Patent Publication Number: US-2017363073-A1

Title: Heat transfer in an energy recovery device

Description:
FIELD 
     The present application relates to the field of energy recovery and in particular to the use of shape memory alloys (SMA) or Negative Thermal Expansion (NTE) materials for same. 
     BACKGROUND 
     Low grade heat, which is typically considered less than 100 degrees, represents a significant waste energy stream in industrial processes, power generation and transport applications. Recovery and re-use of such waste streams is desirable. An example of a technology which has been proposed for this purpose is a Thermoelectric Generator (TEG). Unfortunately, TEGs are relatively expensive. Another largely experimental approach that has been proposed to recover such energy is the use of Shape Memory Alloys. 
     A Shape Memory alloy (SMA) is an alloy that “remembers” its original, cold-forged shape which once deformed returns to its pre-deformed shape upon heating. This material is a lightweight, solid-state alternative to conventional actuators such as hydraulic, pneumatic, and motor-based systems. 
     A heat engine concept is under development which utilises Shape Memory Alloy (SMA) or another Negative Thermal Expansion (NTE) material as the working medium. In such an engine, for example as disclosed in PCT Patent Publication number WO2013/087490 and assigned to the assignee of the present invention, the forceful contraction of such material on exposure to a heat source is captured and converted to usable mechanical work. 
     Thus far, a useful material for such a working mass has been found to be NickelTitanium alloy (NiTi). This alloy is a well-known Shape Memory Alloy and has numerous uses across different industries. 
     For example, a plurality of NiTi wires form the working element of the engine. Force is generated through the contraction and expansion of these elements by application of a hot and cold fluid within the working core, via a piston and crank mechanism. A problem to be solved is that it is desirable to improve the heat transfer from the fluid to each wire element in the working core to ensure the engine works as efficiently as possible. 
     It is therefore an object of the invention to provide a device and method to overcome the above mentioned problem. 
     SUMMARY 
     According to the invention there is provided, as set out in the appended claims, An energy recovery device comprising:
         an engine comprising a plurality of Shape Memory Alloy (SMA) elements or Negative Thermal Expansion (NTE) elements fixed at a first end by a holder element and connected at a second end to a drive mechanism wherein Shape Memory Alloy (SMA) elements or Negative Thermal Expansion (NTE) elements are positioned to from a gap between adjacent elements and configured to improve heat transfer from a fluid to each element.       

     Shape Memory Alloy or other Negative Thermal Expansion (NTE) Material, fixed at one end and free to move at a second end, such that the wires are arranged adjacently and are in friction or interference contact with each other or with a spacing element, positioned so that the wires are kept slightly removed from each other for the purposes of enhancing heat transfer performance of the fluid/wire system. The grouped elements (wires and spacers) are secured, in aggregate at the outer perimeter of wires utilising a suitable bracket. In such arrangement, during the operation of the bundle arrangement in a heat engine system, the plate elements act to transmit the aggregated force generation of the wire grouping and thus usefully recover and transmit power. 
     In one embodiment a spacer element is positioned to urge the elements away from each other to form said gap. 
     In one embodiment there is provided a bracket system configured to maximise the heat transfer surface area of the elements when a fluid is flowing over the elements. 
     In one embodiment the diameter of at least one NTE or SMA element is reduced relative to either or both ends to optimise heat transfer between the fluid and element. 
     In one embodiment there is provided a U-shaped separator. 
     In one embodiment at least one or more of the NTE or SMA element ends are coated with a resin or plastic to maintain a gap towards each center. 
     In one embodiment there is provided a tapered lock mechanism in the shape of a cone adapted to friction lock the elements at one end. 
     In another embodiment there is provided a core for use in an energy recovery device comprising a grouping of wire elements, composed of Shape Memory Alloy or other Negative Thermal Expansion (NTE) Material, fixed at one end and free to move at a second end, such that the wires are arranged adjacently and are in friction or interference contact with each other, positioned so that the wires are kept removed from each other to increase heat transfer surface when a fluid is passed over said wires. 
     In one embodiment a spacer element is positioned to urge the elements away from each other to form a gap between adjacent elements. 
     In one embodiment a bracket system is configured to maximise the heat transfer surface area of the elements when a fluid is flowing over the elements. 
     In one embodiment the diameter of at least one NTE or SMA element is reduced relative to either or both ends to optimise heat transfer between the fluid and element. 
     In one embodiment the core comprises a U-shaped separator. 
     In one embodiment at least one or more of the NTE or SMA element ends are coated with a resin or plastic to maintain a gap towards each center. 
     In one embodiment the core is dimensioned to allow fluid flow through the centre of the engine in operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  illustrates a prior art energy recovery system using SMA or NTE materials; 
         FIG. 2  illustrates an embodiment of a core in operation with a plurality of SMA or NTE elements; 
         FIG. 3  illustrates a schematic representation of the bundle holder; 
         FIG. 4  illustrates a schematic drawing of the bundle holder using a wedging technique to fix SMA wires; 
         FIG. 5  illustrates a snapshot of the fluid circulation in between wedged wires; 
         FIG. 6  illustrates flow paths in between the rows of SMA; 
         FIG. 7  illustrates a friction-fit bundle holder concept (a) obstructing bracket (b) wire twisting effect (c) wire twisting effect detail showing scissors grip effect; 
         FIG. 8  illustrates an adjustable obstructing bracket system, according to one embodiment; 
         FIG. 9  illustrates an obstructing bracket system according to one embodiment; 
         FIG. 10  illustrates SMA or NTE wires comprising butted ends and domes, according to one embodiment; 
         FIG. 11  the addition of an extra butted end at the either end of the wire to improve heat transfer; 
         FIG. 12  illustrates an embodiment using multiple terminals; 
         FIG. 13  illustrates a U-shaped separator for use in a core having a plurality of SMA or NTE elements; 
         FIG. 14  illustrates a plurality of SMA or NTE elements coated with a plastic or resin material; 
         FIG. 15  illustrates how a clamp can be placed around an optimum number of SMA wires and tightened using a screw until the wires are fixed together in a friction lock; 
         FIG. 16  illustrates a tapered press fit unit, pressed from the top downwards, which can be used to create large friction forces between the wires; 
         FIG. 17  illustrates smaller sub-units consisting of taper threaded locks or press fit locks, or standard plumbing compression fittings can be placed on a single cast bundle holder; 
         FIG. 18  illustrates a multi-layered unit consisting of taper cones with SMA wires in between the layers; 
         FIG. 19  illustrates a swaged friction fit embodiment; 
         FIG. 20  illustrates a friction fit embodiment; 
         FIG. 21  illustrates a multiple wire row friction fit embodiment similar to  FIGS. 19 and 20 ; and 
         FIGS. 22 to 24  illustrate an alternative embodiment showing a hollow core to allow fluid flow through the core in use. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The invention relates to a heat recovery system under development which can use either Shape Memory Alloys (SMA) or Negative Thermal Expansion materials (NTE) to generate power from low grade heat. 
     An exemplary known embodiment of an energy recovery device will now be described with reference to  FIG. 1  which provides an energy recovery device employing a SMA engine indicated by reference numeral  1 . The SMA engine  1  comprises an SMA actuation core. The SMA actuation core is comprised of SMA material clamped or otherwise secured at a first point which is fixed. At the opposing end, the SMA material is clamped or otherwise secured to a drive mechanism  2 . Thus whilst the first point is anchored the second point is free to move albeit pulling the drive mechanism  3 . An immersion chamber  4  is adapted for housing the SMA engine and is also adapted to be sequentially filled with fluid to allow heating and/or cooling of the SMA engine. Accordingly, as heat is applied to the SMA core it is free to contract. Suitably, the SMA core comprises a plurality of parallel wires, ribbons or sheets of SMA material. Typically, a deflection in and around 4% is common for such a core. Accordingly, when a 1 m length of SMA material is employed, one might expect a linear movement of approximately 4 cm to be available. Higher displacement can also be obtained. It will be appreciated that the force that is provided depends on the mass of wire used. Such an energy recovery device is described in PCT Patent Publication number WO2013/087490, assigned to the assignee of the present invention, and is incorporated fully herein by reference. 
     For such an application, the contraction of such material on exposure to a heat source is captured and converted to usable mechanical work. A useful material for the working element of such an engine has been proven to be Nickel-Titanium alloy (NiTi). The SMA actuation core is comprised of a plurality of SMA material clamped or otherwise secured at a first point which is fixed 
     In order to secure the NiTi wires in the engine, it is required to develop a system that can anchor each wire at both ends, in such a fashion as will allow it to operate under high load. This system has been designated as the “bundle holder”. The bundle holder should overcome two specific problems: 
     1) Transmit the high-force, low displacement load of the NiTi wires during operation. This is a single degree of freedom (DOF) system whereby one end of the bundle is secured and remains stationary, whilst the opposing end is free to move in one axis of displacement to enable the movement of the piston and the harnessing of the work. 
     2) Enable the close-packing of the wires, insofar as possible, to enable maximum heat transfer from the transiting water to the wire and vice versa. 
     3) From a manufacturing point of view, it has to eliminate the tedious and strenuous process of placing hundreds of these NiTi wires in some sort of support and reduce production time and costs. 
     Such a core is described in UK patent application number 1409679.6, assigned to Exergyn, and is incorporated fully herein by reference. In this application a core engine is described for use in an energy recovery device comprising a plurality of Shape Memory Alloys (SMA) or Negative Thermal Expansion (NTE) elements fixed at a first end and connected at a second end to a drive mechanism. The holder is a holder configured with a plurality of slots adapted to receive the plurality of Shape Memory Alloys (SMA) or NTE elements, for example Nickel Titanium wires. The SMA wires are substantially elongated and arranged in a parallel orientation to make up a core that is housed in a chamber. 
       FIG. 2  illustrates an embodiment of a core in operation with a plurality of SMA or NTE wires  1  arranged in parallel in use in an energy recovery device. The core is housed in a chamber and is connected to a fluid source via valves  10  and manifolds  11 . The SMA wires are secured at both ends by a bottom and top bundle holder  12  and  13 . One end of the core is in communication with a piston  14  that is moveable in response to expansion and contraction of the SMA wires to generate energy. The invention as described improves the heat transfer from the fluid to each wire element in the working core to ensure the engine works as efficiently as possible. 
     First Embodiment 
     In one embodiment there is provided an arrangement of the SMA wires  1  are in straight lines, allowing for a more compact arrangement while guaranteeing equal exposure to the fluid. 
     As shown in  FIG. 3  the top unit is pressed into position, compressing the SMA wires  1  into a friction lock. Compression screws  20  are employed to hold the units together when in their locked state. The top and mid layers can be moulded or cast as the shapes are more basic than conical approaches and they are non-load bearing. The bottom unit acts as the anti-bending bundle support and must be cast from a high tensile material. 
       FIG. 4  is a similar embodiment to  FIG. 3 , in which the splitting element, namely the fixing wedge, is of different shape. This embodiment solves the problem of swaging wires individually. The wire elements  1  are held using a tightening screw  31  and nut  32  in between the top and bottom cap  33 ,  34  to fix the wire elements  1  into place. Coupling two rows of wires might delay the heat transfer or make it unequal so, in order to stop the two rows of wires from touching a fixing wedge  35  can be made longer and in this way will split the rows, i.e. enhancing heat transfer and acting as a turbulators in the core, as shown in  FIGS. 5 and 6 .  FIG. 5  illustrates a snapshot of the fluid circulation in between wedged wires.  FIG. 6  illustrates flow paths in between the rows of SMA wire elements. 
     Second Embodiment 
     The friction fit bundle holders described previously can give rise to the active wire elements being too close together, thus creating a limit to effective fluid interaction and consequently ineffective heat transfer. A means of separating the wires is required. 
       FIG. 7  illustrates a number friction-fit bundle holder concepts, where  FIG. 7 a    shows an obstructing bracket;  FIG. 7 b    illustrates wire twisting effect and  FIG. 7 c    illustrates wire twisting effect in more detail showing a scissors grip effect. 
       FIG. 7 a    illustrates a radius of curvature is introduced in the wires  1  within the bundle below a bundle holder bracket  41  through the use of a strut or other member  42  positioned. The wires and bundle holder are arranged in such way as to allow an outward pressure to be created at the boundary of the wires  1  with the bracket  41 . This pressure is created by the resultant force created by the wedging of the strut element  42  into place. This pressure is in addition to that which is exerted through the use of the basic friction fit as previously described. 
     The wires and the strut  42  are arranged in such way as to provide a resultant force, R, on the bundled wires  43  i.e. the strut is compressed or otherwise forced into the bundle such that there is a net force system acting in the directions denoted in  FIG. 7 . A further advantage is also created. By wedging the wires in this manner, a moderate twisting of the wires in the bundle is created, brought about in large part because of the stiffness of the wire elements (i.e. it is more prominent in larger diameter wires). This twisting gives rise to a type of articulated joint effect  44  (i.e. a “scissors” joint effect) which enables a stronger grip to be exerted due to the exertion of the force at both the top and bottom edge of the bundle holder bracket  41 . This can be seen in  FIG. 7 b    and  FIG. 7   c.    
     In this instance, the bundle holder is left free-floating and unsecured to the core wall. Instead, the core is affixed using the strut. The free-floating nature of the bundle means that, during the tensioning stage of the wire operation, when it is pulling on the piston at the opposing end, the bundled wire has the tendency to pull the bundle holder towards the strut, in essence creating a self-reinforcing, “wedged” friction bond between the wire bundle and the strut. In so doing, volume reductions in the wire bundle brought about through the heating of the wire elements are effectively counteracted through the gripping motion enacted by the interaction of the strut, the wires and the bundle holder. 
     Third Embodiment 
     A means of separating the wires into multiple groups in the friction fit bundle holder is required. 
       FIG. 8  illustrates an adjustable multiple obstructing bracket system, according to one embodiment. In this instance, a plurality of struts  51  is used in a fashion similar to that presented in the previous embodiment. However, in this instance, in addition to creating a stronger frictional bond between the wires and the bundle holder, the additional struts enable the spacing of the wires in such fashion as allows a maximising of heat transfer surface area to be exposed to the transiting fluid flowing over the bundle. 
       FIG. 9  illustrates how the strut might be assembled in the core. The core walls  61  have a hole or opening through which the strut element  62  may be slotted into the core  61 . By inserting the strut  62  through the core walls, it is possible for the bundle holder to be inserted into the core first. This has advantages from an assembly management viewpoint. Because the bundle holder is free-floating, the strut  62  can be added afterwards without difficulty. 
     Fourth Embodiment 
     In one embodiment the SMA or NTE wires  1  can comprise butted ends  71  and domes  72 , as illustrated in  FIG. 10 . The butted ends act to maintain a centre to centre gap between the SMA wires, while the domes act to restrict wire pull through, when wires are placed tightly into a tapered opening or hole  73  and split collet  74  arrangement. 
     When a force is applied to the wires in the bundle, the split collet and taper compact the butted ends together. The diameter of the butted ends is selected to ensure the optimum centre to centre distance between wires. 
     The length of wire interacting with the fluid flow is drawn or rolled to a smaller diameter, hence a gap between the wires is introduced to facilitate fluid flow. The reduced diameter can be optimised to allow for the optimum heat transfer between the fluid and SMA wires. 
       FIG. 11  the addition of an extra butted end  75  at the either end of the wire to ensure that the best heat transfer regime will be met. To better accommodate the second butted end  76 , the design of the collet can be changed by adding an extra butted end on the lower side of the collet the heat transfer would be optimal and the flow paths would be kept in place after fixing the split collet in the tapered mount plate in a pressed fit. 
     Fifth Embodiment 
     In one embodiment a swageless terminal is provided. It uses the terminal as a holder of the wires within the bundle. 
       FIG. 12  outlines using multiples of these terminals. The reasoning behind this being that if one terminal can create a large reaction force due to axial tension then more than one could create larger reactions forces, meaning that the efficiency can be increased. 
     Sixth Embodiment 
     The problem aimed to be solved by this embodiment is how to separate the wires in a compression type bundle holder in such a way to allow each wire to be sufficiently heated and cooled by the fluid being passed through each core. 
     A ‘U’ shaped SMA separator  80  shown in  FIG. 13  makes a gap in the bundle allowing fluid to pass through it and more effectively heat and cool the wires. It is held in place in the same manner as the rest of the wires, by means of a dome swage  81  which is described above. 
     The U shaped wires  80  could also be shape set in this U form so they would contract when heated, this in turn would pull the bottom of the U toward the top of the bundle holder  82  inducing more force on the wire against the bundle holder preventing any slippage. 
     Seventh Embodiment 
     The problem aimed to be solved by this concept is how to separate the wires in a compression type bundle holder in such a way to allow each wire to be sufficiently heated and cooled by the fluid being passed through each core. 
     In one embodiment the SMA or NTE wires  1  are dipped in a plastic or resin  90 , as shown in  FIG. 14 . The thin coating of this plastic or resin  90  adheres to the surface of the SMA wire, as this coating is only applied to the top and bottom of the wires  1  such that a gap  91  remains between them down their entire length.  FIG. 14  illustrates a plurality of SMA or NTE elements coated with a plastic or resin material. 
     Eight Embodiment 
     The problems arising with large bundles of wires using traditional hole and swage techniques are the cost and time involved with drilling multiple holes—from a mass production POV this is not ideal and the time needed to swage and assemble each wire in-situ in the bundle holders. 
       FIG. 15  illustrates how a clamp  100  can be placed around an optimum number of SMA wires  1  and tightened using a screw tightener  101  until the wires are fixed together in a friction lock. A wire separator  102  consisting of a number of slots is then placed under the clamp  100  and slid upwards in order to separate the wires  1  to allow for maximum heat transfer as the fluid passes over them. This approach eliminates the need for multiple openings and swages, and allows for much more rapid assembly of the bundle. Furthermore, since the load is taken by the top clamp  100 , the wire separator  102  can be cast or moulded, reducing manufacturing time. 
     Ninth Embodiment 
     It may not be possible to achieve a compressive load large enough using a screw tightener in the case of using a clamp around an optimum number of wires. This results in smaller maximum bundle sizes or else larger screws—both of which are undesirable. Furthermore, the screw fit protruding from the sides of the clamp means the design is not suitable for placement in a piston. 
       FIG. 16  illustrates a tapered press fit unit  110  pressed from the top downwards, can be used to create large friction forces between the wires  1 . A fixed wall  111  is necessary to absorb the reaction forces as the load is applied. The advantage of this approach is the flexibility afforded in the design to optimise friction surface area for specific bundle sizes. A wire separator may be necessary split the wires for optimised heat transfer. Multiple inline press fit clamps can be used to facilitate rows or circles of SMA wire. The exterior of the fixed wall can be threaded to facilitate insertion into a piston if required. 
     Tenth Embodiment 
     If the maximum bundle size as a result of press fitting is limited to a bundle size that is below the required wire quantity for a particular core, a strategy to utilized multiple multi-wire connections in a single core is required 
       FIG. 17  illustrates smaller sub-units consisting of taper threaded locks  120  or press fit locks, or standard plumbing compression fittings can be placed on a single cast bundle holder, complete with individual wire separator for each sub-unit. Each lock  120  is required to be pressed or locked individually and then slid into a slotted bundle holder  121 . Strategic use of wire separators can be used to ensure equal exposure of each wire to the fluid. 
     Eleventh Embodiment 
     The requirement for multiple locks in a single core can increase the time for assembly, while also increasing cost and complexity of the assembly tooling. A single locking movement can increase assembly efficiency while allowing for a more straight forward volumetric reduction via the reduction of area necessary to mount multiple locking units side by side. 
       FIG. 18  illustrates a multi-layered unit consisting of taper cones  130  with SMA wires  1  in between the layers. A central tapered block  132  is press fitted into position causing the layers to compress together tightly, thereby locking the SMA wires in a friction lock. 
     This method is advantageous as the central tapered block  132  can be press fit at high force to ensure all SMA wires are gripped tightly. The block can then be secured in place using a screw cap, thus maintaining the friction lock. 
     Twelfth Embodiment 
     After the doming process (swaging of the wire ends in the shape of a dome  72 ) is automated being able to place them into a bundle holder with both ends of the wire already domed so as to avoid needing to dome one end while the wire is already in the bundle holder is required. 
     To create a bundle holder that is made up of three main parts, a top and a bottom which shall be the same size and able to slide over each other and a filler block. The top and bottom parts will have slots large enough to fit the wires dome  72  through. Once all the domed wires  1  are placed through the slots the bottom part will be slid over until it hits the side of the wire. The top will then be filled with a block of metal  140  so the dome  72  is supported on two sides. This metal filler block  140  will be held up by the bottom part. This operation is shown in  FIG. 19 . 
     It should be understood that this embodiment is not exclusively applicable to swaged wires, and may be used on swageless wires, where the bottom component is capable of producing enough of a friction force between the SMA wires and the holder wall so as to provide adequate securement of said wire bundle. This embodiment is illustrated in  FIG. 20 , whereby an additional set of filler blocks  140 ,  141  are utilised in order to provide as tight a fit for the wires as possible. 
     In addition to this consideration, the bundle holder concept disclosed in the present invention may also be used to secure pluralities of wire rows (either swaged or swageless) between the top and bottom parts of the assembly, as opposed to a single row as is shown in  FIG. 21 . 
     Thirteenth Embodiment 
       FIGS. 22 to 24  illustrates another embodiment to increase the heat transfer from the fluid to the wire elements and vice versa. Uneven heating of wires in the core such as that described results in stresses within the wires that shorten their fatigue life. 
       FIGS. 22 and 23  show an embodiment that allows for a more evenly distributed flow of liquid to the wires in each core, thereby enhancing even heating of the wires to overcome such difficulties.  FIGS. 22 and 23  shows a ‘Hollow Bundle’ core configured as a ‘donut’ or ring of material  150  that has wires  1  arranged around the outside. In effect the core is made of a plurality of elongated wire elements  1  with a hollow centre  150 . An opening in the middle  160  of the ‘donut’/ring  150  enables fluid to be flowed through the centre of all the wires thereby greatly enhancing dispersion of fluid when compared to fluid being fed from two opposing sides as previously used. 
     The invention also overcomes the effects of a double-opposing entry whereby some wires are heated before others causing potential weaknesses and failure points in the wires. 
       FIG. 24  shows the flow of liquid through the core, highlighting the overall even distribution of such liquid. The ring or donut  150  also has a function of securing the wires arranged around it in place. 
     In the specification the terms “comprise, comprises, comprised and comprising” or any variation thereof and the terms include, includes, included and including” or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation and vice versa. 
     The invention is not limited to the embodiments hereinbefore described but may be varied in both construction and detail.