Material for thermal energy storage, method for manufacture thereof and method for using

Abstract

In accordance with one aspect of the present invention, there is provided a material for thermal energy storage comprising one or more metallic species selected from metals and / or metal alloys encapsulated in a structural host material, wherein the metallic species has a melting point in the range of from 300 to 900˚C and wherein the structural host material comprises at least 90% by mass of a hardened inorganic binder comprising one or more of a hydraulic binder, an alkaline activated material and a phosphate binder. In accordance with another aspect, the invention provides a method of manufacturing said material for thermal energy storage, the method comprising the steps of: - providing a mixture of the metallic species in solid state, and dry unreacted binder capable of reacting with a specified liquid; - adding the specified liquid to the mixture thereby generating a paste of the binder containing the metallic species; - casting the paste in a desired shape; and - allowing the paste to harden, comprising reacting with the specified liquid, whereby the paste is transformed into a structural host material which encapsulated the metallic species. In accordance with still another aspect, there is provided a method of heating a process stream by electric power, comprising: - directly or indirectly electrically heating a quantity of the material for thermal energy storage; and - passing a process stream in direct or indirect heat exchange with the quantity of the material, thereby heating the process stream with heat from the quantity of material.

Description

[0001] MATERIAL FOR THERMAL ENERGY STORAGE, METHOD FOR

[0002] MANUFACTURE THEREOF AND METHOD FOR USING

[0003] Field of the Invention

[0004] In one aspect, the invention relates to a material for thermal energy storage. In another aspect, the invention relates to a method of manufacturing the material. In yet another aspect, the invention relates to a method of using the material, particularly in a method of heating a process stream by electric power. Background to the Invention

[0005] Sustainable energy is an ever-increasing concern in modern society. In this regard, thermal energy storage based on latent heat has shown great potential by using phase change materials (PCM) with high heat capacity. However, application of PCM for thermal energy storage in real-world applications has several practical challenges, such as material dispersion, possible chemical reactions with the environment and low thermal conductivity. Embedding PCM in a shell or porous network known as encapsulated PCM (EPCM) in both micro and nano sizes has been proposed as a promising solution for these issues . A comprehensive review has been published by Kasra Ghasemi et al in Sustainable Energy Technologies and Assessments Vol. 52 (2022) under the title of: "PCM, nano / microencapsulation and slurries: A review of fundamentals, categories, fabrication, numerical models and applications."

[0006] Metals and their alloys have been investigated as PCMs and miscibility gap alloys (MGAs) have been proposed as a solution for thermal energy storage. Miscibility gap alloys comprise a mixture of two immiscible materials, particularly metals or semi-metals, with different melting points. The two materials are pressed and sintered together so that the lower melting temperature component is encapsulated as discrete particles within a matrix of the higher melting temperature component.

[0007] W02014063191 describes such materials, in particular those wherein the higher melting material is a metal, graphite or SiC and the lower melting material is another metal, metal alloy or silicon. The use of such materials comprising a lower metal melting metal (e.g. , zinc) within a graphite encapsulation is further described in "Scaling up Miscibility Gap Alloy Thermal Storage Materials" in Transition Towards 100% Renewable Energy, A. Sayigh (ed. ) 2018 Springer International Publishing. The materials proposed in these systems are generally expensive and are restricted by processing constraints allowing restricted freedom in shaping the composite materials and in the size of blocks that can be used. Further, the use of graphite as the encapsulating material requires an inert gas flow for heat transfer in order to prevent combustion.

[0008] Likesh Kumar Sahu, et al. have published a review on PCM in concrete and various methods of incorporating PCM in concrete, in the Int. Research Journal of Engineering and Technology (IRJET) , Volume 4 (2017) pages 2154-2165 under the title : "A review on thermal and mechanical properties of concrete containing phase change material". The PCM materials in the studies quoted in this review have melting points of generally around room temperature and are proposed for use in concrete walls for building and construction to achieve energy savings for nonindustrial heating, e.g. , for residential and commercial premises. As primary use of these materials is in building and construction, structural strength properties are among the leading requirements and therefore only a small volume percentage can be occupied by the PCM, thereby restricting energy storage capacity .

[0009] The described systems which employ concrete to embed the PCM are not suitable for efficient thermal energy storage at high temperature , such a s temperatures above 600 ° C or 700 ° C .

[0010] There remains a need to produce an adaptable PCM for thermal energy storage at high temperatures . Summary of the Invention

[0011] In accordance with one aspect of the pre sent invention , there is provided a material for thermal energy storage compris ing one or more metallic species selected from metal s and / or metal alloys encapsulated in a structural host material , wherein the metallic species has a melting point in the range of from 300 to 900 ° C and wherein the structural host material comprise s at least 90% by mas s of a hardened inorganic binder comprising one or more of a hydraulic binder , an alkaline activated material and a phosphate binder .

[0012] In accordance with another aspect , the invention provide s a method of manufacturing said material for thermal energy storage , the method compri sing the steps of :

[0013] - providing a mixture of the metallic species in solid state , and dry unreacted binder capable of reacting with a specified liquid;

[0014] - adding the specified liquid to the mixture thereby generating a pa ste of the binder containing the metallic species ;

[0015] - casting the paste in a de sired shape ; and

[0016] - allowing the paste to harden , comprising reacting with the specif ied liquid, whereby the paste i s transformed into a structural host material which encapsulated the metallic specie s . In accordance with still another aspect, there is provided a method of heating a process stream by electric power, comprising:

[0017] - directly or indirectly electrically heating a quantity of the material for thermal energy storage; and

[0018] - passing a process stream in direct or indirect heat exchange with the quantity of the material, thereby heating the process stream with heat from the quantity of material .

[0019] Detailed Description of the Invention

[0020] The person skilled in the art will readily understand that, while the detailed description of the invention will be illustrated making reference to one or more embodiments, each having specific combinations of features and measures, many of those features and measures can be equally or similarly applied independently in other embodiments or combinations.

[0021] When introducing elements of various embodiments of the present disclosure, the articles "a," "an," and "the" are intended to mean that there are one or more of the elements. The terms "comprising," "including," and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements. Additionally, it should be understood that references to "one embodiment" or "an embodiment" of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features.

[0022] We propose a new material for thermal energy storage, which comprises one or more metallic species encapsulated in a structural host material. The structural host material consists of at least 90 %, preferably at least 98 %, by mass of a hardened inorganic binder. The metallic species has a melting point in a range of from 300 °C to 900 °C.

[0023] It has been found that this material provides an excellent low-cost material for thermal energy storage at temperatures of operation in a range of from about 300 °C to about 1000 °C, preferably in a range of from about 400 °C to about 1000 °C, more preferably in the range of from about 600 °C to 1000 °C. In some cases, it is preferred not to exceed a temperature of operation of 950 °C or preferably not to exceed 900 °C.

[0024] The metallic species may function as a phase change material (PCM) within the structural host material, and as such at the phase change temperature it may absorb or release latent heat, depending on which direction the phase change is induced to occur. When the melting point of the metallic species is within the range of operation, heat can be stored both latently and sensitively.

[0025] It has been found that relatively high amounts of metallic species can be encapsulated in the structural host material. This is in contrast to encapsulated PCMs used for building / construction, wherein the strength required in the building material considerably reduces the amount of PCM that may be contained therein. Moreover, the hardened inorganic binder can be cycled over temperatures up to about 1000 °C while staying intact and not spall or form cracks. It has been found, when for example concrete is used as structural host material, such as is the case in certain prior art building and construction materials, spalling and cracking frequently occurs, especially at temperatures above about 600 °C, which may cause leakage of the salt from the material thereby degrading the effectiveness of the material. Concrete typically contains an inorganic binder, but in amounts of well below 90 % by mass. The function of the binder in concrete is to bind significant amounts of sand and / or gravel.

[0026] Moreover, it has been found that prior to hardening (reacting) the mixture of the paste of the (unreacted) inorganic binder, including the metallic species, can be poured and cast in large volumes, very much like regular concrete. This makes it an excellent material to use and process in practice.

[0027] The inventors have found three classes of inorganic binders that can be used for the presently proposed material: hydraulic binders, alkali-activated materials and phosphate binders .

[0028] Hydraulic binders generally react with (liquid) water to harden. Cement is a hydraulic binder. Cement is a finely ground inorganic powder which, when mixed with water, forms a paste which sets and hardens by means of hydration reactions and processes and which, after hardening, retains its strength and stability even under water. In the case of Portland cement, the reactive material is formed through the calcination of limestone and clay. In the case of calcium aluminate cement the reactive materials is generally formed through the sintering of a mixture of a limestone and bauxite. Hydraulic cements are suitable for the present material, as they are generally low in cost. Examples of suitable hydraulic cements include Portland cement, blast-furnace cements, pozzolanic cements, composite cements, and calcium-aluminate cement.

[0029] "Alkali-activated materials" is a generic term which is applied to any binder system derived by the reaction of a solid silicate powder (termed the "precursor") with an alkali metal source (termed the "activator") to produce a hardened material. The precursor can be a predominantly calcium silicate, or a more aluminosilicate-rich precursor such as a metallurgical slag, natural pozzolan, fly ash or bottom ash. The activator is any soluble substance which can supply alkali metal cations, raise the pH of the reaction mixture and accelerate the dissolution of the solid precursor; this includes alkali hydroxides, silicates, carbonates, sulfates, aluminates or oxides. Alkali-activated materials include, but are not limited to geopolymers, mineral polymers, inorganic polymers, inorganic polymer glasses, alkali-bonded ceramics, alkali ash material, soil cements, soil silicates, SKJ-binder, F- concrete, hydroceramics, zeocements and zeoceramics. Geopolymers, for example, are a subset of alkali-activated materials where the binding phase is a pseudo-zeolitic network structure of highly coordinated aluminosilicates. Geopolymers are made by mixing an alkaline solution (often an alkali metal hydroxide or silicate) with solid aluminosilicate materials (often low-calcium fly ash or calcined clays) .

[0030] Phosphate binders, such as phosphate cements, or phosphate bonded cements, are the products of an acid-base reaction between a concentrated aqueous solution of orthophosphoric acid, often containing metal salts, acid and a basic metal oxide or aluminosilicate. The basic metal oxide preferably contains magnesium or aluminium as the metal.

[0031] Cement-forming phosphoric acid liquids nearly always contain cations . The most important of these are aluminium and zinc but other metals may be used. Modifying metals must be moderately soluble in oxide solution and have a high ionic potential. They must have a low coordination number, multiple charge, and small ionic radius. This is necessary to remain in vitreous form in the cement gel. Ions of amphoteric or weakly basic metals such as aluminium, zinc, beryllium and magnesium satisfy this. The metallic species comprises one or more metals and / or metal alloys. Suitable metals include aluminium, lead, magnesium, zinc and alloys thereof. Aluminium is a particularly preferred metallic species. A metal alloy is a combination of two or more elements, of which one must be a metal, which produces an admixture with metal-like properties. One suitable alloy for use herein is an aluminum / silicon alloy. Thus, the metallic species preferably comprises aluminium and / or alloys thereof.

[0032] All significant ingredients in the novel material can be low in cost and therefore this is a very attractive improvement over prior art solutions which either do not contain significant amounts of phase change materials, thus limiting their heat storage capacity, or which do not have the structural strength needed to provide self- supporting material.

[0033] It has been found possible to incorporate much higher amounts of the metallic species in the material than expected from prior art references. The higher the content of metallic species in the material, the more heat can generally be stored per unit of mass of the material. However, the metallic species should not exceed an amount that would cause the metallic species to be leaked from the material at higher temperature. It has been found in the present proposal that an overall mass of metallic species encapsulated in the structural host material in a range of between 30% and 70% is feasible. Preferably the overall mass of metallic species incorporated is between 40% and 70%, more preferably between 50 % and 70 %, of the total mass of the material.

[0034] A balance of the structural host material may further comprise one or more admixtures, up to 10% by mass, which does not include the encapsulated salt. Admixtures, also referred to as "chemical admixtures" are materials added immediately before or during mixing to improve the manufacturing or the properties of the binder .

[0035] Typical admixtures are suitable for hydraulic binders , alkali-activated materials and phosphate binders and include retarders , accelerators , plastici zers , super plasticizers and / or air entrainment admixture s . Such admixtures are beneficial to lower the water content which is helpful in view of the high amounts of salt . Some admixtures may be contemplated to enhance the thermal conductivity of the material .

[0036] The novel material for thermal energy storage can be manufactured by mixing finely divided solid metallic species with dry unreacted inorganic binder capable of reacting with a specified liquid . The specified liquid may be water in case of hydraulic binder . The specified liquid may be an alkaline solution in case of al kali activated binder . In the case of a phosphate binder , the specified liquid is the concentrated aqueous solution of orthophosphoric acid, with the dry unreacted binder compris ing the basic metal oxide or aluminosilicate .

[0037] Preferably the f inely divided solid metallic species are particles with an average diameter in the range of from 2 to 12 mm .

[0038] In one embodiment of the method of manufacturing the material for thermal energy storage , particle s of the metallic specie s are first coated in a sacrif icial species before being mixed with the dry unreacted inorganic binder . Said sacrificial material i s preferably an organic material and more preferably a wax or a plastic . In thi s embodiment , the metal specie s is finely divided into appropriately s ized solid particles and then coated in the sacrificial material . Once the thus -coated particles of the metallic species are mixed with the dry unreacted solid binder , the remaining steps of the method of manufacture may be completed. Upon the first heating of the material for thermal energy storage, the sacrificial material will be burnt off and thus allow space for the metal to expand on heating without damaging the structural host material.

[0039] The specified liquid can be added to the mixture, thereby generating a paste containing the metallic species. The paste can be cast in a desired shape, such as a block, after which the paste should be allowed to harden, whereby forming the structural host material which encapsulates the metallic species. This way, the hardened inorganic binder material, the function of which is to provide the structural properties of the new material, can be kept relatively free from metallic species while the material as a whole can still contain high amounts of metallic species.

[0040] In case of hydraulic binders, the water is preferably added to a mass ratio of water to dry binder in a range of from 0.20 to 0.55. If the water content is below 0.20, the paste is not fluid enough to be shaped in a mold and it would be vulnerable to contain trapped air bubbles and / or unreacted binder. If the water content is above 0.55 the paste is too fluid, thereby creating the risk of the agglomerates to settle to the bottom. Also, the hardened structure would tend to be weaker and vulnerable to developing cracks during the drying process.

[0041] In case of alkali activated materials, the alkaline solution is preferably added to a mass ratio of liquid to dry binder in a range of from 0.20 to 0.55. This provides sufficient liquid for the necessary workability. If the liquid content is below 0.20, the paste is not fluid enough to be shaped in a mold and it would be vulnerable to contain trapped air bubbles and / or unreacted binder. If the liquid content is above 0.55 the paste is too fluid, thereby creating the risk of the agglomerates to settle to the bottom. Also, the hardened structure would tend to be weaker and vulnerable to developing cracks during the drying process.

[0042] In case of phosphate binders, the concentrated aqueous solution of orthophosphoric acid is preferably added to a mass ratio of liquid to dry binder in a range of from 0.09 to 0.20. This provides sufficient liquid for the necessary workability. If the liquid content is below 0.09, the paste is not fluid enough to be shaped in a mold and it would be vulnerable to contain trapped air bubbles and / or unreacted binder. If the liquid content is above 0.20 the paste is too fluid, thereby creating the risk of the agglomerates to settle to the bottom. Also, the hardened structure would tend to be weaker and vulnerable to developing cracks during the drying process.

[0043] The paste is cast into the desired shape and allowed to harden. During casting, additional conductive elements such as metallic bars or wires may be incorporated into the material for thermal energy storage. Such metallic bars or wires suitably comprise a higher melting metal and / or allow so that they remain intact throughout the use of the material for thermal energy storage. The use of such conductive elements serves to increase the thermal conductivity of the material for thermal energy storage.

[0044] The manufacturing may further include a step of drying the structural host material which encapsulates the metallic species. This step is performed after fully reacting the inorganic binder. Drying is preferably performed at a drying temperature in a range of between 70 °C and 150°C, prior to bringing the structural host material which encapsulates the phase change material to its final operating temperature of between 500 °C and 1000 °C. By drying at such temperatures, excessive pressure build up in the material can be avoided thereby also avoiding damage to the hardened binder structure .

[0045] The novel material for thermal energy storage may suitably be used to heat a proces s stream by electric power . Thi s may involve electrically heating a quantity of this material and pas sing the proce s s stream in indirect heat exchange with the quantity of the material whereby heating the proces s stream with heat from the quantity of material . The proces s stream may be a heat transfer fluid, such as an oil or water / steam .

[0046] Reference is made to International publication W02023012250 Al , which describes a thermal energy storage device , compris ing :

[0047] - a powder bed having a relatively high electrical resi stivity;

[0048] - at least two electrodes , embedded in the powder bed and arranged to heat the powder bed by providing an electrical current therebetween ;

[0049] - at least one heat transfer tube arranged to contain a heat transfer fluid, the heat trans fer tube having an inlet and an outlet connectable to a thermal energy consumer, wherein the heat transfer tube and the powder bed are thermally coupled via an electrically insulating material .

[0050] The device described in said application may further comprise a buffer layer , thermally coupled to the powder bed, and separated from the heat transfer tube by at least the powder bed . One or both of the buffer layer and / or the electrically insulating material may comprise the novel material a s des cribed in the present disclosure .

[0051] The electric heating power may be fluctuating over time , such as i s frequently the case when the electric heating power i s derived from a renewable source such a s a solar and / or wind power source . Nevertheles s , the electrically insulating material and the optional buffer layer act as a thermal buffer, which continues to heat up the heat transfer fluid for a certain amount of time during an interruption of the electric heating power.

[0052] The encapsulated metallic species acts as a PCM, having a phase change between sold and liquid at its melting point. The temperature of the material may fluctuate over a certain range, typically increasing when more heat is added than extracted, and decreasing when more heat is extracted than added, as a result of sensible heat being added to or extracted from the material. However, if the predetermined melting point of the metallic species is within that temperature range, then the metallic species will pick up or release latent heat that is associated with the phase change when the temperature reaches the melting point. This means that electrically heating of the material will cause a first phase change in the metallic species from solid to liquid state. Heating of the process stream, in absence of sufficient electrically heating being available, may induce a second phase change in the metallic species, from liquid to solid state.

[0053] The invention is now illustrated with reference to the following non-limiting examples . Examples Example 1

[0054] The heat storage material of Example 1 was comprised of a cementitious binder which encompasses pure aluminium particles. To make the cementitious binder, an alkali activated material comprising fly ash, waterglass, 14M NaOH solution, deionized water, and a superplasticizer (SPL) were used. The SPL was MasterGlenium SKY 648 con. 20%. Aluminium particles comprise 30 volume percent. The constituents were weighed out (see Table 1) . The waterglass, NaOH solution, and deionized water were weighed out and mixed together. The fly ash was placed in the bowl of a Hobart mixer. As the mixer operated at speed 1, roughly 90% of the liquid component was slowly added to the cement over the course of 30 seconds. The remaining 10% of the liquid component was added to the previously weighed out SPL. The liquid / SPL mix was then added to the bowl. The fresh paste was mixed for another 30 seconds. Subsequently the mixer was stopped, the blade, the bottom and the sides of the bowl were scraped to ensure complete mixing and the paste was allowed to rest for 30 seconds . Then the Hobart was turned up to speed 2 and the paste was mixed for an additional 60 seconds. Next the aluminium particles were added to the fresh paste and mixed at speed 1 for 60 seconds. Table 1: Mass fractions of the individual components of the (fresh) heat storage material

[0055] The fresh heat storage material was then cast into moulds. Silicon moulds of 5x5x5 cm were used. The moulds were placed on a vibration table (manufacturer: Tinpeng) and vibrated for 30 - 60 seconds to remove excess air and improve compaction. The moulds were covered and placed in a curing oven at 70°C for 14 days. Subsequently the hardened material was removed from the moulds. The cubes were weighed, and the dimensions were measured. They were then placed in a 105°C oven (manufacturer: Binder) and left until all free water had been removed (11 days) . Once a stable weight had been achieved the weight and the dimensions of the samples were recorded.

[0056] The specimens were placed in the high temperature oven (manufacturer: Western Ovens) and slowly heated up to 500°C over the course of 12 hours. The specimens were held at 500°C for 12 hours to ensure the loss of any remaining free water. Subsequently the specimens were heated to 750°C, held there for 8 hours to allow the entire sample to reach the target temperature and then cooled back down to 550°C. Four specimens were cycled between 750°C and 550°C with 8 hours of heating and 16 hours of coolingError • Reference source not found. ) 9 more times, for a total of 10 thermal cycles. Four specimens were cycled between 750°C and 550°C 49 more times, for a total of 50 thermal cycles. Four specimens were cycled between 750°C and 550°C 99 more times, for a total of 100 thermal cycles .

[0057] After thermal treatment, the specimens were visually inspected weighed, and the dimensions recorded. The compressive strength was determined. Additionally, the aluminium particles from within the heat storage material was extracted and investigated with digital scanning calorimetry and thermal gravimetric analysis.

[0058] The specimen which underwent 10 thermal cycles was stable in regard to shape and dimensions. There was a discoloration of the binder from a greyish brown to a reddish brown. The specimen had an average weight loss of 4.8% based on the dry weight prior to thermal cycling. The average compressive strength was 0.77 MPa.

[0059] The specimen which underwent 50 thermal cycles was stable in regard to shape and dimensions. The specimen maintained the reddish-brown colour observed after 10 thermal cycles. The specimen had an average weight loss of 2.9% based on the dry weight prior to thermal cycling. The average compressive strength was 0.82 MPa.

[0060] The specimen which underwent 100 thermal cycles was stable in regard to shape and dimensions. The specimen maintained the reddish-brown colour observed after 10 thermal cycles. The specimen had an average weight loss of 3.6% based on the dry weight prior to thermal cycling. The average compressive strength was 0.71 MPa. Example 2

[0061] The heat storage material of Example 2 was comprised of a cementitious binder which encompasses aluminium particles. To make the cementitious binder, calcium aluminate cement (ex Caltra Nederland B.V. ) and deionized water were used. The water was used at a water to cement (w / c) ratio of 0.35. Aluminium particles comprise 30 volume percent, see Table 2 for the compiled mass fractions .

[0062] The three constituents were weighed out. The calcium aluminate cement was placed in the bowl of a Hobart mixer. As the mixer operated at speed 1 the water was slowly added to the cement over the course of 30 seconds. The fresh paste was mixed for another 30 seconds. Subsequently the mixer was stopped, the blade, the bottom and the sides of the bowl were scraped to ensure complete mixing and the paste was allowed to rest for 30 seconds. Then the Hobart was turned up to speed 2 and the paste was mixed for an additional 60 seconds . Next the aluminium particles were slowly added to the fresh paste and mixed at speed 1 for 60 seconds . Table 2 : Mass fractions of the individual components of the (fresh) heat storage material Aluminum particles

[0063] The fresh heat storage material was then cast into the moulds. Silicon moulds of 5x5x5 cm were used. The moulds were placed on a vibration table (manufacturer: Tinpeng) and vibrated for 30 - 60 seconds to remove excess air and improve compaction. The moulds were covered and placed in a curing oven at 70°C for 24 hours. Subsequently the hardened material was removed from the moulds. The cubes were weighed, and the dimensions were measured. They were then placed in a 105°C oven (manufacturer: Binder) and left until all free water had been removed (7 days) . Once a stable weight had been achieved the weight and the dimensions of the samples were recorded.

[0064] The specimens were placed in the high temperature oven (manufacturer: Western Ovens) and slowly heated up to 500°C over the course of 12 hours. The specimens were held at 500°C for 12 hours to ensure the loss of any remaining free water. Subsequently the specimens were heated to 750°C, held there for 8 hours to allow the entire sample to reach the target temperature and then cooled back down to room temperature. Subsequently the specimens were heated up to 550°C and then cycled between 750°C and 550°C with 8 hours of heating and 16 hours of cooling for two more cycles.

[0065] After thermal treatment the specimens were visually inspected and weighed. Additionally, the aluminium particles from within the heat storage material were extracted and investigated with digital scanning calorimetry and thermal gravimetric analysis.

[0066] After the first thermal treatment, the samples had an average weight loss of 11.1% based on the dry weight of the samples prior to thermal treatments) . The total weight loss for all samples was less than the weight of water added to the fresh paste indicating the presence of phases containing chemically bound water that are stable up to at least 750°C. The subsequent thermal cycles did not reduce the weight of the samples any further. Small aluminium droplets emerged from the surface pores during thermal treatment. The aluminium droplets remain consistent in shape, amount, size, and placement during and after subsequent thermal cycles.

[0067] Example 3

[0068] The heat storage material of Example 3 was comprised of a cementitious binder which encompassed aluminium particles. To make the cementitious binder, anhydrite (Micro A) and deionized water were used. The water was used at a water to binder (w / b) ratio of 0.40. Aluminium particles comprise 30 volume percent, see Table 3 for the compiled mass fractions. Basalt fibres were added to 1 volume percent.

[0069] First the aluminium particles were coated in wax. This allows for a buffer zone between the aluminium and the hardened binder to compensate for any stresses resulting from different thermal expansion coefficients. The aluminium particles were weighed out and placed inside a cup of SX105 wax in the oven at 110°C. Then the aluminium particles were sieved and cooled in the lab. When the aluminium particles were at room temperature, more molten wax was poured on top of them, and they rested for a minute. Then the material was sieved, and the mass of wax was measured.

[0070] The remaining four constituents were weighed out. The anhydrite and fibres were placed in the bowl of a Hobart mixer and mixed at speed 1 for 60 seconds. As the mixer operated at speed 1 the water was slowly added to the binder over the course of 30 seconds. The fresh paste was mixed for another 30 seconds. Subsequently the mixer was stopped, the blade, the bottom and the sides of the bowl were scraped to ensure complete mixing and the paste was allowed to rest for 30 seconds. Then the Hobart was turned up to speed 2 and the paste was mixed for an additional 60 seconds . Next the wax coated aluminium particles were slowly added to the fresh paste and mixed at speed 1 for 60 seconds.

[0071] Table 3 : Mass fractions of the individual components of the (fresh) heat storage material

[0072] The fresh heat storage material was then cast into moulds. Silicon moulds of 5x5x5 cm were used. The moulds were placed on a vibration table (manufacturer: Tinpeng) and vibrated for 30 - 60 seconds to remove excess air and improve compaction. The moulds were covered and placed in a curing oven at 70°C for 24 hours. Subsequently the hardened material was removed from the moulds. The cubes were weighed, and the dimensions were measured. They were then placed in a 105°C oven (manufacturer: Binder) and left until all free water had been removed (7 days) . Once a stable weight had been achieved the weight and the dimensions of the samples were recorded.

[0073] The specimens were placed in the high temperature oven (manufacturer: Western Ovens) and heated up at l°C / min to 750°C where they were held for 8 hours. Subsequently the specimen was cooled back down to room temperature. After observation the samples were exposed to a second thermal cycle. After the first thermal treatment the specimens had maintained their original shape and form. No cracking was observed and the aluminium particles remained contained in the binder. After the first thermal treatment, the samples had an average weight loss of 5.0% based on the dry weight of the samples prior to thermal treatments.

[0074] After the second thermal treatment the specimens had maintained their original shape and form. No cracking was observed, and the aluminium particles remained contained in the binder. The samples had an average weight loss of 0.47% based on the weight of the samples after the previous thermal treatment. Example 4

[0075] Lead, magnesium, zinc, and aluminium silicon were all tested as metals / metal alloys to function as a phase change material. They were tested in two different methods .

[0076] Sample preparation #1

[0077] First the individual phase change materials were weighed out to replace 30 volume precent of the fresh binder. Then the fresh calcium aluminate cement paste was made by mixing calcium aluminate cement with water at a w / b ratio of 0.35. 30 grams of calcium aluminate paste was cast around each PCM. The specimens were covered and placed in a curing oven at 70°C for 24 hours. Subsequently they were placed in a 105°C oven and dried for 7 days. Sample preparation #2

[0078] Alternatively, containers were made of calcium aluminate cement, water, and basalt sand according to the mass fractions in Table 4. The mix was cast into cubes of 10X10X10 cm with a cavity 8cm deep and an area of 5x5 cm. The blocks were cured at 70°C for 24 hours and then dried at 105°C until a stable weight was achieved. Once the containers were cured and dried 30 grams of each PCM was added to the basin.

[0079] Table 4 : Mass fractions of the individual components of the heat storage material containers

[0080] The specimens were placed in the high temperature oven (manufacturer: Western Ovens) and heated up at l°C / min to 750°C where they were held for 8 hours. Subsequently the specimen was cooled back down to room temperature .

[0081] After thermal treatment the specimens prepared using preparation 1 were split open and the PCM was examined. For all PCMs it was evident that melting had occurred. The lead and magnesium sample had turned from sliver and grey respectively to white. The zinc sample had exerted forces upon the hardened binder and some cracking was observed. The aluminium silicon samples showed agglomeration of the smaller particles within the hardened binder.

[0082] The specimen from preparation 2 showed no negative effects on the binder container. Example 4

[0083] The heat storage material of Example 4 was comprised of a cementitious binder which encompassed aluminium particles. To make the cementitious binder, CURAS 90 PF Component A and Component B (Gouda Refractories) and deionized water were used. Aluminium particles comprise 30 volume percent, see Table 3 for the compiled mass fractions .

[0084] The four constituents were weighed out. The Component A and Component B were placed in the bowl of a Hobart mixer and mixed at speed 1 for 60 seconds. As the mixer operated at speed 1 the water was slowly added to the binder. The fresh paste was mixed for another 30 seconds. Subsequently the mixer was stopped, the blade, the bottom and the sides of the bowl were scraped to ensure complete mixing and the paste was allowed to rest for 30 seconds. Then the Hobart was turned up to speed 2 and the paste was mixed for an additional 60 seconds. Next the aluminium particles were slowly added to the fresh paste and mixed at speed 1 for 60 seconds .

[0085] Table 5 : Mass fractions of the individual components of the (fresh) heat storage material

[0086] The fresh heat storage material was then cast into moulds. Silicon moulds of 5x5x5 cm were used. The moulds were placed on a vibration table (manufacturer: Tinpeng) and vibrated for 30 - 60 seconds to remove excess air and improve compaction. The moulds were covered and left to cure qat 20°C for 24 hours. Subsequently the hardened material was removed from the moulds. The cubes were weighed, and the dimensions were measured. They were then placed in a 105°C oven (manufacturer: Binder) and left until all free water had been removed (7 days) . Once a stable weight had been achieved the weight and the dimensions of the samples were recorded.

[0087] The specimens were placed in the high temperature oven (manufacturer: Western Ovens) and heated up at l°C / min to 750°C where they were held for 8 hours. Subsequently the specimen was cooled back down to room temperature . After the thermal treatment the specimens had maintained their original shape, form, and colour. No cracking was observed, and the aluminium particles remained contained in the binder. After the first thermal treatment, the samples had an average weight loss of 0.7% based on the dry weight of the samples prior to thermal treatments .

[0088] The person skilled in the art will understand that the present invention can be carried out in many various ways without departing from the scope of the appended claims .

Claims

C L A I M S1. A material for thermal energy storage comprising one or more metallic species selected from metals and / or metal alloys encapsulated in a structural host material, wherein the metallic species has a melting point in the range of from 300 to 900°C and wherein the structural host material comprises at least 90% by mass of a hardened inorganic binder comprising one or more of a hydraulic binder, an alkaline activated material and a phosphate binder.

2. The material of claim 1, wherein the metallic species has a melting point in the range of from 600 to 800 °C.

3. The material of claim 1 or claim 2, wherein the metallic species is selected from the group consisting of aluminium, lead, magnesium, zinc and aluminium / silicon alloy.

4. The material of any one of the preceding claims, wherein the overall amount of metallic species encapsulated in the structural host material is in a range of between 20% and 70%, preferably between 25% and 60%, more preferably between 30 % and 50 %, of the total volume of the material.

5. A method of manufacturing the material of any one of claims 1 to 4, the method comprising the steps of:- providing a mixture of the metallic species in solid state, and dry unreacted binder capable of reacting with a specified liquid;- adding the specified liquid to the mixture thereby generating a paste of the binder containing the metallic species ;- casting the paste in a desired shape; and- allowing the paste to harden, comprising reacting with the specified liquid, whereby the paste is transformed into a structural host material which encapsulated the metallic species .

6. A method as claimed in claim 5, wherein finely divided particles of the metallic species are first coated in a sacrificial species, preferably wax, before being mixed with the dry unreacted inorganic binder.

7. A method as claimed in claim 5 or claim 6, wherein additional conductive elements are incorporated into the material for thermal energy storage during casting.

8. The method of any one of claims 5 to 7 , wherein the dry unreacted inorganic binder comprises a hydraulic binder and the specified liquid comprises water, whereby the water is added to the hydraulic binder with mass ratio of liquid to hydraulic binder in a range of from 0.20 to 0.55.

9. The method of any one of claims 5 to 7 , wherein the dry inorganic binder comprises an alkaline activated binder and the specified liquid comprises an alkaline solution, whereby the alkaline solution is added to the alkaline activated binder with mass ratio of liquid to alkaline activated binder in a range of from 0.20 to 0.55.

10. The method of any one of claims 5 to 7 , wherein the dry inorganic binder comprises a basic metal oxide or aluminosilicate and the specified liquid comprises a concentrated aqueous solution of orthophosphoric acid, whereby the concentrated aqueous solution of orthophosphoric acid is added to the basic metal oxide or aluminosilicate with mass ratio of liquid to the basic metal oxide or aluminosilicate in a range of from 0.09 to11. The method of any one of claims 5 to 10, further comprising a step of drying the structural host material, which encapsulates the metallic species, at a drying temperature in a range of between 70 °C and 150 °C, prior to bringing the structural host material which encapsulates the phase change material to its final operating temperature, preferably wherein the final operation temperature is between 500 °C and 1000 °C, more preferably between 500 °C and 900 °C.

12. A method of heating a process stream by electric power, comprising:- directly or indirectly electrically heating a quantity of the material for thermal energy storage as claimed in any one of claims 1 to 5; and - passing a process stream in direct or indirect heat exchange with the quantity of the material whereby heating the process stream with heat from the quantity of material .

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