Clathrate heat storage material

Abstract

Disclosed herein are phase change materials (PCMs) comprising clathrate hydrates and pH modulation agents. More specifically, claimed herein are PCMs comprising tetra- alkylammonium and tetra-alkylphosphonium salt hydrate clathrates and pH modulation agents which comprise common cations or anions to the salt hydrate clathrates. Also disclosed herein are methods of preparing the PCMs and thermal energy storage devices for use with the PCMs.

Description

[0001] Clathrate Heat Storage Material

[0002] Field of the Invention

[0003] The present invention relates to thermal energy storage media. More specifically, the present invention relates to clathrate hydrate thermal energy storage media. More specifically still, the present invention relates to pH modulated clathrate hydrate thermal energy storage media.

[0004] Background of the Invention

[0005] Clathrate hydrates are large hydrogen bonded cage-like structures of water molecules which host secondary guest species. A sub-set of clathrates, the semi-clathrates, exhibit guest molecules and / or ions in both the cage cavity and also form a part of the water cage structure itself. Herein the term clathrate is defined as being inclusive of both clathrate and semiclathrate materials.

[0006] Clathrates may take the form of a salt hydrate.

[0007] Clathrates may take the form of a salt hydrate comprising a large cation comprising alkyl chains, a variety of different anions, and a hydration number between about 20 and 80 (i.e. the saltwater molar ratio is between about 1 :20 and 1 :80). Salt hydrate clathrates exist having an anion displacing water in the cage network, thereby incorporating the ion in the cage network, and the cation filling the cage cavity spaces.

[0008] Clathrates may be used in thermal energy storage systems as phase change materials (PCMs). Clathrates typically have melting and freezing points in the temperature range of -10 to 40 °C. This makes them particularly suitable for use in a variety of cooling and mild heating applications and thermal buffering, where their high latent heat of fusion is advantageous.

[0009] Key properties of thermal performance of a PCM (i.e. it’s energy storage capacity), are phase transition temperature, stability to thermal cycling and maximum and minimum temperature of operation.

[0010] However, clathrate hydrates may have pHs between 9 and 14 or between than 1 and 4 and therefore they can be hazardous for users, difficult to dispose of without causing environmental damage, and difficult to work with as they may corrode equipment with which they come into contact. Specifically, clathrate hydrate PCMs may be unable to be disposed of safely, as the pH is too high or low to be safe to dispose of by normal routes. Clathrate hydrate PCMs may be hazardous to handle by manufacturers or users of the PCM due to the risk of coming into contact with materials with high or low pHs, which may cause skin and eye damage and / or irritation. Clathrate hydrate PCMs may be incompatible with preparation vessel components or thermal energy storage device components, and may degrade, corrode or react with said components in a detrimental manner. Particularly, clathrate hydrate PCMs may be corrosive to various metals, which have particular stability windows of pH which the clathrate may exceed.

[0011] Furthermore, it has been found by the inventors that clathrate hydrates may exhibit subcooling. This is the process by which a material may remain in a liquid state as it is cooled below its phase transition temperature (i.e. it does not freeze), and may remain in a supersaturated state. This is disadvantageous for reliable release of heat from the PCM, and may be disadvantageous for systems comprising a PCM. For example, such a system may not freeze at all over the available temperature range, and thus the latent heat is not accessed.

[0012] It is an object of the present invention to mitigate or obviate one or more of the preceding problems and to provide a PCM which is safe, environmentally benign and which can be disposed of with lessened environmental impact, compatible with any components / equipment with which it comes into contact (i.e. the PCM is non-reactive with common preparation vessels and / or thermal energy storage device components) and which does not exhibit subcooling and freeze reliably at a maximum of temperature.

[0013] Summary of the Invention

[0014] According to the first aspect of the present invention is disclosed a phase change material (PCM) comprising: at least one clathrate hydrate, and at least one pH modulation agent.

[0015] The clathrate hydrate may comprise a salt hydrate. The clathrate hydrate may comprise a cation, and anion and water. The pH modulation agent may comprise at least one of the same cation and / or the same anion as the clathrate hydrate.

[0016] The PCM may further comprise a nucleation agent. The nucleation agent may be a zeolite. Preferably, the zeolite may be one or more materials of a zeolite type selected from a list comprising: 3A, 4A, 5A, 10X, and / or 13X. The PCM may comprise from about 70 wt% to about 99.995 wt% of at least one clathrate hydrate, or from about 80 wt % to about 99.995 wt%, or from about 90 wt% to about 99.995 wt%, or from about 95 wt% to about 99.995 wt%, or from about 96 wt% to about 99.995 wt%, or from about 97 wt % to about 99.995 wt%, or from about 98 wt% to about 99.995 wt%, or from about 99 wt% to about 99.995 wt%, or from about 96 wt% to about 98 wt%, or from about 96 wt% to about 97 wt%, or from about 96 wt% to about 96.5 wt% clathrate hydrate. In preferred embodiments, the PCM may comprise from about 96 wt% to about 99.995 wt% of at least one clathrate hydrate.

[0017] The PCM may comprise from about 0.005 wt% to about 30 wt%% of, or from about 0.005 wt% to about 20 wt%, or from about 0.005 wt% to about 10 wt%, or from about 0.005 wt% to about 5 wt%, or from about 0.005 wt% to about 4 wt%, or from about 0.005 wt% to about 3 wt%, or from about 0.005 wt% to about 2 wt%, or from about 0.005 wt% to about 1 wt%, or from about 0.005 wt% to about 0.5 wt%, or from about 0.005 wt% to about 0.05 wt %, or from about 0.05 wt% to about 5 wt %, or from about 0.5 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 1 wt% to about 4 wt%, or from about 2 wt% to about 4 wt%, or from about 3 wt% to about 4 wt% of at least one pH modulation agent. In preferred embodiments, the PCM may comprise from about 0.005 wt% to about 4 wt% of at least one pH modulation agent.

[0018] The PCM may comprise from about 0.1 wt% to about 20 wt% of a nucleation agent, or from about 0.1 wt% to about 5 wt%, or from about 1 wt% to about 20 wt%, or from about 1 wt% to about 15 wt%, or from about 1 wt% to about 10 wt% or from about 1 wt% to about 5 wt%, or from about 5 wt% to about 10 wt% or from about 5 wt% to about 15 wt%, or from about 10 wt% to about 20 wt%, or from about 10 wt% to about 15 wt%, or from about 15 wt% to about 20 wt%. Preferably, the PCM may comprise from about 1 wt% to about 5 wt% of a nucleation agent, preferably a zeolite nucleation agent.

[0019] The PCM may comprise between about 96 wt% and about 99.995 wt.% of at least one clathrate hydrate, and between about 0.005 and about 4 wt.% of one or more pH modulation agent, and optionally a nucleation agent, further optionally from about 0.1 wt% to about 5 wt% of a nucleation agent.

[0020] The pH modulation agent may be used to change the pH of the PCM to within the range 4-11 , between 5-10, between 6-9, between 7-9, between 7.5-8.5, or about 8. In other words, the (pH modulated) PCM may have a pH of about 4 to about 11 , about 5 to about 10, about 6 to about 9, about 7 to about 9, about 6.5 to about 8.5, or about 8. Advantageously, these pH ranges are most suitable for safe handling, environmental safety, and compatibility with various thermal energy storage device components and preparation vessel components.

[0021] The clathrate hydrate may be a salt-hydrate.

[0022] The clathrate hydrate may be a salt hydrate with a molar ratio of saltwater of from about 1 :20 to about 1 :40, from about 1 :25 to about 1 :35, or about 1 :30.

[0023] The clathrate may be a salt hydrate that having a molar ratio of catiomanion of 1 :1 , and a molar ratio of saltwater from about 1 :20 to about 1 :40, from about 1 :25 to about 1 :35, or about 1 :30.

[0024] The clathrate hydrate may be a salt hydrate with a molar ratio of saltwater of from about 1 :50 and to about 1 :70, from about 1 :55 to about 1 :65, or about 1 :60.

[0025] The clathrate may be a salt hydrate that having a molar ratio of catiomanion of 2:1 , and a molar ratio of saltwater from about 1 :50 to 1 :70, from about 1 :55 to about 1 :65, or about 1 :60.

[0026] The clathrate hydrate may comprise a large cation bearing organic chain groups (e.g. alkyl chains), connected to a central nitrogen or phosphorus atom. The organic chain groups may comprise branched or linear chains. The clathrate hydrate may comprise one or more anions.

[0027] The clathrate hydrate may comprise a tetra-alkyl ammonium and / or tetra-alkyl phosphonium salt hydrate.

[0028] The clathrate hydrate may be a tetra-alkyl ammonium and / or tetra-alkyl phosphonium salt hydrate where the alkyl chains have a carbon chain length of C1 to C6. The carbon chains may be straight or branch chained. The clathrate hydrate may be a tetra-alkyl ammonium and / or tetra-alkyl phosphonium salt hydrate where the alkyl chains are all the same or where at least two of the alkyl chains are different. The carbon chains may be one or more alkyl chains selected from a list comprising: methyl-, ethyl-, propyl-, butyl-, isoamyl-, pentyl-, isopentyl-, and / or hexyl-.

[0029] The clathrate hydrate may be a salt hydrate wherein the salt comprises an anion selected from a list comprising: any deprotonated amino acid anion, carbonate, formate, acetate, propionate, butyrate, caprate, caproate, glycolate, lactate, pyruvate, valerate, acrylate, citrate, oxalate, malonate, succinate, glutarate, malate, adipate, pimelate, fumarate, suberate, azelate, benzoate, salicylate, phthalate, terephthalate, picolinate, methanesulfonate, ethanesulfonate, propanesulfonate, butanesulfonate, benzenesulfonate, toluenesulfonate, methanedisulfonate, ethanedisulfonate, propanedisulfonate, butanedisulfonate, sulfamate and / or isomers thereof, and / or halogenated forms thereof and optionally the salt hydrate may comprise a tetra-alkyl ammonium and / or tetra-alkyl phosphonium cation.

[0030] The clathrate hydrate may be a salt hydrate wherein the salt comprises an anion selected from a list comprising: any halide, chromate, tungstate, sulfate and / or nitrate and optionally the salt hydrate may comprise a tetra-alkyl ammonium and / or tetra-alkyl phosphonium cation.

[0031] It has been found by the inventors that it is advantageous to match the pH modulation agent to either the cation or the anion that comprise the salt in a salt hydrate clathrate hydrate PCM. It has surprisingly been found by the inventors that this allows the pH to be altered without significant negative effects on the thermal performance (for example the melting and crystallisation phase transition temperatures) of the PCM.

[0032] The pH modulation agent may be an acid or base which has a common ion with the clathrate hydrate. The pH modulation agent may be a conjugate acid or conjugate base of the cation and / or or anion which forms part of the clathrate hydrate.

[0033] The pH modulation agent may be an agent which raises the pH of the PCM, (i. e., it may behave as a base). The pH modulation agent may be a base which comprises a tetra-alkyl ammonium and / or tetra-alkyl phosphonium cation. The pH modulation agent may be a tetraalkyl ammonium hydroxide and / or a tetra-alkyl phosphonium hydroxide.

[0034] The pH modulation agent may be an agent which reduces the pH of the PCM, (i.e. it may behave as an acid).

[0035] Said acid may have the general formula Hn[X]m', wherein [X] is an anion with the same identity as an anion present in the clathrate hydrate, H is a proton, and n and m are integers (e.g. 1 , 2, 3, 4, 5, or 6, preferably 1 , 2 or 3, further preferably 1 or 2). The integers m and n may be the same, or may be different, i.e. the acid may be provided in a completely protonated or partially protonated form. The anion [X] of the acid may be selected from the list comprising: any deprotonated amino acid anion, carbonate, formate, acetate, propionate, butyrate, caprate, caproate, glycolate, lactate, pyruvate, valerate, acrylate, citrate, oxalate, malonate, succinate, glutarate, malate, adipate, pimelate, fumarate, suberate, azelate, benzoate, salicylate, phthalate, terephthalate, picolinate, methanesulfonate, ethanesulfonate, propanesulfonate, butanesulfonate, benzenesulfonate, toluenesulfonate, methanedisulfonate, ethanedisulfonate, propanedisulfonate, butanedisulfonate, sulfamate and / or isomers thereof, and / or halogenated forms thereof. .

[0036] The anion [X] of the acid may be selected from the list comprising: any halide, chromate, tungstate, sulfate and / or nitrate

[0037] Preferably, the anion [X] of the acid may be selected from a carboxylate bearing ion, sulfonate bearing ion, nitrate ion, sulfate ion and / or halide ion.

[0038] The pH modulation agent may be an acid selected from a list comprising: any amino acid, carbonic acid, formic acid, acetic acid, propionic acid, butyric acid, capric acid, caproic acid, glycolic acid, lactic acid, pyruvic acid, valeric acid, acrylic acid, citric acid, oxalic acid, malonic acid, succinic acid, glutaric acid, malic acid, adipic acid, pimelic acid, fumaric acid, suberic acid, azelaic acid, benzoic acid, salicylic acid, phthalic acid, terephthalic acid, picolinic acid, methanesulfonic acid, ethanesulfonic acid, propanesulfonic acid, butanesulfonic acid, benzenesulfonic acid, toluenesulfonic acid, methanedisulfonic acid, ethanedisulfonic acid, propanedisulfonic acid, butanedisulfonic acid, sulfamic acid and / or isomers thereof, and / or halogenated forms thereof.

[0039] The pH modulation agent may be an acid selected from a list comprising: sulfuric acid, nitric acid, tungstic acid, chromic acid, hydrochloric acid, hydrobromic acid, hydrofluoric acid, and / or hydroiodic acid.

[0040] Preferably the pH modulator is an acid comprising the protonated form of the salt anion which forms part of the clathrate hydrate. In other words, the pH modulator may be the conjugate acid of the anion part of the clathrate hydrate of the PCM.

[0041] By way of a series of non-limiting examples, where the pH modulation agent is an acid, formic acid is preferably used where the clathrate hydrate salt is a formate salt, acetic acid is preferably used where the clathrate hydrate salt is an acetate salt, sulfuric acid is preferably used where the clathrate hydrate salt is a sulfate salt, nitric acid is preferably used where the clathrate hydrate salt is a nitrate salt, and propionic acid is preferably used where the clathrate hydrate salt is a propionate salt.

[0042] In the PCM, the clathrate hydrate may comprise the salt tetrabutylammonium acetate, and the pH modulation agent may be acetic acid; the clathrate hydrate may comprise the salt tetrabutylammonium oxalate, and the pH modulation agent may be oxalic acid; the clathrate hydrate may comprise the salt tetrabutylammonium glycolate, and the pH modulation agent may be glycolic acid; the clathrate hydrate may comprise the salt tetrabutylammonium formate and the pH modulation agent may be formic acid; the clathrate hydrate may comprise the salt tetrabutylammonium succinate, and the pH modulation agent may be succinic acid; the clathrate hydrate may comprise the salt tetrabutylammonium pimelate, and the pH modulation agent may be pimelic acid; the clathrate hydrate may comprise the salt tetrabutylammonium nitrate, and the pH modulation agent may be nitric acid; the clathrate hydrate may comprise the salt tetrabutylammonium sulfate, and the pH modulation agent may be sulfuric acid; and / or the clathrate hydrate may comprise the salt tetrabutylammonium propionate, and the pH modulation agent may be propionic acid.

[0043] In preferred embodiments, the clathrate hydrate may comprise the salt tetrabutylammonium acetate, and the pH modulation agent may be acetic acid; the clathrate hydrate may comprise the salt tetrabutylammonium nitrate, and the pH modulation agent may be nitric acid; the clathrate hydrate may comprise the salt tetrabutylammonium glycolate, and the pH modulation agent may be glycolic acid; the clathrate hydrate may comprise the salt tetrabutylammonium glutarate, and the pH modulation agent may be glutaric acid, and / or the clathrate hydrate may comprise the salt tetrabutylammonium propionate, and the pH modulation agent may be propionic acid.

[0044] In preferred embodiments, the PCM may comprise from about 97 wt% to about 99.9 wt% clathrate hydrate and from about 0.1 wt% to about 1 wt% pH modulator. For example, the PCM may comprise 97 wt%, , or 97.5 wt%, or 98 wt%, or 98.5 wt%, or 99 wt%, or 99.1 wt%, or 99.2 wt%, or 99. 5 wt % or 99.4 wt%, or 99.5 wt%, or 99.6 wt%, or 99.7 wt%, or 99.8 wt% or 99.9 wt% of clathrate hydrate and 0.1 wt%, or 0.2 wt%, or 0.3 wt%, or 0.4 wt%, or 0.5 wt%, or 0.6 wt%, or 0.7 wt%, or 0.8 wt%, or 0.9 wt%, or 1 wt%, or 1 .5 wt%, or 2 wt%, or 2.5 wt%, or 3 wt% of pH modulator.

[0045] The PCM may further comprise a nucleation agent. The nucleation agent may be a zeolite. The zeolite may be selected from the list comprising: 3A, 4A, 5A, 10X, and / or 13X. PCM may comprise a nucleation agent in a concentration range as discussed above. Preferably the PCM may comprise a nucleation agent in a concentration from about 1 wt% to about 5 wt%.

[0046] According to a further aspect of the present invention there is provided a thermal energy storage device apparatus comprising a PCM as described herein above which is in contact with or substantially in contact with one or more metallic component(s).

[0047] PCMs may be applied within devices known as thermal stores, thermal energy storage devices, heat banks, heat batteries, thermal banks, thermal batteries, thermal banks or buffers. Such devices typically comprise a containment vessel and may comprise various internal components which are in contact or substantially in contact with the PCM. In such a system it is key that the PCM be compatible with the internal components and containment materials with which it is in contact. Should the PCM be incompatible with said components and containment materials, degradation, corrosion, leeching, leaking or other damaging process can occur. This may coincide with the production of noxious, explosive, flammable or otherwise hazardous gases.

[0048] Advantageously, inclusion of a pH modulated clathrate PCM according to the present invention into a thermal energy storage device may avoid or alleviate incompatibilities. This may be due to modulation of the pH into a range where the materials which form the thermal energy storage device are passivated, or otherwise do not undergo degradation.

[0049] The thermal energy storage device may comprise various different metallic components to enable its use. The one or more metallic component(s) may be one or more items selected from a list comprising: a pipe / pipework, a heat exchanger, a valve, a heat spreader, a heating element, a containment material, a sensor, a manifold, a plug, a bolt, a rivet, and / or other linkage.

[0050] Said components may comprise metals which are susceptible to corrosion, which may simultaneously produce hazardous gases.

[0051] The one or more metallic component may comprise one or more metal(s) selected from a list comprising: aluminium, copper, iron, nickel, tin, titanium, steel, brass, zinc, Inconel, Hastelloy.

[0052] Each of the metals disclosed above may have a passivating window of pH (i.e. a pH range over which the material is stable and has negligible ongoing corrosion). The pH of the clathrate PCM would therefore be modulated according to the present invention to be within the passivating window of pH of the metal or metals with which the PCM is in contact in the thermal energy storage device.

[0053] The energy storage device may preferentially comprise a heat exchanger, optionally the heat exchanger may comprise steel, copper and / or aluminium.

[0054] The energy storage device may preferentially comprise copper pipework, which may comprise a part of whole of one or more heat exchanger(s).

[0055] The energy storage device may preferentially comprise a heat exchanger comprising copper and optionally aluminium, and a clathrate PCM according to the present invention wherein the pH has been modulated to between 7 and 10, to between 7.5 and 9.5, to between 7.5 and 9, between 8 and 9, or preferably about 8.

[0056] Methods of preparation of a PCM as disclosed herein are also described.

[0057] According to a further aspect of the present invention is disclosed a method of preparation of a PCM according to the present invention, wherein the method comprises: i) providing the clathrate hydrate in a liquid state; ii) combining the clathrate hydrate with one or more pH modulation agents, which may be acids and / or bases, to tune the pH into a desired range.

[0058] The clathrate hydrates comprising a salt may be prepared from the corresponding acids and bases. Where the salt comprises a cation [Y] and anion [X], the salt may be prepared from an acid form of the anion [X] (i.e. the conjugate acid of the anion [X]), and the basic form of the cation [Y] (e.g. the hydroxide form of the cation [Y]).

[0059] Herein [X] may be used to denote an anion and [Y] may be used to denote a cation. The corresponding reaction of an acid comprising [X] and a base comprising [Y] therefore produces a salt comprising both [X] and [Y], and one or more molar equivalents of water.

[0060] According to another aspect of the present invention there is provided a method of preparation of a PCM as described hereinabove, the method comprising the steps of: i) providing a clathrate hydrate in a liquid state, wherein the clathrate hydrate comprises a cation, and anion, and water; and ii) combining the clathrate hydrate with one or more pH modulation agent, wherein the at least one pH modulation agent comprises at least one of the same cation and / or the same anion as the clathrate hydrate.

[0061] According to a further aspect of the present invention is disclosed method of preparation of a phase change material (PCM) as described hereinabove, comprising the steps of: i) providing one or more acid(s); ii) providing one or more base(s); iii) combining with water the one or more acid(s) and one or more base(s) to form a clathrate hydrate comprising water and a salt having a cation and an anion; and iv) combining the clathrate hydrate with one or more pH modulation agent(s) comprising at least one of the same cation and / or the same anion as the clathrate hydrate, optionally wherein the one or more pH modulation agent(s) is selected from one or more acids; and / or one or more bases.

[0062] According to a further aspect of the present invention is disclosed a method of preparation of a PCM according to the present invention, wherein the method comprises: i) providing one or more acids comprising an anion [X]; ii) providing one or more bases comprising cation [Y]; iii) combining the one or more acids and one or more bases with water to form a clathrate hydrate having a salt comprising anion [X], cation [Y], and water; iii) providing the clathrate hydrate in a liquid state; and iv) modulating the pH by one or more of: adding of one or more acids, wherein at least one acid comprises anion

[0063] [X]; and / or adding one or more bases, wherein at least one base comprises cation

[0064] [Y]-

[0065] The anion [X] used in its protonated (i.e. conjugate acid) form to prepare the clathrate hydrate and optionally to modulate the pH, may be one or more anion selected from a list comprising: any deprotonated amino acid anion, carbonate, formate, acetate, propionate, butyrate, caprate, caproate, glycolate, lactate, pyruvate, valerate, acrylate, citrate, oxalate, malonate, succinate, glutarate, malate, adipate, pimelate, fumarate, suberate, azelate, benzoate, salicylate, phthalate, terephthalate, picolinate, methanesulfonate, ethanesulfonate, propanesulfonate, butanesulfonate, benzenesulfonate, toluenesulfonate, methanedisulfonate, ethanedisulfonate, propanedisulfonate, butanedisulfonate, sulfamate and / or isomers thereof, and / or halogenated forms thereof

[0066] The anion [X} used in its protonated (i.e. conjugate acid) form to prepare the clathrate hydrate and optionally to modulate the pH may be one or more anion selected from a list comprising: any halide, chromate, tungstate, sulfate and / or nitrate

[0067] The cation [Y] used as a basic (e.g. hydroxide) form to prepare the clathrate hydrate and optionally to modulate the pH, may be a tetra-alkyl ammonium and / or tetra-alkyl phosphonium cation. Preferably the cation [Y] may be tetrabutylammonium and / or tetrabutylphosphonium. The PCM may comprise from about 70 wt% to about 99.995 wt% of the clathrate hydrate, and from about 0.005 wt% to about 30 wt% of the at least one pH modulation agent.

[0068] The method may further comprise adding a nucleation agent. The nucleation agent may be added when the clathrate hydrate is formed (e.g. in step iii) in the two methods discussed above). The nucleation agent may be a zeolite. The zeolite may be selected from the list comprising: 3A, 4A, 5A, 10X, and / or 13X. PCM may comprise a nucleation agent in a concentration range as discussed above. Preferably the PCM may comprise a nucleation agent in a concentration from about 1 wt% to about 5 wt%.

[0069] Brief description of the Figures

[0070] Figure 1 shows the Pourbaix diagram of aluminium, showing passivating, corroding and immune windows over various pH ranges at various applied potentials.

[0071] Figure 2 shows a plot of pH vs molar ratio of acid:base which form a salt hydrate clathrate hydrate, showing the step change in PCM pH as the molar ratio of acid: base changes and fitting thereof with a Hill plot.

[0072] Figure 3 shows a melt-freeze cycle of a PCM comprising a clathrate hydrate comprising tetrabutylammonium acetate and a pH modulation agent comprising acetic acid. Point 301 indicates the nucleation temperature of the PCM.

[0073] Figure 4 shows a melt-freeze cycle of a PCM comprising a clathrate hydrate comprising tetrabutylammonium acetate, a pH modulation agent comprising acetic acid and a nucleation agent comprising zeolite 4A. Point 401 indicates the nucleation temperature of the PCM.

[0074] Figure 5 shows a melt-freeze cycle of a PCM comprising a clathrate hydrate comprising tetrabutylammonium acetate, a pH modulation agent comprising acetic acid and a nucleation agent comprising zeolite 13X. Point 501 indicates the nucleation temperature of the PCM.

[0075] Detailed Description

[0076] The present invention concerns clathrate hydrate PCMs, which may be described as large hydrogen bonded cage-like structures of water molecules which host secondary guest species in the cage cavities and which may also form part of the cage structure itself. A sub-set of clathrates, the semi-clathrates hydrates, exhibit guest molecules and / or ions in both the cage cavity and also form a part of the water cage structure itself. These semi-clathrate hydrates are often simply referred to as clathrates in the literature. Herein the term “clathrate hydrate” is defined as being inclusive of both clathrate hydrates and semi-clathrate hydrates. Clathrate hydrate materials exhibit high energy storage capacity, stability to thermal cycling and have melting and freezing temperature which are well-suited to many applications. Their use would be highly beneficial therefore to many thermal energy storage opportunities.

[0077] However, without modification, clathrate hydrate type PCMs may be found to exhibit extremes of pH, and therefore their use as energy storage media is limited. Unmodified clathrate hydrate PCMs may naturally have pHs between 11 and 14 or between than 1 and 4, and therefore may exhibit corrosive, damaging, degrading, or otherwise adverse effects on surfaces with which they are in contact, in particular when said surfaces are metallic.

[0078] Figure 1 shows the Pourbaix diagram of aluminium metal, to illustrate an example of the passivating window for a metal (in this case Al) where the pH is such that the oxide layer is preserved and no or negligible corrosion occurs. This passivating window which, at 0 applied potential, occurs between pH 4.5 and 8.5, could easily be exceeded by some clathrate hydrate PCMs which may exhibit pHs outside this window. It should be noted that similar diagrams are known for various different metals, including the exact conditions of pH which allow immunity or passivation, or result in corrosion. Corrosion reduces the performance of metal components, and can often result in the evolution of noxious, explosive, flammable or otherwise harmful gases.

[0079] Furthermore, PCMs with such high or low pHs can be hazardous to users, potentially causing chemical burns on skin or eye contact, or if ingested or inhaled. Thus, extremes of pH should be avoided in PCMs and systems comprising PCMs for the safety of those involved in the manufacture, transport or use of said materials, or devices comprising said materials.

[0080] Disclosed herein is a clathrate hydrate PCM which is modified with a pH modulation agent to avoid such extremes of pH.

[0081] The PCM may comprise from about 70 wt% to about 99.995 wt% of at least one clathrate hydrate and from about 0.005 wt% to about 30 wt% of at least one pH modulation agent. The PCMs may comprise any proportion of clathrate hydrate and pH modulation agent within said ranges. In addition, the PCM may comprise additives, such as nucleation additives.

[0082] In preferred examples, the PCM comprises:

[0083] Between about 96 and about 99.995 wt.% of one or more clathrate hydrate, and

[0084] Between about 0.005 and about 4 wt.% of one or more pH modulation agent. Optionally the PCM may comprise an additive (e.g. a nucleation agent). The additive may be present in a content between about 0.1 and about 5 wt %.

[0085] Clathrate hydrates which may form part of the PCM may be one or more salt hydrates. Salt hydrates which form clathrate structures typically comprise: a large cation bearing organic chain groups (e.g. alkyl chains), which may be branched or straight chain, connected to a central nitrogen or phosphorus atom; a variety of different anions; and between 20 and 70 molar equivalents of water that form the cage-like structure (e.g. molar ratio of saltwater from about 1 :20 to about 1 :70).

[0086] In a salt hydrate clathrate hydrate structure, the water and anion may form the cage-like structure within which the cation is held. It has been found by the inventors that a clathrate hydrate PCM may, without modification, have a pH between 1 and 4 or between 11 and 14. Such pHs make clathrate hydrate PCMs unsuitable for use with certain materials, as well as a hazard to manufacturing and transport operatives and the environment.

[0087] In a salt hydrate clathrate hydrate as disclosed herein the salt cation may be an ammonium and / or phosphonium ion. The cation may be a tetra-alkyl ammonium and / or tetra-alkyl phosphonium ion comprising organic chain groups (i.e. the alkyl chains), which may be branched or straight chain. The alkyl chains may be between C1-C6 in length, wherein C1 is a carbon chain of one carbon in length, and C2-C6 are to be understood accordingly. Said C1- C6 chains may also be branched (i.e. any isomeric forms of the straight chain form).

[0088] In a salt hydrate clathrate hydrate as disclosed herein the salt cation may be a tetra-alkyl ammonium and / or phosphonium ion, wherein the alkyl groups are one or more of the groups selected from a list comprising: methyl-, ethyl-, propyl-, butyl-, isoamyl-, pentyl-, isopentyl-, and / or hexyl-.

[0089] In a salt hydrate clathrate hydrate as disclosed herein, the salt may be a tetrabutylammonium and / or tetrabutylphosphonium salt.

[0090] In a salt hydrate clathrate hydrate as disclosed herein, the salt may comprise a cation, and an anion which may be one or more carboxylate-bearing ions, sulfonate-bearing ions, halides, tungstate ions, chromate ions, nitrate ions and / or sulfate ions. In a salt hydrate clathrate hydrate as disclosed herein, the salt may comprise a cation, and one or more anion(s) selected from a list comprising: any deprotonated amino acid anion, carbonate, formate, acetate, propionate, butyrate, caprate, caproate, glycolate, lactate, pyruvate, valerate, acrylate, citrate, oxalate, malonate, succinate, glutarate, malate, adipate, pimelate, fumarate, suberate, azelate, benzoate, salicylate, phthalate, terephthalate, picolinate, methanesulfonate, ethanesulfonate, propanesulfonate, butanesulfonate, benzenesulfonate, toluenesulfonate, methanedisulfonate, ethanedisulfonate, propanedisulfonate, butanedisulfonate, sulfamate and / or isomers thereof, and / or halogenated forms thereof.

[0091] In a salt hydrate clathrate hydrate as disclosed herein, the salt may comprise a cation and one or more anion(s) selected from a list comprising: any halide, chromate, tungstate, sulfate and / or nitrate.

[0092] Anions as part of the salt may displace water to form part of the salt hydrate clathrate cage structure.

[0093] It is disclosed herein that the clathrate hydrate may be a salt hydrate comprising a salt and water. The molar ratio of saltwater in said salt hydrate may be between 1 :20 and 1 :40, between 1 :25 and 1 :35, or about 1 :30. The molar ratio of saltwater in said salt hydrate may be between 1 :50 and 1 :70, between 1 :55 and 1 :65, or about 1 :60.

[0094] Where the salt hydrate comprises one or more salts with a molar ratio of cations:anions of 1 : 1 , the molar ratio of saltwater may be between 1 :20 and 1 :40, between 1 :25 and 1 :35, or about 1 :30.

[0095] Where the salt hydrate comprises one or more salts with a molar ratio of cations:anions of 2: 1 , the molar ratio of saltwater may be between 1 :50 and 1 :70, between 1 :55 and 1 :65, or about 1 :60.

[0096] The PCMs as disclosed herein comprises a pH modulation agent. A pH modulation agent is defined as any agent which may be used to alter, adjust, control, tune or otherwise change the pH of a clathrate hydrate from what it would be without intervention.

[0097] A pH modulation agent may be used to modify the pH of a clathrate hydrate material into a specific selected range. Combination of a pH modulation agent with a clathrate hydrate may be used to change the pH of the resulting material into a range which is between about 4-11 , between about 5-10, between about 6-9, between about 7-9, between about 7.5-8.5, or about 8. Where the unmodified pH of the clathrate hydrate is between about 1-4, a pH modulation agent which comprises one or more bases may be used to form the PCM. An amount of a pH modulation agent which comprises one or more bases between about 0.005 and about 4 wt.% may be used to form the PCM. Combination of between about 0.005 and about 4 wt.% of a pH modulation agent having one or more bases may be used to alter the pH from between about 1-4 to between about 5-10, between about 6-9, between about 7-9, between about 7.5- 8.5, or about 8 to form the PCM.

[0098] Where the unmodified pH of the clathrate hydrate is between about 11-14, a pH modulation agent which comprises one or more acids may be used to form the PCM. An amount of a pH modulation agent which comprises of one or more acids between about 0.005 and about 4 wt.% may be used to form the PCM. Combination of between about 0.005 and about 4 wt.% of a pH modulation agent having one or more acids may be used to alter the pH from between about 11-14 to between about 5-10, between about 6-9, between about 7-9, between about 7.5-8.5, or about 8 to form the PCM.

[0099] It is preferable to use acids and / or bases which share a common ion with the salt which forms part of the clathrate hydrate. It is preferable to select an acid which is the protonated form of the anion which forms part of the clathrate hydrate. It is preferable to select an acid which is the conjugate acid of the anion which forms part of the clathrate hydrate. It is preferable to select a base which is the hydroxide salt of the cation which forms part of the clathrate hydrate. Thus the pH modulation agents may be one or more acid(s) and base(s) which may be combined to give the salt which forms a part of the clathrate hydrate, and thus the acid(s) and base(s) may be considered to be combined to give said salt in a non-stoichiometric ratio, and as such alter the pH of the material, with the degree of non-stoichiometry dependent on the degree of pH modulation required.

[0100] It is preferable, due to the potential phase transition temperature depression effect which can result from the colligative properties of solution, to use pH modulation agents which share a common ion with the salt which forms part of the clathrate hydrate. On introduction of further ions which differ from those which are present, a melting point depression effect may be observable. For example, if a salt hydrate comprises the tetrabutylammonium cation and acetate anion, introducing pH modulation agents which do not include either the tetrabutylammonium cation or the acetate anion may cause problematic colligative effects. Melting point depression, if not desired, is detrimental to the performance of the PCM, as the phase transition temperature is typically key in matching the PCM to the application of said PCM. Further problems may be reduction in nucleation and crystal growth rate, reducing the efficiency with which the PCM may be frozen. Finally, should the pH modulation agent be poorly chosen, it may result in the formation of insoluble products, which do not contribute significantly to the PCM performance, and may also not result in any pH modulation.

[0101] Matching the pH modulation agent to the clathrate hydrate is therefore a key consideration for producing PCMs which comprise a clathrate hydrate. Table 1 shows examples of cations which form part of a clathrate hydrate based PCM, and the corresponding bases which may be used as pH modulation agents to raise the PCM pH as part of the present invention.

[0102] Acids which may be used to modulate the pH of a clathrate hydrate PCM as disclosed as part of the present invention may be described with the formula Hn[X]m', wherein [X] is an anion as disclosed herein, H is a proton, and n and m are integers (e.g. 1 , 2, 3, 4, 5, or 6, preferably 1 , 2 or 3, further preferably 1 or 2). Preferably [X] is an anion with the same identity as an anion which forms part of the clathrate hydrate. The acid therefore is preferably a conjugate acid of [X].

[0103] Table 2 shows examples of anions which form part of a clathrate hydrate based PCM, and the corresponding acids which may be used as pH modulation agents to lower the PCM pH as part of the present invention.

[0104] Acids used as pH modulation agents, with examples given in Table 2, may be present as a different isomer, such as a structural isomer and / or stereoisomer. For example, any amino acid used as a pH modulating agent may be used in their L- or D- stereoisomeric forms. For further example, where an unsaturated acid is used, such as fumaric acid, it may be used in its trans or cis form (e.g. it may be maleic acid, the cis isomer of fumaric acid), or mixture thereof.

[0105] Table 3 shows preferred embodiments of salts which form part of a clathrate hydrate based PCM, and the corresponding acids and bases which may be used as pH modulation agents to raise or lower the PCM pH as part of the present invention.

[0106] Acidic and basic pH modulation agents as exemplified in Tables 1-3 or combinations thereof may be used to tune the pH of the PCM into a desired range, preferably such that the pH of the resulting PCM material is in a range between about 4-11 , or between about 5-10, or between about 6-9, or between about 7-9, or between about 7.5-8.5, or about 8. Non-limiting examples of pH adjusted clathrate hydrate PCMs are given in Tables 4-8.

[0107] It was found that addition of the amounts of acetic acid to the clathrate hydrate PCMs as described in Table 4 did not significantly affect the melting and crystallisation transitions of the PCMs, which remained around 15 °C. Thus, the pH may be modulated from about 13, which would exhibit the handling, environmental, compatibility and safety problems described earlier, to between 6-8, which alleviates or overcomes said problems.

[0108] A further non-limiting example is given in Table 5

[0109] It was found that addition of the amounts of propionic acid to the clathrate hydrate PCMs as described in Table 5 did not significantly affect the melting and crystallisation transitions of the PCMs, which remained around 17 °C. Thus the pH may be modulated from about 13, which would exhibit the handling, environmental, compatibility and safety problems described earlier, to about 8, which alleviates or overcomes said problems.

[0110] A further non-limiting example is given in Table 6.

[0111] It was found that addition of the amounts of glycolic acid to the clathrate hydrate PCMs as described in Table 6 did not significantly affect the melting and crystallisation transitions of the

[0112] PCMs, which remained around 5 °C. Thus, the pH may be modulated from about 12, which would exhibit the handling, environmental, compatibility and safety problems described earlier, to about 7, which alleviates or overcomes said problems.

[0113] A further non-limiting example is given in Table 7. It was found that addition of the amounts of tetrabutylammonium hydroxide to the clathrate hydrate PCMs as described in Table 7 did not significantly affect the melting and crystallisation transitions of the PCMs, which remained around 6 °C. Thus, the pH may be modulated from about 4, which would exhibit the handling, environmental, compatibility and safety problems described earlier, to about 7, which alleviates or overcomes said problems. Addition of sodium hydroxide may also be used to raise the pH of the PCM.

[0114] Table 8 shows preferred embodiments of the PCMs disclosed herein, including the pH modulation agents and their relative loadings. It has been found by the inventors that combination of the PCMs disclosed in Table 8 may have a pH of about 7-9 and thereby have improved safety properties, ease of handling and may be combined with metallic components, which may comprise steel, copper and / or aluminium without risking detrimental corrosive effects.

[0115] The PCMs as disclosed herein may be included within a thermal energy storage device apparatus. Thermal energy storage devices may also be known as thermal stores, heat banks, heat batteries, thermal banks, thermal batteries and / or thermal buffers. Such devices typically comprise a containment vessel and may comprise various internal components which may be in contact or substantially in contact with the PCM. Internal components may also be temporarily in contact with the PCM during the use, preparation and / or storage of the system.

[0116] In such an apparatus it is key that the PCM be compatible with the internal components and containment materials with which it is in contact, even if only temporarily. Should the PCM be incompatible with said components and containment materials, degradation, corrosion, leeching, leaking or other damaging process can occur. This may coincide with the production of noxious, explosive, flammable or otherwise hazardous gases.

[0117] The pH modulated PCM may be in contact or substantially in contact with one or more metallic components and / or surfaces which comprise the thermal energy storage device apparatus.

[0118] The pH modulated PCM may be temporarily in contact with one or more metallic components and / or surfaces which comprise the thermal energy storage device apparatus during use, preparation and / or storage of the system.

[0119] The PCM may be in contact or substantially in contact with one or more metallic component and / or surfaces, where without the presence of the pH modulation agent corrosion would occur. Contact with such a metallic surface may occur at least temporarily during use, preparation and / or storage of the thermal energy storage device apparatus. Due to the presence of the pH modulation agent however, lessened, minimal, lower or no corrosive effects may occur.

[0120] The thermal energy storage device may comprise one or more metallic component selected from a list comprising: pipework, a heat exchanger, a valve, a heat spreader, a heating element, a containment material, a sensor, a manifold, a plug, a bolt, a rivet, and / or other linkage which contacts the PCM, at least temporarily during use, preparation and / or storage of the thermal energy storage device apparatus. Modulation of the pH of the clathrate hydrate as described herein allows the compatibility of the PCM to be tuned to the materials used as containment and / or other internal componentry which may come into contact with the PCM. Thus, the PCM materials as disclosed herein may enable the use of materials which may be lower-cost, safer, more sustainable, or otherwise of superior performance.

[0121] By way of non-limiting example, modulating the pH of the PCM into a passivating window for a lower cost metal which may then be used as a containment or internal component material.

[0122] Such components may be used in the use of the thermal energy storage device, to add or remove heat, improve heat transfer throughout the device, provide a volume within which the PCM may be housed, measure various properties such as temperature, pressure or flow of the PCM, allow addition or removal of materials (which may include the PCM), and / or connect various components to one another and / or to the containment vessel.

[0123] The one or more metallic component comprises one or more metals selected from a list comprising: aluminium, copper, iron, nickel, tin, titanium, zinc, steel, brass, Inconel, Hastelloy, and combinations thereof.

[0124] Preferably, the thermal energy storage device may comprise a heat exchanger which aids in the addition of removal of heat from the thermal energy storage device. Heat exchangers providing sanitary water may be comprise copper, and more specifically may comprise copper pipework.

[0125] Heat exchangers may comprise only copper or may be comprise copper and aluminium. Such a heat exchanger comprising copper and aluminium may be configured such that the pipework which forms part of the heat exchanger comprises copper, and a heat spreader or spreaders which form a further part of the heat exchanger comprise aluminium. The whole or a part of the heat exchanger apparatus may contact the PCM and act as a means of adding and / or removing heat from the PCM, and as such the PCM is preferably compatible (i.e. does not exhibit corrosive effects) with the heat exchanger materials. In a preferred embodiment of the present invention the one or more metallic component comprises a heat exchanger, and wherein the heat exchanger comprises copper and optionally aluminium; and the pH of the clathrate hydrate PCM is between 7 and 10, between 7.5 and 9.5, between 7.5 and 8.5, or preferably about 8. These pH ranges are optimal for the minimisation of corrosion of both copper and aluminium, which perform well as heat exchanger materials.

[0126] A PCM according to the present invention may be prepared by the following method: i) providing the clathrate hydrate in a liquid state, ii) combining the clathrate hydrate with one or more pH modulation agents to tune the pH into a desired range.

[0127] Modulation of the pH in this manner may require careful metering of pH modulation agents since a small adjustment in the content of pH modulator (e.g. less than about 1 wt%) may be effective at altering the pH at the clathrate. As shown in Figure 2, the pH swing around the fully neutralised clathrate composition can take place over a narrow compositional range, making the potential for missing passivating windows, or safely handleable pHs, by overshooting the target pH a possibility. It should be noted that the clathrate material used as an example in Figure 2 is comprises a salt with a molar ratio of catiomanion of 1 :1.

[0128] In Figure 2, the ratio of exactly 1 :1 acid to base, provides a clathrate hydrate with a cation:anion ratio of 1 :1 in the resulting salt, and results in a pH of the unmodified clathrate of approximately 10-11 according to the Hill fit. Thus, if the PCM is required to have a pH of less than about 10-11 , an acidic pH modulation agent should be included in the PCM, altering the molar ratio of acid: base to be >1 :1.

[0129] The clathrate hydrate itself may be prepared from the corresponding acids and bases.

[0130] A PCM according to the present invention may be prepared by the following method: i) providing one or more acid and one or more base; ii) combining with water said acid(s) and base(s) to form a clathrate hydrate, and iii) adding one or more pH modulation agent(s).

[0131] Clathrate hydrates comprising a salt may be prepared from the corresponding acids and bases. Where the salt comprises a cation [Y] and anion [X], the salt may be prepared from an acid form of the anion [X] (i.e. the conjugate acid of the anion [X]), and the basic form of the cation [Y] (i.e. the hydroxide form of the cation [Y]) . This results in the salt and water according to the molar ratio of catiomanion (i.e. [Y]:[X]) in the salt. Combination of this reaction with addition of further water amounting to the required saltwater molar ratio may give the salt hydrate which may be a clathrate hydrate.

[0132] Water may be combined with the one or more acids and / or bases in the preparation of the clathrate hydrate, i.e. they may be provided as a solution. The clathrate hydrate may comprise one or more salts and water; and the molar ratio of cations:anions in the salt may be 1 :1.

[0133] Water is combined with the one or more bases and one or more acids such that the saltwater molar ratio is from about 1 :20 to about 1 :40, from about 1 :25 to about 1 :35, or about 1 :30.

[0134] The clathrate may be a salt hydrate that having a molar ratio of catiomanion of 1 :1 , and a molar ratio of saltwater from about 1 :20 to about 1 :40, from about 1 :25 to about 1 :35, or about 1 :30. The clathrate hydrate may be a salt hydrate with a molar ratio of saltwater of from about 1 :50 and to about 1 :70, .from about 1 :55 to about 1 :65, or about 1 :60. The clathrate may be a salt hydrate that having a molar ratio of cation:anion of 2: 1 , and a molar ratio of saltwater from about 1 :50 to 1 :70, from about 1 :55 to about 1 :65, or about 1 :60.

[0135] Water may be combined with the one or more acids and / or bases in the preparation of the clathrate hydrate, wherein: the clathrate hydrate comprises one or more salts and water; the molar ratio of cations:anions in the salt is 2:1 ; and water is combined with the one or more bases and the one or more acids such that the saltwater molar ratio is from about 1 :50 to 1 :70, from about 1 :55 to about 1 :65, or about 1 :60.

[0136] A PCM according to the present invention may be preferably prepared by the following method: i) providing one or more acids comprising anion [X]; ii) providing one or more bases comprising cation [Y]; iii) combining with water with the one or more acids and one or more bases to form a clathrate hydrate comprising a salt comprising anion [X], cation [Y], and water; iv) providing the clathrate hydrate in a liquid state; and v) modulating the pH by one or more of: adding one or more acids, wherein at least one acid is comprises anion [X]; and / or adding one or more bases, wherein at least one base comprises cation [YL For example, a clathrate hydrate PCM according to the present invention comprises tetrabutyl ammonium acetate and a pH modulation agent comprising acetic acid. The clathrate hydrate may further comprise a nucleation agent. The nucleation agent may comprise a zeolite. The zeolite may be selected from zeolite 4A and zeolite 13X.

[0137] A method of preparing a clathrate hydrate PCM comprising tetrabutyl ammonium acetate may comprise: i) providing acetic acid; ii) providing tetrabutyl ammonium hydroxide; iii) combining tetrabutyl ammonium hydroxide, acetic acid and water to form a tetrabutyl ammonium acetate clathrate hydrate PCM; iv) providing the tetrabutyl ammonium acetate clathrate hydrate PCM in liquid state; and v) modulating the pH of the PCM by addition of acetic acid.

[0138] Optionally, the method may comprise adding a nucleation agent (e.g. a zeolite, e.g. xeolite 4A or zeolite 13X) at any stage of the preparation.

[0139] It is disclosed herein that the acids and bases comprising anion [X] and cation [Y] respectively may be provided neat (i.e. without solvent / water) or as a solution. In the methods described herein, one or more acids may be added to one or more bases and combined with water to form the clathrate hydrate, or vice-versa. The one or more acids comprising anion [X] and / or the one or more bases comprising cation [Y] may be provided as a solution in a concentration which comprises the amount of water needed to form the clathrate (i.e. no addition of water is required).

[0140] A preferred method of preparation of a PCM according to the present invention comprises the following steps: i) providing one or more acids comprises an anion [X], as described above; ii) providing a solution of one or more bases selected from tetrabutylammonium hydroxide and / or tetrabutylphosphonium hydroxide; iii) combining with water the one or more bases with the one or more acids to form a clathrate hydrate comprising one or more tetrabutylammonium and / or tetrabutylphosphonium salt with anion [X] and water; iv) retaining the clathrate hydrate in a liquid state; and v) modulating the pH by one or more of: addition of an acid comprising anion [X]; and / or addition of tetrabutylammonium hydroxide and / or tetrabutylphosphonium hydroxide.

[0141] Thus, tetrabutylammonium and / or tetrabutylphosphonium salt hydrate clathrate hydrate PCMs such as those shown in Figures 3, 4, and 5 are prepared and their pH modulated by addition of one or more acids with a common anion [X] and / or addition of one or more bases with a common cation of tetrabutylammonium or tetrabutylphosphonium. The cations and anions which are present may be restricted to the tetrabutylammonium and / or phosphonium cations and anions [X], while also allowing the pH to be adjusted to a desired value or range.

[0142] A further preferred method of preparation of a PCM according to the present invention comprises the following steps: i) providing one or more acids comprising an anion [X], wherein [X] is selected from a list comprising: formate, acetate, propionate, butyrate, glycolate, lactate, valerate, acrylate, sulfate, nitrate, oxalate, malonate, succinate, glutarate, adipate, pimelate, and / or any halide; ii) providing one or more bases comprising cation [Y]; iii) combining with water the one or more bases with the one or more acids to form a clathrate hydrate comprising a salt comprising cation [Y] and anion [X] and water, iv) retaining the clathrate hydrate in a liquid state; and v) modulating the pH of the clathrate hydrate by one or more of: addition of one or more acids, wherein at least one acid comprises anion [X]; and / or addition one or more bases comprising cation [Y],

[0143] Retaining the clathrate hydrate in a liquid state may be achieved by any suitable means (e.g. by maintaining the clathrate hydrate above its melting point). Where necessary, this may be achieved by heating above the melting point of the clathrate hydrate. However, in practice, the clathrate hydrate may have a melting point at around ambient temperature (e.g. from about 15 °C to about 25 °C) and additional heating of the clathrate hydrate may not be required to maintain it in a liquid state. Thus, salt hydrate clathrate hydrate PCMs comprising the anions [X] as disclosed herein may be prepared and their pH modulated by addition of one or more acids with a common anion [X] and / or addition of one or more bases with a common cation [Y], The cations and anions which are present may be restricted to the cations [Y] and anions [X], while also allowing the pH to be adjusted to a desired value or range.

[0144] In preferred embodiments, the cation [Y] is a tetra-alkyl ammonium and / or tetra-alkyl phosphonium cation.

[0145] A further preferred method of preparation of a PCM according to the present invention comprises the following steps: i) providing one or more acids comprises an anion [X], as described above; ii) providing a solution of one or more bases selected from tetrabutylammonium hydroxide and / or tetrabutylphosphonium hydroxide; iii) combining with water the one or more bases with the one or more acids to form a clathrate hydrate comprising one or more tetrabutylammonium and / or tetrabutylphosphonium salt with anion [X] and water; iv) retaining the clathrate hydrate in a liquid state; and v) modulating the pH by one or more of: addition of an acid comprising anion [X] addition of tetrabutylammonium hydroxide and / or tetrabutylphosphonium hydroxide.

[0146] Thus, salt hydrate clathrate hydrate PCMs comprising the tetrabutylammonium and / or tetrabutylphosphonium salts with anions [X] as disclosed herein may be prepared and their pH modulated by addition of one or more acids with a common anion [X] and / or addition of one or more tetrabutylammonium and / or tetrabutylphosphonium bases. The cations and anions which are present may be restricted to the anions [X] and tetrabutylammonium and / or tetrabutylphosphonium cations, while also allowing the pH to be adjusted to a desired value or range.

[0147] The PCMs disclosed herein may also comprise one or more nucleation agents. Nucleation agents aid in the initiation of crystallisation of the PCM, reducing subcooling (i.e. the propensity for a PCM to cool below its phase transition temperature without crystallising and remaining as a supersaturated liquid). It has been found by the inventors that zeolitic nucleation agents may be used to nucleate clathrate hydrates. Therefore, the PCM may further comprise a nucleation agent comprising a zeolite.

[0148] Zeolites, which may be aluminosilicates with various alumina:silica ratios, exhibit porous structures with large surface area. Zeolites may be referred to as molecular sieves. Without wishing to be bound to any particular theory, it is proposed that parts of the clathrate water cage structure, which may be complete cage structures or fragments thereof, may be retained within the zeolite pores when the PCM is heated above its melting point. Thereby these parts of the PCM are held in a state which requires minimal re-arrangement to form the solid (i.e. crystallise) when the PCM is cooled.

[0149] It is disclosed herein that a zeolite of various types may be used as nucleation agents for clathrate hydrate PCMs. Zeolites for use as nucleation agents may be of one or more types selected from a list comprising: 3A, 4A, 5A, 10X, and / or 13X.

[0150] By way of non-limiting example, Figure 3 shows thermal cycle (melt-freeze) of a pH modulated salt hydrate clathrate hydrate PCM comprising the salt tetrabutylammonium acetate and 28 equivalents of water. Herein the pH of the PCM has been modulated by the addition of acetic acid to change the pH from about 14 to about 7.5. In Figure 3 it can be observed that the PCM subcools to about -1.45 °C (point 301 on Figure 3).

[0151] Thermal cycles of the same material modified by the addition of two zeolites, in these examples zeolite 4A and 13X, are shown in Figures 4 and 5.

[0152] In Figure 4, the presence of zeolite 4A provides an improvement of the nucleation temperature to 7.3 °C (point 401 on Figure 4).

[0153] In Figure 5, the presence of zeolite 13X provides an improvement of the nucleation temperature to 6.85 °C (point 501 on Figure 5). Thus, it is demonstrated that the presence of zeolites in a clathrate hydrate PCM as disclosed herein may have a beneficial effect on the nucleation temperature of said PCMs.

Claims

Claims1. A phase change material (PCM) comprising: at least one clathrate hydrate comprising a cation, and anion and water; and at least one pH modulation agent comprising at least one of the same cation and / or the same anion as the clathrate hydrate.

2. A PCM according to claim 1 , wherein the pH modulation agent is a pH reducing agent.

3. A PCM according to claim 1 or 2, wherein the pH modulation agent is an acid; optionally wherein the acid has the formula Hn[X]m-, wherein [X] is an anion with the same identity as an anion present in the clathrate hydrate, H is a proton, and n and m are integers; further optionally wherein the pH modulation agent is a monoprotic, diprotic or triprotic acid.

4. A PCM according to any preceding claim, wherein the pH modulation agent is an acid selected from a list comprising: any amino acid, carbonic acid, formic acid, acetic acid, propionic acid, butyric acid, capric acid, caproic acid, glycolic acid, lactic acid, pyruvic acid, valeric acid, acrylic acid, citric acid, oxalic acid, malonic acid, succinic acid, glutaric acid, malic acid, adipic acid, pimelic acid, fumaric acid, suberic acid, azelaic acid, benzoic acid, salicylic acid, phthalic acid, terephthalic acid, picolinic acid, methanesulfonic acid, ethanesulfonic acid, propanesulfonic acid, butanesulfonic acid, benzenesulfonic acid, toluenesulfonic acid, methanedisulfonic acid, ethanedisulfonic acid, propanedisulfonic acid, butanedisulfonic acid, sulfamic acid and / or halogenated forms thereof.

5. A PCM according to any preceding claim, wherein the pH modulation agent is an acid selected from a list comprising: sulfuric acid, nitric acid, tungstic acid, chromic acid, hydrochloric acid, hydrobromic acid, hydrofluoric acid, and / or hydroiodic acid.

6. A PCM according to any preceding claim, wherein one of: the clathrate hydrate comprises the salt tetrabutylammonium acetate, and the pH modulation agent is acetic acid; the clathrate hydrate comprises the salt tetrabutylammonium oxalate, and the pH modulation agent is oxalic acid;.the clathrate hydrate comprises the salt tetrabutylammonium glycolate, and the pH modulation agent is glycolic acid; the clathrate hydrate comprises the salt tetrabutylammonium formate, and the pH modulation agent is formic acid;. the clathrate hydrate comprises the salt tetrabutylammonium succinate, and the pH modulation agent is succinic acid; the clathrate hydrate comprises the salt tetrabutylammonium pimelate, and the pH modulation agent is pimelic acid;. the clathrate hydrate comprises the salt tetrabutylammonium nitrate, and the pH modulation agent is nitric acid;. the clathrate hydrate comprises the salt tetrabutylammonium sulfate, and the pH modulation agent is sulfuric acid; the clathrate hydrate comprises the salt tetrabutylammonium propionate, and the pH modulation agent is propionic acid; the clathrate hydrated comprises the salt tetrabutylammonium glutarate, and the pH modulation agent is glutaric acid7. A PCM according to claim 1 or 2, wherein the pH modulation agent is a base, optionally wherein the pH modulation agent is a tetra-alkyl ammonium and / or tetraalkyl phosphonium base, further optionally wherein the pH modulation agent is tetra-alkyl ammonium and / or tetra-alkyl phosphonium hydroxide.

8. A PCM according to any preceding claim, wherein the pH of the PCM is between 4-11 , between 5-10, between 6-9, between 7-9, between 7.5-8.5, or about 8.

9. A PCM according to any preceding claim, wherein the clathrate hydrate is a tetraalkyl ammonium and / or tetra-alkyl phosphonium salt hydrate, optionally wherein the salt anion is one or more carboxylate anion and / or sulfonate anion.

10. A PCM according to any preceding claim, wherein the clathrate hydrate is a tetraalkyl ammonium and / or tetra-alkyl phosphonium salt hydrate bearing alkyl chains of C1-C6 in length, optionally wherein the alkyl groups are selected from a list comprising one or more of: methyl-, ethyl-, propyl-, butyl-, isoamyl-, pentyl-, isopentyl-, and / or hexyl-.

11. A PCM according to any preceding claim, wherein the clathrate hydrate comprises a salt comprising a tetra-alkyl ammonium and / or a tetra-alkyl phosphonium cationand one or more anion(s) selected from a list comprising: any deprotonated amino acid anion, carbonate, formate, acetate, propionate, butyrate, caprate, caproate, glycolate, lactate, pyruvate, valerate, acrylate, citrate, oxalate, malonate, succinate, glutarate, malate, adipate, pimelate, fumarate, suberate, azelate, benzoate, salicylate, phthalate, terephthalate, picolinate, methanesulfonate, ethanesulfonate, propanesulfonate, butanesulfonate, benzenesulfonate, toluenesulfonate, methanedisulfonate, ethanedisulfonate, propanedisulfonate, butanedisulfonate, sulfamate and / or isomers thereof, and / or halogenated forms thereof.

12. A PCM according to any preceding claim, wherein the clathrate hydrate comprises a salt comprising a tetra-alkyl ammonium and / or a tetra-alkyl phosphonium cation and one or more anion(s) selected from a list comprising: any halide, chromate, tungstate, sulfate and / or nitrate.

13. A PCM according to any preceding claim comprising: from about 70 wt% to about 99.995 wt% of the at least one clathrate hydrate; and from about 0.005 wt% to about 30 wt% of the at least one pH modulation agent.

14. A PCM according to any preceding claim comprising: from about 96 wt% to about 99.995 wt.% of the one or more clathrate hydrate; and from about 0.005 wt% to about 4 wt.% of the one or more pH modulation agent.

15. A PCM according to any preceding claim, wherein the clathrate hydrate is a salt hydrate with a molar ratio of saltwater of between 1 :20 and 1 :40, between 1 :25 and 1 :35, or about 1 :30; and wherein the cation:anion molar ratio in the salt is 1 :1.

16. A PCM according to any one of claims 1 to 14, wherein the clathrate hydrate is a salt hydrate with a molar ratio of saltwater of between 1 :50 and 1 :70, between 1 :55 and 1 :65, or about 1 :60; and wherein the cation:anion molar ratio in the salt is 2:1.

17. A PCM according to any preceding claim, wherein the PCM further comprises a nucleation agent; optionally wherein the nucleation agent is a zeolite;further optionally wherein the zeolite selected from a list comprising: 3A, 4A, 5A, 10X, and / or 13X.

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

19. Athermal energy storage device apparatus according to claim 18, wherein the PCM is in contact or substantially in contact with one or more metallic component and / or surfaces, optionally wherein contact occurs at least temporarily during use, preparation and / or storage of the thermal energy storage device apparatus.

20. A thermal energy storage device apparatus according to claim 19, wherein at least one of: the one or more metallic component is one or more item selected from a list comprising: pipework, a heat exchanger, a valve, a heat spreader, a heating element, a containment material, a sensor, a manifold, a plug, a bolt, a rivet, and / or other linkage; and / or. the one or more metallic component comprises one or more metals selected from a list comprising: aluminium, copper, iron, nickel, tin, titanium, zinc, steel, brass, Inconel, Hastelloy and combinations thereof.

21. A thermal energy storage device apparatus according to any one of claims 19 to 20, wherein the one or more metallic component comprises a heat exchanger, and wherein the heat exchanger comprises copper and optionally aluminium;, optionally wherein the pH of the clathrate hydrate PCM is between 7 and 10, between 7.5 and 9.5, between 7.5 and 8.5, or preferably about 8.

22. A method of preparation of a PCM according to any one of claims 1 to 17, comprising the steps of: i) providing a clathrate hydrate in a liquid state, wherein the clathrate hydrate comprises a cation, and anion, and water; and ii) combining the clathrate hydrate with one or more pH modulation agents, wherein the at least one pH modulation agent comprises at least one of the same cation and / or the same anion as the clathrate hydrate.

23. A method of preparation of a phase change material (PCM) according to any one of claims 1 to 17, comprising the steps of:i) providing one or more acid(s); ii) providing one or more base(s); iii) combining with water the one or more acid(s) and one or more base(s) to form a clathrate hydrate comprising water and a salt having a cation and an anion; and iv) combining the clathrate hydrate with one or more pH modulation agent(s) comprising at least one of the same cation and / or the same anion as the clathrate hydrate, optionally wherein the one or more pH modulation agent(s) is selected from one or more acids; and / or one or more bases; optionally wherein the PCM comprises from about 70 wt% to about 99.995 wt% of the clathrate hydrate, and from about 0.005 wt% to about 30 wt% of the at least one pH modulation agent.

24. A method of preparation of a PCM according to claim 23, wherein: the one or more acids comprises anion [X], optionally wherein the one or more acid is provided as a solution; the one or more bases comprises cation [Y], optionally wherein the one or more base is provided as a solution; the clathrate hydrate comprises a salt comprising anion [X], cation [Y], and water; and the method comprises maintaining the clathrate hydrate in a liquid state and combining the clathrate hydrate with the pH modulation agent in step iv), wherein at least one acid is comprises anion [X] and wherein at least one base is comprises cation [Y],25. A method of preparation of a PCM according claim 23 or 24, wherein the method comprises: i) providing a solution of one or more bases comprising cation [Y], optionally wherein the one or more bases are selected from tetrabutylammonium hydroxide and / or tetrabutylphosphonium hydroxide; ii) providing one or more acids comprising anion [X], optionally wherein [X] is selected from a list comprising: any deprotonated amino acid anion, carbonate, formate, acetate, propionate, butyrate, caprate, caproate,glycolate, lactate, pyruvate, valerate, acrylate, citrate, oxalate, malonate, succinate, glutarate, malate, adipate, pimelate, fumarate, suberate, azelate, benzoate, salicylate, phthalate, terephthalate, picolinate, methanesulfonate, ethanesulfonate, propanesulfonate, butanesulfonate, benzenesulfonate, toluenesulfonate, methanedisulfonate, ethanedisulfonate, propanedisulfonate, butanedisulfonate, sulfamate and / or isomers thereof, and / or halogenated forms thereof. iii) combining with water the one or more bases with the one or more acids to form a clathrate hydrate comprising one or more salt(s) comprising cation [Y] with anion [X] and water, iv) retaining the clathrate hydrate in a liquid state; and v) adding one or more acids comprising anion [X] and / or adding one or more bases comprising cation [Y] to modulate the pH of the clathrate hydrate optionally wherein the one or more bases are selected from tetrabutylammonium hydroxide and / or tetrabutylphosphonium hydroxide.