Low-melting ternary mixed molten salt heat transfer and heat storage material and preparation method thereof

Abstract

The application discloses a low-melting-point ternary mixed molten salt heat transfer and storage material and a preparation method. The low-melting-point ternary mixed molten salt heat transfer and storage material is a multi-component eutectic salt formed by mixing potassium nitrate, sodium nitrate and calcium nitrate, has an actual measured melting point less than 100 DEG C, a decomposition temperature greater than 580 DEG C, an average specific heat of about 1.5 J / (g*K), a lower limit temperature of normal use of 150 DEG C, an upper limit temperature of use of 550 DEG C, a wide liquid temperature range of the molten salt, low cost, small corrosion and good fluidity. The material has good heat transfer and storage performance, is very suitable for the fields of photothermal power generation, compressed air energy storage, industrial waste heat recovery, flexible transformation of thermal power plants and molten salt heat pump heat storage, and has a good application prospect.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of physical heat transfer and energy storage materials, and particularly relates to a low-melting-point ternary mixed molten salt heat transfer and energy storage material and a preparation method. BACKGROUND

[0002] Energy storage is a key technology for achieving the "3060 double carbon goal". As an excellent heat transfer and heat storage medium, molten salt has been successfully applied in various solar thermal power stations, realizing efficient conversion between heat and electricity. At present, in the solar thermal power system, the main heat transfer and heat storage medium is mixed nitrate salt, for example, the solar two in California, USA, Andasol in Spain, and the tower type solar thermal power station of Delingha and Dunhuang of Zhongkong, all of which use solar salt (60wt% NaNO3+40wt% KNO3) as the heat transfer and heat storage medium. Mixed solar molten salt has good thermal stability and low cost, but its melting point is relatively high, which is 223.2℃, the complete melting temperature is 249.4℃, the decomposition temperature is 586℃, and the recommended use temperature is 280-560℃. The use temperature of solar thermal power is 290-568℃, the normal use liquid temperature range is narrow, and the risk of pipe freezing is high during use, which requires the arrangement of heat tracing.

[0003] In order to reduce the melting point of mixed molten salt, sodium nitrite is added on the basis of solar salt to prepare Hitec salt (53wt% KNO3+7wt% NaNO3+40wt% NaNO2), and the melting point is reduced to 142℃, which is greatly reduced compared with the melting point of solar salt, and the risk of pipe freezing is greatly reduced. However, the decomposition temperature of Hitec salt is 535℃, and the maximum use upper limit temperature is also reduced, the heat storage density is limited, and the presence of sodium nitrite in Hitec makes the corrosion of molten salt to carbon steel pipe increase when the temperature is higher than 400℃, so stainless steel pipe must be replaced to ensure the safe operation of Hitec salt at high temperature.

[0004] In order to further reduce the melting point of mixed molten salt, calcium nitrate is added on the basis of solar salt to form Hitec XL salt (45% KNO3+48% Ca(NO3)2+7% NaNO3), and the melting point is 126.8℃, the complete melting temperature is 140.9℃, the crystallization point is 134.1℃, and the decomposition temperature is 514.5℃. The normal use liquid temperature range is further narrowed. Beijing University of Technology adds calcium nitrate on the basis of Hitec salt to form a low-melting-point quaternary mixed nitrate salt, and the melting point is 83.1℃, the complete melting temperature is 180.9℃, the crystallization point is 163.1℃, and the decomposition temperature is 628.5℃. The normal use liquid temperature range is greatly widened. The melting point, complete melting temperature, crystallization temperature and decomposition temperature of Hitec XL commercial salt and existing low-melting-point mixed molten salt are as follows: Figure 7 andFigure 8 as shown.

[0005] According to the relevant national standards, the temperature range for using the mixed molten salt is 50 DEG C above the melting point to 30 DEG C below the decomposition temperature, while considering the complete melting temperature and crystallization temperature of the mixed molten salt. From the above analysis, it can be seen that the lower limit temperature of the existing low melting point mixed molten salt is difficult to be widened to below 150 DEG C, which limits the application field of the mixed molten salt.

[0006] In recent years, with the promotion of the "3060" double carbon target, compressed air energy storage and flexible regulation of thermal power plants play an increasingly important role in large-scale wind and light consumption regulation of the power grid. However, the storage of compression heat in compressed air energy storage technology and the recovery and storage of steam heat in thermal power plants have higher requirements for the lower limit temperature of the heat storage material, and it is hoped that the normal use temperature of the mixed molten salt heat storage material is lower, that is, the melting point is lower than 100 DEG C, or the crystallization point is less than 120 DEG C, and the lower limit temperature of normal use is lower than 150 DEG C, and the upper limit temperature of use is higher than 550 DEG C. The existing mixed molten salt reported in the literature is difficult to meet this requirement.

[0007] Therefore, the development of low-cost mixed molten salt materials with a lower limit temperature of less than 150 DEG C and an upper limit temperature of more than 550 DEG C is of great importance to the application of compressed air energy storage, flexible modification of thermal power plants, heat pump heat storage and other long-term and large-scale energy storage technologies. SUMMARY

[0008] In view of the deficiencies in the prior art, the present application adjusts the proportion of each component on the basis of Hitec XL mixed molten salt, and proposes a ternary mixed molten salt heat transfer and storage material and a preparation method, which has the advantages of low melting point, low cost, low corrosion, high specific heat, high decomposition temperature and wide liquid working temperature range.

[0009] The application discloses a low-melting-point ternary mixed molten salt heat transfer and storage material, which comprises, by mass percentage:

[0010] KNO3 45~55%;

[0011] NaNO3 10~20%;

[0012] Ca(NO3)2 30~40%。

[0013] As a further improvement of the application, the further optimized addition amount of each component is, by mass percentage:

[0014] KNO3 45~51%;

[0015] NaNO3 14~19%;

[0016] Ca(NO3)2 34~39%.

[0017] The application also discloses a preparation method of the low-melting-point ternary mixed molten salt heat transfer and storage material.

[0018] KNO3, NaNO3 and Ca(NO3)2 are weighed according to preset component proportions, mixed, ground, and uniformly stirred to obtain the solid mixed molten salt.

[0019] The solid mixed molten salt is placed in a drying box for constant-temperature drying treatment.

[0020] The dried solid mixed molten salt is placed in a muffle furnace, the solid mixed molten salt is heated to a set temperature at a preset heating rate, the mixed molten salt is then melted and left to stand to obtain the molten salt.

[0021] The molten salt is taken out of the muffle furnace and placed in the drying box for natural cooling.

[0022] The cooled molten salt is placed in a pulverizer for pulverization to obtain the mixed molten salt powder.

[0023] The mixed molten salt powder is placed in the drying box for drying treatment to obtain the low-melting-point ternary mixed molten salt heat transfer and storage material.

[0024] As a further improvement of the application, the solid mixed molten salt is placed in the drying box at 85-90 DEG C for constant-temperature drying for more than 24 hours.

[0025] As a further improvement of the application, the preset heating rate of the muffle furnace is 5-10 K / min.

[0026] As a further improvement of the application, the solid mixed molten salt is heated to 50-150 DEG C below the decomposition temperature in the muffle furnace and left to stand for more than 12 hours to obtain the molten salt.

[0027] As a further improvement of the application, the molten salt is placed in the drying box at 80-85 DEG C for natural cooling.

[0028] As a further improvement of the application, the particle size of the mixed molten salt powder is 20-200 meshes.

[0029] As a further improvement of the application, the mixed molten salt powder is placed in the drying box at 85-90 DEG C for drying treatment for more than 24 hours to obtain the low-melting-point ternary mixed molten salt heat transfer and storage material.

[0030] As a further improvement of the application, the initial melting and charging temperature of the low-melting-point ternary mixed molten salt heat transfer and storage material is not less than 350 DEG C, so as to reduce the influence of the moisture and corrosiveness of the molten salt.

[0031] As a further improvement of the present application, the application fields of the low-melting ternary mixed molten salt heat transfer and storage material include, but are not limited to, photothermal power generation, compressed air energy storage, industrial waste heat recovery, molten salt heat pump, low-valley electricity storage heating and flexible modification of thermal power plants.

[0032] Compared with the prior art, the present application has the following beneficial effects:

[0033] The low-melting ternary mixed molten salt heat transfer and storage material of the present application is a new type of mixed molten salt heat transfer and storage material with low cost and wide liquid temperature range, with a melting point less than 100 DEG C, which is reduced by more than 110 DEG C compared with commercial solar salt; the decomposition temperature is greater than 580 DEG C, the normal use liquid temperature range is between 150 DEG C and 550 DEG C, the liquid temperature range is wide, the heat storage density is large, the anti-frozen effect is obvious; it can be applied in the fields of compressed air energy storage, flexible adjustment of thermal power plants, molten salt heat pump heat storage, solar photothermal power generation, industrial waste heat recovery, etc., which can increase the stability of the safe operation of the whole system.

[0034] The low-melting ternary mixed molten salt heat transfer and storage material of the present application does not contain noble metal salt, has low cost, large heat storage density, and greatly reduces the cost of the heat transfer and storage system.

[0035] The low-melting ternary mixed molten salt heat transfer and storage material of the present application is formed by mixing three nitrate salts to form a multi-component salt, has low corrosion, low viscosity and good flowability. The preparation process is simple, the requirement for equipment performance is low, and it is suitable for large-scale production. BRIEF DESCRIPTION OF DRAWINGS

[0036] The present application will be further described below with reference to the embodiments and the accompanying drawings, in which:

[0037] Figure 1 The melting point and crystallization temperature diagram of the mixed molten salt sample prepared in Example 1 is shown in the figure;

[0038] Figure 2 The decomposition temperature diagram (calculated according to 3% mass reduction) of the mixed molten salt sample prepared in Example 1 is shown in the figure;

[0039] Figure 3 The specific heat capacity diagram of the mixed molten salt sample prepared in Example 1 is shown in the figure;

[0040] Figure 4 The melting point and crystallization temperature diagram of the mixed molten salt sample prepared in Example 2 is shown in the figure;

[0041] Figure 5 The decomposition temperature diagram (calculated according to 3% mass reduction) of the mixed molten salt sample prepared in Example 2 is shown in the figure;

[0042] Figure 6 The specific heat capacity diagram of the mixed molten salt sample prepared in Example 2 is shown in the figure;

[0043] Figure 7 Melting point and crystallization temperature diagram of existing Hitec XL commercial salt and existing low melting point mixed molten salt;

[0044] Figure 8 Decomposition temperature diagram (calculated according to 3% mass loss) of existing Hitec XL commercial salt and existing low melting point mixed molten salt. DETAILED DESCRIPTION

[0045] In order to make the purpose, technical scheme and advantages of the embodiments of the present application clearer, the technical scheme in the embodiments of the present application will be described clearly and completely below in conjunction with the drawings of the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work belong to the scope of protection of the present application.

[0046] The present application will be further described in detail below in conjunction with the drawings:

[0047] 1. The present application will be further described below in conjunction with Example 1.

[0048] Example 1 provides a low melting point ternary mixed molten salt heat storage and transfer material, which is composed of 50wt% KNO3, 15wt% NaNO3 and 35wt% Ca(NO3)2.

[0049] The present application provides a preparation method of a low melting point ternary mixed molten salt heat storage and transfer material, comprising:

[0050] Step 1. Commercial salts of 50wt% KNO3, 15wt% NaNO3 and 35wt% Ca(NO3)2 are weighed according to the preset component ratio, mixed and ground, and uniformly stirred to obtain a solid mixed molten salt;

[0051] Step 2. The solid mixed molten salt is placed in a constant temperature drying box at 90℃ for constant temperature drying for 36h;

[0052] Step 3. The dried solid mixed molten salt is placed in a muffle furnace, and the solid mixed molten salt is heated to 450℃ at a heating rate of 8 K / min to melt the mixed molten salt, and the mixed molten salt is left to stand for 18h to obtain a molten salt;

[0053] Step 4. The molten salt is taken out and naturally cooled in a drying box at 85℃;

[0054] Step 5. The cooled molten salt is placed in a pulverizer for pulverization to obtain a mixed molten salt powder; wherein the particle size of the mixed molten salt powder is 150 mesh.

[0055] Step 6: Place the mixed molten salt powder in a drying oven at 90°C for 36 hours to obtain a low-melting-point ternary mixed molten salt heat transfer and heat storage material.

[0056] experiment:

[0057] The thermophysical properties of the obtained low-melting-point ternary mixed molten salt heat transfer and storage material were tested; among them,

[0058] like Figure 1 As shown, the melting point of the mixed molten salt heat transfer and heat storage material prepared in Example 1 is 98.5℃, the melting termination point is 136.9℃, the latent heat of phase change is 43.5J / g, and the primary crystallization point is 115.1℃; therefore, the lowest lower limit temperature at which this mixed molten salt can be used is 145.1℃.

[0059] like Figure 2 As shown, the decomposition temperature of the mixed molten salt heat transfer and heat storage material prepared in Example 1 is 582.6℃ (calculated based on a 3% mass reduction), therefore the upper limit of the use temperature of this mixed molten salt is 552.6℃;

[0060] like Figure 3 As shown, the liquid specific heat of the mixed molten salt heat transfer and storage material prepared in Example 1 is 1.36~1.56 J / (g ∙ K) in the temperature range of 150~450℃.

[0061] 2. The present invention will be further described below with reference to Example 2.

[0062] Example 2 provides a low-melting-point ternary mixed molten salt heat transfer and storage material, which is composed of 51wt% KNO3, 15wt% NaNO3 and 34wt% Ca(NO3)2.

[0063] This invention provides a method for preparing a low-melting-point ternary mixed molten salt heat transfer and storage material, comprising:

[0064] Step 1: Weigh 51wt% KNO3, 15wt% NaNO3 and 34wt% Ca(NO3)2 of commercial salt according to the preset component ratio, mix and grind them, and stir them evenly to obtain a solid mixed molten salt.

[0065] Step 2: Place the solid mixed molten salt in a constant temperature drying oven at 90℃ and dry it at a constant temperature for 36 hours;

[0066] Step 3: Place the dried solid mixed molten salt in a muffle furnace and heat the solid mixed molten salt to 450℃ at a heating rate of 8 K / min to melt the mixed molten salt. Let it stand for 18 hours to obtain molten salt.

[0067] Step 4: Remove the molten salt and place it in a drying oven at 85°C to cool naturally;

[0068] Step 5: Place the cooled molten salt in a pulverizer for pulverization to obtain mixed molten salt powder; wherein the particle size of the mixed molten salt powder is 150 mesh;

[0069] Step 6: Place the mixed molten salt powder in a drying oven at 90°C for 36 hours to obtain a low-melting-point ternary mixed molten salt heat transfer and heat storage material.

[0070] experiment:

[0071] The thermophysical properties of the obtained low-melting-point ternary mixed molten salt heat transfer and storage material were tested; among them,

[0072] like Figure 4 As shown, the melting point of the mixed molten salt heat transfer and heat storage material prepared in Example 2 is 98.6℃, the melting termination point is 134.8℃, the latent heat of phase change is 35.8J / g, and the primary crystallization point is 114.7℃; therefore, the lowest lower limit temperature at which this mixed molten salt can be used is 144.7℃.

[0073] like Figure 5 As shown, the decomposition temperature of the mixed molten salt heat transfer and heat storage material prepared in Example 2 is 582.8℃ (calculated based on a 3% mass reduction), therefore the upper limit of the use temperature of this mixed molten salt is 552.8℃;

[0074] like Figure 6 As shown, the liquid specific heat of the mixed molten salt heat transfer and storage material prepared in Example 2 is 1.29~1.59 J / (g ∙ K) in the temperature range of 150~450℃.

[0075] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A low melting point ternary mixed molten salt heat transfer and heat storage material, characterized by, By mass percentage, comprising: KNO3 45-51%; NaNO3 14-19%; Ca(NO3)2 34-39%.

2. A method for producing a low-melting ternary mixed molten salt heat transfer and heat storage material as claimed in claim 1, characterized by, Comprising: By preset component ratio, KNO3, NaNO3 and Ca(NO3)2 are weighed, mixed, ground, and uniformly stirred to obtain solid mixed molten salt; The solid mixed molten salt is placed in a drying box for drying treatment; The dried solid mixed molten salt is placed in a muffle furnace, and the solid mixed molten salt is heated to a set temperature at a preset heating rate, then the mixed molten salt is melted, and is left to stand to obtain molten salt; The molten salt is taken out of the muffle furnace and placed in a drying box for natural cooling; The cooled molten salt is placed in a pulverizer for pulverization to obtain mixed molten salt powder; The mixed molten salt powder is placed in a drying box for drying treatment to obtain low-melting-point ternary mixed molten salt heat transfer and storage material.

3. The preparation method of the low-melting-point ternary mixed molten salt heat transfer and storage material as described in claim 2, characterized in that, The solid mixed molten salt is placed in a drying box at 85-90 DEG C for constant temperature drying for more than 24 hours.

4. The preparation method of the low-melting-point ternary mixed molten salt heat transfer and storage material as described in claim 2, characterized in that, The preset heating rate of the muffle furnace is 5-10 K / min.

5. The preparation method of the low-melting-point ternary mixed molten salt heat transfer and storage material as described in claim 2, characterized in that, The solid mixed molten salt is heated to 50-150 DEG C below the decomposition temperature by the muffle furnace, and is left to stand for more than 12 hours to obtain molten salt.

6. The preparation method of the low-melting-point ternary mixed molten salt heat transfer and storage material as described in claim 2, characterized in that, The molten salt is placed in a drying box at 80-85 DEG C for natural cooling; the particle size of the pulverized mixed molten salt powder is 20-200 mesh.

7. The preparation method of the low-melting-point ternary mixed molten salt heat transfer and storage material as described in claim 2, characterized in that, The mixed molten salt powder is placed in a drying box at 85-90 DEG C for drying treatment for more than 24 hours to obtain low-melting-point ternary mixed molten salt heat transfer and storage material.

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