Phase change material for thermal energy storage

Abstract

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

Description

[0001] This application claims priority to Australian Provisional Patent Application No. 2023903870, filed on November 30, 2023, the contents of which shall be deemed to be incorporated herein by reference. Technical Field

[0002] This 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 the phase change material, a system for thermal energy storage, and a method for storing thermal energy. Background Technology

[0003] As the world transitions from fossil fuel power generation to increasingly abundant renewable solar and wind power, there is a pressing need for technologies that can address the mismatch between renewable energy supply and electricity demand. Energy storage systems capable of storing mechanical energy (such as pumped-storage hydropower) or chemical energy (such as electrochemical batteries) can be deployed to temporarily store excess renewable electricity when available and release it when needed.

[0004] Thermal energy storage is another promising technology for temporarily storing energy from renewable energy generation or intermittent heat sources or waste heat sources (such as solar energy or industrial waste heat). When needed, the heat can be released in a useful form, such as through power generation, or used for thermal applications, such as providing hot water.

[0005] Of particular interest is the recently developed concept of the Carnot battery, in which thermal energy storage is integrated with a reversible heat pump / organic Rankine cycle system. In the charging mode of such a Carnot battery, renewable electricity drives a heat pump to pump thermal energy from a cold storage unit to a heat storage unit, where the heat is stored in a thermal energy storage material. As is typical for heat pumps, the generated heat can be two to three times the electrical input driving the pump. On discharge, the thermal energy storage material transfers heat to a working fluid, which drives the heat pump in reverse mode, thus operating as an organic Rankine cycle (ORC) engine to generate electricity. Optionally, waste heat in the working fluid can be used to provide lower-grade heat for various thermal applications, such as hot water heating. To date, such Carnot batteries have demonstrated an energy-to-energy round-trip efficiency exceeding 70%, not including any use of residual low-grade heat. It is predicted that once this new technology is optimized to include additional waste heat applications, a round-trip energy ratio approaching 100% will be achievable.

[0006] The selection of thermal energy storage materials is crucial for the design and performance of thermal energy storage technologies, including Carnot batteries. Thermal energy storage materials are typically classified according to their heat storage mechanisms, which involve sensible heat storage, latent heat storage, or thermochemical energy storage.

[0007] Solids (such as sand), molten salts, and liquid metals can be used as sensible heat storage materials. These materials can advantageously store energy at very high temperatures, but due to their limited heat capacity, they typically only provide low energy density storage.

[0008] Higher energy densities can be achieved by using thermal energy storage materials that store latent heat, typically through a solid-to-liquid phase change (i.e., melting). The stored latent heat is then released by reversing the phase change (i.e., solidification / crystallization). Such materials are called phase change materials. In practice, phase change materials often provide both sensible and latent heat storage because they typically cycle between lower temperatures below their melting point and higher temperatures above their melting point.

[0009] Phase change materials suitable for a given thermal energy storage application must fully meet a set of performance criteria, which typically include one or more of the following: heat of fusion (ΔH) f High toxicity, good matching between phase change temperature and desired exothermic temperature, high chemical stability during thermal cycling, acceptable phase change kinetics (e.g., low supercooling), no phase separation, low toxicity, and low flammability. Many reported phase change materials, such as organic compounds (paraffin, fatty acids, etc.) or inorganic salt hydrates, are inadequate in one or more of these requirements.

[0010] The melting temperature is in the intermediate temperature range of 120°C to 220°C (e.g., 120°C to 170°C for ORC engines) and ΔH f Greater than 300 J g -1 High-performance phase change materials are particularly difficult to identify.

[0011] In principle, thermal energy storage materials that absorb and release thermochemical heat (enthalpy of reaction) through reversible chemical reactions can provide much higher energy densities. A key challenge in this approach is identifying materials capable of undergoing fully reversible reactions multiple times within the desired operating temperature range. Boric acid (H3BO3) has been studied as a potential thermochemical phase change material because it undergoes endothermic dehydration above 140°C to metaboric acid (HBO2), and further dehydration at even higher temperatures to produce anhydrous boron oxide (B2O3). These processes are known to require a large amount of thermal energy (1554 J g). -1 However, for pure boric acid, the dehydration reaction is not reversible enough, making it unsuitable for use in practical thermochemical energy storage systems.

[0012] Therefore, there remains a need for phase change materials for thermal energy storage that at least partially address one or more of the aforementioned drawbacks or provide useful alternatives.

[0013] References to patent documents or other matters given in this document as prior art should not be construed as an admission that the documents or matters are known or that the information contained therein is part of common general knowledge from the priority date of any claim. Summary of the Invention

[0014] This invention is based on the discovery that mixtures of boric acid and succinic acid (preferably in or near a eutectic composition) provide phase change materials capable of storing thermal energy at unexpectedly high energy densities. Therefore, it has been found that a mixture of 60 mol% boric acid and 40 mol% succinic acid (44 wt.% boric acid and 56 wt.% succinic acid) at a melting point of 150°C yields approximately 400 J / g. -1 ΔH f Melting. The molten phase was then re-solidified at 138°C, releasing approximately 360 J g. -1 The heat energy.

[0015] The observed abnormally high ΔH f This can be attributed to a reversible thermochemical process, which experiments show occurs at approximately the same temperature as the phase transition of the mixture. When the phase change material melts, a portion of the boric acid (orthoboric acid; H3BO3) undergoes endothermic dehydration to form metaboric acid (HBO2). Therefore, in addition to the latent heat corresponding to the phase transition, the phase change material also absorbs and stores heat corresponding to the enthalpy of the dehydration reaction; both heat storage modes contribute to the observed ΔH. f The released water is largely retained in the molten phase. When the molten phase subsequently solidifies, metaboric acid undergoes exothermic rehydration to form orthoboric acid, releasing heat corresponding to both the enthalpy of hydration and the latent heat of crystallization. Surprisingly, the latent heat storage process and the thermochemical energy storage process occur simultaneously or almost simultaneously during the heating and cooling cycles, so most of the heat is absorbed and released within a narrow temperature range during the phase transition.

[0016] Considering the thermochemical contribution of boric acid to the total heat storage capacity, 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.% boric acid. In contrast, binary eutectic crystals of boric acid 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 lower temperatures below the melting point and higher temperatures above the melting point, boric acid-succinic acid phase change materials store and release energy through all three thermal storage modes: sensible heat, latent heat, and thermochemical heat storage mechanisms.

[0018] The study found that a single-melting-point mixture of boric acid and succinic acid provides highly stable performance during multiple thermal cycles, with an observed ΔH value of [missing value] over 1000 cycles. f The loss was less than 6%, and no irreversible chemical decomposition was observed. This is considered surprising because: (i) succinic acid itself is unsuitable as a phase change material due to its poor chemical stability above its melting point; (ii) the dehydration of boric acid has previously been found to be insufficiently reversible for thermochemical energy storage materials; and (iii) the loss of aqueous products (e.g., by distributing the aqueous products into the vapor phase above the molten phase change material) is expected to limit the reverse reaction. The inventors have discovered that, despite the high temperature, the water released during dehydration remains strongly retained in the molten phase and is therefore readily reacted with the metaboric acid in the molten phase upon re-solidification.

[0019] According to a first aspect, the present 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 boric acid and succinic acid in a ratio (mol / mol) between 50:50 and 75:25, such as a ratio (mol / mol) between 55:45 and 70:30.

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

[0022] In some embodiments, based on the total amount of phase change components in the phase change material, the phase change material comprises boric acid and succinic acid in an amount of at least 90 wt.% or at least 95 wt.% (e.g., at least 99 wt.%, such as 100 wt.%).

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

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

[0025] In some embodiments, the curing composition comprising boric acid and succinic acid melts at 150°C ± 5°C upon heating.

[0026] In some embodiments, the molten phase comprising boric acid and succinic acid is cured upon cooling at a temperature greater than 135°C, such as 138°C or higher.

[0027] In some embodiments, based on the combined mass of boric acid and succinic acid, the phase change material is produced at a concentration greater than 350 J g. -1 or greater than 360 J g -1 or greater than 370 J g -1 If greater than 380 J g -1 For example, greater than 390 J g -1 enthalpy (ΔH) f ) melt.

[0028] In some embodiments, when cooled from the molten phase, the phase change material yields a concentration greater than 310 J g based on the combined mass of boric acid and succinic acid. -1 or greater than 320 J g -1 or greater than 330 J g -1 If greater than 340 J g -1 For example, greater than 350 J g -1 enthalpy (ΔH) c ( ) Curing.

[0029] In some embodiments, the phase change material further comprises non-molten solid particles, optionally thermally conductive solid particles, to improve the thermal conductivity of the phase change material.

[0030] According to a second aspect, the present 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 (mol / mol) of boric acid and metaboric acid to succinic acid in the molten phase is between 50:50 and 75:25, such as between 55:45 and 70:30.

[0032] In some embodiments, the ratio (mol / mol) of boric acid and metaboric acid to succinic acid in the molten phase 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 is cured upon cooling at a temperature greater than 135°C, such as 138°C or higher.

[0034] In some embodiments, based on the combined mass of boric acid, metaboric acid, succinic acid, and water, the molten phase is in a concentration greater than 310 J g. -1 or greater than 320 J g -1 or greater than 330 J g -1 If greater than 340 J g -1 For example, greater than 350 J g -1 enthalpy (ΔH) c It solidifies upon cooling.

[0035] According to a third aspect, the present invention provides a system for thermal energy storage, the system comprising a phase change material according to any 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 container.

[0037] In some embodiments, the system includes a power generation device for generating electricity from fluid.

[0038] In some embodiments, the system includes an organic working fluid, wherein the energy generating device is powered by a hot organic gas in an organic Rankine cycle, the hot organic gas being generated by vaporizing the organic working fluid as heat is transferred from a phase change material to the organic working fluid via at least one heat exchange element.

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

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

[0041] According to a fourth aspect, the present invention provides a method for 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 generating a heated fluid.

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

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

[0044] In some embodiments, based on the total amount of phase change components in the phase change material, the phase change material comprises boric acid and succinic acid in an amount of at least 90 wt.% or at least 95 wt.% (e.g., at least 99 wt.%, such as 100 wt.%).

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

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

[0047] In some embodiments, the molten phase is solidified 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 container.

[0049] In some embodiments, transferring heat to the phase change material includes 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 includes 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 energy and / or electrical energy from a heated fluid.

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

[0052] In some embodiments, transferring heat to the phase change material includes heating an organic working fluid to generate a hot organic liquid, and transferring heat from the hot organic liquid to the phase change material. In some embodiments, the organic working fluid is heated using a heat pump.

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

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

[0055] In some embodiments, based on the combined mass of boric acid and succinic acid, the phase change material is produced at a concentration greater than 350 J g. -1 or greater than 360 J g -1 or greater than 370 J g -1 If greater than 380 J g -1 For example, preferably greater than 390 J g -1 enthalpy (ΔH) f ) melt.

[0056] In some embodiments, based on the combined mass of boric acid and succinic acid, the molten phase is at a concentration greater than 310 J g. -1 or greater than 320 J g -1 or greater than 330 J g -1 If greater than 340 J g -1 For example, greater than 350 J g -1 enthalpy (ΔH) c ( ) Curing.

[0057] When the terms “comprise”, “comprises” and “comprising” are used in this specification (including the claims), they should be interpreted as specifying the stated feature, integer, step or component, but do not exclude the presence of one or more other features, integers, steps or components or groups thereof.

[0058] Further aspects of the invention are described in the following detailed description of the invention. Attached Figure Description

[0059] This document will illustrate embodiments of the invention by way of example only, with reference to the accompanying drawings, in which:

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

[0061] Figure 2 A Carnot cell comprising a phase change material according to an embodiment of the invention is schematically depicted.

[0062] Figure 3 DSC scans of a binary mixture of boric acid and succinic acid, as obtained in Example 1, are shown, wherein boric acid is in excess relative to the binary eutectic composition.

[0063] Figure 4 DSC scans of a mixture of 60 mol% boric acid and 40 mol% succinic acid, as obtained in Example 1, are shown, the mixture being in or near a eutectic composition.

[0064] Figure 5 DSC scans of a binary mixture of boric acid and succinic acid, as obtained in Example 1, are shown, wherein succinic acid is in excess relative to the binary eutectic composition.

[0065] Figure 6 It is shown in Figure 3-5 A graph of the main endothermic transitions of each mixture analyzed in DSC scans.

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

[0067] Figure 8 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, is shown.

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

[0069] Figure 10 Raman spectra of a mixture of 60 mol% boric acid and 40 mol% succinic acid, as measured in Example 4, are shown before thermal cycling and after 1000 heat-cooling cycles, and compared with the spectra of pure boric acid and pure succinic acid.

[0070] Figure 11 Powder X-ray diffraction patterns of a mixture of 60 mol% boric acid and 40 mol% succinic acid, as measured in Example 4, are shown before thermal cycling and after 1000 heat-cooling cycles, and compared with the spectra of pure boric acid and pure succinic acid. Detailed Implementation

[0071] Phase change materials for thermal energy storage

[0072] This invention relates to a phase change material comprising boric acid and succinic acid. As used herein, a phase change material is a material that absorbs heat energy as latent heat during a solid-to-liquid phase change and releases latent heat during a reverse liquid-to-solid phase change. Preferably, the phase change of such materials is repeatable, allowing the phase change material to undergo multiple melting (endothermic) and solidification (exothermic) cycles with little or no loss of endothermic capacity. Therefore, phase change materials are suitable for a variety of thermal energy storage applications.

[0073] Boric acid, also known as orthoboric acid, has the molecular formula H3BO3. Succinic acid is the common name for 1,4-succinic acid.

[0074] Boric acid and succinic acid can, 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 cured composition containing boric acid and succinic acid as a close mixture. As used herein, a cured composition of a phase change material refers to a solid composition or phase formed when the 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 cured 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, for example, as a mixture of discrete solid particles of boric acid and succinic acid. When heated in a first heating cycle, these two discrete components can melt and combine to form a molten phase comprising each component in desired proportions.

[0075] Phase change materials contain at least two phase change components, including boric acid and succinic acid. As used herein, a phase change component is one of those components that can transform between a solid form (e.g., a cured composition as defined herein) and a molten phase in the phase change material, and therefore excludes non-meltable components such as non-meltable solid particles. Preferably, the phase change material contains boric acid and succinic acid as the primary or only phase change components. In some embodiments, boric acid and succinic acid are present in a combined amount of at least 90 wt.%, such as at least 95 wt.%, such as at least 99 wt.%, based on the total amount of phase change components in the phase change material. In some embodiments, boric acid and succinic acid are the only phase change components. Neglecting the contribution of any trace impurities, the cured form of such a phase change material can be considered as a binary mixture of boric acid and succinic acid.

[0076] Boric acid and succinic acid can be present in a ratio suitable for providing an effective phase change material. Ideally, boric acid and succinic acid can be present in a specific proportion, thereby curing the composition into a eutectic composition, i.e., a single homogeneous mixture with a single, well-defined melting point. The eutectic composition of a binary system can be identified by studying 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, which can be clearly seen from the single endothermic peak of the composition at 150 °C in differential scanning calorimetry (DSC) scans.

[0077] In practice, the ratio of boric acid to succinic acid in phase change materials may deviate from the ideal eutectic ratio, yet still provide excellent thermal storage capacity. For example, a binary mixture of 65 mol% boric acid and 35 mol% succinic acid also exhibits near-eutectic characteristics in DSC scans. Therefore, in some embodiments, boric acid and succinic acid are present in a ratio (mol / mol) 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.

[0078] When the cured composition is heated, the melting point of the phase change material can be in the range of 150°C ± 5°C. Such a melting point is particularly useful for certain heat storage applications, such as Carnot batteries operating with organic Rankine cycle engines. When the cured composition is heated to the point where it is completely melted, the melting point can be identified as the unique or dominant endothermic peak in a DSC scan. In some embodiments, when the cured composition is heated, more than 90% of the latent heat of absorption is absorbed in the range of 150°C ± 5°C.

[0079] Upon cooling, the molten phase of the phase change material can solidify at temperatures above 135°C, preferably at 138°C or higher. When the molten phase is cooled to allow the composition to fully solidify, the solidification temperature (also known as the crystallization temperature) can be identified as the unique or dominant exothermic peak in a DSC scan. In some embodiments, when the molten phase is cooled, more than 90% of the latent heat of release is released at temperatures above 135°C, preferably at 138°C or higher.

[0080] As those skilled in the art will understand, curing temperature is typically dependent on the kinetics of the crystallization process and can therefore be influenced by variables such as sample size, cooling rate, thermal conductivity of the phase change material, and the presence of any nucleating agents. In practice, curing temperature is below the melting point, and the difference represents the degree of supercooling in the system. Supercooling is preferably minimized to maximize the energy release efficiency from the phase change material. Advantageously, at least some embodiments of the phase change materials disclosed herein are cured at a single, well-defined crystallization temperature and at a supercooling not exceeding about 12°C. Further reductions in supercooling are expected in commercial-scale implementations due to the use of larger samples and the use of solid microparticles to improve thermal conductivity and / or nucleation.

[0081] Phase change materials can have high heat storage capacity, as determined by: (i) the endothermic enthalpy (ΔH) measured when the phase change material is heated to melt the solidified composition. f (i) and / or (ii) the exothermic enthalpy (ΔH) measured when the phase change material is cooled to solidify the phase that has completely melted it. c ). ΔH fThe area of ​​the endothermic peak during the heating phase (preferably in the second or later heating and cooling cycle) based on DSC scanning can be used to ensure that the solid form of the phase change material is a cured composition. ΔH c It can also be determined by the area of ​​the exothermic peak in the cooling phase of DSC-based scanning. ΔH f and ΔH c All of these can be quantified using methods known to those skilled in the art, such as ASTM E793-06 (2018). As will be explained in more detail below, the ΔH of the phase change materials disclosed herein... f and ΔH c The measured values ​​can include contributions from latent heat storage mechanisms and thermochemical heat storage mechanisms.

[0082] In some embodiments, based on the combined mass of boric acid and succinic acid, the phase change material is produced at a concentration greater than 350 J g. -1 or greater than 360 J g -1 or greater than 370 J g -1 If greater than 380 J g -1 For example, greater than 390 J g -1 enthalpy (ΔH) f Melting. Experiments showed that a eutectic or near-eutectic composition of 60 mol% boric acid and 40 mol% succinic acid melts at approximately 400 J / g. -1 Enthalpy melting.

[0083] In some embodiments, when cooled from the molten phase, the phase change material yields a concentration greater than 310 J g based on the combined mass of boric acid and succinic acid. -1 or greater than 320 J g -1 or greater than 330 J g -1 If greater than 340 J g -1 For example, greater than 350 J g -1 enthalpy (ΔH) c ( ) Curing. Experiments showed that a eutectic or near-eutectic composition of 60 mol% boric acid and 40 mol% succinic acid cured at approximately 360 J / g. -1 Enthalpy crystallization.

[0084] The superior heat storage capacity of the phase change material disclosed in this paper can be attributed to a reversible thermochemical process that occurs at approximately the same temperature as the phase change of the mixture. When the phase change material melts, a portion of the boric acid (orthoboric acid; H3BO3) undergoes endothermic dehydration to form metaboric acid (HBO2). Therefore, in addition to the latent heat corresponding to the phase change, the phase change material also absorbs and stores heat corresponding to the enthalpy of the dehydration reaction; both heat storage modes contribute to the observed ΔH fThe water released during the dehydration reaction is strongly retained in the molten phase. Therefore, the molten phase 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 cured 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).

[0085] When the molten phase subsequently solidifies, metaboric acid undergoes exothermic rehydration through a reaction with the retained water to form orthoboric acid, simultaneously or almost simultaneously releasing heat corresponding to both the enthalpy of hydration and the latent heat of crystallization. Surprisingly, the latent heat storage process and the thermochemical energy storage process occur simultaneously or almost simultaneously during the heating and cooling cycles, so that most of the heat is absorbed and released within a narrow temperature range during the phase transition.

[0086] Considering the thermochemical contribution of boric acid to the total thermal storage capacity, it is particularly advantageous that the compositions disclosed herein can work effectively with relatively high amounts of boric acid. For example, the eutectic composition of boric acid and succinic acid contains about 44 wt.% boric acid. Therefore, in some embodiments, the phase change material contains at least 40 wt.% boric acid, such as at least 42 wt.% of the phase change component.

[0087] When thermally cycled between lower temperatures below the melting point and higher temperatures above the melting point, boric acid-succinic acid phase change materials store and release energy through all three thermal storage modes: sensible heat, latent heat, and thermochemical heat storage mechanisms. For example, based on the known heat capacities (Cp) of the two materials... 硼酸 = 1.392 J g -1 K -1 And Cp 琥珀酸 = 1.39 J g -1 K -1 If the eutectic or near-eutectic composition (60 mol% boric acid and 40 mol% succinic acid) is cycled between 115 °C and 165 °C, the sensible heat storage capacity can be expected to be approximately 65 J g. -1 (In addition to the observed ΔH) f (In addition to the latent heat and thermochemical heat storage capacity of the combined system).

[0088] The three energy storage modes provided by the phase change materials disclosed herein Figure 1 Visualization was performed, showing a DSC scan of a eutectic or near-eutectic mixture of 60 mol% boric acid and 40 mol% succinic acid (heated and cooled at 10 °C / min between approximately 103 °C and 173 °C). Potential energy storage (LES) and thermochemical energy storage (TCES) are provided in combination during phase transitions, such as through ΔH. f and ΔH cThe value is quantified. Furthermore, due to the operating temperature range between lower and higher temperatures of the phase change material, sensible thermal storage (SES) is provided.

[0089] As disclosed herein, phase change materials are capable of thermal cycling between a low-energy form and a high-energy form, in which the phase change component is primarily or entirely present in the cured composition, and in the high-energy form, in which the phase change component is primarily or entirely present in the molten phase. Advantageously, at least some embodiments of the phase change materials are capable of such thermal cycling in numerous heating-cooling cycles without significant loss of thermal storage capacity due to irreversible processes such as chemical degradation.

[0090] In some embodiments, the phase change material is capable of undergoing at least 100 heating and cooling cycles, preferably at least 1000, between a lower temperature below the curing temperature and a higher temperature approximately equal to the melting temperature, wherein the heat storage capacity loss does not exceed 10%, preferably not more than 7%, as indicated by ΔH. f or ΔH c Measured. For example, it has been found that eutectic or near-eutectic compositions (60 mol% boric acid and 40 mol% succinic acid) can undergo 1000 thermal cycles between 110°C and 165°C, and ΔH f The measurement loss is less than 6%.

[0091] In use, the phase change material will typically be contained in a sealed container. Furthermore, it has been found that water released during the boric acid dehydration reaction is strongly retained in the molten phase, despite the high temperatures. Therefore, the reversibility of the melt and the repeatability during thermal cycling are not expected to be significantly affected by the loss of water from the phase change composition. However, if necessary, water can be added to the phase change material to replace any loss.

[0092] In some embodiments, the phase change material comprises a non-molten component, such as non-molten solid particles. As used herein, a non-molten component is a solid component that does not melt during thermal cycling of the phase change material. The non-molten solid particles may be present in an amount of less than 5 wt.%, such as less than 2 wt.%.

[0093] In some embodiments, the phase change includes thermally conductive solid microparticles to improve the thermal conductivity of the phase change material. Improved thermal conductivity can advantageously facilitate the absorption and release of thermal energy from the phase change material, thereby improving its performance in thermal energy storage applications. Examples of such materials can include various forms of particulate carbon, such as graphite, graphene, and reduced graphene oxide, as well as particulate metals, such as metal sheets.

[0094] In some embodiments, the phase change comprises solid microparticles as nucleating agents. The nucleating agent provides a solid surface on which the molten phase of the phase change composition can nucleate upon cooling, thereby improving the kinetics of the solidification process. Therefore, the use of nucleating agents can advantageously reduce supercooling in phase change materials. Suitable solid microparticle nucleating agents may include finely fragmented (optionally in nanoparticle form) inert inorganic compounds such as metal oxides (e.g., TiO2, SiO2, Al2O3, CaO) or carbon materials such as carbon nanoparticles, nanotubes, nanosheets (e.g., graphene), etc.

[0095] Any non-molten solid particulate component, such as thermally conductive solid particles or nucleating agents, is preferably dispersed throughout the phase change material, even when fully molten, in order to improve the overall thermal conductivity and / or nucleation of the material.

[0096] Phase change materials may also include small amounts of other additives (such as antioxidants) to enhance chemical stability.

[0097] Systems for thermal energy storage

[0098] The present invention further relates to a system for thermal energy storage. The system includes a phase change material and means for transferring heat to and / or from the phase change material. Specifically, the system may include 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 gas).

[0099] A heat exchange element or heat exchanger may contain a solid thermally conductive partition that physically separates the phase change material and the fluid, but allows heat transfer between these media. For example, a heat exchange element may contain one or more heat-conducting pipes, such as metal pipes, that pass through a volume of phase change material to facilitate heat exchange of the phase change material as a whole.

[0100] Phase change material (PCM) can be enclosed in a sealed container. Therefore, the sealed container can retain the heat generated during the thermal cycling of the PCM and any gaseous components, such as water vapor. The container can be insulated to prevent unwanted heat loss, thereby maximizing the efficiency of heat storage and recovery. Heat exchange elements can be integrated with the sealed container, for example as one or more pipes passing through the wall of the sealed container, allowing fluid to be introduced into the container for heat exchange while remaining physically separated from the PCM. Alternatively or additionally, the wall of the sealed container can form part of one or more heat exchange elements, allowing heat to be transferred across the wall between the PCM enclosed by the container wall and the fluid outside the container wall. Since the PCM transitions between a solid and a molten form, it may be preferable that it remains stationary within the sealed container during thermal cycling. The fluid can flow through the heat exchange elements for a sufficient contact time to allow the necessary heat exchange.

[0101] The system as a whole, and particularly the heat exchange elements, can be configured to transfer heat from the molten phase of the phase change material to the fluid, thereby heating the fluid. If the fluid is a liquid working fluid, the working fluid can therefore vaporize to produce a hot gas. Alternatively, heat transfer can simply heat a liquid, such as water, to produce a hot liquid, such as hot water.

[0102] Alternatively or additionally, the system as a whole, and particularly the heat exchange element, can be configured to transfer heat from a fluid to the phase change material, thereby melting at least a portion of the cured composition present in the phase change material. Thus, a hot liquid or gas at a temperature above the melting point of the phase change material can contact the heat exchange element to transfer heat to the phase change material. In some embodiments, a single working fluid is used to transfer heat to the phase change material via the heat exchange element when storing thermal energy, and to receive heat from the phase change material via the heat exchange element when recovering thermal energy.

[0103] In some embodiments, the system includes a power generation device for generating electricity from the working fluid. The working fluid can be vaporized at a heat exchange element by heat transferred from the phase change material, and the resulting hot gas is used to generate electricity, for example, by passing it through an expansion device such as a turbine.

[0104] Given the phase change temperature of a phase change material, organic working fluids with boiling points below that of water are particularly suitable for converting the thermal energy released by the phase change material into electricity. Therefore, the system can contain organic working fluids such as pentane or isobutane. Thus, the energy generation device is powered in an organic Rankine cycle by a hot organic gas, which generates electricity by vaporizing the organic working fluid as heat is transferred from the phase change material to the organic working fluid via a heat exchange element.

[0105] The system can be configured to heat the phase change material, thereby storing the thermal energy as disclosed herein using any suitable thermal energy source. For example, the system may include one or more electrically heated elements to heat the phase change material, for example, by resistance heating. However, in some preferred embodiments, the system is configured to transfer heat from a thermal fluid to the phase change material via a heat exchange element, as disclosed herein. For example, the thermal fluid may be a hot liquid or gas containing waste heat provided by an industrial process. Alternatively, the thermal fluid may be heated by a solar heater.

[0106] In some embodiments, the system includes a thermal energy source configured to provide a thermal fluid for transferring heat to the phase change material via a heat exchange element. For example, the thermal energy source could be a heat pump or a solar heater.

[0107] In some embodiments, the system is configured as a Carnot battery, instances of which are... Figure 2The diagram is schematically depicted. The Carnot battery 100 receives electricity for storage, such as excess electricity intermittently generated by the renewable power source 102, and discharges it when needed, for example, during periods of high demand, to the local power grid 104 or industrial end-user 106. The Carnot battery 100 includes a sealed container 108 housing 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, and an electromechanical device 110 configured to operate as a heat pump during the heat storage phase and as a generator during the heat release phase.

[0108] In charging mode, electricity from renewable power source 102 drives heat pump 110. The heat pump compresses warm organic gas 112 (temperature, for example, 25-50°C), thereby condensing and heating it to form hot organic working liquid 114 (temperature, for example, 155-165°C). Hot organic working liquid 114 flows to sealed container 108, where it passes through an internal heat exchanger and transfers heat to phase change material 109. According to the principles disclosed herein, the heat transfer causes at least a portion of the phase change material to melt, thereby forming a molten phase and storing thermal energy within the phase change material. Cooled organic working liquid 116 (temperature close to the melting point temperature of the phase change material, for example, 150-155°C) leaving the heat exchanger then expands to form cold organic gas 118 (temperature, for example, 0-20°C). The cold organic gas 118 then flows to a cryogenic storage tank 120, where it passes through a second heat exchanger (not shown) in thermal contact with a heat source 121 and is thus heated to produce a warm organic gas 112 (at a temperature, for example, 25-50°C), which is then supplied to the heat pump 110 again. The heat source 121 can be a storage container for a liquid such as polyethylene glycol, or, in a ground source design, simply the ground (i.e., soil). The loading cycle can continue until the phase change material in the sealed container 108 is completely melted and heated to its higher operating temperature.

[0109] In discharge mode, and using the same working fluid, organic working fluid 116a (temperature, for example, 25-50°C) flows to a sealed container 108, where it passes through an internal heat exchanger, thereby absorbing heat from the phase change material 109. According to the principles disclosed herein, the heat transfer solidifies at least a portion of the phase change material, thereby forming a solidified composition and releasing thermal energy from the phase change material. The heat transfer heats and vaporizes the organic working fluid 116a in the internal heat exchanger, thereby forming a hot organic gas 114a (temperature close to the freezing point of the phase change material, for example, 145-150°C) under increased pressure. The hot organic gas 114a drives a generator 110, thereby generating electricity, and then the expanding outflow gas 112a (temperature, for example, 50-70°C) flows to a cryogenic storage tank 120, where it passes through a second heat exchanger to transfer residual heat to the heat exchange material 121 within the storage tank. Gas 112a is thus cooled and condensed to form a warm organic working liquid 118a (temperature, for example, 25-50°C), which is then recirculated through generator 110 to form a warm organic working liquid 116a. The discharge cycle can continue until the phase change material in the sealed container is completely solidified and cooled to its lower operating temperature.

[0110] Methods of storing thermal energy

[0111] The present invention further relates to a method for storing thermal energy. The method includes 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 release step, heat is transferred from the phase change material to a fluid, thereby solidifying at least a portion of the molten phase and generating a heated fluid.

[0112] Phase change materials can generally be implemented according to any of the embodiments disclosed in the section describing phase change materials for thermal energy storage. In some embodiments, the phase change material is melted at a temperature of 150°C ± 5°C. In some embodiments, the molten phase is solidified at a temperature greater than 135°C, preferably at 138°C or higher.

[0113] During the heat storage step, at least a portion of the boric acid can be dehydrated to form metaboric acid and water. The dehydration reaction can occur when the phase change material melts, causing the phase change material to absorb heat simultaneously or nearly simultaneously as enthalpy and latent heat of the thermochemical reaction. Therefore, the molten phase comprises (and can consist substantially of) boric acid (H3BO3), metaboric acid (HBO2), succinic acid, and water. In embodiments where the only phase change components of the cured composition are boric acid and succinic acid, the molten phase can consist of boric acid, succinic acid, and their reaction products (i.e., metaboric acid, water, and any trace degradation products).

[0114] 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 a higher temperature of greater than 155°C, such as greater than 160°C. In this way, in addition to thermochemical and latent heat storage mechanisms, the phase change material can also store a significant amount of thermal energy as sensible heat. However, it will be understood that, in principle, heat can be transferred to and from the phase change material without significantly altering its temperature to deviate from its melting temperature. This may occur if the phase change material is not fully melted before switching from the heat storage step to the heat release step, or is not fully solidified before switching from the heat release step to the subsequent heat storage step.

[0115] In some embodiments, based on the combined mass of boric acid and succinic acid, the phase change material is produced at a concentration greater than 350 J g. -1 or greater than 360 J g -1 or greater than 370 J g -1 If greater than 380 J g -1 For example, greater than 390 J g -1 enthalpy (ΔH) f Melting. When heated from an initial fully solidified state to a subsequent fully molten state during the heat storage step, the phase change material can absorb such a large amount of heat.

[0116] During the exothermic step, metaboric acid can be rehydrated to form orthoboric acid, preferably in a quantitative or near-quantitative yield. The dehydration reaction can occur during solidification of the molten phase, causing the phase change material to release heat simultaneously or almost simultaneously as thermochemical 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.

[0117] In some embodiments, the phase change material is cooled from a higher temperature greater than 155°C, such as greater than 160°C, to a lower temperature less than 140°C or less than 130°C, such as less than 120°C. In this way, the phase change material releases heat energy as sensible heat in addition to thermochemical and latent heat release mechanisms.

[0118] In some embodiments, based on the combined mass of boric acid and succinic acid, the molten phase is at a concentration greater than 310 J g. -1 or greater than 320 J g -1 or greater than 330 J g -1 If greater than 340 J g -1 For example, greater than 350 J g -1 enthalpy (ΔH) c Solidification. Phase change materials can release such a large amount of heat when cooled from an initial fully molten state to a subsequently fully solidified state during the exothermic step.

[0119] Methods for storing thermal energy can be performed in systems for thermal energy storage according to any embodiment disclosed herein. Therefore, the phase change material is preferably enclosed in a sealed container during use. Heat can be transferred from the phase change material to the fluid via at least one heat exchange element. Optionally, heat can also be transferred to the phase change material via a heat exchange element.

[0120] The fluid to which heat is transferred can be a liquid or a gas. The heating fluid can be a liquid or a gas. In some embodiments, transferring heat from the phase change material to the fluid involves vaporizing at least a portion of the fluid from its initial liquid state to form a hot gas. In other embodiments, the fluid is water, and the heating fluid is heated water, such as hot water and / or steam.

[0121] The method may further include generating mechanical energy and / or electrical energy from a heated fluid, preferably a hot gas.

[0122] In some embodiments, the fluid is a liquid organic working fluid. Heat transferred from the phase change material during the energy emission step causes at least a portion of the liquid organic working fluid to vaporize to form a hot organic gas. The method may then further include generating electrical energy from the hot organic gas within an organic Rankine cycle.

[0123] In some embodiments, an organic working fluid is heated to generate a hot organic liquid, and then heat is typically transferred from the hot organic liquid to a phase change material via at least one heat exchange element as disclosed herein. In some embodiments, the organic working fluid is heated using a heat pump.

[0124] In some embodiments, the phase change material is circulated through multiple thermal cycles, each cycle comprising a heat storage step and a heat release step. The phase change material may be circulated through at least 100 such cycles, preferably at least 1000 such cycles. In some embodiments, the heat storage capacity loss of the phase change material caused by the cycling does not exceed 10%, preferably not exceeding 7%, as indicated by ΔH. f or ΔH c The measurements were taken.

[0125] Example

[0126] The present invention is described with reference to the following examples. It should be understood that the examples are illustrative and do not limit the invention described herein.

[0127] Example 1. Phase diagram of a binary mixture

[0128] Phase diagrams of binary mixtures of boric acid and succinic acid were constructed to identify the eutectic composition of the mixtures. This was accomplished by grinding various binary mixtures of boric acid and succinic acid together in a mortar and pestle to produce homogeneous mixtures. The mixtures were then tested by differential scanning calorimetry (DSC) (Perkin Elmer DSC 8000, 2–4 mg sample, aluminum DSC disk, at 10 °C min in increments of 10 °C between 100–200 °C). -1 (The rate of heating and cooling was subjected to multiple heating and cooling cycles). The following molar ratios of boric acid to succinic acid were studied: 10:90, 20:80, 30:70, 35:65, 40:60, 45:55, 50:50, 60:40, 70:30, 80:20, and 90:10.

[0129] Figures 3 to 5 The DSC trace (second heating / cooling cycle) in the data indicates that, in addition to Figure 4 Apart from the mixture shown containing 60 mol% boric acid and 40 mol% succinic acid (44 wt.% boric acid and 56 wt.% succinic acid), all compositions exhibited multiple endothermic peaks during heating cycles. Figure 6 This can also be clearly seen in the text. Figure 6 The major endothermic transitions for each mixture are shown. The 60 mol% boric acid and 40 mol% succinic acid mixture was identified as a eutectic or near-eutectic mixture, as indicated by a single endothermic peak in the DSC trace. Hereinafter, the 60 mol% boric acid and 40 mol% succinic acid binary mixture is referred to as the “eutectic mixture”. The eutectic mixture yielded 395 ± 5% J g at 150 ± 1 °C. -1 Very high enthalpy (ΔH) f It melts. It crystallizes at 138±1℃ and releases 356±5% J g during cooling cycles. -1 The energy. This indicates a favorable low degree of supercooling; supercooling is a common problem in many phase change materials because it undesirably increases the temperature range at which energy is released. Supercooling is expected to be reduced by using larger samples, lower cooling rates, or the use of nucleating particles.

[0130] A mixture containing 65 mol% boric acid and 35 mol% succinic acid also approximates a eutectic composition. This composition reacts with 375 J g during heating cycles. -1 ΔH f Melt, and in the cooling cycle at 340 J g -1 ΔH c crystallization.

[0131] Example 2. Heat absorption and release mechanisms

[0132] As shown in Example 1, the eutectic mixture of boric acid and succinic acid exhibits a surprisingly high ΔH. f The value cannot be reasonably explained based solely on sensible and latent heat storage mechanisms. It is assumed that boric acid undergoes partial dehydration during the melt phase transition to produce metaboric acid. Dehydration is an endothermic reaction, which promotes overall energy absorption through an additional thermochemical heat storage mechanism. Further proposed is that during the solidification phase transition, metaboric acid is rehydrated to form boric acid as orthoboric acid. Hydration is an exothermic reaction, therefore, in addition to the heat energy released as sensible and latent heat, the heat energy is released as reaction enthalpy from the molten eutectic mixture during solidification.

[0133] Raman spectroscopy was used (Renishaw inVia visual Raman microscope equipped with a 488 nm laser, working range 20-4000 cm⁻¹). -1 This study investigates chemical species formation during thermal cycling. Boric acid (in the form of orthoboronic 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 corresponds to a 598 cm⁻¹ timescale for the ring breathing vibration. -1 and 819 cm -1 Two characteristic Raman peaks are generated at this point (Bertoluzza et al., Journal of Molecular Structure, 1980, 64, 123-136; Servoss et al., The Journal of Chemical Physics, 1957, 26(5), 1175-1178). These are characteristic bands of metaboronic acid and are unique in distinguishing pure orthoboronic acid from metaboronic acid. However, the 584 cm⁻¹ peak corresponding to the carbonyl group of succinic acid... -1 The weak zone at that location may be related to 598 cm. -1 The metaboronic acid peak overlaps at 819 cm⁻¹. In this case, the peak at 819 cm⁻¹... -1 The metaboric acid peak at that point is only related to metaboric acid and is used here as an indicator of metaboric acid formation.

[0134] Raman spectra of pure boric acid and pure succinic acid were obtained on glass slides at room temperature. Raman spectra of the eutectic mixture at different temperatures were obtained on a thin layer of the mixture between two glass slides.

[0135] Raman spectroscopy studies showed that metaboric acid was formed upon heating, thus proving that the dehydration process was feasible. Figure 7The Raman spectra of the eutectic mixture are shown, exhibiting the following characteristics: (i) solid at room temperature, (ii) molten liquid at 150 °C, and (iii) solid at room temperature after 1000 heating (melting) / cooling (solidification) cycles. The Raman spectrum of the liquid state clearly shows the formation of metaboric acid, as indicated by the 812 cm⁻¹ spectrum. -1 The presence of new peaks indicates this. In the Raman spectrum of the solid sample after 1000 cycles, no peaks corresponding to metaboric acid were observed, and all peaks were identical to those of the original solid sample before heating. This indicates that the formation of metaboric acid upon heating and the reforming of orthoboric acid upon cooling occur reversibly, and in high yield over many cycles.

[0136] Example 3. Water distribution after dehydration

[0137] When the eutectic mixture is present in a closed system, most of the water formed during dehydration remains in the solution of the eutectic liquid. This was confirmed by measuring the pressure in a sealed vial after melting the eutectic mixture at 150°C. Measurements were performed in an Anton Paar Monowave 50, with a sample size of 1 g, a vial volume of 10 ml, a temperature of 150°C, and a holding time of 3 hours. Figure 8 As shown, the observed pressure is < 2 bar, while if the water were to evaporate completely into the vapor space within the vial, the amount of water involved would be expected to generate a pressure of 25 bar.

[0138] Example 4. Long-term loop

[0139] To test long-term chemical and thermal reliability, the eutectic mixture underwent 1000 heating and cooling cycles. This cycling was conducted in a gold-plated high-pressure DSC pan to ensure that water obtained from dehydration of the eutectic mixture was retained in the pan and could be used for the rehydration process. The eutectic mixture was heated at 10°C for [time missing]. -1 The mixture was repeatedly heated and cooled at rates between 110°C and 165°C. After cycling, the mixture was analyzed using powder X-ray diffraction (PXRD) and Raman spectroscopy to investigate any chemical changes that occurred during the cycling.

[0140] like Figure 9 As shown, after 1000 heating-cooling cycles, no change in the position and shape of the melt peak was observed by DSC. ΔH f From 415 J g -1 Reduced to 395 J g -1 (A decrease of 5.9%), which mainly occurred in the first 300 cycles, with ΔH decreasing in the next 700 cycles. f The change is small or not significant. ΔH c The initial value in the first cycle was 395 J g. -1After 200 cycles, it decreased to 378 J g. -1 And thereafter it remains essentially constant (decreases by about 4%). During the initial cycle, the mixture homogenizes, adjusts its composition, and reaches a stable equilibrium between the dehydration and rehydration reactions of boric acid, and once equilibrium is reached, the enthalpy and melting point remain almost unchanged.

[0141] Figure 10 The following Raman spectra are shown: (i) eutectic mixture before cycling, (ii) eutectic mixture after cycling, (iii) boric acid, and (iv) succinic acid. Figure 11 The following PXRD diffraction patterns are shown: (i) eutectic mixture before cycling, (ii) eutectic mixture after cycling, (iii) boric acid, and (iv) succinic acid. No signs of decomposition or irreversible chemistry or phase transition were observed.

[0142] Those skilled in the art will understand that variations and modifications may be made to the invention described herein, in addition to those specifically described. It should be understood that the invention includes all such variations and modifications falling within the spirit and scope of the invention.

Claims

1. A phase change material for thermal energy storage, said 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) 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) between 57:43 and 63:

37.

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

5. The phase change material according to any one of claims 1 to 4, wherein it is melted 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 the cured composition comprising the boric acid and the succinic acid melts at 150°C ± 5°C upon heating.

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

8. The phase change material according to any one of claims 1 to 7, wherein the phase change material, based on the combined mass of boric acid and succinic acid, has a mass greater than 350 J g. -1 enthalpy (ΔH) f ) melt.

9. The phase change material according to any one of claims 1 to 8, wherein, upon cooling from the molten phase, the phase change material yields a yield greater than 310 J g based on the combined mass of boric acid and succinic acid. -1 enthalpy (ΔH) c ( ) Curing.

10. The phase change material according to any one of claims 1 to 9, further comprising non-molten solid particles.

11. A molten phase of a phase change material, said molten phase 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 of claim 12, wherein the phase change material is enclosed in a sealed container.

14. The system of claim 12 or claim 13, comprising a power generation device for generating electricity from the fluid.

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

16. A method for storing thermal energy, the method comprising: We provide phase change materials containing boric acid and succinic acid; Heat is transferred to the phase change material, thereby melting at least a portion of the phase change material to form a molten phase; and Heat is transferred from the phase change material to the fluid, thereby solidifying at least a portion of the molten phase and generating a heated fluid.

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

25.

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

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

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

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