Fuel battery system

Abstract

To prevent a harmful substance of a regulation concentration or larger is generated in a burning part while being corresponded to a request power generation output.SOLUTION: A fuel battery system comprises: a burning part that is housed in an adiathermancy case with the fuel battery, and burns an unused off gas from a fuel battery; a power consumption device that consumes the power generated in the fuel battery; and a control part, that controls, in the case where a power generation output of the fuel battery is a threshold value or less, an operation of the fuel battery so as to generate a power with the power generation output exceeding the threshold value of the fuel battery, and consumes an exceed amount exceeding a request power generation output requested by the fuel battery system.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] This specification discloses a fuel cell system. [Background technology]

[0002] Conventionally, there have been proposals for reducing emissions of harmful substances (particularly NOx) by adjusting the ignition characteristics of fuel (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Special Publication No. 2019-501357 Summary of the Invention [Problem to be solved by the invention]

[0004] In a fuel cell system that includes a fuel cell and a combustion section, the temperature of the fuel cell is maintained at an appropriate temperature by burning off-gas from the fuel cell in the combustion section. However, if too much off-gas is supplied to the combustion section, the temperature of the combustion section will rise excessively, and there is a risk of generating large amounts of harmful substances such as NOx. In particular, if the combustion section is concentrated in one place, such as a burner, the combustion section will become locally hot, making it easier for harmful substances to be generated.

[0005] The main object of the fuel cell system of the present disclosure is to prevent harmful substances from being generated in a combustion section at a concentration greater than a specified level while meeting the required power generation output. [Means for solving the problem]

[0006] The fuel cell system of the present disclosure employs the following measures to achieve the above-mentioned main object.

[0007] The fuel cell system of the present disclosure is a fuel cell system comprising a fuel cell, the system being housed in an insulated case together with the fuel cell and comprising: a combustion section that combusts unused off-gas from the fuel cell; a power consuming device that consumes power generated by the fuel cell; and a control section that, when the power generation output of the fuel cell is below a threshold value, controls the operation of the fuel cell so that the fuel cell generates power in excess of the required power generation output required for the fuel cell system, and controls the power consuming device so that surplus power in excess of the required power generation output is consumed.

[0008] In the fuel cell system disclosed herein, when the power output of the fuel cell is equal to or lower than a threshold value, the fuel cell is controlled to generate power in excess of the required power output, and the power consuming device is controlled to consume the surplus power in excess of the required power output. When the power output of the fuel cell decreases, the fuel utilization rate (the ratio of fuel used for power generation to fuel supplied to the fuel cell) decreases, and the amount of off-gas supplied to the combustion section increases. This causes the temperature of the combustion section to rise excessively due to the combustion of the off-gas, and harmful substances (NOx) at a specified concentration or higher may be generated. In the present disclosure, when the power output of the fuel cell is lower than a threshold value, the fuel cell is controlled to generate power in excess of the required power output, so that the amount of off-gas supplied to the combustion section can be adjusted to an appropriate amount, thereby suppressing an excessive temperature rise in the combustion section. In addition, the surplus power in excess of the required power output is consumed by the power consuming device, so that the fuel cell system can supply power commensurate with the required power output. As a result, it is possible to prevent harmful substances at a specified concentration or higher from being generated in the combustion section while responding to the required power output. [Brief description of the drawings]

[0009] [Figure 1] 1 is a schematic diagram of a fuel cell system according to an embodiment of the present invention; [Diagram 2] FIG. 2 is a schematic configuration diagram of a unit including a reformer and a combustor. [Diagram 3] 4 is a flowchart showing an example of a power generation control routine. [Figure 4]10 is a flowchart illustrating an example of a target surplus power setting process. [Diagram 5] 4 is an explanatory diagram showing an example of a surplus power basic value setting map and a target surplus power ΔP. FIG. [Figure 6] FIG. 4 is an explanatory diagram showing an example of a combustion section reference temperature setting map and a temperature difference α. [Figure 7] FIG. 11 is an explanatory diagram showing an example of the relationship between generated power and combustion section temperature T7. [Figure 8] FIG. 4 is an explanatory diagram showing an example of the relationship between generated power and NOx concentration. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present disclosure will be described with reference to the drawings.

[0011] Fig. 1 is a schematic diagram of a fuel cell system 10 of this embodiment, and Fig. 2 is a schematic diagram of a unit including a reformer 23 and a combustor 24. As shown in Fig. 1, the fuel cell system 10 of this embodiment 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, an electric heater 67 as a power consumption device that consumes a part of the power generated by the fuel cell stack 21, and a control device 100 that controls the entire system.

[0012] 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, all of which are housed in a module case 29 having thermal insulation properties.

[0013] 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.

[0014] The evaporator 22 , reformer 23 and combustor 24 (units) of the power generation module 20 are disposed above the fuel cell stack 21 in a module case 29 .

[0015] The evaporator 22 has many balls (ceramic or alumina balls) filled therein for storing heat, and when reforming water is supplied to the evaporator 22 in a state in which heat has been stored in the balls by heat from the combustor 24, the reforming water is evaporated 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 of the evaporator 22 (evaporation section temperature T1).

[0016] The reformer 23 has a Ru-based or Ni-based reforming catalyst 233 filled therein, and generates hydrogen gas and carbon monoxide by a reaction (steam reforming reaction) of the mixed gas from the evaporator 22 by the reforming catalyst 233 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, and the like. The anode gas generated by the reformer 23 is supplied to the anode of each unit cell S from the reformer outlet 235 through the anode gas pipe 71.

[0017] 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.

[0018] 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.

[0019] 2, in this embodiment, the reformer 23 is disposed to surround the combustor 24 so as to enable heat transfer from the combustor 24. That is, the reformer 23 has a cylindrical inner wall 231 with a bottom that serves as a partition between the reformer 23 and the combustor 24, a cylindrical outer wall 232 with a bottom that is disposed to cover the inner wall 231 across a space, a reforming catalyst 233 provided in an annular space defined between the inner wall 231 and the outer wall 232, a reformer inlet 234 for introducing a mixed gas containing a raw fuel gas and steam, and a reformer outlet 235 for discharging an anode gas.

[0020] 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 241 defined by the inner wall 231 of the reformer 23, an off-gas supply unit 242 that supplies the anode off-gas and the cathode off-gas to the combustion chamber 241, and an ignition unit 25 that ignites the mixed gas introduced into the combustion chamber 241. The combustor 24 is provided with a temperature sensor 113 for detecting the temperature inside the combustion chamber 241 (combustion unit temperature T7).

[0021] The outer wall 232 of the reformer 23 is configured with a double wall, and the combustion exhaust gas generated by the combustion of the off-gas in the combustion chamber 241 passes through a combustion exhaust gas passage 232a formed between the double walls, and heats the reformer 23 (reforming catalyst 233) from the outside. As a result, the reformer 23 is heated from the inside and outside by the combustion heat of the combustor 24 and the combustion exhaust gas, and the heat required for the reaction is efficiently provided.

[0022] 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.

[0023] 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).

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] Further, the exhaust heat recovery device 60 has a radiator 65 incorporated in the circulation pipe 63, a radiator fan (electric fan) 66 that sends air to the radiator 65, an electric heater (e.g., a ceramic heater) 67 that consumes a portion of the power generated by the fuel cell stack 21 to heat the hot and cold water in the circulation pipe 63, and a temperature sensor (thermistor) 68 that detects the temperature of the hot and cold water heated by the electric heater 67. The radiator 65 is incorporated in the circulation pipe 63 so as to be located between the condenser 62 and the circulation pump 64, and the electric heater 67 is incorporated in the circulation pipe 63 so as to be located between the circulation pump 64 and the radiator 65. Moreover, the temperature sensor 68 is installed downstream of the electric heater 67 so as to be close to the electric heater 67.

[0029] When surplus power is generated in the power generation module 20 (fuel cell stack 21) relative to the required power generation output required for the power generation module 20, the electric heater 67 is operated to consume the surplus power. At this time, the circulation pump 64 is duty controlled so that the water temperature detected by the temperature sensor 68 becomes a predetermined temperature, and the radiator fan 66 is operated as necessary. This makes it possible to continue the operation of the power generation module 20 while the condenser 62 recovers the exhaust heat of each fuel cell stack 21 and the electric heater 67 consumes the surplus power of the power generation module 20. In addition, by appropriately operating the radiator fan 66 to send air to the radiator 65, the hot water circulating through the circulation pipe 63 is cooled (heat is released), and the temperature of the hot water in the hot water storage tank 61 can be prevented from rising more than necessary.

[0030] 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.

[0031] 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 68, 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 machinery 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.

[0032] 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 68, 111, 112, 113, current sensor 114, voltage sensor 115, etc. are input to the control device 100 via an 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 an 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.

[0033] Next, the operation of the fuel cell system 10 thus configured, particularly the operation during power generation, will be described. Figure 3 is a flow chart showing an example of a power generation control routine executed by the CPU 101 of the control device 100. This routine is repeatedly executed at predetermined time intervals after the system is started up.

[0034] In the power generation control routine, the CPU 101 inputs data such as the required power generation output Prep required for the system, the power generation power P currently being generated by the power generation module 20, the gas flow rate Fg, and the combustion section temperature T7 (step S100). The power generation power P is calculated based on the stack current detected by the current sensor 114 and the stack voltage detected by the voltage sensor 115 and is input. The gas flow rate Fg is inputted as detected by the flow rate sensor 39. The combustion section temperature T7 is inputted as detected by the temperature sensor 113. Next, the CPU 101 determines whether the power generation power P is less than the threshold value Pref (step S102). If the CPU 101 determines that the power generation power P is equal to or greater than the threshold value Pref, it sets the target surplus power ΔP to a value of 0 (step S104). On the other hand, if the CPU 101 determines that the power generation power P is less than the threshold value Pref, it sets the target surplus power ΔP by a target surplus power setting process described later (step S106).

[0035] After setting the target surplus power ΔP, the CPU 101 sets the target power generation output Ptag to the required power generation output Preq plus the target surplus power ΔP (step S108). If the target surplus power ΔP is set to a value of 0, the target power generation output Ptag is set to the required power generation output Preq as is. Next, the CPU 101 sets a target current Itag of the fuel cell stack 21 based on the target power generation output Ptag and the stack voltage from the voltage sensor 115 (step S110). Then, the CPU 101 sets a target gas flow rate Fgtag, which is the supply amount of raw fuel gas required to output the target current Itag from the fuel cell stack 21 (step S112). The target gas flow rate Fgtag is set based on the target current Itag so that the fuel utilization rate Uf coincides with the target utilization rate Uftag. 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.

[0036] Next, the CPU 101 sets a target reforming water flow rate Fwtag, which is the supply amount of reforming water required for reforming the raw fuel gas, based on the target gas flow rate Fgtag (step S114). The target reforming water flow rate Fwtag is set 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. 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. Furthermore, the CPU 101 sets a target air flow rate Fatag, which is the required supply amount of air (step S116). The target air flow rate Fatag is set based on the stack temperature T4 so that the stack temperature T4 detected by the temperature sensor 112 coincides with the target temperature T4tag.

[0037] After setting the target gas flow rate Fgtag, the target reforming water flow rate Fwtag, and the target air flow rate Fatag in this way, the CPU 101 duty controls the gas pump 34 with the target gas flow rate Fgtag (step S118), duty controls the reforming water pump 43 with the target reforming water flow rate Fwtag (step S120), and duty controls the air pump 53 with the target air flow rate Fatag (step S122).Then, the CPU 101 controls the electric heater 67 so as to consume the target surplus power ΔP set in step S104 or step S106 (step S124), and ends the power generation control routine.

[0038] When the power generation power P of the power generation module 20 (fuel cell stack 21) decreases, the fuel utilization rate Uf decreases, and the amount of off-gas (anode off-gas and cathode off-gas) supplied to the combustor 24 increases. If the amount of off-gas increases too much, the combustion of the off-gas causes the inside of the combustion chamber 241 to become excessively hot, and the nitrogen contained in the cathode off-gas (air) is oxidized in the high-temperature atmosphere, which may result in the generation of NOx at a specified concentration or higher. In particular, in this embodiment, since the combustor 24 burns the off-gas using a burner, the combustion range is narrow, and the inside of the combustion chamber 241 is likely to become locally hot, so that the concentration of NOx is likely to become the specified concentration or higher. In this embodiment, when the power generation power P of the power generation module 20 is less than the threshold value Pref, the operation of the power generation module 20 is controlled so as to generate power exceeding the required power generation output Preq, so that the decrease in the fuel utilization rate Uf is suppressed, and the amount of off-gas supplied to the combustor 24 is made appropriate, thereby suppressing an excessive temperature rise in the combustion chamber 241. As a result, the generation of NOx at a specified concentration or higher can be prevented. Moreover, since the excess over the required power output Preq is consumed by the electric heater 67 as a power consuming device, it is possible to supply power corresponding to the required power output Preq from the fuel cell system 10. This makes it possible to prevent the generation of NOx at a specified concentration or higher in the combustor 24 while satisfying the required power output Preq.

[0039] Next, the target surplus power setting process executed in step S106 will be described in detail. Fig. 4 is a flowchart showing an example of the target surplus power setting process executed by the CPU 101 of the control device 100.

[0040] In the target surplus power setting process, the CPU 101 first sets a surplus power basic value ΔPb based on the generated power P input in step S100 of the power generation control routine (step S200). The surplus power basic value ΔPb is set by previously determining the relationship between the generated power P and the surplus power basic value ΔPb through experiments or the like and storing it in the ROM 102 as a map for setting the surplus power basic value, and when the generated power P is given, deriving the corresponding surplus power basic value ΔPb from the map. An example of the map for setting the surplus power basic value is shown in FIG. 5. As shown in the figure, the surplus power basic value ΔPb is set so as to be larger as the generated power P is lower when it is less than the threshold value Pref. In this embodiment, the surplus power basic value ΔPb is set based on the generated power P [W], but it may be set based on the stack current [A]. In this case, the relationship between the stack current and the surplus power basic value ΔPb may be stored in the ROM 102 as the map for setting the surplus power basic value.

[0041] Next, the CPU 101 sets the combustion section reference temperature T7b based on the power generation power P input in step S100 of the power generation control routine (step S202). The combustion section reference temperature T7b specifies the expected temperature in the combustion chamber 241 when the power generation module 20 (fuel cell stack 21) is operating with the power generation power P. The combustion section reference temperature T7b is set by determining the relationship between the power generation power P and the combustion section reference temperature T7b in advance by experiments or the like and storing it in the ROM 102 as a combustion section reference temperature setting map, and when the power generation power P is given, deriving the corresponding combustion section reference temperature T7b from the map. An example of the combustion section reference temperature setting map is shown in FIG. 6. The combustion section reference temperature T7b is set so that it is higher as the power generation power P is lower. In this embodiment, the combustion section reference temperature T7b is set based on the power generation power P [W], but it may also be set based on the stack current [A]. In this case, the relationship between the stack current and the combustion unit reference temperature T7b may be stored in the ROM 102 as a combustion unit reference temperature setting map.

[0042] Next, CPU 101 calculates temperature difference α by subtracting combustion section reference temperature T7b from combustion section temperature T7 from temperature sensor 113 input in step S100 of the power generation control routine (step S204). Temperature difference α is calculated from the difference between combustion section reference temperature T7b estimated from power generation power P and combustion section temperature T7 detected by temperature sensor 113 (see FIG. 6). CPU 101 then sets surplus power correction value ΔPc based on the set temperature difference α (step S206), sets the target surplus power ΔP to the surplus power basic value ΔPb plus surplus power correction value ΔPc (step S208), and ends the target surplus power setting process. In this embodiment, surplus power correction value ΔPc is calculated by a linear function (A·α+B; A and B are constants (A>0)) with temperature difference α as a variable. Since the surplus power correction value ΔPc increases as the combustion section temperature T7 increases with respect to the combustion section reference temperature T7b, the target surplus power ΔP is set to increase as the combustion section temperature T7 increases. The surplus power correction value ΔPc may be set using a map.

[0043] As a result of extensive research, the inventors of the present application have found that as the generated power of the power generation module 20 (fuel cell stack 21) decreases, the combustion section temperature T7 increases (see FIG. 7), and as the generated power and stack current of the power generation module 20 decrease, the NOx concentration increases (see FIG. 8). In light of this, when the generated power P is less than the threshold value Pref, the target surplus power ΔP is set so that it increases as the generated power P and stack current decrease, and the target power generation output Ptag is set by adding the target surplus power ΔP to the required power generation output Preq. This reduces the amount of off-gas supplied to the combustor 24, and makes it possible to suppress an excessive temperature rise in the combustor 24. Furthermore, due to factors such as deterioration of the fuel cell stack 21, as shown in Figures 7 and 8, variations occur in the combustion section temperature T7 and NOx concentration relative to the generated power P, so by setting a target surplus power ΔP corrected using the surplus power correction value ΔPc based on the combustion section temperature T7 from the temperature sensor 113 provided in the combustion chamber 241, the temperature inside the combustion chamber 241 can be maintained at an appropriate temperature with good accuracy.

[0044] In the above-described embodiment, the CPU 101 sets the target surplus power ΔP as the sum of the surplus power basic value ΔPb, which is set based on the generated power P (stack current), and the surplus power correction value ΔPc, which is set based on the generated power P (stack current) and the combustion section temperature T7. However, the CPU 101 may set the target surplus power ΔP based only on the generated power P (stack current), or may set the target surplus power ΔP based only on the combustion section temperature T7.

[0045] In the above-described embodiment, the electric heater 67 incorporated in the circulation pipe 63 of the exhaust heat recovery device 60 is used as a power consuming device that consumes surplus power generated by the power generation module 20 (fuel cell stack 21). However, this is not limited to this, and any device that can consume surplus power, such as consuming surplus power by driving a ventilation fan, may be used.

[0046] 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]

[0047] The present disclosure is applicable to the fuel cell system manufacturing industry. [Explanation of symbols]

[0048] 10 fuel cell system, 21 fuel cell stack (fuel cell), 231 inner wall (housing), 24 combustor (combustion section), 29 module case (case), 67 electric heater (power consumption device), 100 control device (control section), 113 temperature sensor (detection section), 242 off-gas supply section (supply section).

Claims

1. A fuel cell system including a fuel cell, a combustion section that is housed in a heat-insulating case together with the fuel cell and combusts unused off-gas from the fuel cell; a power consuming device that consumes the power generated by the fuel cell; a control unit that, when the power generation output of the fuel cell is equal to or lower than a threshold value, controls the operation of the fuel cell so that the fuel cell generates power in excess of a required power generation output required for the fuel cell system, and controls the power consumption equipment so that surplus power in excess of the required power generation output is consumed; A fuel cell system comprising:

2. 2. The fuel cell system according to claim 1, the control unit sets a target surplus power, which is a target value of the surplus power, so that the target surplus power becomes larger as the current power generation output of the fuel cell becomes lower, and controls the operation of the fuel cell so as to generate power based on power obtained by adding the target surplus power to the required power generation output, and controls the power consumption device so as to consume power based on the target surplus power. Fuel cell system.

3. 2. The fuel cell system according to claim 1, A detection unit for detecting the temperature of the combustion unit is provided, the control unit sets a reference temperature of the combustion unit so that the reference temperature increases as the current power generation output of the fuel cell decreases, sets a target surplus power that is a target value of the surplus power based on a difference between the temperature of the combustion unit detected by the detection unit and the reference temperature, controls the operation of the fuel cell to generate power based on power obtained by adding the target surplus power to the required power generation output, and controls the power consumption device to consume power based on the target surplus power. Fuel cell system.

4. 4. The fuel cell system according to claim 1, The combustion unit is a burner-type combustor having a housing and a supply unit that supplies off-gas into the housing. Fuel cell system.

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