A heat pump with enhanced heat transfer

Abstract

A heat transfer apparatus comprising: a quantity of a caloric material; a flow arrangement to direct fluid along a flow path, in which the fluid is in thermal contact with the caloric material; a stress arrangement adapted to apply force to the caloric material to cause mechanical stress in the caloric material, and remove the force from the caloric material; and an ultrasonic transducer positioned to deliver ultrasonic vibrations to fluid within the flow path.

Description

[0001] A HEAT PUMP WITH ENHANCED HEAT TRANSFER

[0002] Background

[0003] This invention relates to a heat transfer apparatus, and in particular relates to a heat transfer apparatus which makes use of caloric materials, such as shape memory alloy (SMA) components.

[0004] The use of SMA components in heat transfer applications has become well known in recent years. SMA components may change their crystalline state and shape by an external force below a critical temperature, but resume their original state and shape after being heated above the critical temperature. An SMA component is generally fabricated at a high temperature, above the critical temperature, to have a predetermined shape.

[0005] One example of an SMA material is the NiTi family of alloys, which includes titanium nickel alloy, and also alloys which contain titanium, nickel and one or more further elements. Other SMA alloys include CuAINi, CuAIZn, NiAl and MnCu. The skilled reader will be aware of other alloy compositions that can be used to form SMA components.

[0006] SMA components have potential for use in many heat transfer applications, such as heat pumps, which may be used for heating and I or cooling in heating, ventilation and air-conditioning & refrigeration (HVAC - R) applications. The use of SMAs promises many advantages with respect to conventional technology, such as increased efficiency, reduced emissions, enhanced performance stability, and the ability to avoid chemical refrigerants.

[0007] In known laboratory and prototype SMA heat pump systems, elongate tubes of SMA material, or stacks of plates of SMA material, are positioned within a flow chamber, through which a fluid such as water or air may flow during operation of the heat pump. In one phase of operation, an external force is applied to the SMA material, causing the material to change its phase from austenite to martensite. This is an exothermic change, and large quantities of heat are released during this process. The flow over and through the SMA material absorbs this heat energy during the phase change.

[0008] In a further stage of the cycle, the external force is removed from the SMA material, allowing the material to change its phase from martensite back to austenite. This is an endothermic process, and significant quantities of heat energy will be absorbed from the surrounding fluid into the SMA material during the phase change, causing the fluid to cool.

[0009] A cycle which makes use of an SMA material may have four stages: (1 ) compressing the material to release heat; (2) pre-cooling, where the temperature of the material starts to fall; (3) removing the load and allowing the material to resume its original crystalline structure and shape, leading to absorption of heat and cooling of the surrounding fluid; and (4) pre-heating, where the temperature of the material starts to rise.

[0010] Equipment utilising this principle may have a plurality of cylinders or stacks of SMA material, with one or more of these cylinders / stacks performing one of the four stages of the cycle at any moment. This arrangement provides capacity for constant heating and also constant cooling.

[0011] Whilst some properties of SMAs hold potential for high efficiency and power, their thermal conductivity is relatively low. This presents an impediment to the balance of system components of a heat pump in making the most efficient use of the thermal energy of the material. Techniques to improve heat transfer coefficients of SMA heat pump systems are needed to increase power density and efficiency and also to decrease cycle time and unit size.

[0012] It is an object of the present invention to provide an improved heat transfer apparatus of this type.

[0013] Accordingly, one aspect of the present invention provides a heat transfer apparatus, and a method of operating a heat transfer apparatus, in accordance with the independent claims appended hereto.

[0014] Preferred features of the invention are set out in the dependent claims.

[0015] In order that the invention may be more readily understood, embodiments thereof will now be described, by way of example, with reference to the accompanying drawings, in which:

[0016] Figure 1 is a first view of major components of a heat transfer apparatus embodying the invention;

[0017] Figure 2 shows the heat transfer apparatus of figure 1 from a different perspective; and

[0018] Figure 3 is a schematc diagram 21 which shows an example of a flow passage in ann example of the present invention.

[0019] Detailed description

[0020] With reference to figures 1 and 2, major components of a heat transfer apparatus 1 embodying the invention are shown. Figure 1 is an end-on view of the heat transfer apparatus 1 , and figure 2 shows a side-on view of the same heat transfer apparatus 1 . The apparatus includes a stack of SMA plates 8. In the example shown the SMA plates 8 are stacked together such that they are generally parallel with each other, as is known from (for instance) WO2021 / 219667. Some or all of the SMA plates may be identical or substantially identical to each other in at least cross-sectional shape, which has advantages relating to manufacturing and construction of the apparatus 1 , but this is not essential.

[0021] The SMA plates 8 may have holes (not shown) passing therethrough, and stiffening rods or other structural elements may extend through the holes to provide mechanical stiffness to the stack, and prevent the stack from buckling under the forces that are applied to it during the cycle.

[0022] The invention is not limited to the provision of elongate SMA tubes or a stack of SMA plates. Any form of suitable SMA component(s) may be used. In other examples, a single SMA component may be provided. The single component may be elongate, and may for instance take the form of a cylinder.

[0023] In yet further examples, an SMA component which comprises one or more elongate SMA wires or ribbons, which may be twisted or otherwise formed together, may be used.

[0024] Returning to the arrangement shown in the figures, the stack of SMA plates has an inlet aperture 2, which is preferably positioned at or near a first end 3 of the stack. The stack also has an outlet aperture 4, which is preferably positioned at or near a second, opposite end 5 of the stack.

[0025] In this example the SMA stack has a flow passage (not shown), which passes through an interior of the stack, along a part of the length of the stack. The flow passage preferably passes along a significant part of the length of the stack (e.g. 90% or more of the total length), for reasons which will become clear.

[0026] The inlet aperture 2 provides a fluid communication path between the flow passage and the exterior of the stack. The outlet aperture 4 similarly provides a fluid communication path between the flow passage and the exterior of the stack. It should be understood that fluid may flow into the stack through the inlet aperture 2, along the length of the flow passage in the direction indicated by the arrows in figure 2, and then leave the stack through the outlet aperture 4. Suitable conduits or other flow arrangements (not shown) are provided to guide fluid into the inlet aperture 2 and out of the outlet aperture 4.

[0027] As can be seen in figure 1 , in the example shown there are two inlet apertures 2, provided on opposite sides of the stack. There are also two outlet apertures 4. However, this is not essential. Any suitable number of inlet and outlet apertures 2, 4 may be provided. The apparatus 1 comprises an upper housing 6 or manifold, which receives the first end 3 of the stack, and supports or forms part of conduits that supply fluid to the inlet apertures 2. The apparatus further comprises a lower housing 7 or manifold, which receives the second end 5 of the stack, and supports or forms part of conduits that remove fluid from the outlet apertures 4. The upper and lower housings 6, 7 may take any suitable form. The upper and lower housings 6, 7 are preferably formed from a robust material, which is resistant to corrosion and thermal fluctuations, such as a sturdy plastics material. Delrin® (i.e. polyoxymethylene (POM)) is one example of such a material.

[0028] A cover 12 or bellows extends between the upper and lower housings 6, 7, and surrounds or substantially surrounds the stack. In the example shown the cover 12 has a generally circular cross section, but may take any other suitable form. The cover 12 may be formed from the same material as the upper and lower housings 6, 7, or may be formed from a different material. The cover 12 is preferably flexible and resilient, as will be apparent from the discussion below.

[0029] In the example shown, first and second telescoping supports 14, 15 are positioned within the cover 12, surrounding the stack. Each of the supports 14,15 preferably has a cross-sectional shape which is round or generally round on its outer side, to fit within the cover, and has a square or generally square aperture passing therethrough, which fits closely around the outer side of the stack. In the example shown, the first and second supports are elongate, with the first support 14 being positioned closer to the inlet end 3, and the second support 15 being positioned closer to the outlet end 5. When the SMA material is in a relaxed state, the first and second supports 14, 15 fill almost all of the space between the cover 12 and the stack. However, there is a gap 16 between the supports 14, 15. This gap is preferably positioned near the middle of the length of the stack, but this is not essential.

[0030] The first support 14 has an elongate extension 13, which protrudes towards the second support 15. The second support 15 has a corresponding recess 17 into which the extension 13 is received. In the example shown the extension 13 takes the form of a tube which surrounds and lies close to the outer surface of the stack, and the recess 17 takes the form of a corresponding space formed in the second support 15, around the outer surface of the stack. The recess 17 is of sufficient length that, when the stack is in a relaxed state, the extension 13 may be received in the recess, with additional space in the recess 17 beyond the free end of the extension 13, to allow further movement of the extension 13 into the recess 17.

[0031] The details of the supports 14, 15 as described above are not essential, and any configuration which allows the supports to slide with respect to each other may be used. Indeed, in some examples no special features of the supports 14, 15 are provided for this purpose. The first and second supports 14, 15 are preferably formed from a relatively hard plastic material, but any suitable material can be used.

[0032] First and second compression components 9 are positioned at opposing ends of the stack of SMA plates 8. A drive mechanism (not shown), which may for instance be hydraulic, is operable to exert forces on the stack of SMA plates 8, to compress the stack between the compression components at certain stages of the thermal cycle, as discussed above. In preferred embodiments one of the compression components 9 is fixed in place, while the other one is driven by the drive mechanism, although in other examples both of the compression components 9 may be driven by the drive mechanism.

[0033] As the SMA material is compressed, the gap 16 between the first and second supports 14, 15 partially or completely closes. The extension 13 of the first support 14 slides further into the recess 17 of the second support 15. The cover 12 may bulge or otherwise deflect outwardly to accommodate this motion.

[0034] The invention is not limited to the use of a linear compressive force to the SMA material, and any suitable mechanical stress (whether compressive or tensile) may be applied, which causes the SMA material to change phase in the required manner. A stress arrangement may, for instance, apply any combination of tension, axial compression, lateral compression, bending stress and / or torsional stress.

[0035] It should also be understood that the drive mechanism that is used to apply stress to the SMA material can take any suitable form, and is not limited to hydraulic mechanisms. This includes nonmechanical drive mechanisms, such as magnetic or electrical mechanisms. It is also envisaged that barocaloric systems, in which multiaxial pressure is applied to a caloric, may be used.

[0036] Thus far the components of the heat transfer apparatus 1 shown in figure 1 are known.

[0037] The arrangement shown in figure 1 further comprises a plurality of ultrasonic transducers 10. The ultrasonic transducers 10 are arranged so that, in use, they can create ultrasonic vibrations in the fluid within the flow passage.

[0038] In the example shown in the figures, one or more of the ultrasonic transducers 10 are placed in engagement with one of the outer side walls of the cover 12. It is envisaged that this will be the most practical placement of the ultrasonic transducers 10, because they are separated from the stack, and will therefore not be exposed to significant temperature fluctuations during the thermal cycle. In this location the transducers 10 are also readily accessible for inspection, repair or maintenance. In the example shown in the figure, four transducers 10a are positioned in contact with outer side walls of the cover 12. These transducers 10a are supported by mounting brackets 1 1 , but this is not essential and the transducers 10a may be held in position in any suitable manner.

[0039] In the example shown, one or more of the ultrasonic transducers 10 are placed in direct contact with the SMA material. In the embodiment shown in the figures, two transducers 10b are placed in direct contact with the SMA material near the inlet aperture 2, and two further transducers 10c are placed in direct contact with the SMA material near the outlet aperture 4. This may provide advantages in that the SMA material itself may be caused to vibrate in a significant manner, which will in turn lead to vibrations in the fluid surrounding the SMA material.

[0040] In the example shown the transducers 10b, 10c that are positioned near the inlet and outlet apertures 2, 4 are held in position by the upper and lower housings 6, 7, respectively, but this is not essential.

[0041] It is also envisaged that one or more ultrasonic transducers may be positioned within the flow passage, through which fluid flows during operation of the heat transfer apparatus 1 .

[0042] In embodiments of this type, the ultrasonic transducer(s) 10 will be able to produce vibrations within the fluid in the flow passage in a more direct manner.

[0043] If an ultrasonic transducer 10 is positioned within the flow passage, the ultrasonic transducer 10 may be placed in contact with the SMA material.

[0044] In further embodiments, however, the ultrasonic transducer 10 may be placed within the flow passage but not in direct contact with the SMA material.

[0045] Overall when positioning a transducer, the benefits and disbenefits of any one position will need to be taken into account. Benefits will include the degree of turbulence and interruption of the boundary layer achieved, and disbenefits the amount of energy input needed to get the ultrasonic energy to the target location.

[0046] The ultrasonic transducers 10 may, however, be engaged with, or positioned in proximity to, any other component or components of the apparatus 1 , including components that are not shown in the schematic representation shown in figure 1 . For instance, the ultrasonic transducer 10 may be placed in contact with, or in close proximity to, pipework of the heat transfer apparatus. Overall the purpose of providing the ultrasonic transducers 10 is to cause vibrations in the fluid within the flow passage, to increase the rate of heat transfer between the SMA material and the fluid.

[0047] There are two main mechanisms by which this will occur. One is acoustic cavitation, which is the formation, growth and collapse of gas bubbles in liquid, caused by ultrasonic vibrations. Acoustic cavitation gives rise to very high levels of turbulence, particularly during the collapse of the bubbles, which takes place in a violent manner.

[0048] A second effect that arises from the application of ultrasonic vibrations to fluid is acoustic streaming, i.e. the dissipation of acoustic energy within the fluid giving rise to gradients in momentum, and hence to currents within the fluid.

[0049] Without wishing to be bound by theory, it is the inventors’ understanding that these are the major effects that will arise from the application of ultrasonic vibrations to the fluid, that will be of relevance to the invention. However, the inventors anticipate that other factors may arise from the application of ultrasonic vibrations, such as direct heating and / or nebulisation, that may also give rise to noticeable effects.

[0050] A result of acoustic cavitation and acoustic streaming taking place within the fluid will be an increase in the rate of heat transfer between the SMA material and the fluid. Overall, both of these effects will result in disruption in the boundary layer of the fluid flow, which exposes more fluid molecules to the heat from the SMA and continuously restores the heat gradient between SMA and fluid to enable faster heat transfer. On this basis, in preferred embodiments the region to which the ultrasonic waves should be targeted is the boundary layer of the flow adjacent to the surface of the SMA material.

[0051] Where the SMA material includes holes or channels formed therethrough, as shown in the figures, it will be advantageous for the ultrasonic waves to be targeted to the boundary layer of fluid that is adjacent to the surfaces of the holes or channels.

[0052] The application of ultrasonic vibration to fluid in an arrangement such as shown in the figures is expected to allow an increase in heat transfer by a factor of two to five, compared to a corresponding arrangement in which no ultrasonic vibration is provided.

[0053] A further advantage of ultrasonic vibrations within the fluid is to avoid the build-up of contaminants on surfaces of the SMA material, and also on other surfaces of the heat transfer apparatus 1 . Such build-up can reduce the rate of heat transfer, by forming an insulating layer between the SMA material and the surrounding fluid. The turbulence arising from the effects described above will help to prevent contaminants from settling or growing on surfaces of the heat transfer apparatus.

[0054] Ultrasonic waves can also disrupt and / or destroy biological matter or cells which are in contact with fluid. Fouling of systems of this kind may often take the form of a biofilm which builds up over time. The biofilm can provide a breeding ground for harmful bacteria etc. to include legionella and other pathogens. Other fouling may comprise inorganic matter, such as scale.

[0055] Pressure waves generated within the fluid by the ultrasonic transducer will generally disrupt formation of such layers, and also act to scour off any layers which have already formed.

[0056] The ultrasonic transducer may be of any suitable form, and may for instance comprise a piezoelectric transducer or a capacitive transducer.

[0057] It is also envisaged that an ultrasonic horn may be provided in conjunction with the ultrasonic transducer 10. An ultrasonic horn will amplify oscillations, and help to transfer the oscillations more effectively to the relevant component(s) of the heat transfer apparatus 1 . As the skilled reader will be aware, an ultrasonic horn may take the form of a tapering metal bar, having a round crosssection and a varying shape along its length, but the invention is not limited to this and any suitable type or configuration of ultrasonic horn may be used. In the accompanying figures, each transducer is represented by a horn, and it should be understood that each horn would be attached to a respective transducer.

[0058] In this document references to an ultrasonic transducer should be interpreted as encompassing a combination of a transducer and a horn (as well as a transducer without a horn).

[0059] A controller (not shown in the figures) is connected to the ultrasonic transducers 10, to drive the transducers 10 to generate vibrations at a suitable frequency.

[0060] As the skilled reader will be aware, ultrasound waves between around 20 kHz and 100 kHz are often referred to as “low frequency ultrasound” or “power ultrasound”.

[0061] It is anticipated that vibrations in the low frequency ultrasound range are likely to be of greatest utility in increasing heat transfer in applications such as that shown in the figures.

[0062] Current indications are that the preferred range of frequencies for heat transfer enhancement is 15- 60kHz, and that frequencies within the range of 40-60kHz are most effective for cleaning purposes. The use of frequencies within these ranges are preferred. The use of frequencies that fall within both ranges (i.e. 40-60kHz) are particularly preferred. However, the invention is not limited to this and ultrasound in other frequency ranges - such as high frequency ultrasound (generally defined between 100 kHz and 1 MHz) and very high frequency ultrasound (generally defined as having frequencies above 1 MHz) - is also anticipated for use with the invention.

[0063] In the arrangement shown in the figures, several ultrasonic transducers 10 are used.

[0064] However, in other embodiments, only one ultrasonic transducer is provided. This transducer may be placed in any of the locations that are shown in the figures, or may be provided in a different location. The positioning of the single transducer may be selected depending on the location in which it is most advantageous to provide ultrasonic vibrations.

[0065] Any suitable number of transducers may be used. The apparatus shown in the figures includes eight transducers, but any suitable number below this (including a single transducer) or above this may be used, depending on the size and scale of the apparatus, and the requirements of the particular application.

[0066] Where a plurality of ultrasonic transducers 10 is provided, all of the transducers may be connected to the same controller, and adapted to be driven to vibrate by the controller, although this need not be the case. In other examples two or more controllers are provided, and for instance a separate controller may be provided to control each transducer.

[0067] In embodiments of the invention, two or more ultrasonic transducers 10 are provided to deliver vibrations to the fluid within the flow passage. These ultrasonic transducers 10 may be activated simultaneously, to deliver ultrasonic waves having greater energy to the fluid, thus increasing the rate of thermal transfer between the fluid and the SMA material.

[0068] In such embodiments, the ultrasonic transducers 10 may be placed such that, in the vicinity of the surface of the SMA material, the waves produced by the transducers tend to interfere constructively, thus increasing the amplitude of the ultrasonic waves. This is likely to lead to advantages relating to the efficiency of the system, and with regard to minimising the power consumption of the ultrasonic components.

[0069] In other embodiments, the ultrasonic transducers are arranged such that each transducer 10 primarily delivers ultrasonic vibrations to one region of the SMA material, which is different from regions of the SMA material to which ultrasonic vibrations are primarily delivered by other ultrasonic transducers. However, such measures are not essential, and a plurality of transducers may simply be applied to the apparatus 1 in any convenient way, with the vibrations generated by the transducers all contributing to vibrations within the fluid.

[0070] In further embodiments, a plurality of ultrasonic transducers 10 are provided, but only one, or a subset, of the transducers are activated at any one time. For instance, during operation of the heat transfer apparatus 1 , the ultrasonic transducer(s) which are activated to deliver vibrations are cycled, and this cycling may be correlated with the overall thermal cycle, or may be operated on a different timing scheme.

[0071] Changing the transducer(s) that deliver ultrasonic vibrations may, for instance, avoid the generation of localised heating or excessive vibration at any particular part of the heat transfer apparatus 1 .

[0072] Providing a plurality of transducers 10 will also allow a degree of redundancy, in that if a particular transducer 10 fails for any reason, the apparatus 1 will still have one or more further transducers 10 available to generate vibrations.

[0073] As discussed above in relation to the first heat transfer apparatus 1 , ultrasonic transducers 10 may be placed in a variety of locations, e.g. against an outer side wall 15 of the cover 12, against an outer surface of the SMA material, or within the flow passage inside the SMA material. Where a plurality of ultrasonic transducers are provided, all of the transducers may be in similar locations. In other embodiments, the transducers may be placed in different locations, e.g. as shown in the figures. An arrangement of this kind may allow a greater level of control over the manner in which ultrasonic vibrations are delivered to fluid within the flow passage.

[0074] Figure 3 is a schematc diagram 21 which shows an example of a flow passage 23 which extends between the inlet 2 and the outlet 4. The flow passage is at least part of the flow arrangement to direct fluid along a flow path, in which the fluid is in thermal contact with the caloric material; In this example, the flow passage is a linear path from inlet 2 to outlet 4. In other examples the flow passage may be of a different shape.

[0075] The examples described above involve the use of SMA material. However, it should be understood that embodiments of the invention may make use of any kind of caloric material. Caloric materials are those that display reversible thermal effects, triggered by magnetic or electrical fields, and / or by mechanical stress. Caloric materials include elastocaloric materials (which are triggered by direct mechanical stress), magnetocaloric materials (which are triggered by magnetic fields), electrocaloric materials (which are triggered by magnetic fields) and barocaloric materials (which are triggered by the application of pressure). All of these types of caloric materials may be used in embodiments of the invention. References in this document to the caloric material experiencing mechanical stress should be understood as mechanical stress within the caloric material itself, without this necessarily arising from externally applied mechanical forces. For instance, if magnetocaloric material is used, a magnetic field may be applied to the material, causing the material to experience mechanical stress.

[0076] The skilled reader will appreciate that the details of how the ultrasonic waves are delivered may depend on the type of caloric material that is used. However, implementing a technique to deliver ultrasonic waves in an appropriate manner, for any kind of caloric material, will be within the capability of a person of ordinary skill in the art.

[0077] The examples given herein make use of SMA material, which is an example of an elastocaloric material. For the avoidance of doubt, the material used in some embodiments of the invention may be any type of elastocaloric material, including but not limited to SMA material.

[0078] As the skilled reader will appreciate, the SMA material will have a “natural frequency”. For instance, in the case of the stack of SMA plates 8 shown in figures 1 and 2, which may include stiffening rods or other structural elements, the stack will have a natural frequency at which it will vibrate.

[0079] The skilled reader will understand that the natural frequency will depend on the medium that surrounds the SMA material, and also the medium that is within the flow passage. The natural frequency will, for instance, be different if the stack of SMA plates 8 has air within the flow passage, compared to a situation where water is within the flow passage.

[0080] In advantageous embodiments of the invention, the ultrasonic waves that are delivered by the transducer(s) 10 are tuned to the natural frequency of vibration of the SMA material.

[0081] As discussed above, during the thermal cycle the SMA material is placed under significant mechanical stress. At other times, the forces producing the stress are entirely or substantially removed. The skilled reader will understand that the natural frequency of the SMA material will be different in these two states and in states in-between.

[0082] In embodiments of the invention, the ultrasonic transducers may be configured to emit vibrations at a single frequency throughout the thermal cycle, where this frequency matches or substantially matches the natural frequency of the SMA material in either the stressed or the “relaxed” state. In yet further embodiments of the invention, the transducer(s) may be configured to emit ultrasonic vibrations at changing frequencies, such that, at different points in the cycle, the ultrasonic vibrations are tuned to the frequency of the SMA material in both the stressed and relaxed states.

[0083] In order to achieve this, one or more transducers may be controlled to adjust the frequency of the vibrations that are imparted to the fluid over time, such that the frequency of the imparted vibrations matches or substantially matches the natural frequency of the SMA material throughout the cycle from the stressed to relaxed state and back again at different times.

[0084] In other embodiments, at least two ultrasonic transducers are provided, with one ultrasonic transducer being configured to emit vibrations at a frequency which matches or substantially matches the natural frequency of the SMA material in the stressed state, and a further ultrasonic transducer being configured to emit vibrations at a frequency which matches or substantially matches the natural frequency of the SMA material in the relaxed state.

[0085] In such embodiments, the transducers are controlled so that they emit vibrations at the corresponding part of the thermal cycle, i.e. the first transducer emits ultrasonic waves during parts of the cycle where the SMA material is stressed, and the second transducer emits vibration during parts of the cycle where the SMA material is relaxed. When either one of the two transducers is activated, the other transducer may be substantially or entirely deactivated.

[0086] Imparting vibrations which are tuned to the natural frequency of the SMA material will lead to the vibrations striking a resonance with the SMA material, causing it to vibrate with a relatively large amplitude. This will in turn significantly increase the amplitude of the vibrations that are delivered to the fluid immediately surrounding the SMA material, and thus increase the rate of heat transfer between the SMA material and the fluid, for the reasons discussed above.

[0087] It is anticipated that the most effective way of achieving this effect will be for the natural frequency of the SMA material in both the stressed and relaxed states to be established during a configuration or set-up phase of the heat transfer apparatus.

[0088] The natural frequency of the SMA material may be measured directly, for instance through the use of one or more accelerometers, and the skilled reader will readily understand how this may be carried out. Alternatively, the natural frequency of the SMA material in the stressed and relaxed states may be calculated through (for example) fast Fourier transform (FFT) analysis.

[0089] In either case, the natural frequency of the SMA material in both states is established, and is stored in a memory which is accessible by the controller. Vibrations of these frequencies are then produced during at appropriate times during the thermal cycle. In further embodiments, the natural frequency of the SMA material is calculated or approximated as the thermal cycle proceeds. This could be carried out, for instance, by “sweeping” the vibrations produced by the transducer(s) across a range of frequencies, and observing the response of the SMA material. The response may, for example, be measured by placing one or more sensors such as accelerometers on the SMA material itself, or by measuring the amplitude of ultrasonic vibrations within the fluid, and noting when this amplitude increases, indicating that the amplitude is being increased by resonance of the SMA material.

[0090] While such approaches may not be as simple as solutions discussed above in which the natural frequencies are pre-established and stored in a memory, it is anticipated that the natural frequency of the SMA material may vary with age, or with other changing parameters of the heat transfer apparatus, such as changes in density of the fluid, or the temperature of the heat source. Embodiments in which the natural frequency of the SMA material is directly measured as the thermal cycle proceeds may therefore allow greater reliability of the system over time.

[0091] As discussed above the transducer(s) 10 may be driven to produce vibrations at a frequency which matches or substantially matches the natural frequency of the SMA material. It is anticipated that producing vibrations that are at a frequency which is within 10% of the natural frequency will be sufficient to give rise to the advantageous effects discussed above. In further examples the transducer(s) are driven to produce vibrations at a frequency which is within 5% of the natural frequency. In still further examples the transducer(s) are driven to produce vibrations at a frequency which is within 2% of the natural frequency. Overall, however, the vibrations produced by the transducer(s) 10 must be sufficiently close to the natural frequency to produce a resonance in the SMA material. One of skill in the field will readily be able to discern when vibrations produced by the transducer(s) are sufficiently matched to the natural frequency of the SMA material to produce a resonance.

[0092] In some embodiments, ultrasonic vibrations will be applied to the apparatus when the SMA material is in the stressed state, and also in the relaxed state, but not in the intervening periods in which the SMA material is transitioning between these two states.

[0093] However, in other embodiments, ultrasonic vibrations are also applied to the SMA material while it is being transitioned between the stressed and relaxed states.

[0094] In examples where the ultrasonic vibrations are not tuned to the natural frequency of the SMA material, in these embodiments ultrasonic vibrations may be applied continuously throughout all parts of the cycle. In examples where the sonic vibrations are tuned to the natural frequency of the SMA material, a different approach will be needed. As discussed above, the SMA material will have a first natural frequency when it is in the stressed state, and a second natural frequency when it is in the relaxed state. As the mechanical stress is applied to, or removed from, the SMA material for it to change from one state to the other, this natural frequency will change. Due to the nature of the change, the natural frequency may not be expected to change in a smooth and linear manner during this process.

[0095] Once again, the change of natural frequency over time during the transitions may be measured directly, for instance through one or more sensors which are attached to the SMA material, or may be calculated through a modelling process, for instance using FFT analysis. In either case the way in which the natural frequency of the SMA material changes may be established beforehand, and stored in a memory that is accessible by the controller which is used to drive the transducer(s).

[0096] Alternatively, the natural frequency can be calculated in real time, during the cycle, and techniques by which this may be achieved are discussed above.

[0097] The application of ultrasonic vibrations during a thermal cycle, as described above, will require input of power, to drive the ultrasonic transducer(s), and also other associated components such as a controller. However, this parasitic energy will be outweighed by the increase in efficiency of the system overall.

[0098] In systems embodying the invention the ultrasonic vibrations may be supplied all or substantially all of the time when the apparatus is operating.

[0099] However, this is not essential. In other embodiments, ultrasonic vibrations may be supplied for only part of the time when the apparatus is operating.

[0100] In some examples, the supply of ultrasonic vibrations may be pulsed, in that vibrations are supplied intermittently while the apparatus is operating. Any appropriate timing for the pulsing may be used, although it is anticipated that there will be two main parameters, which are the duration of each pulse, and the duty cycle, i.e. the proportion of the duration of the pulse for which vibrations are supplied. The duration of each pulse need not be the same as, or be any multiple of, the length of the thermal cycle itself.

[0101] The pulsing of the vibrations may continue for extended periods of operation of the apparatus.

[0102] In some embodiments the duration of the pulses, and / or the duty cycle, may be altered during the operation of the apparatus. The pulsing of the vibrations may have the benefit of allowing greater control over the amount of vibrational energy that is supplied to the fluid.

[0103] In other examples, the supply of vibrations may be turned on and off as required, for instance vibrations could be applied during peak times, or times of increased need for heating and / or cooling,

[0104] The apparatus 1 shown in figures 1 and 2 involves a stack of SMA plates 8 with flow passages passing through an interior of the stack. However, the invention is not limited to this, and any other suitable arrangement may be used. For example, an alternative apparatus includes a flow chamber, through which fluid may flow during operation of the apparatus.

[0105] The SMA material (e.g. a stack of SMA plates, similar to that shown in figures 1 and 2) is positioned within the flow chamber. In use of the apparatus, fluid flows through the flow chamber, and as this occurs the fluid flows over the outer surface of the SMA material.

[0106] In such embodiments, one or more ultrasonic transducers may be placed in engagement with one of the side walls of the flow chamber. It is envisaged that this will be the most practical placement of the ultrasonic transducer, because it is separated from the fluid within the flow chamber, and will therefore not be exposed to fluid flow during operation.

[0107] It is also envisaged that one or more ultrasonic transducers may be positioned within the flow chamber, i.e. within the volume defined by the side walls thereof, through which fluid flows during operation of the heat transfer apparatus.

[0108] In embodiments of this type, the ultrasonic transducer(s) will be able to produce vibrations within the fluid in the flow chamber in a more direct manner.

[0109] If an ultrasonic transducer is positioned within the flow chamber, the ultrasonic transducer may be placed in contact with the SMA material. This may provide advantages in that the SMA material itself may be caused to vibrate in a significant manner, which will in turn lead to vibrations in the fluid surrounding the SMA material. In such arrangements it is envisaged that a seal will need to be provided around the transducer (or around a horn attached to the transducer) in order to prevent leaks.

[0110] In further embodiments, however, an ultrasonic transducer may be placed within the flow chamber but not in direct contact with the SMA material. It is also envisaged that the SMA material may be positioned within a flow chamber, such that fluid flows over outer surfaces of the SMA material, and fluid also flows through a flow passage within the SMA material, as shown in figures 1 and 2. Such arrangements may maximise the area of thermal contact between the SMA material and the fluid.

[0111] These are only examples, and any other arrangement of SMA material, past, through or over which fluid may flow along a flow path to exchange heat with the SMA material, may be used.

[0112] The skilled reader will appreciate that heat transfer systems embodying the present invention will display significant advantages with respect to conventional systems, particularly with regard to the rate of thermal transfer, and COP (coefficient of performance).

[0113] When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

[0114] The invention may also broadly consist in the parts, elements, steps, examples and / or features referred to or indicated in the specification individually or collectively in any and all combinations of two or more said parts, elements, steps, examples and / or features. In particular, one or more features in any of the embodiments described herein may be combined with one or more features from any other embodiment(s) described herein.

[0115] Protection may be sought for any features disclosed in any one or more published documents referenced herein in combination with the present disclosure.

[0116] Although certain example embodiments of the invention have been described, the scope of the appended claims is not intended to be limited solely to these embodiments. The claims are to be construed literally, purposively, and / or to encompass equivalents.

Claims

CLAIMS1 . A heat transfer apparatus comprising: a quantity of a caloric material; a flow arrangement to direct fluid along a flow path, in which the fluid is in thermal contact with the caloric material; a stress arrangement adapted to apply force to the caloric material to cause mechanical stress in the caloric material, and remove the force from the caloric material; and an ultrasonic transducer positioned to deliver ultrasonic vibrations to fluid within the flow path.

2. A heat transfer apparatus according to claim 1 , wherein the flow path comprises a flow passage which passes through an interior of the caloric material.

3. A heat transfer apparatus according to claims 1 or 2, wherein the flow path comprises a flow chamber, wherein at least a part of the caloric material is positioned within the flow chamber.

4. A heat transfer apparatus according to claim 2, wherein the ultrasonic transducer is placed in contact with, or proximity to, a cover or housing which surrounds the caloric material.

5. A heat transfer apparatus according to claim 3, wherein the ultrasonic transducer is positioned outside the interior volume of the flow chamber.

6. A heat transfer apparatus according to claim 5, wherein the ultrasonic transducer is positioned in contact with a side of the flow chamber.

7. A heat transfer apparatus according to any one of claims 3 to 6, wherein the ultrasonic transducer is positioned within the interior volume of the flow chamber.

8. A heat transfer apparatus according any preceding claim, wherein the ultrasonic transducer is arranged to be in contact with the caloric material.

9. A heat transfer apparatus according to claim 2 or any clause dependent thereon, wherein the ultrasonic transducer is positioned within the flow passage.

10. A heat transfer apparatus according to any preceding claim , further comprising an ultrasonic horn which is arranged to amplify vibrations produced by the ultrasonic transducer.

11. A heat transfer apparatus according to any preceding claim, comprising a single ultrasonic transducer.

12. A heat transfer apparatus according to any one of claim 1 to 10, comprising a plurality of ultrasonic transducers.

13. A heat transfer apparatus according to claim 12, wherein the ultrasonic transducers are arranged so that, in the region of at least one surface of the quantity of caloric material, vibrations in the fluid produced by the ultrasonic transducers interfere constructively.

14. A heat transfer apparatus according to claim 13, wherein the ultrasonic transducers are controlled such that, during operation of the apparatus, only one of the ultrasonic transducers is driven to produce vibrations at any one time, with the remaining ultrasonic transducers being deactivated or substantially deactivated.

15. A heat transfer apparatus according to claim 13, wherein the ultrasonic transducers are controlled such that, during operation of the apparatus, two or more of the ultrasonic transducers are driven to produce vibrations simultaneously.

16. A heat transfer apparatus according to any preceding claim, wherein: the caloric material has a natural frequency of vibration; and the or each ultrasonic transducer is driven to produce vibrations which are at, or substantially at, the natural frequency.

17. A heat transfer apparatus according to claim 16, wherein: the caloric material has a first state, in which the mechanical stress results from the stress arrangement, and a second state, in which the stress arrangement does not operate; and the caloric material has a first natural frequency of vibration when it is in the first state, and a second natural frequency of vibration when it is in the second state, the second natural frequency of vibration being different from the first natural frequency of vibration.

18. A heat transfer apparatus according to claim 17, wherein the or each ultrasonic transducer is operable to deliver ultrasonic vibrations which are tuned or substantially tuned to the first natural frequency of vibration, and is also operable to deliver ultrasonic vibrations which are tuned or substantially tuned to the second natural frequency of vibration.

19. A heat transfer apparatus according to claim 18, wherein the or each ultrasonic transducer is operable, in a first mode, to deliver ultrasonic vibrations which are tuned or substantially tuned tothe first natural frequency of vibration, and is also operable, in a second mode, to deliver ultrasonic vibrations which are tuned or substantially tuned to the second natural frequency of vibration.

20. A heat transfer apparatus according to claim 17, comprising two ultrasonic transducers, wherein a first one of the two ultrasonic transducers is operable to deliver ultrasonic vibrations which are tuned or substantially tuned to the first natural frequency of vibration, and wherein a second one of the two ultrasonic transducers is operable to deliver ultrasonic vibrations which are tuned or substantially tuned to the second natural frequency of vibration.

21. A heat transfer apparatus according to claim 20, wherein the ultrasonic transducers are controlled such that, when the mechanical stress is caused in the caloric material, the first of the two ultrasonic transducers is operated to deliver ultrasonic vibrations which are tuned or substantially tuned to the first natural frequency of vibration, and when the mechanical stress is not caused in the caloric material, the second of the two ultrasonic transducers is operated to deliver ultrasonic vibrations which are tuned or substantially tuned to the second natural frequency of vibration.

22. A heat transfer apparatus according to any preceding claim, wherein the caloric material is an elastocaloric material.

23. A heat transfer apparatus according to claim 22, wherein the elastocaloric material is an SMA material.

24. An HVAC-R system incorporating a heat transfer apparatus according to any preceding claim.

25. A method of operating a heat transfer apparatus, comprising the steps of: providing a heat transfer apparatus according to any one of clauses 1 to 24; directing fluid along the flow path; and operating the ultrasonic transducer to deliver ultrasonic vibrations to the fluid within the flow path.

26. A method according to clause 25, when dependent upon claim 16, further comprising the steps of: in a first phase of a cycle, applying the stress to the caloric material; in the first phase, delivering ultrasonic vibrations which are tuned or substantially tuned to the first natural frequency of vibration; and in a second phase of the cycle, in which the mechanical stress is not applied to the caloric material, delivering ultrasonic vibrations which are tuned or substantially tuned to the second natural frequency of vibration.

27. A method according to claim 26, when dependent upon clause 20, comprising the steps of: in the first phase of the cycle, delivering ultrasonic vibrations which are tuned or substantially tuned to the first natural frequency of vibration using the first one of the two ultrasonic transducers; and in the second phase of the cycle, delivering ultrasonic vibrations which are tuned or substantially tuned to the second natural frequency of vibration using the second one of the two ultrasonic transducers.

28. A heat transfer apparatus comprising: a quantity of a caloric material; a flow arrangement to direct fluid along a flow path, in which the fluid is in thermal contact with the caloric material; a stress arrangement adapted to apply force to the caloric material to cause mechanical stress in the caloric material, and remove the force from the caloric material; and an ultrasonic transducer positioned to deliver ultrasonic vibrations to fluid within the flow path, wherein: the caloric material has a natural frequency of vibration; and the or each ultrasonic transducer is driven to produce vibrations which are at, or substantially at, the natural frequency.

29. A heat transfer apparatus according to claim 28, wherein: the caloric material has a first state, in which the mechanical stress results from the stress arrangement, and a second state, in which the stress arrangement does not operate; and the caloric material has a first natural frequency of vibration when it is in the first state, and a second natural frequency of vibration when it is in the second state, the second natural frequency of vibration being different from the first natural frequency of vibration.

30. A heat transfer apparatus according to claim 29, wherein the or each ultrasonic transducer is operable to deliver ultrasonic vibrations which are tuned or substantially tuned to the first natural frequency of vibration, and is also operable to deliver ultrasonic vibrations which are tuned or substantially tuned to the second natural frequency of vibration.31 . A heat transfer apparatus according to claim 30, wherein the or each ultrasonic transducer is operable, in a first mode, to deliver ultrasonic vibrations which are tuned or substantially tuned to the first natural frequency of vibration, and is also operable, in a second mode, to deliver ultrasonic vibrations which are tuned or substantially tuned to the second natural frequency of vibration.

32. A heat transfer apparatus according to claim 29, comprising two ultrasonic transducers, wherein a first one of the two ultrasonic transducers is operable to deliver ultrasonic vibrations which are tuned or substantially tuned to the first natural frequency of vibration, and wherein a second one of the two ultrasonic transducers is operable to deliver ultrasonic vibrations which are tuned or substantially tuned to the second natural frequency of vibration.

33. A heat transfer apparatus according to claim 32, wherein the ultrasonic transducers are controlled such that, when the mechanical stress is caused in the caloric material, the first of the two ultrasonic transducers is operated to deliver ultrasonic vibrations which are tuned or substantially tuned to the first natural frequency of vibration, and when the mechanical stress is not caused in the caloric material, the second of the two ultrasonic transducers is operated to deliver ultrasonic vibrations which are tuned or substantially tuned to the second natural frequency of vibration.

34. A heat transfer apparatus according to any of claims 28 to 33, wherein the caloric material is an elastocaloric material.

35. A heat transfer apparatus according to claim 33 wherein the elastocaloric material is an SMA material.

36. A heat transfer apparatus according to any of claims 28 to 35, further comprising an ultrasonic horn which is arranged to amplify vibrations produced by the or each ultrasonic transducer.

37. An HVAC-R system incorporating a heat transfer apparatus according to any of claims s 28 to 36.

38. A method of operating a heat transfer apparatus, comprising the steps of: providing a heat transfer apparatus according to any one of clauses 28 to 36; directing fluid along the flow path; and operating the ultrasonic transducer to deliver ultrasonic vibrations to the fluid within the flow path.

39. A method according to claim 38, further comprising the steps of: in a first phase of a cycle, applying the stress to the caloric material; in the first phase, delivering ultrasonic vibrations which are tuned or substantially tuned to the first natural frequency of vibration; and in a second phase of the cycle, in which the mechanical stress is not applied to the caloric material, delivering ultrasonic vibrations which are tuned or substantially tuned to the second natural frequency of vibration.

40. A method according to claim 39, when dependent upon clause 32, comprising the steps of: in the first phase of the cycle, delivering ultrasonic vibrations which are tuned or substantially tuned to the first natural frequency of vibration using the first one of the two ultrasonic transducers; and in the second phase of the cycle, delivering ultrasonic vibrations which are tuned or substantially tuned to the second natural frequency of vibration using the second one of the two ultrasonic transducers.

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