Note: Descriptions are shown in the official language in which they were submitted.
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Description
Solid electrolyte high temperature ~uel cell module and
method for its operation
The invention relates to a solid electrolyte high
temperature fuel cell module and a method for its oper-
ation.
It is known that during the electrolysis o~
water, the water molecules are decomposed into hydrogen
and oxygen by an electric current. In the fuel cell, this
procedure runs in the opposite direction. During the
electrochemical joining of hydrogen and oxygen to form
water, electric current is produced: with a high effi-
ciency and - i~ pure hydrogen is used as fuel gas -
without the emission o~ pollutants and carbon monoxide.
Even using industrial fuel gases, for example natural
gas, and using air instead of pure oxygen, the fuel cell
produces distinctly fewer pollutants and less CO2 than
other technologies of fossil energy carriers. The indus-
trial implementation o~ this principle has led to very
different solutions, having various types o~ electrolytes
and having operating temperatures between 80~C and
1 0 0 0 ~ C .
In the solid electrolyte high temperature ~uel
cell (solid oxide fuel cell; SOFC), natural gas is used
as the primary energy source. The power density of
1 MW/m3 enables a very compact construction. The heat
which is additionally produced has a temperature of over
9 0 0 ~ C .
In the case o~ a solid electrolyte high tempera-
ture ~uel cell module, a fuel cell module is alsoreferred to as a "stack" in the technical literature,
underneath an upper bipolar covering plate there are, in
order, a window ~ilm, a solid electrolyte electrode
element, a further window ~ilm, a ~urther bipolar plate,
and so on, one on another. In this case,
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a solid electrolyte electrode element lying between two
adjacent bipolar plates, including the window films
resting directly on both sides of the solid electrolyte
electrode element and those sides of each o~ the two
bipolar plates resting on the window films, together form
a solid electrolyte high temperature fuel cell.
This type and further types o~ fuel cell modules
are known, for example from the "Fuel Cell Handbook" by
A.J. Appelby and F.R. Foulkes, Van Nostraud Reinhold,
pages 442 to 454, or from the article "Brennstoffzellen
als Energiewandler" [Fuel Cells as Energy Converters],
Energiewirtschaftliche Tagesfragen, June 1993, issue 6,
page 382 to 390.
German Laid-Open Publications 39 35 722 and
40 09 138 disclose solid electrolyte high temperature
fuel cell modules which comprise a plurality of solid
electrolyte high temperature fuel cells which are con-
nected in series, are planar and rest firmly on one
another. Installed between directly adjacent cells
connected in series is a bipolar plate which electrically
conductively connects the cathode of one cell to the
anode of the directly adjacent cell, and ensures a supply
o~ operating gas by means of ducts let in on both sides.
In this case, the ducts conveying the operating gas are
arranged parallel to the longitudinal axis of the solid
electrolyte high temperature fuel cell module and extend
through the entire solid electrolyte high temperature
~uel cell module.
A signi~icant problem during the operation of a
solid electrolyte high temperature fuel cell module
consists in dissipating the heat produced during the
reaction out o~ the solid electrolyte high temperature
fuel cell module. Part of this heat is dissipated via the
operating medium. The amount of heat dissipated via the
3 5 operating medium is greater the higher the temperature
dif~erence between the inflowing and outflowing operating
media. A higher temperature difference
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can lead to a lower operating temperature o~ the ~uel
cells, if the operating media ~low into the solid elec-
trolyte high temperature ~uel cell module at a lower
temperature, which leads to a reduction in the power
capacity of the solid electrolyte electrode element. At
the same time, because o~ the temperature di~erences in
the active cell region, there is the risk that the
mechanically sensitive electrolytes will be damaged.
A ~urther problem arises during operation with
operating media to be re~ormed. As a result o~ the
re~orming reaction, the region ~or the ~eeding in o~ the
operating media is additionally cooled, which likewise
leads to mechanical stresses in the electrolytes.
Furthermore, German Laid-Open Specification
42 17 892 discloses a ~uel cell arrangement in which a
waste gas ~rom a stack chamber, which comprises a multi-
plicity o~ solid electrolyte ~uel cells, is burned
outside the stack chamber in a combustion chamber. The
heat o~ the combustion gas is trans~erred to an oxidation
gas and a reaction gas in a heat exchanger connected
downstream of the stack chamber, said gases subsequently
being fed to the stack chamber in order to operate the
solid electrolyte ~uel cells.
A high temperature ~uel cell stack with inte-
grated heat exchanger arrangement ~or the dissipation ofthe heat released ~rom the same is also disclosed by the
German Laid-Open Speci~ication 41 37 968. In that docu-
ment, ~or ~urther use in a gas turbine connected down-
stream, a waste gas is heated to the necessary input
temperature o~ the gas turbine within the high tempera-
ture ~uel cell stack. In addition, an oxidation gas and
a reaction gas ~or the high temperature ~uel cell stack
are heated outside the high temperature ~uel cell stack,
using the heat released.
-
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The invention is thus based on the object of
specifying a method of operating a solid electrolyte high
temperature fuel cell module in which the heat is used
efficiently and with a low technical outlay to heat the
operating media. In addition, it is intended to specify
a solid electrolyte high temperature fuel cell module for
carrying out the method.
The object mentioned first is achieved with the
features of patent claim 1. The object mentioned second
is achieved with the features of patent claim 2.
In this method of operating a solid electrolyte
high temperature fuel cell module, which is composed of
a plurality o~ fuel cells stacked on one another, and to
which an operating medium necessary for operating the
fuel cells is fed, according to the invention the heat
produced in the combustion process in the fuel cells is
used for heating the operating medium before it is fed
into the fuel cells, in which the entire operating means
flows through a first duct section arranged inside the
solid electrolyte high temperature fuel cell module, in
order to pick up the heat. Some of the heat produced in
the fuel cells is thus transferred to the operating
medium outside the fuel cells but within the solid
electrolyte high temperature fuel cell module. Given a
suitable design of the temperature of the operating
medium before it is fed into the solid electrolyte high
temperature fuel cell module, the operating medium is
heated by this heat transfer to the temperature necessary
for the operation of the fuel cells, so that virtually no
reduction in the power capacity of the solid electrolyte
high temperature fuel cell module occurs. The operating
medium can thus be used for cooling the ~uel cell module,
as a result of a low input temperature, without a reduc-
tion in the power occurring. Because of the heating o~
the operating medium and the resultant reduction in the
temperature
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differences in the active cell region, the mechanically
sensitive electrolyte is also not damaged. Since the
temperature at the introduction of the operating media
into the solid electrolyte high temperature fuel cell
5 module is lower than in the case of the known methods,
the mass flow of operating media which is necessary for
the dissipation of heat is reduced. By means of the
method claimed, a compact construction of the solid
electrolyte high temperature fuel cell module is made
possible.
In the case of the solid electrolyte high tem-
perature fuel cell module having a plurality of fuel
cells stacked on one another, to which an operating
medium necessary for operating the fuel cells is fed,
15 according to the invention means are provided, arranged
inside the solid electrolyte high temperature fuel cell
module, for heating the operating medium before it is fed
into the fuel cells.
Preferably, a first duct section is arranged at
2 0 the edge of the solid electrolyte high temperature fuel
cell module, approximately parallel to the longitudinal
axis of the latter. Said section extends over the entire
length o~ the solid electrolyte high temperature fuel
cell module and opens into a second duct section, which
25 is arranged approximately parallel to the first duct
section and communicates with the fuel cells. The operat-
ing medium is thus heated on a path which approximately
corresponds to the length of the solid electrolyte high
temperature fuel cell module, before it is fed into the
3 0 fuel cells.
In a further configuration, the first duct
section comprises a first subsection, which is arranged
approximately parallel to the longitudinal axis at the
edge of the solid electrolyte high temperature fuel cell
35 module. The first subsection, which extends over the
entire length of the solid electrolyte high temperature
fuel cell module, opens into a second subsection running
approximately vertically with respect to the longitudinal
axis,
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which subsection opens into a third subsection, which is
arranged approximately parallel to the longitudinal axis
on the opposite edge and extends over the entire length
of the solid electrolyte high temperature fuel cell
5 module. This section opens into a second duct section,
which is arranged approximately parallel to the third
subsection of the first duct section and communicates
with the fuel cells. In this particular configuration,
the operating medium is thus heated on a path which
corresponds to more than twice the length of the solid
electrolyte high temperature fuel cell module, before it
is fed into the fuel cells.
In a further preferred configuration, the first
and/or second duct section in each case comprises at
15 least two parallel duct sections. By means of increasing
the number of duct sections, the surface for picking up
the heat is enlarged. This leads to an improved heat
dissipation from the solid electrolyte high temperature
fuel cell module.
2 O The duct sections have, in particular, a circular
cross section.
In a further configuration, the first and second
duct section for feeding and heating the operating medium
are coated with catalytic material for reforming a
25 combustion medium contained in the operating medium. As
a result, the combustion medium is already reformed
before it is fed into the fuel cells, and the mechanical
stresses in the solid electrolyte electrode element,
which occur because of the temperature differences during
the reforming in the fuel cell, are largely avoided.
In order to explain the invention further,
reference is made to the exemplary embodiments of the
drawing, in which:
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FIG l shows a cross section through a solid electrolyte
high temperature fuel cell module according to
the invention in a schematic representation.
FIG 2 shows a plan view of a bipolar plate of the solid
electrolyte high temperature fuel cell module
from FIG 1 according to the invention in a sche-
matic representation.
FIG 3 shows a cross section through a solid electrolyte
high temperature fuel cell module according to
the invention in a schematic representation.
FIG 4 shows a plan view of a bipolar plate of a solid
electrolyte high temperature fuel cell module
according to the invention in a schematic repre-
sentation.
According to FIG l, a solid electrolyte high
temperature fuel cell module 2 comprises a multiplicity
of rectangular, plate-like fuel cells 4. The solid
electrolyte high temperature fuel cell module 2 is closed
off at the top and at the bottom using two cover plates
6 and 8 and a baseplate 10. A first duct section 12 is
arranged at the edge of the solid electrolyte high
temperature fuel cell module 2, approximately parallel to
the longitudinal axis 14 of the latter, and extends over
the entire length of the solid electrolyte high temp-
erature fuel cell module 2.
Arranged approximately parallel to the first duct
section 12 is a second duct section 16, which communi-
cates with the fuel cells 4. The first duct section 12
and the second duct section 16 open into a cut-out 18,
which is arranged in the cover plate 6. An operating
medium, for example hydrogen or oxygen, whose flow
direction is indicated by an arrow 20, flows through the
baseplate 10 into the solid electrolyte high temperature
fuel cell module 2. It passes, via the first duct section
12 and the cut-out 18, into the second duct
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section 16, which communicates with the ~uel cells 4.
After the reaction carried out there, in other words
combustion, the operating medium is led away through the
baseplate 10 via a third duct section 22. The flow
direction of the emerging operating medium is indicated
by an arrow 24.
Some of the heat produced in the fuel cells 4 is
thus transferred to the operating medium outside the fuel
cells 4 but within the solid electrolyte high temperature
fuel cell module 2, before it is fed into the fuel cells
4.
The first and second duct sections 12 and 16 are
separated in this embodiment by a partition 26 made of
insulating ceramic, ~or example A1203, ZrO2 (YSZ) or
MgA1204-Spinel.
FIG 2 shows, in a plan view, the construction of
a bipolar plate 32 constructed according to the cross-
cocurrent principle. This is designed in one piece.
Arranged at the edge of the bipolar plate 2 are slot-like
apertures, which belong to the first and second duct
section 12 and 16, respectively, ~or a 2x2 cell arrange-
ment 34a,b,c and d for an operating medium. Said aper-
tures communicate with a grooved area 36, which supplies
the solid electrolyte electrode element, the actual
reaction space, with the operating medium.
This grooved area 36, which is composed of
directly adjacent grooves, covers virtually the entire
area o~ the bipolar plate 32, with the exception of an
edge region. It communicates with the slot-like apertures
of the third duct section 22 ~or the discharge of the
operating medium. The slot-like apertures of the first
and second duct section 38 and 40, respectively, convey
the second operating medium to the solid electrolyte
electrode element and are deflected in parallel through
the slot-like aperture of the duct section 42.
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The grooved area, which communicates with the duct
sections 40, 42 and 44 and is arranged at right angles to
the grooved area 36, hence the term cross-cocurrent
principle, is not illustrated. It is located on the
remote side of the bipolar plate 32.
A further embodiment is depicted in FIG 3. The
solid electrolyte high temperature fuel cell module 2 in
this embodiment comprises a multiplicity of rectangular,
plate-like fuel cells 4 and is closed off at the top and
bottom with two cover plates 6 and 8 and two base plates
52 and 56.
The introduction and discharge of the operating
media, indicated by the arrows of the flow directions of
the operating media 20 and 24, respectively, is carried
out on the same side of the solid electrolyte high
temperature fuel cell module 2, through the base plates
52 and 56. In this configuration, the first duct section
12 comprises a first subsection 12a, which is arranged
approximately parallel to the longitudinal axis 14 at the
edge of the solid electrolyte high temperature fuel cell
module 2. It is arranged parallel to the third duct
section 22, which carries away the operating medium a~ter
the reaction has been carried out in the ~uel cells.
The first subsection 12a, which extends over the
entire length o~ the solid electrolyte high temperature
~uel cell module 12, opens into a second subsection 12b,
running approximately vertically with respect to the
longitudinal axis 14 through a cut-out in the upper cover
plate 6, said subsection opening into a third subsection
12c, which is arranged approximately parallel to the
longitudinal axis 14 on the opposite edge and extends
over the entire length of the solid electrolyte high
temperature fuel cell module 2. The latter subsection
opens via a cut-out 54 in a base plate 56 into a second
duct section 16, which is arranged approximately parallel
to the third
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subsection 12c of the first duct section 12 and communi-
cates with the ~uel cells 4.
In this preferred configuration, the operating
medium is thus heated on a path which corresponds to more
than twice the length of the solid electrolyte high
temperature fuel cell module 2, before it is fed into the
fuel cells 4.
FIG 4 shows, in a plan view, the construction of
a bipolar plate 62, which is constructed according to the
same principle as that in FIG 2. In the present embodi-
ment, the first duct section comprises, for the first and
second operating medium 12 and 38, respectively, in each
case a number of a plurality of parallel duct sections
120 and 380, respectively, of circular cross section.
By enlarging the number of duct sections 120 and
380, respectively, the surface for picking up the heat is
enlarged. This leads to an improved dissipation of heat.
