U.S. patent application number 14/742113 was filed with the patent office on 2015-10-08 for fuel cell system and method for controlling same.
This patent application is currently assigned to NISSAN MOTOR CO , LTD.. The applicant listed for this patent is NISSAN MOTOR CO , LTD.. Invention is credited to Kenichi GOTO, Yasushi ICHIKAWA, Keigo IKEZOE, Mitsunori KUMADA, Ken NAKAYAMA, Yousuke TOMITA.
Application Number | 20150288008 14/742113 |
Document ID | / |
Family ID | 42198187 |
Filed Date | 2015-10-08 |
United States Patent
Application |
20150288008 |
Kind Code |
A1 |
ICHIKAWA; Yasushi ; et
al. |
October 8, 2015 |
FUEL CELL SYSTEM AND METHOD FOR CONTROLLING SAME
Abstract
A fuel cell system 100 includes: a fuel cell 1 for generating a
power by causing an electrochemical reaction between an oxidant gas
supplied to an oxidant electrode 34 and a fuel gas supplied to a
fuel electrode 67; a fuel gas supplier HS for supplying the fuel
gas to the fuel electrode 67; and a controller 40 for controlling
the fuel gas supplier HS to thereby supply the fuel gas to the fuel
electrode 67, the controller 40 being configured to implement a
pressure change when an outlet of the fuel electrode 67 side is
closed, wherein based on a first pressure change pattern for
implementing the pressure change at a first pressure width
.DELTA.P1, the controller 40 periodically changes a pressure of the
fuel gas at the fuel electrode 67.
Inventors: |
ICHIKAWA; Yasushi;
(Atsugi-shi, JP) ; IKEZOE; Keigo; (Atsugi-shi,
JP) ; GOTO; Kenichi; (Atsugi-shi, JP) ;
NAKAYAMA; Ken; (Atsugi-shi, JP) ; KUMADA;
Mitsunori; (Atsugi-shi, JP) ; TOMITA; Yousuke;
(Atsugi-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NISSAN MOTOR CO , LTD. |
Yokohama-shi |
|
JP |
|
|
Assignee: |
NISSAN MOTOR CO , LTD.
Yokohama-shi
JP
|
Family ID: |
42198187 |
Appl. No.: |
14/742113 |
Filed: |
June 17, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13129986 |
Aug 2, 2011 |
|
|
|
PCT/JP2009/069425 |
Nov 16, 2009 |
|
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|
14742113 |
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Current U.S.
Class: |
429/446 |
Current CPC
Class: |
H01M 2008/1095 20130101;
Y02E 60/50 20130101; H01M 8/04365 20130101; H01M 8/04388 20130101;
H01M 8/04753 20130101; H01M 8/04179 20130101; H01M 8/04104
20130101; H01M 8/04395 20130101 |
International
Class: |
H01M 8/04 20060101
H01M008/04 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 21, 2008 |
JP |
2008-298191 |
Nov 27, 2008 |
JP |
2008-302465 |
Claims
1. A fuel cell system comprising: a fuel cell configured to
generate electric power by causing an electrochemical reaction
between an oxidant gas supplied to an oxidant electrode and a fuel
gas supplied to a fuel electrode; a fuel gas supplier configured to
supply the fuel gas to the fuel electrode of the fuel cell; an
output takeout device configured to take out an output from the
fuel cell; and a controller configured to control the output
takeout device to thereby take out from the fuel cell, an output
corresponding to a required load required for the fuel cell system,
and to control the fuel gas supplier to thereby supply the fuel gas
to the fuel electrode in such a manner as to change a pressure of
the fuel gas at the fuel electrode with a predetermined pressure
change range, wherein the controller is programmed to set the
pressure change range such that the pressure change range in a case
where the required load is high is larger than the pressure change
range in a case where the required load is low.
2. The fuel cell system according to claim 1, wherein the
controller sets an operation pressure of the fuel cell such that
the higher the required load is, the higher the operation pressure
is.
3. The fuel cell system according to claim 1, wherein the
controller sets an upper limit pressure and a lower limit pressure
of the pressure of the fuel gas at the fuel electrode based on an
operation pressure of the fuel cell, and change the pressure of the
fuel gas at the fuel electrode between the upper limit pressure and
the lower limit pressure to thereby change the pressure of the fuel
gas at the fuel electrode with the predetermined pressure change
range.
4. The fuel cell system according to claim 3, wherein a rate of
increase of the lower limit pressure relative to an increase of the
required load is set such that the rate of increase of the lower
limit pressure in a case where the required load is high is larger
than the rate of increase of the lower limit pressure in a case
where the required load is low.
5. A fuel cell system comprising: a fuel cell configured to
generate electric power by causing an electrochemical reaction
according to a load of the fuel cell system between an oxidant gas
supplied to an oxidant electrode and a fuel gas supplied to a fuel
electrode and to consume the fuel gas in the fuel electrode; a
non-recirculating type fuel gas system comprising: a fuel gas
supplier configured to supply the fuel gas to an inlet of the fuel
electrode of the fuel cell: a capacity device provided on an outlet
side of the fuel electrode of the fuel cell; and a purge valve
provided on the outlet side of the fuel electrode of the fuel cell;
and a pressure increase-decrease controller programmed to have a
pressure of the fuel gas at the fuel electrode increased with an
increase of the load, while controlling the fuel gas supplier to
increase/decrease the pressure of the fuel gas at the fuel
electrode with a predetermined pressure increase/decrease range at
a given load, wherein the pressure increase-decrease controller
sets, in a case where the load is high, a large pressure
increase/decrease range as compared to a case where the load is
low.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 13/129,986, filed Aug. 2, 2011, which is the National Stage of
Application No. PCT/JP2009/069425 filed Nov. 16, 2009, which is
based upon and claims the benefit of priority from prior Japanese
Application No. 2008-298191, filed Nov. 21, 2008 and Japanese
Application No. 2008-302465, filed Nov. 27, 2008; the entire
contents of all of which are incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention relates to a fuel cell system.
BACKGROUND ART
[0003] Conventionally, such a fuel cell system is known as is
provided with a fuel cell where a fuel gas (for example, hydrogen)
is supplied to a fuel electrode and an oxidant gas (for example,
air) is supplied to an oxidant electrode to thereby make an
electrochemical reaction of these gases, thus implementing a power
generation.
[0004] With respect to the fuel cell system of the above type,
nitrogen included in the air is permeated to the fuel electrode
side, so that the fuel electrode has a portion having a high
nitrogen concentration, that is, a portion having a low hydrogen
concentration. The thus caused gas unevenness is a cause for
deteriorating members included in the fuel cell. Then, Patent
Literature 1 discloses a method of changing gas pressures of the
fuel electrode and oxidant electrode to thereby purge the water of
the fuel cell and the accumulated unreactive gas.
CITATION LIST
Patent Literature
[0005] [Patent Literature 1]
[0006] Japanese Patent Publication No. 2007-517369
{JP2007517369(T)}
SUMMARY OF INVENTION
Technical Problem
[0007] However, with respect to the method disclosed in the Patent
Literature 1, a pressure change with a relatively large pressure
width is necessary for purging the liquid water and unreactive gas.
Thereby, a large stress may be applied to electrolyte membranes
included in the fuel cell, thus causing such a possibility as may
deteriorate durability of the fuel cell.
[0008] The present invention has been made in view of the above
circumstances. It is an object of the present invention to suppress
unevenness of reactive gas while suppressing durability
deterioration of the fuel cell.
[0009] Moreover, it is another object of the present invention to
suppress the stress caused in the fuel cell or fuel gas supply
components to thereby suppress deterioration of the fuel cell
system.
Solution to Problem
[0010] A fuel cell system according to an aspect of the present
invention comprises: a fuel cell for generating a power by causing
an electrochemical reaction between an oxidant gas supplied to an
oxidant electrode and a fuel gas supplied to a fuel electrode; a
fuel gas supplier for supplying the fuel gas to the fuel electrode;
and a controller for controlling the fuel gas supplier to thereby
supply the fuel gas to the fuel electrode, the controller being
configured to implement a pressure change when an outlet of the
fuel electrode side is closed, wherein based on a first pressure
change pattern for implementing the pressure change at a first
pressure width, the controller periodically changes a pressure of
the fuel gas at the fuel electrode.
[0011] A method of controlling a fuel cell system according to the
aspect of the present invention comprises: generating a power by
causing an electrochemical reaction between an oxidant gas supplied
to an oxidant electrode and a fuel gas supplied to a fuel
electrode; supplying the fuel gas to the fuel electrode; and
controlling the supplying operation of the fuel gas to thereby
supply the fuel gas to the fuel electrode, and implementing a
pressure change when an outlet of the fuel electrode side is
closed, wherein based on a first pressure change pattern for
implementing the pressure change at a first pressure width, the
controlling operation periodically changes a pressure of the fuel
gas at the fuel electrode.
[0012] A fuel cell system according to the aspect of the present
invention comprises: a fuel cell for generating a power by causing
an electrochemical reaction between an oxidant gas supplied to an
oxidant electrode and a fuel gas supplied to a fuel electrode; a
fuel gas supplying means for supplying the fuel gas to the fuel
electrode; and a means for controlling the fuel gas supplying means
to thereby supply the fuel gas to the fuel electrode, the
controlling means being configured to implement a pressure change
when an outlet of the fuel electrode side is closed, wherein based
on a first pressure change pattern for implementing the pressure
change at a first pressure width, the controlling means
periodically changes a pressure of the fuel gas at the fuel
electrode.
Advantageous Effects of Invention
[0013] According to the present invention, periodically changing a
pressure of a fuel gas at a fuel electrode based on the first
pressure change pattern which implements pressure change at the
first pressure width can agitate the fuel electrode side gas. With
this, the fuel electrode side gas can be made even.
[0014] Moreover, according to the present invention, the fuel gas
supply quantity in the implementation period of one control pattern
is increased, thus it is possible to suppress increase in the
number of implementations of the pressure rise-fall per unit
period. With this, a stress applied to the fuel cell or fuel gas
supply components can be relieved, thus it is possible to suppress
deterioration of the fuel cell system.
BRIEF DESCRIPTION OF DRAWINGS
[0015] FIG. 1(a) is a block diagram schematically showing a
structure of the fuel cell system according to the first
embodiment. FIG. 1(b) is a block diagram schematically showing
another structure of the fuel cell system according to the first
embodiment.
[0016] FIG. 2(a) is explanatory view showing a state of hydrogen on
the fuel electrode side in the fuel cell, showing hydrogen
streamlines in the fuel electrode side gas flow channel. FIG. 2(b)
shows the hydrogen concentration distribution in the fuel electrode
side gas flow channel. FIG. 2(c) shows the hydrogen concentration
distribution on the fuel electrode side reaction surface.
[0017] FIG. 3 (a) is an explanatory view schematically showing the
fuel cell, assuming eight current measurement points. FIG. 3(b)
shows time-series transition of the current distribution at an
individual measurement point.
[0018] FIG. 4 is a cross sectional view schematically showing the
structure of the fuel cell.
[0019] FIG. 5 is an explanatory view showing a leak nitrogen
quantity relative to nitrogen partial pressure difference between
the oxidant electrode and the fuel electrode.
[0020] FIG. 6 is an explanatory view showing the relation between
an ambient humidity and a leak nitrogen quantity according to an
ambient temperature.
[0021] FIG. 7(a) is an explanatory view schematically showing an
agitation state of hydrogen with the unreactive gas. FIG. 7(b)
shows a timing for stopping the hydrogen supply (valve closing
operation).
[0022] FIG. 8(a) is an explanatory view showing a liquid water
discharge state. FIG. 8(b) shows a timing for stopping the hydrogen
supply (valve closing operation). FIG. 8(c) shows another example
of the timing for stopping the hydrogen supply (valve closing
operation). FIG. 8(d) shows still another example of the timing for
stopping the hydrogen supply (valve closing operation).
[0023] FIG. 9 is an explanatory view showing current distribution
in the power generation surface.
[0024] FIG. 10 is a flowchart showing process procedures of a
method of controlling the fuel cell system according to the second
embodiment.
[0025] FIG. 11 is an explanatory view showing control patterns by
the first control method.
[0026] FIG. 12 is an explanatory view showing control patterns by
the second control method.
[0027] FIG. 13 is an explanatory view showing control patterns by
the third control method.
[0028] FIG. 14 is an explanatory view showing a transition of
pressure rise-fall in the fuel electrode.
[0029] FIG. 15 is an explanatory view of the first keeping time
Tp1.
[0030] FIG. 16 is an explanatory view of the second keeping time
T.
[0031] FIG. 17 is an explanatory view showing the load relative to
each of the first keeping time Tp1 and the second keeping time
Tp2.
[0032] FIG. 18 is an explanatory view showing the load relative to
each of the first keeping time Tp1 and the second keeping time
Tp2.
[0033] FIG. 19 is an explanatory view showing the upper limit
pressure P1 and lower limit pressure P2 relative to the load
current.
[0034] FIG. 20(a) is an explanatory view schematically showing the
fuel electrode side capacity Rs in the fuel cell stack and the
capacity Rt of the capacity portion. FIG. 20 (b) shows that new
hydrogen flowed into the fuel cell stack in an amount of around 1/4
of the capacity of the fuel system.
[0035] FIG. 21 is an explanatory view of the upper limit pressure
P1 and lower limit pressure P2.
[0036] FIG. 22 is an explanatory view of a pressure fall speed.
DESCRIPTION OF EMBODIMENTS
First Embodiment
[0037] FIG. 1(a) is a block diagram schematically showing a
structure of a fuel cell system 100 according to the first
embodiment of the present invention. The fuel cell system 100 is
installed, for example, in a vehicle that is a mobile object, where
the vehicle is driven by an electric power supplied from the fuel
cell system 100.
[0038] The fuel cell system 100 is principally provided with a fuel
cell stack 1 including a plurality of stacked fuel cells. Each of
the fuel cells included in the fuel cell stack 1 is so formed that
a fuel cell structure is sandwiched between a pair of separators,
where the fuel cell structure has such a structure that a fuel
electrode 67 (refer to after-described FIG. 4) and an oxidant
electrode 34 (refer to after-described FIG. 4) sandwich
therebetween a solid polymer electrolyte membrane.
[0039] In the fuel cell stack 1, corresponding to each of the fuel
gas and the oxidant gas, a pair of internal flow channels are so
formed as to extend in a stack direction of the fuel cell. Of the
pair of the internal flow channels (manifolds) corresponding to the
fuel gas; with respect to a supply internal flow channel as the
first internal flow channel, a fuel gas is supplied to each of the
fuel electrode 67 side reaction surfaces via the fuel electrode 67
side gas flow channels (cell flow channels) of the individual fuel
cells, while with respect to a discharge internal flow channel as
the second internal flow channel, a gas (hereinafter referred to as
"fuel electrode off-gas") discharged from each of the fuel
electrode 67 side gas flow channels of the individual fuel cells
flows into the discharge internal flow channel. Likewise, of the
pair of the internal flow channels corresponding to the oxidant
gas; with respect to a supply internal flow channel as the first
internal flow channel, an oxidant gas is supplied to each of the
oxidant electrode 34 side reaction surfaces via the oxidant
electrode 34 side gas flow channels (cell flow channels) of the
individual fuel cells, while with respect to a discharge internal
flow channel as the second internal flow channel, a gas
(hereinafter referred to as "oxidant electrode off-gas") discharged
from each of the oxidant electrode 34 side gas flow channels of the
individual fuel cells flows into the discharge internal flow
channel. The fuel cell stack 1 according to the first embodiment
adopts what is called a counter flow method where the fuel gas and
the oxidant gas flow in directions opposite to each other.
[0040] In each of the individual cells of the fuel cell stack 1,
electrochemically reacting the fuel gas and the oxidant gas with
each other, which gases are respectively supplied to the fuel
electrode 67 and the oxidant electrode 34, generates an electric
power.
[0041] According to the first embodiment, an explanation is made
based on the case of using hydrogen as a fuel gas and air as an
oxidant gas. In addition, in this specification, the languages
"fuel cell," "fuel electrode" and "oxidant electrode" are not to be
used only for designating a single fuel cell or its fuel electrode
or oxidant electrode, but are also to be used for unanimously
designating each of the fuel cells of the fuel cell stack 1 or
their fuel electrodes or oxidant electrodes.
[0042] The fuel cell system 100 further includes a hydrogen system
for supplying hydrogen to the fuel cell stack 1 and an air system
for supplying air to the fuel cell stack 1.
[0043] In the hydrogen system, hydrogen as the fuel gas is stored
in the fuel tank 10 (for example, a high pressure hydrogen
cylinder), and is supplied from the fuel tank 10 to the fuel cell
stack 1 via a hydrogen supply flow channel (fuel electrode inlet
flow channel) L1. Specifically, the hydrogen supply flow channel L1
has the first end portion connected to the fuel tank 10 and the
second end portion connected to an inlet side of the fuel gas
supply internal flow channel of the fuel cell stack 1. In the
hydrogen supply flow channel L1, a tank source valve (not shown in
FIG. 1) is disposed at a downstream of the fuel tank 10. Rendering
the tank source valve in an open state allows the high pressure
hydrogen gas from the fuel tank 10 to be mechanically
pressure-reduced to a predetermined pressure by means of a
pressure-reducing valve (not shown in FIG. 1) disposed at the
downstream of the fuel tank 10. The thus pressure-reduced hydrogen
gas is further pressure-reduced by means of a hydrogen pressure
adjusting valve 11 disposed at the further downstream of the
pressure-reducing valve, and then is supplied to the fuel cell
stack 1. The hydrogen pressure supplied to the fuel cell stack 1,
that is, the hydrogen pressure in the fuel electrode 67 can be
adjusted by controlling opening degree of the hydrogen pressure
adjusting valve 11. According to the first embodiment, the fuel
tank 10, the hydrogen supply flow channel L1 and the hydrogen
pressure adjusting valve 11 which is disposed in the hydrogen
supply flow channel L1 constitute a hydrogen supplier HS (fuel gas
supplier HS) for supplying hydrogen to the fuel electrode 67 of the
fuel cell stack 1.
[0044] According to the first embodiment, the fuel cell stack 1 has
such a structure that an outlet side of the fuel gas discharge
internal flow channel is basically closed, thus restricting the
fuel electrode off-gas's discharge from the fuel cell stack 1, that
is, the fuel cell stack 1 is included in the fuel cell system 100
which adopts what is called a closed system. Herein, the closed
system does not mean an exact closed state. For discharging, from
the fuel electrode 67, impurities such as inactive gas (nitrogen
and the like) and liquid water, there is disposed, as an exception,
a discharge system capable of opening the outlet side of the fuel
gas discharge internal flow channel. Specifically, a fuel electrode
off-gas flow channel (discharge flow channel) L2 is connected to
the outlet side of the fuel gas discharge internal flow channel.
The fuel electrode off-gas flow channel L2 has the second end
portion connected to an after-described oxidant electrode off-gas
flow channel L6.
[0045] In the fuel electrode off-gas flow channel L2, a capacity
portion (capacity device) 12 having a predetermined capacity Rs
(see after-described FIG. 20) as a space is disposed, where the
predetermined capacity Rs is, for example, equivalent to or about
80% of the fuel electrode 67 side capacity with respect to all fuel
cells included in the fuel cell stack 1. The capacity portion 12
functions as a buffer for primarily storing impurities included in
the fuel electrode off-gas entering from the fuel electrode 67
side. In FIG. 1, a discharge water flow channel L3 having an open
first end portion is connected to the capacity portion 12's lower
portion in a vertical direction, and a discharge water valve 13 is
provided for the discharge water flow channel L3. The impurities
(mainly, liquid water) contained in the fuel electrode off-gas
entering the capacity portion 12 is stored in the lower part of the
capacity portion 12. Controlling the open-closed state of the
discharge water valve 13 can discharge the thus stored impurities.
Moreover, in the fuel electrode off-gas flow channel L2, a purge
valve (shutter) 14 is disposed on a downstream of the capacity
portion 12. The fuel electrode off-gas entering the capacity
portion 12, specifically, the gas including the impurities (mainly,
inactive gas such as nitrogen) and unreacted hydrogen can be
discharged by controlling the open-closed state of the purge valve
14.
[0046] The fuel electrode off-gas flow channel (discharge flow
channel) L2, the capacity portion (capacity device) 12 and the
purge valve (shutter) 14 form a limiter 70.
[0047] Meanwhile, the air as the oxidant gas of the air system is
to be set forth. For example, air is compressed when an atmosphere
is taken in by means of a compressor 20, thereby supplying the air
to the fuel cell stack 1 by way of an air supply flow channel L5.
The air supply flow channel L5 has the first end portion connected
to the compressor 20 and the second end portion connected to the
inlet side of an oxidant gas supply internal flow channel of the
fuel cell stack 1. Moreover, an air supply flow channel L5 has a
humidifier 21 for humidifying the air supplied to the fuel cell
stack 1.
[0048] In the fuel cell stack 1, an oxidant electrode off-gas flow
channel L6 is connected to the outlet side of the oxidant gas
discharge internal flow channel. With this, the oxidant electrode
off-gas from the oxidant electrode 34 in the fuel cell stack 1 is
discharged outside by way of the oxidant electrode off-gas flow
channel L6. The oxidant electrode off-gas flow channel L6 has the
above-described humidifier 21, thus removing the water generated by
the generation (this removed water is used for humidifying the
supply air). Moreover, in the oxidant electrode off-gas flow
channel L6, an air pressure adjusting valve 22 is disposed on the
downstream of the humidifier 21. Adjusting the opening degree of
the air pressure adjusting valve 22 can control the air pressure
supplied to the fuel cell stack 1, that is, the air pressure of the
oxidant electrode 34. According to the first embodiment, the
compressor 20, the air supply flow channel L5, and the air pressure
adjusting valve 22 which is disposed in the oxidant electrode
off-gas flow channel L6 constitute an oxidant gas supplier OS for
supplying the air to the oxidant electrode 34 of the fuel cell
stack 1.
[0049] Moreover, an output takeout device 30 for controlling an
output (for example, current) taken out from the fuel cell stack 1
is connected to the fuel cell stack 1. By way of the output takeout
device 30, the power generated in the fuel cell stack 1 is
supplied, for example, to a vehicle-driving electric motor (not
shown in FIG. 1), a secondary battery and various accessories
necessary for the generation operation of the fuel cell stack 1.
Moreover, the power generated by the output takeout device 30 is
also supplied to the secondary battery (not shown in FIG. 1). This
secondary battery is provided for supplementing shortage of the
power supplied from the fuel cell stack 1 in such occasions as to
start the fuel cell system 100 or in a transient response of the
fuel cell system 100.
[0050] A controller (control device) 40 functions to
administratively control the entire fuel cell system 100. By
operating according to a control program, the controller 40
controls operation conditions of the fuel cell system 100. A
microcomputer including main components such as CPU, ROM, RAM and
I/O interface can be used as the controller 40. According to the
control program stored in the ROM, the controller 40 implements
various calculations. Then, to various actuators (not shown in FIG.
1), the controller 40 outputs such calculation results as control
signals. With this, the controller 40 controls various elements
such as the hydrogen pressure adjusting valve 11, the discharge
water valve 13, the purge valve 14, the compressor 20, the air
pressure adjusting valve 22 and the output takeout device 30, to
thereby implement the generation operation of the fuel cell stack
1.
[0051] For detecting conditions of the fuel cell system 100, sensor
signals from various sensors and the like are input to the
controller 40. According to the first embodiment, the above various
sensors include a hydrogen pressure sensor 41, an air pressure
sensor 42, and a stack temperature sensor 43. The hydrogen pressure
sensor 41 detects the hydrogen pressure supplied to the fuel cell
stack 1, the air pressure sensor 42 detects the air pressure
supplied to the fuel cell stack 1, and the stack temperature sensor
43 detects the temperature of the fuel cell stack 1.
[0052] According to the first embodiment, the controller 40
controls the fuel cell system 100 in the following manner. Firstly,
the controller 40 supplies air and hydrogen to the fuel cell stack
1, to thereby implement the generation by the fuel cell stack 1.
The pressure (operation pressure) of each of the air and the
hydrogen which are supplied to the fuel cell stack 1 is set in
advance either at a certain standard value which is constant
irrespective of operation load or at variable values which are
variable according to the operation load. Then, the controller 40
supplies the air and hydrogen at a predetermined operation
pressure, to thereby implement the generation of the fuel cell
stack 1. Herein, as one feature of the first embodiment, when
supplying the hydrogen to the fuel electrode 67 of the fuel cell
stack 1, the controller 40 periodically changes the hydrogen
pressure in the fuel electrode 67 of the fuel cell stack 1, based
on the first pressure change pattern for implementing the pressure
change at the first pressure width (differential pressure) and the
second pressure change pattern for implementing the pressure change
at the second pressure width (differential pressure) larger than
the first pressure width. Specifically, the controller 40
repeatedly implements basic control patterns, that is, a plurality
of the first pressure change patterns, followed by the second
pressure change pattern. When implementing the pressure change, the
controller 40 stops hydrogen supply to the fuel cell stack 1, and
on the condition that the hydrogen pressure in the fuel electrode
67 of the fuel cell stack 1 is decreased by the predetermined
pressure width (first pressure width or second pressure width), the
controller 40 restarts the hydrogen supply to the fuel cell stack
1, to thereby allow the hydrogen pressure in the fuel electrode 67
of the fuel cell stack 1 to return to the operation pressure.
Opening and closing of the hydrogen pressure adjusting valve 11
accomplish the stop and restart of the hydrogen supply to the fuel
cell stack 1. Referring to the value detected by the hydrogen
pressure sensor 41 can monitor the hydrogen pressure drop which is
equivalent to the pressure width.
[0053] Moreover, FIG. 1(b) is a block diagram schematically showing
another structure of the fuel cell system 100 according to the
first embodiment of the present invention. Herein, the structure
abolishes the discharge water valve 13, leaving the purge valve 14
only. With the above structure, controlling the open-close
condition of the purge valve 14 can discharge the gas included in
the fuel electrode off-gas, that is, the gas including the
impurities (mainly, inactive gas such as nitrogen, and liquid
water) and unreacted hydrogen.
[0054] Hereinafter, concept of the fuel cell system 100 adopting
the above structure and control method is to be set forth.
[0055] In view of improved fuel economy and decrease of driving
power of various accessories for operating the fuel cell stack,
operating the fuel cell system 100 at a low stoichiometric ratio
(otherwise referred to as "low reactive gas supply excess ratio")
and at a low flow rate lowers the flow velocity of the reactive gas
(hydrogen or air) flowing in the gas flow channel (cell flow
channel) in each of the fuel cells of the fuel cell stack 1. With
this, impurities unnecessary for the generation reaction, for
example, liquid water or an unreactive gas (mainly, nitrogen) are
likely to be accumulated in the gas flow channel, which may prevent
distribution of the reactive gas necessary for the generation. In
this case, the output of the fuel cell stack 1 is lowered and the
generation is disabled, in addition, the catalyst necessary for
reaction may possibly be deteriorated.
[0056] For example, a condition for the fuel cell stack 1 to
implement the generation by the following operations is to be taken
into account: supplying air to the oxidant electrode 34 of the fuel
cell stack 1; restricting the fuel electrode off-gas's discharge
from the fuel cell stack 1; and constantly supplying hydrogen by an
amount equivalent to hydrogen consumed in the fuel electrode 67. In
the individual fuel cell, nitrogen in air makes a cross leak to the
fuel electrode 67 side gas flow channel from the oxidant electrode
34 side gas flow channel by way of the solid polymer electrolyte
membrane included in the fuel cell. Meanwhile, to the fuel
electrode 67 side gas flow channel, hydrogen in equivalent to
hydrogen consumed by the generation reaction flows by convection
current. However, since the outlet side of the fuel gas discharge
internal flow channel is closed, the thus cross-leaked nitrogen is
pushed into the downstream side (outlet side) of the gas flow
channel by the convection of hydrogen. Nitrogen of the fuel
electrode 67 is not consumed by the generation reaction. On top of
that, nitrogen leak from the oxidant electrode 34 continuously
increases the nitrogen in the fuel electrode 67 until the oxidant
electrode 34 side partial pressure is equal to the fuel electrode
67 side partial pressure.
[0057] FIG. 2(a) to FIG. 2(c) are explanatory views showing states
of the fuel electrode 67 side hydrogen in the fuel cell. FIG. 2(a)
shows hydrogen streamlines in the fuel electrode 67 side gas flow
channel. Herein, the abscissa axis denotes a distance (in gas flow
channel direction) of the gas flow channel, where the left side of
the abscissa axis corresponds to the inlet side of the gas flow
channel and the right side of the abscissa axis corresponds to the
outlet side of the gas flow channel. Meanwhile, the ordinate axis
denotes a height of the gas flow channel, where the lower side of
the ordinate axis corresponds to the reaction surface. Moreover,
FIG. 2(b) shows hydrogen concentration distribution in the fuel
electrode 67 side gas flow channel. Like FIG. 2(a), the abscissa
axis denotes the distance (in gas flow channel direction) of the
gas flow channel, while the ordinate axis denotes the height of the
gas flow channel. In FIG. 2(b), an area al denotes a hydrogen
concentration range of 93% to 100%, an area a2 denotes the hydrogen
concentration range of 83% to 93%, and an area a3 denotes the
hydrogen concentration range of 73% to 83%. Moreover, an area a4
denotes the hydrogen concentration range of 63% to 73%, an area a5
denotes the hydrogen concentration range of 53% to 63%, an area a6
denotes the hydrogen concentration range of 43% to 53%, and an area
a7 denotes the hydrogen concentration range of 33% to 43%.
Moreover, FIG. 2(c) shows the hydrogen concentration distribution
on the fuel electrode 67 side reaction surface. Herein, the
abscissa axis denotes the distance of the gas flow channel, where
the left side of the abscissa axis corresponds to the inlet side of
the gas flow channel while the right side of the abscissa axis
corresponds to the outlet side of the gas flow channel. Meanwhile,
the ordinate axis denotes the hydrogen concentration.
[0058] As stated above, the cross leaked nitrogen's inflow and the
inflow hydrogen allow the fuel electrode 67 to have a portion where
the nitrogen concentration is high, i.e., a portion where the
hydrogen concentration is low. Specifically, in the fuel cell, the
further downstream side (outlet side) of the gas flow channel has a
tendency to further decrease the hydrogen concentration. Moreover,
continuing the generation from such a state further decreases the
hydrogen concentration of the portion where the hydrogen
concentration is low.
[0059] FIG. 3 is an explanatory view schematically showing the fuel
cell. As shown in FIG. 3(a), along the flow of the reactive gas,
eight current measurement points #1 to #8 are respectively assumed
in the power generation surface of the fuel cell. FIG. 3(b) shows
time-series transition of the current distribution at the
individual measurement point #1 to #8. Specifically, as denoted by
a broken line arrow, the current distribution transition in each of
the measurement points #1 to #8 is shifted from the one-dot chain
line to the broken line and to the solid line. That is, in the
initial generation step, the hydrogen concentration in the gas flow
channel is substantially even, therefore, as denoted by the one-dot
chain line, the current values at the measurement points #1 to #8
are substantially equal to each other. However, continuously
implementing the generation decreases the hydrogen concentration on
the outlet side of the gas flow channel, thus, as denoted by the
broken line or the solid line, the current values on the outlet
side of the gas flow channel drop and a current concentration is
caused on the inlet side of the gas flow channel. In such states,
it is difficult to continue the stable generation and the
generation may possibly be finally disabled. Moreover, since the
above local current drop is difficult to detect, as the case may
be, the output from the fuel cell stack is continuously taken with
the current drop unnoticed.
[0060] FIG. 4 is a cross sectional view schematically showing the
structure of the fuel cell. The fuel cell structure 150 included in
the fuel cell has such a structure that the solid polymer
electrolyte membrane 2 is sandwiched between the fuel electrode 67
and the oxidant electrode 34 which two electrodes (reactive
electrodes) are pairwise. The solid polymer electrolyte membrane 2
includes, for example, an ion conductive macromolecular membrane
such as a fluorine resin ion exchange membrane, and functions as an
ion conductive electrolyte membrane through water saturation. The
oxidant electrode 34 includes a platinum-based catalytic layer 3
carrying thereon a catalyst such as platinum and a gas diffusion
layer 4 including a porous body such as carbon fiber. The electrode
67 includes a platinum-based catalytic layer 6 carrying thereon a
catalyst such as platinum and a gas diffusion layer 7 including a
porous body such as a carbon fiber. Moreover, the separators (not
shown in FIG. 4) sandwiching therebetween the fuel cell structure
150 from both sides respectively have gas flow channels 5, 8 for
supplying the reactive gases (hydrogen and air) to the individual
reactive electrodes.
[0061] When the generation is continued, oxygen simultaneously with
nitrogen leak from the oxidant electrode 34 side to the fuel
electrode 67 side, thereby oxygen moves to the fuel electrode 67
side. Moreover, water generated by the generation reaction is
present in the oxidant electrode 34 side. Moreover, the gas
diffusion layer 4 or the separator (not shown in Fig.), that is,
the members included in the gas flow channel in the fuel cell or
the members for supporting the catalyst mainly include carbon. With
this, the following reactions are promoted in the area (area B in
FIG. 4) where the hydrogen is in short supply:
Fuel electrode 67 side:
O.sub.2+4H.sup.++4e.sup.-.fwdarw.2H.sub.2O
Oxidant electrode 34 side:
C+2H.sub.2O.fwdarw.CO.sub.2+4H.sup.++4e.sup.- [Equation 1]
[0062] Referring to the equation 1, carbon in the structure of the
fuel cell reacts with water generated on the oxidant electrode 34
side, to thereby generate carbon dioxide on the oxidant electrode
34 side. This signifies that the structure in the fuel cell is
eroded. Carbon included in each of an element forming the gas flow
channel, a structure carrying thereon a catalyst for causing the
reaction, a structure of the gas diffusion layer 4, and a structure
of the separator changes to carbon dioxide, thus leading to
deterioration of the fuel cell.
[0063] Moreover, the following operations are also seen on the fuel
electrode 67. A reverse diffusion phenomenon moves the generation
reaction water from the oxidant electrode 34 side to the solid
polymer electrolyte membrane 2, or the condensed water in the
hydrogen which is humidified and supplied is, as the case may be,
stored in the gas flow channel. In the case where the liquid water
in a form of water drop is present in the gas flow channel, no
substantial problem is caused. However, in the case where the
liquid water in a form of membrane spreads widely to thereby cover
a gas flow channel face of the gas diffusion layer 7, the liquid
water prevents the hydrogen supply to the reaction surface, thus
causing portions with low hydrogen concentration. This may lead to
the deterioration of the fuel cell, like the above case on the
oxidant electrode 34 side.
[0064] The inconvenience caused by the liquid water in the gas flow
channel is generally recognized, and a method for discharging the
liquid water is implemented. However, without the liquid water, the
fuel cell is deteriorated. That is, the deterioration phenomenon of
the fuel cell (catalyst) is caused by a shortage of hydrogen in the
fuel electrode 67, and therefore it is important to suppress
occurrence of such a hydrogen shortage portion (for example, a
portion of about 5% or less in volume concentration). Herein, a
cause for lowering the hydrogen concentration in the gas on the
fuel electrode 67 side is that nitrogen contained in the gas on the
oxidant electrode 34 side permeates to the fuel electrode 67 side.
Thereby, it is necessary to properly obtain nitrogen permeation
quantity. Therefore, at first, nitrogen permeation quantity (leak
nitrogen quantity permeating through solid macromolecular membrane)
per unit time relative to each of physical quantities (nitrogen
partial pressure, temperature, and humidity) was checked through
experimentations or simulations, with the results shown in FIG. 5
and FIG. 6.
[0065] FIG. 5 is an explanatory view showing leak nitrogen quantity
relative to nitrogen partial pressure difference between the
oxidant electrode 34 and the fuel electrode 67. FIG. 6 is an
explanatory view showing the relation between an ambient humidity
and a leak nitrogen quantity according to ambient temperatures,
where as denoted by a broken line arrow, the leak nitrogen quantity
relative to the ambient humidity is increased according to an
increase in the ambient temperature, that is, Temp1, Temp2, Temp3
and Temp4. As shown in FIG. 5, the nitrogen quantity permeating
from the oxidant electrode 34 side to the fuel electrode 67 side
(leak nitrogen quantity) is larger as the nitrogen partial pressure
difference is larger. Moreover, as shown in FIG. 6, the nitrogen
quantity permeating from the oxidant electrode 34 side to the fuel
electrode 67 side (leak nitrogen quantity) is larger as the
humidity and temperature at the fuel electrode 67 are higher.
[0066] As set forth above, in the fuel cell, the nitrogen permeated
to the fuel electrode 67 rides on the flow of the supplied hydrogen
and then stays in such a manner as to be pushed into the downstream
side (outlet side). Then, according to the present first
embodiment, causing a forced convection current to agitate hydrogen
with nitrogen suppresses occurrence of the shortage portion where
the hydrogen concentration is locally low.
[0067] FIG. 7 is an explanatory view schematically showing an
agitation state of hydrogen with the unreactive gas (mainly,
nitrogen). As a method for implementing agitation by the forced
convection current, for example, the hydrogen pressure on the fuel
electrode 67 side of the fuel cell stack 1 is rendered lower than
the hydrogen supply pressure, to thereby cause a predetermined
differential pressure between inside and outside of the fuel cell
stack 1. Then, momentarily releasing the predetermined differential
pressure can momentarily secure a large supply quantity (flow
velocity) of hydrogen flowing into the fuel cell stack 1. With
this, as shown in FIG. 7(a), the agitation between hydrogen and
nitrogen becomes possible. When a turbulent flow is obtained, an
effect of the agitation is larger although such effect depends on
the size of the gas flow channel in the fuel cell. Moreover, even
in the case of a laminar flow, since nitrogen is pushed to the
capacity portion 12 disposed at a downstream of the fuel cell stack
1 in the hydrogen system, the gas in the fuel cell is replaced with
hydrogen. Moreover, since the pressure is lowered in the entire gas
flow channel, hydrogen can be distributed to the entire area of the
gas flow channel until the pressure of the fuel electrode 67
becomes equal to the supply pressure.
[0068] For obtaining a constant differential pressure, it is also
possible to supply hydrogen to the fuel cell stack 1 in generating
power while momentarily causing a large pressure. However, for more
easily obtaining the differential pressure, as shown in FIG. 7(b),
the hydrogen supply is stopped by means of the hydrogen pressure
adjusting valve 11 (closing valve operation) at a timing T1 while
continuing the generation of the fuel cell stack 1. Then, a keeping
time is set until a predetermined differential pressure (pressure
width) .DELTA.P1 is obtained, to thereby secure the differential
pressure. After the predetermined differential pressure .DELTA.P1
is obtained (timing T2), hydrogen is supplied by means of the
hydrogen pressure adjusting valve 11 (opening valve operation).
With this, a large supply quantity (flow velocity) is momentarily
caused, which can implement the agitation. Moreover, repeating the
above pressure change patterns (first pressure change pattern) at a
period C implements the closing valve operation at a timing T3 and
the opening valve operation at a timing T4. With this, hydrogen can
be pulsatorily supplied. The differential pressure .DELTA.P1 is,
for example, in a range of 5 kPa to 8 kPa. In view of the fuel cell
stack characteristics, the gas's agitation characteristics, and the
like, experiments or simulations can set the optimum value of the
differential pressure .DELTA.P1. The differential pressure
.DELTA.P1 necessary for the gas agitation is set smaller than the
differential pressure necessary for an after-described liquid water
discharge.
[0069] The above gas agitation can suppress the occurrence of the
hydrogen shortage portion. However, in the case of the generation
continuing for a long time, the generated water or condensed water
is accumulated, thus blocking the fuel electrode 67 side gas flow
channel in the fuel cell. Then, according to the present first
embodiment, flowing hydrogen into the fuel electrode 67 discharges
the liquid water which blocks the gas flow channel out of the fuel
cell.
[0070] FIG. 8 is an explanatory view showing a liquid water
discharge state. As a method of implementing the liquid water
discharge by supplying hydrogen, for example, the hydrogen pressure
on the fuel electrode 67 side of the fuel cell stack 1 is rendered
lower than the hydrogen supply pressure, to thereby cause a
predetermined differential pressure between inside and outside of
the fuel cell stack 1. Then, momentarily releasing the constant
differential pressure can momentarily secure a large supply
quantity (flow velocity) of the fuel gas which flows into the fuel
cell stack 1. With this, as shown in FIG. 8(a), the liquid water
can be discharged from the gas flow channel.
[0071] The differential pressure necessary for the liquid water
discharge is required to be larger than the differential pressure
necessary for the above gas agitation. Meanwhile, the frequency
required for the liquid water discharge is lower than the frequency
required for the gas agitation. Then, as shown in FIG. 8(b), a
plurality of pressure change patterns required for the gas
agitation are implemented, then, at a timing Tm, the hydrogen
supply is stopped by means of the hydrogen pressure adjusting valve
11 (closing valve operation). Then, a keeping time is set until a
predetermined differential pressure (pressure width) .DELTA.P2 is
obtained, to thereby secure the differential pressure. After the
differential pressure .DELTA.P2 is obtained (timing Tn), hydrogen
is supplied by means of the hydrogen pressure adjusting valve 11
(opening valve operation). With this, a large flow velocity is
momentarily caused, thus the liquid water discharge can be
implemented. Herein, the above pressure change pattern (second
pressure change pattern) is periodically repeated, like the first
pressure change pattern required for the gas agitation. However,
compared with the first pressure pattern required for the gas
agitation, the second pressure change pattern required for the
liquid water discharge has lower implementation frequency. The
differential pressure .DELTA.P2 is, for example, in a range of 20
kPa to 30 kPa. In view of the fuel cell stack 1's characteristics,
the liquid water discharge characteristics and the like,
experiments or simulations can set the optimum value of the
differential pressure .DELTA.P2. The differential pressure
.DELTA.P2 required for the liquid water discharge is set larger
than the differential pressure .DELTA.P1 required for the above gas
agitation.
[0072] Moreover, as shown in FIG. 8(c), a plurality of the pressure
change patterns required for the gas agitation are implemented and
then, at the timing Tm, the hydrogen supply is stopped by means of
the hydrogen pressure adjusting valve 11 (closing valve operation).
Then, a keeping time is set until the predetermined differential
pressure (pressure width) .DELTA.P1 is obtained, to thereby secure
the differential pressure. After the differential pressure
.DELTA.P1 is obtained (timing Tn), the opening degree of the
hydrogen pressure adjusting valve 11 is rendered larger than that
at the timing Tm, to thereby supply the hydrogen (opening valve
operation). With this, the gas is supplied at a pressure higher
than the pressure at the timing Tm, to thereby cause the
predetermined differential pressure (pressure width) .DELTA.P2
(timing To). Then, at a timing Tp, the hydrogen supply is stopped
by means of the hydrogen pressure adjusting valve 11 (closing valve
operation). Then, a keeping time is set until the predetermined
differential pressure (pressure width) .DELTA.P2 is obtained, to
thereby secure the differential pressure. After the differential
pressure .DELTA.P2 is obtained (timing Tq), hydrogen is supplied by
means of the hydrogen pressure adjusting valve 11 (opening valve
operation). At that time, it is preferable that hydrogen is
supplied at the opening degree same as that at the timing Tm. Then,
at a timing Tr, the pressure returns to the same pressure as that
at the timing Tm. After the timing Tr, the pressure change patterns
same as those before the timing Tm are implemented. Even in the
case of the above operations, a large flow velocity is momentarily
caused, so that the liquid water discharge can be implemented.
[0073] Moreover, as shown in FIG. 8(d), a plurality of pressure
change patterns required for the gas agitation are implemented and
then, at the timing Trn, the hydrogen supply is stopped by means of
the hydrogen pressure adjusting valve 11 (closing valve operation).
Then, a keeping time is set until a differential pressure larger
than the predetermined differential pressure (pressure width)
.DELTA.P1 is obtained. When a differential pressure larger than the
differential pressure .DELTA.P1 is obtained (timing Tn), the
opening degree of the hydrogen pressure adjusting valve 11 is
rendered larger than that at the timing Tm, to thereby supply the
hydrogen (opening valve operation). With this, the gas is supplied
at the pressure higher than that at the timing Tm, to thereby cause
the predetermined differential pressure (pressure width) .DELTA.P2
(timing To). Next, at the timing Tp, the hydrogen supply is stopped
by means of the hydrogen pressure adjusting valve 11 (closing valve
operation). Then, a keeping time is set until a predetermined
differential pressure (pressure width) .DELTA.P3 is obtained, to
thereby secure the differential pressure. Herein, it is preferable
that the lower pressure limit at the obtaining of the differential
pressure .DELTA.P3 is set to the lower pressure limit at the
obtaining of the differential pressure .DELTA.P1. Next, after the
differential pressure .DELTA.P3 is obtained (timing Tq), hydrogen
is supplied by means of the hydrogen pressure adjusting valve 11
(opening valve operation). At that time, it is preferable that
hydrogen is supplied at the opening degree same as that at the
timing Tm. Then, at the timing Tr, the pressure returns to the same
pressure as that at the timing Tm. After the timing Tr, the
pressure change patterns same as those before the timing Tm are
implemented. Even when the above operations are implemented, a
large flow velocity can be momentarily caused, to thereby implement
the liquid water discharge.
[0074] As set forth above, according to the first embodiment, the
controller 40 controls the fuel gas supplier HS (10, 11, L1), to
thereby supply hydrogen to the fuel electrode 67 of the fuel cell
stack 1, and based on the first pressure change pattern which
implements the pressure change at the first pressure width
.DELTA.P1 and on the second pressure change pattern which
implements the pressure change at the second pressure width
.DELTA.P2, the controller 40 periodically changes the hydrogen
pressure in the fuel electrode 67 of the fuel cell stack 1.
[0075] With the above structure, the first pressure change pattern
having a small pressure width is used in addition to the second
pressure change pattern, to thereby be able to agitate the fuel
electrode 67 side gas without applying a large stress to the
individual fuel cell of the fuel cell stack 1. With this, the fuel
electrode 67 side gas can be made even. Thereby, the fuel cell
stack 1's deterioration attributable to the partial decrease of the
hydrogen concentration can be suppressed. Moreover, providing the
second pressure change pattern can discharge the liquid water and
the like which cannot be discharged by the first pressure change
pattern. With this, the fuel cell stack 1's deterioration
attributable to the liquid water can be suppressed.
[0076] Moreover, the fuel cell system 100 of the first embodiment
adopts the closed system where the fuel electrode off-gas
discharged from the fuel electrode 67 side of the fuel cell stack 1
is restricted. With the above structure, impurities are likely to
decrease the hydrogen concentration in the fuel electrode 67 side
gas flow channel. However, implementing the above control can make
the fuel electrode 67 side gas even.
[0077] Moreover, according to the first embodiment, the controller
40 implements the second pressure change pattern after implementing
a plurality of first pressure change patterns. With the above
structure, the frequency of applying a large stress to the
individual cell of the fuel cell stack 1 can be decreased, while
compatibly implementing the gas agitation and liquid water
discharge on the fuel electrode 67 side. Moreover, since the
implementation frequency of the first pressure change pattern which
implements the gas agitation is high, the gas agitation can
effectively be implemented even when the generation is continuously
implemented. With this, as shown in FIG. 9, even when the
generation is continuously implemented, the current value in the
power generation surface is substantially equal, thus the current
value drop on the outlet side of the gas flow channel and the
current concentration on the inlet side of the gas flow channel can
be suppressed.
[0078] Moreover, according to the first embodiment, the controller
40 stops the hydrogen supply to the fuel cell stack 1 in a state
that the generation of the fuel cell stack 1 is implemented by
supplying hydrogen at the predetermined operation pressure,
moreover, on a condition that the hydrogen pressure of the fuel
electrode 67 is decreased by the predetermined pressure width
(.DELTA.P1, .DELTA.P2), the controller 40 restarts the hydrogen
supply, to thereby change the hydrogen pressure in the fuel
electrode 67. With the above structure, the hydrogen pressure
adjusting valve 11 can easily implement the pressure change, so
that a simple control system can be accomplished.
[0079] Moreover, the fuel cell system 100 of the first embodiment
has the fuel electrode off-gas flow channel L2, the capacity
portion 12 and the purge valve 14. In this case, the capacity
portion 12 functions as a space (capacity Rs: after-described FIG.
20) for storing the fuel electrode off-gas from the fuel electrode
67 side, that is, nitrogen or liquid water. With this, though the
fuel cell system 100 has substantially a closed system, opening the
purge valve 14 when necessary can also discharge the impurities
(such as nitrogen which is relatively increased) outside. That is,
the nitrogen leak is caused until the nitrogen partial pressure
difference is removed. However, when the hydrogen concentration is
to be kept at more than or equal to the predetermined value on the
fuel electrode 67 side, the flow rate corresponding to the leak
quantity can be discharged outside. Herein, the flow rate in this
case is sufficiently small, thus unlikely to cause an influence on
the pressure change necessary for the gas agitation in the fuel
electrode 67, and in addition, diluting by the oxidant electrode 34
off-gas can be easily implemented. However, the entire pressure on
the fuel electrode 67 side may be increased such that the
generation can be implemented even when the nitrogen partial
pressure is brought into an equilibrium state. In this case, a
simple closed system can be adopted.
[0080] Moreover, when the hydrogen supply is stopped, the speed at
which the hydrogen pressure in the fuel electrode 67 is decreased
is determined by the flow channel capacity in the fuel cell stack
1. When a rapid pressure decrease is not desired due to a request
associated with controlling of the fuel cell system 100, changing
the capacity of the hydrogen supply flow channel L1 to the fuel
cell stack 1 or the capacity of the capacity portion 12 of the fuel
electrode off-gas flow channel L2 can control the pressure change
time.
Second Embodiment
[0081] Hereinafter, the fuel cell system 100 according to the
second embodiment of the present invention is to be set forth. The
fuel cell system 100 according to the second embodiment is
different from the fuel cell system 100 according to the first
embodiment in terms that the hydrogen quantity which is supplied to
the fuel electrode 67 of the fuel cell stack 1 attributable to the
pressure change by the pressure change pattern is variably set
according to the operation condition of the fuel cell system 100.
In addition, the structure of the fuel cell system 100 according to
the second embodiment is the same as that according to the first
embodiment, therefore repeated explanations are to be omitted and
differences are to be mainly set forth below.
[0082] FIG. 10 is a flowchart showing a control method of the fuel
cell system 100 according to the second embodiment of the present
invention, specifically, showing process procedures of a method of
supplying hydrogen to the fuel electrode 67. The controller 40
implements the processes shown in this flowchart.
[0083] At first, at a step 1 (S1), the controller 40 detects the
operation conditions of the fuel cell stack 1. The operation
conditions detected at this step 1 include an operation load of the
fuel cell stack 1, an operation temperature of the fuel cell stack
1, and an operation pressure of the fuel cell stack 1 (operation
pressure of the oxidant electrode 34). In view of the vehicle side
required power specified from the vehicle speed or acceleration
opening degree, the required power of accessories, and the like,
the operation load of the fuel cell stack 1 can be calculated.
Moreover, the operation temperature of the fuel cell stack 1 can be
detected by the stack temperature sensor 43. In terms of the
operation pressure of the fuel cell stack 1, a certain standard
value irrespective of the above operation load is set in advance,
or variable values according to the operation load are set in
advance. Therefore, by referring to these values, the operation
pressure of the fuel cell stack 1 can be detected.
[0084] At a step 2 (S2), the controller 40 determines whether or
not the operation condition thus detected at this time is changed
compared to the operation condition detected in advance. When the
determination is positive, that is, when the operation condition is
changed, the routine proceeds to a step 3 (S3). Meanwhile, when the
determination is negative in the step 2, that is, when the
operation condition is not changed, the routine skips the process
at the step 3, to thereby proceed to a step 4 (S4).
[0085] At the step 3, the controller 40 sets the pressure change
pattern based on the operation condition. As set forth according to
the first embodiment, the controller 40 implements a plurality of
first pressure change patterns necessary for the gas agitation and
then implements the second pressure change pattern necessary for
the liquid water discharge. By repeating the first and second
pressure change patterns as one set, the controller 40 implements
the hydrogen supply. By the way, in the supply manner involving the
pressure change, the hydrogen quantity supplied to the fuel
electrode 67 attributable to the pressure change pulsatorily
varies, thus applying repeated loads to the solid polymer
electrolyte membrane 2, which acts as a stress. Then, in a scene
where the cross leak from the oxidant electrode 34 is small, it is
preferable that the hydrogen quantity supplied to the fuel
electrode 67 attributable to the above pressure change is made
small to thereby decrease the load applied to the solid polymer
electrolyte membrane 2. Meanwhile, in a scene where the cross leak
is large, it is preferable to positively implement the pressure
change to thereby pulsatorily vary the hydrogen quantity supplied
to the fuel electrode 67 attributable to the pressure change, thus
implementing the gas agitation and liquid water discharge.
[0086] Ordinarily, the smaller the operation load of the fuel cell
stack 1 is, the lower the operation temperature of the fuel cell
stack 1 is, and the lower the operation pressure of the fuel cell
stack 1 (specifically, operation pressure of the oxidant electrode
34) is; the smaller the cross leaked nitrogen quantity is. Then,
when the operation condition is changed according to any of the
above cases, the hydrogen quantity supplied to the fuel electrode
67 attributable to the pressure change is decreased. On the
contrary, the larger the operation load of the fuel cell stack 1
is, the higher the operation temperature of the fuel cell stack 1
is, and the higher the operation pressure of the fuel cell stack 1
(specifically, operation pressure of the oxidant electrode 34) is;
the larger the cross leaked nitrogen quantity is. Then, when the
operation condition is changed according to any of the above cases,
the hydrogen quantity supplied to the fuel electrode 67
attributable to the pressure change is increased.
[0087] For setting small the hydrogen quantity supplied to the fuel
electrode 67 attributable to the pressure change, the basic control
patterns are to be modified in the following manner.
[0088] As the first control method, as shown in FIG. 11, a valve
closing time T of the hydrogen pressure adjusting valve 11 is set
longer than the valve closing time of the basic control pattern. In
other words, the basic control pattern is to be so modified that
the implementation period of the pressure change is set longer.
[0089] As the second control method, as shown in FIG. 12,
differential pressures (pressure widths) .DELTA.P11, .DELTA.P21 of
the pressure control pattern are set smaller than the differential
pressures (pressure widths) .DELTA.P1, .DELTA.P2 of the pressure
control pattern in the basic control pattern.
[0090] As the third control method, as shown in FIG. 13, the
implementation frequency of the second pressure change pattern
(necessary for the liquid water discharge) relative to the first
pressure change pattern (necessary for the gas agitation) is
decreased compared with the implementation frequency of the second
pressure change pattern of the basic control pattern.
[0091] Contrary to this, in the case of setting large the hydrogen
quantity supplied to the fuel electrode 67 attributable to the
pressure change, each of the first to third control methods is to
be controlled in the opposite direction.
[0092] According to the changed operation conditions, the
controller 40 modifies the basic control pattern based on any one
of the first to third control methods or a combination thereof.
Then, the controller 40 sets the thus modified control pattern as a
present control pattern.
[0093] At the step 4, the controller 40 implements the hydrogen
supply based on the control pattern which is presently set.
[0094] At a step 5 (S5), the controller 40 determines whether or
not to end the operation of the fuel cell system 100. Specifically,
the controller 40 determines whether or not an off-signal is input
from an ignition switch. When the determination is positive at the
step 5, that is, when the operation of the fuel cell system 100 is
to be ended, the present control is ended. Meanwhile, when the
determination is negative at the step 5, that is, when the
operation of the fuel cell system 100 is not to be ended, the
routine returns to the processes at the step 1.
[0095] As set forth above according to the second embodiment, with
respect to the fuel cell system 100, the hydrogen quantity supplied
to the fuel electrode 67 attributable to the pressure change is set
small based on the operation condition of the fuel cell system 100.
With the above structure, while the gas agitation and liquid water
discharge of the fuel electrode 67 are implemented, it is possible
to decrease the repeated loads to the individual fuel cell of the
fuel cell stack 1.
Third Embodiment
[0096] Hereinafter, the fuel cell system 100 according to the third
embodiment of the present invention is to be set forth. Herein, the
structure of the fuel cell system 100 according to the third
embodiment is like those according to the first and second
embodiments, therefore repeated explanations are to be omitted and
differences are to be mainly set forth.
[0097] The controller 40 controls the fuel cell system 100 in the
following manner. The controller 40 supplies air and hydrogen to
the fuel cell stack 1, to thereby implement the generation by the
fuel cell stack 1. In this case, the controller 40 supplies air and
hydrogen such that the pressure of each of air and hydrogen which
are supplied to the fuel cell stack 1 becomes a predetermined
operation pressure. This operation pressure is set, for example, as
a certain standard value irrespective of the power generated by the
fuel cell stack 1, or set as variable values according to the power
generated by the fuel cell stack 1.
[0098] According to the third embodiment, in terms of the air
supply to the oxidant electrode 34, the controller 40 implements
the pressure control according to the predetermined operation
pressure. Meanwhile, in terms of the hydrogen supply to the fuel
electrode 67, the controller 40 controls the supply-stop of
hydrogen according to the control patterns for implementing the
pressure rise-fall within the range between an upper limit pressure
P1 and a lower limit pressure P2. Then, the controller 40 repeats
operations according to the control pattern, to thereby as shown in
FIG. 14, supply hydrogen to the fuel electrode 67 while
periodically changing the hydrogen pressure in the fuel electrode
67 of the fuel cell stack 1.
[0099] Specifically, on the condition that the hydrogen pressure of
the fuel electrode 67 reaches the upper limit pressure P1 and the
hydrogen concentration sufficient for implementing the generation
is secured in the fuel electrode 67, the controller 40 controls the
hydrogen pressure adjusting valve 11 to the minimum opening degree,
to thereby stop the hydrogen supply to the fuel cell stack 1. When
from the fuel cell stack 1 by way of the output takeout device 30,
the controller 40 continues to take out a load current which
corresponds to the load required by the fuel cell system 100,
hydrogen is consumed by the generation reaction, to thereby lower
the hydrogen pressure of the fuel electrode 67.
[0100] Next, on the condition that the hydrogen pressure of the
fuel electrode 67 is decreased to the lower limit pressure P2, the
controller 40 controls the hydrogen pressure adjusting valve 11 to
the maximum opening degree, to thereby restart the hydrogen supply
to the fuel cell stack 1. With this, the hydrogen pressure in the
fuel electrode 67 is increased. Then, on the condition that the
hydrogen pressure reaches (comes back to) the upper limit pressure
P1, the controller 40 controls the hydrogen pressure adjusting
valve 11 to the minimum opening degree, to thereby stop again the
hydrogen supply. By repeating the above series of processes as
one-cycle control pattern, the controller 40 supplies hydrogen to
the fuel electrode 67 of the fuel cell stack 1 while periodically
changing the hydrogen pressure.
[0101] Herein, the upper limit pressure P1 and the lower limit
pressure P2 are respectively set based on, for example, a specified
operation pressure. It is possible to monitor the hydrogen pressure
of the fuel electrode 67 of the fuel cell stack 1 by referring to
values detected by the hydrogen pressure sensor 41. Moreover, for
increasing the pressure, it is desired that the hydrogen pressure
on the upstream side of the hydrogen pressure adjusting valve 11 is
set sufficiently high in advance to thereby increase a
pressure-increasing speed as high as possible. For example, the
pressure increase period from the lower limit pressure P2 to the
upper limit pressure P1 is set to be in a range from 0.1 sec to
about 0.5 sec. Meanwhile, the time from the upper limit pressure P1
to the lower limit pressure P2 is in a range from 1 sec to about 10
sec, however, the above time depends on the upper limit pressure
P1, the lower limit pressure P2 and the current value taken out of
the fuel cell stack 1, that is, the hydrogen consumption speed.
[0102] In the hydrogen supply control involving the above
periodical pressure rise-fall, as one of the features of the third
embodiment, a first keeping time Tp1 and a second keeping time Tp2
for keeping the pressure of the fuel electrode 67 respectively at
the upper limit pressure P1 and the lower limit pressure P2 can be
set to the control pattern. The controller 40 can arbitrarily set
the first keeping time Tp1 and second keeping time Tp2 in a range
from zero to a predetermined value.
[0103] As shown in FIG. 15, the first keeping time Tp1 is a time
for keeping the pressure of the fuel electrode 67 at the upper
limit pressure P1 before implementing the first process for
decreasing the pressure of the fuel electrode 67 from the upper
limit pressure P1 to the lower limit pressure P2. Specifically, on
the condition that the pressure of the fuel electrode 67 is
decreased to the lower limit pressure P2, the controller 40
controls the opening degree Ot of the hydrogen pressure adjusting
valve 11 to the maximum opening degree O1, to thereby restart the
hydrogen supply to the fuel cell stack 1, thus increasing the
pressure of the fuel electrode 67. On the condition that the
pressure of the fuel electrode 67 reaches the upper limit pressure
P1, the controller 40 decreases the opening degree Ot of the
hydrogen pressure adjusting valve 11 from the maximum opening
degree O1 to a predetermined opening degree, to thereby keep the
pressure of the fuel electrode 67 at the upper limit pressure P1.
Then, on the condition that the first keeping time Tp1 elapsed from
the timing at which the pressure of the fuel electrode 67 reaches
the upper limit pressure P1, the controller 40 controls the opening
degree Ot of the hydrogen pressure adjusting valve 11 to the
minimum opening degree O2, to thereby stop the hydrogen supply to
the fuel cell stack 1.
[0104] Contrary to the above, as shown in FIG. 16, the second
keeping time Tp2 is a time for keeping the pressure of the fuel
electrode 67 at the lower limit pressure P2 before implementing the
second process for increasing the hydrogen pressure of the fuel
electrode 67 from the lower limit pressure P2 to the upper limit
pressure P1. Specifically, on the condition that the pressure of
the fuel electrode 67 reaches the upper limit pressure P1, the
controller 40 controls the opening degree Ot of the hydrogen
pressure adjusting valve 11 to the minimum opening degree O2, to
thereby stop the hydrogen supply to the fuel cell stack 1. On the
condition that the hydrogen pressure of the fuel electrode 67 is
deceased to the lower limit pressure P2, the controller 40
increases the opening degree Ot of the hydrogen pressure adjusting
valve 11 from the minimum opening degree O2 to a predetermined
opening degree, to thereby keep the pressure of the fuel electrode
67 at the lower limit pressure P2. Then, on the condition that the
second keeping time Tp2 elapsed from the timing at which the
pressure of the fuel electrode 67 reaches the lower limit pressure
P2, the controller 40 controls the opening degree Ot of the
hydrogen pressure adjusting valve 11 to the maximum opening degree
O1, to thereby restart the hydrogen supply to the fuel cell stack
1, thus increasing the pressure of the fuel electrode 67.
[0105] FIG. 17 is an explanatory view showing the load relative to
each of the first keeping time Tp1 and the second keeping time Tp2.
For example, in the case of a low load (for example, a condition of
taking out the load current up to about 1/3 of a rated load
current) as an operation scene of the fuel cell system 100, each of
the first keeping time Tp1 and the second keeping time Tp2 is set
at zero. Then, in the case of an intermediate load (for example, a
condition of taking out the load current larger than about 1/3 to
smaller than about 2/3 of the rated load current), the first
keeping time Tp1 is set at zero while the second keeping time Tp2
is so set as to be increased as the load is higher with zero as a
start point. Moreover, in the case of a high load (for example, a
condition of taking out the load current larger than or equal to
about 2/3 of the rated load current), the first keeping time Tp1 is
so set as to be increased as the load is higher with zero as a
start point while the second keeping time Tp2 is set constant. In
this way, the controller 40 can determine the first keeping time
Tp1 and the second keeping time Tp2 according to the load
conditions. In other words, according to the load, the controller
40 can select whether to keep the pressure of the fuel electrode 67
at the upper limit pressure P1 or at the lower limit pressure
P2.
[0106] As set forth above, according to the third embodiment, as
shown in FIG. 17, when the required load is high (load current is
large), the controller 40 increases the hydrogen supply quantity in
the implementation period of one control pattern, compared with
when the required load is low (load current is small). In the
operation scene such as high load, the hydrogen consumption
quantity is likely to be large. Therefore, for covering the
hydrogen supply, the number of implementations of the pressure
rise-fall corresponding to one control pattern may be increased.
However, according to the third embodiment, the hydrogen supply
quantity in the implementation period of one control pattern is
increased, thus the increase of the number of implementations of
the pressure rise-fall per unit time can be suppressed. With this,
the stress applied to the fuel cell stack 1 or hydrogen-associated
components can be relieved, thus the deterioration of the fuel cell
system 100 can be suppressed.
[0107] Moreover, according to the third embodiment, as shown in
FIG. 16, the first keeping time Tp1 for keeping the pressure of the
fuel electrode 67 at the upper limit pressure P1 before
implementing the first process and the second keeping time Tp2 for
keeping the pressure of the fuel electrode 67 at the lower limit
pressure P2 before implementing the second process can be set to
the control pattern. Then, the higher the required load is, the
longer the controller 40 sets the first keeping time Tp1 or the
second keeping time Tp2. With the required load being high, the
hydrogen consumption quantity is increased, to thereby increase
pressure drop speed in the first process. However, according to the
third embodiment, the larger the required load is, the longer the
first keeping time Tp1 and second keeping time Tp2 are set. With
this, the period from the timing at which the pressure of the fuel
electrode 67 reaches the upper limit pressure P1 to the timing at
which the pressure of the fuel electrode 67 is returned from the
lower limit pressure P2 to the upper limit pressure P1 can be set
long. That is, setting long the first keeping time Tp1 and second
keeping time Tp2 can elongate the implementation period of one
control pattern, thus suppressing the increase in the number of
implementations of the pressure rise-fall per unit time. With this,
the stress applied to the fuel cell stack 1 or hydrogen-associated
components can be relieved, thus suppressing the deterioration of
the fuel cell system 100.
[0108] Especially, it is preferable that the higher the required
load is, the longer the controller 40 sets the first keeping time
Tp1. With the required load increased, as the case may be, it is
difficult to secure the hydrogen partial pressure in the fuel
electrode 67. Therefore, setting long the first keeping time Tp1
for the upper limit pressure P1 can bring about an effect that the
hydrogen partial pressure can be secured with ease even when the
required load is high.
[0109] Moreover, according to the third embodiment, the higher the
required load is in the required load's region from the low load to
the intermediate load, the longer the second keeping time Tp2 is
set (lower in FIG. 17). From the low load to the intermediate load,
the liquid water is likely to be stored in the fuel electrode 67.
Setting long the second keeping time Tp2 for the lower limit
pressure P2 can enhance accuracy of implementing the liquid water
discharge process. Moreover, it is preferable that the higher the
required load is in the required load's region from the
intermediate load to the high load, the longer the controller 40
sets the first keeping time Tp1 (upper in FIG. 17). When the
required load is increased, securing the hydrogen partial pressure
in the fuel electrode 67 is, as the case may be, difficult.
Therefore, setting long the first keeping time Tp1 for the upper
limit pressure P1 can bring about an effect that the hydrogen
partial pressure can be secured with ease even when the required
load is high.
[0110] In addition, as shown in FIG. 18, the hydrogen partial
pressure may be secured in the following manner: the higher the
impurity concentration such as the nitrogen concentration in the
fuel electrode 67 is (namely, immediately after the fuel cell
system 100 is started), the longer the first keeping time Tp1 for
keeping the upper limit pressure P1 is set. In this case, the
longer the time until the fuel cell system 100 restarts after stop,
the higher the inactive gas concentration in the fuel electrode 67
is. Therefore, the first keeping time Tp1 for keeping the upper
limit pressure P1 may be made variable by measuring the stop period
of the fuel cell system 100 or by measuring the nitrogen
concentration in the fuel electrode 67 at the start of the fuel
cell system 100.
[0111] Moreover, in the fuel cell system 100 that adopts no idling
(or idle reduction) which, at the low load and the like,
temporarily stops generation of the fuel cell stack 1 and allows
traveling by means of a power of a secondary battery, the nitrogen
concentration in the fuel electrode 67 is high even immediately
after the recovery from the no idling (or idle reduction). Then, in
such a scene as well, the first keeping time Tp1 may be set
long.
Fourth Embodiment
[0112] Hereinafter, the fuel cell system 100 according to the
fourth embodiment of the present invention is to be set forth.
Herein, the structure of the fuel cell system 100 according to the
fourth embodiment is like those according to the first to third
embodiments, therefore repeated explanations are to be omitted.
According to the fourth embodiment, a method of setting the upper
limit pressure P1 and lower limit pressure P2 is to be set
forth.
First Setting Method
[0113] With respect to the first setting method, the upper limit
pressure P1 and the lower limit pressure P2 can be set according to
the load current. Based on the vehicle speed, the acceleration
operation quantity of the driver, and the information about the
secondary battery, the controller 40 determines the fuel cell stack
1's target generation power as the required load for the fuel cell
system 100. Based on the target generation power, the controller 40
calculates the load current which is a current value to be taken
out from the fuel cell stack 1.
[0114] FIG. 19 is an explanatory view showing the upper limit
pressure P1 and lower limit pressure P2 relative to the load
current Ct. An operation pressure Psa for supplying the reactive
gas necessary for taking out the load current Ct from the fuel cell
stack 1 can be defined through experiments or simulations in view
of the fuel cell system 100's characteristics such as the fuel cell
stack 1, hydrogen system, air system and the like. Cr in FIG. 19
denotes a rated load current Cr {likewise, in an after-described
FIG. 20(b)}.
[0115] For supplying air to the oxidant electrode 34, the operation
pressure Psa is set as a target operation pressure.
[0116] Contrary to this, for supplying hydrogen to the fuel
electrode 67, the upper limit pressure P1 and the lower limit
pressure P2 are respectively set based on the operation pressure
Psa. Herein, the upper limit pressure P1 and the lower limit
pressure P2 are so set that the larger the load current Ct is, the
larger the differential pressure between the upper limit pressure
P1 and the lower limit pressure P2 is, that is, the larger the
pressure change width in the gas supply operation is.
[0117] With the above structure, the higher the required load is,
the more the hydrogen supply quantity in the implementation period
of one control pattern can be increased. With this, the increase in
the number of implementations of the pressure rise-fall per unit
time can be suppressed. With this, the deterioration of the fuel
cell system 100 can be suppressed.
Second Setting Method
[0118] As the second setting method, the upper limit pressure P1
and the lower limit pressure P2 may be set in view of the
generation safety of the fuel cell stack 1. In the case of the low
load, that is, when the load current is small, the differential
pressure between the upper limit pressure P1 and the lower limit
pressure P2 is so set as to be relatively small, for example, about
50 kPa. In this case, the average hydrogen concentration in the
individual fuel cell is about 40%. Contrary to this, in the case of
the high load, that is, when the load current is large, the supply
pressure on each of the oxidant electrode 34 side and the fuel
electrode 67 side is to be entirely increased since the gas
pressure made larger can increase the generation efficiency. In
addition, the difference between the upper limit pressure P1 and
the lower limit pressure P2 is set at about 100 kPa. In this case,
the fuel cell stack 1 is operated with the average hydrogen
concentration of about 75% in the individual fuel cell.
[0119] According to the fourth embodiment which implements the
periodical pressure rise-fall, the atmosphere in the fuel cell
stack 1 (fuel electrode 67) is in a condition that the hydrogen
concentration is low at the timing of the lower limit pressure P2
while the hydrogen concentration is high at the timing of the upper
limit pressure P1. That is, increasing the pressure from the lower
limit pressure P2 to the upper limit pressure P1 introduces a high
hydrogen concentration gas to the fuel electrode 67, to thereby
push a low hydrogen concentration gas from the fuel cell stack 1 to
the capacity portion 12. Moreover, the high hydrogen concentration
gas agitates the gas in the fuel electrode 67.
[0120] FIG. 20(a) and FIG. 20(b) are explanatory views
schematically showing the fuel electrode 67 side capacity Rs and
the capacity Rt of the capacity portion 12 in the fuel cell stack
1. For example, in the case where the upper limit pressure P1 is
set at 200 kPa (absolute pressure) and the lower limit pressure P2
is set at 150 kPa (absolute pressure), the pressure ratio P1/P2
between the upper limit pressure P1 and the lower limit pressure P2
is about 1.33. In this case, as shown in FIG. 20(a), the pressure
increased from the lower limit pressure P2 to the upper limit
pressure P1 allows an inflow of additional hydrogen to about 1/4 of
the capacity (specifically, the capacity of the fuel cell stack 1
and the capacity of the capacity portion 12) of the fuel system
(=hydrogen system), that is, to 50% point of the fuel cell stack 1
[hereinafter, this condition is expressed as hydrogen exchange
ratio 0.5 {refer to FIG. 20(b)}].
[0121] In the case of the low load, the hydrogen consumption speed
is low, therefore, the hydrogen exchange ratio of around the above
degree can implement the generation of the fuel cell stack 1. In
this scene, for example, the hydrogen concentration of the
time-averaged hydrogen electrode off-gas is about 40%. Contrary to
this, in the case of the high load, the pressure ratio P1/P2 (for
example, 2 or more) which replaces the entire fuel electrode 67 of
the fuel cell stack 1 with the additional hydrogen is preferable,
that is, the hydrogen exchange ratio of about 1 is preferable.
Although the discharged hydrogen concentration is preferably
suppressed low, the hydrogen concentration greater than or equal to
a predetermined value is necessary for stably implementing the
generation (for example, about 75% or more is necessary) since the
hydrogen consumption speed is high.
[0122] In the above cases, for adjusting the hydrogen
concentration, the purge valve 14 opens the fuel electrode off-gas
flow channel L2. With this, such a minor amount of gas (flow rate)
can be continuously or intermittently discharged from the purge
valve 14 as not to prevent the hydrogen supply attributable to the
periodical pressure rise-fall. Since the gas (flow rate) discharged
from the purge valve 14 is minor, the gas is diluted by a cathode
side exhaust (off gas) and then is safely discharged out of the
system. Opening of the purge valve 14 is implemented for
discharging the impurities (nitrogen or steam) from the fuel
electrode 67, however, hydrogen is mixed in the fuel electrode 67.
Therefore, it is preferable to effectively discharge the impurities
by suppressing the hydrogen discharge.
[0123] Then, according to the fourth embodiment, in the hydrogen
supply, the purge valve 14 is controlled to the open state
corresponding to the process for increasing the hydrogen pressure
from the lower limit pressure P2 to the upper limit pressure P1
(second process), to thereby open the purge valve 14 (purge
process). Specifically, the controller 40 monitors the pressure of
the fuel electrode 67 of the fuel cell stack 1, and then controls
the purge valve 14 to the open state according to a timing at which
the monitored pressure reaches the lower limit pressure P2,
moreover, the controller 40 controls the purge valve 14 to the
closed state according to a timing at which the monitored pressure
reaches the upper limit pressure P1 (basic control pattern). With
this, the low hydrogen concentration gas is pushed into the
capacity portion 12 from the fuel cell stack 1, and then, the low
hydrogen concentration gas is discharged from the capacity portion
12 by way of the purge valve 14 before the high concentration
hydrogen gas reaches the purge valve 14. With this, many impurities
can be efficiently discharged.
[0124] However, the opening-closing control of the purge valve 14
is not limited to this basic control pattern. Provided that the
purge valve 14 is so controlled to the open state as to include at
least the process of increasing the pressure from the lower limit
pressure P2 to the upper limit pressure P1 (second process), the
opening-closing control of the purge valve 14 is sufficient.
Therefore, the timing for controlling the purge valve 14 to the
closed state can be modified also to a timing which is later than
the timing (hereinafter, referred to as "basic closing timing") at
which the hydrogen pressure reaches the upper limit pressure P1.
For example, in view of a diffusion speed, a boundary between the
high concentration hydrogen and the low concentration hydrogen can
be determined as a constant face within a short time. Then, with
respect to the fuel cell stack 1 and capacity portion 12 during the
hydrogen supply operation, how long time it takes for a boundary
face (what is called a hydrogen front) to reach and up to which
position the boundary face reaches are to be estimated in advance
through experiments or simulations. Then, until the boundary face
reaches the purge valve 14, the timing of controlling the purge
valve 14 to the closed state can be further delayed than the basic
closing timing.
[0125] Moreover, it is not necessary to implement the purge
treatment for each implementation of the control pattern,
specifically, for every pressure increasing process (second
process). For example, on the condition that the hydrogen
concentration in the fuel electrode 67 reaches less than or equal
to a predetermined determination threshold, the purge valve 14 may
be opened according to the subsequent pressure increasing
process.
[0126] Moreover, since the liquid water also is regarded as a
factor for disturbing the generation reaction, the liquid water can
also be discharged. However, compared with the presence of the
inactive gas, the time for the liquid water to cause an influence
is longer. Therefore, it is preferable to implement the liquid
water discharge treatment once in a plurality of periodical
pressure rise-fall operations or at predetermined time intervals,
instead of every periodical pressure rise-fall operation. It is
sufficient that the liquid water be removed from inside the fuel
cell stack 1. Therefore, the discharging of the liquid water from
the fuel cell stack 1 to the capacity portion 12 is to be taken
into account. In this case, since increase of the flow velocity is
necessary, the differential pressure between the upper limit
pressure P1 and the lower limit pressure P2 is preferably set about
100 kPa.
[0127] Moreover, in terms of the upper limit pressure P1 and the
lower limit pressure P2, the following additional methods can be
set in addition to the thus-far described method of varying the
upper limit pressure P1 and the lower limit pressure P2 according
to the required load.
[0128] At first, as the first additional method, the upper limit
pressure P1 and the lower limit pressure P2 may be set according to
an allowable differential pressure between the oxidant electrode 34
and fuel electrode 67 in the fuel cell.
[0129] Moreover, as the second additional method, in the fuel cell
system 100 for implementing the purge treatment for discharging the
inactive gas accumulated in the fuel electrode 67, the upper limit
pressure P1 and the lower limit pressure P2 may be so restricted as
to secure the minimum pressure for securely implementing the
purging.
[0130] Moreover, as the third additional method, the upper limit
pressure P1 is set larger as the nitrogen concentration (impurity
concentration) in the fuel electrode 67 is higher, and the lower
limit pressure P2 is set to a small value in a condition that the
liquid water staying quantity or liquid water generation quantity
in the fuel electrode 67 is expected to be large. With this, a
large differential pressure is already secured when it is
determined that the liquid water is actually stored, to thereby be
able to securely implement the liquid water discharge.
[0131] Moreover, as the fourth additional method, in a scene where
the liquid water quantity staying in the fuel cell stack 1 is
assumed to be large, as shown in FIG. 21, the upper limit pressure
P1 and the lower limit pressure P2 are so set as to allow the
pressure ratio (P1/P2) between the upper limit pressure P1 and the
lower limit pressure P2 is temporarily large (P1 w/P2w). The
pressure width .DELTA.P2 P1w-P2w) necessary for discharging the
liquid water in the fuel electrode 67 is, for example, more than or
equal to 100 kPa, and the pressure width .DELTA.P1 (=P1-P2) for
discharging the inactive gas in the fuel electrode 67 is, for
example, more than or equal to 50 kPa. As stated above, since the
pressure widths of the two are different from each other, the upper
limit pressure P1 and the lower limit pressure P2 are set as
described above in view of the liquid water discharge.
[0132] Herein, when the upper limit pressure P1 is set high, that
is, to P1w, as stated in the third and fourth additional methods,
the speed of lowering the pressure from the upper limit pressure P1
to the lower limit pressure P2 is decreased since the hydrogen
consumption speed is small in the low load region. In this case,
since the time is required until the pressure reaches the lower
limit pressure P2, as the case may be, the second process for
increasing the pressure from the lower limit pressure P2 to the
upper limit pressure P1 cannot be implemented for a while.
[0133] Then, as shown in FIG. 22, when the upper limit pressure P1
is set high (for example, pressure P1w) in the low load condition,
it is permitted that the controller 40 temporarily increases the
current taken out from the fuel cell stack 1, to thereby increase
the pressure drop speed. For example, when the current is not
increased, the time required for decreasing the pressure from the
upper limit pressure P1w to the lower limit pressure P2 is a time
Tm2. Meanwhile, increasing the current allows the time required for
decreasing the pressure from the upper limit pressure P1w to the
lower limit pressure P2 to be a time Tm3 (=Tm1) which is shorter
than the time Tm2. With this, an interference to the pressure
rise-fall control for the inactive gas discharge or an interference
to the pressure rise-fall control for the subsequent liquid water
discharge can be suppressed.
[0134] In addition, when the generation condition may possibly be
made unstable attributable to a temporary increase of the current
taken out of the fuel cell stack 1, which temporary increase is
implemented in such a scene that the voltage of the fuel cell stack
1 is lowered, or in the case where the charge level of the
secondary battery for storing the taken-out current is high,
another method may be used for increasing the pressure drop speed,
instead of the method of increasing the taken-out current.
[0135] As the other method for increasing the pressure drop speed,
for example, the flow rate of the fuel electrode off-gas discharged
from the purge valve 14 is to be increased. Moreover, the pressure
drop speed may be increased by enlarging the capacity of the fuel
electrode 67. As a method for enlarging the capacity of the fuel
electrode 67, the liquid water control level in the fuel electrode
67 is lowered, to thereby discharge the liquid water in the fuel
electrode 67.
[0136] In addition, as a method of estimating the liquid water
staying quantity in the fuel electrode 67, an estimation method by
accumulating the load current based on the feature that the liquid
water generation quantity is substantially proportional to the load
current can be considered. Moreover, the liquid water staying
quantity may be estimated by the time elapsed from the timing of
the liquid water discharge implemented in advance. Moreover, by
measuring the voltage of the fuel cell, estimating, based on the
fuel cell's voltage which is abnormally lowered, that the liquid
water staying quantity is large is allowed. Moreover, in the
estimation of the liquid water staying quantity, the temperature of
the coolant water for cooling the fuel cell stack 1 can be used for
correcting the liquid water staying quantity. The reason therefor
is that even when the load current is the same, the lower the
coolant water temperature is, the more the liquid water (quantity)
stays. Likewise, the number of pressure pulsations or the cathode's
air quantity can also correct the liquid water staying
quantity.
Fifth Embodiment
[0137] Hereinafter, the fuel cell system 100 according to the fifth
embodiment of the present invention is to be set forth. According
to the third embodiment, the ordinary operation process for
implementing the generation according to the load current in the
fuel cell stack 1 has been set forth. Meanwhile, according to the
fifth embodiment, the process of each of at the start and stop of
the fuel cell system 100 is to be set forth. Herein, the structure
of the fuel cell system 100 according to the fifth embodiment is
like those according to the first to fourth embodiments, therefore
repeated explanations are to be omitted and differences are to be
mainly set forth.
Start Process
[0138] At first, the start process of the fuel cell system 100 is
to be set forth. In the case where after the stop of the fuel cell
system 100, the fuel cell stack 1 is left as it is for a while
instead of being started immediately, the low hydrogen
concentration gas is filled in the fuel electrode 67. In the case
of starting the system 10 in the above state, the low hydrogen
concentration gas is to be discharged from the fuel electrode 67 of
the fuel cell stack 1. Therefore, the high hydrogen concentration
gas is to be momentarily supplied from the fuel tank 10 at a
predetermined starting upper limit pressure, to thereby increase
the gas pressure in the fuel electrode 67. In this case, the purge
valve 14 is also controlled to the open state. With this, the
passage of the hydrogen front which is the boundary face between
the low hydrogen concentration gas and the high hydrogen
concentration gas can be accelerated, and also the hydrogen front
can be pushed out of the fuel electrode 67.
[0139] Then, before the timing at which the hydrogen front reaches
the purge valve 14, the hydrogen pressure adjusting valve 11 and
the purge valve 14 are controlled to the closed state, to thereby
implement the generation and consume hydrogen, thus reducing the
hydrogen pressure in the fuel electrode 67. Then, when the hydrogen
pressure reaches a predetermined starting lower limit pressure, the
hydrogen pressure is again increased to the predetermined starting
upper limit pressure. Then, the above pressure rise-fall operations
are to be repeated until the hydrogen concentration of the fuel
electrode 67 of the fuel cell stack 1 reaches the predetermined
average hydrogen concentration.
[0140] In addition, an actual vehicle, as the case may be, starts
moving during the period that the above start process is being
implemented. In this case, the output from the installed secondary
battery may be used.
Stop Process
[0141] Then, the stop process of the fuel cell system 100 is to be
set forth. As a start scene after stopping the fuel cell system
100, a low temperature environment is assumed. In this case, when
the liquid water is present in the fuel cell stack 1, hydrogen
pressure adjusting valve 11, discharge water valve 13, purge valve
14 and the like at the stop of the fuel cell system 100, as the
case may be, freezing and the like disenables starting of the fuel
cell system 100. Therefore, it is necessary to establish a process
for removing the liquid water at the stop of the fuel cell system
100. At first, air is to be supplied to the oxidant electrode 34
while implementing the generation in the low load condition. On the
fuel electrode 67 side, the pressure rise-fall operations are to be
repeatedly implemented according to the control pattern, like the
third embodiment. In this case, for example, with the upper limit
pressure P1 at 200 kPa (absolute pressure) and the lower limit
pressure P2 at 101.3 kPa, sufficient values should be set in
advance for discharging the liquid water from the fuel electrode
67. Moreover, the number of repetitions of pressure rise-fall
operations for sufficiently discharging the liquid water are to be
obtained in advance through experiments or simulations. Based the
thus obtained numbers, the pressure rise-fall operations should be
repeated. With this, the generation is ended.
[0142] Then, with the discharge water valve 13 controlled to the
open state, the discharge liquid water from the fuel cell stack 1
to the capacity portion 12 is discharged. Then, the power which was
generated immediately before the discharge operation is used, to
thereby operate heating devices such as heater and the like after
the above discharge operation, thus heating the purge valve 14 and
the discharge water valve 13, to thereby dry the discharge liquid
water.
[0143] According to the fifth embodiment, in the fuel cell system
100, the stop process can accomplish startability at the start, in
addition, even the process at the start can discharge impurities
more preferentially than hydrogen.
[0144] The entire contents of the Japanese Patent Application
Laid-Open No. 2008-298191 (filed on Nov. 21, 2008) and Japanese
Patent Application Laid-Open No. 2008-302465 (filed on Nov. 27,
2008) are incorporated herein by reference in order to take
protection against translation errors or omitted portions.
[0145] As set forth above, the contents of the present invention
have been set forth based on the embodiments. However, it is
obvious to a person skilled in that art that the present invention
is not limited to the above embodiments and various modifications
and improvements thereof are allowed.
INDUSTRIAL APPLICABILITY
[0146] According to the present invention, based on the first
pressure change pattern for implementing the pressure change at the
first pressure width, the pressure of the fuel gas in the fuel
electrode is periodically changed, to thereby be able to agitate
the fuel electrode side gas. With this, the fuel electrode side gas
can be made even.
* * * * *