U.S. patent application number 13/405361 was filed with the patent office on 2012-09-06 for method for controlling fuel cell system.
This patent application is currently assigned to HONDA MOTOR CO., LTD.. Invention is credited to Yuji MATSUMOTO, Koichiro Miyata.
Application Number | 20120225365 13/405361 |
Document ID | / |
Family ID | 46613438 |
Filed Date | 2012-09-06 |
United States Patent
Application |
20120225365 |
Kind Code |
A1 |
MATSUMOTO; Yuji ; et
al. |
September 6, 2012 |
METHOD FOR CONTROLLING FUEL CELL SYSTEM
Abstract
A method includes determining, if an instruction to stop a
operation of a fuel cell is detected, whether an in-stop-mode power
generating process has been executed, a fuel gas being to be
stopped and an oxide gas being to be supplied to the fuel cell to
generate power from the oxide-gas supply apparatus in the
in-stop-mode power generating process, and shortening a time for a
diluting process to be executed by a scavenging apparatus when it
is determined that the in-stop-mode power generating process has
been executed, as compared with a case where it is determined that
the in-stop-mode power generating process has not been
executed.
Inventors: |
MATSUMOTO; Yuji; (Wako,
JP) ; Miyata; Koichiro; (Wako, JP) |
Assignee: |
HONDA MOTOR CO., LTD.
Tokyo
JP
|
Family ID: |
46613438 |
Appl. No.: |
13/405361 |
Filed: |
February 27, 2012 |
Current U.S.
Class: |
429/429 |
Current CPC
Class: |
H01M 8/04303 20160201;
H01M 8/04223 20130101; H01M 8/04388 20130101; Y02E 60/50 20130101;
H01M 8/0432 20130101; H01M 8/04231 20130101; H01M 8/0267 20130101;
H01M 8/04574 20130101; H01M 8/2483 20160201 |
Class at
Publication: |
429/429 |
International
Class: |
H01M 8/04 20060101
H01M008/04 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 4, 2011 |
JP |
2011-048006 |
Claims
1. A method for controlling a fuel cell system having a fuel cell
to generate power according to an electrochemical reaction of an
oxide gas supplied to a cathode side with a fuel gas supplied to an
anode side, an oxide-gas supply apparatus to supply the oxide gas
to the fuel cell, a fuel-gas supply apparatus to supply the fuel
gas to the fuel cell, and a scavenging apparatus to execute a
diluting process of diluting a concentration of the fuel gas in the
anode of the fuel cell, the scavenging apparatus being configured
to supply a scavenging gas into the anode for scavenging when an
outside temperature is equal to or lower than a predetermined
temperature at a time of stopping an operation of the fuel cell,
the method comprising: determining, if an instruction to stop the
operation of the fuel cell is detected, whether an in-stop-mode
power generating process, in which the fuel gas is no longer
supplied and the oxide gas is supplied to the fuel cell to generate
power from the oxide-gas supply apparatus, has been executed; and
shortening a time for the diluting process to be executed by the
scavenging apparatus when it is determined that the in-stop-mode
power generating process has been executed, as compared with a case
where it is determined that the in-stop-mode power generating
process has not been executed.
2. The method according to claim 1, wherein the longer an elapsed
time till activation of the scavenging apparatus after the
in-stop-mode power generating is, the shorter the time for the
diluting process is set.
3. The method according to claim 1, wherein the diluting process is
executed together with a scavenging process for the cathode.
4. The method according to claim 1, wherein whether the
in-stop-mode power generating process has been executed is
determined based on an anode pressure when power generation of the
fuel cell is stopped.
5. The method according to claim 1, wherein whether the
in-stop-mode power generating process has been executed is
determined based on an accumulated current value from a point of
detecting the instruction to stop the operation of the fuel cell to
a point of stopping the power generation of the fuel cell.
6. The method according to claim 1, wherein in the determining of
the in-stop-mode power generating process, if a pressure on the
anode side is lower than a threshold pressure, it is determined
that the in-stop-mode power generating process has been executed,
and in the determining of the in-stop-mode power generating
process, if the pressure on the anode side is equal to or higher
than the threshold pressure, it is determined that the in-stop-mode
power generating process has not been executed.
7. The method according to claim 1, wherein the shortening of the
time for the diluting process includes setting a first time as the
time for the diluting process when it is determined that the
in-stop-mode power generating process has been executed, and
setting a second time as the time for the diluting process when it
is determined that the in-stop-mode power generating process has
not been executed, the first time being shorter than the second
time.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority under 35 U.S.C.
.sctn.119 to Japanese Patent Application No. 2011-048006, filed
Mar. 4, 2011, entitled "Control Method For Fuel Cell System." The
contents of this application are incorporated herein by reference
in their entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present disclosure relates to a method for controlling a
fuel cell system.
[0004] 2. Discussion of the Background
[0005] A fuel cell system acquires DC electric energy according to
an electrochemical reaction of a fuel gas (gas essentially
containing hydrogen, such as hydrogen gas) and an oxide gas (gas
essentially containing oxygen, such as air) respectively supplied
to an anode electrode and a cathode electrode. This system is of a
stationary type, or is mounted in a fuel cell vehicle as an
on-vehicle fuel cell system.
[0006] For example, a solid polymer fuel cell has an electrolyte
membrane/electrode assembly (MEA) having an anode electrode and a
cathode electrode provided on the respective side of an electrolyte
membrane formed by a polymer ion-exchange film; the electrolyte
membrane/electrode assembly is sandwiched by a pair of separators.
A fuel gas passage for supplying a fuel gas to the anode electrode
is formed between one of the separators and the electrolyte
membrane/electrode assembly. An oxide gas passage for supplying an
oxide gas to the cathode electrode is formed between the other
separator and the electrolyte membrane/electrode assembly.
[0007] When the fuel cell is stopped, supply of the fuel gas and
oxide gas is stopped. However, the fuel gas remains in the fuel gas
passage, and the oxide gas remains in the oxide gas passage. When
the operation-stop period of the fuel cell becomes long, therefore,
the fuel gas and the oxide gas may pass through the electrolyte
membrane, so that the fuel gas is mixed with the oxide gas to react
therewith, thereby deteriorating the electrolyte membrane/electrode
assembly.
[0008] To cope with the problem, a fuel cell system disclosed in,
for example, Japanese Unexamined Patent Application Publication No.
2004-22487 (FIG. 1 and paragraph [00029]) shuts off the supply of a
reaction gas to the anode side, and shuts off the supply of the
reaction gas to the cathode side when the operation of the fuel
cell is stopped. Further, the exhaust gas on the anode side is
circulated to the upstream side through an anode-side circulation
line, and the exhaust gas on the cathode side is circulated to the
upstream side through a cathode-side circulation line, so that an
electrochemical reaction in the fuel cell is maintained to generate
power, thereby charging the battery with the generated power.
Hydrogen in the exhaust gas on the anode side is consumed and
oxygen in the exhaust gas on the cathode side is consumed this way,
and a nitrogen gas is stored in a tank. The gases in the anode and
cathode of the fuel cell are replaced with the nitrogen gas stored
in the tank.
SUMMARY OF THE INVENTION
[0009] According to one aspect of the present invention, a method
is for controlling a fuel cell system having a fuel cell to
generate power according to an electrochemical reaction of an oxide
gas supplied to a cathode side with a fuel gas supplied to an anode
side, an oxide-gas supply apparatus to supply the oxide gas to the
fuel cell, a fuel-gas supply apparatus to supply the fuel gas to
the fuel cell, and a scavenging apparatus to execute a diluting
process of diluting a concentration of the fuel gas in the anode of
the fuel cell, the scavenging apparatus being to supply a
scavenging gas into the anode for scavenging when an outside
temperature is equal to or lower than a predetermined temperature
at a time of stopping an operation of the fuel cell. The method
includes: determining, if an instruction to stop the operation of
the fuel cell is detected, whether an in-stop-mode power generating
process has been executed, the fuel gas being to be stopped and the
oxide gas being to be supplied to the fuel cell to generate power
from the oxide-gas supply apparatus in the in-stop-mode power
generating process; and shortening a time for the diluting process
to be executed by the scavenging apparatus when it is determined
that the in-stop-mode power generating process has been executed,
as compared with a case where it is determined that the
in-stop-mode power generating process has not been executed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] A more complete appreciation of the invention and many of
the attendant advantages thereof will be readily obtained as the
same becomes better understood by reference to the following
detailed description when considered in connection with the
accompanying drawings.
[0011] FIG. 1 is a schematic configurational diagram of a fuel cell
system which is a target of a control method according to an
exemplary embodiment of the disclosure.
[0012] FIG. 2 is an explanatory diagram of circuits included in the
fuel cell system.
[0013] FIG. 3 is a timing chart illustrating the control
method.
[0014] FIG. 4 is an explanatory diagram of a hydrogen-gas volume
part and an air volume part in the fuel cell system.
[0015] FIG. 5 is an explanatory diagram of the anode pressure v.s.
soak time characteristics corresponding to whether or not
low-oxygen stoichiometric power generation is executed.
[0016] FIG. 6 is an explanatory diagram of the timing for an anode
scavenging process which is executed after the operation of the
fuel cell system is stopped.
[0017] FIG. 7 is a flowchart used to explain the anode scavenging
process which is executed after the operation of the fuel cell
system is stopped.
[0018] FIG. 8 is a flowchart used to explain the anode scavenging
process which is executed in a soak period after the operation of
the fuel cell system is stopped.
DESCRIPTION OF THE EMBODIMENTS
[0019] The embodiments will now be described with reference to the
accompanying drawings, wherein like reference numerals designate
corresponding or identical elements throughout the various
drawings.
[0020] As shown in FIG. 1, a fuel cell system 10 which is a target
of a control method according to an exemplary embodiment of the
disclosure includes a fuel cell stack 12, an oxide-gas supply
apparatus 14 that supplies an oxide gas to the fuel cell stack 12,
a scavenging apparatus 15 that supplies a replacement gas to the
fuel cell stack 12, a fuel-gas supply apparatus 16 that supplies a
fuel gas to the fuel cell stack 12, a battery (electric storage
device) 17 connectable to the fuel cell stack 12, and a controller
(control apparatus, control unit) 18 that performs the general
control of the fuel cell system 10.
[0021] The controller 18 is a computer including a microcomputer,
and has a CPU (Central Processing Unit), a ROM (including EEPROM)
as a memory, a RAM (Random Access Memory), input/output units, such
as an A/D converter and a D/A converter, and a timer 19 serving as
a clock or time-measuring unit. When the CPU reads a program stored
in the ROM and executes it, the controller 18 functions as various
functional parts, such as a control unit, an arithmetic operation
unit, and a processing unit.
[0022] The fuel cell system 10 is mounted in a fuel cell vehicle
like a fuel cell car. The battery 17 permits the fuel cell vehicle
to run normally, and has a capacity of 20 A and as high as about
500 V, a higher voltage and higher capacity than a 12-V power
supply 98 to be described later.
[0023] The fuel cell stack 12 has a stack of a plurality of fuel
cells (also called "cells" or "cell pairs") 20. Each fuel cell 20
includes an electrolyte membrane/electrode assembly (MEA) 28 which
has a solid polymer electrolyte membrane 22 sandwiched between a
cathode electrode 24 and an anode electrode 26. The solid polymer
electrolyte membrane 22 is formed by a thin film of
perfluorosulfone impregnated with water.
[0024] The cathode electrode 24 and the anode electrode 26 each
have a gas diffusion layer formed by carbon paper or the like, and
an electrode catalyst layer formed by applying porous carbon
particles each carrying a platinum alloy (or Ru or the like) on its
surface to the surface of the gas diffusion layer uniformly. The
electrode catalyst layer is formed on both sides of the solid
polymer electrolyte membrane 22.
[0025] The electrolyte membrane/electrode assembly 28 is sandwiched
by a cathode-side separator 30 and an anode-side separator 32. The
cathode-side separator 30 and anode-side separator 32 are made of,
for example, carbon or a metal.
[0026] An oxide gas passage 34 is provided between the cathode-side
separator 30 and the electrolyte membrane/electrode assembly 28. A
fuel gas passage 36 is provided between the anode-side separator 32
and the electrolyte membrane/electrode assembly 28.
[0027] The fuel cell stack 12 provided with an oxide-gas inlet hole
38a for supplying an oxide gas, e.g., an oxygen-containing gas
(hereinafter also referred to as "air"), a fuel-gas inlet hole 40a
for supplying a fuel gas, e.g., a hydrogen-containing gas
(hereinafter also referred to as "hydrogen gas"), a coolant inlet
hole (not shown) for supplying a coolant, an oxide-gas outlet hole
38b for discharging the oxide gas, a fuel-gas outlet hole 40b for
discharging the fuel gas, and a coolant outlet hole (not shown) for
discharging the coolant. The oxide-gas inlet hole 38a, the fuel-gas
inlet hole 40a, the coolant inlet hole, the oxide-gas outlet hole
38b, the fuel-gas outlet hole 40b, and the coolant outlet hole
communicate with one another in the stack direction of the fuel
cells 20.
[0028] The oxide-gas supply apparatus 14 includes an air pump 50
that compresses atmospheric air and supplies it. The air pump 50 is
disposed in an air supply passage 52. A humidifier 54 that
exchanges moisture and heat between a supplied gas and an exhaust
gas is disposed in the air supply passage 52. The air supply
passage 52 communicates with the oxide-gas inlet hole 38a of the
fuel cell stack 12.
[0029] The oxide-gas supply apparatus 14 further includes an air
discharge passage 56 communicating with the oxide-gas outlet hole
38b. The air discharge passage 56 communicates with the a
humidifying-medium passage (not shown) of the humidifier 54. The
air discharge passage 56 is provided with a back-pressure control
valve (hereinafter also referred to simple as "back pressure
valve") 58 whose degree of opening is adjustable, such as a
butterfly valve, to adjust the pressure of air supplied to the fuel
cell stack 12 from the air pump 50 via the air supply passage 52.
The back-pressure control valve 58 is preferably a normally-closed
control valve (which is closed when not energized). The air
discharge passage 56 communicates with a dilution box 60.
[0030] The scavenging apparatus 15 shares the air pump 50, which
compresses and supplies atmospheric air, the air supply passage 52
and the air discharge passage 56 with the oxide-gas supply
apparatus 14. The scavenging apparatus 15 further includes an air
inlet passage 53 provided between a hydrogen supply passage 64
downstream of an ejector 66 and the air supply passage 52, and an
air inlet valve 55 disposed in the air inlet passage 53.
[0031] The air inlet valve 55 is an on-off valve which is opened at
the time of what is called an anode-side air scavenging process or
the like in order to supply compressed air from the air pump 50 to
the fuel gas passage 36 through the fuel-gas inlet hole 40a via the
air supply passage 52 and the air inlet passage 53. The anode-side
air scavenging process differs from the anode-side air replacing
process in that it involves a large airflow rate enough to blow out
water droplets, and is common to the anode-side air replacing
process in that the fuel gas on the anode side is replaced with
air.
[0032] The fuel-gas supply apparatus 16 includes a hydrogen tank 62
that stores high-pressure hydrogen and is integrally provided with
an in-tank solenoid valve 63 which is an on-off valve. The hydrogen
tank 62 communicates with the fuel-gas inlet hole 40a of the fuel
cell stack 12 via the hydrogen supply passage 64.
[0033] The hydrogen supply passage 64 is provided with a shutoff
valve 65 which is an on-off valve, and the ejector 66. The ejector
66 supplies the hydrogen gas supplied from the hydrogen tank 62 to
the fuel cell stack 12 through the hydrogen supply passage 64, and
sucks an exhaust gas containing an unused hydrogen gas, which has
not been used in the fuel cell stack 12, from a hydrogen
circulation path 68 and supplies the exhaust gas to the fuel cell
stack 12 again as a fuel gas.
[0034] An off-gas passage 70 communicates with the fuel-gas outlet
hole 40b. The hydrogen circulation path 68 communicates with a
halfway portion of the off-gas passage 70, which is connected with
the dilution box 60 via a purge valve 72. The outlet side of the
dilution box 60 is connected with a discharge passage 74. The
discharge passage 74 is provided with a storage buffer (not shown)
to which an exhaust passage communicating with the atmosphere is
connected.
[0035] The controller 18 acquires signals from a pressure sensor
102 provided in the hydrogen supply passage 64, a pressure sensor
103 provided in the vicinity of the oxide-gas outlet hole 38b, a
temperature sensor 104 provided in the vicinity of the fuel-gas
outlet hole 40b, an outside temperature sensor 105, a temperature
sensor 106 provided in the coolant inlet hole (not shown), a
voltage sensor 108, and a current sensor 110, and controls the ON
(closing) and OFF (opening) actions of an FC contactor 86 to be
described later, the opening/closing of and the degrees of opening
of valves such as the shutoff valve 65, and controls an actuator
for, for example, regulating the flow rate (airflow rate) of the
air pump 50, based on the acquired signals. The pressure sensor 102
detects an anode pressure Pa. The pressure sensor 103 detects a
cathode pressure Pk. The temperature sensor 104 detects a hydrogen
temperature Th. The outside temperature sensor 105 detects an
outside temperature Te outside a vehicle. The temperature sensor
106 detects a coolant temperature Tc. The voltage sensor 108
detects the voltage of each fuel cell (called "cell voltage" or
"cell pair voltage"). The current sensor 110 detects a current
value Io of the current flowing out from the fuel cell stack
12.
[0036] As shown in FIG. 2, the fuel cell stack 12 is connected with
one end of a main power line 80 whose other end is connected to an
inverter 82. The inverter 82 is connected with a three-phase drive
motor 84 for driving a vehicle. Substantially, two main power lines
80 are used, but are described as a single main power line 80 for
the sake of descriptive convenience. The same is applied to other
lines to be described hereinafter.
[0037] The main power line 80 is provided with the FC contactor
(main-power-supply ON/OFF unit, fuel cell stack ON/OFF unit) 86,
and is connected with the air pump 50. The main power line 80 is
connected with one end of a power line 88 to which the battery 17
is connected via a DC/DC converter 90 and a battery contactor
(electric-storage-unit ON/OFF unit) 92. The power line 88 is
provided with a branched power line 94 to which the 12-V power
supply 98 is connected via a down converter (DC/DC converter) 96.
The voltage of the 12-V power supply 98 has only to be lower than
the voltage of the battery 17, and is not limited to 12 V.
[0038] The operation of the fuel cell system 10 with the foregoing
configuration will be described below.
[0039] First, at the time of the normal operation of the fuel cell
system 10 (also referred to as "at the time of normal power
generation" or "at the time of a normal power generating process"),
air is supplied to the air supply passage 52 via the air pump 50
included in the oxide-gas supply apparatus 14. The air is
humidified through the humidifier 54, and is supplied to the
oxide-gas inlet hole 38a of the fuel cell stack 12. The air moves
along the oxide gas passage 34, provided in each fuel cell 20 in
the fuel cell stack 12, to be supplied to the cathode electrode
24.
[0040] The used air is discharged into the air discharge passage 56
from the oxide-gas outlet hole 38b, and is supplied to the
humidifier 54 to humidify air newly supplied. The air is then
supplied to the dilution box 60 via the back pressure valve 58.
[0041] When the in-tank solenoid valve 63 and the shutoff valve 65
in the fuel-gas supply apparatus 16 are opened, the hydrogen gas
from the hydrogen tank 62 is depressurized by a depressurization
control valve (not shown), and is then supplied to the hydrogen
supply passage 64. The hydrogen gas is supplied to the fuel-gas
inlet hole 40a of the fuel cell stack 12 through the hydrogen
supply passage 64. The hydrogen gas supplied into the fuel cell
stack 12 moves along the fuel gas passage 36 of each fuel cell 20
to be supplied to the anode electrode 26.
[0042] The used hydrogen gas is sucked by the ejector 66 from the
fuel-gas outlet hole 40b via the hydrogen circulation path 68, and
is supplied to the fuel cell stack 12 again as a fuel gas.
Therefore, the air supplied to the cathode electrode 24
electrochemically reacts with the hydrogen gas supplied to the
anode electrode 26 to generate power.
[0043] An impurity is likely to be mixed in the hydrogen gas that
circulates in the hydrogen circulation path 68. Accordingly, the
impurity-containing hydrogen gas is supplied to the dilution box 60
via the purge valve 72 opened. This hydrogen gas is mixed with an
air-off gas in the dilution box 60 to reduce (dilute) the hydrogen
concentration, and is then discharged into the storage buffer (not
shown).
[0044] During normal power generation, the scavenging apparatus 15
is not activated, and the air inlet valve 55 is kept closed. The
air inlet valve 55 is preferably a normally closed on-off valve
(which is closed when not energized).
[0045] Next, the "in-stop-mode power generating process of the fuel
cell system 10" will be described, followed by a description of an
anode scavenging process after the operation of the fuel cell
system 10 is stopped. The description of this anode scavenging
process will be described separately for two cases: one case where
the anode scavenging process is executed immediately after the
operation of the fuel cell system 10 is stopped (FC contactor 86 is
set off) as a "first embodiment", and the other case where the
anode scavenging process is executed after the soak time elapses
after the operation of the fuel cell system 10 is stopped (FC
contactor 86 is set off) as a "second embodiment". The in-stop-mode
power generating process is also referred to as a "discharge
process", "low-oxygen stoichiometric power generating process",
"O.sub.2 lean process" or "O.sub.2 lean power generating
process".
[0046] The anode scavenging process according to the first
embodiment is executed when the outside temperature Te, which is
continuously detected (actually, intermittently detected for every
predetermined time) by the outside temperature sensor 105, becomes
a subfreezing temperature, or is predicted to be likely to become a
subfreezing temperature during the normal power generation of the
fuel cell system 10 or during the in-stop-mode power generating
process.
[0047] The anode scavenging process according to the second
embodiment is executed when the outside temperature Te, which is
continuously detected (actually, intermittently detected for every
predetermined time) by the outside temperature sensor 105, does not
become a subfreezing temperature, or is not predicted to be likely
to become a subfreezing temperature during the normal power
generation or during the in-stop-mode power generating process, but
when the outside temperature Te becomes a subfreezing temperature,
or is predicted to be likely to become a subfreezing temperature
during soaking after the operation of the fuel cell system 10 is
stopped (the FC contactor 86 is set off).
(Description of In-Stop-Mode Power Generating Process of Fuel Cell
System 10)
[0048] First, the in-stop-mode power generating process of the fuel
cell system 10 will be described below referring to a timing chart
illustrated in FIG. 3.
[0049] The fuel cell system 10 mounted in a fuel cell vehicle (not
shown) executes the normal power generation in the foregoing
manner, the vehicle runs desirably. When an unillustrated ignition
switch is set off, the controller 18 detects the turn-off action as
an operation stop instruction (time t1 in FIG. 3), and initiates
the operation stopping process for the fuel cell system 10.
[0050] First, after a discharge process (in-stop-mode power
generating process), the supply pressure of the hydrogen gas (fuel
gas) is set beforehand so that the fuel-gas pressure (anode
pressure Pa) in the fuel cell stack 12 is kept at a set pressure.
Specifically, as shown in FIG. 4, a hydrogen-gas volume part 200
which is closed after being filled with the hydrogen gas includes
the fuel gas passages 36, the fuel-gas inlet hole 40a and the
fuel-gas outlet hole 40b in the fuel cell stack 12, a downstream
region downstream of the ejector 66 in the hydrogen supply passage
64, the hydrogen circulation path 68, an upstream region upstream
of the purge valve 72 in the off-gas passage 70.
[0051] An air volume part 202 which replaces an air atmosphere with
a nitrogen atmosphere or an inactive gas includes the oxide gas
passages 34, the oxide-gas inlet hole 38a and the oxide-gas outlet
hole 38b in the fuel cell stack 12, the air supply passage 52, the
air discharge passage 56, the humidifier 54, the dilution box 60
and the storage buffer (not shown).
[0052] At the time of the discharge process, air is supplied with
an oxygen stoichiometric ratio lower than the oxygen stoichiometric
ratio in normal power generation mode. Specifically, the low oxygen
stoichiometric ratio is set around a value of 1. It is preferable
that the oxygen stoichiometric ratio should fall between 1.2 to 1.8
in normal power generation mode. The supply of the hydrogen gas is
stopped at the time of the discharge process.
[0053] Accordingly, nO.sub.2 or the number of moles of the residual
oxygen in the air volume part 202 which are to be set to a nitrogen
atmosphere in the fuel cell stack 12, n'O.sub.2 or the number of
moles of the oxygen in the humidifier 54, the dilution box 60 and
the storage buffer which are to be set to a nitrogen atmosphere by
the low oxygen stoichiometric ratio, and nH.sub.2 or the number of
moles of the residual hydrogen in the hydrogen-gas volume part 200
are set to have a relation of 2(nO.sub.2+n'O.sub.2)=nH.sub.2.
[0054] The supply pressure (anode pressure), Pa1, of the hydrogen
gas is calculated from the set number of moles of the residual
hydrogen nH.sub.2 using an equation of n=P.times.V/R.times.T where
n represents the number of moles, P represents the pressure, V
represents the volume, R represents a gas constant, and T
represents the absolute temperature (see FIG. 3). It is to be noted
that the anode pressure Pa1 is set so as to be kept equal to or
higher than a constant pressure Pa2 when the discharge process is
completed. The constant pressure Pa2 is low enough to prevent
hydrogen from running short or being present excessively.
[0055] When a relation of (the volume of the air volume part
202)>>(the volume of the hydrogen-gas volume part 200) is
fulfilled, it is possible to employ a method of increasing the
pressure to the anode pressure Pa1, or a method of supplying
hydrogen gas for the deficit hydrogen in order to increase the
volume of the hydrogen-gas volume part 200.
[0056] When a relation of (the volume of the hydrogen-gas volume
part 200)>>(the volume of the air volume part 202) is
fulfilled, on the other hand, a method of reducing the pressure to
the anode pressure Pa1 is employed to decrease the volume of the
hydrogen-gas volume part 200.
[0057] When the ignition switch (operation switch) is set off (time
t1), as shown in FIG. 3, the hydrogen gas is supplied to the fuel
cell stack 12 with the in-tank solenoid valve 63 and the shutoff
valve 65 being opened, so that the pressure in the fuel cell stack
12 rises to the anode pressure Pa1 (times t1 to t2: boosting
process). This anode pressure Pa1 is calculated by the
aforementioned equation.
[0058] When the boosting process is terminated (time t2), the
in-tank solenoid valve 63 is closed, and the processing shifts to a
process of detecting a failure of the in-tank solenoid valve 63. In
the failure detecting process, a failure in the in-tank solenoid
valve 63 is detected according to the present/absence of a change
in pressure directly below the in-tank solenoid valve 63. When the
pressure drops, the in-tank solenoid valve 63 is regarded as
normal. That is, it is determined that the in-tank solenoid valve
63 is closed properly.
[0059] When the failure detecting process for the in-tank solenoid
valve 63 is terminated (time t3), a cathode scavenging process is
executed. In the cathode scavenging process, a scavenging process
based on air (using the oxide-gas supply apparatus 14) is performed
to blow off liquid droplets or the like containing water droplets
on the cathode side. In this process, power insufficient to drive
the air pump 50 which is set to have a high number of rotations
[rpm] is supplemented (times t4 to t5).
[0060] After the cathode scavenging process, control on the degree
of opening of the back pressure valve 58 is temporarily stopped,
and the back pressure valve 58 is opened to communicate with the
atmosphere, so that the cathode pressure Pk is set to PK=0 [kPag]
where g means the gauge pressure (times t6 to t7). Further, when
the cathode scavenging process is terminated (time t6), the number
of rotations of the air pump 50 included in the oxide-gas supply
apparatus 14 is reduced considerably as compared with that in
normal operation mode, so that the oxide gas is supplied to with
the oxygen stoichiometric ratio being lower than the oxygen
stoichiometric ratio in normal operation mode. Specifically, the
oxygen stoichiometric ratio is preferably set around 1. Then, a
learning process (compensation for 0 point) for the pressure sensor
103 is executed.
[0061] Thereafter, the degree of opening of the back pressure valve
58 is adjusted at times t7 to t8 to set the cathode pressure Pk
detected by the pressure sensor 103 to a predetermined low pressure
Pkl corresponding to the low oxygen stoichiometric ratio, and a
learning process for the back pressure valve 58 at the low pressure
Pkl is executed (times t7 to t8). Thereafter, the cathode pressure
Pk is kept set at the low pressure Pk1 until the air pump 50 is
turned off (time t9).
[0062] Meanwhile, the fuel cell stack 12 keeps generating power
(times t1 to t8).
[0063] In the low-oxygen stoichiometric power generating process
(also referred to as "O.sub.2 lean power generating process" or
simply as "O.sub.2 lean process"; times t8 to t9) after the
learning process for the back pressure valve 58 (times t7 to t8),
the current (FC current) to be acquired from the fuel cell stack 12
is set to a value which inhibits the hydrogen gas as the fuel gas
from passing through the solid polymer electrolyte membrane 22 to
move toward the cathode from the anode. At this time, the FC
contactor 86 and the battery contactor 92 are set on in FIG. 2, and
power obtained when the fuel cell stack 12 generates power is
reduced by the DC/DC converter 90 to be charged in the battery
17.
[0064] As described above, while air with a low oxygen
stoichiometric ratio is supplied to the fuel cell stack 12, the
fuel cell stack 12 is generating power with supply of the hydrogen
gas being stopped by the closure of the shutoff valve 65 (time t3).
The purge valve 72 is also closed. The power generated by the fuel
cell stack 12 is supplied to the battery 17 to be discharged (DCHG
(O.sub.2 lean process) in FIG. 3). When the power generated by the
fuel cell stack 12 drops to a predetermined voltage, i.e., to a
voltage which cannot be supplied to the battery 17 (substantially
the same voltage as the voltage of the battery 17), therefore, the
generated power is supplied only to the air pump 50.
[0065] Accordingly, while the hydrogen concentration on the anode
side drops in the fuel cell stack 12 during the O.sub.2 lean
process (times t8 to t9), the oxygen concentration on the cathode
side drops. Therefore, when the hydrogen pressure (anode pressure
Pa) becomes equal to or lower than the predetermined pressure Pa2,
for example, the air pump 50 is turned off, and the battery
contactor 92 is set off (time t9).
[0066] Accordingly, the fuel cell stack 12 generates power
according to the reaction of the hydrogen gas and air present
inside the fuel cell stack 12 with each other (times t9 to t10).
The power that is generated by the power generation of the fuel
cell stack 12 is reduced via the down converter 96 to be charged in
the 12-V power supply 98 (D/V DCHG in FIG. 3), and the power is
supplied to a radiator fan or the like (not shown) as needed.
Further, when the voltage generated by the fuel cell stack 12 drops
down to near the operational limit voltage of the down converter
96, the FC contactor 86 is set off (time t10). As a result, the
fuel cell system 10 goes to an operation stopped state or what is
called a soaked state.
[0067] As described above, when the ignition switch is set off
(time t1), the anode pressure Pa in the fuel cell stack 12 rises to
the anode pressure Pa1 before supply of the hydrogen gas is stopped
(time t2), and then the back pressure valve 58, the air pump 50,
the in-tank solenoid valve 63 and the shutoff valve 65 are
actuated. Therefore, the fuel cell stack 12 generates power
according to the reaction of the hydrogen gas and air with a
low-oxygen stoichiometric ratio, which remain inside the fuel cell
stack 12, with each other, and the generated power is supplied to
the battery 17 to be discharged (times t2 to t9).
[0068] Accordingly, the hydrogen concentration decreases on the
anode side in the fuel cell stack 12, and the oxygen concentration
decreases on the cathode side, thereby increasing the nitrogen
concentration. As a result, a nitrogen gas with a high
concentration is produced as an exhaust gas on the cathode side,
and the nitrogen gas is supplied to the dilution box 60.
[0069] Therefore, the air volume part 202 including the fuel cell
stack 12 shown in FIG. 4 can be filled with the nitrogen gas which
is an inactive gas.
[0070] In addition, the supply pressure of the hydrogen gas to be
supplied to the fuel cell stack 12 is increased to the anode
pressure Pa1 (time t2) before supply of the hydrogen gas is
stopped. This can bring about an effect that low-oxygen
stoichiometric power generation (O.sub.2 lean process) is carried
out properly with the fuel cell stack 12 being filled with an
adequate amount of hydrogen, surely preventing an excessive
hydrogen gas from remaining inside the fuel cell stack 12 or a
hydrogen gas in the fuel cell stack 12 from running short, after
discharging is completed.
[0071] Further, with the air pump 50 being stopped (time t9), the
fuel cell stack 12 is caused to generate power according to a
reaction of only hydrogen and oxygen remaining in the fuel cell
stack 12 with each other (D/V DCHG in FIG. 3).
[0072] Therefore, the nitrogen-gas replacement range in the system
is limited within the fuel cell stack 12 when the power generation
of the fuel cell stack 12 is carried out while supplying air via
the air pump 50, whereas the nitrogen-gas replacement range is
expanded to the inlet side of the fuel cell stack 12 when the power
generation of the fuel cell stack 12 is carried out after the air
pump 50 is stopped. This brings about an advantage that even when
the fuel cell system 10 is stopped for a comparatively long period
of time, deterioration of the fuel cells 20 on the cathode side can
be prevented as much as possible.
[0073] An anode pressure characteristic 150 indicated by a solid
line in FIG. 5 represents the decreasing characteristic of the
anode pressure Pa when the low-oxygen stoichiometric power
generation (O.sub.2 lean power generation) is executed. An anode
pressure characteristic 152 indicated by a dotted line in FIG. 5
represents the decreasing characteristic of the anode pressure Pa
when the low-oxygen stoichiometric power generation (O.sub.2 lean
power generation) is not executed (when the FC contactor 86 is set
off at the ignition-off time (time t1)).
[0074] When the low-oxygen stoichiometric power generation (O.sub.2
lean power generation) in which the shutoff valve 65 and the purge
valve 72 are closed is executed, hydrogen is consumed by the power
generation. As a result, the anode pressure drops abruptly, and
drops down to or below a threshold pressure Path at a
power-generation stop time (time t10) when the FC contactor 86 is
set off.
[0075] When the low-oxygen stoichiometric power generation (O.sub.2
lean power generation) is not executed, on the other hand, power is
not generated, and the anode pressure Pa just drops due to
diffusion or the like of the fuel gas (hydrogen). Therefore, the
anode pressure does not fall below the threshold pressure Path.
[0076] The threshold pressure Path takes a value which has been
confirmed beforehand through simulation and/or experiments or the
like such that the anode pressure Pa falls below the threshold
pressure Path when the low-oxygen stoichiometric power generation
(O.sub.2 lean power generation) is executed, but does not fall
below the threshold pressure Path when the low-oxygen
stoichiometric power generation (O.sub.2 lean power generation) is
not executed.
[0077] The deterioration characteristic 150 for the case where
low-oxygen stoichiometric power generation (O.sub.2 lean power
generation) is executed at or after the time when the FC contactor
86 is set off (time t10) shows that the anode pressure Pa slowly
drops and the amount of hydrogen decreases. As a result, at an
elapsed time (t11) of a soak time (also called "first soak time")
Tsa, a hydrogen concentration Dp on the anode side reaches a low
concentration of about 2%, for example, at which hydrogen need not
be diluted to be discharged outside.
[0078] By way of contrast, the deterioration characteristic 152 for
the case where low-oxygen stoichiometric power generation (O.sub.2
lean power generation) is not executed at or after the time when
the FC contactor 86 is set off (time t10) shows that the anode
pressure Pa drops comparatively fast. It is to be noted however
that because the anode pressure Pa at the time when the FC
contactor 86 is set off (time t10) is high, a soak time for the
anode pressure Pa to reach the low hydrogen concentration Dp at
which hydrogen dilution is not needed (also called "second soak
time") Tsb (t12-t10) becomes significantly longer than the first
soak time Tsa (Tsb>>Tsa).
First Embodiment
Description of Anode Scavenging Process to be Executed Immediately
After the Operation Stops (FC Contactor 86 is Set Off)
[0079] When the outside temperature Te, which is continuously
detected (actually, intermittently detected for every predetermined
time) by the outside temperature sensor 105, becomes a subfreezing
temperature, or is predicted by the controller 18 to be likely to
become a subfreezing temperature during the normal power generation
of the fuel cell system 10 (until time t1 in FIG. 3) or during the
in-stop-mode power generating process (times t8 to t9 in a narrow
sense, and times t1 to t10 in a broad sense), the anode scavenging
process according to the first embodiment based on a timing chart
in FIG. 6 and a flowchart in FIG. 7 is executed immediately after
the operation of the fuel cell system 10 is stopped (FC contactor
86 is set off).
[0080] In the anode scavenging process according to the first
embodiment, the hydrogen diluting process starts immediately after
the operation of the fuel cell system 10 is stopped (FC contactor
86 is set off) (time t10), and the cathode scavenging process is
executed. That is, the back pressure valve 58 is opened to drive
the air pump 50, and the air inlet valve 55 is opened (time
t10).
[0081] Under this condition, as shown in FIG. 6, the purge valve 72
is intermittently opened to set the concentration of the discharged
hydrogen to or lower than a predetermined concentration in a
hydrogen diluting period Tsc.
[0082] In this case, on the cathode side, compressed air is
supplied to the cathode of the fuel cell 20 via the air supply
passage 52, the humidifier 54 and the oxide-gas inlet hole 38a,
passes through the cathode electrode 24 of the fuel cell 20, flows
through the oxide-gas outlet hole 38b into the dilution box 60 via
the humidifier 54, the back pressure valve 58 and the air discharge
passage 56.
[0083] On the anode side where the shutoff valve 65 is closed, on
the other hand, compressed air flows to the anode of the fuel cell
20 through the fuel-gas inlet hole 40a via the air inlet valve 55,
and an air-hydrogen mixed gas or the compressed air that is mixed
with the fuel gas in the anode is discharged through the fuel-gas
outlet hole 40b from the anode side of the fuel cell 20, and flows
into the dilution box 60 via the purge valve 72 and the off-gas
passage 70. In this manner, the hydrogen gas in the anode of the
fuel cell 20 is intermittently and gradually diluted. The
air-hydrogen mixed gas is eventually discharged outside from the
dilution box 60 through the discharge passage 74.
[0084] A description will now be given of the difference (time
difference) between the hydrogen diluting period (time for the
hydrogen diluting process) Tsc shown in FIG. 6 when low-oxygen
stoichiometric power generation (O.sub.2 lean power generation) is
executed and the hydrogen diluting period Tsc when low-oxygen
stoichiometric power generation (O.sub.2 lean power generation) is
not executed.
[0085] When low-oxygen stoichiometric power generation (O.sub.2
lean power generation) is executed, the amount of hydrogen at the
time the in-stop-mode power generating process is completed (time
t10) is less than that when low-oxygen stoichiometric power
generation (O.sub.2 lean power generation) is not executed. It is
therefore possible to reduce the number of purging processes, thus
shortening the hydrogen diluting period Tsc.
[0086] Whether low-oxygen stoichiometric power generation (O.sub.2
lean power generation) has been executed during the process of
stopping the fuel cell system 10 is determined by executing, for
example, the processing of the flowchart illustrated in FIG. 7 at
the time the in-stop-mode power generating process is completed
(time t10).
[0087] In step S1, it is determined whether the anode pressure Pa
is lower than the threshold pressure Path (FIG. 5). When the anode
pressure Pa is lower than the threshold pressure Path, in which
case it is considered that the low-oxygen stoichiometric power
generation (O.sub.2 lean power generation) has been executed. Then,
in step S3, the hydrogen diluting period Tsc associated with anode
scavenging is selected to be short (the number of times the purge
valve 72 is opened is reduced).
[0088] When the decision in step S1 is negative (including a case
where the anode pressure Pa is not measured), it is determined in
step S2 whether an accumulated current value .SIGMA.Io in a period
which is the sum of the time for low-oxygen stoichiometric power
generation (DCHG (O.sub.2 lean power generation): t9-t8) and the
power generation time using the down converter 96 (D/V DCHG:
t10-t9) in FIG. 3 is greater than a threshold current value Moth.
When the accumulated current value .SIGMA.Io is greater than the
threshold current value Moth, in which case it is considered that
the low-oxygen stoichiometric power generation (O.sub.2 lean power
generation) has been executed. Then, in step S3, the hydrogen
diluting period Tsc associated with anode scavenging is selected to
be short (the number of times the purge valve 72 is opened is
reduced). At least one of the decisions in steps S1 and S2 has only
to be made.
[0089] When both of the decisions in steps S1 and S2 are negative,
in which case it is considered that the low-oxygen stoichiometric
power generation (O.sub.2 lean power generation) has not been
executed. Then, in step S4, the hydrogen diluting period Tsc
associated with anode scavenging is selected to be long.
[0090] In steps S3 and S4 in the flowchart in FIG. 7, there is a
relation of dilution time for executing O.sub.2 lean power
generation<dilution time without O.sub.2 lean power
generation.
[0091] When low-oxygen stoichiometric power generation (O.sub.2
lean power generation) is executed, the time for generating the
scavenging gas after power generation of the fuel cell system 10 is
stopped can be shortened by setting the drive time for the air pump
50 corresponding to the hydrogen diluting period Tsc for dilution
by the scavenging apparatus 15 shorter than the generation time
when low-oxygen stoichiometric power generation (O.sub.2 lean power
generation) is not executed. Because the scavenging apparatus 15
needs to be driven to generate the scavenging gas, the time of
driving the scavenging apparatus 15 can be made shorter, thus
reducing consumed energy after power generation of the fuel cell
system 10 is stopped.
[0092] Next, at a time when the hydrogen diluting process is
completed (time t21) in the timing chart in FIG. 6, the number of
rotations of the air pump 50 is temporarily reduced to execute a
process of opening the scavenging valve (times t21 to t22). In the
scavenging-valve opening process, the back pressure valve 58 is
closed, and the purge valve 72 as the scavenging valve is opened
with the air inlet valve 55 closed.
[0093] Next, after the scavenging-valve opening process (time t22),
air whose airflow rate is increased by increasing the number of
rotations of the air pump 50 is discharged from the air pump 50 to
scavenge the anode side with the air for removing liquid droplets.
In this case, the air with the increased airflow rate is supplied
to the anode side through the fuel-gas inlet hole 40a via the air
inlet passage 53 and the air inlet valve 55. The air supplied to
the anode side blows off liquid droplets in the anode. The blown
liquid droplets are discharged via the dilution box 60 through the
fuel-gas outlet hole 40b, the purge valve 72 and the off-gas
passage 70.
[0094] After the anode scavenging process for removing liquid
droplets with a high airflow rate is completed (time t23), the
airflow rate of the air pump 50 is reduced, after which the anode
scavenging process for accelerating drying is executed for a
predetermined time. Thereafter, the air inlet valve 55 and the
purge valve 72 are closed at the same time driving the air pump 50
is stopped, completing the anode scavenging process for
accelerating drying (time t24).
[0095] The oxide gas passage 34 on the cathode side and the fuel
gas passage 36 on the anode side can be dried sufficiently to have
a predetermined water content (same as the water containing ratio)
in the above manner. As a result, the fuel cell system 10 can be
reliably restarted even under a next low-temperature environment
such as a subfreezing environment.
Second Embodiment
Description of Anode Scavenging Process to be Executed After Elapse
of the Soak Time After the Operation Stops (FC Contactor 86 is Set
Off)
[0096] When the outside temperature Te, which is continuously
detected (actually, intermittently detected for every predetermined
time) by the outside temperature sensor 105, becomes a subfreezing
temperature, or is predicted to be likely to become a subfreezing
temperature during soaking after the operation of the fuel cell
system 10 is stopped (FC contactor 86 is set off; time t10 in FIG.
3), the anode scavenging process illustrated in FIG. 6 is changed
based on whether low-oxygen stoichiometric power generation
(O.sub.2 lean power generation) has been executed, and the soak
time, and is executed based on a flowchart in FIG. 8.
[0097] In step S11 and step S12, it is determined whether the anode
pressure Pa is lower than the threshold pressure Path (see FIG. 5)
as done in step S1 and step S2. When the anode pressure Pa is lower
than the threshold pressure Path (Pa<Path), it is considered
that low-oxygen stoichiometric power generation (O.sub.2 lean power
generation) has been executed, and a no-dilution-time determination
soak time is which is used in the following description is set to a
short soak time Tsa (see FIG. 5) in step S13.
[0098] When the decision in step S11 is negative (including a case
where the anode pressure Pa is not measured), it is determined in
step S12 whether the accumulated current value .SIGMA.Io in the
period which is the sum of the time for low-oxygen stoichiometric
power generation (DCHG (O.sub.2 lean power generation): t9-t8) and
the power generation time using the down converter 96 (D/V DCHG:
t10-t9) in FIG. 3 is greater than the threshold current value Moth.
When the accumulated current value .SIGMA.Io is greater than the
threshold current value Moth, in which case it is considered that
the low-oxygen stoichiometric power generation (O.sub.2 lean power
generation) has been executed, and the no-dilution-time
determination soak time is set to the short soak time Tsa (see FIG.
5) in step S13.
[0099] When both of the decisions in steps S11 and S12 are
negative, in which case it is considered that the low-oxygen
stoichiometric power generation (O.sub.2 lean power generation) has
not been executed, and the no-dilution-time determination soak time
is set to a long soak time Tsb (see FIG. 5) in step S14.
[0100] Next, when the outside temperature Te, which is continuously
detected (actually, intermittently detected for every predetermined
time) by the outside temperature sensor 105, becomes a subfreezing
temperature, or is predicted to be likely to become a subfreezing
temperature during soaking after the operation of the fuel cell
system 10 is stopped (FC contactor 86 is set off; time t10 in FIG.
3), it is determined that there is a request for execution of
low-temperature scavenging.
[0101] Next, an elapsed time since stopping of power generation (FC
contactor 86 has been set off) when the decision in step S15 is
positive, i.e., the soak time Ts, is acquired from the timer
19.
[0102] Then, it is determined in step S16 whether the soak time is
larger than the no-dilution-time determination soak time Tsa,
Tsb.
[0103] When the soak time is long and Ts>Tsa or Ts>Tsb,
dilution is carried out only for a dilution time (time for the
diluting process) Tsk from the time t10 shown in FIG. 6 (in this
case, the time at which the decision in step S15 is positive),
i.e., the cathode scavenging time Tsk, and the diluting process
from time t20 to time t21 is skipped to shift to the anode
scavenging process at and after time t21 (step S17).
[0104] When the soak time Ts is short and Ts.ltoreq.Tsa or
Ts.ltoreq.Tsb, the diluting process is carried out for a dilution
time Tsk+Tsd=Tsc from time t10 shown in FIG. 6 (also the time at
which the decision in step S15 is positive in this case), and the
processing is shifted to the anode scavenging process at and after
time t21 (step S18).
(Overview of Embodiments)
[0105] As described above, the foregoing embodiments provide a
control method for the fuel cell system 10 having the fuel cells 20
that generate power according to an electrochemical reaction of an
oxide gas supplied to the cathode side with a fuel gas supplied to
the anode side, the oxide-gas supply apparatus 14 that supplies the
oxide gas to the fuel cells 20, the fuel-gas supply apparatus 16
that supplies the fuel gas to the fuel cells 20, and the scavenging
apparatus 15 that executes the diluting process of diluting the
concentration of the fuel gas in the anode of each fuel cell 20,
and then supplies a scavenging gas such as compressed air into the
anode for scavenging, when the outside temperature Te is equal to
or lower than a predetermined temperature, e.g., the freezing
point.
[0106] The control method includes determining, upon detection of
an instruction to stop the operation of the fuel cells 20 (time
t1), whether the in-stop-mode power generating process (times t8 to
t9) of stopping supply of the fuel gas, and supplying the oxide gas
to the fuel cells 20 to generate power therefrom has been executed,
and shortening the time for the diluting process to be executed by
the scavenging apparatus 15 (dilution time Tsa) when it is
determined that the in-stop-mode power generating process has been
executed, as compared with a case where it is determined that the
in-stop-mode power generating process has not been executed.
[0107] When the in-stop-mode power generating process is executed,
the hydrogen concentration of the fuel gas drops. Therefore, the
hydrogen diluting process which is executed by the scavenging
apparatus 15 does not use much scavenging gas (dilution gas). When
it is determined that the in-stop-mode power generating process has
been executed, the time for generating the scavenging gas after
power generation of the fuel cell system 10 is stopped can be
shortened by setting the time (Tsc) for the diluting process by the
scavenging apparatus 15 shorter as compared with the case where it
is determined that the in-stop-mode power generating process has
not been executed. It is therefore possible to reduce energy needed
to drive the air pump 50 or the like.
[0108] In this case, when the elapsed time (soak time Ts) till
activation of the scavenging apparatus 15 after the in-stop-mode
power generating becomes longer, the residual hydrogen gas is
gradually diffused from the anode side during the elapsed time.
Therefore, the time (Tsc) for the diluting process may be set
shorter.
[0109] Because the diluting process does not need much dilution
gas, executing the diluting process together with the cathode
scavenging process can shorten the time for automatically
activating the fuel cell system 10 after the fuel cell system 10 is
stopped (FC contactor 86 is set off), thereby improving the
merchantability.
[0110] Furthermore, according to the control method, whether the
in-stop-mode power generating process has been executed is
preferably determined based on the anode pressure Pa when power
generation of the fuel cell 20 is stopped. In this case, a
phenomenon that the anode pressure Pa is decreased by execution of
the in-stop-mode power generating process is used. Because the
pressure sensor 102 which detects the pressure on the anode side is
normally mounted in the fuel cell system 10, the use of the
phenomenon does not raise the cost.
[0111] Moreover, according to the control method, whether the
in-stop-mode power generating process has been executed is
preferably determined based on the accumulated current value
.SIGMA.Io from the point of detecting the instruction to stop the
operation of the fuel cell to the point of stopping power
generation (accumulated value of the current Io between times t8
and t9 or between times t8 and t10. In this case, a phenomenon that
the accumulated current value .SIGMA.Io is increased by execution
of the in-stop-mode power generating process. Because the current
sensor 110 normally detects the current value Io in the fuel cell
system 10, the use of the phenomenon does not raise the cost for
calculating the accumulated current value .SIGMA.Io of the current
value Io.
[0112] According to the embodiments, in the case where the
scavenging process for scavenging the anode side with a scavenging
gas (anode scavenging process) is executed after the operation of
the fuel cell system 10 is stopped (FC contactor 86 is set off)
when the outside temperature Te becomes a predetermined low
temperature like the subfreezing point, or is likely to become
equal to or lower than the predetermined low temperature during the
operation of the fuel cell system 10, it is determined whether the
in-stop-mode power generating process of stopping supply of a fuel
gas, and supplying an oxide gas with a low-oxygen stoichiometric
ratio to the fuel cells 20 to generate power therefrom has been
executed, and when it is determined that the in-stop-mode power
generating process has been executed, the time (Tsc) for the
diluting process of diluting the concentration of the fuel gas in
the anode with the scavenging apparatus 15 is set shorter than that
in the case where it is determined that the in-stop-mode power
generating process has not been executed. This makes it possible to
reduce the energy loss of the fuel cell system 10 accordingly.
[0113] The disclosure is not limited to the foregoing embodiments,
and may take various configurations based on the contents of the
present specification.
[0114] According to the embodiments, there is provided a control
method for a fuel cell system having a fuel cell that generates
power according to an electrochemical reaction of an oxide gas
supplied to a cathode side with a fuel gas supplied to an anode
side, an oxide-gas supply apparatus that supplies the oxide gas to
the fuel cell, a fuel-gas supply apparatus that supplies the fuel
gas to the fuel cell, and a scavenging apparatus that executes a
diluting process of diluting a concentration of the fuel gas in the
anode of the fuel cell, and then supplies a scavenging gas into the
anode for scavenging, when an outside temperature is equal to or
lower than a predetermined temperature at a time of stopping an
operation of the fuel cell.
[0115] The control method includes determining, upon detection of
an instruction to stop the operation of the fuel cell, whether an
in-stop-mode power generating process of stopping supply of the
fuel gas, and supplying the oxide gas to the fuel cell to generate
power therefrom has been executed, and shortening a time for the
diluting process to be executed by the scavenging apparatus when it
is determined that the in-stop-mode power generating process has
been executed, as compared with a case where it is determined that
the in-stop-mode power generating process has not been
executed.
[0116] In the above control method, the longer an elapsed time till
activation of the scavenging apparatus after the in-stop-mode power
generating is, the shorter the time for the diluting process may be
set.
[0117] With the method according to the embodiments, when the
in-stop-mode power generating process is executed, the hydrogen
concentration of the fuel gas drops. Therefore, the hydrogen
diluting process (diluting process for making the concentration of
hydrogen to be discharged to below a predetermined value before the
hydrogen is discharged) on the anode side with the scavenging
apparatus does not use much scavenging gas (dilution gas). When it
is determined that the in-stop-mode power generating process has
been executed, the time for generating the scavenging gas after
power generation of the fuel cell system is stopped can be
shortened by setting the time for the diluting process by the
scavenging apparatus shorter as compared with the case where it is
determined that the in-stop-mode power generating process has not
been executed. This can shorten the drive time for driving the
scavenging apparatus which is needed to generate the scavenging
gas, thereby reducing the energy consumption after power generation
of the fuel cell system is stopped.
[0118] Since the oxygen concentration on the cathode side is low
after the in-stop-mode power generating process is stopped, the
amount of oxygen transmitted to the anode is small, so that the
hydrogen concentration on the anode side is reduced slowly. Because
of the slow reduction in hydrogen concentration, the dilution time
can be set short.
[0119] Shortening the dilution time can suppress the cathode side
from becoming dry. The suppression of the dry state of the cathode
prevents the IV (current-voltage) characteristic from dropping at
the time of the next activation, thus improving the fuel
consumption and the power performance. It is known that when the
cathode side becomes too dry, the reaction is not accelerated,
which reduces the cell voltage per the amount of unit current from
the fuel cell (reduction in IV characteristic).
[0120] In case of scavenging the anode side after the in-stop-mode
power generating process, the scavenging process is carried out
with the amount of hydrogen on the anode side being reduced.
Therefore, deterioration of the electrolyte membrane/electrode
assembly which is caused by the scavenging-originated reaction of
oxygen with hydrogen gas is reduced as compared with the case where
the in-stop-mode power generating process is not executed.
[0121] In the control method, the diluting process may be executed
together with a scavenging process for the cathode.
[0122] Because a small amount of the scavenging gas as a dilution
gas is needed, the scavenging process for the anode can be executed
at the same time as the scavenging process for the cathode, and the
time for operating the fuel cell system after the operation of the
fuel cell system is stopped (after an operation stopping switch
(ignition switch) is set off) can be shortened. This can reduce
uncomfortable feeling given to a user (awkward feeling caused by
activation of the fuel cell system for scavenging after the
operation of the fuel cell system is stopped), thus improving the
merchantability.
[0123] It is to be noted that the process of supplying the
scavenging gas into the anode to scavenge the anode after executing
the diluting process of diluting the concentration of the fuel gas
in the anode of the fuel cell when the outside temperature is equal
to or lower than a predetermined temperature at the time of
stopping the operation of the fuel cell blows off moisture from
inside the anode to prevent the fuel cell from being frozen at a
low temperature.
[0124] In the control method according to the embodiments, whether
the in-stop-mode power generating process has been executed may be
determined based on an anode pressure when power generation of the
fuel cell is stopped.
[0125] In the control method according to the embodiments, whether
the in-stop-mode power generating process has been executed may be
determined based on an accumulated current value from the point of
detecting the instruction to stop the operation of the fuel cell to
the point of stopping power generation.
[0126] With the method according to the embodiments, the conditions
for determining whether the in-stop-mode power generating process
has been executed. When the in-stop-mode power generating process
is executed, nitrogen spreads to the rear portion of a cathode
pipe, so that the concentration of oxygen in the cathode becomes 0%
for a long period of time. Therefore, whether the in-stop-mode
power generating process has been executed can be grasped by the
reduction in the oxygen concentration of the cathode. However, many
fuel cell systems are not provided with a cathode oxygen sensor.
Therefore, whether the in-stop-mode power generating process has
been executed may be determined by detecting the anode pressure
that is reduced by the in-stop-mode power generating process in
which supply of the fuel gas is stopped, using the anode pressure
sensor which is generally provided, or may be determined based on
an accumulated current value which is generally calculated. This
makes it possible to determine whether the in-stop-mode power
generating process has been executed, without requiring an extra
cost.
[0127] In this case, the state where the in-stop-mode power
generating process is executed is when the decision on the anode
pressure or the accumulated current value is satisfied.
Interruption in the in-stop-mode power generating process in which
those decisions are not satisfied is not said to be the
in-stop-mode power generating process being executed. Practically,
it is determined whether the in-stop-mode power generating process
should be executed from the time at which the ignition switch is
set off, or the in-stop-mode power generating process should be
interrupted before being completed.
[0128] With the method according to the embodiments, in case of
executing the scavenging process for scavenging the anode side with
a scavenging gas (anode scavenging process) after stopping power
generation of the fuel cell system when the outside temperature is
equal to or lower than a predetermined low temperature during
operation of the fuel cell system, it is determined whether the
in-stop-mode power generating process of stopping supply of a fuel
gas, and supplying an oxide gas to a fuel cell to generate power
therefrom has been executed, and the time (dilution time) for a
diluting process of diluting the concentration of the fuel gas in
the anode with the scavenging apparatus is shortened when it is
determined that the in-stop-mode power generating process has been
executed, as compared with a case where it is determined that the
in-stop-mode power generating process has not been executed.
[0129] Obviously, numerous modifications and variations of the
present invention are possible in light of the above teachings. It
is therefore to be understood that within the scope of the appended
claims, the invention may be practiced otherwise than as
specifically described herein.
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