GB2587070A

GB2587070A - Phase change material

Info

Publication number: GB2587070A
Application number: GB2008933.0A
Authority: GB
Inventors: Burton Geoff
Original assignee: Phase Change Mat Products Ltd
Current assignee: Phase Change Mat Products Ltd
Priority date: 2019-06-12
Filing date: 2020-06-12
Publication date: 2021-03-17
Anticipated expiration: 2040-06-12
Also published as: GB2587070B; GB201908384D0; GB202008933D0

Abstract

A low temperature phase change material comprises an aqueous solution of lithium chloride, the aqueous solution consisting of lithium chloride in an amount between 30 to 40 wt.% and water in an amount between 70 to 60 wt.%. The phase change material may further comprise a nucleating agent e.g. inorganic Group 1 salt, inorganic Group 2 salt and/or an inorganic ammonium salt. Also shown is a method of forming the low temperature phase change material and its use.

Description

PHASE CHANGE MATERIAL

This invention relates generally to phase change materials for use in the storage and release of thermal energy. More specifically, although not exclusively, this invention relates to phase change materials having a phase change temperature of below -40°C Phase change materials (PCM) are substances that are usable to store and release latent heat for future use. Useful PCMs typically have a high heat of fusion, which, upon changing phase via melting and freezing at certain temperatures, can store and release large to amounts of thermal energy. This technology is well known and has been used for many heating and cooling applications, including transport of frozen or perishable goods such as foodstuffs, medicinal products, and biological materials; chilled water applications; means of providing passive cooling in buildings and maintenance of ambient temperature; waste heat rejection storage; and solar applications.

Numerous materials and compositions have been described in the prior art as potential PCMs having different phase change temperatures suited to specific applications. PCMs may be broadly separated into three categories: (i) inorganic salt-based solutions; (ii) clathrates; and (iii) organic materials. Salt based materials may be salt hydrates, which generally freeze and melt at temperatures above 0°C, or eutectic solutions, which may freeze and melt at low temperatures, Le. temperatures below 0°C. Clathrates have high latent heat storage capacities and have some uses as PCMs. There are a limited number of materials that form clathrates with water, and most of these require extreme pressures to do so. Some materials are able to form clathrates at atmospheric pressures, but the choice of phase change temperature available by these materials is limited, meaning that they cannot be used for most of the aforementioned applications. Additionally, the materials used to form clathrates at atmospheric pressures tend to be expensive and the solutions are prone to supercooling, i.e. it is necessary to cool the clathrate below its formation temperature before any clathrate is produced. There are a wide range of organic materials used as PCMs, such as fatty acids, fatty acid esters, polyols, and waxes. Of these, waxes are particularly suitable, given that they are available over a wide range of operating temperatures, have relatively high latent heat capacities, are not prone to supercooling, are stable over the long term, are non-corrosive to metals, and are relatively inexpensive. Such examples of these waxes include candle waxes, fully and partially refined paraffins, n-paraffins, microcrystalline waxes, scale waxes, slack unrefined/recycled waxes, natural waxes, polyethylene and polypropylene waxes, petrolatum, amide waxes, synthetic waxes, white oils or many different types of mineral or vegetable-based oils.

There are many known examples of aqueous salt-based eutectic PCM solutions for use in many operational applications, including temperature-controlled transport. Ideally, these solutions are non-hazardous, not regulated for transport, and should freeze and melt cleanly over a narrow temperature range with the release and absorption of large amounts of thermal energy in the form of latent heat or heat of fusion.

At present, eutectic PCM solutions having a phase change temperature of between 0°C to -40°C are an attractive option for low temperature transportation of goods because the eutectic PCM solutions can easily be contained or encapsulated in suitable packages, e.g. bottles, ice packs, pouches, and so on, and frozen in standard or low temperature freezers or cold stores. Eutectic PCMs can be kept in the frozen state until required for use and incorporated into the packaging used to transport the temperature sensitive material. In transit, the temperature of the frozen eutectic PCM solution slowly rises until it reaches its phase change temperature. At this point, the solution starts to melt and absorbs thermal energy in the form of latent heat in the process. The melting process occurs at a constant temperature and thus maintains the desired temperature of the temperature sensitive material. The amount of eutectic PCM solution required to maintain a constant temperature throughout a transportation process may be calculated by considering the amount of temperature sensitive material to be transported, as well as other factors such as ambient temperature and length of journey time. At the end of the journey, the packaging containing the eutectic PCM solution can simply be recharged by placing in a suitable freezer or cold store to re-freeze the eutectic PCM solution for re-use when frozen. In this way, the packages containing eutectic PCM solution can be used multiple times.

Whilst there are suitable aqueous salt-based eutectic PCM solutions available for utilisation in temperature control applications between 0°C and AO°C, there are very few suitable eutectic PCM solutions suitable for use below -40°C, and even fewer suitable eutectic PCM solutions for use below -55°C. There are currently several categories of materials or products that benefit from transportation in a temperature regulated environment below -55°C. For example, many vaccines, biological materials, medicines and pharmaceuticals are required to be kept below -60°C during transport, and up until the point of usage. The most widely used method of temperature regulation currently is packing in solid carbon dioxide (dry ice). Dry ice sublimes at -77°C, which absorbs thermal energy and thus maintains the contents of any package or transport container at a constant temperature for a long period of time. However, the use of dry ice is disadvantageous for several reasons. Firstly, it is a single use application. Furthermore, there may be issues of practically in some instances. For example, vaccination programmes in, for example, sub-Saharan Africa, have been adversely affected due to the lack of locally available solid carbon dioxide making equipment or facilities.

Several materials have been proposed for use as PCMs with phase change temperatures io below -40°C. However, many of these materials have drawbacks that limit their utility.

One drawback is that many aqueous salt-based eutectic PCM solutions have a tendency to supercool, in that the liquid solution can be subjected to temperatures significantly below its freezing temperature without crystallising or solidifying. This is a disadvantageous property because the surrounding temperature must be further decreased to the eutectic PCM solution to initiate freezing. Furthermore, in some instances, a potential eutectic PCM solution may fail to freeze at all.

For example, a eutectic mixture of potassium acetate in water has a eutectic temperature of -60°C. However, this material does not freeze and, instead, sets to a glass, which does not absorb latent heat energy when thawed. Therefore, there is no heat of fusion involved in the process, so any energy storage capability would be in the form of sensible heat over a wide temperature range, rather than the much higher latent heat of fusion involved in changing phase from liquid to solid crystalline material and back again. A eutectic mixture of zinc chloride in water is reported to have a eutectic temperature of -62°C. However, this material is toxic, hazardous to the environment, and is subject to classification as hazardous for transport regulations. Furthermore, this material sets to glass, does not freeze or melt cleanly over a narrow temperature range, and does not utilise any appreciable latent heat storage.

Some organic materials have been identified, particularly primary alcohols and alcohol blends, which have suitable phase change temperatures, and which can be frozen completely in an ultra-low temperature freezer at -85°C. Furthermore, these materials freeze and melt cleanly over a narrow temperature range and reproducibly over repeated thermocycling. However, primary alcohols are flammable and are classed as hazardous for transport. Therefore, it is only permitted to carry very small quantities on regular modes of transport, e.g. passenger planes, non-licensed couriers and haulage companies. Therefore, the use of these materials in low temperature transport applications is extremely limited.

It is therefore a first non-exclusive object of the invention to provide a PCM having a phase change temperature of below -40°C, and that freezes and melts cleanly over a narrow temperature range and completely, for use in low temperature applications, e.g. the transportation of temperature-sensitive material in a temperature regulated environment io below -40°C, the PCM being economical, reusable, non-hazardous, and not subject to any transport regulations.

The PCM may have a phase change temperature of below -40°C, e.g. at or below -50°C, -60°C or -65°C, e.g. at or between -40° and -100°C, or between -40°C and -80°C, e.g. between -60°C and -70°C, e.g. -65°C.

Accordingly, a first aspect of the invention provides a composition suitable for use as a phase change material, preferably a low temperature phase change material, the composition comprising an aqueous solution of lithium chloride, the aqueous solution consisting of lithium chloride in an amount between 30 to 40 wt.%, e.g. between 31 to 33 wt.%, and water in an amount between 70 to 60 wt.%, e.g. between 67 to 69 wt.%.

In this specification between 30 wt.% to 40 wt.% is intended to mean 30 wt.°/0 [LiC1].40wt.c/0 and the amount of water is to be correspondingly understood. For example, lithium chloride may be present in 30, 31, 32, 33, 34, 35, 36, 37, 38, 39 or 40 wt.%, and water may be present in 70, 69, 68, 67, 66, 65, 64, 63, 62, 61 or 60 wt.%.

It is known that lithium chloride forms several hydrates with water, for example the mono-, tri-, and penta-hydrates. It has been reported that lithium chloride has a eutectic composition of 7.86 mol/kg lithium chloride to water (equivalent to 24.98% w/w lithium chloride) with a eutectic temperature of 199K (-74°C) (J. Chem. Eng. Data; 2002, 47, 6, 1331-1336). We are unaware of any proposal to use lithium chloride as a low temperature (sub-zero degree centigrade) PCM even though the reported eutectic temperature of lithium chloride may suggest its use as an alternative to solid carbon dioxide. In its support, lithium chloride is not classified as hazardous, it is non-toxic, non-corrosive, non-flammable, and is not subject to any transport regulations. However, our investigations have concluded that the eutectic composition does not crystallise, even when subjected to cooling to -190°C, and does not release or absorb any significant energy in the form of latent heat of fusion at any temperature above -190°C. Therefore, the eutectic composition of lithium chloride is not considered to exhibit the required properties for use as a PCM in low temperature transport applications, to. below -40°C.

It has been surprisingly found that a composition comprising an aqueous solution of lithium chloride according to the invention freezes and melts cleanly at -65°C to store and release io latent heat energy. This is surprising because the eutectic solutions described above do not freeze and melt cleanly over a narrow temperature range and, instead, the eutectic solutions supercool and/or no crystallisation is observed. Without wishing to be bound any theory, it is believed that the composition of the invention is stoichiometrically equivalent to lithium chloride pentahydrate.

In embodiments, the aqueous solution may comprise or consist of lithium chloride in less than 39 wt.%, e.g. less than 38 wt.%, less than 37 wt.%, less than 36 wt.%, less than 35 wt.%, less than 34 wt.%, less than 33 wt.%, less than 32 wt.%, or less than 31 wt.%; and water in greater than 61 wt.%, e.g. greater than 62 wt.%, greater than 63 wt.%, greater than 64 wt.%, greater than 65 wt.%, greater than 66 wt.%, greater than 67 wt.%, greater than 68 wt.%, or greater than 69 wt.%.

In embodiments, the aqueous solution may comprise or consist of lithium chloride in an amount between 30 to 40 wt.% and water in an amount between 60 to 70 wt.%. In embodiments, the aqueous solution may comprise or consist of lithium chloride in greater than any one of 30, 31, 32, 33, 34, 35, 36, 37, 38, or 39 wt.%, and less than any one of 40, 39, 38, 37, 36, 35, 34, 33, 32, 31 wt.%; and water in greater than any one of 60, 61, 62, 63, 64, 65, 66, 67, 68, or 69 wt.%, and less than any one of 70, 69, 68, 67, 66, 65, 64, 63, 62, or 61 wt.% In embodiments, the composition further comprises a nucleating agent.

It has been surprisingly found that the addition of a nucleating agent enables the composition of the invention to crystallise and melt cleanly at a temperature below -60°C, e.g. below -65°C, with release and absorption of thermal energy in the form of latent heat of fusion. Although it is known that nucleating agents promote crystallisation of aqueous solution and minimise supercooling, we are not aware of a report of the use of a nucleating agent that is effective in controlling supercooling of a lithium chloride composition to promote effective and consistent crystallisation over repeated thermocycling, thus ensuring the composition freezes completely in a freezer at -85°C.

By nucleating agent, we mean a substance that promotes the formation of the solid phase of the composition when subjected to temperatures at or below the freezing temperature of the composition, by acting as a seed crystal on which the composition may initiate crystallisation.

The nucleating agent may be selected from one or more of an inorganic Group 1 salt, an inorganic Group 2 salt, and/or an inorganic ammonium salt. For example, the cationic species of the nucleating agent may be or comprise one or more of a lithium ion, a sodium ion, a potassium ion, a rubidium ion, a caesium ion, a beryllium ion, a magnesium ion, a calcium ion, a strontium ion, a barium ion, or ammonium ion. In embodiments, the nucleating agent may comprise an inorganic halide salt. For example, the anionic species of the nucleating agent may be one or more of a fluoride ion, a chloride ion, a bromide ion, or an iodide ion. In embodiments, the nucleating agent may be selected from one or more of an inorganic Group 1 halide salt, an inorganic Group 2 halide salt, and/or an ammonium halide. In embodiments, the anionic species of the nucleating agent may comprise a sulphate ion, a phosphate ion, and/or a sulphide ion. In embodiments, the nucleating agent may comprise a Group 2 sulphate salt, a Group 2 phosphate salt, and/or a Group 2 sulphide salt.

In embodiments, the nucleating agent may be selected from one or more of sodium fluoride, sodium chloride, sodium bromide, potassium bromide, ammonium bromide, calcium sulphate, barium sulphate, calcium phosphate, and/or barium sulphide.

Without wishing to be bound by any theory, the inventor believes that the most preferable nucleating agents are those having an FCC cubic lattice crystal structure, e.g. alkali metal halides, metal oxides, and metal sulphides. Many of these compounds are freely soluble in water. However, if added to the aqueous solution consisting of lithium chloride and water according to the invention, the degree of solvation is significantly reduced, such that a relatively small addition may exceed the saturation limit of the nucleating agent and undissolved material may remain in the composition so that it is able to act as a nucleating agent.

In embodiments, the composition may further comprise a first nucleating agent and a second nucleating agent, i.e. two or more different nucleating agents.

It has been surprisingly found that a combination of two different nucleating agents has a synergistic effect on the phase change material of the invention upon freezing and/or melting by suppressing supercooling and promoting crystallisation when compared to the io action of singular nucleating agents under the same conditions.

In embodiments, the composition may comprise one or more nucleating agent(s) in less than 10 wt.% of the total composition, e.g. less than 9 wt.%, 8 wt.%, 7 wt.%, 6 wt.%, 5 wt.%, 4 wt.%, 3 wt.%, 2 wt.%, or less than 1 wt.% of the total composition. In embodiments, the composition may comprise one or more nucleating agent(s) in an amount between any one of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, or 9.0 wt.% to any one of 10.0, 9.0, 8.0, 7.0, 6.0, 5.0, 4.0, 3.0, 2.0, 1.0, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, or 0.2 wt.% of the total composition. Most preferably the composition may comprise one or more nucleating agent(s) in less than 7 wt.% of the total composition, e.g. less than 6 wt.%, 5 wt.%, 4 wt.%, 3 wt.%, 2 wt.%, or less than 1 wt.% of the total composition, say from any one of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0 to any one of 7.0, 6.0, 5.0, 4.0, 3.0, 2.0, 1.0, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, or 0.2 wt.% of the total composition.

Preferably, the nucleating agent comprises sodium chloride, for example, in less than or equal to 6 wt.% of the composition as set out above, e.g. in less than or equal to 3 wt.%, 4 wt.%, or 5 wt.%.

It has been surprisingly found that the nucleating agent or nucleating agents may be sufficiently sparingly soluble in lithium chloride solutions that undissolved nucleating agent remains after the composition is exposed to temperatures of 50°C for considerable periods of time.

Advantageously, the composition may be frozen in an ultra-low temperature freezer, which are widely commercially available. This is because the composition freezes when subjected to a temperature not lower than -90°C, e.g. at -85°C.

In embodiments, the composition may further comprise additives. For example, suitable additives may include or more of a thickening agent, a gelling agent, a colourant, a dye, a biocide, a protective agent to prevent degradation or spoilage, and/or a thermal property modifier such as a thermal conductivity improver, e.g. a graphite-based material.

to In embodiments, the composition may be micro-or macro-encapsulated.

In embodiments, the composition may be incorporated into any suitable adsorbent material, for example, but not limiting to, fumed silica, precipitated silica, activated charcoal or carbon-based adsorbent, expanded or otherwise, vermiculite, kieselgel, diatomaceous earth, or clay.

The composition may be located in a container, for example a flexible walled container (e.g. a flexible plastic pouch) or a rigid walled container (e.g. a rigid plastics container), or a contained with rigid and flexible walls. The composition may be located in the container with an adsorbent material.

A further aspect of the invention provides a method of forming a phase change material, the method comprising forming a composition, the composition comprising an aqueous solution of lithium chloride, e.g. by dissolving lithium chloride in water and/or by diluting a stock solution of aqueous lithium chloride with water, the aqueous solution consisting of lithium chloride in an amount between 30 to 40 wt. % and water in an amount between 70 to 60 wt.%.

A yet further aspect of the invention provides a method of forming a phase change material, the method comprising forming a composition, the composition comprising an aqueous solution of lithium chloride, e.g. by dissolving lithium chloride in water and/or by diluting a stock solution of aqueous lithium chloride with water, the aqueous solution consisting of between 30 wt.% and 40 wt.% lithium chloride and between 70 wt.% and 60 wt.% water.

In this specification between 30 wt.% and 40 wt.% is intended to mean 30 wt.%. [LiC1]4Owt.% and the amount of water is to be correspondingly understood.

Advantageously, the aqueous solution of the composition may be prepared by the direct dissolution of solid lithium chloride in water or, alternatively, may be produced by further diluting commercially available lithium chloride solutions to the desired concentration with water.

The method may further comprise addition of one or more nucleating agents to the composition. The nucleating agents may be added to less than or equal to 6 wt.% of the composition.

The method may further comprise freezing the composition at a temperature lower than -60°C, e.g. lower than or at -65°C, for example, using liquid nitrogen and/or in an ultra-low temperature freezer.

For the avoidance of doubt, any of the features described herein apply equally to any aspect of the invention.

Advantageously, the composition of the invention may be frozen completely in an ultra-low temperature freezer with a cooling temperature of -85°C if the composition further comprises a suitable nucleating agent. The composition melts consistently and reproducibly at a temperature of -65°C during which it absorbs large quantifies of thermal energy. The composition of the invention freezes and melts cleanly over a narrow temperature range and reproducibly over repeated thermocycling. The composition is stable, non-hazardous, and is not subject to any restrictive regulations for transport.

Advantageously, the composition of the invention may be incorporated into a vessel and/or may be incorporated into the packaging of a temperature-sensitive product.

Once sealed within a suitable container, the composition may be re-used many times as a PCM in a temperature-controlled transportation process. In use, once frozen, the composition gradually melts, which maintains the temperature-sensitive materials at a constant temperature. At the end of the transportation process, the composition may then be re-frozen for further use as a PCM.

Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. For the avoidance of doubt, the terms "may", "and/or', "e.g.", "for example" and any similar term as used herein should be interpreted as non-limiting such that any feature so-described need not be present. Indeed, any combination of io optional features is expressly envisaged without departing from the scope of the invention, whether or not these are expressly claimed. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

To further exemplify the invention, reference is also made to the following non-limiting Examples.

Comparative Example 1 A solution of lithium chloride was prepared in a eutectic composition by dissolving anhydrous lithium chloride (24.98g) in potable water (75.02g). A portion of this solution was cooled in liquid nitrogen and did not display any evidence of crystallisation or freezing when cooled to -190°C. Furthermore, when allowed to return to ambient temperature, there was no evidence of absorption of energy in the form of latent heat.

The addition of nucleating agents did not alter this behaviour, i.e. no crystallisation or freezing was observed, and the solution set to glass. It can therefore be assumed that the eutectic composition of lithium chloride and water does not freeze when subjected to cooling to -190°C, and therefore is unsuitable for use as a PCM, for example, which is suitable for low temperature transport applications.

Example 1

A solution of lithium chloride was prepared in a lithium pentahydrate composition by dissolving anhydrous lithium chloride (64.04g) in potable water (135.96g). A portion of the resulting composition was cooled in liquid nitrogen to -190°C and then allowed to gradually return to ambient temperature. The composition froze completely when cooled. The composition melted completely and cleanly at -65°C when allowed to return to ambient temperature.

A thermocouple positioned in the centre of the composition during the process showed that during melting the temperature remained constant for a substantial time, indicating that energy was being absorbed in the form of latent heat.

io This demonstrates that a composition comprising lithium chloride pentahydrate is a useful PCM for use as an alternative to solid carbon dioxide.

A portion of the composition was cooled in an isopropyl alcohol jacketed bottle in liquid nitrogen such that the temperature of the test portion did not fall below -90°C. Once the centre of the composition had reached -90°C, it was allowed to gradually return to ambient temperature. During the cooling process, the composition did not display any evidence of crystallisation or freezing, and when allowed to return to ambient temperature there was no evidence of absorption of energy in the form of latent heat.

Therefore, a composition consisting of lithium chloride pentahydrate is only suitable for use as a PCM when cooled using very low temperatures, e.g. -190°C using liquid nitrogen. If the composition is cooled using temperatures similar to those provided by an ultra-low temperature freezer, e.g. -90°C, then the composition undergoes a supercooling process and does not freeze. Although the composition of Example 1 is useful as a PCM, it would be further advantageous if the composition was able to be frozen in a ultra-low temperature freezer rather than using liquid nitrogen.

Examples 2 to 5

The following calcium/barium salts were added to separate compositions of Example 1 to provide Examples 2 to 5 as follows: i. Example 2: Calcium sulphate (1 wt.%); ii. Example 3: Barium sulphate (1 wt.%); iii. Example 4: Calcium phosphate (1 wt.%); iv. Example 5: Barium sulphide (1 wt.%).

A portion of each of the compositions of Examples 2 to 5 was cooled in an isopropyl alcohol jacketed bottle in liquid nitrogen such that the temperature of each composition did not fall below -90°C. Once the centre of the composition had reached -90°C, it was allowed to gradually return to ambient temperature. Each of the compositions of Examples 2 to 5 froze completely when cooled. Each of the compositions of Examples 2 to 5 melted completely and cleanly at -65°C when allowed to return to ambient temperature.

Example 6

io Sodium chloride (0.3 wt.%) was added to the composition described for Example 1 to

provide Example 6.

A portion of the composition of Example 6 was cooled in an isopropyl alcohol jacketed bottle in liquid nitrogen such that the temperature of the composition did not fall below -90°C.

Once the centre of the composition had reached -90°C, it was allowed to gradually return to ambient temperature. The composition of Example 6 froze completely when cooled. The composition of Example 6 melted completely and cleanly at -65°C when allowed to return to ambient temperature.

Examples 7 to 10

The following Group 1 metal halide salts were added to separate compositions of Example 1 to provide Examples 7 to 10 as follows: i. Example 7: Sodium chloride (3 wt.%); ii. Example 8: Sodium fluoride (3 wt.%); iii. Example 9: Sodium bromide (3 wt.%); iv. Example 10: Potassium bromide (3 wt.%); A portion of each of the compositions of Examples 7 to 10 was cooled in an ultra-low temperature freezer at -85°C. After several hours, the freezer was switched off and the compositions were allowed to warm to ambient temperature. Each of the compositions of Examples 7 to 10 froze completely when cooled. Each of the compositions of Examples 7 to 10 melted completely and cleanly at -65°C when allowed to return to ambient temperature.

Example 11

Ammonium bromide (6 wt.%) was added to the to the composition described for Example 1 to provide Example 11.

A portion of the compositions of Example 11 was cooled in an ultra-low temperature freezer at -85°C. After several hours, the freezer was switched off and the composition was allowed to warm to ambient temperature. The compositions of Example 11 froze completely when cooled. The compositions of Example 11 melted completely and cleanly at -65°C when allowed to return to ambient temperature.

Examples 12 to 15

The following halide salts were added to separate compositions of Example 1 to provide Examples 12 to 15 as follows: i. Example 12: Sodium chloride (6 wt.%); ii. Example 13: Sodium fluoride (6 wt.%); iii. Example 14: Sodium bromide (6 wt.%); iv. Example 15: Potassium bromide (6 wt.%); v. Example 16: Ammonium bromide (6 wt.%).

A portion of each of the compositions of Examples 12 to 16 was heated in a water bath at 50°C for 40 hours. The compositions of Examples 12, 15, and 16 contained substantial portions of undissolved salt after the heating process. In contrast, the sodium fluoride and sodium bromide of Examples 13 and 14 respectively had completely dissolved during and after the heating process.

The resulting compositions of Examples 12 to 16 were cooled in an ultra-low temperature freezer at -85°C and held at this temperature for several hours before being allowed to gradually return to ambient temperature.

The compositions of Examples 12 and 15 froze completely when cooled. The compositions of Examples 12, 15, and 16 melted completely and cleanly at -65°C when allowed to return to ambient temperature.

In contrast, the compositions of Examples 13 and 14 did not display any evidence of freezing when cooled to -85°C. However, when cooled using liquid nitrogen then complete freezing and cooling at -65°C was observed.

Therefore, this demonstrates that the halide salts of Examples 12, 15, and 16 act as nucleating agents. If there is no undissolved salt, e.g. for Examples 13 and 14, then no nucleation is able to take place and, instead, a supercooling process is observed.

Examples 17 to 19

The following salt combinations were added to separate compositions of Example 1 to provide Examples 17 to 19 as follows: i. Example 17: Sodium chloride (3 wt.%) and ammonium bromide (3 wt.%); ii. Example 18: Sodium chloride (3 wt.%) and potassium bromide (3 wt.%); iii. Example 19: Sodium chloride (6 wt.%).

The compositions of Examples 17 to 19 were cooled in an ultra-low temperature freezer at -85°C at the same time. The compositions of Examples 17 to 19 each froze completely when cooled. However, Examples 17 and 18 containing a combination of two different salts in addition to the lithium chloride solution initiated freezing more quickly. Moreover, a lesser amount of supercooling (that is, the temperature change between the phase change temperature and the actual, lower temperature of crystallisation) was observed for Examples 17 and 18 in contrast to Example 19, which contains sodium chloride as the additional salt only.

Therefore, this demonstrates that the combination of two different additional salts of Examples 17 and 18 act synergistically with each other, i.e. the nucleating effect of a mixture of 3 wt.% sodium chloride and 3 wt.% ammonium bromide (Example 17), or a mixture of 3 wt.% sodium chloride and 3 wt.% potassium bromide (Example 18), is greater than that observed for 6 wt.% sodium chloride (Example 19) or 6 wt.% ammonium bromide (Example 16), or 6 wt.°/0 potassium bromide (Example 15) alone.

Example 20

A first composition of lithium chloride was prepared at the composition of lithium chloride pentahydrate by mixing a commercially available 35 w/w% lithium chloride solution (Leverton Clarke Ltd, UK) (91.5g) with potable water (8.5g).

A second composition of lithium chloride was prepared at the composition of lithium chloride pentahydrate by mixing a commercially available 40 w/w% lithium chloride solution (Leverton Clarke Ltd, UK) (80.0g) with potable water (20.0g).

A portion of each composition was then dosed with sodium chloride (3 wt.%) as nucleating agent and cooled to -85°C in the ultra-low temperature freezer. Both solutions froze completely and then melted completely and cleanly at -65°C when allowed to gradually return to ambient temperature.

io This demonstrates that a composition according to the invention may be produced by dilution of commercially available aqueous solutions of lithium chloride. This is advantageous because lithium chloride is a highly hygroscopic material, which, if exposed to the atmosphere, will absorb moisture from the air to the extent that it will dissolve in the absorbed water. Therefore, compositions according to the invention may be obtained in an accurate and simple way using commercially available solutions rather than anhydrous solid lithium chloride.

It will be appreciated by those skilled in the art that several variations to the aforementioned embodiments are envisaged without departing from the scope of the invention.

It will also be appreciated by those skilled in the art that any number of combinations of the aforementioned features and/or those shown in the appended drawings provide clear advantages over the prior art and are therefore within the scope of the invention described herein.

Claims

CLAIMS1 A low temperature phase change material, the phase change material comprising an aqueous solution of lithium chloride, the aqueous solution consisting of lithium chloride in an amount between 30 to 40 wt.% and water in an amount between 70 to 60 wt.%.
2 A low temperature phase change material according to Claim 1, wherein the aqueous solution consists of lithium chloride in an amount between 31 to 33 wt.% io and water in an amount between 67 to 69 wt.%.
3 A low temperature phase change material according to any preceding Claim, wherein the aqueous solution consists of 32 wt.% lithium chloride and 68 wt.% water.
4. A low temperature phase change material according to any preceding Claim, further comprising a nucleating agent.
A low temperature phase change material according to Claim 4, wherein the nucleating agent comprises one or more of an inorganic Group 1 salt, an inorganic Group 2 salt, and/or an inorganic ammonium salt.
6 A low temperature phase change material according to Claim 4 or 5, wherein the cationic species of the nucleating agent is one or more of a lithium ion, a sodium ion, a potassium ion, a rubidium ion, a caesium ion, a beryllium ion, a magnesium ion, a calcium ion, a strontium ion, a barium ion, or ammonium ion.
7. A low temperature phase change material according to any one of Claims 4 to 6, wherein the nucleating agent comprises an inorganic halide salt.
8 A low temperature phase change material according to any one of Claims 4 to 7, wherein the anionic species of the nucleating agent is one or more of a fluoride ion, a chloride ion, a bromide ion, or an iodide ion.
9 A low temperature phase change material according to any one of Claims 4 to 8, wherein the nucleating agent is selected from one or more of an inorganic Group 1 halide salt, an inorganic Group 2 halide salt, and/or an ammonium halide.
10. A low temperature phase change material according to any one of Claims 4 to 6, wherein the anionic species of the nucleating agent is a sulphate ion, a phosphate ion, and/or a sulphide ion.
11.A low temperature phase change material according to Claim 10, wherein the to nucleating agent comprises a Group 2 sulphate salt, a Group 2 phosphate salt, and/or a Group 2 sulphide salt.
12. A low temperature phase change material according to Claims 4 to 11, wherein the nucleating agent is selected from one or more of sodium fluoride, sodium chloride, sodium bromide, potassium bromide, ammonium bromide, calcium sulphate, barium sulphate, calcium phosphate, and/or barium sulphide.
13. A low temperature phase change material according to any of Claims 4 to 12, further comprising a second, different, nucleating agent.
14. A low temperature phase change material according to any one of Claims 4 to 13, wherein the composition comprises the one or more nucleating agent(s) in less than 7 wt.% of the total composition, e.g. less than 6 wt.%, 5 wt.%, 4 wt.%, 3 wt.%, 2 wt.%, or less than 1 wt.% of the total composition.
A low temperature phase change material according to Claim 14, wherein the composition comprises one or more nucleating agent(s) in an amount between any one of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 or 7.0 wt.% to any one of 7.0, 6.0, 5.0, 4.0, 3.0, 2.0, 1.0, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, or 0.2 wt.% of the total composition.
16. A low temperature phase change material according to any preceding Claim, further comprising additives including one or more of a thickening agent, a gelling agent, a colourant, a dye, a biocide, a protective agent to prevent degradation or spoilage, and/or a thermal property modifier such as a thermal conductivity improver, e.g. a graphite-based material.
17. A low temperature phase change material according to any preceding Claim, wherein the composition freezes and melts at less than -40°C and more than -80°C, for example, between -60°C to -70°C, e.g. -65°C.
18. A low temperature phase change material according to any preceding Claim, wherein the composition is micro-or macro-encapsulated.
19. A low temperature phase change material according to any preceding Claim, wherein the composition is incorporated into an adsorbent material, e.g. fumed silica, precipitated silica, activated charcoal or carbon-based adsorbent, expanded or otherwise, vermiculite, kieselgel, diatomaceous earth, or clay.
A method of forming a phase change material, the method comprising forming a composition, the composition comprising an aqueous solution of lithium chloride, e.g. by dissolving lithium chloride in water and/or by diluting a stock solution of aqueous lithium chloride with water, the aqueous solution consisting of between 30 wt.% and 40 wt.% lithium chloride and between 70 wt.% and 60 wt.% water.
21. A method according to Claim 20, further comprising addition of one or more nucleating agents to the composition.
22. A method according to Claim 20 or 21, further comprising freezing the composition at a temperature lower than -60°C, e.g. lower than or at -65°C, for example, using liquid nitrogen and/or in an ultra-low temperature freezer.
23. Use of a composition as a low temperature phase change material, the composition comprising an aqueous solution of lithium chloride, the aqueous solution of lithium chloride consisting of lithium chloride in an amount between 30 to 40 wt.% and water in an amount between 70 to 60 wt.%.
24. Use according to Claim 23, wherein the composition further comprises one or more nucleating agents, said nucleating agents preferably comprising less than 7 wt% of the composition.
25. Use according to Claim 23 or 24, wherein the composition further comprises one or more of a thickening agent, a gelling agent, a colourant, a dye, a biocide, a protective agent to prevent degradation or spoilage, and/or a thermal property modifier such as a thermal conductivity improver, e.g. a graphite-based material.