Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF MEASURING CONCENTRATION OF FUEL
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] The present invention generally relates to a method of measuring a
concentration of the fuel, and more particularly a simple and high-accuracy
method of
measuring the concentration of the fuel.
2. Description of Related Art
[0002] With the progress of industry, consumption of traditional energy
sources,
such as coal, petroleum, and natural gas, are continuously growing. Due to the
limited
reserves of natural energy, it is necessary to develop new alternative energy
sources to
replace the traditional energies, and among them, fuel cells are the important
and
practical one.
[0003] In brief, the fuel cells are basically a power generation device that
converts
chemical energy into electric energy through the inverse reaction of water
electrolysis.
For example, a proton exchange membrane fuel cell (PEMFC) mainly includes a
membrane electrode assembly (MEA) and two electrode plates. The MEA includes a
proton exchange membrane, an anode catalyst layer, a cathode catalyst layer,
an anode
gas diffusion layer (GDL), and a cathode GDL. The anode catalyst layer and the
cathode catalyst layer are respectively disposed at two sides of the proton
exchange
membrane. The anode GDL and the cathode GDL are respectively disposed on the
anode catalyst layer and the cathode catalyst layer. Furthermore, the two
electrode
plates include an anode and a cathode respectively disposed on the anode GDL
and the
cathode GDL.
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[0004] Currently, the common proton exchange membrane fuel cell is the direct
methanol fuel cell (DMFC), which directly uses a methanol solution as a fuel
supply
source to generate current through the relevant electrode reaction of methanol
and
oxygen. The reaction formulae of the direct methanol fuel cells are expressed
as
follows:
Anode: CH3OH+H20 -* C02+6H++6e-
Cathode: 3/202+6H++6e---* 3H20
[0005] During the reaction, the concentration of the methanol solution
introduced to
the anode has great impact on the stability of the output of the direct
methanol fuel cell.
If the concentration of the methanol solution introduced to the anode is not
properly
controlled, disadvantages of poor power generation efficiency and unstable
output
power may be generated, and further, the MEA may be damaged. Therefore, it has
become the important subject in the research and development of the direct
methanol
fuel cells how to properly supplement methanol so as to control the
concentration of the
methanol solution introduced into the anode in a suitable range.
[0006] The most direct way to control the concentration of the fuel in a fuel
cell is
to directly measure the concentration of the fuel by a sensor and determine
the
supplement amount of the fuel and water. This method has been disclosed in TWI
228591, US 6,589,671 B1, US 6,488,837, US 2002/076589 Al, US 2003/0196913 Al,
WO 01/35478. US 6,488,837 and US 2003/0196913 Al have disclosed the MEA
serving as a sensor for directly measuring the concentration of methanol. It
should be
noted that the accuracy of the above method is liable to be affected by the
impurities in
the fuel, aging or instability of the MEA.
[0007] In the prior art, for example, in US 6,698,278 B2, the measured
temperature
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and the current value are brought into an empirical formula to calculate the
concentration of the fuel. This method can directly measure the concentration
of the
fuel without using a sensor. However, this method must be adjusted according
to
different fuel electrode systems so as to calculate the concentration of the
fuel. US
6,589,679 and TW 1282639 have also disclosed the methods of directly measuring
the
concentration of the fuel without using a sensor.
[0008] Furthermore, since the concentration of the methanol solution is
somewhat
related to physical properties such as the transmission speed of the sound in
the
methanol solution, the dielectric constant of the methanol solution and the
density of the
methanol solution, the transmission speed of the sound in the methanol
solution is used
to calculate the concentration of the methanol solution in some prior arts. It
is noted
that TWI 251954 has disclosed that the dielectric constant and the density of
the
methanol solution can be used to calculate the concentration of the methanol
solution.
However, the sensor used in this concentration calculating method is expensive
and the
precision of measurement is significantly affected by bubbles in the methanol
solution,
the methanol solution inside the sensor must be kept still and have no bubbles
when
measuring, which increase the difficulties in the measurement.
[0009] In view of the above concentration measuring methods, the problems of
difficult measurement, high measurement cost, unstable measurement accuracy
exist.
Therefore, it is urgent to find a simple and high-accuracy method of measuring
concentration of the fuel currently in this field.
SUMMARY OF THE INVENTION
[0010] Accordingly, an aspect of the present invention is directed to a
simple, low-cost, and
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stable method capable of accurately measuring the concentration of a fuel.
[0011] An aspect of the present invention is directed to a method of measuring
the concentration of the fuel, which includes the following steps. First, a
fuel cell unit
having at least an anode, a cathode, and a membrane electrode assembly (MEA)
is
provided. Next, a fuel is supplied to the anode, while a reactive gas is
supplied to the
cathode. Then, the amount of the reactive gas supplied to the cathode is
adjusted
and the concentration of the fuel is estimated in accordance with the
consumption
rate of the reactive gas in the fuel cell unit.
According to one aspect of the present invention, there is provided a
method of measuring concentration of a fuel, comprising: providing a fuel cell
unit
comprising at least an anode, a cathode, and a membrane electrode assembly
(MEA); establishing at least one reactive gas concentration vs. time curve
corresponding to at least one known concentration of a fuel, wherein the
method of
establishing the at least one reactive gas concentration vs. time curve
comprises:
(a) supplying the fuel having one known concentration to the anode; (b)
supplying a
reactive gas to the cathode; (c) stopping supplying the reactive gas to the
cathode
and measuring concentration of the reactive gas in the cathode so as to obtain
one
reactive gas concentration vs. time curve; (a)' supplying the fuel having an
unknown
concentration to the anode; (b)' supplying the reactive gas to the cathode;
(c)'
stopping supplying the reactive gas to the cathode and measuring concentration
of
the reactive gas in the cathode so as to obtain a new reactive gas
concentration vs.
time curve thus considerably reducing a measurement time; and comparing the
new
reactive gas concentration vs. time curve with the at least one reactive gas
concentration vs. time curve, so to estimate the concentration of the fuel
having the
unknown concentration.
According to another aspect of the present invention, there is provided
a method of measuring concentration of a fuel, comprising: providing a fuel
cell unit
comprising at least an anode, a cathode, and a membrane electrode assembly
(MEA); establishing at least one gaseous product concentration vs. time curve
corresponding to at least one known concentration of a fuel, wherein the
method of
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establishing the at least one gaseous product concentration vs. time curve
comprises: (a) supplying the fuel having one known concentration to the anode;
(b) supplying a reactive gas to the cathode; (c) stopping supplying the
reactive gas to
the cathode and measuring concentration of a gaseous product in the cathode so
as
to obtain one gaseous product concentration vs. time curve; (a)' supplying the
fuel
having an unknown concentration to the anode; (b)' supplying the reactive gas
to the
cathode; (c)' stopping supplying the reactive gas to the cathode and measuring
concentration of the gaseous product in the cathode so as to obtain a new
gaseous
product concentration vs. time curve thus considerably reducing a measurement
time; and comparing the new gaseous product concentration vs. time curve with
the
at least one gaseous product concentration vs. time curve, so to estimate the
concentration of the fuel having the unknown concentration.
According to a further aspect of the present invention, there is provided
a method of measuring concentration of a fuel, comprising: providing a fuel
cell unit
comprising at least an anode, a cathode, and a membrane electrode assembly
(MEA); establishing at least one open circuit voltage vs. time curve
corresponding to
at least one known concentration of a fuel, wherein the method of establishing
the at
least one open circuit voltage vs. time curve comprises: (a) supplying the
fuel having
one known concentration to the anode; (b) supplying a reactive gas to the
cathode;
(c) stopping supplying the reactive gas to the cathode and measuring open
circuit
voltage of the MEA so as to obtain one open circuit voltage vs. time curve;
(a)'
supplying the fuel having an unknown concentration to the anode; (b)'
supplying the
reactive gas to the cathode; (c)' stopping supplying the reactive gas to the
cathode
and measuring open circuit voltage of the MEA so as to obtain a new open
circuit
voltage vs. time curve thus considerably reducing a measurement time; and
comparing the new open circuit voltage vs. time curve with the at least one
open
circuit voltage vs. time curve, so to estimate the concentration of the fuel
having the
unknown concentration.
According to still a further aspect of the present invention, there is
provided a method of measuring concentration of a fuel, comprising: providing
a fuel
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cell unit comprising at least an anode, a cathode, and a membrane electrode
assembly (MEA); establishing at least one output current vs. time curve
corresponding to at least one known concentration of a fuel, wherein the
method of
establishing the at least one output current vs. time curve comprises: (a)
supplying
the fuel having one known concentration to the anode; (b) supplying a reactive
gas to
the cathode; (c) stopping supplying the reactive gas to the cathode and
measuring
output current of the MEA so as to obtain one output current vs. time curve
thus
considerably reducing a measurement time; (a)' supplying the fuel having an
unknown concentration to the anode; (b)' supplying the reactive gas to the
cathode;
(c)' stopping supplying the reactive gas to the cathode and measuring output
current
of the MEA so as to obtain a new output current vs. time curve; and comparing
the
new output current vs. time curve with the at least one output current vs.
time curve,
so to estimate the concentration of the fuel having the unknown concentration.
According to yet another aspect of the present invention, there is
provided a method of measuring concentration of a fuel, comprising: providing
a fuel
cell unit comprising at least an anode, a cathode, and a membrane electrode
assembly (MEA); establishing at least one output voltage vs. time curve
corresponding to at least one known concentration of a fuel, wherein the
method of
establishing the at least one output voltage vs. time curve comprises: (a)
supplying
the fuel having one known concentration to the anode; (b) supplying a reactive
gas to
the cathode; (c) stopping supplying the reactive gas to the cathode and
measuring
output voltage of the MEA so as to obtain one output voltage vs. time curve
thus
considerably reducing a measurement time; (a)' supplying the fuel having an
unknown concentration to the anode; (b)' supplying the reactive gas to the
cathode;
(c)' stopping supplying the reactive gas to the cathode and measuring output
voltage
of the MEA so as to obtain a new output voltage vs. time curve; and comparing
the
new output voltage vs. time curve with the at least one output voltage vs.
time curve,
so to estimate the concentration of the fuel having the unknown concentration.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The accompanying drawings are included to provide a further
understanding of the invention, and are incorporated in and constitute a part
of this
specification. The drawings illustrate embodiments of the invention and,
together
with the description, serve to explain the principles of the invention.
[0013] FIGs. 1A and 1 B are schematic views of a method of measuring a
concentration of a fuel according to the first embodiment of the present
invention.
[0014] FIG. 2 shows a curve of a relationship between an open circuit voltage
output by a fuel cell unit and a time when the supply of the reactive gas is
stopped.
[0015] FIG. 3 shows a curve of a relationship between an open circuit voltage
output by the fuel cell unit and a time when the supply of the reactive gas is
stopped
in the case of different fuel concentrations.
[0016] FIGs. 4A and 4B are schematic views of a method of measuring a
concentration of a fuel according to the second embodiment of the present
invention.
[0017] FIG. 5 shows a curve of a relationship between a current of a fuel cell
unit
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outputting electric energy under a constant voltage and a time when the supply
of the
reactive gas is stopped in the case of different fuel concentrations.
[0018] FIG. 6 shows a curve of a relationship between a voltage of a fuel cell
unit
outputting electric energy under a constant current and time ceasing the
supply of the
reactive gas at different fuel concentrations.
[0019] FIG. 7 is a schematic view of a method of measuring a concentration of
a
fuel according to the third embodiment of the present invention.
DESCRIPTION OF THE EMBODIMENTS
[0020] Reference will now be made in detail to the present embodiments of the
invention, examples of which are illustrated in the accompanying drawings.
Wherever
possible, the same reference numbers are used in the drawings and the
description to
refer to the same or like parts.
[0021] FIGs. IA and I B are schematic views of a method of measuring a
concentration of a fuel according to the first embodiment of the present
invention.
Referring to FIG. I A, first, a fuel cell unit 100 having an anode 110 and a
cathode 120
is provided. In this embodiment, the fuel cell unit 100 may be a direct
methanol fuel
cell unit. In detail, the, fuel cell unit 100 at least has a membrane
electrode assembly
(MEA) 130 between the anode 110 and the cathode 120. The MEA 130 includes, for
example, a proton exchange membrane 131, an anode catalyst layer 132, a
cathode
catalyst layer 133, an anode gas diffusion layer (GDL) 134 and a cathode GDL
135.
The anode catalyst layer 132 and the cathode catalyst layer 133 are
respectively
disposed at two sides of the proton exchange membrane 131, and the anode GDL
134
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and the cathode GDL 135 are respectively disposed on the anode catalyst layer
132 and
the cathode catalyst layer 133. Definitely, the fuel cell unit 100 in this
embodiment
may be a fuel cell unit of any form, such as a single cell or the fuel cell
stack itself, and
those of ordinary skill in the art can select a suitable fuel cell unit
according to the
actual requirements. It is noted that the above-mentioned fuel cell unit 100
may be a
fuel cell stack installed in a fuel cell system.
[0022] Then, a fuel 140 is supplied to the anode 110 of the fuel cell unit
100, and a
reactive gas 150 is supplied to the cathode 120 of the fuel cell unit 100. In
this
embodiment, the fuel 140 supplied to the anode 110 is, for example, a methanol
solution
of an uncertain concentration. Definitely, the fuel 140 supplied to the anode
110 may
also be another fuels, for example, ethanol solution and formic acid solution,
and those
of ordinary skill in the art may select a suitable fuel according to the
actual requirements.
In addition, the reactive gas 150 supplied to the cathode 120 is, for example,
air, oxygen,
or other suitable gases. When the fuel 140 and the reactive gas 150 are
continuously
supplied to the fuel cell unit 100, a part of the fuel at the anode 110
reaches the cathode
catalyst layer 133 of the MEA 130 by a crossover phenomena (e.g. the crossover
fuel
160 shown in FIG. 1A), and is consumed by the combustion reaction with oxygen,
and
the reaction formula is expressed as follows:
3/2 02 + CH3OH 4 CO2 + 2H2O
[0023] The combustion reaction consumes the oxygen at the cathode 120, and the
fuel cell unit 100 maintains a suitable open circuit voltage OCV.
[0024] Referring to FIG. 1B, the supply of the reactive gas 150 to the cathode
120
may be reduced or stopped by controlling a gas transmission component or
switching a
valve in this embodiment. Since the amount of the crossover fuel 160 reaching
the
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cathode 120 is in direct proportion to the concentration of the fuel 140 at
the anode 110,
the amount of the crossover fuel 140 reaching the cathode 120 directly
influences the
consumption rate of the reactive gas 150 at the cathode 120. In detail, when
the
amount of the reactive gas 150 supplied to the cathode 120 is limited, since
the reactive
gas 150 in the cathode 120 may react with the crossover fuel 160 and be
gradually
consumed by the combustion reaction, the consumption rate of the reactive gas
150 may
be estimated in accordance with the open circuit voltage of the fuel cell unit
100. In
this embodiment, when the reactive gas 150 supplied to the cathode 120 is
stopped, the
reactive gas 150 in the cathode 120 can only allow maintaining the open
circuit voltage
OCV of the fuel cell unit 100 for a while, and the duration of the time period
is closely
related to the concentration of the fuel 140. In detail, the higher the
concentration of
the fuel 140 is, the higher the capability of crossover of the fuel 140 from
the anode 110
to the cathode 120 will be, and meanwhile, the consumption rate of the
reactive gas 150
is greater. Otherwise, the lower the concentration of the fuel 140 is, the
lower the
capability of crossover of the fuel 140 from the anode 110 to the cathode 120
will be,
and meanwhile, the consumption rate of the reactive gas 150 is lower.
[0025] Accordingly, since the concentration of the fuel 140 is related to the
consumption rate of the reactive gas 150, the present invention may rapidly
calculate
the concentration of the fuel 140 in accordance with the consumption rate of
the
reactive gas 150.
[0026] FIG. 2 shows a curve of a relationship between an open circuit voltage
OCV
output by a fuel cell unit 100 and a time when the supply of the reactive gas
150 is
stopped, and FIG. 3 shows a curve of a relationship between an open circuit
voltage
OCV output by the fuel cell unit 100 and a time when the supply of the
reactive gas 150
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is stopped in the case of different fuel concentrations. This embodiment may
estimate
the consumption of the reactive gas 150 in accordance with the measured open
circuit
voltage OCV. In detail, when the reactive gas 150 is exhausted, the open
circuit
voltage OCV will drop, and the time point at which the open circuit voltage
OCV drops
.5 may be used to calculate the concentration of the fuel 140.
[0027] It would be apparent from FIG. 3 that, when the concentrations of the
used
methanol solution are 1%, 3%, 5%, 7%, or 9%, the time points at which the open
circuit
voltage OCV drops are apparently different. In other words, this embodiment
may
calculate the concentration of the fuel in accordance. with the time points at
which the
open circuit voltage OCV drops. This method has good measurement sensitivity.
[0028] In the above embodiment, the time points at which the open circuit
voltage
drops are used to calculate the concentrations of the fuel. However, the
present
invention is not limited thereto, and may also calculate the concentration of
the fuel in
accordance with the speed at which the open circuit voltage drops. In
addition, this
embodiment may further calculate the concentration of the fuel in accordance
with the
time period for the open circuit voltage OCV to drop to a certain value. As
shown in
FIG. 3, the higher the concentration of the methanol solution is, the shorter
the time
period for the open circuit voltage OCV to drop to a certain value will be.
Otherwise,
the lower the concentration of the methanol solution is, the longer the time
period for
the open circuit voltage OCV to drop to a certain value will be. It should be
noted that
this embodiment may enlarge the time period for the open circuit voltage OCV
to drop
to the certain value corresponding to different concentrations of the fuel by
controlling
the amount of the introduced reactive gas, thus achieving an easier estimation
of the
concentration of the fuel.
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[0029] It should be noted that the fuel cell unit 100 of the embodiment may be
directly used as a fuel concentration sensor and may also be connected in a
fuel
circulation loop of a fuel cell system. The fuel concentration sensor when
installed
does not need to be connected in branch, and the fuel concentration sensor
when
operated will not affect the operation of the fuel cell system. In detail,
since the
present invention utilizes the fuel cell unit 100 to measure the concentration
of the fuel,
the present invention may use one or more units in the fuel cell stack as the
fuel
concentration sensor to measure the concentration of the fuel directly. At
this time, the
fuel cell stack does not need an additional fuel concentration sensor.
[0030] FIGs. 4A and 4B are schematic views of a method of measuring a
concentration of a fuel according to the second embodiment of the present
invention.
Referring to FIG. 4A, a fuel cell unit 400 of this embodiment is similar to
the fuel cell
unit 100 of the first embodiment except that the fuel cell unit 400 in FIG. 4A
outputs the
electric energy to an external load 170 directly.
[0031] Referring to FIG. 4B, the fuel cell unit 400 may reduce or stop the
supply of
the reactive gas 150 to the cathode 120 by controlling a gas transmission
component or
switching a valve (not shown). When the amount of the reactive gas 150
supplied to
the cathode 120 is limited, the reactive gas 150 in the cathode 120 is
consumed by the
reduction reaction of the fuel cell unit 400 and the combustion reaction with
the
crossover fuel 160. Since the amount of the crossover fuel 160 is in direct
proportion
to the concentration of the fuel 140 at the anode 110, the concentration of
the fuel 140
may be calculated in accordance with the consumption rate of the reactive gas
150.
[0032] FIG. 5 shows a curve of a relationship between a current of a fuel cell
unit
outputting electric energy at a constant voltage and a time when the supply of
the
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reactive gas is stopped in the case of different fuel concentrations. FIG. 6
shows a
curve of a relationship between a voltage of a fuel cell unit outputting
electric energy at
a constant current and a time when the supply of the reactive gas is stopped
in the case
of different fuel concentrations. It would be apparent from FIGs. 5 and 6 that
the
measurement observer may rapidly calculate the concentration of the fuel 140
in
accordance with the voltage variation rate or the current variation rate.
[0033] Referring to FIG. 5 again, based on the current-time curve in FIG. 5,
the
measurement observer may calculate the concentration of the fuel in accordance
with
the time period for the current to drop to a certain value. In detail, the
higher the
concentration of the fuel is, the shorter the time period for the current to
drop to a
certain value will be. Otherwise, the lower the concentration of the fuel is,
the longer
the time period for the current to drop to a certain value will be. In other
embodiments,
the measurement observer may also calculate the concentration of the fuel in
accordance with the drop amount of the current in a specific time. In detail,
the higher
the concentration of the fuel is, the greater the drop amount of the current
in a specific
time will be. Otherwise, the lower the concentration of the fuel is, the less
the drop
amount of the current in a specific time will be.
[0034] Referring to FIG. 6, based on each voltage-time curve in FIG. 6, the
measurement observer may calculate the concentration of the fuel in accordance
with
the time period for the voltage to drop to a certain value. In detail, the
higher the
concentration of the fuel is, the shorter the time period for the voltage to
drop to a
certain value will be. Otherwise, the lower the concentration of the fuel is,
the longer
the time period for the voltage to drop to a certain value will be. In other
embodiments,
the measurement observer may also calculate the concentration of the fuel in
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accordance with the drop amount of the voltage in a specific time. In detail,
the higher
the concentration of the fuel is, the greater the drop amount of the voltage
in a specific
time will be. Otherwise, the lower the concentration of the fuel is, the less
the drop
amount of the voltage in a specific time will be.
[0035] Accordingly, in the measurement methods in FIGs. 5 and 6, the
concentration of the fuel may be easily estimated by controlling the
introduced reactive
gas.
[0036] FIG. 7 is a schematic view of a method of measuring a concentration of
a
fuel according to the third embodiment of the present invention. Referring to
FIG. 7,
the fuel cell unit 500 of the present embodiment is similar with the fuel cell
unit 100 of
except that the fuel cell unit 500 shown in FIG. 7 further includes a sensor
180.
Specifically, the concentration of the reactive gas 150 in the cathode 120 is
measured
through the sensor 180 first, for example. Then, the concentration of the fuel
140 is
estimated in accordance with a relationship between the concentration of the
reactive
gas 150 and time. In the present embodiment, the sensor 180 is a sensor for
measuring
oxygen (02 sensor) or a pressure gauge, for example.
[0036] Since the concentration variation of the reactive gas 150 is measured
through
the sensor 180 directly, it is more convenience to estimate the concentration
of the fuel
140 precisely.
[0037] Similarly, the concentration of the fuel 140 can be estimated in
accordance
with the time period for the concentration of the reactive gas 150 to drop to
a certain
value, or in accordance with the speed at which the concentration of the
reactive gas
150 drops.
[0038] In the third embodiment, the sensor 180 is a sensor for measuring
carbon
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dioxide (CO2 sensor). Since the consumption rate of the reactive gas at the
cathode is
proportional to the generation rate of the gaseous product such as carbon
dioxide (CO2),
the CO2 sensor may also be used in the present embodiment to measure the
concentration of the carbon dioxide such that the concentration of the fuel
140 can also
be estimated. Specifically, the CO2 sensor is used to measure the speed at
which the
concentration of the carbon dioxide increases or the time period for the
concentration of
the carbon dioxide to increase to a certain value.
[0039] In view of the above, as the present invention may estimate the
concentration
of the fuel in accordance with the consumption rate of the reactive gas at the
cathode,
the method of measuring a concentration of a fuel of the present invention is
simple and
accurate. Furthermore, the method of measuring a concentration of a fuel of
the
present invention has a high stability and will not be easily affected by the
flow of the
fuel and the bubbles in the fuel. Especially, one can just use operational
fuel cell stack
in the system to measure the fuel concentration.
[0040] It will be apparent to those skilled in the art that various
modifications and
variations can be made to the structure of the present invention without
departing from
the scope or spirit of the invention. In view of the foregoing, it is intended
that the
present invention cover modifications and variations of this invention
provided they fall
within the scope of the following claims and their equivalents.
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