MAGNETOCALORIC REFRIGERATOR OR HEAT PUMP COMPRISING AN EXTERNALLY ACTIVATABLE THERMAL SWITCH

Abstract

Magnetocaloric refrigerator or heat pump comprising an externally activatable thermal switch for transferring heat from a heat source to a heat sink, comprising: an insulator cage with thermally conductive windows for the source and sink; a magnetic nanofluid, comprised within said cage, wherein said magnetic nanofluid is able to flow under a magnetic field inside the insulator cage between a contact of the thermally conductive window of the heat source and a contact of the thermally conductive window of the heat sink; and a activatable magnet placed at either one of the thermally conductive windows, such that the produced magnetic field is aligned substantially parallel to the temperature gradient from heat source to heat sink. The apparatus alternates between: activating the magnet, such that the nanofluid flows to establish a thermal contact with the thermal source but not with the sink; deactivating the magnet, such that the nanofluid flows to establish a thermal contact with the thermal sink but not with the source.

Claims

1. A magnetocaloric refrigerator or heat pump apparatus, comprising an externally activatable thermal switch for transferring heat from a heat source to a heat sink, said switch comprising: an insulator cage having thermally conductive windows having a contact to the heat source and a contact to the heat sink; a magnetic nanofluid within said insulator cage, wherein said magnetic nanofluid flows under a magnetic field inside the insulator cage between the contact of the thermally conductive window to the heat source and the contact of the thermally conductive window to the heat sink; and a first activatable magnet placed at either one of the thermally conductive windows, such that the magnetic field produced by the magnet is aligned substantially parallel to a temperature gradient from heat source to heat sink.

2. The magnetocaloric apparatus according to claim 1, wherein the activatable thermal switch is arranged such that, when the activatable thermal switch is activated, the apparatus alternates between the following two states: activating the first activatable magnet, such that the magnetic nanofluid flows to establish a thermal contact with the thermal source and not with the thermal sink; and deactivating the first activatable magnet, such that the magnetic nanofluid flows to establish a thermal contact with the thermal sink and not with the thermal source.

3. The magnetocaloric apparatus according to claim 2, wherein the activatable thermal switch is arranged such that a frequency of the alternating between the two states is between 5 and 30 Hz.

4. The magnetocaloric apparatus according to claim 1, wherein the magnetic nanofluid is a colloidal mixture of ferromagnetic nanoparticles or is a ferromagnetic nanoparticle dispersion.

5. The magnetocaloric apparatus according to claim 4, wherein the first activatable magnet is an electromagnet.

6. The magnetocaloric apparatus according to claim 1, wherein the first activatable magnet is a permanent magnet movable between a proximal position and a distal position in respect of its thermally conductive window.

7. The magnetocaloric apparatus according to claim 1, further comprising a second activatable magnet, placed at the other of the thermally conductive windows in respect of the thermally conductive window of the first activatable magnet, such that the produced magnetic field is aligned substantially parallel to the temperature gradient from heat source to heat sink.

8. The magnetocaloric apparatus according to claim 7, wherein the second activatable magnet is an electromagnet.

9. The magnetocaloric apparatus according to claim 7, wherein the second activatable magnet is a permanent magnet movable between a proximal position and a distal position in respect of its thermally conductive window.

10. The magnetocaloric apparatus according to claim 1, wherein the insulator cage is tubular.

11. The magnetocaloric apparatus according to claim 1, wherein the insulator cage is made of a polymer, a ceramic or another material that limits thermal contact between the two windows of the thermal switch.

12. The magnetocaloric apparatus according to claim 1, wherein the thermally conductive windows are made of a thermally conductive or thermally semi-conductive material, metal, alloy, ceramic or composite.

13. The magnetocaloric apparatus according to claim 1, further comprising a plurality of the externally activatable thermal switches, wherein said switches are connected in series, parallel, or in combinations thereof.

14. The magnetocaloric apparatus according to claim 1, wherein there are one or more of said externally activatable thermal switches, and wherein said one or more of the externally activatable thermal switches is configured for thermal energy storage, for refrigeration, for heating, or for combinations thereof.

15. The magnetocaloric apparatus of claim 1, further comprising two magnetocaloric material layers, wherein the externally activatable thermal switch is a layer between the two magnetocaloric material layers.

16. The magnetocaloric apparatus of claim 1, further comprising a plurality of the externally activatable thermal switches and a plurality of magnetocaloric material layers arranged in alternating layers.

17. A method for operating a magnetocaloric apparatus of the type comprising an externally activatable thermal switch for transferring heat from a heat source to a heat sink, wherein the switch comprises: an insulator cage having thermally conductive windows having a contact to the heat source and a contact to the heat sink; a magnetic nanofluid within said insulator cage; and a first activatable magnet placed at either one of the thermally conductive windows, the method comprising the steps of: activating the first activatable magnet, such that the magnetic nanofluid flows to establish a thermal bridge between heat source and heat sink, when the switch is activated; deactivating the first activatable magnet, such that the magnetic nanofluid flows to disrupt a thermal bridge between heat source and heat sink, when the switch is deactivated.

18. The method for operating the externally activatable thermal switch according to claim 17, comprising the step of alternating between the following two states when the switch is activated: activating the first activatable magnet, such that the magnetic nanofluid flows to establish a thermal contact with the thermal source and not with the thermal sink; and deactivating the first activatable magnet such that the magnetic nanofluid flows to establish a thermal contact with the thermal sink and not with the thermal source.

19. The method according to claim 17, wherein a frequency of the alternating between the two states is between 5 and 30 Hz.

20. The method according to claim 17, wherein a predefined level of electric current, electric field, pressure or light is used to trigger the externally activatable thermal switch.

21. The method of claim 17, further comprising the step of providing an electronic circuit or electronic controller which includes the magnetocaloric apparatus.

22. A non-transitory storage media including computer program instructions for implementing a magnetic thermal apparatus, the program instructions including instructions executable to carry out the method of claim 17.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0074] The following figures provide preferred embodiments for illustrating the description and should not be seen as limiting the scope of invention.

[0075] FIG. 1: Schematic representation of a liquid wettability tuning through the application of an electric voltage.

[0076] FIG. 2: Schematic representation of contact control between two surfaces through the tuning of a drop wettability.

[0077] FIG. 3: Schematic representation of the front view of an embodiment of the externally activated thermal switch and respective illustration the working method using a fluid of flexible material or metallic fillings as thermal bridge or thermal carrier;

[0078] FIG. 4: Schematic representation of 3D representation of an embodiment of the externally activated thermal switch structure;

[0079] FIG. 5: Schematic representation of front view of an embodiment of the externally activated thermal switch and respective illustration of the working method using a solid as thermal bridge or thermal carrier;

[0080] FIG. 6: Schematic representation of an embodiment of the apparatus used to prove the externally activated thermal switch concept.

[0081] FIG. 7: Schematic representation of an embodiment of the apparatus having a cascade of magneto caloric material and a thermal switch.

DETAILED DESCRIPTION

[0082] With reference to the drawings and more specifically FIG. 5, the externally activated thermal switch (EATS) is represented and associated to the heat sink 51 and heat source 35. The EATS structure is comprised by a 333 cm (FIG. 6) Poly(methyl methacrylate) (PMMA) thermal insulator cage 53, with a transversal cavity with 1.5 cm diameter and 3 cm of height 61. In the top and bottom two sheets of copper 52 are fixed to the PMMA body using a polyurethane (PU) based glue.

[0083] The interior of the PMMA thermal insulator, contains magnetic nanofluid (MNF) 54. The MNF used is a colloidal mixtures of 4.35 vol. % of 10 nm Fe.sub.3O.sub.4 nanoparticles dispersed in poly--olefin oil.

[0084] The present device uses a permanent magnet 61 to apply a magnetic field to the MNF 65, as depicted in FIG. 6. In this way the magnetic field is aligned parallel to the temperature gradient. This mechanism allows the enhancement of the MNF thermal conductivity up to 300% when compared to the systems where the magnetic field is applied perpendicular to the temperature gradient, as demonstrated in [7,8].

[0085] Adjusting the quantity of the MNF inside the insulator cage and the magnetic field, it is possible to have two different mechanisms of thermal transport: [0086] 1The fluid is pulled by the magnetic field to form a bridge between the heat source and the heat sink (FIG. 1). [0087] 2The fluid is pulled from the heat source and migrates to the heat sink interrupting the previous contact with the heat source (FIG. 5).

[0088] These two mechanisms are following described for further understanding. [0089] 1When an external magnetic field is applied using a permanent magnet 61 the MNF is attracted to the top in the direction of the permanent magnet. Therefore, the MNF establishes a thermal bridge between the heat sink 51 and the heat source 35. The nanoparticles contained in the MNF are forced to travel inside the nanofluid, transporting heat through the fluid and injecting it into the top copper sheet 52 that then conducts to the heat sink. After losing heat to the heat sink, the magnetic field is removed or reversed and the already cooled MNF travels down in the direction of the heat source, reinitiating the process. [0090] 2If the magnetic field felt by the MNF is sufficiently strong, it will be totally pulled from the bottom (heat source) to the top (heat sink). In this case, we have no longer the formation of a thermal bridge and the system will behave as a ferrofluid thermal contact switch.

[0091] To assess the performance of the present invention the apparatus schematized in FIG. 6 was put into place. Here the EATS 64 containing the MNF 65 was attached to a Peltier element 67 (heat source). In each side of the EATS and in contact to the copper sheets 62 a thermocouple was fixed 63 and 66, in order to record the changes in temperature. Using a permanent magnet 61, the magnetic field was alternately applied and removed. It was found that our device, using the ferrofluid thermal contact switch mechanism, can operate between 5 and 20 Hz, reducing the span temperature between the hot and cold sides in .sup.70%.

[0092] By decreasing the thermal switch thickness to 1 cm, it was possible to increase the device operational frequency, with no loss of efficiency. Therefore, it was assessed a reduction in the temperature span of .sup.80% for frequencies of 5 Hz and .sup.70% for frequencies between 10-30 Hz.

[0093] Using the apparatus described above (FIG. 6), we tested the substitution of the MNF for an iron disk with 1.5 cm of diameter and 1 mm of thickness. The results showed a reduction (in comparison with the MNF case) in the temperature span between the two sides (hot and cold) in 56%. This experiment attests the versatility of the presented thermal switch and the capability to work with other filling materials, rather than MNF, allowing it to be tailored for a specific application.

[0094] FIG. 7 illustrates a representation of of an embodiment of the apparatus having a cascade of magneto caloric material and a thermal switch, where the following are represented: 71Heat sink; 72Magneto caloric material (MCM); 73Thermal switch; 74Heat source.

[0095] While specific embodiments of this invention have been shown and described, it should be understood that many variations thereof are possible. The description of the present invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form released. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.

[0096] The term comprising whenever used in this document is intended to indicate the presence of stated features, integers, steps, components, but not to preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0097] It will be appreciated by those of ordinary skill in the art that unless otherwise indicated herein, the particular sequence of steps described is illustrative only and can be varied without departing from the disclosure. Thus, unless otherwise stated the steps described are so unordered meaning that, when possible, the steps can be performed in any convenient or desirable order.

[0098] It is to be appreciated that certain embodiments of the invention as described herein may be incorporated as code (e.g., a software algorithm or program) residing in firmware and/or on computer useable medium having control logic for enabling execution on a computer system having a computer processor, such as any of the servers described herein. Such a computer system typically includes memory storage configured to provide output from execution of the code which configures a processor in accordance with the execution. The code can be arranged as firmware or software, and can be organized as a set of modules, including the various modules and algorithms described herein, such as discrete code modules, function calls, procedure calls or objects in an object-oriented programming environment. If implemented using modules, the code can comprise a single module or a plurality of modules that operate in cooperation with one another to configure the machine in which it is executed to perform the associated functions, as described herein.

[0099] The disclosure should not be seen in any way restricted to the embodiments described and a person with ordinary skill in the art will foresee many possibilities to modifications thereof.

[0100] The above described embodiments are combinable.

[0101] The following claims further set out particular embodiments of the disclosure.