Low-melting ternary mixed molten salt heat transfer and heat storage medium

Abstract

A low-melting-point ternary mixed molten salt heat transfer and storage medium belongs to the field of physical heat transfer and energy storage technology in high-tech applications. The high decomposition temperature mixed molten salt heat transfer and storage medium has the following component ratio: 52.25 wt% potassium nitrate, 42.75 wt% sodium nitrite, and 5 wt% calcium nitrate. This mixed molten salt has a decomposition temperature of approximately 601.5℃ and a melting point of 120.3℃, exhibiting a wide operating temperature range. Furthermore, compared to common heat storage materials, its average specific heat is 1.59 J / (g·K), and its thermal diffusivity is approximately 0.19 mm². 2 / s, thermal conductivity of about 0.56W / (m·K), viscosity and density are not significantly different from existing heat storage media.

Description

Technical Field

[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] Energy is the most important foundation for national economic development, but the large-scale development and utilization of conventional fossil fuels has led to their gradual depletion. As the world's second-largest energy producer and consumer, my country faces a particularly severe energy crisis. Currently, my country's per capita coal and hydropower resources are only 50% of the world average, while its per capita oil and natural gas resources are only about 1 / 15 of the world average. Uneven distribution of energy resources, difficulties in development, and outdated and inefficient energy utilization technologies contribute to my country's faster energy consumption rate than other countries. The ever-increasing energy demand forces us to find a way to break through the energy crisis. While vigorously developing energy-saving and energy-efficient technologies, we must also focus on developing renewable energy sources such as solar and wind power, and pursue a path of sustainable development—this is an inevitable choice.

[0003] Concentrated solar thermal power generation (CSP) is widely recognized as one of the most promising forms of power generation because it can be combined with large-scale thermal storage technology to provide continuous, stable, and adjustable high-quality electricity. Currently, the development of CSP technology is slow due to its short development history, high technical barriers, and an immature market for auxiliary equipment. In the future, when renewable energy generation completely replaces conventional energy generation, CSP can fully assume the functions of current thermal power generation and solve the economic problems of long-distance transmission of wind and solar power. It is precisely this characteristic that makes CSP a focus of international renewable energy research.

[0004] Molten salt energy storage technology uses raw materials such as nitrates as heat transfer media to store or generate energy by converting the heat energy generated by new energy sources with the internal energy of molten salt. It is generally combined with solar thermal power generation systems to enable solar thermal power generation systems to have energy storage and nighttime power generation capabilities, meet the peak shaving needs of the power grid, and has strong economic advantages. It has been commercially applied in developed countries such as Spain, Italy and other European regions and parts of North America.

[0005] Currently, commercially available concentrated solar power (CSP) technology uses solar salt (NaNO3-KNO3) as the heat transfer and storage medium (with a maximum operating temperature of 565℃) and employs the Rankine cycle for heat-to-work conversion. However, due to the special properties of the water medium (critical point: 22MPa, 374℃) and limitations in material strength and processing, the biggest challenge facing solar thermal power generation is its high cost. Increasing the molten salt storage / release temperature and decreasing the melting temperature are effective technical approaches to improve the energy conversion efficiency and economics of electrothermal molten salt storage for peak shaving, combined heat and power (CHP), heat pump energy storage, deep peak shaving in thermal power plants, and solar thermal power generation. Several typical common mixed molten salt systems are shown in Table 1-1. Hitec XL has the lowest melting point of 130℃, but the low-melting-point ternary mixed molten salt in this invention has a melting point reduced to 120℃, which can more effectively improve economic efficiency.

[0006] Table 1-1

[0007]

[0008] Summary of the Invention

[0009] The technical problem to be solved by this invention is to eliminate the presence of precious metal salts, reduce the melting point, lower the cost of solar thermal power generation and industrial heat storage, and maximize its thermal stability.

[0010] To address the aforementioned technical problems, this invention provides a formulation for a mixed molten salt.

[0011] The low-melting-point ternary mixed molten salt heat transfer and heat storage medium is characterized by comprising 52.25 wt% potassium nitrate, 42.75 wt% sodium nitrite, and 5 wt% calcium nitrate.

[0012] The beneficial effects of this invention are as follows:

[0013] The mixed molten salt prepared by the technical solution of the present invention has a lower melting point. When applied to a solar thermal power generation system, it will greatly reduce the cost of the heat transfer and heat storage system, simplify the initial operation procedure of the system, and increase the safety and stability of the entire system.

[0014] 2. The melting point of the mixed molten salt prepared by the technical solution of the present invention is lower than that of common heat storage material Hitec salt. The melting point of KNO3-NaNO2-Ca(NO3)2 is reduced, about 100°C lower than that of Solar Salt and about 22°C lower than that of HITEC. Attached Figure Description

[0015] Figure 1 TG curve of mixed molten salt.

[0016] (The mass ratio is KNO3:NaNO2:Ca(NO3)2 = 0.5225:0.4275:0.05)

[0017] Figure 2 DSC curves of mixed molten salts.

[0018] (The mass ratio is KNO3:NaNO2:Ca(NO3)2 = 0.5225:0.4275:0.05)

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

[0020] Figure 4 Example 1: Measurement results of the thermal diffusivity of a ternary mixed molten salt.

[0021] Figure 5 Example 1: Measurement results of the thermal conductivity of a ternary mixed molten salt.

[0022] Figure 6 Example 1: Viscosity measurement results of ternary mixed molten salt. Detailed Implementation

[0023] The present invention will now be described in further detail with reference to specific embodiments.

[0024] 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 nitrite, and calcium nitrate. The mixed molten salt formulation contains 52.25 wt% potassium nitrate, 42.75 wt% sodium nitrite, and 5 wt% calcium nitrate. This mixed molten salt has a melting point of approximately 120.3℃, a decomposition temperature of approximately 601.5℃, an average specific heat of approximately 1.59 J / (g·K), and a thermal conductivity of approximately 0.56 W / (m·K).

[0025] The mechanism by which this technology lowers the melting point is mainly as follows: Single-component molten salts have excessively high melting points, while mixing several molten salts to form a eutectic molten salt mixture can significantly lower the melting point of the eutectic molten salt mixture. This eutectic molten salt mixture can ensure stable and uniform thermophysical properties of the phases 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 in the mixed molten salt should be carefully selected.

[0026] The specific implementation method of this series of ternary mixed molten salts is as follows:

[0027] First, a ternary mixed nitrate (KNO3, NaNO2, Ca(NO3)2) was prepared according to different molar ratios. These molar ratios were then converted to mass ratios to initially prepare mixed molten salts with eight different component proportions. The specific operation method was as follows: the three 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 90°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 400°C for 12 hours to ensure complete melting and homogeneous mixing; this is the static melting method. After the mixed molten salt cooled, it 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, thereby guaranteeing experimental accuracy. Finally, the pulverized molten salt was placed in a drying oven for constant temperature drying before use in experiments. After extensive experimental analysis, a optimal molten salt formulation was selected. A total of 10 ternary mixed molten salts were prepared, with salt No. 1 having the lowest melting point; therefore, salt No. 1 was chosen as the final research subject.

[0028] Serial Number <![CDATA[Mass ratio (KNO3: NaNO2: Ca(NO3)2)]]> Melting point (°C) 1 52.25：42.75：5 120.3 2 49.5：40.5：10 —— 3 46.75：48.25：15 —— 4 44：36：20 153.9 5 41.25：33.75：25 169.96 6 38.5：31.5：30 165.8 7 35.75：29.25：35 164.2 8 33：27：40 166.1 9 30.25：24.75：45 160.9 10 27.5：22.5：50 158.74

[0029] Example 1

[0030] This mixed molten salt consists of 52.25 wt% potassium nitrate, 42.75 wt% sodium nitrite, and 5 wt% calcium nitrate. DSC (Differential Scanning Calorimetry) analysis shows its melting point to be 120.3℃, and TG (Thermogravimetric Analysis) analysis shows its decomposition temperature to be 601.5℃. Figure 1 The TG curve of this sample is shown. Figure 2 The DSC curve of this sample is shown. Figure 3 This is the specific heat test result for this sample. Figure 4 This is the result of the thermal diffusivity measurement of the sample. Figure 5 This is the measurement result of the thermal conductivity of the sample. Figure 6 This is the viscosity measurement result for the sample.

[0031] Compared to Solar Salt, this formulation has a melting point 100°C lower and a decomposition temperature nearly 40°C higher. Compared to Hitec Salt, its melting point is nearly 21°C lower, resulting in a wider operating temperature range. Compared to common thermal storage materials, its average specific heat is 1.59 J / (g·K), making this ternary mixed molten salt relatively cost-effective. Its thermal conductivity is approximately 0.56 W / (m·K), exhibiting good thermal conductivity, and its viscosity and density are not significantly different from existing thermal storage media.

[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 low-melting-point mixed molten salt heat transfer and storage medium, characterized in that: Its composition includes 52.25 wt% potassium nitrate, 42.75 wt% sodium nitrite, and 5 wt% calcium nitrate.

2. The method for preparing the low-melting-point mixed molten salt heat transfer and storage medium according to claim 1, characterized in that: Weigh the three components and mix and grind them thoroughly. Then place them in a drying oven for constant temperature drying. Set the heating temperature to 90℃ 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 400℃ for 12 hours to completely melt and mix the mixture evenly. This is the static melting method. After the mixed molten salt cools, take it out and use an ultrafine pulverizer to pulverize the mixture to a fineness of 20-200 mesh.

3. The application of the low-melting-point mixed molten salt heat transfer and storage medium as described in claim 1, for solar thermal power generation and industrial heat storage.

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