JP2007141744A - Fuel cell system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell system capable of warming up a fuel cell even in a low-load state to a necessary and sufficient level. <P>SOLUTION: When warming up the fuel cell, the fuel cell is operated while an air theoretical ratio is set to less than 1.0 (theoretical value) (see the dotted line in Fig. 2). When the air theoretical ratio is set to a low value for operation, power loss (namely, thermal loss) in energy taken out by the reaction of hydrogen and oxygen increases positively, thus warming up the fuel cell rapidly. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a fuel cell system.

  In general, fuel cells have poor startability compared to other power sources. The power generation efficiency of such a fuel cell decreases due to a decrease in internal temperature or the like, and there may be a case where a device cannot be started without supplying a desired voltage / current.

  For example, in a fuel cell using a polymer electrolyte membrane, water molecules existing in the polymer electrolyte or in the vicinity of the electrode are frozen while stopped when kept in a low temperature range of 0 ° C. or lower, and the reaction gas ( In some cases, the fuel cell or the oxidizing gas) cannot be started after the fuel cell is stopped, for example, by inhibiting gas diffusion of the fuel gas or oxidizing gas.

  In view of such circumstances, when the operation (power generation) of the fuel cell is stopped, the fuel cell is heated (warmed up) by performing a power generation process in a high load state, and moisture present in the cell is effectively removed. A method has been proposed (see, for example, Patent Document 1).

Japanese Patent Laid-Open No. 2004-3277

  However, in order to warm up the fuel cell by the above method, it is necessary to operate in a high load state (for example, power supply to the electric heater or vehicle motor drive in a high torque state). In other words, when the supply destination of the electric power generated during the warm-up operation cannot be secured, the fuel cell cannot be heated to a desired temperature, and the fuel cell cannot be warmed up sufficiently and sufficiently. was there.

  The present invention has been made in view of the circumstances described above, and an object thereof is to provide a fuel cell system capable of sufficiently and sufficiently warming up the fuel cell even in a low load state.

  In order to solve the above-described problem, the fuel cell system according to the present invention is configured such that when a fuel cell and an instruction to stop the operation of the fuel cell are given, the operation is performed after a low-efficiency operation with a larger power loss than a normal operation. And an operation control means for stopping the operation.

  According to such a configuration, the fuel cell operation is stopped after performing a low-efficiency operation with a power loss larger than that in the normal operation. By performing the low-efficiency operation, the power loss (that is, the heat loss) is actively increased, so that the fuel cell can be sufficiently warmed up even in a low load state. Here, “when there is a stop instruction” means that the control device responds to a predetermined parameter such as time (timer operation) or power generation amount in addition to a case where a signal indicating stop is directly input by a button operation by the user. It is also intended to include cases where it is determined whether or not to stop and a positive determination result is obtained.

  In the above configuration, the fuel cell generates power using the reaction gas supplied to the anode and the cathode, and the operation control means sets the stoichiometric ratio of at least one of the reaction gases to the normal operation. The low-efficiency operation may be realized by setting it lower than the hour.

  Further, it is preferable that the apparatus further includes a scavenging control means for scavenging by supplying a reaction gas to the cathode after stopping the operation. Further, it is preferable that the operation control means sets the stoichiometric ratio of the reaction gas supplied to the cathode lower than that during the normal operation. Furthermore, a cathode gas supply passage, a cathode gas discharge passage, bypass means for bypassing the fuel cell to a part of the cathode gas flowing through the cathode gas supply passage to the cathode gas discharge passage, and the low efficiency operation An aspect further comprising adjusting means for adjusting the bypass amount of the cathode gas by the bypass means is preferable.

  As described above, according to the present invention, the fuel cell can be warmed up sufficiently and sufficiently even in a low load state.

  Embodiments according to the present invention will be described below with reference to the drawings.

A. First Embodiment FIG. 1 is a diagram showing a main configuration of a fuel cell system 100 according to a first embodiment. In the present embodiment, a fuel cell system mounted on a vehicle such as a fuel cell vehicle (FCHV), an electric vehicle, or a hybrid vehicle is assumed. It can also be applied to stationary power sources.

The fuel cell system 100 includes a fuel gas circulation supply system and an oxidizing gas supply system.
The fuel gas circulation supply system includes a fuel gas supply source 30, a fuel gas supply path 21, a fuel cell 40, a fuel gas circulation path 22, and an anode off-gas flow path 23, and the oxidizing gas supply system includes an air compressor 60, an oxidation gas The gas supply path 11 and the cathode off-gas flow path 12 are included.

  The fuel cell 40 is a means for generating electric power from supplied reaction gas (fuel gas and oxidant gas), and has a stack structure in which a plurality of single cells including MEAs (membrane / electrode assemblies) are stacked in series. Have. Specifically, various types of fuel cells such as a solid polymer type, a phosphoric acid type, and a molten carbonate type can be used.

  The fuel gas supply source 30 is means for supplying a fuel gas such as hydrogen gas to the fuel cell 40, and is constituted by, for example, a high-pressure hydrogen tank, a hydrogen storage tank, or the like. The fuel gas supply path 21 is a gas flow path for guiding the fuel gas discharged from the fuel gas supply source 30 to the anode electrode of the fuel cell 40. The gas flow path includes a tank valve H1, hydrogen gas from upstream to downstream. Valves such as a supply valve H2 and an FC inlet valve H3 are provided. The tank valve H1, the hydrogen supply valve H2, and the FC inlet valve H3 are shut valves for supplying (or shutting off) the fuel gas to the gas flow paths 21 to 23 or the fuel cell 20, and are constituted by, for example, electromagnetic valves. Yes.

The fuel gas circulation path 22 is a return gas flow path for recirculating unreacted fuel gas to the fuel cell 40. The gas flow path includes an FC outlet valve H14, a hydrogen pump 50, and a check valve 51 from upstream to downstream. Are arranged respectively. The low-pressure unreacted fuel gas discharged from the fuel cell 40 is appropriately pressurized by the hydrogen pump 50 and guided to the fuel gas supply path 21. The backflow of the fuel gas from the fuel gas supply path 21 to the fuel gas circulation path 22 is suppressed by the check valve 51.
The anode off gas passage 23 is a gas passage for exhausting the anode off gas containing the hydrogen off gas discharged from the fuel cell 40 out of the system, and a purge valve H5 is disposed in the gas passage.

  The air compressor 60 supplies oxygen (oxidizing gas) taken from outside air via an air filter (not shown) to the cathode electrode of the fuel cell 40. Cathode off-gas is discharged from the cathode of the fuel cell 40. The cathode off gas includes pumping hydrogen generated on the cathode side in addition to oxygen off gas after being subjected to the cell reaction of the fuel cell 40 (details will be described later). This cathode off gas is in a highly moist state because it contains moisture generated by the cell reaction of the fuel cell 40.

  The humidification module 70 exchanges moisture between the low-humidity oxidizing gas flowing through the oxidizing gas supply path 11 and the high-humidity cathode offgas flowing through the cathode offgas flow path 12, and the oxidation supplied to the fuel cell 40. Humidify the gas moderately. The back pressure of the oxidizing gas supplied to the fuel cell 40 is regulated by a pressure regulating valve A1 disposed in the vicinity of the cathode outlet of the cathode offgas channel 12.

  Here, the oxidizing gas supply path 11 from the air compressor 60 to the humidification module 70 and the cathode offgas flow path 12 from the humidification module 70 to the diluter 80 are connected by a bypass valve B1. The bypass valve (bypass means) B1 is a means for guiding a part of the oxidizing gas output from the air compressor 60 to the cathode off-gas flow path 12 without supplying a part of the oxidizing gas to the fuel cell 40 (that is, bypassing the fuel cell 40). The amount of oxidizing gas bypassed by the control device (adjusting means) 160 is adjusted. In the following description, the bypassed oxidizing gas is referred to as a diluting oxidizing gas.

  The diluter 80 dilutes the hydrogen gas discharge concentration so that it falls within a preset concentration range (such as a range determined based on environmental standards). The diluter 80 communicates with the downstream of the cathode offgas flow channel 12 and the downstream of the anode offgas flow channel 23. The hydrogen offgas, the pumping hydrogen, the oxygen offgas, and the oxidizing gas for dilution are mixed and diluted and exhausted outside the system. .

A part of the DC power generated by the fuel cell 40 is stepped down by the DC / DC converter 130 and charged to the battery 140.
The battery (capacitor) 140 is a chargeable / dischargeable secondary battery, and is composed of various types of secondary batteries (for example, a nickel metal hydride battery). Of course, instead of the battery 140, a chargeable / dischargeable capacitor other than the secondary battery, for example, a capacitor may be used.

The traction inverter 110 and the auxiliary inverter 120 are pulse width modulation type PWM inverters, and convert DC power output from the fuel cell 40 or the battery 140 into three-phase AC power in accordance with a given control command, thereby obtaining a traction motor. Supply to M3 and auxiliary motor M4.
The traction motor M3 is a motor for driving the wheels 150L and 150R, and the auxiliary motor M4 is a motor for driving various auxiliary machines. The auxiliary motor M4 is a generic term for a motor M1 that drives the hydrogen circulation pump 50, a motor M2 that drives the air compressor 60, and the like.

  The control device (operation control means, scavenging control means) 160 includes a CPU, a ROM, a RAM, and the like, and centrally controls each part of the system based on each input sensor signal. Specifically, an accelerator pedal sensor s1 that detects the accelerator pedal opening, an SOC sensor s2 that detects a state of charge (SOC) of the battery 140, and a T / C motor rotational speed that detects the rotational speed of the traction motor M3. Based on the sensor signals input from the detection sensor s3, the voltage sensor s4 that detects the output voltage, output current, and internal temperature of the fuel cell 40, the current sensor s5, the temperature sensor s6, etc., the output pulses of the inverters 110 and 120 Control width etc. Further, when the control device 160 receives an operation stop instruction from the operation switch, for example, when the ignition key is turned off, the control device 160 supplies the fuel cell 40 to the cathode in order to warm up the fuel cell 40 (that is, raise the temperature of the fuel cell 40). Operation with low power generation efficiency is performed by reducing the oxidizing gas.

  FIG. 2 is a diagram showing the relationship between the output current (FC current) and the output voltage (FC voltage) of the fuel cell, and shows a case where an operation with high power generation efficiency (normal operation) is performed, indicated by a solid line. The dotted line shows the case of low operation (low efficiency operation). The horizontal axis represents the FC current, and the vertical axis represents the FC voltage.

  Normally, when operating the fuel cell 40, the fuel cell 40 is operated in a state where the air stoichiometric ratio is set to 1.0 or more (theoretical value) so that high power generation efficiency can be obtained while suppressing power loss (see FIG. (See solid line part 2). Here, the air stoichiometric ratio means an oxygen surplus rate, and indicates how much oxygen is supplied to oxygen necessary for reaction without excess or deficiency.

  On the other hand, when the fuel cell 40 is warmed up, the fuel cell 40 is set with the air stoichiometric ratio set to less than 1.0 (theoretical value) in order to increase the power loss and raise the temperature of the fuel cell 40. (See the dotted line portion in FIG. 2). When the air stoichiometric ratio is set to a low value, the power loss (ie, heat loss) out of the energy that can be extracted by the reaction between hydrogen and oxygen is actively increased. Pumping hydrogen is generated at the cathode.

FIG. 3 is a diagram for explaining the generation mechanism of pumping hydrogen, FIG. 3A is a diagram showing a battery reaction during normal operation, and FIG. 3B is a diagram showing a battery reaction during low-efficiency operation.
Each cell 4 includes an electrolyte membrane 4a, and an anode electrode and a cathode electrode that sandwich the electrolyte membrane 4a. A fuel gas containing hydrogen (H ₂ ) is supplied to the anode, and an oxidizing gas containing oxygen (O ₂ ) is supplied to the cathode. When fuel gas is supplied to the anode, the reaction of the following formula (1) proceeds and hydrogen is separated from hydrogen ions and electrons. Hydrogen ions generated at the anode permeate the electrolyte membrane 4a and move to the cathode, while electrons move from the anode to the cathode through an external circuit.

Here, when the supply of the oxidizing gas to the cathode is sufficient (air stoichiometric ratio ≧ 1.0), the following formula (2) proceeds to generate water from oxygen, hydrogen ions, and electrons (see FIG. 3A). ). On the other hand, when the supply of the oxidizing gas to the cathode is insufficient (air stoichiometric ratio <1.0), the following equation (3) proceeds according to the insufficient oxidizing gas amount, and the hydrogen ions and the electrons are recombined. Thus, hydrogen is generated (see FIG. 3B). The produced hydrogen is discharged from the cathode together with the oxygen off gas. Note that hydrogen generated at the cathode by recombination of dissociated hydrogen ions and electrons, that is, the anode gas generated at the cathode is called pumping hydrogen.
Anode: H ₂ → 2H ⁺ + 2e ⁻ (1)
Cathode: 2H ⁺ + 2e ⁻ + (1/2) O ₂ → H ₂ O (2)
Cathode: 2H ⁺ + 2e ⁻ → H ₂ (3)

  Thus, in the state where the supply of the oxidizing gas to the cathode is insufficient, the pumping hydrogen is contained in the cathode offgas. Therefore, in this embodiment, the flow rate of the oxidizing gas for dilution is adjusted according to the amount of pumping hydrogen contained in the cathode offgas. This controls the concentration of hydrogen discharged to the outside. Hereinafter, the operation of the fuel cell system 100 during the low-efficiency operation will be described.

FIG. 4 is a flowchart showing the operation of the fuel cell system during low-efficiency operation.
When receiving the operation stop instruction from the operation switch, the control device 140 refers to an air stoichiometric ratio map (not shown) stored in the memory to warm up the fuel cell 40 by the low efficiency operation, and for the low efficiency operation. An air stoichiometric ratio (<1.0) is determined (step S1 → step S2). Here, the air stoichiometric ratio set during the low-efficiency operation may be constant, but may be appropriately changed according to the system power requirement.

When the air stoichiometric ratio is determined, the control device 140 controls the opening degree of the pressure regulating valve A1 and the bypass valve B1 (step S3). As described above, pumping hydrogen is generated on the cathode side when the low-efficiency operation is performed. Therefore, it is necessary to suppress the exhaust hydrogen concentration in consideration of this pumping hydrogen. Therefore, in this embodiment, the opening degree of the bypass valve B1 is controlled to adjust the flow rate of the oxidizing gas for dilution flowing from the oxidizing gas supply passage 11 to the cathode offgas passage 12. Note that the flow rate of the oxidizing gas for dilution flowing into the cathode off-gas channel 12 may be determined using a map that associates the amount of generated pumping hydrogen with the flow rate of the oxidizing gas for dilution. The amount of generated pumping hydrogen may be measured using a hydrogen concentration meter or the like, but may be estimated using the following theoretical formula (4) or the like, and which method is adopted is arbitrary. .
Vt = (1-St) * Ifc * [n / (2 * F)] * 22.4 * 60 (4)
Vt; theoretical generation amount of pumping hydrogen St; air stoichiometric ratio Ifc; FC current (power generation characteristics)
F: Faraday constant n: Number of cells

  Thereafter, the control device 140 refers to the internal temperature of the fuel cell 40 detected by the temperature sensor s6 and determines whether or not the low-efficiency operation should be terminated (that is, whether or not the warm-up operation of the fuel cell 40 should be terminated). Judgment is made (step S4). For example, if the internal temperature of the fuel cell 40 exceeds a preset reference temperature, it is determined that the low efficiency operation should be terminated. Of course, the present invention is not limited to this, and it may be determined whether or not the low-efficiency operation should be terminated based on the heat generation amount, the operation time of the low-efficiency operation, or the like. By performing such warm-up operation, the amount of water taken away from each off gas increases, and the water content of the fuel cell 40 can be efficiently reduced.

  When finishing the warm-up operation based on the above determination, the control device 140 executes a scavenging process (step S5). Specifically, after the supply of the reaction gas to the fuel cell 40 is stopped, the air compressor 60 is driven in a state where the internal temperature of the fuel cell 40 is high, and the scavenging is performed by sending the oxidizing gas into the fuel cell 40. End driving. By performing this scavenging process, the water content of the fuel cell 40 can be more efficiently reduced in a short time. Of course, only the warm-up operation may be executed without executing the scavenging process.

  As described above, according to the present embodiment, when the operation of the fuel cell is stopped, the fuel cell is quickly warmed up by performing the low efficiency operation with low power generation efficiency, and the system operation is terminated. It becomes possible to sufficiently remove moisture present in the battery.

Here, when the fuel cell is caused to generate power with sufficient oxidizing gas supplied to the cathode, the reaction shown in the above formula (3) proceeds and water is generated. Due to this generated water, the reaction shown by the following formula (5) proceeds at the anode electrode, and elution of Pt, which is a catalyst component of the cathode electrode, and corrosion of the carbon support progress, thereby reducing catalyst performance and battery performance. I will invite you.
Cathode: C + 2H ₂ O → CO ₂ + 4H ⁺ + 4e ⁻ (5)

On the other hand, in this embodiment, by generating power in the fuel cell in a state where the oxidizing gas supplied to the cathode is insufficient, the reaction shown in the above formula (2) proceeds to generate pumping hydrogen. Due to the generation of pumping hydrogen, the electrode potential of the cathode electrode is lowered, so that it is possible to suppress a decrease in catalytic activity and a decrease in battery performance.
In the above example, the case where the fuel cell is caused to generate power in a state where the oxidizing gas supplied to the cathode is insufficient has been described. Instead of (or in addition to) this, the amount of fuel gas supplied to the anode is less than usual. In this state, the fuel cell may be generated by reducing the voltage.

B. Second Embodiment In the first embodiment described above, low-efficiency operation is realized by generating power in the fuel cell in a state where the oxidizing gas supplied to the cathode is insufficient. However, in this embodiment, the output power of the fuel cell is controlled. To achieve low-efficiency operation.
FIG. 5 is a diagram showing changes in output power when the operating point is shifted, with the horizontal axis representing FC current and the vertical axis representing FC voltage and output power. In FIG. 5, for convenience of explanation, the IV characteristic of the fuel cell is represented by a straight line (hereinafter referred to as “IV line”). As shown in FIG. 5, the output power Pfc of the fuel cell 40 is centered around the maximum output operation point (Ifcmax, Vfcmax) at which the maximum output power Pfcmax is obtained. For example, in (Ifc1, Vfc1)), the output power Pfc increases as the FC voltage Vfc decreases. On the other hand, the FC voltage Vfc decreases at the operating point on the IV line 11 shown on the right side of the figure (for example, (Ifc2, Vfc2)). As a result, the output power Pfc decreases. Here, regarding the power loss, the driving at the driving operation point on the IV line 11 shown on the right side of the maximum output driving operating point operates at the driving operation point on the IV line 11 shown on the left side of the maximum output driving operating point. Bigger than. Therefore, in the following description, an operation with a small power loss in which the output power Pfc increases with a decrease in the FC voltage Vfc is called a normal operation, and a power loss with a large power loss in which the output power Pfc decreases with a decrease in the FC voltage Vfc. Operation is called low efficiency operation.

  In the present embodiment, when the operation of the fuel cell 40 is stopped, the output power is controlled to shift from the normal operation point to the low efficiency operation point, thereby realizing the low efficiency operation. Here, the output power of the fuel cell 40 is stored in the battery 140, but the output power varies greatly according to the shift of the operation point of the IV line 11 (see the power line pl1), so the remaining capacity of the battery 140 is It is necessary to have a margin for storing output power. In addition, after rapid warm-up by low-efficiency operation, the system operation may be stopped after performing the scavenging process. Further, after performing the scavenging process (or without performing the scavenging process), pumping hydrogen may be generated in a deficient state of oxidizing gas, and the system operation may be stopped after the electrode potential of the cathode electrode is lowered. Thus, low efficiency operation may be realized by controlling the output power of the fuel cell.

It is a figure which shows the principal part structure of the fuel cell system which concerns on 1st Embodiment. It is a figure which shows the relationship between FC electric current and FC voltage which concern on the same embodiment. It is a figure for demonstrating the generation | occurrence | production mechanism of pumping hydrogen concerning the embodiment. It is a figure for demonstrating the generation | occurrence | production mechanism of pumping hydrogen concerning the embodiment. It is a flowchart which shows the operation | movement at the time of the low efficiency driving | operation which concerns on the same embodiment. It is a figure which shows the change of output electric power when the driving | operation operating point which concerns on 2nd Embodiment is shifted.

Explanation of symbols

30 ... Fuel gas supply source, 40 ... Fuel cell, 50 ... Hydrogen circulation pump, 60 ... Air compressor, 70 ... Humidification module, 80 ... Diluter, 110 ... Traction Inverter, 120 ... Auxiliary inverter, 130 ... DC / DC converter, 140 ... Battery, 150L, 150R ... Wheel, 160 ... Control device, H1 ... Tank valve, H2 ... -Hydrogen supply valve, H3 ... FC inlet valve, H4 ... FC outlet valve, H5 ... Purge valve, 51 ... Check valve, A1 ... Pressure adjustment valve, B1 ... Bypass valve, DESCRIPTION OF SYMBOLS 11 ... Oxidation gas supply path, 12 ... Cathode off-gas flow path, s1 ... Accelerator pedal sensor, s2 ... SOC sensor, s3 ... T / C motor rotation speed detection sensor, s4 · Voltage sensor, s5 ... current sensor, s6 ... temperature sensor, 100 ... fuel cell system.

Claims (5)

A fuel cell;
An operation control means for stopping the operation after performing a low-efficiency operation with a larger power loss than in a normal operation when an instruction to stop the operation of the fuel cell is given. The fuel cell performs power generation using a reaction gas supplied to an anode and a cathode,
2. The fuel cell system according to claim 1, wherein the operation control unit realizes the low-efficiency operation by setting a stoichiometric ratio of at least one of the reactant gases to be lower than that during the normal operation.   The fuel cell system according to claim 2, further comprising a scavenging control means for scavenging by supplying a reaction gas to the cathode after stopping the operation.   4. The fuel cell system according to claim 2, wherein the operation control unit sets a stoichiometric ratio of a reaction gas supplied to the cathode lower than that during the normal operation. A cathode gas supply passage, a cathode gas discharge passage, bypass means for bypassing the fuel cell to a part of the cathode gas flowing through the cathode gas supply passage to the cathode gas discharge passage, and the low-efficiency operation, The fuel cell system according to claim 1, further comprising an adjusting unit that adjusts a bypass amount of the cathode gas by the bypass unit.

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