U.S. patent application number 11/700854 was filed with the patent office on 2007-08-16 for hybrid heat pump / refrigerator with magnetic cooling stage.
This patent application is currently assigned to Bruker BioSpin AG. Invention is credited to Johannes Boesel, Robert Schauwecker.
Application Number | 20070186560 11/700854 |
Document ID | / |
Family ID | 38050136 |
Filed Date | 2007-08-16 |
United States Patent
Application |
20070186560 |
Kind Code |
A1 |
Schauwecker; Robert ; et
al. |
August 16, 2007 |
Hybrid heat pump / refrigerator with magnetic cooling stage
Abstract
A device for transporting heat from a cold reservoir to a warm
reservoir, in which at least two cyclic processes are employed for
transporting heat thereby absorbing work, of which at least one is
a regenerative cyclic process, and at least one is a magnetocaloric
cyclic process, wherein the regenerative cyclic process has a
working fluid and a heat storage medium, is characterized in that
the heat storage medium of the regenerative cyclic process
comprises a magnetocaloric material for the magnetocaloric cyclic
process, wherein the magnetocaloric material is in a regenerator
area with a cold end and a warm end, the working fluid of the
regenerative cyclic process additionally serving as a heat transfer
fluid for the magnetocaloric cyclic process. This produces a
compact device with low apparative expense, wherein the power
density and also the efficiency of the device are increased. The
device may advantageously be used for cooling a superconducting
magnet configuration.
Inventors: |
Schauwecker; Robert;
(Zuerich, CH) ; Boesel; Johannes; (Neuheim,
CH) |
Correspondence
Address: |
KOHLER SCHMID MOEBUS
RUPPMANNSTRASSE 27
D-70565 STUTTGART
omitted
|
Assignee: |
Bruker BioSpin AG
Faellanden
CH
|
Family ID: |
38050136 |
Appl. No.: |
11/700854 |
Filed: |
February 1, 2007 |
Current U.S.
Class: |
62/3.1 ;
62/335 |
Current CPC
Class: |
F25B 21/00 20130101;
Y02B 30/00 20130101; Y02B 30/52 20130101; Y02B 30/66 20130101; F25B
9/145 20130101; F25B 9/10 20130101; F25B 2309/003 20130101; F25B
9/14 20130101; F25B 2321/0021 20130101; F25B 25/00 20130101 |
Class at
Publication: |
62/3.1 ;
62/335 |
International
Class: |
F25B 21/00 20060101
F25B021/00; F25B 7/00 20060101 F25B007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 11, 2006 |
DE |
10 2006 006 326.0 |
Claims
1. A device for transporting heat from a cold reservoir to a warm
reservoir using at least two cyclic processes for transporting the
heat, thereby absorbing work, the device comprising: means for
transporting heat via a working fluid in a regenerative cyclic
process; means for exchanging heat via a magnetocaloric material in
a magnetocaloric cyclic process; means for storing heat with the
magnetocaloric material during said regenerative cyclic process;
and means for transferring heat with the working fluid during said
magnetocaloric cyclic process.
2. The device of claim 1, wherein said regenerative cyclic process
has a heat storage medium comprising said magnetocaloric material,
said magnetocaloric material being disposed in a regenerator area
having a cold end and a warm end, wherein said working fluid of
said regenerative cyclic process additionally serves as a heat
transfer fluid for said magnetocaloric cyclic process.
3. The device of claim 2, wherein a cold reservoir has a
temperature which is below ambient temperature, and a warm
reservoir has a temperature which is at or above ambient
temperature.
4. The device of claim 2, wherein said regenerative cyclic process
is based on a Stirling, a Vuilleumier, a Gifford-McMahon, or a
pulse tube gas cycle.
5. The device of claim 2, wherein said magnetocaloric material
comprises different components with different Curie temperatures
which are disposed next to each other in layers at said regenerator
area in order of decreasing Curie temperature, wherein a component
of said magnetocaloric material with a highest Curie temperature is
disposed at said warm end and a component of said magnetocaloric
material with a lowest Curie temperature is disposed at said cold
end of said regenerator area.
6. The device of claim 2, further comprising means for providing a
magnetic field and/or for shielding a magnetic background field to
provide and/or shield a magnetic field at least at a location of
said magnetocaloric material.
7. The device of claim 6, wherein said means for providing a
magnetic field and/or shielding a magnetic background field
comprise a permanent magnet.
8. The device of claim 6, wherein said means for providing a
magnetic field and/or shielding a magnetic background field
comprise a magnet coil winding with a normally conducting and/or a
superconducting wire.
9. The device of claim 6, wherein a magnetic shielding of soft
magnetic material is provided for shielding said magnetic
background field.
10. A superconducting magnet configuration comprising the device
for transporting heat from a cold reservoir to a warm reservoir of
claim 1.
11. The superconducting magnet configuration of claim 10, wherein
the superconducting magnet configuration is part of an apparatus
for magnetic resonance (MR), nuclear magnetic resonance imaging
(MRI), or nuclear magnetic resonance spectroscopy (NMR).
12. The superconducting magnet configuration of claim 10, wherein
the superconducting magnet configuration is part of an apparatus
for ion cyclotron resonance spectroscopy (ICR) or electron spin
resonance (ESR, EPR).
13. A method for transporting heat from a cold reservoir to a warm
reservoir using at least two cyclic processes for transporting the
heat, thereby absorbing work, the method comprising the steps of:
a) transporting heat via a working fluid in a regenerative cyclic
process; b) exchanging heat via a magnetocaloric material in a
magnetocoleric cyclic process; c) storing heat with the
magnetocaloric material during step a); and d) transferring heat
with the working fluid during step b).
14. The method of claim 13, wherein the working fluid passes
through a compression phase, a phase of heat release, an expansion
phase and a heat absorbing phase in the regenerative cyclic
process.
15. The method of claim 13, wherein a field strength of a magnetic
field is periodically varied at least at a location of the
magnetocaloric material.
16. The method of claim 15, wherein the field strength is varied
through cyclic change of a relative position of the magnetocaloric
material relative to a permanent magnet.
17. The method of claim 15, wherein the field strength is varied by
changing a current flow in a normally conducting and/or
superconducting magnet coil.
18. The method of claim 13, wherein a strength of a magnetic field
shielding in a magnetic background field is varied periodically at
least at a location of the magnetocaloric material.
19. The method of claim 13, wherein, in the regenerative cyclic
process, a heat absorbing phase and a compression phase are
executed with a high magnetic field in the magnetocaloric material,
and a heat release phase and expansion phase are executed with a
low magnetic field in the magnetocaloric material.
Description
[0001] This application claims Paris Convention priority of DE 10
2006 006 326.0 filed Feb. 11, 2006 the complete disclosure of which
is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] The invention concerns a device for transporting heat from a
cold reservoir to a warm reservoir, in which at least two cyclic
processes are employed for transporting heat, thereby absorbing
work, of which at least one is a regenerative cyclic process, and
at least one is a magnetocaloric cyclic process, wherein a working
fluid and a heat storage medium are provided for the regenerative
cyclic process.
[0003] A device of this type has been disclosed in the document "A
multi-stage continuous-duty adiabatic demagnetization refrigerator"
(P. J. Shirron et al., Adv. Cry. Eng., Vol. 45B, page 1629). It
combines a regenerative, cyclic gas refrigeration process with
magnetic cooling by using a Gifford-McMahon gas refrigerator for
precooling several, series-connected magnetic cold stages in order
to produce extremely low temperatures in the mK range.
[0004] A further device is disclosed in the document "Performance
testing of a 4 K active magnetic regenerative refrigerator" (S. F.
Kral et al., Adv. Cry. Eng., Vol. 45A, page 329). This arrangement
suggests a combination of a Gifford-McMahon cooler with a separate
magnetic cold stage in order to generate temperatures in the range
of 4 K. The Gifford-McMahon cooler thereby precools the magnetic
cold stage to approximately 10 K.
[0005] A further device of this type is described in the document
"Prospects of magnetic liquefaction of hydrogen" (Barclay, J. A. Le
froid sans frontieres, vol. 1, page 297, 1991). It proposes a
magnetic cold stage for liquefying hydrogen (at 20 K). The magnetic
cold stage usually does not work between room temperature and 20 K
but e.g. between approximately 80 K and 20 K. The magnetic stage
may be precooled to approximately 80 K either via liquid nitrogen
or a refrigerator, e.g. a regenerative gas refrigerator. The active
regenerator bed of the magnetic cold stage thereby consists of
several ferromagnetic materials which have different magnetic
transition temperatures (Curie temperature) and are disposed in
layers next to each other from the maximum to the minimum Curie
temperature.
[0006] Common to all these examples is that precooling of the
magnetic cold stage is effected either via a gas refrigerator, such
as a Stirling, pulse tube or Gifford-McMahon cooler or liquid
nitrogen. The gas refrigerators often have a multi-stage
construction. In each stage thereby a gas refrigeration process is
employed which is divided into several process phases such as
compression, heat release, expansion and supply of heat. Heat is
intermediately stored between the compression and expansion phases
in a (passive) regenerator matrix to cool the gas. After expansion
and supply of heat from the outside (="release of cold"), the
stored heat is once more absorbed by the gas.
[0007] In the following magnetic cold stage, the working medium
which passes through a thermodynamic process is not gas, but
magnetocaloric material, a solid. Heat is exchanged between the
magnetocaloric material and a heat source or heat sink via a
thermal connection (in the form of a thermal switch) that can be
connected or disconnected, or an additional heat transfer fluid.
The temperature in the process is increased and decreased through
magnetization or demagnetization of the magnetocaloric material,
e.g. by a permanent magnet. The temperature change is maximum when
the average temperature of the material corresponds to its Curie
temperature. In order to bridge large temperature differences
between the heat source and sink, several materials having
different Curie temperatures may be used, which are disposed in
layers next to each other in bulk form. The magnetocaloric material
provided at each location in the regenerator bed runs through its
own cycle between different temperatures (active magnetic
regeneration) using the heat transfer fluid. Coupling to the
outside is provided at the ends of the regenerator bed, where the
heat transfer fluid flows through a heat exchanger. Heat is thus
finally transported from a cold heat source to a warm heat sink.
The magnetocaloric material may be introduced into a magnetic field
in cycles or a magnetic field may be periodically switched on and
off. The heat transfer fluid (a liquid or a gas depending on the
application) must be supplied at the right moment through the
magnetocaloric material, e.g. using a pump.
[0008] Refrigeration at low (4-20 K) or very low temperatures
(<4 K) is primarily suited as an application of such devices for
transporting heat from a cold reservoir to a warm reservoir,
wherein at least two cyclic processes are performed for
transporting heat thereby absorbing work (a gas cycle and a
magnetic cycle), in order to liquefy e.g. gases having a low
boiling temperature, such as hydrogen or helium. It is, however,
also possible to use such an arrangement as a heat pump, i.e. for
heating. One common feature of a refrigerator and a heat pump is
that heat is transported from a colder to a warmer reservoir by
supplying work. For the refrigerator, the heat is absorbed at a
temperature below the ambient temperature. For a heat pump, heat is
released at a temperature above the ambient temperature. The
physical principle, however, remains the same.
[0009] Since there are always two independent cooling mechanisms in
the conventional devices of this type, the apparative expense is
relatively large. Thus, a separate drive mechanism must be provided
for the precooling stage(s) with a gas cooling cycle or with liquid
nitrogen, and also for the magnetic cooling stage, e.g. in the form
of a compressor or an apparatus which moves the magnetic material
in and out in cycles in a permanent magnet. A heat transfer fluid
must also be supplied through the active regenerator bed of the
magnetic cooling stage. The working fluid of the regenerative
cyclic process and the heat transfer fluid of the magnetic cooling
stage are two different media or at least hydraulically separated
from each other. The (passive) heat storage medium of the
regenerative process and the active regenerator bed are also not
identical.
[0010] Two-stage cryocoolers which are merely based on a gas
refrigeration cycle have also recently been used for liquefying
gases having a low boiling point, such as helium. A particularly
interesting application is the use of a pulse tube cooler for
reliquefying evaporated helium in an apparatus with a
superconducting magnet, as is described e.g. in the patent document
U.S. 2002/0002830A1. The magnet coil is still cooled through
evaporating helium. However, in contrast to conventional systems,
no helium or other cryogen is lost to the outside. Magnetic
materials are also used in the heat storage medium (passive
regenerator) of the second cold stage of these coolers, but for a
completely different reason than in active regenerator beds of
magnetic coolers. These materials have a large heat capacity in the
area of their magnetic transition compared to the working gas,
which is a prerequisite for running the cooling process.
Frequently, the magnetic materials must even be shielded from the
magnetic field of the superconducting magnet in order to maintain
their efficiency. An external magnetic field is thereby undesired
in this case. Moreover, a pulse tube cooler does not work very
efficiently in contrast to magnetic coolers, with the consequence
that the operating costs for cooling the magnet system are
relatively high. A thermodynamically more efficient method for
producing cold would therefore be advantageous.
[0011] It is therefore the object of the present invention to
improve a device for transporting heat from a cold reservoir to a
warm reservoir, wherein at least two cyclic processes for
transporting heat, thereby absorbing work, are employed in the
device in such a manner that the apparative expense of the device
is considerably reduced, and the efficiency and the power density
of the device increased.
SUMMARY OF THE INVENTION
[0012] This object is achieved in accordance with the invention in
that the heat storage medium of the regenerative cyclic process
comprises a magnetocaloric material for the magnetocaloric cyclic
process, wherein the magnetocaloric material is in a regenerator
area with a cold end and a warm end, the working fluid of the
regenerative cyclic process additionally serving as a heat transfer
fluid for the magnetocaloric cyclic process. The inventive device
may be part of a multi-stage arrangement for transporting heat from
a cold reservoir to a warm reservoir, in particular, when the heat
is transported through greatly differing temperatures.
[0013] The advantage of an inventive device compared to
conventional devices consists in that the apparative expense is
greatly reduced. The advantages of the inventive device compared to
prior art may be explained by means of example by the arrangement
of the document "Performance testing of a 4 K active magnetic
regenerative refrigerator" (S. F. Kral et al., Adv. Cry. Eng., Vol.
45A, page 329). When the working fluid of a cooler of a
regenerative cyclic process, e.g. of the Gifford-McMahon cooler
that is used in the conventional device is used in accordance with
the invention as a heat transfer fluid of the magnetocaloric cold
stage, both may be advantageously transported via the same drive
mechanism. In the same way, the inventive use of the active
material of the magnetocaloric cold stage as a regenerator material
in the Gifford-McMahon cooler represents a simplification of the
apparatus. Moreover, the inventive combined process is
thermo-dynamically more efficient than a conventional regenerative
gas cycle alone (as used e.g. for a gas refrigerator in a pulse
tube cooler or Gifford-McMahon cooler). The cooling or heating
performance may be increased without considerably increasing the
volume of the machine, thereby increasing the power density.
Moreover, an existing external magnetic field (as described e.g. in
the patent document U.S. 2002/0002830A1) is not disturbing, but
even advantageous, since it can be utilized for the magnetocaloric
cycle.
[0014] A particularly preferred embodiment of the inventive device
for transporting heat from a cold reservoir to a warm reservoir is
characterized in that the cold reservoir has a temperature below
the ambient temperature, and the warm reservoir has a temperature
which is equal to or larger than the ambient temperature. Thus, a
refrigerator may generate a temperature below the ambient
temperature, which provides a plurality of possible applications.
It is, however, also feasible to use the principle with a heat pump
for heating.
[0015] The inventive device is particularly advantageous when the
regenerative cyclic process is based on a Stirling, a Vuilleumier,
a Gifford-McMahon or a pulse tube gas cycle. All processes are
mainly used for producing cold, and in the present case, at
temperatures preferably below 100 K. The machines based on these
processes (in particular for a Gifford-McMahon or pulse tube
cooler) in combination with the magnetocaloric process are
generally more efficient and have a larger power density.
[0016] In another advantageous fashion, the magnetocaloric material
has different components with different Curie temperatures which
are disposed next to each other in layers in the order of
decreasing Curie temperature in the regenerator area, such that the
component of the magnetocaloric material with the highest Curie
temperature comes to rest at the warm end and the component of the
magnetocaloric material with the lowest Curie temperature comes to
rest at the cold end of the regenerator area. This joining of
different components permits active magnetic regeneration, and
relatively large temperature differences (e.g. 60 K) can be covered
with only little change in magnetic field strength (e.g. 2T).
[0017] The inventive device is particularly advantageous when a
means for providing a magnetic field and/or shielding a magnetic
background field is provided which provides and/or shields a
magnetic field at least at the location of the magnetocaloric
material. In this fashion, the magnetocaloric material may be
alternately magnetized and demagnetized to change the temperature
in the material.
[0018] In a further advantageous embodiment of the inventive
device, the means for providing a magnetic field and/or shielding a
magnetic background field comprise a permanent magnet. The relative
position of the permanent magnet compared to the magnetocaloric
material is varied through a suitable device such that the
magnetocaloric material can run through the intended process. This
provides a simple and robust embodiment of the inventive
device.
[0019] In an alternative embodiment, the means for providing a
magnetic field and/or shielding a magnetic background field
comprises a magnet coil winding with a normally conducting and/or a
superconducting wire. The magnetocaloric material may then
alternately be magnetized and demagnetized (and thereby heated and
cooled) via a variable current, without moving parts. This is
particularly advantageous when the device is to be used in
environments which are sensitive to vibration.
[0020] In a special embodiment of the inventive device, a magnetic
shielding of soft magnetic material is provided as a means for
shielding the magnetic background field. Through suitable variation
of the relative position of the magnetic shielding compared to the
magnetocaloric material, the magnetocaloric material may be
efficiently magnetized and demagnetized with little additional
expense.
[0021] The invention also concerns a superconducting magnet
configuration with an inventive device for transporting heat from a
cold reservoir to a warm reservoir. In a superconducting magnet
configuration, at least the area of superconducting windings must
be cooled down to low temperatures, and the configuration provides
a magnetic field which can be variably shielded with time in the
magnetocaloric material of an inventive device, thereby
facilitating the magnetocaloric process required for cooling.
[0022] With particular advantage, the superconducting magnet
configuration is thereby part of an apparatus for magnetic
resonance (MR), in particular, for nuclear magnetic resonance
imaging (MRI) or nuclear magnetic resonance spectroscopy (NMR).
These are analysis methods which usually use liquid cryogens for
cooling the superconducting magnet configuration, such that direct
cooling (without refilling the otherwise evaporating cryogens) is
very attractive for the user.
[0023] It is also, however, feasible for the superconducting magnet
configuration to be part of an apparatus for ion cyclotron
resonance spectroscopy (ICR) or electron spin resonance (ESR, EPR).
The user-friendliness is thereby also improved by autonomous
cooling of the superconducting magnet configuration.
[0024] The invention also concerns a method for transporting heat
from a cold reservoir to a warm reservoir, wherein at least two
cyclic processes are employed for transporting heat, thereby
absorbing work, of which at least one is a regenerative cyclic
process in which heat is transported via a working fluid, and at
least one is a magnetocaloric cyclic process in which heat is
exchanged via a magnetocaloric material. The inventive method is
characterized in that the magnetocaloric material is also used as a
heat storage medium in the regenerative cyclic process and the
working fluid is also used as a heat transfer fluid for the
magnetocaloric cyclic process.
[0025] In one variant of the inventive method, the working fluid
runs through a compression phase, a heat release phase, an
expansion phase and a heat absorbing phase in the regenerative
cyclic process. This provides a thermodynamic cycle, wherein heat
is transported from a cold reservoir to a warm reservoir, thereby
absorbing work.
[0026] In a further advantageous variant, the field strength of a
magnetic field is periodically varied at least at the location of
the magnetocaloric material. The magnetocaloric material is thus
heated and cooled and may run through a thermodynamic cycle, in
which heat is transported from a cold reservoir to a warm
reservoir.
[0027] It is, however, also feasible to vary the field strength
through cyclic change of the position of the magnetocaloric
material relative to a permanent magnet. This is a simple, robust
and inexpensive variant.
[0028] The field strength may moreover vary through changing the
current flow in a normally conducting and/or superconducting magnet
coil. In this case, no parts are moved. This provides a variant of
the inventive method that has particularly little vibration.
[0029] In a particularly advantageous variant of the inventive
method, the strength of magnetic field shielding in a magnetic
background field is periodically varied at least at the location of
the magnetocaloric material. The magnetocaloric material can
thereby be magnetized and demagnetized with little effort.
[0030] The regenerative cyclic process moreover includes the heat
supply phase and the compression phase with a high magnetic field
in the magnetocaloric material, the phase of heat release and
expansion phase with a low magnetic field in the magnetocaloric
material. In this fashion, the method permits both a regenerative
and a magnetocaloric working cycle in one single device, which
provides an associated machine having high power and
efficiency.
[0031] The inventive method is essentially advantageous when the
heat is transported within a superconducting magnet configuration,
wherein the superconducting magnet configuration is part of an
apparatus for magnetic resonance (MR), in particular, for nuclear
magnetic resonance imaging (MRI) or nuclear magnetic resonance
spectroscopy (NMR) or is part of an apparatus for ion cyclotron
resonance spectroscopy (ICR) or for electron spin resonance (ESR,
EPR). In this fashion, such an apparatus can be cooled efficiently
and in a user-friendly fashion. Compared to conventional cooling
using liquid cryogens, the user friendliness of the apparatus and
the cooling costs can be reduced, in particular, when the price for
cryogens increases in the near future.
[0032] Further advantages of the invention can be extracted from
the description and the drawing. The features mentioned above and
below may be used in accordance with the invention either
individually or collectively in arbitrary combination. The
embodiments shown and described are not to be understood as
exhaustive enumeration but have exemplary character for
illustrating the invention.
[0033] The invention is shown in the drawing and is explained in
more detail with reference to embodiments.
BRIEF DESCRIPTION OF THE DRAWING
[0034] FIG. 1a shows a schematic construction of a device for
transporting heat from a cold reservoir to a warm reservoir in a
regenerative, cyclic process in accordance with Stirling (prior
art);
[0035] FIG. 1b shows a schematic construction of a device for
transporting heat from a cold reservoir to a warm reservoir in a
magnetocaloric cyclic process (prior art);
[0036] FIG. 1c shows a schematic construction of an inventive
device for transporting heat from a cold reservoir to a warm
reservoir;
[0037] FIG. 2a shows the different process phases in a device for
transporting heat from a cold reservoir to a warm reservoir in a
regenerative, cyclic process in accordance with Stirling (prior
art);
[0038] FIG. 2b shows the different process phases in a device for
transporting heat from a cold reservoir to a warm reservoir in a
magnetocaloric cyclic process (prior art);
[0039] FIG. 2c shows the different process phases in an inventive
device for transporting heat from a cold reservoir to a warm
reservoir;
[0040] FIG. 3 shows an embodiment of an inventive device for
cooling a superconducting magnet system;
[0041] FIG. 4 shows the process phases III->IV and VI->I in
an inventive device for transporting heat from a cold reservoir to
a warm reservoir with volume elements of the working fluid;
[0042] FIG. 5a shows all process phases of an inventive device for
a first volume element of the working fluid in a
temperature-entropy diagram (T-S diagram);
[0043] FIG. 5b shows all process phases of an inventive device for
any volume element of the working fluid in a temperature-entropy
diagram (T-S diagram);
[0044] FIG. 5c shows all process phases of an inventive device for
a last volume element of the working fluid in a temperature-entropy
diagram (T-S diagram); and
[0045] FIG. 6 shows the power increase and the exergetic efficiency
in dependence on the pressure ratio for an exemplary inventive
hybrid Stirling refrigerator.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0046] FIG. 1a schematically shows the structure of a (one-stage)
regenerative Stirling gas refrigerator (or heat pump) according to
prior art. The Stirling machine represents the basic form of all
machines which are based on a regenerative cycle. The other gas
refrigerators, such as Gifford-McMahon coolers or pulse tube
coolers, are derived from this basic form.
[0047] The machine consists of a heat storage medium, a so-called
(passive) regenerator 1 which is limited at its warm end 2 by a
warm heat exchanger 3 (for a refrigerator approximately at ambient
temperature) and is limited at its cold end 2' by a cold heat
exchanger 3' (for a refrigerator below ambient temperature). The
regenerator 1 consists of a finely distributed solid, e.g. in the
form of woven metal screens or bulk particles, and has a high heat
capacity compared to the working fluid, e.g. helium gas. The
regenerator 1 absorbs heat from the working fluid during passage,
and releases it after flow reversal to the working fluid without
considerably changing the temperature distribution in the
regenerator 1. The working fluid is compressed by a warm piston 4
in a compression space 5 and is expanded (after transferring it) in
an expansion space 5' by a cold piston 4'. In order to perform the
various steps one after the other, the motion of the pistons 4, 4'
is controlled by suitable drive mechanisms 6, 6' in such a manner
that the warm piston 4 leads the cold piston 4' by a quarter of a
period (90.degree. phase shift).
[0048] In a one-stage magnetic refrigerator or heat pump (FIG. 1b),
there is an (active) regenerator 1' which consists of a
magnetocaloric working medium, mostly in the form of bulk particles
in a regenerator bed. The regenerator 1' is, in turn, bordered by
two heat exchangers 3, 3': at its warm end 2 by the warm heat
exchanger 3, and at its cold end 2' by the cold heat exchanger 3'.
In order to bridge large temperature differences between the two
heat exchangers 3, 3', several components of magnetically active
materials having different Curie temperatures may be used in the
regenerator 1', which are disposed next to each other in layers,
such that the component of the magnetocaloric material with the
highest Curie temperature comes to rest on the warm heat exchanger
3 and the component of the magnetocaloric material with the lowest
Curie temperature comes to rest on the cold heat exchanger 3' (not
shown). The two pistons 4, 4' transport a heat transfer fluid (gas
or liquid) through the (active) regenerator 1'; its motion via the
drive mechanism 6, 6' is performed in phase and the heat transfer
fluid is neither compressed nor expanded. The working medium
(magnetocaloric material) in the regenerator 1' is magnetized via a
magnet 7. The magnet 7 may be designed as a permanent magnet which
is cyclically shifted over the regenerator bed, or as a coil of
normally and/or superconducting wire. Then the magnetocaloric
material in the regenerator 1' is magnetized or demagnetized
through charging and discharging the magnet coil. The motion of the
two pistons 4, 4' and the motion of the permanent magnet or
charging and discharging of the magnet coil must be matched in time
such that the magnetocaloric material runs through a magnetocaloric
cycle.
[0049] FIG. 1c schematically shows the construction of the
inventive device. Components of both machines in accordance with
FIGS. 1a and 1b are combined into a one-stage refrigerator or heat
pump. The (passive and active) regenerator 1'' comprises the
magnetocaloric working medium, e.g. in the form of bulk particles
of preferably several magnetically active materials with different
Curie temperatures, wherein the component of the magnetocaloric
material with the highest Curie temperature comes to rest at the
warm end 2 of the regenerator 1'' and the component of the
magnetocaloric material with the lowest Curie temperature comes to
rest at the cold end 2' of the regenerator 1''. The regenerator 1''
then also provides intermediate storage of heat (as in the Stirling
machine of FIG. 1). The working fluid is compressed in the
compression space 5 by the compression piston 4, as in the
regenerative process of the machine in accordance with FIG. 1a, and
is expanded in the expansion space 5' by the expansion piston 4',
and also serves as a heat transfer fluid for the magnetocaloric
process. The two pistons 4, 4' transport the heat transfer fluid
(working fluid of the regenerative process) through the regenerator
1'' where it releases or absorbs heat. The magnetocaloric material
in the regenerator 1'' is magnetized via magnet 7. The magnet 7 may
thereby also be a permanent magnet which is cyclically shifted over
the regenerator bed, or a coil of normally and/or superconducting
wire. Then the magnetocaloric material in the regenerator 1'' is
magnetized and demagnetized through charging and discharging the
magnet coil. Another possibility is to shield a magnetic field,
that is already present in the environment, by a suitable device
(e.g. of soft metal or a further magnet coil). Since a regenerative
gas cycle and a magnetocaloric process are combined in one machine,
the process phases (compression, expansion, transferring the
working fluid through the regenerator 1'', magnetization,
demagnetization) must be thoroughly matched to each other in
time.
[0050] Prior to a more detailed description of the process phases
that are employed in an inventive device, the process phases of a
Stirling machine (according to FIG. 1a) or a magnetocaloric machine
(according to FIG. 1b) are considered separately.
[0051] FIG. 2a schematically shows the different process phases of
a regenerative gas cycle in a one-stage Stirling machine for
transporting heat in a simplified form. In detail, the following
process steps are performed:
I->II Isothermal Compression
[0052] The working fluid in the compression space 5 is isothermally
compressed by the piston 4, thereby releasing heat. The released
heat 8 may be absorbed e.g. by a cooling medium at a constant
temperature.
II->III Isochore Cooling (Heat Release)
[0053] The working fluid in the compression space 5 is transferred
at a constant volume into the expansion space 5' through the
regenerator 1. In the regenerator 1, the working fluid releases
heat to the heat storage medium of the regenerator 1 (matrix of the
regenerator 1) for intermediate storage, and is cooled.
III->IV Isothermal Expansion
[0054] The working fluid in the expansion space 5' is isothermally
expanded by the piston 4', thereby absorbing heat. The absorbed
heat 9 is thereby supplied from the outside by the object/space to
be cooled.
IV->I Isochore Heating (Absorption of Heat)
[0055] The working fluid in the expansion space 5' is transferred
at a constant volume into the compression space 5 through the
regenerator 1. The working fluid absorbs the intermediately stored
heat from the heat storage medium in the regenerator 1 and is
thereby heated. The initial state is reached again.
[0056] The temperature profile 10 in the regenerator 1 remains
unchanged during all process phases. In the machine net work is
exerted and the released heat 8 exceeds the absorbed heat 9 by this
energy amount.
[0057] Under ideal conditions, a Stirling machine has the maximum
coefficient of performance COP of the Carnot process (COP.sub.Car)
and thereby an efficiency .eta. (COP/COP.sub.Car) of 1. Other
coolers, such as e.g. pulse tube coolers do not work reversibly,
not even in the ideal case, and are therefore not that
efficient.
[0058] FIG. 2b shows the different process phases of a one-stage
magnetocaloric refrigerator or heat pump in a simplified and
schematic fashion.
I->II Demagnetization
[0059] The magnetocaloric material in the (active) regenerator 1'
is demagnetized, i.e. a magnetic field of a strength B, that is
present in state I, is reduced (e.g. by removing the permanent
magnet, discharging of the magnet coil or activating the
shielding). The magnetocaloric material of the regenerator 1'
(matrix of the regenerator 1') thereby cools at each location over
its length by a certain temperature difference, the temperature
profile 11 in the regenerator 1' before demagnetization differs
therefore from the temperature profile 11' in the regenerator 1'
after demagnetization.
II->III Cooling (Release of Heat)
[0060] The warm heat transfer fluid is transferred together with
the pistons 4, 4' through the magnetocaloric material of the
regenerator 1'. It is thereby cooled (i.e. the magnetocaloric
material absorbs heat). When it leaves the regenerator 1', the heat
transfer fluid is initially colder than the environment. The heat
transfer fluid may thereby absorb heat 9' from the (cold)
environment. During transfer, the temperatures in the regenerator
1' change and, in state III, the temperature profile 11 has been
restored (as in state I).
III->IV Magnetization
[0061] The magnetocaloric material of the regenerator 1' is
magnetized to a field strength B (with permanent magnets, through
charging a coil or removing a shielding). The magnetocaloric
material of the regenerator 1' is heated at any location over its
length by a certain temperature difference such that the
temperature profile 11'' is achieved.
IV->I Heating (Absorption of Heat)
[0062] The cold heat transfer fluid is shifted back by the pistons
4, 4' through the magnetocaloric material of the regenerator 1'. It
is thereby heated (i.e. the magnetocaloric material releases heat).
When exiting the regenerator 1', the heat transfer fluid is
initially warmer than the environment. The heat transfer fluid
releases heat 8' to the (warm) environment. During transfer, the
temperatures in the regenerator 1' change and the original
temperature profile 11 is restored in state I.
[0063] Work (by moving a permanent magnet or charging of a magnet
coil) must also be exerted in the magnetic refrigerator or heat
pump. The released heat 8' exceeds the absorbed heat 9' by this
energy amount. High efficiencies can be realized as in a Stirling
machine.
[0064] The inventive device is based on a combination of processes
shown e.g. in FIGS. 2a and 2b. FIG. 2c schematically shows the
different process phases of such a refrigerator or heat pump
combination. In the simplest case of only one cooling stage, a
device of this type (e.g. for a hybrid Stirling refrigerator)
comprises the compression space 5 and expansion space 5' with an
intermediate regenerator 1''. As in a magnetic machine, it is
thereby also possible to switch on or off an external magnetic
field B. At the start, the temperature profile 11 is prevailing in
the regenerator 1''. This results in the following cycle:
I->II Isothermal Compression
[0065] When an external magnetic field B is present, the working
fluid in the compression space 5 is isothermally compressed by the
piston 4, thereby releasing heat. The released heat 8 may e.g. be
absorbed by a cooling medium at a constant temperature.
II->III Demagnetization
[0066] The magnetocaloric material in the regenerator 1'' is
demagnetized, i.e. a magnetic field of a strength B which is
present in states I and II is reduced (by removing the permanent
magnet, discharging the magnet coil or activating the shielding).
The magnetocaloric material of the regenerator 1'' is thereby
cooled at any location over its length by a certain temperature
difference. The temperature profile 11 in the regenerator 1'' prior
to demagnetization is therefore different from the temperature
profile 11' in the regenerator 1'' after demagnetization.
III->IV Cooling (Release of Heat)
[0067] The warm heat transfer fluid (working fluid in process step
I->II) is transferred with the piston 4, 4' through the
magnetocaloric material of the regenerator 1''. It is thereby
cooled (i.e. the magnetocaloric material absorbs heat). When it
leaves the regenerator 1'', the heat transfer fluid is initially
colder than the environment at that location. The heat transfer
fluid may thereby absorb heat 9' from the (cold) environment.
During transfer, the temperatures in the regenerator 1'' change
and, in state IV, the temperature profile 11 is restored (as in
states I and II).
IV->V Isothermal Expansion
[0068] The heat transfer fluid in the expansion space 5' is
isothermally expanded by the piston 4', thereby absorbing heat. The
absorbed heat 9 is thereby supplied from the outside by the
object/space to be cooled.
V->VI Magnetization
[0069] The magnetocaloric material of the regenerator 1'' is
magnetized by applying a magnetic field with a field strength B
(with permanent magnets, by charging a coil or removing a
shielding). The magnetocaloric material of the regenerator 1'' is
heated at any location over its length by a certain temperature
difference to yield the temperature profile 11''.
VI->I Heating (Absorption of Heat)
[0070] The cold heat transfer fluid (working fluid in the process
step IV->V) is passed back by the pistons 4, 4' through the
magnetocaloric material of the regenerator 1''. It is thereby
heated (i.e. the magnetocaloric material releases heat). When it
leaves the regenerator 1'', the heat transfer fluid is initially
warmer than the environment at that location. The heat transfer
fluid releases heat 8' to the (warm) environment. During passage,
the temperatures in the regenerator 1'' change and in state I, the
original temperature profile 11 has been restored.
[0071] The heat (heat 9', 9) is thereby supplied from the space to
be cooled (useful cold) during two process steps (III->IV and
IV->V). The heat (heat 8', 8) is released to the warm
environment during the process steps VI->I and I->II. The
required work increases correspondingly. The coefficient of
performance (COP) and the efficiency of the device are, however,
high. The volume of the device corresponds approximately to the
individual volume of one of the non-combined machines from FIGS. 2a
and 2b, which increases the power density.
[0072] Other machines may also be used which are based on a
regenerative gas process, e.g. a Gifford-McMahon cooler or a pulse
tube cooler. In order to obtain very low temperatures (<20 K),
such a device will moreover be composed of several stages. It is
then feasible to use combined cooling in accordance with the
invention only at the coldest stage.
[0073] FIG. 3 shows a multi-stage embodiment of the inventive
device based on a magnetic pulse tube cooler for cooling a
superconducting magnet configuration. A first magnet coil 12 is
thereby located in a helium container 14 filled with liquid helium
13. The helium container 14 is connected to an outer shell 16 via
at least one suspension tube 15. A two-stage cold head 20 of a
magnetic pulse tube cooler is installed into a neck tube 17 whose
upper warm end 18 is connected to the outer shell 16 and whose
lower cold end 19 is connected to the helium container 14. The
helium container 14 is moreover surrounded by a radiation shield
21, which is connected both to the suspension tubes 15 and the neck
tube 17 in a thermally conducting fashion. A heat-conducting solid
connection 23 is provided between the first cold stage 22 of the
cold head 20 and the neck tube 17, via which heat is conducted from
the radiation shield 21 to the first cold stage 22 of the cold head
20. Evaporated helium from the helium container 14 is reliquefied
at the second cold stage 24 of the cold head 20. In contrast to a
conventional two-stage pulse tube cooler, magnetocaloric material
is provided in the regenerator tube 25 of the second cold stage 24.
The regenerator tube 25 is within the stray field of the first
magnet coil 12 without magnetic shielding. The stray field may be
shielded or increased by the second magnet coil 26 in such a manner
that the magnetocaloric material may be used for producing cold in
a magnetocaloric cyclic process in accordance with the invention,
and also as a heat storage medium in the regenerative gas cycle.
The working fluid of the regenerative gas cycle (helium gas) is
moreover the heat transfer fluid for the magnetocaloric cyclic
process. The power and efficiency of the cooler may thereby be
increased, such that the cold head 20 may be decreased in size and
the energy consumption during operation may be reduced.
[0074] The use of an inventive cooling device is therefore
advantageous when the cold head 20 is located in the already
existing stray field of a superconducting magnet, wherein the
magnetic field can then be suitably shielded. Apparatus comprising
a superconducting magnet configuration, such as e.g. for nuclear
magnetic resonance spectroscopy, nuclear magnetic resonance imaging
(MRI), ion cyclotron resonance spectroscopy (ICR) or electron spin
resonance (ESR, EPR) can thereby be efficiently cooled in a
user-friendly way.
[0075] The invention is explained below with reference to an
exemplary calculation and further drawings.
[0076] An ideal inventive hybrid Stirling refrigerator is initially
shown as an example during the process phases III->IV and
VI->1 (FIG. 4). The (ideal) working fluid (e.g. helium gas) in
the compression space 5 is divided into n small volume elements.
Each of these volume elements a, i, n runs through its own
thermodynamic cycle which is represented for the first volume
element a in FIG. 5a in a temperature-entropy diagram (T-S
diagram). During displacement into the expansion space 5', the
volume element a cools from the temperature T.sub.h (state III) to
a temperature T.sub.c. Due to change in temperature of the
magnetocaloric material, the volume element a is further cooled and
finally exits the regenerator 1'' with T.sub.c-.DELTA.T.sub.ca,
(state IV' in FIG. 5a). External heat is then supplied to the
volume element a (at a constant volume) such that it is heated to a
temperature T.sub.c (state IV). Another arbitrary inner volume
element i (see T-S diagram in FIG. 5b) also enters the regenerator
1'' with a temperature T.sub.h, and exits it with temperature
T.sub.c.DELTA.T.sub.ci, which is higher than the exiting
temperature of the first volume element a, since the regenerator
1'' has previously absorbed heat from the volume elements. After
leaving the regenerator 1'', the volume element i is also heated to
the temperature T.sub.c by absorbing heat from the outside. The
heat supplied to the volume element i, however, is then smaller
than for the first volume element a. The last volume element n (see
T-S diagram in FIG. 5c) also enters the regenerator 1'' with the
temperature T.sub.h, but leaves it with the temperature T.sub.c,
since the original temperature profile 11' has been shifted to the
temperature profile 11. The volume element n can therefore no
longer absorb any external heat.
[0077] The heat supplied to the volume elements a, i, n, during
change of states IV'->IV can be represented in the T-S diagram
as areas below the curve.
[0078] The heat Q.sub.i,IV'-IV absorbed by the volume element i
(having the mass .DELTA.m.sub.i) during exiting the regenerator 1''
can be calculated as follows:
Q.sub.i,IV'-IV=.DELTA.m.sub.ic.sub.v,i.left
brkt-bot.T.sub.c-(T.sub.c-.DELTA.T.sub.c,i).right
brkt-bot.=.DELTA.m.sub.ic.sub.v,i.DELTA.T.sub.c,i,
wherein c.sub.v,i is the specific heat capacity of the working
fluid at a constant volume. The overall heat Q.sub.IV'-IV
(corresponds to heat 9' in FIG. 4) absorbed by the working fluid
(having a total mass M) is then
Q IV ' - IV = i .DELTA. m i c v , i .DELTA. T c , i = .DELTA. m c v
i .DELTA. T c , i = .DELTA. m c v n .DELTA. T c ' = M c v .DELTA. T
c ' , ##EQU00001##
wherein .DELTA.T.sub.c' is the average value of temperature changes
.DELTA.T.sub.c,i and .DELTA.m.sub.i=.DELTA.m for all i.
[0079] The subsequent process step IV.fwdarw.V (isothermal
expansion) is the same for all volume elements a, i, n. The overall
heat supplied to the working fluid at a temperature T.sub.c is:
Q.sub.IV-V=MT.sub.c(s.sub.V-s.sub.IV),
wherein s.sub.IV and s.sub.V are the specific entropies of the
working fluid in states IV and V.
[0080] During subsequent shifting to the compression space 5, the
volume element n is heated from a temperature T.sub.c to a
temperature T.sub.h (VI.fwdarw.I in FIG. 5c). Due to the
temperature change of the magnetocaloric material, the volume
element n is further heated and finally exits the regenerator 1''
with T.sub.h+.DELTA.T.sub.h,n (state I'). The volume element n
subsequently releases heat to the outside (at a constant volume)
such that it is cooled again to a temperature T.sub.h (state 1).
Another arbitrary inner volume element i also enters the
regenerator 1'' at a temperature T.sub.c, and leaves it at a
temperature T.sub.h+.DELTA.T.sub.h,i, which is less than the
exiting temperature of the last volume element n, since the
regenerator 1'' has previously released heat to the volume element
a and the volume elements i (FIG. 5b). After leaving the
regenerator 1', the volume element i is also cooled to a
temperature T.sub.h by releasing heat to the outside. The heat
released by the volume element i is now, however, smaller than for
the last volume element n. The first volume element a finally also
enters the regenerator 1'' at a temperature T.sub.c and leaves it
at a temperature T.sub.h (FIG. 5a), since the original temperature
profile 11'' has shifted to temperature profile 11. The first
volume element a can therefore no longer release heat to the
outside. The heat released by the volume elements during the change
of state I'.fwdarw.I can also be represented in the T-S diagram as
areas below the curve.
[0081] The heat Q.sub.i,I'-I released by the volume element i
(having the mass .DELTA.m.sub.i) during exiting the regenerator 1''
(which is therefore negative in accordance with the normal
conventions in thermodynamics) can be calculated as follows:
Q.sub.i,I'-I=.DELTA.m.sub.ic.sub.v,i.left
brkt-bot.T.sub.h-(T.sub.ch+.DELTA.T.sub.h,i).right
brkt-bot.=-.DELTA.m.sub.ic.sub.v,i.DELTA.T.sub.h,i,
[0082] The heat Q.sub.I'-I (which corresponds to the heat 8' in
FIG. 4) released in total by the working fluid (having the overall
mass M) is:
Q I ' - I = - i .DELTA. m i c v , i .DELTA. T h , i = - .DELTA. m c
v i .DELTA. T h , i = - .DELTA. m c v n .DELTA. T h ' = - M c v
.DELTA. T h ' , ##EQU00002##
wherein .DELTA.T.sub.h' is the average value of temperature changes
.DELTA.T.sub.h,i and .DELTA.m.sub.i=.DELTA.m for all i.
[0083] The subsequent step I.fwdarw.II (isothermal compression) is
the same for all volume elements a, i, n. The overall heat released
by the working fluid at a temperature T.sub.h is thereby:
Q.sub.I-II=-MT.sub.h(s.sub.I-s.sub.II),
wherein s.sub.I and s.sub.II are the specific entropies of the
working fluid in states I and II.
[0084] Since the isochores (V=const) in the T-S diagram are
equidistant lines in case of an ideal working fluid (as assumed
herein), the entropy changes during the state changes I.fwdarw.II
and IV.fwdarw.V are equal:
s.sub.I-s.sub.II=s.sub.V-s.sub.IV=.DELTA.s.
[0085] During passage through the regenerator 1'', no heat is
transferred to the outside, such that the following total amount of
heat is supplied to the working fluid in the overall process:
Q.sub.zu=M[T.sub.c.DELTA.s+c.sub.v.DELTA.T.sub.c']
and the following total amount of heat is released from the working
fluid:
Q.sub.ab=-M[T.sub.h.DELTA.s+c.sub.v.DELTA.T.sub.h'].
[0086] In accordance with the first law of thermodynamics, the
following applies for a working cycle with work W:
[0087] W+(Q.sub.zu+Q.sub.ab)=0, such that the work (with
.DELTA.T=T.sub.h-T.sub.c) is calculated as shown below:
W=-(M[T.sub.c.DELTA.s+c.sub.v.DELTA.T.sub.c']-M[T.sub.h.DELTA.s+c.sub.v.-
DELTA.T.sub.h'])=M(.DELTA.T.DELTA.s+c.sub.v(.DELTA.T.sub.h'-.DELTA.T.sub.c-
'))
[0088] The heat supplied in a "pure" Stirling cooling process at a
temperature T.sub.c is:
Q.sub.zu,Stir.=MT.sub.c.DELTA.s,
such that the increase in cooling power can be calculated as
follows:
Q zu Q zu , Stir . = 1 + c v .DELTA. T c ' T c .DELTA. s .
##EQU00003##
[0089] In contrast to a "pure" ideal Stirling cooling process, even
the ideal hybrid process contains irreversibilities which can be
determined through exergy loss, as is common in thermodynamics.
Below, it is assumed that the temperature T.sub.h corresponds to
the ambient temperature. Then, the exergy of the working fluid
would change as follows, through supply of heat during process
phase IV'.fwdarw.IV:
E IV ' - IV = Q IV ' - IV - MT h ( s IV - s IV ' ) = Q IV ' - IV -
MT h c v ln ( T c T c - .DELTA. T c ' ) = M c v ( .DELTA. T c ' - T
h ln ( T c T c - .DELTA. T c ' ) ) . ##EQU00004##
[0090] The following exergy changes occur during the process phase
IV.fwdarw.V (VI):
E.sub.IV-V=Q.sub.IV-V-MT.sub.h.DELTA.s=-M.DELTA.s.DELTA.T.
[0091] The exergy of the working fluid changes through heat release
during the process phase I'.fwdarw.I:
E I ' - I = Q I ' - I - MT h ( s I - s I ' ) = Q I ' - I - MT h c v
ln ( T h T h + .DELTA. T h ' ) = M c v ( - .DELTA. T h ' - T h ln (
T h T h + .DELTA. T h ' ) ) . ##EQU00005##
[0092] Since heat is released during phase I.fwdarw.II (III) at
ambient temperature (T.sub.h) the exergy does not change.
[0093] An overall exergy balance may now be calculated:
E.sub.VI'-VI+E.sub.IV-V+E.sub.VI-I+E.sub.I'-I+E.sub.I-II+E.sub.III-IV+E.-
sub.loss+W=0
[0094] Since the exergy changes of the working fluid cancel during
passage through the regenerator 1'' (in both directions) (i.e.
E.sub.VI-I+E.sub.III-IV=0), the exergy loss E.sub.lossr related to
work can be calculated after substitution and in a simplified
fashion:
E loss W = c v T h ( ln ( T c T c - .DELTA. T c ' ) + ln ( T h T h
+ .DELTA. T h ' ) ) .DELTA. T .DELTA. s + c v ( .DELTA. T h ' -
.DELTA. T c ' ) . ##EQU00006##
[0095] We obtain the following exergetic efficiency or efficiency
factor .eta. (which permits a statement about the "quality" of the
process):
.eta. = 1 - E ver W ##EQU00007##
[0096] As derived from the document "Prospects of magnetic
liquefaction of hydrogen" (Barclay, J. A, Le froid sans frontieres,
vol. 1, page 297, 1991), the following applies for a magnetic
refrigerator and also for the inventive hybrid machine:
T h T c = .DELTA. T h .DELTA. T c .apprxeq. .DELTA. T h ' .DELTA. T
c ' , ##EQU00008##
such that an example can be calculated. With the given boundary
values FIG. 6 shows the (cooling) power increase and the exergetic
efficiency of a hybrid Stirling refrigerator compared to a "pure"
Stirling refrigerator. The actually occurring irreversibilities are
not taken into consideration in these two machines ("ideal"
machines). It is obvious that the power can be considerably
increased mainly with small pressure ratios without considerably
reducing the process quality. For small pressure ratios, the two
isochores (see e.g. FIG. 5b) move closer together, such that the
area Q.sub.i,IV-V is reduced compared to the area Q.sub.i,IV' IV.
The exergy loss is still tolerable, since the temperature changes
.DELTA.T.sub.h, .DELTA.T.sub.c are still small compared to the
absolute temperatures T.sub.h, T.sub.c. A small pressure ratio in a
machine is always advantageous, since the alternating loads are
reduced and the operating life of the machine is increased. This is
another reason why the inventive hybrid machine is superior to a
"pure" gas refrigerator.
[0097] In summary, we obtain a simple device with little apparative
expense for transporting heat from a cold reservoir to a warm
reservoir, in which at least two cyclic processes are employed for
transporting heat thereby absorbing work, of which at least one is
a regenerative cyclic process, and at least one is a magnetocaloric
cyclic process. The device has high power density (mainly with
small pressure ratio) and efficiency and can advantageously be used
for cooling a superconducting magnet configuration, since a
magnetic stray field, which is already present at that location,
can be used for the magnetocaloric cycle.
LIST OF REFERENCE NUMERALS
[0098] 1 passive regenerator (heat storing medium) [0099] 1' active
regenerator (magnetocaloric material) [0100] 1'' passive and active
regenerator of the inventive device (magnetocaloric material)
[0101] 2 warm end of the regenerator [0102] 2' cold end of the
regenerator [0103] 3 warm heat exchanger [0104] 3' cold heat
exchanger [0105] 4 warm piston [0106] 4' cold piston [0107] 5
compression space [0108] 5' expansion space [0109] 6 warm drive
mechanism [0110] 6' cold drive mechanism [0111] 7 magnet (permanent
magnet, magnet coil) [0112] 8 released heat in the regenerative gas
cycle [0113] 8' released heat in the magnetocaloric cycle [0114] 9
absorbed heat in the regenerative gas cycle [0115] 9' absorbed heat
in the magnetocaloric cycle [0116] 10 temperature profile in the
passive regenerator [0117] 11 temperature profile in the active
regenerator prior to demagnetization [0118] 11' temperature profile
in the active regenerator after demagnetization [0119] 11''
temperature profile in the active regenerator after magnetization
[0120] 12 first magnet coil [0121] 13 liquid helium [0122] 14
helium container [0123] 15 suspension tube(s) [0124] 16 outer shell
[0125] 17 neck tube [0126] 18 warm end of the neck tube [0127] 19
cold end of the neck tube [0128] 20 two-stage cold head [0129] 21
radiation shield [0130] 22 first cold stage of the cold head [0131]
23 heat conducting solid connection [0132] 24 second cold stage of
the cold head [0133] 25 regenerator tube of the second cold stage
[0134] 26 second magnet coil [0135] a first volume element [0136] i
any inner volume element [0137] n last volume element
* * * * *