U.S. patent application number 13/134427 was filed with the patent office on 2012-02-09 for magnetocaloric refrigerator.
Invention is credited to Jan Vetrovec.
Application Number | 20120031109 13/134427 |
Document ID | / |
Family ID | 45555052 |
Filed Date | 2012-02-09 |
United States Patent
Application |
20120031109 |
Kind Code |
A1 |
Vetrovec; Jan |
February 9, 2012 |
Magnetocaloric refrigerator
Abstract
The invention is for an apparatus and method for a refrigerator
and a heat pump based on the magnetocaloric effect (MCE) offering a
simpler, lighter, robust, more compact, environmentally compatible,
and energy efficient alternative to traditional vapor-compression
devices. The subject magnetocaloric apparatus alternately exposes a
suitable magnetocaloric material to strong and weak magnetic field
while switching heat to and from the material by a mechanical
commutator using a thin layer of suitable thermal interface fluid
to enhance heat transfer. The invention may be practiced with
multiple magnetocaloric stages to attain large differences in
temperature. Key applications include thermal management of
electronics, as well as industrial and home refrigeration, heating,
and air conditioning. The invention offers a simpler, lighter,
compact, and robust apparatus compared to magnetocaloric devices of
prior art.
Inventors: |
Vetrovec; Jan; (Larkspur,
CO) |
Family ID: |
45555052 |
Appl. No.: |
13/134427 |
Filed: |
June 7, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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61397246 |
Jun 7, 2010 |
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61397175 |
Jun 7, 2010 |
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Current U.S.
Class: |
62/3.1 |
Current CPC
Class: |
Y02B 30/66 20130101;
F25B 2321/0021 20130101; F25B 21/00 20130101; Y02B 30/00
20130101 |
Class at
Publication: |
62/3.1 |
International
Class: |
F25B 21/00 20060101
F25B021/00 |
Claims
1. An apparatus for transferring heat from a cooler reservoir to a
warmer reservoir while expending mechanical energy in the process;
said apparatus comprising a member made of magnetocaloric effect
(MCE) material, a first thermal conductor, a second thermal
conductor, a means for producing a region of strong magnetic field
and a region of weak magnetic field, and a thermal interface fluid
(TIF); said first thermal conductor being arranged to be in a good
thermal communication by means of said TIF with a portion of said
MCE material when said portion of said MCE material is immersed in
said weak magnetic field; and said second thermal conductor being
arranged to be in a good thermal communication by means of said TIF
with a portion of said MCE material when said portion of said MCE
material is immersed in said strong magnetic field.
2. The apparatus of claim 1, wherein said first thermal conductor
and said member are arranged to form a first gap therebetween and
said first gap is substantially filled with said TIF; and said
second thermal conductor and said member are arranged to form a
second gap therebetween and said second gap is substantially filled
with said TIF.
3. The apparatus of claim 2, wherein the width of said first gap
and said second gap are each chosen to be between about 50
micrometers and 500 micrometers.
4. The apparatus of claim 2, wherein said TIF is selected from the
family consisting of liquid metal, gallium-based liquid metal
alloy, gallium-indium-tin liquid metal alloy,
gallium-indium-tin-zinc liquid metal alloy, nanofluid, and
nanofluid substantially comprising carbon nanotubes.
5. The apparatus of claim 1, wherein said member is arranged to be
in motion relative to each said first thermal conductor and said
second thermal conductor.
6. The apparatus of claim 5, wherein said motion is causing said
TIF to flow in a shear flow regime.
7. The apparatus of claim 5, wherein said motion is causing a
portion of said member to be cyclically exposed to said weak
magnetic field and said strong magnetic field.
8. The apparatus of claim 1, wherein said means for producing said
region of strong magnetic field is selected from the family
consisting of a permanent magnet, electromagnet, and
superconducting coil.
9. A staged magnetocaloric refrigerator (MCR) comprising a
plurality of MCR stages; a) each said MCR stage comprising a member
made of magnetocaloric effect (MCE) material, a first thermal
conductor, a second thermal conductor, a means for producing a
region of strong magnetic field and a region of weak magnetic
field, and a thermal interface fluid (TIF); b) within each said MCR
stage, said first thermal conductor of that MCR stage being
arranged to be in a good thermal communication by means of said TIF
with a portion of said MCE material of that stage when said portion
of said MCE material of that stage is immersed in a weak magnetic
field; c) within each said MCR stage, said second thermal conductor
of that MCR stage being arranged to be in a good thermal
communication by means of said TIF with a portion of said MCE
material of that MCR stage when said portion of said MCE material
of that MCR stage is immersed in a strong magnetic field; d) the
thermal conductor of the first MCR stage being thermally coupled to
a lower heat reservoir; e) for each subsequent said MCR stage, the
first thermal conductor of that MCR stage being coupled to the
second thermal conductor of the preceding MCR stage; and f) said
second thermal conductor of the last MCR stage being thermally
coupled to an upper heat reservoir.
10. The staged MCR of claim 9, wherein the temperature of said
lower heat reservoir is substantially lower than the temperature of
said upper heat reservoir.
11. The staged MCR of claim 9, wherein within each said MCR stage
said member of that MCR stage is arranged to be in motion relative
to each said first thermal conductor of that MCR stage; and within
each said MCR stage said member of that MCR stage is arranged to be
in motion relative to each said second thermal conductor of that
MCR stage.
12. The staged MCR of claim 11, wherein said motion is causing said
TIF to flow in a shear flow regime.
13. The staged MCR of claim 9, wherein within each said MCR stage,
said first thermal conductor of that MCR stage and said member of
that MCR stage are arranged to form a first gap therebetween and
said first gap is substantially filled with said TIF; and said
second thermal conductor of that MCR stage and said member of that
MCR stage are arranged to form a second gap therebetween and said
second gap is substantially filled with said TIF.
14. The staged MCR of claim 13, wherein the width of said first gap
and said second gap are each chosen to be between about 50
micrometers and 500 micrometers.
15. The staged MCR of claim 9, wherein said TIF is selected from
the family consisting of liquid metal, gallium-based liquid metal
alloy, gallium-indium-tin liquid metal alloy,
gallium-indium-tin-zinc liquid metal alloy, nanofluid, and
nanofluid substantially comprising carbon nanotubes.
16. A method for pumping heat comprising the steps of: a) providing
a magnetocaloric effect (MCE) material; b) providing a first
thermal conductor at a first temperature; c) providing a second
thermal conductor at a second temperature; d) arranging said MCE
material to be in close proximity of said first conductor with a
first gap therebetween; e) arranging said MCE material to be in
close proximity of said second conductor with a second gap
therebetween; f) substantially filling said first gap and said
second gap with a thermal interface fluid (TIF); g) moving said MCE
material with respect to said first thermal conductor; h) moving
said MCE material with respect to said second thermal conductor; i)
flowing said TIF in said first and said second gap in a shear flow
regime; j) exposing said MCE material to a weak magnetic field; k)
forming a good thermal communication between said MCE material and
said first thermal conductor through said TIF; l) exposing said MCE
material to a strong magnetic field; m) forming a good thermal
communication between said MCE material and said second thermal
conductor through said TIF.
17. The method of claim 16, wherein said second temperature is
higher than said first temperature.
18. The method of claim 16, wherein said steps of (j) exposing said
MCE material to a weak magnetic field and (k) forming a good
thermal communication between said MCE material and said first
thermal conductor through said TIF, are performed concurrently.
19. The method of claim 16, wherein said steps of (l) exposing said
MCE material to a strong magnetic field and (m) forming a good
thermal communication between said MCE material and said second
thermal conductor through said TIF, are performed concurrently.
20. The method of claim 16, wherein said step of forming a good
thermal communication between said MCE material and said first
thermal conductor through said TIF further comprises flowing of
heat from said first thermal conductor through said TIF to said MCE
material.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS:
[0001] This application claims priority from U.S. provisional
patent application U.S. Ser. No. 61/397,246, filed on Jun. 7, 2010
and entitled "Magneto-Caloric Refrigerator" and from U.S.
provisional patent application U.S. Ser. No. 61/397,175, filed on
Jun. 7, 2010 and entitled "Staged Magneto-Caloric Refrigerator,"
the entire contents of all of which are hereby expressly
incorporated by reference.
FIELD OF THE INVENTION
[0002] This invention relates generally to magnetocaloric machines
and more specifically to heat pumps based on magnetocaloric
effect.
BACKGROUND OF THE INVENTION
[0003] The subject invention is an apparatus and method for
magneto-caloric refrigerator (MCR) offering improved energy
efficiency, and reduced emissions of pollutants and greenhouse
gases.
[0004] According to the U.S. Department of Energy, refrigeration
and air conditioning in buildings, industry, and transportation may
account for approximately 10.sup.19 joules of yearly primary energy
consumption in the U.S.A. Air conditioning is also a major
contributor to electric utility peak loads, which incur high
generation costs while generally using inefficient and polluting
generation turbines. In addition, peak loads due to air
conditioning may be a major factor in poor grid reliability. Most
of the conventional air conditioning, heat pumps, and refrigerators
may achieve cooling through a mechanical vapor compression cycle.
The thermodynamic efficiency of the vapor compression cycle is
today much less than the theoretical maximum, yet dramatic future
improvements in efficiency are unlikely. In addition, the
hydrofluorocarbon refrigerants used by vapor compression cycle
today are deemed to be strong contributors to the green house
effect. Hence, there is a strong need for innovative approaches to
cooling with high efficiencies and net-zero direct green house gas
emissions.
[0005] The magneto-caloric effect (MCE) describes the conversion of
a magnetically induced entropy change in a material to the
evolution or absorption of heat, with a corresponding rise or
decrease in temperature. In particular, MCE material may heat up
when it is immersed in magnetic field and it may cool down when
removed from the magnetic field, see FIG. 1.
[0006] All magnetic materials, to a greater or lesser degree, may
exhibit an MCE. However, some materials, by virtue of a unique
electronic structure or physical nanostructure, may display a
significantly enhanced MCE, which may potentially be harnessed for
technological application. In contrast to the MCE found in
paramagnetic materials, the large MCE exhibited by ferromagnetic
materials near their magnetic phase transition temperature (also
known as the Curie temperature or Currie point) may render them
suitable as working materials for magnetic cooling at the target
temperatures appropriate for commercial, industrial, and home
refrigeration application and heat pump devices, namely 200 to 400
degrees Kelvin. For example, gadolinium (Gd) is a ferromagnetic
material known to exhibit a significant MCE near its Curie point of
about 293 degrees Kelvin. In recent years, a variety of other MCE
materials potentially suitable for operation at near room
temperature have been discovered. See, for example, "Chapter 4:
Magnetocaloric Refrigeration at Ambient Temperature," by Ekkes
Bruck in "Handbook of Magnetic Materials," edited by K. H. J.
Buschow, published by Elsevier B.V., Amsterdam, Netherlands, in
2008.
[0007] One of the very promising novel MCE materials is the
intermetallic compound series based on the composition
Gd.sub.5(Si.sub.xGe.sub.1-x).sub.4, where 0.1.ltoreq.x1.ltoreq.0.5,
disclosed by K. A. Gschneider and V. K. Pecharsky in U.S. Pat. No.,
5,743,095 issued on Apr. 28, 1998 and entitled "Active Magnetic
Refrigerants based on Gd--Si--Ge Materials and Refrigeration
Apparatus and Process," which is hereby incorporated by reference
in its entirety. See also and article by V. K. Pecharsky and K. A.
Gschneider, "Tunable Magnetic Refrigerator Alloys with a Giant
Magnetocaloric Effect for Magnetic Refrigeration from .about.20 to
.about.290K," published in Applied Physics Letters, volume 70, Jun.
16, 1997, starting on page 3299. MCE produced by this family of
compounds, also referred to as GdSiGe, has been labeled as "giant"
because of its relatively large magnitude (reported as 4 to 6
degrees C. per Tesla of magnetic flux density). In particular, the
MCE of the GdSiGe alloys may be reversible. Another noteworthy
characteristic of the GdSiGe family is that the Curie temperature,
may be tuned with compositional variation. This feature allows the
working temperature of the magnetic refrigerator to vary from 30
degrees Kelvin to 276 degrees Kelvin, and possibly higher, by
adjusting the Si:Ge ratio. For the purpose of this disclosure, an
MCE material is defined as a suitable material exhibiting a
significant MCE.
[0008] A magneto-caloric refrigerator (MCR) is a refrigerator based
on MCE. MCR offers a relatively simple and robust alternative to
traditional vapor-compression cycle refrigeration systems. MCR
devices may have reduced mechanical vibrations, compact size, and
lightweight. In addition, the theoretical thermodynamic efficiency
of MCR may be much higher than for a vapor compression cycle and it
may approach the Carnot efficiency. An MCR may employ an MCE
material (sometimes referred to as a magnetic refrigerant working
material) that may act as both as a "coolant" producing
refrigeration and a "regenerator" heating a suitable heat transfer
fluid. When the MCE material is subjected to strong magnetic field,
its magnetic entropy may be reduced, and the energy released in the
process may heat the material. With the MCE material in magnetized
condition, a first stream of heat transfer fluid directed into a
thermal contact with the MCE material may be warmed in the process
and the heat may be carried away by the flow. When substantial
portion of the heat is removed from the MCE material, the fluid
flow may be terminated. As the next step, the magnetic field may be
reduced, which may cause an increase in magnetic entropy. As a
result, the MCE material may cool. A second stream of heat transfer
fluid may be directed into a thermal contact with the MCE material
where may deposit some of its heat and it may be cooled in the
process. When substantial portion of the heat is deposited into the
MCE material, the fluid flow may be terminated. Repeating the above
steps may result in a semi-continuous operation. One disadvantage
of such an MCR is the need for multiple flow loops typically
involving pumps, heat exchangers, and significant plumbing.
[0009] Despite the apparent conceptual simplicity, there are
significant challenges to the development of a practical MCR
suitable for commercial applications. This is in-part due to the
relatively modest temperature changes (typically few degrees Kelvin
per Tesla of magnetic flux density) of the MCE material undergoing
MCE transition. In addition, at present time the magnetic field
produced by permanent magnets is limited to about 1.5 Tesla
maximum. As a result, an MCR using permanent magnets and a single
step MCE process may produce only a few degrees Kelvin temperature
differential. Many important practical applications such as
commercial refrigeration and air conditioning may require
substantially higher temperature differentials, typically 30
degrees Kelvin and higher.
[0010] One approach to achieving commercially desirable temperature
differentials from MCR may use multiple MCR stages (also known as
cascades). Heat flow between stages may be managed by heat
switches. Each stage contains a suitable MCE material undergoing
magnetocaloric transition at a slightly different temperature.
While the temperature differential achieved by one stage may be
only a few degrees Kelvin, the aggregate operation of multiple
stages may produce very large temperature differentials. See, for
example, "Thermodynamics of Magnetic Refrigeration" by A.
Kitanovski, P. W. Egolf, in International Journal of Refrigeration,
volume 29 pages 3-21 published in 2006 by Elsevier Ltd., the entire
contents of which are hereby expressly incorporated by
reference.
[0011] A variety of heat switching approaches have been proposed
but none has won commercial acceptance. For example, Ghoshal, in
U.S. Pat. No., 6,588,216 entitled "Apparatus and methods for
performing switching in magnetic refrigeration systems," issued on
Jul. 8, 2003, and incorporated herein by reference in its entirety,
discloses switching of thermal path between MCR stages by
mechanical means using micro-electro-mechanical systems (MEMS),
and/or electronic means using thermoelectric elements. Ghoshal's
thermal path switching by MEMS is inherently limited by the poor
thermal conductivity of bare mechanical contacts. Ghoshal's
thermoelectric switches have very limited thermodynamic efficiency
which substantially increases the heat load to the MCR and reduces
the overall MCR efficiency.
[0012] In summary, there is a need for 1) reducing or eliminating
moving parts and pumped fluid loops in MCR systems, 2) simpler and
more reliable MCR operation, and 3) means for attaining
commercially desirable temperature differentials from MCR. A
specific need exists for reliable, low-thermal resistance means for
switching of the heat flow to and from the MCE material in staged
(cascaded) MCR.
SUMMARY OF THE INVENTION
[0013] The present invention provides a magnetocaloric refrigerator
(MCR) having one or more stages. An MCR stage in accordance with
the subject invention comprises a body made of a suitable
magnetocaloric effect (MCE) material positioned in proximity of a
first thermal conductor and in proximity of a second thermal
conductor with only small gaps therebetween. The gaps between the
MCE material and the thermal conductors may be filled with a
suitable thermal interface fluid (TIF) having acceptably high
thermal conductivity. TIF may significantly enhance thermal
communication between the MCE material and the thermal conductors.
The MCE material may be arranged to be in relative motion with
respect to the thermal conductors, which may cause the TIF layer in
the gaps between MCE material and the thermal conductors to flow in
a regime known as a shear flow and also known as a Couette flow.
TIF flowing in a shear flow regime may further significantly
enhance thermal communication between the MCE material and the
thermal conductors. The relative motion of the MCE material may
cause the MCE material to be alternately exposed to regions of
strong magnetic field and weak magnetic field. A portion of MCE
material exposed to a weak magnetic field may become cooler. The
portion of MCE material in a cooler state may be arranged to be in
a good thermal communication through the TIF with the first thermal
conductor and it may receive heat from it. A portion of MCE
material exposed to a strong magnetic field may become warmer. The
portion of MCE material in a warmer state may be arranged to be in
a good thermal communication through the TIF with the second
thermal conductor and it may transfer heat to it. As a result, the
inventive MCR stage may transfer heat from the first thermal
conductor to the second thermal conductor against a temperature
gradient and, therefore, it may act as a heat pump. The inventive
MCR stages may be thermally connected in series to produce higher
temperature differential than possible in a single MCR stage.
[0014] For the purposes of this disclosure, the term "strong
magnetic field" is defined as a magnetic field having an absolute
value of magnetic flux density of at least 0.5 Tesla (5,000 Gauss),
and the term "weak magnetic field" is defined as a magnetic field
having an absolute value of magnetic flux density of at least 0.1
Tesla (1,000 Gauss) lower than the "strong magnetic field" flux
density. In particular, the range of weak magnetic field may
include magnetic flux density of essentially zero (0) Tesla (i.e.,
no field).
[0015] The thermal interface fluid (TIF) is a key material for
facilitating efficient heat transfer between MCE material and
thermal conductors in the MCR stage of the subject invention. For
the purpose of this disclosure, TIF may be a liquid or a paste.
Preferably, suitable TIF has a good thermal conductivity, surface
wetting capability, lubrication properties, low melting point,
acceptably low viscosity, low or no toxicity, and low cost. The
inventor has determined that TIF should preferably have a thermal
conductivity of at least as 1 W/m-degree K and most preferably at
least 3 W/m-degree K. In some embodiments of the invention the TIF
may be a liquid metal. Suitable liquid metal may be an alloy of
gallium (Ga) such as a non-toxic eutectic ternary alloy known as
galinstan and disclosed in the U.S. Pat. No. 5,800,060. Galinstan
(68.5% gallium, 21.5% indium, and 10% tin) is reported to have
thermal conductivity of about 16 W/m-degree K (about 27 times
higher than water), a melting point of minus 19 degrees Centigrade,
low viscosity, and excellent wetting properties. Brandeburg et al.
in the U.S. Pat. No. 7,726,972 discloses a quaternary gallium alloy
having a melting point of minus 36 degrees Centigrade, which may be
also suitable for use with the subject invention. Other suitable
gallium alloys may include those disclosed in the U.S. Pat. No.
5,792,236.
[0016] Other suitable variants of the TIF may also comprise a fluid
containing nanometer-sized particles (nanoparticles) also known as
nanofluid. Nanofluids are engineered colloidal suspensions of
nanoparticles in a base fluid. The nanoparticles used in nanofluids
may be typically made of metals, oxides, carbides, carbon,
graphite, graphene, graphite nanotubes, or carbon nanotubes. Common
base fluids may include water, alcohol, and ethylene glycol.
Nanofluids may exhibit enhanced thermal conductivity and enhanced
convective heat transfer coefficient compared to the base fluid
alone. In yet other embodiments of the invention the TIF may not be
strictly a fluid but rather a paste comprising mainly of
micro-scale and/or nano-scale particles made of high thermal
conductivity materials such as silver, copper, or graphite in
suitable base liquid or paste.
[0017] In one preferred embodiment of the present invention, the
MCR stage comprises an MCE disk, a first thermal conductor, a
second thermal conductor, and a magnet assembly. The MCE disk
further comprises a suitable MCE material. The MCE disk is arranged
to rotate about its axis of rotational symmetry. The magnet
assembly is arranged to produce a region of strong magnetic field.
A portion of the MCE material of the MCE disk may be exposed to a
strong magnetic field while another portion of the MCE material of
the MCE disk may be exposed to a weak magnetic field. The first
thermal conductor is arranged to be in close proximity of the
portion of the MCE material exposed to weak magnetic field with
only a small gap therebetween. The second thermal conductor is
arranged in close proximity of the portion of the MCE material
exposed to strong magnetic field with only a small gap
therebetween. A portion of the MCE disk may be submerged in a
suitable TIF. The TIF is selected to provide good wetting of the
disk surface. When the disk is rotated about its axis, some of the
TIF is entrained by the disk surface and carried into the gap
between the MCE disk the first thermal conductor, thereby
establishing a good thermal communication therebetween. In a
similar fashion, some of the TIF entrained by the MCE disk surface
is also carried into the gap between the MCE disk the second
thermal conductor, thereby establishing a good thermal
communication therebetween.
[0018] When a specific portion of the MCE material is rotated into
the region of strong magnetic field and exposed to the strong
magnetic field, the entropy of the specific portion may be reduced
and the temperature of the material of the specific portion may be
increased to a temperature higher than the temperature of the
second thermal conductor. This may establish a thermal gradient
which may transport heat from the specific portion of the MCE
material through the TIF into the second thermal conductor. As a
result, the specific portion of the MCE material may cool down
toward the temperature of the second thermal conductor.
[0019] When the specific portion of the MCE material is
subsequently rotated out of the region of strong magnetic field and
it is exposed to a weak magnetic field, the entropy of the specific
portion of the MCE material may be increased. This may cause
reduction in temperature of the specific portion to a temperature
that may be lower than the temperature of the first thermal
conductor. Further rotation of the MCE disk may deliver the
specific portion of the MCE material into proximity of the first
thermal conductor. The temperature gradient between the first
thermal conductor and the specific portion of the MCE material may
transport heat from the first thermal conductor through the TIF
into the specific portion of the MCE material portion. This process
may cause the temperature of the specific portion to rise toward
the temperature of the first thermal conductor. Yet further
rotation of the disk may bring the specific portion of the MCE
material back into the region of the strong magnetic field, and the
whole cycle may be repeated again. As a result, heat is transported
(pumped) from the first thermal conductor to the second thermal
conductor against a thermal gradient that may exist between the two
thermal conductors.
[0020] The MCR stages can be thermally connected in series into an
MCR assembly by connecting the second thermal conductor of the
first stage to the first thermal conductor of the second stage, the
second thermal conductor of the second stage to the first thermal
conductor of the third stage, the second thermal conductor of the
third stage to the first thermal conductor of the fourth stage, and
so on. In this fashion the inventive MCR assembly can produce much
higher temperature differential than possible in a single MCR
stage.
[0021] Accordingly, it is an object of the present invention to
provide an MCR that is relatively simple and robust alternative to
traditional vapor-compression cycle refrigeration systems, while
attaining comparable or even higher thermodynamic efficiency.
[0022] It is another object of the invention to provide an MCR for
general refrigeration and air conditioning while improving energy
efficiency and reducing emissions of pollutants and greenhouse
gases.
[0023] It is yet another object of the invention to provide an MCR
having one or more stages to achieve commercially useful
temperature differentials.
[0024] It is still another object of the subject invention to
provide an MCR having low mechanical vibrations, compact size, and
lightweight coupled with a thermodynamic efficiency exceeding that
of thermo-electric coolers.
[0025] It is a further object of the subject invention to provide
efficient switching of heat to and from an MCE material.
[0026] These and other objects of the present invention will become
apparent upon a reading of the following specification and
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is an illustration of the magneto-caloric effect.
[0028] FIG. 2 is a partial side cross-sectional side view of an MCR
stage in accordance with one embodiment of the subject
invention.
[0029] FIG. 3 is a plan view of an MCR stage in accordance with one
embodiment of the subject invention.
[0030] FIG. 4 is a view 4-4 of an MCR stage of FIG. 2.
[0031] FIG. 5 is an enlarged view of portion 5 of the MCR stage of
FIG. 3 showing the relative arrangement of the disk and the thermal
conductor.
[0032] FIG. 6 is an exemplary arrangement of the magnet
assembly.
[0033] FIG. 7 is an illustration of the MCR operation showing
exemplary temperatures.
[0034] FIG. 8 is a side cross-sectional side view of an MCR
assembly with four MCR stages.
[0035] FIG. 9 is a plan view of the MCR assembly with four MCR
stages of FIG. 8.
[0036] FIG. 10A is a view of an alternative MCE ring for reduced
parasitic heat flow in azimuthal direction.
[0037] FIG. 10B is a cross-sectional view 10B-10B of the
alternative MCE ring of FIG. 10A.
[0038] FIG. 11A is a view of another alternative MCE disk having
portions made of material having high thermal conductivity.
[0039] FIG. 11B is an enlarged view of portion 11B of the another
alternative MCE ring of FIG. 11A.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0040] Selected embodiments of the present invention will now be
explained with reference to drawings. In the drawings, identical
components are provided with identical reference symbols in one or
more of the figures. It will be apparent to those skilled in the
art from this disclosure that the following descriptions of the
embodiments of the present invention are merely exemplary in nature
and are in no way intended to limit the invention, its application,
or uses.
[0041] Referring now to FIGS. 2, 3, 4 and 5, there is shown an MCR
stage 10 in accordance with one preferred embodiment of the present
invention. The MCR stage 10 comprises an MCE disk 102, a first
thermal conductor 106, a second thermal conductor 108, a reservoir
114, thermal interface fluid (TIF) 110, and a magnet assembly 112.
The MCE disk 102 further comprises a suitable MCE material. For
example, the MCE disk 102 may be made in its entirety from a
suitable MCE material. Alternatively, only selected portions of the
MCE disk (for example, the perimetral portion) may be made from MCE
material. Suitable MCE materials include but are not limited to
gadolinium (Gd) and a family of gadolinium-silicon-germanium
(GdSiGe) alloys disclosed in the above noted U.S. Pat. No.
5,743,095.
[0042] The MCE disk 102 is rotatably mounted on a shaft 104 having
an axis of rotation 103. The magnet assembly 112 is configured to
produce strong magnetic field in a region 118 overlapping a portion
of the MCE disk 102. The MCE disk 102 is constructed and arranged
so that at any given time, a portion of the disk MCE material is
immersed in the strong magnetic field of the region 118 while other
portion of the MCE disk 102 is immersed in a weak magnetic field
outside the region 118. The magnet assembly 112 may further
comprise permanent magnets 122, a yoke 126 made of soft magnetic
material, and flux focusing pole pieces 124 as shown in FIG. 6. The
permanent magnets 122 are preferably of the rare earth type such as
neodymium-iron-boron (NdFeB), most preferably having a remanent
field in excess of 1.4 Tesla. However, the invention may be
practiced with alternative magnets. The yoke 126 and flux focusing
pole pieces 124 are preferably made of soft magnetic material
having a high magnetic saturation such as, but not limited to, mild
steel, low carbon steel, silicon steel, iron, iron-cobalt-vanadium
alloys, Consumet.RTM. electrical iron, and Hyperco.RTM. 50.
Consumet.RTM. electrical iron and Hyperco.RTM. 50 are available
from Carpenter Technology Corporation in Wyomissing, Pa.
[0043] The reservoir 114 contains suitable TIF 110. The reservoir
114 is arranged so that a portion of the MCE disk 102 is immersed
in and wetted by the TIF 110 within the reservoir. TIF 110 is
selected to provide good wetting of the disk surface material. If
TIF 110 comprises gallium, the MCE disk and the thermal conductors
may require protective coating to prevent corrosion. Suitable
protective coatings may include but they are not limited to
titanium nitride (TiN) and the diamond-like coating (DLC) Titankote
C11 available from Richter Precision, Inc. in East Petersburg,
Pa.
[0044] The first thermal conductor 106 is arranged to be in close
proximity of the portion of MCE disk 102 exposed to a weak magnetic
field outside the region 118. The second thermal conductor 108 is
arranged to be in close proximity of the disk portion exposed to
strong magnetic field in the region 118. Preferably, only a small
gap 120 exists between the MCE disk 102 and the thermal conductors
106 and 108. FIG. 5 shows an enlarged view of the gap 120 between
the MCE disk 102 and the second thermal conductor 108. Preferably,
the gap 120 should be between about 50 and about 500 micrometers
wide. The gap 120 is filled with TIF 110. The TIF 110 is selected
to have a good thermal conductivity (compared to the MCE material)
to facilitate a good thermal communication path between the disk
102 and the thermal conductors 106 and 108. The thermal conductors
106 and 108 are preferably made of material having high thermal
conductivity. Suitable materials for the thermal conductors 106 and
108 may include but are not limited to copper, aluminum, tungsten,
silicon, silicon carbide, aluminum nitride, and beryllium
oxide.
[0045] In operation, the MCE disk 102 is arranged to rotate on the
shaft 104 about its axis 103 in the direction of the arrow 116
(FIG. 2) so that at any given time a portion of the MCE material of
the MCE disk 102 is exposed to a strong magnetic field in the
region 118 while another portion of the MCE material of the MCE
disk 102 is exposed to a weak magnetic field. In particular, a
specific portion 128 of the MCE material of the MCE disk 102 may
rotate into and out of the region 118 of strong magnetic field.
Therefore, the specific portion 128 of the MCE material of the disk
102 is alternately exposed to the strong magnetic field in the
region 118 and weak magnetic field outside the region 118.
[0046] As the MCE disk 102 is rotated in the direction of arrow 116
(FIG. 2), some of the TIF 110 in the reservoir 114 is entrained by
the disk surface. Entrained TIF 110 is carried into the gap 120
between the MCE disk 102 the first thermal conductor 106, thereby
establishing a good thermal communication therebetween. In a
similar fashion, some of the TIF 110 entrained by the disk surface
is also carried into the gap 120 between the disk 102 the second
thermal conductor 108, thereby establishing a good thermal
communication therebetween.
[0047] When the specific portion 128 of the MCE material of the MCE
disk 102 is rotated in the direction of the arrow 116 into the
region 118 of strong magnetic field and exposed to the strong
magnetic field, the entropy of the MCE material portion may be
reduced. As a consequence, the temperature of the material may be
increased to a temperature higher than the temperature of the
second thermal conductor 108. The resulting thermal gradient may
transport heat from the specific portion 128 of the MCE material
through the TIF 110 in the gap 120 into the second thermal
conductor 108. Consequently, the specific portion 128 of the MCE
material may cool down toward the temperature of the second thermal
conductor 108. As the specific portion 128 of the MCE material is
subsequently rotated in the direction of arrow 116 out of the
region 118 of strong magnetic field, it is becoming exposed to a
weak magnetic field. Thus, the entropy of the specific portion 128
of the MCE material may be increased. As a consequence, the
temperature of the specific portion 128 of the MCE material may be
decreased to a temperature lower than the temperature of the first
thermal conductor 106. Further rotation of the disk 102 in the
direction of arrow 116 delivers the specific portion 128 of the MCE
material into proximity of the first thermal conductor 106. The
temperature gradient between the first thermal conductor 106 and
the specific portion 128 of the MCE material may transport heat
from the first thermal conductor 106 through the TIF 110 in the gap
120 into the specific portion 128 of the MCE material. As a result,
the temperature of the specific portion 128 may rise toward the
temperature of the first thermal conductor 106. Yet further
rotation of the disk 102 in the direction of arrow 116 brings the
specific portion 128 of the MCE material back into the region 118
of the strong magnetic field, and the whole process may be repeated
again. As a result, heat may be transported (pumped) from the first
thermal conductor 106 to the second thermal conductor 108 against a
thermal gradient that may exist between the two thermal
conductors.
[0048] FIG. 7 shows exemplary relative temperatures that may exists
in various portions of the disk 102 and the thermal conductors 106
and 108 during the operation of the MCR stage 10. The thermal
conductors 106 and 108 may be thermally coupled to heat reservoirs
having temperatures T.sub.1 and T.sub.2, respectively. With the MCE
disk 102 rotating in the direction of the arrow 116, a specific
portion 128 of the MCE material may terminate its thermal
communication with the first thermal conductor 106 and it may have
a temperature approximately equal to the temperature T.sub.1. When
the specific portion 128 enters the region 118, its temperature may
increase to the temperature about T.sub.H=T.sub.1+.DELTA.T, where
the temperature increase .DELTA.T is due to MCE. The temperature
T.sub.H is arranged to be higher than the temperature T.sub.2
(namely, T.sub.H>T.sub.2). For example, T.sub.H may be increased
by increasing the strength of the magnetic field in the region 118
and/or by choosing an MCE material having a strong MCE. In
addition, the temperatures T.sub.1 and T.sub.2 may be selected so
that .DELTA.T>T.sub.2-T.sub.1. Therefore, heat may flow from the
portion 128 into the thermal conductor 108 (through TIF 110 in the
gap 120) and the temperature of the portion 128 may be reduced
toward the temperature T.sub.2. For example, the temperature of the
portion 128 may arranged to be approximately equal to the
temperature T.sub.2. When the portion 128 further rotates in the
direction of the arrow 116 out of the region 118, its temperature
may decrease due to MCE to about T.sub.L=T.sub.2-.DELTA.T. The
portion 128 is subsequently rotated in the direction of the arrow
116 into a thermal communication with the thermal conductor 106.
Because T.sub.L is arranged to be smaller than T.sub.1, (namely,
T.sub.L<T.sub.1), heat may flow from the first thermal conductor
106 into the specific portion 128 and the temperature of specific
portion 128 may rise toward the temperature T.sub.1. For example,
the temperature of the portion 128 may arranged to be approximately
equal to the temperature T1.sub.2. The specific portion 128 is now
rotated further in the direction of arrow 116, and the cycle may be
repeated.
[0049] Multiple MCR stages 10 may be thermally connected in series.
FIGS. 8 and 9 show an MCR assembly 100 having four (4) MCR stages.
In particular, the MCR assembly 100 is constructed by thermally
connecting MCR stages 10a, 10b, 10c, and 10d. More specifically,
the second thermal conductor 108a of the first stage 10a is
thermally coupled to the first thermal conductor 106b of the second
stage 10b, the second thermal conductor 108b of the second stage
10b to the first thermal conductor 106c of the third stage 10c, the
second thermal conductor 108c of the third stage 10c to the first
thermal conductor 106d of the fourth stage 10d. Additional stages
may be added in a similar fashion to obtain desired temperature
differential between the cold and hot ends. In this fashion the
inventive MCR assembly can produce higher temperature differential
than possible in a single MCR stage.
[0050] Heat conduction in the azimuthal direction within the MCE
disk may be undesirable because it may reduce the efficiency of the
MCR stage 10. FIG. 10A shows an alternative MCE disk 102' having
radial slots 132 for restricting parasitic flow of heat in
azimuthal direction. The slots 132 may be empty or filled with a
suitable thermally insulating material. FIG. 10B is a
cross-sectional view of the MCE disk 102' showing that the slots
132 may penetrate through the full thickness of the MCE ring
material. An alternative slots (not shown) may not be necessarily
radial and/or may not necessarily penetrate through the full
thickness of the MCE ring material.
[0051] MCE materials may have only a limited thermal conductivity
generally in the range of about 10 Watts/meter-degree Kelvin and
often lower. This makes it challenging to conduct heat to and from
the interior of the MCE disk 102. FIG. 11A shows another
alternative MCE disk 102'' having portions 134 made of suitable MCE
material and portions 130 (FIGS. 11B and 11C) made of material
having high thermal conductivity. For example, portions 130 may be
made of copper, silver, aluminum, graphite, graphite fiber,
graphene, or other suitable material. The transverse dimension "X"
of portions 130 is preferably made comparable to or smaller than
the thickness "T" of the MCE disk 102''. Portions 130 may be formed
as a cylinder, prism, parallel-piped, cones, or pyramids, or other
suitable shapes. Portions 130 may enhance the conductive heat
transfer between the interior of the MCE material of the MCE disk
102'' and the flat surfaces of the MCE disk 102'', thus mitigating
the limited thermal conductivity of typical MCE materials. This may
beneficially allow for a substantial increase of the thickness "T"
of the MCE disk 102'', and/or substantial increase of the speed of
rotation of the MCE disk 102''. In either case, an increased
refrigeration power may be obtained.
[0052] The above description of the embodiments of the present
invention are merely exemplary in nature and are in no way intended
to limit the invention, its application, or uses. For example,
other embodiments of the invention may use linearly moving strips
or plates of MCE material rather than rotating rings. Suitable
linear motion may be continuous or reciprocating. As another
example, yet other embodiments of the invention may use
electromagnets or superconducting magnets instead (or in a
combination with) permanent magnets.
[0053] The terminology used herein is for the purpose of describing
particular embodiments only and is not intended to be limiting of
the invention. As used herein, the singular forms "a", "an" and
"the" are intended to include the plural forms as well, unless the
context clearly indicates otherwise. It will be further understood
that the terms "comprises" and/or "comprising," and "includes"
and/or "including" when used in this specification, specify the
presence of stated features, integers, steps, operations, elements,
and/or components, but do not preclude the presence or addition of
one or more other features, integers, steps, operations, elements,
components, and/or groups thereof.
[0054] The terms of degree such as "substantially", "about" and
"approximately" as used herein mean a reasonable amount of
deviation of the modified term such that the end result is not
significantly changed. For example, these terms can be construed as
including a deviation of at least .+-.5% of the modified term if
this deviation would not negate the meaning of the word it
modifies.
[0055] The term "suitable," as used herein, means having
characteristics that are sufficient to produce a desired result.
Suitability for the intended purpose can be determined by one of
ordinary skill in the art using only routine experimentation.
[0056] Moreover, terms that are expressed as "means-plus function"
in the claims should include any structure that can be utilized to
carry out the function of that part of the present invention. In
addition, the term "configured" as used herein to describe a
component, section or part of a device includes hardware and/or
software that is constructed and/or programmed to carry out the
desired function.
[0057] Different aspects of the invention may be combined in any
suitable way.
[0058] While only selected embodiments have been chosen to
illustrate the present invention, it will be apparent to those
skilled in the art from this disclosure that various changes and
modifications can be made herein without departing from the scope
of the present invention as defined in the appended claims.
Furthermore, the foregoing description of the embodiments according
to the present invention are provided for illustration only, and
not for the purpose of limiting the present invention as defined by
the appended claims and their equivalents. Thus, the scope of the
present invention is not limited to the disclosed embodiments.
* * * * *