WO2019121343A1

WO2019121343A1 - Magnetocaloric stack and magnetocaloric device

Info

Publication number: WO2019121343A1
Application number: PCT/EP2018/084830
Authority: WO
Inventors: Gerhard Martinek; Florian SCHARF; Ralf Böhling; Ulrich Kalck
Original assignee: Basf Se
Priority date: 2017-12-18
Filing date: 2018-12-13
Publication date: 2019-06-27

Abstract

Described is a magnetocaloric stack (200) with at least two magnetocaloric layers (211, 212, 213) comprising magnetocaloric material, wherein at least one stabilization layer (221, 222) comprising soft magnetic material is sandwiched between the at least two magnetocaloric layers (211, 212, 213).

Description

Magnetocaloric stack and magnetocaloric device

The invention relates to a magnetocaloric stack and its use in magnetocaloric devices as a magnetocaloric regenerator, a magnetocaloric heat pump, a cooling device and a magnetocaloric power generator.

Magnetocaloric materials change their temperature upon an application and removal of an external magnetic field. This behavior is the basis for development of magnetic cooling systems. Furthermore, the magnetocaloric effect can be used for power generation.

In the design of magnetocaloric devices, challenging and in some cases even opposing requirements have to be considered, inter alia maximum magnetocaloric density for the sake of efficient use of the magnetic field volume, as well as mechanical stability against the stress exerted from the cycle of introduction into and removal from the magnetic field. For heating and cooling devices additionally a maximum interface area between solid (magnetocaloric material) and fluid (heat transfer fluid) phase in order to enhance the heat transfer as well as a minimum resistance against the flow of the heat transfer fluid for reducing the pressure drop and an enhanced stability against stress exerted from the flow of the heat transfer fluid are required.

These problems are solved by a magnetocaloric stack and devices based on the magnetocaloric stack according to the invention. In a first aspect the invention provides a magnetocaloric stack. A second aspect is formed by a magnetocaloric device as a magnetocaloric regenerator; a magnetocaloric heat pump, a cooling device or a magnetocaloric power generator. According to a first aspect the invention relates to a magnetocaloric stack with at least two magnetocaloric layers comprising magnetocaloric material, wherein at least one stabilization layer comprising soft magnetic material is sandwiched between the at least two magnetocaloric layers. The magnetocaloric stack of the invention is based on the recognition that by using a sandwiched, at least mainly metallic stabilization layer, in particular a layer comprising soft magnetic material in-between two layers of magnetocaloric material, the mechanical stability of the magnetocaloric stack is enhanced due to the stability of the soft magnetic material. By using soft magnetic material in the stabilization layer the efficiency of the magnetocaloric stack is only slightly or not affected compared to a magnetocaloric element without stabilization layer. Thus with the stabilization layer according to the invention a stable and efficient stack is achieved. Furthermore production, forming and processing of soft magnetic material are well known technologies and the implementation of further structuring, like building of channels or the like, can be easily realized in a stabili- zation layer comprising soft magnetic material. The invention thus encloses the recognition that necessary structuring within a magnetocaloric element can be facilitated, when stabilizing or guiding structures can be implemented in a stabilization layer comprising soft magnetic material instead of implementing them within the magnetocaloric layers. Magnetocaloric materials are usually brittle and thus their processability poses restrictions to forming processes.

Soft magnetic materials are materials that magnetize to saturation and experience a reversal in polarity in relatively weak magnetic fields, this means soft magnetic materials are easy to magnetize and demagnetize, showing a small hysteresis area. Soft-magnetic materials are characterized by high values of magnetic permeability and low coercivity. According to the present invention, the at least one stabilization layer comprising the soft magnetic material is sandwiched between the at least two magnetocaloric layers. This means that the stabilization layer comprising soft magnetic material is arranged between the two magnetocaloric layers, being directly adjacent to both of them, without any further layer inbetween. In the following, embodiments of the magnetocaloric stack according to the first aspect of the invention are described. The at least one stabilization layer is advantageously in direct mechanical contact to the at least two magnetocaloric layers. This enhances heat transfer through the stabilization layer and improves overall efficiency of the magnetocaloric stack.

Preferably the soft magnetic material has a saturation polarization of at least 0.5 T, pref- erably higher than 1.4 T, preferably higher than 2 T. With such a high saturation polarization, losses in the performance of the magnetocaloric stack compared to a magnetocaloric element without a stabilization layer comprising a soft magnetic material are minimized. The soft magnetic material can either be a ferrous or a non-ferrous metal. The term ferrous metal includes pure iron as well as alloys containing iron, for example steel. Preferred ferrous metal are for example Fe, FeCo or steel, in particular 1.1 141 (CK15), 1.0038, 1 .0403, 1.0737 or martensitic chrome steel as for example 1 .4021 or 1.4104 . FeCo shows Js = 2.4T, whereas pure iron has saturation polarization of Js = 2.2T and the martensitic chrome steel 1.4021 or 1.4104 show a saturation polarization of Js = 1.7 respectively of Js = 1.6T. The invention further encloses the recognition that the higher the saturation polarization of a soft magnetic material is, the higher an open porosity of the stabilization layer may be without negatively affecting the achievable field strength. This means the higher the saturation polarization of a soft magnetic material is, the higher the open volume, or the open porosity, of the stabilization layer may be, through which the fluid can flow. The relation between an advantageous highest effective porosity e in a sectional plane of the stabilization layer perpendicular to the stacking direction of the magnetocaloric stackand the targeted magnetic flux density B within in the magnetocaloric stack with a given saturation polarization Js of the soft magnetic material complies with J_s (1— e) ³ B .

This means the highest advantageous effective porosity achieved in a sectional plane perpendicular to the stacking direction of the magnetocaloric stack with the smallest cross section of the stabilization layer is given as s = ₁1 - B and thus depends on targeted

magnetic flux density and saturation polarization of the soft magnetic material.

In a preferred embodiment at least one fluid channel is arranged in the stabilization layer. This allows transport of a heat transfer fluid through the magnetocaloric stack. The im- plementation of fluid channels in the stabilization layer is further advantageous because usually the soft magnetic material has an enhanced stability against stresses caused by the heat transfer fluid than most magnetocaloric material. Furthermore the implementation of the fluid channels is simplified compared to an implementation within the magneto- caloric layers. This embodiment is based on the further recognition that implementing channels within a stabilization layer allows fluid transport in a magnetocaloric stack without affecting the volume of the magnetocaloric material, to which the magnetic field is applied. As the channels are implementd in the stabilization layer, the magnetocaloric layers are unaffected, in contrast to an implementation of channels in the magnetocaloric layers, which would reduce the volume to which the magnetic field is applied. This results in a more effective use of the magnetic field.

Nevertheless also embodiments in which parts of the heat transfer fluid or the whole heat transfer fluid are guided through the magnetocaloric layers are under the scope of the invention.

The magnetocaloric stack may contain a plurality of stabilization layers comprising soft magnetic material and a plurality of magnetocaloric layers, wherein the stabilization layers and the magnetocaloric layers are arranged in alternating order and wherein an uppermost layer and a bottommost layer of the magnetocaloric stack are magnetocaloric lay- ers. Preferably the magnetocaloric stack comprises between 3 and several hundreds of layers, depending on the specific application, for which the stack is designed and on reasonable thicknesses of the individual layers for manufacturing. With an increased number of layers, an enhanced stability of the stack, and in the case of fluid channels within the stabilization layers, an enhanced heat transfer is possible while maintaining an overall thickness of the stack. This is achievable with thinner individual magnetocaloric layers stabilized by the respective stabilization layer.

The soft magnetic material forms a bulk structure or alternatively a porous structure. Bulk structures are advantageous in view of stability, whereas porous structures allow fluid transport through the soft magnetic material and thus through the stabilization layer. In some embodiments the soft magnetic material forms packed bead, a metallic webbing or a metal foam. These embodiments allow fluid transport across the stabilization layer.

In further embodiments the stabilization layer comprises pillars of the soft magnetic material. The pillars may have a round, oval, rectangular or hexagonal cross section for example and may border on the at least one fluid channel or a fluid channel system. Pillars are advantageous as they provide a reduced conducting cross section compared to a bulk metal sheet and thus reduce a heat flow through the stabilization layer perpendicular to the stacking direction. This results in supporting a heat flow through the pillars from one magnetocaloric layer to the other or from the pillars to a surrounding fluid. In other embodiments the stabilization layer comprises elongated ridges of the soft magnetic material. Elongated ridges are advantageous as wall structures for fluid channels for guiding a heat transfer fluid through the at least one stabilization layer.

A channel cross section of the at least one fluid channel is in some embodiments diamond-shaped, circular or rectangular. The width of the channel depends on the applica- tion cases for the stack. For some embodiments, a channel width below 25 pm is advantageous, while in other embodiments the channel width can be larger than 100 pm.

The fluid channel may be fully embedded in the soft magnetic material of the stabilization layer, in other words side walls, a bottom wall and a top wall, thus all walls, of the at least one fluid channel comprise soft magnetic material. This embodiment is advantageous as it prevents direct contact between a heat transfer fluid within the channel and the magnetocaloric material. Thus possible undesired chemical reactions between the magnetocaloric material and the fluid are prevented. Alternatively the at least one fluid channel has side walls which comprise the soft magnetic material, whereas at least one of a bottom and a top wall of the fluid channel are formed of one of the at least two magnetocaloric layers. These embodiments allow direct contact between fluid and magnetocaloric material and thus may enhance heat transfer from the magnetocaloric material to the fluid. In this context bottom wall and top wall mean the walls of the fluid channel which are facing the two magnetocaloric layers between which the stabilization layer is sandwiched, whereas the side walls face either further channels or other parts of the respective stabili- zation layer.

Preferably insulating structures are arranged in the stabilization layer between structures built of the soft magnetic material such that a heat flow through the stabilization layer in a direction perpendicular to a stacking direction of the stack interrupted or reduced. The insulating structures suppress a heat flow through the stabilization layer perpendicular to the stacking direction and thus support heat transfer from the soft magnetic material into the fluid in the channel in embodiments having at least one fluid channel or from the soft magnetic material into the adjacent magnetocaloric material. The insulating structures are either recesses in elongated structures of the ferrous material or structures built of an insulation material like for example a polymer. The soft magnetic material forms in some embodiments a deposit on one of the at least two magnetocaloric layers. Thus the stabilization layer can be built by depositing the soft magnetic material in predefined structures like for example pillars and subsequently the other of the two magnetocaloric layers between which the stabilization layer is sandwiched can be stacked on top of this layer compound. Alternatively the stabilization layer is built of a pre-structured foil of soft magnetic material. This allows structuring of the stabilization layer in advance to a stacking with the magnetocaloric material and thus either deposition of the magnetocaloric material on the foil or a simple stacking of layers without the need of post-processing of the soft magnetic materi- al when stacked on at least one of the magnetocaloric layers.

In some embodiments in at least one of the magnetocaloric layers the magnetocaloric material is a bulk material, for example arranged in form of bulk sheet. This is especially advantageous for embodiments in which fluid channels are provided within the stabilization or in which no fluid channel are needed. In some embodiments the magnetocaloric material is provided in the form of a composite bulk sheet comprising the magnetocaloric material bound with polymer to bulk sheet. Alternatively the magnetocaloric material in at least one of the magnetocaloric layers is a porous material. In these embodiments a large interface area between magnetocaloric material and fluid is provided and thus provides an advantageous heat transfer from the magnetocaloric material to the fluid. For example at least one of the magnetocaloric layers is a packed bed of particles of the magnetocaloric material. The magnetocaloric material is arranged in form of fibres in at least one of the magnetocaloric layers in other embodiments.

Advantageously at least one of the magnetocaloric layers comprises at least two magnetocaloric materials with at least two different compositions. In some embodiments at least one of the magnetocaloric layers comprises a magnetocaloric cascade of at least three different magnetocaloric materials with different Curie temperatures, which are arranged alongside each other in succession by ascending or descending Curie temperature in a direction perpendicular to a stacking direction. The cooling or heating effect of a device based on the magnetocaloric stack can be increased by designing the magnetocaloric material as a sequence of elements with decreasing or ascending Curie temperatures, or, in other words, as a magnetocaloric cascade containing two or more magnetocaloric material layers in succession by descending Curie temperature. In such a magnetocaloric cascade during cooling, the first magnetocaloric material cools down the second magnetocaloric material to a temperature near the Curie temperature of the second magnetoca- loric material, and so on with any further magnetocaloric material contained in the cascade. This way, the cooling or heating effect achieved can be greatly increased in comparison with the use of a single magnetocaloric material.

According to a second aspect the invention relates to a magnetocaloric device comprising a magnetocaloric stack according to the first aspect of the invention or one of its embodi- merits, wherein the magnetocaloric device is one of a magnetocaloric regenerator, a magnetocaloric heat pump, a cooling device or a magnetocaloric power generator.

The magnetocaloric device according to the second aspect of the invention shares the advantages described in the context of the magnetocaloric stack according to the first aspect.

It shall be understood that the magnetocaloric stack of the first aspect of the invention, also defined in claim 1 and the magnetocaloric device of the second aspect, also defined in claim 14, especially the magnetocaloric regenerator, the magnetocaloric heat pump, the cooling device and the magnetocaloric power generator have similar or identical embodiments.

Further embodiments will be described below with reference to the enclosed drawings.

In the drawings:

Fig. 1 shows an embodiment of a magnetocaloric device according to the second aspect of the invention, Fig. 2 shows an embodiment of a magnetocaloric stack according to the first aspect of the invention,

Figs. 3a, b show another embodiment of a magnetocaloric stack according to the first aspect of the invention,

Fig. 4 shows a further embodiment of a magnetocaloric stack according to the first aspect of the invention,

Fig. 5 shows a cross-sectional view of an of a magnetocaloric stack according to the first aspect of the invention,

Figs. 6a, b show cross-sectional views of further embodiments of a magnetocaloric stack according to the first aspect of the invention. In the figures, different embodiments are shown. Identical elements or elements with substantially identical functions are indicated with the same reference signs in the drawings.

Fig. 1 is an illustrations of an embodiment of a magnetocaloric device 1000 according to the second aspect of the invention. The magnetocaloric device 1000 in this embodiment is a magnetocaloric heat pump comprising a magnet assembly 1001 for applying an external magnetic field to a magnetocaloric stack 100, which is arranged within the magnet assembly 1001 . The magnetocaloric stack 100 comprises two magnetocaloric layers 1 1 1 , 1 12 comprising magnetocaloric material and a stabilization layer 120, which is sandwiched between the two magnetocaloric layers 1 1 1 , 1 12. The stabilization layer 120 comprises soft magnetic material. The soft magnetic material forms in the shown embodiment a bulk structure, in particular a bulk sheet, which is especially advantageous in providing stability to the magnetocaloric stack. The magnetocaloric layers 1 1 1 , 1 12 in this embodiment are packed beds of particles of magnetocaloric material, so that a heat fluid 130 can pass through the magnetocaloric stack 100 through the porous magnetocaloric layers 1 1 1 , 1 12 and thus transport heat to further elements.

Fig. 2 shows an embodiment of a magnetocaloric stack 200 according to the first aspect of the invention. The magnetocaloric stack 200 comprises three magnetocaloric layers 21 1 , 212, 213 of magnetocaloric material with two stabilization layers 221 , 222, wherein each of the stabilization layers is arranged and sandwiched between two respective magnetocaloric layers. The stabilization layers 221 , 222 are in direct mechanical contact to the respective magnetocaloric layers 21 1 , 212, 213. In the shown embodiment the stabilization layers 221 , 222 comprise porous ferrous material. Thus fluid can pass through the stabilization layers 221 , 222 whereas the magnetocaloric layers 21 1 , 212 and 213 are in the form of bulk sheets of magnetocaloric material. The overall effective porosity of the e of each stabilization layer 221 , 222 complies to the formula J_s (1— e) ³ B depending on a targeted magnetic flux density B within the magnetocaloric stack 200 with a given saturation polarization Js of the soft magnetic material. The embodiments of a magnetocaloric stack according to a first aspect of the invention shown in the further figures differ from the embodiment of Fig. 2 mainly in the embodiments of the respective stabilization layers. Thus only the differences with respect to the embodiment of Fig. 2 will be described.

Figs. 3a, b show another embodiment of a magnetocaloric stack 300 according to the first aspect of the invention, wherein Fig. 3b shows a cross-sectional view of parts of the magnetocaloric stack 300 along sectional plane A-A in Fig. 3a. The two stabilization layers 321 , 322 of the magnetocaloric stack 300 comprise ferrous material in form of elongated ridges 340, which edges fluid channels 350. The side walls of the fluid channels 350 are built of the elongated ridges 340 of ferrous material, whereas bottom wall 351 and top wall 352 of each fluid channel are formed by the respective adjacent magnetocaloric layers 21 1 , 212, 213. Thus a fluid passing through the fluid channels 350 is in direct contact to the magnetocaloric material of the respective magnetocaloric layers 21 1 , 212, 213. The cross sections of the fluid channels 350 in this embodiment is rectangular. In an alternative embodiment, which is not shown in the drawings, the bottom wall can additionally be formed by soft magnetic material. This embodiment can be easily realized by using a pre-structured foil with pre-structured elongated ridges for example.

Fig. 4 shows a further embodiment of a magnetocaloric stack 400 according to the first aspect of the invention. In this embodiment the fluid channels 450 in the stabilization layers 421 , 422 are fully embedded in the soft magnetic material of the stabilization layers 421 , 422 and have a round cross section. In this embodiment a direct contact of fluid in the channels 450 with the magnetocaloric material of the magnetocaloric layers 21 1 , 212, 213 can be prevented.

Fig. 5 shows a cross-sectional view of a magnetocaloric stack 500 according to the first aspect of the invention. The sectional plane corresponds to sectional plane A-A in Fig. 3a for the shown embodiment. This means the sectional plane is located in a stabilization layer 520 of the magnetocaloric stack 500. In the shown embodiment the stabilization layer 520 comprises pillars 540 of ferrous material, wherein the pillars have an oval cross- section. Other possible cross section geometries of the pillars are for example rectangular or round. The pillars 540 border a fluid channel system for a fluid 530. The fluid 530 is in direct contact to a magnetocaloric layer 212 which forms the bottom of the fluid channel system.

Figs. 6a, b show cross-sectional views of further embodiments of a magnetocaloric stack 601 , 602 according to the first aspect of the invention. The sectional plane corresponds to sectional plane A-A in Fig. 3a for the respective embodiment. In the stabilization layers 620, 620a of the magnetocaloric stacks 601 , 602 insulating structures are arranged between structures 640 built of the soft magnetic material so that a heat flow through the stabilization layer 620 in a direction 660 perpendicular to a stacking direction of the stack is interrupted. The insulating structures suppress a heat flow through the stabilization layer perpendicular to the stacking direction and thus support heat transfer from the soft magnetic material structures 640 into the fluid in the channels 650. The insulating struc- tures 671 of stabilization layer 620 are built of an insulation material, whereas the insulation structures 671a of stabilization layer 620 a partly disruptions in the elongated structures 640 reducing the conducting cross section.

Claims

1. A magnetocaloric stack with at least two magnetocaloric layers comprising magnetocaloric material, wherein at least one stabilization layer comprising soft magnetic material is sandwiched between the at least two magnetocaloric layers.

2. The magnetocaloric stack according to claim 1 , wherein the at least one stabilization layer is in direct mechanical contact to the at least two magnetocaloric layers.

3. The magnetocaloric stack according to at least one of the previous claims, wherein the soft magnetic material has a saturation polarization of at least 0.5 T, preferably at least 1.4 T, preferably higher than 2 T.

4. The magnetocaloric stack according to at least one of the previous claims, wherein the relation between an overall effective porosity e of the stabilization layer and the targeted magnetic flux density B with a given saturation polarization Js of the soft magnetic material complies J_s · (1— e) > B .

5. The magnetocaloric stack according to at least one of the previous claims, wherein at least one fluid channel is arranged in the stabilization layer.

6. The magnetocaloric stack according to at least one of the previous claims with a plurality of stabilization layers comprising soft magnetic material and a plurality of magnetocaloric layers, wherein the stabilization layers and the magnetocaloric layers are arranged in alternating order and wherein an uppermost layer and a bottommost layer of the magnetocaloric stack are magnetocaloric layers.

7. The magnetocaloric stack according to at least one of the previous claims, wherein the stabilization layer comprises pillars of the soft magnetic material or elongated ridges of the soft magnetic material.

8. The magnetocaloric stack according to at least one of the previous claims, wherein insulating structures are arranged in the stabilization layer between structures built of the soft magnetic material such that a heat flow through the stabilization layer in a direction perpendicular to a stacking direction of the stack is interrupted or reduced.

9. The magnetocaloric stack according to at least one of the claims 5 to 8, wherein the at least one fluid channel has side walls which comprise soft magnetic material, whereas at least one of a bottom and a top wall of the fluid channel are formed of one of the at least two magnetocaloric layers.

10. The magnetocaloric stack according to at least one of the claims 5 to 8, wherein side walls, bottom wall and top wall of the at least one fluid channel comprise soft magnetic material.

11. The magnetocaloric stack according to at least one of the previous claims, wherein at least one of the magnetocaloric layers comprises at least two magnetocaloric materials with at least two different compositions.

12. The magnetocaloric stack according to at least one of the previous claims, wherein at least one of the magnetocaloric layers comprises a magnetocaloric cascade of at least three different magnetocaloric materials with different Curie temperatures, which are arranged in succession by ascending or descending Curie temperature.

13. The magnetocaloric stack according to at least one of the previous claims, wherein at least one of the magnetocaloric layers comprises a magnetocaloric material with different composition than the composition of a magnetocaloric material of at least one other of the magnetocaloric layers.

14. A magnetocaloric device comprising a magnetocaloric stack according to at least one of the claims 1 to 13, wherein the magnetocaloric device is one of a magnetocaloric regenerator, a magnetocaloric heat pump, a cooling device or a magnetocaloric power generator.