Heat storage material

Abstract

Disclosed herein is a sodium acetate trihydrate-based phase change material (PCM) with improved thermal stability over repeated cycles. More explicitly, disclosed herein is a sodium acetate trihydrate-based phase change material comprising a polymer additive to improve the phase stability of the PCM over repeated melt-freeze cycles by maintaining the bulk PCM in a substantially homogeneous state at all temperatures.

Description

[0001] Heat Storage Material 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. Said secondary phases may decrease the energy storage capacity of the PCM, by, for instance, not undergoing a phase change in the temperature range applied to the PCM during use and thereby only contributing specific heat to the energy storage capacity of the material. 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 formulaCH3COONa·3H2O, orfrom about CH3COONa·2.5H2O to about CH3COONa·3.5H2O. 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. Furthermore, the PCM may remain homogeneous or substantially homogeneous at all temperatures over extended periods of time and over repeated thermal cycles. The polymeric additive may prevent, delay or otherwise hinder 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. If crystalline sodium acetate anhydrous was to form the polymeric additive would cause the crystals to form in a crystal habit which would remain distributed throughout the PCM and thereby the PCM would be retained in a homogeneous state. The polymeric additive may cause any crystalline of sodium acetate anhydrous to form in a crystal habit which is smaller, finer, has a higher aspect ratio, is buoyant and / or forms a self- supporting network and is thereby resistant to sinking through the PCM, compared to PCM without the polymeric additive. 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 be a biopolymer. The polymeric additive may be a biopolymer comprising one or more functional group selected from Carboxylate, Sulfate, Sulfonate, Phosphate, and / or phosphonate. The polymeric additive comprise a biopolymer backbone wherein the biopolymer has been modified to include one or more functional groups selected from Carboxylate, Sulfate, Sulfonate, Phosphate, and / or phosphonate. 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 biopolymer (e.g. at least one naturally-derived polymer). The biopolymer may be any suitable biopolymer capable of withstanding the conditions in which the PCM is used. The biopolymer may be based on humin, lignin, and / or a polysaccharide such as cellulose, chitin, or chitosan. However, it has been found by the inventors that for such polymers to be effective, they require a structural modification to include one or more acid groups, which may be deprotonated in the PCM becoming anionic in nature. The structural (e.g. acidic) modification of the biopolymer may be a carboxylation, phosphorylation, phosphonation, sulfation, and / or sulfonation, resulting in a natural polymer featuring carboxylate, phosphate, phosphonate, sulfate and / or sulfonate functional group. Such a polymer may then be applied as an additive for hydrated sodium acetate-based PCMs according to the present invention. In other words, the PCM material of the present invention may comprise biopolymer selected from humin, humic acid, lignin and / or a polysaccharide (e.g. chitin, chitosan, cellulose), comprising at least one carboxylate, phosphate, phosphonate, sulfate and / or sulfonate functional group. The polymeric additive may comprise at least one polymer selected from a list comprising: Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated chitin, Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated chitosan, Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated lignin; Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated cellulose; Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated humin; Phosphorylated, phosphonated, 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. NH4+forms of the polymers may also be used. 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. phosphate, phosphonate, sulfate, sulfonate, carboxylate) partially or fully charge balanced with one or more group I and / or group II metal cation and / or ammonium. 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. 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. 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 at a temperature below its melting point while the polymeric additive prevents, hinders or delays crystallisation of the unwanted species (sodium acetate anhydrous) at any temperature. This combination may be particularly effective at maintaining the PCM in a homogeneous state over extended use of the PCM. The at least one polymer preferably may be a group I or group II metal salt of a polymer selected from a list comprising: Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated chitin, Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated chitosan, Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated lignin; Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated cellulose; Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated humin; Phosphorylated, phosphonated, sulfated and / or sulfonated Humic acid, and / or humic acid, and / or a salt or derivative 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 acetate and water) throughout the PCM volume. 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. It is key to the performance of the PCM that the PCM remains homogeneous during use, which may comprise repeated melt and freeze cycles, extended periods of being retained molten, repeated partial melt and / or freeze cycles, and / or repeated extended periods of being retained molten with repeated partial 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%. It is an advantage of the present invention that a minimum or no additional water may be required to stabilise the hydrated sodium acetate based PCM. If a PCM of around the trihydrate composition is desired, additional water may be beneficial for the stability of the PCM as it hinders the crystallisation of sodium acetate anhydrous, however additional water decreases the energy storage density of the PCM and also decreases the probably of nucleation of the trihydrate. Inclusion of the polymers as disclosed herein allows a minimum or no excess water to be used in sodium acetate trihydrate based PCMs without the disadvantageous formation of anhydrous sodium acetate. 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: Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated chitin, Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated chitosan, Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated lignin; Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated cellulose; Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated humin; Phosphorylated, phosphonated, sulfated and / or sulfonated Humic acid, and / or humic acid, and / or a salt or derivative thereof. And may exhibit delayed 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 polymer may be a lignosulfonate, particularly 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. 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, or it may provide the inhibition of sodium acetate anhydrous nucleation with minimal thickening effect. 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 or other biomass. 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: Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated chitin, Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated chitosan, Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated lignin; Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated cellulose; Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated humin; Phosphorylated, phosphonated, sulfated and / or sulfonated Humic acid, and / or humic acid, and / or a salt or derivative thereof. and at least one nucleation agent. The polymeric additive may be preset in salt form, optionally as a Group I or Group II metal salt or as a NH4+salt of the polymer. 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 polymer selected from a list comprising: Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated chitin and / or a salt or derivative thereof, Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated chitosan and / or a salt or derivative thereof, Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated lignin and / or a salt or derivative thereof; Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated cellulose and / or a salt or derivative thereof; Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated humin and / or a salt or derivative thereof; Phosphorylated, phosphonated, sulfated and / or sulfonated Humic acid, and / or humic acid, and / or a salt or derivative thereof. The polymer may be preset in salt form, optionally as a Group I or Group II metal salt or as a NH4+salt of the polymer. 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, more than 10,000 times, more than 20,000 times or more than 40,000 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 be present in the PCM bulk. If SAA is formed and crystallised from the molten SAT bulk, it will sink due to the density of SAA being greater than the liquid density of molten SAT, finally depositing at the bottom of the PCM container as shown in Figure 2. In Figure 2 a volume of SAT-based PCM

[0201] is shown segregating into a SAT layer

[0202] , a segregated SAA rich layer

[0203] and a water rich layer

[0204] 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 in terms of composition between the lower and upper portions of the PCM volume. 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 available water to form the trihydrate (and thereby release heat). 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. Should a gradient of concentration form, water rich compositions will be found towards the top of the PCM volume and water poor compositions will be found towards the bottom of the PCM volume, both performing poorly compared to the homogeneous composition. 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, and may improve cycle stability, 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 SAA in SAT-based PCMs. The need for additional water is thereby reduced or avoided without reducing the stability of the PCM. 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. The inventors have surprisingly found that biopolymers comprising one or more anionic groups on the biopolymer backbone may be added into the SAT-based PCM to provide such a stabilising effect. The anionic groups may be selected from a list comprising: Carboxylate, Sulfonate, Sulfate, Phosphate, And, Phosphonate Said anionic groups may be present on the biopolymer backbone as anions, or may be present as their protonated forms (i.e. their acid forms). Carboxylation is the introduction of R-COOH groups (or R-COO- groups in their anionic / deprotonated form), where R represents the biopolymer backbone. Sulfonation is the introduction of R-SO3H groups (R-SO3- groups in their anionic / deprotonated form), where R represents the biopolymer backbone. Sulfation is the introduction of R-OSO3H groups (R-OSO3- groups in their anionic / deprotonated form), where R represents the biopolymer backbone. Phosphorylation is the introduction of R-OPO3H2 groups (R-OPO3H- and / or R-OPO32-in their anionic / mono and di-deprotonated forms respectively), where R represents the biopolymer backbone. Phosphonation is the introduction of R-PO3H2 groups (R-PO3H- and / or R-PO32-in their anionic / mono and di-deprotonated forms respectively), where R represents the biopolymer backbone. Any of such modifications to biopolymers produces an additive which may be used as part of a PCM which comprises hydrated sodium acetate, resulting in superior stability of the PCM. Biopolymers which may be used as a backbone for said modifications may be selected from a list comprising: chitin, chitosan, cellulose, lignin, humin, and, humic acid Said biopolymer backbones may be selected for their cost and / or environmental sustainability. The structure of chitin is shown below: The structure of chitosan is shown below: The structure of cellulose is shown below: A section of the approximate structural formula of lignin is shown below:

[0002] 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 plurality of different ways, giving a variety of different polymeric structures. 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 humin type polymer. A section of the approximate structure of humic and lignohumic acids showing typical functionalities present in said materials is shown below: 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 S.S. Al-Faiyz, CPMAS13C 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. In preferred embodiments of the present invention, the above structures are modified by phosphorylation, phosphonation, carboxylation, sulfation and / or sulfonation, giving modified polymers which may be applied as additives to hydrated sodium acetate-based PCMs. The phosphorylation, phosphonation, carboxylation, sulfation and / or sulfonation of the above structures may take place at one or a plurality of different locations on the polymer chain, and may concurrently cause functional groups present in the structure to be cleave and / or re-arrangement of the polymer chain. Typically, the polymers activity for inhibition of formation of sodium acetate anhydrous and crystal habit modification of sodium acetate anhydrous requires the polymer to be at least partially soluble in the PCM (e.g. at least partially soluble in hydrated sodium acetate). Thus, the polymer may be used as a salt, or may form a salt in the PCM. Hydrated sodium acetate based PCMs are typically alkaline in nature, and therefore may deprotonate the anionic groups forming part of the polymer forming a salt of the polymer should they be added in their acid form. In particular, polymers may be one or more group I and / or group II metal salts of polymers selected from a list comprising: Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated chitin, Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated chitosan, Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated lignin; Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated cellulose; Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated humin; Phosphorylated, phosphonated, sulfated and / or sulfonated Humic acid, and / or humic acid, and / or a salt or derivative thereof. Without wishing to be bound by any particular theory, it is proposed that selecting a natural polymer such as chitin, chitosan, lignin, cellulose, humin and / or humic acid, provides a backbone for anionic groups such as those as described above. The biopolymers have improved sustainability, due to their natural origins, and act as a suitable scaffold for the inclusion of anionic functionalities. Said functionalities have an affinity for sodium acetate anhydrous nuclei surface, adhering to the surface and inhibiting growth. As such the polymers disclosed herein inhibited sodium acetate anhydrous nuclei from reaching the critical size necessary for spontaneous particle growth, therefore macroscopic crystals are inhibited from forming. Due to the probabilistic nature of nucleation, it is possible that some macroscopic sodium acetate anhydrous crystals do form under certain conditions of temperature, pressure, or over long durations. The polymers disclosed herein have a secondary function in addition to the inhibition of sodium acetate anhydrous nucleation, in that, if nucleated, sodium acetate anhydrous crystal growth is altered. Without modification, sodium acetate anhydrous crystals grow in plate- or block-like morphologies, however in the presence of the polymers disclosed in the present invention, sodium acetate anhydrous, where successfully nucleated, grows into fine needles, hairs, rods, or other similar acicular morphology. Such morphologies have improved surface to volume ratios and tend to form loose aggregates which are retained in a well distributed manner throughout the PCM and thereby redissolve easily on heating. A series of non-limiting examples of the polymers disclosed herein are given below, showing the structural features present in the polymers and suggested structural alterations caused by the addition of the one or more anionic groups to the polymer backbones. These structures may be altered in terms of the size of the polymer, the number, position, connectivity and arrangement of functionalities present, and may further be altered in terms of the number, position and arrangement of the anionic groups (e.g. the carboxylate, phosphonate, phosphate, sulfate and sulfonate groups) which form part of the polymer according to the present invention. These features vary dependent on the biopolymer source used and processing applied. Lignin may be phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated for use in the PCMs disclosed herein. Examples of such are given below. Sulfonated lignins and lignosulfonates exhibit similar subunits, connected in similar or the substantially the same manner, with the addition of sulfonate (-SO3-) groups, which may be at various points on the lignin polymer backbone. Sulfonation may occur in varying amounts, (i.e. various amounts of sulfonate groups may be substituted onto the lignin polymer backbone). For example, a section of the approximate structural formula of sodium lignosulfonate is shown below:

[0003] 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. By way of further example, a section of the approximate structural formula of carboxylated lignin is shown below: Sulfation of lignin involves the inclusion of (-OSO3-) groups into one or a plurality of points in the lignin polymer chain structure. Sulfated lignin may be understood accordingly, where the sulfate group (-OSO3-) 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 (-OSO3-) may be present at any point in the polymer 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. Alterations to the lignin structure have also been suggested in sulfated lignins, exemplified by Raghuraman et. al. in Biomacromolecules. 2007 Apr 17;8(5):1759–1763, as shown below: Phosphonation and phosphorylation of lignin involve the inclusion of phosphonate (-PO32-) and phosphate groups (-O-PO32-) onto the polymer backbone respectively. An example of phosphorylation processing of lignin proposed by Gao et. al. (International Journal of Biological Macromolecules Volume 162, 1 November 2020, Pages 1642-1652), showing various sites on the lignin chain where phosphate groups may be inserted, is shown below:

[0004] In addition to addition of a phosphate group, polyphosphate functionalities may be present in phosphorylated polymers, as can be exemplified by the above structures. Sulfation, sulfonation, phosphonation, phosphorylation 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. Cellulose may be phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated for use in the PCMs disclosed herein. Examples of such are given below. Examples of 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 sulfated cellulose structure is given below: 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. Phosphorylated cellulose may be used as part of the present invention. An example of phosphorylated cellulose proposed by Benhamou et. al. (Cellulose 28, 4625–4642 (2021)) is given below: Addition of phosphate groups may occur at a variety of positions around the cellulose ring structure, for example at one or both of the methyl alcohol groups (the above example shows substitution at a single methyl alcohol group). In the above example, the acid is shown, however any salt and / or derivative thereof may be used as part of the present invention. Further phosphorylation at the phosphate group to give polyphosphates is also possible. Chitin and chitosan may be phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated for use in the PCMs disclosed herein. Often the processing of chitin and chitosan to produce said modifications proceeds similarly, and modification of one proceeds in the same manner as the other. Examples of phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated chitin and chitosan are given below. Phosphorylation of chitin and chitosan has been shown by Jayakumar et. al. (International Journal of Biological Macromolecules Volume 43, Issue 3, 1 October 2008, Pages 221-225) to proceed in two positions on the chitin / chitosan ring system dependent on the conditions used: Phosphorylated chitin and chitosan can therefore be present in the PCM where the phosphorus containing moiety is bound to the methyl-oxygen or the amine group. Carboxylation of chitin and chitosan has been suggested to proceed according to Zhong et. al. (Nanotechnology Reviews, vol.11, no.1, 2022, pp.2673-2713) to produce polymers as shown below: Where R = H in the case of chitosan and R = N-acetyl in the case of chitin. According to Khattak et. al. (Appl Microbiol Biotechnol 103, 1989–2006 (2019)) sulfation and sulfonation of chitosan may produce a polymer as shown below: Where sulfate groups may be attached to alcohol functionalities around the chitosan ring system and / or produce sulfamate and / or sulfonate functionalities at the ring amine functionality. Similarly, Wang et. al. (Cellulose, Volume 29, pages 7099–7109, (2022)) suggest that sulfation and sulfonation of chitin may produce polymers of the type shown below: Such example polymers given above are suitable for use as stabilising additives in the hydrated sodium acetate-based PCMs. 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 connectivity and interconnectivity of the polymer chains may also 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 phosphate, phosphonate, 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 chitosan, chitin, lignin, cellulose and humin without the presence of the phosphonate, phosphate, carboxylate, sulfate and / or sulfonate groups have no activity in stabilising hydrated sodium acetate based PCMs. Thus, the presence of one or more anionic 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

[0301] with a fill level

[0303] in a PCM container

[0302] to produce a homogeneous distribution of SAA and SAT in the PCM volume

[0304] up to the fill volume

[0305] in the PCM container

[0306] 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: Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated chitin, Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated chitosan, Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated lignin; Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated cellulose; Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated humin; Phosphorylated, phosphonated, sulfated and / or sulfonated Humic acid, and / or humic acid, and / or a salt or derivative thereof. 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 1,000 100,000 salt 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 750 150,000 salt Carboxylated chitin sodium 1,000 3,000,000 salt Carboxylated chitosan 1,000 1,000,000 sodium salt Sulfated chitin sodium salt 1,500 1,000,000 Sulfated chitosan sodium 1,500 1,000,000 salt Phosphorylated chitin 1,500 700,000 sodium salt Phosphorylated chitosan 1,500 700,000 sodium salt It has been found that low molecular weight polymers of the types disclosed herein may be preferable for application as stabilising agents for hydrated sodium acetate-based PCMs. Lower molecular weights, for example below 1,000,000, below 500,000, below 250,000, below 100,000, below 75,000, below 50,000, below 40,000, below 30,000, below 20,000 or below 10,000 Da in average molecular weight. It is thought by the inventors that lower molecular weight polymers have improved dispersibility and solubility in the PCM, and therefore perform better. 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 ºC 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 ºC 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 ºC 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 ºC 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 ºC 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 ºC 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 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.

Claims

Claims 1. A phase change material (PCM) comprising: hydrated sodium acetate; and a polymeric additive comprising a biopolymer backbone wherein the biopolymer has been modified to include one or more functional groups selected from: Carboxylate, Sulfate, Sulfonate, Phosphate, And / or Phosphonate.

2. A phase change material (PCM) according to claim 1, wherein the polymeric additive comprises a polymer selected from a list comprising: Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated chitin, Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated chitosan, Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated lignin; Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated cellulose; Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated humin; Phosphorylated, phosphonated, sulfated and / or sulfonated Humic acid, and / or humic acid, and / or a salt or derivative thereof.

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

4. A PCM according to any preceding claim, wherein the PCM comprises a salt of the at least one polymer, optionally 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, optionally 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, further optionally wherein the polymer is sodium lignosulfonate with an average molecular weight of about 52,000 Da.

8. 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.

9. 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.

10. 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.

11. A PCM according to any preceding claim wherein the PCM has a phase transition temperature at around 58 °C.

12. A PCM according to any preceding claim, wherein the PCM does not comprise a thickener.

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

14. 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: Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated chitin, Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated chitosan, Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated lignin; Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated cellulose; Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated humin; Phosphorylated, phosphonated, sulfated and / or sulfonated Humic acid, and / or humic acid, and / or a salt or derivative thereof. optionally wherein the polymer is a group I or a group II metal salt of the polymer.

15. 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.

16. 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.

17. 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.

18. The PCM according to claim 17, wherein the PCM comprises from about 3 wt% to about 40 wt% of the melting point depression agent.

19. 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.

20. 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: Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated chitin, Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated chitosan, Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated lignin; Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated cellulose; Phosphorylated, phosphonated, carboxylated, sulfated and / or sulfonated humin; Phosphorylated, phosphonated, sulfated and / or sulfonated Humic acid, and / or humic acid, and / or a salt or derivative thereof.; and water to balance; and optionally 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 / or from 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.

21. 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; and water to balance.

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