Patent Application
20240141221
The invention relates to thermal energy storage, specifically thermal energy storage at temperatures below 0° C. More specifically, the invention relates to the use of inorganic phase change materials (PCMs) as cold storage media and the nucleation thereof.
Cooling systems lack thermal inertia and thermal mass. One solution is to add in a buffer tank, for example a glycol-water mixture, in order to add thermal mass and inertia to the system. However, these systems are undesirable because they have high heat gains from ambient (low efficiency), high purchasing and maintenance costs and low energy density. Such systems require the use of chillers, which use a refrigerant to remove heat from the water-glycol circuit via a heat exchanger, and this is a significant cause of low efficiency.
The use of phase-change materials (PCMs) to store thermal energy is a high energy density alternative to water/glycol tanks. Such materials store energy using the latent heat of a phase change (i.e. solid-liquid, solid-gas, liquid-gas), including polymorphic changes (i.e. solid-solid).
Materials which exhibit phase changes below zero may be organic in nature (i.e. carbon based) or inorganic salt-water eutectics. Compared to organics, inorganic PCMs are typically cheaper, have lower flammability/combustibility and may have higher energy density. However, they exhibit subcooling, a phenomenon where a material which will remain liquid below its thermodynamic phase change temperature and therefore require nucleation aids or extreme low temperatures to initiate crystallisation.
It is an object of at least one aspect of the present invention to obviate or at least mitigate one more of the aforementioned problems.
It is an object of at least one aspect of the present invention to provide a sub-zero phase change material with multiple crystallisation events.
It is an object of at least one aspect of the present invention to provide a sub-zero phase change material with one or more nucleation agents that act to reduce subcooling in one of the crystallisation events specifically.
It is a further object of at least one aspect of the present invention to provide an improved phase change material for cold storage media.
It is a further object of at least one aspect of the present invention to provide a sub-zero phase change material with one or more nucleation agents chosen to reduce subcooling in one crystallisation event, combined with one or more other nucleation agents chosen to reduce subcooling in the other crystallisation event.
It is a benefit of the present invention that the PCM undergoes both crystallisation processes with minimal cooling below the thermodynamic phase transition temperature (i.e. the temperature at which the phase transition could occur with no subcooling).
It is a benefit of the present invention that the PCM may be frozen with a minimum of cooling power, i.e. is frozen at high temperature.
It is a benefit of the present invention that the PCM undergoes both crystallisation phase transitions at a temperature close to (e.g. between 0 and 20° C. below) the thermodynamic phase transition temperature.
According to a first aspect of the present invention there is provided a phase change material (PCM) with a melting point below 0° C. which exhibits two crystallisation events on cooling, comprised of:
Disclosed herein are compositions of PCMs which exhibit multiple crystallisation events on cooling.
Typically, the salt(s) may be comprised of:
Typically, the PCM may be comprised of one or more nucleation agent(s) comprising one or more of the following:
Preferably, the PCM according to the invention is comprised of one or more nucleation agent(s) selected from one or more of:
It is a preferred embodiment of the present invention that the PCM comprises one or more salts of group I and/or group II metals.
It is a preferred embodiment of the present invention that the PCM comprises one or more lithium, sodium, potassium, magnesium, calcium, strontium and/or ammonium salts.
It is a preferred embodiment of the present invention that the PCM comprises one or more halide, sulfate, nitrate, carbonate and/or carboxylate salt.
Herein it is defined that the first and second crystallisation events are defined by the order in which they occur chronologically as the PCM is cooled from its liquid state.
The nucleation agent may act to induce nucleation of the first crystallisation transition.
The nucleation agent may act to induce nucleation in the second crystallisation transition.
A plurality of nucleation agents may be used to nucleate both crystallisation events.
It is a preferred embodiment of the present invention to use two or more nucleation agents, with at least one acting to reduce subcooling in the first crystallisation event and at least one acting to reduce subcooling in the second crystallisation event.
In particular embodiments of the invention on cooling nucleation at higher temperature precedes nucleation at a low temperature.
One of the crystallisation events may be a solid-solid phase transition.
The nucleation agent may be selected from at least one oxide, carbonate, carbide, silicate and/or halide of the following: Silicon;
The nucleation agent may be at least one material selected from a group comprised of:
The nucleation agent may be a ceramic composite comprised of more than one oxide and/or carbide.
The nucleation agent may be present at a loading of at least 0.01 wt. %, at least 0.1 wt. %, at least 1 wt. %, at least 5 wt. %, at least 10 wt. %, or at least 20 wt. %.
The nucleation agent may be present at a loading of about 0.5 wt. %.
The nucleation agent may be present at a loading greater than the solubility limit of the nucleation agent in the salt solution.
Other aspects of the present invention are set out in the appended claims.
According to further aspect of the present invention there is provided use of a PCM according to the first aspect, wherein the PCM is retained in a solid state after its first crystallisation event before the second crystallisation event occurs. The second crystallisation event can be considered to be a solid-solid phase transition.
A further disclosure as part of the present invention are nucleation agent materials which act to reduce subcooling in one of the crystallisation events exhibited by salt-water eutectic PCMs.
As part of the present invention, it is disclosed that further to subcooling, salt-water eutectics often crystallise at two stages and at two different temperatures.
It is also disclosed herein that the two or at least two crystallisation events in salt-water eutectic PCMs may have distinctly different thermal energies. Control over these two or at least two crystallisation events, including nucleation and crystal growth are highly advantageous towards producing a sub-zero PCM with reliable cyclability, taking advantage of the full thermal capacity of the material.
It is a preferred embodiment of the present invention to use silicon carbide, phyllosilicate materials (e.g. vermiculite, talc or mica) and combinations thereof as nucleation agents to nucleate the second crystallisation event of a salt-water eutectic PCM.
In addition, the application of said control of crystallisation to the operation of a thermal storage device is described.
Embodiments of the present invention will now be described, by way of example only, with reference to the following Figures:
The present invention relates to the use of inorganic phase change materials (PCMs) as cold storage media and the nucleation thereof.
Herein is disclosed a range of salt-water eutectics that have phase transition temperatures between about 0 and about −100° C. and exhibit a two-stage crystallisation event.
A two-stage crystallisation event is described pictorially in
Herein it is defined that the first and second crystallisation events are defined by the order in which they occur chronologically as the PCM is cooled from its liquid state.
However, experimentally several different potential regimes may be observed depending on the composition of the PCM.
Subcooling may be more extreme than is noted in
Furthermore, the maximum output temperature of a sub-zero PCM with multiple nucleation events may be weighted more towards the crystallisation process initiated by the first or second crystallisation events.
For example,
Another eventuality is that both crystallisation stages produce the same output temperature after each crystallisation event.
A further key disclosure which is demonstrated in
It is disclosed herein that salts of halides, nitrates, sulfates and carboxylic acids (such as acetates or formates), and their mixtures exhibit said multiple crystallisation events.
The PCMs disclosed herein may be comprised of at least one salt with at least one or moreof cations selected from a group comprised of:
Lithium;
Any halide;
Sulfate;
It is a preferred embodiment of the present invention that the PCM comprises one or more salts of group I and/or group II metals.
It is a preferred embodiment of the present invention that the PCM comprises one or more lithium, sodium, potassium, magnesium, calcium, strontium and/or ammonium salts.
It is a preferred embodiment of the present invention that the PCM comprises one or more halide, sulfate, nitrate, carbonate and/or carboxylate salt.
It is noted herein that this double crystallisation effect can be observed in salt-water eutectics where the salt in question has one or more known hydrates. For example the sodium acetate-water eutectic described in
Furthermore, it is disclosed herein that even salts with a plurality of hydrate forms tend to exhibit only two distinct crystallisation events. In
The invention discloses materials which may be used to aid nucleation of one of the crystallisation events of a salt-water eutectic. Providing materials which aid in nucleation ensure that both crystallisation stages are completed before the material is warmed, and therefore allows the full use of the PCM as a thermal energy storage medium. Said nucleation agents also decrease the temperature below which the PCM must be cooled to ensure nucleation for one, or both, of the phase transitions. It is a further disclosure of the present invention that multiple nucleation agents that each individually act to reduce subcooling in one of the crystallisation transitions may be combined to overcome subcooling in both crystallisation transitions.
Herein, it is disclosed that metal oxides, carbides, silicates, halides, and combinations thereof are effective nucleation agents for at least one of the phase transitions observed in sub-zero salt-water eutectic PCMs.
The nucleation agent may be effective on one of the two phase transitions but not the other. For example, as shown in
Nucleation agents may also affect only the first crystallisation event. For example, a ceramic composite comprised of alumina and silica is disclosed as an effective nucleation agent for the first, high temperature nucleation event of the magnesium nitrate-water eutectic.
Ice nucleating proteins are disclosed herein as effective nucleation agents for the first crystallisation event.
Nucleation is known to be a stochastic process, and thus crystallisation is generally improved by increasing the sample size as the probability of a stable nucleation point being forming increases with increasing sample size. Considering this, it could be expected that the double nucleation profile of sub-zero salt-water eutectic PCMs would differ at large scales. However, it is disclosed herein that this is not the case, and that even at very large (i.e. >1 L) scales that such PCMs still exhibit two distinct crystallisation events.
By way of further non-limiting example, a PCM comprised of magnesium sulfate, water and one or more nucleation agents may be considered.
By way of further non-limiting example, a PCM comprised of sodium bromide, water and one or more nucleation agents are considered. It has been found by the inventors that aluminium oxide, silica and calcium carbonate may be used as a nucleation agent to trigger the first crystallisation event (
Further testing was carried out on other 1+ cation halides, such as the KCl-water and NH4Cl-water eutectic, and it was found that silver iodide would act as a nucleation agent for the first crystallisation event.
In summation, Table 1 discloses nucleation agents which tend to, but do not in every case, act to reduce subcooling on their first and second crystallisation events.
More specifically, Table 2 shows the types of salts, in accordance with the present invention, used in sub-zero salt-water eutectic PCMs with nucleation agents which act on the first and second crystallisation event.
The types of salts defined in Table 1 and 2 are representative of the preferred nucleating agent(s). Further, more specific, detail is given in Tables 3 and 4.
Table 3 discloses preferred embodiments of the present invention.
Table 4 shows more specific further preferred embodiments of the present invention.
Table 5 details various nucleation agents and the salts and concentrations with which they may be used.
Further specific embodiments of the present invention are further exemplified in
It is disclosed herein that corrosion of metal components in contact with a salt-water eutectic PCM may be reduced by reduced subcooling by nucleation agent addition. Corrosion is increased where liquid PCM is in contact with metal components, whereas by contrast the solid phase of the same material will have significantly reduced corrosion. Improving the nucleation characteristics of a salt-water eutectic PCM such that less contact is made between liquid PCM and any metallic components, and thereby overall corrosion is limited.
Furthermore, it is disclosed herein that sub-zero PCMs featuring one or more nucleation agent(s) which act to suppress subcooling in one of the PCMs crystallisation events may constitute part of a heat battery apparatus. Thereby, energy storage at subzero temperatures may be achieved with reliable nucleation and the potential for minimal cooling below the thermodynamic phase transition of the PCM component. Cooling of such a system to induce both crystallisation events may proceed via a heat exchanger, or via the addition of a cooling material such as dry ice or liquid nitrogen.
Determination of the state of crystallisation of a sub-zero PCM is also complicated by their double crystallisation characteristics. As they appear solid after the first crystallisation event, it could be concluded at that point that the material has been fully crystallised and may be used in cooling applications. However, it is known to the inventors that to access the full latent heat of the material both crystallisation events must occur. This leads to issues when using such materials, for example in a heat battery apparatus, where the crystallisation state of the material must be known to determine the cooling potential available (i.e. the state of charge of a heat battery comprising such a PCM). Further to the disclosures herein that one or more nucleation agents may be used to ensure that both crystallisation events occur, it is disclosed that full crystallisation may be determined by various means such as, but not limited to, determination of the amount of free water content (i.e. water not bound in a solid form) and optical means. It is disclosed herein that a sub-zero PCM sample may be optically distinct after its first and second crystallisation events. In
It is disclosed herein that a PCM with a phase change temperature around −30° C. may be formed by the addition of magnesium nitrate to water to produce about a 29.9 wt. % solution. For this purpose, a hydrated form of magnesium nitrate (e.g. magnesium nitrate hexahydrate) may be used. This solution may then be combined with silica and/or alumina in an amount corresponding to more than 0.1 wt. %, or preferably at least about 0.5 wt. % to aid in the nucleation of the first crystallisation event. This solution may also be combined with silicon carbide in an amount corresponding to more than 0.1 wt. %, or preferably at least about 0.5 wt. % to aid in the nucleation of the second crystallisation event. This material may then be used by thermally cycling it across the phase change at around −30° C. Using this nucleation agent system, the first crystallisation event may occur around −30° C., where the second crystallisation event may occur between about −30° C. and about −40° C.
It is disclosed herein that a PCM with a phase change temperature around −26° C. may be formed by the addition of strontium bromide to water to produce about a 41 wt. % solution. For this purpose, a hydrated form of strontium bromide (e.g. strontium bromide hexahydrate) may be used. This solution may then be combined with calcium carbonate in an amount corresponding to more than 0.1 wt. %, or preferably at least about 0.5 wt. % to aid in the nucleation of the first crystallisation event. This solution may also be combined with silver iodide in an amount corresponding to more than 0.1 wt. %, or preferably at least about 0.5 wt. % to aid in the nucleation of the second crystallisation event. This material may then be used by thermally cycling it across the phase change at around −26° C. Using this nucleation agent system, the first and second crystallisation events may occur between about −26° C. and about −32° C.
It is disclosed herein that a PCM with a phase change temperature around −25° C. may be formed by the addition of sodium bromide to water to produce about a 39 wt. % solution. This solution may then be combined with calcium carbonate in an amount corresponding to more than 0.1 wt %, or preferably at least about 0.5 wt. % to aid in the nucleation of the first crystallisation event. This solution may also be combined with silicon carbide in an amount corresponding to more than 0.1 wt. %, or preferably at least about 0.5 wt. % to aid in the nucleation of the second crystallisation event. This material may then be used by thermally cycling it across the phase change at around −25° C. Using this nucleation agent system, the first and second crystallisation events may occur between about −25° C. and about −33° C.
It is disclosed herein that a PCM with a phase change temperature around −22° C. may be formed by the addition of lithium nitrate to water to produce about a 25 wt. % solution. For this purpose, a hydrated form of lithium nitrate (e.g. lithium nitrate trihydrate) may be used. This solution may then be combined with iron oxide in an amount corresponding to more than 0.1 wt. %, or preferably at least about 0.5 wt. % to aid in the nucleation of the first crystallisation event. This solution may also be combined with silicon carbide in an amount corresponding to more than 0.1 wt. %, or preferably at least about 0.5 wt. % to aid in the nucleation of the second crystallisation event. This material may then be used by thermally cycling it across the phase change at around −22° C. Using this nucleation agent system, the first and second crystallisation events may occur between about −22° C. and about −27° C.
It is disclosed herein that a PCM with a phase change temperature around −21° C. may be formed by the addition of sodium chloride to water to produce about a 22 wt. % solution. This solution may then be combined with calcium carbonate in an amount corresponding to more than 0.1 wt. %, or preferably at least about 0.5 wt. % to aid in the nucleation of the first crystallisation event. This solution may also be combined with silicon carbide and/or vermiculite in an amount corresponding to more than 0.1 wt. %, or preferably at least about 0.5 wt. % to aid in the nucleation of the second crystallisation event. This material may then be used by thermally cycling it across the phase change at around −21° C. Using this nucleation agent system, the first and second crystallisation events may occur between about −21° C. and about −28° C.
It is disclosed herein that a PCM with a phase change temperature around −18° C. may be formed by the addition of sodium acetate to water to produce about a 20-30 wt. % solution, preferably about 23 wt. % or 27 wt. % sodium acetate in water. For this purpose, a hydrated form of sodium acetate (e.g. sodium acetate trihydrate) may be used. This solution may then be combined with calcium carbonate in an amount corresponding to more than 0.1 wt. %, or preferably at least about 0.5 wt. % to aid in the nucleation of the first crystallisation event. This solution may also be combined with silver iodide in an amount corresponding to more than 0.1 wt. %, or preferably at least about 0.5 wt. % to aid in the nucleation of the second crystallisation event. This material may then be used by thermally cycling it across the phase change at around −18° C. Using this nucleation agent system, the first and second crystallisation events may occur between about −18° C. and about −30° C.
It is disclosed herein that a PCM with a phase change temperature around −17° C. may be formed by the addition of sodium nitrate to water to produce about a 35 wt. % solution. This solution may then be combined with iron oxide and/or silica in an amount corresponding to more than 0.1 wt. %, or preferably at least about 0.5 wt. % to aid in the nucleation of the first crystallisation event. This solution may also be combined with aluminium oxide and/or vermiculite in an amount corresponding to more than 0.1 wt. %, or preferably at least about 0.5 wt. % to aid in the nucleation of the second crystallisation event. This material may then be used by thermally cycling it across the phase change at around −17° C. Using this nucleation agent system, the first and second crystallisation events may occur between about −17° C. and about −25° C.
It is disclosed herein that a PCM with a phase change temperature around −16° C. may be formed by the addition of strontium chloride to water to produce about a 20 wt. % solution. For this purpose, a hydrated form of strontium chloride (e.g. strontium chloride hexahydrate) may be used. This solution may then be combined with aluminium oxide in an amount corresponding to more than 0.1 wt. %, or preferably at least about 0.5 wt. % to aid in the nucleation of the first crystallisation event. This solution may also be combined with vermiculite and/or silver iodide in an amount corresponding to more than 0.1 wt. %, or preferably at least about 0.5 wt. % to aid in the nucleation of the second crystallisation event. This material may then be used by thermally cycling it across the phase change at around −16° C. Using this nucleation agent system, the first and second crystallisation events may occur between about −16° C. and about −25° C.
It is disclosed herein that a PCM with a phase change temperature around −15° C. may be formed by the addition of sodium formate to water to produce about a 24 wt. % solution. This solution may then be combined with calcium carbonate in an amount corresponding to more than 0.1 wt. %, or preferably at least about 0.5 wt. % to aid in the nucleation of the first crystallisation event. This solution may also be combined with vermiculite in an amount corresponding to more than 0.1 wt. %, or preferably at least about 0.5 wt. % to aid in the nucleation of the second crystallisation event. This material may then be used by thermally cycling it across the phase change at around −15° C. Using this nucleation agent system, the first and second crystallisation events may occur between about −15° C. and about −20° C.
It is disclosed herein that a PCM with a phase change temperature around −14° C. may be formed by the addition of ammonium chloride to water to produce about a 19 wt. % solution. This solution may then be combined with silver iodide in an amount corresponding to more than 0.1 wt. %, or preferably at least about 0.5 wt. % to aid in the nucleation of the first crystallisation event. This solution may also be combined with vermiculite and/or silicon carbide in an amount corresponding to more than 0.1 wt. %, or preferably at least about 0.5 wt. % to aid in the nucleation of the second crystallisation event. This material may then be used by thermally cycling it across the phase change at around −14° C. Using this nucleation agent system, the first and second crystallisation events may occur between about −14° C. and about −20° C.
It is disclosed herein that a PCM with a phase change temperature around −10° C. may be formed by the addition of potassium chloride to water to produce about a 20 wt. % solution. This solution may then be combined with titanium dioxide and/or silver iodide in an amount corresponding to more than 0.1 wt. %, or preferably at least about 0.5 wt. % to aid in the nucleation of the first crystallisation event. This solution may also be combined with vermiculite in an amount corresponding to more than 0.1 wt. %, or preferably at least about 0.5 wt. % to aid in the nucleation of the second crystallisation event. This material may then be used by thermally cycling it across the phase change at around −10° C. Using this nucleation agent system, the first and second crystallisation events may occur between about −10° C. and about −20° C.
It is disclosed herein that a PCM with a phase change temperature around −5° C. may be formed by the addition of magnesium sulfate to water to produce about a 19 wt. % solution. For this purpose, a hydrated form of magnesium sulfate (e.g. magnesium sulfate heptahydrate) may be used. This solution may then be combined with aluminium oxide and/or silver iodide in an amount corresponding to more than 0.1 wt. %, or preferably at least about 0.5 wt. % to aid in the nucleation of the first crystallisation event. This solution may also be combined with silicon carbide and/or calcium carbonate in an amount corresponding to more than 0.1 wt. %, or preferably at least about 0.5 wt. % to aid in the nucleation of the second crystallisation event. This material may then be used by thermally cycling it across the phase change at around −5° C. Using this nucleation agent system, the first and second crystallisation events may occur between about −5° C. and about −10° C.
It is disclosed herein that a PCM with a phase change temperature around −1° C. may be formed by the addition of sodium sulfate to water to produce about a 4 wt. % solution. For this purpose, a hydrated form of sodium sulfate (e.g. sodium sulfate decahydrate) may be used. This solution may then be combined with silver iodide in an amount corresponding to more than 0.1 wt. %, or preferably at least about 0.5 wt. % to aid in the nucleation of the first crystallisation event. This solution may also be combined with silicon carbide in an amount corresponding to more than 0.1 wt. %, or preferably at least about 0.5 wt. % to aid in the nucleation of the second crystallisation event. This material may then be used by thermally cycling it across the phase change at around −5° C. Using this nucleation agent system, the first and second crystallisation events may occur between about −5° C. and about −10° C.
Whilst various exemplary embodiments have been disclosed, it shall be understood that variations, modifications and combinations of the phase change materials disclosed herein disclosed herein may be made without departing from the scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2102248.8 | Mar 2021 | GB | national |
| 2111449.1 | Aug 2021 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2022/050541 | 3/2/2022 | WO |
