Ternary mixed molten salt heat transfer and storage medium and preparation method thereof

Abstract

The invention discloses a ternary mixed molten salt heat transfer and storage medium and a preparation method thereof, and belongs to the technical field of physical heat transfer and energy storage, the ternary mixed molten salt heat transfer and storage medium comprises 30-60wt% of potassium nitrate, 20-40wt% of sodium nitrite and 20-40wt% of lithium nitrate. According to the ternary mixed molten salt heat transfer and storage medium and the preparation method thereof, the melting point of the prepared ternary mixed molten salt is 86.167, the decomposition temperature is 599, the liquid temperature range is wide, the operation and maintenance cost of a molten salt heat storage system is greatly reduced, and the safety of the system is improved.

Description

Technical Field

[0001] This invention relates to the field of physical heat transfer and energy storage technology, and in particular to a ternary mixed molten salt heat transfer and storage medium and its preparation method. Background Technology

[0002] Molten nitrate salts are widely used in high-temperature sensible heat storage applications such as concentrated solar power (CSP), renewable energy storage for peak shaving, and flexible retrofitting of thermal power plants due to their wide availability, low price, low corrosivity, high thermal density, and low system pressure. Currently, commonly used mixed molten nitrate salts include Solar salt, Hitec salt, and Hitec XL salt. Among them, Solar salt (60wt% NaNO3 + 40wt% KNO3) has a melting point of 220°C. Decomposition temperature 586 Hitec salt (53wt% KNO3 + 7wt% NaNO3 and 4wt% NaNO2) has a melting point of 142°C. Decomposition temperature 535 And the upper limit of long-term thermal stability is 454. Hitec XL salt (48wt% Ca(NO3)2 + 7wt% NaNO3 + 45wt% KNO3) melting point 130°C Upper limit of operating temperature: 500°C These molten salts operate at temperatures between 130 and 586 degrees Celsius. It can meet the needs of medium and high temperature sensible heat storage.

[0003] However, conventional molten salt dual-tank sensible heat storage has significant problems: when the system stops operating, the molten salt easily solidifies and clogs the pipes and heat exchangers, requiring additional heat tracing and insulation systems, leading to a double increase in energy consumption and investment. For example, the Hebei Xinji molten salt thermal storage off-peak electricity green heating demonstration project converts off-peak electricity at night into 180-390 kWh of electricity. Molten salt thermal energy can be used for heating and medium- and low-temperature industrial steam, but due to the high melting point of molten salt, two-stage heat exchange is required for domestic water supply, significantly increasing system complexity. Therefore, developing low-melting-point, wide-temperature-range molten salt thermal storage and heat transfer materials has become a key direction for reducing the operation and maintenance costs of molten salt thermal storage systems, improving system safety, and expanding the application areas of molten salt technology. Summary of the Invention

[0004] The purpose of this invention is to provide a ternary mixed molten salt heat transfer and storage medium and its preparation method. The prepared ternary mixed molten salt has a low melting point and a wide liquid temperature range, which greatly reduces the operation and maintenance cost of the molten salt heat storage system and improves the system's safety.

[0005] To achieve the above objectives, the present invention provides a ternary mixed molten salt heat transfer and storage medium, comprising potassium nitrate, sodium nitrite and lithium nitrate.

[0006] Preferably, it includes 30-60 wt% potassium nitrate, 20-40 wt% sodium nitrite and 20-40 wt% lithium nitrate.

[0007] Preferably, the mass fractions of potassium nitrate, sodium nitrite, and lithium nitrate are obtained by solving the ternary phase diagram of potassium nitrate-sodium nitrite-lithium nitrate system using FactSage 8.3 software based on the phase diagram thermodynamics theory.

[0008] This invention also provides a method for preparing a ternary mixed molten salt heat transfer and storage medium, comprising the following steps: S1. According to the eutectic point composition of the ternary mixed molten salt calculated based on the phase diagram thermodynamic theory, potassium nitrate, sodium nitrite and lithium nitrate are mixed to obtain a premixed molten salt; S2. The premixed molten salt obtained in S1 is pretreated and then melted to obtain molten salt; S3. Cool and crush the molten salt obtained in S2 to obtain a ternary mixed molten salt.

[0009] Preferably, in S2, the pretreatment process is carried out at 100-110... Dry under the specified conditions for 96-100 hours.

[0010] Preferably, in S2, the melting heating rate is 5-10. The melting temperature is 450-500 min. The melting time is 12-15 hours.

[0011] Preferably, in S3, the fineness of the ternary mixed molten salt is 20-200 mesh.

[0012] Therefore, the present invention employs the above-mentioned ternary mixed molten salt heat transfer and storage medium and its preparation method, which has the following beneficial effects: (1) The melting point of the ternary mixed molten salt of the present invention is 86.167. The decomposition temperature is 599. The wide liquid temperature range greatly reduces the operation and maintenance costs of molten salt thermal storage systems and improves system safety.

[0013] (2) The mixed molten salt of the present invention is composed of two pure nitrates and one nitrite. Potassium nitrate is inexpensive and easy to purchase and store, which can reduce the overall preparation cost of the mixed molten salt and balance performance and economy. Lithium nitrate can lower the melting point of the system and improve thermal conductivity, thus expanding the operating temperature range. After the nitrate molten salt is operated at high temperature, it will contain a certain amount of nitrite. Increasing the nitrite content in the nitrate will further reduce the melting point and freezing point of the molten salt. Therefore, the present invention also adds some sodium nitrite.

[0014] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0015] Figure 1 The differential scanning calorimetry (DSC) curve of the ternary mixed molten salt prepared in Example 1 of this invention is shown. Figure 2 The thermogravimetric analysis (TG) curve of the ternary mixed molten salt prepared in Example 1 of this invention is shown. Detailed Implementation

[0016] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0017] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0018] In this invention, unless otherwise specified, the test materials and instruments are all conventional test materials in the field and can be purchased through commercial channels.

[0019] Example 1 This invention provides a ternary mixed molten salt heat transfer and storage medium, comprising 45.14 wt% potassium nitrate, 26.74 wt% sodium nitrite and 28.12 wt% lithium nitrate.

[0020] In this invention, the ternary mixed molten salt is based on the phase diagram thermodynamics theory. The phase diagram of the potassium nitrate-sodium nitrite-lithium nitrate ternary system is solved using FactSage 8.3 software to determine the composition of the eutectic point of the system, which has high screening efficiency.

[0021] This invention also provides a method for preparing the above-mentioned ternary mixed molten salt heat transfer and storage medium, comprising the following steps: S1. Using a high-precision balance, accurately weigh 45.14g of potassium nitrate, 26.74g of sodium nitrite and 28.12g of lithium nitrate, place them in a crucible and mix them evenly to form a premixed molten salt; S2. Place the premixed molten salt obtained in S1 at a temperature set to 100°C. Dry in a drying oven at a constant temperature for 96 hours, then melt in a muffle furnace, set to 10 The temperature rise time is set to 450 degrees Celsius. The melting time is 12 hours to allow the premixed molten salt to completely melt and mix evenly, thus obtaining molten salt. S3. Take the molten salt obtained in S2 out of the muffle furnace, place it in a drying oven to cool, put the cooled molten salt into an ultrafine pulverizer, pulverize it until the fineness of the molten salt powder reaches 200 mesh, then place it in a crucible and dry it at a constant temperature in a drying oven to obtain a ternary mixed molten salt.

[0022] The ternary mixed molten salt prepared in Example 1 was characterized by differential scanning calorimetry, and the results are as follows: Figure 1 As shown, from Figure 1 As can be seen, the melting point of the ternary mixed molten salt is 86.167. The latent heat of fusion is 89.68 J. g, primary crystallization point is 85 .

[0023] The ternary mixed molten salt prepared in Example 1 was characterized by thermogravimetric analysis, and the results are as follows: Figure 2 As shown, from Figure 2 As can be seen from this, based on a 3% decrease in mass, the decomposition temperature of the ternary mixed molten salt is 599°C. .

[0024] Therefore, the present invention employs the above-mentioned ternary mixed molten salt heat transfer and storage medium and its preparation method, and the resulting ternary mixed molten salt has a melting point of 86.167°C. The decomposition temperature is 599. The wide liquid temperature range greatly reduces the operation and maintenance costs of molten salt thermal storage systems and improves system safety.

[0025] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A ternary mixed molten salt heat transfer and storage medium, characterized in that: This includes potassium nitrate, sodium nitrite, and lithium nitrate.

2. The ternary mixed molten salt heat transfer and storage medium according to claim 1, characterized in that: It includes 30-60 wt% potassium nitrate, 20-40 wt% sodium nitrite, and 20-40 wt% lithium nitrate.

3. The method for preparing a ternary mixed molten salt heat transfer and storage medium according to any one of claims 1-2, characterized in that: Includes the following steps: S1. According to the eutectic point composition of the ternary mixed molten salt calculated based on the phase diagram thermodynamic theory, potassium nitrate, sodium nitrite and lithium nitrate are mixed to obtain a premixed molten salt; S2. The premixed molten salt obtained in S1 is pretreated and then melted to obtain molten salt; S3. Cool and crush the molten salt obtained in S2 to obtain a ternary mixed molten salt.

4. The method for preparing a ternary mixed molten salt heat transfer and storage medium according to claim 3, characterized in that: In S2, the preprocessing process is from 100 to 110. Dry under the specified conditions for 96-100 hours.

5. The method for preparing a ternary mixed molten salt heat transfer and storage medium according to claim 3, characterized in that: In S2, the melting heating rate is 5-10. The melting temperature is 450-500 min. The melting time is 12-15 hours.

6. The method for preparing a ternary mixed molten salt heat transfer and storage medium according to claim 3, characterized in that: In S3, the fineness of the ternary mixed molten salt is 20-200 mesh.

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