JP2013026179A - Sofc power generation system and output voltage recovery method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To recover an output voltage of a cell whose output voltage is reduced by a high fuel utilization rate and low load operation.SOLUTION: An SOFC power generation system comprises: a plurality of power generation modules 1; a voltage measuring unit 14 measuring output voltages of the power generation modules 1 per power generation module 1; a switch 15 connecting or blocking an output terminal of the power generation module 1 and a load R of a later step per power generation module 1; a supplying valve 16 controlling a flow rate of fuel gas; and a control part 17 controlling the switch 15 to separate the output terminals of the power generation modules 1 whose output voltage becomes not more than a threshold from the load R, and control the supplying valve 16 to fill a fuel gas passage contacting a fuel electrode of the power generation module 1 whose output voltage becomes not more than the threshold with the fuel gas, when the output voltage of at least one of the plural power generation modules 1 becomes not more than a predetermined threshold.

Description

  The present invention relates to a method for operating a power generation system using a solid oxide fuel cell, and more particularly to a technique for recovering the output voltage of the fuel cell when the output voltage decreases during power generation.

Power generation using solid oxide fuel cells (SOFC), which is attracting attention as one of the new distributed power sources, has higher efficiency and lower CO ₂ emissions than other power generation methods, regardless of power generation capacity. It is characterized by its ability to generate electricity, and is attracting attention as a distributed generator in various fields, together with the ability to supply high-quality hot water through high-temperature exhaust heat. Among several types of SOFCs currently under development, the flat plate type SOFC is composed of a flat fuel electrode, an electrolyte, and an air electrode, and a reducing fuel is provided on the fuel electrode side of the cell. Electric power is generated by supplying gas and supplying oxygen and air to the air electrode side. Flat type SOFC can form a stack by stacking flat fuel cells, so that there are few wasted voids inside the stack, and it is possible to reduce the size and energy density of the stack. Patent Document 1).

H.Arai, et al., “3kW SOFC Power Generation Module Developed by NTT / THG / SPP”, ECS Transactions, 25 (2), pp.125-132, 2009

  However, in the conventional flat plate type SOFC, when the low load operation is continued while maintaining the high utilization rate of the fuel gas, there is a problem that the output voltage gradually decreases and the SOFC does not function finally. This problem was particularly noticeable when trying to obtain small power in a system using a large SOFC cell. Such a problem is caused by the fact that fuel gas is supplied to the entire surface of the fuel cell side of the flat plate cell via the fuel gas passage with a limited thickness. Along with this, the reducing gas concentration in the fuel gas decreases as it flows in the direction from the inlet of the flow path to the outlet, and this is considered to occur because a sufficient fuel gas concentration cannot be secured in the vicinity of the outlet of the flow path.

  If more fuel gas is supplied than the amount necessary for power generation so that a sufficient fuel gas concentration is ensured even near the outlet of the flow path, the output voltage does not decrease, but one of the fuel gases supplied to the cell does not occur. The part is discharged unreacted in the exhaust gas, leading to a decrease in fuel utilization.

  The present invention has been made to solve the above problems, and provides a system and an output voltage recovery method capable of recovering the output voltage of an SOFC cell whose output voltage has decreased due to high fuel utilization and low load operation. For the purpose.

  The SOFC power generation system of the present invention includes a power generation module having an SOFC stack using one or more solid oxide fuel cell (SOFC) cells, fuel supply means for supplying fuel gas to the power generation module, and the power generation Voltage measuring means for measuring the output voltage of the module, switching means for connecting or disconnecting between the output terminal of the power generation module and a subsequent load, and when the output voltage of the power generation module falls below a predetermined threshold A control means for controlling the switching means to disconnect the output terminal of the power generation module from the load, and for controlling the fuel supply means to fill a fuel gas flow path in contact with the fuel electrode of the power generation module. It is characterized by comprising.

  The SOFC power generation system of the present invention also includes a plurality of power generation modules each having a SOFC stack using one or more solid oxide fuel cell (SOFC) cells, and a fuel supply for supplying fuel gas to each power generation module Means, voltage measuring means for measuring the output voltage of the power generation module for each power generation module, switching means for connecting or shutting off between the output terminal of the power generation module and the subsequent load for each power generation module, When the output voltage of at least one power generation module among the power generation modules is equal to or lower than a predetermined threshold value, the switching means is controlled so that the output terminal of the power generation module whose output voltage is equal to or lower than the threshold value is removed from the load. At the same time, the fuel supply means is controlled to contact the fuel electrode of the power generation module whose output voltage is below the threshold value. It is characterized in that a control means to meet the fuel gas flow passage to the fuel gas that.

Further, in one configuration example of the SOFC power generation system of the present invention, the control means controls the fuel supply means when the output voltage of the power generation module falls below a predetermined threshold value, and supplies the power to the power generation module. It is characterized in that the flow rate of hydrogen is less than the amount necessary for normal power generation.
In addition, one configuration example of the SOFC power generation system of the present invention further includes fuel discharge means for discharging fuel gas used for power generation by the power generation module, and the control means has a predetermined output voltage of the power generation module. The fuel supply means and the fuel discharge means are controlled when the fuel gas becomes below a threshold value, and the fuel gas is sealed in the fuel gas flow path in contact with the fuel electrode of the power generation module. .
In addition, one configuration example of the SOFC power generation system of the present invention further includes fuel discharge means for discharging fuel gas used for power generation by the power generation module, and the control means has a predetermined output voltage of the power generation module. The fuel supply means and the fuel discharge means are controlled to circulate the fuel gas from the fuel gas outlet of the power generation module to the fuel gas inlet when the threshold value is below the threshold value. .
In one configuration example of the SOFC power generation system of the present invention, the threshold value is 0.2 V per single cell.

  Further, the output voltage recovery method of the SOFC power generation system of the present invention includes a voltage measurement step of measuring an output voltage of a power generation module having a SOFC stack using one or a plurality of solid oxide fuel cell (SOFC) cells, When the output voltage of the power generation module is equal to or lower than a predetermined threshold, the output voltage of the power generation module is disconnected from the subsequent load and the fuel gas flow path in contact with the fuel electrode of the power generation module fills the fuel gas. And a recovery step.

  Further, the output voltage recovery method of the SOFC power generation system according to the present invention provides the output voltage of a plurality of power generation modules each having a SOFC stack using one or more solid oxide fuel cell (SOFC) cells for each power generation module. A voltage measurement step for measuring, and when an output voltage of at least one power generation module among the plurality of power generation modules is equal to or lower than a predetermined threshold, an output terminal of the power generation module whose output voltage is equal to or lower than the threshold is And an output voltage recovery step for filling the fuel gas flow path in contact with the fuel electrode of the power generation module whose output voltage is equal to or lower than the threshold value.

  According to the present invention, the output voltage can be recovered even if the output voltage of the power generation module decreases due to high fuel utilization and low load operation.

  In the present invention, even if the output voltage of at least one power generation module among the plurality of power generation modules is reduced, the output voltage can be recovered, and even one power generation module whose output voltage is larger than the threshold value can be recovered. If so, it is possible to recover the output voltage of the power generation module whose performance has deteriorated due to high utilization and low load operation without stopping the power generation of the entire system.

It is a block diagram which shows the structure of the SOFC electric power generation system which concerns on embodiment of this invention. The figure which shows the time-dependent change of the output voltage and output current of a single cell stack when the single cell stack is operated at a high fuel utilization rate and a low load and when it is recovered by the output voltage recovery processing according to the embodiment of the present invention. It is. It is a figure which shows the current-voltage curve before recovering with the output voltage recovery process which concerns on embodiment of this invention before operating a single cell stack by a high fuel utilization factor and low load.

[Principle of the Invention]
In the present invention, when the cell output voltage drops below a certain threshold due to high utilization and low load operation, power generation is stopped, the output terminal is opened, and the surface of the fuel electrode is kept in a reducing atmosphere. To restore the function of the cell.
In addition, the present invention measures the output voltage of each module at any time in an SOFC power generation system composed of two or more modules capable of individually controlling the generated power and the flow rate of the supplied gas. The power generation of only the module that is below a certain threshold is stopped, the output terminal is set to an open circuit, and the fuel electrode surface in the module is left in a reducing atmosphere to recover the module output voltage.

  As a method of making the surface of the fuel electrode into a reducing atmosphere, a small amount of fuel gas is kept flowing, or after the power generation is stopped, the fuel gas is once flowed into the fuel gas passage and the gas in the passage is replaced with unreacted fuel gas. Later, it is conceivable to close the outlet and inlet of the gas flow path and fill the gas flow path with fuel gas. The fuel gas used to reduce the anode material is only a part of the gas introduced into the module. Therefore, when a small amount of fuel gas is kept flowing, the fuel gas discharged from the module outlet is supplied to the module inlet. It can be returned and circulated.

  The reason why the output voltage of the SOFC cell decreases due to high fuel utilization and low load operation is that the electrode material is oxidized due to the reduction of the reducing gas partial pressure in the fuel gas, and there is a substance with low conductivity in the fuel electrode. This is probably because a voltage drop (IR drop) due to the resistance component occurs. When fuel gas is introduced with the output terminal of the SOFC cell open, the highly reducible fuel gas is supplied to the vicinity of the outlet of the fuel gas flow path with almost no change in concentration, so it is oxidized during power generation. The fuel electrode material is reduced again, and no voltage drop occurs.

  The reason for stopping power generation when the output voltage of the SOFC cell drops below a certain threshold is that if the voltage is lowered further, cracks will occur in the electrolyte, and the cell output voltage will not recover even if reduction is performed. Because. When the fuel electrode material is oxidized, volume expansion occurs. However, while the oxidation range is limited, the porous fuel electrode itself absorbs the material expansion, so that the stress applied to the solid electrolyte thin film is not so great. However, since the fuel electrode begins to expand as a whole when the oxidation proceeds to a wider range, cracks occur in the solid electrolyte thin film formed on the fuel electrode. Therefore, it is necessary to stop the power generation in a range where the oxidation of the fuel electrode material is limited and cracks do not occur in the electrolyte. On the other hand, if the power generation is stopped at a higher voltage, the output voltage is recovered by a short recovery process, but the frequency of the recovery process increases and the operation becomes complicated.

  In addition, in an SOFC power generation system composed of one module, power generation of the system stops when the operation of the module is stopped for recovery. However, if the SOFC power generation system is composed of two or more modules that can individually control the generated power and the flow rate of the gas to be supplied, the output voltage of each module is measured at any time. When the voltage falls below a certain threshold, power generation of only that module is stopped, the output terminal is opened, and fuel gas is present in the fuel gas flow path in contact with the fuel electrode in the module. By standing still, it becomes possible to recover the output voltage of the module without stopping the power generation as a system.

[Embodiment]
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing a configuration of a SOFC power generation system according to an embodiment of the present invention.
The SOFC power generation system includes a power generation module 1 having an SOFC stack using one or more SOFC cells, an air supply manifold 12 that supplies air to the power generation module 1, and hydrogen that is a fuel gas to the power generation module 1. A fuel supply manifold 13, a voltage measuring device 14 for measuring the output voltage of the power generation module 1, a switch 15 serving as switching means for connecting or disconnecting the output terminal of the power generation module 1 and the load R at the subsequent stage, and fuel gas It comprises a supply valve 16 for controlling the flow rate of the gas and a control unit 17.

  The SOFC stack of the power generation module 1 is electrically connected to the electrolyte 2, the air electrode 3 disposed on one surface of the electrolyte 2, the fuel electrode 4 disposed on the opposite surface of the electrolyte 2, and the air electrode 3. The plate-shaped air electrode side separator 5 in contact, the plate-shaped fuel electrode side separator 6 in electrical contact with the fuel electrode 4, and the insulation that electrically insulates the air electrode side separator 5 and the fuel electrode side separator 6 from each other. And material 7. In this embodiment, an SOFC stack using one SOFC cell composed of the electrolyte 2, the air electrode 3, and the fuel electrode 4, ie, a single cell stack will be described as an example. However, a plurality of single cell stacks are stacked. The SOFC stack may be used.

  An air supply path 8 for supplying air and a fuel supply path 9 for supplying fuel gas are formed in the air electrode side separator 5, the fuel electrode side separator 6, and the insulating material 7. Further, the air electrode side separator 5 is formed with an air supply channel 10 for supplying air from the air supply path 8 to the air electrode 3, and the fuel electrode side separator 6 is supplied with fuel gas from the fuel supply path 9. A fuel supply channel 11 for supplying the fuel electrode 4 is formed. The inlet of the air supply channel 10 is connected to the air supply channel 8, and the inlet of the fuel supply channel 11 is connected to the fuel supply channel 9. The fuel supply path 9, the fuel supply manifold 13, and the supply valve 16 constitute fuel supply means. In the present embodiment, two power generation modules 1 configured as described above are installed.

  In FIG. 1, the air discharge channel provided in the air electrode side separator 5, the fuel discharge channel provided in the fuel electrode side separator 6, and the outlet of the air discharge channel are connected to discharge the air to the outside. The description of the air discharge path and the fuel discharge path that is connected to the outlet of the fuel discharge flow path and discharges the fuel gas to the outside is omitted. A fuel discharge path, a discharge valve (not shown) provided in the fuel discharge path, and a connection valve (not shown) provided in the fuel discharge path constitute fuel discharge means.

  Next, the normal operation of the SOFC power generation system will be described. Air is supplied from the air supply manifold 12 to the air electrode 3 of each SOFC cell via the air supply path 8 and the air supply flow path 10 provided for each SOFC cell. Hydrogen is supplied from the fuel supply manifold 13 to the fuel electrode 4 of each SOFC cell via the fuel supply path 9 and the fuel supply channel 11 provided for each SOFC cell. An air electrode side output terminal 18 is connected to the air electrode side separator 5, and a fuel electrode side output terminal 19 is connected to the fuel electrode side separator 6, and the air electrode side output terminal 18 and the fuel electrode side output terminal 19 are connected. A load R is connected between the two. In normal times, the switch 15 is in the closed state, so that the electric power generated by the SOFC power generation system is supplied to the load R. In the present embodiment, since two power generation modules 1 are used, two power generation modules 1 are connected in parallel.

  The air that has been used for power generation passes through an air discharge passage provided for each SOFC cell, and is discharged to the outside through an air discharge path. The hydrogen used for power generation passes through the fuel discharge channel provided for each SOFC cell and is discharged to the outside through the fuel discharge path.

Next, the operation when the output voltage of the power generation module 1 decreases due to the high fuel utilization rate / low load operation will be described.
The voltage measuring instrument 14 constantly measures the output voltage of the power generation module 1 (the voltage between the air electrode side separator 5 and the fuel electrode side separator 6).
When the output voltage of at least one power generation module 1 out of the two power generation modules 1 falls below a certain threshold value, the control unit 17 sets the switch 15 corresponding to the power generation module 1 whose output voltage falls below the threshold value. In an open state, the power generation module 1 is disconnected from the load R, and the supply valve 16 corresponding to the power generation module 1 whose output voltage is equal to or lower than the threshold value is controlled, so that the hydrogen flow rate to the power generation module 1 Less than the amount required for power generation.

In this way, power generation is stopped only in the power generation module 1 whose output voltage is below the threshold, the output terminal is opened, and the fuel electrode surface in the power generation module 1 is left in a reducing atmosphere to generate power. The output voltage of module 1 is restored.
For the power generation module 1 whose output voltage is equal to or less than the threshold value, the control unit 17 connects a connection valve (not shown) that connects the fuel discharge path on the outlet side of the power generation module 1 and the fuel supply manifold 13 on the inlet side. ) And returning the hydrogen discharged from the fuel discharge path of the power generation module 1 to the fuel supply manifold 13, the hydrogen may be circulated.

  Further, the control unit 17 supplies hydrogen to the power generation module 1 for a while when the output voltage of the power generation module 1 becomes a threshold value or less, so that the fuel supply flow path 11 and the fuel discharge flow path of the power generation module 1 After replacing the gas with unreacted hydrogen, a discharge valve (not shown) provided in the fuel discharge path on the outlet side of the power generation module 1 and a supply valve 16 provided on the fuel supply manifold 13 on the inlet side, The fuel supply channel 11 and the fuel discharge channel may be filled with hydrogen by closing.

  In this embodiment, the SOFC power generation system using two power generation modules 1 is described as an example. However, the present invention is not limited to this, and the SOFC power generation system is configured by one power generation module. Needless to say.

  FIG. 2 shows a case where a single cell stack using a flat plate type SOFC cell having a diameter φ = 120 mm is operated at a high fuel utilization rate and a low load (fuel utilization rate 95%, discharge current 25 A), and the output of the present embodiment. It is a figure which shows the time-dependent change of the output voltage V and the output current I of a single cell stack when it is made to recover by a voltage recovery process. Here, the threshold value is 0.2V. When operating at high fuel utilization and low load, the output voltage V gradually decreases even when operating at a constant current value. In the example of FIG. 2, the output voltage V reaches the threshold value of 0.2 V around 480 minutes. When the power generation is stopped at this time and the single cell stack is recovered by the output voltage recovery processing of this embodiment, 0 is output. It can be seen that the output voltage V recovers to around .9V. Furthermore, when the output voltage recovery was continued, the output voltage V rose to about 1.2 V in about 30 minutes.

  FIG. 3 shows the output voltage of the present embodiment before operating at a high fuel utilization and low load (fuel utilization 95%, discharge current 25 A) in a single cell stack using a flat plate SOFC cell having a diameter φ = 120 mm. It is a figure which shows the electric current-voltage curve after making it recover by a recovery process. 3, 30 is a current-voltage curve before the single cell stack is operated at a high fuel utilization rate and low load, and 31 is a current-voltage after the single cell stack is recovered by the output voltage recovery processing of the present embodiment. It is a curve. There is almost no change in the two curves, and even in a single cell stack in which the output voltage V has once decreased to 0.2 V, the current-voltage characteristics similar to those before the operation are obtained by recovering with the output voltage recovery processing of the present embodiment. It can be seen that That is, FIG. 3 shows that the performance of the cell in which the output voltage V has decreased can be recovered by the output voltage recovery process of the present embodiment.

  When the threshold value for stopping power generation was examined, when the threshold value was lower than 0.2 V, such a reliable recovery of power generation performance was not seen. Table 1 shows the ratio of cells whose output voltage has been recovered by the output voltage recovery process of the present embodiment and the ratio of cells in which cracks have been confirmed in the electrolyte after the output voltage recovery process for each threshold value. The test at this time was implemented using four cells for each threshold.

  When the threshold value for stopping power generation was 0.2 V or more, the output voltage was 100%, and even when the SOFC cell was observed after supplying hydrogen, the occurrence of cracks in the electrolyte was not confirmed. On the other hand, when the threshold value was set to 0.1V, only one cell out of the four cells recovered the output voltage, and when the threshold value was set to 0V, no recovery of the output voltage was observed. In addition, when the threshold value was set to 0 V, occurrence of cracks was observed in the electrolytes of all cells. Since the threshold value 0.2V is a value per unit cell, when stacking n (n is an integer of 2 or more) single cells, the threshold value may be 0.2 × nV.

  The control unit 17 according to the present embodiment can be realized by, for example, a computer including a CPU, a storage device, and an interface, and a program that controls these hardware resources. The CPU executes the processing described in the present embodiment in accordance with a program stored in the storage device.

  The present invention can be applied to a technique for operating a power generation system using a solid oxide fuel cell.

  DESCRIPTION OF SYMBOLS 1 ... Power generation module, 2 ... Electrolyte, 3 ... Air electrode, 4 ... Fuel electrode, 5 ... Air electrode side separator, 6 ... Fuel electrode side separator, 7 ... Insulating material, 8 ... Air supply path, 9 ... Fuel supply path, DESCRIPTION OF SYMBOLS 10 ... Air supply flow path, 11 ... Fuel supply flow path, 12 ... Air supply manifold, 13 ... Fuel supply manifold, 14 ... Voltage measuring device, 15 ... Switch, 16 ... Supply valve, 17 ... Control part, 18 ... Air Electrode side output terminal, 19 ... Fuel electrode side output terminal.

Claims (8)

A power generation module having a SOFC stack using one or more solid oxide fuel cells (SOFC) cells;
Fuel supply means for supplying fuel gas to the power generation module;
Voltage measuring means for measuring an output voltage of the power generation module;
Switching means for connecting or disconnecting between the output terminal of the power generation module and the subsequent load;
When the output voltage of the power generation module falls below a predetermined threshold, the switching means is controlled to disconnect the output terminal of the power generation module from the load, and the fuel supply means is controlled to control the fuel of the power generation module. A SOFC power generation system comprising: a control means for filling a fuel gas in a fuel gas flow path in contact with an electrode. A plurality of power generation modules each having a SOFC stack using one or more solid oxide fuel cells (SOFC) cells;
Fuel supply means for supplying fuel gas to each power generation module;
Voltage measuring means for measuring the output voltage of the power generation module for each power generation module;
Switching means for connecting or blocking each power generation module between the output terminal of the power generation module and the load on the subsequent stage;
When the output voltage of at least one power generation module among the plurality of power generation modules is equal to or lower than a predetermined threshold value, the switching means is controlled so that the output terminal of the power generation module whose output voltage is equal to or lower than the threshold value And a control means for controlling the fuel supply means to fill the fuel gas flow path in contact with the fuel electrode of the power generation module whose output voltage is equal to or lower than a threshold, while disconnecting from the load. SOFC power generation system. The SOFC power generation system according to claim 1 or 2,
The control means controls the fuel supply means when the output voltage of the power generation module falls below a predetermined threshold value, so that the flow rate of hydrogen to the power generation module is higher than the amount necessary for normal power generation. SOFC power generation system characterized by a reduction. The SOFC power generation system according to claim 1 or 2,
Furthermore, it comprises a fuel discharge means for discharging the fuel gas used for power generation in the power generation module,
The control means controls the fuel supply means and the fuel discharge means when the output voltage of the power generation module becomes a predetermined threshold value or less, and in the fuel gas flow path in contact with the fuel electrode of the power generation module. An SOFC power generation system characterized by sealing the fuel gas at the bottom. The SOFC power generation system according to claim 3,
Furthermore, it comprises a fuel discharge means for discharging the fuel gas used for power generation in the power generation module,
The control means controls the fuel supply means and the fuel discharge means when the output voltage of the power generation module falls below a predetermined threshold, and the fuel gas inlet to the fuel gas inlet of the power generation module SOFC power generation system characterized by circulating fuel gas to The SOFC power generation system according to any one of claims 1 to 5,
The SOFC power generation system, wherein the threshold is 0.2 V per unit cell. A voltage measurement step of measuring an output voltage of a power generation module having a SOFC stack using one or more solid oxide fuel cell (SOFC) cells;
When the output voltage of the power generation module is equal to or lower than a predetermined threshold, the output voltage of the power generation module is disconnected from the subsequent load and the fuel gas flow path in contact with the fuel electrode of the power generation module fills the fuel gas. And a recovery step. An output voltage recovery method for an SOFC power generation system, comprising: a recovery step. A voltage measurement step of measuring output voltages of a plurality of power generation modules each having a SOFC stack using one or a plurality of solid oxide fuel cells (SOFC) cells for each power generation module;
When the output voltage of at least one power generation module among the plurality of power generation modules is equal to or lower than a predetermined threshold, the output terminal of the power generation module whose output voltage is equal to or lower than the threshold is disconnected from the subsequent load, and An output voltage recovery method for an SOFC power generation system, comprising: an output voltage recovery step for filling the fuel gas flow path in contact with the fuel electrode of the power generation module whose output voltage is equal to or less than a threshold value to fill the fuel gas.

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