U.S. patent application number 14/170940 was filed with the patent office on 2014-05-29 for fuel cell.
This patent application is currently assigned to Murata Manufacturing Co., Ltd.. The applicant listed for this patent is Murata Manufacturing Co., Ltd.. Invention is credited to Naoki IWAMURA, Hirofumi KAN, Shunsuke KIMURA, Hidenori SUZUKI, Daisuke WATANABE.
Application Number | 20140147761 14/170940 |
Document ID | / |
Family ID | 43921637 |
Filed Date | 2014-05-29 |
United States Patent
Application |
20140147761 |
Kind Code |
A1 |
IWAMURA; Naoki ; et
al. |
May 29, 2014 |
FUEL CELL
Abstract
A fuel cell has a fuel cell main body, a fuel supply unit, a
voltage sensor, a supply speed determining unit, a fuel supply
control unit, and a connecting unit. The voltage sensor measures
the open-circuit voltage of the fuel cell main body. The supply
speed determining unit determines the fuel supply speed of the fuel
supply unit, on the basis of the results obtained from the
measurement performed by the voltage sensor, in the case where the
voltage measured by the voltage sensor is smaller than a
predetermined value. The fuel supply control unit controls, on the
basis of the supply speed thus determined, the fuel supply from the
fuel supply unit. The connecting unit connects a load to the fuel
cell main body, in the case where the voltage measured by the
voltage sensor is larger than the predetermined value.
Inventors: |
IWAMURA; Naoki;
(Kawasaki-shi, JP) ; KAN; Hirofumi; (Kawasaki-shi,
JP) ; WATANABE; Daisuke; (Chigasaki-shi, JP) ;
SUZUKI; Hidenori; (Saku-shi, JP) ; KIMURA;
Shunsuke; (Ota-ku, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Murata Manufacturing Co., Ltd. |
Nagaokakyo-shi |
|
JP |
|
|
Assignee: |
Murata Manufacturing Co.,
Ltd.
Nagaokakyo-shi
JP
|
Family ID: |
43921637 |
Appl. No.: |
14/170940 |
Filed: |
February 3, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13423893 |
Mar 19, 2012 |
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14170940 |
|
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PCT/JP2010/006363 |
Oct 28, 2010 |
|
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13423893 |
|
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Current U.S.
Class: |
429/432 |
Current CPC
Class: |
H01M 8/04388 20130101;
H01M 8/04201 20130101; H01M 8/1011 20130101; Y02E 60/50 20130101;
H01M 8/04544 20130101; H01M 8/04559 20130101; H01M 8/04753
20130101; H01M 8/04626 20130101; Y02E 60/523 20130101 |
Class at
Publication: |
429/432 |
International
Class: |
H01M 8/04 20060101
H01M008/04 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 28, 2009 |
JP |
2009-248100 |
Claims
1-9. (canceled)
10. A fuel cell, comprising: a fuel cell main body configured to
generate power by using a liquid fuel; a fuel supply unit
configured to supply the fuel to the fuel cell main body; a fuel
supply control unit configured to control the supply of the fuel
supply unit by alternately repeating ON time and OFF time, the fuel
supply unit supplying the fuel during the ON time, and the fuel
supply unit not supplying the fuel during the OFF time; a voltage
sensor configured to measure an open-circuit voltage of the fuel
cell main body; a determining unit configured to determine duty
ratio being the ratio of the ON time to the sum of the ON time and
the OFF time, the determining unit increasing the duty ratio when
the measured voltage is smaller than a first voltage, the
determining unit decreasing the duty ratio or keeping the duty
ratio constant when the measured voltage is larger than the first
voltage; and a connecting unit configured to turn on a connection
between a load and the fuel cell main body when the measured
voltage is larger than a second voltage larger than the first
voltage.
11. The fuel cell according to claim 10, wherein the determining
unit increases the duty ratio by increasing the ON time.
12. The fuel cell according to claim 10, wherein the determining
unit decreases the duty ratio by increasing the OFF time.
13. The fuel cell according to claim 10, wherein the determining
unit decreases the duty ratio and then keeps the duty ratio
constant when the measured voltage is larger than the first
voltage.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of prior International
Application No. PCT/JP2010/006363, filed on Oct. 28, 2010 which is
based upon and claims the benefit of priority from Japanese Patent
Application No. 2009-248100 filed on Oct. 28, 2009; the entire
contents of all of which are incorporated herein by reference.
FIELD
[0002] Embodiments described herein relate generally to a fuel
cell.
BACKGROUND
[0003] Attempts are being made to downsize electronic equipment
such as cellular phones, personal digital assistants and the like.
In addition to the downsizing of the electronic equipment, attempts
are also being made to use a fuel cell for the power source of the
electronic equipment. The fuel cell can generate electricity by
merely supplying fuel and air and can continuously generate
electricity by changing and replenishing the fuel only. Therefore,
if the fuel cell can be downsized, it is effective as a power
source for compact electronic equipment.
[0004] As the fuel cell, a direct methanol fuel cell (hereinafter
referred to as DMFC) attracts attention. The DMFC is classified
according to a liquid fuel supplying method, and the supplying
method includes an active method such as a gas supply type, a
liquid supply type or the like and a passive method such as an
inside vaporization type which supplies the liquid fuel from a fuel
storing unit to a fuel electrode by vaporizing in the cell. Between
them, the passive method is advantageous for downsizing of the
DMFC.
[0005] It is usual that the fuel cell is activated and put into a
steady operation state before it is connected to a load.
[0006] It was found that the startup of the DMFC becomes instable
occasionally due to an external environment, and particularly a
temperature. That is, the fuel tank inner pressure increases in a
high temperature environment, and the fuel supply amount from the
fuel tank to the DMFC increases easily. On the other hand, the fuel
tank inner pressure decreases in a low temperature environment, and
the fuel supply amount from the fuel tank to the DMFC decreases
easily. Therefore, the supply of the fuel becomes excessive in the
high temperature environment, and the DMFC temperature rises
sharply, resulting in possibility of an occurrence of overshooting.
On the other hand, the supply of the fuel becomes insufficient in
the low temperature environment, and it might take time to startup
the DMFC.
[0007] To remedy the instable startup of the DMFC due to the
influence of the outside temperature, it is considered to measure
the outside temperature and to control the fuel supply depending on
the outside temperature.
[0008] But, it is not preferable to dispose a sensor for measuring
the outside temperature because the DMFC will have a complex
equipment structure. The temperature of the DMFC itself can be
measured by a temperature sensor disposed within the DMFC. On the
other hand, the measurement of an outside temperature of the DMFC
requires the temperature sensor disposed outside the DMFC, and the
equipment structure of the DMFC becomes complex.
[0009] In view of the above, it is desired to have a fuel cell
startup method which has excellent robustness with respect to an
outside temperature and does not require a sensor for measuring the
outside temperature.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a block diagram showing a schematic configuration
of the fuel cell system according to an embodiment of the
invention.
[0011] FIG. 2 is a sectional view of a fuel cell main body 1.
[0012] FIG. 3 is a perspective view of a fuel distribution
mechanism 105.
[0013] FIG. 4 is a flow chart showing a comparative example of a
startup procedure of a fuel cell.
[0014] FIG. 5 is a flowchart showing an example of the startup
procedure of the fuel cell.
[0015] FIG. 6 is a flow chart showing another example of the
startup procedure of the fuel cell.
[0016] FIG. 7 is a schematic view showing an example of preferable
ON time and OFF time.
[0017] FIG. 8 is a schematic view showing an example of a temporal
change of ON time and OFF time.
[0018] FIG. 9 is a schematic view showing an example of a temporal
change of ON time and OFF time.
[0019] FIG. 10 is a schematic view showing an example of a temporal
change of ON time and OFF time.
DETAILED DESCRIPTION
[0020] The fuel cell of an embodiment has a fuel cell main body, a
fuel supply unit, a voltage sensor, a supply speed determining
unit, a fuel supply control unit, and a connecting unit. The
voltage sensor measures the open-circuit voltage of the fuel cell
main body. In a case where the voltage measured by the voltage
sensor is smaller than a predetermined value, the supply speed
determining unit determines the fuel supply speed of the fuel
supply unit on the basis of the results obtained from the
measurement performed by the voltage sensor. On the basis of the
supply speed thus determined, the fuel supply control unit controls
the fuel supply from the fuel supply unit. The connecting unit
connects a load to the fuel cell main body in the case where the
voltage measured by the voltage sensor is larger than the
predetermined value.
[0021] Embodiments are described below with reference to the
drawings. The fuel cell shown in FIG. 1 is provided with a fuel
cell main body (DMFC) 1, a pump drive unit 2, a DC-DC converter 3,
a control unit 4, a temperature sensor SS1, a voltage sensor SS2,
and a switch SW.
[0022] The fuel cell main body 1 has a power generation unit 101, a
fuel storing unit 102, a passage 103, a pump 104, and the
temperature sensor SS1. The power generation unit (cell) 101
generates power by combustion of fuel and configures an
electromotive portion of a fuel cell system. The fuel storing unit
102 stores the liquid fuel used by the power generation unit 101.
The passage 103 connects the fuel storing unit 102 and the power
generation unit (cell) 101. The pump 104 is a fuel supply means for
transferring the liquid fuel from the fuel storing unit 102 to the
power generation unit (cell) 101.
[0023] As shown in FIG. 2, the power generation unit 101 has a
membrane electrode assembly (MEA) which comprises an anode (fuel
electrode) 13 having an anode catalyst layer 11 and an anode gas
diffusion layer 12, a cathode (air electrode/oxidant electrode) 16
having a cathode catalyst layer 14 and a cathode gas diffusion
layer 15, and a proton (hydrogen ion) conductive electrolyte
membrane 17 held between the anode catalyst layer 11 and the
cathode catalyst layer 14.
[0024] The catalyst contained in the anode catalyst layer 11 and
the cathode catalyst layer 14 includes, for example, a sole
platinum group element such as Pt, Ru, Rh, Ir, Os or Pd, an alloy
containing the platinum group elements, or the like. Pt--Ru or the
like having high resistance to methanol and carbon monoxide is
preferably used for the anode catalyst layer 11. Pt, Pt--Co or the
like is preferably used for the cathode catalyst layer 14. The
catalyst is not limited to the above, but various types of
substances having catalyst activity can be used. The catalyst may
be a supported catalyst using a conductive carrier such as carbon
material, or a non-supported catalyst.
[0025] Examples of a proton conductive material configuring the
electrolyte membrane 17 include an organic material such as a
fluorine-based resin (Nafion (trade name, a product of DuPont),
Flemion (trade name, a product of Asahi Glass Co., Ltd.), etc.)
such as a perfluorosulfonic acid polymer having a sulfonic group, a
hydrocarbon-based resin having the sulfonic group, or the like, or
an inorganic material such as tungstic acid, phosphotungstic acid
or the like. But, the proton conductive electrolyte membrane 17 is
not limited to the above.
[0026] The anode gas diffusion layer 12 stacked on the anode
catalyst layer 11 serves to uniformly supply the fuel to the anode
catalyst layer 11 and also serves as a power collector of the anode
catalyst layer 11. The cathode gas diffusion layer 15 stacked on
the cathode catalyst layer 14 serves to uniformly supply an oxidant
to the cathode catalyst layer 14 and also serves as a power
collector of the cathode catalyst layer 14. The anode gas diffusion
layer 12 and the cathode gas diffusion layer 15 are configured of a
porous base material.
[0027] If necessary, a conductive layer is stacked on the anode gas
diffusion layer 12 and the cathode gas diffusion layer 15. As the
conductive layer, there is used, for example, a porous layer (such
as mesh), a porous film, a foil made of an electrically conductive
metal material such as Au or Ni, or a composite material having a
good conductive metal such as gold or carbon coated on an
electrically conductive metal material such as stainless steel
(SUS) or Cu.
[0028] A rubber O-ring 19 is interposed between the electrolyte
membrane 17 and a fuel distribution mechanism 105 and a cover plate
18 both described later to prevent a fuel leak or an oxidant leak
from the power generation unit 101.
[0029] The cover plate 18 has an opening (not shown) for taking in
air as an oxidant. If necessary, a moisture retaining layer or a
surface layer is disposed between the cover plate 18 and the
cathode 16. The moisture retaining layer suppresses water
evaporation by partially impregnating water generated by the
cathode catalyst layer 14 and also promotes uniform diffusion of
air into the cathode catalyst layer 14. The surface layer adjusts
an air intake amount and has plural air introduction ports of which
number, size and the like are adjusted depending on the air intake
amount.
[0030] The fuel distribution mechanism 105 is arranged on the side
of the anode (fuel electrode) 13 of the power generation unit 101.
The fuel distribution mechanism 105 is connected to the fuel
storing unit (fuel tank) 102 through the passage 103 for the liquid
fuel, such as piping.
[0031] The fuel storing unit 102 stores the liquid fuel suitable
for the power generation unit 101. The liquid fuel includes
methanol fuels such as various concentrations of aqueous methanol
solutions, pure methanol and the like. The liquid fuel is not
necessarily limited to the methanol fuel. For example, the liquid
fuel may be an ethanol fuel such as an aqueous ethanol solution or
pure ethanol, a propanol fuel such as an aqueous propanol solution
or pure propanol, a glycol fuel such as an aqueous glycol solution
or pure glycol, dimethyl ether, formic acid, or the like. At any
event, the fuel storing unit 102 stores the liquid fuel suitable
for the power generation unit 101.
[0032] The fuel is introduced into the fuel distribution mechanism
105 from the fuel storing unit 102 through the passage 103. The
passage 103 is not limited to a pipe which is independent of the
fuel distribution mechanism 105 and the fuel storing unit 102. For
example, in a case where the fuel distribution mechanism 105 and
the fuel storing unit 102 are stacked into one body, the passage
103 may be a fuel passage which connects them. It is adequate when
the fuel distribution mechanism 105 is connected to the fuel
storing unit 102 through the passage 103.
[0033] As shown in FIG. 3, the fuel distribution mechanism 105 is
provided with a fuel distribution plate 23 having at least one fuel
inlet opening 21, into which the fuel flows through the passage
103, and plural fuel output openings 22 which discharge the fuel
and its vaporized component. As shown in FIG. 2, the fuel
distribution plate 23 has therein a space unit 24 which forms a
fuel passage for the liquid fuel introduced through the fuel inlet
opening 21. The plural fuel output openings 22 each are directly
connected to the space unit 24 serving as the fuel passage.
[0034] The fuel introduced from the fuel inlet opening 21 into the
fuel distribution mechanism 105 is entered into the space unit 24
and introduced into the plural fuel output openings 22 via the
space unit 24 serving as a fuel passage. For example, a
vapor-liquid separator (not shown) which allows passage of only the
vaporized component of the fuel but not the liquid component may be
disposed at the plural fuel output openings 22. Thus, the vaporized
component of the fuel is supplied to the anode (fuel electrode) 13
of the power generation unit 101. The vapor-liquid separator may be
disposed as a vapor-liquid separation film or the like between the
fuel distribution mechanism 105 and the anode 13. The vaporized
component of the fuel is discharged from the plural fuel output
openings 22 toward the plural portions of the anode 13.
[0035] The fuel output openings 22 are disposed in plural in the
plane of the fuel distribution plate 23, which is contacted to the
anode 13, so that the fuel can be supplied to the entire power
generation unit 101. The number of the fuel output openings 22 is
appropriately two or more, but it is preferable that the fuel
output openings 22 of 0.1 to 10/cm.sup.2 are formed in order to
uniformize the fuel supply amount in the plane of the power
generation unit 101.
[0036] The pump 104 as a fuel transfer control means is disposed at
a position on the passage 103 which connects the fuel distribution
mechanism 105 and the fuel storing unit 102. This pump 104 is not a
circulation pump for circulation of the fuel but a fuel supply pump
which transports the fuel from the fuel storing unit 102 to the
fuel distribution mechanism 105. By sending the fuel by the pump
104 if necessary, controllability of the fuel supply amount is
enhanced. In this case, as the pump 104, a rotary vane pump, an
electro-osmotic pump, a diaphragm pump, a tubing pump or the like
is preferably used from a view point that a small amount of fuel
can be sent under control and further reduction in size and weight
can be made. The rotary vane pump rotates the vanes by a motor to
send the fuel. The electro-osmotic pump uses a sintered porous body
of silica or the like to cause an electro-osmotic flow phenomenon.
The diaphragm pump sends the fuel by driving a diaphragm by an
electromagnet or piezoelectric ceramics. The tubing pump presses
partly a fuel passage having flexibility to send the fuel by
squeezing. Among them, it is more preferable to use the
electro-osmotic pump or the diaphragm pump having the piezoelectric
ceramics from a view point of driving power, size and the like.
[0037] The fuel stored in the fuel storing unit 102 is transferred
through the passage 103 by the pump 104 and supplied to the fuel
distribution mechanism 105. And, the fuel discharged from the fuel
distribution mechanism 105 is supplied to the anode (fuel
electrode) 13 of the power generation unit 101.
[0038] The fuel storing unit 102 may be disposed between the pump
104 and the fuel distribution mechanism 105 to transfer the liquid
fuel by pressurizing the fuel storing unit 102 by the pump 104. In
this case, it is configured to dispose a fuel cutoff valve between
the fuel storing unit 102 and the fuel distribution mechanism 105,
so that it becomes possible to control the supply of the liquid
fuel by means of the passage.
[0039] Within the power generation unit 101, the fuel is diffused
in the anode gas diffusion layer 12 and supplied to the anode
catalyst layer 11. When methanol fuel is used as the fuel, the
internal reforming reaction of methanol expressed by the following
formula (1) occurs in the anode catalyst layer 11. When pure
methanol is used as the methanol fuel, water produced by the
cathode catalyst layer 14 and water in the electrolyte membrane 17
are reacted with methanol to cause the internal reforming reaction
of the formula (1). Otherwise, the internal reforming reaction is
caused by another reaction mechanism not requiring water.
CH.sub.3OH+H.sub.2O.fwdarw.6H.sup.++CO.sub.2+6e.sup.- (1)
[0040] Electrons (e.sup.-) generated by the above reaction are
guided to outside via the power collector, supplied as so-called
output to a load side, and then guided to the cathode (air
electrode) 16. And, protons (H.sup.+) generated by the internal
reforming reaction of the formula (1) are guided to the cathode 16
through the electrolyte membrane 17. Air is supplied as an oxidant
to the cathode 16. The electrons (e.sup.-) and the protons
(H.sup.+) having reached the cathode 16 react with oxygen contained
in the air within the cathode catalyst layer 14 according to the
following formula (2), and water is generated by the reaction.
6H.sup.++(3/2)O.sub.2+6e.sup.-.fwdarw.3H.sub.2O (2)
[0041] The pump drive unit 2 controls the drive of the pump 104.
The pump drive unit 2 controls on/off and the like of the pump 104
according to an instruction from the control unit 4.
[0042] The DC-DC converter 3 has an unshown switching element and
energy accumulation element. Electrical energy generated by the
fuel cell main body 1 is accumulated/discharged by the switching
element and energy accumulation element, and a relatively low
output voltage from the fuel cell main body 1 is increased to an
appropriate voltage before it is outputted.
[0043] The temperature sensor SS1 is a sensor, such as a thermistor
or a thermocouple, which is disposed near the cathode 16 and
measures a temperature Tmp (cathode temperature, or DMFC
temperature) of the cathode 16. A signal (temperature signal)
indicating the measured result of the temperature Tmp from the
temperature sensor SS1 is sent to the control unit 4 and used to
control the fuel supply.
[0044] The fuel cell according to this embodiment does not use the
outside temperature to control and does not require the measurement
of the outside temperature, so that the device configuration is
simplified. For example, it is not necessary to externally expose a
sensor for measuring the outside temperature.
[0045] The voltage sensor SS2 is connected to a terminal of the
power generation unit 101 and measures an output voltage from the
power generation unit 101. Here, when the switch SW is opened to
disconnect the load from the power generation unit 101, the voltage
sensor SS2 can measure an open circuit voltage (OCV) Voc of the
power generation unit 101. The voltage sensor SS2 functions as "the
voltage sensor to measure the open-circuit voltage of the fuel cell
main body".
[0046] The voltage sensor SS2 may measure either a voltage of the
power generation unit 101 alone or a voltage of a plurality (e.g.,
four) of the power generation units 101 connected in series (e.g.,
voltage of four in series). Since a sufficient voltage cannot be
necessarily obtained when the power generation unit 101 is single,
it is normal to have the plural power generation units 101 in the
fuel cell. Here, the voltage sensor SS2 is determined to measure a
voltage of four in series (provided that predetermined values V1
and V2 described later are also voltages of four in series).
[0047] The switch SW is driven by the control unit 4, and the
connection between the power generation unit 101 and the DC-DC
converter 3 (and therefore the connection between the power
generation unit 101 and the load) is turned ON/OFF. The switch SW
functions as "a connecting unit to connect a load to the fuel cell
main body when the voltage measured by the voltage sensor is larger
than the predetermined value".
[0048] The control unit 4 has a supply speed determining unit 41, a
fuel supply control unit 42, a load connection control unit 43, and
an abnormal supply judgment unit 44.
[0049] The supply speed determining unit 41 determines a fuel
supply speed (Duty ratio D) both at the time of startup and steady
operations. At the startup, the supply speed determining unit 41
determines the fuel supply speed (Duty ratio D) on the basis of the
open-circuit voltage Voc measured by the voltage sensor SS2. On the
other hand, at the steady operation, the supply speed determining
unit 41 determines the fuel supply speed (Duty ratio D) such that
the temperature Tmp becomes a target temperature Tt.
[0050] The supply speed determining unit 41 can directly determine
the fuel supply speed. The supply speed determining unit 41 may
also determine the fuel supply speed as a result of determination
of both an operation time and a stop time of the pump 104 described
later.
[0051] Here, it is assumed that the pump drive unit 2 controls
whether or not the fuel is supplied to the power generation unit
101 by the pump 104. That is, the pump drive unit 2 does not
directly control the fuel supply speed. The pump drive unit 2
controls temporally whether or not the fuel is supplied by the pump
104, and as a result, can control the fuel supply amount in a
predetermined time. In other words, the fuel supply speed can be
adjusted by controlling the operation time (hereinafter referred to
as ON time) and stop time (hereinafter referred to as OFF time) of
the pump 104.
[0052] At this time, a fuel supply speed V [g/sec] is represented
by the Duty ratio D as expressed by the following formula (11).
V=Av*D
D=t.sub.on/(t.sub.on+t.sub.of)
=t.sub.on/t.sub.al formula (11)
Av: proportional constant t.sub.on: time of fuel supply by pump 104
(ON time (operation time)) t.sub.of: time of no fuel supply by pump
104 (OFF time (stop time)) t.sub.al=t.sub.on+t.sub.of: total of ON
time and OFF time
[0053] The fuel supply control unit 42 controls the pump 104 via
the pump drive unit 2 so that the fuel is supplied at the Duty
ratio D determined by the supply speed determining unit 41.
Specifically, the pump 104 is controlled so that fuel supply during
the ON time t.sub.on and a stop of the fuel supply during the OFF
time f.sub.of are repeated.
[0054] The fuel supply speed V might be varied due to a temperature
or the like even if the Duty ratio D is constant as described
later. That is, the proportional constant Av is variable depending
on the temperature or the like.
[0055] The load connection control unit 43 controls the switch SW
to turn ON/OFF the connection between the power generation unit 101
and the DC-DC converter 3 (and therefore the connection between the
power generation unit 101 and the load). Specifically, the switch
SW is kept OFF until the open-circuit voltage Voc reaches the
predetermined value V1 (Voc<V1) to continue a state that the
load is not connected to the power generation unit 101. When the
open-circuit voltage Voc reaches the predetermined value V1
(Voc.gtoreq.V1), the switch SW is turned ON, and the load is
connected to the power generation unit 101.
[0056] When the open-circuit voltage Voc does not reach the
predetermined value V1 even if the number of supply times (Cp) of
the fuel to the power generation unit 101 exceeds a predetermined
number of times (Cth) (Cp.gtoreq.Cth), the abnormal supply judgment
unit 44 judges that the fuel supply has suffered from an
abnormality and stops the startup of the fuel cell. In this case,
some means (image, voice, etc.) may be used to give warning to a
user. The abnormal supply judgment unit 44 functions as a "judging
unit to judge that the fuel supply from the fuel supply unit is
abnormal in a case where the number of times of determining the
supply speed by the supply speed determining unit is a
predetermined number or more".
(Operation of Fuel Cell)
[0057] Operation of the fuel cell is described below. FIG. 4 shows
a startup procedure of the fuel cell according to a comparative
example. FIG. 5 and FIG. 6 show startup procedures of the fuel
cells according to the embodiment and a modified embodiment.
I. Startup Procedure of Fuel Cell According to Comparative
Example
[0058] First, a startup procedure of the fuel cell according to the
comparative example is described. In the comparative example, ON
time t.sub.on and OFF time t.sub.of are respectively determined to
be constant values (t.sub.on=t.sub.on0 and t.sub.of=t.sub.of0, Step
S11). And, the pump 104 is operated with the ON time t.sub.on0 and
OFF time t.sub.of0 until the open-circuit voltage Voc reaches the
predetermined value V1 (Steps S12 to S14). When the open-circuit
voltage Voc reaches the predetermined value V1 (Voc.gtoreq.V1), a
load is connected to the fuel cell, and the procedure moves to a
steady operation state according to PID control. If the
open-circuit voltage Voc does not reach the predetermined value V1,
even when the number of times of operation Cp of the pump 104
becomes the predetermined value Cth or more (Cp.gtoreq.Cth), it is
judged as an abnormal supply of fuel (Steps S15 and S16).
[0059] As described above, the fuel cell is started with the
constant ON time t.sub.on0 and OFF time t.sub.of0 in the
comparative example. But, it was found that the startup of the fuel
cell was occasionally instable with the constant ON time t.sub.on
and OFF time t.sub.of (constant Duty ratio D). That is, it is
preferable that the Duty ratio D is changed appropriately at the
time of startup. It is described below.
(1) Behavior of Comparative Example in High Temperature
Environment
[0060] Even if the ON time is constant in a high temperature
environment, an inner pressure of the fuel storing unit (fuel tank)
102 increases, and a liquid sending amount (fuel supply speed V)
from the pump 104 increases. Here, the open-circuit voltage Voc
increases with a time lag after the fuel is sent. The temperature
Tmp of the cathode 16 increases further late. In this case, when
the ON time and OFF time optimized in a normal temperature
environment (outside temperature of 25.degree. C.) are used, there
is a possibility that the temperature Tmp overshoots in the high
temperature environment. It is because the fuel is supplied more
than necessary when the open-circuit voltage Voc exceeds the
predetermined value V1.
(2) Behavior of comparative example in low-temperature
environment
[0061] In a low temperature environment, the inner pressure of the
fuel storing unit (fuel tank) 102 decreases and a liquid sending
amount (fuel supply speed V) from the pump 104 decreases even if
the ON time is constant. As a result, the increase rate of the
open-circuit voltage Voc lowers, and it takes time to start the
fuel cell.
[0062] And, since the liquid sending amount decreases, the number
of operations Cp of the pump 104 increases inevitably. Therefore,
the number of operations Cp reaches the predetermined number of
times Cth before the open-circuit voltage Voc reaches the
predetermined value V1, and there is a possibility that it is
judged as abnormal fuel supply (such as out of fuel) even though
the fuel storing unit 102 still has the fuel. On the other hand,
when the number of operations Cp exceeds largely the predetermined
number of times Cth, it takes time to judge the abnormal supply of
the fuel, and a user's convenience is spoiled.
(3) Coping with High and Low Temperature Environments
[0063] Examples of preferable ON time and OFF time in high
temperature environment and low temperature environment are shown
in FIG. 7 and Table 1. FIG. 7 shows ON times and OFF times coping
with a high temperature environment, a normal temperature
environment (25.degree. C.), and a low temperature environment.
Table 1 shows preferable ON times and OFF times in the high
temperature environment and the low temperature environment with
respect to the ON times and OFF times in the normal temperature
environment (25.degree. C.).
TABLE-US-00001 TABLE 1 ON Time t.sub.on OFF Time t.sub.of High
Temperature Environment Short Long (Fuel Tank Inner Pressure: High)
Low Temperature Environment Long as short as (Fuel Tank Inner
Pressure: possible Low)
[0064] As shown in FIG. 7 and Table 1, it is preferable that the ON
time is decreased and the OFF time is increased in the high
temperature environment in a case where the ON time and OFF time
are optimized in the normal temperature environment. To decrease
the Duty ratio D, it is enough by varying only one of the ON time
and the OFF time, but it is preferable that the ON time is
decreased preferentially in order to prevent the fuel from being
supplied in an excessive amount. It is because if the fuel is
excessively supplied to the power generation unit 101, it is
difficult to remedy a state of excessive amount of fuel other than
the combustion of the excessive amount of fuel. That is, the
increase of the OFF time may be determined as an incidental
element.
[0065] It is preferable that the ON time is increased in the low
temperature environment when the ON time is optimized in the normal
temperature environment. On the other hand, it is preferable that
the OFF time is adjusted to the same level or to be shorter in the
low temperature environment when the OFF time is optimized in the
normal temperature environment. The more the OFF time is shortened,
the more the time required for startup becomes short. But, when a
delay of the increase of the open-circuit voltage Voc and the
increase of the temperature Tmp with respect to the liquid sending
is taken into consideration, it is not preferable to shorten the
startup more than necessary.
[0066] At the time of startup in this embodiment, the ON time and
the OFF time are determined on the basis of the open-circuit
voltage Voc measured by the voltage sensor SS2. Thus, it becomes
possible to control the ON time and the OFF time coping with the
high temperature environment and the low temperature environment
shown in FIG. 7 and Table 1 without measuring the outside
temperature. Details are described later.
[0067] As described above, it is assumed that the fuel cell does
not have a sensor for measuring the outside temperature, and the
outside temperature is not measured directly. What is measured by
the temperature sensor SS1 is the temperature Tmp of the DMFC
itself, and it is not assured that the measured temperature has a
value near the outside temperature. If the fuel is charged
excessively, the inner pressure of the fuel storing unit 102 (fuel
tank) might increase even at normal temperature, and it is
preferable not to depend on the measurement of the outside air
temperature. That is, it is preferable that startup is made stably
in a wide environment.
II. Activation Procedure of Fuel Cell According to Embodiment
[0068] FIG. 5 is a flow chart showing an example of the startup
procedure of the fuel cell. As described above, at the time of
startup, the supply speed determining unit 41 determines the fuel
supply speed (Duty ratio D) on the basis of the open-circuit
voltage Voc measured by the voltage sensor SS2. Here, setting of
the ON time and OFF time is changed when the open-circuit voltage
Voc is smaller or larger than the predetermined value V2.
A. Initial Setting (Step S21)
[0069] ON time t.sub.on is set to a minimum value (t.sub.onmin, for
example, 1 sec) (t.sub.on=t.sub.onmin). And, a pump operation
counter Cp is set to 0 (Cp=0). The pump operation counter Cp shows
a number of times that the pump 104 has operated.
B. Determination of ON Time and OFF Time
[0070] It is judged whether the open-circuit voltage Voc is a
predetermined value V1 (e.g., 1V) or more (Step S22). It is judged
whether or not the startup is terminated. When the open-circuit
voltage Voc is smaller than the predetermined value V1, it means
the fuel cell is during startup, and the fuel supply speed (ON time
and OFF time) is determined. Specifically, the open-circuit voltage
Voc and the predetermined value V2 (V2<V1, for example, 0.5V)
are compared (Step S23). The ON time t.sub.on and OFF time t.sub.of
are determined on the basis of the compared results (Steps S24 and
S25).
(1) "Open-Circuit Voltage Voc<Predetermined Value V2" (Step
S25)
[0071] When "open-circuit voltage Voc<predetermined value V2",
the ON time t.sub.on is increased on the basis of a predetermined
function (t.sub.on=f(t.sub.on)). For example, it is increased by a
predetermined multiple n (for example, two times)
(t.sub.on=n*t.sub.on). And, the OFF time t.sub.of is determined on
the basis of a predetermined function (t.sub.of=f(t.sub.of)). For
example, it is set to a minimum value (for example, 30 sec)
(t.sub.of=t.sub.ofmin).
[0072] It is considered that the state "Voc<V2" continues just
after the startup. Thus, when the state "Voc<V2" continues, the
Step S25 is performed repeatedly, and the ON time t.sub.on
increases every time it is repeated. For example, when the Step S25
is performed i times after the startup, the ON time t.sub.on is
multiplied by n.sup.i and expressed by the following formula
(12).
t.sub.on=n.sup.i*t.sub.onmin formula (12)
Meanwhile, the OFF time t.sub.of is held to the minimum value
(t.sub.ofmin).
[0073] FIG. 8 is a schematic view showing an example of a change
over time of the ON time t.sub.on and OFF time t.sub.of at that
time. It is seen that the ON time t.sub.on increases from t.sub.on
11 to t.sub.on14 by a predetermined function (here, n times
(specifically, 2 times). Meanwhile, the OFF time t.sub.of is kept
at a predetermined value (here, a minimum value (t.sub.ofmin)) for
all of t.sub.of11 to t.sub.of14.
[0074] As described above, when the state "Voc<V2" continues,
the ON time t.sub.on increases, and the Duty ratio is increased.
The continuation of the state "Voc<V2" after the startup means
that the startup takes time, for example, an outside temperature is
low, and the substantial supply amount of the fuel is small. Thus,
when the fuel supply is insufficient and the startup takes time
because the outside temperature is low, the startup time can be
shortened by increasing the Duty ratio.
(2) "Open-Circuit Voltage Voc.gtoreq.Predetermined Value V2" (Step
S24)
[0075] When "open-circuit voltage Voc.gtoreq.predetermined value
V2", the ON time t.sub.on is kept constant (t.sub.on=t.sub.on).
And, the OFF time t.sub.of is set to a maximum value (t.sub.ofmax,
for example, 120 sec (four times of minimum value t.sub.ofmin (30
sec))) (t.sub.of=t.sub.ofmax).
[0076] It is considered that when a certain time passes after the
startup, the state "Voc<V2" changes to a state "Voc.gtoreq.V2".
Thus, it is considered that the state "Voc.gtoreq.V2" indicates
that the startup process has proceeded to some extent, and the Duty
ratio D is decreased by increasing the OFF time t.sub.of.
[0077] Here, it is also considered that when the state
"Voc.gtoreq.V2", both of the ON time t.sub.on and the OFF time
t.sub.of are kept constant, namely the Duty ratio is kept constant.
Here, the OFF time t.sub.of is increased to secure a time for
consumption of the fuel by considering a possibility that a final
Duty ratio D in the state "Voc<V2" is excessive.
[0078] FIG. 9 and FIG. 10 are schematic views showing examples of
changes over time of the ON time t.sub.on and the OFF time t.sub.of
in the above case.
[0079] In FIG. 9, it is assumed that "Voc.gtoreq.V2" is established
at time t2 just after initiating the startup (just after liquid
sending at the ON time t.sub.on21). In this case, the ON time
t.sub.on is kept at t.sub.on22 to t.sub.on24 and a value
(t.sub.on21) just after initiation of the startup. On the other
hand, the OFF time t.sub.of is changed from the minimum value
(t.sub.ofmin) just after the initiation (t.sub.of21) of the startup
to and kept at t.sub.of22 to t.sub.of24 and the maximum value
(t.sub.ofmax). For example, since an outside temperature is high,
"Voc.gtoreq.V2" might be established just after the initiation of
the startup when a substantial fuel supply amount is large.
[0080] In FIG. 10, it is determined that "Voc.gtoreq.V2" is
established at time t3 when a certain time has passed after the
initiation of the startup. In this case, the ON time t.sub.on
increases from t.sub.on31 to t.sub.on33, and kept at a constant
value after the time t3. Meanwhile, it is determined that the OFF
time t.sub.of is the minimum value (t.sub.ofmin, 30 sec) until
t.sub.of31 to t.sub.of33, and t.sub.of34 and t.sub.of35 after the
time t3 are maximum values (t.sub.ofmax, 120 sec).
(3) Control of Pump 104
[0081] The pump 104 is controlled according to the ON time t.sub.on
and OFF time t.sub.of determined as described above (Steps S26 and
S27). The pump 104 operates for the ON time t.sub.on to supply the
fuel, and then it is stopped for the OFF time t.sub.of to stop the
fuel supply. As a result, the fuel is supplied at the Duty ratio D.
As described above, the Duty ratio D is determined by the time
(cycle of ON time t.sub.on and OFF time t.sub.of) when the state
"Voc<V2" continues from
C. Termination of Startup (Step S22)
[0082] When the open-circuit voltage Voc reaches the predetermined
value V1 or more, the startup of the fuel cell is terminated
(movement to the steady operation). That is, a load is connected to
the power generation unit 101, and the supply speed determining
unit 41 determines a fuel supply speed (Duty ratio D) by PID
control or the like such that the temperature Tmp becomes a target
temperature Tt.
[0083] Here, the termination of the startup is judged before the
supply speed is determined. But, the time of judgement may be
changed to judge, for example, the termination of the startup
during the operation or the stop of the pump 104.
[0084] The supply speed determining unit 41 can determine the Duty
ratio D by using the following formula (13).
D=A(Tmp-Tt)+B.intg.(Tmp-Tt)dt+Cd(Tmp-Tt)/dt formula (13)
[0085] The formula (13) shows so-called PID (Proportional Integral
Differential) control and determines the Duty ratio D on the basis
of a proportional term (A(Tmp-Tt)), an integral term
(B.intg.(Tmp-Tt)dt) and a differential term (Cd(Tmp-Tt)/dt) of a
deviation (Tmp-Tt) between a current temperature Tmp and the target
temperature Tt. PI (Proportional Integral) control may be used
instead of the PID control.
[0086] The Duty ratio D is determined periodically by using the
formula (13) or the like, and the fuel supply speed by the pump 104
is controlled, so that the fuel cell can be controlled to agree the
temperature Tmp with the target temperature Tt. D. Judgement of
abnormal fuel supply (Steps S28 and S29)
[0087] When an ON/OFF cycle (pump operation counter Cp) of the pump
104 reaches a predetermined number of times Cth (for example, 5
times), the startup stops. It is considered that the startup takes
too much time, and the fuel supply has abnormality or the like
(e.g., the fuel storing unit (fuel tank) 102 is out of fuel).
III. Startup Procedure of Fuel Cell According to Modified
Embodiment
[0088] FIG. 6 is a flow chart showing another example (modified
embodiment) of the startup procedure of the fuel cell. The
procedure of this modified embodiment is not substantially
different from the procedure of the embodiment of FIG. 5, and
differences are mainly described.
(1) Separate Determination of ON Time t.sub.on and OFF Time
t.sub.of
[0089] In FIG. 5, ON time t.sub.on and OFF time t.sub.of are
determined integrally in Steps S23 to S25. On the other hand, in
the modified embodiment, the ON time t.sub.on and the OFF time
t.sub.of are determined separately in Steps S33 to S36 and Steps
S45 to S48.
[0090] It is because the ON time t.sub.on and the OFF time t.sub.of
have a different determination condition. That is, to determine the
ON time t.sub.on, it is judged whether or not the temperature Tmp
is larger than a predetermined value Tth (e.g., 40.degree. C.)
(Step S34). When the temperature Tmp is smaller than the
predetermined value Tth (when judgement is No in Step S34), the ON
time t.sub.on is increased on the basis of a predetermined function
(t.sub.on=f(t.sub.on)) even when "Voc.gtoreq.V2". For example, it
is increased by a predetermined multiple n (for example, 2 times)
(Step S36). It is because to judge the necessity of an increase of
the fuel supply more correctly by addition of the temperature Tmp.
If the temperature Tmp is low, the fuel supply might be
insufficient even if "Voc.gtoreq.V2".
(2) Unification of Determination of OFF Time t.sub.of and Execution
of Pump OFF
[0091] In FIG. 5, the determination of the OFF time t.sub.of (Steps
S23 to S25) and the stop of the pump 104 (Step S27) are separated
from each other. On the other hand, in the modified embodiment, the
determination of the OFF time t.sub.of and the stop of the pump 104
are executed integrally (Steps S44 to S48). That is, after the pump
104 is stopped (Step S44), the OFF time t.sub.of is determined, and
a time lapse is judged (Steps S46 to S48). It is because the
determinations of the ON time t.sub.on and the OFF time t.sub.of
are separated. But, the determination of the OFF time t.sub.of and
the execution of the pump OFF may be made separately.
(3) Judgement of Termination of Startup
[0092] In FIG. 5, judgement to terminate the startup is made only
one time in one cycle (Step S22). On the other hand, in the
modified embodiment, the termination of the startup is judged
before the determination of the OFF time t.sub.of (Step S32),
during the operation of the pump 104 (Step S42), and during the
stop of the pump 104 (Step S45). To quickly terminate the startup
to supply power to a load, a number of times to judge the
termination of the startup is increased. The termination of the
startup is repeatedly judged while the pump 104 is not in
operation.
(4) Judgement of Abnormal Fuel Supply
[0093] In FIG. 5, abnormal fuel supply is judged according to the
ON/OFF cycle (pump operation counter Cp) of the pump 104. On the
other hand, in the modified embodiment, when the ON time t.sub.on
is smaller than a predetermined value t.sub.onth (for example, 64
sec) (t.sub.on<t.sub.onth) and the pump operation counter Cp0
has a predetermined value Cth0 (for example, five times) or less,
it is judged that the fuel supply is free from an abnormality (Step
S37). That is, when the pump operation counter Cp0 becomes larger
than the predetermined value Cth0 and the ON time t.sub.on is the
predetermined value t.sub.onth or more, it is judged that the fuel
supply has an abnormality. The case that the ON time t.sub.on is
the predetermined value t.sub.onth or more is subject to judgement,
and the ON time t.sub.on is prevented from being increased
excessively (excessive fuel supply).
[0094] Here, counting by the pump operation counter Cp0 is
performed only when the fuel supply becomes constant (Step S35).
Meanwhile, the ON time t.sub.on depends on the number of times that
Step S36 is executed as expressed by the formula (12). As a result,
the case where the fuel supply becomes constant (Step S35) and the
case where the fuel supply is increased (Step S36) are counted
separately in the modified embodiment, and when the fuel supply
exceeds the respective predetermined values (Cth0 and Cth1=LOG n
(t.sub.onth/t.sub.onmin)), it is judged that the fuel supply has an
abnormality. That is, a number of operation times of the pump 104
is counted as a whole in FIG. 5, and when "Voc<V2" and
"Voc.gtoreq.V2" in the modified embodiment, they are counted
separately.
[0095] Thus, the cases that the fuel supply is increased and not
increased are counted separately, so that the accuracy of the
judgment of abnormalities can be improved.
[0096] When the ON time of the pump 104 exceeds a predetermined
time at the startup, the pump drive frequency may be decreased
(specifically, 1/2 of an initial frequency) on the basis of a
predetermined function. It is because the pressure in the fuel
storing unit 102 is decreased by suctioning the fuel from the fuel
storing unit 102 when the pump 104 is turned ON to send well the
fuel to the fuel distribution mechanism. That is, a fuel discharge
pressure of the pump 104 is increased by lowering the pump drive
frequency, so that good liquid sending can be made even when the
pressure in the fuel storing unit 102 is decreased to a low
level.
Example
[0097] A result of starting the fuel cell by the startup procedure
according to the above-described modified embodiment is described
below.
(1) Check of Operation in High Temperature Environment
[0098] As a result of a startup test in a high temperature
environment (outside temperature of 38.degree. C.), a temperature
Tmp was little under 50.degree. C. at maximum, and the overshoot of
the temperature Tmp did not occur.
[0099] In the high temperature environment where the pump flow rate
becomes large, "open-circuit voltage Voc.gtoreq.predetermined value
V2" is true when the ON time t.sub.on is short, and the OFF time
t.sub.of becomes a maximum value (t.sub.ofmax, 120 sec). That is,
the ON time t.sub.on and the OFF time t.sub.of change as shown in
FIG. 9.
(2) Check of operation in low temperature environment
[0100] As a result of a startup test in a low temperature
environment (outside temperature of 5.degree. C.), it takes about 5
min to 6 min when the open-circuit voltage Voc rises to the
predetermined value V1 (completion of startup), which is shorter
than the startup time (20 min) in the comparative example.
[0101] In the low temperature environment where the pump flow rate
becomes small, the ON time t.sub.on becomes long, "open-circuit
voltage Voc.gtoreq.predetermined value V2" becomes true, and the ON
time t.sub.on continues. That is, the pump behavior becomes as
shown in FIG. 10. Since the ON time t.sub.on is extended to some
extent, the fuel supply amount can be secured, and a rise time of
the open-circuit voltage Voc becomes shorter than that in the
startup method of the comparative example.
[0102] As described above, the operation of the pump 104
corresponding to the preferable ON time t.sub.on and OFF time
t.sub.of shown in FIG. 7 and Table 1 can be realized both in a high
temperature environment and in a low temperature environment. That
is, it becomes possible to start the fuel cell which has secured
robustness with respect to an outside temperature and a fuel tank
inner pressure.
[0103] And, the completion of startup can be promoted by increasing
the Duty ratio at a low temperature, or the like. It means that
there is no problem even if the reference values (Cth and Cth0) at
the time of the judgement of an abnormal fuel supply are decreased.
As a result, the time required for the judgement of an abnormality
which took about 20 minutes in the comparative example can be
shortened to five minutes (1/4).
[0104] While certain embodiments have been described, these
embodiments have been presented by way of example only, and are not
intended to limit the scope of the inventions. The novel
embodiments described herein may be embodied in a variety of other
forms; and various omissions, substitutions and changes may be made
without departing from the spirit of the inventions. Such
embodiments or modifications are intended to be included within the
scope and spirit of the inventions and also covered by the
accompanying claims and their equivalents.
* * * * *