Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
FUEL CELL SYSTEM
TECHNICAL FIELD
The present invention relates to a fuel cell system installed as a power
source for driving a fuel cell vehicle, where the fuel cell system has a
control
which is improved in moving from an idle stop state to an idle state.
BACKGROUND ART
Conventional technologies known for allowing a fuel cell system on a fuel
cell vehicle to move from an idle stop state to an idle state include the
following:
Japanese Patent Unexamined Publication No. 2001-359204 discloses a
device for controlling idle of fuel cell vehicle, in which, with an idle stop
mode is
started according to a traveling state of a- fuel cell vehicle, an auxiliary
unit and the
like (such as an air compressor) for driving the fuel cell is stopped and
output of
the fuel cell body is also stopped, followed by stopping of certain loads such
as an
auxiliary unit and the like excluding various controllers, thus improving fuel
economy.
In addition, Japanese Patent Unexamined Publication No. 2004-014159
discloses a power supply device, in which, with a low load, a fuel cell system
is
detached from a circuit and a power is supplied to the fuel cell system from
a.
capacitor, to thereby implement a low-load operation of the fuel cell system,
thus
preventing decreased energy efficiency of an entire power system.
As described above, with the fuel cell system having an idle stopping
function for stopping power generation of the fuel cell stack in a small load
area
requiring a small driving force, continuing the idle state for a long time
results in
repetitions of the idle states and the idle stop states, due to restriction on
capacity
of a battery which is installed on the vehicle together with the fuel cell
system and
which supplies an initial power to the fuel cell system's auxiliary unit and
the like.
Specifically, as shown in Fig. 1-(b), when the fuel cell system enters an
idle stop state with the battery at an idle stop allowable SOC (state of
charge) level,
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the battery's SOC starts decreasing. Then, when the battery's SOC is decreased
to
an idle stop unallowable SOC level, the idle stop state will be canceled to
thereafter start the power generation in the idle state, to thereby increase
the
battery's SOC with the thus generated power.
In the above operation, the fuel cell stack moves from a low-voltage state
of the idle stop state to a high-voltage state of the idle state, as shown in
Fig. 1-(a).
The above change in voltage of the fuel stack moving below and beyond a
certain
voltage {referred to as "a deterioration prompting potential" shown in Fig. 1-
(a)}
inconveniently promotes deterioration of the fuel cell stack.
DISCLOSURE OF INVENTION
It is therefore an object of the present invention to provide a fuel cell
system which prevents deterioration of a fuel cell stack by suppressing a
voltage
of the fuel cell stack when the fuel cell system moves from an idle stop state
to an
idle state.
According to a first aspect of the present invention, there is provided a
fuel cell system for generating a power by an electrochemical reaction between
a
fuel gas and an oxidizing gas, the fuel cell system comprising: 1) a fuel cell
stack
for receiving the fuel gas and the oxidizing gas, the fuel cell system
implementing
the following operations: i) when the power is required, supplying to the fuel
cell
stack the fuel gas and the oxidizing gas each having a quantity and a pressure
which correspond to a current from the fuel cell stack, to thereby generate
the
power of the fuel cell stack, and ii) when the power is not required, stopping
the
supplying of the fuel gas and the oxidizing gas to the fuel cell stack, to
thereby
stop the generating of the power of the fuel cell stack, leading to an idle
stop state
of the fuel cell system; and 2) a controller for controlling the current from
the fuel
cell stack such that the fuel cell stack has a voltage less than or equal to a
certain
voltage, the controlling being implemented when the idle stop state is
cancelled
and thereafter the fuel cell system moves to an idle state.
According to a second aspect of the present invention, there is provided a
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method for generating a power by an electrochemical reaction between a fuel
gas
and an oxidizing gas, the method comprising: 1) when the power is required,
supplying to a fuel cell stack the fuel gas and the oxidizing gas each having
a
quantity and a pressure which correspond to a current from the fuel cell
stack, to
thereby generate the power of the fuel cell stack; 2) when the power is not
required, stopping the supplying of the fuel gas and the oxidizing gas to the
fuel
cell stack, to thereby stop the generating of the power of the fuel cell
stack,
leading to an idle stop state of the fuel cell system; and 3) controlling the
current
from the fuel cell stack such that the fuel cell stack has a voltage less than
or equal
to a certain voltage, the controlling being implemented when the idle stop
state is
cancelled and thereafter the fuel cell system moves to an idle state.
According to a third aspect of the present invention, there is provided a
fuel cell system for generating a power by an electrochemical reaction between
a
fuel gas and an oxidizing gas, the fuel cell system comprising: 1) means for
receiving the fuel gas and the oxidizing gas, the fuel cell system
implementing the
following operations: i) when the power is required, supplying to the
receiving
means the fuel gas and the oxidizing gas each having a quantity and a pressure
which correspond to a current from the receiving means, to thereby generate
the
power of the receiving means, and ii) when the power is not required, stopping
the
supplying of the fuel gas and the oxidizing gas to the receiving means, to
thereby
stop the generating of the power of the receiving means, leading to an idle
stop
state of the fuel cell system; and 2) means for controlling the current from
the
receiving means such that the receiving means has a voltage less than or equal
to a
certain voltage, the controlling being implemented when the idle stop state is
cancelled and thereafter the fuel cell system moves to an idle state.
Other and further features, advantages and benefits of the present
invention will become apparent from the following description in conjunction
with
the following drawings.
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BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows diagrams of an idle stop state and an idle state after the idle
stop state being cancelled, according to a related art, in which;
Fig. 1-(a) shows a change of a voltage of a fuel cell stack, and
Fig. 1-(b) shows a change of an SOC (state of charge) of a
secondary battery.
Fig. 2 shows a structure of a fuel cell vehicle provided with a fuel cell
system, according to an embodiment of the present invention.
Fig. 3 shows a structure of the fuel cell system, according to the
embodiment of the present invention.
Fig. 4 shows a flow chart of operations, according to the embodiment of
the present invention.
Fig. 5 shows a flow chart of operations for the fuel cell system to move to
the idle stop state, according to the embodiment of the present invention.
Fig. 6 shows a flow chart of operations for the fuel cell system to be
canceled from the idle stop state, according to the embodiment of the present
invention.
Fig. 7 shows diagrams of an idle stop state and an idle state after the idle
stop state being cancelled, according to the embodiment of the present
invention,
in which
Fig. 7-(a) shows a change of a voltage of a fuel cell stack, and
Fig. 7-(b) shows a change of a current from the fuel cell stack.
Fig. 8 shows voltage-current characteristics of the fuel cell stack
corresponding to a temperature of the fuel cell stack, according to the
embodiment
of the present invention.
Fig. 9 shows voltage-current characteristics of the fuel cell stack
corresponding to a stoichiometric ratio of a cathode of the fuel cell stack,
according to the embodiment of the present invention.
Fig. 10 shows diagrams of the idle stop state and the idle state after the
idle stop state being cancelled, according to the embodiment of the present
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invention, in which;
Fig. 10-(a) shows the change of the voltage of the fuel cell stack,
Fig. 10-(b) shows the change of the current from the fuel cell
stack, and
5 Fig. 10-(c) shows a change of rotary speed of an air compressor.
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter described referring the drawings is the best mode for carrying
out the invention. For convenience sake, the term "state of charge" is denoted
by
"SOC."
Fig. 2 shows a basic structure of a fuel cell vehicle on which a fuel cell
system 102 is installed, according to an embodiment of the present invention.
Fig.
3 shows a structure of the fuel cell system 102, according to the embodiment
of
the present invention.
In Fig. 2, the fuel cell vehicle includes a vehicular body 101 on which the
fuel cell system 102 is installed as a driving power source. Moreover, the
fuel cell
vehicle is provided with an inverter 103, a driving motor 104, a driving wheel
105,
a vehicle speed sensor 106, a secondary battery 107, a relay 108 and a
controller
109. Moreover, the fuel cell vehicle is provided with a shift position sensor
111
(for sensing a position of a vehicular shift), a brake sensor 112 (for sensing
whether or not a braking is implemented), and an accelerator opening sensor
113
(for sensing an opening of an accelerator).
.In the fuel cell system 102, a hydrogen pressure adjusting valve 203 (refer
to an after-described Fig. 3), an air pressure adjusting valve 213 (refer to
the
after-described Fig. 3), an air compressor 212 (refer to the after-described
Fig. 3)
and the like control a pressure, a flowrate and the like of i) hydrogen of a
fuel gas
and ii) air of an oxidizing gas which are supplied to a fuel cell stack 201
(refer to
the after-described Fig. 3), so as to generate i) a power consumed by the
driving
motor 104 and ii) a power necessary for charging the secondary battery 107.
Converting a direct current power (generated by the fuel cell system 102)
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to an alternating current power, the inverter 103 controls the driving motor
104
such that the thus converted alternating current power serves as an output
torque
which is instructed from the controller 109 for driving the driving motor 104.
The driving wheel 105 is mechanically connected to the driving motor 104.
With a drive torque transmitted from the driving motor 104 to the driving
wheel
105, the driving wheel 105 brings about a driving force, thereby driving the
vehicle. The vehicle speed sensor 106 senses a rotary speed of the driving
wheel
105.
In a state where no power is supplied from the fuel cell system 102,
specifically, in such a state as an idle stop and the like of the vehicle, the
secondary battery 107 supplies a power to auxiliary units such as the hydrogen
pressure adjusting valve 203, the air pressure adjusting valve 213 and the air
compressor 212 which are necessary for generating the driving motor 104 and
the
fuel cell system 102. The secondary battery 107 is provided with i) a voltage
sensor 114 for sensing a voltage of the secondary battery 107 and ii) a
current
sensor 115 for sensing a current of the secondary battery 107. Based on the
thus
sensed voltage and current, a charging quantity of the secondary battery 107
can
be estimated.
Based on an instruction from the controller 109, a relay 108 connects the
fuel cell system 102 with a load or cuts the fuel cell system 102 from the
load.
The controller 109 functions as a control center for controlling an
operation of the fuel cell system 102. The controller 109 is realized, for
example,
by a microcomputer and the like provided with resources such as CPU, memory,
input/output device and the like which are necessary for a computer
controlling
various operations based on a program. The controller 109 reads in a signal
from
each of the above sensors of the fuel cell vehicle, then sends instructions to
each
of the structural elements of the fuel cell vehicle based on the thus read
various
signals and on a pre-retained control logic (program). Then, the controller
109
administratively controls all operations necessary for operating and stopping
the
fuel cell vehicle, including operations of the fuel cell system 102 in a
process of
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moving to the idle stop state of the fuel cell vehicle, to be described
afterward.
Hereinafter described referring to Fig. 3 is the fuel cell system 102.
In Fig. 3, the fuel cell system 102 includes: i) the fuel cell stack 201 for
power generation, ii) a hydrogen supply system for supplying to the fuel cell
stack
201 hydrogen (or hydrogen rich gas) as a fuel gas, and iii) an air supply
system for
supplying to the fuel cell stack 201 an air including oxygen as an oxidizing
gas.
The fuel cell stack 201 has a multilayer structure of generator cells in
which a hydrogen electrode (to which hydrogen is supplied) and an air
electrode
{to which oxygen (air) is supplied} are overlapped with an electrolyte-
electrode
catalyst complex sandwiched therebetween, and includes a generating portion
for
converting chemical energy to electrical energy through electrochemical
reaction
between the hydrogen and the oxygen.
To the hydrogen electrode of the fuel cell stack 201, the hydrogen is
supplied, to thereby dissociate hydrogen ion from electron. Then, the hydrogen
ion
passes through the electrolyte while the electron passes through an outer
circuit, to
thereby generate a power, thereafter, the hydrogen and the electron
respectively
move to the air electrode. In the air electrode, the oxygen (in the thus
supplied air),
the hydrogen ion and the electron react, to thereby bring about water, to be
exhausted outward.
In view of higher energy concentration, lower cost, lighter weight and the
like, the fuel cell stack 201 uses, for example, a solid high molecular
electrolyte.
The solid high molecular electrolyte is made of ion (proton)-conductive high
molecule film such as fluorine resin ion exchange film and the like, serving
as an
ion-conductive electrolyte by hydrous saturation.
To the hydrogen electrode of the fuel cell stack 201 via a hydrogen
electrode passage, the hydrogen supply system leads the hydrogen supplied from
a
hydrogen supplier. Specifically, the hydrogen supply system has i) a hydrogen
tank
202 which is a hydrogen supplier for storing the hydrogen at a high pressure,
2)
the hydrogen pressure adjusting valve 203 for adjusting the pressure of the
hydrogen supplied to the fuel cell stack 201 such that the hydrogen necessary
for
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the power generation by the fuel cell stack 201 can be supplied to the fuel
cell
stack 201, 3) a hydrogen circulating pump 206 for circulating a hydrogen-off
gas
(which is exhausted from the fuel cell stack 201) through a hydrogen
circulating
pipe 205, to thereby return the hydrogen-off gas to an inlet side of the fuel
cell
stack 201 via an ejector 204, and 4) a hydrogen supply pipe 207 serving as the
hydrogen electrode passage.
Moreover, in the vicinity of an inlet of the hydrogen electrode of the fuel
cell stack 201, there are provided i) a hydrogen pressure sensor 208 for
sensing
pressure of the hydrogen supplied to the fuel cell stack 201, and ii) a
hydrogen
concentration sensor 209 for sensing the hydrogen concentration.
The hydrogen gas supplied from the hydrogen tank 202 (hydrogen supply
source) is sent to the hydrogen supply pipe 207 via the hydrogen pressure
adjusting valve 203, to thereafter be supplied to the hydrogen electrode of
the fuel
cell stack 201. In this case, the pressure of the thus supplied hydrogen gas
is
adjusted by the hydrogen pressure adjusting valve 203 in such a manner as to
be
controlled based on the hydrogen pressure sensed by a hydrogen pressure sensor
208, and such that pressures in the hydrogen electrode and hydrogen electrode
passage of the fuel cell stack 201 can be varied according to the load.
In the fuel cell stack 201, all the thus supplied hydrogen gases are not
completely consumed. Specifically, the hydrogen-off gas exhausted from the
fuel
cell stack 201 without being consumed is circulated by means of the hydrogen
circulating pump 206 via the hydrogen circulating pipe 205, and then is mixed
with a fresh hydrogen gas supplied by the ejector 204, to be thereafter
supplied to
the hydrogen electrode of the fuel cell stack 201. With this, stoichiometric
ratio
(supply flowrate/consumption flowrate) of the hydrogen can be kept more than
or
equal to 1, thus stabilizing a cell voltage.
On an outlet side of the fuel cell stack 201 of the hydrogen supply system,
there is provided a purge valve 210 and a purge pipe 211. The purge valve 210
is
ordinarily closed, however, is opened with a decrease in cell voltage sensed
which
decrease is attributable to the fuel cell stack 201's failures such as water
clogging,
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stored inactive gas and the like. Circulating the hydrogen gas may store
impurity,
nitrogen and the like in the hydrogen circulating pipe 205, thereby decreasing
a
partial pressure of the hydrogen, leading to a possible decrease in generation
efficiency of the fuel cell stack 201. Therefore, providing the outlet side of
the fuel
cell stack 201 with the purge valve 210 and the purge pipe 211, and when
necessary, opening the purge valve 210 for purging can remove the impurity,
the
nitrogen and the like from the hydrogen circulating pipe 205.
To the air electrode of the fuel cell stack 201 through the air electrode
passage, the air supply system of the fuel cell stack 201 leads the air from
an air
supplier. Specifically, the air supply system includes i) the air compressor
212 as
the air supplier, ii) the air pressure adjusting valve 213, and iii) an air
pressure
supply pipe 214 serving as an air electrode passage.
The air compressor 212 sends the air to the air electrode of the fuel cell
stack 201. For example, an air compressed by driving a motor is supplied to
the air
electrode of the fuel cell stack 201 via the air pressure supply pipe 214.
The air pressure adjusting valve 213 adjusts the air supplied by the air
compressor 212 to the fuel cell stack 201, and is disposed on an exhaust pipe
215
outside the air electrode of the fuel cell stack 201.
In the vicinity of an inlet of the air electrode of the fuel cell stack 201,
there is provided an air pressure sensor 216. In this case, the pressure of
the air
supplied by the air compressor 212 is adjusted by the air pressure adjusting
valve
213 in such a manner as to be controlled based on the air pressure sensed by
the air
pressure sensor 216, and such that the pressures in the air electrode and air
electrode passage of the fuel cell stack 201 can be varied according to the
load.
The oxygen and other components of the air which are not consumed by
the fuel cell stack 201 are exhausted from the fuel cell stack 201 via the air
pressure adjusting valve 213 and the exhaust pipe 215.
The fuel cell stack 201 using the above solid high molecular electrolyte
film has a proper operating temperature about 80 C (relatively low), and
needs
cooling when overheated. Therefor, a cooling mechanism for maintaining the
fuel
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cell stack 201 at a proper temperature is provided for the fuel cell stack
201. The
cooling mechanism ordinarily cools the fuel cell stack 201 by circulating
cooling
water in the fuel cell stack 201.
Specifically, the cooling mechanism includes: i) a cooling water pump 217
5 as a cooling water supplier, ii) a radiator 218 for properly cooling the
cooling
water, and iii) a cooling water pipe 219 serving as a passage of the cooling
water.
Moreover, in the vicinity of a cooling water's inlet of the fuel cell stack
201, there
is provided a cooling water temperature sensor 220 for sensing a temperature
of
the cooling water supplied to the fuel cell stack 201. Based on the cooling
water
10 temperature sensed by the cooling water temperature sensor 220, the cooling
water
pump 217 is controllably driven, adjusting the flowrate of the cooling water
in
such a manner that the temperature of the cooling water distributed in the
cooling
water pipe 219 is kept at about 80 C.
At least any one of the following can be regarded as the temperature of the
fuel cell stack 201:
I) a temperature of the hydrogen-off gas exhausted from the
hydrogen electrode of the fuel cell stack 201,
II) a temperature of the air-off gas exhausted from the air electrode,
III) the cooling water's temperature sensed by the cooling water
temperature sensor 220, and
iv) an outer temperature.
For the item I) above, in the vicinity of an outlet of the hydrogen electrode,
there is provided a temperature sensor 221 for sensing the temperature of the
hydrogen-off gas exhausted from the fuel cell stack 201.
For the item II) above, in the vicinity of an outlet of the air electrode,
there is provided a temperature sensor 222 for sensing the temperature of the
air-off gas exhausted from the fuel cell stack 201.
For the item 111) above, in the vicinity of the fuel cell stack 201, a
temperature sensor (not shown) for sensing the outer temperature is to be
provided.
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Moreover, the fuel cell system 102 is provided with a voltage sensor 223,
a current sensor 224, and a system controller 225. The voltage sensor 223
senses a
stack voltage generated by the fuel cell stack 201. The current sensor 224
senses a
current I201 from the fuel cell stack 201.
The system controller 225 functions as a control center for controlling the
operation of the fuel cell system 102. The system controller 225 is realized,
for
example, by a microcomputer and the like provided with resources such as CPU,
memory, input/output device and the like which are necessary for a computer
controlling various operations based on a program. Specially, the system
controller
225 is realized, for example, as part of a function of the controller 109 in
Fig. 2.
The system controller 225 reads in the signal from each of the above sensors
of the
fuel cell system 102, then sends instructions to each of the structural
elements of
the fuel cell system 102 based on the thus read various signals and on the
pre-retained control logic (program). Then, the system controller 225
administratively controls all operations necessary for operating and stopping
the
fuel cell system 102, including operations of the fuel cell system 102 in the
process of moving between the idle state and the idle stop state, to be
described
afterward.
(Controlling operation)
Hereinafter described referring to a flow chart in Fig. 4 is a controlling
operation between the idle state and the idle stop state of the fuel cell
system 102.
The controlling operation is implemented by the system controller 225. Herein,
controlling operation in Fig. 4 is to be repeatedly implemented at a preset
period.
(S301) In Fig. 4, for implementing the controlling operation of the idle stop
of the
fuel cell system 102, a routine determines whether or not the fuel cell system
102
is in the idle stop state.
(S302) When No at S301, the routine determines whether or not an allowable
condition (for example, the SOC of the secondary battery 107) is established
for
moving the fuel cell system 102 to the preset idle stop state.
(S303) When Yes at S302, the routine implements a treatment for moving the
fuel
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cell system 102 to the idle stop state.
(S304) The fuel cell system 102 enters the idle stop state.
For moving the fuel cell system 102 to the idle stop state, the routine
implements operations shown by a flow chart in Fig. 5.
(S41) In Fig. 5, at first, the routine closes the hydrogen pressure adjusting
valve
203, to thereby stop supplying the hydrogen.
(S42) Then, the routine determines whether or not the hydrogen pressure of the
fuel cell stack 201 is less than or equal to a certain pressure, for example,
a certain
negative pressure lower than an atmospheric pressure.
(S43) When Yes at S42, the routine closes the purge valve 210 and stops
driving
the hydrogen circulating pump 206.
(S44) Then, the routine stops driving the air compressor 212 and closes the
air
pressure adjusting valve 213, to thereby stop supplying the air.
(S45) Then, the routine stops driving the cooling water pump 217.
Thereby, the routine stops generating the fuel cell stack 201, to thereafter
move the fuel cell system 102 to the idle stop state.
In sum, the routine moving the fuel cell system 102 to the idle stop state
stops the operation of the auxiliary units such as the hydrogen circulating
pump
206 and the air compressor 212, thereby improving fuel economy, improving
noise-vibration performance and decreasing power consumption.
(S305) Back to Fig. 4, when No at S302, the routine continues generating the
fuel
cell stack 201 instead of allowing the fuel cell system 102 to enter the idle
stop
state.
(S306) On the other hand, when Yes at S301, the routine determines whether or
not the idle stop state is to be canceled (determination 1). The determination
1
determines whether or not the vehicle requires the fuel cell system 102 for a
driving force.
(S307) When Yes at S306, the routine cancels the idle stop state and then
starts
generating the fuel cell stack 201, to thereby implement an ordinary
generation
control.
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Fig. 6 shows a flow chart of operations for restarting the fuel cell system
102 from the idle stop state.
(S51) In Fig. 6, at first, the routine opens the hydrogen pressure adjusting
valve
203, to thereby start supplying the hydrogen.
(S52) Then, the routine drives the cooling water pump 217.
(S53) Then, the routine drives the air compressor 212.
(S54) Then, the routine opens the air pressure adjusting valve 213, to thereby
start supplying the air.
(S55) Then, the routine starts the generation, to thereby take out the power
from
the fuel cell stack 201. In addition, after restarting the generation, the
routine
opens the purge valve 210 to thereby aggressively exhaust impurities such as
the
nitrogen and the like.
Then, the routine returns to an ordinary purge control.
(S308) Back to Fig. 4 again, when No at S306, the routine determines whether
or
not the idle stop state is to be canceled (determination 2). The determine 2
determines whether or not the idle stop is to be canceled regardless of the
vehicle's
requirement for the driving force. For example, the routine determines whether
or
not the SOC of the secondary battery 207 is decreased to such an extent as to
become lower than an idle stop state cancellation level which is preset
through an
experiment, study and the like.
(S309) When Yes at S308, the routine cancels the idle stop state to thereafter
move the fuel cell system 102 to the idle state, thus controlling the
generation in
the idle state.
(S310) On the other hand, when No at S308, the routine continues the idle stop
state.
For moving the fuel cell system 102 from the idle stop state to the idle
state, a power generation quantity is to be controlled in the following
manner:
With the fuel cell system 102 moved to the idle state and then starting the
generation, the fuel cell stack 201 has a voltage less than or equal to a
certain
voltage (Vdp), specifically, less than or equal to a deterioration-promoting
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potential Vdp (for example, about 0.7 V) which promotes deterioration of the
fuel
cell stack 201.
In the above controlling of the power generation quantity, as shown in Fig.
7-(a), for the generation in the idle state after the idle stop state being
canceled,
the current I201 from the fuel cell stack 201 is so set as to allow the fuel
cell stack
201 to have the voltage less than or equal to the deterioration-promoting
potential
Vdp, contrary to the ordinary idle state showing the voltage more than the
deterioration-promoting potential Vdp. In other words, the current I201 taken
out
from the fuel cell stack 201 in the idle state after the idle stop state being
canceled
is, as shown in Fig. 7-(b), larger in quantity than the current I201 in the
generation
in the ordinary idle state in which the fuel cell stack 201 has the voltage
more than
the deterioration-promoting potential Vdp.
Specifically, the current I201 from the fuel cell stack 201 is set based on a
voltage-current characteristic of the fuel cell stack 201 in Fig. 8. The
voltage-current characteristic in Fig. 8 is set in advance through an
experiment and
the like according to the temperature of the fuel cell stack 201, and is
memorized
in the system controller 225. Therefore, the current 1201 from the fuel cell
stack
201 can be variably set according to the temperature of the fuel cell stack
201. As
is seen in Fig. 8, the higher the temperature of the fuel cell stack 201 is,
the more
the current 1201 is. Moreover, the taken-out current I201 causing the voltage
of the
fuel cell stack 201 to be less than or equal to the deterioration-promoting
potential
Vdp is feedbacked and is set according to the voltage of the fuel cell stack
201
based on the voltage-current characteristic in Fig. 8.
Moreover, in the idle state after the idle stop state being cancelled, an air
quantity (oxidizing gas quantity) supplied to the fuel cell stack 201 is so
rendered
as not to increase according to the power generation quantity, that is, the
taken-out
current I201. In other words, as shown in Fig. 1 0-(c), the above air quantity
is like
the one in the ordinary idle state, namely, the air quantity in the idle state
other
than the idle state after the idle stop being cancelled.
As shown in Fig. 10-(c), in the idle state after the idle stop state being
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cancelled, the rotary speed of the air compressor 212 is set as low as that in
the
ordinary idle state. In other words, the air stoichiometric ratio (supply
flowrate/consumption flowrate) of the cathode is decreased within such an
extent
that a difference between the air pressure in the cathode and the hydrogen
pressure
5 in the anode is allowable, to thereby generate the power with decreased
generation
efficiency.
For generating the power with the cathode's stoichiometric ratio decreased,
the current I201 from the fuel cell stack 201 is set based on a voltage-
current
characteristic of the fuel cell stack 201 shown in Fig. 9. The voltage-current
10 characteristic in Fig. 9 is set in advance through an experiment and the
like
according to increase or decrease in the stoichiometric ratio of the cathode
of the
fuel cell stack 201, and is memorized in the system controller 225. Therefore,
the
current I201 from the fuel cell stack 201 can be variably set according to the
stoichiometric ratio of the cathode. As is seen in Fig. 9, the less the
stoichiometric
15 ratio of the cathode is, the less the above current Iaol is.
In sum, decreasing the stoichiometric ratio of the cathode can decrease the
rotary speed of the air compressor 212 in the idle state, thereby preventing
repetitions of calm states (the idle stop state) and noisy states (the idle
state) of the
air compressor 212, to thereby suppress deterioration of the noise-vibration
performance. Moreover, decreasing the generation efficiency decreases the
power
generated in the idle state, thereby suppressing increase in the SOC of the
secondary battery 107 when the secondary battery 107 is charged with the power
through the above generation, to thereby increase the power generation
quantity,
that is, the taken-out current I201.
Moreover, in the idle state after the idle stop being cancelled, an allowable
SOC (in other words, upper limit of SOC) of the secondary battery 107 is to be
set,
for example, about 5% to 10% more than the one in the ordinary idle state.
With
this, the charge quantity to the secondary battery 107 is increased, thereby
increasing the taken-out current I201, leading to an increase in the power
generation quantity. Moreover, in the idle state after the idle stop being
cancelled,
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another idle stop state is highly probable thereafter. Therefore, when the SOC
of
the secondary battery 107 is high in the idle state, and then the fuel cell
system
102 moves to the idle stop state in the above high-SOC idle state, the idle
stop
state can be maintained for a long time. Moreover, even when the idle stop
state is
cancelled with the SOC of the secondary battery 107 being high, the secondary
battery 107 can be charged.
The embodiment of the present invention can be summarized below:
The operation according the embodiment is implemented in a state where
the vehicle does not require the driving force. When the fuel cell system 102
moves from the idle stop state to the idle state to start the generation,
increasing
the current I201 (from the fuel cell stack 201) to more than the current 1201
in the
ordinary idle within such an extent as to keep the voltage of the fuel cell
stack 201
less than or equal to the deterioration-promoting potential Vdp can prevent
the
deterioration of the fuel cell stack 201 from being promoted when the fuel
cell
system 102 moves to the idle state.
Moreover, increasing the taken-out current I201 to thereby increase the
power generation quantity, and charging the secondary battery 107 with the
thus
generated power can rapidly increase the SOC of the secondary battery 107,
thus
preventing in a short time the charge quantity of the secondary battery 107
from
being low immediately after the idle stop is cancelled.
With this, the fuel cell system 102 can be more often moved to the idle stop
state.
The entire contents of Japanese Patent Application No. 2005-128274 with
its filing date of April 26, 2005 in Japan are incorporated herein by
reference.
Although the present invention has been described above by reference to a
certain embodiment, the present invention is not limited to the embodiment
described above. Modifications and variations of the embodiment described
above
will occur to those skilled in the art, in light of the above teachings.
INDUSTRIAL APPLICABILITY
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Under the present invention, when the fuel cell system moves from the
idle stop state to the idle state, setting the current I201 from the fuel cell
stack in
such a manner as to keep the voltage of the fuel cell stack less than or equal
to the
certain voltage, that is the deterioration-promoting potential, can eliminate
the fuel
cell stack's potential change between the idle stop state and the idle state,
thereby
preventing the deterioration of the fuel cell stack from being promoted both
in the
fuel cell system's periods of i) moving to the idle state and ii) in the idle
state.
The scope of the present invention is defined with reference to the
following claims.
