Fuel battery system

Abstract

To provide a fuel battery system capable of improving a fuel utilization rate in an entire system.SOLUTION: A fuel battery system 1 comprises: a first fuel battery CS1 that outputs an electric energy by an electric chemical reaction of a fuel gas and an oxidant gas; and a first off fuel channel 51 in which an off fuel exhausted from the first fuel battery CS1 flows. The fuel battery system 1 comprises: a second fuel battery CS2 that outputs the electric energy by using the fuel gas contained in the off fuel flowing in the first off fuel channel 51; and a second off fuel channel 61 in which the off fuel exhausted from the second fuel battery CS2 flows. The fuel battery system 1 comprises a downstream circulation channel 71 that is connected to both of the first off fuel channel 51 and the second off fuel channel 61, and returns one part of an off as flowing in the second off fuel channel 61 to the first off fuel channel 51.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to fuel cell systems. [Background technology]

[0002] 2. Description of the Related Art Conventionally, a multi-stage fuel cell system has been known that includes two fuel cells, in which off fuel discharged from one fuel cell is regenerated and supplied to the other fuel cell (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2021-136169 A Summary of the Invention [Problem to be solved by the invention]

[0004] The multi-stage fuel cell system described above is based on the idea of ​​making effective use of the fuel gas contained in the off-fuel of the first-stage fuel cell, but the off-fuel from the second-stage fuel cell is not used for power generation, leaving room for improvement in terms of fuel utilization rate in the entire system.

[0005] An object of the present disclosure is to provide a fuel cell system that is capable of improving the fuel utilization rate in the entire system. [Means for solving the problem]

[0006] The invention described in claim 1 is 1. A fuel cell system comprising: a first fuel cell (CS1) that outputs electrical energy by an electrochemical reaction between a fuel gas and an oxidant gas; a first off-fuel flow path (51) through which off-fuel discharged from the first fuel cell flows; a second fuel cell (CS2) that outputs electrical energy by utilizing a fuel gas contained in the off-fuel flowing through the first off-fuel flow passage; a second off-fuel flow path (61) through which the off-fuel discharged from the second fuel cell flows; The fuel cell further includes a downstream circulation flow path (71) connected to both the first off-fuel flow path and the second off-fuel flow path for returning a portion of the off-gas flowing through the second off-fuel flow path to the first off-fuel flow path.

[0007] In this way, if a portion of the off-fuel discharged from the second fuel cell is returned to the second fuel cell via the first off-fuel flow path, the unreacted fuel contained in the off-fuel discharged from the second fuel cell can be consumed by the second fuel cell. Therefore, the fuel cell system of the present disclosure can improve the fuel utilization rate compared to a conventional system that does not use the off-fuel discharged from the second fuel cell for power generation.

[0008] The reference symbols in parentheses attached to each component indicate an example of the correspondence between the component and specific components described in the embodiments described below. [Brief description of the drawings]

[0009] [Figure 1] 1 is a schematic configuration diagram of a fuel cell system according to a first embodiment. [Diagram 2] FIG. 4 is an explanatory diagram for explaining a fuel utilization rate in a fuel cell system serving as a comparative example of the first embodiment. [Diagram 3] FIG. 2 is an explanatory diagram for explaining a fuel utilization rate in the fuel cell system according to the first embodiment. [Figure 4] FIG. 11 is an explanatory diagram for explaining a problem that occurs when the fuel utilization rate of the second fuel cell becomes excessive. [Diagram 5] 4 is a flowchart showing the flow of a control process executed by a control device of the fuel cell system of the first embodiment. [Figure 6] 5 is a flowchart showing the flow of a setting process executed by a control device of the fuel cell system of the first embodiment. [Figure 7] FIG. 4 is a schematic configuration diagram of a fuel cell system according to a modified example of the first embodiment. [Figure 8] 6 is a flowchart showing the flow of a control process executed by a control device of a fuel cell system according to a second embodiment. [Figure 9] FIG. 9 is an explanatory diagram for explaining a fuel utilization rate in the fuel cell system when the control process shown in FIG. 8 is executed. [Figure 10] FIG. 11 is a schematic configuration diagram of a fuel cell system according to a third embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, the embodiments of the present disclosure will be described with reference to the drawings. In the following embodiments, the same reference numerals are used for parts that are the same as or equivalent to those described in the preceding embodiments, and the description thereof may be omitted. In addition, in the embodiments, when only a part of the components is described, the components described in the preceding embodiments can be applied to the other parts of the components. In the following embodiments, each embodiment can be partially combined with each other, even if not specifically stated, as long as it does not cause any trouble in combination.

[0011] (First embodiment) This embodiment will be described with reference to Figures 1 to 6. In this embodiment, a fuel cell system 1 of this embodiment includes a first fuel cell CS1 and a second fuel cell CS2, as shown in Figure 1. The fuel cell system 1 is configured as a multi-stage fuel cell system that supplies off fuel discharged from the first fuel cell CS1 to the second fuel cell CS2 and generates power in the second fuel cell CS2 using unreacted gas contained in the off fuel.

[0012] Each of the fuel cells CS1 and CS2 is composed of a solid oxide fuel cell, which is generally called SOFC (short for Solid Oxide Fuel Cell), and has a high operating temperature (for example, 500°C to 1000°C).

[0013] Each of the fuel cells CS1 and CS2 has a stack structure in which a plurality of power generating cells are stacked, and outputs electrical energy through an electrochemical reaction between a fuel gas and an oxidizing gas (oxygen from the air in this example).

[0014] In each of the fuel cells CS1 and CS2 of this embodiment, the number of power generating cells of the first fuel cell CS1 located upstream is greater than the number of power generating cells of the second fuel cell CS2 located downstream. For example, the number of power generating cells of the first fuel cell CS1 is about five times the number of power generating cells of the second fuel cell CS2. The shape of the power generating cells may be either flat plate type or cylindrical type.

[0015] The power generation cell is composed of a solid oxide electrolyte, an air electrode (i.e., the cathode CA), and a fuel electrode (i.e., the anode AN). The power generation cell uses hydrogen and carbon monoxide produced by reforming city gas (i.e., gas mainly composed of methane), which is a hydrocarbon fuel, as fuel gas. Note that the fuel used does not have to be a hydrocarbon gas, as long as it can be used to generate electricity in the power generation cell.

[0016] Each of the fuel cells CS1 and CS2 outputs electric energy through an electrochemical reaction between hydrogen and oxygen as shown in the following reaction formulas F1 and F2.

[0017] [Anode AN] 2H2+2O2 - →2H2O+4e - …(F1)

[0018] [Cathode CA] O2+4e - →2O2 - …(F2) Each of the fuel cells CS1 and CS2 outputs electric energy through the electrochemical reaction of carbon monoxide and oxygen shown in the following reaction formulas F3 and F4.

[0019] [Anode AN] 2CO+2O2 - →2CO2+4e - …(F3)

[0020] [Cathode CA] O2+4e- →2O2 - …(F4) Each of the fuel cells CS1 and CS2 has a characteristic that oxygen ions generated by an electrochemical reaction between a fuel gas and an oxidant gas permeate from the cathode CA to the anode AN. Each of the fuel cells CS1 and CS2 is disposed inside a housing HG having thermal insulation properties together with a reformer 32, a combustor, etc., which will be described later.

[0021] An air flow path is connected to the air introduction portion of each of the fuel cells CS1 and CS2. That is, a first air supply flow path 20A, which is an air flow path, is connected to the first fuel cell CS1. Also, a second air supply flow path 20B, which is an air flow path, is connected to the second fuel cell CS2. Each of the air supply flow paths 20A and 20B is formed of piping or the like. Although not shown, each of the air supply flow paths 20A and 20B is provided with a pressure blower that pressure-feeds air to each of the fuel cells CS1 and CS2, an air preheater that heats the air to be supplied to each of the fuel cells CS1 and CS2, and the like.

[0022] In addition, a fuel supply flow path 30, which is a distribution path for the reforming material and the fuel gas, is connected to the fuel gas introduction portion of the first fuel cell CS1. The fuel supply flow path 30 is composed of piping, etc. A fuel pump 31, a desulfurizer (not shown), and a reformer 32 are provided in this order from the upstream side in the fuel supply flow path 30.

[0023] The fuel pump 31 is a pump for supplying city gas, which is a raw material for reforming, from outside the system, etc., to the first fuel cell CS1 side. The fuel pump 31 is configured as an electric pump whose operation is controlled by a control signal from the control unit 100.

[0024] The desulfurizer (not shown) is a device for removing sulfur components contained in the reforming raw material. For example, a hydrodesulfurizer is used as the desulfurizer, which reacts the sulfur components contained in the reforming raw material with hydrogen to remove the sulfur components from the reforming raw material.

[0025] The reformer 32 reforms the reforming raw material to generate fuel gas. The reformer 32 is configured to include a steam reforming catalyst containing a precious metal such as rhodium or ruthenium. Specifically, the reformer 32 heats a mixed gas of the reforming raw material and steam, and generates fuel gas (hydrogen, carbon monoxide) by a reforming reaction shown in the following reaction formula F5 and a shift reaction shown in the following reaction formula F6.

[0026] CH4+H2O→CO+H2…(F5) CO+H2O→CO2+H2…(F6) Here, the steam reforming in the reformer 32 is an endothermic reaction, and has the characteristic that the reforming rate improves under high temperature conditions. For this reason, the reformer 32 is used in a heated state. For example, the reformer 32 may be disposed around each of the fuel cells CS1, CS2 so as to absorb heat released to the surroundings when each of the fuel cells CS1, CS2 generates power, or may be configured to be heated using combustion gas generated in a combustor (not shown) as a heat source.

[0027] A water supply passage 40 is connected to the fuel supply passage 30 between the fuel pump 31 and the reformer 32, and this water supply passage 40 is provided with a water pump 41 that supplies water outside or inside the system, and a carburetor (not shown).

[0028] The first fuel cell CS1 outputs electric energy by an electrochemical reaction between the fuel gas supplied via the fuel supply passage 30 and the oxygen supplied via the first air supply passage 20A. At this time, in the first fuel cell CS1, oxygen ions generated by the electrochemical reaction between the fuel gas and the oxidant gas permeate from the cathode CA to the anode AN.

[0029] A first off-gas flow path 50 including a first off-fuel flow path 51 and a first off-air flow path 52 is connected to the off-fuel discharge portion and the off-air discharge portion of the first fuel cell CS1. The first off-gas flow path 50 is configured by piping or the like. The off-fuel is off-gas discharged from the anode side of the power generation cell.

[0030] The first off-fuel flow path 51 serves as a path for supplying unreacted fuel gas contained in the off-fuel to the second fuel cell CS2. The first off-fuel flow path 51 is connected to the second fuel cell CS2.

[0031] The second fuel cell CS2 outputs electric energy by an electrochemical reaction between the fuel gas supplied via the first off-fuel flow path 51 and oxygen supplied via the first air supply flow path 20A. At this time, in the second fuel cell CS2, oxygen ions generated by the electrochemical reaction between the fuel gas and the oxidant gas permeate from the cathode CA to the anode AN.

[0032] Further, a second off-gas passage 60 including a second off-fuel passage 61 and a second off-air passage 62 is connected to the discharge portion of the off-fuel and the discharge portion of the off-air of the second fuel cell CS2. The second off-gas passage 60 is constituted by piping or the like.

[0033] Although not shown, a combustor that generates combustion gas by combusting the unreacted gas that has passed through the second fuel cell CS2 is disposed in the second off-gas passage 60. The fuel gas generated in the combustor is used for warming up the fuel cells CS1 and CS2, raising the temperature of the reformer 32, preheating the oxidant gas, etc.

[0034] The fuel cell system 1 of this embodiment is provided with a recycle unit 70 that returns a portion of the off fuel that has passed through the second fuel cell CS2 to the upstream side of the second fuel cell CS2. The recycle unit 70 includes a downstream circulation flow path 71, a downstream ejector 72, and a removal unit 73.

[0035] The downstream circulation flow path 71 is connected to both of the off-fuel flow paths 51, 61, and is a flow path that returns a portion of the off-fuel flowing through the second off-fuel flow path 61 to the first off-fuel flow path 51. One end of the downstream circulation flow path 71 is connected to the second off-fuel flow path 61, and the other end is connected to the first off-fuel flow path 51. A flow rate sensor 130 that detects the flow rate of the off-fuel flowing through the downstream circulation flow path 71 is disposed in the downstream circulation flow path 71.

[0036] The downstream ejector 72 is provided in the first off-fuel flow path 51, and uses the off-fuel discharged from the first fuel cell CS1 as a driving flow to suck the off-fuel flowing through the downstream circulation flow path 71 into the first off-fuel flow path 51. Specifically, the downstream ejector 72 is provided at the junction of the first off-fuel flow path 51 with the downstream circulation flow path 71.

[0037] The downstream ejector 72 has a nozzle portion that sprays fluid, a suction portion that sucks fluid from the other end side of the downstream circulation flow path 71, and a discharge portion that mixes the fluid sprayed from the nozzle portion with the fluid sucked from the suction portion and discharges it toward the second fuel cell CS2.

[0038] The downstream ejector 72 has a characteristic that the flow rate of the suction fluid sucked in from the suction part increases with an increase in the mass flow rate of the fluid flowing into the nozzle part as the driving flow. Therefore, by increasing the mass flow rate of the raw reforming material flowing into the nozzle part of the downstream ejector 72, it is possible to increase the suction flow rate of the off-fuel sucked in from the suction part.

[0039] The removal unit 73 is provided in the first off-fuel flow path 51, and removes, as impurities, a portion of substances that do not contribute to power generation in the second fuel cell CS2, such as water, contained in the off-fuel flowing through the first fuel cell CS1. The removal unit 73 is provided downstream of the downstream ejector 72 in the first off-fuel flow path 51.

[0040] Specifically, the removal unit 73 includes a water condensation unit 731 that separates impurities from the off-fuel and a storage unit 732 that stores the impurities. The water condensation unit 731 is a radiator that radiates heat from the off-fuel flowing through the first off-fuel flow path 51 to condense the water contained in the off-fuel. The storage unit 732 is a tank that stores the water generated in the water condensation unit 731.

[0041] Next, we will explain the control unit 100 that constitutes the electronic control device of the fuel cell system 1. The control unit 100 is composed of a microcomputer including a processor 101 and a memory 102, and its peripheral circuits. The control unit 100 performs various calculations and processes based on the control programs stored in the memory 102, and controls the operation of various control devices connected to the output side. The memory 102 is composed of a non-transient substantial storage medium.

[0042] Connected to the input side of the control unit 100 are various sensors including current sensors 110A, 110B that detect the output current of each fuel cell CS1, CS2, temperature sensors 120A, 120B that detect the temperature of each fuel cell CS1, CS2, and a flow rate sensor 130. As a result, the control unit 100 receives detection signals that correspond to the detection results of the various sensors.

[0043] Although not shown, an operation panel is connected to the control unit 100. The operation panel is provided with operation switches for turning on and off the power generation of each of the fuel cells CS1, CS2, a display for displaying the operating status of each of the fuel cells CS1, CS2, and the like.

[0044] On the other hand, a pressure blower (not shown), a fuel pump 31, a current regulator CNV, etc. are connected to the output side of the control unit 100 as control devices. The operations of these control devices are controlled in response to control signals output from the control unit 100. The control unit 100 controls the amount of reforming material supplied to the reformer 32 by the fuel pump 31, for example.

[0045] Here, the current adjustment unit CNV adjusts the current extracted from each of the fuel cells CS1, CS2. The current adjustment unit CNV is, for example, configured with a DC / DC converter. The control unit 100 controls the current adjustment unit CNV so that the current extracted from each of the fuel cells CS1, CS2 becomes a target current value. The current extracted from each of the fuel cells CS1, CS2 by the current adjustment unit CNV is sometimes called a sweep current.

[0046] Next, we will explain the basic operation of the fuel cell system 1. When the operation switch on the operation panel of the fuel cell system 1 is turned on and each of the fuel cells CS1, CS2 is started, the control unit 100 executes a power generation process to cause each of the fuel cells CS1, CS2 to output electric energy. In this power generation process, for example, the pressure blower and fuel pump 31 are controlled so that the first fuel cell CS1 is supplied with the oxidant gas and fuel gas in amounts suitable for power generation.

[0047] When the first fuel cell CS1 is supplied with the oxidant gas and the fuel gas, it outputs electric energy through the reactions shown in the above-mentioned reaction formulas F1 to F4, and then the first fuel cell CS1 discharges off-fuel and off-air.

[0048] When the second fuel cell CS2 is supplied with the oxidant gas and the off fuel that has passed through the first fuel cell CS1, the second fuel cell CS2 outputs electrical energy through the reactions shown in the above-mentioned reaction formulas F1 to F4, and then the second fuel cell CS2 discharges the off fuel and the off air.

[0049] 2 shows a fuel cell system CE as a comparative example of this embodiment. Unlike the fuel cell system 1 of this embodiment, this fuel cell system CE does not include a recycle section 70.

[0050] For example, when the amount of fuel gas supplied to the first fuel cell CS1 is "100" and the fuel utilization rate in each of the fuel cells CS1 and CS2 is "70%, " in the fuel cell system CE of the comparative example, as shown in FIG. 2, 70% of the fuel gas is consumed in the first fuel cell CS1. In this case, the fuel consumption in the first fuel cell CS1 is "70", which is the amount of fuel gas supplied, "100", multiplied by the fuel utilization rate in the first fuel cell CS1 (hereinafter, also referred to as the first fuel utilization rate Uf1), "70". Furthermore, the fuel gas not consumed in the first fuel cell CS1 is supplied to the second fuel cell CS2 as off fuel. In this example, the amount of fuel gas supplied to the second fuel cell CS2 is "30". The fuel utilization rate is the ratio [%] of the fuel gas consumed in each of the fuel cells CS1 and CS2 to the amount of fuel gas supplied to the anode AN.

[0051] In the second fuel cell CS2, 70% of the fuel gas is consumed. In this case, the fuel consumption in the second fuel cell CS2 is "21", which is the product of "30", the amount of fuel gas supplied, and "70", the fuel utilization rate in the second fuel cell CS2 (hereinafter also referred to as the second fuel utilization rate Uf2). The fuel gas not consumed in the second fuel cell CS2 is supplied to the combustor, etc. as off fuel.

[0052] 2, the fuel utilization rate Uf of the entire system of the comparative fuel cell system CE is "91", which is the sum of the fuel consumption in the first fuel cell CS1, "70", and the fuel consumption in the second fuel cell CS2, "21". In this example, the fuel gas discharged from the second fuel cell CS2 is "9".

[0053] In the comparative example of the fuel cell system CE, the off fuel from the first fuel cell CS1 (the first stage) is effectively utilized, but the off fuel from the second fuel cell CS2 (the second stage) is not used for power generation, and there is room for improvement in terms of the fuel utilization rate Uf of the entire system.

[0054] In contrast, the fuel cell system 1 of this embodiment is provided with a recycle unit 70 for effectively utilizing the off fuel from the second fuel cell CS2 for power generation, and can improve the fuel utilization rate Uf in the entire system.

[0055] For example, when the amount of fuel gas supplied to the first fuel cell CS1 is "100" and the fuel utilization rate in each of the fuel cells CS1 and CS2 is "70%," in the fuel cell system 1 of this embodiment, 70% of the fuel gas is consumed in the first fuel cell CS1, as shown in Fig. 3. In this case, the fuel consumption is "70", which is the amount of fuel gas supplied, "100", multiplied by the first fuel utilization rate Uf1, "70". Furthermore, the fuel gas not consumed in the first fuel cell CS1 is supplied to the second fuel cell CS2 as off fuel.

[0056] Here, in this example, a portion of the off-fuel of the second fuel cell CS2 is returned to the second fuel cell CS2 via the downstream circulation flow path 71 of the recycle unit 70. When the amount of fuel gas supplied to the second fuel cell CS2 via the downstream circulation flow path 71 is "6", the amount of fuel gas supplied to the second fuel cell CS2 is "36", which is the sum of the amount of fuel gas supplied from the first fuel cell CS1, "30", and the amount of fuel gas supplied from the downstream circulation flow path 71, "6".

[0057] In the second fuel cell CS2, 70% of the fuel gas is consumed. In this case, the fuel consumption is "25.2" obtained by multiplying the fuel gas supply amount "36" by the second fuel utilization rate Uf2 "70". A portion of the fuel gas that is not consumed in the second fuel cell CS2 is reused for power generation via the downstream circulation flow path 71, and the remainder is supplied to the combustor, etc. as off fuel.

[0058] 3, the fuel utilization rate Uf of the entire system of the comparative fuel cell system CE is "95.2", which is the sum of "70", the fuel consumption in the first fuel cell CS1, and "25.2", the fuel utilization rate in the second fuel cell CS2. In this example, the fuel gas discharged from the second fuel cell CS2 is "4.8".

[0059] In this way, the fuel cell system 1 of this embodiment effectively utilizes the off fuel of the second fuel cell CS2 for power generation, and therefore can improve the fuel utilization rate Uf of the entire system compared to the fuel cell system CE of the comparative example.

[0060] Here, it may be possible to set the current drawn from each fuel cell CS1, CS2 so that the fuel utilization rate Uf of the entire system becomes a predetermined target value. In this case, however, the second fuel utilization rate Uf2 of the second fuel cell CS2 will be left to chance.

[0061] In the fuel cell system 1 of this embodiment, for example, when the amount of fuel gas supplied to the second fuel cell CS2 via the downstream circulation passage 71 is small, the second fuel utilization rate Uf2 is increased. Conversely, when the amount of fuel gas supplied to the second fuel cell CS2 via the downstream circulation passage 71 is large, the second fuel utilization rate Uf2 is decreased.

[0062] 4, suppose that the amount of fuel gas supplied to the first fuel cell CS1 is "100," the first fuel utilization rate Uf1 is "70%," and the target value of the fuel utilization rate Uf of the entire system is "95.2." In this case, if the amount of fuel gas supplied to the second fuel cell CS2 via the downstream circulation passage 71 is "3," the amount of fuel gas supplied to the second fuel cell CS2 becomes "33," and the second fuel utilization rate Uf2 becomes "76.3."

[0063] If the fuel utilization rate in each of the fuel cells CS1 and CS2 is excessively high, a partial fuel gas shortage tends to occur on the fuel outlet side of each of the fuel cells CS1 and CS2, which is undesirable because it reduces the reliability of each of the fuel cells CS1 and CS2.

[0064] Considering these points, in the fuel cell system 1 of this embodiment, the control unit 100 executes a control process for improving the fuel utilization rate Uf in the entire system while ensuring the reliability of each of the fuel cells CS1, CS2. An example of the control process executed by the control unit 100 will be described below with reference to Fig. 5. Note that the control process shown in Fig. 5 is executed by the control unit 100 periodically or irregularly, for example, when each of the fuel cells CS1, CS2 is in a power generation state.

[0065] As shown in Fig. 5, the control unit 100 executes a setting process in step S100. The setting process is a process for setting various parameters including the output and power generation efficiency of each fuel cell CS1, CS2 based on the system requirements. An example of the various setting processes will be described with reference to Fig. 6.

[0066] 6, in step S101, the control unit 100 determines the output from the fuel cell system 1 to a load device (not shown) based on a system request. The system request is a required output based on the power required by the load device (not shown).

[0067] Next, in step S102, the control unit 100 sets target values ​​for the currents to be extracted from each of the fuel cells CS1, CS2 by the current adjustment unit CNV. The control unit 100 sets the target values ​​for the currents to be extracted from each of the fuel cells CS1, CS2 so that the ratio of the currents to be extracted from each of the fuel cells CS1, CS2 is the same as the ratio of the numbers of the power generation cells constituting each of the fuel cells CS1, CS2.

[0068] Next, in step S103, the control unit 100 estimates the fuel consumption in each of the fuel cells CS1, CS2. The control unit 100 estimates the fuel consumption in each of the fuel cells CS1, CS2, for example, by using a formula or a control map that predefines the relationship between the current drawn from each of the fuel cells CS1, CS2 and the fuel consumption.

[0069] Next, in step S104, the control unit 100 sets the power generation efficiency of each of the fuel cells CS1, CS2 based on the system requirements. The control unit 100 obtains the power generation efficiency of each of the fuel cells CS1, CS2 using, for example, a formula or a control map that defines the relationship between the system requirements and the power generation efficiency of each of the fuel cells CS1, CS2. The power generation efficiency is the ratio of the energy consumed by each of the fuel cells CS1, CS2 to the energy generated by each of the fuel cells CS1, CS2.

[0070] Next, in step S105, the control unit 100 sets a target value for the fuel utilization rate Uf of the entire system based on, for example, the results of estimating the fuel consumption amount and the power generation efficiency of each of the fuel cells CS1 and CS2.

[0071] Next, in step S106, the control unit 100 determines the amount of fuel gas to be supplied to the first fuel cell CS1. The control unit 100 controls various control devices of the fuel cell system 1 based on the set values ​​and the like obtained through such various setting processes.

[0072] Returning to FIG. 5, the control unit 100 proceeds to step S110, where it determines the amount of fuel gas supplied to the second fuel cell CS2 via the downstream circulation flow path 71 based on the detection result of the flow rate sensor .

[0073] Next, in step S120, the control unit 100 calculates the second fuel utilization rate Uf2. For example, the control unit 100 multiplies the amount of fuel gas supplied to the first fuel cell CS1 by the first fuel utilization rate Uf1 to calculate the amount of fuel gas discharged from the first fuel cell CS1. The control unit 100 also calculates the amount of fuel gas supplied to the second fuel cell CS2 via the downstream circulation passage 71 by adding the amount of fuel gas discharged from the first fuel cell CS1. The control unit 100 then calculates the second fuel utilization rate Uf2 based on the expected fuel consumption in the second fuel cell CS2 and the amount of fuel gas supplied to the second fuel cell CS2.

[0074] Next, in step S130, the control unit 100 determines whether the second fuel utilization rate Uf2 is within a predetermined set range. The set range is set in advance. For example, the set range is set to a range of 65% to 75%.

[0075] If the result of the determination process in step S130 is that the second fuel utilization rate Uf2 is above the set range, the control unit 100 reduces the amount of air supplied to the first fuel cell CS1 in step S140, and returns to step S110, thereby increasing the temperature of the first fuel cell CS1.

[0076] When the temperature of the first fuel cell CS1 rises, the temperature of the off-fuel discharged from the first fuel cell CS1 also rises, and the off-fuel expands and increases in speed, which increases the kinetic energy of the driving flow of the downstream ejector 72 and increases the flow rate of the fuel gas sucked from the downstream circulation passage 71 by the downstream ejector 72, thereby decreasing the second fuel utilization rate Uf2.

[0077] On the other hand, if the result of the determination process in step S130 indicates that the second fuel utilization rate Uf2 is below the set range, the control unit 100 increases the amount of air supplied to the first fuel cell CS1 in step S150, and returns to step S110, thereby decreasing the temperature of the first fuel cell CS1.

[0078] When the temperature of the first fuel cell CS1 decreases, the temperature of the off fuel discharged from the first fuel cell CS1 also decreases, and the volume and velocity of the off fuel decrease, which reduces the kinetic energy of the driving flow of the downstream ejector 72 and decreases the flow rate of the fuel gas sucked from the downstream circulation passage 71 by the downstream ejector 72, thereby increasing the second fuel utilization rate Uf2.

[0079] Furthermore, if the result of the determination process in step S130 is that the second fuel utilization factor Uf2 is within the set range, it can be determined that the fuel utilization factor Uf in the entire system is improved while ensuring reliability, and therefore this process is terminated.

[0080] The fuel cell system 1 described above includes a first fuel cell CS1, a first off-fuel flow path 51, a second fuel cell CS2 that outputs electric energy by utilizing fuel gas contained in the off-fuel flowing through the first off-fuel flow path 51, and a second off-fuel flow path 61. In addition, the fuel cell system 1 includes a downstream circulation flow path 71 that is connected to both the first off-fuel flow path 51 and the second off-fuel flow path 61 and returns a portion of the off-gas flowing through the second off-fuel flow path 61 to the first off-fuel flow path 51.

[0081] In this manner, if a portion of the off fuel discharged from the second fuel cell CS2 is returned to the second fuel cell CS2 via the first off fuel flow path 51, the unreacted fuel contained in the off fuel discharged from the second fuel cell CS2 can be consumed by the second fuel cell CS2. Therefore, in the fuel cell system 1, the fuel utilization rate Uf of the entire system can be improved compared to a conventional system that does not use the off fuel discharged from the second fuel cell CS2 for power generation.

[0082] Moreover, the fuel cell system 1 of this embodiment has the following features.

[0083] (1) The first off-fuel flow passage 51 is provided with a downstream ejector 72 that uses the off-fuel discharged from the first fuel cell CS1 as a driving flow to draw the off-fuel flowing through the downstream circulation flow passage 71 into the first off-fuel flow passage 51. In this manner, by using an ejector that is a simple fluid pump without a power unit to return the off-fuel flowing through the second off-fuel flow passage 61 to the first off-fuel flow passage 51, the configuration is less affected by power consumption than when a fluid pump with a power unit is used. This allows for high energy efficiency.

[0084] (2) The first off-fuel flow path 51 is provided with a removal unit 73 that removes, as impurities, a portion of the substances that do not contribute to power generation in the second fuel cell CS2 and are contained in the off-fuel flowing through the first off-fuel flow path 51. This can increase the fuel utilization rate in the second fuel cell CS2, thereby improving energy efficiency.

[0085] (3) The removal unit 73 is provided downstream of the downstream ejector 72 in the first off-fuel flow path 51. By providing the removal unit 73 downstream of the downstream ejector 72 in this manner, it is possible to supply off-fuel containing impurities to the downstream ejector 72 to ensure the flow rate of the driving flow of the downstream ejector 72, while supplying the off-fuel from which the impurities have been removed to the second fuel cell CS2.

[0086] (4) The removal unit 73 includes a storage unit 732 that stores impurities. In this way, if the impurities are stored in the storage unit 732, the impurities can be used for other purposes. This contributes to energy saving and simplification of the fuel cell system 1.

[0087] (5) The first fuel cell CS1 and the second fuel cell CS2 have the characteristic that oxygen ions generated by the electrochemical reaction between the fuel gas and the oxidant gas permeate from the cathode CA to the anode AN. In each of the fuel cells CS1 and CS2, the oxygen ions from the cathode CA permeate to the anode AN and are used as the driving flow of the downstream ejector 72, so that the flow rate of the driving flow of the downstream ejector 72 can be sufficiently secured and the fuel utilization rate of the second fuel cell CS2 can be increased.

[0088] (6) The fuel cell system 1 includes a control unit 100 that determines the fuel utilization rate in the second fuel cell CS2 and controls the fuel utilization rate in the entire system including both the first fuel cell CS1 and the second fuel cell CS2.

[0089] When the fuel utilization rate in the second fuel cell CS2 exceeds a predetermined set range, the control unit 100 increases the temperature of the first fuel cell CS1 to increase the kinetic energy of the off fuel supplied to the downstream ejector 72 as the driving flow. In this manner, if the configuration is such that the kinetic energy of the off fuel supplied to the downstream ejector 72 as the driving flow is increased when the fuel utilization rate in the second fuel cell CS2 is high, the amount of off fuel supplied to the second fuel cell CS2 can be appropriately adjusted. This makes it possible to prevent the fuel utilization rate in the second fuel cell CS2 from increasing excessively.

[0090] (7) When the fuel utilization rate in the second fuel cell CS2 falls below a set range, the control unit 100 lowers the temperature of the first fuel cell CS1 to reduce the kinetic energy of the off fuel supplied as the driving flow to the downstream ejector 72. This makes it possible to appropriately increase the fuel utilization rate in the second fuel cell CS2.

[0091] (Modification of the first embodiment) The fuel cell system 1 of the first embodiment described above is configured so that the fuel gas generated in the reformer 32 is supplied to the first fuel cell CS1, but is not limited to this. The fuel gas can also be generated by reforming the fuel inside the first fuel cell CS1. For this reason, the fuel cell system 1 may omit the reformer 32, for example, as shown in FIG. 7. Note that the reformer 32 can also be omitted from the fuel cell system 1 configured so that the fuel gas is supplied as fuel.

[0092] In the first embodiment described above, when the second fuel utilization rate Uf2 falls outside the set range, the amount of oxidant gas supplied to the first fuel cell CS1 is changed to change the temperature of the first fuel cell CS1, but the method of changing the temperature of the first fuel cell CS1 is not limited to this. For example, if the fuel cell system 1 includes a temperature regulation circuit that adjusts the temperature of the first fuel cell CS1, the temperature of the first fuel cell CS1 may be changed using the temperature regulation circuit when the second fuel utilization rate Uf2 falls outside the set range.

[0093] Second embodiment Next, a second embodiment will be described with reference to Figures 8 and 9. In this embodiment, differences from the first embodiment will be mainly described.

[0094] In the fuel cell system 1 of this embodiment, the control unit 100 executes a control process for changing the current drawn from each of the fuel cells CS1, CS2 so that the second fuel utilization rate Uf2 falls within a set range. An example of the control process executed by the control unit 100 will be described below with reference to Fig. 8. Note that the control process shown in Fig. 8 is executed by the control unit 100 periodically or irregularly, for example, when each of the fuel cells CS1, CS2 is in a power generation state.

[0095] 8, the control unit 100 executes various setting processes in step S100A. Since this various setting process is similar to that described in the first embodiment, the description thereof will be omitted.

[0096] The control unit 100 proceeds to step S110A, and calculates the amount of fuel gas supplied to the second fuel cell CS2 via the downstream circulation passage 71 based on the detection result of the flow rate sensor 130. Then, the control unit 100 calculates the second fuel utilization factor Uf2 in step S120A. The method of calculating the second fuel utilization factor Uf2 is the same as in the first embodiment.

[0097] Next, in step S130A, the control unit 100 determines whether or not the second fuel utilization rate Uf2 is within a predetermined set range. The set range is set in advance. For example, the set range is set to a range of 65% to 75%.

[0098] If the result of the determination process in step S130A is that the second fuel utilization rate Uf2 is above the set range, the control unit 100 in step S140A gradually increases the current drawn from the first fuel cell CS1 and gradually decreases the current drawn from the second fuel cell CS2.

[0099] The amount of change in current here is desirably determined according to the number of power generating cells in each of the fuel cells CS1 and CS2. For example, if the number of power generating cells in the first fuel cell CS1 is five times the number of power generating cells in the second fuel cell CS2, the amount of change in current in each of the fuel cells CS1 and CS2 is determined so that the current drawn from the second fuel cell CS2 is five times the current drawn from the first fuel cell CS1.

[0100] When the current drawn from the first fuel cell CS1 is increased, the first fuel utilization rate Uf1 increases. When the first fuel utilization rate Uf1 increases, the amount of water produced in the first fuel cell CS1 and the amount of oxygen ions permeating from the cathode CA to the anode AN of the first fuel cell CS1 increase. This increases the mass flow rate of off-fuel discharged from the first fuel cell CS1 to the first off-fuel flow path 51, and increases the flow rate of fuel gas sucked from the downstream circulation flow path 71 by the downstream ejector 72. As a result, the second fuel utilization rate Uf2 decreases. Furthermore, when the current drawn from the second fuel cell CS2 is decreased, the second fuel utilization rate Uf2 decreases.

[0101] On the other hand, if the result of the determination process in step S130A shows that the second fuel utilization rate Uf2 is below the set range, in step S150A, the control unit 100 gradually reduces the current drawn from the first fuel cell CS1 and gradually increases the current drawn from the second fuel cell CS2.

[0102] When the current drawn from the first fuel cell CS1 is reduced, the first fuel utilization rate Uf1 is reduced. When the first fuel utilization rate Uf1 is reduced, the amount of water produced in the first fuel cell CS1 and the amount of oxygen ions permeating from the cathode CA to the anode AN of the first fuel cell CS1 are reduced. This reduces the mass flow rate of off-fuel discharged from the first fuel cell CS1 to the first off-fuel flow path 51, and reduces the flow rate of fuel gas drawn from the downstream circulation flow path 71 by the downstream ejector 72. As a result, the second fuel utilization rate Uf2 is increased. Furthermore, when the current drawn from the second fuel cell CS2 is increased, the second fuel utilization rate Uf2 is increased.

[0103] For example, as shown in FIG. 9, the amount of fuel gas supplied to the first fuel cell CS1 is set to "100", the target value of the fuel utilization rate Uf of the entire system is set to "95.2", and further, the first fuel utilization rate Uf1 is increased to "71%" by increasing the sweep current of the first fuel cell CS1.

[0104] At this time, 71% of the fuel gas is consumed in the first fuel cell CS1. The fuel consumption in this case is "71", which is the amount of fuel gas supplied ("100") multiplied by the first fuel utilization rate Uf1 ("71"). Furthermore, the fuel gas not consumed in the first fuel cell CS1 is supplied to the second fuel cell CS2 as off fuel.

[0105] Although the supply of fuel gas from the first fuel cell CS1 to the second fuel cell CS2 decreases, the mass flow rate of the fluid flowing into the downstream ejector 72 increases due to an increase in the amount of water produced by power generation and the amount of oxygen ions permeating from the cathode CA to the anode AN.

[0106] In addition, a portion of the off-fuel in the second fuel cell CS2 is returned to the second fuel cell CS2 via the downstream circulation flow path 71 of the recycle unit 70. As the mass flow rate of the fluid flowing into the downstream ejector 72 increases, the kinetic energy of the driving flow of the downstream ejector 72 increases, and the amount of fuel gas supplied to the second fuel cell CS2 via the downstream circulation flow path 71 increases.

[0107] When the amount of fuel gas supplied from the first fuel cell CS1 to the second fuel cell CS2 is "29" and the amount of fuel gas supplied to the second fuel cell CS2 via the downstream circulation flow path 71 is "5," the amount of fuel gas supplied to the second fuel cell CS2 is "34." When the amount of fuel gas supplied to the second fuel cell CS2 increases to "34," the second fuel utilization factor Uf2 decreases to "71."

[0108] In addition, when the result of the determination process in step S130A indicates that the second fuel utilization ratio Uf2 is within the set range, the control unit 100 proceeds to step S160A and determines whether the first fuel utilization ratio Uf1 is within a predetermined reference set range. This reference set range may be the same as or different from the "set range" described in step S130A.

[0109] If the result of the determination process in step S160A indicates that the first fuel utilization ratio Uf1 is above the reference setting range, the control unit 100 increases the amount of fuel gas supplied to the first fuel cell CS1 in step S170A, thereby decreasing the first fuel utilization ratio Uf1.

[0110] On the other hand, if the result of the determination process in step S160A indicates that the first fuel utilization ratio Uf1 is below the reference setting range, the control unit 100 reduces the amount of fuel gas supplied to the first fuel cell CS1 in step S180A, thereby increasing the first fuel utilization ratio Uf1.

[0111] Furthermore, if the result of the determination process in step S160A is that the first fuel utilization factor Uf1 is within the reference setting range, it can be determined that the fuel utilization factor Uf in the entire system is improved while ensuring reliability, so this process is terminated.

[0112] The rest is the same as in the first embodiment. The fuel cell system 1 of the present embodiment can obtain the same effects as in the first embodiment, which are achieved by a configuration common to the first embodiment or an equivalent configuration.

[0113] Moreover, the fuel cell system 1 of this embodiment has the following features.

[0114] (1) The second fuel cell CS2 has a characteristic that oxygen ions generated by an electrochemical reaction between the fuel gas and the oxidant gas permeate from the cathode CA to the anode AN. The control unit 100 reduces the current drawn from the second fuel cell CS2 when the fuel utilization rate of the second fuel cell CS2 exceeds a set range. In addition, the control unit 100 increases the current drawn from the first fuel cell CS1 to increase the amount of oxygen ions permeating from the cathode CA to the anode AN, thereby increasing the mass flow rate of the off fuel supplied to the downstream ejector 72. This also makes it possible to appropriately adjust the amount of off fuel supplied to the second fuel cell CS2 via the downstream circulation passage 71, thereby preventing the fuel utilization rate in the second fuel cell CS2 from increasing excessively.

[0115] (2) When the fuel utilization rate of the second fuel cell CS2 falls below a set range, the control unit 100 increases the current drawn from the second fuel cell CS2 and decreases the current drawn from the first fuel cell CS1. This reduces the amount of oxygen ions passing from the cathode CA to the anode AN and reduces the mass flow rate of the off fuel supplied to the downstream ejector 72. This makes it possible to appropriately increase the fuel utilization rate in the second fuel cell CS2.

[0116] (3) The control unit 100 determines the fuel utilization rate in the second fuel cell CS2 as the second fuel utilization rate Uf2, and determines the fuel utilization rate in the first fuel cell CS1 as the first fuel utilization rate Uf1. Then, when the second fuel utilization rate Uf2 is within a set range and the first fuel utilization rate Uf1 exceeds a reference predetermined range, the control unit 100 increases the amount of fuel gas supplied to the first fuel cell CS1. This makes it possible to keep the fuel utilization rate in the second fuel cell CS2 within an appropriate range while preventing the fuel utilization rate in the first fuel cell CS1 from increasing excessively.

[0117] (4) When the second fuel utilization rate Uf2 is within the reference set range and the first fuel utilization rate Uf1 is below the reference predetermined range, the control unit 100 reduces the amount of fuel gas supplied to the first fuel cell CS1. This makes it possible to appropriately increase the fuel utilization rate of the first fuel cell CS1 while keeping the fuel utilization rate of the second fuel cell CS2 within an appropriate range.

[0118] Third embodiment Next, a third embodiment will be described with reference to Fig. 10. In this embodiment, differences from the first embodiment will be mainly described.

[0119] 10, the fuel cell system 1 is provided with a gas circulation unit 80 that returns a portion of the off-fuel that has passed through the first fuel cell CS1 to the upstream side of the first fuel cell CS1. The gas circulation unit 80 includes an upstream circulation flow path 81, an upstream ejector 82, and a flow rate adjustment unit 83.

[0120] The upstream circulation flow path 81 is connected to both the fuel supply flow path 30, through which the fuel gas to be supplied to the first fuel cell CS1 flows, and the first off-fuel flow path 51, and is a flow path that returns a portion of the off-fuel flowing through the first off-fuel flow path 51 to the fuel supply flow path 30. One end of the upstream circulation flow path 81 is connected to the first off-fuel flow path 51, and the other end is connected to the fuel supply flow path 30.

[0121] Specifically, the upstream circulation flow path 81 is connected to the first off-fuel flow path 51 between the downstream ejector 72 and the removal unit 73. Although not shown, the upstream circulation flow path 81 is connected to the fuel supply flow path 30 upstream of the desulfurizer.

[0122] The upstream ejector 82 is provided in the fuel supply flow passage 30, and uses the fuel gas supplied to the first fuel cell CS1 as a driving flow to suck the off-fuel flowing through the upstream circulation flow passage 81 into the fuel supply flow passage 30. Specifically, the upstream ejector 82 is provided at the junction of the fuel supply flow passage 30 with the upstream circulation flow passage 81. The specific configuration of the upstream ejector 82 is similar to that of the downstream ejector 72, and therefore a description thereof will be omitted.

[0123] The flow rate adjustment unit 83 is provided in the upstream circulation flow path 81, and adjusts the flow rate of the off-fuel flowing through the upstream circulation flow path 81. The flow rate adjustment unit 83 is configured to include, for example, a variable throttle mechanism that can change the throttle opening continuously or discontinuously. The flow rate adjustment unit 83 is configured to be able to fully close the upstream circulation flow path 81. The flow rate adjustment unit 83 is configured with an electric valve whose throttle opening is adjusted based on a control signal from the control unit 100. Note that the flow rate adjustment unit 83 may be configured with a fixed throttle and an electromagnetic opening / closing valve.

[0124] The flow rate adjustment unit 83 can function not only as an adjustment means for adjusting the amount of fuel sucked from the upstream circulation passage 81 to the upstream ejector 82, but also as an adjustment means for adjusting the amount of off-fuel supplied from the first off-fuel passage 51 to the second fuel cell CS2.

[0125] The rest is the same as in the first embodiment. The fuel cell system 1 of the present embodiment can obtain the same effects as in the first embodiment, which are achieved by a configuration common to the first embodiment or an equivalent configuration.

[0126] Moreover, the fuel cell system 1 of this embodiment has the following features.

[0127] (1) The fuel cell system 1 includes an upstream circulation flow path 81 that returns a portion of the off-fuel flowing through the first off-fuel flow path 51 to the fuel supply flow path 30. This improves the fuel utilization rate of the first fuel cell CS1, thereby making it possible to achieve even higher efficiency.

[0128] (2) The fuel cell system 1 includes an upstream ejector 82 that uses the fuel gas supplied to the first fuel cell CS1 as a driving flow to draw the off-fuel flowing through the upstream circulation flow path 81 into the fuel supply flow path 30. With this, if the ejector, which is a simple fluid pump without a power unit, is used to return the off-fuel flowing through the first off-fuel flow path 51 to the fuel supply flow path 30, it is not affected by power consumption compared to the case where a fluid pump with a power unit is used. Therefore, it is possible to achieve high energy efficiency.

[0129] (3) In the fuel cell system 1, the upstream circulation flow path 81 is provided with a flow rate adjuster 83 that adjusts the flow rate of the off-fuel flowing through the upstream circulation flow path 81. This makes it possible to adjust the fuel utilization rate of the first fuel cell CS1 to an appropriate amount by adjusting the flow rate of the off-fuel returned from the first off-fuel flow path 51 to the fuel supply flow path 30. In particular, when the upstream circulation flow path 81 is connected downstream of the downstream ejector 72, the flow rate adjuster 83 makes it possible to adjust the flow rate of the driving flow of the downstream ejector 72 to an appropriate amount.

[0130] (Other embodiments) Representative embodiments of the present disclosure have been described above, but the present disclosure is not limited to the above-described embodiments and can be modified in various ways, for example, as described below.

[0131] As in the above-described embodiment, it is desirable for the fuel cell system 1 to have the upstream ejector 82 disposed in the fuel supply passage 30 and the downstream ejector 72 disposed in the first off fuel passage 51, but is not limited thereto. For example, the fuel cell system 1 may have a fluid pump having a power unit disposed in the fuel supply passage 30 and the first off fuel passage 51 instead of the upstream ejector 82 and the downstream ejector 72.

[0132] As in the above-described embodiment, it is preferable that the fuel cell system 1 is provided with a removal section 73 including a storage section 732 in the first off fuel flow passage 51, but this is not essential.

[0133] As in the above-described embodiment, it is desirable that each fuel cell CS1, CS2 is configured to have the property of allowing oxygen ions generated by the electrochemical reaction between the fuel gas and the oxidant gas to permeate from the cathode CA to the anode AN, but this is not necessarily required.

[0134] The system configuration of the fuel cell system 1 shown in the above embodiment is an example, and some of the configuration may be different from that described above. The same applies to the control processing executed by the control unit 100.

[0135] It is desirable that the fuel cell system 1 is configured so that the control unit 100 executes a control process for controlling the second fuel utilization rate Uf2 described in the first and second embodiments within a set range, but this is not necessarily required.

[0136] In the above-described embodiments, it goes without saying that the elements constituting the embodiments are not necessarily essential, except in cases where it is particularly expressly stated that they are essential or where they are clearly considered to be essential in principle.

[0137] In the above-described embodiments, when numerical values ​​such as the number, values, amounts, ranges, etc. of components of the embodiments are mentioned, they are not limited to the specific numbers unless it is specifically stated as required or unless it is clearly limited to a specific number in principle.

[0138] In the above-described embodiments, when referring to the shapes, positional relationships, etc. of components, etc., there is no limitation to those shapes, positional relationships, etc., unless specifically stated or in principle limited to a specific shape, positional relationship, etc.

[0139] The control unit and the method of the present disclosure may be implemented in a special-purpose computer provided by configuring a processor and a memory programmed to execute one or more functions embodied in a computer program. The control unit and the method of the present disclosure may be implemented in a special-purpose computer provided by configuring a processor with one or more dedicated hardware logic circuits. The control unit and the method of the present disclosure may be implemented in one or more special-purpose computers configured by a combination of a processor and a memory programmed to execute one or more functions and a processor configured with one or more hardware logic circuits. The computer program may also be stored in a computer-readable non-transitory tangible recording medium as instructions executed by a computer.

[0140] [Points of view of the present disclosure] [First viewpoint] 1. A fuel cell system comprising: a first fuel cell (CS1) that outputs electrical energy by an electrochemical reaction between a fuel gas and an oxidant gas; a first off-fuel flow path (51) through which off-fuel discharged from the first fuel cell flows; a second fuel cell (CS2) that outputs electrical energy by utilizing a fuel gas contained in the off-fuel flowing through the first off-fuel flow passage; a second off-fuel flow path (61) through which off-fuel discharged from the second fuel cell flows; a downstream circulation flow path (71) connected to both the first off-fuel flow path and the second off-fuel flow path and returning a portion of the off-gas flowing through the second off-fuel flow path to the first off-fuel flow path.

[0141] [Second viewpoint] The fuel cell system according to a first aspect, further comprising: a downstream ejector (72) provided in the first off-fuel flow path that uses off-fuel discharged from the first fuel cell as a driving flow to draw off-fuel flowing through the downstream circulation flow path into the first off-fuel flow path.

[0142] [Third Perspective] The fuel cell system according to a second aspect, further comprising a removal section (73) provided in the first off-fuel flow path that removes, as impurities, a portion of substances that do not contribute to power generation in the second fuel cell and are contained in the off-fuel flowing through the first off-fuel flow path.

[0143] [Fourth viewpoint] The fuel cell system according to a third aspect, wherein the removal unit is provided downstream of the downstream ejector in the first off-fuel flow channel.

[0144] [Fifth viewpoint] The fuel cell system according to the third or fourth aspect, wherein the removal unit includes a storage unit (732) that stores the impurities.

[0145] [Sixth viewpoint] The fuel cell system according to any one of the second to fifth aspects, wherein the first fuel cell and the second fuel cell have a characteristic that oxygen ions generated by an electrochemical reaction between a fuel gas and an oxidant gas are transmitted from the cathode to the anode.

[0146] [Seventh viewpoint] a control unit (100) that determines a fuel utilization rate in the second fuel cell and controls a fuel utilization rate in an entire system including the first fuel cell and the second fuel cell, The fuel cell system according to any one of the second to sixth aspects, wherein the control unit increases the temperature of the first fuel cell when a fuel utilization rate in the second fuel cell exceeds a predetermined set range, thereby increasing the kinetic energy of the off fuel supplied to the downstream ejector as a driving flow.

[0147] [Eighth viewpoint] The fuel cell system described in a seventh aspect, wherein the control unit reduces the temperature of the first fuel cell when a fuel utilization rate in the second fuel cell falls below the set range, thereby reducing the kinetic energy of the off fuel supplied to the downstream ejector as a driving flow.

[0148] [Ninth viewpoint] a control unit (100) for determining a fuel utilization rate in the second fuel cell and controlling a fuel utilization rate in an entire system including the first fuel cell and the second fuel cell, the second fuel cell has a characteristic that oxygen ions generated by an electrochemical reaction between a fuel gas and an oxidant gas permeate from a cathode to an anode; The fuel cell system according to any one of the second to eighth aspects, wherein the control unit reduces the current extracted from the second fuel cell and increases the current extracted from the first fuel cell when a fuel utilization rate of the second fuel cell exceeds a predetermined set range, thereby increasing the amount of oxygen ions passing from the cathode to the anode, thereby increasing the mass flow rate of off fuel supplied to the downstream ejector.

[0149] [10th viewpoint] The fuel cell system of a ninth aspect, wherein the control unit increases the current extracted from the second fuel cell and decreases the current extracted from the first fuel cell when a fuel utilization rate of the second fuel cell falls below the set range, thereby reducing the amount of oxygen ions passing from the cathode to the anode, thereby reducing the mass flow rate of off fuel supplied to the downstream ejector.

[0150] [Eleventh viewpoint] The control unit is configured to calculate a fuel utilization rate in the second fuel cell as a second fuel utilization rate and to calculate a fuel utilization rate in the first fuel cell as a first fuel utilization rate, and when the second fuel utilization rate is within the set range, if the first fuel utilization rate exceeds a predetermined reference range, the control unit increases the amount of fuel gas supplied to the first fuel cell.

[0151] [12th viewpoint] A fuel cell system as described in an eleventh aspect, wherein the control unit reduces the amount of fuel gas supplied to the first fuel cell when the second fuel utilization rate is within the set range and the first fuel utilization rate falls below the reference specified range.

[0152] [13th viewpoint] The fuel cell system according to any one of the first to twelfth aspects, further comprising an upstream circulation flow path (81) connected to both a fuel supply flow path (30) through which fuel gas to be supplied to the first fuel cell flows and the first off-fuel flow path, and configured to return a portion of the off-fuel flowing through the first off-fuel flow path to the fuel supply flow path.

[0153] [14th viewpoint] A fuel cell system according to a thirteenth aspect, further comprising an upstream ejector (82) provided in the fuel supply flow path and configured to draw off-fuel flowing through the upstream circulation flow path into the fuel supply flow path using a fuel gas supplied to the first fuel cell as a driving flow.

[0154] [15th viewpoint] The fuel cell system according to a thirteenth or fourteenth aspect, further comprising a flow rate regulator (83) provided in the upstream circulation flow path to regulate a flow rate of off-fuel flowing through the upstream circulation flow path. [Explanation of symbols]

[0155] 1. Fuel cell system 51 First off fuel passage 61 Second off fuel passage 71 Downstream circulation channel CS1 1st fuel cell CS2 2nd fuel cell

Claims

1. 1. A fuel cell system comprising: a first fuel cell (CS1) that outputs electrical energy by an electrochemical reaction between a fuel gas and an oxidant gas; a first off-fuel flow passage (51) through which off-fuel discharged from the first fuel cell flows; a second fuel cell (CS2) that outputs electric energy by utilizing a fuel gas contained in the off-fuel flowing through the first off-fuel flow passage; a second off-fuel flow passage (61) through which off-fuel discharged from the second fuel cell flows; a downstream circulation flow path (71) connected to both the first off-fuel flow path and the second off-fuel flow path for returning a portion of the off-gas flowing through the second off-fuel flow path to the first off-fuel flow path.

2. 2. The fuel cell system according to claim 1, further comprising a downstream ejector (72) provided in the first off-fuel flow path, which uses the off-fuel discharged from the first fuel cell as a driving flow and draws the off-fuel flowing through the downstream circulation flow path into the first off-fuel flow path.

3. 3. The fuel cell system according to claim 2, further comprising a removal section (73) provided in the first off-fuel flow path that removes, as impurities, a portion of substances contained in the off-fuel flowing through the first off-fuel flow path that do not contribute to power generation in the second fuel cell.

4. 4. The fuel cell system according to claim 3, wherein the removal unit is provided downstream of the downstream ejector in the first off-fuel flow channel.

5. 5. The fuel cell system according to claim 3, wherein the removal section includes a storage section (732) for storing the impurities.

6. 4. The fuel cell system according to claim 2, wherein the first fuel cell and the second fuel cell have a characteristic that oxygen ions generated by an electrochemical reaction between a fuel gas and an oxidant gas permeate from the cathode to the anode.

7. a control unit (100) for determining a fuel utilization rate in the second fuel cell and controlling a fuel utilization rate in an entire system including the first fuel cell and the second fuel cell, 5. The fuel cell system according to claim 2, wherein the control unit increases the temperature of the first fuel cell to increase the kinetic energy of the off fuel supplied to the downstream ejector as a driving flow when a fuel utilization rate in the second fuel cell exceeds a predetermined set range.

8. 8. The fuel cell system of claim 7, wherein the control unit reduces the temperature of the first fuel cell to reduce the kinetic energy of the off fuel supplied to the downstream ejector as a driving flow when a fuel utilization rate in the second fuel cell falls below the set range.

9. a control unit (100) for determining a fuel utilization rate in the second fuel cell and controlling a fuel utilization rate in an entire system including the first fuel cell and the second fuel cell, the second fuel cell has a characteristic that oxygen ions generated by an electrochemical reaction between a fuel gas and an oxidant gas permeate from a cathode to an anode; 5. The fuel cell system according to claim 2, wherein the control unit reduces the current drawn from the second fuel cell and increases the current drawn from the first fuel cell when the fuel utilization rate of the second fuel cell exceeds a predetermined set range, thereby increasing the mass flow rate of off fuel supplied to the downstream ejector by increasing the amount of oxygen ions passing from the cathode to the anode.

10. 10. The fuel cell system of claim 9, wherein the control unit increases the current drawn from the second fuel cell and decreases the current drawn from the first fuel cell when a fuel utilization rate of the second fuel cell falls below the set range, thereby reducing the mass flow rate of off fuel supplied to the downstream ejector by reducing the amount of oxygen ions passing from the cathode to the anode.

11. 10. The fuel cell system of claim 9, wherein the control unit is configured to calculate a fuel utilization rate in the second fuel cell as a second fuel utilization rate and to calculate a fuel utilization rate in the first fuel cell as a first fuel utilization rate, and when the second fuel utilization rate is within the set range, if the first fuel utilization rate exceeds a predetermined reference range, the control unit increases the amount of fuel gas supplied to the first fuel cell.

12. 12. The fuel cell system according to claim 11, wherein the control unit reduces the amount of fuel gas supplied to the first fuel cell when the second fuel utilization rate is within the set range and the first fuel utilization rate falls below the reference predetermined range.

13. 3. The fuel cell system of claim 1, further comprising an upstream circulation flow path (81) connected to both a fuel supply flow path (30) through which fuel gas to be supplied to the first fuel cell flows and the first off-fuel flow path, for returning a portion of the off-fuel flowing through the first off-fuel flow path to the fuel supply flow path.

14. 14. The fuel cell system according to claim 13, further comprising an upstream ejector (82) provided in the fuel supply flow path and configured to draw off-fuel flowing through the upstream circulation flow path into the fuel supply flow path using the fuel gas supplied to the first fuel cell as a driving flow.

15. 15. The fuel cell system according to claim 14, further comprising a flow rate adjusting section (83) provided in the upstream circulation flow passage and adjusting a flow rate of the off-fuel flowing through the upstream circulation flow passage.

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