JP4945938B2 - Fuel cell system - Google Patents

Description

  The present invention relates to a fuel cell system having a fuel cell stack in which a plurality of fuel cells that generate power by receiving supply of fuel gas and oxidant gas to a fuel electrode and an oxidant electrode, respectively, are stacked.

In general, when a fuel cell system is started in a sub-freezing environment, the water generated during power generation freezes, and the supply of oxidant gas to the oxidant electrode is hindered by ice. The continuous time is limited until the supply of the oxidant gas is completely shut off. From such a background, it is possible to continue power generation in a sub-freezing environment for a vehicle fuel cell system in which it is time and energy good to raise the temperature of the fuel cell stack by reaction heat generated by power generation. In order to start up, a fuel cell system having a mechanism for holding water generated by power generation is proposed until the temperature of the fuel cell stack is raised to a temperature at which water can be discharged, such as 0 [° C.] or higher. (See Patent Document 1). In addition, the temperature of the fuel cell stack at the next start-up is predicted when the operation is stopped, and the generated water holding amount that can start the fuel cell stack is calculated based on the predicted temperature, and the fuel up to the calculated generated water holding amount is calculated. A fuel cell system for drying the cell stack has also been proposed (see Patent Document 2).
JP 2004-158387 A JP 2004-192852 A

  However, when the fuel cell system is used as a driving power source for a vehicle such as an automobile, it is difficult to accurately predict the usage status of the fuel cell system and the surrounding weather. It is difficult to accurately predict the temperature of the stack. For this reason, the temperature of the fuel cell stack at the time of startup cools to the predicted temperature or higher, so that the gas supply is hindered by the generated water before the temperature of the fuel cell stack rises to a temperature at which water can be discharged. The battery stack may not be able to continue power generation.

  In order to solve such a problem, a method of providing a plurality of generated water holding mechanisms as described above in consideration of temperature prediction accuracy is also conceivable. However, when this method is used, the fuel cell stack is The time required to stop the operation for drying becomes longer, and the fuel efficiency of the fuel cell system is lowered because a large amount of energy is used for the operation and temperature increase of the compressor.

  The present invention has been made to solve the above-described problems, and its purpose is to continue power generation even when the temperature of the fuel cell stack at the time of startup cools to a temperature that is predicted to be higher than when the operation is stopped. It is to provide a fuel cell system capable of performing the above.

In order to solve the above-described problem, the fuel cell system according to the present invention determines whether to heat up the temperature of the fuel cell stack during startup based on the temperature of the fuel cell stack and the startable temperature, and When heating and raising the temperature of the fuel cell stack, the operating conditions including whether the heating and heating of the fuel cell stack are completed before the start of power generation of the fuel cell stack or started substantially simultaneously are determined, and the determined operation is determined. Start the fuel cell stack under conditions.

  According to the fuel cell system of the present invention, since the operating condition of the fuel cell stack is determined based on the temperature of the fuel cell stack and the startable temperature at the time of startup, the temperature of the fuel cell stack at the time of startup is Power generation can be continued even when the temperature is lower than the predicted temperature.

  Hereinafter, the configuration and operation of a fuel cell system according to an embodiment of the present invention will be described with reference to the drawings.

[Configuration of fuel cell system]
A fuel cell system according to an embodiment of the present invention is used as a driving power source of a vehicle such as an automobile, and supplies hydrogen and air as fuel gas and oxidant gas to an anode and a cathode, respectively, as shown in FIG. A fuel cell stack 2 in which a plurality of fuel cells 1 that receive and generate electric power are stacked is provided. In this embodiment, the end portion of the fuel cell stack 2 is sandwiched by the end plate 3, and the generated power of the fuel cell stack 2 can be taken out via the end plate 3. The generated power extracted via the end plate 3 is supplied to the external circuit 4. Moreover, the electrochemical reaction in the anode and the cathode and the electrochemical reaction of the fuel cell stack 2 as a whole are based on the following formulas (1) to (3).

[Anode] H ₂ → 2H ⁺ + 2e ⁻ (1)
[Cathode] 1/2 O ₂ + 2H ⁺ + 2e ⁻ → H ₂ O (2)
[Overall] H ₂ +1/2 O ₂ → H ₂ O (3)
[Configuration of hydrogen system]
The fuel cell system includes a hydrogen supply device 5 and a three-way valve 6, and hydrogen supplied from the hydrogen supply device 5 is supplied to the anode of the fuel cell stack 2 through the three-way valve 6. The hydrogen discharged from the fuel cell stack 2 is discharged out of the system from the discharge flow path 7 or is circulated to the anode of the fuel cell stack 2 through the three-way valve 6 by the hydrogen circulation pump 8. By discharging the hydrogen discharged from the fuel cell stack 2 out of the system in this way, it is possible to prevent the power generation efficiency from being lowered due to the impurity gas that has permeated from the cathode side mixed into the hydrogen. Further, by circulating unused hydrogen discharged from the fuel cell stack 2 to the anode, the fuel efficiency of the fuel cell system can be improved. The hydrogen supply device 5 may supply hydrogen directly from a hydrogen storage device (not shown) such as a high-pressure gas tank, a liquefied hydrogen tank, a hydrogen storage alloy tank, or hydrogen containing natural gas, methanol, gasoline, or other hydrogen. Hydrogen may be supplied after reforming the contained gas.

[Air system configuration]
The fuel cell system includes an air filter 9, an air compressor 10, and a humidifier 11, and air taken from outside the system is purified by the air filter 9 and then pumped to the humidifier 11 by the air compressor 10. The air pumped from the air compressor 10 is humidified by the humidifier 11 and then supplied to the cathode of the fuel cell stack 2. The air discharged from the fuel cell stack 2 is discharged out of the system from the discharge path 7. The In this fuel cell system, a bypass air flow path that bypasses the humidifier 11 and supplies air from the air compressor 10 so that air that has not been humidified by the humidifier 11 can be supplied to the cathode of the fuel cell stack 2. 12 is provided.

[Cooling system configuration]
The fuel cell system includes a refrigerant circulation pump 13 that pumps the refrigerant to the fuel cell stack 2 and a radiator 15 that returns to the refrigerant circulation pump 13 via a radiator 15 that cools the refrigerant using outside air supplied from a radiator fan 14. Discharged from the fuel cell stack 2 between the side flow path 16, the bypass flow path 18 that returns to the refrigerant circulation pump 13 via the heating device 17 that heats the refrigerant, and the radiator side flow path 16 and the bypass flow path 18. A three-way valve 19 for switching the refrigerant flow path and a three-way valve 20 for controlling the flow rate of the refrigerant supplied to the refrigerant circulation pump 13 from each of the radiator side flow path 16 and the bypass flow path 18 are provided.

[Control system configuration]
The fuel cell system includes a temperature sensor 21 that detects the internal temperature of the fuel cell stack 2, a temperature sensor 22 that detects the temperature of the refrigerant supplied to the fuel cell stack 2, and the refrigerant discharged from the fuel cell stack 2. A temperature sensor 23 for detecting temperature, a moisture sensor 24 for detecting a partial pressure of water vapor in the air supplied to the fuel cell stack 2, and a moisture sensor for detecting a partial pressure of water vapor in the gas discharged from the fuel cell stack 2 25 and a controller 26 for controlling the operation of the entire fuel cell system. In this embodiment, the controller 26 includes a microprocessor having a CPU, a program ROM, a working RAM, and an input / output interface, and the CPU executes a control program stored in the program ROM. Various functions are realized.

[Configuration of fuel cell]
As shown in FIG. 2, the fuel cell 1 has a configuration in which an electrolyte membrane 31 such as a solid polymer electrolyte membrane is sandwiched between an anode 32 and a cathode 33. The anode 32 and the cathode 33 are composed of a carbon-supported platinum catalyst. It has a catalyst layer 34 and a gas diffusion layer (GDL) 35 formed by applying a paste mixed with a Nafion solution. The fuel cells 1 are separated from each other by a carbon graphite separator 36 in which grooves serving as reaction gas supply channels are formed.

  In the fuel cell system having such a configuration, the temperature of the fuel cell stack at the time of start-up is cooled to be higher than the temperature predicted at the time of operation stop by the controller 26 performing the stop process and the start-up process shown below. Even in cases, power generation can be continued. The operation of the controller 26 when executing the stop process and the start process will be described below with reference to the flowcharts shown in FIGS.

[Stop processing]
First, the operation of the controller 26 when executing the stop process will be described with reference to the flowchart shown in FIG.

  The flowchart shown in FIG. 3 starts in response to a stop command input to the controller 26, and the stop process proceeds to step S1.

  In the process of step S1, the controller 26 supplies air fed from the air compressor 10 to the fuel cell stack 2 via the bypass flow path 12, so that dry air having a predetermined water vapor partial pressure or less is supplied to the fuel cell stack. 2 and a purge process for removing water remaining in the fuel cell stack 2 is started. The controller 26 may remove water remaining in the fuel cell stack 2 by supplying a gas having a predetermined water vapor partial pressure or less to the anode. Thereby, the process of step S1 is completed and a stop process progresses to the process of step S2.

  In the process of step S2, the controller 26 detects the temperature, flow rate, and pressure of the air flowing in the fuel cell stack 2 and the partial pressure of water vapor in the air on the inlet side and the outlet side of the fuel cell stack 2 and detected. Record data as purge conditions. The temperature of the air flowing in the fuel cell stack 2 may be the temperature of the fuel cell stack 2 detected by the temperature sensor 21 or may be estimated from the refrigerant temperature detected by the temperature sensors 22 and 23. Good. Further, the flow rate and pressure of air flowing through the fuel cell stack 2 can be estimated from the number of rotations of the air compressor 10. Further, the water vapor partial pressure in the air can be detected by the moisture sensors 24 and 25. Thereby, the process of step S2 is completed, and the stop process proceeds to the process of step S3.

  In the process of step S3, the controller 26 removes the water remaining in the fuel cell stack 2 in accordance with the water vapor partial pressure in the air on the outlet side of the fuel cell stack 2 becoming a predetermined partial pressure or less. The purge process is terminated. Thereby, the process of step S3 is completed, and a stop process progresses to the process of step S4.

In the process of step S4, the controller 26 calculates the next startable temperature Tp based on the purge condition recorded in the process of step S2, and records the data of the calculated startable temperature Tp in the nonvolatile memory. Specifically, the water removal amount Δm removed by the purge process can be calculated by the following formulas 1 and 2. The parameter in the equation _{1 Δh s (t), V} (t), respectively _{t p,} the absolute humidity difference between the inlet side and the outlet side of the fuel cell stack 2 [g ^{/ m} 3], the gas flow rate ^[m 3 / s] and purge time [s]. In addition, parameters ρ, α, θ, p _w , and p in Equation 2 represent the water vapor density, the expansion coefficient, the gas temperature, the water vapor partial pressure, and the wet air total pressure, respectively.

Further, assuming that the heat capacity C of the fuel cell stack 2 is known, and the temperature of the fuel cell stack 2 at the time of startup and the temperature at which power generation can be continued are θ _b and θ ₀ , respectively, the output current and temperature of the fuel cell stack 2 over time Since the change is as shown in FIG. 4, the generated water amount m _w in the fuel cell stack 2 can be calculated from the integrated current value (the hatched portion shown in FIG. 4) according to the following formula 3, and the following formula 4 Thus, the calorific value Q by power generation can be calculated from the output voltage and output current of the fuel cell stack 2. Note that the parameters i, F, and _MH20 in Equation 3 indicate the current, the Faraday constant, and the molecular weight of water, respectively. Further, parameters E _cell and E _HHV in Equation 4 indicate the output voltage of the fuel cell stack 2 and the theoretical electromotive force based on the higher heating value, respectively.

Therefore, the controller 26 calculates the next startable temperature Tp by substituting the heat capacity C and the total heat generation amount Q of the fuel cell stack 2 into the following formula (5), and the calculated startable temperature Tp data. Is recorded in the nonvolatile memory. Thereby, the process of step S4 is completed and a series of stop processes are completed.

[Start process]
Next, the operation of the controller 26 when executing the startup process will be described with reference to the flowchart shown in FIG.

  The flowchart shown in FIG. 5 starts in response to an activation command input to the controller 26, and the activation process proceeds to step S11.

In the process of step S <b> 11, the controller 26 determines whether or not the temperature T _cell of the fuel cell stack 2 detected by the temperature sensor 21 is 0 ° C. or less. If the temperature T _cell of the fuel cell stack 2 is not equal to or lower than 0 ° C. as a result of the determination, the controller 26 activates the system by a normal activation sequence and ends a series of activation processes. On the other hand, when the temperature T _cell of the fuel cell stack 2 is 0 ° C. or less, the controller 26 advances the startup process to the process of step S12.

In the process of step S12, the controller 26 determines whether or not the temperature T _cell of the fuel cell stack 2 detected by the temperature sensor 21 is equal to or lower than the startable temperature Tp recorded in the nonvolatile memory. As a result of the determination, if the temperature T _cell of the fuel cell stack 2 is not equal to or lower than the startable temperature Tp, the controller 26 advances the start process to step S16. On the other hand, when the temperature T _cell of the fuel cell stack 2 is equal to or lower than the startable temperature Tp, the controller 26 advances the start process to the process of step S13.

In the process of step S13, the controller 26 calculates the absolute value of the difference value between the temperature T _cell of the fuel cell stack 2 and the startable temperature Tp. Further, the controller 26 calculates the power generation continuation time of the fuel cell stack 2 from the amount Δm of water removed from the inside of the fuel cell stack 2 by the purge process and a predetermined starting current density, and within the calculated power generation continuation time Next, the temperature of the fuel cell stack 2 that can be raised by the heating device 17 is calculated as a temperature riseable temperature Tup.

  Then, the controller 26 determines whether or not the calculated absolute value is equal to or lower than the temperature riseable temperature Tup. If the absolute value is equal to or lower than the temperature riseable temperature Tup, the controller 26 proceeds to the process of step S14. On the other hand, if the absolute value is not equal to or lower than the temperature riseable temperature Tup, the controller 26 advances the activation process to the process of step S15.

  In step S14, the controller 26 switches the refrigerant flow path to the bypass flow path 18 side, thereby supplying the refrigerant heated by the heating device 17 to the fuel cell stack 2 to raise the temperature of the fuel cell stack 2. At the same time, the fuel cell stack 2 is started to generate power by supplying hydrogen and air to the fuel cell stack 2. Thereby, the process of step S14 is completed and a series of starting processes are complete | finished.

  In step S15, the controller 26 switches the refrigerant flow path to the bypass flow path 18 side, thereby supplying the refrigerant heated by the heating device 17 to the fuel cell stack 2 to raise the temperature of the fuel cell stack 2. Thereby, the process of step S15 is completed and the activation process proceeds to the process of step S16.

  In the process of step S <b> 16, the controller 26 starts power generation of the fuel cell stack 2 by supplying hydrogen and air to the fuel cell stack 2. Thereby, the process of step S16 is completed and a series of starting processes are complete | finished.

As is apparent from the above description, according to the fuel cell system according to the embodiment of the present invention, at the time of start-up, the controller 26 is activated based on the temperature T _cell of the fuel cell stack 2 and the startable temperature Tp. Since the operating conditions are determined and the fuel cell stack 2 is started up under the determined operating conditions, power generation can be continued even when the temperature T _cell of the fuel cell stack at the time of starting is cooled to the startable temperature Tp temperature or higher. .

Further, according to the fuel cell system according to the embodiment of the present invention, the controller 26 calculates the amount Δm of water removed from the inside of the fuel cell stack 2 by the purge process, and the temperature T _cell of the fuel cell stack 2 is the freezing point. Since the amount _mw of water generated by power generation is calculated while raising the temperature from a temperature lower than the temperature to the temperature at which power generation can be continued, and the startable temperature Tp is estimated based on the calculation result, the startable temperature Tp is accurately determined. Can be estimated.

Further, according to the fuel cell system according to the embodiment of the present invention, when the temperature T _cell of the fuel cell stack 2 is lower than the startable temperature Tp, the controller 26 raises the fuel cell stack 2 to the startable temperature Tp. After that, since the power generation of the fuel cell stack 2 is started, the energy required for heating can be saved, and the time required for raising the temperature of the fuel cell stack 2 can be shortened. In addition, the amount of retained water generated in the fuel cell stack 2 can be increased, and deterioration of the fuel cell stack 2 can be suppressed. Further, since the current density can be increased, the temperature of the fuel cell stack 2 can be raised in a short time.

Further, according to the fuel cell system according to the embodiment of the present invention, the controller 26 continues the power generation of the fuel cell stack 2 from the amount Δm of water removed from the inside of the fuel cell stack 2 by the purge process and a predetermined starting current density. available time is calculated, and by the heating device 17 within the calculated power generation continuable time to calculate the temperature Tup of the fuel cell stack 2 can be heating, the temperature T _cell of the fuel cell stack 2 is lower than the bootable temperature Tp, and When the temperature Tup of the fuel cell stack 2 that can be raised by the heating device 17 within the power generation duration is larger than the difference between the temperature T _cell of the fuel cell stack 2 and the startable temperature Tp, the fuel device stack 2 is moved by the heating device 17. Since power generation of the fuel cell stack 2 is started almost simultaneously with the temperature rise, the startup time can be shortened.

  Further, according to the fuel cell system according to the embodiment of the present invention, the controller 26 removes water remaining in the fuel cell stack 2 by supplying air having a predetermined water vapor partial pressure or less to the cathode. Therefore, water remaining on the cathode side of the fuel cell stack 2 can be effectively removed. Further, according to the fuel cell system according to the embodiment of the present invention, the controller 26 removes water remaining in the fuel cell stack 2 by supplying hydrogen having a predetermined water vapor partial pressure or less to the anode. The water remaining on the cathode side through the electrolyte membrane 31 can be drawn to the anode side, and the water remaining on the cathode side of the fuel cell stack 2 can be effectively removed. In addition, when the hydrogen flow path and the air flow path are opposed to each other, a uniform dry state can be obtained by canceling the dry distribution of both the anode and the cathode, and the generated current density distribution at the start-up can be reduced. it can.

  In addition, according to the fuel cell system according to the embodiment of the present invention, the controller 26 heats the fuel cell stack 2 by circulating the heated refrigerant in the refrigerant flow path of the fuel cell stack 2 to increase the fuel temperature. Since the temperature of the fuel cell stack 2 is raised without changing the configuration of the battery stack 2, the cost of the fuel cell system can be suppressed using the existing fuel cell stack.

  Further, according to the fuel cell system according to the embodiment of the present invention, when the refrigerant is circulated in the refrigerant flow path of the fuel cell stack 2, the controller 26 bypasses the radiator flow path by the bypass flow path 18. Therefore, the amount of refrigerant to be heated can be reduced, and the temperature raising time can be shortened. Further, since the flow path length of the bypass flow path 18 is short, the loss due to heat dissipation can be reduced, and the energy required for heating the refrigerant can be saved. In addition, since the heating device 17 for heating the refrigerant is provided in the bypass flow path 18, the cooling performance can be lowered without increasing the pressure loss during normal room temperature operation without using the bypass flow path 18. Absent. Further, the electric power required for driving the refrigerant circulation pump 13 is not increased.

  As mentioned above, although the embodiment to which the invention made by the present inventor is applied has been described, the present invention is not limited by the description and the drawings that form part of the disclosure of the present invention according to this embodiment. For example, in the above embodiment, when the fuel cell system is connected to an external power source, it is desirable that the controller 26 keeps the fuel cell stack 1 at a temperature equal to or higher than the startable temperature Tp using electric power from the external power source. Thereby, starting time can be shortened, without using much energy. As described above, it is a matter of course that all other embodiments, examples, operation techniques, and the like made by those skilled in the art based on the above embodiments are included in the scope of the present invention.

It is a block diagram which shows the structure of the fuel cell system used as embodiment of this invention. It is sectional drawing which shows the internal structure of the fuel cell shown in FIG. It is a flowchart figure which shows the flow of the stop process used as embodiment of this invention. It is a figure which shows the time-dependent change of the output current and temperature of a fuel cell stack. It is a flowchart figure which shows the flow of the starting process used as embodiment of this invention.

Explanation of symbols

1: Fuel cell 2: Fuel cell stack 3: End plate 4: External circuit 5: Hydrogen supply device 6, 19, 20: Three-way valve 7: Discharge flow path 8: Hydrogen circulation pump 9: Air filter 10: Air compressor 11: Humidifier 12: Bypass air channel 13: Refrigerant circulation pump 14: Radiator fan 15: Radiator 16: Radiator side channel 17: Heating device 18: Bypass channels 21, 22, 23: Temperature sensors 24, 25: Moisture sensor 26 :controller

Claims (7)

A fuel cell system having a fuel cell stack in which a plurality of fuel cells that generate power by receiving supply of fuel gas and oxidant gas to a fuel electrode and an oxidant electrode, respectively, are stacked,
Heating means for heating and heating the fuel cell stack;
Temperature detecting means for detecting the temperature of the fuel cell stack;
Water removing means for removing water remaining in the fuel cell stack when the operation is stopped;
Startable temperature estimating means for estimating a startable temperature at the next start of the fuel cell stack after removing water remaining in the fuel cell stack;
Whether the temperature of the fuel cell stack is heated by the heating means during startup based on the temperature of the fuel cell stack detected by the temperature detection means and the startable temperature estimated by the startable temperature estimation means during startup. And in the case of heating and raising the temperature of the fuel cell stack, the operating conditions including whether the heating and heating of the fuel cell stack are completed before the start of power generation of the fuel cell stack or are started substantially simultaneously are determined. And a control means for starting the fuel cell stack under the determined operating conditions. The fuel cell system of the placing serial to claim 1,
When the temperature of the fuel cell stack is lower than the startable temperature, the control means starts power generation of the fuel cell stack after raising the temperature of the fuel cell stack to the startable temperature by the heating means. system. The fuel cell system according to claim 1 or 2 , wherein
The water removing means removes water remaining in the fuel cell stack by supplying an oxidant gas having a predetermined water vapor partial pressure or less to the oxidant electrode. The fuel cell system according to any one of claims 1 to 3 , wherein
The water removing means removes water remaining in the fuel cell stack by supplying a fuel gas having a predetermined water vapor partial pressure or less to the fuel electrode. The fuel cell system according to any one of claims 1 to 4 , wherein
The heating means heats and raises the temperature of the fuel cell stack by circulating the refrigerant heated by the heating element in the refrigerant flow path of the fuel cell stack. The fuel cell system according to claim 5 , wherein
The heating means, when circulating the refrigerant in the refrigerant flow path of the fuel cell stack, bypasses the refrigerant flow path from a heat exchanger that cools the refrigerant. The fuel cell system according to any one of claims 1 to 6 , wherein
When the fuel cell system is connected to an external power source, the heating means keeps the fuel cell stack at a temperature higher than the startable temperature by using electric power from the external power source.

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