Note: Descriptions are shown in the official language in which they were submitted.
CA 02348861 2001-04-30
Method for regulating the fuel concentration in a fuel
mixture of a fuel cell which contains alcohol or ether
as fuel and water, and fuel cell system
Description
The invention relates to a method for
regulating the fuel concentration in a fuel mixture for
a fuel cell which is formed by an alcohol or an ether
as fuel and water, and to a fuel cell system,
containing at least one fuel cell, which can be
operated with a fuel mixture consisting of an alcohol
or an ether as fuel and water, and at least one mixing
space, which is connected to in each case one
controllable fuel inlet, and at least one fuel-mixture
feedline, which connects the at least one mixing space
to in each case the at least one fuel cell.
At the date of this application, fuel cell
systems of the abovementioned type have already been
known for over 30 years (cf. for example W. Vielstich;
"Brennstoffelemente" [Fuel Elements] Verlag Chemie,
Weinheim 1965, pages 73-93 and L. Oniciu~ "Fuel Cells",
Abacus-Press, Kent, 1976, pages 93-98). The alcohol,
for example methanol, as fuel, mixed with water to form
a fuel mixture, is fed to a fuel cell, where it is
directly converted into electrical energy. Alcohol
contents of between 1$ by volume and 5$ by volume in
the fuel mixture have proven particularly advantageous
for operation of a fuel cell of this type. One problem
with the operation of fuel cells of this type is that
of keeping the alcohol concentration of the fuel
mixture as constant as possible. This applies equally
to fuel cells which use alcohol and fuel cells which
use ether as the fuel.
DE 197 O1 560 A1 has disclosed a fuel cell
system in which a liquid fuel mixture comprising a fuel
and a coolant is fed to the anode space of a fuel cell.
DE 196 28 888 Cl discloses a fuel cell which can be
operated, inter alia, with a fuel mixture of fuel and
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water. In this case, the efficiency of the fuel cell is
increased by an alternating operating pressure.
DE 44 25 634 C1 has disclosed a method and a device for
the metered supply of methanol and/or water from a
reservoir, via a conveyor pipe, to a fuel cell system.
In this case, a delivery pump is used to convey a
constant mass flow rate from the reservoir into the
conveyor line, and the differential pressure between
conveyor line and fuel cell system is set to a
predetermined value with the aid of a differential
pressure regulator, so that the quantity of methanol
and/or water supplied can be set, for example, by
varying the opening and closing times of a solenoid
valve. The abovementioned prior art fails to disclose
either a method or a device for regulating the fuel
concentration in the fuel mixture.
DE 35 08 153 has disclosed the regulation of
the concentration of the fuel within a fuel mixture
using a fuel cell which is operating in idling mode.
This document exploits the fact that the idling
potential of a fuel cell is dependent on the fuel
concentration. However, this type of regulation is
highly complex and therefore very expensive.
It is now an object of the present invention to
provide a method of the type described in the
introduction which is simpler than the prior art and at
the same time is effective, and to provide a fuel cell
system which is suitable for carrying out this method.
With regard to a method for regulating the fuel
concentration in a fuel mixture of a fuel cell which
contains an alcohol or ether and water, this obj ect is
achieved by the fact that the fuel is fed via a
controllable fuel inlet to a mixing space, from where
the fuel mixture is fed, via a fuel-mixture feedline,
to the fuel cell via a membrane which is arranged
downstream of the fuel inlet, as seen in the direction
of flow, delimits a measurement chamber and is
selectively permeable to water and the fuel, a liquid
or gaseous measurement mixture with a fuel
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concentration of less than 1~ by volume or more than 5~
by volume, depending on the quantity of fuel which
permeates into the measurement chamber from the fuel
mixture per unit time, being produced in the
measurement chamber, whereupon the fuel concentration
in the measurement mixture is determined and the fuel
inlet is controlled as a function of the fuel
concentration in the measurement mixture.
Therefore, the regulation is achieved by the
fact that it is not directly the fuel concentration in
the fuel mixture which is determined, but rather the
fuel concentration in a measurement mixture in which
the fuel concentration is within a range which can be
measured with sufficient accuracy and speed using known
sensors. The fuel concentration in the measurement
mixture is directly dependent on the fuel concentration
in the fuel mixture, so that it can be controlled by
measuring the fuel content in the measurement mixture.
The method according to the invention can be
carried out in such a way that, to produce the
measurement mixture in the measurement chamber, a
carrier liquid or a carrier gas is used to dilute
and/or remove the permeated fuel. The permeated fuel
can also be consumed at electrodes in the measurement
chamber, so that it is not imperative that the fuel be
removed by the carrier liquid or the carrier gas. The
specified fuel concentrations can also be achieved even
without a carrier liquid or a carrier gas, by means of
suitable membranes: for example, the membrane may be
more permeable to the fuel than to water by more than
one order of magnitude, so that a measurement mixture
with a fuel concentration of less than 1~ by volume
even down to the ppm range can be formed in the
measurement chamber from the permeated fuel and the
permeated water. Conversely, if the permeability of the
membrane is higher for water than for the fuel, it is
possible to form measurement mixtures in which the fuel
concentration is over 5, preferably over 10~ by volume.
Membranes which are suitable for this purpose are known
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(cf. for example H. Strahtmann; "Membranes and Membrane
Separation processes", B. Elvers, S. Hawkins, G. Schulz
(Ed. ) ; "UIImanns Encyclopedia of Industrial Chemistry",
Vol. A. 16. VCH, Weinheim, 1990, pages 187-263) .
The method according to the invention may also
be implemented in such a way that the fuel
concentration of the measurement mixture, in the case
of values above 10 or 5$ by volume, is determined by
means of a liquid sensor.
The method according to the invention may also
be carried out in such a way that the fuel
concentration of the measurement mixture is determined
by means of density or viscosity measurements.
The method according to the invention may also
be carried out in such a way that the fuel
concentration of the measurement mixture is determined
by means of optical methods, for example by means of
infrared absorption. Optical sensors which are suitable
for this purpose, in particular for assessing the CH
vibrations of the fuel, are also known (cf. for example
A. Brittain et al. (Ed.) "Optically Based Methods for
Process Analysis", S. PIE Proceedings Vol. 1681,
Somerset, NJ, USA, 1992) .
The method according to the invention may also
be carried out in such a way that the fuel
concentration of the measurement mixture, in the case
of values below 1.0 or 0.1$ by volume, is determined by
means of a gas sensor. A gas sensor of this type has a
semiconductor element which changes its electrical
properties as a function of the concentration of the
fuel. A gas sensor of this type and its use in a
measurement chamber delimited by a silicone membrane is
likewise known in connection with bioreactors (cf. FMC-
Handbuch der Biotechnologie [FMC Biotechnology Manual)
Kempe GmbH, Berlin). Furthermore, it is in this case
possible to carry out the determination by recording
the conductivity and by infrared absorption.
In a fuel cell system of the type described in
the introduction, the abovementioned object is achieved
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by the fact that the fuel cell system has at least one
measurement probe, which is arranged downstream of the
fuel inlet, as seen in the direction of flow of the
fuel mixture, and comprises
a) a measurement chamber,
b) a membrane which delimits the measurement chamber,
is in contact with the fuel mixture, is
selectively permeable to the fuel and/or water and
is used to produce a liquid or gaseous measurement
mixture with a fuel concentration of less than 1~
by volume or more than 5~ by volume depending on
the quantity of fuel which has permeated into the
measurement chamber from the fuel mixture per unit
time, and
c) a sensor for determining the fuel concentration in
the measurement mixture,
and that the fuel cell system has means for controlling
the fuel inlet as a function of the fuel concentration
in the measurement mixture.
The invention furthermore provides for the
sensor to be a gas, liquid, infrared absorption,
density measurement, optical or conductivity
measurement sensor.
In the text which follows, an embodiment or a
configuration is described, by way of example, for the
methods according to the invention and the fuel cell
systems according to the invention in connection with
methanol combustion. In the drawing:
Fig. 1 diagrammatically depicts the structure of a
fuel cell system which uses methanol as fuel,
and
Fig. 2 diagrammatically depicts an enlarged excerpt
from the arrangement shown in Fig. 1, with a
measurement probe coupled to a fuel-mixture
feedline.
The fuel cell system shown in Fig. 1, which
uses methanol as fuel, comprises a fuel cell 1 with a
fuel chamber 2 and a reactant chamber 3. The fuel
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mixture, which consists of methanol and water, is fed
to the fuel chamber 2 via the fuel-mixture feedline 4,
the methanol concentration in the fuel mixture
amounting to approximately 4$ by volume. The reactant
chamber 3 is supplied with air via a reactant feedline.
The exhaust gas, which is a water/air mixture, is
discharged from the reactant chamber 3 via the exhaust-
gas line 6.
The fuel mixture, in which the methanol level
has been reduced by the amount consumed, is discharged
from the fuel chamber 2 via the residual fuel discharge
line 7 and is passed to an intermediate store 8. A
recycling pump 9 is connected to the intermediate store
8, via which pump the fuel mixture of reduced methanol
content is pumped into the fuel-mixture feedline 4 and
therefore to the fuel chamber 2. The fuel mixture is
therefore circulated.
To enable the fuel mixture which enters the
fuel chamber 2 to have the requisite methanol
concentration, methanol is pumped out of a methanol
store 10, via a methanol pump 11, into a mixing space
12, where it is mixed with the fuel mixture of reduced
methanol content from the intermediate store 8. The
mixing space 12 is symbolically represented in enlarged
form in Fig. 1 but may actually also simply be a part
of the fuel-mixture feedline 4. The pumping capacity of
the methanol pump 11 is controllable.
To be able to ensure that the methanol content
of the fuel mixture which enters the fuel chamber 3 is
as constant as possible, the methanol concentration is
regulated. To do this, a parameter which is directly
dependent on the methanol concentration in the fuel
mixture is measured. The area surrounded by a dashed
circle in Fig. 1 is shown on an enlarged scale in Fig.
2. Fig. 2 shows a measurement probe 13 which is fitted
to the fuel-mixture feedline 4. The measurement probe
13 has a membrane 14 which is in direct contact with
the fuel mixture flowing past. The membrane 14 is
selectively permeable to water and methanol, in such a
CA 02348861 2001-04-30
manner that a measurement mixture which consists of
water and methanol and has a methanol content in the
ppm range is formed in the measurement chamber 16 which
is delimited by the walls 15 of the measurement probe
13 and of the membrane 14. In the measurement chamber
16 there is a methanol sensor 17 which is highly
effective in this concentration range. The measured
variable which is generated by the methanol sensor 17
as a function of the methanol concentration in the
measurement mixture is transmitted via the measurement
line 18 to a control unit 19 (Fig. 1) which therefore
controls the methanol pump 11 as a function of the
methanol concentration in the measurement mixture.
Since the methanol concentration in the measurement
mixture is directly dependent on the methanol
concentration in the fuel mixture in the fuel feedline
4 at the location of the measurement probe 13, this
control arrangement can be used to maintain a
substantially constant methanol concentration in the
fuel mixture.
CA 02348861 2001-04-30
List of reference numerals
1 Fuel cell
2 Fuel chamber
3 Reactant chamber
4 Fuel-mixture feedline
Reactant feedline
6 Exhaust-gas line
7 Residual fuel discharge line
8 Intermediate store
9 Recycling pump
Methanol store
11 Methanol pump
12 Mixing space
13 Measurement probe
14 Membrane
Wall
16 Measurement chamber
17 Methanol sensor
18 Measurement line
19 Control unit
