WO1987007630A1

WO1987007630A1 - Activation of phase change medium

Info

Publication number: WO1987007630A1
Application number: PCT/GB1987/000389
Authority: WO
Inventors: Andrew James Matthews; Deborah Elaine Glover
Original assignee: Edeco Holdings Limited
Priority date: 1986-06-04
Filing date: 1987-06-04
Publication date: 1987-12-17
Also published as: DK55288D0; DK55288A; GB2198449A; AU7483587A; FI880502A0; GB8801170D0; FI880502A; GB8613520D0; EP0269689A1; JPS63503463A

Abstract

An actuator for inducing phase change to a lower temperature phase in a salt hydrate that exhibits a transformation from a high temperature phase to a low temperature phase at a transition temperature. The actuator comprises a coiled spring that is flexed to induce nucleation, a piezoelectric vibration or a thermoelectrically induced cold spot.

Description

ACTIVATION OF PHASE CHANGE MEDIUM

This invention relates to activation of phase change energy storage media.

There has been much recent interest in utilising the latent heat of phase changes, most notably between different hydrated states of salt hydrates. The latent heat can be regarded as stored while the material is in the upper temperature phase, and this stored energy is released when the transformation to the lower temperature phase occurs. The transition from one phase to the other occurs, with corresponding absorption or release of heat, at the transition temperature. Various salt hydrates have transition temperatures within, the ra-nge of 0 to 100°C, one of the most favoured being sodium acetate that has a transition temperature of 58.4°C.

A development from investigations of these materials has been the stabilisation of supercooling phenomena in phase change materials in order to produce media that contain a phase change material in the higher energy state but supercooled below the transition temperature so that the latent heat of the transition can be 'stored' at ambient temperatures. In order to stabilise the supercooled state various additives may be included with the phase change material. For example sodium acetate in aqueous solution will undergo a transformation from an anhydrous or less hydrated phase to a more hydrated phase, and if the solution is made very dilute then it can be supercooled well below the normal transition temperature. A more efficient medium, that is with a greater proportion of phase change material, is described in our United Kingdom patent 2134532 in which a polysaccharide additive stabilises the supercooled state.

Once a phase change material is supercooled it is necessar to have methods of inducing the phase change that are reliable over many cycles, and preferably convenient to use. Dilute solutions of sodium acetate have been found to respond well to activation by snap deformable discs immersed in the solution, the discs being provided with apertures or slits which appear to act as nucleation sites as the disc is snapped. However when these discs are subjected to repeated use they become unpredictable, often becoming more sensitive and causing nucleation for decreasing levels of manipulation with increasing number of snap cycles and yet at^" other times ceasing to create nucleation until subjected to vigorous and repeated snapping, after which they may become effective again but at a different sensitivity level. Neither the reason for the variation in behaviour nor the basic mechanism of nucleation is understood.

Latent heat storage appliances may take many forms varying from sachets of heat storage medium to complete central heating systems, and even the sachets have many uses varying from simple handwarmers to medical and lifesaving equipment. In some applications such as a hand warmer it may be acceptable to utilise an activator that is a little erratic or requires repeated manipulation, but for more high technology applications involving remote or automatic triggering of nucleation a variation in the sensitivity of the nucleation activator can create problems with control circuits and increase complexity. For example in the event of failure to initiate activation a repeat signal has to be given and the mechanism has either to have a back up activator or be able to cope with varying sensitivity. On the other hand if an actuator has become over sensitive accidental triggering can occur by shock, such as knocking or dropping the appliance, and this can have very serious consequences if for example a life saving or medical appliance is rendered useless due to premature and unnoticed nucleation.

The present invention is directed towards the provision of consistent activation means. The invention may also be utilised in conjunction with non-supercooled media in systems operating without supercooling in order to ensure consistent or uniform nucleation, or indeed to inhibit supercooling.

Accordingly the present invention provides apparatus for inducing phase change to a lower temperature phase in a salt hydrate that exhibits a transformation from a high temperature phase to a low temperature phase at a transition temperature, the salt being in the higher temperature phase and at or below the- transition temperature, the apparatus comprising a coiled spring capable of flexure.

The invention is now described by way of example with reference to the accompanying drawings in which:

Figure 1 shows a spring for use in a first embodiment of the invention;

Figure 2 is schematic cross section through the first embodiment;

Figure 3 is a perspective view of a second embodiment of the invention, and

Figure 4 is a perspective view of a fourth embodiment of the invention.

In one aspect of the present invention nucleation of latent heat storage appliance is instigated by flexing a coiled metallic spring that is immersed -in the medium within the appliance. The medium is preferably sodium acetate trihydrate, most preferably suspended in a xanthan gum gel without a great excess of water. The spring may be flexed manually by manipulation through a flexible portion of the casing or housing of the appliance or the spring may be remotely flexed for example through a mechanical or electromagnetic link. The spring may be made of a variety of materials chosen to suit the medium, expected lifetime and sensitivity requirement of the heat storage appliance. For a Sodium acetate based storage medium the following materials may be used for the spring: carbon and alloy steels, copper and copper alloys, stainless steels, nickel alloys and titanium alloys.

If a ferrous metal is used, e.g. carbon steel, then the spring will rust which may lead to contamination of the storage medium and creation of spontaneous nucleation sites. For this reason stainless steel and titanium or nickel alloys are preferred, the latter being especially suitable if highly sensitive activators are required, that is activators that reliably induce nucleation for very small flexure.

The structure of the spring may vary from a tightly wound helical structure in which each turn touches or almost touches its adjacent turns to a looser structure in which the- spaces between turns is greater than ^'the wire diameter or a helix in which the diameter, varies between turns.

Figure 1 shows a tightly wound helical spring which may for example be made from circular section wire of diameter 1mm wound to an outside diameter of 3.75mm and having an overall length of about 30mm. With this structure which is of generally elongate format the spring is most easily flexed laterally. Whilst it is possible to provide a spring activator that is free within the body of a container of storage medium and operable by manipulation through a flexible portion of the container, such as is possible if the medium is encapsulated in a flexible plastics sachet, it is generally preferred to locate the activator in a housing to minimise the risk of accidental flexure of the spring by handling the appliance. Preferably the spring is located in a rigid or near rigid housing such as that shown in Figure 2 having a window of more flexible material through which the spring can be deformed. The housing which contains phase change material that is contiguous with the main body of phase change material to be activated, serves to protect the spring from accidental flexure, to positively locate the spring and to retain it in the position most favourable for. consistent performance.

Conveniently the housing for the spring may be combined with a port through which phase change material is introduced into its container. Such a port is shown in Figure 2, a side wall 2 forming a housing for spring 1 and an upper flexible seal 3 serving to close the port after filling is complete (the port itself also being full) and provide the flexible window through which the spring 1 can be flexed. It will be noted that the spring ends are supported on inward projections '4 and thus when pressed from above (as viewed) essentially three point bending occurs and the spring is flexed laterally. The activator and housing shown is primarily intended for manual operation although the flexure of the spring could be done automatically, for example utilising a solenoid.

It is possible to utilise configurations of spring other than that shown in Figure 1. For example a looser coil may be used or one having varying turn diameters to form a spiral or other non-uniform structure. If the coil is flat, that is having a diameter of the same order as or greater than the length the spring, it may be flexed by longitudinal pressure, this being aided by having a less closely wound configuration. It should be noted that the longitudinal compression may- be applied to the complete spring or to one side. The cross section of the wire from which the springs are formed need not be circular and may be a composite material, single or multi stranded. It has been found preferable to utilise springs of elongate format having relatively tightly wound configurations in which each turn touches a part of the adjacent turn. The sensitivity of the activator in terms of movement required to induce nucleation can be selected by choice of tightness of the coil and ratio of wire diameter to coil diameter. In general increasing the ratio of wire diameter to coil diameter increases sensitivity (less flexing required) as also does tighter winding.

Pre-stressing the spring can also influence the sensitivity and a particularly preferred configuration is an elongate helical coil having a preformed core member that holds the spring in a bent configuration. The greater the degree of bending the greater the sensitivity. If the spring is held in a U shape by the core the movement for inducing nucleation is imparted by urging the ends of the U closer together. Another structure of interest is a spring that is closed into a loop without free ends, squeezing opposite sides of such a loop

05 towards each other provides the required flexure. The closed loop has the particular advantage that it eliminates free ends that can otherwise be relatively sharp ends that could puncture the container, and for this purpose it is particularly favoured for use within sachets when the

10 activator is not retained in a housing.

With mechanical actuation such as that described with reference to Figures 1 and 2, there is always a chance of accidental triggering. In order to avoid this chance, and

15. also for use with larger or more sophisticated apparatus when manual activation would not be appropriate, electrical operation is preferred. This may be achieved with an electromechanical link to the spring mechanism or by utilising the electrical excitation to induce activation in

20 some other way so that the system is fail-safe in the sense of no electrical excitation, no activation.

Figure 3 shows, schematically, a piezo-electric crystal 6 and its associated power supply and drive circuit 7. The power

25 supply comprises a 1.5V DC source and may be provided by a smal 1 battery such as those used in hearing aids or watches and the drive circuit includes an AC/DC converter and frequency multiplier. The crystal 6 is subjected to an oscillating electric field, typically of a frequency of

30 around 20 to 50 KHz bu may be up to lOOKHz, which sets up an ultrasonic vibration in the crystal, one face of which is arranged to contact the body of phase change material that is to be triggered. In the embodiment of Figure 3 the power suppl y and drive circuit are compl etely enc l osed in a pl astics moul ding 8 with the piezo-electric crystal 6 being attached to the moul ding 8 by a rubbery pol ymer 9 , with a single surface exposed. Thus the entire device is protected from the environment except for the crysta l surface. The moulding may be immersed in phase change material without deleterious effect, part of the moulding being secured to the container for the phase change material , or even fabricated integral ly with it. A typical size for the moulding housing the circuitry is 20mm x 10mm x 10mm.

In another embodiment the piezoel ectric osci l lation is transmitted to the phase change material via a point. This may be achieved by an elongate finger or needle of zirconium titanate ceramic projecting into the phase change medium or a needle of another material may be attached to the rectangle to provide a sharper point. The finger may have a cross section of the order of lmm^ώ and have an aspect ratio of the oreder of 5:1. This type of assembly may also be protected by a covering.

A third method of activating supercoo l ed phase change material is by inducing a localised temperature gradient by forming a 'cold spot '. Figure 4 shows a third embodiment of the invention in which power circuitry 11 for a Pe l tier ef fect device 12 i s housed in a mou lding 13. The Pe l tier device 12 may optiona l l y be mounted on a meta l insert 14 , which acts as a heat sink, and the cold face of the device is exposed. This device is be mounted with the col d face in contact with the phase change material, or in contact with a part of the container wa l l that is suf ficiently thermal ly conductive to enable a sufficient thermal gradient to be set up. In order to set up a gradient suff icient to induce nucleation a current through the Peltier element of the order of 100mA to 10A is used which causes a temperature drop of about 70°C within a few seconds. In a preferred embodiment the Peltier device consists of a p-n thermoelectric heat pump. A device of this nature having ceramic faces suitable for contact with the phase change material and with a face cross section of 4mm"¹ and a thickness of 3mm has a pumping capacity of about 0.3 Watts and can be run directly from a small 1.5 V battery. A thermally conductive point may be used to project into the phase change medium to induce a cold spot.

It will be ralised that with both the piezoelectric nucleation and the thermoelectric nucleation activators it is not necessary for the devices to be in actual contact with the phase change material, all that is required is for the physical effect of the vibration or cold to be transferred to the material, that is for an 'operational' contact, thus the devices themselved can be protected from harmful effects of direct contact.

Claims

1. Apparatus for inducing phase change to a lower temperature phase in a salt hydrate that exhibits a transformation from a high temperature phase to a low temperature phase at a transition temperature, the salt being ^"5 in the higher temperature phase and at or below the transition temperature, the apparatus comprising a coiled spring capable of flexure.

2. Apparatus according to Claim 1 in which the spring 0 comprises a single strand coil.

3. Apparatus according to Claim 1 in which the spring comprises a plurality of strands.

5 4. Apparatus according to any preceding claim in which the spring is wound on to a pre-formed core.

5. pparatus according to any preceding claim in which the spring is a helical spring the turns of which are in 0 substantial contact with adjacent turns.

6. Apparatus according to any preceding claim in which the spring is elongate and adapted to be laterally flexed.

5 7. Apparatus according to any preceding claim in which the flexure of the spring is remotely controlled.

8. Apparatus for inducing phase change to a lower temperature phase in a salt hydrate that exhibits a transformation from a high temperature phase to a low 0 temperature phase at a transistion temperature, the salt being in the higher temperature phase and at or below the transition temperature, the apparatus comprising a piezoelectric or thermoelectric device mounted with a surface in operational contact with the salt hydrate.

9. Apparatus according to claim 8 in which the device is powered by battery.

10 Apparatus according to claim 8 or claim 9 in which the device is provided with a point that projects into the salt hydrate.

11. Apparatus according to any preceding claim in which the salt hydrate is sodium acetate trihydrate.