fuel cell system

Abstract

To suppress the deformation of a fuel cell due to a differential pressure between an anode side pressure and a cathode side pressure and also improve the operating condition of the fuel cell.SOLUTION: A fuel cell system includes a fuel cell, a reforming part, a raw fuel gas supply part, a cathode gas supply part, a temperature detection part, and a control part. The temperature detection part detects a battery correlated temperature correlated with the temperature of the fuel cell. When a battery correlated temperature detected by the temperature detection part is low, the control part sets a differential pressure upper limit, which is an allowable upper limit of a differential pressure between an anode side pressure and a cathode side pressure, so as to become larger than when the battery correlated temperature is high, and controls the raw fuel gas supply part and the cathode gas supply part such that the fuel cell is operated within the range of the differential pressure upper limit.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] A conventional fuel cell system of this type includes a fuel cell stack, a hydrogen gas supply pipe that supplies hydrogen gas to the anode of the fuel cell stack, a hydrogen gas pressure regulating valve provided in the hydrogen gas supply pipe, an anode pressure gauge provided on the anode inlet side of the hydrogen gas supply pipe, an air supply pipe that supplies air gas to the cathode of the fuel cell stack by a compressor, an air gas pressure regulating valve provided in the air supply pipe, a cathode pressure gauge provided on the cathode inlet side of the air supply pipe, and a control device that controls the hydrogen gas pressure regulating valve and the air gas pressure regulating valve so that the pressure difference between the hydrogen gas pressure and the air gas pressure is maintained within a predetermined pressure difference range depending on the required output (see, for example, Patent Document 1). In this system, the aperture of the hydrogen gas pressure regulating valve is controlled by the anode pressure measured by the anode pressure gauge, and hydrogen gas supplied through the hydrogen gas supply pipe is adjusted to a predetermined pressure within the range in which the fuel cell system can operate, and then supplied to the anode of the fuel cell stack. In addition, by controlling the opening of the air gas pressure regulating valve and the rotation speed of the compressor based on the cathode pressure measured by the cathode pressure gauge, the air supplied through the air supply pipe is adjusted to a predetermined pressure within the range in which the fuel cell system can operate, and then supplied to the cathode of the fuel cell stack. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-278046 Summary of the Invention [Problem to be solved by the invention]

[0004] However, if the predetermined pressure difference range is uniformly set, in some cases the constraints on the operation of the fuel cell will be too great, making it impossible to operate the fuel cell appropriately.

[0005] The fuel cell system of the present disclosure has as its main object to suppress deformation of the fuel cell due to the pressure difference between the anode side pressure and the cathode side pressure, and to improve the operating state of the fuel cell. [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 comprises: a fuel cell that generates electricity based on an anode gas supplied to the anode side and a cathode gas supplied to the cathode side; a reforming unit that reforms the raw fuel gas into the anode gas; a raw fuel gas supply unit that supplies the raw fuel gas to the reforming unit; a cathode gas supply unit that supplies the cathode gas to the fuel cell; a temperature detection unit for detecting a cell-correlated temperature that correlates with the temperature of the fuel cell; a control unit that sets a differential pressure upper limit value, which is an allowable upper limit value of the differential pressure between the anode side pressure and the cathode side pressure, so that the cell correlation temperature detected by the temperature detection unit is higher when the cell correlation temperature is low than when the cell correlation temperature is high, and controls the raw fuel gas supply unit and the cathode gas supply unit so that the fuel cell is operated within the range of the differential pressure upper limit value; The gist of the project is to provide the following:

[0008] Separators, a component of a fuel cell, separate the cathode gas from the anode gas between adjacent cells. Their rigidity varies with temperature, depending on the material. Therefore, operating a fuel cell system within a range that does not exceed the upper limit of the differential pressure can prevent separator deformation caused by the pressure difference between the anode and cathode sides. Furthermore, setting the upper limit of the differential pressure based on the cell-related temperature can prevent excessive restrictions on the operating state based on the differential pressure, thereby maintaining a good operating state of the fuel cell system.

[0009] In the fuel cell system of the present disclosure, during power generation, the control unit may set a target flow rate of the raw fuel gas based on the required output of the system, and set a target flow rate of the cathode gas based on the target flow rate of the raw fuel gas within a range not exceeding the upper differential pressure limit, and control the raw fuel gas supply unit and the cathode gas supply unit. In this way, deformation of the separator can be suppressed and the fuel cell can generate power in a good condition.

[0010] In the fuel cell system of the present disclosure, the control unit may control the cathode gas supply unit by setting a target flow rate of the cathode gas for cooling the fuel cell within a range that does not exceed the upper limit of the differential pressure when the system is shut down. This makes it possible to quickly cool the fuel cell when the system is shut down, while suppressing deformation of the separator, thereby shortening the time required for shutting down the system.

[0011] Furthermore, in the fuel cell system of the present disclosure, the control unit may set an upper limit or lower limit of the allowable pressure on the cathode side based on the differential pressure upper limit value, and control the cathode gas supply unit so that the cathode gas is supplied within a range that does not exceed the upper limit value or within a range that does not fall below the lower limit value. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a schematic diagram of a fuel cell system according to an embodiment of the present invention; [Figure 2]4 is a flowchart illustrating an example of a power generation control routine. [Figure 3] 10 is a flowchart illustrating an example of an air flow rate upper and lower limit value setting process. [Figure 4] FIG. 4 is an explanatory diagram showing an example of a differential pressure upper limit value setting map. [Figure 5] 10 is a flowchart illustrating an example of a stop control routine. [Figure 6] FIG. 10 is an explanatory diagram showing how the estimated separator temperature and the upper limit value of the differential pressure change over time when the system is stopped. [Figure 7] 10A and 10B are explanatory diagrams showing the time-dependent changes in the target gas flow rate and the target air flow rate when the system is stopped. DETAILED DESCRIPTION OF THE INVENTION

[0013] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present disclosure will be described with reference to the drawings.

[0014] 1 is a schematic diagram of a fuel cell system 10 according to this embodiment. As shown in the figure, the fuel cell system 10 according to this embodiment includes a power generation module 20 including a fuel cell stack 21 that generates power through 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 raw fuel gas (e.g., natural gas or LP gas) that serves as a raw material for the anode gas to the power generation module 20 via a raw fuel gas supply pipe 31, a reforming water supply device 40 that supplies reforming water necessary 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, a reflux device 80 that refluxes a portion of unused fuel that was not used in the electrochemical reaction (power generation) in the fuel cell stack 21 back to the raw fuel gas supply pipe 31, and a control device 100 that controls the entire system.

[0015] The power generation module 20 includes a fuel cell stack 21, a vaporizer 22, a reformer 23, a combustor 24, and two heat exchangers 26 and 27, all of which are housed in a module case 29 with thermal insulation properties.

[0016] The fuel cell stack 21 is configured as a flat-plate solid oxide fuel cell stack in which flat unit cells and separators are alternately stacked in the thickness direction. Each unit cell has an electrolyte such as zirconium oxide, and an anode electrode and a cathode electrode sandwiching the electrolyte. The anode electrode of each unit cell is formed with an anode gas passage through which anode gas flows. The cathode electrode of each unit cell is formed with a cathode gas passage through which cathode gas flows. The separator is made of, for example, a press-formed stainless steel plate. It separates adjacent unit cells in the thickness direction and blocks the anode gas flowing through the anode electrode of one unit cell from the cathode gas flowing through the cathode electrode of the other unit cell. Furthermore, a temperature sensor 112 is installed near the fuel cell stack 21. The temperature sensor 112 detects a temperature (stack-correlated temperature Tst) correlated with the temperature of the fuel cell stack 21.

[0017] The vaporizer 22 and reformer 23 of the power generation module 20 are disposed above the fuel cell stack 21 within the module case 29 at a distance. A combustor 24 is disposed between the fuel cell stack 21 and the vaporizer 22 and reformer 23 to generate heat required for the operation of the fuel cell stack 21 and the reactions in the vaporizer 22 and reformer 23. An ignition device 25 is installed in the combustor 24.

[0018] The vaporizer 22 heats the raw fuel gas from the raw fuel gas supply device 30 and the reforming water from the reforming water supply device 40 using heat from the combustor 24, preheating the raw fuel gas and evaporating the reforming water to generate steam. The raw fuel gas preheated by the vaporizer 22 is mixed with steam, and the mixed gas flows from the vaporizer 22 into the reformer 23. A temperature sensor 111 is installed near the inlet of the reformer 23 to detect the temperature of the mixed gas flowing into the reformer 23 (vaporizer temperature).

[0019] 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 vaporizer 22 with the reforming catalyst in the presence of heat from the combustor 24. The reformer 23 also 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 flows through the anode gas piping 71 into the anode gas passage of each unit cell and is supplied to the anode electrode.

[0020] Air as a cathode gas flows into the cathode gas passage of each unit cell through a cathode gas pipe 72 and is supplied to the cathode electrode. At the cathode electrode of each unit cell, oxide ions (O 2- ) is generated, and the oxide ions pass through the electrolyte and react with hydrogen and carbon monoxide at the anode electrode, generating electrical energy.

[0021] Anode gas (hereinafter referred to as "anode offgas") that has not been used in the electrochemical reaction (power generation) in each unit cell is supplied to the condenser 62 through an anode offgas pipe 73. The anode offgas is cooled by the condenser 62 to remove water vapor from the anode offgas, and then the anode offgas is supplied to the combustor 24 through an anode offgas pipe 74. A heat exchanger 26 is installed in the anode offgas pipes 73, 74, and the anode offgas that flows through the anode offgas pipe 74 (anode offgas after passing through the condenser 62) is heated in the heat exchanger 26 by heat exchange with high-temperature anode offgas that flows through the anode offgas pipe 73 from the fuel cell stack 21 (anode offgas before passing through the condenser 62). Cathode gas (hereinafter referred to as "cathode offgas") that has not been used in the electrochemical reaction (power generation) in each unit cell is supplied to the combustor 24 through a cathode offgas pipe 75.

[0022] The anode off-gas that flows into the combustor 24 is a combustible gas containing fuel components such as hydrogen and carbon monoxide, and is mixed with the cathode off-gas that flows into the combustor 24 and contains oxygen. When the mixed gas is ignited in the combustor 24 by the ignition device 25, the combustion of the mixed gas generates heat necessary for operating the fuel cell stack 21, preheating the raw fuel gas and generating steam in the vaporizer 22, and the steam reforming reaction in the reformer 23. The combustor 24 also generates combustion exhaust gas containing unburned fuel. This combustion exhaust gas passes through the combustion exhaust gas piping 76, the heat exchanger 27, and the combustion catalyst 28 before being discharged into the outside air. The combustion catalyst 28 is an oxidation catalyst for re-burning the unburned fuel in the combustion exhaust gas.

[0023] The raw fuel gas supply device 30 includes a raw fuel gas supply pipe 31 that connects a raw fuel supply source 1 that supplies raw fuel gas to a vaporizer 22, and on-off valves (dual valves) 32 and 33, an orifice 34, a zero governor (pressure equalizing valve) 35, a gas pump 36, and a desulfurizer 38 that are installed in the raw fuel gas supply pipe 31. By operating the gas pump 36, the raw fuel gas is pressure-fed (supplied) from the raw fuel supply source 1 to the vaporizer 22 via the desulfurizer 38. In addition, a flow rate sensor 39 is installed between the orifice 34 and the zero governor 35 of the raw fuel gas supply pipe 31 to detect the flow rate per unit time of the raw fuel gas flowing through the raw fuel gas supply pipe 31 (gas flow rate Qg).

[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 vaporizer 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 vaporizer 22.

[0025] The air supply device 50 has an air supply pipe 51 connected to a cathode gas pipe 72 installed inside the module case 29, an air filter 52 provided at the inlet of the air supply pipe 51, and an air pump 53 installed in the air supply pipe 51. By operating the air pump 53, air as 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 electrode) through the cathode gas pipe 72. The air flowing through the cathode gas pipe 72 exchanges heat with high-temperature combustion exhaust gas flowing through the combustion exhaust gas pipe 76 in the heat exchanger 27, thereby increasing its temperature.

[0026] The exhaust heat recovery device 60 includes 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 the 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, where it is heated by heat exchange with the anode off-gas and then returned to the hot water storage tank 61.

[0027] Furthermore, a condensed water pipe 44 and an anode off-gas pipe 74 are connected to the passage outlet on the anode off-gas side of the condenser 62, and the condensed water obtained by condensing the water vapor in the anode off-gas through heat exchange with hot water from the hot water storage tank 61 is introduced into the reforming water tank 42 through the condensed water pipe 44. The reforming water tank 42 is equipped with a water purifier (not shown) that purifies the condensed water that has passed through the condensed water pipe 44. As described above, the anode off-gas from which the water vapor has been removed in the condenser 62 is supplied to the combustor 24 through the anode off-gas pipe 74.

[0028] Furthermore, the exhaust heat recovery device 60 has a radiator 65 incorporated in the circulation piping 63, a radiator fan (electric fan) 66 that sends air to the radiator 65, and an electric heater 67 that consumes electricity generated by the power generation module 20 to heat the water in the circulation piping 63. The radiator 65 is installed so as to be located between the circulation pump 64 and the condenser 62 on the circulation piping 63. The electric heater 67 is installed so as to be located between the radiator 65 and the circulation pump 64 on the circulation piping 63.

[0029] The reflux device 80 includes a reflux pipe 81 that branches off from the anode offgas pipe 74 and is connected to the raw fuel gas supply pipe 31 between the zero governor 35 and the gas pump 36, a solenoid valve 82 that is installed in the reflux pipe 81, and an orifice 83 that is formed in the reflux pipe 81. The solenoid valve 82 is a normally closed on-off valve, and the reflux line of the anode offgas from the anode offgas pipe 74 to the raw fuel gas supply pipe 31 is blocked when the solenoid valve 82 is closed and is opened when the solenoid valve 82 is opened.

[0030] An input terminal of a power conditioner 90 is connected to an output terminal of the fuel cell stack 21, and the output terminal of the power conditioner 90 is connected via a relay to a power line 3 from the power grid 2 to a load 4. The power conditioner 90 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 250 V to 300 V), and an inverter that converts the converted DC power into AC power of a voltage (e.g., AC 200 V) that can be connected to the power grid. 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 power supply board 91 is connected to the power conditioner 90. The power supply board 91 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 auxiliary equipment such as the gas pump 36, the reforming water pump 43, the air pump 53, and the circulation pump 64, sensors such as the flow sensor 39, temperature sensors 111 and 112, the current sensor 113, and the voltage sensor 114, and the control device 100. In addition, a cooling fan and a ventilation fan (not shown) for cooling the power conditioner 90 and the power supply board 91 are disposed in the auxiliary equipment room where the power conditioner 90 and the power supply board 91 are disposed. The cooling fan sends air to heat-generating parts of the power conditioner 90 and the power supply board 91. The air that has cooled the heat-generating parts and has been heated is discharged into the atmosphere by the ventilation fan.

[0031] The control device 100 is configured as a microprocessor centered around a CPU 101, and in addition to the CPU 101, is equipped with a ROM 102 that stores processing programs, a RAM 103 that temporarily stores data, and input / output ports (not shown). Various detection signals are input to the control device 100 via the input port from a current sensor 113 that detects the current (output current I) output from the fuel cell stack 21, a voltage sensor 114 that detects the voltage (output voltage V) output from the fuel cell stack 21, a flow rate sensor 39, temperature sensors 111, 112, and the like. The control device 100 also outputs various control signals via the output port to the solenoids of the on-off valves 32, 33, the pump motor of the gas pump 36, the pump motor of the reformed water pump 43, the pump motor of the air pump 53, the pump motor of the circulation pump 64, the ignition device 25, the solenoid of the solenoid valve 82, and the like. A remote control (not shown) is also 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 controller operated by the user of the fuel cell system 10 .

[0032] In the fuel cell system 10 of this embodiment, the fuel cell stack 21 is a stack of flat plate-type single cells, which has the advantage of a higher power density per unit area compared to cylindrical cells. On the other hand, flat plate-type cells tend to be at a disadvantage in terms of mechanical strength compared to cylindrical cells, and if the pressure difference between the anode gas pressure and the cathode gas pressure (inter-electrode pressure difference) becomes too large, there is a risk of damaging the cells (electrolyte). Therefore, in the fuel cell system 10 of this embodiment, the operation of the system is controlled so that the inter-electrode pressure difference does not exceed an allowable range, thereby preventing cell damage. The operation of the fuel cell system 10 will be described below.

[0033] 2 is a flowchart 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 (for example, every few msec or every few tens of msec) when the system is started.

[0034] When the power generation control routine is executed, the CPU 101 of the control device 100 first inputs the required output (required power) Preq required by the load 4, the output voltage V from the voltage sensor 114, the stack correlation temperature Tst from the temperature sensor 112, the gas flow rate Qg from the flow rate sensor 39, etc. (Step S100). Next, the CPU 101 sets a target current Itag, which is a target value for the current to be output from the fuel cell stack 21, based on the input required output Preq and output voltage V (Step S110). The target current Itag is set so that the drop in output voltage V in response to a sudden increase in the required output Preq falls within an allowable range.

[0035] After setting the required current Ireq, the CPU 101 sets the target gas flow rate Qgtag, which is the target flow rate of the raw fuel gas to be supplied from the raw fuel gas supply device 30, based on the target current Itag so that the fuel utilization rate Uf (the ratio of the amount of fuel gas used for power generation to the amount of fuel gas supplied to the anode) becomes the target utilization rate Uftag (step S120).

[0036] Next, the CPU 101 sets the target reforming water flow rate Qwtag, which is the target flow rate of the reforming water to be supplied from the reforming water supply device 40, based on the target gas flow rate Qgtag so that the steam-carbon ratio SC in the reformer 23 (the molar ratio of carbon contained in hydrocarbons in the raw fuel gas to steam added for steam reforming) becomes the target ratio SCtag (step S130).

[0037] Next, the CPU 101 sets a tentative air flow rate Qatmp, which is a tentative value of the flow rate of air to be supplied by the air supply device 50 (step S150). The tentative air flow rate Qatmp is set by setting the air utilization rate Ua based on the stack correlation temperature Tst so that the temperature of the fuel cell stack 21 becomes the target temperature, and by setting the tentative air flow rate Qatmp based on the target gas flow rate Qgtag so that the set air utilization rate Ua is achieved. Next, the CPU 101 acquires upper and lower air flow rate limits Qamax and Qamin, which are the upper and lower limits of the flow rate of air that can be supplied by the air supply device 50 (step S140). The CPU 101 then sets the target air flow rate Qatag, which is the target flow rate of air to be supplied by the air supply device 50, to the smaller of the tentative air flow rate Qatmp and the air flow rate upper limit Qamax, or the air flow rate lower limit Qamin (step S150). That is, the target air flow rate Qatag is set within a range that does not exceed the air flow rate upper limit Qamax and does not fall below the air flow rate lower limit Qamin. Here, the upper and lower air flow rate limits Qamax and Qamin are set by executing the air flow rate upper and lower limit value setting process illustrated in Figure 3. Hereinafter, the description of the power generation control routine will be interrupted and the air flow rate upper and lower limit value setting process will be described in detail.

[0038] In the air flow rate upper and lower limit value setting process, the CPU 101 first estimates the temperature of the separators of the fuel cell stack 21 (separator temperature Tsp) based on the stack correlation temperature Tst (step S200). The separator temperature Tsp can be estimated based on the stack correlation temperature Tst and the thermal conductivity of the fuel cell stack 21 including the separators, etc. Next, the CPU 101 sets a differential pressure upper limit ΔPmax, which is the upper limit of the differential pressure (inter-electrode differential pressure) between the pressure of the anode gas flowing on the anode electrode side and the pressure of the cathode gas flowing on the cathode electrode side across the separator, based on the separator temperature Tsp (step S210). The differential pressure upper limit ΔPmax is the upper limit of the inter-electrode differential pressure that is allowable within a range that does not cause creep deformation of the separator. In this embodiment, the differential pressure upper limit ΔPmax is set by previously determining the relationship between the separator temperature Tsp and the differential pressure upper limit ΔPmax and storing it as a differential pressure upper limit setting map. When a stack correlation temperature Tst is given, the corresponding differential pressure upper limit ΔPmax is derived from the map. An example of the differential pressure upper limit setting map is shown in FIG. 4. Separators, particularly metal separators, have temperature-dependent stiffness that decreases at high temperatures. Therefore, the higher the separator temperature Tsp, the more likely they are to creep due to the load caused by the inter-electrode differential pressure. On the other hand, when the separator temperature Tsp is low, the separator has sufficient stiffness and is therefore less susceptible to creep deformation. Taking this into consideration, the differential pressure upper limit ΔPmax is set to a relatively small value when the separator temperature Tsp is high, and a relatively large value is set when the separator temperature Tsp is low so that a relatively large differential pressure can be tolerated. In this embodiment, the differential pressure upper limit ΔPmax is set to a larger value as the separator temperature Tsp decreases.

[0039] Next, the CPU 101 estimates the anode pressure Pa, which is the pressure on the anode electrode side of the fuel cell stack 21 (step S220). The anode pressure Pa can be estimated, for example, based on the gas flow rate Qg detected by the flow rate sensor 39 and the stack-correlated temperature Tst, or based on the duty or rotation speed of the gas pump 36 and the stack-correlated temperature Tst. Next, the CPU 101 sets a cathode pressure upper limit Pcmax, which is the upper limit of the pressure on the cathode electrode side of the fuel cell stack 21, and a cathode pressure lower limit Pcmin, which is the lower limit of the pressure (step S230). The cathode pressure upper limit Pcmax can be calculated by adding the differential pressure upper limit ΔPmax to the anode pressure Pa, and the cathode pressure lower limit Pcmin can be calculated by subtracting the differential pressure upper limit ΔPmax from the anode pressure Pa. Then, the CPU 101 sets an air flow rate upper limit Qamax, which is the upper limit flow rate of air supplied from the air supply device 50, so that the cathode pressure does not exceed the cathode pressure upper limit Pcmax, and sets an air flow rate lower limit Qamin, which is the lower limit flow rate of air supplied from the air supply device 50, so that the cathode pressure does not fall below the cathode pressure lower limit Pcmin (step S240), and terminates the air flow rate upper and lower limit value setting process.

[0040] Returning to the power generation control routine, the CPU 101 sets the target gas flow rate Qgtag and the target reforming water flow rate Qwtag, and also sets the target air flow rate Qatag within the range of the air flow rate upper and lower limits Qamax and Qamin. Then, the CPU 101 controls the gas pump 36 so that the raw fuel gas is supplied at the target gas flow rate Qgtag (step S160), controls the reforming water pump 43 so that the reforming water is supplied at the target reforming water flow rate Qwtag (step S170), and controls the air pump 53 so that air is supplied at the target air flow rate Qatag (step S180), and ends the power generation control routine. The gas pump 36 is controlled by setting a duty by feedback calculation (e.g., proportional-plus-integral control) based on the deviation between the target gas flow rate Qgtag and the gas flow rate Qg from the flow sensor 39, and controlling the pump motor of the gas pump 36 at the set duty. The reforming water pump 43 is controlled by setting a duty based on the target reforming water flow rate Qwtag, and controlling the pump motor of the reforming water pump 43 at the set duty. Furthermore, the air pump 53 is controlled by setting a duty based on the target air flow rate Qatag and controlling the pump motor of the air pump 53 at the set duty.

[0041] In this way, by supplying air to the cathode of the fuel cell stack 21 within the upper and lower air flow rate limits Qamax and Qamin so as not to exceed the upper limit ΔPmax of the inter-electrode pressure difference, it is possible to cool the fuel cell stack 21 by supplying air while preventing creep deformation of the separator due to the inter-electrode pressure difference. As a result, the temperature of the fuel cell stack 21 can be maintained at an appropriate temperature, thereby improving power generation efficiency. Since the upper limit ΔPmax of the differential pressure is set to a relatively small value when the separator temperature Tsp is high and a relatively large value when the separator temperature Tsp is low, it is possible to optimize the supply of air within a range that does not cause creep deformation of the separator, even for temperature-dependent separators. This allows the heated fuel cell stack 21 to be efficiently cooled, for example, when the fuel cell stack 21 is deteriorated.

[0042] Next, a description will be given of the operation when shutting down the fuel cell system 10. Fig. 5 is a flowchart showing an example of a shut down control routine executed by the CPU 101 of the control device 100. This routine is executed when a request to shut down the system is made.

[0043] When the stop control routine is executed, the CPU 101 first inputs the stack correlation temperature Tst from the temperature sensor 112 (step S300) and determines whether the stack correlation temperature Tst is greater than a first threshold value T1 (step S310). The first threshold value T1 is a threshold value for determining whether the anode of the fuel cell stack 21 is in a temperature state at which oxidation degradation will not occur in an oxygen atmosphere. If the CPU 101 determines that the stack correlation temperature Tst is greater than the first threshold value T1, it sets the target gas flow rate Qgtag to a predetermined flow rate Qgset (step S320) and sets the target reforming water flow rate Qwtag to a predetermined flow rate Qwset (step S330). Next, the CPU 101 obtains the air flow rate upper limit value Qamax set by the air flow rate upper / lower limit value setting process described above (step S340) and sets the smaller of the air flow rate upper limit value Qamax and the predetermined flow rate Qaset to the target air flow rate Qatag (step S350). Then, the CPU 101 controls the gas pump 36, the reforming water pump 43, and the air pump 53 with the target gas flow rate Qgtag, the target reforming water flow rate Qwtag, and the target air flow rate Qatag (steps S360 to S380), and returns to step S300.

[0044] In the fuel cell stack 21, if the anodes are exposed to an oxygen atmosphere at high temperatures, the anodes will oxidize, resulting in a decrease in power generation performance. For this reason, when the system is shut down, raw fuel gas and reforming water are supplied from the raw fuel gas supply device 30 and the reforming water supply device 40, respectively, so that the supply of fuel gas to the anodes continues until the fuel cell stack 21 is sufficiently cooled. Here, the target gas flow rate Qgtag and the target reforming water flow rate Qwtag are set to relatively small flow rates Qgset and Qwset so that the anodes are not exposed to an oxygen atmosphere. On the other hand, the target air flow rate Qatag is set to a relatively large flow rate Qaset to quickly cool the fuel cell stack 21. However, to prevent creep deformation of the separators, the target air flow rate Qatag is set to an upper limit of the air flow rate upper limit Qamax so as not to exceed the differential pressure upper limit ΔPmax.

[0045] If the CPU 101 determines in step S310 that the stack correlation temperature Tst is equal to or lower than the first threshold value T1, it stops the supply of raw fuel gas and reforming water (step S390). Next, the CPU 101 inputs the stack correlation temperature Tst from the temperature sensor 112 (step S400) and determines whether the stack correlation temperature Tst is greater than a second threshold value T2 (step S310). The second threshold value T2 is a threshold for determining whether the fuel cell system 10 can be completely shut down, and is set to a temperature lower than the first threshold value T1. If the CPU 101 determines that the stack correlation temperature Tst is greater than the second threshold value T2, it obtains the air flow rate upper limit value Qamax set by the air flow rate upper and lower limit value setting process described above (step S420), and sets the smaller of the air flow rate upper limit value Qamax and a predetermined flow rate Qaset as the target air flow rate Qatag (step S430). Then, CPU 101 controls air pump 53 at the target air flow rate Qatag (step S440), and returns to step S400.

[0046] When CPU 101 determines in step S410 that stack-correlated temperature Tst is equal to or lower than second threshold value T2, it stops the supply of air (step S450) and ends the stop control routine.

[0047] In the stop control routine, the target air flow rate Qatag is set to the smaller of the air flow rate upper limit Qamax based on the differential pressure upper limit ΔPmax and the flow rate Qaset for quickly cooling the fuel cell stack 21. When cooling air is supplied to the fuel cell stack 21, the separator temperature Tsp decreases, and the differential pressure upper limit ΔPmax gradually increases as the separator temperature Tsp decreases, as shown in FIG. 6. Because the air flow rate upper limit Qamax increases as the differential pressure upper limit ΔPmax increases, the target air flow rate Qatag gradually increases as the separator temperature Tsp decreases, as shown in FIG. 7. Thus, by controlling the air supply by setting the target air flow rate Qatag within a range that does not exceed the differential pressure upper limit ΔPmax, creep deformation of the separators can be prevented. Furthermore, in the latter half of the stop control when the fuel cell stack 21 has cooled to a certain extent, the amount of air supply can be increased to further promote cooling of the fuel cell stack 21. As a result, the time required to shut down the system can be shortened.

[0048] In the fuel cell system 10 of the present embodiment described above, the fuel cell system 10 is operated within a range that does not exceed the differential pressure upper limit ΔPmax, so the differential pressure between the anode side pressure and the cathode side pressure can be well controlled to prevent creep deformation of the separator. Furthermore, because the differential pressure upper limit ΔPmax is set based on the stack correlation temperature Tst, excessive restriction of the cathode gas supply based on the differential pressure upper limit ΔPmax can be suppressed, thereby maintaining a good state (temperature state) of the fuel cell system 10.

[0049] During power generation, the target gas flow rate Qgtag is set based on the required output Preq required for the system, and the target air flow rate Qatag is set based on the target gas flow rate Qgtag within a range not exceeding the upper limit value ΔPmax of the differential pressure, thereby controlling the raw fuel gas supply device 30 and the air supply device 50. This makes it possible to prevent creep deformation of the separators while allowing the fuel cell stack 21 to generate power efficiently.

[0050] Furthermore, when the system is shut down, the target air flow rate Qatag for cooling the fuel cell stack 21 is set within a range that does not exceed the upper limit value ΔPmax of the differential pressure, and the air supply device 50 is controlled. This makes it possible to prevent creep deformation of the separators while quickly cooling the fuel cell stack 21 when the system is shut down, thereby shortening the time required for shutting down the system.

[0051] In the above-described embodiment, during power generation, the CPU 101 sets the target gas flow rate Qgtag based on the required output Preq, and also sets the target air flow rate Qatag based on the target gas flow rate Qgtag so as not to exceed the differential pressure upper limit value ΔPmax based on the stack-correlated temperature Tst. However, it is also possible to set the target air flow rate Qatag that is independent of the differential pressure upper limit value ΔPmax based on the target gas flow rate Qgtag, and then limit the target gas flow rate Qgtag based on the target air flow rate Qatag so as not to exceed the differential pressure upper limit value ΔPmax.

[0052] In the above-described embodiment, the CPU 101 sets the upper limit value ΔPmax of the differential pressure based on the stack-correlated temperature Tst during power generation and when the system is stopped, and sets and controls the target flow rates of the raw fuel gas and the anode gas so that they do not exceed the upper limit value ΔPmax of the differential pressure. However, the same control may also be performed when the system is started up.

[0053] The correspondence between the main elements of the embodiment and the main elements of the invention described in the "Means for Solving the Problem" section will be described below. In the embodiment, the fuel cell stack 21 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 "cathode gas supply section," the temperature sensor 112 corresponds to the "temperature detection section," and the control device 100 corresponds to the "control section."

[0054] The correspondence between the main elements of the embodiments and the main elements of the invention described in the "Means for Solving the Problem" section does not limit the elements of the invention described in the "Means for Solving the Problem" section, since the embodiments are examples for specifically explaining the mode for implementing the invention described in the "Means for Solving the Problem" section. In other words, the interpretation of the invention described in the "Means for Solving the Problem" section should be based on the description in that section, and the embodiments are merely specific examples of the invention described in the "Means for Solving the Problem" section.

[0055] The above describes the form for carrying out the present disclosure using embodiments, but the present invention is not limited to these embodiments in any way, and it goes without saying that the present invention can be carried out in various forms within the scope that does not deviate from the gist of the present invention. [Industrial Applicability]

[0056] The present invention can be used in the fuel cell system manufacturing industry and the like. [Explanation of symbols]

[0057] 1 raw fuel supply source, 2 power system, 3 power line, 4 load, 10 fuel cell system, 20 power generation module, 21 fuel cell stack, 22 vaporizer, 23 reformer, 24 combustor, 25 ignition device, 26, 27 heat exchanger, 28 combustion catalyst, 29 module case, 30 raw fuel gas supply device, 31 raw fuel gas supply pipe, 32, 33 on-off valve, 34 orifice, 35 zero governor, 36 gas pump, 38 desulfurizer, 39 flow sensor, 40 reforming water supply device, 41 reforming water supply pipe, 42 reforming water tank, 43 reforming water pump, 44 condensate water pipe, 50 air supply device, 51 air supply pipe, 52 air filter, 53 air pump, 60 exhaust heat recovery device, 61 hot water storage tank, 62 condenser, 63 circulation pipe, 64 Circulation pump, 65 radiator, 67 electric heater, 71 anode gas piping, 72 cathode gas piping, 73, 74 anode off-gas piping, 75 cathode off-gas piping, 76 combustion exhaust gas piping, 80 reflux device, 81 reflux piping, 82 solenoid valve, 83 orifice, 90 power conditioner, 91 power supply board, 100 control device, 101 CPU, 102 ROM, 103 RAM, 111, 112 temperature sensors, 113 current sensor, 114 voltage sensor.

Claims

1. A fuel cell including an electrolyte, an anode, a cathode, and a separator having temperature dependency, and generating electricity based on an anode gas supplied to the anode side and a cathode gas supplied to the cathode side; an anode gas supply unit that supplies the anode gas to the fuel cell; a cathode gas supply unit that supplies the cathode gas to the fuel cell; a temperature detection unit for detecting a cell-correlated temperature that correlates with the temperature of the fuel cell; a control unit that sets a differential pressure upper limit value, which is an allowable upper limit value of the differential pressure between the anode side pressure and the cathode side pressure, so that the cell correlation temperature detected by the temperature detection unit is higher when the cell correlation temperature is low than when the cell correlation temperature is high, regardless of the required output required for the system, and controls the anode gas supply unit and the cathode gas supply unit so that the fuel cell is operated within the range of the differential pressure upper limit value; A fuel cell system comprising:

2. 2. The fuel cell system according to claim 1, the control unit, during power generation, sets a target flow rate of the anode gas based on a required output of the system, and sets a target flow rate of the cathode gas based on the target flow rate of the anode gas within a range not exceeding the differential pressure upper limit value, and controls the anode gas supply unit and the cathode gas supply unit. Fuel cell system.

3. 3. The fuel cell system according to claim 1, the control unit, when the system is stopped, sets a target flow rate of the cathode gas for cooling the fuel cell within a range that does not exceed the differential pressure upper limit value, and controls the cathode gas supply unit. Fuel cell system.

4. 4. The fuel cell system according to claim 1, the control unit sets an upper limit or a lower limit of an allowable pressure on the cathode side based on the differential pressure upper limit value, and controls the cathode gas supply unit so that the cathode gas is supplied within a range not exceeding the upper limit value or a range not falling below the lower limit value. Fuel cell system.

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