Fuel cell system

Abstract

To quickly stop a fuel cell system by more properly determining the end of intermittent stop control.SOLUTION: A fuel cell system comprises: a fuel cell which generates power based on fuel gas supplied to an anode and oxidant gas supplied to a cathode; a reforming part which steam-reforms raw fuel gas so as to generate the fuel gas; a raw fuel gas supply part which supplies the raw fuel gas to the reforming part; an oxidant gas supply part which supplies the oxidant gas to the cathode; a combustion part which burns off-gas from the fuel cell; a temperature sensor which detects a temperature of the combustion part; and a control part which executes, when stopping the system, intermittent stop control for intermittently stopping the supply of the raw fuel gas and the oxidant gas so that the burning in the combustion part may be stopped, and ends the intermittent stop control if a gradient of decrease in the temperature of the combustion part detected by the temperature sensor is equal to or higher than a predetermined value.SELECTED DRAWING: Figure 2

Description

[Technical field]

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

[0002] Conventionally, as this type of fuel cell system, a fuel cell system has been known that includes a fuel cell that generates electricity using fuel gas and oxidant gas, a reformer that generates fuel gas by steam reforming raw fuel gas, and a combustion section that combusts gas (off-gas) after power generation in the fuel cell. For example, Patent Document 1 describes a system in which, when the system is stopped, intermittent stop control is executed to intermittently stop the supply of raw fuel gas and oxidant gas in order to stop the combustion in the combustion section and promote cooling of the fuel cell, and the execution of the intermittent stop control is terminated when the temperature of the fuel cell reaches a predetermined temperature. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2013-186945 A Summary of the Invention [Problem to be solved by the invention]

[0004] Here, due to individual differences in fuel cell systems and variations in the measurement accuracy of the temperature sensor, a discrepancy may occur between the actual temperature and the temperature measured by the temperature sensor. Therefore, if the above-mentioned predetermined temperature is set to a high value, the intermittent stop control may end in a state where the temperature has not actually decreased and the combustion in the combustion section has not stopped, and the system stop is delayed because cooling does not progress due to the continuation of combustion in the combustion section. On the other hand, if the predetermined temperature is set to a low value, the intermittent stop control may continue longer than necessary even if the combustion in the combustion section has stopped, and cooling by the flow of oxidant gas does not progress during the intermittent stop, so the system stop is delayed.

[0005] A primary object of the present disclosure is to more appropriately determine the end of intermittent stop control and to quickly stop the system. [Means for solving the problem]

[0006] In order to achieve the above-mentioned main object, the present disclosure has adopted the following means.

[0007] The fuel cell system of the present disclosure comprises: a fuel cell that generates power based on a fuel gas supplied to the anode and an oxidant gas supplied to the cathode; a reforming unit that generates the fuel gas by steam reforming a raw fuel gas; a raw fuel gas supply unit that supplies the raw fuel gas to the reforming unit; an oxidant gas supply unit that supplies the oxidant gas to the cathode; a combustion section that combusts off-gas from the fuel cell; A temperature sensor for detecting the temperature of the combustion section; a control unit that, when the system is stopped, executes an intermittent stop control for intermittently stopping the supply of the raw fuel gas and the oxidant gas so that combustion in the combustion unit is stopped, and terminates the execution of the intermittent stop control when a gradient of a decrease in the temperature of the combustion unit detected by the temperature sensor is equal to or greater than a predetermined value; The gist of the project is to provide the following:

[0008] In the fuel cell system of the present disclosure, when the system is stopped, intermittent stop control is executed to intermittently stop the supply of fuel gas and oxidant gas so that the combustion in the combustion section is stopped, and the execution of the intermittent stop control is terminated when the gradient of the decrease in the temperature of the combustion section detected by the temperature sensor is equal to or greater than a predetermined value. By using the gradient of the decrease in the temperature of the combustion section, the effect of individual differences in the fuel cell system and variations in the measurement accuracy of the temperature sensor can be suppressed, and the stop of combustion in the combustor can be accurately detected. Therefore, the end of the intermittent stop control can be more appropriately determined, and it is possible to prevent cooling from slowing down due to the intermittent stop control being continued longer than necessary, and therefore the system can be stopped quickly. [Brief description of the drawings]

[0009] [Figure 1] 1 is a schematic diagram of a fuel cell system 10. FIG. [Diagram 2] 13 is a flowchart illustrating an example of a system shutdown process. [Diagram 3] FIG. 11 is an explanatory diagram showing an example of a transition of temperature when the system is stopped. [Figure 4] 13 is a flowchart showing a system shutdown process according to a modified example. [Diagram 5] FIG. 11 is an explanatory diagram showing an example of the relationship between a combustion part temperature T7 and a threshold value αref. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] An embodiment of the present disclosure will be described with reference to the drawings. Fig. 1 is a schematic diagram of a fuel cell system 10. As shown in the figure, the fuel cell system 10 includes a power generation module 20 including a fuel cell stack 21 that generates power by an electrochemical reaction between hydrogen in an anode gas and oxygen in a cathode gas, a raw fuel gas supply device 30 that supplies a hydrocarbon-based raw fuel gas (e.g., natural gas or LP gas) that is a raw material for the anode gas to the power generation module 20, a reforming water supply device 40 that supplies reforming water required for reforming (steam reforming) the raw fuel gas to the power generation module 20, an air supply device 50 that supplies air as a cathode gas to the power generation module 20 (fuel cell stack 21), an exhaust heat recovery device 60 that recovers exhaust heat generated in the power generation module 20, and a control device 100 that controls the entire system.

[0011] The power generation module 20 includes a fuel cell stack 21, an evaporator 22, a reformer 23, a combustor 24, and a heat exchanger 26, which are housed in a thermally insulating module case 29. The evaporator 22, the reformer 23, and the combustor 24 (units) are disposed above the fuel cell stack 21 inside the module case 29.

[0012] As shown in FIG. 1, in this embodiment, the fuel cell stack 21 is configured as a flat-plate type solid oxide fuel cell stack in which a plurality of flat-plate type unit cells are stacked in the plate thickness direction. Each unit cell S has an electrolyte, an anode arranged on one side of the electrolyte, and a cathode arranged on the other side of the electrolyte. One end of an anode gas pipe 71 is connected to the anode inlet of the fuel cell stack 21, and the other end of the anode gas pipe 71 is connected to the reformer 23. One end of a cathode gas pipe 72 is connected to the cathode inlet of the fuel cell stack 21, and the other end of the cathode gas pipe 72 is connected to the air supply device 50 (air supply pipe 51). In addition, one end of an anode off-gas pipe 73 is connected to the anode outlet of the fuel cell stack 21, and the other end of the anode off-gas pipe 73 is connected to the combustor 24. One end of a cathode offgas pipe 74 is connected to the cathode outlet of the fuel cell stack 21, and the other end of the cathode offgas pipe 74 is connected to the combustor 24. In addition, a temperature sensor 112 is installed near the fuel cell stack 21. The temperature sensor 112 is a temperature sensor that detects a temperature (stack temperature T4) that serves as a substitute for the temperature of the fuel cell stack 21, and in this embodiment, is installed near the cathode outlet.

[0013] The evaporator 22 has many balls (ceramic or alumina balls) filled therein for storing heat, and is supplied with reforming water in a state in which heat has been stored in the balls by heat from the combustor 24, thereby evaporating the reforming water to generate water vapor. The water vapor generated in the evaporator 22 is mixed with the raw fuel gas, and the mixed gas flows from the evaporator 22 into the reformer 23. The evaporator 22 is provided with a temperature sensor 111 for detecting the temperature inside the evaporator 22 (evaporator temperature T1).

[0014] The reformer 23 has a Ru-based or Ni-based reforming catalyst filled therein, and generates hydrogen gas and carbon monoxide by a reaction (steam reforming reaction) of the mixed gas from the evaporator 22 with the reforming catalyst in the presence of heat from the combustor 24. Furthermore, the reformer 23 generates hydrogen gas and carbon dioxide by a reaction (carbon monoxide shift reaction) between the carbon monoxide generated in the steam reforming reaction and steam. As a result, the reformer 23 generates anode gas containing hydrogen, carbon monoxide, carbon dioxide, steam, unreformed raw fuel gas, etc. The anode gas generated by the reformer 23 is supplied to the anode of each unit cell S from the reformer outlet through the anode gas pipe 71.

[0015] Air as a cathode gas is supplied to the cathode of each unit cell S via a cathode gas pipe 72. In the cathode of each unit cell S, oxide ions (O 2- ) are produced, and the oxide ions pass through the electrolyte and react with hydrogen and carbon monoxide at the anode to produce electrical energy.

[0016] Anode gas (hereinafter referred to as "anode off-gas") that has not been used in the electrochemical reaction (power generation) in each unit cell S is supplied to the combustor 24 through an anode off-gas pipe 73, and cathode gas (hereinafter referred to as "cathode off-gas") that has not been used in the electrochemical reaction (power generation) in each unit cell S is supplied to the combustor 24 through a cathode off-gas pipe 74. The anode off-gas is a combustible gas containing fuel components such as hydrogen and carbon monoxide, and is mixed with the cathode off-gas containing oxygen in the combustor 24. Then, the mixed gas is burned to generate heat required for the operation of the fuel cell stack 21, preheating the raw fuel gas and generating steam in the evaporator 22, the steam reforming reaction in the reformer 23, and the like.

[0017] The combustor 24 uses a burner to combust a mixed gas of anode off-gas discharged from the anode outlet of the fuel cell stack 21 and cathode off-gas discharged from the cathode outlet. The combustor 24 has a combustion chamber defined by the inner wall of the reformer 23, and has an ignition section 25 for igniting the mixed gas introduced into the combustion chamber. The combustor 24 is provided with a temperature sensor 113 for detecting the temperature inside the combustion chamber (combustion section temperature T7).

[0018] The combustion exhaust gas generated by the combustion of the mixed gas in the combustor 24 passes through the reformer 23, the heat exchanger 26, and the evaporator 22 in this order, and after supplying the heat required for steam reforming, the heat required for raising the temperature of the cathode gas (air), and the heat required for generating water vapor, respectively, the combustion exhaust gas is supplied to the condenser 62 through the combustion exhaust gas pipe 75. Note that a combustion catalyst 28 (oxidation catalyst) for burning unburned fuel contained in the combustion exhaust gas is provided near the outlet of the combustion exhaust gas pipe 75. The combustion exhaust gas supplied to the condenser 62 is cooled by the condenser 62 to remove at least a portion of the water vapor contained in the combustion exhaust gas, and then is discharged into the atmosphere.

[0019] The raw fuel gas supplying device 30 has a raw fuel gas supplying pipe 31 connecting a raw fuel supply source 1 that supplies raw fuel gas to an evaporator 22, and on-off valves (dual valves) 32, 33, a flow sensor 39, a gas pump 34, and a desulfurizer 35 that are arranged in sequence from upstream to downstream on the raw fuel gas supplying pipe 31. By operating the gas pump 34, the raw fuel gas is pumped (supplied) from the raw fuel supplying source 1 to the evaporator 22 via the desulfurizer 35. The flow sensor 39 detects the flow rate per unit time of the raw fuel gas flowing through the raw fuel gas supplying pipe 31 (gas flow rate Fg).

[0020] The reforming water supply device 40 has a reforming water tank 42 that stores reforming water, a reforming water supply pipe 41 that connects the reforming water tank 42 and the evaporator 22, and a reforming water pump 43 installed in the reforming water supply pipe 41. By operating the reforming water pump 43, the reforming water in the reforming water tank 42 is pumped (supplied) by the reforming water pump 43 to the evaporator 22.

[0021] The air supply device 50 has an air supply pipe 51 connected to a cathode gas pipe 72 installed in the module case 29, an air filter 52 installed at the inlet of the air supply pipe 51, and an air pump 53 and a flow sensor 54 installed at the air supply pipe 51. By operating the air pump 53, air as a cathode gas is sucked into the air supply pipe 51 through the air filter 52, and is pressure-fed (supplied) to the fuel cell stack 21 (cathode) through the cathode gas pipe 72. A heat exchanger 26 is installed in the cathode gas pipe 72, and the air that flows into the heat exchanger 26 is heated to a required temperature by heat exchange with the high-temperature combustion exhaust gas discharged from the combustor 24, and then is supplied to the cathode of the fuel cell stack 21. The flow sensor 54 detects the flow rate (air flow rate Fa) of air flowing through the air supply pipe 51 per unit time.

[0022] The exhaust heat recovery device 60 has a hot water storage tank 61 that stores hot water, a condenser 62 that exchanges heat between the hot water and anode off-gas flowing from the fuel cell stack 21 through an anode off-gas piping 73, and condenses water vapor contained in the anode off-gas, a circulation piping 63 connected to the hot water storage tank 61 and the condenser 62, and a circulation pump 64 incorporated in the circulation piping 63. The hot water stored in the hot water storage tank 61 is introduced into the condenser 62 by operating the circulation pump 64, and is returned to the hot water storage tank 61 after being heated by heat exchange with the anode off-gas in the condenser 62.

[0023] In addition, the condensed water pipe 44 and the combustion exhaust gas pipe 76 are connected to the passage outlet on the combustion exhaust gas side of the condenser 62, and the condensed water obtained by condensing the water vapor in the combustion exhaust gas through heat exchange with the hot water from the hot water storage tank 61 is introduced into the reforming water tank 42 through the condensed water pipe 44. Note that the reforming water tank 42 is provided with a water purifier (not shown) that purifies the condensed water that has passed through the condensed water pipe 44. In addition, as described above, the combustion exhaust gas from which the water vapor has been removed in the condenser 62 is discharged into the atmosphere through the combustion exhaust gas pipe 76.

[0024] An input terminal of a power conditioner 80 is connected to an output terminal of the fuel cell stack 21, and the output terminal of the power conditioner 80 is connected to a power line 3 from the power system 2 to a load 4 via a relay (not shown). The power conditioner 80 has a DC / DC converter that converts the DC power output from the fuel cell stack 21 into DC power of a predetermined voltage (e.g., DC 250V to 300V), and an inverter that converts the converted DC power into AC power of a voltage (e.g., AC 200V) that can be connected to the power system. This makes it possible to convert the DC power from the fuel cell stack 21 into AC power and supply it to a load 4 such as a home appliance. A current sensor 114 that detects a current (stack current) flowing through the fuel cell stack 21 is attached to the output terminal of the fuel cell stack 21, and a voltage sensor 115 that detects a terminal-to-terminal voltage (stack voltage) of the fuel cell stack 21 is attached between the output terminals of the fuel cell stack 21.

[0025] A power supply board 81 is connected to the power conditioner 80. The power supply board 81 converts DC power from the fuel cell stack 21 and AC power from the power system 2 into low-voltage DC power, and supplies it to the drive circuits of the gas pump 34, the reforming water pump 43, the air pump 53, and the circulation pump 64, sensors such as the flow rate sensors 39 and 54, the temperature sensors 111, 112, and 113, the current sensor 114, and the voltage sensor 115, and the control device 100. In addition, a cooling fan and a ventilation fan (not shown) for cooling the power conditioner 80 and the power supply board 81 are arranged in the auxiliary equipment room where the power conditioner 80 and the power supply board 81 are arranged. The cooling fan sends air to the heat generating parts of the power conditioner 80 and the power supply board 81 to cool the heat generating parts by heat exchange with the air. The air that has been heated by cooling the heat generating parts is discharged into the atmosphere by the ventilation fan.

[0026] The control device 100 is configured as a microprocessor centered on a CPU 101, and includes, in addition to the CPU 101, a ROM 102 for storing a processing program, a RAM 103 for temporarily storing data, and an input / output port (not shown). Various detection signals from the flow rate sensors 39, 54, temperature sensors 111, 112, 113, current sensor 114, voltage sensor 115, etc. are input to the control device 100 via the input port. In addition, the control device 100 outputs various control signals to the solenoids of the on-off valves 32, 33, the pump motor of the gas pump 34, the pump motor of the reforming water pump 43, the pump motor of the air pump 53, the pump motor of the circulation pump 64, etc. via the output port. In addition, a remote control (not shown) is connected to the control device 100 via a wireless or wired communication line. The control device 100 executes various controls based on signals from the remote control operated by a user of the fuel cell system 10.

[0027] Next, the operation of the fuel cell system 10 thus configured will be described. The CPU 101 of the control device 100 controls the supply amounts of raw fuel gas, reforming water, and air so that a current (current command Ireq) according to the required output required for the system is output from the fuel cell stack 21, and starts power generation. The supply amount of raw fuel gas is controlled by setting a gas flow rate according to the current command Ireq to a target gas flow rate Fgtag so that the fuel utilization rate Uf coincides with the target utilization rate Uftag, and controlling the gas pump 34 by feedback control so that the gas flow rate Fg detected by the flow rate sensor 39 coincides with the set target gas flow rate Fgtag. The fuel utilization rate Uf is the ratio of the amount of anode gas used for power generation to the amount of anode gas supplied to the anode. The supply amount of reforming water is controlled by setting a target reforming water flow rate Fwtag based on the target gas flow rate Fgtag so that the steam-carbon ratio SC in the reformer 23 coincides with the target ratio SCtag, and controlling the reforming water pump 43 so that reforming water at the set target reforming water flow rate Fwtag is supplied. The steam carbon ratio SC is the molar ratio between carbon contained in the hydrocarbons in the raw fuel gas and steam added for steam reforming. The air supply amount is controlled by setting a target air flow rate Fatag by feedback control so that the stack temperature T4 detected by the temperature sensor 112 coincides with the target temperature T4tag, and by controlling the air pump 53 by feedback control so that the air flow rate Fa detected by the flow rate sensor 54 coincides with the set target air flow rate Fatag.

[0028] Next, the operation when the system is stopped will be described. FIG. 2 is a flow chart showing an example of the system stop processing. This processing is executed by the CPU 101 when the system stop is requested. In the system stop processing, the CPU 101 first supplies raw fuel gas, reforming water, and air for system stop (S100). In S100, the CPU 101 controls the gas pump 34 and the reforming water pump 43 so that a small amount of raw fuel gas and reforming water are supplied from the raw fuel gas supply device 30 and the reforming water supply device 40, respectively, in order to stop power generation in the fuel cell stack 21, and also controls the air pump 53 so that as much air as possible is supplied from the air supply device 50 in order to cool the fuel cell stack 21. Note that, when the anode of the fuel cell stack 21 is exposed to an oxygen atmosphere under high temperature, the anode is oxidized, leading to a decrease in power generation performance. Therefore, when the system is stopped, the minimum amount of raw fuel gas and reforming water is supplied so that the supply of fuel gas to the anode is continued until the fuel cell stack 21 is sufficiently cooled.

[0029] Next, the CPU 101 starts executing intermittent stop control to intermittently stop (intermittently stop) the supply of raw fuel gas, reforming water, and air so as to stop the combustion in the combustor 24 in order to promote cooling of the fuel cell stack 21, etc. (S110). The intermittent stop control is executed by repeatedly stopping the supply of raw fuel gas, reforming water, and air for a second time (intermittent stop time, for example, 30 seconds) when a first time (for example, 15 minutes) has elapsed since the supply of raw fuel gas, reforming water, and air was started, and starting (resuming) the supply of raw fuel gas, reforming water, and air when the second time has elapsed since the supply was stopped.

[0030] Here, in order to promote the cooling of the fuel cell stack 21 and the reformer 23 and to quickly shut down the system, it is necessary to stop the combustion in the combustor 24, that is, to turn off the fire in the combustor 24. To do this, it is necessary to stop the supply of fuel to the combustor 24 and make the fuel concentration in the combustor 24 equal to or lower than the flammability limit concentration. However, when the fuel cell stack 21 is at a high temperature, if air flows into the anode side, the anode may be oxidized and deteriorated, so when the supply of fuel is stopped, the supply of air must also be stopped. In addition, if the supply of fuel and air is stopped for a long period of time, the volume of the gas inside the combustor 24 contracts due to the drop in temperature of the fuel cell stack 21, generating a force that draws the gas inward, and for example, the cathode off-gas (oxygen) remaining in the combustor 24 may flow back to the anode side through the anode off-gas piping 73, causing the anode to be oxidized and deteriorated. For this reason, the intermittent stop control is executed to temporarily stop the introduction of fuel and air into the combustor 24 for a short period of time, thereby stopping the combustion in the combustor 24. Furthermore, when the combustion section temperature T7 is high, re-ignition will occur when the introduction of fuel and air into the combustor 24 is resumed, and so it is necessary to continue the intermittent stop control until re-ignition does not occur. On the other hand, after the combustion in the combustor 24 has completely stopped, it is required to end the intermittent stop control and continuously supply a large amount of air to promote cooling of the fuel cell stack 21. In this embodiment, when the execution of the intermittent stop control is started, the combustion section temperature T7 is monitored as follows, and the end of the execution of the intermittent stop control is determined based on the gradient α of the temperature decrease of the combustion section temperature T7.

[0031] The CPU 101 calculates the gradient α of the combustion section temperature T7 based on the current combustion section temperature T7 detected by the temperature sensor 113 and the temperature T7pre, which is the combustion section temperature T7 detected by the temperature sensor 113 a predetermined time Δt ago and stored in the RAM 103 (S120). In this embodiment, the gradient α is calculated by dividing the temperature difference (temperature change) obtained by subtracting the current combustion section temperature T7 from the temperature T7pre a predetermined time Δt ago by the predetermined time Δt. The predetermined time Δt is set to a time longer than the second time (intermittent stop time) in the intermittent stop control, and is set to a time close to the first time, such as 15 minutes or 20 minutes. During the system stop process, the temperature inside the combustor 24 gradually decreases, so that the temperature T7pre is usually higher than the current combustion section temperature T7, and the temperature difference is a positive value, and therefore the gradient α is also calculated as a positive value. For this reason, the greater the gradient α, the more rapidly the combustion section temperature T7 decreases.

[0032] Next, the CPU 101 determines whether the slope α is equal to or greater than a predetermined threshold value αref (S130). If it is determined that the slope α is equal to or greater than the threshold value αref, it determines whether the state has continued for a certain period of time (for example, several tens of seconds or several minutes) (S140). The threshold value αref is a value determined so that the stop of combustion can be determined by obtaining the temperature transition when the combustion of the combustor 24 stops through an experiment, a simulation, or the like. If the slope α cannot be properly calculated in S120 because the temperature T7pre before the predetermined time Δt is not stored in the RAM 103, the CPU 101 may determine in S130 that the slope α is not equal to or greater than the threshold value αref. If the CPU 101 determines that the slope α is not equal to or greater than the threshold value αref or that the state has not continued for a certain period of time, it determines to continue the execution of the intermittent stop control (S150) and returns to S120.

[0033] When the CPU 101 determines that the state in which the slope α is equal to or greater than the threshold value αref continues for a certain period of time in S130 and S140 while repeating the processes of S120 to S150 during the execution of the intermittent stop control, it determines that the combustion in the combustor 24 has stopped, and ends the execution of the intermittent stop control (S160). After the execution of the intermittent stop control ends, the raw fuel gas, the reforming water, and the air are continuously supplied without intermittent stop.

[0034] Next, the CPU 101 determines whether the stack temperature T4 detected by the temperature sensor 112 has become equal to or lower than a predetermined threshold value T4ref (S170). If the CPU 101 determines that the stack temperature T4 has not become equal to or lower than the threshold value T4ref, it waits for the stack temperature T4 to become equal to or lower than the threshold value T4ref. Then, if the CPU 101 determines that the stack temperature T4 has become equal to or lower than the threshold value T4ref, it stops the supply of the raw fuel gas, the reforming water, and the air (S180) and ends the system shutdown process.

[0035] Fig. 3 is an explanatory diagram showing an example of temperature transition when the system is stopped. Fig. 3 illustrates transitions of stack temperatures T4, T4' and combustion section temperatures T7, T7' after the system stop process is executed. The stack temperature T4 and combustion section temperature T7 shown by the solid line are transitions of a comparative example in which intermittent stop control is continued up to a certain temperature Ta even after the combustor 24 stops combustion. The stack temperature T4' and combustion section temperature T7' shown by the dotted line are transitions of this embodiment in which the execution of intermittent stop control is terminated based on the slope α as described above.

[0036] As shown in the figure, for example, when the combustor 24 stops combustion at time t1, the combustion section temperature T7 drops sharply from time t1 onwards. In this embodiment, in the system stop process, it is determined that the slope α exceeds the threshold value αref after time t1, and the execution of the intermittent stop control is terminated. On the other hand, in the comparative example, the execution of the intermittent stop control continues for a longer period of time after time t1 than in this embodiment, and the intermittent stop control is terminated at time t2, for example, when the combustion section temperature T7 reaches the temperature Ta. For this reason, the temperature drops more quickly in this embodiment, as indicated by the dotted line, than in the comparative example.

[0037] Here, in the comparative example, it is also conceivable to set the temperature Ta to a high value so that the intermittent stop control can be ended early. However, due to individual differences in the fuel cell system 10 and variations in the measurement accuracy of the temperature sensor 113, a deviation may occur between the actual combustion section temperature T7 and the combustion section temperature T7 detected by the temperature sensor 113. Therefore, when the temperature Ta is set to a high value, the intermittent stop control may be ended in a state where the combustion section temperature T7 has not actually decreased and the combustion has not stopped, and the cooling does not progress due to the continuation of the combustion in the combustor 24, and the system stop is delayed. For this reason, it is necessary to set the temperature Ta to a low value so that the intermittent stop control is not ended in a state where the combustion has not stopped, that is, so that the combustion in the combustor 24 can be reliably determined to have stopped even if a relatively large deviation occurs. However, when the temperature Ta is set to a low value, the intermittent stop control may be continued longer than necessary even if the combustion in the combustor 24 has stopped.

[0038] That is, in the comparative example of FIG. 3, the time for which the supply of air is stopped due to the continuation of the intermittent stop control is longer than in this embodiment, and the period during which cooling of the fuel cell stack 21 and the like is not promoted is longer. As a result, in the comparative example, the stack temperature T4 and the combustion section temperature T7 are less likely to fall than in this embodiment, and as a result, it takes longer for the stack temperature T4 to become equal to or lower than the threshold value T4ref. On the other hand, in this embodiment, the slope α is calculated from the temperature change in the combustion section temperature T7 in a predetermined time Δt, so that the slope α is less likely to be affected by individual differences and variations in measurement accuracy. For this reason, by determining the end of execution of the intermittent stop control based on the slope α, it is possible to accurately detect the stop of combustion in the combustor 24 for each fuel cell system 10 and end the intermittent stop control at an appropriate timing. Therefore, by preventing the intermittent stop control from continuing longer than necessary and shortening the time for stopping the supply of air, it is possible to prevent cooling from slowing down, and therefore the system can be stopped quickly.

[0039] In the fuel cell system 10 of the present embodiment described above, when the system is shut down, intermittent stop control is executed to intermittently stop the supply of raw fuel gas and air, and the execution of the intermittent stop control is terminated when the gradient α of the combustion section temperature T7 detected by the temperature sensor 113 is equal to or greater than the threshold value αref (predetermined value). Therefore, the stop of combustion in the combustor 24 can be detected with high accuracy, and the intermittent stop control can be terminated at an appropriate timing, so that the system can be shut down promptly.

[0040] In addition, the slope α of the combustion part temperature T7 for a predetermined time Δt that is longer than the supply stop time (second time) in the intermittent stop control is calculated to determine whether it is equal to or greater than the threshold value αref. Therefore, compared to a determination based on the slope α based on a short-term temperature change of the combustion part temperature T7, it is possible to more accurately determine the stop of combustion in the combustor 24 by excluding the influence of a temporary temperature change.

[0041] In the system shutdown process of the embodiment, the gradient α of the combustion section temperature T7 is compared with a fixed threshold value αref to determine whether or not to terminate the intermittent shutdown control, regardless of the value of the combustion section temperature T7, but the following modified example may be used. Figure 4 is a flowchart showing the system shutdown process of the modified example. In the modified example, the same steps as in the embodiment are assigned the same step numbers, and the description thereof will be omitted.

[0042] In the system stop process of the modified example, when the CPU 101 starts execution of the intermittent stop control in S110, it waits for the combustion unit temperature T7 to become equal to or lower than a first threshold value T7ref1 (predetermined temperature) (S115). When the CPU 101 determines that the combustion unit temperature T7 has become equal to or lower than the first threshold value T7ref1, it performs the process from S120 onwards, that is, determines whether to end execution of the intermittent stop control based on the gradient α of the temperature decrease of the combustion unit temperature T7 (S120 to S160). Furthermore, in the system stop process of the modified example, the CPU 101 sets the threshold value αref based on the combustion unit temperature T7 (S125), and then compares the gradient α with the threshold value αref in S130.

[0043] FIG. 5 is an explanatory diagram showing an example of the relationship between the combustion part temperature T7 and the threshold value αref. As shown in the figure, the threshold value αref is set so that it increases as the combustion part temperature T7 increases. For example, when the combustion part temperature T7 is equal to or lower than the first threshold value T7ref1 and exceeds the second threshold value T7ref2, the threshold value αref is set to the first threshold value αref1, and when the combustion part temperature T7 is equal to or lower than the second threshold value T7ref2, the threshold value αref is set to the second threshold value αref2, which is smaller than the first threshold value αref1. Here, the higher the combustion part temperature T7 is, the larger the temperature difference to the temperature at which the combustion chamber 24 stops burning becomes, so that the temperature tends to decrease with a relatively large gradient α. In the modified example, the threshold value αref that reflects such a tendency is set and compared with the gradient α, so that the end of the intermittent stop control can be more appropriately determined. Note that, although the threshold value αref is set (switched) in two stages as an example, it is not limited to this, and may be set to three or more stages, or may be set to change steplessly according to the combustion part temperature T7.

[0044] In this manner, in the modified example, when the combustion section temperature T7 is equal to or lower than the first threshold value (predetermined temperature) T7ref1, it is determined whether or not the gradient α is equal to or higher than the threshold value αref. Therefore, when the combustion section temperature T7 is in a high temperature state where it has not yet dropped to near the temperature at which the combustion chamber 24 stops combustion, for example, it is possible to prevent the intermittent stop control from being terminated due to an erroneous determination that the gradient α is equal to or higher than the threshold value αref due to some factor. Note that the first threshold value T7ref1 can be set to, for example, a temperature when the above-mentioned temperature Ta is set relatively high, that is, the upper limit temperature of the temperature range at which the combustion chamber 24 is considered to have stopped combustion.

[0045] In addition, in the modified example, the higher the combustion section temperature T7, the larger the value of the threshold value αref that is used, so that the end of the intermittent stop control can be more appropriately determined as the threshold value αref that reflects the tendency of the slope α. Note that, in the modified example of Fig. 4, both the processes of S115 and S125 are added to the flow of Fig. 2, but only one of the processes may be added.

[0046] In the embodiment, in S170 of the system stop processing, when the stack temperature T4 becomes equal to or lower than the threshold value T4ref, the supply of the raw fuel gas, the reforming water, and the air is stopped, but this is not limited to this. For example, when the stack temperature T4 becomes equal to or lower than the threshold value T4ref, the supply of the raw fuel gas may be stopped first, and the supply of the reforming water and the air may be continued and then stopped until the stack temperature T4 further decreases. In addition, the judgment in S170 may be performed using a temperature other than the stack temperature T4.

[0047] The correspondence between the main elements of the embodiment and the main elements of the present disclosure described in the section on Means for Solving the Problems will be described below. The fuel cell stack 21 of the embodiment corresponds to the "fuel cell" of the present disclosure, the reformer 23 corresponds to the "reforming section", the raw fuel gas supply device 30 corresponds to the "raw fuel gas supply section", the air supply device 50 corresponds to the "oxidant gas supply section", the combustor 24 corresponds to the "combustion section", and the control device 100 corresponds to the "control section".

[0048] In addition, the correspondence between the main elements of the embodiment and the main elements of the present disclosure described in the Means for Solving the Problem column does not limit the elements of the present disclosure described in the Means for Solving the Problem column, since the embodiment is an example for specifically explaining the form for implementing the present disclosure described in the Means for Solving the Problem column. In other words, the interpretation of the present disclosure described in the Means for Solving the Problem column should be made based on the description in that column, and the embodiment is merely a specific example of the present disclosure described in the Means for Solving the Problem column.

[0049] The above describes the form for implementing the present disclosure using embodiments, but the present disclosure is not limited to these embodiments in any way, and it goes without saying that the present disclosure can be implemented in various forms without departing from the gist of the present disclosure. [Industrial Applicability]

[0050] The present disclosure is applicable to the fuel cell system manufacturing industry. [Explanation of symbols]

[0051] 10 fuel cell system, 21 fuel cell stack (fuel cell), 23 reformer (reformer section), 24 combustor (combustion section), 30 raw fuel gas supply device (raw fuel gas supply section), 50 air supply device (oxidizer gas supply section), 100 control device (control section), 113 temperature sensor.

Claims

1. a fuel cell that generates power based on a fuel gas supplied to the anode and an oxidant gas supplied to the cathode; a reforming unit that generates the fuel gas by steam reforming a raw fuel gas; a raw fuel gas supply unit that supplies the raw fuel gas to the reforming unit; an oxidant gas supply unit that supplies the oxidant gas to the cathode; a combustion section that combusts off-gas from the fuel cell; A temperature sensor for detecting the temperature of the combustion section; a control unit that, when the system is stopped, executes an intermittent stop control for intermittently stopping the supply of the raw fuel gas and the oxidant gas so that combustion in the combustion unit is stopped, and terminates the execution of the intermittent stop control when a gradient of a decrease in the temperature of the combustion unit detected by the temperature sensor is equal to or greater than a predetermined value; A fuel cell system comprising:

2. the control unit calculates the gradient for a predetermined time that is longer than a supply stop time in the intermittent stop control, and determines whether the calculated gradient is equal to or greater than the predetermined value. The fuel cell system according to claim 1 .

3. The control unit determines whether or not the inclination is equal to or greater than the predetermined value when the temperature of the combustion unit detected by the temperature sensor is equal to or less than a predetermined temperature.

3. The fuel cell system according to claim 1 or 2.

4. The control unit determines whether the gradient is equal to or greater than the predetermined value by using a larger value as the temperature of the combustion unit detected by the temperature sensor is higher.

3. The fuel cell system according to claim 1 or 2.

Images