Note: Descriptions are shown in the official language in which they were submitted.
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Description
High-temperature fuel cell system and a method for its
operation
The invention relates to a high-temperature fuel
cell system, and to a method for its operation.
It is known that, during the electrolysis of
water, the water molecules are decomposed by electrical
current into hydrogen and oxygen. In the fuel cell, this
process takes place in the opposite direction. When
hydrogen and oxygen are electrochemically combined to
form water, electric current is produced, with high
efficiency and - if pure hydrogen is used as the
combustion gas - without any emission of hazardous
materials or carbon dioxide. Even with technical
combustion gases, for example natural gas or coal gas,
and with air or air enriched with ~2 instead of pure
oxygen, a fuel cell produces considerably fewer hazardous
materials and less CO2 than other energy producers which
operate with fossil energy sources. The technical
implementation of the principle of the fuel cell has led
to widely different solutions, to be precise with various
types of electrolyte and with operating temperatures To
between 80~C and 1000~C. Fuel cells are split on the
basis of their operating temperature To into low, medium
and high-temperature fuel cells, which in turn are
distinguished by various technical configurations.
In the case of the high-temperature fuel cell
(Solid Oxide Fuel Cell, SOFC), for example, natural gas
is used as the primary energy source. The very compact
structure allows a power density of 1 MW/m3. The
operating temperatures To are above 900~C.
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A fuel cell block, which is also called a "stack"
in the specialist literature, is, as a rule, composed of
a large number of fuel cells of planar construction and
stacked one above the other.
In order to operate a fuel cell system comprising
at least one fuel cell block at a high, constant
operating temperature To of, for example, more than
900~C, it must be supplied with heat before operation in
order to reach the operating temperature To~ and must be
supplied with heat in order to maintain the required
operating temperature To during brief breaks in
operation. Present-day fuel cell blocks have relatively
low power levels and have dimensions on a laboratory
scale. A furnace is used to raise them to the operating
temperature To of about 600~C for MCFC (Molten Carbonate
Fuel Cell) or about 950~C for SOFC, and are operated in
the furnace. This solution is impracticable for fuel cell
blocks with higher power levels and larger dimensions.
"A Study for a 200 kWe-System for Power and
Heat", by M. R. Taylor, D. S. Beishon, Symposium Report
"First European Solid Oxid Fuel Cell Forum", Lucerne
1994, pages 849 to 864, discloses a method which passes
power-plant gas through the fuel cell block in order to
heat it. This method is disadvantageous since the power-
plant gas pollutes or damages the fuel cells of which thefuel cell block is composed.
DE 42 23 291 Al discloses a fuel cell system unit
which comprises a cell stack composed of a large number
of individual fuel cells. When operation starts, an
apparatus for raising the temperature, which is arranged
outside the fuel cell system unit, heats water passing
through the fuel cell system unit, as
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a result of which the individual fuel cells are preheated
to a predetermined temperature.
DE 40 37 970 A1 discloses a method in which a
consumable equipment for the fuel cell stack is heated by
5 a hot exhaust gas from a fuel cell stack. Such a method
is also disclosed in EP 0 654 838 A1.
The invention is now based on the object of
specifying a high-temperature fuel cell system in which
the high-temperature fuel cells are not polluted or
10 damaged during heating. In addition, it is intended to
specify a method for operation of such a high-temperature
fuel cell system.
The first-mentioned object is achieved by a high-
temperature fuel cell system having at least one high-
15 temperature fuel cell block, for heating of which atleast one electrical heating element is provided, the
heating element being arranged outside the high-
temperature fuel cell block.
The second-mentioned object is achieved by a
20 method for operating a high-temperature fuel cell system
having at least one high-temperature fuel cell block, the
high-temperature fuel cell block being heated from the
outside by at least one electrical heating element.
The use of the electrical heating element ensures
25 good temperature regulation.
The electrical heating element is preferably
arranged inside a high-temperature fuel cell container
having thermal
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insulation. In consequence, only a small amount of heat
is emitted into the environment from the high-temperature
fuel cell container.
In particular, the intermediate space between the
S heating element and the high-temperature fuel cell block
is filled by thermally conductive material. The heat
transfer between the electrical heating element and the
high-temperature fuel cell block is particularly good as
a result of this. The heating element can, of course,
also be fitted closely against the outer wall of the fuel
cell block.
In the case of the method for operating the high-
temperature fuel cell system, the high-temperature fuel
cell block is, according to the invention, heated by at
least one electrical heating element. The electrical
heating element in this case heats the high-temperature
fuel cell block independently of the heat produced during
the reaction process. Thus no power-plant gas is used for
heating. In consequence, there is no pollution or damage
to the high-temperature fuel cells caused by the effects
of power-plant gas. The high-temperature fuel cell block
is not heated in a special furnace, that is to say the
method can be applied to any required configuration of
high-temperature fuel cell blocks. The method is thus
independent of the power levels and dimensions of the
high-temperature fuel cell blocks, and is thus likewise
independent of the dimensions of the high-temperature
fuel cell system.
The high-temperature fuel cell block is
preferably heated from an initial temperature to the
required operating temperature To~ No consumables, for
example hydrogen H2 or oxygen ~2, are required for
heating. In consequence, costs for consumables during the
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heating of the high-temperature fuel cell block are
saved.
In particular, the high-temperature fuel cell
block is kept at the required operating temperature To~
An electrical control loop can be provided for this
purpose. In consequence, power fluctuations resulting
from fluctuations in the operating temperature To are
compensated for or avoided. After relatively short breaks
in operation, the high-temperature fuel cell block no
longer needs to be raised to the required operating
temperature To again, as a result of which costs for
consumables as well as time are saved.
For the further explanation of the invention,
reference is made to the exemplary embodiment in the
drawing, the single figure of which illustrates,
schematically, a high-temperature fuel cell system.
According to the figure, a high-temperature fuel
cell system 2 comprises a high-temperature fuel cell
block 4, which is split into an anode part 6 with anode
gas areas which are not illustrated further, and a
cathode part 8 with cathode gas areas which are not
illustrated further. The high-temperature fuel cell block
4 is composed of a large number of high-temperature fuel
cells which are of planar construction and are not
illustrated further, as are known, for example, from
German Patent P 39 35 722.8. The output of the high-
temperature fuel cell block 4 is connected to an invertor
16, which converts the direct current produced by the
high-temperature fuel cell block 4 into alternating
current for an electrical power system, which is not
illustrated further here.
The high-temperature fuel cell block 4 is
arranged on the inner walls of a high-temperature fuel
cell container 10 having thermal
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insulation 9. In addition, two electrical heating
elements 12, 14 are in each case arranged in the interior
11 of the high-temperature fuel cell container 10,
outside the high-temperature fuel cell block 4. They are
located on two opposite walls. Otherwise, the
intermediate space between the fuel cell block 4 and the
[lacuna] and the heating elements 12, 14 is filled with
thermally conductive material 13, 15. The heat transfer
between the electrical heating elements 12, 14 and the
high-temperature fuel cell block 4 is improved by the
thermally conductive material 13, 15.
The electrical heating elements 12, 14 are in
thermal contact with the high-temperature fuel cell block
4. No power-plant gas is required here for heating. In
consequence, there is no pollution of or damage to the
high-temperature fuel cells because of the effects of
power-plant gas. The method can be applied to any
required configuration of high-temperature fuel cell
blocks. It is thus independent of the power levels and
the dimensions of the fuel cell blocks, and thus likewise
independent of the dimensions of the fuel cell system 2.
By means of this method, the high-temperature
fuel cell block 4 is heated to its operating temperature
To~ or is held at this temperature during brief breaks in
operation. The temperature T of the high-temperature fuel
cell block 4 is regulated for this purpose. The
temperature T as the controlled variable is in this case
detected continuously by a temperature sensor 62 which is
fitted closely against an outer wall of the high-
temperature fuel cell block 4, and is connected via anelectrical signal line 60 to a regulation unit 54. The
operating temperature To is made available as the
reference variable to the regulation unit 54 by a set
value transmitter 56, via an electrical signal line 58.
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The controlled variable T is continuously compared with
the reference variable To in the regulation unit 54. The
electrical heating elements 12, 14 are heated as
appropriate via the electrical cables 50, 52, for the
purpose of matching to the reference variable To~
The cathode part 8 is allocated a cathode system
20, which comprises an inlet path 22 and an outlet path
24. The process gas for the cathode part 8, for example
oxygen ~2, iS fed via the inlet path 22 with a compressor
26 into the high-temperature fuel cell block 4. After the
reaction, the process gas is removed via the outlet path
24. The inlet path 22 has a first heat exchanger 28
arranged in it, in which the process exhaust gas heats
the process gas being supplied for the cathode part 8.
After leaving the first heat exchanger 28, the
process exhaust gas from the cathode part 8 is passed via
the outlet path 24 to a device 38 for processing the
residual gases. From this device 38, the processed gases
are passed out via an exhaust line 40 for further use.
The anode part 6 is assigned an anode system 30
which comprises an inlet path 32 and an outlet path 34.
The process gas for the anode part 6, for example
hydrogen H2, is passed via the inlet path 32. The inlet
path 32 has a second heat exchanger 36 arranged in it, in
which the process exhaust gas removed from the anode part
6 via the outlet path 34 heats the process gas being
supplied to the anode part 6. The outlet path 34 opens
into the device 38 for processing the residual gases.
Alternatively, process gases for operation of the
high-temperature fuel cell system 2, for example
combustion gas and reaction vapor, can be fed into the
inlet path 32 via supply lines 42 and 44 and a mixer 46.
