GB2600510A - Phase change material - Google Patents

Abstract

A phase change material composition comprising an aqueous solution of lithium bromide (LiBr), the LiBr is in amount greater than or equal to 38 wt% and the water in an amount between 0-62 wt%. The PCM further comprises a nucleating agent (e.g. inorganic group 1 or 2 salts, halide salts, hydroxide ion, cholesterol). The composition may contain additives such as thickening agents, biocides, graphite as a thermal conductivity improver. The composition may be incorporated into an adsorbent material (e.g. silica, activated charcoal, vermiculite). Preferably the composition freezes at more than -86°C, and melts at less than -40°C making it suitable for use as an aqueous salt-based eutectic PCM solution in ultra-low temperature thermal storage applications. Forming the latent heat storage PCM comprises dissolving lithium bromide in water or diluting a stock solution of aqueous lithium bromide.

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 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 are concerns with its widespread use in certain forms of transport, for example, aircraft, due to the asphyxiating properties of the gaseous carbon dioxide evolved during sublimation. Finally, there may be issues of practically and availability 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, and in recent years there have io been global shortages in all forms of carbon dioxide due to excess demand and production outages To be a viable alternative to dry ice for low temperature controlled transport, a PCM solution should ideally have several important properties. Firstly, it should not be classified as hazardous for transport, or have any usage restrictions. Furthermore, it should freeze and melt at a clearly defined temperature, with the absorption or release of a large amount of energy in the form of latent heat. It should be able to do this consistently and reproducibly many times. Finally, it should freeze completely and reproducibly when cooled in current commercially available refrigerators or freezers.

For low temperature applications, typically below -40 °C, there are two current ranges of Ultra Low Temperature (ULT) freezers. Firstly, there are what is known as medical ULT freezers, and these typically operate to temperatures as low as -86 °C. Secondly, there are standard commercial ULT freezers, whose minimum operating temperature is typically -80 °C Several materials have been proposed for use as PCMs with phase change temperatures 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 io 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.

Japanese Patent 6623347 discloses compositions based on aqueous solutions of lithium chloride. Lithium chloride solutions have a tendency to severely supercool, to the extent that instead of crystallising at the predicted phase change temperature, they grow increasingly viscous when cooled and ultimately vitrify without freezing. JP6623347B2 claims to overcome this tendency by the addition of several nucleating agents, either singly or in admixture, to promote crystallisation. However, the required addition rate of the nucleating agents claimed is between 7 and 22 weight To, which noticeably diminishes the latent heat capacity of the composition. Furthermore, when examples of the disclosed compositions were prepared and cooled, these compositions cooled to below -80 °C before crystallisation was initiated, and freezing was extremely slow due to the excessive viscosity of the compositions at the crystallisation temperature. Because these compositions supercooled below -80 °C it is unlikely that they would be suitable for use with standard commercial ULT freezers.

UK Patent application GB2587070 discloses other lithium chloride based compositions, primarily using a lithium chloride/water ratio stoichiometrically consistent with lithium chloride pentahydrate. By the addition of nucleating agents, the supercooling tendency of the solution was suppressed somewhat. However, in practice, these compositions require cooling below -80 °C before crystallisation was initiated.

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 to 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 below -40°C, the PCM being economical, reusable, non-hazardous, and not subject to any transport regulations.

It would be advantageous for a PCM to be capable of freezing when exposed to temperatures of -80°C, thereby allowing the use of commercial (rather than medical grade) ULT freezers.

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 bromide, the aqueous solution comprising or consisting of lithium bromide in an amount that is greater than or equal to 38 wt.% and water in an amount that is greater than 0 wt.% and less than or equal to 62 wt.% of the aqueous solution, the composition preferably further comprising a nucleating agent, e.g. in an amount configured to freeze the phase change material at a defined temperature and to release latent heat when thawed.

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. -67°C.

In embodiments, the composition may comprise an aqueous solution of lithium bromide, wherein the lithium bromide is present in an amount greater than or equal to 38 wt.% and less than or equal to 55 wt.%, and water in an amount greater than or equal to 45 wt.% and less than or equal to 62 wt.% e.g. lithium bromide in an amount greater than or equal to 38 wt.% and less than or equal to 50 wt%, and water in an amount greater than or equal to 50 wt.% and less than or equal to 62 wt.% of the aqueous solution.

In embodiments, the lithium bromide in the aqueous solution may be present in an amount between any one of 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54 wt.% to any one of 55, 54, 53, 52, 51, 50, 49, 48, 47, 46, 45, 44, 43, 42, 41, 40, 39 wt.%. In embodiments, the water in the aqueous solution may be present in an amount between any one of 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61 wt.% to any one of to 62,61, 60, 59, 58, 57, 56, 55, 54, 53,52, 51, 50, 49, 48, 47, 46 wt.% of the aqueous solution.

In embodiments, the aqueous solution may consist of lithium bromide in water, wherein the lithium bromide is present in greater than or equal to 38 wt.% in water, e.g. greater than or equal to 39 wt.%, greater than or equal to 40 wt.%, greater than or equal to 41 wt.%, greater than or equal to 42 wt.%, greater than or equal to 43 wt.%, greater than or equal to 44 wt.%, greater than or equal to 45 wt.%, greater than or equal to 46 wt.%, greater than or equal to 47 wt.%, greater than or equal to 48 wt.%, greater than or equal to 49 wt.%, greater than or equal to 50 wt.%, greater than or equal to 51 wt.%, greater than or equal to 52 wt.%, greater than or equal to 53 wt.%, greater than or equal to 54 wt.%.

In embodiments, the aqueous solution may consist of lithium bromide in water, wherein the lithium bromide is present in less than or equal to 55 wt.% in water, e.g. less than or equal to 54 wt.%, less than or equal to 53 wt.%, less than or equal to 52 wt.%, less than or equal to 51 wt.%, less than or equal to 50 wt.%, less than or equal to 49 wt.%, less than or equal to 48 wt.%, less than or equal to 47 wt.%, less than or equal to 46 wt.%, less than or equal to 45 wt.%, less than or equal to 44 wt.%, less than or equal to 43 wt.%, less than or equal to 42 wt.%, less than or equal to 41 wt.%, less than or equal to 40 wt.%, less than or equal to 39 wt.%.

In embodiments, the aqueous solution may consist of lithium bromide in water, wherein the water is present in greater than or equal to 45 wt.%, e.g. greater than or equal to 46 wt.%, greater than or equal to 47 wt.%, greater than or equal to 48 wt.%, greater than or equal to 49 wt.%, greater than or equal to 50 wt.%, greater than or equal to 51 wt.%, greater than or equal to 52 wt.%, greater than or equal to 53 wt.%, greater than or equal to 54 wt.%, greater than or equal to 55 wt.%, greater than or equal to 56 wt.%, greater than or equal to 57 wt.%, greater than or equal to 58 wt.%, greater than or equal to 59 wt.%, greater than or equal to 60 wt.%, greater than or equal to 61 wt.%.

In embodiments, the aqueous solution may consist of lithium bromide in water, wherein the water is present in less than or equal to 62 wt.%, e.g. less than or equal to 61 wt.%, less than or equal to 60 wt.%, less than or equal to 59 wt.%, less than or equal to 58 wt.%, less than or equal to 57 wt.%, less than or equal to 56 wt.%, less than or equal to 55 wt.%, less than or equal to 54 wt.%, less than or equal to 53 wt.%, less than or equal to 52 wt.%, less than or equal to 51 wt.%, less than or equal to 50 wt.%, less than or equal to 49 wt.%, less to than or equal to 48 wt.%, less than or equal to 47 wt.%, less than or equal to 46 wt.%.

It is known that lithium bromide forms several hydrates with water, for example the mono-, di-, tri-, and penta-hydrates. It has been variously reported that lithium bromide has a eutectic composition of between 39.07 and 39.4 wt.% lithium bromide to water with a eutectic temperature of between -67.5 and -73 °C, and that a peritecfic composition corresponding to that of the pentahydrate has a composition of between 47.3 and 49.1 wt.% lithium bromide to water with a freezing temperature of -47 and -53 °C (Fluid Phase Equilibria; 250 (2006), 138-149). We are unaware of any proposal to use lithium bromide as a low temperature PCM, even though the reported eutectic and pentahydrate peritecfic temperatures of lithium bromide may suggest its use as an alternative to solid carbon dioxide. In its support, lithium bromide is not classified as hazardous, it is non-toxic, noncorrosive, 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 below -90 °C, and does not release or absorb any significant energy in the form of latent heat of fusion at any temperature above -90°C. Therefore, in an unmodified state, the eutectic composition of lithium bromide is not considered to exhibit the required properties for use as a PCM in low temperature transport applications, Le. below -40°C.

In embodiments, the aqueous solution of lithium bromide may be a eutectic composition, i.e. between 39.07 and 39.4 wt. % lithium bromide in water. In embodiments, the aqueous solution of lithium bromide may be a peritecfic composition, i.e. between 47.3 and 49.1 wt.% lithium bromide in water.

A further 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 a eutectic (between 39.07 and 39.4 wt.% lithium bromide in water) or peritectic pentahydrate (between 47.3 and 49.1 wt.% lithium bromide in water) aqueous solution of lithium bromide, the composition further comprising a nucleating agent e.g. in an amount configured to freeze the phase change material at a defined temperature and to release latent heat when thawed.

The following statements apply to any aspect of the invention.

In embodiments, the composition may further comprise a eutectic or peritectic (e.g. pentahydrate) aqueous solution of a further lithium halide, e.g. lithium chloride.

A eutectic solution of lithium chloride is defined as a solution of 23.90 to 25.33 wt.%, e.g. 24.98 wt.% (equivalent to 7.86 mol/kg H20) at the eutectic temperature, anhydrous lithium chloride in water (J. Patek, J. Klomfar / Fluid Phase Equilibria 250 (2006) 138-149; Monnin et. al, J Chem Eng Data 2002, 47, 1331-1336).

A peritectic pentahydrate solution of lithium chloride is defined as a solution of between 32 to 33 wt.%, e.g. 32.05 wt.%, anhydrous lithium chloride in water (Monnin et. al, J Chem Eng Data 2002, 47, 1331-1336).

In embodiments, the composition may comprise greater than or equal to 50 wt.% of the aqueous solution of lithium bromide, e.g. greater than or equal to any one of 55, 60, 65, 70, 75, 80, 85, 90, 95, 96, 97, 98, or 99 wt.% aqueous solution lithium bromide in the composition. For example, the composition may consist of an aqueous solution and a nucleating agent, wherein the aqueous solution comprises an aqueous solution of lithium bromide (comprising lithium bromide in an amount that is greater than or equal to 38 wt.% and water in an amount that is greater than 0 wt.% and less than or equal to 62 wt.%) and a further aqueous solution of lithium halide (e.g. eutectic or peritectic aqueous solution of lithium chloride), wherein the aqueous solution comprises greater than or equal to 50 wt.% of the aqueous solution of lithium bromide, and less than or equal to 50 wt.% of the further aqueous solution of lithium halide (e.g. eutectic or peritectic aqueous solution of lithium chloride).

In embodiments, the ratio (x:y) of the volume of the aqueous solution of lithium bromide (x) to the volume of the aqueous solution of the eutectic or peritectic further lithium halide (e.g. lithium chloride) (y) may be present in a ratio (x:y) of between 0.1:99.9 to 99.9:0.1.

In embodiments, the ratio of the volume of the aqueous solution of lithium bromide (x) to the volume of the aqueous solution of the eutectic or peritectic further lithium halide (y) may be present in a ratio (x:y) of any one of 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, or 9:1 e.g. from 1:9 to 9:1, or from 2:8 to 9:1, or 3:7 to 9:1, or 4:6 to 9:1, or 5:5 to 9:1, or 6:4 to 9:1, or 7:3 to 9:1, or 8:2 to 9:1. For example, the ratio (x:y) may be present between any one of 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, or 8:2 to any one of 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, or 2:8.

In embodiments, the composition comprises a eutectic aqueous solution of lithium bromide and a eutectic aqueous solution of lithium chloride, and the ratio (x:y) between the eutectic aqueous solution of lithium bromide (x) and the eutectic aqueous solution of lithium chloride (y) may be between 9:1 to 5:5.

It might be postulated that compositions with greater than 50% LiCI eutectic should, in theory, still produce materials with useful low freezing temperatures, and acceptable latent heats. However, it has been surprisingly found that those materials supercool to such an extent that, even with any of the nucleating agents described herein, they do not freeze in a ULT freezer and are therefore not suitable as PCM components.

Accordingly, the aqueous solution may comprise 50 wt.% or less eutectic aqueous solution of lithium chloride.

It has been surprisingly found that both the eutectic and pentahydrate peritectic lithium bromide compositions may be mixed with other aqueous lithium halide solutions in such proportions that the resulting mixtures have freezing and melting temperatures which are different to those of the component compositions. Furthermore, unlike many other mixtures of aqueous salt solutions, the mixtures retain clean, precise freezing and melting temperatures, and latent heats of fusion similar to the individual component solutions across the range of mixture ratios. Thus, many other compositions suitable for use as a PCM in low temperature transport applications are achieved. These lithium bromide/other lithium halide mixtures display properties similar to those exhibited by the aforementioned lithium bromide compositions, that make them suitable as low temperature PCMs. They freeze and melt consistently and reproducibly at a fixed temperature and release or absorb large amounts of energy in the form of latent heat in the process. Suitable lithium halide solutions that display these properties include lithium chloride The lithium chloride aqueous solutions may be provided as the eutectic composition, or as any of the hydrate compositions.

In embodiments, the composition further comprises a nucleating agent.

It has been surprisingly found that the addition of a nucleating agent enables the io compositions of the invention to crystallise and melt cleanly 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 bromide composition, or an aqueous solution comprising lithium bromide and another lithium halide, to promote effective and consistent crystallisation over repeated thermocycling, thus ensuring the composition freezes completely 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 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 an 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 hydroxide ion. 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.

Many of these compounds are freely soluble in water. However, if added to the aqueous solution consisting of lithium bromide and water, or lithium bromide, another lithium halide 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. Nonetheless, some Group 1 halides and Group 1 hydroxides were found to be still significantly soluble in such lithium bromide or an aqueous composition containing eutectic or peritectic lithium bromide and an aqueous solution of another lithium halide, to the extent io that more than 10% of the Group 1 halide needed to be added in order that some insoluble material remained to act as a nucleating agent. This not only reduces the latent heat of fusion of the overall composition, but alters the freezing and melting properties, such that the composition does not freeze or melt as cleanly, and as at such a precise temperature as without the nucleating agent. For this reason, several Group 1 halides and Group 1 hydroxides are less preferred for commercial deployment, despite their nucleating ability.

Furthermore, the nucleating agent may be a sterol, e.g. cholesterol, stigmasterol and/or diosgenin.

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

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, in some embodiments, 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 action of singular nucleating agents under the same conditions.

In embodiments, the composition may comprise one or more nucleating agent(s) in less than or equal to 10 wt.% of the total composition, e.g. less than or equal to 9 wt.%, 8 wt.%, 7 wt.%, 6 wt.%, 5 wt.%, 4 wt.%, 3 wt.%, 2 wt %, 1 wt %, 0.5 wt.%, 0.1 wt %, 0.05 wt.%, or less than or equal to 0.01 wt.% of the total composition. In embodiments, the composition may comprise one or more nucleating agent(s) in an amount between any one of 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 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, 0.2, 0.1, 0.09, 0.08, 0.07, 0.06, 0.05, 0.04, 0.03, 0.02, or 0.01 wt.% of the total composition.

Preferably, the nucleating agent comprises or consists of one or more of the following: 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 5 wt.%, or 4 wt.%, or 3 wt.%.

Sodium bromide, for example, in less than or equal to 9 wt.% of the composition as set out above, e.g. in less than or equal to 8 wt.%, or 7 wt.%, 6 wt.%, or 5 wt.%.

Sodium fluoride, for example, in less than or equal to 1.0 wt.% of the composition as set out above, e.g. in less than or equal to 0.8 wt.%, or less than or equal to 0.5 wt.%, or less than or equal to 0.1 wt.%. In embodiments, sodium fluoride is present in greater than or equal to 0.01 wt.%, e.g. greater than or equal to 0.02, 0.03, 0.04, or 0.05 wt.%. In embodiments, sodium fluoride is present in greater than or equal to 0.01 wt.% and less than or equal to 0.05 wt.%, e.g. greater than or equal to 0.02 wt.% and less than or equal to 0.05 wt.%. In embodiments, the compositions suitable for use as a phase change material comprises an aqueous solution of lithium bromide, the aqueous solution consisting comprising of lithium bromide in an amount greater than or equal to 38 wt.% and in less than or equal to 55 wt.% (e.g. greater than or equal to 38 wt.% and less than or equal to 40 wt.%), and water in an amount greater than or equal to 45 wt.% and less than or equal to 62 wt.% (e.g. greater than or equal to 60 wt.% and less than or equal to 62 wt.%), the composition further comprising a nucleating agent comprising sodium fluoride in an amount that is greater than or equal to 0.02 wt.% and less than or equal to 0.05 wt.%.

Lithium hydroxide monohydrate, for example, in less than or equal to 1.0 wt.% of the composition as set out above, e.g. in less than or equal to 0.8 wt.%, or 0.5 wt.%, 01 0.1 wt.%. In embodiments, lithium hydroxide monohydrate is present in greater than or equal to 0.05 wt.%, e.g. greater than or equal to 0.06, 0.07, 0.08, 0.09, or 0.1 wt.%. In embodiments, lithium hydroxide is present in greater than or equal to 0.05 wt.% and less than or equal to 0.1 wt.%.

Cholesterol, for example, in less than or equal to 0.1 wt.% of the composition as set out above, e.g. in less than or equal to 0.05 wt.%, or less than or equal to 0.02 wt.%, or less than or equal to 0.01 wt.%. In embodiments, cholesterol is present in greater than or equal to 0.005 wt.%, e.g. greater than or equal to 0.006, 0.007, 0.008, 0.009, 0.010 wt.%. In embodiments, cholesterol is present in greater than or equal to 0.005 wt.% and less than or equal to 0.01 wt.%.

Potassium chloride, for example, in less than or equal to 6 wt.%, or 5 wt.%, or 4 wt.%, or 3 wt.%.

Potassium bromide, for example, in less than or equal to 6 wt.%, or 5 wt.%, or 4 wt.%, or 3 15 wt.%.

Rubidium chloride, for example, in less than or equal to 8 wt.%, or 7 wt.%, or 6 wt.%, or 5 wt.%, or 4 wt.%, or 3 wt.%.

Ammonium fluoride, for example, in less than or equal to 6 wt.%, or 5 wt.%, or 4 wt.%, or 3 wt.%.

Ammonium bromide, for example, in less than or equal to 6 wt.%, or 5 wt.%, or 4 wt.%, or 3 wt. c/0 Calcium sulphate, for example, in less than or equal to 3 wt.%, or 2 wt.%, or 1 wt.%.

Barium sulphate, for example, in less than or equal to 3 wt.%, or 2 wt.%, or 1 wt.%.

Calcium phosphate, for example, in less than or equal to 3 wt.%, or 2 wt.%, or 1 wt.%.

Barium sulphide, for example, in less than or equal to 3 wt.%, or 2 wt.%, or 1 wt.%.

Any of the compositions may include any of the above nucleating agents, for example in the amounts set out above.

It has been surprisingly found that the nucleating agent or nucleating agents may be sufficiently sparingly soluble in lithium bromide solutions and lithium bromide solutions comprising another lithium halide according to the invention that undissolved nucleating agent remains after the composition is exposed to temperatures of 50°C for considerable periods of time. This is advantageous in that the PCM compositions may be stored and utilised in areas of high ambient temperature, without need for permanent refrigerated storage prior to freezing.

Advantageously, the compositions may be frozen in an ultra-low temperature freezer, which are widely commercially available. This is because the compositions freeze when subjected to a temperature not lower than -90°C, e.g. at -85°C, which is within the typical operating temperature specifications of a medical ULT freezer. Furthermore, the compositions freeze when subjected to a temperature not lower than -85°C, e.g. at -80°C which is within the typical operating temperature specifications of a standard ULT freezer. Furthermore, the compositions freeze when subjected to a temperature not lower than -80°C, e.g. at -75°C which is within the typical operating temperature fluctuation of some standard ULT freezers, i.e. the temperature inside the freezer may be +/-5°C of the programmed or specified temperature. Accordingly, the compositions may be freezeable at (i.e. have a freezing point at or lower than) -90, -85, -84, -83, -82, -81 or -80°C, for example at -79, -78, -77, -76, - 75°C, say from -84 to -75°C.

In embodiments, the compositions 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.

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

In embodiments, the compositions 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 compositions 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 compositions 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 bromide, e.g. by dissolving lithium bromide in water and/or by diluting a stock solution of aqueous lithium bromide with water, the aqueous solution comprising io lithium bromide in an amount that is greater than or equal to 38 wt.% and water in an amount that is greater than 0 wt.% and less than 62 wt.% of the aqueous solution, the aqueous solution preferably further comprising a nucleating agent, e.g. in an amount configured to freeze the phase change material at a defined temperature and to release latent heat when thawed.

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

The nucleating agents may be added to less than or equal to 10 wt.% of the composition, e.g. less than or equal to 9 wt.%, 8 wt.%, 7 wt.%, 6 wt.%, 5 wt.%, 4 wt.%, 3 wt.%, 2 wt.%, or 1 wt.%.

The method may further comprise freezing the composition at a temperature lower than - 60°C, e.g. lower than or at -67°C, for example, using liquid nitrogen and/or in a standard 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 -86°C or -80°C if the composition further comprises a suitable nucleating agent. The composition melts consistently and reproducibly at a temperature of between -48°C and -75°C, depending on the composition, during which it absorbs large quantities 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 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 10.0g samples of solutions of lithium bromide in water were prepared to the eutectic compositions reported historically in literature. There were five, broadly similar, compositions which varied slightly in composition, with the lowest lithium bromide eutectic composition being 39.07 wt.%, and the highest being 39.4 wt.% (Fluid Phase Equilibria; 250, (2006), p.139, Table 1).

These solutions were placed in a medical ULT freezer at -86°C and left overnight. None of these solutions displayed any evidence of crystallisation or freezing when cooled to -86°C.

Furthermore, when allowed to return to ambient temperature, there was no evidence of absorption of energy in the form of latent heat.

Comparative Example 2 10.0g samples of solutions of lithium bromide in water were prepared to the peritectic compositions, corresponding to the pentahydrate, reported historically in literature. There were five, broadly similar, compositions which varied slightly in composition, with the lowest lithium bromide pentahydrate peritectic composition being 47.7 wt.%, and the highest being 49.1 wt.% (Fluid Phase Equilibria; 250, (2006), p.139, Table 1).

These solutions were placed in a medical ULT freezer at -86°C and left overnight. None of these solutions displayed any evidence of crystallisation or freezing when cooled to -86°C. Furthermore, when allowed to return to ambient temperature, there was no evidence of absorption of energy in the form of latent heat.

Comparative Example 3 A series of 10.0g samples of solutions of lithium chloride in water, as described in Japanese Patent 6623347, and incorporating the additives disclosed as nucleating agents at the concentrations described, were prepared and were placed in a ULT, set at -80°C, and allowed to stand for 48 hours at this temperature. None of these solutions displayed any evidence of crystallisation or freezing when cooled to -80°C. Furthermore, when allowed to return to ambient temperature, there was no evidence of absorption of energy in the form of latent heat.

It can therefore be assumed that the eutectic composition of lithium chloride and water, even if treated with the materials disclosed in said patent, and at the concentrations disclosed, does not freeze when subjected to cooling to -80°C, and therefore is unsuitable for use as a PCM if the means of freezing the PCM is a standard ULT freezer, capable of a minimum operating temperature of -80°C.

Comparative Example 4 Three separate 10.0g samples of a solution of lithium chloride in water, corresponding stoichiometrically to lithium chloride pentahydrate, as described in UK Patent application GB2587070, and incorporating sodium chloride as the nucleating agent at the concentrations described, were prepared and were placed in a ULT, set at -80°C, and allowed to stand for 48 hours at this temperature. One of these solutions partially froze during this time, whereas neither of the other two samples displayed any evidence of crystallisation or freezing when cooled to -80°C. Furthermore, when allowed to return to ambient temperature, whilst the sample which displayed evidence of partial freezing showed some evidence of the absorption of energy at -65°C, there was no such evidence from the other two samples.

It can therefore be assumed that such a composition of lithium chloride and water, even if treated with the materials disclosed in said patent, and at the concentrations disclosed, does not freeze reproducibly when subjected to cooling to -80°C, and therefore is unsuitable for use as a PCM if the means of freezing the PCM is a standard ULT freezer, capable of a minimum operating temperature of -80°C.

Example 1

10.0g samples of lithium bromide were prepared, according to the concentrations described in Comparative Example 1, and 5 wt.% sodium bromide was added to each as a nucleating agent. The samples were placed in a ULT freezer at -80°C and left to stand overnight at this temperature. All the samples completely froze, and data from thermocouples inserted into the samples showed that they all froze at -72 to -73°C, with no more than 2°C of supercooling. Furthermore, when allowed to return to ambient, they all showed evidence of absorption of thermal energy at a temperature of -67°C.

It can therefore be demonstrated that, regardless of slight variations in the eutectic composition, the eutectic composition of lithium bromide and water, when treated with sodium bromide at an addition rate of 5 wt.% freezes reproducibly when subjected to cooling to -80°C, and therefore is suitable for use as a PCM if the means of freezing the PCM is a standard ULT freezer, capable of a minimum operating temperature of -80°C.

The latent heat absorbed during the melting process of these samples was measured by Differential Scanning Calorimetry (Netzsch DSC 404 Fl, -150 to 0°C, 10°C/min), and was found to be 210 +/-10 kJ/kg. This, combined with the high density of lithium bromide solutions, makes this eutectic composition a very attractive candidate for use in low temperature transport applications, and as a potential alternative to solid carbon dioxide.

Example 2

Example 1 demonstrated that the slight differences in the eutectic composition of lithium bromide in water reported in literature had negligible effect on the thermophysical properties. Therefore a composition of 39.1 wt.% lithium bromide and 60.9 wt.% water was used for all subsequent examples. A stock solution was made by dissolving 117.3g anhydrous lithium bromide 99+% (Alfa Aesar, UK) in 182.7g potable water, and was replenished whenever necessary.

8 wt.% sodium bromide was added to a portion of 39.1 wt.% lithium bromide in water, and was cooled in a ULT freezer at -80°C. The composition froze completely and, when allowed to return to ambient, melted cleanly and completely at -67°C. The sample was placed in an environmental chamber set at 50°C for one week, after which time there was still undissolved material in the sample. The freezing and melting process was then repeated a further 49 times, each time the sample froze completely and melted cleanly at -67°C.

Example 3

wt.% sodium chloride was added to a portion of 39.1 wt.% lithium bromide in water, and was cooled in a ULT freezer at -80°C. The composition froze completely and, when allowed to return to ambient, melted cleanly and completely at -67°C. The sample was placed in an environmental chamber set at 50°C for one week, after which time there was still undissolved material in the sample. The freezing and melting process was then repeated and the sample froze completely and melted cleanly at -67°C.

A DSC scan was performed on a portion of the sample after the 15th freeze/melt cycle, and the latent heat absorbed during the melting process was 220 kJ/kg, and the onset temperature was -66.1°C.

Examples 2 and 3 show that the lithium bromide eutectic can be modified by the addition of nucleating agents so that it freezes and melts reproducibly and reliably, with minimal supercooling, and at an ambient temperature of no lower than -80°C.

Examples 4 to 9

The following halide salts were added to separate compositions of stock lithium bromide solution according to Example 2 to provide Examples 4 to 9 as follows. The salts were selected due to their solubilities being no greater than that of sodium bromide.

i. Example 4: Potassium chloride (5 wt.%); ii. Example 5: Potassium bromide (5 wt.%); iii. Example 6: Rubidium chloride (8 wt.%).

iv. Example 7: Ammonium fluoride (5 wt.%) v. Example 8: Ammonium bromide (5 wt.% vi. Example 9: Sodium fluoride (2 wt.%) A portion of each of the compositions of Examples 4 to 9 was cooled in a ULT freezer at 80°C. All the compositions of Examples 4 to 9 froze completely and melted cleanly and completely at -67°C when allowed to return to ambient.

Example 10

0.1% wt.% cholesterol was added to a portion of stock lithium bromide solution according to Example 2 to provide Example 10, and was cooled in a ULT freezer at -80°C. The composition froze completely and, when allowed to return to ambient, melted cleanly and completely at -67°C.

Example 11

Sodium fluoride was added to separate portions of stock lithium bromide solution according to Example 2 in decreasing quantifies to determine the minimum quantity required to achieve nucleation. It is important to use as little sodium fluoride as possible due to its toxicity. Addition rates of sodium fluoride of 0.5 wt.%, 0.1 wt.%, 0.05 wt.%, and 0.01 wt.% were used, and the compositions were cooled in a ULT freezer at -80°C. All compositions except that with 0.01% wt.% sodium fluoride froze completely, demonstrating a minimum addition rate of between 0.01 wt.% and 0.05 wt.%.

Example 12

Lithium hydroxide monohydrate was added to separate portions of stock lithium bromide solution according to Example 2 in decreasing quantities to determine the minimum quantity required to achieve nucleation. It is important to use as little lithium hydroxide monohydrate as possible due to its toxicity and corrosivity. Addition rates of 1.0 wt.%, 0.5 wt.%, 0.1 wt.%, and 0.05 wt.% were used, and the compositions were cooled in a ULT freezer at -80°C. All compositions except that with 0.05% wt.% lithium hydroxide monohydrate froze completely, demonstrating a minimum addition rate of between 0.05 wt.% and 0.1 wt.%.

to Example 13

Cholesterol was added to separate portions of stock lithium bromide solution according to Example 2 in decreasing quantifies to determine the minimum quantity required to achieve nucleation. It is important to use as little cholesterol as possible due to its high cost. Addition rates of cholesterol of 0.05 wt.%, 0.025 wt.%, 0.01 wt.%, and 0.005 wt.% were used, and the compositions were cooled in a ULT freezer at -80°C. All compositions except that with 0.005% wt.% cholesterol froze completely, demonstrating a minimum addition rate of between 0.005 wt.% and 0.01 wt.%.

Example 14

A portion of the composition of Example 3 was placed in a ULT freezer set at -75°C. The composition froze completely and, when allowed to return to ambient, melted cleanly and completely at -67°C. This demonstrates that the composition will freeze in a standard ULT freezer with a setpoint accuracy of +/-5°C, which is the tolerance included in the specification of some such commercially available instruments.

Example 15 to 18

The following calcium/barium salts were added to separate portions of stock lithium bromide solution according to Example 2 to provide Examples 18 to 21 as follows: i. Example 15: Calcium sulphate (1 wt.%); ii. Example 16: Barium sulphate (1 wt.%); iii. Example 17: Calcium phosphate (1 wt.%); iv. Example 18: Barium sulphide (1 wt.%).

A portion of each of the compositions of Examples 15 to 18 was cooled in a ULT freezer set at -80°C. Each of the compositions of Examples 15 to 18 froze completely when cooled and, when allowed to return to ambient, melted cleanly and completely at -67°C.

Example 19

10.0g samples of lithium bromide in water were prepared, according to the concentrations described in Comparative Example 2, and 5 wt.% sodium bromide was added to each as a nucleating agent. The samples were placed in a ULT freezer at -80°C and left to stand overnight at this temperature. All the samples completely froze, and data from to thermocouples inserted into the samples showed that they all froze at -50 to -52°C, with no more than 12°C of supercooling. Furthermore, when allowed to return to ambient, they all showed evidence of absorption of thermal energy at a temperature of -48°C.

It can therefore be demonstrated that, regardless of slight variations in the peritectic composition, the peritectic composition of lithium bromide and water, when treated with sodium bromide at an addition rate of 5 wt.% freezes reproducibly when subjected to cooling to -80°C, and therefore is suitable for use as a PCM if the means of freezing the PCM is a standard ULT freezer, capable of a minimum operating temperature of -80°C.

The latent heat absorbed during the melting process of these samples was measured using Differential Scanning Calorimetry, and was found to be 200 +/-10 kJ/kg. This, combined with the high density of lithium bromide solutions, makes this composition a very attractive candidate for use in low temperature transport applications, and as a potential alternative to solid carbon dioxide.

Example 20 demonstrated that the slight differences in the pentahydrate peritectic composition of lithium bromide in water reported in literature had negligible effect on the thermophysical properties. Therefore a composition of 49.1 wt.% lithium bromide and 50.9 wt.% water was used for all subsequent examples. A stock solution was made by dissolving 147.3g anhydrous lithium bromide 99+% (Alfa Aesar, UK) in 152.7g potable water, and was replenished whenever necessary.

Examples 21 to 23

The following materials were added to separate compositions of stock lithium bromide solution according to Example 22 to provide Examples 23 to 25 as follows.

i. Example 21: Sodium chloride (5 wt.%); ii. Example 22: Sodium fluoride (0.5 wt.%); iii. Example 23: Cholesterol (0.05 wt.%); A portion of each of the compositions of Examples 21 to 23 was cooled in a ULT freezer at- 70°C. The compositions of Examples 21 and 22 froze completely. The composition of Example 23 did not freeze unless the temperature of the ULT was lowered to -80°C. All examples melted cleanly and completely at -48°C when allowed to return to ambient.

io Example 24

A stock solution of lithium chloride in water, comprising the eutectic composition of 24.98 wt.%, was prepared by dissolving 74.94g anhydrous lithium chloride 99+% (Alfa Aesar, UK) in 225.06g potable water. This was used in this example, and subsequent examples where lithium chloride eutectic solution was required, and was replenished whenever necessary.

Portions of this lithium chloride eutectic stock solution were mixed with portions of the lithium bromide eutectic stock solution described in Example 2. The ratios of these mixtures ranged from 90 wt.% lithium bromide eutectic solution and 10 wt.% lithium chloride eutectic solution, to 10 wt.% lithium bromide eutectic solution and 90 wt.% lithium chloride eutectic solution. A 5 wt.% addition of sodium bromide nucleating agent was added to portions of each mixture and the mixtures were placed in a ULT freezer at -80°C. The mixtures comprising 90 wt.% lithium bromide eutectic solution and 10 wt.% lithium chloride eutectic solution, to 70 wt.% lithium bromide eutectic solution and 30 wt.% lithium chloride eutectic solution froze completely at -80°C, whereas the others did not. The temperature of the freezer was reduced to -86°C, whereupon the mixtures comprising 60 wt.% lithium bromide eutectic solution and 40 wt.% lithium chloride eutectic solution, and 50 wt.% lithium bromide eutectic solution and 50 wt.% lithium chloride eutectic solution froze completely, whereas those compositions with a lithium chloride eutectic composition content of greater than 50 wt.% did not. All those solutions that froze in the freezer melted cleanly and completely when allowed to return to ambient, whereas those that showed no evidence of freezing did not show any evidence of absorption of thermal energy in the form of latent heat at any temperature during the return to ambient. Supercooling observed during the freezing process of any composition that successfully froze in the ULT freezer was no more than 3°C.

The latent heat absorbed during the melting process of these compositions was measured using Differential Scanning Calorimetry. Those compositions with a lithium bromide eutectic composition content of 50 wt.%, or greater, showed an endothermic peak corresponding to absorption of thermal energy during the melting process, whereas those with a content of less than 50 wt.% showed no such peak. Latent heat of fusion of those compositions which showed an endothermic peak was between 120 kJ/kg and 230 kJ/kg, depending on the composition. Of particular interest was the composition comprising 60 wt.% lithium bromide eutectic solution and 40 wt.% lithium chloride eutectic solution, which melted cleanly at -69.6°C, with a latent heat of fusion of 230 kJ/kg.

The freezing and melting temperatures of the nucleated compositions are given in Table 1.

% LiBr eutectic: Freezing Melting temperature % Lid eutectic temperature (°C) (°C) 90:10 -72 -66 80:20 -74 -68 70:30 -78 -69 60:40 -80 -70 50:50 -80 -72 <50:>50 DNF DNF Table 1: LiBr= lithium bromide, Lid I = lithium chloride, DNF = did not freeze Therefore, this demonstrates that, even when nucleated with a suitable nucleating agent, compositions of less than 50 wt.% lithium bromide eutectic solution and greater 50 wt.% lithium chloride eutectic solution tend to display the thermophysical properties of the pure lithium chloride eutectic solution, namely excessive supercooling and inability to freeze at temperatures obtainable with a ULT freezer. For this reason they are considered less suitable candidates for use in low temperature transport applications, as a potential alternative to solid carbon dioxide. However, those compositions of 50 wt.% or greater lithium bromide eutectic solution, and 50 wt.% or less lithium chloride eutectic solution, may be considered as suitable candidates. They have specific melting points between those of lithium bromide eutectic solution alone, and lithium chloride eutectic solution alone, freeze completely when placed in a ULT freezer, and absorb large amounts of thermal energy in the form of latent heat at their melting temperatures.

Example 25

A stock solution of lithium chloride in water, comprising the pentahydrate peritectic composition of 32.05 wt.%, was prepared by dissolving 96.15g anhydrous lithium chloride 99+% (Alfa Aesar, UK) in 203.85g potable water. This was used in this example, and subsequent examples where lithium chloride pentahydrate peritectic solution was required, and was replenished whenever necessary.

Portions of this lithium chloride pentahydrate peritectic stock solution were mixed with portions of the lithium bromide eutectic stock solution described in Example 2. The ratios to of these mixtures ranged from 90 wt.% lithium bromide eutectic solution and 10 wt.% lithium chloride pentahydrate peritectic solution, to 10 wt.% lithium bromide eutectic solution and 90 wt.% lithium chloride pentahydrate peritectic solution. A 5 wt.% addition of sodium bromide nucleating agent was added to portions of each mixture and the mixtures were placed in a ULT freezer at -80°C. The mixtures comprising 90 wt.% lithium bromide eutectic solution and 10 wt.% lithium chloride pentahydrate peritectic solution, to 50 wt% lithium bromide eutectic solution and 50 wt.% lithium chloride pentahydrate peritectic solution froze completely at -80°C, whereas the others did not. The temperature of the freezer was reduced to -86°C, whereupon all the previously unfrozen compositions froze completely. All compositions melted cleanly and completely when allowed to return to ambient.

Supercooling observed during the freezing process of the compositions comprising 50 wt.% lithium bromide eutectic solution or greater was no more than 3°C, whereas those compositions comprising less than 50 wt.% lithium bromide eutectic solution supercooled between 5 and 11°C.

The latent heat absorbed during the melting process of these compositions was measured using Differential Scanning Calorimetry. All compositions showed an endothermic peak corresponding to absorption of thermal energy during the melting process, and the latent heat of fusion of those compositions which showed an endothermic peak was between 200 kJ/kg and 250 kJ/kg, depending on the composition.

The freezing and melting temperatures of the nucleated compositions are given in Table 2.

% LiBr eutectic: Freezing Melting temperature % LiCI.5H20 temperature (°C) (°C) 90:10 -73.7 -67.7 80:20 -73.4 -71.2 70:30 -75.7 -72.7 60:40 -76 -73.2 50:50 -76.4 -74.3 40:60 -75.3 & -76.8 -73.8 30:70 -74.4 & -77.7 -66.1 20:80 -71.1 & -79.8 -64.2 10:90 -67.3 & -80.7 -62.9 Table 2: LiBr= lithium bromide, LiC1.5H20 = lithium chloride pentahydrate peritectic Therefore, this demonstrates that, when nucleated with a suitable nucleating agent, compositions comprising mixtures of lithium bromide eutectic solution, and lithium chloride pentahydrate peritectic solution, in any ratio, may be considered as suitable candidates for use in low temperature transport applications, as a potential alternative to solid carbon dioxide. They have specific melting points, substantially different to those of lithium bromide eutectic solution alone, and lithium chloride pentahydrate peritectic solution alone, which io may be more suitable than either of these compositions for particular temperature applications. They freeze completely when placed in a ULT freezer, and absorb large amounts of thermal energy in the form of latent heat at their melting temperatures.

Example 26

Portions of lithium bromide pentahydrate peritectic stock solution described in Example 20 were mixed with portions of the lithium chloride eutectic stock solution described in Example 24. The ratios of these mixtures ranged from 90 wt.% lithium bromide pentahydrate peritectic solution and 10 wt % lithium chloride eutectic solution, to 10 wt% lithium bromide pentahydrate peritectic solution and 90 wt.% lithium chloride eutectic solution. A 5 wt.% addition of sodium bromide nucleating agent was added to portions of each mixture and the mixtures were placed in a ULT freezer at -80°C. The mixtures comprising 90 wt.% lithium bromide pentahydrate peritectic solution and 10 wt.% lithium chloride eutectic solution, to 40 wt.% lithium bromide pentahydrate peritectic solution and 60 wt.% lithium chloride eutectic solution froze completely at -80°C, whereas the others did not. The temperature of the freezer was reduced to -86°C, whereupon all the previously unfrozen compositions froze completely. All compositions melted completely when allowed to return to ambient. However, compositions comprising between 70 wt.% lithium bromide pentahydrate peritectic solution and 30 wt.% lithium chloride eutectic solution, and 40 wt.% lithium bromide pentahydrate peritectic solution and 60 wt.% lithium chloride eutectic solution each showed two separate melting ranges, i.e. they did not melt cleanly. Supercooling observed during the freezing process of the compositions was no more than 5°C for any composition.

The latent heat absorbed during the melting process of these compositions was measured using Differential Scanning Calorimetry. All compositions showed an endothermic peak corresponding to absorption of thermal energy during the melting process, and the latent heat of fusion was between 115 kJ/kg and 215 kJ/kg, depending on the composition. Those io compositions that displayed two separate melting ranges also displayed a double endothermic peak, rather than a single one.

The freezing and melting temperatures of the nucleated compositions are given in Table 3.

% LiBr.5H20: Freezing Melting temperature % LiCI.eutectic temperature (°C) (t) 90:10 -56.3 -51.3 80:20 -59.8 -54.0 70:30 -65.1 -57.1 60:40 -69.2 -58.8* 50:50 -73.8 62.1" 40:60 -77.4 -72.4* 30:70 -80.2 -74.6 20:80 -80.5 -74.3 10:90 -81.0 -74.0 Table 3: LiBr5H20= lithium bromide pentahydrate peritectic, Lid! = lithium chloride ": temperature of the predominant melting plateau Therefore, this demonstrates that, when nucleated with a suitable nucleating agent, compositions comprising mixtures of lithium bromide pentahydrate peritectic solution, and lithium chloride eutectic solution, in ratios of 70%, or greater, lithium bromide pentahydrate peritectic solution, or 40%, or less, lithium bromide pentahydrate peritectic solution, may be considered as suitable candidates for use in low temperature transport applications, as a potential alternative to solid carbon dioxide. They have specific melting points, that lie between those of lithium bromide pentahydrate peritectic solution alone, and lithium chloride pentahydrate peritectic solution alone, which may be more suitable than either of these compositions for particular temperature applications. They freeze completely when placed in a ULT freezer, and absorb large amounts of thermal energy in the form of latent heat at their melting temperatures.

Example 27

Portions of lithium bromide pentahydrate peritectic stock solution described in Example 20 were mixed with portions of the lithium chloride pentahydrate peritectic stock solution described in Example 25. The ratios of these mixtures ranged from 90 wt.% lithium bromide pentahydrate peritectic solution and 10 wt.% lithium chloride pentahydrate peritectic solution, to 10 wt.% lithium bromide pentahydrate peritectic solution and 90 wt.% lithium io chloride pentahydrate peritectic solution. A 5 wt.% addition of sodium bromide nucleating agent was added to portions of each mixture and the mixtures were placed in a ULT freezer at -80°C. All the compositions froze completely. All compositions melted cleanly and completely when allowed to return to ambient. Supercooling observed during the freezing process of the compositions was between 6 and 10°C.

The latent heat absorbed during the melting process of these compositions was measured using Differential Scanning Calorimetry. All compositions showed an endothermic peak corresponding to absorption of thermal energy during the melting process, and the latent heat of fusion of was between 190 kJ/kg and 230 kJ/kg, depending on the composition.

The freezing and melting temperatures of the nucleated compositions are given in Table 4.

% LiBr5H20: Freezing Melting temperature % LiCI.5H20 temperature (°C) (°C) 90:10 -55.1 -52.9 80:20 -57.3 -54.4 70:30 -59.5 -56.2 60:40 -61.5 -57.5 50:50 -62.9 -58.6 40:60 -64.3 -59.9 30:70 -65.7 -61.5 20:80 -66.3 -62.5 10:90 -66.5 -62.8 Table 4: LiBr5H20= lithium bromide pentahydrate peritectic, LiCI.5H20 = lithium chloride pentahydrate peritectic Therefore, this demonstrates that, when nucleated with a suitable nucleating agent, compositions comprising mixtures of lithium bromide pentahydrate peritectic solution, and lithium chloride pentahydrate peritectic solution, in any ratio, may be considered as suitable candidates for use in low temperature transport applications, as a potential alternative to solid carbon dioxide. They have specific melting points, that lie between those of lithium bromide pentahydrate peritectic solution alone, and lithium chloride pentahydrate peritectic solution alone, which may be more suitable than either of these compositions for particular temperature applications. They freeze completely when placed in a ULT freezer, and absorb large amounts of thermal energy in the form of latent heat at their melting o temperatures.

Example 28

A first composition of lithium bromide was prepared at the composition of the eutectic by mixing a commercially available 55 w/w°/0 lithium bromide solution (Leverton Clarke Ltd, UK) (71.1g) with potable water (28.9g).

A second composition of lithium bromide was prepared at the composition of lithium bromide pentahydrate by mixing a commercially available 55 w/w% lithium bromide solution (Leverton Clarke Ltd, UK) (89.3g) with potable water (10.7g).

A portion of each composition was then dosed with sodium bromide (5 wt.%) as nucleating agent and cooled to -80°C in the ultra-low temperature freezer. Both solutions froze completely and then melted completely and cleanly when allowed to gradually return to ambient temperature. The eutectic composition melted at -67°C, and the pentahydrate composition melted at -48°C.

This demonstrates that a composition according to the invention may be produced by dilution of commercially available aqueous solutions of lithium bromide. This is advantageous because lithium bromide 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 bromide.

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 (25)

1. CLAIMS1 A phase change material, the phase change material comprising an aqueous solution of lithium bromide, the aqueous solution comprising (e.g. consisting of) lithium bromide in an amount that is greater than or equal to 38 wt.%, and water in an amount that is greater than 0 wt.% and less than or equal to 62 wt.% of the aqueous solution, the composition further comprising a nucleating agent, e.g. in an amount configured to freeze the phase change material at a defined temperature and to release latent heat when thawed.
2. 2 A phase change material according to Claim 1, wherein the aqueous solution comprises or consists of lithium bromide in an amount greater than or equal to 38 and less than or equal to 55 wt.% (e.g. greater than or equal to 38 and less than or equal to 50 wt.%) and water in an amount greater than or equal to 45 to less than or equal to 62 wt.% (e.g. greater than or equal to 50 and less than or equal to 62 wt.%).
3. 3 A phase change material according to any preceding Claim, wherein the aqueous solution comprises or consists of lithium bromide in an amount greater than or equal to 38 to less than or equal to 40 wt.% and water in an amount greater than or equal to 60 to less than or equal to 62 wt.%.
4. 4 A phase change material according to any preceding Claim, wherein the aqueous solution comprises or consists of lithium bromide in an amount greater than or equal to 39.07 to less than or equal to 39.4 wt.% and water in an amount greater than or equal to 60.60 to less than or equal to 60.93 wt.%, for example 39.1 wt.% lithium bromide and 60.9 wt.% water.
5. A phase change material according to Claim 1 or Claim 2, wherein the aqueous solution comprises or consists of lithium bromide in an amount greater than or equal to 46 and less than or equal to 50 wt.%, and water in an amount greater than or equal to 50 and less than or equal to 54 wt.%, for example wherein the aqueous solution comprises or consists of lithium bromide in an amount greater than or equal to 47.3 and less than or equal to 49.1 wt.%, and water in an amount greater than or equal to 50.9 and less than or equal to 52.7 wt.%, for example wherein the aqueous solution comprises or consists of 49.1 wt.% lithium bromide and 50.9 wt.% water.
6. 6. A phase change material according to any preceding Claim, further comprising a eutectic or peritectic (e.g. pentahydrate) aqueous solution of a further lithium halide, e.g. lithium chloride.
7. 7 A phase change material according to Claim 6, wherein the ratio (x:y) of the volume of the aqueous solution of lithium bromide (x) to the volume of the aqueous solution io of the eutectic or peritectic further lithium halide (e.g. lithium chloride) (y) is in a ratio (x:y) of between 0.1:99.9 to 99.9:0.1, e.g. from 1:9 to 9:1, or from 2:8 to 9:1, or 3:7 to 9:1, or 4:6 to 9:1, or 5:5 to 9:1, or 6:4 to 9:1, or 7:3 to 9:1, or 8:2 to 9:1.
8. 8 A phase change material according to Claim 6 or 7 wherein the aqueous solution of lithium chloride consists of lithium chloride in an amount between 22 to 34 wt.% and water in an amount between 66 to 78 wt.%, for example wherein the aqueous solution of lithium chloride consists of lithium chloride in an amount between 23 to 26 wt.% and water in an amount between 74 to 77 wt.%.
9. 9. A phase change material according to Claim 8 wherein the aqueous solution of lithium chloride consists of 24.98 wt.% lithium chloride and 75.02 wt.% water.
10. 10. A phase change material according to Claim 6 or 7, wherein the aqueous solution of lithium chloride consists of lithium chloride in an amount between 31 to 33 wt.% and water in an amount between 67 to 69 wt.%, e.g. the aqueous solution of lithium chloride consists of between 32 to 33 wt.%, e.g. 32.08 wt.% lithium chloride and 67.92 wt.% water or 32.05 wt.% lithium chloride and 67.95 wt.% water.
11. 11. A phase change material according to any preceding Claim, wherein the nucleating agent comprises or consists of one or more of an inorganic Group 1 salt, an inorganic Group 2 salt, and/or an inorganic ammonium salt, e.g. wherein the cationic species of the nucleating agent is selected from 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 an ammonium ion.
12. 12. A phase change material according to any preceding Claim, wherein the nucleating agent comprises or consists of an inorganic halide salt, e.g. 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.
13. 13. A phase change material according to any preceding Claim, 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.
14. 14. A phase change material according to any preceding Claim, wherein the anionic species of the nucleating agent comprises or consists of a hydroxide ion, a sulphate ion, a phosphate ion, and/or a sulphide ion.
15. 15. A phase change material according to any preceding Claim, wherein the nucleating agent comprises or consists of a Group 2 sulphate salt, a Group 2 phosphate salt, and/or a Group 2 sulphide salt.
16. 16. A phase change material according to any preceding Claim, wherein the nucleating agent is selected from one or more of sodium fluoride, sodium chloride, sodium bromide, potassium bromide, ammonium bromide, lithium hydroxide monohydrate, calcium sulphate, barium sulphate, calcium phosphate, and/or barium sulphide. 17.
17. A phase change material according to Claim 16, wherein the nucleating agent comprises or consists of sodium fluoride, for example, in an amount greater than 0.01 wt.% and less than 0.05 wt.% of the total composition.
18. 18. A phase change material according to any one of Claims 1 to 10, wherein the nucleating agent is a sterol, e.g. cholesterol.
19. 19. A phase change material according to any preceding Claim, wherein the composition comprises the 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.%, 1 wt.%, 0.5 wt %, 0.1 wt %, 0.05 wt.%, or less than 0.01 wt.% of the total composition.
20. 20 A 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.
21. 21. A phase change material according to any preceding Claim, wherein the composition freezes at more than -86°C, and melts at less than -40°C and more than -86°C, for example, between -48°C to -75°C.
22. 22. A phase change material according to any preceding Claim, wherein the composition is micro-or macro-encapsulated.
23. 23. A 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.
24. 24 A method of forming a phase change material, the method comprising forming a composition, the composition comprising an aqueous solution of lithium bromide, e.g. by dissolving lithium bromide in water and/or by diluting a stock solution of aqueous lithium bromide with water, the aqueous solution comprising lithium bromide in an amount that is greater than or equal to 38 wt.%, and water in an amount that is greater than 0 wt.% and less than or equal to 62 wt.% of the aqueous solution, the composition further comprising a nucleating agent, e.g. in an amount configured to freeze the phase change material at a defined temperature and to release latent heat when thawed.
25. 25. A method according to Claim 24, further comprising freezing the composition at a temperature lower than -40°C, e.g. lower than or at -48°C, for example, in an ultra-low temperature freezer.