A low-melting-point binary molten salt heat transfer and heat storage working medium, its preparation method and application

Abstract

The present invention relates to a low-melting-point binary molten salt heat transfer and heat storage working medium, a preparation method and an application thereof, belonging to the technical field of heat storage and heat transfer, and is prepared by the following steps: preparing a composite molten salt of sodium nitrate and calcium nitrate; grafting graphene oxide on the surface of Al2O3, and then in-situ reducing and depositing metal particles on the surface of graphene oxide to obtain porous rGO-metal / Al2O3 nanoparticles with Al2O3 as the core and graphene oxide-metal composite material as the shell; dispersing the porous rGO-metal / Al2O3 nanoparticles in the composite molten salt of sodium nitrate and calcium nitrate to obtain a molten salt nanofluid; and then using the molten salt nanofluid as the core and a ceramic material as the shell to obtain the product. In the technical solution of the present invention, the ceramic material has good thermal properties. After introducing the ceramic material, the heat storage of the molten salt phase change material has better performance, which can extend the working life of the molten salt phase change material, improve the thermal properties and extend the working temperature.

Description

Technical Field

[0001] The present invention belongs to the technical field of heat storage and transfer, and specifically relates to a low-melting-point binary molten salt heat transfer and storage working medium, a preparation method thereof, and an application thereof. Background Art

[0002] Solar energy is the most abundant renewable clean energy on the earth and is also one of the best choices to solve the energy crisis. The solar thermal utilization system is the main way to utilize solar energy. Due to the intermittency of solar energy, a heat storage system is indispensable. The heat storage material is the core of the heat storage system, which is generally divided into sensible heat storage materials, latent heat storage materials by phase change, and chemical heat storage materials. It is difficult for a single material to simultaneously obtain an ideal phase change temperature and heat storage performance. Therefore, a carrier material is often used to prepare a composite phase change material, which can maintain its solid shape and has no fluidity. The thermal properties of different composite PCMs are one of the research hotspots in the field of solar thermal utilization. Composite phase change materials, such as molten salt-graphite composites, molten salt-metal composites, and molten salt-ceramic composites, have better comprehensive thermal properties, such as increasing the phase change temperature, higher thermal conductivity, excellent heat storage performance, and thermal stability.

[0003] In the prior art, more attention is paid to single PCM or simple composites of PCM. These materials often show disadvantages such as limitations in the working temperature of molten salts, low thermal conductivity, and corrosiveness of metals or alloys. Summary of the Invention

[0004] The purpose of the present invention is to provide a low-melting-point binary molten salt heat transfer and storage working medium, a preparation method thereof, and an application thereof. By reasonably controlling the proportion of the composite molten salt of sodium nitrate and calcium nitrate, a low-melting-point molten salt is prepared; by grafting graphene oxide on the surface of Al2O3 and then in-situ reducing metallic magnesium, porous rGO-metal / Al2O3 nanoparticles are formed, which can not only prevent the agglomeration effect of each nanoparticle but also enhance the thermal conductivity of the low-melting-point molten salt; by using the composite molten salt of sodium nitrate and calcium nitrate mixed with porous rGO-metal / Al2O3 nanoparticles as the core and ceramic material SiO2 as the shell, the melting point of the molten salt is further reduced, and the problem of molten salt corrosion is solved.

[0005] The technical problem to be solved by the present invention: Single PCM or simple composites of PCM often show disadvantages such as limitations in the working temperature of molten salts, low thermal conductivity, and corrosiveness of metals or alloys.

[0006] The purpose of the present invention can be achieved by the following technical solutions:

[0007] A preparation method of a low-melting-point binary molten salt heat transfer and storage working medium, comprising the following steps:

[0008] S1. Prepare a composite molten salt of sodium nitrate and calcium nitrate, specifically:

[0009] Disperse sodium nitrate and calcium nitrate in deionized water to form a homogeneous mixture, and heat it at 200 °C until the water completely evaporates to obtain the composite molten salt; wherein, the mass ratio of sodium nitrate to calcium nitrate is 50 - 60:40 - 50.

[0010] S2. Graft graphene oxide on the surface of Al2O3, and then in-situ reduce and deposit metal particles on the surface of graphene oxide to obtain porous rGO-metal / Al2O3 nanoparticles with Al2O3 as the core and graphene oxide-metal composite as the shell, specifically:

[0011] Disperse Al2O3 nanoparticles in deionized water, add APTES, and react at 80 °C for 2 h, then filter to obtain Al2O3-NH2; wherein, the dosage ratio of Al2O3 nanoparticles to APTES is 0.1 - 0.3 g:1 - 2 mL.

[0012] During the above reaction process, -NH2 functional groups are chemically grafted on the surface of Al2O3 nanoparticles.

[0013] Disperse Al2O3-NH2 in deionized water, add a 10 mg / mL graphene oxide dispersion under stirring, after 1 h, add MgCl2·6H2O, then add a reducing agent, reduce at 50 °C for 3 h, and then centrifuge, wash and dry to obtain porous rGO-Mg / Al2O3 nanoparticles; wherein, the dosage ratio of Al2O3-NH2, deionized water, graphene oxide dispersion, MgCl2·6H2O and the reducing agent is 0.1 - 0.3:20 - 30 mL:10 - 20 mL:0.02 - 0.05 g:0.04 - 0.06 g.

[0014] During the above reaction process, Al2O3-NH2 with -NH2 functional groups on the surface is connected to graphene oxide through an amidation reaction to form a porous core-shell material with graphene oxide as the shell and Al2O3 as the core, and then metallic magnesium and reduced graphene oxide are in-situ reduced on the surface of graphene oxide.

[0015] S3. Disperse the porous rGO-metal / Al2O3 nanoparticles in the composite molten salt of sodium nitrate and calcium nitrate to obtain a molten salt nanofluid; then, using the molten salt nanofluid as the core and a ceramic material as the shell, a low-melting binary molten salt heat transfer and heat storage working medium is obtained, specifically:

[0016] Dissolve the composite molten salt in deionized water, add rGO-metal / Al2O3 powder, then add Span 80 and cyclopentyl methyl ether, and subsequently stir the mixture at 30 °C for 3 h using a magnetic stirrer to form a stable emulsion; wherein, the dosage ratio of rGO-metal / Al2O3 powder, deionized water, Span 80 and cyclopentyl methyl ether is 0.5 g: 1.2 mL: 150 mL: 1.8 mL;

[0017] Add the tetraethyl orthosilicate solution to the emulsion and continuously stir for 1 h, add the NH4OH solution with a concentration of 28%, continuously stir for 3 h at the same temperature to form a SiO2 shell, centrifuge, wash with ethanol and dry at 70 °C for 12 h, and then heat at a temperature of 150 °C for 3 h to obtain a low-melting binary molten salt heat transfer and energy storage working fluid; wherein, the tetraethyl orthosilicate solution is prepared by adding 1.666 g of tetraethyl orthosilicate to 40 mL of ethanol and stirring at room temperature for 15 min; the dosage ratio of the tetraethyl orthosilicate solution, the emulsion and the NH4OH solution is 2 mL: 150 mL: 4 mL.

[0018] In the above reaction process, mix the brine solution and the surfactant with the oil phase solvent. In this step, the hydrophilic group of the surfactant associates with water molecules to form micelles. Continuously stir for 3 h and add TEOS to the emulsion. Subsequently, the ethoxy group of TEOS is connected to the hydrophobic part of the surfactant on the outer surface of the micelle. Next, by adding a catalyst (NH4OH), it provides water and promotes the reaction. Finally, a silica shell is formed.

[0019] Furthermore, the size of the Al2O3 is 10 - 12 nm.

[0020] ]]Furthermore, the metal particles are metallic magnesium.

[0021] Furthermore, the ceramic material is silica and the shell thickness is 23 - 27 nm.

[0022] Furthermore, the reducing agent is N2H4·H2O.

[0023] A low-melting binary molten salt heat transfer and energy storage working fluid prepared by the preparation method as described above.

[0024] An application of a low-melting binary molten salt heat transfer and energy storage working fluid in industrial energy storage or solar thermal power generation.

[0025] The beneficial effects of the present invention:

[0026] (1) In the technical solution of the present invention, a molten salt system is prepared by mixing calcium nitrate and sodium nitrate, which has a low melting point, saves costs, and has stable molten salt performance.

[0027] (2) In the technical solution of the present invention, the Al2O3 nanoparticles can enhance the specific heat capacity and thermal conductivity of the molten salt. By grafting graphene oxide on the surface of Al2O3 to form a porous core-shell material, and then in-situ reducing magnesium metal, the generated porous rGO-metal / Al2O3 nanoparticles solve the dispersion problem of Al2O3, rGO and magnesium metal in the molten salt. In addition, ion clusters can be formed around the Mg particles with sodium ions, further improving the specific heat capacity.

[0028] (3) In the technical solution of the present invention, the ceramic material has good thermal properties. After introducing the ceramic material, the molten salt phase change material has better heat storage performance, can extend the working life of the molten salt phase change material, improve the thermal performance, and extend the working temperature. Specific embodiments

[0029] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the scope of protection of the present invention.

[0030] Among them, 1.666 g of tetraethyl orthosilicate was added to 40 mL of ethanol, and the mixture was stirred at room temperature for 15 min to prepare the tetraethyl orthosilicate solution.

[0031] Example 1

[0032] A preparation method of a low-melting-point binary molten salt heat transfer and heat storage working medium includes the following steps:

[0033] S1. Disperse sodium nitrate and calcium nitrate in deionized water to form a uniform mixture, and heat it at 200 °C until the water completely evaporates to obtain a composite molten salt; wherein, the mass ratio of sodium nitrate to calcium nitrate is 57.5:42.5;

[0034] S2. Disperse Al2O3 nanoparticles with a particle size of 10 nm in deionized water, add APTES, react at 80 °C for 2 h, and filter to obtain Al2O3-NH2; wherein, the dosage ratio of Al2O3 nanoparticles to APTES is 0.1 g:1 mL;

[0035] Disperse Al2O3-NH2 in deionized water, add a graphene oxide dispersion solution with a concentration of 10 mg / mL under stirring. After 1 h, add MgCl2·6H2O, and then add N2H4·H2O. Reduce at 50 °C for 3 h, and then perform centrifugation, washing, and drying to obtain porous rGO-Mg / Al2O3 nanoparticles; among them, the dosage ratio of Al2O3-NH2, deionized water, graphene oxide dispersion solution, MgCl2·6H2O, and N2H4·H2O is 0.1 g:20 mL:10 mL:0.02 g:0.04 g;

[0036] S3. Dissolve the composite molten salt in deionized water, add the rGO-metal / Al2O3 powder, then add Span 80 and cyclopentyl methyl ether. Subsequently, use a magnetic stirrer to stir the mixture at 30 °C for 3 h to form a stable emulsion; among them, the dosage ratio of rGO-metal / Al2O3 powder, deionized water, Span 80, and cyclopentyl methyl ether is 0.5 g:1.2 mL:150 mL:1.8 mL;

[0037] Add tetraethyl orthosilicate solution to the emulsion and continuously stir for 1 h. Add an NH4OH solution with a concentration of 28%, and continuously stir at the same temperature for 3 h to form a SiO2 shell. Perform centrifugation, wash with ethanol, and dry at 70 °C for 12 h. Subsequently, heat at a temperature of 150 °C for 3 h to obtain a low-melting-point binary molten salt heat transfer and energy storage working fluid; the dosage ratio of tetraethyl orthosilicate solution, emulsion, and NH4OH solution is 2 mL:150 mL:4 mL.

[0038] Example 2

[0039] A preparation method of a low-melting-point binary molten salt heat transfer and energy storage working fluid, comprising the following steps:

[0040] S1. Disperse sodium nitrate and calcium nitrate in deionized water to form a homogeneous mixture, and heat at 200 °C until the water completely evaporates to obtain a composite molten salt; among them, the mass ratio of sodium nitrate and calcium nitrate is 55:45;

[0041] S2. Disperse Al2O3 nanoparticles with a particle size of 11 nm in deionized water, add APTES, react at 80 °C for 2 h, and filter to obtain Al2O3-NH2; among them, the dosage ratio of Al2O3 nanoparticles and APTES is 0.2 g:1.5 mL;

[0042] Disperse Al2O3-NH2 in deionized water, add a graphene oxide dispersion solution with a concentration of 10 mg / mL under stirring. After 1 h, add MgCl2·6H2O, and then add N2H4·H2O. Reduce at 50 °C for 3 h, and then perform centrifugation, washing, and drying to obtain porous rGO-Mg / Al2O3 nanoparticles; wherein, the dosage ratio of Al2O3-NH2, deionized water, graphene oxide dispersion solution, MgCl2·6H2O, and N2H4·H2O is 0.1 g: 25 mL: 15 mL: 0.035 g: 0.05 g;

[0043] S3. Dissolve the composite molten salt in deionized water, add rGO-metal / Al2O3 powder, then add Span 80 and cyclopentyl methyl ether, and subsequently use a magnetic stirrer to stir the mixture at 30 °C for 3 h to form a stable emulsion; wherein, the dosage ratio of rGO-metal / Al2O3 powder, deionized water, Span 80, and cyclopentyl methyl ether is 0.5 g: 1.2 mL: 150 mL: 1.8 mL;

[0044] Add tetraethyl orthosilicate solution to the emulsion and continuously stir for 1 h. Add an NH4OH solution with a concentration of 28%, and continuously stir at the same temperature for 3 h to form an SiO2 shell. After centrifugation, ethanol washing, and drying at 70 °C for 12 h, and then heating at a temperature of 150 °C for 3 h, a low-melting binary molten salt heat transfer and energy storage working fluid is obtained; the dosage ratio of tetraethyl orthosilicate solution, emulsion, and NH4OH solution is 2 mL: 150 mL: 4 mL.

[0045] Example 3

[0046] A preparation method of a low-melting binary molten salt heat transfer and energy storage working fluid, comprising the following steps:

[0047] S1. Disperse sodium nitrate and calcium nitrate in deionized water to form a homogeneous mixture, and heat at 200 °C until the water completely evaporates to obtain a composite molten salt; wherein, the mass ratio of sodium nitrate and calcium nitrate is 50:50;

[0048] S2. Disperse Al2O3 nanoparticles with a particle size of 12 nm in deionized water, add APTES, react at 80 °C for 2 h, and filter to obtain Al2O3-NH2; wherein, the dosage ratio of Al2O3 nanoparticles and APTES is 0.3 g: 2 mL;

[0049] Disperse Al2O3-NH2 in deionized water, add a graphene oxide dispersion solution with a concentration of 10 mg / mL under stirring. After 1 h, add MgCl2·6H2O, and then add N2H4·H2O. Reduce at 50 °C for 3 h, and then perform centrifugation, washing, and drying to obtain porous rGO-Mg / Al2O3 nanoparticles; among them, the dosage ratio of Al2O3-NH2, deionized water, graphene oxide dispersion solution, MgCl2·6H2O, and N2H4·H2O is 0.3 g: 30 mL: 20 mL: 0.05 g: 0.06 g;

[0050] S3. Dissolve the composite molten salt in deionized water, add the rGO-metal / Al2O3 powder, then add Span 80 and cyclopentyl methyl ether, and then use a magnetic stirrer to stir the mixture at 30 °C for 3 h to form a stable emulsion; among them, the dosage ratio of rGO-metal / Al2O3 powder, deionized water, Span 80, and cyclopentyl methyl ether is 0.5 g: 1.2 mL: 150 mL: 1.8 mL;

[0051] Add tetraethyl orthosilicate solution to the emulsion and continuously stir for 1 h, add an NH4OH solution with a concentration of 28%, continuously stir at the same temperature for 3 h to form an SiO2 shell, perform centrifugation, wash with ethanol, and dry at 70 °C for 12 h. Subsequently, heat at a temperature of 150 °C for 3 h to obtain a low-melting binary molten salt heat transfer and heat storage working fluid; the dosage ratio of tetraethyl orthosilicate solution, emulsion, and NH4OH solution is 2 mL: 150 mL: 4 mL.

[0052] Comparative Example 1

[0053] The difference between this comparative example and Example 2 is that the mass ratio of sodium nitrate to calcium nitrate is 60:40, and the remaining steps and raw materials are the same as those in Example 1.

[0054] Comparative Example 2

[0055] The difference between this comparative example and Example 2 is that MgCl2·6H2O is not added in step S2:

[0056] S2. Disperse Al2O3 nanoparticles with a particle size of 11 nm in deionized water, add APTES, react at 80 °C for 2 h, and filter to obtain Al2O3-NH2; among them, the dosage ratio of Al2O3 nanoparticles and APTES is 0.2 g: 1.5 mL;

[0057] Disperse Al2O3-NH2 in deionized water, add a graphene oxide dispersion solution with a concentration of 10 mg / mL under stirring. After 1 h, add N2H4·H2O, and reduce at 50 °C for 3 h. Then, perform centrifugation, washing, and drying to obtain porous rGO / Al2O3 nanoparticles. Among them, the dosage ratio of Al2O3-NH2, deionized water, graphene oxide dispersion solution, and N2H4·H2O is 0.1 g: 25 mL: 15 mL: 0.05 g.

[0058] Comparative Example 3

[0059] The difference between this comparative example and Example 2 is that the molten salt nanofluid is not surface-coated with ceramic materials, and the remaining steps and raw materials are the same as those in Example 2.

[0060] Now, conduct heat storage and heat transfer performance tests on the low-melting-point binary molten salt heat transfer and heat storage working fluids prepared in Examples 1-3 and Comparative Examples 1-3:

[0061] After uniformly heating the prepared particles to 320 °C with a heating furnace, quickly take them out and put them into a soaking tank, and start recording the temperature. The soaking tank is wrapped with silicon-aluminum fiber, and only the top is in convective heat transfer with the air. The ceramic particles are stacked in a tank. Heat is transferred vertically and undergoes convective heat transfer with the air layer. Finally, the heat is transferred to the stainless steel shell at the top. The ceramic particles at the highest position are 3.5 cm away from the top of the tank and are in convective heat transfer with the air. Four thermocouples are placed through the small holes left at the top of the tank to measure the temperature, and the measured temperatures are recorded as T1, T2, T3, and T4. Among them, the depths of the thermocouple thermometers of T1 and T3 are different, so the temperatures can be used to compare different air layers. T1 is at the top of the soaking tank, and T3 is below T1 (T3 is 1 cm different from T1). Insert the T2 wire into the stacked ceramic particles to measure their temperature, and measure the temperature of the tank top shell and record it as T4; the test results are shown in Table 1 below.

[0062] Table 1

[0063]

[0064] As can be seen from Table 1 above, in the present invention, due to the use of ceramic material silicon dioxide to coat the molten salt nanofluid, the heat storage and heat transfer performance of the material is better. In addition, the deposition of magnesium metal also has a certain improvement in the heat storage and heat transfer performance of the molten salt.

[0065] Now, use a general differential scanning calorimeter (abbreviated as DSC) to test the lowest melting temperature of the sample molten salts of the low-melting-point binary molten salt heat transfer and heat storage working fluids prepared in Examples 1-3 and Comparative Examples 1-3. The test results are shown in Table 2 below.

[0066] Table 2

[0067]

[0068] As can be seen from Table 2 above, the low-melting-point binary molten salt heat transfer and energy storage working fluid prepared in the embodiments of the present invention has a lower melting point.

[0069] The method of using the low-melting-point binary molten salt heat transfer and energy storage working fluid prepared in the embodiments of the present invention for solar thermal power generation can refer to the method of using a nitrate molten salt heat transfer and energy storage medium for solar thermal power generation in the prior art. In addition, the low-melting-point binary molten salt heat transfer and energy storage working fluid prepared in the present invention can reduce the corrosion of equipment.

[0070] In the description of the specification, the description referring to terms such as "one embodiment", "example", "specific example", etc. means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in a suitable manner in any one or more embodiments or examples.

[0071] The above content is only an example and illustration of the present invention. Those skilled in the art of the present technology can make various modifications or supplements to the described specific embodiments or use similar methods for substitution, as long as they do not deviate from the invention or exceed the scope defined by the claims of the present invention, they shall fall within the protection scope of the present invention.

Claims

1. A preparation method of a low-melting-point binary molten salt heat transfer and heat storage working medium, characterized in that, It includes the following steps: S1. Prepare a composite molten salt of sodium nitrate and calcium nitrate; S2. Graft graphene oxide on the surface of Al2O3, and then in-situ reduce and deposit metal particles on the surface of graphene oxide to obtain porous rGO-metal / Al2O3 nanoparticles with Al2O3 as the core and graphene oxide-metal composite as the shell; the metal particles are metallic magnesium; the size of the Al2O3 is 10-12 nm; S3. Disperse the porous rGO-metal / Al2O3 nanoparticles in the composite molten salt of sodium nitrate and calcium nitrate to obtain a molten salt nanofluid; then, using the molten salt nanofluid as the core and ceramic material as the shell, a low-melting-point binary molten salt heat transfer and energy storage working medium is obtained; Step S3 is specifically as follows: Dissolve the composite molten salt in deionized water, add rGO-metal / Al2O3 powder, then add Span80 and cyclopentyl methyl ether, and then use a magnetic stirrer to stir the mixture at 30 °C for 3 h to form a stable emulsion; Add tetraethyl orthosilicate solution to the emulsion and continuously stir for 1 h, add NH4OH solution with a concentration of 28%, continuously stir at the same temperature for 3 h to form a SiO2 shell, centrifuge, wash with ethanol and dry at 70 °C for 12 h, and then heat at 150 °C for 3 h, and a low-melting-point binary molten salt heat transfer and energy storage working medium is obtained.

2. The preparation method of a low-melting-point binary molten salt heat transfer and heat storage working medium according to claim 1, characterized in that, The ceramic material is silicon dioxide, and the shell thickness is 23-27 nm.

3. The preparation method of a low-melting-point binary molten salt heat transfer and heat storage working medium according to claim 1, characterized in that Step S1 is specifically as follows: Disperse sodium nitrate and calcium nitrate in deionized water to form a homogeneous mixture, and heat at 200 °C until the water completely evaporates to obtain a composite molten salt; The mass ratio of sodium nitrate to calcium nitrate is 50-60:40-50.

4. The preparation method of a low-melting-point binary molten salt heat transfer and heat storage working medium according to claim 1, wherein Step S2 is specifically as follows: Disperse Al2O3 nanoparticles in deionized water, add APTES, react at 80 °C for 2 h, filter to obtain Al2O3-NH2; The dosage ratio of the Al2O3 nanoparticles to APTES is 0.1-0.3 g:1-2 mL; Disperse Al2O3-NH2 in deionized water, add 10 mg / mL graphene oxide dispersion under stirring, after 1 h, add MgCl2·6H2O, then add a reducing agent, reduce at 50 °C for 3 h, and then centrifuge, wash and dry to obtain porous rGO-Mg / Al2O3 nanoparticles; The dosage ratio of the Al2O3-NH2, deionized water, graphene oxide dispersion, MgCl2·6H2O and the reducing agent is 0.1-0.3 g:20-30 mL:10-20 mL:0.02-0.05 g:0.04-0.06 g.

5. The preparation method of a low-melting-point binary molten salt heat transfer and heat storage working medium according to claim 1, characterized in that, The reducing agent is N2H4·H2O.

6. The preparation method of a low-melting binary molten salt heat transfer and heat storage working medium according to claim 1, characterized in that, The dosage ratio of the rGO-metal / Al2O3 powder, deionized water, Span 80 and cyclopentyl methyl ether is 0.5 g:1.2 mL:150 mL:1.8 mL; The tetraethyl orthosilicate solution is prepared by adding 1.666 g of tetraethyl orthosilicate to 40 mL of ethanol and stirring at room temperature for 15 min; The dosage ratio of the tetraethyl orthosilicate solution, the emulsion and the NH4OH solution is 2 mL:150 mL:4 mL.

7. A low-melting-point binary molten salt heat transfer and heat storage working medium prepared by the preparation method according to any one of claims 1-6.

8. An application of the low-melting-point binary molten salt heat transfer and heat storage working medium according to claim 7 in industrial energy storage or solar thermal power generation.

Images