primary stacks 2a, 2b via the recycle line 14. In the example shown in FIG. 1, the recycle blower 15 supplies the fuel gas discharged from the secondary stack 2c to the primary stacks 2a, 2b via the recycle line 14 and the fuel gas supply line 12. The recycle blower 15 can change the flow rate of the fuel gas supplied from the secondary stack 2c to the primary stacks 2a, 2b. The recycle blower 15 is controlled by a control device 40, which will be described later.

[0033] Next, a description will be given of the oxidizing gas supply system 20. The oxidizing gas supply system 20 is configured to supply an oxidizing gas to each fuel cell stack 2. The oxidizing gas supply system 20 includes an oxidizing gas blower 21, an oxidizing gas supply line 22, and an oxidizing gas discharge line 23.

[0034] The oxidant gas blower 21 is configured to supply the oxidant gas to each fuel cell stack 2 (the primary stacks 2a, 2b and the secondary stack 2c) via an oxidant gas supply line 22. More specifically, the oxidant gas blower 21 supplies the oxidant gas to each of the oxidant gas flow paths 4 of the primary stacks 2a, 2b and the secondary stack 2c via the oxidant gas supply line 22.

[0035] The oxidant gas supply line 22 is a pipe through which the oxidant gas supplied to each fuel cell stack 2 flows. The oxidant gas supply line 22 is connected to the oxidant gas blower 21 and each of the oxidant gas flow paths 4 of the fuel cell stack 2. The oxidant gas supply line 22 extending from the oxidant gas blower 21 branches off upstream of each fuel cell stack 2 and is connected to each fuel cell stack 2. The oxidant gas supplied from the oxidant gas blower 21 flows through this oxidant gas supply line 22 and is supplied to each fuel cell stack 2.

[0036] The oxidant gas discharge line 23 is a pipe through which the fuel gas discharged from each fuel cell stack 2 flows. The oxidant gas discharge line 23 is connected to each of the oxidant gas flow paths 4 of the fuel cell stack 2. The oxidant gas discharge line 23 also communicates with the outside atmosphere. The oxidant gas discharge lines 23 extending from each fuel cell stack 2 join at a joining point downstream of each fuel cell stack 2. The oxidant gas discharged from each fuel cell stack 2 flows through this oxidant gas discharge line 23 and is released into the outside atmosphere.

[0037] Next, a description will be given of the cooling water supply system 30. The cooling water supply system 30 is configured to supply cooling water to each fuel cell stack 2 (the primary stacks 2a, 2b and the secondary stack 2c). The cooling water supply system 30 includes a cooling water circulation line 31 and a cooling water pump 32.

[0038] The cooling water circulation line 31 is a pipe through which the cooling water supplied to each fuel cell stack 2 (main stacks 2a, 2b and secondary stack 2c) circulates. The cooling water circulation line 31 is connected to the inlet and outlet of each cooling water flow path 5 of the fuel cell stack 2. The cooling water circulation line 31 branches off on the upstream side of each fuel cell stack 2 and is connected to each fuel cell stack 2. The cooling water flows through this cooling water circulation line 31 and is supplied to each fuel cell stack 2. The cooling water circulation lines 31 extending from each fuel cell stack 2 merge on the downstream side of each fuel cell stack 2. The cooling water discharged from each fuel cell stack 2 flows through this cooling water circulation line 31 and is supplied again to the upstream side of each fuel cell stack 2. In this manner, the cooling water circulates through the cooling water circulation line 31.

[0039] The cooling water pump 32 is disposed in the cooling water circulation line 31. The cooling water pump 32 is configured to supply the cooling water discharged from each of the fuel cell stacks 2 again to each of the fuel cell stacks 2 via the cooling water circulation line 31. In other words, the cooling water pump 32 supplies the cooling water to each of the fuel cell stacks 2 while circulating the cooling water in the cooling water circulation line 31.

[0040] Next, the control device 40 will be described. The control device 40 is configured to control at least the recycle blower 15. The control device 40 may also be configured to control the fuel gas blower 11, the oxidant gas blower 21, and the cooling water pump 32. The control device 40 may also be configured to control each of the fuel cell stacks 2 (the primary stacks 2a, 2b, and the secondary stack 2c). In this case, the control device 40 may be configured to control the primary stacks 2a, 2b, and the secondary stack 2c, respectively, individually. The control device 40 may also be configured to control other auxiliary devices. The control device 40 may also be configured to control the entire fuel cell system 1.

[0041] In this embodiment, the control device 40 is configured to control the recycle blower 15 to change the flow rate of the fuel gas flowing through the recycle line 14. In particular, the control device 40 is configured to control the recycle blower 15 to make the flow rate of the fuel gas flowing through the recycle line 14 greater during output increase than the flow rate of the fuel gas flowing through the recycle line 14 during rated operation, so that the fuel gas utilization rate in the fuel cell stack 2 (main stacks 2a, 2b and secondary stack 2c) during output increase of the fuel cell system 1 is smaller than the fuel gas utilization rate in the fuel cell stack 2 during rated operation of the fuel cell system 1.

[0042] Here, the time when the output of the fuel cell system 1 is increasing may include the time when the fuel cell system 1 is started up. In other words, the control device 40 may be configured to control the recycle blower 15 to make the flow rate of the fuel gas flowing through the recycle line 14 at startup larger than the flow rate of the fuel gas flowing through the recycle line 14 during rated operation, so that the fuel gas utilization rate in the fuel cell stack 2 at startup of the fuel cell system 1 is smaller than the fuel gas utilization rate in the fuel cell stack 2 during rated operation of the fuel cell system 1.

[0043] Furthermore, the time when the output of the fuel cell system 1 increases may include the time when the load of the fuel cell system 1 increases. That is, the control device 40 may be configured to control the recycle blower 15 to make the flow rate of the fuel gas flowing through the recycle line 14 greater during load increase than the flow rate of the fuel gas flowing through the recycle line 14 during rated operation, so that the fuel gas utilization rate in the fuel cell stack 2 during load increase of the fuel cell system 1 is smaller than the fuel gas utilization rate in the fuel cell stack 2 during rated operation of the fuel cell system 1.

[0044] As described above, the fuel gas utilization rate in each fuel cell stack 2 (main stacks 2a, 2b and secondary stack 2c) during rated operation may be 70% or more and 90% or less. Also, the fuel gas utilization rate in each fuel cell stack 2 (main stacks 2a, 2b and secondary stack 2c) during output increase may be 40% or more and 60% or less. In other words, the controller 40 may control the recycle blower 15 to set the flow rate of fuel gas supplied to each fuel cell stack 2 during output increase to 115% or more and 225% or less of the flow rate of fuel gas supplied to each fuel cell stack 2 during rated operation.

[0045] The control device 40 may also determine the flow rate of the fuel gas flowing through the recycle line 14 when the fuel cell system 1 is started based on the stop time of the fuel cell system 1. More specifically, the control device 40 may increase the flow rate of the fuel gas flowing through the recycle line 14 when the fuel cell system 1 is started as the stop time of the fuel cell system 1 becomes longer. For example, the control device 40 may increase the flow rate of the fuel gas flowing through the recycle line 14 when the fuel cell system 1 is started when the stop time of the fuel cell system 1 is longer than a threshold time, compared with the flow rate of the fuel gas flowing through the recycle line 14 when the fuel cell system 1 is started when the stop time of the fuel cell system 1 is shorter than the threshold time. A plurality of such threshold times may be set, and the flow rate of the fuel gas may be changed in stages according to each threshold time.

[0046] Furthermore, the control device 40 may increase the output of the primary stacks 2a, 2b at a rate faster than the rate of increase in the output of the secondary stack 2c when the output of the fuel cell system 1 is increased. In other words, the control device 40 may increase the output of the primary stacks 2a, 2b at a rate faster than the output of the secondary stack 2c when the output of the fuel cell system 1 is increased. For example, the control device 40 may increase the output of the primary stacks 2a, 2b and then increase the output of the secondary stack 2c.

[0047] Next, a method of operating the fuel cell system 1 according to this embodiment will be described with reference to Figures 2 and 3. Here, the operation when the output of the fuel cell system 1 increases will be described. In this embodiment, the operation when the output of the fuel cell system 1 increases includes the start-up of the fuel cell system 1 and the load increase of the fuel cell system 1. Therefore, the operation when the fuel cell system 1 starts up and the operation when the load of the fuel cell system 1 increases will be described separately.

[0048] First, the operation at the start-up of the fuel cell system 1 will be described with reference to Fig. 2. Fig. 2 is a flow chart showing the operation at the start-up of the fuel cell system 1 according to this embodiment.

[0049] 2, when the fuel cell system 1 is started, first, the circulation of the cooling water is started in step S11. More specifically, the cooling water pump 32 is driven, so that the cooling water circulates through the cooling water circulation line 31, and the cooling water is supplied to each fuel cell stack 2 (the primary stacks 2a, 2b and the secondary stack 2c).

[0050] Next, in step S12, the supply of fuel gas is started. More specifically, the fuel gas blower 11 is driven to supply the fuel gas to the main stacks 2a and 2b through the fuel gas supply line 12. In addition, the fuel gas discharged from the main stacks 2a and 2b is supplied to the secondary stack 2c through the fuel gas exhaust supply line 13.

[0051] Next, in step S13, recycling of the fuel gas is started. More specifically, by driving the recycle blower 15, the fuel gas discharged from the secondary stack 2c flows to the upstream side of the main stacks 2a, 2b through the recycle line 14. The fuel gas flowing to the upstream side of the main stacks 2a, 2b is merged with the fuel gas supplied from the fuel gas blower 11, and is supplied to the main stacks 2a, 2b through the fuel gas supply line 12.

[0052] Next, in step S14, the supply of oxidant gas is started. More specifically, the oxidant gas blower 21 is driven, so that the oxidant gas is supplied to each fuel cell stack 2 (the primary stacks 2a, 2b and the secondary stack 2c) via the oxidant gas supply line 22. The oxidant gas discharged from each fuel cell stack 2 is released to the outside atmosphere via the oxidant gas discharge line 23.

[0053] Thereafter, in step S15, power generation is performed by each fuel cell stack 2 (the primary stacks 2a, 2b and the secondary stack 2c). More specifically, each fuel cell stack 2 consumes the supplied fuel gas and oxidant gas to generate power.

[0054] Next, the operation of the fuel cell system 1 when the load increases will be described with reference to Fig. 3. Fig. 3 is a flow chart showing the operation of the fuel cell system 1 according to this embodiment when the load increases. An increase in the load of the fuel cell system 1 can occur, for example, when the fuel cell system 1 is operating and an attempt is made to increase the amount of power generation in response to power demand.

[0055] When the load on the fuel cell system 1 increases, first, in step S21, the supply amount of fuel gas is increased, as shown in Fig. 3. More specifically, by increasing the driving amount of the fuel gas blower 11, more fuel gas is supplied to the main stacks 2a, 2b via the fuel gas supply line 12. Also, more fuel gas is exhausted from the main stacks 2a, 2b and supplied to the secondary stack 2c via the fuel gas exhaust supply line 13.

[0056] Next, in step S22, the amount of recycled fuel gas is increased. More specifically, by increasing the driving rate of the recycle blower 15, more fuel gas is discharged from the secondary stack 2c and flows to the upstream side of the main stacks 2a, 2b via the recycle line 14. The fuel gas that has flowed to the upstream side of the main stacks 2a, 2b is merged with the fuel gas supplied from the fuel gas blower 11, and is supplied to the main stacks 2a, 2b via the fuel gas supply line 12.

[0057] Next, in step S23, the supply amount of the oxidant gas is increased. More specifically, by increasing the driving amount of the oxidant gas blower 21, more oxidant gas is supplied to each fuel cell stack 2 (the primary stacks 2a, 2b and the secondary stack 2c) via the oxidant gas supply line 22.

[0058] Thereafter, in step S24, the current of each fuel cell stack 2 (main stacks 2a, 2b and secondary stack 2c) is increased, and in step S25, the output (power generation amount) of each fuel cell stack 2 is increased.

[0059] Here, in the operating method of the fuel cell system 1 according to this embodiment, in the above-mentioned step S13 and / or step S22, the flow rate of the fuel gas flowing through the recycle line 14 is made larger than the flow rate of the fuel gas flowing through the recycle line 14 during rated operation so that the fuel gas utilization rate in the fuel cell stack 2 (main stacks 2a, 2b and secondary stack 2c) is made smaller than the fuel gas utilization rate in the fuel cell stack 2 during rated operation.

[0060] The fuel gas utilization rate in each fuel cell stack 2 (main stacks 2a, 2b and secondary stack 2c) during rated operation may be 70% or more and 90% or less. Also, the fuel gas utilization rate in each fuel cell stack 2 (main stacks 2a, 2b and secondary stack 2c) during output increase may be 40% or more and 60% or less. In other words, the flow rate of fuel gas supplied to each fuel cell stack 2 during output increase may be 115% or more and 225% or less of the flow rate of fuel gas supplied to each fuel cell stack 2 during rated operation.

[0061] Furthermore, in the above-mentioned step S13, the flow rate of the fuel gas flowing through the recycle line 14 may be determined based on the stop time of the fuel cell system 1. More specifically, the flow rate of the fuel gas flowing through the recycle line 14 may be increased as the stop time of the fuel cell system 1 becomes longer. For example, the flow rate of the fuel gas flowing through the recycle line 14 when the stop time of the fuel cell system 1 is longer than a threshold time may be increased compared to the flow rate of the fuel gas flowing through the recycle line 14 when the stop time of the fuel cell system 1 is shorter than the threshold time. A plurality of such threshold times may be set, and the flow rate of the fuel gas may be changed in stages according to each threshold time.

[0062] Furthermore, when the output of the fuel cell system 1 is increased, the rate at which the output of the primary stacks 2a, 2b is increased may be faster than the rate at which the output of the secondary stack 2c is increased. In other words, when the output of the fuel cell system 1 is increased, the output of the primary stacks 2a, 2b may be increased faster than the output of the secondary stack 2c. For example, the output of the primary stacks 2a, 2b may be increased first, and then the output of the secondary stack 2c may be increased.

[0063] Thus, according to this embodiment, the flow rate of the fuel gas flowing through the recycle line 14 during output increase is set to be greater than the flow rate of the fuel gas flowing through the recycle line 14 during rated operation so that the fuel gas utilization rate in the fuel cell stack 2 during output increase is smaller than the fuel gas utilization rate in the fuel cell stack 2 during rated operation. In a typical fuel cell system, the fuel gas utilization rate in each fuel cell stack is set to be constant, and the fuel gas utilization rate in each fuel cell stack during output increase is also the same as the fuel gas utilization rate in each fuel cell stack during rated operation. However, in this case, there is a risk that the fuel gas will be insufficient in the fuel cell of each fuel cell stack during output increase, resulting in a deterioration in the performance of the fuel cell stack. In particular, in a fuel cell system having a plurality of fuel cell stacks including a main stack and a sub stack, the length of the fuel gas piping connecting each fuel cell stack becomes long, so that it takes time to supply the fuel gas, and there is a high possibility that the fuel gas will be insufficient in the fuel cell. In contrast, according to this embodiment, the flow rate of the fuel gas flowing through the recycle line 14 is increased so that the fuel gas utilization rate during output increase is smaller than that during rated operation, thereby increasing the amount of fuel gas supplied to the main stacks 2a and 2b during output increase. This makes it possible to prevent a shortage of fuel gas in each fuel cell stack 2. As a result, it is possible to prevent a decrease in performance of the fuel cell stack 2 due to a shortage of fuel gas.

[0064] Furthermore, according to this embodiment, the period when the output of the fuel cell system 1 is increasing includes the period when the fuel cell system 1 is started up. In particular, when the fuel cell system 1 is started up, fuel gas is consumed throughout the fuel cell stack 2, and if there are any locations where the supply of fuel gas is insufficient, the function of the catalyst layer may be degraded. In contrast, according to this embodiment, when the fuel cell system 1 is started up, as described above, the flow rate of fuel gas flowing through the recycle line 14 is increased, thereby effectively suppressing the degradation of performance of the fuel cell stack 2 due to a shortage of fuel gas.

[0065] Furthermore, according to this embodiment, the time when the output of the fuel cell system 1 increases includes the time when the load of the fuel cell system 1 increases. When the load of the fuel cell system 1 increases, the consumption of fuel gas in the fuel cell stack 2 also increases, and similarly, there is a risk of the function of the catalyst layer deteriorating. In contrast, according to this embodiment, when the load of the fuel cell system 1 increases, as described above, by increasing the flow rate of fuel gas flowing through the recycle line 14, it is possible to effectively suppress the performance degradation of the fuel cell stack 2 due to a shortage of fuel gas.

[0066] According to this embodiment, the fuel gas utilization rate in the fuel cell stack 2 during rated operation is 70% or more and 90% or less, and the fuel gas utilization rate in the fuel cell stack 2 during output increase is 40% or more and 60% or less. That is, the flow rate of the fuel gas supplied to each fuel cell stack 2 during output increase is set to 115% or more and 225% or less of the flow rate of the fuel gas supplied to each fuel cell stack 2 during rated operation. By setting the flow rate of the fuel gas to 115% or more during rated operation, a sufficient amount of fuel gas can be supplied to each fuel cell stack 2 so that there is no shortage of fuel gas, and the performance degradation of the fuel cell stack 2 due to the shortage of fuel gas can be effectively suppressed. On the other hand, by setting the flow rate of the fuel gas to 225% or less during rated operation, an excessive decrease in the fuel gas utilization rate in each fuel cell stack 2 can be suppressed.

[0067] Furthermore, according to this embodiment, the flow rate of the fuel gas flowing through the recycle line 14 when the fuel cell system 1 is started is determined based on the stop time of the fuel cell system 1. If the fuel cell system is stopped for a long time, more fuel gas will leak out from the piping of the fuel cell system, etc., and a performance degradation of the fuel cell stack 2 due to a shortage of fuel gas will easily occur when the fuel cell system is started. In contrast, according to this embodiment, the flow rate of the fuel gas flowing through the recycle line 14 when the fuel cell system 1 is started is determined based on the stop time of the fuel cell system 1, so that a performance degradation of the fuel cell stack 2 due to a shortage of fuel gas can be effectively suppressed even if the fuel cell system 1 is stopped for a long time.

[0068] Furthermore, according to this embodiment, when the output of the fuel cell system 1 increases, the output increase rate of the main stacks 2a, 2b is made faster than the output increase rate of the secondary stack 2c. In a fuel cell system equipped with a plurality of fuel cell stacks including a main stack and a secondary stack, it takes time for the fuel gas supplied to the upstream main stack to be supplied to the downstream secondary stack, and therefore a fuel gas shortage is particularly likely to occur in the downstream secondary stack. In contrast, according to this embodiment, when the output of the fuel cell system 1 increases, the output increase rate of the main stacks 2a, 2b is made faster than the output increase rate of the secondary stack 2c, so that fuel gas can be quickly supplied from the main stacks 2a, 2b to the secondary stack 2c, and a decrease in performance of the fuel cell stack 2 (secondary stack 2c) due to a shortage of fuel gas can be effectively suppressed.

[0069] According to the embodiment described above, it is possible to suppress a decrease in the performance of the fuel cell stack due to a shortage of fuel gas.

[0070] Although several embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the gist of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are included in the scope of the invention and its equivalents described in the claims. [Explanation of symbols]

[0071] 1: fuel cell system, 2: fuel cell stack, 2a, 2b: primary stack, 2c: secondary stack, 14: recycle line, 15: recycle blower, 40: control device

Claims

1. 1. A fuel cell system comprising: A plurality of fuel cell stacks that generate power by being supplied with a fuel gas and an oxidant gas, the plurality of fuel cell stacks including a primary stack and a secondary stack to which the fuel gas discharged from the primary stack is supplied; a recycle line through which the fuel gas discharged from the secondary stack flows upstream of the primary stack; a recycle blower disposed in the recycle line and configured to supply the fuel gas discharged from the secondary stack to the primary stack via the recycle line; A control device for controlling the recycle blower, The control device controls the recycle blower to make the flow rate of the fuel gas flowing through the recycle line greater during output increase than the flow rate of the fuel gas flowing through the recycle line during rated operation, so that the fuel gas utilization rate in the fuel cell stack during output increase of the fuel cell system is smaller than the fuel gas utilization rate in the fuel cell stack during rated operation of the fuel cell system.

2. 2. The fuel cell system according to claim 1, wherein the time of increasing the output includes a time of starting up the fuel cell system.

3. 2. The fuel cell system according to claim 1, wherein the time when the output increases includes a time when the load of the fuel cell system increases.

4. The fuel gas utilization rate during the rated operation is 70% or more and 90% or less, 2. The fuel cell system according to claim 1, wherein the fuel gas utilization rate during the output increase is equal to or greater than 40% and equal to or less than 60%.

5. 3. The fuel cell system according to claim 2, wherein the control device determines a flow rate of the fuel gas flowing through the recycle line at the time of startup based on a stop time of the fuel cell system.

6. 6. The fuel cell system according to claim 1, wherein the control device, during the output increase, makes the output increase rate of the primary stack faster than the output increase rate of the secondary stack.

7. A method for operating a fuel cell system having a plurality of fuel cell stacks that generate electricity by being supplied with a fuel gas and an oxidant gas, the plurality of fuel cell stacks including a primary stack and a secondary stack to which the fuel gas discharged from the primary stack is supplied, comprising the steps of: a supply step of supplying the fuel gas to the primary stack and the secondary stack; a recycling step of supplying the fuel gas discharged from the secondary stack to the primary stack via a recycling line, A method for operating a fuel cell system, in which, in the recycle process, the flow rate of the fuel gas flowing through the recycle line during output increase is made larger than the flow rate of the fuel gas flowing through the recycle line during rated operation, so that the fuel gas utilization rate in the fuel cell stack during output increase of the fuel cell system is made smaller than the fuel gas utilization rate in the fuel cell stack during rated operation of the fuel cell system.

8. The method of operating a fuel cell system according to claim 7 , wherein the time when the output is increased includes a time when the fuel cell system is started up.

9. The method of operating a fuel cell system according to claim 7 , wherein the time when the output increases includes a time when the load of the fuel cell system increases.

10. The fuel gas utilization rate during the rated operation is 70% or more and 90% or less, 8. The method of operating a fuel cell system according to claim 7, wherein the fuel gas utilization rate during the output increase is equal to or greater than 40% and equal to or less than 60%.

11. 9. The method of claim 8, wherein in the recycling step, a flow rate of the fuel gas flowing through the recycling line at the time of startup is determined based on a stop time of the fuel cell system.

12. 12. The method of operating a fuel cell system according to claim 7, wherein, during the output increase, a rate at which the output of the primary stack increases is made faster than a rate at which the output of the secondary stack increases.

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