Quaternary mixed molten salt heat transfer and storage medium

Abstract

The invention discloses a quaternary mixed molten salt heat transfer and storage medium, and belongs to the technical field of physical heat transfer and energy storage in high and new technologies. The high-decomposition-temperature mixed molten salt heat transfer and storage medium comprises 30-70 wt% of potassium nitrate, 20-60 wt% of sodium nitrate, 5-45 wt% of sodium nitrite and 0.5-20 wt% of sodium carbonate, the decomposition temperature of the mixed molten salt is higher than 700 DEG C, the melting point of the mixed molten salt is lower than 150 DEG C, and the high-decomposition-temperature mixed molten salt heat transfer and storage medium has a wide use temperature range.

Description

Technical field:

[0001] The invention relates to a formula of mixed molten salt for medium- and high-temperature heat transfer and heat storage, belonging to the technical field of physical heat transfer and energy storage in high-tech. Background technology:

[0003] Pumped storage power stations are currently the most important grid energy storage technology, but because they require special terrain conditions, pumped storage alone cannot meet the demand for a high proportion of renewable energy. There is an urgent need to develop new long-term, large-capacity, and low-cost energy storage technologies other than pumped storage.

[0004] Molten salt heat storage uses the absorption of heat by liquid salt when the temperature rises and the release of heat when the temperature drops to achieve heat storage and release. The molten salt used for heat storage is generally a eutectic mixed salt formed by mixing two or more inorganic salts in a certain proportion. It has the advantages of a wide liquid temperature range, large temperature difference, large heat storage density and long life. Molten salt heat storage generally uses a double-tank liquid sensible heat storage solution, which has the advantages of constant inlet and outlet parameters of the heat storage and heat release heat exchanger / electric heater, the outlet temperature of the heat-taking fluid is close to the temperature of the hot salt tank, and simple control. It is a low-cost, large-capacity and long-life long-term energy storage technology, and has broad application scenarios in the fields of solar thermal power generation, thermal power plant heat storage peak regulation, molten salt direct / heat pump heat storage power generation, molten salt heat storage for heat and steam supply, compressed air energy storage and compressed heat storage.

[0005] At present, more than 30 solar thermal power stations with integrated large-capacity heat storage have been put into commercial operation around the world (with a total installed capacity of more than 3 million kilowatts), and the longest molten salt heat storage solar thermal power station has been successfully operating for 18 years.

[0006] Improving the heat storage density of molten salt is the development direction of molten salt heat storage. Molten salt heat storage belongs to sensible heat storage, so increasing the temperature difference of molten salt heat storage can significantly increase the heat storage density of molten salt. Improving the temperature difference of molten salt heat storage requires that the molten salt has a wide liquid temperature range, that is, the melting point of the molten salt should be as low as possible and the decomposition temperature should be as high as possible.

[0007] Increasing the storage / release temperature of molten salt is an effective technical approach to improve the energy conversion efficiency and economy of electric thermal molten salt heat storage peak-shaving combined heat and power, heat pump storage, deep peak-shaving of thermal power plants, and solar thermal power generation. In particular, the development of supercritical carbon dioxide power generation and ultra-supercritical Rankine cycle power generation technology urgently requires the development of high-temperature molten salt heat storage materials, equipment and systems with a decomposition temperature of 700°C. The Energy Storage Grand Challenge Roadmap released by the United States in December 2020 also included heat storage materials and equipment above 700°C in the Grand Challenge of Heat Storage and future key support for heat storage research projects.

[0008] Lowering the minimum operating temperature of molten salt can maximize the absorption of heat from the heat source and meet the lower temperature requirement of the power cycle working fluid of the molten salt thermal storage coupled energy system. Low melting point can also reduce the freezing risk and anti-freezing cost of the molten salt thermal storage system, and improve the reliability of the molten salt thermal storage system.

[0009] At present, binary nitrate (solar salt) is used for molten salt heat storage, which has the defects of melting point up to 220℃, easy freezing and blocking of pipelines, maximum operating temperature within 570℃, small heat storage temperature difference and low heat storage density. A few molten salt heat storage uses ternary nitrate (Hitec) salt with a melting point of 142℃, but it has the defects of low decomposition temperature (nitrite begins to be oxidized above 450℃) and low heat storage density. In order to reduce the risk of freezing and blocking of molten salt heat storage system, domestic and foreign scholars have been conducting research and development of low melting point molten salt in recent years. In particular, Beijing University of Technology has successively developed low melting point binary salt, low melting point ternary salt and low melting point quaternary salt. The melting point of mixed molten salt is reduced to about 90-140℃, the decomposition temperature is between 560-630℃, the long-term minimum operating temperature of molten salt is reduced to 150-200℃, and the maximum operating temperature is between 500-600℃, which greatly improves the heat storage temperature difference and heat storage density, thereby significantly reducing the cost of heat storage materials and heat storage systems. The U.S. Department of Energy has invested $84.51 million to support the development of high-temperature chloride salts through the Coal FIRST initiative, DAYS and third-generation solar thermal power generation programs, but chloride salts have a technical bottleneck of melting point up to 400°C and high corrosiveness. High-temperature mixed carbonates have also been developed at home and abroad, but the formulas all contain precious metal lithium salts, which have the disadvantages of high cost and high viscosity.

[0010] In summary, the decomposition temperature of molten salts currently developed and used at home and abroad is concentrated below 650°C, which cannot meet the requirements of high-parameter power generation such as supercritical carbon dioxide power generation and ultra-supercritical steam power generation for high-temperature molten salts with a wide liquid temperature range and a decomposition temperature ≥ 700°C and a melting point ≤ 150°C. Therefore, it is very necessary to develop a molten salt heat storage medium with a low melting point, high decomposition temperature and a wide temperature range that does not contain lithium salts. Summary of the invention:

[0011] The technical problem to be solved by the invention is that the invention does not contain noble metal salts and improves the thermal stability decomposition temperature.

[0012] In order to solve the above technical problems, the present invention provides a formula of mixed molten salt.

[0013] A quaternary mixed molten salt heat transfer and heat storage medium is a quaternary mixed molten salt heat transfer and heat storage medium with a wide liquid temperature range, comprising 30-70wt% potassium nitrate, 20-60wt% sodium nitrate, 5-45wt% sodium nitrite, and 0.05-20wt% sodium carbonate.

[0014] The beneficial effects of the present invention are:

[0015] 1 The decomposition temperature of the mixed molten salt prepared by the technical solution of the present invention is greatly improved (the decomposition temperature range is: 710-745°C). Its application in the solar thermal power generation system broadens the applicable temperature range of the molten salt and improves the thermal stability.

[0016] 2 The decomposition temperature of the mixed molten salt KNO3-NaNO3-NaNO2-Na2CO3 prepared by the technical solution of the present invention is greatly improved compared with the common thermal storage materials Solar salt and Hitec salt, and the melting point is lower than that of Solar salt. Description of the drawings:

[0017] Figure 1 DSC curve of the mixed molten salt of Example 1.

[0018] Figure 2 Example 1 TG curve of mixed molten salt.

[0019] Figure 3 Example 1 Specific heat measurement results of quaternary mixed molten salt.

[0020] Figure 4 Example 1 Viscosity measurement results of quaternary mixed molten salt.

[0021] Figure 5 DSC curve of the mixed molten salt of Example 2.

[0022] Figure 6 Example 2 TG curve of mixed molten salt. Specific implementation method:

[0023] The present invention is further described in detail below in conjunction with specific implementation modes.

[0024] The invention provides a mixed molten salt formula used as a heat transfer and heat storage medium in a solar thermal power generation system. The formula mainly contains potassium nitrate, sodium nitrate, sodium nitrite and sodium carbonate.

[0025] The main mechanism of this technology to increase the decomposition temperature is that the decomposition temperature of a single component nitrate is too low, and adding a carbonate with a high decomposition temperature to the nitrate to form a eutectic mixed molten salt can significantly increase its decomposition temperature. The eutectic mixed molten salt can ensure stable and uniform thermal properties of phases and components within a wide operating temperature range. The thermophysical properties of mixed molten salts are different due to different components and proportions. Therefore, in the preparation of mixed molten salts, the needs of all parties should be weighed and the types and proportions of components of the mixed molten salt should be carefully selected.

[0026] The preferred specific implementation of this series of quaternary mixed molten salts is:

[0027] First, the quaternary mixed nitrate (KNO3, NaNO3, NaNO2, Na2CO3) is configured according to different mass ratios to obtain a mixed molten salt. The specific operation method is: use a high-precision analytical balance to weigh the above four components, and mix and grind them thoroughly; then place them in a drying oven for constant temperature drying, set the heating temperature to 150°C, and the heating time is 96 hours to allow the water contained in the mixture to overflow, and place the dried molten salt in a muffle furnace and heat it to 500°C for 12 hours to completely melt the mixture and mix it evenly, which is the static melting method; after the mixed molten salt is cooled, it is taken out. Since the molten salt will form a hard solid crystalline salt after being melted and cooled, the present invention uses an ultrafine grinder to crush the mixture, and the sample fineness can reach 20 to 200 meshes. While ensuring the fineness, the mixture is also fully mixed, thereby ensuring the accuracy of the experiment; finally, the crushed molten salt is placed in a drying oven for constant temperature drying for use in the experiment.

[0028] Example 1

[0029] This mixed molten salt is composed of 43wt% potassium nitrate, 38wt% sodium nitrate, 18wt% sodium nitrite, and 1wt% sodium carbonate. Its melting point is 144.69°C as tested by DSC (differential scanning calorimetry), and its decomposition temperature is 745.35°C as tested by TG (thermogravimetric analysis). Figure 1 Shown is the DSC curve of the sample. Figure 2 Shown is the TG curve of the sample. Figure 3 This is the specific heat test result of the sample. Figure 4 is the viscosity measurement result of the sample.

[0030] Compared with Solar Salt, the decomposition temperature of this formula is increased by about 140°C, and compared with Hitec salt, the decomposition temperature of this formula is increased by about 210°C, with a wider operating temperature range. Compared with common thermal storage materials, its average specific heat is 1.3667J / (g·K), and the energy storage density is 710.684kJ / kg, which is significantly higher than the energy storage density of molten salt thermal storage media such as solar salt and Hitec. The cost of sensible heat storage is 18.81 yuan / (kW·h), and the cost of this quaternary mixed molten salt is relatively considerable.

[0031] Example 2

[0032] This mixed molten salt is composed of 48wt% potassium nitrate, 39wt% sodium nitrate, 12wt% sodium nitrite, and 1wt% sodium carbonate. Its melting point is 136.17°C as tested by DSC (differential scanning calorimetry), and its decomposition temperature is 712.95°C as tested by TG (thermogravimetric analysis). Figure 5 Shown is the DSC curve of the sample. Figure 6 The TG curve of the sample is shown in Figure 1. Compared with Solar Salt, the decomposition temperature of this formula is increased by about 110°C, and compared with Hitec salt, the decomposition temperature of this formula is increased by about 180°C, which has a wider operating temperature range.

[0033] The protection scope of the present invention is not limited to the above embodiments, and all technical modifications made according to the technical principles of the present invention fall within the protection scope of the present invention.

Claims

1. A quaternary mixed molten salt heat transfer and heat storage medium, characterized in that: The invention is a mixed molten salt heat transfer and heat storage medium with a wide liquid temperature range, comprising 30-70wt% potassium nitrate, 20-60wt% sodium nitrate, 5-45wt% sodium nitrite and 0.5-20wt% sodium carbonate.

2. The method for preparing a quaternary mixed molten salt heat transfer and heat storage medium according to claim 1, characterized in that: The above four components are weighed, mixed and ground thoroughly; then placed in a drying oven for constant temperature drying, the heating temperature is set to 150°C, the heating time is 96 hours, so that the water contained in the mixture overflows, the dried molten salt is placed in a muffle furnace and heated to 500°C for 12 hours to completely melt the mixture and mix it evenly, which is the static melting method; after the mixed molten salt is cooled, it is taken out; the mixture is crushed with an ultrafine grinder, and the sample fineness reaches 20-200 mesh.

3. Application of the quaternary mixed molten salt heat transfer and heat storage medium described in claim 1 in a solar thermal power generation system.

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