A composite phase change material with long-lasting seed crystals and no phase separation, and its preparation method.

Abstract

This invention relates to the field of phase change materials (PCM) technology, and more particularly to a composite PCM with long-lasting seed crystals and no phase separation, and its preparation method. The composite PCM comprises sodium acetate trihydrate, sodium silicate nonahydrate, and a nucleating agent, disodium hydrogen phosphate dodecahydrate; the mass ratio of sodium acetate trihydrate to sodium silicate nonahydrate is 70-80:30-20; the mass fraction of disodium hydrogen phosphate dodecahydrate is 1-3% of the total mass of sodium acetate trihydrate and sodium silicate nonahydrate. Disodium hydrogen phosphate dodecahydrate is partially dissolved in the molten state of the composite PCM, and the undissolved disodium hydrogen phosphate dodecahydrate settles at the bottom of the PCM as a long-lasting seed crystal. The composite PCM of this invention has simple composition and preparation method, and the prepared composite PCM exhibits no phase separation or supercooling, has a long lifespan, and even after 500 cycles, it still shows no supercooling or phase separation, with a latent heat value remaining above 225 J / g, thus possessing high industrial application value.

Description

Technical Field

[0001] This invention relates to the field of phase change materials technology, and in particular to a composite phase change material with long-lasting seed crystals and no phase separation, and its preparation method. Background Technology

[0002] The information disclosed in the background section of this invention is intended only to enhance the understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] Among various energy storage technologies, thermal storage is the most economical form. Thermal storage can be further divided into sensible thermal storage, latent thermal storage, and thermochemical thermal storage. Sensible thermal storage has low energy density, while thermochemical thermal storage is costly and has poor cycle stability. Thermal storage technologies based on latent heat have lower equipment costs, longer lifespans, and simpler structures, making them suitable for diverse applications and more complex thermal storage environments.

[0004] Thermal energy storage technology is central to various energy systems, addressing their intermittency and volatility. Energy storage devices can store industrial waste heat, low-grade waste heat, and renewable energy heat, releasing or transporting it as needed. Latent heat storage devices utilize the phase change enthalpy of phase change materials (PCMs) to store heat and leverage the device's structure to transfer it. Hydrated salts have higher latent heat and thermal conductivity than organic PCMs at the same melting point, making them more suitable for use in thermal energy storage. However, hydrated salts also have several drawbacks, primarily supercooling and phase separation. Supercooling refers to the phenomenon where the PCM solution temperature drops below its melting point before crystallization. Phase separation refers to the stratification that occurs when the chemical bonds in the water of crystallization break during melting (the top layer contains free water, the middle layer is saturated water and salt, and the bottom layer contains anhydrous salt).

[0005] Existing technologies generally employ methods such as adding nucleating agents, thickeners, or applying a perturbation field (ultrasound, vibration, electric field, or microwave) to address the problems of supercooling and phase separation. However, the inventors discovered that when using nucleating agents in thermal storage devices, after the inhomogeneity of the phase change material's complete melting is effectively reduced, the number of nucleation sites gradually decreases, leading to a high degree of randomness in the nucleation effect of the nucleating agent. When using thickeners in thermal storage devices, phase separation still slowly accumulates during long-term melting and solidification cycles. Furthermore, applying a perturbation field requires a large energy consumption and has a relatively complex structure, making it unsuitable for application in thermal storage devices. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a composite phase change material with long-lasting seed crystals and no phase separation, as well as its preparation method. The method involves melting and blending two hydrated salts, and using the common ion effect to suppress the solubility of the nucleating agent to retain long-lasting seed crystals in the phase change material. The resulting composite phase change material exhibits virtually no supercooling or phase separation, has a high latent heat value, and demonstrates good cycling stability.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a composite phase change material with long-lasting seed crystals and no phase separation, comprising sodium acetate trihydrate, sodium silicate nonahydrate, and a nucleating agent; wherein the mass ratio of sodium acetate trihydrate to sodium silicate nonahydrate is 70-80:30-20; wherein the nucleating agent is disodium hydrogen phosphate dodecahydrate, wherein the mass fraction of disodium hydrogen phosphate dodecahydrate is 1-3% of the total mass of sodium acetate trihydrate and sodium silicate nonahydrate, wherein the disodium hydrogen phosphate dodecahydrate is partially dissolved in the molten state of the composite phase change material, and the undissolved portion serves as a long-lasting seed crystal.

[0009] Preferably, the mass ratio of sodium acetate trihydrate to sodium silicate nonahydrate is 80:20, and the mass fraction of disodium hydrogen phosphate dodecahydrate is 2% of the total mass of sodium acetate trihydrate and sodium silicate nonahydrate.

[0010] This invention uses sodium acetate trihydrate and sodium silicate nonahydrate as the main phase change materials to solve the phase separation problem. This is because: firstly, the mixing of the two hydrated salts, sodium acetate trihydrate and sodium silicate nonahydrate, enhances the ionic strength in the solution. The increased ionic strength leads to increased solubility of the anhydrous salt in the solution, thus preventing phase separation; secondly, the composite of the two hydrated salts expands the aquifer region, and the downward expansion of the aquifer makes it easier for the anhydrous salt to contact the free water, which also helps to alleviate the phase separation problem.

[0011] The latent heat of sodium acetate trihydrate is approximately 265 J / g, while that of sodium silicate nonahydrate is around 210 J / g. The latent heat of the composite phase change material falls between these two values. Therefore, to maintain the highest possible latent heat, the amount of sodium acetate trihydrate added to the composite phase change material is higher than that of sodium acetate nonahydrate. When the proportion of sodium acetate trihydrate is too high, phase separation is likely to occur. Therefore, this application limits the mass ratio of sodium acetate trihydrate to sodium silicate nonahydrate to 70-80:30-20, preferably 80:20.

[0012] In the composite phase change material of this invention, sodium acetate trihydrate, sodium silicate nonahydrate, and the nucleating agent disodium hydrogen phosphate dodecahydrate all contain sodium ions. The common ion effect of sodium ions inhibits the solubility of disodium hydrogen phosphate dodecahydrate, and the undissolved disodium hydrogen phosphate dodecahydrate settles at the bottom of the composite phase change material as a long-lasting seed crystal, becoming the most effective nucleation site. The crystallization process of the phase change material needs to begin at the nucleation site; therefore, rapidly forming nucleation sites is key to solving the problem of overcooling. The appropriate addition of disodium hydrogen phosphate dodecahydrate can act as a nucleating agent for sodium acetate trihydrate, and in its solid state, it can also act as a seed crystal for sodium acetate trihydrate. The addition of the nucleating agent alleviates the overcooling phenomenon, and the surface of the long-lasting seed crystal is the most ideal nucleation site; therefore, its existence solves the problems of overcooling and inconsistent degrees of overcooling.

[0013] Seed crystals are generally solid particles of the phase change material itself or other substances with similar lattice parameters to the phase change material. The inventors discovered that disodium hydrogen phosphate dodecahydrate and sodium acetate trihydrate have similar lattices, and disodium hydrogen phosphate dodecahydrate contains sodium ions, whose solubility is suppressed due to the common ion effect. Therefore, disodium hydrogen phosphate dodecahydrate is suitable as a long-lasting seed crystal for the composite phase change material of this invention.

[0014] The present invention limits the mass fraction of the nucleating agent disodium hydrogen phosphate dodecahydrate to 1-3% of the total mass of sodium acetate trihydrate and sodium silicate nonahydrate. This is because when its mass fraction is less than 1%, disodium hydrogen phosphate dodecahydrate will completely dissolve in the phase change material and cannot play the role of a long-lasting seed crystal; while if its mass fraction is too high, it will cause waste of nucleating agent. Therefore, this application limits its mass fraction to 1-3%.

[0015] This invention employs a compound of sodium silicate nonahydrate and sodium acetate trihydrate. The inventors discovered that when these two are compounded, the sum of sodium ions is near the sodium salt precipitation line, thus exhibiting good compatibility while effectively utilizing the common ion effect. Furthermore, sodium silicate nonahydrate has a melting point of around 47°C, making it relatively easy to melt and not highly hygroscopic; its properties are also suitable for use as a phase change thermal storage material. When a hygroscopic hydrated salt is compounded with sodium acetate trihydrate, it will compete for the water of crystallization of sodium acetate trihydrate, causing it to lose its thermal storage capacity. Even when a salt that is inherently difficult to melt (i.e., loses some water of crystallization upon melting to form a hydrated salt with a higher melting point) is compounded with sodium acetate trihydrate, the problem of infusibility still exists.

[0016] Secondly, the present invention provides a method for preparing the above-mentioned composite phase change material with long-lasting seed crystals and no phase separation, comprising the following steps:

[0017] Step S1: Mix sodium acetate trihydrate and sodium silicate nonahydrate at a mass ratio of 70-80:30-20, and heat to melt under stirring to obtain a composite hydrated salt;

[0018] Step S2: Add 1-3 wt% of disodium hydrogen phosphate dodecahydrate to the composite hydrated salt in step S1, and stir continuously until the disodium hydrogen phosphate dodecahydrate no longer dissolves. The undissolved disodium hydrogen phosphate dodecahydrate settles at the bottom and acts as a long-lasting seed crystal, thus obtaining a composite phase change material with a long-lasting seed crystal and no phase separation.

[0019] In step S1, the heating and melting temperature is 65-80℃, preferably 70℃. The stirring speed is 100-300 rpm, and the stirring time is 20-40 min; preferably, the stirring speed is 150-250 rpm, and the stirring time is 25-35 min. Preferably, the heating and melting are carried out using a constant temperature water bath, and the stirring is done using magnetic stirring.

[0020] In step S2, the stirring temperature is 65-75℃, the stirring time is 1-3 hours, and the stirring speed is 100-300 rpm. Preferably, the stirring temperature is 70℃, the stirring time is 1.5-2.5 hours, and the stirring speed is 150-250 rpm.

[0021] Thirdly, the present invention provides the application of the above-mentioned composite phase change material with long-lasting seed crystals and no phase separation in thermal storage or industrial waste heat recovery.

[0022] The present invention has achieved the following beneficial effects:

[0023] This invention uses a mixed hydrated salt of sodium acetate trihydrate and sodium silicate nonahydrate as the main material, disodium hydrogen phosphate dodecahydrate as the nucleating agent, and undissolved disodium hydrogen phosphate dodecahydrate as a long-lasting seed crystal. The composition and preparation method are simple, and the resulting composite phase change material exhibits no phase separation or supercooling, has a long lifespan, and remains free of supercooling and phase separation even after 500 cycles, with a latent heat value maintained above 225 J / g. Furthermore, the composite phase change material has a melting point of approximately 52°C, making it suitable for recovering energy generated in the form of hot water from various energy systems, and possesses high industrial utilization value. Attached Figure Description

[0024] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation thereof. Obviously, those skilled in the art can obtain other drawings based on these drawings without any inventive effort.

[0025] Figure 1 This is a diagram illustrating the phase separation phenomenon in Embodiment 1 of the present invention;

[0026] Figure 2 These are the water content curves of the upper, middle, and lower layers of the phase change materials in Embodiment 2 and Comparative Example 1 of the present invention.

[0027] Figure 3 This is a schematic diagram illustrating the downward expansion principle of the aquifer in Embodiment 2 of the present invention;

[0028] Figure 4 These are the DSC curves of the phase change materials in Embodiment 2 and Comparative Examples 1 and 2 of the present invention;

[0029] Figure 5 This is a diagram illustrating the crystallization process of the composite phase change material in Embodiment 2 of the present invention;

[0030] Figure 6 This is a cooling curve diagram of the composite phase change material of Embodiment 2 of the present invention at the first and 500 cycles.

[0031] Figure 7 These are the XRD patterns of the phase change materials in Embodiment 2 and Comparative Examples 1-3 of the present invention;

[0032] Figure 8 These are SEM images of the phase change material of Comparative Example 1 of this invention;

[0033] Figure 9 This is a SEM image of sodium silicate nonahydrate from Comparative Example 2 of this invention;

[0034] Figure 10 These are SEM images of the composite phase change material of Embodiment 2 of the present invention;

[0035] Figure 11 These are the cooling curves of the phase change materials in comparative examples 1, 3, and 4 of this invention. Detailed Implementation

[0036] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0037] The abbreviations used in this invention have the following meanings: SAT: Sodium acetate trihydrate (CH3COONa·3H2O); SMN: Sodium silicate nonahydrate (Na2SiO3·9H2O); DSP: Disodium hydrogen phosphate dodecahydrate (Na2HPO4·12H2O); DSC: Differential scanning calorimetry; XRD: X-ray diffraction; SEM: Scanning electron microscopy. This invention does not impose any special restrictions on the source of the compounds described; commercially available products well known to those skilled in the art can be used.

[0038] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0039] Example 1

[0040] This embodiment provides the phase separation of composite hydrated salts of sodium acetate trihydrate and sodium silicate nonahydrate at different mass ratios. The specific experimental steps are as follows:

[0041] With a fixed total mass of 40g, sodium acetate trihydrate and sodium silicate nonahydrate were mixed at mass ratios of 0:100, 13:87, 38:62, 50:50, 70:30, 80:20, 85:15, and 90:10 and placed in a beaker equipped with a rotor. The mixture was then placed in a 70℃ magnetically stirred water bath and stirred continuously until completely melted. After stirring was stopped and the mixture was allowed to stand for two hours, it was removed from the water bath and cooled in air until completely solidified. It was then placed back into a 70℃ constant-temperature water bath for further melting, and the presence of phase separation was observed.

[0042] Experimental results are as follows Figure 1 As shown, from Figure 1 As can be seen, the composite hydrated salts with a mass ratio of sodium acetate trihydrate to sodium silicate nonahydrate of 70:30 and 80:20 do not exhibit phase separation, while phase separation occurs in all other mass ratios.

[0043] Example 2

[0044] This embodiment provides the preparation of a composite phase change material with long-lasting seed crystals and no phase separation. In this composite phase change material, the ratio of sodium acetate trihydrate to sodium silicate nonahydrate is 80:20, and the mass fraction of disodium hydrogen phosphate dodecahydrate is 2% of the total mass of sodium acetate trihydrate and sodium silicate nonahydrate. The specific steps are as follows:

[0045] Mix 32g of sodium acetate trihydrate crystals with 8g of sodium silicate nonahydrate crystals and place them in a beaker with a rotor. Then place the beaker in a 70℃ magnetic stirring constant temperature water bath and stir while melting at a speed of 200 rpm for 30 minutes until completely melted.

[0046] After adding 0.8g of disodium hydrogen phosphate dodecahydrate and stirring continuously for 2 hours, some disodium hydrogen phosphate dodecahydrate remained undissolved and settled at the bottom of the beaker, thus obtaining a composite phase change material with long-lasting seed crystals and no phase separation.

[0047] Example 3

[0048] This embodiment provides the preparation of a composite phase change material with long-lasting seed crystals and no phase separation. In this composite phase change material, the ratio of sodium acetate trihydrate to sodium silicate nonahydrate is 70:30, and the mass fraction of disodium hydrogen phosphate dodecahydrate is 1.5% of the total mass of sodium acetate trihydrate and sodium silicate nonahydrate. The specific steps are as follows:

[0049] Mix 28g of sodium acetate trihydrate crystals with 12g of sodium silicate nonahydrate crystals and place them in a beaker with a rotor. Then place the beaker in a 70℃ magnetic stirring constant temperature water bath and stir while melting at a speed of 220 rpm for 25 minutes until completely melted.

[0050] After adding 0.6g of disodium hydrogen phosphate dodecahydrate and stirring continuously for 1.5h, some disodium hydrogen phosphate dodecahydrate remained undissolved and settled at the bottom of the beaker; thus, a composite phase change material with long-lasting seed crystals and no phase separation was obtained.

[0051] Example 4

[0052] This embodiment provides the preparation of a composite phase change material with long-lasting seed crystals and no phase separation. In this composite phase change material, the ratio of sodium acetate trihydrate to sodium silicate nonahydrate is 75:25, and the mass fraction of disodium hydrogen phosphate dodecahydrate is 3% of the total mass of sodium acetate trihydrate and sodium silicate nonahydrate. The specific steps are as follows:

[0053] Mix 30g of sodium acetate trihydrate crystals with 10g of sodium silicate nonahydrate crystals and place them in a beaker with a rotor. Then place the beaker in a 70℃ magnetic stirring constant temperature water bath and stir while melting at a speed of 180 rpm for 30 minutes until completely melted.

[0054] After adding 1.2g of disodium hydrogen phosphate dodecahydrate and stirring continuously for 2.5h, some disodium hydrogen phosphate dodecahydrate remained undissolved and settled at the bottom of the beaker; thus, a composite phase change material with long-lasting seed crystals and no phase separation was obtained.

[0055] Comparative Example 1

[0056] 40g of sodium acetate trihydrate crystals were placed in a beaker with a rotor and then placed in a 70℃ magnetically stirred constant temperature water bath. The mixture was stirred while melting at a speed of 200 rpm for 30 minutes until it was completely melted, thus obtaining a phase change material.

[0057] Comparative Example 2

[0058] 40g of sodium silicate nonahydrate crystals were placed in a beaker with a rotor and then placed in a 70℃ magnetically stirred constant temperature water bath. The mixture was stirred while melting at a speed of 200 rpm for 30 minutes until it was completely melted, thus obtaining a phase change material.

[0059] Comparative Example 3

[0060] 32g of sodium acetate trihydrate crystals and 8g of sodium silicate nonahydrate crystals were mixed and placed in a beaker with a rotor. The mixture was then placed in a 70°C magnetically stirred constant temperature water bath and stirred while melting at a speed of 200 rpm for 30 minutes until completely melted to obtain a composite phase change material.

[0061] Comparative Example 4

[0062] Mix 32g of sodium acetate trihydrate crystals with 8g of sodium silicate nonahydrate crystals and place them in a beaker with a rotor. Then place the beaker in a 70℃ magnetic stirring constant temperature water bath and stir while melting at a speed of 200 rpm for 30 minutes until completely melted.

[0063] After adding 0.3g of disodium hydrogen phosphate dodecahydrate and stirring continuously for 2 hours, the disodium hydrogen phosphate dodecahydrate was completely dissolved, thus obtaining the composite phase change material.

[0064] Test case

[0065] After the phase change materials obtained in Example 2 and Comparative Example 1 were allowed to stand at room temperature for a sufficient period of time, the upper, middle, and lower layers of the phase change materials were extracted and dried. Their moisture content was measured, and plotted as shown in the figure. Figure 2 The water content curve shown is from... Figure 2 As can be seen, after sodium acetate trihydrate and sodium silicate nonahydrate are combined, the water content between the layers tends to be balanced, and the top aquifer extends downwards. The schematic diagram of this principle is shown below. Figure 3 As shown. The phase separation problem disappears after the two hydrated salts are combined for the following two reasons: First, the ionic strength in the solution increases after the two hydrated salts are combined, which leads to an increase in the solubility of the anhydrous salt; second, after the two hydrated salts are combined, the water layer extends downward, increasing the probability of free water coming into contact with the anhydrous salt.

[0066] DSC measurements were performed on the phase change materials of Example 2 and Comparative Examples 1 and 2. Figure 4 The DSC curves of the three phase change materials are shown. Integrating the curve at the concave points yields the corresponding latent heat value. The latent heat value of sodium acetate trihydrate is 260.5 J / g, that of sodium silicate nonahydrate is 210.3 J / g, and that of the composite hydrated salt is between the two at 232.4 J / g. The latent heat value of the composite hydrated salt is slightly lower than that of sodium acetate trihydrate, but still remains at a high level. The phase change material of Example 2 was subjected to 500 solidification-melting cycles, and its latent heat value was measured to be 228.1 J / g, showing only a slight decrease and remaining within an acceptable range. The melting point of the composite phase change material is between that of the two hydrated salt phase change materials: sodium acetate trihydrate has a melting point of 58.4 °C, sodium silicate nonahydrate has a melting point of 46.8 °C, and the composite phase change material has a melting point of 52.3 °C.

[0067] The crystallization process of the composite phase change material in Example 2 was observed, such as... Figure 5As shown. The crystallization process of phase change materials needs to begin at nucleation sites, therefore, rapid formation of nucleation sites is key to solving the problem of supercooling. When the composite phase change material prepared in Example 2 is first melted, disodium hydrogen phosphate heptahydrate precipitates in the solution. If it is directly cooled in air at this point, the nucleation sites will occur on the surface of the disodium hydrogen phosphate heptahydrate. If the disodium hydrogen phosphate heptahydrate is kept at a constant temperature for 5 hours after precipitation, it will dissolve again. If it is then cooled in air, the nucleation sites will occur on the surface of the disodium hydrogen phosphate dodecahydrate at the bottom. The nucleation process expands from bottom to top and from the outside to the inside.

[0068] The cooling curve of the composite phase change material in Example 2 was measured, such as... Figure 6 As shown in the figure, the composite phase change material with long-lasting seed crystals exhibits virtually no supercooling, remaining almost unchanged even after 500 melting and solidification cycles. The supercooling and latent heat values ​​of the composite hydrated salt after 500 melting and solidification cycles demonstrate that this composite phase change material possesses excellent cycle stability. Figure 11 The graphs show the supercooling curves of phase change materials in Comparative Examples 1, 3, and 4 of this invention. As can be seen from the graphs, the supercooling of SAT is about 14°C, and the supercooling of the SAT+SMN composite phase change material is about 11°C. When completely dissolved disodium hydrogen phosphate dodecahydrate is added to the composite material as a nucleating agent, the supercooling is significantly reduced to about 3°C, but it still has a certain degree of supercooling.

[0069] XRD tests were performed on the phase change materials of Examples 2 and Comparative Examples 1-3, such as... Figure 7 As shown in the figure, the peak values ​​before and after recombination correspond one-to-one, proving that the recombination process is only a physical process and no new substances are produced.

[0070] SEM tests were performed on the phase change materials of Comparative Example 1, Comparative Example 2, and Example 2, as follows: Figure 8 , 9 As shown in Figure 10, the overall crystal size of SAT is between 10-50 μm, while that of SMN is between 130-180 μm. The overall crystal size of the composite hydrated salt SAT+SMN is between 5-30 μm, indicating a significant reduction in overall crystal size and a refinement of the crystal lattice. After SAT and SMN are combined, the large SMN crystals break down into fragments and mix with the granular crystals of SAT, forming a smaller crystal structure. The interaction between the two types of water and salt during crystal growth leads to changes in crystal shape and a reduction in size. The reduction in crystal size means a higher nucleation rate and a slower crystal growth rate. Macroscopically, the two composite hydrated salts are more stable and have a longer exothermic crystallization time.

[0071] As can be seen from the above tests, the present invention effectively avoids the problems of supercooling and phase separation by suppressing the solubility of nucleating agents through the combination of two hydrated salts and the common ion effect, exhibiting good cycle stability. In addition, it also retains a high heat storage capacity and has high industrial application value.

[0072] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A composite phase change material with long-lasting seed crystals and no phase separation, characterized in that, It is composed of sodium acetate trihydrate, sodium silicate nonahydrate, and a nucleating agent; the mass ratio of sodium acetate trihydrate to sodium silicate nonahydrate is 70-80:30-20; the nucleating agent is disodium hydrogen phosphate dodecahydrate, the mass fraction of which is 1-3% of the total mass of sodium acetate trihydrate and sodium silicate nonahydrate, and the disodium hydrogen phosphate dodecahydrate is partially dissolved in the molten state of the composite phase change material; the undissolved disodium hydrogen phosphate dodecahydrate settles at the bottom and acts as a long-lasting seed crystal.

2. The composite phase change material as described in claim 1, characterized in that, The mass ratio of sodium acetate trihydrate to sodium silicate nonahydrate is 80:

20.

3. The composite phase change material as described in claim 1, characterized in that, The mass fraction of the sodium hydrogen phosphate dodecahydrate is 2% of the total mass of sodium acetate trihydrate and sodium silicate nonahydrate.

4. A method for preparing a composite phase change material with long-lasting seed crystals and no phase separation as described in any one of claims 1-3, characterized in that, Includes the following steps: Step S1: Mix sodium acetate trihydrate and sodium silicate nonahydrate at a mass ratio of 70-80:30-20, and heat to melt under stirring to obtain a composite hydrated salt; Step S2: Add 1-3 wt% of disodium hydrogen phosphate dodecahydrate to the composite hydrated salt in step S1, and stir continuously until the disodium hydrogen phosphate dodecahydrate no longer dissolves. The undissolved disodium hydrogen phosphate dodecahydrate settles at the bottom and acts as a long-lasting seed crystal, thus obtaining a composite phase change material with a long-lasting seed crystal and no phase separation.

5. The preparation method according to claim 4, characterized in that, In step S1, the heating and melting temperature is 65-80℃.

6. The preparation method according to claim 4, characterized in that, In step S1, the heating and melting temperature is 70°C.

7. The preparation method according to claim 4, characterized in that, In step S1, the stirring speed is 100-300 rpm and the stirring time is 20-40 min.

8. The preparation method according to claim 7, characterized in that, In step S1, the stirring speed is 150-250 rpm and the stirring time is 25-35 min.

9. The preparation method according to claim 4, characterized in that, In step S2, the stirring temperature is 65-75℃, the stirring time is 1-3 hours, and the stirring speed is 100-300 rpm.

10. The preparation method according to claim 9, characterized in that, In step S2, the stirring temperature is 70℃, the stirring time is 1.5-2.5h, and the stirring speed is 150-250rpm.

11. The application of a composite phase change material with long-lasting seed crystals and no phase separation as described in any one of claims 1-3 in thermal storage or industrial waste heat recovery.

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