SMA stack structure for improved heat transfer and structural stability

Abstract

The present invention relates to a hyperelastic SMA structure with enhanced power density and compressive stability, comprising two or more plate sections connected together to form a substantially closed periphery, the structure having a centrally located opening, the connected plate sections dimensioned to be circularly symmetrical thereby allowing stacking, each section comprising a number of hollow perforated cells between one or more thin vertical walls having a predetermined range of thicknesses, at least one perforated cell defining a fluid passageway within a predetermined range of hydraulic diameters.

Description

[Technical Field]

[0001] The present disclosure relates to shape memory alloy (SMA) stack or plate structures. In particular, the present disclosure relates to superelastic SMA structures with enhanced power density and improved compressive stability. [Background technology]

[0002] Recent research into the elastocaloric (EC) effect has demonstrated the potential of solid-state technologies as an alternative to traditional vapor compression refrigeration and heat pumping. In the EC cycle, the superelastic behavior of shape memory alloys is exploited, and cyclic uniaxial loading and unloading facilitates the absorption of heat from a cold source and its release to a hotter side.

[0003] In recent years, there has been growing interest in using SMA materials to construct energy recovery devices and heat pump / refrigeration systems. One example of the use of SMA plate materials is a heat pump device that includes at least one stack of plates, at least two of which are formed or constructed from SMA material, and the SMA plates have one or more fluid ports that allow a working fluid to pass through the stack. The use of such an SMA stack is disclosed in International Patent Publication No. 2021 / 219667, assigned to Exergyn Ltd.

[0004] Patent document 1 describes a known, generally rectangular SMA plate structure (see Figure 1) with a number of rectangular openings through which the working fluid can flow. Heat transfer from the SMA to the working fluid and vice versa is important for the thermal performance of the stack. The thinner the walls, the faster the heat transfer, but the less stable the stack will be under compressive loads.

[0005] U.S. Patent Application Publication No. 2016 / 0084544 discloses a heating / cooling system with thermoelastic modules. Each module includes one or more structures formed from a shape memory alloy that transforms from austenite to martensite upon application of a first stress, releasing the latent heat associated with the transformation. The system includes SMA tubes. However, the tubes have poor durability, preventing commercial use due to their low buckling strength and the manufacturing costs associated with conventional heating and cooling devices. The tubes are not thermally efficient and do not expand or contract uniformly. Furthermore, the tubes suffer from buckling problems during use.

[0006] Therefore, there is a need for a more stable SMA stack structure that provides improved heat transfer rates to and from the SMA material (and therefore power density) while maintaining structural strength. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2021 / 219667 [Patent Document 2] US Patent Application Publication No. 2016 / 0084544 Summary of the Invention

[0008] The present invention relates to a structure made from SMA material, as claimed in the accompanying claims. The SMA structure is particularly suitable for use in heat pump / refrigeration systems.

[0009] In one embodiment, a superelastic SMA structure with improved power density and compressive stability is provided, having a generally hexagonal or circular shape to define a multitude of hollow perforated cells between one or more thin vertical walls, each perforated cell defining a liquid or gas passageway, where fluid refers to a working heat transfer fluid that can be in gaseous or liquid form.

[0010] In one embodiment, a superelastic SMA structure with improved power density and compressive stability is provided, the structure having a generally hexagonal or circular shape thereby defining a multitude of hollow perforated cells between one or more thin vertical walls having a predetermined range of thicknesses, at least one of the perforated cells defining a fluid passageway within a predetermined range of hydraulic diameters.

[0011] In one aspect of the invention, a hyperelastic SMA structure is provided, comprising two or more plate sections connected together to form a substantially closed periphery having a centrally located opening, the connected plate sections dimensioned to be circularly symmetric and thereby stackable, each section comprising a number of hollow perforated cells between one or more thin vertical walls having a predetermined range of thicknesses, at least one perforated cell defining a fluid passageway within a predetermined range of hydraulic diameters.

[0012] In one embodiment of the present invention, each hollow perforated cell has a generally hexagonal honeycomb or circular shape. Furthermore, the central opening in the structure provides a higher moment of inertia, which provides stability against buckling and allows for longer stack lengths with less support structure. There is a numerical relationship between stack diameter, moment of inertia, stack height or length, and power density. Furthermore, structures with central openings have shown improved heat flux (W / m²) or heat transfer coefficient (W / m² / K).

[0013] In one embodiment, the thickness of one or more vertical walls is selected from the range of 0.75 mm to 1.25 mm.

[0014] In one embodiment, the vertical wall thickness is 1 mm. It has been found that when the distance from the midpoint of the SMA material to the fluid wall is reduced to approximately 0.5 mm, there is an exponential improvement in heat transfer, and therefore power density.

[0015] In one embodiment, the effective hydraulic diameter of the fluid passageway for one or more cells is selected from the range of 2 mm to 3 mm.

[0016] In one embodiment, the effective hydraulic diameter of the fluid passage is 2.4 mm.

[0017] In one embodiment, the structure includes a large opening located near the center of the structure.

[0018] In one embodiment, the opening is sized to accommodate the support element.

[0019] In one embodiment, the support element comprises a central pillar and is dimensioned to support a plurality of superelastic SMA structures.

[0020] In one embodiment, the structure comprises two or more plate sections connected together to form a substantially closed periphery, each section having a generally hexagonal or circular shape thereby defining a number of hollow perforated cells between one or more thin vertical walls, each perforated cell defining a fluid passageway.

[0021] In one embodiment, the number of connected sections is three or six.

[0022] In one embodiment, the connected sections are dimensioned to have circular symmetry, thereby allowing them to be stacked.

[0023] In one embodiment, the connected sections are sized during fabrication from a large sheet to form a fully or partially nested pattern.

[0024] In one embodiment, each section is configured to interlock with adjacent sections to define a superelastic SMA structure.

[0025] In one embodiment, tension elements are located around the periphery of the structure.

[0026] In one embodiment, the tension element is located on the inner periphery of the structural opening.

[0027] In one embodiment, the tension elements are positioned around the perimeter of the structural opening, multiple tension elements are placed on the outside of the stack, or both, to reduce SMA loads during operation.

[0028] In one embodiment, one or more tensioning elements are located internally or externally to reduce the SMA load during operation.

[0029] In one embodiment, the tension element is insulated from or isolated relative to the SMA structure.

[0030] The invention will be more clearly understood from the following description of illustrative embodiments, taken in conjunction with the accompanying drawings, in which: FIG. [Brief explanation of the drawings]

[0031] [Figure 1] 1 illustrates a conventional SMA plate material that can be used in an elastocaloric device or system. [Figure 2] 10 shows a graph and table of power density of an SMA material versus the distance from its midpoint to the fluid channel. [Figure 3] FIG. 1 shows a plan view of a hexagonal superelastic SMA structure according to one embodiment of the present invention. [Figure 4] A plan view of a hexagonal SMA structure with a central opening is shown. [Figure 5] 1 shows a plan view of a hexagonal SMA structure with an opening in the center and tension pillars located around the inner periphery. [Figure 6] A plan view of a hexagonal SMA structure that can be separated into interlocking parts is shown. [Figure 7]1 shows a plan view of a hexagonal SMA structure that can be separated into interlocking parts and has multiple tension carrying elements. [Figure 8] 1 shows diagrams of several hexagonal shaped superelastic SMA structures with different shaped openings. [Figure 9A] 1 shows a hexagonal SMA structure with circular holes according to one embodiment of the present invention. [Figure 9B] 1 shows a hexagonal SMA structure with circular holes according to one embodiment of the present invention. [Figure 10] 1 shows a generally hexagonal SMA structure with circular holes according to one embodiment of the present invention. [Figure 11A] 1 shows a circular SMA structure with hexagonal holes according to one embodiment of the present invention. [Figure 11B] 1 shows a circular SMA structure with hexagonal holes according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0032] Patent document 1 (WO 2021 / 219667, assigned to Exazine Limited) discloses the use of SMA plate stacks. It is desirable for each plate to have a superelastic SMA structure with enhanced power density and compressive strength. For this to be practical, improvements to the plate design are required. Figure 2 shows the distance from the midpoint of the SMA to the fluid versus power density. The present invention can be used to construct a core with a single stack of SMA material or multiple stacks of SMA material mechanically mounted in axial series. The SMA structure described herein is suitable for use in thermodynamic cycles for heating and cooling systems containing SMA material.

[0033] A preferred embodiment of the present invention provides a superelastic SMA structure with enhanced power density and compressive stability. Figure 3 shows an SMA structure 10 with a generally hexagonal honeycomb structure, defining a number of hollow perforated cells 11 formed between one or more thin vertical walls and open at both ends. Each perforated cell 11 defines a fluid passageway through which fluid can flow, thereby activating the SMA structure 10. The hexagonal honeycomb structure is intended to ensure that the distance from the midpoint to the fluid channel is approximately less than 1 mm. Therefore, the overall thickness of the vertical walls is ideally close to 1 mm. As shown in the table in Figure 2, it was found that when the distance from the SMA midpoint to the fluid wall is approximately 0.5 mm or less, significant improvements in heat transfer are observed, thereby improving power density.

[0034] FIG. 4 shows a plan view of a hexagonal superelastic SMA structure 10 with a central opening 12. This embodiment provides a large central open area 12, facilitating manifolding for fluid transport. This embodiment provides a larger moment of area, which provides greater stability against buckling and allows for longer stack lengths with less support structure. The superelastic SMA structure 10 includes two or more plate sections connected together to form a substantially closed perimeter. The structure includes a centrally located opening 12, and the connected plate sections are dimensioned to be circularly symmetrical, allowing stacking. Each section also includes a number of hollow perforated cells between one or more thin vertical walls having a predetermined range of thicknesses, with at least one perforated cell defining a fluid passageway within a predetermined range of hydraulic diameters.

[0035] FIG. 5 shows a top view of a hexagonal, hyperelastic SMA structure with a central opening and a centrally insulated tension pillar 13. In this embodiment, manifolding at each end of the SMA stack structure 10 can be implemented from the exterior and / or interior surfaces of the hexagonal, hyperelastic SMA structure, facilitating fluid flow management. The thick perimeter lines shown in FIGS. 5 and 6 indicate a rigid structure, a flexible structure, or both, ensuring thermal insulation 14, 15 of the tension pillar 13 from the SMA stack and ensuring that any working fluid within the SMA stack is contained only within the activated SMA stack area.

[0036] 6 shows a plan view of a hexagonal hyperelastic SMA structure made up of multiple elements or sections 17. In this embodiment, the hyperelastic SMA structure can be divided into three or six identical parts 17 designed with circular symmetry. When these identical parts 17 are designed to interlock like a "jigsaw puzzle," or interlock with one another, a self-interlocking, self-supporting core assembly is created, resulting in a more robust solution. The interlocking SMA elements allow parts to be cut in a nested pattern from a sheet of SMA material, resulting in cost efficiencies and reduced waste.

[0037] 7 shows a plan view of a hexagonal hyperelastic SMA structure 10 made up of multiple elements or sections 17. Multiple tension elements 18 can be placed on the inner periphery, outer periphery, or both of the SMA stack to reduce applied load forces. One inner element 13 and four outer elements 18 are shown.

[0038] 8 shows diagrams of several hexagonal-shaped superelastic SMA structures 10 with different shaped openings 19. The overall shape of the openings 19 can be related to performance. While hexagonal openings are shown in the previous figures, annular shapes offer advantages in structural stability at the expense of a slight sacrifice in surface area:mass ratio.

[0039] The SMA wall thickness in the honeycomb structure described above was chosen to be 1 mm overall, as it provides an optimal balance between heat transfer rate from the SMA and compressive stability, while still providing sufficient stability for practical stack heights. As a result, multiple SMA structures can be stacked on top of each other. An ideal / optimal range of 0.75 mm to 1.25 mm can be selected to implement superelastic SMA structures with enhanced power density and compressive stability. Note that a smaller value than this range, while maintaining stack stability, would require the use of smaller fluid channels, resulting in a significant pressure drop in the system and reduced efficiency due to pumping losses during operation.

[0040] The hydraulic diameter affects the heat transfer coefficient and pressure drop during operation. The hydraulic diameter varies depending on the internal passage geometry. The example shown in Figure 8 shows a nominal passage size of 2 mm, but each passage has a completely different hydraulic diameter. Considering the pressure drop in the passage, the hydraulic diameter of the fluid channel is preferably in the range of 2 mm to 3 mm. This provides the best balance of heat transfer from the SMA to the fluid, stack stability, and reduced pumping losses.

[0041] 9A and 9B show a hexagonal SMA structure 902 with circular holes 904 according to one embodiment of the present invention. In the hexagonal SMA structure 902, the fluid channel ID ranges from 2.4 mm to 2.5 mm, the hydraulic diameter (Dh) ranges from 2.4 mm to 2.5 mm, the SMA wall thickness ranges from 0.9 mm to 1 mm, the fluid to SMA area ratio ranges from 0.705 to 0.821, and the wetted perimeter ranges from 1176.2 mm to 1225.22 mm.

[0042] FIG. 10 shows a generally hexagonal SMA structure 1002 with circular holes 1004 according to one embodiment of the present invention. In the hexagonal SMA structure 1002, the fluid channel ID ranges from 2 mm to 2.4 mm, the hydraulic diameter (Dh) ranges from 2 mm to 2.4 mm, the SMA wall thickness is 1 mm, the fluid to SMA area ratio ranges from 0.545 to 0.667, and the wetted perimeter ranges from 1005.31 mm to 1147.7 mm. The SMA structure 1002 is composed of multiple SMA elements interconnected in a manner similar to the SMA structure described with reference to FIG. 6. However, other suitable dimensions may be used.

[0043] 11A and 11B show a circular SMA structure 1102 with hexagonal holes 1104 according to one embodiment of the present invention. In the SMA structure 1102, the fluid channel ID ranges from 2 mm to 2.4 mm, the hydraulic diameter (Dh) ranges from 2 mm to 2.425 mm, the SMA wall thickness is 1 mm, the fluid to SMA area ratio ranges from 0.698 to 0.797, and the wetted perimeter ranges from 1271.1 mm to 1327.4 mm. Other suitable dimensions may also be used.

[0044] In the context of the present invention, the following definitions shall apply throughout the specification: Segment - A section of a plate arranged so that two or more segments can be effectively cut, nested, or stacked together to form a planar plate. Plate - An individual SMA element with one or more fluid passages created to facilitate heat transfer by fluid flow in the plate. Stack - Multiple SMA plates consisting of at least two plates assembled together. Housing - A container for the plate stack, for insulation and fluid inflow / outflow. Core - consists of a single stack or multiple stacks mechanically mounted axially in series. Fluid - The heat transfer medium used to absorb heat into or transfer heat away from the SMA plates during operation. The fluid may be gas, liquid, or in a transitional state between solid-liquid or liquid-gas. Wall Thickness - the total distance from one fluid channel to the adjacent fluid channel perpendicular to the wall. This is actually twice the distance from the midpoint of the material to the fluid. Hydraulic diameter - Used for non-circular fluid channels, it is calculated by dividing four times the cross-sectional area of the channel by the wetted perimeter.

[0045] As used herein, the words "comprise, comprise, comprised, and comprising" or any variation thereof and "include, includes, included, and including" or any variation thereof are considered to be fully interchangeable and should be given the broadest possible interpretation, and vice versa.

[0046] The invention is not limited to the embodiments described above, which may be modified both in arrangement and detail. [Explanation of symbols]

[0047] 10 Superelastic SMA structure 11 Perforated Cell 12 Opening 13 Tension pillar / internal element 14. Insulation 15. Insulation 17 sections / parts 18 Tension elements / external elements 19 Opening 902 SMA construction 904 hole 1002 SMA structure 1004 hole 1102 SMA structure 1104 hole

Claims

1. 1. A hyperelastic SMA structure comprising two or more plate sections connected together to form a substantially closed periphery, the structure has a centrally located opening; the connected plate sections are dimensioned to have circular symmetry, thereby allowing them to be stacked; each section comprising a number of hollow perforated cells between one or more thin vertical walls having a predetermined range of thickness; The superelastic SMA structure, wherein at least one perforated cell defines a fluid passageway within a predetermined range of hydraulic diameters.

2. The hyperelastic SMA structure of claim 1 , wherein the thickness of said one or more vertical walls is selected from the range of 0.75 mm to 1.25 mm.

3. The hyperelastic SMA structure of claim 1 or 2, wherein the vertical wall has a thickness of 1 mm.

4. The hyperelastic SMA structure of any of claims 1 to 3, wherein the hydraulic diameter of the fluid passageway for one or more cells is selected from the range of 2 mm to 3 mm.

5. The hyperelastic SMA structure of any of claims 1 to 4, wherein the opening is sized to accommodate a support element.

6. The hyperelastic SMA structure of claim 5 , wherein the support element comprises a central pillar and is dimensioned to support a plurality of hyperelastic SMA structures.

7. The superelastic SMA structure of any one of claims 1 to 6, wherein each section has a generally hexagonal or circular shape.

8. The superelastic SMA structure according to any one of claims 1 to 7, wherein each hollow perforated cell has a generally hexagonal honeycomb shape or a circular shape.

9. The hyperelastic SMA structure of any one of claims 1 to 8, wherein the number of said connected sections is three or six.

10. The hyperelastic SMA structure of any of claims 1 to 9, wherein the connected sections are sized in a fully or partially nested pattern during fabrication from a larger sheet.

11. A hyperelastic SMA structure according to any preceding claim, wherein each section is configured to interlock with adjacent sections to define the hyperelastic SMA structure.

12. The hyperelastic SMA structure of any of claims 1 to 11, wherein tension elements are located on the periphery of the structure.

13. The hyperelastic SMA structure of any of claims 1 to 12, wherein tension elements are located on the inner periphery of the structure opening.

14. 14. The hyperelastic SMA structure of any of claims 1 to 13, wherein the tension elements are located at the periphery of the structural opening, or multiple tension elements are located on the outside of the stack, or both, thereby reducing SMA loads during operation.

15. The hyperelastic SMA structure of any of claims 1 to 14, wherein one or more tension elements are located internally and externally to reduce SMA loads during operation.

16. The hyperelastic SMA structure of any preceding claim, wherein the tension element is thermally insulated or isolated from the SMA structure.

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