JP2024071845A - Fuel Cell Systems - Google Patents

Abstract

[Problem] To appropriately control the operation of a fuel cell without excessively restricting the output voltage. [Solution] A fuel cell system includes a fuel cell, a control unit that controls the operation of the fuel cell so that the output voltage of the fuel cell does not fall below a lower limit voltage, a temperature sensor that detects a temperature that correlates with the temperature of the fuel cell, and a setting unit that sets the lower limit voltage so that it tends to increase as the temperature detected by the temperature sensor increases. [Selected Figure] Figure 3

Description

This specification discloses a fuel cell system.

A conventional fuel cell system of this type is a fuel cell system (SOFC system) that includes multiple solid oxide fuel cell stacks (SOFC stacks), in which the current-voltage characteristics of each solid oxide fuel cell stack are measured in advance as a function of temperature, and a lower voltage limit is set for each solid oxide fuel cell stack based on the measurement data (see, for example, Patent Document 1). Because the temperature during operation of the SOFC system differs for each SOFC stack, the current-voltage characteristics of each SOFC stack are measured as a function of temperature and a lower voltage limit is set, thereby predicting the output characteristics of each SOFC stack, and the amount of fuel and air supplied to each SOFC stack is independently controlled for each stack based on the output characteristics.

JP 2007-059359 A

In the above-mentioned fuel cell system, which includes multiple SOFC stacks, although it is described that the temperature of each stack during operation differs, no consideration is given to situations in which the temperature of the fuel cell stack changes during operation. In such a situation, in order to prevent stack failure, it is necessary to set a minimum voltage limit under the worst-case conditions, which ends up excessively restricting the stack voltage in many situations.

The primary objective of the fuel cell system disclosed herein is to provide a fuel cell system that can appropriately control the operation of a fuel cell without excessively restricting the output voltage.

The fuel cell system disclosed herein employs the following measures to achieve the above-mentioned primary objective:

The fuel cell system of the present disclosure comprises:
a fuel cell that generates electricity using an anode gas and a cathode gas;
a control unit that controls the operation of the fuel cell so that the output voltage of the fuel cell does not fall below a lower limit voltage;
a temperature sensor for detecting a temperature correlated with a temperature of the fuel cell;
a setting unit that sets the lower limit voltage so that the lower limit voltage increases as the temperature detected by the temperature sensor increases;
The gist of the project is to provide the following:

In the fuel cell system disclosed herein, the operation of the fuel cell is controlled so that the output voltage of the fuel cell does not fall below a lower limit voltage, and the lower limit voltage is set so that it tends to increase as the temperature correlated with the temperature of the fuel cell increases. This allows the operation of the fuel cell to be appropriately controlled without excessively restricting the output voltage.

In such a fuel cell system of the present disclosure, the control unit may have a load following operation mode in which the operation of the fuel cell is controlled so that the power generation output changes in accordance with the electrical load. When the fuel cell is controlled in the load following operation mode, the temperature of the fuel cell is likely to change, so it becomes even more important to set the lower limit voltage based on a temperature that correlates with the temperature of the fuel cell.

1 is a schematic diagram of a fuel cell system according to an embodiment of the present invention; 13 is a flowchart illustrating an example of a lower limit voltage setting process. FIG. 4 is an explanatory diagram showing the relationship between stack correlation temperature and stack voltage. FIG. 4 is an explanatory diagram showing the relationship between a stack current and a stack voltage.

The form for implementing this disclosure will be explained with reference to the drawings.

Figure 1 is a schematic diagram of a fuel cell system 10 of this embodiment. As shown in Figure 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 the anode gas and oxygen in the cathode gas, a raw fuel gas supply device 30 that supplies raw fuel gas (e.g., natural gas or LP gas) that is the 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.

The power generation module 20 includes a fuel cell stack 21, a vaporizer 22, a reformer 23, a combustor 24, and a heat exchanger 26, all of which are housed in a thermally insulating module case 29.

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 multiple flat-plate type single cells are stacked in the plate thickness direction. Each single cell S has an electrolyte such as zirconium oxide, and an anode (electrode) and a cathode (electrode) that sandwich the electrolyte. The reformer 23 is connected to the anode of each single cell S via the anode gas pipe 71. In addition, the air supply device 20 (air supply pipe 51) is connected to the cathode of each single cell S via the cathode gas pipe 72. A temperature sensor 111 is installed near the fuel cell stack 21. The temperature sensor 111 detects a temperature (stack-correlated temperature Tst) that correlates with the temperature of the fuel cell stack 21. The temperature sensor 111 may be installed in the cathode off-gas pipe 74 described later.

The vaporizer 22 and reformer 23 of the power generation module 20 are disposed above the fuel cell stack 21 inside the module case 29. In addition, 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. The reformer 23 and combustor 24 may be formed as a single unit.

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 the steam, and the mixed gas flows from the vaporizer 22 into the reformer 23.

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. 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 through the anode gas pipe 71.

In addition, air as a cathode gas is supplied to the cathode of each unit cell S via a cathode gas pipe 72. Oxide ions (O ²⁻ ) are generated in the cathode of each unit cell S, and the oxide ions permeate the electrolyte and react with hydrogen and carbon monoxide at the anode to obtain electric energy.

The anode gas (hereinafter referred to as "anode off-gas") that was not used in the electrochemical reaction (power generation) in each unit cell S is supplied to the combustor 24 through the anode off-gas piping 73, and the cathode gas (hereinafter referred to as "cathode off-gas") that was not used in the electrochemical reaction (power generation) in each unit cell S is supplied to the combustor 24 through the cathode off-gas piping 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 in the vaporizer 22, generating steam, and the steam reforming reaction in the reformer 23. The combustor 24 is provided with an ignition device 25 for igniting the mixed gas of the anode off-gas and the cathode off-gas introduced into the combustor 24.

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 vaporizer 22 in order to supply the heat required for steam reforming, the heat required for raising the temperature of the cathode gas (air), and the heat required for generating steam, respectively, and is then supplied to the condenser 62 through the combustion exhaust gas pipe 75. In addition, 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 discharged into the atmosphere.

The raw fuel gas supply device 30 has a raw fuel gas supply pipe 31 that connects the raw fuel supply source 1 that supplies the raw fuel gas to the vaporizer 22, and on-off valves (two 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 supply pipe 31. By operating the gas pump 34, the raw fuel gas is pumped (supplied) from the raw fuel supply source 1 through the desulfurizer 35 to the vaporizer 22. The flow sensor 39 detects the flow rate per unit time of the raw fuel gas flowing through the raw fuel gas supply pipe 31 (gas flow rate Fg).

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.

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. The flow sensor 54 detects the flow rate per unit time (air flow rate Fa) of air flowing through 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) through the cathode gas pipe 72. A heat exchanger 26 is installed in the cathode gas pipe 72, and the air flowing 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 supplied to the cathode of the fuel cell stack 21.

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 the anode off-gas flowing from the fuel cell stack 21 through the 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, and is heated by heat exchange with the anode off-gas in the condenser 62, and then returned to the hot water storage tank 61.

The condensed water pipe 44 and the combustion exhaust gas pipe 76 are connected to the exhaust gas side passage outlet 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. 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 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.

The input terminal of the power conditioner 80 is connected to the output terminal of the fuel cell stack 21, and the output terminal of the power conditioner 80 is connected to the power line 3 from the power system 2 to the 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 the load 4, such as a home appliance. A current sensor 113 that detects the current flowing through the fuel cell stack 21 is attached to the output terminal of the fuel cell stack 21, and a voltage sensor 114 that detects the terminal-to-terminal voltage of the fuel cell stack 21 is attached between the output terminals of the fuel cell stack 21.

A power supply board 81 is connected to the power conditioner 80. The power supply board 81 converts the DC power from the fuel cell stack 21 and the 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 sensor 111, the current sensor 113, and the voltage sensor 114, and the control device 100. In addition, cooling fans and ventilation fans (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 cooled the heat generating parts and increased in temperature is discharged into the atmosphere by the ventilation fan.

The control device 100 is configured as a microprocessor with a CPU 101 at its core, and includes a ROM 102 for storing processing programs, a RAM 103 for temporarily storing data, and an input/output port (not shown) in addition to the CPU 101. Various detection signals from the flow rate sensors 39, 54, temperature sensor 111, current sensor 113, voltage sensor 114, 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 the user of the fuel cell system 10.

Next, the operation of the fuel cell system 10 of this embodiment configured in this way, particularly the operation during power generation, will be described. The CPU 101 of the control device 100 controls the operation of the fuel cell system 10 in either a rated output operation mode in which the system operates at a rated output or a load following operation mode in which the system follows the power consumption of the load 4. In the load following operation mode, the CPU 101 of the control device 100 sets a target output with the rated output (e.g., 700 W) as the upper limit so as to follow the amount of power of the load 4 detected by a load power meter (not shown). Next, the CPU 101 sets a target stack current Itag based on the set target output so that the stack voltage Vst does not fall below the lower limit voltage Vmin, and controls the supply amounts of the raw fuel gas, reforming water, and air so that the set target stack current Itag is output from the fuel cell stack 21, and also controls the power conditioner (DC/DC converter and inverter) 80.

The supply amount of raw fuel gas is controlled by setting the gas flow rate according to the target command Itag to the 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 the 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 of the set target reforming water flow rate Fwtag is supplied. The steam carbon ratio SC is the molar ratio of carbon contained in the hydrocarbons in the raw fuel gas to the steam added for steam reforming. The amount of air supplied is controlled by setting a target air flow rate Ftag to achieve a predetermined air-fuel ratio relative to the target gas flow rate Fgtag, and controlling the air pump 53 by feedback control so that the air flow rate Fa detected by the flow sensor 54 matches the set target air flow rate Ftag.

Next, the process of setting the lower limit voltage Vmin will be further described. FIG. 2 is a flowchart showing an example of a lower limit voltage setting process routine executed by the CPU 101 of the control device 100. This routine is repeatedly executed at predetermined time intervals during power generation.

When the lower limit voltage setting process routine is executed, the CPU 101 of the control device 100 first inputs necessary data such as the stack correlation temperature Tst from the temperature sensor 111 and the stack current Ist from the current sensor 113 (S100). Next, the CPU 101 sets the lower limit voltage Vmin based on the input stack correlation temperature Tst and stack current Ist (S110), stores the set lower limit voltage Vmin in the RAM 103 (S120), and ends this routine. In this embodiment, the setting of the lower limit voltage Vmin is performed by experimentally determining the relationship between the stack correlation temperature Tst, the stack current Ist, and the lower limit voltage Vmin in advance and storing it in the ROM 102 as a map, and when the stack correlation temperature Tst and the stack current Ist are given, setting the corresponding lower limit voltage Vmin from the map. Figure 3 shows the relationship between the stack correlation temperature Tst and the stack voltage (lower limit voltage Vmin), and Figure 4 shows the relationship between the stack current Ist and the stack voltage (lower limit voltage Vmin). As shown in the figure, the lower limit voltage Vmin is set to be higher as the stack correlation temperature Tst increases and lower as the stack current Ist increases.

In the fuel cell system 10, if the potential of the anode electrode (negative electrode) of the fuel cell stack 21 becomes equal to or higher than the anode oxidation potential, the anode electrode becomes easily oxidized, which may cause the fuel cell stack 21 to break down. Therefore, the operation of the fuel cell stack 21 is controlled so that the stack voltage Vst (potential difference between the anode electrode and the cathode electrode) does not fall below the lower limit voltage Vmin. Since the anode oxidation potential depends on the temperature of the fuel cell stack 21, if the lower limit voltage Vmin is set uniformly, depending on the temperature of the fuel cell stack 21, even if the stack voltage Vst is controlled so as not to fall below the lower limit voltage Vmin, the potential of the anode electrode may exceed the anode oxidation potential, causing the anode electrode to be oxidized. Alternatively, if the lower limit voltage Vmin is set under the worst-case conditions to suppress oxidation of the anode electrode, a higher value will be set for the lower limit voltage Vmin, which will excessively limit the increase in the stack current Ist. In particular, in the fuel cell system 10 of this embodiment, load-following operation is performed to follow the amount of power consumed by the load 4, so when the load fluctuates greatly, the temperature of the fuel cell stack 21 (stack correlation temperature Tst) is likely to change. In addition, as the fuel cell system 10 deteriorates with use over time, the temperature of the fuel cell stack 21 gradually increases. In this situation, in the fuel cell system 10 of this embodiment, by setting the lower limit voltage Vmin based on the stack correlation temperature Tst, it is possible to appropriately control the operation of the fuel cell stack 21 without oxidizing the anode electrode or excessively restricting the increase in the stack current Ist.

Since the stack voltage Vst varies depending on the stack current Ist, in the fuel cell system 10 of this embodiment, the lower limit voltage Vmin is set based on the stack correlation temperature Tst and the stack current Ist. Note that the lower limit voltage Vmin may be set based on the stack correlation temperature Tst regardless of the stack current Ist.

In the fuel cell system 10 of the present embodiment described above, the operation of the fuel cell stack 21 is controlled so that the stack voltage Vst does not fall below the lower limit voltage Vmin, and the lower limit voltage Vmin is set so that it tends to increase as the stack correlation temperature Tst increases. This allows the operation of the fuel cell stack 21 to be appropriately controlled without excessively restricting the stack voltage.

The correspondence between the main elements of the embodiment and the main elements of the invention described in the section on means for solving the problem will be explained. In the embodiment, the fuel cell stack 21 corresponds to the "fuel cell", the control device 100 corresponds to the "control unit", the temperature sensor 111 corresponds to the "temperature sensor", and the CPU 101 of the control device 100 that executes the lower limit voltage setting process routine corresponds to the "setting unit".

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

Although the above describes the form for implementing this disclosure using embodiments, this disclosure is in no way limited to these embodiments, and it goes without saying that this disclosure can be implemented in various forms without departing from the spirit of this disclosure.

This disclosure is applicable to the fuel cell system manufacturing industry.

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 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 gas pump, 35 desulfurizer, 39 flow sensor, 40 reforming water supply device, 41 reforming water supply pipe, 42 reforming water tank, 43 reforming water pump, 44 condensed water pipe, 50 air supply device, 51 air supply pipe, 52 air filter, 53 air pump, 54 flow sensor, 60 exhaust heat recovery device, 61 hot water tank, 62 condenser, 63 circulation pipe, 64 circulation pump, 71 anode gas pipe, 72 Cathode gas piping, 73 combustion exhaust gas piping, 74 anode off gas piping, 75 cathode off gas piping, 76 combustion exhaust gas piping, 80 power conditioner, 81 power supply board, 100 control device, 101 CPU, 102 ROM, 103 RAM, 111 temperature sensor, 113 current sensor, 114 voltage sensor.

Claims (2)

a fuel cell that generates electricity using an anode gas and a cathode gas;
a control unit that controls the operation of the fuel cell so that the output voltage of the fuel cell does not fall below a lower limit voltage;
a temperature sensor for detecting a temperature correlated with a temperature of the fuel cell;
a setting unit that sets the lower limit voltage so that the lower limit voltage increases as the temperature detected by the temperature sensor increases;
A fuel cell system comprising: 2. The fuel cell system according to claim 1,
The control unit has a load following operation mode that controls the operation of the fuel cell so that the power generation output changes in accordance with an electrical load.
Fuel cell system.

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