Note: Descriptions are shown in the official language in which they were submitted.
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22539 ~ Transl. of PCT/DE01/044327
T R A N S L A T I O N
D a s c r i p t i o n
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Technical Field
The invention relates to a fuel cell stack, especially a
control for an operating medium for a fuel cell stack.
A fuel cell comprises a cathode, an electrolyte and an
anode. The cathode is supplied with an oxidizing agent, for
example, air and the anode is supplied with fuel, for example,
hydrogen.
A plurality of fuel cells are, as a rule, electrically
and mechanically connected together by connecting elements to
produce more electrical outputs. An example for such a connecting
element is illustrated as the bipolar plate known from DE 44 10
711 C1. By means of bipolar plates, fuel cells stacked one above
the other can be electrically connected in series. This
arrangement is knows as a fuel cell stack. These are comprised of
the bipolar plates and the electrode-electrolyte units.
In a liquid operated direct-methanol fuel cell, a
water/methanol mixture is fed into the anode compartments. For
producing technologically usable voltages, individual cells are
electrically connected in series. Typical methanol concentrations
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lie, in this case, in the range of 0.1 m to 2 m. As a rule, the
methanol/water mixture is supplied in a substoichiometric amount.
Of special significance for an economical operation of the fuel
cell is the correct adjustment of the methanol concentration. Too
low a methanol concentration gives rise to a reduction is the cell
efficiency because of a methanol deficiency, whereas too high a
concentration reduces the efficiency because of the useless
combustion of diffusing methanol to the cathode side. The methanol
concentration must, therefore, be controlled.
As state of the art there is a control system for
regulating the methanol concentration which comprises a methanol
sensor.
Such an arrangement is described in DS 198 50 720 A1.
For example, a data set can be prograamned into a minicomputer to
provide the optimal methanol concentration for a respective
operating point (in terms of temperature electrical current and
medium throughflow rates). Furthermore, a unit can be provided for
metering the methanol. In conjunction with the methanol sensor and
the methanol metering unit, the methanol concentration is the cell
can be adjusted by means of a control algorithm.
The methanol regulation known from the state of the art
is indeed satisfactory, but it is connected with significant
structural and financial cost. 8specially for small systems in a
power range of the order of 10 to 500 watts, the cost for such a
control system is outside the economical range. Furthermore, the
establishment of a data set for all possible operating conditions
is connected with greater expense.
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Similar problems arise in the control of the operating
medium on the oxidizing side (cathode side) of the fuel cell which
is not limited only to the case of direct methanol fuel cells.
The oxidizing agent, especially oxygen or air, is
supplied to the fuel cell generally under pressure with the aid of
a compressor. On cost grounds here as well a control for an
optimum operating range is advantageous.
Obiect and solution of the Invention
The invention has as its object to provide a simple and
cost effective method of controlling at least one operating medium
for a fuel call stack as well as a fuel cell stack suitable for
this method.
The object is attained with a method according to the
main claim as well as an apparatus for carrying out the method
according to the auxiliary claim.
Advantageous embodiments can be deduced from the
dependent claims which relate back to them.
Subject of the Invention
The invention is based upon the concept that an
individual fuel cell of a fuel cell stack can be so selected and,
optionally, so modified that it requires a somewhat higher
operating medium concentration for a predetermined cell voltage
than the remaining individual cells of the fuel cell stack. For
the same available operating medium concentration for all cells,
the voltage of a so modified individual cell can, as a rule, be
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lower than that of the other cells. When the voltage of this
individual cell falls below a predetermined threshold, utilizing a
simple electronic circuit, an operating medium can be additionally
fed into the operating medium circulation until the cell voltage of
the modified fuel cell rises above threshold or a further
threshold.
The method of the invention according to claim 1 thus is
provided for a fuel cell stack with a multiplicity of fuel cells.
These fuel cells are connected with at least one operating medium
circulation through which the fuel cells are supplied with the
operating medium. The method of the invention provides that the
metering of an operating medium into the operating medium circuit
is effected in dependence upon the detected cell voltage.
In an advantageous embodiment according to claim 2, the
feed of the operating medium is effected directly upstream of the
fuel cell whose cell voltage is detected. The regulating mechanism
thus occurs especially rapidly because the direct feed of the
operating medium into the fuel cell can immediately provide an
increased conversion and thus an increase in the cell voltage.
Advantageously, the metering or feed of the operating
medium according to claim 3 is controlled by a valve. Thus
especially the overstepping or understepping of the threshold for
the cell voltage is directly converted into an electrical signal
which controls the valve.
The control of the metering of the operating medium is
effected according to claim 4, as soon as the cell voltage of the
fuel cell falls below a predetermined value. This simple control
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mechanism allows the operating medium metering to be controlled in
dependence upon the conversion which occurs is this cell and thus
optimize the operating medium metering. Advantageously, the
operating medium metering according to the method of the invention
is effected directly via the conversion of the fuel cell and thus
the cell voltage and not as is customary in the state of the art
via control of the concentration of the operating medium. Con-
sequently, it is also not necessary for the method of the invention
for different operating states and operating parameters to
determine data sets for the concentration of an operating medium.
In order to prevent an overdosing of the operating medium
into the operating medium circulation, a further embodiment
according to claim 5 provides that the feed of the operating medium
is terminated as soon as a call voltage of the fuel cell exceeds a
predetermined value, for example, as upper limiting value.
The monitoring of the cell voltage with respect to the
upper and lower limiting values is advantageously effected in a
single modified fuel cell to minimize the apparatus costs. The
upper and lower limiting values can be either identical or also
slightly different. Even with identical limiting values, a control
mechanism is provided since operating medium dosing and the
resultant increase is the cell voltage occur only with a time
delay. For the operation of the fuel cell stack, as optimum range
of the cell voltage can be established. The closer the feed of the
operating medium to the fuel cell, the smaller is the time delay
and thus the more closely the upper and lower limiting values can
be matched to the optimum range.
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Advantageously, the detection of the cell voltage is
effected for a modified fuel cell according to claim 6. The
modified fuel cell used in the method according to claim 6 has, by
comparison with the remaining fuel cells of the fuel cell stack, a
diffusion resistance which is greater by at least 5% so that an
correspondingly altered cell voltage will be obtained. The
"modification" in the sense of the invention is that this modified
fuel cell at optimal fuel concentration for the remaining fuel
cells of the cell stack produces a call voltage which is at least
10% less. Such a modification can be obtained by the use of a
thicker diffusion layer. Stated otherwise, the modified fuel cell
requires for its optimum operating point, a methanol concentration
which is greater by about 5 to 10% than that of the remaining fuel
cells of the stack.
Advantageously the method is used in a direct methanol
fuel cell with methanol as a fuel.
It is also conceivable to use the method for control of
the oxidizing agent. For example, the air or oxygen feed to the
fuel cells can be effected in dependence upon the cell voltage of a
modified cell. In that case the modified cell serves to control,
for example, an air compressor and its output.
A further advantageous embodiment of the method according
to claim 9 proposes to detect the cell voltage of a further fuel
cell within the fuel cell stack. This fuel call is advantageously
not modified. The dosing of the operating medium is then controlled
additionally as a function of the cell voltage of this fuel cell.
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The method according to the invention is is principle
usable with all direct methanol fuel cell stacks. It can thus
control not only the metering of the fuel (pure methanol or also
highly concentrated methanol-water mixture) but also can be used
for the control of the oxidizing agent. It especially allows
control by regulating a compressor which supplies the requisite
oxidation medium at a corresponding pressure.
For carrying out the method according to the invention, a
direct methanol fuel cell stack with a multiplicity of fuel cell
according to claim 11 is suitable in which at least one of the fuel
calls is provided with a diffusion resistance which is increased by
comparison with that of the remaining fuel cells of the stack.
This means that this modified fuel cell at optimum operating
conditions of the remaining fuel cells will produce as a rule a
cell voltage which is at least 10% less.
According to claim 12, the means [providing the increased
diffusion resistance] can advantageously be a thicker diffusion
layer or an additionally arranged diffusion layer. A thicker
diffusion layer or an additional diffusion layer generally will
reduce the access of methanol to the anode. As a result there is
also usually a reduced conversion. To the extent that the
remaining fuel cells of the stack operate with approximately
optimal conditions, there is a reduction as a consequence of the
cell voltage in this modified fuel cell against that of the
remaining fuel cells.
Advantageously the fuel cell stack according to claim 13
has an electrically switchable valve which is disposed in the feed
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of the operating medium. The valve can thus be located within the
operating medium circulation or also outside it.
In an advantageous refinement of the fuel call stack
according to claim 14, the modified cell is connected with the
electrically switchable valve so that the modified cell is capable
of controlling the valve via an electric signal.
Special Descrit>tive Part
The invention is described below in greater detail in
conjunction with Figures and exemplary embodiments.
Figure 1: A schematic illustration of the automatic
operating medium concentration control, for example for a direct
methanol fuel cell stack.
Figure 2: Current/voltage graphs of different methanol
concentrations for a direct methanol fuel cell.
Figure 1 shows the configuration of the fuel cell stack
according to the invention. From a supply vessel 1, the methanol
is supplied to the anode circulation 2 of the fuel cell stack
comprising the fuel cells BZ with the bipolar plates 5a to 5e. A
circulation tank 4 and a circulation pump 3 serve for the uniforan
flow to the anode. 'Via the bipolar plate 5d, the cell voltage is
additionally detected. With this cell voltage a circuit 6 is
activated which controls the valve (pump) 7 for metering the
methanol.
A direct methanol fuel cell stack is supplied via an
anode circulation, comprised of a circulating pump and circulating
tank with fuel. One cell of the stack has a thicker anodic
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diffusion layer than the other cells. As the methanol concentration
approaches the minimum required methanol concentration within the
cells as a result of the utilization [of methanol], the cell
voltage within the modified cell falls because of the anodic
diffusion over voltage. The cell voltage is detected by an
electronic circuit and sets a pump in operation which feeds
methanol from a vessel into the circulation.
Advantageously the modified cell is protected by a diode
to exclude pole reversal. Furthermore, it is advantageous to meter
the methanol directly into the feed line of the cell. In this
manner a rapid reaction time for the control is ensured. A further
advantageous feature is found in the connection of a resister is
parallel to the poles of the modified fuel cell. When the fuel
cell stack is operated in an idling mode, this can ensure that the
adjustment of a methanol concentration to zero will be avoided and
which could lead to difficulties is startup.
As a modification of the cell, apart from the increase in
the thickness of the diffusion layer, there are other possibilities
for decreasing the effective diffusion coefficient within the
anode. For example with the use of identical electrodes, a
reduction in the active area can serve the purpose as well. The
method advantageously does not require calibration.
In as advantageous embodiment, the cell voltage of a
further fuel cell is detected. The detected cell voltage of the
nonmodified fuel cell thus serves as an additional parameter for
the regulation of the metering. In this manner the metering of
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fuel can be prevented in the case in which, for example, there is
afailure of the supply of the oxidation medium for the entire stack
and a consequent drop in the cell voltage of the modified fuel
cell. The detection of the cell voltage of a further fuel cell
ensures that onlyin the case of a relative drop of the cell voltage
of the modified cell because of a reduction in the fuel
concentration, will there by the addition of fuel. Thus also an
overdosing can be precluded for example since in such a case both
the cell voltage of the modified cell as wall as that of the
nonmodified cell will drop.
The following table shows an advantageous possibility of
the switching logic for a direct methanol fuel cell stack.
Cell Voltage of the Cell voltage of a Metering of
Modified Fuel Cell Further Fuel Cell Additional Fuel
> 0.3 V > 0.3 V No
< 0.3 v > 0.3 v Yes
< 0.3 V < 0.3 V No
In Figure 2, four current/voltage curves are shows for a
direct methanol fuel cell whereby the fuel concentration varies in
the range of 0.5 M to 4 M of methanol in water. Both too low and
too high concentrations of methanol give rise to a drop in the cell
voltage and thus make the system uneconomical. The goal is to
operate the system with an optimal fed methanol concentration
depending upon the operating conditions (for example predetermined
current draw). The term "optimal" as used hare means with the
least possible methanol concentration to produce the highest
possible cell voltage in the fuel cell.
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