Medium-high temperature fused salt phase change heat storage composite material and preparation method thereof

Abstract

The invention relates to the technical field of fused salt phase change heat storage materials, in particular to a medium-high temperature fused salt phase change heat storage composite material and a preparation method thereof. The preparation method of the medium-high temperature molten salt phase change heat storage composite material comprises the following steps: adding water into a mixture of a cementing material and a phase change material to obtain a premix; sequentially carrying out slow stirring and rapid stirring on the premix to obtain a rapidly stirred premix; carrying out compression molding on the rapidly stirred premix to obtain a green body; and sequentially carrying out sealed maintenance and drying on the green body to obtain the medium-high temperature fused salt phase change heat storage composite material. According to the medium-high temperature molten salt phase change heat storage composite material and the preparation method thereof, the binding material is used as a base material, effective packaging of the phase change material can be achieved, and the prepared medium-high temperature molten salt phase change heat storage composite material has the advantages of being high in compressive strength, good in structural stability, excellent in thermal cycle stability, stable in phase change latent heat and phase change temperature and good in forming degree and has excellent heat storage performance.

Description

Technical Field

[0001] The present invention relates to the technical field of molten salt phase change heat storage materials, and particularly to a medium-high temperature molten salt phase change heat storage composite material and a preparation method thereof. Background Art

[0002] With the rapid economic development and continuous population growth, the energy consumption is increasing continuously. However, the reserves of traditional fossil fuels such as coal, oil, and natural gas are limited and cannot meet the long-term future energy demand. At the same time, the large-scale use of fossil fuels has led to serious environmental pollution and climate change problems, making sustainable development face huge challenges. Therefore, developing clean and sustainable energy and reducing the dependence on fossil fuels have become the urgent needs of the global energy transformation.

[0003] As an efficient energy storage method, heat storage technology has received extensive attention. Heat storage technology uses heat storage materials as a medium to convert and store thermal energy such as solar thermal energy, geothermal energy, industrial waste heat, and low-grade waste heat, and then reuse it. It has advantages such as large energy storage capacity, strong stability, and environmental protection. It has important application value in the fields of solar thermal utilization, industrial energy conservation and waste heat recovery, and building energy conservation, and is one of the key technologies to achieve efficient energy utilization and reduce fossil energy consumption.

[0004] According to different heat storage principles, heat storage technology is mainly divided into three methods: sensible heat storage, phase change heat storage, and thermochemical reaction heat storage. Among them, the phase change heat storage method that stores heat through phase changes such as solid-solid, solid-liquid, solid-gas, and liquid-gas of phase change heat storage materials is the most common. In solid-liquid phase change heat storage materials, medium-high temperature molten salt phase change heat storage materials (such as nitrate-based phase change materials) have become a research hotspot due to their wide phase change temperature range, approximate constant temperature during charging and discharging processes, good physical / chemical stability, and low price. However, the molten salt is prone to leakage after the solid-liquid phase change of the medium-high temperature molten salt phase change heat storage materials, which limits its popularization in practical applications.

[0005] Currently, in order to solve the problem of molten salt leakage, the existing technology usually adopts the method of sintering the phase change material with a porous material to prepare an encapsulated composite phase change heat storage material. For example, the Chinese patent technology with the publication number CN 109135683 A discloses a method for preparing a molten salt-ceramic phase change heat storage material. The sol-gel method is used to encapsulate high-temperature molten salt particle phase change materials, and the encapsulated particles are compounded and sintered with ceramic particles. During the sintering process, the ultrafine powder on the surface of the molten salt particles completely coats the molten salt particles therein, and the molten salt particles are completely isolated from the external environment, thus solving the problem of easy leakage of molten salt. However, the sintering process conditions are relatively harsh, and high-temperature treatment is usually required during sintering, resulting in high energy consumption and production costs, and it is difficult to meet the requirements of large-scale industrial applications.

[0006] Therefore, exploring a preparation method for medium- and high-temperature molten salt phase change heat storage materials with simple process and low cost is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0007] The present invention provides a preparation method for a medium- and high-temperature molten salt phase change heat storage composite material. Through this preparation method, a medium- and high-temperature molten salt phase change heat storage composite material with high compressive strength, good structural stability, excellent thermal cycle stability, stable phase change latent heat and phase change temperature, and good formability can be prepared; this preparation method has a simple process and does not require high-temperature treatment, and has the advantages of energy consumption saving and production cost saving, and is suitable for wide promotion and application.

[0008] The present invention also provides a medium- and high-temperature molten salt phase change heat storage composite material, which is prepared by the above preparation method. Therefore, this medium- and high-temperature molten salt phase change heat storage composite material has the characteristics of high compressive strength, good structural stability, excellent thermal cycle stability, stable phase change latent heat and phase change temperature, and good formability, and has excellent heat storage performance.

[0009] The first aspect of the present invention provides a preparation method for a medium- and high-temperature molten salt phase change heat storage composite material, including the following steps:

[0010] Add water to the mixture after mixing the gelling material and the phase change material to obtain a premix;

[0011] Carry out slow stirring and then fast stirring on the premix to obtain the premix after fast stirring;

[0012] Press the premix after fast stirring into a shape to obtain a green body;

[0013] Carry out sealed curing and then drying on the green body, and the medium- and high-temperature molten salt phase change heat storage composite material is obtained.

[0014] For the preparation method of the medium- and high-temperature molten salt phase change heat storage composite material as described above, by mass fraction, in the medium- and high-temperature molten salt phase change heat storage composite material, the mass fraction ratio of the gelling material to the phase change material is (50 - 60) : (40 - 50), and the sum of the mass fractions of the gelling material and the phase change material is 100 parts;

[0015] And / or, the mass fraction of the water is 10 - 15% of the mass fraction of the gelling material.

[0016] For the preparation method of the medium- and high-temperature molten salt phase change heat storage composite material as described above, the gelling material is portland cement or phosphate cement, and the phase change material is sodium nitrate.

[0017] For the preparation method of the medium- and high-temperature molten salt phase change heat storage composite material as described above, the particle size of the sodium nitrate is greater than 0 and less than 0.1 mm.

[0018] The preparation method of the medium- and high-temperature molten salt phase change heat storage composite material as described above, the pressure during the press molding is 5 - 30 MPa;

[0019] The time of the press molding is 1 - 30 min.

[0020] The preparation method of the medium- and high-temperature molten salt phase change heat storage composite material as described above, the press molding is to press the premix after rapid stirring into a cylinder, and the diameter of the cylinder is 18 - 40 mm.

[0021] The preparation method of the medium- and high-temperature molten salt phase change heat storage composite material as described above, the rotation speed of the slow stirring is 100 - 150 r / min and the time is 50 - 100 s.

[0022] The preparation method of the medium- and high-temperature molten salt phase change heat storage composite material as described above, the rotation speed of the rapid stirring is 200 - 300 r / min and the time is 100 - 200 s.

[0023] The preparation method of the medium- and high-temperature molten salt phase change heat storage composite material as described above, the green body is successively subjected to sealed curing and drying, including:

[0024] The green body is sealed and cured at 20 - 30 °C for 336 - 360 h, and then dried at 70 - 130 °C for 2 - 48 h.

[0025] The second aspect of the present invention provides a medium- and high-temperature molten salt phase change heat storage composite material, which is prepared by the preparation method of the medium- and high-temperature molten salt phase change heat storage composite material as described above.

[0026] The solution of the present invention has at least the following effects:

[0027] (1) The preparation method of the medium- and high-temperature molten salt phase change heat storage composite material provided by the present invention, by mixing the gelling material (portland cement) and the phase change material (sodium nitrate) and then successively performing slow stirring, rapid stirring, press molding, sealed curing and drying, with the gelling material (portland cement) as the matrix material, can effectively encapsulate the phase change material (sodium nitrate), and prepare a medium- and high-temperature molten salt phase change heat storage composite material with high compressive strength, good structural stability, excellent thermal cycle stability, stable phase change latent heat and phase change temperature, and good formability; this preparation method has a simple process, does not require high-temperature treatment, has the advantages of energy consumption saving and production cost saving, and is suitable for wide promotion and application.

[0028] (2) The medium- and high-temperature molten salt phase-change heat storage composite material provided by the present invention is generated at a relatively low temperature, which is lower than the temperature required for sintering, and the cementitious material used can bond the granular or block materials into a whole through its own physical and chemical changes, thereby avoiding the high energy consumption caused by the high-temperature preparation process. The direct use of cementitious material as the packaging skeleton material simplifies the packaging process. Compared with the traditional melt impregnation method and cold pressing-sintering method, the process is simpler and more energy-saving; the raw material cost for preparing the medium- and high-temperature molten salt phase-change heat storage composite material is low, which can effectively save production costs and is suitable for large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0030] Figure 1 This is an appearance diagram of the medium-high temperature molten salt phase change thermal storage composite material in Example 1 of the present invention;

[0031] Figure 2 This is a scanning electron microscope (SEM) image of the medium-high temperature molten salt phase change thermal storage composite material in Example 1 of the present invention;

[0032] Figure 3 This is an appearance diagram of the medium-high temperature molten salt phase change thermal storage composite material in Example 1 after one thermal cycle of the present invention;

[0033] Figure 4 This is an appearance diagram of the medium-high temperature molten salt phase change thermal storage composite material in Example 1 after 50 thermal cycles of the present invention;

[0034] Figure 5 This is a SEM image of the medium-high temperature molten salt phase change thermal storage composite material in Example 1 after 50 thermal cycles of the present invention;

[0035] Figure 6 The DSC test result diagram of the medium-high temperature molten salt phase change heat storage composite material in Example 1 of the present invention, the medium-high temperature molten salt phase change heat storage composite material in Example 1 after one thermal cycle, the medium-high temperature molten salt phase change heat storage composite material in Example 1 after 50 thermal cycles, and sodium nitrate (NaNO3);

[0036] Figure 7 These are XRD test graphs of the medium-high temperature molten salt phase change heat storage composite material in Example 1 of the present invention, the medium-high temperature molten salt phase change heat storage composite material in Example 1 after one thermal cycle, and the medium-high temperature molten salt phase change heat storage composite material in Example 1 after 50 thermal cycles. Detailed Embodiments

[0037] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Apparently, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0038] In the following embodiments, the raw materials and reagents used, unless otherwise specified, can be obtained from commercial channels; the processes used, unless otherwise specified, are conventional processes in the art.

[0039] The first aspect of the present invention provides a method for preparing a medium-high temperature molten salt phase change heat storage composite material, comprising the following steps:

[0040] Adding water to the mixture obtained by mixing the gelling material and the phase change material to obtain a premix;

[0041] Sequentially performing slow stirring and fast stirring on the premix to obtain the premix after fast stirring;

[0042] Pressing the premix after fast stirring into a mold to obtain a green body;

[0043] Sequentially performing sealed curing and drying on the green body to obtain the medium-high temperature molten salt phase change heat storage composite material.

[0044] The present invention does not particularly limit the specific amount of water, which can be selected according to actual situations.

[0045] The present invention does not particularly limit the specific equipment for slow stirring and fast stirring. In some embodiments, a mixer can be used to sequentially perform slow stirring and fast stirring on the premix.

[0046] The present invention does not particularly limit the specific equipment for drying. In some embodiments, a hot air dryer can be used to dry the cured green body.

[0047] In the present invention, the above-mentioned pressing the premix after fast stirring into a mold means pressing the premix after fast stirring into a mold by a cold pressing method. It shows that the medium-high temperature molten salt phase change heat storage composite material of the present invention can be formed at a relatively low temperature.

[0048] The object of the present invention is to prepare a medium-high temperature molten salt phase change heat storage composite material. Specifically, first, a gelling material and a phase change material are mixed to obtain a mixture, and then water is added to the mixture to obtain a premix. Water serves as the medium for the hydration reaction of the gelling material. Then, the premix is slowly stirred to obtain the premix after slow stirring, and then the premix after slow stirring is rapidly stirred to obtain the premix after rapid stirring. The purpose of slow stirring is to initially mix the raw materials such as the gelling material, the phase change material, and water evenly. Through slow stirring, the raw materials such as the gelling material, the phase change material, and water can be fully contacted to start forming a relatively uniform premix, laying a foundation for subsequent rapid stirring, and helping to avoid local performance differences caused by uneven raw material distribution. The purpose of rapid stirring is to further improve the uniformity and compactness of the premix after slow stirring. Rapid stirring can generate a stronger shear force between the raw materials, making the contact between the raw materials closer, so as to better disperse and reduce the agglomeration phenomenon. At the same time, rapid stirring can also promote the full hydration reaction between water and the gelling material, making the gelling material better dispersed in the premix to form a uniform slurry. In addition, rapid stirring can also expel the air bubbles in the premix to a certain extent, reduce the porosity, and improve the strength of the material, which is beneficial to the subsequent preparation of a medium-high temperature molten salt phase change heat storage composite material with excellent compressive strength. The premix after rapid stirring is pressed into a shape to obtain a green body. Subsequently, the green body is sealed and cured to obtain the cured green body, and finally the cured green body is dried to obtain a medium-high temperature molten salt phase change heat storage composite material with high compressive strength, good structural stability, excellent thermal cycle stability, stable phase change latent heat and phase change temperature, and good formability. Sealed curing is to prevent the evaporation of water in the medium-high temperature molten salt phase change heat storage composite material during high-temperature heat storage use, and drying is to completely evaporate the free water in the cured green body to prevent the violent evaporation of water in the medium-high temperature molten salt phase change heat storage composite material during high-temperature heat storage use from damaging its structure.

[0049] In a specific embodiment, by mass, in the above medium-high temperature molten salt phase change heat storage composite material, the mass ratio of the gelling material to the phase change material is (50 - 60):(40 - 50), and the sum of the mass parts of the gelling material and the phase change material is 100 parts. Further, the mass ratio of the gelling material to the phase change material can be preferably 50:50, and the sum of the mass parts of the gelling material and the phase change material is 100 parts.

[0050] When the mass ratio of the gelling material to the phase change material in the medium-high temperature molten salt phase change heat storage composite material is within the above range, the gelling material and the phase change material can be more fully combined, so as to prepare a medium-high temperature molten salt phase change heat storage composite material with high compressive strength, good structural stability, excellent thermal cycle stability, stable phase change latent heat and phase change temperature, and good formability.

[0051] In a specific embodiment, the mass fraction of the above-mentioned water is 10-15% of the mass fraction of the gelling material.

[0052] When the mass fraction of the added water is 10-15% of the mass fraction of the above-mentioned gelling material, it is beneficial for a more sufficient hydration reaction to occur between the water and the gelling material.

[0053] In a specific embodiment, the above-mentioned gelling material is portland cement or phosphate cement, preferably portland cement; the above-mentioned phase change material is sodium nitrate.

[0054] As a gelling material, portland cement has the advantages of low price, excellent performance, wide sources, etc.

[0055] As a phase change material, sodium nitrate (NaNO3) has the advantages of no corrosion, excellent thermal stability, low price, etc.

[0056] The present invention can prepare a medium-high temperature molten salt phase change heat storage composite material with high compressive strength, good structural stability, excellent thermal cycle stability, stable phase change latent heat and phase change temperature, and good formability by using portland cement and sodium nitrate as raw materials; portland cement can bond granular or massive materials into a whole through its own physical and chemical changes, eliminating the high energy consumption caused by high-temperature preparation processes, and strengthening the compressive strength of the product during the reaction of the phase change heat storage material.

[0057] In a specific embodiment, the particle size of the above-mentioned sodium nitrate is greater than 0 and less than 0.1 mm.

[0058] When the particle size of sodium nitrate is within the above range, it can avoid the phenomenon of molten salt leakage during pressing and drying. If the particle size of sodium nitrate is too large, the porous structure on the portland cement skeleton particles cannot completely adsorb the molten sodium nitrate, resulting in uneven internal stress of sodium nitrate, and the capillary force and surface tension on both sides of the gas-solid interface cannot suck the residual molten sodium nitrate on the surface into the interior, ultimately leading to sodium nitrate leakage, which is not conducive to the encapsulation of sodium nitrate.

[0059] In a specific embodiment, the pressure during the above die-casting is 5-30 MPa, for example, it can be any one of 10 MPa, 20 MPa, 30 MPa and the range composed of any two of them; the time for the above pressing is 1-30 min, for example, it can be any one of 1 min, 5 min, 10 min, 15 min, 20 min, 25 min, 30 min and the range composed of any two of them.

[0060] When the present invention is subjected to press molding, if the pressure of press molding is high, the time of press molding is short; if the pressure of press molding is low, the time of press molding is long. Those skilled in the art can select the corresponding pressure and time according to process requirements to achieve good pressing effects. If the pressure during press molding is too high, the structure of the prepared medium-high temperature molten salt phase change heat storage composite material is denser, and the porosity decreases. When used at high temperatures, the hydration products of the portland cement in the medium-high temperature molten salt phase change heat storage composite material decompose, and the physical water / bound water overflows, which will damage the structure of the medium-high temperature molten salt phase change heat storage composite material. Moreover, sodium nitrate will expand in volume at high temperatures, damaging the portland cement.

[0061] The present invention does not particularly limit the specific shape of the above-mentioned green body. In some embodiments, the above-mentioned press molding may be to press the premix after rapid stirring into a block, sheet or cylinder. Further, the above-mentioned press molding is preferably to press the premix after rapid stirring into a cylinder, and the diameter of the cylinder is 18 - 40 mm.

[0062] When the premix after rapid stirring is pressed into a cylinder with the diameter parameter within the above range, the combination of the gelling material and the phase change material is tighter, and the formed medium-high temperature molten salt phase change heat storage composite material has good formability and will not show leakage.

[0063] In a specific embodiment, the rotation speed of the above-mentioned slow stirring is 100 - 150 r / min and the time is 50 - 100 s.

[0064] When the parameters of the rotation speed and time of slow stirring are respectively within the above ranges, the raw materials such as the gelling material, the phase change material and water can be initially mixed evenly. Through slow stirring, the raw materials can be fully contacted with each other, and a relatively uniform premix begins to form, laying a foundation for subsequent rapid stirring and helping to avoid local property differences caused by uneven distribution of raw materials.

[0065] Exemplarily, the rotation speed of slow stirring can be any one of 100 r / min, 110 r / min, 120 r / min, 130 r / min, 140 r / min, 150 r / min and the range composed of any two of them;

[0066] The time can be any one of 50 s, 60 s, 70 s, 80 s, 90 s, 100 s and the range composed of any two of them.

[0067] In a specific embodiment, the rotation speed of the above-mentioned rapid stirring is 200 - 300 r / min and the time is 100 - 200 s.

[0068] When the parameters of the rotation speed and time of the rapid stirring are respectively within the above ranges, the uniformity and compactness of the premix after slow stirring can be further improved. The rapid stirring can generate a stronger shear force between the raw materials, making the contact between the raw materials closer, so as to better disperse and reduce the agglomeration phenomenon. At the same time, the rapid stirring can also promote the full hydration reaction between water and the gelling material, making the gelling material better dispersed in the premix to form a uniform slurry. In addition, the rapid stirring can also expel the air bubbles in the premix to a certain extent, reduce the porosity, which is beneficial to the subsequent preparation of the medium-high temperature molten salt phase change heat storage composite material with excellent compressive strength.

[0069] Exemplarily, the rotation speed of the rapid stirring can be any one of 200 r / min, 210 r / min, 220 r / min, 230 r / min, 240 r / min, 250 r / min, 260 r / min, 270 r / min, 280 r / min, 290 r / min, 300 r / min and the range composed of any two of them;

[0070] The time can be any one of 100 s, 110 s, 120 s, 130 s, 140 s, 150 s, 160 s, 170 s, 180 s, 190 s, 200 s and the range composed of any two of them.

[0071] In a specific embodiment, the above green body is subjected to sealed curing and drying in sequence, including:

[0072] The green body is subjected to sealed curing at 20 - 30 °C for 336 - 360 h, and then dried at 70 - 130 °C for 2 - 48 h.

[0073] When the parameters of the temperature and time of the sealed curing are within the above ranges, it can effectively prevent the evaporation of moisture during the high-temperature heat storage use of the medium-high temperature molten salt phase change heat storage composite material.

[0074] When the parameters of the temperature and time of the drying are within the above ranges, it can completely evaporate the free water in the cured green body and prevent the violent evaporation of moisture from damaging the structure of the medium-high temperature molten salt phase change heat storage composite material during the high-temperature heat storage use process.

[0075] The second aspect of the present invention provides a medium-high temperature molten salt phase change heat storage composite material, which is prepared by the preparation method of the above medium-high temperature molten salt phase change heat storage composite material. Therefore, this medium-high temperature molten salt phase change heat storage composite material has the characteristics of high compressive strength, good structural stability, excellent thermal cycle stability, stable phase change latent heat and phase change temperature, and good formability, and has excellent heat storage performance.

[0076] Hereinafter, the present invention will be further introduced through specific examples.

[0077] In the present invention, sodium nitrate is purchased from the National Pharmaceutical Group, and its latent heat of phase change is 184.94 J / g; Portland cement is produced by Jidong Cement Co., Ltd., and its main components are CaO and SiO2.

[0078] Example 1

[0079] This example provides a preparation method of a medium-high temperature molten salt phase change heat storage composite material, comprising the following steps:

[0080] (1) By mass, 50 parts of Portland cement, 50 parts of sodium nitrate with a particle size of 0.09 mm, and 5 parts of water are respectively weighed.

[0081] (2) The Portland cement and sodium nitrate in (1) are mixed evenly to obtain a mixture.

[0082] (3) The water in (1) is added to the mixture in (2) to obtain a premix.

[0083] (4) First, the premix in (3) is slowly stirred at 100 rpm for 60 s to obtain a slowly stirred premix, and then the slowly stirred premix is rapidly stirred at 200 rpm for 120 s to obtain a rapidly stirred premix.

[0084] (5) The rapidly stirred premix in (4) is pressed in a pressing mold at a pressure of 10 MPa for 2 min to be formed into a green body, and the green body is a cylinder with a diameter of 18 mm.

[0085] (6) The green body in (5) is sealed and cured at 25 °C for 336 h, and then dried at 105 °C for 24 h to obtain a medium-high temperature molten salt phase change heat storage composite material.

[0086] Example 2

[0087] The preparation method of the medium-high temperature molten salt phase change heat storage composite material provided in this example is basically the same as that in Example 1, except that:

[0088] (1) By mass, 40 parts of Portland cement, 60 parts of sodium nitrate with a particle size of 0.1 mm, and 4 parts of water are respectively weighed.

[0089] Example 3

[0090] The preparation method of the medium-high temperature molten salt phase change heat storage composite material provided in this example is basically the same as that in Example 1, except that:

[0091] (5) The rapidly stirred premix in (4) is pressed in a pressing mold at a pressure of 20 MPa for 2 min to be formed into a green body, and the green body is a cylinder with a diameter of 18 mm.

[0092] Comparative Example 1

[0093] The preparation method of the portland cement material provided in this comparative example is basically the same as that in Example 1, except that:

[0094] (1) By mass, 100 parts of portland cement and 10 parts of water were weighed respectively.

[0095] The inventor found that the portland cement material in this comparative example was lighter in color than the medium and high temperature molten salt phase change heat storage composite material in Example 1, and raw materials fell off around it; in the case of not using sodium nitrate, the preparation of this portland cement material was mainly cured and hardened through hydration reaction rather than through a phase change process, so its heat storage performance was not tested.

[0096] Comparative Example 2

[0097] The preparation method of the medium and high temperature molten salt phase change heat storage composite material provided in this comparative example is basically the same as that in Example 1, except that:

[0098] (1) By mass, 50 parts of portland cement, 50 parts of sodium nitrate with a particle size of 0.3 mm, and 5 parts of water were weighed respectively.

[0099] The inventor found that the molten salt leakage phenomenon of the medium and high temperature molten salt phase change heat storage composite material in this comparative example was relatively serious. The reason might be that: the particle size of sodium nitrate was too large, and the porous structure on the portland cement skeleton particles could not completely adsorb the molten sodium nitrate, resulting in uneven internal stress of sodium nitrate, and the capillary force and surface tension on both sides of the gas-solid interface could not suck the residual molten sodium nitrate on the surface into the interior, ultimately leading to sodium nitrate leakage, which was not conducive to the encapsulation of sodium nitrate.

[0100] Comparative Example 3

[0101] The preparation method of the medium and high temperature molten salt phase change heat storage composite material provided in this comparative example is basically the same as that in Example 1, except that:

[0102] (5) The premixed material after rapid stirring in (4) was pressed in a pressing mold at a pressure of 40 MPa for 2 min to form a green body, and the green body was a cylinder with a diameter of 18 mm.

[0103] The inventor of the present invention found that the molten salt leakage phenomenon of the medium-high temperature molten salt phase change heat storage composite material in this comparative example was relatively serious after 1 thermal cycle. The reason may be that: the pressure during pressing and forming was too high, the structure of the prepared medium-high temperature molten salt phase change heat storage composite material was denser, the porosity decreased, and when used at high temperatures, the hydration products of the portland cement in the medium-high temperature molten salt phase change heat storage composite material decomposed, and the physical water / bound water overflowed, which would damage the structure of the medium-high temperature molten salt phase change heat storage composite material. In addition, sodium nitrate would expand in volume at high temperatures, damaging the portland cement.

[0104] Comparative Example 4

[0105] The preparation method of the medium-high temperature molten salt phase change heat storage composite material provided in this comparative example was basically the same as that in Example 1, except that:

[0106] (1) By mass, 50 parts of ferroaluminate cement, 50 parts of sodium nitrate with a particle size of 0.1 mm, and 5 parts of water were weighed respectively.

[0107] The inventor of the present invention found that the molten salt leakage phenomenon of the medium-high temperature molten salt phase change heat storage composite material in this comparative example was relatively serious. The reason may be that: the iron-containing substances in the ferroaluminate cement may have reacted with sodium nitrate, and the skeleton structure was damaged, resulting in a significant reduction in the encapsulation effect of the ferroaluminate cement on sodium nitrate, and ultimately leading to the molten salt leakage phenomenon, which was not conducive to the encapsulation of sodium nitrate.

[0108] Performance Test

[0109] 1. Mechanical Properties

[0110] For the medium-high temperature molten salt phase change heat storage composite materials in Examples 1-3 of the present invention and the portland cement material in Comparative Example 1, mechanical property tests were carried out respectively at a loading rate of 0.5 MPa / s and under the test condition of maintaining the temperature at 350 °C for 30 min. The test results are shown in Table 1.

[0111] Table 1 Test Results

[0112] Item Example 1 Example 2 Example 3 Comparative Example 1 Compressive strength (MPa) 85.2 75.3 97.2 10.3

[0113] As can be seen from Table 1, the high-temperature molten salt phase change heat storage composite materials provided in Examples 1-3 of the present invention have the advantage of high compressive strength.

[0114] 2. Heat Storage Performance

[0115] The medium- and high-temperature molten salt phase change heat storage composite materials in Examples 1-3 of the present invention were used as samples for thermal cycling tests: the samples were heated from 20°C to 350°C at a heating rate of 2°C / min and held at 350°C for 60 min, and then naturally cooled to room temperature to obtain the samples after 1 thermal cycle (the medium- and high-temperature molten salt phase change heat storage composite materials after 1 thermal cycle); the samples were heated from 20°C to 350°C at a heating rate of 2°C / min and held at 350°C for 60 min, then cooled to 250°C at a cooling rate of 5°C / min and held at 250°C for 30 min, and then heated from 250°C to 350°C at a heating rate of 5°C / min, which was defined as 1 thermal cycle. According to this process, the thermal cycling was carried out 50 times. When the number of cycles reached 50, the heating was stopped and the sample was naturally cooled to room temperature to obtain the samples after 50 thermal cycles (the medium- and high-temperature molten salt phase change heat storage composite materials after 50 thermal cycles).

[0116] (1) Figure 1 is the appearance diagram of the medium- and high-temperature molten salt phase change heat storage composite material in Example 1 of the present invention, Figure 2 is the scanning electron microscope (SEM) diagram of the medium- and high-temperature molten salt phase change heat storage composite material in Example 1 of the present invention, Figure 3 is the appearance diagram of the medium- and high-temperature molten salt phase change heat storage composite material in Example 1 of the present invention after 1 thermal cycle, Figure 4 is the appearance diagram of the medium- and high-temperature molten salt phase change heat storage composite material in Example 1 of the present invention after 50 thermal cycles, Figure 5 is the SEM diagram of the medium- and high-temperature molten salt phase change heat storage composite material in Example 1 of the present invention after 50 thermal cycles.

[0117] It can be seen from Figure 1 that the medium- and high-temperature molten salt phase change heat storage composite material in Example 1 of the present invention has good formability and no molten salt leakage phenomenon.

[0118] It can be seen from Figure 1 、 Figure 3 and Figure 4 that the medium- and high-temperature molten salt phase change heat storage composite material in Example 1 of the present invention can still maintain good formability after 1 thermal cycle and 50 thermal cycles, the structure remains intact, there are no obvious cracks, and basically no molten salt leakage phenomenon.

[0119] It can be seen from Figure 2 and Figure 5 that the internal structure of the medium- and high-temperature molten salt phase change heat storage composite material in Example 1 of the present invention becomes denser after 50 thermal cycles, and there is no obvious accumulation of phase change material (sodium nitrate), indicating that the high-temperature molten salt phase change heat storage composite material provided in Example 1 of the present invention has good structural stability.

[0120] (2) The medium-high temperature molten salt phase change heat storage composite materials in Examples 1-3 of the present invention, the medium-high temperature molten salt phase change heat storage composite materials in Examples 1-3 after 1 thermal cycle, the medium-high temperature molten salt phase change heat storage composite materials in Examples 1-3 after 50 thermal cycles, and sodium nitrate (NaNO₃) were respectively subjected to DSC (differential scanning calorimetry) tests to obtain the phase change temperature and phase change latent heat. The test results are shown in Table 2.

[0121] Table 2 Test Results

[0122]

[0123] Figure 6 It is a DSC test result diagram of the medium-high temperature molten salt phase change heat storage composite material in Example 1 of the present invention, the medium-high temperature molten salt phase change heat storage composite material in Example 1 after 1 thermal cycle, the medium-high temperature molten salt phase change heat storage composite material in Example 1 after 50 thermal cycles, and sodium nitrate (NaNO₃).

[0124] As can be seen from Table 2, the medium-high temperature molten salt phase change heat storage composite material provided in the embodiments of the present invention has excellent heat storage performance. From Figure 6 and Table 2, it can be known that the phase change temperature and phase change latent heat of the medium-high temperature molten salt phase change heat storage composite material in Example 1 of the present invention are 303.2 °C and 98.30 J / g respectively. Its phase change temperatures after 1 thermal cycle and 50 thermal cycles are 302.73 °C and 303.96 °C respectively, with changes of 0.16% and 0.25% respectively. The phase change latent heats after 1 thermal cycle and 50 thermal cycles are 97.64 J / g and 85.65 J / g respectively, with changes of 0.67% and 12.8% respectively, indicating that the medium-high temperature molten salt phase change heat storage composite material provided in Example 1 of the present invention has the characteristics of excellent thermal cycle stability, stable phase change latent heat and phase change temperature.

[0125] (3) The medium-high temperature molten salt phase change heat storage composite material in Example 1 of the present invention, the medium-high temperature molten salt phase change heat storage composite material in Example 1 after 1 thermal cycle, and the medium-high temperature molten salt phase change heat storage composite material in Example 1 after 50 thermal cycles were respectively subjected to X-ray diffraction (XRD) tests. The test results are as Figure 7 shown.

[0126] From Figure 7 it can be known that the diffraction curves of the medium-high temperature molten salt phase change heat storage composite material in Example 1 of the present invention hardly change after 1 thermal cycle and 50 thermal cycles, and its main diffraction peaks are also the same as those before the thermal cycle. This indicates that during the repeated thermal cycle process, no new bands or chemical structures are formed inside the medium-high temperature molten salt phase change heat storage composite material, further proving that the medium-high temperature molten salt phase change heat storage composite material provided in Example 1 of the present invention has excellent thermal cycle stability at high temperatures.

[0127] In summary, in the embodiments of the present invention, the cementitious material (portland cement) is used as the matrix material, which can effectively encapsulate the phase change material (sodium nitrate). The prepared medium-high temperature molten salt phase change heat storage composite material has the characteristics of high compressive strength, good structural stability, excellent thermal cycle stability, stable phase change latent heat and phase change temperature, and good formability, and has excellent heat storage performance.

[0128] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a medium- and high-temperature molten salt phase-change thermal storage composite material, characterized in that: The following steps are involved: adding water to the mixture of the gelling material and the phase change material to obtain a premix; The premix is ​​sequentially subjected to slow stirring and rapid stirring to obtain a rapidly stirred premix; Pressing the rapidly stirred premix into a shape to obtain a green body; The green body is sealed, maintained and dried in sequence to obtain the medium- and high-temperature molten salt phase-change heat storage composite material.

2. The method for preparing the medium- and high-temperature molten salt phase-change thermal storage composite material according to claim 1, characterized in that: In terms of mass fractions, in the medium-high temperature molten salt phase change thermal storage composite material, the mass fraction ratio of the gelling material to the phase change material is (50-60):(40-50), and the sum of the mass fractions of the gelling material and the phase change material is 100 parts; And / or, the mass fraction of the water is 10%-15% of the mass fraction of the gelling material.

3. The method for preparing the medium- and high-temperature molten salt phase-change thermal storage composite material according to claim 2, characterized in that: The cementitious material is silicate cement or phosphate cement, and the phase change material is sodium nitrate.

4. The method for preparing the medium- and high-temperature molten salt phase-change thermal storage composite material according to claim 3, characterized in that: The particle size of the sodium nitrate is greater than 0 and less than 0.1 mm.

5. The method for preparing the medium- and high-temperature molten salt phase-change thermal storage composite material according to claim 1, characterized in that: The pressure during the compression molding is 5-30MPa; The compression molding time is 1-30 minutes.

6. The method for preparing the medium- and high-temperature molten salt phase-change thermal storage composite material according to claim 5, characterized in that: The compression molding is to compress the rapidly stirred premix into a cylinder, wherein the diameter of the cylinder is 18-40 mm.

7. The method for preparing the medium- and high-temperature molten salt phase-change thermal storage composite material according to claim 1, characterized in that: The slow stirring has a rotation speed of 100-150 r / min and a time of 50-100 s.

8. The method for preparing the medium- and high-temperature molten salt phase-change thermal storage composite material according to claim 1, characterized in that: The rapid stirring has a rotation speed of 200-300 r / min and a time of 100-200 s.

9. The method for preparing the medium- and high-temperature molten salt phase-change thermal storage composite material according to claim 1, characterized in that: The green body is sealed, cured and dried in sequence, including: The green body is sealed and cured at 20-30° C. for 336-360 hours, and then dried at 70-130° C. for 2-48 hours.

10. A medium-high temperature molten salt phase change thermal storage composite material, characterized in that: The composite material is prepared by the method for preparing the medium- and high-temperature molten salt phase-change heat storage composite material according to any one of claims 1 to 9.

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