WO2018051828A1

WO2018051828A1 - Fuel cell system and solid oxide fuel cell

Info

Publication number: WO2018051828A1
Application number: PCT/JP2017/031756
Authority: WO
Inventors: 慎弥宇井; 邦幸高橋
Original assignee: 富士電機株式会社
Priority date: 2016-09-13
Filing date: 2017-09-04
Publication date: 2018-03-22
Also published as: JP6187660B1; JP2018045814A

Abstract

Provided are a fuel cell system and a solid oxide fuel cell which maintain high power generation efficiency while achieving stable operation by minimizing open circuit voltage at startup using a simple configuration while keeping costs low. To this end, a shunt circuit (30) is capable of being connected to a direct current transmission line from a solid oxide fuel cell (10) to a DC/AC converter (20). The shunt circuit (30) has a discharge resistance (31) which takes some of the direct current generated by the solid oxide fuel cell (10), and switching units (32, 33, 35) for switching the discharge resistance (31) between a state of connection to the transmission line and a state of disconnection therefrom.

Description

Fuel cell system and solid oxide fuel cell

The present invention relates to a fuel cell system and a solid oxide fuel cell.

In recent years, solid oxide fuel cells (SOFCs) have been developed. SOFC is a power generation mechanism in which oxide ions generated at the air electrode permeate the electrolyte and move to the fuel electrode, where the oxide ions react with hydrogen or carbon monoxide to generate electrical energy. . The SOFC has the characteristics that the power generation operating temperature is the highest (for example, 900 ° C. to 1000 ° C.) and the power generation efficiency is the highest among the currently known fuel cell configurations.

Generally, when direct current generated by a fuel cell is converted to alternating current and connected to a system power supply, the conversion efficiency is increased by setting the direct current portion to a high voltage and low current. This is realized by increasing the number of stacked fuel cell stacks and connecting the cells in series. The reason why the power generation efficiency of the SOFC is high is that the direct current portion can be made high voltage and low current.

Patent Document 1 discloses a fuel cell system that controls opening and closing of an FC relay that switches connection and disconnection between a fuel cell and a load. In this fuel cell system, the fuel cell voltage is increased from the starting voltage to an operating voltage lower than the open circuit voltage to start the fuel cell, and the fuel cell voltage is lower than the operating voltage and lower than the starting voltage of the fuel cell. An FC relay close command is output between the high first voltage and the second voltage lower than the first voltage and higher than the starting voltage of the fuel cell.

Patent Document 2 discloses a fuel cell system having a DC / DC converter (hereinafter referred to as a converter), a DC / AC inverter (hereinafter referred to as an inverter), and a rectifier circuit. The converter converts the DC voltage from the fuel cell into a predetermined DC current and outputs it to an auxiliary machine that operates the fuel cell. The inverter converts the DC voltage output from the converter into a predetermined AC voltage and outputs it to a power supply line connected to the system power supply, and converts the AC voltage from the power supply line into a predetermined DC voltage and converts it into an auxiliary machine. Output to. The rectifier circuit is provided in parallel with the inverter between the power supply line and the auxiliary machine, converts the AC voltage from the power supply line into a DC voltage, and supplies it to the auxiliary machine. And at the time of starting of a fuel cell, a converter and a rectifier circuit or an inverter are balanced and the voltage of a fuel cell is adjusted.

JP 2010-238538 A JP 2009-48972 A

However, in the conventional fuel cell system including Patent Document 1-2, the open circuit voltage (OCV) at the start-up (no load) of the fuel cell becomes too high, and the cell stack of the fuel cell and thus the compensation. As a result of inducing machine deterioration and damage, there is a problem that stable operation of the fuel cell system becomes difficult. In addition, since the components necessary for voltage control of the fuel cell are large and complicated, the cost increases.

The present invention has been made in view of the above points, and a fuel capable of realizing stable operation by suppressing open circuit voltage at start-up with a simple configuration and low cost while maintaining high power generation efficiency. An object is to provide a battery system and a solid oxide fuel cell.

In one aspect, the fuel cell system of the present embodiment is a solid oxide fuel cell that generates power by an electrochemical reaction between a fuel gas and an oxidant gas, and a direct current generated by the solid oxide fuel cell is an alternating current. A DC / AC converter that converts the current into a DC / AC converter, and a shunt circuit that can be connected to a direct current transmission path from the solid oxide fuel cell to the DC / AC converter. It has a discharge resistor that extracts a part of the direct current generated by the oxide fuel cell, and a switching unit that switches the discharge resistor between a connection state and a non-connection state to the transmission line.

In one aspect, the fuel cell system of the present embodiment is a fuel cell system having a solid oxide fuel cell that generates electric power by an electrochemical reaction between a fuel gas and an oxidant gas, the solid oxide fuel cell being And a cell stack configured by stacking a plurality of cells, and a serial-parallel switching mechanism for switching each cell of the cell stack to be electrically connected in series or connected in parallel. .

In one aspect, the fuel cell system of the present embodiment generates a power by an electrochemical reaction between a fuel gas and an oxidant gas, and has a cell stack formed by stacking a plurality of cells. A DC / AC converter that converts a direct current generated by the solid oxide fuel cell into an alternating current, and a direct current transmission path from the solid oxide fuel cell to the DC / AC converter. A possible shunt circuit, wherein the shunt circuit is a discharge resistor that extracts a part of the direct current generated by the solid oxide fuel cell, and the discharge resistor is disconnected from the connection state to the transmission line. A switching unit that switches to a state, and the switching unit has an open circuit voltage that reaches a predetermined threshold voltage before the solid oxide fuel cell is started or when the solid oxide fuel cell is started. When the discharge When the direct current flowing into the DC / AC converter reaches a predetermined threshold current, the switching unit switches the discharge resistance to the transmission line. The solid oxide fuel cell has a series-parallel switching mechanism that switches whether the cells of the cell stack are electrically connected in series or connected in parallel. At the time of starting the solid oxide fuel cell, the series-parallel switching mechanism electrically connects the cells of the cell stack in parallel, and the open circuit voltage of the solid oxide fuel cell becomes the first threshold voltage. A part of the direct current generated by the solid oxide fuel cell is extracted by at least one of the DC / AC converter and the discharge resistor, and the open circuit voltage of the solid oxide fuel cell is When dropped to less than one threshold voltage second threshold voltage, said series-parallel switching mechanism, electrically connected in series to each cell of the cell stack is characterized in that.

In one aspect, the solid oxide fuel cell of the present embodiment is a solid oxide fuel cell that generates electric power by an electrochemical reaction between a fuel gas and an oxidant gas, and is configured by stacking a plurality of cells. It has a cell stack, and a serial-parallel switching mechanism for switching whether cells of the cell stack are electrically connected in series or connected in parallel.

ADVANTAGE OF THE INVENTION According to this invention, the fuel cell system and solid oxide fuel which can implement | achieve stable operation | movement by suppressing the open circuit voltage at the time of starting with simple structure and low cost, maintaining high electric power generation efficiency. A battery can be provided.

It is a block diagram which shows the fuel cell system of 1st Embodiment. It is a block diagram which shows the case where the surplus electric power transmission line is added to the fuel cell system of FIG. 1A. It is a block diagram which shows the internal structure of the shunt circuit of 1st Embodiment. 3 is a timing chart showing the relationship between the voltage applied to the SOFC from the start of the fuel cell system of the first embodiment to the start of rated operation, the current flowing through the discharge resistor, and the current flowing through the power generation load. It is a flowchart which shows the operation | movement from the time of starting of the fuel cell system of 1st Embodiment to the start of rated operation. It is a block diagram which shows the internal structure of the shunt circuit of 2nd Embodiment. It is a timing chart which shows the relationship between the voltage concerning SOFC from the time of starting of the fuel cell system of 2nd Embodiment to the start of rated operation, the electric current which flows into a discharge resistance, and the electric current which flows into an electric power generation load. It is a flowchart which shows the operation | movement from the time of starting of the fuel cell system of 2nd Embodiment to the start of rated operation. It is a conceptual diagram which shows the structure of the cell stack of SOFC of 3rd Embodiment, and the serial-parallel switching mechanism of each cell. It is a conceptual diagram corresponding to FIG. 8 which shows the state which connected each cell of the cell stack of SOFC in parallel. It is a conceptual diagram corresponding to FIG. 8 which shows the state which connected each cell of the cell stack of SOFC in series. It is a timing chart which shows the relationship between the voltage concerning SOFC from the time of starting of the fuel cell system of 3rd Embodiment to the start of rated operation, the electric current which flows into discharge resistance, and the electric current which flows into an electric power generation load. It is a flowchart which shows operation | movement from the time of starting of the fuel cell system of 3rd Embodiment to the start of rated operation. It is a conceptual diagram which shows the structure of the cell stack of SOFC of 4th Embodiment, and the serial-parallel switching mechanism of each cell.

<< First Embodiment >>
The fuel cell system 1 of the first embodiment will be described in detail with reference to FIGS. In the figure, a solid line indicates the flow of electricity (current, power), and a broken line (inside the SOFC 10) and a one-dot chain line (outside of the SOFC 10) indicate a flow of a fluid such as gas or water.

As shown in FIG. 1A, a fuel cell system 1 includes a solid oxide fuel cell (SOFC) 10, a DC / AC converter 20, a shunt circuit 30, a system power network 40, and a combustor. 50 and an exhaust heat recovery / circulation system 60.

The SOFC 10 has a cell stack in which a plurality of cells are stacked or assembled. Each cell has a basic configuration in which an electrolyte is sandwiched between an air electrode and a fuel electrode, and a separator is interposed between the cells. Each cell of the cell stack is electrically connected in series. The SOFC 10 is a power generation mechanism that generates electric energy when oxide ions generated at the air electrode permeate the electrolyte and move to the fuel electrode, and the oxide ions react with hydrogen or carbon monoxide at the fuel electrode. . Although details will be described later in the third and fourth embodiments, it is possible to switch whether the cells of the cell stack of the SOFC 10 are electrically connected in series or in parallel.

The SOFC 10 has a fuel gas channel (anode gas channel) 12 and an oxidant gas channel (cathode gas channel) 14. Fuel gas is supplied to the fuel gas channel 12 from a fuel gas supplier (not shown), and oxidant gas is supplied to the oxidant gas channel 14 from an oxidant gas supplier (not shown). A direct current is generated by causing an electrochemical reaction between the fuel gas supplied to the fuel gas passage 12 and the oxidant gas supplied to the oxidant gas passage 14. The fuel gas and oxidant gas that have not caused an electrochemical reaction are discharged from the SOFC 10 as exhaust gas. Part of the fuel gas discharged from the SOFC 10 is recirculated to the fuel gas channel 12 via the recycle gas channel 16.

The DC / AC converter 20 converts the direct current generated (generated) by the SOFC 10 into an alternating current.

The shunt circuit 30 is provided so as to be connectable to the direct current transmission line DL from the SOFC 10 to the DC / AC converter 20 (straddling the transmission line DL). In the connected state to the transmission line DL, the shunt circuit 30 extracts a part of the direct current generated by the SOFC 10 from the transmission line DL (releases it from the transmission line DL by being divided and consumed). For this reason, in the transmission line DL, the direct current flowing from the shunt circuit 30 to the DC / AC converter 20 is smaller than the direct current flowing from the SOFC 10 to the shunt circuit 30. On the other hand, the shunt circuit 30 passes the direct current generated by the SOFC 10 as it is in a non-connected state to the transmission line DL. For this reason, in the transmission line DL, the direct current flowing from the shunt circuit 30 to the DC / AC converter 20 has the same magnitude as the direct current flowing from the SOFC 10 to the shunt circuit 30. The configuration and operation of the shunt circuit 30 will be described in detail later.

The generated power of the SOFC 10 passes through the DC / AC converter 20 (the shunt circuit 30 and the DC / AC converter 20 in the connection state of the shunt circuit 30 to the transmission line DL), and passes through the grid interconnection relay 25 to the grid power network 40. It is connected to the. The generated power of the SOFC 10 is connected to the grid power network 40 when the grid connection relay 25 is in an on state, and is disconnected from the grid connection relay 25 when the grid connection relay 25 is in an off state.

During the interconnected operation, the generated power of the SOFC 10 is supplied to the system, and during the independent operation, the generated power is consumed in the apparatus with a load smaller than the rated maximum power.

As shown in FIG. 1B, from the power transmission path between the DC / AC converter 20 and the grid connection relay 25, the grid power network 40 (system grid relay 25) is switched from the grid state to the SOFC 10 to the disconnected state. You may provide the surplus electric power transmission line L which transmits the surplus electric power which is a part of electric power generated with SOFC10 when it switches. The surplus power transmission path L may be connected to a heater (not shown) provided in an exhaust heat recovery / circulation line (not shown) of the exhaust heat recovery / circulation system 60.

In the surplus power transmission line L, a relay switch LS is provided. When the relay switch LS is in the on state, surplus power can be transmitted to the exhaust heat recovery circulation system 60 via the surplus power transmission line L. When the relay switch LS is in the off state, the surplus power transmission line L is cut off and discharged. Surplus power cannot be transmitted to the heat recovery circulation system 60.

The grid interconnection relay 25 and the relay switch LS are controlled so that, for example, when one is on, the other is off. Of course, it may be controlled so that there is a time zone in which both the grid interconnection relay 25 and the relay switch LS are in the on state or the off state. In FIG. 1B, the case where both the grid connection relay 25 and the relay switch LS are an OFF state is drawn.

Although illustration is omitted, a power transmission path different from the surplus power transmission path L described above may be branched from the power transmission path between the DC / AC converter 20 and the grid interconnection relay 25. This other power transmission path is connected to the DC / AC conversion unit 20 and devices (not shown) mounted on the fuel cell system 1 such as a pump, blower, and radiator, and the grid power network 40 is connected via the power transmission path. Regardless of the connection / disconnection state with the power, the power may be supplied from either the SOFC 10 or the grid power network 30 to be driven.

The combustor 50 removes the fuel component remaining in the exhaust gas by burning the exhaust gas discharged from the SOFC 10.

The exhaust heat recovery circulation system 60 recovers the heat of the combustion gas (exhaust gas) from the combustor 50. The exhaust heat recovery / circulation system 60 has an exhaust heat recovery / circulation line (not shown) through which water (hot water) as a heat medium for exhaust heat recovery is circulated. The exhaust heat recovery circulation line is provided with various reactors (all not shown) such as an exhaust heat recovery heat exchanger, a hot water heat exchanger, a heater, a radiator, and a pump. The gas after the exhaust heat recovery by the exhaust heat recovery circulation system 60 is exhausted to the outside of the fuel cell system 1.

FIG. 2 is a block diagram showing the internal configuration of the shunt circuit 30. 2 is different from FIGS. 1A and 1B in the positional relationship between the shunt circuit 30, the SOFC 10, the DC / AC conversion unit 20, and the system power network 40. This is for convenience of drawing (FIG. 1A). 1B and FIG. 2 are equivalent).

The shunt circuit 30 includes a discharge resistor 31, a relay switch (switching unit) 32, and an open / close control unit (switching unit) 33. The discharge resistor 31 and the relay switch 32 are grounded to the earth 34.

The discharge resistor 31 draws a part of the direct current generated by the SOFC 10 from the transmission line DL (releases it from the transmission line DL by being divided and consumed). The resistance value of the discharge resistor 31 is a high voltage (for example, about 750 V to 950 V DC) calculated from the IV characteristics of the SOFC 10 when the open circuit voltage (OCV: Open Circuit Voltage) at the start-up (no load) of the SOFC 10 It is calculated and set based on the load current value that can avoid this.

The relay switch 32 switches the discharge resistor 31 (the shunt circuit 30) between the connected state and the disconnected state to the transmission line DL according to the opening / closing control by the opening / closing control unit 33. That is, when the relay switch 32 is closed, the discharge resistor 31 and the transmission line DL are connected, and when the relay switch 32 is open, the discharge resistor 31 and the transmission line DL are disconnected (FIG. 2 shows the latter). Draws the case).

The open / close control unit 33 performs open / close control of the relay switch 32.

The opening / closing control unit 33 closes the relay switch 32 and connects the discharge resistor 31 to the transmission line DL by the time the SOFC 10 starts up (at the latest, before starting to supply fuel gas and oxidant gas to the SOFC 10). State.

Alternatively, the open / close control unit 33 switches the relay switch 32 from the open state to the closed state when the OCV at the time of activation of the SOFC 10 reaches a predetermined threshold voltage, and the discharge resistor 31 is not connected to the transmission line DL. May be switched to a connected state. The “predetermined threshold voltage” is set to a value (for example, 600 V) lower than the high voltage calculated from the IV characteristics of the SOFC 10.

The open / close control unit 33 switches the relay switch 32 from the closed state to the open state when the direct current flowing into the DC / AC conversion unit 20 reaches a predetermined threshold current, and connects the discharge resistor 31 to the transmission line DL. Switch from state to disconnected state. Here, the “predetermined threshold current” can be set to a value of a direct current drawn by the discharge resistor 31 from the transmission line DL. That is, the open / close control unit 33 switches the relay switch 32 from the closed state to the open state when the direct current flowing into the DC / AC conversion unit 20 and the direct current drawn by the discharge resistor 31 from the transmission line DL become the same. The discharge resistor 31 is switched from the connection state to the transmission line DL to the non-connection state. The “predetermined threshold current” can be set, for example, to a value of 10% with respect to the output current during rated operation.

Referring to the timing chart of FIG. 3 and the flowchart of FIG. 4, the operation from the start of the fuel cell system 1 to the start of the rated operation will be described. In the timing chart of FIG. 3, the solid line indicates the voltage applied to the SOFC 10, the broken line indicates the voltage applied to the SOFC 10 when the shunt circuit 30 is omitted, the one-dot chain line indicates the current flowing through the discharge resistor 31, and the two-dot chain line indicates power generation. The electric current which flows into load (for example, various auxiliary machines) is shown.

In step ST1, before the start of the SOFC 10 (before supply of fuel gas and oxidant gas to the SOFC 10 is started), the open / close control unit 33 closes the relay switch 32 and connects the discharge resistor 31 to the transmission line DL. State.

In step ST2, an air blower (not shown) for sending fuel gas and oxidant gas into the SOFC 10 is turned on. In step ST3, an air heating mechanism (not shown) for heating the fuel gas and the oxidant gas is turned on. In step ST4, it is determined whether the temperature of the SOFC 10 is equal to or higher than a predetermined value. If the temperature of the SOFC 10 is equal to or higher than the predetermined value (step ST4: Yes), the process proceeds to step ST5. If the temperature of the SOFC 10 is less than the predetermined value (step ST4: No), the process waits for the temperature of the SOFC 10 to be equal to or higher than the predetermined value.

In step ST5, supply of fuel gas and oxidant gas to the SOFC 10 is started, and the OCV of the SOFC 10 starts to rise (see FIG. 3). At this time, the relay switch 32 is already closed, and the discharge resistor 31 (the shunt circuit 30) and the transmission line DL are already connected. For this reason, a part of the direct current generated by the SOFC 10 is extracted from the transmission line DL using the discharge resistor 31 (see the one-dot chain line in FIG. 3). As a result, the OCV of the SOFC 10 can be suppressed more than when the shunt circuit 30 is omitted (see the solid line and the broken line in FIG. 3).

In step ST6, it is determined whether or not the OCV of the SOFC 10 is equal to or higher than a predetermined threshold voltage. If the OCV of the SOFC 10 is equal to or higher than the predetermined threshold voltage (step ST6: Yes), the process proceeds to step ST7. If the OCV of the SOFC 10 is less than the predetermined threshold voltage (step ST6: No), it waits for the OCV of the SOFC 10 to be equal to or higher than the predetermined threshold voltage.

In step ST7, it is determined whether or not the supply amounts of the fuel gas and the oxidant gas to the SOFC 10 are equal to or greater than a predetermined value. If the supply amounts of the fuel gas and the oxidant gas to the SOFC 10 are equal to or greater than the predetermined values (step ST7: Yes), the process proceeds to step ST8. If the supply amounts of the fuel gas and the oxidant gas to the SOFC 10 are less than the predetermined values (step ST7: No), it waits for the supply amounts of the fuel gas and the oxidant gas to the SOFC 10 to be equal to or greater than the predetermined values.

In step ST8, the power generated by the SOFC 10 is started to be input to the power generation load (for example, various auxiliary machines). Thereby, the current flowing through the power generation load starts to rise (see the two-dot chain line in FIG. 3). At this time, the relay switch 32 is in a closed state, and the discharge resistor 31 draws a part of the direct current generated by the SOFC 10 from the transmission line DL (the one-dot chain line and the two-dot chain line in FIG. overlapping).

In step ST9, whether or not the direct current flowing into the DC / AC conversion unit 20 is equal to or greater than a predetermined threshold current (or the direct current flowing into the DC / AC conversion unit 20 and the direct current drawn by the discharge resistor 31 from the transmission line DL are determined. Whether or not they are the same). If the direct current flowing into the DC / AC conversion unit 20 is greater than or equal to a predetermined threshold current (step ST9: Yes), the process proceeds to step ST10. If the direct current flowing into the DC / AC conversion unit 20 is less than the predetermined threshold current (step ST9: No), it waits for the direct current flowing into the DC / AC conversion unit 20 to be equal to or greater than the predetermined threshold current.

In step ST10, the open / close control unit 33 switches the relay switch 32 from the closed state to the open state, and switches the discharge resistor 31 (the shunt circuit 30) from the connection state to the transmission line DL to the non-connection state. At this time, since the generated power of the SOFC 10 is input to the power generation load, the voltage of the SOFC 10 does not become excessively high and gradually converges toward the rated voltage.

In step ST11, it is determined whether or not the generated power of the SOFC 10 is a steady operation that converges to the rated maximum power. If it is a steady operation (step ST11: Yes), the process is terminated, and if it is not a steady operation (step ST11: No), it waits for a steady operation.

As shown in the timing chart of FIG. 3, in the first embodiment, before the SOFC 10 is activated, the opening / closing control unit 33 closes the relay switch 32 and connects the discharge resistor 31 to the transmission line DL. . For this reason, simultaneously with the activation of the SOFC 10, the discharge resistor 31 starts to extract a part of the direct current generated by the SOFC 10 from the transmission line DL (starts to escape from the transmission line DL by being divided and consumed) (one point in FIG. 3) See chain line). Thereby, OCV of SOFC10 can be suppressed rather than the case where the shunt circuit 30 is abbreviate | omitted (refer the continuous line and broken line of FIG. 3).

<< Second Embodiment >>
FIG. 5 is a block diagram showing an internal configuration of the shunt circuit 30 ′ of the second embodiment. In the shunt circuit 30 ′ of the second embodiment, in the configuration of the shunt circuit 30 of the first embodiment, the relay switch 32 and the open / close control unit 33 as a switching unit are omitted, and a Zener diode (constant voltage diode) is used instead. 35 is provided.

When the OCV of the SOFC 10 is less than a predetermined threshold voltage, the Zener diode 35 brings the discharge resistor 31 (the shunt circuit 30 ′) into a disconnected state to the transmission line DL, and the OCV of the SOFC 10 is equal to or higher than the predetermined threshold voltage. In some cases, it has element characteristics such that the discharge resistor 31 (the shunt circuit 30 ') is connected to the transmission line DL. The “predetermined threshold voltage” is set to the same value as the avalanche breakdown voltage of the Zener diode 35 or a value slightly smaller than that.

Referring to the timing chart of FIG. 6 and the flowchart of FIG. 7, the operation from the start of the fuel cell system 1 of the second embodiment to the start of rated operation will be described. In the timing chart of FIG. 6, the solid line indicates the voltage applied to the SOFC 10, the broken line indicates the voltage applied to the SOFC 10 when the shunt circuit 30 ′ is omitted, the one-dot chain line indicates the current flowing through the discharge resistor 31, and the two-dot chain line indicates The electric current which flows into an electric power generation load (for example, various auxiliary machines) is shown.

The flowchart of FIG. 7 of the second embodiment is basically the same as the flowchart of FIG. 4 of the first embodiment, but differs in the following points.
(1) Step ST1 in which the relay switch 32 is closed and the discharge resistor 31 is connected to the transmission line DL in the stage before starting the SOFC 10 is omitted. That is, when the SOFC 10 is activated, the relay switch 32 is in an open state, and the discharge resistor 31 is not connected to the transmission line DL.
(2) When the OCV at the time of starting the SOFC 10 is equal to or higher than a predetermined threshold voltage (step ST6: Yes), in step ST6 ′, the discharge resistor 31 (the shunt circuit 30 ′) is connected to the transmission line due to the element characteristics of the Zener diode 35. It is switched from the non-connected state to the DL state to the connected state.
(3) When the direct current flowing into the DC / AC conversion unit 20 is equal to or greater than a predetermined threshold current (step ST9: Yes), in step ST10 ′, the discharge resistor 31 (the shunt circuit 30 ′) due to the element characteristics of the Zener diode 35. ) Is switched from the connection state to the transmission line DL to the non-connection state.

As shown in the timing chart of FIG. 6, in the second embodiment, when the OCV when the SOFC 10 starts up reaches a predetermined threshold voltage, the Zener diode 35 causes an avalanche breakdown, thereby causing the discharge resistor 31 (a shunt circuit). 30 ′) is switched from the non-connected state to the transmission line DL to the connected state. As a result, the discharge resistor 31 draws a part of the direct current generated by the SOFC 10 from the transmission line DL (releases it from the transmission line DL by dividing and consuming it) (see the one-dot chain line in FIG. 6). As a result, the OCV of the SOFC 10 can be suppressed more than when the shunt circuit 30 'is omitted (see the solid line and the broken line in FIG. 6).

«Third embodiment»
FIG. 8 is a conceptual diagram showing the configuration of the cell stack of the SOFC 10 of the third embodiment and the series-parallel switching mechanism of each cell. The SOFC 10 has a cell stack configured by stacking a first cell 10-1 and a second cell 10-2. In FIG. 8, the first cell 10-1 and the second cell 10-2 are functionally depicted as power supply units. When the first cell 10-1 and the second cell 10-2 can each output 25 kW, the SOFC 10 can output 50 kW.

The SOFC 10 functions as a series-parallel switching mechanism of the first transmission line DL1 and the second transmission line DL2, and the first transmission line DL1 and the second transmission line DL2 that are branched again from the direct current transmission line DL and then merged again. A third transmission line DL3 that connects the intermediate portions is provided. A first cell 10-1 is provided on the first transmission line DL1, and a second cell 10-2 is provided on the second transmission line DL2. The third transmission line DL3 connects the upstream portion of the first transmission line DL1 with respect to the first cell 10-1 and the downstream portion of the second transmission line DL2 with respect to the second cell 10-2. The first switch SW1 is provided on the upstream side of the connection portion of the first transmission line DL1 with the third transmission line DL3, and the second switch is provided on the downstream side of the connection portion of the second transmission line DL2 with the third transmission line DL3. A switch SW2 is provided. A third switch SW3 is provided at an intermediate portion of the third transmission line DL3. The SOFC 10 is provided with an open / close control unit (not shown) for switching the open / close state of the first switch SW1 to the third switch SW3.

As shown in FIG. 9A, when the first switch SW1 and the second switch SW2 are closed and the third switch SW3 is opened by the opening / closing control unit, the first cell 10-1 and the second cell 10-2 are opened. Are electrically connected in parallel.

As shown in FIG. 9B, when the first switch SW1 and the second switch SW2 are opened and the third switch SW3 is closed by the opening / closing control unit, the first cell 10-1 and the second cell 10-2 are closed. Are electrically connected in series.

10 will be described with reference to the timing chart of FIG. 10 and the flowchart of FIG. 11 from the startup of the fuel cell system 1 of the third embodiment to the start of rated operation. In the timing chart of FIG. 10, a solid line indicates a voltage applied to the SOFC 10, and a broken line indicates a voltage applied to the SOFC 10 when the control according to the third embodiment is not performed. The control of the third embodiment can be executed together with the control of the first and second embodiments, or can be executed independently of the control of the first and second embodiments. In the former case, the DC current extraction control from the transmission line DL by the shunt circuit 30 (discharge resistor 31) of the first and second embodiments, and the first cell 10-1 by the series-parallel switching mechanism of the third embodiment, The series / parallel switching control of the second cell 10-2 is executed in parallel.

In step ST1 ″, the open / close control unit (not shown) closes the first switch SW1 and the second switch SW2 and opens the third switch SW3, so that the first cell 10-1 and the second cell 10-2 are electrically connected in parallel (see FIG. 9A).

In step ST2 ″, an air blower (not shown) for sending fuel gas and oxidant gas to the SOFC 10 is turned on. In step ST3 ″, an air heating mechanism (not shown) for heating the fuel gas and oxidant gas. ) Is turned on. In step ST4 ″, it is determined whether or not the temperature of the SOFC 10 is equal to or higher than a predetermined value. If the temperature of the SOFC 10 is equal to or higher than the predetermined value (step ST4 ″: Yes), the process proceeds to step ST5 ″. If it is less than the predetermined value (step ST4 ″: No), it waits for the temperature of the SOFC 10 to become equal to or higher than the predetermined value.

In step ST5 ″, the supply of fuel gas and oxidant gas to the SOFC 10 is started, and the OCV of the SOFC 10 starts to rise (see FIG. 10). At this time, the first cell 10-1 and the second cell 10-2 are electrically connected. Therefore, the OCV of the SOFC 10 can be suppressed more than when the control of the present embodiment is not performed (see the solid line and the broken line in FIG. 10).

In step ST6 ″, it is determined whether or not the OCV of the SOFC 10 is equal to or higher than the first threshold voltage. If the OCV of the SOFC 10 is equal to or higher than the first threshold voltage (step ST6 ″: Yes), the process proceeds to step ST7 ″. If the OCV of the SOFC 10 is less than the first threshold voltage (step ST6 ″: No), it waits for the OCV of the SOFC 10 to be equal to or higher than the first threshold voltage. The “first threshold voltage” can be set to 400 V, for example.

In step ST7 ″, a part of the direct current generated by the SOFC 10 is extracted by at least one of the DC / AC converter 20 and the discharge resistor 31 (small load extraction). As a result, the OCV of the SOFC 10 is reduced from the first threshold voltage. It begins to fall gradually (see FIG. 10).

In step ST8 ″, it is determined whether or not the OCV of the SOFC 10 is equal to or lower than a second threshold voltage lower than the first threshold voltage. If the OCV of the SOFC 10 is equal to or lower than the second threshold voltage (step ST8 ″: Yes), the process proceeds to step ST9 ". If the OCV of the SOFC 10 is larger than the second threshold voltage (step ST8": No), the process waits for the OCV of the SOFC 10 to be equal to or lower than the second threshold voltage. The “second threshold voltage” can be set to 300 V, for example.

In step ST9 ″, the open / close control unit (not shown) opens the first switch SW1 and the second switch SW2 and closes the third switch SW3, so that the first cell 10-1 and the second cell are closed. 10-2 are electrically connected in series (see FIG. 9B), so that the OCV of the SOFC 10 rises to enable low-loss operation with high voltage and low current (see FIG. 10).

In step ST10 ", the power generated by the SOFC 10 is started to be input to the power generation load (for example, various auxiliary machines).

In step ST11 ″, it is determined whether or not the power generated by the SOFC 10 is a steady operation that has converged to the rated maximum power. If it is a steady operation (step ST11 ″: Yes), the process is terminated and the steady operation must be performed. If it is (step ST11 ″: No), it waits for a steady operation.

<< Fourth Embodiment >>
FIG. 12 is a conceptual diagram showing the configuration of the cell stack of the SOFC 10 of the fourth embodiment and the series-parallel switching mechanism of each cell. In the fourth embodiment, the termination capacitor EC is provided so as to straddle the first transmission line DL1 to the third transmission line DL3. The termination capacitor EC can suppress a surge voltage when the first switch SW1 to the third switch SW3 are opened and closed for series-parallel switching.

According to the fuel cell system 1 of the first to fourth embodiments described above, only the shunt circuit 30 is provided in the direct current transmission line DL from the SOFC 10 while maintaining the high power generation efficiency of the SOFC 10, and / or Or, it is possible to achieve stable operation by suppressing the OCV at the start-up of the SOFC 10 with a simple configuration and low cost by simply providing a serial / parallel switching mechanism for the first cell 10-1 and the second cell 10-2 of the SOFC 10 can do.

It should be noted that the present invention is not limited to the above embodiment, and can be implemented with various modifications. In the above-described embodiment, the size, shape, function, and the like of the components illustrated in the accompanying drawings are not limited thereto, and can be appropriately changed within a range in which the effects of the present invention are exhibited. In addition, various modifications can be made without departing from the scope of the object of the present invention.

In the first and second embodiments, the case where the relay switch 32 or the Zener diode 35 is used as the “switching unit” for switching the discharge resistor 31 between the connection state and the non-connection state to the transmission line DL has been described as an example. However, the configuration of the “switching unit” is not limited to these. For example, as the “switching unit”, a magnetic contactor, a self-extinguishing semiconductor element, or the like may be used, or a plurality of these may be used in combination (for example, a relay switch is arranged on the upstream side and the downstream side is used). A zener diode may be arranged). That is, as long as it has a function of switching the discharge resistor 31 between the connection state and the non-connection state to the transmission line DL, the configuration of the “switching unit” has a degree of freedom.

In the third and fourth embodiments, the case where the SOFC 10 has a cell stack in which the first cell 10-1 and the second cell 10-2 are stacked has been described as an example. However, the cells constituting the cell stack of the SOFC 10 are described. The number of stacks is not limited to this, and may be a cell stack in which three or more cells are stacked.

In the first to fourth embodiments, the combustor 50 is provided between the SOFC 10 and the exhaust heat recovery circulation system 60. However, the combustor 50 is omitted, and the exhaust gas discharged from the SOFC 10 is directly used. You may lead to the exhaust heat recovery circulation system 60.

The fuel cell system and the solid oxide fuel cell of the present invention are suitable for application to fuel cell systems in all industrial fields such as home use, business use and the like.

1 Fuel Cell System 10 Solid Oxide Fuel Cell (SOFC)
10-1 First cell 10-2 Second cell 12 Fuel gas flow path (anode gas flow path)
14 Oxidant gas channel (cathode gas channel)
16 Recycle gas flow path 20 DC / AC conversion part 25 System interconnection relay 30 30 'Shunt circuit 31 Discharge resistance 32 Relay switch (switching part)
33 Open / close control unit (switching unit)
34 Ground 35 Zener diode (constant voltage diode)
40 grid power network 50 combustor 60 exhaust heat recovery circulation system DL direct current transmission path DL1 first transmission path (series-parallel switching mechanism)
DL2 second transmission line (series-parallel switching mechanism)
DL3 3rd transmission line (series parallel switching mechanism)
SW1 1st switch (series-parallel switching mechanism)
SW2 2nd switch (series-parallel switching mechanism)
SW3 3rd switch (series-parallel switching mechanism)
EC termination capacitor L Surplus power transmission line LS Relay switch

Claims

A solid oxide fuel cell that generates electricity by an electrochemical reaction between a fuel gas and an oxidant gas and has a cell stack configured by stacking a plurality of cells;
A DC / AC converter for converting a direct current generated by the solid oxide fuel cell into an alternating current;
A shunt circuit connectable to a direct current transmission path from the solid oxide fuel cell to the DC / AC converter;
Have
The shunt circuit is
A discharge resistor for extracting a part of the direct current generated by the solid oxide fuel cell;
A switching unit that switches the discharge resistance between a connection state and a non-connection state to the transmission line;
Have
The switching unit connects the discharge resistance to the transmission line until the solid oxide fuel cell is started or when an open circuit voltage at the start of the solid oxide fuel cell reaches a predetermined threshold voltage. Switch from disconnected to connected state,
The switching unit switches the discharge resistance from a connection state to the transmission line to a non-connection state when a direct current flowing into the DC / AC conversion unit reaches a predetermined threshold current,
The solid oxide fuel cell has a series-parallel switching mechanism that switches whether each cell of the cell stack is electrically connected in series or connected in parallel.
When starting up the solid oxide fuel cell, the series-parallel switching mechanism electrically connects the cells of the cell stack in parallel,
When the open circuit voltage of the solid oxide fuel cell reaches the first threshold voltage, a part of the direct current generated by the solid oxide fuel cell is converted into the DC / AC conversion unit and the discharge resistance. Pull out by at least one,
When the open circuit voltage of the solid oxide fuel cell drops to a second threshold voltage lower than the first threshold voltage, the series-parallel switching mechanism electrically connects the cells of the cell stack in series. Connecting,
A fuel cell system.
2. The fuel cell system according to claim 1, wherein the switching unit includes any one or a combination of a magnetic contactor, a relay switch, a Zener diode, and a self-extinguishing semiconductor element.