Note: Descriptions are shown in the official language in which they were submitted.
a , J
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Description
Motor vehicle having a fuel cell system
The invention relates to a motor vehicle having a fuel
cell system comprising at least one fuel cell module.
It is known to use fuel cell systems to supply energy
to new drive concepts in automotive engineering. By way
of example, EP 0 677 412 B1 provides a detailed
description as to how the fuel cell for a drive unit in
a motor vehicle should be arranged at a suitable
location beneath the floor of the vehicle, since this
provides optimum protection for the sensitive fuel cell
module against mechanical influences. Alternative
arrangements for the fuel cell system are, for example,
the vehicle roof, which is suitable in particular for
buses or trucks.
Furthermore, DE 196 29 084 A1 has disclosed an electric
vehicle, the drive battery of which comprises a fuel
cell system having a cooling system, if appropriate a
secondary cooling system, through which a gaseous
cooling medium flows, wherein the fuel cell system is
arranged in such a way that the cooling medium - if
appropriate the secondary cooling medium - is
introduced into the cooling system completely or
partially by the dynamic pressure of the air stream. In
this case, the fuel cell system should be incorporated
in the vehicle in such a way that the plane normals to
the active surfaces of the individual fuel cells are
perpendicular to the direction of travel. Specifically,
DE 196 02 315 has disclosed a liquid-cooled fuel cell
with distribution passages in which the cell surfaces
are supplied with reaction media via axial supply
passages and radial distribution passages. The supply
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and distribution passages in the entire fuel cell stack
are arranged in such a way that the cell surfaces are
uniformly supplied with the operating media.
Finally, US 5,879,826 A has disclosed a PEM fuel cell
arrangement having a stack of repeating units in which
cooling boards with cooling circuits are arranged
between the individual units.
The storage of hydrogen and refueling with hydrogen are
known to present problems. PEM fuel cell systems, in
particular systems with HT-PEM fuel cells, use hydrogen
as fuel, which is generated from standard gasoline,
from methanol or from another higher
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hydrocarbon in a reformer on board the vehicle. The
oxidizing agent used in this case is atmospheric
oxygen, for which purpose, when the vehicle is
travelling, the air from the environment, in particular
from the air stream, is fed to the fuel cell. For this
purpose, there must be suitable means; in the prior
art, forced guides for the air are provided.
Therefore, it is an obj ect of the invention to propose
a fuel cell system for a motor vehicle which is
simplified with regard to means of this type.
According to the invention, the object is achieved by
the features of patent claim 1. Further developments
are given in the subclaims.
By using a suitable arrangement of the fuel cell system
in or on the vehicle, the invention ensures that the
fuel cells operate as far as possible in a self-
aspirating manner. In this context, the term "self-
aspirating" is understood as meaning that there is
always ample oxygen as oxidizing agent from the ambient
air available in the fuel cell system, so that the fuel
cell reaction can take place. There is then no need for
forced guidance of the aspiration air to the cathodes
of the fuel cells.
In the invention, therefore, the fuel cell module is
aspirated as far as possible by the energy of the air
stream. For low vehicle speeds and/or high loads in
operation, there may preferably be an auxiliary blower.
However, in this context it is a critical factor that
there is no need for forced guides for the aspiration
air, i.e. for supplying oxidizing agents to the fuel
cells.
If, in a motor vehicle, the entire fuel cell system or
at least individual fuel cell modules are arranged
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beneath the floor of the vehicle, diverter plates
and/or nozzle arrangements for introducing the
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air stream are advantageously provided. Electrically
actuable valves can also be used to set and control
these devices.
If, in accordance with DE 196 29 084 A1, the air stream
and/or the blower air are also to be used to cool the
fuel cells, it is recommended for the feed air, on the
one hand, and the cooling air, on the other hand, to be
guided in cross-current.
In the invention, the fuel cell system -v operates with
what are known as PEM fuel cells, in particular at high
temperatures, i.e. the fuel cells are advantageously
HT-PEM fuel cells. By suitable use of mechanical means,
e.g. by diverter plates and/or nozzle arrangements, the
air stream can be utilized in such a manner that it
ensures optimum use of the fuel cells. In this case,
the fuel cell module may be of flat design and of
limited height. In particular, it also ensures that the
aerodynamic properties of the motor vehicle, such as
the drag coefficient or the like, are not adversely
affected.
Further details and advantages of the invention will
emerge from the following description of figures
showing exemplary embodiments on the basis of the
drawing in conjunction with the patent claims. In the
drawing:
Figure 1 shows a motor vehicle with an
integrated fuel cell system,
Figure 2 shows a fuel cell module for self-
aspirating operation, and
Figures 3 and 4 show sectional illustrations of
alternative arrangements to that
shown in Figure 2.
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In Figure 1, a motor vehicle is denoted by 1, and its
electric motor drive 3 (not shown in detail)
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is supplied by a fuel cell system. The fuel cell system
substantially comprises a fuel cell module 10 and
corresponding auxiliary equipment, not shown in detail
in Figure 1. At least the fuel cell module 10 is
positioned on or in the motor vehicle 1 in such a way
that it is supplied with air in an appropriate way.
Specifically, the longitudinal axis of the fuel cell
module 10 runs in the direction of the vehicle
longitudinal axis 5. A nozzle-like mechanical
arrangement passes the air stream from a front end
opening of the motor vehicle 1 to the fuel cell module
10 in such a manner that after it has been diverted the
air flows in from above. In addition, a fan supplies
additional ambient air when required.
Fuel cells which operate with a solid electrolyte and
are referred to as PEM (polymer electrolyte membrane)
fuel cells are used for the fuel cell system. Fuel
cells of this type with operating temperatures of
approx. 60°C are known from the prior art; for mobile
applications, fuel cells of this type are
advantageously operated at higher temperatures than
have previously been described. Working temperatures of
between 80°C and 300°C, but in particular in the range
from 120 to 200°C, are used for the HT-PEM fuel cells.
A fuel cell module 10 with HT-PEM fuel cells
11, 11', ... may be of flat design. Specifically, a
multiplicity of fuel cells are stacked, and
consequently in this case one refers to a flat stack. A
flat stack of this type is advantageously arranged
beneath the vehicle floor or, if it is not a passenger
automobile, may alternatively be arranged on the roof
of the vehicle. This ensures that the air stream
reaches the fuel cells in a suitable way with the aid
of the mechanical device 20.
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Figure 2 illustrates a fuel cell module 10 of this type
which comprises individual HT-PEM fuel cells 11, 11',
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.... The mechanical device 20, for example an obliquely
running metal sheet 21 or air diverter plate, ensures
that the air stream is guided and diverted in a nozzle-
like arrangement and therefore reaches the individual
cells 11, 11', ... of the HT-PEM fuel cell module 10
directly. For this purpose, a nozzle 22 with pivoted
flaps 40 and 40' is arranged in the vehicle upstream of
the fuel cell module 10, so that the air which flows in
is directed in targeted fashion onto the fuel cell.
There may also be electrically actuable valves with
associated control devices.
The auxiliary blower 6 is used if, on account of the
speed being too low, insufficient air stream is being
generated or, on account of a particularly high load,
there is a high demand for air. The valve arrangement,
which is formed from the pivoted flaps 40 and 40' and
can be electrically actuated, allows the supply of air
using the air stream and/or blower air to be matched to
the prevailing requirements by means of a suitable
control unit (not shown).
To prevent damage to the fuel cells from cold incoming
air, the aspiration air has to be preheated before it
enters the fuel cells 11, 11', .... The supply of air
to the fuel cell module 10 illustrated in Figure 2 can
be ideally combined with the air preheating.
Figure 3 diagrammatically depicts how air is guided via
the blower 5 and a heat exchanger 30 and, after it has
been diverted, passes into the fuel cells 11, 11' , . . .
of the fuel cell module 10. A corresponding arrangement
in more compact form is shown in Figure 4. In both
cases, a liquid medium, for example oil, as coolant is
passed from the fuel cells 11, 11', ... into the heat
exchanger 30. The length and height of the fuel cells
11, 11', ..., on the one hand, and of the heat
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exchanger 30, on the other hand, are matched to one
another,
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so that the desired overall form, which is as compact
as possible, is achieved.
With a fuel cell module arranged in a motor vehicle as
shown in Figure 2, it is easy to operate in cross-
current mode . What this means in this case is that the
air which is required for operation of the fuel cells
11, 11' , . . . passes through the fuel cell module 10 in
a vertical line, while the cooling air, after it has
been diverted, in each case flows into the fuel cell
module 10 in a direction which is perpendicular to the
vertical. This arrangement allows the quantity of air
to be set in particular in such a way that a suitable ~,
value can be predetermined. In practice, it has been
found that a ~, value of at least 2 should be
maintained.
It can be seen in particular from Figure 2 that in the
arrangement illustrated the air guidance has a dual
function. Firstly, the oxygen in the air is used as
oxidizing agent, and secondly the air is used for
cooling. This enables the quantitative flow of
oxidizing agent or air to be regulated as a function of
the temperature of the fuel cell module 10.
It has been found that an operating temperature of
between 80°C and 200°C can be maintained in particular
for the use of HT-PEM fuel cells. The ideal operating
temperature is in this case approx. 160°C. In this
case, therefore, the air which is introduced into the
fuel cell system in the motor vehicle is used not only
to supply oxidizing agent but also to cool the fuel
cell modules.
