Heat storage material

Abstract

A phase change material (PCM) comprises hydrated sodium acetate and a polymeric additive comprising carboxylated, sulphated and / or sulfonated lignin; carboxylated, sulphated and / or sulphonated cellulose; carboxylated, sulphated and / or sulfonated humin; sulphated and / or sulfonated humic acid; and / or humic acid. The PCM may further comprise a nucleating agent e.g. disodium phosphate. The sodium acetate trihydrate-based phase change material (PCM) has improved thermal stability over repeated cycles.

Description

Field of the Invention The present invention relates to phase change materials (PCMs) for thermal energy storage. More specifically, the present invention relates to PCMs comprising sodium acetate trihydrate which exhibit improved homogeneity and stability over repeated heating and cooling cycles. Background of the Invention Phase change materials (PCMs) may be applied as energy storage media in thermal energy storage systems. In such a system, a PCM will undergo one or more phase transitions, which may be, for example, between a solid phase and a liquid phase, between a liquid phase and a gas phase, between a solid phase and a gas phase, or between two distinct solid phases. Phase changes have a characteristic latent heat, which contributes to the overall energy storage density and typically gives PCMs a higher energy storage capacity than materials exhibiting no phase change over a given temperature range. During the preparation and use of a thermal energy storage system comprising a PCM, the PCM may undergo one or more thermal cycles. A thermal cycle may be defined as heating and then cooling, or cooling and then heating, the material over a temperature range which includes at least in part the phase change. Energy is thereby stored and released as the PCM changes phase. It is often required that the PCM is able to undergo a plurality of thermal cycles. In order to be able to employ a PCM as energy storage medium in a thermal energy storage system, the performance of the PCM should be unchanged over said plurality of thermal cycles, i.e. the PCM should be stable. Salt hydrates are a class of PCMs which may be used in thermal energy storage systems. Some salt hydrates however are not stable over repeated thermal cycles, due to, for example, incongruent melting or competing crystallisation of a secondary phase. The salt hydrate sodium acetate trihydrate is well known for its instability, as on cycling there is a propensity for the anhydrous form of sodium acetate to crystallise and settle at the bottom of the PCM container. This separation of phases is disadvantageous as the PCM is no longer homogeneous, and as such will no longer fully melt and freeze when thermally cycled, reducing the energy storage capacity and thereby the system performance. Attempts to use sodium acetate trihydrate as a PCM energy storage medium typically involve applying various thickening agents to hinder the separation of the anhydrous phase from the bulk of the PCM, physically stopping the anhydrous particles from sinking through the material. This is disadvantageous as thickening the PCM material may hinder the crystallisation of the trihydrate phase, limiting the thermal power output of the system. Furthermore, phase separation may still occur, simply being slowed by the thickening of the PCM rather than fully overcome, and prolonged use may therefore still result in a poorly performing inhomogeneous PCM. It is an object of at least one aspect of the present invention to obviate or mitigate at least one or more of the aforementioned problems in relation to the utility of sodium acetate trihydrate as a potential PCM for use in thermal energy storage systems. Summary of the Invention The present invention relates to phase change materials (PCMs) comprising hydrated sodium acetate and stabilisation thereof. Sodium acetate trihydrate is well known to undergo deleterious phase separation in its use as a phase change material. This is due to incongruent melting, or the formation of anhydrous sodium acetate during the phase transition process, which then sinks through the melt to deposit at the base of the PCM container with the released water remaining at the top of the container and the remaining molten sodium acetate trihydrate remaining therebetween. According to an aspect of the present invention there is provided a phase change material (PCM) comprising hydrated sodium acetate and a polymeric additive. The sodium acetate may have any degree of hydration. For example, the sodium acetate may be di-, tri-, tetra-hydrated, or any level of hydration in between. Preferably the sodium acetate may be about tri-hydrated. The hydrated sodium acetate may be from about 2.5 to about 3.5 hydrated. For example, the sodium acetate may be 2.95 hydrate, or 3.05 hydrate, or 3.15 hydrate, and the like. The hydrated sodium acetate may have a formula CHsCOONaS^O, or from about CH3COONa-2.5H2O to about CH3COONa3.5H2O. The polymeric additive may be configured to prevent phase segregation of the PCM into sodium acetate trihydrate, sodium acetate anhydrous and water. The PCM may remain homogeneous or substantially homogeneous at all temperatures. The polymeric additive may prevent crystallisation of sodium acetate anhydrous. Additionally or alternatively, the polymeric additive may be configured to alter the crystal habit / morphology of sodium acetate anhydrous (e.g. if it forms within the bulk PCM material comprising mostly sodium acetate trihydrate). This may prevent crystals of sodium acetate anhydrous which have formed sinking within the bulk PCM material. The polymeric additive may prevent sodium acetate anhydrous crystalising in plate- or block-like morphologies. The polymeric additive may induce any sodium acetate anhydrous that forms to crystallise into an acicular morphology (e.g. fine needles, hairs, rods). The polymeric additive may produce a homogeneous distribution of sodium acetate anhydrous in a bulk PCM material comprising sodium acetate trihydrate. The polymeric additive may comprise at least one polymer selected from a list comprising: carboxylated, sulfated and / or sulfonated lignin; carboxylated, sulfated and / or sulfonated cellulose; carboxylated, sulfated and / or sulfonated humin; sulfated and / or sulfonated Humic acid, and / or humic acid, and / or a salt or derivative thereof. When the PCM comprises a salt of the at least one polymer, the salt may be a group I metal or a group II metal salt of the polymer. A group I metal salt may comprise a Li+, Na+, K+, Rb+, and / or Cs* ion. A group II metal salt may comprise a Be2+, Mg2+, Ca2*, Sr2*, and / or Ba2+ ion. Group I salts of the polymers may be preferentially used as they may be freely soluble and / or dispersible in the PCM. More preferentially, the polymers may be included in the PCM as a sodium salt, and thus avoid the inclusion of any unnecessary cations which may affect the PCMs phase transition characteristics. In a salt of the polymer, the anion may be the polymer chain. The polymer chain may comprise anionic groups (e.g. sulfonate, carboxylate). The PCM may further comprise at least one nucleation agent. The nucleation agent may be a nucleation agent for the sodium acetate hydrate (e.g. sodium acetate trihydrate). The at least one nucleation agent may be selected from a list comprising: mono-, di- and / or tri-sodium phosphate and / or any hydrate form(s) thereof, mono-, di-, tri- and / or tetra-sodium pyrophosphate and / or any hydrate form(s) thereof. Without wishing to be bound by theory, providing a nucleation agent for the sodium acetate hydrate may promote crystallisation of the hydrated form of the sodium acetate below its melting point while the polymeric additive prevents, hinders or delays crystallisation of the unwanted species (sodium acetate anhydride). This combination may be particularly effective at maintaining the PCM in a homogeneous state. The polymer may be sodium lignosulfonate. The polymer may be sodium lignosulfonate with an average molecular weight of from about 1,000 to about 400,000 Da, between 1,000 and 200,000 Da, or from about 1,000 to about 100,000 Da, or from about 1,000 Da to about 75,000 Da, or from about 10,000 to about 75,000 Da, or from about 20,000 to about 75,000 Da. The polymer may be sodium lignosulfonate with an average molecular weight of about 52,000 Da. The polymer may have an average polymer chain length from about 5,000 units to about 10,000 units, or from about 5,000 units to about 8,000 units, or from about 6,000 units to about 8,000 units, or from about 6,000 units to about 7,000 units, or from about 7,000 units to about 8,000 units. The polymer may have an average polymer chain length of about 7,000 units. It has been surprisingly found by the inventors that phase separation of PCM material comprising sodium acetate trihydrate over one or more heat-cool cycles may be overcome by the addition of at least one polymer (e.g. at least one naturally-derived polymer). The at least one polymer may be a group I or group II metal salt of a polymer selected from a list comprising: lignosulfonic acid, humic acid, lignohumic acid, sulfonated humin, sulfonated humic acid, and sulfonated cellulose, carboxylated cellulose, and / or derivatives or mixtures thereof. Without wishing to be bound by theory, the polymers disclosed herein may have the effect of allowing the PCM to remain homogeneous during use. This may be achieved by hindering the crystallisation of anhydrous sodium acetate, delaying said crystallisation, and / or changing its crystal morphology to prevent movement of the crystals within the PCM container. Herein a homogeneous PCM is defined as a PCM where the chemical composition (i.e. the amounts of the various PCM components including water) does not exhibit any significant changes in chemical composition (e.g. the relative amounts of sodium aetate and water) throughout the PCM volume during use of the PCM. Conversely a heterogeneous PCM is defined as one where the composition does change dependent on the location within the PCM volume. Heterogeneity in a PCM may be caused by instability of the PCM during use. The nucleation agent may be present in the PCM as a solid, and as such may sink through the PCM when the PCM is molten to settle at the bottom of the container. In such a regime, the PCM may still be considered to be substantially homogeneous, as there is no segregation of anhydrous sodium acetate. Advantageously, the PCM described herein may comprise hydrated sodium acetate which is stable over a plurality of thermal cycles (i.e. over repeated melt and freeze cycles). The PCM may comprise water. Without wishing to be bound by theory, water may be added to prevent, delay or hinder formation of sodium acetate anhydrous. The PCM may comprise more water than it is required to form sodium acetate trihydrate, e.g. more than 3 molar equivalents of water per 1 mole of sodium acetate The PCM may comprise water such that the molar ratio of sodium acetate:water is between about 1:3 and 1:3.5, between about 1:3 and 1:3.3, between about 1:3 and 1:3.1, or is about 1:3.05, about 1:3.1, about 1:3.2, or about 1:3.3. The PCM may comprise from about 10wt% to about 60 wt% of water, or from about 25 wt% to about 50 wt% of water, or from about 35 wt% to about 45 wt% of water. Preferably, the PCM may comprise about 40 wt% of water. Sodium acetate trihydrate may comprise about 60 wt.% sodium acetate and about 40 wt.% water. The PCM may comprise a maximum additional water content of 20 wt.% relative to the total content of sodium acetate hydrate (e.g. sodium acetate tri-hydrate). The additional water content in the PCM relative to the total content of sodium acetate hydrate (e.g. sodium acetate trihydrate) may be from about 0 wt% (where the water content in the PCM is stoichiometric to the water content in the sodium acetate trihydrate) to about 20 wt% (e.g. where there is 20% excess water in the PCM over the water content required to form the sodium acetate trihydrate), or from about 0 wt% to about 15 wt%, or from about 0 wt% to about 10 wt%, or from about 0 wt% to about 5 wt%, or from about 0 wt% to about 3 wt%, or from about 2.5 wt% to about 5 wt%, or from about 5 wt% to about 10 wt%, or from about 10 wt% to about 15 wt%, or from about 15 wt% to about 20 wt% Preferably, the water content relative to the total content of sodium acetate hydrate is from about 0 wt% to about 5 wt%, or from about 1 wt% to about 5 wt%, or from about 2 wt% to about 5 wt%, or from about 3 wt% to about 5 wt%, or from about 4 wt% to about 5 wt%, or from about 0 wt% to about 2 wt%. or from about 2 wt% to about 4 wt%, or from about 3 wt% to about 4 wt%. The PCM may have a phase transition temperature at around 58 °C. The polymeric additive (e.g. group I or group II metal salt of group I or group II metal salt of a polymer selected from a list comprising: lignosulfonic acid, sulfated lignin, carboxylated lignin, humic acid, lignohumic acid, sulfonated humin, sulfated humin, carboxylated humin, sulfated humic acid, sulfonated humic acid, sulfated cellulose, sulfonated cellulose, carboxylated cellulose, and / or derivatives or mixtures thereof) may delay nucleation of sodium acetate anhydrous between about 58 °C. and about 77 °C and the PCM may preferentially crystallise in its preferred form of sodium acetate trihydrate below 58 °C. The polymeric additive may delay the crystallisation of molten sodium acetate anhydrous from molten hydrated sodium acetate below about 77 °C by at least 1 hour, at least 4 hours, at least 10 hours, at least 24 hours, at least 48 hours or at least 72 hours. The PCM may not comprise a thickener. For example, the PCM may comprise sodium acetate trihydrate which exhibits minimal or no phase segregation The PCM may comprise hydrated sodium acetate which exhibits minimal or no crystallisation of anhydrous sodium acetate on cooling from a molten state. Additionally or alternatively, the PCM may comprise hydrated sodium acetate which exhibits delayed crystallisation of sodium acetate anhydrous when cooling the PCM from a molten state. The PCM comprising hydrated sodium acetate may exhibit crystallisation of anhydrous sodium acetate in a volume filling regime (i.e. a homogeneous network of sodium acetate anhydrous crystals which fill the PCM volume). The polymers disclosed herein may be configured to change the crystal morphology of any anhydrous sodium acetate that forms in the PCM such that fine crystals which are resistant to sinking through the PCM are generated. This in turn may allow the PCM to remain in a homogeneous state (i.e. without visible or detectable precipitates, e.g. without precipitated anhydrous sodium acetate) during use (e.g. during multiple freeze-melt cycles). This may allow the PCM to perform thermally in a consistent manner over prolonged periods of use, i.e. retain the same melting and freezing temperatures, energy storage capacity, crystallisation rate and / or crystal growth rate. The polymers disclosed herein may maybe configured to cause any crystals of anhydrous sodium acetate which form to arrange themselves into a self-supporting network of crystals which fills the PCM volume. Without wishing to be bound by theory, providing a self-supporting network of anhydrous sodium acetate crystals may substantially prevent crystals from sinking through the PCM and thereby retain the PCM in a homogeneous state. These inventors have surprisingly found that PCMs according to the invention are stable to repeated melt and freeze cycles. Advantageously, the polymers disclosed herein are derived from biological resources. The polymers disclosed herein may be prepared from non-petrochemical sources, such as wood. This may represent a more environmentally sustainable option than petrochemically based alternatives. Additionally, the polymers disclosed herein may be considered to be carbon negative, defined as being derived from materials which remove carbon dioxide from the atmosphere, and therefore gain an environmental advantage. The phase change material (PCM) may comprise: hydrated Sodium acetate; and a polymeric additive comprising a polymer selected from a list comprising: carboxylated, sulfated and / or sulfonated lignin; carboxylated, sulfated and / or sulfonated cellulose; carboxylated, sulfated and / or sulfonated humin; sulfated and / or sulfonated Humic acid, and / or humic acid, and at least one nucleation agent. The at least one nucleation agent may be selected from a list comprising: mono-, di- and / or tri-sodium phosphate and / or any hydrate form(s) thereof, mono-, di-, tri- and / or tetra-sodium pyrophosphate and / or any hydrate form(s) thereof. The PCM may comprise from about 40 wt% to about 70 wt.% of sodium acetate, or from about 40wt% to about 60 wt%, or from about 40 wt% to about 50 wt%, or from about 45 wt% to about 55 wt%, or from about 50 wt% to about 70 wt%, or from about 60 wt% to about 70 wt% of sodium acetate. The PCM may comprise from about 0.1 wt% to about 10 wt.%, or from about 0.1 wt% to about 1 wt%, or from about 1 wt% to about 3 wt%, or from about 0.1 wt% to about 5 wt%, or from about 0.1 wt% to about 3 wt%, or from about 2 wt% to about 5 wt%, or from about 5 wt% to about 10 wt%, or from about 5 wt% to about 7 wt%, or from about 7 wt% to about 10 wt% of one or more group I or group II metal salt of a polymer selected from a list comprising: lignosulfonic acid, humic acid, lignohumic acid, sulfonated humin, sulfonated humic acid, carboxylated cellulose, and sulfonated cellulose. The PCM may comprise from about 0.1 wt% to about 10 wt.%, or from about 0.1 wt% to about 1 wt%, or from about 1 wt% to about 3 wt%, or from about 0.1 wt% to about 5 wt%, or from about 0.1 wt% to about 3 wt%, or from about 2 wt% to about 5 wt%, or from about 5 wt% to about 10 wt%, or from about 5 wt% to about 7 wt%, or from about 7 wt% to about 10 wt% of a nucleation agent. The PCM may comprise water. For example, the PCM may comprise from about 10% to about 60% of water, or from about 10 wt% to about 50 wt%, or form about 10 wt% to about 40 wt%, or from about 10 wt% to about 30 wt%, or from about 10 wt% to about 20 wt%, or from about 20 wt% to about 40 wt%, or from about 40 wt% to about 50 wt%, or from about 50 wt% to about 55 wt% of water. Preferably, the PCM may comprise from about 35 wt% to about 45 wt% of water, more preferably about 42 wt% of water. The PCM may comprise: from about 40 wt% to about 70 wt% of sodium acetate; from about 0.1 wt% to about 10 wt% of a polymer as described herein and / or salt thereof; and water to balance. Optionally, the PCM may further comprise from about 0.1 wt% to about 10 wt% of at least one nucleation agent as described herein. The PCM may further comprise a melting point depression agent. The melting point depression agent may be a salt of sodium and / or an acetate salt other than sodium acetate. For example, the melting point depression agent may be one or more material selected from a list comprising: a group I acetate salt, a group II acetate salt, a sodium halide salt, a sodium carboxylate salt; and sodium nitrate. The PCM may comprise from about 3 wt% to about 40 wt% of the melting point depression agent, or from about 5 wt% to about 10 wt%, or from about 10 wt% to about 20 wt%, or from about 20 wt% to about 30 wt%, or from about 30 wt% to about 40 wt%, or from about 5 wt% to about 15 wt%, or from about 15 wt% to about 30 wt%, or from about 20 wt% to about 40 wt%, or from about 3 wt% to about 10 wt%, or from about 3 wt% to about 5 wt%. It has been found by the inventors that the inclusion of melting point depression agents has no significant effect on the ability of the aforementioned polymers to prevent segregation of sodium acetate anhydrous (SAA) from molten SAT The PCM may be capable of being repeatedly melted and frozen with no phase segregation and presenting no change in thermal properties over multiple cycles. This may be beneficial in the performance of thermal energy storage devices comprising the PCMs disclosed herein, which may exhibit decrease, significantly decreased or substantially no degradation of performance when thermally cycled. The PCM as disclosed herein may be capable of being thermally cycled more than once, more than 50 times, more than 500 times, more than 1000 times, more than 2,000 times, more than 5,000 times or more than 10,000 times without significant change in thermal performance. The PCMs as disclosed herein may be cycled to this extent within a thermal energy storage device apparatus. According to another aspect of the invention there is a thermal energy storage apparatus comprising a PCM as disclosed herein. Brief Description of the Drawings Figure 1 shows a phase diagram of sodium acetate in water at various temperatures, showing the phases present and the incongruency of the melting transition. Figure 2 shows diagrammatically the macroscopic phase separation of a volume of sodium acetate trihydrate-based PCM into a sodium acetate trihydrate layer, a segregated sodium acetate anhydrous-rich layer and a water-rich layer which occurs due to incongruent melting of the PCM material. Figure 3 shows diagrammatically the volume filling crystal networking effect caused by the crystal habit modification of sodium acetate anhydrous by the polymers as disclosed herein, wherein a sodium acetate trihydrate-based PCM with a fill level in a PCM container undergoes crystallisation of SAA in the presence of one or more of the disclosed polymers to produce a homogeneous distribution of sodium acetate anhydrous in the PCM volume up to the fill volume in the PCM container. Detailed Description Sodium acetate trihydrate (SAT) is a material which may be used as a phase change material (PCM). Being low-cost, non-toxic, and having a phase transition temperature of around 58 °C, SAT is well placed to be used in thermal energy storage systems, which may be otherwise known as heat batteries, thermal stores or thermal banks. In such applications, it is beneficial to be able to repeatedly thermally cycle, i.e. repeatedly melt and freeze, the PCM without significant changes to its performance. It is beneficial if the melting transition, energy storage capacity and nucleation characteristics of the PCM do not change, or do not significantly change during use, meaning the device comprising the PCM behaves in a reliable, predictable manner with good performance for an extended period of time. However, the use sodium acetate trihydrate (SAT) as a PCM is not straightforward due to its phase behaviour during use, where there is a propensity for the anhydrous form of sodium acetate (sodium acetate anhydrous, SAA) to form while the bulk of the PCM is in a molten state. This is due to an incongruency in its phase diagram as shown in Figure 1. As a PCM is heated from a solid state to the melting point of sodium acetate trihydrate (Tm, sat = 58 °C), it is desired that the material melts into a single homogeneous liquid. SAT does not do this, instead melting to form a liquid (molten sodium acetate trihydrate) and a solid (precipitated sodium acetate anhydrous). To completely avoid the formation of SAA using heat alone, the PCM must be heated far above its phase transition temperature to about 77 °C, as shown in Figure 1. Thus between 58 °C and about 77 °C there is the potential for SAA to crystallise from the PCM bulk. If SAA is formed and crystallised from the molten SAT bulk, it will sink, finally depositing at the bottom of the PCM container as shown in Figure 2. In Figure 2 a volume of SAT-based PCM

[201] is shown segregating into a SAT layer

[202] , a segregated SAA rich layer

[203] and a water rich layer

[204] due to incongruent melting. This leaves a sodium acetate rich layer at the bottom of the container and a water rich layer at the top of the container. The PCM has become heterogeneous. This effect reduces the thermal performance of the PCM, as sodium acetate-containing material at the top of the container will not freeze when decreasing the temperature below the phase transition point (Tm, sat = 58 °C) due to the excess of water, and the SAA present at the bottom of the container cannot form SAT due to a lack of water. Therefore, neither of these areas of the PCM can contribute to the thermal energy storage capacity of the PCM. It should be noted that while distinct layers can be observed in some cases as exemplified in Figure 2, it is also possible to produce a gradient of concentration of sodium acetate through the PCM volume, with higher concentrations present towards the base of the PCM container. From the phase diagram shown in Figure 1, it can be inferred that a method by which SAA may be avoided is to increase the water content of the PCM. However, while a small amount of excess water is tolerable a significant amount of additional water is disadvantageous as increasing the water content decreases the heat storage capacity of the material and also deleteriously affects the nucleation properties of the PCM, that is to say increased water content makes the PCM as a whole less likely to crystallise. The water content may be such that the molar ratio of sodium acetate:water is about 1:3, about 1:3.05, about 1:3.10, about 1:3.2 or about 1:3.3. The water content may be about 35-45 wt.%. Preferably the water content may be about 42 wt.%. It is disclosed herein that a polymeric additive may be applied to advantageously alter the crystallisation of SAAin SAT-based PCMs. Established means to overcome segregation are the inclusion of thickeners which hinder the ability of formed SAA to sink through the PCM volume and settle at the bottom of the PCM container. This is disadvantageous as thickeners can interfere, slow or otherwise hinder the crystallisation of SAT, and thereby reduce the thermal performance of the material. Furthermore, segregation may still occur, albeit at a reduced rate. Hence there is a requirement in the field of PCMs to provide a SAT-based PCM which is stable to prolonged use, including repeated melt and freeze cycles. It has been surprisingly found by the inventors that the addition polymeric additives, and in particular the addition of one or more group I or group II metal salt of a polymer selected from a list comprising: lignosulfonic acid, carboxylated lignin, sulfated lignin, humic acid, lignohumic acid, sulfonated humin, sulfated humin, carboxylated humin, sulfonated humic acid, sulfonated cellulose, sulfated cellulose and carboxylated cellulose, may advantageously alter the crystallisation of SAAin hydrated sodium acetate-based PCMs. A section of the approximate structural formula of lignin is shown below: H.COH OH Typically, lignin is a polyaromatic polymer made up of subunits of para-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol, which can be interlinked by ether functionalities in a 5 plurality of different ways, giving a variety of different polymeric structures. Sulfonated lignins and lignosulfonates exhibit similar subunits, connected in similar or the substantially the same manner, with the addition of sulfonate groups, which may be at various points on the lignin polymer backbone. A section of the approximate structural formula of sodium lignosulfonate is shown below: 10 In the above structural formula, the sodium salt is given merely by example, and the sodium shown may be exchanged for any group I or group II metal. Sulfonation may occur in varying amounts, (i.e. various amounts of sulfonate groups may be substituted onto the lignin polymer backbone), and at various positions on the lignin polymer backbone. Carboxylated and sulphated lignins may be used as part of the present invention. Carboxylation of lignin involves the inclusion of carboxylate -COOH groups (-COO- when 5 deprotonated) into one or a plurality of points in the lignin polymer chain structure. By way of further example, a section of the approximate structural formula of carboxylated lignin is shown below: Si DOC COCH Sulfation of lignin involves the inclusion of (-OSCh') groups into one or a plurality of points in 10 the lignin polymer chain structure. Sulfated lignin may be understood accordingly, where the sulfate group (-OSOf) is included on the polymer chain at one or more points. An example structure showing sulfation of lignin is shown below: In the above structure, the sulfate group (-OSOs') may be present at any point in the polymer 15 structure, or a plurality of points in the polymer structure. Additionally, the above structure is shown without a cation, and that any cation may be present to balance charge. The cation may be a group I or II metal cation. A section of the approximate structural formula of Humin is shown below: Humin is a polyaromatic polymeric furanic-type structured material exhibiting a variety of ether, hydroxyl, aldehyde and ketone functionalities. These functionalities may be arranged in a variety of ways to form the polymer, i.e. the connectivity may be altered while remaining a 5 humin type polymer. Sulfation, sulfonation and / or carboxylation of various points on the polymeric chain may be used to solubilise the polymer, and such a material may be used as part of PCM disclosed as part of the present invention. A section of the approximate structure of humic and lignohumic acids showing typical functionalities present in said materials is shown below: 10 Alternative structures have been proposed for humic acid fragments, such as that proposed by Steelink as shown below: It is noteworthy that many different structures of humic acids have been suggested (Yasair 15 S.S. Al-Faiyz, CPMAS 13C NMR characterization of humic acids from composted agricultural Saudi waste, Arabian Journal of Chemistry, Volume 10, Supplement 1, 2017, Pages S839-S853). Such humic and lignohumic materials bear the functionalities indicated above in various ratios and connectivities. For example, humic and lignohumic polymers may comprise amide, carboxylate, ether quinone, phenol, aldehyde, alcohols, catechol, and sugar moieties in various ratios and connectivities. Sulfonation and / or sulfation, being the introduction of -SOa" and -OSOa’ groups respectively, of the above structure produces a polymer which is useable as part of the present invention. Sulfonation and / or sulfation may occur at one or a plurality of positions on the polymer chain, or may take the position of one or more of the functional groups shown, to become a sulfonated or sulfated humic acid or sulfonated or sulfated lignohumic acid. Cellulose is a polysaccharide, with ether and alcohol functionalities. It’s repeating unit consisting of P(1 —>4) linked d-glucose units is shown below: This structure may be used in a carboxylated, sulfated and / or sulfonated form when used as part of the present invention. Examples of different carboxylation, i.e. the process of addition of a carboxylate group (-COO or equivalent salt form) to cellulose are shown below: The extent and position of the introduction of carboxyl groups on the cellulose polymer chain may vary as shown in the above structural formulae. Carboxyl containing side chains may also be added to the cellulose backbone (e.g. grafted onto the polymer chain). A section of the approximate structural formula of sulfonated cellulose is shown below: Cellulose is a polysaccharide, with ether and alcohol functionalities. Sulfonation of cellulose leads to the introduction of sulfonate groups into the polymer chain, and may also (as shown above) lead to the breakage of the sugar ring system. Sulfonation may also occur at one or more of the alcohol groups and may not involve breaking the ring structure. Sulfated cellulose may also be used as part of the present invention. An example sulphated cellulose structure is given below: o O=S-ONa < OR R= H or O O=S-ONa 1 Addition of sulfate groups may occur at a variety of positions around the cellulose ring structure. Sulfate groups may be present at one or a plurality of positions around the cellulose ring structure. In the above example, the sodium salt is shown, however any salt and / or derivative thereof may be used as part of the present invention. The structures shown above describe sections, models and specific instances and representations of the polymers disclosed herein, and are merely exemplary in nature and not limiting. The number, location and connectivity of the functional groups can vary. The structures shown give indications of the functionalities present in the polymers disclosed herein, but do not show the only structure of said polymers possible to be used as part of the present invention. The presence of the carboxylate, sulfate and / or sulfonate functionalities on the polymer backbone is key in their activity for providing a PCM which does not exhibit instability during use. It has been found by the inventors that lignin, cellulose and humin without the presence of the carboxylate, sulfate and / or sulfonate groups have no activity in stabilising hydrated sodium acetate based PCMs. Thus, the presence of one or more carboxylate, sulfonate and / or sulfate group is necessary for the polymers disclosed herein to function as stabilising additives for hydrated sodium acetate based PCMs. The first means by which said polymers may beneficially affect the crystallisation of SAA in SAT-based PCMs is to hinder the crystallisation of SAA. Typically, SAA forms rapidly and readily from molten SAT below about 77 °C. However, the addition of the aforementioned polymers has been surprisingly found by the inventors to delay the nucleation of SAA, and as such give the PCM time to crystallise in its preferred form of SAT. The aforementioned polymers may delay the crystallisation of SAA from molten SAT indefinitely. The aforementioned polymers may delay the crystallisation of SAA in SAT by more than 1 hour, more than 4 hours, more than 10 hours, more than 24 hours, more than 48 hours or more than 72 hours. The delay in crystallisation of SAA in SAT may not proceed at temperatures below about 77 °C when the PCM comprises one or more of the polymers as disclosed herein. Further to this, it has been found by the inventors that, if the crystallisation of SAA from SAT does proceed in the PCM composition, that the crystal habit of the SAA is modified by the presence of the aforementioned polymers. Within the context of this disclosure, crystal habit is the favoured growth pattern (external shape or morphology) of an individual crystal or aggregate of crystals. In the absence of the aforementioned polymers, SAA crystallised from the SAT typically forms with plate- or block-like crystal morphologies. These crystals are large and discrete (i.e. do not form a network) and may easily sink through the PCM. Said crystals will tend to collect towards the bottom of the container in which the PCM, resulting in problematic macroscopic phase segregation which causes loss of PCM performance as shown in Figure 2. If phase segregation occurs, raising the temperature of the PCM to for example, 77 °C, may not return the PCM to a homogeneous state due to changes in the local concentration of sodium acetate. As SAA sinks in the PCM volume, the concentration of sodium acetate anhydrous towards the bottom of the container becomes higher than that at the top. Thus, the local temperature required to fully redissolve any sodium acetate anhydrous (SAA) formed within the phase change material rises towards the bottom of the PCM volume according to the phase diagram shown in Figure 1 due to said higher local concentration. Furthermore, this is simply the temperature required to redissolve the material, and does nothing to redistribute (i.e. re-homogenise) the sodium acetate throughout the PCM. Without any redistribution of materials, the SAA will simply reform in the same segregated regime when cooled and no improvement to the performance of the PCM will be obtained. Thus, it is desirable that, should SAA crystalise from the hydrated sodium acetate bulk in the PCM, that the crystals are hindered or prevented from sinking. Altering the crystal habit of SAA by inclusion in the PCM composition the aforementioned polymers has been found by the inventors to be a means by which any SAA which does crystallise from molten SAT may be hindered or prevented from sinking in the PCM bulk material. Inclusion of one or more of the aforementioned polymers in the PCM compositions disclosed herein has been found to change the crystal habit of any crystallised SAA from the plate- or block-like morphologies to fine needles, hairs, rods, or other similar acicular morphology. Said morphologies allow the crystals to form a volume-filling crystal network in the PCM as shown in Figure 3. In Figure 3 the crystallisation of sodium acetate anhydrous is modified by the polymers as disclosed herein causing a SAT-based PCM

[301] with a fill level

[303] in a PCM container

[302] to produce a homogeneous distribution of SAA and SAT in the PCM volume

[304] up to the fill volume

[305] in the PCM container

[306] rather than a segregated layer or gradient system as described in Figure 2 having a layer of precipitated crystalised SAA 203 at the bottom of the container, an intermediate layer of SAT 202, and a layer of water 204 at the top. Such a homogeneous PCM network is self-supporting, and as such the SAA material cannot sink as it is held in place by the macroscopic interparticle network. Thus, the SAA cannot form a concentration gradient in the PCM volume, cannot settle out or segregate on a macroscopic scale and therefore PCM remains substantially homogeneous with the SAA distributed evenly throughout the PCM volume. It should be understood that the mechanisms discussed above are merely non-binding suggestions, and it is not desired that the present invention be bound to any particular theory. It is disclosed herein that, in addition to the inclusion of one or more of the aforementioned polymers, additional water (i.e. beyond the amount required to form the trihydrate form of sodium acetate) may optionally be added to the composition to further hinder the crystallisation of SAA. It is a further disclosure of the present invention a PCM comprising: from about 40 wt% to about 70 wt.% of sodium acetate; from about 0.1 wt% to about 10 wt.% of one or more group I or group II metal salt of a polymer selected from a list comprising: lignosulfonic acid, carboxylated lignin, sulfated lignin, humic acid, lignohumic acid, sulfonated humin, sulfated humin, sulfonated humic acid, carboxylated cellulose, sulfated cellulose and sulfonated cellulose; from about 0.1 wt% to about 10 wt.% of one or more nucleation agents; and, water to balance. The at least one nucleation agent may be included in the PCM composition to aid in the crystallisation of SAT. Such at least one nucleation agent directs the PCM to crystallise at, up to, or substantially near about 58 °C, assuming that no melting point depression agents have also been included in the PCM composition. The at least one nucleation agent may be selected from a list comprising: mono-, di- and / or tri-sodium phosphate and / or any hydrate form(s) thereof, mono-, di-, tri- and / or tetra-sodium pyrophosphate and / or any hydrate form(s) thereof. Optionally, the PCM may further comprise a melting point depression agent. Melting point depression agents may take the form of salts of sodium and / or acetate salts other than sodium acetate. For example, the melting point depression agent may take the form of one or more materials selected from a list comprising: a group I acetate salt, a group II acetate salt, a sodium halide salt, a sodium carboxylate salt; and sodium nitrate. The PCM may comprise from about 3 wt% to about 40 wt% of the melting point depression agent. It has been found by the inventors that the inclusion of melting point depression agents has no significant effect on the ability of the aforementioned polymers to prevent segregation of SAA from molten SAT. Table 1 shows polymeric properties of specific polymers which may be selected in embodiments of the present invention. Table 1 Polymer Types Polymer Min Molecular Weight (Da) Max Molecular Weight (Da) Sodium lignosulfonate 1,000 400,000 Carboxylated lignin sodium salt 1,000 100,000 Sulfated lignin sodium salt 1,000 200,000 Sodium humate 1,500 600,000 Sodium cellulose sulfate 10,000 2,500,000 Sodium cellulose sulfonate 10,000 3,000,000 Sulfated humin sodium salt 750 150,000 Sulfonated humin sodium salt 750 150,000 In a preferred embodiment of the present invention, the polymer selected is sodium lignosulfonate. In a further preferred embodiment of the present invention, the polymer selected is sodium lignosulfonate with an average molecular weight of between 1,000 and 400,000 Da, between 1,000 and 200,000 Da, between 1,000 and 100,000 Da, between 1,000 and 75,000 Da, between 10,000 and 75,000 Da, or between 20,000 and 75,000 Da. In a further preferred embodiment of the present invention, the polymer selected is sodium lignosulfonate with an average molecular weight of about 52,000 Da and an average polymer chain length of about 7,000 units. In a preferred embodiment of the present invention, the polymer selected is humic acid and / or a salt thereof. In a further preferred embodiment of the present invention, the polymer selected is sodium humate with an average molecular weight of between 1000 and 600,000 Da, between 1000 and 500,000 Da, between 2,000 and 200,000 Da, between 2,000 and 100,000 Da, between 10,000 and 75,000 Da, or between 20,000 and 50,000 Da. In a preferred embodiment of the present invention, the polymer selected is the sodium salt of sulfated lignin. In a further preferred embodiment of the present invention, the polymer selected is the sodium salt of sulfated lignin with an average molecular weight of between 500 and 200,000 Da, between 500 and 150,000 Da, between 1,000 and 100,000 Da, between 1,000 and 50,000 Da, between 1,000 and 40,000 Da, between 1,000 and 30,000 Da, between 1,000 and 20,000 Da, between 1,000 and 10,000 Da or between 1,000 and 5,000 Da. In a preferred embodiment of the present invention, the polymer selected is the sodium salt of carboxylated lignin. In a further preferred embodiment of the present invention, the polymer selected is the sodium salt of carboxylated lignin with an average molecular weight of between 1,000 and 100,000 Da, between 1,000 and 50,000 Da, between 1,000 and 25,000 Da, between 1,000 and 15,000 Da, between 1,000 and 10,000 Da, between 1,000 and 7,000 Da, between 1,000 and 5,000 Da, between 1,000 and 4,000 Da or between 1,000 and 3,000 Da. In a preferred embodiment of the present invention, the polymer selected is a salt of cellulose sulfate. In a further preferred embodiment of the present invention, the polymer selected is sodium cellulose sulfate with an average molecular weight of between 10,000 and 2,500,000 Da, between 10,000 and 2,000,000 Da, between 10,000 and 1,500,000 Da, between 10,000 and 1,000,000 Da, between 10,000 and 500,000 Da, or between 10,000 and 100,000 Da. In a further preferred embodiment of the present invention, the polymer selected is sodium cellulose sulfate with an average molecular weight of about 11,000 and / or about 2,000,000. Note, herein the average molecular weights of the polymers disclosed are given as weight average molecular weights, typically denoted as Mw. PCMs as disclosed herein have been found by the inventors to be capable of being repeatedly melted and frozen with no phase segregation and therefore no change in thermal properties. This has been found to be beneficial in the performance of thermal energy storage devices comprising the PCMs disclosed herein, which may exhibit decrease, significantly decreased or substantially no degradation of performance when thermally cycled. The PCMs as disclosed herein may be thermally cycled more than once, more than 50 times, more than 500 times, more than 1000 times, more than 2,000 times, more than 5,000 times or more than 10,000 times without any significant change in thermal performance. The PCMs as disclosed herein may be cycled to this extent within a thermal energy storage device apparatus. Experimental Examples Example 1 A PCM was produced as follows; sodium acetate anhydrous was added to water in a 59:41 mass ratio of sodium acetate anhydrous to water, corresponding to a molar ratio of sodium acetate to water of 1:3.16. To this material was also added sodium lignosulfonate with an average molecular weight of about 52 kDa and an average mass number of 7000. The amount of the sodium lignosulfonate was about 0.8 wt.%. The material was then heated to >58 °C to produce the PCM in a liquid state. Cooling the PCM to ambient temperature resulted in no crystallisation of anhydrous sodium acetate for more than 72 hours. For comparison, a PCM sample prepared in the same way but without the addition of sodium lignosulfonate, anhydrous sodium acetate was found to crystallise within 1-2 hours of the sample reaching ambient temperature. Thus the hindering effect of the sodium lignosulfonate on the crystallisation of anhydrous sodium acetate was demonstrated. Example 2 A PCM was produced as follows; sodium acetate anhydrous was added to water in a 58:42 mass ratio of sodium acetate anhydrous to water, corresponding to a molar ratio of sodium acetate to water of 1:3.3. To this material was also added sodium lignosulfonate with an average molecular weight of about 10 kDa. The amount of the sodium lignosulfonate was about 0.6 wt.%. The material was then heated to >58 °C to produce the PCM in a liquid state. Cooling the PCM to ambient temperature resulted in no crystallisation of anhydrous sodium acetate for more than 12 hours. As in Example 1, a PCM sample prepared in the same way but without the addition of sodium lignosulfonate showed anhydrous sodium acetate to crystallise within 1-2 hours of the sample reaching ambient temperature. Thus the hindering effect of the sodium lignosulfonate on the crystallisation of anhydrous sodium acetate was demonstrated. Once sodium acetate anhydrous was found to crystallise within the sample comprising sodium lignosulfonate, it was observed that the crystal habit of the sodium acetate anhydrous was fine and needle-like in nature, and filled the whole volume of the PCM (as per the scenario detailed in Figure 3). Conversely the sample prepared without sodium lignosulfonate produced block or plate-like crystals which sunk through the bulk of the PCM to deposit on the bottom of the PCM container (as per the scenario detailed in Figure 2). Thus, the homogenising effect of sodium lignosulfonate on any crystallised anhydrous sodium acetate in the PCM is demonstrated. Example 3 A PCM was produced as follows; sodium acetate anhydrous was added to water in a 58:42 mass ratio of sodium acetate anhydrous to water, corresponding to a molar ratio of sodium acetate to water of 1:3.3. To this material was also added sodium lignosulfonate with an average molecular weight of about 20 kDa. The amount of the sodium lignosulfonate was about 1.2 wt.%. To this material was also added disodium phosphate dihydrate in an amount corresponding to about 1.6 wt.% of the total. The material was then heated to >58 °C to produce the PCM in a liquid state. The PCM was then transferred to a heat battery apparatus comprising a container within which was disposed a fin-tube heat exchanger. The heat battery was then sealed and thermally cycled by passing heated and cooled fluid through the heat exchanger tubes. The PCM was found to exhibit a melting transition at about 58 °C and a crystallisation transition between about 58 °C and 50 °C when cooled. The system was found to be able to be cycled about 5000 times without noticeable loss of energy storage capacity or changes to the phase change temperatures. Example 4 A PCM was produced as follows; sodium acetate anhydrous was added to water in a 60:40 mass ratio of sodium acetate anhydrous to water, corresponding to a molar ratio of sodium acetate to water of 1:3.03. To this material was also added sodium lignosulfonate with an average molecular weight of about 100 kDa. The amount of the sodium lignosulfonate was about 0.4 wt.%. To this material was also added disodium phosphate dihydrate in an amount corresponding to about 1.6 wt.% of the total and the melting point depression agent sodium nitrate in an amount corresponding to about 13 wt.% of the total. The material was then heated to >50 °C to produce the PCM in a liquid state. The PCM was then transferred to a heat battery apparatus comprising a container within which was disposed a fin-tube heat exchanger. The heat battery was then sealed and thermally cycled by passing heated and cooled fluid through the heat exchanger tubes. The PCM was found to exhibit a melting transition at about 50 °C and a crystallisation transition between about 50 °C and 40 °C when cooled. Example 5 A PCM was produced as follows; sodium acetate anhydrous was added to water in a 60:40 mass ratio of sodium acetate anhydrous to water, corresponding to a molar ratio of sodium acetate to water of 1:3.03. To this material was also added sodium lignosulfonate with an average molecular weight of about 15 kDa. The amount of the sodium lignosulfonate was about 1.0 wt.%. To this material was also added disodium phosphate dihydrate in an amount corresponding to about 1.6 wt.% of the total and the melting point depression agent monosodium glutamate monohydrate in an amount corresponding to about 15 wt.% of the total. The material was then heated to >56 °C to produce the PCM in a liquid state. The PCM was then transferred to a heat battery apparatus comprising a container within which was disposed a fin-tube heat exchanger. The heat battery was then sealed and thermally cycled by passing heated and cooled fluid through the heat exchanger tubes. The PCM was found to exhibit a melting transition at about 56 °C and a crystallisation transition between about 56 °C and 35 °C when cooled. The system was found to be able to be cycled about 5000 times without noticeable loss of energy storage capacity or changes to the phase change temperatures. Example 6 A PCM was produced as follows; sodium acetate anhydrous was added to water in a 59:41 mass ratio of sodium acetate anhydrous to water, corresponding to a molar ratio of sodium acetate to water of 1:3.16. To this material was also added sodium humate with a molecular weight in the range of 20kDa - 50kDa. The amount of the sodium humate was about 0.9 wt.%. The material was then heated to >58 °C to produce the PCM in a liquid state. Cooling the PCM to ambient temperature resulted in no crystallisation of anhydrous sodium 5 acetate for more than 24 hours. For comparison, as shown in Example 1, a PCM sample prepared in the same way but without the addition of sodium humate, anhydrous sodium acetate was found to crystallise within 1-2 hours of the sample reaching ambient temperature. Thus, the hindering effect of the sodium humate on the crystallisation of anhydrous sodium acetate was demonstrated. 10

Claims

1. A phase change material (PCM) comprising:hydrated sodium acetate;anda polymeric additive comprising a polymer selected from a list comprising: carboxylated, sulfated and / or sulfonated lignin;carboxylated, sulfated and / or sulfonated cellulose;carboxylated, sulfated and / or sulfonated humin;sulfated and / or sulfonated Humic acid, and / or humic acid,And / or a salt or derivative thereof.

2. A PCM according to claim 1, wherein the hydrated sodium acetate is from 2.5 hydrate to 3.5 hydrate, optionally wherein the hydrated sodium acetate is sodium acetate tri hydrate.

3. A PCM according to claim 1 or 2, wherein the PCM comprises a salt of the at least one polymer.

4. A PCM according to claim 3, wherein the salt is a group I metal salt or a group II metal salt of the polymer, optionally wherein the salt is a sodium salt of the polymer.

5. A PCM according to any preceding claim wherein the PCM further comprises a nucleation agent.

6. A PCM according to claim 5, wherein the nucleation agent is selected from a list comprising:mono-, di- and / or tri-sodium phosphate and / or any hydrate form(s) thereof;mono-, di-, tri- and / or tetra-sodium pyrophosphate and / or any hydrate form(s) thereof.

7. A PCM according to any preceding claim, wherein the polymer is sodium lignosulfonate.

8. A PCM according to any preceding claim wherein the polymer is sodium lignosulfonate with an average molecular weight of from about 1,000 to about 400,000, between 1,000 and 200,000 Da, or from about 1,000 to about 100,000 Da, or from about 1,000 Da to about 75,000 Da, or from about 10,000 to about 75,000 Da, or from about 20,000 to about 75,000 Da.

9. A PCM according to any preceding claim, wherein the polymer is sodium lignosulfonate with an average molecular weight of about 52,000 Da.

10. A PCM according to any preceding claim, wherein the polymer has an average polymer chain length from about 5,000 units to about 10,000 units, or from about 5,000 units to about 8,000 units, or from about 6,000 units to about 8,000 units, or from about 6,000 units to about 7,000 units, or from about 7,000 units to about 8,000 units, optionally wherein the polymer has an average polymer chain length of about 7,000 units.

11. A PCM according to any preceding claim, wherein the PCM comprises more water than is required to convert all the sodium acetate present in the PCM into sodium acetate trihydrate, optionally wherein the PCM comprises more than 3 molar equivalents of water per 1 mole of sodium acetate.

12. A PCM according to any preceding claim, wherein the PCM comprises water such that the molar ratio of sodium acetate:water is between about 1:3 and 1:3.5, between about 1:3 and 1:3.3, between about 1:3 and 1:3.1, or is about 1:3.05, about 1:3.1, about 1:3.2, or about 1:3.3.

13. A PCM according to any preceding claim wherein the PCM has a phase transition temperature at around 58 °C14. A PCM according to any preceding claim, wherein the PCM does not comprise a thickener.

15. A PCM according to any preceding claim wherein the PCM comprises: from about 40 wt% to about 70 wt% of sodium acetate.

16. A PCM according to any preceding claim wherein the PCM comprises from about 0.1 wt% to about 10 wt% of a polymer selected from a list comprising: lignosulfonic acid, carboxylated lignin, sulfated lignin, humic acid, lignohumic acid, sulfonated humin, sulfated humin, sulfonated humic acid, sulfonated cellulose, sulfated cellulose, carboxylated cellulose, and / or a salt or derivative thereof, optionally wherein the polymer is a group I or a group II metal salt of the polymer.

17. A PCM according to any preceding claim, wherein the PCM comprises from about 10 wt% to about 60 wt% of water, or from about 25 wt% to about 50 wt% of water, or about 40% water.

18. A PCM according to any preceding claim, wherein the PCM comprises from about 0.1 wt% to about 10 wt% of at least one nucleation agent; optionally wherein the at least one nucleation agent is selected from a list comprising:mono-, di- and / or tri-sodium phosphate and / or any hydrate form(s) thereof;mono-, di-, tri- and / or tetra-sodium pyrophosphate and / or any hydrate form(s) thereof.

19. The PCM according to any preceding claim, wherein the PCM comprises a melting point depression agent, optionally wherein the melting point depression agent is a salt of sodium and / or an acetate salt other than sodium acetate, further optionally wherein the melting point depression agent is one or more material selected from a list comprising: a group I acetate salt, a group II acetate salt, a sodium halide salt, a sodium carboxylate salt; and sodium nitrate.

20. The PCM according to claim 19, wherein the PCM comprises from about 3 wt% to about 40 wt% of the melting point depression agent.

21. The PCM of any preceding claim, wherein the PCM is capable of being thermally cycled more than once, more than 50 times, more than 500 times, more than 1000 times, more than 2,000 times, more than 5,000 times or more than 10,000 times without significant change in thermal performance.

22. A PCM according to any preceding claim wherein the PCM comprises:from about 40 wt% to about 70 wt% of sodium acetate;from about 0.1 wt% to about 10 wt% of a polymer selected from a list comprising: lignosulfonic acid, humic acid, lignohumic acid, sulfonated humin, sulfonated humic acid, sulfonated cellulose, sulfated cellulose, carboxylated cellulose, and / or a salt or derivative thereof.; and water to balance; andoptionally wherein the PCM further comprises at least one of:from about 0.1 wt% to about 10 wt% of at least one nucleation agent; optionally wherein the at least one nucleation agent is selected from a list comprising: mono-, di- and / or tri-sodium phosphate and / or any hydrate form(s) thereof; mono-, di-, tri- and / or tetra-sodium pyrophosphate and / or any hydrate form(s) thereof; and / orfrom about 3 wt% to about 40 wt% of a melting point depression agent, optionally wherein the melting point depression agent is selected from a group I acetate salt, a group II acetate salt, a sodium halide salt, a sodium carboxylate salt; and sodium nitrate.

23. A PCM according to any preceding claim, wherein the PCM comprises:from 40 wt% to 70 wt% of sodium acetate,from 0.1 wt% to 10 wt.% sodium lignosulfonate,from 0.1 wt% to 10 wt.% of disodium phosphate and / or any hydrate form thereof; andwater to balance.

24. A thermal energy storage apparatus comprising a PCM according to any preceding claim.

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