A heat accumulating energy storage molten salt material, a preparation method and application thereof

Abstract

The application relates to the technical field of heat storage and energy storage, and discloses a heat storage and energy storage molten salt material and a preparation method and application thereof, the method comprises raw material pretreatment, molten salt system preparation, additive modification, vacuum melting treatment, step cooling and heat treatment, and crushing and screening; by adding nano-aluminum oxide and carbon nanotubes as the additives into a ternary basic molten salt mixture, the thermal conductivity and the thermal stability of the molten salt material are improved, meanwhile, the introduction of nucleating agents promotes the uniformity of molten salt crystallization, so that the phase change latent heat and the recycling service life of the material are enhanced, and the reliability of the heat storage and energy storage performance is ensured; the preparation process of vacuum melting treatment combined with step cooling and heat treatment is adopted, bubbles and impurities in the molten salt are removed, the high purity and the compactness of the microstructure of the material are ensured, and by controlling the cooling rate and the heat preservation and ripening process, the crystal structure of the molten salt is optimized, and the thermal stability and the use temperature range of the material are improved.

Description

Technical Field

[0001] This invention relates to the field of thermal energy storage technology, specifically to a thermal energy storage molten salt material, its preparation method, and its application. Background Technology

[0002] Salt refers to liquid salt in a molten state. Molten salt used in engineering usually refers to inorganic salt melt. Molten salt has the characteristics of high boiling point, low viscosity, low vapor pressure and high volumetric heat, making it an excellent heat storage medium. Molten salt energy storage technology utilizes the temperature difference during the heating and cooling process of molten salt to achieve heat energy storage.

[0003] Currently, in the preparation and application of thermal energy storage molten salt materials, the limited thermal conductivity and insufficient thermal stability of the basic molten salt system make it prone to heat capacity decay during long-term high-temperature cycling, affecting energy storage efficiency. At the same time, conventional preparation processes cannot effectively avoid the problems of component segregation and uneven crystal structure generated during the melting and cooling process of molten salt, resulting in a short material cycle life and limiting its application in high-temperature energy storage systems.

[0004] Therefore, a thermal energy storage molten salt material, its preparation method, and its application are proposed to solve the above problems. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a thermal energy storage molten salt material, its preparation method, and its application, solving the problems mentioned in the background technology, such as heat capacity decay affecting energy storage efficiency and short material cycle life, which limit its application in high-temperature energy storage systems.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing a thermal energy storage molten salt material, comprising the following steps: Step 1: Raw material pretreatment. Sodium nitrate, potassium nitrate and sodium nitrite are selected as the basic molten salts. Each component is dried at 80-120℃ for 2-6 hours to remove adsorbed water. Step 2: Preparation of molten salt system. By weight percentage, mix 40-60% sodium nitrate, 20-35% potassium nitrate and 15-30% sodium nitrite to obtain a ternary basic molten salt mixture. Step 3: Additive modification, adding 1-5% nano-alumina and 0.5-3% carbon nanotubes by mass to the ternary basic molten salt mixture, and simultaneously adding 0.1-2% nucleating agent by mass. Step 4: Vacuum melting treatment. Place the mixed material in a vacuum induction furnace and melt it at a vacuum level below 1×10⁻⁶. - 2Under the conditions of Pa, heat to 250-350℃ at a heating rate of 2-5℃ / min, hold at the temperature for melting for 1-3 hours, and mechanically stir at a rate of 100-300 rpm. Step 5: Step cooling and heat treatment. The molten material is slowly cooled to 150-200℃ at a cooling rate of 1-3℃ / min, held at the temperature for 2-4 hours, and then cooled to room temperature in the furnace to obtain blocky heat storage molten salt material. Step 6: Crushing and sieving. The blocky thermal energy storage molten salt material is mechanically crushed and sieved to obtain granular thermal energy storage molten salt material with a particle size distribution of 0.5-3mm.

[0007] Preferably, the nano-alumina in step three has a particle size of 20-50 nm and a specific surface area of ​​100-200 m² / g; The carbon nanotubes are multi-walled carbon nanotubes with an outer diameter of 5-15 nm and a length of 1-10 μm. The nucleating agent is at least one of calcium fluoride, zirconium oxide, or boron nitride.

[0008] Preferably, the nucleating agent is added in the following manner: First, the nucleating agent and a dispersant accounting for 0.5-1% of the mass of the ternary basic molten salt mixture are ultrasonically dispersed in anhydrous ethanol for 30-60 minutes to form a suspension. Then, this suspension is mixed with the ternary basic molten salt mixture, nano-alumina and carbon nanotubes. The dispersant is polyethylene glycol 400 or sodium dodecylbenzenesulfonate.

[0009] Preferably, the vacuum melting process in step four specifically includes: First, evacuate the vacuum induction furnace to a vacuum level of 1×10⁻⁶. -2 The pressure is below Pa, and then an inert protective gas, such as argon or nitrogen, is introduced to a slightly positive pressure state before a programmed temperature increase and melting process.

[0010] Preferably, the stepped cooling and heat treatment process in step five specifically includes: After the molten material is held at 350℃, it is first cooled to 250℃ at 2℃ / min, then cooled to 200℃ at 1℃ / min, and then held at 200℃ for 3 hours to mature. Finally, the power is turned off and the furnace is cooled.

[0011] Preferably, in step six, the crushing is performed using a double roll crusher for primary crushing, followed by fine crushing using an air jet mill. The screening is performed using a standard vibrating screen to control the particle size distribution. By mass, the proportion of particles with a particle size between 0.8 mm and 2 mm is not less than 90%.

[0012] A thermal energy storage molten salt material, the molten salt material being composed of the following components: By weight percentage, sodium nitrate 40-60%, potassium nitrate 20-35%, sodium nitrite 15-30%, nano alumina 1-5%, carbon nanotubes 0.5-3%, nucleating agent 0.1-2%.

[0013] Preferably, the latent heat of phase change of the molten salt material is 160-190 kJ / kg, the operating temperature range is 200-550℃, the thermal conductivity is 0.8-1.2 W / (m·K) in the solid state and 0.6-0.9 W / (m·K) in the liquid state, and the heat capacity decay rate is less than 5% after 1000 melting-solidification cycles.

[0014] An application of a thermal energy storage molten salt material, wherein the molten salt material is encapsulated in an energy storage tank or heat exchanger and used as a thermal storage medium.

[0015] Preferably, in a concentrated solar thermal power generation system, the molten salt material operates at a temperature of 280-550°C and is used to store solar energy and drive a steam turbine to generate electricity. In industrial waste heat recovery systems, it is used to recover waste heat from industrial kiln flue gas at 300-500℃. In power peak shaving and energy storage systems, in conjunction with electric heaters, electrical energy is converted into heat energy and stored during off-peak hours, and released during peak hours.

[0016] Compared with the prior art, the present invention provides a thermal energy storage molten salt material, its preparation method and application, which has the following beneficial effects: 1. In this invention, by adding additives such as nano-alumina and carbon nanotubes to the ternary basic molten salt mixture, the thermal conductivity and thermal stability of the molten salt material are improved. At the same time, the introduction of nucleating agents promotes the uniformity of molten salt crystallization, thereby enhancing the latent heat of phase change and cycle life of the material and ensuring the reliability of thermal energy storage performance.

[0017] 2. In this invention, a preparation process combining vacuum melting treatment with stepped cooling and heat treatment is adopted to remove bubbles and impurities in the molten salt, ensuring the high purity and compactness of the microstructure of the material. Furthermore, by controlling the cooling rate and the heat preservation and curing process, the crystal structure of the molten salt is optimized, thereby improving the thermal stability and operating temperature range of the material.

[0018] 3. In this invention, by optimizing the composition and preparation process of the molten salt material, the molten salt material can be used as a high-efficiency thermal storage medium in concentrated solar thermal power generation, industrial waste heat recovery and power peak-shaving energy storage systems, realizing stable storage and release of thermal energy, broadening application scenarios and improving energy storage efficiency. Detailed Implementation

[0019] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0020] Example 1: A method for preparing a thermal energy storage molten salt material, comprising the following steps: Step 1: Raw material pretreatment. Sodium nitrate, potassium nitrate and sodium nitrite are selected as the basic molten salts. Each component is dried at 80°C for 2 hours to remove adsorbed water. Step 2: Preparation of molten salt system. By weight percentage, mix 45% sodium nitrate, 30% potassium nitrate and 25% sodium nitrite to obtain a ternary basic molten salt mixture. Step 3: Additive modification. Add 1% nano-alumina and 0.5% carbon nanotubes by mass to the ternary basic molten salt mixture, and simultaneously add 0.1% nucleating agent by mass. Step 4: Vacuum melting treatment. Place the mixed material in a vacuum induction furnace and melt it at a vacuum level below 1×10⁻⁶. - 2 Under the condition of Pa, heat to 250°C at a heating rate of 2°C / min, hold at the temperature for 1 hour to melt, and mechanically stir at a rate of 100 rpm. Step 5: Step cooling and heat treatment. The molten material is slowly cooled to 150°C at a cooling rate of 1°C / min, held at the temperature for 2 hours, and then cooled to room temperature in the furnace to obtain blocky heat storage molten salt material. Step 6: Crushing and screening. The blocky thermal energy storage molten salt material is mechanically crushed and screened to obtain granular thermal energy storage molten salt material with a particle size distribution of 0.5 mm.

[0021] The nano-alumina in step three has a particle size of 20 nm and a specific surface area of ​​100 m² / g; The carbon nanotubes are multi-walled carbon nanotubes with an outer diameter of 5 nm and a length of 1 μm. The nucleating agent is calcium fluoride.

[0022] The nucleating agent is added in the following manner: First, the nucleating agent and a dispersant accounting for 0.5% of the mass of the ternary basic molten salt mixture are ultrasonically dispersed in anhydrous ethanol for 30 minutes to form a suspension. Then, this suspension is mixed with the ternary basic molten salt mixture, nano-alumina and carbon nanotubes. The dispersant is polyethylene glycol 400 or sodium dodecylbenzenesulfonate.

[0023] The vacuum melting process in step four is as follows: First, evacuate the vacuum induction furnace to a vacuum level of 1×10⁻⁶. -2 The pressure is below Pa, then an inert protective gas is introduced to a slightly positive pressure state. The inert protective gas is argon or nitrogen, and then a programmed temperature rise melting is performed.

[0024] The stepped cooling and heat treatment process in step five is as follows: After the molten material is held at 350℃, it is first cooled to 250℃ at 2℃ / min, then cooled to 200℃ at 1℃ / min, and then held at 200℃ for 3 hours to mature. Finally, the power is turned off and the furnace is cooled.

[0025] In step six, the crushing process uses a double roll crusher for primary crushing, followed by an air jet mill for fine crushing. The screening process uses a standard vibrating screen to control the particle size distribution. By mass, the proportion of particles with a diameter of 0.8 mm is not less than 90%.

[0026] A thermal energy storage molten salt material, the molten salt material being composed of the following components: By weight percentage, sodium nitrate 45%, potassium nitrate 30%, sodium nitrite 20%, nano-alumina 3%, carbon nanotubes 1.5%, nucleating agent 0.5%.

[0027] The latent heat of phase change of the molten salt material is 160 kJ / kg, the operating temperature is 200℃, the thermal conductivity is 0.8 W / (m·K) in the solid state and 0.6 W / (m·K) in the liquid state, and the heat capacity decay rate is less than 5% after 1000 melting-solidification cycles.

[0028] An application of a thermal energy storage molten salt material, in which the molten salt material is encapsulated in an energy storage tank or heat exchanger and used as a thermal storage medium.

[0029] In a concentrated solar thermal power generation system, the molten salt material operates at a temperature of 280°C and is used to store solar energy and drive a steam turbine to generate electricity. In industrial waste heat recovery systems, it is used to recover waste heat from industrial kiln flue gas at 300°C. In power peak shaving and energy storage systems, in conjunction with electric heaters, electrical energy is converted into heat energy and stored during off-peak hours, and released during peak hours.

[0030] Example 2: A method for preparing a thermal energy storage molten salt material, comprising the following steps: Step 1: Raw material pretreatment. Sodium nitrate, potassium nitrate and sodium nitrite are selected as the basic molten salts. Each component is dried at 100°C for 4 hours to remove adsorbed water. Step 2: Preparation of molten salt system. By weight percentage, mix 50% sodium nitrate, 22% potassium nitrate and 28% sodium nitrite to obtain a ternary basic molten salt mixture. Step 3: Additive modification. Add 3% nano-alumina and 1.7% carbon nanotubes by mass to the ternary basic molten salt mixture, and simultaneously add 1.0% nucleating agent by mass. Step 4: Vacuum melting treatment. Place the mixed material in a vacuum induction furnace and melt it at a vacuum level below 1×10⁻⁶. - 2 Under the condition of Pa, the temperature was heated to 300℃ at a heating rate of 3.5℃ / min, held at the temperature for 2 hours to melt, and mechanically stirred at a rate of 200 rpm. Step 5: Step cooling and heat treatment. The molten material is slowly cooled to 225°C at a cooling rate of 2°C / min, held at the temperature for 3 hours, and then cooled to room temperature in the furnace to obtain blocky heat storage molten salt material. Step 6: Crushing and screening. The blocky thermal energy storage molten salt material is mechanically crushed and screened to obtain granular thermal energy storage molten salt material with a particle size distribution of 1.7mm.

[0031] The nano-alumina in step three has a particle size of 35 nm and a specific surface area of ​​150 m² / g; The carbon nanotubes are multi-walled carbon nanotubes with an outer diameter of 10 nm and a length of 5 μm. The nucleating agent is calcium fluoride.

[0032] The nucleating agent is added in the following manner: First, the nucleating agent and a dispersant accounting for 0.75% of the mass of the ternary basic molten salt mixture are ultrasonically dispersed in anhydrous ethanol for 45 minutes to form a suspension. Then, this suspension is mixed with the ternary basic molten salt mixture, nano-alumina and carbon nanotubes. The dispersant is polyethylene glycol 400 or sodium dodecylbenzenesulfonate.

[0033] The vacuum melting process in step four is as follows: First, evacuate the vacuum induction furnace to a vacuum level of 1×10⁻⁶. -2 The pressure is below Pa, then an inert protective gas is introduced to a slightly positive pressure state. The inert protective gas is argon or nitrogen, and then a programmed temperature rise melting is performed.

[0034] The stepped cooling and heat treatment process in step five is as follows: After the molten material is held at 350℃, it is first cooled to 250℃ at 2℃ / min, then cooled to 200℃ at 1℃ / min, and then held at 200℃ for 3 hours to mature. Finally, the power is turned off and the furnace is cooled.

[0035] In step six, the crushing process uses a double roll crusher for primary crushing, followed by an air jet mill for fine crushing. The screening process uses a standard vibrating screen to control the particle size distribution. By mass, the proportion of particles with a diameter of 1.4 mm is not less than 90%.

[0036] A thermal energy storage molten salt material, the molten salt material being composed of the following components: By weight percentage, sodium nitrate 50%, potassium nitrate 28%, sodium nitrite 18%, nano alumina 2.5%, carbon nanotubes 1.0%, nucleating agent 0.5%.

[0037] The latent heat of phase change of the molten salt material is 175 kJ / kg, the operating temperature range is 325℃, the thermal conductivity is 1.0 W / (m·K) in the solid state and 0.75 W / (m·K) in the liquid state, and the heat capacity decay rate is less than 5% after 1000 melting-solidification cycles.

[0038] An application of a thermal energy storage molten salt material, in which the molten salt material is encapsulated in an energy storage tank or heat exchanger and used as a thermal storage medium.

[0039] In a concentrated solar thermal power generation system, the molten salt material operates at a temperature of 390°C and is used to store solar energy and drive a steam turbine to generate electricity. In industrial waste heat recovery systems, it is used to recover waste heat from industrial kiln flue gas at 400℃. In power peak shaving and energy storage systems, in conjunction with electric heaters, electrical energy is converted into heat energy and stored during off-peak hours, and released during peak hours.

[0040] Example 3: A method for preparing a thermal energy storage molten salt material, comprising the following steps: Step 1: Raw material pretreatment. Sodium nitrate, potassium nitrate and sodium nitrite are selected as the basic molten salts. Each component is dried at 120°C for 6 hours to remove adsorbed water. Step 2: Preparation of molten salt system. By weight percentage, mix 55% sodium nitrate, 25% potassium nitrate and 20% sodium nitrite to obtain a ternary basic molten salt mixture. Step 3: Additive modification. Add 5% nano-alumina and 3% carbon nanotubes by mass to the ternary basic molten salt mixture, and simultaneously add 2% nucleating agent by mass. Step 4: Vacuum melting treatment. Place the mixed material in a vacuum induction furnace and melt it at a vacuum level below 1×10⁻⁶. - 2 Under the condition of Pa, heat to 350°C at a heating rate of 5°C / min, hold at the temperature for 3 hours to melt, and mechanically stir at a rate of 300 rpm. Step 5: Step cooling and heat treatment. The molten material is slowly cooled to 200°C at a cooling rate of 3°C / min, held at the temperature for 4 hours, and then cooled to room temperature in the furnace to obtain blocky heat storage molten salt material. Step 6: Crushing and screening. The blocky thermal energy storage molten salt material is mechanically crushed and screened to obtain granular thermal energy storage molten salt material with a particle size distribution of 3mm.

[0041] The nano-alumina in step three has a particle size of 50 nm and a specific surface area of ​​200 m² / g; The carbon nanotubes are multi-walled carbon nanotubes with an outer diameter of 15 nm and a length of 10 μm. The nucleating agent is calcium fluoride.

[0042] The nucleating agent is added in the following manner: First, the nucleating agent and a dispersant accounting for 1% of the mass of the ternary basic molten salt mixture are ultrasonically dispersed in anhydrous ethanol for 60 minutes to form a suspension. Then, this suspension is mixed with the ternary basic molten salt mixture, nano-alumina and carbon nanotubes. The dispersant is polyethylene glycol 400 or sodium dodecylbenzenesulfonate.

[0043] The vacuum melting process in step four is as follows: First, evacuate the vacuum induction furnace to a vacuum level of 1×10⁻⁶. -2 The pressure is below Pa, then an inert protective gas is introduced to a slightly positive pressure state. The inert protective gas is argon or nitrogen, and then a programmed temperature rise melting is performed.

[0044] The stepped cooling and heat treatment process in step five is as follows: After the molten material is held at 350℃, it is first cooled to 250℃ at 2℃ / min, then cooled to 200℃ at 1℃ / min, and then held at 200℃ for 3 hours to mature. Finally, the power is turned off and the furnace is cooled.

[0045] In step six, the crushing process uses a double roll crusher for primary crushing, followed by an air jet mill for fine crushing. The screening process uses a standard vibrating screen to control the particle size distribution. By mass, the proportion of particles with a diameter of 2 mm is not less than 90%.

[0046] A thermal energy storage molten salt material, the molten salt material being composed of the following components: By weight percentage, sodium nitrate 55%, potassium nitrate 25%, sodium nitrite 16%, nano-alumina 2.0%, carbon nanotubes 1.2%, nucleating agent 0.8%.

[0047] The latent heat of phase change of the molten salt material is 190 kJ / kg, the operating temperature is 550℃, the thermal conductivity is 1.2 W / (m·K) in the solid state and 0.9 W / (m·K) in the liquid state, and the heat capacity decay rate is less than 5% after 1000 melting-solidification cycles.

[0048] An application of a thermal energy storage molten salt material, in which the molten salt material is encapsulated in an energy storage tank or heat exchanger and used as a thermal storage medium.

[0049] In a concentrated solar thermal power generation system, the molten salt material operates at a temperature of 550°C and is used to store solar energy and drive a steam turbine to generate electricity. In industrial waste heat recovery systems, it is used to recover waste heat from industrial kiln flue gas at 500℃. In power peak shaving and energy storage systems, in conjunction with electric heaters, electrical energy is converted into heat energy and stored during off-peak hours, and released during peak hours.

[0050] Comparative Example 1: The difference between this comparative example and Example 1 is that no nano-alumina was added during the preparation of this comparative example.

[0051] Comparative Example 2: The difference between this comparative example and Example 1 is that no carbon nanotubes were added during the preparation of this comparative example.

[0052] Comparative Example 3 differs from Example 1 in that no nucleating agent was added during the preparation of this comparative example.

[0053] Comparative Example 4 differs from Example 1 in that it was not subjected to vacuum melting but rather to melting under normal pressure.

[0054] The thermal energy storage molten salt materials prepared in Examples 1-3 and Comparative Examples 1-4 were subjected to performance tests. The test items and test methods are as follows: The latent heat of phase change was tested using a differential scanning calorimeter (DSC) under a nitrogen atmosphere, with the temperature increased from room temperature to 600°C at a rate of 10°C / min. The enthalpy of fusion was measured, and the latent heat of phase change was calculated. Thermal conductivity was tested using a hot-wire thermal conductivity meter, measuring thermal conductivity at 200°C in the solid state and 500°C in the liquid state. Thermal cycling stability test: Molten salt material was encapsulated in a crucible and subjected to 1000 melt-solidification cycles in a temperature range of 200-550℃. Each cycle included heating to 550℃ and holding for 10 minutes, then cooling to 200℃. After the cycle, the change in heat capacity was measured using DSC and the decay rate was calculated. Temperature range testing was used to determine the melting point and decomposition temperature of the molten salt material through thermogravimetric analysis (TGA) and differential thermal analysis (DTA), thereby determining the operating temperature range.

[0055] The test data of the thermal energy storage molten salt materials prepared in Examples 1-3 and Comparative Examples 1-4 are recorded in the table below:

[0056] By comparing and analyzing the data in the table, it can be seen that the thermal energy storage molten salt materials prepared using the processes in Examples 1-3 have significantly superior performance compared to the molten salt materials prepared using the processes in Comparative Examples 1-4. This indicates that the present invention improves the thermal conductivity and thermal stability of the molten salt material by adding additives such as nano-alumina and carbon nanotubes to the ternary basic molten salt mixture. Simultaneously, the introduction of nucleating agents promotes the uniformity of molten salt crystallization, thereby enhancing the latent heat of phase change and cycle life of the material, ensuring the reliability of its thermal energy storage performance. The preparation process using vacuum melting combined with stepped cooling and heat treatment removes bubbles and impurities from the molten salt, ensuring high purity and dense microstructure. Furthermore, by controlling the cooling rate and the heat preservation and ripening process, the crystal structure of the molten salt is optimized, improving the material's thermal stability and operating temperature range. By optimizing the composition and preparation process of the molten salt material, it can be used as a highly efficient thermal storage medium in concentrated solar thermal power generation, industrial waste heat recovery, and power peak-shaving energy storage systems, achieving stable storage and release of thermal energy, broadening application scenarios, and improving energy storage efficiency.

[0057] By comparing and analyzing the relevant data in the table, it can be seen that the thermal energy storage molten salt material prepared by the preparation process of the present invention has high latent heat of phase change, excellent thermal conductivity and good thermal cycling stability.

[0058] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0059] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a thermal energy storage molten salt material, characterized in that: Includes the following steps: Step 1: Raw material pretreatment. Sodium nitrate, potassium nitrate and sodium nitrite are selected as the basic molten salts. Each component is dried at 80-120℃ for 2-6 hours to remove adsorbed water. Step 2: Preparation of molten salt system. By weight percentage, mix 40-60% sodium nitrate, 20-35% potassium nitrate and 15-30% sodium nitrite to obtain a ternary basic molten salt mixture. Step 3: Additive modification, adding 1-5% nano-alumina and 0.5-3% carbon nanotubes by mass to the ternary basic molten salt mixture, and simultaneously adding 0.1-2% nucleating agent by mass. Step 4: Vacuum melting treatment. Place the mixed material in a vacuum induction furnace and melt it at a vacuum level below 1×10⁻⁶. -2 Under the conditions of Pa, heat to 250-350℃ at a heating rate of 2-5℃ / min, hold at the temperature for melting for 1-3 hours, and mechanically stir at a rate of 100-300 rpm. Step 5: Step cooling and heat treatment. The molten material is slowly cooled to 150-200℃ at a cooling rate of 1-3℃ / min, held at the temperature for 2-4 hours, and then cooled to room temperature in the furnace to obtain blocky heat storage molten salt material. Step 6: Crushing and sieving. The blocky thermal energy storage molten salt material is mechanically crushed and sieved to obtain granular thermal energy storage molten salt material with a particle size distribution of 0.5-3mm.

2. The method for preparing a thermal energy storage molten salt material according to claim 1, characterized in that: The nano-alumina in step three has a particle size of 20-50 nm and a specific surface area of ​​100-200 m² / g; The carbon nanotubes are multi-walled carbon nanotubes with an outer diameter of 5-15 nm and a length of 1-10 μm. The nucleating agent is at least one of calcium fluoride, zirconium oxide, or boron nitride.

3. The method for preparing a thermal energy storage molten salt material according to claim 1, characterized in that: The nucleating agent is added in the following manner: First, the nucleating agent and a dispersant accounting for 0.5-1% of the mass of the ternary basic molten salt mixture are ultrasonically dispersed in anhydrous ethanol for 30-60 minutes to form a suspension. Then, this suspension is mixed with the ternary basic molten salt mixture, nano-alumina and carbon nanotubes. The dispersant is polyethylene glycol 400 or sodium dodecylbenzenesulfonate.

4. The method for preparing a thermal energy storage molten salt material according to claim 1, characterized in that: The vacuum melting process in step four is specifically as follows: First, evacuate the vacuum induction furnace to a vacuum level of 1×10⁻⁶. -2 The pressure is below Pa, and then an inert protective gas, such as argon or nitrogen, is introduced to a slightly positive pressure state before a programmed temperature increase and melting process.

5. The method for preparing a thermal energy storage molten salt material according to claim 1, characterized in that: The stepped cooling and heat treatment process in step five is specifically as follows: After the molten material is held at 350℃, it is first cooled to 250℃ at 2℃ / min, then cooled to 200℃ at 1℃ / min, and then held at 200℃ for 3 hours to mature. Finally, the power is turned off and the furnace is cooled.

6. The method for preparing a thermal energy storage molten salt material according to claim 1, characterized in that: In step six, the crushing process uses a double roll crusher for primary crushing, followed by an air jet mill for fine crushing. The screening is performed using a standard vibrating screen to control the particle size distribution. By mass, the proportion of particles with a diameter between 0.8 mm and 2 mm is not less than 90%.

7. A thermal energy storage molten salt material, prepared by the preparation method of a thermal energy storage molten salt material according to any one of claims 1-6, characterized in that: This molten salt material is composed of the following components: By weight percentage, sodium nitrate 40-60%, potassium nitrate 20-35%, sodium nitrite 15-30%, nano alumina 1-5%, carbon nanotubes 0.5-3%, nucleating agent 0.1-2%.

8. The thermal energy storage molten salt material according to claim 7, characterized in that: The latent heat of phase change of the molten salt material is 160-190 kJ / kg, the operating temperature range is 200-550℃, the thermal conductivity is 0.8-1.2 W / (m·K) in the solid state and 0.6-0.9 W / (m·K) in the liquid state, and the heat capacity decay rate is less than 5% after 1000 melting-solidification cycles.

9. An application of a thermal energy storage molten salt material, as described in any one of claims 7-8, in a concentrated solar thermal power generation system, an industrial waste heat recovery system, or a power peak-shaving energy storage system, characterized in that... The molten salt material is encapsulated in an energy storage tank or heat exchanger and used as a heat storage medium.

10. The application of the thermal energy storage molten salt material according to claim 9, characterized in that: In a concentrated solar thermal power generation system, the molten salt material operates at a temperature of 280-550°C and is used to store solar energy and drive a steam turbine to generate electricity. In industrial waste heat recovery systems, it is used to recover waste heat from industrial kiln flue gas at 300-500℃. In power peak shaving and energy storage systems, in conjunction with electric heaters, electrical energy is converted into heat energy and stored during off-peak hours, and released during peak hours.

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