A phase change material for thermal energy storage

Abstract

The invention provides a phase change material for thermal energy storage, the phase change material comprising boric acid and succinic acid.

Description

A phase change material for thermal energy storage

[0001] The present application claims priority from Australian provisional patent application No. 2023903870 filed on 30 November 2023, the contents of which should be considered to be incorporated into this specification by this reference.Technical Field[2] The present invention relates to a phase change material for thermal energy storage, the phase change material comprising boric acid and succinic acid. The invention further relates to a molten phase of a phase change material, to a system for thermal energy storage, and to a method of storing thermal energy.Background of Invention[3] As the world transitions from fossil fuel-derived power to increasing amounts of renewal solar- and wind-derived electricity, there is an urgent need for technologies that can address mismatches between renewable power supply and electricity demand. Energy storage systems capable of storing mechanical energy (e.g. pumped-storage hydroelectricity) or chemical energy (e.g. electrochemical batteries) can be deployed to temporarily store excess renewable electricity when available, and to release it when required.[4] Thermal energy storage is another technology with good potential for temporarily storing energy from renewable power generation or from intermittent or waste heat sources (e.g. solar energy or industrial waste heat). When required, the heat can be discharged in a useful form, for example by generating electricity or for thermal energy applications such as the provision of hot water.[5] Of particular interest is the recently developed concept of the Carnot battery, where thermal energy storage is integrated with a reversible heat pump / organic Rankine cycle system. In the charge mode of such a Carnot battery, renewable electricity drives a heat pump, with the effect of pumping thermal energy from a cold reservoir to a hot reservoir where heat is stored in a thermal energy storage material. As is typical with heat pumps, the quantity of heat generated can be two to three times the electrical energy input to drive the pump. On discharge, the thermal energy storagematerial transfers heat to a working fluid, which drives the heat pump in reverse mode, operating as an organic Rankine cycle (ORC) engine to generate electricity. Optionally, residual heat in the working fluid is used to provide lower grade heat for various thermal energy applications, e.g. water heating. Thus far, power-to-power round trip efficiencies of over 70 % have been demonstrated for such a Carnot battery, excluding any uses of the residual low-grade heat. It is predicted that close to 100 % round-trip energy ratio will be achievable once this new technology is optimised, including the additional waste heat applications.[6] The choice of the thermal energy storage material is critical to the design and performance of thermal energy storage technologies, including Carnot batteries. Thermal energy storage materials are usually categorized by the heat storage mechanism, involving sensible, latent, or thermochemical energy storage.[7] Solids (e.g. sand), molten salts and liquid metals may be used as sensible heat storage materials. Such materials can advantageously store energy at very high temperatures, but typically provide only low energy density storage due to limited heat capacity of the materials.[8] Higher energy densities can be achieved by using a thermal energy storage material which stores latent heat, typically via a solid-to-liquid phase transition (i.e. melting). The stored latent heat is then released by reversing the phase transition (i.e. solidification I crystallisation). Such materials are known as phase change materials. In practice, phase change materials typically provide both sensible heat and latent heat storage, as they are usually cycled between a lower temperature which is below the melting point and an upper temperature which is above the melting point.[9] A phase change material suitable for a given thermal energy storage application must adequately satisfy a range of performance criteria, typically including one or more of: a high heat of fusion (ZI / - / f), a phase transition temperature well matched to the desired heat discharge temperature, high chemical stability when thermally cycled, acceptable phase-change kinetics (e.g. low supercooling), lack of phase separation, low toxicity and low flammability. Many reported phase change materials, such as organic compounds (paraffins, fatty acids, etc) or inorganic salt hydrates, are deficient with respect to one or more of these requirements.

[0010] High performing phase change materials with a melting temperature in an intermediate temperature range of 120 to 220°C, e.g. 120 to 170°C for an ORC engine, and having a AH of greater than 300 J g1, are particularly challenging to identify.

[0011] In principle, much higher energy densities can be provided by thermal energy storage materials which absorb and release thermochemical heat (the reaction enthalpy) via a reversible chemical reaction. A key difficulty with this approach is to identify a material capable of fully reversible reaction over multiple cycles in the required operating temperature range. Boric acid (H3BO3) has been investigated as a possible thermochemical phase change material, since it dehydrates endothermically above 140°C into metaboric acid (HBO2), with further dehydration to produce anhydrous boron oxide (B2O3) occurring at higher temperatures. These processes are known to require large quantities of thermal energy (1554 J g-1), but the dehydration reactions are not adequately reversible for pure boric acid to be used in practical thermochemical energy storage systems.

[0012] There is therefore an ongoing need for phase change materials for thermal energy storage, which at least partially address one or more of the above-mentioned short-comings, or provide a useful alternative.

[0013] A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that the document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.Summary of Invention

[0014] The present invention is based on the finding that a mixture of boric acid and succinic acid, preferably at or close to the eutectic composition, provides a phase change material capable of thermal energy storage with unexpectedly high energy density. A mixture of 60 mol% boric acid and 40 mol% succinic acid (44 wt.% boric acid and 56 wt.% succinic acid) was thus found to melt with a AH of about 400 J g-1at a melting point of 150°C. The molten phase re-solidified at 138°C, releasing about 360 J g1of thermal energy.

[0015] The exceptionally high observed AH can be attributed to a reversible thermochemical process which has been shown by experiment to occur at about the same temperature as the phase transition of the mixture. As the phase change material melts, a portion of the boric acid (orthoboric acid; H3BO3) dehydrates endothermically to form metaboric acid (HBO2). The phase change material therefore takes up and stores heat corresponding to the dehydration reaction enthalpy, in addition to the latent heat corresponding to the phase transition, with both modes of heat storage contributing to the observed AH. The water released is substantially retained in the molten phase. When the molten phase is subsequently re-solidified, the metaboric acid re-hydrates exothermically to form orthoboric acid, simultaneously releasing heat corresponding to both the hydration reaction enthalpy and the latent heat of crystallisation. Surprisingly, the latent and thermochemical energy storage processes occur simultaneously or near-simultaneously in both heating and cooling cycles, so that most of the heat is absorbed and released within a narrow temperature range at the phase transition.

[0016] Given the thermochemical contribution to the total heat storage capacity provided by the boric acid, it is particularly advantageous that a mixture of boric acid and succinic acid with essentially a single melting point can be obtained with about 44 wt.% of boric acid. By contrast, binary eutectics of boric acids with many other organic materials contain a much lower mass fraction of boric acid (e.g. 20 wt.% or less).

[0017] When thermally cycled between a lower temperature which is below the melting point and an upper temperature which is above the melting point, the boric acid- succinic acid phase change material stores and releases energy via all three modes of thermal heat storage, i.e. sensible, latent, and thermochemical heat storage mechanisms.

[0018] The single melting point mixture of boric acid and succinic acid was found to provide highly stable performance over many thermal cycles, with less than 6% loss in the observed AH over 1000 cycles and no evidence of irreversible chemical decomposition. This is considered surprising because: (i) succinic acid is itself unsuitable as a phase change material due to poor chemical stability above its melting point, (ii) the dehydration of boric acid has previously been found insufficiently reversible for thermochemical energy storage materials, and (iii) loss of the waterproduct, e.g. by partitioning to the vapour phase above the molten phase change material, would be expected to limit the reverse reaction. The inventors have found that the water released during dehydration is strongly retained in the molten phase, despite the high temperature, and is thus readily available for reaction with metaboric acid in the molten phase as it resolidifies.

[0019] In accordance with a first aspect, the invention provides a phase change material for thermal energy storage, the phase change material comprising boric acid and succinic acid.

[0020] In some embodiments, the phase change material comprises the boric acid and the succinic acid in a ratio (mol / mol) of between 50:50 and 75:25, such as in a ratio (mol / mol) of between 55:45 and 70:30.

[0021] In some embodiments, the phase change material comprises the boric acid and the succinic acid in a ratio (mol / mol) of between 57:43 and 68:32, such as in a ratio (mol / mol) of between 57:43 and 63:37, for example in a ratio (mol / mol) of about 60:40.

[0022] In some embodiments, the phase change material comprises boric acid and succinic acid in a combined amount of at least 90 wt.%, or at least 95 wt.%, such as at least 99 wt.%, for example 100 wt.%, based on the total amount of phase-change components in the phase change material.

[0023] In some embodiments, the phase change material melts to form a molten phase comprising boric acid (H3BO3), metaboric acid (HBO2), succinic acid and water.

[0024] In some embodiments, the phase change material melts to form a molten phase consisting of boric acid, succinic acid and any reaction products thereof.

[0025] In some embodiments, a solidified composition comprising the boric acid and succinic acid melts at 150°C ± 5°C when heated.

[0026] In some embodiments, a molten phase comprising the boric acid and succinic acid solidifies at a temperature of greater than 135°C, such as at a temperature of 138°C or greater, when cooled.

[0027] In some embodiments, the phase change material melts with an enthalpy (AH) of greater than 350 J g-1, or greater than 360 J g-1, or greater than 370 J g-1, such as greater than 380 J g1, for example greater than 390 J g1, based on the combined mass of boric acid and succinic acid.

[0028] In some embodiments, the phase change material solidifies, when cooled from a molten phase, with an enthalpy (AHC) of greater than 310 J g-1, or greater than 320 J g1, or greater than 330 J g1, such as greater than 340 J g1, for example greater than 350 J g-1, based on the combined mass of boric acid and succinic acid.

[0029] In some embodiments, the phase change material further comprises a nonmelting solid particulate, optionally a thermally conductive solid particulate to improve the thermal conductivity of the phase change material.

[0030] In accordance with a second aspect, the invention provides a molten phase of a phase change material, comprising boric acid (H3BO3), metaboric acid (HBO2), succinic acid and water.

[0031] In some embodiments, the ratio of combined boric acid and metaboric acid to succinic acid in the molten phase (mol / mol) is between 50:50 and 75:25, such as between 55:45 and 70:30.

[0032] In some embodiments, the ratio of combined boric acid and metaboric acid to succinic acid in the molten phase (mol / mol) is between 57:43 and 68:32, such as between 57:43 and 63:37, for example about 60:40.

[0033] In some embodiments, the molten phase solidifies at a temperature of greater than 135°C, such as at a temperature of 138°C or greater, when cooled.

[0034] In some embodiments, the molten phase solidifies when cooled with an enthalpy (AHC) of greater than 310 J g1, or greater than 320 J g1, or greater than 330 J g1, such as greater than 340 J g1, for example greater than 350 J g1, based on the combined mass of boric acid, metaboric acid, succinic acid and water.

[0035] In accordance with a third aspect, the invention provides a system for thermal energy storage, the system comprising a phase change material according toany embodiment of the first aspect and at least one heat exchange element configured to transfer heat between the phase change material and a fluid.

[0036] In some embodiments, the phase change material is enclosed in a sealed vessel.

[0037] In some embodiments, the system comprises an electricity generation device for generating electricity from the fluid.

[0038] In some embodiments, the system comprises an organic working fluid, wherein the energy generation device is powered, in an organic Rankine cycle, by a hot organic gas produced by vaporising the organic working fluid when heat is transferred from the phase change material to the organic working fluid via the at least one heat exchange element.

[0039] In some embodiments, the system comprises a source of thermal energy to heat the phase change material, optionally wherein the source of thermal energy is configured to provide a hot fluid that transfers heat to the phase change material via the at least one heat exchange element. In some embodiments, the source of thermal energy is a heat pump.

[0040] In some embodiments, the system is a Carnot battery.

[0041] In accordance with a fourth aspect, the invention provides a method of storing thermal energy, the method comprising: providing a phase change material comprising boric acid and succinic acid; transferring heat to the phase change material, thereby melting at least a portion of the phase change material to form a molten phase; and transferring heat from the phase change material to a fluid, thereby solidifying at least a portion of the molten phase and producing a heated fluid.

[0042] In some embodiments, the phase change material comprises the boric acid and the succinic acid in a ratio (mol / mol) of between 50:50 and 75:25, such as in a ratio (mol / mol) of between 55:45 and 70:30.

[0043] In some embodiments, the phase change material comprises the boric acid and the succinic acid in a ratio (mol / mol) of between 57:43 and 68:32, such as in a ratio (mol / mol) of between 57:43 and 63:37, for example in a ratio (mol / mol) of about 60:40.

[0044] In some embodiments, the phase change material comprises boric acid and succinic acid in a combined amount of at least 90 wt.%, or at least 95 wt.%, such as at least 99 wt.%, for example 100 wt.%, based on the total amount of phase-change components in the phase change material.

[0045] In some embodiments, at least a portion of the boric acid dehydrates to form metaboric acid and water as the phase change material melts, and the metaboric acid rehydrates to form orthoboric acid as the molten phase solidifies.

[0046] In some embodiments, the molten phase consists of boric acid, succinic acid and reaction products thereof.

[0047] In some embodiments, the molten phase solidifies to form a solidified phase consisting of boric acid and succinic acid.

[0048] In some embodiments, the phase change material is enclosed in a sealed vessel.

[0049] In some embodiments, transferring heat to the phase change material comprises heating the phase change material from a temperature of less than 140°C, such as less than 120°C, to a temperature of greater than 155°C, such as greater than 160°C, and transferring heat from the phase change material to the fluid comprises cooling the phase change material from a temperature of greater than 155°C, such as greater than 160°C, to a temperature of less than 140°C, such as less than 120°C.

[0050] In some embodiments, the method further comprises generating mechanical and / or electrical energy from the heated fluid.

[0051] In some embodiments, the fluid is a liquid organic working fluid. T ransferring heat from the phase change material to the fluid may comprise vaporising at least a portion of the liquid organic working fluid to form a hot organic gas. The method may comprise generating electrical energy from the hot organic gas in an organic Rankine cycle.

[0052] In some embodiments, transferring heat to the phase change material comprises heating the organic working fluid to produce a hot organic liquid andtransferring heat from the hot organic liquid to the phase change material. In some embodiments, the organic working fluid is heated with a heat pump.

[0053] In some embodiments, the phase change material melts at 150°C ± 5°C.

[0054] In some embodiments, the molten phase solidifies at a temperature of greater than 135°C, such as at a temperature of 138°C or greater.

[0055] In some embodiments, the phase change material melts with an enthalpy (AH) of greater than 350 J g1, or greater than 360 J g-1, or greater than 370 J g1, such as greater than 380 J g1, for example preferably greater than 390 J g1, based on the combined mass of boric acid and succinic acid.

[0056] In some embodiments, the molten phase solidifies with an enthalpy (AHC) of greater than 310 J g-1, or greater than 320 J g-1, or greater than 330 J g-1, such as greater than 340 J g-1, for example greater than 350 J g-1, based on the combined mass of boric acid and succinic acid.

[0057] Where the terms “comprise”, “comprises” and “comprising” are used in the specification (including the claims) they are to be interpreted as specifying the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof.

[0058] Further aspects of the invention appear below in the detailed description of the invention.Brief Description of Drawings

[0059] Embodiments of the invention will herein be illustrated by way of example only with reference to the accompanying drawings in which:

[0060] Figure 1 shows a DSC scan of a mixture of 60 mol% boric acid and 40 mol% succinic acid, and schematically depicts the three modes of energy storage obtained when this mixture is used as a phase change material.

[0061] Figure 2 schematically depicts a Carnot battery comprising a phase change material according to embodiments of the invention.

[0062] Figure 3 shows DSC scans of binary mixtures of boric acid and succinic acid, with excess boric acid relative to the binary eutectic composition, as obtained in Example 1 .

[0063] Figure 4 shows a DSC scan of a mixture of 60 mol% boric acid and 40 mol% succinic acid, which is at or close to the eutectic composition, as obtained in Example 1.

[0064] Figure 5 shows DSC scans of binary mixtures of boric acid and succinic acid, with excess succinic acid relative to the binary eutectic composition, as obtained in Example 1 .

[0065] Figure 6 is a graph showing the main endothermic transitions for each mixture analysed in the DSC scans of Figures 3-5.

[0066] Figure 7 shows Raman spectra for a mixture of 60 mol% boric acid and 40 mol% succinic acid at room temperature, in the molten phase at 150°C, and at room temperature after 1000 cycles of heating and cooling, as measured in Example 2.

[0067] Figure 8 shows the vapor pressure above a mixture of 60 mol% boric acid and 40 mol% succinic acid, after melting at 150°C, as measured in Example 3.

[0068] Figure 9 shows DSC scans of a mixture of 60 mol% boric acid and 40 mol% succinic acid at various heating and cooling steps during 1000 heating-cooling cycles, as measured in Example 4.

[0069] Figure 10 shows Raman spectra for a mixture of 60 mol% boric acid and 40 mol% succinic acid before thermal cycling and after 1000 heating-cooling cycles, with comparison against spectra for pure boric acid and pure succinic acid, as measured in Example 4.

[0070] Figure 11 shows a powder X-Ray diffractogram for a mixture of 60 mol% boric acid and 40 mol% succinic acid before thermal cycling and after 1000 heatingcooling cycles, with comparison against spectra for pure boric acid and pure succinic acid, as measured in Example 4.Detailed DescriptionPhase change material for thermal energy storage

[0071] The present invention relates to a phase change material which comprises boric acid and succinic acid. As used herein, a phase change material refers to a material which can absorb thermal energy when it undergoes a solid-to-liquid phase transition, thus storing energy as latent heat, and which can release the latent heat when the material undergoes the reverse liquid-to-solid phase transition. Preferably, the phase transitions of such materials occur repeatably so that the phase change material can undergo multiple cycles of melting (heat absorption) and solidification (heat release) with little or no loss of heat absorption capacity. The phase change material may thus be suitable for various thermal energy storage applications.

[0072] Boric acid, also known as orthoboric acid, has the molecular formula H3BO3. Succinic acid is the common name for 1 ,4-butane-dioic acid.

[0073] The boric acid and succinic acid may in principle be combined in any form capable of providing a reversible phase transition between a solid form and a molten phase. During thermal cycling, the solid form will comprise a solidified composition comprising both boric acid and succinic acid as an intimate mixture. As used herein, a solidified composition of the phase change material refers to a solid composition or phase that forms when a molten phase of the phase change material is cooled and undergoes a liquid-to-solid phase transition. At least a portion, and preferably substantially all, of the solidified composition may be a homogeneous phase comprising both boric acid and succinic acid. However, it is not excluded that the phase change material may comprise boric acid and succinic acid as discrete components, e.g. as a mixture of discrete solid particles of boric acid and succinic acid. When heated in the first heating cycle, the two discrete components can melt and combine to form a molten phase comprising the desired proportions of each component.

[0074] The phase change material comprises at least two phase-change components, including the boric acid and succinic acid. As used herein, the phasechange components are those components that can transition between the solid form (e.g. a solidified composition as defined herein) and the molten phase of the phase change material, and thus excludes non-meltable components such as non-melting solid particulates. Preferably, the phase change material comprises the boric acid andsuccinic acid as the primary or only phase-change components. In some embodiments, the boric acid and succinic acid are present in a combined amount of at least 90 wt.%, such as at least 95 wt.%, for example at least 99 wt.%, based on the total amount of phase-change components in the phase change material In some embodiments, the boric acid and succinic acid are the only phase-change components. Ignoring the contribution of any trace impurities, the solidified form of such a phase change material may be considered a binary mixture of boric acid and succinic acid.

[0075] The boric acid and the succinic acid may be present in a ratio suitable to provide an effective phase change material. Ideally, the boric acid and the succinic acid may be present in a ratio whereby the solidified composition is a eutectic composition, i.e. a single homogeneous mixed phase with a single, well-defined melting point. The eutectic composition of a binary system can be identified by investigating the thermal properties of mixtures with a range of different mixing ratios. For the binary boric acid-succinic acid system, the eutectic composition is close to 60 mol% boric acid and 40 mol% succinic acid, as is evident from the single endothermic peak at 150°C for this composition in a differential scanning calorimetry (DSC) scan.

[0076] In practice, the ratio of boric acid to succinic acid in the phase change material may depart from the ideal eutectic ratio while still providing excellent heat storage capability. For example, a binary mixture of 65 mol% boric acid and 35 mol% succinic acid also exhibits near-eutectic characteristics in a DSC scan. In some embodiments, therefore, the boric acid and the succinic acid are present in a ratio (mol / mol) of between 50:50 and 75:25, or between 55:45 and 70:30, such as between 57:43 and 68:32, for example between 57:43 and 63:37.

[0077] The phase change material may have a melting point in the range of 150°C ± 5°C when the solidified composition is heated. Such a melting point is particularly useful for certain thermal storage applications, such as Carnot batteries operating with an organic Rankine cycle engine. The melting point may be identified as the only or dominant endothermic peak in the DSC scan when the solidified composition is heated to fully melt the solidified composition. In some embodiments, more than 90% of the latent heat absorbed when heating the solidified composition is absorbed in the range of 150°C ± 5°C.

[0078] The molten phase of the phase change material may solidify at a temperature of greater than 135°C, preferably at a temperature of 138°C or greater, when cooled. The temperature of solidification (also known as the crystallisation temperature) may be identified as the only or dominant exothermic peak in the DSC scan when the molten phase is cooled to fully solidify the composition. In some embodiments, more than 90% of the latent heat released when cooling the molten phase is released at a temperature of greater than 135°C, preferably at a temperature of 138°C or greater.

[0079] As the skilled person will appreciate, the temperature of solidification is typically dependent on the kinetics of the crystallisation process, and may thus be affected by variables such as the size of the sample, the rate of cooling, the thermal conductivity of the phase change material, and the presence of any nucleation agents. In practice, the temperature of solidification is lower than the melting point, with the difference representing the extent of supercooling in the system. Supercooling is preferably minimised so as to maximise the energy release efficiency from the phase change material. Advantageously, at least some embodiments of the phase change materials disclosed herein solidify with a single, well-defined crystallisation temperature and with supercooling of no more than about 12°C. It is expected that the degree of supercooling in commercial-scale implementations will be further reduced due to larger samples and the use of solid particulates to improve thermal conductivity and / or nucleation.

[0080] The phase change material may have a high heat storage capacity, as determined by (i) the measured endothermic enthalpy (AH) when heating the phase change material so as to melt the solidified composition thereof and / or (ii) the measured exothermic enthalpy (AH) when cooling the phase change material so as to solidify the fully molten phase thereof. AH may be determined by DSC, based on the area of the endothermic peak(s) in a heating stage of the DSC scan, preferably in a second or later cycle of heating and cooling so as to ensure the solid form of the phase change material is a solidified composition. AH may also be determined by DSC, based on the area of the exothermic peak(s) in a cooling stage of the DSC scan. AH and AH can both be quantified by methods known to those of skill in the art, such as ASTM E793-06 (2018). As will be explained in greater detail hereafter, the measured values of AH and AH forthe phase change materials of the present disclosure may include contributions from both latent heat storage and thermochemical heat storage mechanisms.

[0081] In some embodiments, the phase change material melts with an enthalpy (AH) of greater than 350 J g1, or greater than 360 J g-1, or greater than 370 J g1, such as greater than 380 J g-1, for example greater than 390 J g-1, based on the combined mass of boric acid and succinic acid. It has been found by experiment that a eutectic or near-eutectic composition of 60 mol% boric acid and 40 mol% succinic acid melts with an enthalpy of about 400 J g-1.

[0082] In some embodiments, the phase change material solidifies, when cooled from the molten phase, with an enthalpy (A / -fc) of greater than 310 J g-1, or greater than 320 J g1, or greater than 330 J g1, such as greater than 340 J g1, for example greater than 350 J g-1, based on the combined mass of boric acid and succinic acid. It has been found by experiment that a eutectic or near-eutectic composition of 60 mol% boric acid and 40 mol% succinic acid crystallises with an enthalpy of about 360 J g-1.

[0083] The excellent heat storage capability of the phase change materials disclosed herein can be attributed to a reversible thermochemical process which occurs at about the same temperature as the phase transition of the mixture. As the phase change material melts, a portion of the boric acid (orthoboric acid; H3BO3) dehydrates endothermically to form metaboric acid (HBO2). The phase change material therefore takes up and stores heat corresponding to the dehydration reaction enthalpy, in addition to the latent heat corresponding to the phase transition, with both modes of heat storage contributing to the observed AH. The water released by the dehydration reaction is strongly retained in the molten phase. The molten phase thus comprises (and may consist essentially of) boric acid (H3BO3), metaboric acid (HBO2), succinic acid and water. In embodiments where the only phase-change components of the solidified composition are boric acid and succinic acid, the molten phase consists of boric acid, succinic acid and their reaction products (i.e. metaboric acid, water and any trace degradation products).

[0084] When the molten phase is subsequently re-solidified, the metaboric acid rehydrates exothermically by reaction with the retained water to form orthoboric acid, simultaneously or near-simultaneously releasing heat corresponding to both thehydration reaction enthalpy and the latent heat of crystallisation. Surprisingly, the latent and thermochemical energy storage processes occur simultaneously or near- simultaneously in both heating and cooling cycles, so that most of the heat is absorbed and released within a narrow temperature range at the phase transition.

[0085] Given the thermochemical contribution to the total heat storage capacity provided by the boric acid, it is particularly advantageous that the compositions disclosed herein can operate effectively with a relatively high amount of boric acid. For example, the eutectic composition of boric acid and succinic acid contains about 44 wt.% boric acid. In some embodiments, therefore, the phase change material comprises the boric acid in an amount of at least 40 wt.%, such as at least 42 wt.%, of the phase-change components.

[0086] When thermally cycled between a lower temperature which is below the melting point and an upper temperature which is above the melting point, the boric acid- succinic acid phase change material stores and releases energy via all three modes of thermal heat storage, i.e. sensible, latent, and thermochemical heat storage mechanisms. For example, a sensible heat storage capacity of about 65 J g-1(in addition to the latent and thermochemical heat storage capacity combined in the observed AH), can be expected if cycling a eutectic or near-eutectic composition (60 mol% boric acid and 40 mol% succinic acid) between 115°C and 165°C, based on the known heat capacities of the two materials (Cp boric acid = 1 .392 J g-1K1and Cp succinic acid = 1.39 J g-1K’1).

[0087] The three modes of energy storage provided by the phase change materials of the present disclosure are visualised in Figure 1 , which shows a DSC scan (heating and cooling at 10°C per minute between about 103°C and 173°C) of a eutectic or neareutectic mixture of 60 mol% boric acid and 40 mol% succinic acid. Latent energy storage (LES) and thermochemical energy storage (TCES) are provided, in combination, at the phase transition, as quantified by the values of AH and AHC. In addition, sensible heat storage (SES) is provided by virtue of the operating temperature range between the lower and upper temperatures of the phase change material.

[0088] As disclosed herein, the phase change material is capable of thermally cycling between a low energy form, where the phase-change components are presentprimarily or entirely in a solidified composition, and a high energy form, where the phase-change components are present primarily or entirely in a molten phase. Advantageously, at least some embodiments of the phase change material are capable of such thermal cycling over a large number of heating-cooling cycles without substantial loss of heat storage capacity due to irreversible processes, e.g. chemical degradation.

[0089] In some embodiments, the phase change material is capable of at least 100 cycles, preferably at least 1000 cycles, of heating and cooling between a lower temperature below the solidification temperature and an upper temperature about the melting temperature, with no more than 10% loss in the heat storage capacity, preferably no more than 7% loss in heat storage capacity, as measured by AH or AH. For example, a eutectic or near-eutectic composition (60 mol% boric acid and 40 mol% succinic acid) has been found capable of 1000 thermal cycles between 1 10°C and 165°C with less than 6% loss in the measured value of AH.

[0090] In use, the phase change material will typically be contained in a sealed vessel. Moreover, the water released during the dehydration reaction of boric acid has been found to be strongly retained in the molten phase, despite its high temperature. It is thus expected that the reversibility of melting, and the repeatability during thermal cycling, will not be significantly affected by loss of water from the phase change composition. However, if necessary, water could be added to the phase change material to replace any losses.

[0091] In some embodiments, the phase change material comprises a non-melting component such as a non-melting solid particulate. As used herein, a non-melting component is a solid component does not melt during thermal cycling of the phase change material. The non-melting solid particulate may be present in an amount of less than 5 wt.%, such as less than 2 wt.%.

[0092] In some embodiments, the phase change comprises a thermally conductive solid particulate to improve the thermal conductivity of the phase change material. An improved thermal conductivity may advantageously facilitate absorption and release of thermal energy from the phase change material, thus improving its performance in thermal energy storage applications. Examples of such materials may include variousforms of particulate carbon such as graphite, graphene and reduced graphene oxide, and particulate metal such as metal flakes.

[0093] In some embodiments, the phase change comprises a solid particulate as a nucleation agent. The nucleation agent provides a solid surface on which the molten phase of the phase change composition can nucleate when cooled, thus improving the kinetics of the solidification process. The use of a nucleation agent may thus advantageously reduce the degree of supercooling in the phase change material. Suitable solid particulate nucleation agents may include finely divided (optionally nanoparticle forms of) inert inorganic compounds such as metal oxides (e.g. TiC , SiO2, AI2O3, CaO) or carbon materials such as carbon nanoparticles, nanotubes, nanosheets (e.g. graphene) and the like.

[0094] Any non-melting solid particulate components, such as thermally conductive solid particulate or nucleation agents, are preferably dispersed throughout the phase change material, even when fully molten, so as to improve thermal conductivity and / or nucleation in the bulk of the material.

[0095] The phase change material may also include minor amounts of other additives, such as an anti-oxidant to enhance chemical stability.System for thermal energy storage

[0096] The present invention further relates to a system for thermal energy storage. The system comprises a phase change material and apparatus for transferring heat to and / or from the phase change material. In particular, the system may comprise a phase change material as disclosed herein and at least one heat exchange element. The heat exchange element may be configured to transfer heat between the phase change material and a fluid, such as a liquid or a gas.

[0097] The heat exchange element, or heat exchanger, may comprise a solid, thermally conductive partition which physically separates the phase change material and fluid, but which allows heat transmission between these media. For example, the heat exchange element may comprise one or more thermally conductive pipes, e.g. metal pipes, which pass through a volume of the phase change material so as to facilitate heat exchange in the bulk of the phase change material.

[0098] The phase change material may be enclosed in a sealed vessel. The sealed vessel may thus retain heat and any gaseous components produced during thermal cycling of the phase change material, such as water vapour. The vessel may be insulated to prevent unwanted loss of heat, thus maximising the efficiency of heat storage and recovery. The heat exchange element may be integrated with the sealed vessel, for example as one or more pipes passing through the walls of the sealed vessel, thus allowing the fluid to be introduced into the vessel for heat exchange while remaining physically separated from the phase change material. Alternatively or in addition, walls of the sealed vessel may form part of one or more heat exchange elements, so that heat transfers through the walls between the phase change material enclosed by the vessel walls and the fluid outside the vessel walls. Since the phase change material transitions between solid and molten forms, it may be preferred that it remains static in the sealed vessel during thermal cycling. The fluid may be flowed through the heat exchange element with a sufficient contact time to allow the necessary heat exchange.

[0099] The system as a whole, and the heat exchange element in particular, may be configured to transfer heat from a molten phase of the phase change material to the fluid, thus heating the fluid. If the fluid is a liquid working fluid, the working fluid may thus be vaporised to produce a hot gas. Alternatively, the heat transfer may simply heat a liquid, such as water, to produce a hot liquid, such as hot water.

[0100] Alternatively, or in addition, the system as a whole, and the heat exchange element in particular, may be configured to transfer heat from a fluid to the phase change material, thus melting at least a portion of a solidified composition present in the phase change material. A hot liquid or gas, with a temperature above the melting point of the phase change material, may thus be contacted with the heat exchange element to transfer heat to the phase change material. In some embodiments, a single working fluid is used to transfer heat via the heat exchange element to the phase change material when storing thermal energy, and to receive heat via the heat exchange element from the phase change material when recovering thermal energy.

[0101] In some embodiments, the system comprises an electricity generation device for generating electricity from a working fluid. The working fluid may be vaporised at the heat exchange element by heat transferred from the phase changematerial, and the resultant hot gas is used to generate electricity, for example by passing it through an expansion device such as a turbine.

[0102] Given the phase transition temperature of the phase change material, organic working fluids with boiling points below that of water are particularly suitable for converting thermal energy released by the phase change material into electricity. The system may thus comprise an organic working fluid, such as pentane or isobutane. The energy generation device is thus powered, in an organic Rankine cycle, by the hot organic gas produced by vaporising the organic working fluid when heat is transferred from the phase change material to the organic working fluid via the heat exchange element.

[0103] The system may be configured to heat the phase change material, thereby storing thermal energy as disclosed herein, with any suitable source of thermal energy. For example, the system may comprise one or more electrical heating elements to heat the phase change material, for example by resistive heating. In some preferred embodiments, however, the system is configured to transfer heat from a hot fluid to the phase change material via the heat exchange element, as already disclosed herein. For example, the hot fluid may be a hot liquid or gas containing waste heat provided by an industrial process. Alternatively, the hot fluid may be heated by a solar heater.

[0104] In some embodiments, the system comprises a source of thermal energy configured to provide the hot fluid for transferring heat to the phase change material via the heat exchange element. For example, the source of thermal energy may be a heat pump or a solar heater.

[0105] In some embodiments, the system is configured as a Carnot battery, an example of which is schematically depicted in Figure 2. Carnot battery 100 receives electricity for storage, for example excess electricity intermittently produced by renewable power source 102, and discharges electricity when required, for example to local grid 104 or industrial end user 106 during periods of high demand. Carnot battery 100 includes a sealed vessel 108 containing a reservoir of phase change material 109 as disclosed herein, an internal heat exchanger (not shown) configured to transfer heat between the phase change material and an organic working fluid, andelectromechanical device 1 10 configured to operate as a heat pump during a heat storage step and an electric generator during a heat discharge step.

[0106] In charge mode, electricity from renewable power source 102 drives heat pump 1 10. The heat pump compresses warm organic gas 112 (temperature e.g. 25- 50°C), thereby condensing and heating it to form hot organic working liquid 1 14 (temperature e.g. 155-165°C). Hot organic working liquid 114 flows to sealed vessel 108 where it passed through the internal heat exchanger and transfers heat to phase change material 109. The transfer of heat melts at least a portion of the phase change material, thereby forming a molten phase and storing thermal energy in the phase change material according to the principles disclosed herein. Cooled organic working liquid 1 16 (temperature close to that of the phase change material’s melting point, e.g. 150-155°C) exiting the heat exchanger is then expanded to form cold organic gas 1 18 (temperature e.g. 0-20°C). Cold organic gas 1 18 then flows to low temperature heat reservoir 120, where it passes through a second heat exchanger (not shown) in thermal contact with heat source 121 and is thus heated to produce warm organic gas 1 12 (temperature e.g. 25-50°C) for feeding again to heat pump 1 10. Heat source 121 may be a reservoir of liquid such as polethylene glycol in a vessel, or just the ground (i.e. soil) in a ground sourced design. The charging cycle may be continued until the phase change material in sealed vessel 108 is fully melted and heated to its upper operating temperature.

[0107] In discharge mode, and using the same working liquid, organic working liquid 1 16a (temperature e.g. 25-50°C) flows to sealed vessel 108 where it passes through the internal heat exchanger, thereby absorbing heat from phase change material 109. The transfer of heat solidifies at least a portion of the phase change material, thereby forming a solidified composition and releasing thermal energy from the phase change material according to the principles disclosed herein. The transfer of heat heats and vaporises organic working liquid 1 16a in the internal heat exchanger, thereby forming hot organic gas 1 14a at elevated pressure (temperature close to that of the freezing point of the phase change material e.g. 145-150°C). Hot organic gas 1 14a drives electrical generator 1 10, thereby generating electricity, and the expanded outflowing gas 1 12a (temperature e.g. 50-70°C) is then flowed to low temperature heat reservoir 120 where it passes through the second heat exchanger to transfer residual heat toheat exchange material 121 in the reservoir. Gas 112a is thus cooled and condensed to form warm organic working liquid 118a (temperature e.g. 25-50°C) which is then recycled via generator 1 10 to form warm organic working liquid 116a. The discharge cycle may be continued until the phase change material in sealed vessel is fully solidified and cooled to its lower operating temperature.Method of storing thermal energy

[0108] The present invention further relates to a method of storing thermal energy. The method comprises providing a phase change material comprising boric acid and succinic acid. In a heat storage step, heat is transferred to the phase change material, thereby melting at least a portion of the phase change material to form a molten phase. In a heat discharge step, heat is transferred from the phase change material to a fluid, thereby solidifying at least a portion of the molten phase and producing a heated fluid.

[0109] The phase change material may generally be according to any of the embodiments disclosed herein in the section describing the phase change material for thermal energy storage. In some embodiments, the phase change material melts at a temperature of 150°C ± 5°C. In some embodiments, the molten phase solidifies at a temperature of greater than 135°C, preferably at a temperature of 138°C or greater.

[0110] During the heat storage step, at least a portion of the boric acid may dehydrate to form metaboric acid and water. The dehydration reaction may take place as the phase change material melts, so that the phase change material simultaneously or near-simultaneously absorbs heat both as thermochemical reaction enthalpy and latent heat. The molten phase thus comprises (and may consist essentially of) boric acid (H3BO3), metaboric acid (HBO2), succinic acid and water. In embodiments where the only phase-change components of the solidified composition are boric acid and succinic acid, the molten phase may consist of boric acid, succinic acid and their reaction products (i.e. metaboric acid, water and any trace degradation products).

[0111] In some embodiments, the phase change material is heated from a lower temperature of less than 140°C, or less than 130°C, such as less than 120°C, to an upper temperature of greater than 155°C, such as greater than 160°C. In this manner, the phase change material may store a significant amount of thermal energy as sensible heat, in addition to the thermochemical and latent heat storage mechanisms.However, it will be appreciated that heat may in principle be transferred to and from the phase change material without significantly varying its temperature away from the melting temperature. This may occur if the phase change material is not fully melted before switching from the heat storage step to the heat discharge step, or is not fully solidified before switching from the heat discharge step to a subsequent heat storage step.

[0112] In some embodiments, the phase change material melts with an enthalpy (AH) of greater than 350 J g1, or greater than 360 J g-1, or greater than 370 J g1, such as greater than 380 J g-1, for example greater than 390 J g-1, based on the combined mass of boric acid and succinic acid. The phase change material may absorb such amounts of heat when heated from an initial, fully solidified state to a subsequent, fully molten state during the heat storage step.

[0113] During the heat discharge step, the metaboric acid may rehydrate to form orthoboric acid, preferably in quantitative or near-quantitative yield. The dehydration reaction may take place as the molten phase solidifies, so that the phase change material simultaneously or near-simultaneously releases heat both as thermochemical reaction enthalpy and latent heat. In some embodiments, the molten phase solidifies to form a binary mixture of boric acid and succinic acid, ideally a binary eutectic mixture of boric acid and succinic acid.

[0114] In some embodiments, the phase change material is cooled from an upper temperature of greater than 155°C, such as greater than 160°C, to a lower temperature of less than 140°C, or less than 130°C, such as less than 120°C. In this manner, the phase change material releases thermal energy as sensible heat, in addition to the thermochemical and latent heat release mechanisms.

[0115] In some embodiments, the molten phase solidifies with an enthalpy (AHC) of greater than 310 J g-1, or greater than 320 J g-1, or greater than 330 J g-1, such as greater than 340 J g-1, for example greater than 350 J g-1, based on the combined mass of boric acid and succinic acid. The phase change material may release such amounts of heat when cooled from an initial, fully molten state to a subsequent, fully solidified state during the heat discharge step.

[0116] The method of storing thermal energy may be performed in a system for thermal energy storage according to any embodiment disclosed herein. The phase change material is thus preferably enclosed in a sealed vessel in use. Heat may be transferred from the phase change material to the fluid via at least one heat exchange element. Optionally, heat may also be transferred to the phase change material via the heat exchange element.

[0117] The fluid to which heat is transferred may be a liquid or a gas. The heated fluid may be a liquid or a gas. In some embodiments, transferring heat from the phase change material to the fluid comprises vaporising at least a portion of the fluid from an initial liquid state to form a hot gas. In other embodiments, the fluid is water and the heated fluid is heated water, as hot water and / or steam.

[0118] The method may further comprise generating mechanical and / or electrical energy from the heated fluid, preferably from a hot gas.

[0119] In some embodiments, the fluid is a liquid organic working fluid. Heat transferred from the phase change material during the energy discharge step vaporises at least a portion of the liquid organic working fluid to form a hot organic gas. The method may then further comprise generating electrical energy from the hot organic gas in an organic Rankine cycle.

[0120] In some embodiments, heat is transferred to the phase change material by heating the organic working fluid to produce a hot organic liquid and then transferring heat from the hot organic liquid to the phase change material, typically via at least one heat exchange element as disclosed herein. In some embodiments, the organic working fluid is heated with a heat pump.

[0121] In some embodiments, the phase change material is cycled through multiple thermal cycles, each cycle comprising a heat storage step and a heat discharge step. The phase change material may be cycled through at least 100 such cycles, preferably at least 1000 such cycles. In some embodiments, the cycling causes no more than 10% loss in the heat storage capacity, preferably no more than 7% loss in heat storage capacity, of the phase change material, as measured by H or AHC.EXAMPLES

[0122] The present invention is described with reference to the following examples. It is to be understood that the examples are illustrative of and not limiting to the invention described herein.Example 1. Phase diagram of binary mixtures

[0123] The phase diagram of a binary mixture of boric acid and succinic acid was constructed to identify the eutectic composition of the mixture. This was done by grinding together various binary mixtures of boric acid and succinic acid in mortar and pestle to produce a homogenous mixture. The mixtures were then tested by differential scanning calorimetry (DSC) (Perkin Elmer DSC 8000, 2-4 mg sample, aluminium DSC pan, multiple cycles of heating and cooling between 100-200 °C at the rate of 10 °C min-1). The following molar ratios of boric acid to succinic acid were investigated: 10:90, 20:80, 30:70, 35:65, 40:60, 45:55 50:50, 60:40, 70:30, 80:20, 90:10.

[0124] The DSC traces (second heating / cooling cycle) in Figures 3 to 5 show that all compositions have multiple endothermic peaks in the heating cycle except the mixture containing 60 mol% boric acid and 40 mol% succinic acid (44 wt.% boric acid and 56 wt.% succinic acid), shown in Figure 4. This is also evident from Figure 6 which shows the main endothermic transitions for each mixture. The 60 mol% boric acid and 40 mol% succinic acid mixture is identified as a eutectic or near-eutectic mixture, as indicated by the single endothermic peak in the DSC trace. Hereafter, the 60 mol% boric acid and 40 mol% succinic acid binary mixture is referred to as the "eutectic mixture”). The eutectic mixture melts at 150 ± 1 °C with a very high enthalpy (AH) of 395 ± 5% J g-1. It crystallizes at 138 ± 1 °C, releasing 356 ± 5% J g-1energy in the cooling cycle. This represents an advantageously low degree of supercooling; supercooling is a frequent issue with many phase change materials as it undesirably increases the range of temperatures over which the energy is released. It is expected that the amount of supercooling may be reduced with larger samples, lower rates of cooling, or the use of a nucleation particulate.

[0125] The mixture containing 65 mol% boric acid and 35 mol% succinic acid is also close to a eutectic composition. This composition melts with AH of 375 J g-1in the heating cycle and crystallizes with AHcof 340 J g-1energy in the cooling cycle.Example 2. Heat absorption and release mechanism

[0126] As shown in Example 1 , the eutectic mixture of boric acid and succinic acid had an unexpectedly high value of AH which was difficult to rationalise based on sensible and latent heat storage mechanisms alone. It was hypothesized that boric acid undergoes partial dehydration to produce metaboric acid during the melting phase transition. Dehydration is an endothermic reaction which would contribute towards the overall energy uptake by an added thermochemical heat storage mechanism. It was further proposed that, during the solidification phase transition, the metaboric acid is rehydrated to form boric acid as orthoboric acid. The hydration reaction is an exothermic reaction, so that thermal energy is released from the molten eutectic mixture upon solidification as reaction enthalpy, in addition to the thermal energy released as sensible and latent heat.

[0127] Raman spectroscopy (Renishaw inVia Visible Raman Microscope with a 488 nm laser, operating in the range of 20-4000 cm’1) was used to investigate the chemical speciation during thermal cycling. Boric acid (in the form of orthoboric acid, H3BO3) has three -OH groups attached to the central boron atom in a trigonal planar structure, while metaboric acid has three HBO2 units bonded together to form a six-membered ring structure. This ring structure of metaboric acid gives rise to two characteristic Raman peaks at 598 cm’1and 819 cm’1, corresponding to ring breathing vibrations (Bertoluzza et al, Journal of Molecular Structure 1980, 64, 123-136; Servoss et al, The Journal of Chemical Physics 1957, 26 (5), 1 175-1 178). These are characteristic bands of metaboric acid and are distinctive in differentiating between pure orthoboric acid and metaboric acid. However, a weak band at 584 cm’1corresponding to the carbonyl of succinic acid may overlap with the metaboric acid peak at 598 cm’1. In this context, the metaboric acid peak at 819 cm’1is exclusively linked to metaboric and is used here as an indicator for metaboric acid formation.

[0128] Raman spectra of pure boric acid and pure succinic acid were obtained at room temperature on a glass slide. Raman spectra of the eutectic mixture at different temperatures were obtained on a thin layer of the mixture between two glass slides.

[0129] The Raman spectroscopy investigations showed the formation of metaboric acid upon heating, proving the dehydration process is operational. The Raman spectraof the eutectic mixture (i) as a solid at room temperature, (ii) in the molten liquid state at 150°C, and (iii) as a solid at room temperature after 1000 cycles of heating (with melting) / cooling (with solidification) are shown in Figure 7. The Raman spectrum of the liquid state clearly shows the formation of metaboric acid as indicated by the appearance of a new peak at 812 cm-1. In the Raman spectrum of the solid sample after 1000 cycles, no peaks corresponding to metaboric acid were observed, and all peaks were the same as the original solid sample before heating. This indicates the formation of metaboric acid on heating and the reformation of orthoboric acid on cooling occurs reversibly and in high yield over many cycles.Example 3. Water partitioning after dehydration

[0130] When the eutectic mixture is present in a closed system, water formed during dehydration is mostly retained in solution in the eutectic liquid. This was confirmed via a measurement of pressure in a sealed vial after melting the eutectic mixture at 150°C. The measurement was conducted in an Anton Paar Monowave 50, sample size: 1 g, vial volume: 10ml, temperature: 150 C, Hold time: 3 hours). As seen in Figure 8, the pressure observed was < 2 bar, whereas the quantity of water involved would be expected to produce a pressure of 25 bar if totally evaporated into the vapour space in the vial.Example 4. Long term cycling

[0131] To test long-term chemical and thermal reliability, the eutectic mixture was subjected to 1000 heating and cooling cycles. This cycling was performed in a gold- plated high-pressure DSC pan to ensure that water that is dehydrated from the eutectic mixture is retained in the pan and available for the rehydration process. The eutectic mixture was subjected to repeated heating and cooling between 110 °C and 165 °C at the rate of 10 °C min-1. After cycling the mixture was analysed using powder X-Ray diffraction (PXRD) and Raman spectroscopy to investigate any chemical change during cycling.

[0132] As seen in Figure 9, no changes of the position and the shape of the melting peak were observed by DSC after 1000 heating-cooling cycles. AH dropped from 415 to 395 J g-1(5.9% decrease), which mostly occurred over the first 300 cycles, with little or no change in AH evident for the next 700 cycles. AH was initially 395 J g-1in thefirst cycle, dropping to 378 J g-1after 200 cycles and remaining substantially constant thereafter (c.a. 4% decrease). It is possible that the mixture homogenised, adjusted in composition, and reached a stable equilibrium between dehydration and rehydration reactions of boric acid during initial cycles, and once the equilibrium was achieved the enthalpies and melting points remained nearly unchanged.

[0133] Figure 10 shows the Raman spectra of (i) the eutectic mixture before cycling, (i) the eutectic mixture after cycling, (iii) boric acid and (iv) succinic acid. Figure 1 1 shows the PXRD diffractogram of (i) the eutectic mixture before cycling, (i) the eutectic mixture after cycling, (iii) boric acid and (iv) succinic acid. No evidence of decomposition or irreversible chemical or phase changes was seen.

[0134] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.

Claims

Claims1 . A phase change material for thermal energy storage, the phase change material comprising boric acid and succinic acid.

2. The phase change material according to claim 1 , comprising the boric acid and the succinic acid in a ratio (mol / mol) of between 50:50 and 75:25.

3. The phase change material according to claim 1 , comprising the boric acid and the succinic acid in a ratio (mol / mol) of between 57:43 and 63:37.

4. The phase change material according to any one of claims 1 to 3, comprising boric acid and succinic acid in a combined amount of at least 90 wt.% based on the total amount of phase-change components in the phase change material.

5. The phase change material according to any one of claims 1 to 4, which melts to form a molten phase comprising boric acid (H3BO3), metaboric acid (HBO2), succinic acid and water.

6. The phase change material according to any one of claims 1 to 5, wherein a solidified composition comprising the boric acid and succinic acid melts at 150°C ± 5°C when heated.

7. The phase change material according to any one of claims 1 to 6, wherein a molten phase comprising the boric acid and succinic acid solidifies at a temperature of greater than 135°C when cooled.

8. The phase change material according to any one of claims 1 to 7, which melts with an enthalpy (AH) of greater than 350 J g-1based on the combined mass of boric acid and succinic acid.

9. The phase change material according to any one of claims 1 to 8, which solidifies, when cooled from a molten phase, with an enthalpy (A / -fc) of greater than 310 J g-1based on the combined mass of boric acid and succinic acid.

10. The phase change material according to any one of claims 1 to 9, further comprising a non-melting solid particulate.1 1 . A molten phase of a phase change material, comprising boric acid (H3BO3), metaboric acid (HBO2), succinic acid and water.

12. A system for thermal energy storage, the system comprising a phase change material according to any one of claims 1 to 10 and at least one heat exchange element configured to transfer heat between the phase change material and a fluid.

13. The system according to claim 12, wherein the phase change material is enclosed in a sealed vessel.

14. The system according to claim 12 or claim 13, comprising an electricity generation device for generating electricity from the fluid.

15. The system according to any one of claims 12 to 14, which is a Carnot battery.

16. A method of storing thermal energy, the method comprising: providing a phase change material comprising boric acid and succinic acid; transferring heat to the phase change material, thereby melting at least a portion of the phase change material to form a molten phase; and transferring heat from the phase change material to a fluid, thereby solidifying at least a portion of the molten phase and producing a heated fluid.

17. The method according to claim 16, wherein the phase change material comprises the boric acid and the succinic acid in a ratio (mol / mol) of between 50:50 and 75:25.

18. The method according to claim 16 or claim 17, wherein at least a portion of the boric acid dehydrates to form metaboric acid and water as the phase change material melts, and wherein the metaboric acid rehydrates to form orthoboric acid as the molten phase solidifies.

19. The method according to any one of claims 16 to 18, further comprising generating mechanical and / or electrical energy from the heated fluid.

20. The method according to any one of claims 16 to 19, wherein the phase change material melts at 150°C ± 5°C, and wherein the molten phase solidifies at a temperature of greater than 135°C.

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