Preparation method of medium and low temperature fused salt heat storage material with high thermal conductivity and high stability

Abstract

The invention provides a preparation method of a high-thermal-conductivity high-stability medium-low-temperature fused salt heat storage material, and belongs to the technical field of phase change heat storage materials.The preparation method comprises the steps of vacuum impregnation, centrifugal treatment and resin dip-coating, the preparation method comprises the following steps: preparing a framework, removing fused salt on the surface layer of the framework through a surface heating melting / centrifuging combined process in centrifugal treatment, coating a framework / fused salt compound with composite resin, embedding the composite resin into the framework, forming a continuous heat conduction path with the framework, and keeping low thermal resistance between a resin shell and the internal framework / fused salt compound; the interface bonding strength is better, the resin layer is prevented from being separated from the porous framework when the phase change material is heated to expand, the resin on the surface can prevent molten salt from leaking, volatilizing and absorbing moisture, and the stability of the heat storage material is improved; the material has the characteristic of high thermal conductivity while maintaining high stability.

Description

Technical Field

[0001] This invention belongs to the field of phase change thermal storage materials technology. Background Technology

[0002] Molten salt phase change thermal energy storage materials are core media in fields such as concentrated solar power generation and industrial waste heat utilization. These materials are composed of inorganic salt mixtures and have advantages such as high phase change enthalpy, high thermal stability, and low cost. However, their susceptibility to leakage, volatilization, low thermal conductivity, and strong corrosion limits their applications. Common encapsulation methods include porous media adsorption encapsulation and spherical shell encapsulation. Porous media encapsulation can effectively improve thermal conductivity but cannot effectively suppress leakage and volatilization; spherical shell encapsulation has a pure molten salt interior with low thermal conductivity, and the filling port requires welding, making the process complex. Therefore, there is an urgent need to develop a composite thermal energy storage material with simple processing, high thermal conductivity, and high stability. Summary of the Invention

[0003] To address the aforementioned problems, this invention proposes a method for preparing high thermal conductivity and high stability low-temperature molten salt thermal storage materials. The specific technical solution adopted is as follows: A method for preparing a high thermal conductivity and high stability medium-low temperature molten salt thermal storage material, the specific steps of which are as follows: 1) Vacuum impregnation: The phase change material is heated to melt in a vacuum furnace, and the porous framework is immersed in the molten phase change material. Adsorption is carried out in a vacuum environment of -0.1MPa to -0.7MPa for more than 30 minutes until the internal pores of the porous framework are completely filled with the phase change material. After cooling to solidify the phase change material, it is removed to obtain a framework / molten salt composite. The phase change material has a melting point higher than 90℃ and a maximum temperature under thermal storage conditions lower than 260℃. The porous framework is selected from porous metals, foamed carbon, and porous ceramics. 2) Centrifugation: The centrifugation device consists of a centrifugal shaft 1, an oven 2, and a sample rack 3. The sample rack 3 is placed inside the oven 2 and is fixedly installed at the lower end of the centrifugal shaft 1. The centrifugal shaft 1 drives the sample rack 3 to rotate around the centrifugal shaft 1, fixing the skeleton / molten salt composite onto the sample rack 3. The oven 2 is turned on, and the internal temperature of the oven 2 is heated to 5-10°C above the melting point of the phase change material. The centrifugal shaft 1 is then rotated immediately for centrifugation. The processing time is 0.5-3 minutes, removing the molten salt from the surface of the skeleton to the inward depth x, where x is 0.3-1 times the average pore size of the porous skeleton. 3) Resin impregnation: The centrifuged skeleton / molten salt composite is immersed in a composite resin solution. After removal, the composite resin is uniformly coated on the surface of the skeleton / molten salt composite with a thickness of 1-10 mm. After curing and shaping, a high thermal conductivity and high stability medium-low temperature molten salt thermal storage material is obtained. The composite resin solution is composed of a resin matrix, thermally conductive filler and coupling agent solution. The resin matrix is ​​a thermosetting resin, preferably one of phenolic resin and furfural resin.

[0004] Preferably, the thermally conductive filler is selected from one of carbon powder, carbon fiber, graphene, and silicon carbide fiber.

[0005] Preferably, the phase change material is selected from one of the following: LiNO3–NaNO3–KNO3, LiNO3–KNO3, and KNO3–NaNO3 multi-element salt systems.

[0006] Further preferred, the mass ratio of LiNO3, NaNO3, and KNO3 in the LiNO3–NaNO3–KNO3 multi-element salt system is 20~30:15~20:50~55.

[0007] Preferably, the porosity of the porous skeleton is 70% to 90%; the pore size is preferably 2 to 5 mm.

[0008] The beneficial effects of this invention are: 1. The composite resin coating of the skeleton / molten salt composite can prevent molten salt leakage, volatilization and moisture absorption, and improve the stability of the thermal storage material.

[0009] 2. The molten salt on the surface of the skeleton is removed by a combined process of surface heating melting and centrifugation. The composite resin can be embedded in the skeleton to form a continuous thermal conduction path with the skeleton. The resin shell and the internal skeleton / molten salt composite maintain low thermal resistance and have better interfacial bonding strength, which prevents the resin layer from separating from the porous skeleton when the phase change material expands due to heat.

[0010] 3. The impregnation coating process is used to prepare the leak-proof volatile shell. Compared with the molding process, the process is simple and easy to mass-produce. Attached Figure Description

[0011] Figure 1 This is a schematic diagram of the material preparation process of the present invention; Figure 2 Heated centrifuge device; Figure 3 The following are photographs of the thermal storage material obtained in the embodiments of the present invention: a) porous framework; b) framework molten salt composite; c) surface of the composite after centrifugation; d) thermal storage material. Detailed Implementation

[0012] The technical solution of the present invention will be further explained and described below with reference to specific embodiments. The following embodiments are only preferred embodiments of the present invention. Example

[0013] 1) Mix LiNO3, NaNO3, and KNO3 in a mass ratio of 29:18:53 to prepare a ternary nitrate. Heat the mixture to melt in a vacuum furnace. Place carbon foam (3 mm pore size, 85% porosity) into the molten salt and adsorb it for 30 minutes in a vacuum environment of -0.2 MPa. After cooling to solidify the phase change material, remove the mixture to obtain a framework / molten salt composite.

[0014] 2) Fix the skeleton / molten salt complex on the centrifuge sample holder, turn on the heating to 10°C above the melting point, and immediately centrifuge for 1 minute to remove the molten salt from the area between the skeleton surface and 2 mm inward.

[0015] 3) Immerse the centrifuged skeleton / molten salt composite in composite resin, then remove it and coat the surface of the skeleton / molten salt composite evenly with a thickness of 4 mm. Allow it to cure at 80℃ for 2 hours to set, thus obtaining a high thermal conductivity, high stability, and low-temperature molten salt thermal storage material. The preparation method of the composite resin is as follows: a. After treating short carbon fibers with a length of 3-10 mm at high temperature (400℃) in air for 1 hour, they are immersed in an ethanol solution (1% concentration) of coupling agent KH-550 for 10 hours and then dried at 50℃. b. Mix the treated short carbon fibers with phenolic resin at a mass ratio of 15:85, add 30% of the total mass of anhydrous ethanol, and then mechanically stir at 1500 r / h for 30 minutes at room temperature. c. Finally, the composite resin is obtained by vacuum degassing and drying at 40°C in a vacuum oven to remove anhydrous ethanol.

[0016] The performance of the thermal storage material obtained in this embodiment was tested and found to be as follows: equivalent thermal conductivity 2.1 W / m·K, phase change enthalpy 207 J / g, no structural change observed after 1000 thermal cycles, mass loss rate 2.4%, phase change enthalpy decay rate 3.3%, and thermal conductivity decay rate 2.9%.

[0017] Comparative Example 1: LiNO3, NaNO3, and KNO3 were mixed evenly in a mass ratio of 29:18:53 to prepare a ternary nitrate. The mixture was heated to melt in a vacuum furnace. Foamed carbon (3 mm pore size, 85% porosity) was placed into the molten salt and adsorbed in a vacuum environment of -0.2 MPa for 30 minutes. After cooling to solidify the phase change material, the material was removed and the excess molten salt on the surface was scraped off to obtain the composite thermal storage material.

[0018] The performance of the high-temperature composite thermal storage material obtained by the comparative test is as follows: equivalent thermal conductivity 2.3 W / m·K, phase change enthalpy 207 J / g. After 1000 cycles of heating and cooling, the molten salt is significantly reduced, the mass loss rate is 14.4%, the phase change enthalpy decay rate is 15.8%, and the thermal conductivity decays by 7.9%.

[0019] Comparative Example 2: 1) Mix LiNO3, NaNO3, and KNO3 in a mass ratio of 29:18:53 to prepare a ternary nitrate. Heat the mixture to melt in a vacuum furnace. Place carbon foam (3 mm pore size, 85% porosity) into the molten salt and adsorb it for 30 minutes in a vacuum environment of -0.2 MPa. After cooling to solidify the phase change material, remove the mixture to obtain a framework / molten salt composite.

[0020] 2) The skeleton / molten salt composite was immersed in the composite resin. After removal, the skeleton / molten salt composite was uniformly coated on the surface to a thickness of 4 mm. It was then cured at 80°C for 2 hours to set, thus obtaining a composite heat storage material with a resin shell. The preparation method of the composite resin in this comparative example is the same as that in the example.

[0021] The performance of the thermal storage material obtained in this embodiment was tested and found to be as follows: equivalent thermal conductivity 1.8 W / m·K, phase change enthalpy 207 J / g, mass loss rate 2.9% after 1000 thermal cycles, phase change enthalpy decay rate 3.7%, and thermal conductivity decay rate 5.8%.

[0022] Comparative Example 3: 1) Mix LiNO3, NaNO3, and KNO3 in a mass ratio of 29:18:53 to prepare a ternary nitrate. Heat the mixture to melt in a vacuum furnace. Place carbon foam (3 mm pore size, 85% porosity) into the molten salt and adsorb it for 30 minutes in a vacuum environment of -0.2 MPa. After cooling to solidify the phase change material, remove the mixture to obtain a framework / molten salt composite.

[0023] 2) Fix the skeleton / molten salt complex on the centrifuge sample holder, turn on the heating to 10°C above the melting point, and immediately centrifuge for 80 seconds to remove the molten salt from the area between the skeleton surface and 3 mm inward.

[0024] 3) The centrifuged skeleton / molten salt composite was immersed in the composite resin. After removal, the composite resin was uniformly coated on the surface of the skeleton / molten salt composite with a thickness of 4 mm. It was then cured at 80°C for 2 hours to set, thus obtaining a high thermal conductivity and high stability medium-low temperature molten salt thermal storage material. The preparation method of the composite resin in this comparative example is the same as that in the example.

[0025] The performance of the thermal storage material obtained in this embodiment was tested and found to be as follows: equivalent thermal conductivity 2.1 W / m·K, phase change enthalpy 191 J / g, mass loss rate 2.4% after 1000 thermal cycles, phase change enthalpy decay rate 3.3%, and thermal conductivity decay rate 2.9%.

[0026] Effect verification: Table 1 shows a performance comparison between Comparative Examples 1-3 and the Examples.

[0027] Table 1 It can be observed that when x is less than 0.3 times the pore size (Comparative Example 2), the resin shell and the porous carbon skeleton fail to fit together effectively, resulting in high interfacial thermal resistance. When x is greater than 1 times the pore size (Comparative Example 3), excessive resin intrusion into the porous carbon region reduces the amount of molten salt loaded, leading to an excessively low phase transition enthalpy.

Claims

1. A method for preparing a high-thermal-conductivity high-stability medium-low temperature molten salt heat storage material, characterized in that, The specific steps of the method are as follows: 1) Vacuum impregnation: heat the phase change material to melting in a vacuum furnace, immerse the porous framework in the molten phase change material, adsorb in the environment of-0.1MPa to-0.7MPa for more than 30 minutes, until the internal space of the porous framework is completely filled with the phase change material, and then take out after cooling to the solidification of the phase change material, to obtain a framework / molten salt composite; the melting point of the phase change material is higher than 90℃, and the highest temperature of the heat storage working condition is lower than 260℃, and the porous framework is selected from one of porous metal, foam carbon and porous ceramic; 2) Centrifugal treatment: the centrifugal treatment device is composed of a centrifugal shaft (1), an oven (2) and a sample holder (3), the sample holder (3) is arranged in the oven (2), the sample holder (3) is fixedly installed at the lower end of the centrifugal shaft (1), the sample holder (3) is driven to rotate around the centrifugal shaft (1) through the centrifugal shaft (1), the framework / molten salt composite is fixed on the sample holder (3), the oven (2) is started, the temperature inside the oven (2) is heated to be higher than the melting point of the phase change material by 5-10℃, then the centrifugal shaft (1) is immediately rotated for centrifugal treatment, the treatment time is 0.5-3 minutes, and the molten salt in the region between the surface of the framework and the inward depth x is removed, wherein x is 0.3-1 times the average pore diameter of the porous framework; 3) Resin dipping: immerse the framework / molten salt composite after centrifugal treatment in a composite resin solution, take out, and evenly coat the surface of the framework / molten salt composite with the composite resin with a thickness of 1-10mm, so that after curing and shaping, a high-thermal-conductivity high-stability medium-low-temperature molten salt heat storage material is obtained; the composite resin solution is composed of a resin matrix, a thermal conductive filler and a coupling agent solution, and the resin matrix is a thermosetting resin.

2. The method for preparing high thermal conductivity and high stability low-temperature molten salt thermal storage material according to claim 1, characterized in that, The thermosetting resin is a phenolic resin or a furfural resin.

3. The method for preparing high thermal conductivity and high stability low-temperature molten salt thermal storage material according to claim 1, characterized in that, The thermal conductive filler is selected from one of carbon powder, carbon fiber, graphene and silicon carbide fiber.

4. The method for preparing high thermal conductivity and high stability low-temperature molten salt thermal storage material according to claim 1, characterized in that, The phase change material is selected from one of LiNO3-NaNO3-KNO3, LiNO3-KNO3 and KNO3-NaNO3 multi-component salt systems.

5. The method for preparing high thermal conductivity and high stability low-temperature molten salt thermal storage material according to claim 4, characterized in that, The mass ratio of LiNO3, NaNO3 and KNO3 in the LiNO3-NaNO3-KNO3 multi-component salt system is (20-30):(15-20):(50-55).

6. The method for preparing high thermal conductivity and high stability low-temperature molten salt thermal storage material according to claim 1, characterized in that, The porosity of the porous framework is 70%-90%, and the pore diameter is 2-5mm.

7. A high-thermal-conductivity high-stability medium-low-temperature molten salt heat storage material prepared by the method according to any one of claims 1-6.

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