Cell Stack System

Abstract

To prevent fuel starvation from continuing even if fuel starvation occurs due to drift. [Solution] The cell stack system 1 includes multiple cell stacks, a supply pipe 21, and an exhaust pipe 22. Each cell stack is electrically connected in series to a circuit including a power source or a load, in groups of one or more cell stacks. The supply pipes 21 have multiple inlets 21a to 21c through which supply gases are introduced, and the exhaust pipes 22 have multiple outlets 22a to 22c through which exhaust gases are discharged. The cell stack system 1 is configured to be able to change the supply flow rate ratio, which is the flow rate ratio of the supply gases introduced into the inlets 21a to 21c, and the exhaust flow rate ratio, which is the flow rate ratio of the exhaust gases discharged from the outlets 22a to 22c, based on voltage changes in each circuit.

Description

[Technical Field]

[0001] The present invention relates to a cell stack system including a plurality of cell stacks each including a plurality of solid oxide electrochemical unit cells. [Background technology]

[0002] Conventionally, cell stack systems comprising multiple cell stacks have been known. The multiple solid oxide electrochemical unit cells that make up the cell stack are characterized by performing electrochemical reactions with high efficiency in high-temperature environments, and are broadly classified into solid oxide electrolysis cells (SOECs) and solid oxide fuel cells (SOFCs) (hereinafter, multiple electrochemical unit cells will also be referred to as "electrochemical cells"). SOECs have the function of decomposing fuel gas using electrical energy, while SOFCs have the function of generating electrical energy by combining fuel gas with oxidant gas. When the electrochemical cells are used as SOECs (hereinafter, also referred to as "SOEC mode"), the cell stack system functions as an electrolyzer, and when the electrochemical cells are used as SOFCs (hereinafter, also referred to as "SOFC mode"), the cell stack system functions as a power generation device.

[0003] In a cell stack system, each cell stack is connected in parallel to a gas supply pipe, so that gas introduced into the supply pipe (supply gas) is distributed and supplied to the corresponding cell stack via multiple branch pipes.

[0004] The flow rate of the distributed gas supplied to each cell stack tends to decrease as the cell stack connection position (more precisely, the position where the cell stack is connected to the supply pipe via the branch pipe) increases from the inlet of the supply pipe. This is because the pressure loss in the supply pipe increases the farther the connection position from the inlet. This phenomenon of uneven distribution of gas supplied to multiple cell stacks is called "biased flow." In a cell stack system with uneven flow, if the same amount of current is applied to each cell stack in SOEC mode, cell stacks with low distributed gas flow rates will experience excessive current flow relative to the flow rate, resulting in fuel starvation. Similarly, if an attempt is made to extract the same amount of power from each cell stack in SOFC mode, cell stacks with low distributed gas flow rates will experience excessive power generation relative to the flow rate, resulting in excessive current flow, resulting in fuel starvation. In either mode, continued fuel starvation can have adverse effects on the cell stacks (e.g., significant degradation of the electrochemical cell performance). In order to prevent fuel starvation in a cell stack system where uneven flow occurs, it is conceivable to adjust the applied current value or amount of power to match the cell stack with the lowest flow rate of distributed gas. However, this method is not rational because, for cell stacks with a relatively high flow rate of distributed gas, electrolysis or power generation commensurate with the flow rate is not performed (i.e., electrolysis efficiency or power generation efficiency is low), and the performance of the cell stack system as a whole cannot be maximized.

[0005] For this reason, studies are being conducted to eliminate uneven flow and supply the distributed gas evenly to each cell stack. For example, Non-Patent Document 1 describes that the flow rate of the distributed gas is made uniform by providing a pressure loss adjustment component (typically an orifice) in the branch pipe. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] Yoshinobu Takagi and seven others, "Development of Industrial Power Generation Devices Using Solid Oxide Fuel Cells," Hitz Technical Report, September 2015, Vol. 76, No. 1, pp. 11-15 Summary of the Invention

[0007] However, the flow rate ratio of the distributed gas supplied to each cell stack depends not only on the pressure loss in the supply pipe but also on the flow rate of the supply gas introduced into the supply pipe and the temperature distribution of all the cell stacks included in the cell stack system. Therefore, even if pressure loss adjustment components are installed, the distributed gas may not be supplied evenly if the supply gas flow rate or the temperature distribution of the cell stack changes. Cell stack systems have three operating modes: a mode in which the system operates continuously in SOEC mode only, a mode in which the system operates continuously in SOFC mode only, and a mode in which the system operates reversibly by switching between SOEC mode and SOFC mode as appropriate. However, in all operating modes, the supply gas flow rate and the temperature distribution of the cell stack may change. In particular, in the third reversible operation mode, the supply gas flow rate and the temperature distribution of the cell stack differ significantly between SOEC mode and SOFC mode. For this reason, the technology described in Non-Patent Document 1 has difficulty in adequately eliminating the uneven flow. There is a need for a cell stack system that can prevent fuel starvation from continuing even if it occurs due to uneven flow.

[0008] The present invention has been made to address the above-mentioned problems. That is, one of the objects of the present invention is to provide a cell stack system that can prevent fuel starvation from continuing even if a drift occurs and fuel starvation occurs.

[0009] The cell stack system according to the present invention comprises: a plurality of cell stacks (S), each of which includes a plurality of solid oxide electrochemical unit cells (10) including a solid electrolyte layer (12), an air electrode (14) stacked on the front side of the solid electrolyte layer, and an anode (16) stacked on the back side of the solid electrolyte layer, the plurality of cell stacks (S) having a supply path (Pfi) for supplying gas to a fuel chamber (Sf), which is a space on the anode side, and an exhaust path (Pfo) for exhausting gas from the fuel chamber; a supply pipe (21) through which a supply gas can flow and through which the supply paths of the plurality of cell stacks are connected in parallel; an exhaust pipe (22) through which exhaust gas can flow and to which the exhaust paths of the plurality of cell stacks are connected in parallel; Equipped with The cell stacks are electrically connected in series to a circuit (C) including a power source (DC) or a load (I) for each of a plurality of groups (G) including one or more of the cell stacks, The supply pipe has a plurality of inlets (21a to 21c) through which the supply gas is introduced, The exhaust pipe has a plurality of outlets (22a to 22c) from which the exhaust gas is discharged, The supply flow rate ratio, which is the flow rate ratio of the supply gas introduced into each of the inlets, and the exhaust flow rate ratio, which is the flow rate ratio of the exhaust gas discharged from each of the outlets, are configured to be changeable based on voltage changes in each of the circuits.

[0010] In this cell stack system, the supply gas introduced into the supply pipe is distributed and supplied to the fuel chambers via the supply paths of each cell stack. Hereinafter, the gas distributed and supplied to the fuel chambers of each cell stack will also be referred to as "distributed gas." The flow rate of the distributed gas changes with changes in the temperature of the cell stack. For example, when the temperature of any given cell stack rises, the temperature of the distributed gas supplied to that cell stack rises and its viscosity increases, thereby decreasing the flow rate of the distributed gas. The flow rate of the distributed gas also changes depending on the flow rate of the supply gas introduced into the supply pipe.

[0011] The flow rate of the distributed gas used in the electrochemical reaction in each cell stack depends on the current value flowing through the cell stack (in other words, the current value flowing through the circuit to which each cell stack is connected). Therefore, when the current value is constant, the flow rate of the distributed gas used in the electrochemical reaction is constant. In such a case, if the flow rate of the distributed gas supplied to the cell stack changes due to changes in the temperature of the cell stack and / or the flow rate of the supplied gas, the distribution gas utilization rate also changes. Here, the "distributed gas utilization rate" is the flow rate ratio of the "distributed gas actually used in the electrochemical reaction" to the "distributed gas supplied to each cell stack." Specifically, when the current value is constant, an increase in the flow rate of the distributed gas supplied to the cell stack decreases the distribution gas utilization rate, and a decrease in the flow rate of the supplied distributed gas increases the distribution gas utilization rate. Generally, an upper limit is set for the distribution gas utilization rate to ensure stable long-term operation of the cell stack. If this upper limit is exceeded, fuel starvation begins. When fuel starvation begins, the cell stack voltage changes. Specifically, the voltage increases in SOEC mode and decreases in SOFC mode. If the fuel starvation state continues, the characteristics of the electrochemical cell will deteriorate significantly, causing a sudden change in voltage.

[0012] Therefore, in this cell stack system, the supply pipe has multiple inlets and the exhaust pipe has multiple outlets, and the supply flow rate ratio (the flow rate ratio of the supply gas introduced into each inlet) and the exhaust flow rate ratio (the flow rate ratio of the exhaust gas discharged from each outlet) are configured to be changeable based on voltage changes in the circuits of each group. With this configuration, when a decrease in the flow rate of the distribution gas supplied to a given cell stack causes the utilization rate of the distribution gas in that cell stack to increase and exceed its upper limit, causing a change in circuit voltage, the supply flow rate ratio and / or the exhaust flow rate ratio are changed based on the voltage change. By appropriately changing the supply flow rate ratio and / or the exhaust flow rate ratio, the flow rate of the distribution gas supplied to that cell stack is likely to increase, making it more likely that the utilization rate of the distribution gas can be reduced to below its upper limit. Therefore, the cell stack system according to the present invention can prevent fuel starvation from continuing even if it occurs due to uneven flow. As a result, significant deterioration of the electrochemical cell characteristics can be suppressed. That is, in the present invention, the occurrence of fuel starvation is detected from a change in the circuit voltage, and the supply flow rate ratio and / or the discharge flow rate ratio are changed based on the change in voltage, thereby preventing the continuation of fuel starvation.

[0013] Furthermore, in this cell stack system, instead of all cell stacks being electrically connected in series to a single circuit, multiple groups are electrically connected in series to the circuit. A group includes one or multiple cell stacks (here, "multiple" refers to a number less than the total number of cell stacks in the system). The circuit is configured to include a power source in SOEC mode and a load in SOFC mode. This configuration allows the current value to be set for each circuit based on the supply flow rate ratio and / or the exhaust flow rate ratio. Therefore, compared to a configuration in which all cell stacks are connected to a single circuit and the same current value flows, the electrolysis efficiency or power generation efficiency of each cell stack can be improved. The exhaust flow rate ratio can be changed based on voltage changes because the pressure loss in the supply pipe is also affected by the exhaust flow rate ratio.

[0014] The supply flow rate ratio can be changed by controlling the opening and closing of multiple inlets of the supply pipe or by controlling the opening ratio of the inlets. The supply flow rate ratio may be changed by changing the flow rate ratio of the supply gas introduced into each inlet while maintaining the total flow rate of the supply gas, or by changing the total flow rate of the supply gas (for example, by introducing additional supply gas into a certain inlet). On the other hand, the exhaust flow rate ratio can be changed by controlling the opening and closing of multiple outlets of the exhaust pipe or by controlling the opening ratio of the outlets.

[0015] Furthermore, the task of changing the supply flow rate ratio and the discharge flow rate ratio based on the voltage change of each circuit may be configured to be performed by an operator or by a control device.

[0016] In one aspect of the invention, The cell stack system (1) is The system further includes a control device (20) electrically connected to the circuits (C) of each of the groups (G), the inlets (21a to 21c) and the outlets (22a to 22c). The control device When a first condition is met when the rate of change (Re) of voltage in a predetermined time (ΔTe) of any of the circuits (C) including the power supply (DC) becomes equal to or greater than a predetermined first threshold (Reth), or when a second condition is met when the rate of change (Rg) of voltage in a predetermined time (ΔTg) of any of the circuits (C) including the load (I) becomes equal to or less than a predetermined second threshold (Rgth), the supply flow rate ratio and / or the discharge flow rate ratio are changed by controlling each of the inlet ports and / or each of the outlet ports.

[0017] Because each cell stack that makes up a cell stack system gradually deteriorates with use, over the long term, the voltage increases in SOEC mode and decreases in SOFC mode. According to one aspect of the configuration of the present invention, the control device determines whether the first and second conditions are met based on the rate of change of voltage over a predetermined period of time. In this way, by determining whether the first and second conditions are met based on the rate of change of voltage over a predetermined period of time rather than simply on the rate of change of voltage, it is possible to appropriately distinguish whether a voltage change is due to fuel starvation or aging degradation, thereby improving the accuracy of fuel starvation detection.

[0018] In one aspect of the invention, The control device (20) When the supply flow rate ratio is changed due to the first condition or the second condition being satisfied, each of the plurality of inlets (21a to 21c) is controlled so that the flow rate of the supply gas introduced into the inlet closest to the circuit (C) that satisfies the first condition or the second condition is increased. The inlet closest to the circuit that satisfies the first condition or the second condition is defined as the inlet with the shortest sum of distances from the connection position (pu) where one or more of the cell stacks (S) connected to the circuit are connected to the supply pipe (21) to the inlet.

[0019] According to this configuration, the flow rate of the distributed gas supplied to the cell stack connected to the circuit that satisfies the first or second condition increases quickly, so that the utilization rate of the distributed gas can be quickly reduced, and as a result, the continuation of fuel depletion can be more appropriately suppressed.

[0020] In one aspect of the invention, The supply pipe (21) includes at least a first inlet (21a) that is arranged closer to one end than a connection position (pu11) at which a cell stack (S11) arranged on one end of the plurality of cell stacks (S) is connected to the supply pipe; a second inlet (21b) that is arranged on the other end side of the connection position (pun3) of a cell stack (Sn3) that is arranged on the other end side of the plurality of cell stacks; Equipped with.

[0021] According to this configuration, the supply flow rate ratio and the discharge flow rate ratio can be appropriately controlled.

[0022] In the above description, in order to facilitate understanding of the invention, the symbols used in the embodiments are added in parentheses to the constituent elements of the invention corresponding to the embodiments, but each constituent element of the invention is not limited to the embodiments defined by the symbols. [Brief explanation of the drawings]

[0023] [Figure 1] FIG. 2 is a block diagram of a cell stack system according to an embodiment of the present invention when operated in an SOEC mode. [Figure 2] FIG. 1 is a block diagram of a cell stack system operated in an SOFC mode. [Figure 3] FIG. 2 is a perspective view of a cell stack. [Figure 4] FIG. 4 is a cross-sectional view taken along line IV-IV in FIG. [Figure 5] 4 is a cross-sectional view in the thickness direction of a single cell included in the electrochemical unit of FIG. 3. FIG. [Figure 6] 10 is a graph showing the change over time in the voltage Ve of an arbitrary circuit Ck in the SOEC mode. [Figure 7] 10 is a graph showing the change over time in voltage Vg of an arbitrary circuit Ck in the SOFC mode. DETAILED DESCRIPTION OF THE INVENTION

[0024] A cell stack system according to an embodiment of the present invention will now be described with reference to the drawings. FIGS. 1 and 2 are block diagrams of a cell stack system 1. The system 1 is configured to be reversibly operable by switching between SOEC mode and SOFC mode at predetermined timing. FIG. 1 illustrates a case in which the system 1 is operated in SOEC mode, and FIG. 2 illustrates a case in which the system 1 is operated in SOFC mode. FIG. 2 differs from FIG. 1 only in the configuration of circuit C. Therefore, hereinafter, FIG. 1 will be described first, and then FIG. 2 will be described focusing on the differences from FIG. 1.

[0025] As shown in FIG. 1, the system 1 includes 3n cell stacks S, a control device 20, a supply pipe 21, a discharge pipe 22, n circuits C, and n voltmeters VM. The 3n cell stacks S are divided into n groups Gk (k: an integer from 1 to n), and each group Gk includes three cell stacks Sk1 to Sk3. The three cell stacks Sk1 to Sk3 in each group Gk are electrically connected in series to a circuit Ck. In the SOEC mode, the circuit Ck is configured to include a DC power supply DCk. The DC power supply DCk is an example of a "power supply." In this specification, the letter "k" is added when it is necessary to clarify the correspondence between components, and the letter "k" is omitted otherwise.

[0026] The supply pipe 21 is a pipe for distributing the supply gas to the fuel electrodes 16 (described later) of each cell stack S and supplying it as a distributed gas. In this embodiment, the supply pipe 21 extends in a straight line and has three inlets 21a to 21c. The inlet 21a is provided at one end (the left end in FIG. 1) of the supply pipe 21. The inlet 21b is provided at the other end (the right end in FIG. 1) of the supply pipe 21. The inlet 21c is provided midway along the supply pipe 21 (described later). The opening ratio of each of the inlets 21a to 21c is changeable. The supply gas is introduced into all of the inlets 21a to 21c except for those in a closed state. In the SOEC mode, the supply gas contains a fuel gas and a reducing gas (a gas for reducing oxidation of the catalyst contained in the fuel electrodes 16).

[0027] The exhaust pipe 22 is a pipe for collecting exhaust gas discharged from the fuel electrode 16 of each cell stack S. In this embodiment, the exhaust pipe 22 extends linearly and substantially parallel to the supply pipe 21, and has three discharge ports 22a to 22c. The discharge port 22a is provided at one end (the left end in FIG. 1) of the exhaust pipe 22. The discharge port 22b is provided at the other end (the right end in FIG. 1) of the exhaust pipe 22. The discharge port 22c is provided midway along the exhaust pipe 22 (described later). The opening ratio of each of the discharge ports 22a to 22c is changeable. The exhaust gas is discharged from all of the discharge ports 22a to 22c except for those in a closed state.

[0028] The cell stacks S are arranged in a row such that the cell stacks Si1 to Si3 in group Gi (i: an integer from 1 to n-1) are located at one end of the cell stacks S(i+1)1 to S(i+1)3 in group Gi+1. Each cell stack S is connected in parallel to a supply pipe 21 and a discharge pipe 22. Specifically, the cell stacks Sk1 to Sk3 are connected to the supply pipe 21 at connection positions puk1 to puk3 via upstream branch pipes buk1 to buk3, respectively. The cell stacks Sk1 to Sk3 are connected to the discharge pipe 22 at connection positions plk1 to plk3 via downstream branch pipes blk1 to blk3, respectively.

[0029] That is, inlet 21a is located closer to one end than connection position pu11 where cell stack S11 (of all cell stacks S, the cell stack located on one end) is connected to supply pipe 21. Inlet 21b is located closer to the other end than connection position pn3 where cell stack Sn3 (of all cell stacks S, the cell stack located on the other end) is connected to supply pipe 21. Furthermore, outlet 22a is located closer to one end than connection position pl11 where cell stack S11 is connected to discharge pipe 22, and outlet 22b is located closer to the other end than connection position pln3 where cell stack Sn3 is connected to discharge pipe 22.

[0030] Inlet 21c is provided near connection position pu13 (the position where cell stack S13 is connected to the supply pipe 21). In this specification, the "inlet closest to any given circuit Ck" is defined as the inlet for which the sum of the distances from connection positions puk1 to puk3 at which three cell stacks Sk1 to Sk3 connected to the circuit Ck are respectively connected to the supply pipe 21 to the inlet is shortest. In this embodiment, the sum of the distances from connection positions pu11 to pu13 on the supply pipe 21 of the three cell stacks S11 to S13 in the circuit C1 to inlet 21c is shorter than the sum of the distances from connection positions pu11 to pu13 to other inlets (e.g., inlet 21a). Therefore, the inlet closest to circuit C1 is inlet 21c.

[0031] The outlet 22c is provided near the connection position p113 (the position where the cell stack S13 is connected to the exhaust pipe 22). In this specification, the outlet closest to any given circuit Ck is defined as the outlet for which the sum of the distances from the connection positions p11 to p113 at which the three cell stacks Sk1 to Sk3 connected to the circuit Ck are connected to the exhaust pipe 22, respectively, is the shortest. In this embodiment, the sum of the distances from the connection positions p11 to p113 on the exhaust pipes 22 of the three cell stacks S11 to S13 in the circuit C1 to the outlet 22c is shorter than the sum of the distances from the connection positions p11 to p113 to other outlets (e.g., outlet 22a). Therefore, the outlet closest to the circuit C1 is outlet 22c. Note that FIG. 1 (and FIG. 2) do not show piping for supplying gas to the air electrodes (described below) of each cell stack S, and piping for recovering exhaust gas discharged from the air electrodes.

[0032] The number of inlets and outlets is not limited to three, and any number equal to or greater than two may be used. The number of inlets and outlets may differ from each other. Furthermore, the positions of the inlets in the supply pipe 21 are not limited to those shown in this embodiment. Similarly, the positions of the outlets in the discharge pipe 22 are not limited to those shown in this embodiment.

[0033] The voltmeter VMk is connected to the circuit Ck so as to be electrically parallel to the DC power supply DCk. The voltage value measured by the voltmeter VMk is equal to the sum of the partial voltages of the cell stacks Sk1 to Sk3. Hereinafter, this voltage value will also be referred to as the "voltage of the circuit Ck" or the "circuit voltage."

[0034] In the SOEC mode, the control device 20 is electrically connected to each circuit C (specifically, each voltmeter VM), the inlets 21a to 21c, and the outlets 22a to 22c, and acquires a measurement value (the voltage of the circuit Ck) from each voltmeter VMk. The control device 20 is configured to be able to change the supply flow rate ratio and the exhaust flow rate ratio by controlling the opening ratio of the inlets 21a to 21c and the outlets 22a to 22c based on the circuit voltage acquired from each voltmeter VMk. Here, the supply flow rate ratio refers to the flow rate ratio of the supply gas introduced into each of the inlets 21a to 21c, and the exhaust flow rate ratio refers to the flow rate ratio of the exhaust gas discharged from each of the outlets 22a to 22c. Details of the control device 20 will be described later.

[0035] Next, FIG. 2 will be described. In the SOFC mode, the supply gas includes fuel gas. As shown in FIG. 2, the circuit Ck is configured to include an inverter Ik. That is, when the circuit Ck is switched from the SOEC mode to the SOFC mode, the circuit Ck switches its own elements from the DC power supply DCk to the inverter Ik, and when the circuit Ck is switched from the SOFC mode to the SOEC mode, the circuit Ck switches its own elements from the inverter Ik to the DC power supply DCk. An AC output ACk is connected to the inverter Ik. The inverter Ik corresponds to an example of a "load."

[0036] The voltmeter VMk is connected to the circuit Ck so as to be electrically parallel to the inverter Ik. The voltage value measured by the voltmeter VMk is equal to the sum of the divided voltages of the cell stacks Sk1 to Sk3.

[0037] In the SOFC mode, the control device 20 is electrically connected to each circuit C (specifically, each voltmeter VM), the inlets 21a to 21c, and the outlets 22a to 22c, and acquires a measurement value (the voltage of the circuit Ck) from each voltmeter VMk. The control device 20 is configured to be able to change the supply flow rate ratio and the discharge flow rate ratio by controlling the opening ratio of the inlets 21a to 21c and the outlets 22a to 22c based on the circuit voltage acquired from each voltmeter VMk. The control device 20 will be described in detail later.

[0038] Next, the cell stack S will be described with reference to FIGS. 3 to 5. FIG. 3 is a perspective view of the cell stack S, and FIG. 4 is a cross-sectional view taken along line IV-IV in FIG. 3. As shown in FIGS. 3 and 4, the cell stack S includes an electrochemical unit group formed by stacking a plurality of rectangular flat-plate-shaped electrochemical units U in the thickness direction (vertical direction), and a pair of end plates 2, 3 disposed on the upper and lower surfaces of the electrochemical unit group. In this embodiment, the upper side corresponds to an example of the "front side," and the lower side corresponds to an example of the "rear side." The electrochemical unit U is the smallest unit of an electrochemical device. The electrochemical device includes an electrolyzer and a power generator. The end plates 2, 3 are rectangular flat-plate-shaped members having the same outer shape as the electrochemical unit U and each have a rectangular opening formed in its center. The electrochemical unit group and the end plates 2, 3 are fastened to each other at their four corners by bolts B inserted through them in the thickness direction and nuts (not shown). The end plates 2, 3 are made of metal (e.g., stainless steel). When each electrochemical unit U functions as an electrolyzer, the end plates 2 and 3 serve as an anode and a cathode, respectively, and when each electrochemical unit U functions as a power generator, the end plates 2 and 3 serve as a positive electrode and a negative electrode, respectively. For ease of explanation, the proportions of the components in the drawings may differ from the actual proportions.

[0039] The electrochemical unit U will be described in detail with reference to Fig. 4. As shown in Fig. 4, the electrochemical unit U includes a single cell 10, an interconnector 4, a separator 5, an air electrode frame 6, an anode frame 7, and a current collector 8.

[0040] The unit cell 10 is the smallest unit of a solid oxide electrochemical cell (i.e., an electrochemical unit cell), and includes a solid electrolyte layer 12, an air electrode 14 laminated on the upper surface of the solid electrolyte layer 12, and an anode 16 laminated on the lower surface of the air electrode 14. The air electrode 14 has a smaller outer shape than the solid electrolyte layer 12 and the anode 16, and is disposed in the center of the upper surface of the solid electrolyte layer 12 when viewed from above. Therefore, the upper surface of the outer periphery of the solid electrolyte layer 12 is exposed to the outside.

[0041] The interconnector 4 is a rectangular metal (e.g., stainless steel) member having a rectangular current collecting portion 4a that protrudes downward from the center of its lower surface. A pair of interconnectors 4 is arranged on both sides of the single cell 10 in the thickness direction. Two adjacent electrochemical units U share one interconnector 4. In other words, the interconnector 4 also functions as a separator that separates the two adjacent electrochemical units U. The lower surface of the current collecting portion 4a is in contact with the upper surface of the air electrode 14 of the single cell 10. The lower electrochemical unit U has a pair of interconnectors 4, 9 instead of a pair of interconnectors 4, 4. The interconnector 9 is arranged at the bottom end of the cell stack S and differs from the interconnector 4 in that it does not have a current collecting portion 4a.

[0042] The separator 5 is a rectangular plate-shaped metal (e.g., stainless steel) member with a rectangular opening formed in the center thereof. The periphery of the opening of the separator 5 is brazed to the upper surface of the outer periphery of the solid electrolyte layer 12 of the unit cell 10 with a brazing material (e.g., Ag brazing) not shown.

[0043] The air electrode frame 6 is a rectangular plate-shaped insulating member and may be formed of, for example, a mica sheet. A rectangular opening is formed in the center of the air electrode frame 6. The air electrode frame 6 is disposed so as to be interposed between the interconnector 4 (strictly speaking, the upper interconnector in the electrochemical unit U) and the separator 5.

[0044] The fuel electrode frame 7 is a rectangular plate-shaped metal (e.g., stainless steel) member with a square opening formed in the center. The fuel electrode frame 7 is disposed between the separator 5 and the interconnector 4 (strictly speaking, the lower interconnector in the electrochemical unit U).

[0045] The internal space of the electrochemical unit U is divided into an air chamber Sa and a fuel chamber Sf. The air chamber Sa is the space on the air electrode 14 side and is composed of the upper interconnector 4, separator 5, air electrode frame 6, and unit cell 10. The fuel chamber Sf is the space on the fuel electrode 16 side and is composed of the separator 5, lower interconnector 4, fuel electrode frame 7, and unit cell 10.

[0046] The current collector 8 is a rectangular porous member made of metal (for example, nickel) that is smaller than the fuel electrode 16 in a plan view. The current collector 8 is arranged in the fuel chamber Sf so as to contact the lower surface of the fuel electrode 16 and the upper surface of the lower interconnector 4. Two adjacent unit cells 10 are stacked in the thickness direction so as to share the interconnector 4 via the current collector 8, thereby electrically connecting the unit cells 10 in series.

[0047] 3 and 4, four paths Pfi, Pfo, Pai, and Pao are formed as gas flow paths on the outer periphery of the cell stack S. These paths Pfi, Pfo, Pai, and Pao are all formed to penetrate through the members of the cell stack S in the thickness direction, excluding the "end plate 2" and the "interconnector 4 above the upper electrochemical unit U at the upper end."

[0048] The path Pfi is formed along side E1, one of the four sides that make up the outer periphery of the cell stack S, near one corner of side E1. The path Pfo is formed along side E2, which is opposite side E1, near the other corner of side E2 (the corner located diagonally opposite one corner of side E1). As shown in FIG. 4, the path Pfi communicates with the fuel chamber Sf via a horizontal hole 7a formed in the fuel electrode frame 7 of each electrochemical unit U. A branch pipe bu of the supply pipe 21 (see FIGS. 1 and 2) is connected to the opening of the path Pfi. Meanwhile, the path Pfo communicates with the fuel chamber Sf via a horizontal hole 7b formed in the fuel electrode frame 7 of each electrochemical unit U. A branch pipe bl of the discharge pipe 22 (see FIGS. 1 and 2) is connected to the opening of the path Pfo. The paths Pfi and Pfo correspond to examples of a "supply path" and a "discharge path," respectively.

[0049] The path Pai is formed along the side E2 near one corner of the side E2. The path Pao is formed along the side E1 near the other corner of the side E1. The paths Pai and Pao are each connected to the air chamber Sa via a horizontal hole (not shown) formed in the air electrode frame 6 of each electrochemical unit U.

[0050] Next, the configuration of the unit cell 10 will be described in more detail with reference to FIG. 5. The size and thickness of each layer of the unit cell 10 shown below are examples and are not limited to these values. FIG. 5 is a cross-sectional view of the unit cell 10 in the thickness direction. As shown in FIG. 5, the solid electrolyte layer 12 is a rectangular flat layer measuring 150 mm square and 6 μm thick, and is configured to contain YSZ (yttria-stabilized zirconia). The solid electrolyte layer 12 has high oxide ion conductivity.

[0051] The air electrode 14 is a rectangular, flat layer with a thickness of 108 μm, and is configured to contain a perovskite oxide such as lanthanum strontium cobalt iron oxide (LSCF). The air electrode 14 has a functional layer and a current collecting layer (not shown). The current collecting layer is thicker than the functional layer and is disposed on the upper surface of the functional layer. The air electrode 14 has high electronic conductivity and effectively collects electrons from the current collecting layer. An intermediate layer may be disposed between the solid electrolyte layer 12 and the air electrode 14. The intermediate layer may be configured to contain, for example, a ceria-based oxide. The intermediate layer functions as a diffusion barrier layer that prevents elements contained in the air electrode 14 from diffusing into the solid electrolyte layer 12.

[0052] The anode 16 is a rectangular, flat layer measuring 150 mm on each side, and is formed to have a thickness greater than that of the solid electrolyte layer 12 and the air electrode 14, 400 μm in this embodiment. That is, the unit cell 10 is an anode-supported cell in which the solid electrolyte layer 12 and the air electrode 14 are supported by the anode 16. The anode 16 has a support layer 18a and a functional layer 18b. The support layer 18a is significantly thicker than the functional layer 18b. The functional layer 18b is laminated and disposed between the upper surface of the support layer 18a and the lower surface of the solid electrolyte layer 12.

[0053] The support layer 18a is a cermet of nickel and YSZ, and is configured to be porous, including a plurality of micropores (not shown). The diameter of the micropores is on the order of several μm, which ensures the permeability of the supply gas. The functional layer 18b is similar to the support layer 18a in that it is a cermet of nickel and YSZ, but differs from the support layer 18a in that it is formed more densely than the support layer 18a. The fuel electrode 16 has high electronic conductivity.

[0054] In this embodiment, water vapor is used as the fuel gas in the SOEC mode, and hydrogen and oxygen are used as the fuel gas and oxidant gas in the SOFC mode. In this case, hydrogen is produced by electrolyzing water vapor into hydrogen and oxygen in the SOEC mode, and hydrogen and oxygen are combined in the SOFC mode to generate electricity and water vapor. That is, the electrochemical reactions proceed in reverse order in the SOEC mode and the SOFC mode.

[0055] The operation of the cell stack S when operated alone in SOEC mode will be described. First, the end plates 2 and 3 of the cell stack S are connected to a circuit including a DC power supply, and a voltage is applied from the DC power supply. Next, a high-temperature mixed gas of water vapor and hydrogen is supplied as a feed gas from path Pfi. The mixed gas flows into the fuel chamber Sf of each electrochemical unit U through the horizontal holes 7a. Here, water vapor is the fuel gas and hydrogen is the reducing gas. Furthermore, when high-temperature air is supplied from path Pai, the air flows into the air chamber Sa of each electrochemical unit U through horizontal holes (not shown). The high-temperature air is supplied to control the temperature of the cell stack S.

[0056] The water vapor that flows into the fuel chamber Sf passes through the support layer 18a of the fuel electrode 16 and travels to the functional layer 18b. In the functional layer 18b, the water vapor reacts with electrons (electrons supplied from the end plate 3 via the current collector 8) and is electrolyzed into hydrogen and oxide ions. The hydrogen (specifically, hydrogen generated by electrolysis and hydrogen as a reducing gas) diffuses within the fuel chamber Sf and is discharged through the horizontal hole 7b via path Pfo and collected by a well-known method. At this time, unreacted water vapor is discharged along with the hydrogen via path Pfo. Meanwhile, the oxide ions travel through the solid electrolyte layer 12 to the air electrode 14 in the air chamber Sa and release electrons in the functional layer of the air electrode 14 to become oxygen. The oxygen diffuses within the air chamber Sa and is discharged through path Pao via a horizontal hole (not shown) together with the air that flowed into the air chamber Sa and collected by a well-known method. The electrons emitted from the functional layer are collected by the current collecting portion 4a of the interconnector 4 via the current collecting layer and circulate from the end plate 2 to the end plate 3 via the DC power supply. This causes a current corresponding to the applied voltage to flow through the circuit. Note that carbon monoxide may be used as the reducing gas instead of hydrogen.

[0057] Next, we will explain the operation of the cell stack S when operated alone in SOFC mode. First, the end plates 2 and 3 are connected to a circuit including an inverter. An AC output is connected to the inverter. Next, when high-temperature hydrogen is supplied as a feed gas through path Pfi, the hydrogen flows into the fuel chamber Sf through the horizontal hole 7a. When high-temperature air is supplied through path Pai, the air flows into the air chamber Sa through a horizontal hole (not shown). Oxygen in the air that flows into the air chamber Sa receives electrons (electrons supplied from the end plate 2 via the current collector 4a) at the air electrode 14 and becomes oxide ions. "Gases other than oxygen in the air" and "unreacted oxygen" diffuse within the air chamber Sa and are discharged and collected through path Pao via a horizontal hole (not shown). The oxide ions travel through the solid electrolyte layer 12 to the fuel electrode 16 in the fuel chamber Sf, where they release electrons and combine with hydrogen to form water vapor in the functional layer 18b. The water vapor diffuses within the fuel chamber Sf and is discharged along with unreacted hydrogen through the horizontal holes 7b and the path Pfo for recovery. The emitted electrons are collected by the current collector 8 via the support layer 18a and circulate from the end plate 3 to the end plate 2 via the inverter. Electric power is extracted from the circuit in a predetermined manner.

[0058] An example of the operation of the system 1 when operating in the SOEC mode will be described with reference to FIG. 1. In this example, at the start of operation, the control device 20 controls the inlets 21a to 21c so that the supply flow rate ratio of the inlets 21a to 21c is 10:0:0, and controls the outlets 22a to 22c so that the discharge flow rate ratio of the outlets 22a to 22c is 10:0:0. As a result, at the start of operation, the supply gas is introduced only through the inlet 21a, and the exhaust gas is discharged only through the outlet 22a. The DC power supply DCk is preset so that a current having a predetermined value flows through the circuit Ck. The current value flowing through each circuit Ck can be predetermined based on the flow rate of the supply gas and the distance of the connection positions pk1 to pk3 from the inlet 21a. However, each DC power supply DCk may be electrically connected to the control device 20. In this case, the control device 20 may control each DC power supply DCk to determine the current value to be passed through each circuit Ck.

[0059] The supply gas introduced from the inlet 21a flows through the supply pipe 21 and is distributed as a distribution gas via each branch pipe bu to each cell stack S. Each voltmeter VMk measures the voltage of the circuit Ck and transmits the measurement value to the control device 20 at a predetermined interval.

[0060] When the control device 20 receives a measurement value from each voltmeter VMk, it determines whether a first condition is met for each circuit Ck. The first condition is a condition for determining whether fuel starvation has occurred in at least one cell stack S of the cell stacks Sk1 to Sk3 connected to the circuit Ck. In this embodiment, the first condition is met when the rate of change Re of the circuit voltage over a predetermined time ΔTe is equal to or greater than a predetermined threshold value Reth. The threshold value Reth corresponds to an example of a "first threshold value."

[0061] The first condition will be described in detail with reference to FIG. 6. FIG. 6 is a graph showing the change over time of the voltage Ve (i.e., the circuit voltage) of any circuit Ck in the SOEC mode. The solid line Le shows the behavior of the voltage Ve according to this embodiment, and the dashed line Lep shows the behavior of the voltage Ve according to the conventional example. The voltage Ve is the sum of the voltages of each cell stack Sk1 to Sk3. The flow rate of the fuel gas (water vapor) used in the electrolysis reaction in the cell stacks Sk1 to Sk3 depends on the current value flowing through the circuit Ck. For this reason, when the current value is constant, the flow rate of the water vapor used in the electrolysis reaction is constant. In such a case, when the flow rate of the water vapor supplied to the cell stack S changes due to a temperature change and / or a change in the flow rate of the supply gas of at least one cell stack S, the water vapor utilization rate of the cell stack S also changes. Here, the water vapor utilization rate is the flow rate ratio of "the water vapor actually used in the electrolysis reaction" to "the water vapor supplied to the cell stack S". Generally, an upper limit value for operating the cell stack S stably over a long period is defined for the water vapor utilization rate. The upper limit value varies depending on the configuration of the cell stack S, and the upper limit value of the water vapor utilization rate of the cell stack S used in this embodiment is 80%. When the water vapor utilization rate of the cell stack S exceeds its upper limit value, fuel depletion occurs and the voltage of the cell stack S increases, so the voltage Ve of the circuit Ck including the cell stack S also increases.

[0062] In the graph of FIG. 6, during the period t < T1, the water vapor utilization rates of each of the cell stacks Sk1 to Sk3 are all less than 80%. At the time t = T1, the water vapor utilization rate of one cell stack S reaches 80%, and during the period t > T1, the water vapor utilization rate of the cell stack S continues to increase. In this case, during the period t ≤ T1, the voltage Ve changes at a constant value Ve1, but thereafter, the voltage Ve begins to increase (see the solid line Le and the dashed line Lep). In this embodiment, the control device 20 calculates the change rate Re of the voltage Ve at a predetermined time ΔTe every predetermined period and determines whether the first condition is satisfied. When the change rate Re (= ΔVe / ΔTe) becomes equal to or greater than the threshold value Reth at the time t = T2, the control device 20 determines that the first condition is satisfied.

[0063] If it is determined that the first condition is met, the control device 20 controls the inlets 21a to 21c and the outlets 22a to 22c to change the supply flow rate ratio and the exhaust flow rate ratio, respectively. More specifically, the control device 20 changes the supply flow rate ratio by gradually increasing the opening ratio of the inlet closest to the circuit Ck to increase the flow rate of the supply gas introduced into the inlet. Similarly, the control device 20 changes the exhaust flow rate ratio by gradually increasing the opening ratio of the outlet closest to the circuit Ck to increase the flow rate of the exhaust gas discharged from the outlet. This causes the water vapor utilization rate to decrease, and as a result, the voltage Ve, which had increased to Ve2 at time t=T2, gradually decreases (see solid line Le).

[0064] The control device 20 stops controlling the inlet and outlet (i.e., stops increasing the opening ratio) at the time t=T4 when the voltage Ve has decreased to the original value Ve1 (in other words, when the steam utilization rate has decreased to 80%). As a result, the voltage Ve again remains constant at the value Ve1 during the period t≧T4 (see solid line Le).

[0065] In contrast, in the conventional example, the establishment of the first condition is not determined. In this case, as shown by the dashed line Lep, the water vapor utilization rate continues to increase even during the period t > T2, and therefore the voltage Ve also continues to rise. As a result, when the voltage Ve reaches the limit value Velim at t = T3, the characteristics of the cell stack S deteriorate significantly, causing the voltage Ve to rise sharply. In this way, if fuel starvation is allowed to continue, the deterioration of the characteristics of the cell stack S will make it difficult to operate the cell stack system 1.

[0066] In this embodiment, the threshold value Reth is set to a value greater than the "rate of change in voltage Ve due to aging of the cell stack S" and less than the "rate of change in voltage Ve during a predetermined time ΔTe when the water vapor utilization rate rises above 80%." With this configuration, the first condition is met if there is at least one cell stack S in any circuit Ck whose water vapor utilization rate rises above 80%. This makes it possible to appropriately determine whether fuel starvation has occurred in that circuit Ck. Note that the method for setting the threshold value Reth is not limited to the above-described method. The threshold value Reth can be set appropriately based on the number of cell stacks S connected to the circuit Ck, the upper limit of the water vapor utilization rate, etc. Furthermore, the predetermined time ΔTe can be set in advance based on experiments or simulations. Furthermore, the magnitude relationship between T3 and T4 is not limited to the example shown in FIG. 6.

[0067] 1, when it is determined that the first condition is met (i.e., when fuel starvation is detected in at least one of the cell stacks S11 to S13), the control device 20 gradually increases the opening ratio of the inlet 21c (the inlet closest to the circuit C1) to increase the flow rate of the supply gas introduced into the inlet 21c, and gradually increases the opening ratio of the outlet 22c (the outlet closest to the circuit C1) to increase the flow rate of the exhaust gas discharged from the outlet 22c. For example, when the supply flow rate ratio of the inlets 21a to 21c becomes 9:0:1 and the discharge flow rate ratio of the outlets 22a to 22c becomes 9:0:1, and the measurement value of the voltmeter VM1 drops to its original voltage value, the control device 20 stops controlling the inlet 21c and the outlet 22c at that point.

[0068] This allows the electrolysis of water vapor to be carried out appropriately in each cell stack S. The gases discharged from each cell stack S are joined in the discharge pipe 22 via the branch pipe bl, and then collected as exhaust gas.

[0069] Next, an example of operation of the system 1 in SOFC mode will be described with reference to FIG. 2 , focusing on differences from the SOEC mode. In this example, at the start of operation, the control device 20 controls the inlets 21a to 21c so that the supply flow rate ratio of the inlets 21a to 21c is 10:0:0, and controls the outlets 22a to 22c so that the discharge flow rate ratio of the outlets 22a to 22c is 10:0:0. The inverter Ik is preset so that a current having a predetermined value flows through the circuit Ck. The current value flowing through each circuit Ck can be predetermined based on the flow rate of the supply gas and the distance of the connection positions pk1 to pk3 from the inlet 21a. However, each inverter Ik may be electrically connected to the control device 20. In this case, the control device 20 may control each inverter Ik to determine the current value to flow through each circuit Ck.

[0070] When the control device 20 receives a measurement value from each voltmeter VMk, it determines whether or not a second condition is met for each circuit Ck. The second condition is a condition for determining whether or not fuel starvation has occurred in at least one cell stack S of the cell stacks Sk1 to Sk3 connected to the circuit Ck. In this embodiment, the second condition is met when the rate of change Rg of the circuit voltage over a predetermined time ΔTg is equal to or less than a predetermined threshold value Rgth. The threshold value Rgth corresponds to an example of a "second threshold value."

[0071] The second condition will be described in detail with reference to FIG. 7. FIG. 7 is a graph showing the change over time of the voltage Vg (i.e., the circuit voltage) of an arbitrary circuit Ck in the SOFC mode. The solid line Lg shows the behavior of the voltage Vg according to the present embodiment, and the dashed line Lgp shows the behavior of the voltage Vg according to the conventional example. The voltage Vg is the sum of the voltages of each cell stack Sk1 to Sk3. The flow rate of the fuel gas (hydrogen) used in the chemical reaction (power generation reaction) in the cell stacks Sk1 to Sk3 depends on the current value flowing through the circuit Ck. Therefore, when the current value is constant, the flow rate of hydrogen used in the chemical reaction is constant. In such a case, if the flow rate of hydrogen supplied to a cell stack S changes due to a temperature change and / or a change in the flow rate of the supply gas of at least one cell stack S, the hydrogen utilization rate of the cell stack S also changes. Here, the hydrogen utilization rate is the flow rate ratio of "hydrogen actually used in the chemical reaction" to "hydrogen supplied to the cell stack S". Generally, an upper limit value for the long-term stable operation of the cell stack S is defined for the hydrogen utilization rate. The upper limit value of the hydrogen utilization rate of the cell stack S used in the present embodiment is 80%. If the hydrogen utilization rate of the cell stack S exceeds its upper limit value, fuel depletion occurs and the voltage of the cell stack S decreases, so the voltage Ve of the circuit Ck including the cell stack S also decreases.

[0072] The graph of FIG. 7 shows an example where the hydrogen utilization rates of each cell stack Sk1 to Sk3 are all less than 80% during the period t < T5, the hydrogen utilization rate of one cell stack S reaches 80% at the time t = T5, and the hydrogen utilization rate of the cell stack S continues to increase during the period t > T5. In this case, the voltage Vg remains at a constant value Vg1 during the period t ≤ T5, but then the voltage Vg begins to decrease (see the solid line Lg and the dashed line Lgp). In the present embodiment, the control device 20 calculates the change rate Rg of the voltage Vg at a predetermined time ΔTg every predetermined period and determines whether the second condition is satisfied. When the change rate Rg (= ΔVg / ΔTg) becomes less than or equal to the threshold value Rgth at the time t = T6, the control device 20 determines that the second condition is satisfied.

[0073] If it is determined that the second condition is met, the control device 20 controls the inlets 21a to 21c and the outlets 22a to 22c to change the supply flow rate ratio and the exhaust flow rate ratio, respectively. More specifically, the control device 20 changes the supply flow rate ratio by gradually increasing the opening ratio of the inlet closest to the circuit Ck to increase the flow rate of the supply gas introduced into that inlet. Similarly, the control device 20 changes the exhaust flow rate ratio by gradually increasing the opening ratio of the outlet closest to the circuit Ck to increase the flow rate of the exhaust gas discharged from that outlet. This causes the hydrogen utilization rate to begin to decrease, and as a result, the voltage Vg, which had dropped to Vg2 at time t=T6, gradually increases (see solid line Lg).

[0074] The control device 20 stops controlling the inlet and outlet (i.e., stops increasing the opening ratio) at time t=T8 when the voltage Vg has increased to its original value Vg1 (in other words, when the hydrogen utilization rate has decreased to 80%). As a result, the voltage Vg remains constant at Vg1 again during the period t≧T8 (see solid line Lg).

[0075] In contrast, in the conventional example, the satisfaction of the second condition is not determined. In this case, as shown by the dashed line Lgp, the hydrogen utilization rate continues to increase even during the period t > T6, so the voltage Vg continues to decrease. As a result, when the voltage Vg reaches the limit value Vglim at t = T7, the characteristics of the cell stack S deteriorate significantly, causing the voltage Vg to drop sharply. In this way, if fuel starvation is allowed to continue, the deterioration of the characteristics of the cell stack S will make it difficult to operate the cell stack system 1.

[0076] In this embodiment, the threshold value Rgth (negative value) is set to a value that is less than the "rate of change (negative value) of voltage Vg due to aging of the cell stack S" and greater than or equal to the "rate of change (negative value) of voltage Vg during a predetermined time ΔTg when the hydrogen utilization rate rises above 80%." With this configuration, the second condition is met if there is at least one cell stack S in any circuit Ck whose hydrogen utilization rate rises above 80%. This makes it possible to appropriately determine whether fuel starvation has occurred in that circuit Ck. Note that the method for setting the threshold value Rgth is not limited to the above-described method. The threshold value Rgth can be set appropriately based on the number of cell stacks S connected to the circuit Ck, the upper limit of the hydrogen utilization rate, and the like. Furthermore, the predetermined time ΔTg can be set in advance based on experiments or simulations. Furthermore, the magnitude relationship between T7 and T8 is not limited to the example shown in FIG. 7 .

[0077] 2 is satisfied (i.e., fuel starvation is detected in at least one of the cell stacks S11 to S13), the control device 20 gradually increases the opening ratio of the inlet 21c (the inlet closest to the circuit C1) to increase the flow rate of the supply gas introduced into the inlet 21c, and gradually increases the opening ratio of the outlet 22c (the outlet closest to the circuit C1) to increase the flow rate of the exhaust gas discharged from the outlet 22c. For example, if the supply flow rate ratio of the inlets 21a to 21c becomes 9:0:1 and the discharge flow rate ratio of the outlets 22a to 22c becomes 9:0:1, and the measurement value of the voltmeter VM1 rises to its original voltage value, the control device 20 stops controlling the inlet 21c and the outlet 22c at that point.

[0078] As a result, power generation is properly carried out in each cell stack S by combining hydrogen (fuel gas) and oxygen (oxidant gas).

[0079] As described above, in the cell stack system 1 according to this embodiment, the supply pipe 21 has three inlets 21a to 21c, and the exhaust pipe 22 has three outlets 22a to 22c. The control device 20 is configured to change the supply flow rate ratio and the exhaust flow rate ratio based on voltage changes in the circuit Ck of each group Gk. With this configuration, when the fuel gas utilization rate of a given cell stack S increases and exceeds its upper limit due to a decrease in the flow rate of fuel gas supplied to that cell stack S, causing the voltage of the circuit C to change, the control device 20 changes the supply flow rate ratio and the exhaust flow rate ratio. By appropriately changing the supply flow rate ratio and / or the exhaust flow rate ratio, the flow rate of fuel gas supplied to that cell stack S is likely to increase, increasing the likelihood that the fuel gas utilization rate can be reduced to or below its upper limit. Therefore, the cell stack system 1 can prevent fuel depletion from continuing even if it occurs due to uneven flow. As a result, significant deterioration of the characteristics of each unit cell 10 can be prevented. That is, in this embodiment, the occurrence of fuel starvation is detected from a voltage change in circuit C, and the supply flow rate ratio and the discharge flow rate ratio are changed based on the voltage change, thereby preventing the continuation of fuel starvation.

[0080] Furthermore, in the cell stack system 1, all of the cell stacks S are not electrically connected in series to one circuit, but rather multiple groups Gk are electrically connected in series to circuits Ck. The circuits C are configured to include a direct current power supply DC in SOEC mode and an inverter I in SOFC mode. With this configuration, the current value can be set for each circuit C based on the supply flow rate ratio and the discharge flow rate ratio. Therefore, compared to a configuration in which all of the cell stacks S are connected to one circuit and the same value of current flows, the electrolysis efficiency or power generation efficiency of each cell stack S can be improved.

[0081] Additionally, the control device 20 determines whether the first and second conditions are met based on the rates of change in the voltages Ve and Vg over the predetermined times ΔTe and ΔTg, respectively. In this way, by determining whether the first and second conditions are met based not simply on the rates of change in the voltages Ve and Vg but on the rates of change in the voltages Ve and Vg over the predetermined times ΔTe and ΔTg, it is possible to appropriately distinguish whether the voltage change is due to fuel starvation or aging deterioration, thereby improving the accuracy of fuel starvation detection.

[0082] Furthermore, in the cell stack system 1, when the first condition or the second condition is met, the control device 20 controls each of the inlets 21a to 21c to increase the flow rate of the supply gas introduced into the inlet closest to the circuit C that satisfies the first condition or the second condition. With this configuration, the flow rate of the fuel gas supplied to the cell stack S connected to the circuit C that satisfies the first condition or the second condition increases quickly, so the utilization rate of the fuel gas can be quickly reduced, and as a result, the continuation of fuel starvation can be more appropriately suppressed.

[0083] The cell stack system according to the embodiment has been described above, but the present invention is not limited to the above embodiment, and various modifications are possible without departing from the object of the present invention.

[0084] For example, the operating mode of the system 1 is not limited to a mode in which the system 1 is reversibly operated by switching between the SOEC mode and the SOFC mode. The system 1 may be continuously operated only in the SOEC mode, or may be continuously operated only in the SOFC mode. When continuously operated only in the SOEC mode, the type of fuel gas is not limited to water vapor, but may be a mixed gas of water vapor and carbon dioxide, or may be carbon dioxide. When the fuel gas is the mixed gas, each unit cell 10 constituting the cell stack S electrolyzes the mixed gas to produce a synthesis gas containing hydrogen and carbon monoxide. When the fuel gas is carbon dioxide, each unit cell 10 electrolyzes carbon dioxide to produce carbon monoxide.

[0085] Furthermore, in the above embodiment, when the first condition or the second condition is met, both the supply flow rate ratio and the discharge flow rate ratio are changed, but it may also be configured to change only one of the supply flow rate ratio or the discharge flow rate ratio.

[0086] The number of cell stacks S included in one group G may be one or any number equal to or greater than two. In addition, the number of cell stacks S in each group G may differ from one another.

[0087] Furthermore, the present invention may include the following aspects. [1] a plurality of cell stacks, each of which includes a plurality of solid oxide electrochemical unit cells, each of which includes a solid electrolyte layer, an air electrode stacked on the front side of the solid electrolyte layer, and an anode stacked on the back side of the solid electrolyte layer, and the cell stacks have a supply path for supplying gas to a fuel chamber, which is a space on the anode side, and a discharge path for discharging gas from the fuel chamber; a supply pipe through which a supply gas can flow and through which the supply paths of the plurality of cell stacks are connected in parallel; an exhaust pipe through which exhaust gas can flow and through which the exhaust paths of the plurality of cell stacks are connected in parallel; Equipped with The cell stacks are electrically connected in series to a circuit including a power source or a load, for each of a plurality of groups including one or more of the cell stacks, the supply pipe has a plurality of inlets through which the supply gas is introduced, the exhaust pipe has a plurality of outlets from which the exhaust gas is discharged, a supply flow rate ratio, which is a flow rate ratio of the supply gas introduced into each of the inlets, and a discharge flow rate ratio, which is a flow rate ratio of the discharge gas discharged from each of the discharge outlets, are configured to be changeable based on a voltage change of each of the circuits. Cell stack system. [2] In the cell stack system according to [1], a control device electrically connected to the circuit of each group, each of the inlets, and each of the outlets; The control device when a first condition is satisfied when a rate of change in voltage of any of the circuits including the power supply in a predetermined unit time is equal to or greater than a predetermined first threshold, or when a second condition is satisfied when a rate of change in voltage of any of the circuits including the load in a predetermined unit time is equal to or less than a predetermined second threshold, the supply flow rate ratio and / or the discharge flow rate ratio are changed by controlling each of the inlet ports and / or each of the outlet ports, Cell stack system. [3] [2] The cell stack system according to [2], The control device when the supply flow rate ratio is changed due to the first condition or the second condition being satisfied, each of the inlets is controlled so that a flow rate of the supply gas introduced into an inlet among the plurality of inlets that is closest to the circuit and that satisfies the first condition or the second condition is increased, The inlet closest to the circuit that satisfies the first condition or the second condition is defined as the inlet for which the sum of the distances from the connection positions where one or more of the cell stacks connected to the circuit are connected to the supply pipe to the inlet is the shortest. Cell stack system. [4] The cell stack system according to any one of [1] to [3], The supply pipe includes at least a first inlet that is located closer to one end than a connection position where a cell stack located on one end of the plurality of cell stacks is connected to the supply pipe; a second inlet that is arranged on the other end side of the connection position of a cell stack that is arranged on the other end side of the plurality of cell stacks; Equipped with Cell stack system. [Explanation of symbols]

[0088] 1: cell stack system, 2: end plate, 3: end plate, 4: interconnector, 5: separator, 6: air electrode frame, 7: fuel electrode frame, 8: current collector, 9: (lowest) interconnector, 10: solid oxide electrochemical unit cell, 12: solid electrolyte layer, 14: air electrode, 16: fuel electrode, 18a: support layer, 18b: functional layer, 20: control device, 21: supply pipe, 21a, 21b, 21c: inlet, 22: outlet pipe, 22a, 22b, 22c: outlet

Claims

1. a plurality of cell stacks, each of which includes a plurality of solid oxide electrochemical unit cells, each of which includes a solid electrolyte layer, an air electrode stacked on the front side of the solid electrolyte layer, and an anode stacked on the back side of the solid electrolyte layer, and the cell stacks have a supply path for supplying gas to a fuel chamber, which is a space on the anode side, and a discharge path for discharging gas from the fuel chamber; a supply pipe through which a supply gas can flow and through which the supply paths of the plurality of cell stacks are connected in parallel; an exhaust pipe through which exhaust gas can flow and through which the exhaust paths of the plurality of cell stacks are connected in parallel; Equipped with The cell stacks are electrically connected in series to a circuit including a power source or a load, for each of a plurality of groups including one or more of the cell stacks, the supply pipe has a plurality of inlets through which the supply gas is introduced, the exhaust pipe has a plurality of outlets from which the exhaust gas is discharged, a supply flow rate ratio, which is a flow rate ratio of the supply gas introduced into each of the inlets, and a discharge flow rate ratio, which is a flow rate ratio of the discharge gas discharged from each of the discharge ports, are configured to be changeable based on a voltage change of each of the circuits. Cell stack system.

2. The cell stack system according to claim 1, a control device electrically connected to the circuit of each group, each of the inlets, and each of the outlets; The control device when a first condition is satisfied that a rate of change in voltage of any of the circuits including the power supply over a predetermined time period is equal to or greater than a predetermined first threshold, or when a second condition is satisfied that a rate of change in voltage of any of the circuits including the load over a predetermined time period is equal to or less than a predetermined second threshold, the supply flow rate ratio and / or the discharge flow rate ratio are changed by controlling each of the inlet ports and / or each of the outlet ports. Cell stack system.

3. The cell stack system according to claim 2, The control device when the supply flow rate ratio is changed due to the first condition or the second condition being satisfied, each of the inlets is controlled so that a flow rate of the supply gas introduced into an inlet among the plurality of inlets that is closest to the circuit and that satisfies the first condition or the second condition is increased, the inlet closest to the circuit that satisfies the first condition or the second condition is defined as the inlet for which the sum of distances from the connection positions at which one or more of the cell stacks connected to the circuit are connected to the supply pipe to the inlet is the shortest; Cell stack system.

4. The cell stack system according to any one of claims 1 to 3, The supply pipe includes at least a first inlet that is located closer to one end than a connection position where a cell stack located on one end of the plurality of cell stacks is connected to the supply pipe; and a second inlet that is disposed on the other end side of the connection position of a cell stack that is disposed on the other end side of the plurality of cell stacks; Equipped with Cell stack system.

Images