U.S. patent application number 10/568286 was filed with the patent office on 2006-09-21 for fuel cell system.
This patent application is currently assigned to NISSAN MOTOR CO., LTD.. Invention is credited to Takashi Fukuda.
Application Number | 20060210853 10/568286 |
Document ID | / |
Family ID | 34385994 |
Filed Date | 2006-09-21 |
United States Patent
Application |
20060210853 |
Kind Code |
A1 |
Fukuda; Takashi |
September 21, 2006 |
Fuel cell system
Abstract
A fuel cell system (1) which includes: a fuel cell (2) to be
supplied with a gas for power generation, the gas unused for the
power generation to be discharged out of the fuel cell (2); a
circulation flow path (8) through which the discharged gas is
resupplied to the fuel cell (2); a variable flow rate circulation
pump (6) for circulating the gas through the circulation flow path
(8); a valve (7) for discharging the gas in the circulation flow
path (8) to the outside thereof; a voltage sensor (22) for
measuring voltage of the fuel cell (2); and a controller (32) for
controlling the circulation pump (6) and the valve (7). The
circulation pump (6) and the valve (7) are controlled based on the
voltage (CV) measured by the voltage sensor (22).
Inventors: |
Fukuda; Takashi; (Kanagawa,
JP) |
Correspondence
Address: |
FOLEY AND LARDNER LLP;SUITE 500
3000 K STREET NW
WASHINGTON
DC
20007
US
|
Assignee: |
NISSAN MOTOR CO., LTD.
|
Family ID: |
34385994 |
Appl. No.: |
10/568286 |
Filed: |
August 11, 2004 |
PCT Filed: |
August 11, 2004 |
PCT NO: |
PCT/JP04/11803 |
371 Date: |
February 16, 2006 |
Current U.S.
Class: |
429/415 ;
429/432; 429/444; 429/454 |
Current CPC
Class: |
Y02E 60/50 20130101;
H01M 8/04231 20130101; H01M 8/04156 20130101; H01M 8/04552
20130101; H01M 8/04402 20130101; H01M 8/04365 20130101; H01M
8/04097 20130101; H01M 8/04761 20130101 |
Class at
Publication: |
429/023 ;
429/034 |
International
Class: |
H01M 8/04 20060101
H01M008/04 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 25, 2003 |
JP |
2003333656 |
Claims
1. A fuel cell system comprising: a fuel cell to be supplied with a
gas for power generation, the gas unused for the power generation
to be discharged out of the fuel cell; a circulation flow path
through which the gas discharged out of the fuel cell is resupplied
to the fuel cell; a variable flow rate circulation pump for
circulating the gas through the circulation flow path; a valve for
discharging the gas in the circulation flow path to the outside of
the circulation flow path; a voltage sensor for measuring voltage
of the fuel cell; and a controller for controlling the circulation
pump and the valve, wherein the circulation pump and the valve are
controlled based on the voltage measured by the voltage sensor.
2. The fuel cell system of claim 1, wherein the fuel cell comprises
a plurality of cells stacked on one another, and the voltage sensor
measures voltages of the respective cells, and wherein the
circulation pump is controlled to reduce flow rate of the gas
circulated, and the valve is controlled to increase an amount of
gas to be discharged, as the number of cells with substantial
voltage drops increases.
3. The fuel cell system of claim 1, wherein the fuel cell comprises
a plurality of cells stacked on one another, and the voltage sensor
measures voltages of the respective cells, and wherein the
circulation pump is controlled to reduce flow rate of the gas
circulated, and the valve is controlled to increase an amount of
gas to be discharged, as a variation in the measured voltages
between the cells becomes smaller.
4. The fuel cell system of claim 1, further comprising: a clogging
detector for determining possibility of clogging of a gas passage
in the fuel cell, wherein the circulation pump is controlled to
reduce flow rate of the gas circulated, and the valve is controlled
to increase an amount of gas to be discharged, as the possibility
of the clogging is determined to be low.
5. The fuel cell system of claim 4, wherein the fuel cell comprises
a plurality of cells stacked on one another, and the voltage sensor
measures voltages of the respective cells, and wherein the
possibility of clogging is determined to be lower, as the number of
cells with substantial voltage drops increases.
6. The fuel cell system of claim 4, wherein the fuel cell comprises
a plurality of cells stacked on one another, and the voltage sensor
measures voltages of the respective cells, and wherein the
possibility of clogging is determined to be lower, as a variation
in the measured voltages between the cells becomes smaller.
7. The fuel cell system of claim 1, wherein the valve is controlled
to increase an amount of gas to be discharged, as a rate of
increase in the measured voltage is low, after the circulation pump
is controlled to increase flow rate of the gas circulated more than
that in a normal operation.
8. A method for improving fuel gas consumption in power generation
of fuel cells, wherein the fuel gas unused for the power generation
is resupplied to the fuel cells through a fuel gas circulation
system, the method comprising: monitoring output voltages of the
respective fuel cells; increasing flow rate of the fuel gas in the
fuel gas circulation system, if variation in the output voltages is
larger than a predetermined range; and discharging the fuel gas out
of the fuel gas circulation system, if the variation in the output
voltages is within the predetermined range and an average value of
the output voltages of the respective fuel cells is lower than a
predetermined value.
Description
TECHNICAL FIELD
[0001] The present invention relates to a fuel cell system, more
particularly a purge control in a fuel cell system having an anode
gas circulation system.
BACKGROUND ART
[0002] A fuel cell is an electrochemical device to convert chemical
energy of fuel gas such as hydrogen gas and oxidant gas containing
oxygen supplied thereto, directly to electric energy which is
extracted from electrodes provided on both sides of an electrolyte
thereof. A fuel cell using a solid polymer electrolyte membrane has
low operation temperature and can be easily handled, and therefore,
it has been a focus of attention as a power source for an electric
vehicle.
[0003] The solid polymer electrolyte membrane is required to be
retained in a moderately humidified state in order to exert
sufficient hydrogen ion conductivity. The fuel gas or the oxidant
gas or the both gases are humidified to be supplied to the fuel
cell. Thus, water added for the humidification and water produced
by a power generation reaction in the fuel cell are condensed and
may cause clogging or blocking of a gas passage in the fuel cell,
depending on operating conditions of the fuel cell.
[0004] Moreover, in a fuel cell using air as the oxidant gas,
nitrogen contained in the air passes through a solid polymer
membrane thereof, and accumulates in a fuel gas circulation system.
Consequently, a fuel gas partial pressure decreases at a fuel
electrode of the fuel cell, lowering operation efficiency thereof.
In order to resolve the clogging and the accumulation of nitrogen,
purging is performed for the fuel gas circulation system.
[0005] Japanese Patent Laid-Open Publication No. 2002-243417
discloses a fuel cell system which removes impurities accumulated
in a hydrogen gas circulation system by opening a purge valve
provided in the system and releasing anode off-gas to outside of
the system.
DISCLOSURE OF INVENTION
[0006] In the system described above, however, even in the case
that clogging in the hydrogen gas circulation system is required to
be resolved, hydrogen containing fuel gas is released to the
outside of the system through the purge valve, whereby operation
efficiency thereof is lowered.
[0007] The present invention was made in the light of the problem.
An object of the present invention is to provide a fuel cell system
which removes clogging of gas passages in the fuel cell without
lowering the operation efficiency thereof.
[0008] An aspect of the present invention is a fuel cell system
comprising: a fuel cell to be supplied with a gas for power
generation, the gas unused for the power generation to be
discharged out of the fuel cell; a circulation flow path through
which the gas discharged out of the fuel cell is resupplied to the
fuel cell; a variable flow rate circulation pump for circulating
the gas through the circulation flow path; a valve for discharging
the gas in the circulation flow path to the outside of the
circulation flow path; a voltage sensor for measuring voltage of
the fuel cell; and a controller for controlling the circulation
pump and the valve, wherein the circulation pump and the valve are
controlled based on the voltage measured by the voltage sensor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The invention will now be described with reference to the
accompanying drawings wherein:
[0010] FIG. 1 is a block diagram of a fuel cell system according to
a first embodiment of the present invention;
[0011] FIG. 2 shows relationship between fuel cell load and
operation pressure of a fuel cell of the first embodiment;
[0012] FIG. 3 shows relationship between fuel cell load and average
cell voltage of the fuel cell of the first embodiment;
[0013] FIG. 4 is a flowchart of a purge operation of the first
embodiment;
[0014] FIG. 5 shows a relationship between fuel cell load and
hydrogen gas circulation flow rate Qh of the first embodiment;
[0015] FIG. 6 is a flowchart of a purge operation of a second
embodiment of the present invention;
[0016] FIG. 7 shows a relationship between fuel cell load and purge
flow rate Qp of the second embodiment; and
[0017] FIG. 8 is a flowchart of a purge operation of a third
embodiment of the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0018] Embodiments of the present invention will be described in
detail with reference to the drawings. In all of the embodiments,
described is a fuel cell system suitable for a fuel cell
vehicle.
First Embodiment
[0019] As shown in FIG. 1, a fuel cell system 1 includes a fuel
cell stack 2, a hydrogen gas supply system 1a which supplies
hydrogen gas as fuel gas to the fuel cell stack 2, an air supply
system 1b which supplies air to the fuel cell stack 2, and a
control unit 32.
[0020] The hydrogen gas supply system 1a includes: a hydrogen tank
3 which stores hydrogen gas; a pressure control valve 4 which
regulates pressure of the hydrogen gas taken out of the hydrogen
tank 3; a hydrogen gas supply line 5 through which the pressure
control valve 4 and the fuel cell stack 2 are communicated with
each other; a hydrogen circulation pump 6 which feeds hydrogen gas
discharged from the fuel cell stack 2 back to an inlet of the fuel
cell stack 2 and circulates the hydrogen gas through a hydrogen gas
circulation flow path 8; and a purge valve 7 which discharges the
hydrogen gas discharged from the fuel cell stack 2 to the outside
of the system.
[0021] The air supply system 1b includes: a compressor 9 which
takes in air from the outside of the system and compresses the air;
a humidifier 12 which humidifies the compressed air to supply the
humidified air to the fuel cell stack 2; a condenser 13 which
collects water from the air discharged from the fuel cell stack 2;
a pressure control valve 14 which regulates pressure of the
discharged air; a water tank 17 which stores the water collected by
the condenser 13; and a water pump 18 which sends the water in the
water tank 17 to the humidifier 12.
[0022] The fuel cell stack 2 is formed of a plurality of unit cells
stacked on one another. Each of the cells has an anode AN, a
cathode CA, and a solid electrolyte membrane M sandwiched
therebitween, wherein the hydrogen gas is supplied to the anode AN
and the air is supplied to the cathode CA. In the fuel cell stack
2, provided are: a temperature sensor 21 which detects a
temperature Tc of the fuel cell stack 2; a cell voltage sensor 22
which detects cell voltages CV of the respective cells of the fuel
cell stack 2; and a pressure sensor 34 which detects hydrogen gas
pressure at an outlet of the fuel cell stack 2.
[0023] The hydrogen gas discharged from the fuel cell stack 2 is
pressure-fed and resupplied to the fuel cell stack 2 through the
hydrogen gas circulation flow path 8 by the hydrogen circulation
pump 6. The hydrogen gas from the hydrogen tank 3 is introduced
into the hydrogen gas circulation flow path 8 on the downstream of
the hydrogen circulation pump 6 and supplied to the fuel cell stack
2.
[0024] When impurities such as nitrogen, CO and water are
accumulated in the hydrogen gas circulation flow path 8 or when
starting up the system, the purge valve 7 is opened to release the
circulating hydrogen gas to the outside of the circulation flow
path 8. The operation of this purging operation will be described
later.
[0025] Each cell voltage CV of the fuel cell stack 2 is detected by
the cell voltage sensor 22 and the detected value is sent to the
control unit 32. Moreover, the temperature Tc of the fuel cell
stack 2 and the hydrogen gas pressure Ph are detected by the
temperature sensor 21 and the pressure sensor 34, respectively, and
are sent to the control unit 32.
[0026] The control unit 32 is a controller which controls the fuel
cell system 1 based on the values of CV, Ph and Tc detected by the
sensors 21, 22 and 34 and controls the hydrogen circulation pump 6
and the purge valve 7 based on the cell voltages CV detected by the
cell voltage sensor 22.
[0027] In this embodiment, the control unit 32, although not
particularly limited, is formed of a microprocessor including a
CPU, a program ROM, a work RAM and an input-output interface.
[0028] The purge valve 7 is a valve which allows the hydrogen gas
circulation flow path 8 and the outside of the system to
communicate/non-communicate with each other and has a variable
opening which can be adjusted arbitrarily.
[0029] The compressor 9 compresses air taken in from the outside of
the system. The compressed air is humidified by the humidifier 12
provided on an air supply line 11 and supplied to the fuel cell
stack 2.
[0030] Air discharged from the fuel cell stack 2 contains water
produced in reaction of power generation in the fuel cell stack 2.
The condenser 13 provided downstream of the fuel cell stack 2
collects the water. On a line downstream of the condenser 13,
provided is the pressure control valve 14 which provides the air
supply system 1b with a desired pressure.
[0031] The water condensed and collected by the condenser 13 is
introduced into the water tank 17 via an ON/OFF valve 15 in a water
channel 16.
[0032] The water in the water tank 17 is pressure-fed by the pump
18 and supplied to the humidifier 12 through a feed line 19. When
excessive water is supplied to the humidifier 12, the excess water
is returned to the water tank 17 through a return line 20.
[0033] Next, description will be given to operations.
[0034] A requested output (=required power) of the fuel cell is set
based on a throttle opening of an accelerator operated by a driver,
and the like. The hydrogen gas and air are regulated according to
this requested output and supplied to anode AN side passage and
cathode CA side passage of the fuel cell stack 2, respectively.
[0035] As shown in FIG. 2, a hydrogen gas pressure in the anode AN
and an air pressure in the cathode CA, both of which are
represented as operation pressure, are set to be higher as the fuel
cell load becomes heavier.
[0036] In a normal operation, a closed loop is formed in the
hydrogen gas supply system 1a. Specifically, in the closed loop,
the hydrogen gas discharged from an anode side outlet of the fuel
cell stack 2 is fed to an anode side inlet and circulated through
the hydrogen gas circulation flow path 8 by the hydrogen
circulation pump 6.
[0037] Inside the fuel cell stack 2, nitrogen in the air supplied
to the cathode CA passes through the solid polymer electrolyte
membrane M to the anode AN. Thus, concentration of impurities in
anode gas in the closed loop is gradually increased. Moreover, the
gas passage in the stack 2 is clogged with humidifying water or
produced water. As a result, the cell voltages CV of the fuel cell
stack are lowered.
[0038] When the cell voltage sensor 22 detects the cell voltages CV
and the control unit 32 determines, based on the detected cell
voltages, that the cell voltages are lowered, a purge operation is
performed. Specifically, in the purge operation, the purge valve 7
is temporarily opened and the gas containing impurities in the
hydrogen gas circulation flow path 8 is released to the outside of
the system.
[0039] Next, the purge operation will be described with reference
to the flowchart of FIG. 4. A series of processes shown in the
flowchart is repeatedly carried out every predetermined time.
[0040] First, in S1, the control unit 32 reads each cell voltage CV
of the fuel cell stack 2 from the cell voltage sensor 22 and
computes an average cell voltage AVG. CV of all the cells. Next, in
S2, it is determined whether or not there is a cell with its
voltage lower than the average cell voltage AVG. CV computed in S1
by a predetermined value (for example, 0.1 V) or more.
[0041] When there exists even one lower voltage cell, it is
determined that clogging has occurred in the gas passage in the
fuel cell stack and the processing proceeds to S3. When there
exists no relevant cell, the processing proceeds to S4. In other
words, a clogging detector is thus formed of the cell voltage
sensor 22 and the control unit 32.
[0042] In S3, in order to resolve the clogging, the hydrogen
circulation pump 6 is speeded up and a hydrogen gas circulation
flow rate Qh is increased.
[0043] As indicated by a thick line in FIG. 5, a hydrogen gas
circulation flow rate Qh1 in a normal operation is increased at a
constant rate as fuel cell load (output current) of the fuel cell
stack 2 increases, while in a low load range, regardless of a
change in the fuel cell load, the flow rate Qh is maintained
substantially constant for ensuring even distribution of the
supplied gas. In accordance with characteristics of the fuel cell
to be used, the increase of the hydrogen gas circulation flow rate
Qh can be adjusted. In FIG. 5, the hydrogen gas circulation flow
rate Qh is increased at a substantially constant increase rate as
the fuel cell load increases. However, the flow rate may be
increased by a substantially constant increase amount (indicated by
a thin line in FIG. 5) over the whole fuel cell load range, or
alternatively, the increase rate may be varied along with the
load.
[0044] In S4, the normal hydrogen gas circulation flow rate Qh1
corresponding to the thick line in FIG. 5 is set.
[0045] In S5, it is determined whether or not the average cell
voltage AVG. CV is lower than a table value TV, which is previously
stored in a ROM of the control unit 32, by 0.1 V. Accordingly, if
the average cell voltage AVG. CV is lower than the table value TV
by 0.1 V or more, the processing proceeds to S6, and if not, the
processing proceeds to S7.
[0046] The lowering of the average cell voltage AVG. CV is caused
by accumulation of impurities in the hydrogen gas circulation flow
path 8 due to diffusion of nitrogen from the cathode CA or the
like. If the average cell voltage AVG. CV is lowered by a
predetermined value (for example, 0.1 V) or more, the purge valve 7
is opened for a predetermined period of time (for example, 5
seconds) in S6 and S8. Accordingly, nitrogen and the like are
discharged to the outside of the system together with the hydrogen
gas in the hydrogen gas circulation flow path 8. Thus, the average
cell voltage AVG. CV is restored.
[0047] Here, for detecting lowering of the average cell voltage
AVG. CV, a judgment is performed in the following manner. Voltage
characteristics (I-V characteristics) of each of the fuel cells
with respect to the fuel cell load as shown in FIG. 3 are stored in
the control unit 32 as table data, from which a voltage
characteristic curve in the full range of the fuel cell load,
giving a certain average cell voltage at a certain temperature T1
is obtained. A correction is then made to put this voltage
characteristic as voltage characteristic at the measured fuel cell
temperature Tc (=T1). An estimated average cell voltage AVG. CV1
for a current fuel cell load FCL1 is obtained from the corrected
voltage characteristic and compared with the average cell voltage
AVG. CV in the actual operation.
[0048] As described above, the clogging of gas passages in the fuel
cells can be eliminated by increasing the hydrogen gas circulation
flow rate Qh. Thus, in this embodiment, for the lowering of the
cell voltages due to the clogging, the hydrogen gas circulation
flow rate Qh is increased without opening the purge valve 7.
Consequently, amount of the hydrogen gas to be discharged to the
outside of the system is suppressed and fuel gas consumption is
improved.
[0049] For the lowering of the cell voltages caused by the
increasing concentration of impurities in the circulated hydrogen
gas due to nitrogen diffusion or the like, the purge valve 7 is
opened to discharge the impurities. Thus, the cell voltages can be
surely restored.
[0050] Furthermore, the lowering of the cell voltages due to
long-term factors, such as aged deterioration of the cells, may be
corrected by learning. Thus, the determinations or judgments
described above are possible even if the average cell voltage of
the fuel cell gradually drops.
[0051] Note that, in S2, the cause of the lowering of the cell
voltages is determined based on the cell-to-cell variation in the
cell voltages CV. However, the cause of the lowering of the cell
voltages may be determined based on a hydrogen concentration
detected by a hydrogen concentration sensor provided on a hydrogen
gas circulation system.
[0052] Although, in the fuel cell system 1 of this embodiment, the
hydrogen circulation pump is used as means for circulating
hydrogen, an ejector may be used in conjunction therewith.
Second Embodiment
[0053] A second embodiment of the present invention has the same
configuration as that of the first embodiment shown in FIG. 1 and
is different from the first embodiment only in an operation
thereof. With reference to the flowchart shown in FIG. 6,
description will be given to the only difference. S1 to S7 in FIG.
6 are the same as those of the first embodiment shown in FIG.
4.
[0054] After the purge valve 7 is opened in S6, a hydrogen gas
circulation flow rate Qh2 of the hydrogen circulation pump 6 is
reduced in S21. Opening the purge valve 7 downstream the fuel cell
stack 2 necessarily increases a flow rate of hydrogen gas supplied
to the fuel cell stack 2. This will compensate for the reduction in
the hydrogen gas circulation flow rate Qh2. The flow rate Qh2 is
set to be smaller in order to efficiently discharge high
concentration of impurities of gas in the hydrogen gas circulation
flow path 8 to the outside of the system. Thus, a nitrogen
concentration can be reduced in a shorter period of time.
[0055] In this embodiment, the reduction in the flow rate Qh2 is
set to be approximately equivalent to the increase in the flow rate
of hydrogen gas supplied to the fuel cell stack 2 when the purge
valve 7 is opened. A purge flow rate Qp at the purge valve 7 when
the purge valve 7 is opened changes depending on pressure
difference between upstream and downstream of the purge valve and a
fluid flowing therethrough. In this embodiment, as shown in FIG. 2,
the hydrogen gas pressure (=operation pressure) is changed
according to the fuel cell load. Therefore, if the fuel cell load
is determined, the flow rate Qp can be obtained, whereby table data
of the graph as shown in FIG. 7 is provided.
[0056] The value obtained by subtracting the flow rate Qp from the
hydrogen gas circulation flow rate Qh obtained from the curve of
thick line in FIG. 5 (Qh-Qp) is set as the circulation flow rate
Qh2 of the hydrogen circulation pump 6.
[0057] Moreover, the amount of reduction in the hydrogen gas
circulation flow rate Qh2 in the purge operation may be set to be
smaller, for example, than the purge flow rate Qp, as long as the
fuel cell to be used is not particularly affected thereby. In this
case, efficiency in purging the hydrogen gas circulation flow path
8 is further improved.
[0058] The processing proceeds to S22 after S21 and the purge valve
7 is kept open until valve opening time of the purge valve 7
reaches a predetermined time. After the predetermined time has
elapsed in S22, the processing proceeds to S23 and the purge valve
7 is closed. Subsequently, in S24, the hydrogen gas circulation
flow rate Qh is restored to the normal flow rate Qh1 and the
processing returns.
[0059] In this embodiment, unlike the first embodiment, the
hydrogen gas circulation flow rate Qh is reduced when opening the
purge valve. Thus, the duration of purging can be shortened while
suppressing unnecessary discharge of hydrogen.
[0060] Moreover, in this embodiment, when the cell voltages are not
uniformly lowered, that is, when the voltages are lowered due to
the clogging, the hydrogen gas circulation flow rate Qh is
increased in S2 and S3. By simultaneously opening the purge valve
in this event, the purge operation can be promptly carried out.
[0061] The increase in the hydrogen gas circulation flow rate Qh is
accompanied by an increase in power consumption of the hydrogen
circulation pump 6. In consideration of the power consumption, a
selection may be made between only increasing the flow rate Qh and
the combination of opening the purge valve with increasing the flow
rate Qh.
Third Embodiment
[0062] A third embodiment has the same configuration as that of the
first embodiment shown in FIG. 1 and is different from the first
embodiment only in an operation thereof. With reference to the
flowchart shown in FIG. 8, description will be given to the only
differences. S1 to S4 in FIG. 8 are the same as those of the first
embodiment shown in FIG. 4.
[0063] After the hydrogen gas circulation flow rate Qh is increased
in S3, the processing proceeds to S31 and the circulation flow rate
is kept until a predetermined period of time (for example, 5
seconds) elapses. After the predetermined time has elapsed in S31,
the processing proceeds to S32. Also when the hydrogen gas
circulation flow rate Qh is set to the normal value Qh1 in S4, the
processing proceeds to S32.
[0064] In S32, the average cell voltage AVG. CV of all the cells,
which is computed in S1, is compared with the cell voltage CV of
each cells. Accordingly, it is determined whether or not there is a
cell having voltage lower than the average cell voltage AVG. CV by
0.1 V or more. If even one such cell exists, the processing
proceeds to S33 and the purge valve 7 is opened and kept open until
a predetermined period of time (for example, 5 seconds) elapses in
S35. After the predetermined time has elapsed in S35, the
processing proceeds to S36 and the purge valve 7 is closed.
Subsequently, in S37, the hydrogen gas circulation flow rate Qh is
returned to the value Qh1 in the normal operation. Thereafter, the
processing returns to the start.
[0065] Meanwhile, when there exists no cell which satisfies the
condition described above in S32, the processing proceeds to S34
and the purge valve 7 is closed. Thereafter, the processing returns
to the start.
[0066] In this embodiment, the same logic is used in both of S2 and
S32 to simplify the determination whether nitrogen and the like are
diffused into the anode gas.
[0067] Specifically, the case where lowering of the cell voltages
are caused by nitrogen diffusion is less urgent than the case where
lowering thereof is caused by clogging. Also in this case, as the
concentration of nitrogen increases, there occurs variation in the
cell voltages CV. For the reasons described above; the following
two steps are taken. Specifically, when the variation in the cell
voltages CV is detected, first, the hydrogen gas circulation flow
rate is increased without discharging the hydrogen gas to the
outside of the system, without specifying the cause of the lowered
cell voltages. If the lowering of the cell voltages cannot be
resolved even after taking the step described above, the purge
valve 7 is opened. Consequently, even if the cause of the lowered
cell voltages is the clogging or the nitrogen diffusion and the
like, performance of the fuel cell and the like are not
deteriorated.
[0068] Whether to adopt the method of the first embodiment or to
use the method of the third embodiment may be determined, taking
into consideration the power consumption of the circulation pump
and lowering of efficiency caused by nitrogen diffusion.
[0069] In other words, the fuel cell system 1 according to the
present invention includes: the fuel cell stack 2 which is supplied
with fuel gas to the anode AN thereof and oxidant gas to the
cathode CA thereof for power generation; the anode gas circulation
flow path 8 which returns the fuel gas discharged from the outlet
of the anode gas passage in the fuel cell stack 2, to the inlet of
the anode gas passage; the variable flow rate hydrogen circulation
pump 6 which circulates the gas in the anode gas circulation flow
path 8; the purge valve 7 which discharges the anode off-gas from
the outlet of the anode gas passage to the outside of the system;
the cell voltage sensor 22 for measuring the cell voltages CV of
the fuel cell stack 2; and the controller 32 for controlling the
hydrogen circulation pump 6 and the purge valve 7 based on the cell
voltages CV measured by the cell voltage sensor 22.
[0070] The cell voltage sensor 22 measures voltages of a plurality
of cells included in the fuel cell stack 2, respectively. The
controller 32 controls, in purge operation, the hydrogen
circulation pump 6 to have a smaller circulation flow rate Qh, and
the purge valve 7 to have a larger amount of the gas discharged, as
there are more cells of which voltages are significantly lowered,
or as the variation between the cell voltages is smaller, when the
cell voltages are lowered.
[0071] The fuel cell system 1 further includes clogging detector
for detecting clogging of the anode gas passages in the fuel cell
stack 2. This clogging detector determines possibility of clogging
to be low, as there are more cells of which voltages are
significantly lowered, or as the variation between the cell
voltages is smaller, when the cell voltages are lowered.
[0072] The controller 32 controls, in the purge operation, the
hydrogen circulation pump 6 to have a smaller circulation flow rate
Qh, and the purge valve 7 to have a larger amount of the gas
discharged, as the possibility of clogging becomes lower, when the
cell voltages are lowered.
[0073] Moreover, in the fuel cell system 1, in the beginning of the
purge operation, the controller 32 sets the circulation flow rate
Qh of the hydrogen circulation pump 6 to be larger than the
circulation flow rate Qh1 in the normal operation. Thereafter, if
increase rate of the cell voltages CV are low, the controller 32
sets the amount of the gas discharged from the purge valve 7 to be
large.
[0074] The present disclosure relates to subject matters contained
in Japanese Patent Application No. 2003-333656, filed on Sep. 25,
2003, the disclosure of which is expressly incorporated herein by
reference in its entirety.
[0075] The preferred embodiments described herein are illustrative
and not restrictive, and the invention may be practiced or embodied
in other ways without departing from the spirit or essential
character thereof. The scope of the invention being indicated by
the claims, and all variations which come within the meaning of
claims are intended to be embraced herein.
INDUSTRIAL APPLICABILITY
[0076] In a fuel cell system according to the present invention,
voltages of the respective cells of the fuel cell are measured, and
a gas circulation pump with variable flow rate, provided on an
anode gas circulation flow path, and a purge valve which discharges
anode off-gas to the outside of the system, are controlled based on
the measured cell voltages. Thus, clogging of gas passages in the
fuel cell, causing voltage drops in the cell voltages, can be
detected, and the clogging can be eliminated by increasing the flow
rate of the gas circulation pump. Moreover, frequency of discharge
of hydrogen by the purge valve can be reduced. Thus, the present
invention is industrially applicable as a technology for improving
fuel gas consumption of the fuel cell system.
* * * * *