Method for stopping operation of a fuel cell and fuel cell system

Abstract

This suppresses structural loads caused by thermal stress applied to the fuel cell components during shutdown. [Solution] A method for shutting down a fuel cell, comprising an oxidizer flow path through which oxidizer flows in the order of the second stack and the first stack, and a fuel flow path through which fuel flows in the order of the first stack and the second stack, wherein the fuel cell is mechanically connected to an auxiliary structure, and the method comprises an oxidizer reduction step, a fuel supply stop step, and a temperature difference control step after both of these steps, which controls the temperature difference between the first stack and the second stack, wherein the temperature difference control step restarts fuel supply when the first temperature difference, which is the difference between the first temperature, which is the temperature at the oxidizer inlet portion of the second stack, and the second temperature, which is the temperature at the oxidizer outlet portion of the first stack, widens to an upper limit temperature difference, and controls the flow rates of fuel and oxidizer so that the first temperature difference becomes less than the upper limit temperature difference.

Description

Technical Field

[0004] , , , , , , ,

[0005] , , ,

[0001] The present invention relates to a method for stopping operation of a fuel cell and a fuel cell system.

Background Art

[0002] There is known a fuel cell in which a plurality of fuel cell stacks are mechanically connected to an auxiliary structure including auxiliary equipment such as a combustor. In Patent Document 1, a fuel cell is disclosed in which a first stack and a second stack are arranged in the stacking direction of fuel cell cells via an auxiliary structure, and end plates are provided at ends of the fuel cell stacks on the side opposite to the connection portions with the auxiliary structure, respectively.

[0003] When fixing a fuel cell to an external system (a vehicle body in the case of an in-vehicle fuel cell), it is common to fix an end plate to the external system. For the fuel cell in the above document, either the end plate of the first stack or the second stack, or the end plates of both stacks are fixed to the external system. Further, in the fuel cell in the above document, the fuel cell stack is held by an end plate and an auxiliary structure so as to be always in a state where a compressive load in the stacking direction is applied. Therefore, the first stack and the auxiliary structure, the second stack and the auxiliary structure, or the first stack, the second stack, and the auxiliary structure are in a constrained state.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] During the investigation, it was discovered that in fuel cells with the configuration described above, a temperature difference develops between the first and second stacks over time, especially when supplying an oxidizer at room temperature during shutdown. This temperature difference causes thermal strain at the connection points between the first stack and the auxiliary structure, and between the second stack and the auxiliary structure, due to the difference in expansion rates, resulting in thermal stress. This can then impose structural loads on the fuel cell components due to thermal stress. From a durability standpoint, it is preferable to avoid such structural loads.

[0007] Therefore, the present invention aims to provide a method for operating a fuel cell while it is stopped, which can suppress the structural load caused by thermal stress applied to the fuel cell components during stopped operation. [Means for solving the problem]

[0008] According to one aspect of the present invention, a method for stopping power generation of a fuel cell is provided, comprising an oxidizer flow path through which an oxidizer flows in the order of the second stack and the first stack, and a fuel flow path through which fuel flows in the order of the first stack and the second stack, with the first stack and the second stack mechanically connected to an auxiliary structure. This method comprises an oxidizer reduction step of reducing the amount of oxidizer supplied, a fuel supply stop step of stopping the fuel supply, and a temperature difference control step of controlling the temperature difference between the first stack and the second stack after the oxidizer reduction step and the fuel supply stop step. The temperature difference control step comprises a temperature measurement step of obtaining a first temperature, which is the temperature at the oxidizer inlet portion of the second stack, and a second temperature, which is the temperature at the oxidizer outlet portion of the first stack, and a flow rate control step of restarting the fuel supply and controlling the fuel flow rate and the oxidizer flow rate so that the first temperature difference is less than the upper limit temperature difference when the first temperature difference, which is the difference between the first temperature and the second temperature, widens to an upper limit temperature difference, which is a preset temperature difference. [Effects of the Invention]

[0009] According to the above embodiment, it is possible to suppress the structural load caused by thermal stress applied to the components of the fuel cell during shutdown operation. [Brief explanation of the drawing]

[0010] [Figure 1] Figure 1 is a schematic block diagram showing a fuel cell according to this embodiment. [Figure 2] Figure 2 is a flowchart illustrating the procedure for shutting down a fuel cell. [Figure 3] Figure 3 is a graph showing an example of the relationship between the fuel-oxidizer mixture ratio and combustion temperature. [Figure 4] Figure 4 is a flowchart showing the stopping operation method related to control example 1. [Figure 5] Figure 5 is a graph showing the relationship between time and temperature when using control example 1. [Figure 6] Figure 6 is a graph showing the relationship between time and temperature when using control example 2. [Figure 7] Figure 7 is a graph showing an example of a mixing ratio range used in flow rate control. [Figure 8] Figure 8 is a graph showing another example of the mixing ratio range used in flow rate control. [Figure 9] Figure 9 is a graph showing an example of the relationship between oxidant flow rate and pressure inside the oxidant flow path. [Modes for carrying out the invention]

[0011] Embodiments of the present invention will be described below with reference to the drawings.

[0012] (fuel cell) First, the configuration of the fuel cell warmed by the warming method according to this embodiment will be described. Figure 1 is a schematic block diagram showing the fuel cell 1 according to this embodiment. As shown in Figure 1, this fuel cell 1 has a first stack 3-1, a second stack 3-2, an auxiliary structure 4, an oxidant supply device 6-1 (first oxidant supply device), an oxidant supply device 6-2 (second oxidant supply device), a fuel supply device 7, a partial oxidation catalyst 8, a first temperature sensor 9-1, a second temperature sensor 9-2, an oxidant flow path 10-1, and a fuel flow path 10-2.

[0013] The first stack 3-1 and the second stack 3-2 are the parts that realize the function of a fuel cell, that is, the power generation function. Although not shown in the diagram, each stack is provided with a fuel electrode layer, an air electrode layer, and an electrolyte layer. During operation, fuel is supplied to the fuel electrode layer and an oxidizer is supplied to the air electrode layer.

[0014] In Figure 1, although they are depicted separately for clarity, the first stack 3-1 and the second stack 3-2 are mechanically connected. In this embodiment, the first stack 3-1 and the second stack 3-2 are mechanically connected via the auxiliary structure 4.

[0015] The auxiliary equipment structure 4 houses peripheral equipment such as the combustor 5.

[0016] The oxidant supply device 6-1 is configured to supply an oxidant to the fuel cell 1. For example, air is used as the oxidant. Specifically, the oxidant supply device 6-1 supplies the oxidant to the fuel cell 1 via the oxidant flow path 10-1. The oxidant flow path 10-1 connects the components such that the oxidant flows through the combustor 5, the second stack 3-2, the first stack 3-1, and the combustor 5 in this order. Specifically, the oxidant supplied by the oxidant supply device 6-1 is first heat-exchanged and heated in the combustor 5. The heated oxidant is supplied to the air electrode of the second stack 3-2. Next, the oxidant is sent from the second stack 3-2 to the air electrode of the first stack 3-1. Next, the oxidant is sent from the first stack 3-1 to the combustor 5 and used for fuel combustion in the combustor 5. In this specification, some components within the first stack 3-1 and the second stack 3-2 can also be regarded as part of the oxidant flow path 10-1.

[0017] The fuel supply device 7 is configured to supply fuel to the fuel cell 1. For example, hydrocarbon fuels such as methane are used as the fuel. Specifically, the fuel supply device 7 supplies the fuel to the fuel cell 1 via the fuel flow path 10-2. The fuel flow path 10-2 connects the components such that the fuel flows through the first stack 3-1, the second stack 3-2, and the combustor 5 in this order. Specifically, the fuel is first sent to the fuel electrode layer of the first stack 3-1. Next, the fuel is sent from the first stack 3-1 to the fuel electrode layer of the second stack 3-2. Next, the fuel is sent from the second stack 3-2 to the combustor 5 and burned. In this specification, some components within the first stack 3-1 and the second stack 3-2 can also be said to constitute part of the fuel flow path 10-2.

[0018] The partial oxidation catalyst 8 is provided for fuel reforming and the like. The partial oxidation catalyst 8 is provided on the fuel flow path 10-2. In the example shown in FIG. 1, the partial oxidation catalyst 8 is provided inside the first stack 3-1. Specifically, the partial oxidation catalyst 8 is provided in the fuel electrode layer of the first stack 3-1. However, the partial oxidation catalyst 8 may be provided outside the first stack 3-1. That is, the partial oxidation catalyst 8 may be provided on the fuel flow path 10-2 upstream of the first stack 3-1. Further, the partial oxidation catalyst 8 may be provided together with other internal reforming catalysts and the like.

[0019] The oxidant supply device 6-2 is configured to add an oxidant to the fuel supplied to the partial oxidation catalyst 8. In the example shown in FIG. 1, the oxidant supply device 6-2 is configured to add an oxidant to the fuel flow path 10-2 upstream of the partial oxidation catalyst 8 (upstream of the first stack 3-1). When fuel and an oxidant are supplied to the partial oxidation catalyst 8, an exothermic reaction proceeds on the partial oxidation catalyst 8. The heat generated by this exothermic reaction is used for temperature control in the warm air method described later.

[0020] Note that examples of the exothermic reaction include a partial oxidation reaction (hereinafter sometimes referred to as a POX reaction) and a complete combustion reaction. For example, when the fuel is methane, the partial oxidation reaction is represented by CH4 + 0.5O2 → CO + H2. The complete combustion reaction is represented by CH4 + 2O2 → CO2 + 2H2O. Whether the partial oxidation reaction proceeds or the complete combustion reaction proceeds is determined by the flow rates of the fuel and the oxidant and the like.

[0021] The first temperature sensor 9-1 is configured to measure the temperature at the oxidant inlet portion in the second stack 3-2 as the first temperature T1. The second temperature sensor 9-2 is configured to measure the temperature at the oxidant outlet portion in the first stack 3-1 as the second temperature T2. In the present embodiment, the first temperature T1 is regarded as the temperature of the second stack 3-2, and the second temperature T2 is regarded as the temperature of the first stack 3-1.

[0022] Note that the first temperature T1 and the second temperature T2 do not necessarily need to be measured directly. For example, the temperature of the oxidizer before it is supplied to the second stack 3-2 reflects the temperature of the oxidizer inlet portion of the second stack 3-2. Therefore, the first temperature sensor 9-1 may be configured to measure the temperature of the oxidizer before it is supplied to the second stack 3-2. Similarly, the temperature of the oxidizer after it has been discharged from the first stack 3-1 reflects the temperature of the oxidizer outlet portion of the first stack 3-1. Therefore, the second temperature sensor 9-2 may be configured to measure the temperature of the oxidizer downstream of the first stack 3-1.

[0023] The above describes the configuration of the fuel cell 1 that is warmed up in this embodiment.

[0024] (Method for shutting down a fuel cell system) Next, a method for shutting down the fuel cell system according to this embodiment will be described. The main entity of each operation included in the shutdown operation method described below is not particularly limited. However, in this embodiment, the case where the main entity of each operation is the control device 2 will be described. That is, as shown in Figure 1, the fuel cell 1 is connected to the control device 2. The control device 2 is a computer. The control device 2 performs its functions by executing a control program, which is stored in a storage device such as ROM, by an arithmetic unit such as a CPU. In this specification, the fuel cell 1 and the control device 2 together may be referred to as the fuel cell system.

[0025] Figure 2 is a flowchart illustrating a schematic method for shutting down a fuel cell system. As shown in Figure 2, this shutdown method includes an oxidizer reduction step (S1) to reduce the amount of oxidizer supplied, a fuel supply shutdown step (S2) to stop the fuel supply, and a temperature difference control step (S3) to control the temperature difference between the first stack 3-1 and the second stack 3-2. Steps S1 and S2 may be performed in reverse order or simultaneously. Each step will be described below.

[0026] (Step S1) Reduction in oxidizing agent When stopping power generation from fuel cell 1, the amount of oxidizer supplied to fuel cell 1 is reduced. For example, if a blower is used as the oxidizer supply device 6-1, the rotation speed of the blower is reduced. The supply of oxidizer may be stopped, but in this embodiment, in order to quickly lower the temperature of fuel cell 1, the supply is continued at a reduced rate rather than being stopped. As combustion in the combustor 5 stops due to the stopping of fuel supply, which will be described later, room temperature air (oxidizer) is supplied, and the temperature drop is accelerated.

[0027] (Step S2) Stop fuel supply When the power generation of fuel cell 1 is stopped, the supply of fuel to fuel cell 1 is also stopped. For example, a valve located at the outlet of the fuel tank is closed.

[0028] (Step S3) Temperature difference control After the start of steps S1 and S2, the temperatures of the first stack 3-1 and the second stack 3-2 decrease. At this time, the oxidizer is at room temperature at the inlet of the second stack 3-2, while at the inlet of the first stack 3-1, its temperature rises due to heat absorption by the second stack 3-2. As time passes, the temperature difference ΔT between the first stack 3-1 and the second stack 3-2 increases. The larger this temperature difference ΔT, the greater the structural load caused by the difference in expansion rates in the mechanically connected parts of the first stack 3-1, the second stack 3-2, and the auxiliary structure 4, which may adversely affect durability.

[0029] Therefore, in this embodiment, the temperature difference ΔT is controlled so that a large temperature difference does not occur between the first stack 3-1 and the second stack 3-2.

[0030] In controlling the temperature difference ΔT, the exothermic reaction (combustion) in the combustor 5 is utilized. That is, when the fuel supply is restarted and the oxidizer and fuel are supplied to the combustor 5, an exothermic reaction proceeds on the catalyst in the combustor 5, and the oxidizer is heated through heat exchange with the combustor 5. When the heated oxidizer is supplied, the temperature drop of the second stack 3-2 is suppressed or the temperature rises. The effect of the temperature rise of the oxidizer is also seen in the first stack 3-1. However, this effect is greater in the second stack 3-2, which is located upstream in relation to the flow of the oxidizer. Therefore, if the temperature drop rate of the second stack 3-2 is slowed down to the extent that the temperature drop rates reverse, the temperature difference ΔT between the first temperature T1 and the second temperature T2 can be reduced.

[0031] Furthermore, by controlling the flow rates of fuel and oxidizer supplied to the combustor 5, the amount of heat generated in the combustor 5 can be controlled, and the rate of temperature decrease of the second stack 3-2 can be controlled.

[0032] Figure 3 is a graph showing an example of the relationship between the fuel-oxidizer mixing ratio λ and the combustion temperature in the combustor 5. Figure 3 shows the graph when methane is used as the fuel and air (oxygen) is supplied as the oxidizer. The combustion temperature is maximum when the fuel-oxidizer mixing ratio is the stoichiometric mixing ratio (λ=1), and decreases as it deviates from the stoichiometric mixing ratio. Therefore, by controlling this mixing ratio, that is, by controlling the flow rate of the fuel or oxidizer, the rate of temperature decrease in the second stack 3-2 can be controlled. This makes it possible to control the temperature difference between the first stack 3-1 and the second stack 3-2 so that it remains within a predetermined range.

[0033] In the temperature difference control step S3, the temperature difference is controlled using the principle described above. Specifically, as shown in Figure 2, first, in step S3-1, temperature measurement is performed. That is, the first temperature T1 is measured by the first temperature sensor 9-1. In addition, the second temperature T2 is measured by the second temperature sensor 9-2.

[0034] Next, in step S3-2, the flow rate of fuel or oxidizer supplied to the combustor 5 is controlled based on the first temperature T1 and the second temperature T2. These flow rates are controlled so that the temperature difference ΔT, which is the difference between the first temperature T1 and the second temperature T2, is less than the upper temperature difference, which is a preset temperature difference. For example, if the temperature difference ΔT reaches the upper temperature difference when no fuel is being supplied to the combustor 5 (when combustion is not occurring in the combustor 5), the fuel supply is restarted to restart combustion, and the flow rate of the fuel or oxidizer is further controlled. If the temperature difference ΔT reaches the upper temperature difference when fuel is already being supplied, the amount of heat generated in the combustor 5 is controlled by flow rate control so that the temperature difference ΔT is less than the upper or lower temperature difference.

[0035] The above describes the stopping operation method in this embodiment. As described above, according to this embodiment, the temperature difference ΔT between the first stack 3-1 and the second stack 3-2 is controlled to be less than the upper limit temperature difference, so that structural load due to thermal stress can be suppressed.

[0036] Next, the processing in the temperature difference control step (S3) will be explained in more detail with reference to a control example. However, the processing content in the temperature difference control step (S3) according to this embodiment is not limited to the following control example.

[0037] (Control example 1) Figure 4 is a flowchart showing the stop operation method for Control Example 1, and is a flowchart showing the operation method of the temperature difference control step (S3). Figure 5 is a graph showing the relationship between time and temperature when this control example is adopted. As a reference example, Figure 5 also shows a graph of the first temperature T1 when the temperature difference control step is not performed. As shown in Figure 4, in this control example, the temperature difference control step (S3) includes the processing of steps S10 to S15. The processing in each step will be explained below.

[0038] (Step S10) After steps S1 and S2 are initiated, temperature measurement is performed as shown in step S10 of Figure 4. Specifically, the first temperature T and the second temperature T2 are measured. In the control example shown in Figure 5, steps S1 and S2 are initiated at time t0. Therefore, temperature measurement is performed after time t0.

[0039] (Step S11) Next, in step S11, it is determined whether the temperature difference ΔT is greater than or equal to a preset upper limit temperature difference ΔTup. As shown in Figure 5, the first temperature T1 and the second temperature T2 decrease from time t1 onward. In this case, the first temperature T1 decreases faster than the second temperature T2. Therefore, the temperature difference ΔT widens over time. In this step, it is determined whether this temperature difference ΔT has reached the upper limit temperature difference ΔTup.

[0040] The upper limit temperature difference ΔTup referred to here is a value determined from the perspective of whether or not the deterioration of the constituent members (especially the connecting parts) of each stack (3-1, 3-2) is accelerated by structural loads originating from thermal stress. Specifically, if the coefficient of linear expansion of the constituent members is α, the thermal strain occurring between the stacks is expressed by the following equation 1.

[0041] (Equation 1)

number

[0042] Furthermore, the thermal stress generated between the stacks is expressed by the following formula, using the elastic modulus E of the constituent material of each stack.

[0043] (Equation 2)

number

[0044] Furthermore, if the yield point and thermal stress of each stack component are in the following relationship, the deterioration of the component will not be accelerated.

[0045] (Equation 3)

number

[0046] Therefore, the "upper temperature difference ΔTup" referenced in step S11 is set in advance within the range of temperature difference ΔT such that the relationship described in equation 3 above holds.

[0047] (Step S12) If the temperature difference ΔT exceeds the upper limit temperature difference ΔTup, combustion in the combustor 5 is restarted in step S12. Specifically, the fuel supply is restarted. In the example shown in Figure 5, the temperature difference ΔT reaches the upper limit temperature difference ΔTup at time t2. As a result, combustion is carried out from time t2 onward.

[0048] As a result of restarting combustion, the oxidizer receives heat as it passes through the combustor 5 before being supplied to the second stack 3-2, causing the first temperature T1 to rise and approach the second temperature T2, and the temperature difference ΔT to decrease. Furthermore, the rise in the first temperature T1, which is the upstream temperature, also affects the second temperature T2, which is the downstream temperature, so the rate of decrease in the second temperature T2 slows down after time t2.

[0049] (Steps S13-S14) Even after combustion is restarted, the measurement of the first temperature T1 and the second temperature T2 continues (step S13). Then, it is determined whether or not the temperature difference ΔT has become zero.

[0050] (Step S15) When the temperature difference ΔT becomes zero, that is, when the first temperature T1 becomes equal to the second temperature T2, the fuel supply is stopped (step S15). As a result, combustion in the combustor 5 stops, and the first temperature T1 begins to decrease again. In the example shown in Figure 5, the temperature difference ΔT is zero at time t3. Therefore, combustion has stopped from time t3 onward.

[0051] According to the process described above, the first temperature T1 and the second temperature T2 are controlled so that the temperature difference ΔT does not exceed the upper limit temperature difference ΔTup, thereby suppressing deterioration of the constituent materials due to thermal stress.

[0052] (Control example 2) Next, we will explain Control Example 2. The operation method of the temperature difference control step (S3) in this control example is basically the same as in Control Example 1 (Figure 4). However, the flow rate control in step S12 in Figure 4 is different from that in Control Example 1. Figure 6 is a graph showing the relationship between time and temperature when this control example is used for shutdown operation. As a reference example, Figure 6 also shows a graph of the case when the flow rate control related to this control example is not implemented.

[0053] In the flow rate control (S12) of Control Example 1, the fuel supply is restarted without changing the oxidizer flow rate. However, in the flow rate control (S12) of this Control Example, the oxidizer flow rate is increased along with the restart of the fuel supply. As a result, the amount of heat generated in the combustor 5 becomes larger, and as shown from time t2 onwards in Figure 6, the rate of rise of the first temperature T1 becomes higher, and the temperature difference ΔT decreases more quickly. In this process, the fuel flow rate and the oxidizer flow rate are controlled so that the amount of heat generated in the combustor 5 Qheat is greater than the sum of the heat transferred by the oxidizer Qtransfer and the heat dissipated by the first stack 3-1 and the second stack 3-2 Qloss. The relationship between the above amounts of heat can be expressed as shown in Equation 4 below.

[0054] (Equation 4)

number

[0055] The rate of change of the temperature difference ΔT is expressed by the following equation 5.

[0056] (Equation 5)

number

[0057] Here, Cp is the specific heat and Mass is the mass of the auxiliary components.

[0058] Therefore, if the relationship in Equation 4 holds in Equation 5, the temperature difference ΔT can be reduced more quickly, as described above. However, since the fuel cell 1 has an upper temperature limit determined from the standpoint of heat resistance (hereinafter referred to as the heat resistance upper temperature limit), the flow rates of the oxidizer and fuel are controlled so that the combustion temperature in the combustor 5 is below the heat resistance upper temperature limit. Figure 7 is a graph showing an example of the relationship between the mixing ratio λ of the oxidizer and fuel and the heat generation temperature. In the example in Figure 7, similar to the example in Figure 3, the heat generation temperature is highest when the mixing ratio λ is 1 (i.e., the stoichiometric mixing ratio), and decreases as it moves away from the stoichiometric mixing ratio. Furthermore, the heat generation temperature exceeds the heat resistance upper temperature limit when the mixing ratio λ is in the range of λ1 to λ2. In this case, the condition that the mixing ratio λ is λ1 or less or λ2 or more (hatched area in Figure 7) is added to the flow rate control. This suppresses deterioration of the auxiliary structure, deterioration of the piping and seals connecting the auxiliary structure to each stack 3-1 and 3-2, etc.

[0059] Furthermore, as shown in Figure 8, an additional condition may be added: the mixing ratio λ must be 1 or greater (hatched area in Figure 8). By adding this condition, the generation of unburned fuel during combustion in the combustor 5 can be suppressed, thereby preventing deterioration of the components due to thermal stress without worsening emission performance.

[0060] (Control example 3) Next, control example 3 will be described. The operation method of the temperature difference control step (S3) in this control example is basically the same as in control example 1 (Figure 4). However, the flow rate control in step S12 in Figure 4 is different from control examples 1 and 2. In the flow rate control (S12) of this control example, the flow rate of the oxidizer is increased along with the restart of fuel supply, similar to control example 1. However, the oxidizer flow rate is adjusted so that the heat transfer amount Qtrasfer of the oxidizer is greater than the heat dissipation amount Qloss of the fuel cell 1 at a predetermined mixing ratio λ. In other words, for example, as in control example 1, the oxidizer flow rate is increased while maintaining the mixing ratio λ at which the heat generation amount in the combustor 5 is below the heat resistance upper limit temperature. In this case, as the oxidizer flow rate increases, the heat generation amount Qheat in the combustor 5 increases, and the heat dissipation amount Qloss of the first stack 3-1 and the second stack 3-2 remains constant. Therefore, the larger the flow rate of the oxidizing agent, such that the heat transfer Qtrasfer of the oxidizing agent increases, the larger the amount of heat used to raise the temperature of the oxidizing agent (i.e., the numerator on the right side of equation 5 above) becomes, and the faster the first temperature T1 rises.

[0061] However, increasing the flow rate of the oxidizer increases the internal pressure of the oxidizer channel. Figure 9 is a graph showing the relationship between the oxidizer flow rate and the pressure inside the channel. As shown in Figure 9, the internal pressure increases as the oxidizer flow rate increases, and when the flow rate exceeds F1, the internal pressure exceeds the upper limit pressure. The upper limit pressure here is the pressure determined by the design pressure resistance performance of the oxidizer channel.

[0062] Therefore, in this control example, the oxidizer flow rate is limited to a flow rate F1 or less, which is the flow rate at which the internal pressure falls below the upper limit pressure. This suppresses the deterioration of the fuel cell 1 due to an increase in internal pressure.

[0063] The present invention has been described above using embodiments and control examples 1 to 3. These embodiments and their control examples 1 to 3 are not independent of each other, and can be combined and adopted within a non-contradictory range.

[0064] As described above, in this embodiment, a method for stopping power generation of a fuel cell 1 is provided, which comprises an oxidizer flow path 10-1 through which the oxidizer flows in the order of the second stack 3-2 and the first stack 3-1, and a fuel flow path 10-2 through which the fuel flows in the order of the first stack 3-1 and the second stack 3-2, respectively, with the auxiliary structure 4 being mechanically connected to the fuel cell 1. This method comprises an oxidizer reduction step of reducing the amount of oxidizer supplied, a fuel supply stop step of stopping the supply of fuel, and a temperature difference control step of controlling the temperature difference between the first stack 3-1 and the second stack 3-2 after the oxidizer reduction step and the fuel supply stop step. The temperature difference control step includes a temperature measurement step to obtain a first temperature T1, which is the temperature at the oxidizer inlet portion of the second stack 3-2, and a second temperature T2, which is the temperature at the oxidizer outlet portion of the first stack; and a flow rate control step in which, when the first temperature difference ΔT, which is the difference between the first temperature T1 and the second temperature T2, widens to a preset upper temperature difference ΔTup, the fuel supply is restarted and the fuel flow rate and oxidizer flow rate are controlled so that the first temperature difference ΔT becomes less than the upper temperature difference ΔTup. This makes it possible to suppress structural loads due to thermal stress applied to the fuel cell components during shutdown operation.

[0065] In the flow rate control step of this embodiment, the fuel flow rate and the oxidizer flow rate may be controlled so that the amount of heat generated in the combustor of the auxiliary structure is greater than the sum of the heat transferred by the oxidizer and the heat dissipated by the first and second stacks. This increases the combustion heat in the combustor 5 and increases the heating rate of the second stack, thereby shortening the time required to reduce the temperature difference ΔT between the first stack 3-1 and the second stack 3-2.

[0066] In the flow rate control step of this embodiment, the mixing ratio of fuel and oxidizer may be controlled to a ratio such that the combustion temperature in the combustor is below the upper limit temperature determined by the heat resistance performance of the fuel cell. This makes it possible to shorten the time required to reduce the temperature difference ΔT between the first stack 3-1 and the second stack 3-2 without impairing the heat resistance performance.

[0067] In the flow rate control step of this embodiment, the mixing ratio of fuel and oxidizer may be controlled to a mixing ratio greater than the stoichiometric mixing ratio λ. This makes it possible to shorten the time required to reduce the temperature difference ΔT between the first stack 3-1 and the second stack 3-2 without impairing heat resistance performance and suppressing the generation of unburned fuel due to combustion in the combustor 5.

[0068] In this embodiment, the first temperature and the second temperature are detected using temperature sensors 9-1 and 9-2, respectively. This allows for more accurate temperature detection.

[0069] In the flow rate control step of this embodiment, the fuel flow rate and the oxidizer flow rate may be controlled so that the amount of heat transferred by the oxidizer is greater than the amount of heat dissipated by the fuel cell. This increases the amount of heat used to raise the temperature of the oxidizer, thus shortening the time required to reduce the temperature difference ΔT between the first stack 3-1 and the second stack 3-2.

[0070] In the flow rate control step of this embodiment, the flow rate of the oxidizer may be controlled to a flow rate such that the internal pressure of the oxidizer flow path is below the upper limit pressure determined by the pressure resistance performance. This makes it possible to reduce the time required to reduce the temperature difference ΔT between the first stack 3-1 and the second stack 3-2 while suppressing deterioration of the components of the auxiliary structure 4, the piping and sealing materials connecting the auxiliary structure 4 to each stack 3-1 and 3-2.

[0071] Although embodiments of the present invention have been described above, these embodiments only represent a part of the application examples of the present invention, and are not intended to limit the technical scope of the present invention to the specific configurations of the above embodiments. [Explanation of Symbols]

[0072] 1. Fuel cell, 2. Control device, 3-1. First stack, 3-2. Second stack, 4. Auxiliary structure, 5. Combustor, 6-1. Oxidizer supply device, 6-2. Oxidizer supply device, 7. Fuel supply device, 8. Partial oxidation catalyst, 9-1. First temperature sensor, 9-2. Second temperature sensor, 10-1. Oxidizer flow path, 10-2. Fuel flow path

Claims

1. The auxiliary structure is mechanically connected to a first stack and a second stack, respectively, and includes an oxidizer flow path through which the oxidizer flows in the order of the second stack and the first stack, and a fuel flow path through which the fuel flows in the order of the first stack and the second stack, A method for stopping operation when stopping power generation of a fuel cell equipped with the following: A step of reducing the amount of oxidizing agent supplied, A fuel supply stop step of stopping the supply of the aforementioned fuel, After the oxidizer reduction step and the fuel supply stop step, a temperature difference control step is performed to control the temperature difference between the first stack and the second stack. Equipped with, The aforementioned temperature difference control step is: A temperature measurement step to obtain a first temperature, which is the temperature at the oxidizing agent inlet portion of the second stack, and a second temperature, which is the temperature at the oxidizing agent outlet portion of the first stack. A flow rate control step in which, when the first temperature difference, which is the difference between the first temperature and the second temperature, widens to a predetermined upper temperature difference, the fuel supply is restarted and the flow rate of the fuel and the flow rate of the oxidizer are controlled so that the first temperature difference becomes less than the upper temperature difference, A method for stopping operation of a fuel cell, characterized by comprising the following:

2. In the method for stopping operation of a fuel cell according to claim 1, A method for shutting down a fuel cell, wherein in the flow rate control step, the flow rate of the fuel and the flow rate of the oxidizer are controlled such that the amount of heat generated in the combustor provided by the auxiliary equipment structure is greater than the sum of the amount of heat transferred by the oxidizer and the amount of heat dissipated by the first stack and the second stack, in a mixing ratio.

3. In the method for stopping operation of a fuel cell according to claim 2, A method for shutting down a fuel cell, wherein in the flow rate control step, the mixing ratio of the fuel and the oxidizer is controlled to a mixing ratio such that the combustion temperature in the combustor is below the upper limit temperature determined by the heat resistance performance of the fuel cell.

4. In the method for stopping operation of a fuel cell according to claim 3, A method for shutting down a fuel cell, wherein in the flow rate control step, the mixing ratio of the fuel and the oxidizer is controlled to a mixing ratio greater than the stoichiometric mixing ratio λ.

5. In the method for stopping operation of a fuel cell according to claim 4, A method for shutting down a fuel cell, wherein the first temperature and the second temperature are detected using temperature sensors, respectively.

6. In the method for stopping operation of a fuel cell according to claim 1, A method for shutting down a fuel cell, wherein in the flow rate control step, the flow rate of the fuel and the flow rate of the oxidizer are controlled so that the amount of heat transferred by the oxidizer is greater than the amount of heat dissipated by the fuel cell, in order to achieve a mixing ratio.

7. In the method for stopping operation of a fuel cell according to claim 6, A method for shutting down a fuel cell, wherein the flow rate control step involves controlling the flow rate of the oxidizer to a flow rate such that the internal pressure of the oxidizer flow path is below the upper limit pressure determined by the pressure resistance performance.

8. In the method for stopping operation of a fuel cell according to claim 7, A method for shutting down a fuel cell, wherein the first temperature and the second temperature are detected using temperature sensors, respectively.

9. A fuel cell comprising an auxiliary structure in which a first stack and a second stack are mechanically connected, an oxidizer flow path through which an oxidizer flows in the order of the second stack and the first stack, and a fuel flow path through which fuel flows in the order of the first stack and the second stack, An oxidant supply device that supplies the oxidant to the fuel cell, A fuel supply device that supplies the fuel to the fuel cell, A first temperature detection device for detecting a first temperature, which is the temperature at the oxidizing agent inlet portion of the second stack, A second temperature detection device for detecting a second temperature, which is the temperature of the oxidizer outlet portion in the first stack, A control device for controlling the operation of the fuel cell, In a fuel cell system equipped with, When the control device stops power generation, The amount of the oxidizing agent supplied is reduced, The supply of the aforementioned fuel is stopped, After reducing the amount of the oxidizer and stopping the supply of the fuel, the first temperature and the second temperature are obtained. A fuel cell system characterized in that, when the first temperature difference, which is the difference between the first temperature and the second temperature, widens to a predetermined upper temperature difference, the fuel supply is restarted, and the flow rate of the fuel and the flow rate of the oxidizer are controlled so that the first temperature difference becomes less than the upper temperature difference.

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