Quaternary mixed molten salt heat transfer and storage medium

Abstract

A kind of quaternary mixed molten salt heat transfer and heat storage medium belongs to the field of physical heat transfer energy storage technology in high-tech.The component ratio of high decomposition temperature mixed molten salt heat transfer and heat storage medium is as follows: 30-70wt% potassium nitrate, 20-60wt% sodium nitrate, 5-45wt% sodium nitrite, 0.5-20wt% sodium carbonate.The decomposition temperature of the mixed molten salt is greater than 700℃, the melting point is lower than 150℃, and it has a wide range of use temperature.

Description

Technical fields:

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

[0002] Pumped hydro storage is currently the most important grid energy storage technology. However, due to the need for special terrain conditions, pumped hydro 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, low-cost energy storage technologies other than pumped hydro storage.

[0003] Molten salt thermal energy storage utilizes the absorption of heat when the temperature of liquid salt rises and the release of heat when the temperature falls to achieve the storage and release of heat. The molten salt used in thermal energy storage is generally a eutectic mixture formed by mixing two or more inorganic salts in a certain proportion. It has advantages such as a wide liquid temperature range, large temperature difference, high heat storage density, and long lifespan. Molten salt thermal energy storage typically uses a dual-tank liquid sensible heat storage scheme, which has advantages such as constant inlet and outlet parameters of the heat exchanger / electric heater, a heat extraction fluid outlet temperature close to the hot salt tank temperature, and simple control. It is a low-cost, large-capacity, and long-life long-term energy storage technology with broad application scenarios in fields such as concentrated solar power generation, thermal power plant thermal energy storage for peak shaving, molten salt direct / heat pump thermal energy storage for power generation, molten salt thermal energy storage for heating and steam supply, and compressed air energy storage for compressed heat storage.

[0004] Currently, more than 30 solar thermal power plants with integrated large-capacity thermal storage have been put into commercial operation worldwide (with a total installed capacity of more than 3 million kilowatts), and the longest molten salt thermal storage solar thermal power plant has been operating successfully for 18 years.

[0005] Increasing the thermal density of molten salt is the future direction of molten salt thermal storage. Molten salt thermal storage belongs to sensible heat storage, so increasing the temperature difference in molten salt thermal storage can significantly increase its thermal density. Increasing the temperature difference in molten salt thermal storage requires that the molten salt have 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.

[0006] Increasing the molten salt storage / release temperature is an effective technical approach to improve the energy conversion efficiency and economy of molten salt thermal storage for peak shaving in combined heat and power (CHP), heat pump energy storage, deep peak shaving in 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 technologies urgently requires the development of high-temperature molten salt thermal storage materials, equipment, and systems with decomposition temperatures reaching 700℃. The "Energy Storage Grand Challenge Roadmap" released by the United States in December 2020 also listed thermal storage materials and equipment with temperatures above 700℃ as a key research project for future support in the thermal storage challenge.

[0007] Lowering the minimum operating temperature of molten salt maximizes the absorption of heat from the heat source and meets the lower limit of the working fluid temperature requirement for the power cycle in molten salt thermal storage coupled energy systems. A lower melting point also reduces the risk and cost of freezing in molten salt thermal storage systems, thereby improving their reliability.

[0008] Currently, molten salt thermal storage primarily uses binary nitrates (solar salts), which suffer from drawbacks such as a high melting point of 220℃, susceptibility to freezing and clogging of pipelines, a maximum operating temperature below 570℃, small thermal temperature difference, and low thermal density. A few molten salt thermal storage systems utilize ternary nitrates (Hitec) with a melting point of 142℃, but these have drawbacks including a low decomposition temperature (nitrites begin to oxidize above 450℃) and low thermal density. To reduce the risk of freezing and clogging in molten salt thermal storage systems, researchers both domestically and internationally have been developing low-melting-point molten salts in recent years. In particular, Beijing University of Technology has developed low-melting-point binary, ternary, and quaternary salts. These mixed molten salts have a melting point reduced to approximately 90–140℃, a decomposition temperature between 560–630℃, and a long-term minimum operating temperature reduced to 150–200℃, with a maximum operating temperature between 500–600℃. This significantly improves the thermal temperature difference and thermal density, thereby substantially reducing the cost of thermal storage materials and systems. The U.S. Department of Energy has invested $84.51 million in research and development of high-temperature chlorides through the Coal FIRST initiative, the DAYS program, and the third-generation solar thermal power generation program. However, chlorides face technical bottlenecks due to their high melting point (400°C) and corrosiveness. High-temperature mixed carbonates have also been developed both domestically and internationally, but these formulations all contain precious metal lithium salts, resulting in high costs and high viscosity.

[0009] In summary, current research and applications of molten salts both domestically and internationally focus on decomposition temperatures below 650℃, which cannot meet the requirements of high-parameter power generation such as supercritical carbon dioxide power generation and ultra-supercritical steam power generation, which require high-temperature molten salts with a wide liquid temperature range (decomposition temperature ≥700℃ and melting point ≤150℃). Therefore, it is essential to develop a lithium-free, low-melting-point, high-decomposition-temperature, wide-temperature-range molten salt thermal storage medium. Summary of the Invention

[0010] The technical problem to be solved by this invention is that it does not contain precious metal salts and improves the thermal stability decomposition temperature.

[0011] To address the aforementioned technical problems, this invention provides a formulation for a mixed molten salt.

[0012] A quaternary mixed molten salt heat transfer and storage medium is a wide liquid temperature range quaternary mixed molten salt heat transfer and storage medium, comprising 30-70wt% potassium nitrate, 20-60wt% sodium nitrate, 5-45wt% sodium nitrite, and 0.05-20wt% sodium carbonate.

[0013] The beneficial effects of this invention are as follows:

[0014] The mixed molten salt prepared by the technical solution of the present invention has a significantly increased decomposition temperature (decomposition temperature range: 710-745℃). When applied to solar thermal power generation systems, it broadens the applicable temperature range of molten salt and improves thermal stability.

[0015] 2. The decomposition temperature of the mixed molten salt KNO3-NaNO3-NaNO2-Na2CO3 prepared by the technical solution of the present invention is significantly higher than that of common heat storage materials Solar salt and Hitec salt, while the melting point is lower than that of Solar salt. Attached Figure Description

[0016] Figure 1 Example 1: DSC curve of mixed molten salt.

[0017] Figure 2 Example 1: TG curve of mixed molten salt.

[0018] Figure 3 Example 1: Specific heat measurement results of quaternary mixed molten salt.

[0019] Figure 4 Example 1: Viscosity measurement results of quaternary mixed molten salt.

[0020] Figure 5 Example 2: DSC curve of mixed molten salt.

[0021] Figure 6 Example 2: TG curve of mixed molten salt. Detailed Implementation

[0022] The present invention will now be described in further detail with reference to specific embodiments.

[0023] This invention provides a mixed molten salt formulation for use as a heat transfer and storage medium in a solar thermal power generation system. The formulation mainly contains potassium nitrate, sodium nitrate, sodium nitrite, and sodium carbonate.

[0024] The mechanism by which this technology increases the decomposition temperature is mainly as follows: The decomposition temperature of a single-component nitrate is too low. However, adding a carbonate with a high decomposition temperature to the nitrate to form a eutectic mixed molten salt can significantly increase its decomposition temperature. This eutectic mixed molten salt can ensure stable and uniform thermophysical properties of the phase and components over a wide operating temperature range. Different compositions and proportions of the mixed molten salt result in different thermophysical properties. Therefore, in preparing mixed molten salts, the requirements of all parties should be weighed, and the types and proportions of components should be carefully selected.

[0025] The specific implementation method of this series of quaternary mixed molten salts is as follows:

[0026] First, a mixed molten salt was prepared by mixing four nitrates (KNO3, NaNO3, NaNO2, Na2CO3) in different mass ratios. The specific procedure was as follows: the four components were weighed using a high-precision analytical balance and thoroughly mixed and ground. Then, the mixture was placed in a drying oven for constant temperature drying at 150°C for 96 hours to allow the moisture in the mixture to escape. The dried molten salt was then placed in a muffle furnace and heated to 500°C for 12 hours to ensure complete melting and homogeneous mixing; this is the static melting method. After cooling, the mixed molten salt was removed. Since the molten salt forms a hard solid crystalline salt after melting and cooling, this invention uses an ultrafine pulverizer to pulverize the mixture, achieving a sample fineness of 20-200 mesh. This ensures both fineness and thorough mixing, thus guaranteeing experimental accuracy. Finally, the pulverized molten salt was placed in a drying oven for constant temperature drying before use in experiments.

[0027] Example 1

[0028] This mixed molten salt consists of 43 wt% potassium nitrate, 38 wt% sodium nitrate, 18 wt% sodium nitrite, and 1 wt% sodium carbonate. DSC (Differential Scanning Calorimetry) analysis shows its melting point to be 144.69℃, and TG (Thermogravimetric Analysis) analysis shows its decomposition temperature to be 745.35℃. Figure 1 The DSC curve of this sample is shown. Figure 2 The TG curve of this sample is shown. Figure 3 This is the specific heat test result for this sample. Figure 4 This is the viscosity measurement result for the sample.

[0029] Compared to Solar Salt, this formulation has a decomposition temperature increased by approximately 140℃, and compared to Hitec Salt, it has a decomposition temperature increased by approximately 210℃, offering a wider operating temperature range. Compared to common thermal storage materials, its average specific heat is 1.3667 J / (g·K), and its energy storage density is 710.684 kJ / kg, significantly higher than that of molten salt thermal storage media such as Solar Salt and Hitec. The sensible heat storage cost is 18.81 yuan / (kW·h), making this quaternary mixed molten salt relatively cost-effective.

[0030] Example 2

[0031] This mixed molten salt consists of 48 wt% potassium nitrate, 39 wt% sodium nitrate, 12 wt% sodium nitrite, and 1 wt% sodium carbonate. DSC (Differential Scanning Calorimetry) analysis shows its melting point to be 136.17℃, and TG (Thermogravimetric Analysis) analysis shows its decomposition temperature to be 712.95℃. Figure 5 The DSC curve of this sample is shown. Figure 6 The TG curve of this sample is shown. Compared to Solar Salt, the decomposition temperature of this formulation is increased by about 110℃, and compared to Hitec Salt, the decomposition temperature of this formulation is increased by about 180℃, exhibiting a wider operating temperature range.

[0032] The scope of protection of this invention is not limited to the above embodiments. Any technical modifications made based on the technical principles of this invention fall within the scope of protection of this invention.

Claims

1. A quaternary mixed molten salt heat transfer and storage medium, characterized in that: It is a mixed molten salt heat transfer and storage medium with a wide liquid temperature range, including 43-48 wt% potassium nitrate, 38-39 wt% sodium nitrate, 12-18 wt% sodium nitrite, and 1 wt% sodium carbonate.

2. The method for preparing a quaternary mixed molten salt heat transfer and storage medium according to claim 1, characterized in that: 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℃ and the heating time to 96 hours to allow the moisture in the mixture to escape. Place the dried molten salt in a muffle furnace and heat it to 500℃ for 12 hours to completely melt and mix the mixture evenly. This is the static melting method. After the mixed molten salt cools down, take it out. Use an ultrafine pulverizer to pulverize the mixture to a fineness of 20-200 mesh.

3. The application of the quaternary mixed molten salt heat transfer and storage medium as described in claim 1, for supercritical carbon dioxide power generation and ultra-supercritical steam power generation.

Images