Note: Descriptions are shown in the official language in which they were submitted.
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Description
Cooling for superconducting machines
The invention relates to a device for cooling superconducting
machines. This device has a closed thermal siphon system which
can be filled with a liquid coolant and which has an
evaporator for evaporating the liquid coolant.
DE 102 44 428 Al discloses a machine with a rotor and a stator
in a machine housing which contains an installation for
cooling parts within this housing. This cooling installation
has on at least one face of the machine a closed system of
piping with a condenser located outside the housing, with an
evaporator located inside the housing and with connecting
tubes running between the condenser and the evaporator,
wherein the circulation of a coolant in this system is
effected in accordance with a thermal siphon effect.
WO 2006/082194 Al discloses a machine with a rotor which can
rotate about an axis, the superconducting winding of which has
a heat-conducting coupling, via a winding carrier and a
thermal contact gas to a central cooling agent space of a
stationary thermally conductive solid which projects into a
hollow space in the rotor. Together with conducting parts
connected to its side and a condenser space of a cooling unit
located outside the machine, the cooling agent space forms a
piping system in which a cooling agent circulates due to a
thermal siphon effect. For the purpose of maintaining the
infeed of cooling agent into the central cooling agent space
even when the rotor is out of alignment, the cooling agent
space is provided with a lining of a porous material,
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preferably a sintered material, with a high thermal
conductivity.
The object underlying the invention is to improve the cooling
performance of a device for cooling superconducting machines.
This object is achieved by a device with the features as
claimed in claim 1.
The invention is based on the recognition that for the purpose
of achieving a required cooling performance in a device for
cooling superconducting machines, it is not the absolute
quantity of the liquid coolant available which is decisive but
the size of a surface of the evaporator which can be wetted by
the liquid coolant. The larger the surface of the evaporator
which can be wetted by the liquid coolant, the more coolant
can evaporate, i.e. the more thermal energy can be transferred
to the evaporating coolant via this available wettable
surface. Thus the available cooling performance of the device
for cooling superconducting machines can be effectively raised
by an enlargement of the wettable surface of the evaporator.
An evaporator is usually designed as a hollow space, the
bounds of which are available as the surface of the
evaporator. Depending on the level of filling with the liquid
coolant, a larger or smaller surface of the evaporator is then
available for evaporating the liquid coolant. In order to
enlarge this surface which can be wetted by the liquid
coolant, without the need to increase the quantity of the
liquid coolant, it is proposed that the means for enlarging
the surface of the evaporator which can be wetted by the
liquid coolant have at least one displacer for displacing the
liquid coolant. By this means, there is a saving on coolant
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combined with an enlargement of the surface of the evaporator
which can be wetted by the liquid coolant.
Advantageous embodiments of the device in accordance with the
invention emerge from the dependent claims.
In accordance with one advantageous embodiment of the
invention, the evaporator is arranged in the interior of a
rotor of a superconducting machine. The surplus thermal energy
can thereby be dissipated directly from the rotor. The
enlargement of the surface of the evaporator which can be
wetted by the liquid coolant, achieved by the invention, is
especially advantageous with this embodiment of the invention
because the volume, and with it also the surface, of an
evaporator located in the interior of a rotor is normally
limited by the relatively small dimensions of a rotor.
Constructional advantages are achieved in that, in accordance
with another advantageous embodiment of the invention, the
evaporator and the at least one displacer are cylindrical, in
particular circularly cylindrical, in shape. Such shaping is
simple to manufacture, and nevertheless is efficient in
displacing the liquid coolant.
In accordance with another advantageous embodiment of the
invention, it is proposed that the surface of the evaporator
which can be wetted by the liquid coolant has a surface
structure which is formed in such a way that the surface which
can be effectively used for the transfer of heat is enlarged.
By this means it is possible to achieve a particularly
significant enlargement of the surface of the evaporator which
can be wetted by the liquid coolant, combined with low
construction costs.
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Here, in accordance with another advantageous embodiment of
the invention one surface structure which is particularly
simple to realize, in terms of manufacturing technology, has
elements which are one-dimensional, in particular groove- or
fin-like.
In order to further raise the cooling performance, the surface
structure has, in accordance with another advantageous
embodiment of the invention, elements which are two-
dimensional, in particular hole-like or spiky.
In accordance with another embodiment of the invention, the
liquid coolant is neon. Neon permits a particularly favorable
working point, e.g. in the cooling of high temperature super-
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conductors, but is however relatively expensive so that the
reduction in coolant which is achieved by the invention is
particularly useful.
The invention is described and explained below by reference to
the exemplary embodiments illustrated schematically in the
figures.
These show:
FIG 1 a schematic diagram of a section through a
superconducting machine together with a device for
cooling the superconducting machine,
FIG 2 a schematic diagram of an evaporator in accordance with
the prior art,
FIG 3 an exemplary embodiment of the inventive device, with a
displacer for displacing the liquid coolant,
FIG 4 another exemplary embodiment of the inventive device, in
which the surface of the evaporator which is effectively
usable for the transfer of heat is enlarged, and
FIG 5 an exemplary embodiment of the inventive device, in
which use is made of various means for enlarging the
surface which can be wetted by the liquid coolant.
Figure 1 shows a schematic diagram of a superconducting
machine 1 together with a device for cooling the
superconducting machine 1. This shows a section along the
longitudinal axis of the superconducting machine 1. The
superconducting machine 1 in the case of the exemplary
embodiment shown in FIG 1 is a rotating electrical machine, in
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particular a synchronous machine, for example a motor or a
generator. This has a stator 10 together with a rotor 6. In
addition, it has a housing 11 for accommodating the stator 10
and for the bearing mountings of the rotor 6. The
superconducting machine 1 is cooled by a closed thermal siphon
system, which has an evaporator 4, a condenser 9 together with
elements which connect the evaporator 4 and the condenser 9,
e.g. connecting pipes. The evaporator 4, the connecting
elements and the condenser 9 form the bounds of an enclosed
space, which is provided to accommodate the liquid coolant 3.
The evaporator 4 has a surface 5, which can be wetted by the
liquid coolant 3, via which the thermal energy arising in the
rotor and which is to be dissipated is transferred to the
coolant 3. In this process, the coolant 3 is normally
converted from the liquid state into the gaseous state by the
thermal energy transferred, i.e. the coolant 3 is evaporated
or boils. Due to the lower density of the gaseous form of the
coolant, it rises through the connecting elements to the
condenser 9, which is at a higher geodetic level, and there it
is converted back from the gaseous to the liquid state by
extraction of the thermal energy which it had taken up. Due to
gravity, the coolant 3 which has in this way been re-liquefied
flows back to the evaporator 4, and in particular to the
surface 3 of the evaporator 4 which can be wetted by the
coolant 3. A cooling system of this type utilizes the so-
called thermal siphon effect. The cooling circulation is
maintained solely by the density differences mentioned, or
gravity, as applicable.
FIG 2 shows an axial section through the evaporator 4 of a
superconducting machine with the machine stationary. The other
parts of the machine are not explicitly illustrated in FIG 2.
The evaporator 4 shown in FIG 2 has a circularly cylindrical
cross-section. The evaporator 4 illustrated is known from the
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prior art. The evaporator 4 is at least partially filled with
a liquid coolant 3. Here, the surface of the evaporator 4
which can be or is wetted by the liquid coolant 3 is
identified with the reference mark 5.
When superconducting machines 1 are cooled using a thermal
siphon system, a certain minimum area of the evaporator 4 must
be wetted by the liquid coolant 3 in order to achieve the
required cooling performance. Depending on the precise
geometry of the evaporator 4 combined with the heat transfer,
which during the cooling-down phase is frequently limited by
film boiling, a comparatively large amount of liquid coolant
(e.g. neon, nitrogen or similar) is required in the case of
superconducting machines as presently designed.
Currently, this problem is normally solved by simply filling
up with an appropriate quantity of coolant 3 to be able to wet
a sufficiently large surface in a (normally horizontally
arranged) cylindrical-shaped evaporator 4. In order at the
same time to adhere to the concept of a closed thermal siphon
system with a one-time filling, this method requires a
comparatively large buffer container at room temperatures
(pressurized container), in which liquid coolant 3, which
gradually evaporates when the cooling system is switched off
or fails, can be collected with a tolerable pressure rise.
Alternatively of course, it is also possible to make allowance
for the fact that a lower level of filling with coolant causes
cooling-down to last longer than is really necessary.
FIG 3 shows an evaporator 4 in an exemplary embodiment of a
device in accordance with the invention. The evaporator 4 is
at least partially filled with a liquid coolant 3. By using an
additional (advantageously cylindrical) displacer 7, the
quantity of liquid required for wetting the same evaporator
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surface area can be substantially reduced. The device has, as
the means 7, 8 for enlarging the surface 5 of the evaporator 4
which can be wetted by the liquid coolant 3, a displacer 7 for
displacing the liquid coolant 3. The displacer 7 restricts the
volume available within the evaporator 4 for the liquid
coolant 3, in such a way as to enlarge the surface 5 of the
evaporator 4 which is actually wetted by the coolant 3.
FIG 4 shows an evaporator 4 in another exemplary embodiment of
a device in accordance with the invention. Alternatively or
additionally to the embodiment shown in FIG 3, the
functionally effective surface of the evaporator surface can
itself also be substantially enlarged by the introduction of
an appropriate surface structure 8. Advantageous embodiments
are one-dimensional groove- or fin-like structures, with which
the surfaces can in a simple way be substantially enlarged
(factor 3-5). As shown in the exemplary embodiment
illustrated, the means 7, 8 for enlarging the surface 5 of the
evaporator 4 which can be wetted by the liquid coolant 3 are
in the form of a surface structure 8 on the surface of the
evaporator, wherein the surface structure 8 is arranged so as
to enlarge the surface 5 which is effectively usable for the
transfer of heat. The surface structure 8 in the exemplary
embodiment shown has one-dimensional elements, in this case
groove- or fin-like elements. Two-dimensional variants, rather
more complicated to manufacture, are also advantageous for the
purpose of enlarging the surfaces (such as for example the
introduction of holes or spiky structures), and permit an even
greater enlargement of the effective surface.
As another exemplary embodiment, FIG 5 shows an evaporator 4,
in a device in accordance with the invention, which has a
combination of the means 7, 8 for enlarging the surface 5 of
the evaporator 4 which can be wetted by the liquid coolant 3.
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in the exemplary embodiment shown in FIG 5, the means shown in
FIG 3, i.e. a displacer 7, is combined with the means shown in
FIG 4, i.e. a surface structure 8 for enlarging the surface 5
of the evaporator 4 which can be wetted by the coolant 3.
The embodiments of the invention shown enable a reduction in
the quantity of fluid required for wetting a particular
minimum surface of the evaporator 4 as part of the thermal
siphon cooling circuit. The advantages lie in the directly
associated reduction in the required buffer volume (typically
from several 100 liters to about one tenth of that) and thus
from a smaller space requirement and lower costs. The costs of
the actual filling of the thermal siphon system are also
reduced thereby (less coolant 3).
In summary, the invention relates to a device for cooling
superconducting machines 1, with a closed thermal siphon
system 2 which can be filled with a liquid coolant 3 and which
has an evaporator 4 for evaporating the liquid coolant 3. In
order to improve the cooling performance of the device,
inventive means 7, 8 are provided for enlarging a surface 5 of
the evaporator 4 which can be wetted by the coolant 3.
