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CA 02398727 2002-07-30
21994 Transl. of PCT/DE00/00462
T R A N S L A T I O N
D E 8 C R I P T I O N
CURRENT SOURCE WITB SERIES CONNECTED CELhB
The invention relates to a current source comprised of a
plurality of cells connected electrically in series. The
individual cells generate current or are prepared to generate
current.
Examples for devices of the type described at the outset
are a plurality of batteries connected in series or a plurality of
fuel cells connected in series and thus a fuel cell stack.
A fuel cell comprises a cathode, an electrolyte and an
anode. The cathode is supplied with an oxidizing agent, e.g. air
and the anode is supplied with a fuel, e.g. hydrogen.
Different fuel cell types are known and include, for
example, the SOFC [solid oxide fuel cell] fuel cell as described by
the publication DE 44 30 958 C1.
The SOFC fuel cells are also called high temperature fuel
cells since their operating temperatures are up to 1000°C. At the
cathode of a high temperature fuel cell, oxygen ions are formed in
the presence of an oxidizing agent. The oxygen ions pass through
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21994 Transl. of PCT/DE00/00462
an electrolyte and recombine on the anode side with the hydrogen
arising from the fuel to water. With the recombination, electrons
are liberated and thus electrical energy is generated.
The operating temperature of a PEM fuel cell is about
80°C. At the anode of a PEM fuel cell protons are formed in the
presence of fuel by means of a catalyst. The protons pass through
the electrolyte and combine at the cathode side with oxygen arising
from the oxidizing agent to water. Electrons are thus liberated
and electrical energy is generated.
A multiplicity of fuel cells are as a rule electrically
and mechanically connected together to generate large electrical
power by connecting elements. A sample of such a connecting
element is known from DE 44 10 711 C1 to be a bipolar plate. By
means of bipolar plates, fuel cells stacked one above another are
electrically connected in series. This arrangement is known as a
fuel cell stack.
As a fuel, methane or methanol can, inter al3a be used.
The mentioned fuels are converted b~ reforming or oxidation inter
alia, into hydrogen or hydrogen-rich gas.
Should an individual cell within a group or series
connected cells fail, for example, because of a defect, it becomes
an electrical resistance in the series circuit. The availability
of power is reduced not only by the cell which has dropped out of
the circuit but also because of the additional electric resistance
which has been introduced for which power must be expanded to
overcome the efficiency of the apparatus, thus correspondingly
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CA 02398727 2002-07-30
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CA 02398727 2002-07-30
21994 Transl. of PCT/DE00/00462
reduced. If the individual cells forming the currant-generating
apparatus are PEM fuel cells and one of the fuel cells fails, for
example because of an interruption in the operating medium supply,
the flow of current through electrolysis membrane converts water in
the membrane to hydrogen and oxygen. The oxygen emerges at the
anode side and the hydrogen at the cathode side via the piping
oxygen is passed into the anode spaces of the other cell. There
mixed o curs
a o ential ~s. Oxygen recombines with the hydrogen
generated at the anode side. In this manner the current generation
is reduced also in neighboring cells and finally terminates..
Because of the dropout of one fuel call in a series circuit of fuel
cells, other fuel cells can thus be poisoned. As a result, the
power-generating capacity of such a fuel cell stack can be
significantly reduced.
A solution to such problems for fuel cells has been
proposed according to the publication DE 197 466 16 Al by bridging
the individual fuel cells in the case of failure. For that purpose
corresponding electrical switching means has been provided.
The drawback of this solution that it is technologically
expensive, involves high cost and requires the availabilityof a
corresponding volume [space] for the electrical switching means.
The availability of space and the weight associated therewith is
especially highly disadvantageous in mobile units.
The object of the invention is to make available an
apparatus and the conditions of its use in which in s simple and
economical manner and with reduced need for space, power losses can
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CA 02398727 2002-07-30
21994 Transl. of PCT/DE00/00462
be minimized upon defects arising in individual cells in a series
circuit.
The object of the invention is obtained with an apparatus
with the features of the main claim as well as by a use with the
features of the dependent claims.
In the apparatus according to the claims the diodes are
connected parallel to the individual cells. The diodes are so
poled that the current flow 3s possible through the diodes in the
normal direction. Under "normal direction" a flow direction is to
be understood which is that of the apparatus of an intact current-
generating unit. The diodes are thus so connected that no short
circuit results.
Advantageously each individual cell has a diode connected
in parallel thereto to maximize the results.
The provision of a diode is required to prevent a short
circuit. In the direction opposite the blocking direction. the
diodes have a reduced ohmic resistance. During a defect-free
operation of the apparatus the current flows through the
individual cells since these show no electrical resistance but
rather act as current sources. In such a state there is no current
flow through the diode or practically no current flow therethrough.
If a defect arises in one of the individual cells this
creates a high ohmic resistance as a result of the defeat in
comparison to the ohmic resistance of the diode. In such a case
the current flows through the parallel-connected diodes instead of
through the individual cell. Power losses which have been required
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21994 Transl. of PCT/DE00/00462
to overcome ohmic resistances are thus minimized. As a whole the
apparatus remains capable of delivering power up to the maximum
with comparatively simple means. In addition. poisoning of
neighboring cells, which can arise with a fuel cell stack, is
avoided. The fuel cells with which the present invention are used
are thus fuel cells which not only result in power loss because of
ohmic resistances but also those with which poisoning phenomena can
be limited. With a fuel cell stack, especially high power
maintenance effects have been indicated.
Small, light, economical diodes are commercially
available. The invention can thus be realized with relatively low
cost. An apparatus according to the claims can for example be used
in mobile apparatus like portable computers, portable lamps and the
like since here reduced Weight and volume is especially desirable.
In the Figure a circuit according to the claimed
invention has been sketched. A plurality of fuel cells 1 are
serially connected electrically with one another. For each fuel
cell 1 a respective diode 2 is connected therewith in parallel so
that no short circuit can arise from the diode 2 of the current aye
generated by the fuel cells.
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ACA 02398727 2002-07-30
